Page 154 - Handbook of Plastics Technologies

THERMOSETS

3.24 CHAPTER 3

Casting. Unreinforced polyester is poured into an open mold, typically silicone rub-
ber, and cured to produce bathroom sinks, counters, tubs, showers, and toilets; giftware,
art objects, and “cultured marble and onyx” (Table 3.21). Appearance is controlled by
choice of fillers.

TABLE 3.21 Polyester Casting Formulas
Ingredients Cultured marble Cultured onyx

Polyester 100 100
30-mesh CaCO 3 200
80-mesh CaCO 3 100
Alumina trihydrate 200
MEK peroxide 0.6 1.5
Colorants “To suit” “To suit”

Cast monolithic flooring is seamless, resistant to wear and chemicals, and easy to clean.
“Polymer concrete” is polyester filled with aggregate.
Hand layup. The oldest method is a purely manual operation. (1) The open female
mold is first treated with mold release. (2) Optionally, a pigmented gel coat is applied 15
to 20 mils thick and partly cured. (3) Glass fiber mat, or woven or knitted cloth, is hand-
laid into the mold or onto the gel coat. (4) Thixotropic polyester/styrene liquid, containing
MEK peroxide and activator, is impregnated into the cloth. (5) The assembly stands and
cures at room temperature. This gives a product with one good surface and somewhat ir-
regular thickness.
Spray layup. The layup process is partly mechanized by spraying. (1) Mold release is
applied first. (2) Gel coat is sprayed onto the mold surface and partly cured. (3) Glass rov-
ing and catalyzed liquid resin are fed through a gun, which chops the roving, mixes it with
resin, and sprays the mixture into the mold.
Improvements on hand and spray layup. A better gel coat is an acrylic sheet,
which is vacuum formed to fit into the mold. This is used to make tubs, showers, spas, and
toilets.
Styrene monomer emissions sometimes cause occupational and environmental health
concerns. Evaporation of styrene can be reduced by adding wax to the formulation; it is
immiscible and comes to the surface, forming a barrier layer. Another method of reducing
volatilization is to use a less volatile monomer such as methyl styrene.
Vacuum and pressure bag molding. A plastic film can be used to cover the layup. If
a vacuum is used to pull it down onto the molding, this prevents styrene evaporation and
air inhibition of the cure reaction, and it helps to compress the impregnated fiber and elim-
inate empty spots. If air pressure is further applied above the film, this increases the per-
formance still more.
Resin transfer molding (RTM). This uses a closed mold. (1) Gel coat is first applied
to one or both mold halves. (2) The mold is filled with reinforcing fibers and clamped shut.
(3) Low-viscosity catalyzed liquid resin is pumped through a tube into the mold, impreg-
nating the fibers, until the excess resin comes out of a vent in the top of the mold. Resin
“transfer” can be assisted by attaching a vacuum line to the vent. (4) Cure is usually at
room temperature. A low-exotherm resin is usually preferred.

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