Polymers as Lubricating-Oil Viscosity Modifiers - ACS Symposium Series

May 13, 1991 - DOI: 10.1021/bk-1991-0462.ch015. ACS Symposium Series , Vol. 462. ISBN13: 9780841220096eISBN: 9780841213180. Publication Date ...
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Chapter 15

Polymers as Lubricating-Oil Viscosity Modifiers

Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch015

G. Ver Strate and M.

J . Struglinski

Polymers Group and Paramins Technology Division, Exxon Chemical Company, Linden, NJ 07036

The basic principles of "viscosity modification" of lubricating oils by high molecular weight polymers is discussed. The Newtonian viscosity-temperature characteristics of the oil are not strongly affected by the polymer. The use of polymers simply permits low viscosity basestocks with good temperature charac­ teristics to be employed. A variety of polymer types are used commercially, each having a different balance in cost-performance characteristics. A semiquantitative discussion of the polymer solution physics is presented which explains the general per­ formance of the different polymer types as a function of temperature and strain rate. Certain details of non-Newtonian behavior at low temperature are not completely understood. It i s economically and t e c h n i c a l l y u s e f u l t o employ p o l y m e r c o n t a i n i n g " m i n e r a l " o i l s as l u b r i c a t i n g f l u i d s f o r i n t e r n a l combustion engines. T h e r e a r e many i n t e r e s t i n g s c i e n t i f i c i s s u e s t o be u n d e r s t o o d i n t h e p e r f o r m a n c e o f s u c h f l u i d s as l u b r i c a n t s . The " o r i g i n a l " technology which employed simple o i l s o l u b l e polymers such a s p o l y i s o b u t y l e n e , to create f l u i d s with d e s i r a b l e v i s c o s i t y and viscosity-temperature c h a r a c t e r i s t i c s , has m a t u r e d t o see block, long chain branched, narrow molecular weight d i s t r i b u t i o n and i n t r a m o l e c u l a r l y t a p e r e d c o m p o s i t i o n m a t e r i a l s y i e l d s p e c i f i c tailored properties. The c o n t r i b u t i o n o f v i s c o e l a s t i c e f f e c t s t o t h e b a l a n c e o f f u e l economy and wear i s n o t y e t c o m p l e t e l y r e solved. Some p o l y m e r s a l s o c o n t a i n p o l a r f u n c t i o n a l g r o u p s so t h a t t h e y can a c t as d i s p e r s a n t s f o r s l u d g e e t c . as w e l l as " v i s c o s i t y modifiers."

0097-6156/91/O462-O256$06.00/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

15.

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The b a s i s f o r t h i s t e c h n o l o g y has been r e v i e w e d t o v a r y i n g d e g r e e s i n a number o f a r t i c l e s ( 1 - 1 3 ) . T h e s e r e f e r e n c e s p r o v i d e a s a m p l i n g o f t h e t e c h n o l o g y as i t e x i s t s t o d a y . In c e r t a i n i n s t a n c e s t h e r e a p p e a r s t o be some m i s c o n c e p t i o n as t o what i s g o i n g on. We p r e s e n t a b r i e f r e v i e w o f what we p e r c e i v e t o be t h e facts. Multigrade O i l Formulation. Petroleum r e f i n e r i e s produce a range o f h y d r o c a r b o n " c u t s " f o r use as l u b e o i l b a s e s t o c k s , w h i c h c o n t a i n m o l e c u l e s r a n g i n g i n m o l e c u l a r w e i g h t f r o m a few h u n d r e d t o a few thousand. T h e s e h y d r o c a r b o n f l u i d s a c t as u s e f u l l u b r i c a n t s v i a a hydrodynamic mechanism, a l t h o u g h i n t y p i c a l g a s o l i n e o r d i e s e l f i r e d e n g i n e s some b o u n d a r y l u b r i c a t i o n a l s o o c c u r s . I t s h o u l d be n o t e d t h a t m i n e r a l and s y n t h e t i c b a s e s t o c k s a r e N e w t o n i a n l i q u i d s above t h e i r c l o u d p o i n t , v i s c o s i t y i s i n d e p e n d e n t o f s h e a r s t r e s s . The g l a s s t r a n s i t i o n t e m p e r a t u r e o f t h e s e o i l s i s i n t h e -70°C + 30°C r a n g e and t h e y e x h i b i t a v e r y s i g n i f i c a n t v i s c o s i t y c h a n g e i n t h e t e m p e r a t u r e r a n g e f r o m -40°C t o 170°C where t y p i c a l g a s o l i n e engines operate. A m i n e r a l o i l w h i c h has h i g h enough m o l e c u l a r w e i g h t t o have a d e q u a t e v i s c o s i t y a t h i g h t e m p e r a t u r e , e . g . 2.5-5.0 cP a t 150°C, w i l l have t o o h i g h a v i s c o s i t y , >30,000 cP, a t -25°C. T h i s i s an o r d e r o f m a g n i t u d e t o o h i g h t o p e r m i t e n g i n e c r a n k i n g w i t h b a t t e r i e s and s t a r t e r m o t o r s t y p i c a l l y f o u n d i n t o d a y ' s engines. Some m i n e r a l o i l b a s e s t o c k s have b e t t e r v i s c o s i t y t e m p e r a t u r e c h a r a c t e r i s t i c s t h a n o t h e r s , and s y n t h e t i c b a s e s t o c k s ( u s u a l l y m i x t u r e s o f p o l y a l p h a o l e f i n s and d i - o r t r i e s t e r s o f a d i c a r b o x y l i c a c i d and a p o l y a l k y l e n e g l y c o l ) a r e p a r t i c u l a r l y g o o d , b u t e x p e n s i v e t o make. A l l n a t u r a l p a r a f f i n i c b a s e s t o c k s u s e d as c r a n k c a s e l u b r i c a n t s have m e t h y l e n e s e q u e n c e - b a s e d waxes i n them ( l i n e a r and 2-methyl a l k a n e s h a v i n g c a r b o n numbers between 18 and 3 0 ) , w h i c h can a l s o i n c r e a s e low s t r e s s v i s c o s i t y a t low t e m p e r a ­ t u r e s i f a n i s o t r o p i c wax c r y s t a l s f o r m . The l o w e r m o l e c u l a r w e i g h t b a s e s t o c k s ( e . g . SAE 5W v s . SAE 30) g e n e r a l l y have a l e s s s t e e p t e m p e r a t u r e d e p e n d e n c e o f v i s c o s i t y . T h i s i s b a s i c a l l y a f r e e volume e f f e c t , w i t h T d e c r e a s i n g as m o l e c u l a r w e i g h t d e c r e a s e s , s i n c e t h e y have s i m i l a r c o m p o s i t i o n s apart from m o l e c u l a r weight. As c a n be s e e n i n F i g u r e 1, t h e d i f f e r e n c e between an SAE 5W and an SAE 30 o i l a t 100°C i s a b o u t a f a c t o r o f 2.5 i n v i s c o s i t y , w h e r e a s a t 10°C i t i s a b o u t a f a c t o r o f 10. I f t h e SAE 5W b a s e s t o c k i s t h i c k e n e d w i t h enough p o l y m e r ( i n t h i s c a s e , a random e t h e n e - p r o p e n e c o p o l y m e r ) t o g i v e i t t h e same 100°C v i s c o s i t y as t h e SAE 30 o i l , i t g e n e r a l l y m a i n t a i n s t h e v i s c o s i t y - t e m p e r a t u r e s l o p e o f t h e SAE 5W o i l ( F i g u r e 1 ) . T h i s i s r e f e r r e d t o as a " m u l t i g r a d e " o i l , and g i v e n t h e d e s i g n a t i o n SAE 5W-30, f o r r e a s o n s t o be d i s c u s s e d b e l o w . P o l y m e r i s added t o a c o n c e n t r a t i o n , c, s u c h t h a t ο[η] i s a b o u t 1, c o r r e s p o n d i n g t o t h e r e g i o n where t h e p o l y m e r c o i l s b e g i n t o o v e r l a p as t h e s o l u t i o n moves 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 f r o m t h e d i l u t e t o t h e s e m i d i l u t e regime. The i n t r i n s i c v i s c o s i t y [η] i s 0.5 t o 2 d l / g m f o r p o l y m e r s u s e d as VM's so t h e p o l y m e r i s p r e s e n t a t a b o u t 0.5 t o 2 w e i g h t %. T h i s p o l y m e r c o n c e n t r a t i o n has o n l y a s m a l l e f f e c t on t h e o v e r a l l ry-T r e l a t i o n , u n l e s s some s t r o n g a s s o c i a t i o n o r phase change behavior o c c u r s . The Newtonian v i s c o s i t y o f t h e SAE 5W-30 o i l i s f o u r t i m e s l o w e r t h a n t h a t o f t h e SAE 30 a t 10°C, and t h e g

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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m a r g i n grows f u r t h e r a s t h e t e m p e r a t u r e i s l o w e r e d . T h i s i s why any o i l s o l u b l e p o l y m e r t h a t u n d e r g o e s n o u n u s u a l " p h a s e " c h a n g e b e h a v i o r c a n be a s o - c a l l e d " v i s c o s i t y m o d i f i e r . " We d o n o t show d a t a h e r e below a b o u t 10°C, b e c a u s e o f c o m p l i c a t i o n s due t o wax c r y s t a l l i z a t i o n at lower temperatures. T h i s a l s o does n o t t a k e i n t o account the non-Newtonian shear t h i n n i n g t h a t occurs a t high shear s t r e s s e s f o r the polymerc o n t a i n i n g o i l , r e s u l t i n g i n s t i l l lower v i s c o s i t i e s . I t t u r n s out t h a t a t h i g h s h e a r s t r e s s , t h e SAE 5W-30 o i l d e s c r i b e d above has t h e same v i s c o s i t y a t -25°C as a N e w t o n i a n SAE 5W o i l . T h u s , m u l t i g r a d e o i l s c a n be p r o d u c e d u s i n g v i s c o s i t y m o d i f i e r s , due t o a more f a v o r a b l e v i s c o s i t y - t e m p e r a t u r e r e l a t i o n s h i p from t h e u s e o f a l o w e r m o l e c u l a r w e i g h t m i n e r a l o i l , and s h e a r t h i n n i n g o f t h e p o l y ­ mer a t h i g h s h e a r r a t e s . There a r e important p e r t u r b a t i o n s around t h i s b a s i c p r i n c i p l e . I f t h e r e i s polymer c o i l expansion or c o n t r a c t i o n with temperature due t o ( 1 ) r o t a t i o n a l i s o m e r i c b a c k b o n e e f f e c t s , (2) e x c l u d e d v o l u m e e f f e c t s , ( 3 ) m i c e l l i z a t i o n , ( 4 ) c r y s t a l l i z a t i o n , o r some o t h e r phenomenon, t h e v i s c o s i t y t e m p e r a t u r e c h a r a c t e r i s t i c s w i l l be f u r t h e r m o d i f i e d . G e n e r a l l y t h e s e e f f e c t s are s m a l l , on the o r d e r o f f a c t o r s o f 2, compared t o f a c t o r s o f >10 c h a n g e s due t o d i f f e r ­ e n c e s i n b a s e s t o c k t y p e , o v e r t h e same t e m p e r a t u r e r a n g e . F a c t o r s o f 2 a r e n o t t o be s c o f f e d a t , h o w e v e r . I f one i s t r y i n g t o meet a low t e m p e r a t u r e o i l v i s c o s i t y s p e c i f i c a t i o n ( e . g . , SAE J 3 0 0 ( 1 4 ) ) o f 30,000 cP max a t -25°C, t h e r e i s a b i g d i f f e r e n c e between 35,000 and 17,500 c P . ( T h i s i s r e l a t e d t o o i l p u m p a b i l i t y as m e a s u r e d b y t h e MRV and TP-1 m i n i - r o t a r y v i s c o m e t e r ) ( 1 5 ) . Viscosity Modifier Characteristics. A s F l o r y s a i d l o n g ago ( 1 6 ) , i n c a s e s where s o l v e n t c h a r a c t e r i s n o t c h a n g i n g g r e a t l y w i t h temperature, as i s typical with lube o i l s , d^rel/dT i s c o n t r o l l e d m o s t l y b y d/dT, where rjrel i s t h e r a t i o o f t h e p o l y m e r s o l u t i o n v i s c o s i t y t o t h e b a s e o i l v i s c o s i t y and i s t h e mean square r a d i u s o f g y r a t i o n o f the polymer. There are c u r r e n t l y f i v e polymer types used as commercial viscosity modifiers: p o l y ( e t h e n e - c o - p r o p e n e ) , p o l y a l k y l m e t h a c r y l a t e s (where t h e a l k y l e s t e r s a r e t y p i c a l l y made f r o m C g t o C a l c o h o l s ) , h y d r o g e n ated p o l y ( i s o p r e n e - b - s t y r e n e ) , hydrogenated p o l y ( s t y r e n e - b - ( b u t a diene-r-styrene)), and hydrogenated p o l y i s o p r e n e " s t a r s " . Of these, o n l y t h e m e t h a c r y l a t e s have p o s i t i v e d/dT. Coil d i m e n s i o n s o f t h e o t h e r p o l y m e r b a c k b o n e s a c t u a l l y s h r i n k as Τ i n c r e a s e s ( 1 7 - 1 9 ) , a s shown i n F i g u r e 2. On t h i s b a s i s , t h e a l k y l methacrylates s h o u l d be t h e b e s t p o l y m e r s f o r o b t a i n i n g l o w v i s c o s i t y a t low temperature. N o t i c e t h a t t h e change i n c o i l dimensions i s on the o r d e r o f 50-70% o v e r t h i s temperature range, compared t o more t h a n a n o r d e r o f m a g n i t u d e c h a n g e i n v i s c o s i t y o f an SAE 5W m i n e r a l o i l ( F i g u r e 1 ) . But t h e r e a r e o t h e r p e r f o r m a n c e a n d c o s t c r i t e r i a . S i n c e p o l y m e r s c o s t more t h a n o i l , o n e w o u l d l i k e t o use as l i t t l e polymer as p o s s i b l e . T h i c k e n i n g power i n c r e a s e s w i t h i n c r e a s i n g m o l e c u l a r w e i g h t , and t h e r e f o r e h i g h m o l e c u l a r w e i g h t s y s t e m s w o u l d be p r e f e r r e d . When t h e m o l e c u l e s g e t l o n g enough t h e y c a n b e m e c h a n i c a l l y b r o k e n by t h e s t r e s s e s i n a n e n g i n e ( w h i c h c a n be a t z

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In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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l e a s t on t h e o r d e r o f (10' s e c " ) ( . 0 3 p o i s e ) = 3 x l O dynes/ crrr shear s t r e s s , i n flows t h a t c a n a l s o i n c l u d e complex s q u e e z i n g o r e x t e n s i o n a l c o m p o n e n t s ) . Two b a s i c s h e a r s t a b i l i t y g r a d e s a r e s o l d w i t h e i t h e r 2 0 % o r 10% maximum v i s c o s i t y l o s s i n a b e n c h t e s t ( t h e " K u r t O r b a h n " d i e s e l f u e l i n j e c t o r ) ( 2 0 ) . In t h i s t e s t , a formulated o i l i s c i r c u l a t e d repeatedly through a d i e s e l f u e l i n j e c t o r , f o r a t o t a l o f 30 p a s s e s . The p o l y m e r m o l e c u l e s u n d e r g o some c o m p l i c a t e d e x t e n s i o n a l f l o w t h e s o l u t i o n e n t e r s a n o z z l e a n d upon e x i t a s o i l d r o p l e t s a r e f o r m e d , and b r e a k n e a r the c e n t e r o f each c h a i n . Some p o l y m e r s a r e i n h e r e n t l y more s t a b l e t h a n o t h e r s f o r a g i v e n t h i c k e n i n g power. F o r a l l c a r b o n - c a r b o n bond t y p e p o l y m e r s , i n h e r e n t bond s t r e n g t h s a r e n o t t o o d i f f e r e n t f r o m one p o l y m e r t o the next (21,22). T h i c k e n i n g g o e s a s [η] ~ < S * > ' V M (Zimm m o d e l ) , w h i l e s h e a r s t r e s s p e r p o l y m e r m o l e c u l e g o e s a s [t?]M»y a t c o n s t a n t s h e a r r a t e . The i n t r i n s i c v i s c o s i t y , [η], f o r a g i v e n M, i n c r e a s e s t h e f e w e r t h e number and s i z e o f s i d e g r o u p s and t h e s t i f f e r t h e c h a i n ( i . e . , l a r g e c h a r a c t e r i s t i c r a t i o , C^) a s > C l , where η i s t h e number o f b a c k b o n e bonds and 1 t h e bond l e n g t h . By t h i s m e a s u r e p o l y ( e t h e n e - c o - p r o p e n e ) i s b e s t , followed by styrene-b-HPI or styrene-b-(HPB-r-styrene) f o l l o w e d by polyalkyl methacrylates. The data i n Figure 3 agree reasonably w e l l w i t h t h e s e p r e d i c t i o n s , w i t h t h e e x c e p t i o n o f t h e HPI s t a r s and t h e p o l y ( s t y r e n e - b - H P I ) , w h i c h a r e more s t a b l e t h a n e x p e c t e d f o r t h e i r [η]. T h e y w i l l be d i s c u s s e d i n g r e a t e r d e t a i l b e l o w . P o l y i s o b u t y l e n e i s now e s s e n t i a l l y o f f t h e m a r k e t due t o p o o r mechanical and o x i d a t i v e s t a b i l i t y . Polymethacrylates, while not t e r r i b l y c o s t - e f f e c t i v e , have a d e s i r a b l e v i s c o s i t y - t e m p e r a t u r e relationship and therefore low zero-shear v i s c o s i t y a t low temperatures, a n d s u r v i v e m a i n l y i n m a r k e t s where o u t s t a n d i n g low t e m p e r a t u r e p e r f o r m a n c e a t low s h e a r s t r e s s e s i s d e s i r e d . J

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Recent V i s c o s i t y M o d i f i e r Developments. A g a i n s t t h i s b a s i c s e t o f c o n c e p t s , t h e r e h a v e emerged " h i g h t e c h " v i s c o s i t y m o d i f i e r s . E x a m p l e s a r e shown i n F i g u r e 4. A l t h o u g h t h e S h e l l K r a t o n - t y p e S-b-HPI d i b l o c k p o l y m e r s have been a r o u n d f o r many y e a r s t h e y p r e s e n t an i n t e r e s t i n g case (23). These polymers have m o l e c u l a r w e i g h t f r o m 80,000 t o 120,000, and c o n t a i n a r o u n d 3 0 wt% s t y r e n e . They have extremely narrow m o l e c u l a r weight d i s t r i b u t i o n s , w i t h M /M r f 7 ) M , i . e . a s c/M f o r our s o l u t i o n s ( 3 4 ) . β i s a dimensionless shear r a t e l o c a t i n g t h e o n s e t o f s h e a r r a t e d e p e n d e n c e o f v i s c o s i t y , and i s t y p i c a l l y n e a r 1.5 i n t h i s c o n c e n t r a t i o n r a n g e . T h e s l o p e o f t h e v i s c o s i t y shear r a t e curve i n t h e non-Newtonian r e g i o n s h o u l d be s i m i l a r a t constant c|>]. The f a c t t h a t t h e CCS i s a c t u a l l y r u n a t c o n s t a n t s h e a r s t r e s s s h o u l d make t h e d i f f e r e n c e s even l a r g e r . The S-HPI-1 a n d -2 p o l y m e r s a g a i n have E P - 1 i k e "arms" i n s o l u t i o n extending from t h e s t y r e n e m i c e l l e c e n t e r s . Thus, »? ] v s . Τ i s s i m i l a r a t h i g h t e m p e r a t u r e s t o EP-1, EP-3, o r HPI s t a r 250. T h e MRV o f 9,000 c P i s n e a r l y t h e same a s 200 o r 250, s i g n i f i c a n t l y h i g h e r t h a n E P - 3 o r - 4 . T h e CCS v i s c o s i t y i s measurably b e l o w a l l o t h e r p o l y m e r t y p e s . r;(CCS)/»?(MRV) = 0.27 i s c l o s e t o HPI 200 o r EP-3, b u t t h e a b s o l u t e v i s c o s i t y i s 300 c P lower. T h i s v a l u e c o u l d r e s u l t f r o m one o r b o t h o f t h e f o l l o w i n g reasons. P e r h a p s t h e p o l y s t y r e n e m i c e l l e c o r e becomes somewhat s m a l l e r a t l o w Τ a n d h e l p s keep »7 -| s l i g h t l y s m a l l e r t h a n f o r f r e e EP chains. T h u s t h e 9,000 c P MRV v a l u e i s r e a l l y b e l o w t h e s t a r HPI a t 9,500 and t h e EP-1 a t 12,000 c P . P e r h a p s t h e r e a l s t a r d o e s n o t s h e a r t h i n as much as EP-1 o r t h e S-b-HPI m i c e l l e . A l t e r n a t i v e l y , i t seems r e a s o n a b l e t h a t s t r e s s c o u l d p u l l t h e m i c e l l e s a p a r t t o some e x t e n t . V i s c o s i t y f a l l s o f f d r a m a t i c a l l y f o r S-b-HPI p o l y m e r s above 110°C, where t h e m i c e l l e s a p p e a r t o d i s s o c i a t e . Why c o u l d n ' t t h i s happen t o some e x t e n t u n d e r s t r e s s a t - 2 0 ° C ? P e r h a p s s c a t ­ t e r i n g e x p e r i m e n t s on s h e a r e d s o l u t i o n s , t h e a d d i t i o n o f s m a l l re

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POLYMERS AS R H E O L O G Y MODIFIERS

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In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

15.

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amounts o f s o l v e n t t o " s o f t e n " t h e s t y r e n e d o m a i n s , o r e v e n " h y s t e r e s i s " i n s t r a i n r a t e c y c l e h i s t o r y c o u l d help t o answer t h i s question. The S - b - ( S - r - H P B ) h a s a good c o m b i n a t i o n o f CCS and MRV. P e r h a p s t h e r e i s enough s t y r e n e i n t h e S-r-HPB b l o c k t o c a u s e t h o s e s e g m e n t s t o come o u t o f s o l u t i o n as compared t o t h e HPI s e g m e n t s i n t h e S-b-HPI p o l y m e r . The S-r-HPB segment may f u r t h e r c o l l a p s e i n t o a c e n t e r m i c e l l e a s Τ d r o p s g i v i n g good drç -j/dT, due t o a r e a l s o l v e n t q u a l i t y e f f e c t a n d not r o t a t i o n a l i s o m e r i c e f f e c t s . T h e s h e a r t h i n n i n g o f t h i s s t r u c t u r e i s n o t as g o o d as t h e S-b-HPI s t r u c t u r e , however. F u r t h e r m o r e , we d o n o t know d/dT f o r t h i s polymer. re

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Conclusions Lube o i l t e c h n o l o g y p r o v i d e s a n i n t e r e s t i n g o p p o r t u n i t y t o work c r e a t i v e s c i e n c e from molecule s y n t h e s i s to s e m i d i l u t e s o l u t i o n behavior o f polymers. T h e r e a r e many o p p o r t u n i t i e s r e m a i n i n g t o p r o d u c e an optimum s t r u c t u r e t h a t i s b e s t f o r m e c h a n i c a l s t a b i l i t y , low temperature viscometric properties, fuel economy a n d dispersancy.

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