Mechanical Properties of Surface-Modified Cellulose

Chapter 6

Mechanical Properties of Surface-Modified Cellulose Fiber—Thermoplastic Composites R. G . Raj and Β. V . Kokta

Downloaded by UNIV OF GUELPH LIBRARY on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch006

Centre de Recherche en Pâtes et Papiers, U n i v e r s i t é du Québec à Trois-Rivières, Quebec G9A 5H7, Canada

Recent a t t e n t i o n has examined the use o f n a t u r a l organic fillers, such as c e l l u l o s e f i b e r s , as fillers/reinforcing agents i n elastomers and t h e r m o p l a s t i c polymers. These f i b e r s , when used i n a polymer matrix, cannot f u n c t i o n as an e f f e c t i v e reinforcement system due t o poor adhesion a t the f i b e r - m a t r i x i n t e r f a c e . C e l l u l o s e f i b e r s a l s o tend t o aggregate and thus the f i b e r s do not d i s p e r s e w e l l i n a hydrophobic polymer matrix. The o b j e c t i v e o f t h i s study was to improve the suitability o f c e l l u l o s e f i b e r s i n a hydrophobic polymer matrix (polyethylene) through the use o f v a r i o u s p r o c e s s i n g a i d s / c o u p l i n g agent. S t e a r i c a c i d and mineral o i l were used as a d d i t i v e s and maleated ethylene as a c o u p l i n g agent. T e n s i l e s t r e n g t h and modulus o f the composites i n c r e a s e d with t h e f i b e r conce n t r a t i o n , l a r g e l y because o f improved f i b e r d i s p e r s i o n (with s t e a r i c a c i d ) a t higher filler c o n c e n t r a t i o n s . Increased mechanical p r o p e r t i e s of the composites due t o improved c o m p a t i b i l i t y between the f i b e r and matrix was a l s o achieved. Maleated ethylene improved the adhesion between the f i b e r and polymer matrix. The r u l e o f mixture equation was used t o c a l c u l a t e t e n s i l e modulus o f t h e composites and these values compared with the experimental r e s u l t s . F a c t o r s a f f e c t i n g t h e modulus o f t h e composites a r e discussed. Many s t u d i e s have d e s c r i b e d t h e use o f c e l l u l o s e o r wood f i b e r s as a f i l l e r / r e i n f o r c i n g agents i n t h e r m o p l a s t i c polymer matrices (1-6). These f i b e r s a r e r e l a t i v e l y cheap and l i g h t w e i g h t (lower d e n s i t y ) compared t o i n o r g a n i c f i l l e r s . In a d d i t i o n , the biodegradable nature o f c e l l u l o s i c f i l l e r s o f f e r s a p o t e n t i a l s o l u t i o n t o the growing waste d i s p o s a b l e 0097-6156/92/0476-0076$06.00/0 © 1992 American Chemical Society

In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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Surface-Modified Cellulose Fiber-Thermoplastic Composites 77

problem. In s p i t e of these advantages, wood o r c e l l u l o s e f i l l e r s have not found much use i n t h e r m o p l a s t i c s . I t has been argued t h a t c e r t a i n drawbacks such as i n c o m p a t i b i l i t y to t h e hydrophobic polymer matrix, tendency t o form aggregates, poor r e s i s t a n c e t o moisture, and t h e l i m i t a t i o n of p r o c e s s i n g temperature g r e a t l y reduce t h e p o t e n t i a l o f c e l l u l o s e and/or wood as a f i l l e r / r e i n f o r c e m e n t . Many approaches have been d e s c r i b e d i n the l i t e r a t u r e t o improve adhesion a t the polymer-matrix i n t e r f a c e . One of the methods used t o improve c o m p a t i b i l i t y between the f i l l e r and matrix i s chemical g r a f t i n g . By a t t a c h i n g a s u i t a b l e polymer segment t o the s u r f a c e of a f i l l e r with a s i m i l a r s o l u b i l i t y parameter as t h e polymer matrix, a s i g n i f i c a n t improvement i n bonding between the f i b e r and matrix can be achieved. F o r example, the p o l y m e r i z a t i o n o f methyl methacrylate on sawdust improved the p h y s i c a l and mechanical p r o p e r t i e s o f t h e composite (7). Aspen and b i r c h pulps g r a f t e d with p o l y styrene and i n c o r p o r a t e d i n t o the p o l y s t y r e n e matrix gave a 40% i n c r e a s e i n mechanical p r o p e r t i e s as compared t o unf i l l e d p o l y s t y r e n e (8). M o d i f i c a t i o n o f the f i l l e r - m a t r i x i n t e r f a c e has a l s o been attempted by the a d d i t i o n o f v a r i o u s a d d i t i v e s o r c o u p l i n g agents d u r i n g p r o c e s s i n g . Dalvag e t a l . (9) used maleic anhydride m o d i f i e d propylene t o improve s t r e n g t h and d u c t i l i t y o f polypropylene (PP)-wood and c e l l u l o s e f l o u r composites. C e l l u l o s e f i b e r s t r e a t e d with v i n y l c h l o r i d e , a p l a s t i c i z e r and an isocyanate produced b e t t e r adhesion with p o l y v i n y l c h l o r i d e (10). High d e n s i t y p o l y e t h y l e n e (HDPE) f i l l e d with s i l a n e A-174 t r e a t e d chemithermomechnical pulp (CTMP) o f aspen f i b e r s produced h i g h e r t e n s i l e s t r e n g t h and modulus (11). An i n c r e a s e i n t e n s i l e and impact s t r e n g t h was reported when r o s i n was used i n PP-wood f l o u r composites (12).

Present Technology C e l l u l o s e f i b e r s i n t h e form o f paper have been used with interleaving thermoplastic polymer films, a f t e r hot p r e s s i n g , t o o b t a i n laminates (13). McKenzie and Y u r i t t a (14) reported t h a t p r e t r e a t i n g wood f i b e r s with urea f o r m a l dehyde, followed by sheet formation and hot-press l a m i n a t i o n with polyethylene, gave a product with very good r e t e n t i o n of wet t e n s i l e s t r e n g t h . However, the above f a b r i c a t i o n system i s not p r a c t i c a l f o r t h e manufacture o f t h i c k products. B e t t e r bonding between wood f i b e r and PP matrix was achieved by gamma i r r a d i a t i o n o f wood f i b e r (15). Improvements i n the mechanical s t r e n g t h of wood f i b e r - f i l l e d t h e r m o p l a s t i c composites can a l s o be achieved with the use of c o u p l i n g agents (4,11). The polymer i s c h e m i c a l l y bonded to the f i l l e r p a r t i c l e by a c o u p l i n g agent, which improves i n t e r f a c e adhesion. The degree o f adhesion depends on t h e type o f polymer, c o u p l i n g agent, and f i l l e r combination. Formation o f a strong adhesive bond between t h e polymer


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matrix and c e l l u l o s e f i b e r g i v e s increased s t r e n g t h and s t i f f n e s s t o the composite m a t e r i a l . The f a c t o r s which can i n f l u e n c e the mechanical p r o p e r t i e s of short f i b e r - f i l l e d thermoplastics are summarized i n Table I.


Table I .

F a c t o r s A f f e c t i n g Mechanical P r o p e r t i e s o f Short F i b e r Composites

Tensile strength:

T e n s i l e modulus:

- strong i n t e r f a c e - low s t r e s s concentrations - fiber orientation

- high f i b e r aspect r a t i o - f i b e r wetting - f i b e r concentration

Impact s t r e n g t h : - d u c t i l e matrix - energy absorption Both the matrix and f i b e r p r o p e r t i e s are important t o improve mechanical p r o p e r t i e s of the composite. T e n s i l e s t r e n g t h i s more s e n s i t i v e t o matrix p r o p e r t i e s , while f i b e r p r o p e r t i e s are more important f o r modulus. However, a balance between the f i b e r and matrix p r o p e r t i e s i s r e q u i r e d t o achieve good impact s t r e n g t h . When s h o r t discontinuous c e l l u l o s e f i b e r s are used i n t h e r m o p l a s t i c s , it i s very important to reduce the fiber-to-fiber i n t e r a c t i o n t o achieve a uniform d i s t r i b u t i o n of f i b e r i n the matrix. The s u r f a c e c h a r a c t e r i s t i c s of the r e i n f o r c i n g f i b e r are important i n the t r a n s f e r of s t r e s s from the matrix t o the f i b e r . The pretreatment of c e l l u l o s e f i b e r s with a s u i t a b l e a d d i t i v e p r i o r t o i n c o r p o r a t i n g the f i b e r s with the polymer matrix a i d s d i s p e r s i o n and s i g n i f i c a n t l y improves the mechanical p r o p e r t i e s of the composite (16,17). Many types of wood pulps are a v a i l a b l e (thermomechanical, chemithermo-mechanical and chemical); the p h y s i c a l and mechanical p r o p e r t i e s of these f i b e r s vary widely depending upon the p u l p i n g process. Thus, m o d i f i c a t i o n of the f i b e r s u r f a c e needs t o be s p e c i f i c f o r a p a r t i c u l a r polymer matrix. In an e a r l i e r work, Quick (18) reported t h a t s o f t wood k r a f t pulp f i b e r t r e a t e d with e t h y l e n e - a c r y l i c a c i d co-polymer had good r e i n f o r c i n g e f f e c t on low d e n s i t y polyethylene, but the same f i b e r d i d not perform w e l l with PP and polystyrene (PS). H y d r o l y t i c pretreatment of c e l l ulose f i b e r s with o x a l i c a c i d improved the homogeneity and mechanical p r o p e r t i e s of PP, HDPE and PS which contained v a r i o u s amounts of bleached pulps (19). Current work P r o c e s s i n g a i d s / c o u p l i n g agents. The performance of f i b e r f i l l e d p l a s t i c i s g r e a t l y i n f l u e n c e d by the degree of f i b e r



6. RAJ & KOKTA


d i s p e r s i o n w i t h i n the polymer matrix. Àspen f i b e r with an average aspect r a t i o (L/D) of 12 was used as a r e i n f o r c i n g f i b e r . Maleic anhydride modified HDPE (DuPont Canada; melt index 13.5 dg/min.) was used as a matrix. Two d i f f e r e n t p r o c e s s i n g a i d s and a c o u p l i n g agent were used t o improve the d i s p e r s i o n of f i b e r as w e l l as the c o m p a t i b i l i t y of the f i b e r with the polymer matrix; a) s t e a r i c a c i d ( A l d r i c h ) b) mineral o i l (Sigma) c) maleated ethylene (Epolene C-18, Eastman Kodak) The compounding of p r e d r i e d polymer and f i b e r (0-40% by weight) was done at 170"C i n a t w o - r o l l m i l l . In a t y p i c a l mixing process, about h a l f of the polymer-fiber mixture was added t o the heated c o - r o t a t i n g r o l l s . A f t e r mixing f o r 3 minutes, the p r o c e s s i n g a i d s (0-2% by weight of f i b e r ) and the remaining polymer-fiber mixture was slowly added and mixed throughly t o o b t a i n the wetting of f i b e r by the polymer. The above mixture was compression molded i n a Carver l a b o r a t o r y press t o o b t a i n t e n s i l e specimens (ASTM D-638, Type V ) . A f t e r heating of the mold f o r 15 min. a t 160"C (pressure 3.2 MPa), the samples were slowly cooled t o room temperature with the pressure maintained during the process. T e n s i l e p r o p e r t i e s of the composites were measured with an Instron model 4201. The f u l l - s c a l e l o a d was 500 Ν and the cross-head speed was 10 mm/min. The t e s t r e s u l t s were a u t o m a t i c a l l y c a l c u l a t e d by a HP86B computing system u s i n g the Instron 2412005 General T e n s i l e Test Program. S i x specimens were t e s t e d i n each case and the average was r e p o r t e d . The c o e f f i c i e n t s of v a r i a t i o n of the r e p o r t e d p r o p e r t i e s were: T e n s i l e s t r e n g t h , 1.2-6.9%; E l o n g a t i o n , 3.4-7.6%; T e n s i l e modulus, 2.5-6.7%; and Izod-Impact s t r e n g t h , 4.9-8.6%. The a d d i t i v e e f f e c t on t e n s i l e s t r e n g t h of HDPEc e l l u l o s e f i b e r composites (at 10 and 30% f i b e r concentra t i o n s ) i s presented i n Table I I . The c o n c e n t r a t i o n of a d d i t i v e was 1% by weight of f i b e r . The r e s u l t s show an i n c r e a s e i n t e n s i l e s t r e n g t h and modulus with the use of s t e a r i c a c i d , mineral o i l , and maleated ethylene. T e n s i l e s t r e n g t h of the composite (at 30% f i b e r content) with s t e a r i c a c i d increased t o 35.2 MPa as compared t o 29.8 MPa f o r the c o n t r o l . The r e s u l t s show t h a t s t e a r i c a c i d i s h i g h l y e f f e c t i v e as a d i s p e r s a n t ; i . e . , reducing f i b e r - t o f i b e r i n t e r a c t i o n . Normally, improved f i b e r d i s p e r s i o n r e s u l t s i n higher s t r e n g t h values (9). An i n c r e a s e i n t e n s i l e s t r e n g t h and modulus was a l s o observed with mineral o i l . When compared t o c o n t r o l , t e n s i l e s t r e n g t h i n c r e a s e d by 13% a t 30% f i b e r c o n c e n t r a t i o n . I t was observed t h a t mineral o i l f u n c t i o n s as a l u b r i c a n t which i s adsorbed by the f i b e r , and t h i s f a c i l i t a t e s the disentanglement of i n d i v i d u a l f i b e r s (17). However, the best improvement i n t e n s i l e s t r e n g t h and modulus was achieved with maleated ethylene; i . e . , t e n s i l e s t r e n g t h increased from 29.8 MPa ( c o n t r o l ) t o 36.1 MPa a t 30% f i b e r c o n c e n t r a t i o n . T h i s may be a t t r i b u t e d t o improved bonding between the f i b e r and


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matrix. The c o u p l i n g r e a c t i o n ( e s t e r linkage) between the maleated ethylene and the hydroxyl groups of c e l l u l o s e thus provides a means t o improve the bonding between f i b e r and matrix (20). Table I I . T e n s i l e p r o p e r t i e s composites


Processing Fiber aids (% wt.)

control

stearic acid mineral oil maleated ethylene

Tensile strength (MPa)

of

HDPE-cellulose

Elongation

fiber

(%)

Tensile modulus (GPa)

10

23.7 (1.2)

8.6 (4.5)

1.16 (3.6)

30

29.8 (2.3)

6.0 (3.6)

1.60 (4.9)

10

26.6 (1.4)

8.0 (4.9)

1.28

30

35.2 (6.2)

6.0 (3.8)

1.86 (4.5)

10

26.5 (3.5)

8.6 (5.0)

1.26 (3.9)

30

33.8 (4.7)

6.1 (2.8)

1.84

10

25.9 (2.7)

8.2 (2.1)

1.29 (3.0)

30

36.1 (3.7)

6.0 (5.3)

1.94

(4.7)

(6.5)

(5.8)

( ) values represent c o e f f i c i e n t s of v a r i a t i o n . T e n s i l e modulus of the composites which contained a d d i t i v e s improved as compared t o the c o n t r o l (Table I I ) . With maleated ethylene, the modulus increased from 1.60 GPa t o 1.94 GPa a t 30% f i b e r c o n c e n t r a t i o n . The data show t h a t s t e a r i c a c i d and mineral o i l were a l s o e f f e c t i v e i n improv i n g the modulus of the composites. To r e a l i z e the maximum s t i f f n e s s p o t e n t i a l of the f i b e r , a good contact i s e s s e n t i a l between the matrix and f i b e r phases so t h a t the load can be t r a n s f e r r e d e f f e c t i v e l y from the matrix t o the f i b e r . T e n s i l e modulus i s a l s o a f f e c t e d by the concentrat i o n of f i b e r i n the composite. T h i s i s evident from the data, where the modulus increased s t e a d i l y with a r i s e i n fiber concentration; i . e . , 1.29 GPa a t 10% fiber c o n c e n t r a t i o n t o 1.94 GPa a t 30% f i b e r c o n c e n t r a t i o n . While the a d d i t i o n of the f i b e r increased the s t i f f n e s s of the composite, i t had a negative e f f e c t on e l o n g a t i o n . The r e s u l t s show a decrease i n e l o n g a t i o n with the i n c r e a s e i n f i b e r c o n c e n t r a t i o n . U n l i k e t e n s i l e s t r e n g t h and modulus, a d d i t i v e s had no i n f l u e n c e on e l o n g a t i o n . E a r l i e r s t u d i e s have shown a s i m i l a r r e d u c t i o n i n e l o n g a t i o n with the increase i n f i b e r concentration (11). Figures 1 and 2 show the e f f e c t of a d d i t i v e concentra-


6. RAJ & KOKTA


% change in tensile strength 40

•


•

.

-10

stearic acid maleated ethylene mineral oil

1

1 2 Concentration of additive (%)

F i g u r e 1. E f f e c t of a d d i t i v e c o n c e n t r a t i o n on t e n s i l e s t r e n g t h of HDPE-cellulose f i b e r composites.

% change in tensile modulus

• -

0

• •

stearic acid maleated ethylene mineral oil

1 2 Concentration of additive (%)

3

Figure 2 . E f f e c t o f a d d i t i v e c o n c e n t r a t i o n on t e n s i l e modulus o f HDPE-cellulose f i b e r composites.



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t i o n on t e n s i l e s t r e n g t h and modulus of composites with 30% f i b e r c o n c e n t r a t i o n . The c o n c e n t r a t i o n of s t e a r i c a c i d , mineral o i l , and maleated ethylene v a r i e d from 0 t o 2% (based on f i b e r weight). The r e s u l t s show t h a t the % change i n t e n s i l e s t r e n g t h (compared t o c o n t r o l ) i s s i g n i f i c a n t a t low a d d i t i v e c o n c e n t r a t i o n (Figure 1). A l s o , the i n c r e a s e i n t e n s i l e s t r e n g t h i s a f f e c t e d by the type of a d d i t i v e used. Maleated ethylene seems t o be most e f f e c t i v e a t improving t e n s i l e s t r e n g t h , with a 25% i n c r e a s e i n s t r e n g t h observed at 0.5% c o n c e n t r a t i o n . But i n the case of s t e a r i c a c i d and mineral o i l , maximum i n c r e a s e i n s t r e n g t h was observed at 1% c o n c e n t r a t i o n . A f u r t h e r i n c r e a s e i n a d d i t i v e c o n c e n t r a t i o n r e s u l t e d i n a s t r e n g t h decrease. A s i m i l a r t r e n d was observed i n the case of t e n s i l e modulus (Figure 2). The l o s s i n s t r e n g t h and modulus a t the higher c o n c e n t r a t i o n of a d d i t i v e s may be due t o the p l a s t i c i z i n g e f f e c t on the polymer matrix. Figure 3 shows the Izod-impact s t r e n g t h (un-notched) as a f u n c t i o n of f i b e r c o n c e n t r a t i o n of HDPE-cellulose f i b e r composites c o n t a i n i n g untreated, stearic acidt r e a t e d , and maleated e t h y l e n e - t r e a t e d f i b e r s . Impact s t r e n g t h s t e a d i l y decreased with an i n c r e a s e i n f i b e r c o n c e n t r a t i o n . Compared t o untreated f i b e r composites, maleated e t h y l e n e - t r e a t e d f i b e r s produced s l i g h t l y higher impact s t r e n g t h . For good impact s t r e n g t h , an optimum bonding l e v e l i s necessary. Good bonding may produce poor impact s t r e n g t h because a crack can propagate r a p i d l y from the matrix through a f i b e r and i n t o the matrix again i f the i n t e r f a c e between f i b e r and matrix r e s i s t s s e p a r a t i o n . On the other hand, i f the f i b e r s are not bonded s t r o n g l y with the matrix, they may separate e a s i l y from the matrix and can d i v e r t the crack by absorbing i t s energy. The degree of adhesion, f i b e r p u l l - o u t , and a mechanism t o absorb energy are some of the parameters which can i n f l u e n c e the impact s t r e n g t h of s h o r t f i b e r - f i l l e d composites (4). Fiber dispersion. Surface m o d i f i c a t i o n of c e l l u l o s e f i b e r i s important t o promote high f i b e r l o a d i n g i n the polymer matrix and t o improve polymer c o m p a t i b i l i t y and property enhancement. C e l l u l o s i c f i b e r s tend t o form aggregates i n a polymer matrix due t o t h e i r h y d r o p h i l i c nature. The degree of f i b e r d i s p e r s i o n depends on the i n t e r f a c i a l c h a r a c t e r i s t i c s of the polymer and f i b e r . The a b i l i t y t o break down f i b e r aggregates i s s h a r p l y reduced by poor wetting of the f i b e r s u r f a c e by the polymer. The r e s u l t s show t h a t the a d d i t i o n of 1.0% s t e a r i c a c i d during the compounding of discontinuous c e l l u l o s e f i b e r s g r e a t l y improves the f i b e r d i s p e r s i o n i n the polymer matrix. When compared t o untreated f i b e r s (no a d d i t i v e ) as shown i n Figure 4, the number of aggregates i s g r e a t l y reduced i n s t e a r i c a c i d - t r e a t e d f i b e r composites (Figure 5). The s t e a r i c a c i d s i g n i f i c a n t l y reduces f i b e r - t o - f i b e r i n t e r a c t i o n , and as a r e s u l t , the f i b e r s are b e t t e r d i s p e r s e d i n the polymer matrix. T h i s i s demonstrated by the i n c r e a s e i n


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Surjhce-Modified Cellulose Fiber-Thermoplastic Composites 83

Izod-lmpact strength (KJ/m ) 40

•

untreated

•

stearic acid


maleated ethylene 30

-

0

10

1

1

1

20

30

40

50

Fiber concentration (% weight) Figure 3. E f f e c t of f i b e r c o n c e n t r a t i o n on Izod-impact strength (un-notched) of HDPE-cellulose fiber composites.

Figure 4. F r a c t u r e s u r f a c e of HDPE-untreated c e l l u l o s e f i b e r Composite (30% f i b e r weight), m a g n i f i c a t i o n 190 X



84


t e n s i l e s t r e n g t h and modulus of the composite. The mixing time i s o f t e n i n c r e a s e d t o achieve good d i s p e r s i o n . However, repeated mixing tends t o break the f i b e r due t o high shear f o r c e s . Delvag, e t a l . (9) found t h a t the degree of f i b e r length r e d u c t i o n d u r i n g compounding i n c r e a s e d p r o p o r t i o n a l l y with the mixing time. A d d i t i v e s h e l p t o reduce the mixing time t o minimize the f i b e r breakage d u r i n g the compounding process. Young's modulus o f the s h o r t f i b e r - f i l l e d composites may vary depending on the o r i e n t a t i o n of the f i b e r s , the angle between the f i b e r a x i s , and the t e s t d i r e c t i o n . I t has been suggested t h a t a high degree of o r i e n t a i o n can be achieved by m i l l i n g o r e x t r u s i o n (16). S e v e r a l f a c t o r s can i n f l u e n c e the degree of o r i e n t a t i o n : f i b e r aspect r a t i o , v i s c o s i t y and flow g r a d i e n t of the polymer matrix, and the f i b e r c o n c e n t r a t i o n . F i g u r e 6 shows some of the p o s s i b l e types of f i b e r o r i e n t a t i o n s i n s h o r t d i s c o n t i n u o u s f i b e r composites. The f i r s t case (a) r e p r e s e n t s f i b e r s which are randomly o r i e n t e d i n a plane; the o r i e n t a t i o n f a c t o r k i s < 0.33. I f the f i b e r s are c r o s s e d a t 90° and t e s t e d i n e i t h e r of the two f i b e r d i r e c t i o n s as i n case ( b ) , the o r i e n t a t i o n f a c t o r k i s > 0.5. However, the i d e a l case i s (c) where the f i b e r s are f u l l y a l i g n e d i n the d i r e c t i o n of t e s t (k i s 1.0). In such a case as ( c ) , maximum s t r e n g t h and modulus of the composite would be r e a l i z e d . In p r a c t i c e , however, case (b) i s most l i k e l y . The modulus of the composite can be c a l c u l a t e d from the r u l e of mixtures equation: E

c

= k V E f

f

+ Ε, (1 - V, )

(1)

where E i s modulus of the composite, E and E are moduli of the f i b e r and matrix r e s p e c t i v e l y , V i s the volume f r a c t i o n of f i b e r , and k i s the o r i e n t a t i o n f a c t o r . F i g u r e 7 shows the r e l a t i o n s h i p between the o r i e n t a i o n f a c t o r , volume f r a c t i o n of the f i b e r , and t e n s i l e modulus of the composite. From Eq. 1, the k values were c a l c u l a t e d f o r the f i b e r s t r e a t e d with s t e a r i c a c i d . The data show t h a t as k v a r i e s from 0.27 (V =0.13) t o 0.47 (V =0.26), the modulus value c a l c u l a t e d u s i n g Eq. 1 agrees w e l l with the e x p e r i mental value a t lower c o n c e n t r a t i o n of f i b e r . However, a t higher f i b e r c o n c e n t r a t i o n s , the experimental values were lower than p r e d i c t e d perhaps because o f poor f i b e r o r i e n t a tion. c

f

B

f

f

f

Future needs A d d i t i o n a l r e s e a r c h i s needed before c e l l u l o s e fiber composites can compete with high performance composites such as g l a s s , g r a p h i t e , and k e v l a r f i b e r composites. New t e c h n o l o g i e s need t o be developed f o r the use of c e l l u l o s e based m a t e r i a l s as a source o f reinforcement. M i c r o f i b r i l l a t e d c e l l u l o s e may provide a s t i f f e r r e i n f o r c i n g f i b e r f o r the composites (21). The r e a c t i o n I n j e c t i o n



6. RAJ & KOKTA


Figure 5. F r a c t u r e surface of HDPE-stearic acid t r e a t e d c e l l u l o s e f i b e r composite (30% f i b e r weight), m a g n i f i c a t i o n 190 X

/\-/J

Mechanical Properties of Surface-Modified Cellulose

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