He Yifeng

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: The dropping point, cone penetration, corrosion, oil separation rate and oxidation stability of lithium grease prepared by two different processes at room temperature and high temperature were measured. The rheological properties of the samples were studied and analyzed with the microstructure of soap fiber. The results show that the content of thickener used in grease prepared by the room temperature method is higher in order to obtain the same consistency. The dropping point and evaporation loss of grease samples prepared by two processes are similar, and the oil separation rate of the lubricating grease prepared by the room temperature method is smaller. The preparation process has a great influence on the rheological properties of grease, and the higher content of thickener is beneficial to improving the rheological properties of grease.

Key words: lubricating grease; lithium grease; rheology; viscosity; structure

1 Introduction

Lubricating grease is a mixture consisting of a lubricating base oil, a thickening agent, and additives. The thickening agent is a material that, in combination with the base oil,can produce the solid to semifluid structure[1]. According to different thickeners, lubricating grease can be divided into the soap based grease and the non-soap based grease. The soap based grease accounts for most of the total number of lubricating grease in variety and output.According to the different thickeners used in metal soap base greases, the soap base grease can be divided into simple soap base grease, mixed soap base grease and complex soap base grease, such as calcium base, sodium base, lithium base, calcium sodium base, lithium calcium base and calcium complex base, lithium complex base grease, etc.[2]Lithium grease is still the most important type of greases in the world. In 2018, the global output was 597 900 t, accounting for 50.94% of the total, which was absolutely dominant[3].

Lithium grease has good comprehensive properties, and its rheological properties are of significance for its wide application. Rheology is a science mainly used to study material deformation and flow in terms of stress, strain,shear rate, temperature, and time. Grease has typical double characteristics of elasticity and viscosity. The research on it belongs to rheology. The rheology of grease refers to the flow and deformation characteristics of grease under external force[4-7]. There are many literature reports on rheological research of lithium grease,including composition (base oil, thickener, and additives),microstructure of thickener, operating conditions(temperature, speed, time, etc.), preparation process, etc[8].The preparation process and conditions of lubricating grease often have a great influence on its rheological properties. However, little research has been done in this aspect, mainly aimed at investigating the effects of cooling methods, cooling conditions, post-treatment methods on the rheological properties. Xu Jun, et al. reported that the structure of lithium grease prepared under natural cooling changed reversibly, while the structure of grease prepared in ice water bath was easily damaged irreversibly[9].Delgado, et al. collected samples at different stages of grease preparation process and studied the rheological differences. It was found that the crystalline form and cooling process of lithium grease had a significant influence on the rheological property and microstructure of greases[10]. Franco, et al. studied the mixed rheological properties during the production of lubricating greases.The results indicated that the mechanical properties of lubricating greases depended significantly on some preparation process variables. Lowering the cooling rate during the cooling crystallization of the grease improves the linear viscoelasticity and elasticity of the lithium grease[11]. Hu Zhide, et al. have studied the influence of preparation methods on the rheological properties of lithium-based magnetorheological grease:the magnetorheological grease prepared via cooling and crystallizing in water bath, and saponifying for two hours, prior to refining at a temperature up to 200 °C,followed by adding iron powder during dilution has better rheological properties[12]. Du Jinghuai, et al. investigated the influence of homogeneous pressure on the properties of lithium grease, indicating that a proper homogeneous pressure can improve the physicochemical properties of lithium grease and obtain stable energy state and structural framework of the product[13].

In the present work, lithium grease was prepared at room temperature. Its properties, especially rheological properties, were studied by various analytical methods and were compared with those of lithium grease prepared at high temperature. The influence of preparation temperature on the microstructure and rheological properties of lithium grease was investigated.

2 Experimental

2.1 Materials

Base oil 500SN was commercially obtained from the Panjin North Asphalt Fuel Co., Ltd. The kinematic viscosity at 40 °C and 100 °C is 105.4 cSt and 11.19 cSt,respectively. The viscosity index, flash point, and pour point is 91, 265 °C, and -12 °C, respectively.

12-Hydroxystearic acid (superior grade) was purchased from the Tongliao Tonghua Castor Chemical Co.,Ltd. Lithium hydroxide monohydrate (with a lithium hydroxide content of 56.5%) was purchased from the Sichuan Tianqi Lithium Co., Ltd.

Petroleum ether (60—90 °C, chemically pure) was commercially obtained from the Beijing Yili Fine Chemicals Co., Ltd.

2.2 Main equipment for grease preparation

The main equipment includes a small grease preparation kit (provided with an adjustable-speed stirring device with a scraper, and an automatic temperature controller),a balance, thermometers, and three roll mills.

2.3 Instrument

Rheological measurements were performed with a Physica MCR 301 rotary rheometer (Anton Paar,Austria). The minimum torque, maximum torque, and torque resolution are 0.01 mNm, 200 mNm, and 0.1 nNm,respectively. The frequency range is 10-7—628 rad/s.The normal force range is 0.01―50 N. The temperature control accuracy can reach 0.01 °C. The scanning electron microscope (SEM) images were collected on a S-4800(Hitachi, Japan) electron microscope operating at 5.0 kV.

2.4 Preparation of lithium grease

(1) Preparation of prefabricated lithium 12-hydroxystearate soap: A certain amount of 12-hydroxystearic acid and deionized water were added into the reactor under stirring. When the temperature rose to 80—90 °C, lithium hydroxide monohydrate was added. The saponification reaction lasted for 2 h, and then heating and dehydration were carried out. The product was dried in the oven to obtain a white powder of lithium 12-hydroxystearate for standby.

(2) The traditional method for preparing lithium grease with prefabricated soap (high temperature method, HTM):A certain amount of base oil and the above-mentioned prefabricated soap were added into the kettle. The resultant mixture was stirred and slowly heated to 210 °C.When the mixture became a homogeneous solution, some cold oil was added and the mixture was cooled down to room temperature under agitation. The resultant product was ground for three times with three roll mills to obtain the final sample.

(3) Preparation of lithium grease using the ambient temperature method (ATM): A certain amount of base oil and the above-mentioned prefabricated soap were mixed evenly without heating. The resultant prduct was ground for three times with three roll mills to obtain the final sample.

Sample preparation of lubricating grease soap fiber for SEM: a small amount of lubricating grease sample was dispersed into petroleum ether followed by centrifugation to remove the base oil. The same operation was repeated three times in succession for removing the base oil completely.

3 Results and Discussion

3.1 Fundamental properties of samples

Table 1 shows the typical data of two lithium grease samples prepared by the ambient temperature method(ATM) and the high temperature method (HTM). For two samples with the same consistency, the soap content of the sample made via the ATM is as high as 23%, while that made via the HTM is only 9%. The high temperature method makes the thickener have stronger thickening ability; the dropping points of the two samples are exactly the same, and the evaporation loss is similar; the lithium grease obtained via ATM has smaller oil separation rate,which indicates that its colloidal stability is better. Higher thickener content may be helpful to better colloidal stability.

Table 1 Typical data of ATM and HTM samples

Figure 1 shows the oxidation stability test curve of two samples. The test pressure for both samples was 0.600 MPa at room temperature. After they were put into the 99 °C bath, the pressure of both samples rose rapidly to the highest point, and then the pressure of HTM sample began to decline and gradually became stable after 100 h.However, the pressure of ATM sample began to decline after 35 h, and it was equal to that of HTM sample at about 72 h, and then it successively and rapidly declined. The pressure drop of HTM sample was by 0.149 MPa lower than that of ATM. The results show that the oxidation stability of the two base grease samples without antioxidant is not good, but the ATM sample is better in a short period,and the HTM sample is better in an extended time.

Figure 1 Oxidation stability test for ATM and HTM samples■—ATM; ▲—HTM

3.2 Modulus curve

Lubricating grease possesses viscoelastic properties.When it is subjected to a small external force, it shows elasticity. The shear kinetic energy is completely converted into potential energy and is temporarily stored in the fiber skeleton. However, when the shear stress reaches a critical value, the elastic deformation can no longer convert all the shear kinetic energy. The grease will undergo irreversible deformation and start to flow. The maximum shear stress which it can bear is the strength limitation, also called yield stress. A CP25-SN13229 type rotor test system was selected in the present test. This test system has a d of 0.048 mm, the angular velocity of which is set at 1 rad/s. The strain amplitude is 0.01%—100%.

Figure 2 Modulus curve of ATM and HTM samples at -20 °CATM: ●—G'; ■—G''; HTM: ▼—G'; ▲—G''

Figure 3 Modulus curve of ATM and HTM samples at 25 °CATM: ●—G'; ■—G''; HTM: ▼—G'; ▲—G''

The modulus curves of the samples prepared by two preparation processes at different temperatures were investigated (G' and G'' were storage modulus and loss modulus, respectively). Figure 2 and Figure 3 show the modulus curves of two samples at -20 °C and 25 °C,respectively. Both two samples show obvious linear viscoelastic regions. The linear viscoelastic region of ATM sample at -20 °C was larger than that of HTM sample at 25 °C. It can be seen from Table 2 that at the same temperature, the flow transition indexes (τf/τy) of two samples were close to each other and significantly greater than 1, indicating that the structure of the grease was stable, flexible and malleable, and easy to spread and can enter the raceway surface. When the temperature was lowered, the grease generally became hard and brittle, and the transformation index was also smaller. However, the decrease value at -20 °C was smaller than that of 25 °C,which indicated that the grease had better ductility at low temperature. Under both temperature conditions, the loss factor (tanδ) value is very small (＜1), which indicated that the elasticity played a dominant role in the viscoelasticity of lubricating grease. The tanδ value of HTM sample at-20 °C is larger , which indicated that the role of viscosity of the sample at high temperature began to dominate.

3.3 Complex viscosity under constant shear rate and variable temperature

Figure 4 shows the change of complex viscosity (|η*|)with temperature under a constant shear rate. The overall trend was that the complex viscosity decreased with the increase of temperature. In the temperature range of-20 °C to -12 °C, the complex viscosity of two samples changed nearly linearly with the temperature, mainly because the temperature was lower than the pour point of the base oil, and the base oil dominated the viscosity of the interval. Although the soap content of ATM sample was 2.5 times more than that of HTM, the complex viscosity of ATM sample at -20 °C is only 62% of that of HTM, which demonstrated its better low-temperature starting performance. With the increase of temperature,the complex viscosity of ATM sample changed more greatly. The complex viscosity of ATM sample at 120 °C was only 55% of that of HTM sample.

Figure 4 Complex viscosity of two samples at different temperatures■—ATM; ▲—HTM

3.4 Rheological curve under high shear speed

Lubricating grease is mainly used for lubricationof bearings. Bearings filled with grease are usually accelerated to a higher speed in a short time during startup, which may also involve changes from low temperature to high temperature. In order to explore the rheological change of lubricating grease during the actual operation process, a rheometer was used to simulate the working condition of shear rate rising from 0.01 s-1to 4 000 s-1in a short time at different constant temperature.The curve for change of viscosity with shear time was obtained (Figure 5). From -20 °C to 120 °C, the viscosity of two samples decreased with the increase of shear time,and finally became stable. The viscosity of ATM sample at -20 °C first descended to a platform area with the shear time, then further descended to another platform area after a period of time, and tended to be stable. Table 3 shows the initial viscosity data of samples at different temperatures. The viscosity of the two samples at -20 °C was close at the beginning and the end, mainly because the temperature was lower than the pour point of base oil contained in samples, and the properties of the base oil accounting for a largest proportion in grease were dominant. The viscosity of ATM sample at 25 °C at the beginning and the end was about 3 times that of HTM sample, indicating that the change trend was the same.At 120 °C, the initial viscosity of ATM sample was 1/5 times that of HTM sample, but after a long time of shearing, its viscosity was 4 times higher than that of HTM sample, which indicated that ATM sample with higher soap content was conducive to providing greater oil film thickness and better lubrication protection during the actual use.

Table 2 Storage modulus, loss modulus and corresponding strain value of samples at yield point/flow point at different temperatures

Figure 5 Viscosity curves of samples under high speed shear at different temperatures■—ATM; ▲—HTM

Table 3 Initial and final viscosity of samples at different temperatures

The colloidal structure of lubricating grease has a significant influence on its performance, so it is necessary to characterize its microstructure. The higher the working temperature and shear rate are, the greater the damage to the structure of grease is. Figure 6 shows the SEM images of two grease samples before and after high-speed shearing at 120 °C. Figure 6 (a) and (c) are the original samples of ATM and HTM, respectively. The soap fibers of HTM sample were obvious, forming a spatial network structure,while ATM sample had no obvious fibrous structure and spatial network structure. Figure 6 (b) and (d) are the two samples after high-speed shearing at 120 °C. The soap fibers of HTM sample became shorter obviously, and the soap structure of ATM sample became more homogeneous.In general, the high-speed shear had more obvious damage to the colloidal structure of HTM sample, leading to a much larger viscosity change than that of ATM sample. It further confirmed that ATM sample had better colloidal structure and mechanical stability.

Figure 6 SEM images of different samples before(ATM (a), HTM (c)) and after high speed shear at 120 °C (ATM (b), HTM (d))

3.5 Thixotropic loop curve

Thixotropic loop curve can reflect the structure recovery ability of grease after working or shearing. The larger the area of thixotropic ring is, the worse the structure recovery ability after shearing would be; otherwise,the smaller the area of thixotropic ring is, the stronger the structure recovery ability after shearing would be.Figure 7 shows the thixotropic loop curve of samples at different temperatures. At a constant temperature, the shear rate gradually increased from 0.1 s-1to 100 s-1.After a period of time when the shear rate reached 100 s-1, it gradually decreased from 100 s-1to 0.1 s-1. The viscosity and shear stress of the samples were measured,respectively. Table 4 shows the thixotropic ring data of samples at different temperatures. It can be seen that the final viscosity of the two samples was greater than the initial viscosity at -20 °C, and the final viscosity of HTM sample increased significantly; the initial and final viscosity of the two samples had little change at 25 °C,indicating that the structure of the samples was better retained at this temperature; the final viscosity of ATM sample was 1.6 times the initial viscosity, and the final viscosity of HTM sample was 1/7 times the initial viscosity at 120 °C, indicating that ATM sample had a good ability of viscosity retention. In terms of the thixotropic ring area,ATM sample was smaller at low temperature, indicating that its structure recovery ability was stronger; at room temperature, two samples were close; at high temperature,the structure recovery ability of HTM sample was stronger.

Table 4 Thixotropic ring data of ATM and HTM samples under low-speed shear at different temperatures

Figure 7 Thixotropic loop curves of ATM and HTM samples under low-speed shear at different temperatures■—ATM; ▲—HTM

Figure 8 shows a thixotropic loop curve of the samples at different temperatures during high-speed shearing.At a constant temperature, the shear rate gradually increased from 0.1 s-1to 4 000 s-1after being subject to shearing for a period of time at a shear rate of 4 000 s-1,and then gradually decreased from 4 000 s-1to 0.1 s-1.The viscosity and shear stress of the samples were measured respectively. Table 5 shows the thixotropic ring data of samples during high-speed shearing at different temperatures. The final viscosity of ATM sample was larger than the initial viscosity at -20 °C, while that of HTM sample was the opposite, and the thixotropic ring area of HTM sample was larger; the final viscosity of both samples was greater than the initial viscosity at 60 °C, and the thixotropic ring area of HTM sample was larger, indicating that the structure of ATM sample was better at this temperature; the final viscosity of ATM sample was slightly larger than the initial viscosity at 120 °C. However, the final viscosity of HTM sample was 1/4 of the initial viscosity, and the thixotropic ring area of HTM sample was 5 times that of ATM sample, which showed that the viscosity retention ability of ATM sample was stronger and the structure recovery ability was better.Generally, the AMT samples had better structure retention ability during high-speed shearing, which might be mainly attributed to the high content of thickener.

Table 5 Thixotropic ring data of ATM and HTM samples under high-speed shear at different temperatures

Figure 8 Thixotropic loop curves of ATM and HTM samples under high-speed shear at different temperatures■—ATM; ▲—HTM

4 Conclusions

1) In the present work, the preparation method has no obvious effect on the dropping point of the grease, but it has a great influence on the thickening ability of the thickener. The thickening ability of the thickener at room temperature is poor, and the oxidation stability of the sample is also weak.

2) Under the condition of variable temperature and very low speed shear, the overall trend of ATM and HTM samples shows that the complex viscosity decreases with the increase of temperature. HTM samples have higher complex viscosity at high temperature.

3) At different temperature and high shear rate, the final viscosity of ATM sample is higher, which is beneficial to providing a larger oil film thickness under actual working conditions.

4) The microstructure of the thickener is different between ATM and HTM samples. The HTM samples have obvious soap fiber structure, richly spatial network structure and good thickening ability; the ATM samples have no obvious fiber structure and spatial network structure, and their thickening ability is poor. After high-temperature and high-speed shearing, the structure damage for HTM samples is more significant.

5) The thixotropic ring data at low and high shear rates generally show that the ATM samples have better structure recovery ability.