Degradation of Polymers

Topics Covered

Background

Plastics, Not as Long-Lasting as Once Thought

Recognition of Polymer Degradation

Deterioration of Acrylic Paintings and Pieces of Art

Impact on the Photographic Film Industry

Preserving Plastics Pieces in Museums

Factors Affecting Polymer Degradation

Preserving Plastics

Types of Polymer Degradation

Solutions to Polymer Degradation

Conclusion

Background

While the plastics industry searches for solutions to the problem of plastics waste, there is, surprisingly, a growing band of people trying to save plastics. The classic nightmare that we will all end up beneath a tide of plastics bottles and packaging is evaporating as plastics reveal that they are not the everlasting materials that we thought they were. Although the manufacturers of early ‘plastics’ such as horn buttons, Bois Durci paperweights or celluloid collars would be astonished to see how well many of their goods have survived, many other plastics have begun to show disturbing signs of instability. Every collection of plastics worldwide, from the Science Museum and Tate Gallery to the Comb & Plastics Museum in Oyonnax, has already lost or is losing unique and beautiful pieces through degradation.

AZoM - Metals, Ceramics, Polymer and Composites : Degradation of a celluloid Polymer

Figure 1. Degradation is clearly apparent from this 1930’s shoehorn made of celluloid.

Plastics, Not as Long-Lasting as Once Thought

The crucial fact is that plastics are organic and have been described as a time bomb ticking away since cellulose nitrate based plastics were invented around 130 years ago. It can of course be argued that manufacturers’ foremost intentions have never been to make beautiful objects for museums. However, museums have a duty to preserve their acquisitions. Two conferences on this subject have already been held in Britain during the first quarter of 1995. The tone of the second conference, organised by The National Museums of Scotland in Edinburgh in April, was fatalistic but determined.

Recognition of Polymer Degradation

It was not until the late 1980s that attention was paid to the fact that plastics artefacts had been physically changing, showing signs of acid vapour, tackiness, warping, embrittlement and crazing. Cellulose nitrate and cellulose acetate were particularly affected. By 1991 John Morgan of the Plastics Historical Society had collected enough data to write Conservation of Plastics -An Introduction, a joint PHS /Conservation Unit publication, and the Conservation Unit launched a survey to identify objects at risk with the aim of setting up a research programme. The survey included everything from radios and cables to textiles and sculptures.

Deterioration of Acrylic Paintings and Pieces of Art

By 1992 acrylic based paintings worth millions of pounds by leading artists of the 1960s including David Hockney and Jackson Pollock had begun to suffer discolouration, cracking and greyness due to the absorption of dust and atmospheric pollutants. These paints seemed particularly vulnerable. At room temperature they are relatively soft and attract dirt which becomes embedded. However, to date no method has been found of cleaning them.

Impact on the Photographic Film Industry

The photographic film industry was also badly hit when irreplaceable archive nitrate stock started to decompose. Today, the National Film Archive transfers cellulose nitrate and cellulose triacetate onto more stable polyester at the rate of a million metres a year.

Preserving Plastics Pieces in Museums

The PHS/CU survey unearthed some interesting facts. For example, 40% of museums surveyed contain plastics objects manufactured and collected since 1980, and modem plastics are also showing symptoms of decay. Polyurethane foam appears to be one of the worst victims, and many early video and audio tapes on magnetic media are already unplayable. The curator who has to supervise a collection of high-tech, mixed material products such as space suits is confronted with a conservation dilemma: which material deserves priority treatment when each separate plastic has different requirements?

Factors Affecting Polymer Degradation

The degradation of plastics can be said to begin as soon as the polymer is synthesised, and is increased by residual stresses left by moulding processes. This can be followed by exposure to light (especially UV), humidity, oxygen, heat, bacteria and stress. Plastics can also be contaminated by other materials, including other plastics. A polystyrene camera body, for example, can be attacked by plasticiser migrating from a PVC strap. Ideally the conservator needs information about the history of an object before prescribing treatment, but even before this, the plastics ‘doctor’ must jump another hurdle. Specific conservation action cannot be taken until the polymer has been identified, and this is a technical area full of pitfalls. A 1920s black brooch could be made of at least six different plastics materials, or simply painted as was common practice in the 19th century. Even a patent number, one of the few ‘hallmarks’ found on plastics and an obvious aid to identification, may refer to a fixing mechanism and not to the moulding.

Preserving Plastics

Derek Pullen, a conservator at the Tate Gallery, explains, ‘Plastics are giant molecules held together by forces which can be broken by attacking energy forces such as light. All the conservator can do is to keep mouldings in a very stable, low energy environment (the burial chambers of the Pyramids were ideal)’.

Types of Polymer Degradation

There are two main types of plastics degradation being researched at present: physical and chemical, and both are closely inter-connected. Physical degradation can involve environmental stress cracking and plasticiser migration and loss. Chemical reactions include oxidation and hydrolysis, and are a problem particularly affecting the cellulose esters (cellulose nitrate and cellulose acetate), which emit acidic degradation products. If not removed, these catalyse further reactions and eventually cause serious crazing and total destruction of the object. If degrading cellulose esters are not isolated, the acidic fumes will infect similar objects stored close by and initiate degradation there.

Solutions to Polymer Degradation

As the recognition of polymer degradation improves, conservation guidelines are beginning to emerge. High-tech solutions which could help in theory are prohibitively expensive, but tailor made scavengers such as activated charcoal or Ageless help to create a low oxygen environment. Ageless is a reactive powdered iron and is normally used to prolong the shelf-life of dry foods by absorbing oxygen. Epoxidised soyabean oil (ESBO), has also been tested with encouraging results as an acid absorbing coating on degrading cellulose nitrate.

Conclusion

Compared to traditional materials with long established technologies such as metals and glass, the complex chemical nature of plastics is providing conservators with possibly their most formidable challenge yet.

 

Primary author: Sylvia Katz

Source: Materials World, Vol. pp. 377-78, August 1995.

 

For more information on this source please visit The Institute of Materials, Minerals and Mining.

 

Date Added: Sep 20, 2002 | Updated: Jun 11, 2013
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