*2.1. The Production Stage*

The design of a modern car tire is an intricate and complex structural process that combines a number of rubber, steel, and textile elements. Each of these materials displays different characteristics that impart a specifically selected set of properties, e.g., shape, stiffness, strength, vibration damping, heat, and electrostatic charges dissipation [5]. Each manufactured tire is expected to perform according to the design specifications; hence, the choice and quality of materials are equally important as the manufacturing technology. Tires are elements that play an absolutely critical role in ensuring the safety of the driver and passengers, which is why it is essential that their production process is performed in accordance with applicable requirements and the product undergoes specialist tests [6]. The product damage in manufacturing usually occurs at the interface of two heterogeneous materials and is categorized as inner, outer, sidewall, tread face, or shoulder damage [7]. The tire is a system combining up to 20 different basic and supplementary elements that affect car handling. The components of the tire are shown in Table 1.



Depending on the class of the tire, it is reinforced with a range of supplementary elements, including insulation inserts, zero-degree belt, chafer strip, bead reinforcements, insolation bundle, sidewall reinforcement, and undertread [10,11]. After the designing, documentation, and prototyping stages, the tire components are prepared. With the top product quality in mind, all materials should be carefully selected on the basis of their respective physical and chemical properties, size, thickness, cutting angles, etc. Design specifications must be observed at every production plant, regardless of its location, thus ensuring that the same tire types provide the same performance, regardless of the place of manufacture [10].

The structure of rubber compounds is carefully adjusted to impart desired properties to the tire, so as to ensure its proper functioning and so that it will serve its intended purpose, e.g., a different compound is used in a summer versus winter tire. Moreover, rubber compounds of specially engineered properties build different parts of the tire, such as the sidewalls, the inner liner, or fillers in the bead zone, all of which are made of various types of rubber compounds. The compounds typically include the following materials: synthetic rubbers (butyl, SBR, BR), natural rubbers, oils, fillers (silica, soot), Sulphur, resins, anti-aging agents (antioxidants), vulcanization accelerators, and other agents as per specific requirements. The approximate material consumption involved in producing 1000 kg of tread rubber compound for a passenger car tire is 500 kg of rubber (mainly synthetic), 150 kg of silica, 150 kg of carbon black, 20 kg of Sulphur, 20 kg of resin, 100 kg of oils, and about 60 kg of other components [12,13]. All the ingredients are machine-mixed, their quantity and the adding sequence being crucial, until a homogeneous mass is obtained. The prepared compound is formed into plates or ribbons, whose dimensions facilitate their use in the subsequent stages of production. The compound surface is next coated with a release agent, for easier separation of rubber layers in the later stages, and the rubber compound material itself is subjected to accuracy measurements and analyses to verify its compliance with the design specifications [14]. An integral part of the tire design are the steel and textile cords. They form a carcass of the tire, which guarantees its required stiffness and shape, and which translates into a desired level of performance while driving. A single thread of a textile cord is made of twisted weaves of a large number of ultra-thin viscose, nylon, aramid, and polyester fibers (a single 195/65 R15 tire contains between 1500 and 1800 threads) [15]. The purpose of bead wires is to enable mounting a tire on a rim. There are several technologies for their production. They may come in the form of rings formed by braiding several wires with a round cross-section or by winding several layers of tape. The beads consist of separate wires of a rectangular or polygonal cross-section, coated in a rubber compound. Having all the components collected and prepared in advance, the confection stage may commence. The tire building process consists of arranging the pre-products in a strictly controlled sequence. Depending on the technology, a raw tire is obtained in a one- or two-stage process, which is followed by vulcanization in special molds, and then quality control [16,17]. The flowchart in Figure 1 presents the car tire production process.

**Figure 1.** The tire-manufacturing process flowchart.

As mentioned, the technologies used by tire manufacturers involve one or two stages. In the former case, the entire tire is produced on a single machine; in the latter, the carcass is typically put together in the first stage, and the basic and supplementary layers and technologies (belt, tread, etc.) are fitted consecutively [17]. The layers of the tire are laid flat inside a drum, which is next filled with air, the pressure of which causes the layers to bond. In the next stage, the structure is subjected to rolling, which shapes the tire, ensures the strong bonding of components and guarantees the removal

of excess air that could become trapped between the components. The process output is the raw tire, which undergoes dimensional and shape control tests for errors [15].

After successfully passing the control step, in the next stage of production, the raw tires are placed in vulcanization presses, where vulcanization molds are fitted. The molds typically have a container structure, i.e., their elements form two sides of the mold. The rings form the sidewalls and impress the branding and other inscriptions, while the side protrusions form the groove and sipes in the tread. The two halves of the rings are each responsible for the half of the tread on one side. The tire is treated with high temperatures (in excess of 150 ◦C), while the pneumatic load presses the tire against the sides and the protrusions of the mold. In the thermoplastic melt-freeze process, the rubber transits to fluid (thus acquiring the shape of the mold) and eventually becomes elastic. Following the treatment, the tire remains in the mold for 10 minutes and then removed to cool down [17,18].

The final quality inspection includes a variety of tests. The visual assessment of the tire condition detects any foreign bodies that might have become trapped between the layers. This is followed by an X-ray inspection of the interior structure that aims to ensure that no defects or internal damage is present, and additional analyses that involve testing the mass, shape, and rigidity parameters, heterogeneity criteria, radial force measurements, conicity, and other parameters whose uniformity ensures the safety and comfort of driving. The tires are also cut across to assess the product's compliance with the design specifications, which would otherwise impair their safety and performance properties [17].

Modern tire constructions tend to incorporate composites that combine various material properties, thus eliminating the disadvantages of particular compounds. As a result, new tire designs can conform to the challenges of constantly rising environmental and operational requirements [17,19].

#### *2.2. The Use Stage*

Each element of the tire can have a significant impact on fuel consumption and also on the reduction or increase of the negative environmental impact. On 1 November 2012, the obligation to label car tires was introduced in the European Union. The labels provide information on the most important aspects of tire performance, such as rolling resistance, wet grip, and noise emissions allowing for the quick and easy comparison of different products. They are designed to encourage manufacturers to act towards reducing rolling resistance, which leads to a reduction in carbon dioxide emissions to the atmosphere. Tires, mainly due to the rolling resistance, are responsible for 20–30% of the fuel consumption in passenger cars. Reducing fuel consumption allows for the reduction of the emission of CO2 and other harmful compounds into the environment. The other parameters on the labels are intended to force tire manufacturers to care for all performance metrics equally. Labels are to encourage consumers to make more informed choices and mobilize manufacturers to create products of the highest quality [20].

Tire wear contributes significantly to the flow of (micro) plastics into the environment. The particles generated during their use, emitted on the roads, can be dispersed in the environment through various routes. Smaller ones are usually emitted into the air, while large particles settle on the road surface and due to rainwater runoff, get into the soil, sewage, and surface waters. It is estimated that the relative share of tire wear in the total global amount of plastics ending up in our oceans is 5–10%. In the air, 3–7% of particulate matter (PM2.5) is made up of tire wear, indicating that it can contribute to the global health burden of air pollution, which can only be effectively addressed if awareness increases in this area [21].

#### *2.3. The End of Life Management*

Car tires are the largest source of post-consumer rubber products. For years, however, the number of tires withdrawn from use has increased, causing a negative impact on the environment. However, nowadays, the application of the principles of sustainable development implies a new approach to waste using environmentally friendly concepts, and it is necessary to re-evaluate the possibilities of managing this waste in order to increase its use and reduce the amount that requires disposal or

storage. To this end, a waste hierarchy should be followed, focusing on reuse, recycling, and recovery, and disposal should be interpreted as the last available option corresponding to the highest level of loss and change in resources. The disposal of used tires is a global environmental problem due to the large number of tires produced each year; according to ETRMA, around 3.4 million tons of used tires are produced in Europe annually. There are three main lines of action aimed at solving the problem of used tires: extension of service life through increased durability and retreading, material recycling, and energy recovery. A certain number of tires can also be used in full [22–24].

Due to their durability, used tires constitute nuisance waste and should be used industrially. They do not degrade in the environment within 100 years. They must not be left in landfills because their accumulation in large quantities is a fire hazard. Recycling tires is a difficult process due to their composition and construction. In addition to rubber, they contain textile and steel cords that must be properly separated. Used tires that are no longer suitable for retreading are waste. Therefore, they should be managed in an environmentally safe manner. For this purpose, the most frequently used methods are the ones involving product or material recycling. Whole, compressed, or cut worn tires have many applications, including their shape, material characteristics, and ability to absorb impacts or noise are used [22,25]. Used car tires can be also used by burning them in cement plants or other energy installations. The obtained heat is used in many technological processes, e.g., for burning clinker in cement kilns. The calorific value of rubber can be compared to the calorific value of coal. The use of used tires as fuel in the production of cement is considered a waste-free method of managing large amounts of this type of waste, because the tires burn completely without slag or ash remains. The metals contained in the tire permanently adhere to the clinker, improving its properties [26].

The adoption of three directives by the European Parliament, Landfill 1999/31/EC, End-of-Life Vehicle 2000/53/EC, and Waste Incineration 2000/73/EC have had a major impact on the management of used tires in the EU. The Landfill Directive 1999/31/EC introduced a ban on the disposal of whole used tires from July 2003, and from July 2006, on shredded tires. Moreover, it obliges the member states to create conditions enabling the implementation of this intention. The End-of-Life Vehicle Directive 2000/53/EC requires tires to be removed from vehicles prior to scrapping. The third of the Waste Incineration Directive 2000/73/EC obliges cement plants that use tires as a supplementary fuel to obtain lower limits for the pollutant content in the waste gases [27].

The issues of post-consumer management are more often being considered when analyzing all the earlier stages of the life cycle. Tire manufacturers should, as much as possible, prevent or reduce waste and harmful negative impacts on the environment, not only during the production of tires but also after their end of life [28].
