Water damage in polyester resins

Abstract

The nature of cracks, produced in three cured polyester resins during exposure to water has been studied by the combined techniques of optical microscopy, scanning electron microscopy and electron probe X-ray microanalysis. There are many cracks totally enclosed within the resin and it is shown that these must be attributed to pockets of high pressure produced at impurity inclusions by interaction with water : in most cases, osmotic pressure from water soluble inorganic inclusions. The distribution and orientation of cracks gives evidence of relatively complex laminar stress distributions generated by water exposure in a polyester resin plate. There can be a laterally compressive stress in the interior of a plate, with laterally tensile stress at or nearer to the surface. It is pointed out that the interactions of water with a cured polyester resin have the necessary complexity to generate such stress systems since the first effects, water uptake and hydrolysis, causing swelling, also enhance molecular mobility and can promote further double-bond polymerization, accompanied by shrinkage.