Performance of Viton® fluoroelastomers in ethanol–fuel blends
Two market forces in the automotive fuel systems sector driving developments are the need to lower hydrocarbons emissions to clean up the environment, and the use of alternate, renewable fuels such as ethanol. California's Air Resource Board and the Environmental Protection Agency have new evaporative emissions regulations called LEV II and Tier II respectively.
These regulations have lowered the allowable evaporative emissions for all automotive vehicles, while increasing the longevity requirements of the systems on the vehicle to 15 years or 240,000 kilometers (150,000 miles) of use. At the same time, the need for alternate fuels such as ethyl alcohol, ethanol, is starting to grow as we see demand for flex fuels such as E85 emerging.
While ethanol mixed with gasoline is known to lower engine / tailpipe emissions, previous work has shown that such blended fuels exhibit increased permeation through plastic and rubber. This leads to the question - how do higher percentages of ethanol in fuel blends affect fuel system rubber components such as seals and hoses of fluoroelastomer?
A project was undertaken to document the performance of several common Viton® fluoroelastomers of varying fluorine contents tested in various blends of fuel and ethanol. The types of Viton® tested ranged from 64% fluorine content Viton® GLT-600S to 70.2% fluorine content Viton® GF-600S. Two of the properties measured were volume swell changes and permeation rates.
Figure 1 shows the volume swell results after an immersion of 168 hours at 40°C. Six different fuels were tested ranging from 100% Fuel C hydrocarbon test fuel to 100% ethanol. The data shows that the lower fluorine types of Viton swell more than the higher fluorine types. Also the data shows that the highest swell occurs when CE-25, 25% ethanol in Fuel C, is used as the test fuel. This occurs with all six types of Viton® tested.
Permeation resistance was measured on the same types of Viton® using the same six fuel-ethanol blends. Permeation testing was conducted per ASTM E96, using the Thwing Albert cup method. Testing took place for 672 hours at 40°C with the results being expressed in permeation rate units of grams-mm/m2/day. The results are shown in Figure 2.
The permeation results show much the same trend as was seen in the previously reported volume swell results. The lower fluorine content GLT-600S shows the highest permeation rate ranging from ~25 g-mm/m2/day in Fuel C to a peak of ~100 g-mm/m2/day in CE-25 fuel and ~20 g-mm/m2/day in neat ethanol.
A strong trend was observed for permeation rates where low levels of ethanol in Fuel C appear to exhibit higher permeation rates. The CE-25 fuel, with 25% ethanol, consistently exhibited the highest permeation rate regardless of the FKM polymer tested.
GBLT-600S showed a marked improvement in permeation rate compared to GLT-600S when comparing the low temperature FKM polymers were examined, and the 70.2% fluorine GF-600S showed the lowest permeation rate of any of the FKM polymers tested in this study.
This data and more was reviewed by Ronald Stevens at the Fall 170th Technical Meeting of the Rubber Division, American Chemical Society, Cincinnati, OH, USA, 11 October, 2006, in a paper entitled "Fuel and Permeation Resistance of Fluoroelastomers to Ethanol Blends". For the full paper, click here.