An interview with Dr. Ben Chouchaoui from Windsor Industrial Development Laboratory.

Scrap tires are hazards. They take hundreds of years to decompose naturally, collect rainwater and frequently serve as breeding grounds for mosquitoes. Additionally, they are hard to extinguish if they catch on fire and are made from finite resources such as hydrocarbons. Natural rubber (that’s used in the manufacturing of tires), is concentrated in the Far East and presents major logistical issues—many of which were felt during the recent COVID-19 pandemic and conflicts in the Ukraine and the Middle East.

Current solutions to address retiring tires in most cases are not proving effective and government bodies are looking for more advanced solutions. EcoCa (a spinoff of Windsor Industrial Development Laboratory or WIDL) is proposing devulcanization of thermoset rubbers. This will enable infinitely-recyclable rubber to be turned into useful materials (such as devulcanized rubber pellets to surface roads) or products (and the idea is to develop large products because of the magnitude of the scrap tire problem).

In an exclusive interview with Autosphere, Dr. Ben Chouchaoui, a rubber Product Development Specialist from WIDL, discusses some of the more recent trends in scrap tire disposal and recycling as well as some of the opportunities regarding new practices being considered by the industry.

Autosphere: Can you tell us a little about the current state of scrap tire recycling in Canada?

Dr. Ben Chouchaoui: A recent report by Ottawa’s Materials Technology Laboratory (CanMet), supported by the Mineral and Metals Sector of Natural Resources Canada, gathered data from all Canadian provinces and the Canadian Association of Tire Recycling Agencies (CATRA). The objective was to develop a further picture of tire recovery and recycling in Canada and the related environmental impacts—including greenhouse-gas emissions (GHG)—from both current and future practices.

The study, “Scrap Tire Recycling in Canada”, shows a great market potential for processing retiring tires into new applications and value-added products. Thanks to well-operated tire programs in Canadian provinces, scrap tires across the country are now being processed according to several techniques.

The biggest output for scrap tires in Canada over the past two decades was crumb rubber, representing more than 40% by weight (wt) of the total scrap tire usage, exceeding 0.5M metric tonnes. A further 20% wt was used as tire-derived fuel (TDF) for cement kilns and paper mills; 18% wt was directly recycled into geometrically simple products like mats, interlocking bricks or tiles, etc., by gluing the crumb rubber with epoxy or polyurethane (PU), and 13% were processed as shreds; the balance was baled or stored. Approximately 75% of all scrap tires required processing in ambient and cryogenic recycling plants. Only cement kilns can accommodate whole tires, while civil engineering applications like barrier reefs, require no crushing or grinding of scrap tires. The report indicated that no single solution for Canada was viable, recommending different processing methods to address the huge scrap tire waste streams.

AS: Can you tell us a little about some of the options currently available when it comes to the scrap processing of tires?

BC: The recycling of tires is not that different from addressing other waste products, particularly from the viewpoint of protecting the environment. There are four basic choices in dealing with scrap, commonly referred to as the four Rs (4Rs): Reduce, Reuse, Recycle, and Recover.

First in the hierarchy is Reduce, which means reducing the generation of waste. It is the best method to reduce greenhouse gases, and it also saves natural resources and avoids the upstream generation of solid wastes.

As to the reuse of tires (while limited because of safety issues), there still is a strong market in retreading worn tires. Truck tire retreading is one of the best examples of extending the use of a tire. In fact, most commercial truck and aircraft tire casings are re-treaded several times before being discarded. Truck and aircraft tires are specially designed with retreading in mind. Passenger tires, however, are very different and offer little opportunity for retreading except in small niche markets.

Tire manufacturers, through ever-evolving technologies, have prolonged the life expectancy of automobile and truck tires (the need for new tires is reduced however when automobile manufacturers sell more new cars). The public too, can make useful contributions: It is estimated that tire life can be extended by as much as 30% simply by motorists maintaining proper tire pressures, rotating tires as recommended, keeping wheels properly aligned and practicing good driving habits.

Recycling rubber and other materials from scrap tires damaged beyond repair is the biggest market. Most technologies described in the report focus on material recovery and the generation of new products from it. However, the products seen so far on the market consist of gluing crumb rubber with epoxy or PU for use in playgrounds and sport fields (which are not sustainable solutions) or into simple three-dimensional products like mats or pavers. A paradigm shift is to devulcanize the rubber (breaking the bridges between long molecular chains), with which to make new products (mixed to given ratios with non-recycled rubber, an operation known as compounding in the industry, depending on the stringencies of the applications sought).

AS: How does that play out for rubber sustainability and rubber product circular economy?

BC: Vulcanized rubber cannot be remolten (like thermoplastics) and put to new uses. Today, the main end-of-life-tire (ELT) solutions [and for other rubber products] are landfilling, incineration (i.e., used by cement plants or in waste-to-energy programs), and grinding to fine granules. The latter generates huge quantities of powder and represents a lack of sustainable recycling of valuable rubber.

Additionally, tires have become high-tech products, where the simultaneous improvements of wet traction, rolling avoidance, and abrasion resistance is hard to achieve. A total of 26.3 million motor vehicles were registered for road use in Canada in 2022, which equivalates to about 330,000 metric tonnes of scrap tires each year. Further adding to that are scrap tires from OTR (off-the-road) vehicles, trucks, buses, tractors, farm and construction machinery, etc. By devulcanizing ELTs, it is possible to produce new raw materials with good mechanical properties and a superior environmental footprint over new materials. Devulcanization or the breaking up of sulphur bonds by mechanical, chemical, thermophysical, or biological means, is promising for rubber sustainability and rubber product circular economy.

In vulcanization, sulphur can form bonds between unsaturated polymer chains found in latex to yield natural rubber; the process is also used for synthetic rubbers. Accelerators can be added in the process, at elevated temperatures. Accelerated sulphur vulcanizations are classified into conventional (CV), semi-efficient (semi-EV), and efficient vulcanization (EV), depending on accelerator/sulphur ratios (A/S) between 0.1 and 12.

Vulcanization gives the properties to natural or synthetic rubbers. Devulcanization aims at selectively cleaving the C-S bonds while leaving the C-C bonds intact. The devulcanization of waste rubber applies energy to the material in order to break up, totally or partially, the three-dimensional network formed during vulcanization. The process is difficult to achieve, since the energies needed to break the S-S and C-S bonds (227 and 273 kJ/mol, respectively) are lower but close to the energy required to break the C-C bonds (348 kJ/mol).

The higher the selectivity of the devulcanization process, the better the mechanical properties of the material. Scientists developed a tool for investigating the mechanism of network breakdown in a vulcanized rubber network. Accordingly, the rate of increase of the soluble (sol) fraction of the rubber as a function of the measured cross-link density of the remaining insoluble (gel) fraction is different for cleavage of carbon-sulfur and carbon-carbon bonds. Thus, sol fraction and cross-link density measurements of devulcanized rubber samples yield an indication of the dominant mechanism of network breakdown.

Regarding useful life, there are two types of tires: 1. Reusable tires and 2. Non-reusable ones. Those tires that cannot be re-treaded because of advanced damage, structural deformation, or high degradation are the starting materials for ELT recycling.

For devulcanization, waste rubber tires are typically processed into ground tire rubber (GTR) that can be imbedded into rubberized asphalt, bitumen, cement, concrete, tiles, thermal and acoustic isolations, etc. However, simply mixing untreated GTR into an (elastomeric) matrix greatly decreases its mechanical properties, because the cross-linked rubber particles show poor interfacial adhesion and dispersion. A twin-screw extruder for thermomechanical devulcanization is considered the most practical method, as that type of machinery is commonly used in the polymer industry. In addition, scalability to industrial volumes is seen as the best solution when it comes to extruders. For thermochemical extrusion, the use of supercritical CO₂ (scCO₂) showed benefits. Chemically non-toxic, inactive, non-flammable, and inexpensive, the critical point of CO₂ can be reached easily (at 31.1 C and 7.38 MPa), and residual scCO₂ from the devulcanized rubber removes easily to the ambient.

AS: What are your thoughts on how to implement real scrap tire recycling mechanisms in the industry, including devulcanization?

BC: With an increasing global population, consumption of polymers such as thermoplastics, thermosets, and elastomers has shown significant growth since the 1950s with thermoplastics representing by far the largest group. The worldwide production of plastics reached a staggering 400.3M metric tonnes in 2022. This marks an increase of about 1.6% from the previous year even factoring in a global pandemic, as well as political and economic instability. Plastics production has soared since 1950s with continued growth in production and market share. However, such a steady historic yearly growth rate is expected to flatten in the coming years because of pressures toward recycling plastic products (like water bottles).

Feedstock recycling can be considered the ultimate goal for polymers in that the original monomers are recovered. This seems feasible for some pure polymers, but for complex product mixtures such as rubbers [the bulk of which is made into tires], feedstock recycling back to isoprene and other constituents isn’t feasible today. However, a process to reverse vulcanization (known as devulcanization), makes the elastomeric material meltable and processable again, offering a route to recycling ELTs back into high value-added materials and products. Through devulcanization, natural and synthetic rubbers can be partly replaced and saved, leading to multiple economic and environmental benefits.

AS: Can you tell us a little about how the tire recycling process typically works?

BC: Tires are used on all sorts of vehicles. After several years, they need to be replaced, because their profiles wear out, and/or they become brittle and crack under repeated stressing and the elements. Retreading is done for truck tires, while passenger car tires are mostly single-use items. End-of-life-tires can be mainly recovered through two routes: 1. The recovery of material or 2. The recovery of energy. The calorific value of ELTs is close to that of coal, and they are often used in paper mills and cement kilns. By pyrolysis, oils can be made, though this tends to result in substantial emissions. Another possible outlet is oil spill remediation. Material recovery requires the granulation of ELTs. Grinding encompasses ambient, wet, and cryogenic processes. Most technologies for tire recycling involve the separation of metallic and textile (cord) materials and a grinding process leading to a significant reduction of the tire’s dimensions. During grinding, which typically yields granulates of a few millimetres or below, the temperature can be lower than the glass transition temperature (i.e., cryogenic grinding) of the polymers in the tires at room temperature. The resulting powder can be used as a filler, e.g., in new tire compounds, but only in little amounts. To improve the compatibility between the new rubber compound and ELT powder, the latter must be devulcanized by breaking the three-dimensional cross-linking network, or at least by modifying the surface of the granules.

Twin-screw extrusion for mechanical devulcanization is the most practical way to treat scrap tires, for commonality in the polymer industry. The process uses no chemicals and no heat and, can be scaled up to industrial volumes. The use of supercritical CO₂ (scCO₂) helps swell the rubber and stretch the cross-links between long molecular chains; CO₂ is chemically non-toxic, inactive, non-flammable, inexpensive, and compatible with the ambient. Good rubber is obtained, and extruders can process between 0.5 to 3 metric tonnes of scrap tires per hour, which over three shifts can treat 18,000 metric tonnes or about 1.5 million scrap tires, yearly. The devulcanized rubber can be turned into pellets to mix with asphalt to rubberize roads, or as a master batch for polymers, into moulds or extrude products.

AS: Tell us a little about some of the research findings from your laboratory?

BC: EcoCa, an offshoot of Windsor Industrial Development Laboratory (WIDL), developed technologies to 1. Recover rubber from products containing rubbers like tires, 2. Devulcanize the recovered rubber, 3. Compound the devulcanized rubber, and 4. Manufacture with recycled (devulcanized and compounded) rubber materials and products. Currently, EcoCa offers two lines of products:

1. Recycled Rubber: This can be recycled SBR (styrene butadiene rubber) pellets to replace SBS (styrene butadiene styrene) polymer in asphalt to rubberize roads, parking lots, driveways, etc. EcoCa also offers devulcanized rubber that can be used in mixing, to reduce the cost of new compounds.
2. Recycled Rubber Products: EcoCa is moulding under contract a small solid idler for the mining industry for a heavy machinery manufacturer in the GTA (Greater Toronto Area). EcoCa is also launching a first product of its own, namely a front parking block for passenger vehicles.

EcoCa offers services in terms of receiving and treating post-industrial polymeric wastes involving rubbers as well. One such application is temporary seals around vehicle doors from General Motors at the GM facility in Warren, Mich. Talks are underway with various other vehicle Original Equipment Manufacturers (OEMs) and different level tire suppliers too.

Many products are under developments under the EcoCa brand as well and include various markets and industrial sectors spanning automotive, oil and gas, leisure, etc. Some of the applications that we are present in are:

• Truck front parking blocks
• Bus front parking blocks
• Lane dividers (between bikes and vehicles)
• Barrier-post joints along highways
• Sleeve covers to bollards
• Entrance and anti-fatigue pads
• Fenders for boats
• Fenders for docks

Additionally, EcoCa is open to engineering applications for customers. Developing engineered (functional) products from rubber recouped from scrap tires involves three main technical ingredients: 1. Rubber chemistry (in particular, the devulcanization of vulcanized rubber), 2. Rubber product design and engineering (based on computer-aided design and simulations), and 3. Rubber manufacturing with rheology and tooling design and machine processing (manufacturing), to optimize in a computer then make. The EcoCa team and extended team are made up of veterans in the Rubber Industry and each member offers advanced academic credentials and decades of industrial experiences in material development and characterization, rubber chemistry, product design and optimization, manufacturing processing and fine-tuning, tooling, prototyping, and testing.

AS: If we look at some of the products EcoCa is developing, what are some advantages they offer compared to existing solutions?

BC: EcoCa recycled rubber parking blocks offer:

Weather Resistance—Cement parking blocks crack, break, change colour with the elements, and build mould. They are heavy to move around and damage the pavements below. Rubber blocks eliminate these issues and avoid scratching the underneath of fenders of vehicles when parking close to a block.

Greater Longevity—Rubber blocks resist the weather compared to cement blocks. Water in cracks can freeze to break the cement blocks.

Improved Economics—Cement blocks are steel-reinforced and require many moulds as curing takes around 24 hours. Conversely, one single mould makes several rubber blocks per hour.

Increased Safety—Markings stand out on rubber blocks, making parking at night and in poor weather conditions easier. Rubber absorbs hits, cushions slips and falls, and does not damage vehicles. It is also easy to install and remove, and is light to bring in to and take away from job sites.

Environmental Responsibility—Using scrap tires to make parts re-uses masses of rubber that would otherwise be wasted every year. Recycled rubber blocks eliminate 200 scrap tires in a 60-slot parking lot. It does not take a large project to make a sizable dent in our huge ELT waste problem. Ontario alone discards about 0.17M metric tonnes of scrap tires each year. Three quarters of these can be diverted away from incineration and landfills, into similar product development endeavours.

AS: If applicable, can you tell us about any future developments?

BC: Strategies to treat ELTs in North America mainly include incineration, crumb rubber generation, and land filling, all of which presenting numerous drawbacks, besides a good deal of illegal dumping. EcoCa is offering current technologies three steps forward (through the successive steps of 1. Recovered rubber devulcanization, 2. Devulcanized rubber compounding, and 3. Compounded devulcanized rubber manufacturing into engineered materials and products). The benefits are numerous and include natural resource savings, reductions in greenhouse emissions, drops in energy consumptions, rubber sustainability, and the only path currently plausible to a true rubber product circular economy.

EcoCa’s range of products aim to address the growing health and environmental challenges scrap tires pose, through crumb rubber devulcanization and compounding, and then turning that into engineered materials (pellets for pavements) and products (starting with parking blocks for passenger vehicles and trucks), made locally and generated from locally sourced scrap tires.