ZDDPs thermal decomposition

Acidic conditions accelerate ZDDP decomposition and basic barium sulfonate detergents increase the rate of the ZDDP thermal decomposition reaction pathway. ZDDPs have been reported to form complexes with amines and succinimide dispersants (Gallopoulos, 1964 Heilweil, 1969 Rounds, 1976). 

The (P) L-edge XANES spectra of tribofilms and thermal films generated from the neutral di-isopropyl ZDDP along with the model compounds (zinc metaphosphate and zinc pyrophosphate) are very similar and compare well with model compounds. The surface may also play an important role in catalysis of the thermal decomposition and provide oxygen for phosphate formation. There is also... 

A rearranged ZDDP or LI-ZDDP has been proposed previously as an intermediate in the thermal decomposition of ZDDP in solution studied by IR, and by 31-P NMR spectroscopy. This is the first time this proposed LI-ZDDP has been observed on a metal surface in air after being immersed in a thermally decomposing ZDDP oil solution (Coy and Jones, 1981 Dickert and Rowe, 1967 Fuller et al., 1998 Jones and Coy, 1981). 

The rate of thermal decomposition of ZDDPs can be greatly inhibited by the presence of overbased detergents [47]. Inhibition of volatile alkyl thiol formation due... 

On the gas chromatogram relating to the endothermic peak observed at about 270 °C, a small amount of HjS, tC /nC and a large amount of CgHj SH + 1-Cg are detected. CgHj SH and 1-Cf(,are the main primary products of the thermal decomposition of ZDDP at temperatures lower than 270 °C and are comparably stable in an inert atmosphere (l-Cfo, 1-decene others see Figure 13.13). 

Figure 13.14 DTA-GC curves of thermal decomposition of ZDDP antioxidant additive at 280 "C [13]...

The uncertainty in the mechanism of antiwear tribofilm formation derives in part from observations that exposing metal surfaces to heated ZDDP/oil solution forms films similar to those generated in a tribochemical way. From the utility standpoint, both thermal and tribochemical films seem to provide protection from wear. Thus, the current model involves both a tribochemical and thermooxidative component for the decomposition of ZDDP and tribofilm formation (Aktary et al., 2001 Bancroft et al., 1997 Fuller et al., 1997 and 1998 Martin, 1999 Willermet et al., 1995b Yin et al., 1997a). 

The second step (eq. 4.5) is a thermal oxidative process. This initiates the reaction of ZDDP with oxygen, and enhances the decomposition. Since oxygen and/or hydroperoxide is present in the oil, decomposition is not a pure thermal degradation. The main products on the surface are zinc polyphosphates with minor amounts of zinc sulfides. As the rubbing continues, the polyphosphate layer comes into closer contact with water in oil and is hydrolyzed to give short-chain polyphosphates (eq. 4.6). 

It follows from this that the ZDDPs which are most efficient at anti-wear film formation will also be likely to suffer depletion due to thermal effects. The thermal degradation of ZDDPs in service has often been confirmed by P NMR and IR studies [45] and is not really important unless decomposition proceeds to the stage where significant reduction in phosphorus and sulphur levels as insoluble or volatile products occurs. Indeed, according to [6], the early stage decomposition products are ... 

Figure 13.13 (a) DTA-GC curves of thermal oxidative decomposition of ZDDP anti-oxidant additive... 

According to the information provided by the DTA-GC curves, the mechanism of the thermal oxidative decomposition of ZDDP can be explored [5]. Figure 13.13 indicates that CgHjjSH is unstable at 280-300 °C in air. 

Recent work conducted by Martin [6] has shown that zinc polyphosphates are important intermediate reaction products in the tribochemical reaction of ZDDP. ZDDP decomposition is thought to be through complex chemistry which can be either thermal, catal)dic or oxidative. In this case Martin believes that ZDDP undergoes mainly thermo-oxidative decomposition prior to friction, when the lubricant is heated above 100°C. 

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