Pressure Viscosity Coefficient of Vegetable Oils

The pressure viscosity coefficient (PVC) of ten vegetable oils from commodity and new crops, and two petroleum-based oils [polyalphaolefin (PAO) and hexadecane], were investigated. PVC was estimated using two different methods: from analysis of oil physical properties (viscosity and density); and from analysis of elastohydrodynamic (EHD) film thickness data. Estimated PVC of vegetable and petroleum-based oils decreased with increasing temperature, which was expected. PVC values derived from physical properties were generally much higher than the values from the literature or those derived from EHD film thickness. PVC values increased with increasing viscosity of the tested oils. Based on further examination of PVC data for a wide range of oils from the literature and this work, it is proposed that lubricating oils can be roughly divided into three structural categories, each with defined range of PVC values: low PVC oils (< 10 GPa(-1)) with predominantly layered molecular structures; high PVC oils (> 20 GPa(-1)) with predominantly entangled molecular structures, and intermediate PVC oils (10-20 GPa(-1)) with negligible layering or entangling molecular structures. It is proposed that vegetable oils, along with PAO and hexadecane, have flexible hydrocarbon chain structures, with little or no branching or layering, and, hence, belong to the intermediate PVC category.