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.     

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.     

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     

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.     

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.     

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。     

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...     

Variation and recovery of resistivity of industrial greases — an experimental investigation

In the present work, variation in the resistivity of different greases with time in static conditions, under the influence of applied potential drops, is established. The change in resistivity and physico-chemical characteristics of the greases in electric fields, and recovery of the resistivity within the test period in the absence of applied potential drop, have been found. The properties of the greases are compared, and their correlation with resistivity is analysed. The paper also gives the results of analysis using an SRV tester, IR spectra, chemical analysis and spectrophotometry, of changes that occurred periodically in the greases, normally used in non-insulated rolling-element bearings in the presence of a potential drop across the inner and outer races. The paper highlights the behaviour and mechanism of the process of bearing failure under electric fields.     

纳米莫来石添加量对聚脲润滑脂摩擦学性能的影响

将不同质量分数（0.01%,0.03%,0.05%,0.10%,0.40%）的纳米莫来石添加到聚脲润滑脂中,采用四球摩擦试验机评价纳米莫来石的添加量对聚脲润滑脂摩擦学性能的影响,并研究了纳米莫来石的减摩抗磨机理。结果表明：随着聚脲润滑脂中纳米莫来石添加量的增加,摩擦副的稳定摩擦因数和磨痕宽度均呈先减小后增大的趋势,在纳米莫来石质量分数为0.03%时达到最小值,分别为0.058 7,0.64 mm,相比于未添加莫来石的基础润滑脂润滑条件下分别降低9.3%和11.1%。添加纳米莫来石的聚脲润滑脂优异的摩擦学性能归因于在摩擦过程中形成了由聚脲润滑脂、含C—O键的化合物、氮氧化物与莫来石组成的润滑膜。莫来石中的SiO₂有助于降低接触压力,使摩擦副的摩擦方式向滚动摩擦转变,而Al₂O₃能够填充摩擦副表面的凹坑与凹陷,促进润滑膜的形成;2种成分所发挥的协同作用使得聚脲润滑脂表现出良好的润滑效果。     

Effect of metallic thiophosphates on dropping point and penetration properties of some greases

Thiophosphates of zinc and molybdenum have been used in lubricant formulations to impart extreme pressure and anti-wear properties for some time. Zinc dialkyldithiophosphate has found favour in greases as a multifunctional - antioxidant, extreme pressure and antiwear - additive.     

Investigation of the triboengineering characteristics of plastic railway greases with inorganic polymeric additives

It is proposed to use certain inorganic polymeric phosphates as antiwear and antiscoring additives in plastic greases in heavily loaded units of railway rolling stock. Introduction of sodium molybdophosphate into the plastic greases Buksol and GRO-M considerably reduces the friction coefficient of the steel-steel friction pair and keeps it stable over time. The molecular mechanism of the effect of the additives is considered.     

氟化石墨烯作为聚脲润滑脂添加剂的摩擦学性能*

采用Rtec MFT-5000多功能摩擦磨损试验机研究氟化石墨烯作为聚脲润滑脂添加剂的抗磨减摩性能,利用三维轮廓仪和X射线光电子能谱仪分析试验后钢盘磨斑表面形貌和主要元素的化学状态。结果表明：氟化石墨和氟化石墨烯对聚脲润滑脂的锥入度、钢网分油量和蒸发损失有显著影响,而对聚脲润滑脂的滴点影响较小;相同添加量下,氟化石墨烯较氟化石墨对聚脲润滑脂具有更好的抗磨减摩性能,当氟化石墨烯的质量分数为0.8%时,抗磨减摩效果最好;含氟化石墨烯聚脲润滑脂润滑条件下,钢盘表面生成的摩擦化学反应膜由复合碳氧化物、(C₁₀F₈)_n、Fe₂O₃、FeF₃、碳氮化物和(或)氮氧化物等组成,共同在钢球-钢盘的摩擦过程中起到抗磨减摩作用。