Lubrication of synovial joints The extremely low value of coefficient of friction found in bone joints (~0.01) is due to the combined effect of the cartilage tissue as well as the synovial fluid present. Cartilage tissue at the ends of bones are soft, slippery and deformable. Synovial fluid contains certain molecules that cause the low COF.

Lamina splendens Collagen fibers Proteoglycan aggregate Chondrocyte cells Calcified cartilage Subchondral bone Superficial tangential zone (STZ) Middle zone Deep zone Zone of calcified cartilage Cartilage is made up of a porous material. It contains 70% by volume water. It is therefore considered to be a bi-phasic material. Within the cartilage there are hydrophilic molecules that hold the water within.

Lubrication regimes in a bone joint The bone joint undergoes a combination of lubrication regimes. Due to the deformable nature of cartilage, there is elastohydrodynamic lubrication (EHL). Under standing conditions there is normal squeeze film lubrication. When there is heavy load, the cartilage undergoes weeping lubrication. In weeping lubrication, the fluid that is present within the cartilage is squeezed out into the joint space to provide lubrication.

Synovial fluid- characteristics The lubricant in a bone joint is called synovial fluid. Syn means like and ovial means egg white. The lubricant is therefore similar to egg white. It is non-newtonian in nature. This non-newtonian behavior is provided by a molecule known as Hyaluronic acid containing a chain of the COO - group approximately 1.5 nm apart. In a healthy joint, the molecular weight of the hyaluronic acid is approximately 5 x 10 5 and its concentration is mg/ml. The hyaluronic acid of diseased joints gets depolymerized and the fluid does not have non-newtonian behavior.

5 Production of basic grades of lubricating oils

6 Ref:

7 Atmospheric distillation In an oil refinery, the principal process for converting crude oil into a range of marketable products is distillation After removal of gaseous products, the crude is distilled in a column at atmospheric pressure All materials boiling at temperatures below 340 o C (approximately) are vaporized in stages depending on their boiling temperature These materials are then condensed in order of increasing boiling range, at successively lower points in the column, and are drawn off

8 Atmospheric distillation (contd.) The distillates obtained are the raw materials for production of gas oils, kerosines and motor oils. The residue from the bottom of this column contains materials boiling above about 340 o C, and is the raw material for manufacture of lubricating oils, fuel oil, waxes and bitumen. This residue is generally known as ‘atmospheric residue’.

9 Vacuum distillation The temperature to which the residue from the atmospheric column can be raised is limited by its tendency to decompose or ‘crack’. Thus, further separation is effected in a vacuum distillation column where boiling is achieved at moderate temperatures by operating under reduced pressures. The products from the vacuum distillation column comprise a series of three or four distillate fractions separated according to boiling range, and a vacuum residue. The distillate fractions provide a number of materials with different viscosities, and an oil with a higher viscosity than the distillates is produced from the residue.

10 Vacuum distillation (contd.) Some heavy gas oil is also produced in this process and is removed from the top of the column. The distillate and residual fractions from the vacuum column are not, at this stage, suitable for blending into finished lubricating oils, but require to be treated to remove undesirable constituents. The treatments applied vary according to the properties of the crude.

11 Bitumen Bitumen is a mixture of organic liquids that are highly viscous, black, sticky, entirely soluble in carbon disulfide, and composed primarily of highly condensed polycyclic aromatic hydrocarbons. Naturally occurring or crude bitumen is a sticky, tar-like form of petroleum which is so thick and heavy that it must be heated or diluted before it will flow. At room temperature, it is much like cold molasses. Refined bitumen is the residual (bottom) fraction obtained by fractional distillation of crude oil. It is the heaviest fraction and the one with the highest boiling point, boiling at 525 °C (977 °F). Bitumen is primarily used for paving roads. It is also used for water proofing products, blending with other oils, and preservation of fossils.

12 Solvent de-asphalting The residue from the vacuum distillation column can be used as a blend component for fuel oils, or for bitumen manufacture. It can also be processed to yield a high viscosity lubricating oil. For lubricant production, the vacuum residue is treated with liquid propane, which dissolves the oily and waxy components and leaves the asphaltic residue as a precipitate. After separating the precipitate, the propane is stripped off for re-use and the treated residual oil proceeds to further processing, after which it is generally called ‘Bright stock’.

13 Solvent extraction or acid treatment All of the oils, including the de-asphalted vacuum residue, contains resins and aromatic hydrocarbons which impair stability and oxidation resistance and lower the viscosity index. These materials are removed, either by treatment with concentrated sulphuric acid, or by the more modern process of extraction with solvents. The principal solvents used in the extraction process are furfural and phenol.

14 Solvent extraction The disadvantage of the sulphuric acid process is that acid sludges are formed, which present a disposal problem. Extract from the solvent treatment can be stripped to recover solvent and the residue is suitable for fuel oil blending. Very heavy acid treatments are used to produce speciality products of highly paraffinic character such as medicinal ‘liquid paraffin’.

15 Solvent dewaxing Wax refers to crystalline substance in crude oil which are solids at room temperature. The waxes are normally dissolved in the lubricating oil stocks, but are readily precipitated at low temperatures to form a solid matrix which holds the liquid oil in the interstices. Thus, wax has the undesirable property of preventing oil from flowing freely at low temperatures, and is therefore removed.

16 Solvent dewaxing In this process the oil is dissolved in a mixture of solvents, such as methyl ethyl ketone and toluene, and chilled The wax is precipitated as a crystalline solid and is filtered off The solvent is stripped from the dewaxed oil for re- use, and the wax is treated separately to produce a marketable product

17 Finishing treatments The lubricating oil cuts receive a final treatment to neutralize or remove any acidic materials, to improve the color and increase the oxidation stability. Normally the oil is either percolated (passed through porous substance or small holes) through a bed of clay or mixed with finely divided clay, heated and filtered. The spent clay is removed and either discarded or reactivated for further use, and the oil is passed to storage.

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