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Applications of nanofibrillated cellulose in polymer composites

Zimmermann, T., Tingaut, P., Eyholzer, Ch., Richter, K.

Empa, Swiss Federal Laboratories for Materials Science and Technology, Wood Labora-tory

Corresponding author: tanja.zimmermann@empa.ch

Due to its high strength and stiffness, low thermal expansion and its optical transparency, nanofibrillated cellulose (NFC) is used as a reinforcing component in (bio)polymers of vari-ous polarities. However, the use of this hydrophilic NFC is often limited to aqueous or polar environments, due to well known re-agglomeration and hornification problems. To over-come these problems, chemical modification is needed to obtain dry redispersible NFC powders for further processing or compatibilisation with hydrophobic polymers. Different modification routes were investigated to fulfil these requirements, namely the carboxy-methylation, the acetylation and the silylation reaction.

Carboxymethylation of refined bleached beech pulp followed by high shear mechanical disintegration into NFC was used to reduce hornification upon drying. A drying method was developed, including solvent exchange to alcohol and repeated stirring during drying in the oven. Depending on the substitution degree, the final powders showed good redis-persibility in water and formed stable suspensions for several hours (Eyholzer et al. 2010a). The mechanical properties of composites from carboxymethylated NFC and hy-droxypropyl cellulose (HPC) were the same, regardless whether the NFC was dried and redispersed or not before compounding (Eyholzer et al. 2010b).

The grafting of acetyl moieties on the cellulose surface also prevented NFC hornification upon drying and drastically improved redispersibility of the powdered nanofibers in Chloro-form, a solvent of low polarity. Bionanocomposite materials were successfully prepared us-ing a polylactic acid (PLA) matrix and acetylated NFC as reinforcing agent. These nano-composites showed improved filler dispersion, higher thermal stability, and reduced hygro-scopicity compared to those with unmodified NFC. Results obtained by dynamic mechani-cal analysis (DMA) indicated an improved compatibility at the fiber-matrix interface when using acetylated NFC (Tingaut et al. 2010). The lack of compatibility/adhesion at the interface between the hydrophilic NFC and hy-drophobic polymer matrices is a major problem in compounding. Therefore, commercially available alkoxysilanes known as adhesion promoter and/or coupling agents were used for improved NFC/matrix compatibility. The polarity and/or reactivity of the NFC can be tai-lored by the organofunctional group X of the silicon which can be a compatibility or reac-tive group (Bordeanu et al. 2008). Morphological and mechanical properties of composites prepared from modified and non modified NFC and different kind of polymers in form of films, fibers or hydrogels will be presented. High pressure freezing, followed by freeze drying (Fig. 1), freeze fracturing, freeze etching was used. Nanomechanical investigations were carried out by Hysitron nanoindentation (Fig.2) as well as atomic force microscopy (AFM).

The high potential of various ensuing cellulose nanocomposites for applications in medi-cine, textiles, adhesives, packaging or food will be intensively discussed.

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Fig. 1: SEM image of a freeze dried carboxy-methylated NFC/ hydroxypropyl cellulose (HPC) suspension after high pressure freezing. The NFC network as well as the HPC network is visible.

Fig. 2: Hysitron AFM image showing in-dents on a NFC film: 10 x 10 µm scan size. Pyramidal structures on the fibrils are tip artifacts due to the Berkovich ge-ometry of the nanoindenter tip.

References:

Bordeanu, N., Eyholzer, C., Zimmermann, T. Surface modified cellulose nanofibers. Patent WO 2010/066905 A1

Eyholzer, C, Bordeanu, N, Lopez-Suevos, F, Rentsch, D., Zimmermann, T., Oksman, K. 2010a. Preparation and characterization of water-redispersable nanofibrillated cellulose in powder form. Cellulose 17:19-30

Eyholzer, C., Lopez-Suevos, Tingaut, P, Zimmermann, T., Oksman, K. 2010b. Reinforcing effect of carboxymethylated nanofibrillated cellulose powder on hydroxypropyl cellulose. Cellulose DOI 10.1007/s10570-010-9423-9

Tingaut, P., Zimmermann, T., Lopez-Suevos F. 2010. Synthesis and characterization of bionanocomposites with tunable properties from poly(lactic acid) and acetylated microfibril-lated cellulose. Biomacromolecules 11: 454-464

Applications of nanofibrillated cellulosein polymer composites

Empa, Swiss Federal Institute for Materials Science and Technology, Wood Laboratory, Cellulose NanocompositesGroup, Dübendorf, CH

Tanja Zimmermann, Philippe Tingaut, Christian Eyholzer,Klaus Richter

International Conference on Nanotechnology for the Forest Products IndustryEspoo, Finnland, 27 – 29 Sept. 2010

Materials Science & Technology

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Outline

Cellulosic raw materialsIsolation of nanofibrillated cellulose (NFC)Advantages of NFC and drawbacksDetermination of mechanical properties by hysitronnanoindentationHornification of NFC upon drying

Morphological characterisation using cryo-techniquesDevelopment of dry redispersable NFC

Application in water soluble polymersApplication in hydrogels

Compatibilization of NFC with hydrophobic matricesApplication in bio-nanocomposites

Conclusions

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Refined bleached pulp fibers

- Wood: Sulphite pulp

- Spruce (Picea abies) and White Fir (Abies alba)- Beech (Fagus sylvatica)

- Oat straw (Avena sativa)- Wheat straw (Triticum vulgare)

Cellulosic raw materials

40 µm Zimmermann et al. 2010. Carbohydrate Polymers 79: 1086-1093

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Isolation of nanofibrillated cellulose (NFC)I. Mechanical pretreatment

Separation of fibril bundles from the cell walls by using a closed dispersing system with an ultra-turrax (Inline disperser; IKA, Megatron MT300, 22000 rpm, 15-120 min)

1 µm

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II. Microfluidizer® High shear processingHigh-shear homogenization of the fibrils bundles by application of a high pressure disperser (Microfluidics, M-110y, up to 1500 bar, 3-10 cycles)

⇒ nanofibrillated cellulose

Isolation of nanofibrillated cellulose (NFC)

SEM image of NFC from wood pulp

Aqueous NFCsuspension0.5 – 2.5 wt.%

Zimmermann et al. 2004. Adv. Eng. Mat. 6(9): 754-761

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Advantages of NFC

• Lightweight material• Renewable resource, biodegradable• High strength and stiffness• Transparent, translucent• High surface area and aspect ratio• High reactivity, barrier properties

Drawbacks• High energy consumption during production• Large amount of water (10 – 30% solids content)

• High storage and transportation costs, microbial degradation• Chemical reactions with NFC usually require solvent exchange

• Hornification• Compatibility with hydrophobic polymer matrices due to its high hydrophilic

character

Image: Turbak et al. 1983

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Mechanical properties by hysitron nanoindentationon NFC films

Indent

Scan size: 20x20 µm,Post indentation example

0.0

0.4

0.8

1.2

1.6

12

13

14

Har

dnes

s (G

Pa)

Si substrate

Max. loading 6 mN

A1(22) A2(22) A3(28)

0

50

100

150

200

250

Inde

ntat

ion

mod

ulus

(GPa

)

Si substrate

A1(22) A2(22) A3(14)

A1, A2; A3 =NFC films

Absolute values forhardness and indentationmodulus must be takencarefully due to possibleinfluences by Si substrate(film thickness)

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Refined bleached beech pulp (RBP) RBP undergone hornification(ground to a cube)

drying

Issues for polymer reinforcement:

• Agglomerated fibrils with decreased aspect ratio• Difficulty of dispersion in matrix

Hornification (Irreversible agglomeration of fibrils)

Young et al. 1994Scallan and Tigerström 1992

Laivins and Scallan 1993Hult et al. 2001

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Morphological characterisation using cryo-techniques

Hornification is also an issue for morphological characterisation of NFCPreservation of native structure by using cryo-technique:

High pressure freezing (vitrification)Freeze fracturingFreeze etching, freeze drying, cryo-sem

NFC composite hydrogelNFC/HPC composite

acetylated NFCNFC/clay film

collapsed structure of NFC

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Development of NFC powder by carboxymethylation

1Lindström and Carlsson 19921Laivins and Scallan 1993

• Carboxylate groups can reduce the degree of hornification during drying of pulps1

• Degree of substitution (DS): average number of substituted OH groups per AGU (anhydroglucose unit)

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RBP carboxymethylated NFC powder

carboxymethylated NFC redispersed in water

carboxymethylationmechanical disintegration

drying from 5/3 isopropanol/ethanolunder repeated stirring at 60°C

Water-redispersible NFC powder

redispersion in waterusing a blender

Patent applications: Bahia et al. 1995Excoffier et al. 1999

Cash et al. 2003Bordeanu et al. 2008

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Quality of carboxymethylated NFC powders

Redispersable in water, forming stable suspensions for at least 20 hSEM images of redispersed and freeze-dried powders show that the nanostructure of the NFC network is preserved, hornification prevented

Impact of DS on stability of carboxymethylated NFC suspensions

concentration: 0.2 % w/wtime: 20 h

Eyholzer et al. 2010. Cellulose 17: 19-30

0.05 0.06

0.09

0.15

0.17

0.20

0.26

Increasing DS

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Quality of carboxymethylated NFC powders

Reinforcing potential (determined by DMA and tensile tests) of driedand redispersed carboxymethylated NFC on water soluble polymers isidentical compared to unmodified and never dried NFC

Eyholzer et al. 2010. Cellulose 17: 793-802

Freeze fractured neat HPC unmodified NFC (dried and redispersed)

carboxymethylated-NFC(dried and redispersed)

agglomerates

voids10μm 10μm10μm

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Reinforcing potential of unmodified and carboxymethylated NFC for hydrogels

Aim: to develop a method for the preparation of biocomposite hydrogelswith increased hydrophilicity for biomedical application; crucial are the swelling and mechanical properties (e.g. replacement of nucleus pulposus in the intervertebral discs)

UV-cross-linked N-vinyl pyrrolidone/ trimethacrylate hydrogelNFC and carboxymethylated NFC (c-NFC) as filler

Borges et al. Biomaterials submitted; Eyholzer et al. In preparationNucleus pulposus

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Reinforcing Potential of unmodified and carboxymethylated NFC for hydrogels

Resultsaddition of NFC led to a decrease in swelling ratioaddition of the more hydrophilic c-NFC led to an increase in swelling ratiowith increasing DS compared to hydrogels containing unmodified NFC higher MOE values in compression + lower strain values duringrelaxation tests + more pronounced relaxation during unloading period forc-NFC/hydrogel (1b) composites compared to neat hydrogel or hydrogelswith unmodified NFC (1a)

1 µm1 µm

1a 1b

Borges et al. 2010. Patent application

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Prevention of hornification and compatibilization withhydrophobic polymer matrices

Main drawback of NFC in composites applications involving apolarenvironments : strong hydrophilic character

Non-homogeneous dispersion

Lack of compatibility at the fiber-matrix interface

Find a method to improve the compatibility at the fiber/matrix interfaceand solve the hornification problem at the same time

One solution: decrease the NFC‘s hydrophilicity throughthe chemical modification of its surface hydroxyl groups

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Acetylation of NFC to decrease its surface hydrophilicity

Cell OH CH3 O

O

CH3

O

N

Cell O

O

CH3 CH3

O

OH+ +Solvent

T = 105°C

AA NFC-AA

Impact of the degree of acetylation (%Ac) on:

Dispersion of NFC in low polarity solvents (such as CHCl3)

Properties of nanocomposites reinforced with functionalized NFC (such as poly(lactic acid) nanocomposites)

Prevention of hornification and compatibilization withhydrophobic polymer matrices: acetylation of NFC

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Prevention of hornification and compatibilization withhydrophobic polymer matrices: acetylation of NFC

t = 24h

Impact of %Ac on the dispersion of dried and redispersed NFC-AA

Drying T = 60°C, sonication 3 x 15 min[c] ~ 0.1% (w/w)

NFC 1.5% 4.5% 8.5% 13% 17%

Hornified

Dispersion improved with %Ac

Hornification prevented

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Prevention of hornification and compatibilization withhydrophobic polymer matrices: acetylation of NFC

PLA (2002D, NatureWorks) reinforced with 0, 2.5, 5, 10 and 17.5% (w/w) of NFC-AA (solvent casting from CHCl3 suspensions)

Properties of poly(lactic acid) nanocomposites evaluated

Neat PLA PLA + 10% (w/w) NFC-AA

%Ac = 0% %Ac = 17%

Dispersion improved with %Ac

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Prevention of hornification and compatibilization withhydrophobic polymer matrices: acetylation of NFC

Mechanical properties (DMA, tensile tests measurements)

Tingaut et al. 2010. Biomacromolecules 11(2): 454-464.

Reinforcing potential of NFC not affected by the chemical modification

Increased stiffness and thermal stability

Positive interactions between PLA and NFC-AA

Main conclusion:

Properties of PLA nanocomposites could be tuned with %Ac

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Compatibilization with hydrophobic polymer matrices: silylation of NFC

Water based silane condensation:

OO

OHOH

OSiO OX

* *

OO

OHOH

OHsilane reagent

X:HN

NH2OO

N CCl

O

netc.; ; ; ; ;

hydrophilic hydrophobic

pH, ΔT

NFC Functional NFC

Bordeanu et al. 2008. PCT/EP 067005

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Conclusions

Reinforcing potential of NFC is high, hysitron nanoindentation canbe used to estimate hardness and MOE of fibrils in NFC films

Hornification issue can be solved by using carboxymethylatedNFC with low substitution degree (properties of NFC are notchanged)

Prevention of hornification and compatibilization with hydrophobicpolymers by acetylation or silylation of NFC

Preservation of original NFC or nanocomposite structure by usingcryo-techniques

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Thank you very much for your attention!And the Cellulose Nanocomposites Group for the work!

We are also grateful for participation of

- Fabien Duc and Ana Borges (EPFL Lausanne)- Roger Wepf (EMEZ, ETH Zürich)