Abstract: The cracking phenomenon of aviation perspex YB-4 during weathering is discussed. Various techniques, such as GPC, TMA (Deformation-temperature) ,IR, ESGA, TG, DMA, stress-solvent crazing, and stress-strain testing have been employed to investigate and compare the structure and properties of perspex YB-4 and YB-3, and their changes in aging. The results show that the most important reasons that lead to relatively, easily cracking of YB-4 during weathering are the following; ( 1 ) The average molecular weight of original YB-4 is lower and its molecular weight distribution is wider. They are the inherent weaknesses of YB-4 in quality. ( 2 ) Oxygen and water, especially the latter, have more serious effects on YB-4 than on YB-3.

Key words: aviation perspex, weathering, molecular weight and molecualr weight dis-tribution cracking