Notes on polyurethane
General information on polyurethane
Polyurethane is a general term for an extensive group of polymers containing mixtures of materials with varying properties. Polyurethane elastomers are included in the so-called CASE (Coatings, Adhesives, Sealants and Elastomers) group of polyurethanes, of which elastomers make up the majority. Polyurethane elastomers amount to approx. 5% of all polyurethanes produced.
Properties of polyurethane elastomers
Polyurethane elastomers are block copolymers consisting of alternating elastic chain segments and highly polar hard segments. Polyurethane elastomers have a Poisson’s ratio of 0.5, which means that their volume remains constant during deformation, and boast the following key properties:
• High strength
• Good wear resistance
• Good resistance to gases, oils and aromatic hydrocarbons
• Excellent oxygen and ozone resistance
• Excellent adherence to metals, especially ferrous metals
• The adhesion strength is almost always higher than the intrinsic tear strength of polyurethane elastomers
However, under certain circumstances, polyurethane elastomers can be susceptible to microbial attacks. The combined effect of high temperature (>80°C) and humidity can also shorten the service life of polyurethane. Where high temperature and humidity do not pose a problem, polyurethane elastomers are far superior to rubbers, for example. However, some polyurethane grades withstand moisture and high temperature better than others do.
Polyurethane elastomer properties notable from a chemical point of view:
• No oxidisation in air, not affected by ozone
• Excellent resistance to aliphatic and aromatic hydrocarbons
• Excellent resistance to oils, lubricants, animal and vegetable fats, and resins
• Good or satisfactory resistance against chlorinated solvents
• Moderate resistance to diluted acids and weak acid concentrations
• Usually do not withstand ethers, esters and ketones
• Need to be tested for resistance to oxidants and alcohols
• Under hot and humid conditions, special attention must be paid to choosing the right polyurethane grade
Thermoplastic polyurethanes (thermoplastics) differ significantly from cast vulcanised polyurethane elastomers. Thermoplastic polyurethanes have clearly weaker properties as compared to cast vulcanised urethane elastomers. The properties of rolled urethane rubbers are also inferior to those of cast vulcanised polyurethane elastomers.
Raw materials of PU elastomers
The raw materials used for the manufacture can be divided into four main groups: isocyanates, polyols, cross-linkers, and additives. In addition, fillers are used. The use of additives is constantly being studied, since many additives may have negative side effects.
Polyurethane elastomers are classified into special plastics/thermosetting plastics. Solid polyurethane elastomers are mainly used in technical components subjected to high static and dynamic loads. Typical products include:
• Stone, cast iron, and coal screens
• Heavy-duty tyres
• Switch parts
• Damping elements