Effect of Base Oil and Thickener on Texture and Flow of Lubricating Greases

  • The structure and flow behavior of lubricating greases depend on the base oil and the type and concentration of the dissolved thickener. In this study, the linear viscoelastic properties of greases were characterized by combining oscillatory shear and squeeze flow covering a broad frequency range (0.1–105 rad s−1). Multiple-particle tracking (MPT) microrheology and scanning electron microscopy (SEM) provided further insight into local viscoelastic properties and sample structure on a submicron-length scale. The type and viscosity of the base oil did not affect the absolute value of the complex viscosity and the filament shape formed by a given thickener. High-frequency shear modulus data, however, indicated that the thickener lithium 12-hydroxystearate formed stiffer networks/filaments in poly-α-olefins than in mineral oils. As expected, the viscosity increased with increased thickener concentrations, but microscopy and high-frequency rheometry revealed that the thickness, length, and stiffness of the individual filaments did notThe structure and flow behavior of lubricating greases depend on the base oil and the type and concentration of the dissolved thickener. In this study, the linear viscoelastic properties of greases were characterized by combining oscillatory shear and squeeze flow covering a broad frequency range (0.1–105 rad s−1). Multiple-particle tracking (MPT) microrheology and scanning electron microscopy (SEM) provided further insight into local viscoelastic properties and sample structure on a submicron-length scale. The type and viscosity of the base oil did not affect the absolute value of the complex viscosity and the filament shape formed by a given thickener. High-frequency shear modulus data, however, indicated that the thickener lithium 12-hydroxystearate formed stiffer networks/filaments in poly-α-olefins than in mineral oils. As expected, the viscosity increased with increased thickener concentrations, but microscopy and high-frequency rheometry revealed that the thickness, length, and stiffness of the individual filaments did not change. In mineral oil, the 12-hydroxystearate thickeners yielded higher viscosity than the corresponding stearates with the same metal ion. The filamentous lithium thickeners created stronger networks than the roundish aggregates formed by magnesium and zinc stearate. Network mesh sizes varying between approximately 100 nm and 300 nm were consistently determined from SEM image analysis and MPT experiments. The MPT experiments further disclosed the existence of gel-like precursors of approximately 130 µm at thickener concentrations far below the critical value at which a sample-spanning network resulting in a characteristic grease texture is formed.show moreshow less

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Metadaten
Author:Annika Hodapp, Andreas ConradORCiD, Bernhard Hochstein, Karl-Heinz Jacob, Norbert Willenbacher
DOI:https://doi.org/10.3390/lubricants10040055
ISSN:2075-4442
Parent Title (English):Lubricants
Subtitle (English):Insights from Bulk Rheometry, Optical Microrheology and Electron Microscopy
Publisher:MDPI AG
Document Type:Article
Language:English
Date of first Publication:2022/03/31
Reviewed:Begutachtet/Reviewed
Release Date:2024/07/26
Tag:lubricating grease; rheology; structure; multiple-particle tracking (MPT); scanning electron microscopy (SEM); lithium 12-hydroxystearate
Volume:10
Issue:4
Article Number:55
Pagenumber:17
institutes:Institut für Chemie, Material- und Produktentwicklung
Research Themes:Materialien & Produktionstechnik
Licence (German):Creative Commons - CC BY - Namensnennung 4.0 International
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