polymers Article Discoloration of Historical Plastic Objects: New Insight into the Degradation of b-Naphthol Pigment Lakes Anna Micheluz 1 , Eva Mariasole Angelin 2,* , João Almeida Lopes 3, Maria João Melo 2 and Marisa Pamplona 1,* 1 Conservation Science Department, Deutsches Museum, Museumsinsel 1, 80538 Munich, Germany;
[email protected] 2 Department of Conservation and Restoration and LAQV-REQUIMTE, NOVA School of Science and Technology, NOVA University Lisbon, 2829-516 Lisbon, Portugal;
[email protected] 3 iMed.ULisboa-Research Institute for Medicines, Faculty of Pharmacy, University of Lisbon, Av. Prof. Gama Pinto, 1649-003 Lisbon, Portugal;
[email protected] * Correspondence:
[email protected] (E.M.A.);
[email protected] (M.P.) Abstract: Light is a determining factor in the discoloration of plastics, and photodegradation pro- cesses can affect the molecular structures of both the polymer and colorants. Limited studies focused on the discoloration of heritage plastics in conservation science. This work investigated the discol- oration of red historical polyethylene (PE) objects colored with PR 48:2 and PR 53:1. High-density and low-density PE reference polymers, neat pigment powders, and historical samples were as- sessed before and after accelerated photoaging. The applied methodology provided insight into the individual light-susceptibility of polyethylenes, organic pigment lakes, and their combined effect in the photoaging of historical plastic formulations. After light exposure, both PE references and Citation: Micheluz, A.; Angelin, historical samples yellowed, PR53:1 faded, and PR 48:2 darkened; however, both organic pigments E.M.; Lopes, J.A.; Melo, M.J.; faded severely in the historical samples.