Impact break behaviour of PMMA sheet: When damaged surfaces tell a story

Libby Ireland
Tate Britain, London, England

Polymethyl methacrylate (PMMA) sheet, commonly known as Perspex or Plexiglas, has been used by artists and designers since it was first produced in the 1930s. Present in many art and design collections, this material poses various challenges to the conservator – particularly due to its susceptibility to physical damage. The full extent of cracks and breaks are readily visible at the surface of many transparent or translucent PMMA sheets – this paper discussed how these can be used to gain information about how how the sheet was manufactured.
It has been widely documented that the method by which PMMA is produced can affect the material’s properties, and that the process can differ between manufacturers. The two main methods of PMMA sheet production, casting and extrusion, are commonly known within the industry to have very different properties, lending themselves to different applications. Break testing a variety of opal and transparent 5mm sheet as part of wider research into treatment options for PMMA, gave interesting insights into how the break behaviour of the material under impact can be indicative of how it was made.
Impact break testing was undertaken on sets of new cast opal and transparent sheet, new extruded transparent sheet, and two sets of naturally aged transparent sheet (manufacturing process unknown). The results revealed convincingly consistent behaviours within the different PMMA types, and clear differences between the sets. New cast material was the most impact resistant due to its high molecular weight and 3D network, whilst the extruded material showed a clear weakness in one direction due to the linear alignment of the the polymer chains. Most interestingly, the opal material showed the highest impact resistance despite manufacturers listing it as having the same impact resistance as the cast transparent material. It is unclear why this was so but may be due to the addition of pigment to the material. Break testing the unknown aged transparent material showed that both sets had lower impact resistance but with break patterns similar to those seen in the new cast transparent samples. This may be a reflection of materials having been produced in a less controlled manner than the new samples or be indicative of the material’s degradation by depolymerisation causing a reduction in molecular weight.