Intrinsic Differences on the Photodegradation Mechanisms between Pigmented and Non-Pigmented Coatings Determined by Multi-Scale Analysis

Abstract

Multi-scale analysis of photodegradation are conducted for pigmented coating containing acrylic urethane + TiO2 pigment and for non-pigmented coating containing only acrylic urethane. We discuss the intrinsic differences in the photodegradation mechanism between the pigmented and nonpigmented coatings and the effect of the interface between the pigment and the binder. Photo-aging tests are conducted using artificial ultraviolet (UV) irradiation under conditions of 60 °C and dry atmosphere. The results of Fourier transform infrared spectroscopy (FTIR), solvent swelling experiments, ultrasonic measurements of elastic moduli, and colourimetry used for material characterisation before and after photo-aging. Although the behaviour of E and the carbonyl index (CI) show common trends for both samples, the overall trends of yellowness index (YI) and swelling degree (Q) differ significantly between the pigmented and non-pigmented samples. The results reveal that changes in macroscopic properties may not necessarily correspond with the CI behaviour and that characteristic interfacial effects exist between the pigment and the binder. The onsets of coating erosion and chalking are observed in the pigmented coating as surface topological changes. The different behaviour of YI and Q between the sample types can be attributed to the interfacial effect at pigment/binder interface based on the photocatalytic effect from TiO2 pigment

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