Page 149 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.19

TABLE 3.14 Melamine-Formaldehyde Moldings: Typical
Properties
Specific gravity 1.5
Rockwell hardness M120

Tensile modulus 1,300,000 psi
Flexural modulus 1,100,000 psi
Tensile strength 10,000 psi
Flexural strength 15,000 psi
Compressive strength 39,000 psi
Notched Izod impact strength 0.3 fpi

Mold shrinkage 0.9%
–6 o
Coefficient of thermal expansion 43 × 10 / C
o
Heat deflection temperature 185 C
11
Volume resistivity 10 Ω-cm
Dielectric constant 7

Water absorption 0.5

tance of any plastic, along with resistance to heat and staining. Their major use is in house-
hold dinnerware (plates, bowls, cups, and glasses), where they are much lighter and more
impact resistant than china. Market volume of 7 percent also includes a number of minor
items such as buttons, handles, knobs, small appliances, sinks, and toilets.

3.1.2.4 Furan Resins. Furfuryl alcohol is an agricultural by-product, which is polymer-
ized and cured by acid catalysts, producing a very hard plastic that is very resistant to heat,
flame (low smoke), water, and chemical attack (Table 3.15). Limitations are black color
and brittleness.
Initial polymerization is catalyzed by acid, and very exothermic, so it must be cooled
and neutralized to produce liquid dimers and trimers (Fig. 3.14). These are then cured by
strong acid such as 4 percent of p-toluene sulfonic acid. They may also be copolymerized
with phenol-formaldehyde.
Reported uses include sand molds for metal foundries and linings for chemical plant
equipment such as reaction vessels, tanks, pipes, fume ducts, and sewers.

3.1.3 Vinyl Polymers
Several families of thermosetting plastics are based on cross-linking through the C=C vi-
nyl group. The earliest, of course, were the vegetable oils used in paint for thousands of
years, but their cure by atmospheric oxygen was limited to thin coatings and too slow to be
useful in plastics. The leading family is the unsaturated polyesters, which form the basis of
most reinforced plastics. More specialized families are the so-called “vinyl esters” and the

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