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

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 tw o 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 diff raction 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 composi tion, 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 sh apes with rough surfaces, whereas the granulates had angular shapes with sm ooth surfaces. The larger particles produced by the SSSE process were mainl y agglomerates of smaller particles. The total and external surface areas o f the particles produced by the SSSE process were significantly greater tha n those of a cryogenically ground rubber of a similar size range. The exten t of thermo-oxidation depended on the external and total surface areas of t he samples. However, characteristic temperatures and kinetic parameters of the thermal degradation in the nitrogen environment were not affected by th e size or surface area. The composition of the granulates was the same as t hat of the particles. However, the cross-link density and gel fraction of t he rubber particles were smaller than those of the granulates suggesting th e cleavage of chemical bonds due to the high mechanical stresses and possib le oxidation during the SSSE process. The particle agglomeration during the reprocessing was instrumental in the alteration of the rubber properties. (C) 2001 Elsevier Science B.V. All rights reserved.