Modelling plasticiser loss inside closed environments

Abstract

Plasticiser loss, a fundamental mechanism in the degradation of polymeric materials, leads to material instability and contamination of the environment. The process depends on environmental conditions, but the size of the container in which an object is housed and the thickness of the material also play key roles in the rate of plasticiser loss and the time at which equilibrium is reached. Understanding these dependencies provides valuable insight into the degradation of plastic museum artefacts inside enclosures as typified by museum storage and, more broadly, the deterioration of polymeric materials in closed environments. Migration of low molecular weight plasticisers, like diethyl phthalate or dimethyl phthalate, from plastics has been widely studied in accelerated ageing experiments at elevated temperatures and different airflows. Here, to investigate these effects, we modelled plasticiser loss in a stagnant environment inside an enclosure at room temperature. Our model is one-dimensional and describes loss through a two-phase transient diffusion process, between the solid plastic and the air. The comparison of numerical simulations to FTIR and 1H NMR spectroscopic data from cellulose acetate samples plasticised with diethyl phthalate aged at T = 70 °C and 50% relative humidity indicates that the model is appropriate for thin enclosed plastics. We applied the model over a range of diffusion coefficients [10−21–10−14 m2 s−1] and partition coefficients [103–107] to represent different polymeric materials. Under the investigated scenarios, thin plastics tolerate a maximum total plasticiser loss of 10% and the timescales vary between 1 and 109 years. The model provides insight into the relationship between plasticiser loss, enclosure dimensions and material thickness for different plastics and can suggest how to improve packaging dimensions of thin plastic products to minimise loss, as well as future conservation strategies and guidelines for plastic museum artefacts.