High-Tech Fibrous Materials - American Chemical Society

Chapter 7

Linear Low-Density Polyethylene Filled with Silane-Coated Wood Fibers

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R. G. Raj and B. V. Kokta Centre de Recherche en Pâtes et Papiers, Université du Québec, C.P. 500, Trois-Rivières,QuébecG9A 5H7, Canada

Silane precoated wood fibers were used as f i l l e r in linear low density polyethylene (LLDPE). Chemithermomechanical pulp (CTMP) of aspen was pretreated with silane coupling agents (silane A-172, A-174 and A-1000) having different functional groups. LLDPE f i l l e d with silane A-174 pretreated (0.5% by weight of fiber) wood fibers (50% f i l l e r weight) produced a 31% increase in tensile strength and 124% increase in tensile modulus compared to unfilled LLDPE. The effect of variation in f i l l e r concentration, silane concentration and type of initiator, on mechanical properties of the composites was examined. The r i s i n g c o s t o f r e s i n and t h e need t o enhance mecha n i c a l p r o p e r t i e s a r e i n c e n t i v e s t o use low c o s t f i l l e r s as extenders o r r e i n f o r c e m e n t s i n p l a s t i c s . D i f f e r e n t i n o r g a n i c f i l l e r s such as g l a s s f i b e r , mica, t a l c , c l a y etc. are being incorporated i n t o thermoplastics (1,2). Cellulosic fillers have r e c e n t l y a t t r a c t e d greater a t t e n t i o n due t o t h e i r lower p r i c e , low d e n s i t y , b i o degradable and renewable i n nature (3-5). The use o f wood f i b e r i n u r e a - o r phenol-formaldehyde a r e a l r e a d y common, where t h e f i b e r p r o v i d e s a degree o f toughness which i s n o t a t t a i n a b l e i n t h e r e l a t i v e l y b r i t t l e thermos e t t i n g r e s i n s i n t h e absence o f some r e i n f o r c i n g f i b e r . Thus, t h e use o f wood f i b e r as a f i l l e r o r r e i n f o r c i n g agent i n h i g h volume t h e r m o p l a s t i c r e s i n s seems t o be t h e main area where t h e p o t e n t i a l o f wood f i b e r i s not f u l l y e x p l o i t e d . The s u r f a c e c h a r a c t e r i s t i c o f t h e r e i n f o r c i n g f i b e r i s an important f a c t o r due t o i t s r o l e i n t h e t r a n s f e r o f s t r e s s from m a t r i x t o f i b e r . C h e m i c a l l y , t h e hydroxyl-rich surface o f l i g n o c e l l u l o s i c material i s 0097-6156/91/0457-0102$06.00/0 © 1991 American Chemical Society

In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


7. RAJ & KOKTA

Polyethylene Filled with Silane-Coated Wood Fibers 103

advantageous because i t provides the p o t e n t i a l f o r r e a c t i o n w i t h the c o u p l i n g agent or m a t r i x (phenol o r urea formaldehyde). Another f a c t o r which i s a l s o important i s the p h y s i c a l d i f f e r e n c e between the f i b e r s commonly used as f i l l e r / r e i n f o r c i n g agent i n p l a s t i c s . S y n t h e t i c f i b e r s u s u a l l y have a smooth s u r f a c e and are relatively s t r a i g h t , whereas the normal wood f i b e r i s t w i s t e d and has an i r r e g u l a r s u r f a c e . Thus, the wood f i b e r i s l i k e l y t o e x h i b i t a g r e a t e r r e s i s t a n c e t o withdrawal from the m a t r i x compared t o the s y n t h e t i c f i b e r s ( 6 ) . The main disadvantage of the wood f i b e r i s the c o m p a t a b i l i t y problem between the h y d r o p h i l i c f i b e r and the hydrophob i c polymer. A s u f f i c i e n t degree of adhesion between the f i l l e r and polymer m a t r i x i s a necessary c o n d i t i o n t o maximize the mechanical performance of the composite. For t h i s reason, a c o u p l i n g agent which promotes adhesion between the f i b e r s u r f a c e and m a t r i x i s d e s i r a b l e (7,8). The p r i n c i p a l o b j e c t i v e f o r the use of c o u p l i n g agent i n a composite i s t o e f f e c t a s i g n i f i c a n t improvement i n the s t r e n g t h of f i n i s h e d product. Assuming t h a t the optimum r e a c t i v e groups f o r the f i l l e r s u r f a c e and m a t r i x can be s e l e c t e d , the o b j e c t i v e then i s t o maximize the i n t e r a c t i o n . I n the present work, d i f f e r e n t s i l a n e c o u p l i n g agents were used t o improve the f i b e r - m a t r i x adhesion. The e f f e c t of v a r i a t i o n i n f i l l e r c o n c e n t r a t i o n , s i l a n e c o n c e n t r a t i o n and the type of i n i t i a t o r , on mechanical p r o p e r t i e s of the composites was examined. Experimental LLDPE Navopal LLGR-0118-A was s u p p l i e d by Novacor Chemic a l s L t d . The r e p o r t e d p r o p e r t i e s of LLDPE a r e : melt index: 1 g/10 min; d e n s i t y : 0.918 g/cc. CTMP (aspen) was prepared i n a sund d e f i b r a t o r ( 9 ) . The average f i b e r aspect r a t i o (L/D) was 11.9. Three s i l a n e c o u p l i n g agents (Union Carbide) were used: a) V i n y l t r i ( 2 - m e t h o x y e t h o x y ) s i l a n e ( s i l a n e A-172) b) V- M e t h a c r y l o x y p r o p y l t r i m e t h o x y s i l a n e ( s i l a n e A-174) c) A m i n o p r o p y l t r i e t h o x y s i l a n e ( s i l a n e A-1100) Pretreatment of wood f i b e r The wood f i b e r s were p r e t r e a t e d w i t h d i f f e r e n t s i l a n e c o u p l i n g agents. a) Oven d r i e d wood f i b e r s (50 g) were p l a c e d i n a f l a s k t o which 300 mL of carbon t e t r a c h l o r i d e was added, f o l l o w e d by the a d d i t i o n of dicumyl peroxide (2%) and 2% of s i l a n e A-172 o r A-174. The above mixture was r e f l u x e d f o r 3 hours. A f t e r c o o l i n g , the carbon t e t r a c h l o r i d e was evaporated and the f i b e r s were d r i e d a t 55°C f o r 24 hours before mixing w i t h the polymer. A s e r i e s of p r e t r e a t e d wood f i b e r s were prepared u s i n g o t h e r peroxides ( 2 % ) , benzoyl or d i - t - b u t y l peroxide.



104

HIGH-TECH FIBROUS MATERIALS

b) Pretreatment of wood f i b e r s w i t h s i l a n e A-1100 was c a r r i e d out by a two-stage mixing procedure. I n the f i r s t stage, a mixture of wood f i b e r , s i l a n e A-1100 (2%) and dicumyl p e r o x i d e (2%) was r e f l u x e d f o r 3 hours i n carbon t e t r a c h l o r i d e . I n the second s t a g e , a s m a l l amount of polymer, t o reduce the f i b e r - f i b e r i n t e r a c t i o n , was added to the m i x t u r e . The polymer (LLDPE, 5%) was d i s s o l v e d i n p-xylene, t o which m a l e i c anhydride (2%) and benzoyl pero x i d e (0.05%) were added. The m i x t u r e was r e f l u x e d f o r 3 hours; then the c o n t e n t s of f i r s t stage were added t o the precoated polymer and the r e s u l t i n g mixture was a g a i n r e f l u x e d f o r 2 hours. The mixture was then c o o l e d t o room temperature, f i l t e r e d , washed w i t h d i s t i l l e d water and then d r i e d a t 105°C f o r 12 hours. P r e p a r a t i o n of compogsiteg Compounding of LLDPE and p r e t r e a t e d wood f i b e r s (0-50% by weight) was done i n a C.W. Brabender R o l l M i l l No.065. The above mixture was compression molded i n t o dog-bone shaped t e n s i l e specimens. The molding was done a t 150°C w i t h a p r e s s u r e o f 3.2 MPa. A f t e r h e a t i n g t h e mold f o r 15 min., t h e samples were s l o w l y c o o l e d t o room temperature w i t h the p r e s s u r e maintained d u r i n g the p r o c e s s . Mechanical t e s t s I n s t r o n model 4201 was used t o study the t e n s i l e propert i e s o f the composites. The f u l l - s c a l e l o a d was 500 N 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 t h e I n s t r o n 2412005 General T e n s i l e Test Program. The average c o e f f i c i e n t 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 was l e s s than 6.0%. R e s u l t s and D i s c u s s i o n The mechanical p r o p e r t i e s of thermoplastic-wood f i b e r composites depend upon the p r o p e r t i e s of the polymer m a t r i x , wood f i b e r and of the p o l y m e r - f i b e r i n t e r f a c e . Wood f i b e r s are r e l a t i v e l y s h o r t , f l e x i b l e and coarse i n comparison t o most o f t h e f i l l e r s used f o r r e i n f o r c i n g the t h e r m o p l a s t i c polymers. Thus, bonding of the m a t r i x t o t h e f i b e r i s an important f a c t o r i n d e t e r m i n i n g whether o r not the f u l l s t r e n g t h of wood f i b e r can be u t i l i z e d i n a composite. F i g u r e s 1-3 i l l u s t r a t e t h e i n f l u e n c e of s i l a n e pretreatment of f i b e r s on mechanical p r o p e r t i e s of the composite. The composites c o n t a i n i n g u n t r e a t e d wood f i b e r s ( c o n t r o l ) g r a d u a l l y l o s t t h e i r t e n s i l e s t r e n g t h w i t h an i n c r e a s e i n f i l l e r c o n c e n t r a t i o n . Both c o u p l i n g agents ( s i l a n e A-172 and A-174) provided a considerable increase i n t e n s i l e strength r e l a t i v e t o the u n t r e a t e d f i b e r composites. F i g u r e 1 compares the t e n s i l e p r o p e r t i e s of LLDPE



7.

RAJ & KOKTA

105 Polyethylene Filled with Silane-Coated Wood Fibers

f i l l e d w i t h u n t r e a t e d and s i l a n e A-172 o r A-174 p r e t r e a t e d wood f i b e r s ( 2 % dicumyl p e r o x i d e ) . T e n s i l e s t r e n g t h i n c r e a s e d s t e a d i l y w i t h f i l l e r c o n c e n t r a t i o n i n s i l a n e A172 and A-174 t r e a t e d f i b e r s , which i n d i c a t e s t h a t t h e s i l a n e treatment i n f l u e n c e s t h e bonding a t t h e i n t e r f a c e . At 40% f i b e r l e v e l , t e n s i l e s t r e n g t h of t h e s i l a n e A-174 t r e a t e d wood f i b e r composites i s more than f o u r times g r e a t e r than t h e u n t r e a t e d f i b e r composites. F i g u r e 2 shows t h a t t h e i n c r e a s e i n f i l l e r concentr a t i o n has a negative e f f e c t on t h e e l o n g a t i o n o f t h e composites. The e l o n g a t i o n decreased s h a r p l y w i t h an i n c r e a s e i n f i l l e r content i n t h e polymer. The s i l a n e pretreatment had l i t t l e i n f l u e n c e on t h e e l o n g a t i o n o f the composites. The e f f e c t o f f i b e r treatment on t e n s i l e modulus o f t h e composites i s presented i n F i g u r e 3. The a d d i t i o n o f f i l l e r r e s u l t s i n an a p p r e c i a b l e s t i f f e n i n g e f f e c t o f t h e m a t r i x as was observed i n an e a r l i e r study by Klason e t a l . ( 5 ) . A t 30% f i l l e r l e v e l , modulus rose from 132 MPa ( u n f i l l e d LLDPE) t o 350 MPa. An i n c r e a s e i n modulus was a l s o observed i n t h e case o f s i l a n e A-172 and A-174 p r e t r e a t e d wood f i b e r composites. A p o s s i b l e c o u p l i n g mechanisim o f polymer-coupling agent-wood f i b e r i s shown i n F i g u r e 4. The c o u p l i n g agent can be represented as Y-R-Si-X , 3

where X i s a h y d r o l y z a b l e a l k o x y group, Y i s a f u n c t i o n a l o r g a n i c group ( v i n y l i n s i l a n e A-172 and methacryl i n s i l a n e A-174) and R i s an a l i p h a t i c l i n k a g e . During f i b e r pretreatment, t h e r e a c t i o n of t h e c o u p l i n g agent w i t h t h e hydroxy1 groups of wood f i b e r s u r f a c e produces t h e h y d r o l y s i s product s i l a n o l , - S i ( 0 H ) . A c o u p l i n g agent s i l a n o l can develop e i t h e r c o v a l e n t ( s i l o x a n e bond) o r hydrogen bonds w i t h OH groups o f c e l l u l o s e . Subsequent r e a c t i o n o f t h e f u n c t i o n a l o r g a n i c group ( v i n y l o r methacryl) w i t h t h e polymer completes t h e establishment of a molecular b r i d g e between t h e polymer and f i b e r . The e f f e c t of s i l a n e A-174 c o n c e n t r a t i o n ( 2% benzy o l peroxide) on t e n s i l e p r o p e r t i e s of t h e composites a r e presented i n Tables I and I I . A s i g n i f i c a n t 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 , w i t h t h e i n c r e a s e i n f i l l e r content, was achieved even a t lower c o n c e n t r a t i o n s o f s i l a n e A174. The r e s u l t s a l s o i n d i c a t e t h a t t h e c o u p l i n g agent i s e f f e c t i v e a t very low c o n c e n t r a t i o n s than a t h i g h e r conc e n t r a t i o n s (2 and 4%). E l o n g a t i o n decreased d r a s t i c a l l y a t h i g h e r f i l l e r c o n c e n t r a t i o n s and t h e s i l a n e p r e t r e a t ment had l e s s e f f e c t on e l o n g a t i o n . The i n c o r p o r a t i o n o f wood f i b e r s i n t h e polymer m a t r i x i n c r e a s e d t h e t e n s i l e modulus as can be observed from Table I I . T e n s i l e modulus i n c r e a s e d l i n e a r l l y w i t h f i l l e r c o n c e n t r a t i o n . The r i s e i n t e n s i l e modulus o f t h e composite i s due t o t h e higher modulus o f t h e wood f i b e r . F r a c t u r e energy, c a l c u l a t e d by t h e area under t h e s t r e s s 3


106

HIGH-TECH FIBROUS MATERIALS Tensile strength (MPa)


30

O

untreated

•

silane A-172

A

silane A-174

10

6

20

30

40

50

Fiber weight (%)

F i g u r e 1. E f f e c t o f s i l a n e pretreatment s t r e n g t h o f LLDPE-wood f i b e r composites.

on t e n s i l e

Elongation at break (%) 30

20

10

O

untreated

•

silane A-172

A

silane A-174

I

10

L

20

30

40

50

Fiber weight (%)

F i g u r e 2. E f f e c t o f s i l a n e pretreatment on e l o n g a t i o n of LLDPE-wood f i b e r composites.


7.

Polyethylene Filled with Silane-Coated Wood Fibers 107

RAJ & KOKTA

Tensile modulus

(MPa)

800


600

-

O

untreated

•

silane

A-172

A

silane

A-174

400

200

-

10

20

30

40

50

Fiber weight (%)

F i g u r e 3. E f f e c t of s i l a n e pretreatment on modulus of LLDPE-wood f i b e r composites.

Y

- R - Si X

3

CH = CH Y is

t^^v

2

-

R

_ silOH^

-

CH = C - C HN

_

tensile

0CH.

C^-

F i g u r e 4. P o s s i b l e c o u p l i n g mechanism a t the f i b e r matrix interface.


108


s t r a i n curve, can p r o v i d e a measure o f t h e toughness o f the composite. Table I

Fiber Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch007

(%

Wt.)

EFFECT OF SILANE A-174 CONCENTRATION ON TENSILE PROPERTIES OF LLDPE-PRETREATED WOOD FIBER COMPOSITES T e n s i l e s t r e n g t h (MPa) E l o n g a t i o n (%) 10 20 30 40 10 20 30 40

S i l a n e A-174 0%

12.6

8.6

8.1

8.5

782

242

7.3

3.8

1%

12.7

14.4

20.4

23.0

573

23.7

8.9

5.4

2%

12.6

14.8

19.4

21.1

249

37.1

9.3

6.8

4%

12.3

14.5

18.1

19.4

332

42.3

14.8

10.5

Table I I EFFECT OF SILANE A-174 CONCENTRATION ON TENSILE PROPERTIES OF LLDPE-PRETREATED WOOD FIBER COMPOSITES Fiber (%

Wt.)

F r a c t u r e energy ( J ) 10 20 30 40

T e n s i l e modulus (MPa) 10 20 30 40

S i l a n e A-174 0%

7.3

1.92

0.31

0.19

142

201

337

392

1%

2.7

0.51

0.42

0.34

162

230

298

347

2%

5.8

0.59

0.48

0.40

167

242

291

384

4%

3.6

0.53

0.47

0.31

158

265

326

375

The above r e s u l t s show t h a t t h e f r a c t u r e energy decreased s t e a d i l y w i t h an i n c r e a s e i n f i l l e r concentrat i o n . The l o s s i n f r a c t u r e energy can be a t t r i b u t e d t o an i n c r e a s e i n f i b e r p u l l - o u t , due t o t h e h i g h e r s t r e s s generated a t t h e f i b e r - m a t r i x i n t e r f a c e when t h e f i b e r i s s t r a i n e d i n t e n s i o n , a t t h e i n t e r f a c e ( 6 ) . Improvement i n toughness can be achieved o n l y i f t h e r e i s an i n c r e a s e i n i n t e r f a c i a l bond s t r e n g t h between t h e f i b e r and matrix. D i f f e r e n t types o f peroxides were used d u r i n g t h e


7. RAJ & KOKTA

Polyethylene Filled with Silane-Coated Wood Fibers

pretreatment o f wood f i b e r . Tables I I I and IV show t h e e f f e c t of peroxide on t e n s i l e p r o p e r t i e s o f LLDPEsilane A-174 p r e t r e a t e d wood f i b e r composites. The results show t h a t a c o n s i d e r a b l e g a i n i n t e n s i l e p r o p e r t i e s can be achieved w i t h t h e proper s e l e c t i o n o f the i n i t i a t o r .


Table I I I EFFECT OF PEROXIDE ON TENSILE PROPERTIES OF LLDPE-SILANE A-174 PRETREATED WOOD FIBER COMPOSITES Fiber

T e n s i l e s t r e n g t h (MPa) 10 20 30 40

10

Dicumyl peroxide

12. 6

16. 2

21.5

26 .8

127

39. 2

12 .6

9. 7

Benzoyl peroxide

12. 7

14. 4

20.4

23 .0

573

23. 7

8 .9

5.4

D i - t - b u t - 11.7 y l peroxide

13. 1

16.3

18 .7

684

17. 3

9 .6

5. 1

(%

Wt.)

E l o n g a t i o n (%) 20 30 40

Table IV

EFFECT OF PEROXIDE ON TENSILE PROPERTIES OF LLDPE-SILANE A-174 PRETREATED WOOD FIBER COMPOSITES

Fiber

F r a c t u r e energy ( J ) 10 20 30 40

T e n s i l e modulus (MPa) 10 20 30 40

Dicumyl peroxide

1. 4

0.56

0. 52

0. 47

172

223

324

413

Benzoyl peroxide

2. 7

0.51

0. 42

0. 34

162

230

298

347

D i - t - b u t - 3. 1 y l peroxide

0.47

0. 28

0. 16

141

206

291

322

(%

Wt.)

Higher t e n s i l e s t r e n g t h , w i t h i n c r e a s e i n f i b e r c o n t e n t , was observed when dicumyl peroxide was used as an i n i t i a t o r (Table I I I ) . T e n s i l e modulus seems t o be l e s s i n f l u e n c e d by t h e type o f t h e p e r o x i d e used. As observed e a r l i e r , f r a c t u r e energy decreased w i t h t h e a d d i t i o n o f f i b e r i n t h e m a t r i x (Table I V ) . The chemical


1


110


s t r u c t u r e and the p h y s i c a l c h a r a c t e r i s t i c s o f the peroxide seems t o i n f l u e n c e the r e a c t i o n w i t h f i l l e r s u r f a c e . I t i s a l s o p o s s i b l e t h a t the peroxide can generate f r e e r a d i c a l s on the polymer which can a i d i n the f o r mation of chemical bonding w i t h the c o u p l i n g agent (1J2). Organic peroxides are t h e r m a l l y u n s t a b l e and undergo h o m o l y t i c cleavage a t the oxygen-oxygen bonds t o form r a d i c a l s . The r e l a t i v e s t a b i l i t y and the r e a c t i v i t y of the r a d i c a l s can be c o r r e l a t e d t o the s t r u c t u r e of the peroxide (11). The r a d i c a l s can e x t r a c t hydrogen from p o l y e t h y l e n e and add t o the f u n c t i o n a l group of the c o u p l i n g agent t o form c o v a l e n t bonds. I t i s a l s o p o s s i b l e t h a t the r a d i c a l s can i n i t i a t e v i n y l groups i n s i l a n e which can be l i n k e d t o the polymer. I n a d d i t i o n , t h e r e i s a l s o the p o s s i b i l i t y of c r o s s l i n k i n g of the polyethylene chains which can change the p h y s i c a l p r o p e r t i e s of the m a t r i x . The e f f e c t of s i l a n e pretreatment on the morphology of polymer-wood f i b e r composites was s t u d i e d by SEM. Micrographs of the f r a c t u r e d s u r f a c e s of the samples are shown i n F i g u r e s 5 and 6. As can be seen, the u n t r e a t e d f i b e r composites show a poor d i s p e r s i o n of f i b e r i n the polymer m a t r i x ( F i g u r e 5 ) . A l s o t h e r e i s a l a c k of adhes i o n a t the i n t e r f a c e , which leads t o more f i b e r p u l l out from the m a t r i x . In s i l a n e A-174 p r e t r e a t e d wood f i b e r composites, good bonding a t the f i b e r - m a t r i x i n t e r f a c e g r e a t l y reduces the f i b e r p u l l - o u t ( F i g u r e 6 ) . F i g u r e s 7 and 8 summarize the t e n s i l e p r o p e r t i e s of LLDPE f i l l e d w i t h d i f f e r e n t s i l a n e t r e a t e d (2% by weight) f i b e r s . I t can be seen t h a t 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 n a l l s i l a n e t r e a t e d f i b e r composites i s h i g h e r than the u n t r e a t e d f i b e r composites ( F i g u r e 7 ) . I t was a l s o observed t h a t w i t h 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 , a s i g n i f i c a n t g a i n i n t e n s i l e s t r e n g t h can be achieved. The % i n c r e a s e i n t e n s i l e modulus i s much lower compared t o the t e n s i l e s t r e n g t h of s i l a n e pret r e a t e d f i b e r composites ( F i g u r e 8 ) . T h i s e x p l a i n s why good bonding a t the i n t e r f a c e i s a primary c o n d i t i o n f o r the improvement of t e n s i l e s t r e n g t h , whereas a s i g n i f i c a n t g a i n i n modulus can be achieved s i m p l y by i n c r e a s i n g the f i l l e r content i n the polymer m a t r i x . The r e s u l t s a l s o i n d i c a t e t h a t s i l a n e A-174 (methacr y l f u n c t i o n a l group) performed b e t t e r as a c o u p l i n g agent than s i l a n e A-172 or A-1100. v i n y l group i n s i l a n e A-172 may r e a c t w i t h the m a t r i x through r a d i c a l r e a c t i o n , whereas t h i s may not be the case w i t h aminopropyl groups i n s i l a n e A-1100. In an e a r l i e r study on s i l a n e c o u p l i n g agents, I s h i d a observed t h a t s i l a n e s tend t o be ordered i n the i n t e r p h a s e and the degree of o r g a n i z a t i o n depends l a r g l y on the o r g a n o f u n c t i o n a l i t y of the s i l a n e . The o r i e n t a t i o n and o r g a n i z a t i o n of the s i l a n e a f f e c t s the reinforcement mechanism (12). S i l a n e s w i t h l a r g e and f l e x i b l e f u n c t i o n a l groups tend t o form more c y c l i c s t r u c t u r e s than the s i l a n e s w i t h s m a l l e r and more r i g i d



7.

RAJ & KOKTA

Polyethylene Filled with SUane-Coated Wood Fibers

F i g u r e 5. F r a c t u r e s u r f a c e of LLDPE-untreated f i b e r Composite (30% f i b e r w e i g h t ) .

wood

F i g u r e 6. F r a c t u r e s u r f a c e of LLDPE-silane A-174 p r e t r e a t e d wood f i b e r composite (30% f i b e r w e i g h t ) . In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

112


% increase in T.S.

160 silane A-172

120

YZA


I

si,ane

A-174

| silane A-1100

80

10%

40

Fiber weight (%)

F i g u r e 7. E f f e c t o f d i f f e r e n t s i l a n e pretreatments on t e n s i l e s t r e n g t h o f LLDPE-wood f i b e r composites. % increase in T.M.

40 silane A-172

30

\7A

I

20 -

silane A-174

| silane A-1100

30%

10%

10 -

Fiber weight (%)

F i g u r e 8. E f f e c t o f d i f f e r e n t s i l a n e pretreatments on t e n s i l e modulus o f LLDPE-wood f i b e r composites.


7. RAJ & KOKTA

Polyethylene Filled with Silane-Coated Wood 113 Fibers

substituents. In the present case, the vinyl and methacryloxy groups i n silanes exhibit a stronger interaction with the wood fiber surface and polymer matrix than the aminopropyl group. This may explain the reason for higher tensile strength values observed in silane A-172 and A-174 pretreated wood fiber composites than silane A-1100.


conclusions The potential of wood fiber as a low cost reinforcing f i l l e r in LLDPE was shown to be promising. A significant improvement in tensile properties can be achieved with the pretreatment of wood fiber with silane coupling agent. The concentration of wood fiber in the polymer matrix, the type of the silane coupling agent and their concentration were found to be important factors in achieving optimum strength properties of the composites. Silane A-174 with methacryl functional group was found as a most effective coupling agent for LLDPE-wood fiber composites. Dicumyl peroxide as an i n i t i a t o r gave best improvement in mechanical properties of the composites. SEM studies of silane A-174 pretreated wood fiber composites showed good bonding at the interface with less fiber pull-out from the polymer matrix. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Theberge, J.E.; Hohn, K. Polym. Plast. Technol. Eng. 1981, 16, 41-52. Chun, I; Woodhams, R.T. Polymer Composites 1984, 5, 250-263. Kokta, B . V . ; Chen, R.; Daneault, C.; Valade, J.L. Polymer Composites 1983, 4, 229-232. Nakajima, Y. Japan Patent Kokai 127 632, 1981. Klason, C . ; Kubat, J.; Strömvall, H . - E . Intern. J. polymeric Mater. 1984, 11, 9-38. Mckenzie, A.W.; Yuritta, J.P. Appita 1979, 32, 460465. Morrell, S.H. Plastics and Rubber Processing and Applications 1981, 1, 179-186. Goettler, L . A . U.S. Patent 4 376 144, 1983. Beshay, A . D . ; Kokta, B . V . ; Daneault, C. Polymer Composites 1985, 6, 261-271. Raj, R . G . ; Kokta, B . V . ; Daneault, C. J. Appl. Polym. S c i . 1989, 37, 1089-1103. C a l l a i s , P . A . ; Kazmierczak, R.T. Proc. of SPI 47th Annual Technical Conf., 1989, p 1368. Ishida, H. Polymer Composites 1984, 5, 101-123.

RECEIVED

July 18, 1990


High-Tech Fibrous Materials - American Chemical Society

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