Solid‐state 13C‐NMR and equilibrium‐swelling studies of filled, TBBS‐accelerated sulfur vulcanization of natural rubber

Abstract

The effective network chain density (νe) and the density of the chains between sulfurizations (2[S]chem) for natural rubber vulcanized with N-t-butyl benzothiazole sulfenamide (TBBS) in different levels of HAF–HS (N347) carbon black were determined by equilibrium swelling and solid-state 13C-NMR measurements, respectively. The proportion of the intermolecular crosslinks over a whole range of sulfurized products (Ec) for the gum stock was estimated by the comparison of the chemical network chain density (νchem) calculated from νe using Mullins's approach with 2[S]chem. Ec increases with cure, reaches a maximum at an optimum cure state (trpar;, and decreases during an overcuring. It was observed from the NMR results that the total amount of chemical sulfurization reactions occurring in the rubber is almost independent of the level of carbon black loading. Based on the assumption that the amount of the intermolecular crosslinking reactions is also constant with the carbon black content, the amount of physical crosslinks (vent) in filled formulations was determined by subtracting the estimated νchem from the νe. The linear relationship obtained from the plot of the vent at the optimum cure state (t90) as a function of carbon black content postulates that the physical adsorption plays a major role in the polymer–filler interactions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1391–1399, 1998