Tire Compound Processing

There is an equal emphasis being put into the processing of tire compounds as into the design of the recipes. Mixer technology has, until recently, remained unchanged for many years with major companies reluctant to invest in expensive new plant. Many of today’s compounds, however, require modern mixers with alternative rotors to the tangential designs, intermeshing , interlocking or a combination, together with superior computerised control systems, which help in the temperature control. Much work with computer models and neural networks is now finding application to help in understanding how processes and mixer operations can improve the quality and efficiency of ingredient incorporation and hence the resulting performance of the compound. Downstream, extruders are producing more complex multiple components at faster rates, which
has again led to a revolution in equipment design. Modern equipment manufacturers are now attempting to integrate mixing into the extrusion

process to facilitate a single continuous rather than batch processing technique, even considering cold mixing technology. New ingredients are being developed, where the filler is combined with the polymer at the polymerisation stage to generate a powdered rubber material , suitable for continuous mixing through an extruder rather than an internal mixer.

The concept of precuring components using electron beam radiation (EBR) has been utilised to prevent movement of cords within their bonding compound and to shorten cure times.

All these techniques may be of use, especially with the current move towards integrated modular tire manufacture, which has such acronyms as MIRS (95), Impact and C3M , etc. Here small space and energy efficient, self-contained production units deliver compounds and components to building machines in a fast and highly flexible configuration. Again, the whole subject of tire manufacturing technology could be the subject of another review.

For tire compounders, new process technology opens up fresh opportunities of study in the formulation of tire compounds.

A range of ancillary materials have been developed to support the tire manufacturing process, anti-tack solutions , tack promoters, internal and external release agents , paints and sprays. Here again there are many conflicting needs which material suppliers and users have had to balance. Productivity with reduced scrap needs to be balanced against cost and environmental impact. Over the past few years there has been a strong drive throughout the whole rubber industry to eliminate solvents or volatile organic compounds (VOCs) from the basket of process chemicals. Either substitution with water based ) materials or the complete elimination of these materials has occurred, in a major initiative to create a cleaner healthier environment.

Tire surface quality when it leaves the mould is becoming more important in the eyes of customers. Materials added during processing must not discolour the outer surface of tires. More than that, the moulding process can now even enhance the final
appearance of the tire. Clean black and smooth product surfaces make tires attractive on a vehicle. This has also led to a whole new interest in additives, mould cleaning and techniques of mould surface design.


Compound Properties


Tire compounders carry out a range of conventional compound tests aimed at determining the downstream processability of a compound and to predict final tire performance. The tests include those traditional to the rubber industry with a few more novel tests developed specifically to help tire compounders.

Rheometry

The rheological behaviour of a compound, normally defined by Mooney viscosity will determine the energy required to process a component. If the viscosity is too high there may be a risk of surface scorch thereby reducing component tack and giving rough extrudates . It is vital, however, to know the stiffness of the green compound as measured at typical processing rates of shear, which may be orders of magnitude greater than those found in most conventional laboratory rheometers. Many new tire compounds are becoming more demanding on processing. Stiffer components are required to give support to the tire whilst weight reductions are
dictating thinner and more sharply defined component shapes. Processability in large volumes has produced a gamut of new process aids to support the compounder.

New equipment for assessing processability, like the Rubber Process Analyser (RPA) from Alpha Technologies, are being introduced into the control laboratories of factories to monitor the more sensitive parameters of raw component viscosity and compound elasticity . Interpretation of these traces will indicate many useful factors to help the compounder understand the various physical and chemical actions that take place during mixing. Once processed the tire components must be cured to obtain the final shaped crosslinked tire. A moving die rheometer (MDR, developed by Monsanto now Alpha Technologies) is generally used to track the compound stiffness against time as it cures at an elevated temperature. The trace will monitor the margin of processing safety, the rate at which the compound cures and the final stiffness at optimum cure.
Each parameter can be used firstly to develop a compound cure system
with the desired process characteristics and then within a factory environment to monitor the ongoing consistency of each successive mix. The demand for
high productivity means cure times are becoming ever shorter. Faster curing formulations are needed but they must process safely leading to a balance of slow cure

initiation followed by rapid cure rates. In thick components of truck or earthmover tires the internal components will be critical in reaching the desired state of cure. Long times are sometimes required for the necessary temperature rise to be achieved deep within
a tire section whilst guarding against overcure at the surface which could result in less than optimum performance. Finite element computer models are now available which can call upon non-isothermal cure data, Arrhenius reaction rates, tire shapes and mould
characteristics to predict cure times and carry out ‘what if’ scenarios to help reduce overall cure times.


Ingredient Dispersion

In the early stages of mixing it is important to ensure that all ingredients are well-dispersed , since this will have a critical effect on downstream processing and ultimate tire performance. Good dispersion offers more contact area between the polymer and filler molecules and also influences the mobility of the polymer chains. Optical techniques have been developed using transmission or reflection microscopes to evaluate the dispersion of ingredients within a compound. High dispersion levels are needed for most performance characteristics, whilst poor
dispersion may be indicative of problems in the mixing stage or with raw material handling.

Physical Properties

Compounders still use classical measurements of tensile strength, modulus, tear strength, hardness, abrasion, permeability, heat build-up, fatigue, skid resistance , resilience, etc., to determine the properties of various tire components. These techniques have already been reviewed . The results are key indicators about how a compound is likely to perform but they reveal little about some of the more complex tire performance characteristics that may be experienced in the field. From stress-strain curves, much of the more fundamental data required by finite element analysis models can be obtained.

Newer techniques are available offering a wider range of data, some of which can be used to relate more closely to final tire performance. The techniques for assessing the strain energy density applied and the tear energy as it relates to component fatigue, have led to the introduction of new methods and test rigs such as the tear analyser . This rig can be used to determine more precisely the fracture mechanics data for compounds. Again such data may be required where finite element modelling is being used to assess structural durability.



Wear, or the slow loss of material from the surface of a rubber product is not a simple mechanism, it is the composite effect of several factors. Abrasive wear occurs as the rubber is dragged across sharp asperities that cut or tear the compound. Fatigue wear can occur as compound is repeatedly stressed until it fails though microscopic crack propagation. Adhesive wear occurs under the slip-stick mode of grip where the rubber is momentarily bonded to another surface then, as it is removed, tiny pieces tear away from the main body of material. Each mechanism can occur in the contact patch depending upon the mode under which the tire is operating. Predicting tire wear from a single condition test only is fraught with dangers. Fundamental wear test rigs such as DIN, Taber or PICO , whilst still being used for pre-sorting, are now being superseded by new machines tests like the Lambourn , Grosch , FKK or even whole tire test drums . Each claim to assess wear under a wider range of conditions likely to be experienced in the real world. Properties versus performance maps can be produced to predict wear potential under a wide range of operational conditions. Crossovers in performance have been established which highlight the risk of judging performance from single condition tests.

For sidewall compounds it is valuable to have some understanding of the resistance to ozone under static or dynamic conditions. In some places around the world sunlight or UV light is intense, and tires may be exposed to high concentrations of ozone. Laboratory ozone cabinets can quickly determine whether or not there is likely to be a problem for the sidewall compound.

Permeability is of particular importance when developing the inner liner. If any oxygen or moisture is contained within the inflation air and permeates into the carcass, attack of adhesive bonds may begin to occur. A simple rig with a diaphragm of rubber
separating two sides of a cell can be employed to monitor a pressure gradient or flow rate as a measure of permeability or diffusion. Alternatively, and more realistically, the pressure loss over time of an inflated tire can be used to assess the permeability of the liner assembly itself.

There is an additional test in which a hypodermic needle attached to a pressure gauge is carefully inserted into the tire casing to measure casing pressure build-up.

Adhesion

The integrity of a tire in service depends upon the ability of components to stay bonded to each other even after long severe conditions of heat and stress. The most critical components for adhesion are the compounds that bond to the continuous reinforcing cords of nylon, rayon, polyester and more critically steel (brass coated). Numerous tests have been devised to simulate ageing by heat, humidity, salt, high oxygen concentrations, etc. These have been coupled with many static and dynamic test modes to determine how the cord to rubber interface may weaken. Compounders use many tests to evaluate any new formulations that may offer improved adhesion, particularly after ageing. Studies also involve tests of surface chemistry to explain the mechanisms of adhesion and to show how ingredients can counter the chemical failures that may occur at the interface. Scanning electron microscopy of surface elements indicates how the dendritic structures and chemistry within the interface play a part in determining the durability of the bond.

Here again is another physical property where finite element modelling is gaining credence, through its ability to predict stress concentrations and heat buildup within tire structures. Fatigue and even possible structural failure may now be modelled in the computer so that compound properties can be used to pre-test prior to building tires and carrying out field trials.

Nuclear magnetic resonance (NMR) hydrogen ion imaging is a new technique that is being used to track the rubber ageing process.

Viscoelasticity

The rubber used in products is inherently, but not entirely, elastic. When a strain cycle is applied to a rubber component the behaviour is therefore not completely Hookian. During the application and release of the stress, the strain lags slightly due to hysteresis
losses within the molecular structure.

Using dynamic tests it is possible from the stress-strain curves to resolve the elastic, viscous and hence the complex moduli of the compound. The loss tangent
(Tan d) is the ratio of the elastic to the viscous moduli.

Each of these properties is temperature and frequency dependent as the material changes from a plastic to an elastic form. They are important parameters that strongly influence the performance of the final product.




Many workers have attempted to explain how laboratory measurement of viscoelasticity can offer insights into the potential performance of tires
under various test conditions.

Below a critical temperature or above a critical frequency the ability of side groups on the molecular chain to rotate becomes inhibited. The material then behaves like a
plastic. Conversely at higher temperatures or lower frequencies the chains are more mobile and the material behaves like a rubber. The temperature at which this
change occurs is defined as the glass transition temperature (Tg) of the polymer or compound. Passing though the transition, the hysteretic energy loss, as
defined by Tan d, goes through a maximum.

In winter environments where temperatures are well below 0 °C, polymers like natural rubber offer the best elasticity retention. As ambient temperatures rise, the
SBR polymers work more effectively to provide a good balance of properties. Some of the new polymers are either high in their glass transition temperature or possess a broad transition over quite a wide temperature range.

There are several types of equipment on the market to evaluate the viscoelastic properties of compounds, each operating in its own way. They use various modes of
motion, cyclic or pulsed (100), different sample shapes and each has its own options for strain modes, strain rates, amplitudes, etc. Various modes: tensile, compression and shear, have been used to try and improve the correlation with tire performance. The
classic Williams, Landel and Ferry (WLF) transposition of temperature and frequency into single master curves requires that, for simplicity of operation,
many test rigs evaluate compound viscoelastic properties over a range of temperatures either side of the glass transition point. The transposition theory defines a decrease in temperature as being related to a corresponding increase in frequency. The test principle
is to measure the changes in elastic and viscous stiffness levels over a range of strains and temperatures and assume these are equivalent to appropriate changes in
frequency of deformation.

Various models have been proposed in the literature, but generally it is accepted that the lower the hysteresis, as measured by the Tan d value at tire running temperatures (60 °C to 70 °C), the lower the energy loss. This in turn leads to a lower drag force as the tire rolls and hence a lowering of the fuel consumption for the vehicle. Making compounds more elastic therefore improves rolling resistance. Additionally a low Tg compound may exhibit good wearing characteristics under normal operating conditions.

A proposed mechanism for wet grip is one where the tread compound envelops road surface asperities. The more the enveloping power and the slower the compound
is to release its hold on the asperity, the greater the grip. Here the mechanism is a very high frequency phenomenon (MHz), related to the tread surface moving across the small microtexture of the road surface. In order to achieve this the compound needs to exhibit high hysteresis or Tan d high frequencies, hence lower test temperatures are used (–20 °C to 0 °C). A higher glass transition temperature for improved wet grip at high
frequencies may therefore be detrimental to rolling resistance and wear. Other workers claim that modelling molecular relaxation processes is another important aspect in predicting tire performance or, alternatively, an understanding of the polymer to filler interactions .The characteristics of a filler may be used to explain its reinforcing properties.

Compounders are continually seeking new polymer structures; novel fillers like silica and improved filler to polymer coupling in order to enhance each of the critical viscoelastic performance criteria with little or no compromise.