Date of Award

December 2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Tien-Chien Jen

Second Advisor

Michael R. Lovell

Committee Members

Illya V. Avdeev, Junhong Chen, Pradeep L. Menezes, Dexuan Xie, Kurt Beschorner

Keywords

Biolubricants, Friction, Ionic Liquids, Lubrication, Natural Oils, Wear

Abstract

Over the last several decades the lubrication industry has been striving to bring bio-based lubricants known as biolubricants to prominence. The reasons for the increased environmental initiatives are due to depletion of oil reserves, increases in oil price, stringent government regulations on petroleum-based oils, and most importantly, concerns for protecting the environment. With an estimated, 50% of all lubricants entering the environment and much of these being composed of toxic mineral oils, biolubricants have begun to witness a resurgence. This experimental investigation seeks to develop a new class of ecofriendly biolubricants that are less toxic to the environment, derived from renewable resources, and provide feasible and economical alternatives to traditional petroleum-based lubricants. Advantages of biolubricants include their higher lubricity, lower volatility, higher shear stability, higher viscosity index, higher load carrying capacity, and superior detergency and dispersancy when compared to petroleum-based lubricants.

This work highlights the evolution of biolubricants derived from natural oils and fats to green lamellar solid additives to a new class of "greener" functional fluids known as room temperature ionic liquids (RTILs). The attraction to biolubricants began with natural oils due to their low friction and wear characteristics owing to fatty acid monolayers that enable high lubricity. Despite these accolades, natural oils suffer from thermal-oxidative instability and high pour points. To improve upon the tribological properties, natural oils were combined with solid powders additives. Currently, RTIL lubricants derived from bio-based feedstock represent a more promising potential solution to many of the problems associated with previous biolubricants. In a final study the RTILs where shown to benefit from the use of solid powder additives to further improve upon their tribological performance.

In this experimental investigation, friction and wear tests were carried out using a pin-on-disk tribometer under ambient and high temperature conditions to evaluate the tribological performance of the various natural and synthetic biolubricants. A thermogravimetric analysis (TGA) was conducted to study the thermal response of the lubricants in a high temperature oxygen-free environment. Scanning electron microscopy and surface profilometry studies were performed to assess the surface roughness.

These experiments investigated the performance of natural oils as neat bio-based lubricants to understanding the effects that long chain fatty acids have on the tribological performance of natural oils. The experiments revealed that natural oils with low unsaturation numbers due to high concentrations of oleic acid demonstrated to have the superior friction and wear properties as well as high thermal-oxidative stability. Extensive testing of multiple natural oils with various particulate additives composed of a variety of types, sizes, and shapes revealed that natural oils benefit tremendously from nanometer-sized spherical shaped particles. However, this is not without its complexities as surface roughness, sphericity, particle size, and capillary effects all influenced the use and performance of particulate additives.

In an effort to refine the natural oils composed of particulate additives, RTILs were chosen because of their ability to lubricate in boundary lubrication due to their inherent polar molecules; their ability to be tuned for specific applications; and most notably their lack of vapor pressure providing new opportunities for liquid lubricants. Investigations into the ionic liquid lubricants revealed that longer alkyl chains on the cations with aromatic carboxylate anions exhibited the most lubricity as neat lubricants i.e. lubricants with negligible additives. Again, these lubricants were subjected to particulate additivation and it was revealed that smaller nanometer sized particles independent of the particle type provided the greatest benefit to lowering friction and minimizing wear. The performance of the ionic liquids improved with the particulate additives and it was further verified that phosphonium and imidazolium cations combined with food grade carboxylate anions such as saccharinate, salicylate, or benzoate formed biolubricants that maintained superior tribological properties as well as maintained a high degree of thermal-oxidative stability. This experimental investigation has illuminated the potential of RTIL biolubricants to satisfy the growing environmental, health, economic, and performance concerns of modern lubricants. The mechanisms governing the chemical compositions, improved tribological performance, and thermal response of the lubricants are extensively discussed along with their viability as sustainable and renewable biolubricants.

Download

Share

COinS