Phosphate esters with a complex alkyl‐aryl structure considered as tribological fluids

This paper presents results of reserch carried out to produce synthetic ester oils with adequate tribological properties, which could also act as antiwear additives. By varying the molar relationship between aliphatic alcohols of variable chain length and a special alcohol with complex alkyl‐aryl structure, namely 2‐[(o‐sec‐butyl)phenoxy]ethanol, two series of new phosphate esters have been synthesised. The influence of the aryl content and the effect of the length of the aliphatic chain on the characteristics of these esters as base oils, and on their qualities as antiwear additives, have been investigated.     

Use of a non-conventional synthetic basestock in the formulation of high-quality engine oils

Research has been undertaken to identify new synthetic basestocks, to be used alone or with polyalphaolefins, for the formulation of high-quality engine oils. Basestock selection was limited to products not normally used in the synthesis of ester-type basestocks. A non-conventional product was identified: a carbonic ester of synthetic alcohols. A complete engine oil has been formulated using this basestock and submitted to a full test programme. The results are reported.     

水溶性复合磷酸酯添加剂抗磨性能研究

合成了水溶性复合磷酸酯,利用四球试验机考察了添加到水溶液中的抗磨性能,并优化了复合磷酸酯的配比,用扫描电子显微镜对钢球磨斑形貌和摩擦表面的元素组成进行了分析,四球结果表明,磷酸酯与硫化物硫脲,油性剂油酸三乙醇胺具有良好的协同抗磨效应,并存在一个最佳配比,电镜能谱分析表明,复合磷酸酯水溶液在摩擦表面与Ｆｅ发生化学反应,生成一种含磷富硫的化学反应膜,而且表面膜中硫,磷元素的含量比为２：１时极压抗磨性能     

Tribological study and mechanism of B–N and B–S–N triazine borate esters as lubricant additives in mineral oil

The tribological study of N-containing heterocyclic borate esters as lubricating additives had been the research hotspot. In this work, B–N and B–S–N triazine borate esters were synthesized and their antiwear/extreme pressure (AW/EP) properties were studied. Results showed the synthetical additives had good AW performance. However, B–S–N triazine borate ester showed excellent EP property while B–N triazine borate ester hardly owned EP property. The hydrolytic stability of borate ester additives was improved by the formation of coordination of nitrogen to boron. The XANES spectroscopy analysis showed that there was a layer of borate–oxygen–iron inorganics in the tribofilms. The existence of iron sulfate and iron sulfide guaranteed good AW/EP properties of B–S–N triazine borate ester additive in mineral oil.     

Antiwear additives for poly(chlorophenyl methyl) siloxane fluids

An additive package based on the chlorendate and thiadiazole molecular structures has been found to impart antiwear properties to poly(chlorophenyl methyl) siloxane fluids which are comparable with those of such fluids as petroleum oils, synthetic hydrocarbons, diesters and phosphate esters.     

A tribological study of an S,P-containing benzotriazole derivative as an additive in pentaerythritol ester

The load-carrying capacity, antiwear and friction reduction properties of an S,P-containing benzotriazole derivative (BMDDP) added to a synthetic lubricant (pentaerythritol ester) were evaluated using a four-ball test machine. The results when compared with those of tricresyl phosphate (TCP) and zinc dialkyldithiophosphate (ZDDP) show that, in pentaerythritol ester, the novel compound (BMDDP) is an excellent multifunctional additive, which possesses better load-carrying capacity, antiwear and friction reduction properties than either TCP or ZDDP. The rubbed surfaces were analysed using XPS and EPMA, and the antiwear mechanism is discussed.     

水溶性聚乙二醇磷酸酯的合成及润滑与腐蚀性能研究

有机磷酸酯具有良好的润滑性能,被广泛用作润滑油和水基润滑剂的添加剂。以P2O5和聚乙二醇(400)为原料,采用缓释法合成水溶性聚乙二醇磷酸酯,研究聚乙二醇磷酸酯的极压、抗磨和减摩性能。试验结果表明,添加聚乙二醇磷酸酯可以明显提高基础水基润滑剂的极压、抗磨和减摩性能,水基润滑剂的抗腐蚀性能也能满足要求。     

Antiwear Additives for Ester Oils

Esters for lubrication can be classified into two major groups. One is the neopentyl type or the so‐called hindered type. The other is the triglyceride type, which includes vegetable oils. The improvement of the antiwear properties of carboxylic esters by additive technology under boundary lubrication conditions is studied in this paper. Two strategies are considered, based on the structure of the esters. Triglyceride‐type esters are mainly used as rapidly biodegradable fluids. They have carbon‐carbon unsaturated bonds in their molecules, which are susceptible to auto‐oxidation, and this auto‐oxidation has deleterious effects on the antiwear properties. A synergistic effect of antioxidants and antiwear additives on wear reduction was observed. The mechanism is discussed and practical solutions proposed. Neopentyl‐type esters have thermal and oxidative stability. For these oils, the antiwear properties depend on the polarity of the additive and base oil. The effect of molecular structure on the antiwear properties is discussed. Computer simulation is used to understand the mechanism of action. Examples of molecular design of antiwear additives for neopentyl‐type esters are also proposed.     

Molecular Design of Environmentally Adapted Lubricants: Antiwear Additives Derived from Natural Amino Acids

Novel environmentally adapted lubricant additives were synthesized from cystine (Cys ₂ ), an essential amino acid obtained from natural sources. The structural feature of cystine is a dimeric amino acid with a central disulfide bond. The carboxyl groups in Cys ₂ were converted to corresponding esters by reaction with long-chain alcohols. The resultant diesters were soluble in poly-alpha-olefin (PAO) and ester-type synthetic oils. The structural features of the new additives include multifunctional groups on the same molecule, such as disulfide as a tribologically active moiety and polar functional groups as anchors to friction surfaces. The additives consist of hydrogen, carbon, nitrogen, oxygen, and sulfur; they are free of phosphorus, chlorine, and metals. The tribological properties of the additives in a solution of synthetic oil were evaluated by performing laboratory tribotests under boundary conditions. The Cys ₂ -derived additives exhibited comparable antiwear properties to the conventional additive zinc dialkyldithiophosphate (ZnDTP). The additives showed good antiwear properties even at low concentrations of sulfur (160 ppm) in synthetic hydrocarbons such as PAOs. A high concentration of sulfur (640 ppm) was required to obtain an optimized antiwear performance of the synthetic esters as a base oil. The new additives reduced the friction coefficient of PAOs and synthetic esters. A saturated ester of Cys ₂ reduced the friction of PAOs and synthetic esters up to oil temperatures of 150°C. The lubrication mechanism was discussed with respect to the role of functional groups in the additive molecule. The use of amino acids as versatile building blocks for the synthesis of environmentally adapted additives was also pointed out.     

Tribological properties of phosphate esters as additives in rape seed oil

With the growing concern about environmental pollution, vegetable oils have begun to be applied as base oils for environmentally friendly lubricants. Additives containing phosphorus can be used for their excellent antiwear properties and low toxicity. In view of this, a study of a series of phosphate esters as additives in rape seed oil (RSO) was carried out using a four‐ball tester. The results indicate that phosphate esters possess very good load‐carrying capacity and good antiwear and friction‐reducing properties compared with RSO by itself. Surface analysis of the worn balls was carried out using X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The tribological mechanism of the additives is discussed on the basis of the experimental results.     

X-Ray Absorption Study of the Effect of Calcium Sulfonate on Antiwear Film Formation Generated From Neutral and Basic ZDDPs: Part 1—Phosphorus Species

Zinc dialkyldithiophosphates (ZDDPs) from very effective antiwear films in boundary lubrication applications. In most cases, however, the ZDDPs do not work alone. They are formulated with many other additives to provide the performance required by today's modern oils. X-ray absorption near-edge spectroscopy (XANES) has been used to study the antiwear films formed from the commonly used combination of ZDDP and calcium sulfonate in both neutral and basic forms. The results are presented in two papers: Part 1 for the phosphorus species and Part 2 for the sulfur species. XANES showed conclusively that in the presence of LOB (low overbased) or HOB (high overbased) calcium sulfonate under sliding conditions, ZDDPs do not form long-chain polyphosphates that have been associated with antiwear action. Instead, short-chain polyphosphates calcium phosphate are formed. The relative amounts of calcium phosphate formed depend on the ester group of the ZDDP: aryl > n-alkyl > sec-alkyl. Interestingly, this order of ester groups is inversely related to the antiwear effectiveness of the ZDDPs. Thus, it is probable that the addition of either LOB or HOB calcium sulfonate to ZDDP will result in a decrease in antiwear effectiveness of the additive mixture compared to the ZDDP by itself. Wear data support this conclusion. It is suggested that the elimination of long-chain polyphosphates and the formation of calcium phosphates in the tribofilm leads to this decrease in antiwear effectiveness, the latter by abrasion of the antiwear film.     

A laboratory evaluation of partial synthetic automatic transmission fluids

Blending synthetic fluids with conventional base stocks gave large reductions in low temperature viscosity without appreciable reductions in high temperature viscosity or deterioration in properties such as volatility or flash point. Ten percent blends of a polyalpbaolefin or an ester synthetic reduced the viscosity at — 27.9°C by approximately 50%. These blends gave improved oxidation stability as measured by the Aluminum Beaker Oxidation Test, but poorer antiwear performance as measured by the Four Ball Test. The latter result indicated that the antiwear additive package should be optimised to accommodate composition differences between conventional fluids and conventional/synthetic blends.     

Amine Phosphates as Antiwear Additives in Neopentyl Polyol Esters

Dialkyl acid phosphates are effective antiwear agents in neopentyl polyol ester lubricants applied to 440C stainless steel sliding surfaces at low loads, but they promote ester deterioration. Various amine salts of three representative dialkyl phosphates were studied to determine if the adverse effect could be reduced without sacrificing antiwear properties. The amine phosphates were generally highly effective in the low load region in four-ball wear tests. They were at least as effective as the parent acid phosphates and appreciably more effective than tricresyl phosphate. Little effect of amine structure on performance was observed. At a higher load, both the acid phosphates and their amine salts had no beneficial effect on wear. The amine phosphates promoted significantly less deterioration of the base ester under four-ball test conditions (266F) than did the dialkyl phosphates. In oxidation-corrosion tests at 3253, however, they were corrosive to copper and adversely affected ester stability.     

Modification of the chemical structure of an environmentally‐friendly castor oil lubricant

Widely used mineral‐oil based lubricants are often released into the environment and cause pollution. Therefore importance is attached to developing environmentally‐friendly lubricants. Vegetable oils have been used as lubricants since ancient times. These materials, similar to synthetic esters, have better biodegradability than mineral oils. They are also renewable. Vegetable oils have some shortcomings, such as a higher pour point and a lower viscosity index than synthetic esters. They have a limited viscosity range and lower oxidative stability due to the presence of unsaturated bonds. Therefore vegetable oils as such cannot satisfy all the requirements of modern machine lubrication. In this paper, the chemical structure of natural castor oil is modified by an isomerisation reaction and by extending the carbon chain, so as to lower the pour point and improve the viscosity index. The results show that structurally modified castor oil has very good lubrication characteristics. Its viscosity at 40°C is 150 mm²/s, its pour point is −40°C, and its viscosity index is improved. The antiwear and friction characteristics are better than those of mineral oil of the same viscosity and comparable to pentaerythritol esters and diisocapryl sebacate.     

The tribological behaviour and mechanism of action of sulphur-containing borate esters in rape seed oil

Three sulphur-containing borate esters and one borate ester based on boric acid and n-octanol were synthesised. The antiwear and friction-reducing properties of these synthetic compounds as additives in rape seed oil were examined using a four-ball machine. The relationship between the additive structure and tribological properties was explored, and the lubrication mechanisms of the additives investigated. The worn surfaces of a steel ball lubricated with the additive-containing rape seed oil were analysed using X-ray photoelectron spectroscopy and scanning electron microscopy. The elemental compositions of, and distributions in, the worn steel surfaces were determined with an electron probe micro-analyser. It was found that sulphur-containing borate esters as additives in rape seed oil at appropriate concentrations can be effective in improving antiwear and friction-reducing behaviour. The rape seed oil base stock experiences competitive adsorption and tribochemical reactions with additives on the rubbing steel surface. The introduction of the active element sulphur into the borate ester additives functions to improve the tribological behaviour of the base stock by increasing the rate of formation of a protective layer on the rubbing steel surface. However, the tribological behaviour of the additives is not dependent on the number of sulphur atoms in the additive molecules, and further work is needed to investigate the possible dependence of this behaviour on the ratio of boron to sulphur in the additive molecules.     

Additive influence on wear and friction performance of environmentally adapted lubricants

In this paper, the influence of concentration level and chemical composition of three different additive types on friction and wear coefficient are presented for a synthetic ester base fluid and a mineral base oil. One extreme-pressure (EP), two antiwear (AW) and two yellow metal passivator (Cu-passivators) additives were used. Factorial experimental design was used as the basis for a systematic evaluation of wear rates under mixed and boundary lubrication conditions. A total of 33 different lubricant blends were tested in a Plint and Partner High Frequency Friction Machine. For the synthetic ester, the extreme-pressure (EP) additive, containing phosphorus and nitrogen, was found to be much more effective in reducing wear than either of the two antiwear (AW) additives tested. In fact, the AW and Cu-passivator additives made little or no contribution to the wear protection in most of the cases studied. A synergy effect between the three additive combinations was observed only for the reference mineral oil blend. A significant difference between the antiwear performance of the test lubricants was found. This study suggests that the traditional “AW” and “EP” labels associated with commonly used additives are poor aids when designing of ester based lubricants.     

纳米碳球作为合成酯抗磨剂的摩擦学性能研究

采用表面化学改性的方法得到硬脂酸修饰的纳米碳球,在四球摩擦试验机上考察纳米碳球在合成酯类润滑油中的抗摩擦性能,探讨纳米碳球的抗磨与润滑机制。结果表明,通过表面改性,显著提高纳米碳球在润滑油中的分散稳定性;纳米碳球作为合成酯润滑油抗磨剂表现出优良的抗磨减摩性能;在转速1.200 r/min,载荷150 N的实验条件下,质量分数0.07%的改性纳米碳球可使三羟甲基丙烷混酸酯平均摩擦因数减小幅度达到30%左右,磨损率降低33%,质量分数0.15%的改性纳米碳球可使季戊四醇油酸酯的平均摩擦因数降低幅度达到50%以上,磨损率降低14.6%     

Borate esters used as lubricant additives

Borate esters possess friction‐reducing, antiwear, and anti‐oxidant characteristics when blended in lubricating oils. However, borate esters are susceptible to hydrolysis. The formation of a stable five‐member ring structure in the ester molecules, involving coordination of nitrogen with boron, contributes substantially to the resistance to hydrolysis of borate esters. The susceptibility of borates to hydrolysis can be reduced by introducing N,N‐dialkylaminoethyl groups with alkyl radicals containing more than three carbon atoms. X‐ray photo‐electron spectroscopy and X‐ray diffraction reveal that the borate ester can be adsorbed on the rubbing surface, and some of the adsorbed borate film degrades and forms boron nitride. Four‐ball wear tests indicate that the combination of oil‐soluble copper, tin, and cadmium compounds with organoborates gives better antiwear properties than the components separately. An antiwear synergistic mechanism is postulated in which borates with electron‐deficient boron p orbitals catalyse the tribo‐reduction of the metal compounds on the rubbing surfaces, producing elemental metals.     

Structures of adsorbed zinc dithiophosphates and their relationship to engine wear

Zinc dithiophosphates (ZnDTPs) have been used extensively as anti-wear agents for over 30 years. The mechanism of action of these remarkable materials is not fully understood. In particular, the molecular composition of the antiwear film produced by the ZnDTPs remains an area requiring investigation. In this paper, the molecular composition of adsorbed ZnDTPs on aluminium oxide (Al₂O₃) surfaces has been studied using Inelastic Electron Tunnelling Spectroscopy (IETS), a highly sensitive molecular spectroscopy specifically applicable to surface species at low coverages. As such, it complements other surface science techniques for determining surface compositions. A vibrational spectroscopy, IETS utilises the literature on group characteristic frequencies available from the very extensive correlations developed for infrared and Raman spectra. Thus, functional groups may be identified that are formed by the reaction of a lubricating oil additive with a metal surface covered by a native oxide. Using IETS, we show that ZnDTP aryl esters are dissociatively chemisorbed, and do not appear to form P=S groups bonded to the surface. By contrast, primary and secondary ZnDTPs retain the phosphate ester groups and bond to the surface by the P=S bond to different degrees. These spectral differences are qualitatively correlated with the observed antiwear performance of these materials.     

The relationships between structure and rheological properties of hydrocarbons and oxygenated compounds used as base stocks

The growing interest shown by industry in synthetic lubricants for severely stressed engines and machines has led the author's organisation to investigate the best hydrocarbon and oxygenated structures which serve to achieve a good compromise between viscosity index and pour point. Several analyses of structure and property relationships have been published. The American Petroleum Institute's project 42¹ has given the characteristics of many light hydrocarbons from C₁₀ to C₃₅ belonging to different chemical families. During the Second World War, American and German research centres investigated a large number of esters and selected the best diacid and polyol ester structures. The compromise between the viscosity index and pour point has been studied for hydrocarbons and oxygenated products as to their prospects for use as base stocks. With hydrocarbons having a long main chain which is substituted either by a very branched chain, or by a chain ending with a saturated ring, it is possible to reach a viscosity index of around 160 for a pour point of −20°C. The introduction of ester functions in a hydrocarbon chain improves the compromise between viscosity index and pour point. Diacid esters constituted of linear diacids and branched monoalcohols are very favourable, but the best compromise is achieved by polyalkoxy -ether-esters and polyalkoxy diesters. Unhappily these are miscible to hydrocarbons only if the hydrocarbon chains are sufficiently long. Of course for formulation of lubricants, other properties also linked to chemical structure should be taken into account, such as thermal and oxidative stability, antiwear behaviour, hydrolytic stability, and compatibility with additives. The data presented here and other data derived from research projects conducted in the author's laboratory, some jointly with Produits Chimiques Ugine Kuhtmann, have been analysed to determine the possibilities for main synthetic lubricant base stocks.