In filled rubber compounds carbon-black particles form a network of mutually interactive agglomerates. These effects can me measured and quantified using a simple D-RPA 3000 Matrix test. Storage shear modulus (G’) results at low strains (e.g. +/- 1%) are typically high and get reduced after a larger strain amplitude (e.g. +/-50%) is applied for a short period of time. With lower strain amplitudes applied over time, the reduced Storage shear modulus (G’) will partially recover. This effect relates to the Van der Waals forces linking the agglo-merates, getting broken at higher strain amplitudes and the partially recovering over time.
The extent of recovery of the Storage shear modulus (G’) directly relates to the Dispersion Rating (DR) of the rubber compound. If the carbon black is poorly dispersed, the recovery of the Storage shear modulus (G’) will be much lower indicating a much weaker filler structure and therefore reduced mechanical performance properties.
A simple CBDI performed by a Rubber Process Analyzer allows consistent testing and quality control on the Carbon Black Dispersion rating: CBDI = Inital storage shear modulus G’ (50°C, 1Hz, 1%) / Final storage shear modulus G’ (50°C, 1Hz, 1%). The higher the CBDI value, the better the carbon black dispersion.