(37d) Degradation Kinetics of a Liquid Mineral Oil

Heat plays a key role behind the degradation of paraffinic mineral oils used to prepare various metalworking and hydraulic fluids [1]. We have measured and modeled thermal degradation to provide first steps toward understanding the practical degradation kinetics as a mechanism of elementary reactions.

In the present study, fresh and degraded samples of a popular commercial paraffinic mineral oil (CAS number 64742-52-5) are used. The degraded mineral oil sample had undergone accelerated degradation at 370°C for 100 hours, above its rated operating temperatures. Each sample (0.5 μl) is analyzed with two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC x GC/ToFMS; LECO Pegasus 4D), giving excellent resolution of species. Initially, the species are identified solely by similarity with respect to the reference library mass spectra of the tentatively identified species. For the long linear and branched alkane chains, comparison of measured spectra is typically 70-75% similar. Adding GC elution times of standards will provide greater certainty.

The fresh mineral-oil sample mainly consisted of linear and branched alkanes having 15 to 28 carbons. Additional species were identified in the degraded mineral oil, notably smaller linear alkanes, branched alkanes having 5 to 14 carbon numbers, and cycloalkanes with 5, 6, and 8 carbons.

To probe the possible reactions underlying this degradation, the Reaction Mechanism Generator (RMG) code of Gao et al. [2] was used. Pyrolysis kinetics of 8-butyloctadecane, a possible major compound in this mineral oil, was estimated and tested at 370°C and 1 atm. Breakage of the branched alkane was predicted to occur through different radical-forming reactions, principally disproportionation, H abstraction, internal H transfer, and radical recombination.

[1] James E. Anderson, Byung R. Kim, Sherry A. Mueller, Tiffany V. Lofton, “Composition and Analysis of Mineral Oils and Other Organic Compounds in Metalworking and Hydraulic Fluids.” Environmental Science and Technology 33:1 (2010) 73-109. https://doi.org/10.1080/10643380390814460

[2] Connie W. Gao, Joshua W. Allen, William H. Green, Richard H. West, “Reaction Mechanism Generator: Automatic construction of chemical kinetic mechanisms.” Computer Physics Communications 203 (2016) 212-225. https://doi.org/10.1016/j.cpc.2016.02.013