Cellulose Based Composites (New Green Nanomaterials) || Fabrication and Evaluation of Cellulose-Nanofiber-Reinforced Green Composites

27

2Fabrication and Evaluation of Cellulose-Nanofiber-ReinforcedGreen CompositesHitoshi Takagi

2.1Introduction

In the recent past, there have been many reports on natural fiber-reinforced com-posites aiming to develop polymer-based composite materials that can substituteconventional glass-fiber-reinforced plastics (GFRPs) synthesized from petroleum[1–5]. The recent progress in cellulose nanofibers or cellulose microfibrils iso-lation from various cellulose sources such as wood pulp and tunicin cellulosehas made it possible to blend these materials with polymers to improve theirproperties. Young’s modulus of the cellulose microfibrils has been estimated ataround 140 GPa [6] and the tensile strength is generally above 1.7 GPa [7]. Forthese reasons as well as for being sustainable and ‘‘green,’’ cellulose nano- andmicrofibrils are of great interest in examining the possibilities and limitations ofnanoscale reinforcement [8, 9]. In addition to its excellent mechanical properties, ithas become apparent that cellulose-nanofiber-reinforced composite materials haveunique functions in optical and thermal performances that cannot be achievedusing conventional natural fibers.

In this chapter, the basic features of cellulose nanofibers are described. In addi-tion, various extraction methods for producing cellulose nanofibers are presented,and then the fabrication methods as well as some interesting characteristics ofcellulose-nanofiber-reinforced composites are described.

2.2Cellulose Nanofiber

Cellulose is one of the most widely and abundantly spread natural substancesand has the potential to become one of the most important renewable materialsof the twenty-first century. While the major sources of cellulose are plant fibers(cotton, hemp, ramie, etc.), the minor ones are from microbes (acetobacter) andanimals (ascidian). Cellulose is an organic polymer with a formula (C6H12O6)n anda linear carbohydrate polymer chain consisting of more than 10 000 β-1,4-linked

Cellulose Based Composites: New Green Nanomaterials, First Edition.Edited by Juan P. Hinestroza and Anil N. Netravali.c© 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

28 2 Fabrication and Evaluation of Cellulose-Nanofiber-Reinforced Green Composites

glucopyranose units. These cellulose chains aggregate to form nanofibrils(nanofiber) and microfibrils, long threadlike bundles of molecules stabilized byhydrogen bonds [10]. The cellulose microfibril bundles have transverse dimensionsthat range from 20 to 200 nm depending on their origin [11].

The advantages of cellulose nanofibers are threefold: cellulose nanofibers haveexcellent mechanical properties because of the linear structure with high crys-tallinity; they possess stable thermal properties and heat resistance because of theabsence of hemicellulose, lignin, and so on; and they can be easily blended asthe reinforcing phase for small-scale composites in applications such as micro-electromechanical systems (MEMS) and medical devices, for example, scaffold andstent.

2.3Preparation of Cellulose Nanofibers

Several kinds of preparation methods for extracting cellulose nanofibers from plantfibers have been proposed [12–19]. These methods can be classified into threegroups on the basis of the extraction mechanisms: chemical extraction methods[12–14], enzymatic extraction methods [15, 16], and physical extraction methods[17–19]. Numerous reports are available on the preparation of cellulose nanofibers.Table 2.1 summarizes the reported work on the preparation method of cellulosenanofibers. As can be seen from the table, the majority of the work is on chemicalextraction, with a few reports on other fibers such as jute, sisal, and kenaf.

2.3.1Chemical Extraction Method

In the chemical extraction method, Bondeson et al. [12] have reported that cellulosenanowhisker (CNW) can be obtained by homogenizing treatment of acid-treatedmicrocrystalline cellulose (MCC). MCC is now a commercially available cellulosematerial, which is derived from natural plant fiber and wood fiber, and is mainlyused as an inert binder for tablets. They also found that cellulose whiskers canbe easily dispersed in an organic medium, dimethylacetamide/lithium chloride(DMAc/LiCl), and that the whisker size is measured to be between 200 and 400 nmin length and less than 12 nm in width.

Table 2.1 Recent works on the preparation method of cellulose nanofiber.

Types Remarks References

Chemical extraction DMAc/LiCl [12]TEMPO-mediated oxidation [13, 14]

Enzymatic extraction Trichoderma viride cellulose [15, 16]Physical extraction Homogenizer [17–19]

2.3 Preparation of Cellulose Nanofibers 29

Cellulose nanofibers dispersed in water were successfully prepared using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation of hardwoodbleached kraft pulp. The C6 primary hydroxyl groups of polysaccharides includingβ-glucose in cellulose are selectively oxidized to carboxylate groups via the alde-hyde structure by the TEMPO-mediated oxidation [13, 14]. An advantage of theTEMPO-mediated oxidation method is that significant amounts of carboxylate andaldehyde groups can be introduced into native celluloses, maintaining their fibrousmorphologies and crystallinities [13, 14]. Recent study by Saito et al. showed thatbecause the original cellulose crystallinity is maintained during TEMPO-mediatedoxidation and the successive stirring, cellulose nanofibers with crystallinities of65–95% can be obtained. The cellulose nanofibers obtained by this method are3–4 nm in width and a few micrometers in length, that is, aspect ratio of more thanseveral hundred [14]. Such high aspect ratios are highly advantageous and desiredin the case of polymer reinforcing.

2.3.2Enzymatic Extraction Method

Hayashi et al. [15, 16] reported that the algal-bacterial type cellulose and the cotton-ramie type cellulose can be degraded preferentially by Trichoderma viride celluloseand that the residues become cellulose Iβ phase dominant. Relatively short cellulosenanofibers with lateral dimension of 40 nm and aspect ratio of 10–50 were obtainedby this enzymatic extraction method. This type of cellulose nanofiber seems to besuitable for reinforcement in polymer-based composites. Because of their smallaspect ratio, it is easy to obtain uniform dispersion of nanofibers in polymers.However, there has been no report on the application of this type of extractedcellulose nanofiber for reinforcement in composite materials.

2.3.3Physical Extraction Method

Several physical extraction methods, such as mechanical homogenization process[17, 18] and mechanical grinding process [19], have been reported. These methodshave been developed in order to produce food additives and thickening agents infood industry [20]. Basically, high shearing force is applied to kraft pulp by using ahigh-pressure homogenizer. This results in splitting and unraveling of the cellulosefibers to expose smaller microfibrils and nanofibrils having diameters in the range10–100 nm [18]. One of the most important features of this method is that thecellulose nanofibers have a web-like network structure consisting of nano-orderunits. In other words, the cellulose nanofibers are not independent single fibers butan entangled mass containing nanofibers having approximately the same aspectratio.

Taniguchi et al. [19] have developed a mechanical grinding process in order toobtain cellulose microfibrils from wood pulp without using any organic solvent. Asmall commercial grinder with a specially designed disk was used to microfibrillate


natural fibers. A slurry containing 5–10% wood pulp and 90–95% water werepassed through the grinder disk 10 times. Each pass took only a few seconds;therefore, the total processing time was less than 1 min revealing a very efficientprocessing method. Scanning electron microscopy (SEM) observation of the sampleair dried at room temperature showed the surface structure of the film to be verysmooth at the submicrometer level and that the individual microfibrils were notidentified as being tightly bonded [19]. They also demonstrated that uniform andhigh-strength microfibrils were obtained from a wide variety of raw materials suchas wood pulp and tunicin cellulose.

2.4Fabrication of Cellulose-Nanofiber-Reinforced Composites

Favier et al. [21] prepared CNWs by the so-called chemical extraction method.They fabricated CNW-reinforced latex composites by film casting method. Thecomposite samples were slowly dried at room temperature for 1 month resultingin 2 mm thick plates. Petersson and Oksman [22] also prepared biopolymer-basednanocomposite film by using similar film casting method. While the film castingmethod makes it easy to obtain a nanocomposite film, it is intrinsically difficult toget thick composite material because of the long drying times and possible warpingof the specimens because of uneven drying.

Nakagaito and Yano [18, 23] prepared microfibrillated cellulose (MFC) by anumber of passes through the homogenizer using kraft pulp. The MFC was mixedin water at a fiber content of 0.2 wt% and stirred for 48 h, resulting in uniformdispersion of the MFC. This water suspension was vacuum filtered, producing athin paperlike mat. This mat was dried and then immersed in phenol-formaldehyde(PF) resin diluted in methanol. Finally, the sample was stacked in layers of about25 sheets, put in a metal mold, and hot pressed at 160 ◦C for 30 min at 30–100 MPapressure, resulting in a 1.5 mm thick specimen.

In order to make relatively thick nanocomposites easily, Takagi and Asano [24]used a starch-based dispersion-type biodegradable polymer as resin. The resinis made from an esterified starch and exhibits thermoplasticity. Fine particles of∼6 μm diameter are mixed in water forming an emulsion (dispersion), and the resincontent is 40 wt%. This dispersion-type resin and commercially available cellulosenanofibers, prepared using the homogenizing process on wood pulp, were mixedthoroughly for 20 min using a kitchen blender. Some specimens were further mixedusing a low-speed stirrer at 300 rpm for 24 h to form what is designated simply asstirrer-treated composite. The excess water was removed using a filter paper and themixture was subsequently dried at 105 ◦C in air, to form a control composite, or in avacuum, to form a vacuum-treated composite. Figure 2.1 shows the effect of process-ing conditions on flexural strength of the starch-based cellulose nanocomposites,indicating the effectiveness of high-pressure molding and stirrer treatment.

Mathew et al. [25] tried to fabricate cellulose nanocomposites using twin screwextrusion method. They used polylactic acid (PLA) as the resin and MCC and

2.5 Properties of Cellulose-Nanofiber-Reinforced Composites 31

0 20 40 600

20

40

60

80F

lexu

ral s

tren

gth

(M

Pa)

Molding pressure (MPa)

ControlVacuum treated

Stirrer treated

Figure 2.1 Effect of molding pressure on the flexural strength of cellulose-nanofiber-reinforced composites [24].

Table 2.2 Typical mechanical properties of PLA and extruded composites [25].

Materials Tensile strength (MPa) E-modulus (GPa) Elongation (%) Toughness (kJ m−2)

PLA 58± 6 2.0± 0.2 4.2± 0.6 35± 8PLA-MF 58± 5 2.6± 0.1 2.8± 0.5 24± 8PLA-CNW 57± 2 2.4± 0.3 3.3± 0.4 31± 7PLA-PEG 51± 3 2.1± 0.6 >50 —PLA-PEG-MF 59± 2 2.3± 0.1 3.3± 0.2 27± 2PLA-PEG-CNW

47± 5 2.1± 0.3 5.4± 1.8 49± 19

CNW as the reinforcement. In addition, polyethylene glycol (PEG) was usedas a processing aid to decrease the viscosity of the polymer. Typical mechanicalproperties of nanocomposites with 5 wt% reinforcement are presented in Table 2.2.It can be seen from these results that the extruded nanocomposites have slightlyhigher mechanical properties compared to those of pure PLA. They pointed out thatthe reason for such low increase in the mechanical properties was the nonuniformdispersion and considerable agglomeration of reinforcement. Further modificationin extrusion processing or the nanowhisker surface chemistry may be needed toobtain uniform dispersions and thus to get significant increase in the mechanicalproperties of nanocomposites.

2.5Properties of Cellulose-Nanofiber-Reinforced Composites

2.5.1Mechanical Properties

Wu et al. [26] fabricated polyurethane/cellulose nanocomposites containing3–10 wt% nanocellulose fibers. They prepared the cellulose nanofibers byhydrolysis using sulfuric acid and reported considerable increases in mechanical


properties, for example, 60% increase in Young’s modulus, 100% increasein tensile strength, and 50% increase in tensile elongation as compared tothose of near polyurethane. For polymer composites, therefore, the dispersionof cellulose nanofiber in the composites has considerable effects on theirmechanical properties. Zimmermann et al. [27] synthesized two kinds ofcellulose-microfibril-reinforced polymer composite films in which the cellulosenanofibers were dispersed in polyvinyl alcohol (PVA) or hydroxypropyl cellulose(HPC). The cellulose nanofibers were obtained from sulfite pulp by chemicaland/or mechanical isolation process. They reported that the stiffness of the PVAcomposites was higher than that of HPC and explained this result by the higherstiffness of neat PVA resin compared to that of neat HPC resin. They also pointedout that the use of mechanically isolated cellulose nanofibers with comparativelyhigh degree of polymerization (DP) showed a better mechanical performancethan the chemically isolated ones. Better mechanical performance is the highernanofiber strength resulting from the higher DP as could be expected. The effectof processing on the DP of nanofibers was also discussed, indicating that themechanically treated cellulose nanofibers showed higher DP (i.e., less chainscission) compared to that of chemically treated ones.

Nakagaito and Yano [18] also studied the performance of cellulose-nanofiber-reinforced PF composites. High bending strength of up to 400 MPa was reportedfor the composite films reinforced by MFC [18] or bacterial cellulose (BC) nanofiber[28]. In the case of MFC composites, their mechanical properties such as bendingstrength and bending modulus were strongly dependent on the degree of fibrilla-tion of the pulp fiber. The degree of fibrillation of kraft pulp was evaluated indirectlyby water retention, which was measured as moisture content after centrifuging2% fiber content pulp treated by a refiner or by the refiner and a high-pressurehomogenizer. Figures 2.2 and 2.3 show the variation of modulus and bendingstrength of nanocomposites as a function of water retention, respectively. Whilethere was no significant change in modulus (Figure 2.2), the bending strengthshowed substantial increases with increasing water retention (Figure 2.3). Theyconcluded that the refining process of more than 30 passes through the homog-enizer led to sudden and large increases in bending strength compared withuntreated pulp-based composites and that any higher number of homogenizerpasses causes reduction in bending strength.

Mechanical properties of BC nanofiber composites with fiber content of 3–22 wt%have also been reported by Nakagaito et al. [28]. Bending strength and Young’s mod-ulus of BC nanofiber composites were compared with those of the nanocompositesreinforced with fibrillated cellulose nanofibers. The effect of compressing pressureon the mechanical properties was evaluated for BC-based nanocomposites as wellas MFC-based nanocomposites. Figure 2.4 presents the variation of mechanicalproperties as a function of compressing pressure. It should be noted that the cleardifference between BC-based nanocomposites and MFC-based nanocompositesexists in their stiffnesses or Young’s moduli. However, a similar tendency ofbending strength with compressing pressure was also observed and is presentedin Figure 2.4. This is in spite of the fact that all samples exhibited similar densities,


10010

14

18

22

150 200 250 300

Water retention (%)

350 400 450 500

P R2 R4

R8

R16

R30

H2H6

H22

H30

H14

Low resin content (2.4–3.9%)

Medium resin content (6.8–10.5%)

High resin content (14.4–27.9%)

E (

GP

a)

10010

14

18

22

150 200 250 300

Water retention (%)

350 400 450 500

100 150 200 250 300

Water retention (%)

350 400 450 500

E (

GP

a)

10

14

18

22

E (

GP

a)

P R2R4

R8 R16

R30

H2H6

H22

H30

H14

P R2 R4

R8 R16

R30

H2

H6 H30

H14

H22

Figure 2.2 Effect of water retention on modulus of nanocomposites [18].

for example, 1.44–1.53 g cm−3 for BC-based composites and 1.44–1.49 g cm−3 forMFC-based composites. They have pointed out that the increase in Young’s modu-lus could be attributed to a high planar orientation of the ribbon-like elements aftercompression and to the changes in fiber morphology, namely, fibers in BC-basedcomposites showed relative straightness, continuity, and uniformity. The bendingstrength of 400 MPa and Young’s modulus of 28 GPa are comparable to thoseof engineering GFRPs indicating that nanocellulose fiber composites show greatpromise for use as high-strength materials in structural applications.


100150

P R2R4

R8 R16

R30

H2 H6

H14H22

H30

200

250

300

350

400

150 200 250 300

Water retention (%)

350 400 450 500

σ b (

MP

a)

100150

200

250

300

350

400

150 200 250 300

Water retention (%)

350 400 450 500

σ b (

MP

a)

100150

200

250

300

350

400

150 200 250 300

Water retention (%)

350 400 450 500

σ b (

MP

a)

Low resin content (2.4–3.9%)

Medium resin content (6.8–10.5%)

High resin content (14.4–27.9%)

P

R2

R4R8 R16

R30H2 H6

H14H22

H30

P

R2 R4 R8

R16

R30

H2

H6

H14H22

H30

Figure 2.3 Effect of water retention on bending strength of nanocomposites [18].

2.5.2Thermal Properties

A few studies have dealt with the thermal conductivity behavior of cellulose-nanofiber-reinforced composites. Recently, Shimazaki et al. [29] have reportedexcellent thermal conductive properties and optical transparent properties ofcellulose nanofiber/epoxy resin nanocomposites, which contained 58 wt% of cel-lulose nanofiber. The nanocomposites showed anisotropic thermal conductivity,


0

(a)

(b)

10

15

20

E (

GP

a)

σ b (

GP

a)

25

30

100

Compressing pressure (MPa)

50 150 200

0150

200

250

300

350

400

450

100

Compressing pressure (MPa)

50 150 200

Figure 2.4 (a) Modulus (E) and (b) bending strength (𝜎b) against compressing pressureof BC-based composites (O) and MFC-based composites (Δ) [28].

in-plane thermal conductivity of 1.1 W (mK)−1 and out-of-plane (normal to samplesurface) thermal conductivity of 0.23 W (mK)−1, whereas epoxy resin withoutnanofibers showed isotropic thermal conductivity of 0.15 W (mK)−1. The thermalconductivity value of the nanocomposites was approximately five times largerthan that of neat epoxy resin without nanofiber. They also reported relativelylow thermal conductivity of 0.7 W (mK)−1 for the same epoxy-based nanocom-posites with nanofiber content of 39 wt%, showing that a larger amount ofcellulose nanofibers can transport more phonons through the nanocomposites.On the other hand, the out-of-plane thermal conductivity value is close to thatof neat epoxy resin. They explained that low thermal conductivity value in out-of-plane direction was derived from the in-plane orientation of nanofibers innanocomposites.

Thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis(DMTA) were carried out in order to investigate the thermal properties of PLA/CNWnanocomposites [30]. Five kinds of samples used for the thermal analysis arepresented in Table 2.3. Typical TGA results for whiskers and nanocomposites areshown in Figure 2.5. As can be seen from this figure, all materials were thermally


Table 2.3 Compositions of five samples [30].

Materials PLA (wt%) CNW (wt%) Surfactant (wt%)

PLA 100 — —PLA/S 80 — 20PLA/CNW 95 5 —PLA/B-CNW 95 5 —PLA/S-CNW 75 5 20

0

(a)

(b)

20

40

60

Resid

ual w

eig

ht (%

)R

esid

ual w

eig

ht (%

)

80

100

100 200

Temperature (°C)

300 400

2092

94

96

98

100

102

70 120

Temperature (°C)

170 200

MCC

CNW

B-CNW

S-CNW

PLA/SPLA

PLA/CNW

PLA/B-CNW

PLA/S-CNW

Figure 2.5 TGA analysis of (a) whiskers and (b) nanocomposites [30].

stable in the region below 220 ◦C. It was concluded that there was no degradationtaking place in either whiskers or composites resulting in large weight reductionsin the temperature region of 25–220 ◦C. However, the TGA results show only theweight loss of the materials during high temperature exposure. As no physicalinformation, such as changes in strength and stiffness, is available from TGA,


7.515 35 55

Temperature (°C)

75 95

15

(b)

(a)

35 55

Temperature (°C)

75 95

8

8.5

9

9.5

0

0.04

0.08

0.12

0.16L

ogE′ (

Pa

)

7.5

8

8.5

9

9.5

Lo

gE′ (

Pa

)

tan

δta

nδ

0

0.05

0.1

0.15

0.2

0.25

PLAPLA/CNWPLA/B-CNW

PLAPLA/SPLA/S-CNW

Figure 2.6 (a,b) Storage modulus curves and tan 𝛿 peaks from DMTA analysis [30].

further assessment is needed to clarify the physical stability of cellulose nanofibersat around 200 ◦C. The storage modulus as a function of temperature and thetan 𝛿 peak for nanocomposites and neat PLA are shown in Figure 2.6. It can beseen that all whiskers are able to improve the storage modulus of pure PLA athigher temperatures and that this improvement in storage modulus is also alteredby the type of whisker embedded, that is, CNW or B-CNW as well as type ofmatrix modification; that is, PLA or PLA/S. Similarly MCC/PLA nanocompositesshowed improved storage modulus compared to pure PLA. This increase in storagemodulus was most significant at higher temperatures, that is, between 40 and90 ◦C [31].


2.5.3Optical Properties

Another important aspect is optical properties of cellulose nanocomposites, partic-ularly in the case of applications requiring optical clarity. The cellulose nanofiberhas possibilities as a reinforcing fiber for a transparent resin, such as epoxy andunsaturated polyester, because of the principle in physics that a substance with thesize less than one-tenth of wavelength of light does not produce light scattering.In the case of visible light, the wavelength ranges between 400 and 800 nm; thusthe embedded object whose typical dimension less than 80 nm cannot be visible.In these cases, such an object becomes a transparent reinforcement for trans-parent polymers. Similar phenomena have also been reported on nanoscale claycomposites [32] and electrospun Nylon-fiber-reinforced epoxy composites [33].

Yano et al. [34] have used the fact that a cellulose nanofibrils obtained from BChave dimensions in the range of 50 nm compared with the wavelength of visiblelight. They reported the first example of transparent composites reinforced withBC nanofibers. In making these composites, predried BC sheets were impregnatedwith thermoset epoxy resin with the help of vacuum to fabricate the transparentnanocomposites materials. The BC nanofiber content of the nanocomposites wasin the range of 60–70 wt%. The relationship between light transmittance andwavelength for the BC/epoxy nanocomposites with 65 wt% BC fiber is presentedin Figure 2.7. It can be seen from this graph that the BC/epoxy nanocompositestransmit higher than 80% of the incident light in the range of 500–800 nmwavelength and that the loss of light transmittance because of the BC fibers is less

2000

10

20

30

40

50

60

70

80

90

100

400 600

Wavelength (nm)

800 1000

Tra

nsm

itta

nce

(%

)

Epoxy resin

BC/epoxy sheet

BC sheet

Figure 2.7 Relationship between light transmittance and wavelength for the BC/epoxynanocomposites [34].

2.6 Summary 39

40060

70

80

Tra

nsm

itta

nce

(%

) 90

100

450 500 550

Wavelength (nm)

600 650 700

Composite A (24/76)

Composite B (43/57)

Composite C (59/41)

Figure 2.8 Transmittance of all-cellulose composites to visible light. Numbers in bracketsdenote the estimated cellulose I/cellulose II ratio [35].

than 10% compared with that of neat epoxy resin without nanofiber. On the otherhand, the light transmittance of BC sheet itself is very low (less than 50%), andtherefore, it does not transmit the light, that is, it is opaque. Such a high lighttransmittance property of nanocomposites was mainly derived from the selection ofepoxy resin that had refractive index similar to that of BC nanofibers. For example,refractive index of BC nanofiber is 1.618 along the longitudinal direction of fiberand 0.544 in the transverse direction and that of impregnated epoxy resin was setto be 1.522 at wavelength of 587.6 nm and at 23 ◦C.

To avoid combining different refractive indexes of resin and fiber, Gindl andKeckes [35] fabricated all-cellulose nanocomposites by means of partial dissolutionof MCC powder in lithium chloride/N,N-dimethylacetamide and subsequent filmcasting. By this way, both resin and fibers were cellulose with the same refractiveindex. However, the cellulose used as resin was type II and the cellulose usedas fibers was type I. They produced three types of all-cellulose nanocompositesin which the ratio of cellulose I and cellulose II varied. Figure 2.8 presentsthe relationship between transmittance of visible light and wavelength showingexcellent transparency to visible light. There is a small effect of the ratio of celluloseI and cellulose II on the transmittance of visible light. However, higher cellulose IIcontent showed higher transparency. Very high transmittance rates of up to 95%in all composites also suggest that there is a perfect interface bonding betweenmatrix and fiber.

2.6Summary

This chapter describes in brief the cellulose nanofibers and the cellulose-nanofiber-reinforced composites including their fundamental features, extraction methods,and fabrication processes as well as multilateral characteristics. Cellulose nanofiberscan be obtained from fully sustainable plant-based cellulose fibers that are


abundantly available worldwide. This renewable resource has sufficient physicalproperties to be considered as useful reinforcing constituents in polymer com-posites. It should be noted that cellulose-nanofiber-reinforced composites provideimprovements in both mechanical and functional properties. However, this areaof research is still in its infancy and further significant work is required to fullyunderstand the fundamental mechanisms in strengthening of composites and toincrease their mechanical, thermal, and optical performances. The technology infuture will be utilized to obtain advanced eco-friendly materials.

Abbreviations

BC Bacterial celluloseCNW Cellulose nanowhiskerDMAc/LiCl Dimethylacetamide/lithium chlorideDMTA Dynamic mechanical thermal analysisDP Degree of polymerizationGFRP Glass-fiber-reinforced plasticsHPC Hydroxypropyl celluloseMCC Microcrystalline celluloseMEMS Microelectromechanical systemsMFC Microfibrillated cellulosePEG Polyethylene glycolPF Phenol-formaldehydePLA Polylactic acidPVA Polyvinyl alcoholSEM Scanning electron microscopyTEMPO 2,2,6,6-Tetramethylpiperidine-1-oxyl radicalTGA Thermogravimetric analysis

References

1. Netravali, A.N. and Chabba, S. (2003)Composites get greener. Mater. Today,22, 22–29.

2. Okubo, K., Fujii, T., and Yamashita,N. (2005) Improvement of interfacialadhesion in bamboo polymer compos-ite enhanced with micro-fibrillatedcellulose. JSME Int. J., Ser. A, 48,199–204.

3. Takagi, H. and Ichihara, Y. (2004) Effectof fiber length on mechanical propertiesof ‘‘green’’ composites using a starch-based resin and short bamboo fibers.JSME Int. J., Ser. A, 47, 551–555.

4. Gomes, A., Goda, K., and Ohgi, J. (2004)Effects of alkali treatment to reinforce-ment on tensile properties of curauafiber green composites. JSME Int. J.,Ser. A, 47, 541–546.

5. Nishino, T., Hirano, K., Kotera, M.,Nakamae, K., and Inagaki, H. (2003)Kenaf reinforced biodegradable com-posites. Compos. Sci. Technol., 63 (9),1281–1286.

6. Sakurada, I., Nukushima, Y., and Ito,T. (1962) Experimental determinationof the elastic modulus of crystallineregions in oriented polymers. J. Polym.Sci., 57 (165), 651–660.

References 41

7. Page, D.H. and EL-Hosseiny, F. (1983)The mechanical properties of singlewood pulp fibres. Part VI. Fibril angleand the shape of the of the stress straincurve. J. Pulp Pap. Sci., 9 (4), 99–100.

8. Orts, W.J., Shey, J., Imam, S.H., Glenn,G.M., Guttman, M.E., and Revol, J.(2005) Application of cellulose microfib-rils in polymer nanocomposites. Polym.Environ., 13 (4), 301–306.

9. Yano, H. and Nakahara, S. (2004)Bio-composites produced from plantmicrofiber bundles with a nanometerunit web-like network. J. Mater. Sci., 39(5), 1635–1638.

10. Gacitua, W., Ballerini, A., and Zhang,J. (2005) Polymer nanocomposites:synthetic and natural fillers a review.Maderas Cienc. Technol., 7 (3), 159–178.

11. Bhattacharya, D., Germinariob, L.T., andWinter, W.T. (2008) Isolation, prepa-ration and characterization of cellulosemicrofibers obtained from bagasse.Carbohydr. Polym., 73 (3), 371–377.

12. Bondeson, D., Kvien, I., and Oksman,K. (2005) in Cellulose NanocompositesProcessing, Characterization and Properties(eds K. Oksman and M. Sain), AmericanChemical Society, Washington, DC, pp.10–32.

13. Saito, T. and Isogai, A. (2004) The effectof oxidation conditions on chemical andcrystal structures of the water-insolublefractions. Biomacromolecures, 5 (5),1983–1989.

14. Saito, T., Kimura, S., Nishiyama, Y., andIsogai, A. (2007) Cellulose nanofibersprepared by TEMPO-mediated oxidationof native cellulose. Biomacromolecures, 8(8), 2485–2491.

15. Hayashi, N., Sugiyama, J., Okano, T.,and Ishihara, M. (1997) The enzymaticsusceptibility of cellulose microfibrils ofthe algal-bacterial type and the cotton-ramie type. Carbohydr. Res., 305 (2),261–269.

16. Hayashi, N., Sugiyama, J., Okano, T.,and Ishihara, M. (1997) Selective degra-dation of the cellulose Iα component inCladophora cellulose with Trichodermaviride cellulose. Carbohydr. Res., 305 (1),109–116.

17. Turbak, A.F., Snyder, F.W., andSandberg, K.R. (1983) Microfibrillated

cellulose a new cellulose product: prop-erties, uses and commercial potential. J.Appl. Polym. Sci.: Appl. Polym. Symp, 37,815–827.

18. Nakagaito, A.N. and Yano, H. (2004)The effect of morphological changesfrom pulp fiber towards nano-scalefibrillated cellulose on the mechanicalproperties of high-strength plant fiberbased composites. Appl. Phys. A, 78 (4),547–552.

19. Taniguchi, T. and Okamura, K. (1998)New films produced from microfibril-lated natural fibres. Polym. Int., 47 (3),291–294.

20. Herrick, F.W., Casebier, R.L., Hamilton,J.K., and Sandberg, K.R. (1983) Micro-fibrillated cellulose: morphology andaccessibility. J. Appl. Polym. Sci.: Appl.Polym. Sym., 37, 797–813.

21. Favier, V., Chanzy, H., and Cavaille,J.Y. (1995) Polymer nanocompositesreinforced by cellulose whiskers. Macro-molecules, 28 (18), 6365–6367.

22. Petersson, L. and Oksman, K. (2005)in Cellulose Nanocomposites Processing,Characterization and Properties (edsK. Oksman and M. Sain), AmericanChemical Society, Washington, DC, pp.132–150.

23. Nakagaito, A. and Yano, H. (2005) inCellulose Nanocomposites Processing,Characterization and Properties (edsK. Oksman and M. Sain), AmericanChemical Society, Washington, DC, pp.151–168.

24. Takagi, H. and Asano, A. (2008) Effectsof processing conditions on flexuralproperties of cellulose nanofiber rein-forced ‘‘green’’ composites. CompositesPart A, 39 (4), 685–689.

25. Mathew, A.P., Chakraborty, A., Oksman,K., and Sain, M. (2005) in CelluloseNanocomposites Processing, Characteriza-tion and Properties (eds K. Oksman andM. Sain), American Chemical Society,Washington, DC, pp. 114–131.

26. Wu, Q.J., Henriksson, M., Liu, X., andBerglund, L.A. (2007) A high strengthnanocomposite based on microcrys-talline cellulose and polyurethane.Biomacromolecules, 8 (12), 3687–3692.

27. Zimmermann, T., Pohler, E., andGeiger, T. (2004) Cellulose fibrils


for polymer reinforcement. Adv. Eng.Mater., 6 (9), 754–761.

28. Nakagaito, A.N., Iwamoto, S., and Yano,H. (2005) Bacterial cellulose: the ulti-mate nano-scalar cellulose morphologyfor the production of high-strengthcomposites. Appl. Phys. A, 80 (1), 93–97.

29. Shimazaki, Y., Miyazaki, Y., Takezawa,Y., Nogi, M., Abe, K., Ifuku, S., andYano, H. (2007) Excellent thermalconductivity of transparent cellulosenanofiber/epoxy resin nanocomposites.Biomacromolecules, 8 (9), 2976–2978.

30. Petersson, L., Kvien, I., and Oksman, K.(2007) Structure and thermal propertiesof poly(lactic acid)/cellulose whiskersnanocomposite materials. Compos. Sci.Technol., 67 (11–12), 2535–2544.

31. Petersson, L. and Oksman, K. (2006)Biopolymer based nanocompos-ites: comparing layered silicates and

microcrystalline cellulose as nanorein-forcement. Compos. Sci. Technol., 66 (13),2187–2196.

32. LeBaron, P.C., Wang, Z., and Pinnavaia,T.J. (1999) Polymer-layered silicatenanocomposites: an overview. Appl. ClaySci., 15 (1–2), 11–29.

33. Bergshoef, M.M. and Vancso, G.J.(1999) Transparent nanocompositeswith ultrathin, electrospun nylon-4,6fiber reinforcement. Adv. Mater., 11 (16),1362–1365.

34. Yano, H., Sugiyama, J., Nakagaito, A.N.,Nogi, M., Matsuura, T., Hikita, M., andHonda, K. (2005) Optically transparentcomposites reinforced with networks ofbacterial nanofibers. Adv. Mater., 17 (2),153–155.

35. Gindl, W. and Keckes, J. (2005) All-cellulose nanocomposite. Polymer, 46(23), 10221–10225.

Documents

Cellulose Based Composites (New Green Nanomaterials) || Fabrication and Evaluation of Cellulose-Nanofiber-Reinforced Green Composites