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RUBBER COMPOUND (1): POLYMER
A tire is mainly made of "rubber" and cords. Rubber in this context means the rubber compound to be exact. The compound is made by mixing polymer, reinforcement material, softener and various chemicals.
Different characteristics are required for every type of tire or tire part. For example, a TBR tire requires heat, wear and cut resistance for the tread rubber, while the sidewall requires good weather resistance. Therefore many kinds of rubber compounds are needed.
Polymers, their types, characteristics and uses, which all perform very important functions, are explained.
The main polymers used for tires are as follows:
1. Natural Rubber (NR)
2. Styrene Butadiene Rubber (SBR)
3. Butadiene Rubber (BR)
4. Isoprene Rubber (IR)
5. Halogenated Butyl Rubber
All of the above except natural rubber are synthetic.
1. Natural Rubber (NR)
| NR is made from latex taken from rubber trees, mainly grown in Southeast Asia. Although there are now various kinds of synthetic rubber available, natural rubber is still used extensively in tires. |
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2. Styrene Butadiene Rubber (SBR)
SBR is now the most common synthetic rubber being used in tires. It is made by polymerizing Styrene and Butadiene together, it is also possible by changing styrene content and polymerization process to make various types of SBR's with different characteristics. |
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3. Butadiene Rubber (BR)
Like SBR, BR is a common synthetic rubber used in tires. However unlike NR or SBR there is less interaction among the molecules, and for this reason, a compound of BR only or high BR content has high flexibility but poor elongation resistance. BR however has good resistance to both wear and low-temperatures and therefore is generally used by mixing with either NR or SBR to compensate for its disadvantages. |
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4. Isoprene Rubber (IR)
IR is produced by artificially synthesizing Isoprene which is a principle constituent of NR. Its characteristics are naturally quite similar to those of NR. The main difference between the two being, that the quality of IR is more consistent than NR since it dose not contain natural impurities. The downside however is the cost is higher than NR.
5. Halogenated Butyl Rubber
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Halogenated Butyl Rubber is made by halogenating (adding chlorine or bromine) to Butyl Rubber. The molecular structure of Butyl Rubber gives a high air impermeability. Its disadvantage is it needs a long vulcanization time, however by halogenated the Butyl Rubber this can be overcome. |
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| Main Use for Tire | ||
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| 1 | Natural Rubber (NR) | General Use, TB Tread |
| 2 | Styrene Butadiene Rubber (SBR) | PC Tread |
| 3 | Butadiene Rubber (BR) | Sidewall |
| 4 | Isoprene Rubber (IR) | Partially Used to Replace NR |
| 5 | Halogenated Butyl Rubber | Inner Liner |
| Characteristics of Various Polymer | ||||
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| Polymer |
Natural Rubber / Isoprene Rubber |
Styrene Butadiene Rubber |
Butadiene Rubber |
Halogenated Butyl Rubber |
| Impact Resilience |  |
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| Tear Strength | |
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| Wear Resistance | |
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| Fatigue Resistance | |
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| Aging Resistance | |
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| Weather Resistance | |
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| Ozone Resistance | |
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| High Air Impermeability |
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: Excellent : Good : Fair : Inferior
| Use for Tire | |||||
|---|---|---|---|---|---|
| PC | Tread(Cap) | |
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| Tread(Base) | |
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| Sidewall | |
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| Belt | |
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| Carcass Ply | |
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| Inner Liner | |
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| TB | Tread(Cap) | |
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| Tread(Base) | |
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| Sidewall | |
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| Belt | |
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| Carcass Ply | |
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| Inner Liner | |
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RUBBER COMPOUND (2): ANTIOXIDANTS AND WAXES
As commonly known, rubber is a main constituent of tires and, regardless whether natural or synthetic, it suffers aging like any other substance as a result of usage over a long period. Aging is recognized as deterioration in physical properties. Namely, aged rubber becomes either hardened or softened causing cracking or loss of adhesion. The main causes of rubber aging in tires are Ozone, Heat and Deflection. To protect rubber from aging, Antioxidants and Waxes are generally used in the rubber compounding.
Functions of Antioxidants and Waxes as Anti-Aging Agents in Tires
| Cause | Antioxidant | Wax |
|---|---|---|
| Ozone | Antioxidants come out to the surface of the tire where chemical reactions take place with the attacking ozone. Thus ozone is rendered inactive to age rubber.*
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Waxes migrate to the tire surface and form a thin film covering the surface. Thus the rubber is physically protected from the attack of ozone.** |
| Heat/Deflection | Tire deflection and heat generation in the tire weaken and finally cut the polymer (rubber molecule) linkage which results in aging. Antioxidants in the rubber work to chemically bond with polymers in order to prevent the polymer linkage from being extensively cut. |
Wax has no effect against heat and deflection aging. |
**When you see the tire with a "whitish" color, this is evidence of a layer of wax protecting the tire from ozone. (However the effectiveness of the wax layer is lost once the tire becomes rolling because the wax layer is broken.)
Type and Property Requirements of Antioxidants
Tire parts where wax is used are as follows: Tread, Sidewall, Rim Strip and Tread Strip.
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STEEL CORDS
Tire cords play an important role in the "reinforcement of rubber". They are an important factor in tire construction and have a major influence upon the tire's performance. The below chart shows the trends in tire cord material and organic fibers such as cotton, rayon, nylon and polyester, were used for tire cord for some time. Within the last 25 years the increase in steel cord can be seen and this is due to the increase in radial tire manufacture.
The steel cords are mainly used in the following parts of the tires.
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Some basic knowledge of steel cords currently used in the "carcass" of LT/TB tires and "belt" of PC/LT/TB tires is explained.
1. The Material of Steel Cords
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In general, high level carbonaceous steel wire is used. |
2. Manufacturing Method of Steel Cords
(1) The manufacturing process runs high level carbonaceous steel wire through a die numerous times.
But in the middle of this process, the steel manufacturer has to carry out the following;
→The heat treatment must be carried out once or twice because steel wire becomes hard by running it through the die.
→The plating with "brass" must be carried out so that the steel wire can easy adhere to rubber.*
*Brass plating: In general, copper (Cu) and zinc (Zn) are used. The ratio is usually between 60:40 to 70:30
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(2) By repeatedly drawing the wire through the extruder die the wire finally |
3. Characteristics of Steel Cords
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(1) Excellent tensile strength (2) High modulus of elasticity per uniform section area By utilizing the characteristics of steel cord, it is easy to see where it is suitable for certain tire categories, components and operating conditions. |
Steel cord construction and the effect on requirement performance is shown below
Notes
*General diameter of filament: 0.2 to 0.38m/m per filament
**Pitch of twist (turn/cm): The length of cord, strand or wrapping per one twist. Strand is like a rope where several filaments are twisted while wrapping, known as "fret wire", is the filament wrapped around the cord bundle in a spiral.
The main purpose that the steel wire is twisted is to keep the wire's form. This form prevents the twist of the steel wire from becomeing loose. (Wrapping also assists.)
The popular construction of steel cord used in the belt of PC tire and the belt and carcass of TB tire is shown below.
A) Characteristics of Steel Cord for Passenger Tire (Belt)
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(a) Diameter of filament is smaller. (b) Pitch of twist (turn/cm) is longer. |
B) Characteristics of Steel Cord for Truck Tire (Belt)
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(a) Diameter of filament is bigger. (b) Pitch of twist (turn/cm) is longer. |
C) Characteristics of Steel Cord for Truck Tire (Carcass)
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(a) Diameter of filament is smaller. (b) Pitch of twist is shorter. |
As with most things in the world steel cord has its good and weak points. The most commonly known weak point of steel cord is "rust". Rust resistance can be improved by improving rubber penetration.
D) High-Tensile* Open Construction Carcass-Cord*
Regular steel cord
Open-type steel cord
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Regular steel cord: There is a very small opening between filaments.
Open-type steel cord: There is an opening between filaments.
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*High-Tensile Steel Cord (High strength steel cord):
Carbon content of high level carbonaceous steel wire (steel cord material) is increased to about 0.8%. (Regular carbon contents is around 0.7%) This results in a High-Tensile Steel Cord with greatly improved strength as a result of the increasing carbon content.
