Devulcanization of Waste Tire Rubber via Microwave and Biological Methods: A Review

This paper presents a thorough literature review on devulcanization methods applied to waste tire rubber: "microwave devulcanization" and "biological desulfurization". To do so, 80 papers published from the year 1990 to 2024 in journals with subscription and open access status across 12 databases were reviewed. This paper compares the efficacy and reviews the basic concepts, advantages, processes, and variable parameters of these two methods. In microwave devulcanization, microwave energy breaks the sulfur crosslinks between polymer chains. The latter breakage is mainly enabled by the presence of carbon black in the tire, which is an excellent microwave absorbent. In biological desulfurization, bacteria or fungi convert the crosslinks to elemental sulfur substances or sulfate. In general, microwave devulcanization of rubber leads to a lower crosslink density and thus a higher degree of devulcanization. On the one hand, breaking the crosslinks requires a significantly shorter time than biological desulfurization. Crosslink scission occurs throughout the sample in microwave devulcanization but only on the sample surface in biological desulfurization. Microwave devulcanization is not sensitive to rubber additives and does not require detoxification before devulcanization. On the other hand, biological desulfurization requires detoxification before devulcanization since it involves living organisms that may not tolerate certain rubber additives.