Accelerated test procedures for hydrothermal aging

It is inevitable that insulation systems have to interact with their environment. They have to withstand atmospheric stresses such as temperature and humidity. Under these conditions, many polymers show degradation due to hydrolysis effects. Hydrolysis describes the chain scission of the polymer as a result of chemical reaction with water. For the simulation of the aging processes during a complete lifetime, it is usual to perform accelerated tests. The maximal acceptable degradation for polymers has to be defined in specifications. State of the art for accelerated testing is climatic chamber storage under increased temperatures and humidity. Aging samples in a pressure cooker is an even faster test method. Due to a rise of the atmospheric pressure, higher temperature and humidity can be reached. Under these conditions, the aging process can be accelerated considerably. This paper deals with the different available test methods for accelerated hydrothermal aging. For the quantification of the aging state of the insulation system, the dielectric constant, the loss factor, the specific resistivity and the dielectric strength of polyamide samples are investigated during the testing procedure. Further the water absorption during the aging process is measured and the cracking of the surface is evaluated optically. Finally, a comparison between the different developments of the determined parameters is performed leading to an evaluation of the two aging procedures.