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2 Tyre types, manufacture and requirements

2 Tyre types, manufacture and requirements

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Natural rubber (NR) for the tyre industry



Steel cord



327



Filler Polymer



Organic fibre cord



Bead wire

500 nm

Compound (polymer + filler + chemicals)



12.2 ‘Microscopic’–‘macroscopic’ composites.



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 tyre

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 tyre itself.

An example of the basic construction of a typical tyre is shown in Fig.

12.3. The exterior is a layer of rubber, which can be divided into four areas:

tread, shoulder, sidewall and bead. The interior of the tyre is reinforced by

long, rubber-coated fibres and wires called carcass cords, belt cords and

bead wires. The tyre’s air compartment confines the inflating air either by

the use of a low air-permeating layer of rubber – the inner-liner – or a tyre

tube in the case of tubed tyre construction. The following list discusses the

functions and requirements of the major tyre parts.





Tread: The part of the tyre which physically touches the road surface. Its

functions are to protect the inner casing from road hazards and impacts,

grip the road surface, and sustain usability through wear resistance. The

surface of the tread usually has an engineered pattern in order to displace

water and provide sufficient braking and traction in wet or other difficult

conditions. Materials for the tread vary widely by the application, and

polymers such as SBR, NR, and BR can be used alone or as blends.

∑ Shoulder: The zone between the tread and the sidewall, which protects

the inner casing from damage. It also helps to dissipate heat generated

by the rolling of the tyre.

∑ Sidewall: The zone between the shoulder and the bead, which protects



328



Chemistry, Manufacture and Applications of Natural Rubber

Tread

Shoulder

Rubber

Belt (breaker belt)

Sidewall

Carcass layer



Inner-liner



Bead wire

Bead



Rim



Rim valve



12.3 Basic construction of a typical tyre.



















the inner casing from damage. It also functions as an information label

for the tyre, describing the size, number of carcass layers, name of design

pattern, manufacturer, serial number, etc. It is usually composed of a

blend of NR and the synthetic butadiene rubber (BR).

Carcass layer: The main reinforcing layer, which sustains internal air

pressure. It is composed of strong, precisely aligned cords (organic

fibre yarns or steel cords, depending on tyre type) covered with rubber

compound coating. In radial tyres, the cords are aligned in the radial

direction, while in bias tyres the cords are aligned with a bias angle to

the radial direction. The strength of the carcass layer is defined by cord

types and density. The rubber compound is composed mainly of NR.

Belt: The reinforcing layer between tread and carcass in a radial tyre. It

serves as a main reinforcing hoop underneath the tread, and strengthens

the rigidity of the tread area. It also protects the carcass from road hazards.

It is usually composed of steel cords covered with rubber compounds.

The rubber compound is composed mainly of NR. (In a bias tyre there is

a layer of cords and rubber called the ‘breaker belt’ in a similar location

to the belt in a radial tyre. It protects the carcass layer from road hazards

and prevents the delamination of the tread.)

Inner-liner: The layer of rubber compound in the innermost surface

of a tyre, which functions as an air-retaining layer. Low permeability

of inflating air is the key function. The rubber compound is composed

mainly of butyl (or halogenated butyl) rubber in developed countries.

Bead wire: A ring or bundle of steel wires covered with rubber compound,

which fixes the tyre to the wheel rim when the tyre is inflated with air.

The rubber compound is typically composed mainly of NR.



Natural rubber (NR) for the tyre industry



329



The typical weight composition of a tyre is shown in Fig. 12.4. Materials

used in rubber compounds, such as NR, synthetic rubber, filler and rubber

chemicals, make up almost 80% of a tyre. Typical types of synthetic rubber

include styrene butadiene rubber (SBR), butadiene rubber (BR), butyl rubber

(IIR), and synthetic polyisoprene (IR). The filler is still usually carbon black,

although the use of silica is gradually increasing. 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.

Natural rubber (or NR) comprises about 30% by weight of a tyre in the

example shown in Fig. 12.4, which corresponds to slightly less than 60%

of the polymer component (or total natural and synthetic rubbers) of a

tyre. This number used to be lower, as more SR used to be used for tyres

in relation to NR. Reasons for the recent increased share of NR in tyres

include the expansion in the use of radial tyres and heavy-duty tyres. This

trend is depicted in Fig. 12.5, which explains the history of the NR ratio

in all polymer components for tyre usage in Japan. The trend may also be

attributed to NR’s specific properties, such as durability under heavy loads

or adhesion with steel cords. There is no clear technical reason why the NR/

SR ratio may drastically shift in the future.

Table 12.1 shows the raw material composition of a typical passenger car

radial tyre (PCR) and a truck and bus radial tyre (TBR).1 The ratio of NR is

larger in TBR than in PCR. This may be due to the composition of the tread,

Steel cord 10%



Bead wire 5%



Organic fibre cord 3%



Rubber

chemicals 6%



Synthetic

rubber 21%



Filler 26%

Natural

rubber 29%



12.4 Weight composition of a tyre.



330



Chemistry, Manufacture and Applications of Natural Rubber

60



%



50

40



2005



2000



1995



1990



1985



1975



20



1980



30



12.5 NR ratio in all rubber components for tyre usage in Japan.

Table 12.1 Typical weight composition of a tyre

Raw materials



PCR

(195/65R15)



TBR

(275/80R22.5)



General use Fuel efficient



General use



Fuel efficient



Polymeric components



100.0



100.0



100.0



100.0



Natural rubber



39.0



46.4



77.0



78.8



Synthetic rubber



61.0



53.6



23.0



21.2



50.0



41.3



52.0



47.3



Silica



1.0



16.9



1.0



2.8



Process oil



8.0



9.6



2.0



1.8



Organic rubber

chemicals



8.0



13.1



10.0



8.3



Inorganic chemicals



Carbon black



7.0



22.8



9.0



9.9



Zinc oxide



3.0



3.4



5.0



4.4



Sulphur



3.0



2.5



3.0



2.7



Organic fibre cord



10.0



8.0



0.0



0.4



Steel cord



15.0



14.1



33.0



31.5



Bead wire



8.0



9.5



11.0



13.3



206.0



218.4



217.0



212.5



Total



the heaviest rubber part in each tyre. PCR tread is typically composed mainly

of SR, while TBR tread contains more NR. Off-road tyres and aircraft tyres

must sustain even heavier loads in demanding conditions and rely even more

on NR. This will be explained in Sections 12.5.2 and 12.5.3, respectively.



12.2.3 Manufacturing process of tyres

An example of the general tyre manufacturing process is shown in Fig. 12.6.

The three general steps are as follows.



Mixing process

Mix materials

Natural/synthetic

rubber

Carbon black

Sulphur and

other chemical

agents



Tread extruding process

Cool

Apply heat to make rubber elastic



Form into

sheet strips



Building process

Tread, sidewall



Cool

Body ply, steel belt

Extrude rubber



Cut to tyre length



Bead



Begin with body ply



Cord manufacturing/calendering process

Cutting process



Twist of nylon, polyester, and others

into tyre cord



“Green” tyre



Cut at proper angle into specific

length and width



Weave into cord fabric



Steel belt manufacturing process



Coat fabric with rubber



Improve strength after

treating fabric in dip solution



Cutting process



Bead wire



Align bead wires



Attach bead



Vulcanizing process



Coat steel cord with rubber



Bead-making process



Apply sidewall,

steel belt, and

tread



Cut at proper angle into

specific length and width

Apply heat and pressure to green tyre

Cool to form bead

Inspection process



Coat with rubber



12.6 Production process for a passenger car radial tyre.



Trim and perform

appearance inspection and

balance/uniformity check



332



Chemistry, Manufacture and Applications of Natural Rubber



1. Rubber mixing: A process that produces uncured rubber compound by

mixing raw materials, as described in Section 12.2.2.

2. Shaping and forming parts (extruding tread and sidewall, calendaring and

cutting, forming bead): The process described in Section 12.2.2, using

the uncured rubber compound prepared in the first step. The tread and

sidewall are usually prepared by extruding uncured rubber compound into

the desired cross-section shape, then cutting a length that corresponds

with the appropriate length for the tyre.

3. Assembling parts and finishing: The parts prepared in the second step

are assembled into a tyre, then cured and inspected.

It should be pointed out that properties of NR can affect workability of

materials, as explained above. The requirements for NR in manufacturing

tyres will be discussed in Section 12.3 in relation to each manufacturing step:

to ease understanding, however, three examples can also be included here.

Firstly, gel content and molecular weight may contribute to viscosity and

other rheological attributes of NR, which may impact compound viscosity

and filler dispersion in rubber mixing. The odour of NR can also become

a significant issue. A third potential factor is NR’s tendency to crystallize

upon stretching, even before crosslinking has contributed to the strength of

uncured rubber compounds. This affects the handling of rubber compounds

in many processes.



12.2.4 Basic functions and desired performances of

tyres

Tyres must be able to fulfil the following basic functions:







support the vehicle weight,

accelerate the vehicle by transferring traction force to the road

surface,

∑ steer the vehicle by generating cornering force,

∑ stop the vehicle by generating braking force between the tyre and the

road surface,

∑ insulate the vehicle from shocks from road conditions and vehicle

motion. Technical advances in tyre manufacture have improved these

basic functions and contributed to better mobility and safer and more

comfortable motoring, as well as providing additional value to society.

For example, tyres with an improved capability to stop the vehicle on

wet or icy roads and tyres that can sustain mobility when inflating air

pressure is lost (tyres with ‘run-flat’ technology) have already been

developed.



Natural rubber (NR) for the tyre industry



333



12.2.5 Environmental aspects of tyres

Global environmental issues such as global warming have begun to be more

clearly understood in recent years, and action to combat these trends should

not be delayed. It is therefore becoming more important to develop tyres with

reduced environmental impact, which nevertheless satisfy market demands

for safety and comfort, thus contributing to the realization of the ‘sustainable

society’. This concept is summarized in Fig. 12.7. When dealing with the

environmental aspects of an industrial product, the total environmental

impact in the life cycle of the product needs to be considered, not only in the

product manufacturing and usage phases, but also the raw materials phase,

the logistics phase, and the end-of-life phase (disposal and recycling).

Figure 12.8 gives some examples of life cycle analyses of the greenhouse

gas (GHG) emissions of two typical PCR tyres (‘general use’ and ‘fuel

Environment

Fuel efficiency

Weight



Noise



Durability

Handling

Traction/braking

Comfortable mobility



Safety



12.7 Desired performance of tyres.

GHG emissions in the life cycle of a general-use tyre = 300.6 kgCO2e

Raw material

25.0 kgCO2e

8.3%



Production

7.8 kgCO2e

2.6%



Logistics

1.6 kgCO2e

0.5%



Customers’use

263.4 kgCO2e

87.6%



Disposal and

recycle

2.9 kgCO2e

1.0%



GHG emissions in the life cycle of a fuel efficient tyre = 243.9 kgCO2e

Raw material

23.9 kgCO2e

9.8%



Production

7.0 kgCO2e

2.9%



Logistics

1.5 kgCO2e

0.6%



Customers’use

210.8 kgCO2e

86.4%



Disposal and

recycle

0.7 kgCO2e

0.3%



12.8 GHG emissions in the life cycle of a passenger car tyre.



334



Chemistry, Manufacture and Applications of Natural Rubber



efficient’), based on the recent study carried out by the Japan Association of

Tyre Manufacturers (JATMA).1 The ‘fuel-efficient’ tyre, according to this

study, reduces CO2-equivalent GHG emissions in its life cycle by approximately

57 kg compared to the ‘general use’ tyre. Of this reduction, the usage phase

(reduction of rolling resistance) contributes 53 kg, while reductions in the

raw materials and end-of-life phases account for the rest. There is a higher

content of NR in the fuel-efficient tyre than in the general-use tyre, as

shown in Table 12.1. Because NR is largely reliant on photosynthesis for

its production, it is considered ‘carbon neutral’ in the end-of-life phase, in

which a thermal recycling model was employed in the Japanese study, as

shown in Table 12.2. The CO2 emission of NR compared to SR was lower

in the raw material phase for similar reasons.

In addition to reducing GHG emissions, developments to improve the

efficiency of resource usage are necessary. For example, technology is being

developed and marketed to enable the reuse of a worn tyre by applying and

bonding a new tread rubber onto the worn tyre after cleaning and resurfacing

(‘retreading’), as shown in Fig. 12.9. The capability of a worn tyre to serve

society again as a ‘retreaded tyre’ may be determined by the remaining

integrity of the internal casing, including the carcass and the belt, if no

significant external damage is present. The durability of rubber compounds

in these areas, where NR is mainly used, is important in order to maintain

the integrity of these parts after tyre use. In other words, NR contributes to

the reusability of worn tyres, in addition to its being a renewable material

(photosynthesized polymer). From these perspectives, it is desirable to develop

Table 12.2 GHG emission factors of NR and SR (kgCO2e/kg)

Raw materials phase

NR

SR



Production



Logistics



0.64

2.40



0.923

0.092



Worn tyre



12.9 Retread.



Waste disposal Total

0

3.30



‘Retreading’ process



1.56

5.79



Retread tyre



Natural rubber (NR) for the tyre industry



335



technologies for even more efficient utilization of NR in tyres for the future,

taking full advantage of NR’s mechanical strength and other properties while

overcoming its tendency to degrade in conditions of high temperature more

easily than most synthetic rubbers. Different approaches may contain both

macroscopic (mechanical/structural) and microscopic (chemical/material)

design technologies.



12.3



Natural rubber (NR) properties required in tyre

manufacture



As was briefly explained in Section 12.2.2, the properties of natural rubber

(NR) contribute to typical tyre manufacturing processes in many ways.

However, some properties may require more attention from NR manufacturers

and shippers. In addition to the general review given above, this section

attempts to address these points.



12.3.1 Rubber mixing process

Rubber mixing is a process that produces a rubber compound from raw

materials, blending, softening, homogenizing, and dispersing the ingredients

by applying mechanical shear forces (and the resultant heat).

In general, NR has a tendency to increase its viscosity during long storage.

When NR is used in a location distant from NR producing sites, this becomes

a significant issue due to the inevitability of long periods of storage during

transportation. In order to ensure satisfactory dispersion of ingredients and

processability of the rubber compound, a process called ‘pre-mastication’ can

be applied if necessary. This reduces the viscosity of NR to a satisfactory level

for rubber mixing by applying mechanical shear forces in internal mixers or

on open mills, occasionally with rubber chemicals classified as ‘peptizers’,

which help to break down the chain. In this process, macroscopic gel in

the NR is broken down to a microscopic scale, and molecular weight and

chain-to-chain interactions are reduced. Pre-mastication is also beneficial to

reduce deviations in the viscosity of NR – which is much greater than that of

typical synthetic rubbers due to the agricultural origin of the material – into

a narrower range before rubber compounding. As an alternative solution, a

class of NR called constant viscosity (CV) is generally produced for users

who wish to avoid the pre-mastication process. CV-grade NR is produced by

adding a viscosity-stabilizing agent in the manufacturing process in order to

prevent post-production increase in viscosity. Examples of viscosity-stabilizing

agents include hydroxylamine sulphate, semicarbazide and hydrazides.

The rubber mixing process seeks to achieve fine and uniform dispersion

of ingredients as well as establishing strong (often chemical) interactions

between the filler surface and the matrix polymers (NR and SR). In order to



336



Chemistry, Manufacture and Applications of Natural Rubber



achieve these goals, mechanical shear force is controlled by process parameters

such as mixing time, temperature, fill factor (volume of ingredients divided

by internal volume of the internal mixer), and rotor speed or mill speed,

as well as by design parameters of the mixers such as mill gap. Reducing

viscosity deviations in NR will be beneficial in stabilizing this process.

The specific odour of NR that is emitted to air in pre-masticating is another

critical issue in tyre manufacturing. This odour can affect the quality of the

workplace environment inside the manufacturing sites as well as the quality

of living in nearby residential areas. Although tyre industries have been taking

action to deodorize the air, including adding deodorizing equipment or using

deodorant materials, enough cases remain to suggest that these actions are

insufficient. In other words, NR with lower odour generation is increasingly

desired by the industry. NR odour is known to be the result of biological

decomposition of non-rubber components such as proteins and lipids, which

leads to the generation of low-fat acids such as valeric acid 2 and aromatic

cyclic amines such as indole and skatole, which are components in many

odorants.



12.3.2 Parts shaping process (extrusion and calendaring)

In the extrusion process, the rubber compound obtained in rubber mixing

(Section 12.3.1) is shaped into parts that are used in the tyre forming process

(Section 12.3.3). In the calendaring process, reinforcing fibres such as steel

cords, or the fabrics of organic reinforcing fibres are covered with appropriate

rubber compound. Because the dimensional precision of the intermediate

products of this step partly defines the dimensional precision of a finished

tyre, reproducible properties such as flowability and dimensional stability of

the rubber compound are required in this process. The molecular weight and

gel content of NR have strong influences upon its rheology. Tighter control

of these NR parameters may contribute to more stable and efficient operation

of this process. When shaping thin-gauge parts by extrusion or calendaring,

foreign bodies in rubber compounds that are larger than a certain size can

cause unwanted slitting of extrudates or other problems, which can seriously

impact the productivity of the process. NR free from foreign materials is

therefore highly desirable.



12.3.3 Tyre forming process

The parts obtained in the shaping processes (Section 12.3.2) are assembled

to produce uncured (green) tyre. In this process, tackiness and green strength

are the most desirable attributes of the rubber compounds. These properties

are improved when a greater proportion of NR is used in the compound. In

some compounds, NR is blended only to improve tackiness and green strength



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