RC can test for signs of 'Tyre Softening'

RC can test for signs of 'Tyre Softening'

F1 racing carA number of motor racing series require the entrants to drive on identical tyre formulations, producing close racing and highlighting driver skill. However, this can lead to the doctoring of tyres, in an effort to give a driver an unfair advantage over their competitors. The usual way of achieving this is through the addition of tyre softeners. There is also the possibility of a rogue batch of otherwise fair tyres being supplied, which may have a marked effect on car performance. At Rubber Consultants we are able to test for both of these situations and have been asked to supply evidence in a number of cases.

Tyre softening agents usually consist of some sort of ‘plasticiser’ in a solvent base, which is either applied to the surface of the tyre or allowed to soak into the bulk of the tyre some time prior to racing.F1 racing tyres If there is any suspicion that a tyre has been illegally doctored it is usually tested, either at trackside or shortly after, using a hand-held photo ionisation detector (PID). Whilst these detectors can test for the presence of volatile organic compounds (VOC) down to parts-per-million (ppm) levels, they cannot distinguish between different VOCs. This can lead to disputed results as it is easy for tyres to pick up contamination from the race track or pit area, for example from leaking fuel, oil or other fluids. Also, common tyre lubricating agents such as tyre soap or WD40 will also give a positive result without imparting any added benefit to the tyre itself. There is also the problem of processing oils and anti-degradants, which are integral to the tyre manufacturing process, blooming to the surface of the tyre due to the heating it undergoes during racing. It is therefore preferable that any testing should be able to discriminate between different VOCs, as well as determining their levels. By using Headspace Gas Chromatography-Mass Spectrometry (GC-MS), Rubber Consultants is able to achieve this.

Headspace GC-MS is usually carried out by heating a small sample (~0.5g) of tyre in a sealed vial (normally 20 mL) to a temperature that will cause all the volatile components in the rubber to move into the vapour phase. A small amount of this vapour is then injected onto a GC column and is detected by mass spectrometry. By matching the separated components to a library of mass spectra held on computer, it is possible to distinguish a large number of different VOCs that may be present in the tyre. Figure 1 shows the different chromatograms that are obtained when an untreated tyre and an identical one, soaked in a mixture of xylene and di-octyl phthalate (a common plasticiser), are analysed. The ability of the computer to match all the peaks that are present in the chromatogram means that components that would normally be present in a tyre formulation can be discounted, avoiding the risk of false-positive results. By running the GC-MS in Single Ion Monitoring (SIM) mode, using suitable standards, it is possible to quanitify the identified components as well. The detection limit for a large number of common VOCs is frequently below ppm levels, easily outperforming a normal PID device.

Figure 1.

Sometimes, due to unwitting manufacturing errors, a rogue batch of tyres may be produced, with marked effects on the overall performance of the car. Rubber Consultants’ extensive experience in rubber formulation analysis, enables us to quickly identify many problems that may have occurred during manufacture.

One of the largest detrimental effects on tyre performance may be caused by the use of the wrong rubber, resulting in the tyre being harder or softer than expected. Polymer type analysis is usually carried out by Pyrolysis Infrared (PIR) analysis. A small sample (~0.3g) is burnt in nitrogen at ~500ºC and the resultant liquid is analysed by FT-IR. Figure 2 shows the IR spectra of two, apparently identical, tyres. By matching these spectra to reference spectra, the polymer type for both of these tyres was identified as styrene-butadiene rubber (SBR). It is possible to calculate the amount of styrene in the blend, by using the ratio of the styrene peak to the CH 2 peak and comparing that to a standard SBR rubber with known styrene content. When this was carried out, it was shown that the good tyre was ~29% styrene and the suspect tyre was ~35% styrene. By increasing the styrene content in SBR you increase the hardness of the rubber. This observation was borne out by hardness measurements carried out in our Physical Testing Unit. The styrene content of SBR rubber is intrinsic and cannot be affected by track use, however hard you try. It was quite clear that the suspect tyre had been manufactured using the wrong rubber grade.

Figure 2.

Occasionally, analysis of this type is not as straightforward as a problem with a single component of the tyre formulation. Figure 3 shows FTIR spectra obtained on two further tyres suspected of being different formulations.

Figure 3.

Again, they were both shown to be SBR with different amounts of styrene in them. The good tyre was ~24% styrene and the suspect tyre was ~29% styrene. However, hardness measurements showed the good tyre to be harder than the suspect tyre despite the styrene content. Tyre formulations – indeed all rubber formulations – are more complex than just the polymer type and many different factors contribute to their overall properties. By carrying out Thermogravimetric Analysis (TGA) on the rubber and GC-MS on the extractable processing oils and anti-degradants, it was shown that the formulation was completely different in a number of areas, including filler type and levels and processing agents. When all this information was taken as a whole, the properties of the two tyres could be explained. Additional analysis using a new technique available to the Analytical Unit of Rubber Consultants – Coupled Thermogravimetric Analysis Infrared Spectroscopy (TG-IR) – also confirmed that the levels of styrene had been analysed accurately.

If you are interested in any of the above techniques or think that any of our services may be of use to you do not hesitate to contact us. We also have some of the most comprehensive tyre testing facilities in the world and can test a wide range of different tyres to many different regulations and standards.

If you require any of our testing services or would like some more information do not hesitate to contact us.


© Rubber Consultants 2005
February 2007
Experts in Elastomer & Polymer Testing