The biodegradable and bio-based lubricants market is growing at a rate well in excess of traditional lubricants. Most of this growth is occurring in Europe and has been spurred by government mandates, legislation and regulation originating in the 1980s, expanding in the middle to late 1990s and continuing today. Public opinion and current initiatives such as REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) will continue to drive markets and end-users in the direction of lubricants that have less negative impact on the environment.

Industries and applications where contact with water or environment are inevitable can benefit from the use of biodegradable and bio-based lubricants due to the reduction or minimization of negative environmental impact from unintentional spills or leaks. Applications utilizing once-through or total loss lubrication can also benefit from the use of modern biodegradable lubricants. Industries including forestry, mining, wastewater treatment, railroads, off-shore drilling and other marine operations have many applications suitable for these kinds of lubricants.

Given that many biodegradable lubricants are derived from edible plants (bio-based), it seems as if the food and beverage industry is a natural fit with its incidental lubricant contact applications. Common applications for biodegradable and bio-based lubricants across various industries include grease, hydraulic fluid, gear oil, chain saw oil, way lubricants and cutting fluids, among others.

History and Science Lessons
The ancient Egyptians used vegetable-based oils as lubricants to reduce friction while moving stone building blocks. Lack of availability, war, and an increasing desire to use and purchase products made from renewable, environmentally friendly resources - coupled with increasingly demanding applications - have led to the development of bio-based lubricants that perform many times better than those of yesterday. Thanks in part to modern genetic engineering, it is possible today to purchase bio-based lubes with performance matching or exceeding that of mineral oils.

The first bio-based lubricant basestocks were unsaturated ester (triglycerides) oils of vegetable and animal origin such as olive and rapeseed oil, tallow and sperm whale oil. Their good lubricating properties were well-known. Triglycerides are fairly inexpensive and have good fire resistance. They lack with regard to low-temperature performance, thermal and oxidative stability, and could be prone to hydrolytic degradation.

Physical properties such as low temperature characteristics depend upon the composition of fatty acids attached to the glycerol backbone of the triglyceride. Increasing the saturation of fatty acids has been demonstrated to improve low-temperature performance. The more significant problem of oxidative stability has been addressed by increasing the content of oleic acid. Natural soybean oil has oleic content of roughly 20 percent. Modern genetic engineering has provided canola, soybean and sunflower oil with oleic content exceeding 80 percent. This increase in oleic content translates into oxidative life that is approximately three to four times what was possible with conventional soybean oil. These oils seem well-suited for all but the most demanding applications. Many types of these basestocks do provide lower coefficients of friction than mineral oils.

Synthetic Esters
Given technology's progression, tomorrow's gearboxes and other mechanical components will be charged with doing more work in the same space or less. Operating speeds, loads and temperatures will likely increase.

Synthetic esters - including diesters and polyolesters - are constructed from organic acids and alcohols and may offer the next step up with regard to the performance combination of increased thermal, oxidative and hydrolytic stability of biodegradable lubricants. Synthetic esters offer excellent high- and low-temperature performance combined with low volatility, and good lubricity and hydrolytic stability. Synthetic ester fluids are an excellent choice for rotary screw air compressor applications and can provide service life in excess of other synthetic fluid technologies, biodegradable or not.

Diesters are known to be rather aggressive to elastomers. Take care when considering seal compatibility with them. Polyolesters are used to formulate fire-resistant hydraulic oils. Multiple types of synthetic esters are available in a wider viscosity range than other types of bio-based lubricants, thus expanding their suitability for various applications.

Polyglycols can offer good biodegradability and can be produced in a wide viscosity range. Higher molecular weight polyglycols degrade at slower rates, and it appears that there is a need for additional study of their biodegradation.

Polyglycols offer many of the same benefits as polyolesters, although they are more likely to affect some seals and paints. They have excellent lubricity and high viscosity indexes that allow very good characteristics at temperature extremes. They are not miscible with mineral or polyalphaolefin oils. They may offer greater hydrolytic stability than polyolesters. In gear applications, poly-glycol gear oil may offer benefits over other gear lubricants such as increased lubricity that may lower amp draw when compared to mineral oil and other synthetics, resulting in energy savings and possibly lower carbon dioxide emissions.

Two common families of polyglycols are polyethylene glycols and polypropylene glycols. Polyethylene glycols are water soluble; polypropylene glycols are not. A few of the more common methods for assessing biodegradability are applicable to only non-water soluble lubricants.

Polyalphaolefins (PAO) with kinematic viscosities under 6 centistokes (cSt) are readily biodegradable and offer good low-temperature performance, hydrolytic stability and low volatility. They make an excellent choice for barrier fluids and seal lubricants in applications where low viscosity fluid is necessary. Higher-viscosity PAO fluids exist and perform extremely well, but they shift from readily biodegradable to inherently biodegradable fluids.

Terms and Tests
No discussion of environmentally friendly or biodegradable lubricants can be complete without giving meaning to what is meant or implied by the terminology in the market and how lubricants are classified as this or that. While products positioned as environmentally friendly, acceptable or compatible are labeled by mostly subjective means, properties such as toxicity and biodegradability can be measured, and results of specific test methods determine into which classification given lubricant products fall. Biodegradable products can be broken down by fungi or bacteria found in nature from complex molecules (original material) to simple molecules or compounds, right down to basic materials of construction such as carbon, oxygen, nitrogen, water or carbon dioxide.

Lubricants can be either readily biodegradable or inherently biodegradable. Inherently biodegradable lubricants will degrade to some extent in some undefined (longer) period of time. They may have the potential to bioaccumulate; this persistence in the environment may or may not result in pollution or environmental damage. Readily biodegradable lubricants will biodegrade to a determined extent in a short period of time.

There is currently no industry or performance standard to assess biodegradability, but there are a number of test methods in existence. What is biodegradable by one test method may not be so under another. Two common methods for assessing rapid biodegradation are the Coordinating European Council (CEC) L-33-A-93 tests (at least 80 percent within 21 days) and the Organization for Economic Cooperation and Development (OECD) 301 tests (at least 60 percent in 28 days). The CEC tests measure primary degradation only. They do not measure the ultimate fate of lubricant molecules. They are concerned with how much of the original material degrades into something else. These primary degradation products may not be the final products on the path to complete degradation, and some degradation products may pose their own concerns. OECD 301 tests measure aerobic degradation by checking loss of carbon, consumption of oxygen and evolved carbon dioxide. These tests are a better measure of ultimate biodegradability.

Eco-labeling such as the European Union Eco-Label considers both biodegradability of the lubricant combined with toxicity hazards to humans, aquatic environments and the overall environment. The fully formulated lubricant and individual components are subject to scrutiny. In addition, for hydraulic fluids and some other fluid types, there are technical performance standards such as oxidation stability, demulsibility, seal swell and wear tests that must be met.

Important Reminders
Selection of a biodegradable, environmentally friendly or environmentally compatible lubricant would seem to be best done just as it would with any other lubricant. Keep in mind what the main goals, objectives and requirements are. Fully understand terminology, test methods and criteria used to assign performance or compliance under any given standard. And most importantly, always bear in mind that there are many different kinds of lubricants available. All of them have their own unique combinations of strengths and weaknesses. Often, it will be necessary to select lubricants by taking the positives most important to you along with some possible negatives with which you can live.

References

1. Mang and Dresel (2007), Lubricants and Lubrication, second edition, Chapter 7, Wiley-VCH Verlag GmbH & Co. KgaA.

2. Rostro, B. "Everything Old Is New Again" Tribology & Lubrication Technology, December 2007.

3. Sullivan, T. "Cultivating a Market" Lubes-n-Greases, May 2001.

About the Author

Gene Finner has accumulated more than 10 years of experience in the operation and lubrication of mechanical equipment in his role in applied engineering technical service for Dow Corning Corporation's Molykote lubricants. Finner's job responsibilities include process and product optimization, troubleshooting and lubrication training, along with North and South American lubricants product stewardship for Dow Corning. Finner earned a bachelor of science degree from the University of Michigan and is a Certified Lubrication Specialist through the Society of Tribologists and Lubricating Engineers. Dow Corning is a corporate member of the National Lubricating Grease Institute (NLGI). Learn more about NLGI by visiting www.nlgi.org.