Contribution of physico-chemical properties of interfaces on dispersibility, adhesion and flocculation of filler particles in rubber

Reinforcing fillers are added to elastomeric compounds to improve and adjust several mechanical, dynamical, tribological, etc. properties with respect to different applications, i.e. for automotive tires, or technical rubber goods. Carbon black and precipitated silica are widely used as rubber reinforcing fillers; however, some new classes of nanosized substances like organophilic modified clay or carbon nanotubes are presently intensive studied as possible future filler systems in combination with carbon black or silica.An important parameter for the dispersibility and compatibility of the filler in the polymer matrix of rubber compounds is the surface energy and surface polarity of the solid filler particles. Therefore, we systematically measured and compared the dynamic contact angles of a collection of different filler types (carbon blacks, silica, carbon nanotubes and organoclays) using the Wilhelmy method, whereby the particles were fixed as a thin layer at a double-sided adhesive tape. From the contact angle values the polar and disperse part of the surface energies of the filler particles were calculated by fitting Fowkes formula. For an estimation of the compatibility of the fillers with different types of rubber polymers we additionally analyzed the surface energy and polarity of the gum (unfilled) elastomers. From the evaluated surface energies and polarities, thermodynamic predictors for the dispersibility (enthalpy of immersion), the adhesion between filler particles and polymer matrix in the nanocomposite, and for the flocculation behaviour of the particles in a rubber matrix (difference in the works of adhesion) were derived. These thermodynamic predictors improve considerably the compounding process of novel rubber nanocomposites with respect to target-oriented adjustment of rubber properties.