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Biolubricant

Danyang Cao1, Leonidas Matsakas2, Jie Zhang1, Lisong Dong1, Yijun Shi3, Jiahua Zhu1, Xin Feng1, Xiaohua Lu1 and Liwen Mu1,3*

1 State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, PR China

2 Biochemical Process Engineering, Division of Chemical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, Luleå, Sweden

3 Division of Machine Elements, Luleå University of Technology, Luleå, Sweden

Abstract

This chapter introduces the current situation of the global lubricant market and the potential application of bio-lubricants in future. The sources, properties and applications of vegetable oil and waste edible oil as the lubricant base oil were summarized. In addition, the prospect of bio-refining base oil from biomass was put forward. The upgrading process of vegetable oil, such as hydrogenation, epoxidation, esterification, transesterification and isomerization, were reviewed. In this chapter, some research achievements on the development of environmental-friendly additives through organic conversion of biological materials are reviewed. Finally, the current strategic goals of bio-lubricant are prospected: (1) to reduce pollution caused by lubricating industry, (2) to decrease the safety risk of using or producing dangerous lubricating oil, (3) to improve the utilization rate of vegetable oil raw materials through more effective conversion technologies and reaction routes, and (4) to give priority to using abundant carbon neutral biomass as raw materials.

Keywords: Biolubricant, biodegradable, non-toxic, vegetable oil, biomass, additives, tribological performance

1.1 Introduction

Statistics show that friction consumes one-third of the world's primary energy, wear causes about 80% of machine parts to fail, and more than 50% of machine equipment accidents are caused by lubrication failure and excessive wear [1]. The resulting waste of resources has become an important issue for destroying the environment. Since the last century, with the significant increase in transportation and various industrial activities, non-renewable petroleum resources have been introduced into industrial activities as a lubricant (Figure 1.1(a)), thereby minimizing the friction between the surfaces in contact with each other. The lubricants ultimately provide smooth running, as well as expand the life of the machine component by reducing the heat generated when the surfaces have relative motion between them [2].

Generally, researchers think that lubricants are only used to make the parts 'slippery', but in real life, lubricants play a very important role because they [3]:

• Reduce the wear of parts with relative motion
• Reduce the friction between rotating and stationary parts

  Figure 1.1 (a) Global lubricant market in terms of application; (b) CAGR of global lubricant market in terms of type and region; (c) Global lubricant market in terms of raw materials type [4]. Copyright © 2021, the royal society of chemistry, reprinted with permission.

• Absorb shock
• Minimize operating temperature
• Prevent the corrosion of metal surfaces
• Prevent contamination of the system.

However, about 50% of all lubricants in the world are scattered in the environment due to their volatility, spills, accidents, or application losses. More than 95% of these lubricants are based on mineral oil. These lubricants are now a considerable threat to the environment as well as human health because of their low biodegradability and high eco-toxicity [5].

Depending on the contact time, exposure to mineral oil-based lubricants can cause a series of skin problems, such as itching, skin discoloration, eczema, and skin thickening. In addition, as the exposure time increases, it can also cause respiratory diseases such as bronchitis, bronchopneumonia, asthma, emphysema, and tuberculosis [5]. Besides, the currently used lubricants can pose a carcinogenic risk to humans. Polycyclic aromatic hydrocarbons (PAHs) which have 3-7 rings, such as benz-a-anthracene, benz-a-pyrene, and chrysene, are the main components causing this carcinogenic effect. During the combustion cycle of the engine, toxic chemicals such as PAHs are formed [6].

The mineral oil-based lubricants are very hazardous for plants and animals as well as groundwater bodies because of their low biodegradability [7]. The lubricants that are produced from crude oil are also significantly dangerous for aquatic ecosystems. Water containing as low as one part per million of lubricant is considered to be polluted [8]. Additionally, the lubricant film on water causes a severe obstacle for exchanging oxygen between water and the atmosphere. It also reduces the amount of light that passes through the water. For these reasons, the photosynthesis process of aquatic plants will be reduced and the temperature of the water will be increased by absorbing solar radiation. These limitations cause a critical disruption in the metabolic system of the aquatic organisms [9].

In recent years, more and more attention has been paid to biolubricants. In order to protect the environment from serious damage and reduce economic losses, the use of biolubricants is increasing. Biolubricants are composed of components which are biodegradable and non-toxic to humans as well as other living organisms. Rapid biodegradability indicates that these substances are eco-friendly [10, 11]. Biolubricants can be classified into two main categories based on their chemical composition: natural oils and synthetic oils. The main ingredients of natural oils are vegetable oils or animal fats. On the other hand, researchers use various natural oils as starting substances to form synthetic oils and they show better thermo-oxidative stability, wear resistance, and lubrication properties than mineral-based lubricants, which can be used as advanced biolubricants [12, 13].

As people's awareness of environmental protection has become stronger, plant-based lubricants have gradually received widespread attention. Plant-based lubricating oil is synthesized by the internal catalytic recombination of plant raw materials, which is similar to light crude oil. Compared with traditional mineral-based lubricants, plant-based lubricants have unique properties such as environmental performance, excellent lubricity, and higher viscosity index and anti-corrosion performance due to their chemical structure [14]. Moreover, they have a higher flash point that makes them suitable for high-temperature uses (over 300°C) [15]. Therefore, the development of bio-lubricants can also alleviate the fossil energy crisis. More importantly, bio-lubricants are biodegradable with little to zero toxic properties. Furthermore, their lower volatility resulted in decreased exhaust emissions [16]. Biolubricants have their own negative side. They have poor oxidative stability, which means the lubricants will oxidize quickly during the application process if untreated and become thick and polymerizing to a plastic-like consistency [16]. The free acid generated due to oxidation may increase corrosion. Chemical modification may lessen this problem but it will increase the cost [17].

It now appears that biolubricants are mainly used in total loss and high-risk industrial applications, such as railway grease and hydraulic oil. According to market research company Kline & Co, the compound annual growth rate of biolubricants is about 5%. From this, it can be seen that biobased lubricants are showing a steady growth momentum and the market potential is huge.

1.2 Biolubricant Base Oil

Lubricating base oils are mainly divided into three categories: mineral base oils, synthetic base oils, and vegetable oil base oils [8]. A large amount of mineral base oils are widely used. However, in some applications, synthetic base oils from biomass and vegetable base oils must be used for blending products, so these two base oils have developed rapidly.

Base oil accounts for more than 70% of lubricant. The degradability of base oil determines the biodegradability of green lubricants. At present, degradable lubricant base oils mainly include synthetic oils and vegetable oils [8]. Vegetable oil is a component of natural organic long hydrocarbon chains containing polar groups, which make vegetable oil essentially amphiphilic surfactant and can be used as boundary lubricants [14]. Polar molecules have strong affinity and interaction with metal surfaces and long hydrocarbon chains are oriented away from the metal surface to form a monomolecular layer with excellent boundary lubricating properties. Therefore, the application of vegetable oil and synthetic base oils from biomass to make it a green lubricant base oil has become the main trend of environmental protection lubrication.

1.2.1 Edible and Non-Edible Oils

Originally, vegetable and animal oil were used as lubricants for machinery and transportation [16]. At present, with the depletion of petroleum resources and people's demand for green lubrication, bio-oil is proposed as a lubricant base oil again. However, due to the complex source of biooil [8], poor oxidation stability, and low-temperature stability, it is now only used for niche...