Bio-based (edible) oils: feedstock for lubricants of the future
By Raj Shah, Nathan Aragon, and John Calderon
- The global market for bio-lubricants is expected to grow by 20% in the next five years.
- Recent growth in the healthcare and food-processing industries demonstrates a need for lubricants that are less toxic and more biodegradable.
- Research aimed at enhancing the performance of current bio-lubricants will be key to meeting this need.
There has recently been a shift in research toward improving bio-lubricants that use base oils made from biodegradable feedstocks. Vegetable oils are attractive base oils for bio-lubricants because they are mostly biodegradable and are made from edible feedstocks. However, vegetable oil-based lubricants are not capable of completely replacing the more standard petroleum-based lubricants because vegetable oils lack the physical properties (oxidative stability, thermal stability, and viscosity range) that give petroleum-based lubricants their high performance. The physical properties of bio-lubricants can be enhanced using additives, blending, or modification by chemical means, but such enhancements can increase the total cost and toxicity, and decrease biodegradability . Due to enhanced environmental guidelines and regulations in the food- processing industry, the global market for bio-lubricants is expected to grow from $2 billion to $2.4 billion in the next five years . The estimated growth is seen in Figure 1. For that to occur, there is a wide gap that must be bridged between the petroleum-based lubricants and bio-lubricants, and this seems to be where the bulk of the research will go. Recent growth in the healthcare and food processing industries demonstrates a need for lubricants that are less toxic and more biodegradable, so research aimed toward enhancing current bio-lubricants will be key in the near future .
The term “vegetable oil” was introduced previously, but this is a broad term that encompasses several oils made from different feedstocks and used for different applications. Canola oil is used to make food-grade lubricants, hydraulic oils, and metal working fluids. Coconut oil is mainly used in gas engines. Palm oil is used in the steel industry and is used to make greases. Rapeseed oil is used in farm equipment and in the fabrication of biodegradable greases. Soybean oil has a wide variety of uses, including fabrication of soaps, shampoos, detergents, and pesticides, and acts as an ingredient in various lubricants, biodiesel fuel, and hydraulic oil. Sunflower oil is used for greases and diesel fuel . As such, the versatility of vegetable oils spans a multitude of industries, and many express key interests in utilizing vegetable oils as bio-lubricants—especially industries that are environmentally aware and face strict environmental regulations.
Key industries that apply bio-lubricants include the marine, food processing, agricultural, and pharmaceutical industries, as well as underground mining. Other industries that show a large interest are commercial transportation and the automotive industry . This is due to the biodegradability and low toxicity of bio-lubricants in addition to the benefit of lower emissions from using them . However, there are some factors to consider when comparing vegetable oils and mineral, or petroleum-based, oils. Mineral oils tend to have better stability with hydrolysis and oxidation than vegetable oils. They also tend to have superior low-temperature performance . For example, one study found the pour points of soybean oil and 90% sunflower oil to be -9°C and -12°C, respectively, while the pour point of mineral oil was -21°C. The same study also found the percentage of micro-oxidation after 30 minutes of soybean oil and 90% sunflower oil to be 48% and 13%, respectively, while the percentage of micro-oxidation of mineral oil after the same time was found to be 5% . The extant of micro-oxidation is an important factor, because any oil that undergoes more oxidation will be less stable.
Economic and social concerns about the effects of large-scale usage of edible oils on seasonal-dependent food markets are a major concern as well. The food markets are examples of environments where non-toxic and biodegradable lubricants are required. Research and development of improved edible oil-based bio-lubricants is driven by these sensitive environments and the push for safe and sustainable lubricants. Consequently, many industries are poised to benefit from such research.
Depending on the industry, the production and use of lubricants must follow specific standards for performance and environmental regulations established by numerous international standards organizations and environmental protection regulators. ASTM issues international standards for lubricants through their D02 Committee . Other institutions that set standards for lubricants and the petroleum industry include SAE International, the American Petroleum Institute (API), the International Lubricant Standardization and Approval Committee (ILSAC), and the International Organization for Standardization (ISO) . The US Environmental Protection Agency (EPA) lists different categories of environmentally acceptable lubricants, which include vegetable oils, synthetic esters, polyalkylene glycols, and water . The US Department of Agriculture (USDA) and the National Sanitation Foundation (NSF) also have food grades specifically for lubricants categorized as H1–H3, with each category representing the lubricant’s acceptable exposure levels for food as shown in Table 1 . The H1 category is for lubricants that may come into direct contact with food, the maximum limit of lubricant in food being 10 ppm. The H2 category applies to lubricants that are used in equipment where there is no possibility of direct contact with food, while the H3 category is for lubricants used for cleaning and preventing rust on equipment .
|Description and Use|
Although performance and environmental/toxicity specifications for food-grade lubricants are well defined and heavily regulated, what specifically constitutes a “bio-lubricant” may not be defined and can change depending on the industry and country . Some organizations may consider only biodegradability as a criterion, while others may also incorporate renewable feedstocks as a requirement. Consequently, biodegradable synthetic-based lubricants may be classified as “bio-lubricants.” According to the EPA’s designation of Environmentally Acceptable Lubricants (EALs) used to determine environmental limits for lubricants used in marine environments, at least 90% of the formulation must be readily biodegradable for oil lubricants and 75% for grease lubricants, with the rest being either inherently biodegradable or non-biodegradable but non-bio accumulative . The EU’s Ecolabel has similar requirements but separates lubricants into the following classes: Total Loss Lubricant (TLL), Partial Loss Lubricant (PLL), and Accidental Loss Lubricant (ALL). Each class represents lubricants based on their application and have different biodegradable specifications/limits as summarized in Table 2 . As such, bio-lubricants and bio-lubricant manufacturers must have these specifications in mind when forming bio-lubricant compositions alongside general lubricant specifications, such as ISO/SAE grades.
|ALL||PLL||TLL||Greases (ALL, PLL, TLL)|
|Readily aerobically biodegradable||>90||>75||>95||>80|
|Inherently aerobically biodegradable||≤10||≤25||≤5||≤20|
|Non-biodegradable and non-bioaccumulative||≤5||≤20||≤5||≤20|
|Non-biodegradable and bioaccumulative||≤0,1||≤0,1||≤0,1||≤0,1|
Advancements in bio-lubricant research
As mentioned before, it is well known that bio-lubricants, in comparison to mineral oils, show a large degree of biodegradability, a low amount of toxicity, high flash points, high viscosity indices, and better adherence to metal, but suffer from low pour points and oxidative stability. Table 3 compares the viscosities and pour points of many common mineral oil-based lubricants with numerous edible and non-edible natural oil-based bio-lubricants. Higher grades of petroleum lubricants possess significantly lower pour points down to -54°C, whereas the lowest pour points of the bio-lubricants only reached -27°C, indicating reduced low temperature performance. Oxidative stabilities of a few petroleum lubricants were orders of magnitude better than those of the bio-lubricants as well .
|Lubricant||Viscosity 40°C (cSt)||Viscosity 100°C (cSt)||Viscosity Index1||Pour Point (°C)||Flash Point (°C)||Oxidative Stability (min)||Coecient of Friction||Wear (mm)|
|Passion fruit oil||31.78||–||–||–6||228||7.5||–||–|
|Rice bran oil||40.6||8.7||169||–13||318||–||0.073||0.585|
|1Viscosity index: (<35) low, (35–80) medium, (80–110) high, (>110) very high.|
To take advantage of the high viscosities, biodegradability, and flash points, much of the research done so far has attempted to improve the weaknesses of the vegetable oil base, i.e., the thermal and oxidative stabilities through chemical modification or additives. The inherent unsaturation found in plant oils makes them perform poorly in extreme temperature conditions, so chemical modification, such as epoxidation of the double bonds, has been used to overcome this. Epoxides are generated by forming a peroxy acid and then reacting the peroxy acid with the carbon-carbon double bond. The epoxidation has been shown to increase resistance to heat in vegetable oil-based polymer PVC plasticizers/additives, allowing for high-temperature applications . Further esterification of the epoxides by a ring-opening reaction using fatty acids and p-toluenesulfonic acid (PTSA) has been shown to improve low-temperature activity due to long mid-chain esters disrupting macrocrystallization at low temperatures. Some fatty acids used in this reaction include octanoic, non-anoic, lauric, and myristic acids.
Transesterification has also been used to synthesize biodegradable organic polyesters using plant oils and molecules such as trimethylolpropane (TMP) and pentaerythritol (PE). Biodegradable TMP was formed from rapeseed oil fatty acids using sodium methylate as a catalyst. Transesterification of TMP with esters from palm oil was done using sodium methoxide as a catalyst. Polyesters made from PE and erucic acid were formed using p-PTSA in xylenes as a catalyst. All these polyesters have displayed improved low-temperature properties and oxidative stabilities .
Various antioxidants have been used as additives, but more research is needed to find antioxidants with lower toxicity. Antioxidants, such as vitamin C, vitamin E, esters from gallic acid, and derivatives of citric acid, are all naturally occurring and can serve as possible replacements to more toxic antioxidants, but more investigation is needed on the effectiveness of these replacements . Other additives, such as plant-derived cysteine Schiff base esters, have shown excellent anticorrosion, antiwear, and antifriction properties, while non-toxic inorganic nanoparticle additives, such as ZnO and CuO, provide similar benefits in regards to antiwear and friction performance and are already used in many petroleum based lubricant formulations .
Regarding the commercial application of bio-lubricants, many current advancements have attempted to bridge the wide gap between the bio-lubricants and mineral oils. An American company named Novvi LLC has developed a cycloalkane base oil using lignocellulose derived from sugar cane that is completely renewable and meets the H1 certification from the NSF . Biomass-based lubricants made from the condensation of ketones were synthesized that have good pour points, but they still need improvement with respect to the viscosity index values . Researchers at the Catalysis Center for Energy Innovation (CCEI) at the University of Delaware have synthesized renewable base oils from biomass and natural oils that have specialized functional groups and molecular architectures. Their pour points and viscosity indices are comparable to commercially available base oils, but their extent of biodegradation needs to be analyzed . A recent collaboration between Neste and AT-Tuote claims to have the world’s first 100% bio-based industrial lubricant. The lubricant uses Neste’s MY Renewable IsoalkaneTM as a key component and can comply with the OECD 31 test for biodegradability . Similar commercial investments will only continue to grow as bio-lubricants improve in performance and research and development and environmental regulations grow stricter.
More research needed
Significant progress has been made with regards to the performance and commercial investment of bio-lubricants. Although petroleum-based lubricants are still preferred for low-temperature performance and oxidative stability, numerous methods have been explored and developed to allow bio-lubricants to compete with them. Chemical modification of plant oil-derived fatty acids by saturating the carbon-carbon double bonds through epoxidation and esterification has shown to be effective in the improvement of extreme temperature properties and oxidative stabilities of bio-lubricants. Possible replacements for antioxidant additives used in bio-lubricants that are more natural and less toxic have been proposed, which can preserve the enhancement of oxidative stability. More focus has also been placed on renewable sources of base oils, and this has brought base oils made from sugar cane and other types of biomass.
For the future, more research needs to be done to determine the actual biodegradability of new renewable base oils and on the cost-effectiveness of the currently known chemical modifications. There is also much consideration regarding the use of bio-lubricants in the automotive industry but for this to move forward, further studies on the tribological performance of bio-lubricants need to be completed . While the greater financial cost of bio-lubricants is a concern, newer bio-lubricant formulations will be safe to use in industries with many environmental regulations and can be applied to total-loss systems without harming the environment. Continued commercial investment and advances in bio-lubricant performance and lower-cost synthesis methods will enable wider bio-lubricant applications and contribute to a more sustainable and clean future.
Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 years. With a PhD in Chemical Engineering from Penn State University and a Fellow from the Chartered Management Institute, London, Raj has been an active member of AOCS for the last 2 decades. An Adjunct Professor at State University of New York, Stony Brook, Shah has been elected a Fellow by his peers at STLE, AIC, NLGI, INSTMC, CMI, IChem E, The Energy Institute, and The Royal Society of Chemistry. He is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute, A Chartered Chemist with the Royal Society, and a Chartered Engineer with the Engineering council, UK. He has over 225 publications and is a co-editor of various books related to lubricants. He can be reached at firstname.lastname@example.org. Read more about Dr. Shah in this article from Petro Industry News.
John Calderon and Nathan Aragon are chemical engineering students at Stony Brook University, where Shah is the Chair of the external advisory board of directors. Both students are also part of a thriving internship program at Koehler instrument Company in Holtsville, New York, USA.
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