Photopinking of white PVC products: A kind of magic

Abstract

Short-term photoeffects like photograying, photobluing, and photopinking are well-known in the polyvinyl chloride (PVC) industry. While photobluing and photograying are well understood regarding their mechanisms of occurrences there are still some missing facts. Furthermore, there are three different types of pinking effects with three different mechanisms and consequently three different ways to avoid this discoloration of mainly white PVC products. The recent paper is focusing on one hand of the photopinking of PVC products, which are stabilized with nitrogencontaining, organic stabilizer whose mechanism was not explained until today. In the past there was neither a method to analyze nor to simulate this type of photopinking. However, the discovery of a charge transfer complex between the nitrogen-containing substance and nitric acid gave the basic idea to investigate this phenomenon by photo-electromotive force investigations of titanium dioxide and the nitrogen-containing substance and by cyclo voltammetry. On the other hand, the authors are attempting to supplement the findings regarding the mechanism of the photopinking of white window profiles which are lead stabilized and which were installed north-facing in cooler areas with high humidity. Again, a simulation and the analysis of the color-giving substance of the north-faced photopinking were impossible in the past. The authors assumed that antioxidants might play an important rule for this type of discoloration. They supported their hypothesis by synthesis of quinoid structures based on antioxidants, spectroscopic investigations, artificial weathering, and electrochemical calculations.FIG. 12. VIS spectra of a "north-faced pinked" window profile in comparison with a quinoid structure based on CAS: 2082-79-3 and a PVC specimen containing this quinoid structure [23].