Heritage PVC objects: Understanding the diffusion-evaporation of plasticizers

Abstract

The predominance of diffusion-evaporation of plasticizers in heritage collections is investigated and an analytical method of studying surface exudates based on dry swabbing is introduced for an in-situ museum survey. Most plasticized heritage PVC objects exhibit no visible surface exudates, indicating plasticizer loss is governed by the diffusion-evaporation process and not phase-separation. A new approach to analyzing the diffusion-evaporation process based on experimental data is presented to model plasticizer loss at room conditions. Experiments conducted with historical PVC objects exposed to accelerated degradation for up to 8 weeks at moderate temperatures enable the monitoring of plasticizer loss. Evaluation of the mass loss and the chromatographically determined plasticizer content allows us to determine the temperature dependence of the diffusion coefficient and the surface emission coefficient. Three representative groups of heritage PVC are identified and the activation energies for the coupled diffusion-evaporation processes are used to determine the rate-limiting step and introduce a general model for predicting plasticizer loss from the time of storage, temperature, and thickness of an object. The model can be used as part of risk assessment in heritage PVC collections.