| During the service life of epoxy coatings, due to the influence of environmental factors, it is easy to be aged. Under outdoor conditions, photoaging is one of the main factors leading to the failure of the epoxy coating. Currently, the studies of photoaging of epoxy coating are mainly focused on the evolution of chemical structures under ultraviolet (UV) irradiation conditions, such as formation of amide groups, scission of carbon-nitrogen and other chemical bonds, generation of carbonyl groups including carboxylic acids, aldehydes, etc. While changes in the chemical structures of the coating are happening, the microstructure of the coating, such as free volume, defects always evolutes simultaneously. Study on the process of microstructure evolution of epoxy coating under UV irradiation conditions, is of great importance for understanding the mechanism of photoaging mechanism.On the basis of the study on variation of chemical structures under UV irradiation conditions, this paper investigated water resistance of the coating during the early immersion stage in detail. By analyzing the changes of water resistance of the coating in different irradiation stages, combined with the variation characteristics of the S-parameters, the process of microstructure evolution of epoxy coating under UV irradiation was reasonably speculated. Based on above study, the effects of amount of curing agent on variation of chemical structures of epoxy coating and the evolution of microstructure under UV condition were investigated. Moreover, taking into account additions of ultraviolet absorbers and phenolic epoxy may be effective ways to improve UV aging resistance properties of epoxy coatings, we studied the UV aging process of these composite coatings in order to obtain informations of microstructure evolution, and provide a theoretical basis for completing photoaging mechanism of epoxy coating in some extent. The main contents of this paper are summarized as follows:1) Apolyamide-cured bisphenol A epoxy coating system was chosen, to study the variation of its chemical structures and the process of microstructure evolution under UV-A irradiation condition. At the early irradiation stage (<88hours), S parameter and diffusion coefficient of water in epoxy coating reduce, which may be due to the generation of more compact structure. Meanwhile, characteristic absorption peak of epoxy end groups in the FTIR spectrum also evidently decreases, demonstrating that short-term UV-A irradiation may lead to post-curing process of the epoxy coating. After208hours of UV irradiation, a new time constant appears in the high frequency region of the impedance spectra. At this time, the diffusion coefficient of water also increases. This may be related with the generation of surface layer with micro-defects. As UV irradiation proceeds, overlap of two time constants appears in medium-high frequency (18Hz,3.7×103Hz) region of the impedance spectra after a relatively short time (1.33hours) of immersion, indicating that the micro-defect layer grows towards the coating bulk and forms more characteristic layers with microporous structure. After relatively long time of UV irradiation (399,543hours), slow positron beam results reveal that an aged layer with thickness of about0.20μm forms near the coating surface. The formation of the characteristic layer is associated with the existence of a large amount of carbonyl groups, in addition, radicals generated during the aging process participate in the cross-linking reactions and form compact structure near the coating surface, which also facilitates the generation of the aged layer. At the early immersion stage, EIS results basically confirm the existence of the compact structure. However, the structure only shows relatively excellent water resistance properties at the early immersion stage.2) The effects of the amount of curing agent on variation of epoxy coating chemical structures and the evolution of the microstructure under UV-A irradiation condition were studied. By investigating the variation of the relative peak area of carbonyl groups with irradiation time, we qualitatively described the effects of the amount of curing agent on evolution of the chemical structures. It is found that the formation rate of the carbonyl group is relatively low when curing agent amount is close to the theoretical value. At the relatively late stage of UV irradiation (543hours), slow positron beam results reveal that aged layer generates in all sample surface region. The thickness of the aged layer is affected by curing agent amount as well. By studying the water resistance of all samples at relatively late stage of UV irradiation, the effects of the content of curing agent on evolution of microstructure of epoxy coating were speculated. It is found that the impedance spectra reveal electrochemical characteristics associated with the existence of micropores when curing agent content is relatively low; the spectra show features related with the dense layer when curing agent content is close to the theoretical value; and the water resistance properties decrease drastically, namely, the protective properties basically lose when the curing agent content is relatively high. The calculation results of coating porosity and water uptake of coatings at saturation state during immersion are coincident with coating features those showed in impedance spectra.3) The effects of the addition of ultraviolet absorber on the variation of chemical structures of epoxy coating and the evolution of microstructure under UV-B irradiation condition were studied. The results show that the addition of ultraviolet absorber UV-531reduces the formation rate of carbonyl group under UV-B radiation condition. After a relatively long time of UV irradiation, aged layer generates in all samples. And the addition of ultraviolet absorber effectively reduces the region of aged layer. EIS results show that, the water resistance property of the coating decreases after the addition of ultraviolet absorber, which may be due to ultraviolet absorber affects the cross-linking density of the cured epoxy structure. After a relatively short time of UV irradiation (≤360hours), the coating exhibits features related with post-curing, due to the ultraviolet absorber can convert ultraviolet energy into thermal energy. With increases of the irradiation time, the formation of microscopy defects becomes the dominant process of microstructure evolution. In the early stage of irradiation, pure epoxy coating does not show features associated with post-curing. At the relatively late irradiation stage (≥528hours), EIS spectra of all samples exhibit impedance characteristics associated with dense structure. The effects of the content of ultraviolet absorber on the water resistance properties at late stage of UV irradiation were investigated. It is found that the addition of appropriate amount of ultraviolet absorber can effectively improve the water resistance properties at late stage of UV irradiation.4) The effects of the addition of novolac epoxy resin (EPN) on the variation of chemical structures of epoxy coating and the evolution of microstructure under UV-B irradiation condition were investigated. The results show that the addition of the novolac epoxy resin effectively improves the structural stability of the coating at the relatively late irradiation stage (≥528hours):the generation rate of carbonyl groups is relatively low, the aged layer is thinner as well. EIS results reveal that the epoxy/EPN composite coating has relatively excellent water resistance properties before UV irradiation, demonstrating that the addition of novolac epoxy resin effectively enhances the cross-linking density of the cured epoxy structure. During UV-B irradiation, the microstructure evolution of epoxy/EPN composite coating shows a very similar tendency with the pure epoxy coating. Since UV-B can break chemical bonds with high bond-energy directly, the composite coating does not exhibit more excellent water resistance properties. Taking into account that the ultraviolet absorber can convert ultraviolet energy into thermal energy, we investigated the effects of the content of the ultraviolet absorber on the water resistance of composite coatings at relatively late irradiation stage. The results show that the water resistance properties of composite coatings are greatly enhanced, after the addition of ultraviolet absorber. The best water resistance properties appears when the content of ultraviolet absorber is1.5wt%, after20days of immersion, the coating resistance remains at2.37×108Ω cm2, indicating that there is synergy between novolac epoxy resin and ultraviolet absorber under UV-B irradiation condition. |
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