The tires used in modern automobiles are fabricated from advanced materials and use systems designed to take into account the full range of a tire’s operating conditions. The tread compound is made up of various materials and includes additives that help to balance such factors as wear rate, traction, strength, noise, ride quality, and rolling resistance.
These additives are important as while some materials will improve some aspects of the ride, this will come at the expense of others. Finding the right balance and optimizing the properties is crucial. The mechanical properties of the tread compound have a strong relationship with temperature, keeping the proper operational range of the tire over a small temperature window.
It is vital that the local mechanical properties are characterized so that the distribution and impact of filler particles and additives across various areas of the tire’s tread is fully understood. In this article, a winter tire is investigated over its entire operating range, from -60 °C to 40 °C, to measure its glass transition temperature and mechanical behavior.
Figure 1. Tire structure and engineered layers.
The properties of a tire are controlled at the nanoscale by altering the level of crosslinking of the polymer, and can then be tuned even further by incorporating varying amounts of fillers, such as carbon black, clays, and silicates. In between these size scales, other methods like micro-porosity and compositional gradients may be employed to further enhance the tire’s performance.
At the macroscale it is the pattern of the tread and the processing parameters used in the production of the tire that have the final say on how the tire behaves in the service conditions. The local mechanical properties are strongly reflected in the how particles are distributed, which makes this of particular interest to engineers. For nanoscale behavior, volumes of several hundred cubic nanometers are probed with a sharp indenter and an instrument with high force sensitivity.
This can be scaled up to micro level by using larger indenters with an increased force and penetration depth, so that volumes of several hundred cubic microns can be accessed. The stiffness and damping behavior of the tire at the nano and micro scales is the arbiter of its behavior under various conditions. Such characteristics as noise abatement, wear resistance, and friction are mostly determined by small-scale properties whereas the ability of the tire to flex under load and dampen harshness from road imperfections is reliant on how the material is structured at a larger scale.
The tread compound of a commercial winter tire was tested at temperatures between -60 °C and 40 °C: reflecting a complete range of temperatures it is likely to face. The tire’s properties were investigated using a Hysitron® TI 950 TriboIndenter® nanomechanical test instrument equipped with an xSol® Environmental Control Stage and nanoDMA® III dynamic testing module.
To control the micro-environment around the sample surface and prevent condensation or frost forming, evaporated nitrogen gas was continuously flowed over the sample. The controlled evaporation of liquid nitrogen in conjunction with PID-controlled heaters on both sides of the sample kept a constant environment. The storage modulus (E’) and loss tangent (tan δ) of the material was measured using dynamic indentation tests performed with a Berkovich indenter probe at an oscillation frequency of 75 Hz at various temperatures.
Figure 2. Storage Modulus of the tread compound of a winter tire measured at temperatures between -60°C and 40°C.
Figure 3. tan(δ) at a frequency of 75 Hz of a tread compound of a winter tire measured at temperatures between -60°C and 40°C.
The material properties greatly vary over the temperature range, compare a storage modulus of around 10 GPa measured at -60 °C to less than 0.1 GPa at 40 °C. The material displayed a strong tan δ peak at -5 °C, indicating that this is the point where the glass transition begins and why the sharp change in the properties happens there. The peak at -5 °C indicates a maximum in dissipation and accompanies a marked change in the material’s stiffness. With temperatures much lower than -5 °C, the high stiffness and thus low damping will cause increased noise and harshness, but at temperatures well above -5 °C, the compound will soften and wear life and mechanical stability will be compromised.
To ensure that advanced materials are characterized correctly it is vital that the investigator has good control of the sample environment as the service conditions under which the sample operates can vary significantly to those of a laboratory.
The xSol Environmental Control Stage ensures that even for those samples with a low thermal conductivity, a uniform sample temperature is maintained, as well as providing atmospheric control to prevent condensation even at very low temperatures. The xSol has been developed for nanoscale dimensional stability, allowing measurement of individual microstructural components and, at larger scales, overall composite response.
By combining the nanoDMA III with the xSol stage a powerful DMTA characterization tool is created, one that requires a minimal quantity of material and little sample preparation. On top of this, it can rapidly adjust and equalize temperature, and provides exceptional stability for fast and reliable investigation of material properties across a wide range of environmental conditions and orders of length scale. This experiment successfully measured the glass transition temperature and mechanical behavior of a winter tire tread compound over its entire operating range.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.