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A Survey of Advancements in Tire Technology

by

Dan Schwarz

School of EngineeringGrand Valley State University

Term Paper

EGR 250 – Material Science & EngineeringSection 01

Instructor: Dr. P.N. Anyalebechi

March 17, 2006

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Abstract

Tires are one of the most important parts of an automobile because they are the only part

of the car that is intended to touch the ground. In view of this fact, tires have been designed to

provide traction and shock absorption while protecting the wheels from wear. A brief history of

the developments in tire technology is given for the past two centuries including the discovery of

vulcanization through the invention of the radial tire. Functional requirements and

corresponding material properties of a tire are discussed to ascertain the appropriate materials for

manufacturing tires. Common rubber, metal, textile, and filler materials are reviewed with

respect to their desirable properties. The current manufacturing processes that assemble these

materials into a tire are explained chronologically. The entire discussion is intended to offer a

brief survey of the advancements made in tire technology and is concluded with recent

developments in the tire industry.

1. Introduction

For centuries wheels have been used to reduce large frictional forces that oppose the

movement of heavy objects. Most of these early wooden wheels were implemented on horse

drawn carriages. The first development resembling the design and functionality of a tire was

steel bands fastened to carriage wheels along their outer circumference. The steel bands offered

an increase in wear resistance but did not offer any shock absorption. In order to make carriages

more comfortable for long rides, leather was placed on the contact surface of the wheels.

Eventually, leather wheel coverings were replaced by natural rubber and the concept of

the tire, as it is known today, began to develop. The first obstacle in tire development was

overcome by Charles Goodyear when he developed the process of vulcanization in 1839 [1].

Good year discovered that heating a mixture of rubber and sulfur improved the rubber’s physical

properties. Vulcanized rubber held its shape better than plain rubber and was not as susceptible

to temperature changes.

Several years after vulcanization was discovered, the first tire companies produced

extremely heavy solid rubber tires. In order to make the tires lighter, John Boyd Dunlop

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developed the air filled “pneumatic” tire in 1888 [1]. Dunlop’s pneumatic tire was lighter and

absorbed shock much better than the early solid tires. The tire consisted of an outer casing that

was supported by an inner tube filled with air. The outer casing provided protection for the inner

tube and traction for gripping the road.

Fifty years passed without any significant changes in tire technology until the bias ply tire

was invented [1]. A bias ply tire contained layers of plies that strengthen the casing. Each ply

layer had fabric cords embedded in the rubber that ran diagonally from one bead (i.e. edge) of

the tire casing to the other. All of the cords within a single ply ran in the same diagonal direction

which is where the name “bias ply” came from [1]. Each successive ply had cords running in the

opposite direction than the previous ply to make the tire rigid.

The current radial tire design was introduced to the market by Michelin in France, 1948

[2]. Radial tires differ from the bias ply design in that the cords embedded in the rubber run

perpendicular to the direction of the tire treads instead of diagonal. Radials also have steel belts

underneath the treads to square the treads with the surface of the road. Although radial tires are

twice as expensive to make, they improve the life of the treads and decrease rolling resistance

[1]. Rolling resistance refers to the amount of force required to turn the wheel. The rigidity of

the belts in radial tires reduces rolling resistance by decreasing the amount of tread in contact

with the road (i.e. contact patch). By decreasing roll resistance, radials improve the fuel

economy of the consumer’s automobiles. Figure 1 provides a cross-sectional comparison of

radial and bias ply tire designs.

Figure 1: The cross-section of a bias ply tire (left) [21] is compared to the cross section of a radial (right) [22].

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2. Components of a Modern Tire

At first glace, the modern tire appears to be nothing more than a round piece of rubber

that fits onto a wheel. However, close examination will reveal that tires are actually very

complex products. Figure 2 provides a cross-sectional view of a radial tire, showing its many

hidden components. The most familiar part of a tire is the tread which has different patterns of

grooves depending on its intended use. Beneath the tread are several layers of rubber called the

undertread. The undertread is intended to fuse the tread to the belts and absorb heat that builds up

in the belts. The belts are circumferential strips of rubber with wire suspended in it to offer

support for the contact surface of the tread. Beneath the belts lies the carcass which is the sum of

all body plies in the tire. The body plies of a radial tire are layers of rubber with polyester fibers

suspended perpendicular to the direction of the tread. Bead bundles on either side of the tire are

made of steel wires that hold the body plies in place. The bead also creates a tight fit between

the tire and the wheel to ensure that there is no slippage between the two. The most important

layer of a pneumatic tire is the inner liner that prevents air leakage.

Figure 2: The cross-sectional view of a radial tire and its components [19].

Tire materials include rubber, steel, fabric, and fillers materials as shown in Table 1.

Steel wire and fabric fibers are used to create various ply layers as described above. The rubber

compound contains filler materials such as carbon black to increase wear resistance and decrease

rolling resistance in the tire. Accelerator materials are included in the rubber compound to aid in

the vulcanization process of manufacturing tires.

Table 1: Percent composition of a tire [23].

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Natural rubber 14%

Synthetic rubber 27%

Carbon black 28%

Steel 14 - 15%Fabric, fillers, accelerators, antiozonants, etc.

16 - 17%

Average weight New 25 lbs, Scrap 20 lbs.

2.1. Functional Requirements

The most basic function of the tire is to protect the wheels from wear. This requirement

suggests that tires must be the only part of the wheel in contact with the ground. Since the entire

weight of a vehicle will be resting on the tires, they must be rigid enough to support the vehicle’s

weight. Normal operation of a vehicle requires the tires to roll across both paved and unpaved

surfaces. Consequently, tires must be made of robust materials that are not susceptible to being

damaged by these surfaces. The materials must also possess enough heat resistance to remain

unaffected by high temperatures produced from friction between the ground and tires.

Tires must also produce enough traction to allow the automobile to effectively transfer

power to the road. The tire must be able to maintain this traction even when the roads are

covered with water or snow. At the same time, it is important that the tires do not produce high

roll resistance that would cause the vehicle to burn excessive amounts of fuel.

Another important consideration is the comfort of a vehicle’s occupants. No matter how

smoothly paved a road may be, there is always debris that can transfer shock into the cabin of the

vehicle. Tires must be able to absorb the shock and vibrations produced by roughness of the

road. In order to absorb the shock of passing over bumps in the road, tire materials must be

flexible. The tire must also be made of a material that can retain a constant air cushion in the

tube.

2.2. Required Properties

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The intended functionality of the tire is achieved though careful considerations of the

properties of candidate tire materials. A balance between rigidity and flexibility of the tire is

important to simultaneously provide both structure and shock absorption. This necessary

balance between rigidity and flexibility is achieved by reinforcing the relatively soft rubber with

rigid fibers. The rubber must have a low modulus of elasticity such as styrene-butadiene rubber

(i.e. SBR) which has a modulus of 1.5 GPa [24]. Conversely, tread reinforcing fibers such as

ASTM 1070 steel wire offer an elastic modulus of 205 GPa [26].

When the tire experiences strain as it rolls over uneven pavement, it must possess

elastomer properties to absorb the shock. This means that the material should be able to

successfully elongate over 200% of its original length without breaking and then return to its

normal shape [11]. Once again, SBR is an excellent candidate material because it offers up to

480% elongation before it breaks at its ultimate tensile strength of 28.5MPa [24]. The tire should

also be resistant to fatigue as it will be strained sporadically.

Friction between the tire and the road creates heat that tends to build up in the belts of

radial tires. The edge of a belt can reach temperatures of up to 100°C as the tire rolls down the

road [9]. Accordingly, it is important to design the rubber compound in such a way that the

melting temperature of the tire’s undertread is high enough to withstand operating temperatures.

SBR has been tested to operate safely at 110°C which is slightly higher than the safe operating

temperature of 100°C for natural rubber [25].

The tire tread material must provide enough traction to prevent the tire from spinning or

sliding on the pavement. A high coefficient of friction will allow the weight of the vehicle to

keep the tires adhered to the ground. Tire tread materials must also be resistant to abrasion wear

and cuts from road debris. The addition of carbon black fillers to SBR increases its wear

resistance and produces Shore hardness values of approximately 75 [6].

2.3. Tire Materials

As tire designs have progressed throughout history they have addressed each functional

requirement more successfully. However, it wasn’t until the last few decades that tire

manufacturers have directed their focus on improving tire functionality with new materials. The

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first tires where made only using natural isoprene rubber which has high abrasion resistance

compared to many other rubbers [4]. More recently, synthetic rubbers such as styrene-butadiene

copolymers have been used for their fatigue resistance [4]. Random configuration styrene-

butadiene copolymers are commonly used because they are more pliable than block

configurations (i.e. styrene-butadiene-styrene or SBS). Chlorobutyl rubber can also be used in

tire compounds to make them more resistant to high temperatures [4]. Another important rubber

in the tire industry is polyisobutylene because it is impermeable to gases. Polyisobutylene is

used to create the inner liner of the tire that prevents air leakages [5]. Today, most tires are made

of a compound of rubbers and other materials to obtain a balance of desired material properties.

Table 2 shows the composition of four common tire compounds and their respective locations

within a tire.

Table 2: Composition of common tire compounds [23].

 Tread (PHR)

Base (PHR)

Sidewall (PHR)

Innerliner (PHR)

Natural Rubber 50.0  100 75 -Styrene-Butadiene Rubber 50 - 25 -Isobutylene-Isoprene Rubber - - - 100Carbon Black (Grade N110) 50 15 20 -Carbon Black (Grade N330) - 25 35 -Carbon Black (Grade N765) - - - 50Processing Oil 7.5 5 5 3

Antioxidant  1 0.75 1 1Antioxidant Wax - - 2 -Stearic Acid 2 4 3 1.5Zinc Oxidant 5 5 5 5Accelerator (High) - 1 0.7 -Accelerator (Middle) 1.25 - - 0.4Accelerator (Low) - - - 0.4Sulfur 2.5 3 2.8 2*PHR = Per Hundred Rubber *Carbon grade = ASTM grading: Particle size and structure of carbon are different.

Fillers are a very important part of tire rubber compounds because they offer abrasion

resistance and reinforcement. Fillers are solid granular materials that are suspended in the rubber

compound such as carbon black. Carbon black not only increases the abrasion resistance of the

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rubber but also increases the rate of vulcanization [4]. Smaller granules of carbon black are used

to extend the tread life of tires but if the granules are too small it is difficult to attain proper

dispersion of the filler [4]. Vulcanization can also occur prematurely if the granules of carbon

black are too small [4].

In 1992, Michelin patented a new tire tread compound using silica as the primary filler

instead of carbon black [6]. The silica filler is more expensive to use than carbon black but it

produces less roll resistance. For a long time silica had been used in small quantities to increase

tear resistance and decrease cut growth in tires. The reason it was never used as the primary

filler was a lack of compatibility between silica and the other polymers used in tire treads. In

addition, silica did not disperse very well during the mixing process. To overcome this problem,

Michelin developed a new high dispersal silica and added an organosilane as a coupling agent

[6]. The new silica/silane filler still does not disperse as well as carbon black but it has proven to

be the best filler for reducing roll resistance [7].

One of the most essential components of any tire compound is the sulfur which allows

the rubber to be vulcanized. The sulfur creates cross-links between the carbon backbones of

adjacent polymer chains when the compound is heated [3]. Cross-linking increases the strength

of the rubber by preventing polymer chains from slipping past one another. Tire compounds

typically contain low amounts of sulfur (1-5% wt. S) to prevent too many cross-links from

forming which causes the rubber to become hard [3].

Tire rubber is given structure and support by fibers embedded in the rubber. In early tires

cotton was used because it bonded well with the rubber and prevented the tire plies from

separating during operation [8]. Synthetic rayon eventually replaced cotton but after a few years

nylon replaced rayon because of its superior strength and low cost [8]. Today the most common

fiber used in tire production is polyester due to its low cost per unit strength [8].

Radial tires also implement the use of metal fibers to form the circumferential belts that

provide structure for the tread. The metal fibers are made of woven ASTM 1070 steel wires that

are coated with brass. Brass allows the steel wire to bond with the rubber since steel alone

cannot to bond with rubber. ASTM 1070 steel is also used to form the bead bundles that support

the inner diameter of the sidewalls. Table 3 shows the composition of ASTM 1070 steel and the

brass compositions used to coat the steel.

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Table 3: Composition of metal fibers used in tire belts and bead bundles [23].

  STEEL BELTS BEAD WIRECarbon 0.67 - 0.73% 0.60% min.Manganese 0.40 - 0.70% 0.40 - 0.70%Silicon 0.15 - 0.03% 0.15 - 0.30%Phosphorus 0.03% max. 0.04% max.Sulfur 0.03% max. 0.04% max.Copper Trace TraceChromium Trace TraceNickel Trace Trace

COATING66% Copper 98% Brass

34% Zinc 2% Tin

2.4. Tire Manufacturing

All of the materials described above come together in the manufacturing process to form

a tire. A tire has several components including the inner liner, bead bundle, bead filler, bead

chafer, body plies, carcass, side wall, belts, edge covers, cap plies, undertread, and tread. The

cross-sectional diagram in Figure 2 illustrates the position of each component in the tires

construction.

The first step in manufacturing a tire is to create the rubber compounds for different parts

of the tire. Compounds for the tread are designed on the basis of the intended use of the tire such

as wet, dry, and snow covered surfaces while also taking the desired performance into

consideration. Tread compounds are typically made with either natural rubber, styrene-

butadiene rubber, polybutadiene rubber, or a combination of these rubbers [12]. Fillers are also

incorporated into the compounds based on the desired performance of the tire. Degussa, for

example, distributes fillers such as Corax HP 160 carbon black for high performance passenger

car treads or Utrasil 7005 for low roll resistance wet passenger tires [13]. Curative and

accelerator materials are added to the compound to encourage elasticity of the rubber. Sidewall

compounds contain different concentrations of materials than tread compounds because of the

difference in required properties. In order to ensure that the rubber compounds contain the

correct proportions of materials, machines are used to distribute precise amounts of each material

into every batch of rubber compound.

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Once the compounds are made, they are mixed at temperatures of 160-170°C without the

curative package being added [12]. Then the curative package is added after reducing the

temperature to approximately 100°C which minimizes the likelihood of scorching [12]. Some of

the mixed compounds are extruded to form tread and sidewall strips while others are milled into

thick sheets for calendaring.

Extrusion is the process of forcing the rubber through an extruder head to produce a

particular shape. Since the tire tread and undertread are made of different compounds, multiple

extruder barrels feed the individual compounds to the same extruder head. This process joins the

necessary layers of the tread together.

Calendaring is the process of embedding fibers into the sheets of rubber. Calendaring

processes differ slightly depending on what part of the tire must be produce. During calendaring

of the carcass and body plies, adhesive coated cords are pressed into the rubber strips by a series

of rollers. The calendaring process for the belts involves pressing brass coated steel cords into

the rubber. The inner lining is also produce by a calendaring process using an impermeable

halogenated butyl rubber. Halogen atoms (normally bromine or chlorine) create a better bond

between the butyl rubber and the carcass [8].

Once the wire bead bundles are wrapped, all of the components of the tire are ready to be

assembled. The first stage of assembling the tire takes place on a large drum that is

approximately the same diameter as a tire. Figure 3 shows a simple illustration of a tire building

drum. Initially, the inner liner is wrapped on the drum followed by the body plies with bead

bundles positioned at either end. A tire-shaped air bladder is inflated at the center of the drum to

allow the side walls to be added. At the same time, the edges of the plies are wrapped over the

bead bundles and then the sidewalls are pressed on either side. A second machine is used to put

the belts, edge covers, nylon cap, and tread in place. The final assembly is referred to as a

“green tire” until it has been cured.

Green tires are placed in tire molds where they receive tread patterns as shown in Figure

4. Once the mold is closed an air bladder inflates within the tire pressing it against the tread

pattern plates. The tire is subjected to heat and pressure while additional rubber flows into the

mold to produce sidewall markings and tread patterns.

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Figure 3: A tire building drum [20].

Figure 4: The tire mold creates tread patterns and vulcanizes the rubber [20].

After the tire has been heated long enough for vulcanization to reach completion, the tires

are removed for inspection. The mold flash is trimmed off and a visual inspection of the tire is

performed. An x-ray examination determines if there are any unseen or internal defects in the

tire. Finally, the tire is subjected to durability and balance testing before it can be shipped to

retailers.

3. Tire Disposal

After tires have completed their lifecycle, it is difficult to known what to do with them.

Recycling used tires is a formidable task because the polymer cross-linking that occurs during

vulcanization prevents the rubber from being melted and reused. Studies have shown that cross-

linked polymer chains can be separated by ultrasonic treatment but this technology has only been

used experimentally [14].

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In the early twentieth century, rubber recycling was very common because it was, ounce-

for-ounce, as expensive as silver [15]. However, by the 1960’s, rubber was becoming

significantly less expensive and the steel belted radial tires were more expensive to recycle than

bias ply tires. This caused the rubber recycling industry to steadily decline until only 2% of

scrap tires were being recycled in 1995 [15].

In response to the increasing number of scrap-tire dumps, the Rubber Manufacturers

Association has helped to organize a new scrap tire market. Parts of scrap tires are being used

for re-treading tires to extend their lifecycles. Tires are also being burned as an alternative to

coal in electric plants and cement kilns [16]. Many unique products such as rubber garden mulch

are being made from ground scrap tires [16]. The Rubber Manufacturers Association is also

working on several other scrap tire markets for the future including playground and athletic

surfaces.

4. Tire Innovations for the Future

Many innovative tire designs for the future are currently in the prototype stage. Figure 5

shows a polyethylene prototype tire designed by Amerityre. Urethanes have many advantages

over rubber including lower material cost, ease of mixing, and they are chemically inert in their

solid state. Polyurethane can also be recycled more easily than rubber since it does not require

vulcanization.

Amerityre claims that the manufacturing processes required for polyurethane tires are so

advantageous that in a few years polyurethane will replace rubber in tire manufacturing.

Polyurethane manufacturing is faster and less expensive because it requires fewer machines and

personnel to construct the tires. Manufacturing facilities would only require 20% of the floor

space required by the average rubber facility [17]. Amerityre also estimates that the initial

capital investment required for a polyurethane plant would be approximately 7% of the capital

investment for a rubber plant [17].

The tire prototype itself is advantageous due its solid construction which means it cannot

be punctured like a pneumatic tire. Polyurethane tires can be produced to very precise

dimensions making them ride smoother than rubber tires. They have lower operating

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temperatures and better abrasion resistance which translates into long tread life. They also create

less roll resistance and will improve the fuel economy of vehicles.

Figure 5: Amerityre’s Arcus polyurethane prototype tire [17].

Another interesting innovation in tire products that has recently hit the market is run-flat

tires. These tires have extra rigid side walls and steel belts that can safely support the vehicle

when the tire goes flat for approximately 50 miles at speeds of up to 55 mph [18].

Since the invention of vulcanization in 1839, tires have developed from solid rubber rings

to complex radial tires. As technology continues to progress it will be interesting to see what the

next generation of tires will look like. If Amerityre is successful in selling their products, then

there is a good chance that polyurethane tires will be the next generation.

References

[1] “History of the Passenger Tire,” General Tire,

<http://www.generaltire.com/generator/www/us/en/generaltire/automobile/themes/

generaluniversity/history/tire_history_en.html>.

[2] R. Rajan et al.: “The Eclipse of the U.S. Tire Industry,” Proc. of NBER Conf. on Mergers and

Productivity, edited by unknown, 1997, pp. 3.

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[3] W. Callister: “Fundamentals of Materials Science and Engineering,” John Wiley & Sons,

Hoboken, pp.105-107.

[4] W. Frates: “Tire Technology,” Tire Accidents,

<http://www.tireaccidents.com/tire_technology.htm>.

[5] L. Mathias: “Kinds of Polymers,” Polymer Science and Learning Center,

<http://www.pslc.ws/mactest/kidsmac/rubber.htm>.

[6] J. Byers: Rubber Chemistry and Technology, 2002, vol. 75, no. 3, pp.527-548.

[7] C. Wang, et al.: Rubber Chemistry and Technology, 2005, vol. 78, no. 1, pp.17.

[8] Wikipedia contributors: “Tire manufacturing,” Wikipedia, The Free Encyclopedia,

<http://en.wikipedia.org/w/index.php?title=Tire_manufacturing&oldid=39141943>.

[9] S. Laube: “Improving Carcass Durability Trough Filler Selection” Proc. of Conf. on Fillers,

edited by unknown, 2002, pp. 4.

[10] D. Hall: Rubber Chemistry and Technology, 2004, vol. 77, no. 5, pp.891-914.

[11] P. N. Anyalebechi: “Essentials of Materials Science and Engineering,” Padnos College of

Engineering and Computing, Grand Rapids, 2006, pp. 82.

[12] “How a Tire is Made,” Maxxis,

<http://www.maxxis.com/Services/how_a_tire_is_made.asp#tire_curing>.

[13] “Our Products for the Tire Industry,” Degussa,

<http://www.degussa-fp.com/en/anwendungen/reifen.html>.

[14] A. Isayev, et al.: Rubber Chemistry and Technology, 2005, vol. 78, no. 4, pp.606-620.

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[15] “Recycling Tires,” Ohio Department of Natural Resources,

<http://www.dnr.state.oh.us/recycling/awareness/facts/tires/rubberrecycling.htm>.

[16] S. Ahluwalia: Real Answers, 2006, vol. 10, no. 3, pp. 22-25.

[17] R. Steinke: “A Summary of Amerityre Corporation’s Polyurethane Elastomer Tire

Technology and Its Advantages Over Existing Rubber Tire Technology,” Amerityre

Corporation,

<http://www.amerityre.com/news/Summary_of_Amerityre_PU_Tire_Technology.pdf>.

[18] R. Demeis: Design News, 1998, vol. 53, no. 9, pp. 80-82.

[19] “Construction,” Yokohama Tire, <http://www.yokohamatire.com/utconstruct.asp>.

[20] R. Miller: “Tire,” How Products are Made,

<http://www.madehow.com/Volume-1/Tire.html>.

[21] “Bias-ply Tire,” Your Dictionary, <http://www.yourdictionary.com/ahd/b/b0226700.html>.

[22] “Aircraft Tire Construction Radial Ply,” Desser Tire and Rubber Company,

<www.desser.com/ tech/radial.html>.

[23] E. Yamaguchi: “Anatomy of a Tire,” Waste Tire Recycling,

<http://www.p2pays.org/ref/11/10504/html/intro/tire.htm>.

[24] “Dow Buna SB 1500 cold polymerized emulsion styrene butadiene rubber E-SBR,”

Matweb, <http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDOW1017>.

[25] W. Hofmann: “Rubber Technology Handbook,” Hanser, Cincinnati, 1980, pp.162-163.

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[26] “AISI 1070 Steel, cold drawn, spheroidized, annealed, 19-32 mm (0.75-1.25 in) round,”

Matweb, <http://www.matweb.com/search/SpecificMaterial.asp?bassnum=M1070A>.