*2.1. Rheological Measurements*

The steady shear measurements were performed with MCR301 and 702 rheometers from Anton Paar (Ostfildern-Scharnhausen, Germany). A plate-plate geometry with 25 mm in diameter (PP25) made of stainless steel was used as measuring system with a gap of 1 mm. In the temperature range between 20 and −40 ◦C the Peltier unit was purged with dry air (dew point: −80 ◦C) to prevent condensation and freezing of humidity. Measurements below −40 ◦C were performed using a PP25 geometry covered by a low-temperature CTD450 cell and an EVU10 controller for liquid nitrogen, both from Anton Paar (Ostfildern-Scharnhausen, Germany). Temperature-dependent oscillatory shear measurements were performed at an angular frequency of *ω* = 10 rad s<sup>−</sup><sup>1</sup> and an amplitude of *γ* = 0.05%. Strict care was taken to ensure that the linear viscoelastic (LVE) range was maintained in the temperature range investigated. All rheological experiments were performed in triplicate with fresh samples for each measurement.

#### *2.2. Differential Scanning Calorimetry (DSC)*

Heat flow measurements during the cooling and heating cycles were performed with a DSC 204 F1 Phoenix® (Netzsch, Selb, Germany) in a pierced aluminum pan with a sample weight of approx. 10 mg to detect glass transition, crystallization, and melting of the lubricating greases. The measurements were carried out in the temperature range from 25 to −60 ◦C with heating and cooling rate of 2 K min−1. For the extended temperature range down to −180 ◦C, a Netzsch DSC 204 cell with a CC 200 L controller for liquid nitrogen was used. The lubricating grease samples were cooled with −20 K min−<sup>1</sup> from 25 ◦C to −180 ◦C, held for 5 min, and then heated with a rate of +10 K min−1. All thermoanalytical experiments were performed in triplicate with fresh samples for each measurement.

#### **3. Results and Discussion**

#### *3.1. Crystallization and Melting of Lubricating Greases Based on Mineral Oil (Group I)*

Figure 1 shows the absolute value of the complex viscosity |*η*\*| as a function of the temperature for the pure mineral oil (MO) and two mineral oil greases, with different Li- (a) and Ca-12-hydroxystearate (b) concentrations during cooling (empty symbols) and heating (filled symbols). Upon cooling, the complex viscosity increases steadily until the crystallization temperature *<sup>T</sup>*c,rheo is reached. Below *<sup>T</sup>*c,rheo, the slope of the complex viscosity curve increases sharply. The complex viscosity increases levels off below −20 ◦C. On reheating, the complex viscosity of the greases first decreases sharply until the melting temperature *<sup>T</sup>*m,rheo is reached, which is always higher than *<sup>T</sup>*c,rheo. Above *<sup>T</sup>*m,rheo, the absolute values of the complex viscosities of the cooling and heating cycles overlap. The reason for the sharp change in slopes on cooling and heating, respectively, is the formation and dissolution of paraffin crystals. Previous investigations on the mineral oil MO have shown that the MO changes from a Newtonian liquid to a shear-thinning suspension below *<sup>T</sup>*c,rheo [3].

**Figure 1.** Absolute value of the complex viscosity |*η*\*| as a function of temperature from cooling (empty) and heating (filled) of pure mineral oil and two lubricating greases based on mineral oil of different proportions of (**a**) Li and (**b**) Ca-12 hydroxystearate at a heating and cooling rate *β* of 2 K min−1, angular frequency *ω* of 10 rad s<sup>−</sup><sup>1</sup> and deformation *γ* of 0.05%. The dashed line marks the crystallization temperature *<sup>T</sup>*c,rheo and the melting temperatures *<sup>T</sup>*m,rheo are marked by arrows.

Above the melting temperature *<sup>T</sup>*m,rheo, the temperature range in which the oil behaves Newtonian, the level of the complex viscosity increases with increasing thickener concentration from 0.5 Pas without thickener to approx. 10<sup>4</sup> Pas with 15.0 wt.% Li-12- hydroxystearate and 15.8 wt% Ca-12-hydroxystearate as thickener, respectively. Below the temperature of −20 ◦C, the viscosity reaches a level of about 10<sup>5</sup> Pas regardless of thickener type and concentration. The complex viscosity of the greases at temperatures lower than −20 ◦C is affected mainly by the presence of paraffin crystals.

Figure 2 depicts the crystallization temperatures *<sup>T</sup>*c,rheo and melting temperatures *<sup>T</sup>*m,rheo of mineral oil greases with Li- (a) and Ca-12-hydroxystearate (b) as a function of thickener concentration. *<sup>T</sup>*c,rheo and *<sup>T</sup>*m,rheo were obtained by intersecting two tangents above and below the significant slope change in Figure 1. The mineral oil without thickener exhibits a crystallization temperature *<sup>T</sup>*c,rheo of −11.7 ± 0.15 ◦C and a melting temperature *<sup>T</sup>*m,rheo of −0.8 ± 1 ◦C. Regardless of Li- or Ca-12-hydroxystearate greases, the crystallization temperatures increase to values between −8 and −10 ◦C, and the melting temperatures

decrease to values between −5 and −7 ◦C. The supercooling (Δ*T* = *T*c − *T*m) decreases from approx. 11 K without thickener to approx. 3 K for Li-12-hydroxystearate and approx. 5 K for Ca-12-hydroxystearate greases at a thickener concentration of about 5 wt.% and essentially remains constant when the fraction of thickener is further increased.

**Figure 2.** Crystallization- (*T*c,rheo) and melting temperature (*T*m,rheo) as a function of thickener concentration *w* for Li-12-hydroxystearate (**a**) and Ca-12-hydroxystearate (**b**) based on mineral oil (MO), obtained from small amplitude oscillatory shear rheometry cooling and heating cycles (see Figure 1) with an angular frequency *ω* = 10 rad s<sup>−</sup>1, deformation *γ* = 0.05% cooling and heating rate of *β* = 2 K min−1. *<sup>T</sup>*c,rheo, and *<sup>T</sup>*m,rheo are obtained by intercepting two tangents above and below the significant slope change in Figure 1. The supercooling (Δ*T* = *T*c − *T*m) decreases from approx. 11 K without thickener to approx. 3 K with Li-12-hydroxystearate and approx. 5 K with Ca-12-hydroxystearate. A dashed line marks the pour point of −12 ◦C.

The increase in crystallization temperature *<sup>T</sup>*c,rheo is presumably caused by the thickener particles in the base oil. The thickener particles provide crystal nuclei, favoring the formation of paraffin crystals at higher temperatures, here about −8 ◦C [9,10]. Consequently, the crystallization temperature of the lubricating greases is 4 K above the pour point of the mineral oil (−12 ◦C). A small amount of dissolved thickener in the base oil presumably causes the decrease in *T*m [19].

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

Figure 3a displays the absolute value of the complex viscosity and Figure 3b the specific heat flow .*q* as a function of temperature for the grease with the synthetic base oil PAO8 and 22 wt.% Li-12-hydroxystearate (PAO8-22) as a thickener. In the logarithmic plot, the complex viscosity increases approximately linearly with decreasing temperature up to the onset of the glass transition (*T*G,PAO8-22 = −83 ± 0.2 ◦C). The absolute value of the complex viscosity increases sharply in the temperature interval between −66 ◦C and *T*G, but remains constant at a high level of 10<sup>7</sup> Pas below *T*G. The lubricating greases based on KR-008 (Figure A1) and PAG (Figure A2) behave similarly, see Appendix A. Notably, the temperature at which the two tangents fitted to the sections of the |*η*\*| curve with different slope agrees with the pour point and the end of the section in which |*η*\*| steeply increases corresponds to the glass transition temperature. According to the nucleation theory, for

oils such as PAO8, the maximum temperature of crystal growth rate is below the glass temperature *T*G [10,11].

**Figure 3.** (**a**) Absolute value of complex viscosity |*η*\*| as a function of temperature for the lubricating grease made from the base oil PAO8 with 22 wt.% Li-12-hydroxystearate as a thickener (PAO8-22), measured in oscillatory shear with the deformation amplitude *γ* = 0.05%, angular frequency *ω* = 10 rad s<sup>−</sup><sup>1</sup> and cooling rate of −10 K min−1. Lines are to guide the eye. (**b**) Specific heat flow . *q* of the PAO8-22 grease at a heating rate of 10 K min−1. PAO8 shows a glass transition temperature *T*G at −83 ± 0.2 ◦C, marked by a solid line. The dashed line marks the pour point for the base oil PAO8 of −66 ◦C.

Figure 4 compares the glass transition temperatures *T*G of the base oils PAO8, KR-008 and PAG (abscissa) and the corresponding lubricating greases (ordinate) with Li and Ca-12- hydroxystearate as a thickener. Glass transition temperature of the lubricating greases is always slightly higher ( ≈2 K) than that of the corresponding base oil but hardly depends on the thickener concentration at least in the investigated concentration range and stays always below −70 ◦C. The solidification behavior of greases based on non-crystallizing synthetic base oils does not change significantly due to the added thickener. The pour point as well as the glass transition temperature for these base oils and lubricating greases, respectively, are good indicators for their lowest application temperature.

#### *3.3. Greases Based on Crystallizing Synthetic Base Oils (Group III)*

The absolute value of the complex viscosity of the lubricating greases based on the trimellitate EO increases uniformly on cooling down to the crystallization temperature *T*c and then rises abruptly. The ester crystallizes to such an extent that the rheometer blocks, and no further measurements are possible [3]. For this reason, the crystallization behavior of trimellitate greases was investigated primarily using differential scanning calorimetry (DSC).

Figure 5 shows the heat flux from DSC measurements during cooling (a) and reheating (b) for Ca-12 hydroxystearate lubricating greases with thickener concentrations between 0.5 and 13 wt.%. During cooling, in the temperature range between −10 ◦C and −50 ◦C, an exothermic signal arises at thickener concentrations higher than 1 wt.%, which increases in magnitude with increasing thickener concentration and the peak shifts to higher temperatures. The onset temperature increases slightly.

**Figure 4.** Comparison of the glass transition temperatures of pure base oils PAO8, KR-008, and PAG (abscissa) and lubricating greases with different Li and Ca-12-hydroxystearate concentrations (ordinate) obtained from DSC-measurements at 10 K min−1. PAO8 with 22 wt.% (PAO8-Li-22, PAO8-Ca-22), KR-008 with 10 and 11 wt.% (KR-008-Li-10, KR-008-Li-11, KR-008-Ca-11) and PAG with 11 and 15 wt.% (PAG-Li-11, PAG-Li-15, PAG-Ca-11). The dashed line represents the angle bisector on which the *T*G's of the pure oils lie (PAO8, PAG, and KR-008).

**Figure 5.** Heat flow . *q* as obtained from DSC measurements during cooling (**a**) and reheating (**b**) for Ca-12 hydroxystearate greases with thickener concentrations of 0.5, 1, 2, 7, 9, and 13 wt.% with trimellitate (EO) as base oil at a cooling (**a**) and heating (**b**) rate of 2 K min−1. The integral of the exothermic signals during cooling and heating represent the crystallization enthalpy <sup>Δ</sup>*h*c,cooling and <sup>Δ</sup>*h*c,heating, respectively. The integral of the endothermic peak represents the enthalpy of fusion (i.e., melting) Δ*h*f. *<sup>T</sup>*c,cooling and *<sup>T</sup>*c,heating are the onset temperatures of the crystallization peaks. *T*m is the offset of the melting peak.

In addition, an exothermic peak is observed during heating in the temperature range between −45 ◦C and −30 ◦C. The crystallization enthalpy during heating <sup>Δ</sup>*h*c,heating decreases with increasing thickener concentration and is not measurable anymore for thickener concentrations >9 wt.%; peak temperatures and peak widths remain the same. Above −10 ◦C, melting of the sample begins. Melting temperature and area of the endothermic peak do not change with thickener concentration. However, the peak becomes somewhat flatter and broader with increasing thickener concentration.

Figure 6 shows melting temperature *T*m and crystallization temperature during cooling (*T*c,cooling) and heating (*T*c,heating) of the trimellitate (EO) based Li- and Ca-12-hydroxystearate greases as a function of thickener content. The melting temperature *T*m = 7.8 ± 0.4 ◦C is independent of thickener type and concentration and the same is true for the crystallization temperature determined during the heating cycle *<sup>T</sup>*c,heating = −46.1 ± 1.9 ◦C.

**Figure 6.** Crystallization temperatures obtained during cooling (*T*c,cooling) and heating cycles (*T*c,heating) and melting temperature *T*m, as a function of thickener concentration *w* for Li-12- hydroxystearate (**a**) and Ca-12-hydroxystearate greases (**b**) based on trimelltiate (EO), obtained from the onset temperature of the exothermic peaks of DSC measurements with 2 K min−<sup>1</sup> cooling and heating rate (Figure 5). The crystallization temperature at heating *<sup>T</sup>*c,heating is independent of the thickener fraction at about −45 ◦C, and the melting temperature is about 7.8 ± 0.4 ◦C. The dashed line marks the pour point of −57 ◦C.

The pure ester (EO) does not crystallize upon cooling but only upon heating (*T*c,heating = −43 ± 0.3 ◦C). The presence of thickener causes the oil to crystallize during both, cooling and heating cycles. For Li-12-hydroxystearate contents between 1 and 11 wt.% crystallization can be observed upon cooling and heating, for thickener contents above 11 wt.% only during cooling. Up to a thickener concentration of 11 wt.% the crystallization temperature *<sup>T</sup>*c,cooling increases linearly from −34 ◦C to −17 ◦C, but the crystallization temperature during heating *<sup>T</sup>*c,heating remains constant at −46.1 ± 1.9 ◦C. Above 11 wt.% the Li-12-hydroxystearate greases crystallize only during cooling and the crystallization temperature is *<sup>T</sup>*c,cooling = −16.7 ± 0.7 ◦C.

The trimellitate (EO) in Ca-12-hydroxystearate greases crystallizes at thickener concentrations between 0.5 and 9 wt.% during cooling and heating and above 9 wt.% only during cooling. With increasing thickener concentration, the crystallization temperature upon cooling increases slightly from −22.1 ± 1.1 ◦C to −19.0 ± 1.0 ◦C, whereas the crystallization temperature upon reheating remains constant at −43.7 ± 0.4 ◦C, irrespective of thickener concentration.

The enthalpy of fusion Δ*h*f = 92.5 ± 3.0 J g<sup>−</sup><sup>1</sup> (endothermic, Figure 5b) is larger than the crystallization enthalpy Δ*h*c = 73.5 ± 6.0 J g<sup>−</sup><sup>1</sup> because crystal growth is to some extent too slow to cause a detectable heat flux signal. Furthermore, the total enthalpy of fusion Δ*h*f and crystallization Δ*h*c are independent of the thickener concentration, indicating that only the base oil crystallizes.

Figure 7 shows the specific enthalpies of crystallization of the greases based on trimellitate (EO) during cooling <sup>Δ</sup>*h*c,cooling (a) and heating <sup>Δ</sup>*h*c,heating (b) relative to the total enthalpy of crystallization Δ*h*c = <sup>Δ</sup>*h*c,cooling + <sup>Δ</sup>*h*c,heating as a function of Li- and Ca-12- hydroxystearate concentration.

**Figure 7.** Specific enthalpies of crystallization during cooling <sup>Δ</sup>*h*c,cooling (**a**) and heating <sup>Δ</sup>*h*c,heating (**b**) normalized to the total enthalpy of crystallization Δ*h*c = <sup>Δ</sup>*h*c,cooling + <sup>Δ</sup>*h*c,heating of the trimellitate (EO) based greases vs. concentration of Li- and Ca-12-hydroxystearate as determined from DSC measurements (see Figure 5) at cooling/heating rates of 2 K min−1. The total enthalpy of crystallization remains constant at Δ*h*c = 73.5 ± 6.0 J g<sup>−</sup>1for both types of thickeners. Ranges (1–3) highlight the shift in the supercooling effect. Range (1) indicates predominantly cold crystallization, (2) crystallization upon cooling and heating, and (3) predominantly crystallization upon cooling.

According to ISO 11357-7 [20] the exothermic peaks' total area during cooling and heating corresponds to complete crystallization, and thus the percentage of crystallization during cooling and heating can be calculated. Figure 7 depicts that the percentage of crystallizing base oil upon cooling increases with increasing thickener concentration. As already shown in Figure 5, this indicates a monotonically increasing proportion of heterogeneous crystallization. At thickener concentrations >9 wt.% Ca-12-hydroxy stearate and >11 wt.% Li-12-hydroxystearate, respectively, only crystallization during cooling occurs because heterogeneous nucleation is predominant.

In the case of the pure trimellitate (EO), nuclei form below the temperature range in which crystal growth is optimal. In case of greases, the thickener creates interfaces at which nuclei can form in the temperature range in which crystal growth takes place. Since nuclei are present, crystal growth is detectable as an exothermic DSC signal [9–11].

At low thickener concentrations (0–9 wt.% for Ca-12-hydroxystearate and 0–11 wt.% Li-12-hydroxystearate) only a few nuclei are present. Therefore, only a few crystals start to grow, and the crystal growth rate is not fast enough for complete crystallization before reaching the lowest measuring temperature (see Figure 5a).

At thickener concentrations >9 wt.% for Ca-12-hydroxystearat and >11 wt.% for Li-12-hydroxystearate, sufficient amounts of nuclei are present to allow for complete crystallization of the ester upon cooling.

Due to heterogeneous crystallization, the lubrication greases crystallize approximately 37 K above the pour point. Thus, the pour point is not suitable to judge the low temperature application behavior of greases based on trimellitate (EO) with linear alkyl chains (C8–C10).
