Modification of polysulphide rubber using butadiene—acrylonitrile copolymer/poly(vinyl chloride) blend

Abstract

Blending of polymers for property improvement or for economic advantage has gained considerable importance in the field of polymer science in the last decade, Polysulphide rubber is a costly speciality rubber which is well known for its outstanding resistance to solvents such as ketones, alcohols, acids, hydrocarbon solvents, water, etc. However, processing of polysulphide rubber is difficult, its mechanical properties are poor and it has an unpleasant odour [1-3]. Hence it would be worthwhile to attempt to improve the processing and mechanical properties of polysulphide rubber without sacrificing much of its excellent solvent resistance. Attempts have already been made in this direction, by blending polysulphide lattices by other synthetic lattices [4]. One of the commercially important and miscible polymer blends is that of NBR and PVC [5]. Compared to hydrocarbon rubbers, polysuiphide rubber is more polar and hence likely to form successful blends with NBR and PVC. In this study up to 50% by weight of polysulphide rubber is replaced with 50/50 NBR/PVC blend and the properties are compared. MgO/ZnO combination in the presence of stearic acid has recently been shown to be an efficient stabilizer system for PVC, especially in PVC-based blends [6-8]. Hence this combination was used to stabilize the PVC phase in this study. Polysulphide rubber (FA type, Mooney viscosity, ML 1 +4 at 100°C, 82), NBR (33.5% acrylonitrile, Mooney viscosity, ML 1+4 at 100°C, 40.9), PVC (powder, suspension polymer, K = 65), MgO (light magnesia), ZnO (white seal), Stearic acid, dibenzothiazyl disulphide (MBTS), diphenyl guanidene (DPG), tetramethyl thiuram disulphide (TMTD), and sulphur (all rubber grade) were the materials used. 50/50 NBR/PVC blend was made in a Brabender plasticorder model PL3S employing a rotor speed of 30 r.p.m. at the optimum temperature [8]. In addition to the equal weights of NBR and PVC, stabilizers for PVC (MgO 4 parts per 100 parts of PVC resin (p.h.r.), ZnO 4p.h.r. and stearic acid 3p.h.r.) were added at this stage. Blending of the polysulphide rubber and the NBR/PVC blend along with the additives for the rubber was done in a tight warm (about 60°C) laboratory mixing mill. The formulations used for the various mixtures are shown in Table I. The cure characteristics of the mixtures were determined in a Monsanto rheometer model R100 at 150°C. The cure characteristics are shown in Table II. The mixtures were vulcanized up to the respective optimum cure times in an electrically heated press at 150° C in a specially designed mould so that the mould with the sample inside could be cooled immediately after moulding, keeping the sample still under compression. The tensile properties of the vulcanizates were determined as per ASTM D412-80 [9] test method, at 28° C using dumb-bell-shaped test pieces at a crosshead speeed of 500 mm min-' using a Zwick universal testing machine. The ageing characteristics of the samples were determined by keeping them at 100°C for 24 h in an air oven and then measuring the retention in the tensile properties. The hardness of the vulcanizates was determined according to ASTM 2240