Tribological and rheological performance of lithium grease with poly-a-olefin and alkyl-tetralin as base oils

A new category of lithium greases was synthesized by using poly-a-olefin(PAO8) and alkyl-tetralin as base oil, where the alkyl-tetralins were synthesized by the alkylation of tetralin and olefins. The influence of thickener concentration, alkyl-tetralin content and type of blend oils on the rheological and tribological performance of lithium grease was investigated. The microstructures of soap fibers were measured to reveal the structure-property correlations. The concentration of thickener and alkyl-tetralin content obviously affect the lubricating performance of lithium grease, while the molecular structure of alkyltetralin has no obvious impact on their properties. It was found that alkyl-tetralin could significantly enhance the thickening ability of PAO8 base oils, and decrease the amount of thickeners by 1.5%(mass).Lithium greases prepared using 20%(mass) alkyl-tetralin as co-base oil exhibited high colloidal stability,excellent rheological behaviors and tribological properties.

Numerical Visualization of Grease Flow in a Gearbox

Lubricating greases are widely used in e.g.open gear drives and gearboxes with difficult sealing conditions.The efficiency and heat balance of grease-lubricated gearboxes depend strongly on the lubrication mechanisms channeling and circulating,for which the grease flow is causal.The computational fluid dynamics opens up the possibility to visualize and understand the grease flow in gearboxes in more detail.In this study,a single-stage gearbox lubricated with an NLGI 1-2 grease was modeled by the finite-volume method to numerically investigate the fluid flow.Results show that the rotating gears influence the grease sump only locally around the gears.For a low grease fill volume,the rotation of the gears is widely separated from the grease sump.For a high grease fill volume,a pronounced geargrease interaction results in a circulating grease flow around the gears.The simulated grease distributions show good accordance with high-speed camera recordings.

Well-Dispersed Graphene Enhanced Lithium Complex Grease Toward High-Efcient Lubrication

Graphene as a lubricating additive holds great potential for industrial lubrication. However, its poor dispersity and compatibility with base oils and grease hinder maximizing performance. Here, the infuence of graphene dispersion on the thickening efect and lubrication function is considered. A well-dispersed lubricant additive was obtained via trihexyl tetradecyl phosphonium bis(2-ethylhexyl) phosphate modifed graphene ([P_(66614)][DEHP]-G). Then lithium complex grease was prepared by saponifcation with 12-OH stearic acid, sebacic acid, and lithium hydroxide, using polyalphaolefn (PAO20) as base oil and the modifed-graphene as lubricating additive, with the original graphene as a comparison. The physicochemical properties and lubrication performance of the as-prepared greases were evaluated in detail. The results show that the as-prepared greases have high dropping point and colloidal stability. Furthermore, modifed-graphene lithium complex grease ofered the best friction reduction and anti-wear abilities, manifesting the reduction of friction coefcient and wear volume up to 18.84% and 67.34%, respectively. With base oil overfow and afux, well-dispersed [P_(66614)][DEHP]-G was readily adsorbed to the worn surfaces, resulting in the formation of a continuous and dense graphene deposition flm. The synergy of deposited graphene-flm, spilled oil, and adhesive grease greatly improves the lubrication function of grease. This research paves the way for modulating high-performance lithium complex grease to reduce the friction and wear of movable machinery.

Effect of Synthetic Conditions and Thickener Formulation on the Rheological and Tribological Properties of Polypropylene Grease

Polypropylene(PP)grease holds great potential for special industrial applications.In this study,synthetic conditions,thickener content,and the ratio of two different molecular weight PPs were investigated systematically using a rheometer,scanning electron microscope,X-ray diffraction,Fourier transform infrared spectrometer,oscillating tribometer,and 3D surface profiler measurements.The results showed that suitable synthetic conditions are two quenching cycles,and the synthetic temperature and time is 240℃for 12 h.The rheological analysis showed that thickener content and different proportions of the two PP molecular weights have a significant influence on the rheological properties of PP grease.High molecular weight PP(H-PP)has a stronger thickening ability than low molecular weight PP(L-PP).The higher the amount of H-PP in the fixed thickener content or the higher the thickener content with a specific proportion,the higher the viscoelasticity of PP grease.The tribological performance is related to the rheological properties.The proportion of two different molecular weight PPs in the thickener content should be appropriate;excessive H-PP content leads to lubrication failure.

Research Status and Prospects of Bio-based Materials for Grease Barrier Coatings on Paper Food Packaging

Increased environmental and health concerns over the use of plastic packaging or fluorine-containing coatings,in combination with increased market demand for products with a longer shelf life,make bio-based materials one of the most important research candidates for alternative paper packaging materials for oil resistance.These bio-based materials have excellent oxygen and oil barriers,which are critical for food packaging.Moreover,they are biodegradable,naturally renewable,and safe.In this artical,two main groups of bio-based oil repellents for paper food packaging,including polysaccharide-based biopolymers and protein-based biopolymers,are enumerated,and the advantages and weaknesses of bio-based oil repellents are discussed,and effective solutions are proposed.Finally,research status and prospects on the development of bio-based oil-resistant coatings for the food packaging industry are presented.

Influence of Two Preparation Methods on Rheological Properties of Lithium Grease

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.

Comparative Studies on Low Noise Greases Operating under High Temperature Oxidation Conditions

Oxidation induction time(OIT) testing by differential scanning calorimetry(DSC) was used to evaluate the oxidation resistance of lubricating greases. Under the high temperature condition, bearing noise was detected when grease passed the initial stable stage of oxidation. The chemical and physical structure of grease samples before and after high temperature oxidation were also analyzed by FT-IR spectrometry and scanning electron microscopy(SEM), then the effects of oxidation at high temperature on bearing noise were investigated. It is found out that for lithium greases, oxidation of base oil and thickener is the main reason responsible for the increasing bearing noise. As regards the polyurea greases, the change of fiber microstructure at high temperature is the main reason for the increasing bearing noise.

Impact of polydimethylsiloxanes on physicochemical and tribological properties of naphthenic mineral oil(KN 4010)-based titanium complex grease

Titanium complex greases were prepared by using naphthenic mineral oil and polydimethylsiloxane as the mixed base oil. The effect of polydimethylsiloxane molecular weight and polydimethylsiloxane content in mixed base oil on the physicochemical and tribological properties of titanium complex greases was investigated.As compared to the sole mineral oil-based titanium complex grease, the use of polydimethylsiloxane(H201-350)as a co-base oil increased the dropping point from 310 to 329℃, decreased the oil separation from 3.7% to 2.3%,reduced the corrosion extent, and obviously improved the tribological properties. When the mixed oil-based titanium complex grease was used as a lubricant, lubricating films of polydimethylsiloxane were probably formed on the surfaces of friction pairs, giving good lubricating property.

Tribological Characteristics of Graphene as Lithium Grease Additive

The tribological properties of graphene(GN) and graphite(G) as lubricant additives in lithium greases were investigated with a four-ball tribotester. The micro-morphology as well as the content and chemical state of elements on the worn surfaces was characterized by scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy(XPS). The results showed that the tribological performance of lithium grease could be significantly improved by the addition of graphene. During the friction process, an adhesion film and a deposition film consisted of graphene as well as a tribochemical reaction film composed of FeO, Fe_2O_3, FeOOH and LiOH could be formed on the tribosurface, and these complex films were responsible for the improved tribological performance of lithium grease.

Consistency-dependent optical properties of lubricating grease studied by terahertz spectroscopy

The optical properties of four kinds of lubricating greases （urea, lithium, extreme pressure lithium, molybdenum disulfide lithium greases） with different NLGL （National Lubricant Grease Institute of America） numbers were investigated using terahertz time-domain spectroscopy. Greases with different NLGL grades have unique spectral features in the terahertz range. Comparison of the experimental data with predictions based on Lorentz Lorenz theory exhibited that the refractive indices of each kind of lubricating grease were dependent on the their consistency. In addition, molybdenum disullfide （MoS2） aa a libricant additive shows strong absorption from 0.2 to 1.4 THz, leading to higher absorption of MoS2-1ithium grease than that of lithium grease.

Preparation and Rheological Properties of Vacuum Lubricating Grease

A kind of vacuum lubricating grease was prepared by using perfluoropolyether as base oil,perfluorinated polymer as thickener,and self-made additives.The colloidal stability of the grease was greatly improved by introducing a colloidal structure modifier.The rheological properties of the self-made grease(SMG),such as viscosity versus time,thixotropy,etc.,were studied by a rheometer at different temperatures and were compared with those of foreign brand grease(FBG).The results show that the performance of SMG reached the level of similar FBG,and some properties such as mechanical stability,colloidal stability,extreme pressure and antiwear properties were better.It can be used for long life lubrication of moving parts in a vacuum environment.

Discussion on the Mechanism of Boric Acid and Phosphoric Acid to Improve the Hardening of Complex Calcium Lubricating Grease

In order to solve the hardening problem of complex calcium lubricating grease, the water absorption test of several calcium salts was carried out, and it was found that calcium 12-hydroxystearate did not absorb water, and calcium acetate, calcium phosphate and calcium borate had different degrees of water absorption. Calcium acetate has the highest water absorption rate, while calcium phosphate and calcium borate show comparable water absorption rates. Upon using the molecular simulation technology, it is found that in the complex calcium grease system, calcium phosphate and calcium borate tend to combine with water, which inhibits the water absorption of calcium acetate and alleviate the hardening problem.

Surface Modified TiO2 Nanoparticles-an Effective Anti-wear and Anti-friction Additive for Lubricating Grease

The surface modified TiO_2 nanoparticles were prepared by using 12-hydroxystearic acid chemically modified on the TiO_2 surface. The average size of the TiO_2 particles is about 30 nm. The optimum ratio of tetrabutyl titanate to 12-hydroxystearic acid was 1/0.5. The bonding form between 12-hydroxystearic acid and TiO_2 nucleus was investigated by FTIR, DSC, TGA and XRD techniques. The lubricating grease containing the surface modified TiO_2 nanoparticles possesses excellent anti-wear and anti-friction properties. Compared with the grease without TiO_2, the PB value can be increased by 52% as the best performance of the grease containing surface modified TiO_2 nanoparticles, while the friction coefficient can be reduced by 33% with the addition of a small amount of TiO_2 nanoparticles, and meanwhile the wear scar diameter decreases by 25%.

Comparative Study of the Impact of Corundum Particle Contaminants Size on Wear and Fatigue Life of Grease Lubricated Ball Bearings

Ball bearings’ rating life is reduced with the presence of hard particle contaminants in the lubricant. This life reduction is taken into account when calculating the modified rating life by using the contamination factor. The contamination factor is based on a general characterization of the lubrication conditions but the impact of contaminant’s variables such as size, hardness and concentration level is not determined in detail. In this work, greases contaminated with hard corundum (alumina, Al2O3) particles of different sizes are tested aiming at finding a pattern in the relationship between particle sizes’ and wear’s progress. A laboratory rig is utilized for these tests and vibration analysis tools regarding bearings’ condition and estimated residual life are being assessed. After the tests, optical inspections using a stereoscope verify the vibration analyses results. The experimental results show that wear is progressing faster when smaller particles are used. The crystalline structure of the corundum and the different degree of brittleness of the contaminants seem to be the reasons for this behavior, whereas severe abrasion and deformation of the raceways have been detected.

Effect of Fluorides and Cerium Dioxide Additives onEP and Antiwear Performance of Grease

CeF3. CaF2, (CF)n, SbF3, and CeO2 were selected as the high temperature solid additives in a lithium grease, and their friction, wear, and extreme pressure properties were evaluated on an Optimol-SRV wear tester. The effect of additive on the fiber structure of soap was studied by means of the transmission electron microscopy(TEM). The corrosive property of grease containing additive was evaluated by copper strip corrosion test. Thermal decomposition behaviors of the additives were studied by thermogravimetry(TG) tester. The adhesive strength of rubbing surface film was measured by automatic scratch tester and the wear surface was detected by means of X-ray photoelectron spectroscopy (XPS). The test results showed that the grease containing CeF3 has good anti-wear, anti-friction, and EP performances, and has high adhesive strength of rubbing surface film.The greases containing CaF2 or (CF)n are good in antiwear and anti-friction properties, but poor in extreme pressure function. The tribological properties of the grease containing SbF3 are decreased.And the addition of CeO2 in grease was useless. Finally the mechanism of fluoride additive was proposed in this paper.

Running Temperature and Mechanical Stability of Grease as Maintenance Parameters of Railway Bearings

Spherical roller bearings in railway car wheels are critical components whose failure may have catastrophic consequences. The present study aims to analyse the mechanical stability of greases and temperature of bearings as indicators for condition-based bearing maintenance. The performed case study includes nine fully-formulated commercial greases examined in the wheel bearings of five ore cars operated in northern Scandinavia. The studied ore cars travelled a distance of about 300 000 km during a period of three years. Small samples of the greases were taken on eight occasions to test their mechanical stability. In addition, the temperatures of the bearings were continuously recorded. After the test period, the wear, electrical damage, and corrosion of the bearings were examined. One of the findings is that the shear stress of the grease at a certain shear velocity （the certain yieldstress （CEY） value） is a good maintenance indicator and is highly dependent on the grease type. The bearing＇s wear, electrical damage and corrosion also depend on the grease type. However, no oxidation of the greases was identified. The paper also outlines a systematic methodology to determine an overall maintenance indicator for railway roller bearings which is based on the field measurements.

Preparation of Overbased Calcium Alkylbenzene Sulfonate for Formulating Complex Sulfonate Grease

Six kinds of alkylbenzene sulfonic acids were selected to prepare the sulfonates S1—S6. Among them, the sulfonates S3, S4, and S6 could be incorporated into lubricating grease with good performance in comparison with the grease produced from commercial sulfonates T106-1 and T106-2. The optimized conditions for synthesis of the sulfonates S3, S4, and S6 were explored by using different mass ratios of methanol, water and the type of copromoters. It was found out that the appropriate conditions for synthesis of the sulfonate S3 included a methanol to M(M is the total mass of alkylbenzene sulfonic acid and base oil) mass ratio of 16%, a water to M mass ratio of 4%, and a copromoter A to M mass ratio of 2%; the appropriate conditions for synthesis of the sulfonate S4 included a methanol to M mass ratio of 24%, a water to M mass ratio of 2%, a copromoter B to M mass ratio of 2%; and the optimized conditions for synthesis of the sulfonate S6 included a methanol/ M mass ratio of 8%, a water/M mass ratio of 4% and a copromoter B/M mass ratio of 2%. The new sulfonates S3, S4, and S6 produced under the optimized conditions exhibited higher TBN and better antiwear property in camparison with the previous products. Grease samples G9, G10, and G11 were prepared with new sulfonates S3, S4, and S6 successfully and exhibited improved water stability and high temperature performance.

Preparation of graphite nanosheets in different solvents by sand milling and their enhancement on tribological properties of lithium-based grease

Graphite nanosheets with the average thicknesses ranging from 24.4 to 48.9 nm were prepared with the use of expanded graphite as the raw material by sand milling in deionized water,anhydrous ethanol,glycerol,and 1,4-butanediol,respectively.Anhydrous ethanol favored the formation of graphite nanosheets with a smaller average thickness.When the graphite nanosheets with the content of 2 wt%were added in lithium-based grease,the average friction coefficient decreased by 27%as compared with the pure lithium-based grease.The weld point and load wear index were 1.6 and 1.4 times those of the pure lithium-based grease,respectively.The tribological properties of the graphite nanosheet-containing lithium-based grease were comparable with those of the graphene-containing lithium-based grease.

Study on Preparation and Properties of Grease Based on Ultrafine Bentonite Powder

The feasibility for preparation of ultrafine bentonite powder by different milling methods was studied. And the comparison of comprehensive performance between ultrafine bentonite grease and traditional bentonite grease was also investigated. The results indicated that the statistic Z-average size of ultrafine bentonite prepared by sand milling was 250 nm with a narrow size distribution and the lattice structure of ultrafine bentonite maintained good character despite a slight distortion occasioned. The mechanical stability, colloid stability, antiwear ability and friction-reducing property of ultrafine bentonite grease were superior to the traditional one.

Study on Rheological Characteristics of a Grease Used in High Speed Bearing

On a self-made super-high shear strain rate rheometer, the rheological characteristics and apparent viscosity curves of a high-speed bearing grease were obtained under different working conditions. A new grease rheological model suited to a shear strain rate range of 0—3.5×10~6s^(-1) was presented. The results showed that the shear stress increased linearly at first and then increased nonlinearly with the increase in shear strain rate up to 1.5×10~6s^(-1), and finally the shear stress decreased slightly with the successive increase in shear strain rate. The shear stress increased with a decreasing rolling speed and an increasing contact pressure. The apparent viscosity decreased rapidly with the increase of shear strain rate at beginning and could approach the viscosity of the base oil if the shear strain rate surpassed 1.5×10~6s^(-1). The fits between the test data and the predicted values by the new model were fairly good.