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Plastics for posterity

May 25, 2007 By Katharine Sanderson This article courtesy of Nature News.

How do you prevent valuable collectors' items from degrading?

Many environmentalists would tell you that plastics, such as those that litter our oceans and landscapes, last for ever. But ask the collectors and museum curators who have gathered at London's Victoria and Albert Museum this week, and they will say the opposite.

They have met to discuss ways to prevent damage to culturally important plastic objects such as museum pieces and collectibles — created to last for ever, but doomed to degrade if not properly cared for.

Compared with paintings and sculptures, plastics have a short cultural history. But some plastics are now more than a century old, and preservation of some ageing items is becoming a headache. "You can't prevent degradation; you can only slow down the process," says Yvonne Shashoua from the National Museum of Denmark, Copenhagen.

Polyvinyl chloride, or PVC, is one of the big problems. "It is one of the plastics used the most in society," Shashoua says. Plastic figurines from cult movies such as Star Wars can attract whopping price tags if sold in pristine condition, but the Force isn't strong enough to keep the plastic from inevitable degradation.

Black to white

The trouble arises from the plasticizing additives used to prevent the PVC from turning out brittle. These plasticizers generally belong to a class of chemicals called phthalate esters and are only attached physically, rather than chemically, to the PVC polymer chain. Over time, this weak attachment breaks down and the plasticisers leak out to the surface of the material.

If the phthalate esters are exposed to an acidic environment — such as that present in many packaging materials — they form crystals of phthalic acid. And eventually that pristinely cared-for Darth Vader doll will end up with these crystals growing out of his famously fearsome black helmet, which rather ruins the effect. "Culturally, the figurines look a lot different if they've got a white head," says Shashoua. Not surprisingly, this reduces their value both commercially and culturally.

The best way to prevent such objects from deteriorating is to store them in a household freezer, Shashoua advises.

Role for charcoal

Other common plastics include cellulose acetate and cellulose nitrate. The artist Naum Gabo made several cellulose acetate sculptures, renowned glass artist René Lalique worked with cellulose nitrate in the 1920s, and early plastic dolls were made from cellulose nitrate. Both of these plastics degrade via an unstoppable chain reaction, which is initiated by light. "When I found out I immediately had the windows in the plastics gallery masked out," says Susan Mossman, plastics curator at the Science Museum in London.

Mossman is investigating the extent to which adsorbent materials placed with plastic exhibits affect degradation. Charcoal cloth, for example, has a huge surface area and can take up gases given out during the degradation process — including nitric acid or acetic acid, depending on the polymer. These by-products can themselves catalyse the polymer-breaking reaction in a process called autocatalysis.

The gases are easy to detect, says Amy Anderson from the National Trust's Museum of Childhood in Sudbury, UK, "You can smell a great waft of acids as you open the exhibiting case," she says.

Removing the emitted by-products from the system can therefore halt autocatalysis. Mossman also suggests that the main mechanism by which charcoal cloth slows the plastic's degradation is by removing oxygen from the system. Others say that the charcoal is more likely to be simply absorbing anything that is given off — including the acids.

One thing is clear: the problem of preserving plastics objects is not easy to solve, and the responsibility to maintain these preservation steps will fall to curators, says Mossman "It requires good housekeeping," she says.

Shashoua agrees that plastics conservation is not set to get any easier, even with developments in new manufacturing processes. "Plastics made today are still made to last a particular length of time. We'll see the same problem when those plastics are old enough," she says. In the meantime, it might be worth renewing the insurance on your irreplaceable original Darth Vader before he starts to lose both his menace and his value.


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