Waterborne Polymers for Aircraft Coatings - ACS Symposium Series

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Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 19, 2015 | http://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch009

Waterborne Polymers for Aircraft Coatings LOREN W. HILL Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105 DANIEL E. PRINCE Air Force Materials Labortory A F M L / M B E , Wright-Patterson Air Force Base, O H 45433 Exterior aircraft c o a t i n g s in c u r r e n t u s e c o n t r i b u t e to air p o l l u t i o n by emmision of volatile o r g a n i c c o m p o u n d s that a r e p r e s e n t as s o l v e n t s . T h e Air Force h a s begun a t h r e e phase program f o r r e d u c t i o n of emmisions. T h e program c o n s i s t s of r e p l a c i n g air s p r a y a p p l i c a t i o n by airless electrostatic spray, u s e of high solids c o a t i n g s , and d e v e l o p m e n t of w a t e r b o r n e p o l y m e r s f o r e v e n t u a l u s e in aircraft c o a t i n g s . T h e first phase is o p e r a t i o n a l . Airless electrostatic s p r a y is b e i n g u s e d to a p p l y s o l v e n t b o r n e t o p c o a t s and primers w i t h s u b s t a n t i a l r e d u c t i o n s in e m i s s i o n s resulting from elimination of m o s t of the o v e rs p r a y . The high solids a n d w a t e r b o r n e c o a t i n g s a p p r o a c h e s a r e b e i n g p u r s u e d t h r o u g h contractural p r o g r a m s and a n i n h o u s e research program at the Air Force Materials L a b o r a t o r y (AFML). T h e p u r p o s e of the w o r k r e p o r t e d h e r e w a s to o b t a i n a b e t t e r u n d e r s t a n d i n g of the b a s i c m e c h a n i c a l p r o p e r t i e s and curing b e h a v i o r of c u r r e n t aircraft c o a t i n g s so that e v a l u a t i o n of p o t e n t i a l w a t e r b o r n e r e p l a c e m e n t s c o u l d be p e r f o r m e d on a m o r e r i g o r o u s b a s i s than u s i n g paint t e s t m e t h o d s a l o n e . M e t h o d s c h o s e n i n c l u d e d characterization o f u n p i g m e n t e d f r e e f i l m s by d y n a m i c m e c h a n i c a l a n a l y s i s ( D M A ) and u s e of F o u r i e r t r a n s f o r m i n f r a r e d s p e c t r o s c o p y ( F T - I R ) f o r f o l l o w i n g the d i s a p p e a r a n c e of i s o c y a n a t e g r o u p s d u r i n g p o l y u r e t h a n e c u r e . A l though c h a r a c t e r i z a t i o n of the c u r r e n t topcoat i s e m p h a s i z e d , p r e l i m i n a r y results for s e v e r a l potential waterborne r e p l a c e ment systems are also presented. Background C u r r e n t Solventborne C o a t i n g s . The topcoat i s a t w o p a c k a g e p o l y u r ethane c o n s i s t i n g of an a l i p h a t i c i s o c y a n a t e c o m 0-8412-0567-l/80/47-132-083$05.50/0 © 1980 American Chemical Society In Resins for Aerospace; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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ponent and a polyester polyol component. The impetus for finding a w a t e r b o r n e o r h i g h s o l i d s r e p l a c e m e n t i s , of c o u r s e , r e d u c t i o n of the solvent content. U n p i g m e n t e d f o r m u l a t i o n s con tain approximately 6 5 % solvent by weight. The gloss white pigmented formulation contains approximately 4 8 % solvent by weight. T h e polyurethane s y s t e m i s n o r m a l l y appliecMn two c o a t s , t o a t o t a l d r y f i l m t h i c k n e s s o f 51 - 8 Mm ( 2 . 0 - 0 . 3 m i l s ) . The current p r i m e r i s a two-package epoxy/polyamide con s i s t i n g of a b i s p h e n o l A type epoxy and an a m i n e t e r m i n a t e d d i m e r a c i d p o l y a m i d e component. T h e solvent content ofthe m i x e d components i s approximately 6 1 % by weight i n f o r m u l a t i o n s c o n t a i n i n g c o r r o s i o n i n h i b i t i n g p i g m e n t s . N o r m a l l y one coat i s u s e d a t a d r y f i l m t h i c k n e s s of 15 to 23 μ ι η ( 0 . 6 to 0 . 9 mils). Potential replacements w i l l have to cure at ambient temper a t u r e s a n d be s u i t a b l e f o r s p r a y a p p l i c a t i o n , p r e f e r a b l y a i r l e s s e l e c t r o s t a t i c spray. T h e size and i r r e g u l a r shape o f a i r c r a f t make other application methods i m p r a c t i c a l . The size also e l i m i n a t e s oven c u r e , and the i r r e g u l a r shape e l i m i n a t e s radiation c u r e , at least w i t h c u r r e n t l y available technology. A i r c r a f t P e r f o r m a n c e R e q u i r e m e n t s . The m a i n function of a i r c r a f t coatings i s to m i n i m i z e c o r r o s i o n . H i g h level per f o r m a n c e i s r e q u i r e d t o i n s u r e that the c o a t i n g s y s t e m r e m a i n s i n t a c t so that i t c a n m a i n t a i n i t s p r o t e c t i v e f u n c t i o n i n a l l a i r craft environments. Flexibility at low temperatures and impact resistance a r e r e q u i r e d . Resistance to softening i n fluids u s e d in aircraft, such as hydraulic fluids, lubricating oils, and h y d r o c a r b o n s (fuel) i s a l s o r e q u i r e d . T h e f l e x i b i l i t y a n d f l u i d r e s i s t a n c e r e q u i r e m e n t s a r e often i n conflict because the f o r m e r i s favored by a r e l a t i v e l y l o w l e v e l of c r o s s l i n k i n g andthe latter by a high level of cros slinking. Thus, the combination of flexibility and fluid r e s i s t a n c e , obtained with the current poly urethane topcoat, i s a difficult challenge for potential r e p l a c e ments. Paint test methods involving procedures w e l l described i n A S T M o r federal test method publications a r e used to determine flexibility, fluid resistance and other coatings performance characteristics. These tests a r eused to establish aircraft coatings specifications. H o w e v e r , as i s often true, the s p e c i f i cations tend to r e f l e c t the p e r f o r m a n c e c h a r a c t e r i s t i c s of the c u r r e n t s y s t e m rather than actual in-use p e r f o r m a n c e r e q u i r e ments. Therefore, evaluation of potential replacements using paint test m e t h o d s and c u r r e n t s p e c i f i c a t i o n s c o u l d be m i s -

In Resins for Aerospace; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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l e a d i n g . U s e of D M A m a y p e r m i t m o r e v a l i d e v a l u a t i o n . DMA r e s u l t s m a y a l s o s u g g e s t h o w m o d i f i c a t i o n of p o l y m e r s t r u c t u r e m i g h t c o n t r i b u t e to i m p r o v e d p e r f o r m a n c e . P o l y u r e t h a n e C u r e . P a i n t t e s t r e s u l t s c a r r i e d out o v e r m a n y y e a r s f o r q u a l i f i c a t i o n p u r p o s e s i n d i c a t e that c u r e con t i n u e s at l e a s t f o r s e v e r a l d a y s a f t e r a p p l i c a t i o n of the c u r r e n t t o p c o a t . Q u a l i f i c a t i o n t e s t s a r e r u n a f t e r a m i n i m u m of s e v e n d a y s a m b i e n t c u r e . In t h i s study, F T - I R has b e e n u s e d to f o l l o w the r e a c t i o n of i s o c y a n a t e g r o u p s i n u n p i g m e n t e d f i l m s d u r i n g this extended c u r e p e r i o d . A n attempt i s m a d e to r e l a t e i s o c y a n a t e r e a c t i o n w i t h the d e v e l o p m e n t of f i l m p r o p e r t i e s . The f e a s i b i l i t y of u s i n g t h i s F T - I R technique f o r studying c u r e of p o t e n t i a l r e p l a c e m e n t s i s being i n v e s t i g a t e d . Expe rimental Following Isocyanate Reaction by F T - I R . C l e a r coating f i l m s of the t w o - p a c k a g e p o l y u r e t h a n e ( S u p e r D e s o t h a n e , DeSoto, Inc.) w e r e p r e p a r e d f o r F T - I R a n a l y s i s by a i r spray a p p l i c a t i o n onto e i t h e r a p o l y m e r i c s u b s t r a t e ( c l e a r p o l y e t h y l e n e , i . e . c o m m e r c i a l Glad Wrap, Union Carbide) o ra tinfoil substrate. The p o l y m e r i c s u b s t r a t e p e r m i t t e d r e c o r d i n g of s p e c t r a i m m e d i a t e l y a f t e r s p r a y a p p l i c a t i o n . Substrate a b s o r p t i o n i n the r e g i o n o f t h e i s o c y a n a t e b a n d a t 4.4 μπι w a s s l i g h t . T h e t i n f o i l s u b s t r a t e w a s u s e d t o o b t a i n f r e e f i l m s . A f t e r 17 h r . a m b i e n t c u r e , the f i l m s w e r e s u f f i c i e n t l y c u r e d so that f r e e f i l m s c o u l d be r e m o v e d b y m e r c u r y a m a l g a m a t i o n . — T h e f i r s t s p e c t r u m f o r s a m p l e s r e m o v e d f r o m t i n f o i l w e r e r e c o r d e d a b o u t 17 t o 18 h r . a f t e r a p p l i c a t i o n . B o t h f r e e f i l m s and f i l m s o n polyethylene w e r e c u t ( 2 . 0 χ 2.0 c m ) a n d m o u n t e d o n c a r d b o a r d m a s k s s i z e d for i n s e r t i o n i n t o the I R s p e c t r o p h o t o m e t e r s a m p l e h o l d e r . Mounted samples were stored under ambient conditions or i n a d e s s i c a t o r , and I R s p e c t r a w e r e r e c o r d e d a f t e r v a r i o u s c u r e times. T h e e x t i n c t i o n c o e f f i c i e n t o f t h e i s o c y a n a t e b a n d a t 4.4 μπι w a s 1 4 4 l i t e r g " c m " a s d e t e r m i n e d b y η-butylacetate d i l u t i o n s of the i s o c y a n a t e c o m p o n e n t . 1

1

The F T - I R s p e c t r a w e r e r e c o r d e d u s i n g a W i l l e y M o d e l 318S F o u r i e r T r a n s f o r m S p e c t r o p h o t o m e t e r ( W i l l e y , I n c . , s e e r e f . 2). The m a i n advantage of F T - I R o v e r c o n v e n t i o n a l I R to t h i s study was g r e a t e r a c c u r a c y i n absorbance data a s s o c i a t e d with averaging multiple scans. T h e computer system also facilitated obtaining difference spectra.



86


Dynamic Mechanical Analysis (DMA). Samples for D M A w e r e cut (3.0 χ 0 . 4 c m ) f r o m t i n f o i l s u b s t r a t e s a p p r o x i m a t e l y one h o u r b e f o r e t e s t i n g . A f t e r s e p a r a t i o n b y a m a l g a m a t i o n , the f r e e f i l m s w e r e m o u n t e d i n the j a w s (1.5 c m between j a w s ) o f the D M A i n s t r u m e n t . F i l m t h i c k n e s s r a n g e d f r o m 43 to 56 μ π ι (1.7 to 2 . 2 m i l s ) . F o r c u r e studies the f i r s t p r o p e r t y / t e m p e r a t u r e c u r v e s w e r e d e t e r m i n e d a f t e r 17 h r . a m b i e n t c u r e , a n d subsequent d e t e r m i n a t i o n s w e r e m a d e at a p p r o x i m a t e l y 24 h r . i n t e r v a l s w i t h a d i f f e r e n t s a m p l e e a c h t i m e . T h e effect of a m b i e n t v e r s u s d e s s i c a t o r s t o r a g e between d e t e r m i n a t i o n s w a s investigated. T h e e f f e c t of m o i s t u r e o n D M A p l o t s w a s a l s o studied u s i n g s a m p l e s w h i c h h a d c u r e d f o r 35 d a y s o r m o r e . M o i s t u r e a b s o r p t i o n w a s v a r i e d by s t o r i n g s a m p l e s at a m b i e n t r e l a t i v e humidity (RH), i n a dessicator (RH = 0%), or i n a dessicator c o n t a i n i n g l i q u i d w a t e r a n d n o d e s s i c a n t ( R H = 100%). The tinfoil substrate sample p r e p a r a t i o n method was u s e d f o r the c u r r e n t p o l y u r e t h a n e topcoat, the c u r r e n t epoxy p o l y a m i d e p r i m e r , a n d one p o t e n t i a l w a t e r b o r n e topcoat r e p l a c e m e n t , w h i c h w a s a p o l y u r e t h a n e aqueous d i s p e r s i o n ( P o l y v i n y l C h e m i c a l ' s N e o R e z R-960) c r o s s - l i n k e d w i t h 0 . 5 to 2. 0 wt % of polyfunctional aziridine c r o s s - l i n k e r (Cordova C h e m i c a l s X A M A 7 ) . T h e s e s a m p l e s w e r e p r e p a r e d at the A F M L . Clear f i l m s of s e v e r a l p o t e n t i a l w a t e r b o r n e r e p l a c e m e n t s y s t e m s w e r e s u p p l i e d b y D e S o t o , I n c . O f the g r o u p of f i l m s s u p p l i e d , D M A c u r v e s w e r e o b t a i n e d f o r the f o l l o w i n g : a s e c o n d type of aqueous p o l y u r e t h a n e d i s p e r s i o n ( W . R . G r a c e ' s H y p o l W B 4000), two coatings based on water soluble a c r y l i c c o p o l y m e r s ( B . F . G o o d r i c h ' s C a r b o s e t 514 and U n i o n C h e m i c a l ' s A M S C O R e s 200) and a n o t h e r s a m p l e p r e p a r e d w i t h P o l y v i n y l C h e m i c a l ' s N e o R e z R-960. 1

A R h e o v i b r o n D i r e c t R e a d i n g D y n a m i c V i s c oe l a s tomete r ( T o y o - B a l d w i n , L t d . ) was u s e d f o r D M A . T h i s instrument and i t s o p e r a t i o n h a v e b e e n d e s c r i b e d . — B r i e f l y , the t e s t c o n s i s t s of a p p l y i n g a n o s c i l l a t i n g s t r a i n to a r e c t a n g u l a r s a m p l e u n d e r t e n s i o n . T h e r e s u l t i n g s t r e s s and the l a g of p e a k s t r e s s b e h i n d p e a k s t r a i n (phase d i f f e r e n c e ) a r e o b t a i n e d . In t h i s study the data a r e p r e s e n t e d i n t e r m s of s t o r a g e m o d u l u s , Ε ' , a n d the tangent of the angle c o r r e s p o n d i n g to the p h a s e d i f f e r e n c e , t a n k Ε ' i s a m e a s u r e of s t o r e d e n e r g y o r e l a s t i c r e s p o n s e w h i l e t a n δ i s a m e a s u r e of e n e r g y l o s s e s t h r o u g h v i s c o u s r e s p o n s e of a v i s c o e l a s t i c s a m p l e that i s s u b j e c t e d to a n o s c i l l a t i n g s t r a i n . T h e o s c i l l a t i n g f r e q u e n c y w a s 11 H z f o r a l l data reported h e r e .


9.

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Woterbome

87

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R e s u l t s and D i s c u s s i o n P o l y u r e t h a n e C u r e , F T - I R . C o m p a r i s o n of p e n c i l h a r d n e s s r e s u l t s b e f o r e and a f t e r e x p o s u r e of the p o l y u r e t h a n e topcoat to f l u i d s i n d i c a t e s that c u r e c o n t i n u e s f o r s e v e r a l d a y s a f t e r a p p l i c a t i o n . T h e p o s s i b i l i t y that this change i s r e l a t e d to c o n t i n u e d c u r i n g i n v o l v i n g i s o c y a n a t e g r o u p s w a s i n v e s t i g a t e d by F T - I R . A s i n d i c a t e d i n F i g u r e 1 A , the i s o c y a n a t e b a n d at 4 . 4 μιτι i s s t i l l p r o m i n e n t a f t e r 18 h r s . a m b i e n t c u r e . U s i n g the e x t i n c t i o n c o e f f i c i e n t and f i l m t h i c k n e s s , the c o n c e n t r a t i o n of i s o c y a n a t e g r o u p s r e m a i n i n g a f t e r 18 h r s . i s c a l c u l a t e d to be 1.56 e q u i v a l e n t s / l i t e r . A f t e r 41 h r s . a n d 65 h r s . the c o n c e n t r a t i o n h a d d e c r e a s e d to 0 . 3 5 e q . / I and 0 . 0 9 e q . / l , r e s p e c t i v e l y . T h e a m b i e n t r e l a t i v e h u m i d i t y (RH) d u r i n g t h i s p e r i o d w a s h i g h , r a n g i n g f r o m 65 % to 7 5 % . F i g u r e I B shows that when the f r e e f i l m i s s t o r e d i n a d e s s i c a t o r f o l l o w i n g a m b i e n t c u r e f o r 18 h r s . , the d e c r e a s e i n i s o c y a n a t e a b s o r b a n c e i s g r e a t l y r e d u c e d . A f t e r 185 h r s . i n the d e s s i c a t o r , the 18 h r . c o n c e n t r a t i o n of 1.56 e q . /I h a s o n l y d e c r e a s e d to 0 .87 e q . / I . T h e ambient exposure experiment was repeated under l e s s h u m i d c o n d i t i o n s as shown i n F i g u r e 2. In t h i s c a s e a b s o r b a n c e i s p l o t t e d on a l i n e a r s c a l e so the r e l a t i o n s h i p to c o n c e n t r a t i o n i s m o r e d i r e c t . T h e r e l a t i v e h u m i d i t y w a s c l o s e to 50% t h r o u g h o u t . A f t e r 17 h r . a m b i e n t c u r e the c o n c e n t r a t i o n of i s o c y a n a t e r e m a i n i n g was 1.88 e q . / l . A f t e r 45 h r s . and 65 h r s . the c o n c e n t r a t i o n had d e c r e a s e d to 1.38 and 1.03 e q . / l , r e s p e c t i v e l y . A t a l l c o m p a r a b l e c u r e t i m e s , the c o n c e n t r a t i o n s w e r e c o n s i d e r a b l y h i g h e r when the R H w a s l o w e r ( F i g u r e 2). T h e d o m i n a n t c u r i n g r e a c t i o n f o r the topcoat w a s p r e v i o u s l y a s s u m e d to be u r e t h a n e f o r m a t i o n a s shown i n s i m p l i f i e d f o r m i n r e a c t i o n (1). T h e d e p e n d e n c e of r a t e of i s o c y a n a t e

P-NCO + P

!

-OH -

Η Ο P-A-Ô-OP

1

urethane

(1)

d i s a p p e a r a n c e o n R H and the slow l o s s of i s o c y a n a t e i n de s s i c a t e d s a m p l e s i n d i c a t e that a m o i s t u r e c u r e r e a c t i o n i s of m u c h g r e a t e r i m p o r t a n c e than p r e v i o u s l y r e a l i z e d . It i s w e l l k n o w n " that i s o c y a n a t e c o n t a i n i n g s y s t e m s c a n u n d e r g o m o i s t u r e c u r e a s shown i n r e a c t i o n (2). B e r g e r - also noted a strong d e pendence of the r a t e of i s o c y a n a t e l o s s o n R H i n p o l y e s t e r p o l y o l / i s o c y a n a t e s y s t e m s . R e a c t i o n of i s o c y a n a t e w i t h w a t e r




lOOj

2

40

3

4

5

6

3

4

5

Wavelength (microns)

Figure 1. F TIR spectra of a two-package solventborne polyurethane unpigmented film: (A) ambient cure for the indicated times, (B) comparison of 18-hr ambient cure and a film cured for 18 hr under ambient conditions and then stored in a dessicator until 185 hr after application; film thickness = 38 μΜ (1.5 mils); RH = 65-75%

5

4

5 4

5

4

5 4

5

4

WAVELENGTH, μπ\ Figure 2. FTIR spectra of a two-package solventborne polyurethane unpigmented film after various cure times under ambient conditions at an RH of approximately 50%; film thickness = 48 μπι (1.9 mils)


9.

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Waterborrw Polymers

P - N C O +H 0

" H O P-N-C-OH

2


P-NH

2

+ P-NCO

-

P-NH

89

2

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z

(2)

Η Ο H P-N-C-r>r-P

-

urea p r o d u c e s a c a r b a m i c a c i d i n t e r m e d i a t e w h i c h d e c o m p o s e s to f o r m a n a m i n e a n d c a r b o n d i o x i d e . In a s e c o n d step the a m i n e g r o u p r e a c t s w i t h another i s o c y a n a t e g r o u p to f o r m a s u b stituted u r e a c r o s s l i n k . O f c o u r s e , the a c t u a l c u r e i s m o r e c o m p l e x than i n d i c a t e d h e r e b e c a u s e b o t h Ρ a n d P a r e m u l t i f u n c t i o n a l a n d b e c a u s e e i t h e r the u r e a o r the u r e t h a n e c a n u n d e r go s e c o n d a r y r e a c t i o n s at the r e m a i n i n g - N H - sites Λ R e c o m m e n d e d m i x i n g v o l u m e s a r e r e p o r t e d ^ to r e s u l t i n a n e x c e s s of - N C O g r o u p s w h i c h c o u l d c o n t r i b u t e to m o i s t u r e c u r e a s shown i n r e a c t i o n (2). 1

A n e s t i m a t e o f the f r a c t i o n of i s o c y a n a t e that r e a c t s b y m o i s t u r e c u r e ( r e a c t i o n 2) c a n be o b t a i n e d f r o m a b s o r b a n c e d e t e r m i n a t i o n s f o r d e s s i c a t e d s a m p l e s . F o r e x a m p l e , the dessicated sample considered i n F i g u r e 1 had a concentration of 0. 87 e q . /I a f t e r 185 h r s . Since i t i s l i k e l y that a v e r y h i g h p e r c e n t a g e of the h y d r o x y l g r o u p s o r i g i n a l l y p r e s e n t h a v e r e a c t e d a f t e r 185 h r s . (7 .7 d a y s ) , the i s o c y a n a t e r e m a i n i n g i n the d e s s i c a t e d s a m p l e m u s t r e p r e s e n t that w h i c h n o r m a l l y i s l o s t t h r o u g h r e a c t i o n w i t h a m b i e n t m o i s t u r e . B y u s i n g the r e s i n c h a r acte r i stic s p r o v i d e d b y the s u p p l i e r , one c a n c a l c u l a t e that the i n i t i a l c o n c e n t r a t i o n o f i s o c y a n a t e i n the m i x e d c o m p o n e n t s , c o r r e c t e d f o r s o l v e n t l o s s , i s a pp r oxi m a t e l y 2 . 7 e q . / l . There f o r e , the m i n i m u m p e r c e n t a g e of i s o c y a n a t e that r e a c t s b y m o i s t u r e c u r e i s a p p r o x i m a t e l y 32%. T h i s i s a l o w e r l i m i t f o r m o i s t u r e c u r e b e c a u s e the s a m p l e w a s c u r e d u n d e r a m b i e n t c o n d i t i o n s f o r 18 h r s . b e f o r e r e m o v a l f r o m the t i n f o i l a n d w a s only d e s s i c a t e d t h e r e a f t e r . T o follow isocyanate l o s s by F T - I R beginning i m m e d i a t e l y a f t e r s p r a y i n g , a p o l y m e r i c f i l m w a s u s e d a s the s u b s t r a t e i n p l a c e of t i n f o i l . A s shown i n F i g u r e 3, the a b s o r b a n c e of the 4 . 4 μ ΐ η band d e c r e a s e d l i n e a r l y o v e r the 6 h r . p e r i o d . T h e i n i t i a l r a t e of di s appe a r anc e of i s o c y a n a t e o b t a i n e d f r o m the s l o p e w a s 0 . 1 2 e q . I" h r " . L i n e a r i t y i n d i c a t e s that the r a t e is independent of i s o c y a n a t e c o n c e n t r a t i o n . T h i s r e suit w o u l d be e x p e c t e d i f the slow step i s a b s o r p t i o n a n d d i f f u s i o n of w a t e r . A t l o n g e r t i m e s the r a t e d e c r e a s e d p o s s i b l y due to a c o m b i n 1

1



90


a t i o n of: a d e c r e a s e i n d i f f u s i o n r a t e of w a t e r as c r o s s l i n k i n g i n c r e a s e s , a d e c r e a s e i n s e g m e n t a l m o b i l i t y so that the s e c o n d step of r e a c t i o n (2) i s r e t a r d e d , and d e p l e t i o n of i s o c y a n a t e g r o u p s . A f t e r 24 h r s . the i s o c y a n a t e c o n c e n t r a t i o n r e m a i n i n g was 0.20 e q . / l . T h e c o n c e n t r a t i o n of 1 . 8 e q . / l o b t a i n e d at z e r o t i m e i n F i g u r e 3 i s l o w e r than 2 . 7 e q . /I c a l c u l a t e d f r o m r e s i n c h a r a c t e r i s t i c s because some isocyanate reacts during a one h o u r d i g e s t i o n p e r i o d " between m i x i n g and a p p l i c a t i o n . 11

P o l y u r e t h a n e C u r e , D M A . Since we felt that the m o i s t u r e r e l a t e d c r o s s l i n k i n g w o u l d be i n d i c a t e d by c h a n g e s i n d y n a m i c m e c h a n i c a l p r o p e r t i e s , D M A d e t e r m i n a t i o n s w e r e m a d e at a p p r o x i m a t e l y the s a m e c u r e t i m e s as the F T - I R d e t e r m i n a t i o n s i n F i g u r e 1 A . C u r i n g c o n d i t i o n s w e r e not i d e n t i c a l h o w e v e r , b e c a u s e F T - I R s a m p l e s w e r e e x p o s e d to the a t m o s p h e r e o n both s i d e s w h i l e the D M A s a m p l e s w e r e not r e m o v e d f r o m the t i n f o i l s u b s t r a t e s u n t i l about 1 h r . b e f o r e e a c h D M A r u n . N e v e r t h e l e s s , D M A r e s u l t s , shown i n F i g u r e 4 , a r e c o n s i s t e n t w i t h the o c c u r r e n c e of the c r o s s l i n k i n g r e a c t i o n . T h e change i n t e m p e r a t u r e of the o n s e t of the m a i n t r a n s i t i o n f r o m about - 5 ° C to about 1 0 ° C f o r s a m p l e s c u r e d f o r 17 h r . as c o m p a r e d to those c u r e d f o r 41 h r . , and the shift i n p o s i t i o n a n d height of the t a n δ p e a k i n d i c a t e that the a d d i t i o n a l c u r e w h i c h o c c u r s a f t e r 17 h r . h a s a s i g n i f i c a n t e f f e c t on p r o p e r t i e s . S a m p l e s c u r e d f o r 41 and 65 h r . gave an E p l o t w i t h a r u b b e r y p l a t e a u at about 1 χ 10 dyne s / c m w h e r e a s the 17 h r . c u r e s a m p l e d i d not e x h i b i t a p l a t e a u . S i n c e the l o w e r l i m i t of m e a s u r e m e n t f o r s a m p l e s of the d i m e n s i o n s u s e d w a s about 5 χ 10 d y n e s / c m , i t i s not k n o w n w h e t h e r the 17 h r s a m p l e w a s s u f f i c i e n t l y c r o s s l i n k e d to p r o d u c e a r u b b e r y p l a t e a u below t h i s d e t e c t i o n l i m i t . S i n c e i t i s w e l l e s t a b l i s h e d that f l u i d r e s i s t a n c e i s r e l a t e d to c r o s s l i n k d e n s i t y - and that the s t o r a g e m o d u l u s i n the r u b b e r y p l a t e a u region increases with i n c r e a s i n g c r o s s l i n k density,""the i m p r o v e m e n t i n f l u i d r e s i s t a n c e of the topcoat o v e r s e v e r a l d a y s i s v e r y l i k e l y a r e s u l t of the m o i s t u r e r e l a t e d c r o s s l i n k i n g i n d i c a t e d by the a p p e a r a n c e and i n c r e a s e i n m o d u l u s i n the rubbery region. 1

8

2

7

2

E f f e c t of A b s o r b e d W a t e r on D M A C u r v e s . In a d d i t i o n to the e f f e c t of a m b i e n t m o i s t u r e on c u r e , t h e r e i s a l s o a s e c o n d m o i s t u r e effect which was o b s e r v e d for fully c u r e d polyurethane f i l m s (35 d a y s o r m o r e a m b i e n t c u r e ) . In F i g u r e 5 A r e s u l t s a r e g i v e n f o r a s a m p l e e x p o s e d to 60% R H (ambient) w h i l e F i g u r e 5 Β g i v e s r e s u l t s f o r a s a m p l e o b t a i n e d f r o m the s a m e s u b s t r a t e


HILL AND PKINCE

Woterborne Polymers


45 h

0

1

3

2

4

6

5

Time (hrs) Figure 3. Isocyanate absorbance, corrected for baseline absorbance, as a function of time after spray application for a two-package solventborne polyurethane applied as a clear on a polyethylene substrate; film thickness = 18 pm (0.7 mil); RH = 15-80%

1

5 χ ÎO-LU

1

1

c 1

χ 10

5

χ ίο*

\ 32%) of i s o c y a n a t e l o s s r e s u l t s f r o m m o i s t u r e c u r e . T h e slow a b s o r p t i o n and d i f f u s i o n of w a t e r , e s p e c i a l l y at l o w R H , l i m i t s the c u r e r a t e and r e s u l t s i n quite l o n g t e r m c h a n g e s i n p r o p e r t i e s . D M A r e s u l t s o b t a i n e d o v e r the s a m e t i m e i n t e r v a l ( s e v e r a l days) as the F T - I R r e s u l t s i n d i c a t e that i s o c y a n a t e l o s s i s a c c o m p a n i e d by an i n c r e a s e i n crosslink density. A second m o i s t u r e related effect was o b s e r v e d by D M A for f u l l y c u r e d (>35 days) f i l m s of the s o l v e n t b o r n e p o l y u r e t h a n e . F i l m s conditioned under ambient o r high R H exhibited plateau r e g i o n s i n the m i d d l e of the g l a s s t r a n s i t i o n . T h e s e p l a t e a u s w e r e not o b s e r v e d f o r d r y s a m p l e s . T h e s t o r a g e m o d u l u s of "wet" versus d r y " s a m p l e s i n s o m e c a s e s d i f f e r e d by a f a c t o r of f i v e n e a r r o o m t e m p e r a t u r e . Τ hi s e f f e c t of a b s o r b e d m o i s t u r e o n p r o p e r t i e s c o u l d be a c a u s e of p o o r r e p r o d u c i b i l i t y of paint t e s t r e s u l t s . 11

S e v e r a l p o t e n t i a l w a t e r b o r n e r e p l a c e m e n t s have a l s o b e e n s t u d i e d by D M A . N o n e of the c l e a r w a t e r b o r n e s y s t e m s gave r u b b e r y p l a t e a u s w i t h i n the range of d e t e c t i o n w h e r e a s c l e a r f i l m s of both the c u r r e n t topcoat a n d c u r r e n t p r i m e r do e x h i b i t s u c h p l a t e a u s at 1 χ 10 and 6 . 4 χ 10 dyne s / c m , r e s p e c t i v e l y . T h i s c o m p a r i s o n i n d i c a t e s that the w a t e r b o r n e s y s t e m s h a v e a r e l a t i v e l y low c r o s s l i n k d e n s i t y . O n e p a i n t t e s t w h i c h i s s e n s i t i v e to c r o s s l i n k d e n s i t y i s f l u i d r e s i s t a n c e . The waterborne s y s t e m s a r e a l l m o r e s e n s i t i v e to softening d u r i n g f l u i d i m m e r s i o n i n S k y d r o l 5 0 0 Β and w a t e r than the s o l v e n t b o r n e p o l y u r e t h a n e . W o r k d i r e c t e d at e l i m i n a t i n g t h i s d e f i c i e n c y i s underway. 8

7

2

T h e r e l a t i o n s h i p of D M A r e s u l t s to i m p a c t r e s i s t a n c e and f l e x i b i l i t y i s s t i l l b e i n g i n v e s t i g a t e d . U s e of l o n g e r s a m p l e s w i l l p e r m i t o b t a i n i n g m o r e a c c u r a t e data at l o w t e m p e r a t u r e s . D M A c u r v e s f o r s e v e r a l of the f i l m s gave i n d i c a t i o n s of l o w t e m p e r a t u r e s e c o n d a r y t r a n s i t i o n s , w h i c h a r e often o b s e r v e d for materials having high impact resistance. Preliminary re s u l t s w i t h p i g m e n t e d f i l m s i n d i c a t e that p i g m e n t a t i o n of the c u r r e n t topcoat and p r i m e r c a u s e s s u b s t a n t i a l i n c r e a s e s i n E ' i n the r u b b e r y p l a t e a u r e g i o n .


9.

HILL AND PRINCE

Waterborne Polymers

103


Acknowledgements The authors wish to recognize and express appreciation to C . J . H u r l e y , R . L . V i s s o c , and D . W. Hamilton of the University of Dayton Research Institute for their assistance i n operation of the F T - I R and DMA instruments. Recognition and appreciation is also extended to DeSoto, Inc. for supplying some of the samples and paint test data. Literature Cited 1. Yaseen, M. and Ashton, H.E., J. Coatings T e c h n o l . , 1977, 49, (629), 50. 2. W i l l e y , R . R . , Applied Spectroscopy, 1973, 30, (6), 593. 3. A k a y , M.; B r y a n , S . J.; and White, E.F.T., J.Oil C o l . C h e m . A s s o c . , 1973, 56, 86. 4. Saunders, K . J., "Organic Polymer C h e m i s t r y , " Chapman and H a l l : London, 1973, p.342. 5. B e r g e r , W . , Federation d'Association de Techniciens des Industries des Peintures, V e r n i s , Emaux et E n c r e d'Imprimerie de l'Europe Continentale ( F A T I P E C ) Congress, 1962, p.300. 6. Vanderhoff, J. W., "Water-Base Coatings," Technical Report A F M L - T R - 7 4 - 2 0 8 , A i r F o r c e Materials Laboratory, Wright-Patterson AFB, 1976, p a r t II, p. 85, (Available through National Technical Information Service). 7. C a l a , J . A . and Lapkin, M., A m . C h e m . S o c . D i v . O r g . C o a t . P l a s t . C h e m . P a p . , 1976, 36, (2), 431. 8. Tobolsky, A. and Mark,H., "Polymer Science and M a t e r i a l s , " Wiley-Interscience: New Y o r k , 1971, p.216. 9. D e a n i n , R . D . and Nalepa, S. M., Am.Chem.Soc.Div.Org. P l a s t . C h e m . P a p . , 1976, 36, (2), 811. 10. B o l o n , B . A . ; O l s o n , D . R . ; L u c a s , G . M . and Webb,K.K., A m . C h e m . S o c . D i v . O r g . C o a t . P l a s t . C h e m . P a p . , 1978, 39, 512 and 518. 11. B i l l m e y e r , F.W., "Textbook of Polymer Science," Second Edition, Wiley-Interscience: New Y o r k , 1971, p.435. 12. R e i m s c h u e s s e l , H . K . , J.Poly.Sci., Chem.Ed., 1978, 16, 1229. 13. Hill, L.W., P r o g . O r g . C o a t i n g s , 1977, 5, 277. 14. S e n i c h , G . A . and MacKnight, W. J. in "Multiphase P o l y m e r s , " R.F. Gould, Ed., Advances in C h e m . S e r i e s 176, A m e r i c a n Chemical Society:Washington, D.C., 1979, p . 9 7 . 15. K i m , S.C.; Klempner, D.; Frisch, K. C.; Radigan, W . ; and Frisch,H.L., Macromolecules, 9, 1976, 258.



104

16. 17.

Lagasse,R.R., Hoy,K.L., J.

8, 1980.


RECEIVED J a n u a r y

J.Applied Poly.Sci., 2 1 , 1977, 2489. C o a t i n g s T e c h n o l . , 1979, 51., (651), 2 7 .


Waterborne Polymers for Aircraft Coatings - ACS Symposium Series

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