Extending the Utility of Silicone Lubricants through Structural Modifications.

Abstract

SILICONE polymers are semiorganic compounds that can cover a wide range of polymer composition. The fluid polymers display excellent viscosity-temperature properties coupled with very good heat stability. These polymers have successfully fulfilled many lubrication requirements in the last decade. The excellent viscositytemperature and stability properties of silicones are inherent in the inorganic siloxane backbone as shown in Figure 1. The properties of the siloxane polymer are varied largely through the use of a number of different types of organic substituents on the silicon atom. Methyl substitution has been found to be the most stable alkyl group, and it best retains the viscosity-temperature relationship imparted by the siloxane backbone. This type of substitution also imparts low volatility characteristics to the oils. Longer alkyl groups can be substituted in place of methyl with a resultant sacrifice in stability. A methylethyl-substituted polysiloxane retains most of the desirable properties of the dimethyl substitution, but has better organic solubility. A new series of oils is produced when phenylmethyl is substituted for dimethyl on the silicon atom (Figure 1). A polymer with low phenyl content has very good low temperature properties. Increased phenyl content adds to the heat stability of the siloxane polymer at the expense of low temperature and viscosity-temperature properties. The phenyl group adds to both thermal and oxidation stability ( I 4 ) , but has its largest effect on the latter. The alkyland phenyl-substituted polysiloxanes have proved to be good lubricants in both rolling and sliding contact for many types of bearing metal combinations (3 , 4 , 7, 9). The use of these lubricants has been restricted, to some extent, by their inherent lack of boundary lubricity for sliding ferrous metal bearings (9). A step towards overcoming this lubricity deficiency was made when halophenyl groups were substituted for methyl or phenyl (10-12). In general, incorporation of the halophenyl group has an effect similar to that of substituting phenyl, while additionally imparting better antiwear characteristics. Introduction of a large number of halophenyl groups will enhance the extreme pressure properties of the oil a t the expense of some low temperature and antiwear properties. The halogen on the phenyl can be chosen to have sufficient stability to withstand bulk oil temperatures, but have enough reactivity a t boundary surface temperatures to form a lubricating film (2). Typical properties of a low chlorophenyl-substituted fluid (60) are compared to a low phenyl-substituted fluid in Table I. The lubricants described above have been compounded into a variety of greases usin$ suitable thickeners. Both oils and greases find use in applications requiring a wide temperature range or long life lubricant. With the proper choice of oil, the requirements for either a very low or a very high temperature lubricant can be met.