Nanotechnology in Tires
Lewis B Tunnicliffe, James JC Busfield
Soft Matter Group, Queen Mary University of London
The passenger tire is a complex composite device. There are many ways in which it is possible to
improve a single aspect of a tires' performance - for example improving rolling resistance by simply
changing the dimensions of the tire. However, this is usually detrimental to other performance
parameters. In this sense the advancement of tire science and technology involves a constant
balancing between apparently mutually exclusive performance demands.
Taken as a whole, the typical tire is a composite of several parts: side walls, inner liner and the tire
tread - which itself is a rubber component comprising a steel chord reinforcement of a particulatereinforced rubber network. In our group at Queen Mary University of London (QMUL) it is the physics
of this rubber material which is our main focus. In fact when one considers that the tire tread is the
only part of the motor car in contact with the road, it becomes apparent that any sources of energy
dissipation (essentially heat generation) within the tread material can have a major influence on total
fuel efficiency and overall braking performance.
A key challenge facing the industry is how to develop new tread compounds which are able to improve
upon and balance the requirements for tire safety, fuel efficiency and wear resistance. The legislative
drive for this has been evident over the past few years with the introduction of tire labelling schemes
throughout Europe, the USA and other regions by which tires are ranked according to wet braking and
rolling resistance performance amongst other factors.
The main component of the tire tread is the rubber which is a remarkable material whose viscoelastic
behaviour is defined predominantly by the glass transition phenomenon. Through time-temperature
equivalence of polymer mechanics, it is clear to see that a rubbery material becomes more viscoelastic
(less elastic) at either higher deformation frequencies or lower temperatures - see Figure 1.
Figure 1: Schematic of the frequency and temperature dependence of crosslinked rubber. Through
time-temperature superposition it possible to see that high frequency deformations are broadly
equivalent to the dynamic response from the polymer at low temperatures. Rolling resistance and wet
grip properties are found at opposing ends of the viscoelastic spectrum.
Essentially the tread material is required to be as viscoelastic as possible under harsh braking
conditions in order to rapidly dissipate the kinetic energy of the vehicle (the high pitched screeching
noise hear under breaking shows that the frequencies of this behaviour are in the kHz range) and as
elastic as possible under steady driving conditions in order to reduce dissipation of energy and
therefore rolling resistance and fuel consumption (deformation frequencies in the Hz range).
Traditionally this balancing act between elastic and viscoelastic behaviour has been accomplished
through the careful selection and optimisation of the base polymer blend, the vulcanising system and
the carbon black filler.
The dramatic improvements in rubber properties achieved through the incorporation of carbon black
were first noted at the start of the last century [1] although the precise mechanisms behind the
reinforcing mechanism(s) are even now not completely resolved. The incorporation of filler materials
into rubber fundamentally changes the viscoelastic behaviour of the material and brings with it both
challenges and opportunities. At a technological level, the selection of carbon black filler based on
particle size, surface area, morphology and surface activity allows for a substantial degree of tailoring
of the tread performance. Since the early 1990's, precipitated silica has also become an increasingly
popular reinforcing material in tire tread compounding. The use of highly dispersible silica in
combination with a silane surface modification has allowed developers to achieve improvements in
rolling resistance and wet grip simultaneously without reducing the wear properties, expanding the
so-called 'Magic Triangle' of tire performance.
Looking to current and future research trends, there is an apparent drive towards innovative filler
materials with organoclays [2,3], carbon nanotubes [4] and more recently graphene-like fillers being
widely evaluated and investigated [5]. In addition to functionalising the filler surface for improved
interaction with the rubber, there have also been commensurate developments in functionalised
rubber polymer materials. Advances in experimental techniques such as NMR are bringing new
insights into the microstructural details of filled rubber materials [6]. Furthermore, the precise role of
filler aggregates and particles in defining the dynamics of polymer in proximity to the interface and its
relation to reinforcement of rubber is currently a very active area of research [7-10].
At QMUL we are interested in probing the fundamentals of the filler reinforcement phenomenon. This
entails working towards an understanding of the microstructural origins of viscous dissipation in filled
rubbers and non-linearities in the mechanical properties of filled rubbers such as the Payne effect.
It is well known that simple continuum-level descriptions of reinforcement - the hydrodynamic
analogy - fail to describe the levels of stiffening observed in commercial compounds (11). As such we
have tried to understand why this description breaks down by observing and modelling the
reinforcement evident in very simple microcomposite systems such as rubbers reinforced with
spherical microspheres or macrofibres of high aspect ratio (12, 13). Other researchers have taken this
a step further and implemented FEA analyses of actual carbon black and silica filler microdispersions
and have demonstrated clear departures from simple hydrodynamic reinforcement on the
microstructural level (13).
In fact mesoscale simulation of reinforced rubbers has become an increasingly popular and
informative technique over the past decade [15-17]. Work on this topic performed at Queen Mary
[16, 17] has focussed on the prediction of the stress-strain behaviour of rubber filled with low
structure carbon blacks (Mid Thermal N990) using a microstructural finite element simulation
approach (Figure 2). Here a representative volume element of the filled rubber microstructure is
produced through a random distribution of rigid spherical inclusions throughout a hyperelastic matrix.
Predictions can then be made of the elastic behaviour of the composite material. In practise it is found
that a slippage constraint must be introduced at the filler surface in order to accurately describe the
corresponding experimental data which is in line with mechanisms previously proposed in the
literature [18].
With continually advancing computational power it is plausible that we could one day see continuum
and mesoscale simulations supported with molecular-level information from atomistic or coarse
grained molecular dynamic simulations. The ultimate goal being to simulate tire behaviour by
considering a range of length scales from the molecular to the macroscopic.
Figure 2: Mesoscale FEA simulation of the stress-strain behaviour of a filled rubber from
microstructural volume elements [16, 17]. A) A representative volume element under tensile
deformation. B) The predicted and experimental stress-strain behaviour. C) Simulating slippage at the
filler-rubber interface: the red dot is a label for a specific point on the rubber surface. Under strain the
node can slide relative to the filler surface resulting in a better correlation with experimental data
A key factor when considering tire tread formulation and performance is the magnitude, temperature
and frequency dependence of the Payne effect. The conventional interpretation of the Payne effect is
the progressive disruption of 'filler structure' resulting in a decrease in dynamic storage modulus and
a peak in loss modulus [19]. One issue defining the magnitude of the Payne effect is the thixotropic
behaviour of the green rubber compound during processing. Thixotropy is underpinned by shear or
temperature-induced flocculation of filler aggregates. This is well known for the case of silica-silane
technology but we have recently also investigated the effects of surface modifications of carbon black
on the flocculation process [20].
Traditionally the Payne effect in vulcanised compounds is measured using mechanical instrumentation
such as a Dynamic Mechanical Analyser (DMA) or a rheometer. Work by our group and other
institutions has shown that it is also possible to track the Payne effect and filler network dynamics for
rubbers reinforced with carbon black and other conductive filler particles using combined DMAbroadband dielectric spectroscopy (BDS) techniques [21, 22]. Figure 3 shows the setup of our DMABDS rig in tension mode. Example mechanical and permittivity data are presented in Figure 4 and
clearly demonstrate the observation of an 'electrical Payne effect'. This results from the breakdown
and reorientation of filler structure and provides a new way of analysing this phenomenon focussing
on the changes to the dipoles under strain associated with the conductive filler network.
Figure 3: Combined DMA-BDS setup. A practical issue when measuring dielectric properties of
deformable materials is using a compliant electrode. A) Here the electrodes are made from deposition
of a silver nanowire film which can deform reversibly up to 100 % without significant changes in
conductivity. B) the electrodes are applied to the rubber sample surface and the sample is mounted
in a commercial DMA with attached dielectric spectrometer.
Figure 4: Example data showing the Payne effect measured by DMA (top) and the simultaneous
permittivity response of the material (bottom) at fixed field frequency of 10 kHz. The permittivity data
gives us a new way to probe the structural dynamics of the Payne effect - for example by changing the
electrical field frequency it is possible to selectively probe dipoles at specific length scales active during
filler structure breakdown.
Fillers and filler-polymer interactions also have a role to play in defining tire wear properties. Crack
growth in elastomeric materials is suppressed through the dissipation of energy ahead of the crack
tip. Incorporation of filler materials leads to an increase in viscous dissipation at finite strains and
dramatic improvements in toughness and wear behaviour depending on the filler loading, aggregate
size and surface activity. A current project in our group is investigating the role of silicas and carbon
black fillers in the abrasion process and relating this back to the fundamental fracture properties of
the material by prediction of the tearing energy developed at asperity-rubber contact through
simulation as illustrated in Figure 5 [23].
Figure 5: Abrasion of filled rubber tackled through an FEA-fracture mechanics approach. A) abrasion
patterns found on filled rubber materials after unidirectional abrasion. B) the correlation of the
intrinsic material toughness with predictions of tearing energies from FEA simulation (black symbols)
and the empirically-derived equation for tearing energy (hollow symbols). C) An FEA simulation of the
unidirectional abrasion process used to calculate the tearing energy
A clear theme in current research is that the understanding of filler reinforcement processes at a
microstructural level across a range of service conditions (strains, temperatures, frequencies...) is a
fundamental prerequisite for improvements in aspects of tire performance. Continual advances in
innovative experimental techniques and modelling capabilities offer new ways to achieve this goal. At
the same time, nano-materials and tailored polymer systems may offer more immediate material
selection solutions outside of the traditional filler-polymer combinations.
References:
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[13] L.B. Tunnicliffe, PhD Thesis, Queen Mary University of London, 2015
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[19] It should be noted here that a number of alternative explanations for the Payne effect have also
been proposed. See for example: P.G. Maier, D. Göritz, Kaut. Gummi Kunst., (1996) 49, 18-21, and
reference [7] of this article.
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(2014) 299, 1474-1483
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[23] H.C. Liang, Y. Fukahori, A.G. Thomas, J.J.C. Busfield, Wear, (2009) 266, 288-296