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3 Natural rubber (NR) properties required in tyre manufacture

3 Natural rubber (NR) properties required in tyre manufacture

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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


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

Natural rubber (NR) for the tyre industry


properties. Figure 12.10 depicts the correlation between green strength and

blend ratio of NR and synthetic polyisoprene (IR). The data indicate that NR

provides greater green strength than IR. The example also illustrates how

the tendency of NR towards stretch-induced crystallization is exploited in

industry: this tendency augments not only the performance of cured products

but also the processability of uncured material.

12.3.4 Tyre curing (vulcanization) process

Green tyre is heated in the compartment shaped by the mould and the bladder,

in which the vulcanization reaction takes place. In this process, different tyre

parts should be cured in a unified process as a whole tyre. Curing speed and

crosslink density are important factors to be controlled. Rubber compounds

are usually designed considering potential deviations in some major nonrubber compounds in NR which can affect cure characteristics. The major

contributors are protein, which accelerates the cure reaction, and fatty acids,

which increase crosslink density. For example, stearic acid can be added in a

much higher proportion than is found in NR, thus making deviation in fatty

acids contained in NR a minor component in the curing process.


NR properties required in tyre products

As already discussed in Sections 12.2.3 and 12.2.4, a tyre is principally required

to offer safety, reduced environmental impact, and comfort of mobility (see


NR/IR = 100/0

NR/IR = 50/50

Stress (MPa)


NR/IR = 15/85











Strain (%)




12.10 Stress–strain curves for unvulcanized NR/IR blending 35 phr

carbon black, tested at room temperature at a strain speed of 100



Chemistry, Manufacture and Applications of Natural Rubber

Fig. 12.7). ‘Safety’ may include structural integrity or durability, as well as

sufficient braking and steering ability in different road conditions. ‘Reduced

environmental impact’ may relate to fuel efficiency (mostly reduction of

rolling resistance), wear life (improved wear resistance and durability), and

road noise. ‘Comfort of mobility’ may refer to the level of noise/vibration/

harshness, handling (steering and centre feel, predictability of vehicle

motions), and in-vehicle road noise. In this section, durability, braking, and

fuel efficiency are discussed in relation to NR properties. We also discuss

quality stability of NR.

12.4.1 Durability

Improvement of tyre durability is important not only because of safety aspects

but also from an environmental perspective by supporting longer service life

and a greater number of retreads, thus contributing to more efficient resource

usage and greater opportunities for reuse.

Basic processes behind tyre durability

Tyre failure can be classified into many types. External damage is one major

category. However, failure of internal skeleton (carcass and belt) is less visible

and requires more attention. Such parts are composed of rubber and very

hard material (organic fibres and steel) as reinforcing fibres, resulting in a

significant gap in elasticity (in the order of 103–105) around the interface of

the rubber and these hard materials. This elasticity gap and the major strains

generated by tyre rolling may lead to concentration of local strains at the

vicinity of the cut ends of the reinforcing fibres buried in the tyre.

Rubber compounds covering the reinforcing fibres are composed of

NR as a main polymer compound, combined with a class of carbon black

that provides low heat generation and sufficient reinforcement, as well as

a relatively high dosage of sulphur to achieve sufficient bonding with the

surface of the reinforcing fibres. NR is selected for its outstanding resistance

to mechanical fracture, possibly thanks to its tendency towards strain-induced

crystallization. These rubber compounds are exposed to a great number of

strain cycles and heat generated by tyre rolling. These complex physical

and thermal inputs are believed to lead to a physicochemical change in the

rubber compound structure. In other words, durability of a tyre must be

considered the result of interaction between multi-scale phenomena that

are tightly linked with each other. The elasticity gap at the interfaces in the

rubber-fibre (macroscopic) composite creates locally concentrated strain

in the rubber compound. The rubber compound (microscopic composite)

also has microscopic inhomogeneity and is exposed to repeated and locally

concentrated strain together with internally generated heat, which may lead

Natural rubber (NR) for the tyre industry


to concentrated physicochemical change in the material in the vicinity of the

strain-concentrated area. This will further promote a greater concentration

of the strain at that point. This interplay of microscopic and macroscopic

fatigue phenomena leads to the mechanical breakdown of the macroscopic

composite structure.

The microscopic physicochemical changes described above may involve

(1) physical processes such as locational rearrangements of filler particles and

changes in the amount and shape of physically bound (occluded) polymers,

and (2) chemical processes such as breakdown of the main chain of NR

and its re-combinations, oxidation of the unsaturation on the backbone and

chemical changes at the crosslinking sites including changes in cross-link

density and rearrangements of organo-sulphur bonds.

Durability and NR quality

The changes described above may be affected by market conditions, tyre

types and the situation of rubber compounds in a tyre. However, some general

points can be made. Firstly, rubber compounds encased in the carcass and the

belt areas harden (the modulus increases) during usage, resulting in reduced

tensile strength or elongation at the break.3 This hardening is mainly a result

of principally chemical changes inside the compound, such as oxidation of

the unsaturation as well as rearrangements of the organo-sulphur linkage

between and within chains. The decrease in tensile strength and elongation at

the break may be explained by chemical and physical changes that generate

localization of defects such as chain scissions, rearrangements of the carbon

black network, and lost linkage between carbon black and NR, in addition

to the reasons suggested for the hardening.

Among the changes mentioned above, chemical changes are more observable

in the rubber compounds that contain more sulphur, and accelerated in the

presence of oxygen. Oxygen can be interpreted as a reactant because oxygen

content is chemically bound to the rubber compound, and is increased by

long-term usage. Some studies have examined the kinetics of these ‘oxygen

intake’ phenomena as a descriptor of the reduction of elongation at the

break during ageing of the rubber compound at different temperatures. 4 In

the study by Terrill et al., the half-decay time of the elongation at the break

at room temperature was estimated from the oxygen intake at different

ageing temperatures, and its relationship with the elongation data at elevated

temperatures (accelerated) was depicted using an Arrhenius plot. Half-decay

time of elongation at break was estimated to be approximately 100 years at

24°C based on oxygen intake at 50°C and above, while about ten years at

24°C was estimated based on room temperature oxygen intake. Durability

in real market conditions should involve even more variables, which makes

it more difficult to predict.


Chemistry, Manufacture and Applications of Natural Rubber

In the tyre development process, rubber compounds are designed to

control the changes discussed in this section, relying on mechanistic analyses

of potential changes during service (microscopic design); meanwhile, tyre

structure is designed to reduce the sensitivity of the tyre to such changes in

rubber compounds (macroscopic design). Figure 12.11 depicts an example

of such structural design, in which the distribution of ply cord tension is

controlled so that the strain is less localized at the ply cord ends when the

tyre is rolling, leading to improved tyre durability. However, large foreign

bodies in the rubber compound can promote unwanted strain localization in

the material. High quality of NR, especially in terms of freeness from foreign

materials, is a foundation for controlling all durability issues in tyres.

12.4.2 Traction/braking and fuel efficiency

It is a fundamental function of a tyre to transfer forces from a vehicle’s

drivetrain or braking system onto the road. Braking performance of a tyre

on wet roads is an important part of a tyre’s safety function. It is desirable

to improve this performance through macroscopic design, for example in

terms of water removal and optimizing contact conditions, as well as through

microscopic design to maximize the friction coefficient between the tread

rubber compound and the wet road surface.

As a central element of the ‘environmental’ performance of tyres, fuel

efficiency has become increasingly important recently. As discussed in Section

12.2.5 and Fig. 12.8, GHG emissions from usage of a tyre correspond to about

85% of its life cycle emissions, based on the assumption that the contribution

of a tyre’s rolling resistance to the fuel consumption of the vehicle is 1/8,

although this contribution factor is affected by vehicle type and driving mode.

Reducing rolling resistance, as well as improving wet braking performance,

should be achieved by concerted efforts in macroscopic control, for example



Direction of main strain

Local deformation of

ply-end rubber





12.11 The strain was less localized at the ply cord ends.

Natural rubber (NR) for the tyre industry


controlling the macroscopic deformations of a tyre caused by rolling (to

which maintaining proper inflation is a large contributor), and in microscopic

control, for example designing the material’s viscoelastic response to take

into account microscopic deformations of the tyre in the vicinity of the point

where the tyre is in contact with the road. The contribution of the rubber

compound in the tread to wet braking performance is clearly significant,

because it generates friction force with the wet road surface. However, tread

is also the largest contributing part of a tyre to rolling resistance, as shown in

Plate XV (between pages 198 and 199), which describes the greatest strain

energy loss contributions of the tread in a finite element analysis (FEA)

model of a rolling tyre. It is safe to conclude that rubber compound design

of tread is critically important to provide satisfactory rolling resistance and

wet braking performance.

Among various mechanisms, viscoelastic energy dissipation of the tread

rubber is considered one of the most significant contributors to the friction

forces generated by sliding of tread rubber on wet road surfaces when braking.

In this situation, the tread rubber is excited by relatively high frequency

inputs in the range of 105 Hz caused by the fine texture of the road surface

sliding underneath the tread rubber. A rubber compound that generates

higher energy loss under such excitation will provide greater wet braking

performance. On the other hand, the contribution of tread rubber to the rolling

resistance of a tyre can be understood as similar to the viscoelastic energy

dissipation, but in a much slower range at around 101 Hz. In other words, in

general, a rubber compound that generates lower energy dissipation under

such slow excitation will provide better fuel efficiency of a tyre, as shown

in Fig. 12.12.

The viscoelastic response of the tread rubber originates with the

polymers used in the tread rubber. According to the ‘time–temperature

superposition’ theory (TTS), which is specific to the polymeric material,

the viscoelastic response of tread rubber at higher frequency at a given

temperature is equivalent to the viscoelastic response of the same material

at lower frequency and lower temperature. Taking advantage of this theory,

viscoelastic response that corresponds to wet braking (high frequency) and

rolling resistance (low frequency) can be represented by different regions in

a single temperature sweep viscoelastic spectrum, in which the wet braking

response can be represented by lower temperature (and low frequency) in

the spectrum. If the input frequency of a viscoelastic test is in the order of

101 Hz, wet braking performance of a rubber compound can be correlated

with hysteresis loss at around 0°C (wet braking is better when hysteresis

loss is greater at this temperature), while rolling resistance can be correlated

with the same frequency at around 50–60°C (rolling resistance is smaller

when the hysteresis loss is smaller at this temperature). As described in Fig.

12.12, it is desirable to satisfy both of these viscoelastic attributes.


Chemistry, Manufacture and Applications of Natural Rubber

Rolling resistance

Wet breaking performance


104 ~ 105 Hz

104 ~ 102 Hz

tan d

Wet breaking performance

Rolling resistance

Temperature (°C) –20



10,000 Hz




100 Hz

12.12 Temperature/frequency dependence of compound tan d in

relationship with required tyre performances.

The viscoelastic response of NR may be characterized by the relatively

low glass transition temperature (Tg) and relatively long chain entanglement

molecular weight, as well as milder heat generation. NR is not necessarily the

best polymer to provide wet braking performance in all applications. On the

other hand, the flexibility of NR at very low temperatures may not surpass

that of high-cis BR. Popular approaches to improving both wet braking

and rolling resistance of a tyre include blending polymers with different

Tg in order to meet viscoelastic targets and other criteria by supplementing

the shortcomings of one type of polymer with different polymers. In this

context, one of the most important roles of NR is to supplement resistance

to breakdown of a rubber compound, either before or after curing, taking

advantage of its tendency to undergo strain-induced crystallization.

12.4.3 Control of tyre quality

In order to ensure tyre performance as described in Sections 12.4.1 and 12.4.2,

it is important to stabilize the quality of rubber compounds and their raw

materials. Firstly, contamination of rubber compounds with foreign materials

must be tightly controlled in order to ensure quality of tyre, especially in

terms of safety. Due to its agricultural origin, contamination of NR with

Natural rubber (NR) for the tyre industry


foreign materials is one of the most important tyre quality issues. In order

to achieve tyres of stable and superior quality, it is necessary to keep foreign

material contamination as low as possible.

The other important quality issue for NR is viscosity. Deviation in NR

viscosity could lead to deviations in the state of filler dispersion after

compounding, potentially impacting on the stability of tyre performance.

Viscosity of uncured rubber compound is known to be influenced by NR

viscosity, which is further affected by features of the backbone such as gel,

molecular weight, distributions, branching, and so on. Controlling these

elements of NR viscosity should be very beneficial in improving tyre quality.

Deviations in test results based on official specifications of NR, which will

be discussed in Section 12.6, can lead to deviations in the critical properties

of NR, which can cause issues in tyre quality control.


Examples of NR use in demanding tyre


12.5.1 Winter passenger car tyres

Winter tyres provide critical support in terms of safety of road mobility in

areas with a cold, snowy winter climate. Although performance requirements

of winter tyres differ by regions because of the differences of the climate

and road conditions, it can be said that a winter tyre is demanded to perform

satisfactorily in many different conditions. Even though winter performance is

the most important function, some adaptability to different road conditions is

also required. For example, on city roads in snowy regions of Japan, in winter

it is common to encounter icy road surfaces at near-melting temperature (–4

to 0°C), on which an extremely low friction coefficient with tyres is probable

due to the significant lubrication of water film on the ice. Such water film

can be generated by heating of ice through friction with a tyre. Another

challenging aspect of the winter market of Northern Japan is the relatively

high density of the traffic. These circumstances combined, a winter tyre

is required to be highly reliable in stopping or accelerating the vehicle on

icy roads. In recent years, it has become important to find further technical

sophistication to maintain performance on dry roads alongside advances in

winter performance.

Tread pattern design is a viable macroscopic design technology for wellbalanced winter performance. For snow performance, the typical pattern

is blocks with a large number of deep grooves surrounding them, so that

compaction of snow by the rolling tyre can create ridges of snow that run

in multiple directions and are firm enough to generate the forces necessary

for braking, traction, or steering of the vehicle. For ice performance, the

same pattern can be equipped with ‘sipes’, or thin slits cut into each block.


Chemistry, Manufacture and Applications of Natural Rubber

Sipes with specific shapes and locations in a block create ‘edges’ that help

to scratch the icy surface when braking or acceleration forces are applied,

also providing an escape channel for the water between the ice and the tyre,

which leads to greater friction forces on ice. However, overly ‘sipped’ blocks

become less rigid and can easily collapse under applied forces, resulting in

unsatisfactory snow performance. In order to overcome this, advanced sipe

designs involving complex three-dimensional shapes have been developed,

which prevent blocks from collapsing upon braking by providing an additional

supporting mechanism only when large horizontal stress is applied to the

block, as shown in Fig. 12.13.

Microscopic design technology has made great contributions to the

performance of winter tyres, in addition to the macroscopic design described

above. In order to more efficiently remove the water generated on ice by

ice–tyre friction, technology was developed that uses foamed rubber in

the tread of winter tyres. Figure 12.14 shows an example of the texture of

such a foamed rubber compound. However, at the same time it is desirable

for the tread compound to be soft at lower temperatures, which allows the

tread surface to deform flexibly and establish good contact with the ice.

Such requirements can be met by a microscopic design, employing highcis polybutadiene rubber (HCBR) for very low Tg (–110°C), and a flexible

polymer component in an immiscible blend with NR for the high Tg polymer

component. In this example, silica is selected as filler in order to compensate

low wet grip performance due to the low Tg of HCBR. However, normal

mixing of these ingredients will lead to silica localization to the NR-rich

domain (with higher Tg), whereby the compound’s viscoelastic response

becomes hard in relation to low strain inputs at low temperatures, and

performance in icy conditions is reduced.

In recent years, new technology has been developed that enables a functional

group with affinity to silica to be attached to the end of a low Tg HCBR.

When the resulting end-modified HCBR is employed in the HCBR-NRsilica system, silica becomes localized and microscopically well dispersed

in the HCBR-rich domain (the component with greater low-temperature

Outer shape

Cross section

12.13 The sipes such as complex three-dimensional shape.

Natural rubber (NR) for the tyre industry


300 µm

12.14 Foamed rubber.


ice grip






Conventional HCBR

End-modified HCBR







Deformation of tread compound (%)


Storage modulus (index)

Storage modulus (index)





Conventional HCBR





dry handling

End-modified HCBR






Deformation of tread compound (%)

12.15 Benefit of morphological control to improve trade-offs between

ice grip and dry handling performances.

flexibility), as shown schematically in Plate XVI (between pages 198 and

199). With these morphological controls to lower the Tg of the polymer

component touching the silica surface and reduce filler–filler interactions,

under conditions of small strain and low temperature the storage modulus

becomes softer; maintaining the storage modulus under conditions of large

strain and high temperature, as shown in Fig. 12.15, leads to highly balanced

ice and dry performance with less trade-off. In this example, NR contributes

to tyre performance through morphological control of the immiscible blend,


Chemistry, Manufacture and Applications of Natural Rubber

in which each polymer component plays a specific role in order to provide

tyre performance under a range of environmental conditions with fewer tradeoffs. The specific role of NR in this example may include providing enough

tensile strength despite the low filler content in the domain, which may be

related to the tendency of NR to promote strain-induced crystallization.

12.5.2 Off-the-road tyres

‘Off-the-road’ (OTR) tyres are defined as tyres for ground vehicles driving

primarily away from public roads; these range from tyres for cargo-handling

machines at ports to gigantic earthmover tyres with an outer diameter

ranging from 60 cm to 4 m (7 t in weight), largely depending on purpose.

Some examples are shown in Fig. 12.16, including medium-sized to large

OTR tyres. These tyres are used on a wide range of ground surfaces, from

crushed stone and rock bed to mud and paved roads, and under extremely

demanding conditions. As a result, such tyres need to meet a wide range of

performance requirements, including durability-related (anti-wear, anti-cut,

low heat generation), traction, all-terrain performance, and manoeuvrability,

in addition to the basic functions.

There are two types of construction of OTR tyres: bias and radial. The

radial construction is becoming more popular because of its superior durability

(anti-wear, anti-cut, low heat generation) compared to bias construction.

Most markets that require relatively high speeds are occupied by radial tyres.

Earthmover tyres that are bigger and can support greater loads are being

developed as bigger earthmovers are introduced by mining companies to

improve the efficiency of mining operations. Some recently developed tyres

can support more than 100 metric tons per tyre. Durability has become the

most critical performance aspect for OTR tyres, not only because service

conditions are so demanding but also because tyre size is increasing. As tyre

size increases, less surface area becomes available for heat radiation into

the air in relation to the weight and volume of the tyre, which increases the

12.16 Off-the-road tyre.

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