Polyurethane rubbers casting

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. 

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. 

Adiprene [Du Pont], TM for a polyurethane rubber, the reaction product of diisocyanate and polyalkylene ether glycol. In its raw polymer form, it is a liquid of honey-like color and consistency, which is compounded chemically (to polymerize it further) and converted into products by casting and other techniques. 

Vulkollan-type rubbers suffer from the disadvantage that the prepolymers are unstable and must be used within a day or two of their production. Moreover, these rubbers cannot be processed with conventional rubber machiners, so the products are usually made by a casting process. Attempts were then made to develop other polyurethane rubbers which could be processed by conventional techniques. 

Casting. Epoxy, polysulfide, polyester, polyurethane, and silicone A-stage liquids can be poured into a mold, cured, and removed from the mold as finished products. Typical products are simulated wood frames, figurines, and furniture decoration electrical and electronic products and solid polyurethane rubber tires for industrial equipment. 

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... 

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

Thermoplastic polyurethanes do not require compounding as do the conventional rubbers, and exhibit the same general properties as the cast and millable types. 

---