High temperature greases based on polyurea gellants: A review

There is increasing demand for high temperature greases capable of operating in the range of 175–235°C. In this paper, one type of grease, polyurea, is described, and its preparation given, in terms of raw materials, temperature, solvents, base fluids including synthetics, catalysts, additives and thickeners. Two typical preparation procedures are described. The properties of polyurea greases are then reviewed, with detailed tabular matter given.     

The influence of lubricating fluid type on the properties of biodegradable greases

The lubricating ability of a grease depends on both the base oil and the thickener. As a result of their intrinsic properties and/or because of their com‐patibility with thickeners and specific additives, base fluids have different influences upon the properties of grease formulations. It is well known that mineral oils are the most widely used lubricant bases due to their inherent lubricity and lower cost, but recent environmental concern has led to consideration of the use of vegetable oils and readily biodegradable synthetic fluids as raw materials in lubricating grease formulations. As well as the base materials, the additives for biodegradable greases should also be biodegradable. This requirement limits the kind of products that may be used in environmentally friendly greases. This paper presents comparative data concerning the tribological and physico‐chemical properties of biodegradable greases formulated with certain vegetable oils, such as rape seed oil, castor oil, and soybean oil or their mixtures, and synthetic esters. The improvement of the load‐carrying properties of biodegradable greases and the antioxidative effect of some suitable additives have also been studied, and the results are presented here.     

Synthetic polyurea extreme‐pressure greases

This paper describes the preparation of synthetic polyurea greases and evaluates their performance. The polyurea greases were prepared in situ by reacting diisocyanate with monoamines and ethylene diamine in the presence of a synthetic base oil (polyalphaolefin). Two different extreme‐pressure (EP) additives blended with the prepared polyurea greases were evaluated for their EP activity in four‐ball and Timken tests. The additivated greases gave higher loads, and higher values of weld load in the four‐ball test as compared to the polyurea grease without additive. The blended greases were also found to pass rust, corrosion, oxidative stability, and shear stability tests. The topography of the wear‐scar surfaces was investigated using scanning electron microscopy.     

Tribological characteristics of bisphenol S bis(diphenyl phosphate) as a high‐performance antiwear additive in lubricating greases at elevated temperature

Bisphenol S bis(diphenyl phosphate) (BSDP) was synthesised and characterised, and its tribological behaviours as additives in polyurea grease and lithium complex grease were evaluated for steel/steel contact at 200 °C. The results indicated that BSDP could dramatically reduce the friction and wear of sliding pairs in the base grease of polyurea, and the tribological performances of BSDP in polyurea grease were significantly superior to the normally used molybdenum disulfide‐based additive package. Furthermore, BSDP in polyurea grease has better tribological behaviour than that in lithium complex grease at a constant load of 100 N. X‐ray photoelectron spectroscopy analysis indicated that boundary lubrication films composed of Fe(OH)O, Fe₂O₃, Fe₃O₄ and FePO₄ compounds containing the P–O bonds and nitride compounds were formed on the worn surface, which resulted in excellent friction reduction and antiwear performance. Copyright © 2016 John Wiley & Sons, Ltd.     

Total vegetable‐oil based greases prepared from castor oil

Total vegetable oil greases are those in which both the lubricant and gellant are formed from vegetable oil. Vegetable oil may be used as a lubricant as such or, with appropriate structural modification, as an ester or alkylated ester. Total vegetable oil greases have the advantage of good biodegradability and conform to the requirements for high performance. Total vegetable‐oil based greases can be formed by mixing preformed soap and lubricant in the required proportions to form a grease with the desired properties. Alternatively, a grease can be formed from the same vegetable oil by simultaneous alcoholysis and saponification of the oil to form the lubricant and soap. The alkali used as a catalyst for the alcoholysis reaction serves as a reactant for the saponification reaction. The use of the appropriate proportion of oil, alcohol, and alkali will thus form a grease of the desired composition. A simultaneous reaction scheme to form sodium and lithium grease using castor oil has been studied. The alcoholysis reaction is separately studied for its kinetics, and the kinetics of the alcoholysis and saponification reaction are reported. Greases with 20% soap have been formed and their properties evaluated.     

Comparative study of the tribological properties of ionic liquids as additives of the attapulgite and bentone greases

Three kinds of ionic liquids (1‐butyl‐3‐methylimidazolium hexafluorophosphate (L‐P104), 1‐hexyl‐3‐methyl imidazolium hexafluorophosphate (L‐P106) and 1‐octyl‐3‐methylimidazolium tetrafluoroborate (LB108)) were added to the attapulgite base grease and the bentone base grease to investigate and compare the tribological behaviours of the ionic liquids with the two base greases at room temperature and 150°C. Tribological tests were performed using a ball‐on‐plate reciprocating tribometer. The attapulgite base grease showed better wear resistance properties than that of bentone base grease by adding ionic liquids as additives. At same time, the attapulgite base grease showed excellent friction‐reducing and wear resistance properties at high temperature (150°C). Also, we discussed the tribological mechanism of the attapulgite base grease at both room temperature and 150°C from the aspect of the structure of the grease thicker. Copyright © 2012 John Wiley & Sons, Ltd.     

Electrical resistivity and conductivity of greases: An initial study

Investigations of railway wagons fitted with rolling‐element bearings have shown damage to the bearings from the passage of electric current. This also occurs in railway wagons without an electric energy consumption of their own, because of residual currents from electric locomotives. This phenomenon gives rise to technical, financial, and even safety problems. This paper reviews research in the field of rolling‐element bearings exposed to electric currents. The paper also reports on an ongoing study undertaken to understand the mechanism of current flow and the role of lubricants in the associated damage, and to identify the possibilities of minimising bearing damage. The obvious solution to this problem is to use optimised greases, i.e., with high electrical conductivity and good lubricating ability. In the preliminary work reported here a number of greases for railway use were tested to determine their electrical conductivity in static and dynamic situations. These tests were performed using specially designed apparatus, and significant initial information about the electrical properties of greases has been obtained in this research, for example, that there is a correlation between the electrical conductance under static and dynamic conditions. All the greases tested showed comparatively high resistivity values, which could be one explanation for the bearing damage observed.     

Rheological properties of greases in ehd contacts

A rheological model of four characteristic parameters is proposed to describe the effective viscosity of greases in ehd contacts. Experimental determinations of film thickness have been achieved in line and point contacts. The proposed model fits the viscometric results better than a Herschel-Bulkley model and allows calculation of the grease viscosity at the high shear rates found in a typical ehd contact. Rules are given to find the effective viscosity of greases in a practical situation     

Impact of polyurea structure on grease properties

The thickener structure of polyurea grease has a crucial effect on its properties. Therefore, it is very important to study the relationship between a thickener structure and physical and performance properties. In this study, polyurea greases were synthesised, having a different number of urea groups in per polyurea thickener molecule. Properties such as dropping point, penetration, oil separation, shear stability and four‐ball wear and extreme pressure (EP) were evaluated. The result shows that properties of polyurea grease vary with the number of urea group per polyurea molecule. Based on molecular theory, the mechanisms explaining these effects are briefly discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

New fluorinated greases: A tribological evaluation

New fluorinated greases based on perfluoropolyether (PFPE) fluids and perfluorinated polymer thickeners were prepared. Both the type of fluid (branched and linear PFPE) and the thickener (polytetrafluoroethylene, PTFE) were varied, and different oil/thickener ratios were also evaluated. The greases were characterised in terms of penetration, oil separation, and wear. Some relationships regarding the influence of both the amount and the type of thickener used were found. Torque values of ball bearings lubricated with these greases were investigated at low temperature and at high bearing speed. Specifically, the use of a new PTFE grade obtained through micro‐emulsion polymerisation technology allowed the formulation of greases with a substantially reduced amount of thickener. These greases show improved antiwear properties as well as lower friction coefficients under high‐speed conditions, thus paving the way to their use in applications not generally fulfilled by fluorinated greases.     

Total vegetable oil greases

A grease has two major constituents, namely, a lubricant, that performs the function of lubrication; and a gellant, that provides a solid continuous phase, occludes the lubricant, and gives apparent physical structure to the grease. Generally, the gellant is 5–30% and the lubricant 65–90%, additives and fillers making up the rest. In conventional greases, the gellant is a vegetable oil soap, and the lubricant is a liquid oil of petroleum origin or is a synthetic. Such greases have limited biodegradability, because the major constituent, i.e., the lubricant, is normally not biodegradable. In total vegetable oil grease, both the gellant and the lubricant are derived from vegetable oils, giving a grease of potentially high eco‐compatibility. Esters, dibasic acid esters, and alkylated esters of vegetable oil are known to be high‐quality lubricants. These can be used with soap stocks prepared from vegetable oils to give a grease of total vegetable oil origin. The vegetable‐oil based lubricants and soaps are prepared separately and combined in appropriate proportions to give a grease of the required specifications. Alternatively, esterification and saponification can be carried out simultaneously to give a grease of the desired specifications, where an alkali will be the catalyst for esterification, and reactant for saponification. In this paper, the process parameters, and kinetics of these simultaneous reactions are discussed. The results of experimental evaluation of some of these greases are also presented.     

Antifriction plastic greases based on intermediate products of oil refining and liquid crystalline compounds

The rheological and triboengineering properties are presented of hydrated calcium plastic greases based on intermediate products of oil refining (hydrofined vacuum gas oil and vacuum distillate). It is shown that application of liquid crystalline cholesterol compounds as an additional component of the dispersion medium of plastic greases permits significant reduction of the wear and losses for friction and boosts the load—bearing capacity of tribocouples.     

The influence of lubricating greases on rolling bearing failure

Significant savings can be made by the correct use of lubricants in terms of energy consumption, replacement parts, maintenance costs and the reduction of machinery downtime, because the service life of machines is one of the most important aspects of engineering. Planned preventive maintenance means the minimisation of failures and of maintenance costs; the technology should allow identification of the tribomechanical systems in all operating conditions and of the tribological regime and duration of operation. Despite the enormous advances that have been made over the last thirty years in the theory of rolling bearing lubrication, the successful operation of rolling bearings remains highly dependent on empirical knowledge. The aim of the present study is to investigate the influence of lubricating grease on rolling bearing failure; the experimental work was carried out at a metalworking factory.     

Self-diffusion of oil in lubricating greases by NMR

Self-diffusion of base oil in six model lubricating greases and in the base oils themselves was measured using pulsed-field gradient nucleic magnetic resonance (NMR) at 23, 40, 70, and 90°C. Three of the greases were based on naphthenic mineral oils, and three were based on synthetic polyalphaolefin oils. Soaps of 12-hydroxy stearic acid were used as thickener agents in all six greases. The purpose of the investigation using NMR was to see if structure varies with temperature, and if NMR is a viable method for structure determination. The ratio between the diffusion coefficient of the base oil in the grease and the neat base oil in itself was evaluated as a function of temperature. This ratio was taken as a direct indicator of the obstruction effect that the thickener has on the oil. The thickener showed a stronger obstruction effect on the base oil in the synthetic grease than in the mineral-based grease. This was related to the soap content, which, for these greases, is roughly twice as high in the synthetic greases as in the mineral ones. The obstruction effect was constant in the temperature range 40–90°C for mineral- as well as synthetic-based greases. NMR was judged to be a promising method for investigating how the gellant affects the diffusion coefficient of oil in a lubricating grease.     

稠化剂组成对聚脲润滑脂性能的影响

为探究聚脲润滑脂高温下长期工作容易发生硬化现象的原因,从稠化剂原料有机胺的结构与组成等方面分析其对聚脲润滑脂高温性能的影响;通过使用锥入度测试、热重分析、红外测试等表征手段,对制备的二脲基、四脲基、六脲基润滑脂高温硬化的机制进行探究。结果表明：多脲基稠化剂的热稳定性要明显高于普通二脲基润滑脂,其中六脲基润滑脂在高温下硬化程度最小;多芳烃胺类对聚脲润滑脂的高温硬化现象有明显改善作用,不同的有机单胺中,使用芳香胺和烷基胺复配得到的六脲基润滑脂,其耐高温性能更好;不同有机二胺中,以二氨基二苯甲烷制备的润滑脂样品耐高温性能最好。随着工作温度的升高,聚脲的N-H和C＝O官能团会被破坏,脲基结构改变是导致聚脲润滑脂在高温下出现硬化现象的原因。     

Tribological Performance and Surface Analysis of a Borate Calcium as Additive in Lithium and Polyurea Greases

In this work, a borate calcium additive was added to lithium and polyurea greases to investigate the tribological performance. Friction and wear tests were conducted on a four-ball machine under higher load and a reciprocating tribometer under lower load. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analyses were performed on the worn surface after the tests. It was found that the tribological performance affected the boundary layers formed by the additive and the soap fibers. The boundary films in lithium grease mainly consist of ferrum hydroxide, and more oxide can be found in polyurea-based films. SEM analyses of soap fibers show that the soap fibers in polyurea-based grease were more separated than those in lithium-based grease. Compared to the base grease, the soap shows smaller and shorter fibers.     

聚脲润滑脂的研究现状

简述了聚脲润滑脂的结构、性能以及研究现状 ,并介绍了改善聚脲润滑脂剪切安定性的研究。     

Tribological characteristics of greases with and without metallo-organic friction-modifiers

ABSTRACT

An important focus of grease development is to minimize friction and wear while improving load bearing capacity. ASTM D2266 test method is commonly used to evaluate performance of grease at 75°C, 40?kg and 1200?rpm for 1 hour. However, actual applications may require bearings to be subjected to cyclic loading and variable frequency conditions wherein rotations per minute (rpm), load and duration of test are variables. Five different blends of greases were formulated using ZDDP (3?wt.%), PTFE (2?wt.%), MoDTC (2?wt.%), combination of ZDDP/PTFE in a weight ratio of 3:2 and a combination of ZDDP/PTFE/MoDTC in 3:2:2 weight ratios. They were tested under ASTM D2266 test method as well as under cyclic loading and variable frequency conditions where loads, frequency and duration of the tests were treated as variables. It was found that the combination of ZDDP/PTFE/MoDTC results in significant improvement in the wear and friction under cyclic loading as well as ASTM D2266 test conditions. It was also demonstrated that MoDTC accelerated the tribochemical degradation of ZDDP that resulted in the formation of a protective tribofilm layer on the interacting surfaces. The analysis of the tribofilm formed indicated that when MoDTC was used together with ZDDP and PTFE, a combination of MoS₂, phosphates and sulfates of Zn and Fe are formed whereas when only ZDDP and PTFE was used the tribofilms were largely composed of phosphates and sulfates of Zn and Fe.     

The effect of space irradiation on the lubricating performance of perfluoropolyether greases in simulated space environment

In this paper, 2 kinds of commercial perfluoropolyether (PFPE) greases were coated on the polyimide (PI) blocks, which were placed within simulated space environment including atomic oxygen (AO), proton (Pr), ultraviolet (UV), and electron (El) irradiations, and then the tribological performance has been investigated with a ball‐on‐disc tribometer. Results indicated that the MoS₂‐grease showed better lubrication performance than the PTFE‐grease. The changes in infrared spectroscopy induced by Pr and El irradiations were more obvious than that by AO and UV irradiations. Results of energy dispersive X‐ray spectroscopy indicated that Pr and El irradiations caused carbonation of greases, and AO and UV irradiations induced oxidation of greases. Referred to the tribological properties of PI coated with PFPE oil, PI coated with PFPE greases showed minor changes in friction coefficient and wear rate, and the MoS₂ additives could significantly improve the lubrication properties of PFPE greases in simulated space environment.     

聚脲润滑脂稠化剂结构与其性能的关系

以脂肪胺、芳香胺、脂环胺的不同组合和异氰酸酯为稠化剂,在不同的工艺条件下制备一系列的聚脲润滑脂;利用扫描电子显微镜(SEM)对润滑脂皂纤维微观结构进行表征;探讨不同的稠化剂组成、不同制备工艺对聚脲润滑脂的微观结构和性能的影响。结果表明:单纯均匀颗粒状稠化剂比纤维与颗粒状复合的稠化剂所制备的聚脲脂具有更优的性能;纤维结构以单根形式存在为主的聚脲润滑脂性能优于纤维结构为多根纤维聚集为一股,各股之间交织的聚脲润滑脂。