Abrasion of Elastomer Materials

Collaborators: J.J.C. Busfield, H. Liang and Y. Fukahori

For tyres the rate of abrasion is clearly important when determining the product life. Abrasion tests have been done for the first time in parallel with a coupled finite element / fracture mechanics approach. For typical SBR tyre tread compounds the principal abrasion rate determining mechanisms for the loss of rubber can be seen to result from tearing phenomena occurring at the root of individual asperities in the contact patch and this has allowed the complex abrasion processes to be understood. This has shown that the rate of abrasion under a wide range of friction conditions can be predicted from measurements made on independent fatigue test pieces. With this knowledge it is possible to develop simple tests to determine the suitability of particular compounds for rubber abrasion applications. It is hoped to extend this work into other elastomers and elastomer blends that are encountered in tire tread applications.

Blade Abrasion Testing Facility

Modelling of Abrasion

Dynamic Behaviour of Rubber Materials

Collaborators: J.J.C. Busfield, A.G. Thomas and N. Suphadon

This project is examining how the viscoelastic properties of different elastomer materials are varied by compounding, temperature and speed of loading. It is hoped that relatively simple approaches can then be adopted to model the behaviour using a finite element analysis approach.

Dynamical Mechanical Testing of Rubber

Oil Extraction Using a Soxhlet extractor

Electrically conductive elastomers

Collaborators: J.J.C. Busfield, V.K. Jha and A.G. Thomas

This work examines the changes in electrical properties under strain of elastomers filled with traditional conducting fillers such as carbon black and novel conducting fillers such as carbon nanotubes at volume fractions above the conduction percolation threshold for the filler. The electrical behaviour allows the different models of the filler reinforcement to be validated by independent tests. One interesting application that has developed from this work is the discovery that the changes in conductivity of the material with strain could be used to create an effective pressure sensor or strain measuring device derived directly from the rubber materials used within the tyre.

The orientation of carbon black aggregates under the application of a strain

Fatigue life prediction of elastomer components

Collaborators: J.J.C. Busfield, A.G. Thomas, Y. Fukahori and S. Asare

This project measures the cyclic fatigue crack growth for specific tearing energies in a variety of elastomer compounds using simple test pieces. Then finite element techniques are applied to calculate the tearing energy relationships for cracks of different sizes located in real components. From this it is possible to calcuate the fatigue life of the component. This work is being validated with a range of different components, such as tyres, engine mounts and vehicle suspension components.

A double bonded elastomer engine mount used to characterise fatigue

Tensile loading of a double bonded elastomer engine mount containing a crack

Friction behaviour of rubber

Collaborators: J.J.C. Busfield, A.G. Thomas and P. Gabriel

This project is investigating the fundamental frictional properties between rubber materials and rigid surfaces. The project has examined how friction behaviour depends upon compounding, surface roughness, sliding speed, temperature and surface preparation. The project uses a range of finite element analysis modelling techniques and other experimental methods to observe how phemomenon such as the microvibrations at the surface and the Schallamach waves are formed during sliding.

Close up of the rubber surface and the rigid indentor.

Plint friction measuring device.

Microstructural modelling of filler reinforcement in elastomers

Collaborators: J.J.C. Busfield, A.G. Thomas and V.Jha

Materials such as carbon black and silica can significantly improve the mechanical properties measured in terms of strength and fatigue resistance. In addition, fillers typically impart an increase in the stiffness of the material. This work uses micro-structural models of fillers in a rubber matrix for a range of different filler volume fractions, filler shapes and surface interactions between the filler and the elastomer matrix. These approaches have been used to investigate not only the stiffening but also changes to the hysteresis encountered with the incorporation of fillers. Micro-structural models that use finite element techniques have been developed to allow the precise micromechanics to be investigated. In addition, molecular modelling simulations can be used to tackle the problem at a different length scale. The continued development of these different techniques and their validation by state of the art micro-tomography and advanced TEM techniques which can be used to visualise precisely what is happening to the filler network structure under strain will allow the existing theories of filler reinforcement to be evaluated much more critically than in the past. This will eventually result in tailored filler structures being developed with enhanced mechanical properties for a wide range of applications.

The stresses developed around a filler particle under a tensile load.

Mixing a masterbatch of filler into a gum stock prior to vulcanisation.

Modelling of Foamed Rubbers

Collaborators: J.J.C. Busfield and R. Shorter

Foamed rubber materials can be classified as open cell (i.e. porous like a sponge) or closed cell. Much work has been conducted into the behaviour of materials with either type of cell, but mainly theoretically and as a complete system. A known phenomenon occurs when elastic foam materials are compressed, they exhibit non-linear behaviour in three distinct phases; cell wall bending, cell wall buckling and then densification. However, a third type of rubber foam cell exists, one where a hollow filler material is used to create the closed cells, and for this foam the full behaviour of foamed material is not well understood.

This work will examine the mechanical behaviour of hollow microspheres when they are used as the filler material to create a closed cell rubber foam. The behaviour will be observed in transparent elastomer materials using microscopy techniques to observe the process of bending, buckling and collapsing as well as to investigate any de-wetting at the rubber filler interface.

The behaviour will be further examined by modelling using Abaqus FEA software to predict the interactions between the rubber matrix and the microsphere filler particles.

The project is funded by DSTL

A model of an expancel under compression.

Spherical expancel bubbles mixed in as a filler in a rubber.

Recycling of Elastomers

Collaborators: J.J.C. Busfield and P. Kumar

Environmental legislation that will shortly come into force will prevent the disposal of tyres and some other rubber products using landfill. This presents the elastomer manufacturers and users with the problem of finding alternative strategies on how to deal with this significant problem. Techniques for reclaiming oil and carbon black have been considered as well as a strategy for making use of crumb tyre material.

The ever increasing tyre mountain.

Rubber crumb that has been ground up from waste materials.

The reinforcement of rubber using organoclays

Collaborators: J.J.C. Busfield and D. Lowe

Filler with exceptionally large aspect ratios are being investigated incorporated into a range of rubbers. The types of fillers considered include carbon nanotubes and nanoclays. Typically the nanoclays studied are layered silicates like montmorillonite, which can be readily modified into organoclays. This has the duel effect of making them more compatible with rubber and also increases the interlayer separation. This facilitates the delamination and exfoliation of the clays into very thin layers with a high aspect ratio. The organoclays have a dramatic effect on the rubber modulus and other properties such as the gas permeability even at low volume fractions of below 2%. Novel TEM techniques are being used in collaboration with TARRC to directly observe how the polymer filler interactions are changing with specific processing conditions. These techniques allow the effectiveness of other additives such as silane coupling agents to be observed in a moulded sample.

Filler network visualisation using TEM