When Plastics Aren’t Forever: Part II

Only 20 years old, this polyurethane-based plastic has already lost flexibility and yellowed considerably. This sort of discoloration and decay is common among plastics, although exact problems vary considerably by the type of polymer.

Photo: Tom Learner. © The J. Paul Getty Trust, 2013.  All rights reserved.

Plastics weep. Plastics bleed and bloom. Plastics flake and darken and crumble, all because of their unique chemistry.

Down on a nanoscale, plastics are long chains of a single molecule repeated again and again: they’re polymers. This arrangement often produces quite brittle structures, but chemists can get around this limitation by adding plasticizers. These molecules (phthalates being the most common example) wriggle between the chains and soften the plastics up, making them much more pliable and malleable.

Chemists created some of the very first plastics by injecting nitric acid into cellulose, a natural polymer that makes up the cell walls of plants. Unfortunately, this combination—called cellulose nitrate—proved alarmingly flammable. Cinema companies used cellulose nitrate to make reel-to-reels, for instance, and not a few early movie theaters went up in flames when the hot projection bulbs fried the film. Billiard balls made from cellulose nitrate sometimes exploded when cigar ash hit them or even when they knocked into each other during game play.

After sitting around in a museum for decades, cellulose nitrate objects can even catch fire spontaneously. And good luck putting the flames out: because the combustion of this plastic produces its own oxygen, it can continue to burn even underwater (a property you don’t much see outside the alkali metals). Overall, it’s one of the worst materials a curator can have the misfortune of being entrusted with.

Plastics also “dry out” and become brittle over time, mostly because their long chains of molecules have a bad habit of squeezing the plasticizers out—hence the terms “weeping” and “bleeding.” The plasticizers then pool near the surface of the objects, causing dust and dirt to adhere. Other deplasticized plastics feel sugary to the touch or their surfaces break out in tiny cracks like a rash. The migration of plasticizers can change the color of objects as well. What should be translucent, for instance, can look nicotine-stained, and light no longer reflects properly from the surface (a defect called “bloom”).

Yvonne Shashoua, an expert in plastic conservation from the National Museum of Denmark, works to arrest the degradation of this 1970s crash-test dummy, which is weeping plasticizer. Museums like the Museum of Science and Industry in London, which owns the dummy, must struggle with how to preserve its plastic collections.

Image courtesy of Shashoua.

The oddest consequence of migrating plasticizers is the odors. Some deteriorating plastics smell like swimming pools, thanks to weeping chlorine. Others smell like vinegar because of bleeding acetate or like ammonia or new cars. Smells redolent of burnt milk, celery, raspberry jam, “muscle rub,” cinnamon, and (worst) burnt hair can also appear.

Often, these strange smells are the first signs of deterioration—a tickle in the nose that shouldn’t be there. But by the time you can smell something, it’s often too late, for the deterioration never stops with mere odors. Next week we’ll look at some well-known works of art that have suffered, as well as the little that curators can do to salvage them.

Related: When Plastics Aren't Forever: Part I; When Plastics Aren't Forever: Part III

 

Sam Kean is the best-selling author of The Disappearing Spoon and The Violinist’s Thumb. His work has appeared in the New York Times Magazine, Mental Floss, Slate, Psychology Today, and the New Scientist as well as on NPRs “Radio Lab” and “All Things Considered,” among other programs.  

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