PMMA

PMMA is the transparent plastic chosen when clarity and weathering matter more than impact strength. It transmits more visible light than glass, does not yellow in sunlight, and can be cast in thicknesses that stay optically clear — which is why the walls of large aquaria are acrylic and not glass.

Against polycarbonate it is the harder, clearer, cheaper, more brittle option. Polycarbonate absorbs vastly more impact energy and scratches easily; PMMA resists scratching and cracks. Which of the two is the safety material depends entirely on the threat.

It shatters into relatively blunt fragments rather than the shards glass makes, which is the reason it displaced glass in vehicle rear lights and in glazing where people might fall against it.

Made two ways that give genuinely different materials. Cast sheet is polymerised between glass plates and has the higher molecular weight, the better optical quality and the cleaner laser-cut edge. Extruded sheet is cheaper, more consistent in thickness and easier to thermoform, and is what most signage is.

It thermoforms beautifully — heated to around 160 °C it drapes into a mould — and it is the standard material for a machinist learning to work plastic, because it cuts and polishes predictably. It also cracks readily around a drilled hole under stress, which is the first thing anyone working with it learns.

Aquarium and viewing panels, aircraft canopies and cabin windows, illuminated signage, museum and display cases, vehicle rear light lenses, sanitary ware, and the light guides in older flat panel displays.

In medicine as bone cement in joint replacement, and as the material of intraocular lenses — a use discovered because RAF surgeons in the Second World War noticed that acrylic canopy splinters left in pilots' eyes did not provoke rejection.

Commercialised in 1933 by Otto Röhm as Plexiglas and independently by ICI as Perspex. Its first mass use was military: aircraft canopies, gun turrets and bomb-aimer windows, in quantities that made it one of the war's strategic plastics.

The intraocular lens follows directly from that. Harold Ridley, having observed that acrylic fragments in airmen's eyes were tolerated indefinitely, implanted the first artificial lens in 1949 — an operation his profession rejected for years before it became one of the most common in surgery.

Unusual among common polymers in that it depolymerises cleanly. Heated to around 400 °C it unzips back to methyl methacrylate monomer at high yield, which makes genuine closed-loop chemical recycling technically straightforward — the recovered monomer is indistinguishable from new.

What is missing is collection, not chemistry. Acrylic arrives as scattered sheet offcuts and end-of-life products rather than a stream, so most of it is landfilled despite being one of the few polymers that could be returned to monomer economically.