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Chapter
16
Thermal Stress Development in Thick Epoxy Coatings 1
D. King and J . P. Bell Chemical Engineering Department and Institute of Materials Science, Box U-136, University of Connecticut, Storrs, CT 06268
The thermal stress development in thick (60 mils), solventless, amine-cured epoxy coatings on aluminum was investigated. The stress level as a function of temperature was measured by the bending beam technique. The results obtained showed increased deviation from the behavior of thinner coatings with increasing Tg and elastic modulus epoxies. The measured stress was greatly affected by increasing coating thickness up to eleven mils thick, with only slight effect after eleven mils. The curing schedule was found to control the temperature at which the epoxy-metal system is at zero stress. Plasticization by moisture was found to only slightly relieve the stress developed.
The thermal shock resistance of epoxies i s an increasingly important f i e l d of research because of the widening use of epoxies i n a p p l i c a t i o n areas where the conditions f o r thermal shock e x i s t . The Handbook of EPOXV Resins (1) defines thermal shock as "the l i k e l i h o o d of stress cracking during cure or thermal c y c l i n g " , and thermal shock i s "a function of the e l a s t i c modulus of the r e s i n , the thermal expansion c h a r a c t e r i s t i c s , and the stresses imposed by the curing temperature". An understanding of the development of thermal stresses i n epoxy-metal systems would a i d i n designing thermal shock-resistive epoxies. The i n t e r e s t i n g recent work i n thermal shock by McCoy (2-3) and Rauhut (4) gives q u a l i t a t i v e r e s u l t s , but no i n d i c a t i o n of the Current address: A. O. Smith Corporation, Corporate Technology Center, 12100 West Park Place, Milwaukee, WI 53224-3006 0097-6156/88/0367-0221 $06.00/0 © 1988 American Chemical Society Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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stress l e v e l s achieved. Dewey (5) applied a l i n e a r finite-element analysis to the McCoy work, showing where the maximum area of stress was located and an i n d i c a t i o n of problem areas, but with no absolute values. There has been extensive work i n understanding the o r i g i n and l e v e l of stresses that develop with coating systems due to curing and/or a thermal gradient. For solventless cured epoxy systems, curing stresses are small (6-8); these can be neglected i n the present study since the concern i s with solventless cured epoxy systems. Dannenberg (9) and also Shimbo, Ochi and A r a i (10) present work on 2-5 m i l t h i c k coatings of epoxy on aluminum. Their work indicates that above the glass t r a n s i t i o n temperature, Tg, of the epoxy, there are no stresses present because the epoxy w i l l r e l a x to a l l e v i a t e any stresses developed. Upon cooling to temperatures below Tg, the stresses increase because the epoxy i s a glassy material contracting at a rate greater than the aluminum. The stress l e v e l achieved at some temperature T
