Lecture. The Price of Pristine PMMA.
Although all surveys of the condition of plastics conducted in museums in the United Kingdom and Scandinavia since the 1990s conclude that approximately 75% of collections require cleaning, few treatments have been developed. This is mainly due to the high risk of damaging plastics mechanically or chemically with an invasive cleaning treatment. Plastics with a glass transition temperature close to ambient, including polyethylene and plasticized polyvinyl chloride, are readily abraded by contact with brushes, cloths and sponges. Solvents and detergents in commercial cleaning products can extract additives from plastics or induce environmental stress cracking in rigid plastics such as polystyrene, polycarbonate and polymethyl methacrylate. Despite the high risks, removal of oily fingerprints, carbonaceous dirt or crystalline degradation products is necessary because they reduce the significance and commercial value of artworks and objects.
Polymethyl methacrylate (PMMA), known widely as Plexiglas or Perspex has been used commercially to manufacture designer furniture, aircraft windscreens, mathematical instruments, contact lenses and dental appliances as well as artworks. As a result, PMMA is found in industrial, modern history, technical museum and art collections. Although literature addressing conservation of PMMA is sparse, cleaning agents and procedures have been developed as aftercare for industrial products constructed from the plastic. These were studied.
The physical and chemical changes induced by applying mechanical, aqueous and solvent-based cleaning techniques to cast and extruded model PMMA were examined in order to examine any degradation introduced. Mechanical cleaning techniques included vacuum cleaning, microfibre cloths, synthetic and natural hair brushes, washing leather, various woven cloths, cotton wool and lens tissue. Aqueous cleaning involved application of water, detergents and commercial solutions, pastes and sprays. A range of single solvents were also applied to investigate any changes induced at PMMA surfaces.
In order to clean effectively, liquid agents need to make close contact or wet plastics surfaces. Determining the contact angle between the leading edge of a droplet of cleaning agent and PMMA was measured and demineralised water was found to make the poorest contact while ethanol wetted readily. Physical degradation induced by cleaning was examined using optical microscopy, change in gloss and colour measurements while chemical changes at surfaces including the deposition of residues were examined using Attenuated Total Reflection- FTIR spectroscopy. The change in stresses introduced by cleaning was examined on macro and micro scales using photoelasticity also known as polarized stress analysis.
Initial results suggest that microfibre cloths readily scratch new PMMA while lens tissue produces no measurable degradation. Some commercial cleaning sprays deposit a visible residue while solvents, particularly acetone, introduce stress cracking. This project is at an early stage. Future research will investigate the use of new cleaning agents such as CO2 snow and supercritical liquid and the effectiveness of non-damaging cleaning treatments to remove oily and carbonaceous soils.
The third section of Future Talks 009 will be chaired by Yvonne Shashoua.
Curriculum Vitae. Yvonne Shashoua has been a Senior Researcher at the National Museum of Denmark investigating the degradation and conservation of plastics since 2001. After graduating in industrial chemistry she worked as a paint technologist for Berger Paints in England. She joined the British Museum as a conservation scientist in 1988, specialising in the deterioration reactions and conservation of cellulose nitrate, cellulose acetate and rubber. She has 70 publications including a monograph ´Conservation of Plastics-materials science, degradation and conservation´ published by Elsevier in 2008. Yvonne Shashoua was coordinator of ICOM-CC´s working group Modern Materials and Contemporary Art until 2008.