Lego bricks, clarinets and keyboard keys share one tough three-part plastic
Since 1963, Lego bricks have mostly been moulded from ABS, a plastic built from three ingredients that each contribute something different. The same material turns up in drainpipes, golf club heads, recorders, trombones, computer keycaps and 3D-printer filament. Sunlight is its weakness: degraded ABS seatbelt buttons triggered one of America's biggest car recalls.
ABS stands for acrylonitrile butadiene styrene. Chemists make it by polymerising styrene and acrylonitrile around rubbery polybutadiene, producing long rubber chains crisscrossed by shorter, stiffer ones. Recipes vary, from roughly 15 to 35 percent acrylonitrile, 5 to 30 percent butadiene and 40 to 60 percent styrene. Acrylonitrile's polar groups pull neighbouring chains together, adding strength, hardness and chemical resistance; styrene provides a glossy, impervious surface; and the elastic butadiene particles absorb knocks. More butadiene means better impact resistance, which holds up well even in the cold.
The plastic was patented in 1948 and brought to market by the Borg-Warner Corporation in 1954. It has no sharp melting point, since its chains vary in length, and softens at around 105 °C. Makers tune it during processing: moulding hot improves gloss and heat resistance, while moulding cool maximises toughness. Raw ABS is a translucent ivory, and most of it is opaque because its components bend light differently; adding methyl methacrylate makes a transparent version that is still tougher than acrylic.
Its drawbacks are real. Ultraviolet light and heat break its chains, causing yellowing and brittleness. It dissolves in acetone and struggles against alcohols and chlorinated solvents. In a fire it melts, boils and then burns fiercely, releasing carbon monoxide and hydrogen cyanide, and above 400 °C it can break back down into its three starting chemicals. Producing a kilogram in Europe uses about 95 megajoules of energy from natural gas and petroleum, and not every recycling plant accepts it.
Its strengths keep it everywhere else. Hobbyists print with it because it is cheap, strong and easy to sand or paint, though it warps as it cools unless printed on a heated bed or in an enclosure. Hospitals use it for inhalers and tracheal tubes, Lamy makes its Safari fountain pen from it, and finely ground ABS even colours some tattoo inks.
Source: Acrylonitrile butadiene styrene
