Polystyrene (PS)

On the morning of 30 January 1933, a crowd packed the street outside the Reich President’s palace in Berlin to watch Adolf Hitler’s car arrive. Paul von Hindenburg had just appointed him Chancellor, and the men waving their hats below the windows had no way of knowing they were watching the last transfer of power the German republic would ever make peacefully. Six weeks later they would know. On 27 February the Reichstag burned; on 23 March the Enabling Act handed Hitler the power to rule by decree without it. Germany’s constitutional government ended within eight weeks of starting the year.

Plate I

Plate II

A hundred and fifty miles to the southwest, on the Rhine at Ludwigshafen, none of this was news to the chemists at IG Farben’s central laboratory. A year earlier, in 1932, the plant’s managing director had taken Herman Mark aside and told him, plainly, that whatever Hitler was about to do would not be survivable for a foreigner whose father had been born Jewish, whatever Mark’s standing in the lab. Mark left for a professorship in Vienna before the Reichstag ever caught fire. He did not need to see 1933 to know what it would bring; he had already been warned by someone who did.

Plate III

What Mark left behind was the process he had spent five years building: a way of turning styrene⬡, a simple oil that Germany could make from its own coal-tar benzene without importing anything, into a hard, clear plastic on an industrial scale. Ludwigshafen had been the world’s only polystyrene plant since around 1930, and it stayed that way through the mid-1930s. The chemistry did not change in 1933. The politics around it did. The new government’s drive to make German industry independent of foreign raw materials rewarded exactly this kind of product (a plastic built from what the country already had, not from an insect’s secretion or a colonial import), and demand for it grew accordingly through the years that followed.

An Accident, Then an Explanation

The molecule itself was almost a century old by then. In 1839, a Berlin apothecary named Eduard Simon distilled an oily substance from storax, the resin of the Oriental sweetgum tree, and called it styrol. Days later he noticed the liquid in his storage jar had thickened into a jelly, and assumed, wrongly, that it had absorbed oxygen from the air; he named the jelly “styrol oxide” and moved on. It took until 1866 for Marcellin Berthelot to recognise what Simon had actually watched happen: not oxidation, but polymerization, molecules of styrol joining end to end into long chains. Even then, nobody could say why a liquid would spontaneously do that. The explanation came from Hermann Staudinger, who argued from 1920 onward that such substances were not loose aggregates of small molecules but genuine giant ones, held together by ordinary chemical bonds. This was a claim the chemical establishment resisted for years before it won him the Nobel Prize in 1953. Polystyrene was one of the materials Staudinger used to make his case.

Plate IV

The Shape of the Molecule

Once the theory existed, the structure was easy to draw. Polystyrene is a plain carbon backbone, two carbons at a time, with a flat six-sided benzene ring hanging off every other one: a long chain wearing a ring on alternating links, like charms threaded at even intervals along a bracelet. Those rings are bulky and rigid, and because there are so many of them crowded along the chain, they stop neighbouring chains from sliding past each other or folding into any regular, repeating pattern. Left alone, the commercial polymer is atactic: the rings sit at random along the backbone, with no order to them at all, which is exactly why it never crystallizes and always sets as a clear glass rather than a cloudy solid. A different catalyst can force the rings into a strict alternating pattern instead (syndiotactic polystyrene), and that version does crystallize, at the cost of the transparency the ordinary material is known for.

What the Numbers Mean

Ordinary polystyrene is denser than water, though not by much, and it is one of the clearest plastics made: light passes through it with almost nothing scattered or absorbed, which is why it was an early choice for lenses and optical parts before it was ever a coffee cup. That clarity comes with brittleness: it is stiff enough to hold a sharp edge or a crisp corner, but it will snap rather than bend, taking almost no stretch before it fractures. Heat does not treat it kindly either: it softens at a temperature well below boiling water, and because the ordinary grade is amorphous it has no true melting point at all; push it hot enough and it simply flows, rather than passing through a sharp transition the way a crystalline plastic does. It burns readily in open air rather than smothering itself, which is one reason foam grades are often treated with a flame retardant before they reach a building site. Chemically it is a mixed bag: dilute acids and alkalis barely touch it, but oils, greases, and the aromatic and chlorinated solvents that dissolve grease will also dissolve or craze polystyrene on contact. This is a fact anyone who has set a plastic fork down in gasoline can confirm. It absorbs almost no water, which is part of why it insulates so reliably even in damp conditions.

From Monomer to Object

The route from raw material to finished plastic starts with ethylbenzene, made from benzene⬡ and ethylene⬡ and then stripped of two hydrogen atoms in a high-temperature catalytic step to yield styrene. From there, free-radical initiators set off polymerization in bulk or in suspension, stitching styrene molecules end to end into chains thousands of units long. Manufacturers can also graft the growing polystyrene onto dissolved polybutadiene rubber first, scattering microscopic rubber droplets through the finished plastic. This is the basis of high-impact polystyrene, tougher and cloudier than the crystal-clear general-purpose grade.

From there, the finished resin becomes an object by one of a handful of routes: injection molding forces the molten polymer into a precision cavity, extrusion pushes it continuously through a shaped die to make sheet or film, and thermoforming reheats that sheet just enough to drape it over a mold. Foaming is its own trade: small polystyrene beads are impregnated with a low-boiling hydrocarbon such as pentane, then heated with steam until each bead swells into a cluster of gas-filled cells many times its original size. How fast the plastic cools after any of these steps decides what you get: a rapid quench locks in the amorphous clarity that makes a clear plastic cup or a CD case, while a slower, warmer cycle relieves the internal stresses that make a molded part brittle at the corners. Both routes start from the same resin; only the schedule differs.

One Molecule, Several Careers

The general-purpose, crystal-clear grade is what most people picture, but it is only the starting point. Blend in rubber and the same backbone becomes high-impact polystyrene, tough enough for the housing of a television or a kitchen appliance instead of the display case around it. Foam it, and it becomes something else again: Dow’s chemists stumbled onto a way of foaming polystyrene by accident in the early 1940s, spent the rest of that decade turning the accident into a product, and sold it from 1954 as Styrofoam, a trademark the public has used ever since for almost any foam plastic, whether Dow made it or not. Expanded polystyrene, made from pre-foamed beads fused together in a mold, is mostly trapped air held in a fine plastic scaffold; extruded polystyrene, made by a continuous process instead, packs that same trapped air into smaller, more even cells, which is why it insulates a little better and takes moisture a little less readily.

Applications: From Coffee Cups to Cell Cultures

That range of forms explains why polystyrene turns up in such different places. The foam cup that keeps a coffee hot is trapped air doing the insulating, at a fraction of the weight of anything else that would do the same job; the same principle, in board form, lines the walls and roofs of buildings, where an inch of foam does the work of several inches of older insulation. The clear, rigid grade goes into jewel cases and food packaging that has to show what is inside it, while the rubber-toughened grade becomes the housing around a television or a printer, molded to close tolerances and able to survive a drop. In laboratories, surface-treated polystyrene dishes and culture plates gave biology a cheap, optically clear, sterile surface to grow cells on. This was an unglamorous but genuine contribution to a century of research.

Plate V

Looking Forward

Ninety years on, polystyrene sits at an uncomfortable crossroads: cheap, useful, and durable in exactly the ways that make it a problem once it is thrown away. That durability has drawn attention from an unexpected direction: researchers have found that certain mealworms and their gut bacteria can actually digest polystyrene foam, breaking the chain back down into smaller fragments the insect can process. It is a long way from a recycling plant, but it is a real crack in a material once assumed to be permanent, and it points toward the kind of chemistry that might eventually close the loop Simon opened by accident in a Berlin storeroom.