Monitoring with reflectance spectroscopy the colour change of PVC plastisol coated strip steel due to weathering

Abstract

Weathering can cause colour changes in pigmented PVC plastisol coatings. The rate of colour change is dependent on a number of factors, such as PVC resin, pigment type and concentration, and local climate. In this paper, an investigation into the mechanisms that cause these colour changes is reported. The investigation involves a set of 350 paints, based on a typical plastisol formulation, that has been exposed to natural weathering at sites in Lancashire and Sussex in the UK, and to accelerated weathering using QUV testers with UVA-340 lamps and condensation cycles. Colour changes were measured using a Gretag SPM50 reflectance spectrophotometer. It was found that the reflectance spectra measured in this way provided a more potent tool for investigating the chemical reactions that cause discolouration in coatings during weathering than the CIE L*a*b*parameters that are normally used to describe colour changes.

Reflectance spectroscopy has been shown to facilitate the correlation between natural weathering and accelerated weathering, by giving a better understanding of the prevalent chemical processes that take place in the coatings during different accelerated weathering programmes (with and without condensation) and with natural weathering at different locations (high UV climates and higher rainfall climates). Phenomena such as PVC dehydrochlorination and organic pigment degradation could be analysed conveniently using reflectance spectrophotometry.