Polyurethane cross-linked rubbers

Thermoplastic elastomers (TPE s) are characterized by the exceptional property that, without vulcanization, they behave as cross-linked rubbers. They are block-copolymers, in which blocks of the same nature assemble in hard domains, acting as cross-links between the rubbery parts of the chain. These hard domains lose their function when they reach their softening temperature, so that the material can then be processed as a thermoplast. One of the oldest member of the family of TPE s is SBS (styrene-butadiene-styrene block copolymer), but several other TPE s have been developed, i.a. on the basis of polyesters, polyurethanes and polyolefins. In their properties these polymers cover a broad range between conventional rubbers and soft thermoplastics. 

Properties and Applications of Cross-linked Polyurethane Rubbers... 

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. 

Properties and applications of cross-linked polyurethane rubbers... 

Where fumes or deposits which act as anti-oxidants are present, no orthodox paint which dries by oxidation can give satisfactory service. Instead, a coating which dries either by evaporation (e.g. a selected chlorinated rubber paint), or by a cross-linking reaction (e.g. a catalysed epoxy or two-component polyurethane paint) must be used. 

Castable polyurethanes can be formulated to contain conventional crosslinks, as in rubbers and epoxies. They may be either in the form of the liquid system or in the form that requires conventional rubber-processing equipment. The introduction of conventional cross-links will break the formation of the hydrogen bonds and normally reduce the hardness of the finished product. In very hard grades, they are used to increase the compression resistance. 

The millable materials have a diene group (-CH=CH-) included in the polymer to allow for cross-linking by sulfur or by peroxides. However, the capital costs to use millable polyurethanes are high because rubber mills, presses, and heavy-duty molds are needed to produce an article. 

Sealants are often based on butyl rubber, polyisopropylene, polyurethane, or silicone rubber. Reactive sealants may be vulcanized at room temperature (RTV) or at high temperature (HTV). Silicone sealants are chemically-hardening, single-component systems in which cross-linking of the substrate is initiated by moisture from the air. This separates the reaction products. After rapid formation of a skin on the surface, cross-linking continues inside the mixture until it is fully hardened. 

Figure 3.8 Storage and loss moduli versus reduced frequency in small-amplitude oscillatory deformation for a cross-linked polyurethane rubber, Sorbothane 70, at 20°C. The imposed deformation was uniaxial extension, yielding tensile moduli E and E" which were converted to shear moduli using G = E /3 and G" = E"/3. The data at 16 different temperatures ranging from —81°C to 80°C were collapsed to a master curve using methods described in Section 3.5.2, (From Larson et al. 1996, reprinted with permission from Steinkopff Publishers.)...

Use Dye manufacture synthetic fibers, resins, films UV absorption pharmaceuticals rubber chemicals cross-linking agent for polyurethanes and polysulfides. 

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