Particulate Reinforcement of Elastomers at Small Strains.

Abstract

A series of particulate reinforced natural rubber composites are prepared using both model (glass sphere) and commercial (carbon black and precipitated silica) reinforcing filler materials having a range of surface activities. Small strain reinforcement and viscoelastic behaviour of the model (glass sphere-filled) microcomposites are found to be well described by hydrodynamics and temperature-insensitive stiffening mechanisms such as strain amplification and elastomer occlusion. This means that the energy applied to the model materials during small strain deformations is entirely stored and dissipated within the elastomer phase. For carbon black-filled natural rubbers such mechanisms are no longer found to completely describe the levels of reinforcement and viscoelastic behaviour. This is the case particularly for high surface area carbon blacks of small aggregate size. For carbon blacks, additional mechanisms of reinforcement are identified and associated with the formation of a filler network and with effects at the polymer-filler interface. For all the compounds considered in this study, no direct experimental evidence is found for the formation of significant volumes of interphase polymer exhibiting retarded chain dynamics near the filler surface. The observation of a secondary dissipation process in rubbery region small strain dynamic mechanical and creep measurements of carbon black-natural rubber compounds where the polymer filler interaction is particularly poor (where the carbon black surface is graphitised) indicates that there may be a significant slippage of polymer chain segments at the filler-elastomer interface. There is some limited evidence from small strain creep testing to indicate that this process also occurs in commercial carbon black-filled compounds but to a much reduced extent. To the best of the author's knowledge this is the first time that such processes have been observed in carbon black filled elastomers.