Semi-flexible foams

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

The structural semi-flexible and rigid urethane foams, combining high strength with low density and low cost, have found wide interest and early acceptance in divergent markets although this development is still quite new. The semi-flexible foams are used primarily in the transportation industry. At medium densities,... 

The polyisocyanates which can be used for preparing isocyanate-based foams are mainly aromatic compounds and some aliphatic or aralkyl polyisocyanates. TDI (toluene diisocyanate) is widely used for flexible foams. Pure MDI (diphenylmethane diisocyanate) is used for elastomers and coatings. Modified TDI and modified MDI are used for high-resilience flexible foams. Polymeric isocyanates (polymeric MDI or oligomeric MDI) are mostly used for preparing rigid urethane and isocyanurate foams, and in part, for preparing flexible and semi-flexible foams. 

The major industries which utilize flexible or semi-flexible foams are ... 

Other Polyols, Hydroxyl-containing vegetable oils such as castor oil were used for producing semi-flexible foams in the initial stage of the urethane foam industry, but they have not been used much in recent years. 

Semi-flexible foams (or semi-rigid foams) are sometimes classified as subdivisions of flexible foam because the foams have higher load-bearing properties and good compression recovery. Microcellular flexible foams and integral-skin flexible foams may also be classified in this category. In some classifications, however, microcellular foams are classified as elastomers. 

The foam for the construction industry is made mainly without plasticiser. Such an additive is introduced in the manufacturing of flexible or semi-flexible foams. 

Polyurethane foams cellular plastics basically of three types flexible urethane foams used for upholstery, matresses, textile linings, acoustic insulation, etc rigid urethane foams used for heat insulation (eg refrigeration), cold stores, building-panel cores, etc and semi-flexible foams used for arm rests, packaging, etc. 

This section can reasonably be divided into (a) conventional semi-flexible foam moulding, where PVC or ABS skins are first formed and then filled with the semi-flexible... 

Tables. Typical properties of conventional semi-flexible foam.

Integral-skin semi-flexible foams (ISSF)... 

This newer process allows the foam expansion to be controlled so that a cellular core and a non-cellular outer skin are obtained in one moulding process. The raw materials used are similar to those used in conventional semi-flexible foam but water is ommitted. The foam reaction is provided solely by the exothermic heat causing an auxiliary blowing agent (eg Refrigerant 11 or methylene chloride) to vaporise. The heat of reaction will be removed at the mould surfaces, retarding foam formation and producing a coherent urethane skin. 

The isocyanate used is of the less toxic type and may be Table 6. Typical properties of integral skin semi-flexible foam. 

The most interesting development in rigid polyurethane foams recently has been the advent of integral-skin rigid foams. These foams are formed in a similar way to integral-skin semi-flexible foams (discussed earlier) and are finding application in decorative mouldings, furniture pieces, domestic appliance cabinets and the like. In future they may even find application in the automotive industry. 

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