Detailed characterisation of sulphur and nitrogen components in a multifunctional,ashless lubricant additive by spectroscopic and chromatographic techniques

The work reported here used infrared (IR), multinuclear nuclear magnetic resonance (NMR), and chromatographic techniques to identify and characterise a commercial sulphur‐ and nitrogen‐containing ashless multifunctional additive. The proper assignment of signals in the ¹H/¹³C NMR and IR spectra of the sample has facilitated the identification of different types of sulphur‐ and nitrogen‐containing components. The methodology involves investigation by NMR and IR spectroscopies, thin‐layer chromatography (TLC) and high‐performance liquid chromatogaphy (HPLC) techniques to elucidate the types of components present. This requires the generation of NMR and IR spectral data for standard compounds of alkyl sulphides with different sulphur content and alkyl chain, and nitrogen and sulphur components such as thiadiazole, imidazoline, triazole, etc., and spectral comparison with the spectra of the unknown sample. Further, these components have been separated by silica/alumina column chromatographic and preparative TLC techniques and subsequently analysed for their exact chemical structure by spectroscopic techniques. The combined spectroscopic and chromatographic analyses have yielded the presence of four types of components: di‐t‐octyl polysulphide, di‐t‐octyl thiadiazole, a component containing amine ether functionality, and alkyl amine salts of mono/dialkyl phosphoric acid.     

Anti wear and anti friction characteristics of tribochemical films of alkyl octa‐decenoates and their derivatives

The wear, friction and film forming characteristic of ethyl octadecenoate and methyl 12‐hydroxy ocatadecenoates, their thio and thiopyrophosphoro derivatives have been studied on HFRR friction and wear machine. The structural characteristics of chemisorbed films formed on the iron surfaces by these derivatives have been investigated using various analytical techniques such FT‐IR, Microreflectance Absorption and Proton Magnetic Resonance. It has been inferred that these films are primarily organic in nature. The constituents of these films are hydroxy alkyl and aryl ethers, unsaturated and condensed ring hydroxy ketones, aromatic hydroxy quinones and highly condensed aromatic structures chemically linked to iron sufaces through hydroxy, oxy, thia, oxyphosphoro and thiophosphoro moieties forming chelated and coordinated complexes. The antiwear and antifriction characteristic of alkyl octadecenoates increase with increasing polar linkages in the alkyl ocatadecenoate backbone. This is primarily due to the increase in number of sites amenable to chemisorption on the surfaces and consequently their reactivity. The antiwear and antifriction characteristic of derivatives of ethyl octadecenoates are inferior to the corresponding 12‐hydroxy methyl octadecenoates derivatives. This can be attributed to increased reactivity of 12‐hydroxy methyl octadecenoates derivatives at the surfaces due to an additional hydroxyl moiety. The films formed are highly condensed ring aromatic layered structures coordinating to iron atoms of the surface particularly in case of thiopyrophosphoro derivatives of methyl 12‐hydroxy octadecenoates. These films, as a consequence, provide lower wear and friction characteristics as compared to other types of organic films. These studies have led to much clear evidence of the intimate relationship between the chemical structure of the additives and their film forming characteristics. Copyright © 2006 John Wiley & Sons, Ltd.     

Tribological properties of alkyl and hydroxyl groups on the imidazolium tetrafluoroborate ionic liquids

We present an investigation of effect of alkyl chain length and hydroxyl group functionalisation on the tribological properties of imidazolium tetrafluoroborate ionic liquids. Four kinds of hydroxyl‐functionalised ionic liquids with the alkyl chain of C₁–C₁₀ were synthesised, and the relationships between their structure and physicochemical properties such as viscosity, thermal stability and corrosion, before and after incorporation of a hydroxyl group, were measured. In particular, the studies on their lubrication properties as ionic liquid (IL) lubricants for steel–steel contacts, including the friction coefficient, the wear volume and so on, were particularly emphasised. Moreover, an in‐depth exploration about the function mechanism and failure mode of the ILs during the friction process were studied using the scanning electron microscope and X‐ray photoelectron spectroscope technologies, and a proposed interaction model between ILs and steel substrate was presented. Copyright © 2012 John Wiley & Sons, Ltd.     

An ESCA study of the effectiveness of antiwear and extreme‐pressure additives based on substituted phosphorodithioate derivatives,and a comparison with ZDDP

Ashless substituted dithiophosphoric acid derivatives (ADPs) are a new generation of multifunctional additives with promising antiwear (AW) and extreme‐pressure (EP) characteristics. Three such additives synthesised in the authors' laboratory have been evaluated for their AW and EP properties by standard four‐ball friction and wear tests. The friction‐reducing properties of these additives were compared with those of a commercial zinc dialkyldithiophosphate (ZDDP). It was found that the phosphorodithioate compounds studied here possessed excellent AW/EP properties. Their AW characteristics were found to be comparable to those of ZDDP at low loads. However, at higher loads they show inferior AW characteristics in comparison to ZDDP. Nevertheless, ADP derived from cashew nut shell oil had a higher load‐carrying capacity than ZDDP. The mechanism of the AW and EP behaviour exhibited by the different additives was investigated using X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) of the worn surfaces formed during friction. XPS and AES analyses of the worn surfaces reveal that the tribochemical film formed on the ADP‐tested surfaces consisted mainly of metal phosphates and only a small amount of metal sulphides, even though the ADPs contained twice the number of sulphur atoms than phosphorus atoms. The ZDDP‐tested surface showed a mixture of metal sulphides and metal phosphates. Alkylamino substitution appeared to have no significant effect on the AW/EP properties of the additive. XPS and AES analyses also revealed that the tribochemical film formed on an ADP‐tested surface was thicker than that present on the ZDDP‐tested surface at low loads, whereas at higher loads the reverse was true. The higher weld load obtained for the blend containing cashew nut shell oil‐derived ADP is attributed to the thicker adsorbed reaction film formed on the surface due to the long alkyl groups present in the original additive structure. Short‐chain alkyl groups, however, form only a thin adsorbed layer, which may get rubbed off during the friction at high load. The low sulphide formation on ADP‐tested surfaces was attributed to the absence of any metal atom in the additive, which would help in the formation of metal sulphides during tribofragmentation and further tribochemical reactions.     

The hydrocarbon/water interfacial behaviour of alkyl aryl sulphonates

The interfacial tension and solubilisation characteristics of the alkane/water interfaces of a series of alkyl aryl sulphonates synthesised with 1‐butanol as a co‐surfactant have been studied. It was observed that there is a narrow range of carbon numbers of these sulphonates at which maximum solubilisation of a particular alkane occurred. This range shifted progressively to a higher value, with a corresponding increase in the carbon number of the n‐alkane. The extent of solubilisation of the alkane is highly dependent on the structure of the hydrocarbon portion of the sulphonate. Unexpectedly, an ultra‐low interfacial tension seems not to be an important prerequisite for the solubilisation of hydrocarbon in the aqueous surfactant phase. The free energy decrease required for solubilisation seems to be far more dependent on the energetic interactions connected with entropy parameters. The conformational aspects of the hydrocarbon portion of sulphonate molecules, as a consequence, seem to play a decisive role in molecular interactions, leading to solubilisation of a large quantity of a particular hydrocarbon in aqueous solution of a given sulphonate. The present work aims at understanding the selection criteria for sulphonates in applications involving solubilisation and ultra‐low interfacial tension at the hydrocarbon/water interface, such as those in insecticidal/pharmaceutical formulations, fire‐resistant hydraulic fluids and enhanced oil recovery.     

New unsymmetrical complex diesters of adipic acid considered as tribological fluids

The paper presents results concerning the synthesis and characterisation of some unsymmetrical diesters of adipic acid with a mixed, complex structure in order to produce synthetic ester oils with specific tribological properties. These complex diesters have been synthesised by taking into account medium and superior aliphatic alcohols, such as n‐butanol, n‐hexanol, 2‐ethyl‐hexanol, isodecanol and isotridecanol along with special alcohols of a complex alkyl–aryl nature, namely 2‐phenoxy‐ethanol, 2‐([o‐sec butyl] phenoxy) ethanol and 2‐([p‐nonyl] phenoxy) ethanol, respectively. By varying the length of the aliphatic alcohols and also by modifying the length and position of the alkyl pendant group on the aromatic ring of the complex alkyl–aryl alcohols, three series of products have been realised. On the basis of this programme, it has been possible to obtain fluid products that show valuable tribological features with adequate and versatile characteristics. Copyright © 2010 John Wiley & Sons, Ltd.     

New citric esters with aromatic content and a complex structure considered for use as tribological fluids

This paper presents the results of research carried out to produce synthetic ester oils with a mixed structure, based on citric acid, with beneficial tribological properties and potential biodegradability. Two series of unsymmetrical triesters have been synthesised by varying the molar relationship between aliphatic alcohols from C₁, to C₁₃ along with an alcohol of a complex alkyl‐aryl structure, namely 2‐[(p‐nonyl)phenoxy]ethanol. The variation of the principal properties as a function of the length of the aliphatic chain introduced by the aliphatic alcohol and the aromatic content introduced by the alkyl‐aryl alcohol has been assessed. The wide range of viscosities of these triesters demonstrates their versatility, and the natural citric acid origin holds potential for their having good ecological characteristics.     

Effect of fatty-acid composition and structure of alkyl radicals of plant oil triglycerides on their tribotechnical characteristics

Sunflower, corn, rapeseed, olive, and linseed oils have been tested on a four-ball friction machine. The fatty-acid composition of oils has been determined. It has been shown that the similarity of their antiwear characteristics and the holding range of surface-active substances is related to the equivalence of the chain lengths of alkyl radicals. It has been determined that the activation of unsaturated bonds as a result of the oxidation and thermomechanical exposure upon friction has a positive influence on the friction interaction due to tribopolymerization and film formation on the abrasive surfaces. It has been stated that, with change in the average chain length of alkyl radicals and the number of unsaturated bonds by the addition of corresponding triglycerides, one can control the tribotechnical characteristics of plant oils.     

New Mixed Esters with a Complex Alkyl‐Aryl Structure Considered as Tribological Fluids

This paper presents the results of a study carried out to produce synthetic ester oils of mixed structure and based on phthalic anhydride with beneficial tribological properties. Three series of unsymmetric diesters were synthesised using aliphatic alcohols such as n‐propanol, n‐butanol, n‐hexanol, 2‐ethylhexanol, isodecanol, and isotridecanol, along with special alcohols of a complex alkyl‐aryl structure, namely 2‐phenoxyethanol, 2‐((o‐sec‐butyl)phenoxylethanol, and 2‐ftp‐nonyl)phenoxylethanol. Also synthesised were symmetric esters based on these special aliphatic‐aromatic alcohols. The variation in the principal properties as a function of both the length of the aliphatic chain introduced by the aliphatic alcohol and the length and position of the aliphatic pendant chain on the aromatic nucleus of the complex alkyl‐aryl alcohol was assessed. Based on this, it has been possible to obtain fluid products with variable properties. The introduction of the aromatic function in the structure of these diesters leads to interesting features, and the wide range of viscosity values demonstrates their versatility.     

The antiwear mechanism of zddp's. Part II

The solution breakdown of zddp's to zinc polyphosphates and mixed alkyl sulphides has been discussed. In this paper, their interaction with metal surfaces is pursued using esca. Zinc phosphate is demonstrated to arrive in the surface oxide layer of engine cams and tappets by physical adsorption and FeS is formed via the oxide layer and elemental sulphur generated from alkyl sulphides with Fe₂O₃. It is hypothesized that their presence there as fusible glassy compounds, phosporus as polyphosphate and iron sulphide as a ternary eutectic with iron oxide, provides the antiwear function of zddp     

Alkylated naphthalenes as high‐performance synthetic fluids

The physical and chemical properties for a series of structurally well‐defined, alkylated naphthalenes have been compared. Structural characteristics, such as the alkyl chain length, alkyl chain branching, and number of alkyl groups on the naphthalene ring, have been correlated with physical properties, such as viscosity, viscosity index, pour point, and aniline point. The thermo‐oxidative stability of the alkylated naphthalenes was evaluated by the rotating pressure vessel oxidation test (RPVOT), a bulk oxidation test (BOT), and pressure differential scanning calorimetry (PDSC). Although all of the alkylated naphthalenes showed better thermo‐oxidative stability than other traditional hydrocarbon‐based fluids, the relative performance of the structurally different alkylated naphthalenes was found to be test dependent. Initial RPVOT studies with a linear polyalkylated naphthalene show that it gives good additive response to the incorporation of traditional antioxidants.     

Synthetic lubricants based on sebacic complex esters

This paper presents the results of a study carried out in order to produce synthetic ester oils of a mixed structure based on sebacic acid with beneficial tribological properties. These series of unsymmetrical diesters were synthesized using superior aliphatic alcohols such as 2‐ethyl‐hexanol, isodecanol and isotridecanol, along with special alcohols of a complex alkyl‐aryl structure, namely 2‐phenoxy‐ethanol, 2‐[(o‐sec butyl) phenoxy] ethanol and 2‐[(p‐nonyl) phenoxy] ethanol. Also synthesized were symmetric esters based on the complex aliphatic‐aromatic alcohols with pendant aliphatic chain. The variation in the principal properties as a function of both the length of the aliphatic chain introduced by the aliphatic alcohol, and the length and position of the aliphatic pendant chain on the aromatic nucleus of the complex alkyl‐aryl alcohol was assessed. Based on this, it has been possible to obtain fluid products with variable properties. The introduction of the aromatic function in the structure of these diesters led to interesting features, and the superior values recorded for the main tribological characteristics demonstrated their versatility. Copyright © 2006 John Wiley & Sons, Ltd.     

EP/AW performance evaluation of some zinc alkyl/dialkyl/alkylaryl‐dithiocarbamates in four‐ball tests

Tribochemistry, the chemistry of interacting surfaces under the influence of a lubricant, helps in the appropriate selection of suitable lubricant additives for specific uses. Modern lubricants are usually formulated from a range of petroleum base oils or synthetic fluids incorporating a variety of chemical additives for performance enhancement. Extreme‐pressure (EP) and anti‐wear (AW) additives are used extensively in lubricants for hypoid gears and metal cutting and forming operations to reduce wear, modify friction, and prevent scuffing of moving metallic parts. The present paper includes the synthesis and the evaluation of the tribological properties of 0.5% (w/v) solutions of some zinc bis‐(alkyl/dialkyl/alkylaryldithiocarbamates) in paraffin oil using 12.7 mm diameter steel bearing ball specimens in four‐ball tests. All the synthesised zinc dithiocarbamate additives in general, and zinc bis‐(morpholinodithio‐carbamate) (A₄) in particular, exhibited good AW, EP, and friction‐reducing properties. Additive A₄ especially gave low values of wear‐scar diameter and coefficient of friction at higher loads and higher values of load wear index and flash temperature parameter during EP tests (ASTM D 2783) and afforded lower values of wear‐scar diameter in a one‐hour wear test (ASTM D 2266–67). The surface topography of the wear‐scar matrix of the used ball specimens was investigated by scanning electron microscopy.     

Polyolefin and poly(alkyl methacrylate) mixed additives as mineral lubricating oil rheology modifiers

Some rheological properties of mineral lubricating oils containing polyolefin (OCP) (ethylene/propylene copolymer) and poly(alkyl methacrylate) (PAMA) mixed additives over a wide composition range have been examined. Increasing the OCP content significantly increases the viscosity and shear stability of solutions, whereas the viscosity index is directly proportional to the PAMA content. The pour point values of the additive solutions investigated are lower than −30°C even for the smallest concentration of PAMA in the polymer mixture of 15 wt.% (at a total polymer mixture concentration in oil of 2 wt.%). The viscosities of dilute mixed additive solutions obey the typical Krigbaum‐Wall equation and, due to the negative viscometric interaction parameter values (Δb₁₂ < 0), the OCP/PAMA mixture is found to be immiscible. The immiscibility becomes more pronounced on increasing the overall polymer concentrations. The viscosities of concentrated mixed additive solutions decrease significantly with an increase of OCP content, showing a minimum of half the initial value at a polymer ratio of about 50/50. On decreasing the polymer concentration to practical levels as well as increasing the temperature, the viscosity values approach the ideal additivity law. Based on the ratio of specific viscosities at 40 and 100°C (Q), the OCP additive solution shows a better thickening effect at lower temperatures (Q < 1). On the other hand, the PAMA additive solution shows a better effect at higher temperatures (Q > 1) with more coherent thickening with temperature change. The mixed polymer additive, containing ∼10 wt.% OCP and ∼90 wt.% PAMA, is a nearly optimal viscosity‐index improver showing the same thickening effect at different temperatures. The results obtained are promising as regards the design of lubricant formulations with mixed polymeric additives.     

Chemical characterization of tribochemical and thermal films generated from neutral and basic ZDDPs using X-ray absorption spectroscopy

X-ray absorption near edge structure (XANES) spectroscopy at the phosphorus L-edge and sulphur L-edge has been used to characterize the chemical nature of tribochemical and thermally generated films from several ZDDP antiwear agents in the neutral and basic forms. Using the P and S L-edge XANES spectra of model compounds with known structure as fingerprints, the chemical structures of P and S species in the films have been identified. P appears in all the films as polyphosphates in different proportions of short and long chain polyphosphates. In some films, polyphosphates are accompanied by unchanged ZDDP. Generally films generated from neutral and basic ZDDPs show similar P and S chemistry (polyphosphates and sulphides) but contain different proportions of unchanged ZDDP. However, the aryl ZDDP films have different polyphosphate structure compared to the alkyl ZDDP films. The sulphur proportion in the tribochemical films is decreased a great deal, but remains in the reduced form. However, S in the thermo-oxidatively generated films, appears both in the reduced and oxidized form, depending on the ZDDP and the temperature.     

Base oils of high aromatic,long branched aliphatic pyromellitic esters

The present paper deals with the synthesis of some tetraesters with a mixed structure of pyromellitic dianhydride, and with their characterisation as potential base oils. These esters are made on the basis of a long branched aliphatic alcohol, such as isotridecanol, and an alcohol with aromatic content, such as 2‐phenoxy‐ethanol. The variation of the principal properties as a function of both the length of the aliphatic chain introduced by the aliphatic alcohol, and the aromatic content introduced by the aryl‐alkyl alcohol has been assessed and the potential for viable products with a wide range of viscosity values has been examined. The enhanced aryl content is shown to induce superior thermal behaviour. These products present good test results and, thereby, demonstrate their versatility.     

Tribological properties of aqueous solutions of alkyl polyglucosides

This research was inspired by increasing interest in ecological lubricants. Aqueous solutions of alkyl polyglucosides (APGs) were chosen as potential lubricant compositions. Commercially provided products were tested for surface tension, wetting angle, light diffusion and viscosity. Additionally, atomic force microscopy, polarized light microscopy and X-ray spectroscopy were employed. The tests were supposed to identify micellar and liquid-crystalline structures formed in bulk and surface phases. The tribological tests were performed with a four-ball machine and with a ring-on-disc apparatus. The friction couples were made of steel.On the basis of the results obtained via four-ball machine tests, the influence of additive concentration on friction coefficient (μ), wear and seizure load was assessed. As compared to water, the friction coefficient was reduced even seven times and the seizure twice. Also anti-seizure properties significantly changed. For instance, for some of the compositions applied, seizure did not occur, even at a maximum load of 8 kN. The dependence of tribological quantities on concentration was not monotonic. One can note, however, a reduction of μ and wear and a significant improvement of anti-seizure properties, at concentrations of about 0.1%. The influence of the additive applied on the friction coefficient was similar for the ring-on-disc test.The changes in the tribological properties observed could be explained by high APG affinity to the surface, while the effect of concentration on the tribological performance is caused by changing the solution structure. The compounds applied can form various types of micelles and mesophases in bulk and surface phases.     

Influence of the alkyl radical on the optical activity of cholesteric liquid-crystal nanomaterials

The influence of the number of carbon atoms in the alkyl radical of cholesteric liquid-crystal nanomaterials and their concentration on the optical properties of mineral and synthetic lubricants is investigated. It is found that adding cholesteric liquid-crystal nanomaterials to lubricants changes their optical properties, as indicated by a switch from right rotation to left rotation of the polarization plane of transmitted light. It is shown that the angle of rotation of the polarization plane depends practically linearly on the concentration of cholesteric liquid-crystal nanomaterials, in semilogarithmic coordinates.     

Additives for Rape Seed Oil 3: Synthesis and Tribological Behaviour of Boron‐Sulphur‐Modified Rape Seed Oil Lubricant Additive

A type of boron‐sulphur‐modified rape seed oil for use as a lubricant additive was prepared. The tribological properties of the additive in rape seed oil were evaluated using a friction tester. The morphologies and tribochemical species of the worn surfaces of the test balls were analysed by means of X‐ray photo‐electron spectroscopy. The results show that the additive is very good in increasing the load‐carrying capacity and improving the antiwear and friction‐reducing properties of a rape seed base oil. The inferred lubrication mechanism is that a high‐strength adsorption film and a tribochemical reaction film form on rubbing surfaces due to the carrier effect of the long‐chain fatty acid molecules, to the high reactivities of sulphur and boron, and to the electron deficiency of boron, and their synergism.     

Study of the tribological performance of a rare earth naphthenate as a lubricant additive

The tribological behaviour of a rare earth naphthenate (REN) as a lubricant oil additive in VG26 white oil and the complexes of REN and organo‐sulphur or organo‐phosphate compounds have been evaluated with a four‐ball friction and wear tester. The chemical features and elemental composition of the boundary lubricating film were examined by means of Auger electron spectroscopy (AES) and X‐ray photoelectron spectrometry (XPS). The results show that REN exhibits good antiwear, load‐carrying, and friction‐reducing properties in the base stock. When 2.0% REN is added, the wear‐scar diameter value reduces to 54.7% of that for the base stock alone and the maximum non‐seizure load increases 2.95 times. A synergistic effect is found for the load‐carrying capability of the complex of REN and organo‐sulphur while poor compatibility is exhibited for the complex of REN and organo‐phosphate. The analytical results of AES and XPS indicate that the good performance of REN is attributable to the formation of a boundary lubricating film mainly composed of naphthenic acid, rare earth oxide, and complexes of rare earth metals, which is formed on a rubbed surface when lubricated by oil containing the REN additive.