Page 139 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.9

TABLE 3.5 Polyurethane Elastomers: Typical Properties
Property Polyurethane Natural rubber
Shore A hardness 81 71

300% modulus, psi 2000 2200
Tensile strength, psi 6300 3800
Elongation,% 600 440

bility. In aerospace, they offer resistance to rain erosion. And in cloth coating, the are su-
perior to PVC for adhesion and freedom from plasticizers.

Adhesives and Sealants. Polyurethanes can be conveniently applied in liquid or
paste form and then polymerized/cured in place without evolution of volatile by-products,
a very convenient feature in making enclosed adhesive bonds. Their mechanical strength,
flexibility, adhesion, and chemical resistance make them attractive in many applications.
Typical applications of polyurethane sealants are in expansion joints, aerospace, architec-
tural, electronic, and marine products.

3.1.2 Formaldehyde Copolymers
Formaldehyde reacts readily with several types of active-hydrogen monomers (phenol,
urea, and melamine) to form highly cross-linked thermoset plastics. They form a family in
their fundamental chemistry, and they form complementary families in terms of materials
properties, markets, and practical applications.
3.1.2.1 Phenol-Formaldehyde. Phenol-formaldehyde resins were the first commercial
synthetic plastics. Since their invention in 1908, they have grown and matured into the
second most important family of thermoset plastics, with a U.S. market volume of 4 bil-
lion lb/yr (see Table 3.6).

TABLE 3.6 Phenolic Resin Markets

Market %
Plywood 49
Adhesives and bonding 30
Laminates 6
Molding compounds 5
Protective coatings 1
Other 9

3.1.2.1.1 Chemistry (Figure 3.6). The phenolic hydroxyl group activates the ortho-
and para-hydrogens. Formaldehyde adds readily to these positions, forming methylol

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