A Critical Review of Cellulose Solvent Systems - ACS Publications

2 A Critical Review of Cellulose Solvent Systems A.F.TURBAK, R. B. HAMMER, R. E. DAVIES, andN.A.PORTNOY

Downloaded by UNIV OF BATH on June 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0058.ch002

ITT Rayonier, Inc., Eastern Research Div., Whippany,N.J.07981

C e l l u l o s e is the most abundant renewable, organic raw material a v a i l a b l e in the world today. Y e t ,forallit'savailability, i t has still not reached its potential utility in many areas of application. One o f the major reasons f o r t h i s is t h a t many end-use a p p l i c a t i o n s r e q u i r e t h a t c e l l u l o s e be in a different form from that found in nature. I n most of these applications, it is necessary first to dissolve cellulose in some manner and then t o re-form it from such s o l u t i o n s i n t o the d e s i r e d products. It is this very important d i s s o l v i n g step which has proved to be either cumbersome o r expensive compared t o alternate m a t e r i a l s which compete for market p o s i t i o n s . In many cases only c e l l u l o s e has the d e s i r a b l e p r o p e r t i e s r e q u i r e d f o r end product use and, in these i n s t a n c e s , the methods r e q u i r e d t o achieve cellulose s o l u t i o n present p o t e n t i a l hazards and pollution control problems. Thus, improved techniques f o r d i s s o l v i n g cellul o s e are u r g e n t l y needed if cellulose is to continue t o occupy a c o m p e t i t i v e market position. This is particularly t r u e o f the v i s c o s e i n d u s t r y where pollution control continues to p l a c e ever i n c r e a s i n g restraints on the process. Scientists have long recognized the need f o r more efficient cellulose s o l v e n t systems and hundreds of p u b l i c a t i o n s have been i s s u e d covering a wide range of approaches. S e v e r a l e x c e l l e n t review articles on s w e l l i n g and dissolving cellulose by Warwicker (1,2) Jayme ( 3 ) , Phillip ( 4 , 5 ) , P o l y o l a (6) and Brandrup (7) have been p u b l i s h e d and we shall not attempt simply t o resubmit t h e i r data. Rather, t h i s paper shall attempt t o consider the v a r i o u s processes from the commercial as well as the scientific viewpoint t o emphasize potential areas f o r c o n t r i b u t i o n which still e x i s t , particularly f o r systems capable of producing cellulosic fibers through the use of organic s o l v e n t s . Fundamentally, a l l o f the known methods f o r d i s s o l v i n g c e l l u l o s e can be summarized under f o u r main c a t e g o r i e s : A. C e l l u l o s e As A Base B. C e l l u l o s e As An A c i d C. C e l l u l o s e Complexes D. C e l l u l o s e D e r i v a t i v e s 12

Turbak; Solvent Spun Rayon, Modified Cellulose Fibers and Derivatives ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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While each of the above areas has r e c e i v e d e x t e n s i v e study and some have been commercialized, s e v e r a l of the approaches are r e a l l y not completely understood. S t i l l others i n v o l v e mixtures of many i n g r e d i e n t s and could never be v a l u a b l e i n d u s t r i a l l y . In t r y i n g to e x p l a i n why c e r t a i n m a t e r i a l s do i n f a c t d i s s o l v e c e l l u l o s e , s e v e r a l authors have s i m p l i s t i c a l l y i n d i c a t e d that what i s r e q u i r e d i s that the s o l u b i l i t y parameter of the s o l v e n t must be the same as t h a t of c e l l u l o s e . While such s t a t e ments are d i r e c t i o n a l l y c o r r e c t , they are incomplete and, perhaps, even a l i t t l e m i s l e a d i n g , s i n c e many s o l v e n t s having the proper s o l u b i l i t y parameter values w i l l not d i s s o l v e c e l l u l o s e . For example, DMF and DMSO both have s o l u b i l i t y parameters i n the range c a l c u l a t e d f o r c e l l u l o s e but n e i t h e r o f these d i s s o l v e c e l l u l o s e under any known c o n d i t i o n s . The s o l u b i l i t y parameter ( i ) or cohesive energy d e n s i t y (CED) values r e l a t e d i r e c t l y to the energy of v a p o r i z a t i o n which i s e a s i l y o b t a i n a b l e f o r many l i q u i d s , and w i l l apply d i r e c t l y t o the m i s c i b i H t y of l i q u i d s o r amorphous polymers where (&) values w i l l r e l a t e to simple heats of m i x i n g . When d e a l i n g w i t h polymers i n g e n e r a l , other f a c t o r s may be e q u a l l y as important, o r even more important than s i n g l e s o l u b i l i t y parameter v a l u e s . For example, w i t h c r y s t a l l i n e polymers, the heat o f f u s i o n or m e l t i n g energy, must a l s o be considered as an e n t i r e l y separate f a c t o r . Furthermore, superimposed on these f a c t o r s i s the hydrogen bonding c a p a b i l i t y of the polymer i n q u e s t i o n which may not be c o n s e q u e n t i a l f o r polymers such as c r y s t a l l i n e polypropylene, but which i s extremely important w i t h n a t u r a l polymers such as p r o t e i n s and c e l l u l o s e i n p a r t i c u l a r . These f a c t o r s , as w e l l as second order e f f e c t s were recognized many years ago by Spur l i n , B u r r e l l , and Barton and s e v e r a l exc e l l e n t reviews are a v a i l a b l e f o r f u r t h e r reference (8,9,10)· A combination of such f a c t o r s must be considered i n t r y i n g t o f i n d a s o l v e n t f o r c e l l u l o s e and help to g i v e guidance as t o c o n d i t i o n s as w e l l as compounds which must be employed i f usable c e l l u l o s e s o l u t i o n s are to be o b t a i n e d . Even i f a m a t e r i a l i s a s a t i s f a c t o r y s o l v e n t f o r c e l l u l o s e , i t i s necessary to emphasize two other important f a c t o r s which must be s e r i o u s l y evaluated f o r any p o t e n t i a l l y commercial c e l l u l o s e s o l v e n t system, these are "recovery" and " r e c y c l e " . The recovery and r e c y c l e f a c t o r s , perhaps more than any o t h e r s , are the most important ones which, i n the f i n a l a n a l y s i s , decide whether or not any p a r t i c u l a r approach w i l l be economically feasible. One f i n a l f a c t o r must a l s o be kept i n mind r e l a t i v e t o e v a l u a t i n g a l l o f the d a t a t h a t appear i n the l i t e r a t u r e r e garding c e l l u l o s e s o l v e n t s . S p e c i f i c a l l y i t must be emphasized t h a t v a s t d i f f e r e n c e s e x i s t between being able to coagulate o r


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p r e c i p i t a t e a s o l i d mass, as compared to t r u l y spinning f i b e r s w i t h reasonable p h y s i c a l p r o p e r t i e s . Keeping these f a c t o r s i n mind, l e t us examine the f o u r c a t e g o r i e s l i s t e d above which encompass e s s e n t i a l l y a l l of the c e l l u l o s e s o l v e n t systems reported i n the l i t e r a t u r e .


A.

C e l l u l o s e As A Base

A l l a l c o h o l s , whether polymeric or not, can be f o r c e d t o act as acids or bases r e l a t i v e to other reagents, depending upon the s t r e n g t h of the reagent employed. Thus, c e l l u l o s e w i l l act as a base and become protonated by p r o t o n i c a c i d s or w i l l act as a "Lewis" base and supply e l e c t r o n s from i t s oxygens t o e l e c t r o n r e c e p t i v e centers on "Lewis" a c i d s . 1. P r o t o n i c Acids (phosphoric, s u l f u r i c , n i t r i c ) . The a b i l i t y of 78% phosphoric a c i d , 68% n i t r i c a c i d , 42% h y d r o c h l o r i c a c i d or 70% s u l f u r i c a c i d to d i s s o l v e c e l l u l o s e r a p i d l y at room temperature i s w e l l known. While the l i t e r a t u r e i s f u l l of r e p o r t s i n which the r e s u l t i n g s o l u t i o n s are r e f e r r e d to as hyd r a t e s " , they can a l s o be regarded as protonated hydroxyIs where the s p e c i f i c reagent concentrations employed were the ones needed to provide s u f f i c i e n t a c i d s t r e n g t h to protonate the c e l l u l o s e so that the r e s u l t i n g p o s i t i v e l y changed ROHj c e l l u l o s e moiety i s able to d i s s o l v e i n the excess reagent. I n these cases, recovery and r e c y c l e of the concentrated a c i d s i s the l i m i t i n g f a c t o r , economically. For example, e x c e l l e n t c e l l u l o s e s o l u t i o n s can be r a p i d l y prepared a t room temperature using phosphoric a c i d . However, no one has yet suggested a f e a s i b l e approach to c a s t i n g the c e l l u l o s e and recovering the phosphoric a c i d without n e u t r a l i z a t i o n . In our l a b o r a t o r y , we have t r i e d to p r e c i p i t a t e such phosphoric a c i d s o l u t i o n s i n t o g l a c i a l a c e t i c a c i d i n the a n t i c i p a t i o n that the a c e t i c a c i d could be v o l a t i l i z e d , recovered and r e c y c l e d , along w i t h the r e l e a s e d pure phosphoric a c i d . The i n i t i a l r e s u l t s were not too s u c c e s s f u l s i n c e the p r e c i p i t a t i o n was too slow to be u s e f u l under the p a r t i c u l a r c o n d i t i o n s emp l o y e d . However, the concept of using a weaker, v o l a t i l e a c i d i c m a t e r i a l to coagulate strong a c i d c e l l u l o s e s o l u t i o n s does r e present a n o v e l approach to developing a t o t a l r e c y c l a b l e c e l l u l o s e s o l v e n t system. lf

2. Lewis Acids ( z i n c c h l o r i d e , thiocyanates. iodides» b r o ~ mides). The a b i l i t y of c e r t a i n s a l t s o l u t i o n s to s w e l l and d i s s o l v e c e l l u l o s e i s a l s o reported (11). At the high concentrat i o n s normally necessary to achieve s o l u t i o n , these s a l t s not only provide necessary a c i d f u n c t i o n a l i t y but a l s o s i g n i f i c a n t l y a l t e r the i o n i c nature of the aqueous mediums so as to f u r t h e r a s s i s t d i s s o l u t i o n . The need f o r having a c i d s a l t s i s demonstrated i n the case of thiocyanates where the sodium, potassium and ammonium thiocyanates do not d i s s o l v e c e l l u l o s e w h i l e the calcium and


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s t r o n t i u m s a l t s do g i v e c e l l u l o s e s o l u t i o n s up to about the 400 D.P. l e v e l i n d i c a t i n g t h a t i t i s the t h i o c y a n i c a c i d formed i n s o l u t i o n which i s a c t u a l l y i n v o l v e d i n the d i s s o l v i n g a c t i o n . B.

C e l l u l o s e As An A c i d


Perhaps more work has been undertaken on s w e l l i n g c e l l u l o s e w i t h bases than w i t h any other c l a s s of chemical compounds. The hydrogen atoms on the hydroxyIs of c e l l u l o s e , l i k e most a l c o h o l s , have a degree o f a c i d c h a r a c t e r and t h e r e f o r e r e a d i l y i n t e r a c t w i t h reasonably strong i n o r g a n i c and organic bases. 1. Inorganic Bases (sodium z i n c a t e , hydrazine, i n o r g a n i c hydroxides). I t should be immediately noted that the conc e n t r a t i o n of base needed f o r o b t a i n i n g maximum s w e l l i n g of c e l l ulose by osmotic a c t i o n i s not the same c o n c e n t r a t i o n that i s r e q u i r e d to e f f e c t a c t u a l compound formation. Thus w h i l e 8-10% NaOH e x h i b i t s the maximum s w e l l i n g o f c e l l u l o s e f i b e r s , concent r a t i o n s of about 18% NaOH are a c t u a l l y r e q u i r e d t o form sodium c e l l u l o s a t e s t r u c t u r e s . Other i n o r g a n i c bases l i k e potassium and l i t h i u m hydroxide have a l s o been used s i m i l a r l y f o r t h e i r s w e l l i n g a c t i o n . Various a d d i t i v e s t o the c a u s t i c soda, such as z i n c oxide, have been used to make sodium z i n c a t e s o l u t i o n s which enhance the a c t i o n of c a u s t i c so higher D.P. m a t e r i a l s can be d i s s o l v e d . The temperatures o f these aqueous bases p l a y a f u r t h e r dominant r o l e i n the s w e l l i n g and d i s s o l v i n g phenomenon; w i t h the c o l d e r c o n d i t i o n s g i v i n g more s w e l l i n g and d i s s o l u t i o n . While a c o n s i d e r a b l e amount o f e f f o r t has been devoted t o aqueous a l k a l i systems, they normally do not d i s p l a y s u f f i c i e n t d i s s o l v ing power to completely overcome the c r y s t a l and hydrogen bonding energies of c e l l u l o s e t o g i v e acceptable s o l u t i o n s of h i g h e r D.P. m a t e r i a l s of i n t e r e s t f o r d i r e c t commercial conversion. I n a d d i t i o n , these aqueous a l k a l i n e systems present s e v e r a l r e covery and r e c y c l e problems. The use of the i n o r g a n i c base, h y d r a z i n e , f o r s w e l l i n g c e l l ulose was considered by Hess and Trogus many years ago.(12) However, they were never able to o b t a i n c e l l u l o s e s o l u t i o n s under the c o n d i t i o n s they employed. More r e c e n t l y , L i t t (13) has r e ported c o n d i t i o n s under which he has been able t o o b t a i n comp l e t e s o l u t i o n s of high D.P. c e l l u l o s e i n hot (150°-200°C) hydrazine and 207-345 kPa (30-50 p s i ) p r e s s u r e . These r e s u l t s serve to demonstrate t h a t s e v e r a l f a c t o r s are important f o r a s w e l l i n g reagent t o become a s o l v e n t . The o r i g i n a l i n v e s t i g a t i o n s i n 1931 by Hess employed hydrazine hydrate but d i d not employ the more extreme c o n d i t i o n s of heat and temperature r e c e n t l y employed by L i t t i n 1976 t o o b t a i n s o l u t i o n s . L i t t was able to get s o l u t i o n s of c e l l u l o s e u s i n g e i t h e r pure hydrazine or hydrazine hydrate p r o v i d i n g he employed elevated temperatures and p r e s s u r e s . I t was noted p r e v i o u s l y that c r y s t a l l i n e f o r c e s


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and hydrogen bonding could be important f a c t o r s and t h i s case c l e a r l y demonstrates t h a t even a m a t e r i a l having the proper s o l u b i l i t y parameter had t o be employed under f o r c i n g c o n d i t i o n s t o overcome these f a c t o r s t o achieve s o l u t i o n . I t has y e t t o be demonstrated, however, i f t e x t i l e q u a l i t y yarns o r packaging q u a l i t y f i l m s can be prepared from such hydrazine s o l u t i o n s . C e r t a i n l y , t o our knowledge no data along these l i n e s have y e t been p u b l i s h e d . 2. Organic Bases ( T r i t o n - q u a t e r n a r y h y d r o x i d e s , amines, DMSO/CH^NH?, amine o x i d e s ) . The a b i l i t y of c e l l u l o s e t o a c t as an a c i d towards v a r i o u s o r g a n i c bases has a l s o been e v a l u a t e d . Perhaps the b e s t known organic bases a r e the v a r i o u s " T r i t o n " bases which are quaternary ammonium hydroxides and the best s o l vent of t h i s s e r i e s i s " T r i t o n B" o r benzyltrimethylammoniurn hyd r o x i d e . I t i s i n t e r e s t i n g again t o note here t h a t the b e n z y l grouping i s f a r more e f f e c t i v e than other groups such as m e t h y l , e t h y l o r p r o p y l and t h i s Improvement has been a t t r i b u t e d t o the f a c t t h a t the b u l k i e r b e n z y l group behaves l i k e a "wedge" t o e f f e c t i v e l y separate the c e l l u l o s e chains once i t has entered the c r y s t a l l i n e region. A c o n s i d e r a b l e amount of work has a l s o been r e p o r t e d by S e g a l and others (14) r e l a t i v e t o the use of v a r i o u s amines and diamines f o r s w e l l i n g c e l l u l o s e . None o f these systems were claimed t o cause s u f f i c i e n t s w e l l i n g t o g i v e c e l l u l o s e s o l u t i o n s . More r e c e n t l y , P h i l l i p and h i s co-workers (15,16) have r e p o r t e d t h a t 16.5% methylamine i n DMSO gave b e t t e r c e l l u l o s e s o l u t i o n s than any other amine o r any other c o n c e n t r a t i o n o f methylamine used. The c e l l u l o s e i s claimed t o d i s s o l v e by r e a c t i o n w i t h an equimolar complex of CH3NH2/DMSO and 80% of the i n t r o d u c e d c e l l u l o s e d i s s o l v e d under c o l d anhydrous c o n d i t i o n s . As exc i t i n g as t h i s seems a t f i r s t g l a n c e , d i s s o l u t i o n of only 80% of the c e l l u l o s e may not be o f v a l u e f o r commercial c o n s i d e r a t i o n . Unless the d i s s o l u t i o n of c e l l u l o s e i s a c t u a l l y b e t t e r than 99%, the system would c e r t a i n l y be too expensive t o use r e l a t i v e t o y i e l d and f i l t r a t i o n c o s t s . The s p e c i f i c i t y of CH3NH2 and i n p a r t i c u l a r the 16.5% c o n c e n t r a t i o n i s i n t r i g u i n g and deserves more study. Perhaps, t h i s system might more p r o p e r l y belong under c o n s i d e r a t i o n as an organic complex r a t h e r than as a pure base system, but more data are needed t o f i r m l y decide the exa^t nature of t h e r e a c t i o n . One other organic base system which d i s s o l v e s c e l l u l o s e as an a c i d i n v o l v e s compounds not normally considered as bases, b u t which i n f a c t , are v e r y good "Lewis" bases. I n 1939, Graenacher and Sallman (17) reported t h a t a l i p h a t i c and c y c l o a l i p h a t i c amine oxides such as t r i e t h y l a m i n e oxide o r c y c l o h e x y l d i m e t h y l amine o x i d e gave 7-10% s o l u t i o n s of c e l l u l o s e a t 50-90°C. More r e c e n t l y , Johnson (18) has r e p o r t e d t h a t a l i c y c l i c amine oxides such as N-methylmorpholine oxide g i v e up t o 6% s o l u t i o n s o f c e l l u l o s e a t 110°C. These m a t e r i a l s are most i n t e r e s t i n g and could


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o f f e r commercial p o s s i b i l i t i e s i f adequate recovery and r e c y c l e procedures c o u l d be e s t a b l i s h e d . To our knowledge, no one has yet p u b l i s h e d any a c t u a l f i b e r o r f i l m d a t a on products d e r i v e d from such systems. Thus, a wide range of i n o r g a n i c and organic bases have been examined f o r d i s s o l v i n g and s w e l l i n g c e l l u l o s e . S e v e r a l systems have achieved a s u f f i c i e n t l y good balance of p r o p e r t i e s t o pro v i d e good c e l l u l o s e s o l u t i o n s and i t i s expected t h a t f u r t h e r e f f o r t s w i l l be forthcoming t o achieve a c c e p t a b l e r e c o v e r y / r e c y c l e and p h y s i c a l p r o p e r t i e s which could l e a d t o i n d u s t r i a l acceptance. C.

C e l l u l o s e Complexes

1. I n o r g a n i c Complexes (cuene, cadoxene, EWNN, cuprammonium). The use of v a r i o u s copper complexes t o d i s s o l v e c e l l u l o s e i s w e l l known. More r e c e n t l y , complexes of cadmium and i r o n have been added t o the l i s t t o reduce the s e n s i t i v i t y of such c e l l u l o s e s o l u t i o n s t o degradation by exposure t o a i r . T h i s work i s reviewed r a t h e r completely by Jayme (3) who, w i t h h i s c o workers, has c o n t r i b u t e d e x t e n s i v e l y t o t h i s a r e a . I n o r g a n i c m e t a l l i c complexes such as cuprammonium have met w i t h o n l y minimal commercial success f o r p r o d u c t i o n f i b e r s and f i l m s f o r two reasons. F i r s t , complete recovery of m e t a l l i c e f f l u e n t contaminants i s d i f f i c u l t a t the extremely low ppm l e v e l s needed t o meet p o l l u t i o n requirements and secondly the o v e r a l l economics and f i b e r p r o p e r t i e s (of the cuprammonium p r o c e s s , a t l e a s t ) were not as good as those of the v i s c o s e p r o c e s s . The second f a c t o r would today be l e s s important than the recovery and p o l l u t i o n aspect and, u n t i l t h i s aspect i s s o l v e d , metal i n o r ganic complexes w i l l continue t o have l i m i t e d u t i l i t y f o r f i l m and f i b e r p r o d u c t i o n . 2 . Organic Complexes (DMSO/CH3NH2, (KOCH2CHOHCH9S) 2. The p o s s i b l e use o f organic complexes t o d i s s o l v e c e l l u l o s e appears to be extremely l i m i t e d . Of a l l the systems r e p o r t e d , o n l y two seem to q u a l i f y as organic complexes. The f i r s t of these uses DMSO/16.5% CH3NH2 and was p r e v i o u s l y d i s c u s s e d under category Β (above) s i n c e i t s a c t i o n may be r e l a t e d more t o i t s b a s i c nature than t o a t r u e complex f o r m a t i o n . The second system was r e p o r t e d by P e t r o v i n 1965 (19) who c l a i m s t h a t c e l l u l o s e d i s s o l v e s d i r e c t l y a t 110°C i n a n e u t r a l s o l v e n t - b i s (φ-Y dihydroxypropyl) d i s u l f i d e - prepared by o x i d a t i o n of t h i o g l y c e r o l . T h i s s o l v e n t works w e l l f o r d i s s o l v i n g cellophane f i l m s a t 300-600 D.P., but i s not good f o r d i r e c t l y d i s s o l v i n g h i g h e r D.P. c o t t o n l i n t e r s o r r e g u l a r p u r i f i e d wood p u l p s . I n any case, i t i s i n t r i g u i n g t o f i n d a n e u t r a l s o l v e n t t h a t appears to be capable o f d i r e c t l y d i s s o l v i n g reason ably h i g h D.P. c e l l u l o s e and f u r t h e r work should be undertaken t o a s c e r t a i n why t h i s p a r t i c u l a r s t r u c t u r e seems t o be e f f e c t i v e .


SOLVENT SPUN RAYON, MODIFIED CELLULOSE FD3ERS

18

D.

Cellulose Derivatives

1. S t a b l e D e r i v a t i v e s ( e s t e r s , e t h e r s ) , A wide v a r i e t y of s t a b l e c e l l u l o s e d e r i v a t i v e s are known and used commercially. The e s t e r s and e t h e r s have found scores of commercial uses but only the acetate and n i t r a t e have ever been u t i l i z e d f o r producing regenerated c e l l u l o s e products; the acetate t o g i v e F o r t i s a n and the n i t r a t e t o make regenerated t u b u l a r c e l l u l o s e f i l m s .


2,

Unstable D e r i v a t i v e s , a.

Sulfur

(xanthates, SC^/amines - s u l f i t e s )

b.

Nitrogen

(DMF/N2O4, DMSO/N 0 - n i t r i t e s )

c.

Carbon

(carbonates, formates, DMSO/(CH 0)x me thy l o i )

2

4

2

Unstable c e l l u l o s e d e r i v a t i v e s have been and are being a c t i v e l y i n v e s t i g a t e d i n depth f o r use i n p r e p a r i n g regenerated f i b e r s and f i l m s . This paper w i l l c o n s i d e r such " t r a n s i e n t " d e r i v a t i v e s under three main headings: a) s u l f u r - b) n i t r o g e n and c) carbon-containing i n t e r m e d i a t e s . a) S u l f u r Intermediates: The use of CS to form xanthates some 90 years ago s t i l l forms the backbone of the present day rayon i n d u s t r y . Improved processes are u r g e n t l y needed which can not only overcome the v a r i o u s p o l l u t i o n problems a s s o c i a t e d w i t h the v i s c o s e process, but which might a l s o h o p e f u l l y lower both the i n i t i a l c a p i t a l investment and subsequent o p e r a t i n g c o s t s a s s o c i a t e d w i t h present day rayon p l a n t s . One attempt along these l i n e s i s reported by Kimura et a l (20) who used a m o d i f i e d organic type system f o r x a n t h a t i o n . They used DMSO/CS2/amine to d i s s o l v e c e l l u l o s e and reported f i b e r s having c o n d i t i o n e d and wet t e n a c i t y / e l o n g a t i o n of 3.0 g/d/14% and 1.8 g/d/25% r e s p e c t i v e l y . T h e i r c o a g u l a t i o n and r e g e n e r a t i o n steps d i d not i n v o l v e the use of a c i d s and thus would s i g n i f i c a n t l y reduce part of the p o l l u t i o n l o a d normally a s s o c i ated w i t h rayon p r o d u c t i o n . However, i n i t i a l l a b o r a t o r y attempts to reproduce the r e p o r t e d process gave r i s e to an extremely noxious odor and t h i s could represent s u f f i c i e n t detriment t o o f f s e t other p o s s i b l e advantages. As another p o s s i b l e approach to d i s s o l v i n g c e l l u l o s e , seve r a l i n v e s t i g a t o r s have s t u d i e d the use of S02/amine s o l v e n t s y s tems. E x t e n s i v e work i n t h i s area i s reported by P h i l l i p and h i s coworkers (21) by Yanazaki and Nakao (22) and by Hata and Yokota (23-29). T h e i r e f f o r t s covered a wide range of amines and organic s o l v e n t d i l u e n t s which were both p o l a r and non-polar. 2


2.

TURBAK E T A L .


19


While i n i t i a l l y i t was thought t h a t some type of complex was being formed w i t h the c e l l u l o s e , i t i s now reasonably establ i s h e d that t h i s treatment r e s u l t s i n the formation of amine s a l t s of the r a t h e r unstable c e l l u l o s e s u l f i t e e s t e r . Again f i l a m e n t s r e p o r t e d l y were spun, but no f i b e r p h y s i c a l p r o p e r t i e s were r e p o r t e d . The use of s u l f i t e e s t e r s f o r p r e p a r i n g rayon i n preference t o the v i s c o s e system would c r i t i c a l l y depend on good recovery of a l l s t a r t i n g m a t e r i a l s , and t h i s may be an area worthy of both chemical and engineering e f f o r t . b) Nitrogenous Intermediates: The use o f u n s t a b l e n i t r o genous i n t e r m e d i a t e s f o r a c h i e v i n g c e l l u l o s e s o l u t i o n i n organic s o l v e n t s i s concentrated mainly i n the area of systems i n v o l v i n g n i t r o g e n oxides. I n i t i a l l y n i t r o g e n oxides were used t o o x i d i z e c e l l u l o s e t o the 6-carboxy d e r i v a t i v e . The o r i g i n a l work o f Kenyon, Y a c k e l , Unruh, Fowler, and McGee i n the 40 s stands as a milestone i n t h i s a r e a . (30-34) More r e c e n t l y , Pavlyuchenko and Ermolenko and coworkers have presented f u r t h e r d e t a i l e d s t u d i e s on the use of N2O4 f o r c e l l u l o s e o x i d a t i o n . (35-39) I n 1947, Fowler e t a l (34) reported t h a t v a r i o u s s o l v e n t s , when used i n c o n j u n c t i o n w i t h N2O4, gave d i f f e r e n t responses t o the a c t i o n of t h i s reagent on c e l l u l o s e and found t h a t many s o l v e n t s a c t u a l l y gave r i s e to s o l u t i o n s o f c e l l u l o s e without h i g h degrees o f o x i d a t i o n o c c u r r i n g . They used over 40 d i f f e r e n t s o l v e n t s where the N2O4 t o s o l v e n t r a t i o s were a t l e a s t 1/1 or h i g h e r and f i n a l l y concluded that the amount of o x i d a t i o n v s . c e l l u l o s e s o l u t i o n was r e l a t e d t o the p o l a r i t y of the s o l v e n t employed. With non-polar s o l v e n t s such as CCI4 e t c . the N2O4 produced mostly o x i d a t i o n w h i l e the more p o l a r s o l v e n t s gave r i s e t o c e l l ulose s o l u t i o n s w i t h g r e a t l y diminished o x i d a t i o n . I f no s o l vents were employed and l a r g e excesses of c o l d l i q u i d N2O4/N2O3 mixtures were used, then c e l l u l o s e d i s s o l v e d w i t h v e r y l i t t l e o x i d a t i o n and could be recovered from such s o l u t i o n s e s s e n t i a l l y c h e m i c a l l y unchanged except f o r a D.P. l o s s . T h i s was f i r s t r e ported by H i a t t and Crane i n 1949 (40) and subsequently s t u d i e d i n great d e t a i l by Chu i n 1970.(41) F o l l o w i n g these c l a s s i c d i s c l o s u r e s by Fowler, other r e searchers began t o examine even more p o l a r s o l v e n t s f o r use w i t h N2O4. Thus, W i l l i a m s (42) s t u d i e d the use o f DMSO/N2O4 as a s o l v e n t system f o r c e l l u l o s e . I t was l a t e r demonstrated r a t h e r c l e a r l y by Hergert and Z o p o l i s (43) t h a t the DMSO/N2O4 system would d i s s o l v e c e l l u l o s e more e f f e c t i v e l y i f t h e r e was a s m a l l amount of water present t o keep the c e l l u l o s e s t r u c t u r e open f o r r e a c t i o n . S u r p r i s i n g l y very l i t t l e more work has been done w i t h the DMSO/N2O4 system up t o the present time. Subsequently, Nakao obtained patent coverage on the use of DMF/N2O4 mixtures t o d i s s o l v e c e l l u l o s e (44,45) and on the a d d i t i o n s o f a wide range o f other polymers i n DMF t o such c e l l u l o s e s o l u t i o n s to produce s p e c i a l types of products. Mahomed (46) 1



20

S O L V E N T S P U N R A Y O N , MODIFIED C E L L U L O S E FIBERS

d e s c r i b e s the use of cellulose/DMF/N2O4 s o l u t i o n s as c o a t i n g m a t e r i a l s f o r g l a s s f i b e r s t o improve product performance. During t h i s p e r i o d s e v e r a l other workers began to appreciate that very p o l a r s o l v e n t s gave c e l l u l o s e s o l u t i o n s from which c e l l u l o s e could be recovered i n an e s s e n t i a l l y unchanged s t a t e r a t h e r than i n the h i g h l y o x i d i z e d s t a t e p r e v i o u s l y a s s o c i a t e d w i t h N2O4 treatments. A c o n s i d e r a b l e amount of research was p u b l i s h e d t r y i n g to d e f i n e what was o c c u r r i n g when the N2O4 was used w i t h the p o l a r s o l v e n t s . Clermont and Bender and t h e i r a s s o c i a t e s r e p o r t e d s t u d i e s where c e l l u l o s e was d i s s o l v e d i n both c o l d and hot s o l u t i o n s w i t h N2O4/DMF mixtures. (47-49) They found t h a t e s s e n t i a l l y unchanged c e l l u l o s e w i t h no added n i t r o g e n or c a r b o x y l l e v e l s was obtained from c o l d d i s s o l u t i o n , w h i l e waters o l u b l e c e l l u l o s e r e s u l t e d from i n c r e a s e d d i s s o l v i n g temperatures apparently due to the formation of v a r i o u s D.S. c e l l u l o s e n i t r a t e s . They subsequently t r i e d to use t h i s procedure t o d i s s o l v e l i g n o c e l l u l o s e removed from wood chips.(50) In a s e r i e s of a r t i c l e s d a t i n g from 1969-1976, Schweiger ha§ been r e p o r t i n g s t u d i e s aimed at t r y i n g to e l u c i d a t e what was happening i n the DMF/N2O4 treatment of c e l l u l o s e and t r i e d to use such s o l u t i o n s f o r forming other d e r i v a t i v e s . (51-56) He was a b l e to i s o l a t e a product from a p y r i d i n e - m o d i f i e d dope which appeared to be an u n s t a b l e c e l l u l o s e n i t r i t e i n t e r m e d i a t e s i n c e i t could produce a l k y l n i t r i t e s when decomposed by lower m o l e c u l a r weight a l c o h o l s . While t h i s i s not a d i r e c t s t r u c t u r a l proof i t c e r t a i n l y i s s u f f i c i e n t t o s u b s t a n t i a t e h i s proposal t h a t c e l l u l o s e r e a c t s w i t h N2O4 to g i v e c e l l u l o s e n i t r i t e and HNO3 r a t h e r than forming some type of N 2 O 4 / c e l l u l o s e a s s o c i a t i o n complex. The c e l l u l o s e n i t r i t e i s subsequently r a p i d l y decom posed by p r o t o n i c s o l v e n t s to regenerate the c e l l u l o s e and g i v e HNO2 along w i t h HNO3 f o r recovery and r e c y c l e . The s t r u c t u r e and r e a c t i o n s of N2O4 i n general have been reviewed by Gray (52) w h i l e the r e a c t i o n s of N 0^ to n i t r a t e and n i t r o s a t e a l c o h o l s and amines are reported by White and Feldman. (58) Pasteka and M i s l o v i c o v a s t u d i e d the e f f e c t s of v a r i o u s d i s s o l v i n g c o n d i t i o n s on the D.P. l o s s of c e l l u l o s e i n the DMF/N2O4 system. (59-61) They noted that the moisture content of the system and even the r a t e of s t i r r i n g caused a drop i n D.P. and t h a t the presence of p y r i d i n e or (ΰ2Η5)βΝ d i d not i n h i b i t such l o s s . W h i l e moisture may w e l l r e l a t e to D.P. l o s s , i t i s d i f f i c u l t to understand how s t i r r i n g r a t e could have such an e f f e c t unless i t was r e f l e c t i n g l o c a l temperature r i s e s t h a t occurred under the c o n d i t i o n s employed f o r s t i r r i n g . I n any case, these i n v e s t i g a t o r s again confirmed no i n c r e a s e i n e i t h e r n i t r o g e n or c a r b o x y l content f o r the regenerated c e l l u l o s e . The N2O4 system has a l s o been i n v e s t i g a t e d by s e v e r a l Russians (62-64) who a c t u a l l y r e p o r t p h y s i c a l p r o p e r t i e s f o r f i b e r s spun from DMF/N2O4 and EtOAc/^O^ systems. These f i b e r s are about 120 d e n i e r and have t e n a c i t i e s of 1.6 g/d w i t h 5-6% 2


2.

TURBAK E T A L .


21

e l o n g a t i o n s . Obviously more d e f i n i t i o n of the ^ O ^ o r g a n i c s o l vent system i s needed t o determine i t s p o t e n t i a l as a s o l v e n t based rayon process to s u b s t i t u t e f o r v i s c o s e and t h i s work w i l l be r e p o r t e d l a t e r i n t h i s symposium.


f

c. Carbon Intermediates: The use of non-hetero atom-cont a i n i n g m o i e t i e s to make c e l l u l o s e i n t e r m e d i a t e s o f t r a n s i e n t s t a b i l i t y has r e c e i v e d r e l a t i v e l y l i t t l e a t t e n t i o n i n s p i t e of the f a c t that such d e r i v a t i v e s might u l t i m a t e l y o f f e r the b e s t prospects f o r n o n - p o l l u t i n g systems. C e l l u l o s e carbonate has been prepared and r e p o r t e d . However, i t e v i d e n t l y i s so uns t a b l e as to have e s s e n t i a l l y no u t i l i t y as an i n t e r m e d i a t e . A l s o , as prepared from phosgene, i t would f a c e other i n d u s t r i a l problems. C e l l u l o s e formate i s w e l l known and has been reported as being r a p i d l y prepared by the a c t i o n o f 95% f o r m i c a c i d on c e l l u l o s e . The cleavage of c e l l u l o s e formate by hot steam a l s o r e presents an i n t e r e s t i n g approach f o r "dry s p i n n i n g " t h i s d e r i v a t i v e . While f o r m i c a c i d would not be considered a " m a t e r i a l o f c h o i c e " under most circumstances f o r i n d u s t r i a l use, i t i s c e r t a i n l y no worse than "hydrazine" i n most s a f e t y and h e a l t h cons i d e r a t i o n s and t h i s approach, o r one s i m i l a r t o i t , w i l l undoubtedly see f u r t h e r e f f o r t i n the f u t u r e . R e c e n t l y , N i c h o l s o n and D.C. Johnson reported on t h e i r work on d i s s o l v i n g c e l l u l o s e i n mixtures o f DMSO w i t h paraformaldehyde. (65) I n i t i a l i n d i c a t i o n s are that a c e l l u l o s e m e t h y l o l compound i s formed which i s s t a b l e under the e l e v a t e d temperatures of s o l u t i o n p r e p a r a t i o n and i s subsequently s t a b l e f o r days under storage i n open a i r at room temperature. The extreme s p e c i f i c i t y of t h i s combination of reagents i s p a r t i c u l a r l y n o t a b l e . For example, Seymour and E.L. Johnson (66,67) noted t h a t n e i t h e r DMF, DMAc, acetone, HMPA, nitromethane, a c r y l o n i t r i l e , a c e t o n i t r i l e , nor s u l f o l a n e can be s u b s t i t u t e d f o r the DMSO. Thus DMSO and o n l y DMSO has been found to be e f f e c t i v e t o date f o r a c h i e v i n g c e l l u l o s e s o l u t i o n s w i t h formaldehyde. This extreme s p e c i f i c i t y may w e l l r e l a t e i n some way t o the f a c t t h a t DMSO i t s e l f breaks down i n t o DMS and paraformaldehyde on h e a t i n g (68) or may poss i b l y be i n t e r a c t i n g t o s t a b i l i z e the proposed c e l l u l o s e m e t h y l o l i n t e r m e d i a t e whereas no other reagent i s e v i d e n t l y a b l e t o do so. I t should be f u r t h e r noted that w h i l e o n l y one mole of paraformaldehyde i s r e q u i r e d to h o l d one mole of the c e l l u l o s e i n s o l u t i o n , l a r g e molar excesses of 5/1 (CH20)x/cellulose must be employed i n i t i a l l y f o r d i s s o l u t i o n to occur. Thus t h i s system which appears t o i n i t i a l l y o f f e r an easy route t o c e l l u l o s e s o l u t i o n s , may o f f e r c o n s i d e r a b l e d i f f i c u l t y commercially from the aspects o f s p i n n i n g , recovery and r e c y c l e . F u r t h e r data from t h i s work are to be presented l a t e r i n t h i s symposium.


22

S O L V E N T SPUN R A Y O N , MODIFIED C E L L U L O S E

FIBERS

Throughout t h i s review, an attempt has been made t o h i g h l i g h t the need f o r a low investment c o s t , n o n - p o l l u t i n g s o l v e n t system f o r c e l l u l o s e t o s u b s t i t u t e f o r the v i s c o s e process. None o f the systems known t o date meet the necessary requirements f o r commercial e x p l o i t a t i o n . However, as has always been the case, whenever a need l i k e t h i s e x i s t s some outstanding s c i e n t i s t s w i l l develop methods t o produce the d e s i r e d r e s u l t s - and t h i s case w i l l be no e x c e p t i o n . The c e l l u l o s e chemists are equal to the c h a l l e n g e and the rewards f o r success w i l l be l a r g e .


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

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