Thin film testing of biodiesel degradation residues and their solubility

With broader application of biodiesel, reports about residues, varnish and other solid deposits in engines emerge more frequently. A thin film degradation test, originally developed for lubricant studies, was employed to investigate fuel vaporization, acid buildup and solubility of degradation products. Films of 25, 100 and 500 μm thickness of rapeseed and palm fatty acid methyl esters (FAME) as well as hexadecane were exposed to 50 °C and 90 °C for up to 3000 h in dry or humid atmosphere on steel surface. Hexadecane produced clean steel surfaces, while FAME films would leave at least 10% residues or more in thicker films. Simple soaking in acetone or tetrahydrofuran dissolved FAME films nearly completely, if degradation was shorter than 1000 h at 90 °C. But in non-polar solvents FAME films became insoluble after much less degradation. As little as 50 h degradation at 50 °C was enough to produce a viscous bottom layer, which would not dissolve in fresh FAME or heptane. Other solvents were tested as well, further showing that FAME degradation products are poorly compatible with fuel-like solvents, such as xylene, heptane or fresh FAME. Formation of immiscible bottom phase may promote buildup of residues, deposits and varnish in fuel systems.