الطرق الحديثة لتصنيع انواع الشحوم المعدنية المختلفة

THE MODERN PROCESS FOR GREASE PRODUCTIONS

BY

TECHNOLAB EL-BAHAA GROUP

GENERAL.DR

BAHAA BADR

CHEMICAL ADVISOR

A solid or semisolid lubricant consisting of a thickening agent (soap or other additives) in a fluid lubricant (usually petroleum lubricating oil) is called grease.

Grease is a lubricant which has been thickened in order that it remains in contact with moving surfaces and not leak out under gravity or centrifugal action.

ASTM defines lubricating grease as “A solid to semi-solid product consisting of dispersion of a thickening agent in a liquid lubricant.”

There has been a need since ancient times for lubricating greases.

The Egyptians used mutton fat and beef tallow to reduce axle friction in chariots as far back as 1400 B.C.

Good lubricating greases were not available until the development of petroleum based oils in the late 1800’s.

Today there are many different types of lubricating greases but the basic structure of these greases is similar.

Greases are used where a mechanism can only be lubricated infrequently and where a lubricating oil would not stay in position.

In general greases contain 70-95% of base oils, 5-20% of thickening agent, and 0-10% of additives.

Depending on type of thickening agents different types of greases are classified as follows.

Calcium

Lithium

Titanium

Sodium

Aluminium

Clay

Polyurea and others

There are two different methods by which grease can be manufactured.

Batch process

Continuous process

The manufacture of lubricating greases has shown constant progress with time.

This holds true for raw materials, equipments, processes, and formulations.

RAW MATERIAL

As this definition indicates, there are three components that form lubricating grease.

These components are oil, thickener and additives.

The base oil and additive package are the major components in grease formulations, and as such, exert considerable influence on the behavior of the grease.

The thickener is often referred to as a sponge that holds the lubricant (base oil and additives).

BASE OIL

Most greases produced today use mineral oil as their fluid components.

These mineral oil-based greases typically provide satisfactory performance in most industrial applications.

In extreme temperature conditions (low or high), grease that utilizes synthetic base oil provide better stability.

When formulating grease the selection of base fluid is not only about product properties, it’s also about production costs.

And a significant proportion of the production cost is the amount of soap required to achieve a certain NLGI grade.

The solvating power of the base fluid affects the amount of soap needed.

The following test was performed to determine the difference in amounts of soap needed between naphthenic (high solvating power) and paraffinic (lower solvating power) oils.

NLGI grade 2 greases were produced using three naphthenic oils with increasing degrees of refining and a paraffinic oil, all of approximately the same viscosity.

As can be seen, the naphthenic oils with higher solvating power result in a saving of as much as 25% on soap consumption if you compare the oil with the lowest aniline point with the paraffinic oil.

This would obviously have a significant impact on production costs as 25% less soap would be needed to produce the same NLGI grade.

Another production cost to consider is energy consumption.

When “cooking” the grease, the temperature must be raised until the fatty acids are dissolved.

Obviously the higher the temperature needed, the more energy is consumed.

Higher temperatures also increase the risk of soap oxidation.

The difference in the solution temperature of hydroxystearic acid in three naphthenic oils with different degrees of refining and one paraffinic oil.

The concentration of hydroxystearic acid in each oil was 30 wt%, which is representative of the typical concentration during grease cooking.

As can be seen, the temperature at which the fatty acid dissolves is significantly lower for all the naphthenic oils than for the paraffinic oil.

The lower temperatures needed in greases with naphthenic oils is due to their higher solvating power.

Oils with higher solvating power by definition have a higher capability of dissolving additives.

The additives are dissolved at lower temperatures and smaller amounts of them are required to achieve the same grades.
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Effect of base oil on grease properties

Due to the higher solvating power of naphthenic oils they display a higher affinity towards the soap.

In naphthenic-based greases there is a prevalence of physiochemical interaction between the oil and the soap, as opposed to paraffinic-based greases where most of the oil is physically rather than physiochemically trapped in the soap structure.

This means the naphthenic oil is more intimately bonded with the soap structure and displays a lower tendency to separate or bleed from the grease.

THICKENER

The thickener is a material that, in combination with the selected lubricant, will produce the solid to semi fluid structure.

The primary type of thickener used in current grease is metallic soap.

These soaps include lithium, aluminum, clay, Polyurea, sodium and calcium. Lately, complex thickener-type greases are gaining popularity.

They are being selected because of their high dropping points and excellent load-carrying abilities.
Complex
greases are made by combining the conventional metallic soap with a complexing agent

. The most widely used complex grease is lithium based.

These are made with a combination of conventional lithium soap and a low- molecular-weight organic acid as the complexing agent.

Non-soap thickeners are also gaining popularity in special applications such as high-temperature environments.

Smectonite or Bentonite and silica aerogel are examples of thickeners that do not melt at high temperatures.

There is a misconception, however, that even though the thickener may be able to withstand the high temperatures, the base oil will oxidize quickly at elevated temperatures, thus requiring a frequent relube interval.

ADDITIVES

Additives can play several roles in lubricating grease.

These primarily include enhancing the existing desirable properties, suppressing the existing undesirable properties, and imparting new properties.

The most common additives are oxidation and rust inhibitors, extreme pressure, antiwear, and friction-reducing agents.

In addition to these additives, boundary lubricants such as molybdenum disulfide or graphite may be suspended in the grease to reduce friction and wear without adverse chemical reactions to the metal surfaces during heavy loading and slow speeds

Different Types of Additives With Their Functions Are As Follows
FUNCTIONS

TYPE OF ADDITIVES
Antioxidant

Phenols,
Amines,
Phosphorous Compound,
Sulfur Compound
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Extreme Pressure & Corrosion Inhibitor

Tricrysylphospate,
Amine Phosohate
Triphenylthiphosphate

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Rust Inhibitor

Barium & Calcium Sulphonates

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Corrosion Inhibitor

Benzotrizoles,
Mercapto Enzothiozoles,
Dimercaptothiozoles,
Alkyl Benzene Sulphonates

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Vi Improvers

Methacrylates.

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Antiwear

ZDDP,
Antimony Di Alkyl Dithio Phosphate

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Water Repelling Agent

Fatty Oils
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Tackiness Agent

Polymers (Methacrilate)
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Friction Modifiers

MoS2, Graphite
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SOAP BASED GREASE

Soap based grease contains organic or inorganic thickeners.

It is formed when the fatty acid or ester (from either animals or vegetables) is mixed with an alkali (such as lithium) and then heated under pressure with agitation.

The process of this chemical reaction, which takes place is known as saponification.

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LITHIUM GREASE.

(1)

Lithium grease is smooth, buttery-textured and by far the most popular when compared to all others.

The normal grease contains lithium 12-hydroxystearate soap.

It has a dropping point around 205°C and can be used at temperatures up to about 150°C It can also be used at temperatures as low as (-)35°C.

It has good shear stability and a relatively low coefficient of friction, which permits higher machine operating speeds.

It has good water-resistance, but not as good as that of calcium or aluminum base. Pumpability and resistance to oil separation are good to excellent.

It does not naturally inhibit rust, but additives can provide rust resistance.

Anti-oxidants and extreme pressure additives are also responsive in lithium greases.

(2)

Lithium complex grease and lithium soap grease have similar properties except the
complex grease has superior thermal stability as indicated by a dropping point of 260°C.

It is generally considered to be the nearest thing to true multipurpose grease.

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CALCIUM GREASE.

(1)

Calcium or lime grease, the first of the modern production greases, is prepared by reacting mineral oil with fats, fatty acids, small amount of water, and calcium hydroxide (also known as hydrated lime).

The water modifies the soap structure to absorb mineral oil.

Because of water evaporation, calcium grease is sensitive to elevated temperatures.

It dehydrates at temperatures around 79°C at which its structure collapses, resulting in softening and, eventually, phase separation.

Greases with soft consistencies can dehydrate at lower temperatures while greases with firm consistencies can lubricate satisfactorily to temperatures around 93°C.

In spite of the temperature limitations, lime grease does not emulsify in water and is excellent at resisting “wash out.

” Also, its manufacturing cost is relatively low.

If calcium grease is prepared from 12-hydroxystearic acid, the result is an anhydrous (waterless) grease.

Since dehydration is not a concern, anhydrous calcium grease can be used continuously to a maximum temperature of around 110°C

(2)

Calcium complex grease is prepared by adding the salt calcium acetate.

The salt provides the grease with extreme pressure characteristics without using an additive.

Dropping points greater than 260°C can be obtained and the maximum usable temperature increases to approximately 177 °C.

With the exception of poor pumpability in high-pressure centralized systems, where caking and hardening sometimes occur calcium complex greases have good all-around characteristics that make them desirable multipurpose greases.

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SODIUM GREASE.

Sodium grease was developed for use at higher operating temperatures than the early hydrated calcium greases.

Sodium grease can be used at temperatures up to 121°C but it is soluble in water and readily washes out.

Sodium is sometimes mixed with other metal soaps, especially calcium, to improve water resistance.

Although it has better adhesive properties than calcium grease, the use of sodium grease is declining due to its lack of versatility.

It cannot compete with water-resistant, more heat-resistant multipurpose greases.

It is, however, still recommended for certain heavy-duty applications and well-sealed electric motors.
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ALUMINUM GREASE.

(1)

Aluminum grease is normally clear and has a somewhat stringy texture, more so when produced from high-viscosity oils.

When heated above 79°C this stringiness increases and produces a rubber like substance that pulls away from metal surfaces, reducing lubrication and increasing power consumption.

Aluminum grease has good water resistance, good adhesive properties, and inhibits rust without additives, but it tends to be short-lived.

It has excellent inherent oxidation stability but relatively poor shear stability and pumpability.

(2)

Aluminum complex grease has a maximum usable temperature of almost 100 °C higher than aluminum-soap greases.

It has good water-and-chemical resistance but tends to have shorter life in high-temperature, high-speed applications.

NON-SOAP BASED GREASE

A non-soap based grease consists of inorganic like molybednum disulphide and graphite, as well as organic thickeners.

The organic thickeners are considered as non-abrasives, which have high capacity to absorb and "hold" the base oil.

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POLYUREA GREASE.

Polyurea is the most important organic non soap thickener.

It is a low-molecular-weight organic polymer produced by reacting amines (an ammonia derivative) with iso cyanates, which results in an oil soluble chemical thickener.

Polyurea grease has outstanding resistance to oxidation because it contains no metal soaps (which tend to invite oxidation).

It effectively lubricates over a wide temperature range of -20 to 177 °C and has long life. Water-resistance is good to excellent, depending on the grade.

It works well with many elastomer seal materials.

It is used with all types of bearings but has been particularly effective in ball bearings.

Its durability makes it well suited for sealed-for-life bearing applications

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ORGANO-CLAY.

Organo-clay is the most commonly used inorganic thickener.

Its thickener is modified clay, insoluble in oil in its normal form, but through complex chemical processes, converts to platelets that attract and hold oil.

Organo-clay thickener structures are amorphous and gel-like rather than the fibrous, crystalline structures of soap thickeners.

This grease has excellent heat-resistance since clay does not melt.

Maximum operating temperature is limited by the evaporation temperature of its mineral oil, which is around 177 °C.

However, with frequent grease changes, this multipurpose grease can operate for short periods at temperatures up to its dropping point, which is about 260 °C.

A disadvantage is that greases made with higher-viscosity oils for high thermal stability will have poor low temperature performance.

Organo-clay grease has excellent water-resistance but requires additives for oxidation and rust resistance. Work stability is fair to good.

Pumpability and resistance to oil separation are good for this buttery textured grease.

CHARACTERISTICS

As with oil, grease displays its own set of characteristics that must be considered when being chosen for an application.

The characteristics commonly found on product data sheets include the following:
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PUMPABILITY

Pumpability is the ability of a grease to be pumped or pushed through a system.

More practically, pumpability is the ease with which pressurized grease can flow through lines, nozzles and fittings of grease-dispensing systems.
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WATER RESISTANCE

This is the ability of grease to withstand the effects of water with no change in its ability to lubricate.

Soap/water lather may suspend the oil in the grease, forming an emulsion that can wash away or, to a lesser extent, reduce lubricity by diluting and changing grease consistency and texture.
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CONSISTENCY

Grease consistency depends on the type and amount of thickener used and the viscosity of its base oil.

Grease’s consistency is its resistance to deformation by an applied force.

The measure of consistency is called penetration.

Penetration depends on whether the consistency has been altered by handling or working.

ASTM D 217 and D 1403 methods measure penetration of unworked and worked greases.

To measure penetration, a cone of given weight is allowed to sink into a grease for five seconds at a standard temperature of 25°C.

The depth, in tenths of a millimeter, to which the cone sinks into the grease, is the penetration.

A penetration of 100 would represent solid grease while a penetration of 450 would be semi fluid.
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DROPPING POINT

Dropping point is an indicator of the heat resistance of grease.

As grease temperature increases, penetration increases until the grease liquefies and the desired consistency is lost.

The dropping point is the temp which grease becomes fluid enough to drip.

The dropping point indicates the upper temperature limit at which grease retains its structure, not the max temperature at which grease may be used.
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OXIDATION STABILITY

This is the ability of grease to resist a chemical union with oxygen.

The reaction of grease with oxygen produces insoluble gum, sludge and lacquer-like deposits that cause sluggish operation, increased wear and reduction of clearances.

Prolonged exposure to high temperatures accelerates oxidation in greases.

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HIGH-TEMPERATURE EFFECTS

High temperatures harm greases more than they harm oils.

Grease, by its nature, cannot dissipate heat by convection like circulating oil.

Consequently, without the ability to transfer away heat, excessive temperatures result in
accelerated oxidation or even carbonization where grease hardens or forms a crust.

Effective grease lubrication depends on the grease's consistency.

High temperatures induce softening and bleeding, causing grease to flow away from needed areas.

The mineral oil in grease can flash, burn or evaporate at temperatures greater than 177°C
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LOW-TEMPERATURE EFFECTS

If the temperature of grease is lowered enough, it will become so viscous that it can be classified as hard grease.

Pumpability suffers and machinery operation may become impossible due to torque limitations and power requirements.

As a guideline, the base oil's pour point is considered the low-temperature limit of grease.

MANUFACTURING
PROCESS

Lithium based grease is generally manufactured with two methods.

1.Batch process
2.Continuous process

Between these two methods Batch process is preferable because, it is more advantageous over Continuous process

STEPS INVOLVED DURING BATCH PROCESS

1. Saponification

2. Dehydration

3. Dilution, additive addition

4. Homogenization /milling

5. Check for suitability

6. Packaging

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TYPICAL FLOWCHART FOR BATCH PROCESS

Batch production is the most common manufacturing method. The steps of manufacturing include the following.

1.

Bulk ingredients are metered or weighed into the processing reactor.

For soap-based greases made by saponification (the process of forming soap by splitting a fat with an alkali), the fatty ingredient, alkali and a portion of the oil are added to the reactor.

By heating (300 - 450°F) and mixing, the fat is converted to soap, and the soap is dispersed throughout the mixture.

This may be done in open kettles or in closed pressure kettles.

After completion of saponification and dehydration (removal of water), the remaining oil is added to the batch to lower the temperature.

Next, the grease is milled or homogenized.

2.

This step of homogenization or milling is very important, because it will produce a uniform crystal and gel structure that will not change when the grease is used.

Homogenizing the grease will break down the solid particles or fibers and will disperse the resultant small particles in the liquid.

It also breaks up lumps, eliminates graininess and produces a smooth product.

Homogenization of certain types of greases will stiffen the grease producing lower penetration value.

Homogenization can improve texture and “brighten” grease’s appearance.

In many cases this homogenization process is carried out at temperatures greater than 200°F (93°C).

3.

After homogenization, the grease is further cooled, desecrated and packaged.

Of course, it is understood that there are many different grease manufacturing methods depending on the type of grease and the manufacturer.

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CONTINUOUS PROCESS

The saponifiable material, lubricating oil & metal base flow into inlet of saponification zone of tubular reactor.

In saponification zone pressure is about 100-300 psig and temperature up to 180°F.

The reactant stream is passed through saponification zone at a velocity which is preferably sufficient to maintain turbulent flow within the tubular reactor.

Reactant mixture flow velocity is sufficient for producing highly turbulent flow with Reynolds number in the range of about 4000 to 100000.

Flow rates required for obtaining the degree of turbulence are generally within the range of about 0.6-12.0ft3/min. of reaction mixture.

For obtaining such high flow rates of reactant mixture through saponification zone reactor outlet may be recycled to reactor via reactor inlet.

It is desirable to minimize the water injected into the grease with the additives. Temperature of the reaction mixture which comes out of the saponification zone is maintained in the range of about 250-350°F.

A temperature of about 350°F is sufficient to provide the necessary amount of heat to such combined mixtures.

The temperature of the combined mixture should not exceed the melting point of soap component of the grease.

Additional heat is imparted to the combined mixture in heating means to restore or increase the temperature of the combined mixture to about 250°F and to prevent condensation of water.

Dehydration zone comprises a vertical cylindrical vessel having a volume sufficient to receive combined mixture and water vapour and provide residence time of about 1 to 20 minute.

Dehydration zone is maintained under vacuum conditions.

Due to such conditions all the liquid water present in the combined mixture flash vaporizes.

The grease mixture is recycled continuously from the bottom of the dehydration zone with the pressure up to 10-200 psi.

Recycling of the grease mixture is preferably carried out at a rapid rate such that the turnover rate in the dehydration zone is at least equivalent to about the average volume of grease therein per minute.

The recycle rate and average residence time in dehydration zone are sufficient to provide a soap conditioning period of at least about 5 minute.

By such conditioning the soap of grease mixture is reduced to a consistency which contributes to the desired consistency for the product grease.

The base oil which is added in grease mixture is at lower temperature than that of the grease mixture.

This is done for cooling purpose.

In case if additional cooling is desirable then the grease may be passed through cooler and the grease mixture may be recycled for obtaining multiple passes.

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