The Physical Modification of a Natural Rubber-Polypropylene Thermoplastic Elastomer Blend by Azobisformamide Blowing Agent

The decomposition of azobisformamide (ABFA) blowing agent and its expansion in a soft grade of thermoplastic natural rubber composed of a 70/30 natural rubber (NR)/polypropylene (PP) was monitored using a Haake plasticorder. The aim of the study was to evaluate the effect of different concentrations of the blowing agent and to investigate its eventual interactions with a sulfur-based curing system that was used for the dynamic vulcanization. The plastograms allowed to detect variations in the initial mixing torque as well as the level of the final equilibrium torque for both unvulcanized and dynamically vulcanized blends. The plastograms of the dynamically vulcanized blends were characterized by the appearance of a second peak that corresponds to the crosslinking reaction which followed the blowing agent decomposition reaction. Since changing the order of the addition of the curatives with respect to the blowing agent did not cause any major effect on the general trend of the plastograms, it was concluded that the azobisformamide blowing agent did not alter the chemical action of the curatives and vice versa. The effects on the plastograms caused by variations in the blowing agent concentration were also reflected through the density measurements.