As a thermoplastic polyurethane (TPU), Desmopan® is a multiphase block copolymer.
It is selectively produced by a polyaddition reaction from the three basic raw materials polyol (long-chain diol), short-chain diol – also known as chain extender – and diisocyanate, in a specially coordinated process.
The polyols used are generally polyester, polyether or polycarbonate diols with molecular weights of between 1000 and 4000.
The short-chain diols are generally butane diol, hexane diol or aromatic diols.
The diisocyanate used is generally diphenyl methane diisocyanate or hexamethylene diisocyanate.
The diisocyanate provides the chemical bond between the diols. Chemists call the bond “urethane”. The frequency of these bonds in the long-chain polymer molecule then gives the material its name of “polyurethane”.
By controlling the reaction in a targeted manner, it is possible to achieve a bigger or smaller block structure, which is determined by the frequency (separation) of flexible polyol groups and rigid (ordered) chain extenders. This order in the chain-extender blocks is made possible by a physical interaction with the chain-extender blocks of the neighboring polymer chain. These interactions constitute the nodes for the elastic behavior of TPU. At the same time, these reversibly melting nodes are a condition for the thermoplastic behavior of TPU.
The flexible polyol blocks then form the so-called flexible segments (shown as loops in the diagram) in three-dimensional macroscopic space, while the rigid chain extender blocks form the rigid segments (depicted as rods in the diagram).
These rigid and flexible segments form different morphological phases. It is the shape and distribution of the phases, which can also comprise mixed phases of both rigid and flexible segments, which are ultimately the chief determining factors when it comes to the application properties of a TPU.
The non-ordered flexible segments are responsible for the material’s chemical properties and for its resistance to hydrolysis, microbes and chemicals. The key characteristic of low-temperature flexibility can also be adjusted via the flexible segment.
The ordered “crystalline” rigid segments determine the processing behavior, such as the melting and solidification behavior, and also the thermal properties, such as the heat distortion temperature.
These properties of the material can similarly be influenced to a great extent through the production process, i.e. by the order and strength in which the basic raw materials are mixed and by the mixing temperature.
The hardness can be varied over a broad range (60 Shore A to 70 Shore D) through the quantitative ratio of rigid segment to flexible segment, as can the moduli in mechanical tests.
The chain length of the TPU polymer is adjusted via the ratio of diisocyanate to the sum total of diols. TPU molecular weights of Mn 20 000 to 150 000 can be achieved.
It is similarly possible to influence the melting behavior, heat deflection temperature and mechanical strength in this way.