Recycling vehicle tires is complicated because the rubbers used (including butadiene and polyisoprene) are highly cross-linked, and because pyrolysis-based recycling of the rubbers generates environmentally hazardous compounds like benzene and dioxins. Now, researchers at the University of North Carolina at Chapel Hill (UNC; www.unc.edu) have developed a streamlined chemical process that offers a potential alternative to recycling used tires.
Source: UNC
The polymer laboratory of UNC chemistry professor Alex Zhukhovitskiy recently published work describing a method for deconstructing waste tire rubber into precursor compounds that can be used to make epoxy resins. The researchers developed a sulfur diimide reagent that introduces an amine group to the allylic position (adjacent to C=C double bonds) in the polymer chain. After C-H amination, the method induces an aza-Cope rearrangement (ACR), which inserts a nitrogen atom from the previously added amine group into the polymer backbone. ACRs have been used in other organic synthesis applications, but Zhukhovitskiy’s lab is the first to develop a method for employing ACR for recycling waste tires.
“We thought that if we could introduce chemical entities into the polymer backbone that are readily cleavable with water, that would offer a straightforward way to deconstruct the waste rubber,” explains Zhukhovitskiy. After experimenting with the amination, followed by the ACR, using small polymers and house-made rubbers, the researchers found that the approach also worked on crumb rubber from used vehicle tires.
The two-step reaction, which occurs in the same vessel, broke down used rubber into a soluble material with amine groups that could be broadly useful in making materials like epoxy resins, according to the research team. These resins are widely used for adhesives, coatings and composite materials. The researchers point out the contrast between their new recycling method, which occurs at mild conditions, and traditional recycling techniques, which often require high temperatures or expensive catalysts.
This study, published in a recent issue of Nature, shows that amine-modified poly-dienes, produced using the UNC researchers’ method, can create epoxy materials with strength similar to commercial resins.
While so far, the reaction has been carried out at small scales, the team is thinking about engineering challenges of scaleup, such as the design of a reactor in which the recycling process could happen, as well the efficient handling of solid crumb rubber and the separation of byproducts, such as carbon black, that are used in tires.