Uracil Pinking in Polyvinylchloride

ABSTRACT Harsh environmental conditions, such as extreme temperatures, high humidity, and sunlight, are the leading causes of PVC items' degradation during their service life. In particular, light can trigger not only severe weathering of outdoor PVC articles, leading to degradation of the PVC matrix, but also milder phenomena that primarily affect color without compromising the material's mechanical properties. Those phenomena are not of secondary importance because they often require the substitution of damaged items, which impacts the costs and durability (and therefore sustainability) of the article. One of these issues is uracil pinking that occurs when the thermal stabilizer used in the finished item contains 1,3‐dimethyl‐4‐amino uracil. This substance is commonly used as a primary stabilizer; moreover, in items such as PVC pipes, fittings, and roofing membranes. Specifically, PVC pipes containing 1,3‐dimethyl‐4‐aminouracil can undergo severe discoloration, compromising their use. Understanding uracil pinking in terms of its triggering and chemistry, it is essential to prevent or mitigate it. Therefore, in this paper, the aging of several rigid PVC formulations containing 1,3‐dimethyl‐4‐amino uracil was performed. The aging was carried out using natural outdoor and accelerated weathering, both direct and modulated by filters. Specifically, cut‐off filters were used to understand which wavelengths are capable of triggering the uracil pinking through colorimetric measurements according to the CIE colorimetric system of ASTM E308‐22. In the current literature, uracil pinking has always been associated with the presence of titanium dioxide. Our findings show that the phenomenon occurs even in the absence of titanium dioxide. The investigation on the chemical mechanisms responsible for the uracil pinking is outside the scope of our work and still to be identified, but for sure they do not involve only titanium dioxide.