Acrylic Rings

This transparent hoop is made of acrylic and was probably injection moulded to get a perfectly clear, bubble-free object, although one sneaky bubble has made it into this example! The plastic is introduced into the mould in its liquid state with an added catalyst that causes it to ‘cure’ or set to become the solid ring we see before us. It heats up to about 85°C as it hardens in a process called thermosetting. This acrylic resin is a widely used, stable, strong and versatile material, which can be cast into almost any shape, colour or finish, and re-formed using heat. These rings might well be sold wholesale as display components to hang silk scarves from, or sold by the packet in craft shops for a variety of uses.

We rarely notice this naturally transparent, cheap, lightweight and shatter-proof substitute for glass, whose pseudonym – Perspex – derives from the Latin ‘to see through’. Acrylic is a material of many names, otherwise known as Perspex, Plexiglas, Lucite, Acrylite, Polycast and Altuglas, to name but a few of its alter egos. These substances are all essentially variants of the same plastic made from a chemical called methacrylate.

Poly(methyl methacrylate) - or PMMA for short for us lazy materials enthusiasts - is the most common acrylic plastic. It was first developed in the early 1900s by a German chemist. Since then, the process for making acrylic has been refined to give us a highly transparent thermoplastic that is relatively scratch resistant (for a clear plastic), very stable in UV light (it doesn’t yellow in sunlight like the celluloid that came before it), and although it’s quite brittle by comparison to more impact-resistant plastics like polycarbonate, much less likely to shatter than glass.

Because it is one of the clearest plastics we have, acrylic first found favour in the 1930s as a glass substitute in safety goggles, as well as World War Two military applications like gas masks, submarine periscopes and cockpit windows. Because of this superb clarity, acrylic is also the material of choice for enclosures around exhibits in museums and galleries, including Damien Hirst’s array of unfortunate perspex-encapsulated and formaldehyde-pickled animals.

You probably also have acrylic to thank for your next endoscopy: PMMA is a cheaper, more flexible and more robust alternative to glass optical fibres, which comes in handy when we need to shine a light inside our bodily cavities to see what’s going on in there. Check out our Materials Library entry on plastic optical fibres for more info.

Acrylic is also quite well-suited to being implanted into our bodies. Although the first clinical application of PMMA was in 1930s, when it was used to repair cranial defects in monkeys, acrylic wasn’t used to repair human bodies until the 1940s. Its biocompatibility was discovered accidentally when World War Two pilots whose eyes were injured by shards of broken acrylic from plane cockpits fared much better than those affected by shards of shattered glass. These tragic injuries led directly to the invention of the first prosthetic intraocular lens: a dome of acrylic that was implanted into the eye to replace the natural lens as part of cataract surgery.

Acrylic is also used extensively used in dentistry, where its similarity to human dentine has seen it take over from ivory, animal bone and human teeth as the denture, dental implant and dental crown material of choice. Orthopaedic surgeons have also caught on to acrylic’s advantages, developing PMMA ‘bone cement’: a kind of space-filling plastic ‘grout’ that sits between our bones and an artificial knee or hip joint, absorbing shock and distributing pressure.

As a group 7 plastic, acrylic is not easily recycled and is not often collected for recycling, despite being widely used throughout the world, so large producers are looking for more sustainable alternatives.

To learn more about acrylic’s many talents, including its use in piano keys, Cardi B’s glorious nail art, David Hockney’s technicolour scenes of Californian suburbia and more, read our long-form blog post.