Mechanical and Chemical Stability of Polymer Latices - ACS

9 Mechanical and Chemical Stability of Polymer Latices D. C. BLACKLEY

Downloaded by PURDUE UNIVERSITY on June 13, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch009

National College of Rubber Technology, The Polytechnic of North London, Holloway, London, N7 8DB, England

The purpose of this paper is to summarise results which have recently been obtained for the effects of various soaps and surfactants upon the mechanical and chemical stability of natural rubber latex, and to indicate the inferences which have been drawn in the course of endeavouring to interpret these observations. Some of these results have already been published and discussed in detail elsewhere; it is intended that the others will be published and discussed in detail elsewhere in due course. Experimental procedure All the results described in this paper were obtained using a high-ammonia centrifuged natural rubber latex concentrate. As yet, no investigations have been made using low-ammonia centrifuged natural rubber latex concentrates. Although generally similar effects will be expected in the case of low-ammonia concentrates, it is possible that the presence of secondary preservatives, such as sodium pentachlorophenate or zinc oxide, will cause some perturbation of the observed effects. It is also the case that no investigations have as yet been made using natural rubber latices from which some of the non-rubber constituents have been removed, nor have any investigations as yet been made using synthetic rubber latices. It seems likely that further progress in understanding the interesting effects which have been observed so far will require that measurements now be made upon rubber latices which have a more closely-defined composition than the high-ammonia centrifuged natural rubber latex concentrate which is widely used in industry. In particular, there is now a need to elucidate the effects of proteins and protein-degradation products in influencing the mechanical stability of natural rubber latex, as well as for further elucidation of the role of indigenous and added soaps and surfactants. Mechanical stabilities were determined at 35 C and 55% total solids content by means of a Klaxon high-speed-stirring mechanical stability test apparatus. The tests were carried out according to the procedure specified in either BS 1672:Part 2:1954 or o

0097-6156/81/0165-0171$05.00/0 © 1981 American Chemical Society

In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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BS 1672:1972. E a c h m e c h a n i c a l s t a b i l i t y was d e t e r m i n e d i n d u p l i c a t e , a n d t h e a v e r a g e o f t h e two v a l u e s r e p o r t e d . The r e p r o d u c i b i l i t i e s o f t h e d e t e r m i n a t i o n s were s a t i s f a c t o r y i n a l l c a s e s . The a b i l i t y o f a s o a p o r s u r f a c t a n t t o enhance t h e c h e m i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x was a s s e s s e d b y a s c e r t a i n i n g i t s e f f e c t upon the m e c h a n i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t i c e s whose s t a b i l i t i e s h a d b e e n r e d u c e d b y v a r i o u s c h e m i c a l m o d i f i c a tions. N a t u r a l r u b b e r l a t i c e s o f r e d u c e d s t a b i l i t y were p r o d u c e d i n t h r e e d i f f e r e n t ways a s f o l l o w s : ( i ) by the a d d i t i o n o f s u f f i c i e n t potassium c h l o r i d e t o reduce the m e c h a n i c a l s t a b i l i t y t i m e o f t h e l a t e x t o a p p r o x i m a t e l y half i t s i n i t i a l value; ( i i ) by the a d d i t i o n o f s u f f i c i e n t a c e t i c a c i d t o reduce t h e mechanical s t a b i l i t y time o f the l a t e x t o approximately h a l f i t s i n i t i a l value; ( i i i ) b y d e - a m m o n i a t i o n t o c a . pH 8.5 b y a e r a t i o n . I n t h e c a s e o f ( i ) , t h e p o t a s s i u m c h l o r i d e was a d d e d t o t h e l a t e x as a 10% aqueous s o l u t i o n . The amount o f p o t a s s i u m c h l o r i d e w h i c h was r e q u i r e d i n o r d e r t o e f f e c t t h e d e s i r e d r e d u c t i o n o f s t a b i l i t y was e q u i v a l e n t t o a p p r o x i m a t e l y 1 p a r t b y w e i g h t p e r 100 p a r t s o f latex solids. I n t h e c a s e o f ( i i ) , t h e a c e t i c a c i d was a d d e d t o t h e l a t e x a s a 5% a q u e o u s s o l u t i o n . The r e q u i r e d amount was a p p r o x i m a t e l y 0.2 p a r t s b y w e i g h t p e r 100 p a r t s o f l a t e x s o l i d s . I n t h e c a s e o f ( i i i ) , d e - a m m o n i a t i o n was e f f e c t e d b y b l o w i n g a i r a c r o s s t h e s u r f a c e o f t h e l a t e x w h i l s t t h e l a t e x was g e n t l y warmed and s t i r r e d . As f a r a s p o s s i b l e , f a t t y - a c i d s o a p s a n d s u l p h a t e / s u l p h o n a t e s u r f a c t a n t s o f h i g h p u r i t y were u s e d . The f u l l d e t a i l s o f t h e g r a d e s a n d p r e p a r a t i v e p r o c e d u r e s a r e g i v e n e l s e w h e r e . The e t h y l ene o x i d e c o n d e n s a t e s were c o m m e r c i a l m a t e r i a l s p r o d u c e d b y r e a c t i n g a m i x t u r e o f c e t y l a n d o l e y l a l c o h o l s w i t h v a r i o u s amounts o f e t h y l e n e o x i d e . The a v e r a g e m o l e r a t i o o f e t h y l e n e o x i d e t o f a t t y a l c o h o l i n t h e s e c o n d e n s a t e s r a n g e d f r o m 2 t o 60. I n endeavouring to i n t e r p r e t t h e e f f e c t s o f t h e s e condensates upon the s t a b i l i t y o f n a t u r a l r u b b e r l a t e x , i t must be b o r n e i n m i n d t h a t c o m m e r c i a l ethylene oxide condensates are u s u a l l y heterogeneous w i t h r e s p e c t to the l e n g t h o f the p o l y e t h y l e n e oxide c h a i n . Furthermore, t h e e x t e n t o f t h e h e t e r o g e n e i t y i n c r e a s e s a s t h e o v e r a l l mole r a t i o o f e t h y l e n e o x i d e t o hydrophobe base i n c r e a s e s . W h e r e v e r p o s s i b l e , t h e s o a p s a n d s u r f a c t a n t s were a d d e d t o the n a t u r a l r u b b e r l a t e x a s d i l u t e aqueous s o l u t i o n s . The c a s e s where t h i s was n o t p o s s i b l e were ( a ) e t h y l e n e o x i d e - f a t t y a l c o h o l condensates o f l o w e t h y l e n e o x i d e : f a t t y a l c o h o l mole r a t i o , and (b) s p a r i n g l y - s o l u b l e f a t t y - a c i d soaps such as l i t h i u m l a u r a t e and c a l c i u m soaps. The f o r m e r were a d d e d a s p a s t e s w i t h w a t e r , t h e l a t t e r a s d r y p o w d e r s . I n a l l c a s e s , t h e l a t e x s a m p l e s were a l l o w e d t o m a t u r e f o r a b o u t t h r e e d a y s a t room t e m p e r a t u r e b e f o r e t h e i r m e c h a n i c a l s t a b i l i t i e s w e r e d e t e r m i n e d . T h i s a l l o w e d some opportunity f o r the attainment o f a d s o r p t i o n e q u i l i b r i u m .


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E f f e c t s o f added f a t t y - a c i d s o a p s u p o n m e c h a n i c a l and c h e m i c a l s t a b i l i t y of n a t u r a l rubber l a t e x ( 1 , 2 , 3 ) S a t u r a t e d s t r a i g h t - c h a i n f a t t y - a c i d soaps ( 1 ) . F i g u r e 1 shows t h e e f f e c t s o f i n c r e a s i n g l e v e l s o f v a r i o u s p o t a s s i u m s a t u r a t e d s t r a i g h t - c h a i n f a t t y - a c i d soaps upon the m e c h a n i c a l s t a b i l i t y of n a t u r a l rubber l a t e x . F o r convenience of making comparisons b e t w e e n t h e v a r i o u s s o a p s , t h e l e v e l s o f added s o a p a r e e x p r e s s e d a s m o l e s p e r 1 0 0 g. o f l a t e x s o l i d s . These r e s u l t s show two f e a t u r e s o f e s p e c i a l i n t e r e s t : ( i ) V e r y l a r g e enhancements o f m e c h a n i c a l s t a b i l i t y c a n b e b r o u g h t a b o u t b y t h e a d d i t i o n o f amounts o f f a t t y - a c i d soap w h i c h a r e s m a l l r e l a t i v e t o t h e amounts o f soap i n d i g e n o u s l y present i n the l a t e x . An i n d i c a t i o n of the c o n c e n t r a t i o n o f i n d i g e n o u s soaps p r e s e n t i n the l a t e x can be g a i n e d from the d i f f e r e n c e b e t w e e n t h e VFA and KOH numbers o f t h e l a t e x , f o r t h e r e a r e good reasons(4) f o r s u p p o s i n g t h a t t h e c o n c e n t r a t i o n o f i n d i g e n o u s s o a p s i s e q u i v a l e n t t o a p p r o x i m a t e l y one t h i r d of t h i s d i f f e r e n c e . U s i n g t h i s index, the c o n c l u s i o n i s reached t h a t the a d d i t i o n o f , say, potassium l a u r a t e i n a n amount e q u i v a l e n t t o c a . o n l y 5-10% o f t h e i n d i g e n o u s s o a p s c a n enhance t h e m e c h a n i c a l s t a b i l i t y b y a f a c t o r o f 3 · (ii) The a b i l i t y o f a n a d d e d f a t t y - a c i d soap t o enhance t h e mecha n i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x depends v e r y much upon the c h a i n l e n g t h o f i t s a l k y l group. F o r any g i v e n m o l a l l e v e l o f a d d i t i o n o f s o a p , t h e optimum enhancement i s o b s e r v e d when t h e a l k y l c h a i n o f t h e s o a p c o n t a i n s a p p r o x i m a t e l y 11 c a r b o n a t o m s . I n F i g u r e 2 , t h e d a t a o f F i g u r e 1 a r e r e - p r e s e n t e d i n s u c h a way as t o d e m o n s t r a t e t h i s p o i n t more c l e a r l y t h a n i s e v i d e n t f r o m F i g u r e 1. I n t h i s d i a g r a m , the mechanical s t a b i l i t y at v a r i o u s m o l a l l e v e l s of a d d i t i o n o f soap h a s b e e n p l o t t e d a s a f u n c t i o n o f t h e number o f c a r b o n atoms i n t h e a l k y l c h a i n o f t h e s o a p . The r e s u l t s p l o t t e d i n t h i s way g i v e a p i c t o r i a l i n d i c a t i o n o f t h e m o l e c u l a r e f f i c i e n c i e s o f t h e v a r i o u s soaps i n e n h a n c i n g t h e mechanical s t a b i l i t y of n a t u r a l rubber l a t e x . The r e s u l t s s u m m a r i s e d i n T a b l e I show t h e e f f e c t o f e q u a l p a r t s b y w e i g h t o f each o f the p o t a s s i u m f a t t y - a c i d soaps upon t h e mechanical s t a b i l i t y of each o f the t h r e e c h e m i c a l l y - d e s t a b i l i s e d latices. F o r convenience i n making comparisons, e s t i m a t e s of the corresponding r e s u l t s f o r unmodified n a t u r a l rubber l a t e x are also included. I t i s c l e a r from these r e s u l t s t h a t the a b i l i t y o f a d d e d p o t a s s i u m f a t t y - a c i d s o a p s t o enhance t h e s t a b i l i t y o f chemi c a l l y - d e s t a b i l i s e d n a t u r a l rubber l a t e x roughly p a r a l l e l s t h e i r a b i l i t i e s t o enhance t h e m e c h a n i c a l s t a b i l i t y o f u n m o d i f i e d n a t u r a l rubber l a t e x . In endeavouring t o e x p l a i n these observations - i n p a r t i c u l a r , the o b s e r v a t i o n that the molal e f f i c i e n c y of a s t r a i g h t - c h a i n f a t t y - a c i d soap i n e n h a n c i n g the m e c h a n i c a l s t a b i l i t y o f n a t u r a l r u b b e r i s a maximum when t h e a l k y l c h a i n o f t h e soap c o n t a i n s a p p r o x i m a t e l y 11 c a r b o n atoms - we h a v e come t o t h e c o n c l u s i o n


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40001

I 0

ι 1

ι 2

ι 3

ι 4

1 5

level of added potassium fatty-acid soap 4

(moles per 100 grams of latex solids x I 0 )

Plastics and Rubber: Materials and Applications

Figure 1. Effect of added straight-chain potassium fatty-acid soaps upon mechani cal stability of natural rubber latex (I). Numbers appended to curves are number of carbon atoms in alkyl chain of soap.


BLACKLEY

Stability of Polymer Latices


4000 r

7

9

II

13

η in C H2 +|C02K n

n


Figure 2. Effect of alkyl chain length of added soap upon mechanical stability of natural rubber latex at four molal levels of addition: (A) 0.84 X 10' ; (B) 2 χ 10 ; (C) 336 X 10 ; (D) 4.20 χ 10 mol/100 g of latex solids (I) 4

4

4


A


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t h a t a d d e d f a t t y - a c i d s o a p s enhance t h e m e c h a n i c a l a n d c h e m i c a l s t a b i l i t y o f n a t u r a l rubber l a t e x p r i n c i p a l l y by making the i n d i g enous s o a p s more e f f e c t i v e a s s t a b i l i s e r s , r a t h e r t h a n b y i n c r e a s i n g t h e t o t a l amount o f a d s o r b e d s o a p p r e s e n t i n t h e l a t e x a n d t h e r e b y i n c r e a s i n g t h e s u r f a c e p o t e n t i a l a t t h e i n t e r f a c e between rubber and water. According t o o u rpresent view, t h e e x i s t e n c e o f t h e optimum s o a p a l k y l c h a i n l e n g t h a r i s e s f r o m t h e b a l a n c e o f t h e f o l l o w i n g two o p p o s i n g t e n d e n c i e s a s t h e a l k y l c h a i n l e n g t h o f t h e added s o a p i s i n c r e a s e d : (i) a n i n c r e a s i n g t e n d e n c y f o r t h e a d d e d s o a p a n i o n t o be a d s o r bed a t t h e r u b b e r - w a t e r i n t e r f a c e ; and (ii) a d e c r e a s i n g a b i l i t y o f t h e a d d e d s o a p a n i o n s t o make t h e i n d i g e n o u s s o a p a n i o n s more e f f e c t i v e a s s t a b i l i s e r s . I n s o f a r a s some o f t h e a d d e d s o a p s may be v e r y e f f i c i e n t i n " a c t i v a t i n g " t h e i n d i g e n o u s s o a p s , i n t h e s e n s e t h a t a s m a l l number o f m o l e c u l e s o f a d d e d s o a p i s a b l e t o a c t i v a t e a l a r g e number o f m o l e c u l e s o f i n d i g e n o u s s o a p , t h e n we h a v e a r e a d y e x p l a n a t i o n f o r t h e a b i l i t y o f some a d d e d s o a p s t o e f f e c t l a r g e enhancements o f m e c h a n i c a l s t a b i l i t y e v e n a l t h o u g h t h e y a r e p r e s e n t i n a n amount w h i c h i s s m a l l compared t o t h e c o n c e n t r a t i o n o f i n d i g e n o u s s o a p s i n the latex. T h e r e t h e n a r i s e s t h e q u e s t i o n o f t h e mechanism b y w h i c h s m a l l amounts o f a d d e d s o a p m i g h t make t h e i n d i g e n o u s s o a p s more e f f e c t i v e as s t a b i l i s e r s . Our p r e s e n t s u g g e s t i o n i s t h a t t h e i n digenous soap a n i o n s a r e m a i n l y p r e s e n t a s c o h e r e n t c l u s t e r s o r " i s l a n d s " a d s o r b e d a t t h e r u b b e r - w a t e r i n t e r f a c e , a n d t h a t mechani c a l déstabilisation o c c u r s b y way o f m e c h a n i c a l l y - i n d u c e d i n t e r a c t i o n s between those r e g i o n s o f t h e s u r f a c e s o f c o n t i g u o u s r u b b e r p a r t i c l e s w h i c h a r e r e l a t i v e l y b a r e o f a d s o r b e d s o a p a n i o n s . The b a r e r e g i o n s t h r o u g h w h i c h t h e i n i t i a l i n t e r a c t i o n o c c u r s may b e e s s e n t i a l l y t h o s e w h i c h must e x i s t i f ( a ) t h e a d s o r b e d s o a p a n i o n s a r e c l u s t e r e d a n d ( b ) ( a s i s known t o be t h e c a s e ) t h e l a t e x p a r t i c l e s a r e n o t s a t u r a t e d w i t h soap. A l t e r n a t i v e l y , theextent of t h e b a r e r e g i o n s i n t h o s e p a r t s o f t h e p a r t i c l e s u r f a c e s where i n t e r a c t i o n i s o c c u r r i n g may be augmented b y e l e c t r o s t a t i c r e p u l s i o n s between t h e c l u s t e r s o f soap a n i o n s on d i f f e r e n t p a r t i c l e s as t h e y a p p r o a c h one a n o t h e r . T h e s e r e p u l s i o n s a r e e n v i s a g e d a s c a u s i n g t h e c l u s t e r s t o move away f r o m t h e r e g i o n s o f i m p e n d i n g contact, ( i n t h i s l a t t e r connection, i t i s i n t e r e s t i n g t o note that N a p p e r h a s p o s t u l a t e d that c o l l o i d p a r t i c l e s which are s t a b i l i s e d b y a n a d s o r b e d s t e r i c s t a b i l i s e r c a n become d e s t a b i l i sed by t h e s t a b i l i s e r molecules moving l a t e r a l l y around t h e s u r face o f the p a r t i c l e w h i l s t remaining attached t o the p a r t i c l e . We a l s o n o t e t h a t c a r b o x y l a t e i o n s w h i c h a r e c h e m i c a l l y c o m b i n e d a t t h e p o l y m e r - w a t e r i n t e r f a c e a r e known t o be c o n s i d e r a b l y more e f f e c t i v e i n c o n f e r r i n g m e c h a n i c a l s t a b i l i t y upon a l a t e x t h a n a r e carboxylate ions which are h e l d a t t h e i n t e r f a c e by a d s o r p t i o n . P r e s u m a b l y t h i s i s b e c a u s e t h e l a t t e r a r e a b l e t o move l a t e r a l l y i n t h e p a r t i c l e s u r f a c e , w h e r e a s t h e f o r m e r a r e n o t . ) We p r o p o s e t h a t a g i v e n number o f a d s o r b e d s o a p a n i o n s i s more e f f e c t i v e i n c o n f e r r i n g m e c h a n i c a l s t a b i l i t y i f a b l e t o move i n d e p e n d e n t l y o f



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each o t h e r than i f aggregated i n t o coherent c l u s t e r s . This i s e i t h e r because the normal e q u i l i b r i u m d i s t r i b u t i o n o f bare patches i s d i f f e r e n t , o r because the e l e c t r o s t a t i c r e p u l s i o n s between p a r t i c l e s c a r r y i n g the adsorbed a n i o n s are g r e a t e r , o r because the s u r f a c e osmotic p r e s s u r e a r i s i n g from the presence o f the adsorbed anions i s greater. A p l a u s i b l e a c t i v a t i o n mechanism i s t h e n one i n w h i c h t h e added soap a n i o n s adsorb a t t h e r u b b e r - w a t e r i n t e r f a c e , m i x w i t h t h e i n d i g e n o u s a d s o r b e d s o a p a n i o n s , and t h e r e b y encourage the coherent c l u s t e r s t o d i s p e r s e . According to this v i e w , the soaps o f s h o r t a l k y l c h a i n l e n g t h a r e r e l a t i v e l y i n e f f i c i e n t a s e n h a n c e r s o f m e c h a n i c a l and c h e m i c a l s t a b i l i t y b e c a u s e t h e y a r e n o t r e a d i l y a d s o r b e d a t t h e r u b b e r - w a t e r i n t e r f a c e . On the o t h e r hand, the soaps o f l o n g a l k y l c h a i n l e n g t h a r e i n e f f i c i e n t because, a l t h o u g h they are s t r o n g l y adsorbed, they are s i m i l a r i n n a t u r e t o t h e i n d i g e n o u s soaps, and t h e r e f o r e have l i t t l e e f f e c t upon the coherence o f the s o a p - a n i o n c l u s t e r s . A soap s u c h as p o t a s s i u m l a u r a t e i s v e r y e f f i c i e n t a s a n enhancer o f mechanic a l a n d c h e m i c a l s t a b i l i t y b e c a u s e o n t h e one h a n d t h e l e n g t h o f i t s a l k y l c h a i n i s s u f f i c i e n t t o ensure t h a t t h e soap a n i o n i s reasonably s t r o n g l y adsorbed a t the rubber-water i n t e r f a c e , and on t h e o t h e r h a n d t h e a l k y l c h a i n i s s u f f i c i e n t l y s h o r t s e r i o u s l y to d i s r u p t the coherence o f the c l u s t e r s o f indigenous adsorbed s o a p a n i o n s when t h e a d d e d a n i o n s m i x w i t h them. S t r a i g h t - c h a i n C J A c a r b o x y l a t e soaps o f v a r i o u s hydrophobe s t r u c t u r e s ( 2 ) . F i g u r e 1 and T a b l e I show t h a t p o t a s s i u m stéarate i s r e l a t i v e l y i n e f f i c i e n t as a n enhancer o f the mechanical and chemical s t a b i l i t y o fnatural rubber l a t e x . T h i s o b s e r v a t i o n was r a t h e r s u r p r i s i n g i n view o f the w i d e l y - h e l d b e l i e f that waters o l u b l e stéarate s o a p s a r e e f f i c i e n t s t a b i l i s e r s f o r a n i o n i c l a t i c e s a t a l k a l i n e pH. I t was t h e r e f o r e o f i n t e r e s t t o i n v e s t i g a t e t o what e x t e n t t h i s i n e f f i c i e n c y i s a g e n e r a l f e a t u r e o f p o t a s s i u m c a r b o x y l a t e s o a p s w h i c h c o n t a i n 18 c a r b o n atoms i n a l l . E x p e r i ments w i t h a r a n g e o f p o t a s s i u m s t r a i g h t - c h a i n C - j g - c a r b o x y l a t e s o a p s h a v e shown t h a t t h e a b i l i t y o f s u c h s o a p s t o e n h a n c e t h e m e c h a n i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x depends m a r k e d l y u p o n t h e c h e m i c a l s t r u c t u r e o f t h e C-jγ h y d r o p h o b e c h a i n ; thus, f o r i n s t a n c e , p o t a s s i u m o l e a t e i s much more e f f e c t i v e i n e n h a n c i n g s t a b i l i t y t h a n i s p o t a s s i u m stéarate. The r e s u l t s f o r e f f e c t s u p on m e c h a n i c a l s t a b i l i t y a r e s u m m a r i s e d i n F i g u r e 3· They show t h a t , b r o a d l y s p e a k i n g , t h e more t h e s t r u c t u r e o f t h e a d d e d s o a p d e v i a t e s f r o m t h a t o f p o t a s s i u m stéarate, t h e g r e a t e r i s t h e a b i l i t y t o enhance m e c h a n i c a l s t a b i l i t y . The e f f e c t s o f t h e C - | Q s o a p s u p o n c h e m i c a l s t a b i l i t y ( a s a s s e s s e d b y t h e methods u s e d i n t h i s work) b r o a d l y p a r a l l e l t h e e f f e c t s o f t h e soaps upon m e c h a n i c a l stability. These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e v i e w t h a t s t a b i l i t y i s enhanced b y r e d u c t i o n o f the coherence o f c l u s t e r s o f i n d i g e n o u s a d s o r b e d soap a n i o n s , i f i t i s a l s o assumed t h a t t h e i n d i g e n o u s s o a p a n i o n s have a c h e m i c a l s t r u c t u r e w h i c h i s s i m i l a r t o t h a t o f t h e stéarate a n i o n .



178

EMULSION POLYMERS AND EMULSION POLYMERIZATION

level of added potassium C

( 8

soap 4

(moles per 100 grams of latex solids x I 0 )


Figure 3. Effect of various straight-chain potassium C carboxylate soaps upon mechanical stability of natural rubber latex (2): (KC ) potassium stéarate; (KC ) potassium oleate; (KC ") potassium elaidate; (KC J potassium linoleate; ( KC ~~) potassium linolenate; (KC ) potassium 12-hydroxy stéarate; (KC ) potassium 9,10-dihydroxystéarate; (KCje^ "*) potassium ricinoleate. Results also are included for potassium laurate (KC ). 18

=

1S

18

=

1H

18

12(0H)

lg

9J0(OH)

0

18

12



9.

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Latices

179

C o u n t e r i o n e f f e c t s (5). E x p e r i m e n t s w h i c h have been c a r r i e d o u t u s i n g l i t h i u m , s o d i u m , p o t a s s i u m , ammonium and m o r p h o l i n i u m l a u r a t e s h a v e shown t h a t t h e e f f e c t s o f t h e s e f i v e l a u r a t e s u p o n m e c h a n i c a l and c h e m i c a l s t a b i l i t y a r e b r o a d l y s i m i l a r , a l t h o u g h such d i f f e r e n c e s as are observed are s t a t i s t i c a l l y s i g n i f i c a n t . The r e s u l t s f o r e f f e c t s u p o n m e c h a n i c a l s t a b i l i t y a r e s u m m a r i s e d i n Table I I . That l i t h i u m l a u r a t e behaves s i m i l a r l y t o , say, p o t a s s i u m l a u r a t e i s p e r h a p s s u r p r i s i n g , i n t h a t i t i s known t h a t a l i t h i u m s a l t i s m o r e ^ f f e c t i v e i n r e d u c i n g the mechanical s t a b i l i t y o f n a t u r a l r u b b e r ^ -ftian i s t h e c o r r e s p o n d i n g p o t a s s i u m s a l t (j6). The i n f e r e n c e h a s b e e n drawn t h a t t h e c o u n t e r i o n o f t h e c a r b o x y l a t e soap h a s a n e g l i g i b l e e f f e c t u p o n t h e a b i l i t y o f t h e soap t o enhance m e c h a n i c a l s t a b i l i t y , r e l a t i v e t o t h e e f f e c t o f t h e anion, a tl e a s t f o r those c a t i o n s f o r which s p e c i f i c a d s o r p t i o n e f f e c t s are absent. T a b l e I I shows t h a t m o r p h o l i n i u m l a u r a t e i s m a r k e d l y l e s s e f f e c t i v e i n enhancing mechanical s t a b i l i t y than are the o t h e r l a u r a t e s w h i c h have been i n v e s t i g a t e d . This i s attributed t o s p e c i f i c c o u n t e r i o n a d s o r p t i o n , w i t h a consequent r e d u c t i o n o f the e f f e c t i v e s u r f a c e p o t e n t i a l a t the rubber-water i n t e r f a c e . The a b i l i t i e s o f t h e f i v e l a u r a t e s t o p r o t e c t n a t u r a l r u b b e r l a t e x a g a i n s t c h e m i c a l déstabilisation a p p e a r t o b e b r o a d l y p a r a l l e l t o t h e i r e f f e c t s upon m e c h a n i c a l s t a b i l i t y . We have a l s o r e c e n t l y d i s c o v e r e d t h a t a d d e d c a l c i u m l a u r a t e i s a b l e m a r k e d l y t o enhance t h e m e c h a n i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x (2,)· T h i s o b s e r v a t i o n i s s u r p r i s i n g , p a r t l y b e c a u s e o f t h e l o w s o l u b i l i t y o f c a l c i u m l a u r a t e i n w a t e r , and p a r t l y b e c a u s e c a l c i u m i o n s a r e known t o b e p o w e r f u l d e s t a b i l i s e r s o f n a t u r a l rubber l a t e x ( 6 j . I t i n d i c a t e s that the s t a b i l i s i n g e f f e c t o f t h e l a u r a t e a n i o n i s much g r e a t e r t h a n t h e d e s t a b i l i s i n g e f f e c t o f the c a l c i u m c a t i o n . I t i s i m p o r t a n t t o p o i n t out t h a t our i n v e s t i g a t i o n o f count e r i o n e f f e c t s i n c a r b o x y l a t e s o a p s has s o f a r b e e n c o n c e r n e d almost e x c l u s i v e l y w i t h l a u r a t e soaps. L a u r a t e soaps were chosen p a r t l y because they are g e n e r a l l y convenient t o handle i n t h a t many o f them a r e r e a d i l y s o l u b l e i n w a t e r t o g i v e s o l u t i o n s o f l o w v i s c o s i t y , and p a r t l y b e c a u s e , a s h a s b e e n shown a b o v e , l a u r a t e s o a p s a r e v e r y e f f e c t i v e i n e n h a n c i n g t h e m e c h a n i c a l and c h e m i c a l s t a b i l i t y of n a t u r a l rubber l a t e x . I t must t h e r e f o r e be borne i n mind t h a t the c o n c l u s i o n s w h i c h have been drawn from t h i s i n v e s t i g a t i o n concerning effects attributable to counterion v a r i a t i o n i n l a u r a t e s o a p s may n o t b e g e n e r a l l y v a l i d f o r c a r b o x y l a t e s o a p s as a f a m i l y . E f f e c t s o f a d d e d s u l p h a t e and s u l p h o n a t e s u r f a c t a n t s u p o n t h e m e c h a n i c a l and c h e m i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x The e f f e c t s o f a r a n g e o f s o d i u m n - a l k y l s u l p h a t e s and s o d i u m n - a l k y l sulphonate s upon the m e c h a n i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x a r e s u m m a r i s e d i n F i g u r e s 4 and. 5 r e s p e c t i v e l y . As i n t h e case o f added p o t a s s i u m f a t t y - a c i d soaps, s m a l l a d d i t i o n s o f


180


Table Is

E f f e c t o f 0.1 p a r t by weight p e r 100 p a r t s l a t e x s o l i d s of v a r i o u s potassium f a t t y - a c i d soaps upon mechanical s t a b i l i t y o f unmodified and c h e m i c a l l y - d e s t a b i l i s e d n a t u r a l rubber l a t i c e s ΓΠ


number of carbon atoms in alkyl chain of soap

mechanical s t a b i l i t y time (seconds) unmodified latex

l a t e x con t a i n i n g 0.2 p a r t by weight acetic acid per 100 p a r t s of l a t e x solids

500 910 I36O 1210 1140 740

754 1200 2195 2620 2330 1768 1300

530 980 1580 2367 1685 1420 1240

455

490

350

-

5 7 9 11

1310* 3190* 3540 2060 1650 1320

13 15 17 MST o f latex i n absence of f a t t y a c i d soap

860

965

* obtained by

l a t e x deammoniated to pH 8.5

l a t e x con taining 1 p a r t by weight potassium c h l o r i d e per 100 p a r t s o f latex solids

aeration

extrapolation

Table I I ; E f f e c t s o f added l a u r a t e soaps o f v a r i o u s counterions upon mechanical s t a b i l i t y o f n a t u r a l rubber l a t e x (3)

l e v e l o f added soap (moles p e r 100 g o f l a t e x solids)

0.84 2.10

χ 10-4

10~ 3.36 χ 10~ 4.20 χ 10" χ

4

4

4

mechanical s t a b i l i t y time (seconds) i n presence of l a u r a t e soap o f c o u n t e r i o n i n d i c a t e d lithium 2280

2670 3390 3990

sodium

246Ο 3120 364Ο 4080

potassium ammonium morpholinium

2540 3080

2080

29ΟΟ

3880

3280

4220

364Ο


I65O 1950 2 580 3090

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181

Latices

5400 Γ

4600 h

3800h


3000 r

22001

1400 r

fiOOl

4

•

•

•

>

ι

1

1

6

8

10

12

14

16

18

Figure 4. Effect of alkyl chain length of added sodium n-alkyl sulfate upon me chanical stability of natural rubber latex at four levels of addition: (A) 20; (B) 60; (C) 100; (D) 120 mg/100 g of latex sol ids (8)

η in C H , ,OS0 Na n

2f

+

3

5000

r

20001 4

ι 6

ι 8

ι 10

η in C H n

1 12 2n+

1 14

1— 16

,S0 Na 3

Figure 5. Effect of alkyl chain length of added sodium n-alkyl sulfonate upon mechanical stability of natural rubber latex at four levels of addition: (A) 20; (B) 40; (C) 60; (D) 80 mg/100 g of latex solids (S)



182

EMULSION POLYMERS

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b o t h n - a l k y l s u l p h a t e s and n - a l k y l s u l p h o n a t e s c a n cause marked enhancements o f t h e m e c h a n i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x , and t h e m a g n i t u d e o f t h e enhancement a t a n y g i v e n l e v e l o f a d d i t i o n depends u p o n t h e l e n g t h o f t h e a l k y l c h a i n i n t h e s u r f a c t a n t . As i n t h e c a s e o f t h e p o t a s s i u m f a t t y - a c i d s o a p s , a n optimum a l k y l chain l e n g t h i s observed as t h ea l k y l c h a i n l e n g t h o f the s u r f a c tant i s increased. The optimum a l k y l c h a i n l e n g t h i n t h e c a s e o f t h e s u l p h a t e s a n d s u l p h o n a t e s i s a p p r o x i m a t e l y 10 c a r b o n atoms, and t h i s i s v e r y s i m i l a r t o t h e optimum a l k y l c h a i n l e n g t h o f a p p r o x i m a t e l y 11 c a r b o n atoms o b s e r v e d f o r t h e c a r b o x y l a t e s . The e x p l a n a t i o n w h i c h we o f f e r f o r t h e e x i s t e n c e o f optimum a l k y l c h a i n l e n g t h s f o r t h e s u l p h a t e s and sulphonates i s s i m i l a r t o t h a t proposed f o r t h e c a r b o x y l a t e s . The a b i l i t y o f t h e s e s u r f a c t a n t s t o enhance t h e m e c h a n i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x i s a t t r i b u t e d p r i m a r i l y t o t h e i r b e i n g adsorbed a t t h e rubber-water i n t e r f a c e , and t h e r e m i x i n g w i t h t h e anions o f t h e indigenous soaps, thereby a l t e r i n g t h e p h y s i c a l nature o f t h e monolayer o f adsorbed anions. The a b i l i t i e s o f t h e s u l p h a t e s a n d s u l p h o n a t e s t o p r o t e c t n a t u r a l r u b b e r l a t e x a g a i n s t c h e m i c a l déstabilisation a r e a g a i n b r o a d l y p a r a l l e l t o t h e i r a b i l i t i e s t o enhance m e c h a n i c a l s t a b i l ity. Some d a t a a r e a l s o a v a i l a b l e f o r t h e e f f e c t o f t h e c o u n t e r i o n o f a d o d e c y l s u l p h a t e u p o n i t s a b i l i t y t o enhance t h e m e c h a n i c a l s t a b i l i t y o f natural rubber l a t e x . As i n t h e case o f t h e l a u r a t e s t h e l i t h i u m , s o d i u m , p o t a s s i u m a n d ammonium s a l t s a r e s i m i l a r i n behaviour, b u t the morpholinium s a l t i s s l i g h t l y l e s s e f f e c t i v e . Again, thel a t t e r e f f e c t i s a t t r i b u t e d t o s p e c i f i c adsorption o f the morpholinium c a t i o n . C a l c i u m a n d magnesium d o d e c y l s u l p h a t e s are a l s o e f f e c t i v e i n enhancing mechanical s t a b i l i t y , t h e i r a b i l i t i e s being s i m i l a r t o that o f morpholinium dodecyl sulphate. E f f e c t s o f a d d e d n - a l k y l t r i e t h y l ammonium b r o m i d e s u p o n t h e mechanical s t a b i l i t y o f n a t u r a l rubber l a t e x (9) Some i n t e r e s t i n g r e s u l t s h a v e r e c e n t l y become a v a i l a b l e f o r t h e e f f e c t s o f a r a n g e o f n - a l k y l t r i e t h y l ammonium b r o m i d e s u p o n the mechanical s t a b i l i t y o f n a t u r a l rubber l a t e x . The number o f c a r b o n atoms i n t h e a l k y l g r o u p v a r i e d f r o m 6 t o 18. F i g u r e 6 summarises t h e r e s u l t s . I ti susually believed that the addition of c a t i o n i c s u r f a c t a n t s t o an a n i o n i c l a t e x such as n a t u r a l rubber l a t e x i n v a r i a b l y leads t o a reduction i n c o l l o i d s t a b i l i t y , the e f f e c t b e i n g a t t r i b u t e d t o a d s o r p t i o n o f t h e cations w i t h consequent p a r t i a l n e u t r a l i s a t i o n o f t h e p a r t i c l e charge and reduction of thecounterion cloud surrounding the p a r t i c l e s . W h i l s t t h e r e s u l t s s u m m a r i s e d i n F i g u r e 6 show t h a t t h i s e x p e c t a t i o n i s o f t e n r e a l i s e d , t h e y a l s o show t h a t s m a l l a d d i t i o n s o f a n n - a l k y l t r i e t h y l ammonium b r o m i d e whose a l k y l g r o u p c o n t a i n s b e t w e e n a p p r o x i m a t e l y 8 a n d 12 c a r b o n atoms c a n b r i n g a b o u t a small increase i nmechanical s t a b i l i t y . I n such cases, t h e i n c r e a s e i n s t a b i l i t y cannot p o s s i b l y be a t t r i b u t e d t o enhance-



BLACKLEY

Stability of Polymer

1

Latices

1600r

0

4

θ

12

16

level of added quaternary ammonium surfacant 4

(moles per 100 grams of latex solids x I0 )

Figure 6. Effect of added n-alkyl triethyl ammonium bromides upon mechanical stability of natural rubber latex (9). Numbers appended to curves are numbers of carbon atoms in n-alkyl chain of surfactant.


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EMULSION POLYMERS

AND



merit o f t h e c h a r g e c a r r i e d b y t h e p a r t i c l e s , s i n c e p r e s u m a b l y t h e p a r t i c l e charge c a n o n l y be r e d u c e d b y t h e a d d i t i o n o f a c a t i o n i c surfactant. I t appears t h a t i n these cases t h e d e s t a b i l i s i n g e f f e c t o f t h e r e d u c e d p a r t i c l e c h a r g e i s more t h a n o f f s e t b y some o t h e r e f f e c t which accompanies t h e a d s o r p t i o n o f the s u r f a c e a c t i v e c a t i o n a t t h e p a r t i c l e s u r f a c e and which causes t h e s t a b i l i t y t o increase. One p o s s i b l e e f f e c t w h i c h m e e t s t h e s e r e q u i r e ments i s t h e m i x i n g e f f e c t , w i t h r e d u c t i o n o f l a t e r a l c o h e r e n c e w i t h i n t h e adsorbed l a y e r o f soap a n i o n s , w h i c h h a s been p o s t u l a t e d above. To t h i s e x t e n t , t h e o b s e r v a t i o n s o n t h e e f f e c t s o f added c a t i o n i c s u r f a c t a n t s a r e c o n s i s t e n t w i t h t h e s u g g e s t i o n s w h i c h h a v e b e e n made a b o v e , a n d s o a d d w e i g h t t o t h e i r c r e d i b i l i t y . E f f e c t s o f e t h y l e n e o x i d e - f a t t y a l c o h o l c o n d e n s a t e s u p o n t h e mecha n i c a l and c h e m i c a l s t a b i l i t y o f n a t u r a l r u b b e r l a t e x (10) The r e s u l t s s u m m a r i s e d i n F i g u r e 7 show t h a t s m a l l a d d i t i o n s of ethylene o x i d e - f a t t y a l c o h o l condensates t o n a t u r a l rubber l a t e x g e n e r a l l y cause the mechanical s t a b i l i t y o f the l a t e x t o fall. T h i s phenomenon i s a t t r i b u t e d t o t h e d i s p l a c e m e n t o f adsorbed p r o t e i n a c e o u s molecules by the condensate molecules. A l t h o u g h t h e l a t t e r a r e more s u r f a c e a c t i v e t h a n t h e f o r m e r , t h e y are presumably l e s s e f f e c t i v e i n c o n f e r r i n g mechanical s t a b i l i t y upon the r u b b e r p a r t i c l e s , perhaps because, u n l i k e t h e p r o t e i n a ceous m o l e c u l e s , t h e y a r e n o t i o n i s e d . F i g u r e 7 a l s o shows t h a t , w i t h i n c r e a s i n g a d d i t i o n s o f e t h y lene o x i d e - f a t t y a l c o h o l condensates, the mechanical s t a b i l i t y p a s s e s t h r o u g h a minimum a n d t h e n i n c r e a s e s . However, o n l y i f t h e o v e r a l l mole r a t i o o f e t h y l e n e o x i d e t o f a t t y a l c o h o l i n t h e condensate exceeds about 3 0 does t h e m e c h a n i c a l s t a b i l i t y i n c r e a s e above t h a t o f t h e i n i t i a l l a t e x ^ a t l e a s t a t l e v e l s o f a d d i t i o n w h i c h would n o r m a l l y be u s e d i n p r a c t i c e . An i n t e r e s t i n g f a c t emerges when, a s i n F i g u r e 8, m e c h a n i c a l s t a b i l i t y i s p l o t t e d a s a f u n c t i o n o f the l e v e l o f a d d i t i o n o f condensate expressed as m o l e s o f e t h y l e n e o x i d e u n i t s a d d e d p e r u n i t mass o f l a t e x s o l i d s . T h i s i s t h a t , f o r condensates f o r w h i c h t h e o v e r a l l mole r a t i o o f e t h y l e n e o x i d e t o f a t t y a l c o h o l i s w i t h i n the approximate range 6 - 3 0 , t h e a b i l i t y t o enhance t h e s t a b i l i t y o f n a t u r a l r u b b e r l a t e x a f t e r t h e minimum i n s t a b i l i t y h a s b e e n p a s s e d depends p r i m a r i l y u p o n t h e t o t a l number o f e t h y l e n e o x i d e u n i t s w h i c h h a v e b e e n a d d e d t o t h e l a t e x a s t h e c o n d e n s a t e . The l e n g t h o f t h e b l o c k s i n w h i c h t h e u n i t s a r e added i s o f secondary i m p o r t a n c e . We i n t e r p r e t t h i s o b s e r v a t i o n a s i m p l y i n g t h a t , f o r t h e s e c o n d e n s a t e s , t h e e f f e c t upon m e c h a n i c a l s t a b i l i t y i s determined p r i m a r i l y by the b i n d i n g o f water t o t h eethylene oxide u n i t s which a r e anchored t o t h e rubber-water i n t e r f a c e by the f a t t y - a l c o h o l moiety o f the condensate. I n the case o f condensates f o r which t h e o v e r a l l mole r a t i o o f e t h y l e n e o x i d e t o f a t t y a l c o h o l exceeds c a . 3 0 , t h e e f f e c t u p o n m e c h a n i c a l s t a b i l i t y i s much g r e a t e r t h a n w o u l d b e e x p e c t e d o n t h e b a s i s o f t h e t o t a l amount o f e t h y l e n e o x i d e w h i c h h a s been added t o t h e l a t e x , a s e v i d e n c e d b y t h e



9.

BLACKLEY

Stability

of Polymer

Latices

185


Figure 7. Effect of added ethylene oxide-fatty alcohol condensates upon mechanical stability of natural rubber latex (10). Levels of condensate are expressed in parts by weight. Numbers appended to curves indicate overall mole ratio ethylene oxideifatty alcohol in condensate.


186

EMULSION POLYMERS

AND EMULSION POLYMERIZATION


2500 r

I— 0

1

1

1

1

5

10

15

20

ι 25

I 0

1

5

ι 10

ι 15

ι 20

ι 25

ι 30

level of ethylene oxide-fatty alcohol condensate (moles of ethylene oxide units per 100 grams of latex solids x I0 ) 5


Figure 8. Effect of added ethylene oxide-fatty alcohol condensates upon mechani cal stability of natural rubber latex (10). Levels of condensate are expressed in moles of ethylene oxide units. Numbers appended to curves indicate overall mole ratio ethylene oxideifatty alcohol in condensate.


,i 35

9.

BLACKLEY


TableBI:

Stability of Polymer

187

Latices

E f f e c t o f 1 p a r t b y w e i g h t p e r 100 p a r t s l a t e x s o l i d s o f v a r i o u s e t h y l e n e o x i d e - f a t t y a l c o h o l condensates upon mechanical s t a b i l i t y o f unmodified and c h e m i c a l l y d e s t a b i l i s e d natural rubber l a t i c e s

overall mole ratio of ethylene oxide t o fatty alcohol

mechanical s t a b i l i t y time unmodified latex containing 1 part by latex weight potassium c h l o r i d e per 1 0 0 p a r t s of l a t e x solids

(seconds)

latex containing 0 . 2 part by weight acetic acid per 1 0 0 p a r t s of l a t e x solids

l a t e x deammoniated t o pH 8 . 5 b y aeration

2

757

710

623

520

6

415

247

325

500

10

479

364

461

590

14

482

575

558

630

24

531

850

728

708

30

612

1041

802

785

45

1050

1993

II30

1256

60

1810

2578

2003

2015

970

440

499

374

MST o f l a t e x i n absence of ethylene oxide-fatty alcohol condensate




188

results for condensates containing between 6 and 30 moles of ethylene oxide per mole of fatty alcohol. We suggest that in such cases steric stabilisation makes an important contribution to the mechanical stability in addition to stabilisation by hydration. Data for the abilities of these condensates to protect natural rubber latex against chemical déstabilisation (as assessed by the methods used in this work) are given in Table III* The effects upon chemical stability broadly parallel effects upon mechanical stability. An interesting and significant observation is that the chemically-destabilised latices to which the ethylene oxide condensates had been added were in most cases more mechanically stable than the unmodified latex to which the same level of the same condensate had been added. Thus it appears that the ability of an ethylene oxide condensate to enhance the mechanical stability of natural rubber latex is itself enhanced by the presence of factors (such as increased ionic strength) which normally tend to reduce the stability of the latex. In the case where that factor is increased ionic strength, a possible explanation is that the adsorption tendency of the condensate is increased by a saltingout effect. But whatever the explanation, we note that our data provide objective support for the view, widely held in the latexusing industry, that ethylene oxide condensates of suitable composition are very efficient enhancers of the chemical stability of natural rubber latex. Literature cited 1.

Blackley, D.C.; Nor Aisah, bt. A.A.; Twaits, R., Plastics and Rubber : Materials and Applications,1979,4,77

2. Blackley, D.C.; Azas, Μ., Plastics and Rubber : Materials and Applications, 1980, 5, 57 3.

Blackley, D.C.; Haynes, A.C., Plastics and Rubber : Process ing and Applications, in press

4.

Calvert, K.O., Plastics and Rubber : Materials and Applica tions, 1977, 2, 59

5.

Napper, D.H., J . Colloid Interface Sci., 1977, 58, 390

6.

Blackley, D.C.;

7.

Blackley, D.C.; Pieris, H.S.M., unpublished results

8.

Blackley, D.C.; Emengo, F.N., unpublished results

Loha, S.;

Twaits, R., unpublished results

9. Blackley, D.C.; Zahari bin Hashim, unpublished results 10. Blackley, D.C.; Salleh, b. N.B.; Twaits, R., Plastics and Rubber : Materials and Applications, 1977, 2, 117 RECEIVED

April 6, 1981.


Mechanical and Chemical Stability of Polymer Latices - ACS

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