Polyurethane

Polyurethane is a linkage rather than a single material. A diisocyanate reacts with a polyol, and by varying both — chain length, functionality, branching — the same chemistry produces a soft flexible foam, a rigid insulating foam, a tough elastomer, a coating, an adhesive or a fibre.

That range is why it is everywhere and why the name tells you almost nothing about the properties. A mattress and a skateboard wheel are both polyurethane, and no useful single figure describes both.

The foams are made by letting water react with some of the isocyanate, which releases carbon dioxide and blows the mixture as it cures. The gas is generated by the reaction itself rather than added, which is a genuinely elegant piece of chemistry.

Two liquid streams metered and mixed at the point of use, which is unusual and consequential: the material is made at the same moment as the part. Slabstock foam is poured onto a moving conveyor and rises into a continuous loaf; moulded foam cures in the shape of a car seat; spray foam is mixed at the gun and expands against the surface it lands on.

Cast elastomers are poured into moulds and cured — that is how a skateboard wheel and an industrial roller are made. Thermoplastic polyurethane is the exception to all of this: a linear grade that can be injection moulded and recycled like any thermoplastic.

Flexible foam: mattresses, upholstery, car seats, carpet underlay. Rigid foam: the insulation in refrigerators, cold stores, sandwich panels and insulation board, where it outperforms mineral wool per unit thickness because the cells hold a low-conductivity gas rather than air.

Elastomers: skateboard and rollerblade wheels, industrial rollers, conveyor scrapers, shoe soles, and the flexible parts of ski boots. Coatings: floor and deck varnish, vehicle clear coats. Adhesives, including most construction foam in a can. And as spandex, which is a segmented polyurethane fibre.

Developed by Otto Bayer's group in Germany from 1937, initially as an answer to DuPont's nylon patents — the goal was a fibre, and the foams and elastomers that followed proved far more important.

Flexible foam transformed furniture and bedding in the 1950s, replacing horsehair, feather and spring construction. Rigid foam did the same for refrigeration a decade later, and the resulting improvement in insulation is a substantial and rarely credited part of the fall in domestic refrigerator energy use.

Thermoset foams cannot be melted and reprocessed. Flexible foam is rebonded — shredded, glued and pressed into carpet underlay, which is a genuine second life and a downgrade. Rigid foam largely is not recycled at all. Thermoplastic polyurethane, being linear, recycles conventionally.

The blowing agents have their own history. Rigid foams used CFCs until the Montreal Protocol, then HCFCs, then HFCs — each with a smaller ozone impact and, for the HFCs, a large global warming potential of their own. The industry is now moving to hydrocarbons and hydrofluoroolefins. Three decades of insulation in existing buildings still contains the earlier agents, and releases them when demolished.

Flexible polyurethane foam is also a significant fire load in furniture. Combustion produces hydrogen cyanide as well as carbon monoxide, which is why furniture fire regulations exist and why the flame retardants used to satisfy them became a controversy of their own.