nylons 66 and 6. Where a thermoplastic for light engineering purposes is required with a low water absorption, nylon 11, acetal resins and, in certain instances, polycarbonates are cheaper and, usually, at least as satisfactory.

The resulting “prepolymer” can then be chain extended with water, glycols or amines by linking cross terminal isocyanate groups:

The water reaction evolves carbon dioxide and is to be avoided

cross-linking is increased by incorporating triols in the original polyester. In contrast to results with hydrocarbon rubbers the greater the degree of cross-linking the lower is the tensile strength and tear resistance but the higher the elasticity. This effect is believed to be due to the fact that in rubbers of the Vulkollan type much of the strength of the material is derived from secondary forces, hydrogen bonding in particular. Cross­linking not only interferes with the effectiveness of such secondary forces but it also restricts crystallization. Corroborating evidence for this is that at elevated temperatures, where secondary forces are greatly reduced, the more highly cross-linked polymers are rather stronger. The fact that the chain consists of blocks of hard polyurethane segments separated by soft polyol segments is also a contributing factor.

A serious disadvantage of the Vulkollan system is the necessity to use the prepolymer almost immediately after manufacture. In 1958 Du Pont introduced Adiprene С prepolymers (now marketed by Uniroyal). In this case the polyol was the polyether polytetramethylene glycol with tolylene diisocyanate as the isocyanate, the latter ingr edient providing the key to the storage stability. Somewhat inferior products were obtained by using glycols as extenders, with better results being obtained using amines such as 3,3'- dichloro-4,4'-diaminodiphenyl methane (MOCA), although concern about the toxicity of this additive has led to the search for alternative materials.

7.2

15

700

16

460

128

363

5.6

In a typical manufacturing process a prepolymer is first produced by reacting a linear polyester with terminal hydroxyl groups, or a similar polyether with molecular weights in the range 800-2500, with an excess of di-isocyanate (usually of the MDI type) to give an isocyanate-terminated polyol prepolymer plus free (unchanged) isocyanate. This blend is then reacted with a chain extender, usually a glycol such as 1,4-butane glycol, to give a polymer with hard polyurethane segments whose block length depends on the extent of excess isocyanate (and corresponding stoichiometric glycol). It is possible that an excess of isocyanate may react with urethane groups in the chain to produce allophanate cross-links. These do not, however, destroy the thermoplastic nature of the polymers because of their thermal lability, breaking down on heating and reforming on cooling. However, where amines have been used as chain extenders urea groups are produced and these on reaction with excess isocyanate give the more stable biuret cross-links.

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

(which contains approximately three hydroxyl groups in each molecule).

Three general processes are available known as one-shot, prepolymer or quasi-prepolymer processes. These variations lead to 27 basic types of product or process, all of which have been used commercially. This section deals only with flexible foams (which are made only from polyesters and polyethers). Since prepolymers and quasi-prepolymer processes are no longer important with polyesters, the four following types only will be considered here: