WATER-INDUCED NUCLEATION OF DISK CRACKS IN SELECTED THERMOSETS

Orthophthalic polyester, isophthalic polyester and vinyl ester resins exposed to water and to saturated salt solutions at 40, 65 and 90-degrees-C were studied to determine if disc cracks formed. The time of initiation of disc cracks was determined and related to the water absorption curves. The morphology was studied by optical and scanning electron microscopy. Disc cracks were found in orthophthalic and vinyl esters but not in isophthalic resins. The two-component phase diagram of polymer and water is the key to the prediction of behaviour. A new mechanism for disc crack initiation is presented. It states that disc cracks are the result of a thermodynamically driven phase separation that results in nucleation of a water phase within a supersaturated polymer. The mechanism is supported by the observation that, when the temperature of a nearly, but not completely, water-saturated polymer is reduced, bringing the polymer in to a region of supersaturation, massive disc crack initiation results. The reverse process-the disappearance or healing of disc cracks-predicted by the phase diagram and reported earlier in the literature, was confirmed in this work when a water-saturated, heavily disc-cracked specimen was placed into an environment of lower relative humidity. The growth rate and ultimate coalescence of disc cracks depend on the nature of the polyester. The hydrolysis of the polyester by the disc crack liquid with chain breakdown by de-esterification at the water polymer interface is a key step in blister formation. It results in a concentrated solution, which generates osmotic pressure. The orthophthalic polyester is susceptible to hydrolysis; the other two resins are more resistant. Examples are noted of disc crack formation in polymers not saturated. This is explained by the presence of a water-soluble second phase, which was present initially or is generated in the polymer during heat treatment. The ability of the water polymer phase diagram to predict the appearance and disappearance of disc cracks strongly supports the interpretation of disc cracking as a thermodynamic process.