dissolved wax was shifted to the lower region relative to
the raw wax. The peak intensities were normalized with
the maximum peak intensity species of C31H64 to investigate
the difference in the molecular weight distributions of
the raw and dissolved waxes in detail. Figs. 7 and 8 are the
relative molecular weight distributions of waxes dissolved
in the unfilled rubber samples using the wax solution and
molten wax, respectively. The relative intensities of the
lower molecular weight waxes in the dissolved waxes were
larger than those of the raw wax, while the relative
intensities of the higher molecular weight waxes in the
dissolved waxes are smaller than those of the raw wax, as
shown in Fig. 7. This implies that the lower molecular
weight waxes dissolved more in the rubber sample than
the higher ones. This may have been because of the
difference in the mobilities of n-alkanes according to their
size. This trend was more clearly shown in the NR sample.
The relative molecular weight distributions of the dissolved
waxes in the SBR and BR samples were nearly the same. As
for the dissolved waxes using the wax solution, the relative
molecular weight distributions of dissolved waxes using
the molten wax were different from the raw wax as shown
in Fig. 8. However, the difference in the molecular weight
distributions between the raw and dissolved waxes
declined. This can be explained by evaporation of waxes
with low molecular weights. Amounts of evaporated wax
increased as the wax size decreased and the temperature
increased [9]. Measurement of wax solubility using the
molten wax was carried out at 100 C.
4. Conclusions
The uncorrected wax solubilities (Swaxs) of the unfilled
samples were much higher than those of the filled ones.
The Swaxs of the NR vulcanizates were higher than those of
the SBR and BR ones. This may have been caused by solubility
parameters. The Swaxs were corrected with the
solvent swelling ratios and rubber fraction. The solvent
swell-corrected wax solubilities (Swaxswell) of the NR
vulcanizates were also larger than those of the SBR and BR
ones. The solvent swell and rubber fraction corrected wax
solubilities (Swaxswell-rubber) of the unfilled NR and SBR
samples were lower than the filled ones due to wax
adsorption by free filler. The Swaxswell-rubbers of the carbon
black-filled rubber vulcanizates were larger than those of
the silica-filled ones. The melt wax solubilities (Smeltwax)
were much higher than the wax solubilities (Swax) using the
wax solution in toluene. The Smeltwaxs of the silica-filled NR
and BR samples were higher than the carbon black-filled
ones because of the poor dispersion of silica. The lower
molecular weight waxes dissolved more in the rubber
samples than the higher ones because of the difference in
the mobilities of n-alkanes.