Lubricants serve primarily to reduce friction between, and prevent wear of, contacting surfaces as they move past each other. Lubes must do this under varying loads, speeds, temperatures and contamination levels. Typical lubricants consist of more than 90% of a base fluid, with various additives such as corrosion inhibitors and extreme-pressure (EP) wear protectants to upgrade functional properties or stabilize the base fluid against degradation.
The federal government defines biolubes, or bio-based lubricants, as “composed, in whole or in significant part, of biological materials or renewable domestic agricultural materials (including plant, animal and marine materials) or forestry materials.” These lubricants are considered more environmentally responsible than those based on mineral oil derived from the fossil sources of crude oil or coal; their use is meant to reduce the carbon footprint.
A manifestation of green
European countries have been world leaders in the environmental movement for decades, through government regulations and consumer pressure. Initially, the criteria for a product’s environmental acceptability were based on just biodegradability and toxicity to aquatic organisms. Biodegradability is a measure of naturally-occurring microorganisms’ ability to decompose a material into harmless end-products. These bacteria metabolize organic (carbon-based) substances through stepwise oxidation, obtaining the energy they need and producing water and carbon dioxide as their major waste products.
There are several degrees of biodegradability (ready, inherent, primary and ultimate), and a number of official tests that can be used to demonstrate them. These 28-day bacteria-contact tests in aqueous solution measure variables such as dissolved organic carbon, carbon dioxide evolution and chemical or biochemical oxygen demand.
Standard tests for aquatic toxicity determine concentrations of a material needed for inhibition of algae growth, immobilization of Daphnia (water fleas) and acute toxicity to fish. Most lubricant base fluids and additive packages typically show low aquatic toxicity. In some cases this is because their lack of solubility in water makes them unavailable to the test organisms. Many of these official environmental test procedures are designated as Organization for Economic Cooperation and Development (OECD) methods.
Renewability is a more recent “green” criterion. U.S. government agencies are required to purchase and use qualified products containing specified levels of bio-based content wherever possible. This program, known as BioPreferred, includes several classes of lubricants, such as chain and cable lubricants, forming lubricants, hydraulic oils, gear lubricants, penetrating oils and greases. Numerous federal incentives exist for renewable-resource partnerships aimed at enhancing the value of crop-based materials. Agricultural states in the United States have had increasing success in promoting lubricants based on soybeans and other crops.
For the private sector, the primary motivation to use non-mineral-oil lubricants remains focused on avoidance of the risks and costs associated with environmental release of conventional oils. These include remedial spill clean-up costs, waste-disposal costs, administrative procedures and punitive fines under RCRA, the Federal Resources Conservation and Recovery Act, as well as long-term liability and litigation. There continues to be more pressure to use environmentally-benign lubricants from the supply side than from the demand side. The bio-based content of a product, such as a lubricant, can be determined by the same testing used in radiocarbon dating. It’s valuable to remember, though, that biodegradability is related to molecular structure rather than merely source of origin.
An even more recent manifestation of environmental responsibility is sustainability, the cradle-to-grave consideration of a product’s total interaction with the environment. This life-cycle assessment considers the overall energy and resources required to manufacture a lubricant, as well as the mass balance (potential for waste generation), compared to the value that the product brings to society. Sustainability also considers emissions to the environment, disposal and transportation demands during a product’s lifetime. Extending a lubricant’s lifetime before its disposal is probably the best way to minimize harm by reducing overall resource consumption. Product-lifetime extension also reduces replacement costs.
By far, the most common lubricant base fluid is mineral oil, a complex mixture of hydrocarbons whose molecules have between 15 and 50 carbon atoms. It often contains some low level of sulfur and nitrogen compounds, as well. Mineral oil is the least expensive lubricant base fluid, it’s relatively stable to hydrolytic and oxidative (to about 200°F) degradation, and it won’t swell or shrink the elastomers in gaskets and seals, thus avoiding leaks.
However, in addition to its adverse environmental reputation, some can be lost through evaporation over time (and add to atmospheric emissions). The disappearance of the more volatile components with their relatively low flash point and fire resistance leads to unwanted increases in viscosity. Compared to more recently developed lubricant base stocks, mineral oil has a relatively low viscosity index, changing viscosity noticeably with temperature and a relatively high coefficient of friction. Moreover, mineral-oil pollution is readily evident because it leaves a visible floating sheen on top of water. It’s estimated that, depending on concentration, mineral oil will biodegrade in about three years.