shielded by bound rubber molecules so wax will not be
dissolved in the filler particles. Therefore, the filler fraction
of a filled rubber composite must also be corrected to
calculate the wax solubility. The solvent swell and rubber
fraction-corrected wax solubilities (Swaxswell-rubber) were
obtained by dividing the Swaxswell by the rubber fraction.
Since the filler content was 50.0 phr, the rubber fraction of
the filled sample was 0.667. Fig. 4 shows the Swaxswell-rubber
variations of the rubber samples. For the NR and SBR
samples, the Swaxswell-rubbers of the unfilled samples were
lower than the filled ones. Especially for the NR samples,
the Swaxswell-rubber of the carbon black-filled sample was
much higher than that of the unfilled one. This indicates
that some filler was not shielded by bound rubber and
existed in a free state. The free filler can absorb some wax.
A rubber compound is made by mixing rubber with
fillers and ingredients using a mixer or a two roll mill at
high temperature. Wax is also physically mixed with rubber
and filler. Thus, we also measured wax solubility using
molten wax at 100 C. Fig. 5 shows the melt wax solubilities
(Smeltwax) of the unfilled and filled specimens. The melt wax
solubilities (Smeltwax) were much higher than the wax
solubilities (Swax) using the wax solution in toluene. Like
the Swaxs, the Smeltwaxs of the unfilled samples were also
higher than the filled ones, but the difference reduced. For
the unfilled specimens, the order of wax solubility was
NR > BR z SBR. For NR and BR specimens, the Smeltwaxs of
the silica-filled samples were higher than those of the
carbon black-filled ones. This may be due to the poor
dispersion of silica. Dispersion of silica in a rubber
compound is much worse than that of carbon black and
agglomerates of silica can be easily formed in a rubber
compound [19–21]. Agglomerates of silica can provide
space for wax to penetrate.
Waxes dissolved in the rubber vulcanizates were
extracted with toluene and analyzed using GC. Fig. 6 shows
GC chromatograms of the raw and dissolved waxes which
show normal alkanes. The most intense n-alkane was
C31H64. The n-alkane distributions of the raw and dissolved
waxes were different. Molecular weight distribution of the