Page 138 - Handbook of Plastics Technologies

THERMOSETS

3.8 CHAPTER 3

Medium-density and semi-rigid foam is produced by polyols of medium molecular
weight and medium functionality, and less foaming agent. These foams are used for crash
padding and packaging.
3.1.1.3.3 Reaction Injection Molding (RIM). This high-speed low-cost process
mixes liquid polyisocyanate and liquid polyol, injects them into a light-weight mold, and
polymerizes/cures rapidly to form large, tough, durable products. It is used to make auto
bumpers, front ends, and other auto parts; furniture and other imitation wood products; ap-
pliance cabinets; and shoe soles.
The process pumps liquid polyisocyanate, liquid polyol, and auxiliary ingredients in-
cluding catalysts, foaming agents, and polyamine for faster cure, through an impingement
mixer at 2000 to 3000 psi, at viscosity up to 3000 Cp, and injects them rapidly into a mold
at 50 to 100 psi, where they polymerize/cure rapidly to structural foam or tough elastic
products. (See Table 3.4.) For rubber tires on industrial equipment, the addition of glass fi-
bers gives reinforced RIM (RRIM) with greater durability under rough conditions.

TABLE 3.4 RIM: Typical Properties

Density 60 pcf
Shore D hardness 60
Flexural modulus 25 kpsi
Tensile strength 6 kpsi
Elongation 250%

Elastomers. Polyurethane elastomers are outstanding for their strength and for resis-
tance to abrasion, oxygen, ozone, and gasoline. This combination of properties has proved
particularly useful in shoe soles and heels, oil seals, industrial tires and wheels, chute lin-
ings, drive belts, shock absorption and vibration damping, medical products, and miscella-
neous industrial applications.
They are made from long, flexible polyols with a light degree of cross-linking. They
may be cast as liquids and polymerized/cured directly to solid rubber products, or they can
be polymerized to linear, melt-processable rubber and then cross-linked by polyurethane
chemistry or conventional rubber vulcanization chemistry. (More recently, they have also
been produced as thermoplastic elastomers, in which hydrogen-bonding and/or crystallin-
ity provide thermoplastic “cross-links,” but that is another story.) This range of process-
ability is attractive to both the thermoset plastics and rubber industries. Cast polyurethanes
give the best properties (see Table 3.5).

3.1.1.3.4 Coatings. Coatings based on polyurethanes can be applied from solution,
from emulsion, or as self-curing liquid systems. The use of low-solvent or nonsolvent sys-
tems is a big help to the coatings industry in meeting the demand for better protection of
the environment. In addition to simple polyurethane homopolymers, their cure reactions
permit coatings technologists to copolymerize them with alkyds, epoxies, and other estab-
lished coatings polymers to produce improved balance of properties.
Their adhesion, mechanical strength, flexibility, abrasion resistance, and chemical and
aging resistance make them particularly useful in steel and industrial products for corro-
sion resistance, on wood for decoration and preservation of furniture and flooring, on ships
for salt-water resistance, and on leather and textiles to upgrade their appearance and dura-

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