Using Mechanical Constants to Predict Pour Points of Environmental Friendly Lubricants

Mary Biddy, University of Wisconsin

Vegetable oils are a very promising alternative to traditional petroleum-based lubricants. Of particular importance is the environmentally friendly nature of vegetable oils; unlike petroleum oils they are a naturally occurring, renewable, and biodegradable resource. However, the low temperature properties of vegetable oil lubricants are less desirable than those of petroleum-based lubricants. A molecular-level understanding of how vegetable oils behave in this low temperature regime will help in engineering vegetable oils suitable for the most demanding applications.

Industrially applied oils are typically characterized by their viscosity, stability, and degradation temperature. One property of particular interest is the pour point, which is an industrial standard that characterizes the temperature at which a lubricant no longer flows and ceases to properly function for certain applications. It is believed that the pour point of vegetable oils is similar to a liquid to gel transition point. Previous molecular simulation studies have modeled the gel points of several different systems; however, these studies have employed very simplified models. Our united atom model, which is based on quantum mechanical calculations and experimental values, is more realistic and detailed than other widely used course grained gel models.

In this work we have accurately predicted the pour point temperature of several vegetable oil lubricants by performing molecular dynamics simulations to obtain elastic mechanical constants as a function of temperature. We also show that at temperatures below the pour point transition vegetable oils form physical gel structures. Dipole-dipole interactions are found to play a critical role in gel formation by leading to the clustering of molecules. The size and frequency of these dipole clusters are significantly reduced at temperatures above the pour point. These findings can lead to the rational design of vegetable oil lubricants with thermophysical properties that make them competitive with petroleum-based lubricants for widespread application.

Abstract Author(s): Mary J. Biddy<br />Yioryos J. Papakonstantopoulos<br />Juan J. de Pablo