Page 153 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.23

TABLE 3.19 Polyester Properties Increase with Glass Fiber Length
Process BMC SMC Layup Filament winding
Fiber length 1/4 in 1 in Woven fabric Continuous filament
Flexural modulus 1750 1600 2000 6000 kpsi

Flexural strength 16 26 60 175 kpsi
Impact strength 7 15 18 50 fpi

Viscosity must be controlled for most processes. For fluid processes like spraying and
impregnation, it can be decreased by using lower molecular weight or higher styrene con-
tent. For many processes, it is increased by adding 0.1 to 2.0 percent of thixotropes such as
silica, clay, and polyols. For leather-like tack-free BMC and SMC compounds, the polyes-
ters are made with acid end-groups and then reacted with CaO or MgO to link them into
higher-MW organometallic oligomers.
Profile is a problem when cure and shrinkage of the polymer matrix leave glass fibers
at the surface, giving a rough profile. This is reduced by dissolving thermoplastic poly-
mers such as polyvinyl acetate in the liquid system; since it does not react, it reduces the
overall shrinkage of the system and thus retains a smoother profile.
Ultraviolet stability for outdoor use can be improved by opaque pigments that reflect
the UV light before it can penetrate the polymer; titanium dioxide and aluminum flake are
frequently used for this purpose. Ultraviolet absorbers (UVAs) such as hydroxybenzotriaz-
oles and hydroxybenzophenones are sometimes used, and hindered amine light stabilizers
(HALS) are becoming more popular.

3.1.3.1.3 Processes. Unsaturated polyesters are usually reinforced by glass fibers.
This complicates conventional plastic processing, and has led to a great variety of special-
ized processes.
Viscosity. Different processes require different viscosity. This is most easily lowered
by adding styrene monomer to the polyester oligomer (Table 3.20). It is raised by increas-
ing polyester molecular weight and by adding thixotropic fillers. In addition, it is raised to
the point of gelation by adding group II metal oxides to react with the acid end groups of
the polyester, thus dramatically increasing molecular weight.

TABLE 3.20 Polyester Viscosity Is
Controlled by Styrene Content

Styrene monomer Viscosity, cP

25 5500
30 2000
35 550
40 200
43 100

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