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Ind. Eng. Chem. Prod. Res. Dev. 1983,
22, 280-286
Heterogeneous Reactions of Aluminum and Copper Surfaces with Stearic Acid Robert A. Ross' and Anlko Takacs A l a n International Limited, Kingston Laboratories, Kingston, Ontario, Canada K7L 424
Reactions of stearic acid with the contacting surfaces presented by underlying copper and aluminum metals as balance pans and fine powders have been examined by thermal measurements of mass and enthalpy changes in air and in argon. These data have been interpreted to include dehydrogenation, decarboxylation, and dehydration steps with supportlng evMence gathered from evolved gas analysis by gas chromatography and from solid structural analysis by X-ray diffraction, electron diffraction, Fourier transform infrared spectroscopy, and electron microscopy. Although no metal soaps were detected in aluminum/stearic acid reactions, it seems likely that a copper stearate complex was formed in air at 200 O C . The material absorbed at 1590 cm-' but did not exhibit a stable crystalline configuration in diffraction experiments.
Introduction The applications of stearic acid in surface processes range from its use as an adsorbate in surface area measurements on silicas (Yarnto, 1970) to an antiwear additive in lubricating oils (Wills, 1980). In practically all of these systems the acid is maintained in a solvent-type environment which may be regarded as a competing component regarding any adsorption/interaction effects with solid surfaces. Thus for surface area evaluations the acid is in alcohol solution and for lubrication purposes it is usually present in mineral oils. Such heterogeneous compositions can lead to several types of acidlmetal or acidlmetal oxide reactions of a complex nature. In adhesion and boundary lubrication, stearic acid is believed to interact with the contacting solid surface to form a protective coating (Buckley, 1981) and it has been suggested that metal soaps were formed a t the surfaces of both copper and aluminum from studies of their workhardening rates as affected by stearic acid concentrations in paraffin oils (Kramer, 1961). However, when aluminum or copper stearates were added directly to the oil no change was noted in the rate of work-hardening of the metals. In a surface chemical context, many common questions arise from these various applications which are concerned with the nature of the species formed and with the mechanism and rates of their formation. In the present work an attempt has been made to help clarify these questions by reducing the components of the system to stearic acid with aluminum or copper and examining the reactions from ambient to 700 OC by thermal measurements of mass and enthalpy changes supplemented with product analysis data obtained by gas chromatographic, spectroscopic, micrographic, and diffraction techniques. Experimental Section Materials. Stearic, octadecanoic, acid was specially pure grade (B.D.H. >99%) with a particle size range of about 0.2 to 2 111111,long axis, and 0.05 to 0.6 mm, short axis. The samples of copper and aluminum stearates for comparative structural analyses were at the highest purities commercially available. Copper stearates were around 95% (Pfaltz and Bauer Inc.) and 90%, (Chemical Services Inc.), while aluminum hydroxystearate (K and K Laboratories) was 98% and aluminum stearate (Fischer Scientific Co., Ltd.) was a technical grade. The copper and platinum thermal balance pans (Du Pont) were 99.999% pure, aluminum was AA-1145 alloy, >99% Al, quartz 99.997% pure (Warden Quartz Products)
Table I. Enthalpies of Reactions, A H , , from Room Temperature to 700 "C for Stearic Acid in Copper and Aluminum Pans for Both Air and Argon Flowing at 30 mL min-' (NTP) AH,,
atm
kJ mol-'
aluminum aluminum copper
air argon air
-950 120 -2400
(fusion
argon
pan material
-60a)
Literature value 58 (Raznjevic, 1976).
and copper powder (Fisher) 99.6% pure with zinc, 0.25%, and iron, 0.056%, as the principal impurities determined by atomic absorption spectroscopy. The oxide film thickness prior to reaction as determined by 5 KeV argon etching was 42 A on the aluminum surface and from 40 to 50 A on the copper pans. Profilometer readings with a modified Tencor Alpha Step instrument showed that the areas of both aluminum and copper pans were very close to their apparent values, copper to 0.5% and aluminum to 0.3%. Slight surface bowing was noted with both materials, probably related to the pan cupping process. Profiles were digitized at a 1-pm sampling length interval. The copper powder had a surface area of 0.24 m2 g-' as determined by nitrogen adsorption and particle size range limits from around 10 to 100 pm as shown by scanning electron microscopy. Apparatus and Procedure. The principal operating instrument was the Du Pont 900 Thermal Analyzer in both TGA and DSC modes. A Varian 6000 gas chromatograph equipped with flame ionization and thermal conductivity detectors was used for effluent gas analysis with a 2 m X 3 mm separating column packed with Poropak N. Infrared analysis, KBr disk, was by a Nicolet 7199 FT-IR singlebeam instrument; electron diffraction patterns and scanning electron micrographs were obtained with Philips 400 TEM and 500 SEM instruments and X-ray analysis conducted with both Guinier and Philips diffractometers using Cu Kcu radiation. Results Differential Scanning Calorimetry. Runs were carried out in dynamic conditions at 10 "C min-' in both air and argon at 30 mL min-' (NTP). Scans for the decomposition of the acid held in copper pans are shown in Figure 1for both atmospheres. Similar data were obtained for aluminum pans. The overall enthalpies of reaction
0196-4321/83/1222-0280$01.50/00 1983 American Chemical Society
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Table 111. Summary of Dynamic TG Results for Stearic Acid in Air and Argon, 30 mL min-' (NTP), with Platinum, Aluminum, and Quartz Balance Pans balance Pan quartz platinum aluminum platinum aluminum
dH
100
ambient atm air air air argon argon
'
200
400
300
Ti, "C 180 195 185 170 170
0
TC ~ ,
Tp,,, C 275 293 295 315 290
285 310 310 310 305
°
1
500
T (OC)
Figure 1. Dynamic DSC scans of stearic acid in copper pans in (a) air and (b) argon.
\
75
3
dw dl
Weight
(%I
100
0
2
300
200
50
400
500
600
T('C1
Figure 4. Dynamic TG and DTG scam of stearic acid in copper pan in air, 30 mL min-' (NTP).
I
25'
T(OC)
Figure 2. Dynamic TG and DTG scans of stearic acid in aluminum pan in argon, 30 mL min-' (NTP).
Weight (%)
75
-
50
-
dw dt
TIME (min.)
Figure 5. Isothermal TG and DTG scans of stearic acid in aluminum pan at 300 "C in air.
25 -
0
100
I
I
I
200
300
400
500
TIOCI
Figure 3. Dyanmic TG and DTG scans of stearic acid in aluminum pan in air, 30 mL min-' (NTP).
50
t
\
Table 11. Enthalpy Data for Decomposition of Stearic Acid by Copper Powder in Air a t 30 mL min-' (NTP)
i2
-w
AH % Cu,
w/w 5.3
sample wt, mg
temp region, "C
cal mg-'
4.0
155-350 350-5 1 0 155-350 350-510 155-350 350-510
-390 -880 -470 -1170 -580 -1500
9.8
4.6
19.5
5.0
AH,,
(total), kJ mol-' -1500 -1950 -2470
computed from the scans (Boersma, 1955) are given in Table I. Experiments with copper powder were carried out in air at metal concentration levels of around 5,10, and 20% by weight. The results are summarized in Table 11. Thermogravimetric Analysis. Those instrumental experimental parameters which could influence reaction rate values were evaluated, including buoyancy effects, heating, and flow rates. As a result, all samples were compared at a heating rate of 10 "C min-' and a flow rate of 30 mL min-' (NTP). Figure 2 shows dynamic TG scans on stearic acid in aluminum pans in flowing argon. The
0
2
4
6
S 1 TIME lmin.)
0
1
2
Figure 6. Isothermal TG and DTG scans of stearic acid in aluminum pan at 300 OC in argon.
differential of the curve (DTG) is included in the figure. Data for similar measurements in air are shown in Figure 3. The results of dynamic TG experiments in which both the ambient atmosphere and the balance pan material were varied are summarized in Table 111. The key features of the thermograms have been identified as Ti, T-, and T,, which represent, respectively, the temperatures at which the initial, maximum, and termination rates were first detected. The runs in copper pans could not be analyzed in this way. No distinct enough transitions were observed (Figure 4). This figure also illustrates that chemical reaction between the copper and fatty acid has occurred. In order to evaluate reaction rate constants and the effect of temperature on the reaction rate, isothermal
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 5-
300OC
4 -
00 3 2 -
-0 4 -
I -
Ink
-
-0 8
0 -
-I -
200oc
-2 -
0
08
04 log
12
-3
-
-4
-
wo
Figure 7. Van't Hoff plots for the stearic acid reaction in aluminum pans in argon, method 1,calculation of rates based on initial sample weight, w P
0.0013
0.0015
0.0017
0.0019
0.0021
T (K-I)
Figure 9. Arrhenius plots for stearic acid reaction with aluminum in argon, methods 1 and 2.
Table IV. Kinetic Parameters for the Thermal Reaction of Stearic Acid on Aluminum Pans from 200 to 400 "C in Argon a t 30 mL min-' (NTP) temp, "C
log k
logA
E,, kJ mol-'
1
200 250 300 400
-1.77 -1.29 0.021 1.61
9.8
107
2
200 250 300 400
-1.70 -1.14 0.061 1.58
9.5
103
method
Table V. Kinetic Parameters for the Thermal Reaction of Stearic Acid on Aluminum Pans from 200 to 400 " C in Air at 30 mL min-' (NTP) 0
08
04 109
12
Figure 8. Van't Hoff plots for the stearic acid reaction in aluminum pans in argon, method 2, calculation of rates based on sample weight at time t , ut.
measurements were conducted with aluminum pans in air at 200,225,250,275,300, and 400 O C and in argon at 200, 250, 300, and 400 "C. Figures 5 and 6 show both the weight-loss and the differential weight-loss curves determined at 300 "C for air and argon. The acid decomposed at a more rapid rate in air than in argon. Before reaction rate constants and related kinetic parameters from experimental data were evaluated, correlation tests were carried out on the applicability of established kinetic expressions used elsewhere to describe similar heterogeneous reactions (Henry and Ross, 1962). The simplest relationship was that based on a standard rate law log [ - d ~ / d t ] = log It
+ n log w
te0mp, C
loa k
loaA
E,, kJ mol-'
1
200 225 25 0 27 5 300 400
-2.15 -1.48 -0.701 -0.324 -0.703 2.14
12.6
134
2
200 225 250 27 5 300 400
-2.05 -1.38 -0.634 0.239 0.665 2.04
11.9
126
method
w,
(1)
Thus a plot of log (rate) against log w should give n, the reaction "order", from the slope and log 8, where It is the reaction rate constant, from the intercept. Two methods of calculating rates were used. In one, the rate of reaction at time t was based on the change of the initial sample
weight, and in the other, the rate was related to a small increment of change around the sample weight at time t-essentially the differential of the TG curve. Figures 7 and 8 show the results obtained from these Van't Hoff or "conventional" plots using the alternate methods of rate derivation. The influence of temperature on the reaction rate in argon is shown by the Arrhenius plot in Figure 9. The activation energy and frequency factor for the reaction were derived from these lines in the usual way. Table IV shows the principal kinetic data for the decomposition of stearic acid in argon with aluminum pans using both methods for rate calculations. The principal
I d . Eng. Cham. Rod. Res. Dev.. Vol. 22. No. 2. 1983 283
+ In-') Figure 10. Arrheniua plots for the reaction of atearic acid with copper in air.
kinetic results for the reaction in air are summarized in Table V. The rate order varied with temperature from 0.5 at 200 OC to 1at 250 "C to 0 a t 400 OC with respect to the acid in argon and from 1at 200 OC to 0.5 a t 250 'C to -0.5 a t 400 "C in air. Other, more complex, mathematical expresaions were observed to describe the present rate results, notably the Prout-Tompkins law which was derived initially to explain the thermal decomposition of potassium permanganate (Prout and Tompkins, 1946) and subsequently generalid to include a number of decomposition reactions which yield both solid and gaseous products. The variation in the reaction rate with temperature for stearic acid on copper pan surfaces indicated that two principal processes could be distinguished, one from -200 to 275 "C and the other from 325 to 450 "C. These rate data were also analyzed for best fit relationships and eq 1 gave log k values with correlation coefficients in Arrhenius plots of 0.99,200, -300 OC, and 0.98,325-450 OC, Figure 10. From the slopes, experimental activation energies of 169 and 35 !dmol-' were determined, respectively, for the low and high temperature ranges with allied log A values of 16.4 and 23.0. Again, the value of n varied, and within both temperature ranges,but strikingly, it was almost one a t 200 to 225 'C and closely approached zero from 350 to 450 OC, thus suggesting a fvstorder reaction with respect to stearic acid a t the lowest temperatures and zero order a t high temperatures. Product Analysis. Gaseous product analysis of the reaction between the acid and aluminum surfaces in air showed large amounts of carbon dioxide and smaller quantities of carbon monoxide and water at 200 "C. The same gases were the main products at 300 and 400 OC with detectable amounts of methane, ethane, and hydrogen aLS0 present. A t 300 "C, a similar range was detected by gas chromatography in the effluent from the reaction of 10.9% copper powder in stearic acid in air. The experiments were conducted in a quartz tube reador in a horizontal furnace. Scanning electron micrographs were taken of several copper/stearic acid systems. The reproduction of Figure 11shows the product of reaction of the acid with a copper pan after heating from ambient to 350 "C in 18 min in flowing air a t 30 mL min-' (NTP) followed by an imme-
Figure 11. SEM micrographs of stearic acid in copper pan after heating to 350 'C at 20 "C mi& in air. (Magnification, 125ox; inset, 5000x1
I
diate quench to room temperature. Analysis of the residues of the reactions of the acid with 10% copper powder at 100,200,225,250,300,400,and 500 OC in air was completed largely by X-ray diffractometry and infrared spectroscopy. Samples were heated to each temperature at 10 "C m i d and then quenched to ambient. The 100 and 200 "C samples gave almost identical X-ray patterns indicative of copper, ol-stearic acid and some 0 isomer. A trace of an unknown was noted in the 200 O C sample, a second at 225 "C, and a third at 250 O C . A t 250 OC, the fmt pattem showing copper(1) oxide was obtained. The copper metal pattern persisted at 250,300,400, and 500 "C. Copper(1) oxide was present also a t the highest temperatures while copper(I1) oxide was not evident until 500 "C. An amorphous material was clearly present at 300 OC.
No s i g n i i m t variation in these observations was noted with several selected samples heated in air for an extended period to 110 min prior to quenching. The limits of sensitivity of the procedure range from 1to 5% depending on the nature of the material examined. The powder diffraction file from the International Centre for Crystallographic Data contained no information on copper stearate. None of the unknowns, however, gave the same diffraction pattem as that determined with the two separate samples of the substance obtained from different suppliers. Since small amounts of copper stearate soaps might he formed below the limits of detection of the diffradameter or even present in larger amounts which were X-ray amorphous, a series of infrared scans on selected samples was completed using Fourier transform. These showed marked peaks in the spectra of the standard stearic acid and copper stearate at 1710 and 1590 cm-', respectively. The green solid formed on heating 10% copper powder with the acid in air at 200 "C for 110 min gave a significant soap peak and a lower intensity signal for the acid. Spectra for samples heated to 200 and 225 O C in air at 10 O C m i d
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No 2, 1983
and then quenched showed a less prominent soap peak and a more intense acid one. Electron diffraction was used to examine the product formed between the acid and copper pan surfaces after observing the formation of polygonal crystals at 325 "C. The crystals were very stable in the electron beam and seven good diffraction patterns were obtained which gave d spacings (f4%) of 5.48,4.35, 4.13, 3.10, 2.48, 2.36, and 2.03. These data were close to those published for a-stearic acid (5.65, 4.36, 4.11, 3.66, 2.47, 2.31, 2.05) and not in agreement with either the X-ray (3.83, 4.13,4.22,4.75, 5.18, 6.65, 7.42, and 7.82) or electron diffraction results (2.62, 3.12,4.17,4.26, and 4.81) on the pure copper stearate. The latter crystals resembled flat needles or plates which were extremely sensitive in the electron beam. In diffraction, clear patterns were initially obtained but they faded rapidly within 3-4 s to amorphous haloes because of electron beam damage. Nevertheless, five diffraction patterns were obtained with d values in reasonable agreement with the X-ray results. Less extensive analyses were necessary on the products resulting from stearic acid reactions on aluminum surfaces. No evidence of aluminum stearate soap formation was obtained from either X-ray or infrared analysis. Samples of acid heated in air to 175 "C for 1 h before quenching gave only the cy-stearic acid diffraction pattern as did those heated to 200 "C and immediately quenched. Samples of both aluminum stearate and aluminum hydroxystearate were also analysed to obtain comparison diffraction patterns. Infrared analysis of reaction products at 200, 225, and 250 "C which were air-quenched in the DSC cells showed only the acid peak at 1710 cm-' for the 200 "C sample. The slight shoulder noted around 1740 cm-' at 225 "C became more pronounced at 250 "C. The products were all solids with a color gradation varying from yellow to gold as the temperature was increased. Extended heating at 200 "C for 110 min yielded a liquid-like preparation which also absorbed strongly at 1740 cm-', possibly indicative of a polymeric or associated ketonic system. Solvent extraction experiments were confined to the products formed in the reactions of 10% copper powder with the acid at 175 "C for 120 min and at 200 "C for 45 min. Tetrahydrofuran was a suitable solvent for the nonmetallic phase showing that 5.4 and 5.2% of the original amount of copper remained unreacted at 175 and 200 "C, respectively.
Discussion The mode of attachment of lubricant, adhesive, or protective coating to metal or oxide surfaces can be directly related to the nature of the repulsion/attraction forces at the interface. The magnitude of the forces will depend on the type and condition of the interactive materials. Specifically, for a metal/oil lubricant system important parameters to consider include: (a) the temperature, structure, roughness, form, impurity content, and chemical reactivity of the metal surface, (b) the viscosity, stability, purity, temperature, chemical and surface properties of the oil, and (c) the nature and condition of the ambient atmosphere surrounding the system. The DSC scans show clearly the influence of the ambient atmosphere on the reactivity of aluminum toward stearic acid. Thus from an accumulation of the enthalpies of the reactions taking place between room temperature and 700 O C , a value of -950 kJ mol-' in air contrasts markedly with +119 kJ mol-' in argon. The exothermicity in air occurs in stages related to chemical changes and thermal gradients which ensue across the air/(stearic acid + products) /alu-
minum interfaces. Since these conditions are much steadier in argon-some heat is absorbed by the acid-the air driving force is likely to be associated with a surface sensitive oxidation or oxidative degradation of stearic acid. The calorimetric data gathered from runs in which the acid was placed in an aluminum pan initially relate to the interactions of a plane surface in contact with a liquid film after melting occurred at 69 "C. Changing the metal contact surface in air to copper gave a significant shift in the enthalpy difference from -950 to -2400 kJ mol-'. This indicates a much greater chemical reactivity of copper toward the fatty acid with differences anticipated in both quality and quantity of reaction products from that with aluminum. The results in Table I1 for the reaction enthalpies with powdered samples of copper confirm the conclusions regarding reactivity. Plots of the enthalpy differences for the separate reaction stages against the three concentrations to the first or half power of copper powder, in gram-atoms, were linear over the entire range and a logarithmic plot of the total reaction enthalpy changes vs. concentration gave a straight line with a gradient of 0.5. These and related data might be analyzed for kinetic information (Borchardt and Daniels, 19571, but because of the possible influence of temperature gradients within the reacting systems, the use of isothermal thermogravimetric data was preferred (Sharp and Wentworth, 1969). The effect of contact surface and ambient atmosphere was also noted in the thermogravimetric experiment (Table 111). Subsequent isothermal kinetic measurements with aluminum pans in air and in argon showed an interesting variation in the value of n, the reaction "order", with temperature in both atmospheres. In argon, as the temperature was raised, p changed from 0.5 to 1 to 0 with respect to stearic acid and from 1 to 0.5 to -0.5 in air, over the same temperatures, 200 to 400 "C. The results are consistent with degradation occurring in air a stage ahead of that in argon which did not show the final rate retardation or inhibition effect demonstrated by the negative one half "order" value in air around 400 "C. Typically, reactions which show such variations in n values are complex involving changes in mechanisms or slow steps (Swinbourne, 1971). For the acid in air the falling value of n in the primary stages suggests reaction inhibition perhaps exerted through a poisoning step initiated by a product which would involve its direct participation in reaction or it may be that intervention takes the form of negative catalysis. The excellent fit of the kinetic results for both air and argon to a Prout-Tompkins relationship is consistent with a reaction description involving an initial acceleratory period followed by deceleration in the latter stages (Figure 12). The correlation coefficients of these lines ranged from 0.95 to 0.99. This particular expression has been related previously to reactions which show a variation in the rate of branching with time. The initial stages of decomposition in all systems seem to be. dominated by decarboxylation, consistent with the preponderance of carbon dioxide in the effluent gases and with previous proposals regarding the photochemical decomposition of the bulk acid (Pilpel and Hunter, 1970). Reactions leading to the formation of carbon monoxide and water must also occur, but since hydrogen was not detected until 300 "C, and then in trace quantities only, the possibility of the initial stages involving the intervention of the water-gas shift reaction may be excluded. Reactions of the acid on aluminum produced fewer complexities than with copper. No evidence for metal soap
Ind. Eng. Chem. Prod. Res. o4
log
400T
3OOOC
I -a
200OC
250%
(Y
275O
04
225OC
0
-04
-0 8
-02
02
06
IO
14
18
22
log t
Figure 12. Kinetic results for the aluminum surface reaction with stearic acid in (a) argon and (b) air, plotted according to the equation: log[a/(l - a)] = k log t + constant.
formation was obtained from any of the analytical techniques. Probably reactions proceed by mechanisms related to the catalytic selectivity of the aluminum surface and its oxide film. No kinetic data on the catalytic decomposition of stearic acid were available although activation energies similar to those here have been reported for formic acid decomposition on alumina surfaces (Galwey, 1977). The products show that the overall processes are complex and likely to include contributions from dehydration, dehydrogenation, and dissociation reactions. The individual steps and sequences in these would be expected to have several features in common and also similarities to mechanisms proposed previously for autoxidation (Brodnitz, 1968) and the photochemical decomposition. Thus species such as RCOO, RCO, H, and OH are probable in initiation with the additional possibility of OH- and RCOO- ions forming during a surface-activated process through charge-transfer steps. In this regard, an excellent linear relationship has been noted between the ionization potential of the major aluminum alloying elements and the heats of formation of the surface complexes formed with stearic acid (Ross and Takacs, 1982). Several propagation steps are feasible involving interaction of initial species with stearic acid. Two possible contributors would be RCOOH + OH -* RCOO + H20 (2) RCOOH + H -* RCOO + H2 (3) with (3) becoming increasingly likely at higher temperatures. Entities like RCOO and RCO might dissociate, respectively, through decarboxylationand decarbonylation with the volatile paraffinic hydrocarbons created through dissociative rearrangement of the aliphatic group in termination sequences. The latter stops short of cracking and alkane dehydrogenation since no evidence of the formation of unsaturated moieties was obtained. Reaction rates were significantly faster in air than in argon above 250 "C, inplying either a significant influence exerted on the slow step in the mechanism or possibly the intervention of an added oxidative dehydrogenation reaction which could form H02 and related propagating species. Reactions of stearic acid with copper were more readily defined. Thus the progress of the reaction with temperature was consistent with chemical interactions leading to
Dev., Vol. 22, No. 2, 1983 285
the formation of copper(1) oxide and then copper(I1) oxide accompanied by evolved gases indicative of acid decomposition. The principal uncertainty rested with the possible formation of a metallic soap product or intermediate related to a copper stearate in structure. Strong evidence in favor of soap formation was noted in the infrared experiments which pointed substantially to the presence of a stearate system increasingly with increase in time and temperature of preparation in the vicinity of 200 to 225 OC. The absence of confirmatory evidence from X-ray diffractometry and electron diffraction might be then assumed to depend on the sensitivity of these techniques. In electron diffraction, known samples of copper stearate were noticeably damaged by the electron beam yielding amorphous patterns, and hence it may be that the background in both the X-ray and electron diffraction records of the unknown samples was related to the presence of a noncrystalline form of the copper soap. Significant also in this regard are the observations that no details of a copper(I1) stearate unit cell are available and that this soap forms a deformable meso-phase around 116 OC and an isotropic liquid phase around 120 "C (Burrows and Ellis, 1982). Proton magnetic resonance studies suggest that the hydrocarbon chain in copper(I1) stearate is free to rotate around several axes at 120 OC even though it did not appear to form a clear-blue freely-flowing liquid until 250 OC (Grant, 1964). Calculations of the heats of reactions involved in the formation of copper(1) and copper(I1) stearates and copper(1) and copper(I1) oxides from copper and stearic acid were inconclusive, largely because of a dearth of literature data on the relevant thermodynamic properties of the stearates. Thus the experimental enthalpy values could not be used to assess reaction proposals.
Conclusions Aluminum surfaces do no form detectable amounts of metal soaps in direct contact with stearic acid at temperatures up to 250 "C and times to 110 min in air. A liquid results which absorbs in the infrared at 1740 cm-l and is thought to be a highly associated ketonic system. Copper surfaces are believed to react with stearic acid in air at around 200 "C to form a copper stearate soap complex which absorbs noticeably at 1590 cm-' but which does not show a stable crystalline configuration in either X-ray or electron diffraction experiments. The product gas distribution arising from the acid decomposition is temperature sensitive and includes mainly carbon monoxide, carbon dioxide, and water at 300 "C with the addition of hydrogen and lower boiling alkanes at 400 OC and above. No evidence for the formation of unsaturated fatty acids in air or argon on either surface was obtained by Fourier transform infrared spectroscopy. Although the principal reaction paths for the decomposition of the acid on aluminum surfaces are likely to involve catalysis, for copper both calorimetric and kinetic measurements indicate that considerable direct interaction occurs consistent with structural evidence for increased formation of copper(1) oxide initially, followed by copper(I1) oxide around 500 "C. Kinetic measurements in both air and argon of the stearic acid/aluminum reaction to 700 O C gave data which were consistent with a mathematical description of a decay mechanism showing a variation in the rate of branching with time. All of the data indicate that the nature of the products formed and the mechanism of action of stearic acid in boundary lubrication and related coating applications is
Ind. Eng. Chem. Prod. Res. Dev. 1983,22,286-290
286
considerably influenced by the chemistry and condition of the surface involved. Registry No. Al,7429-90-5; Cu, 7440-50-8; stearic acid, 57-11-4.
Literature Cited Boersma. S. L. J . Am. Ceram. SOC. 1955, 38, 281. Borchardt, H. J.; Daniels, F. J. Am. Chem. SOC. 1957, 7 9 , 41. Brodnltz, M. H. J. Agric. FoodChem. 1968, 16, 994. Buckley, D. H. "Surface Effects in Adhesion, Friction, Wear and Lubrication", Elsevier: New York. 1981; p 542. Burrows, H. D.; Ellis, H. A. Thermochim. Acta 1962, 52. 121. Galwey, A. K. Adv. Catal. 1977, 26, 247. Grant, R. F. Can. J . Chem. 1964, 42, 951. Henry, N.; Ross, R. A. J. Chem. SOC.1962, 4265.
Kramer, I. R. Trans. AIM€ 1961, 221, 989. Pllpel, N.; Hunter, 8. F. J. J. ColloM Interface Sci. 1970, 3 3 , 615. Prout. E. G.; Tompkins, F. C. Trans. Faraday SOC. 1946, 42, 482. Raznjevic, K. "Handbook of Thermodynamic Tables and Charts"; McGrawHill: New York. 1976: p 4. Ross, R. A,; Takacs, A. Unpublished work, Alcan International LimRed, 1982. Sharp, J. H.; Wentworth, S. A. Anal. Chem. 1969, 4 7 , 2060. Swinbourne. E. S. "Analysis of Kinetic Data"; Nelson: London, 1971; Chapter 3. Wills. J. G. "Lubrication Fundamentals"; Marcel Dekker: New York, 1980; p 33. Yarnto, D. Metallurgica 1970, 82, 1959.
Received for review November 23, Accepted January 20,
1982 1983
Influence of Condensed Silica Fume on the Pore-Size Distribution of Concretes Plerre Delage and Pierre-Claude Altcln' D6partement de g5nie civil, Facult5 des sciences appliquws, Universit6 de Sherbrooke, Sherbrooke P.0.J 1K 2R 1 Canada
Condensed silica fume is a quite new pozzolanic material used in concrete. I t is composed of very fine spherical glassy spheres of quite pure silica, a hundred times finer than cement particles. I t is known that condensed silica fume increases the compressive strength and decreases the permeability of concrete in a very spectacular manner. This action has always been related to the pozzolanic reaction. I n this paper it is shown that the action of condensed silica fume results also not only in the closing of most of the pores of the concrete having a diameter between 0.05 and 0.5 pm but also in a decrease of the size of the micropores (r < 0.05 pm). This physical action, in addition to the chemical one previously advanced, can explain more completely the beneficial action of this very reactive pozzolan on the concrete properties.
Introduction Condensed silica fume is a quite new pozzolanic material used in cement and concrete. The addition of a small amount of condensed silica fume, 5 to 15% by weight of the cement, changes drastically most of the properties of the fresh and hardened concrete. The workability of the fresh concrete is generally increased, there is no bleeding, the compressive strength of the hardened concrete is significantly increased, and its permeability is reduced considerably (Loland, 1981; Aitcin and Pinsonneault, 1981b). Those effects are quite similar to those obtained with natural pozzolanic materials or fly ashes, but the compressive strength increase is obtained quite rapidly (14 to 28 days) and the reduction of permeability is more significant (Aitcin and Pinsonneault, 1981a). It has been shown by Buck and Burkes (1981) and Regourd et al. (1981) that from a chemical point of view the particles of condensed silica fume were reacting with the lime liberated during the hydration of the cement to produce a very dense secondary C-S-H. In this paper the effect of the introduction of condensed silica fume on the porosity of concrete will be discussed. Generation of Condensed Silica Fume Condensed silica fume is a byproduct of silicon and ferrosilicon alloy manufacturing. These alloys are produced in an electric arc furnace by reducing quartz by coal (see Figure 1). Iron is added when ferrosilicon is produced. The reduction of quartz to silicon is not direct, and a gazeous suboxide Si0 is produced in the upper part of the furnace. Some of this S i 0 gas is entrained in the furnace hood and condenses into ultrafine vitreous spherical
Table I. Physicochemical Properties of the Condensed Silica Fume Used (Silicon Furnace) chemical compn, %
SiO,
Fe,O,
MgO
Na,O
K,O
C
S
L.O.I.
93.1
0.55
0.56
0.13
1.6
2.1
0.2
3.5
average grain size specific surface area specific gravity bulk density color in bulk color in slurry
0.1 pm
(BET)
2 0 000 m2/kg 2.2 0.2 to 0.3 t/m3
gray black
particles, Figure 2, when in contact with air. As can be seen in Table I, the average diameter of those spheres is 0.1 km, and their specific surface area is about 20000 m2/kg, as compared to 250 to 450 m2/kg for an ordinary portland cement or a fly ash. Condensed silica fume contains from 85 to 95% Si02 depending on the type of silicon alloy which is produced. Its chemical composition is generally very constant because of the high purity of the two main raw materials used in the process (quartz and coal). Its carbon content is generally less than 2 % . The typical chemical analysis of the condensed silica fume used in this project is given in Table I. This condensed silica fume was coming from a silicon furnace. Condensed silica fume has generally a more or less pale grey color according to its carbon and iron content. However, when the electric arc furnace is equipped with a heat recovery system, the leaving gas has a temperature about 800 "C and most of the carbon is burnt. The produced silica fume by that time is white. Without a heat
0196-432118311222-0286$01.50/00 1983 American Chemical Society
