Network evolution based on general-purpose diene rubbers/sulfur/TBBS system during vulcanization (I)

Abstract

In order to investigate the rule of network evolvement during vulcanization of diene rubber, generally used diene rubbers, including natural rubber (NR), polybutadiene rubber (BR), and styrene butadiene rubber (SBR), were cured with the same curing agents but at different time. The cured rubbers were then analyzed with Rubber Process Analyzer (RPA2000), dissolution/swell, and Magnetic Resonance Crosslink Density Spectrometer (MR-CDS 3500). Through data analysis on torques obtained from RPA2000, gel contents from dissolution/swell experiment and crosslink densities obtained from MR-CDS 3500, a new concept about the formation of rubber network during induction period of vulcanization was proposed. The experiments showed that even in scorch delay period, crosslink density, gel content and torque of the three rubbers increased as curing time went. The increase of these parameters indicated the formation of primary crosslink among several macromolecules which could be called local primary network-the first stage of network development. After this stage, both crosslink density and gel content of these three rubbers increased abruptly. The change percent of torque, crosslink density and gel content increased from 5% to 15%, 13%–33% and 2.5%–20% respectively which meant that almost one third of the vulcanization had been carried out during this period. Corresponding photographs of dissolution/swell experiments showed that whole gels which only swell but didn't solve in their solvents were formed. So the mutation point can be taken as the second stage of network development – a fundamental network had been formed. For NR, BR and SBR, the critical average change percent for crosslink density, gel content and torque were 33%, 20% and 15% respectively. BR, NR and SBR formed local primary network during induction period and a continuous fundamental network at the beginning of crosslinking period. This is different from traditional vulcanization theory.