The rubber compounds used for evaluation of properties include
(1) SBR-0F: unfilled SBR, (2) SBR-30CB: SBR filled with 30 phr of
carbon black, (3) SBR-30LSCB1090: SBR filled with 30 phr of LS-CB
hybrid filler consisting of LS and CB in 10:90 ratio by weight, and (4)
SBR-30LSCB2080: SBR filled with 30 phr of LS-CB 20:80 hybrid filler
consisting of LS and CB in 20:80 ratio by weight.
The tensile strength at failure of the rubber vulcanizates reflects
the true reinforcement behavior of the hybrid fillers as the tensile
strength and elongation at break are the failure properties while
the tensile modulus captures only the effect of stiffness of the filler
particles and may not be as discriminatory as tensile strength or
elongation at break. The values of tensile strength of cured compounds
of carbon black and hybrid filler particles are presented in
Fig. 8.
As seen in Fig. 8, the tensile strength of the compounds of LS-CB
hybrid filler with 10:90 LS:CB by weight is close to that of the
compound of only carbon black. This closeness in values may be
attributed to the fractal morphology of the hybrid filler particles
seen in Fig. 3(c). However, the tensile strength drops dramatically
for the compound of hybrid filler with 20:80 weight ratio of LS to
carbon black. The SEM images in Fig. 9 present some valuable
insights.
Fig. 9(a) and (b) shows that carbon black aggregates dispersed
very well in the rubber compound. The CB particle aggregates
visible in these images show size of about 100 nm. The SEM image
of the compound of 10:90 hybrid filler (Fig. 9(ced)) shows 100 nm
carbon black aggregates dispersed in rubber matrix in addition to
hybrid filler domains consisting of carbon black particles separated
by lignin. The concentration of lignin in this hybrid filler was much
smaller than the concentration of carbon black. Thus, as supported
by SEM image in Fig. 4(b), we can infer that not enough lignin was
present to completely coat all carbon black particles. Instead, it is
quite possible that some lignin entered the space between the
primary particles and prevented carbon black particles from
forming a network of its own. The presence of large particle aggregates
in SEM images in Fig. 9(e) also provides evidence of strong
interactions between lignin and carbon black, which resist
breakage of the networks and prevent dispersion by shear forces
during compounding.
The SEM images of compounds containing LS and carbon black
in the ratio of 20:80 show large number of particles of size
20e50 mm. Note in this case that greater number of CB particles
were coated and became stiffer in this case than in the case of LS:CB
ratio of 10:90. These aggregates could not break and easily disperse
in the rubber. This is reflected in low values of tensile strength, in
Fig. 8. Thus, it is critical to use an optimum amount of lignin in
the design of hybrid fillers. The lignin to carbon black ratio should
be enough to prevent carbon black filler networking and at the
same time not affect the reinforcing behavior of carbon black. As is
evident from the tensile strength and filler flocculation data, the
10:90 weight ratio of lignin to carbon black is acceptable as it
provides a balance between reinforcement and suppression of
flocculation.
The values of representative mechanical and dynamic mechanical
properties of carbon black and hybrid filler with 10:90
LS:CB ratio are listed in Table 3. The method of measurement of