Room Temperature Production of Polyurea-Based Lubricants: Using L-Serine Derivatives, 1,5 Pentamethylene Diisocyanate, and a Planetary Ball Mill

Abstract

In this work, we produced a new polyurea (PU)-based thickener based on serine derivatives (ethanolamine or L-Serine ethyl ester) and 1,5 pentamethylene diisocyanate (PDI), using castor oil as base oil and methylene diphenyl diisocyanate (MDI) as a reference. Polymerization was carried out in a planetary ball mill at room temperature for 75 min. The polymerization degree of the PU thickener was examined via ¹H NMR, which ranged between 1.8 and 14.6 repeating units after the extraction of the base oil. Rheological analysis showed gel formation for ten out of twelve samples, which was strongly dependent on the polymerization degree and thickener amount. The decomposition temperature of the MDI-based PU greases was consistently roughly 20 °C higher than that of PDI-based systems. The lubricants were further evaluated through rheology experiments before and after the gels underwent an annealing process at 100 °C for 1 h (amplitude and frequency test), indicating a strong increase in the storage modulus G’, whereas the yield point γ_F remained constant or decreased.

1. Introduction

Lubricants are used to reduce friction and, thus, the heating or wear between two machine parts, which can lead to their destruction [1,2,3]. Depending on their application, liquid (lubricating oils), semi-solid (lubricating greases), solid, and gaseous lubricants can be distinguished [4]. Lubricating greases are viscoelastic materials which consist of 65–95% base oil, 5–30% thickener, and 0–10% additives [5]. The thickener is of great importance as it is responsible for the formation of fibers and a network structure that encloses the base oil, impacting the consistency of the grease and preventing oil leakage [6,7]. The purpose of the additives is to improve the antioxidant, corrosion-inhibiting, and wear-reducing properties of lubricating greases [8]. Currently, most commercially available greases are based on metal soap thickeners such as lithium, sodium, aluminum, calcium, and barium soaps [9].

In addition, polyurea-based lubricating greases are playing an increasingly important role due to their chemical inertness and resistance to higher temperatures and mechanical stress [10,11,12]. While most raw materials for such greases were of petrochemical origin in the past, bio-based raw materials are increasingly becoming the focus of industry, with the aim of making lubricants more environmentally friendly and sustainable [13,14,15]. Recently, we demonstrated that polyurea-based thickeners, which are typically based on methylene diphenyl diisocyanate (MDI)/4,4′-methylenedianiline (MDA), can also be produced by employing bio-based diamines such as 1,5-diaminopentane or 2,5-bis(aminomethyl)furan (BAMF) and 1,5 pentamethylene diisocanate (PDI) [16]. 1,5-Pentamethylene diisocyanate (PDI) is a commercial product obtained from the gas-phase phosgenation of 1,5-diaminopentane, which itself can be produced from lysine after decarboxylation [17,18]. The resulting PDI has a bio-based carbon content of approximately 70%.

With the aim of further increasing the proportion of bio-based raw materials in polyurea-based lubricants and making the process more sustainable, we employed two serine derivatives—i.e., 2-ethanolamine (SerD) as a decarboxylated serine and L-serine ethylester (SerOEt) as a bifunctional reaction component—and reacted them with 1,5-pentamethylene diisocyanate (PDI); methylene diphenyl diisocyanate (MDI) was used as a reference diisocyanate. Beyond raw materials, a second aspect that has received less attention to date is the manufacturing method of PU-based lubricants. The synthesis of PU-thickening materials typically occurs at high temperatures (e.g., between 100 and 160 °C) [10,11,12,16]. This means that more sustainable methods that are less energy-intensive are of great interest, especially when it is known that individual components of the reaction mixture are highly sensitive to oxidation at temperatures above 100 °C, as is known for castor oil [19]. Although vegetable oils are attracting increasing interest as base oils, they are only of limited use in low-temperature applications because they exhibit very complex crystallization behavior that depends on both the shear rate and the cooling conditions [20].

The aim of this research is therefore twofold: Firstly, PU-based thickeners were produced based on bio-based raw materials, i.e., serine derivatives (SerD and SerOEt) and PDI as the biobased diisocyanate in castor oil (as the base oil), for which methylendi(phenylisocyanate) (MDI) will be used as a reference. Secondly, we synthesized these PU-based grease samples through the in situ polymerization of all raw materials in castor oil in a planetary ball mill at room temperature (Scheme 1). To date, a planetary ball mill has only been used to incorporate additives into fats but not to produce the fats themselves [21,22,23]. The resulting PU thickeners were characterized using ¹H NMR and infrared spectroscopy. Furthermore, rheological properties (e.g., storage modulus, yield point, sol–gel transition) of the grease samples were investigated before and after annealing at 100 °C for 1 h using amplitude and frequency tests. Finally, regarding thermal analysis, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out on the fats.

Scheme 1. Synthesis of polyurea thickener materials based on serine derivatives and 1,5 pentamethylene diisocyanate (PDI) or methylendi(phenylisocyanate) (MDI).

Scheme 1. Synthesis of polyurea thickener materials based on serine derivatives and 1,5 pentamethylene diisocyanate (PDI) or methylendi(phenylisocyanate) (MDI).

2. Materials and Methods

2.1. Materials

2.1.1. Desalination of L-Serine Ethyl Ester Hydrochloride

An Amberlyst^® A26 ion exchanger (SigmaAldrich, Darmstadt, Germany) was first washed for one hour each with Miili-Q water and ethanol. After filtration using a nutsche filter, the ion exchanger was dried under high vacuum for two days. The dried A26 ion exchanger (14.74 g, 1.125 eq.) was transferred to a 600 mL beaker, suspended in an ethanol/water (5:1) solvent mixture, and stirred for 30 min. The solvent was removed with a syringe, and l-serine ethyl ester hydrochloride (5.0 g, 29.5 mmol, 1 eq.) was dissolved in 100 mL of ethanol/water (5:1) solvent mixture (0.29 mmol/mL). The reaction mixture was stirred for 90 min at RT. The ion exchanger was removed using a nutsche filter and washed with 100 mL of the solvent mixture. The ion exchanger was then stirred again for 30 min in ethanol/water (5:1). After a second filtration, the filtrate was concentrated in a rotary evaporator, and the residue was freeze-dried. The product l-serine ethyl ester (2.32 g, 17.43 mmol, 93%) was obtained as a white solid.

Elemental Analysis	Carbon: 42.1% (45.1%); hydrogen: 7.9% (8.3%); nitrogen: 11.5% (10.5%)
1 H-NMR	(600 MHz, D2O) δ = 1.25 (t, 3H, CH3), 3.61 (t, 1H, CH), 3.71–3.96 (dd, 2H, CH2), 4.17 (m, 2H, CH2) ppm.

2.1.2. Preparation of the Fats

All fats examined in this study were prepared in the same manner. First, grinding balls (94.6 g, Ø 6 mm), made of steel, were placed in a 50 mL steel grinding beaker. The reactants were always added in the same order: First, the n-stearylamine and serine derivatives were added. Then, the base oil castor oil was added, and finally, the diisocyanate component was added to avoid premature polymerization. The sealed grinding beaker was circulated in a PM 100 planetary ball mill from Retsch for 75 min at 400 rpm. The fats produced were then homogenized three times using an Exakt roller (type 35). During the first rolling process, the balls were separated from the grease. Finally, the fats were degassed under high vacuum for one hour.

2.1.3. Synthesis of SerD-MDI Greases

Methylenediphenyl isocyanate (MDI) and 2-aminoethanol (SerD) were used as monomers for the SerD-MDI thickening system. The monoamine stearylamine was used as an end group.

VA [wt.%]	Theor.  ${\stackrel{\mathbf{-}}{\mathit{P}}}_{\mathbf{n}}$	Component	M [g/mol]	Eq	m [g]	n [mmol]
20	5	MDI	250.26	1.00	2.560	10.23
		SerD	61.08	0.83	0.521	8.53
		Stearylamine	269.52	0.33	0.919	3.41
		Castor oil			16
20	10	MDI	250.26	1.00	2.822	11.27
		SerD	61.08	0.91	0.626	10.25
		Stearylamine	269.52	0.18	0.553	2.05
		Castor oil			16
20	15	MDI	250.26	1.00	2.934	11.72
		SerD	61.08	0.94	0.671	10.99
		Stearylamine	269.52	0.13	0.395	1.47
		Castor oil			16

The NMR spectroscopic data of the ¹H NMR spectrum are listed below, and the number of protons is given for a repeating unit.

1 H-NMR

(500 MHz, D2O): δ = 0.64 (t, 6H), 0.71–2.00 (m, 64 H), 2.27–3.36 (m, 4H), 3.58 (s, 2H), 3.85 (s, 2H), 4.26 (s, 2H), 5.80 (s, 3H), 6.85–7.78 (m, 8H) ppm.

2.1.4. Synthesis of SerOEt-MDI Fats

VA [wt.%]	Theor.  ${\stackrel{\mathbf{-}}{\mathit{P}}}_{\mathbf{n}}$	Component	M [g/mol]	Eq.	m [g]	n [mmol]
20	5	MDI	250.26	1.00	2.219	8.87
		SerOEt	133.15	0.83	0.984	7.39
		Stearylamine	269.52	0.33	0.797	2.96
		Castor oil			16
20	10	MDI	250.26	1.00	2.382	9.52
		SerOEt	133.15	0.91	1.152	8.65
		Stearylamine	269.52	0.18	0.466	1.73
		Castor oil			16
20	15	MDI	250.26	1.00	2.449	9.79
		SerOEt	133.15	0.94	1.222	9.17
		Stearylamine	269.52	0.13	0.330	1.22
		Castor oil			16

The NMR spectroscopic data of the ¹H NMR spectrum are listed below, and the number of protons is given for a repeating unit.

1 H-NMR

(500 MHz, D2O): δ = 0.64 (t, 6H), 0.71–2.00 (m, 64 H), 2.38–3.34 (m, 4H), 3.63–4.72 (m, 6H), 5.80 (s, 3H), 6.66–7.67 (m, 8H) ppm.

2.1.5. Synthesis of SerD-PDI Fats

VA [wt.%]	Theor.  ${\stackrel{\mathbf{-}}{\mathit{P}}}_{\mathbf{n}}$	Component	M [g/mol]	Eq.	m [g]	n [mmol]
20	5	PDI	154.17	1.00	2.091	13.56
		SerD	61.08	0.83	0.69	11.3
		Stearylamine	269.52	0.33	1.219	4.52
		Castor oil			16
20	10	PDI	154.17	1.00	2.384	15.46
		SerD	61.08	0.91	0.859	14.06
		Stearylamine	269.52	0.18	0.758	2.81
		Castor oil			16
20	15	PDI	154.17	1.00	2.516	16.32
		SerD	61.08	0.94	0.934	15.30
		Stearylamine	269.52	0.13	0.550	2.04
		Castor oil			16

The NMR spectroscopic data of the ¹H NMR spectrum are listed below, and the number of protons is given for a repeating unit.

1 H-NMR

(500 MHz, D2O): δ = 0.65 (t, 6H), 0.70–2.00 (m, 64 H), 3.10 (s, 4H), 3.55 (s, 2H), 4.28 (s, 2H), 5.75 (s, 3H) ppm.

2.1.6. Synthesis of SerOEt-PDI Fats

VA [wt.%]	Theor.  ${\stackrel{\mathbf{-}}{\mathit{P}}}_{\mathbf{n}}$	Component	M [g/mol]	Eq.	m [g]	n [mmol]
20	5	PDI	154.17	1.00	1.173	11.27
		SerOEt	133.15	0.83	1.250	9.39
		Stearylamine	269.52	0.33	1.012	3.76
		Castor oil			16
20	10	PDI	154.17	1.00	1.902	12.34
		SerOEt	133.15	0.91	1.493	11.22
		Stearylamine	269.52	0.18	0.607	2.24
		Castor oil			16
20	15	PDI	154.17	1.00	1.972	12.79
		SerOEt	133.15	0.94	1.597	11.99
		Stearylamine	269.52	0.13	0.431	1.60
		Castor oil			16

The NMR spectroscopic data of the ¹H NMR spectrum are listed below, and the number of protons is given for a repeating unit.

1 H-NMR

(500 MHz, D2O): δ = 0.65 (t, 6H), 0.70–2.10 (m, 64 H), 2.50–3.30 (m, 4H), 4.15–4.73 (m, 3H), 5.08 (s, 2 H), 5.76 (s, 1H), 6.78 (s, 1 H) ppm.

2.2. Methods

2.2.1. NMR Spectroscopy

The NMR spectra used in this study were measured using Bruker AVANCE-400 (400.28 MHz) and AVANCE-NEO 500 (500 MHz) devices. The samples were measured at room temperature, the spectra were calibrated with D₂O (δ = 4.75 ppm), the NMR data were evaluated using the ACD LABS 12.0 1D NMR Processor program, and the chemical shift δ is given in parts per million (ppm). The following abbreviations were used to indicate signal multiplicities: s = singlet, d = doublet, t = triplet, quart = quartet, m = multiplet.

For the NMR spectroscopic analysis of the thickeners, 10 mg of sample was dissolved in 0.5 mL of concentrated H₂SO₄ and placed in NMR tubes with a D₂O-filled capillary as a reference.

For the NMR spectroscopic characterization of the amino acid ester SerOEt, 10 mg of sample was dissolved in 0.6 mL D₂O.

2.2.2. Rheology

Oscillation tests were performed on a rheometer from Anton Paar (type: MCR 102). A plate–plate system with a diameter of 25 mm was used. The measuring and trim distances were 1.000 mm and 1.025 mm, respectively. Excess fat was removed with a spatula. All measurements were performed in duplicate, and the evaluation was carried out using the RheoCompass program.

The amplitude measurements were performed at 25 °C with a circular frequency of 1.59 Hz (10 rad/s) in the deformation range γ of 0.01–100%. The yield point was determined in accordance with DIN 51810-2 through the intersection of the storage and loss modulus. Frequency tests were carried out after sample preparation and a 30 min waiting period at an amplitude γ of 0.1% in the frequency range ω of 0.01–100 rad/s at 25 °C.

2.2.3. Thermogravimetric Analysis

Thermogravimetric analysis was performed using a TGA Q50 measuring device from TA Instruments (Hüllhorst, Germany). Under a nitrogen atmosphere, the samples were heated from 20 °C to 800 °C at a heating rate of 20 °C/min. For the measurement, 4–6 mg of sample was applied. The evaluation was performed using the Universal Analysis 2000 Version 4.5A program.

2.2.4. Dynamic Differential Calorimetry

The measurements were performed using the Q200 device (Hüllhorst, Germany) from TA Instruments with the RCS90. For this purpose, 5–7 mg of sample was weighed and placed into an aluminum crucible with a lid (manufacturer: ThePro, Heinsberg, Germany). The measurements were performed under nitrogen purging with an additional empty reference crucible in two heating cycles with a heating and cooling rate of 10 °C/min. The evaluation was performed using the Universal Analysis 2000 Version 4.5A program.

2.2.5. Infrared Spectroscopy

A Tensor 27 FTIR spectrometer (Bruker Optics GmbH, Karlsruhe, Germany) with a platinum diamond ATR unit A225/Q was used for infrared spectroscopy (IR). The evaluation was performed with the corresponding OPUS program (version 7.0). The spectra were recorded in transmission mode at a wave number range of ν = 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹ and 96 scans per sample.

3. Results and Discussion

3.1. Synthesis of PU-Fats in Castor Oil Using a Planetary Ball Mill

The amino acid ester serine ethyl ester (SerOEt) was present as HCl salt and had to be desalted with the ion exchanger Amberlyst^®A26 before use as a monomer. The yields obtained were 59–69% for SerOEt with a purity of 89–92%. The decarboxylated serine derivative 2-ethanolamine (SerD) could be used without further pretreatment.

The theoretical molar mass of the polyurea thickener was calculated based on the so-called equivalent approach, which was described by Ren et al. [24]. This approach represents the ratio of the reactants as the stoichiometric ratio of the monomers in the polymer of the desired average polymerization degree. The ratio of functional groups and amount of starting materials are therefore given by Equations (1) and (2), where P_n is the calculated degree of polymerization, and n_AA, n_BB, and m_B represent the molar amounts of the starting compounds, given in Section 2 (Materials and Methods) for theoretical P_n values of 5, 10, and 15 repeating units.

$$N_{AA} ; N_{BB }: N_B=1: {\displaystyle \frac{{\overline{P}}_n}{{\overline{P}}_n+1}} : {\displaystyle \frac{1}{{\overline{P}}_n+1}}$$

(1)

$$n_{AA} : n_{BB} :n_B=1 : {\displaystyle \frac{{\overline{P}}_n}{{\overline{P}}_n+1}} : {\displaystyle \frac{2}{{\overline{P}}_n+1}}$$

(2)

The samples were produced for all serine derivatives through in situ polymerization of the thickener in castor oil, used as the base oil. First, the grinding balls (94.6 g, Ø 6 mm), made of steel, were placed in a 50 mL steel grinding beaker. Stearyl amine and the serine derivatives were then added, followed by castor oil (the base oil) and finally the diisocyanate component to prevent premature polymerization. The sealed grinding beaker was placed in a planetary ball mill and circulated for 75 min at 400 rpm. The samples produced were then homogenized three times using an EXAKT roller (type 35, EXACT Advanced Technologies GmbH, Norderstedt). A thickener content of approx. 20% by weight was targeted for all samples, and the degrees of polymerization were varied. Figure 1 shows the structures of the polyurea thickeners produced with this method.

Figure 1. (A) Steel balls in a 50 mL container, (B) crude fat after ball mill synthesis (SerD-PDI), (C) planetary ball mill PM100 and (D) EXAKT roller (type 35) to separate and homogenize the fat samples, and (E) structures of the fabricated polyureas, SerD-PDI and SerOEt-PDI, and SerD-MDI and SerOEt-MDI.

After the extraction of the castor oil, the polymer thickeners of the lubricating greases were examined for their composition and degree of polymerization using ¹H NMR spectroscopy. An example of an ¹H NMR spectrum for the SerD-PDI-2 thickener with signal assignment is shown in Figure 2.

Figure 2. ¹H NMR spectrum of the SerD-PDI-2 thickener with a ${\stackrel{-}{P}}_{\mathrm{n}}$ of 7.2 and a thickener content of 13.7 wt.%, measured in H2SO4 with D₂O as a reference at 500 MHz.

The average degrees of polymerization determined from the ¹ H-NMR spectra and the resulting molar masses are summarized in Table 1. The values determined are below the theoretical degrees of polymerization for all thickeners. A reason for this could be the reaction of the diisocyanate monomer with air humidity. Importantly, only a small fraction of the thickener was extracted with ethyl acetate as the solvent, due to the high polarity of the thickener molecule. For most PU greases, thickener content reached between 13 and 17.7 wt%, below the theoretical value of 20 wt%. During the production of SerOEt-MDI greases with theoretical average degrees of polymerization of 10 and 15, larger quantities of solid residues were observed in the grease. An examination of the residues using FTIR spectroscopy showed that the solids were unreacted SerOEt. All ¹H NMR spectra are shown in Appendix A, Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8, Figure A9, Figure A10 and Figure A11.

Table 1. Theoretical and experimentally obtained VA thickener proportions, as well as average degrees of polymerization and average molar masses of the thickeners in the fat systems based on serine derivatives with MDI or PDI as diisocyanate.

Sample	VA [wt.%] [a]	DP [b]	${\stackrel{\mathbf{-}}{\mathit{M}}}_{\mathbf{n}}$ [103 g/mol] [c]
SerD-PDI-1	14.5 (20)	4.2 (5)	1.4 (1.6)
SerD-PDI-2	13.7 (20)	8.1 (10)	2.3 (2.7)
SerD-PDI-3	13.0 (20)	14.6 (15)	3.7 (3.8)
SerD-MDI-1	17.7 (20)	1.8 (5)	1.0 (2.1)
SerD-MDI-2	16.0 (20)	4.5 (10)	1.9 (3.7)
SerD-MDI-3	16.6 (20)	5.4 (15)	2.3 (5.2)
SerOEt-PDI-1	13.5 (20)	2.5 (5)	1.3 (2.0)
SerOEt-PDI-2	15.2 (20)	3.5 (10)	1.5 (3.4)
SerOEt-PDI-3	14.4 (20)	4.0 (15)	1.7 (4.8)
SerOEt-MDI-1	13.1 (20)	3.6 (5)	2.4 (3.1)
SerOEt-MDI-2	6.7 (20)	6.8 (10)	3.9 (6.1)
SerOEt-MDI-3	7.2 (20)	10.4 (15)	5.6 (9.2)

[a] Thickener content determined via extraction with the theoretical thickener content in brackets. [b] Average degrees of polymerization determined via ¹H NMR end group analysis with the theoretical average degrees of polymerization in brackets. [c] Average molar masses calculated from the average degrees of polymerization and theoretical average molar masses in brackets.

3.2. Rheological Characterization of Fats

The amplitude and frequency were measured to determine the linear viscoelastic region (LVE) and flow point of the samples (Table 2). In Figure 3, the storage and loss moduli of the amplitude measurements are plotted logarithmically against the applied shear deformation for the grease samples from Table 1.

Figure 3. Double logarithmic plot of storage and loss modulus G′ and G″ as a function of shear deformation γ for greases based on PDI, SerD-PDI (A) and SerOEt-PDI (B), and on MDI, SerD-MDI (C) and SerOEt-MDI (D), as a function of the average degree of polymerization obtained from the amplitude test.

SerD-PDI(1–3) showed only a slight upward trend in strength with an increasing degree of polymerization, with values for G′_LVE between 4.1 and 5.5 kPa. In contrast, when MDI was employed as diisocyanate, the strength was significantly increased compared to the aliphatic PDI-based PUs (G′_LVE between 17.7 and 64.4 kPa) and the values for G′_LVE decreased with an increasing degree of polymerization. The differences in strength can be attributed to the flexibility of the polymer chain and have also been observed for other polyurea thickeners [12] Aliphatic PDI-based polyureas are significantly more flexible than aromatic MDI-based polyureas. Moreover, the latter may exhibit π-π interactions as an additional effect. When employing the serine ethylester SerOEt, the G’_LVE values for the SerOEt-PDI series were significantly higher at 151 to 319 kPa than those for the SerD-PDI series. One reason for this is probably the low degree of polymerization, as short-chain polyureas generally exhibit higher strengths than longer-chain polyureas [12]. In addition, H-bonds can be formed between the ester functionality of serine ethyl ester and the OH-group of castor oil, which could significantly contribute to the strength of the fats. Interestingly, the flow point for the SerD-PDI series and SerOEt-PDI remained nearly constant and independent of the gel strength. The SerOEt-MDI series showed only gel formation for the shortest PU thickener with P_n = 3.9. When the thickener content was only 6.7 and 7.2 wt.%, as determined for SerOEt-MDI-2 and SerOEt-MDI-3, respectively, a viscous sol was obtained (Figure 4). The reason for this could be the rather low thickener content in the samples, as shown in Table 1, which is known to be critical in gel formation [6]. For the SerD-MDI and SerOEt-MDI samples, there was a uniform decrease in strength with an increasing degree of polymerization, suggesting that the strength of these fats was dominated by the distance between the hydrophobic end groups of the thickeners.

Figure 4. Plot of shear stress τ as a function of shear deformation γ obtained from the amplitude test for samples based on PDI, SerD-PDI (A) and SerOEt-PDI (B), and on MDI, SerD-MDI (C) and SerOEt-MDI (D), as a function of the average degree of polymerization.

Figure 4. Plot of shear stress τ as a function of shear deformation γ obtained from the amplitude test for samples based on PDI, SerD-PDI (A) and SerOEt-PDI (B), and on MDI, SerD-MDI (C) and SerOEt-MDI (D), as a function of the average degree of polymerization.

Table 2. Values of the storage modulus G′ in the LVE range, yield points γ_F and τ_F, and the frequency-dependent gel–sol transition ω_co of samples SerD-PDI and SerOEt-PDI, SerD-MDI, and SerOEt-MDI as a function of the average degree of polymerization.

Sample	DP [a]	G′LVE [103 Pa] [b]	γF [%] [c]	τF [Pa] [c]	ωco [rad/s] [d]
SerD-PDI	4.2	4.1 ± 0.3	5.2 ± 0.5	73.7 ± 0.3	Gel
	8.1	5.5 ± 0.2	4.2 ± 0.3	50.6 ± 0.8	Gel
	14.6	4.9 ± 0.8	3.6 ± 0.3	59.1 ± 5.1	Gel
SerD-MDI	1.6	64.4 ± 8.6	20.2 ± 0.4	2154.9 ± 44.5	Gel
	4.5	23.0 ± 5.6	8.6 ± 0.4	556 ± 86.6	Gel
	5.5	17.7 ± 0.5	9.9 ± 0.1	501 ± 10.4	Gel
SerOEt-PDI	2.5	151.2 ± 5.1	3.4 ± 0.1	1014.8 ± 39.8	Gel
	3.5	284.8 ± 43.6	2.7 ± 0.2	1533.7 ± 157.3	Gel
	4.0	319.6 ± 38.4	2.8 ± 0.0	1795.2 ± 210.4	Gel
SerOEt-MDI	3.6	19.5 ± 1.5	12.2 ± 0.0	517.6 ± 21.1	Gel
	6.8	0.082 ± 0.002	Sol	Sol	Sol
	10.4	0.25 ± 0.01	Sol	Sol	Sol

[a] Determined via ¹ H NMR end group analysis. [b] Linear fit of the LVE range with G′ ± 10% (amplitude test). [c] Flow point and intersection of G′ and G″ (amplitude test). [d] Intersection of G’ and G″ (frequency test).

Table 2. Values of the storage modulus G′ in the LVE range, yield points γ_F and τ_F, and the frequency-dependent gel–sol transition ω_co of samples SerD-PDI and SerOEt-PDI, SerD-MDI, and SerOEt-MDI as a function of the average degree of polymerization.

Sample	DP [a]	G′LVE [103 Pa] [b]	γF [%] [c]	τF [Pa] [c]	ωco [rad/s] [d]
SerD-PDI	4.2	4.1 ± 0.3	5.2 ± 0.5	73.7 ± 0.3	Gel
	8.1	5.5 ± 0.2	4.2 ± 0.3	50.6 ± 0.8	Gel
	14.6	4.9 ± 0.8	3.6 ± 0.3	59.1 ± 5.1	Gel
SerD-MDI	1.6	64.4 ± 8.6	20.2 ± 0.4	2154.9 ± 44.5	Gel
	4.5	23.0 ± 5.6	8.6 ± 0.4	556 ± 86.6	Gel
	5.5	17.7 ± 0.5	9.9 ± 0.1	501 ± 10.4	Gel
SerOEt-PDI	2.5	151.2 ± 5.1	3.4 ± 0.1	1014.8 ± 39.8	Gel
	3.5	284.8 ± 43.6	2.7 ± 0.2	1533.7 ± 157.3	Gel
	4.0	319.6 ± 38.4	2.8 ± 0.0	1795.2 ± 210.4	Gel
SerOEt-MDI	3.6	19.5 ± 1.5	12.2 ± 0.0	517.6 ± 21.1	Gel
	6.8	0.082 ± 0.002	Sol	Sol	Sol
	10.4	0.25 ± 0.01	Sol	Sol	Sol

[a] Determined via ¹ H NMR end group analysis. [b] Linear fit of the LVE range with G′ ± 10% (amplitude test). [c] Flow point and intersection of G′ and G″ (amplitude test). [d] Intersection of G’ and G″ (frequency test).

3.3. Thermal Analysis of the Greases

Possible influences of the polymerization degree on the thermal properties of the lubricating greases were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The decomposition of the grease samples proceeds in two stages. The greases are temperature-resistant up to approximately 200 °C. At higher temperatures, the thickener begins to decompose first, followed by the castor oil, until the samples have decomposed almost completely. When using the diisocyanate PDI (a), decomposition temperatures of 198–230 °C were reached. However, the degree of polymerization had only a minor influence on the determined decomposition temperature. The samples of the SerOEt-PDI series exhibited the lowest decomposition temperatures of 198 to 205 °C, while the SerD-PDI samples exhibited the highest decomposition temperatures of approximately 220 °C (see Figure 5 and Table 3).

Figure 5. Plot of mass loss m as a function of temperature T as a result of TGA measurements for fats based on PDI, SerD-PDI (A) and SerOEt-PDI (B), and on MDI, SerD-MDI (C) and SerOEt-DDI (D).

Table 3. Temperature T and enthalpy change ∆H for exothermic (exo) phase transitions from the first cooling, determined via DSC, and decomposition temperatures at a 2% mass loss T_Z,2%, determined via TGA, for the different fat samples: SerD-PDI/MDI and SerOEt-PDI/MDI.

Sample	DP	TZ,2% [°C]	Texo,b [°C]	∆Hexo,b [J/g]
SerD-PDI	4.2	221.4	125.28	9.23
	8.1	220.1	111.64, 123.18	8.48
	14.6	218.2	105.20, 123.80	10.22
SerD-MDI	1.6	248.8	58.4; 91.3; 158.9	1.84; 0.15; 0.14
	4.5	245.8	44.3, 161.2	0.39; 0.22
	5.5	244.1	28.2; 160.3	0.08; 0.64
SerOEt-PDI	2.5	205.7	110.01	2.45
	3.5	198.3	94.69	1.12
	4.0	198.3	84.31	0.94
SerOEt-MDI	3.6	221.8	32.4	0.20
	6.8	216.1	56.5; 150.2	0.18; 0.87
	10.4	226.0	--	--

In contrast to the PDI-based materials, the SerD-MDI series reached decomposition temperatures of 244–248 °C, which are significantly higher than those of the SerD-PDI series. This is most likely due to the stronger π-π interactions of the aromatic rings.

The DSC results show pronounced melting and recrystallisation peaks only for the SerD-PDI-based samples in the temperature range of 105 to 125 °C (Table 3) with an enthalpy of approx. 8–10 J/K.

In some cases, a splitting of the peaks was observed during the exothermic transitions, which is why two temperatures are given in Table 3. The thermal transitions between 40 and 80 °C were rather weak with ΔH ≈ 0.2–2 J/K.

Comparing the peak with the thermal transition of the extracted thickener reveals that this thermal transition can be attributed to the melting and recrystallization process of the polymer thickener (see Figure A12). The MDI-based gels showed only rather weak transitions in the temperature range from 20 to 200 °C (Figure 6C,D) with enthalpies of approx. 0.2 to 1.8 J/K. Clear melting/recrystallisation peaks were not be observed; this is in agreement with the literature that reports melting peaks for segmented polyurea based on MDI at temperatures of 320–340 °C [25].

Figure 6. Plot of heat flow dQ/dt as a function of temperature T as a result of DSC measurements during the first cooling and second heating for fats based on PDI, SerD-PDI (A) and SerOEt-PDI (B), and on MDI, SerD-MDI (C) and SerOEt-DDI (D), as a function of the average degree of polymerization.

3.4. Temperature Treatment of Fats

The thermoanalytical investigations using DSC showed thermal transitions in the temperature range of 100–160 °C and in the range of 40–80 °C. Since these transitions may influence the flow properties of the fats, the manufactured lubricating greases were subjected to a temperature treatment in which the samples were first heated to 100 °C for one hour and then cooled to 60 °C within one hour. After the greases had been tempered at 60 °C for one hour, they were cooled to room temperature overnight. The greases obtained in this way were then examined rheologically once again using an amplitude test, a frequency test, and infrared spectroscopy. The results are summarized in Table 4.

Table 4. Values of the storage modulus G′ in the LVE range, yield points γ_F and τ_F, and the frequency-dependent gel-sol transition ω_co of the SerD-PDI/MDI and SerOEt-PDI/MD fats as a function of the average degree of polymerization before (vT) and after (nT) tempering.

Sample	DP [a]	G′LVE,bT [103 Pa] [b]	G′LVE,aT [103 Pa] [b]	γF,bT [%] [c]	γF,aT [%] [c]	ωco,bT [rad/s] [d]	ωco,aT [rad/s] [d]
SerD-PDI	4.2	4.1 ± 0.3	26.6 ± 0.6	5.2 ± 0.5	8.0 ± 0.5	Gel	Gel
	8.1	5.5 ± 0.2	4.4 ± 0.5	4.2 ± 0.3	7.5 ± 0.3	Gel	Gel
	14.6	4.9 ± 0.8	3.1 ± 0.5	3.6 ± 0.3	6.2 ± 0.3	Gel	Gel
SerD-MDI	1.6	64.4 ± 8.6	262.8 ± 20.7	20.2 ± 0.4	8.1 ± 0.2	Gel	Gel
	4.5	23.0 ± 5.6	136.6 ± 4.8	8.6 ± 0.4	6.9 ± 0.6	Gel	Gel
	5.5	17.7 ± 0.5	93.1 ± 3.3	9.9 ± 0.1	7.5 ± 0.8	Gel	Gel
SerOEt-PDI	2.5	151.2 ± 5.1	597.3 ± 78.2	3.4 ± 0.1	1.8 ± 0.0	Gel	Gel
	3.5	284.8 ± 43.6	963.9 ± 112.1	2.7 ± 0.2	1.6 ± 0.,2	Gel	Gel
	4.0	319.6 ± 38.4	654.8 ± 52.7	2.8 ± 0.0	2.1 ± 0.0	Gel	Gel
SerOEt-MDI	3.6	19.5 ± 1.5	87.7 ± 0.96	12.2 ± 0.0	8.1 ± 0.1	Gel	Gel
	6.8	0.082 ± 0.002	0.21 ± 0.02	Sol	Sol	Sol	Sol
	10.4	0.25 ± 0.01	2.68 ± 0.12	Sol	5.9 ± 0.2	Sol	30.2± 0.6

[a] Determined via ¹H NMR end group analysis. [b] Linear fit of the LVE range with G′ ± 10% (amplitude test). [c] Intersection of G′ and G″ (amplitude test). [d] Intersection of G′ and G″ (frequency test).

Evidently, the temperature treatment caused a change in the viscoelastic properties of the samples. All fats with short-chain polymer thickeners (P_n = 3.7–5.7) showed a significant increase in the storage modulus in the linear viscoelastic range G′_LVE. The value for G′_LVE increased by a factor of 6.5 for the grease SerD-PDI-1 (P_{n =} 4.2) with decreasing G′_LVE values as the degree of polymerization increased. For example, although the trends were comparable for the SerD-MDI series, the values for G′_LVE were significantly higher than for the SerD-PDI series after the annealing procedure and decreased with an increasing degree of polymerization from 263 kPa to 93 kPa. Conversely, tor the fats of the SerOEt-PDI series, the increases in the storage modulus were significantly lower, ranging from 3.9 to 2.1. SerOEt-MDI samples 2 and 3 remained a sol after the annealing procedure, most likely due to the low thickener content in the sample, as mentioned before. The increase in the storage modulus after the temperature treatment may be due to the polymer chains becoming more flexible at higher temperatures and having more opportunities to form new intermolecular interactions, e.g., through hydrogen bonding, thereby increasing the storage modulus [26]. This effect works particularly well with short-chain polymer thickeners, while the temperature of 100 °C and the duration of 1 h may still be too low and too short for longer-chain polymer thickeners such as SerD-PDI-3 with a degree of polymerization of 14. For the SerOEt-PDI series, a counteracting effect can be observed, because although the storage modulus increases by a factor of 2.1 to 3.9, the yield point remains almost constant or even decreases slightly, i.e., the force required to make the material flow does not change.

Compared to our data, Courtonne et al. reported decomposition temperatures for MDI-based di- and tetraurea lubricants between 240 and 266 °C for 7 wt% and 14 wt. thickener content. Yield stress depends strongly on the thickener content and the number of urea groups decreasing from 680 to 105 Pa when increasing the number of urea groups from 2 to 4 with 7 wt% thickener content. However, it quickly increases from 105 Pa to 980 Pa for the tetraurea sample when the thickener content was increased from 7 to 14 wt%. Also, the storage modulus G’ depends on the number of urea units; it decreased from 21,000 Pa (diurea, 7 wt.%) to 9500 Pa (tetraurea, 7 wt%) and increased again to 22,500 Pa (tetraurea, 14 wt.%) with increasing thickener content [27]. Wang et al. showed an MDI-based diurea thickener with different amine end groups and thickener content of 10 wt%, reporting that the base oil had a dramatic effect on rheological properties, making comparisons difficult. The order of yield stress can be summarized as follows: PG (PAO40) (153.11 Pa, 23%) >AG (Alkyl naphthalene oil) (118.36 Pa, 13.7%) > OG (polyether oil) (73.07 Pa, 7.5%). Decomposition starts at around 225 °C for the PU-OG sample, whereas the two other polyurea samples showed that the decomposition of the sample started at around 280–290 °C for the PG and AG samples. The storage modulus G’_LVE for all three samples is rather low with values between 3000 and 5500 Pa [28].

3.5. Infrared Spectroscopy

All FTIR spectra were recorded on a Tensor 27 FTIR spectrometer (Bruker Optics GmbH) with a platinum diamond ATR unit A225/Q in transmission mode. Spectra for the SerD-MDI samples are depicted in Figure 7. Since these FTIR spectra are dominated by the base oil (i.e., castor oil), the base oil was removed via extraction and only the thickener is examined in Figure 8. The polymeric thickener is a polyurea urethane molecule. The absence of a signal at 2270 cm⁻¹ indicates complete conversion of the isocyanate groups for the SerD-MDI (Figure 7) and SerOEt-MDI samples (Figure A14). In contrast, PDI-based thickener samples always showed residual isocyanate groups, suggesting an incomplete polymerization reaction (see Figure A13 and Figure A15). The band at 3324 cm⁻¹ can be assigned to the H-bonded N-H stretching vibration, whereas the shoulder at 3440 cm⁻¹ can be assigned to the free N-H bond [29,30]. The band at 3007 cm⁻¹ represents the -C-H stretching vibrations of the cis-double bond from castor oil [29], whereas the bands at 2922 and 2855 cm⁻¹ are characteristic of asymmetrical and symmetrical vibrations of aliphatic –CH₂ groups from the fatty acid hydrocarbon chain. The strong carbonyl bond (C = O) at 1741 cm⁻¹ can be assigned to the castor oil ester functional group but also to the free carbonyl group of the urethane functional group (Figure 8) [31]. Moreover, the bands at 1515–1565 cm⁻¹ (amide II, bending N-H and stretching C-N), 1220–1240 cm⁻¹ (amide III, C-N stretching and N-H bending), and 1140–1120 cm⁻¹ (C-O-C asymmetric stretching) confirm the formation of the urethane group [30,32].

Figure 7. FTIR spectra of the SerD-MDI(1–3, A,B) samples (left) and their superimposed spectra (right).

Figure 8. FTIR spectra of the SerD-MDI-3 sample (blue) and the pure thickener (red) after base oil extraction.

The annealing reaction of the SerD-MDI-3 sample results in a strengthening of the band at 3340 cm⁻¹ (ordered N-H bond) and those at 1633 and 1598 cm-1 (Figure 9). This suggests an increase in ordered N-H bonds and H-bonded urea groups after annealing, which would also explain the increase in stiffness after the annealing reaction (Table 4) [31].

Figure 9. (Left) FTIR spectra of SerD-MDI-3 before (blue) and after annealing (red) (left) and their superimposed spectra (right).

In the case of the SerD-PDI samples, the band at 2270 cm-1 disappears after the annealing reaction, and the bands at 1614 and 1585 cm⁻¹ display an increase in intensity (Figure A17), which can also be attributed to the increase in ordered H-bound urea groups. However, the SerOEt samples show no change in the FTIR spectrum for either MDI or PDI after annealing (Figure A17 and Figure A18).

4. Conclusions

New polyurea-based greases based on bio-based raw materials (ethanolamine or serine ethylester + 1.5 pentamethylene diisocyanate or methylendi(phenylisocyanate) (MDI)) were successfully produced in castor oil for the first time. The in situ production took place in a planetary ball mill at room temperature for 75 min. After extraction of the base oil, the PU thickeners were examined for their composition and degree of polymerization using ¹H NMR spectroscopy. The rheological characterization of the fats showed an almost constant yield point regardless of the degree of polymerization of the PU thickener, while the storage modulus in the linear viscoelastic range remained almost constant for the SerD-PDI series but decreased with an increasing degree of polymerization for the SerD-MDI series. Thermogravimetric analysis (TGA) showed decomposition temperatures of 198–230 °C for the fats with PDI-based thickener, whereas the SerD-MDI series resulted in an even higher decomposition temperature of up to 248 °C. Thermal transitions were observed via DSC analysis, revealing melting and recrystallization processes between 100 and 160 °C for the SerD-PDI samples; thus, the samples were subjected to an annealing process (1 h at 100 °C and then at 60 °C for 1 h). The resulting materials showed a significant increase in storage modulus in the G′_LVE range, with the yield point also increasing for the SerD-PDI series and remaining constant or even decreasing slightly for the SerOEt-PDI series with side-chain ester functionalities. In summary, the ball mill represents an interesting alternative to conventional manufacturing processes for PU-based greases. However, further work is needed to optimize the procedure to better control the thickener content and degree of polymerization of the final grease. Moreover, tribology data are necessary to compare the performances of the new materials with those of commercially available materials.