Durability of Acrylic Products during Heat Aging

Currently, the areas of application of polymethyl methacrylate plastic (acrylic) owing to its unique properties (most notably, lightness, plasticity, exceptional transparency and high impact resistance) range widely from modeling, lighting technology and medicine, automotive industry and watchmaking to aviation, shipbuilding, rocket engineering and production of military equipment. Mechanical characteristics of polymethyl methacrylate plastic, like most polymers, change significantly over time when exposed to temperature. The temperature impact extends to all volume of the material and leads to its “heat aging”, which should be considered when designing the products made of polymethyl methacrylate plastic. The process of thermal destruction is long enough, so it is reasonable to predict the changes in the properties of polymethyl methacrylate plastic using accelerated methods of material specimen aging with subsequent testing under simple loading. The paper deals with the experimental study of the process of changing of physical and mechanical characteristics of polymethyl methacrylate plastic during thermal destruction. Heat aging of the material is identified with the process of the thermal oxidative destruction occurring at a constant rate and specified temperature. It is assumed that the change in polymer’s mechanical properties is proportional to the change in number of its functional groups. The tests were performed on cylindrical specimens with fillets at the ends at three levels of temperature: 40, 70 and 100°C. Specimens were held at each temperature for 2; 5; 10; 20 and 30 days. According to results of statistical processing of experiments, we obtained the average values of ultimate strains for each temperature-time regime. The obtained ultimate strain values were the basis for the construction of long-term aging curves. For the minimum allowable value of ultimate strain in operation, the period of operation of polymethyl methacrylate plastic was determined for different aging temperatures. At the operating temperature of 20oС the period of operation was 12 years. During this period, the ultimate deformability of polymethyl methacrylate plastic decreases to 3%, which is approximately equal to elastic deformations. The results will allow us to predict the period of operation of polymethyl methacrylate plastic products for different values of the ultimate strain and operating temperature.