Pulverization of rubber granulates using the solid state shear extrusion process: Part II. Powder characterization

Abstract

Rubber powder obtained from the solid state shear extrusion (SSSE) process and the unprocessed rubber granulates were analyzed using physical, thermal, and chemical characterization methods. A portion of the granulates and two different size fractions of the powder produced by several passes of the rubber through the extruder were sampled by sieves and then characterized. Particle size distribution of the samples was determined using a laser diffraction technique. The shape of the particles was observed with an optical microscope, and the details of the particle surfaces were visualized using a scanning electron microscope. The total surface area was obtained using a BET method. Thermal analysis techniques were used to determine the composition, thermal, and thermo-oxidative degradation characteristics. The cross-link density and gel fraction of the rubber were determined using swelling and Soxhlet extraction methods, respectively. The microscopy study revealed that the particles generally had irregular shapes with rough surfaces, whereas the granulates had angular shapes with smooth surfaces. The larger particles produced by the SSSE process were mainly agglomerates of smaller particles. The total and external surface areas of the particles produced by the SSSE process were significantly greater than those of a cryogenically ground rubber of a similar size range. The extent of thermo-oxidation depended on the external and total surface areas of the samples. However, characteristic temperatures and kinetic parameters of the thermal degradation in the nitrogen environment were not affected by the size or surface area. The composition of the granulates was the same as that of the particles. However, the cross-link density and gel fraction of the rubber particles were smaller than those of the granulates suggesting the cleavage of chemical bonds due to the high mechanical stresses and possible oxidation during the SSSE process. The particle agglomeration during the reprocessing was instrumental in the alteration of the rubber properties.