Study On The Photo-Oxidation Of PVC-Coated Membrane Material

The PVC-coated membrane material is widely used in the field of outdoor constructions, because of its cheap price and excellent performance. According to the research of foreign scholar, the PVC-coated membrane material is vulnerable to age by the UV irradiation under the outdoor conditions. Domestic scholars have done primary research on the life prediction of the PVC-coated membrane material. The photo-oxidation aging performance of pure PVC film also had an in-depth study. Using the PVC-coated membrane material as the research object, the photo-oxidation is studied.In the experiment, the UVB-313ultraviolet fluorescence tube is chosen as the light source, which based on the wavelength-sensitive of PVC. Four tubes are used to enhance the irradiation intensity. The material is placed in fluorescent light apparatus for UV exposure of nonmetallic materials to irradiate for3000hours, the sampling cycle is200hours. In the course of the experiment, macroscopic examination was applied to the surface color and mechanical properties of the material. The microstructure was analyzed through the Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscope (SEM) to explain the changes of macroscopic properties in the aging process. The result and analysis of the artificial accelerated ageing are as follows:When the cumulative UV radiation energy reached91.29×103kJ/m2(1200hr), a large number of small cracks appeared on the PVC film surface. The degradation of the PVC coating generates conjugated double bond in the UV-irradiation, which made the yellowness index increase rapidly. But the carbonyl index is descending, the main reason is that the plasticizer contains carbonyl. In addition, the evaporation rate of plasticizer is much greater than the carbonyl generation rate of the PVC degradation. In the stress-strain curve, the breaking elongation has suddenly increased, but the tensile strength hasn’t been changed much after the cumulative UV radiation energy reached76.03×103kJ/m2(1000hr). Combining with Scanning Electron microscopic observation, we found that the cracks on the surface of the PVC coating had extended to the interface between the PVC coating and the polyester base fabric. This shows that with the deepening of ageing, the adhesion effect of PVC coating to the polyester fabric is getting smaller. When the cumulative UV radiation energy between91.29×103kJ/m2and167.61×103kJ/m2(1200-2200hr), the yellowness index continues to rise, but the speed slows down. The carbonyl index begins to rise rapidly, mainly because most of the plasticizer has volatilized. At the same time, the PVC coating continues to generate the conjugate double bond and carbonyl. The breaking elongation and the tensile strength remain unchanged, when the cumulative UV radiation energy reached152.64×103kJ/m2(2000hr). The main reason is that most of the UV-irradiation energy was absorbed by the PVC coating, the polyester fabric hadn’t been effected.The yellowness index is no longer rising and stabilizing, after the cumulative UV radiation energy reached167.61×103kJ/m2. We know that the polyester does not contain conjugated double bond, the photo-oxidation also not produce the double bond. This shows that the PVC coating has aged completely. For this reason the carbonyl index will not rise, but the curve is declining. This is because residual plasticizers continue to volatilize. The PVC has lost the ability of Ultraviolet absorption, Ultraviolet light works on the polyester directly. This made the polyester be brittle and rigid, and then the breaking elongation and strength began to descend.The photo-oxidation of the PVC-coated membrane material has been understood sufficiently under different cumulative UV radiation energy by macroscopic and microstructure examination of the artificial accelerated ageing. Due to the actual working environment of the product is under the natural, we need to conduct the natural ageing test.We found that the trend of photo-oxidation of the PVC-coated membrane material is consistent, by the artificial accelerated ageing compared with the natural ageing. The Schwarzschild’s law is valid when introducing a constant p on the basis of the reciprocity law. It was found that, when p=3.67, the correlation between the ageing time and the UV intensities is good. The equations of whiteness degree, ageing time and the average radiation intensity, indicating the intensity of UV radiation, are developed. y75.37-0.45x Where, x=I3.67t, I is the average radiation intensity and t is the ageing time in hours.