The reduction of the torque difference (MHdML) after
the 5.0 phr loading, i.e. the E/7.0 NR compound, is most probably due to the softening or lubricating effect of the
excessive ALK, which caused a lower crosslink density.
Thiscan be attributed to the phenomenon of dissolving a part of
the elemental sulphur and silica particles into the excessive
ALK and, as a consequence, less sulphur and silica were
attached to the NR chains. The ALK is a unique molecule
which is structured by a non-polar hydrocarbon chain and
polar terminal groups.The non-polar hydrocarbon chain
dissolved in the non-polar NR, forming a monolayer andlubricating the NR; whereas the polar terminal groups
together with other curatives and silica formed intermediatecomplexes that remained insoluble. The excessive
amount of ALK caused a more pronounced lubricating effect,and produced boundary layers which coated and
absorbed not only the curatives but also the silica fillerinside the layers. Through this mechanism, the ability of
sulphur to form a sulphide crosslink and silica to form a physical crosslink would be less. This explanation could be
supported by a morphological study of the tensile fractured surface of the E/7.0 NR compound. The fractured surface
seemed smoother than that of the D/5.0 NR compound due to excessive ALK loading. ALK was synthesized from the RBDPS, a palm oil fraction. Palm oil has the effect of plasticizing or lubricating rubbers [18].