Rubber Recycling: How Scrap Tyre Rubber Gets a Second Life

By:     Conor Murphy

Rubber’s Challenge: Why Rubber Recycling Requires More Than Melting

Recycling rubber from tyres is fundamentally more complex than recycling glass, aluminium, or paper, and understanding why explains both the processing routes used and the end markets available for recycled rubber products.

The challenge is vulcanisation. During tyre manufacture, the rubber compound is vulcanised: sulphur bridges form between the polymer chains, creating a three-dimensional cross-linked network that gives the rubber its strength, elasticity, and heat resistance. This cross-linking is what makes a tyre perform under the demanding conditions of road contact, temperature variation, and mechanical stress. It also makes the rubber impossible to melt and reprocess into new rubber compound the way thermoplastics can be remelted and remoulded.

This means rubber recycling, unlike plastic or glass recycling, cannot simply involve melting and reprocessing. The vulcanisation must either be accepted (and the rubber used in particulate form as crumb rubber), reversed through energy-intensive devulcanisation processes, or broken down through pyrolysis into simpler chemical compounds.

Each of these approaches produces a different output with different applications and different commercial values. Understanding the processing routes and their outputs is essential context for anyone involved in tyre collection, processing, or equipment specification.

Gradeall International manufactures tyre processing equipment that sits at the front end of the rubber recycling chain. The tyre recycling equipment range from Gradeall’s Dungannon, Northern Ireland facility includes balers, sidewall cutters, splitters, and rim separators that prepare tyres for onward processing through whichever recycling route the downstream processor uses. With nearly 40 years of manufacturing experience and equipment operating in over 100 countries, Gradeall’s equipment processes tyres for rubber recycling operations globally.

Route One: Mechanical Size Reduction to Crumb Rubber

The most commercially established rubber recycling route is mechanical size reduction: the progressive shredding and granulation of whole or pre-processed tyres into crumb rubber, a particulate rubber material with particle sizes typically ranging from 0.5mm to 10mm depending on the application.

The processing sequence. Whole tyres or pre-cut tyre sections enter the processing line at the primary shredder. Large, high-torque shredders reduce tyres to chips of 50 to 100mm. Magnetic separators then extract the steel wire reinforcement (which is sold as scrap metal). The rubber chips pass through secondary shredders and granulators that progressively reduce particle size. Air separation systems (aspirators) remove the nylon and polyester textile fibres that remain in the rubber. The output is crumb rubber at the target particle size, along with the recovered steel and fibre by-products.

Cryogenic processing. An alternative to ambient mechanical shredding is cryogenic processing, where tyres are cooled with liquid nitrogen to below -120°C, making the rubber brittle. The brittle rubber is then fractured by mechanical impact into smaller particles, and the steel and fibre are separated by screening and air separation. Cryogenic processing produces a more liberated, cleaner crumb rubber granulate with better separation of rubber from steel and fibre than ambient processing, but requires significant energy input for cooling and is typically used for producing fine-particle crumb rubber for specialist applications.

Crumb rubber particle sizes and their applications. The particle size of crumb rubber determines which applications it is suitable for:

Coarse granulate (4 to 10mm) is used in playground safety surfaces, equestrian arena surfaces, rubber-modified asphalt for road surfacing, and as an infill material in sports surfaces. Medium granulate (1 to 4mm) is used in artificial turf infill, rubber flooring tiles, and moulded rubber products. Fine crumb (under 1mm, approaching rubber powder) is used in new rubber compound production where partial substitution of virgin rubber is acceptable, and in some coating and waterproofing applications.

Route Two: Devulcanisation

Devulcanisation reverses, at least partially, the chemical cross-linking that makes rubber non-meltable. If the sulphur cross-links can be selectively broken without degrading the polymer backbone, the resulting devulcanised rubber can be reprocessed into new rubber compound, potentially substituting a significant proportion of virgin rubber in new tyre or rubber product manufacture.

The challenge of devulcanisation is selectivity. The sulphur cross-links must be broken without also breaking the polymer backbone; if the polymer degrades, the resulting product has inferior mechanical properties. Several devulcanisation approaches are used commercially or in development:

Chemical devulcanisation uses chemical reagents to break sulphur bonds. The selection of reagents and processing conditions determines the selectivity of the bond-breaking process. Chemical devulcanisation has been in commercial use for some applications, particularly for reclaiming rubber from off-specification factory scrap, but large-scale application to post-consumer tyre rubber remains technically challenging.

Microwave devulcanisation uses microwave energy to selectively heat the polar sulphur bonds, breaking them preferentially over the non-polar carbon backbone. Microwave processing is faster and more selective than some thermal approaches, and pilot and commercial-scale systems have been developed.

Ultrasonic devulcanisation uses high-frequency ultrasonic energy applied through a flow-through die to mechanically and thermally break cross-links. Ultrasonic systems have been demonstrated at pilot scale with promising results for producing devulcanised rubber that can replace a meaningful proportion of virgin rubber in new compound.

The commercial reality of devulcanisation is that it remains an area of active development rather than a fully mature technology at the scale needed to process the UK’s tyre stream. Devulcanised rubber currently serves niche applications where its economics and properties align; it is not yet a mainstream route for the majority of scrap tyre rubber. Development continues, and the route’s commercial significance is likely to grow as the technology matures.

Route Three: Pyrolysis

Pyrolysis is the thermal decomposition of organic materials in the absence of oxygen. When tyre rubber is heated to 400 to 700°C in an oxygen-free environment, the complex rubber polymers break down into simpler compounds that leave the reactor as gases (which condense to a liquid oil fraction and a non-condensable gas fraction) and a solid residue (char, which contains carbon black and inorganic material from the tyre).

The three main products of tyre pyrolysis and their applications:

Pyrolysis oil (tyre-derived oil, TDO). The liquid fraction from tyre pyrolysis is a complex mixture of hydrocarbons with a composition broadly similar to light fuel oil. It can be used directly as a fuel in industrial burners, as a blending component in fuel oil, or as a feedstock for further chemical processing to produce higher-value petrochemical products. The energy content of TDO is high; each tonne of tyres processed produces approximately 400 to 600 litres of oil equivalent.

Carbon black (recovered carbon black, rCB). The solid char from tyre pyrolysis contains carbon black, the fine carbon particles that are used as a reinforcing filler in new tyre manufacture. Recovered carbon black from pyrolysis has the potential to substitute for virgin carbon black in rubber and plastic applications. The challenge has been that rCB from tyre pyrolysis is less consistent in quality than virgin carbon black, containing residual inorganic material and variability in particle structure that makes direct substitution in demanding applications difficult. Significant work has been done on rCB cleaning and characterisation, and several producers now offer certified rCB grades for specific applications.

Steel. The steel wire reinforcement from tyres is recovered from the pyrolysis char as clean steel, suitable for sale as scrap metal.

Gas. The non-condensable gas fraction from pyrolysis has a high calorific value and is typically used within the pyrolysis facility as a fuel to heat the reactor, improving the overall energy efficiency of the process.

Route Four: Civil Engineering Applications Through Baling

Before the rubber reaches any of the processing routes described above, a significant portion of UK tyres takes a different path: whole tyre use in civil engineering applications through PAS 108-compliant tyre baling.

PAS 108 tyre bales contain approximately 100 whole passenger car tyres compressed together, wrapped in geotextile fabric. These bales are used as a lightweight, durable structural fill material in civil engineering projects. The rubber of the tyres is not chemically altered; it is used in its original state as a structural material exploiting its physical properties of high compressibility under load followed by elastic rebound, durability in wet conditions, and relatively low weight compared to conventional fill materials.

This route consumes a large volume of whole tyres and gives them a genuinely functional second life as an engineering material. A tyre bale used in a road embankment or coastal protection scheme has a service life measured in decades, locking the rubber material out of the waste stream for a substantial period.

Gradeall’s MKII tyre baler is the primary production tool for PAS 108 bales, producing bales at the dimensional and density specifications required by the standard. The equipment is used by tyre processors, civil engineering contractors, and waste management operators across the UK and internationally.

The Commercial Reality of Rubber Recycling Markets

The rubber recycling market is subject to price volatility driven by the price of virgin rubber, the volume of used tyres available, and the demand for specific crumb rubber particle sizes in end-use markets.

Crumb rubber for artificial turf infill has been the largest end market by volume in recent years, but this market is under regulatory scrutiny in several European countries due to concerns about chemical leaching from rubber granulate in contact with players. Regulatory decisions on artificial turf infill will affect crumb rubber market volumes and prices.

Road surfacing with rubber-modified asphalt (RMA) is a growing application that improves road durability and reduces road noise. UK Highways England specifications include RMA options, and local authority road resurfacing programmes using RMA consume increasing volumes of crumb rubber. This market is generally less price-volatile than the artificial turf market and is supported by government procurement of road maintenance.

“The end markets for recycled rubber are more diverse and more robust than they were ten years ago,” says Conor Murphy, Director of Gradeall International. “The processing industry has matured, the applications have diversified, and the regulatory pressure to divert tyres from landfill and from fly-tipping has supported investment across the chain. Our equipment at the front end of that chain is part of making the whole system work.”

Contact Gradeall International for tyre processing equipment that prepares rubber for its most commercially valuable next application.

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