Historical Summary

The interest in biobased lubricants and particularly greases are on the rise. Interestingly, the original introduction of environment friendly lubricants began in Europe during the early 1980s. US researchers and lubricants experts followed Europe’s lead and the 1990s saw a huge developmental activity in the United States. Companies like The Lubrizol Corporation invested significant amount of resources to develop additive packages for vegetable oil based hydraulic oils and focused on high oleic and ultra-high

oleic sunflower oils as base oils. In the early 1990s, the giant US agricultural equipment manufacturer Deere and Company introduced a Universal Tractor Transmission Hydraulic Fluid called Bio-Hy-Gard which had the research cooperation of The Lubrizol Corporation’s additive technology. It was specifically designed to accommodate prevailing mandates in the Black Forest areas in Germany. Caterpillar too later introduced, a hydraulic fluid called Bio-Hydo.

Introduction

In 1991, this author founded a biobased research center at the University of Northern Iowa with support from the Iowa Soybean Promotion Board (ISPB) and the US Department of Agriculture among many other funding agencies. In 1997 a soybean oil-based version of Bio-Hy-Gard was introduced as a soybean oil-based universal tractor transmission hydraulic fluid with funding support from ISPB. This product has been under the ELM brand (Figure 1). Europe’s interest peaked again during the current century after research and developmental activities in the US had blossomed into a growing business. Most importantly, the US government initiated purchase preference programs for federal purchasing agents in order to create usage history and a model for the private  industry to emulate. The United State’s Department of Agriculture in cooperation with the United Soybean Board created the terms Biobased and Bio-preferred and has an extensive program that lists available biobased products and their manufacturers. www.biopereffered.com and www.usb.org. Figure 2 illustrates a number of retail packaged biobased products that are sold nationwide in large big box stores. These products are sold based on performance and price without overtly emphasizing the environmental adders that are only visible upon inspection of the packaging.

While in Europe the emphasis has been primarily on biodegradability, in the US the focus is on renewability and petroleum substitution to the extent possible. As a result, since PAOs are considered biodegradable, in Europe they are preferred over non-biodegradable lubricants. But, PAOs are not biobased since they are not derived from renewable materials and in the US, they are not considered bio-preferred. Biobased lubricants in most cases are also biodegradable; whereas not all biodegradable lubricants are biobased.

Biobased Base Oils

There appears to be a general misunderstanding among the end users about the base oils used in environmentally aware products. The expectations give the impression that all biobased products are supposed to be vegetable oil based and/or that all lubricants can be made from vegetable oils.

Vegetable oils offer many inherently advantageous properties when used as lubricants, but they also have inherent limitations when compared with mineral or synthetic oils. As a result, there are vegetable oil derivatives such as simple and complex esters, estolides, and an array of other chemically modified oils that offer base oils for different end use applications. A thorough understanding of the properties of the base oil along with an appreciation of the performance requirements of the end use are essential in preparing a superior biobased product. Table 1 shows a list of vegetable oils with their select properties. For example, castor oil shows a superior
oxidation stability and low temperature performance. The Oil Stability Index (OSI) is an American Oil Chemists’ Society (AOCS) official method Cd 12b-92 re-approved 2017. For castor oil the OSI is 105.13 hours which is the highest stability of all oils listed in this table. But, castor oil also has the lowest viscosity index of all vegetable oils at 85. So, if used as hydraulic fluid, for instance, it would present good oxidation stability and cold temperature performance, but it would also thin down excessively at high operating temperatures. This makes it unsuitable as hydraulic fluid if used by itself.

Vegetable oils are often chemically modified, mixed together, or extracted from genetically enhanced seed oils with higher built-in stability. So, a combination of vegetable oils could enhance the properties of the mixture of one or more vegetable oil. An understanding of the properties of each vegetable oil alone or as a mixture when considered as base oil for lubricants is important when formulating biobased lubricants.

Base oil developers have often sought out oils that could be used across the application spectrum. As a result, there are several biobased derived or chemically modified base oils available with the intention of addressing the shortcomings of natural oils. Examples of these oils include a line of biobased derived esters from Zschimmer & Schwarz that offer cold temperature performance down to -70C with high levels of oxidation stability, and hydrolytic stability. Under the brand Lexolube, these esters come in viscosities ranging from 4 cSt ro 220 or higher.

Croda offers a line of complex esters, under the brand Priolube, that present performance and stability for use as hydraulic oil and other liquid lubricants. Ranging in viscosities from these esters offer highly stable base oils for various lubricants. The biggest barrier in using these biobased derived, chemically modified oils are that they are 3x-5x more expensive than the vegetable oils used to synthesise them.

Applications

As biobased lubricant and grease technologies mature, their ultimate success will depend on matching the performance and price of conventional mineral oil products. High quality and high-performance products that are expensive find success in niche markets. But, to reach the mainstream lubricant markets or to capture a significant portion of select lubricant fields, the end use application must closely match the performance and price of conventional lubricants.