Production and Performance Evaluation of Shea Butter-Based Lubricating Grease

. The overwhelming cost of conventional lubricants has instigated the need for alternatives in the engineering industry. This study identified the physicochemical properties of Shea butter made from the shea nut and their application as a bio-grease in the food processing and industrial sectors. Some preliminary tests were conducted to ascertain the combination of materials that will produce grease with the right consistency as the commercial product. Results on its physicochemical properties showed that shea butter was suitable for use as lube base oil. Best bio-grease was obtained with mass combinations of 80.0 g of Shea butter, 0.5 g of resin, 1.0 g of P.K.O, 1.5 g of calcium hydroxide, 8 g of stearic acid, 1.5 g of silicate, 1.5 g of sodium carbonate, 4 g of biochar, and 2 g of cellulose. The physicochemical properties observed were evaporation loss (1.2%), ash content (4%), moisture content (0%), flash point (146.5℃), fire point (155℃), pour point (25.9℃), and density (0.97 g/cm3). Most of the properties of the bio-grease were found to be comparable with conventional grease. In addition, since the bio-grease was produced from majorly edible sources and based on its properties, it could be classified as H3 food-grade grease.


Introduction
Most dynamic machine operations run on bearings, requiring grease for lubrication to reduce or eliminate friction [1].These machines' effective and efficient operation requires sufficient lubricants in their moving parts [2].Lubricating greases are semi-solid colloidal dispersion of a thickening agent in a liquid lubricant matrix.They owe their consistency to a gel-forming network where the thickener is dispersed in the lubricating base fluid.Greases usually contain chemical additives channeled to enhance specific properties [3].A typical grease composition includes 60-95 wt.% base fluid (mineral, synthetic, or vegetable oil), 5-25 wt.% thickener (fatty acid soaps of alkali or alkaline metals), and 0-10 wt.% additives (antioxidants, corrosion inhibitors, anti-wear/extreme pressure, antifoam, tackiness agents) [4].The base oil induces the lubricating property into the grease constituents, while the thickener helps hold the matrix together and gives the grease jelly-like properties [3,4].The advantages of grease include sealing out contaminants, and greases do not need circulation systems, decreasing dripping, splattering, and leakage, and suspending solid additives easily.Greases are suitable for intermittent operations by working under extreme operating conditions, greases seal for life, reduce noise in machine operations, and greased machinery tends to need less power.While some of the disadvantages of grease are that; it may not reach all places in need of lubrication, does not have any cleaning effect, does not work as a cooling agent, and cannot be used at as high speeds as liquids [1,5].The purpose of lubrication is to prevent direct metallic contact between various rolling and sliding components.This is achieved by forming a thin oil (or grease) film on the contact surfaces [6].For rolling surfaces, lubrication has many merits, including reduction of friction and wear, protection against harmful elements to the materials, prolonged material reliability period, dissipation of friction heat, and prevention of rust [7].To achieve the stated purpose of lubrication, a proper amount of quality lubricants that match service conditions must be employed [6].
Currently, most of the lubricants in the market are mineral oil-based and, thus, nonenvironmentally friendly [8].Moreover, due to the depletion of traditional fossil fuels reserves, increase in greenhouse effects due to emissions of combustion-generated pollutants, unstable price of mineral oil in the world markets, and the possibility of contamination of processed food products.The search for environmentally friendly materials that have the potential to substitute mineral oil in various industrial lubricants remains a top priority in fuel and energy research [8,9].Eco-friendly grease from renewable sources has posed many challenges posed by fossil fuels-based grease, which is non-renewable, expensive, and not readily available in every part of the country [10].
Over the centuries, all manner of materials has been employed as lubricants.For example, in Sweden, black slugs were used as grease to lubricate wooden axle tress or carts [11].A bearing manufacturer developed an oil mist principle in Europe during the 1930s.This development was nurtured due to the inability to efficiently lubricate high-speed spindle bearings on grinders and similar equipment [12].Due to increased environmental interest over the past two decades, a renewed interest in biodegradable lubricants, such as vegetable oil-based lubricants, arose [13].In Europe during the 1980s, various mandates and regulations were placed on petroleum products necessitating biodegradable lubricants [14].Vegetable oils with high oleic content are potential substitutes for conventional petroleumbased lubricating and synthetic esters.They are preferred as lubricants over synthetics because, unlike mineral-based oils, they are bio-degradable, non-toxic, renewable, and relatively inexpensive [14,15].Therefore, this article aims to formulate and evaluate the performance of vegetable oil-based grease, which can be used as a substitute for conventional grease in the market.

Preliminary study 1
In this preliminary study, three samples of bio-grease (A1, B1, and C1) with different formulations were produced.In this typical experimental run, the shea butter was initially heated in a beaker at 150℃ for 2 h to eliminate impurities and moisture.It was then allowed to cool for about 1 h before commencing the process of bio-grease production.From the literature, a typical grease composition contains 60-95 wt.% base fluid (mineral, synthetic, or vegetable oil), 5-25 wt.% thickener (fatty acid soaps of alkali or alkaline metals), and 0-10 wt.% additives (antioxidants, corrosion inhibitors, anti-wear/extreme pressure, antifoam, tackiness agents) [4].In a mixing operation, shea butter, calcium hydroxide (Ca(OH)2), stearic acid, silicate, sodium carbonate (Na2CO3), palm kernel oil (PKO), and resin were added respectively in proportion as shown in Table 1.The mixture was stirred constantly at 120℃ for 1 h on a magnetic stirrer and then cooled to room temperature.The three different formulations that were produced were compared with conventional grease.The investigation compared the physicochemical properties of the three bio-grease formulated with conventional petroleum-based grease.The main physicochemical properties of the bio-grease determined were oxidation stability test with an area of control (AOC) of 120℃, dropping point test, cone penetration test, copper corrosion test, oil separation test, and pour point test according to ASTM standards ASTM D-566 [16], ASTM D-217 [17], ASTM D-4048 [18], ASTM D-1742 [19], and ASTM D97 [20], respectively.Colour illustrations You are free to use colour illustrations for the online version of the proceedings, but any print version will be printed in black and white unless special arrangements have been made with the conference organiser.Please check whether or not this is the case.If the print version will be black and white only, you should check your figure captions carefully and remove any reference to colour in the illustration and text.In addition, some colour figures will degrade or suffer loss of information when converted to black and white, and this should be considered when preparing them.

Preliminary study 2
This study needed to improve on Study 1, and the number of samples was increased to seven (A2, B2, C2, D2, E2, F2, G2) (Table 2).The pour point of the 7 samples was then determined and compared with the conventional grease again.After this study, some hardness and stiffness still showed in the final product, so there was a need for a preliminary study 3.

Preliminary study 3
The constituents of the bio-grease were increased by adding biochar (coloration and filler) and cellulose (binder) in four samples (A3, B3, C3, D3).The amounts of the two previously included constituents (biochar and cellulose) were increased significantly to enhance the edibility of the bio-grease, as it is one of the objectives of this research to produce a foodgrade bio-grease.Based on this, the other four runs, as shown in Table 3, were selected and investigated.Furthermore, PKO was removed from the formulation of B3 and C3.The D3 was compared with conventional grease after testing the pour point, cloud point, and pH value using pH multiparameter equipment.Table 4 shows the final experimental design carried out on products consistent with the commercial product.

Comparison of optimized bio-grease with standards
The optimized bio-grease was further tested for eight different parameters (oxidation stability test, cloud point test, cone penetration test, dropping point test, oil separation test, and chemical composition analysis), then compared with Nigeria Industrial Standard for biogrease production.

Oxidation stability test
Generally, oxidation reduces the service life of bio-grease.Hence, an oxidation stability test was done to investigate the chemical reaction of the combination of the lubricating oil and oxygen.A thin film of the produced grease was dropped on steel and exposed to air and moisture to promote grease oxidation.The oxidation was accelerated by heating the grease at 99 ℃ in oxygen at a pressure of 7.6 bar.The amount of oxygen absorbed by the grease was recorded in terms of pressure drop over 100 hours following standards ASTM D942 [21].

Cloud point test
It was used to detect the temperature at which the bio-grease began to cloud from the separation of wax from cooling.A test tube with a thermometer was filled with a sample of the produced grease.The grease was cooled at a 2 ℃/min rate and continuously monitored until a white cloud appeared on the thermometer bulb.The temperature corresponding to the first formation of the cloud in the oil was recorded.This agrees with the standard test method for cloud points of petroleum products and liquid fuels (constant cooling rate method) ASTM D5773 [22].

Cone penetration test
This was done to determine the consistency and shear stability of the bio-grease.A cone of 10g was allowed to sink in the produced grease for 5 seconds under its weight while the temperature of the grease was held at 25 ℃.The penetration depth of the cone in the grease was recorded in tenths of a millimeter.

Dropping point test
The dropping point test determined the bio-grease's cohesiveness and thickening nature.A sample of the bio-grease was poured into a cup suspended in a test tube and heated in an oil bath at 40℃.The temperature at which the grease fell from the hole in the bottom of the cup was averaged with the temperature of the oil bath and recorded.This was done to satisfy the standard test method for the dropping point of lubricating grease ASTM D566 [16].

Oil separation test
It was used to determine the tendency of oil to separate from the bio-grease.This test is one of the most important tests in bio-grease production.The bio-grease produced was stored at 25 ℃ at an applied air pressure of 1.72kN/m2 (0.25 psi) for 24 hours.This was done to satisfy the standard test method for oil separation from lubricating grease during storage ASTM D1742 [19].

Chemical composition analysis
A gas Chromatography-Mass Spectrometer was used to determine the chemical composition of the bio-grease produced.This was done using Agilent 5975 inert XL MSD coupled to a 7890A gas chromatography.

Results
Results showed bio-grease properties from preliminary studies compared with conventional grease (Tables 5-7) and physicochemical properties of the produced bio-grease (Table 8) compared with Nigeria Industrial Standard for bio-grease production.
After preliminary study 1 (Table 5), it was observed that the bio-greases produced were hard and stiff at room temperature (similar to bar soap) when compared with the conventional grease, thus the need for preliminary study 2. The results of preliminary study 2 (Table 6) showed an improvement compared to preliminary study 1.However, some hardness and stiffness still showed in the final product, and preliminary study 3 was performed.Following study 3 (Table 7), sample D3 showed a significant result close to the conventional grease.

Discussions
The physicochemical properties of the shea butter observed in this study are consistent with those reported by Yami and Ogiri [23].The dropping point, cone penetration, pour point, and water wash tests were performed on preliminary produced bio-grease, and the results are shown in Table 5.The values of C1 were close to the conventional grease.However, further formulations in line with C1 were investigated due to the grease's hardness.Biochar and cellulose were also added to help improve the properties of the bio-grease.
To overcome the hardness problem of the formulated bio-grease, as experienced and reported by Ebisike et al. [24], the amount of caustic soda, Ca(OH)2 in the formulation was reduced (1.6% -3.0%), after which cone penetration and pour point tests were determined for the new seven formulations.There was a significant improvement in the produced biogrease, as shown in Table 6.However, the edibility, coloration, and binding properties needed to be improved since the production aims to produce bio-grease to be classified as H3 grade.The third preliminary formulation was then carried out with cellulose as an edible binder and carbonized rice husk as a colorant.
Table 7 presents the properties of the produced bio-grease from the four formulations.The results were promising, especially D3, with a pH value close to 7 (neutral).This is important because the bio-grease will be used in food processing equipment.Also, the cloud and pour point results were closer when compared to the conventional grease, indicating that sample D3 might be an alternative to conventional grease.Hence, due to these promising results, further optimization studies were performed using D3 as the zero-coded level in design expert software to investigate additional parameters like ash, moisture content, and evaporation loss.The percentage of shea butter, P.K.O., and resin were kept constant to maintain the edibility and cost of materials.
Table 8 shows the physicochemical properties of the produced bio-grease, such as oxidation stability, moisture content, ash content, evaporation loss, thermal stability, oil separation, dropping point, cone penetration, and pour point, and ash content for the four biogreases which were stable during optimization when carried out.These properties fall within the Nigeria Industrial Standard (NIS) specified limit.Sample B4 and D4 were discovered not to be within the NIS for oil separation and dropping point, making them unsuitable.The moisture content investigation on bio-greases A4 and C4 gave a promising result.However, bio-grease C4 tends to be below the NIS standard for cone penetration and dropping point.Also, the oxidation stability of bio-grease A4 gave a better result than bio-grease C4.A similar result was reported by Jolanta and Trzos [25].This result shows that bio-grease A4 would have a longer life span than bio-greases B4, C4, and D4.The flash and fire point of the bio-grease were investigated and then compared with the values obtained from the base oil (shea butter).The results showed that the value of the flash point of bio-grease A4 tends to increase while the fire point decreases.A higher flash point of the produced bio-grease indicates better thermal stability during use [13].Thus, narrowing down the best-produced bio-grease to bio-grease A4 as it is the closest to the Nigeria Industrial Standard.A similar result was obtained by Akshai et al. [26] when they produced bio-grease using waste cooking oil as feedstock.

Conclusion
Shea butter alone in this study demonstrated most properties comparable with conventional grease, but the excessive hardness limited its use as grease.This suggests that shea butter can be used as an alternative to conventional grease when biochar and cellulose are added to improve the properties of conventional grease.With these properties exhibited, the bio-grease produced could be classified as H3 food-grade grease since shea butter is edible.Hence, there is a prospect for developing grease that meets Nigeria's Industrial Standards using shea butter.

Table 1 .
Formulation of three samples in Study 1

Table 2 .
Formulation of the seven samples in Study 2

Table 3 .
Formulation of the four runs in Study 3Components

Table 4 .
The design layout for the experimental runs

Table 5 .
Properties of the bio-grease produced from preliminary study 1 compared with conventional grease.

Table 6 .
Properties of the bio-grease produced from preliminary study 2 compared with conventional grease

Table 7 .
Properties of the bio-grease produced from preliminary study 3 compared with conventional grease

Table 8 .
The physicochemical properties of the produced bio-grease