Long Live Plastics

Because plastic products can be mass-produced cheaply, they have long been considered the poster child of a throwaway culture. Plastics are versatile: Some are soft and flexible, but others are completely rigid. A few mimic natural substances; some are infused with colors rarely found in nature. Others are as clear as glass. And some polymer substances composing plastics can be molded into shapes impossible to reproduce with materials such as wood.

Perhaps because they are so versatile, some objects made from plastics have become highly collectible. Some museum collections, in fact, specialize in items commonly made of plastic — toys, games and dolls, for example. Other museums couldn’t avoid the polymers if they tried: Plastics show up in everything from fabrics to furniture, sequins to sculpture.

Though often praised for their chemical stability, plastics don’t last forever. Vinyl can crack, polyurethane can get cloudy and flexible tubing can become stiff. Even Ken and Barbie, like anyone approaching 50, can succumb to blemishes, age spots and loss of skin tone.

A decade ago, a survey of museum collections in London confirmed the ephemeral nature of polymers:  About one out of every eight plastic items showed signs of physical degradation such as cracking, discoloration or deformation, says Bertrand Lavédrine, an analytical chemist at the National Museum of Natural History in Paris. That doesn’t mean the other plastic objects will stay in good shape. Even items that appear fine for long periods can suddenly deteriorate once chemical changes start, he notes.

Most chemical changes triggering polymer degradation are irreversible. But given the right conditions, the demise of plastics can be slowed. Often, however, the challenge is to find those conditions.

“The museum world, in particular, has suffered badly from a lack of detailed understanding about the materials and techniques used for the manufacturing, the conservation and the restoration of artifacts that are now in critical condition,” says Lavédrine.

Hence the need for the POPART project. This 42-month, multimillion-dollar program — whose name is a shortened version of “Preservation of Plastic ARTefacts in museum collections” — was launched in October to address many of the problems that curators now face. Funded by the European Commission, researchers from the dozen participating museums, government agencies and universities in eight countries will survey museum collections, study how certain polymers deteriorate, develop techniques to display and clean plastic items and design equipment that can quickly discriminate one type of plastic from another.

Chain, chain, chain

Plastics are a type of polymer, a class of materials that gets its name from the Greek words for “many parts.” Researchers create polymers by chemically stringing together large numbers of simple carbon-based units called monomers (“single parts”).  The near-endless variety of plastics stems from the diversity of monomers—esters, amides and imides, to name a few—and the degree of linkage that exists between polymer chains: In general, the more bonds there are, the stiffer the plastic.

Materials scientists can use a variety of additives to further tailor a plastic’s physical properties, says Brenda Keneghan, a polymer scientist at the Victoria and AlbertMuseum in London. Plasticizers such as phthalates, phosphates and glycerols add flexibility and make the plastic workable, she explained in July at the Euroscience Open Forum in Barcelona. Lead and other metallic substances can chemically stabilize the material. Other additives, including pigments and flame retardants, slow degradation that results from exposure to heat, light and atmospheric oxygen.

The problem, says Keneghan, is that many of these components aren’t chemically bound to the polymer chains. Thus, over time and under certain conditions, the additives can ooze out of the plastic. Studies suggest that many of these substances—including phthalates (SN: 6/4/05, p. 355), flame retardants (SN: 3/26/05, p. 206) and bisphenol A (SN: 9/13/08, p. 15)—leach from consumer products  and can cause significant health problems for humans.

Loss of these additives doesn’t help the plastic either. When liquids leach from the material, surfaces can become covered with bloom—the same sort of powdery coating that forms on chocolate when sugar and fat migrate to the surface after the chocolate is stored in cool or wet conditions, says Keneghan. If the oozing liquids are oily or sticky, they attract dirt that often can’t be easily removed.

Such physical changes in plastic materials are just the beginning. Any number of factors—including exposure to ultraviolet light, ozone or even atmospheric oxygen—can trigger chemical changes that can cause the plastic to crack or become discolored. Vinyl car roofs, a popular option on automobiles in the 1970s, are a good example, says R. Scott Williams, an analytical chemist at the Canadian Conservation Institute in Ottawa. The roofs “looked fine for a while after they were purchased, and then they suddenly degraded” when chemical reactions accelerated by long-term exposure to sunshine and pollutants consumed the vinyl’s stabilizers and antioxidants.

There is no standard way to preserve damaged plastic items. Any attempt to ameliorate their deterioration often causes more problems than doing nothing, says Keneghan. At present, she notes, the most effective interventions are preventive—displaying at-risk items in a cool, low-light environment with stable humidity, for example. Other tricks include using materials such as activated charcoal and other absorbent materials to scavenge oxygen or acidic fumes from the atmosphere surrounding the objects.

In the POPART project, researchers will explore ways to conserve plastics. One possibility, says Lavédrine, might be to use gamma rays to repolymerize the material in fragile plastics such as polyurethane foam, often used in furniture cushions or to pad museum drawers. The same technique could be used to form a protective veneer on some types of plastics. Scientists also will evaluate various solutions and cleaning techniques to see how well they clean an object without causing long-term damage.

Hiding in plain sight

Soon after Graham Martin, an analytical chemist who is now head of science at the Victoria and AlbertMuseum, arrived at the V&A in the early 1980s, he asked curators to inventory the plastic items in their collections. “They all said that they had none, and they were all wrong,” he says. “Today curators are much more aware of polymers, but then again there was a lot of room for improvement.”

As the United Kingdom’s national museum of art and design, the V&A contains many items composed of at least some plastic parts. While some plastics are obvious, others turn up in unexpected places, says Keneghan. A partial survey of the museum’s holdings recently found more than a thousand polymer-containing objects in the metalwork collection — many of them plastic baubles in jewelry — and dozens of items with plastic components in the furniture and woodwork collection. Unsurprisingly, the curators tallied more than 6,000 plastic artifacts related to childhood, including dolls, toys and games. In the fashion collection, which wasn’t part of the recent inventory, plastic items include everything from vinyl boots and accessories to fabrics, jewelry and sequins, she told the audience in Barcelona.

Of nearly 8,000 items identified by the inventory, researchers deemed 15 percent to be in poor or unacceptable condition. Another 13 percent of the objects showed signs of chemical changes; more than half of those had become brittle, and one-fifth of the items were discolored.

One of the first tasks of the POPART project will be to conduct similar inventories of other national collections in Europe, says Lavédrine. By assessing the frequency and type of degradation among a wider sampling of museums, the project’s scientists can better tailor research priorities for later phases of the effort.

Determining the current condition of an object, plastic or not, is critical to figuring out how to preserve it, says Keneghan. Perhaps more important, researchers also need to know what type of plastic the object is made of. For 94 percent of the objects surveyed in the Victoria and Albert collections, the identity of the plastic — and therefor how the material might degrade — is unknown. More than half of the rest are known to be made of an unstable material, she says.

Pinning down a plastic’s identity typically involves sending a piece back to a lab for analysis, but museum curators often are loath to mar an artifact to gain even a small sample. Nondestructive methods such as infrared spectroscopy — shining low-level light on an object and then scrutinizing the intensities and wavelengths of the light that bounces back to a detector — is a promising technique, says Williams, who is trying to develop and refine such equipment.

Using a portable unit and a database of reflectance spectra that are signatures of various plastics, researchers could swoop into a museum and efficiently identify the composition of hundreds of artifacts in the course of an afternoon, he says.

POPART aims to develop other nondestructive analytical techniques for identifying plastics, says Lavédrine. Researchers will also work to enhance current analytical methods, such as gas chromatography and mass spectrometry, making them quicker, more accurate and able to take smaller samples.

Incomplete census

Even an accurate tally of a museum’s artifacts wouldn’t include all of its polymers. Drawers and storage boxes can be lined with foam cushions, and the tubing that circulates air through display cases sometimes is made of plastic, as are parts of the display cases themselves. Photographic slides and negatives often are stored in plastic sleeves. Degradation of these auxiliary materials can damage a museum artifact as surely as deterioration of the item itself.

Williams has long crusaded against the use of flexible polyvinyl chloride  tubing to circulate air through exhibits. As much as 40 percent of the material by weight can be plasticizers, not polymer. Even slight changes in humidity can affect the material’s stability, driving the oily additives out where they damage nearby objects. The tubes also can emit vapors such as hydrogen chloride, which when dissolved in water droplets creates hydrochloric acid.

As they degrade, many polymers give off corrosive byproducts—which has led chemists to dub them “malignant” plastics. One of the most prevalent is cellulose acetate, a material used in jewelry to simulate natural materials such as tortoiseshell, ivory and mother-of-pearl but most commonly used as photographic film. When cellulose acetate deteriorates, it gives off acetic acid — the ingredient that gives vinegar its pungent odor and taste. Not only does the acetic acid eat away at the surface of the object that’s deteriorating, its vapors can chew away at any artifacts nearby. “When you open up a box and smell vinegar, you know your items are in trouble,” says Keneghan.

POPART scientists will study the degradation of cellulose nitrate — a forerunner of cellulose acetate that gives off nitric acid as it deteriorates — and of polyurethane, says Lavédrine. Researchers will look to identify chemical markers for polyurethane degradation and measure how variations in humidity, temperature and atmospheric oxygen affect the material’s rate of deterioration.

The POPART project is all about preserving cultural heritage, says Martin. “One of the big questions is, ‘How can we keep what we have longer?’” he says. “A lot of the damage we see might be difficult to reverse, but it still needs to be assessed.”

Challenges in preserving plastics will not be small, and figuring out how to preserve one type of plastic won’t necessarily solve all a curator’s problems. “I’ve always been jealous of film archivists,” Martin notes. “They only have to deal with one type of plastic, and we’ve got a multitude of them.… A plastic chair might have a nylon cover over a polyurethane foam cushion over a polypropylene base,” he bemoans. “That’s three plastics right there in just one item.”