**4. Conclusions**

This study investigates the influence of thickener type and concentration on the crystallization, melting and glass transition of greases made from three different base oil types (Groups I–III) using rheological and thermoanalytical measurements. Group I is represented by a mineral oil (MO), Group II includes amorphously solidifying synthetic lubricating oils (PAG, KR-008, PAO8), and Group III comprises the crystallizing synthetic lubricating ester oil (EO).

#### *4.1. Lubricating Greases Based on Mineral Oil (Group I)*

During cooling below the crystallization temperature *<sup>T</sup>*c,rheo, mineral oils precipitate paraffin crystals. Upon further cooling the complex viscosity reaches an absolute value of about 10<sup>5</sup> Pas, regardless of the thickener type and concentration. The suspension formed by the deposition of paraffin crystals is responsible for the increasingly stiff consistency of the lubricating greases. On reheating, above the melting temperature *<sup>T</sup>*m,rheo, the absolute value of the complex viscosity returns to the initial level. The different thickener types (Li- or Ca-12-hydroxystearate) have a significant effect on crystallization (+4 K) and melting temperature ( −4 K). The pour point ( −12 ◦C) of the mineral oil is 3 K lower than the crystallization temperature of the lubricating grease and 7 K lower than the melting temperature. This indicates that the pour point does not properly define the lowest application temperature for mineral oil (MO) based greases.

#### *4.2. Lubricating Greases Based on Non-Crystallizing Synthetic Oils (Group II)*

Group II oils do not crystallize but solidify glass-like at temperatures below −70 ◦C. This group includes polyalphaolefin (PAO8), alkylated naphthalene (KR-008), and polypropylene glycol (PAG). The addition of Li-12-hydroxystearate does not change the glass transition temperature significantly. The absolute value of the complex viscosity of these greases increases steadily with decreasing temperature. At about 20 K above *T*G, however, the absolute value of the complex viscosity increases sharply, i.e., the slope of the |*η*\*| vs. *T* curve changes significantly. The onset of this steep viscosity increase is equal to the pour point of the corresponding oil and the latter thus is an appropriate measure for the lowest application temperature of these greases.

#### *4.3. Lubricating Greases Based on Crystallizing Synthetic Oils (Group III)*

Group (III) oils crystallize with strong supercooling effects up to cold crystallization. Trimellitate (EO) with linear C8-C10 alkyl chains belongs to this group. This ester oil (EO) shows that adding minor thickener concentrations (about 0.5 wt.% Ca-12-hydroxystearate, 1 wt.% Li-12-hydroxystearate) causes the ester to crystallize partly upon cooling and partly upon heating. The fraction of crystallization during cooling rises with increasing thickener concentration until the ester oil crystallizes only upon cooling. This is due to heterogeneous

nucleation, which becomes more significant with increasing thickener concentration. As a result of heterogeneous nucleation, the crystallization temperature rises from −45 ◦C to −20 ◦C. In contrast, the melting temperature *T* m remains constant at 7 ◦C independent of the thickener concentration and thickener type. The investigations show that using lubricating greases based on this ester oil below the melting temperature is not advisable. Especially for lubricating greases based on trimellitate (EO), the pour point of −57 ◦C is inappropriate as indicator for the lowest application temperature.

**Author Contributions:** Conceptualization, A.C. and K.-H.J.; methodology, A.C., A.H., B.H., K.-H.J. and N.W.; validation, A.C., A.H. and B.H.; formal analysis, A.C. and A.H.; investigation, A.C. and A.H.; resources, K.-H.J. and N.W.; data curation, A.C.; writing—original draft preparation, A.C.; writing—review and editing, A.C., A.H., B.H., K.-H.J. and N.W.; visualization, A.C.; supervision, K.-H.J. and N.W.; project administration, B.H., K.-H.J. and N.W.; funding acquisition, K.-H.J. and N.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Arbeitsgemeinschaft industrieller Forschungsvereinigungen "Otto von Guericke" e.V. (AiF), Grant Number 20001N and Forschungsvereinigung Antriebstechnik e.V. (FVA), Grant Number 829I.

**Acknowledgments:** The authors give thanks to the members of the FVA, especially to Fuchs Schmierstoffe GmbH, Castrol Germany GmbH, and King Industries, for providing additional information and the base oils. In addition, we wish to thank the students Johanna Scheller, Daniel Weiß, and Maximilian Enhuber for their contribution to the manuscript as part of their bachelor thesis.

**Conflicts of Interest:** The authors declare no conflict of interest.
