Most rubber products are vulcanized, a process that involves heating with a prescribed small amount of a cross linking agent to stabilize the polymer chains over a wide range of temperatures. Vulcanizable materials include natural rubber, however the range spread much wider since each polymer becomes elastomeric above its glass transition temperature (below which rubber becomes brittle). As the elastomeric state is unstable with chains slipping past one another resulting in creep or stress relaxation under static or dynamic loads, chemical crosslinks add the stability to the network needed for most practical applications.
Finding a viable solution for the environmental pollution created by waste rubber is a challenge for the Rubber Industry. The problem is exacerbated when waste rubbers are not properly disposed of. Waste rubbers can be grinded into crumb rubber. Typically, CR is rigid and not adequate for revulcanization because of surplus crosslinks. Unvulcanized virgin rubber is conversely composed of long flexible chains and can easily be compounded, processed, and vulcanized. As a result, rubber reclamation and devulcanization processes help convert CR into a secondary polymeric material referred to as reclaimed, devulcanized, or recovered rubber, to be re-used to produce rubber products.
An environmentally friendly possibility to recycle thermoset rubber consists of breaking its spatial structure with the selective scission of sulfur-sulfur (S-S) and carbon-sulfur (C-S) chemical bonds without breaking the backbone network. Waste thermoset rubber is a valuable source of secondary raw materials for circular economy. Moreover, legislations call for the increased application of waste rubbers to manufacture value-added products. Sustainable development of rubber devulcanization technologies and appropriate design of cradle-to-cradle loops for thermoset rubber goods are the most promising strategy to achieve a higher level of thermoset rubber recovery.
Increased numbers of waste tires generated to the environment each year is a huge and global ecological problem. This situation enforced industry and researchers to develop existing technologies and search for new directions to recycle waste rubber, the starting element supporting circular economy in the tire industry. Considering pros and cons of various methods applied for reclaimed rubber characterization a basic standard Mooney viscosity should be determined along with tensile strength according to ASTM D1646 and D312. However, final values of these parameters are affected not only by reclaiming/devulcanization efficiency but also by curing system composition and vulcanization conditions of revulcanizates.
Reviews at WIDL indicate that rubber recycling by suitable grinding and treatment technologies of waste tire rubber and the characterization of obtained products will continue to grow in the future. The most promising avenues for further investigations fall into three main directions. First come grinding, modification, functionalization of CR (or combinations of these methods), preferably through continuous reactive extrusion or high-shearing mixing. The approach allows tailoring of matrix-CR interfacial interactions and preparation of novel rubber recycling based products, which found applications as reinforcing or semi-reinforcing fillers, bitumen modifiers, or engineering materials dedicated to construction and building projects.
Second, further research should focus on quantitative and qualitative characterization of liquid by-products and volatile organic compounds formed and emitted during devulcanization, modification, or functionalization of CR. The impact of such compounds (some being carcinogenic) on final product properties and stability (possible migration of degradation products to the surface or evaporation during storage), and the environment or human health should also be investigated.
Third, deals with solutions to convert promising batch methods of CR modification to continuous processes including profitability estimation of such conversions. Optimization and up-scaling laboratory and prototype lines dedicated for CR recycling should be performed to verify the results at larger semi-technical or industrial scale.
Dr. Ben Chouchaoui
Operations Manager
WIDL Inc.
January 1, 2023
Monday – Friday
8:00 am – 5:00 pm
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Disclaimer
Dr. Chouchaoui and WIDL do not do engineering services regulated by PEO (Professional Engineers Ontario). If you need services as such please contact PEO for referrals to an engineer who does such.
