Antioxidant Action of Zinc Di-iso-propyldithiophosphate in the Presence of Detergent-Dispersant

Abstract

Zinc dialkyldithiophosphates (ZDP) are widely used as antiwear additives, corrosion inhibitors and oxidation inhibitors in lubricating oils. The ZDP play an important role particularly as oxidation inhibitors in automotive lubricating oils which are often in service at high temperatures. It is generally believed that ZDP are effective oxidation inhibitors because they can effect hydroperoxide decomposition1)-5) and/or radical scavengings6), 7) in engine oils. However, their actual role has not yet been clarified.
In this paper, zinc di-iso-propyldithiophosphate(IPZDP) as hydroperoxide decomposers and/or radical scavengers to bring about the oxidation inhibiting effects in engine oils will be studied in the presence of such detergentdispersant additives as bis (dodecenyl succinimide) of triethylenetetramine (BDD. TETA) and calcium dinonylnaphthalenesulfonates (CaDNNS), which always coexist with ZDP in engine oils.
Oxidation stability tests were carried out in the white oil without a catalyst as described in on previous paper13). As seen in Fig. 1, the oxidation stability of the white oil increases with increasing IPZDP concentration whereas the addition of detergent and dispersant additives lowers the oxidation stability of IPZDP-added oils. The mechanism of oxidation inhibitor effected by IPZDP at low concentrations with or without detergent-dispersant additives can be considered to be due to the effect of radical scavenging since induction periods are obviously observed in the oxygen absorption curves. However, they can't be observed when the concentration of IPZDP is increased, thus, the dominant mechanism of the oxidation inhibition by IPZDP becomes equivocal.
The ability of IPZDP to decompose hydroperoxide is determined in the presence of detergent-dispersant additives to clarify the mechanism of oxidation inhibition. Cumene hydroperoxide (CHPO) was used as a model hydroperoxide in this study. The role of a hydroperoxide decomposer is to ionically decompose the hydroperoxides, which are produced and accumlated in the oil during oxidation, into inactive non-radical compounds. The ability of IPZDP for ionic decomposition is remarkable when its concentrations are low relative to those of CHPO as shown in Fig. 3. However, as soon in Figs. 5 and 6, the ionic decomposition hardly occurs in the presence of ashless dispersant. It is reported4) that hydroperoxide decomposition may be due to the acidic, active species, such as SO₂ or H₂SO₄, produced in the course of decomposition of ZDP itself. Therefore, the results obtained in the presence of ashless dispersants can be explained as follows: the active species are trapped or inactivated by such basic compounds as ashless dispersants. The same effects results also for over-basic metallic detergents. (Fig. 7)
The results of radical scavenging ability of IPZDP at 115°C in the presence of detergent-dispersant additives are shown in Table 2. The radical used at 115°C was 1-1-Diphenyl-2-picrylhydrazyl (DPPH). The radical scavenging ability was calculated from the reaction rate of DPPH which can be determined from visible spectrum. The results shown in Table 2 that the radical scavenging ability of IPZDP is not affected by the presence of detergent-dispersant additives.
Based on the above results, it can be assumed that oxidation inhibition by IPZDP in the presence of detergent-dispersant additives is due mainly to the effect of radical scavenging and no due to that of hydroperoxide decomposition.