Biodegradable Lubricants

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The existence of synthetic lubricants ways days back to 1877 when Friedel and Crafts synthesized hydrocarbons using aluminium trichloride as catalysts. Between the 1930s and early 1940s, organizations such as Zurich Aviation Congress, United States Naval Research Laboratories, etc., started to develop ester-based lubricant technologies. In the mid-70s, the lubricant giants mainly focused on synthetic lubricants; however, with the increase in industrial production, the usage of synthetic lubricants also increased, and so are their demands. In recent years, the effects of lubricants on the environment have been in focus. The breakdown products of the synthetic lubricants affect water, air and soil. Thus, using environmentally friendly lubricants in industrial applications is the need of the hour.

Present significant lubricant base stocks available:

  1. Highly unsaturated or high oleic vegetable oils (HOVOs): Primarily recommended for applications exposed to an environment where toxicity is of primary concern, such as sawmill blades, ambient temperature chain drives, small gearboxes etc.
  2. Low viscosity polyalphaolefins (PAOs): Finds their application in several industrial applications, particularly the gearboxes, due to their excellent tribological properties.
  3. Polyalkylene glycols (PAGs): Suitable for industrial gearboxes
  4. Dibasic acid esters (DEs): Recommended for compressors and turbines
  5. Polyol esters (PEs)

What are biodegradable lubricants?

According to sources [1], biodegradable lubricants can be categorized into ready bio degradable (60% biodegradable in 28 days) and inherently biodegradable (biodegradable 20% within 28 days). Biodegradable lubricants must meet ISO 9439 or OECD 301B standards. These tests consist of treating a lubricant sample in the presence of oxygen with microorganisms and measuring the amount of CO2 they produce. Petroleum-based lubricants naturally break down between 15-35% over 28 days, but not enough to reach the required 60%. So, petroleum-based lubricants are called inherently biodegradable lubricants.

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To be a proper biodegradable lubricant, the lubricant must also be “low toxic.” Many tests can determine this. These tests include fish, daphnia and other organisms. Mineral oil and vegetable oils are safe in their purest form; however, they can be toxic because lubricants are not just pure oil. The toxicity of lubricants increases as additives are added to formulations.

Due to strict regulations from the government and increasing awareness of using environmentally friendly lubricants, lubricant manufacturers are facing challenges in meeting the demands of industries. However, it is still yet in the discussion whether the concept of biodegradable lubricants is still realistic and cost-effective. In Western Europe, formulations containing 50:50 (ester: PAO) are used for gasoline engine oils [2].

Vegetable oil-based lubricants are being introduced due to their ease of availability and non-toxicity. Genetically engineered vegetable oils are also being used as lubricant base stock. Researchers have reported the superior tribological properties of various vegetable oils [3] over conventional lubricants. Rapeseed oil, canola oil, castor oil, sunflower oil, etc., are among the most commonly reported oils with excellent tribological properties. Diesters are technically a good choice for biodegradable lubricant, but they are expensive; hence, synthetic esters and vegetable oils are used. It has been seen that a combination of synthetic ester and lithium calcium thickener has shown better results in terms of various tribological properties, including corrosion resistance [4]. Vegetable oils are not expensive, but due to their poor oxidation properties, they are not being used in large quantities for industrial applications; however, genetically modified vegetable oils are now being used as base stocks.

Problems faced by the manufacturers in producing the biodegradable lubricants:

Concern about the environment is now the primary focus for all Governments; hence, equipment manufacturers are looking forward to bio-degradable and environment-friendly solutions. But it is not easy for the manufacturers to produce biodegradable lubricants that meet similar standards as the non-biodegradable industrial lubricants. Several researchers are working towards this common goal of achieving environment-friendly lubricants with comparable properties to those of the present industrial lubricants.

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Though research on vegetable oils has shown promising results, but all vegetable oils can not be used for industrial applications. Though it is difficult to predict how much mineral oil and synthetic oils will be replaced with genetically modified vegetable oil-based lubricants, at least 30% replacement might be possible within the next few years [2]. Research on rapeseed oil, castor oil, and palm oils has shown promising results [3]. However, the problem of the thermo-oxidative stability of vegetable oils still needs to be focused on.

Low oxidative stability is a sign that the oil will oxidize if not treated. Oils can become thicker and have a more plastic-like consistency. This problem can be addressed by chemical modification of vegetable oils or the use of antioxidants. However, it will increase the price.

Chemical modification can involve partial hydrogenation and the shifting of fatty acids. Hydrogenation can be difficult because it is hard to know when the process should stop. The oil’s required liquidity and pour point will determine the optimal hydrogenation. Biotechnology has made it possible to create genetically modified oil seeds that are stable and non-toxic.

The other challenge faced by the manufacturers is the source of vegetable oils. Due to varied climate conditions, the nature of the vegetable oils also changes; hence, getting a uniform composition with similar physico-chemical properties may not always be possible in several situations.

Several manufacturers blend PAO with diesters, making them biodegradable to a large extent. These blends also exhibit good solubility, lesser oxidation properties and a good temperature–viscosity relationship. Also, the blend of canola oil with diesters is common. The choice of the blend depends on the type of application and recyclability.

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Applications of biodegradable lubricants:

Even with the challenges, the lubricant manufacturers and researchers have identified several applications where products derived from vegetable oils are being used: Transmission hydraulic fluid, Industrial hydraulic fluids for process and machinery, Metalworking oils and coolants, Food grade coolants, Chainsaw bar oil, Gear lubricants, Greases [2]. Biodegradable greases (based on lithium calcium thickener) are also available for applications where metal to metal contact is present such as agriculture and forestry types of equipment, construction and earthmoving equipment, mining and conveying equipment, water treatment and irrigation, reservoirs, bridges, etc. [4,5].
Reported results [2] on the usage of vegetable oils, particularly soybean oil, can be seen under the names BioSOYTM, a patented electrical transformer fluid named BioTRANSTM, chainsaw bar oil called SoyLINKTM, a rail curve lubricant called SoyTrakTM, and Soy TRUCKTM, a semi-truck fifth-wheel grease has been reported by Honary.

Thus, it can be seen that biodegradable lubricants will gradually pave their way in various industrial applications, which will ultimately help in reducing the harmful effects of industrial lubricants on the environment.

References:

1.www.global.mobil.com/en/lubricant-expertise/resources/understanding-biodegradable-lubricants

2. Ponnekanti Nagendramma, Savita Kaul, 2012, Development of ecofriendly/biodegradable lubricants: An overview, Renewable and Sustainable Energy Reviews, 16 (1), 764-774.

3.Shubrajit Bhaumik, M. Kamaraj, 2021, Artificial neural network and multi-criterion decision making approach of designing a blend of biodegradable lubricants and investigating its tribological properties, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 235 (8), 1575-1589.

4. www.machinerylubrication.com/Read/577/biodegradable-grease-bearing

5. www.skf.com/in/products/lubrication-management/lubricants/biodegradable

About the Author:

Dr.Shubrajit Bhaumik has been working in the area of tribology for past 15 years, He has published several scientific articles in reputed journals of tribology and has also a patent to his credit. He can be reached at shubrajit.projects@gmail.com.

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The article is Published by the Editorial Team of Industrialubricants.com. Ms. Riya Veluri is one of the editorial team member & after her graduation, Riya works as a website developer & SEO specialist in Lubrication & Tribology Industry & writes technical articles on Lubricants, Lubrication, Reliability & sustainability.

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