Lube-Tech
cooking oils and other natural oils like microalgae oils. The quality and performance of an additive depends largely on the physical properties of the oils (especially degree of unsaturation) and their chemical derivatives. The derivatives of natural oils which are used as multifunctional additives for biolubricants should have higher viscosity index, flash point, thermo-oxidation stability, shear stability, lubricity, and lower pour point and cloud point. ASTM International methods are applied to evaluate these properties.
Common approaches to synthesise additives from VO are transesterification, hydrolysis, conversion of the olefinic functional groups into epoxides, cyclic carbonates, incorporation of different nano-particles in the backbone of VO, polymerisation (homopolymerisation and copolymerisation with suitable comonomers), preparation of polymer composites by reinforcing different organic and inorganic nanofillers in the vegetable oil polymer matrix, formation of ionic liquids, derivatisation of vegetable fatty esters into fatty amines, fatty amides, fatty alcohols.
Transesterification In transesterification, the VO triglyceride esters are converted into fatty acid alkyl esters and glycerol by reacting with smaller alcohols like methanol, ethanol, or propanol in presence of a base or acid catalyst. These fatty acid methyl esters (FAME) or ethyl esters have great importance in the lubricant industry. They are also the key components of biodiesel, biolubricant, and bio-additives. The FAME of canola oil was used as an excellent lubricity additive for biodiesel. FAMEs can be easily further transesterified with longer chain alcohols, polyols like TMP, pentaerythritol etc. which have enhanced additive performances. Garlapati et al. reported the use of transesterified Olax scandens oil as biobased additive in petroleum oil1
. The blends
showed better engine performance, higher flash points, and higher lubricity. The fatty acid alkyl esters are also easily converted into fatty amines,
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.143 page 3
amides, alcohols, polyols, epoxides, or other essential compounds (by various reactions like oxidation, hydrogenation, metathesis, isomerisation, etc.) that can be used as additives for lubricants.
The type of catalyst used in the transesterification reaction may be homogeneous, heterogeneous, or enzymatic. Although the selection of catalyst i.e. acid or base, depends on the quality of the feedstock, the use of homogeneous alkali catalysts (KOH/NaOH) is very common. In the base-catalysed process, the fat and oils must be of good quality with least amounts of free fatty acids (FFAs). In the case of low-quality oils (i.e.: non-edible or spoiled VO, waste cooking oils, grease, animal fats that contain a significant amount of FFAs) base catalysts cannot be used in the transesterification process because of the formation of a soap-like material. In such a case, the FFAs present in the feedstock should be converted into esters first by an acid catalyst to obtain a mixture of fatty acid alkyl esters and triglycerides. The esters in the second step are then transesterified with suitable longer chain alcohol catalysed by a base to obtain fatty acid esters.
For conversion of high-FFA unrefined palm oil, the acid-catalysed process in combination with high temperature is required to break down thick yellow grease. Moreover, due to the corrosive nature of acids, costly acid-resistant reactors will have to be used. Recently, the use of heterogeneous solid catalysts, photocatalysts, enzyme catalysts, and non-catalytic transesterification in subcritical and supercritical conditions is gaining popularity due to disadvantages of homogeneous catalysts like high energy consumption, corrosive nature (acids are more corrosive than base catalysts) and difficulty in their recovery.
Hydrolysis In the hydrolysis of vegetable oil, the triglyceride esters of fatty acids are converted into FFAs using catalysts like acid, base, or enzymes. The rate of the
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