Polyurethane resin (PUR) is an extremely versatile and durable plastic that may be thermoplastic (TPU – formed by melting) or thermosetting (PUR – formed by one-way chemical reaction). By adjusting the chemistry of the reaction, it is possible to make everything from highly elastic to rigid plastic, and memory foam to durable coatings. This incredible range of properties means PUR has ended up in a very diverse range of applications: technical coatings, coated textiles (artificial leather), adhesives, soft energy-absorbing foam, rigid insulating foam, moulded plastics, synthetic rubber and fibre reinforced composites.

PUR is characterised by excellent resistance to water, oil and grease. It adheres very well to a range of other materials and surfaces, useful in coating and composite applications. And it is more durable and resilient that TPU, due to the permanent cross-links formed in the polymer structure. As a two-part cast in-situ material, uses range from one-offs and prototypes through to mass-production projects. It is equally well suited to very small items as it is to parts weighing tons. It can be coloured, and there are additives available to enhance UV shielding, conductivity and mechanical properties.

Polyurethane is a block copolymer made up of alternating hard and soft segments. The soft segment is made up of polyol, and the hard segment is diisocyanate combined with chain extender. The soft segment provides elasticity, toughness and resilience. And the hard segment contributes strength, hardness and temperature stability. Performance is defined by the chemistry of the hard and soft segments, as well as their ratio and chain length.

In the case of thermosetting plastics, the polymer structure is formed in the final shape of the product being made, such as by casting. Cross-links are formed, which restrict the movement of the segments. The more cross-links there are, then the higher the rigidity of the material, and the fewer there are then the more elastic it is. There are a range of options and therefore, polyurethane may be tailored to a diversity of uses.

The polyol (organic compound containing multiple hydroxyl groups) is typically polyether, polyester, acrylic, polycaprolactone or polycarbonate. Each have their own qualities and uses. Polyether and polyester polyols are the most common in PUR plastics, used for flexible and rigid plastics, respectively; acrylic polyols are good for weather resistant coatings; polycaprolactone in applications that require enhanced flexibility, high durability, performance over a broad temperature range and water resistance; and polycarbonate polyol is used in the most demanding applications, such as glass coating and 3D printing, where it offers superior strength, temperature stability and chemical resistance. Bio-based PUR (bio-PUR) is produced with a polyol derived from castor oil, for example.

The isocyanate is typically diisocyanate methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI), or a combination of the two. While MDI is generally used to make rigid parts, such as insulation foam, TDI is mainly used in the production of lightweight flexible foams. These are extremely poisonous and explosive chemicals. Isocyanates are given off in the reaction during forming. The MDI system produces fewer than the TDI method, but both are considered hazardous.

As a thermosetting plastic, PUR cannot be recycled into new material. It tends to be incinerated, or ground up into chips that are used as a filler, such as in new PUR foam products. It is possible to recycle the polyol from post consumer PUR by chemical means. While it is being done commercially, it is not widespread