About the Set Properties of TPUs

The network structure of linear and branched TPUs is obviously different. The branched thermoplastics are capable of forming allophanate and/or possibly biuret crosslinks under suitable conditions. These conditions are partly met by the processing temperatures but usually a subsequent hot-air cure is necessary to achieve the optimum set properties. Inevitably, therefore, the branched TPUs have considerably lower compression and tension set properties than the truly all-linear thermoplastic TPUs, and in this respect approximate to the castable polyurethanes. 

These have become commercially important as a means of obtaining a flexible membrane of controlled water-vapour permeability which finds applications as a replacement for leather, laminates for waterproof garments and various medical uses. Polyurethanes are superior to other plastics and rubbers due to their following combination of properties  

The majority of plastics are eliminated because of low flexibility. As the polymer is required to be used in a coagulation process, solubility is essential. However, in many instances this poses problems nylons, for example, are soluble only in inconvenient solvents such as phenols and polyolefins and then only at very high temperatures. Unmodified PVC is too rigid and, although plasticization confers flexibility, the mechanical properties of plasticized PVC are relatively poor. In a coagulation process loss of plasticizer will cause difficulties. 

Conventional elastomers only develop acceptable mechanical properties when they are crosslinked. This would preclude the possibility of a recovery process for scrap. In addition, for the type of product envisaged, the snappy recovery of a conventional rubber would be inappropriate as a leather substitute. 

Permeability to gases and liquids can be introduced into polymeric materials in a number of ways, including  

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