Page 136 - Handbook of Plastics Technologies

THERMOSETS

3.6 CHAPTER 3

economical. In large-scale commodity production, this is usually the ultimate develop-
ment.
The alternative is a two-stage process. In the first stage, polyol is mixed with excess
isocyanate to form a low-molecular-weight polyurethane with isocyanate end-groups. In
the second stage, the isocyanate end-groups are reacted with the stoichiometric amount of
polyol to finish the polymerization reaction, or with water to link them into polyurea
groups.
A more extreme two-stage process is called “quasi-prepolymer.” Here, all the isocyan-
ate is mixed with a small amount of polyol in the first stage. Then, the remaining polyol is
added for the second-stage polymerization to high molecular weight.
These two-stage processes give the processor more control over the reaction and the
product.

3.1.1.3 Polyurethane Products (Table 3.1)

TABLE 3.1 Polyurethane Markets

Material % %
Flexible foam 51
Furniture 18
Transportation 13
Rug underlay 11
Bedding 5
Other 4
Rigid foam 26
Building insulation 14
Home and commercial refrigeration 5
Industrial insulation 2
Packaging 2
Transportation 1
Other 2
Reaction injection molding 6
Transportation 4
Other 2
Cast elastomers 2
Other (sealants, adhesives, coatings, etc.) 15

Total 100

3.1.1.3.1 Flexible Foam. Compared to foam rubber, polyurethane is stronger and
much more resistant to oxidative aging and embrittlement. Compressive stress-strain be-
havior can be matched to that of natural rubber, which established the preferred “feel”
long ago. The largest amount of flexible foam is used for cushions in furniture, auto seat-
ing and crash-padding, rug underlay, and mattresses. Smaller amounts are used in shoe
soles, winter clothing, and packaging.
Most flexible foam is manufactured by mixing 80/20 TDI with a high-molecular-
weight polyether polyol, a small amount of triol for cross-linking, amine and organotin

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