World Journal of Engineering
Processing Effect of Injection Molded Nanoparticle/Polymer
Composites on Their Electrical Properties
Haihong Wu[1], Shaokang Ma[1], Aiyun Jiang[2], Baofeng Zhang[2]
[1] School of Mechanical and Electrical Engineering, Henan University of Technology, Zhongyuan Road 195,
Zhengzhou 450007,China.
[2] Huanghe College of Science and Technology, Zhengzhou, 450063,China.
Introduction
Polymer is often endowed some outstanding
properties with mixing different fillers. The
development of nanotechnology stimulated research
to create multi-functional composites by integrating
nano-particles into polymer matrices. Nanoscale CB
particles, a kind of filler, have generated practical
application in manufacturing nanoparticle-reinforced
polymer composites due to their low cost and good
moldability. Many researchers focused on electrical
property of CB/polymer composites[1-3]. For a
given composite, electrical conductivity is
determined by not only the CB type and
concentration, but also the specific polymer matrix
used and the generated morphology[4-6]. The
microstructural evolution on injection molded CB
particles/polymer composites is more complex
because CB particles migrate from the region of high
shear stress to the region of low shear stress. Both
morphology of the polymer matrices and the
distribution of CB particles should be significantly
deformed by the high pressure-driven molding flows,
especially in the area near the surface of the
moldings.
However, it is not clear how processing conditions
affect the mobility of CB particles and their
distribution in the matrix at skin-core areas of the
molding. From the view of application, it is
necessary to investigate the conductivities of the
skin, sub-skin and core areas with the change of
processing parameters to obtain better electrical
property of the molding. In this study, we will move
attentions to the distribution of CB particles in the
polymer matrix at different positions of the moldings
with the change of processing conditions. The
objective was to systematically investigate two
questions. One is about the effect of injection
molding conditions on the distribution of CB
particles and the skin-core microstructural
characteristics of the molding. Another is to
understand the relationship among microstructure,
processing conditions and conductivity of the
molding to find out the practicable channel to
improve the conductivity of composites filled by CB
particles.
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Experimental procedure
Materials
CB particles used in the experiments were made
from Huanguang Chemical Factory in Zibo, China.
The resistivity is 0.45Ω.cm and diameters of CB
particles are (28-32)nm. Nitrogen BET surface area
is about 1080 m2 /g. PP used is obtained in a
pelletized form from Sinopec Group. The melting
point of PP is 165℃ and its melt flow index
12g/10min.
Apparatus and Procedures
CB particles were mixed with PP in a ME100LA
mixer at a temperature of 190℃; the concentration of
CB was 25wt%. Pellets were obtained by SHJ35
twin screw plastic pelleter. The sample was cut off
three layers along the direction of thickness of the
molding, and distribution of CB particles in PP
matrix at each layer was observed with Quanta FEG
650 Scanning Electron Microscope. Removed
thickness of each layer is 0.2mm, 0.4mm, 0.8mm
respectively, which approximately denotes the three
zones: skin, sub-skin and core. The volume
resistivity was measured by using a two-terminal
standard resistor under DC condition at room
temperature in the thickness direction of the
specimen.
Results and Discussion
For tested CB /PP composite, we found that the
orientation of CB particles in the matrix, shown in
Fig.1, differs from injection molded PP, especially at
core area where CB particles have strong oriented
distribution and formed continuous conductive path.
The results above showed that CB particles move
from the region loaded by high shear stress to the
region with low shear stress. As a result, the skin
areas near the surface were depleted of CB particles
as migrated particles accumulate at core zones. It
should be illustrated by stretched molecular chain
model which CB particles easily accumulate at areas
where molecular chain was loaded with less tension.
World Journal of Engineering
Because dispersed CB particles are capable of
self-agglomeration to form conductive networks.
Skin zone
Core zone
Fig.1 The microstructure of different layers of
Skin zone
sample under 70MPa injection pressure and 50MPa
packing pressure
The results proved that nano-particles can exhibit
self-assembly into highly anisotropic structures
because the immiscible particle core and grafted
polymer layer attempt to phase separate but
areconstrained by chain connectivity[4]. For injected
7050 samples, CB particles at skin area agglomerate
into approximate smaller spherical cluster. And at
sub-skin area, CB particles form obvious oriented
morphology. The electrical resistivities of different
areas were shown in Table 1.
Fig.2 The microstructure of different layers of sample
under 110MPa injection pressure and 50MPa packing
pressure
Conclusions
For injection molded CB particles/PP composite,
injection pressure and packing pressure have
considerably influence on the resistivity of the part.
The amount of CB particles in the matrix decreases
at sub-skin and core zones of the part with increase
of injection pressure under the same packing
pressure, which lead to the increase of resistivity of
the part. On the other hand, the resistivity decreases
with increase of packing pressure because higher
packing pressure is beneficial to form effective
conductive path in the part. It was speculated that the
best electrical property may be obtained under
processing conditions with lower injection pressure
matched higher packing pressure. Because of
self-aggregation of CB particles, the microstructural
orientation of CB particles/PP composite during
processing differs from PP molding, especially at
higher packing pressure. CB particles have strong
oriented dispersion at core zones because CB
particles migrate from the skin zone to core area,
which form effective conductive path at the core
zones, especially under higher packing pressure. The
results showed that core zones have better electrical
property than skin zones.
Table 1 The electrical resistivity of different
layers in different processing conditions(10×Ω.cm)
Processing
conditions
70/50
110/50
Skin
Sub-skin
427
386
274
589
Core zone
Core
289
493
Table 1 shows the electrical resistivity of different
layers in different processing conditions. These
experimental results could be explained from two
points. Apart from migration of CB particles, one
was speculated that slow cooling at the zones close
to the center led to higher crystallinity of PP and
improved conductive paths because CB particles are
easily localized into the amorphous matrix of
polymer[6]. Another point is that dispersed CB
particles acted as nucleating additive and became
nucleation centre leading to crystal growth in the
process of crystallization which took place during
cooling of a molten polymer sample.
In order to investigate the effect of injection pressure
on the microstructure and resistivity of the molding,
another group of specimen was injected with higher
injection pressure 110MPa. The microstructures of
different layers were shown in Fig.2. And electrical
resistivity of each layer is measured.
However, the amount of CB particles in the matrix
decreased at sub-skin zones and core zones
according to their densities measured. Similar
conclusion was obtained by repeating experiments.
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