<p> An electrically conductive thermoplastic composite (ECTPC) consists of
electrically conductive filler(s) at a concentration above percolation threshold
distributed in an insulating polymer matrix. The high concentration of the filler
required to achieve high electrical conductivity for ECTPC is usually
accompanied with the deterioration of mechanical properties and a large increase
in the viscosity which prevents feasible processing of these materials in common
polymer processing equipments such as injection molding machinery. The initial
focus of this work was to control these drawbacks by using combinations of
conductive fillers namely carbon fiber (CF) and carbon black (CB) to create a
hybrid-filler composite. Cyclic olefin copolymer (COC), an amorphous
polyolefin, was used as the matrix material. It was found that carbon black and
carbon fiber synergistically contribute to the transport of electrons through the
matrix. The synergism exists at various filler concentrations including when one
of the fillers was present below its percolation threshold, but not at high carbon
fiber content. Results showed that where the concentration of CF was several fold
higher than carbon black a good trade-off between viscosity and conductivity can
be achieved so that the obtained composites can be reasonably processed tn
common processing equipment e.g. in an injection molding machine </p> <p> Carbon fiber is preferred to carbon black as it leads to ECTPC with higher
electrical conductivity and lower viscosity. However, the high aspect ratio fibers
preferentially align in the flow direction leading to ECTPCs which have electrical
conductivity several orders of magnitude greater in the in-plane rather than
through-plane. We focused on foaming as a strategy to reorient the fibers toward
the through-plane direction in foam injection molding. Through a fractional
factorial experimental design, the effect of injection rate, melt temperature and
mold temperature on electrical conductivity was screened at two levels for foam
and nonfoam COC/CF(lO vol%)-CB(2 vol%) injection molded composites. It was found that foaming significantly enhanced the through-plane fiber orientation and
through-plane conductivity of the hybrid composite at low injection rate and high
melt temperature. The concurrence of the melt flow and bubble growth was
considered to be the key mechanism for fiber reorientation while the cell size and
shape should not disrupt the conductive path spanning the bulk of the material. </p> <p> The importance of the relative length scale of the fillers on cell size and
subsequently, electrical conductivity was investigated by injection molding.
Results showed that where the length scale of the filler was comparable to the cell
size, as for foamed COC/CF composites, the conductivity considerably decreases
with foaming. The drop was greater in the through plane direction and smaller in
the in-plane direction for the composites with larger average fiber length. Also
smaller cells led to a larger drop in the composite conductivity. It was observed
that where the length scale of the filler was much smaller than the cell size as such
for COC/CB composites, foaming enhanced the electrical conductivity
particularly in the through-plane directions and its effects became more
pronounced at lower carbon black concentrations. It was proposed that induced
carbon black coagulation by foaming was the main reason for the observed
improvement in conductivity. For COC/CF-CB hybrid composites, enhancement
in through-plane conductivity, particularly at CB concentration below percolation,
via foaming inferred that CB aggregates significantly contributed in improving
fiber-fiber contacts. </p> <p> Reorientation of the fibers by foaming was found to be very dependent on
processing conditions. High viscosity and fiber- fiber interactions can hinder fiber
rotation. The general understanding of the investigation was that fiber
reorientation may occur where the cells are much larger than the fibers. In
comparison, a series of nonfoam injection molded composites containing CF, CB
and CF-CB were foamed in a batch process to avoid flow effects. The
insignificant change in fiber orientation with foaming proved that fibers can not rotate by the growth of an adjacent cell in the absence of shear. Also, a large drop
in electrical conductivity with foaming as compared to the foam injection molded
composites suggested that particle relocalization can not occur in batch foaming. </p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19797 |
| Date | 09 1900 |
| Creators | Motlagh, Ghodratollah |
| Contributors | Thompson, Michael, Hrymak, Andrew, Chemical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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