Polysiloxane-Silica Hybrid Resins as Abrasion-Resistant Coatings for

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Downloaded by UNIV OF MELBOURNE on October 14, 2014 | http://pubs.acs.org Publication Date: September 12, 1985 | doi: 10.1021/bk-1985-0287.ch009

Polysiloxane-Silica Hybrid Resins as Abrasion-Resistant Coatings for Plastic Substrates H . L . Vincent, D . J. Kimball, and R. R. Boundy Dow Corning Corporation, Midland, MI 48601

A coating prepared by the hydrolysis of an alkytri-alkoxysilane in the presence of an acidic silica sol exhibits a number of new and interesting features. It is soluble in an alcohol-water solution, can be readily applied to transparent plastic articles, and cured at temperatures below the Tg of the plastic to form a clear, hard, chemical and abrasion resistant coating. The cured film consists of small silica particles chemically bonded within a silicone resin matrix. The use o f t r a n s p a r e n t p l a s t i c m a t e r i a l s i n p l a c e o f g l a s s i s becoming more w i d e s p r e a d i n a p p l i c a t i o n s such as t r a n s p o r t a t i o n , ( e . g . , a u t o m o t i v e s u n r o o f s ) g l a z i n g f o r t r a i n s , buses and a i r c r a f t , windows i n p u b l i c b u i l d i n g s , s a f e t y e n c l o s u r e s i n h i g h r i s k a r e a s and e y e g l a s s l e n s e s . W h i l e polymers such as t h e a c r y l i c s and p o l y c a r b o n a t e s can be r e a d i l y formed i n t o t h e d e s i r e d shapes, a r e more r e s i s t a n t t o breakage than g l a s s and w e i g h t l e s s , t h e i r r e s i s t a n c e t o a b r a s i o n and c h e m i c a l a t t a c k i s r e l a t i v e l y low. T h i s l a c k o f s u r f a c e hardness and a b r a s i o n r e s i s t a n c e has i n t h e p a s t s e v e r e l y r e s t r i c t e d t h e use o f these t r a n s p a r e n t p o l y m e r i c materials. A b r a s i o n r e s i s t a n t c o a t i n g s such as p o l y s i l i c i c a c i d f l u o r i n a t e d copolymer c o m p o s i t i o n s (1) d i d f i n d l i m i t e d commercial use as a b r a s i o n r e s i s t a n t c o a t i n g s . However, i n 1976, H. A. C l a r k , (2,3) d e s c r i b e d a new type o f c o a t i n g t h a t p r o v i d e d t h e c o m b i n a t i o n o f adequate s h e l f l i f e , ease o f a p p l i c a t i o n , low temperature cure and e x c e l l e n t r e s i s t a n c e t o a b r a s i o n and c h e m i c a l a t t a c k . These n o v e l c o a t i n g s a r e based on t h e h y d r o l y s i s o f a l k y l t r i a l k o x y s i l a n e s i n the p r e s e n c e o f an a c i d i c c o l l o d i a l s i l i c a . The term " c o l l o d i a l s i l i c a " refers to stable dispersions or sols of discrete p a r t i c l e s of amorphous s i l i c a . B o t h i n o r g a n i c and o r g a n i c c o a t i n g s have been p r e p a r e d u s i n g c o l l o d i a l s i l i c a a s a component o f t h e c o a t i n g ( 4 ) . T h i s new f a m i l y o f p o l y s i l o x a n e / s i l i c a r e s i n s i s c o n s i d e r e d unique 0097-6156/85/ 0287-0129S06.00/ 0 © 1985 American Chemical Society

In Polymer Wear and Its Control; Lee, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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P O L Y M E R W E A R A N D ITS C O N T R O L

because i t forms an e x t r e m e l y h a r d , c l e a r p r o t e c t i v e c o a t i n g comprised o f v e r y s m a l l s i l i c a p a r t i c l e s c h e m i c a l l y bonded w i t h i n a s i l i c o n e r e s i n m a t r i x . T h i s paper d e s c r i b e s H. A. C l a r k ' s p i o n e e r i n g work on t h e s e a b r a s i o n r e s i s t a n t c o a t i n g s . I t a l s o c o n t a i n s a d d i t i o n a l i n f o r m a t i o n on f i l m f o r m a t i o n , cured f i l m p r o p e r t i e s and c o n c l u d e s t h a t t h e e x c e p t i o n a l performance i s due t o a t i g h t l y c r o s s l i n k e d s t r u c t u r e i n which the c o l l o d i a l s i l i c a p a r t i c l e s a c t as a p a r t o f t h e c r o s s l i n k i n g system.


SYNTHESIS S m a l l samples o f t h e s e h y b r i d r e s i n s can be p r e p a r e d by s i m p l y m i x i n g t h e a l k o x y s i l a n e and aqueous s i l i c a s o l u s i n g t h e w a t e r p r e s e n t i n t h e s o l s as t h e h y d r o l y s i s medium. We have found t h a t m a i n t a i n i n g t h e pH on t h e a c i d s i d e r e s u l t s i n an adequate h y d r o l y s i s r a t e (IA) w i t h o u t an accompanying a c c e l e r a t i o n o f s i l a n o l c o n d e n s a t i o n (IB) t h a t c o u l d r e s u l t i n t h e f o r m a t i o n o f h i g h l y c r o s s l i n k e d r e s i n and g e l p a r t i c l e s .

IA

-SiOR

IB

+

\ '

H 0 — 4 -SiOH + ROH r 2 ι 2-SiOH — ^ - S i O S i - + H 0 ι

o

o

W h i l e many a c i d s can be u s e d , ( 5 ) , a c e t i c a c i d has been found t o be v e r y e f f e c t i v e a t promoting t h e h y d r o l y s i s r e a c t i o n w h i l e m i n i m i z i n g t h e s i l a n o l s e l f - c o n d e n s a t i o n r e a c t i o n and thus p r o d u c i n g more of t h e d e s i r a b l e s i l a n o l e n d b l o c k e d m o n o a l k y l p o l y s i l o x a n e r e s i n . S y n t h e s i s Example. M e t h y l t r i m e t h o x y s i l a n e (50.0 grams) was a c i d i f i e d w i t h g l a c i a l a c e t i c a c i d (1.0 gram). An aqueous d i s p e r s i o n o f a commercial c o l l o d i a l s i l i c a (66.7 grams) h a v i n g a pH o f 9.8 and c o n t a i n i n g 50% w e i g h t s i l i c a o f 13-14 m i l l i m i c r o n s p a r t i c l e s i z e was added t o t h e a c i d i f i e d s i l a n e t o form a methanol-water d i s p e r s i o n o f s i l i c a and m o n o m e t h y l p o l y s i l o x a n e p a r t i c l e condensate. The s i l i c a t o m e t h y l p o l y s i l o x a n e r a t i o was 60/40 ( w e i g h t ) . A f t e r d i l u t i n g t o 22.5 w e i g h t % w i t h i s o p r o p a n o l , the pH was a d j u s t e d t o 5.35 w i t h a c e t i c a c i d . CATALYSIS C o n t r o l o f t h e spontaneous c o n d e n s a t i o n o f t h e s i l a n o l s formed d u r i n g and a f t e r t h e h y d r o l y s i s r e a c t i o n i s v e r y i m p o r t a n t . I f too many s i l a n o l s condense d u r i n g h y d r o l y s i s , t h e s h e l f l i f e w i l l be v e r y s h o r t and g e l f o r m a t i o n w i l l r e a d i l y o c c u r . Therefore, the h y d r o l y s i s o f t h e a l k o x y s i l a n e i s c a r r i e d out under m i l d c o n d i t i o n s and t h e r e s u l t i n g p a r t i a l condensate i s d i l u t e d w i t h w a t e r s o l u b l e s o l v e n t s t o a lower s o l i d s l e v e l . The r e s i n i s cured by f u r t h e r s i l a n o l c o n d e n s a t i o n w h i c h can o c c u r w i t h heat a l o n e . This, however, i s a r e l a t i v e l y slow p r o c e s s t h a t r e q u i r e s c u r e tempera t u r e s above t h e g l a s s t r a n s i t i o n temperature (Tg) o f t h e p l a s t i c s u b s t r a t e . W h i l e many c a t a l y s t s can be u s e d , i n c l u d i n g a c i d s ,


9.

VINCENT E T AL.

Polysiloxane-Silica

Hybrid Resins

131

b a s e s , and o r g a n i c compounds o f m e t a l s , i t has been found t h a t s m a l l amounts o f the s a l t s of weak o r g a n i c a c i d s and s t r o n g bases p r o v i d e the d e s i r e d b a l a n c e o f s h e l f l i f e and cure r a t e . Such c a t a l y s t s i n c l u d e sodium a c e t a t e , p o t a s s i u m a c e t a t e , and the a c e t a t e s a l t s o f q u a t e r n a r y ammonium compounds.


COATING The s u b s t r a t e must be c l e a n and f r e e from p a r t i c u l a t e m a t t e r . A d h e s i o n t o a c r y l i c p l a s t i c s i s n o r m a l l y e x c e l l e n t . We have found t h a t the c o m b i n a t i o n o f i s o p r o p a n o l and a c e t i c a c i d w i l l g e n t l y a t t a c k and s w e l l a p o l y m e t h y l m e t h a c r y l a t e (PMMA) s u r f a c e and i s more a g g r e s s i v e as the m o l e c u l a r w e i g h t o f the PMMA i s decreased. E x c e l l e n t a d h e s i o n t o p o l y c a r b o n a t e can be o b t a i n e d by h e a t i n g the p o l y c a r b o n a t e f o r two hours a t 125°C b e f o r e the c o a t i n g is applied. The p l a s t i c a r t i c l e s can be c o a t e d u s i n g c o n v e n t i o n a l d i p , f l o w o r spray t e c h n i q u e s . However, i n o r d e r t o a c h i e v e optimum f i l m f o r m a t i o n and performance, c o n t r o l s on the t e m p e r a t u r e , r e l a t i v e h u m i d i t y and c l e a n l i n e s s o f the environment d u r i n g s o l v e n t removal are necessary. The c o a t i n g s c o n t a i n f a s t e v a p o r a t i n g s o l v e n t s such as m e t h a n o l , i s o p r o p a n o l and n - b u t a n o l and the s o l v e n t m i x t u r e can f u r t h e r c o n t a i n 10-15 w e i g h t p e r c e n t w a t e r . The s o l v e n t removal must be s u f f i c i e n t l y slow enough, e s p e c i a l l y a t g r e a t e r than 50% r e l a t i v e h u m i d i t y so t h a t the c o a t i n g s u r f a c e i s not c o o l e d below the dew p o i n t o f the s u r r o u n d i n g atmosphere. T h i s would r e s u l t i n the f o r m a t i o n of w a t e r d r o p l e t s known as l a c q u e r b l u s h . H i g h a i r speeds a c r o s s the s u r f a c e o f the a r t i c l e can a l s o induce t h i s f i l m b l u s h i n g . Benard c e l l s (6) can a l s o be formed by a v o r t e x f l o w b e i n g c r e a t e d by the f a s t e v a p o r a t i n g s o l v e n t i n the wet f i l m . H u m i d i t y changes can d r a s t i c a l l y a l t e r e v a p o r a t i o n r a t e s and the s o l v e n t b a l a n c e of w a t e r / s o l v e n t m i x t u r e s . High humidity prolongs d r y i n g time by s l o w i n g down the r a t e o f w a t e r e v a p o r a t i o n from the s o l v e n t b l e n d ( 7 ) . F i l m f o r m a t i o n a t about 25°C and l e s s than 50% r e l a t i v e h u m i d i t y e l i m i n a t e s these k i n d s o f f i l m d e f e c t s . CURE CONDITIONS I t i s n e c e s s a r y t o t h e r m a l l y cure the c o a t e d p l a s t i c i n o r d e r t o a c h i e v e f u l l c r o s s l i n k i n g o f the a i r d r i e d f i l m . The degree o f c r o s s l i n k i n g a f f e c t s not o n l y the a b r a s i o n r e s i s t a n c e o f c o a t i n g , but a l s o the a d h e s i o n and the l o n g term performance under e n v i r o n m e n t a l c o n d i t i o n s . The temperature and time o f c u r e i s a l s o d i c t a t e d by the t h e r m a l s t a b i l i t y and p r o p e r t i e s o f the p l a s t i c s u b s t r a t e . The f o l l o w i n g t a b l e l i s t s recommended c u r e s f o r v a r i o u s plastics.


132


TABLE I - RECOMMENDED CURE PROFILE

PLASTIC

TEMPERATURE

polycarbonate acrylics

125°C 75-95°C

TIME

(hours) 4 4-8


SURFACE PROPERTIES OF COATED PLASTICS T r a n s p a r e n t p l a s t i c g l a z i n g i s o f t e n s u b j e c t e d t o w i p i n g and c l e a n i n g and t h e maintenance o f o p t i c a l q u a l i t y i s i m p o r t a n t . One of t h e b e s t known methods o f measuring such a b r a s i o n o r wear by a Taber a b r a s e r and t o e v a l u a t e t h e degree o f wear by d e t e r m i n i n g t h e p e r c e n t a g e o f l i g h t t h a t i s d i f f u s e d by t h e abraded specimen ( 8 ) , ( 9 ) . The r e s u l t s o f comparing c o a t e d and uncoated a c r y l i c and p o l y c a r b o n a t e t o p l a t e g l a s s u s i n g t h i s t e s t a r e shown i n T a b l e I I . The r e s i s t a n c e o f g l a z i n g m a t e r i a l s t o common c h e m i c a l s , s o l v e n t s and c l e a n i n g agents i s a l s o i m p o r t a n t . A t w e n t y - f o u r hour spot t e s t i n which 1 m l o f t h e reagent was p l a c e d upon t h e s u r f a c e o f the c o a t e d p l a s t i c and covered w i t h a watch g l a s s was used t o measure c h e m i c a l r e s i s t a n c e . A f t e r t h e exposure p e r i o d , t h e spot was wiped c l e a n and examined i m m e d i a t e l y f o r s i g n s o f d e g r a d a t i o n . The c h e m i c a l s l i s t e d i n T a b l e I I I had no e f f e c t upon t h e s u r f a c e o f the cured p o l y s i l o x a n e / s i l i c a c o a t i n g .

TABLE I I ABRASION

RESISTANCE DELTA % HAZE 0.5%

SUBSTRATE Plate Glass Uncoated A c r y l i c Coated A c r y l i c

20% 2.5%

Uncoated P o l y c a r b o n a t e Coated P o l y c a r b o n a t e

40% 1.7%

T e s t Method:

Taber A b r a s e r , 500 c y c l e s , 500 gram l o a d , CS-10F wheels


9.

Polysiloxane-Silica

VINCENT ET AL.

133

Hybrid Resins

TABLE I I I CHEMICAL RESISTANCE


The f o l l o w i n g c h e m i c a l s had no e f f e c t upon the s u r f a c e o f c o a t e d a c r y l i c o r p o l y c a r b o n a t e p l a s t i c s when t e s t e d f o r 24 h o u r s , ASTM D1308, spot t e s t , c o v e r e d . Windex G l a s s C l e a n e r Formula 409 Household C l e a n e r G a s o l i n e - R e g u l a r Grade SAE-30 Motor O i l VM&P Naphtha Isopropanol B u t y l Ether o f Ethylene G l y c o l

Ammonia (A&P sudsy brand) Kerosene Methanol Toluene L i g h t e r f l u i d - Ronsonal Sodium h y d r o x i d e 10% S u l f u r i c a c i d - 90%

DISCUSSION U n l i k e c o n v e n t i o n a l s i l i c o n e r e s i n s , the monomethylpolysiloxane s i l i c a h y b r i d r e s i n s e x h i b i t e x c e l l e n t r e s i s t a n c e t o a b r a s i o n and chemical attack. I t i s f a i r l y w e l l e s t a b l i s h e d that s i l a n e c o u p l i n g agents form m e t a l - o x y g e n - s i l i c o n bonds w i t h m e t a l s u r f a c e s (11). The s i l a n o l groups on the f r e s h l y h y d r o l y z e d m e t h y l t r i m e t h o x y s i l a n e a r e e s s e n t i a l l y the same a s the s i l a n o l s on n e u t r a l s i l a n e c o u p l i n g agents. T h e r e f o r e the s u r f a c e o f the s i l i c a s o l p a r t i c l e s can r e a c t w i t h the monomethy1silane t r i o l (HA).

IIA

-SiOH

Me HOSiOH

+

^

Me -SiOHSiOH

?

' ?

H

H

+

H 0 o

As the r e s i n f i l m undergoes c u r e , these monomethyl r e a c t i v e s i t e s on the s i l i c a s u r f a c e can c r o s s l i n k w i t h t h e p o l y s i l o x a n e s con t a i n e d i n the s u r r o u n d i n g media ( I I B ) . ι

IIB

ψ

-SiOSiOH + MeSiO ( 0 H ) ι ç n/2 /o

Me feiO (OH) _ n/2

3

H0 2

Η M i c r o p h o t o g r a p h s show t h a t the s i l i c a p a r t i c l e s a r e not agglomerated as they pass from the i n i t i a l aqueous s o l t h r o u g h the h y d r o l y s i s w i t h m e t h y l t r i m e t h o x y s i l a n e t o the cured f i l m s t a g e . T o r s i o n a l b r a i d a n a l y s i s s t u d i e s o f c u r e d 50/50 weight % MeSiO^/o/ 2 show v e r y l i t t l e l o s s o f modulus up t o 300°C ( l 2 ) . The modulus o f c o n v e n t i o n a l s i l i c o n e r e s i n s would be reduced by 50-70% o v e r t h i s temperature span. The a b r a s i o n r e s i s tance i s i n c r e a s e d a s the s i l i c a l e v e l i s i n c r e a s e d (Table I V ) . si0

r

e

s

i

n

f

i

l

m

s


134


TABLE I V SAMPLE NO.

SUBSTRATE


1 2 3 4 5 6

PMMA PMMA PMMA PMMA PMMA PMMA

MeSi0

3 / 2

:Si0

2

DELTA % HAZE

100/0 90/10 80/20 70/30 60/40 50/50

4.2 3.0 1.4 0.6 1.5 0.6

The o b s e r v a t i o n s , coupled w i t h t h e cured f i l m s e x c e l l e n t s o l v e n t r e s i s t a n c e , i n d i c a t e s a t i g h t l y c r o s s l i n k e d s t r u c t u r e f r e e from t h e p l a s t i c i z i n g e f f e c t s o f low m o l e c u l a r w e i g h t p o l y s i l o x a n e s and i n which the c o l l o i d a l s i l i c a p a r t i c l e s a c t as part of the c r o s s l i n k i n g system.

Acknowledgments

We would l i k e t o thank H a r o l d A. C l a r k f o r h i s p i o n e e r i n g e f f o r t on t h i s new c l a s s o f s i l i c o n e r e s i n s , f o r h i s c o n t i n u a l o p t i m i s m and the i d e a s r e c e i v e d a f t e r h i s r e t i r e m e n t .

Literature Cited 1. 2. 3. 4.

Engelhardt, Ε. H.; U.S. Patent 3,390,203 (1968). Clark, Η. Α.; U.S. Patent 3,986,997 (1976). Clark, Η. Α.; U.S. Patent 4,027,073, (1977). Iler, R. K.; "The Chemistry of Silica", Wiley-Interscience, 312 (1979). 5. Noll, W.; "Chemistry and Technology of Silicones", Academic Press (1968). 6. Patton, T. C.; Paint Flow and Pigment Dispersion, Wiley Interscience, 593, (1979). 7. Rocklin, A. L . ; Journal of Coatings Technology, 50, No. 646, 46, (1978). 8. ASTM D1044, Resistance of Transparent Plastic Materials to Abrasion. 9. Haluska, L. Α.; Wear Testing of Abrasion - Resistant Coated Plastics, Special Technical Publication 769, American Society for Testing and Material, (1982). 10. ASTM D1308, Effect of Household Chemicals on Clear and Pig mented Organic Finishes. 11. Plueddemann, E. P.; "Silane Coupling Agents", Pluenum Press, 114, 120, (1982). 12. Baney, R.; Unpublished data. RECEIVED January 23, 1985


Polysiloxane-Silica Hybrid Resins as Abrasion-Resistant Coatings for

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