Filler-Polymer Interactions in Filled Rubber Compounds

Development of bound rubber phase in silica filled rubber compounds was discussed in connection with filler-polymer interactions. The amount of bound rubber increased with increasing the averaged size of agglomerate formed by silica particles, independent of chemical structure of rubber molecules. In addition, no trace of bound rubber was found in silica-filled rubber compounds in which no agglomerate existed. This suggests that the bound rubber phase corresponds to the entrapped rubber phase in the agglomerate. NMR results revealed that chemical interactions were negligible between silanol groups and non-polar rubber molecules such as IR and SBR. In that case, the averaged size of agglomerate was dependent on the silanol number per unit surface area of silica particles. For polar rubber molecules such as NBR, infra-red spectra suggested the existence of hydrogen bonding between silanol groups and nitrile groups of NBR. With increasing the nitrile group content of NBR, the averaged size of agglomerate as well as the amount of bound rubber decreased. The hydrogen bonding suppressed the development of agglomerate in the compound, leading to the decrease in the amount of bound rubber entrapped in the agglomerate. Further, the hydrogen bonding had no effect on the formation of bound rubber on the silica particles. Such formation mechanism of bound rubber in silica filled rubber compound is quite different from the case of carbon black filled rubber compounds.