Synthetic fuels should significantly improve the climate footprint of conventional drives. New results from materials research at Freudenberg Sealing Technologies now show what influence the OME, which is being discussed as a diesel replacement, has on the long-term stability of seals. Accordingly, seals based on the already widespread material fluororubber (FKM) are basically also suitable for renewable fuels. But there are surprisingly large differences between the individual FKM types.

To reduce CO2 emissions from road traffic, the automotive industry is not relying solely on electric drives. Even conventional internal combustion engines should become more climate-friendly through the use of synthetic fuels. These are liquid or gaseous fuels that are not produced on the basis of fossil crude oil, but with the help of solar and wind energy from coal and hydrogen. They can be mixed with conventionally produced fuels to immediately reduce CO2 emissions as long as the carbon used has either been previously removed from the air or recovered from unavoidable industrial emissions. So far, however, there is little experience of how the new fuels affect the long-term stability of seals in fuel systems.  

For the first time tested sealing behavior

In extensive tests, the supplier Freudenberg Sealing Technologies has now for the first time investigated the behavior of sealing materials that have long been exposed to OME ("oxymethylene ethers"), one of the most discussed synthetic fuels. The test fuels consisted of a standard diesel admixed with OME in concentrations ranging from 10 to 30% volume fraction. Further experiments were done with pure OME and as a reference with 100% fossil diesel.

The volume expansion as well as the change in the mechanical strength of various seal materials, namely FKM with different fluorine content, FFKM, NBR and EPDM, were stored in the test media for a period of 168 h at a temperature of 125 ° C. With individual fuel combinations, an outsourcing trial was conducted over a period of 1000 h to study long-term effects. The more the material swells during this time, the more the strength decreases, which is particularly noticeable in a drop in the elongation at break.

Protective shield for the base material

Why is the expert in the evaluation of this new fuel system equal to a relatively expensive polymer such as FKM? Dr. Boris Traber, head of global materials development at Freudenberg Sealing Technologies, explains the selection: "Fluorine atoms are significantly larger than carbon atoms, forming a kind of protective shield for the base material of the gasket. In addition, fluorine-carbon compounds are highly symmetrical, neutralizing the charge centers. The effect we observed is similar to a non-stick frying pan, where both water and fat roll off. "Applied to the fuels, this means shielding not only nonpolar fractions such as hydrocarbons but also polar fractions such as water and ether and therefore cause only a slight swelling.

The results surprised even the experienced materials specialists: Pure OME resulted in almost all the tested sealing materials in a volume expansion, which was far above the usual swelling values. Surprising was the picture that emerged in the FKM mixtures. One would have expected that with increasing fluorine content, the volume swelling decreases. But that was not the case, so the volume expansion is not linear with the fluorine content. This anomaly is due to the correlation to the hydrogen content in the FKM type: A high hydrogen content also leads to high swelling values. As a result, the perfluororubber (FFKM), which was also tested, had very good values, but this is considerably more expensive. In pure OME also shows an EPDM low swelling.

Fuel quality may vary

The test series also showed that with an OME admixture up to 30% volume fraction of all tested FKM materials had a swelling at a significantly lower level. At the same time, Traber sees a positive message: "With materials available today, we could immediately replace some of the fossil diesel fuel with OME." At the same time, the expert warns: "Our experience shows that - above all outside Europe - the actual fuel quality is due to uncontrolled admixtures can vary greatly. Therefore, the selection of sealing materials should always be done in cooperation with specialists. "

For more than a decade, the seal specialist has been conducting test series to explore how fluctuating fuel compositions affect seal materials. In our own laboratories, the swelling behavior was investigated in soybean methyl esters and hydrogenated vegetable oils. In addition, the researchers have developed their own test bench concept, with which the permeability of the fuels can be examined by the sealing material, which is continuously exposed to a certain fuel for a long time. In doing so, the door-opener effect was observed that small polar molecules such as methanol have on otherwise impermeable hydrocarbons: The permeability changes with time.

All in all, FKM materials proved to be superior in all test series, whereby not only the fluorine content, but the entire material structure decided on strength and tightness. Traber therefore sees no fundamental obstacle to the introduction of regenerative fuels: "If properly designed, suitable materials are already available today."