13 Application of GPC to Studies of the Viscose Process Part V . Effect of Heat on Rayon Properties JOHN DYER F M C Corp., Fiber Division, R & D , Marcus Hook, Penn. 19061
Summary In this p a p e r , r e s u l t s from s t u d i e s o f the e f f e c t of h e a t i n g regenerated cellulose f i b e r s having all-skin and all-core (not p o l y n o s i c ) s t r u c t u r e s on some fiber p r o p e r t i e s are d i s c u s s e d . The s t u d i e s were made by h e a t i n g the f i b e r s at 184°C in the presence o f air f o r 0, 1, 3 and 16 h o u r s . The p r o p e r t i e s measured i n c l u d e d wet and c o n d i t i o n e d tenacity, single fiber flex life, water imbibition, moisture r e g a i n , basic degree o f p o l y m e r i z a t i o n , cross-section staining characteristics and the m o l e c u l a r weight distribution by g e l p e r m e a t i o n chromatography. Results indicated t h a t fiber p r o p e r t i e s were i n f l u e n c e d by s t r u c t u r e , b a s i c DP and m o l e c u l a r weight distribution; i.e. by the s i z e of cellulose m o l e c u l e s and the way they are a r r a n g e d and h e l d t o g e t h e r in the fiber. Many o f the p r o p e r t y changes c o u l d be r e l a t e d to s t r u c t u r a l change and d e g r a d a t i o n t h a t o c c u r r e d on h e a t i n g . F o r most o f the h e a t e d s a m p l e s , the weight average DP determined by GPC was significantly g r e a t e r t h a n the v a l u e determined viscosimetrically. E x p l a n a t i o n s are suggested f o r these o b s e r v a t i o n s . Introduction Rayon i s a manufactured f i b e r composed o f r e g e n e r ated c e l l u l o s e . Many t y p e s h a v i n g d i f f e r e n t p r o p e r t i e s can be made from c e l l u l o s e wood p u l p by the v i s c o s e rayon p r o c e s s . Some examples o f commercial f i b e r s and t h e i r p r o p e r t i e s are shown i n T a b l e 1. The p r o p e r t i e s are d e t e r m i n e d by the s i z e o f the c e l l u l o s e m o l e c u l e s and by the way they are a r r a n g e d and h e l d t o g e t h e r i n the f i b e r . Changes i n the f i b e r s t r u c t u r e caused by c o n d i t i o n s o f end-use a l t e r the p r o p e r t i e s and 181
CELLULOSE TECHNOLOGY RESEARCH
ω H EH
w P-i
Ο P-i
Ο
< < Ο Η
Ρ-4 >Η
Ο
m
13.
DYER
Viscose Process
183
i n f l u e n c e the u s e f u l l i f e o f the p r o d u c t . The most u s u a l p r o p e r t i e s determined from the r e sponse to t e n s i l e s t r e s s are the t e n a c i t y , extensib i l i t y , modulus and b r e a k i n g energy - the work to r u p t u r e c a l c u l a t e d from the a r e a beneath the s t r e s s s t r a i n curve. As might be e x p e c t e d , t h e s e can be r e l a t e d to s t r u c t u r a l parameters such as the degree o f p o l y m e r i z a t i o n (DP), c r y s t a l l i n i t y , and m o l e c u l a r orientation. A d d i t i o n a l i n f o r m a t i o n about s t r u c t u r e can be o b t a i n e d by d i f f e r e n t i a l s t a i n i n g o f c r o s s s e c t i o n s and from water i m b i b i t i o n and m o i s t u r e r e g a i n measurements. But the c o r r e l a t i o n of f i b e r p r o p e r t i e s w i t h s t r u c t u r e i s extremely complex. It i s v i r t u a l l y i m p o s s i b l e t o change one v a r i a b l e w i t h o u t a f f e c t i n g others. In many end-use a p p l i c a t i o n s , heat i s produced by v a r i o u s m e c h a n i c a l p r o c e s s e s such as b e n d i n g , f l e x i n g , s t r e s s i n g and a b r a d i n g , and by s u n l i g h t and o t h e r irradiation. A p p r e c i a b l e t h e r m a l exposure can a l s o o c c u r d u r i n g the c a r e c y c l e such as i n l a u n d e r i n g and drying. In i n d u s t r i a l and commercial a p p l i c a t i o n s , e . g . conveyor b e l t s , d r i v e b e l t s and t i r e s , the f i b e r s are o f t e n s u b j e c t e d to much h i g h e r temperature and without p r o p e r i n s u l a t i o n and p r o t e c t i o n w i l l d e g r a d e . In s t u d i e s o f the e f f e c t o f heat on r a y o n p r o p e r t i e s , i t has been the u s u a l p r a c t i c e to f o l l o w weight l o s s e s under d i f f e r e n t c o n d i t i o n s w i t h or w i t h o u t i d e n t i f i c a t i o n o f the d e g r a d a t i o n p r o d u c t s . This i n f o r m a t i o n has then been used w i t h measurements on v a r i o u s p h y s i c a l and c h e m i c a l p r o p e r t i e s o f the f i b e r to e x p l a i n the s t r u c t u r a l and c h e m i c a l changes t h a t o c c u r on h e a t i n g . Without a t e d i o u s f r a c t i o n a t i o n , o n l y v e r y l i m i t e d i n f o r m a t i o n about the s i z e o f the c e l l u l o s e m o l e c u l e s can be o b t a i n e d from v i s c o s i t y measurements. G e l p e r m e a t i o n chromatography has been used t o measure the DP d i s t r i b u t i o n o f c e l l u l o s e at v a r i o u s s t a g e s o f the v i s c o s e r a y o n p r o c e s s and the changes t h a t occur under c o n d i t i o n s s i m u l a t i n g e n d - u s e . (1,2,3) In t h i s study o f the e f f e c t o f heat on r a y o n p r o p e r t i e s , e v i d e n c e has been o b t a i n e d t h a t d a t a from the GPC measurement can be i n f l u e n c e d by d i f f e r e n c e s in fiber structure. The e x i s t e n c e o f two s t r u c t u r a l l y d i f f e r e n t r e g i o n s i n the f i b e r c r o s s s e c t i o n o f T a b l e 1 has been r e v e a l e d by a d i f f e r e n t i a l s t a i n i n g t e c h n i q u e (4_). They are known as s k i n and c o r e . Skin i s normally found at and near the s u r f a c e o f the f i l a m e n t and surrounds the c o r e . I t i s p o s s i b l e to v a r y the s t r u c t u r e from a l l - c o r e to a l l - s k i n , depending on the
184
CELLULOSE TECHNOLOGY RESEARCH
c o n d i t i o n s used to make the f i b e r . The q u e s t i o n a r o s e whether s k i n and c o r e would have d i f f e r e n t t h e r m a l response. I t was r e c o g n i z e d t h a t the response o f the f i b e r to v a r i o u s t r e a t m e n t s would a l s o depend on o t h e r f a c t o r s such as m o l e c u l a r o r i e n t a t i o n , m o l e c u l a r s i z e , the t o t a l c r y s t a l l i n i t y and d i s t r i b u t i o n o f o r d e r e d and d i s o r d e r e d r e g i o n s . In t h i s p a p e r , o n l y the a l l core ( e x c l u d i n g p o l y n o s i c ) and a l l - s k i n f i b e r s w i l l be compared w i t h r e s u l t s b e i n g o b t a i n e d f o r t h r e e samples o f each t y p e . I t i s emphasized t h a t the e x p e r i m e n t a l f i b e r s , which are at the extremes o f the s t r u c t u r a l l i m i t s , d e v i a t e i n p r o p e r t i e s from commercial t y p e s . Experimental The experiments were made by h e a t i n g the f i b e r at 1 8 4 ° C i n an a i r o v e n . The sample as a c o n t i n u o u s f i b e r bundle (214 f i l a m e n t s , 330 t o t a l d e n i e r ) was h e l d between two 7 - i n c h s t a i n l e s s s t e e l d i s c s p l a c e d 12 i n c h e s a p a r t by l o o p i n g over a s e r i e s o f 240 notches cut i n the edge o f the d i s c s . Before l o a d i n g , 1300 gram compression was a p p l i e d t o the s p r i n g - l o a d e d top d i s c which was then clamped i n p l a c e . After l o a d i n g , the top d i s c was r e l e a s e d and the system a l l o w e d t o e q u i l i b r a t e b e f o r e p l a c i n g i n an a i r oven set at l 8 4 ° C . There was some heat l o s s on p l a c i n g the sample i n the o v e n , the system t a k i n g about 20 minutes to r e g a i n thermal e q u i l i b r i u m . T h i s temperature p r o f i l e was s i m i l a r f o r a l l the samples. After treat ment, the sample was a l l o w e d t o c o o l t o room tempera ture . Samples f o r a n a l y s i s and t e s t i n g were cut as 1 0 - i n c h l e n g t h s from between the d i s c s , d i s c a r d i n g the f i b e r t h a t had been i n c o n t a c t w i t h the m e t a l . F o r the m o l e c u l a r weight d i s t r i b u t i o n measurement, the samples were n i t r a t e d and d i s s o l v e d i n t e t r a h y d r o furan. F r a c t i o n a t i o n was a c h i e v e d on the b a s i s o f s i z e alone u s i n g a Waters 100 GPC u n i t w i t h a s e r i e s o f f o u r columns c o n t a i n i n g c r o s s l i n k e d p o l y s t y r e n e r e s i n beads o f Ι Ο , Ι Ο , 10 * and 1 0 A ° pore s i z e . The equipment and p r o c e d u r e s have been d e s c r i b e d i n d e t a i l elsewhere ( 1 ) . V a l u e s f o r the number, w e i g h t , ζ and z+1 average D P s were c a l c u l a t e d from the c h r o m â t o g r a m s u s i n g a c a l i b r a t i o n curve based on the DP o f c e l l u l o s e samples determined v i s c o s i m e t r i c a l l y . I t i s p o i n t e d out t h a t t h i s p r o c e d u r e does not g i v e a b s o l u t e v a l u e s o f the DP s but does measure the r e l a t i v e changes r e s u l t i n g from the v a r i o u s t r e a t m e n t s t o which the samples have been s u b j e c t e d . 6
5
1
1
1
3
13.
185
Viscose Process
DYER
R e s u l t s and D i s c u s s i o n A comparison o f the p r o p e r t i e s f o r round and a l l - c o r e f i b e r s i s g i v e n i n T a b l e 2. Table 2
Some P r o p e r t i e s
Tc
Ec
M
Tw
all-skin
o f A l l - s k i n and A l l - c o r e Samples
Ew
SM
Flex
H0 2
H0 2
Retn Reg
S k i n 4.58 10.9 Core 2.71
127 3.10 28.5 2.82 6055 79.0 12.4 760 4.9 126 1.21 11.7 8.36 423 86.2 11.3 735
I t has been e s t a b l i s h e d t h a t s k i n c o n t a i n s numerous s m a l l c r y s t a l l i t e s and the core fewer and l a r g e r c r y s t a l l i t e s (5.). The same r e l a t i o n s h i p h o l d s f o r the amorphous o r low o r d e r r e g i o n s i n t h e s e structures. S i n c e the average c e l l u l o s e m o l e c u l a r c h a i n l e n g t h (DP) i s s i m i l a r f o r b o t h samples i n T a b l e 2, the m o l e c u l e s w i l l pass t h r o u g h many more o r d e r e d r e g i o n s i n the a l l - s k i n f i b e r . The g r e a t e r s t r e n g t h o f the a l l - s k i n f i b e r i s thus a t t r i b u t e d t o the number o f o r d e r e d r e g i o n s w i t h which each c e l l u lose molecule i s a s s o c i a t e d . The arrangement o f c e l l u l o s e m o l e c u l e s between numerous s m a l l o r d e r e d and d i s o r d e r e d r e g i o n s produces a s t r u c t u r e t h a t i s more e x t e n s i b l e t h a n the c o r e . Not o n l y are the s m a l l e r c r y s t a l l i t e s more a b l e t o s l i p p a s t one a n o t h e r , because w i t h r e l a t i v e l y s m a l l c o n t a c t s u r f a c e a r e a , t h e r e w i l l be fewer hydrogen b o n d s ; but t i e s between the c r y s t a l l i t e s w i l l be l e s s l i k e l y t o r e s t r i c t t h e i r movement r e l a t i v e t o t h e i r l e n g t h . Skin swells l e s s than c o r e . Because the c e l l u l o s e m o l e c u l e s are f i x e d i n the c r y s t a l l i t e s at more f r e quent i n t e r v a l s , t h e r e w i l l be l e s s freedom o f the s m a l l amorphous a r e a s t o s w e l l . S i n c e i t i s i n the more open amorphous r e g i o n s o f the s t r u c t u r e t h a t water w i l l be r e t a i n e d on c e n t r i f u g i n g , water i m b i b i t i o n i s lower i n s k i n than f o r the c o r e . On the other hand, moisture r e g a i n i s h i g h e r . This i s e x p l a i n e d by an i n c r e a s e d number o f h y d r o x y l groups a v a i l a b l e f o r b o n d i n g w i t h water as a r e s u l t o f the l a r g e r t o t a l s u r f a c e a r e a o f the more numerous smaller c r y s t a l l i t e s . The r a t i o o f the wet t o c o n d i t i o n e d t e n a c i t y has been used t o e s t i m a t e the p r o p o r t i o n o f a c c e s s i b l e
186
CELLULOSE TECHNOLOGY RESEARCH
hydrogen bonds i n the amorphous r e g i o n s (6) . I f the i n i t i a l s t r u c t u r e s have s i m i l a r m o l e c u l a r o r i e n t a t i o n and c r y s t a l l i n i t y , t h e n t h i s r a t i o w i l l a l s o q u a l i t a t i v e l y r e f l e c t the r e l a t i v e amount o f s k i n and core i n the s t r u c t u r e . In the wet s t a t e , s w e l l i n g d i s r u p t s the s t r u c t u r e at the s u r f a c e o f the o r d e r e d r e g i o n s and causes c h a i n ends t o become detached r e d u c i n g the number o f t i e s between the c r y s t a l l i t e s . Core s w e l l s more t h a n s k i n and s i n c e t h e r e are fewer l a r g e r o r d e r e d r e g i o n s , the r e d u c t i o n i n t e n a c i t y i s much greater than f o r s k i n . The water i m b i b i t i o n and the m o i s t u r e r e g a i n f o r the samples used i n the study are shown as a f u n c t i o n o f Tw/Tc i n F i g u r e 1. The l e s s s k i n , the lower Tw/Tc and m o i s t u r e r e g a i n and the g r e a t e r water i m b i b i t i o n . A r a t h e r s t r i k i n g o b s e r v a t i o n t h a t was made c o n c e r n e d the s t a i n i n g c h a r a c t e r i s t i c s o f the f i b e r s . A f t e r 16 hours h e a t i n g , every sample s t a i n e d as a l l skin. S t a i n e d c r o s s s e c t i o n s f o r f o u r o f the samples are shown i n F i g u r e 2. The change i n s t a i n i n g c h a r a c t e r i s t i c s was r e l a t e d t o the h e a t i n g time w i t h s k i n f o r m a t i o n p r o g r e s s i n g from the o u t s i d e o f the filament. A f i r s t r e a c t i o n t o t h i s o b s e r v a t i o n was t h a t an a l l - s k i n f i b e r c o u l d be made from an a l l - c o r e f i b e r s i m p l y by h e a t i n g . But on e x a m i n a t i o n o f the m e c h a n i c a l p r o p e r t i e s , i t became e v i d e n t t h a t the t r a n s f o r m a t i o n i n v o l v e d more than a s i m p l e c o n v e r s i o n o f c o r e to s k i n . The r a t i o o f wet t o c o n d i t i o n e d t e n a c i t y was used as a measure o f the s k i n - c o r e r a t i o . The e f f e c t o f h e a t i n g on t h i s measurement f o r a l l - s k i n and a l l - c o r e f i b e r s i s g i v e n i n T a b l e 3. Heating Table 3
Hours at p.84°C
H e a t i n g ; E f f e c t on the Wet t o C o n d i t i o n e d Tenacity Ratio 1
Tw/Tc
1
All
Skin
2
3
J
1
All
2
Core
1
3
0
.677
.653
.605
.445
.458
.446
j
1
.642
.628
.502
.444
.470
• 591 .516
.428
.403
.389
.375 .384
.461
.465 .422
j
.602
.380
.324
i
3 16
caused a d e c r e a s e i n the r a t i o f o r b o t h f i b e r t y p e s , c o n t r a r y t o the expected i n c r e a s e i f core were b e i n g
j
DYER
Viscose Process
Figure 1.
Water imbibition, moisture regain and wet to conditioned tenacity ratio
Figure 2.
Stained cross sections
188
CELLULOSE TECHNOLOGY RESEARCH
converted to s k i n . I n i t i a l l y , the r a t i o was much g r e a t e r f o r the a l l - s k i n f i b e r , but a f t e r h e a t i n g f o r 16 h o u r s , the r a t i o s were s i m i l a r f o r b o t h t y p e s , the most d e g r a d a t i o n h a v i n g o c c u r r e d i n the a l l - s k i n f i b e r . The water i m b i b i t i o n f o r t h e s e same samples i s given i n Table 4 . The measurement i s made by Table 4
H e a t i n g ; E f f e c t on Water I m b i b i t i o n
Hours at
% H 0 Retention 2
All
184°C 1
All
Skin 2
3
1
2
Core 3
0
79.0
76.3
79.2
93.6
87.7
86.2
1
76.8
76.8
77.8
81.0
79.2
85.3
3
74.9
81.2
80.8
74.6
69.2
77.8
16
70.5
74.0
75.6
61.8
61.6
61.9
c e n t r i f u g i n g the wet sample under s t a n d a r d c o n d i t i o n s t o determine the amount o f water h e l d by the s t r u c t u r e . The c r y s t a l l i n e areas w i l l have a v e r y low water r e t e n t i o n ; a s s o c i a t i o n between c e l l u l o s e m o l e c u l e s i n t h e s e o r d e r e d r e g i o n s w i l l l i m i t the a c c e s s i b i l i t y and swelling. I t i s i n the d i s o r d e r e d or amorphous r e g i o n s , where s w e l l i n g i s p o s s i b l e , t h a t m o i s t u r e w i l l be r e t a i n e d . The water i m b i b i t i o n can thus be used as an e s t i m a t e o f the c r y s t a l l i n i t y w i t h low values corresponding to higher o r d e r . Because o f the i n c l u s i o n o f a l a r g e number o f s m a l l m i c r o s c o p i c v o i d s , the s k i n s t r u c t u r e i s l e s s dense than c o r e . T h i s i s one o f the reasons I t s w e l l s l e s s and s h r i n k s more. A higher moisture r e t e n t i o n , such as shown i n i t i a l l y f o r the a l l - c o r e f i b e r , i s usually associated with greater swelling. But, a f t e r h e a t i n g , the a l l - c o r e f i b e r s e x h i b i t a lower water r e t e n t i o n than the a l l - s k i n f i b e r s . T h i s suggests t h a t s t r u c t u r a l changes such as c r y s t a l l i t e growth and c r o s s l i n k i n g can o c c u r more e a s i l y and to a g r e a t e r e x t e n t w i t h an a l l - c o r e s t r u c t u r e . M o i s t u r e r e g a i n on the o t h e r hand measures the number o f f r e e h y d r o x y l groups i n the s t r u c t u r e . These can be the h y d r o x y l groups exposed on the s u r f a c e o f the c r y s t a l l i t e and h y d r o x y l groups on the " t i e " c h a i n s and m o l e c u l e s i n the amorphous r e g i o n s . C r y s t a l l i t e growth and c r o s s l i n k i n g d u r i n g h e a t i n g w i l l i n v o l v e m o s t l y the c h a i n s i n the amorphous a r e a s
13.
189
Viscose Process
DYER
and cause a r e d u c t i o n i n the m o i s t u r e r e g a i n . The e x t e n t o f t h i s r e d u c t i o n w i l l depend on t h e s i z e and number o f c r y s t a l l i t e s i n t h e o r i g i n a l s t r u c t u r e . Table 5
Heating; Effect
Hours at
on M o i s t u r e R e g a i n
M o i s t u r e R e g a i n (*) All
108°C 1
All
Skin 2
3
1
Core
2
3
0
12. 4
12.1
12.2
10.8
10.8
11. 3
1
12.1
13.5
12.1
10.5
10.4
10. 6
3
11.9
11.6
10.9
10.2
10.1
10. 6
16
11.2
10.6
10.9
9.4
10.4
9. 7
The m o i s t u r e r e g a i n d a t a g i v e n i n T a b l e 5 i s consistent with t h i s i n t e r p r e t a t i o n . In the a l l - s k i n s t r u c t u r e , the c r y s t a l l i t e s a r e s m a l l e r but more numerous. There i s l e s s m a t e r i a l a v a i l a b l e i n t h e amorphous a r e a s and t h e r e f o r e c r y s t a l l i t e growth on h e a t i n g i s r e s t r i c t e d and a c r o s s l i n k e d s t r u c t u r e c o n t a i n i n g many s m a l l v o i d s w i l l be f o r m e d . Under t h e same c o n d i t i o n s , an a l l - c o r e s t r u c t u r e which c o n t a i n s fewer but l a r g e r o r d e r e d and d i s o r d e r e d r e g i o n s w i l l form a more compact s t r u c t u r e by c r y s t a l l i z a t i o n and crosslinking. The s h r i n k a g e t h a t o c c u r r e d d u r i n g h e a t i n g i n some cases caused the a l l - c o r e f i b e r s t o break. Accompanying t h e s e s t r u c t u r a l changes caused by h e a t i n g , t h e t e n a c i t y and e x t e n s i b i l i t y i n b o t h c o n d i t i o n e d and wet s t a t e s d e c r e a s e . Since c r y s t a l l i t e growth and c r o s s l i n k i n g i n c r e a s e t h e s t r e n g t h , t h e s e o b s e r v a t i o n s suggest t h a t o t h e r p r o c e s s e s such as m o l e c u l a r d e g r a d a t i o n have a l s o occurred. The a r e a beneath the s t r e s s - s t r a i n curve i s reduced and t h e b r e a k i n g energy o r toughness o f t h e f i b e r i s lowered. A consequence o f t h i s i s reduced s i n g l e f i b e r f l e x - l i f e ; with increased h e a t i n g , both a l l - s k i n and a l l - c o r e f i b e r s become b r i t t l e . This i s shown i n T a b l e 6 .
190
CELLULOSE TECHNOLOGY RESEARCH
Table 6
Heating; Effect
Hours at 184°C
0 1
on S i n g l e F i b e r F l e x
Flex L i f e All
2
6055
3750
3037
4488
927
1210
63
20
3 16
(Cycles A l l Core
Skin
1
Life
1
2
3
5570
250
4687 4018
113 Br
423
102
8
8
Br
Br
3
Br
95 3 Br
H e a t i n g caused a s u b s t a n t i a l r e d u c t i o n i n the degree o f p o l y m e r i z a t i o n (DP). The measurements g i v e n i n T a b l e 7 were determined v i s c o s i m e t r i c a l l y u s i n g copper e t h y l e n e d i a m i n e as a s o l v e n t . The r e l a t i o n s h i p between the time o f h e a t i n g and the r e c i p r o c a l DP was Table 7
H e a t i n g : E f f e c t on B a s i c Degree o f P o l y m e r i z a t i o n ( V i s c o s i t y Method)
Hours at 184°C
DP All
All
Skin
1
Core
2
3
1
2
3
0
760
790
720
580
600
735
1
580
580
570
515
525
3 16
440
460
425
175
165
155
395 150
405 160
605 460 140
1.53
1.60
1.74
1.75
1.61
2.04
Rate Sec x x
10
7
I !
l i n e a r i n d i c a t i n g t h a t the d e g r a d a t i o n was random and obeyed f i r s t o r d e r k i n e t i c s . The r a t e s shown i n t h i s t a b l e were c a l c u l a t e d from the e q u a t i o n : 1
1
- k *
They appear to be r e a s o n a b l y independent o f the s t r u c t u r e and degree o f p o l y m e r i z a t i o n o f the o r i g i n a l samples. T h i s i s i n agreement w i t h e a r l i e r
13.
DYER
191
Viscose Process
observations that i r r a d i a t i o n depolymerization occurs e q u a l l y i n r e g i o n s o f low and h i g h o r d e r . D u r i n g the i n i t i a l h e a t i n g , water was d r i v e n from the sample. The s t r u c t u r e was annealed p r o b a b l y by a p r o c e s s o f c r y s t a l l i z a t i o n and the s t r e n g t h i n c r e a s e d w i t h some l o s s i n e x t e n s i b i l i t y . This i s a charac t e r i s t i c o f a l l r a y o n f i b e r s , the s t r e n g t h i n c r e a s i n g and e x t e n s i b i l i t y d e c r e a s i n g as water i s removed from the s t r u c t u r e . At h i g h t e m p e r a t u r e , the presence o f m o i s t u r e w i l l cause a p p r e c i a b l e h y d r o l y t i c d e g r a d a t i o n . Under the c o n d i t i o n s used i n t h e s e e x p e r i m e n t s , h e a t i n g at 184°C i n an a i r oven f o r up t o 16 h o u r s , d e g r a d a t i o n o c c u r r e d as e v i d e n c e d by the y e l l o w / b r o w n c o l o r o f the samples a f t e r h e a t i n g . The weight l o s s , e s t i m a t e d from d e n i e r measure ments, was l e s s t h a n 2% a f t e r 3 hours at l 8 4 ° C f o r both core and s k i n f i b e r s . A f t e r 16 hours h e a t i n g , a 5% weight l o s s was e s t i m a t e d f o r the a l l - s k i n f i b e r but the a l l - c o r e f i b e r was extremely b r i t t l e and d e n i e r measurements c o u l d not be made. The weight average degree o f p o l y m e r i z a t i o n , Mw, from GPC measurements i s g i v e n i n T a b l e 8. Table 8
H
°
U
r
184°
H e a t i n g , E f f e c t on Weight Average Degree o f P o l y m e r i z a t i o n (GPC)
Mw, (100
S
C
/
/
2
1
1234
( 6 2 )
1
1059
( 8 3 )
3
1026 3 1 1
I
A l l Skin
0
16
(Mw - DP)/DP)
( 1 3 3 )
(78)
148
1 1 7 6
9 ( 8 8 )
1 3 M
1155 i
3
(
1
3
2
( 1 5 1 )
* (200) 95
)
1 0 4 8
(
(
976 4 1 0
6
8
3
)
4
)
1 618 673
( 1 3 0 ) 7 2 0
(164)
2 5 6
All
Core
2
3
( 7 )
720
( 2 Q )
873
( 3 1 )
760
( 4 5 )
915
(80) (7D
9 2
3
\l28)
* (1ΐ6) 5
8 7 6
2 i , 1
(
1
9
;
( 5 7 ;
(90: (72:
These v a l u e s are a l l g r e a t e r t h a n the b a s i c degree of p o l y m e r i z a t i o n , DP (Table 7) by the p e r c e n t a g e shown in parenthesis. T h i s can be a t t r i b u t e d to s e v e r a l causes. T h u s , the DP was o b t a i n e d on each sample by a v i s c o m e t r i c method. Theoretically, viscosity m o l e c u l a r weight d a t a i s o n l y v a l i d f o r d i s t r i b u t i o n s which are G a u s s i a n . The GPC d a t a which i s c a l c u l a t e d from the measured d i s t r i b u t i o n i s not s e n s i t i v e t o
CELLULOSE TECHNOLOGY RESEARCH
192
t h i s parameter. Consequently, only with a Gaussian d i s t r i b u t i o n w i l l Mw and DP be the same. In o t h e r work, i t has been shown t h a t e l e c t r o n beam i r r a d i a t i o n o f c e l l u l o s e y i e l d s a p r o d u c t s u s c e p t i b l e to a l k a l i n e d e g r a d a t i o n . An a d d i t i o n a l d e p o l y m e r i z a t i o n amounting t o about a 10% DP l o s s on d i s s o l v i n g the i r r a d i a t e d p r o d u c t i n a l k a l i n e cuene c o u l d be p r e v e n t e d by sodium b o r o h y d r i d e r e d u c t i o n . I t i s p r o b a b l e t h a t the e x t e n t o f a l k a l i n e d e g r a d a t i o n w i l l be i n f l u e n c e d by the number o f c a r b o n y l and c a r b o x y l groups formed d u r i n g i r r a d i a t i o n . D i f f e r e n c e s i n the v i s c o m e t r i c and GPC d a t a are to be expected i f the samples c o n t a i n s t r u c t u r a l u n i t s t h a t are r e s i s t a n t t o a c i d h y d r o l y s i s and u n s t a b l e i n alkali. F o r the unheated s a m p l e s , the d i f f e r e n c e s between Iw and ÏÏP f o r the a l l - c o r e f i b e r s are much s m a l l e r t h a n f o r the a l l - s k i n f i b e r s ( T a b l e 8 ) . T h i s suggests t h a t s o l u b i l i z i n g the samples by n i t r a t i o n i s i n f l u e n c e d by c r y s t a l l i t e s i z e and number which are known t o be d i f f e r e n t f o r s k i n and c o r e s t r u c t u r e s . Incomplete s e p a r a t i o n o f the m o l e c u l e s from the c r y s t a l l i t e s d u r i n g d i s s o l u t i o n would r e s u l t i n h i g h Mw v a l u e s . The e x t e n t t o which t h i s can be caused by poor n i t r a t i o n t e c h n i q u e s or by the presence o f i m p u r i t i e s i n the sample has yet to be c l a r i f i e d . The GPC d a t a showed a d i f f e r e n t response to h e a t i n g f o r the a l l - s k i n _ a n d a l l - c o r e f i b e r s . In the t h r e e a l l - s k i n s a m p l e s , Mw d e c r e a s e d on h e a t i n g . With the a l l - c o r e f i b e r s , Mw i n c r e a s e d d u r i n g the f i r s t few hours and t h e n d e c r e a s e d . These d i f f e r e n t responses appear t o be the r e s u l t o f the amount o f m a t e r i a l i n the amorphous areas o f the o r i g i n a l s t r u c t u r e a v a i l a b l e f o r c r y s t a l l i t e g r o w t h , c r o s s l i n k i n g , and o t h e r s t r u c t u r a l changes. In a l l c a s e s , h e a t i n g i n c r e a s e d the d i f f e r e n c e betweenJ§w and DT w i t h the d i f f e r e n c e , as a p e r c e n t a g e o f the DP, b e i n g g r e a t e s t f o r the a l l - s k i n f i b e r s . High Hw v a l u e s are t o be expected i f the s o l u t i o n o f n i t r a t e d c e l l u l o s e i n j e c t e d i n t o the chromatograph c o n t a i n s c r y s t a l l i t e fragments or g e l s . In some c a s e s , the l a r g e r p a r t i c l e s would not be f r a c t i o n a t e d i n the chromatograph, the pore s i z e s i n the column p a c k i n g h a v i n g been chosen t o g i v e the b e s t f r a c t i o n a t i o n for discrete molecules. T h i s would r e s u l t i n the appearance o f a "prehump" o r peak i n the c h r o m â t o g r a m p r e c e d i n g e l u t i o n o f the f r a c t i o n a t e d m a t e r i a l (7.). P a r t i c l e s s m a l l e r than the l a r g e s t pore s i z e would be f r a c t i o n a t e d as l a r g e m o l e c u l e s . Prehump s were not observed f o r any o f the s a m p l e s , 5
13.
193
Viscose Process
DYER
i n d i c a t i n g t h a t any s t r u c t u r a l r e s i d u e s s o l u t i o n s were b e i n g f r a c t i o n a t e d . Table 9
i n the
H e a t i n g ; E f f e c t , o n DP D i s t r i b u t i o n
Hours at 184°C
DP D i s t r i b u t i o n A l l Skin DP Mn
Mw
A l l Core Mz Mz+1 DT
Μη
Mw
Mz
873 1 9 6 3 4647
0
760
422
1234
3723
7590
735
406
1
580
355
1059
3741
8042
605
354
915
440
3 1 1 1026 4078 8845 460 119 311 868 2177140
305
8 7 6 3292
107
241
3 16
175
2741
479
Mz+1
6635
7872 832
The DP d i s t r i b u t i o n s c a l c u l a t e d as t h e f i r s t moments o f t h e p a r t i c u l a r d i s t r i b u t i o n f u n c t i o n s such as number, w e i g h t , ζ and z + 1 d i s t r i b u t i o n s f o r an a l l s k i n and an a l l - c o r e f i b e r o f s i m i l a r o r i g i n a l DP are compared i n T a b l e 9 . I t can be shown t h e o r e t i c a l l y t h a t t h e number a v e r a g e , Mn, i s i n s e n s i t i v e t o changes i n t h e number o f l a r g e m o l e c u l e s . This, indeed, i s the o b s e r v a t i o n t h a t can be made on t h e s e r e s u l t s , Mn b e i n g s i m i l a r f o r b o t h s t r u c t u r a l t y p e s . The o r i g i n a l s a m p l e s , b e f o r e h e a t i n g , s t a i n e d as a l l - s k i n and a l l - c o r e . . A f t e r 1 6 hours h e a t i n g c a u s i n g d e g r a d a t i o n and s t r u c t u r a l c h a n g e s , the samples b o t h s t a i n e d as a l l - s k i n . The change o f Mz and Mz+1 on h e a t i n g i n d i c a t e s d i f f e r e n c e s i n the number o f l a r g e m o l e c u l e s and p a r t i c l e s i n t h e sample. On t h e one h a n d , t h e more numerous and s m a l l e r c r y s t a l l i t e s o f the a l l - s k i n f i b e r r e s t r i c t t h e s t r u c t u r a l changes and v e r y l i t t l e change i s o b s e r v e d i n Mz and Mz+1 before degradation. On the o t h e r h a n d , the l a r g e r amorphous r e g i o n s i n t h e a l l - c o r e f i b e r a l l o w e x t e n s i v e s t r u c t u r a l changes and Mz and Mz+1 i n c r e a s e c l o s e t o the v a l u e s o b s e r v e d i n the a l l - s k i n f i b e r before degradation. Conclusions Changes i n the m o l e c u l a r weight d i s t r i b u t i o n on h e a t i n g a s e r i e s o f a l l - c o r e and a l l - s k i n r a y o n f i b e r s i n the p r e s e n c e o f a i r i n d i c a t e d t h a t s t r u c t u r a l r e arrangements o c c u r r e d d u r i n g t h e r m a l d e g r a d a t i o n . The e x t e n t o f s t r u c t u r a l rearrangement was r e l a t e d t o
194
CELLULOSE TECHNOLOGY RESEARCH
the c r y s t a l l i t e s i z e and number and to the amount o f m a t e r i a l i n the amorphous a r e a s o f the o r i g i n a l structures. F o r a l l - c o r e f i b e r s which have more e x t e n s i v e d i s o r d e r e d r e g i o n s than a l l - s k i n f i b e r s , the c r o s s s e c t i o n s s t a i n e d as a l l - s k i n a f t e r h e a t i n g . C h e m i c a l and m e c h a n i c a l p r o p e r t i e s o f the f i b e r s were a l s o a f f e c t e d by h e a t i n g i n agreement w i t h the i n d i cated s t r u c t u r a l changes. The r a t e o f d e g r a d a t i o n c a l c u l a t e d from the DP l o s s was s i m i l a r f o r b o t h f i b e r types. Large d i f f e r e n c e s between the b a s i c degree o f p o l y m e r i z a t i o n measured v i s c o s i m e t r i c a l l y and the weight average DP from GPC were a t t r i b u t e d to d e v i a t i o n from a G a u s s i a n d i s t r i b u t i o n o f m o l e c u l a r w e i g h t , t o the p r e s e n c e o f s t r u c t u r a l u n i t s t h a t were r e s i s t a n t t o a c i d h y d r o l y s i s and u n s t a b l e i n a l k a l i and t o i n c o m p l e t e s e p a r a t i o n o f the m o l e c u l e s from the c r y s t a l l i t e s o r s t r u c t u r e s d u r i n g d i s s o l u t i o n o f the n i t r a t e d c e l l u l o s e sample. Literature 1. 2. 3.
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(1971)
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4. Morehead, F . F., ASTM Bull. 163, 54 (1950) 5. S i s s o n , Wayne A . , T e x t i l e R e s . J., 30., 153 (1960) 6. Bingham, B.E.M., Makromol. Chem., 77, 139 (1964) 7.
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