Changes in Cellulose Structure during Manufacture and Converting of

11 Changes in Cellulose Structure during Manufacture and

Downloaded by KTH ROYAL INST OF TECHNOLOGY on December 2, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0048.ch011

Converting of Paper E. L. AKIM Leningrad Technological Institute of Pulp and Paper Industry, Leningrad, U.S.S.R.

A c h a r a c t e r i s t i c f e a t u r e o f development o f t h e p u l p and paper i n d u s t r y d u r i n g t h e l a s t decades was a wide u s e o f achievements o f c h e m i s t r y , physico-chemi s t r y and t e c h n o l o g y o f polymers and c l o s e r e l a t i o n s h i p between t h e t e c h n o l o g y o f paper manufacture and c o n v e r t i n g and t h e t e c h n o l o g y o f polymer m a t e r i a l s i n g e n e r a l and o f c h e m i c a l f i b e r s and f i l m s i n p a r t i c u l a r . The p r i n c i p a l t r e n d s i n t h i s f i e l d were: f i r s t , t h e p r e p a r a t i o n o f composite m a t e r i a l s " p a p e r - s y n t h e t i c polymer**, such as **paper-f ilm** complexes and l a m i n a t e d p l a s t i c s , s e c o n d l y , t h e manufacture o f s y n t h e t i c p a p e r s which i n c l u d e p a p e r s from s y n t h e t i c f i b e r s , s y n t h e t i c paper f i l m s and papers from t h e s o - c a l l e d ' s y n t h e t i c pulp" ( i . e . , a synthetic material imitating fibrous s t r u c t u r e o f c e l l u l o s e ) and, f i n a l l y , c h e m i c a l m o d i f i c a t i o n o f p a p e r . As a r e s u l t , t h e p u l p and p a p e r i n d u s t r y o f today, a p a r t f r o m t r a d i t i o n a l p r o c e s s e s and equipment, i s c h a r a c t e r i z e d by p r o c e s s e s and equipment s i m i l a r t o t h o s e i n c h e m i c a l i n d u s t r y , such as manuf a c t u r e o f p h o t o g r a p h i c and motion p i c t u r e f i l m and magnetic t a p e , manufacture and p r o c e s s i n g o f c h e m i c a l f i b e r s and p l a s t i c s . F i b r o u s and n o n - f i b r o u s s y n t h e t i c and n a t u r a l h a I f - f i n i s h e d p r o d u c t s used f o r paper manufacture as w e l l as methods o f treatment and c o n v e r t i n g o f paper are very v a r i e d . N e v e r t h e l e s s , they a r e a l l based on c h e m i s t r y and p h y s i c o c h e m i s t r y o f p o l y m e r s . The p r o g r e s s i n t h i s f i e l d i s based on t h e i n v e s t i g a t i o n and s o l u t i o n o f r e l a t i v e l y few f u n d a m e n t a l p r o b l e m s . One o f them i s t h e s t r e n g t h development i n a paper s h e e t . I r r e s p e c t i v e o f the type o f the h a l f - f i n i s h e d 153

In Cellulose Chemistry and Technology; Arthur, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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p r o d u c t s used ( n a t u r a l o r s y n t h e t i c ) , paper ( w i t h t h e e x c e p t i o n o f s y n t h e t i c paper f i l m ) i s a f i b r o u s porous m a t e r i a l i n which not o n l y f i b e r s t h e m s e l v e s , b u t a l s o f i b e r - t o - f i b e r bonds e n s u r e p h y s i c o m e c h a n i c a l p r o p e r t i e s of the material. Paper s t r e n g t h i s m a i n l y d e t e r mined by t h e s t r e n g t h o f f i b e r - t o - f i b e r bonds and t h e r e f o r e t h e i r n a t u r e s h o u l d be c o n s i d e r e d i n d e t a i l . For p a p e r s from n a t u r a l c e l l u l o s e f i b e r s , t a k i n g i n t o account t h e i r long s u c c e s s f u l manufacture, t h i s prob]sn i s mainly t h e o r e t i c a l . However, f o r p a p e r s from s y n t h e t i c f i b e r s o r " s y n t h e t i c c e l l u l o s e " t h i s problem i s o f p r i m a r y p r a c t i c a l importance, s i n c e i t s s o l u t i o n determines s u c c e s s f u l s o l u t i o n o f p u r e l y t e c h n o l o g i c a l problems. I t i s advisable to begin the c o n s i d e r a t i o n o f t h i s problem w i t h o r d i n a r y p a p e r . For s e v e r a l decades t h e development o f paper s t r e n g t h was o f g r e a t i n t e r e s t f o r s c i e n t i s t s o f many c o u n t r i e s . V a r i o u s a s p e c t s o f t h i s problem have been i n v e s t i g a t e d more o r l e s s t h o r o u g h l y and t h e e x i s t i n g t h e o r i e s have been c o n s i d e r e d i n d e t a i l i n many monog r a p h s . N e v e r t h e l e s s , one a s p e c t o f t h i s problem has h a r d l y been d e a l t w i t h f o r a long t i m e . T h i s i s t h e problem o f changes i n t h e p h y s i c a l ( r e l a x a t i o n a l ) s t a t e of c e l l u l o s e i n t h e c o u r s e o f paper manufacture. For h i s t o r i c r e a s o n s some problems o f p h y s i c a l c h e m i s t r y o f polymers v i r t u a l l y have n o t been c o n s i d ered f o r c e l l u l o s e . T h i s was c a u s e d by an a r t i f i c i a l d i v i s i o n o f p h y s i c o - c h e m i s t r y and polymers i n g e n e r a l and t h a t o f c e l l u l o s e i n p a r t i c u l a r . Many p a p e r s and books o p e n l y o r i n a v e i l e d form adhered t o t h e i d e a t h a t c e l l u l o s e i s an e x c e p t i o n a l polymer s y n t h e s i z e d by n a t u r e i t s e l f and t h a t t h e o r e t i c a l c o n c e p t s d e v e l o p ed i n d e t a i l f o r s y n t h e t i c polymers a r e i n a p p l i c a b l e t o it. A l l t h i s c e r t a i n l y c o n c e r n s t h e problems o f changes i n t h e p h y s i c a l ( r e l a x a t i o n a l ) s t a t e o f c e l l u l o s e , i t s g l a s s t r a n s i t i o n t e m p e r a t u r e s and p o s s i b i l i t i e s of i t s transition into a highly e l a s t i c state. N e v e r t h e l e s s , b e f o r e c o n s i d e r i n g problems r e l a t e d t o the p h y s i c a l s t a t e o f c e l l u l o s e , we s h o u l d b r i e f l y d e a l w i t h i t s phase s t a t e . A t p r e s e n t t h e r e a r e s e v e r a l v i e w p o i n t s on t h i s problem. A c c o r d i n g t o one o f them, c e l l u l o s e i s r e g a r d e d as a f a u l t y c r y s t a l ( i n t h i s c a s e i t i s i m p o s s i b l e t o speak about any p h y s i c a l transitions i n cellulose). A c c o r d i n g t o t h e most



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w i d e l y h e l d and e x p e r i m e n t a l l y c o n f i r m e d v i e w p o i n t c e l l u l o s e i s a c r y s t a l l i z i n g polymer w i t h c o m p a r a t i v e l y w e l l d e f i n e d c r y s t a l l i n e and amorphous r e g i o n s . The p r e s e n c e o f amorphous r e g i o n s p e r m i t s us t o speak about the g l a s s t r a n s i t i o n temperature o f c e l l u l o s e and about a p o s s i b i l i t y o f i t s t r a n s i t i o n i n t o a h i g h ly e l a s t i c state. There a r e v e r y few works d e a l i n g w i t h the g l a s s t r a n s i t i o n temperature o f c e l l u l o s e . Nevertheless, a l l s e r v i c e p r o p e r t i e s o f polymer m a t e r i a l s and t h e i r behavior i n the process o f mechanical, physico-chemi c a l and c h e m i c a l treatment a r e c l o s e l y r e l a t e d t o tem p e r a t u r e ranges o f p h y s i c a l s t a t e s . C o n s e q u e n t l y , t h e d e t e r m i n a t i o n o f t h e s e ranges, the i n v e s t i g a t i o n o f p o s s i b i l i t i e s o f t r a n s i t i o n o f c e l l u l o s e from one phys i c a l s t a t e i n t o another i s o f d e c i s i v e importance f o r cellulose materials also. In 1949 i n the work o f K a r g i n and co-workers (1) i t was shown t h a t t h e g l a s s t r a n s i t i o n temperature o f c e l l u l o s e i s 220° C. T h i s f i g u r e was o b t a i n e d i n an i n d i r e c t way by e x t r a p o l a t i n g t h e dependences o f g l a s s t r a n s i t i o n temperatures o f c e l l u l o s e samples on amounts o f p l a s t i c i z e r t o i t s z e r o amount. L a t e r t h i s v a l u e has been c o n f i r m e d by a d i r e c t method (2J . The r e s u l t s o b t a i n e d by K a r g i n s u g g e s t e d t h a t under normal c o n d i t i o n s c e l l u l o s e i s i n a g l a s s y s t a t e and s i n c e the g l a s s t r a n s i t i o n t e m p e r a t u r e i s h i g h e r than t h e tem p e r a t u r e o f degradation of c e l l u l o s e m a t e r i a l s i n a i r , i t f o l l o w s that i t i s impossible to transform i t i n t o a h i g h l y e l a s t i c s t a t e without degradation. However, i f i t i s i m p o s s i b l e to a c h i e v e t h i s by h e a t i n g , i n p r i n c i p l e i t i s p o s s i b l e t o do t h i s by p l a s t i f i c a t i o n . One o f t h e methods o f p l a s t i f i c a t i o n i s t o b r i n g t h e polymer sample i n t o c o n t a c t and i n t o a s o r p t i o n e q u i l i b r i u m w i t h t h e p l a s t i c i z i n g l i q u i d medium. Thus, Thus, Bryant and W a l t e r have shown t h a t f o r c e l l u l o s e m a t e r i a l s i n water t h e v a l u e s o f e l a s t i c modulus c o r r e s p o n d t o v a l u e s c h a r a c t e r i s t i c o f polymers i n a h i g h l y e l a s t i c s t a t e (.3). At t h e end o f the s i x t i e s we have i n v e s t i g a t e d the e f f e c t o f l i q u i d media on t h e temperature o f t r a n s i t i o n from a g l a s s y t o a h i g h l y e l a s t i c s t a t e f o r a number o f p o l y m e r s ( 4 ) . For c e l l u l o s e t h e s e i n v e s t i g a t i o n s have been c a r r i e d out i n c o l l a b o r a t i o n w i t h


CELLULOSE CHEMISTRY AND TECHNOLOGY

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transition: wet c e l l u l o s e - d r y c e l l u l o s e i s o f p a r t i c u l a r importance. Wet c e l l u l o s e i s i n a h i g h l y el a s t i c s t a t e whereas d r y c e l l u l o s e i s i n a g l a s s y state. Hence, a change i n t h e p h y s i c a l s t a t e o f c e l l u lose occurs i n the course o f i t s d r y i n g . Conventional methods o f c e l l u l o s e d r y i n g combine p r o c e s s e s o f d r a i n age (water removal) and d r y i n g p r o p e r (removal o f t h e l i q u i d wetting c e l l u l o s e ) . The g l a s s t r a n s i t i o n o f c e l l u l o s e caused by t h e removal o f p l a s t i c i z e r and water o c c u r s under c o n d i t i o n s o f a c o n s i d e r a b l e s h r i n k age s t r e s s . In t h e manufacture o f d i s s o l v i n g p u l p shrinkage s t r e s s e s play a negative part leading t o the f o r m a t i o n o f a dense ( " h o r n i f i e d " ) l a y e r on t h e f i b e r s u r f a c e which makes f u r t h e r p r o c e s s i n g d i f f i c u l t . Table I G l a s s t r a n s i t i o n temperatures o f c e l l u l o s e p r e p a r a t i o n s i n d i f f e r e n t media T of T of No. P l a s t i c i z i n g medium viscose cotton, fiber, °C. °C. g

1 2 3 4 5 6 7 8 9 ΙΟ 11 12 13 14 15 16 17 18 19

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220-230 80 >80 140 >140 >140 60 100 125 40 120 140 >55

c

220-230

Changes in Cellulose Structure during Manufacture and Converting of

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