MANUFACTURING OF GREASE

The manufacture of a grease involves dispersion of a thickener in the oil. The thickener or jelling agent in over 90% of greases is a metallic soap. The usual method by which a metallic soap thickener is formed can be described as follows:

• Fat (Animal – vegetable) + Metal Hydroxide
• Calcium Hydroxide
• Sodium Hydroxide
• Lithium Hydroxide
• Monohydrate Metal Soap + Glycerine

This reaction is carried out by cooking the fatty material and alkali with normally some oil in order to get the soap well dispersed and also to make it softer and easier to stir. Such a procedure produces a wet soap partially dispersed in the mineral oil. It is then cut back with additional mineral oil to produce consistency desired in the finished grease. The soap oil mixture is a plastic mass with a tough fibrous structure. After the cut back operation. It is necessary to further process the grease by kettle milling or homogenization to modity its structure. The grease is then deaerated to remove entrained air and then filtered to remove any contaminants they may have been picked up during manufacture.

The manufacture of greases therefore involves the following steps, some of which may be accomplished simultaneously.

1. SAPONIFICATION
2. DE-HYDRATION
3. CUT – BACK
4. MILLING
5. DEAERATION
6. FILTERATION

The operations are carried out in vessel known as grease kettle, which is equipped with some form of agitation and with heating facility. The agitation is usually of the double acting type that is it has a set of paddles rotating in one direction with a second set rotating in the opposite direction. The rate of cooling is very important in the development of the proper structure for many types of grease. Hence, close temperature control is required to manufacture a grease of desired properties. The structure of the grease can be modified by milling which can be carried out in the kettle itself during the cooling period or may be done outside the kettle. For out side milling, a high shear rate gear pump, homogenizer or colloid mill is normally used. Depending upon the types of grease, the total time required for manufacture of a grease batch in a kettle varies from 10 / 20 hours.

GREASE CHARACTERISTIC
Grease differs from an oil in that it is usually plastic. It is the plasticity that gives a grease its special property of staying in a place where an oil would leak out. Greases can be of different kinds. They can be light or dark, hard, or soft or buttery or fibrous.

TESTS

Following tests are generally carried out on the greases.

1. Appearance, Ash % by Wt. Nature of ASH Colour visual
2. Copper strip Corrosion Test.
3. Consistency : Cone Penetration, unworked Worked 60 X 10000 strokes, 100,000 strokes
4. Dropping point ºC
5. Free Alkali % by Wt. as – free acid as oleic % by wt.
6. Heat stability
7. Filler Content % By Wt.
8. Mineral oil Content % By wt. K. Viscosity at 40ºC K. Viscosity at 100ºC Viscosity Index
9. Flash Point C-o-C ºC
10. Soap content % By wt. type of soap
11. Water content

Performance tests

1. Roll stability test
2. Timken OK Load Test – EP
3. Four Ball EP test
4. Tilted wheel bearing test
5. EMCOR – Bearing Corrosion
6.  Test Oxidation stability

NLGI Grade Numbers
NLGI GRADE NUMBERS, DEFINATION AND METHODS OF APPLICATION

It will be observed that there is a difference of 30 units in each NLGI grade Numbers. And in between 2 grades there is a difference of 45 units.

National lubricating grease institute – U.S.A.
Consistency: Is a measure of the relative hardness of a grease. Working refers to the shear that takes place in a grease when one grease moves relative to another layer. This occurs whenever a grease is handled, stirred, mixed or moved as in a bearing. The consistency of grease usually changes with working, and the direction and degree of change depend upon the type and amount, of thickener in the grease and the shear rate. Normally, greases soften with working – they loose consistency.

RAW MATERIAL
Lubricating fluid most commonly used is mineral oil. The choice of oil depends upon the intended application, and anything from the lightest spindle oil to the heaviest cylinder oil can be used, but usually a light or medium machine oil is chosen. Other fluids used for special purposes include dibasic esters such as dioctyl sebacate, the different types of silicone oils, and polyalkylene glycols.

The fatty material used to form grease making soaps are mainly of animal origin but also include vegetable and marine oils and fats. They can be used as the original glycerides or as the fatty acids split from the glycerides or fractions thereof recovered by filteration distillation or solvent crystallization. Hydrogentated oils are also used especially fish oils and castor oil, the latter being important as the source of 12- hydroxyl stearic acid. Most fats contain both saturated and unsaturated acids in the range C 12 to C 18 chain length, and in some cases (especially fish oil) to C 20 and C 22. The C 16 and C 18 acids preponderate. The type of material affects the melting point of the soap, and the degree of unsaturation may affect grease texture.

Fatty Material
Fatty materials can be characterized chemically for grease manufacturing purposes by their:

1. Saponification value
2. Neutralization number
3. Iodine value

Saponification value: Is an indication of average molecular weight and hence approximate composition (if no unsaponifiable matter is present) and is a measure of the amount of alkali needed to convert it to soap.

Neutralisation Number: Measures the amount of the fatty acid present.
Saponification value and Neutralization number are expressed as Mg KCR/GM.

Iodine Value: Indicates the extent of unsaturation. Too much unsaturation leads to poor oxidation stability, so high Iodine values (0ver 60-70), are avoided and values of 35 or less are preferred.
Iodine value is expressed as No. of gms. Of Iodine absorbed by 100 gms. of product.

Alkalies
Many different alkalies have been used or proposed for saponifying the fatty material but calcium sodium and lithium hydroxides are the most commonly used.

Calcium Hydroxide: is used in the form of hydrated lime, which should be finely ground to pass 200 mesh and with a low content of carbonate which does not saponify easily, limits of 95% Ca(OH)2 minimum and 2.0 % CaCo3 maximum have been recommended.

Sodium Hydroxide: Usually supplied as solid or flake caustic soda but usually used in concentrated aqueous solution: should be at least 99% purity.

Lithium Hydroxide is supplied as the crystalline monohydrate LioH, H2o with LiOH 55% minimum and Carbonate and other alkali metals less than 1.0% by wt. each.

While most greases are made from soaps formed in situ, pre-formed or pre-made soaps supplied by chemical manufacturers are used to some extent.
The commonest are:

1. Lithium Stearate
2. Aluminium Stearate
3. Non stop thickners are also similarly available from manufacturers.

Additivies
Various additives are used in greases in much the same way as in lubricating oils in order to improve the characteristics in particular respect.

Oxidation: Oxidation inhibitiors are widely used in greases in much the same way as in lubricating oils in order to improve the characteristics in particular respect e.g. to obtain longer storage and service life.
It is interesting to note that greases are apparently oxidised in different ways statically as in storage and dynamically as in service.
Amine and phenolic derivatives are effective inhibitors under static conditions and for service are effective high temperatures.
Dilauryl selenide and phenothiazine are effective in high temperature service.

Rust Inhibitors
Rust Inhibitors are mostly surface active materials of which a wide variety are used, and exert their function by forming absorbed films on the metallic surfaces.

EP – Additives
EP Additives are necessary in cases where the greases has to sustain heavy or shock loading as in Roll-neck bearing of steel mill. Lead soaps, sulphurized oils alone or in combination are used for this purpose. Other additives containing sulphur, phosphorous or chlorine may be used.
Additives are also used to improve:

• Structure stability
• Resistance to bleeding
• Adhesiveness
• Stringiness
• Water resistance and so on.

Solid Additives (Fillers) Graphite.
Molybdenum disulphide are also used under very heavy loads or at high temperatures when the normal hydrodynamic lubrication provided by grease has for some reasons ceased to operate.