Petroleum based lubricants can be used under abnormal conditions like extremely high temperatures, chemically reactive atmosphere etc,. By employing certain specific additives. However synthetic lubricants have been developed which alone can meet the most drastic and sever conditions such as those existing in aircraft engine, in which the same lubricants may have to used in the temperature range of -50 to 250 deg centigrade. Such a lubricant should posses’ low freezing point, high viscosity index and also should be non-inflammable.
Modern synthetic lubricants possess in general the following distinguishing characteristic:
- Non-inflammable
- High flash point
- High thermal stability at high operating temperature
- High viscosity index
- Chemical stability
Important synthetic lubricants are given below:
1) Polymerized hydrocarbons like polyethylene, polypropylene, polybutylene in the molecular weight range of 500-50,000 are residue free light in colour free from non hydrocarbon impurities chemically non reactive and high temperature lubricants.
2) Polyglycols and related compounds like polyethylene glycol polypropylene glycol, polyglycidyl ether and higher polyalkylene oxides can be used was water soluble as well as water insoluble lubricants in rubber bearing and joints polyglycidyl ether and higher polyalkylene oxides are water insoluble but they can absorb a considerable amount of water their viscosity index is high and these are used in roller bearing of sheer glass manufacturing machine. it may be pointed that polyethylene oxides undergo thermal decomposition at high temperature to evolve volatile oxidisable products , so these are not useful as lubricants at temperatures.
3) Organic amines, imines and amides are good synthetic lubricants, since they possess low pour points and high viscosity index. They can be used under temperature conditions of – 50 deg centigrade to 250 deg centigrade.
4) Silicones are very good synthetic lubricants, because are not oxidised below 2000C and possess high viscosity index. These are frequently used for low temperature lubricants purposes. It may be pointed have that silicones are oxidised quickly above 2000C and undergo cracking process at about 2300C, so they are not employed for high temperature applications.
5) Fluorocarbons are not decomposed by heat and not easily oxidisable it is chemically inert in nature and have resistant to chemical, except molten sodium.