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 and
lubricating the NR; whereas the polar terminal groups
together with other curatives and silica formed intermediate
complexes 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 filler
inside 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].
The reduction of the torque difference (MHdML) afterthe 5.0 phr loading, i.e. the E/7.0 NR compound, is mostprobably due to the softening or lubricating effect of theexcessive ALK, which caused a lower crosslink density. Thiscan be attributed to the phenomenon of dissolving a part ofthe elemental sulphur and silica particles into the excessiveALK and, as a consequence, less sulphur and silica wereattached to the NR chains. The ALK is a unique moleculewhich is structured by a non-polar hydrocarbon chain andpolar terminal groups. The non-polar hydrocarbon chaindissolved in the non-polar NR, forming a monolayer andlubricating the NR; whereas the polar terminal groupstogether with other curatives and silica formed intermediatecomplexes that remained insoluble. The excessiveamount of ALK caused a more pronounced lubricating effect,and produced boundary layers which coated andabsorbed not only the curatives but also the silica fillerinside the layers. Through this mechanism, the ability ofsulphur to form a sulphide crosslink and silica to form aphysical crosslink would be less. This explanation could besupported by a morphological study of the tensile fracturedsurface of the E/7.0 NR compound. The fractured surfaceseemed smoother than that of the D/5.0 NR compound dueto excessive ALK loading. ALK was synthesized from the RBDPS, a palm oil fraction. Palm oil has the effect of plasticizingor lubricating rubbers [18].
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