Surface reactions of ethyl stearate and stearic acid with zinc, manganese and their oxides

Surface Technology, 21 (1984) 361 - 377

361

SURFACE REACTIONS OF E T H Y L S T E A R A T E AND STEARIC ACID WITH ZINC, MANGANESE AND T H E I R OXIDES

ROBERT A. ROSS and ANIKO M. TAKACS Kingston Laboratories, Alcan International Ltd., Box 8400, Kingston, Ontario K7L 4Z4 (Canada)

(Received August 5, 1983)

Summary The reactions of stearic acid with zinc and manganese powders have been studied from ambient t em pe r at ur e to 600 °C mainly using differential scanning calorimetry and thermogravimetry. Related, but less extensive, work has been carried out with ethyl stearate replacing the acid and with powders o f zinc oxide, MnO, Mn304, Mn203 and MnO2 replacing the metals. The reaction products were analysed by Fourier transform IR spectroscopy, X-ray diffraction, scanning electron microscopy and gas chromatography. For zinc and zinc oxide the first reaction zone began at about 160 °C and e x t e nd ed to 280 - 290 °C. Water and zinc stearate were formed. From 280 to 375 °C, zinc stearate was present in the reaction products of the acid with b o t h zinc metal and zinc oxide. At 600 °C, no acid or soap was detected. The reactions o f zinc with the ethyl ester were considerably less exothermic than those with the acid, consistent with the higher a m o u n t of soap formed in the acid reaction and with the larger heat evolution arising from the decomposition o f any excess acid when c om par e d with that from excess ester. In the acid reactions with manganese powder, the stearate was believed to commence f o r matio n at 135 °C. Its presence was detected in residues quenched f r o m 200 °C in reactions of all f our manganese oxides with stearic acid as deduced from the IR signals at 1562, 1467 and 1438 cm -1. Only a slight trace of manganese stearate was detected in the quenched residues from reactions between ethyl stearate and manganese up to 200 °C. No evidence o f the presence o f soap was obtained when the time o f exposure at 200 °C was increased to 30 rain or when the t e m perat ure was raised in stages up to 600 °C. Samples from these experiments gave IR signals at 1630 and 3400 cm -1 consistent with the presence o f considerable amounts of molecular water. No clear relationship could be established for the kinetics of any o f the reactions over the complete range o f t em perat ure because of the difficulties in separating the contributions o f a n u m b e r o f simultaneous and overlapping heterogeneous and hom ogeneous processes. Within the initiation stage o f the metal-a c i d reactions to 300 °C, good correlations were obtained 0376-4583/84/$3.00

© Elsevier Sequoia/Printed in The Netherlands

362 with a Prout-Tompkins type of expression which led to experimental activation energies of 85 kJ mo1-1, In A = 18.3, and 78.0 kJ mo1-1, In A = 17.0, for zinc and manganese respectively. The oxide film thicknesses on the metals were shown by ion etching to be a maximum of 150 A on zinc powder and just above 1000 A on the manganese.

1. Introduction The presence of thin surface films and adsorbed layers on metals and alloys can exert a profound effect on the chemical, physical and mechanical properties' of the materials. Often, these films and layers have escaped notice because available measuring procedures lacked sufficient sensitivity to detect extremely small concentrations of surface compounds or complexes thinly spread on essentially smooth planes. An important example of the significance of these surface layers in practice can be observed in the tribological behaviour of solids [1]. In addition to oxide layers or regions, the surface chemical composition of metals may be related to the presence of species created in forming or finishing operations which often use long-chain fatty acids and their esters as lubricants or lubricant components. The development of an understanding of the mode of interaction of such substrates with alloys and metals with varying surface structures and compositions thus affords the opportunity for studies of recognizable utilitarian significance. The present work is concerned with the surface reactions of aluminium alloys with fatty acid systems and, as a foundation for the design, related investigations have been carried out using pure aluminium and its principal alloying elements. Thus, structural and kinetic features of the reactions of stearic acid with aluminium and copper have been evaluated [2] followed by thermal and surface studies of reactions of the acid with magnesium, iron, iron(III) oxide and stainless steel [3]. No metal soaps were detected in the experiments with aluminium but stearates were formed in air at temperatures ranging from 165 to 300 °C on copper, magnesium and iron surfaces mainly by reaction of the acid with the metallic oxide layers. On iron and magnesium oxides, it was proposed that sites of octahedral symmetry amenable to direct or coordination transfer of anion species were important participants in initiation steps [3]. The work now described focuses largely on reactions of stearic acid and its ethyl ester with the surfaces presented by zinc and manganese metals from ambient temperature to 600 °C. The principal monitoring procedures were based on thermal analysis of enthalpy and mass changes, IR spectroscopy, X-ray diffractometry and electron microscopy. Some comparative observations have been included on the reactions of the substrates with the bulk oxides of zinc and manganese.

363 2. Experimental details

2.1. Materials The stearic (or octadecanoic) acid was BDH specially pure grade (purity, better than 99%) with a particle size range largely from 0.2 to 2 mm (long axis) and from 0.05 to 0.6 mm (short axis). The ethyl stearate was supplied by Pfaltz and Bauer Inc. The zinc powder (Fisher Scientific Company) was confirmed by spectrographic analysis to be 99.9% pure with silicon and manganese as the main impurities. The ion etch time required to reduce the surface oxygen intensity by 50% was 5 min in X-ray photoelectron spectroscopy analysis of zinc powder compacts and the oxide film was estimated to have a maximum thickness of 150 A. The surface area of the powder from nitrogen adsorption data was 0.33 m 2 g-1 and the size range of the largely spherical particles was from 2 to 10 pm as shown by scanning electron microscopy. The manganese powder (Fisher Scientific Company) was around 99.9% pure; silicon and iron were the principal impurities. The oxide film on manganese was more difficult to analyse by ion etching since the powder did not compact readily. The etch time for about 50% reduction in the oxygen intensity was 40 min and an oxide thickness somewhat above 1000 A was estimated. The surface area of the powder was 0.10 m 2 g-~. The particle shapes were irregular, often with jagged edges. The maximum dimension of the largest particle observed was about 70 gm in a range which extended downwards to about 2 #m. MnO2 was obtained from Mallinckrodt Inc. (purity, 99.75%); the other manganese oxides and the metal stearates were purchased from Pfaltz and Bauer Inc. Impurities detected by X-ray diffractometry were (i) iron and MnO2 in the MnO and (ii) Mn203 in the Mn304. Zinc oxide (General Chemical Corporation) had a minimum purity of 99%. The particle size ranges of the oxides lay mostly between 10 and 100 pm. All other reagents and materials used were at the highest purities commercially available.

2.2. Apparatus and procedure Details of the equipment and the preparative and measuring procedures have been reported [2, 3].

3. Results

3.1. Differential scanning calorimetry and structural analysis Runs were carried out with an air flow rate of 30 ml min -1 (at normal temperature and pressure (NTP)) from ambient temperature to 600 °C at a programmed temperature rise of 10 °C min -1. For a 1:2 stearate complex and 100% conversion to this product, the stoichiometric amounts of zinc and zinc oxide for reaction were calculated to be 10.3 wt.% and 12.8 wt.% respectively. Studies were conducted with a

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range of zinc powder concentrations from 5.1 to 20.7 wt.%, keeping the sample mass as close to 4.6 mg as practice allowed. Only one c o n c e n t r a t i o n level of bulk zinc oxide addition to the acid was evaluated as shown in Fig. 1 which also includes a differential scanning calorimetry (DSC) scan for 10.2 wt.% zinc powder. The curve areas of the thermal displacements in these and related scans were measured by planimetry and converted to A H changes as described earlier [2, 3 ]. Table 1 summarizes these results for the experiments related to zinc reactions. T em p erat ure regions were defined from the minima on DSC curves. The data for zinc stearate are included in the table. Quenched residues from the reaction of zinc with the acid up to 200 °C gave clear IR spectra of zinc stearate (1539, 1465 and 1399 cm -1) and stearic acid (Fig. 2). The soap was also present in samples quenched from 200 °C after 30 rain exposure at that t em pe rat ure while signal intensities associated with stearic acid were noticeably less intense. Residues quenched f r o m 300 and 400 °C showed only a mere trace of the acid. Sharp and distinct zinc stearate peaks were present in the IR spectrum of the sample quenched from 300 °C. These peaks became broad and diffuse in spectra of samples quenched f r om 400 °C. In a similar series of preparations using bulk zinc oxide at one concentration, 13.1 wt.%, the spectrum of the metal stearate was marked in samples b o th quenched immediately on reaching 200 °C and after 30 min exposure at this temperature. The f a t t y acid spectrum was n o t observed in

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the residues from 300 and 400 °C. The residue from 400 °C also showed peak broadening similar to that noted in the reactions with zinc powder. X-ray measurements on solid residues quenched from 600 °C showed mostly zinc oxide, and some zinc, present from the reactions of the metal powder with stearic acid. The principal gases detected by gas chromatography in the reactions of both zinc and manganese powders with stearic acid at 300 °C were hydrogen, water, carbon monoxide, carbon dioxide, ethane and propane. DSC scans of the reaction of ethyl stearate with 15.5 wt.% zinc powder gave A H values o f - - 2 5 5 kJ mo1-1, --345 kJ mo1-1 and --185 kJ mo1-1 in the respective temperature regions 170 - 235 °C, 235 - 415 °C and 415 485 °C. The non-volatile products formed in the reactions of ethyl stearate with zinc and zinc oxide were analysed by Fourier transform IR spectroscopy and the spectra were compared with standards for the reactants and for zinc stearate. In residues of the reaction of zinc powder with ethyl stearate heated from ambient temperature to 200 °C at 10 °C min -1 and then quenched back to ambient temperature, no soap spectrum was detected. In residues that had been held at 200 °C for 30 min prior to quenching, distinct zinc stearate peaks were present. In samples formed from zinc oxide and the ester, zinc stearate was detected similarly after reaction at 200 °C for 30 min.

367 The solid residue quenched from 600 °C after reaction of ethyl stearate with zinc powder gave X-ray diffraction patterns for both zinc and zinc oxide. On the basis of a 1:2 metal:ligand complex, the theoretical a m o u n t of manganese required for complete conversion was 8.8 wt.%. Scans were then carried out at manganese concentrations from 4.9 to 19.9 wt.% and Table 2 summarizes the results obtained in terms of A H changes over the three temperature regions. The data are also included for the reactions of the four manganese oxides with the acid. Manganese stearate was present in the residue quenched from reaction at 200 °C as deduced from the presence of Fourier transform IR peaks at 1562, 1467 and 1438 cm -1. The spectrum of stearic acid was still evident in this sample. The manganese soap was present in products quenched from 300 and 400 °C and also in products which had been maintained at 200 °C for 30 min prior to quenching. Stearic acid was detected in all these residues to a noticeably diminishing extent as both the reaction time and the temperature were increased. The solid product from the reaction of the fatty acid with MnO2 at 200 °C for 30 min gave clear IR spectra for both the acid and the manganese soap. Scanning electron micrographs of residues from the reaction of the manganese powder with the acid which were quenched from 200 and 300 °C were similar, and the residues from the reaction at 600 °C of stearic acid and manganese powder gave distinct X-ray patterns for both Mn203 and Mn304. The A H data for the series of experiments with ethyl stearate and the manganese oxides, usually at two concentration levels, are given in Table 3. Figure 3 shows DSC scans of the reactions of the metal and the oxides with the ester and the acid. IR spectra indicated that a trace of manganese stearate was present in the quenched residue from the reaction of the metal powder with the ester up to 200 °C. Sustaining the reaction at 200 °C for 30 min yielded a product which showed no metal soap peaks but which had significant IR activity at 1630 and 3400 cm -1. This was taken to be indicative of the presence of molecular water [4]. The residue formed in the reaction of MnO 2 with ethyl stearate after 30 min at 200 °C also gave peaks characteristic of molecular water. X-ray diffraction measurements on the residues quenched from 600 °C of the reactions of manganese powder with ethyl stearate showed that both manganese and MnO were present.

3.2. Thermogravimetric analysis Preliminary tests were carried out [2] to evaluate and optimize the experimental procedures regarding the effects of sample mixing, buoyancy, heating and flow rates on the form of the various thermogravimetric curves. Figure 4 shows dynamic thermogravimetry (TG) and differential TG (DTG) scans for the reaction of stearic acid with 10.2 wt.% zinc powder in air to about 900 °C. Regions of temperature related to the maxima in the

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DTG curve may be identified which are similar to those n o t e d in the DSC experiments. Isothermal TG scans were determined for t he same system at regular t e m p e r a t u r e intervals from 200 to 500 °C (Fig. 5). The rate data are summarized in Table 4 in terms of the time required to achieve ~ values of 0.25, 0.50 and 0.75, where ~ is the fraction of reactant mass converted to products at each temperature. In a similar fashion, rate data were established for the reaction of the acid with 9.5 wt.% manganese powder. A series of the isothermal scans is r e p r o d u ced in Fig. 6 and the ~ values are recorded in Table 5 for temperatures from 200 to 500 °C.

4. Discussion The recognized m e t h o d s for preparing zinc stearate include the direct reaction o f stearic acid with zinc oxide or h y d r o x i d e [5] and thus it is n o t surprising that the Fourier transform IR spectra show t hat soap form at i on

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film 150 A thick from the metal powder to form water and zinc stearate ( [ CH3(CH2) 16COO ] 2Zn). If the stearate were formed only by this reaction then it can be calculated from the surface area and thickness measurements that the a m o u n t would be in the microgram range even on the assumption of 100% conversion of oxide to soap. This would not be entirely consistent with the significant signal strengths [3] observed in the IR spectrum and indicates that the stearate must also form by additional reactions. One plausible possibility is suggested by the presence of hydrogen in the gaseous products, which can be related to soap formation by the direct replacement reaction of the carboxyl hydrogens with the metal. This suggestion is compatible with

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reaction: o, AHI; x, AH2; ra, AHa; A, AH4; v, AHtota 1. the known propensity of metals to dehydrogenate carboxylic acids while metal oxides promote dehydration (see for example ref. 6). In the zone from about 280 to 375 °(3, zinc stearate was present substantially in the products of the reactions of the acid with both zinc powder and zinc oxide. Significantly, no acid was detected by IR spectroscopy in the products from the oxide experiments although traces of acid were noted in the residues from the reactions with the metal powder. Similar results were obtained with samples from 400 °C with the added feature that soap decomposition appeared to have begun in the oxide experiments. At 600 °(3 no acid or soap was formed in either of the reactions; the principal c o m p o n e n t was zinc oxide with some zinc observed in the residues from the metal powder reactions. Figure 7 illustrates the variation in A H values with the initial concentration of zinc powder for the overall reaction and for each of its stages. The first three stages show little or no dependence of A H on the metal concentration while the near-exponential trend of the overall AH values simply reflects this property in the AH4 curve which corresponds to the highest temperature reaction zone where the exothermicity below 0.01 mg atom of zinc clearly varies with the metal concentration. The reaction of ethyl stearate with 15.5 wt.% zinc powder gave a AH value of --785 kJ mo1-1 from 170 to 485 °C, considerably less than the value of --2005 kJ mo1-1 from 160 to 545 °(3 with 15.1 wt.% Zn in stearic acid. The reaction with the ester showed only three temperature zones compared with four observed with the acid. The major difference in the AH data lay in the region above 400 °C where the value associated with the acid reaction was about seven times greater than that for the ester. In this temperature

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region the solid residue from both reactions was largely composed of zinc and zinc oxide. Table 1 shows that zinc stearate by itself gave a large AH value in the highest temperature region, which probably accounts for the d o m i n a n t contribution to the exothermicity associated with the zinc-stearic acid and zinc-ethyl stearate reactions in this zone. The difference in AH values for the two systems is consistent with a greater concentration of soap formed in the f a t t y acid reaction and with the larger heat evolution associated with the decomposition of any excess stearic acid compared with that contributed by unreacted ester. The differences in thermal behaviour of the acid and ester in the absence of additives can be noted in the DSC scans reproduced in Fig. 8. The variations in A H with increasing concentration of manganese powder for the three reaction stages are shown in Fig. 9. The surface configuration of the manganese powder after compacting for X-ray photoelectron spectroscopy analysis prevented the unambiguous interpretation of the oxide stoichiometry. Elsewhere, however, it has been determined [7] that manganese metal probably supports an inner oxide film approximating in composition to MnO, which changes progressively to Mn304 at the outermost region of the surface. The initial reaction of the acid with manganese powder must then involve the participation of a non-stoichiometric oxide film to yield manganese stearate at 200 °C, and probably within the range 135 - 275 °C. AHtota 1 for the reaction with Mn3Oa was --2040 kJ mo1-1, at 9 10 wt.% as metal, compared with --3320 kJ mo1-1 for the metal powder, which probably supports an outermost oxide of composition approaching

375 M n 3 0 4. The acid reaction with MnO2 was slightly more exothermic than that with Mn304, while MnO and Mn203 were the least reactive of the manganese oxides. In terms of the earlier proposal [3] that vacant sites of octahedral symmetry were important in the initiation of metal soap formation reactions it may be relevant that the highly reactive MnO2 has an 06 octahedral arrangement and Mn304 (hausmannite) a distorted spinel structure [8]. MnO is normally a very stable and insulating oxide, consistent with the low reactivity towards soap formation. Anion vacancies in octahedral positions would not be numerous unlike Mn203 which normally contains missing oxygen atoms in a distorted cubic crystal [9]. The low reactivity of Mn203 was difficult to reconcile in terms of the trends shown by other oxides but its comparative inactivity was also observed in the sequence of reactions with ethyl stearate. Variations in A H values, exhibited by a related series of materials in their reactions with stearic acid, p r o m p t attempts to relate any trends to the known physical and chemical properties of the participating reactants. From established data for aluminium and its principal alloying elements [2, 3, 10] an interesting relationship was noted between the first ionization potential of the element and the overall reaction enthalpy at 10 wt.% metal concentration. This value could reasonably be taken to lie in the "levelled-out" region for A H with all additives (Fig. 10). The low value of enthalpy for aluminium which appears not to form a complex stearate under these experimental conditions [2] contrasts with the high values generated by manganese and iron. The linear pattern from aluminium to silicon and then to manganese is particularly striking. Only zinc appears to negate the model. The products from the first reaction stage in the ethyl stearatemanganese powder reactions showed only a hint of an IR spectrum attrib-

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376 utable to manganese stearate after the sample had been quenched from 200 °C. Sustaining the reaction for 30 min at this t em perat ure gave a p r o d u c t which showed no soap spectrum but did display IR signals at about 1630 and 3400 cm -1 which were consistent with the presence of considerable amounts of molecular water. The contrasts in the spectroscopic and t h e r m o d y n a m i c results observed in the respective reactions of the manganese systems with the acid and its ethyl ester are in accordance with the widely held view that, in boundary lubrication processes, f a t t y acids probably attach to metal or oxide surfaces via the carboxyl group [11]. This chemisorption m ay lead to the form at i on of identifiable soaps as observed, provided that the metal and its oxide film are sufficiently reactive. The energies involved in the ester reactions with the manganese systems were only a b o u t 15% - 20% of t hat for the acid as judged by the AH'values (Table 3) and thus much weaker adsorption forces would result probably as a main consequence of the blocking action of the ethyl group. A metal soap is thus unlikely to be formed. Comparison of the experimental reaction rate data in Tables 4 and 5 shows that a given ~ value for each isotherm was achieved in less time with manganese than with zinc. There was no fixed mathematical relationship between the two sets on close analysis of all results but, for the limits from = 0.25 at 200 °C to a = 0.75 at 500 °C, the time ratio given by ~Zn/~Mn approached a value o f 2 as it did also for the intermediate state, u = 0.5 at 300 °C. However, t he ratio lay between 1 and 1.2 for the 225 °C isotherm. The range in the value of the ratio is regarded as a function of activity and selectivity factors inherent in the modes of action of zinc and manganese. Several attempts were made to fit the thermogravimetric rate data to established kinetic expressions bot h classical and empirical [12] but no clear and consistent relationship was f o u n d to describe the reaction of the acid with either zinc or manganese over the whole temperature range of the experiments. This was believed to be caused by an inability to separate the contributions to the rates of both simultaneous and overlapping reactions which would be expect ed to occur by both heterogeneous and hom ogeneous mechanisms. The only stage in the reactions which might reasonably be interpreted from kinetic trends was the initiation portion within the range 200 - 300 °C. Tests of these rate results in a range of kinetic expressions showed that the relationship

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16

18

Fig. 11. Semilogarithmic Prout-Tompkins plots and Arrhenius plot (inset) of the reaction of stearic acid with zinc. Fig. 12. Semilogarithmic Prout-Tompkins plots and Arrhenius plot (inset) of the reaction of stearic acid with manganese.

figures. For the reaction of zinc with the acid, the plot gave an experimental activation energy of 85 kJ tool -1, In A was 18.3 and the correlation coefficient was 0.965. The corresponding results for the manganese reaction were 78 kJ mo1-1, 17.0 and 0.965.

References 1 D. H. Buckley, Surface Effects in Adhesion, Friction, Wear and Lubrication, Elsevier, New York, 1981, p. 553. 2 R. A. Ross and A. M. Takacs, Ind. Eng. Chem., Prod. Res. Dev., 22 (1983) 280. 3 R. A. Ross and A. M. Takacs, Surf. Technol., 20 (1983) 219. 4 G. Wirzing, Naturwissenschaften, 50 (1963) 466. 5 E. S. Lower, Pigm. Resin Technol., 11 (1982) 9. 6 R. T. Morrison and R. N. Boyd, Organic Chemistry, Allyn and Bacon, Boston, MA, 1967, p. 589. 7 F. Bouillon, C. Deville and M. Jardinier-Offergeld, C.R. Acad. Sci., 252 (1961) 3986. 8 J. P. Suchet, Crystal Chemistry and Semiconduction in Transition Metal Binary Compounds, Academic Press, New York, 1971, p. 123. 9 A. F. Wells, Structural Inorganic Chemistry, Oxford University Press, Oxford, 1967, p. 472. 10 R. A. Ross and A. M. Takacs, unpublished work, 1982. 11 J.J. O'Connor and J. Boyd, Standard Handbook o f Lubrication Engineering, McGrawHill, New York, 1968, pp. 2 - 11. 12 N. Henry and R. A. Ross, J. Chem. Soc., (1962) 4265. 13 E. G. Prout and F. C. Tompkins, Trans. Faraday Soc., 42 (1946) 482.