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DISPERSION OF CELLULOSE NANOFIBERS
IN BIOPOLYMER BASED
NANOCOMPOSITES
By
Bei Wang
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Faculty of Forestry
University of Toronto
© Copyright by Bei Wang 2008
ii
DISPERSION OF CELLULOSE NANOFIBERS IN BIOPOLYMER
BASED NANOCOMPOSITES
Ph.D., 2008
Bei Wang
Faculty of Forestry
University of Toronto
ABSTRACT
The focus of this work was to understand the fundamental dispersion mechanism of cellulose
based nanofibers in bionanocomposites. The cellulose nanofibers were extracted from soybean
pod and hemp fibers by chemo-mechanical treatments. These are bundles of cellulose
nanofibers with a diameter ranging between 50 to 100 nm and lengths of thousands of
nanometers which results in very high aspect ratio. In combination with a suitable matrix
polymer, cellulose nanofiber networks show considerable potential as an effective reinforcement
for high quality specialty applications of bio-based nanocomposites.
Cellulose fibrils have a high density of –OH groups on the surface, which have a tendency
to form hydrogen bonds with adjacent fibrils, reducing interaction with the surrounding matrix.
The use of nanofibers has been mostly restricted to water soluble polymers. This thesis is
focused on synthesizing the nanocomposite using a solid phase matrix polypropylene (PP) or
polyethylene (PE) by hot compression and poly (vinyl alcohol) (PVA) in an aqueous phase by
film casting. The mechanical properties of nanofiber reinforced PVA film demonstrated a 4-5
fold increase in tensile strength, as compared to the untreated fiber-blend-PVA film.
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It is necessary to reduce the entanglement of the fibrils and improve their dispersion in the
matrix by surface modification of fibers without deteriorating their reinforcing capability.
Inverse gas chromatography (IGC) was used to explore how various surface treatments would
change the dispersion component of surface energy and acid-base character of cellulose
nanofibers and the effect of the incorporation of these modified nanofibers into a biopolymer
matrix on the properties of their nano-composites. Poly (lactic acid) (PLA) and
polyhydroxybutyrate (PHB) based nanocomposites using cellulose nanofibers were prepared by
extrusion, injection molding and hot compression. The IGC results indicated that styrene maleic
anhydride coated and ethylene-acrylic acid coated fibers improved their potential to interact
with both acidic and basic resins. From transmission electron micrograph, it was shown that the
nanofibers were partially dispersed in the polymer matrix. The mechanical properties of the
nanocomposites were lower than those predicted by theoretical calculations for both nanofiber
reinforced biopolymers.
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ACKNOWLEDGEMENTS
I would like to acknowledge Professor Mohini Sain, my supervisor, for his guidance,
encouragement and patience during all stages of this project.
I am also grateful to Professor Kristiina Oksman for following the progress and providing her
technical guidance and valuable contributions throughout the research program.
Special thanks to my graduate committee members: Prof. Martin Hubbes, Prof. D.N. Roy, Prof.
Christine Allen, Dr. R. S. Jazi Hamid and Dr. Hamdy Khalil, for providing a motivating and
enthusiastic environment during many discussions and their helpful feedbacks. My Ph.D.
comprehensive exam committee member, Prof. Paul Cooper, is acknowledged.
I acknowledge the financial support from Natural Sciences and Engineering Research Council
(NSERC) and the University of Toronto Faculty Scholarship. I also would like to acknowledge
the Departments of Chemical Engineering and Applied Chemistry, Botany, Geology, Pulp and
Paper Centre at the University of Toronto.
I am grateful to the faculty, staff and my colleagues in the Faculty of Forestry for their
assistance in every aspect of my program.
v
DEDICATION
I dedicate this thesis to my loving family who encouraged me to excel in a scholarly career.
I wish to express my gratitude to my parents, Jiajun Wang and Xue Fang for their unconditional
love and support throughout the tenure of my doctoral studies.
vi
TABLE OF CONTENTS
1. INTRODUCTION ................................................................................................................. 1
2. OBJECTIVES........................................................................................................................ 5
3. LITERATURE REVIEW ...................................................................................................... 6
3.1 Properties of Cellulose Nanofibers.................................................................................. 6
3.1.1 Chemical composition of natural fibers..................................................................... 6
3.1.2 Defining nanofibers and microfibers for structural application ................................ 8
3.1.3 Isolation of Cellulose Nanofibers ............................................................................ 10
3.2 Dispersion of Cellulose Nanofibers in Plastic Matrix ................................................... 13
3.2.1 Challenges of dispersion of nanofibers in plastic matrix ........................................ 14
3.2.2 Chemical modifications of nanofibers..................................................................... 14
3.2.3 Physical modifications of nanofibers....................................................................... 22
3.2.4 Other methods to improve the dispersion................................................................ 25
3.3 Development of Bionanocomposites............................................................................. 26
3.3.1 Challenges of the nanocomposites development..................................................... 26
3.3.2 Characteristics of biopolymers ................................................................................ 28
3.3.3 Application of bionanocomposites .......................................................................... 31
3.4 Summary........................................................................................................................ 34
4. STUDY OF STRUCTURAL MORPHOLOGY OF CELLULOSE FIBER FROM THE
MICRO TO THE NANOSCALE................................................................................................ 35
4.1 Introduction ................................................................................................................... 35
4.2 Experimental.................................................................................................................. 39
4.2.1 Materials .................................................................................................................. 39
4.2.2 Individualization process ......................................................................................... 39
4.2.3 Microscopy characterization.................................................................................... 42
4.2.4 Chemical characterization of fibers ......................................................................... 43
4.2.5 Spectroscopy............................................................................................................ 43
4.2.6 X-ray analysis .......................................................................................................... 43
4.2.7 Estimation of average degree of polymerization of the cellulose in nanofibers...... 44
4.3 Results and Discussion .................................................................................................. 44
4.3.1 Individualization of cellulose nanofibers................................................................. 44
vii
4.3.2 Chemical characterization of individualized nanofibers ......................................... 53
4.3.3 X-Ray Diffraction.................................................................................................... 59
4.4 Conclusions ................................................................................................................... 64
5. DISPERSION MECHANISM OF NANOFIBERS ............................................................ 66
5.1 Introduction ................................................................................................................... 66
5.2 Dispersion of Nanofibers in the Polymers..................................................................... 68
5.2.1 Dispersion of nanofibers in water soluble polymer-PVA........................................ 70
5.2.2 Dispersion of nanofibers in commodity polymers .................................................. 77
5.2.3 Chemical dispersion of nanofibers in biopolymers ................................................. 80
5.2.4 Physical dispersion of nanofibers in biopolymers ................................................. 105
5.3 Conclusions ................................................................................................................. 111
6. NANOFIBERS REINFORCING CAPABILITY ON POLYMERS ................................ 113
6.1 Introduction ................................................................................................................. 113
6.2 Novel High-strength Nanofiber Thin Mats.................................................................. 114
6.2.1 Nanofiber thin mat production............................................................................... 114
6.2.2 Tensile test ............................................................................................................. 114
6.2.3 Mechanical properties of nanofiber thin mat......................................................... 114
6.3 Mechanical Behaviour of Nanofiber/PVA Film.......................................................... 116
6.3.1 Tensile test ............................................................................................................. 116
6.3.2 Mechanical performance of nanofilm.................................................................... 116
6.3.3 Dynamic mechanical analysis (DMA)................................................................... 118
6.3.4 DMA mechanical properties.................................................................................. 118
6.4 Thermogravimetric Analysis ....................................................................................... 120
6.5 Mechanical Behaviour of Nanocomposites ................................................................. 121
6.5.1 Mechanical testing................................................................................................. 121
6.5.2 Mechanical performance of nanocomposites ........................................................ 121
6.6 Mechanical Behaviour of Bio-nanocomposites........................................................... 123
6.6.1 Tensile testing........................................................................................................ 123
6.6.2 Mechanical properties of bio-nanocomposites ...................................................... 123
6.7 Dynamic Mechanical Analysis of Bio-nanocomposites.............................................. 128
6.8 Conclusions ................................................................................................................. 130
7. CONCLUSIONS ............................................................................................................... 132
viii
LIST OF FIGURES
Figure 1. Hydrolysis of silane..................................................................................................... 16
Figure 2. Hypothetical reaction of fiber and silane .................................................................... 16
Figure 3. Reaction of biofiber with MAPP................................................................................. 18
Figure 4. A possible hypothetical representation of the esterfication of cellulose ..................... 19
Figure 5. Schematic sketch of surfactant molecule .................................................................... 20
Figure 6. Schematic sketch of surfactant molecules in water..................................................... 21
Figure 7. Chemical structure of PVA. ........................................................................................ 29
Figure 8. Chemical structure of PLA.......................................................................................... 29
Figure 9. Chemical structure of PHB.......................................................................................... 31
Figure 10. Hierarchal structure of cellulose to fiber bundle. ...................................................... 37
Figure 11. Isolation of nanofibers............................................................................................... 40
Figure 12. Scanning electron micrographs of: (a) untreated fiber, (b) after acid and alkaline
treatment, (c) after cryo-crushing, (d) after defibrillation, (e) after bleaching, (f) bleaching
followed by cryo-crushing, (g) bleaching followed by cryo-crushing and defibrillation.... 46
Figure 13. The impact head set of homogenizer......................................................................... 47
Figure 14. 3D version of the impact head and the passenger head of homogenizer. ................. 47
Figure 15. Scanning electron micrographs of: bleached hemp nanofibers after 5 (a), 10 (b), 15
(c) and 20 (d) passes during the defibrillation. .................................................................... 48
Figure 16. Transmission electron micrographs of hemp nanofibers (a) unbleached, (b) bleached
under the same magnification (15,000×). ............................................................................ 50
Figure 17. Determination of relative viscosities at different concentrations (a) unbleached, (b)
bleached nanofibers. ............................................................................................................ 51
Figure 18. Atomic force micrographs of unbleached hemp nanofibers (a) force mode image, (b)
height mode image............................................................................................................... 52
Figure 19. Atomic force micrographs of SBN............................................................................ 52
Figure 20. Size distribution of hemp nanofibers (a) unbleached, (b) bleached. ......................... 53
Figure 21. Chemical changes of soybean stock.......................................................................... 56
Figure 22. Lignin content changes of soybean stock.................................................................. 56
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Figure 23. Transmission electron micrographs of hemp nanofibers (a) under 17.5 % alkali
extraction, (b) under 12 % alkali extraction (15,000×). ...................................................... 57
Figure 24. FTIR spectra of hemp fibers (a) untreated, (b) after acid and alkaline treatment and
(c) after bleaching. ............................................................................................................... 59
Figure 25. X-ray diffractometry and crystallinity estimation after each stage of chemo-
mechanical treatment for hemp fiber................................................................................... 62
Figure 26. X-ray diffractometry and crystallinity estimation after each stage of chemo-
mechanical treatment for soybean stock.............................................................................. 63
Figure 27. X-ray crystallographs to demonstrate the crystallinity of (a) soybean stock nanofiber,
(b) hemp nanofiber. ............................................................................................................. 64
Figure 28. The hydrogen bonds between the cellulose chains. .................................................. 66
Figure 29. Dispersion of nanofibers in the plastic matrix........................................................... 67
Figure 30. In aqueous phase - film casting: PVA/nanofibers. .................................................... 70
Figure 31. Scheme of film formation.......................................................................................... 71
Figure 32. A possible chemical scheme for the hydrophobization............................................. 72
Figure 33. Scanning electron micrographs of freeze-dried SBN samples: (a) uncoated and (b)
ethylene-acrylic oligomer coated......................................................................................... 75
Figure 34. Transmission electron micrograph of diluted suspension of SBN: (a) in water and (b)
in ethylene-acrylic oligomer emulsion under the same magnification (20,000×). .............. 76
Figure 35. Processing of nanocomposites. ................................................................................. 78
Figure 36. Optical microscopy (125×) showing the solid phase nanocomposite: (a) soybean
stock as reinforcement without coating and (b) soybean stock coated with ethylene-acrylic
oligomer............................................................................................................................... 79
Figure 37. An overview of the PE/SBN nanocomposites taken with SEM................................ 80
Figure 38. Chemical structure of guanidine hydrochloride. ....................................................... 83
Figure 39. A possible chemical scheme for the EAA-dispersion mechanism............................ 83
Figure 40. A suggested chemical scheme for the reaction of styrene maleic anhydride modified
cellulose. .............................................................................................................................. 84
Figure 41. Visual comparison of freeze-dried HPN samples: (a) uncoated; (b) SMA coated. .. 85
Figure 42. Processing of bio-nanocomposites. ........................................................................... 86
Figure 43. Scanning electron micrographs of freeze-dried HPN samples: (a) uncoated and (b)
SMA coated. ...................................................................................................................... 103
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Figure 44. Transmission electron micrograph of the PLA/SMA-coated HPN composites: (a) an
overview and (b) detailed view.......................................................................................... 104
Figure 45. Scanning electron micrograph of freeze dried HPN: (a) after air-plasma treatment
(×800) and (b) after the sonification process (×700). ........................................................ 108
Figure 46. Processing of bio-nanocomposites. ......................................................................... 109
Figure 47. Storage modulus curves from DMA analysis of the nanocomposites..................... 120
Figure 48. TGA curves under nitrogen for PVA and PVA/SBN nanocomposites................... 121
Figure 49. Experimentally measured tensile modulus data compared to theoretical predictions
by Halpin-Tsai. .................................................................................................................. 126
Figure 50. Storage modulus curves from DMA analysis of the nanocomposites..................... 129
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LIST OF TABLES
Table 1. Chemical composition and structural parameters of natural fibers ................................ 7
Table 2. Fibers dimension and strength chart for inorganic fillers, long fibers and cellulose
nanofibers ............................................................................................................................ 10
Table 3. Percent concentration required to reduce the surface tension of water to indicated
values ................................................................................................................................... 20
Table 4. Degree of polymerization of unbleached and bleached HPN....................................... 51
Table 5. Chemical analysis of hemp fibers after selective chemical treatments......................... 54
Table 6. Physicochemical properties of the IGC probes used in the present study .................... 88
Table 7. Dispersion component, γsd, of the surface energy of lignocellulosic particles at different
temperatures......................................................................................................................... 96
Table 8. Free energy of adsorption, ∆GAB, of the acid-based probes at different temperatures. 98
Table 9. Enthalpy of adsorption, ∆HAB....................................................................................... 99
Table 10. Surface acid-based characteristics, KA and KD. .......................................................... 99
Table 11. Values of specific interaction parameter..................................................................... 99
Table 12. Dispersion component, γsd, of the surface energy of lignocellulosic particles at
different temperatures........................................................................................................ 110
Table 13. Surface acid-based characteristics, KA and KD. ........................................................ 111
Table 14. Values of specific interaction parameter................................................................... 111
Table 15. Mechanical properties of nanofiber thin mat. ........................................................... 116
Table 16. The mechanical properties of nanocomposites in aqueous phase film casting......... 118
Table 17. DMA results of the nanocomposites......................................................................... 119
Table 18. Mechanical properties of nanocomposites in solid phase melt-mixing. ................... 122
Table 19. Tensile properties of the injected bio-nanocomposites............................................. 124
Table 20. Tensile properties of the extruded bio-nanocomposites. .......................................... 128
Table 21. DMA results of the bio-nanocomposites. ................................................................. 130
xii
LIST OF ABBREVIATIONS
AFM Atomic force microscopy BHPN Bleached hemp nanofiber BSBN Bleached soybean stock nanofiber COONa Sodium carboxylate DMA Dynamic mechanical analysis DP Degree of polymerization EAA Eethylene acrylic acid FTIR Fourier transform infrared spectroscopy GC Gas chromatograph HPN Hemp nanofibers HVF High viscosity formula IGC Inverse gas chromatography LVF Low viscosity formula MAPP Maleated polypropylene/Maleic anhydride grafted polypropylene MM Microfibrillated materials NSERC Natural sciences and engineering research council PCL Polycaprolactone PE Polyethylene PGA Propylene glycol alginates PHA PolyhydroxyalkanoatePHB Poly(β-hydroxybutyrate) PHBV Polyhydroxybutyrate-valerate PLA Poly (lactic acid) PP Polypropylene PPMA Polypropylene-maleic anhydride copolymer PVA Poly (vinyl alcohol) PVC Poly (vinyl chloride) PXRD Powder X-ray diffraction SBN Soybean stock nanofiber S.D. Standard deviation. SEM Scanning electron microscope SESBS-MA Maleic-anhydride-grafted styrene-ethylene-butylene-styrene SMA Styrene maleic anhydride T Temperature TEM Transmission electron microscopy Tg Glass transition temperature TGA Thermogravimetric analysis THF Tetrahydrofuran UHP Untreated hemp fiber UHPN Untreated hemp nanofiber UNF Untreated fiber USB Untreated soybean stock fiber USBN Untreated soybean stock nanofiber
1. INTRODUCTION
Miniaturization is a continuing trend in the development of technology. The prefix “nano”
has become applied to new classes of materials intended for manufacturing, e.g. nano-materials
and nanocomposites. Unfortunately, not many of the most recent developments of this nature
are able to satisfy the core concept of sustainability. One way to address issues related to
sustainability is to incorporate renewable materials as miniaturized elements of construction
materials (Sain and Oksman 2006). The backbone of a plant or tree is a polymeric carbohydrate
with an abundance of tiny structural entities known as “cellulose fibrils”. These fibrils are
comprised of different hierarchical microstructures commonly known as nano-sized microfibrils
with high structural strength and stiffness (Wang and Sain 2007). Biopolymers from renewable
resources have attracted much attention lately. Renewable sources of polymeric materials offer
an answer to maintain sustainable development of economically and ecologically attractive
technology. In recent years, scientists and engineers have been working together to use the
inherent strength and performance of these nano-fibrils, combined with natural green polymers,
to produce a new class nano-materials.
The cellulose molecules are always biosynthesized in the form of nanosized fibrils; up to
100 glucan chains aggregate together to form cellulose nano-sized microfibrils or nanofibers
(McCann et al. 1990; 1993; Clowes and Juniper 1968; Stamboulis et al. 2001). The mechanical
performance of cellulose nanofibers in terms of the tensile strength and Young’s modulus is
comparable to other engineering materials such as glass fiber, carbon fiber, etc. Therefore, the
cellulose nanofibers can be considered to be an important structural element of natural cellulose
in a number of applications such as plastic reinforcement, gel forming and thickening agents
1
2
(Eichhorn et al. 2001; Dinand et al. 1999; Yoshinaga et al. 1997). Furthermore, a cellulose
nanofiber has more than 200 times the surface area of isolated softwood cellulose (Krieger 1990)
and possesses higher water holding capacity, higher crystallinity, higher tensile strength, and a
finer web-like network. In combination with a suitable matrix polymer, cellulose nanofiber
networks show considerable potential as an effective reinforcement for high quality specialty
application of bio-based composites. Conventional polymers derived synthetically from
petroleum constitute a growing concern in view of problems associated with their disposal.
Fully biodegradable synthetic polymers have been commercially available since 1990, such as
PVA, PLA and PHB. In view of better environmental characteristics at the end of service life,
biodegradable plastics obtained from plant sources are becoming increasingly attractive. Typical
examples of such biopolymers include plastics derived from natural sources like cellulose,
soybean and starch. The fairly new idea of bionanocomposites, in which the reinforcing material
has nanometer dimensions, is emerging to create the value-added materials with superior
performance and extensive applications. The cellulose nanofiber reinforced nanocomposites can
be used in medical devices such as biocompatible drug delivery system, blood bags, cardiac
devices, and valves as reinforcing biomaterials. Biological tissues are made from nano-sized
materials and given an interest in manufacturing synthetic nanocomposites (Hepworth and
Bruce 2000a). Due to their lightweight and high strength; they also can be utilized as high
strength components in aerospace and automotive sector.
The use of cellulose nanofibers as nanoreinforcement is a new field in nanotechnology, and
as a result there are still some disadvantages. Firstly, the separation of nanoreinforcement
components from natural materials and the associated processing techniques have been limited
to the laboratory scale (Oksman et al. 2006). Secondly, the fiber isolation process consumes a
large amount of energy, water, and chemicals. The production is time consuming and is still
3
associated with low yields. Thirdly, due to their strong hydrogen bonding between cellulose
chains, the nanofibers obtained after chemo-mechanical treatments are kept in water
suspensions. Water is the most widely used carrier to disperse cellulose nanofibers (Dufresne et
al. 2000). Therefore the use of nanofibers has been mostly restricted to water soluble polymers.
Cellulose nanofibers have not been used extensively in the common thermoplastics, as poor
dispersion of the filler in the matrix of a composite material seriously affects its mechanical
properties. But to expand the horizon of bio-based nanocomposites for high-end applications, it
is necessary to reduce the entanglement of the fibrils and improve their dispersion in the solid
phase polymer matrix by surface modification of nanofibers without deteriorating their
reinforcing capability. It has been reported that the surface modifications of cellulose nanofibers
to make them compatible with non-polar solvents/non-polar polymers, such as polyolefins or
other commodity polymer has been attempted (Goussé et al. 2004). The treatment of the fibers
may be bleaching, grafting of monomers, acetylation, and so on. Various processing
aids/coupling agents such as stearic acid, mineral oil, and maleated ethylene have been used
(Saheb and Jog 1999). The compatibilizer can be polymers with functional groups grafted onto
the chain of the polymer for effective stress transfer across the interface. In this way, high
performance composite materials can be processed with a good level of dispersion. Interaction
of cellulose with surfactants has been another way to stabilize cellulose suspensions into non-
polar systems (Heux et al. 2000). In some approaches, corona or plasma discharges have been
used in achieving acceptable dispersion levels (Dong et al. 1993).
This work is based on a novel leading to the generation and isolation of nanofibers from
soybean pod and hemp fiber and their dispersion in a number of polymers, including
biopolymers derived from natural sources. The cellulose nanofibers were extracted from
cellulose fibers by chemo-mechanical treatments. These are bundles of cellulose nanofibers with
4
a diameter ranging between 50 to 100 nm and lengths of thousands of nanometers. Microscopy
techniques, chemical analysis and X-ray diffraction were used to study the structure and
properties of the prepared micro and nanofibers. A series of steps including extrusion and plastic
molding techniques are used to develop the final product. Poor interfacial adhesion between
nanofibers and the polymer matrix leads to a decline in mechanical properties of the
nanocomposites. The surface energy of a material can be described by the sum of a dispersion
component and a specific interaction component (Gulati and Sain 2006). IGC was used to
investigate the effect of the incorporation of modified nanofibers into a biopolymer matrix on
the properties of their surface energy. The fundamental tensile properties of these composites
were then studied in details to investigate the effect of the nanofibers on the tensile properties of
the bionanocomposites.
This project was the first attempt to add solid phase nanofibers into a polymer matrix (non-
polar system) for improving the mechanical properties significantly and was the first attempt to
quantify the surface thermodynamic parameters. The target use of the bionanocomposites in the
present study is in the area of packaging, particularly, in food packaging. The objectives of this
thesis follow the introduction. The chapter three contains a review of published literature
relating to this subject. The literature review was used to guide this thesis in terms of its design,
analysis, and expected trends. Study of structural morphology of cellulose fiber from the micro
to the nanoscale is detailed in chapter four. The dispersion mechanism of nanofibers is proposed
in chapter five. Nanofibers reinforcing capability on polymers follows. This thesis ends with
conclusions and recommendations for future work.
5
2. OBJECTIVES
Nanocomposites incorporating cellulose microfibrils are increasingly of interest in the
composite science community. In this context, poor understanding of dispersion mechanism of
nanofibers in polymer matrix (non-polar system) is occurred. The main purpose of this project
was targeted to add modified nanofibers into a solid phase polymer matrix for improving the
mechanical properties significantly and to quantify the surface thermodynamic parameters. This
information can help discover the reinforcing potential of nanofibers to be used as a value added
product into bionanocomposite applications.
This thesis addresses the following issues:
1. Isolation and extraction cellulose nanofibers from soybean pod and hemp by chemo-
mechanical treatment;
2. Determination of the various levels of high pressure defibrillation, purification and
individualization state effect on fiber surface morphologies;
3. Better understanding the mechanism of surface thermodynamics and interfacial
properties between nanofiber and a polymer matrix; and
4. Investigation the dispersion mechanism of cellulose nanofibers in a solid phase polymer
matrix.
6
3. LITERATURE REVIEW
This chapter presents a review of literature on the dispersion of cellulose nanofibers in a
biopolymer matrix and evaluation their reinforcing potential in nanocomposite manufacturing.
In addition, the details of extraction of cellulose nanofibers, characteristics of biopolymers,
composite manufacturing and the effects of fiber distribution on the composites are discussed.
3.1 Properties of Cellulose Nanofibers
The structural hierarchy of cellulose is investigated in order to analyze the structure of a
microfibril in a logical way. Subsequently, the concept of a fiber as structural material is
discussed. Finally, a formal definition of the term "cellulose microfiber" and "cellulose
nanofiber" is proposed.
3.1.1 Chemical composition of natural fibers
Natural fibers are complex in structure. They are generally lignocellulosic, consisting of
helically wound cellulose microfibrils in an amorphous matrix of lignin and hemi-cellulose.
Table 1 shows natural fibers and their chemical and structural composition. The chemical
composition of natural fibers varies depending upon the type of fiber. Mechanical properties are
determined by the cellulose content and microfibril angle. A high cellulose content and low
microfibril angle are desirable properties of a fiber to be used as reinforcement in polymer
composites (Williams and Wool 2000).
Primarily, fibers contain cellulose, hemicellulose, pectin, and lignin. The properties of each
constituent contribute to the overall properties of the fiber. Cellulose is the common material of
plant cell walls and was first noted as such in 1838 (Dufresne et al. 2000). Cellulose forms the
7
framework of the cell wall which is the primary structural component of plants. Cellulose is a
natural polymer with high strength and stiffness per weight, and it is the building material of
long fibrous cells. Lignin is a polymer of various derivatives of phenyl propane. Hemicelluloses
are branched polymer chains of sugars that include xylose, arabinose, rhamonose, galactose,
glucose and mannose while hemicelluloses cross-link non-cellulosic and cellulosic polymers.
These are amorphous and para-crystalline structures. Pectic substances are amorphous, plastic
and highly hydrophilic substances. Pectins provide cross-links and structural support to the cell
wall. Both the hemicellulosic and pectic materials play important roles in fiber bundle
integration, fiber bundle strength and individual fiber strength as well as water absorbency,
swelling, elasticity and wet strength (Mustată 1997).
Table 1. Chemical composition and structural parameters of natural fibers (Mohanty et al. 2000).
Fiber Cellulose (%)
Hemi-cellulose
(%)
Lignin (%)
Extra-ctives (%)
Ash (%)
Pectin (%)
Wax (%)
Microfibril/spiral
angle (º)
Moisture content
(% w. b.)
BAST Jute 61-71.5 13.6-20.4 12-13 - - 0.2 0.5 8.0 12.6 Flax 71-78.5 18.6-20.6 2.2 2.3 1.5 2.2 1.7 10.0 10.0
Hemp 70.2-74.4 17.9-22.4 3.7-5.7 3.6 2.6 0.9 0.8 6.2 10.8 Ramie 68.6-76.2 13.1-16.7 0.6-0.7 - - 1.9 0.3 7.5 8.0 Kenaf 31-39 15-19 21.5 3.2 4.7 - - - - LEAF Sisal 67-78 10-14.2 8-11 - - 10.0 2.0 20.0 11.0 PALF 70-82 - 5-12 - - - - 14.0 11.8
Henequen 77.6 4-8 13.1 - - - - - - SEED Cotton 82.7 5.7 - - - - 0.6 - - FRUIT
Coir 36-43 0.15-0.25 41-45 - - 3-4 - 41-45 8.0 WOOD
Soft 40-44 25-29 25-31 5 0.2 - - - - Hard 43-47 25-35 16-24 2-8 0.4 - - - -
Each fiber cell consists of a primary cell wall and three secondary cell walls. The primary
8
cell walls are formed during the growth cycle and the secondary cell walls are formed when the
cell has ceased lengthening and begin to thicken. Typical primary plant cell wall is composed of
cellulose microfibrils (9-25 %), an interpenetrating matrix of hemicelluloses (25-50%) and
pectin (10-35 %). Primary cell wall composition as cellulose fibers bound together by molecules
made of sugar units. Approximately 90 % of the cell wall consists of carbohydrates (mostly
pentose and hexose units). Cellulose microfibrils arrangement in the primary wall is random.
Secondary walls are derived from the primary walls by thickening and inclusion of lignin into
the cell wall matrix and occur inside the primary wall. Secondary cell walls of plants contain
cellulose (40-80 %), hemicellulose (10-40 %) and lignin (5-25 %) where cellulose microfibrils
are embedded in lignin. Lignin is the most important constituent in the secondary cell wall.
Cellulose and hemicelluloses appear to be more structurally organized in the secondary cell wall
than in the primary cell wall. It is clear that second cell wall is richer in cellulose than the
primary wall (Brett and Waldron 1996).
3.1.2 Defining nanofibers and microfibers for structural application
Natural fibers exhibit considerable variation in diameter along with the length of individual
filaments. The hydrogen bonds and other linkage provide the necessary strength and stiffness to
the fibers. Each cell wall contains a lignin hemicellulose matrix surrounded by cellulose nano-
sized microfibrils, which are oriented in different directions in the different wall layers
(www.empa.ch). They have diameters in the range of 5-50 nm and lengths of thousands of
nanometers (McCann et al. 1990; Reiter 1998; Hepworth and Bruce 2000). These nanofibers
provide strength to the fibers. A cellulose fiber is made up of bundles of single cellulose fiber,
which has a diameter of 25-30 µm. The single cellulose fiber is made up of bundles of
microfibers, which have diameters of 0.1-1 µm. The microfiber consists of bundles of
9
nanofibers, which has a diameter in the range of 10-70 nm and lengths of thousands of
nanometers (McCann et al. 1990; 1992). Nanofiber is made up of cellulose chains bound by
hydrogen bonding. These cellulose chains have a repeating period along the fiber axis of 1.03
nm (Clowes and Juniper 1968). The plant possesses cell walls where cellulose nanofibers are
embedded in a gel-like matrix of hemicelluloses and pectic polysaccharides (Reiter 1998). The
cellulose molecules are always biosynthesized in the form of nanosized fibrils; up to 100 glucan
chains are grouped together to form elementary fibrils, which aggregate together to form
cellulose nano-sized microfibrils or nanofibers (McCann et al. 1990; 1992; Clowes and Juniper
1968; Stamboulis et al. 2001). These nanofibers provide strength to the plant stem fiber with a
very high aspect ratio. Cellulose nanofibers have great potential to be used as reinforcement.
Their theoretical stiffness has been estimated up to 130 GPa and strength up to 7 GPa (Kroon-
Batenburg et al. 1986; Mark 1967). This would give them a greater energy absorbing capability
than the best synthetic fibers. The mechanical performance of the cellulose nanofibers is
comparable to other engineering materials such as glass fibers, carbon fibers, etc. The cellulose
nanofibers can be considered to be an important structural element of natural cellulose in
engineering application.
The microfiber has been defined as a fiber consisting of continuous cellulose chains with
negligible lignin and hemicellulose content, and having a diameter of 0.1-1 µm, with a
minimum corresponding length of 2-20 µm (Chakraborty 2004). The microfiber with length-to-
diameter ratio (L/D) more than 20 shows a good reinforcing potential in the structural
application. Cell wall of an individual fiber consists of bundles of macro fibrils, which are
strands of nano-sized microfibrils. The molecular arrangements of these fibrillar bundles are so
small that the average diameter of bundle is about 10 nm. These strong linear ribbons like
structures are very stiff and form stable fibrils. Traditionally cellulose nanofibers have been
10
defined as purely crystalline cellulose chains having diameters within the range of 5 to 40 nm
with the lengths of few microns (Preston 1951; Rydholm 1965).
Table 2 shows a comparison of size and mechanical properties of inorganic fibers, natural
fibers and cellulose nanofibers. This table helps us to visualize that micro and nano scale of the
cellulose fibrils can have almost equivalent mechanical strength and modulus as carbon
nanofibers or glass fibers, if extracted properly form the cell walls without degrading the
cellulose chains.
Table 2. Fibers dimension and strength chart for inorganic fillers, long fibers and cellulose nanofibers (http://www.gov.mb.ca/agriculture/crops/hemp/bko07s02.html).
Mean Length of Fiber (mm)
Mean Width of Fiber
Theoretical Tensile Strength
(MPa)
Theoretical Young’s
Modulus (GPa)
Flax 30 20 µm 1081 100 Hemp 20 22 µm 902 69 Wood 3.6 35 µm - 40 Glass Fiber E - 9 µm 1700 70-85 Kevlar Fiber - - 2800 60-200 Carbon Fiber - 70-500 nm 3445 230-490 Large nano-sized microfibrils - 20-100 nm - 130
Nano-sized microfibrils (cellulose nanofibers)
36-40 molecular chains 5-50 nm 7500 130
Elementary fibril 100 glucan units 35 A° - - Cellobiose unit - 10.3 A° - 250
3.1.3 Isolation of cellulose nanofibers
Natural fibers possess moderately high specific strength and stiffness and can be used as
reinforcement in polymeric resin matrix to make useful composite materials. Natural fibers in
the form of wood flour have also been often used for preparation of natural fiber composites
11
(Saheb and Jog 1999). High tensile strength of plants fibers is due to cellulose nanofibers in the
cell wall. The properties of the nanoscopic fibrous components cannot be physically measured
without extracting them from the cell wall, which may result in significant chemical or
mechanical damage. Therefore it is worthwhile to explore the potential of extracting these
nanofibers from the cell wall and analyze their reinforcing potential in composite
manufacturing.
3.1.3.1 Pre-treatment of Fibers: Fiber Swelling
The cell wall structure (the wall of the fiber cylinder) is built up from cellulose molecules
which are extremely hydrophillic. The magnitude of this swelling effect is very dependent upon
the fiber structure. Wood fibers will readily increase their diameter by up to as much as 50 % in
response to the swelling tendency. Fiber swelling is unquestionably a principal cause of many of
the characteristics acquired by paper when made from a beaten wood pulp (www.sewanee.edu).
Hemp bast, flax bast and wheat straw fibres were soaked in 17.5 % sodium hydroxide
solution at room temperature overnight to swell the cell wall to enable chemical molecules to
penetrate through the crystalline region of the cellulose (Bhatnagar 2004). Alkali soaking
increased fiber swelling, reduced internal bonding, enhanced the pliability of the fibers, as well
as increased the potential bonding areas between fibers (www.tfri.gov.tw).
3.1.3.2 Acid Hydrolysis
The treatment of cellulosic, starch, or hemicellulosic materials using acid solutions to break
down the polysaccharides to simple sugars is called acid hydrolysis. Lignocellulosic fibers
contain between 20 % and 40 % hemicellulose which is a hetero polysaccharide consisting
mainly of pentoses and hexoses. These sugars can be obtained as monomers by acid hydrolysis.
The hemicelluloses are amorphous in contrast to cellulose. Therefore the oxidation and
12
degradation reactions take place more rapidly in hemicelluloses than in celluloses. Hydrolysis of
glycosidic bonds occurs in both acid and alkaline medium, but much faster at lower pH. Pectin
occurs in a small degree in the middle lamella, especially in the pith and young tissue and
consists of polygalacturonic acid. Pectins are naturally soluble in aqueous media. To remove
pectin and hemicellulose from the pulp, a 1M hydrochloric acid solution was prepared and the
sample was submerged into this acid solution (Bhatnagar 2004). High temperature (80 ºC) was
favored for acid hydrolysis. Yu et al. (1998) reported the method of preparing microcrystalline
cellulose from wood pulp through acid hydrolysis process. The diameter of the resulting
nanocrystals was in the range of 10-30 nm.
3.1.3.3 Mechanical Treatment
A purely mechanical process can produce more refined, finer fibrils of several micrometers
long and between 50 to 1000 nm in diameter. Hempworth and Bruce (2000b) suggested that the
reinforcing capacity of these nanofibers could be utilized without having to extract them from
the cell wall. In their work, fragments of cell wall were extracted from vegetable parenchyma
tissue and pressed with PVA to make a composite sheet. Taniguchi and Okamura (1998)
processed microfibrillated materials (MM) from natural fibers such as wood pulp fiber, cotton
fiber, tunicin cellulose, chitosan, silk fibers and collagen by a super-grinding method. The
method consists of a unique, simple mechanical treatment, which is designed to give shearing
stress to the longitudinal fiber axis of the fibrous samples. By the above method,
microfibrillated cellulose having diameters in the range 20-90 nm could be obtained.
3.1.3.4 Chemo-mechanical Treatment
As compared to purely mechanical methods, cellulose nanofibers from the primary and
secondary cell walls can be extracted by chemo-mechanical treatments without degrading the
13
cellulose. A chemo-mechanical process can achieve even finer fibrils of cellulose (cellulose
nanofibers), between 5 and 50 nm diameter range.
Natural fibers are constituted of cellulose, hemicellulose, pectin, lignin and waxes. Selective
removal of non-cellulosic compounds constitutes the main objective of fiber chemical treatment.
Chemical treatments have been extensively used for the removal of lignin/pectins surrounding
cellulose and destroying its crystalline structure. Wang (2004) reported that flax fibers were
subjected to sequential extraction with 1: 2 mixture of ethanol and benzene for 72 h at 50ºC,
followed by washing with double distilled water and air drying to remove waxes and water
soluble ingredients prior to chemical treatments. Research by Dufresne and Vignon (1998) also
suggested a method to extract the nanofibers from agricultural sources by chemo-mechanical
treatments. In Bhatnagar's project (2004), cellulose nanofibers were extracted by chemical and
mechanical treatments, from the long fibers without degrading the cellulose content. The aspect
ratio was more than 100, when used as filler in a polymer matrix. These nanofibers can produce
lightweight materials with high strength.
Developing a method to extract the cellulose nanofibers from natural fibers will enable
their use as filler in manufacturing ultra lightweight composites. The resulting end product will
demonstrate very high mechanical properties and may be used in many applications.
3.2 Dispersion of Cellulose Nanofibers in Plastic Matrix
The nanofibers should demonstrate a reinforcing potential as a filler for composite
materials. One of the problems encountered in the use of such filler lies in the difficulty in
ensuring good dispersion of the filler in the composite material. The phenomenon of
agglomeration of the filler is observed in particular with the cellulose fibers used as filler for
matrix made of thermoplastic resin. Poor dispersion of the filler in the matrix of a composite
14
or to hydrolyze the
ellulose fibers.
3.2
effective modification method
ill enhance the interfacial property in natural fiber composites.
3.2
material seriously affects its mechanical properties. In order to improve the dispersion of the
fibers, it has been proposed to chemically modify, physically modify
c
.1 Challenges of dispersion of nanofibers in plastic matrix
Cellulose fibrils have a high density of –OH groups on the surface, which try to bond with
adjacent –OH groups by weak hydrogen bonding. This results in agglomeration or entanglement
of the nanofibers; therefore the nanofibers obtained after chemo-mechanical treatments were
kept as water suspension. Mostly water is used as a carrier to disperse cellulose nanofibers.
Cellulose nanofibers have a tendency to form hydrogen bonds with adjacent fibrils and hence
dispersion is an important challenge in non-polar solvents. In Bhatnagar (2004) present work,
PVA which is water soluble and polar was used as matrix. Therefore, the dispersion of
nanofibers in the polymer was not an issue. But to expand the horizon of bio-based
nanocomposites for high-end applications, it is necessary to reduce the entanglement of the
fibrils and improve their dispersion in the polymer by surface modification, without
deteriorating their reinforcing capability. The modification of the cellulose nanofiber surface to
make them compatible with non-polar polymers has been attempted. Several studies showed
better dispersion of the fibrils in the non-polar solvents after treatment with silane (Goussé et al.
2004). Normal methods of modifying the interface are usually not applicable in natural fibers
for many reasons, cost being the most important. Therefore, an
w
.2 Chemical modifications of nanofibers
The modification of the cellulose nanofiber surface has achieved acceptable dispersion
levels. Chemical modifications may activate -OH groups or can introduce new moieties that can
15
f some important fiber chemical
odifications are summarized in the following sub-sections.
oups grafted onto the chain of the polymer for effective stress
ansfer across the interface.
Sila
ction with hydroxyl group of the fibers. The reaction
sch
effectively interlock with the matrix. Brief descriptions o
m
3.2.2.1 Dispersion/Coupling Agents
The current extraction techniques however use water as carrier and the cellulose
microfibrils cannot be dispersed easily in non-polar solvent. Such restriction is detrimental if
one wants to use these microfibrils to reinforced non-polar polymers, such as polyolefins or
other commodity polymers. Various processing aids/coupling agents such as stearic acid,
mineral oil, and maleated ethylene have been used (Saheb and Jog 1999). The compatibilizer
can be polymers with functional gr
tr
ne Coupling Agents
Coupling agents usually improve the degree of cross-linking in the interface region and
offer a perfect bonding result. Silane coupling agents were found to be effective in modifying
the natural fiber-matrix interface. Various silanes were effective in improving the interface
properties of wood-polypropylene (Coutinho et al. 1997), mineral-filled elastomers (González et
al. 1997), fiber-reinforced epoxies (Culler et al. 1986) and phenolics composites (Ghatge and
Khisti 1989). Alkoxy silanes are able to form bonds with hydroxyl groups. Coupling agents
such as toluene dissocyanate, triethoxyvinyl silane, aminopropylmethyldiethoxy silane,
aminopropyltrimethoxy sliane were tested in fiber treatment in order to improve the interface
properties. Silanes undergo hydrolysis, condensation and bond formation stage. Silanols can
form polysiloxane structures by rea
emes are given in Figure 1 and 2.
In the presence of moisture, hydrolyzable alkoxy group leads to the formation of silanols.
16
igure 1. Hydrolysis of silane (Sreekala et al. 2000). F
CH2=CH-Si-OC2H5 CH2=CH-Si-O-H
OC2H5 O-H
H2O
O-HOC2H5
igure 2. Hypothetical reaction of fiber and silane (Sreekala et al. 2000).
F
Cellulose -O Fibers Hemicellulose -O H + CH2=CH-Si-O-H Lignin -O H H
O-H
Cellulose -O Si-CH==CH2
Fiber Hemicellulose -O Si-CH==CH2
Lignin - O Si-CH==CH2
O-H
O-H
O-H
O-H
O-H
O-H
O-H
González et al. (1997) investigated the effect of silane coupling agent on the interface
performance of henequén fiber-reinforced high-density polyethylene composites. The fiber-
surface silanization resulted in better interfacial load transfer efficiency but did not improve the
wetting of the fiber. Hydrogen and covalent bonding mechanisms could be found in the natural
fiber-silane system. It was assumed that the hydrocarbon chains provided by the silane
17
application influenced the wettability of the fibers, thus improving the chemical affinity to
polyethylene. Silane treatment of cellulosic fibers can increase the interfacial strength and
therefore the mechanical properties of the composite (George et al. 1998; Bataille et al. 1989).
Silane treatment also enhanced the tensile strength of the composite (Joseph et al. 2000).
Several studies showed better dispersion of the fibrils in the non-polar solvents after treatment
with silane (Goussé et al. 2004). 0.5 wt% 3-aminopropyltriethoxysilane was used to modify the
surface of the microfibrils and reduce the hydrogen bonding between the adjacent fibrils.
Maleated Polypropylene/Maleic Anhydride Grafted Polypropylene (MAPP)
It has been widely used as a coupling agent or a compatibilizer in natural fiber reinforced
polypropylene/polyethylene composites (Mohanty and Nayak 2006; Khan and Bhattacharia
2007). The treatment of cellulosic natural fibers with MAPP copolymer (Figure 3) provides
covalent bonds across the interface. Through such treatment, the surface energy of the fibers
was increased, thereby providing better wettability and high interfacial adhesion. The details of
the interphase chemistry of this universal MAPP coupling agent in biocomposites were yet to be
fully understood. Researchers were exploring this mechanism through comparison of the
spectroscopic analysis of the interphase region and the bulk matrix which gave information on
the amount and reactivity of different types of compatibilizers in the interphase. The combined
analysis of the chemical interaction and the physical strength of the interphase would also allow
for the full characterization of the mechanism of the natural fibers-polypropylene composite
system. MAPP is now a well-known coupling agent for PP-based composites from natural fiber.
Figure 3. Reaction of biofiber with MAPP (Mohanty et al. 2001).
Remarkable improvements in strength properties in jute-polypropylene (Ray et al. 1998)
and wood flour-polypropylene composites were observed by the action of compatibilizers
(Oksman and Lindberg 1998). A maleic-anhydride-grafted styrene-ethylene-butylene-styrene
(SESBS-MA) triblock copolymer had been used as a compatibilizer in low-density
polyethylene-wood flour composite. Vignon et al. (1996) reported the PP/hemp fibers composite
films with various amount of fibers were prepared and their mechanical properties were
improved, in particular when fibers treated with PPMA were used. In order to improve the
interfacial interaction of the cellulosic fiber with the hydrophobic polymer, the influence of
pretreating the fibers with a coupling agent or wetting agents such as polypropylene-maleic
anhydride copolymer (PPMA), on the tensile properties of the composite is also reported.
The MA reacts with wood through etherification and hydrogen bonding and also possibly
through interaction between the styrene and wood (Joseph et al. 2000). Figure 4 shows a
possible hypothetical representation of reaction of cellulosic–OH groups with those of maleic
anhydride modified polypropylene at the interface.
18
Figure 4. A possible hypothetical representation of the esterfication of cellulose (Joseph et al. 2000).
Adhesives
The primary plant cell wall has a woven structure that allows rotation rather than stretching
of components during deformation. Therefore it is likely to have a low modulus. If adhesive or
filler can be infiltrated into the cell wall structure, this would lock the structure and allow
microfibrils to be stretched rather than re-orientated when the material was deformed. This
could be achieved using adhesives that are either water-soluble or are able to form an emulsion
of nanometre sized particles in water and enter the cell walls (Hepworth and Bruce 2000). The
water could be removed by evaporation. The adhesion of hydrophilic cellulose to hydrophobic
polymer matrix has been increased by the use of coupling reagents (Dalvag et al. 1985; Maldas
et al. 1989; Felix and Gatenholm 1991).
3.2.2.2 Surfactant/ Surface Active Agents
Interaction of cellulose with surfactants has been another way to stabilize cellulose
suspensions into non-polar systems (Holbery and Houston 2006). A surfactant is briefly defined
as a material that can greatly reduce the surface tension of water when used in very low
concentrations. Table 3 shows that Softanol 90 reduces the surface tension of water from 73 to
30 dynes per centimeter when used at a concentration of 0.005 percent. Ethanol when used at a
concentration of 20 percent; however, only reduced tension of water to 38 dynes per centimeter.
19
Table 3. Percent concentration required to reduce the surface tension of water to indicated values (www.chemistry.co.nz/surfactants.htm).
Surface Tension (dynes per cm) 73 50 40 30 22
Softanol 90 0 0.003 0.0008 0.005 --- Ethanol 0 9 18 40 100
Figure 5. Schematic sketch of surfactant molecule (www.chemistry.co.nz/surfactants.htm).
(Figure 6).
A particular type of molecular structure performs as a surfactant (Figure 5). This molecule
is made up of a water soluble (hydrophilic) and a water insoluble (hydrophobic) component.
The hydrophobe is usually the equivalent of an 8 to 18 carbon hydrocarbon, and can be
aliphatic, aromatic, or a mixture of both. The hydrophilic groups give the primary classification
to surfactants, and are anionic, cationic and non-ionic in nature. In each case, the hydrophilic
end of the surfactant is strongly attracted to the water molecules and the force of attraction
between the hydrophobe and water is only slight. As a result, the surfactant molecules align
themselves at the surface and internally so that the hydrophile end is toward the water and the
hydrophobe is squeezed away from the water
This internal group of surfactant molecules is referred to as a micelle. Because of this
characteristic behavior of surfactants to orient at surfaces and to form micelles, all surfactants
perform certain basic functions. However, each surfactant excels in certain functions and has
others in which it is deficient.
20
21
Figure 6. Schematic sketch of surfactant molecules in water (www.chemistry.co/nz/surfactants.htm).
Foaming agents, emulsifiers, and dispersants are surfactants which suspend respectively, a
gas, an immiscible liquid, or a solid in water or some other liquid. Although there is similarity in
these functions, in practice the surfactants required to perform these functions differ widely. In
emulsification, as an example - the selection of surfactant or surfactant system will depend on
the materials to be used and the properties desired in the end product. An emulsion can be either
oil droplets suspended in water, an oil in water (O/W) emulsion, water suspended in a
continuous oil phase, water in oil (W/O) emulsion, or a mixed emulsion. The surfactants form
what amounts to a protective coating around the suspended material, and these hydrophilic ends
associate with the neighboring water molecules. In addition to surfactant effects the stability of
these suspensions is related to the particle size and density of the suspended material.
Dispersions with liquid droplets smaller than 0.2µ are microemulsions, which exhibit unique
properties. A dispersion agent always has to be compatible with the polymer. Selection and
proper utilization of the dispersion agent is very important to achieve the optimum dispersibility
in an aqueous system.
3.2.2.3 Luwax and Poligen–Waxes and Wax–Emulsions
Vinyl acetate is an appropriate monomer for incorporation in ethylene copolymer waxes.
22
Copolymers of these substances-the Luwax EVA types-contain functional groups which
influence the solubility of the wax. The properties of ethylene co-polymer wax-emulsions are
surface free emulsion, high molecular weight and transparent films. They have good adhesion
properties on various substrates which include paper, glass, metal, wood and plastics. They can
effective in improving the compatibility between non-polar plastic such as PE, PP and EVA and
polar additives such as pigments, chalk, cellulose, etc. Recently, Luwax EAS (ethylene co-
polymer waxes) and Poligen ES (ethylene co-polymer wax-emulsions) were introduced by
BASF as an innovative application in improving surface properties. Nanofibers are rarely
dispersed well in plastic in their original powder form. The ethylene waxes make the nanofiber
easier to disperse, enabling it to be dispersed more homogeneously. More wax has to be used to
disperse nanofibers that are difficult to dispersed, because the strong cohesive forces that cause
nanofibers to agglomerate have to be overcome. The wax is able to wet and encapsulate the
pigments more effectively.
3.2.3 Physical modifications of nanofibers
Physical treatments change structural and surface properties of the fiber and thereby
influence the mechanical bonding with the matrix. Physical methods involve surface fibrillation,
electric discharge (corona, cold plasma), ultrasonic, irradiation, electric currents, etc (Belgacem
et al. 1994; 1995).
3.2.3.1 Low Temperature Plasma Treatment/Electrical Corona Treatment
In some approaches, corona or plasma discharges have been used (Bataille et al. 1989;
Dong et al. 1993; Yuan et al. 2002). Surface modification by discharge treatment such as low
temperature plasma, sputtering and corona discharge is of great interest in relation to the
improvement in functional properties of fibers.
23
Low temperature plasma treatment causes mainly chemical implantation, etching,
polymerization, free radical formation, crystallization, whereas sputter etching brings about
chiefly physical changes such as surface roughness and this leads to increase in adhesion and
decreases light reflection (Vander Wielen et al. 2006). Low temperature plasma is a useful
technique to improve the surface characteristics of the fiber and polymeric materials by utilizing
the ingredients such as electron, ion, radical, excited molecules produced by electrical discharge.
Low temperature plasma can be generated under atmospheric pressure in the presence of helium
(Vander Wielen and Ragauskas 2006). The action of these plasmas involves abstraction of
protons and creation of unstable radicals that convert functional groups such as alcohols,
aldehydes, ketones and carboxylic acids. Cold plasma environments offer a unique way for
modifying the chemical and physical structures of both fiber and polymer surfaces without
altering the bulk structures and characteristics of these materials. The plasma environment has
three effects on the fiber surface: ablation of the surface, cleaning of the surface and chemical
modification of the surface by roughening the surface of fibers to increase mechanical
interlocking between the fiber and the matrix.
Electrical discharge methods are used for cellulose fiber modification to decrease the melt
viscosity of cellulose polyethylene composites (Dong et al., 1992) and to improve the
mechanical properties of composites. Corona treatment is one of the most interesting techniques
for surface oxidation activation. It changes the surface energy of the cellulosic fibers, which in
turn affect the melt viscosity of composites (Belgacem et al. 1995). Mechanical and rheological
properties of cellulose-PP composites subjected to corona treatment were reported by Dong et
al. (1992). Corona treatment modifies the surface composition and therefore the surface
properties of the composite components (Kim et al. 1970; Goring 1967).
24
atment intensity.
3.2.3.2 Dielectric-Barrier Discharge
The surface chemistry of cellulosic fibers treated with an atmospheric cold plasma
generated by dielectric-barrier discharge. The invention of the dielectric-barrier discharge
device which applies an electric current for plasma formation is introduced the concept of
dielectric-barrier discharge treatment of air for ozone generation in 1857. The potential
application of dielectric-barrier discharge to the surface modification of lignocellulosic fibers is
focusing on the surface chemistry, physical strength properties and water affinity of
lignocellulosic fibers. When the high-energy electrons reach the surface of substrates, they have
energies sufficient to break molecular bonds resulting in the creation of ions, free radicals and
other species on treated surfaces. Dielectric-barrier discharge-initiated surface treatments is
associated with increased surface energy and wettability as evidenced by the decreased contact
angles of water on polymeric and lignocellulosic surfaces. Decreased contact angles of water on
the surface of cellophane with increased dielectric-barrier discharge treatment have been
reported (George et al. 2001). Both hydrophilic and hydrophobic behaviors among dielectric-
barrier discharge-treated lignocellulosic fibers depend on tre
3.2.3.3 UV Radiation/Gamma Radiation/Electron Radiation
When cellulose fibers are heated enough to color them, whether by conduction, convection,
or radiation of any kind, water is eliminated from the structure (the cellulose is "dehydrated").
When water is eliminated, C-OH chemical bonds are broken. The C free radicals formed are
extremely reactive, and they will combine with any material in their vicinity. In cellulose, other
parts of the cellulose chains may be the closest reactants (Zahran et al. 1980; Kenaga et al.
1962). The chains crosslink changes the crystal structure of the cellulose, and you can see the
effect with a polarizing microscope (Bergstrom and Tiberg 1976).
25
3.2.3.4 Sonification
A simple and versatile approach for dispersion bionanofibers from natural materials using
the ultrasonic technique was reported. The cellulose suspension was sonicated with a Branson
sonifier (Samir et al. 2004), behaved as non-flocculating and non-sedimenting aqueous
suspensions of cellulose microfibrils. It is made up of individual cellulose fragments consisting
of slender rods that have a broad distribution in size.
3.2.4 Other methods to improve the dispersion
Sassi et al. (2000) reported a patent on surface-modified cellulose microfibrils which are
useful in particular as reinforcing filler or structuring agents for composite materials. The
hydroxyl functions present at their surface are esterified with at least one organic compound
comprising at least one function which can react with the hydroxyl groups of cellulose. The
organic residues originating from the esterifying organic compounds bound to the surface of the
microfibrils ensure compatibility of the cellulose microfibril with the medium in which it is
dispersed. As a result, when the organic compound is a compound that includes acetyl groups
such as acetic acid, said microfibrils are used as reinforcing filler in a material comprising
cellulose acetate as the polymeric matrix. The filled composite material may be shaped to
provide films, moldings, fibers or yarns. As activating agents for the esterification reaction of
cellulose, mention may be made, by way of example, of trifluoroacetic anhydride and
trichloroacetic anhydride. The microfibrils can also be added to the solution of matrix-forming
material, in the form of dispersion in a liquid which is advantageously identical to the solvent
for the matrix. Another advantageous process consists in introducing the microfibrils into the
material in the molten state.
Nanocrystals were prepared by acid hydrolysis of bacterial cellulose microfibrils. There
26
were topochemically trimethylsilylated, in an attempt to reduce their hydrophilicity. Composites
were made by dispersing either native or silylated crystals in cellulose acetate butyrate matrix
and solution casting of the dispersions (Grunert and Winter 2002). Goussé et al. (2004) reported
that when mild silylation conditions were applied, the microfibrils retained their morphology,
but could be dispersed in a non-foucculating manner into organic solvents. Such stabilization
was also achieved with surface grafting or derivation (Cavaillé et al. 2005; Ladouce et al. 2000;
Araki et al. 2001). The challenge has been to keep the integrity of the core of the cellulose
microfibrils while modifying only the polarity of their skin. As shown in previous reports,
partial surface silylation was one way to achieve this goal (Ladouce et al. 2000; Goussé et al.
2002). Surface silylated cellulose whiskers from tunicin-cellulose from sea animal origin-
maintained a high dispersion character in non-polar solvents such as Tetrahydrofuran (THF)
(Goussé et al. 2002).
3.3 Development of Bionanocomposites
Cellulose nanofibers embedded in the cell walls have a great reinforcing potential and it is
predicted that nano reinforcements of the polymer matrix are poised to create the next
generation of value-added novel eco-friendly nanocomposites (www.empa.ch).
3.3.1 Challenges of the nanocomposites development
In the last decade, there has been growing interest in the manufacture of composite materials
that are reinforced with nanofibers (Hepworth and Bruce 2000). The exceptional mechanical
and physical properties demonstrated for cellulose nanofibers, combined with their low density
make this new form of fibers an excellent candidate for composite reinforcement. In the past
few years, there has been a growing interest in composites reinforced with inorganic nanofibers
(Thostenson et al. 2001; Hernandez et al. 1999). The potential for nanocomposites reinforced
27
with carbon tubes having extraordinary specific stiffness and strength represent tremendous
opportunity for application in the 21st century (Falvo et al. 1998). However, these synthetic
nanofibers have several disadvantages such as high-energy consumption, end of life disposal,
expensive and harmful to the environment.
Obtaining the fibers from the plant is usually a fairly straightforward though smelly and
labor intensive process. Many studies were done to prepare composites using cellulose
microfibrils as reinforcement from sugar beet, tunicin etc. (Chanzy et al. 1999; 2000; Ishihara
and Yamanaka 2002; Dufresne et al. 1997). But cellulose microfibrils are very expensive and
can cause contamination problems in alimentary applications. Cellulose microfibrils from
primary cell walls as described in almost all the literature cited can be obtained only from the
sources which are principally constituted of parenchyma cells; therefore the raw material choice
is very limited. Nanocomposites based on cellulose nanofibers are of interest due to high
modulus and the small lateral dimensions. Other raw materials can also be used to obtain
cellulose nanofibers such as root crop, rutabaga, agro-based fibers, wheat straws, etc. Depending
upon the raw materials and defibrillation techniques, degree of polymerization, morphology and
aspect ratio of the nanofibers will differ.
Before these extraordinary properties observed at the nano-scale are realized in a
macroscopic composite, considerable basic research is necessary. Fully understanding of the
thermo-mechanical behavior of nanocomposites requires knowledge of the elastic and fracture
properties of nanofibers as well as of interactions at the nanofiber/matrix interface. The nano-
meter scale of the reinforcement presents additional challenges in mechanics research since we
now must account for interactions at the atomic-scale.
Critical to the use of nanofibers as a structural material, there is a need for development of
nanofiber production techniques at the scale needed for producing macroscopic composites that
28
are cost-effective. Fundamental work in processing, characterization, and functional properties
of this new class of nanocomposites can be optimized. But in view of better utilization of
renewable resources, there is a strong need to explore other renewable green sources, which can
be utilized in developing high strength lightweight nanocomposites for high-end applications.
Another major area of concern is a large-scale production of the nanofibers so that they can be
produced and used commercially.
3.3.2 Characteristics of biopolymers
Biodegradable polymers include a wide variety of materials derived from renewable
resources such as starch, cellulose and polyhydroxy alkanoates and from synthetic means such
as polylactic acid and polycaprolactone. Loose-fill packaging and compost bags have been the
mainstay of biodegradable polymers; however, new markets are emerging led by diapers, plastic
bags/liners, nonwoven hygenics, etc. Fully biodegradable synthetic polymers have been
commercially available since 1990, such as polyvinyl alcohol (PVA), poly (lactic acids) (PLA),
polycaprolactone (PCL), and polyhydroxybutyrate-valerate (PHBV).
3.3.2.1 Polyvinyl Alcohol (PVA)
Polyvinyl alcohol (Figure 7) is a medium viscosity, fully hydrolyzed grade of polymer.
White and granular, it is soluble in hot water but insoluble in cold water and common organic
solvents. For many applications polyvinyl alcohol is prepared in water solutions. On
evaporation of water, transparent films are formed which have high tensile strength and tear
resistance. The binder characteristics of polyvinyl alcohol offer excellent adhesion to porous,
water-absorbent surfaces (www.dupont.com). It offers a combination of excellent film forming
and binder characteristics, along with insolubility in cold water and organic solvents. This
combination of characteristics is useful in a variety of applications. In adhesive applications
polyvinyl alcohol can be used alone or often in combination with extenders such as starch,
dextrin or clay. As the proportion of polyvinyl alcohol increases, the adhesive strength and
water resistance also improve.
Figure 7. Chemical structure of PVA.
3.3.2.2 Polylactic Acid (PLA)
Poly(lactic acid) (PLA) is a class of crystalline polymers with relatively high melting point
(Mohanty et al. 2000). Recently PLA has been highlighted because of its availability from
renewable resources such as corn and sugar beets. PLA (Figure 8) is synthesized by the
condensation polymerization of D- or L-lactic acid or ring opening polymerization of the lactide
(Lunt 2000). Advanced industrial technologies of polymerization have been developed to obtain
high molecular weight pure PLA, which leads to a potential for structural materials with enough
lifetime to maintain mechanical properties without rapid hydrolysis.
Figure 8. Chemical structure of PLA.
O CH C
CH3
n
O
PLA was extensively studied in medical implants, suture, and drug delivery systems since
80s due to its biodegradability. PLA has been attractive for disposable and biodegradable plastic
29
30
substitutes due to its better mechanical properties. The elongation for pure PLA is about 6 %
(www.ecologycenter.org); however, PLA is still more expensive than conventional plastics.
Also, the degradation rate is still slow as compared to the waste accumulation rate.
PLA has many of the advantages of both synthetic and natural fibers. Perhaps most
distinctive among these, though, is the fact that, like natural fibers, its raw material is both
renewable and non-polluting. This eliminates the often-underestimated problems associated
with using a finite supply of oil as a raw material. Beyond having a renewable raw material,
though, PLA is also compostable. After hydrolysis at 98 % humidity and 60 °C or higher, PLA
is readily consumed by microbes and its component atoms are converted for possible re-use in
growing more corn, beets, rice, etc. for future conversion to PLA. Thus PLA is less
environmentally costly than polymers that are recyclable, because there is a limit to the number
of recycling iterations that can occur before the material loses its usefulness. PLA is even less
environmentally costly than other biodegradable thermoplastics, since the entire mass of PLA
can eventually be re-converted into new PLA, whereas many other biodegradable
thermoplastics incorporate at least some material derived from fossil fuels. This ability of PLA
to be completely recycled at the atomic level and by natural processes is summed up in the term
“sustainability.” PLA is not a perfectly sustainable polymer, since some energy must be
irretrievably used in its polymerization and in converting the polymer into fibers and fabrics.
But it offers superior sustainability and lower environmental impact than any other non-
cellulosic synthetic fiber, and possibly even superior to some natural fibers (www.fitfibers.com).
In the U.S., Cargill Dow Polymers (CDP) makes PLA by converting corn starch into lactic acid,
which is then polymerized (Lunt 2000).
3.3.2.3 Poly(β-hydroxybutyrate) (PHB)
31
window (Zhang et al. 1997).
Poly(β-hydroxybutyrate) (PHB) is a biotechnologically produced polyester that constitutes
a carbon reserve in a wide variety of bacteria and has attracted much attention as a
biodegradable thermoplastic polyester (Holmes 1988); however, it suffers from some
disadvantages compared with conventional plastics, for example, brittleness and a narrow
processability
Figure 9. Chemical structure of PHB.
Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA), a polymer belonging to the
polyesters class that was first isolated and characterized in 1925 (Amass et al. 1998). PHB
(Figure 9) is a polyester produced by bacterial fermentation. It is unique in being both a
thermoplastic and melt processable, and in having a biological origin; this gives PHB the
potential to replace existing oil-based commodity polymers when the price of oil rises (Barham
and Keller 1986).
O CH CH2 C
OCH3
n
3.3.3 Application of bionanocomposites
Since 1941, the study on composites, particularly natural fiber-reinforced plastics has
gained increasing attention of researchers and manufacturers (Joseph et al. 2000). The increased
interest in natural fiber-reinforced composites is due to the high performance in mechanical
properties, significant processing advantages, excellent chemical resistance, low cost and low
density. They have long served many useful purposes but the application of material technology
32
tomotive sector.
for the utilization of natural fibers as reinforcement in polymer matrix has taken place in recent
years. Biocomposite consists of a polymer as the matrix material and a natural fiber as the
reinforcing element. The use of fibers derived from annually renewable resources, such as
reinforcing fibers, provide positive environmental benefits with respect to ultimate disposability
and raw material utilization.
Cellulose nanofibers reinforced polymer could be used for applications where a
combination of biodegradability, high mechanical strength and visual transparency is required.
Nanofibers demonstrated high reinforcing potential in composite films (Bhatnagar and Sain
2005). Nanofibers as a new type of raw material can be used in a number of high-end
applications such as plastic reinforcement, gel forming and thickening agent. Since these
nanocomposites can be made biodegradable with tremendous stiffness and strength, these
nanocomposites can be used in medical devices such as biocompatible drug delivery system,
blood bags, cardiac devices, and valves as reinforcing biomaterials. Biological tissues are made
from nanocomposite materials and given an interest in manufacturing synthetic nanocomposites.
Due to their lightweight and high strength, they also can be utilized as high strength components
in aerospace and au
Sassi et al. (2000) in their patent described the method of making surface-modified
cellulose nano-sized microfibrils and use as filler in composite materials. Another patent by
Chanzy et al. (2000) reported the making of cellulose-reinforced polymers and their applications
with tunicin nano-sized microfibrils. Vaslin et al. (2002) used cellulose nano-sized microfibrils
as additives in food formulations. Dufresne et al. (2000) described a method using chemo-
mechanical methods to extract cellulose nano-sized microfibrils from sugar beet pulp making
composite films using them as filler. Other research shows the process of preparing bacterial
cellulose microfibrils filled film in cellulose acetate butyrate matrix and solution casting of the
33
dispersions (Grunert and Winter 2002).
Bhatnagar’s study (2004) demonstrated that the properties of cellulose nanofibers obtained
from plant fibers are much better than the conventional micro scale cellulose composites. The
reasons are higher aspect ratio, smaller diameters and specific matrix morphology effects. It was
observed that even 10 % nanofibers provide a remarkable reinforcing potential. Dufresne and
Vignon (1998) also reported the improvement of starch film performance using cellulose
microfibrils. They used the cellulose microfibril suspension to process composite materials with
a high level of dispersion by mixing with an aqueous suspension of gelatinized starch as matrix.
Chanzy et al. (1999) demonstrated a method to extract the nanofibers from sugar beet pulp by
chemo-mechanical treatments. The properties of the film are determined by measuring the
corrected Young’s modulus, which gives the stiffness of the system. The results showed that the
cellulose nanofibers improved the film forming and their physical properties, in particular their
tensile strength. Chanzy et al. (2000) investigated the cellulose microfibril-reinforced polymers
and their applications. Polymer/cellulose composites are prepared using individualized cellulose
microfibrils as the reinforcement. The property of reinforcement is also connected with their
rigidity, which is itself strictly connected with their crystallinity, whose value increases as their
surface area/volume ratio decreases. Dufresne et al. (1997) evaluated the mechanical behavior
of sheets prepared from sugar beet cellusulose microfibrils through tensile tests according to
their purification level, individualization state, and moisture content, differences in tensile
modulus are observed. The tensile modulus increases with the duration of the mechanical
treatment of the pulp. Taniguchi and Okamura (1998) explored the new films produced from
microfibrillated natural fibers. The mechanical properties of the films were evaluated in terms of
tensile strength. These films were homogeneous, strong and translucent. The film from tunicin
cellulose showed the highest tensile strength of these films.
34
3.4 Summary
Preliminary research in nanotube-based composites has indicated that there is potential in
nanofibers for reinforcement, but, most important, it has illustrated the significant challenges
that must be overcome before the potential is realized. The nanofibers can be used widely as
reinforcement if they could be separated and dispersed more efficiently within the other
thermoplastics. We can expect even better reinforcing effect at a much lower cellulose nanofiber
loadings. In view of better utilization of renewable resources, there is a strong need to explore
other renewable greener sources, which can be utilized in developing high strength lightweight
nanocomposites for high-end applications. Further research work is required to better
understand the dispersion mechanism of nanofibers in the polymer matrix.
35
4. STUDY OF STRUCTURAL MORPHOLOGY OF CELLULOSE FIBER
FROM THE MICRO TO THE NANOSCALE
4.1 Introduction
Lately, there have been considerable interests in the isolation and study of novel
nanomaterials manufactured from renewable resources. An important class of nanomaterials has
been nanofibers and fibrils from different cellulose sources and cellulose crystals (whiskers)
(Bhatnagar and Sain 2005; Chakraborty et al. 2005; 2006; Eichhorn et al. 2001). These novel
nanofibers, fibrils and crystals have been shown to result in unique properties when
incorporated in different polymers (Bhatnagar and Sain 2005; Nakagaito and Yano 2004; 2005;
Sain and Bhatnagar 2003; Hepworth and Bruce 2000). The sources of these nanomaterials have
been wheat straw, bacterial cellulose, kraft pulp, sugar beet pulp, potato, and swede root
(Chakraborty et al. 2006; Nakagaito and Yano 2004; Sain and Bhatnagar 2003; Hepworth and
Bruce 2000; Gindl and Keckes 2004; Dinand et al. 1999).
The chemical constituents of the plant’s cell wall consist not only of cellulose, but also of
hemicellulose, pectin and lignin. The properties of each constituent contribute to the overall
properties of the fiber (Saheb and Jog 1999). Figure 10 illustrates the hierarchical microstructure
of cellulosic fiber bundles. A cellulose fiber is made up of bundles of single cellulose fiber,
which has a diameter of 25-30 µm. The single cellulose fiber is made up of bundles of
microfibers, which have diameters of 0.1-1 µm. The microfiber consists of bundles of
nanofibers, which has a diameter in the range of 10-70 nm and lengths of thousands of
nanometers (McCann et al. 1990; 1993). Nanofiber is made up of cellulose chains bound by
hydrogen bonding. The smallest building element of the cellulose skeleton is considered by
36
some to be an elementary fibril. The fibril can be about 5-10 nm in diameter and its length
varies from 100 nm to several micrometers depending of the source of cellulose (Oksman et al.
2006). The cellulose molecules are always biosynthesized in the form of nanosized fibrils; up to
100 glucan chains aggregate together to form cellulose nano-sized microfibrils or nanofibers
(McCann et al. 1990; 1993; Clowes and Juniper; Stamboulis et al. 2001). A cellulose nanofiber
has more than 200 times the surface area of isolated softwood cellulose (Krieger 1990) and
possesses higher water holding capacity, higher crystallinity, higher tensile strength, and a finer
web-like network. In combination with a suitable matrix polymer, cellulose nanofiber networks
show considerable potential as an effective reinforcement for high quality specialty applications
of bio-based composite. Another type of nanoreinforcement that can be obtained from cellulose
fibers are nanowhiskers. The elementary fibril is made up of amorphous and crystalline parts.
The crystalline parts can be isolated by various treatments producing the cellulose nanowhiskers
(Chakraborty et al. 2005; Bondeson et al. 2006).
Many studies have been done on extracting cellulose microfibrils from various sources and
on using them as reinforcement in composite manufacturing (Bhatnagar and Sain 2005;
Chakraborty et al. 2006; Nakagaito and Yano 2005; Sain and Bhatnagar 2003; Wan et al. 2006;
Ishihara and Yamanaka 2002; Oksman et al. 2006). These microfibrils can be extracted from the
cell walls by four types of isolation processes: simple mechanical methods, a combination of
chemical and mechanical methods, an enzymatic approach or an ultrasonic technique. A purely
mechanical process can produce refined, fine fibrils several micrometers long and between 20 to
90 nm in diameter (Taniguchi and Okamura 1998); however, this nano-scalar web-like structure
of fibrils causes a reduction of strength (Nakagaito and Yano 2004). In contrast, chemo-
mechanical treatments can extract cellulose nanofibers from the primary and secondary cell
walls without degrading the cellulose. A chemo-mechanical process can also achieve finer
fibrils of cellulose, ranging between 5 and 60 nm diameter (Bhatnagar and Sain 2005; Sain and
Bhatnagar 2003). The obtained nanofibers have uniform diameters in the range of 25-120 nm
using the ultrasonic technique (Zhao et al. 2007). The separation of nanoreinforcement from
natural materials and the processing techniques have been limited to laboratory scale (Oksman
et al. 2006). Therefore, it is important to develop new processing techniques which will be at
use in large scale production.
Figure 10. Hierarchal structure of cellulose to fiber bundle.
Removal of lignin left after chemical treatment of fibers is the goal of the bleaching
process. Chlorine-based processes still dominate, but more environmentally-friendly non-
chlorine processes are becoming more prevalent. A bleaching treatment using a sodium chlorite
solution was performed to remove phenolic compounds or molecules having chromophore
37
38
groups, in order to whiten the fibers (Bhatnagar and Sain 2005). This is a popular technique at
the laboratory scale to remove lignin from plants. Lignin is rapidly oxidized by chlorine and
chlorites. Lignin oxidization leads to lignin degradation and to dissolution in an alkaline
medium (Bhatnagar and Sain 2005). A further treatment is often required to fully bleach the
suspension. Removal of a part of the flax fibers’ noncellulosic compounds by sodium chlorite
was reflected in the mechanical and physical characteristics of the surface state (Mustată 1997).
Key issues are the size and the dispersion of the nano-sized reinforcements and the effect
of this fine structure on fiber properties. Transmission electron microscopy (TEM) and atomic
force microscopy (AFM) aid the interpretation of structures from the nm to the µm size scale.
Typical information obtained from conventional TEM is length, aspect ratio, shape and the
aggregated or isolated state of fibers (Kvien et al. 2005). Thin evaporative carbon coatings were
used for TEM sample preparation in this study. Recently, AFM has also been used to examine
plant cell walls at a similar resolution to that of the TEM (Kirby et al. 1996; van der Wel et al.
1996; Thimm et al. 2000). This type of microscopy has the important advantage of reducing the
risks of introducing artefacts resulting from the preparative techniques.
In order to investigate the potential of cellulose nanofiber as reinforcement in polymer
composites, this study was focused on the development of a new isolation technique to extract
cellulose nanofibers from hemp and soybean pod by chemo-mechanical treatments. This
research aims to clarify how the various levels of high pressure defibrillation affect the
morphology from long cellulose fiber towards nano-scale fibrillated cellulose and to compare
the morphology of bleached and unbleached fibers at the different stage of the individualization
of the nanofibers. The structural details were studied with SEM, TEM and AFM. The
crystallinity was determined before and after different stages of the chemo-mechanical
treatments of cellulose fibers. The changes of the chemical composition of fibers after different
39
treatments were studied. Infrared measurements were performed to identify the removal of the
pectins.
4.2 Experimental
4.2.1 Materials
The raw material used in this study were hemp fibers (Cannabis sativa L.) from
southwestern Ontario, Canada (Hempline Inc., ON) and soybean pods from Ontario soybean
producers. These fibers have diameters of approximately 22-25 µm and lengths of 15 – 25 mm.
Reagent grade chemicals were used for fiber surface modifications and bleaching, namely,
sodium hydroxide, hydrochloric acid, sodium chlorite, chlorine dioxide, peroxide and sulfuric
acid.
4.2.2 Individualization process
The indidualization process of nanofibers is a multi-step process, shown in Figure 11.
Chemical and mechanical treatments together are applied onto the fibers to individualize
nanofibers. The chemical treatments include pre-treatment, acid hydrolysis, alkaline treatment
and bleaching. The mechanical treatments include cryo-crushing by liquid nitrogen and high-
pressure defibrillation (Bhatnagar and Sain 2005).
40
Raw Material (hemp, soybean pod)
Pretreatment (12 % w/w NaOH, 2h)
Acid Hydrolysis (1M HCl, 80°C, 1.5h)
Alkaline Treatment (2 % w/w NaOH, 2h, 80°C)
Successive Bleaching
Chlorine Dioxide Stage
Extraction Stage
Acid Stage
Cryo-crushing in Liquid Nitrogen
Peroxide Stage
High Pressure Defibrillation
Figure 11. Isolation of nanofibers
4.2.2.1 Chemical Treatments
The main objective of the chemical treatments is to remove the starch, hemi-cellulose and
lignin/pectins surrounding cellulose. Generally, the first step for all of the fiber surface
treatments is mercerization (pretreatment process), which changes the crystal structure of
cellulose (Gassan and Bledki 1997). The essence of mercerizing fiber is that in the swelling of
cellulose fibers due to exposure to alkalis. The natural crystalline structure of the cellulose
41
.
relaxes under an appropriate tension and the dimensions can be set by the conditions (Shimbun
1985).
Cellulose fibers were soaked in a sodium hydroxide solution of 12 % w/w at room
temperature for 2 h, enabling chemical molecules to penetrate through the crystalline region of
the cellulose. Acid hydrolysis with 1 M hydrochloric acid followed by alkaline treatment with 2
% w/w sodium hydroxide was applied to remove the undesired components.
After the successive chemical treatments, lignin was still remained within the fibers and
removed by multi-stage bleaching. The bleaching was done in four different stages:
1) Chlorine dioxide stage (D), where the fiber consistency was adjusted to 3.5 %. Sodium
chlorite solution was applied based on the Kappa number of the fibers. Therefore the Kappa
number was determined and then chlorine dioxide was calculated based on lignin content in the
sample. The retention time was 1 h.
2) Extraction stage (E), where the consistency was adjusted to 10 % using boiling water.
Sodium hydroxide and peroxide was added to the fiber stock based on 1 % O.D. fiber under
mechanical stirring.
3) Acid stage (A), where the consistency was adjusted to 4 % and sulfuric acid was added
to fiber mixing well for 1 h
4) Peroxide stage (P), where the consistency was adjusted to 10 % and peroxide was added
to fiber. After filtration, the fiber was washed and air dried.
Process for chemical pulping was obtained from literature (Annergren et al. 1998).
However, at this purification level, microfibrils are not individualized and further steps of
mechanical treatments are needed.
42
d to dry.
4.2.2.2 Mechanical Treatments
The fibers were first cryo-crushed with liquid nitrogen to reduce the length and size. The
objective of the cryo-crushing is to form ice crystals within the fiber cell wall. When high
impact is applied on the frozen fiber, ice crystals exert pressure on the cell wall, causing a
rupture and thereby liberating the microfibrils (Sain and Bhatnagar 2004). The chemically
treated and cryo-crushed fibers were then diluted in water and dispersed evenly in a
disintegrator (CRAMER) for 10 minutes. The disintegrator is used to disperse the fibers
uniformly in the water suspension before the high pressure defibrillation process. The water
suspension with higher concentration of fibers (1-2 %) was subsequently passed through the
defibrillator (purchased from GEA-modified by the Centre for Biocomposites and Biomaterials
Processing Lab, University of Toronto, ON). The pressure was above 500 bar and several passes
were needed to crush the cell wall and fully release the nanofibers. The detailed method for
mechanical treatment to produce the nanofibers is described in recent patents (Sain and
Bhatnagar 2004; Sain 2006).
4.2.3 Microscopy characterization
Scanning electron microscope (JEOL JSM-840, Tokyo, Japan) was used as a routine for
microstructural analysis of the fibers after various stages of chemical and mechanical
treatments. All images were taken at an accelerating voltage of 15 kV. The sample surfaces
were coated with a thin layer of gold using an Edwards S150B sputter coater (BOC Edwards,
Wilmington, MA) to provide electrical conductivity.
Transmission electron microscopy (TEM) observations were achieved with a Philips
CM201 (Philips, Eindhoven, The Netherlands) operated at 80 kV. A drop of a dilute cellulose
nanofiber suspension was deposited on carbon-coated grids and allowe
43
Atomic force microscope (AFM) study was obtained using a Digital Instruments
Dimension 3100 AFM (Veeco Metrology Group, Santa Barbara, CA) with a nanoscope IIIa
controller. The system was operated in tapping mode at room temperature with DI tapping mode
tips having a resonant frequency of 280 kHz. A droplet of the aqueous nanofiber suspension was
allowed to dry on a cleaved mica surface.
4.2.4 Chemical characterization of fibers
Over the different stages of nanofiber development, untreated fibers, acid/alkali treated
fibers, bleached fibers and nanofibers were chemically analyzed for hemicellulose, lignin and
cellulose content. The procedure used here for cellulose determination was given by Zobel et al.
(1966). Lignin content was determined based on TAPPI T 222 om-02.2002 (acid-insoluble
lignin in wood and pulp) and TAPPI Useful Method UM250 (raw material and pulp-
determination of acid-soluble lignin).
4.2.5 Spectroscopy
Fourier Transform Infrared Spectroscopy (FTIR), (TensorTM 27, Bruker Optics Inc.,
Billerica, MA) was used to identify the removal of pectins at different purification levels by
measuring the transmitted radiation of various infrared light wavelengths of pectin functional
groups in the sample. The TENSOR 27 standard FTIR spectroscopy was used to obtain spectra
for the fibers after each chemical treatment. Fibers were ground and mixed with KBr
(sample/KBr ratio, 1/99) to prepare pastilles. FTIR spectra were recorded in a spectral range of
4000-400 cm-1 with a resolution of 4 cm-1.
4.2.6 X-ray analysis
The crystallinity determination was made using a powder X-ray diffraction method
44
nsmission mode.
(PXRD). X-ray crystallography was carried out to investigate the relative crystallinity after
various stages of the chemo-mechanical treatment and of nanofibers obtained by air drying of
the nanofiber suspension. A D8 Advance Bruker AXS diffractometer (BRUKER AXS Inc.
Madison, WI), Cu point focus source, Graphite monochromator, and 2D-area detector GADDS
system were used. Samples were analyzed in tra
4.2.7 Estimation of average degree of polymerization of the cellulose in nanofibers
With linear macromolecules, viscosity number measurements can provide a method for the
rapid determination of molecular weight when the relationship between viscosity and molecular
weight has been established. To estimate the average degree of polymerization of extracted
cellulose nanofibers, the method of ASTM D 1795-96 was used. This test gives the value of
intrinsic viscosity which may be converted to degree of polymerization (DP).
4.3 Results and Discussion
4.3.1 Individualization of cellulose nanofibers
The individualization step of nano-sized fibers from the plant cell walls requires chemical
and mechanical treatments. The properties of the nanomaterials cannot be physically measured
without separating them from the plant cell wall. These treatments may result in significant
chemical or mechanical damage on the fibers.
Figure 12 shows how the fiber morphology is changed from the micro to the nanoscale
during the individualization process. Figure 12a shows an untreated hemp fiber bundle where
the individual fibers are bound together by lignin. The size of the bundle is around 100-200 µm.
In Figure 12b, it is clearly visible that the chemical treatments are reducing the bundle size and
the surface roughness compared to the fibers in Figure 12a. Figure 12c shows how the
morphology is affected by the cryo-crushing. This process imparted sufficient energy to break
45
the bundles into single fibers which are around 20 µm in width. In 12d, the single fibers are
defibrillated showing a web-like structure. No individual hemp fibers are visible after the
defibrillation step and the size is reduced to the nm level. The fibrillar structure of individual
fibers was revealed from the Figure 12e after the fiber bleaching and may be due to the leaching
out of waxes and pectic substances. In Figure 12f, it was observed that the diameter of cryo-
crushed fibril with bleaching is much smaller compared to cryo-crushed fibril without
bleaching. High pressure defibrillation provided high turbulence and shear that created an
efficient mechanism of reduction in size. Figure 12g shows the structure after the high pressure
defibrillation, showing nanoscale fibrils and microfibril bundles contributed a unique
morphology of the interconnected web-like structure of fibrils. This combination of forces
promoted a high degree of microfibrillation of cellulose fibers, resulting in cellulose nanofibers.
The homogenizer is normally used in order to obtain fine particle dispersion giving a
uniform and homogeneous end product. Piston pump homogenizers force the sample material
through small tubes via piston pump action. The homogenizer consists of a 3 cylinder positive
piston pump and homogenizing valve. The pump is turned by electric motor through connecting
rods and crankshaft. As it first enters the valve, liquid velocity is about 4 to 6 m/s. It then moves
into the gap between the valve and the valve seat and its velocity are increased to 120 meter/sec
in about 0.2 millisec. The liquid then moves across the face of the valve seat (the land) and exits
in about 50 microsec. The homogenization phenomena is completed before the fluid leaves the
area between the valve and the seat, and therefore emulsification is initiated and completed in
less than 50 microsec. The whole process occurs between 2 pieces of steel in a steel valve
assembly (Figure 13 and Figure 14).
a
20µm
46
c db
20µm 20µm20µm
e
20µm
f
20µm
g
20µm
Figure 12. Scanning electron micrographs of: (a) untreated fiber, (b) after acid and alkaline treatment, (c) after cryo-crushing, (d) after defibrillation, (e) after bleaching, (f) bleaching followed by cryo-crushing, (g) bleaching followed by cryo-crushing and defibrillation.
Figure 13. The impact head set of homogenizer.
Figure 14. 3D version of the impact head and the passenger head of homogenizer.
Figure 15 demonstrates how the number of passes through a defibrillator is affecting the
individualization of hemp nanofibers. Figure 15a shows the morphology after 5 passes which
47
did not result in nanofiber structure. The fibers were still entangled with each other and the size
was in the range of microns. In 15b, after 10 passes, the fibers were split apart into smaller
bundles. Figures 15c and 15d show a large extent of defibrillation after 15 passes and 20 passes,
these small bundles were additionally separated into thinner fibril bundles increasing the
exposed surface area of the cellulose (15d). High pressure and high energy were needed to
defibrillate hemp fibers and achieve acceptable dispersion level. The separated fiber bundles
were shown to create small entanglements that were fibrillated into smaller entities as the
number of passes through the defibrillator was increased (Nakagaito and Yano 2004).
48
ba
20µm20µm
20µm20µm
c d
Figure 15. Scanning electron micrographs of: bleached hemp nanofibers after 5 (a), 10 (b), 15 (c) and 20 (d) passes during the defibrillation.
Figure 16, compares the structure of unbleached and bleached nanofibers. Figure 16a
shows that the unbleached nanofibers were either individual or bundles and coarser compared to
the bleached nanofibers, shown in Figure 16b. The Figure 16b shows that bleached nanofibers
49
were thinner and shorter than unbleached nanofibers in diameter and length. It is likely that
harsh chemicals used for bleaching will reduce the chain length of the cellulose which can result
in cutting the length and weakening cellulose nanofibers. Aspect ratio (fibril length to diameter
ratio) is one of the most important parameters in determining reinforcing capability of the
nanofibers. Aspect ratios of the extracted cellulose nanofibers were estimated from transmission
electron micrographs. In some cases, total fibril length was not visible, therefore only the visible
portion was considered for the calculation in TEM graphs (provide statistical significance of this
assumption). The aspect ratio of these bleached nanofibers (82) is comparable to unbleached
nanofibers (88). We can expect a high reinforcing capability from both nanofibers. There is a
direct relation between degree of polymerization and length of the nanofibers, as cellulose
synthesizes in extended chain conformation. Figure 17 shows the determination of relative
viscosities at different concentrations for bleached and unbleached nanofibers. To obtain the
approximate degree of polymerization (DP) of cellulose nanofibers, the value of intrinsic
viscosity was multiplied by 190 (ASTM D1795-96). Unbleached nanofiber has a similar valve
of DP (1155) compared with that of bleached nanofibers (1138) (Table 4). It is noteworthy here
that the value obtained has relative, not absolute significance. The cellulose nanofibers obtained
from chemo-mechanical methods may certainly find applications, because of higher DP and
better reinforcing capacity due to high aspect ratio.
The nanofiber suspension obtained after the high-pressure defibrillation was also analyzed to
determine the width using AFM, shown in Figure 18. It is seen that the hemp fibers are indeed
nano-sized and the width is within the range of 30-100 nm. The length is estimated to be at
micrometer level. The size of the soybean nanofibers was estimated to be within the range of
50-100 nm in width and several µm in length (Figure 19); therefore these microfibrils have been
referred to as cellulose nanofibers. The network of nanofibers can also be seen in force mode
image Figure 18(a) by interwoven microfibrils overlapping each other. As evident from the
TEM and AFM images, high-pressure defibrillation leads to individualization of the cellulose
nanofibers from the cell wall without degrading them. Figure 20 shows the width distribution of
the nanofibers obtained through chemo-mechanical treatments. In Figure 20 (a), it was observed
that the widths of unbleached nanofibers were estimated between 50-100 nm and most of them
had a diameter range of 70 to 100 nm. Bleached nanofibers had smaller widths (30-100 nm)
compared with that of unbleached nanofibers as shown in Figure 20 (b). Most of bleached
nanofibers had a diameter range of 30-50 nm. By the fiber image statistical analysis, the average
width of unbleached nanofiber was 87.5 nm and of bleached nanofiber was 54 nm based on
averaging 50 individual fibers. It is expected that better reinforcing effect will be obtained by
using bleached nanofibers. It is clearly shown from The TEM picture (Figure 16) and the fiber
size distribution chart (Figure 20) that the size is less in bleached nanofibers, but more surface
areas will be contributed from these fibrils.
Figure 16. Transmission electron micrographs of hemp nanofibers (a) unbleached, (b) bleached under the same magnification (15,000×).
50
500 nm500 nm
ba
Figure 17. Determination of relative viscosities at different concentrations (a) unbleached, (b) bleached nanofibers.
(a)
)
able 4. Degree of polymerization of unbleached and bleached HPN.
Unbleached HPN Bleached HPN
y = - 106. 3x + 5. 9888R2 = 0. 995
0
1
3
4
5
6
0 0. 005 0. 01 0. 015 0. 02 0. 025Concent r at i on
scos
ity
51
2Vi
Unbl eached HPN
y = - 104. 58x + 6. 078R2 = 0. 9902
0
1
2
3
4
5
6
7
0 0. 005 0. 01 0. 015 0. 02 0. 025Concent r at i on
Visc
osit
y
Bl eached HPN
(b
T
Viscosity 5.9888 6.078 Polymerization 1137.872 1154.82
Figure 18. Atomic force micrographs of unbleached hemp nanofibers (a) force mode image, (b) eight mode image.
Figure 19. Atomic force micrographs of SBN.
h
a b
500 nm 500 nm
500nm
52
53
igure 20. Size distribution of hemp nanofibers (a) unbleached, (b) bleached.
a)
F
(
0
0. 1
0. 2
0. 3
0. 4
Wi dt h Range ( nm)
Freq
uenc
y of
Wid
th
<30 30- 50 50- 70 70- 100 >100
(b)
0
0. 1
0. 2
0. 3
0. 4
0. 5
Wi dt h Range ( nm)
Freq
uenc
y of
Wid
th
<30 30- 50 50- 70 70- 100 >100
4.3.2 Chemical characterization of individualized nanofibers
The fibers obtained after the chemical treatment contained mainly alpha-cellulose with
some hemi-cellulose and lignin. As shown in Table 5, the α-cellulose content in the chemically
treated fibers was 94 % as compared to the original 75 % and the hemicellulose content was
54
reduced to 1.6 %. Chemical analysis of these fibers after each stage of the purification showed a
drastic increase in cellulose content and a decline in hemicellulose and lignin content.
Lignocellulosic fibers contain a bit amount of hemicellulose, which is a hetero polysaccharide
consisting mainly of pentoses and hexoses. The treatment of cellulosic, starch, or hemicellulosic
materials using acid solution to break down the polysaccharides to simple sugars allows the
solubilization of both pectins and hemicelluloses. Dilute sodium hydroxide treatment of
lignocellulosic fibers causes separation of structural linkages between lignin and carbohydrate
and disruption of lignin structure (Annergren et al. 1998). Fiber samples from hemp appeared
color even after carrying out the acid and alkali treatments. Chemically treated
hemp fibers were bleached before proceeding to the mechanical treatment to ensure that most of
the lignin was removed from the fibers. After bleaching treatment, it was found that nanofibers
contain both soluble and insoluble lignin. The lignin content of hemp significantly decreased
from 6.6 % to 3.18 %. According to the result of successive bleaching extractions, the
nanofibers lost most of their non-cellulosic constituents. Cellulose can be also partially degraded
during the bleaching process.
brownish in
Table 5. Chemical analysis of hemp fibers after selective chemical treatments.
Holocellulose (%)
α-cellulose (%)
Hemicellulose (%)
Insoluble lignin (%)
Soluble lignin (%)
Total lignin (%)
Untreated fibers 86.22 75.56 10.66 4.89 1.72 6.61
Acid treated 91.42 85.66 5.76 4.49 0.66 5.15 Acid and Alkaline treated
92.82 89.78 3.04 4.40 0.53 4.93
Bleached 95.72 93.87 1.85 2.83 0.35 3.18 Nanohemp 96.12 94.53 1.59 2.53 0.18 2.71
55
y treated fibers was 61 % as compared to the original 41 % and the hemicellulose
ontent reduced to 9.9 %. After chemical treatment, it was found that nanofibers contained both
cantly decreased
from 20 % to 6.5 %; Figure 22 confirm remov c n. ical
analys tage of purification showed a drastic increase in cellulose
co e declin emicellulose and lignin content. After chemical and m ical
treat e nanofi ost most of the non-cellulosic consti s. The fine st
cell rials is sed of c ine and a hous reg It is desirable to retain
ure cellulose, whose crystalline form and high packing density result in a stronger composite,
while removing components such as hemicellulose and lignin, which are amorphous, can easily
abs
Chemically treated soybean stock fibers were analyzed for hemicellulose, lignin and
cellulose contents. As shown in Figure 21, it was observed that α-cellulose content in the
chemicall
c
soluble and insoluble lignin. The high lignin content of soybean stock signifi
al of a signifi ant portion of ligni Chem
is of these fibers after each s
ntent and th e in h echan
ments, th bers l tuent ructure of
ulose mate compo rystall morp ions.
p
orb chemicals and tend to reduce the mechanical strength. Cellulose nanofibers are
embedded in an abundant matrix consisting of hemicellulose and pectin where acid and alkali
treatments are applied. These lead to almost pure cellulose fibers which ensure the high stiffness
and strength of the fibers, removing not only the lignin, hemicellulose and pectin, but also the
surface impurities, waxy substances and hydrophilic hydroxyl groups.
56
Figure 21. Chemical changes of soybean stock.
F
igure 22. Lignin content changes of soybean stock.
Chemical treatments lead to almost pure cellulose fibers, which ensure the high stiffness
mp reinforced composites changes as a
function of hemp cellulose microfibrils purity level (Nakagaito and Yano 2004). Although
and strength. The mechanical behavior of nanohe
57
cellulose possesses excellent strength and good stability, it can be partially degraded due to the
harsh chemical and mechanical treatment. The alkali extraction is expected to hydrolyze pectin
by a β-elimination process and solubilize it (Bhatnagar and Sain 2005; Sain and Bhatnagar
2003). Figure 23 shows the transmission electron micrographs of hemp nanofibers under two
controlled concentrations of NaOH solution for the fiber pretreatment. 17.5 % of sodium
hydroxide induced undesirable reactions to cut down the cellulose chains, therefore reducing the
aspect ratio of nanofibers. Too harsh treatments led to the loss of the microfibrillar morphology
as seen in Figure 23(a). In Figure 23(b), it was shown that the hemp fibers released their
nanofibers either individually or in bundles at a lower alkali content leading to the formation of
icrofibrils.
ouping of acidic structural polysaccharides. The untreated,
hemically treated and bleached hemp fibers were characterized by FTIR, shown in Figure 24.
a strong network of m
Figure 23. Transmission electron micrographs of hemp nanofibers (a) under 17.5 % alkali extraction, (b) under 12 % alkali extraction (15,000×).
Pectin is a heterogeneous gr
a b
500 nm 500 nm
c
By this technique, it was possible to follow the removal of pectins due to the vanishing of the
58
0 cm-1, acetyl and methyl ester group at
1590 cm-1 and 1240 cm-1 (Sain and Bhatnagar 2003). In the transmittance spectra, the
absorption band assigned to the pectin carboxylic groups was observed at 1739 cm-1 in untreated
fibers, but disappeared upon chemical treatments and the successive bleaching. This is because
the carboxylic groups were partially removed by alkali treatment through a process called de-
esterification. During de-esterification, the ester groups on the pectin can be removed as well.
The alkali treatment allows the ionization of pectin carboxylic groups (-COOH) and the
formation of the corresponding sodium carboxylate (COONa), which decreases the ability of
hydrogen-type intermolecular bonds t Sain tnagar 2003) and the solubility of the
pectins. Reduction in the peak intensity found at around 1631-1633 cm-1 in chemically treated
and
-1
-1
characteristic bands for the carboxylate groups at 174
o form ( and Bha
bleached fibers indicates the partial reaction of the C=O bonds of hemicelluloses. The
intensity of the 1259 cm peak is sharply weakened after the bleaching treatment, due to the
removal of hemicellulose materials. The peak observed at 895 cm in both untreated and
chemically treated fibers indicates the presence of the glycosidic linkages between the
monosaccharides and disappears in the spectrum for bleached fibers.
400 600 800 1000 1200 1400 1600 1800 2000 2200
Wavenumber cm-1
Tran
smitt
ance
(%)
Untreated Chemically treated Bleached
1739
1631
1259895
(c) after bleaching.
.3.3 X-Ray Diffraction
graphy was used to investigate the crystallinity of the sample after different
eatments. X-ray powder diffraction photographs from untreated, acid and alkali treated fibers
nd nanofibers are shown in Figure 25. The percentage crystallinity of these samples was
alculated based on X-ray analysis by Equation (1) and they are given in Figure 25 as well. The
ain diffraction intensity was at about 2θ = 21° for each sample. The peak observed close to 2θ
22.4º is from cellulose. Untreated fiber exhibited very low crystallinity (57.4 %) and a single
eak at about 2θ = 22.75º and a broad hump showing amorphous nature. It can be seen that acid
eated and acid/alkali treated fibers show peaks at 2θ = 22.8º and 2θ = 21.4º respectively. In the
ase of acid treated fibers, an additional peak was seen at 2θ = 20.4º with a small hump. It is
ossible that acid treated fiber contains some residual lignin and hemicellulose that contribute to
lower crystallinity (61.9 %). Further the hump has disappeared showing that
Figure 24. FTIR spectra of hemp fibers (a) untreated, (b) after acid and alkaline treatment and
4
X-ray crystallo
tr
a
c
m
=
p
tr
c
p
the slightly 59
60
morphous chains have been rearranged to crystalline regions. Acid/alkali treated fiber shows
nly one prominent peak and exhibits more crystalline nature (69.7 %) than acid treated fiber.
stalline feature with a narrow strong peak at 2θ = 21.6º. This peak
can
a
o
Nanofibers showed a high cry
be attributed to cellulose crystals. It was observed that the hemp nanofibers had an increased
crystallinity from 57.4 % of untreated hemp fibers to 71.2 % of nanofibers. As shown in Figure
25, the relative crystallinity of the samples increased after each stage of chemical treatments.
Consecutive chemical treatments and processing of cellulose fibers give different X-ray
patterns. According to X-ray testing on cellulose, cellulose is not made up of single perfect
crystals. Disordered cellulose molecules as well as hemicelluloses and lignin are located in the
spaces between the microfibrils. The hemicelluloses are considered to be amorphous although
they apparently are oriented in the same direction as the cellulose microfibrils. Lignin is both
amorphous and isotropic. The crystallites have length of about 60 nm and width ranged from 5
to 10 nm. Each are perfectly aligned cellulose chains. It is believed that these crystallites are
connected to each other by disoriented amorphous zones (Stamm 1964). The crystalline nature
of the cellulose nanofibers is not only influenced by the chain conformation but also by the
packing of adjacent chains. Nanofibers are pure cellulose chains having different arrangements
of the glucose chains as in the native cellulose of untreated fibers. The hemp nanofiber, after
successive chemical treatments, only possessed 4.3 % lignin and hemicellulose combined. The
x-ray powder diffraction pattern showed a narrow peak which was more prominent and sharp
for nanofiber, indicating the crystalline nature of this reinforcement and higher relative
crystallinity.
(1)
100⋅+
=amorphousIecrystallinI
ecrystallinC,%I
61
t 2θ = 22.0 º and these peaks are not sharp. In Figure 27, it is the pictorial view of
e actual x-ray pattern of the samples, the sharper patterns corresponding to the high crystalline
pea hs. o
ery low crystallinity of the soybean nanofibers. The soybean stock nanofiber, even after
chemical treatments, possesses 16 % lignin and hemicellulose combined. These probably hinder
the complete separation of nanofibers and also add to the amorphousness of the fibrils. The X-
ray powder diffraction pattern is indicative of a crystalline material and shows peaks at 2θ =
20.3 º and 22.2 º which are less prominent and weak, indicating lower crystallinity. It was
observed that the soybean nanofiber gave a relative crystallinity calculated at 48.4 %. As shown
in Figure 26, the crystallinity of the samples increases after each stage of chemical and
mechanical treatments. Nanofibers are pure cellulose chains having different arrangement of the
glucose chains as in the native cellulose of untreated fibers. During the chemical treatment, the
cellulose chain breakage would occur at the amorphous regions first. The amorphous regions in
the cellulose chains are susceptible to water or chemical penetration and degrade before the
A dominant feature of most of the cellulose is the molecular orientation in cellulose fibrils
and along the cell wall axis of fibers. Strong orientation along these axes is associated with high
tensile strength and has an intense effect on the mechanical properties of the cellulose fiber-
reinforced composites. The presence of disordered cellulose in all cellulosic materials has a
profound technological significance, as these regions are those which are most susceptible to
sorption and chemical modification.
X-ray powder diffraction photographs from untreated, acid and alkali treated fibers and
nanofibers are shown in Figure 26. Untreated fibers exhibit an amorphous nature observing a
narrow peak at 2θ = 19.0 º and a broad hump. The acid treated and acid/alkali treated fibers
show peaks at 2θ = 19.2 º and 2θ = 20.1 º. In the case of acid/alkali treated fibers, additional
peak is seen a
th
ks in the grap S ybean nanofibers show a diffused and very dull pattern, indicative of
v
crystalline region. As a result, the structure has an increasing crystallinity throughout the
chemical treatment. During the mechanical treatment, the amorphous region is less stiff when
compared to the crystalline region and is more susceptible to shear and stain under high
pressure. Therefore, fiber disintegrates mostly at the amorphous region and the crystallinity of
the final product increases.
Figure 25. X-ray diffractometry and crystallinity estimation after each stage of chemo-mechanical treatment for hemp fiber.
10 15 20 25 30 35
Inte
sity
.u.)
Diffraction angle 2θ (degree)
n (a
Untreated (57.4%) Acid treated (61.9%)Acid & Alkaline treated (69.7%) Nanofiber (71.2%)
62
63
Figure 26. X-ray diffractometry and crystallinity estimation after each stage of chemo-mechanical treatment for soybean stock.
10 15 20 25 30 35Di f f r act i on angl e 2θ ( degr ee)
Inte
nsit
y (a
.u.)
Unt r eat ed Aci d t r eat edAci d & Al kal i ne t r eat ed Nanof i ber
46. 6347. 20
47. 29
48. 41
64
Figure 27. X-ray crystallographs to demonstrate the crystallinity of (a) soybean stock nanofiber, (b) hemp nanofiber.
(a)
.4 Conclusions
This study has been concerned how the degree of individulization affects the cellulose fiber
orphology from the micro to the nanoscale. The chemo-mechanical treatment resulted in hemp
iameter range of 50-100 nm. The used chemical treatments resulted in the individualized hemp
(b)
4
m
nanofibers having a width in the range of 30-100 nm; soybean pod nannofibers having a
d
65
ofibers and further mechanical treatment formed a network structure of hemp nanofibers.
he high pressure defibrillation contributed a unique morphology of the interconnected web-like
tructure of nanofibers.
Chemical analysis of the cellulose fiber after each stage of purification showed an increase
cellulose content and a decrease in lignin and hemicellulose content. Successive bleaching
elped with the cellulose purification. FTIR graph indicated the partial removal of the pectins
uring the fiber extraction. It was also seen that the relative crystallinity of the cellulose fibers
creased after each stage of chemical treatments.
micr
T
s
in
h
d
in
Cellulose Chain
5. DISPERSION MECHANISM OF NANOFIBERS
5.1 Introduction
The focus of this work is the study of the dispersion mechanism of cellulose nanofibers in a
plastic matrix. Cellulose fibrils have a high density of -OH groups on the surface, which have a
form hydrogen bonds with adjacent fibrils, reducing interaction with the
urrounding matrix. Agglomeration is the formation of groups of cellulose fibers due to the
ydrogen bonds between each of them (Figure 28). This formation of hydrogen bonds accounts
bers. As a result of the tendency for cellulose
bers to group together, it is problematic to combine the hydrophilic nanofiber with a non-polar
olymer matrix. The cellulose fibers are not compatible with the hydrophobic polymer matrix
erate, deteriorating their reinforcing capability. Overcoming strong hydrogen bonds
quires high energy. The high pressure and high energy are imparted to the cellulose fibers to
efibrillate fibrils intensively and improve the dispersion of fibers.
The hydrogen bonds between the cellulose chains.
For demonstrating a reinforcing potential, the cellulose nanofiber should not get
tendency to
s
h
for the hydrophilic properties of the cellulose fi
fi
p
and agglom
re
d
Figure 28.
66
67
agglomerated o n imized. There
hould be proper dispersion of the fibers in the polymer matrix. Fibers should be uniformly
d not concentrated in some specific zones (Figure 29). The fibers and the
pol
Due to their small diameters, the nanofibers have a very high surface area. This increases
the probability of the hydrogen bonds to make them collapse back into agglomerates or to form
n entangled mass. This poses the greatest problem in retaining the cellulose nanofibers with
eir nano-level diameters after their isolation. Nanofibers are much more agglomerated in the
bsence of water, demonstrating the role of hydrogen bond in interfibrillar dispersion.
r e tangled. In other words, interfibrillar attraction should be min
s
spread in the matrix, an
ymer should be chemically compatible. This is necessary to ensure sufficient adhesion
between the matrix and the fiber. Low interfacial properties between fiber and polymer matrix
often reduce the potential of natural fibers as reinforcing agents (Mohanty et al. 2001).
Interfaces play an important role in the physical and mechanical properties of composites
(Joseph et al. 2000). All these factors are related to the aspect of dispersion and are described in
detail in this chapter.
Figure 29. Dispersion of nanofibers in the plastic matrix.
Good Dispersion Agglomeration
Matrix
Fiber
a
th
a
68
pplication of shear alone is sufficient to disperse the fibers in the polymer if the reinforcing
hase is compatible with the matrix. For example, if the matrix is hydrophobic (as in the case of
ost derived polymers), the fibers should be hydrophobic as well to obtain proper interfacial
adh
anofibers in the Polymers
Due to high interfibrillar hydrogen bond density among nanofibers and high surface area,
y reducing the effective aspect ratio.
This, in turn, reduces the mechanical properties of the resulting composite. Therefore, it is
critical to keep the nanofibers well apart during the composite processing stage.
This work describes the processing of two types of composite. The first type deals with
film casting in water soluble polymers. In order to reduce the interaction between hydroxyl
groups, the nanofibers obtained after chemo-mechanical treatments are kept in water
suspension. Water is the most widely used carrier to dis Therefore the
use of n tly restricted to water soluble po ncorporated
to a cellulose aqueous suspension. The advantage of these polymers is that the nanofibers in
sus
A
p
m
esion; however, cellulose fibers are hydrophilic and have high surface energy. Therefore, the
surface energy of cellulose fibers often has to be lowered through surface modification to make
them compatible with hydrophobic polymers. This is particularly important for cellulose
nanofibers because of their high surface area of interaction with the matrix. On the other hand, if
the matrix is hydrophilic (with PVA and the naturally derived biopolymers), the problem with
incompatibility is not encountered. However, the relative hydrophilicity/surface energy of the
two phases is still a concern and the strong interfibrillar hydrogen bonding often impedes
sufficient dispersion of the cellulose fibers in hydrophilic matrix as well.
5.2 Dispersion of N
these fibers have a strong tendency to agglomerate, thereb
perse cellulose nanofibers.
anofibers has been mos lymers, which are i
in
pension can be easily mixed with the polymer by solution casting. In addition, the presence
69
er required which was
very time-consuming method of adding the aqueous phase fiber suspension mixing with
sible method where the two phases could be
me
sion with poly
(vin
of adequate amount of water prevents fiber agglomeration in solution casting. However,
solution casting is not a commercially viable process, and is used only in biomedical
applications where the cost associated with the process is not a concern. Moreover, the water
soluble thermoplastics available are not derived from natural sources. Nevertheless, solution
casting is discussed in this work in order to demonstrate the reinforcing potential of the
nanofibers.
Cellulose nanofibers have not been used extensively in the common thermoplastics, as poor
dispersion of the filler in the matrix of a composite material seriously affects its mechanical
properties. But to expand the horizon of bio-based nanocomposites for high-end applications, it
is necessary to reduce the entanglement of the fibrils and improve their dispersion in the
polymer without deteriorating their reinforcing capability. The second type of dispersion in
composites deals with water insoluble polymers. Typically, this requires the fibers to be added
to the molten matrix in a solid form. Researchers (Oksman et al. 2006) have been able to
develop stable suspensions of cellulose whiskers to pour into the twin-extruder mixing with
polymer matrix directly. To keep the fibers apart, a certain amount of wat
a
polymer matrix. That was not a commercially fea
lt-mixed directly. But if the fibers are dried before adding them into the polymer mass, as in
conventional melt-mixing, the fibers agglomerate right away. This will result in poor fiber
dispersion in the matrix, and consequently, poor mechanical properties of the resulting
composites. Chazeau et al. (2000) mixed cellulose whiskers in water suspen
yl chloride) obtained by emulsion polymerization, freeze-dried the mixture, made it into a
powder, and then prepared the composite by hot-mixing.
In this work, it is focused on synthesizing the nanocomposite using a solid phase matrix
70
®
®
5.2.1.2 Nanocomposites Processing
To manufacture the biocomposite, Elvanol PVA powder was dissolved in water at 80-90 °C
with continuous stirring by magnetic stirrer (Figure 30). When all the solids were dissolved,
nanofiber suspension was added to the solution. The cellulose nanofibers content was 5 % or 10
%. After the solution became viscous, the solution was poured onto levelled Pyrex glass Petri
dishes and then heated in a 50 °C oven for overnight until dry. The films were finally removed
shows the film formation processing.
polypropylene (PP) or polyethylene (PE) by hot compression. Poly (lactic acid) (PLA) and
polyhydroxybutyrate (PHB) based nanocomposites using cellulose nanofibers were prepared by
extrusion and injection molding.
5.2.1 Dispersion of nanofibers in water soluble polymer-PVA
5.2.1.1 Materials
The raw material used in this study was soybean stock from Ontario soybean producers.
PVA (Elvanol 70-06, Dupont) was used as matrix. Ethylene-acrylic oligomer aqueous
emulsion (Poligen WE 4, BASF) was used as a dispersant.
(by peeling) from the trays and placed in desiccators to avoid moisture exchange. Figure 31
Figure 30.
In aqueous phase - film casting: PVA/nanofibers.
Biocomposite film~ 0.1 mm thick
Evaporation
PVA soluble in water80 Cº, 24 h
71
nd the non-polar characteristics of
ost thermoplastics result in difficulties in compounding the filler and the matrix (Dufresne et
al. 2000). It is possible to reduce the entanglement of the fibrils and improve their dispersion in
the
rent ways by using limited reaction conditions. In case I,
here the amount of oligomer used is limited and the extent of hydrophobization is low, the
orming chemical linkages
with hydroxyl groups of cellulose and ester groups of the oligomer. In that case the dispersion
would be incomplete, only half of the -OHs of the surface cellulose chains are accessible in a
Figure 31. Scheme of film formation.
5.2.1.3 Dispersion of Cellulose Nanofibers
The inherent polar and hydrophilic nature of nanofiber a
m
polymer by fiber surface coating. In this project, ethylene-acrylic oligomer was used as a
dispersant. One of the properties of the oligomer is to coat the cellulose nanofibers so that most
of them do not stick to one another. A suggested scheme for this process can be represented as
shown in Figure 32. The hydroxyl functions present on the nanofiber surface are esterified with
at least one organic compound comprising at least one functional group which can react with the
hydroxyl groups of the cellulose (Sassi et al. 2000). The cellulose chains may associate with
ethylene-acrylic oligomer in two diffe
w
acrylic oligomer may only partially disperse the cellulose fibers by f
72
given cellulose nanofiber; however in case II where the concentration of oligomer is high
enough, the acrylic oligomer could block all effective hydroxyl groups of
by fo s and repel both lulose ch d by
urface cha void agglomeration of nanofibers. In the latter case, since the oligomer has
hobic surface, the coated nanofibers can maintain a high
ispersion character in polymer matrix.
igure 32. A possible chemical scheme for the hydrophobization.
cellulose molecules
rming chemical bond
rges to a
sides of the cel ains that is stabilize
s
conferred to these nanofibers a hydrop
d
F
R = (H, aryl, alkyl, etc.)
The issue of nanofiber dispersion is critical to efficient reinforcement; the aggregates of
nanofibers effectively reduce the aspect ratio (length/diameter) of the reinforcement. Such fibers
are used only to a limited extent in industrial practice by difficulties in achieving acceptable
dispersion levels (Dufresne et al. 1997). Fig. 33 presents typical pictures of freeze-dried SBN.
73
in Figure 34. A drop of a dilute cellulose nanofiber
uspension was deposited on carbeon-coated grids and allowed them to dry. Figure 34a shows
ased their nanofibers either
dividual or in bundles in water suspension. Figure 34b shows in the homogenized sample, the
anofibers are completely disentangled and well individualized in oligomer emulsion. TEM
ages (Figure 34) of cellulose nanofiber suspension show that the soybean stock nanofiber
uspension with acrylic oligomer emulsion has better dispersion than the suspension without
crylic oligomer emulsion. The acrylic oligomer is an aqueous emulsion which is free of
mulsifiers. It is miscible with fiber water suspension in all properties. The use of the acrylic
ligomer made the nanofibers easier to disperse, enabling them to obtain a uniform dispersion in
e polymer matrix. This is because the acrylic oligomer reduces interaction between hydroxyl
roups by hydrophobization. The dispersibility can be explained by the acrylic groups present
n their surface, in the case of coated nanofibers, these groups making the nanofibers
ompatible with the matrix. More oligomer has to be used to disperse nanofibers that are
difficul the strong interfiber hydrogen bonds that cause fibers to
Fig. 33a is a typical SEM image of uncoated SBN. Each particle of SBN is an aggregation of
cellulose fibers due to the strong hydrogen bonds of adjacent molecules. The size of the fiber
bundle is at µm level. Fig. 33b shows a picture of ethylene-acrylic oligomer coated SBN with a
well-organized web-like structure. The morphology of coated SBN appears distinguishable
compared to uncoated SBN. The acrylic oligomer coated fibers formed loose networks during
freeze drying. It is proved that ethylene-acrylic oligomer could reduce the entanglement of the
nanofibers by inducing ethylene and acrylic acid functional groups on the SBN soaked in
dispersant solutions.
To confirm the separation of individual SBN in the dispersant solution, a diluted suspension
was observed using TEM and is shown
s
the chemo-mechanical treatment of soybean stock fibers rele
in
n
im
s
a
e
o
th
g
o
c
t to disperse, because
74
agg
lomerate have to be overcome. The strength of these hydrogen bonds depends on the polarity
of the nanofibers. Polar cellulose fibers can be dispersed more easily with polar oligomer,
because the oligomer is able to wet the fiber surface very effectively and to prevent
agglomerates of fiber.
75
ethylene-acrylic oligomer coated.
b)
Figure 33. Scanning electron micrographs of freeze-dried SBN samples: (a) uncoated and (b)
(a)
(
20µm
20µm
76
igure 34. Transmission electron micrograph of diluted suspension of SBN: (a) in water and (b) ethylene-acrylic oligomer emulsion under the same magnification (20,000×).
a)
b)
Fin
(
500nm
(
100nm
77
.2.2 Dispersion of nanofibers in commodity polymers
The matrix has to be present in a molten state to achieve proper mixing with the fibers. The
bers, however, may be introduced into the system in a couple of different ways: a) they can be
dded directly as a solid phase into the molten polymer, or b) may be introduced into the system
n this case, and
of dispersing the nanofibers in the matrix through solid phase dispersion were explored.
5.2.2.1 Materials
The raw material used in this study was soybean pods from Ontario soybean growers
eorgetown, ON). The soybean seeds were removed by hand. Mature soybean pods were taken
ff from the soybean stock. PP (DOW Polypropylene 5A10) and PE (DOWLEX™ 2021D) were
sed as matrix. Ethylene-acrylic oligomer aqueous emulsion (Poligen ES) was supplied by
ASF as a dispersant.
5.2.2.2 Nanocomposites Processing
A solid phase melt-compounding of nanocomposites was used to mix the nanofiber with PE
(Figure 35). 2.5 % and 5 % by weight of the coated nanofibers were added to molten PE
r PP in a laboratory C.W. brabender (Instruments, Inc.) at 170 °C. After the sample was well-
ixed, test samples were compression moulded with WABASH Hot Press into sheet form at
80 °C and under a pressure of 50 MPa.
5
fi
a
as a suspension in a liquid phase. These options define the principles of solid-phase dispersion
nd liquid-phase dispersion respectively. Solid phase dispersion was applied ia
ways
(G
o
u
B
or PP
o
m
1
78
igure 35. Processing of nanocomposites.
n)
as used as a routine for microstructural analysis of the nanofibers with/without surface
coatings. All images were taken at an accelerating voltage of 15 kV.
Optical microscopy (Figure 36) compares the solid phase nanocomposite with and without
using the acrylic oligomer during processing. For solid phase nanocomposites, it was found that
coating the nanofibers introduced into the molten PE phase improved their dispersion. In Figure
36a, which corresponds to not using acrylic oligomer, some nanofibers are not fully
individualized and form small aggregates. Many agglomerated fibers of micrometer size are
visi n-uniform aspect. In contrast, in Figure 36b, it is
obs
prove the compatibility and dispersion of
F
Polymer
Biocomposite film~ 0.1 mm thick
5.2.2.3 Characterization
Optical microscopy (Polyvar Operating Instructions) was carried out to clearly show the
dispersion of nanofibers in solid phase polymer matrix. SEM (JEOL JSM-840, Tokyo, Japa
w
Internal mixer Compression molding
High pressure High temperature High temperature
Nanofibers in freeze-dried phase
ble. The nanofiber films display a no
erved that there is a very limited amount of microfibers, the fiber diameters being much
smaller. This demonstrates that the nanofibers were dispersed more effectively in the composite
in achieving acceptable dispersion level when they were coated with the acrylic oligomer which
acts as a bridge between the hydrophilic and hydrophobic components. In this project, the use of
ethylene-acrylic oligomer aqueous emulsion can im
79
ellulose nanofibers in non-polar polymers such as PE, PP for the development of transparent,
olorless coatings with fade-resistant, antistatic, antimicrobial, conductive and heat-blocking
roperties.
gomer coated nanofibers and PE or PP are not
ompletely miscible under hot compression. Fig. 37 shows an overview picture of the PE/SBN
omposite. It is difficult to see any SBN in this sample. These white small spots indicate that the
ellulose nanofibers were not uniformly dispersed in the PE matrix and th
egrade processing. Further work is required to better understand the dispersion
echanism of nanofibers into the solid phase melt-mixing.
igure 36. Optical microscopy (125×) sh ase nanocomposite: (a) soybean tock as reinforcement without coating and (b) soybean stock coated with ethylene-acrylic
olig
c
c
p
Unlike PVA/SBN nanocomposite, oli
c
c
c e cellulose may be
d d during
m
F owing the solid phs
omer.
a b
20µm 20µm
80
(lactic acid) (PLA), and the other is
olyhydroxybutyrate (PHB). These are the biopolymers most commonly available in the market.
y for
olid phase dispersion.
een nanofibers and the polymer matrix leads to a decline in
mechanical properties of the nanocom osites. In recent years a deeper understanding has been
achieved related to surface phenomen . This has led to an introduction of more sophisticated
approaches, which allow for a study of thermo-dynamic and kinetic information. One technique,
which has been shown to be very valuable, is inverse gas chromatography (IGC). In IGC, a solid
material under investigation is used as the stationary phase. An empty column is filled with the
orous) material (adsorbent) and the adsorbate molecules in the mobile phase probe the surface
f the adsorbent (Thielmann 2004). The surface energy of a material can be described by the
um of a dispersion component and a specific interaction component (Gulati and Sain 2006).
Figure 37. An overview of the PE/SBN nanocomposites taken with SEM.
5.2.3 Chemical dispersion of nanofibers in biopolymers
In this work, two bioplastics are considered - one is Poly
P
2µm
The properties of these two polymers were carefully considered to study their suitabilit
s
Poor interfacial adhesion betw
p
a
(p
o
s
81
he dispersion component refers to London dispersion forces, and the specific component refers
the polar, ionic, electrical, magnetic, metallic, and acid-base interactions. Fowkes and
ostafa (1978) proposed that dispersion forces and acid-base interactions are the primary forces
the changes in the
ermodynamic properties of a nanofiber surface after treatment and for estimating the London
ispersion component of the surface free energy of nanofibers (before and after treatment).
ulati and Sain (2006) reported that alkalization and acetylation made the hemp fibers
mphoteric, thereby improving their potential to interact with both acidic and basic resins.
The goal of this work was to explore how various surface treatments would change the
ispersion component of surface energy and the acid-base character of hemp nanofibers, using
C. The cellulose nanofibers were extracted from hemp by chemo-mechanical treatments. PLA
nd PHB based nanocomposites using cellulose nanofibers were prepared by injection molding
nd hot compression. The cellulose nanofibers used in this study were treated by five different
c and SEM were used to investigate
the
5.2.3.1 Materials
Matrix
Poly (lactic acid) (PLA), Nature WorksTM 4031D, was supplied by Cargill Dow LLC,
Minneapolis, USA. The material has a density of 1.25 g/cm , a glass transition temperature (Tg)
of 58 °C, and a melting point of 160 °C. Polyhydroxybutyrate (PHB), Biomer-P226
biodegradable polymer, was supplied by Biomer, Krailling, Germany. The material has a
T
to
M
operating across the interface. IGC is an alternative for measuring
th
d
G
a
d
IG
a
a
chemi als. Uncoated cellulose was used as a reference. TEM
nano-structure of the nanocomposites and the dispersion of fibers within the matrix. The
potential use of chemically coated nanofibers as reinforcing agents in biocomposites was also
explored.
3
82
Chemicals
Reagent grade chemicals were used for fiber isolation and bleaching, namely, sodium
hydroxide, hydrochloric acid, sodium chlorite, chlorine dioxide, peroxide, and sulfuric acid.
Michem® Prime EAA (ethylene acrylic acid) copolymer dispersions-4983R (Michelman, Inc.,
Cincinnati, OH) was the dispersant, which exhibits excellent adhesion to cellulosic substrates.
Styrene Maleic Anhydride resins (SMA®) from Sartomer Company (Exton, PA) are low
molecular weight styrene/maleic anhydride copolymers. Hydrophobic SMA resins are used as
surface sizing compounds for paper and cross-linking agents for powder coatings. Kelcoloid
HVF and LVF are stabilizers used for fiber coating. Kelcoloids (International Specialty Products,
Wayne, NJ) are made of propylene glycol alginates (PGA), copolymers of mannuronic and
guluronic acids. The key function of PGAs is to help stabilize an emulsion or high-solids
suspension. Guanidine hydrochloride (Figure 38), 50940 BioChemika (Fluka Chemie AG,
Buchs, Switzerland) was used for the fiber coating. Strong chaotropic agent is useful for the
tion and subsequent refolding of proteins or enzymes into their active form. Fig 39 and
Fig
suggested chemcial scheme for the reaction of styrene maleic anhydride modified cellulose.
density of 1.17 g/cm3 and melting point of 173 °C.
Reinforcement
The raw material used in this study was hemp fibers (Cannabis sativa L.) from southwestern
Ontario, Canada (Hempline Inc., ON). These fibers have diameters of approximately 22-25 µm
and lengths of 15-25 mm. The cellulose nanofibers were extracted from hemp fiber by chemo-
mechanical treatments. Isolated nanofibers were shown to have diameters between 50-100 nm
and lengths in the micrometer scale, which results in a very high aspect ratio (87.5).
denatura
40 show the possible chemical scheme for the EAA-dispersion mechanisms and the
83
ofiber Chemical Coating
fibers were
eeze-dried in a Multi-Drier freeze-drying machine (Frozen in Time, Ltd.). Figure 41 shows the
parison of freeze-dried uncoated and SMA coated HPN samples. The freeze drying
stage helps the change in texture of the nanofib
cellulose nanofibers with chemical coating to clearly differentiate the separation of fiber bundles.
Figure 38. Chemical structure of guanidine hydrochloride.
H NH2
NH
· HCl
2N
5.2.3.2 Nan
Cellulose nanofibers were stored in water suspension after the chemo-mechanical isolation.
Different types of chemicals were added to the suspension containing nanofibers in proportion
of 1:2 (w/w), using an estimated weight of the cellulose nanofibers. In order to improve the
dispersion of the coated nanofibers, the suspensions were prepared with continuous stirring by
magnetic stirrer for 24 h at a room temperature. The suspensions containing nano
fr
visual com
ers. Figure 41b shows the loose powder form of
Figure 39. A possible chemical scheme for the EAA-dispersion mechanism.
OHH2C CH2
H2C CH
C OR
O
mn + H2C CH2
H2C CH
C O
O
mn
84
cellulose.
Figure 40. A suggested chemical scheme for the reaction of styrene maleic anhydride modified
O
HC CH2 CH mn
CO
CH
OC O
HC CH2 C mn
CCOOH
C
C OCellulose OH
Cellulose
85
igure 41. Visual comparison of freeze-dried HPN samples: (a) uncoated; (b) SMA coated.
)
F
(a
b)
(
86
5.2.3.3 Processing of Bio-nanocomposites
This project was focused on synthesizing the bionanocomposite using PLA and PHB in the
olid phase by injection molding or hot compression (Figure 42). A solid phase compounding
ixer
rner and Pfleiderer Gelimat) at 3200 rpm with tip speed of 23 m/s. Product was discharged
at a pre-set temperature of 150 °C. Test samples were compression molded with a WABASH
Hot Press into sheet form. The mold temperature was 180 °C, and the pressure was 50 MPa.
PLA composites containing 5 wt.% SMA coated nanofibers were prepared by melt-blending the
polymer with the fiber using a Brabender mixer (C.W. Brabender Instruments Inc., NJ). The
compounding temperature was 170 °C, and the rotating screw speed was 60 rpm for 5 min.
Then the compound was granulated using a C.W. Brabender Granulator (C.W. Brabender
Instruments Inc., NJ). The granulates were then pre-heated to 100 °C for 1 h and injection
molded using an Engel Injection molder (Model ES-28, ON, Canada) equipped with a standard
ASTM mold for tensile, flexural, and impact test specimens. The typical inject olding
onditions were: injection temperature 180 °C, injection time: 8 s, cooling time 25 s, and mold
ing time 2 s. All composites contained 5 wt.% loading of nanofibers with respect to total
weight of the composite.
Figure 42. Processing of bio-nanocomposites.
s
method was used to mix the freeze dried nanofibers with PHB in a high-intensity kinetic m
(We
ion m
c
open
Cellulose nanofibers
C.W. Brabender/K-Mixer
Freeze dried nanofiber
compounding Compression molding/Injection molding
high temp. high pressure
chemical treatment on the surface
+ PLA/PHB
87
5.2.3.4 Column Preparation and IGC Procedure
IGC measurements were done with a Perkin-Elmer Autosystem XL Gas Chromatograph
(GC) fitted with a flame ionization detector. To ensure flash vaporization, the injection port was
kept at 423 K. All stationary phases, including 2-4g uncoated hemp nanofibers (HPN) or coated-
HPN, were dried in an oven at 70 °C for 24h and packed under vacuum with a vibrator into a
copper column (length 33 cm and internal diameter of 4 mm) of which the end was plugged
with glass wool. The columns were maintained overnight at 105 °C in a nitrogen stream to
remove moisture and other volatiles from the cellulose fibers before each experiment. The
columns were first cleaned with acetone before use to get rid of greases used in copper
processing.
were chromatography grade solvents (Sigma-
Ald
5.2.3.5 Microscopy Characterization
The nanostructure of the composites was examined in a transmission electron microscope
(TEM), Hitachi H-7000 TEM at an acceleration voltage of 100 kV. To examine the
nanocomposites, the samples were cut and polished to rectangular sheets, embedded in epoxy,
and allowed to cure overnight. The final ultra-microtoming was performed with a diamond knife
The IGC probes used in the present study
rich). The probes were used without further treatment. Their physicochemical properties are
listed in Table 6. Helium was used as the carrier gas. The corrected flow rate of helium was 10
mL/min. Small quantities of probes were injected into the column using Hamilton syringes.
Peaks were found to be symmetrical and the area under each peak is directly related the amount
adsorbed/desorbed. In the present study, the temperature dependence was determined within the
temperature range 40 to 100 °C. Averages of three measurements were taken to calculate
retention volumes, with air as the marker.
88
microscope (JEOL JSM-840, Tokyo, Japan) (SEM) was used as a
utine for microstructural analysis of the nanofibers with and without surface coatings. All
15 kV. The sample surfaces were coated with a
thi fa ds Edwards,
Wilmington, MA) to provide electrical conductivity.
Tab roperties of 1987; Guttmann 1983).
2) DN AN Character
at room temperature, generating foils approximately 90 nm in thickness. These foils were
gathered onto Cu grids.
A scanning electron
ro
images were taken at an accelerating voltage of
n layer of gold on the sur ce, using an Edwar S150B sputter coater (BOC
le 6. Physicochemical p the IGC probes used in the present study (Schultz et al.
Probe Area (Aº 2) γld (mJ/m
Hexane 51.5 18.4 0 0 Neutral Heptane 57 20.3 0 0 Neutral Octane 62.8 21.3 0 0 Neutral Nonane 68.9 22.7 0 0 Neutral
Chloroform 44 25.9 0 23.1 Acidic Ethyl Acetate 48 16.5 17.1 9.3 Amphoteric Ethyl Ether 47 15 19.2 3.9 basic
Tetrahydrofuran 45 22.5 20.1 8 basic (THF) Acetone 42.5 16.5 17 12.5 Amphoteric
5.2.3.6 Background
Determination of the Acid-Base Characteristics of Lignocellulosic Surfaces by IGC
The surface energy of a material can be described by the sum of dispersive component and
specific interactions. Thus work of adhesion can be written as,
W = W d + W AB (2)
where W , W d and W AB are the total work of adhesion, work of adhesion due to dispersion
forces and acid-base interactions, respectively. Acid-base interactions are the useful chemical
interactions available for modification (Dwight et al. 1990). Hence, in order to design new
a a a
a a a
89
plored surface modification for lignocellulosic fibers
nd their compatibilization with biopolymers.
Bac
njected at infinite dilution were used to calculate the dispersion
com
ents is the specific retention volume, Vn,
efined as the volume of carrier gas required to elute a probe from a column. Vn is related to
Vn = F*(Tr-To) (3)
where o are the retention times of the probe and the air ma , res * is
the corrected flow rate of the car as, define
F* = FJ
where F is the corrected gas rate in in; J corre fa as
com
modification methods for improving fiber-matrix adhesion and meaningful interpretation of the
existing ones, quantitative determination of surface acid-base characteristics of natural fibers is
important. Data generated in this study ex
a
kground of IGC
IGC has become a widely used technique to characterize the surface properties of organic
and inorganic materials. Acid-base probes are used to measure the acid-base characteristics of
the solid surface, and saturated n-alkane probes are used to measure the dispersion component
of the surface energy of interaction. In the present study, retention times of saturated n-alkane
and acid-base probes i
ponent (γsd) of the surface energy, the free energy of adsorption (∆GAB), and the enthalpy of
adsorption (∆HAB) corresponding to acid-base surface interactions. Papirer’s approach, as
described by Schultz et al. (1987; 1991) was used to estimate the acceptor (KA) and donor (KD)
parameters of the test substrates.
The fundamental parameter in the IGC measurem
d
experimental variables by the following equation,
Tr and T rker p Fectively;
rier g d as,
(4)
flow mL/m is the ction ctor for the g
pressibility,
90
J = 1 -1]/[(Pi-Po)3-1]
as pressure at the column outlet, and Pi is the carrier gas pressure at
raction. Molar free energy of
lowin relati ,
d 1/2 d 1/2
d the surface energy of the probe; and γsd is the
ispersive component of the total surface energy of the interacting solid. Combining equation (6)
ture) should yield a straight line with intercept equal to ∆HAB.
.5 [(Pi/Po)2 (5)
where Po is the carrier g
the column inlet.
The interaction of neutral probes, such as saturated n-alkanes, with the substrate material is
dominated by the van der Waals dispersion forces of inte
adsorption is related to net retention volume by the fol g on
∆G = RT ln(Vn) + C (6)
where R is the gas constant, T is the column absolute temperature, and the value of C
depends on the reference state. The free energy of adsorption is related to work of adhesion by
the following relation (Mukhopadhyay and Schreiber 1995),
∆G = NaWa = 2Na(γs ) (γl ) + C (7)
where N is Avogadro’s number; a is the surface area of a single probe; Wa is the work of
adhesion; γl is the dispersive component of
d
and (7), we get:
RT ln(Vn) = 2Na(γsd)1/2(γl
d)1/2 (8)
A plot of RT ln(Vn) versus 2Na(γld)1/2 should give a straight line with slope (γs
d)1/2 in the
case of probes interacting only due to dispersion component of surface energy. From the slope
of the straight line γsd can be calculated.
The free energy of adsorption (∆GAB) corresponding to the specific acid-base interactions is
related to the enthalpy of adsorption (∆HAB) by,
∆GAB = ∆HAB - T∆SAB (9)
where ∆SAB is the entropy of adsorption corresponding to the specific acid-base interactions.
A plot of ∆GAB versus T (tempera
91
The
versus DN/AN should yield a straight line
wit ltz e al. (1987), the speci raction
par
KAm KD
f (11)
atrix, respectively.
n will be small if the
mo rea c
by
difference between the molecules.
The nt s a measure of this m um work.
Wh
enthalpy of adsorption corresponding to the specific acid-base interaction is related to the
acceptor and donor parameters, KA and KD of the fibers. According to Saint-Flour and Papirer
(1982),
∆HAB = KADN + KDAN (10)
where, DN and AN are the donor and acceptor numbers, respectively of the acid-base probe
as defined by Guttmann (1983). A plot of ∆HAB/AN
h slope KA and intercept KD. According to Schu t fic inte
ameter, I, for acid-base interactions can be defined as,
I = KAf KD
m +
where the superscripts f and m refer to fiber and m
Interfacial Tension
Interfacial tension is the tension that is present at the interface of two immiscible phases and
it has the same unites as surface tension. The value of interfacial tension generally lies between
the surface tension of two immiscible phases. The interfacial tensio
lecules of the two phases are similar. Expansion of the interface by unit a an be achieved
the movement of enough molecules from bulk to the interface. However, the potential energy
difference between the interface molecules and bulk molecules hinders this move. A minimum
amount of work is required to overcome this potential energy
interface free energy per unit area or i erfacial tension i inim
en surfactant is added in such a system, surfactant molecules move towards the interface.
The surfactant molecules destroy the cohesive forces between polar and non-polar molecules
and replace the polar and non-polar molecules at the interface. This phenomenon lowers the
92
ten ly de tera tions e stro than the
inte
Wh
rom each other. Solid-solid interfaces
sep for th mech nical b aterials. A
disp
is so large that their behavior is completely determined by surface properties.
rs uch as tress, the
inte
m deposition, chemical reactions, or
rem urfaces and
it i olymer can bind with
any atoms to the surface and even if the binding energy is easily exceed the thermal energy
Depends on the chemical structures of charged polymers, the more groups
are
sion across the interface because the new veloped in c ar nger
raction between the non-polar and polar molecules (Farn 2006).
5.2.3.7 Results and Discussion
y Surface Energy is Very Important
An interface is the area, which separates two phases f
arate two solid phases. They are important e a ehavior of solid m
ersion is a two-phase system which is uniform on the macroscopic but not on the
microscopic scale. Different kinds of dispersion can be formed. Most of them have important
application. In some cases the distinction is obvious between the continuous, dispersing
(external) phase and the dispersed (inner) phase. For nanocomposites, the interface-to-volume
relation
Determining surface energy paramete s the surface tension, the surface s
rnal surface energy, etc. is a difficult task. The interfaces play an essential role in the
stability of materials. Surface modification is essential for many applications, to reduce friction
and wear, and to make implants biocompatible (Yoshinaga et al. 1997). Solid surfaces can be
changed by various means such as adsorption, thin fil
oval of material. Macromolecules like polymers usually adsorb irreversibly to s
s difficult to remove them after adsorption. The reason is that each p
m
(Thielmann 2004).
dissociated, the higher the charge of the polymer.
Surfactants are strongly enriched at the surface, which lowers the surface tension. This
change of surface tension upon adsorption of substances to the interface is described by the
93
beneficially in the coating
ompositions of the present research. Useful surfactants, for example, block copolymers may be
em eight of the composition (Sassi et al. 2000). Besides the
lar head group and one nonpolar tail, dimeric and
olig
surfactant,
call
Gibbs adsorption isotherm. If a molecularly hydrophilic surface is rough, the appearance is that
of an even more hydrophobic surface (polymer surface). If a hydrophilic surface is roughened it
becomes more hydrophilic (fiber surface). The wetting behavior of a network of fibers is
important. Wetting plays an essential role in industrial coating processes where a thin layer of
liquid is deposited continuously onto a moving solid surface (Schultz et al. 1987). Molecules at
a surface are arranged in a different way from molecules in the bulk. When a solid is deformed
by small external forces, it reacts elastically. After the chemical treatment or mechanical
treatment applied on the fiber surface, the surfaces are chemically or mechanically changed and
the clean crystalline surfaces were generated.
Surfactants, which are either anionic or nonionic, may be used
c
ployed at about 1% to 5% by dry w
conventional surfactants with one po
omeric surfactants have attracted considerable interest in academia and industry (Mohanty et
al. 2001). A dilute solution of ethylene acrylic acid copolymer dispersion, applied as a thin
primer coating to the porous substrates, penetrates and impregnates the surface fibers or pores
and, when dried, binds and toughens the surface of the substrate. When the thermoplastic
adhesive is applied with heat and pressure sufficient to cure the primer on the surface, the
resultant adhesive bond is much stronger, more moisture-resistant and more heat resistant than
that without the primer. The ethylene acrylic acid copolymer is one type of polymeric
ed block copolymer, consists of at least two parts. One part is made of monomer-ethylene,
the other part is made of monomer-acrylic acid. There is only one blockcopolymer-acrylic acid
will be strongly surface active with –OH groups and show many properties of a conventinal
surfactant.
94
ructure formation at 5wt % fiber content
was
environment are used to overcome the problems
pre
The dispersing ability of hydrolyzed styrene-maleic anhydride (SMA) copolymers for
dispersion of nanofibers into polymer matrix was studied. The modified SMA copolymers were
found to be effective dispersants for cellulose nanofibers. The aim was to achieve a reactive
coupling between the maleic anhydride functionality of SMA at the surface of the cellulose
nanofiber’s –OH groups during the coating. The modified nanocomposites exhibited a better
fiber dispersion. The formation of “network-like” st
observed in nanocomposites. The cellulose nanofibers having active –OH groups on their
surface may partially be blocked with a blocking agent. Such a modified fiber has substantially
no free –OH groups.
Propylene glycol alginate is an emulsifier, stabilizer, and thickener used in food products.
Originally derived from brown algae and since mixed with a few other goodies, the chemical
has been used for almost a century in one form or another. Propylene glycol alginate is an ester
of alginic acid in which some of the carboxyl groups are esterified with propylene glycol, some
neutralized with an appropriate alkali and some remain free (Maldas et al. 1989).
Cellulose nanofibers and propylene glycol alginate are contacted by hydrating. Nanofiber
tends to settle out as a sediment in a nanofiber suspension. Protein stabilizing agents that
stabilize proteins as a suspension in an aqueous
sented by protein insolubility. Kelcoloids provide a beverage such as orange juice that is
clouded by a suspension of soy protein particles, where the protein particles are prevented from
aggregating to the point of settling out by Kelcoloids. HVF and LVF inhibit the fiber from
settling by adsorbing to individual fiber and imparting an overall negative charge to the fiber,
resulting in repulsion of the cellulose chains from one another, and thereby preventing the
cellulose nanofibers from aggregating and settling out of the suspension. Kelcoloids also
95
s are provided which may readily be applied as an aqueous suspension to coating
sub
chloride has been added to the protein to increase the thermostability
(Re
increase the viscosity of the nanofiber suspension, which helps stabilize cellulose nanofibers
against gravitational forces.
Propylene glycol alginate, provides important film-forming characteristics required to
provide an elegant coating which is particularly useful in aqueous media (Maldas et al. 1989).
Propylene glycol alginate by itself is known to be a film forming hydrocolloid in an aqueous
dispersion. When a low viscosity propylene glycol alginate is utilized in high concentrations in
combination with a suitable surface-active agent, elegant and high performance coating
formulation
strates. The propylene glycol alginate used in the present research is a low viscosity
propylene glycol alginate which, when present at 1% in water at 25°C produces an aqueous
solution. The high viscosity propylene glycol alginate is difficult to formulate into suitable
coatings and requires numerous additives to produce satisfactory coatings. It tends to be too
viscous for many practical applications.
Guanidine hydrochloride (GuHCl), the well known protein denaturant, acts, at well defined
mild concentrational ranges, as a stabilizer of the “molten globule” intermediates with the final
renaturation as well as aggregation of the denatured protein (Krieger 1990). It was also reported
that guanidine hydro
iter 1998). As a dispersing medium – guanidine hydrochloride was most promising among
several kinds of dispersing solvents. The inner structure of protein was not destroyed in
guanidine hydrochloride media.
Dispersion Component of the Surface Energy
Preliminary experiments were performed on the coated and uncoated cellulose powders to
determine the optimum chromatographic conditions for reproducible measurements of the
96
llulose. In the present study, the temperature dependence was
det
γ d (mJ/m2)
retention times of the probes. The chromatographic peak shape of each probe had to be as
symmetrical as possible. The dispersion component of uncoated and chemically coated hemp
fibers was calculated from a plot of RT ln(Vn) versus 2Na(γld)1/2. The values for the dispersion
component, γsd, of the surface energy at different temperatures are summarized in Table 7.
The dispersion component of resin was also calculated similarly. The linear relationship in
case of n-alkanes illustrates that this technique works well in case of natural fibers. Chemical
treatments had the effect of increasing their respective γsd values toward that of the cellulose
powder. This is due to the dissolution of low energy surface impurities and surface exposure of
relatively higher energy ce
ermined in the temperature range 40 to 100 °C. Chemically coated fibers showed a negative
temperature coefficient over this entire range due to chemical rearrangements. The London
dispersion component was affected by the type of polymers and the treatment of fibers.
Table 7. Dispersion component, γsd, of the surface energy of lignocellulosic particles at different
temperatures.
sMaterial 313K 333K 353K 373K
Cellulose (Dorris and Gray 1979) 48 44 40 36 Uncoated HPN 42 40 34 28
SMA-Coated HPN 44 41 39 36 HVF-Coated HPN 46 44 43 38 LVF-Coated HPN 44 42 40 33 EAA-Coated HPN 46 43 41 37
Guanidium Hydrochloride Coated HPN 50 47 44 41
PLA 32 29 28 27 PHB 51 47 43 40
HPN: hemp nanofibers
97
able 8. The corresponding values of the enthalpy of adsorption,
HAB, determined from the plots of ∆GAB as a function of temperature (T) for all the probes in
s showed a negative acid-base free energy and
ent
ween
the
Acid-base Interactions of the Surface Energy
Values of the free energy of adsorption, ∆GAB, corresponding to surface acid-base
interactions are summarized in T
∆
all cases, are given in Table 9. Some probe
halpy of adsorption on the cellulose. For example, the acid-base interaction between HVF-
coated HPN (hemp nanofibers) and the ethyl ether probe was not favorable to adsorption.
Considering that ethyl ether is basic (DN = 19.2), and the HVF-coated HPN used in this study
was found to have a basic characteristic (KD = 0.22), this result is not surprising. A comparison
of the enthalpy of adsorption between uncoated and coated fibers indicated that the interactions
between the probes and SMA- and EAA-coated HPN were greater than those observed bet
probes and uncoated HPN. The uncoated HPN had relatively low donor (KD = 0.31) and
acceptor (KA = 0.19) parameters compared to the donor (KD = 0.77) and acceptor (KA = 0.34)
parameters of SMA-coated HPN. The significant increase in the acceptor parameter KA suggests
that coated fibers may have stronger interactions with a matrix (Marcovich et al. 1996).
98
Table 8. Fre n, ∆GAB, of the acid-based probes at dif nt temperatures.
∆G
e energy of adsorptio fere
AB (KJ.mol-1) Substrate/Probe 313K 333K 353K 373K Uncoated HPN
Chloroform 3.76 1.50 -0.86 -2.37 Ethyl Acetate 5.36 4.61 3.25 1.66 Ethyl Ether 3.15 -0.20 -0.52 -3.06
Tetrahydrofuran (THF) 4.40 2.88 1.52 -1.32 Acetone 4.61 2.23 0.07 -2.37
SMA-Coated HPN Chloroform 12.86 12.68 8.89 5.55
Ethyl Acetate 12.10 10.79 8.76 6.58 Ethyl Ether 8.20 7.45 6.39 5.42
Tetrahydrofuran (THF) 13.07 12.55 12.21 11.43 Acetone 12.86 11.91 11.28 9.21
HVF-Coated HPN Chloroform 2.30 1.10 -1.63 -4.38
Ethyl Acetate 2.07 0.90 -0.80 -1.64 Ethyl Ether -1.05 -3.43 -7.34 -9.93
Tetrahydrofuran (THF) 2.31 0.41 -0.05 -0.97 Acetone 1.64 -0.14 -1.63 -4.63
LVF-Coated HPN Chloroform 3.59 2.64 -0.17 -1.49
Ethyl Acetate 1.79 0.86 0.36 -0.73 Ethyl Ether -0.07 -1.20 -3.40 -4.53
Tetrahydrofuran (THF) 1.86 0.95 0.33 -0.26 Acetone 2.73 1.76 1.35 0.27
EAA-Coated HPN Chloroform 9.83 12.70 11.33 10.66
Ethyl Acetate 11.71 10.99 9.99 9.08 Ethyl Ether 10.29 8.13 7.00 6.37
Tetrahydrofuran (THF) 13.56 12.47 11.54 10.78 Acetone 9.96 9.35 8.79 8.71
Guanidium Hydrochloride Coated HPN Chloroform -1.87 -3.41 -4.81 -5.09
Ethyl Acetate -3.05 -3.57 -3.89 -4.04 Ethyl Ether -4.26 -4.54 -4.81 -5.09
Tetrahydrofuran (THF) -3.47 -3.48 -3.49 -3.50 Acetone 2.96 2.87 2.71 2.62
99
Table 9. Enthalpy of adsorption, ∆HAB.
∆HAB (KJ.mol-1)
Probe Uncoated HPN
SMA- Coated HPN
HVF- Coated HPN
LVF- Coated HPN
EAA- Coated HPN
Guanidium Hydrochloride Coated HPN
Chloroform 5.70 16.43 5.04 5.65 13.45 1.04 Ethyl Acetate 6.83 14.20 3.34 2.58 12.67 2.81 Ethyl Ether 4.58 9.21 2.20 1.59 11.17 3.98
Tetrahydrofuran (THF) 6.50 13.63 3.00 2.47 14.41 3.46
Acetone 6.91 14.21 3.88 3.47 10.28 3.09
Material u.)
Table 10. Surface acid-based characteristics, KA and KD.
KA (a.u.) KD(a.Uncoated HPN 0.19 0.31
SMA-Coated HPN 0.34 0.77 HVF-Coated HPN 0.07 0.22LVF-Coated HPN 0.04 0.23EAA-Coated HPN 0.49 0.45
Guanidium HydroHPN
chloride Coated 2 0.20 0.0
PLA 2 0.18 0.1PHB 9 0.22 0.6
Table 11. Value teraction para .
+ KAmKD PLA PHB
s of specific in meter
I = KAfKD
m f
Uncoated HPN 0.08 0.20SMA-Coated HPN 0.18 0.41HVF-Coated HPN 0.05 0.10LVF-Coated HPN 0.05 0.08EAA-Coated HPN 0.14 0.44
Guanidium Hydrochloride Coated HPN 0.03 0.14
The K spectiv s were ted fr slope tercept
of the respec lines of ∆ N as a tion of N. The lues are
summarized in litatively, SM ted H wed relatively hig id-base
characteristics t HPN. A sim nd wa rved in ase of -coated
A and KD values for the re e r fibe aestim o em th and in
tive linear regression HAB/A func DN/A se va
Table 10. Qua A-coa PN sho her ac
han uncoated ilar tre s obse the c EAA
100
HPN com . The u d, SM ed, H d LVF d HPN
show KD values appear to be consistent with the
molecular structure of cellulose, where th rogen in the hydroxyl groups act as
electron acceptors and the oxygen atoms in ycosid ages a roxyl act as
lectron donors. The EAA-coated HPN showed an amphoteric surface characteristic and
uanidium hydrochloride coated HPN showed a predominantly acidic characteristic. The
latively high KA value indicates a surface that is rich in hydroxyl groups.
fibers were enriched by different class of
chemicals and extractives. Hemp fibers were found to be basic, which is probably due to
presence ycerid exhi on ba rac ha The
remova es and ellulo hem reat had ec g the
disper nt of th ace e f t N. T lym
study was found to have an acidic ch By rast, sho predom ly basic
haracter, according to the KA D values.
ined by Schultz et al. (1987), were
calculated for each type of fiber and resin combination. These values are shown in Table 11.
Acid-base interactions with y SMA- and E ed HPN, and a very similar
trend was observed for P polymers were used as a dispersant in this study,
bringing together in one product the benefits of both ethyl nd acry id. The crystalline
structure of ethylene provides rrier properties, flex , and resistance to water and
chemicals. The acrylic acid co er imparts improved adhesion, hot-tack strength, and
ng EAA dispersant remains homogeneous
definitely. It has excellent adhesion to cellulose and other polar substrates due to the high
content of acrylic acid in the base copolymer. The EAA ersant a hibits outstanding
pared with uncoated HPN ncoate A-coat VF, an -coate
ed a basic surface characteristic. KA and
e hyd atoms
the gl ic link nd hyd groups
e
g
re
The surfaces of uncoated and chemically coated
of trigl es, which bit a pr ounced sic cha
m
ter (Ts balala 1997).
t of asinl of extractiv hemic se by c ical t ents the eff incre
s eion compon e surf n oergy he HP he po er matrix PLA used in this
aracter. cont PHB wed a inant
c and K
Values of the specific interaction parameter, as def
PLA increased b AA-coat
HB matrix. EAA co
ene a lic ac
the ba ibility
monom
optical clarity. The nanofiber suspension containi
in
disp lso ex
101
adhesion to polyethylen Styrene ma nhydrid A) resins are low
molecular weight styre e copolymers. Altering the styrene to maleic
anhydride ratio c ba of the polymer. At their most
ydrophilic, SMA resins form high solids solutions and can be used to produce fiber dispersions.
The
s
ritical prerequisite for the formation of uniformed
nan
e and other plastics. leic a e (SM
ne/maleic anhydrid
changes the hydrophilic/hydrophobi lance
h
se results are of special practical importance because surface acid-base interactions may be
implicated in the adhesion of coatings and fini hes to polymer and other lignocellulosic fibers.
Adsorption occurred only when there was an exothermic interracial acid-base interaction (Wang
and Sain 2007).
The Effect of Chemically Coated Nanofiber on Dispersion and Surface Energy
It is well recognized that the successful dispersion of nanoparticles requires surface
treatment of the filler to promote wetting by hydrophobic polymers; however, the prior art
methods of forming polymer nanocomposites have failed to produce polymer nanocomposites
that exhibit the expected performance characteristics suggesting that such methods are
fundamentally flawed (Petersson and Oksman 2006).
Some reasons for the deficiencies of the prior art appear to be (1) the failure of the prior art
to appreciate the relationship between the spatial distribution of charge within the crystal lattice
of a filler (e.g., fiber) and self-assembled surfactant structures, and (2) the failure of the prior art
to appreciate the correlation between the surface energy (i.e., critical surface tension) of the
modified filler surface and wetting by the polymer. The IGC surface energy test is used to
confirm whether the filler are homogeneously distributed in the polymer matrix (Saint-Flour and
Papirer 1982). The specific interactions are a c
ocomposites by melt-mixing or melt-compounding. In this project, the use of interfacial
tension forces to estimate the nature and extent of interaction energies has been described.
102
ith a filler to effect adsorption of the surfactant on at least a
ortion of the surface of the filler; and (3) freeze-drying the blend to make the solid surface-
e-modified filler
wit
gh the application of water-immiscible,
amorphous surfactants onto the filler surface.
Accordingly, there is a need for the design and formation of new surface-modified fillers (fibers)
that enable significant improvements in the ease of dispersion nanofibers into the polymer
matrix. The resulting nanocomposites will demonstrate significant improvements in physical
properties, mechanical properties, and barrier performance. Furthermore, there is a need for
methods to control cation charge distribution within the surface-modified fillers to facilitate
greater control over the surface hydrophilic/lipophilic balance of the surface-modified fillers.
The present project provides novel methods of forming surface-modified fillers, polymer
composites and polymer nanocomposites. In other aspects, the present project provides
nanocomposites comprising: a filler having an amorphous coating on at least a portion of the
surface thereof dispersed in the matrix of a crystalline or semi-crystalline polymer; a surface-
modified filler dispersed in a PLA matrix displaying substantially high values of specific
interaction parameter in EAA and SMA coated filler (nanofiber) cases. The preferred surfactants
comprise a surfactant selected from the group consisting of a blocking agent, which can react
with –OH groups on the fiber surface. The present research also provides a method of forming a
surface-modified filler comprising: (1) emulsifying an amorphous surfactant with water; (2)
blending the emulsified surfactant w
p
modified filler. Producing a nanocomposite comprising compounding a surfac
h a polymer to achieve nano-dispersed filler into the matrix was reported. The compounding
is performed in the absence of any one or more of compatibilizers, activators, hydrotropes, or
solvents. Selecting a polymer and the suitable chemical treatments on the fiber surfaces was
reported. The nanocomposites with SMA and EAA-modified fillers possessed exceptionally
high values of specific interaction parameter throu
103
20µm 20µm
a b
terial Structure
Fig. 43 presents typical pictures of freeze-dried HPN. Fig. 43a is a SEM image of uncoated
HPN. Each particle of HPN is an aggregation of cellulose fibers due to the strong hydrogen
bonds of adjacent molecules. The size of the fiber bundle is at the µm level. Fig. 43b shows a
picture of SMA-coated HPN with a well-organized web-like structure. The morphology of
coated HPN appears distinguishable compared to uncoated HPN. The SMA-coated fibers
formed loose networks during freeze drying. It is proved that SMA could reduce the
entanglement of the nanofibers.
Figure 43. Scanning electron micrographs of freeze-dried HPN samples: (a) uncoated and (b) SMA coated.
Ma
The processing of cellulose nanocomposites renders several challenges. The major difficulty
is to achieve uniformly dispersed nanofibers in the polymer matrix. The nanofibers have a very
large surface-to-volume ratio and have a tendency to aggregate when dried. The injected
composites were examined using a transmission electron microscope (TEM) to study the
composite morphology at nanoscale. Fig. 44a shows an overview picture of the PLA/SMA-
coated HPN composite. It was difficult to see any cellulose nanofibers in this sample. There are
some dark spots, indicating that the nanofibers were not uniformly dispersed in the PLA matrix,
and it is possible that the cellulose was degraded during processing. In Fig. 44b, a more detailed
view of the composite with PLA is shown. It can be seen that the nanofibers were partly
dispersed in PLA. Agglomerates were present in the PLA/SMA-coated HPN nanocomposite.
The structure can therefore not be described as fully networked. The dispersion and distribution
of nanofibers can be affected and improved by optimizing the chemical surface treatments and
the compounding process. Figure 44b shows the presence of a non-homogeneous structure of
nanofibers in the PLA based nanocomposite
104
1 µm 200 nm
a b
rovement in properties of the polymer matrix.
Figure 44. Transmission electron micrograph of the PLA/SMA-coated HPN composites: (a) an overview and (b) detailed view.
s. This fact will be reflected in the mechanical
ere is a strong link between the morphology of nanocomposites and the
imp
properties, since th
105
5.2.4 Physical dispersion of nanofibers in biopolymers
tely, inspired by the growing environmental awareness and new standards, there is a
deliberate attempt and interest to develop environmentally friendly and biodegradable systems
with improved performance. The production of "green materials" based on raw materials
derived from natural resources of plant or animal origin is of great interest both in the academic
and industrial fields. One drawback using cellulose nanofibers is the difficulty to disperse them
uniformly in non-polar medium because of their polar surface. Until now, no reports are
available on processing of bio-nanocomposites with matrix like PLA reinforced with cellulose
elt-compounding technique.
Today, there is a large interest to use PLA in packaging, medical but also in automotive
applications where other properties than strength and stiffness are important. It is therefore
interesting to study if the incorporation of nanofibers can improve the toughness, thermal
stability and barrier properties of the PLA. The processing of novel biomaterials sets new
challenges in the field of plastics engineering, and especially the processing of nanocomposites
sets new requirements for processing equipment, optimization and control.
Generally, the main difficulty in melt-compounding of nanocomposites is to achieve well
dispersed nano-reinforcements in the polymer matrix; however, when using unmodified
nanofibers, it exists as aggregates which are difficult to separate during the extrusion process.
Although chemical treatment of fiber surface has been somewhat successful in improving
interfacial bonding, there are environmental concerns related to the disposal of chemical after
the treatment. Plasma is defined as an ionized gas with an equal density of positive and negative
charges (Yuan et al. 2002). The present study employs a cold plasma system where the gas is
usually at room temperature. Cold plasma environment offers a unique way for modifying the
chemical and physical structures of both fiber and polymer surfaces without altering the bulk
La
nanofibers by m
106
tructures and characteristics of these materials. Air-plasma treatment has been used to modify
ir-plasma treated nanofibers and PLA was increased. Sonification is the other physical
dispersion approach to separate cellulose nanofibers. A simple and versatile approach was
reported using the ultrasonic technique. The diluted cellulose nanofiber suspension did not
sediment or flocculate. It is made up of individual cellulose fragments consisting of slender rods
that have a broad distribution in size.
The main focus of this work has been to identify new physical modifications to separate the
cellulose nanofibers, and to develop a new processing technique for a larger scale production of
cellulose nanocomposites. This study is the first attempt to prepare cellulose nanocomposites by
melt-extrusion technique using e nanofibers in a biodegradable poly ix.
5.2.4.1 Physical Dispersion of Cellulose Nanofibers
Sonification
The 1 wt% cellulose nanofiber suspension was stirred for 24 h with a magnetic stirrer at
room temperature. The solution was then subjected to 1.5 h sonification over 2 days in 10 min
s
the surface of cellulose nanofibers. Under optimal treatment, the interfacial bonding between
a
cellulos ester matr
Air-plasma Treatment
The plasma treatment was carried out using a modified plasma cleaner, Model PDG-32G
from Harrick Scientific Corporation, Ossining, NY, USA. The induction coils were connected to
a 13.56 MHz radio-frequency generator that produced a power-output of 60 W (medium). 100-
150 mg freeze dried nanofiber powder was filled in 5 ml polystyrene round-bottom tube with
cell-strainer cap (BD Falcon) to avoid being sucked by the vacuum. The tubes were then placed
inside the chamber of the plasma cleaner for 2 mins.
107
intervals in order to loosen up the nanofibers. It was subjected to sonification at 20 kHz by
Branson digital sonifier (1510 Branson) and cooled in an ice bath between steps.
5.2.4.2 Material Structure
To confirm the separation of individual crystallites or nanofibers, the freeze dried nanofiber
powder was observed using SEM and is shown in Figure 45. The Figure 45a and 45b show
separated cellulose nanofibers in nanometer scale and how these large aggregates have broken
down after the treatments. It was expected that the sonification process would further contribute
to the separation of the cellulose nanofibers. Figure 45a reveals that the fiber surface treated by
air-plasma for 2 mins has some very obvious cracks. These cracks increase the interface area
which may improve the bonding between the air-plasma treated fiber and polymer matrix.
108
igure 45. Scanning electron micrograph of freeze dried HPN: (a) after air-plasma treatment (×8
5.2.4.3 Extrusion
The composite materials were compounded using a co-rotating twin screw extruder
(ONYXTEC-25/40, ONYX P. M. Inc. Technology, Machinery and Tooling, Toronto, ON,
F00) and (b) after the sonification process (×700).
(a)
2 µm
(b)
2 µm
109
anada) with a gravimetric feeding system for dry materials. The cellulose content of 5 wt% is
the final composition. The extrusion was carried out in the temperature range of 170–200 °C
nd the screw speed was held constant at 150 rpm. The total throughput was 5 kg/h and the
aximum capacity of this extruder is 20 kg/h. The cellulose freeze dried nanofiber powder was
d into the melted-PLA at zone 1 (Figure 46). The extruded strands were then pelletized and
jection moulded.
igure 46. Processing of bio-nanocomposites.
Analyses
Cellulose fibers are usually characterized by a high degree of crystallinity and it is
le to assume that the effects observed in their IGC examination can be related to
urface phenomena. The possible bulk sorption and diffusion of probe compounds into these
aterials can be neglected with the experimental conditions chosen here. Chromatographic
were sharp, symmetrical and reproducible. The
tter feature was tested in terms of retention time values, for methane marker and probes and
ave very modest fluctuations, viz. about 0.5 %. It was also found that the volume of injected
fect the retention time, in the range of 1-10 µl. These observations refer to
xperiments conducted at all temperatures and confir plicability of the IGC for the
C
in
a
m
fe
in
F
Cellulose nanofibers
Twin screw extruder
Freeze dried nanofibers
Injection molding
physical treatment on the surface
+ PLA
5.2.4.4 Results and Discussion
IGC
reasonab
s
m
peaks of polar as well as for non-polar probes
la
g
probes did not af
e m the ap
110
e characterization of physically treated cellulose nanofibers. The values of the dispersive
omponent of the substrate, γsd, are given in Table 12. The γs
d for uncoated HPN is 42 mJ/m2; for
lasma treated and sonification treated HPN increases by about 50 % (65 mJ/m2 and 51 mJ/m2)
because of an increase of fiber availability at the surface after removal
f the low-molecular-weight impurities. After the plasma treatment, it seems reasonable to
late this increase to the oxidation caused by plasma discharge. Degradation of fiber surface
lace. Extensive effects of plasma modification altered the composition of fiber surface.
able 12. Dispersion com , γsd, of the surface energy of lignocellulosic particles at
ferent temperatures.
γsd (mJ/m2)
surfac
c
p
at 313K. Probably, it was
o
re
takes p
T ponentdif
Material 313K 333K 353K 373K
Uncoated HPN 42 40 34 28 Plasma Treated HPN 65 54 37 28
Sonification Treated HPN 51 49 47 44 PLA 32 29 28 27
The acid-base properties of cellulose surfaces were determined by applying polar probes
with the characteristics presented in Table 6. The uncoated HPN had relatively low donor (KD =
0.31) and acceptor (KA = 0.19) parameters compared to the donor (KD = 0.27) and acceptor (KA
= 0.40) parameters of plasma treated HPN (Table 13). It can be seen that both the acidity (KA)
and the basicity (KD) of cellulose fibers increased with plasma treatment. The plasma treatment
mplifies the acidity of the fiber surface more effectively than its basicity. This can be related to
similar trend was observed in the case of
sonification treated HPN compared centration of acid/basic groups
o rese ble co strate.
Values of the specific interaction parameter are shown in Table 14. Acid-base interactions with
a
an efficient creation of carboxylic groups. A
with uncoated HPN. The con
nted in the removan the fiber surface, either p mpounds or grafted on the sub
111
LA increased by plasma and sonification treated HPN. Physical treatments enhanced the
acid e, with a more pronounced effect on the former.
Tab
A D
P
ity and the basicity of fibers' surfac
le 13. Surface acid-based characteristics, KA and KD.
Material K (a.u.) K (a.u.) Uncoated HPN 0.19 0.31
Plasma Treated HPN 0.40 0.27 Sonification Treated HPN 0.24 0.31
PLA 0.18 0.12
Table 14. Values of specific interaction parameter.
I = KAfKD
m + KAmKD
f PLA Uncoated HPN 0.08
Plasma Treated HPN 0.10 Sonification Treated HPN 0.09
5.3 Conclusions
Scanning electron micrographs showed oligomer coated nanofibers formed loose networks
during freeze drying. Improved dispersion of nanofibers was revealed in ethylene-acrylic
oligomer emulsion, whereas agglomerated nanofibers were observed in water suspension. There
was a difference in the interaction with the SBN in PVA, PP and PE nanocomposites. Oligomer
coated nanofibers had a large surface area which allowed the nanoreinforcement to interact with
polymer better.
Inverse gas chromatography (IGC) at infinite dilution has proven to be a convenient tool for
er
atrix. Changes in final properties of the composites due to the effect of various chemical and
mechanical treatm fiber surfac lso b ined u is tech e
interactions ed by SMA- and EAA-coated HPN, and the same trend was
measurement of surface energy and acid-base characteristics of natural fibers and polym
m
ents on the e can a e expla sing th nique. Acid-bas
with PLA were increas
112
observed for PHB m lasma and soni tion trea nt on the ofibes al nhanced the
inte
atrix. P fica tme nan so e
ractions between PLA and nanofibers.
SEM pictures showed SMA-coated HPN having a well-organized web-like structure and
proved that the size of nanofibers is indeed in the nano-level. Current TEM pictures showed the
presence of a non-homogeneous structure of nanofibers in the PLA based nanocomposites. The
properties shown here will most probably be improved if it is possible to disperse the nanofibers
more evenly within the polymer matrix. The uniform nanofiber dispersion in a matrix, coupled
with the high aspect ratio of the nanofibers will indicate a strong potential for the use of these
bionanocomposites.
113
e and environmentally-friendly nanocomposites is a new field in
nanotechnology. This project focused on synthe
6.1 Introduction
In the last decade there has been growing interest in the manufacture of composite materials
that are reinforced with nanofibers (Hepworth and Bruce 2000). In combination with a suitable
matrix polymer, cellulose nanofiber networks show considerable potential as an effective
reinforcement for high quality specialty applications of bio-based composites. The small
dimensions of cellulose fibrils enable direct contact between cellulose and matrix polymers,
allowing for a large contact surface and thus excellent adhesion. Nanotechnology is the
manipulation of materials measuring 100 nanometers or less in at lease one dimension. In recent
years, scientists and engineers have been working together to use the inherent strength and
performance of these nano-fibrils, combined with natural green polymers, to produce a new
-materials.
6. NANOFIBERS REINFORCING CAPABILITY ON POLYMERS
The fairly new idea of bionanocomposites, in which the reinforcing material has nanometer
dimensions, is emerging to create the value-added materials with superior performance and
extensive applications. The use of cellulose nanofibers as nanoreinforcement for the
manufacture of cost effectiv
sizing the nano-biocomposite using commodity
polymer (PE, PP) and biopolymer (PLA and PHB) in the solid molten phase and PVA in an
aqueous phase as the polymer matrix. The mechanical properties of the nanocomposites were
studied according to the tensile testing.
class nano
114
.2 Novel High-strength Nanofiber Thin Mats
The microfibrillated cellulose (nanofibers) is a new form of expanded high-volumn cellulose,
oderately degraded and greatly expanded in surface area, obtained through a homogenization
rocess. Due to the unique characteristics of nanofibers, a considerable improvement in the
inforcement-matrix interface interactions is expected. This study was aimed at developing a
ovel concept to produce a new kind of high-strength composite using nanofibers from soybean
ods and hemp fibers. The mechanical properties of the new composites have been quite
pressive due to the nanofibers having nano-order-unit web-like network structure.
.2.1 Nanofiber thin mat production
Nanofibers were kept in water at a fiber content of 0.2 % in weight and stirred for 48 h. A
ter of the nanofiber water suspension was vacuum filtered, producing a thin mat 10 cm in
iameter. About 5 mats, separated by filter papers, were sandwiched between metal plates and
oven ried
t 70 ºC for 5h.
6.2.3 Mechanical properties of nanofiber thin mat
The huge gap between the mechanical properties of USB and USBN mat observed (Table
6
m
p
re
n
p
im
6
li
d
dried at 70 ºC for 48h. In order to assure complete drying, they were further vacuum d
a
6.2.2 Tensile test
The mechanical behavior of nanofiber thin mats was tested by an Instron 5860 (Grove City,
PA) in tensile mode with a load cell of 2 kN in accordance with ASTM D 638. The specimens
were cut in a dumbbell shape with a die ASTM D 638 (type V). Tensile tests were performed at
a crosshead speed of 2.5 mm/min. The values reported in this work result from the average of at
least 5 measurements.
115
as no damage to the elements, the nanofibers having better interactions
be
It creates a composite capable of absorbing a large amount of load before
ilure, resulting in a very high stiffness.
It should be noted that the USBN was obtained by microfibrillation from the same USB;
ucture. Novel high-strength nanocomposites
ba
15). As long as there w
tween them were expected, leading to enhance strength properties. The mechanical properties
demonstrated an increase in tensile strength from 9.85 MPa of USB to 50.72 MPa of USBN and
in stiffness from 1.10 GPa of USB to 6.24GPa of USBN. The huge strength gap between USBN
and BSBN based nanomat remained as a result of the different structures of these two materials.
BSBN thin mat has a highest peak stress at yield. This suggests that the reason for the difference
might reside in the unique structure of the microfibrillated cellulose. In Table 15, the data
compares the tensile strength and stiffness for hemp fiber vs. hemp nanofiber thin mats as well.
There is a remarkable difference concerning the failure behavior between these two thin mats.
The web-like structure of interconnected microfibrils produces an increased bond density,
preventing the cracks.
fa
both have the same nature but differ only in their str
sed on microfibrillated cellulose have nano-order-unit web-like network structure. As a
consequence of this unique structure, the ratio of surface area to volume of nanofiber elements
is dramatically increased.
116
iber thin mat.
Peak Stress (MPa) S.D. Modulus (GPa) S.D.
Table 15. Mechanical properties of nanof
USB 9.85 0.6 1.10 0.2 USBN 50.72 0.5 6.24 0.8
UHP 9.91 0.1 1.00 0.6 BSBN 87.98 0.4 6.33 0.1
UHPN 36.96 0.5 5.00 0.7 55.96 0.2 5.03 0.3 BHPN
USB: untreated soybean stock fiber; USBN: untreated soybean stock nanofiber; BSBN: leached soybean stock nanofiber; UHP: untreated hemp fiber; UHPN: untreated hemp
ched hemp nanofiber; S.D.: standard deviation.
6.3
a dumbbell shape with a die ASTM D 638 (type V). Tensile tests were
erformed at a crosshead speed of 10 mm/min. The values reported in this work result from the
bnanofiber; BHPN: blea
Mechanical Behaviour of Nanofiber/PVA Film
6.3.1 Tensile test
The mechanical behavior of nanofiber-blend-PVA film was tested by a Sintech-1 machine
model 3397-36 in tensile mode with a load cell of 50 lb in accordance with ASTM D 638. The
specimens were cut in
p
average of at least 5 measurements.
6.3.2 Mechanical performance of nanofilm
High performance nanocomposites are obtained by film casting from a mixture of PVA and
cellulose nanofiber suspension. The mechanical properties demonstrated an increase in tensile
strength from 21 MPa of PVA/UNF5 and 65 MPa of pure PVA to 110 MPa of PVA/SBN5. The
increased stiffness of PVA nanocomposites was also very promising; it was 7.3 GPa compared
to 2.3 GPa of pure PVA and 1.5 GPa of PVA/UNF5. Table 16 shows when 10 % SBN was used
as reinforcement, tensile strength of the film increased to 122 MPa. A three to fourfold increase
in E-modulus was observed in both PVA/SBN5 and PVA/SBN10 film compared to PVA/UNF
film. This shows the advantage of nanocomposite materials is their superior mechanical
117
gions (Wan et al. 2006). The main inter- and intra-
mo
te, BSBN is very hydrophilic, and due to its large aspect ratio is
expected to experience strong hydrogen bonding with PVA polymeric chains as well. However,
Table 16 shows the mechanical properties of PVA/BSBN nanocomposites were slightly
decreased (10-20 %). The reason for this behavior might be the BSBN as reinforcement is
affecting the mechanical properties. Harsh chemicals used for bleaching treatment may reduce
the chain length of the cellulose which results in reducing the reinforcing capability of these
nanofibers. The same trend was observed for PVA/HPN and PVA/UHPN composites. The E-
modulus of HPN5-based composites was significantly high even at very low fiber content, while
ulus was much lower (4.6 GPa) at low
fiber content compare to at high fiber content (5.8 GPa).
By crosslinking the fragments of cell wall with PVA, we have been able to use some of the
ing potentia lulose mic rils. The probably penetrates into the cell walls
n the microfib f the cellulose micro-fibrils could be com tely extracted and aligned
eing conce to a high volume fraction, then mat s of high strength could
nts; for
xample, it provides a readymade crosslinked-structure into which adhesives can penetrate.
Thus a physical as well as a chemical interaction can facilitate effective stress transfer. In PVA–
properties even at very low reinforcement content (~5 wt%). It is known that PVA crystallites
are formed through localization of polymer-rich regions, with a high degree of organization,
surrounded by water-rich amorphous re
lecular interactions are hydrogen bonds and van der Waals forces. The strong bonding
between the SBN and the PVA matrix allows the matrix to successfully transfer the load to the
fiber. There are strong interactions between hydroxyl groups at the cellulose surface which lead
to the formation of a rigid network, resulting in having improved mechanical properties. In the
PVA/BSBN nanocomposi
for bleached hemp nanofiber based composites, E-mod
reinforc l of cel rofib PVA
betwee rils. I ple
before b ntrated erial
probably be produced. However, there may be advantages in using cell wall fragme
e
118
l if the requirement is for a structural
Materials to break S.D. S.D. Modulus S.D.
cellu ose composites, water absorption is a problem,
composite.
Table 16. The mechanical properties of nanocomposites in aqueous phase film casting.
Elongation
(%)
Max. stress (MPa)
E-
(GPa) PVA 1.3 0.5 64.8 0.2 2.3 0.6 PVA/UNF5 1.6 0.3 20.6 0.5 1.5 0.5 PVA/UNF10 1.6 0.4 33.3 0.7 2.7 1.0
110.1 0.4 7.3 0.6 PVA/SBN5 1.7 0.1 PVA/SBN10 2.1 0.4 122.2 0.1 9.3 0.4 PPVA/BSBN10 2.0 0.9 94.2 0.8 7.0 0.9
VA/BSBN5 1.2 0.4 85.0 0.7 6.4 0.5
PVA/HPN5 1.8 0.3 100.6 0.5 5.5 0.7 PVA/HPN10 2.3 0.2 103.6 0.3 6.0 0.8 PVA/BHPN5 1.4 0.4 89.3 0.3 4.6 0.6 PVA/BHPN10 1.9 0.7 97.4 0.7 5.8 0.7
UNF: untreated fiber; SBN: soybean stock nanofiber; BSBN: bleached soybean stock nanofiber; HPN: hemp nanofiber; BHPN: bleached hemp nanofiber; S.D.: standard deviation.
6.3.3 Dynamic mechanical analysis (DMA)
Dynamic mechanical properties of the nanocomposite films were measured using a TA
Q800 Dynamic Mechanical Analyzer (New Castle, DW) in tensile mode. The measurements
were carried out at a constant frequency of 1 Hz, strain amplitude of 0.05 %, a temperature
range of 32-150 °C, a heating rate of 3 °C/min.
6.3.4 DMA mechanical properties
The storage modulus for the nanocomposite materials as a function of temperature is given in
Figure 47. The DMA of PVA/SBN composites was performed to investigate if the addition of
the SBN would improve the thermal properties and also to see the possible interaction between
the PVA matrix and SBN. As can be seen, the storage modulus of the two PVA/SBN
119
g
bility to restrict the motion of the
PVA chai
than the PVA/SBN5, a 76 % improvement vs. a 32 % improvement compared to pure PVA, see
Table 17. The difference in reinforcement effect at 70 °C is due to the difference in available
surface area, interaction or adhesion between the nanofiber and matrix. The tan δ peaks recorded
for both nanocomposites have been shifted to higher temperatures compared with that of pure
PVA. The PVA/SBN10 showed a slight shift in the tan δ peak, 5 °C, indicating the segmental
motions of the PVA matrix. The tan δ peak is slightly shifted to higher temperature with
increased SBN content. The shift to higher temperature usually indicates restricted molecule
movement because of improved interaction in filled polymers (Avérous et al. 2001).
Materials E'at 32 °C[GPa]
C
tan δ peak [
nanocomposites improved over the entire temperature span compared to pure PVA. This
indicates that the nanocomposite materials have been able to increase the temperature of use of
PVA. The improvement in storage modulus was most significant between 30 °C and 70 °C,
where the polymer relaxation occurs for PVA as function of temperature. The storage modulus
for PVA shows a significant drop after 42 °C. For the nanocomposite films, it occurs at higher
temperature (49 °C) and then the curve flattens out. It could be noted that the addition of SBN in
PVA increased the softening temperature from 42 °C to 49 °C. The PVA/SBN10 shows the
highest storage modulus. The glass transition temperature (T ) of pure PVA has been reported to
be 85 °C by Dupont. At higher temperatures, where the PVA matrix softens, the reinforcement
effect of the SBN reinforcement will increase due to their a
ns. At 70 °C, the PVA/SBN10 showed a significant improvement in storage modulus
Table 17. DMA results of the nanocomposites
E'at 70 °[GPa] °C]
PVA 52 +3. 0.1 0.37 + 0.03 60. 8 PVA/SBN5 5.54 + 0.1 +0.49 0.01 61.3
5.67 +PVA/SBN10 0.2 +0.65 0.04 65.6
120
F age modulus curves from DM nalysis of the nanocomposi
igure 47. Stor A a tes.
830 50 70 90 110 130 150
8. 5
Temper at ur e [ º C]
9
Log
E
9. 5]10
'[Pa
PVAPVA N5/ SBPVA N10
.4 Thermogravimetric Analysis
A) was performed using a thermogravimetric analyzer-
TG
/ SB
6
Thermogravimetric analysis (TG
A-Q500 (TA Instruments, Inc., New Castle, DE). Few milligrams of the sample were heated
from room temperature up to 500 °C at 10 °C/min under nitrogen flow.
The potential use of nanofiber/PVA nanocomposites in packaging applications requires
thermal stability. Pure PVA and related composites filled with various nanofiber content were
tested using thermogravimetric analysis (Figure 48). Thermal degradation phenomena of PVA
have been reported and a complex mechanism of PVA degradation was proposed. Fig. 48 shows
that the degradation of PVA starts around 75 °C. No significant influence of the presence of
cellulose nanofibers on the thermal stability was reported, despite strong cellulose/PVA
interactions.
121
Figure 48. TGA curves under nitrogen for PVA and PVA/SBN nanocomposites.
95
96
97
101
25 50 75 100 125 150Temper at ur e
Wei
Los
s
98
99
100
ght
PVA/ SBN5PVA/ SBN10Pur e PVA
6.5 Mechanical Behaviour of Nanocomposites
6.5.1 Mechanical testing
The mechanical behavior of prepared materials was tested by a Sintech-1 machine model
3397-36 in tensile mode with a load cell of 50 lb followed by ASTM D 638. Tensile tests were
6 Mechanical performan mpo
p dro of n bers and the non-polar characteristics of
ost thermoplastics result in difficulties in compounding the filler and the matrix (Dufresne et
l. 2000). Poor dispersion of the filler in the matrix of a composite material seriously affects its
mechanical properties. Table 18 summarizes the mechanical properties of PP or PE/SBN
nanocomposites in solid phase melt-mixing. The acrylic/PE and acrylic/PP were used as
reference material to see how the addition of SBN is affecting the mechanical properties for the
nanocomposites. In PE/SBN nanocomposite, both 2.5 wt% and 5 wt% nanofiber reinforced
composites exhibited better mechanical properties in E-modulus and tensile strength, but
performed at a crosshead speed of 10 mm/min.
.5.2 ce of nanoco sites
The inherent olar and hy philic nature anofi
m
a
122
elongation to break was slightly decreased. The ductility of the acrylic/PE film is nearly 235 %.
Because of the hardness and toughness of the acrylic copolymer that it contains, PE/SBN
nanocomposit forms dry-bright films with excellent slip resistance. It is clearly that the addition
of SBN had a positive effect for this composite and the effect was most visible on the composite
strength. The similar results have been obtained in PP/SBN nanocomposite. The E-modulus and
tensile strength were significantly increased with increased nanofiber content. This demonstrates
that the nanofibers were dispersed evenly in the composite in achieving acceptable dispersion
level when they were coated with the acrylic oligomer which acts as a bridge between the
hydrophilic and hydrophobic components. Generally, the main difficulty in melt-mixing of
nanocomposites is to achieve well dispersed nano-reinforcements into thermoplastic polym .
he dispersion level of cellulose fibers within a thermoplastic matrix is naturally subordinated
et al. 1997). Unlike PVA/SBN nanocomposites,
oligom
Tab
ers
T
to the processing technique used (Dufresne
er coated nanofibers and PE or PP are not completely miscible under the hot
compression.
le 18. Mechanical properties of nanocomposites in solid phase melt-mixing.
Materials Elongation
at break (%)
S.D.Max. stress (MPa)
S.D. E-
Modulus (GPa)
S.D.
Acrylic/PE (wt%: 50/50) 235.51 4.3 11.44 1.13 0.21 0.06 SBN/acrylic/PE (wt%: 2.5/47.5/50) 212.24 1.8 12.18 1.06 0.23 0.07
(wt%: 5/45/50) 226.04 3.1 14.20 1.17 0.34 0.11 SBN/acrylic/PE
Acrylic/PP (wt%: 50/50) 282.58 5.6 10.73 2.02 0.25 0.06 SBN/acrylic/PP (wt%: 2.5/47.5/50) 237.07 9.1 15.43 0.59 0.42 0.10
SBN/acrylic/PP 255.79 3.1 26.37 8.36 0.48 0.13 (wt%: 5/45/50) SBN: soybean stock nanofiber; S.D.: standard deviation.
123
6.6 Mechanical Behaviour of Bio-nanocomposites
6.6.1 Tensile testing
The mechanical behavior of nanofiber-blend-PHB film or nanofiber-blend-PLA bio-
nanocomposite was tested by an Instron 5860 (Grove City, PA) in tensile mode with a load cell
of 2 kN or 30 kN in accordance with ASTM D 638. The specimens were cut in a dumbbell
shape with a die ASTM D 638 (type V). Tensile tests were performed at a crosshead speed of
2.5 mm/min. The values reported in this work result from the average of at least five
measurements.
6.6.2 Mechanical properties of bio-nanocomposites
s of cellulose nanofibers were studied with the aim of
improving their inte
properties of the ensuing composites. The mechanical properties of the prepared
re some improvements in the properties of
improvements were similar for both nanoreinforcements. The PHB/SMA-coated HPN
strength and a 37 % increase in modulus relative to pure PHB. The PLA/SMA-coated HPN
compared to PLA/uncoated HPN. There was a 8.6 % increase in tensile strength and a 10 %
nanofibers PLA/ SMA-coated HPN showed an increase in elongation to break around 200 %.
The PLA/SMA-coated SBN nanocomposite showed a 21 % increase in tensile strength and a
The chemical surface modification
rfacial compatibility with PLA and PHB, that is, to enhance the mechanical
nanocomposites are presented in Table 19. There we
the nanocomposite materials, compared to pure PLA and PHB. Table 19 also shows that the
nanocomposite showed a 17 % increase in the yield strength and a 24.5 % increase in modulus
in comparison to PHB/uncoated HPN nanocomposite. There was a 35 % increase in the yield
nanocomposite showed only a 3 % increase in tensile strength and a 7 % increase in modulus
increase in modulus, compared to pure PLA. The composite with partly dispersed cellulose
124
Materials S.D. S.D. Modulus S.D.
14.7 % increase in modulus compared to PLA/uncoated SBN. The above results were lower
than expected. Theoretical calculations were therefore performed in order to better understand
the results and to see the potential effect of both nanoreinforcements. The mechanical properties
are affected by the degree of dispersion and resulting interfacial area between polymer matrix
and nanofibers. SMA-coated SBN nanocomposites showed the best compatibility between
polymer and nanofibers.
Table 19. Tensile properties of the injected bio-nanocomposites.
Elongation at break (%)
Max. Stress (MPa)
E-
(GPa) PHB 10.3 0.3 15.32 1.00 1.41 0.16 PHB/Uncoated HPN 17.8 0.1 17.68 1.68 1.55 0.11
PHB/Uncoated SBN 15.6 0.3 13.79 0.35 1.64 0.37 PHB/SMA-coated HPN 21.5 0.1 20.68 6.66 1.93 1.25
PHB/SMA-coated SBN 19.7 0.2 19.23 1.25 1.88 1.02 1.9 0.1 65.49 0.21 2.72 0.09 PLA
PLA/Uncoated HPN 2.7 0.3 68.97 0.40 2.80 0.06 LA/SMA-coated HPN 3.8 0.2 71.14 0.64 2.99 0.01
01 0.12 PPLA/Uncoated SBN 3.3 0.4 74.86 1.45 3.PLA/SMA-coated SBN 4.1 0.5 79.28 3.34 3.12 0.11
The H quation wa lc the cal
n Eq. (1 (15 arw rou 199
)/(1-ηΦ) (12)
Young’s modulus of the ma f re s Yo mo of th r,
ζ dent up r geo tion, and η
i
(13)
ζ = 2 × Length/Diameter (14) Φ = volume fraction (15)
alpin-Tsai e s used to ca ulate theoreti tensile modulus for the two
anocomposite materials, see 2) – Eq. ) (Ag al and B tman 0),
E = Em (1 + ζηΦ
where Em is the trix, E present ung’s dulus e fille
is a shape parameter depen on fille metry, orientation, and loading direc
s given by,
η = (Ef/Em-1) / (Ef/Em+1)
125
The Halpin-Tsai equation is normally used to predict the modulus for aligned fiber
composites, but it has been used before to predict the modulus of nanocomposites (Wu et al.
2004; Fornes and Paul 2003). It was chosen, since it demanded the least amount of assumptions
to be made about the materials. The Halpin-Tsai equation can only be applied to predict the
modulus of fiber/matrix nanocomposites in the range of low fiber volume fractions. At high
filler concentration, the predicted value is lower than the experimental data. It is assumed that
the filler apparent volume is related to the dispersion of filler, and that the larger apparent
volume may originate in better dispersion, which results in a higher modulus of the composite.
s of less than 6 %, it is well fitted to the
exp
3 3
3 3
experimental results can be seen in Figure 49. When comparing the results, one has to keep in
When the predicted values at filler volume concentration
erimental data (Wu et al. 2004). By comparing model predictions with the two-dimensional
finite element calculations for discontinuous oriented square fiber-reinforced composites,
Ashton et al. (1969) deter-mined that ζ = 2 (length/diameter of the fiber) = 2 × aspect ratio
provided good agreement for longitudinal modulus.
The volume fraction of each nanoreinforcement was calculated using Eq. (16) (Luo and
Daniel 2003),
Φf = (wf/ρf)/( (wf/ρf) + (1 – wf)/ρm) (16)
where, wf = 5 %, ρcellulose = 1.58 g/cm (Ganster and Fink 1999), ρSMA-coated HPN =1.70 g/cm ,
ρPLA = 1.25 g/cm , and ρPHB = 1.17 g/cm . The volume fractions for the PLA/uncoated HPN and
the PLA/SMA-coated HPN were determined to be 4 % and 3.7 %, respectively. The volume
fractions for the PHB/uncoated HPN and the PHB/SMA-coated HPN were 3.75 % and 3.5 %,
respectively. The following data were used in the calculation: EPLA = 1.7 GPa, EPHB = 1.0 GPa,
Ecellulose = 167.5 GPa (Petersson and Oksman 2006), aspect ratio of uncoated HPN is 88, aspect
ratio of SMA-coated HPN is 82 (Wang et al. 2007). A comparison between the theoretical and
126
eoretical value. When it comes to the PLA/uncoated HPN and the
LA/SMA-coated HPN system, the uncoated nanofiber PLA system should have higher
nofiber PLA system, due to its
lower volume fraction. In mical tr he fiber surfa increased the
interfacial adhesion between trix; the exp l results showed the PLA/SMA-
having hi nsile p rties.
meas ensile m lus d pared eoret edicti
mind that theoretical calculations are based on PLA/cellulose and PHB/cellulose systems, where
the nanoreinforcement is aligned in the longitudinal direction and has perfect interfacial
adhesion to the matrix. From Fig. 49, we can draw the conclusion that both systems have large
potentials that this experiment was unable to reach. One can also see that the PLA system has
the largest potential and the PLA/nanofiber system, due to its agglomerated structure, was
farthest away from its th
P
theoretical tensile strength value, compared to the SMA coated na
contrast, the che
fiber and ma
eatments on t
erimenta
ce
coated HPN system as gher te rope
Figure 49. Experimentally u tred odu ata com to th ical pr ons by Halpin-Tsai.
8.00
4.00
6.00
GP
a]E
[
0.00
2.00
0 0.01 0.02 0.03 0.04 0.05
Volume Fraction
Halpin Tsai-PLA/Cellulose- Halpin-Tsai-PHB/Cellulose PL /Uncoate HPNA d-PLA/Coated-HPN PHB/Uncoated-HPN PH /Coated PN
B -H
127
The extrusion process of cellulose nanocomposites renders several challenges. The major
difficulty is to feed the nanofiber into the extruder and achieve uniformly dispersed nanofibers
in the polymer matrix. The nanofibers have a very high surface area and have a tendency to
aggregate when dried. Therefore, to avoid the re-aggregation during drying, the freeze dried
nanofibers were treated by sonification and plasma before fed into the polymer melt processing.
The composite materials were slightly brown after the processing. Table 20 shows the
composites mechanical properties compared with PLA in more details. The PLA was used as
reference material to see how the addition of physical treated nanofibers is affecting the
mechanical properties. In this case the results were much more positive, both composites
showed better mechanical properties and the composite with partly dispersed cellulose
nanofibers PLA/Plasma treated HPN and PLA/sonification HPN showed an increase in tensile
strength around 20 % and 22.5 %. The E-modulus was improved from 1.76 GPa to 2.37 and
2.27 GPa. It is clear that the addition of nanofibers had a positive effect for this composite and
the effect was most visible on the composite strength. Plasma treatment improves fiber-matrix
adhesion either by introducing polar or excited groups that can form strong covalent bonds
between the fiber and the matrix or by roughening the surface of fibers to increase mechanical
terlocking between the fiber and the matrix. The dispersion and distribution of nanofibers can
also be affected and improved by optimizing the compounding process (screw design,
temperature and speed), which will be done in future studies.
in
128
(%) (GPa)
Table 20. Tensile properties of the extruded bio-nanocomposites.
Materials Elongation
at break S.D. Max. Stress (MPa) S.D.
E-Modulus S.D.
PLA 1.9 0.1 44.50 3.56 1.76 0.48 PLA/Plasma treated
PLA/Sonification
(wt%: 95/5)
HPN (wt%: 95/5) 2.5 0.1 53.40 0.26 2.37 0.26
treated HPN 2.3 0.2 54.51 1.21 2.27 0.42
6.7 Dynamic Mechanical Analysis of Bio-nanocomposites
For modulus measurements, the purest form of evaluation occurs when using 3-point bending
and rectangular samples. One of the most common experiments performed in dynamic
mechanical analysis is heating with continuous mechanical oscillation of the test specimen at
onstant frequency and amplitude. This experiment provides valuable information about the
h
temperature, as well as qualitative information about cross-linking, molecular weight, etc.
A stiff composite test specimen, 12.7 mm in width and 3.2 mm in thickness, is placed on the
clamp taking care to ensure that it is parallel to the length axis of the clamp. The sample is
heated at 3 ºC/min to ensure that the 50 mm long sample achieves temperature equilibrium.
The storage modulus for the bio-nanocomposite materials as a function of temperature is
given in Figure 50. The DMA of PLA/HPN composites was performed to investigate if the
addition of the HPN would improve the thermal properties and also to see the possible
interaction between the PLA matrix and HPN. As can be seen, the storage modulus of the SMA-
coated and mechanical treated HPN reinforced bio-nanocomposites improved over the entire
temperature span compared to pure PLA. This indicates that the nanocomposite materials have
been able to increase the temperature of use of PLA. The improvement in storage modulus was
c
glass transition temperature (Tg), the change in modulus and stiffness of the material wit
129
mo
the nanofiber and matrix. The tan δ peaks recorded for both nanocomposites have been shifted
to higher temperatures compared with that of pure PLA. The PLA/Plasma treated HPN showed
a slight shift in the tan δ peak, 1.4 °C, indicating the segmental motions of the PLA matrix.
st significant between 45 °C and 75 °C, where the polymer relaxation occurs for PLA as
function of temperature. The storage modulus for PLA shows a significant drop after 54 °C. For
the bio-nanocomposite samples, it occurs at a higher temperature (62 °C) and then the curve
flattens out. It could be noted that the addition of HPN in PLA increased the softening
temperature from 56 °C to 64 °C. At higher temperatures where the PLA matrix softens, the
reinforcement effect of the HPN will increase due to their ability to restrict the motion of the
PLA chains. At 42 °C the PLA/Plasma treated HPN sample showed a significant improvement
in storage modulus compare to the PLA/Sonification treated HPN, a 114 % improvement vs. a
101 % improvement compared to pure PLA, see Table 21. The difference in reinforcement
effect at 70 °C is due to the difference in available surface area, interaction or adhesion between
Figure 50. Storage modulus curves from DMA analysis of the nanocomposites.
6
7
8
9
10
'[P
]
45 55 65 75 85 95Temper at ur e [ º C]
Log
Ea
PLAPLA/ SMAcoat edPLA/ Pl asmaPLA/ Soni f i cat i on
130
Materials E'at 42 °C [GPa] E'at 70 °C [GPa] tan δ peak [°C]
Table 21. DMA results of the bio-nanocomposites.
PLA 1.39 + 0.1 0.24 + 0.02 64.2 PLA/SMAcoated 2.82 + 0.2 0.47 + 0.01
2.65.3
PLA/Plasma 97 + 0.1 0.41 + 0.03 2.
65.6 PLA/Sonification 80 + 0.1 0.34 + 0.01 65.1
6.8 Conclusions
The nanocomposites were synthesized using two different processing methods, solution
casting with a water soluble polymer PVA and melt blending with a common thermoplastic
polyolefin polymer PE or PP using ethylene-acrylic oligomer emulsion as a dispersant. There
was a difference in the interaction with the SBN in PVA, PP and PE nanocomposites. Oligomer
coated nanofibers had a large surface area which allowed the nanoreinforcement to interact with
polymer. This resulted in a large difference in the mechanical properties between three
nanocomposite systems. The PVA system showed great improvements in both E-modulus and
stress. The tensile strength of PVA/SBN5 increased by five-fold compared with that of
PVA/UNF5. The increased stiffness of PVA/SBN5 was also very promising; about 269 %
increase was obtained in PVA/SBN5 compared with pure PVA film. The bleached nanofiber
addition sacrificed the mechanical properties of nanocomposites. The PP or PE system showed
tendency to improve the mechanical properties in solid phase melt-mixing. Unlike PVA based
system, the strength and stiffness enhancement for PE or PP/SBN nanocomposites was not
significant and elongation was reduced. The DMA study showed that the incorporated nanofiber
was able to hinder the motions of the PVA chains in the matrix and thereby increasing the
temperature of use of the PVA. The nanofibers can provide a remarkable reinforcing potential if
they are uniformly dispersed in the polymer matrix. This study was carried out as an initial step
towards the use of cellulose nanofiber as reinforcement in the solid phase manufacturing.
131
In both PLA and PHB systems, the SMA-coated HPN as the reinforcement enhanced the
mechanical properties over the systems containing uncoated HPN or pure polymer. The
theoretical calculations made in this article showed that the PLA has the largest potential to
improve the mechanical properties, compared to the PHB system. This experiment was a step in
the direction of creating fully renewable biopolymer based nanocomposites.
132
7. CONCLUSIONS
onclu made ab presented i thesis:
1 paren rity in ex re, there is a ing need to clearly
define the concept of nanofibers of cellulose. In this work, based on cellulose anatomy and
structural reinforcement capacity, cellulose nanofibers have diameter ranging between 50 to
100 nm and lengths of thousands of nanometers which results in very high aspect ratio.
2. A new concept in high-strength composite production was developed by utilizing
microfibrillated cellulose as reinforcement. This notion is based on three primary assertions.
1) plant-derived cellulose is composed of extremely strong stretched molecule chain; 2)
microfibrillated cellulose can be considered a nano-material, the elements of which are
highly expanded in terms of surface area; 3) nanofiber has a web-like structure formed by
fibrils and microfibrils continuously interconnected as opposed to discrete fibers. Based on
these foundations, it was possible to reconstitute cellulose to make high-strength composites
by taking advantage of the unique structure of nanofibers. The peak stress of BSBN-based
and BHPN-based thin mats was 88 MPa and 56 MPa; nonetheless the E-modulus was 6.3
GPa and 5.0 GPa, respectively.
3. This study has been concerned how the degree of individulization affects the cellulose fiber
morphology from the micro to the nanoscale. The chemo-mechanical process resulted in
hemp nanofibers having a width in the range of 30-100 nm; soybean pod nannofibers
having a diameter range of 50-100 nm. The used chemical treatments resulted in the
individualized hemp microfibers and further mechanical treatment formed a network
The following c sions can be out the work n this
. In view of an ap t lack of cla isting literatu press
133
the partial removal
of the pectins during the fiber extraction. It was also seen that the relative crystallinity of
the cellulose fibers increased after each stage of chemical treatments.
. The nanocomposites were synthesized using two different processing methods, solution
casting with a water soluble polymer PVA and melt blending with a common thermoplastic
polyolefin polymer PE or PP using ethylene-acrylic oligomer emulsion as a dispersant.
Scanning electron micrographs showed oligomer coated nanofibers formed loose networks
during freeze drying. Improved dispersion of nanofibers was achieved by adding ethylene-
acrylic oligomer emulsion as a dispersant. There was a difference in the interaction with the
SBN in PVA, PP and PE nanocomposites. Oligomer coated nanofibers had a large surface
area which allowed the nanoreinforcement to interact with polymer. This resulted in a large
difference in the mechanical properties between three nanocomposite systems. The PVA
system showed great improvements in both E-modulus and stress. The tensile strength of
PVA/SBN5 increased by fivefold compared with that of PVA/UNF5. The increased
stiffness of PVA/SBN5 was also very promising; about 269 % increase was obtained in
PVA/SBN5 compared with pure PVA film. The bleached nanofiber addition sacrificed the
mechanical properties of nanocomposites. The PP or PE system showed tendency to
improve the mechanical properties in solid phase melt-mixing. Unlike PVA based system,
the strength and stiffness enhancement for PE or PP/SBN nanocomposites was not
significant and elongation was reduced. The DMA study showed that the incorporated
structure of hemp nanofibers. The high pressure defibrillation contributed a unique
morphology of the interconnected web-like structure of nanofibers.
4. Chemical analysis of the cellulose fiber after each stage of purification showed an increase
in cellulose content and a decrease in lignin and hemicellulose content. Successive
bleaching helped with the cellulose purification. FTIR graph indicated
5
134
nanofiber was able to hinder the motions of the PVA chains in the matrix and thereby
increasing the temperature of ibers can provide a remarkable
reinforcing potential if they are uniformly dispersed in the polymer matrix. This study was
e solid phase
6.
fiber dispersion in a matrix, coupled with the high aspect
7.
use of the PVA. The nanof
carried out as an initial step towards the use of nanofiber as reinforcement in th
matrix. Further research work is required for an improved understanding of dispersion
mechanisms of nanofibers in the solid polymer matrix. Processing, characterization, and
functional properties of this new class of nanocomposites can be further optimized.
Inverse gas chromatography (IGC) at infinite dilution has proven to be a convenient tool for
measurement of surface energy and acid-base characteristics of natural fibers and polymer
matrix. Changes in final properties of the composites due to the effect of various chemical
treatments on the fiber surface can also be explained using this technique. Acid-base
interactions with PLA were increased by SMA- and EAA-coated HPN, and the same trend
was observed for PHB matrix. SEM pictures showed SMA-coated HPN having a well-
organized web-like structure and proved that the size of nanofibers is indeed in the nano-
level. Current TEM pictures showed the presence of a non-homogeneous structure of
nanofibers in the PLA based nanocomposites. The properties shown here will most
probably be improved if it is possible to disperse the nanofibers more evenly within the
polymer matrix. The uniform nano
ratio of the nanofibers will indicate a strong potential for the use of these bio-
nanocomposite films.
In both PLA and PHB systems, the SMA-coated HPN as the reinforcement enhanced the
mechanical properties over the systems containing uncoated HPN or pure polymer. The
theoretical calculations made in this project showed that the PLA has the largest potential to
135
nocomposites.
improve the mechanical properties, compared to the PHB system. This experiment was a
step in the direction of creating fully renewable biopolymer based na
136
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