Microwave Technology Applied to a Vulcanized Model Rubber Compound: Reclamation and Recycling Ability

The objective of this work was to investigate the effects of a thermal treatment induced by microwaves (MW) and applied to a simple rubbery model system. This has to be intended as the first step of a study whose final goal is to find an effective, industrially applicable, way to induce partial devulcanization, and improve recycling capabilities, in more complex systems like end-use tyres, whose disposal constitutes a severe environmental issue. The chosen system was based on a styrene-butadiene rubber (SBR) mixed with vulcanization agents. Such “uncured rubber” (UR) was vulcanized under static load to give the reference “vulcanized rubber” (VR), and finally processed in a MW oven to obtain a “treated rubber” (TR). All the materials were characterized in terms of gel-percentage and mechanical properties. In order to evaluate the effects of MW treatment on the recycling capability of the rubber, the UR was loaded with various levels of TR, in a range from 20 to 100 phr, and then subjected to re-vulcanization. Besides the reference VR, also samples obtained by re-vulcanization of UR loaded with cryo-milled vulcanized rubber (CR) were considered for comparison. Rheological, calorimetric, stress-strain and dynamic-mechanical characterizations were performed on all the obtained compounds. By this way it was possible to assess how the addition of either TR or CR to the virgin material influences both the curing process and the properties of the final product. While loaded systems generally showed less attractive mechanical properties than the original VR, the TR-loaded compounds had better performances than those with analogous contents of cryo-milled VR. Moreover, a significant increase of the glass transition temperature was observed in the final material, indicating that the “recycled” part was still able to participate in the second vulcanization process.