Page 137 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.7

catalysts, polyol/silicone surfactant, and a measured amount of water in a one-step pro-
cess, and it is then poured onto a moving belt. Foam rise takes about 1 min. Optimum soft
properties depend on open (interconnecting) cells; these are produced by choice of surfac-
tant, gas expansion while the molecular weight (melt strength) is still low, and mechanical
crushing and re-expansion. This produces continuous slab stock, which is then cut into the
desired individual products. About 70 percent of flexible foam is made in this way. The
other 30 percent is poured into molds to make the finished products directly. This is used
especially for auto and furniture seating. (See Table 3.2.)

TABLE 3.2 Flexible Polyurethane
Foams: Typical Properties
Density 2 pcf
Modulus 1–10 psi

Tensile strength 28 psi
Elongation 300%
CLD/25% 0.7 psi

3.1.1.3.2 Rigid Foam. Rigid foam is used primarily for thermal insulation. Whereas
polystyrene foam must be molded and/or cut to shape before it can be used in finished
products, liquid polyurethane ingredients are mixed, poured or sprayed in place, and poly-
merize/cure directly to the finished insulation. In addition, polyurethane foam has high ad-
hesion to most surfaces in which it is used so, when it is poured into a sandwich structure
and cured, it contributes to mechanical strength as well. Its largest use is in building, and
the second largest in refrigeration. Other applications include pipes, tanks, trucks, railcars,
packaging, and filling empty space in shipbuilding for flotation purposes.
Rigid foam is produced by mixing polymeric MDI with low-molecular-weight poly-
ether polyol, high-functionality polyol for cross-linking, catalysts, and surfactants as
above. Chlorofluorocarbons are technically the best foaming agents, but, because of their
negative effect on the environment, they have been replaced by hydrocarbons or carbon di-
oxide.
Optimum insulation is achieved by low-density, small, closed cells. This foam struc-
ture is produced by choice of surfactants and by control of the temperature and the balance
between rate of polymerization/cure (viscosity = melt strength) versus the rate of gas evo-
lution. (See Table 3.3.)

TABLE 3.3 Rigid Polyurethane Foams:
Typical Properties
Density 2 pcf
Flexural modulus 70 psi
Tensile strength 50 psi
Flexural strength 60 psi
Compressive strength 50 psi

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