Liquid lubricants

Liquid lubricants or lube oils

Vegetable and Animal oils
These are glycerides of higher fatty acids have very good oiliness. However, these oils cannot be used effectively, because
(i) They undergo oxidation at higher temperature and forms gummy and acidic products.
(ii) Also, they get hydrolysed easily under moist conditions. Actually, they are used as “blending agents” with other lubricating oils.

Mineral oils (or) Petroleum oils
It is obtained by fractional distillation of crude petroleum oil. The length of the hydrocarbon chain varies from C12 to C50. It is cheap and quite stable under normal operating condition. But it posses poor oilness. The oilness of which can be improved by mixing it with animal (or) vegetable oils.
The mineral oil obtained cannot be used as such because it contains a lot of impurities such as wax asphalt oxidisable impurities etc. These impurities have to remove from the mineral oil before using it as a lubricant. The impurities are generally removed by the following methods.

(a) Removal of waxes
Waxes get separated at lower temperature and interfere with lubricating properties.
The wax can be removed by dewaxing process in which the petroleum oil is mixed with a suitable solvent (propane trichloroethyl etc.,) and then cooled. The wax crystallises out and is removed by filtration.

(b) Removal of asphalt
Asphaltic and naphthenic materials tend to leave carbon deposits on the engine parts. These materials can be removed by acid refining process, in which the dewaxed oil is treated with con. H2SO4 and then agitated. Some of the unwanted impurities get dissolved in acid while other are converted into sludges. The sludges are removed by filtration. The filtrate is neutralized with calculated quantity of NaOH to neutralize the acid.

(c ) Removal of sulphur
Sulphur can be removed from the oil by desulphurization process in which the oil is treated with hydrogen in the presence of nickel as catalyst. During this process the unsaturated compounds are converted to satured compounds.

(d) Removel of coloured substance
The coloured and micro – crystalline waxes can be removed by filtration through Fuller’s earth.

Synthetic lubricants
Under severe operating condition ( -50 C to 250 C ) for example in air craft the lubricants are pumped at – 50 C but during take off and landing they get heated upto 120 – 150 C. Petroleum oils canot be effectively used because they tend to get oxidized at higher temperature while waxseparation will occur at lower temperatures. So synthetic lubricants have been developed range of ( -50 C to 250 C ) .

Example
Silicones, polyglycol, ethers, etc.

Blended (or) Compounded oils (or) Additives for lubricating oils
To improve the properties of the lubricating oils, certain substances called additives are added to the lubricating oils. The oil thus prepared are known as “ Blended oils (or) Compounded oils” .

Important additives and their functions

(i) Oiliness carriers - Fatty acids such as stearic acid palmatic acid, oleic acid. They increase the oilness adhering property of lubricants.
(ii) Extreme pressure additives - Organic chlorine compounds, organic sulphur compounds, organic phosphorous compounds. They react with metal surface film of lower shear strenge and high melting point.
(iii) Viscosity index improvers - n-hexanol, poly isobutlene, poly alkyl benzene. They prevent the oil from thinning higher temperatures and thickening at lower temperatures.
(iv) Pour-point depressants - Phenols, poly alkyl benzene. They prevent separation of wax from the lubricating oil.
(v) Thickeners - Polyesters, polystyrene. They increase the viscosity of the lubricants.
(vi) Anti oxidants - Aromatic amoino compounds, phenoilc, compounds. They retard the oxidation of the oil and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenols salts of carboxylic acids, sulphonates. They prevent foreign particles and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenls, salts of carboxylic acids, sulphonates. They prevent foreign particles and carbon deposits in engines which block the passage of oil.
(viii) Corrosion preventors or Corrosion inhibitors - Tricaresyl phosphates, organic compounds phosphorous antimony. They are adsorded on metal surfaces there by protecting the surface from attack by moisture.