Calixarene-based viscosity index improvers and detergents for lubricants

Abstract

Lubricants are made of two components: the base oils and the additives. Depending on the type of application, additives may be present from very small percentages up to the case of engine lubricants where the total additive content is usually somewhere between 5% and 30%. Additives, although present in lower percentages, can substantially improve the existing properties of the base oils and, moreover, can add new fundamental properties such as detergency, dispersancy and prevention from wear and corrosion. In order to try to answer to the growing requirements of the market both in terms of performances and environmental sustainability, the research in the field of additives is strongly pushing towards the discovery of novel molecules easy to be synthesised and possessing improved functions and properties. This study investigate the use of an innovative class of macrocycles, calixarenes, as starting point for the preparation of Viscosity Index Improvers and Detergents. Calixarenes, the cyclic oligomers obtained by the condensation of p-substituted phenols and formaldehyde, were used for the synthesis of Star Polymers (SP) to be applied as Viscosity Index Improvers, that are polymers able to modulate the viscosity of the base oils as a function of the temperature. SPs are structurally formed by two parts: a central core and a variable number of peripheral arms linked to the central structure. The approach used in this thesis for the preparation of SPs is the so-called “Arm-First approach” which consists to first prepare polymer arms via the Living Anionic Polymerisation and then to connect these preformed arms to a central core based on a p-tert-butylcalix[8]arene structure properly functionalised. After a purification step, the final product was tested and compared with other Star Polymers of well-known properties showing very good thickening power and good mechanical shear stability, a property very important for this kind of additives. Other calixarenes cores, able to react via a radical polymerisation with preformed arms were obtained and will be used in the preparation of Star Polymers with improved performances. In the second part of this thesis, novel macrocycles derived from para-dodecylphenol and formaldehyde were synthesised, isolated and characterised by a combination of chromatographic separation and spectroscopy/spectrometry methods. The APPI-MS studies of the reaction products show the presence of calixarenes with different sizes and prevalence of calix[8]-, -[6]-, and -[5]arenes together with a smaller amount of linear oligomers. Also due to the long and inhomogeneous alkyl chains present at the upper rim of the macrocycle, the final products are liquid at room temperature. This prevents the easy separation of the macrocyclic components from the linear oligomers by simple precipitation. However, the yield of p-dodecylcalixarenes from the rather cheap starting materials, is rather high (> 80%) and the mixture of macrocyclic (80%) and linear compounds (20%) was successfully used for the preparation, on a large scale, of overbased-detergents. This class of additives is used to neutralise the acid species and keep in suspension the polar compounds produced during the usage of the lubricants in the engine. Thanks to these new macrocycles it is possible to obtain detergents having very high values of TBN (Total Base Number) and very good detergency properties, indicating a rather high quality of these calixarene-based additives.