lubricants Article In-Situ Epoxidation of Waste Cooking Oil and Its Methyl Esters for Lubricant Applications: Characterization and Rheology Atanu Kumar Paul 1 , Venu Babu Borugadda 2 and Vaibhav V. Goud 1, *   Citation: Paul, A.K.; Borugadda, V.B.; Goud, V.V. In-Situ Epoxidation of Waste Cooking Oil and Its Methyl Esters for Lubricant Applications: Characterization and Rheology. Lubricants 2021, 9, 27. https:// doi.org/10.3390/lubricants9030027 Academic Editor: Buyung Kosasih Received: 20 January 2021 Accepted: 25 February 2021 Published: 2 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India; atanu.2013@iitg.ac.in 2 Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, SK S7N 5A9, Canada; vbb123@mail.usask.ca * Correspondence: vvgoud@iitg.ac.in; Tel.: +91-361-2582272; Fax: +91-361-2582291 Abstract: In the present investigation, in-situ epoxidation of waste cooking oil and its methyl esters was prepared, and the rheological behavior was analyzed for biolubricant applications. Rheological properties of the prepared epoxides were measured at a temperature of 25–100 ◦ C, at a shear rate ranging from 5 to 300 s −1 . As viscosity is one of the critical parameters for potential biolubricant applications, in the present study, the power-law model was used to investigate the flow behavior of the epoxides. The viscosity of epoxidized waste cooking oil and its methyl ester epoxides showed Newtonian flow behavior in the studied temperature range. Different shear rates (5–100, 5–300, 100–300 s −1 ) were studied to determine the shear rate dependency of the epoxidized waste cooking oil and its methyl ester epoxides at different temperatures. From the average viscosity values, it was shown that the epoxides show identical results at all shear rates. The dynamic viscosities of the epoxidized waste cooking oil and its methyl ester epoxides were found to be dependent on fatty acid chain length, unsaturation, and temperature. Detailed physicochemical characterization for epoxide waste cooking oil (EWCO) and epoxide waste cooking oil methyl esters (EWCOME) were carried out to evaluate the properties for suitable biolubricant applications using standard American Society for Testing and Materials (ASTM) and American Oil Chemists’ Society (AOCS) methods. Based on the viscosity for EWCO (278.9 mm 2 /s) and EWCOME (12.15 mm 2 /s) and viscosity index for EWCO (164.94) and EWCOME (151.97) of the prepared epoxides, they could complement the standard ISO vegetable grade (VG) lubricants in the market. Keywords: rheology; waste cooking oil; epoxide; physicochemical properties; viscosity 1. Introduction In recent years, eco-friendly products such as biolubricants have played an important role over conventional products. Renewable feedstocks are bound to gradually comple- ment sources of fossil origin (oil, gas, and coal), both as fuel and as raw materials for the chemical industry [1]. Over the 20th century, great efforts were made by the chemical industry to replace petroleum-based feedstock with renewable resources to reduce the en- vironmental impact [2,3]. Among the several renewable sources, non-edible oils and edible waste oils have gained much attention owing to their potential to produce several value- added domestic and industrial-grade products. Chemical modification of double bonds (unsaturation) offers an appropriate method for producing commercial value-added fuels, chemicals, and lubricants from renewable raw materials [4]. Figure 1 shows the molecular structure of a typical triglyceride molecule for structural modification via epoxidation to prepare biolubricant base stocks. Lubricants 2021, 9, 27. https://doi.org/10.3390/lubricants9030027 https://www.mdpi.com/journal/lubricants