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Conservation of Plastics
Materials Science, Degradation and Preservation Review by Brenda Keneghan
The degradation of plastic objects in collections has been a growing problem for museums and galleries over the last 20 years; their sudden and spectacular disintegration confounding conservators more familiar with dealing with the gradual decline of traditional materials. As plastic has become ever more ubiquitous this problem is set to escalate. The 1991 CCI conference Saving the Twentieth Century started the ball rolling on international discussions and a great variety of case histories have been presented at conferences over the intervening years. On the publications front Anita Quye’s & Colin Williamson’s Plastics: Collecting and Conserving (1999), immediately springs to mind as a turning point on the subject. But this excellent little book is almost 10 years old and is targeted towards a general audience: the amateur collector as well as the museum professional. There has been a distinct gap in the literature on this subject ever since. Now Yvonne Shashoua’s Conservation of Plastics, has come along and filled that gap in a most clear and comprehensive manner. The book is a welcome addition to the Butterworth-Heinemann serie of conservation and museology publications, and continues the same high standards. Yvonne Shashoua is eminently qualified to compile such a book, having specialised in modern materials conservation for several years in the British Museum before moving to the National Museum of Denmark to undertake PhD research work into the degradation of PVC. Her (2001) PhD thesis, 'Inhibiting the deterioration of plasticized poly(vinyl chloride) – a museum perspective' has become a cornerstone in conservation science literature, applying rigorous science to a practical heritage problem. A major problem with the conservation of plastics is that a certain knowledge of chemistry is required before attempting either an interventive or inhibitive (Yvonne’s preferred term) treatment. This book is intended to appeal to a wide readership, including practical conservation students and practitioners as well as conservation scientists. This is a tough target to meet without frustrating or disappointing someone, but the structure of the book facilitates the gradual introduction of technological and chemical principles in a neither an over-simplistic nor patronising manner. Each chapter is preceded by a very useful summary of its contents allowing the reader to decide whether they wish to go more fully into this area or not. Almost by way of a summary, Chapter 1 tells us where plastics are found in collections and finishes with a brief word on the current status of plastic conservation. The following three chapters form a complete introduction to plastics, beginning appropriately with their historical development (Chapter 2) and followed by their technological production (Chapter 3) and their properties (Chapter 4). Each of these three chapters is illustrated with attractive colour photographic images and extremely comprehensive tables. In fact, the quality and content of the images and tables throughout the book is exceptionally high. Chapter 3 introduces the chemical structures of monomers and describes the polymerisation processes and resulting polymers in the most painless manner possible. The different types of polymerisation reactions are described. There is also an extremely useful table (Table 3.3) summarising the types, functions and examples of frequently used additives. These tables are a most valuable resource for both the newcomer to the field and those more experienced. The chapter ends with descriptions of the various industrial processes by which the plastics are transformed from granules or liquids into various types of everyday objects. Chapter 4 presents an overview of the chemical, optical and thermal properties of plastics which are most relevant to conservation. Chapters 5, 6 &7 (identification, degradation & conservation) are, perhaps, the chapters that non-scientists will immediately turn to when first confronted with a degrading plastic object. Identification by means of various tests, e.g. hot pin, density, burn tests and solubility are described (along with warnings as to their dangers). Chemical spot tests are also described as giving rapid identification, however never having had any success with these myself, I must disagree with this statement. The chapter concludes with a description of the various types of instrumental analysis useful for polymer identification. The opening sentence of the summary to Chapter 6 states "It is essential to understand the factors causing degradation prior to developing an approach to the conservation of plastics". This is probably the most important sentence in the entire book as experience has shown that the "conservation" of plastics cannot be tackled in the same manner as the conservation of more traditional materials. This is due to differences in chemical composition of individual plastics and/or combinations of different factors being responsible for their degradation. Chapter 6 describes in detail the degradation pathways of the four most problematic plastics in museum collections: cellulose nitrate, cellulose acetate, plasticised PVC and polyurethane foam. Table 6.2 charts clearly and comprehensively the effects of radiation, heat, oxygen and water on the major plastics found in collections. Again this is a most useful resource. Photographic illustrations of the results of physical and chemical degradation bring the message home. Chapter 7 follows on logically from Chapter 6 by describing current practice in the conservation of plastics - inhibitive and interventive. The scientific principles behind the use of various adsorbents and scavengers are explained. Hopefully, this will put an end to repeated recommendations for the inappropriate use of Ageless that have been heard over the previous few years. The pros and cons of low temperature storage are outlined and explained. The cautious approach that has been adopted towards the seemingly simple procedures such as cleaning and adhering plastics is explained and supported by tables of solubility parameters and surface tension values for solvents and plastics. Having said this, the interventive treatments that have been attempted are also described or referenced. Finally, Chapter 8 looks forward to the future of plastics conservation and suggests that a way forward might be to develop an information interface between manufacturers and users as well as increased specialised training. The main text is supplemented by three appendices including a very useful mini-dictionary of degradation terms (with accompanying photographic images). In summary, the book is very clearly and thoughtfully laid out. An enormous amount of painstaking work has gone into sourcing the information from both the scientific and conservation literature and presenting it in such an accessible manner. It is well illustrated with tables and photographic examples. The references at the end of each chapter are comprehensive and up to date. Conservation of Plastics does not teach one how to conserve plastics but it does provide a background that will enable a conservator or conservation student to make informed decisions on the advisability of potential treatments. By providing information previously difficult to obtain in a readily accessible format perhaps more interventive treatments will be attempted. Yvonne states in the introduction that the purpose of the book is "to distil the extensive knowledge produced by the polymer and plastics industries, designers, environmental and conservation professionals into a single publication focussing on the preservation and conservation of plastics". I think she has succeeded beyond expectation in this mammoth task.
Reviewed by Brenda Keneghan, Polymer Scientist Conservation Department - Victoria & Albert Museum London SW7 2RL
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