Filler–filler interaction is commonly measured by the
Payne effect. A comparison of filler–filler interaction in
the NR–silica compounds in both the absence and presence
of silane coupling agent TESPT, in the uncured state, is
shown in Fig. 2. The Payne effect of silica compounds with
TESPT is noticeably lower than for compounds without
TESPT, except for skim rubber. This is in agreement with
work by Luginsland et al. [6] that the Payne effect of silica-
filled compounds is influenced by the degree of silanization
which depends on the structure of the silane and
its amount. This has also been reported by Ramier et al.
[20] in silica-filled styrene–butadiene rubber where the
surface treatment of silica with coupling agents gives more
reduction in the Payne effect amplitude compared to covering
agents. This indicates that the hydrophobation of
the silica surface by coupling agent TESPT greatly reduces
the filler–filler interaction by creating covalent bonds between
rubber and fillers. Blume [21] has shown by Inverse
Gas Chromatography experiments that the dispersive component
of silica reduces from 65 mJ/m2 to 29–48 mJ/m2
after silane treatment. The influence of non-rubber constituents
in the NR, in particular proteins, on silica–silica
interaction can also be seen, where the low protein DPNR
compound exhibits a higher Payne effect and the high protein
skim rubber compound has the lower Payne effect.
The proteins in NR have a similar effect as silane in
hydrophobizing the silica surfaces, although not as strong,
as shown in Fig. 3(a). For unvulcanized samples, the Payne
effect for compounds without silane decreases with
increasing content of proteins in the NR. The role of the
proteins is to disrupt the silica–silica network resulting
in lower filler–filler interaction. The physical interaction
between silica and proteins through hydrogen bonding