Experimental and theoretical analyses of pulverization and recycling of vulcanized rubber

A two-stage process for recycling of vulcanized rubber was proposed. In the first stage, the vulcanized rubber was shredded into granulates, and then the granulates were pulverized into fine particles using a single screw extruder in the solid state shear extrusion (SSSE) process. In the second stage, the produced rubber powder was compression molded to produce new items. The major objective of this research is to understand the fundamental aspects of the pulverization and the compression molding of the rubber powder.; Fine rubber particles were obtained using the SSSE process when the granulates were compressed sufficiently, and loss of strain energy due to viscoelastic stress relaxation was minimized by significant cooling in the pulverization zone. Agglomeration of rubber particles was found to be competing with the pulverization process. A sharp temperature gradient in the rubber was experimentally determined, which was qualitatively predicted by our heat transfer computer simulation.; The rubber particles produced by the SSSE process and the unprocessed rubber granulates were analyzed using physical, thermal, and chemical characterization methods. The characterization study showed that the particles had irregular shapes with convoluted surfaces, and that the powder had a larger specific surface area compared with a cryogenically produced powder. The particles had lower crosslink density than the granulates indicating the breakage of sulfur crosslinks in the rubber, which makes the SSSE powder very suitable for many recycling applications.; In the second stage, the produced powder within several size ranges was compression molded at various processing conditions. Rubber slabs with low-medium tensile strength were obtained without using any virgin rubber. It was found that the strength of the slabs is strongly dependent on the degree of particle bonding.; Inhomogeneous shearing deformations of a homogeneous thermoelastic slab and a non-homogeneous rubber-like slab were analyzed to gain an insight into the pulverization phenomenon. Both the temperature gradient and the material non-homogeneity are shown to be driving forces for the inhomogeneity in the stress-strain fields. A homogeneous rubber-like slab subjected to shear and a temperature gradient can actually develop high normal stresses, which might lead to Mode I type crack propagation.