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. 1. Introduction Thermoplastic elastomers (TPE) are considered as a unique class of materials which play a great role in the polymer industry because they combine the advantages of the processing of thermoplastics with the outstanding mechanical properties and elastic recovery of vulcanized rubbers [1]. Different types of TPEs have been developed. Among them, those prepared by melt mixing natural rubber (NR) with a polyolefin such as polypropylene (PP) and which are classified as thermoplastic natural rubber (TPNR) have gained more acceptance because of the ease of preparation [2]. Research on TPNR is very extensive and can be summarized in three main categories which are the development of different types of TPNR by varying blend components [3–5], the investigation of different crosslinking agents for the dynamic vulcanization [6–8], and study of the effects of the chemical modification of the NR and/or the polyolefin part in the blend [9–11]. These chemical modifications can enhance blend component compatibilization and improve some useful properties of the material. But it is also possible to physically modify TPNRs through the use of blowing agents. Blowing agents are substances that give up a great amount of gas or vapor resulting in a cellular structure. The resulting cellular structure of the material
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