Technological breakthroughs

Scientists in Italy and Bangladesh have developed a cold-calendering process that transforms end-of-life tires into 100% recycled rubber sheets without chemical additives or external heating, offering a potential new route to reduce microplastic pollution.

A research team led by Anna Gobetti from the University of Brescia, Italy, in collaboration with the University of Chittagong, Bangladesh, has demonstrated that ground rubber from discarded tires can be converted into solid rubber sheets using a cold-calendering process. The study was recently published in the journal Recycling.

Unlike conventional recycling methods, the process requires neither chemical additives, devulcanizing agents, nor external heat. Instead, friction generated during rolling raises the material's temperature to around 70°C, allowing the rubber particles to bond into a cohesive sheet.

More than one billion tires reach the end of their service life worldwide each year, including nearly 3.9 million tonnes in Europe alone. Because tire rubber is vulcanized, it cannot be remelted like thermoplastics, making recycling particularly challenging. Most waste tires are currently either used for energy recovery or mechanically ground into rubber granules for secondary applications.

However, this recycling pathway is facing increasing regulatory pressure. Under the European Union's restrictions on intentionally added microplastics, recycled rubber infill used in artificial turf will be phased out from 2031, prompting the tire recycling industry to seek alternative, higher-value applications.

In the study, mechanically ground tire rubber was separated into three particle-size ranges before being processed through a two-roll calender. After approximately 40 minutes, or 450–500 rolling cycles, the loose particles formed solid rubber sheets that could be handled without cracking or delamination.

The researchers found that particle size had a significant influence on the final material properties. Sheets produced from finer particles exhibited approximately 18% higher crosslink density than those made from coarser particles, along with greater hardness and elastic modulus.

Scanning electron microscopy (SEM) images revealed that the individual rubber particles had merged into a continuous matrix with minimal porosity and no major structural defects. Although the tensile strength of the recycled sheets reached only 1.6–1.8 MPa—lower than that achieved through thermal sintering—the material is considered suitable for applications that do not require high mechanical strength.

The study also identified an important limitation. Leaching tests showed that zinc release increased substantially as particle size decreased, likely because finer particles provide a larger surface area for metal dissolution. The authors therefore recommend further environmental assessments before the material is used in products that come into direct contact with soil or water.

According to the researchers, converting recycled rubber from loose granules into solid sheets could help reduce the direct release of microplastics compared with granular applications. Nevertheless, they caution that the material may still generate microplastics under severe abrasion during service.

Potential applications include non-structural construction panels, vibration-damping components, protective barriers, and other engineering products where moderate mechanical performance is sufficient. Compared with thermal sintering, the cold-calendering process offers lower energy consumption, a simpler manufacturing route, and maintains a composition of 100% recycled rubber.

The researchers conclude that cold calendering represents a promising and sustainable approach for recycling end-of-life tires, supporting both circular economy goals and increasingly stringent environmental regulations on microplastics.

Source: Gobetti, A.; Cornacchia, G.; Dey, K.; Ramorino, G. "Impact of Particle Size on Properties of 100% Recycled End-of-Life Tire Rubber Sheets from Calendering." Recycling 2025, 10, 207. https://doi.org/10.3390/recycling10060207 (Creative Commons Attribution CC BY 4.0 license)