When plastics are precious

doi:10.1126/science.373.6550.40

GSTDTAP > 气候变化

DOI	10.1126/science.373.6550.40
	When plastics are precious
	Sam Kean
	2021-07-02
发表期刊	Science
出版年	2021
英文摘要	To prevent museum artifacts from falling apart, conservators experiment with preservation methods. Leanne Tonkin still remembers the ruined coats. She was doing a fellowship at the Metropolitan Museum of Art's Costume Institute in the mid-2010s when she saw a red mackintosh from the 1960s. The raincoat was so rigid it could stand up on its own, as though inhabited by a ghost. Another mackintosh was hardly recognizable as clothing. “You could make out a button on it, but it was completely melted,” she says. Nothing out of the ordinary had happened to the coats. They didn't get caught in fires or exposed to caustic chemicals. Instead, they were doomed for a simpler reason: They contained inherently unstable plastics. People often grumble that plastics are too durable. Water bottles, shopping bags, and other trash litter the planet, from Mount Everest to the Mariana Trench, because plastics are ubiquitous and don't break down easily. But some plastic materials change over time. They crack and frizzle. They “weep” out additives. They melt into sludge. All of which creates huge headaches for institutions, such as museums, trying to preserve culturally important objects. Until recently, museums only had to worry about traditional materials. “We know how to approach the restoration of paintings, books, and materials like wood, metals, and glass,” says Anna Laganà, a research specialist at the Getty Conservation Institute. “But for plastics, our knowledge is still limited.” Tonkin, now a doctoral researcher in fashion conservation at Nottingham Trent University, agrees: “We're now scuttling around trying to figure out how to conserve” plastics, she says. The variety of plastic objects at risk is dizzying: early radios, avant-garde sculptures, celluloid animation stills from Disney films, David Bowie costumes, the first artificial heart. Nearly every museum in the world has plastic items, and even well-cared-for objects can fall apart alarmingly quickly. Joana Lia Ferreira, an assistant professor of conservation and restoration at the NOVA School of Science and Technology, recalls an exhibit with a lamp that had a black polyurethane foam shade. One day, conservators noticed the shade had started to collapse, so they removed the lamp from the exhibit to save it. But it was too late: The shade began to crumble and soon collapsed completely. “From one week to another,” Ferreira says, “it was on the floor.” Museums are doing everything they can to save culturally important items from similar fates. Over the past decade, conservators have developed better tools to identify vulnerable objects. Some conservators have also started to run experiments to shore up preservation practices and arrest decay. Plastics have existed for roughly 150 years at this point, and curators want their plastic treasures to still be recognizable 150 years into the future. SOME PLASTICS , such as polycarbonate and acrylic, are quite stable. But the long-chain polymer molecules in other plastics can break down when exposed to oxygen or light, among other potential threats. Unfortunately, some of the most “malignant” plastics—cellulose nitrate, cellulose acetate, polyurethane, polyvinyl chloride (PVC)—were common historically, finding use in items as diverse as photography film, billiard balls, and high-end clothing and furniture. The additional ingredients in those plastics, including plasticizers, dyes, and fire retardants, can introduce their own problems. PVC, for instance, forms both sturdy plumbing pipes and supple shower curtains, depending on the additives. And those mixtures can naturally unmix over time, especially when temperatures fluctuate. One common challenge for conservators is figuring out what materials they're even working with. Techniques such as Raman spectroscopy and pyrolysis–gas chromatography–mass spectrometry can create a molecular “fingerprint” from a sample of material. By looking for a match in a database of spectra posted online by a European-led project called POPART (Preservation of Plastic Artefacts in museum collections), museums can determine what materials they have. Still, many small museums lack access to fancy lab equipment, which can cost tens of thousands of dollars. So some institutes have developed low-tech approaches based on touching, tapping, and sniffing. Depending on the type, plastics can feel glasslike or waxy. After decay begins, they can also feel sticky as internal plasticizers migrate to the surface. Auditory tests are available, too: Some plastics sound tinny when tapped; others, stolid and dull. And deteriorating plastics can release an astounding array of odors—chlorine, vinegar, pine, burnt hair, used sneakers, mothballs, car tires, sulfur. Once conservators identify the plastic, they can take steps to preserve it. The general rule is “keep it cold, keep it dark, keep it dry, [and] keep it without oxygen in some cases,” says Odile Madden, a senior scientist at the Getty. But the optimal conditions can differ even within the same family of material; it all depends on the specific ingredients and their proportions. Testing new preservation techniques on unique objects is risky, so some conservators have turned to controlled experiments. Tonkin, for instance, simulated clothing by adding stitches and metal studs to 20-centimeter squares of PVC fabric (common in modern raincoats). Then, she stored them for months at temperatures ranging from −17.8°C to 21.1°C and humidities between 40% and 50%. Intuitively, deep-freeze storage might seem the most promising way to preserve a material. But in the coldest conditions, Tonkin found, PVC molecules crystallized, squeezing out the plasticizers that make the fabric supple. The fabric ended up brittle and pitted with minute holes. Storing plastics at 4.4°C avoided that problem. In addition, Tonkin found that when transferring the material into or out of chilled storage, keeping it at an intermediate temperature of about 15.5°C for 24 hours could help avoid damage from quick temperature changes. Other conservators have run experiments to artificially age plastics, trying to understand exactly how they break down—and glean clues about how to arrest decay. Xenon arc lamps bombard materials with photons, collapsing years of light exposure into hours. Researchers can use weather chambers to study heat and humidity. Madden has also considered exposing plastics to pollutants to study the effects of dirty air. ![Figure][1] GRAPHIC: V. ALTOUNIAN/ SCIENCE Such experiments face a limiting factor: a lack of historically accurate plastics. For safety and stability, today's manufacturers add different ingredients to plastics than in decades past. So the results of experiments on modern polymers might not hold for historic ones. To skirt that limitation, in 2019 the Getty purchased its own plasticmaking equipment: an extruder and injection-molding system. Staff members then flipped through old issues of magazines such as Modern Plastics to find representative recipes. The Getty can now re-create historic plastics with the exact ingredients used originally. Scientists there are already running stress tests on small discs and bars, with an eye toward performing artificial aging tests on them later. Certain artifacts are especially vulnerable because some pioneers in plastic art didn't always know how to mix ingredients properly, says Thea van Oosten, a polymer chemist who, until retiring a few years ago, worked for decades at the Cultural Heritage Agency of the Netherlands (RCE). “It's like baking a cake: If you don't have exact amounts, it goes wrong,” she says. “The object you make is already a time bomb.” And sometimes, it's not the artist's fault. In the 1960s, the Italian artist Piero Gilardi began to create hundreds of bright, colorful foam pieces. Those pieces included small beds of roses and other items as well as a few dozen “nature carpets”—large rectangles decorated with foam pumpkins, cabbages, and watermelons. He wanted viewers to walk around on the carpets—which meant they had to be durable. Unfortunately, the polyurethane foam he used is inherently unstable. It's especially vulnerable to light damage, and by the mid-1990s, Gilardi's pumpkins, roses, and other figures were splitting and crumbling. Museums locked some of them away in the dark. So van Oosten and colleagues at RCE began to study ways to protect polyurethane. First, they took foam samples similar to the nature carpets and infused some with stabilizing and consolidating chemicals that modern manufacturers often use. Van Oosten calls those chemicals “sunscreens” because their goal was to prevent further light damage and rebuild worn polymer fibers. Then the team used xenon lamps to artificially age both treated and untreated samples, and examined them under high-powered microscopes. The results were encouraging. Samples that lacked sunscreen had withered under the barrage of photons: The molecular “struts” shoring up the foam were 42% thinner and notably more brittle than before the lamp treatment. The struts in samples with sunscreen decreased by as little as 12.5%. Armed with that knowledge, conservators working with RCE infused several Gilardi sculptures, including two nature carpets, with the sunscreen to stabilize them. Van Oosten is proud that several have even gone on display again, albeit sometimes beneath protective cases. Long called the “queen of plastics,” in 2012, van Oosten was knighted in the Netherlands for her efforts to preserve plastic objects and spread knowledge to other institutes. DESPITE SUCH SUCCESS stories, preservation of plastics will likely get harder. Old objects continue to deteriorate. Worse, biodegradable plastics, designed to disintegrate, are increasingly common. And more is at stake here than individual objects. Ferreira notes that archaeologists first defined the great material ages of human history—Stone Age, Iron Age, and so on—after examining artifacts in museums. We now live in an age of plastic, she says, “and what we decide to collect today, what we decide to preserve … will have a strong impact on how in the future we'll be seen.” Future archaeologists examining the leavings of the 21st century will likely find scads of toxic crud, along with plenty of plastic trash. But if museum preservation efforts succeed, maybe those scholars will also see that plastic items today can be culturally meaningful—and even cherished. [1]: pending:yes
领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/334153
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Sam Kean. When plastics are precious[J]. Science,2021.
APA	Sam Kean.(2021).When plastics are precious.Science.
MLA	Sam Kean."When plastics are precious".Science (2021).