Permeabilities of Model Coatings - American Chemical Society

10 Permeabilities of Model Coatings: Effect of Cross-link Density and Polarity W. J. Muizebelt and W. J. M . Heuvelsland

Downloaded by MONASH UNIV on April 30, 2013 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch010

Akzo Research, Corporate Research Department, P.O. Box 60, 6800 AB Arnhem, the Netherlands

Oxygen and water vapour permeabilities have been measured for a number of model coatings. The coatings consist of pure esterdiols (oligomeric iso/terephthalates of glycol, butanediol or neopentylglycol), crosslinked with hexamethoxymethyl melamine or polyfunctional isocyanate. By varying the length of the esterdiol, the crosslink density was varied. Differences in chemical composition resulted in variations in polarity. Differences in permeability were largely due to differences in solubility; hence diffusion through the polymeric film was not noticeably affected by crosslink density or polarity.

Water vapour and oxygen p e r m e a b i l i t y o f c o a t i n g s i s a n i m p o r t a n t parameter g o v e r n i n g t h e i r c o r r o s i o n p r o t e c t i o n ( 1 - 6 ) · Many f a c t o r s i n f l u e n c e t h e p e r m e a b i l i t y , such a s p o l a r i t y , c r y s t a l l i n i t y and the presence o f f u n c t i o n a l groups (7-9). C r o s s l i n k d e n s i t y i s a l s o mentioned i n t h i s r e s p e c t (10-13). Funke and Carfagna (10) demons t r a t e d t h e e f f e c t o f c u r i n g temperature on p e r m e a b i l i t y but they a s c r i b e d the e f f e c t t o d i f f e r e n c e s i n g l a s s t r a n s i t i o n temperature. F r i t z w a t e r (12) d i s c u s s e d t h e mechanism o f t r a n s p o r t o f w a t e r and oxygen t h r o u g h p o r e s i n c r o s s l i n k e d m a t e r i a l s . Gordon and Ravve (13) s t u d i e d oxygen t r a n s m i s s i o n o f h i g h l y c r o s s l i n k e d m a t e r i a l s . They c o n c l u d e d t h a t p e r m e a b i l i t y d e c r e a s e d w i t h i n c r e a s i n g c r o s s l i n k d e n s i t y and t h e l e a s t permeable membrane was composed o f a c r o s s l i n k e d s t r u c t u r e o f optimum space f i l l i n g c h a r a c t e r and n e t work t i g h t n e s s . We have i n v e s t i g a t e d t h e e f f e c t o f c r o s s l i n k d e n s i t y on p e r m e a b i l i t y o f water vapour and oxygen o f h i g h s o l i d c o a t i n g s . F o r t h i s purpose we have s y n t h e s i z e d a number o f model c o a t i n g s , i . e . c o a t i n g s w i t h a w e l l - d e f i n e d c h e m i c a l s t r u c t u r e . These m a t e r i a l s c o n s i s t o f pure o l i g o m e r i c e s t e r s o f t e r e - o r i s o p h t h a l i c a c i d w i t h t h e d i o l s g l y c o l , 1 , 4 - b u t a n e d i o l o r n e o p e n t y l g l y c o l . The o l i g o m e r s were t h e n r e a c t e d by t h e i r t e r m i n a l OH groups w i t h t h e 0097-6156/86/0322-0110S06.00/0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

10.

MUIZEBELT AND

HEUVELSLAND

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c r o s s l i n k e r s (hexa)methoxymethylmelamine (HMMM, Cymel 303) o r p o l y f u n c t i o n a l i s o c y a n a t e (Desmodur N ) . C r o s s l i n k d e n s i t y o f t h e o b t a i n e d m a t e r i a l s w i l l be dependent on t h e l e n g t h o f t h e o l i g o mer. Because t h e c h e m i c a l c o m p o s i t i o n was changed s i m u l t a n e o u s l y t h e c o a t i n g s a l s o showed s l i g h t d i f f e r e n c e s i n p o l a r i t y . By d e t e r m i n i n g t h e water vapour and oxygen p e r m e a b i l i t y o f t h e f r e e f i l m s as w e l l a s t h e w a t e r s o l u b i l i t y i n t h e c o a t i n g s , t h e c o e f f i c i e n t s o f d i f f u s i o n o f water c o u l d be e s t a b l i s h e d .


Experimental S y n t h e s i s o f o l i g o m e r s . The o l i g o m e r s were e s t e r s o f t e r e - o r i s o p h t h a l i c a c i d (T o r I ) w i t h t h e d i o l s g l y c o l ( G ) , 1,4-butaned i o l ( B ) o r n e o p e n t y l g l y c o l ( N ) . U s i n g these symbols t h e m a t e r i a l s c a n be i n d i c a t e d s i m p l y a s , f o r i n s t a n c e GTG ( f i r s t o l i g o m e r of e t h y l e n e t e r e p h t h a l a t e ) o r ( B I ^ B ( t h i r d o l i g o m e r o f butylène isophthalate). The o l i g o m e r s BTB and ( B T ^ B were p r e p a r e d a c c o r d i n g t o Hâsslin e t a l . ( 1 4 ) and i s o l a t e d by means o f f r a c t i o n a l c r y s t a l l i z a t i o n from e t h a n o l . The i s o p h t h a l a t e o l i g o m e r s NIN and BIB were p r e p a r e d s i m i l a r l y and p u r i f i e d by m o l e c u l a r d i s t i l l a t i o n ( l e a v i n g the n o n - v o l a t i l e h i g h e r o l i g o m e r s i n t h e r e s i d u e ) and c r y s t a l l i z a t i o n . (BI)3B was p r e p a r e d from i s o p h t h a l o y l c h l o r i d e and excess BIB. GCG ( d i g l y c o l e s t e r o f 1,4 c y c l o h e x a n e d i c a r b o x y l i c a c i d ) was made by c a t a l y t i c hydrogénation o f GTG. The p u r i t y o f t h e m a t e r i a l s was checked by means o f GPC and NMR. P r e p a r a t i o n o f c o a t i n g s a s f r e e f i l m s . The o l i g o m e r i c e s t e r d i o l s were mixed w i t h t h e c r o s s l i n k e r s HMMM o r p o l y f u n c t i o n a l i s o c y a n a t e . The molar r a t i o esterdiol/HMMM was 2:1 l e a d i n g t o an OH/OCH3 r a t i o o f 4:6. The OH/NCO r a t i o was 1:1. Some 1 wt% d i e t h a n o l a m i n e s a l t o f ρ-toluene s u l p h o n i c a c i d , r e s p e c t i v e l y 0.2 w t % Dabco were used a s c a t a l y s t . The c o a t i n g s were a p p l i e d t o Bonder 101 p l a t e s which had been sprayed w i t h a t h i n l a y e r (1-2 \im) o f t e f l o n . C u r i n g was e f f e c t e d a t 135°C f o r 30 minutes (HMMM) and one day a t room temperature ( i s o c y a n a t e ) , r e s p e c t i v e l y . The c o a t i n g s c o u l d e a s i l y be removed from t h e t e f l o n by means o f a r a z o r b l a d e . F i l m t h i c k n e s s was g e n e r a l l y i n t h e range 30-50 pm. The e x t e n t of t h e c r o s s l i n k r e a c t i o n w i t h HMMM was checked by i n f r a r e d and 1 3 s o l i d s t a t e NMR. The methoxy band a t 915 cm" d i s a p p e a r e d l a r g e l y r e l a t i v e t o t h e 815 cm" t r i a z i n e r i n g a b s o r p t i o n . However, t h e methoxy group i s p r e s e n t i n excess r e l a t i v e t o the group o f t h e e s t e r d i o l and i t may a l s o d i s a p p e a r i n s i d e r e a c t i o n s o t h e r t h a n t h e c r o s s l i n k r e a c t i o n . Thus t h e amount o f methoxy groups r e m a i n i n g a f t e r c u r e i s n o t a measure o f t h e e x t e n t o f t h e c r o s s l i n k r e a c t i o n . S o l i d s t a t e 1 3 NMR s p e c t r a o f the c u r e d f i l m s showed t h a t t h e group had d i s a p p e a r e d v i r t u a l l y c o m p l e t e l y . The c r o s s l i n k r e a c t i o n i s t h e r e f o r e n e a r l y complete and t h e m o l e c u l a r weight between t h e c r o s s l i n k s i s de t e r m i n e d by t h e m o l e c u l a r weight o f t h e e s t e r d i o l u s e d . 1

C

1

-CH2OH

-CH2OH

C


POLYMERIC MATERIALS FOR CORROSION CONTROL

112

P e r m e a b i l i t y and s o l u b i l i t y measurements. P e r m e a b i l i t y o f t h e f r e e f i l m s f o r w a t e r vapour was measured by means o f t h e wet cup method ( 1 5 ) . Oxygen p e r m e a b i l i t y was measured u s i n g t h e Polymer P e r m e a t i o n A n a l y s e r o f Dohrmann E n v i r o t e c h ( 1 6 , 1 7 ) . R e s u l t s a r e summarized i n T a b l e I . A l s o i n c l u d e d i n the t a b l e a r e s o l u b i l i t i e s o f water i n t h e c o a t i n g s . These s o l u b i l i t i e s were d e t e r m i n e d from t h e w e i g h t d i f f e r e n c e o f a p i e c e o f m a t e r i a l a f t e r d r y i n g over i n vacuo f o r a number o f days and a f t e r s t o r a g e i n w a t e r . B e f o r e w e i g h i n g , the wet c o a t i n g was c a r e f u l l y wiped w i t h t i s s u e i n o r d e r t o remove any a d h e r i n g w a t e r .

P2O5

T a b l e I . Oxygen and w a t e r vapour p e r m e a b i l i t i e s

(P(>2 * Η2θ) anc


w i t h mean d e v i a t i o n (m.d.), w a t e r s o l u b i l i t y

Ρ

(Sj^o)

and d i f f u s i o n c o e f f i c i e n t ( D o ) a t 21°C. H 2

coating

PQ

L 9

Χ 10

1 2

P

L H 9

o Χ 10

1 1

S

H 0 x 10

L

2

9

cc(STP)cm g_çm g cm (cm Hg)sec cm (cm Hg)sec cm^(cm Hg) 2

ΒIB/HMMM (BI) B/HMMM BTB/HMMM BTB + (BT) B/HMMM 3

2

(BT) B/HMMM GTG/HMMM GCG/HMMM NIN/HMMM BIB/isocyanate GTG/isocyanate NIN/isocyanate B/isocyanate N/isocyanate 2

D

x

9

HoO i ° *· (cmV ) 1

2

25 31 47

5.0 4.2 5.2

+ 0.6 + 0.6 + 1.0

1.6 1.3 1.0

3.2 3.3 5.2

-

5.4

+ + + + + + + + + +

0.8

1.2

4.5

1.1 0.2 0.2 0.3 0.5 1.1 0.5 3 4

1.4 1.7 1.8 2.1 4.8

3.4 2.7 3.1 2.1 1.4

30 15 4.2

7.1

3.4

4.7 4.6 5.5 4.4 6.7 11.0 3.5 21 17

10 11

2.0 1.5

R e s u l t s and D i s c u s s i o n Water vapour p e r m e a b i l i t y . The most n o t a b l e phenomenon o v e r l o o k i n g t h e d a t a p r e s e n t e d i n T a b l e I i s t h a t t h e w a t e r vapour p e r m e a b i l i t i e s o f t h e HMMM-based c o a t i n g s a r e n o t w i d e l y d i f f e r e n t . The i s o c y a n a t e c o a t i n g s show somewhat l a r g e r d i f f e r e n c e s . GTG/isocyanate and t h e c o a t i n g s made from b u t a n e d i o l and n e o p e n t y l g l y c o l a r e more permeable. The e x p e r i m e n t a l p e r m e a b i l i t y i s t h e p r o d u c t o f t h e c o e f f i c i e n t o f d i f f u s i o n and s o l u b i l i t y (P - D χ S ) . When t h e measured s o l u b i l i t i e s a r e t a k e n i n t o c o n s i d e r a t i o n i t appears t h a t t h e d i f f e r e n c e s i n p e r m e a b i l i t y o b s e r v e d c a n m a i n l y be a t t r i b u t e d t o t h i s f a c t o r . The c a l c u l a t e d d i f f u s i o n c o e f f i c i e n t s d i f f e r a t most a f a c t o r o f t h r e e . However, i f i t i s r e a l i z e d t h a t t h i s c o e f f i c i e n t i s d e r i v e d from two e x p e r i m e n t a l l y observed v a r i a b l e s and t h a t t h e



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s o l u b i l i t y measurements were somewhat l e s s r e p r o d u c i b l e t h a n t h e p e r m e a b i l i t i e s , i t i s q u e s t i o n a b l e whether t h e d i f f e r e n c e s i n d i f fusion coefficients are significant. We t h e r e f o r e tend t o c o n c l u d e t h a t t h e d i f f e r e n c e s i n permeab i l i t y observed a r e due t o d i f f e r e n c e s i n s o l u b i l i t y r a t h e r t h a n v a r i a t i o n s i n the d i f f u s i o n c o e f f i c i e n t . Differences i n s o l u b i l i t y o f w a t e r may be a t t r i b u t e d t o d i f f e r e n c e s i n p o l a r i t y o f t h e medium. I t w i l l be c l e a r t h a t t h e i s o c y a n a t e c o a t i n g s made from but a n e d i o l o r n e o p e n t y l g l y c o l c o n t a i n a h i g h e r c o n c e n t r a t i o n o f pol a r urethane l i n k a g e s t h a n those made from t h e o l i g o m e r s (although these c o n t a i n p o l a r e s t e r groups). A l s o isocyanate coatings a r e more p o l a r t h a n those w i t h HMMM, w h i c h c o n t a i n l e s s p o l a r e t h e r groups. The main c o n c l u s i o n we would l i k e t o advance i s t h a t t h e d i f f u s i o n o f w a t e r i n t h e c o a t i n g i s n o t n o t i c e a b l y a f f e c t e d by t h e c r o s s l i n k density of the f i l m s . This conclusion i s i n contrast t o t h o s e o f Gordon and Ravve ( 1 3 ) , who found l a r g e e f f e c t s o f c r o s s l i n k d e n s i t y on oxygen p e r m e a b i l i t y o f a c r y l a t e s . A l s o t h e permeab i l i t y o f n a t u r a l v u l c a n i z a t e s f o r v a r i o u s gases was found t o be s t r o n g l y dependent on t h e amount o f s u l f u r used (18,19). I t must be c o n c l u d e d t h a t a l t h o u g h t h e c r o s s l i n k d e n s i t i e s o f o u r m a t e r i a l s a r e i n t h e range o f t h e a c r y l a t e s s t u d i e d by Gordon and Ravve, the d i f f e r e n c e s i n c r o s s l i n k d e n s i t i e s do n o t l e a d t o s i m i l a r e f f e c t s on space f i l l i n g c h a r a c t e r o r network t i g h t n e s s . The d i f f e r e n c e w i t h t h e v u l c a n i z a t e s (18,19) c o u l d c o n c e i v a b l y be a matt e r o f 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 . Our measurements were c a r r i e d out below Tg whereas t h e o b s e r v a t i o n s on t h e v u l c a n i z a t e s were c a r r i e d o u t above Tg. Oxygen p e r m e a b i l i t y . Oxygen p e r m e a b i l i t y measurement r e q u i r e d a l a r g e r p i e c e o f c o a t i n g w i t h a g r e a t e r chance o f l e a k s . T h e r e f o r e i t was o f t e n n o t p o s s i b l e t o p e r f o r m t h e s e measurements. The fewer d a t a f o r oxygen p e r m e a b i l i t y i n Table I i n d i c a t e s m a l l e r v a l u e s f o r t h e i s o c y a n a t e c o a t i n g s than f o r t h o s e based on HMMM. T h i s w i l l be due t o t h e d i f f e r e n c e i n p o l a r i t y , w h i c h i n f l u e n c e s t h e s o l u b i l i t y t h e o p p o s i t e way as i n t h e case o f w a t e r . Oxygen, as a n o n - p o l a r m o l e c u l e , d i s s o l v e s b e t t e r i n media w i t h l o w e r p o l a r i t y i n c o n t r a s t t o w a t e r . T h e r e f o r e t h e p e r m e a b i l i t y o f oxygen i s a l s o l a r g e r i n media o f lower p o l a r i t y . S a l t spray t e s t . The model c o a t i n g s o f T a b l e I a r e o f t h e h i g h s o l i d type used i n a u t o m o t i v e t o p c o a t s . T h e i r p r i m a r y f u n c t i o n i s not c o r r o s i o n p r o t e c t i o n s i n c e t h i s i s f i r s t o f a l l a m a t t e r o f phosphate l a y e r , e l e c t r o c o a t and/or p r i m e r . However, t h e t o p c o a t s may c o n t r i b u t e t o c o r r o s i o n p r o t e c t i o n by t h e i r b a r r i e r f u n c t i o n f o r w a t e r , oxygen and s a l t s . T h e r e f o r e t h e i r p e r m e a b i l i t y i s i m p o r t a n t as one o f t h e f a c t o r s i n t h e c o r r o s i o n p r o t e c t i o n by t h e t o t a l c o a t i n g system. We f e e l t h a t a s a l t s p r a y t e s t o f t h e model coatings d i r e c t l y a p p l i e d t o a s t e e l surface i s of l i t t l e r e l e vance f o r t h e i r c o r r o s i o n p r o t e c t i o n performance i n a r e a l system. N e v e r t h e l e s s we d i d a number o f t e s t s o f o u r model c o a t i n g s d i r e c t l y a p p l i e d t o Bonder 101 p a n e l s . The p a n e l s were g i v e n a s t a n d a r d s c r a t c h j u s t below t h e m e t a l s u r f a c e a f t e r w h i c h they


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POLYMERIC MATERIALS FOR CORROSION CONTROL

were exposed i n the s a l t spray t e s t . The corrosion protection performance was at best moderate and no s i g n i f i c a n t differences between the various coatings could be seen. This i s i n accord with the small differences i n permeability observed. On t h i s basis we do not expect s i g n i f i c a n t differences when the coatings are tested on panels provided with a proper electrocoat primer, although the corrosion protection by the complete system may be expected to be on a much higher l e v e l . Acknowledgment Experimental assistance was given by Rianne Willems and Mark Buurman·


Solid state

NMR spectra were taken by I r . H. Angad Gaur.

Literature Cited 1. W. Funke, J.O.C.C.A. 62, 63 (1979). 2. W. Funke and H. Haagen, Ind. Eng. Chem. Prod. Res. Dev. 17, 50 (1978). 3. H. Haagen and W. Funke, J.O.C.C.A. 58, 359 (1975). 4. F . L . Floyd, R.G. Groseclose and C.M. Frey, J.O.C.C.A. 66, 329 (1983). 5. M. Yaseen and K.V.S.N. Raju, J.O.C.C.A. 67, 185 (1984). 6. S. Guruviah, J.O.C.C.A. 63, 669 (1970). 7. D.Y. Perera and S. Pelier, Progr. Org. Coat. 1, 57 (1973). 8. P.W. Morgan, Ind. Eng. Chem. 2296 (1953). 9. W.L.H. Moll, Kolloid Zeitschr. 195, 43 (1964). 10. W. Funke and C. Carfagna, J.O.C.C.A. 67, 102 (1984). 11. K.A. v. Oeteren, Fette, Seife, Anstrichmittel 84, 242 (1982). 12. J . E . Fitzwater, J . Coat. Techn. 53 (683) 27 (1981). 13. G.A. Gordon and A. Ravve, Polymer Eng. and Sci. 20, 70 (1980). 14. H.W. Hässlin, M. Dröscher and G. Wegner, Makromol. Chem. 181, 301 (1980). 15. M. Yaseen and W. Funke, J.O.C.C.A. 61, 284 (1978). 16. M. Lomax, Polymer Testing 1, 105 (1980). 17. P.E. Cassidy, T.M. Aminabhari and C.M. Thompson, Rub. Chem. Techn. 56, 594 (1983). 18. R.M. Barrer and G. Skirrow, J. Pol. Sci. 3, 549 (1948). 19. A. Aitken and R.M. Barrer, Trans. Farad. Soc. 51. 116 (1955).


Permeabilities of Model Coatings - American Chemical Society

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