Liquid phase oxidation quantitative analysis of biodiesel/diesel blends by differential TG and DTA

the oxidative stability of biodiesel/diesel blends is usually evaluated from the released gaseous phase change characteristics, without analyzing what happens in liquid phase. depending on the storage and/or heating conditions, gums and insoluble organic particulates may be formed in liquid phase by oxidation. to contribute to the evaluation of this oxidized heavier components formation in liquid phase, a new thermal analysis quantitative method was developed by the authors, which was applied to diesel oil/soybean biodiesel blends having from 5 to 20 vol% of the latter. the method consists on obtaining differential thermogravimetric curves (diftg), subtracting the tg curve obtained in inert atmosphere from the tg curve of the same sample obtained in air, which, actually, shows how and how much accumulated oxidized mass is being formed in liquid phase during analysis. corresponding thermal effects are quantified by differential dta (difdta) curves, subtracting respective dta curves in n2 from those in air. the results show that, as the biodiesel content is increased, a higher amount of heavy oxidized products remains in liquid phase of the blend, up to the respective much higher decomposition temperatures. this fact is confirmed by estimating the activation energies as a function of the mass loss conversion degree, which shows that the higher is the formation rate of the liquid phase oxidized mass in a blend, the higher is the activation energy needed for its decomposition and the higher is the blend ignition temperature, which is a polynomial function of second order of the biodiesel content.