The Xanthate Method of Grafting - ACS Symposium Series (ACS

17 The Xanthate Method of Grafting Part IX: Grafting of Acrylonitrile onto High-Yield Hardwood Pulp

Downloaded by UNIV OF TEXAS AT AUSTIN on August 29, 2017 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch017

Β. V. KOKTA, R. C. LO, and C. DANEAULT Centre de Recherche en Pâtes et Papiers, Université du Québec à Trois-Rivières, C.P. 500, Trois-Rivières, Québec, Canada G9A 5H7 Hardwood pulp was copolymerized with a c r y l o n i t r i l e as a monomer using the xanthate grafting method. Grafting has been i n i t i a t e d using a redox system of ferrous ion and hydrogen peroxide. The effect of operating conditions on grafting was investigated. Among the factors studied, the most important were initial pH, the concentration of hydrogen peroxide and the pulp/monomer r a t i o . Grafting parameters has been compared to that of softwood pulps. In addition, the effect of different residual concen tration of lignin in pulp was examined. Sodium chlorite served as the delignification agent. The results showed minimum grafting at 6-8% lignin l e v e l . Finally, the optimum set of reaction conditions, which gives almost 100% grafting efficiency at very high conversion level has been proposed.

The xanthate method of g r a f t i n g , d i s c o v e r e d by Faessinger and Conte (1) i s one of the most promising f o r i n d u s t r i a l a p p l i c a t i o n . In our l a b o r a t o r y we have been working on the o p t i m i z a t i o n of xanthate method. I n order t o develop an e f f i c i e n t method we have t r i e d to reduce the homopolymer formation t o a minimum and consequently t o i n c r e a s e g r a f t i n g e f f i c i e n c y . Maximum monomer conversion was another important o b j e c t i v e in the developing of a cheap g r a f t i n g method. We have shown t h a t f o r low l i g n i n pulps ( 2 ) , f o r h i g h y i e l d pulps (3,4.), as w e l l as f o r mechanical (5) and thermomechanical pulps ( 6 , 7 ) , the xanthate method could be o p t i mized by p r o p e r l y a d j u s t i n g the r e a c t i o n c o n d i t i o n s l i k e the hydrogen peroxide c o n c e n t r a t i o n , the initial pH and the pulp/monomer r a t i o . The pulps used in the above mentioned r e f e r e n c e s (27) were softwood ones (spruce and balsam f i r ) . I t was shown t h a t , in g e n e r a l , the paper p r o p e r t i e s could be improved by the g r a f t i n g of acrylamide onto low s t r e n g t h pulps ( l i k e h i g h y i e l d and mechanical pulps) ( 8 ) . I t i s w e l l known that hardwood pulp f i b e r s are c o n s i d e r a b l y s h o r t e r (about 1 m i l l i ©

0097-6156/82/0187-0269$6.00/0 1982 American Chemical Society

Hon; Graft Copolymerization of Lignocellulosic Fibers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


270

GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS

meter) compare to softwood f i b e r s (about 3 m i l l i m e t e r s ) . I n ad d i t i o n , hardwood f i b e r s , being l e s s compressable than the s o f t wood ones give g e n e r a l l y low s t r e n g t h p u l p s . Therefore, i t seems l o g i c a l that g r a f t i n g on hardwood pulps would improve t h e i r mecha n i c a l p r o p e r t i e s ( 9 ) . U n f o r t u n a t e l y very l i t t l e have been p u b l i shed on g r a f t i n g hardwood pulps using the xanthate method w i t h the exception of work done by Young (10). He has found, that g r a f t i n g e f f i c i e n c i e s in case of aspen could be c l o s e t o 53% f o r both the styrene and a c r y l o n i t r i l e monomers. Conversion values obtained were 76.5% and 17% f o r a c r y l o n i t r i l e and styrene, r e s p e c t i v e l y . The o b j e c t i v e of t h i s work was to optimize g r a f t i n g parame t e r s f o r hardwood pulp in general and aspen in p a r t i c u l a r . The monomer used was a c r y l o n i t r i l e . Experimental a) M a t e r i a l s ; P u l p : the high y i e l d b i s u l f i t e pulp of aspen, y i e l d 93%, b r i g h t n e s s 46.6%, C.S.F. 325, l i g n i n 14.8%. Monomer: a c r y l o n i t r i l e (Eastman Chemical Grade) was p u r i f i e d by d i s t i l l a t i o n ; the c e n t r a l f r a c t i o n was c o l l e c t e d and s t o r e d in a r e f r i g e r a t o r in dark b o t t l e s . A l l other chemicals employed in the work were a n a l y t i c a l grade and were used as s u p p l i e d by manufacturers. b) D e l i g n i f i c a t i o n of pulps : Sodium c h l o r i t e was used as the b l e a c h i n g agent at pH 4. The c o n c e n t r a t i o n of c h l o r i t e r e q u i r e d to o b t a i n a given degree of de l i g n i f i c a t i o n was determined by p r e l i m i n a r y experimentation. I n a b l e a c h i n g o p e r a t i o n , the r e q u i r e d q u a n t i t y of pulp (150 g a i r - d r i e d ) was d e s i n t e g r a t e d and placed i n t o a polyethylene bag. The c a l c u l a t e d amount(from 2 to 12%) of NaC10 d i s s o l v e d in 300 ml water was subsequently added and the mixture was homogeni zed by t r i t u r a t i o n . The pH was adjusted to 4 w i t h s u l f u r i c a c i d , and the mixture was once again homogenized and the pH readjusted if necessary. The bag, placed in a controlled-temperature bath was maintained at 50°C f o r 120 minutes. A f t e r b l e a c h i n g , the pulp was f i l t e r e d , washed 5 times w i t h 1000ml a l i q u o t s of water and then subjected to a l k a l i n e e x t r a c t i o n by sodium hydroxide (1.5% by weight in respect t o dry pulp) at 80 C during 1 hour. The e x t r a c t e d pulp was thoroughly washed u n t i l pH 7 and then vacuum f i l t e r e d to approximately 20% of c o n s i s t e n c y . A l l pulps were preserved in a r e f r i g e r a t o r . The pulps prepared by t h i s procedure as w e l l as the non bleached h i g h y i e l d b i s u l f i t e pulp have been c h a r a c t e r i z e d by the standard methods of the Canadian Pulp and Paper A s s o c i a t i o n (CPPA T e c h n i c a l Section) as shown in Table I . c) Procedures used: The technique of pulp p r e - c o n d i t i o n n i n g as w e l l as the p o l y m e r i z a t i o n c o n d i t i o n s have been described in a previous paper (11). S p e c i f i c c o n d i t i o n s used in t h i s work were as f o l l o w s : Pulp c o n d i t i o n i n g : Pulp 9.0 ί O.02 g (oven-dried w e i g h t ) ; p a r t i a l m e r c e r i z a t i o n at 25°C., 45 min. in 300 ml O.75 Ν NaOH; (so l u b i l i t y in NaOH:3.5%); x a n t h a t i o n , 2 hrs in presence of CS v a 2

?



17.

KOKTA ET AL.

Xanthate Method

111

TABLE I P r o p e r t i e s of Pulps Prepared by C h l o r i t e D e l i g n i f i c a t i o n .

Copper b index

Carboxyl index

Pulp

Kappa

ΒΊ-0

86.3

14.8

2.2

16.4

BT-1

74.5

12.8

2.1

22.4

BT-2

62.0

10.7

1.9

24.8

BT-3

52.8

9.1

2.0

24.5

BT-4

37.2

6.4

1.8

23.7

BT-5

31.8

5.5

1.8

23.2

BT-6

25.4

4.4

1.5

22.5

a

a

CPPA standard G.18.

b

CPPA standard G.22.

C

Lignin %

TAPPI standard T237 su.63.


272

GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS

p o r s , 25°C.; washing, 300 ml of a c i d i f i e d (pH:4.5) d i s t i l l e d water-, i o n exchange, 2 min in 150 ml of O.004% (NH,) Fe(S0,) ; washing, 250 ml of d i s t i l l e d water (pH: 6.5). Copolymerization; Monomer, a c r y l o n i t r i l e , 9.09 g; impregna t i o n 15 min, 25°C.; s u r f a c t a n t (Tween-80), O.9 g; d i s t i l l e d water, 210 ml; ^ °2 ( - ) 2 °4 t i m e , up to 19 h r s ; atmosphere, n i t r o g e n ; r e a c t i o n temperature, 25 C.; quenching of p o l y m e r i z a t i o n : washing of product w i t h d i s t i l l e d water and 1% of K S 0 ^ , 5 min; washing w i t h 1000 ml o f d i s t i l l e d water. E x t r a c t i o n s ; The apparent l e v e l of g r a f t i n g , polymer l o a d i n g and conversion was determined by Soxhlet e x t r a c t i o n u s i n g dime t h y l formamide. The oven dry weight was c o r r e c t e d f o r pulp l o s t in the m e r c e r i z a t i o n s t e p . G r a f t i n g parameters were d e f i n e d as follows : Polymer l o a d i n g , %: (A-B)/(B) χ 100 G r a f t i n g e f f i c i e n c y , %: (A-B)/(D-B) χ 100 Degree of c o n v e r s i o n , %: (D-B)/(C) χ 100 where A i s weight of product a f t e r c o p o l y m e r i z a t i o n and e x t r a c t i o n , Β i s weight of p u l p , C i s weight of monomer charged; and D i s weight of product a f t e r c o p o l y m e r i z a t i o n . ?

c o n c

3 0 %

5

m l

H

S

t o

p H :

5 ;

r

e

a

c

t

i

o

n

2


2

2

R e s u l t s and D i s c u s s i o n 1) E f f e c t of Operating C o n d i t i o n s on G r a f t i n g E f f i c i e n c y and Conversion. a) Dependence of g r a f t i n g on initial pH. Dependence of g r a f t i n g on initial pH i s i l l u s t r a t e d in F i g u re 1. G r a f t i n g e f f i c i e n c y i s not much a f f e c t e d by pH, even though the optimum v a l u e seems t o be between pH 4 and 7. This i s a l i t t l e lower than the v a l u e s obtained f o r mechanical softwood p u l p , f o r which maximum g r a f t i n g e f f i c i e n c y was in the p r o x i m i t y of pH 6-9 ( 4 ) . I n case of softwood k r a f t semibleached pulp (2% l i g n i n ) , g r a f t i n g e f f i c i e n c y was d i r e c t l y p r o p o r t i o n a l to pH up to the v a lue of pH 10 when i t reached 83% ( 2 ) . As f a r as c o n v e r s i o n i s concerned, there i s a sharp i n c r e a s e from 5% c o n v e r s i o n at pH= 1 to 70% conversion a t pH=4 where the conversion stays constant up t o pH*7. This compares w e l l to the values f o r softwood mechanical pulp f o r which the c o n v e r s i o n was h i g h e s t at pH«5 (67.8%), ( 4 ) . On the other way, c o n v e r s i o n v a lues f o r softwood semibleached k r a f t p u l p were h i g h e s t a t pHr2 (87%) and than s t e a d i l y decreased to the 55% v a l u e a t ρΗ=8 and to 0% a t pH=10 (2). From the r e s u l t s shown above, i t seems apparent that the e f f e c t of pH on g r a f t i n g i s s i m i l a r f o r h i g h l i g n i n pulps e i t h e r hardwood as in t h i s study o r softwood mechanical pulp ( 4 ) . C o n s i d e r i n g t h a t both g r a f t i n g e f f i c i e n c y as w e l l as conver s i o n in t h i s study shows o p t i m a l values f o r pH 4 - 7, we have used pH 5 which f a v o r s the attachment of f e r r o u s i o n on p a r t i a l c e l l u l o s e xanthate and other i o n exchange groups p r e s e n t . (The term " p a r t i a l c e l l u l o s e x a n t h a t e s " stands f o r xanthates bound s t a t i s t i c a l l y t o any component of the pulp ( c e l l u l o s e , h e m i c e l l u l o s e , l i g n i n ) . The importance of the presence of f e r r o u s i o n s on g r a f t i n g i s shown in Diagrams I - I I - I I I , which show r e a c t i o n mechanism sug-


17.

KOKTA ET AL.

Xanthate Method

273

100

r


6 ζ g oc

+

Ρ

80 À

100

80

6oJ

60

I LU

O it

LU

LU

O 40

40

O 5 i-

LL

The Xanthate Method of Grafting - ACS Symposium Series (ACS

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