1

A B S T R A C T This work investigated the tribological (lubrication) properties of four

vegetable oils - groundnut oil (GNO), red palm oil (RPO), palm kernel oil (PKO) and refined cottonseed oil (RCSO). Their temperature-dependent density and viscosity were investigated alongside other properties namely: pH value, pour point, flash point, specific gravity and specific heat capacity at constant pressure. Tests were conducted up to 80 oC. All the four oils were sourced locally (South West Nigeria).

The Ostwald Viscometer was employed for viscometry, the density bottle for density determination; while other tests were conducted on equipment available in some laboratories in Nigeria. The pH values range between 3.37 for GNO at 27.8 oC and 5.01 for PKO at 27.9 oC. Their pour points are from 0 oC for GNO to 21oC for PKO. Flash points range between 318 oC for PKO and 325 oC for RCSO. The oils’ pH value, pour point and flash point do not vary with temperature. Their Specific heat capacities are between 2.0839 J/g oC for PKO to 3.7677 J/g oC for RPO. Their densities (and specific gravities) decrease with increasing temperature, ranging from 908.5 kg/m3 for PKO at 30 oC to 863.4 kg/m3 for RPO at 80 oC. 2

ABSTRACT The density values within the test range (30 to 80 oC) were

fitted to a straight line model with 99% correlation. Mathematical models were developed for density and specific gravity respectively; thus density and specific gravity values at any temperature of interest could be evaluated. The kinematic and absolute viscosities of the oils decrease with increasing temperature. For GNO, kinematic viscosities range from 35.9552 centistokes (cS) at 30 oC to 7.8617 cS at 80 oC; for PKO, from 25.2219 to 5.2434 cS; RPO, from 33.3787 to 7.0183 cS and for RCSO, from 30.4482 to 7.0208 cS. The viscosity indices (VI) of the four oils range from 183 for GNO to 206 for PKO.

Further investigations will consider the performance characteristics of the oils in lubrication systems. The potentials of our vegetable oils are thus presented for appropriate industrial application.

3

1.1(a - g) General Background

1.2 Objectives of the Research

1.3 Purpose and Scope of Study

1.4(a, b) Significance of the Study

1.5(a - c) Properties of Lubricating Oils

4

1.1a General Background Tribology is the science and technology of interacting surfaces in relative

motion and of related subjects and practices [1]. The word ‘tribology’

originates from the Greek word “tribos” which means “rubbing”. The

science includes sub-areas such as friction, wear and lubrication [2].

The subject (tribology) generally deals with the technology of lubrication,

control of friction and prevention of wear of surfaces having relative motion

under load [3]. The field of tribology includes analysis of friction, wear,

lubrication phenomena and the application of such principles to mechanical

design, product development, manufacturing processes and machine

operation [4].

Friction is the resistance to bodies moving against each other and is always

present when bodies are in motion. Friction can either be dry or viscous and

in the former case we make a distinction between static and dynamic

friction; and in the latter case friction develops due to molecular forces

between adjacent fluid layers. Wear is a destructive process where surface

material is removed from one or both of the two bodies in relative motion.

Lubrication is a way of controlling both friction and wear [2].

5

1.1b General Background Lubrication is the introduction of a substance between the contact surfaces

of moving parts to reduce friction and to dissipate heat. A lubricant may be

oil, grease, graphite, or any substance - gas, liquid, semisolid, or solid - that

permits free action of mechanical devices and prevents damage by abrasion

and “seizing” of metal or other components through unequal expansion

caused by heat.

In machining processes lubricants also function as coolants to forestall heat-

caused deformities [6]. Lubricants can be either solid or fluid type, and their

main purpose is to reduce the friction and protect the surfaces against wear

thus providing smooth running and a satisfactory operational life for

machine elements.

Lubricants also transfer heat, carry away contaminants and debris, transmit

power, and prevent corrosion. Most lubricants are liquids (such as mineral

oils, synthetic esters, silicone fluids), but they may be solids for use in dry

bearings, greases for use in rolling-element bearings, or gases (such as air)

for use in gas bearings. Fluid film lubrication occurs when opposing

bearings surfaces are completely separated by a lubricant film [2].

6

1.1c General Background The function of tribological research is to bring about a reduction in the

adhesion, friction and wear of mechanical components to prevent their

failure and provide long, reliable component life through the judicious

selection of materials, operating parameters and lubricants [7].

Mechanical systems such as bearings, gears and seals are examples of

components involving tribology. They are technically referred to as

‘tribosystems’. Wherever and however, two or more solid surfaces are in

contact with relative motion between the surfaces, tribology is involved.

This requires the design and failure analysis of machine components -

bearings, gears, seals, etc. - and a study of the effects of pressure,

temperature, humidity, viscosity and other variables on its performance

under appropriate lubrication regime [9].

The significant lubricating-fluid properties are: density, viscosity, specific

heat and thermal conductivity [10]. Base stock (oil) functions important to

tribology include viscosity and its variation with temperature, pressure and

shear rate, traction, visco-elasticity, bulk modulus and thermal properties.

7

1.1d General Background Recent developments has brought about the new field of “environmentally friendly

tribology”, “tribology for the environment” or simply “green tribology”. Thus, the

current focus in lubrication science and technology is on renewable, earth-friendly

and environmentally benign fluids for tribological applications.

The way we use energy: fuels and lubricants; should not threaten our planet. Bio-

based fuels and lubricants are being researched to roll back the spectre of a warming

planet. Vegetable oils (VOs) are among the group of environmentally acceptable and

renewable lubricants for some interacting surfaces.

VOs are used in various industrial applications such as lubricants, emulsifiers,

plasticizers, surfactants, plastics, solvents and resins. Research and development

approaches take advantage of the natural properties of these oils. They have superb

environmental credentials, such as being inherently biodegradable, having low eco-

toxicity and low toxicity towards humans, being derived from renewable resources

and contributing no volatile organic chemicals.

These oils are extracted from the seeds of cotton, groundnut, oil palm, soybean, etc.

Some VOs used industrially include: palm, palm kernel, coconut, cotton, groundnut,

castor, tung, soybean, linseed from flax and rapeseed oils. Consequently, the

relatively low cost and the dependable supply of certain vegetable oils make them

important sources of industrial oils [11].

8

1.1e General Background Selecting the proper lubricant is important for sharp reduction of long-term

costs. The best-fit product selection can mean longer lubricant life, reduced

machine wear, reduced incipient power losses and improved safety.

Suitable base stocks and additives reduce environmental impact. This is important

because there will be leaks, spills and eventual disposal of lubricant [14].

Modern industry rests on a layer of lubricant which separates moving machine

elements from each other. The condition of oil used as a lubricant affects the

working condition of the machine significantly. The chemical and physical

properties of a lubricant have a direct effect on the lubrication situation [15].

Interest in VO based lubricants (bio-lubricants) emerged, in recent time, due to

environmental concerns. Petroleum based lubricants (currently been used) represent

a large source of soil and water pollution. About half of these lubricants are being

spilled off in the environment, deliberately or accidentally. These reasons stimulate

the use of biodegradable, non-toxic lubricants.

Biodegradable lubricants, mostly from vegetable oils, represent the

technical and environmental alternative for conventional lubrication. It has

been assessed that over 90% of all lubricants could be replaced by bio-

lubricants [11, 16 and 67].

9

1.1f General Background Petroleum-based lubricants are carcinogenic and constitute environmental

hazard when spilled or disposed. One of such is water system pollution.

Bio-based lubricants offer significant health and environmental benefits

including resource renewability, biodegradability, as well as providing

satisfactory performance in a wide array of industrial applications.

There is a growing worldwide trend of promoting VO as base oil for

automotive lubricants, metal working lubricants, quenching oils, hydraulic

oils, oilfield applications for avoidance of aquatic pollution, etc.

Many tests and researches are being conducted to understand the potential

of renewable lubricants based on VO to replace the current mineral oil

based lubricants. VOs are recognized as rapidly biodegradable and are thus

promising candidates as base fluid in environmental-friendly lubricants and

tribosystems. Tribological (lubrication) properties of olive oil, coconut oil,

soya oil, canola oil and rapeseed oil have been investigated [16, 17, 18, 19,

20, 21 and 22].

10

1.1g General Background Groundnut oil, palm kernel oil, red palm oil and

refined cotton seed oil are available vegetable oils in Nigeria; these four have been selected for investigation.

Adequate knowledge of the tribological (lubrication) properties of these oils is needed for accurate analysis and simulation in the design of lubricants and fluid lubricated systems, where they may be adopted as base oil to replace mineral oils.

Tribological (lubrication) properties such as density, viscosity, pour point, flash point, heat capacity, etc. are the major input data for lubricant design.

11

1.2 Objectives of the Research

The followings are the specific objectives of the research:

1.To determine experimentally the density, viscosity, flash

point, pour point, specific gravity, heat capacity and pH

value of groundnut oil, palm kernel oil, red palm oil and

refined cotton seed oil.

2. To investigate the influence of temperature on density

and viscosity of the selected oils. It is expected that the

density-temperature relationship will be modelled

empirically. The viscosity indices (which relate viscosity

with temperature) will also be determined for each oil.

12

1.3 Purpose and Scope of Study

The purpose of this research is the investigation of some tribological properties of selected vegetable oils with respect to temperature variation.

The selected oils are (1) groundnut oil, (2) palm kernel oil, (3) red palm oil and (4) refined cotton seed oil.

This research study shall determine some physical properties – pH value, melting point, flashpoint and flammability, and heat capacity.

These tribological properties – density and viscosity of the four oils will be investigated within the temperature range of 30 to 80oC at 10oC intervals. Empirical equations relating these properties with temperature would be developed.

13

1.4a Significance of the Study In view of current environmental realities and demands, both engine fuel and lubricant need to

compliment one another towards reducing emission of green house gases and toxic chemicals. Bio-

based or biodegradable fuels and lubricants are being developed from vegetable oils.

Vegetable oil (VO) lubricants are being developed as alternatives to petroleum-based oil: they can

be circulated in internal combustion engines, or sprayed in one-time applications like those

necessary for lubricating train rails.

Reasons for the current research and development activities on VO lubricants include: they do not

produce toxic fluids or volatile organic compounds, they are biodegradable, renewable and

recyclable and have a higher boiling point than petroleum-based oils.

They can endure harsh and hot engines, offer less ash build-up and better engine performance over

mineral oil-based lubricants. VOs produce less green house gases (GHGs) compared to

conventional mineral oil-based lubricants. VOs have excellent lubricity, they have favorable

viscosity-temperature characteristics and high flash points. They are compatible with mineral oil

and additive molecules, exhibit relatively low lubricant consumption and longer oil drain intervals

and have good energy efficiency combined with public health. They are fire resistant, i.e. safer to

handle in relation to petroleum-based oils.

VOs are food grade oils, applicable in the food processing industry. They offer performance equal

or better than petroleum-based oils at similar cost. Bio-based fluids are being developed from VOs

to eliminate the hazardous pollution caused by accidental oil spillage, which is especially

important in environmentally sensitive applications such as construction, hydropower, marine, oil

and gas, etc. [11, 16, 18, 21 and 58].

14

1.4b Significance of the Study Local sourcing of alternative lubricants (or base oils) is considered an effort in

the right direction. Apart from potential savings from reduced imports, it will

improve income and living standard of local producers. Locally available VOs

are applicable as lubricants for some interacting surfaces. But they are rather

tribologically unknown. The lubrication properties of local VOs are needed for

appropriate industrial application.

It is important to take lubrication into account early in the product design

process. This will make it possible to optimize lubricant choice, manufacturing

method, surface hardening processes, etc., in order to obtain reduced wear and

friction [23]. Therefore, adequate lubrication design tools should be provided

for the engineer in order to enable predictions of friction and the risk of wear. A

lubricant’s application depends, among other factors, on desirable properties in

tandem with the design of a piece of machinery.

The choice of locally available VO as base-oil for lubricant development would

be enhanced by the product of this work. Thus, future lubricants can be

developed from locally available VOs. This will make machinery operation

become more environmentally acceptable.

15

1.5a Properties of Lubricating Oils Viscosity : The viscosity of a fluid is defined qualitatively as its resistance to

flow. This resistance is primarily due to internal friction. For engineering

applications the oil viscosity is usually chosen to give optimum performance at

the required temperature.

Kinematic viscosity (Pas or centistokes) is a measure of a liquid's flow under the influence of gravity. It's handy to think of lubricant's kinematic viscosity as it's "I.D. card”.

Viscosity index (VI): A numbering scale that indicates the changes in oil

viscosity with changes in temperature. Viscosity index can be classified as

follows: low VI - below 35; medium VI - 35 to 80; high VI - 80 to 110; very

high VI - above 110. A high viscosity index indicates small oil viscosity changes

with temperature, i.e. Stable viscosity. A low viscosity index indicates high

viscosity changes with temperature.

Pour point: The pour point is the lowest temperature at which oil will flow

under specified conditions. It does not vary with temperature. Its importance lies

in the ability of the oil to flow at low temperature. This property facilitates

storage and supports the starting of engines especially during cold climatic

condition.

16

1.5b Properties of Lubricating Oils Flash point: The flash point is the lowest temperature, to which a lubricant must be heated

before its vapour, when mixed with air, will ignite but not continue to burn. It remains constant

for oil irrespective of the operating temperature. This property reveals the extent of fire risk to

which the lubricant could be subjected [30]. A good lubricant should have a high flash point

hence lower fire risk.

(pH) number : pH is the negative logarithm of the effective hydrogen-ion concentration or

hydrogen-ion activity in gram equivalents per litre of the lubricant [31]. It is a number on a

scale on which a value of 7 represents neutrality; lower numbers indicate increasing acidity

and higher numbers increasing alkalinity. On the pH scale, each unit of change represents a

tenfold change in acidity or alkalinity.

Density and specific gravity: The density of a substance is the mass of a unit volume of it at a

standard temperature and pressure [33]. The specific gravity or relative density is the density

of a substance divided by that of water at the same temperature and pressure. Specific gravity

is dimensionless. Most lubricating oils have specific gravities in the range 0.85 to 0.95 [24].

The density of a fluid is required for flow rate calculations and for the conversion of kinematic

viscosity to dynamic viscosity. Density is used in lubrication to identify an oil or oil fraction.

The density of a lubricant in g/cm3 is very nearly numerically equal to its specific gravity.

Density is sensitive to temperature and pressure. The density – temperature relationship, i.e.

the thermal expansion, influences the pressure distribution as well as the energy dissipation

due to compression. The density – temperature relationship is especially important for the

performance of hydrodynamic parallel surface thrust bearings since it is the origin of the

density wedge.

17

1.5c Properties of Lubricating Oils Specific heat capacity: The specific heat capacity (J kg-1 C-1) of a substance is

the heat required to warm unit mass of it through 1 degree; it is the heat capacity

per unit mass of the substance [34]. Heat capacity is one of the basic thermo

physical and thermodynamic properties which characterize a liquid. They are

directly linked with temperature derivatives of basic thermodynamic functions

and are therefore indispensable for the calculation of differences in these

functions between different temperatures. It is an important property when the

oil acts as a coolant or heat transfer medium.

Thermal conductivity: the thermal conductivity, k, is the quantity of heat, ∆Q,

transmitted during time ∆t through a thickness x, in a direction normal to a

surface, of area A, due to a temperature difference ∆T, under steady state

conditions and when the heat transfer is dependent only on the temperature

gradient. Thermal conductivity varies linearly with temperature and is affected

by polarity and hydrogen bonding of the molecules. The thermal conductivity of

most of the mineral and synthetic hydrocarbon based lubricants is in the range

between 0.14 W / m K at 0 oC and 0.11 W / m K at 400 oC.

18

2.0a Literature Review

2.0b Literature Review (Contd.)

2.0c Literature Review (Contd.)

19

2.0a LITERATURE REVIEW

One of the interesting recent developments is a growing realization that bio-

resources present practical alternatives to fuels and lubricants derived from liquid

fossil fuels. The advantages of vegetable oils (VOs) as base fluids in lubricants are

perceived to be the following: non-toxicity, biodegradability, resource renewable,

affordable application cost, good lubricity and high viscosity index.

In bio-based formulations, vegetable oils replace a mineral oil as the base, which is

typically 90% of a lubricant. Fatty acids make VOs naturally more slippery than

mineral oils, and their polar molecules make them stick to metal surfaces better.

VOs are used in various industrial applications such as: lubricants, emulsifiers,

plasticizers, surfactants plastics, solvents and resins. Research and development

approaches take advantage of the natural properties of these oils.

These oils are extracted from the seeds of cotton, groundnut, oil palm, etc.

Consequently, the relatively low cost and the dependable supply of certain VOs

make them important sources of industrial oils.

20

2.0b LITERATURE REVIEW (Contd.) Larsson et al. [23] investigated the properties of a number of lubricants namely:

naphthenic and paraffinic mineral oils, blends of the aforementioned oils,

polyalphaolefins and a polyglycol. Properties measured are: the viscosity,

elastohydrodynamic lubrication (EHL) friction coefficient, density, thermal

conductivity and heat capacity per unit volume. These were measured within

relatively broad pressure and temperature ranges.

Hassan et al. [40], worked on the possibility of producing lubricating oil from

vegetable oil with palm olein (oil) as a case study. Some of the properties such as

viscosity, flash/fire point, pour point and specific gravity were analysed.

Afeti et al. [45] investigated viscosity, density, thermal conductivity, specific heat

capacity, flash point, pour point, melting point and oxidation resistance of four oils

namely: coconut oil, palm kernel oil, groundnut oil and shea butter oil. Kinematic

viscosity and density readings were taken between 25 and 80oC. Thermal

conductivity and specific heat capacity were determined without reference to

temperature and pressure changes. It was found out that all the oils investigated have

a higher flash point compared to SAE40 engine oil.

Abramovic and Klofutar [46] determined dynamic viscosities for the following VOs:

unrefined sunflower oil, refined sunflower oil, olive oil, refined corn oil unrefined

pumpkin oil, a mixture of refined VO and unrefined pumpkin oil; at temperatures

from 298.15 K (25oC) to 328.15 K (55oC).

21

2.0c LITERATURE REVIEW (Contd.) Fundamental properties of six ester base fluids, suitable for formation of

environmentally adapted lubricants were investigated by Pettersson [36]. All the

esters have high thermal conductivity and specific heat capacity in comparison with

the mineral oil studied.

Fox and Stachowiak [44] investigated VOs as a potential source of environmentally

favourable lubricants, due to a combination of biodegradability, renewability and

excellent lubrication performance.

Gitis [49] presented a multi-sensing technology, effective for tribology testing of

oils. Oils have to be characterised based on their properties as well as their

operational performance. Tribological properties such as density, viscosity, thermal

conductivity, and others are determined by adopting appropriate laboratory

equipment.

Performance characteristics of oils often require tribometers and test engines.

Application-specific tribometers are usually developed for lubrication system testing

and simulation. For instance: aerospace tribological testing require spiral orbit

tribometer, automotive tribological testing would need engine test bed and the wear

testing machine is needed to investigate anti-wear additives in oils. Each level of

tribometry has its instrumentation challenge.

22

3.1 Materials

3.2 Experimental Procedures

23

3.1 Materials Four locally available vegetable oils were investigated,

namely: Groundnut Oil (GNO),

Palm Kernel Oil (PKO),

Red Palm Oil (RPO), and

Refined Cottonseed Oil.

Samples of groundnut oil and palm kernel oil were collected from Ado – Ekiti. Refined cottonseed oil was bought from Lagos, while red palm oil sample (first grade) was collected from the Nigeria Institute for Oil Palm Research (NIFOR), Benin City.

24

3.2 Experimental Procedures

Properties determined are

pH value,

pour point,

flash point,

specific gravity, and

specific heat capacity.

• density and viscosity were investigated within temperature range of 30 to 80oC at 10oC interval.

• Apparatus for density and viscosity were improvised.

• Other properties were determined using available laboratories including the Petroleum Analysis Laboratory (PAL), Petroleum Training Institute (PTI), Warri. The experimental procedures adopted for determination of investigated properties are presented in this section.

25

3.2.1 pH Value.

The test method used was that for pH determination for oil

and water samples. The apparatus consists of the Mettler

Toled (MX300) pHmeter and 50ml beakers. The pHmeter

was calibrated with buffers 4, 7 and 9. Oil samples were

poured into four beakers. The clean electrode of the

pHmeter was placed in the oil sample. When the reading

on the pHmeter got stable, the respective pH value was

recorded along with the temperature of oil sample.

26

3.2.2 Pour Point.

The test method is the American Society for Testing and

Materials (ASTM) D97-85 standard test method for pour

point of petroleum oils. The apparatus consists of a test

jar, thermometer, cork, thermostatic bath maintained by

refrigeration and a refrigeration jacket. The test started by

heating of the oil sample. The oil sample was cooled and

examined at intervals of 3oC for flow characteristics. The

lowest temperature at which movement of the oil is

observed would be recorded as the pour point.

27

3.2.3 Flash Point.

The test method was the ASTM D93 standard test method

for flash point. Apparatus consists of the Pensky-Martens

closed flash tester and a thermometer. The Pensky-

Martens closed flash tester is shown in Figure 3.1. Oil

sample would be heated at a slow constant rate with

continual stirring. A small flame would be directed into

the cup. The lowest temperature at which application of

the flame causes the vapor above the oil sample to ignite

would be recorded as the oil’s flash point.

28

Figure 3.1: Pensky-Martens Flash Point Tester.

29

3.2.4 Specific Gravity.

The test method was the ASTM D1298 - 85 standard test

method for specific gravity or relative density.

Apparatus consists of a hydrometer, 100ml measuring

cylinder and a thermometer. The oil sample would be

transferred into a cylinder. A hydrometer would be

lowered into the oil sample.

After temperature equilibrium is reached, the hydrometer

scale would be read and the temperature of the sample

noted.

30

3.2.5 Density.

Apparatus include electrically heated thermostatic (water)

bath, density bottle, digital mass balance, thermal

insulator interface between hot density bottle and digital

mass balance sensor and thermometer.

The fixed volume density bottle was filled with oil sample

to its brim. The mass was determined at ambient

temperature (30oC). With the aid of the water bath, oil

sample in the bottle was heated to desired temperature,

and its mass recorded. Average values (mass) were noted

for oil within the temperature range of interest.

31

3.2.6 Viscosity.

Apparatus for the experiment are Ostwald viscometer

(Figure 3.2), thermostatic (water) bath, stop watch,

thermometer, hand suction pump, electrical heater with

stirrer and holders.

32

Figure 3.2: Ostwald Viscometer.

33

3.2.6a Viscosity (contd.) A sample oil of fixed volume is charged to the lower receiving

vessel and the viscometer is placed in a thermostatic bath.

After time is allowed for the sample oil to reach thermal equilibrium (about 5 minutes), the sample is drawn up into the efflux vessel by suction until the level is above the upper etched index line.

The fluid is then permitted to flow down through the capillary by releasing the suction.

When the fluid surface passes the upper etched index line, a stopwatch is started. The stop watch is stopped when the surface passes the lower etched index line of the efflux vessel.

From this efflux time (t), the kinematic viscosity of the fluid is calculated by multiplying it by the viscometer calibration constant.

34

3.2.7 Specific Heat Capacity.

The electrical method was employed for determination of

specific heat capacities of the four oils at atmospheric

pressure.

Apparatus include well-lagged aluminium calorimeter,

thermometer, stirrer, heating coil, voltmeter, ammeter,

rheostat, 12V accumulator, switch, and stop-watch.

The electrical circuit was connected as shown in

Figure 3.3.

35

Figure 3.3: Circuit Diagram for the Electrical Method.

36

Experimental results and empirical analyses are presented for:

1. pH Value.

2. Pour Point

3. Flash Point

4. Specific Gravity

5 Density

6. Viscosity

7. Viscosity Index

of the FOUR oils investigated.

37

Table 4.1: pH Value

Oil pH Value

Groundnut oil 3.37 at 27.8oC

Palm kernel oil 5.01 at 27.9oC

Red palm oil 4.36 at 28.5oC

Refined cottonseed oil 4.28 at 28.1oC

38

4.1 pH Value From the above results, the pH values of the oils show that they

are acidic; groundnut oil having the lowest (3.37), while palm kernel oil has the highest (5.01). Since acids are formed in use by contamination or oxidation in lubrication systems, the oil life of vegetable oils reduce significantly in use as the acidic content increases. To make these oils have a longer life, alkalinity could be introduced for special properties and neutralization of fuel combustion products that are acidic [55]. Acidic and alkalinity (pH value) does not vary with temperature. It is influenced by contamination or oxidation of the oil.

Temperatures at which readings were taken were around the ambient temperature during the experiment, which is 27oC. The temperatures noted are for record purposes, oil pH remains constant irrespective of operating temperature.

39

Table 4.2: Pour Point

Oil Pour Point

Groundnut oil 0oC

Palm kernel oil 21oC

Red palm oil 13oC

Refined cottonseed oil 5.5oC

40

4.2 Pour Point The standard range of pour point for lubricating oils is

between -45 oC and 30 oC [24, 29]. As observed from the

results, all the selected oils can be used as lubricants. The

high pour points of palm kernel oil (21oC) and red palm oil

(13 oC) is mainly due to the presence of high percentages of

wax in them compared to the other two oils. The lower

pour points of groundnut and refined cottonseed oils make

them suitable for low temperature (not cryogenic)

applications. The pour point of SAE 40 engine oil is -27 oC

[56]. SAE implies the Society of Automotive Engineers. The

oils have relatively higher pour points compared to SAE 40.

41

Table 4.3: Flash Point.

Oil Flash Point

Groundnut oil 320oC

Palm kernel oil 318oC

Red palm oil 322oC

Refined cottonseed oil 325oC

42

4.3 Flash Point The standard range of flash point for lubricating oils is

between 40 and 360 oC [28]. The flash point of SAE 40 engine oil is 260 oC [56]. From the results, flash points of the four oils are, not only within acceptable range, but well above that of SAE 40 engine oil. Flash point is clearly related to safety. It is an indication of the combustibility of the vapour of a lubricant. It is a measure of the fire hazards. It is also useful in determining whether oil has been contaminated [24, 29]. Thus the vegetable oils investigated present lesser fire hazards compared to SAE 40.

43

Table 4.4: Specific Heat Capacity

Oil Specific Heat Capacity,

J/goC

Groundnut oil 2.6127

Palm kernel oil 2.0839

Red palm oil 3.7677

Refined cottonseed oil 2.3863

44

4.4 Specific Heat Capacity Afeti et al. [45] presented specific heat capacities of

groundnut and palm kernel oils as 2.03 and 1.61 J/g-K respectively. The trend is similar to experimental results, i.e. groundnut oil has higher heat capacity compared to palm kernel oil. Heat capacity is one of the basic thermo-physical and thermodynamic properties which characterize a liquid. It is an important property when the oil acts as a coolant or heat transfer medium [36]. In lubrication applications, these oils are promising candidates, as they could transfer heat, among other functions, in fluid lubricated machinery or tribosystem.

45

Table 4.5: Density, kg/m3

Temp-

erature, oC

Groundnut

oil

Palm

kernel oil

Red palm

oil

Refined

cottonseed

oil

Water

30 901.4 908.5 900.0 907.0 989.7

40 893.5 900.7 891.1 896.8 985.4

50 887.4 894.9 882.2 889.8 981.0

60 878.6 887.2 875.7 883.1 975.5

70 872.3 881.0 869.6 876.8 970.0

80 864.7 874.9 863.4 870.5 963.5

46

4.5 Density From the results in Table 4.5, densities of the oils

decrease as temperature increases. When compared with the standard range of values for lubricating oils (between 700.0 and 980.0 kg/m3), the densities of the oils indicate their acceptability as lubricants. Table 4.5 as well shows that all the oils have densities lower than that of water; this indicates a good demulsibility property of the oils, i.e. being able to separate readily from water when used as lubricants in circulatory systems and in other lubricating systems. Figure 4.1 shows the variation of density with temperature for the four oils.

47

860

865

870

875

880

885

890

895

900

905

910

915

0 20 40 60 80 100

De

nsi

ty,ρ

in

kg

/m3

Temperature in Degree Celcius (oC).

Figure 4.1: Density - Temperature Curves for the Four Oils.

Groundnut Oil

Palm kernel Oil

Red Palm Oil

Refined Cottonseed Oil

48

Empirical Density – Temperature Relationship

1. Groundnut Oil:

ρGNO (T) = -0.73224 (T) + 923.1962 . . . . . (6)

2. Palm kernel oil:

ρPKO (T) = -0.67086 (T) + 928.0971 . . . (7)

3. Red palm oil:

ρRPO (T) = -0.72571(T) + 920.2476 . . . . (8)

4. Refined cottonseed oil:

ρRCSO (T) = -0.71200(T) + 926.4933 . . . . (9)

49

Table 4.7: Specific Gravity.

Oil Specific Gravity (S.G.)

Groundnut oil 0.8630 at 28.9oC

Palm kernel oil 0.8550 at 43.3oC

Red palm oil 0.8700 at 28.9oC

Refined cottonseed oil 0.8770 at 28.9oC

50

4.6a Specific Gravity The standard range of specific gravity for lubricating oils is

0.7000 to 0.9800 [28]. Hassan et al. [40] got the specific gravity of crude palm oil as 0.8651; the temperature was not specified. Dorfman [57] presented specific gravities (at 15.5oC) of cottonseed oil, palm oil, palm kernel oil and groundnut (peanut) oil as 0.9246 – 0.9280, 0.924 – 0.9279, 0.924 – 0.9258 and 0.917 – 0.9209 respectively; whereas at 25oC, the specific gravities of cottonseed and groundnut oils are 0.915 – 0.921 and 0.912 – 0.920 respectively. Like density, it reduces with increasing temperature. From the results (Table 4.7), all the oils have specific gravities within acceptable range. Specific gravity of mineral oil is commonly presented as API gravity in the petroleum industry. API implies the American Petroleum Institute. Specific gravity is often measured at 15.5oC. A number of derivations are based on specific gravity of respective oil at 15.5oC. From the results, all the oils have specific gravities within acceptable range.

51

4.6b Specific Gravity Specific gravity, like density, is affected by temperature change.

The hydrometer used earlier could not take readings at low temperature (less than 25 oC), since the oils under study would have solidified. As well it could be damaged at temperatures higher than 50 oC. Therefore, the relationship between density and specific gravity was explored to resolve this challenge. There are cases, for instance, where specific gravity of oil at 15.5 or 15.6 oC is required. From the experimental measurements of density of the oils and water at selected temperatures and atmospheric pressure (Table 4.5); and based on the relationship between density and specific gravity as shown below:

Specific gravity = ρoil / ρwater …………….. (21)

Specific gravities of the oils were calculated from results shown in Table 4.5: “Densities of the four vegetable oils at varying temperature”. This is shown in Table 4.8.

52

Table 4.8: Specific Gravities of the Four Oils at Varying Temperature

Temperature,

oC

Groundnut

oil

Palm kernel

oil

Red palm

oil

Refined

cottonseed oil

30 0.9108 0.9180 0.9094 0.9164

40 0.9067 0.9140 0.9043 0.9101

50 0.9046 0.9122 0.8993 0.9070

60 0.9007 0.9095 0.8977 0.9053

70 0.8993 0.9082 0.8965 0.9039

80 0.8975 0.9080 0.8961 0.9035

53

54

0.895

0.9

0.905

0.91

0.915

0.92

0 50 100

Sp

eci

fic

Gra

vit

y

Temperature, Degrees Celsius.

Figure 4.1: Specific Gravity - Temperature Curves for the Oils.

4.6.1 Empirical Specific Gravity – Temperature Relationship

(i) Groundnut oil (GNO):

sGNO (T) = -0.00026 (T) + 0.91782 . . . . . . . . . . (23)

(ii) Palm kernel oil (PKO):

sPKO (T) = -0.00020 (T) + 0.92267 . . . . . . . . . (24)

(iii) Red palm oil (RPO):

sRPO (T) = -0.00026(T) + 0.91493 . . . . . . . . . . (25)

(iv) Refined cottonseed oil (RCSO):

sRCSO (T) = -0.00024(T) + 0.92103 . . . . . . . . . (26)

Using equations 23 to 26 to calculate the specific gravities for the four oils at 15.5oC results in the following:

Specific gravity of Groundnut Oil at 15.5oC is 0.92185,

Specific gravity of Palm Kernel Oil at 15.5oC is 0.92577,

Specific gravity of Red Palm Oil at 15.5oC is 0.91896,

And Specific gravity of Refined Cottonseed Oil at 15.5oC is 0.92475.

55

Table 4.11: Kinematic Viscosities of the Oils (cst).

Temperature, oC

Groundnut

oil Palm kernel

oil Red palm oil Refined

cottonseed oil

30 35.9552 25.5519 33.3787 30.4482

40 24.0244 16.2079 21.5281 21.0761

50 17.8914 12.1892 15.9077 14.4263

60 12.6054 8.7641 11.8318 11.1623

70 9.9248 6.6165 9.0020 8.7717

80 7.8617 5.2434 7.0183 7.0208

56

Table 4.12: Absolute Viscosities of the Oils.

Temperature, oC

Groundnut

oil Palm kernel

oil Red palm oil Refined

cottonseed oil

30 32.4100 23.2136 30.0408 27.6165

40 21.4658 14.5985 19.1837 18.9010

50 15.8768 10.9081 14.0337 12.8365

60 11.0751 7.7755 10.3611 9.8574

70 8.6574 5.8291 7.8281 7.6910

80 6.7980 4.5875 6.0596 6.1116

57

4.7 Viscosity The relationship between absolute and kinematic viscosities is expressed

below:

Absolute Viscosity = Kinematic Viscosity x Density

Using the density values for oils at the respective temperature (Table 4.5)

and values of kinematic viscosity shown in Table 4.11, the absolute

viscosity was calculated. Table 4.12 shows the absolute viscosity values for

the oils.

The standard kinematic viscosity for lubricating oils [28] is between 2 and

300 centistokes. Kinematic viscosity of SAE 40 engine oil at 40 oC is 119.8

and 13.0 at 100 oC [56]. Viscosity decreases with increasing temperature.

From the results, the four oils under study can be used as lubricants.

Performance characteristics of the oils in lubrication systems are needful for

effective industrial application. The temperature influence on a lubricant’s

viscosity (absolute and kinematic) is conventionally presented as its

viscosity index (VI).

58

4.8 Viscosity index Viscosity index indicates how much a lubricant's viscosity will change

according to changes in temperature between 40°C and 100°C, which

roughly define the normal temperature range of most operations. The

viscosity index is an entirely empirical parameter that compares the

kinematic viscosity of the oil of interest to the viscosities of two reference

oils that have a considerable difference in sensitivity of viscosity to

temperature. It is an arbitrary numbering scale that indicates the changes in

oil viscosity with changes in temperature. Viscosity index can be classified

as follows: low VI - below 35; medium VI - 35 to 80; high VI - 80 to 110;

very high VI - above 110.

A high viscosity index indicates small oil viscosity changes with

temperature. A low viscosity index indicates high viscosity changes with

temperature. Therefore, a fluid that has a high viscosity index can be

expected to undergo very little change in viscosity with temperature

extremes and is considered to have a stable viscosity. A fluid with a low

viscosity index can be expected to undergo a significant change in viscosity

as the temperature fluctuates. The viscosity index can be calculated using

the mathematical expression given by Stachowiak and Batchelor [5].

59

Table 4.13: Viscosity Indices (VI) of the Oils.

S/N Oil U V* L** H** VI

1 Groundnut Oil 24.0244 16.465 349.005 171.7400 183.3304

2 Palm Kernel

Oil

16.2079 8.341 107.843 63.5133 206.7127

3 Red Palm Oil 21.5281 9.704 140.204 79.3176 194.9140

4 Refined

Cottonseed Oil

21.0761 11.718 193.822 104.334 193.0381

60

4.8a Viscosity Index The viscosity index is an inverse measure of the decline in oil

viscosity with temperature. High values indicate that the oil shows

less relative decline in viscosity with temperature.

The viscosity index of most of the refined mineral oils available on

the market is about 100, whereas multi-grade and synthetic oils have

higher viscosity indices of about 150. [47]. The viscosity index of

SAE 40 engine oil is 102 [56].

From Table 4.12, the vegetable oils studied possess relatively higher

viscosity indices. High viscosity index lubricants protect better in

operations with temperature variations. Failure to use oil with the

proper viscosity index when temperature extremes are expected may

result in poor lubrication and equipment failure [19].

Thus these vegetable oils are highly potent environmentally friendly

lubricants and base fluids for bio-lubricant development.

61

5.1 (a,b & c) Conclusion

5.2 Areas of Further Work

62

5.1a Conclusion Earth’s environmental future rests in the use of renewable materials.

Vegetable oils are bio-resources, readily available on a renewable basis.

The resulting data on locally available vegetable oils are useful for

lubrication analysis, toward appropriate application of vegetable oils as

base-oils or bio-lubricants.

The pH values of the oils show that they are acidic; groundnut oil having

the lowest (3.37), while palm kernel oil has the highest (5.01). The high

pour points of palm kernel oil (21oC) and red palm oil (13oC) is mainly due

to the presence of high percentages of wax in them compared to the other

two oils.

The lower pour points of groundnut oil (0oC) and refined cottonseed oil

(5.5oC) make them suitable for low temperature (not cryogenic)

applications. The pour point of SAE 40 engine oil is -27oC [56]. The oils

have relatively higher pour points compared to SAE 40. The flash point of

SAE 40 engine oil is 260oC [56].

63

5.1b Conclusion (contd.) The flash points of the four oils are, not only within acceptable range, but

well above that of SAE 40 engine oil. All the oils have specific gravities

within acceptable range (0.7000 to 0.9800).

The empirical equations developed for specific gravity of the oils gave

results close to that of Dorfman [57]. Specific gravity of Groundnut Oil at

15.5oC is 0.92185, that of Palm Kernel Oil is 0.92577 and those of Red

Palm Oil and Refined Cottonseed Oil are 0.91896 and 0.92475

respectively.

When compared with the standard range of values for lubricating oils

(between 700.0 and 980.0 kg/m3), the densities of the oils indicate their

acceptability as lubricants. The empirical equations developed can be

employed to evaluate the density of each oil at required temperature.

The experimental results, i.e. groundnut oil having higher heat capacity

compared to palm kernel oil agree with the trend of similar results (as

shown in Afeti et al. [45]).

64

5.1c Conclusion (contd.) The viscosities of the oils are within the range 4 to 35

centistokes for temperature of 30 to 80 degrees Celsius.

The viscosity index of SAE 40 oil is 102 [56]. The vegetable

oils studied possess relatively higher viscosity indices (in the

range 180 to 210) compared to mineral (engine) oils (between

100 and 150).

With the current attention on environmentally acceptable,

biodegradable or renewable lubricants, local vegetable oils are

worth being considered.

65

GREEN TRIBOLOGY Green Tribology is defined as

the science and technology of the tribological aspects of ecological

balance and of environmental and biological impacts. (Prof. P. Jost, Fifth World Tribology Conference, Kyoto, Japan, September 2009.)

The specific field of green or environment-friendly tribology emphasizes

the aspects of interacting surfaces in relative motion, which are of

importance for energy or environmental sustainability or which have impact

upon today’s environment. This includes tribological technology that

mimics living nature (biomimetic surfaces) and thus is expected to be

environment-friendly, the control of friction and wear that is of importance

for energy conservation and conversion, environmental aspects of

lubrication and surface modification techniques, and tribological aspects of

green or renewable applications such as the wind-power turbines, tidal

turbines, or solar panels. It is clear that a number of tribological problems

could be put under the umbrella of ‘green tribology’ and is of mutual benefit

to one another. [iMechanica , 2010]

66

5.2 Areas of Further Work As a follow-up to this work, the effects of anti-wear additives in vegetable oils can

be studied using 3-ball and 4-ball wear testing machines. The oils could be evaluated

for functionality, depending on the desired application: hydraulic, automotive,

aerospace, hydro power generation, marine, mechanized agriculture, oil field and

food processing.

Blends of the oils can be considered in other to take advantage of qualities possessed

by one relative to another. Bleaching of the oils can be investigated in relation to

their tribological properties and functional characteristics. The effect of ageing on

oil properties and performance could be investigated in other to determine their

optimal life span and eventual shelf life.

The Application of vegetable oils as biodegradable and non-toxic lubricants in

Nigeria could be appraised; especially in the agricultural, food processing, marine

and offshore environment where aquatic pollution persists.

With test engines, a comparative study can be conducted on wear, friction, viscosity,

lubricant degradation and exhaust emissions on vegetable oils and conventional

SAE grade engine oils.

67

68