Phosphorus Compounds Chemical Reactions and Additive Action

Phosphorus Compounds Chemical Reactions and Additive Action 

Compounds with phosphorus as the key element that have been investigated for extreme-pressure additive activity add up to an impressively long list. But only a few of them have turned out to be of practical utility and these are predominantly esters of phosphoric acid or derivatives of thiophosphoric acid. In this section we shall be concerned only with those compounds in which phosphorus in combination with oxygen constitutes the key structure dithiophosphates and related derivatives are discussed in Section 11.2.6. The four main types of phosphorus oxyacid esters treated in the present section are shown in Table 11-12. Phosphoric and phosphorous acids are trifunctional hence there are three or-ganophosphorus structures to be considered the neutral triester, the monoacidic diester and the diacidic monoester. 

In Section 10.6 of the preceding chapter it was shown that tricresyl phosphate, a widely used organophosphorus additive, left an interaction film of ferrous phosphate on the steel surface that it lubricated. In terms of a mechanism generally applicable to the esters of phosphorus oxyacids, two reaction paths immediately suggest themselves. One is the thermal decomposition of the ester linkage, catalyzed by the activated metal interface  

The other is a hydrolytic mechanism involving water vapor in the ambient atmosphere  

The acid phosphate diester then reacts with the iron surface. 

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