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“Research Outline
(Enumeration of Current Projects)”(3) Contribution of Filler-properties to Rheology for
Particle-filled Resin Composite
“Why
does filler property play a dominant role in rheology?”
[ Contents ]
[2-1 Scheme of
the observation]
[2-2 Conceivable
explanation of particular mechanism of rheological property]
[2-3 Origin of
the optical anisotropy]
[ Research Examples ]
Flame Synthesis of Aluminum Nitride Filler-powder
Functions of New
Aerosol Multilayered Gas Sensor
Semiconductor
packaging material encapsulates IC chips etc., and necessitates
a high thermal conductivity, a low thermal expansion and a good moldability. Higher the packing content of filler,
higher the thermal conductivity (lower the thermal expansion); but then, the
moldability degrades.
There is a keen
interest to clarify the relevance of primary properties of silica particles
onto rheological characteristics of the resin polymer composite filled with SiO2
particles. As a rule of thumb, a
largish SiO2 primary particle size or a smallish specific surface
area decreases the viscosity of the composite system, because the resin
supplied on the particle surface per unit weight apparently increases. These works could empirically show some
intrinsic SiO2 filler primary-properties for lowering viscosity;
such as the broader size distribution, the lower viscosity. However, that
was highly stereotyped, and there might be remained other intrinsic
silica-properties which affected the viscosity.
A cause of the lack of understanding was presumably a deficiency of appropriate observation method of this material system having the liquid typed-matrix of epoxy resin. Observing directly the SiO2 internal particle-aggregated structures must be promising to consolidate the scientific foundation. Thus far, that was very few, although some polymeric materials without silica filler, having much rather solidified matrix, were investigated with transmission microscopy. Our previous papers were described the principle of transparent microscopy using the photo-elasticity of resin polymer covered around SiO2 agglomerates, which locally provided a stress toward the resin distributed in the surrounding areas, and structured the rearrangement of polymer molecules. Also concerned its validity to clarify the particle-size distribution or coupling treatment influence on rheological property.
Transparent optical microscopy visualized
directly the particle-flocculated group in resin by using the photo-elasticity
phenomena of resin polymer. Epoxy
resin composite system filled with silica particle characteristically contained
the elliptical shaped features observed as brightened domains at crossed
polarization, which were affected strongly with particulate structural
surfaces. The elliptical feature
exhibited optical anisotropic properties, and was often observed in silica
particle system with smaller amounts of fine-particle on the surface of SiO2
filler core-particle. Elliptical
features observed in epoxy system were assigned to the primary and/or secondary
flocculated groups which were predicted at previous theoretical studies.
It is conceivable conclusion that particular mechanism of
viscosity is still unknown solely from SiO2 primary properties. The epoxy resin composite is predicted
to have a primary or a secondary flocculated group of each SiO2
particles before shearing, and the groups break up as the shear velocity
increases at a thixotropic softening and hardening range. Particle flocculated groups in the
composite system is supposed previously to contain a solvent among them before
applying shear force. The shearing
breaks the flocculation, disperses the solvent into the system, and lowers
apparently the SiO2 filler concentrations in the structure. This means the reduction of
viscosity. The amount of the
flocculation expects to increase with the enlargement of the extent of
non-spherical particle shape, narrow size distribution and reinforced
interaction of particle and solvent.
The larger amount of flocculation means the higher apparent
concentration of SiO2 filler particle in the structure. At broad perspective, tendency of
apparent viscosity of present experiments might be understood by seeing the
explanations. The optical
anisotropy in transmission microscopy showed in increase at the
interspersed-system, although the values of anisotropy were almost the same as
intact and stratified-systems.
Clearly, it could be concluded a linkage role of transmission optical
microscopy for correlating with experimental results of viscosity and the
theoretical model of flocculation.
Origin of the optical anisotropy is urgent for understanding
the rheological properties. Recent study in ceramics showed that the boundary
of the granules of corundum particle of elongated shape made by powder
compaction process could be a polarization point, which enabled to pass the
incident linearly-polarized light though the analyzer-filter of microscope.
This study reported that non-uniformity in the structure, likely as the
elongated particles aligned to one direction, had a relatively larger potential
for the polarization. Polarized microscopy can detect the mismatching of
refractive index at grain boundary and stuff like that. The SiO2
particle used in this experiment were not completely spherical, more likely
elliptical. Similarly as previous study, the elongated shape of particles
(their aggregates) and the mismatching of refractive index at the boundaries of
SiO2/resin are allocated to their optical anisotropic properties in
the SiO2 filled epoxy resin composite system.
As another
possibility of the origin of this material system’ optical anisotropy, it
should be pointed out photo-elasticity phenomena. The photo-elasticity of resin
polymer is fundamental characteristics used in liquid crystal displays (LCD).
Polymer molecules impressed voltage or pressure are rearranged to specific
direction, and they can be detected by polarized microscopy. The SiO2
filled epoxy resin composite system is also a polymer material system, and the
photo-elasticity should be considered as a candidate. The optical anisotropic
features in this experiment had the positive optical character of elongation,
as shown in Fig. 5. This result strongly implied the characteristic orientation
of polymer molecules. As one of assumable explanation, a heterogeneously packed
SiO2 aggregates locally provided a stress toward the resin polymers
distributed in their surrounding areas. Previously, the role of
interrelationship between particle and resin was noted as dominant for the filler-powder
filled epoxy resin composite system. Such kind of “locally induced
photo-elasticity” might structure the rearrangement of
polymer molecules, and make the polarization.
