8 Optimization of Cure Conditions During Processing of Acrylic Latex Coatings
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CHOR HUANG and EDWARD J. LEESON Technical Service Center, BFGoodrich Chemical Division, Avon Lake, OH 44012
Thermosetting acrylic latexes are commonly used as vehicles for industrial water-borne coatings (1). These latexes contain polymers with reactive side groups (such as carboxyls, amides and hydroxyls) which serve to link the polymer chains together during "cure" - a process generally achieved by the application of heat to the coating. Crosslinking of the polymer chains results in a coating with superior physical and chemical properties (1-6). These properties are especially enhanced when the latex polymers are heated in the presence of crosslinking agents such as alkylated melamine formaldehyde compounds (4,5). The crosslinking reaction is catalyzed by the presence of acids (6,7). The rate at which the crosslinking reaction occurs is of major interest to a coating manufacturer since it dictates the amount of energy and the time required to cure the coating to a set of desired physical properties. A number of physical methods for assessing the degree of crosslinking achieved during cure of acrylic polymers have been previously described (7,8). However, very l i t t l e of the information generated in these studies can be directly applied by a coatings manufacturer to predict the conditions required to cure the coating during processing. A method for predicting cure conditions would be very useful to the coatings manufacturer since it will give him the flexibility of adjusting simultaneously the time and temperature for cure in such a way as to minimize the cost of production. The s t u d i e s r e p o r t e d i n t h i s paper were aimed a t f i n d i n g a r e l a t i o n s h i p between time and temperature f o r cure o f an a c r y l i c l a t e x c o a t i n g system. S i n c e t h e c r o s s l i n k i n g r e a c t i o n s t u d i e d i s an a c i d - c a t a l y z e d r e a c t i o n (6,7), t h e e f f e c t o f pH o f t h e c o a t i n g on t h e time-tenperature r e l a t i o n s h i p f o r cure was a l s o s t u d i e d . The s i t u a t i o n o f a c o a t i n g b e i n g conveyed through an oven ( s e t a t a constant temperature) i s a n a l y z e d t o take i n t o account t h e changing temperature o f t h e c o a t i n g w i t h time. The a n a l y s i s l e a d s t o a g e n e r a l procedure f o r p r e d i c t i n g the cond i t i o n s f o r cure o f c o a t i n g s d u r i n g p r o c e s s i n g .
0-8412-0509-4/79/47-107-081$05.00/0 © 1979 American Chemical Society
In Energy Conservation in Textile and Polymer Processing; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
ENERGY
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CONSERVATION
IN TEXTILE
A N DPOLYMER
PROCESSING
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Analysis D e f i n i t i o n o f Minimum Cure Température (MCT). The word "cure" i s used t o d e s c r i b e the process by which polymer c h a i n s which c o n t a i n r e a c t i v e s i d e groups a r e l i n k e d t o each o t h e r , e i t h e r w i t h o r without t h e a i d o f a c u r i n g agent (Figure l a ) . When a sample o f uncross l i n k e d polymer i s p l a c e d i n an oven, s e t a t seme e l e v a t e d temperature, t h e number o f c r o s s l i n k s t h a t a r e formed w i t h i n t h e sample i n c r e a s e s w i t h time (Figure l b ) . The r a t e a t which t h i s number i n c r e a s e s i s dependent on t h e oven temperature. G e n e r a l l y , t h i s r a t e i n c r e a s e s as t h e t a t p e r a t u r e i n c r e a s e s (Figure 2 ) . When a l l p o s s i b l e c r o s s l i n k i n g r e a c t i o n s w i t h i n the polymer have been completed, the polymer i s s a i d t o be " f u l l y c r o s s l i n k e d " (Figure 1 ) . Before t h i s o c c u r s , however, t h e r e i s a p o i n t d u r i n g the c u r i n g process when a l l the polymer chains a r e l i n k e d t o each o t h e r by a t l e a s t one c r o s s l i n k . T h i s p o i n t , c a l l e d t h e " g e l p o i n t " (Figure l a ) i s u s u a l l y c h a r a c t e r i z e d by the a b i l i t y o f the polymer sanple t o r e s i s t being d i s s o l v e d by a good s o l v e n t . Since polymer c h a i n s i n l a t e x e s are u s u a l l y o f h i g h molecular weight, t h e nunfcer o f c r o s s l i n k s r e q u i r e d t o l i n k a l l the chains together i s s m a l l . Hence, g e l p o i n t u s u a l l y occurs q u i t e e a r l y d u r i n g t h e cure process (Figure l b ) . T h i s i s i n c o n t r a s t t o t h e cure o f thermosetting r e s i n s such as alkyd(2) and epoxy (10) r e s i n s where t h e s t a r t i n g r e s i n c o n s i s t s o f m u l t i f u n c t i o n a l monomers and "cure" i n v o l v e s l i n k i n g these monomers t o form a r e l a t i v e l y t i g h t network. G e l p o i n t i n these r e s i n s occur q u i t e c l o s e to t h e p o i n t where a l l t h e c r o s s l i n k i n g r e a c t i o n s have been completed (9, 10). The temperature r e q u i r e d t o cure a polymer t o i t s g e l p o i n t , f o r an a r b i t r a r i l y s e t cure time, i s d e f i n e d here a s t h e "Minimum Cure Temperature" o r MCT. F i g u r e 2 i l l u s t r a t e s t h e f a c t t h a t i f the cure r a t e i n c r e a s e s w i t h tenperature, then MCT decreases as the cure time i n c r e a s e s . Note t h a t t h e d e f i n i t i o n o f MCT here assumes t h a t t h e polymer i n c r e a s e s i n tenperature t o i t s cure temperature i n s t a n t a n e o u s l y a t the beginning o f "cure" and t h a t t h i s tenperature i s maintained throughout t h e cure time. T h i s i s c e r t a i n l y n o t t r u e i n a s i t u a t i o n where t h e c o a t i n g , a p p l i e d t o a s u b s t r a t e , i s conveyed through an oven. The tenperature o f the c o a t i n g , i n t h i s case, i n c r e a s e s w i t h time i n t h e oven and may never reach t h e oven tenperature as i t e x i t s from the oven. The use o f t h e term, MCT f o r c u r i n g under such a s i t u a t i o n i s ambiguous. To a v o i d t h i s arabiguity, a method f o r determining cure has been developed i n o u r l a b o r a t o r y , where t h e c o a t i n g tenperature i n c r e a s e s almost i n s t a n t a n e o u s l y t o i t s cure tatper^a t u r e . The use o f MCT w i l l be r e s t r i c t e d e x c l u s i v e l y t o t h i s method o f determining cure (see Experimental S e c t i o n ) .
In Energy Conservation in Textile and Polymer Processing; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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HUANG
A N D LEESON
Acrylic
(UNCROSSLINKED)
Latex Coatings
83
(OEL POINT) (FULLY CROSJUNKEO)
(b)
TIME Figure 1. (a) Diagram of the "cure" of coatings: (O) reactive groups and (^) crosslinks; (b) number of crosslinL· formed vs. time during the "cure process.
Figure 2. Number of crosslinks formed (n) vs. time during "cure" of coatings at various temperatures. For cure times, t t , and t , the corresponding minimum cure temperatures are T T , and T . u
t
s
u
t
3
In Energy Conservation in Textile and Polymer Processing; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
ENERGY
84
CONSERVATION
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S o l u t i o n o f t h e Rate Equation f o r Cure. c r o s s l i n k s a r e formed i s g i v e n by:
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f
=
A N D POLYMER
PROCESSING
The r a t e a t which
k f (n)
(1)
where k i s the o v e r a l l r a t e constant and f (ji) i s seme f u n c t i o n o f n, the number o f c r o s s l i n k s formed a t time, t . F o r the pur pose o f t h i s a n a l y s i s , i t i s not necessary to knew what form f (n) takes. Ine o n l y assumption made i s t h a t t h i s f u n c t i o n does not change as the c x o s s l i n k i n g r e a c t i o n proceeds. The i n t e g r a t e d form o f Equation 1 takes t h e form:
F
(η)
=/
where F (η) i s another i n Equation 2 r e q u i r e s k, which may v a r y w i t h I n the case where a c i d , the o v e r a l l r a t e sum o f two r a t e terras: k
=
k dt
(2)
f u n c t i o n o f n. S o l u t i o n o f t h e i n t e g r a l a knowledge o f the o v e r a l l r a t e constant, time. the r e a c t i o n i s c a t a l y z e d by a s t r o n g constant can be considered (11) t o be t h e
+
*N
k
C
[
H +
( 3 )
]
where ^ i s the r a t e constant f o r the r e a c t i o n i n an uncatalyzed system and k i s t h e r a t e constant f o r the r e a c t i o n i n t h e p r e sence o f a s t r o n g c a t a l y s t . Assuming the approximate r e l a t i o n s h i p between the hydrogen i o n c o n c e n t r a t i o n and the pH o f t h e s o l u t i o n (12), Equation 3 can be w r i t t e n a s : c
k
=
+
k
Xp^
c
(4)
I f we new assume t h a t each o f the r a t e constants, k^ and k v a r y w i t h temperature a c c o r d i n g t o t h e A r r h e n i u s r e l a t i o n s h i p , Equation 4 then becomes: c
k
=
A
.
Ν
e-V*
1
+ Α>
1
C
ΙΟ"** e - V
5 3
"
(5)
where E ^ and EL, a r e a c t i v a t i o n e n e r g i e s and A ' and A' a r e t h e c o l l i s i o n f a c t o r s a s s o c i a t e d w i t h each o f the r a t e terms. N
S u b s t i t u t i n g Equation 5 i n t o Equation 2 produces t h e equation: F(n) = ^
t
^ N
e
"
V
R
T
+
A
'c 1 0 - ^ e - V * r |
dt
In Energy Conservation in Textile and Polymer Processing; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
6
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8.
HUANG
AND
Acrylic
LEESON
Latex
Coatings
85
Equation 6 can be used t o p r e d i c t t h e time, t r e q u i r e d t o cure a c o a t i n g t o a predetermined s e t o f p h y s i c a l p r o p e r t i e s , corresponding t o a c e r t a i n number o f c r o s s l i n k s , η i n t h e c o a t i n g . In o r d e r t o s o l v e t h e equation, i t i s necessary t o determine how the pH and t h e tenperature o f t h e c o a t i n g v a r y w i t h time. I f t h e c o a t i n g does n o t c o n t a i n any v o l a t i l e a c i d o r base, t h e pH o f the c o a t i n g c a n be considered t o be a constant (see D i s c u s s i o n S e c t i o n ) . The s o l u t i o n o f Equation 6,then, depends on the v a r i a t i o n o f t h e tenperature o f c o a t i n g w i t h time. Two s p e c i a l cases w i l l be considered below: Case 1: Τ = Tq, a constant; η = η , t h e number o f c r o s s l i n k s r e q u i r e d t o b r i n g the polymer t o g e l p o i n t . T h i s c o r r e s ponds t o the c o n d i t i o n s used t o determine MCT, as d e f i n e d above and hence, T i s e q u i v a l e n t t o MCT. S o l u t i o n o f Equation 6 i s simple i n t h i s case s i n c e t h e i n t e g r a n d i s now a constant, and can be taken o u t o f t h e i n t e g r a l t o g i v e t h e r e s u l t : c
1
=
A^e-V^C
+
K, l O ' P V V ^ C
(7)
where
and
F o r r e a c t i o n s o c c u r r i n g i n t h e uncatalyzed l a t e x polymer, the f i r s t term on t h e r i g h t hand s i d e (RHS) o f Equation 7 prédominâtes so t h a t : "V^C
(7a)
When the l a t e x i s h i g h l y a c i d i f i e d (to below pH 3.5), t h e second term on t h e RHS o f Equation 7 predominates so t h a t : I
=
Ac l ( f
p H
e " V ^ C
(7b)
By determining MCT as a f u n c t i o n o f cure time under these two extreme c o n d i t i o n s , i t i s p o s s i b l e t o estimate the v a l u e s o f A^, A , E ^ and E from Arrhenius p l o t o f Equation 7a and 7b (see^estllts Section). Case 2: Τ = T (t,Τ ) , η = η , where η > η . I n t h i s case, t h e c o a t i n g temperature i s cf f u n c t i o n o f both t h e time, t , i n t h e oven and s e t oven tenperature, Τ . The number o f c r o s s l i n k s formed, i s g r e a t e r than t h a t r l q u i r e d t o reach g e l p o i n t . p
In Energy Conservation in Textile and Polymer Processing; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
ENERGY
86
CONSERVATION
IN
TEXTILE
AND
POLYMER
PROCESSING
S o l u t i o n o f Equation 3 under these c o n d i t i o n s g i v e :
r^.-v't'V ν°~ * v-^vj* - s, +
ρΗ
