Changes in Crosslink Density of Structural Silicone Sealants Due to Ozone and Moisture

摘要

The changes in the crosslink density, elastically effective chains and entanglements in the network, and the molecular weights between crosslinks of three different commercially available structural silicone sealants used in glazing applications were monitored as a function of specific aging factors. The aging variables included simultaneous exposure to ozone and moisture in which the pH of the moisture was maintained at discrete levels in the acid and the alkaline range. The simultaneous effect of external loading on the performance of these sealants was also studied. Unexposed and unstressed test samples kept at ambient laboratory conditions were used as the control. Results of these weathering studies indicate that the crosslink density of the sealants will decrease during the initial period as a result of their exposure, and some surface cracking was observed. However, following this initial exposure period, the sealants showed an increase in the crosslinking activity. All three formulations of the silicone sealants appeared to crack at angles of 90#xB0; across the line of tension (loading). The acid and ozone combination especially appears to act synergistically in the deterioration of these sealants. However, the test coupons exposed to moisture at the higher, or caustic, pH level did not show similar behavior. The medium-modulus material exhibited good overall resistance to the synergetic effects which can cause surface cracking of the sealant. However, significant changes were observed in the elastically effective chains in the network and for the molecular weights between crosslinks. The effect of external loading on the sealants was significant only for the high-modulus formulation. Even though the attack of ozone and acid on these silicone sealants was not classified as aggressive, the slow progressive attack and surface cracking should still be of concern. It is hoped that these test results can lead to more informed decisions concerning silicone sealant life following their exposure to ozone and acid precipitation.