Rubber and beyond: What are tires made of?
As manufacturers move toward the circular economy, they’re increasing the use of recycled materials.
In the tire and automotive industry, one of the solutions is to extract raw materials from recycled tires to produce new tires with a higher percentage of recycled materials.
Contec is one of the supporters of this movement toward circularity, supplying raw materials from recycled tires and allowing manufacturers to keep resources circulating within the industry.
However, tires are complex products, and to fully understand how a tire-to-tire circular model can function properly, it’s essential to know the composition and components of the tires.
Let’s start with understanding the composition of tires: what are they made of?
What are tires made of?
Tires are complex structures made of rubber, steel, and fabric components. Rubber is the primary material in any tire, and four main types of rubber are used in various tire components (Grammeli).
The primary source of natural rubber is Hevea trees (or rubber trees), whose latex is about 40 per cent rubber. The rubber is extracted by coagulating the latex with formic acid. Natural rubber is self-reinforcing and has high mechanical strength and medium elasticity but has low viscosity and other disadvantages.
Natural rubber needs further treatments before it can be used in tire manufacturing, such as vulcanisation, mixing with Carbon Black fillers, and other processes (Deng).
Due to the high cost and scarcity of natural rubber, synthetic rubber polymers produced from fossil-fuel-based hydrocarbons are also used (Deng). All synthetic rubbers are highly elastic and have good wear resistance, but variations in heat generation and hysteretic loss exist. There are four main types of synthetic rubber:
- Styrene Butadiene Rubber
- Polybutadiene Rubber
- Isobutylene-isoprene Rubber
- Isobutylene-isoprene Halogenated Rubber
In tire manufacturing, synthetic and natural rubber are cut and mixed in fixed ratios with other ingredients.
Different recipes or precise mixes of materials produce tires with unique properties suitable for diverse vehicles. Moreover, tire rubber composition can differ based on national or regional regulations (Grammelis), as shown in Table 1.
Composition of tires: beyond rubber
Even though rubber is the primary component in any tire, it’s essential to recognise that tires are complex products, and their composition extends to many other materials that vary according to their use and country of origin.
Besides rubber, the different materials in a tire are steel, textiles, fillers, and chemical additives required for structure, strength, longevity, and durability, as we can see in the table below:
Table 1: The composition of tires from different regions will vary. Materials are listed according to the percentage of the total tire weight. (Credits: Progress in used tyres management in the European Union: A review)
Let’s look at each tire component in more detail:
- Rubber (natural and synthetic) comprises 41-45 per cent of tire materials and has structural and strain functions. Rubber determines tread tensile and tear strength, elasticity, and elongation. Truck tires have more natural rubber, and car tires have more synthetic rubber. Trucks carry heavy loads and travel intensively, so the tires are subject to wear and tear. Natural rubber’s abrasion properties are superior to synthetic rubber, so more natural rubber is used for trucks. A higher proportion of synthetic rubber is enough for less wear and tear in car tires due to lower load and mileage.
- Fillers make up nearly 30 per cent of tire materials. Fillers such as Carbon Black, silica, carbon, and chalk, among others, are reinforcing materials. Different fillers provide varying tire strength, wear resistance, tear resistance, rolling resistance, puncture resistance, etc.
- Steel makes up 13-25 per cent of tires, and as part of belts, beads, and plies act as a structural skeleton.
- Textiles (polyester, rayon, nylon) as fabric cords comprise around 5-15 per cent of tires and provide structure and reinforcement.
- Antioxidants, antiozonants, plasticisers, and curing chemicals comprise the remainder. The substances and percentage of these minor ingredients depend on each manufacturer.
Antioxidants like phenols and secondary naphthylamines protect against the effects of temperature and oxygen, and antiozonants shield tires against ozone. Curing chemicals like sulphur, zinc, lead, magnesium, and cadmium oxide are used for vulcanisation. Plasticisers, such as oils and resins, are added to the rubber mixture to reduce friction in the tire.
Differences in tire composition will affect the nature of secondary material recovered after recycling.
Tire recyclers like Contec have a strict truck-to-passenger-tires ratio to maintain consistency in their product profile and characteristics of products like medium-grade recovered Carbon Black (rCB). However, it’s still challenging to standardise rCB since the tires produced in different regions have varying quality and properties due to regional tire composition variations.
Contec’s involvement with ASTM International is important to developing industry rCB standards and industry applications. Learn more about our involvement in ASTM.
What are the main parts of a tire?
The main parts of a tire are the bead, bead filler, inner liner, carcass, sidewall, belts, and tread—you can see it in detail in the image below.
Bear in mind that, depending on the type of tire, there can be more components.
Figure 1: Tire structure. (Credits: What is in a Tire)
The various components used in making a tire are also crucial for material recovery. Several materials can go into making a single component depending on its function. Let’s discuss some components common to all tires below:
- Bead has steel cord in rubber bundles to secure the tire to the wheel rim and prevent wear and tear by rubbing against the edge.
- Bead Filler is a synthetic rubber component wrapped on the top and around the bead and between body plies to tune the ride.
- Innerliner is made of butyl rubber and is necessary to maintain inflation pressure.
- Carcass or body ply is made of textile, fiberglass, and aramid cords to retain tire shape and prevent the tire from bursting during inflation.
- Sidewall is made of natural rubber, protects the carcass, and can withstand bending and aging.
- Belts are composed of steel cords encased in rubber. Belts prevent carcass damage, stabilise the tread, and reduce rolling resistance.
- Tread and tread patterns can have synthetic or natural rubber, depending on their use. It’s the part that comes in contact with the road and provides grip and abrasion resistance.
The cross-linked nature of rubber structures with other materials like fabrics or steel makes recycling challenging, as the different components and materials must be separated before processing.
How are tires manufactured?
Tire manufacturing is a complex process. Each tire manufacturer follows unique and proven procedures, from raw materials selection to quality management.
There are five main stages in tire manufacturing, which, according to Weyessenhoff, are as follows:
- Sourcing good quality material: The first step in tire manufacturing. Choosing reliable and standardised recycled secondary materials allows manufacturers to make tires circular. Manufacturers can get recovered medium-grade Carbon Black, recovered steel, and feedstocks to produce fine-grade virgin Carbon Black. They can also source carbon materials and chemical additives from other recycled biological materials. Choosing materials depends on their properties and interactions with each other because the end goal is to produce a strong and stable tire. The mixed chemical composition makes tires resistant to decomposition by chemicals or high temperatures during recycling.
- Manufacturing components: A step that requires mixing different materials to produce various tire components. For example, steel cords and rubber for beads, etc.
- Tire assembly: This is the stage where the components of prepared belts are wound and glued together. The end-product of this confectioning stage is called a “green tire.”
- Vulcanisation: This step completes the process and gives the tires their final shape, including the tread and its patterns. Since the EU stipulates that the minimum tread groove is at 1.6 mm, some manufacturers add a tread depth indicator: 3 mm for summer tires and 4 mm for winter tires.
- Quality control: Tires must pass quality tests to meet stringent safety standards (UNECE, SAE). Improper quality control during manufacturing can produce hidden defects in materials and tires. Most of these tend to occur at the interface of different materials and can be at the shoulder, internal, external, side, or tread. Quality checks during manufacturing are vital to reducing user risks and financial burdens on customers and producers.
Other questions on how a tire is made
Here are answers to three FAQs for a quick and brief understanding of the multifaceted tire manufacturing process.
Where does rubber come from for tires?
The latex of a tropical tree, Hevea brasiliensis, also known as the “rubber tree”, is the source of natural rubber used in tires.
Around 90 per cent is obtained from Asian plantations. The tire industry is the largest consumer of natural rubber, using 76 per cent of the annual rubber production. The rubber tree is currently the only commercial source of natural rubber, though efforts are being made to identify other easily renewable crops or wildflowers like dandelions to produce rubber.
The four types of synthetic rubber made from hydrocarbons derived from petroleum products are mixed with natural rubber to make tires.
What is used to make tires?
Tires are made of several materials, including natural rubber, synthetic rubber, steel, textiles (rayon, polyester, aramid, and nylon), fillers (carbon black, silica from sand, carbon), chemicals including hazardous compounds like lead and cadmium oxide, and fossil fuels as feedstock for producing many of the synthetic materials.
Are tires made of natural rubber?
Tires are made of natural rubber.
Aviation tires are made entirely of natural rubber as they withstand abrasion better than synthetic rubber. For the same reason, trucks and heavy vehicles also have more natural rubber in their tires. Passengers or light vehicles have more synthetic rubber. Due to the scarcity of natural rubber, synthetic rubber use is high, making up around 60 per cent of tire rubber, while natural rubber makes up about 40 per cent.
Adding recycled materials to tire composition
Manufacturers want to maximise circularity in tires. Therefore, tire manufacturing, especially the sourcing and assembly stages, is undergoing a paradigm shift to fit a circular economy.
Besides recycling and sourcing bio-based renewable materials for significant tire components, manufacturers are also rethinking tire design to make disassembly, material separation, and recycling easier to reduce tire waste.
Contec has one of the few tire pyrolysis plants where they recover 85 per cent of the material in tires and 15 per cent of energy to tackle the growing end-of-life tire waste problem. Contec is promoting tire-to-tire production by providing manufacturers with rCB, recovered steel, and feedstocks to realise manufacturers’ vision.
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