Specimens
The materials used for specimens in this study were bisphenol
A type epoxy composites filled with spherical
silica particles.
The matrix resin was a blend of the diglycidyl ether of
bisphenol A type epoxy resin (Asahi Kasei E-Materials, AER
2603), with methyl-tetrahydro-phthalic anhydride (New
Japan Chemical, RIKACID MH-700) as the curing agent and
2,4,6-tris (dimethyl aminomethyl) phenol (Mitsubishi
Chemical, jER BMI12) as the accelerator. The equivalent
weight of the epoxide resin was 188 g/equivalent and the
acid anhydride equivalent weight of the curing agent was
162 g/equivalent. The stoichiometric-mixture-weight ratio of
the epoxide resin, the curing agent, and the accelerator was
100:86:0.5.
The mixture ratio of the epoxide resin and the curing
agent was changed to consider the interaction between the
silica particles and the network structures in the matrix
resin. In this paper, the changed mixture ratio is hereafter
expressed as the epoxy-equivalent-weight ratio (EEWR)
defined by the epoxide resin weight over the stoichiometric
epoxide resin weight in the mixture. In the experiment, the
EEWR ranged from 1.0 to 3.2 and the accelerator was kept
constant at 0.5, as listed in Table 1. The neat epoxy resins
manufactured for the EEWR over 3.2 were too weak to
measure the viscoelastic properties mentioned below. We
did not use any epoxy-equivalent-weight ratios below 1.0
to manufacture the resin or the composites because unreacted
curing agents might leak out of the cured resins.
Silica particles with a median diameter of 240 nm
(Tatsumori, SO-C1) were used as the filler of the composite.
The volume fraction of the silica particles was 0.2
for every composite. The surfaces of the particles were not
chemically coated.
The silica particles were added to the epoxy resin containing
the curing agent and accelerator. After stirringsufficiently to disperse the particles without agglomeration,
the mixture was stored in a vacuum vessel to remove voids
and then poured into an aluminum mold coated with a
Teflon sheet. The mold was 260 mm long, 5 mm wide, and
120 mm deep. The mixture in the mold was heated for
curing in an oven. The curing procedure was performed in
two steps: first, the mixture was kept at 373 K for 2 h to gel
the matrix resin (pre-curing), and second, the post-curing,
which greatly affects the crosslinking reaction of the resin,
was performed at 403 K for 15 h. The heating rate from
pre-curing to post-curing was kept constant at 72 K h-1
.
Cured epoxy resins without silica particles were also prepared
in the same process as those for the composites.
Hereafter, the cured epoxy resins without the silica particles
are referred to as neat epoxy resins. Both the neat
epoxy resins and the composites were used in the experiment
listed in Table 1.
Densities of the cured materials were measured in
accordance with ASTM standard 792-08. The results are
listed in Table 1. The densities of both the neat epoxy
resins and the composites were confirmed to be independent
of the EEWR