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404 Lubricant Additives: Chemistry and Applications

16.5 FUNCTIONS OF SURFACTANTS IN LUBRICATION

Surfactants play a number of critical roles in lubrication and, thus, are important ingredients in lubri-

cant formulations. Different types of surfactants are used to attain different lubrication objectives.

Examples of lubrication objectives attained by surfactants include solubilization, antifriction, demul-

si cation, dispersion of  nes and debris, anticorrosion, defoaming, antiwear, and antimicrobes.

Sometimes, the performance objectives that surfactants are supposed to provide the lubricant

are contradictory. Examples of contradictory objectives include emulsi cation versus demulsi ca-

tion, biodegradability versus bioresistance, and agglomeration versus dispersion of debris and  nes.

These contradictions occur because of the changing requirements of lubricant performance over

time or process. For example, the performance requirements of the lubricant during use are differ-

ent from that during waste treatment, disposal, or recycling. Another example is the difference in

performance requirements of the lubricant during the lubrication process (e.g., hot rolling) versus

lubricant handling process (e.g.,  ltration).

In water

In oil

Liquid crystals

(lamellar)

Liquid crystals

(hexagonal)

(a) (b)

Vesicles

Air Air

Air

(e) (f)

Foams

FIGURE 16.4 Microstructures of surfactant assemblies in solution: (a,b) lamellar liquid crystals, (c,d) hexagonal

liquid crystals, (e) vesicles, and (f) foams.

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Surfactants in Lubrication 405

In this section, a brief description of some of the surfactant functions is given. Detailed descrip-

tion of two of the functions, solubilization and boundary friction, is provided in subsequent sec-

tions. It is clear that surfactants have numerous functions in lubrication, and it is beyond the scope

of this chapter to provide detailed discussions of each. Readers are encouraged to go through the

various references cited in this chapter for more detail.

Emulsi cation is one of the most important lubrication functions of surfactants and is discussed

in detail in Section 16.6. Emulsi cation allows for preparing a stable dispersion of the lubricant

formulation in water, so that the lubricant satis es both lubrication and cooling functions of the pro-

cess. The surfactants used in these functions are selected so as to provide the appropriate emulsion

properties without interfering with the lubrication function of the formulation.

Demulsi cation is one of the requirements for waste treatment of used lubricants. In this pro-

cess, a surfactant is used to help the oil and water in the lubricant to cleanly separate into their

respective phases, so that they can be properly processed and disposed.

Dispersion of  nes and debris is an important function of surfactants in lubrication processes

that generate a lot of metal  nes and debris. The function of the surfactant is to prevent  ne metal

particles and other debris from aggregating into large mass. If not dispersed, the  nes and debris

will agglomerate and accumulate on tool and workpiece surface. This can cause undesirable out-

comes such as poor product quality (e.g., debris rolled into the workpiece), accelerated tool wear,

and unsafe work conditions due to accumulation of debris on equipment and work area.

Agglomeration of  nes and debris may be important in some processes where removing the

 nes and debris by  ltration is important. Dispersed  nes and debris are hard to  lter out and might

require the use of expensive  ltration equipment and media to attain the degree of lubricant cleanli-

ness required for the process. The function of the surfactant here is to allow the  nes and debris to

attract to each other, aggregate, and grow into large particles. Thus, the application of the proper

surfactant to help the debris and  nes agglomerate for easy  ltration will save cost and result in

cleaner lubricant.

Bioresistance is the property of a lubricant to resist attack by bacteria and fungi (mold and yeast)

microbes. Attack by microbes will result in a number of undesirable outcomes (e.g., changes in pH,

depletion of critical lubricant ingredients, bad odor, tool, and workpiece corrosion) that compromise

the proper functioning of the lubricant. The function of the surfactant here is to protect the lubricant

from attack by microbes. Surfactants used in such application are called biocides.

Biodegradability is the property of a lubricant to be easily and naturally degraded when disposed.

This requires that the lubricant ingredients are easy to digest by microbes employed in composting

and similar processes. Lubricants that are not biodegradable require expensive waste treatment pro-

cessing before disposal to comply with tightening environmental laws. Surfactants that are easily

biodegradable will improve the biodegradability of the lubricant in which they from a part.

16.6 SOLUBILIZATION WITH SURFACTANTS

Liquid lubricants are broadly classi ed into two groups: oil-based and water-based. Oil-based lubri-

cants comprise a base oil of appropriate viscosity with various solubilized additives. The function

of the additives is to reduce friction and wear as well as to impart the lubricant other important

characteristics such as oxidation stability, low pour point, and bioresistance. Oil-based lubricants

are used in processes where the heat generated during lubrication does not cause concern about tool

wear, product quality, productivity, safety, and other issues. Another reason for the use of oil-based

lubricants is when the presence of water causes undesirable outcomes such as poor product quality

(e.g., water stain), corrosion of tools and machine elements, and rust.

Water-based lubricants comprise 0–20% of all or portions of the oil-based lubricant ingredi-

ents mentioned above, solubilized in 80–99% water. Water-based lubricants are employed when

the lubrication process generates excessive heat capable of damaging tools, negatively affecting

product quality, causing  re, and other undesirable results. Examples of such lubrication processes

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406 Lubricant Additives: Chemistry and Applications

include metal forming (e.g., hot rolling of aluminum and steel) and metal removal (e.g., grinding

and machining).

Various types of surfactants (e.g., cosurfactants and coupling agents) are employed to achieve

proper dissolution of the oil phase in water. Water-based lubricants are classi ed into the following

categories depending on their composition and physical properties [12,13].

1. Soluble oils. These are o/w emulsions with up to 20% of the oil phase in water. They are

produced by vigorously mixing the oil and water phases in the presence of surfactants.

These emulsions comprise large droplets of oil stabilized by surfactants in water. Unlike

o/w microemulsions, the o/w emulsions have large diameter of oil droplets. As a result,

o/w emulsions re ect visible light and hence appear milky to the naked eye. Another major

difference between o/w emulsions and microemulsions has to do with their stability. O/w

emulsions are thermodynamically unstable and, hence, separate into the respective oil and

water phases over time. A schematic comparing o/w emulsions and o/w microemulsions is

given in Figure 16.5.

2. Semisynthetic oils. These are water-based lubricants with 1–5% (w/w) of solubilized oil

phase. These are the o/w microemulsions that have been discussed previously (Section 16.4)

and depicted in Figure 16.3e. Semisynthetic oils are clear and transparent solutions. This is

attributed to the fact that the size of the oil droplets is smaller than the wavelength of light.

As mentioned earlier, semisynthetic oils are thermodynamically stable (see Figure 16.5 for

comparison of o/w emulsions versus o/w microemulsions).

FIGURE 16.5 Emulsions versus microemulsions.

Surfactant

mix

Time

Oil

Water

2-phase

Emulsion

milky

Time

Surfactant/

co-surfactant

mix

2-phase

Water

Oil

Microemulsion

clear/transparent

Microemulsion

clear/transparen

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Surfactants in Lubrication 407

3. Synthetic oils. These are normal micellar solutions discussed previously (Section 16.4) and

depicted in Figure 16.3c. Synthetic oils contain no solubilized oil. However, they contain

additives solubilized in the continuous water phase and the lipophilic core of the normal

micelle, to provide the necessary friction, wear, and other properties to the synthetic oils.

Because of the presence of solubilized additives, synthetic oils can be considered as swol-

len micelles. Again, because of the small diameter of the swollen micelles relative to the

wavelength of light, synthetic oils are transparent and clear solutions. They are also thermo-

dynamically stable, that is, various additives will not appear as a separate phase over time.

16.7 BOUNDARY LUBRICATION PROPERTIES OF SURFACTANTS

Boundary lubrication occurs in processes where the friction surfaces are rubbing at relatively low

speeds and under very high load [13]. Under such conditions, no lubricant  lm is formed to separate

the rubbing surfaces from each other. The rubbing surfaces contact each other, but direct contact

is prevented by surfactants adsorbing on the friction surfaces. This is another important function

of surfactants in tribology. The effectiveness of surfactants at reducing friction and wear under

boundary conditions is highly dependent on the chemistries of their polar and nonpolar segments,

concentrations, properties of the friction surfaces, and process conditions (e.g., temperature). This

will be illustrated by a brief review of an extensive study into the boundary properties of agricul-

ture-based surfactants.

Proteins [14,15], starches [16,17], and vegetable oils [18–22], obtained from plant-based agricul-

tural products, with or without further modi cation (through enzymatic, chemical, or other methods),

have been found to possess surface-active properties. This has dramatically increased the potential

application areas where these materials can be used to develop new uses from surplus agricultural

crops. Surface-active materials are used in a wide range of consumer and industrial products includ-

ing food, cosmetics, pharmaceuticals, paints, coatings, adhesives, polymers, and lubricants [1–9].

Among the various ag-based products, vegetable oils are the most widely investigated raw materials

for lubricant applications [18–23]. This is because most vegetable oils are liquid at room temperature.

This, along with the fact that they are surface active, allows vegetable oils to be used in lubrication pro-

cesses that occur in all the lubrication regimes, that is, boundary, hydrodynamic, and mixed- lm.

Vegetable oils have two types of chemical structures: triglycerides and waxy esters (Figure 16.6).

However, most vegetable oils are triglycerides. In both triglycerides and waxy esters, the structure

(a)

(b)

FIGURE 16.6 Chemical structures of vegetable oils: (a) triglycerides, and (b) waxy esters.

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408 Lubricant Additives: Chemistry and Applications

of the polar segment of the molecules comprises one or more ester linkages of fatty acid with glyc-

erol or long-chain fatty alcohols. In all cases, various structures of fatty acids are involved. The

structural variations of the fatty acids include chain length; degree, position, and stereochemistry

of unsaturation; and presence of additional functional groups (such as hydroxy or epoxy groups), on

the fatty acid chain. As a result, triglycerides and waxy esters can have a multitude of structures. In

addition, the triglycerides and waxy esters from speci c crops (e.g., soybean oil) are mixtures of vari-

ous structures. Despite these enormous possible structural variations, the structural composition of

triglycerides from a speci c crop (e.g., soybean oil) is  xed and well-known [23]. Thus, for example,

triglycerides of regular soybean oil comprise 23% oleic acid and 54% linoleic acid, whereas regular

canola oil comprises 61% oleic acid and 23% linoleic acid [23].

The boundary lubrication properties of vegetable oils were investigated in steel/steel and steel/

starch contact. In addition to varying the properties of the contacting surfaces (steel/steel versus

steel/starch), vegetable oils of varying chemical structures were employed. Also, simple methyl esters

of fatty acids were included in these studies as model compounds to the more complex triglycerides

and waxy esters. A summary of these studies along with subsequent data analysis is reviewed next.

16.7.1 BOUNDARY FRICTION OF VEGETABLE OILS IN METAL/METAL CONTACT

The boundary lubrication properties of vegetable oils, in metal/metal contact, were investigated

using a ball-on-disk tribometer. A schematic depiction of the ball-on-disk tribometer is shown in

Figure 16.7. In these studies, the steel ball and steel disk were immersed in a lubricant of hexadec-

ane, with varying concentrations of vegetable oils. Friction between the stationary steel disk and

the rotating steel ball were measured under the following conditions: speed, 5 rpm or 6.22 m/s;

load, 1788N or 400 lb; temperature, room temperature; test duration, 15 min. The concentration of

the vegetable oils in hexadecane varied in the range 0.0–0.6 M. A typical data from a ball-on-disk

tribometer are shown in Figure 16.8. The data show that the coef cient of friction (COF) increases

with time and levels off after ∼6 min. The COF for the speci c lubricant is obtained by averaging

the COF values in the steady-state region.

The effect of vegetable oil concentration on boundary COF from a ball-on- at tribometer is

shown in Figure 16.9. The data in Figure 16.9 have four important features that need pointing out:

(1) the COF without vegetable oil is very high (>0.5); (2) addition of vegetable oil results in a sharp

reduction of COF; (3) the COF decreases with increasing concentration of vegetable oil and reaches

a minimum value; and (4) once the minimum value is attained, the COF remains constant and inde-

pendent of any further increase in vegetable oil concentration. These observations are important for

understanding the mechanism of boundary lubrication by vegetable oils. It also provides the basis

for proper analysis of the friction data to elucidate the effect of vegetable oil and substrate properties

on boundary friction.

FIGURE 16.7 After schematic of ball-on-disk tribometer. (From Biresaw, G., Adharyu, A., Erhan, S.Z.,

Carriere, C.J., J. Am. Oil Chem. Soc., 79(1), 53–58, 2002. With permission.)

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Surfactants in Lubrication 409

FIGURE 16.8 After typical data from a ball-on-disk tribometer. (Biresaw, G., Adharyu, A., Erhan, S.Z.,

Carriere, C.J., J. Am. Oil Chem. Soc., 79(1), 53–58, 2002. With permission.) (SBO, Soybean oil.)

0.20

0.15

0.10

0.05

0.00

COF

12:00 AM

1/1/04

12:04 AM 12:08 AM 12:12 AM 12:16 AM

Run time

0.005 M SBO in hexadecane, COF = 0.208

COF

0.4

0.3

0.2

0.1

0.0

COF

0.60.50.40.30.20.10.0

[Cottonseed] (M)

[Cottonseed] in hexadecane versus COF

COF

FIGURE 16.9 After effect of vegetable oil concentration in hexadecane on steel/steel boundary friction.

(Adhvaryu, A., Biresaw, G., Sharma, B.K., Erhan, S.Z., Ind. Eng. Chem. Res., 45(10), 3735–3740, 2006. With

permission.)

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410 Lubricant Additives: Chemistry and Applications

16.7.2 BOUNDARY FRICTION OF VEGETABLE OILS IN STARCH/METAL CONTACT

Vegetable oils can be encapsulated into a starch matrix by a steam-jet cooking process called

Fantesk

[24]. The resulting starch–oil composite can be redispersed in water and applied on sheet

metal surfaces as a dry  lm lubricant [25–28]. Dry  lm lubricants are those that are applied on sheet

metals to help protect the surface from damage (dents, rust, and corrosion) during transportation,

storage, or as lubricants in subsequent forming processes [13]. Studies were conducted to investigate

the friction properties of vegetable oils in starch–oil composites. In this investigation, composites

with varying concentrations of vegetable oil (0–40 parts per hundred) were redispersed in water,

applied (using various methods) onto steel surfaces, and its friction properties evaluated. Bound-

ary friction of the dry  lm was measured using a ball-on- at tribometer (Figure 16.10) at room

temperature and the following conditions: speed, 2.54 mm/s; load, 14.7 N (1500 gf); temperature,

room temperature; test duration, 24 s. A typical data from a ball-on- at tribometer are shown in

Load

cell

Platen

Sled

Steel

balls

Flat

specimen

FIGURE 16.10 After schematic of a ball-on- at tribometer. (Biresaw, G., Erhan, S.M., J. Am. Oil Chem.

Soc., 79, 291–296, 2002. With permission.)

200

150

100

Friction force (g)

Run time (s)

Starch−oil composite with10 pph oil

Friction force (g)

Static friction = 183.6 g

<Kinetic friction> =150.3 +/− 5.0 g (N = 4608)

FIGURE 16.11 After typical data from a ball-on- at tribometer. (Biresaw, G., Erhan, S.M., J. Am. Oil

Chem. Soc., 79, 291–296, 2002. With permission.)

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Surfactants in Lubrication 411

Figure 16.11. As can be seen in Figure 16.11, initially, the friction force rose sharply to a maximum

and immediately went down to a steady-state value for the remainder of the test period. The maxi-

mum value corresponds to the static friction of the steel ball against the starch, whereas the steady-

state value corresponds to the kinetic friction. The COF for the dry  lm lubricant is obtained by

dividing the average of the steady-state friction force data by the load.

The effect of vegetable oil concentration in starch matrix on starch-steel boundary friction is

illustrated in Figure 16.12. The data show that, in the absence of vegetable oil, the COF between

starch and steel is very high (∼0.8). However, incorporation of vegetable oil into the starch matrix

results in a sharp decrease in COF, which eventually levels off to a constant value. Further addition

of vegetable oil will cause no change in COF. The vegetable oil concentration versus COF pro le

in starch/metal contact shown in Figure 16.12 is similar to that discussed before for metal/metal

contact (Figure 16.9). Thus, it is reasonable to assume that a similar mechanism may be responsible

for the boundary friction properties of vegetable oils in both systems.

16.7.3 MECHANISM OF BOUNDARY LUBRICATION OF VEGETABLE OILS

The effect of vegetable oil concentration on metal/metal and starch/metal boundary COF can be

explained using an adsorption model [29,30]. According to this model, in the absence of vegetable

oil, high COF is observed because of rubbing between two high-energy surfaces (metal/metal or

starch/metal), which are in direct contact (boundary conditions). When vegetable oil is introduced

into the lubricant, the COF is reduced because the vegetable oil adsorbs on the rubbing surfaces

(metal or starch) and prevents direct contact between the two high-energy surfaces.

Adsorption occurs due to a strong polar–polar interaction between the polar groups of the veg-

etable oils and the polar adsorption sites on the starch or metal surfaces. According to the adsorp-

tion model, there are a  nite number of adsorption sites (S

) on the surfaces, and once all adsorption

sites are occupied, full surface coverage is attained, and no further adsorption occurs and friction

remains unchanged. The adsorption sites could be oxides or hydroxides in the metal surfaces and

hydroxyl groups on starch. Depending on the concentration of the vegetable oil in hexadecane or

1.0

0.8

0.6

0.4

0.2

0.0

COF

[SBO], pph of dr

PFGS

Effect of [SBO] on COF

COF

FIGURE 16.12 After effect of vegetable oil concentration in starch matrix on starch/steel boundary fric-

tion. (Biresaw, G., Erhan, S.M., J. Am. Oil Chem. Soc., 79, 291–296, 2002. With permission.) (PEGS, puri ed

food grade starch.)

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412 Lubricant Additives: Chemistry and Applications

starch matrix, the adsorption sites on the steel or starch surface may be fully or partially occupied

by the vegetable oils. The surface concentration of vegetable oils is expressed in terms of fractional

surface coverage, θ, which is de ned as follows:

⫽

















(16.2)

where [S

] = [S

] + [S

]; S

and S

are occupied and unoccupied adsorption sites, respectively.

The fractional surface coverage is obtained from boundary friction data as follows [17–21,28–30]:

⫽

⫺

(16.3)

where

= COF without vegetable oil

= minimum COF or COF at full surface coverage

= COF at various vegetable oil concentrations

Note that in Equation 16.3, without added vegetable oil, f

= f

and θ = 0; at full surface cover-

age, f

= f

and θ = 1. Thus, the value of θ varies in the range of 0–1.

The adsorption model also stipulated that adsorption of vegetable oil is an equilibrium process

between vegetable oil in solution (O

) and that on the surface as follows:

OS S

⫹

→

←

(16.4)

where K

is the equilibrium constant in moles per liter, de ned as follows:

⫽

























(16.5)

in Equation 16.5 can be used to calculate the free energy of adsorption (ΔG

ads

) of vegetable

oils using an appropriate adsorption model. However, to be able to do that, two things have to be

accomplished. First, an adsorption isotherm must be constructed. An adsorption isotherm shows

the relationship between the concentration of vegetable oil in solution (moles per liter) and that on

the surface, expressed as fractional surface coverage, θ. Second, the adsorption isotherm must be

analyzed using an appropriate adsorption model. An adsorption model is a mathematical expression

describing how surface concentration is related to solution concentration. The equilibrium constant

is obtained by analyzing the adsorption isotherm data with the adsorption model.

Typical adsorption isotherms (vegetable oil concentration in hexadecane or starch versus fractional

surface coverage, θ), calculated from boundary coef cient of friction data using Equation 16.3, are

given in Figures 16.13 and 16.14. θ represents the concentration of the vegetable oil on the metal or

starch surface and the COF data are used in the calculation of θ (Figures 16.13 and 16.14). As can

be seen in Figures 16.13 and 16.14, the concentration versus θ pro le (or the adsorption isotherm)

is a mirror image of the concentration versus COF pro le.

Various models are available for analyzing adsorption isotherms [5,6]. The simplest such model

is called the Langmuir adsorption model [31], which assumes that adsorption is only due to primary

interaction between the vegetable oil and the surface. The model assumes no contribution to adsorp-

tion from lateral interaction between the adsorbed molecules. The Langmuir model predicts the

following relationships between surface and solution concentrations of vegetable oils:



⫽⫹

{[]}K













(16.6)

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Surfactants in Lubrication 413

According to Equation 16.6, a plot of [O

]

−1

versus θ

−1

should give a straight line with a slope of

−1

and an intercept of 1. The free energy of adsorption, ΔG

ads

, is then obtained by substituting the

value of K

obtained from Equation 16.6 in Equation 16.7:

ΔG

ads

= ΔG

= –RT[ln(K

)] (kcal/mol) (16.7)

0.4

0.3

0.2

0.1

0.0

COF

0.60.50.40.30.20.10.0

[Cottonseed] (M)

1.0

0.8

0.6

0.4

0.2

0.0

FractCov

Effect of [Cottonseed] in hexadecane on

COF

FractCov

FIGURE 16.13 Concentration in hexadecane versus fractional surface coverage, θ, of vegetable oil from

boundary friction data on a ball-on-disk tribometer.

FIGURE 16.14 Concentration in starch versus fractional surface coverage, θ, of vegetable oil from boundary

friction data on a ball-on- at tribometer.

0.8

0.6

0.4

0.2

0.0

COF

0.300.250.200.150.100.050.00

[Jo

oba] (mol/k

)

1.0

0.8

0.6

0.4

0.2

0.0

pfg-jojoba

COF

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