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.

Effects of washing agents on the mechanical and biocompatibility properties of water-washable 3D printing crown and bridge resin

“…Conversely, the present study found that alcohol-free agents could better preserve the original mechanical properties or even enhance the Vickers hardness. A previous study showed that even when the polymer was immersed in water for a long period of up for 30 days, although water continued to be absorbed throughout the immersion period, neither relaxation behavior nor plastic deformation could be detected 26 . Although both alcohol and water are known to be absorbed by polymer networks, alcohols would weaken the mechanical properties of the contacted polymer and act more aggressively 31 , potentially even causing visible cracks on the polymer surface 26 .…”

“…A previous study showed that even when the polymer was immersed in water for a long period of up for 30 days, although water continued to be absorbed throughout the immersion period, neither relaxation behavior nor plastic deformation could be detected 26 . Although both alcohol and water are known to be absorbed by polymer networks, alcohols would weaken the mechanical properties of the contacted polymer and act more aggressively 31 , potentially even causing visible cracks on the polymer surface 26 . This may explain why the mechanical properties of the samples washed with water were more stable and superior than those washed in alcohol.…”

“…This lowers the glass transition temperature, leading to phenomena such as plasticization and crazing of the PMMA polymer 28 . A previous study showed that contact with free alcohol causes softening of PMMA and mechanical deformations such as the swelling of sample edges 26 . The mechanical properties would be weakened by the solvent molecules, reducing polymer entanglements and destroying interchain bonds, which cause degradation of the matrix–filler interface 29 , 30 .…”

“…Previous studies have shown that increasing the postwashing time with commonly used organic-based solvents improves the cytocompatibility of printed objects 16 , 24 , but this also loosens the polymer network structure and hence degrades the mechanical properties 25 . In contrast, polymethyl methacrylate (PMMA) was found to not exhibit relaxation behavior and plastic deformation after long-term immersion in water 26 . Therefore, extended water washing of the printed object might preserve its original mechanical properties and ensure biocompatibility.…”

Influence of Post-Processing on the Degree of Conversion and Mechanical Properties of 3D-Printed Polyurethane Aligners

“…The solvent choice, its interaction with polyurethane, and its effectiveness in removing uncured resin and impurities are crucial factors, as stated by Gomez et al [ 32 ]. Ethanol, a less aggressive solvent compared to isopropanol, has shown promise in preserving the material’s integrity during the post-processing of PMMA materials [ 33 ]. When combined with an inert nitrogen polymerization environment, it contributes to higher conversion rates, potentially enhancing the aligner’s performance [ 34 ].…”