Polymer Materials for Electronic Applications - American Chemical

15 Characterization of Cured Epoxy Powder Coatings by Solvent Absorption W. A . R O M A N C H I C K

and J . F . G E I B E L

Downloaded by COLUMBIA UNIV on June 20, 2013 | http://pubs.acs.org Publication Date: April 7, 1982 | doi: 10.1021/bk-1982-0184.ch015

Western Electric Co., Inc., Engineering Research Center, Princeton, N J 08540

Crosslinks are extremely important in determining the physical properties of thermoset polymers because they increase the molecular weight as well as limit the motion of chains with respect to one another.(1) Although insoluble, a polymeric network w i l l usually absorb and be swelled by solvents in which the uncrosslinked (uncured) polymer is soluble. The tendency to absorb solvents decreases as the degree of crosslinking is increased. This paper w i l l describe a standard procedure for measuring MEK (Methylethyl Ketone) absorption of cured epoxy powder coatings. The test has proven to be a sensitive measure of both material properties and the effectiveness of manufacture of powder coatings. The effects of time and temperature of MEK, homogeneity of extrusion, state of cure, Epoxy Equivalent Weight (EEW), and CTBN (Carboxy-Terminated Butadiene-Acrylonitrile Copolymer) elastomer concentration on MEK absorption are discussed.

DISCUSSION OF TEST PARAMETERS Although the detailed procedure for the MEK absorption test is given in the experimental section, certain parameters require additional elaboration. In particular, the temperature of the MEK must be tightly controlled and the coating thickness must be kept above a minimum value i f solvent absorption data are to be reproducible. As shown in Table I, the MEK absorption changes drastically for small changes in temperature. As a result, standard test samples are run in a constant temperature bath at 23C.

0097-6156/82/0184-0199$05.00/0 ©

1982 American Chemical Society

In Polymer Materials for Electronic Applications; Feit, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

200

POLYMER

Table

I.

T e m p e r a t u r e o f MEK

MATERIALS

MEK A b s o r p t i o n

(°C)

versus


18 23 28 35


Note:

FOR ELECTRONIC

m

APPLICATIONS

Temperature 2

( g/cm , 5 min. d i p ) 2.996 3.866 4.298 4.862

L i n e a r epoxy r e s i n , EEW = 1185 8/eq., 10% CTBN r u b b e r , c u r e d w i t h P-108 a t 200°C f o r 15 m i n u t e s .

The s e c o n d c r i t i c a l p a r a m e t e r w h i c h must be c o n t r o l l e d i s c o a t i n g t h i c k n e s s . The c o a t i n g must be s u f f i c i e n t l y t h i c k on t e s t samples t o p r e c l u d e t h e p o s s i b i l i t y o f t h e s o l v e n t d i f f u s i n g through t o the polymer-metal i n t e r f a c e . In a given EEW r a n g e (600-2000 8 / e q . ) , 6 m i l s o f c o a t i n g h a s p r o v e n t o be s u f f i c i e n t t o p r e v e n t s a t u r a t i o n . A s t a n d a r d powder c o a t i n g (EEW = 1173 8/eq.) was c o a t e d a t v a r i o u s t h i c k n e s s e s and c u r e d w i t h P-108 a t 200°C f o r 15 m i n u t e s . The MEK a b s o r p t i o n f o r a 5 m i n u t e i m m e r s i o n was t h e n m e a s u r e d . The r e s u l t s a r e shown i n T a b l e I I and F i g u r e 1.

Table

II.

MEK A b s o r p t i o n v e r s u s

Coating Thickness 0.4 I. 0 2.0 4.0 5.0 6.0 6.2 10.0 II. 5 17.5 20.0

(mils)

Coating

Thickness


m

2

( 8/cm )

0.24 0.91 1.93 4.35 4.80 4.90 5.00 5.10 5.10 5.28 5.01

C l e a r l y , t h e MEK a b s o r p t i o n l e v e l s o u t a t c o a t i n g t h i c k n e s s e s g r e a t e r than 6 m i l s . Sample c o a t i n g s l e s s t h a n 6 m i l s l e a d t o erroneous results.




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202


MEK

MATERIALS

FOR

ELECTRONIC

APPLICATIONS

ABSORPTION VERSUS TIME

The a b s o r p t i o n o f MEK by c u r e d epoxy r e s i n s f o l l o w s a s q u a r e r o o t o f time d e p e n d e n c e . F i g u r e 2 shows a t y p i c a l p l o t o f MEK absorbed versus square root of time. T h i s epoxy r e s i n (EEW = 1060 8/eq.) was c u r e d w i t h P-108 a t 200°C f o r 15 m i n u t e s . Each of t h e d a t a p o i n t s r e p r e s e n t s one s e p a r a t e e x p e r i m e n t . In t h i s c a s e , f o u r s a m p l e s were immersed i n MEK, and s a m p l e s w i t h d r a w n a t I , 3, 5, and 10 m i n u t e s . E a c h sample was w e i g h e d as d e s c r i b e d i n the experimental s e c t i o n . The l i n e drawn t h r o u g h t h e d a t a does not p a s s t h r o u g h t h e o r i g i n as one would e x p e c t . This i s probably a r e s u l t o f w a i t i n g 45 s e c o n d s f r o m t h e t i m e the sample i s w i t h d r a w n f r o m t h e MEK t o t h e t i m e t h e w e i g h t i s r e c o r d e d . The w e i g h t o f t h e s w o l l e n samples c o n s t a n t l y d e c r e a s e s o w i n g t o e v a p o r a t i v e l o s s o f MEK. T h u s , we s y s t e m a t i z e t h i s e v a p o r a t i v e e r r o r by w e i g h i n g t h e sample e x a c t l y 45 s e c o n d s a f t e r w i t h d r a w i n g i t f r o m the s o l v e n t . Presumably the s w o l l e n s u r f a c e i s s i m i l a r f o r a l l times subsequent t o t h e time t h a t the s u r f a c e a t t a i n s e q u i l i b r i u m s w e l l i n g . T h e r e f o r e , t h e e v a p o r a t i v e l o s s s h o u l d be s i m i l a r f o r the I , 3, 5, and 10 m i n u t e s a m p l e s , s i m p l y d i s p l a c i n g the p l o t downward. This r e s u l t s i n a n e g a t i v e i n t e r c e p t shown i n F i g u r e 2.

MEK

ABSORPTION AS

A MEASURE OF

HOMOGENEITY

I n the e v a l u a t i o n o f f u n c t i o n a l powder c o a t i n g s , i t must be n o t e d t h a t the f i n a l m a t e r i a l p r o p e r t i e s depend n o t o n l y on the f o r m u l a t i o n b u t on t h e h o m o g e n e i t y o f t h e m a t e r i a l . Optimum d i s p e r s i o n o f r e s i n and c u r i n g a g e n t i s n e c e s s a r y t o f o r m a u n i f o r m l y c r o s s l i n k e d n e t w o r k . The most w i d e l y u s e d i n d u s t r i a l method f o r m i x i n g s o l i d r e s i n and c u r i n g a g e n t i s e x t r u s i o n . S o l v e n t a b s o r p t i o n h a s p r o v e n t o be a s e n s i t i v e t o o l i n e v a l u a t i n g the e f f e c t i v e n e s s o f e x t r u s i o n ( h o m o g e n e i t y ) . T a b l e I I I shows the MEK a b s o r p t i o n o f t w e l v e d i f f e r e n t powder f o r m u l a t i o n s a f t e r one, two, and sometimes t h r e e e x t r u s i o n s . As c a n e a s i l y by s e e n i n a l l c a s e s s o l v e n t a b s o r p t i o n i s r e d u c e d by t h e s e c o n d e x t r u s i o n w h i l e t h e t h i r d e x t r u s i o n does n o t seem t o l o w e r i t f u r t h e r . T h u s , MEK a b s o r p t i o n c a n be u s e d as a t e s t t o o p t i m i z e e x t r u d e r c o n d i t i o n s . I f t h e s o l v e n t a b s o r p t i o n c a n be m i n i m i z e d f o r a p a r t i c u l a r f o r m u l a t i o n i n one e x t r u s i o n , q u a l i t y i s p r e s e r v e d w h i l e t h e m a n u f a c t u r i n g sequence i s o p t i m i z e d .


ROMANCHICK A N D GEIBEL

Cured Epoxy

Powder

Coatings


7 r-

TIME * 1

Figure 2.

(MINUTES ) 15

112

MEK absorption vs. time .


203

POLYMER

204

Table

III.

MEK

Absorption

MEK


Powder 1 2 3 4 5 6 7 8 9 10 11 12

MEK

1 Extrusion

MATERIALS

versus

Absorption

ABSORPTION VERSUS THE

ELECTRONIC

APPLICATIONS

t h e Number o f E x t r u s i o n s

(mg/cm^

2 Extrusions

4.44 5.14 3.86 5.10 2.94 1.72 4.42 4.30 4.00 0.58 3.21 5.30

FOR

h.08

4.09 4.83 3.66 4.47 2.58 1.65 3.95 4.00 3.55 0.26 3.05 4.70

STATE OF

3 Extrusions

CURE

I t i s w e l l known t h a t c u r i n g t i m e s and temperatures i n f l u e n c e the f i n a l p r o p e r t i e s o f t h e r m o s e t epoxy r e s i n s . (2^) We h a v e f o u n d t h a t t h e MEK a b s o r p t i o n f o r a l i n e a r e p o x y r e s i n c o n t a i n i n g 12% c a r b o x y l - t e r m i n a t e d r u b b e r and c u r e d w i t h P-108 i s v e r y s e n s i t i v e t o c u r e c o n d i t i o n s . The MEK a b s o r p t i o n was m e a s u r e d i n t h e s t a n d a r d manner f o r a 5 m i n u t e i m m e r s i o n as a f u n c t i o n of s e v e r a l cure schedules. The r e s u l t s o f t h e s e e x p e r i m e n t s a r e summarized i n T a b l e IV and F i g u r e 3. As t h e s t a t e o f c u r e a d v a n c e s ( i . e . , t h e d e g r e e o f c r o s s l i n k i n g i n c r e a s e s ) , t h e MEK a b s o r p t i o n d e c r e a s e s . Note t h a t a t t h e h i g h e r c u r e t e m p e r a t u r e s , t h e u l t i m a t e MEK a b s o r p t i o n i s a c h i e v e d more r a p i d l y . I t i s not n e c e s s a r i l y a d v a n t a g e o u s t o use h i g h e r c u r e t e m p e r a t u r e s , as t h i s c a n l e a d to s i d e r e a c t i o n s ( e . g . , o x i d a t i v e d e g r a d a t i o n , decomposition of d i c y , changes i n r u b b e r p a r t i c l e morphology) which can a l s o i n f l u e n c e the f i n a l c u r e d p r o p e r t i e s . I t i s , however, c l e a r t h a t t h e e x t e n t o f c u r e c a n be e a s i l y f o l l o w e d by s o l v e n t a b s o r p t i o n measurements. A necessary c o r o l l a r y i s that c o m p a r i s o n o f t h e MEK a b s o r p t i o n s o f two o r more f o r m u l a t i o n s must be done a t c o n s t a n t c u r i n g c o n d i t i o n s . To compare s o l v e n t a b s o r p t i o n s o f two f o r m u l a t i o n s c u r e d a t d i f f e r e n t c o n d i t i o n s i s m e a n i n g l e s s b e c a u s e any d i f f e r e n c e s i n s o l v e n t a b s o r p t i o n s c a n n o t be d e f i n i t i v e l y a s c r i b e d t o e i t h e r t h e c u r e s c h e d u l e o r t h e c o m p o s i t i o n o f t h e f o r m u l a t i o n . T h u s , t o be a f o r m u l a t i n g a i d , s o l v e n t a b s o r p t i o n s t u d i e s mandate a h i g h d e g r e e o f c o n s i s t e n c y i n the g e n e r a t i o n o f samples.


ROMANCHICK AND GEIBEL

Cured Epoxy

Powder

Coatings

CM

e o

60 B

u 3 o

CO

m o O O CO fO 1 ^

H

in ^

Polymer Materials for Electronic Applications - American Chemical

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