Bio-lubricants are generally defined as being both biodegradable and produced from renewable sources. However, biodegradable mineral oil lubricants, produced from petroleum, are sometimes classed as bio-lubricants. Bio-lubricants are used to lubricate machinery in just the same way as other lubricants but are safer and more sustainable. They of particular interest in the food and drinks industry, as well as the production of medicines, cosmetics and animal feed. These food-grade lubricants require demonstrable safety and purity.
Bio-lubricants are also important in applications with a significant risk of environmental contamination. Examples include hydraulic fluids for agricultural and construction machinery, lubricants for chainsaws, mould release oils used in concrete construction, and greases for chains in dams and canals.
Globally, over 37 million tonnes of lubricant are used annually. Most of these are based on environmentally-damaging mineral oil. Thirty percent of the lubricant used finds its way into the environment, causing significant harm to ecosystems. This is leading to interest in extending the application of bio-lubricants to a much wider range of applications.
Many countries are now introducing legislation to regulate the use of mineral oil in environmentally sensitive areas. For example, the EU Dangerous Substances Directive includes aquatic toxicity, biodegradability and bioaccumulation in its criteria for hazard potential. An executive order in the USA (EQ 12873) encourages the use of biodegradable oils, while Austria has banned mineral oils for chainsaws. Researchers at Fuchs have determined that 90% of all lubricants could be made to be rapidly biodegradable (https://is.gd/pivuge).
VEGETABLE OIL
Simple vegetable oils have been used as lubricants since antiquity. They continued to be the main lubricant until the 19th century. As the rate of industrialisation accelerated, mineral oils provided a low-cost source to meet the increasing demand for lubricants. Vegetable oils remain much more expensive than petroleum-derived mineral oil, although competitive with synthetic oil.
Vegetable oils adsorb to metallic surfaces, forming a durable monolayer in which polar heads adhere to the surface while the hydrocarbon chains orientate themselves perpendicular to the surface. This makes them extremely effective lubricants. Vegetable oils are also rapidly biodegradable, but they have poor thermal, oxidative and hydrolytic stability. In other words, their properties change when exposed to extreme temperatures, or when exposed to the air or humidity over a period of time. Other issues with pure vegetable oils include increased wear at high temperatures, poor cold flow, and a limited range of viscosities.
Pure unmodified vegetable oils continue to be used for some lubrication applications. Rapeseed oil and sunflower oil are esters of glycerine and triglycerides (three long-chain fatty acids attached by the ester groups). Almost all vegetable oils have the same glycerine heads, but the fatty acids are plant-specific, differing in chain length and number of double bonds. Other vegetable oils that are used as lubricants include cotton seed, sesame, coconut, date palm, castor, jatropha, and soybean.
Although pure vegetable oils can have attractive properties, they typically cannot match the full performance profile required in demanding applications. For example, cotton seed oils have been trialled in engines, where they have a lower coefficient of friction than SAE 20W50 and SAE 20W40. Despite the superior friction performance, the cotton seed oil resulted in increased engine wear.
“For automotive applications, any fluids including additives and base oils need to go through an approvals process involving self-certification to demonstrate they meet OEM standards. There are some companies actively seeking certification in joint projects with OEMs, but so far none have achieved this. The issue is stability at different temperatures and pressures,” says David Wright, director general, UK Lubricants Association.
VEGETABLE-OIL BASED LUBRICANTS
Vegetable products and modified vegetable oil esters can also be used to produce more sophisticated bio-lubricants. Additives, chemical modification and thermal modifications can all be used to enhance lubricant performance. They may also be blended with mineral oils.
Wright observes: “We’re seeing bio-lubricants being used in non-automotive applications – some industrial applications, metalworking applications and some hydraulics. This is driven by environmental concerns, where the users can access similar performance characteristics. Some projects have European Ecolabel or German Blue Angel certification. Some organisations in the public sector now require this type of environmental certification, especially in Germany.
“There is a relatively small cost premium for bio-lubricants of 10-15%, which is largely because they are currently produced in smaller quantities. It’s also important to consider that these oils still need to be carefully disposed of, just as you would with petrochemical lubricants,” Wright adds.
LAND USE COMPETITION
As with biofuels, a major issue with bio-lubricants is competition with food production. More vegetable oil used to produce lubricant means less for food. We are already seeing high levels of deforestation resulting from land-use pressure, with forests being cleared to rear animals and plant crops. Bio-fuels could increase this pressure and result in even more deforestation, if this happens they could do more harm than good. There are many different land uses that will need to be balanced as the global economy becomes more sustainable. These include food production, carbon sequestration, energy crops, production of raw materials and habitat preservation.
“One concern with bio-lubricants is that many use corn as a base, which means that food is being taken out of the food supply system,” contends Wright.
FOOD GRADE LUBRICANTS
Food grade lubricants are essential in industries such as food and drinks processing, pharmaceuticals and cosmetics. They may be formulated using high-purity white mineral oils, synthetic base oils, or vegetable oils.
Bio-lubricants can meet the food grade requirements while offering additional sustainability. As the EU does not have specific guidance for food-grade lubricants, the US Food and Drugs Administration (FDA) classifications are therefore normally used. This defines three classes:
- H1 lubricants may be used in applications where incidental food contact may potentially occur. Incidental contact means only a trace amount of lubricant may be present in food: it must not exceed 10 parts per million. Typically only H1 lubricants are considered “food-grade”.
- H2 lubricants may be used in machinery within food-processing facilities that will not come into contact with food. Many standard lubricants meet the H2 requirements. They must not contain carcinogens, mutagens, teratogens [causing birth defects], mineral acids or intentionally include heavy metals such as antimony, arsenic, cadmium, lead, mercury or selenium.
- H3 lubricants can be used as additives within foods: they may only contain edible oils. They are used to clean and prevent rust on hooks, trolleys and other equipment that comes into direct contact with food.
HYDRAULIC OILS
Hydraulic oils are an important application for bio oils as they often involve significant loss of lubricant to the environment. Examples include construction, forestry and agriculture. Major suppliers such as Crown Oil and Panolin now produce vegetable-based hydraulic oil which is not toxic to plants and provides high viscosity.
In conclusion, bio lubricants can offer environmental advantages with lower toxicity and no dependence on petroleum. However, they are currently only suitable for certain applications, they still require careful disposal, and there are issues with increasing pressure on food production.