Page 133 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.3

Isocyanates also react with water. The intermediate carbamic acid is so unstable that it
decomposes immediately to form amine plus carbon dioxide. This reaction is important
for two reasons: (1) carbon dioxide bubbles foam the polyurethane as it forms; this is the
leading process for making foam, and (2) the amine by-product reacts to form more urea
groups, which therefore strengthen the final polymer.
Isocyanates have several more reactions that are important in some more specialized
applications (Fig. 3.2). Cyclotrimerization produces the isocyanurate ring, which is ex-
tremely stable, and can be used to build more heat resistance into polyurethanes. Excess
isocyanate can react with the N-H group in polyurethanes to produce allophanate cross-
links, which add to the cure of the polyurethane. And excess isocyanate can similarly react
with the N-H groups in polyureas to produce biuret cross-links, which add to the cure of
the polyurea.

FIGURE 3.2 Specialized isocyanate reactions.

3.1.1.2 Raw Materials. The versatility of polyurethanes is due to the variety of raw ma-
terials that can be used to build different structures into the polymers.
3.1.1.2.1 Isocyanates (Figure 3.3). Toluene diisocyanate (TDI) is a mixture of
mostly 2,4- plus some 2,6-isomer. Two commercial ratios are 80/20 and 65/35. The 4- po-
sition is more reactive; the 2- and 6- positions are sterically hindered. This gives the pro-
cessor the ability to make prepolymers (oligomers) and run two-stage reactions.
Methylene diisocyanate (MDI) in the pure form gives a symmetrical structure that per-
mits the processor to build some crystallinity, and thus greater strength, into the polymer.

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