Tires are critical components of the vehicles on which they are used. They are used on automobiles, trucks, buses, farm tractors, earth-moving equipment, military vehicles, bicycles, motorcycles, and aircraft. Tires support the weight of the vehicle and the passengers and cargo on board; they transmit the motor torque to propel the vehicle (except on aircraft); and they absorb vibrations and shock to provide a comfortable ride.
A tire is a textile/rubber composite. The tire composite is in the form of a network of textile cord structures arranged in a parallel configuration and imbedded in a rubber matrix. Rubber defined as an elastomer compounded with various fillers and chemical ingredients . Most rubber compounds are thermosets, and they are turned into finished molded parts by compression, transfer or injection molding.
The tire is made up of numerous different rubber compounds, many different types of carbon black, fillers like clay and silica, and chemicals & minerals added to allow or accelerate vulcanization. The tires also have several types of fabric for reinforcement and several kinds and sizes of steel. Some of the steel is twisted or braided into strong cables.
Table of Contents
1. Introduction
2. Tire Feedstock Materials
2.1. Elastomers
2.2. Reinforcing fillers
2.3. Reinforcing fibers
2.4. Additives
3. Tire Construction
4. Tire Manufacturing
4.1. Preforming of Components
4.2. Building the Carcass (Tire assembly)
4.3. Molding and Curing
5. Tire Properties
5.1. Mechanical properties of tire composite material
5.2. Performance properties of tires.
6. Tire Testing
7. Tire Degradation
8. Types, cost analysis and market of automotive tires.
8.1. Types of tires
8.2. Cost of automotive tire
8.3. Market of automotive tires
9. Tire Recycling
9.1. 8.1 Retreading of tires.
9.2. Recovery of tire materials
9.3. Use of recovered tire rubber
10. Future of Tires
11. References
Editor's note: For copyright reasons, a large number of the illustrations are not included in this publication. However, they can be researched on the basis of the source references.
1. Introduction
Tires are critical components of the vehicles on which they are used. They are used on automobiles, trucks, buses, farm tractors, earth-moving equipment, military vehicles, bicycles, motorcycles, and aircraft. Tires support the weight of the vehicle and the passengers and cargo on board; they transmit the motor torque to propel the vehicle (except on aircraft); and they absorb vibrations and shock to provide a comfortable ride [12].
A tire is a textile/rubber composite. The tire composite is in the form of a network of textile cord structures arranged in a parallel configuration and imbedded in a rubber matrix. Rubber defined as an elastomer compounded with various fillers and chemical ingredients [1]. Most rubber compounds are thermosets, and they are turned into finished molded parts by compression, transfer or injection molding [5].
The tire is made up of numerous different rubber compounds, many different types of carbon black, fillers like clay and silica, and chemicals & minerals added to allow or accelerate vulcanization. The tires also have several types of fabric for reinforcement and several kinds and sizes of steel. Some of the steel is twisted or braided into strong cables [2].
2. Tire Feedstock Materials
Over 200 raw materials go into tire composition. Researchers draw on this extensive array to combine tire components, each of which has a role to play, depending on the type of tire produced. The rubber compounds are made up of elastomers, reinforcing fillers, reinforcing fibers and additives [6].
2.1. Elastomers
Two elastomers used in tire manufacturing: Natural rubber (NR) and synthetic rubber (SR).
2.1.1. Natural rubber (NR)
Milky, white latex, containing rubber globules, is obtained by making an incision into the bark of rubber trees, the cultivation of which requires specific climatic conditions and rainfall. In compound formulations, natural rubber reduces internal heat generation in tires, whilst offering high mechanical resistance. It is used in many parts of the tire, mainly used for truck and earthmover tire tread [6].
2.1.2. Synthetic rubber (SR)
60% of rubber used in the tire industry is synthetic rubber, produced from petroleum-derived hydrocarbons, although natural rubber is still necessary for the remaining 40%.Synthetic elastomers deform under stress and return to their original shape when the stress is removed (hysteresis).This property is extremely valuable for the manufacture of high-grip tires. Synthetic rubber also provides other specific properties, most notably in the areas of longevity and rolling resistance. It’s mainly used for passenger car and motorcycle tire as it gives them good grip performances [6].
2.2. Reinforcing fillers
2.2.1. Carbon black
Carbon black added to the rubber compound produces a tenfold increase in wear resistance, tensile strength and resistance to abrasion and tearing of the tires. It represents 25 to 30% of the rubber composition and gives tires their distinctive color. Indeed, this color is very effective in acting against ultraviolet rays to prevent the rubber from fissuring and cracking [6].
2.2.2. Silica
Silica, obtained from sand, has properties that have long been recognized, including the improved resistance of rubber compounds to tearing. In 1992, Michelin took a major step forward by combining an original silica and a specific elastomer with a special bonding agent using a special “mixing” process. The compounds obtained make tires with a low rolling resistance, good grip on a cold surface and exceptional longevity. This innovation is at the origin of the green tires with low rolling resistance [6].
2.3. Reinforcing fibers
A tire’s reinforcing materials are the predominant load carrying members of the cord-rubber composite. They provide strength and stability to the sidewall and tread as well as contain the air pressure [9]. The tire needs metal and textile reinforcements in addition to the rubber compounds. These are the real framework of the tire, ensuring its geometry and rigidity. They also provide the flexibility required for tire contact with the road [6].
2.3.1. Rayon cord
Rayon is an excellent choice for passenger tires, because of it low heat shrinkage, high modulus and good adhesion properties [15]. Advantages: Stable dimensions; heat resistant; good handling characteristics. Disadvantages: Expensive; more sensitive to moisture; environmental manufacturing [9].
2.3.2. Polyester cord
Polyester has high modulus, low heat shrinkage. It is relatively cheap making it a good choice for passenger and light truck tires [15]. Advantages: High strength with low shrinkage and low service growth; low heat set; low cost. Disadvantages: Not as heat resistant as nylon or rayon. [9].
2.3.3. Nylon cord
Two types are used in tires: Nylon 6 and Nylon 66. Nylon 6 is less expensive than Nylon 66, but it is more sensitive to moisture and subject to loss in tensile strength if moisture is present during curing. Nylon improves toughness, strength, bruise and impact resistance. Also, it has light weight and low heat generation. It is used in heavy-duty trucks, off-road vehicles and aircrafts [15]. Advantages: Good heat resistance and strength; less sensitive to moisture. Disadvantages: Heat set occurs during cooling (flat spotting); long term service growth [9].
2.3.4. Aramid cord
The most common commercial aramid materials used are Kevlar and Twaron. Those have very high strength, high modulus and low elongation. The major thing to slow adoption as a general radial belt material is relatively high cost. It suits well to replace steel as a belt material where weight is important like in the belts of radial aircraft tires [15]. Advantages: Very high strength and stiffness; heat resistant. Disadvantages: Cost; processing constraints (difficult to cut) [9].
2.3.5. Glass cord
Glass fiber specific stiffness and strength are equal to steel. Some attempts have made to use glass fiber in the belt material, but those have ended premature failure. It works as a belt material if proper tread design and latex adhesive dips are used. However, in cold weather use and with inappropriate tread it fails when it is put into compression [15].
2.3.6. Polyvinyl alcohol (PVA) cord
Polyvinyl alcohol (PVA) fibers have properties similar to rayon and polyester. They have higher tenacity than rayon and lower shrinkage than polyester. This textile has been used successfully in the carcass and belts. A major drawback has been the lack of suppliers [15].
2.3.7. Steel cord
In passenger car tires the steel cord is used in the belt. In truck radial tires steel cord is used both in the belt and in the carcass and in OTR tires it is used as a breaker for puncture protection [15]. Advantages: High belt strength and belt stiffness improves wear and handling. Disadvantages: Requires special processing and more sensitive to moisture [9].it is function includes: helps forming the desired tire profile, generates handling forces and improves tire performance, improves traction, influences footprint shape, influences wear performance and protects and survives against impacts and protruding objects [15].
2.4. Additives
2.4.1. Softeners:
Petroleum oils, pine tar, resins and waxes are all softeners that are used in compounds principally as processing aids and to improve tack or stickiness of unvulcanized compounds [9].
2.4.2. Antidegradents:
Waxes, antioxidants, and antiozonants are added to rubber compounds to help protect tires against deterioration by ozone, oxygen and heat [9].
2.4.3. Curatives:
During vulcanization or curing, the polymer chains become linked, transforming the viscous compounds into strong, elastic materials. Sulfur along with accelerators and activators help achieve the desired properties [9]. Sulphur is a vulcanizing agent that transforms the rubber from a plastic to an elastic state. Its action is accompanied by retarding and accelerating products used simultaneously during production which optimize the action of heat when the tire is cured [6].
3. Tire Construction
A tire is an assembly of many parts, whose manufacture is unexpectedly complex. A passenger car tire consists of about 50 individual pieces; a large earthmover tire may have as many as 175. To begin with, there are three basic tire constructions: (a) diagonal ply, (b) belted bias, and (c) radial ply, pictured in Figure 1. In all three cases, the internal structure of the tire, known as the carcass, consists of multiple layers of rubber-coated cords, called plies. The cords are strands of various materials such as nylon, polyester, fiberglass, and steel, which provide inextensibility to reinforce the rubber in the carcass [12].
The diagonal ply tire has the cords running diagonally, but in perpendicular directions in adjacent layers. A typical diagonal ply tire may have four plies. The belted bias tire is constructed of diagonal plies with opposite bias but adds several more layers around the outside periphery of the carcass. These belts increase the stiffness of the tire in the tread area and limit its diametric expansion during inflation. The cords in the belt also run diagonally, as indicated in the sketch. A radial tire has plies running radially rather than diagonally; it also uses belts around the periphery for support. Asteel-belted radial is a tire inwhich the circumferential belts have cords made of steel. The radial construction provides a more flexible sidewall, which tends to reduce stress on the belts and treads as they continually deform on contact with the flat road surface during rotation. This effect is accompanied by greater tread life, improved cornering and driving stability, and a better ride at high speeds. Thus, radial tires are the most common type used [12].
In each construction, the carcass is covered by solid rubber that reaches a maximum thickness in the tread area. The carcass is also lined on the inside with a rubber coating. For tires with inner tubes, the inner liner is a thin coating applied to the innermost ply during its fabrication. For tubeless tires, the inner liner must have low permeability because it holds the air pressure; it is generally a laminated rubber [12].
Abbildung in dieser Leseprobe nicht enthalten
Figure 1 : Three principal tire constructions: (a) diagonal ply, (b) belted bias, and (c) radial ply [12].
The components of a modern radial tire for passenger cars contain diverse ingredients in differing amounts. These ingredients vary by tire size and tire type (summer or winter tire). Figure 2 shows the ingredients used in the summer tire [8].
Due to copyright reasons, this figure is not part of the publication.
Figure 2 : Breakdown of tire ingredients [8].
Tires are composed of layers of ‘macroscopic’ composites with long reinforcing fibres or wires surrounded by rubber material. This rubber material is made of highly engineered ‘microscopic’ composites of finely mixed then vulcanized raw materials such as polymers (natural rubber (NR ) and/or synthetic rubber (SR)), reinforcing fillers (carbon black and/or silica) and other rubber chemicals. Distribution and dispersion of key ingredients in this engineered rubber is controlled on a nanometre scale. Today’s tire technology realizes a product that satisfies many different requirements by taking advantage of ‘multi-scale’ design technology, or by employing interacting layers of design technologies dealing with different spatial scales, from molecular size to the size of the tire itself [3].
The two major ingredients in a rubber compound are the rubber itself and the filler, combined in such a way as to achieve different objectives. Depending on the intended use of the tire, the objective may be to optimize performance, to maximize traction in both wet and dry conditions, or to achieve superior rolling resistance. The desired objective can be achieved through the careful selection of one or more types of rubber, along with the type and amount of filler to blend with the rubber [4].
In general, there are four major rubbers used: natural rubber, stirene-butadiene rubber (SBR), polybutadiene rubber (BR), and butyl rubber (along with halogenated butyl rubber). The first three are primarily used as tread and sidewall compounds, while butyl rubber and halogenated butyl rubber are primarily used for the innerliner, or the inside portion that holds the compressed air inside the tire [4].
The most popular fillers are carbon black and silica, and there are several types of each. The selection depends on the performance requirements, as they are different for the tread, sidewall, and apex. Other ingredients also come into play to aid in the processing of the tire or to function as anti-oxidants, anti-ozonants, and anti-aging agents. In addition, the “cure package”—a combination of curatives and accelerators—is used to form the tire and give it its elasticity [4]. Figure 3 illustrates the major tire components and Table 1 discusses the functions and requirements of the major tire parts.
Due to copyright reasons, this figure is not part of the publication.
Figure 3 : Basic construction of a typical tire [8].
Table 1 : Functions and requirements of the major tire parts [8].
Abbildung in dieser Leseprobe nicht enthalten
Many types of rubber chemicals are used: crosslinking agents such as sulphur, crosslinking aids such as accelerators, zinc oxides, stearic acid, antioxidants that prevent rubber degradation by oxygen and ozone, plasticizers that adjust compound processability and hardness, and silane coupling agents that help dispersion and rubber bonding of silica. ratio may drastically shift in the future. Table 2 shows the raw material composition of a typical passenger car radial tire (PCR) and a truck and bus radial tire (TBR) [3].
Table 2 : Typical weight composition of a typical passenger car radial tire (PCR), and a truck and bus radial tire (TBR) [3].
Abbildung in dieser Leseprobe nicht enthalten
4. Tire Manufacturing
Tire production can be summarized in three steps:
(1) Preforming of components,
(2) Building the carcass and adding rubber strips to form the sidewalls and treads, and
(3) Molding and curing the components into one integral piece.
The descriptions of these steps that follow are typical; there are variations in processing depending on construction, tire size, and type of vehicle on which the tire will be used [12].
4.1. Preforming of Components
As Figure 1 shows, the carcass consists of a number of separate components, most of which are rubber or reinforced rubber. These, as well as the sidewall and tread rubber, are produced by continuous processes and then pre-cut to size and shape for subsequent assembly. The components, labeled in Figure 1 and Figure 3, and the preforming processes to fabricate them are:
- Bead coil:
Continuous steel wire is rubber-coated, cut, coiled, and the ends joined.
- Plies:
Continuous fabric (textile, nylon, fiber glass, steel) is rubber coated in a calendaring process and pre-cut to size and shape.
- Inner lining:
For tube tires, the inner liner is calendared onto the innermost ply. For tubeless tires, the liner is calendared as a two-layered laminate.
- Belts:
Continuous fabric is rubber coated (similar to plies), but cut at different angles for better reinforcement; then made into a multi-ply belt.
- Tread:
Extruded as continuous strip; then cut and pre-assembled to belts.
- Sidewall:
Extruded as continuous strip; then cut to size and shape.
4.1.1. Calendaring
There are two functions of calendaring process. The first it is a process in tire manufacturing where steel or fabric is coated with rubber as depicted in Figure 4 (a). Steel belts, body plies, cap plies, and belt edge covers are all produced using the calendaring process. Steel or fabric is placed between two large heavy rollers. Rubber is also inserted between the rollers at the top and bottom of the material being coated. As the material moves through the rollers the pressure forces the rubber to permeate and adhere to the steel or fabric [10]. The second process used to press rubber compounds between two or more rotating rolls to form thin, flat sheets of rubber to specified thickness as depicted in Figure 4 (b) [11].
Abbildung in dieser Leseprobe nicht enthalten
Figure 4 : Skectches of the calebdering processes in tire manufacturing. (a) steel or fabric is coated with rubber. (b) pressinng rubber compounds between two or more rotating rolls to form thin, flat sheets of rubber to specified thickness [14].
4.1.2. Extrusion
Extrusion is a process used to manufacture shaped solid rubber. Treads, sidewalls, and bead fillers are manufactured using extruders. An example of extruding is cake decorating. To decorate the cake, the decorator squeezes icing through tips shaped to form different shapes. The decorator is extruding icing [10].
4.2. Building the Carcass (Tire assembly)
The carcass is traditionally assembled using a machine known as a building drum as shown in Figure 5 and in Figure 6, whose main element is a cylindrical arbor that rotates. Pre-cut strips that form the carcass are built up around this arbor in a step-by-step procedure. The layered plies that form the cross section of the tire are anchored on opposite sides of the rimby two bead coils. The bead coils consist of multiple strands of high-strength steel wire. Their function is to provide a rigid support when the finished tire is mounted on the wheel rim. Other components are combined with the plies and bead coils. These include various wrappings and filler pieces to give the tire the proper strength, heat resistance, air retention, and fitting to the wheel rim. After these parts are placed around the arbor and the proper number of plies have been added, the belts are applied. This is followed by the outside rubber that will become the sidewall and tread. At this point in the process, the treads are rubber strips of uniform cross section—the tread design is added later in molding. The building drum is collapsible, so that the unfinished tire can be removed when finished. The form of the tire at this stage is roughly tubular, as portrayed in Figure 5 [12] . Figure 7 shows a complete assembled tire. At this stage the tire is called “green tire” [13].
Abbildung in dieser Leseprobe nicht enthalten
Figure 5 : Tire just before removal from building drum, before molding and curing [12].
Due to copyright reasons, this figure is not part of the publication.
Figure 6 : (a) A machine rolled strip (thread rubberized steel), around the tire while it spins synchronously creating a spiral. (b) building the carcass in the building drum machine [13].
Due to copyright reasons, this figure is not part of the publication.
Figure 7 : Assembly of tire components using the building drum machine. After completing this stage the tires is called “green tire” [13].
4.3. Molding and Curing
Tire molds are usually two-piece construction (split molds) and contain the tread pattern to be impressed on the tire. The mold is bolted into a press, one half attached to the upper platen (the lid) and the bottom half fastened to the lower platen (the base). The uncured tire is placed over an expandable diaphragm and inserted between the mold halves, as in Figure 8 and Figure 9. The press is then closed and the diaphragm expanded, so that the soft rubber is pressed against the cavity of the mold. This causes the tread pattern to be imparted to the rubber. At the same time, the rubber is heated, both from the outside by the mold and from the inside by the diaphragm. Circulating hot water or steam under pressure are used to heat the diaphragm. The duration of this curing step depends on the thickness of the tire wall. A typical passenger tire can be cured in about 15 minutes. Bicycle tires cure in about 4 minutes, whereas tires for large earth-moving equipment take several hours to cure. After curing is completed, the tire is cooled and removed from the press [12].
4.3.1. Curing
Is the process where a green tire is the assembled components that make up a tire before it is cured. Curing bonds all the different rubber components of a tire together. The tread design and sidewall nomenclature are also molded into the tire during the curing process [10].
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