100 Years of Cellulose Fiber Finishing Research and Development in

10 100 Years of Cellulose Fiber Finishing Research and Development in Switzerland HEINRICH ZOLLINGER Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology (ΕΤΗ), 8006 Zurich, Switzerland

This paper contains a historical review of 100 years of basic and applied cellulose research in Switzerland and a discussion of recent, partly unpub lished results concerning the crosslinking of cellu lose fibers obtained in academic and industrial re search laboratories in Switzerland. An investigation of the mechanism of catalysis of crosslinking by me tal salts is reported. 1) History of Cellulose Research in Switzerland Cellulose research in Switzerland has always concentrated principally on cellulose fibers, parti cularly cotton fabrics. In addition, however, our present understanding of the concept of macromolecules in general and the submicroscopic arrangement of cel lulose molecules in cell walls owes much to investi gations made in Switzerland. The title of this paper "100 years of cellulose research and development in Switzerland" is not abso lutely correct - strictly speaking it should be 99 years. The reason for this is that in 1877 Carl Nägeli and Simon Schwendener, two Swiss professors of botany published the second edition of their book "Das Mikroskop, Theorie und Praxis" (The Microscope, Theory and Practical Application) [1] in which they use for the first time the word micelles, i.e. ordered particles of submicroscopic dimensions. They postulated that water may enter between these particles and by means of this concept they explained swelling and other properties. Their idea was not accepted generally, however, for many years until X-ray structure determinations in the Twenties demonstrated that their hypothesis was, at least in principle, correct. It is important to note that the word micelle 197

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was c r e a t e d i n 1 8 7 7 ; t h e c o n c e p t , h o w e v e r , i s o l d e r , b u t i t was a l s o d e v e l o p e d by one o f t h e a u t h o r s , namel y C a r l N à g e l i . I n t h e book m e n t i o n e d t h e d e f i n i t i o n o f a m i c e l l e i s g i v e n on page 4 2 4 : "Die M o l e c u l e g r u p p i r e n s i c h zu k l e i n e n , j e d o c h m i k r o s k o p i s c h n i c h t wahrnehmbaren K r y s t a l l e n , d i e w i r M i c e l l e n * nennen w o l l e n . " (The m o l e c u l e s group t h e m s e l v e s i n s m a l l , m i c r o s c o p i c a l l y u n d e t e c t a b l e c r y s t a l s , w h i c h we w i l l call micelles). Prom t h e p o i n t o f v i e w o f t h e h i s t o r y o f s c i e n c e t h e f o o t n o t e a t t a c h e d t o t h e word " M i c e l l e n " i s e v e n more i n t e r e s t i n g : *"Wir haben f r i ï h e r d i e f r a g l i c h e n G-ruppen k l e i n s t e r Theilchen a l s "Molecule" bezeichnet. S e i t dem j e d o c h d i e s e r A u s d r u c k i n d e r Chemie e i n e ganz a n d e r e und v o r a u s s i c h t l i c h b l e i b e n d e B e d e u t u n g e r h a l t e n h a t , f u h r t d i e genannte B e z e i c h nung n o t h w e n d i g zu M i s s v e r s t à n d n i s s e n , d e r e n B e s e i t i g u n g am b e s t e n d u r c h Aenderung des Namens geschieht." I n t h i s f o o t n o t e the authors r e f e r to the f a c t t h a t N à g e l i , i n h i s book "Die S t à r k e k o r n e r " (The s t a r c h g r a i n s ) , p u b l i s h e d i n Z u r i c h i n 1858 [2] c a l l e d t h e s e p a r t i c l e s n o t m i c e l l e s , b u t m o l e c u l e s . He changed t h e n o m e n c l a t u r e i n 1877 b e c a u s e t h e n t h e t e r m m o l e c u l e became t o mean what i t does t o d a y . I n 1 8 5 8 , N â g e l i w r o t e [ r e f . 2 , p. 332] t h a t atoms a r e e i t h e r e l e m e n t s o r compounds, i . e . s i m p l e o r "comb i n e d " a t o m s . He d e f i n e d s t a r c h a s an atom c o n s i s t i n g o f 12 c a r b o n a t o m s , 10 h y d r o g e n atoms and 10 oxygen atoms. T h i s i s of course i n c o n t r a d i c t i o n to our p r e s e n t , more p r e c i s e d e f i n i t i o n o f a n atom. L a t e r , i n t h e same b o o k , he u s e s t h e e x p r e s s i o n " m o l e c u l e " f o r t h e s u b m i c r o s c o p i c u n i t w h i c h , i n 1 8 7 7 , he and Schwend e n e r renamed m i c e l l e . I t i s obvious from the elementary r a t i o 1 2 : 1 0 : 1 0 f o r c a r b o n , h y d r o g e n and oxygen t h a t i n 1858 t h e c o r r e c t a t o m i c masses were n o t known, h o w e v e r , t h i s p r o b lem w i l l n o t be d i s c u s s e d h e r e . For our p r e s e n t l e c t u r e i t i s i m p o r t a n t to note t h a t the macromolecular nature of c e l l u l o s e , s t a r c h and s y n t h e t i c p r o d u c t s was f i r s t r e a l i z e d o n l y i n t h e T w e n t i e s by Hermann S t a u d i n g e r , a t t h a t t i m e P r o f e s s o r o f G e n e r a l and A n a l y t i c C h e m i s t r y a t t h e S w i s s P e d e r a l I n s t i t u t e o f T e c h n o l o g y ( Ε Τ Η ) i n Z u r i c h . The c o n c e p t o f m a c r o m o l e c u l e s was d e v e l o p e d by S t a u d i n g e r i n Z u r i c h i n s p i t e o f t h e s c e p t i c i s m and d i s b e l i e f o f t h e whole s c i e n t i f i c w o r l d . The f i r s t p a p e r i n w h i c h S t a u d i n g e r d e s c r i b e d the macromolecular nature of s o - c a l l e d

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c o l l o i d a l compounds was p u b l i s h e d i n 1920 [3]. I n t h e P h . D . t h e s i s o f K. P r e y [4] made u n d e r S t a u d i n g e r s s u p e r v i s i o n [5j the macromolecular nature of c e l l u l o s e was d e m o n s t r a t e d . S t a u d i n g e r s p i o n e e r i n g work on m a c r o m o l e c u l e s was h o n o r e d by t h e N o b e l P r i z e i n 1 9 5 2 . A f t e r S t a u d i n g e r s p i o n e e r i n g work t h e c o n f o r m a t i o n , i . e . the arrangement i n the t h r e e d i m e n s i o n a l space o f c h a i n - l i k e and g l o b u l a r m o l e c u l e s , was s t i l l an u n r e s o l v e d problem. Here we have t o d i s t i n g u i s h between t h e a r r a n g e ment o f s u c h m o l e c u l e s i n s o l u t i o n and i n t h e s o l i d state. I n 1 9 3 4 , Werner Kuhn, a S w i s s c h e m i s t w o r k i n g a t t h e I n s t i t u t e o f T e c h n o l o g y i n K a r l s r u h e i n Germany d e v e l o p e d t h e c o n c e p t o f d i s s o l v e d c h a i n m o l e c u l e s hav i n g a s t a t i s t i c a l l y c o i l e d s t r u c t u r e [ 6 ] . On t h i s b a s i s he was a b l e t o s o l v e q u a n t i t a t i v e l y t h e p r o b l e m o f r u b b e r e l a s t i c i t y [2] a n d , i n t h e P o r t i e s , when he was P r o f e s s o r of P h y s i c a l Chemistry a t the U n i v e r s i t y of B a s l e i n S w i t z e r l a n d , he and Hans Kuhn were a b l e t o e x p l a i n the c o r r e l a t i o n of the l e n g t h of c h a i n mole c u l e s w i t h t h e v i s c o s i t y and t h e f l o w b i r e f r i n g e n c e o f t h e i r solutions [8]. P o r c e l l u l o s e t e c h n o l o g y and f i b e r r e s e a r c h i n p a r t i c u l a r the arrangement of m o l e c u l e s i n the s o l i d s t a t e i s v e r y i m p o r t a n t . P a r t o f our p r e s e n t knowledge i n t h i s f i e l d o r i g i n a t e d i n S w i t z e r l a n d . In s p i t e of t h e f a c t t h a t N a g e l i ' s c o n c e p t o f m i c e l l e s was v e r y h y p o t h e t i c a l and c r u d e a t t h e t i m e o f i t s d i s c o v e r y , i t i s a s t o n i s h i n g l y t h a t he p o s t u l a t e d t h e p r i n c i p l e o f a n i d e a w h i c h c a n be r e c o g n i z e d i n t h e r e s u l t s o f more r e c e n t r e s e a r c h on t h e s u b m i c r o s c o p i c s t r u c t u r e o f p l a n t c e l l w a l l s , i . e . n a t i v e c e l l u l o s e . Here t h e p i o n e e r i n g work o f P r e y - W y s s l i n g and M u h l e t h a l e r , two b i o l o g i s t s a t the Swiss F e d e r a l I n s t i t u t e of Technolo gy i n Z u r i c h , c o n c e r n i n g t h e e l e c t r o n m i c r o s c o p y r e c e i v e d w o r l d - w i d e r e c o g n i t i o n : I n 1 9 4 8 , t h e y were a b l e to get, f o r the f i r s t time, e l e c t r o n m i c r o s c o p i c p i c tures of m i c r o f i b r i l l s i n c e l l u l o s e w i t h a diameter of 200 t o 300 I [2J. A t t h a t t i m e i t was g e n e r a l l y assumed t h a t m i c r o f i b r i l l s were t h e s m a l l e s t s u b m i c r o s c o p i c u n i t s above t h e m o l e c u l a r l e v e l . I t was t h e r e f o r e a s e n s a t i o n when M u h l e t h a l e r d i s c o v e r e d i n I 9 6 0 [ 1 0 , 11] t h e e l e m e n t a r y f i b r i l l s , i . e . u n i t s w i t h a d i a m e t e r o f 30 X , c o n t a i n 1

1

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I t may be m e n t i o n e d h e r e t h a t no l e s s t h a n 6 c h e m i s t s and 3 p h y s i s i s t s who were p r o f e s s o r s a t t h e Ε Τ Η have r e c e i v e d N o b e l P r i z e s , t h e l a s t b e i n g P r o f e s s o r V. P r e l o g i n 1 9 7 5 .

200


i n g a b o u t 42 c e l l u l o s e c h a i n s o n l y . A combined c h e m i c a l and e l e c t r o n - m i c r o s c o p i c i n v e s t i g a t i o n by M u g g l i , E l i a s and M u h l e t h a l e r [ 1 2 , l^J demonstrated l a t e r t h a t the molecules i n these elementary f i b r i l l s are not f o l ded a s i n many m a c r o m o l e c u l e s o b t a i n e d f r o m m e l t s o r s o l u t i o n s i n the l a b o r a t o r y . N à g e l i s term " m i c e l l e " disappeared from the n o menclature of submicroscopic u n i t s of c e l l u l o s e . I t reappeared, however, i n the p h y s i c a l c h e m i s t r y of aqueous s o l u t i o n s o f p a r t l y h y d r o p h o b i c compounds, e.g. s o l u t i o n s o f soaps and s y n t h e t i c d e t e r g e n t s . I t was i n t r o d u c e d t h e r e by v a n Bemmelen [l±], M c B a i n [15.] and others [16]. 1

2) H i s t o r y

o f "Swiss C o t t o n " R e s e a r c h and Development

L e t u s r e v i e w now b r i e f l y some i n d u s t r i a l d e v e lopments i n the f i n i s h i n g of c e l l u l o s i c f i b e r s i n t e x t i l e m i l l s i n S w i t z e r l a n d ! The t e r m "Swiss c o t t o n " h a s become a s t a n d a r d e x p r e s s i o n i n s p i t e o f t h e f a c t t h a t t h e r e i s n o t a s i n g l e b a l e o f c o t t o n grown i n S w i t z e r l a n d . The t e r m r e f e r s t o l i g h t w e i g h t c o t t o n f a b r i c s o f very high q u a l i t y standards w i t h s p e c i a l f i n i s h i n g eff e c t s . Where d i d i t o r i g i n a t e ? I n -I84I i n England, John Mercer d i s c o v e r e d t h a t t h e l u s t r e and o t h e r p r o p e r t i e s o f c o t t o n c a n be changed by a t r e a t m e n t w i t h c a u s t i c s o d a . I t t o o k , however, almost h a l f a c e n t u r y u n t i l i n d u s t r i a l mercer i s a t i o n was i n t r o d u c e d i n E n g l a n d by Thomas and P r é v o s t . Swiss t e x t i l e m i l l s s t a r t e d to produce m e r c e r i zed y a r n a t t h e end o f t h e c e n t u r y . V e r y soon two S w i s s f i r m s , H e b e r l e i n & Co. AG- i n W a t t w i l and AG- C i lander i n Herisau r e a l i z e d that treatments with other c h e m i c a l s r e s u l t e d i n v e r y i n t e r e s t i n g and permanent changes i n t h e c o t t o n f a b r i c : A t r e a t m e n t w i t h r e l a t i v e l y c o n c e n t r a t e d s u l f u r i c a c i d had a p a r t c h m e n t i z i n g e f f e c t a n d , when combined w i t h m e r c e r i z a t i o n , y i e l d e d t r a n s p a r e n t f a b r i c s o f a u n i q u e l u s t r e [ 1 8 ] · The p r o d u c t was c a l l e d Organdy. L a t e r a v a r i e t y o f o t h e r t r e a t m e n t s w i t h n i t r i c a c i d and w i t h z i n c and o t h e r s a l t s r e s u l t e d i n new e f f e c t s w h i c h were welcomed by the f a s h i o n i n d u s t r y [ 1 9 ] . A n o t h e r method o f a p p r o a c h was t o c r o s s l i n k t h e c e l l u l o s e f i b e r s . O r i g i n a l l y developed f o r regenerated c e l l u l o s e f i b e r s a t the b e g i n n i n g of the century as a treatment w i t h formaldehyde, c r o s s l i n k i n g f i r s t gained some i m p o r t a n c e i n t h e T w e n t i e s , when T o o t a l B r o a d h u r s t L t d . i n England used r e s i n s of the phenol-form2

D e t a i l s o f t h e h i s t o r y o f m e r c e r i s a t i o n have b e e n d e s c r i b e d by D o h l e [ 1 7 ] .

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201

a l d e h y d e t y p e [ 2 0 , 211. L a t e r , u r e a - f o r m a l d e h y d e s were f o u n d t o be s u p e r i o r . I t was n o t u n t i l t h e m i d - T h i r t i e s , however, t h a t an e s s e n t i a l b r e a k - t h r o u g h took p l a c e i n t h i s f i e l d when more permanent e f f e c t s were o b t a i n e d by t h e p a d - d r y - c u r e p r o c e s s . T h i s p r o c e s s was d i s c o v e r e d s i m u l t a n e o u s l y and i n d e p e n d e n t l y by C. B e n e r a t Raduner t e x t i l e m i l l i n S w i t z e r l a n d [22] and by L. A . L a n t z and A . L. M o r r i s o n a t C a l i c o P r i n t e r s i n E n g l a n d [22.]. P r o c e s s e s o f t h i s t y p e had t h e i r f i r s t boom p e r i o d i n t h e l a t e r F o r t i e s and e a r l y F i f t i e s when t h e f a s h i o n t r e n d s o f t h e t i m e demanded t h r e e d i m e n s i o n a l e f f e c t s on f a b r i c s s u c h as E v e r g l a z e . T h e i r r e a l i m p o r t a n c e became e v i d e n t , h o w e v e r , when t h e y were a p p l i e d t o a l l t y p e s o f e a s y - c a r e c o t t o n f a b r i c s ; t h i s i s , on a w o r l d - w i d e b a s i s , by f a r t h e most i m p o r t a n t development i n f i n i s h i n g p u r e c o t t o n , r a y o n and c o t t o n / p o l y e s t e r b l e n d s i n t h e l a s t twenty y e a r s . I n s p i t e o f t h e enormous amount o f r e s e a r c h and development work p u t i n t o t h i s f i e l d a l l o v e r t h e world i t i s a s t o n i s h i n g t h a t there are s t i l l improvements p o s s i b l e w h i c h r e a c h t h e p r o d u c t i o n s t a g e . Two r e l a t i v e l y r e c e n t examples f r o m S w i t z e r l a n d a r e t h e MS ( m i c r o s t r e t c h ) p r o c e s s o f Raduner [2±] i n w h i c h l i g h t weight f a b r i c s are s t r e t c h e d across the f a b r i c i n r e g i o n s o f l e s s t h a n 0 . 5 cm by a c o m b i n a t i o n o f s p e c i a l r o l l s before the c r o s s l i n k i n g proper takes p l a c e . The o t h e r new development i s t h e s o - c a l l e d MA proc e s s o f T r i a t e x I n t e r n a t i o n a l i n Z u r i c h [2^J. By d i p ping fabrics with crosslinking solutions containing s u i t a b l e s u r f a c t a n t s i t i s p o s s i b l e t o g e t a n even p i c k - u p o f o n l y a b o u t 35 t o 40 96 (w/w). In order to i n v e s t i g a t e the b a s i c p r i n c i p l e s of t h i s process, a s p e c i a l microscopic technique which u t i l i z e s f l u o r e s c e n t compounds a s s t a i n i n g r e a g e n t s was d e v e l o p e d i n o u r Department [26., 2 7 ] . I t demons t r a t e d t h a t the u s u a l p a d - d r y - c u r e t e c h n i q u e has the d i s a d v a n t a g e t h a t i n f a b r i c s c o n t a i n i n g 80 t o 90 % (w/w) o f t h e f i n i s h i n g l i q u o r d i f f u s i o n o f t h e f i n i s h i n g c h e m i c a l s t o t h e s u r f a c e t a k e s p l a c e when w a t e r d i f f u s e s o u t o f t h e f i b e r and f a b r i c i n t h e d r y i n g s t a g e . We made, however, t h e i n t e r e s t i n g o b s e r v a t i o n t h a t t h i s d i f f u s i o n of c r o s s l i n k i n g chemicals d i s s o l ved i n w a t e r becomes n e g l e g i b l e as soon a s t h e f i b e r s c o n t a i n l e s s t h a n a b o u t 35 % w a t e r . The c r i t i c a l phase i s t h e r e f o r e the removal of the f i r s t h a l f of the usual 80 io p i c k - u p o f t h e f i n i s h i n g l i q u o r . Removal o f t h e r e m a i n i n g 35 % w a t e r i s , h o w e v e r , n o t c r i t i c a l . The MA p r o c e s s a v o i d s t h i s d i f f u s i o n by a p p l y i n g o n l y 30 t o 40 io l i q u o r f r o m t h e b e g i n n i n g . I n p r i n c i p l e we f o u n d , however, t h a t e i t h e r by e x t r e m e l y s l o w d r y i n g o f a f a b r i c padded i n t h e n o r m a l way, i . e . w i t h

202


a b o u t 80 t o 90 % p i c k - u p o r by f r e e z e d r y i n g a d i f f u s i o n o f t h e c h e m i c a l s t o t h e s u r f a c e c a n be a l m o s t com p l e t e l y avoided. Unfortunately these processes are, however, not e c o n o m i c a l . The o b s e r v a t i o n t h a t by e x t r e m e l y s l o w d r y i n g no or very l i t t l e d i f f u s i o n of the f i n i s h i n g chemicals t a k e s p l a c e i s u n d e r s t a n d a b l e on t h e b a s i s o f t h e f u n damental law of d i f f u s i o n of m o l e c u l e s , i . e . the S t o k e s - E i n s t e i n e q u a t i o n ( 1 ) . T h i s e q u a t i o n s t a t e s that D = ——— 6

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t h e d i f f u s i o n c o e f f i c i e n t - w h i c h measures t h e r a t e o f d i f f u s i o n - i s p r o p o r t i o n a l to Boltzmann's constant k and t h e a b s o l u t e t e m p e r a t u r e Τ and i n v e r s e l y p r o p o r t i o n a l t o t h e v i s c o s i t y o f t h e medium ( η ) and t h e r a d i u s r o f t h e p a r t i c l e , a s s u m i n g t h e p a r t i c l e t o be a s p h e r e . The r a d i u s o f s u c h a p a r t i c l e , on t h e o t h e r h a n d , i s p r o p o r t i o n a l t o t h e cube r o o t o f t h e m o l e c u l a r mass M. The d i f f u s i o n i s t h e r e f o r e relatively l i t t l e dependent on t h e s i z e o f t h e d i f f u s i n g m o l e c u l e s . I n p r a c t i s e d i f f e r e n c e s i n the d i f f u s i o n r a t e o f s m a l l p a r t i c l e s s u c h a s w a t e r m o l e c u l e s and l a r g e r p a r t i c l e s such as c r o s s l i n k i n g reagents w i l l t h e r e f o r e be more n o t i c e a b l e when t h e o v e r a l l d i f f u s i o n i s s l o w . 0

3) The O r i g i n o f Y a r n T e x t u r i z i n g Processes f o r t e x t u r i z i n g yarns of t h e r m o p l a s t i c f i b e r s have become v e r y i m p o r t a n t i n t h e l a s t t w e n t y y e a r s . One s h o u l d n o t f o r g e t , however, t h a t t h e o r i g i n a l i n v e n t i o n o f t e x t u r i z e d y a r n s was made w i t h r a y o n f i b e r s . The p r i n c i p l e was d e s c r i b e d i n a p a t e n t [ 2 8 ] i n 1932 by t h e H e b e r l e i n company: I t was d e m o n s t r a t e d t h a t w o o l - l i k e y a r n c a n be o b t a i n e d by o v e r t w i s t i n g , m o i s t e n i n g and d r y i n g i n t h e o v e r t w i s t e d s t a t e and r e m o v i n g t h e o v e r t w i s t a f t e r w a r d s . As t h i s t r e a t m e n t was n o t permanent a t a l l , t h e m o i s t e n i n g was l a t e r Γ291 combined w i t h a f o r m a l d e h y d e t r e a t m e n t d u r i n g o v e r t w i s t i n g . The y a r n was s t a b i l i z e d i n t h i s s t r a i n e d s t a t e . The p r i n c i p l e o f f a l s e t w i s t i n g , i . e . a c o m b i n a t i o n o f two y a r n s w i t h s - and z - t w i s t was a l s o d e v e l o p e d a t t h i s t i m e . D u r i n g t h e Second W o r l d War a c o n s i d e r a b l e p r o d u c t i o n of such w o o l - l i k e f i b e r s took p l a c e i n S w i t z e r l a n d when g e n u i n e w o o l was n o t a v a i l able i n s u f f i c i e n t q u a n t i t i e s . The permanency o f t h e f a l s e - t w i s t e f f e c t was, how e v e r , n o t c o m p l e t e l y s u f f i c i e n t and t h e a b r a s i o n s t r e n g t h o f t h e s e t e x t u r i z e d r a y o n f i b e r s was n o t s a t i s f a c t o r y . F o r t u n a t e l y , however, t h e t h e r m o p l a s t i c

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p o l y a m i d e f i b e r s became a v a i l a b l e and H e b e r l e i n d i s c o vered t h a t , w i t h these f i b e r s , the f a l s e - t w i s t process c o u l d be r e a l i z e d by a s i m p l e t h e r m a l t r e a t m e n t ; no c h e m i c a l r e a c t i o n was n e c e s s a r y . I t i s w e l l known t h a t t h i s i n v e n t i o n i n i t i a t e d the world-wide success of t e x t u r i z e d y a r n , the f i r s t product b e i n g H e b e r l e i n s Helanca. 1

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I n t h e f o l l o w i n g p a r t o f t h i s p a p e r t h e mechanism o f r e a c t i o n s i n c r o s s l i n k i n g and t h e i r r e l a t i o n t o t e x t i l e m e c h a n i c a l p r o p e r t i e s w i l l be d i s c u s s e d i n more d e t a i l . The p u r p o s e o f t h i s r e p o r t w i l l be t o demon s t r a t e t h a t a c a r e f u l i n v e s t i g a t i o n of the chemical re a c t i o n s t h e m s e l v e s c a n be h e l p f u l i n o p t i m i z i n g t h e p r o p e r t i e s of f i n i s h e d f a b r i c s , c o n s i s t i n g p a r t l y or completely of c e l l u l o s e f i b e r s . I t may be remembered t h a t , i n s p i t e o f t h e f a c t t h a t f o r m a l d e h y d e t r e a t m e n t s o f c e l l u l o s e f i b e r s have b e e n made f o r 70 y e a r s and w i t h u r e a - f o r m a l d e h y d e r e s i n s f o r 40 y e a r s , i t was s t i l l d e b a t e d i n t h e e a r l y S i x t i e s i f a c h e m i c a l r e a c t i o n between c e l l u l o s e and t h e c h e m i c a l a p p l i e d t o i t t a k e s p l a c e a t a l l . J . T. M a r s h , one o f t h e p i o n e e r s i n t h e t e c h n o l o g y o f t h i s f i e l d , had d o u b t s a b o u t t h e p r o b l e m u n t i l 1962 [ 2 1 ] . The f i r s t c l e a r e v i d e n c e was g i v e n by A b e n d , Stamm and Z o l l i n g e r [30] i n 1966 f o r c e l l u l o s e c r o s s l i n k e d w i t h d i v i n y l s u T f o n e : C o t t o n and r a y o n f i b e r s t r e a t e d w i t h d i v i n y l s u l f o n e gave, a f t e r a c i d t r e a t m e n t s , d e g r a d a t i o n p r o d u c t s w h i c h c o n t a i n e d two g l u c o s e u n i t s a t t h e two ends o f mono- and d i m e r i c s u l fone d e r i v a t i v e s . Por c e l l u l o s e t r e a t e d w i t h c r o s s l i n k i n g reagents which are not s t a b l e to degradation i n s t r o n g l y a c i d i c m e d i a , a p e r m e t h y l a t i o n t e c h n i q u e was d e v e l o p e d by means o f w h i c h i t was p o s s i b l e t o d e t e r m i n e even t h e r e l a t i v e r e a c t i v i t y o f t h e t h r e e h y d r o x y l g r o u p s on a n a n h y d r o g l u c o s e u n i t o f c e l l u l o s e Γ5l1. M o s t o f our r e s e a r c h i n t h e l a t e S i x t i e s and earfy S e v e n t i e s was c a r r i e d o u t w i t h f o r m a l d e h y d e . There a r e s e v e r a l f a c t o r s which i n d i c a t e t h a t formaldehyde might be an i d e a l , c o m m e r c i a l c r o s s l i n k i n g a g e n t f o r c o t t o n f i n i s h i n g : 1 mole o f f o r m a l d e h y d e i s 14 t o 30 t i m e s c h e a p e r t h a n 1 mole of t h e v a r i o u s c y c l i c e t h y l e n e u r e a s ; c e l l u l o s e f o r m a i s have a n e x c e l l e n t s t a b i l i t y t o w a r d s h y d r o l y s i s i n w a s h i n g o p e r a t i o n s ; t h e y do n o t r e t a i n c h l o r i n e and show no s i g n s o f y e l l o w i n g when i r o n e d . There a r e m i l l s i n Europe w h i c h u s e f o r m a l d e hyde, a l t h o u g h c o t t o n c r o s s l i n k i n g w i t h formaldehyde i s s a i d t o have t h e d i s a d v a n t a g e o f e x t e n s i v e strength

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l o s s e s and l o w r e p r o d u c i b i l i t y . We d e c i d e d t h e r e f o r e t o compare t h e r e s i l i e n c e / s t r e n g t h l o s s r e l a t i o n s h i p of formaldehyde c r o s s l i n k e d f a b r i c s r e l a t i v e to f a b r i c s c r o s s l i n k e d w i t h d i m e t h y l o l e t h y l e n e u r e a (DMEU). The l a t t e r was t a k e n a s r e p r e s e n t a t i v e of h i g h l y r e a c t i v e c y c l i c N-dimethylol urea compounds. F o r t h i s p u r p o s e we c r o s s l i n k e d a s t a n d a r d c o t t o n p o p l i n i n p a d - d r y - c u r e p r o c e s s e s w i t h f o r m a l d e h y d e and w i t h DMEU, v a r y i n g t h e r e a c t i o n t i m e , t e m p e r a t u r e and t h e c o n c e n t r a t i o n s o f t h e c r o s s l i n k i n g r e a g e n t and t h e c a t a l y s t ( M g C l 2 ) . B a s i c a l l y , we compared t h e d r y crease r e c o v e r y a n g l e s (a measure o f r e s i l i e n c y ) o f a l l samples w i t h t h e t e n s i l e s t r e n g t h . I n a d d i t i o n we d e t e r m i n e d t h e degree o f p o l y m e r i s a t i o n (DP) o f c r o s s l i n k e d c o t t o n ( a f t e r r e m o v a l o f the c r o s s l i n k s ) i n order to separate the t o t a l l o s s i n t e n s i l e s t r e n g t h i n t o t h e s o - c a l l e d permanent damage Γ521 w h i c h i s due t o t h e d e c r e a s e i n DP o f c e l l u l o ô e c a u s e d by h y d r o l y s i s d u r i n g c u r i n g , and t h e t e m p o r a r y s t r e n g t h l o s s w h i c h i s c a u s e d by t h e s t i f f e n i n g o f t h e m a c r o m o l e c u l a r n e t w o r k by t h e c r o s s l i n k s . C a r e f u l r e moval o f the c r o s s l i n k s n u l l i f i e s t h i s s t r e n g t h l o s s . F o r c o t t o n c r o s s l i n k e d w i t h N - m e t h y l o l compounds t h i s d i v i s i o n of the t o t a l t e n s i l e s t r e n g t h l o s s i n t o t e m p o r a r y and permanent damage h a s b e e n made s e v e r a l t i m e s i n t h e p a s t , a s i t i s r e l a t i v e l y e a s y t o remove s u c h c r o s s l i n k s by m i l d a c i d t r e a t m e n t s . F o r c e l l u l o s e f o r m a i s , h o w e v e r , t h i s h a s n o t b e e n made w i t h o u t d a maging t h e p o l y m e r i t s e l f . However, L e w i n and Weinstein [33] f o u n d t h a t t h e d e t e r m i n a t i o n of" t h e DP o f t r i n i t r o c e l l u l o s e c a n a l s o be a p p l i e d t o c e l l u l o s e f o r m a i s : I n c o t t o n c r o s s l i n k e d w i t h f o r m a l d e h y d e t h e oxymethyl e n e b r i d g e s a r e removed on n i t r a t i o n w i t h o u t h y d r o l y t i c degradation of the c e l l u l o s e c h a i n . S e v e r a l s e r i e s o f p a d - d r y - c u r e a p p l i c a t i o n on a 100 io c o t t o n p o p l i n ^ v a r y i n g c a t a l y s t and c r o s s l i n k e r c o n c e n t r a t i o n and t e m p e r a t u r e y i e l d e d t h e r e s u l t s w h i c h a r e summarized i n F i g u r e 1. One r e c o g n i z e s t h a t t h e c r e a s e r e c o v e r y / t e n s i l e s t r e n g t h r e l a t i o n s h i p o f DMEU a p p l i c a t i o n s a r e s i g n i f i c a n t l y b e t t e r t h a n t h o s e w i t h f o r m a l d e h y d e : A 7 . 0 io l o s s i n t e n s i l e s t r e n g t h i s o b s e r v e d p e r 2 0 ° (W + F) increase i n crease recovery angle. C r o s s l i n k i n g w i t h f o r m a l d e h y d e g i v e s a p o o r e r r e l a t i o n s h i p . The f o r m a l dehyde c u r v e c a n be d e s c r i b e d by a n i n i t i a l l o s s o f 16 io i n t e n s i l e s t r e n g t h and by a n a d d i t i o n a l 7 . 5 $ l o s s p e r 2 0 ° (W + F) i n c r e a s e i n d r y c r e a s e r e c o v e r y a n g l e . Compared t o DMEU a p p l i c a t i o n s , f o r m a l d e h y d e h a s D e t a i l s are d e s c r i b e d elsewhere

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Figure 1. Dry crease recovery angle/tensile strength rela tionship for cotton poplin crosslinked with formaldehyde (Φ and O) and with DMEU (A and A). Ο and Δ : Pad ding solution containing more than 20% w/v of formalde hyde and DMEU, respectively.

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t h e d i s a d v a n t a g e o f t h i s i n i t i a l l o s s and a s l i g h t l y g r e a t e r l o s s per 20° i n c r e a s e i n crease recovery angle. The u p p e r l i m i t s o f d r y c r e a s e r e c o v e r y a n g l e s f o r DMEU and f o r m a l d e h y d e ( 3 0 0 ° and 2 8 0 ° , r e s p e c t i v e l y ) a g r e e w i t h t h o s e r e p o r t e d i n t h e l i t e r a t u r e [.28]. A b e t t e r crease recovery a n g l e / t e n s i l e strength r e l a t i o n i s o b t a i n e d i f p a d d i n g s o l u t i o n s w i t h more t h a n 20 % DMEU a r e u s e d . W i t h f o r m a l d e h y d e , however, no s u c h i n f l u e n c e c a n be o b s e r v e d . What a r e t h e f u n d a m e n t a l c a u s e s o f t h e p o o r e r crease recovery a n g l e / t e n s i l e strength r e l a t i o n of formaldehyde c r o s s l i n k e d cotton? T h i s q u e s t i o n c a n be answered i f t h e t e n s i l e s t r e n g t h l o s s i s s e p a r a t e d i n t o permanent and t e m p o r a ry l o s s . I n F i g u r e 2 the t o t a l ( d i r e c t d e t e r m i n a t i o n ) and permanent Tfrom DP measurements) t e n s i l e s t r e n g t h l o s s e s o f DMEU- and f o r m a l d e h y d e c r o s s l i n k e d samples a r e p l o t t e d a g a i n s t d r y c r e a s e r e c o v e r y a n g l e . Prom a c o m p a r i s o n o f t h e DMEU and f o r m a l d e h y d e c u r v e s i t i s q u i t e o b v i o u s t h a t t h e permanent l o s s i s much g r e a t e r i n the l a t t e r case. I f we a n a l y s e , on t h e o t h e r h a n d , t h e t e n s i l e s t r e n g t h l o s s d a t a f o r t h e t e m p o r a r y damage f r a c t i o n , i t c a n be d e m o n s t r a t e d [J24, 351 t h a t i n t h i s r e s p e c t f o r m a l d e h y d e i s no worse t h a n D M E U - t r e a t e d c o t t o n . I n t e r m s o f p r a c t i c a l a p p l i c a t i o n t h e range w i t h d r y c r e a s e r e c o v e r y a n g l e s h i g h e r t h a n 2 5 0 ° i s t h e most i m p o r t a n t . I n t h i s range t h e r e s u l t s show t h a t f o r DMEU t r e a t m e n t s o f t h e t o t a l t e n s i l e s t r e n g t h l o s s o f 7 io p e r 2 0 ° (W + P) i n c r e a s e i n d r y c r e a s e r e c o v e r y a n g l e o n l y 0 . 5 i i s due t o t h e permanent damage. P o r formaldehyde t r e a t m e n t s , of the t o t a l l o s s of 1 0 . 5 6 . 3 i a r e due t o t h e permanent damage. The h i g h e r permanent s t r e n g t h l o s s o b t a i n e d w i t h f o r m a l d e h y d e i s due t o t h e f a c t t h a t f o r m a l d e h y d e i s a b o u t 8 t i m e s l e s s r e a c t i v e t h a n DMEU. Formaldehyde t r e a t m e n t s need a b o u t 8 t i m e s more c a t a l y s t (MgCl2) t h a n t h o s e w i t h DMEU i n o r d e r t o o b t a i n t h e same r e s i l i e n c y . The h i g h e r c a t a l y s t c o n c e n t r a t i o n i s t h e c a u s e o f t h e h i g h e r permanent l o s s i n f o r m a l d e h y d e t r e a t ments. Our r e s u l t s d e m o n s t r a t e t h e r e f o r e t h a t u n d e r v e r y m i l d c r o s s l i n k i n g c o n d i t i o n s t h e f a b r i c i s p e r se a l most n o t damaged by DMEU. A f t e r a l a r g e number Ô T w a s h i n g s s u c h a f a b r i c w i l l t h e r e f o r e have a b o u t t h e same t e n s i l e s t r e n g t h a s a f a b r i c w h i c h was n o t c r o s s l i n k e d . Observations which support t h i s c o n c l u s i o n have b e e n made by Du B o i s [ 3 9 J . Therefore i t i s worthwiTe to i n v e s t i g a t e f u r t h e r i f m i l d t r e a t m e n t s improve the r e s i l i e n c e / d a m a g e r a t i o . The a p p l i c a t i o n o f some new c a t a l y s t s w i t h h i g h e r a c t i v i t i e s a p p e a r e d t o u s t o be p a r t i c u l a r l y i n t e r e s t i n g

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ZOLLINGER

207


i n t h i s c o n t e x t , as t h e y c a n be u s e d a t l o w e r c u r i n g temperatures. Most of these c a t a l y s t s are m i x t u r e s of m e t a l s a l t s w i t h organic a c i d s such as t a r t a r i c a c i d and c i t r i c a c i d [40, 411. A n o t h e r t y p e c o n s i s t s o f mixt u r e s o f magnesium s a l t s and t e t r a f l u o r o b o r a t e [ 4 2 ] . The a p p l i c a t i o n o f t h e s e h i g h l y a c t i v e c a t a l y s t s i n c r o s s l i n k i n g w i t h formaldehyde a t temperatures b e t ween 80°C and 125°C gave a v e r y s m a l l , b a r e l y s i g n i f i c a n t improvement i n t h e r e s i l i e n c e / m e c h a n i c a l s t r e n g t h r e l a t i o n s h i p r e l a t i v e to experiments w i t h conventional c a t a l y s t s a t 160°C [ 3 4 - 5 7 1 * Y e t f o r t h e a p p l i c a t i o n o f DMEU a t l o w e r t e m p e r a t u r e s t h e u s e o f h i g h l y a c t i v e c a t a l y s t s s i g n i f i c a n t l y improves the r e l a t i o n s h i p b e t ween c r e a s e r e c o v e r y and t e n s i l e s t r e n g t h . I n F i g u r e 3 t h e e x p e r i m e n t s w i t h M g C l p / t a r t a r i c a c i d a t 85°C and 105°C a r e compared w i t h t h e r e s u l t s o f e x p e r i m e n t s w i t h M g C l ç a l o n e a t 130 t o 160°C ( s o l i d l i n e ) . The improvement i s due, a s shown e l s e w h e r e [ 3 4 , 3 5 ] , t o a d e c r e a s e i n t h e t e m p o r a r y damage, i . e . TEe ïïamage w h i c h i s c a u s e d by t h e s t i f f e n i n g o f t h e m o l e c u l a r n e t w o r k by c r o s s l i n k s . Type, l e n g t h o r l o c a t i o n o f c r o s s l i n k s w i t h DMEU a r e t h e r e f o r e f a v o r a b l y i n f l u e n c e d by a d e c r e a s e i n t e m p e r a t u r e . I n a d d i t i o n c u r i n g a t lower temperatures i s advantageous from the p o i n t of view of c o n s e r v a t i o n of energy. I n the case of formaldehyde, however, the perman e n t damage w h i c h , as F i g u r e 2 shows, i s a s i g n i f i c a n t p a r t o f t h e t o t a l damage, i s a b o u t t h e same o v e r t h e whole t e m p e r a t u r e range o f 85°C t o 1 6 0 ° C . As t h e s e r e s u l t s w i t h f o r m a l d e h y d e i n d i c a t e t h a t a d e c r e a s e i n t e m p e r a t u r e does n o t change t h e r a t i o o f r a t e s o f c r o s s l i n k i n g and c e l l u l o s e h y d r o l y s i s , we d e c i d e d t o measure t h e t e m p e r a t u r e dependence o f t h e s e r a t e s d i r e c t l y . I n an o p t i m a l c r o s s l i n k i n g p r o c e s s , t h e r a t i o between t h e r a t e of c r o s s l i n k i n g , r and t h e r a t e o f h y d r o l y s i s , r , s h o u l d be as l a r g e as possible. B o t h r e a c t i o n s a r e v e r y complex i n t h e sense t h a t s e v e r a l p h y s i c a l processes ( s w e l l i n g of c e l l u l o s e , d i f f u s i o n o f c a t a l y s t s and c h e m i c a l s i n t o t h e f i b r e , a d s o r p t i o n i n t h e f i b e r ) a r e combined w i t h c h e m i c a l p r o c e s s e s ( m e t h y l o l and f o r m a l f o r m a t i o n , p r o t o n a t i o n and d i s s o c i a t i o n o f g l u c o s i d i c bonds between a n h y d r o g l u c o s e u n i t s ) [43, ί ΐ ] · F u r t h e r m o r e , one must c o n s i der t h a t c r o s s l i n k i n g i s r e v e r s i b l e , but h y d r o l y s i s i s n o t ; the r a t e r a t i o ^ decreases therefore w i t h increasing time. The t e m p e r a t u r e dependence o f a c h e m i c a l r e a c t i o n i s u s u a l l y e x p r e s s e d by t h e a c t i v a t i o n e n e r g y r e l a t i o n s h i p . T h i s e q u a t i o n i s u s e d h e r e b u t we s h o u l d l i k e t o emphasize t h a t t h e v a l u e s f o r E a r e , s t r i c t l y s p e a k i n g , o n l y apparent a c t i v a t i o n e n e r g i e s as they c x

H

a

208


Figure 3. Dry crease recovery angle and tensile strength for crosslinking with 15 DMEU using MgCl at 80-160°C ( Δ , solid line) and MgCl /tartaric acid at 85°Candl05°C(A)

95% Confidence Limits

2

Dry Crease Recovery Angle ( w+f ) — * -

2

250°

300°

10.


ZOLLINGER

209

r e f e r to o v e r a l l r a t e c o n s t a n t s , i . e . r a t e constants which, i n c l u d e t e r m s f o r p h y s i c a l and c h e m i c a l s t e p s such as a d s o r p t i o n e q u i l i b r i u m which are a l s o tempera t u r e d e p e n d e n t . (A t r u e a c t i v a t i o n e n e r g y i n m o l e c u l a r terms i s one w h i c h i s r e s t r i c t e d s t r i c t l y t o one i s o l a t e d s t e p on t h e r e a c t i o n p a t h ) . k = A e E A R Τ

= = = =

a

RT

(2)

A c t i v a t i o n energy Preexponential factor G-as c o n s t a n t Absolute temperature

L i t e r a t u r e data f o r the a c t i v a t i o n energy of c r o s s l i n k i n g w i t h f o r m a l d e h y d e and f o r t h e h e t e r o g e neous h y d r o l y s i s o f c e l l u l o s e v a r y g r e a t l y . I t was t h e r e f o r e i m p o r t a n t t o have a r e a c t i o n sys tem where t h e c o n d i t i o n s c o u l d be c a r e f u l l y c o n t r o l l e d . I n p a r t i c u l a r , a l l p a r a m e t e r s e x c e p t t i m e and t e m p e r a t u r e must be c o n s t a n t i n a l l e x p e r i m e n t s . The method u s e d was as f o l l o w s : Samples o f c o t t o n f a b r i c were c r o s s l i n k e d i n an aqueous s o l u t i o n o f form a l d e h y d e , a c i d i f i e d w i t h H C 1 t o pH 1 . 3 0 , i o n i c s t r e n g t h ( K C 1 ) I = 0 . 1 0 a t 6 7 . 4 ° C , 7 7 . 6 0 C and 8 7 . 0 ° C . Indivi d u a l samples were t a k e n out a t v a r i o u s t i m e s and washed. The c e l l u l o s e f o r m a l group c o n t e n t and t h e d e g r e e o f p o l y m e r i s a t i o n o f t h e samples was d e t e r m i n e d and u s e d f o r t h e c a l c u l a t i o n o f t h e o v e r a l l ^ - r a t e c o n s t a n t s k^-ç and k^. The system and t h e k i n e t i c equations used are d e s c r i b e d elsewhere [ 3 4 ] . The e v a l u a t i o n o f t h e k i n ë T i c d a t a y i e l d e d t h e f o l l o w i n g apparent a c t i v a t i o n energies (with 9 5 $ confidence l i m i t s ) : Ε ( c r o s s l i n k i n g ) = 1 1 0 . 8 ± 5.2 k J / m o l (26.5 ± 1.2 k c a l / m o l ) E (hydrolysis) = 1 2 5 . 1 ± 9.3 kJ/mol ( 2 9 . 9 ± 2.2 k c a l / m o l ) The r a t i o o f r a t e c o n s t a n t s k c : k g i s 0 . 0 4 4 a t 6 7 . 4 ° C , but 0 . 0 3 3 a t 8 7 . 0 ° C . A low r e a c t i o n tempera t u r e s h o u l d t h e r e f o r e improve t h e c r e a s e r e c o v e r y a n g l e / t e n s i l e s t r e n g t h r e l a t i o n s h i p . I t c a n be c a l c u l a t e d from the d i f f e r e n c e E ( h y d r o l y s i s ) - E ( c r o s s a

a A

X

a

a

•^cx ^ H c a l l e d overall rate constants be cause, i n c o n t r a s t to k and k g , t h e y r e f e r n o t t o t h e c h e m i c a l r a t e p r o p e r , b u t t o t h e complex r a t e o f d i f f u s i o n , s o r p t i o n and r e a c t i o n . a n (

k

a

r

e

c x

210


l i n k i n g ) = 1 4 . 3 k J / m o l , however, t h a t t h e t e m p e r a t u r e must be l o w e r e d f r o m 160°C t o 95°C t o i n c r e a s e t h e r a t i o k c x · k g by a f a c t o r o f 2 . Such a f a c t o r i s t o o s m a l l t o be r e a l l y s i g n i f i c a n t . 5) The Mechanism o f M e t a l I o n C a t a l y s i s i n t h e C r o s s l i n k i n g of Cotton As m e n t i o n e d b e f o r e , c r o s s l i n k i n g o f c e l l u l o s e i s a complex p r o c e s s c o n s i s t i n g o f p h y s i c a l p r o c e s s e s and a t l e a s t two c h e m i c a l r e a c t i o n s , namely t h e r e a c t i o n o f t h e c r o s s l i n k i n g r e a g e n t s w i t h a h y d r o x y l group o f one c e l l u l o s e c h a i n a n d , s e c o n d , t h e c r o s s l i n k i n g p r o per, i . e . the r e a c t i o n w i t h another c e l l u l o s e c h a i n . I n t h e f o l l o w i n g we w i l l c o n c e n t r a t e on t h e r a t e - d e t e r m i n i n g p a r t of c r o s s l i n k i n g w i t h formaldehyde i n t h e p a d - d r y - c u r e p r o c e s s u s i n g M g C l and M g C l / t a r t a r i c a c i d m i x t u r e s as c a t a l y s t s . C r o s s l i n k i n g of c e l l u l o s e w i t h formaldehyde c o n s i s t s e s s e n t i a l l y o f two c h e m i c a l r e a c t i o n s t e p s , n a mely t h e f o r m a t i o n o f t h e h e m i a c e t a l ( o r 0 - m e t h y l o l , e q u a t i o n 3) and the c r o s s l i n k i n g p r o p e r , i . e . t h e f o r m a t i o n o f t h e f o r m a l ( o r oxymethylene b r i d g e , e q u a t i o n 4). 2

2

C e l l - O H + CH 0 * C e l l - 0 C H 0 H 2

(3)

2

Cell-0CH 0H + Cell -OH * Cell-OCH -0-Cell 1

2

2

+ H 0

1

2

(4)

Many i n v e s t i g a t i o n s have shown t h a t t h e f o r m a t i o n and h y d r o l y s i s o f a c e t a l s i s s u b j e c t t o s p e c i f i c a c i d c a t a l y s i s . T h i s c a n be a c c o u n t e d f o r by t h e machanism ( e q u a t i o n s 5 - 8 ) p o s t u l a t e d by B a r k e r and V a i l Γ451> fast + C e l l - 0 C H 0 H + H+ « »Cell-0CH 0H 2

+ Cell-0CH 0H 2

+ Cell-0CH

2

2

2

(5)

2

slow + + •« [Cell-0CH Cell-0=CH ] 2

+ Cell -OH 1

fast

2

+ Cell-(0H)CH -0-Cell 2

+ , fast Cell-(0H)CH -0-Cell « » Cell-0CH -0-Cell 2

+ H 0

2

2

1

(7)

1

+ H

(6)

+

(8)

C o n c e r n i n g c a t a l y s i s by m e t a l s a l t s , no c l e a r e x p e r i m e n t a l evidence f o r or a g a i n s t the e x i s t e n c e of s p e c i f i c m e t a l i o n complexes as e f f e c t i v e c a t a l y s t s has b e e n p r e s e n t e d p r e v i o u s l y , a l t h o u g h a s p e c i f i c me t a l i o n c a t a l y s i s h a s been p o s t u l a t e d . I t was a r g u e d

10.


ZOLLINGER

211

Γ461 t h a t t h e t h e r m a l d e c o m p o s i t i o n o f MgCl2 i n t o MgX*0H)Cl + HC1 (where HC1 i s t h e c a t a l y s t ) o c c u r s o n l y a t t e m p e r a t u r e s h i g h e r t h a n 1 8 1 ° C and t h a t t h e e q u i l i b r i u m l i e s on t h e s i d e o f t h e h y d r o x i d e s and o x i d e s o n l y a t t e m p e r a t u r e s h i g h e r t h a n 8 0 0 ° C . On t h e o t h e r h a n d , however, a r e l a t i o n s h i p between t h e c a t a l y t i c ac t i v i t y o f m e t a l i o n s and t h e i r L e w i s a c t i v i t y was a s sumed Γ 47 Ί . For mixed c a t a l y s t s of t h e M g C l 2 / t a r t a r i c a c i d t y p e , P i e r c e e t a l . 140] and R a v i k r i s h n a n Γ411 assume t h a t complexes formedH5etween t h e m e t a l i o n and t h e o r ganic a c i d a r e t h e c a t a l y s t s proper. Both emphasize, however, t h a t t h e mechanism o f c a t a l y s i s i s s t i l l u n known. F o r o u r i n v e s t i g a t i o n [ 3 7 ] we chose a k i n e t i c a p p r o a c h . We measured t h e r a t e o f c r o s s l i n k i n g i n t h e presence of v a r i o u s p o t e n t i a l c a t a l y t i c s p e c i e s , alone and i n c o m b i n a t i o n s , a s s u m i n g t h a t , i n t h e l a t t e r case, t h e o b s e r v e d r a t e i s t h e sum o f t h e i n d i v i d u a l r a t e s . Equation 9 describes the c r o s s l i n k i n g r e a c t i o n i n t h e p r e s e n c e o f M g C l , whereby t h e t e r m kjy[g2+[Mg ] r e p r e s e n t s a l l Mg^+ c o n t a i n i n g s p e c i e s w h i c h c a t a l y s e 2+

2

k

SgCl

2

= H 0+rH 0+] k

3

3

+

k

Mg2+

[Mg ] 2+

(9)

t h e r e a c t i o n . S p e c i f i c m e t a l i o n o r m e t a l complex c a t a l y s i s o n l y o c c u r s i f t h e second term i s c o n s i d e r a b l y l a r g e r than t h e term k Q + [ H ^ 0 ] . In order t o i n v e s t i g a t e t h i s question experimen t a l l y one h a s t o t a k e i n t o a c c o u n t t h a t p H - m e a s u r e ments on t h e f a b r i c u n d e r c u r i n g c o n d i t i o n s , i . e . w i t h a v e r y l o w w a t e r c o n t e n t a r e i m p o s s i b l e . On t h e o t h e r hand i t seemed r e a s o n a b l e t o assume t h a t t h e p r e s e n c e of a b u f f e r would a l l o w hydroxonium i o n c o n c e n t r a t i o n i n t h e f a b r i c t o be k e p t a l m o s t c o n s t a n t d u r i n g c u r i n g even i f , by m e t a l complex f o r m a t i o n o r o t h e r r e a s o n s , h y d r o x o n i u m i o n s a r e f o r m e d . I n a d d i t i o n , we t o o k i n t o a c c o u n t t h e p o s s i b i l i t y t h a t i o n i c s t r e n g t h m i g h t be an i m p o r t a n t f a c t o r i n c r o s s l i n k i n g . T h e r e f o r e e x p e r i ments w i t h 0.050 M KC1 were c a r r i e d o u t a t t h e same t i m e a s e x p e r i m e n t s w i t h 0.025 M MgCl2 ( i o n i c s t r e n g t h i n b o t h c a s e s : I = 0.050). The k i n e t i c r e s u l t s i n T a b l e I show t h a t t h e r a t e o f f o r m a l d e h y d e c r o s s l i n k i n g w i t h MgCl2 i s a t l e a s t f i f t y t i m e s g r e a t e r t h a n t h a t o f t h e system w i t h K C 1 . The mere p r e s e n c e o f M g i o n s , however, i s n o t a s u f f i c i e n t c o n d i t i o n f o r c a t a l y s i s : I n the presence o f a b u f f e r w h i c h k e e p s t h e pH a t a b o u t 5, t h e r a t e o f c r o s s l i n k i n g i n t h e p r e s e n c e o f Mg2+ i o n s i s n o t s i g n i f i c a n t l y h i g h e r t h a n t h a t w i t h K+ i o n s and a b u f f e r . Thus, i t i s o b v i o u s t h a t t h e p r o c e s s i s g o v e r n e d b y the +

H

2

I.

2

2

4.7

potassium propionate

4.6

0.5 M HProp -5

-5

?

?

5.1 5.1

+0.5 M K P r o p 0.5 M HProp +0.5 M K P r o p

-

HProp, KProp = P r o p i o n i c a c i d ,

2

6.5 # CH 0/0.200 M KC1

2

2

2

6.5 1o CH 0/0.025 M M g C l 6.5 % CH 0/0.050 M KC1 6.5 $> C H 0 / 0 . 1 0 0 M M g C l

pH on the fabric during curing

2. 6

370 7.3 3.5

X

X

X

X

k

^sec

^sec

10""•5

-1

.5 io- ^sec -1 -1 10"•5 ^sec -1 10*".5

•bs

of c r o s s l i n k i n g c o t t o n w i t h

a t 160°C )

Q t ) S

pH o f t h e padding liquor

(pad-dry-cure

formaldehyde Buffer i n padding liquor

constants k

Observed r a t e

Padding l i q u o r

Table

10.

213


ZOLLINGER

p r o t o n c o n c e n t r a t i o n . T n i s means t h a t a s p e c i f i c a c i d c a t a l y s i s occurs i n the case of MgClg. I t i s i n t e r e s t i n g to discuss b r i e f l y p o t e n t i a l reasons f o r t h i s i n c r e a s e i n hydroxonium i o n concent r a t i o n i n h i g h l y c o n c e n t r a t e d c r o s s l i n k i n g systems c o n t a i n i n g MgClg. 2+ The magnesium aquo i o n Mg[HoO] i s a v e r y weak a c i d ( p K = 9 . 8 a t 100°C [ 4 8 ] ; * t h e r e f o r e not an e f f e c t i v e c a t a l y s t . T h e r m a l d e c o m p o s i t i o n o f MgGl2 i s n o t p r o b a b l e e i t h e r , a s i t h a s been o b s e r v e d o n l y above 1 8 1 ° C [^9]. Complex f o r m a t i o n w i t h c e l l u l o s e , however, a n d , i f u s e d , w i t h N - m e t h y l o l c r o s s l i n k i n g a g e n t s , may r a i s e t h e a c i d i t y d u r i n g c u r i n g . Complexes o f z i n c i o n s w i t h p o l y h y d r o x y l compounds a r e known [ 5 0 ] . T h i s e f f e c t may be enhanced w i t h b l e a c h e d o r d a maged c o t t o n where c a r b o x y l i c groups are p r e s e n t . T h i s type of c a t a l y s i s i s a l s o the cause of the i n c r e a s e d r e a c t i v i t y of M g C l 2 / t a r t a r i c a c i d mixtures i n c r o s s l i n k i n g : I n a manner s i m i l a r t o t h a t d e s c r i b e d above f o r t h e systems c o n t a i n i n g MgCl2 a l o n e , we d e termined the i n d i v i d u a l r a t e c o n s t a n t s f o r the v a r i o u s s p e c i e s p r e s e n t i n s u c h systems [^4, 26]. rate c o n s t a n t s a r e shown i n t h e k i n e t i c e q u a t i o n ( 1 0 ) . a

a

n

d

s

T

k

2îture

= H ot 2°] k

+

k

H

2

\ θ ^

+

H Tart[ 2 H

2

T a r t

]

3

Ε

+

k

0

+

]

+

\

MgTart[ S M

g

i

l

e

s

2 ^

T a r t

e

2

+

]

^

< > 10

The k i n e t i c e x p e r i m e n t s showed t h a t t h e f o l l o w i n g sequence o f c a t a l y t i c a c t i v i t y o c c u r s : obs *HC1 k

. , obs *MgCl /H Tart

β

>2400 :

2

670

2

9

. , obs *H Tart 2

:

9.8

#

. ,obs TigClg :

5.5

#

. , obs MgTart :

1.0

HC1 i s , a t a g i v e n m o l a r c o n c e n t r a t i o n , by f a r t h e most e f f e c t i v e c a t a l y s t i n c r o s s l i n k i n g w i t h f o r m a l d e h y d e by t h e p a d - d r y - c u r e p r o c e s s . HC1 i s a t l e a s t 2400 t i m e s more r e a c t i v e t h a n magnesium t a r t r a t e 430 t i m e s more r e a c t i v e t h a n magnesium c h l o r i d e 240 t i m e s more r e a c t i v e t h a n t a r t a r i c a c i d and 3.6 t i m e s more r e a c t i v e t h a n t h e m i x t u r e M g C ^ / ^ T a r t One c a n a l s o c o n c l u d e t h a t t h e o b s e r v e d c a t a l y t i c a c t i v i t y o f t h e m i x t u r e o f MgCl? and t a r t a r i c a c i d i s 44 t i m e s g r e a t e r t h a n t h e sum o f t h e c a t a l y t i c a c t i v i t i e s o f t h e two components. T h i s f a c t o r i s a measure of the " s y n e r g e t i c e f f e c t of mixed c a t a l y s t s " previous-

214


l y d i s c u s s e d by t h e d i s c o v e r e r s

[40, 41].

of these

catalysts

Our i n v e s t i g a t i o n d e m o n s t r a t e s t h e r e f o r e t h a t c a t a l y s i s by h y d r o x o n i u m i o n a c c o u n t s f o r 74 ( o r more) percent of the t o t a l i n the c a t a l y s i s of c r o s s l i n k i n g w i t h formaldehyde u s i n g mixed m e t a l s a l t c a t a l y s t s . We a l s o s t u d i e d t h e complex e q u i l i b r i a w h i c h a r e r e s p o n s i b l e f o r the h i g h hydroxonium i o n c o n c e n t r a t i o n i n m i x t u r e s o f MgCl2 and t a r t a r i c a c i d . p H - v a l u e s o f s o l u t i o n s o f t a r t a r i c a c i d , o f m i x t u r e s o f MgCl2 and t a r t a r i c a c i d i n a m o l a r r a t i o 1:1, and o f h y d r o c h l o r i c a c i d were measured up t o a c o n c e n t r a t i o n o f 5 . 0 M and compared w i t h v a l u e s c a l c u l a t e d a s s u m i n g t h a t a l l n i n e p a r t i c l e s a r e p r e s e n t i n t h e s y s t e m : Mg2+, CI , H T a r t , H T a r f , T a r t " , MgHTart+, M g T a r t , H+ and OH" ? The e x p e r i m e n t a l p H - v a l u e s d i d i n d e e d d e c r e a s e s t r o n g l y w i t h i n c r e a s i n g c o n c e n t r a t i o n and c o i n c i d e d f a i r l y w e l l w i t h t h e c a l c u l a t e d v a l u e s b a s e d on t h e t y p e o f complex f o r m a t i o n shown i n e q u a t i o n s (11)-(12). R

2

2

Mg

2 +

+ H Tart

MgHTart

2

+

*

MgHTart

*

MgTart + H

+

+ H

+

+

(11) (12)

These d a t a s u p p o r t o u r c o n c l u s i o n s t h a t , f i r s t , d i r e c t c a t a l y s i s by t h e magnesium i o n o r i t s c o m p l e x e s i s n e g l i g i b l e r e l a t i v e to hydroxonium i o n c a t a l y s i s , and s e c o n d , t h a t magnesium i o n s and t h e i r c o m p l e x e s with carboxylic a c i d s have a n i n d i r e c t c a t a l y t i c e f f e c t by i n c r e a s i n g t h e h y d r o x o n i u m i o n c o n c e n t r a t i o n . T h i s c o n c e n t r a t i o n i n c r e a s e d when t h e c o n c e n t r a t i o n o f the m e t a l s a l t / c a r b o x y l i c a c i d system was i n c r e a s e d , but to an e x t e n t g r e a t e r than expected from a l i n e a r r e l a t i o n s h i p . This i s e x a c t l y the process which takes p l a c e i n c u r i n g : Here a d e c r e a s e i n t h e w a t e r c o n t e n t of the dry f a b r i c to the s o - c a l l e d bone-dry f a b r i c t a k e s p l a c e . T h i s i s e q u i v a l e n t t o an i n c r e a s e i n t h e c o n c e n t r a t i o n o f M g C l 2 , t a r t a r i c a c i d and t h e i r comp l e x e s i n t h e l i t t l e w a t e r w h i c h i s s t i l l p r e s e n t and t h e r e f o r e c u r i n g l e a d s to a d r a s t i c i n c r e a s e i n the hydroxonium i o n c o n c e n t r a t i o n . Would i t n o t be e a s i e r t o g e t t h i s h i g h h y d r o x o nium i o n c o n c e n t r a t i o n by s i m p l y a d d i n g g r e a t e r amounts of h y d r o c h l o r i c a c i d to the padding l i q u o r i n s t e a d of MgClg and t a r t a r i c a c i d ? I t i s t r u e t h a t t h e a c t i v i t y r e q u i r e d f o r t h e c r o s s l i n k i n g r e a c t i o n c o u l d be o b t a i n e d ; s i m u l t a n e o u s l y , however, the r a t e of h y d r o l y s i s , t h e c a u s e o f t h e permanent damage, w o u l d be i n Complex e q u i l i b r i u m c o n s t a n t s f o r t h i s system known [ 5 1 J .

are

10.

215


ZOLLINGER

c r e a s e d . The r a t i o k ^ : k g d i s c u s s e d i n t h e t h i r d sec t i o n o f t h i s p a p e r w o u l d be i n f l u e n c e d u n f a v o r a b l y b e cause, as mentioned before, t h i s r a t i o decreases w i t h time as c r o s s l i n k i n g i s a r e v e r s i b l e r e a c t i o n , b u t the hydrolysis of cellulose i s i r v e r s i b l e . I n crosslinking w i t h h y d r o c h l o r i c a c i d alone the h i g h hydroxonium i o n c o n c e n t r a t i o n i s p r e s e n t b e f o r e and a f t e r c u r i n g , i . e . f o r a l o n g t i m e . T h i s i s n o t t h e c a s e w i t h MgClp o r MgCl2/tartaric a c i d mixtures which are only h i g h l y a c i d i c i n the extremely concentrated s o l u t i o n s present i n c u r i n g . A f t e r c u r i n g t h e f a b r i c w i l l adsorb mois t u r e a g a i n and t h u s t h e a c i d i t y w i l l d e c r e a s e . W i t h these metal i o n c a t a l y s t s i t w i l l decrease t o an ex t e n t g r e a t e r than expected from a l i n e a r r e l a t i o n w i t h t h e m o i s t u r e c o n t e n t ; w i t h HC1, however, t h e d e c r e a s e is linear. What i s t h e r e a s o n f o r t h e f a c t t h a t t a r t a r i c a c i d o r c i t r i c a c i d a r e much b e t t e r c o c a t a l y s t s t h a n u n s u b s t i t u t e d c a r b o x y l i c a c i d s such as a c e t i c a c i d o r d i c a r b o x y l i c a c i d s s u c n a s s u c c i n i c a c i d ? The m e t a l i o n c o m p l e x e s w i t h c a r b o x y l i c a c i d s and d i c a r b o x y l i c a c i d s a r e more s t a b l e i f t h e a c i d s c o n t a i n h y d r o x y l i c groups i n a p o s i t i o n alpha t o t h e c a r b o x y l i c group(s). The c o m p l e x e q u i l i b r i a a r e s h i f t e d t o t h e s i d e o f t h e complex (and t h e r e f o r e more h y d r o x o n i u m i o n s a r e formed) i f a c h e l a t e complex s u c h a s 1 i s f o r m e d . The s t a b i l i s a t i o n o f m e t a l complexes by c h e l a t e f o r m a t i o n was i n v e s t i g a t e d on a q u a n t i t a t i v e b a s i s f o r t h e f i r s t t i m e b y S c h w a r z e n b a c h a t t h e U n i v e r s i t y o f Z u r i c h Γ521; He f o u n d t h a t complex e q u i l i b r i u m c o n s t a n t s i n c r e a s e by f a c t o r s o f up t o 1θ6 on c h e l a t e f o r m a t i o n . He c a l l e d t h i s f a c t o r t h e c h e l a t e e f f e c t . We c o n c l u d e t h a t t h i s effect i s the essential basis of the a c t i v i t y of the c o c a t a l y s t s i n c r o s s l i n k i n g d i s c o v e r e d b y P i e r c e and P r i c k [40], and b y R a v i k r i s h n a n e t a l . [41]· x

V

HÇ-0H HC-OH ι CL 0* 0H N

V

H

Mg

+

M g

2+

„ 2

s t e

P

s

HÇ-0 HC-OH ι J3. o' 0H S

ι

+ 2 H

Y2:1

+Mg

+

2+

complex

(13)

216


6) C o n c l u d i n g Remarks I n summary, we have d e s c r i b e d f i r s t some b a s i c i n v e s t i g a t i o n s of m a c r o m o l e c u l e s c a r r i e d out i n t h e s e c o n d p a r t of the 1 9 t h and the f i r s t h a l f of the 2 0 t h c e n t u r y w h i c h became the b a s i s o f a p p l i e d r e s e a r c h on l i g h t - w e i g h t f a b r i c s c a l l e d "Swiss c o t t o n " (second s e c t i o n ) and w h i c h i n f l u e n c e d a l s o the i n v e n t i o n o f t e x t u r i z e d y a r n , d i s c u s s e d i n the t h i r d s e c t i o n . The f o u r t h s e c t i o n r e f e r s a l s o t o a p p l i e d r e s e a r c h , namely t o the o p t i m i z a t i o n r e s e a r c h i n c r o s s l i n k i n g c o t t o n f a b r i c s . The l a s t s e c t i o n c o n t a i n s r e s u l t s of b a s i c r e s e a r c h , namely on the mechanism o f c a t a l y s i s i n c r o s s l i n k i n g ; e x p l a n a t i o n s a r e g i v e n f o r the e m p i r i c a l o b s e r v a t i o n s made i n i n v e s t i g a t i o n s of the o p t i m i z a t i o n of the c r o s s l i n k i n g p r o c e s s . Acknowledgement R e s e a r c h p r o j e c t s of our Department w h i c h a r e desc r i b e d i n t h i s p a p e r were s u p p o r t e d by the I n t e r n a t i o n a l I n s t i t u t e f o r C o t t o n and by the S w i s s F o u n d a t i o n f o r the S u p p o r t of S c i e n t i f i c R e s e a r c h as w e l l as by the S w i s s c h e m i c a l and t e x t i l e i n d u s t r y . Literature Cited

[l] Nägeli C. and Schwendener S. "Das Mikroskop, Theorie und Anwendung desselben", second edition, Engelmann-Verlag, Leipzig 1877. [2] Nägeli C. "Die Stärkekörner", Friedrich Schulthess, Zürich 1858. [3] Staudinger Η., Ber. deutsch. chem. Ges. (1920) 53, 1073. [4] Frey K. "Ueber die Konstitution der Polysaccha ride", Ph.D. Thesis ΕΤΗ Zürich, 1926. [5] Staudinger Η., Frey K., Signer R., Starck W. and Widmer G., Ber. deutsch. chem. Ges. (1930) 63, 2308. [6] Kuhn W., Kolloid-Z. (1934) 68, 2. [7] Kuhn W., Kolloid-Z. (1936) 76, 258. [8] Kuhn W. and Kuhn Η., Helv. Chim. Acta (1943) 26, 1394; (1946) 29, 71. [9] Frey-Wyssling A., Mühlethaler K. and Wyckoff R. W. G., Experientia (1948) 4, 475. [10] Mühlethaler K., Beiheft Z. Schweiz. Forstverein (1960) 30, 55. [11] Mühlethaler Κ., The fine structure of the cellu lose microfibril. In "Cellular Ultrastructur of Woody Plants", pp. 191-198, Ed. Coté W. Α., Syra cuse University Press, Syracuse N.Y. 1965.

[12] Muggli R . , C e l l u l o s e Chem. Technol. (1968) 2, 549. [13J Muggli R .Cellulose , Elias Fiber H.-G . and Minu Switzerhnd h l e t h a l e r K., Makro10. ZOLLINGER Finishing 217 mol. Chem. (1969) 121, 290. [14] van Bemmeler J. Μ., Z. anorg. Chem. (1896) 13, 333. [15] McBain J. W., Trans. Faraday Soc. (1913) 9, 99. [ 1 6 ] Reychler Α . , Kolloid-Z. (1913) 12, 277. [17] Döhle W., Melliand Textilberichte (1943) 24, 430. [18] Heberlein & Co. AG, LRP 280 134 (1913), Heberlein G. (inventor). [19] See summary by Weiss E., Textilveredlung (1968) 3, 447. [20] Tootal, Broadhurst, Lee Co. Ltd., DRP 499 818 (1917),Foulds R. P., Marsh J. T. and Wood F. C. (inventors). [21] See summary by Marsh J. T. "Self Smoothing Fab rics", Chapman and Hall Ltd., London 1962. [22] Raduner & Co. AG, US P . 2 161 223 (1933), Bener C. ( i n v e n t o r ) . [23] Tootal, Broadhurst, Lee Co. Ltd., Brit. P. 425 032 (1933), Lantz L. A. and Morrison A. L. (inventors). [24] Raduner & Co. AG, Swiss P. 419 043 (1962), Lauchenauer A. E. (inventor). [25] Schwemmer Μ., Bors H. and Götz Α., Textilveredlung (1975) 10, 15. [26] Kokot S., Matsuoka M., Meyer U. and Zürcher J., Textilveredlung (1975) 10, 127. [27] Meyer U. and Zürcher J., Textilveredlung (1976) 11, 325. [28] Heberlein & Co. AG, DRP 618 050 (1932), Kägi R. H. (inventor). [29] Heberlein & Co. AG, Austrian P. 167 114 (1940), Heberlein G. (inventor). [30] Abend Th., Stamm O. A. and Zollinger Η., Helv. Chim. Acta (1966) 49, 1391. [31] Patel S., Rivlin J., Samuelson T., Stamm O. A. and Zollinger Η., Textile Res. J. (1968) 38, 226. [32] Rüttiger W., Rümens W. and Feinauer Α., Melliand Textilberichte (1963) 44, 296. [33] Lewin M. and Weinstein S., Textile Res. J. (1967) 37, 751. [34] Müller K. "Lie Ursache der schlechten EffektEestigkeitsrelation formaldehydvernetzter Baumwollgewebe". Ph.D. Thesis ΕΤΗ Zürich 1974. [35] Meyer U., Müller K., Rouette Η. K. and Zollinger Η., Textile Res. J. (1976) 46, 691. [36] Meyer U., Müller K. and Zollinger Η., Textile Res. J. (1976) 46, 756. [37] Meyer U., Müller K. and Zollinger Η., Textile Res. J. (1976) 46, 813.

218

CELLULOSE AND FIBER SCIENCE [38] Frick J. G . , Andrews Β. A . K . and Reid J. D . , Textile Res. J. (1960) 30, 495. [39] du Bois W. F., M e l l i a n d T e x t i l b e r i c h t e (1972) 53, 802. [40] P i e r c e , J r . A . G. and Frick, Jr. J. G., Amer. Dyestuff Rep. (1968) 57, 864. [41] R a v i k r i s h n a n M. R., Wadia M. J. and C h i p a l k a t t i H. R. " C a t a l y s t f o r Resin F i n i s h i n g of C e l l u l o s e Textiles", SIRTEC, Institut Textil de France, Paris 1969. [42] Kravetz L. and Ferrante G. R., Textile Res. J. (1970) 40, 362. [43] Annen O., Rouette Η. Κ., Rys P. and Zollinger H., Textilveredlung (1972) 7, 528. [44] Annen O . , Rys P., Terada Y . and Z o l l i n g e r Η . , M e l l i a n d T e x t i l b e r i c h t e (1973) 54, 387. [45] Barker R. H . and Vail S. L., T e x t i l e Res. J . (1967) 37, 1077. [46] Rys P. and Zollinger Η., Textilveredlung (1969) 4, 895. [47] Klein E. and Bingham Β. Ε. Μ., Textile Res. J. (1964) 34, 585. [48] Perrin D. D., Pure A p p l . Chem. (1969) 20, 133. [49] Ullmann's Enzyklopädie, vol. 12, p. 120, Urban and Schwarzenberg, München-Berlin 1960. [50] Richards N. J. "The Aqueous Zink Chloride System and its Complex Formation with Cellulose-Related Compounds", Ph.D. Thesis, Lawrence University, Appleton, Wisc. 1969. [51] Cannen R. K. and Kibrick Α., J. Amer. Chem. Soc. (1938) 60, 2314. [52] Schwarzenbach G., Helv. Chim. Acta (1952) 35, 2344.

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