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Physico-Mechanical Properties of Cellulose Acetate Butyrate/ Yellow Poplar Wood Fiber Composites as a Function of Fiber Aspect Ratio, FiberLoading, and Fiber Acetylation

Insan Akademika

Publications

INTERNATIONAL JOURNAL

OF BASIC AND APPLIED SCIENCE

P-ISSN: 2301-4458

E-ISSN: 2301-8038

Vol. 01, No. 02

Oct 2012www.insikapub.com

385

Physico-Mechanical Properties of Cellulose Acetate Butyrate/ Yellow Poplar

Wood Fiber Composites as a Function of Fiber Aspect Ratio, Fiber Loading,

and Fiber Acetylation

G. N. Onyeagoro1, M. E. Enyiegbulam

2

1Department of Polymer and Textile Engineering

Federal University of Technology, Owerri, Imo State, Nigeria.

[email protected] of Polymer and Textile E ngineering

Federal University of Technology, Owerri, Imo State, Nigeria.

[email protected]

Key words Abstract

Acetylation,

Cellulose acetate butyrate,

Physico-mechanical

properties,

Yellow poplar wood fibers.

In order to achieve completely biodegradable thermoplastic polymer composite,

the development of yellow poplar wood fiber (YPWF)-reinforced cellulose acetate

butyrate (CAB) composites was carried out. The CAB/YPWF composite was

prepared using a two-roll mill. Composite samples were prepared with two

different fiber types with fiber content of 0, 10, 20, 30, and 40wt %. While the

untreated fibers (UTF) were used as obtained without treatment and chemical

modification, the other fiber type, Acetylated Alkali-Extracted Steam Exploded

Fibers (AAEF) were subjected to different physical and chemical treatments.

Some physico-mechanical properties of the composites as well as the swelling

characteristics in dimethylformamide were investigated. Also, scanning electron

microscope (SEM) was used to investigate the morphological characteristics of

the tensile fracture surfaces of the composites. The modified wood fibers (AAEF)

enhanced both the physico-mechanical properties and the swelling characteristics

of the composites studied.

2012 Insan Akademika All Rights Reserved

1 Introduction

Composites with desired properties can be made by incorporating particulate fillers into a polymer matrix to

suit different applications (Bledzki and Gassan, 1999). In recent times, a great deal of research effort has

been directed towards the use of lignocellulosic materials in place of synthetic materials as reinforcing

fillers. Among the possible alternatives, the development of composites using lignocellulosic materials as

reinforcing fillers and thermoplastic polymers as matrices is currently receiving increasing attention.

Compared with talc, silica, glass fiber, carbon fiber and other synthetic fibers, lignocellulosic materials are

inexpensive because they come from an abundant and renewable resource. This guarantees continuous fiber

supply at all times, and provides a significant material cost saving to the plastic industry. They offer superior


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Onyeagoro & Enyiegbulam

386 Insan Akademika Publications

quality with respect to their light weight (Jacobson et al, 1995). Synthetic fibers, such as glass and carbon

fibers are brittle and are often broken into smaller fragments during processing. This potentially makes the

fracture resistance of the composite poor during processing (Hancox, L 2001). In contrast, lignocellulosic

fibers are flexible and will not fracture when processed over sharp curvatures. This enables the fibers to

maintain the desired aspect ratio for good performance of the composites (Tserki et al, 2005; Shaw, 2002).Lignocellulosic fibers are also non-abrasive. This permits a high volume fraction filling during processing,

which results to high mechanical properties without the usual machine wear problems associated with

abrasive synthetic fibers, especially glass and ceramic (Jana Sadhan and Prieto Alberto, 2002). Cellulose

fibers are non-toxic, very easy to handle and present no health problems like most synthetic fibers that cause

skin irritations and respiratory disease, especially when the fibrous dust is inhaled (Yang et al, 2004). Again,

unlike synthetic fibers, cellulose fibers offer a high ability for surface modification, and they required low

amounts of energy for processing (Hattotuwa et al, 2002). Furthermore, cellulose fibers are biodegradable.

Composites made with cellulose fibers will therefore, help to reduce the impact of non-biodegradable

materials including synthetic polymers on our solid waste stream. This is in line with global environmental

concerns which have suggested a need for materials that are biodegradable because of the decreasing

availability of landfill space, as well as increasing cost of municipal solid waste disposal (Rana et al, 2003;

Joshi et al, 2004).

However, a major problem encountered during the incorporation of natural cellulosic materials into polymer

matrix is the lack of good interfacial adhesion between the two components, due to the incorporation of the

hydrophilic natural fibers that are used as reinforcement with the hydrophobic polymer matrix (Saheb and

Jog, 1999; Medalia and Kraus, 1994). Furthermore, cellulose fiber composites are often associated with

agglomeration as a result of insufficient fiber dispersion, caused by the tendency of fibers to also form

hydrogen bonds with each other during processing (Klason et al, 1984; Raj et al, 1990; Joshi et al, 2004).

Moreover, the polar hydroxyl groups on the surface of the lignocellulosic material have difficulty in forming

a well-bonded interface with a non-polar matrix, as the hydrogen bonds tend to prevent the wetting of the

fiber surfaces (Bledzki and Gassan, 1999; Hattotuwa et al, 2002).

These problems are often addressed by the use of adhesion-promoting agents such as a coupling (orcompatibilizing) agent, or a fiber-surface modifying treatment. Many types of fiber surface modifiers have

been employed in order to improve the interfacial adhesion between cellulose fibers and the matrix material.

Cellulose-enzyme treatment has been used in modifying the surface characteristics of cellulose fillers for

application in low-density polyethylene (Sui et al, 2008), as well as mechanical treatments to increase the

aspect ratio of the cellulose fibers for use in polypropylene (Wu et al, 2002).

In a study by Moon and Kokta, 2002 on the use of coupling agent to improve the interfacial adhesion

between cellulose fibers and polystyrene (PS), it was found that a PS-pre-coated fiber treated with isocyanate

was a superior fiber treatment over the use of conventional silane coupling agent. Isocyanate is thought to

participate in the formation of covalent bonds between cellulose and PS, while the silane coupling agents by

comparison produced weaker hydrogen bond and van der Waals interaction. Silane treatments decreased or

only slightly increased properties such as tensile strength, elongation, as well as tensile modulus at 0.1%

strain, whereas isocyanate treated fiber composites with fiber pre-coat exhibited large increases in these

properties over the neat resin.

Lacroix and Joly, 2005, reinforced polystyrene with benzoylated sisal fibers. The results revealed improved

compatibility between treated cellulose fibers and polystyrene matrix, and this resulted in enhanced tensile

properties of the resulting composite. SEM micrographs revealed evidence of improved fiber wetting and

fiber-matrix adhesion between the components. These improvements were attributed to the similarity

between the phenyl-structure present in both benzoylated sisal fibers and polystyrene, which makes them

thermodynamically compatible with each other.

Fiber aspect ratio, i.e, the length to diameter ratio of a fiber is a critical parameter in a composite. For anycomposite system, there is a critical fiber aspect ratio defined as the minimum fiber aspect ratio in which

maximum allowable fiber stress can be achieved for a given load (Wu, 2000). For maximum reinforcement,


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Vol 01, No. 02, Oct 2012, pp. 385-397

www.insikapub.com 387

the fiber aspect ratio of any composite system should be above its critical value (Vovor, 2004). This will

ensure maximum stress transfer to the fiber before the composite fails. If the fiber aspect ratio is lower than

its critical value, insufficient stress will be transferred and reinforcement by the fibers will be poor, i.e, the

fibers are not loaded to their maximum stress value. By contrast, if the fiber aspect ratio is too high, the

fibers may get entangled during mixing, causing problems with fiber dispersion (Wu, 2000). Synthetic fibers,such as glass and carbon fibers are brittle, and are often broken into smaller fragments during processing.

This potentially makes the fracture resistance of the composite poor during processing (Patel and Lee, 2003).

In contrast, cellulose fibers are flexible and resistance to fracture during processing can be expected. This

enables the fibers to maintain a desirable fiber aspect ratio after processing (Shaw, 2002). Excellent literature

reports by Wu, 2000 and Shaw, 2002, have suggested that an aspect ratio in the range of 100-200 after

processing is essential for high performance short-fiber composites.

The incorporation of natural lignocellulosic materials into synthetic thermoplastic matrix does not create a

completely biodegradable composite material. Among bio-polymers, cellulose and cellulose derivatives

enjoy widespread use and remain the single largest bio-polymers. Examples of complete bio-based polymer

composite systems studied include the use of natural fibers as reinforcing agents in natural polymer matrices

such as natural rubber and poly (hydroxylbutyrate) (Geethamma et al, 1998; Avella and Jain, 2003). Byexercising judicious selection method, lignocellulosic materials (or thermoplastic derivatives) serving as both

fiber and matrix components can enjoy favorable interfacial interactions.

Many authors (Bledzki and Gassan, 1999; Lee et al, 2004; Kim et al, 2004; Yang et al, 2004; Jayaraman,

2003) have carried out studies on mechanical properties of natural fiber-filled thermoplastics. They had

indicated that the incorporation of natural fibers into thermoplastic polymer matrix could improve some

desired properties. These studies centered on synthetic thermoplastic matrices that could not produce

completely biodegradable composites. Bio-based fiber/bio-based matrix systems have received relatively

little attention of late. However, the effects of fiber content and fiber acetylation on yield strength, modulus

and elongation at break of cellulose acetate butyrate/yellow poplar wood fiber composites have been

reported by Onyeagoro, 2012, who revealed that improvements in these properties were as a result of

improved fiber-matrix adhesion due to fiber acetylation. Similarly, the dispersion characteristics of yellowpoplar wood fiber in cellulose acetate butyrate/yellow poplar wood fiber composites have been studied by

Onyeagoro, 2012. The quality of fiber dispersion was determined by measuring the standard deviation of

gray level of the composites, which is a measure of the quality of fiber dispersion. It was found that the

acetylated fibers showed the lowest standard deviation of gray level, i.e, the best fiber dispersion due to

fiber acetylation, as well as a result of differences in post-treatment of each fiber.

In the present study, cellulose acetate butyrate (a bio-polymer)/yellow poplar wood fiber composites were

prepared with acetylated and non-acetylated fibers. The effects of fiber aspect ratio, fiber loading and fiber

acetylation on mechanical properties of the composites were investigated. Scanning Electron Microscope

(SEM) was used to examine the effect of acetylation on interfacial bonding.

2 Materials and Method

2.1 Materials

The cellulose acetate butyrate (plasticized), used in this study as polymer matrix was obtained from Lagos,

Nigeria, as TENITE Butyrate formula 285. The manufacturer did not provide information on the type of

plasticizer(s) used or acetyl or butyl contents of the material (proprietary information).

Yellow poplar wood chips were obtained from forest reserve areas in Umuokanne and Obinze in Imo State,

Nigeria. The wood chips were thoroughly washed several times with tap water followed by three times with

de-ionized water to remove any adhering undesired soil and dust, and finally dried at 1020C overnight. Twotypes of fibers were used namely, Acetylated Alkali-Extracted Steam Exploded Fibers (AAEF), and

Untreated Fibers (UTF). The dried wood chips were steam-exploded at a severity of 4.23, which was


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performed on a steam explosion machine at the Federal Research Institute, Umudike, Abia State, Nigeria.

The steam explosion process was followed by fiber fractionation, which included washing with water at

500C to obtain water-washed steam-exploded fibers (WEF). Alkali-extracted steam-exploded fibers (AEF)

were obtained by extracting WEF with 20% aqueous alkali, at an 8:1 liquor-to-fiber ratio at 700C. AAEF

were prepared by esterification of AEF with a mixture of acetic acid, butyric acid, and butyric anhydrideusing sulfuric acid catalyst.

AAEF and UTF were each ground mechanically in rotary cutting mill operated at 10,000 rpm speed to the

range of 2-4.5mm and then sieved automatically into classification according to their aspect ratios. The

investigated fiber aspect ratios were short aspect ratio (


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2.2.2 Compression Moulding

The milled samples obtained from the two-roll mill were compression molded with dimensions of 100mm x

100mm x 2.5mm at 1950C, according to ASTM D 1489. This was followed by cooling at room temperature

for 15 minutes.

2.2.3 Tensile Test

Dumbbell-shaped cutter was employed to punch out the tensile specimens from the compression molded

composite samples and according to the fiber orientations (longitudinal-transverse). Tensile measurements

were carried out at a crosshead speed of 50mm/min in a Zwich 1425 tensile testing machine following

procedures from ASTM D 412-98a). The tensile strength, elongation at break (ultimate elongation) were

determined at room temperature of the examined composite samples (Dzyura, 1980; Bigg, 1987).

2.2.4 Swelling Measurements

Swelling test (ASTM D 471) was performed on a uniform cut from the compression-molded composite

samples with 5mm diameter and 2mm thick by the immersion/weight gain method (Sobhy et al, 1997; Ibarra

and Chamorro, 1991) in dimethylformamide (DMF) at room temperature for two days to allow the swelling

to reach diffusion equilibrium. The swelling value (S.V) was calculated as follows (Hussain et al, 2010):

S.V = [(W1-W0)/W0] ...(1)

Where: W0 = weight before swelling

W1 = weight at time-t

2.2.5 Impact Test

Notched Izod impact tests were conducted according to ASTM D 256-97 at room temperature. Each value

obtained represented the average of five samples.

2.2.6 Morphology

Studies on the morphology of the tensile fracture surfaces of the composites were carried out using a JEOL

Model 5610 LV scanning electron microscope.

3 Results and Discussion

3.1 Effect of Fiber Aspect Ratio on Mechanical Properties

The modulus of elasticity and elongation at break values of CAB/AAEF and CAB/UTF composites

containing a constant amount (30 wt %) of fiber at different aspect ratios (SF, MF, and HF) are shown in

Fig.1 and Fig.2, respectively. The results show that the MF composite samples of medium fiber aspect ratio

(105-250) have higher values than other samples of SF of short aspect ratio (< 105) and HF of high aspect

ratio (250-450) for the measured parameters. This is attributed to both better and uniform dispersion of the

MF fibers in the CAB matrix. The HF sample shows a lower value of modulus than samples SF and MF due

to its relatively higher aspect ratio and poor fiber dispersion. The untreated fiber (UTF) composite shows amodulus than the AAEF composites due to higher lignin content on its surface which tends to agglomerate

through the formation of hydrogen bonding (Ishmail, 2002; Tserki et al, 2005). Thus, with increasing


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hydrogen bonding between the fiber surfaces, the area of contact with the CAB matrix is reduced. The

reduction in elongation at break for HF composites is attributed to decrease in the deformation of the rigid

interface between the CAB matrix and the fiber.

Fig.1: Modulus of CAB/AAEF and CAB/UTF composites at different aspect ratios

3.2 Effect of Fiber Loading on Mechanical Properties

The modulus of elasticity of AAEF and UTF composites of medium fiber aspect ratio (MF) containing 0, 10,

20, 30, and 40 wt % fiber is shown in Fig.3. The results show an increase in modulus with increase in fiber

content rising from 10-40 wt % after showing an initial decrease from 0 - 10 wt % fiber content. At low fiber

loading, the fibers tend to act as flaws instead of as reinforcement. This reduces the strength of the

composites. This informs the initial decrease in modulus of elasticity. Similar results have been reported by

several researchers (Murty and De, 1982; Ishmail et al, 1997; Bigg, 1987). CAB/AAEF composites gave

higher modulus than CAB/UTF composites. This is because of improved fiber-matrix interfacial adhesion in

CAB/AAEF composites due to fiber acetylation.

The elongation at break of AAEF and UTF composites decreases with increasing fiber loading as shown in

Fig.3. This is attributed to higher restriction to molecular motion of the matrix with increasing fiber content.

Increase in fiber content imposes extra resistance to flow which leads to lower resistance to break (Mayer

and Hill, 2004; Raj and Beshay, 2004). Since elongation is the reciprocal of stiffness of a material (Raj et al,

1990), the results show that the fiber imparts a greater stiffening effect than that of the unfilled one. AAEF

composites show a higher elongation at break values than the untreated fiber (UTF) composite due to fiber

acetylation which improves the fiber-matrix interfacial adhesion.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

SF MF HF

M

odulus(MPa)

Fiber Aspect Ratio

AAEF composites

UTF composites


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Fig.2: Elongation at Break of CAB/AAEF and CAB/UTF composites at different

aspect ratios

Fig.3: Modulus of CAB/AAEF and CAB/UTF composites at different fiber loadings

0

100

200

300

400

500

600

700

800

900

1000

SF MF HF

ElongationatBreak(%)

Fiber Aspect Ratio

AAEF composites

UTF composites

0

0,5

1

1,5

2

2,5

0 10 20 30 40

Modulusofelasticity(MPa)

Fiber Loading (wt %)

AAEF composites

UTF composites


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Fig.4: Elongation at Break of CAB/AAEF and CAB/UTF composites at different fiber

loadings

Fig.5: Effect of Fiber loading on Swelling of CAB/AAEF and CAB/UTF composites

in DMF

3.3 Effect of Fiber Loading on Swelling Characteristics

The effect of fiber content on swelling characteristics of CAB/AAEF and CAB/UTF composites (at constant

fiber aspect ratio, MF = 105-250) was investigated. The test specimens were subjected to swelling in

dimethylformamide (DMF) at room temperature for 48 hours. The resulting swelling was determined bymeasuring the changes in mass, before and after immersion in DMF. The result presented in Fig.5 shows that

0

100

200

300

400

500

600

700

800

0 10 20 30 40

ElongationatBreak(%)


AAEF composites

UTF composites

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50

SwellinginDMF(%

)


UTF composites

AAEF composites


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that the modified fibers (AAEF) increase the resistance of the composite samples to swelling in DMF. This is

attributed to improved fiber/matrix interfacial adhesion due to fiber acetylation.

3.4 Effect of Fiber Loading on Impact Strength

The Izod impact tests were conducted at room temperature. The notched specimens of AAEF and the

untreated fiber (UTF) composites containing fibers with medium aspect ratio were tested and Fig.6 shows

the Izod impact strengths of the composites at different fiber contents. The Izod impact strengths of the

composites decreased as the filler content increased (Clemons, 1995). AAEF composites showed higher

impact strengths than the UTF composites due to fiber acetylation which improves the fiber-matrix

interfacial adhesion. The poor interfacial bonding between the matrix polymer and the untreated fiber causes

micro-cracks to occur at the point of impact, which cause the cracks to easily propagate in the untreated fiber

(UTF) composite (Yang et al, 2004). These micro-cracks cause decreased impact strength of the composite.

Fig.6: Plot of Izod Impact Strength versus Fiber loading for CAB/AAEF and

CAB/UTF composites containing medium fiber (MF) aspect ratio

3.5 Morphological Characteristics

The tensile fracture surfaces of AAEF and UTF composites at 30wt % fiber content are shown in Fig.7. In

the case of the composite containing untreated fiber (UTS composite), fiber particles are to be seen at the

tensile fracture surfaces and some cavities are to be seen where the fiber has been pulled-out (Fig.7a). The

presence of these cavities means that the interfacial bonding between the fiber and the matrix polymer is

poor and weak.

In the case of the composite made with acetylated fiber (AAEF composite), the interfacial bonding between

the fiber and the matrix polymer is strong (Fig.7b). Few pulled-out traces of fiber particles are to be seen,

and this is due to the strong interfacial bonding between the fiber and matrix polymer.

The tensile fracture surfaces of the AAEF composite at different fiber loadings are shown in Fig.7. The

composite containing 10wt % fiber loading shows highly uniform dispersion of the fiber, which indicatesstrong interfacial bonding between the fiber and the matrix polymer. As the fiber content increases (Fig.8b-

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50

IzodImpactStrength(KJ/m)


AAEF composites

UTF composites


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8d), localized dispersion of the fiber particles was seen in the composites with cavities which resulted in the

reduction of tensile property (decreased impact strength and increased modulus). This is because the poorly

bonded interfaces and the brittleness of the fiber affect the tensile property. At 40wt % fiber content, fiber

particles become the main component and some traces are to be seen where the fiber has been pulled-out

(Fig.8d).

Fig.7: SEM micrographs of tensile fracture surfaces of CAB/AAEF and CAB/UTF composites at 30wt %

fiber content obtained at magnification of 2000X. A = CAB/UTF, B = CAB/AAEF.

Fig.8: SEM micrographs of tensile fracture surfaces of CAB/UTF and CAB/AAEF composites at different

fiber loadings obtained at magnification of 2000X. A = 10wt %, B = 20wt %, C = 30wt %, and D =

40wt %.


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4 Conclusion

To reduce the impact of non-biodegradable materials including synthetic polymers on our solid waste stream,

bio-based biodegradable thermoplastic polymer composite was prepared by using cellulose acetate butyrate(CAB) as thermoplastic matrix and yellow poplar wood fiber as reinforcement. This is in line with global

environmental concerns which have suggested a need for materials that are biodegradable because of the

decreasing availability of land fill space, as well as increasing cost of municipal solid waste disposal. The

acetylation modification of the wood fiber improved its dispersibility in the matrix polymer and thus,

improved the physico-mechanical properties of the composites with respect to the unmodified one. The

modified fiber composite offered better swelling properties than the unmodified one as it provided greater

resistance to swelling in dimethylformamide.

Morphological investigation of the tensile fracture surfaces of the untreated and modified fiber composites

show fractured fiber particles in the Scanning electron micrographs (SEM) of the untreated fiber composite

due to poor matrix-fiber interfacial bonding. On the other hand, only few pulled-out traces of fiber particles

are noticed in the SEM micrographs of the modified fiber composite, and this is due to the strong interfacialbonding between the fiber and the matrix polymer.

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