Cast polyurethane rubbers

Poiyurelhane raw rubber + black Polyurethane casting rubber Natural rubber + black Nitrile rubber + black... 

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. 

Latex rubber can be poured like polyurethanes, but they generally are only poured in thin sheets. Even thicker sheets such as Linatex require a very long cure time. Pouring is the method of choice for cast polyurethanes. Normally, rubbers require a compression molding press or injection molding machine to produce parts. This has high capital and mold costs. 

Surface modification also enables excellent interfacial bonding between the rubber particles and the polyurethane. The effectiveness of this surface modification in facilitating adhesion is demonstrated by comparing the bond strength of strips of rubber with polyurethane cast on them. In T-peel tests, it was found that where polyurethane was cast onto untreated strips of rubber, the bond strength was 5N/cm. In analogous... 

Fig. 35. Dissipation factor vs frequency for polar polymers at 25°C. A, Clear cast phenolic B, plasticized PVC C, poly(vinyl chloride-co-vinylidene chloride) (saran) D, unplasticized PVC E, poly(methyl methacrylate) G, poly(hexamethylene adipamide) (nylon) H, poly(2-chlorobutadiene) I, plasticized ethyl cellulose J, cast epoxy K, methyl silicone rubber L, polyurethane foam (d = 33 g/L) and M, 50% polystyrene-50% chlorinated biphenyl.

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 ... 

It is generally more convenient to use the thermoplastic polyurethane rubbers discussed in Section 27.4.4 for those applications where a cast process is not appropriate. 

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