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.
[pullquote]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.
Green lubricants, in general, are more readily biodegradable than mineral oil and less toxic to fish and marine organisms. Not all of them are based on renewable ingredients, though. Poly-alpha-olefins, or PAOs, the basis for synthetic motor oils, and polyalkylene glycols, or PAGs, widely used in aqueous hydraulic systems, are examples of synthetic lubricants that show biodegradability and aquatic toxicity advantages over mineral oil, but aren’t derived from annually-renewable resources.
Naturally occurring materials
Biolubes such as olive and palm oils, wool fat and tallow were used centuries before the less-expensive, more stable and more reliably available crude-oil products took over.
Biolube production starts with crushing the seeds of oil-bearing plants and separating the oils from the husks either by mechanical pressing or solvent extraction. The plants most used for their oils worldwide are the palm tree, soybean, sunflower and canola (related to rapeseed in Europe). Its ready availability makes soybean the oil of choice in the United States. Other useful sources of vegetable oils include corn, cottonseed, castor bean and coconut.
Note that the vast majority of these oils are used for food applications, while a minor percentage is used for biodiesel, and lubricants comprise an even smaller fraction of these and other oilseed crops. Naturally, their human toxicity is low, making them an especially good choice for food-processing machinery lubricants.
Unmodified vegetable oils, chemically known as triglycerides, offer more advantages over mineral oils than just renewability, improved biodegradability and low aquatic toxicity. Their chemical structure results in better lubricity than the hydrocarbon makeup of mineral oil; improved lubricity results in less energy demand. Additionally, their viscosity indexes are higher (leading to less viscosity change with temperature) and their volatility is lower (resulting in less evaporative loss and higher flash point) compared to mineral oil. However, straight vegetable oils show high pour points and can solidify at low temperatures. They also exhibit poor hydrolytic and thermo-oxidative stability.
They have, though, been used in Europe since the 1970s as lubricants for chainsaws and two-stroke outboard engines. Their best use is in these moderate-temperature, total-loss lubrication applications as well as in concrete mold-release agents and railway-track greases.
Other sensitive lubrication applications that offer significant potential for accidental direct environmental contact include military materiel, earthmoving and construction equipment, waterway machinery, mining equipment, hydraulic systems, agricultural and forestry machinery, submersible pumps and power-generating wind turbines. These, and related applications, are the most appropriate for environmentally-friendly lubricants.
Avoiding degradation
The stability shortcomings of using environmentally-desirable vegetable oils as lubricant base stocks can be overcome, in large part, by chemically deconstructing the molecules of mixed triglycerides (their chemical structure is that of a tri-ester), isolating the lubricious molecular fragments (fatty acids) and reassembling the most useful of these naturally-occurring organic acids into more stable ester molecules. Reassembled ester chemistry has been used in jet aircraft engine lubricants for decades. The fatty-acid fragments themselves also can be modified chemically, before or after reassembly, to further enhance stability while retaining lubricant and environmental desirability. Of course, the more chemical reactions used to produce a base fluid, the greater its cost.
In the mixture of fatty acids obtained from vegetable oils, the 18-carbon lubricant molecule known as oleic acid provides the best stability compromise between solidification at low temperatures and susceptibility to oxidation, which can form sludge and degrade lubrication performance.
While some vegetable oils might naturally contain as much as 50% to 60% of this most useful component, genetically modified seed-oil plants have increased the oleic acid content to between 85% and 90%. Increases also have been achieved through conventional selective-breeding techniques. This higher oleic content increases cost efficiency in lubricant production, while maintaining the renewable property.
In a finished lubricant, additives normally constitute a sufficiently low fraction that their function can be the primary consideration, rather than their environmental behavior. Moreover, renewable base fluids can provide sufficiently greater metal-surface protection compared to mineral oil to permit lower levels of anti-wear and corrosion protection additives. Naturally, conventional lubricant additives are soluble in mineral-oil base fluids, but there are sometimes solubility issues with such additives in the newer synthetic and renewable base fluids.
Researchers continue to develop antioxidants that further stabilize renewable lubricant base fluids against decomposition under conditions of heat and agitation in the presence of air. Some synthetic polymers can improve the low-temperature fluidity of vegetable oils by retarding crystal formation.
Other additives further enhance corrosion protection for metal surfaces, stabilize against hydrolytic base-fluid decomposition and improve viscosity indexes to retard viscosity changes caused by temperature fluctuations. Moreover, other additives can be used to swell seals that might otherwise shrink and cause leakage.
Blending vegetable oils into other compatible lubricant base fluids can enhance overall biodegradability and reduce the system’s coefficient of friction. Reassembled esters can be blended into PAOs to improve lubricity, elastomer compatibility and environmental acceptability.
Although motor oils are the major segment of the lubricants market, vehicle engines present the most severe challenge to lubricants in terms of performance requirements, viscosity index, and stability against thermal and oxidative decomposition into sludge and deposits.
Cost question
While bio-based vehicle lubricant products aren’t yet sufficiently robust to be cost-effective, the technology to replace industrial mineral-oil lubricants with greener alternatives is available. It’s apparent, though, that the cost differential between mineral oil lubricants and alternatives remains a significant deterrent. Another consideration is the lack of widely documented and credible success stories about replacing mineral oil with biolubes. It’s possible that expanded programs of government subsidies to help defray the higher cost or higher taxes on mineral-oil products could provide a financial stimulus to help level the playing field.
Technological progress in the development of alternative base fluids and efficient additives can be expected to reduce overall costs, while the market price for crude oil, as well as perceptions about its long-term availability, remain an unknown in the replacement decision.
Although regulations and restrictions vary considerably among regions of the world, the global movement toward more environmentally-friendly materials such as biolubes appears to be irreversible. Only the rate and degree of this change remain uncertain.
Dr. Alan C. Eachus is an independent consultant in Villa Park, Ill. Contact him at [email protected] and (630) 632-2675.