A Mechanistic Approach to EPDM Devulcanization

The extensive utilization of rubber in a wide variety of products causes a problem in terms of waste. Reclaiming of end-of-life products or production scrap is a potential solution. A high quality reclaim would preferably be obtained by devulcanization, to leave the polymer chains intact. Reclaiming of natural rubber is common practice, although the fundamental knowledge about de- and re-crosslinking is rather limited. EPDM-based rubber is even more unexplored in terms of reclaiming and re-utilization. This paper gives a mechanistic insight into the thermal reclaiming of two differently vulcanized EPDM compounds, using hexadecylamine (HDA) as devulcanization agent. Reclaim from conventionally vulcanized EPDM, mainly polysulfidic of nature, shows the largest decrease in remaining crosslink density with increasing HDA concentration and at lower reclaim temperatures. After reclaiming at the lower limit of the experimental temperature window applied: 225 °C, the concentration of remaining di- and polysulfidic crosslinks is higher than the concentration of monosulfidic bonds, while at the upper temperature level: 275 °C, the concentration of monosulfidic bonds is highest. For efficiently vulcanized EPDM with primarily monosulfidic crosslinks, HDA again has a positive effect on the reclaim efficiency at low applied reclaiming temperatures of max. 225 °C. At higher temperatures, the crosslink density increases with increasing concentrations of HDA. Application of a treatment proposed by Horikx shows, that conventionally vulcanized EPDM devulcanizes to a larger extent by crosslink scission compared to the efficiently vulcanized material, which primarily shows main-chain scission. Both reclaimed materials can be added up to 50 wt% to a virgin masterbatch, with limited decrease in properties. This is a high amount compared to max. 15 wt% of ground powder rubber, commonly known to be the practical limit.