紫外光对涂层的老化作用

UV PHOTO-DEGRADATION OF COATINGS

用人工加速的方法研究了紫外光对涂层的影响 .用石英晶体微天平 (QCM )测试发现老化后涂层对H2 O和SO2 的吸附率明显增大 .电化学阻抗 (EIS)测定发现孔隙率随光照射时间增加而增大 ,涂层电阻随光照时间增加而减小 .傅利叶变换红外光谱 (FTIR)分析表明 ,光照使涂层分子碳链断裂 ,醇酸涂层发生在芳香酯的C -O键 ,聚氨酯涂层发生在芳香酯的C -O键和氨酯键C -N ,同时有一些亲水基团生成 .用扫描电子显微镜 (SEM )观察涂层表面产生了许多孔穴 .涂层吸水率增加是由亲水基团增加和孔隙增加共同引起的 ,但孔隙增加的贡献更大 .而涂层对SO2 吸附率的增加是由双键α -H过氧化物的增加和孔隙的增加共同引起的 ,α -H过氧化物的增加的贡献更大 .

Photo-oxidation initiated by ultraviolet ray (UV) is a pivotal factor which affects degradation of the coatings.Quartz crystal microbalance (QCM) testing results show that the absorbance of water and SO 2 in the coatings increases remarkably after exposure to UV.Changes in porosity after exposure were determined by means of electrochemical impedance spectroscopy (EIS),and the results show that the porosity increases with the increase of exposure time.Fourier transform infrared spectroscopy (FTIR) analysis shows that breakdown occurs in the C-chain of the coatings after exposure to UV,which occurs at C-O bond of aromatic ester for the alkyd resin and at C-O bond of aromatic ester and C-N bond of urethane for the polyurethane resin.Some hydrophilic groups may be formed simultaneously.Scanning electron microscope (SEM) was employed to analyze the morphology of the coatings and the results show that many pores and holes were formed after exposure to UV.It was suggested that the increase of water is the cooperation of the increases both of the amount of hydrophilic groups and of the porosity of the coatings,but the contribution of the increase of porosity may be greater.The increase of absorbance of SO 2 is the cooperation of the increase of α -H peroxide and the increase of porosity,but the contribution of the increase of α-H peroxide may be higher.