Oxidation Chemistry of Primary and Secondary Antioxidants

Abstract

Inhibition of oil oxidation is the key to long life of synthetic lubricants operating in thermal stress and boundary lubrication environments [1]. Bench-scale tests to screen oil formulations provide a rapid means for optimizing formulations prior to longer running verification tests done with the oil in the end-use application [2]. The ultimate goal of accelerated oil life tests is to link the sample combustion temperature, or induction time, at a given heating rate, or temperature, to the estimated lifetime under normal use temperatures. A first order reaction model has recently been employed to derive kinetic parameters from the heating rate dependence of the combustion temperature in the non-isothermal pressure DSC (NIPDSC) test by Adhvaryu et al. [3]. The first order reaction model [4] is also employed here, but we show that a more detailed scheme is needed to fit the heat flow during the combustion exotherm. The detailed kinetic model also provides the link between the NIPDSC test and the isothermal pressure DSC test, as well lifetime estimation at temperatures closer to the end-use conditions. Although isothermal PDSC is useful on grease [5], it does not provide a sharp exotherm for the unthickened base oil [6]. The NIPDSC test provides a reasonably sharp exotherm for formulated base oil in a relatively short amount of time. In the NIPDSC test, 10–12 mg of oil is placed in an open DSC pan. The sample chamber is pressurized with oxygen. The sample temperature is linearly increased with time until the occurrence of the combustion exotherm. The exotherm peak temperature and total heat flow did not exhibit any regular dependence on oxygen pressure between 0.55 and 3.4 MPa. The base oil was an (average) C7 ester of pentaerythritol. Primary antioxidants were hindered phenol and aromatic amines, and Zn-dialkyldithiocarbamate (ZDTC) and Zn-dialkyldithiophosphate (ZDDP) were used as secondary antioxidants. For some of the tests, soluble catalyst [2] was incorporated as iron (III) 2-ethylhexanoate. The reduced heat flow thermograms during the NIPDSC test on the base oil are shown in Fig. 1. The curves are normalized by the peak heat flow rate as Q/Qp, and the symbols denote curves from the first order reaction model, discussed below. The exotherm temperature increases and the exotherm sharpens with increased heating rate. At the 20 °C/min heating rates, the internal heating upon combustion noticeably skews the shape of the exotherm.