Assessment of the photo-stability of plastics found in heritage collections to visible light using FTIR spectroscopy and colorimetry

Anna Pokorska / Lindsay MacDonald / Elise Talgorn / Boris Pretzel / Stuart Robson / Katherine Curran

Plastics have a relatively short history within heritage and research into their conservation is still limited compared to traditional materials such as wood or metal. Despite a wealth of academic and industrial literature on the subject of polymer photostability, the majority of research is focused on the effect of UV radiation, and therefore sensitivity of plastics to visible light remains largely unexplored.
In fact, institutional guidelines for display of those artefacts can be rather vague and tend to treat different plastic materials as one. However, as more plastic objects are entering museums and galleries there is a compelling need for more research that addresses their conservation requirements. In this experiment, photo-stability of plastic materials commonly found in museums and galleries to visible light exposure (400-700nm) was assessed through accelerated photodegradation over 8 weeks at two different light intensity levels: (i) between 33-38 klux (116-133 W/m2) and (ii) at 8.3 klux (31 W/m2). Chemical stability was investigated with ATR-FTIR, while visual appearance was measured using reflectance spectrophotometry.
In general, most samples were found to be resilient to the visible light exposure used in this experiment, with the exception of some polyurethane foams, ABS, cellulose nitrate and polycarbonate samples, which showed discolouration and chemical changes consistent with literature. LED and fluorescent light sources were also compared for their damaging potential, with the latter causing similar degradation rate despite a much lower overall light dose.
However, the effect of different temperature fluctuations as well as spectral power distribution in both of those setups would have to be further investigated before firm conclusions can be drawn. This study highlights that the light sensitivity of plastics is complex and needs more in-depth study in order to improve museum guidelines. In particular, the effect of colourants on the stability of plastics when exposed to visible light will be explored in future research.