Dissolving old tyres to make new

Materials World magazine
14 Jan 2020
McMaster University Professor, Michael Brook, with Graduate Student, Sija Zheng, analysing polymeric oils used in tyres. Credit: Georgia Kirkos, McMaster University

A process to recover materials from tires has been developed that reutilises organic polymers found in automobile and other sulphur-crosslinked rubbers.

Chemists from McMaster University, Canada, have presented an easy and efficient alternative for the product, which are typically prepared for single use from non-renewable resources. 

The research conducted by the team challenges a long-standing problem among the tire industry regarding their recyclability and environmental impact. Some tyres may be later used as fuel in the cement industry or broken down into crumbs to use as fillers for asphalt, cement or turf.

However, there is no existing method that can recover the petroleum-based polymers after production making it difficult to effectively reuse, repurpose or recycle. 

‘The chemistry of the tire is very complex and does not lend itself to degradation - for good reason,’ said McMaster Department of Chemistry & Chemical Biology Professor, Michael Brook. ‘The properties that make tires so durable and stable on the road also make them exceptionally difficult to break down and recycle.’

Breaking the bond 

In the paper, Dissolving rubber tires, published in Green Chemistry in January 2020, the researchers described a process to efficiently break down the polymeric oils by breaking the sulphur-to-sulphur bond. Brook likens the structure to a piece of fishnet.

‘We have found a way to cut all the horizontal lines so instead of having a net, you now have a large number of ropes, which can be isolated and reprocessed much more easily,’ Brook said.

The study presented that during temperature ranges of room temperature to 100ºC, recoveries of organic polymers as oils from tires ranged from 56% for complex mixtures of rubber crumb from ground tires to 93% for butyl rubber.

 After the team removed the inorganic material via filtration, the recovered polymeric oils were radically, or oxidatively, crosslinked. This process generated new elastomers that can optionally reinforced with the solids recovered during the initial reduction procedure. 

‘The products are polymeric, silyl-protected thiolated organic oils that are readily separated from the accompanying, unreactive solids, such as fillers, fibre and metal reinforcements, pigments, etc. simply by filtration or centrifugation,’ the paper read. ‘The products, sulphur-containing polymeric oils, may be converted back into reinforced rubbers using simple oxidative or radical processes.’ 

Eliminating the stockpile risks 

The researchers have developed this method in an attempt to eliminate and prevent the major dangers and concerns that stockpiling tires poses to the environment. The team acknowledged the method’s promising potential, as well as some of the limitations due to the potentially high costs for implementing in industrial applications. 

‘We're working on it, but this is the first major step,’ said Brook. ‘This process closes the loop on automotive rubber, allowing old tires to be converted into new products.