Elastomer Compatibility of Blends of Biodiesel and Fischer-Tropsch Diesel

Previous studies of blends of biodiesel (fatty acid methyl ester) with Fischer-Tropsch (FT) diesel demonstrated that less than 0.5 volume-% biodiesel is required to improve the lubricity of FT diesel to meet specified requirements. Further studies of various blends of biodiesel with FT diesel demonstrated that the biodiesel does not impact negatively on the FT diesel and that a 20 volume-% biodiesel blend (B20) with FT diesel is stable for up to two years. The effect of biodiesel and FT diesel blends on the physical properties of fuel system elastomer seals has however not been reported previously.

Material compatibility in fuel systems is a concern whenever fuel composition changes. It is known that alternate exposure to severely hydrogenated fuel and conventional diesel leads to variation in the degree of seal swell caused by the change in the aromatic content of the fuel. Neat biodiesel, or high percentage blends thereof with conventional diesel can soften and degrade certain types of elastomers over time. The objective of this study was to compare the shrinkage and swelling nature of Standard Nitrile Butadiene Rubber (NBR), preconditioned in highly aromatic crude-oil derived diesel and then subsequently exposed to FT diesel, with changes exhibited by identical preconditioned elastomers that had been exposed to B20 and B50 FT diesel blends.

Subsequent ageing of pre-conditioned NBR in FT diesel caused the swollen nitrile rubber to shrink and to lose weight. Where the leaching of aromatics from elastomer seals are to be expected upon changing from high aromatic fuel to low aromatic content fuel, blends of biodiesel with FT diesel reduced the net change in physical properties of the pre-conditioned nitrile rubber exposed to such a mixed fuel scenario. Elastomer compatibility is therefore not regarded as a blending constraint for B20 blends of biodiesel with FT diesel.