The Role of Moisture Transportation in Varying the Effect of Accelerated Weathering

摘要

It is increasingly realized that water is one of the most important factors in the durability behavior of coatings. The polar nature of a typical coating polymer means that it will absorb moisture that may have a substantial effect on degradation chemistry, leaching and physical properties. Mechanical properties are time dependent, due to intrinsic relaxation processes in polymers and these processes are changed by the plasticizing properties of water. Thermogravimetric analysis shows the content of moisture in the sample is determined by both the exposure temperature and rate of change of temperature. Diffusion coefficients data was fitted and activation energy was obtained via Arrhenius equation. Diffusion of moisture is typically slow and limits the possibility of complete drying or saturation in an exposure cycle thus it is one reason for the discrepancy between natural and accelerated weathering. Yellowing of an epoxy resin, for example, in accelerated weathering, indicates that resin deterioration is dependent on thermal cycling frequency with the intervention of moisture. A longer cycle length allows slower degradation. The eventual concentration and location of the water depends on the duration and temperature of a cycle and results from the cumulative effect of many cycles will be different depending on the conditions.