Cold-flow properties of estolides: The older (D97 and D2500) versus the mini-(D5773 and D5949) methods

Authors: Bantchev, Grigor; Ngo, Helen; CHEN, YUNZHI - University Of Utah; Winfield, Demichael; Cermak, Steven - Steve

Submitted to: Lubricants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2024
Publication Date: 4/23/2024
Citation: Bantchev, G.B., Lew, H.N., Chen, Y., Winfield, D.D., Cermak, S.C. 2024. Cold-flow properties of estolides: The older (D97 and D2500) versus the mini-(D5773 and D5949) methods. Lubricants. https://doi.org/10.3390/lubricants12050141.
DOI: https://doi.org/10.3390/lubricants12050141

Interpretive Summary: Cold flow is a term to describe how viscous a liquid is at lower temperature – usually the colder the more viscous or thicker a material becomes. Cold flow properties of biobased lubricants can be measured with small-volume methods instead of using outdated and large-volume methods. When evaluating new materials as a potential lubricant, the cold flow properties of the material are a very important factor to consider. The established methods required to measure cold flow properties require large volumes (at least 50 mL) of the prospective lubricants, which may be hard to prepare. When new materials are developed, they are usually made on a smaller scale to save resources and limit environmental impacts. ARS researchers compared older dated methods to new, small-volume automatic or “mini” methods for a series of biobased oils. The results indicated that these mini-methods can replace the outdated methods for the biobased materials. These small-volume methods will help everyone interested in the cold-flow properties of lubricants when acquiring large-volume samples that are challenging.

Technical Abstract: There is growing research on developing new and sustainable lubricants. Sustainble lubricants with adequate cold-flow properties are of particular interest for many applications. One limitation for the established methods for measuring cold-flow properties is the large volume needed to test samples. This makes initial screening of many hard-to-synthesize samples difficult. In the current study, we compared the results of the older, widely accepted ASTM methods D97 (pour point, PP) and D2500 (cloud point, CP) to the newer, smaller volume, and easier-to-perform methods D5949 and D5773 for bio-based base oils (estolides and iso-estolides). The CP results were in good agreement for less colored samples, but D5773 gave lower values for some darker (Gardner color >8) samples, especially esters. The D5949 showed a tendency to report slightly higher PP, especially for the lower values. Viscosities and densities in a wide temperature range (15 to 120 °C) were also measured. The surface tensions were estimated by a literature group method. Viscosity and density effects can only partially explain the differences in the PP values from the two methods. In conclusion, the newer mini-methods are an acceptable substitution when larger volumes are not accessible, unless the sample is too dark.