EVOLUTION OF BOUND RUBBER DURING THE STORAGE OF UNCURED COMPOUNDS

Bound rubber has been assessed in a variety of general purpose rubber compounds (NR, BR, EPM and EPDM) with different carbon blacks (N326, N330 and N347). It has been found that, within one month after the preparation of rubber compounds, bound rubber is not a constant property but significantly evolves until a constant value is reached. There is thus a maturation effect of bound rubber, likely to be due to the slow substitution of the smaller elastomer chains initially adsorbed by longer ones. When dividing the bound rubber values by the cubic root of the di-butyl phthalate adsorption number of the carbon black used, it is found that the bound rubber ageing curves of compounds with equal level of blacks of different structures reduce to a single master curve. Furthermore, the bound rubber variation in the earlier ageing period (around 30 days) is linear with respect to the square root of time. A set of two relationships is therefore proposed to represent the observed behavior. An immediate practical implication of the reported observations is that the well known variations of the rheological properties of uncured rubber compounds upon storage are likely to stem from this ageing behavior of bound rubber.