Desarrollo de nuevos agentes espesantes y/o gelificantes de aceites vegetales a partir de diferentes fracciones lignocelulósicas modificadas químicamente mediante epoxidación

Abstract

During the last decades, the world is really concerned about the pollution and the negative effects that most chemicals and/or end-used products are causing on the environment, especially those derived from crude oil. In this sense, the lubricant industry has also become more sensitive to the needs of the environment and it is fostering the replacement of non-renewable raw materials by others coming from natural resources. The first objective of this tendency was the substitution of mineral oils by other more eco-friendly lubricating base oils, by using vegetable oils or some derivatives, whose properties make them promising candidates to be employed as biodegradable lubricants. However, regarding lubricating grease formulations, these are generally composed of non-natural thickeners, like metallic soaps and poliureas, with the subsequent impact on the biodegradable characteristics of the final product. In this sense, in order to produce completely renewable and biodegradable lubricating grease formulations, there is an open research field aiming to find new bio-thickeners based on natural resources, whose characteristics provide suitable properties to the final biolubricating grease. With this aim, this work claims to develop biodegradable gellike dispersions constituted by a vegetable oil (castor oil) and chemically modified lignocellulosic materials able to act as efficient thickeners in these formulations to be applied as lubricating greases. Lignocellulosic materials such as lignin, which is considered a residual fraction of cellulose pulping and bioethanol production, with a great global manufacture; and cellulose pulp, composed of cellulose, hemicellulose and lignin, and constituting a renewable, abundant and inexpensive raw material for many applications, have been selected as biopolymers to replace the metallic soaps traditionally employed as thickeners in lubricating greases. For this purpose, these biopolymers have been chemically modified by using epoxy compounds, such as epichlorohydrin and glycidyl ether derivatives, by varying both the nature of the epoxide and the proportions used in the epoxidation reaction, and afterwards dispersing them into castor oil, in order to obtain physically stable chemical gels. With the aim of assessing the extent of the biopolymers epoxidation and the properties of resulting biolubricating greases, different characterization techniques have been used. Thus, epoxy index determination, infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry tests were carried out to verify the chemical modification of biopolymers. Moreover, oleogels were fully characterized from a rheological and tribological point of view, also studying the microstructure of some of them. In general, a higher epoxy index, i.e. a higher epoxidation degree of the lignocellulosic materials studied, improves the compatibility with castor oil and favours the physical stability of the resulting oleogels, as a consequent of the chemical cross-linking produced between the free epoxy groups and the hydroxyl groups of castor oil. These chemical interactions are also responsible for the final properties of these oleogels. Thus, more suitable rheological properties are obtained for their use as bio-lubricating greases when the lignin or cellulose pulp have a high epoxy index. The degree of modification of these lignocellulosic materials can be controlled by varying the conditions of the epoxidation reaction (temperature, time and proportion of reagents). On the other hand, the use of aromatic epoxides as modifying agents provides, in general, more convenient rheological and tribological properties compared to their aliphatic counterparts, for the same epoxidation degree of the lignocellulosic material, due to a higher level of cross-linking achieved in the three-dimensional network of chemical oleogels. In this way, the rheological behaviour of most oleogels developed was very similar to that found in traditional lubricating greases. On the other hand, the formulations thickened with epoxidized cellulose pulp show excellent thermal stability, without significant changes in rheological properties up to 150 ºC. In addition, both friction and wear, evaluated in a tribological contact, were reduced by introducing the biothickeners consisting of epoxidized lignocellulosic material in the formulations, in comparison to the castor oil as the sole lubricant. Overall, it may be concluded that all formulations synthetized from completely renewable materials, showed suitable properties to be proposed as promising alternatives to conventional lubricating greases.