I have a fascination with plastics, I guess it’s partly because the array of materials that can be chemically engineered seems to have infinite possibilities. Objects of many textures, shapes, colors and applications exist because of plastics. Unfortunately, their existence creates challenges to both preservation and sustainability. As works of art or material culture, conservators want to make them last for as long as possible, but the most long-lived plastics also pose the problem of disposal. The types of plastics that are most likely to break down in the environment are also crumbling to bits on the shelves of collectors and institutions.
This year’s AIC meeting featured a workshop presented by Yvonne Shashoua and Thea van Oosten, two well-known experts in the field of plastics in museums. Shashoua’s book, Conservation of Plastics: Materials Science, Degradation and Preservation, is a good reference. Both Shashoua and van Oosten were part of the 2008 European POPART initiative, (Preservation of Plastic Artifacts in museum collections), which selected a few types of plastics used in artwork, studied their deterioration pathways, and possible methods for their preservation, cleaning and repair. http://www.bartlett.ucl.ac.uk/graduate/csh/research/projects/popart
To begin the workshop, we were presented with a historic overview of many types of plastic materials encountered in collections. From gutta percha to polyester we learned of the properties and uses of different polymers. Van Oosten had an entertaining way of categorizing plastic types by their properties into three snack food groups; gummy worm, chocolate bar, or cookie. Gummy worm plastics are in the elastomer category, which includes both natural and polyurethane rubber. These materials are stretchy and flexible at room temperature. Chocolate plastics are the thermoplastic category, which includes polyethylene. These materials polymerize through addition and can be melted and reformed into new shapes. Cookie plastics are in the thermosetting category, which includes Bakelite (phenol formaldehyde), melamine formaldehyde and Vulcanite. These plastics are formed by condensation reaction with water being lost, and they cannot be reformed into new shapes with heat.
We learned that it is important to know what type of plastic you have before you attempt any repairs, because an adhesive that might work with one polymer will dissolve another. To help determine the appropriate adhesive, one should consult the Hansen or Hildebrand solubility parameter for the given plastic. The strength of bond needed, the viscosity of the adhesive and the elasticity of the plastic are other factors to consider. For lightweight polyurethane foam, water based adhesives commonly used in conservation are often adequate. Clear plastics, like polystyrene or polyester may require consideration of the refractive index of the adhesive in order to make an invisible joint.
In the afternoon we split into two groups. We had time to experiment with adhering and mending a variety of plastics, and test cleaning cloths, pads and swabs for cleaning plastics. According to results obtained from the POPART study, it is important to clean plastics as soon as possible when they become soiled, since particles may migrate into the plastics and become impossible to remove in a few short weeks. At the same time a soft cleaning cloth must be used that won’t cause abrasion to the plastic being cleaned. My experiences in this workshop highlighted the importance of testing on mock-ups!
The four plastics at greatest risk of deterioration are cellulose nitrate, cellulose acetate, plasticized (flexible) polyvinyl chloride, rubbers, and foams. Cold storage is typically recommended for these materials. The leaders of this workshop also recommended use of an oxygen scavenger in encapsulated packaging for preservation of rubber. Rubber in collections is rapidly deteriorating by oxidation, which causes it to turn yellow and brittle.
Along with POPART a number of research projects have brought the needs of plastics collections into the spotlight in recent years; however, it is clear that more research on active conservation methods is necessary. There is so much more to learn about fascinating plastics!