**1. Introduction**

Concerning the lubrication conditions in operation, the higher viscosity of the base oils and higher consistency of the greases at low temperatures, the friction factor increases slightly [1]. In the case of mineral oils, as the proportion of paraffin crystals increases with decreasing temperature, the sliding friction also increases. Nonetheless, the outflow of mineral oil provides adequate lubrication during sliding [2]. Unlike mineral oil, ester oils crystallize to the extent of blocking the rheometer [3]. Thus, it would also block a tribological contact during operation. Practical test methods such as the low-temperature torque test for wheel bearings (ASTM D4693 [4]) or ball bearings (ASTM D1478 [5]) determine the suitability of greases for low temperatures. Although these standards are close to practical experience, they do not provide information on whether the base oil in lubricating greases precipitate crystals and thus change their flow properties upon cooling. In practice, the pour point according to ASTM D97 [6] is measured for this purpose. The pour point indicates the temperature when the base oil stops flowing as a sample vessel is tilted. Previous research

**Citation:** Conrad, A.; Hodapp, A.; Hochstein, B.; Willenbacher, N.; Jacob, K.-H. Low-Temperature Rheology and Thermoanalytical Investigation of Lubricating Greases: Influence of Thickener Type and Concentration on Melting, Crystallization and Glass Transition. *Lubricants* **2022**, *10*, 1. https://doi.org/10.3390/ lubricants10010001

Received: 15 November 2021 Accepted: 18 December 2021 Published: 22 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

has shown that crystallization and viscosity increase are responsible for oils stop flowing at temperatures below the pour point. However, some oils can crystallize at temperatures above the pour point if given enough time [3].

Mineral oils precipitate paraffin crystals below the crystallization temperature ( *T*c), turning Newtonian mineral oils into shear-thinning suspensions. The transition from Newtonian to viscoelastic behavior is indicated by a significant slope increase in an Arrhenius diagram (ln(*η*)- *<sup>T</sup>*−1) below the crystallization temperature [7]. The necessary supercooling (Δ *T* = *T*c − *T* m) for precipitation of paraffin crystals is hardly dependent on the shear rate and the cooling conditions and is almost constant at about Δ *T* = −10 K. The crystallization temperature can be equated with the pour point of mineral oils [3].

Unlike mineral oils, which are mixtures of paraffinic, naphthenic, and aromatic hydrocarbons, synthetic oils consist of chemically uniform compounds with a comparatively narrow molecular weight distribution. The synthetic oils polyalphaolefin, polyalkylenglycole, alkylated naphthalene, and tris-(2-ethylhexyl)trimelltiate solidify glass-like below −70 ◦C. Upon cooling, the viscosity of these base oils increases steadily and follows a Williams– Landes–Ferry Equation (WLF) down to 20 K above the glass transition temperature [3].

Ester oils with linear alkyl chains and a narrow molecular weight distribution crystallize with strong supercooling effects [8]. Using the example of a trimellitate with linear alkyl chains (C8–C10), the viscosity increases steadily up to the crystallization temperature. At the crystallization temperature, the viscosity rises abruptly. However, crystallization does not lead to shear-thinning suspensions as in mineral oils but to a solid [3].

The crystallization temperatures of a base oil in a lubricating grease may differ from the pure base oil due to the catalytic effect the thickener on crystallization, i.e., heterogeneous crystallization. From a thermodynamic point of view, nucleation in a base oil is spontaneous when the size of nuclei corresponds to a critical size, which decreases with increasing supercooling ( Δ *T* = *T*c − *T* m). Particles in a liquid can act as nuclei with a larger critical nucleation radius, resulting in a lower energy barrier for nucleation and less supercooling [9]. The necessary condition for particles to catalyze nucleation is that they have melting temperatures far higher than the melting point of the lubricating oil and remain solid, which is the case for common thickeners such as Li- and Ca-12-hydroxystearate [10].

From a kinetic point of view, the rate of nucleus formation depends on viscosity, i.e., the lower the temperature, the lower the nucleation rate. Approaching the glass-transition temperature *T*G, the nucleation rate becomes infinitely small. Since the temperatures for a maximum nucleation rate and maximum crystal growth rate are not identical, lubricating oil composition and cooling rate significantly influence supercooling. When the maximum crystal growth rate temperature is below the glass temperature *T*G, nuclei are absent, and glass-like solidification takes place upon cooling [10,11].

If such base oils are processed into lubricating greases with Li- or Ca-12-hydroxy stearate, the thickener type and concentration determine the consistency of the metal soap greases. The metal soap must be melted in the base oil and then cooled under defined conditions. During this process, a thickener structure forms, which is responsible for the viscoelasticity of the lubricating greases [12]. With Li-12-hydroystearate as a thickener, a network of platelets is formed at low concentrations, which changes into fine and dense fibril-like structures at higher concentrations [13]. Calcium complex soaps, for example, build globular structures [14]. Even at relatively low thickener concentrations, metallic soaps cause viscoelasticity of the lubricating greases and, above a specific concentration, forming a viscoelastic liquid with a yield point [15].

In the context of base oils and associated lubricating greases, the question arises whether and what influence the thickener type, and concentration have on crystallization, melting, and glass transition temperatures. These questions are discussed in detail below, based on rheological and thermoanalytical measurements for various lubricating greases. The relevance of the base oils' pour point for the low-temperature behavior of corresponding greases will also be addressed.

#### **2. Materials and Methods**

Table 1 lists the base oils used with the kinematic viscosities at 40 and 100 ◦C, the viscosity index, and the respective pour point. The crystallization behavior and lowtemperature rheology is previously examined in detail and the base oils are classified in three groups [3]. Group I contains a mineral oil (MO), Group II includes amorphously solidifying synthetic lubricating oils (PAG, KR-008, PAO8), and Group III comprises a crystallizing synthetic lubricating oil (EO).

**Table 1.** Classification (Group I–III), kinematic viscosity (*ν*), viscosity index (VI), pour point (ASTM D7346 [16]), and chemical nature of the base oils.


\* with linear C8–C10 alkyl groups.

From the base oils listed in Table 1, lubricating greases with Li- and Ca-12-hydroxystearate were prepared. Ca-12-hydroxystearate greases were prepared by melting the Ca-12- hydroxystearate in the base oil at 120 ◦C for 30 min, while the Li-12-hydroxystearate lubricating greases were prepared by melting Li-12-hydroxystearate above the melting point of 212 ◦C. After cooling to room temperature, homogenization of the cooled suspensions was carried out on a three-roll mill (Exakt Advanced Technologies GmbH, 50I, Norderstedt, Germany). Greases with thickener concentrations lower than 5 wt.% were homogenized with an Ultra-Turrax (IKA GmbH, Staufen, Germany). Table A1 in the appendix lists the exact composition of the lubricating greases with the respective worked penetration *P* w (DIN ISO 2137 [17]) and corresponding NLGI class (DIN 51818 [18]).
