The Chemistry of the Thermal Degradation of Zinc Dialkyldithiophosphate Additives

Abstract

A mechanism for the thermal decomposition of zinc dialkyldithiophosphate additives has been proposed that accounts for the major degradation products and their dependence on the nature of the alkyl group. The proposed mechanism generally follows well-known rules of phosphorus chemistry. The major points are as follows.

(1) The decomposition is initiated by a general migration of alkyl groups from oxygen atoms to sulfur atoms. The rate of this reaction is governed by the ease with which nucleophilic substitution can take place at the α-carbon atom of the alkyl group. The well-established correlation between decreasing number of β-hydrogen atoms and increasing thermal stability is, therefore, explicable in terms of increasing steric hindrance, by the β-subtituents, of nucleophilic attack on the α-carbon atom.

(2) Olefin elimination successfully competes with substitution where the alkyl group structure is favorable (i.e. secondary alkyl groups), leading to the formation of phosphorus acids.

(3) Nucleophilic substitution of one phosphorus species by another lends to formation of P-O-P type structures and of zinc mercaptide (Zn(SR)₂) as reaction intermediate. Reaction of mercaptide with an alkylating agent generates dialkyl sulfide; reaction of mercaptide with dithiophosphate species leads to formation of trithiophosphates and eventually tetrathiophosphates.

(4) In acidic media, mercaptide is protonated to give alkyl mercaptan as a major product.

(5) The oil-insoluble deposit is a mixture of zinc thiophosphates and zinc pyro- and polypyro-thiophosphates.

Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Dayton, OhiO, October 16–18, 1979

Notes

Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Dayton, OhiO, October 16–18, 1979