I

Figure 5

high temperature lubrication. It has been found that poly alpha olefins when they oxidize form a deposit (22). The deposit has a tendency to be unstable because some of the polymer linkages remain in the backbone chain of the deposit. The phosphate esters exhibit a high level of stability, suggesting that for these materials the ester linkage is more stable than the ester linkage in an organic acid ester. The tributyl and tricresyl phosphate esters appear to be thermally stable to their boiling points. The values shown on Table 5 for the phosphate esters are based on the temperature required to produce a measurable thermal decomposition film on a target in the vapor deposition test in a 5 minute interval. In all cases, the temperature required to form a deposit from thermal decomposition is considerably higher than that required to form a deposit by oxidation. These data suggest the desirability of minimizing oxidation by limiting oxygen access to high temperature bearing systems.

Liquid Lubricant Thermal Stability

In the adiabatic diesel, temperatures proposed are high enough to involve substantial thermal reactions in addition to the oxidative reactions discussed previously. Table 5 shows the incipient thermal stability of the various lubricants considered in this paper. Where the C-C single bond dominates in an organic compound such as paraffins and naphthenic hydrocarbons, the temperature where thermal degradations becomes significant is about 375°C. This also applies to aromatic hydrocarbons with significant paraffinic side chains. However, when the aromatic structure consists entirely of condensed ring structures with nothing larger than methyl side chains, the thermal decomposition becomes significant in the range of 400 to 450°C. The addition of the ether group to the polypheny! ethers does not appear to adversely affect the thermal stability. Esters of organic acids appear to decompose thermally at temperatures as low as 200°C in the presence of ferrous alloys which appear to act as catalysts for this reaction. Polymers of olefin monomers such as PAO tend to be less stable than naturally occurring hydrocarbons of a similar structure. This unzippering tendency of polymers in general has been noted. Indeed, this property may be desirable in

Table 5

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