Effects of selected solvents on PMMA after prolonged exposure: unilateral NMR and ATR-FTIR investigations

Abstract

Exposure of heritage plastics to solvents can cause several changes to their surface layers and bulk. The amorphous nature of certain plastics, particularly polymethyl methacrylate (PMMA), allows penetration and diffusion of low-molecular liquids, which can lead to swelling, cracking, plasticisation or stiffening, extraction and dissolution of polymer and additive components. Such phenomena compromise the visual characteristics, chemical and physical structure of heritage collections and remain, for the most part, unexplored. Research studies present contradicting evidence concerning the use of solvents and their safe application on PMMA. The main discrepancy is about PMMA being permeable to water or not. Alcohols have also triggered debates about whether they cause leaching and/or depolymerisation. This paper investigates the potential effects that selected free polar and non-polar solvents—employed in different media as cleaning systems—can have on PMMA after prolonged contact. The Hildebrand solubility theory was used to select solvents based on PMMA’s miscibility. The effects of deionised water, ethanol, isopropanol and petroleum ether are examined through a 30-day immersion study of PMMA via microscopic examination, weight change measurements, ATR-FTIR, peak height ratios, and NMR MOUSE. All four solvents extracted soluble components, i.e. unreacted residual monomer, and increased PMMA’s surface sensitivity to abrasions. Water and petroleum ether did not cause any measurable chemical or physical changes. Alcohols caused surface crystallisation and crazing, with isopropanol leading to stiffening and ethanol to plasticisation of the material. These effects, although extreme and absent in most routine cleaning timeframes, demonstrate the cumulative damage these solvents can potentially induce to PMMA.