Volume 89 Issue 29 | pp. 29-31
Issue Date: July 18, 2011

Preserving Plastic Art

Chemistry of polymer-based creations presents unique problems for conservators
Department: Science & Technology
News Channels: Materials SCENE
Keywords: plastics, art, conservation
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Weeping Plastics
Shashoua treats a 1970s-era crash-test dummy used to test seat belt effectiveness. Plasticizer is pouring out of the PVC mannequin.
Credit: Courtesy of Yvonne Shashoua
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Weeping Plastics
Shashoua treats a 1970s-era crash-test dummy used to test seat belt effectiveness. Plasticizer is pouring out of the PVC mannequin.
Credit: Courtesy of Yvonne Shashoua

Dutch artist Madeleine Berkhemer began making sculptures from stockings in 1997, using up to 1,000 pairs of pantyhose to make each of her taut, ethereal pieces. Initially, she didn’t worry about the durability of the nylon- and spandex-based leg-wear in her art.

Then Berkhemer visited a museum showing a work by the French-American artist Louise Bourgeois, who also used stockings in her sculptures from the 1940s and onward. The stockings in Bourgeois’ art had lost their elasticity and were sagging. “That’s when I got concerned,” Berkhemer says. “I thought, ‘Maybe this will happen with my sculptures.’ ”

Berkhemer contacted Thea van Oosten, a conservation scientist at the Cultural Heritage Agency of the Netherlands. Van Oosten specializes in plastic artifacts and is an expert on polyurethane, the ingredient in spandex that makes stockings stretchy. The two women have started working together to find a way to prevent Berkhemer’s art from sagging in the decades to come.

Like Berkhemer, many artists are inspired to use plastics without being aware of their limited lifetime. Unfortunately, “plastic objects are among the most vulnerable found in museums and galleries,” says Matija Strlič, a chemist at the Centre for Sustainable Heritage at University College London.

Environmentalists worry about the long lifetimes of plastic packaging in city landfills and the pristine waters of the deep ocean. But museum conservators are racing against time to save plastic objects in their collections, whose several-decades-long lifetimes don’t compete with the multiple-millennia-long stability of ancient bronze or stone artifacts, explains Yvonne R. Shashoua, a conservation scientist at the National Museum of Denmark, in Copenhagen.

Almost all plastics darken with time, as harsh light conditions transform their polymeric ingredients into yellow, orange, or brown molecules. Furthermore, light and temperature fluctuations combine with oxygen and water in the air to break apart the polymers that make up the plastic. The degradation products tend to leach, joining an exodus of plastic additives included in recipes to make the material malleable or to protect it from ultraviolet light or heat. As essential components exit the plastic pieces, artwork begins to crack and crumble—just as a plastic bottle does when left in the sun.

Adding insult to injury, the degradation of some types of plastic isn’t a problem for just the plastic artifact itself. Sometimes the breakdown molecules float over to nearby artifacts, inciting corrosion, staining, or degradation. Researchers and conservators are figuring out how to diagnose the damage happening to plastic art and artifacts before it is even visible, and they are developing strategies to help these valuable objects last longer.

Since the invention of plastics in the late 1800s, artists and designers have been using them to make everything from high-fashion hair combs and intricate sculptures to moon-mission spacesuits. But “the number of plastics used by artists increased dramatically in the 1960s,” Shashoua explains, during that era’s love affair with all things plastic and the corresponding increased availability of polymers. Yet it took until the 1990s before the museum world got over what conservators refer to as “plastics denial syndrome”—denying that plastics in a museum’s collection have short lifetimes and degrade—and woke up to the fact that many pieces of plastic-containing art were in grave danger of being lost, van Oosten says.

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PANTYHOSE HEAVEN
Dutch artist Berkhemer made these stocking sculptures, called “Milly’s Chandelier” (2009) and “Milly’s Chandelier II” (2010), from nylon- and spandex-based pantyhose, which have a lifetime of 10 to 15 years.
Credit: Courtesy of Madeleine Berkhemer
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PANTYHOSE HEAVEN
Dutch artist Berkhemer made these stocking sculptures, called “Milly’s Chandelier” (2009) and “Milly’s Chandelier II” (2010), from nylon- and spandex-based pantyhose, which have a lifetime of 10 to 15 years.
Credit: Courtesy of Madeleine Berkhemer

Four kinds of plastics are particularly vulnerable to degradation, she explains. Two of these problematic plastics were among the first developed: cellulose acetate and cellulose nitrate. The third is polyvinyl chloride, one of the top five plastics produced in the world. The fourth is polyurethane foam, whose many pores increase the material’s surface area and thus exposure to oxygen, light, and water in air—all of which make the plastic susceptible to crumbling.

There’s no one-size-fits-all remedy to deal with plastics in a museum collection—each type of plastic has its own particular chemical quirks that need to be accommodated, van Oosten says. For example, she has found that a spray-on light stabilizer called Tinuvin can protect polyurethane artwork from light damage and extend its life by decades. The downside is that it can turn some artwork’s matte look to glossy, which is a deal breaker for many artists and curators, van Oosten adds.

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Heaven
Credit: Courtesy of Madeleine Berhemer
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Heaven
Credit: Courtesy of Madeleine Berhemer

That polyurethane actually has a life-extension option is an exception to the rule in plastics conservation, however. “Once a plastic is made,” Shashoua says, “it’s very difficult to get an antiaging additive into it,” primarily because most plastics are not very porous.

Instead, conservators extend the lives of plastic objects by monitoring and modifying the environment around them. This is particularly true of cellulose acetate and cellulose nitrate objects. A famous replacement for ivory and tortoise shell in jewelry of the late 1800s and early 1900s, these plastics were also used in early movie and photographic film. Exposure to light, heat, and air breaks down the polymers so that they release nitric acid and acetic acid. If these acids aren’t removed from the environment around the plastics, they will catalyze further degradation of the cellulose acetate and cellulose nitrate.

Furthermore, the acidic off-gassing is so harmful to other artifacts in museum collections—it can destroy metals and textiles—that some conservators refer to cellulose acetate and cellulose nitrate as malignant plastics. In fashion display cases that present cotton clothing together with these plastic accessories, Shashoua explains, conservators can find acid-corroded cloth and metal clasps.

Since the mid-1990s, the main weapon against these acids has been adsorbents—activated carbon for nitric acid and zeolites for acetic acid—which conservators typically place near plastic artifacts to trap the acids.

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Di(2-ethylhexyl) phthalate (DEHP)
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Di(2-ethylhexyl) phthalate (DEHP)

But little research on the adsorbents’ efficacy has been done since about 2000, mostly “because there aren’t very many scientists working on plastics in museums,” Shashoua says. “We put adsorbents into the storage box because it’s convenient, and usually we see that it improves the situation, but it could be that the cardboard box that we put it in with the adsorbent is actually the important factor that is taking out the acid. We just don’t know.” Shashoua is planning to spend the next year looking at the efficacy of adsorbents and searching the medical, fuel production, and food industry literature for alternative ways to sequester these acids.

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Fashion Faux Pas
Acids floating from cellulose nitrate and cellulose acetate fashion combs can destroy nearby textiles or corrode metal artifacts.
Credit: Courtesy of Matija Strlic
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Fashion Faux Pas
Acids floating from cellulose nitrate and cellulose acetate fashion combs can destroy nearby textiles or corrode metal artifacts.
Credit: Courtesy of Matija Strlic

In sharp contrast to objects made with cellulose acetate and cellulose nitrate, which are preserved by separating them from leachates to prevent further breakdown, objects made from polyvinyl chloride are protected by keeping them sealed in with their leachates. That’s because one of the biggest problems with PVC museum objects—which can include dolls, sculptures, and furniture—is that its plasticizer, typically di(2-ethylhexyl) phthalate, easily migrates out. And we’re not talking just a bit. Some PVC objects “weep plasticizer,” Shashoua says, leaving the plastic brittle and vulnerable to cracking. But if the plastic artifact is sequestered from the outside atmosphere, then the partial pressure of the exiting plasticizer in the enclosed air counteracts further migration.

Besides destabilizing the structure of PVC museum objects, loss of the plasticizer leaves the objects’ surfaces wet and sticky, making them attractive to dust and grime. In the case of the white Apollo moon-mission space suits, plasticizer leaching out of the PVC life-support tubing has crystallized on the surface of the plastic and stained the nearby white nylon textiles an orangey color, says Lisa Young, a space suit conservator at the Smithsonian National Air & Space Museum in Washington, D.C. Young and other conservators have had to quarantine the PVC from the spacesuits to avoid further unpleasant discolorations of the culturally important suits.

And it’s not just plasticizer that leaches out of PVC. Stearic acid, which is commonly used in many plastics as a lubricant to keep the object from sticking to its mold, also tends to migrate out and harden, coating the surface of objects with a white powdery substance.

When conservators want to clean PVC and other artifacts from these leaching constituents, they have to be extremely careful what chemical cleaners they use to remove dust, grime, and any leaking additives. Organic solvents are generally considered a really bad idea, mostly because they tend to dissolve or crack plastics. Use of acetone on polystyrene, for example, turns the transparent plastic an opaque white.

Yet water and soap don’t always dissolve problematic dirt that conservators want to remove. “The process of cleaning doesn’t just involve dissolving the dirt—it involves moving it off the surface with a mechanical agent,” Shashoua says. “For example, just rinsing Tupperware with soap and water doesn’t remove the red stain of tomato paste. You also need a scourer or a sponge. In museums, the scourer is going to cause more damage than the water.” Conservators formerly used cotton cloths to clean plastic objects, but increasingly they are trying out new microfibers such as polyester or polypropylene or the use of ultrasound to remove dirt via sound waves.

Although cellulose acetate, cellulose nitrate, PVC, and polyurethane are the most problematic plastics, museum objects made from polyester and polypropylene are also beginning to show problems, Shashoua says.

Because any conservation or cleaning strategy is specific to a plastic type, museum staff need to know an object’s precise plastic makeup. But few plastic art objects come with a chemical ingredient list, and not all museums have laboratories with the tools—primarily infrared and other spectroscopic equipment—to help analyze them. Conservators sometimes have to rely on their sense of smell to guide their plastic diagnosis, Shashoua says. Leaching phthalate plasticizers give PVC the smell of a new car, she explains, while cellulose acetate smells like vinegar, and polyester has the odor of raspberry jam, cinnamon, and burning rubber.

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Devil Inside
A new noninvasive technology makes it possible to identify the nylon (blue) and cellulose acetate (red) makeup of a plastic statue (left).
Credit: Anal. Chem.
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Devil Inside
A new noninvasive technology makes it possible to identify the nylon (blue) and cellulose acetate (red) makeup of a plastic statue (left).
Credit: Anal. Chem.

Strlič thinks it might be possible to replace the human nose with portable devices such as those developed by the military to detect trace amounts of toxic gases. His team is hoping that antiterrorism technology—from portable mass spectrometers to tiny lab-on-a-chip devices—can be tweaked to analyze the smells of plastics in museums.

Strlič has also recently built another tool that conservators can use to identify the type of plastic in a museum object without harming the piece. The tool marries near-infrared spectroscopy with a digital camera to produce two-dimensional chemical maps from which conservators can identify the chemical makeup of many plastic artifacts. The device can also follow the onset of plasticizer migration before the object shows external signs of leaching and breakdown (Anal. Chem.,DOI: 10.1021/ac200986p).

As museum staff around the world increasingly realize that plastics in their collection are vulnerable, a growing amount of work is being done to find solutions to a problem that is unlikely to go away. For example, conservators from the Smithsonian Institution museums and galleries in the U.S. are trying to create a plastics working group to research and share solutions to plastic degradation problems. In Denmark, museum curators, conservators, the plastics industry, and artists are also starting to work together to fight plastic art degradation under a program called Plastics Research & Innovation for Museums & Industry.

Although researchers are making progress on the protection of plastic pieces, many artists are still unaware of the materials’ short life even as it becomes easier for them to mix plastics in their ateliers. “There are a lot of ready-to-make plastics available,” van Oosten says. It’s as easy as “putting two cans of liquid material together so polymerization takes place in a mold.” The problem with these do-it-yourself techniques is that the artists don’t realize they need to add the light and heat stabilizers that are often present in industrially produced plastics. “We are seeing problems with art objects made only 10 to 15 years ago,” she notes.

Art schools are not filling the education gap. For example, a recent study of art schools in Europe found that only one—in Munich—gives any training about the inherent instability of plastics, Shashoua says. “Often art schools feel they shouldn’t inhibit the creative skills of artists by saying to them at the beginning: ‘If you use this sort of plastic it will last for 10 years and if you use this other sort of plastic it’s only going to last for three years.’ But the fact is, many museums and art galleries are concerned with the stability of the objects they buy. That means some artists might never get their work in art galleries or museums,” she says.

It’s a dilemma. “As an artist, I shouldn’t be too concerned” about the lifetime of materials—“it stops your creativity,” Berkhemer says. But she thinks more scientific information can also be “a big inspiration.” Stronger, more durable plastics in the stockings would “allow me to make new forms,” she says.

As some artists begin to consider the chemistry of their supplies and as conservation scientists come up with better strategies to protect plastic art and artifacts, museum visitors may be able to enjoy these polymeric art forms for centuries to come. Eventually, van Oosten speculates, museum visitors may also start to appreciate the look of aged plastic, just as they find aged stone and bronze appealing. It’s probably just a matter of time. ◾

 
Chemical & Engineering News
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