The Advantages and Disadvantages of Biodegradable Lubricants

Vegetable oils can be used as lubricants in their natural forms. They have several advantages and disadvantages when considered for industrial and machinery lubrication. On the positive side, vegetable oils can have excellent lubricity, far superior to that of mineral oil. In fact, their lubricity is so potent that in some applications, such as tractor transmissions, friction materials must be added to reduce clutch slippage.

Vegetable oils also have a very high viscosity index (VI). For example, a VI of 223 is common for vegetable oil, compared to 90 to 100 for most mineral oils, about 126 for polyalphaolefin (PAO) and 150 for polyglycol. Viscosity index can be defined as a frequently used measure of a fluid’s change of viscosity with temperature. The higher the viscosity index, the smaller the relative change in viscosity with temperature. In other words, oil with a high VI changes less with temperature than oil with a low VI.

Another important property of vegetable oils is their high flash points. Typically, this might be 326 degrees C (610 degrees F) for a vegetable oil, compared to a flash point of 200 degrees C (392 degrees F) for most mineral oils, 221 degrees C for polyalphaolefin (PAO) and 177 degrees C for polyglycol.

Flash point can be defined as the temperature to which a combustible liquid must be heated to give off sufficient vapor to momentarily form a flammable mixture with air when a small flame is applied under specified conditions, according to ASTM D92.

More importantly, vegetable oils are biodegradable, generally less toxic, renewable and reduce dependency on imported petroleum oils.

On the negative side, vegetable oils in their natural form lack sufficient oxidative stability for lubricant use. Low oxidative stability means the oil will oxidize rather quickly during use if untreated, becoming thick and polymerizing to a plastic-like consistency. Chemical modification of vegetable oils and/or the use of antioxidants can address this problem, but it will increase the cost. Chemical modification may involve partial hydrogenation of the vegetable oil and a shifting of its fatty acids.

The challenge with hydrogenation is determining at what point the process should cease. Depending on the required liquidity and pour point of the oil, optimum hydrogenation is established. Recent advances in biotechnology have led to the development of genetically enhanced oil seeds that are naturally stable and do not require chemical modification and/or use of antioxidants.

Employing tests developed by the American Society for Testing and Materials (ASTM) and the Organization for Economic Cooperation and Development (OECD), oil is inoculated with bacteria and kept under controlled conditions for 28 days.

The percentage of oxygen consumption or carbon-dioxide evolution is monitored to determine the degree of biodegradability. Most vegetable oils have shown to biodegrade more than 70 percent within that period, as compared to petroleum oils biodegrading at nearly 15 to 35 percent. For a test to be considered readily biodegradable, there must be more than 60-percent degradation in 28 days.

Similarly, by using a variety of tests involving fish, daphnia and other organisms, the toxicity of vegetable oils can be measured. In this case, both mineral oil and vegetable oil in their pure forms show little toxicity, but when additives are included, the toxicity increases.

Another disadvantage of using vegetable oils is their high pour point. Pour point is defined as the lowest temperature at which an oil or distillate fuel is observed to flow when cooled under conditions prescribed by test method ASTM D97. The pour point is 3 degrees C (5 degrees F) above the temperature at which the oil in a test vessel shows no movement when the container is held horizontally for 5 seconds.

This problem also can be addressed by winterization, the addition of chemical additives (pour point suppressants) and/or blending with other fluids possessing lower pour points. Various synthetic oils can be used for this purpose.

If a high degree of biodegradability is required, then biodegradable synthetic esters are added to improve cold-temperature properties. On the other hand, if the goal is to maintain the so-called biobased property, where at least 51 percent of the lubricant is made of natural biomaterials, then a portion of the blend could be light mineral oil with low pour points. The latter will show a higher degree of toxicity and a lower degree of biodegradability.