Elsevier

Bioresource Technology

Volume 101, Issue 1, January 2010, Pages 245-254
Bioresource Technology

Lubricants from chemically modified vegetable oils

https://doi.org/10.1016/j.biortech.2009.08.035Get rights and content

Abstract

This work reports laboratory results obtained from the production of polyols with branched ether and ester compounds from epoxidized vegetable oils pertaining to annual, temperate climate crops (soybean, sunflower and high-oleic sunflower oils), focusing on their possible use as components of lubricant base stocks. To this end, two different opening reactions of the epoxide ring were studied. The first caused by the attack with glacial acetic acid (exclusively in a single organic phase) and the second using short-chain aliphatic alcohols, methanol and ethanol, in acid media. Both reactions proceed under mild conditions: low synthesis temperature and short reaction times and with conversions above 99%. Spectroscopic (NMR), thermal (DSC) and rheological techniques were used to characterize the oils, their epoxides and polyols, to assess the impact of the nature of the vegetable oil and the chemical modifications introduced, including long-term storage conditions. Several correlations were employed to predict the viscosity of the vegetable oils with temperature, and good agreement with the experimental data was obtained.

Introduction

New technologies aimed at the development of products from renewable sources have emerged during the last decade, due to increased concerns over the use of petroleum-based products caused by the progressive depletion of the world reserves of fossil fuels, but also owing to concerns on their environmental impact.

In this regard, vegetable oils (VO) constitute a suitable alternative for replacing ‘mineral oils’, as they are wholly biodegradable, non-toxic, and ‘Generally Regarded as Safe’ (GRAS) products (Erhan and Perez, 2002, Erhan and Asadauskas, 2000). Indeed, VOs possess most of the desirable lubricity properties, such as good contact lubrication, high viscosity index (i.e., minimum changes in viscosity with temperature), high flash-point and low volatility. They also have drawbacks, however, that must be overcome, including poor low-temperature properties (such as opacity, precipitation, poor flowability and/or solidification at relatively moderate temperature), their susceptibility to oxidative degradation and their propensity to undergo hydrolysis in acid media. The latter, nevertheless, can be attenuated with relative ease through the use of additives (Hwang and Erhan, 2001, Hwang et al., 2003).

Deliberate modification of the chemical structure of VOs is a sound alternative to allow their direct use as lubricant basestocks. For instance, unsaturated alkene groups of the fatty acid chains in triglyceride molecules can be deliberately altered to give more complex structures that improve both the low-temperature and oxidative stability properties (Erhan and Perez, 2002, Lathi and Mattiasson, 2006, Hwang and Erhan, 2001, Hwang et al., 2003, Adhvaryu et al., 2005, Sharma et al., 2006, Sharma et al., 2008).

Epoxidized vegetable oils are promising intermediates for the utilisation of VO given that the epoxide, or oxirane, group can be easily functionalized. Erhan and co-workers (Erhan and Perez, 2002, Hwang and Erhan, 2001, Hwang et al., 2003, Erhan et al., 2008) recently worked on the production of lubricant base stocks from acid-catalyzed oxirane ring-opening reactions using epoxidized soybean oil, employing several alcohols and later esterifying the resulting hydroxyl group with an acid anhydride. They also synthesized a biolubricant from di-hydroxylated soybean oil, obtained by acid hydrolysis of the epoxide group followed by esterification of said hydroxyl groups (Adhvaryu et al., 2005, Sharma et al., 2006). More recently, they also produced acyl derivatives of soybean oil via the opening of the oxirane ring using acid anhydrides and employing BF3 etherate as catalyst. These products appear to be promising lubricants (Sharma et al., 2008).

In recent years, there has been an increased interested in the production of biodiesel (BD) as a more benign fuel. BD is usually produced by the transesterification of vegetable oil triglycerides with an aliphatic alcohol (such as, methanol) employing sodium hydroxide as a catalyst. Fatty acid methyl ester (FAME) are obtained as the main product of this reaction. Thus, FAMEs have become extensively available and are produced with high purity. This has open new pathways to the synthesis of oleochemical products, such as epoxidized FAMEs (Campanella et al., 2008) and, also, lubricants (Moser and Erhan, 2008, Moser et al., 2007a, Moser et al., 2007b, Moser and Erhan, 2007). Moser et al., 2007a, Moser et al., 2007b, Moser and Erhan, 2008 have recently been producing branched chain derivatives, partially hydrogenated and, also, diesters through the chemical modification of FAMEs and fatty acids.

In this work we propose different synthesis alternatives that can be carried out prior to the esterification step (reaction with an acid anhydride). Several epoxidized vegetable oils obtained from temperate climate crops (soybean, sunflower and high-oleic sunflower oil) were used for the production of polyols. Two types of ‘degradation’ reactions of the epoxide group were studied (Scheme 1). In the first approach, we employed glacial acetic acid (i.e., in organic phase exclusively), whereas the second reaction involved the opening of the oxirane ring with a short-chain aliphatic alcohol (methanol and ethanol) in aqueous, acid media. Only epoxidized soybean oil was used in the second study. Various thermal (differential scanning calorimetry, DSC) and rheological characterization studies were performed with the VOs, their epoxides and polyol derivatives, to evaluate each natural oil and the impact of introducing chemical modifications – the results of which expand the available database on these novel products.

Section snippets

Materials

Well-refined, edible quality (i.e., degummed, neutralized, bleached and deodorized) sunflower oil (Natura brand, from Aceitera General Deheza S.A., Córdoba, Arg.) and high-oleic sunflower oil (Ecoop brand, from SEDASA, Buenos Aires, Arg.) were used as received. Crude soybean oil (from Oleaginosa Humboldt, Santa Fe, Arg.) was refined in our laboratory. The main fatty acids composition of the oils was determined by gas chromatography (GC) after derivatization of the triglycerides to fatty acid

Oxirane ring-opening reactions

The first reaction studied was the opening of the oxirane ring of the epoxidized oils caused by the attack of glacial acetic acid (i.e., exclusively in the organic phase). For ESBO both the reaction temperature and the molar ratio of epoxide groups (Ep) to acetic acid (AA) were varied (Table 2 and Fig. 1). It can be readily appreciated that the final conversion could be achieved at moderate to short reaction times. For epoxidized sunflower and high-oleic sunflower oils a single concentration of

Conclusions

Two new synthetic routes to polyol-derived intermediates, proposed as an alternative way for the production of lubricants from vegetable oils obtained from temperate climate crops (soybean and sunflower seeds), are proposed in this work. The oils are first epoxidized, and then the oxirane ring is opened with either acetic acid or a low chain aliphatic alcohol (methanol or ethanol). These routes are promising in that they demand short reaction times and low reaction temperatures, while ca. 99%

Acknowledgements

The authors thank CONICET, ANPCyT and UNL (CAID 2006 grant 29-161) for their financial support. Thanks are given to Pablo Vicentini and Adolfo Larese for their skilled assistance in the preparation of epoxidized vegetable oils and to Daniel De Pianti for his technical assistance to use the rheometer. A.B. thanks UBA (X089) and ANPCyT (PICT 2005, 32735) for partial financial support. AB and MAB are Research Members of CONICET.

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