Professor James Economy's Group
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[Carbon/BN Composite] [AlB2 Composite] [Polymer Composites] Fiber
Reinforced Thick Section Copolyester Composites Thick
section composite laminates have typically been expensive
and difficult to manufacture because of problems with
porosity, residual strains and the large exothermic
reaction of epoxies. Further, mismatches between the
coefficient of thermal expansion of the fiber and matrix
can lead to residual strains as a result of shrinkage and
thermal stress during cure. Also, in bismaleimides, any
volatile products that form during the cure process have
a potential to form voids between plies, which can lead
to delimitation. Typically, a stage curing process has
been used to reduce the effect of these problems, but it
requires multiple cure processes and is time consuming.
More recently, a method in which a temperature gradient
is applied through the thickness has been used to cure
thick composites without stage curing. The
copolyester materials developed here can eliminate these
problems. Through interchain transesterification
reactions, individual plies can be bonded with each other
in the solid state through heat and pressure (as seen the
background in the Polymer
section). Hence, little or no volatile products are given
off, residual strains are localized to the individual
plies and there is no need to worry about exothermic
reactions. Furthermore, oure Liquid Crystal Polymer
Thermoset (LCP-TS), can potentially match the coefficient
of thermal expansion of carbon fibers at the fiber/matrix
interface. This is important in high temperature
composite applications where many thermal cycles are
experienced. Small strains at the interface can lead,
over many heating and cooling cycles, to fatigue crack
growth and delaminating. Our
copolyesters excel compared to current resins. They have
a high glass transition temperature of about 250oC,
compared to high grade epoxies which can reach
temperatures of about 175o Celsius, and high
thermal stability (degradation in air does not initiate
until about 350oC). Past research has also
shown high lap shear strengths for temperatures up to 300oC.
For thin bond thicknesses of about 4 micrometers, lap
shear strengths of about 3000 Psi have been observed. Current
work is in the primary stages: studies optimizing the
time and temperature required to cure and bond lamina,
literature research on different methods to coat fibers
and fabrics with the solid polyester powders (at room
temperature) and manufacturing thin and thick laminates.
Since we are dealing with a new type of thermosetting
copolyester resin, these processes need to be
characterized if they are to be viable for use in
structural composites. In
the very near future, the tensile properties and flexure
properties will be tested. Dynamic Mechanical Analysis
(DMA) of solid state bonded specimens has been done on
45% fiber volume fraction composites made from plain
woven carbon fibers. The storage modulus at room
temperature averages about 35 GPa in the 0 direction (for
3 ply samples). |
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