Page 165 - Engineering Plastics Handbook

138 Engineering Plastics

10 8
Flexural modulus (MPa × 10 3 ) 6 4 PBT (neat) PBT (GF 30%)

0 2 HDT (18.6 MPa)
0 50 100 150 200
Temperature (°C)
Figure 7.5 Heat deflection temperature and modulus for PBT.

compounding and molding process, glass fiber breakage occurs and
results in an average size of 0.2 to 0.4 mm in the molded product. The
properties of short glass-fiber-reinforced grades show a much higher flex-
ural modulus and tensile strength but lower elongation at breakage
compared to the unfilled PBT. The reinforcements of glass fiber have
some drawbacks such as reduced flow, poor surface appearance, and
increase of part densities. The overall shrinkage values of glass-fiber-
reinforced grades will be decreased [7]. Usually, reinforced glass fibers
are not randomly oriented in a molded sample and so result in some
directional part anisotropy. The part shrinkage values are different in
the flow and perpendicular directions, with the highest orientation in
the flow direction, which can lead to warpage problems.
Dimensional stability and anisotropy are the main reasons why mineral
filler or glass fiber/mineral hybrid reinforced PBT grades have been devel-
oped. Common mineral fillers are talc, mica, silica, clay, wollastonite, glass
sphere, glass flake, and milled glass fiber. Mineral fillers are added to
PBT to improve stiffness and dimensional stability and to reduce shrink-
age. Mineral reinforcements increase the flexural modulus and strength
but are inferior to fiber-reinforced compounds. Mineral fillers reduce ther-
mal shrinkage values as a function of loading amount and are added to
enhance dimensional stability. Combinations of glass fiber and plate-
shaped mineral fillers, hybrid reinforced grades, are often used to bal-
ance warpage characteristics and mechanical properties. These properties
are in between those of glass-fiber-reinforced and mineral-reinforced
grades. Table 7.3 shows summary of physical properties for typical types
of reinforced PBT resins.