Cast polyurethanes Properties

Figure 8.11 Variation of cast polyurethanes properties as a function of hardness...

The thermoplastic rubbers have properties similar to those of the cast polyurethane rubbers but, because of the absence of covalent cross-links, have rather higher values for compression set, a common problem with thermoplastic rubbers. Their main uses are for seals, bushes, convoluted bellows and bearings. 

It is most important to realize that at this time the full properties of the material are not yet developed, and the molded polyurethane has to be returned to the curing oven for the period recommended by the system supplier. The curing time varies depending on the chemistry of the system. The maximum properties of the cast polyurethanes are developed after a further 1 to 2 weeks at ambient temperature. 

Polyurethanes find applications in a wide range of industries in competition with other plastics, ceramics, and metals. Cast polyurethanes are tough, abrasion resistant, have load bearing properties, and are chemical resistant. That makes them the first-choice materials in certain situations. The ability to be easily prototyped and have short production runs is a major positive factor for polyurethanes. 

In general, polishing pads are composed of either a matrix of cast polyurethane foam with filler material to control hardness or polyurediane impregnated felts. Polyurethane is utilized because urethane chemistry allows the pad characteristics to be tailored to meet specific mechanical properties needs. Jairath et al. describe the manufacture of pad material as follows ... 

To examine the effect of various HER chain extenders on the physical, mechanical and dynamic mechanical properties of cast polyurethanes made with MDI-terminated prepolymers. 

The following test methods were used to determine the properties of cast polyurethane elastomers ... 

The main objective of this work was to investigate and study the properties and performances of hard cast polyurethane elastomers obtained from the HER chain extenders. 

C. Zhang and S. Feng, Effect of glycols on the properties of polyester polyols and of room-temperature-curable casting polyurethanes , Polym Int, 2004, 53, 1936-40. 

Some principal suppliers of commercial TPU ranges are given in Table 9.6 and some typical property ranges in Table 9.7. From this data and a consideration of the chemical compositions of the polymer it is obvious that TPUs are very similar in general properties to the cast polyurethanes. 

The chemical properties of cast polyurethane elastomers are similar to those of flexible foams (Section 14.5.1.2). Of particular importance is the outstanding resistance to aliphatic hydrocarbon fuels and oils and to oxygen and ozone these properties make the elastomers useful as seals. Aromatic and chlorinated solvents cause some swelling. 

Saraf, V. P., and Glasser, W. G., 1984, Engineering plastics from lignin. III. Structure property relationships in solution cast polyurethane films. J. Appl Polym. ScL 29 1831-1846. 

The solution-cast polyurethane-semicarbazide films are elastic and transparent, and are characterized by high mechanical properties. The stability of these polymers at low temperatures grows with the increase of the macrochain flexibility, because of the weakening of the intermolecular interactions at lower concentrations of urethane and semicarbazide groups. 

Some typical properties of a Vulkollan-type polyurethane cast rubber and a black-reinforced polyurethane rubber processed by conventional techniques are compared with black-reinforced natural and nitrile rubbers in Table 27.2 ... 

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