Chemistry Reference
In-Depth Information
The highest modulus and tensile strength obtained in this study were
26.7 GPa and 315 MPa for the fiber of PET/80HBA spun at 275
◦
C and
a draw-down ratio of 400.3. The properties of the copolymer with 80 mol-%
HBA were always higher than that of the copolymer with 60 mol-% HBA.
5.5. COMPOSITES REINFORCED BY LIQUID
CRYSTALLINE POLYMERS
Composites are multiphase materials of two or more components with
different physical and chemical properties. The history of polymer com-
posites is almost as old as the history of polymers. The success of the
phenol-formaldehyde resin achieved in the beginning of this century by
L. H. Baekeland was largely based on the addition of an inert filler (wood
powder or asbestos) into the resin, together with an advanced curing pro-
cess of heat and pressure. While the heat and pressure allowed the curing
reaction to be completed in a short time without the gassing, bubbling
and distortion, the inert filler improved strength, reduced shrinkage, and
moderated the curing reaction. The inert filler was also the cheaper com-
ponent so that the cost of production was reduced. Properties and cost
have since been the two major concerns in production of polymeric com-
posites. Glass and graphite (carbon) fibers have been widely used as fillers
in polymer composites for structural applications. The high-strength high-
modulus liquid crystalline polymer fibers can be used as well in this field.
The lower density (thus higher specific mechanical property) and the high
heat resistance make liquid crystalline polymer fibers very attractive. In
Table 5.13 are representative data of epoxy composites reinforced by liquid
crystalline polymer and graphite fibers. The tensile properties of the liquid
crystalline polymer reinforced composites are good, but the compressive
strength is much poorer than the graphite fiber composite. The poor com-
pressive properties of liquid crystalline polymer fibers have limited their
utilization in composites. However, the liquid crystalline polymer fibers
were found to be very valuable in hybrid composite containing graphite
fibers. The excellent compressive tensile properties of the carbon fiber com-
bined with the inherent damage tolerance of the liquid crystalline polymer
fibers make a desirable system.
It has been concluded that in fiber reinforced composites the aspect ratio
L/D of the fiber is one of the most important parameters in controlling the
uniaxial modulus and ultimate tensile strength (Kardos and Raisoni, 1975).
