10 Permeabilities of Model Coatings: Effect of Cross-link Density and Polarity

W. J. Muizebelt and W. J. M . Heuvelsland

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 of coatings i s an important parameter governing 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 the p e r m e a b i l i t y , such as p o l a r i t y , c r y s t a l l i n i t y and the presence of 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 respect (10-13). Funke and Carfagna (10) demon­s t r a t e d the e f f e c t of 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 to 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) discussed the mechanism of transp o r t of water and oxygen through pores 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 transmission of 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 concluded that p e r m e a b i l i t y decreased 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 the l e a s t permeable membrane was composed of a c r o s s l i n k e d s t r u c t u r e of optimum space f i l l i n g character and net­work t i g h t n e s s .

We have i n v e s t i g a t e d the e f f e c t of c r o s s l i n k d e n s i t y on per­m e a b i l i t y of water vapour and oxygen of high s o l i d c o a t i n g s . For t h i s purpose we have synthesized a number of model c o a t i n g s , i . e . coatings w i t h a w e l l - d e f i n e d chemical 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 of pure ol i g o m e r i c e s t e r s of t e r e - or i s o p h t h a l i c a c i d w i t h the d i o l s g l y c o l , 1 , 4-butanediol or n e o p e n t y l g l y c o l . The oligomers were then reacted by t h e i r t e r m i n a l OH groups w i t h the

0097-6156/86/0322-0110S06.00/0 © 1986 American Chemical Society

Dow

nloa

ded

by M

ON

ASH

UN

IV o

n A

pril

30, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: O

ctob

er 1

4, 1

986

| doi

: 10.

1021

/bk-

1986

-032

2.ch

010

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

10. MUIZEBELT AND HEUVELSLAND Permeabilities of Model Coatings 111

c r o s s l i n k e r s (hexa)methoxymethylmelamine (HMMM, Cymel 303) or p o l y f u n c t i o n a l isocyanate (Desmodur N). C r o s s l i n k d e n s i t y of the obtained m a t e r i a l s w i l l be dependent on the length of the o l i g o ­mer. Because the chemical composition was changed simultaneously the coatings 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 determining the water vapour and oxygen p e r m e a b i l i t y of the fr e e f i l m s as w e l l as the water s o l u b i l i t y i n the c o a t i n g s , the c o e f f i c i e n t s of d i f f u s i o n of water could be e s t a b l i s h e d .

Experimental

Synthesis of oligomers. The oligomers were e s t e r s of t e r e - or i s o p h t h a l i c a c i d (T or I ) w i t h the d i o l s g l y c o l (G), 1,4-butane-d i o l (B) or neopentyl g l y c o l (N). Using these symbols the m a t e r i ­a l s can be i n d i c a t e d simply as, f o r instance GTG ( f i r s t oligomer of ethylene t e r e p h t h a l a t e ) or ( B I ^ B ( t h i r d oligomer of butylène i s o p h t h a l a t e ) .

The oligomers BTB and (BT^B were prepared according to Hâsslin et a l . (14) and i s o l a t e d by means of 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 oligomers NIN and BIB were prepared s i m i l a r l y and p u r i f i e d by molecular 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 higher oligomers i n the residue) and c r y s t a l l i z a ­t i o n . (BI)3B was prepared 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 of 1,4 cycl 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 of GTG. The p u r i t y of the m a t e r i ­a l s was checked by means of GPC and NMR.

Pre p a r a t i o n of coatings as f r e e f i l m s . The ol i g o m e r i c e s t e r d i o l s were mixed w i t h the c r o s s l i n k e r s HMMM or p o l y f u n c t i o n a l i s o c y a ­nate. 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 of 4:6. The OH/NCO r a t i o was 1:1. Some 1 wt% diethanolamine s a l t of ρ-toluene sulphonic a c i d , r e s p e c t i v e l y 0.2 wt% Dabco were used as c a t a l y s t . The coatings were a p p l i e d to 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) of t e f l o n . Curing was e f f e c t e d at 135°C f o r 30 minutes (HMMM) and one day at 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 coatings could e a s i l y be removed from the t e f l o n by means of a r a z o r blade.

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 the range 30-50 pm. The extent of the 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 C s o l i d s t a t e NMR. The methoxy band a t 915 cm"1 disappeared l a r g e l y r e l a t i v e to the 815 cm"1 t r i a z i n e r i n g a b s o r p t i o n .

However, the methoxy group i s present i n excess r e l a t i v e to the -CH2OH group of the e s t e r d i o l and i t may a l s o disappear i n si d e r e a c t i o n s other than the c r o s s l i n k r e a c t i o n . Thus the amount of methoxy groups remaining a f t e r cure i s not a measure of the extent of the 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 C NMR spectra of the cured f i l m s showed that the -CH2OH group had disappeared v i r t u a l l y completely. The c r o s s l i n k r e a c t i o n i s therefore n e a r l y complete and the molecular weight between the c r o s s l i n k s i s de­termined by the molecular weight of the e s t e r d i o l used.

Dow

nloa

ded

by M

ON

ASH

UN

IV o

n A

pril

30, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: O

ctob

er 1

4, 1

986

| doi

: 10.

1021

/bk-

1986

-032

2.ch

010

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

112 POLYMERIC MATERIALS FOR CORROSION CONTROL

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 of the f r e e f i l m s f o r water vapour was measured by means of the wet cup method (15). Oxygen p e r m e a b i l i t y was measured using the Polymer Permeation Analyser of Dohrmann Envirotech (16,17). Res u l t s are summarized i n Table I .

Also i n c l u d e d i n the t a b l e are s o l u b i l i t i e s of water i n the co a t i n g s . These s o l u b i l i t i e s were determined from the weight d i f ­ference of a piece of m a t e r i a l a f t e r d r y i n g over P2O5 i n vacuo f o r a number of days and a f t e r storage i n water. Before weighing, the wet coa 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 order to remove any adhering water.

Table I . Oxygen and water vapour p e r m e a b i l i t i e s (P(>2 anc* ΡΗ2θ) w i t h mean d e v i a t i o n (m.d.), water s o l u b i l i t y (Sj^o)

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

P Q 9 Χ 1 0 1 2 P H 9 o Χ 1 0 1 1 SH 90 x 1 0 2 DHoO x i° 9

coating L L L *· cc(STP)cm g_çm g ( c m V 1 )

cm 2(cm Hg)sec cm 2(cm Hg)sec cm^(cm Hg)

ΒIB/HMMM 25 5.0 + 0.6 1.6 3.2 (BI)3B/HMMM 31 4.2 + 0.6 1.3 3.3 BTB/HMMM 47 5.2 + 1.0 1.0 5.2 BTB + (BT)2B/HMMM - 5.4 + 0.8 1.2 4.5 (BT)2B/HMMM - 4.7 + 1.1 1.4 3.4 GTG/HMMM - 4.6 + 0.2 1.7 2.7 GCG/HMMM 30 5.5 + 0.2 1.8 3.1 NIN/HMMM 15 4.4 + 0.3 2.1 2.1 BIB/isocyanate 4.2 6.7 + 0.5 4.8 1.4 GTG/isocyanate - 11.0 + 1.1 - -NIN/isocyanate 7.1 3.5 + 0.5 - -B/isocyanate - 21 + 3 10 2.0 N/isocyanate 3.4 17 + 4 11 1.5

Result s and Di s c u s s i o n

Water vapour p e r m e a b i l i t y . The most notable phenomenon over­l o o k i n g the data presented i n Table I i s that the water vapour p e r m e a b i l i t i e s of the HMMM-based coatings are not widely d i f f e r ­ent. The isocyanate coatings show somewhat l a r g e r d i f f e r e n c e s . GTG/isocyanate and the coatings made from butanediol and neopentyl g l y c o l are more permeable.

The experimental p e r m e a b i l i t y i s the product of the c o e f f i ­c i e n t of 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 the measured s o l u b i l i t i e s are taken i n t o c o n s i d e r a t i o n i t appears that the d i f ­ferences i n p e r m e a b i l i t y observed can mainly be a t t r i b u t e d to 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 of three. However, i f i t i s r e a l i z e d that 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 experimentally observed v a r i a b l e s and that the

Dow

nloa

ded

by M

ON

ASH

UN

IV o

n A

pril

30, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: O

ctob

er 1

4, 1

986

| doi

: 10.

1021

/bk-

1986

-032

2.ch

010

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

10. MUIZEBELT AND HEUVELSLAND Permeabilities of Model Coatings 113

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 than the p e r m e a b i l i t i e s , i t i s questionable whether the d i f f e r e n c e s i n d i f ­f u s i o n c o e f f i c i e n t s are s i g n i f i c a n t .

We th e r e f o r e tend to conclude that the d i f f e r e n c e s i n permea­b i l i t y observed are due to 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 than 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 . D i f f e r e n c e s i n s o l u b i l i t y of water may be a t t r i b u t e d to d i f f e r e n c e s i n p o l a r i t y of the me­dium. I t w i l l be c l e a r that the isocyanate coatings made from bu-t a n e d i o l or neopentyl g l y c o l c o n t a i n a higher concentration of po­l a r urethane l i n k a g e s than those made from the oligomers (although these c o n t a i n p o l a r e s t e r groups). Also isocyanate coatings are more p o l a r than those w i t h HMMM, which c o n t a i n l e s s p o l a r ether groups.

The main c o n c l u s i o n we would l i k e to advance i s that the d i f ­f u s i o n of water i n the co a t i n g i s not n o t i c e a b l y a f f e c t e d by the c r o s s l i n k d e n s i t y of the f i l m s . This c o n c l u s i o n i s i n con t r a s t to those of Gordon and Ravve (13), who found l a r g e e f f e c t s of 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 of a c r y l a t e s . A l s o the permea­b i l i t y of 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 to be s t r o n g l y dependent on the amount of s u l f u r used (18,19). I t must be concluded that although the c r o s s l i n k d e n s i t i e s of our m a t e r i ­a l s are i n the range of the 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 not lead to s i m i l a r e f ­f e c t s on space f i l l i n g character or network t i g h t n e s s . The d i f ­ference w i t h the v u l c a n i z a t e s (18,19) could conceivably be a mat­t e r of g l a s s t r a n s i t i o n temperature. Our measurements were c a r r i e d out below Tg whereas the observations on the v u l c a n i z a t e s were c a r r i e d out 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 required a l a r g e r piece of coating w i t h a greater chance of l e a k s . Therefore i t was o f t e n not p o s s i b l e to perform these measurements. The fewer data 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 values f o r the isocyanate coatings than f o r those based on HMMM. This w i l l be due to the d i f f e r e n c e i n p o l a r i t y , which i n f l u e n c e s the s o l u b i l i t y the opposite way as i n the case of water. Oxygen, as a non-polar molecule, d i s s o l v e s b e t t e r i n media wit h lower p o l a r i t y i n c o n t r a s t to water. Therefore the p e r m e a b i l i t y of oxygen i s a l s o l a r g e r i n media of lower p o l a r i t y .

S a l t spray t e s t . The model coatings of Table I are of the high s o l i d type used i n automotive top coats . Their primary 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 of a l l a matter of phosphate l a y e r , e l e c t r o c o a t and/or primer. However, the topcoats may c o n t r i b u t e to 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 water, oxygen and s a l t s . Therefore t h e i r p e r m e a b i l i t y i s im­portant as one of the f a c t o r s i n the c o r r o s i o n p r o t e c t i o n by the t o t a l c o a t i n g system. We f e e l that a s a l t spray t e s t of the model coatings d i r e c t l y a p p l i e d to 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.

Nevertheless we d i d a number of t e s t s of our model coatings d i r e c t l y a p p l i e d to Bonder 101 panels. The panels were given a standard s c r a t c h j u s t below the metal surface a f t e r which they

Dow

nloa

ded

by M

ON

ASH

UN

IV o

n A

pril

30, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: O

ctob

er 1

4, 1

986

| doi

: 10.

1021

/bk-

1986

-032

2.ch

010

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

114 POLYMERIC MATERIALS FOR CORROSION CONTROL were exposed i n the salt spray test. The corrosion protection per­formance was at best moderate and no significant differences be­tween the various coatings could be seen. This i s in accord with the small differences i n permeability observed. On this basis we do not expect significant 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 level.

Acknowledgment

Experimental assistance was given by Rianne Willems and Mark Buurman·

Solid state NMR spectra were taken by Ir. 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).

Dow

nloa

ded

by M

ON

ASH

UN

IV o

n A

pril

30, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: O

ctob

er 1

4, 1

986

| doi

: 10.

1021

/bk-

1986

-032

2.ch

010

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