During the last decade, efficient new technologies aimed at new renewable products have been developed to replace petroleum-derived chemicals due to increasing sustainability and environmental-health concerns. In this context, vegetable oils represent suitable substitutes for replacing conventional mineral oil-based lubricants due to their biodegradability and non-toxicity. In fact, compared to mineral oils, biolubricants have a higher flash point, viscosity index and lubricity, and lower evaporative loss [1
]. On the other hand, the applicability of biolubricants is partially limited due to their thermo-oxidative hydrolytic instabilities, as well as their low-temperature fluidity. Nevertheless, there are many methods for improving these undesirable properties, such as the genetic modification of vegetable oil fatty acids; the direct addition of antioxidants, viscosity modifiers, pour point depressants and emulsifiers; and the chemical modification of vegetable oils [3
]. Among these methods, the last seems to be the most interesting for improving thermal stability. Chemical modifications mainly involve altering the acyl (C=O) and alkoxy (O–R) functional groups and unsaturations of the triglyceride molecules.
The epoxidation reaction of the double bond contained in vegetable oils, followed by oxirane ring opening by an acid or alkaline catalyzed reaction with organic acid or alcohol, improves the poor stability of oils caused by the presence of unsaturations. Moreover, modification of the molecular structure triglicerides can improve the chemical and physical properties of the final products.
Epoxidized vegetable oils can be produced via conventional reaction of the double bonds of oil with peracids, which act as oxidizing agents. The latter are obtained in situ by the reaction of corresponding carboxylic acids with hydrogen peroxide in presence of soluble mineral acids or, alternatively, acidic ion-exchange resins, as catalysts [5
]. Epoxidized vegetable oils show better performances than vegetable oils for application as lubricants in certain temperature ranges. Indeed, they show a better thermal and oxidative stability due to their better acidity value and lubricity, increased viscosity, and higher pour-point temperature [8
]. Adhvaryu and Erhan [8
] reported epoxidized soybean oil (ESO) as a potential candidate for high-temperature lubrication, considering the good thermal, oxidative and frictional behavior observed. Moreover, epoxidized vegetable oils are promising intermediates, since the oxirane group is easily functionalized by reaction with different nucleophilic reagents, leading to a wide number of products that are interesting from the point of view of biodegradable lubricant formulations. Chemically modified soybean oils, with improved thermal and oxidative stability, were obtained by the ring-opening reaction of epoxidized soybean oil with alcohols in presence of acid catalysts, followed by esterification of the hydroxyl group in the opened-ring product with anhydride [9
]. Sulphuric acid has been widely employed as a homogeneous catalyst in the modification of epoxidized oil. Campanella [12
] proposed an alternative synthesis route, consisting of ring-opening reaction of the epoxidized soybean oils with acetic acid or short-chain aliphatic alcohol (methanol or ethanol) in presence of fluoroboric acid as catalyst in aqueous media. However, the use of acidic homogenous catalysts, mainly mineral acids, poses severe problems due to the potential corrosion of the reactor vessel, as well as waste stream-handling problems. Heterogenous catalysts or alternative methods of solid removal based on magnetic nanostructured materials [13
] allow these limitations to be overcome. To this end, acidic ion exchange resin catalysts have been proposed for epoxidation and for the following modification reaction [6
]. In this work, an optimization of the prepation of biolubricants through the acid-catalysed ring-opening reaction of epoxidized soybean oil with alcohols, using SAC-13 as solid acid catalyst, is proposed. The effect of the addition of alcohols, with different chain length and type, on the tribological features of the final products is investigated. Finally, after having selected the product with the highest viscosity, some reaction parameters—such as catalyst loading and temperature—were studied, in order to detect the best reaction conditions and to present a kinetic model that could be useful for the eventual design of an industrial process.