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Polyurethane resin (PUR)
1.5-5.8 usd/kg
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
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Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Low circularity potential
Microplastics
Polyurethane resin (PUR) coating provides durability, resistance to chemicals, water and abrasion, as well as an attractive glossy, matte, transparent or coloured finish. While they are similar to epoxy coatings in many ways, what sets them apart is enhanced toughness, resistance to dents and scratches, good temperature range and very good adhesion to a wide range of materials.
There are four main families of PUR coating: one-part moisture curing, two-part, oven-curing, and powder coating.
Moisture curing PUR does not need to be mixed, and is applied directly to the substrate. This makes it convenient for coating exterior structures, such as steel bridges, cranes and concrete.
Two-part PUR systems are either waterborne or solvent-based. They are used for demanding applications, such as aerospace, marine and automotive. They often provide part of a multi-layer coating system, such as the clear topcoat covering an epoxy base layer. In addition, multiple layers of PUR may be required in some areas to provide greater durability. For example, one layer may be enough for light vehicle traffic in a parking garage, but multiple layers will be required in areas subject to more abrasion, such as turn lanes or winter tyres.
PUR coatings are commonly used in automotive bodywork and interior applications. For exterior parts, like car bumpers, a primer, base layer and clear coat are typically used. While the primer and base coat may be water-based, the clear coat is typically high-solids. Waterborne PUR coatings reduce volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions by 50–90%. This is achieved by replacing most or all of the VOC and HAP used in conventional solvent-borne PUR coatings with water as the carrier. Similar coating technology is used for furniture and household items, sports equipment, agricultural and industrial equipment, floors (from engineered wood to busy commercial spaces), and engineering structures.
Oven curing PUR is a one-component systems used to coat metals, such as automotive bodywork. They work by blocking the isocyanate at room temperature. Heating to 100-200 degC unblocks the isocyanate and allows cross-linking to occur.
Polyurethane powder coating is oven-cured to a durable and resistant finish. It is more expensive and higher performing than polyester. It is used to coat metal for automotive, furniture and home appliances.
An alternative approach to achieve a PUR coating is to over-mould an existing material. It is a mix of moulding and coating, carried out inside a mould cavity, which creates a precise finish that may be textured. For example, injection moulded car interior parts are over-moulded with PUR to create a mirror-like glossy finish, or soft touch haptics. This technique is known as direct skinning (Bayer), CoverForm (Krauss Maffei and Evonik Rohm) and Flow-coating (Engel), for example.
