Effect of synthetic antioxidants on storage stability of Calophyllum inophyllum biodiesel I. M. Rizwanul Fattah * , H. H. Masjuki, M.A. Kalam and B. M. Masum Biofuels, especially biodiesels derived from renewable sources, are becoming increasingly important because of environmental and energy concerns. However, biodiesels composed of long chain unsaturated fatty acid esters are prone to oxidation. One such biodiesel is non-edible high acid value Calophyllum inophyllum oil-based biodiesel produced through a two-stage esterification process and one-stage transesterification process. In this study, the oxidation stability of biodiesel treated with three prominent antioxidants, namely pyrogallol, propyl gallate and tert-butylhydroxyquinone was evaluated. The induction period of biodiesel with or without antioxidant was measured according to the EN14112 standard using a Rancimat instrument. Antioxidants were added at 500 ppm, which in general improved the induction period. The samples were kept for 70 days and different properties that change during storage, namely induction period, density and kinematic viscosity were monitored. For all samples, oxidation stability decreased and kinematic viscosity increased because of the formation of oxidation products. Pyrogallol showed the best effect in retaining oxidation stability of Calophyllum inophyllum biodiesel. Keywords: Storage stability, Biodiesel, Antioxidants, Calophyllum inophyllum Introduction Biodiesel refers to the fatty acid alkyl esters (FAAEs) derived from lipid substances originated from vegetable oil, animal fats, waste greases, recycled cooking oils, etc. 1–3 Vegetable oils of the edible origin are one of the most promising feedstocks for biodiesel production. Triglyceride molecules that are the main constituents of these oils are transesterified with an alcohol, such as methanol, in the presence of a catalyst to form FAAEs. 4 However, owing to criticism on edible oil use for fuel production, other sources e.g. non-edible oils of plant origin and waste fats with high free fatty acid (FFA) content are now being used for biodiesel pro- duction. 5,6 Biodiesel production from these feedstocks usually employs a pre-treatment step that converts the FFAs to ester that can be used for tranesterification. 7 Biodiesels are more susceptible to degradation com- pared to fossil diesel because of fatty acid chain unsatura- tion (carbon double bonds C=C). 8 The mechanisms of degradation are (a) autoxidation in the presence of atmos- pheric oxygen; (b) thermal or thermal-oxidative degra- dation from excess heat; (c) hydrolysis in the presence of moisture or water during storage and in fuel lines; and (d) microbial contamination of the fuel from contact with dust particles or water droplets containing fungi or bacteria. This degradation is exacerbated if two or more carbon double bonds (polyunsaturation) are present in the fatty acid chain. 9 Fuel instability is the sus- ceptibility of fuel to degradation processes that form undesirable species resulting in deterioration of fuel prop- erties. The fatty acid profile, i.e. the chain length and the level of unsaturation of the FAAE corresponds to that of the parent oil. 10 The level of unsaturation is the major factor that influences the fuel property. Although biodie- sel is thermodynamically stable, its instability primarily occurs from contact with oxygen, present in the ambient air, and is referred to as oxidative instability. It is a general term that involves oxidative degradation, which may occur during the extended storage period, transportation and end use. 11 Accelerated oxidation of biodiesel results in an increase in viscosity, density and polymer content, which initiates the formation of gums, sediments and corrosion of engine components. 12 Biodiesel admixed in the lubricating oil during crankcase dilution tends to be persistent within it due to less vola- tility and begins to degrade and oxidise. This causes a sig- nificant increase in viscosity of the sump oil and, in turn, results in loss of performance, greater engine wear and necessitates a premature oil change. 13 Antioxidants (AHs) significantly slow down the biodiesel degradation process. According to their mode of action, AHs are classified as free radical terminators, metal ion chelators capable of catalysing lipid oxidation, or as oxygen scaven- gers that react with oxygen in closed systems. 14 These are Centre for Energy Sciences, Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia *Corresponding author, email rizwanul.buet@gmail.com © W. S. Maney & Son Ltd 2014 DOI 10.1179/1432891714Z.000000000936 Materials Research Innovations 2014 VOL 18 SUPPL 6 S6-90