Лако-красочные материалы — производство

During cure of epoxies, especially systems with a short pot life or large mixes, considerable heat is evolved. This accelerates the cure, leading to even greater heat evolution. Mixes larger than 5 kg can reach excessive temperatures even with systems that have relatively long pot lives in quantities of 100 g. In addition to shortening the pot life dramatically, exothermic reactions can push the peak temperature to the point where thermal degrada­tion occurs, or at least to a level that creates excessive stresses in the curing matrix, causing it to crack on cooling. Except in certain circumstances, peak exotherm temperature should be limited by formulation to 150° C or preferably less in the mix quantities used. Cured epoxy resins may be formulated to be extremely hard, with Shore D hardeners of 80 or more, or soft, flexible products that barely produce a reading on the Shore A scale. When cured at approximately stoichiometric ratios and unmodified with diluents or plasticizers, however, they are generally hard and tough to brittle, especially DGEBA and epoxy novolac types.

Heat distortion temperature (HDT) or deflection temperature (DT) is a measure of the tendency of cured product to soften when heated. It is a feature of the inherent thermo­plasticity in cured epoxy compounds as a result of the relatively low cross-linking density, and may be any value from below 50°C to about 250°C, depending on formulation and cure cycle. Resins and hardeners of high functionality tend to have higher HDT. Postcuring at elevated temperature can increase HDT significantly.