Long-term stable materials for regenerative fuels

Synthetic fuels are intended to significantly improve the carbon footprint of conventional drives. New results from materials research by Freudenberg Sealing Technologies now show what influence OME, which is being discussed as a diesel replacement, has on the long-term stability of Seals Accordingly, seals based on the already widely used material fluororubber (FKM) is also generally suitable for renewable fuels. However, there are surprisingly large differences between the individual FKM types.

In order to reduce CO2 emissions from road traffic, the automotive industry is not only relying on electric drives. Conventional Internal combustion engines are to become more climate-friendly through the use of synthetic fuels. These are liquid or gaseous fuels that are not made from fossil crude oil, but from coal and hydrogen using solar and wind energy. They can be mixed with conventionally produced fuels and thus immediately reduce CO2 emissions, provided that the carbon used was either previously separated from the air or obtained from unavoidable industrial exhaust gases. However, there is little experience to date of the influence of the new fuels on 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 been exposed to OME (Oxymethylene ether), one of the most discussed synthetic fuels. The test fuels consisted of a standard diesel to which OME was added in concentrations of 10 to 30% by volume. Further tests were carried out with pure OME and as a reference with 100% fossil diesel.

The volume expansion and the change in the mechanical strength of various sealing materials were measured, namely FK extension with different fluorine contents, FFKM, NBR and EPDM, which were stored in the test media in accordance with standards for a period of 168 hours at a temperature of 125 °C. Additional storage tests were carried out with individual fuel combinations over a period of 1000 hours in order 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 does the expert immediately resort to a relatively expensive polymer such as FKM when evaluating this new fuel system? Dr. Boris Traber, Head of Global Materials Development at Freudenberg Sealing Technologies, explains the selection as follows: “Fluorine atoms are significantly larger than carbon atoms and thus form a kind of protective shield for the base material of the seal.

Also are Fluorocarbon compounds highly symmetrical, which neutralizes the charge centers. The effect we observed is similar to a coated pan, on which both water and fat roll off." Applied to fuels, this means that not only non-polar components such as hydrocarbons but also polar components such as water and ether are shielded, thus leading to only slight swelling.

The Results surprised even the experienced material specialists: Pure OME led to a volume expansion in almost all of the sealing materials tested that was far above the usual swelling values. The picture that emerged with the FKM mixtures was surprising. One would have expected that the volume swelling would decrease with increasing fluorine content.

However, this was not the case, so the volume expansion does not behave linearly with the fluorine content. This anomaly can be traced back to the correlation with the hydrogen content in the FKM types: a high hydrogen content also leads to high swelling values. Accordingly, the tested Perfluoro rubber (FFKM) has very good values, but it is significantly more expensive. In pure OME, EPDM also shows little swelling.

Fuel quality may vary

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

For more than ten years, the sealing specialist has been conducting series of tests to research how fluctuating fuel compositions affect sealing materials. In its own laboratories, the swelling behavior was investigated in soy methyl ester and hydrogenated vegetable oils, among other things. The researchers also have their own Test bench concept developed to investigate the permeability of fuels through the sealing material that are continuously exposed to a specific fuel for a long period of time. The door-opening effect that small polar molecules such as methanol have on otherwise impermeable hydrocarbons was observed: the permeability changes over 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."