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Where did the coffee cup go?
It is a demonstration that never fails to draw oohs and ahhs from students. Pour a small amount of acetone into the bottom of a beaker and drop in a foamed plastic coffee cup. It instantly seems to melt into the liquid and within seconds just vanishes.
Of course, matter cannot just vanish, but it can change from one form into another. Like sugar dissolving in water, the plastic dissolves in the acetone. How can so much plastic dissolve so quickly in so little acetone? Because there is actually very little plastic in that coffee cup.
Yet that little plastic has been expanded by being filled with gas bubbles, much like blowing up a balloon. As the plastic dissolves, the air is released, and we have an apparent magical effect.
That demo is a great way to get into the fascinating story of polystyrene, a plastic that when foamed is used to make coffee cups, packaging peanuts and home insulation materials. In its rigid version, it is found in clear plastic glasses, disposable cutlery, laboratory Petri dishes and toy model kits.
Our story begins in 1839, when Berlin apothecary Eduard Simon distilled an oily substance from the sap of the sweetgum tree and days later found that it had turned into a thick jelly-like material. He assumed that the oil had reacted with oxygen in the air, but by 1845, German chemist August Wilhelm von Hofmann had shown that the reaction would occur even in the absence of air.
After the concept of molecules had been introduced by Englishman John Dalton, and the Italians Amedeo Avogadro and Stanislao Cannizzaro, French chemist Marcellin Berthelot suggested in 1866 that small molecules in Simon鈥檚 oily extract had joined to make a novel substance.聽
Although he did not use the term 鈥減olymer,鈥� Berthelot can be credited with the concept of many small molecules linking together through some chemical reaction. While he was on the right track, it wasn鈥檛 until the 1920s that German chemist Hermann Staudinger proposed that substances such as starch, cellulose, proteins and, yes, Simon鈥檚 jelly, are composed of long chains of repeating molecular units linked by chemical bonds. He called these large molecules 鈥減olymers鈥� made of 鈥渕onomers鈥� linked together like chains of paper clips.
By 1931, the small molecules that Simon has distilled from the sap of the sweetgum tree had been identified as styrene, and chemists at the I.G. Farben Company in Germany found a way to link styrene molecules to produce polystyrene. The required styrene was in turn produced by reacting benzene distilled from coal tar with ethylene isolated from coke-oven gas.聽
About a decade later, chemists at the Dow company found a way to expand polystyrene into a foam by introducing a gas that functioned as a blowing agent. This foamed polystyrene, named 鈥淪tyrofoam鈥� by Dow, proved to be a great material for insulation. At first, the blowing agent was Freon, but it was replaced by pentane or carbon dioxide when Freon was found to be harmful to the ozone layer.
While plastics are an integral and indispensable part of modern life, their use, including that of polystyrene, raises some concerns. The benzene and ethylene needed for the production of polystyrene come from petroleum, which brings up the question of whether we are squandering a non-renewable resource. Hardly. Less than a fraction of one per cent of our petroleum reserves is needed to produce all the polystyrene products we use.
There has been concern over styrene being labeled as a potential carcinogen, based on some rare cancers found among workers in the polystyrene industry. Calculations have shown that during an eight-hour shift, a worker inhales roughly 100,000 micrograms of styrene. Compare that with the 2.5 micrograms leaching out from a coffee cup, a good portion of which would not be absorbed from the digestive tract. Furthermore, styrene occurs naturally in coffee beans. There is more natural styrene in the coffee than what leaches out from the cup. Beer contains even more styrene than coffee, and cinnamon has thousands of times more.聽
If you have concerns about styrene, stay away from that cinnamon bun.
When we crunch the numbers, it is pretty clear that there is no risk that can be attributed to styrene leaching out from polystyrene cups or plates.聽
A more significant issue is the disposal of polystyrene items. Throw a foamed polystyrene coffee cup on the compost pile and it will remain untouched. Bacteria do not consider it a tasty morsel, meaning that polystyrene is not biodegradable. However, when exposed to weather conditions, it will break down into smaller pieces and be a source of microplastics that are becoming an environmental nightmare.
Discarded polystyrene items end up in a landfill, where they may sit for hundreds of years, but they do so harmlessly. And they don鈥檛 take up much space: Less than one per cent of the volume in landfills is occupied by the easily compressible polystyrene products.聽
Also, modern landfills are constructed in such a way as to prevent anything from leaching out. Ideally, all plastics should be recycled, and that includes polystyrene, which is marked with the number 鈥�6鈥� in the recycling logo. However, not much of polystyrene ends up being recycled.聽
Transporting foamed polystyrene is uneconomical because it is mostly air and is likely to be contaminated with food residues. Rigid polystyrene can be melted down and recycled but separating it from other plastics is difficult and expensive.聽
If you don鈥檛 use disposable polystyrene cutlery or foamed coffee cups, you don鈥檛 have to worry about disposal.
That in a nutshell is the story I tell to accompany my dissolving coffee cup demo. But the demo also brings up the question of what to do with the contents of the beaker. I explain that I will certainly not dump it down the sink. A better approach is to allow the acetone to evaporate and leave the plastic behind. That evaporation takes many hours, so I show the results from a previous performance.聽
Seeing the foamed coffee cup transformed into a thin disk of rigid plastic elicits a final round of oohhs and aahhs.
