Tire Ingredients

It is not an uncommon misconception for the lay person to consider that a tire is simply black and round and made of rubber. Tires are conventionally black, almost. They are almost round and rubber is the most suitable material for their manufacture. Rubber is an elastic material that can undergo high levels of deformation with low heat generation. It can generate high levels of frictional grip on most surfaces and it is impermeable to gases. Its lack of ultimate strength means that the raw polymer alone is insufficient to meet the other demanding requirements for tires. Fillers, chemicals and special reinforcing cords are all required to produce the
necessary strength and performance.

In fact typical tires are only around 45% rubber with about 25% inert filler and 15% speciality chemicals. The remaining 15% is fabric although in the case of truck tires, which are predominantly reinforced with steel wires, this level may be somewhat higher on a weight basis.

The main objective of a compounder is to develop a formulation with optimum properties to perform in the required component, at minimum cost and in such a way that it can be efficiently processed. Each of the compounding ingredients plays a vital role in generating the desired properties and so no single ingredient has been left undeveloped over the past century since the tire was first patented. However, the polymers and fillers have had the closest attention paid to their chemistry, both as ingredients and within the mixing process, to extract the last element of performance. The work goes on as witnessed by the many new materials being presented at
conferences and in the literature.

Polymers

All polymeric materials used in tires are elastomers which are high molecular weight ingredients consisting of long chains of one or more repeating types of molecules, known as monomers. The selection of monomers, the polymerisation technique and the architecture of the polymer backbone all play a crucial role in determining the final properties of the compound. Within the rubber industry there is a wide range of polymer types, but tire technologists have found value from just a few basic elastomer types.

Natural Rubber


Natural rubber is a renewable polymer from a natural source, the Hevea brasiliensis tree. Natural rubber is still the traditional workhorse material used in many tire components. It is a very high molecular weight material offering excellent fatigue performance. The other advantages over synthetic polymers are reduced build-up of heat from flexing and greater resistance to tearing when hot. Truck tire treads and internal tire components, particularly bonding compounds, are predominantly natural rubber based. Controlled viscosity natural rubber is now being employed, at a limited level, to aid processing. In some cases the synthetic version of natural rubber, polyisoprene, is being seen as offering some benefits as a full or partial
substitute due to its close similarity to natural rubber.

Stirene-Butadiene Rubber

Research was originally carried out in the 1930s to produce the first synthetic polymers.

Nowadays, copolymers of stirene and butadiene (SBR) are the most commonly used synthetic elastomers in tires, because of their suitable properties to meet performance needs particularly in treads of passenger car tires. The normal technique for producing SBR is by radical polymerisation in emulsion. SBRs can be produced with different levels of stirene. As the stirene level increases from about 20% to 40% so the material becomes less rubbery as the glass transition temperature (Tg) rises.

Another technique of polymerising is in solution using organic metal catalysts. This opens up many further opportunities to modify the architecture of the polymer backbone. The macrostructure of a polymer is defined by the molecular weight, the molecular geometry (linear or branched), the order in which the monomers are incorporated (block or random), the functionality along the chain and how the polymerising chain is terminated. Through the solution polymerisation process it is possible to vary the position in which the butadiene monomer polymerises. The ratio of the 1,2 vinyl to the 1,4 cis-polymerised content in the butadiene part of the polymer can be adjusted. Some critical performance characteristics, such as rolling
resistance and wet grip, can thereby be improved without detriment to others, such as tire wear.







To further improve wet grip, a parameter of vital importance particularly to European carmakers, the industry has developed terpolymers based on SBR. Here a third monomer such as isoprene (SIBR) (263) or acrylonitrile (NSBR), is incorporated into the SBR backbone to raise and broaden the Tg, so offering improved wet grip with no loss in wear rate. Such terpolymer technology can be used to design so called integral or multi-Tg polymers, which can significantly affect the balance of tire properties. The acrylonitrile may alternatively be incorporated as a copolymer with butadiene in the form of acrylonitrile-butadiene rubber, (NBR) which can be added at low loadings to an SBR polymer to supplement tire performance .
Even at around 5 parts per hundred rubber loading, the CN group on the acrylonitrile will have a greater affinity for the hydroxyl groups on the surface of the silica filler so improving dispersion, reducing viscosity and enhancing final properties. Additionally, since the acrylonitrile is basic in nature it will counter the negative effects of the inherently acidic silica surface to accelerate a slow cure whilst retaining scorch safety.

Another new polymer type, brominated isobutylene-p-methylstirene, (BIMS) is finding applications as blends in tread compounds, especially for winter conditions , and also in tire sidewalls.

Butadiene Rubber

Butadiene rubber (BR) is another elastomer that finds interest for tire compounders. In tread compounds, BR is usually blended at about 10% to 20% with SBR to obtain a good balance of performance characteristics. Using various catalysts it is possible to control the levels of 1,4 cis-polybutadiene in the polymer. There are BR grades available with a very high (>98%) cis level , which can be used to minimise treadwear whilst maintaining maximum resilience through their low Tg values. However, ultimate traction and braking depends upon energy absorption in higher
Tg polymers. BR is also used in higher loadings as a blend with NR in sidewall or truck tread compounds to enhance damage resistance.

Other technologies now involve the manufacture of higher 1,2 vinyl polymerised butadiene rubber leading to higher Tg polymers with the potential to
balance improved wet grip and rolling resistance.


Butyl Rubbers

Butyl rubber (IIR), first produced in 1940, is a copolymer of an isobutylene and a very low percentage of a diene, such as isoprene, to facilitate sulphur crosslinking. These
rubbers are valuable because of their good heat resistance and low gas permeability. Improved impermeability butyls now incorporate a low percentage of halogen
atoms like chlorine (ClIIR) or bromine (BrIIR) along the polymer backbone . These polymers are more difficult to process but offer significant improvements in pressure retention and hence the potential to employ lighter, thinner gauge components.


Fillers

Inorganic fillers are traditionally added to reinforce the properties of the gum polymers ). The filler types vary from low reinforcing clays, to mineral fillers like
aluminium trihydrate , to carbon black and more recently silica . Stiffness at low strains comes from a combination of:

• the strength of the cured rubber network

• the hydrodynamic effect of mixing solid particles
in an elastomeric medium

• elastomer entanglements

• polymer to filler interactions and

• any residual filler to filler networking.

As higher strains (perhaps above 8% to 10%) are applied to the matrix, any initial filler to filler links are broken leaving polymer to filler links then finally, before breakage at high strain amplitudes, the strength comes from the covalent links in the elastomeric chain.

Carbon Black

Traditionally carbon black started its life in China by burning a flame deficient in oxygen to produce soot or lamp black. Technology has moved on a long way since carbon black was first recognised as a useful filler for rubber by Mote in 1904. Now the elemental carbon particles produced have a wide range of structures, shapes, particle sizes and surface activity. The type and range of properties offered make it a very desirable choice of filler in many tire compounds. There are various grades of carbon black defined by a three numeric scale (ASTM D-1765). The first digit defines the particle size of the carbon, the second and third digits define the surface area or structure of the particles. The viscoelastic properties of a polymer-carbon black network depend strongly on the size, shape and structure of the carbon black particles, their interaction with each other and the polymer chain . Carbon black is effective in its reinforcement
since the polymer to filler interaction is greater than the filler to filler interaction. For each grade there will be an optimum loading (318) depending on which properties are most critical. Blending different carbon blacks may not realise the benefits of each .

Large particle size, low structure blacks can be used in the inner liner at quite high loadings to offer good processing with reinforcement and enhanced impermeability.

Within the casing, medium structure and medium surface area blacks are easy to process and also offer low hysteresis loss for cooler running. These are even finding their way into tread compounds to improve rolling resistance.

In the sidewall and treads, high abrasion resistant fillers are used. These very fine particle blacks offer tread compounds good grip and wear, but as the loading increases to provide higher reinforcement, so there is a trade off in poorer processing and increased energy
loss hence rolling resistance. More recently, as silica has forged a new role in tire compounding, carbon black makers have been generating many new grades to compete with the benefits of silica. The nanostructure of the carbon black surfaces together with the surface activity have been modified to improve the ability of the filler to interact with the polymer chain

Silica

Silica filler is prepared through the reaction of a soluble sodium silicate with an acid. The precipitated amorphous silica particles of the order 10 to 100 nanometres in
diameter aggregate together as structured particles.

Silicas have now been used in tires for many years with the aim to improve damage resistance in applications such as on and off road truck treads. There was an early unsuccessful attempt to introduce silica into tires as an alternative to carbon black. However, it was not until the late 1980s when efficient bifunctional organo-silane coupling agents , such as bis-(triethoxysilyl-propyl)tetrasulphane (TESPT) or the more heat resistant bisulphide variants were produced, that silica found new levels of success . Unlike carbon black, the filler to filler interaction of the hydrophilic silica can be stronger than the polymer to filler interaction. Additives enable the non-polar rubber molecules to physically bond or couple with active polar elements on the silica surface offering a stronger polymer-filler interaction. Improvements through the substitution of carbon black by silica have been explained by the decrease in the loss modulus of the compound above room temperature, a factor related to the tire’s rolling resistance. The benefits of significant improvements in rolling resistance led to the so-called ‘green’ tire revolution, because the use of silica was regarded as a means of reducing energy consumption,
also silica is not an oil-based product.

Having recognised the difficulties in manufacturing of incorporating silica into the polymer matrix, modern highly dispersible silicas have been developed that can be more readily incorporated into the compound mix. This in turn offers further improvements in rolling resistance, wear and other tire properties. These highly dispersible silicas have successfully been used in car tire applications, such as winter tires where improved ice and snow grip can be achieved.

The ability to produce tire compounds without the traditional black filler opened up a whole new aesthetic opportunity for compounders to produced tires with coloured treads, although there will be an additional requirement here to use non-staining antidegradants . This happened in the bicycle field many years ago, but now there is a trickle of marketing ploys that offer tire colours matched to the vehicle bodywork. The concept could be attractive at vehicle exhibitions
but may never take on wholesale for many reasons, not least of which is the logistics nightmare it would present in replacement tire warehouses.

Every benefit in the industry appears to bring with it a downside. Silica compounds are expensive and notoriously difficult to process. The silica structure and surface area will influence the viscosity of the compound and hence its processability. Conventional internal mixers were designed to mechanically blend ingredients. With silica a time and temperature dependent chemical reaction , known as silanisation, must take place to couple the filler to the polymer whilst guarding against unwanted precrosslinking. It has also been reported that moisture is
a critical variable influencing the silanisation reaction . Water or moisture inhibits the silica to silica interactions whilst promoting the reaction with silanol
coupling agents.

These manufacturing problems have led to a range of new processing techniques, equipment and chemical aids, discussed in a later section, highlighting again the ingenuity of raw material suppliers and compounders to exploit any new development no matter how negative it appears on first evaluation.

In the finished product where full replacement of carbon black by silica has been introduced, the resultant treads are highly resistant to the dissipation of electric charges that build up in vehicles . This is prevalent now that many more static generating electronics are being employed on board vehicles. Typical levels of material resistivity for antistatic tires are less than 107 (whereas silica filled tread compounds can have values as high as 1012). Novel solutions and patents have been
published on dealing with static. Some other ideas include the use of conductive inserts built in between the tread surface and the internal components. Others include thin films of rubber or paint coatings on the tread surface running out to the sidewall. Still others describe conductive bridges. Each method is intended to allow static electricity to dissipate from the vehicle through the tire to the road surface .

Many tire makers have opted to partially substitute carbon black with silica to gain the benefits from both fillers. Suppliers of filler materials are also offering hybrid or dual phase fillers with a modified morphology that contains elements of both carbon black and silica in one structure. Advantages are claimed both for tread compounds and for improved adhesion in wire skim compounds.

Other Fillers

Silica may have been the new filler at the end of the 20th century but even now there are rumblings in the industry that a corn starch biopolymeric filler could be the next generation to silica. Will this be the filler of the new millennium? Only time will tell.

In an attempt to gain extra performance many companies have explored the idea of adding short strand fibres to a rubber mix. Materials such as aramid, natural fibres like rice husks, chopped steel strands and many others, incorporated into a polymer matrix have been used in order to offer isotropic reinforcement.

Other filler types include reinforcing resins . Here an application may be in the triangular apex component that offers a graduated reduction in stiffness from the bead coil to the sidewall. This is a component that must be extremely hard and is hence difficult to process. Reinforcing resins or short strand fibre may be of use in generating the required component properties.

Process Aids

When high filler loadings are required by the formulation, oil is one substance typically used as a process aid. In addition it may offer other advantages, such as wet grip enhancement, to tread compounds. This is particularly so for wet grip motorsport compounds where oil levels can be extremely high. Many polymers are sold in the oil extended form mitigating the problems of adding too much free oil in the first stage of mixing. Masterbatches of oil and carbon black or oil, carbon black and polymer may be used to help incorporate very high loadings of oil.

Most oils are of the paraffinic, naphthenic or aromatic types. The use of distilled aromatic extract (DAE) oils in tire treads is attractive due to the similarity in glass transition temperature to the SBR polymer, especially when this polymer is to be oil extended. However aromatics have attracted close environmental scrutiny of late. The oils used in the rubber industry are predominantly classed under the EU dangerous substances directive as, R45, carcinogenic. This is due to the traces of polycyclic aromatics (PCAs) that they contain. It is claimed that these may present a potential hazard to human health, since if they can be leached out of
worn tire debris they may pass through crops in nearby fields and enter the food chain.
Although not proven, as a responsible industry the manufacturers of oils, oil filled polymers and tires have produced, by several techniques, new ‘non-labelled oils’. These are considered environmentally safer and contain acceptably low levels of PCAs as measured according to the dimethylsulphoxide extraction method IP 346.

Other process aids are used to help the manufacturing process enabling faster mixing and processing of thinner, sharper edged and more accurate components. There has been a revolution in the field of process aids for silica filled tread compounds, although the process conditions and the timing of additions is critical . Many additives are now available which offer improved or faster filler dispersion, enhancements to the silica coupling process and help with the downstream processability, to provide more accurate and consistent tire components.

Antidegradants

The industry has been using antioxidants and antiozonants since it was first realised that chemicals can attack the covalent bonds along the polymer chains. Various grades are widely used in both internal and external components to protect the whole tire . A problem lies in the requirement for these additives to be active over very long product cycles, but without the risk of surface discolouration. In the tire sidewall where antidegradants are most necessary, a sufficient sink or reserve of antidegradants is required which slowly migrate to the surface over the life of the product. This means thicker than desirable components must be designed. Could time-delayed microencapsulation of these ingredients find a future application here?

Companies have experimented with blends of saturated terpolymers of ethylene and propylene (EPDM) together with a small amount of a diene to eliminate the need for antidegradants in sidewall compounds. This also offers the opportunity to apply a thin veneer to the casing, an application that has seen use in the retreading of tires.

Antidegradants can also improve treadwear by improved stabilisation of the tread compound. They can enhance high temperature performance in aircraft tires and help prevent reversion when high temperature curing is required .

Adhesion Promoters

Of vital importance to the safety of tire users is the structural integrity of the product. For retreaders, their concern is the value of the casing at the end of its first life. To be of value it must be capable of being retreaded once or even several times. There is a legal requirement for aircraft tires to have holographic validation of their internal integrity. In the end casing life is determined by the longevity of the bonds that exist between the rubber compounds and the textile or steel cords.

The metal wires used in tire structures are brass coated with copper concentrations in the brass at around 60% to 70%. A compound that bonds to the brass will rely on sulphur to form copper sulphide links between the rubber molecules and the copper on the wire surface. For this reason careful consideration must be given to the reaction rates, the choice of higher levels of sulphur (heat stable insoluble grades to prevent bloom) and the type of accelerator used. In addition, many types of adhesion promoters can be used to control the reaction and preserve the integrity of the bonds throughout the life of the tire.

Much activity has been devoted, especially for steel cords, to understanding and improving the way in which the brass coating on the steel wire surface and rubber adhere. Heat, fatigue, moisture, penetration of foreign bodies, salt and many other aggressive environments may attack the rubber to brass bond. Organic cobalt salts are now generally added to promote adhesion, whilst silica is also known to favourably influence the adhesion stability during ageing . New adhesion promoters often appear in the market and tire compounders evaluate these to seek improvements of the brass to rubber bond. However, there is always a careful reluctance to change such critical compounds without first carrying out extensive laboratory then field tests.

One novel approach to improve adhesion to zinc plated steel cords is the use of argon plasma etching followed by plasma polymerisation to coat the surface and promote improved adhesion to the skim compound.


Curatives

Vulcanisation, or curing, has been reviewed elsewhere . This process produces chemical links between sulphur and the loosely coiled polymeric chains. Elasticity occurs because the chains can be stretched whilst the crosslinks prevent chain slippage such that
they can spring back into place when the stress is released.

The traditional chemistry of rubber vulcanisation dates back to 1838 when Charles Goodyear discovered the reaction between sulphur and rubber. A few years later in 1905 George Oenslager discovered the improved benefits for vulcanisation of using accelerators. Since then faster, safer accelerators have been developed for the industry but generally the chemistry in this area has remained much as it was. The health and safety requirements to eliminate chemicals that generate N-nitrosamines have meant the elimination of some older grades of accelerator in favour of newer materials with novel benefits .

The compound designer has to be aware of the cure kinetics of each compound. The cure system needs to process safely with no risk of surface scorch or early curing which may negate tack or adhesion. Once in the press, however, the compounds throughout the tire need to cure rapidly and consistently to shorten cure cycles and improve tire productivity. With a continual drive within the industry to improve productivity and where tire curing is the critical process in the chain, higher curing temperatures with shorter cure times are being demanded. This leads to the need for more stable compounds or even the use of novel curing systems like peroxides . Sulphur, together with the activators and accelerators, reacts with the covalent bonds along the polymer backbones to form sulphur links. The number of sulphur atoms across each link and their density along the chains determine the final properties of the compound and in some critical components, the resistance to damage or ageing. Novel curing systems are being introduced which are claimed to control the crosslink length and thereby improve
reversion resistance.

Fabric

The field of reinforcing materials themselves is outside the scope of this present review but it is worth mentioning the important role these materials play in achieving the desired tire erformance. Fibres have existed naturally for thousands of years but over the past 50 years or so man-made fibres have been developed which offer significant advantages. In the case of tire fabric, clusters of individual fibres are twisted together to form yarns and one, two or more of these are twisted again to produce cords. These are lain up parallel to each other as warp cords with little or no weft cords between them. Any weft cords used are only there to retain cord spacing until the fabric is calendered.

Car tire casing materials in the USA and Asia tend to be polyester. In the case of many European tire makers and especially when it comes to very high performance car tires, the dimensional stability of natural rayon finds more favour. The polyester makers are now marketing new high modulus low shrinkage (HMLS) polyesters aimed to match more closely the benefits of rayon.

The concept of using steel in the belts of tires as a tread stiffening layer for radial tires was introduced by Michelin in the 1940s. In the belt area of all radial tires and in the casing of truck tires, steel is still the dominant reinforcing material. Despite all the possible demerits of fatigue, fretting and poor adhesion at cut ends or where moisture ingress occurs, the alternatives of aramid or other more exotic fibres have still not seen great inroads, perhaps due to cost. Steel cords are basically made from high carbon steel rods, coated in brass and drawn into fine wires. Technology has evolved over the years to improve the tenacity of the original steel rod, the drawing process, the stranding methods, the configuration of the filaments, the fatigue resistance
of the cords and, more importantly, the chemistry of the brass surface. Recent studies have investigated the option of using other more novel coatings . The way in which cord-reinforcing layers are constructed, the cord density and the angles at which the cords are set all control the behaviour of tires. Much of the new technology here is seeking simpler but stronger constructions offering both weight and cost benefits.

Tire makers have been evaluating the potential of newer cord types like polyethylene naphthalate (PEN) (430) or polyketone (POK) as sources of material for tire reinforcement, but as yet only small market penetration has been identified.