**1. Introduction**

Lubricant industry develops and produces lubricating oils and greases for the lubrication of machine elements such as rolling bearings. Lubricants are essential to reduce friction and thus energy consumption on the one hand, and to prevent premature failure of machine elements due to wear. Lubricants also play an important role in damping the vibrations in bearings and decreasing the dissipated energy by decreasing the friction coefficient in the contact. Most lubricants available on the market today are still produced petrochemically, i.e., on the basis of fossil raw materials. With considering the expected shortage of fossil raw materials and the associated increase in the cost of products based on fossil raw materials, the industry is increasingly focusing on products made from renewable raw materials and the necessary adapted manufacturing processes [1,2]. Furthermore, in Germany, approximately 50% of the mineral oil-based lubricants used are released, each year, to the environment as a result of leakage or accidents, which causes environmental pollution due to chemical materials [3]. The first step toward developing biodegradable lubricants is to design sustainable lubricants, which are made of bio-based materials as an alternative to chemical resources [1].

Bio-based lubricants, such as ester or castor oil, are already available on market, which can be applied as an alternative to mineral oils [4]. In a recent study a bio lubricant is applied in a journal bearing and the friction behavior is compared to a mineral and a synthetic oil. It was found that, the bio lubricant showed lower friction compared to the mineral lubricant, however the synthetic oil showed the lowest friction [5]. In particular, there has been a focus on vegetable oils, such as palm oil and sunflower oil., to find a

**Citation:** Vafaei, S.; Fischer, D.; Jopen, M.; Jacobs, G.; König, F.; Weberskirch, R. Investigation of Tribological Behavior of Lubricating Greases Composed of Different Bio-Based Polymer Thickeners. *Lubricants* **2021**, *9*, 80. https:// doi.org/10.3390/lubricants9080080

Received: 21 June 2021 Accepted: 13 August 2021 Published: 17 August 2021

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**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/).

suitable bio-based lubricant. Vegetable oils in comparison with mineral oils are renewable, easy to obtain, environmentally friendly, economically cheaper and hence, sustainable [6]. There was also a suggestion for application of bio lubricants based on soybean oil improved by epoxy compounds, shown in [7].

Usually, a bearing grease is based on around 70 to 97% of base oil, up to 10% of additives and around 3 to 30 wt.% [8]. According to literature, it is accepted to call a material biogenic or bio-based if it is biodegradable or it is produced partly or completely from renewable raw materials, as defined in [9]. The thickener system can be either based on lithium soaps of fatty acids or petrochemical based oligomers or polymers such as polyureas. Although, bio-based lubricants can be an alternative to commercial petrochemical greases [10], they still need to be developed and a reliable application must be approved. Regarding data from Agency for Renewable Resources (Fachagentur Nachwachsende Rohstoffe e.V.-FNR) the available greases on market are maximum 85% bio-based, since the thickener system is normally not bio-based. To develop a full bio-based grease, base oil and thickener system must be made from renewable materials. Recently, there has been some works on the development of a complete bio-based greases. The results demonstrated in [11] reveal, that a cellulose pulp from the Eucalyptus Globulus plant was obtained as the raw material to produce a thickener system for a bio-based grease. The cellulose pulp was also optimized in another study as a suggestion for bio-based thickener production [12]. Moreover, in [13] it was shown that bio-based greases were developed based on castor oil and high-oleic sunflower oil with cellulose fibers as thickener. In other studies, friction coefficients of developed bio-based greases were comparable to the petrochemical greases [14–16]. Even the wear in bearing with application of bio-based greases were shown to be lower than petrochemical greases in [17]. Although, some biobased greases have been proposed recently, it becomes more and more difficult to consider economic limits and ecological goals at the concurrently Therefore, new suggestions for biobased greases are always needed [18]. An alternative for producing bio-based thickeners is to develop polymer-based thickeners, such as polyurea, from sustainable materials, due to their high abilities compared to raw materials. If the thickener system is a lithium soap of conventional high molecular weight fatty acids, it represents a bio-based thickener. The main differences between biogenically thickeners to non-biogenic thickeners is the origin of the raw materials.

Promising bio-based feedstocks for the production of lubricating greases include 1,5-pentamethylene diisocyanate (PDI) [19], 2,5-bisaminomethylenefuran (BAMF), 1,5- pentamethylene diamine (PDA) and 4,4-diaminodiphenylmethane (MDA). PDA can already be obtained from glucose as reported in [20] and can be reacted with phosgene to give PDI. In this way, PDI with a biocarbon content of up to 70% can be obtained. BAMF can also be synthesized on the basis of glucose [21]. Hydroxymethylfurfural (5-HMF) is first obtained as the base chemical and converted to BAMF in several steps. At the present time, MDA cannot ye<sup>t</sup> be produced bio-based on a large scale. However, MDA is produced on the basis of aniline, the sustainable production of which is already being worked on [22]. Thus, a substitution by bio-MDA is foreseeable in the near future.

Therefore, in this study we present the synthesis of bio-based polyurea thickener systems as well as the production method and chemical characterization of bio-based greases. Moreover, the focus of this study is to elucidate the tribological performance of the produced bio-based greases in comparison to a petrochemical reference grease. To evaluate the tribological performance, the results of film thickness and friction measurements on a ball on disc tribometer are discussed.

#### **2. Lubricants and Methods**

In the following the method of development and production of the bio-based greases are explained. Then to characterize the produced bio-based greases, methods of qualifications regarding tribological characteristics of the bio-based greases are explained.

#### *2.1. Bio-Based Grease Development and Production*

To produce the polyurea thickener systems, an in-situ polymerization of the monomers in castor oil was carried out. The reaction follows the mechanism of a polyaddition reaction. Three polyureas were prepared based on pentamethylene diisiocyanate (PDI) that was reacted with one of the diamines (a–c) (Scheme 1). As reaction vessel an ULTRA TURRAX® Tube Drive with ST-20 mixing vessel was used. The monomers were heated to melt and mixed in the oil at 100 ◦C. The PDI was added to the reaction mixture as the last component under stirring. After the addition of all components was completed, stirring was continued for another 2 min. A gel-like structure was typically formed within 10 s. The reaction progress was controlled by FTIR spectroscopy (Bruker Tensor 27, Berlin, Germany) shown in Figure 1. If the conversion was incomplete after two minutes and isocyanate bands were still detectable with the C-N band at 2270 cm<sup>−</sup>1, more stearyl amine was added to the reaction mixture until the C-N band completely disappeared. All three thickeners have a weight share of 15% of the grease. After cooling, the lubricating greases were homogenized by means of a three-roll mill.

**Scheme 1.** Reaction scheme of polyurea grease production with all components.

For all three polyurea systems the same monomer ratio was chosen, i.e., diisocyanate:diamine:monoamine was 1:0.75:0.25. This results in a degree of polymerization of 9 for the chain length of the polymer thickeners. The theoretical chain length was calculated using the CAROTHERS equation [23].

**Figure 1.** IR-spectroscopy of all three bio-based greases. Bio-based grease 1: PDI-BAMF (orange), bio-based grease 2: PDI-PDA (red) and bio-based grease 3: PDI-MDA (blue).

The successful production of the desired polymeric thickener structures was verified by 1H-NMR spectroscopy (Bruker DPX-400, Berlin, Germany). First, the respective grease was extracted with ethyl acetate in a Soxhlet extractor for three hours. The solid residue was dried under reduced pressure and the thickeners were obtained as yellow powder. Since polyureas are nearly insoluble in organic solvents, the powder was dissolved in concentrated sulfuric acid (98%). To determine the chemical shift *δ*, a capillary with deuterium oxide phase-separated was used as reference in 1H-NMR spectroscopy.

Using the bio-based grease 1 (PDI-BAMF system) as an example, the analysis of the 1H-NMR spectrum is shown below. Using the 1H-NMR spectrum of the bio-based grease 1 thickener (Figure 2) the successful synthesis of the chemical structure can be confirmed. With 1H-NMR the real number of protons for a structure can be determined, if the amount for one signal is exactly known. As can be seen from Figure 2 with exception of the protons (5) between the urea group and the furan ring, all protons are visible in the spectrum. These protons are overlaid by the signal of the reference D2O. Since the FTIR spectra (Figure 1) showed that no C-N band is present anymore in the greases, a complete reaction with the termination reagen<sup>t</sup> stearyl amine can be assumed. Thus, there are two alkyl end groups per polymer chain and the protons of the methyl end group (1) can be seen at 0.66 ppm. Since this signal is baseline separated it can be used as an internal reference for the integration of all signals of the spectrum and refers to six protons per polymer chain, due to the fact that each methyl end group has three protons. This makes it possible to determine the exact number of protons for the other signals in the spectrum. For this purpose, the integral of the methyl group protons (1) is set to six. The protons of the CH2 groups of the alkyl chains (4) in the immediate vicinity of the urea groups also give a baseline separated signal. Therefore, this integral allows an exact determination of the number of repeating units within the polymer chain. For the PDI-BAMF system, four protons must therefore be subtracted from the integral (4), since they belong to the end group. This results in 36 protons. Since there are four protons for (4) per repeat unit, this value is divided by four. This results in an average repeat unit of nine for the polymer chains of the PDI-BAMF thickener. The experimentally determined value of the mean repeating units were in good agreemen<sup>t</sup> with the previously calculated using the Carothers equation [23].

**Figure 2.** 1H-NMR spectrum of the PDI-BAMF thickener (bio-based grease 1).

Furthermore, the relative flow limit of the three lubricating grease systems was investigated by using an MCR 302 rheometer (Anton Paar, Ostfildern, Germany). Oscillation

measurements according to DIN 51810-2 at 25 ◦C were performed. A plate-plate system with a geometry diameter of 25 mm was used. Forty-one data points were recorded for a shear deformation between 0.01 and 100%. The measurements were performed at a constant angular frequency of 1.59 Hz and a constant gap size of 1.000 mm. The flow limit was determined by the intersection of the logarithmically plotted curves from the storage and loss modulus. It was shown that castor oil is a suitable candidate regarding stability of produced grease and tribological characteristics such as friction coefficient [13,14,24]. Thus, castor oil was chosen as the base oil to produce the bio-based greases.

#### *2.2. Qualification of Bio-Based Greases*

The thickener systems and the stable bio-based greases were synthesized at TU Dortmund. The produced bio-based greases and grease properties are listed in Table 1. All bio-based greases had NLGI-class 2 as the result of investigations on penetration depth given in Table A3 in Appendix A. Furthermore, a petrochemical grease is also used for comparison the bio-based greases, which is also listed in Table 1.


**Table 1.** Summary of the bio-based greases and tested lubricants. PAO = polyalphaolefine.

Tribological tests were performed at the Institute for Machine Elements and Systems Engineering (MSE) of RWTH Aachen University to characterize the greases. For this purpose, the film thickness in the EHD contact was measured on a steel ball-on-glass disc EHD2-Tribometer from PCS Instruments as shown in Figure 3. A polished steel ball with a diameter of 19.05 mm was pushed against a glass disc with 47 N. The resulting Hertzian pressure was aimed to be 700 MPa and the temperature during the measurements was set to 40 ◦C. The film thickness measurements were conducted based on the interferometry concept, which is described in [25]. This method of measurements was also used in the literature [26]. Film thickness measurements were performed under pure rolling conditions, where the sliding effects were not present in the contact.

**Figure 3.** EHD2-ball-disc tribometer of PCS Instruments.

As preparation for the film thickness measurements, the grease was distributed on the disc with a grease film height of approximately 0.1 mm using a grease distributer following the measurement procedure described in previous work by the authors [27]. Before the actual measurement, the grease was uniformly distributed on the rolling track in the contact area by applying a 15 min running-in phase at a constant rolling speed of 30 mm/s. During the film thickness measurement, the rolling speed was continuously increased from 4 mm/s to 1000 mm/s in approximately 45 min. For oils, the relation of film thickness over rolling speeds is known to be linear in double logarithmic diagrams [8]. Therefore, to compare the film thickness of greases over rolling speed to base oil, the film thickness was measured six times at 61 logarithmically distributed rolling speeds under pure rolling condition, i.e., slide-to-roll-ratio (SRR) of 0%.

As the next characterization method, developed bio-based greases were tested by mean of friction losses in the EHD contact. In order to do so, the friction measurements were conducted using the EHD2-ball-on-disc tribometer. In contrast to the film thickness measurements, the steel ball was pressed against a polished steel disc and a torque transducer measured the traction forces acting on the ball. The friction coefficients were calculated by dividing the traction forces by the normal forces given to the ball. The schematic representation of this procedure is shown in Figure 4.

**Figure 4.** Schematic of the steel ball on steel disc friction measurements on EHD2 tribometer.

The material and roughness properties of the glass disc and steel disc and steel balls for measurements are listed in the Table 2.



The lubricant conditions in the rolling contacts can be determined based on the definition of lambda ratio as in [29,30]:

$$
\lambda = h\_0 / \sqrt{\left(R\_{q,1}^2 + R\_{q,2}^2\right)} \tag{1}
$$

where *h*0 is the film thickness and *Rq* is root-mean-square roughness of contact bodies.

According to [29,30], for values of higher than 3 for lambda, fluid regime and separation of both contact bodies is expected. Knowing the roughness of glass disc and steel ball from Table 2, corresponding film thickness value of lambda 3 was calculated equal to 18 nm. Analog to steel all and glass disc, corresponding film thickness of steel ball and steel disc can be calculated as 23 nm. The values show very small film thickness corresponding to lambda = 3. Therefore, it was assumed that no mixed lubrication regime occurs during the film thickness and friction measurements for the whole rolling speed range.

A relative speed between ball and disc was set to induce friction in the contact for friction measurements. Therefore, an SRR of 15% was adjusted. The greases are evenly distributed on the steel disc. Then, a running in phase was set before the actual friction measurement in analogy to the film thickness measurements. The EHD contact pressure for film thickness measurements was also 700 MPa and 40 ◦C. For each grease, both positive and negative SRR values were measured. This means that both scenarios of faster ball rotation than the disc and vice versa were measured. For each of the positive and negative SRR values, three independent measurements have been performed. In each test the rolling speed was increased from 15 mm/s to 1000 mm/s. The results were averaged to represent the trend of friction coefficient dependent on rolling speed.

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

In this section, three subsections are defined to demonstrate the results of characterization of bio-based greases. First, chemical characterization of the greases after production in labor is included. In the second subsection, results of film thickness measurements are explained. The three bio-based greases and the reference grease were compared and evaluated regarding the film formation capability and the occurrence of starvation. In the third subsection, the friction coefficients of the different greases were evaluated dependent on rolling speeds.

## *3.1. Chemical Characterisation*

Through the knowledge of the exact chemical structure of the thickeners, it is possible to determine the proportion of bio-based carbons for all three thickener systems (Table 3). The castor oil used was completely bio-based and accounts for 85 weight percent of the total grease. The remaining 15 weight percent was thickener, as no additives were used. The PDI used has a bio-carbon content of 70%. The BAMF and PDA used were bio-based. Stearyl amine and MDA were used as petrochemical products. For the bio-based grease 1 thickener (PDI-BAMF), this resulted in a value of 64% bio-based carbon. Taking the weight percentages into account, the total value was 95% bio-based carbon. Similarly, for the bio-based grease 3 thickener (PDI-PDA), this resulted in a value of 62 percent bio-based carbon for the thickener and a total value of 94% bio-based carbon by weight for the system. For the bio-based grease 2 thickener (PDI-MDA), only 20% bio-based carbon was obtained for the thickener which results in 88% bio-based carbon by weight. In all three systems, the maximum proportion of bio-based carbon can thus be increased by at least 3%, and in the best case by as much as 10%, compared commercially available systems that are currently used.


**Table 3.** Overview of all analytical data of the three thickener systems.

A flow limit of 16% was determined for the bio-based grease 1. Bio-based grease 3 had a flow limit of 25% shear deformation, which was the highest value of the three systems. Bio-based grease 2 showed the lowest value with a flow limit of only 5% (Figure 5). The flow limit of the bio-based greases can be correlated with the chemical structure of their thickeners. Both bio-based grease 1 and 2 contain aromatic thickener systems. The presence of aromatics in the polymer backbone results in a reduction of the chain mobility and stiffening of the polymer chain that may also facilitate <sup>π</sup>-<sup>π</sup>-stacking between different chains [31]. The chain stiffening increases even further for the PDI-MDA system, hence the lowest yield point is expected here. The bio-based grease 3 does not contain aromatics in the polymer backbone of the thickener and thus has the highest chain mobility. This allowed the system to withstand shear stress from the geometry for longer, resulting in a higher flow limit. The correlation of chain mobility as a function of polymer backbone has already been observed in rheology for other applications [32] and is in agreemen<sup>t</sup> with the literature.

Additionally, the dropping points of all systems were determined (Table 3). For the bio-based grease 1 a dropping point of 226 ◦C could be determined. The bio-based grease 3 had a dropping point of 252 ◦C. For the bio-based grease 2 a dropping point of 193 ◦C was determined. Class 2 was determined for the NLGI class of all three systems.

**Figure 5.** Flow limit determination for bio-based grease 1 (purple), bio-based grease 2 (green) and bio-based grease 3 (blue).

#### *3.2. Film Thickness Measurements*

In the first set of tribological characterization we measured the film thickness on ball-on-disc tribometer by varying the rolling speed to identify the onset of starvation for each grease. The results of film thickness measurements are shown in Figure 6.

As a reference, film thickness measurement of castor base oil with a measured kinematic viscosity of 254 mm2/s at 40 ◦C is also shown. As stated before, three measurements were averaged to represent the trend of film thickness dependent on rolling speed. The aim of the development of bio-based greases was to identify the bio-based thickener and base oil combinations, which can be used in industry instead of potentially harmful petrochemical based greases and thus reduce harmful environmental influences. Therefore, a petrochemical reference grease was also used to carry out film thickness measurements in comparison to the bio-based greases.

As shown in the upper left diagram in Figure 6, up to the speed of 100 mm/s, film thickness of bio-based grease 1 was higher than base oil. This is caused by thickener effects, as already described in [33–36]. From a rolling speed of 100 mm/s, film thickness increases with further increasing rolling speed, similar to base oil behavior. This effect was the hydrodynamic film formation which mainly depends on the rolling speed and base oil viscosity. In the speed range between 100 mm/s to 1000 mm/s, the base oil dominates the film formation, and thickener effects, assumingly, play a minor role [37]. In slower speeds, up to 100 mm/s, it was shown that bio-based grease 1 had the same film thickness as the reference grease. From 100 mm/s, it was shown that the bio-based grease even higher film thickness than the reference grease, and with increasing the speed the film thickness difference also increases. The hydrodynamic behavior was not captured in reference grease. In upper right diagram in Figure 6, the film thickness of bio-based grease 2 was smaller than base oil curve up to 20 mm/s. The hydrodynamic behavior of bio-based grease 2 ranges from 20 mm/s to 150 mm/s. Above that speed, the film thickness decreases with increasing rolling speed. This may happen due to an insufficient amount of lubricant replenishment in contact [8]. As presented in lower left diagram in Figure 6, hydrodynamic behavior of bio-based grease 3 starts at 20 mm/s and no decrease below the base oil curve can be detected.

In the lower right diagram in Figure 6, three bio-based greases are compared. It can be seen that the bio-based grease 2 shows the smallest film thickness up to 70 mm/s among all bio-based greases. Probably, the effect of thickener in bio-based grease 2 was the least distinctive. Up to 70 mm/s, bio-based grease 1 shows around 100 to 300 nm higher film thickness compared to bio-based grease 3. Thus, bio-based greases 1 and 3 with thickener systems PDI-BAMF and PDI-PDA show a tendency to form higher separating films over

the tested range of rolling speeds in comparison to bio-based grease 2 with thickener system PDI-MDA.

**Figure 6.** Film thickness of bio-based greases over rolling speed compared to reference grease and base oil at hertzian contact pressure of 700 MPa at 40 ◦C with SRR = 0 on steel ball on glass disc tribometer: (**a**) film thickness of bio-based grease 1, reference grease and base oil over rolling speed; (**b**) film thickness of bio-based grease 2, reference grease and base oil over rolling speed; (**c**) film thickness of bio-based grease 3, reference grease and base oil over rolling speed; (**d**) film thickness of bio-based grease 1, 2 and 3 and base oil over rolling speed.

The results of the lubricant film thickness measurement also correlate with the chemical structure of the thickeners. As previously shown with the rheology measurements, the chain mobility also has a decisive influence on the formation of the lubricant film. The polymer chains of the PDI-MDA system (bio-based grease 2) have the highest chain stiffness, from which a small contribution of the thickener structure to the formation of the lubricating film can be deduced. The PDI-BAMF system (bio-based grease 1) also contains an aromatic structure, but has a higher chain mobility. The PDI-PDA system (bio-based grease 3) has the highest chain mobility and is, therefore, the most effective in film thickness formation.

## *3.3. Friction Measurements*

The second method of characterization of the bio-based greases in this paper was measuring the produced friction coefficients on EHD contact on steel ball on steel disc measured by EHD2 tribometer by varying the rolling speed. Friction measurements were also conducted on the petrochemical reference grease to have a comparison with the bio-based greases.

The results of friction measurements are shown in Figure 7. In the upper left diagrams of Figure 7, it can be seen that friction coefficient in higher speeds, up to 800 mm/s, was same between bio-based grease 1 and the reference grease. The higher scatter in results was also observed in lower speed due to small grease particles passing through the contact, as was also captured in [38]. As shown in the upper right diagrams of Figure 7, the bio-based grease 2 shows similar friction coefficients to the reference grease up to 30 mm/s. In average speeds, from 30 mm/s to 400 mm/s, friction coefficient of the bio-based grease 1 was lower than the reference grease, and by higher speeds, from 30 mm/s, show both greases again the same friction coefficients. From the lower left diagrams of Figure 7, it can be seen that the friction coefficient of bio-based grease 3 is lower than the reference grease in all speed ranges up to 1000 mm/s. The friction coefficient of bio-based grease 3 decreases even further with increasing rolling speed.

**Figure 7.** Average friction coefficient of bio-based greases over rolling speed compared to reference grease at hertzian contact pressure of 700 MPa at 40 ◦C with SRR = 15% on steel ball on steel disc tribometer: (**a**) average friction coefficient of bio-based grease 1 and reference grease over rolling speed; (**b**) average friction coefficient of bio-based grease 2 and reference grease over rolling speed; (**c**) average friction coefficient of bio-based grease 3 and reference grease over rolling speed; (**d**) average friction coefficient of bio-based grease 1, 2 and 3 over rolling speed.

The bio-based greases are compared among each other in lower right diagram in Figure 7. It can be generalized that bio-based grease 3 results in the smallest friction coefficient. Up to 200 mm/s, bio-based grease 2 and 3 lead to similar values. The increase of friction at higher speeds for bio-based grease 3 can be linked directly to the decrease of film thickness and the occurring starvation at those speeds.
