PUR (Polyurethanes, aka Urethanes, Polyurethane Rubber)
The unique thing about plastics is that, having such vastly differing properties, they can be applied to every conceivable type of product. Unlike a carpenter or ceramicist, who works directly with their materials, the problem for a product- or industrial designer who works with plastics is that to really understand the different grades, they will have to go through the entire process of specifying, using and applying these materials. I will try to simplify this large family.
Urethanes are one of the five major groups of polymer classifications; the others are ethylenes, styrenes, vinyl chlorides and esters. Part of the reason they can be converted into so many different forms is that, like PVC, they can be produced as thermosets, thermoplastics and in rubbery forms. The versatility of polyurethanes means that there are many forms and grades that can be specified. These can be split into three broad areas, which are coatings, rigid and flexible foams, and rubbers. In the context of design, the most useful form is rubbers, or PUs, for their high abrasion resistance and overall toughness.
In terms of abrasion resistance, they can be compared with Nylons and acetals, with some elastomeric forms of polyurethanes being 20 times more resistant to scratching than metals. In terms of flexibility, they are similar to TPEs but without the adaptability of a range of moulding techniques; however, they are much less expensive than silicones or EVAs.
Image: Mesa Table by Zaha Hadid and Patrik Schumacher
Key features
•Excellent toughness
•Excellent flexibility
•Excellent abrasion resistance
•Excellent cut resistance
•Excellent chemical resistance
•Good resistance to weathering
•High flex life
•Good impact strength
•Generally not recyclable
Sources
Widely available from multiple global suppliers.
Cost
TPU: £2.55 ($4) per kg.
Sustainability issues
Low temperature moulding saves energy and is the most common form of reusing PUs. Low temperature moulding is when the material is re-ground and bound together.
Production
In order to understand the processing of PUs, the various forms need to be understood. As a thermoset material – foams – it is limited to reaction injection moulding. As a TPU, it is suitable for a range of production methods, including injection moulding, compression moulding, casting, extrusion and also spraying.
Typical applications
PU foams are used as insulation in buildings, and in a different form as furniture cushioning and bed mattresses. In its rubber form, it is used in squeegee blades, vandal-proof street bollards, rollers for castor wheels, springs and shock absorbers. In consumer applications it is used for the soles of running shoes and shoe heels, fabric coatings and furniture.
Derivatives
–Polyurethane foams
–Memory foams
–Spandex® and Lycra®
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–Widely available –Enormous range of applications –Strong and resistant –Hardwearing |
–Understanding the different grades of this vast materials family can be a minefield –Not generally recyclable |