Improvement of elastomer swelling measurement for sulfur cross-link density determination

Abstract

Due to growing demand from both environmental and industrial sectors the extent of rubber recycling is constantly increasing. Recycled rubber ranges from slabs of thermochemical reclaim, resembling virgin rubber, to fine rubber powders of diverse surface morphologies. Cross-Link Density (CLD) changes may characterize the effectiveness of rubber processing, but their measurements are often problematic. Most difficulties arise during determination of the volume fraction of polymer in the swollen rubber gel, also known as the Vr parameter (comparable to swell ratio), which closely relates to CLD. Vacuum, blotting and centrifugation were used to remove a swelling solvent (benzene) from the surface of the swollen rubber matrix. Centrifugation was more accurate for evaluating Vr of morphologically different types of rubber. Mechanical shear was used to produce a porous rubber sheet, while addition of acetamide and NaCl generated powder of devulcanized rubber. These samples were tested along with tire buffings and SBR sheets to show that extraction duration and time, needed to establish the swelling equilibrium between rubber and benzene, could be reduced from 72 h to less than 24 h in the case of porous or powdered rubbers. While compressed rubber might need longer durations, the overall time consumption for the CLD analysis could be shortened significantly, which helps in evaluating recycled rubber more thoroughly.