Ind. Eng. Chem. Res. 2000, 39, 2837-2842
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Reaction Pathways and Kinetics in the Degradation of Forging Lubricants Sreekumar Natarajan,† Walter W. Olson,‡ and Martin A. Abraham* Departments of Chemical and Environmental Engineering and Mechanical, Industrial, and Manufacturing Engineering, The University of Toledo, Toledo Ohio 43606
The stability of a hot forging lubricant determines its effectiveness during operation. Aqueous lubricants, which are increasingly popular because of environmental issues, contain an organic ingredient, a stabilizer, and other species including biocides and corrosion inhibitors. The stability of a model forging lubricant containing dipotassium o-phthalate and sodium silicate in water has been measured over a wide range of temperatures and initial concentrations. The oxidation of the organic species is correlated using kinetic analysis, which reveals that decomposition occurs through a first-order reaction with an activation energy of ≈58.2 kJ/mol. The kinetic result is consistent with a proposed free radical reaction mechanism that is also shown to be consistent with theoretical calculations and results reported for other types of lubricants. Introduction One of the most basic manufacturing operations in industry is the forging process. Forging can be defined as the plastic deformation of a bulk material into a desired shape. The process involves placing the heated metal between the two halves of a die and forcing the die together by impact force, hydraulic force, or other mechanical forces. This results in a metal of desired shape and excellent quality. One of the important components used in the forging operation is the lubricant, which plays an active role in controlling the rate of deformation and in protecting the die. It is highly necessary that this lubricant be very effective in reducing wear and friction between the surfaces. It must also be seen that the lubricant does not corrode the surfaces or the dies. Graphite lubricants in which water or oils carry the graphite to the active surfaces have been widely used. Oil-based lubricants are generally found to be very effective at high temperatures and pressures where forging is normally used to form metals into a desired complex shape. However, graphite- and oil-based lubricants have undesirable environmental consequences that have led to the use of water-based lubricants. An additional advantage in using water-based lubricants is that the die can be cooled by evaporation and no separate cooling process is required. The stability of forging lubricants is an important criterion in the choice of lubricants used for the process, as it determines the useful life of the fluid. Thermally induced oxidation is one of the primary degradation processes occurring during forging operation. Thermal oxidative stability has also been found to be a major requirement in many applications because of the extreme temperatures involved in all these type of operations. As the lubricant degrades, the friction-reducing * To whom correspondence should be addressed. Tel.: 419530-8092. Fax: 419-530-8086. E-mail: martin.abraham@ utoledo.edu. † Department of Chemical and Environmental Engineering. ‡ Department of Mechanical, Industrial, and Manufacturing Engineering.
properties are diminished, limiting the effectiveness of the lubricant. A typical water-based lubricant consists of the following: (1) the active ingredient (0.5%-20%); (2) enhancers, which are optional and are used to assist in wetting the surface of the dies (1%-2%); (3) stabilizers, such as sodium molybdate or sodium silicate,1 that are effective in increasing the lifetime of the lubricant by reducing the rate of degradation of the lubricant (1%5%); and (4) corrosion inhibitors and biocides (0.5%1%). Bertell2 disclosed an aqueous lubricant composition that is typically used for hot forging operations. The active ingredient includes water and phthalate salts made by reacting phthalic acids with alkali metal hydroxides. The working range composition of the ingredient was suggested to be between 0.5% and 20%. Although numerous experimental programs have conducted lubrication degradation studies over the years, none have focused on forging lubricants. Several groups have determined that pentaerythritol esters3,4 thermally degrade to form low-molecular-weight acids at moderate temperatures. Degradation occurs through a combination of physical and chemical steps including oxidation, hydrolysis, thermolysis, and evaporation.5,6 Stabilizers, including sodium stearate,7 zinc dialkyl dithiophosphate,8 and sodium silicate or molybdate,1 have been effective inhibitors of lubricant degradation. These materials act by modifying the degradation pathways. Combinations of inhibitors, including the addition of alkali metal salts9,10 with traditional antioxidants, provide a synergistic effect, providing increased stability for the lubricant. Recently, a free radical mechanism was developed to describe the degradation of organic lubricants.11 The overall reaction was described by a series of elementary steps such as
Initiation 2RH + O2 f 2R• + H2O2 ROOH f RO• + •OH
10.1021/ie9909237 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/22/2000
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Propagation R• + O2 f ROO• ROO• + RH f ROOH + R• Termination 2R• f R-R R• + ROO• f ROOR 2ROO• f ROOR + O2 Using this proposed mechanism, the kinetics can be worked out to provide
d[ROOH] ) kp(ki/kt)1/2[RH][ROOH]1/2 dt The rate constants can be evaluated from experimental data by determining the concentrations of the respective species and plotting it as a function of time. Similar free radical mechanisms have been proposed for other organic lubricants.6,8 Experimental Section Small batch reactors have been used to simulate the thermal degradation of the forging fluid. The reactors were constructed from a 3/8-in. o.d. 316 SS port connector closed at both ends with 3/8-in. caps and provided an internal volume of ≈1.3 mL. There was no provision for the addition of gaseous reactants; rather, the reactor was filled with the desired volume of the liquid reactant and ambient air and then was sealed. The reactors were placed in a high-temperature furnace capable of achieving temperatures up to 1200 °C. The furnace temperature was varied by a controller with a temperature resolution of 1 °C starting at a minimum temperature of 200 °C. The reactors were agitated continuously to minimize the mass-transfer resistance and ensure that intrinsic kinetics was measured. A limited number of high-temperature (short residence time) experiments were performed in a flow system. The system consisted of two closed containers, two pumps with minimum and maximum flow rates in the desired range, the flow reactor, constructed from 1/8in. o.d. stainless steel tubing, K-type thermocouples, cooling and sample-collection vessels, and the hightemperature furnace with a controller. The reactant liquid was placed in one of the containers while the other container was filled with hydrogen peroxide solution of the desired concentration. More details for each reactor system can be found in Natarajan.12 The reactant, dipotassium o-phthalate (DPO), and the reaction products were separated and analyzed using a Shimadzu, Class VP high-performance liquid chromatograph (HPLC). The HPLC was equipped with a photodiode array detector. The mobile phase was a combination of water:methanol:phosphoric acid in the ratio of 60:40:0.5 with a flow rate of 0.5 mL/min and the column temperature was maintained at ambient conditions. A 25-cm × 4.6-mm × 5-µm Microsorb C-18 reverse-phase column was used for the analysis. Minor products were identified using a HewlettPackard, model G-1800B GC mass spectrometer, equipped with an electron ionization detector (EID). The samples were initially extracted with dichloromethane
Figure 1. Conversion of DPO and yield of products as a function of reaction temperature. Initial conditions: CDPO ) 0.041 mol/L; T ) 400 °C. Table 1. Summary of Experimental Conditions temperature (K)
DPO concn. (wt %)
491 508 523 673 1173
0.5, 1 0.5, 1 0.5, 1, 8, 15 1, 15 1
stabilizer (wt %)
oxidant
reactor system
0.5, 1 1 1
air air air air, H2O2 H2O2
batch batch batch batch flow
to remove water and then passed through alumina to further prevent water-absorbing compounds from entering the GC column. The carrier gas used was helium and an HP-1 25-m × 0.2-mm × 0.33-µm cross-linked methyl siloxane column was used. The oven temperature was maintained at 50 °C for 3 min and then raised to 250 °C at a rate of 7 °C/min. A few experiments were done with small-scale batch reactors fitted with valves to analyze gas products. The valve was opened at the end of the reaction and the gasphase samples were sent to a Hewlett-Packard model 5890 gas chromatograph for analysis. The GC was equipped with a thermal conductivity detector (TCD) and helium was used as the carrier gas with a flow rate of 25 mL/min. An Alltech 6-ft × 1/8-in. × 0.085-in. Porapak Q column was used for the analysis. The inhibitor chosen for the work was sodium silicate as it is a commonly used industrial stabilizer. The inhibitor was analyzed with a wet chemical analyzer that converted the soluble sodium silicate into an insoluble quinoline molybdosilicate through reaction with 8-hydroxyquinoline in hydrochloric acid solution. Results A series of experiments was performed to quantify the oxidative decomposition of DPO as a function of temperature. The effect of temperature and initial DPO concentration on the rate of degradation was investigated in terms of conversion of DPO and yields of products obtained. The role of the stabilizer was measured and evaluated as a function of the stabilizer concentration. The experiments were performed in a batch system and extrapolation to elevated temperatures was confirmed with additional experiments on the flow system. The experimental conditions are summarized in Table 1. Figure 1 shows typical results for conversion of DPO and yields of products benzene, acetic acid, and carbon dioxide, respectively. Benzene and acetic acid were identified on the HPLC by the addition of a small
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Figure 2. Analysis of observed products from DPO decomposition for development of reaction pathways.
amount of pure compounds to the original sample. The area of the peaks already existing increased, indicating that benzene and acetic acid were present as products. This experiment was performed with an initial DPO concentration of 1 wt % and a reaction temperature of 400 °C. It can be seen that nearly 90% conversion was obtained at a residence time of 30 min. At the same time, the yields of benzene, acetic acid, and carbon dioxide are 0.87, 0.76, and 0.25, respectively. This corresponds to a material balance of 0.98, indicating the good quality of the results obtained. Multiple experiments were done at the same experimental conditions. These experiments, which were replicates, were done independent of each other. The data points in Figure 1 are the average of the replicates and the error bars indicate the range of values of these measurements within which the values lie. Delplot analysis13 was used as the basis to determine the primary products. Because a primary product is formed directly from the reactant, it can be shown that the yield of the product divided by the conversion should have a finite value in the limit of zero conversion. Moreover, the value of the intercept represents the number of moles of the product that is formed from the conversion of each mole of reactant. Thus, a plot of yield/ conversion versus conversion would give a nonzero intercept for a primary product, whereas a zero intercept is obtained for higher order products. Figure 2 confirms that benzene and acetic acid are formed as primary products and that a single mole of each of these products is formed from the decomposition of 1 mol of DPO. Figure 2 also indicates that (Y/X) for both primary products decreases below 1 at higher conversions, whereas the value for CO2 appears to increase. Thus, we conclude that the reaction pathway for the decomposition of DPO to complete oxidation products can be written as
DPO f benzene + acetic acid f CO2 Pseudo-first-order rate constants were evaluated for each data set obtained at a fixed initial DPO concentration and reaction temperature. The values are tabulated in Table 2, along with the 95% confidence intervals. Confidence intervals are generally around 1% of the absolute values, indicating that the simple first-order model adequately represents the experimental data. The effect of initial concentration is indicated in Figure 3; the solid curves represent the first-order model fit with the values of the rate constants reported in Table 2. Clearly, the first-order model fits the data reasonably well at each initial concentration. However, DPO conversion increases with increasing initial con-
Figure 3. Conversion of DPO as a function of initial DPO loading at a constant temperature of 250 °C. The curves are predictions of the first-order rate model using the constants reported in Table 2. Table 2. Summary of First-Order Rate Constants for DPO Decomposition oxidizing DPO concn. temperature source system (wt %) (K) air air air air air air air air air H2O2 H2O2 H2O2
batch batch batch batch batch batch batch batch batch batch batch flow
0.5 1 0.5 1 0.5 1 8 15 1 8 8 1
491 491 508 508 523 523 523 523 673 523 673 1173
rate constant (h-1) 0.0831 ( 0.0014 0.1357 ( 0.0287 0.1443 ( 0.0096 0.1937 ( 0.0204 0.1439 ( 0.0111 0.3042 ( 0.0078 0.3849 ( 0.0127 0.4185 ( 0.0093 3.633 ( 1.021 0.3762 ( 0.0138 4.028 ( 1.150 548.0 ( 90.3
centration, suggesting that this reaction is not truly first order. A more precise estimate of the reaction order can be obtained by separation of the concentration dependence into two terms:
dCA ) kCn-1 AO CA ) k′CA dt
(1)
In this analysis, the pseudo-first-order rate constant can be evaluated as
k′ ) kCn-1 AO
(2)
Thus, a logarithmic analysis of the pseudo-first-order rate constant as a function of initial concentration should give a straight line, the slope being n - 1 and the intercept the true rate constant. Completion of this analysis reveals a reaction order in DPO of 1.25 ( 0.44. Although this value is greater than 1, the confidence limits indicate that this result is not statistically different from unity. Thus, additional modeling was completed, assuming that the reaction obeys first-order kinetics. The effect of temperature on the reaction was investigated by evaluation of DPO conversion as a function of time at temperatures between 218 and 900 °C. The experiments at 900 °C were performed in a flow reactor in which the residence time was varied up to 5 s. In this case, hydrogen peroxide was introduced as the oxidant. Previous experiments at 250 °C in the batch reactor revealed that the reaction behaved identically using either air or hydrogen peroxide (compare values in Table 2), justifying our choice of oxidant at the elevated temperatures. The temporal conversion data were fit to the first-order kinetic model, and these rate
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Figure 4. Evaluation of the effect of temperature on the firstorder rate constant for DPO oxidation at an initial concentration of 0.041 mol/L. Table 3. Effect of Sodium Silicate on First-Order Decomposition Rate Constants temperature (K)
DPO concn. (wt %)
silicate concn. (wt %)
rate constant (h-1)
523 523 523 523 523 523 673 673 1173 1173
1 1 1 1 8 8 1 1 1 1
0 0.5 1 2 0 1 0 1 0 1
0.3042 ( 0.0078 0.1994 ( 0.0258 0.1201 ( 0.0404 0.1027 ( 0.0431 0.3849 ( 0.0127 0.1337 ( 0.0505 3.633 ( 1.021 1.695 ( 0.143 548.0 ( 1.15 29.83 ( 3.70
constants were evaluated versus an Arrhenius fit, as indicated in Figure 4. The data yielded a straight line over the entire temperature range and provided an estimate of the activation energy as 58.0 ( 7.4 kJ/mol. This value is within the range of those previously reported for the oxidation of similar types of compounds.14,15 Sodium silicate was chosen as the oxidation inhibitor for this investigation. A brief investigation of sodium silicate decomposition at 400 °C revealed
