The reaction products of sulfurized Mohwa oil with iron powder in hydrocarbon medium at 150°C for 8 h were studied to investigate the type of lubricant films formed during their application as antiwear and extreme pressure additives. The main reaction product was isolated on the basis of its solubility in mixed solvent. Surface characterization was carried out using ultraviolet-visible spectroscopy (UV), fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray diffraction spectroscopy (XRD), scanning electron microscope (SEM), and high frequency reciprocal rig (HFRR). An examination of their elemental analysis and instrumental analysis data reveals that there is reduction in the length of the alkyl chains and carbonyl ester groups with formation of inorganic iron sulfides. Polymerized product with a number of ketonic and aldehydic groups containing iron and sulfur in the polymeric films in the form of unsaturated cyclic rings was also formed. The films are organo-inorganic in nature, unlike the purely inorganic iron sulfide type. The load-carrying characteristic of this product is strongly influenced by the type of the film formed on the iron surfaces.
The development of coating and lubricating materials for the surface modification, functionalization of metallic surfaces has become one of the major areas of research. Since the functionalization of iron (Fe) surface also depends upon the reactivity of the surface and strength of organic species. In addition, the surface functionalization is also the function of thermal stress, mechanical stress, catalytic activity, and the emissions of low exoelectrons. Functionalization of iron (Fe) surface in the chemical environment of oxygen, chlorine, nitrogen, aluminum, boron, nickel, and carbon has been studied earlier also. However, study of the sulfur, phosphorus, and many other chemical species has been scanty in literature. The application range of organic sulfides is spread throughout the chemical, pharmaceutical, medicinal, fertilizers, electrical, electronic, biological, and biochemical sectors of scientific and technological appliances. These are directly or indirectly involved in most of the industrial, research, and domestic application.
In the present paper, it is aimed to conduct the model study for synthesis of organic sulfides and the surface functionalization of metallic iron (Fe) particles in the chemical environment of organic sulfides for their industrial application in lubricating fluid industry/metal working fluid industry, coating industry, paint industry, and the industry for the automotive and industrial lubricating fluids and its formulations protocol. The concept of functionalities can also be applicable to different categories of research and analytical studies such as process optimization, material conservation, energy utilization, environmental pollution, natural diseases, metrological devices, medical appliances, surface/interfacial reaction, and petroleum exploration.
However, the concept of layer development and their microanalysis appeared more explicitly in the late eighties. Kajdas [
Under the conditions of heavy loads and high sliding speed the extreme pressure (EP) zone formed at metal-to-metal junction. The mechanism of lubrication under the EP conditions by sulfurized additives is said to depend on the formation of a film of iron sulfide due to controlled chemical reactions of these additives with the rubbing surfaces. This reduces the wear and damage to rubbing surfaces [
According to Allum and Ford [
It was pointed out that the formation of organic-inorganic films is chiefly responsible for providing the EP characteristics [
The surface films formation is important to protect the surfaces. There are three ways of film formation: physical adsorption, chemisorptions, and chemical reaction. The surface behavior of films is due to energy binding of the film molecules to the surface. Physical adsorption involves intermolecular forces involved in condensation of vapors to liquids. Liquid molecules attached to the surfaces of the solids, and this provides a modest protection. Physical adsorption is usually rapid, reversible, and nonspecific. There is no electron transfer in this process. An example is of physical adsorption of sulfide on iron (Figure
Adsorption of hexadecanol and sulfide.
The chemical analysis of the absorbed sulfide film indicates the purity of sulfides film around 98% with 2% of unreacted hydrocarbon, which is not detectable in the extraction stage of analysis. But on final analysis by sulfated ash, it was revealed that it is not possible to remove the entire unreacted hydrocarbon from sulfides. However, the correction for purity was made during the calculation and conclusion. Absence of inorganic content and water was an advantage. For characterization of the absorbed sulfide films the results of instrumental analysis, UV, XRD, SEM, IR and NMR, are important.
Electrolyte grade iron (Fe) particles (100–200 mish size), carrier fluid (light and medium paraffin’s liquid). acetone, toluene, xylene, elemental (crystalline) sulfur powder (99.9%), metal (alkali) sulfides (M2S), heating system/oil bath arrangement., soxhlet apparatus arrangement, separating funnels, vacuum pump for filtration, vacuum oven (reduced pressure), evaporating arrangement, reflux arrangement.
Crude Mohwa oil [
The materials used involve Sulfurized Mohwa Oil (SMO), chloroform (CHCl3), dimethylformamide (C3H7ON), xylene (C9H12), n-heptane (C7H16), 2N-hydrochloric acid (HCl), electrolyte grade iron (Fe) particles (100–200 mesh size powder) SD fine chemicals 38597, K-50 (Figure
Elemental analysis, molecular weight, and empirical formula of SMO and SMO Film.
C | H | S | O | Fe | Molecular weight | Empirical formula | |
---|---|---|---|---|---|---|---|
SMO | 68.21 | 10.95 | 6.74 | 10.11 | — | 950 | C54H104O6S2 |
SMO Film | 58.12 | 9.22 | 5.74 | 8.6 | 18.32 | 1115 | C55H102.8O5.99S2Fe3.64 |
Electrolyte grade iron particles for reaction process.
The analysis and characterization of organic sulfides and chemisorbed reaction film (CRF). The organic sulfides are mainly characterized by elemental analysis. Elemental Analysis of organic sulfides takes place by Perkins Elmer Equipment (2400 series II CHNSO analyzer). This analysis describes the percentage, molecular weight, and Empirical formula of the organic compounds mainly organic sulfides. The percentages of carbon, hydrogen, sulfur, phosphorous oxygen, nitrogen, and iron are given in Table
Product properties of sulfurized Mohwa oil.
Characteristics | Value |
---|---|
Copper corrosion Test at 120°C for 3 hours | 1a to 1b |
Sulfur, wt% | 12-13 |
Wear scar diameter on HFRR (in mm) | 0.408 |
Friction coefficient on HFRR | 0.009 to 0.018 |
Film’s contact potential on HFRR (in mv) | 0.4 to 0.8 |
Weld load (Kg) | 455 |
In the present work, XRD spectrum of absorbed sulfide film was analyzed on Brucker Germany Powder XRD-D8 Advance. The XRD spectra as recorded are reproduced in Figure
XRD Graph of SMO.
UV spectrum of absorbed sulfide film was analyzed on Lambda ULTRA-VIOLET VIS. N.K. SPECTROPHOTOMETER. The recorded UV spectrum is reproduced in Figure
UV analysis spectrograph of SMO reaction film.
FT-IR spectrum of absorbed sulfide film was run as either thin film between KBr plates or as KBr pellets from 4000 cm−1 to 400 cm−1 on Perkin Elmer 1760X FT-IR spectrometer. The recorded IR data are given in Figure
FTIR spectrograph of SMO reaction film.
The 1H NMR spectrum of sulfide was recorded on Bruker DRX 300 Spectrometer in CdCl3 solvent with TMS as internal standard. Signal positions (
1H NMR spectrum of SMO reaction film.
13 C NMR spectrum of SMO reaction film.
SEM micrograph of absorbed sulfide film was run on Quanta 200F-FE-SEM-EDS-EESD system, FEI The Netherlands. The recorded SEM microanalyses are given in Figures
SEM of SMO (EP 2.01).
SEM of SMO (EP 2.03).
SEM photo of SMO reaction film.
SEM photo SMO reaction film.
It is not easy to predict the exact nature of SMO chemical reactions taking place between sulfide and iron. However, attempt has been made to increase the knowledge regarding reactions and extracted sulfide film, which was formed on iron surface. It is evident that in primary reactions, ionization occurs by capture of exoelectrons to form thiolate-type ions. Thermal decomposition of SMO produces R-S or R-S2 radicals. Catalytic oxidation of hydrocarbon solvent gives aldehyde, ketones, acids, unsaturated hydrocarbons, polyketones, hydroxy ketones, carbanions, and conjugated ketonic and enolic species. In secondary reactions it occurs at the iron surface to form mercaptides. Other reactions are decomposition of radicals into further radicals, disproportionation of radicals to products and oxidative attack of thio radicals [R-S] on unsaturated hydrocarbons, ketones, aldehydes, acids, hydroxy ketones, polyketones, and so forth. Attack of reactive sulfur species on hydrocarbon solvent and other derivatives to produce olefinic derivatives. Cyclization to heterocyclic derivatives. Formation of iron sulfides by decomposition of mercaptides. Direct reaction with reactive sulfur and hydrogen abstraction from hydrocarbon solvent to form thiols [
Tertiary reactions occur at the iron surface to produce highly absorbed film-like organic species by interaction of ions, radicals, and other derivatives produced by primary and secondary reactions. More iron sulfide is formed by decomposition of mercaptides and reaction with sulfur-containing secondary reaction products. Several
The UV spectrum of SMO has lot of bending in curve. The important changes are at wavelength (
Under chemical environment, sulfides reacted to iron surface with the help of exo-electron and catalytic effect of metal. The films formed may be indicator to the reactions taking place at the metal surface. An XRD spectrum of films generated from SMO is plotted in Figure
Film of SMO was analyzed under SEM. The microanalysis in Table
The
The
On the sulfurization, sulfur attacks the unsaturation/double bond of Mohwa oil. Mohwa oil is triglycerides having more than three double bonds and gives polysulfides with sulfur substitution in the allylic position to the double bond. The FTIR analysis of SMO reaction product shows new bands at 1010 cm−1, 1020 cm −1, 1170 cm −1, 1453 cm −1, 1514 cm −1, 1540 cm −1, 1649 cm −1, 1742 cm −1, and 3344 cm −1. These are due to ketonic, aldehydic carbonyl, and hydroxyl groups and increase in the unsaturation. It also indicates the elimination of esters and reduction in the length of the alkyl side chain. The reduction in hydrogen carbon ratio indicates the increase in the unsaturation. The changes in elemental composition show an increase in the sulfur content and iron as an additional element. The increase in percentage of oxygen and sulfur shows the presence of carbonyl and hydroxyl group. Band at 3344 cm−1 shows the presence of cyclic ketones and hydroxyl group reduction. This is confirmed by XRD. XRD further indicates the presence of Fe–S–O, sulphates, and other inorganics. Film is amorphous in nature and Mohwa oil was fully degraded. UV analysis indicates that major component of cyclic reaction product is monocyclic. The insignificant change in hydrogen carbon ratio indicates that sulfoxides are produced rather than oxidation of hydrocarbon. NMR indicates that organic sulphide or sulfoxides are present and S–O, C–O or C–S linkage is present. The SEM indicates for the presence of organic-inorganic mixture with unreacted hydrocarbon solvent. The film is nonhomogenous and mixture of all types of compounds, for example, crystalline, amorphous, hydrocarbons, and so forth. Film is not continuous. So, it may be possible that FeS, FeSO4, Fe–S–O–R, Ar–S, and so forth are present. The probable sequence of reaction, indicated from the analysis, involves the reaction of iron on the S-S bond, which leads to formation of iron sulfides and elimination of ester group along with oxidation of the alkyl side chain to form ketone, aldehyde, and carboxylic group (giving FTIR band at 1540 and 1170 cm−1) by oxidation. The prominent reaction is oxidation to convert sulfide group into aldehyde to carboxylate and then formation of polymeric iron carboxylate film. The film formed from SMO has an unsaturated cyclic ring system which is nonaromatic in nature, therefore, the strength of the film formed by the SMO is greater.
In the case of these films, sulfur is present in several oxidation states as indicated by XRD peaks. The peaks could be attributed to oxidized form of sulfur as the sulfoxide. The XRD spectrum for SMO with the appearance of only one or no peak corresponds to iron sulfate. And the relative intensity of SMO is stronger. This means the bulk of film generated from the additives is composed of Fe–S–, and the bulk layer of SMO is thinner. The produced RS· or RSS· radical of alkyl polysulfide reacted with the metal or iron oxide to form FeSO4. The higher sulfur content in the sulfides leads to the higher content of radical, accelerating the rate of reaction. As to the inner surface of the film, the content of iron oxide and oxygen decreases, which results in the variation of the film composition, that is, the bulk consists of FeSO4, subsurface consists of a mixture of FeSO4, sulfoxide, and FeS2, and the out surface consists of adsorbed alkyl disulfide. For the SMO, due to the polar group in the molecular structures, it is adsorbed strongly onto the metal surface and a thick protective film formed, which hinders the reaction of mercaptyl radical with the metal surface. So a film consisting of FeS2 from the bulk to the subsurface is formed. Therefore, FeS2 is one of the most probable forms of sulfides in the film. Fe3S4, which can be the same crystal system as Fe3O4, is another candidate although it is a less common compound. This is confirmed by XRD. XRD further indicates the presence of Fe–S–O, sulphates, and other inorganics. Film is amorphous in nature. UV analysis indicates that major component of cyclic reaction product is monocyclic. The insignificant change in hydrogen carbon ratio indicates that sulfoxides are produced instead of oxidation of hydrocarbon. NMR indicates that organic sulphide or sulfoxides are present and S–O, C–O or C–S linkage is present. SEM indicating for the presence of organic-inorganic mixture with unreacted hydrocarbon solvent. The film is nonhomogenous and mixture of all types of compounds, for example, crystalline, amorphous, hydrocarbons, and so forth. Film is not continuous. Presence of FeS, FeSO4, Fe–S–O–R, Ar–S, and so forth are indicated.
The oxygen atom transfer (electron transfer) mechanism revealed that the iron-oxo complex radical cation, Fe(IV)=O, is active oxidant. When the oxidation of a phenyl/alkyl sulfides occured the corresponding sulfoxides were produced. Use of H2 18O or H2 18O2 clearly indicates that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of an SMO whose radical cation is expected to undergo cleavage of the
Several authors erroneously regard SMO as possessing inferior surface protector properties despite the fact that experimental surface properties data obtained by several workers and at the authors’ laboratory quite clearly show that the surface characteristics of SMO are not inferior and comparable with those of lower SMO at equivalent sulfur concentrations up to 0.5%. At equivalent sulfur concentrations above 0.75% the surface properties are definitely superior. This contention has arisen because SMO gives the least amount of iron sulfide. On the other hand, the above results are consistent with the observation that the decomposition of SMO (35 wt%) at 150°C for 8 hours in the presence of hydrocarbon solvent is about 10 wt% higher than that of dibenzyl disulfide (DBDS) at 1% equivalent sulfur concentration [
On the basis of the studies, the following conclusions can be drawn. The sequence of reactions taking place on the rubbing surface involved the reaction of iron with the S–S bonds to form from iron sulfide, with simultaneous oxidation and elimination of alkyl chains, finally leading to the formation of polymeric film with repeating units of unsaturated cyclic rings joined together by iron sulfide functional group. The nature of the ring system produced from the sulfurized Mohwa oil is nonaromatic. The surface protective capacities of the various sulfides depend upon the combined strength of the organic film and the iron sulfide formed. The structural constituents of the organic film profoundly influence its strength. The film which is aromatic in nature has better protective capacities than the non-aromatic one. The films formed are organo-inorganic in nature. A polymeric organic film is superimposed on inorganic iron sulfide. A variety of strongly absorbed organic species are formed as films on metal surfaces under various chemical environments owing to interaction between the organic/inorganic sulfides, hydrocarbon media and metal. The quantity and quality of these organic films have profound protective properties. Films containing aryl derivatives of iron possess better protective capacities than the corresponding alkyl derivatives. The quantity and composition of iron sulfide alone do not seem to have a crucial property than aryl disulfides. The strength of the C–S and S–S bonds and the reactivity of the subsequent radicals produced with iron have a pronounced protection on the metal surface. The films formed from the sulfides are mainly composed of Fe–S–O, and alkyl disulfide also exists in the subsurface and bulk of film generated from alkyl sulfides. The film generated from the SMO is composed of alkyl disulfides in the out surface, a mixture of FeSO4, sulfoxide and FeS2 in the subsurface, and FeSO4 in the bulk.