Market and product

Compiled by Bao Hien

An Indian research team has used a mixture of choline chloride and urea to break sulfur bridges in waste tire rubber. The solvent can be reused and is relatively harmless to bacteria. When combined with bacteria, the amount of rubber degraded after 28 days increased nearly eightfold.

Two studies were conducted by a research team led by Utpal Roy at the Birla Institute of Technology and Science, Goa campus, India. The first study was published in Results in Engineering on February 18, 2026. The second, building on the first, was published in RSC Advances on May 18, 2026.

Tires are treated with sulfur to give rubber high strength and elasticity. However, sulfur bridges link polymer chains into a tightly connected network, making rubber difficult to recycle and degrade. According to sources cited by the researchers, between 800 million and more than 1.5 billion tires are discarded each year.

One possible approach is devulcanization, which involves breaking sulfur bridges while preserving the main polymer chains as much as possible. If successful, this process can turn waste rubber back into a usable raw material.

A solvent made from salt and urea

The researchers used a special type of solvent known as a deep eutectic solvent, made by mixing choline chloride and urea at a 1:2 molar ratio. When gently heated to 65–70°C, the two solid substances turned into a clear liquid.

Both choline chloride and urea are relatively inexpensive, have low toxicity, and are biodegradable.

The waste tire rubber was ground into particles approximately 0.6 mm in size and then soaked in the solvent at a mass ratio of 1:40 for 24 hours. The mixture was subsequently treated with ultrasound for two hours and heated at 180°C for 45 minutes.

According to the researchers, 180°C was an appropriate temperature. At lower temperatures, the reaction proceeded slowly; at higher temperatures, devulcanization increased, but the reaction became much less selective and risked breaking the main polymer chains.

Sulfur bridges are broken while the rubber chains remain largely intact

After treatment, the crosslink density of the rubber network decreased by 30.3%, from 10.0 to 6.97 × 10⁻⁵ mol/cm³. The amount of sulfur on the surface of the rubber particles fell by about 51%, while the zinc content decreased by nearly 90%.

When the entire material was analyzed, the sulfur content decreased by approximately 31–33%. Thermal analysis also showed that the energy required to decompose the material fell from 212 to about 188 kJ/mol, indicating that the rubber network had become less tightly connected.

Analysis using a Horikx plot showed that the solvent primarily broke sulfur bridges rather than the main polymer chains. The particle size of the rubber also remained almost unchanged.

The solvent also removed several additives from the rubber. Dicyclohexylamine, a vulcanization accelerator, decreased by 85%. The antioxidant BHT also decreased by 85%, while dehydroabietic acid fell by nearly 70%. By contrast, IPPD, an antioxidant from the same phenylenediamine group as 6PPD, was largely unaffected.

However, the researchers did not determine whether the removed substances had been completely degraded or converted into other compounds.

The solvent can be reused

After treating the rubber, the researchers filtered and recovered the solvent for use with a new batch of rubber. The results were similar to those obtained in the first cycle: the crosslink density decreased by 29.7%, while the zinc content fell by about 93%.

According to calculations at laboratory scale, reusing the solvent reduced the energy consumption and cost of the subsequent treatment batch by approximately 63%.

The researchers said this was the first study to evaluate the reusability of this type of solvent for tire-rubber devulcanization. However, the solvent could only be used effectively for two cycles. Its performance then declined, and the researchers proposed aerobic biodegradation as a final treatment method.

Tests also showed that the solvent did not produce inhibition zones around cultures of E. coli and Shigella flexneri. The minimum inhibitory concentration was above 128 mg/mL.

According to the OECD 301D biodegradation test, the fresh solvent achieved 74% biodegradation after 28 days, while the used solvent reached 71%.

Combining the solvent with bacteria increases rubber degradation nearly eightfold

In the second study, the researchers continued by using the bacterium Rhodococcus rhodochrous RPK1 to degrade rubber over 28 days.

Untreated rubber lost only 1.63% of its mass. In contrast, rubber that had first been treated with the solvent lost 12.53% of its mass, or 7.68 times more.

The crosslink density of the rubber network decreased by 43.6% in the combined treatment, compared with 15.8% when bacteria alone were used. The fraction of soluble rubber also increased from 6.3% to 17.3%.

After 14 days, the number of viable bacteria on the rubber pretreated with the solvent was 43.9% higher than that on untreated rubber.

The results suggest that breaking some of the sulfur bridges with the solvent first may make the rubber easier for bacteria to access and degrade.

Many challenges remain

Both studies were conducted at laboratory scale. The solvent-to-rubber ratio was 40:1, meaning that treating 1 kg of rubber required as much as 40 kg of solvent, equivalent to approximately 21.5 kg of choline chloride and 18.5 kg of urea.

According to the laboratory chemical prices cited by the researchers, these raw materials alone would cost about USD 880. Therefore, the ability to recover and reuse the solvent will be an important factor determining the economic feasibility of the process.

The studies also did not fully assess the mechanical properties of the rubber after devulcanization and revulcanization. It therefore remains unclear whether the resulting rubber could be used to manufacture new tires or would mainly be suitable for products with lower performance requirements.

In the second study, most of the rubber remained undegraded after 28 days. The researchers therefore said that further work is needed to optimize the process, test it at larger scales, and evaluate other solvents and microorganisms.

In addition, sulfur and zinc measurements differed depending on the analytical method used. Both studies were conducted by the same research group, and the findings have not yet been independently verified.