Bio-lubricant from the olive mill waste through a new biocatalyst immobilized on ZnFe2O4

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1 Lubricant and Bio-lubricant 
Abstract 
 Lubricants are in fact one of the many products derived from the oil industry's processing cycles 
that, perhaps more than others, require a constant commitment to research and innovation. The need 
to respond to different needs and ensure adequate quality standards are the result of the commitment 
and effort, often little known, of many people and considerable resources. Lubricating oils are 
widely used in a wide range of sectors, industrial and non-industrial, ranging from heavy industry to 
food and healthcare and meeting many of our daily needs. However, we must never forget that it is 
a product with a strong environmental impact if not treated in the right way. In this regard, bio-
lubricants can be classified as fluids of a non-oil nature and with high biodegradability 
characteristics as well as products of vegetable origin or obtained by an esterification reaction 
between an alcohol and a fatty acid. 
Lubrication is the operation by which, through the interposition of a substance with suitable 
characteristics, friction and wear between surfaces in contact with solids in the relative movement 
are reduced. Friction and wear are distinct phenomena. Friction is a strong force resulting from the 
sum of the force required to break the adhesions between two bodies that tend to move one on top 
of the other; it is associated with energy consumption and heat production[1]. 
 
Figure 1 Lubricant. 
 
1.1 Lubricants
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Wear, on the other hand, is a process of progressive loss of material caused by the movement of 
surfaces. In general, their reduction is a common requirement of many applications and lubrication, 
in its various forms, is the way to achieve it. It is understandable how lubrication, due to the 
phenomena it involves, can have a significant impact on the dynamic behavior of machines and 
mechanical systems. If correctly implemented, it favors their good functioning, improving the 
overall performance, reducing the energy dissipated and reducing the wear and tear of the 
components. On the other hand, unsuitable or absent lubrication can lead in a short time to the 
decommissioning of a machine, with significant damage [2].  Well-Oriented lubrication is 
accompanied by significant economic benefits through energy savings, the conservation of 
materials and the reduction of maintenance interventions, with positive effects also on the 
environmental impact. A good lubricant has the following characteristics: high viscosity index, high 
boiling point, thermal stability, low freezing point, ability to prevent corrosion and high resistance 
to oxidation [3]. 
 
Lubricants can be classified according to the following criteria: 
Physical state: 
Ø Solid: The film of solid material is composed of an inorganic material or organic 
compounds such as graphite, molybdenum disulfide and cadmium disulfide. 
Ø Semi-solid: the liquid is suspended in a solid matrix of thickener or additives.  
Ø Liquid: examples are oils such as petroleum, vegetable, animal or synthetic oils 
Base oil: 
Ø Natural oils: from animal fats or vegetable oils 
Ø Refined oils: from crude or petroleum reserves such as paraffinic, naphthenic and 
aromatic oils 
Ø Synthetic oils: synthesized from reaction end products such as esters, silicones, and 
polyalphaolefins (PAOs). 
Applications: 
Classification of lubricants
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Ø Automotive oil: used for vehicles and transport industry. Examples are engine oils, 
transmission fluids, brake, and hydraulic fluids. 
Ø Industrial oils: used for industrial purposes. Examples are machine oils, compressor 
oils, metalworking fluids, and hydraulic oils. 
Ø Special oils: used for special purposes according to specific operations; examples are 
process oils, white oils and instrument oils. 
The first problem to be addressed when studying bio-lubricants is the lack of a clear definition of 
the term, with which it means:  
- lubricants of fossil origin with high biodegradable and non-toxic properties;  
- lubricants of vegetable origin 
Given this ambiguity, there is no precise data on European consumption of bio-lubricants of plant 
origin. The values usually used indicate 100,000 tonnes of consumption of bio-lubricants in Europe, 
less than 2% of the European lubricants market, without knowing the precise percentage of 
consumption of bio-lubricants of vegetable origin. When we think about lubricants today, the first 
thing that comes to mind is oil. Petroleum oil components continue to make up the majority of 
lubricants. Although oil-based lubricants possess many useful physical properties, they are also 
non-renewable and toxic to the environment. Rising oil prices, the depletion of the world's crude oil 
reserves and the demand to protect the environment from pollution caused by lubricating oils that 
contaminate water bodies, causing infections and significantly affecting the survival of the aquatic 
ecosystem, have renewed interest in developing and using alternative lubricants such as bio-
lubricating oils, which can be efficient and inexpensive substitutes for oil-based oils. Bio-lubricants 
can also be produced from synthetic esters and petroleum oils that meet the established criteria of 
biodegradability and toxicity. A lubricant is classified as biodegradable if its percentage degradation 
in a standard test exceeds a certain marked level. Bio-lubricants are generally considered to be 
lubricants with high biodegradability and low toxicity to humans and the environment [4]. 
 
Bio-lubricants offer several advantages over typical mineral oil-based lubricants. The most 
important parameter is viscosity; it measures the resistance of a liquid to the forces that tend to slide 
1.2 Bio-lubricants 
1.2.1 Properties of Bio-lubricants
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it and is, therefore, an index of internal friction. There are different types of viscosity: dynamic or 
absolute, kinematic, relative. Other important parameters are : 
• Fluidity: The term is sometimes used when rheological problems are prominent. Fluidity is 
the reciprocal of dynamic viscosity.  
• Viscosity Index: It is a practical number used to quantify the extent to which the viscosity 
of a lubricant is affected by temperature changes. The viscosity index is high when the 
viscosity varies little with temperature.  
• Density Or Density: It is an indicative parameter of the lubricant constitution; the density is 
low, for the same viscosity, for paraffinic products and higher for aromatic or cyclo-
paraffinic products; for the same type it increases with viscosity and decreases with 
increasing viscosity index.  
• Point Of Flammability: This is the temperature at which, as a result of the presence of 
flammable products present, a flame causes the surface of the lubricant to ignite.  
• Intorbidation Point: This is the temperature at which the lubricant begins to lose 
transparency due to the appearance of solid paraffin particles.  
• Scrolling Point: This is the temperature at which the lubricating oil still shows a certain 
smoothness under particular operating conditions.  
• Thermal Stability: It indicates the resistance of a lubricant to change due to temperature 
increases that cause large molecules to break into smaller fragments at high vapor pressure.  
• Specific Heat And Thermal Conductivity: These are important parameters when 
lubricants also have a cooling effect. Specific heat increases with temperature, while thermal 
conductivity decreases with increasing temperature and viscosity.  
 
Despite the advantages, the widespread use of bio-based lubricants is still limited today due to 
major performance challenges. In fact, they have poor thermo-oxidative properties that limit their 
use as a lubricant at high temperatures, the scale of production and the lack of encouragement from 
the authorities. Some bio-lubricants, particularly crude vegetable oils, have lower oxidative stability 
than standards and low-temperature characteristics. However, these deficiencies can be addressed 
by appropriate chemical modifications and the addition of appropriate additives. These 
disadvantages can be overcome by the esterification of vegetable oils with polyols and branched-
1.2.2 Disadvantages of using bio-lubricants
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chain alcohols. The presence of the hydrogen atom in the triglyceride leads to a partial 
fragmentation of the resulting molecule into an unsaturated compound that undergoes 
polymerization and the formation of precipitated particles. The reaction with polyol eliminates the 
presence of the hydrogen atom in the triglyceride position, thus increasing the stability of the bio-
lubricating base [5].  
 
If one thinks of lubricants today, the first thing that flashes in mind is petroleum oil. Petroleum oil 
components continue to form the major proportion of lubricants. The major source of almost all 
lubricants is the lube fraction which is obtained from crude petroleum [6,7]. A major portion of the 
lubricants consumed worldwide ends up in polluting the environment, many efforts are made to 
minimize spillages and evaporation. These high lubricant losses into the environment were behind 
the development of environmentally friendly bio-based lubricants [7]. As an alternative, we are 
considering using vegetable oils that have natural merits and demerits, they have outstanding 
physical properties which justifies them as lubricants but have a poor thermo-oxidative property 
which restricts them to be used as a lubricating agent at elevated temperatures [7]. Much research is 
being carried out to improve the thermo-oxidative property, so that they may compete as an 
economical alternative with petrol-based lubricants. Research conducted to date indicates that 
chemically and genetically modified vegetable oils have excellent potential to perform adequately 
as lubricants. Compared to petroleum-based lubricants, vegetable oils, in general, possess high flash 
point, high viscosity index, higher lubricity, low evaporative losses, and good metal adherence. The 
presence of a polar group with a long hydrocarbon chain makes vegetable oil amphiphilic surfactant 
by nature, allowing it to be used as a boundary lubricant [8]. The molecules have a strong affinity 
for and interact strongly with metal surfaces. The long hydrocarbon chain is oriented away from the 
metal surface to form a monomolecular layer with excellent boundary lubrication properties 
[9,10,11]. A number of plant-based lubricants have been developed for various sectors of industry. 
Various routes of chemical modification of vegetable oils have been developed with an aim to 
prepare a perfect biodegradable lubricant. Chemical modification of vegetable oil enhances its 
thermal as well as oxidation stability, which helps them to withstand within wide operating 
conditions. The methods for the development of vegetable oil-based lubricants are as follows. Low 
oxidative stability and poor low-temperature properties are the factors limiting the full potential of 
bio-based lubricants, especially the ones originating from vegetable oils. There are various methods 
1.2.3 Production of a bio-lubricant
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that can be implemented in order to enhance these unwanted properties, such as the addition of 
additives (antioxidants and pour point depressant), emulsification, and chemical modification. 
Between these methods, chemical modification has the potential to enhance the physical properties 
and oxidation stability of vegetable oils for a wide range of temperatures. Double bonds present on 
the unsaturated FA alkyl chains are reactive and susceptible to atmospheric oxygen. Therefore, 
chemical modification techniques are mainly focusing on these double bonds. Figure 3 shows the 
general pathways of some common chemical modification methods which are transesterification, 
estoile formation, epoxidation along the successive ring-opening and acetylation reaction. A 
comprehensive review of these chemical modifications are not provided in this study due to the 
presence of several published review focusing on chemical modification techniques for bio 
lubricant development. Research trends regarding chemical modifications nowadays are focusing 
on increasing the efficiency of reaction in terms of higher yield rate and lower overall cost. These 
include catalyst development and the utilization of waste materials such as waste cooking oil and 
palm oil mill effluent. Utilization of biodiesel as a bio-based lubricant component received a 
growing interest as it opens the opportunity for biodiesel manufacturers in extending their 
production supply and the potential sales markets [12].  
 
The sustainable production of vegetable oil is the subject of heated debate around the world because 
first-generation mineral oil, obtained mainly from petroleum products, has been in use for over a 
century. 
Besides being limited in reserves, it has many drawbacks such as non-biodegradability (the level of 
biodegradability for mineral oil is not higher than 30%), the relatively low flash point, the non-
renewability; these factors are the cause of continuous environmental pollution. However, industrial 
progress has led to a huge increase in the demand for mineral oils in recent decades; to comply with 
this, it is increasingly essential to find a viable alternative to mineral oils. Research in this direction, 
which began in the mid-1990s, has focused on the use of vegetable oils. 
Vegetable oils, widely available in nature as a natural resource, have properties such as to be used 
as lubricants: in fact, they show characteristics far superior to those shown by mineral oils such as 
[1;2]: 
1.3 Vegetable oils