I n d . Eng. Chem. Res. 1992,31,1810-1813
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Literature Cited Bell, R. P.; Evans, P. G. Kinetics of the Dehydration of Methylene (London),Ser A. 1966, Glycol in Aqueous Solution. h o c . R.SOC. 291, 297. Gorrie, T. M.; Raman, S. K.; Rouette, H. K.; Zollinger, H. NMRInvestigation of the Formaldhyde Addition and Oligomerisation Equilibria in the System Formaldehyde/ Water/Acetic Acid/ Hydrochloric Acid. Helv. Chim. Acta 1973,56,(Fasc. 1 (a)),175. Hoq, M. F.; Ernst, W. R.;Gelbaum, L. T.NMR Procedure for Determining Methanol and Formic Acid in Chlorine Dioxide Plant Solutions. TAPPI J. 1991a, 74,217. Hoq, M. F.; Indu, B.; Emst, W. R.;Neumann, H. M. Kinetics of the Reaction of Chlorine with Formic Acid in Aqueous Sulfuric Acid. J. Phys. Chem. 1991b,95,681. Indu, B.;Hoq, M. F.; Ernst, W. R.; Neumann, H. M. Kinetics of the Reaction of Chlorine with Formaldehyde in Aqueous Sulfuric
Acid. Znd. Eng. Chem. Res. 1991,30,1077. Lowry, T. H.; Richardson, K. S. Reactions of Carbonyl Compounds. Mechaniem and Theory in Organic Chemistry; Harper and Row: New York, 1987;pp 661-664. Masechelein, W. J. Industrial Synthesis. Chlorine Dioxide; Ann Arbor Science: Ann Arbor, MI, 1979; p 120. Norell, M. US. Patent No. 4,770,868,1988. Silverstein, R. M.; Baesler, G. C. Nuclear Magnetic Resonance Spectroscopy. Spectrometric Identification of Organic Compounds. Wiley: New York, 1967;p 118. Sjostrom, L.; Tormund, D. Determination of Inorganic Chlorine Compounds and Total Chlorine in Spent Bleaching Liquors.
Sven. Papperstidn. 1978,4,114. Received for review October 25, 1991 Revised manuscript received March 30, 1992 Accepted April 22, 1992
Oxidation and Weight Loss Characteristics of Commercial Phosphate Esters Sundeep G. Shankwalkar* and Douglas G. Placek Process Additives Division, FMC Corporation, P.O. Box 8, Princeton, New Jersey 08543
Neutral phosphate ester compounds find wide application as plasticizers and flame-retardant additives in the polymer industry. They are also used extensively in the lubrication industry as fire-resistant functional fluids and as lubricant additives. In these industrial applications, the oxidative stability and volatility of the phosphate ester is of critical importance. This paper evaluates the oxidative stability and relative volatility of several commercially available phosphate esters. The techniques of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) have been used to evaluate these thermal properties.
Introduction Phosphate esters find use as flame retardants in polymeric and engineeringplastic applications. They are used in place of flammable organic plasticizers when fire-resistance properties are desired. In the lubrication field, phosphate esters are used in high-temperature applications which warrant the use of fire-resistant fluids. Typically, they are used as hydraulic fluids in applications where molten metal, or open flames, preclude the use of hydrocarbon fluids. Phosphate esters are also used in a variety of lubricant formulations as antiwear additives. Any application at a high temperature in the presence of air could affect the thermal and oxidative stability of a phosphate ester. Under these conditions, oxidation predominates thermal decomposition (Gunsel et al., 1988), and these effects along with volatility can severely impact on the performance and useful life of the phosphate ester. Oxidative stability can be characterized at room temperature or under isothermal conditions; however, for repeatability and ease of operation, it should be measured at higher temperatures and under dynamic conditions (Koski and Saarela, 1982). This study evaluates the onset of oxidation and the weight loss characteristia of commercially available triaryl, trialkyl, and alkyl-aryl phosphate ester products. A discussion of the relationship between oxidative stability and structure of the phosphate ester is also included in this paper. Experimental Technique All DSC measurements for determining the onset of oxidation were made using a Mettler DSC 25. In this technique (ASTM E537), the heat exchanged with the sample over a defined temperature range is measured as 0888-5885/92/2631-1810$03.00/0
the difference between the heat flow to the sample and that to the reference cell. This difference in heat flow is recorded as an exothermic or endothermic peak on the DSC scan, which relates to a physical or chemical change taking place in the material. Weight loss measurements were made using a Mettler TG 50 system. In this technique (ASTM D3850), the change in the mass of the sample is measured over a defiied temperature range. The sample weight loss can be related to volatility, or decomposition, taking place in the material. All measurements were made between 30 and 400 OC, at a heating rate of 10 OC/min with the sample enclosed in a aluminum container with a perforated (one hole) lid. Initial testing of a few phosphate ester samples in air and oxygen did not show any significant differences in their thermal profiles. For the purpose of this study, all DSC/TGA measurements were made in oxygen at 50 mL/min. Data acquisition and analysis were carried out using a Mettler TA 72 thermal analysis software system. The data presented in this paper represent the average values obtained after evaluating three to five different commercial samples, of similar composition. Sixteen different types of commercial phosphate esters were selected for testing. They are categorized in Table I. The basic molecular structure of the phosphate esters evaluated in this study are shown in Figure 1. A more detailed review of the composition of commercial phosphate ester products is covered in the work by Marino and Placek (1992).
Results and Discussion A summary of the onset of oxidation and weight loss data, as determined by DSC/TGA measurements in oxygen, is presented in Table 11. 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1811 Table I. Description of Phosphate Ester Compounds phosphate ester IS0 viscosity grade triphenyl (TPP) 32 tricresyl (TCP) trixylyl (TXP) 46 isopropylphenyl (IPPP-22) 22 32 isopropylphenyl (IPPP-32) isopropylphenyl (IPPP-46) 46 isopropylphenyl (IPPP-68) 68 22 tert-butylphenyl (TBPP-22) tert-butylphenyl (TBPP-32) 32 tert-butylphenyl (TBPP-46) 46 tert-butylphenyl (TBPP-100) 100 tributyl (TBP) 2 tris(butoxybthyl) (TBEP) 7 7 trioctyl" (TOP) 2-ethylhexyl diphenyl (EHDPP) isodecyl diphenyl (IDDPP) a
viscosity, 20 O C , solid 80 90 70 90 155 245 75 104 160 448 3.7 12.2 14.1 25.6 19.6
CP
sp gr, 20°/20 1.200 1.170 1.140 1.180 1.165 1.125 1.100 1.200 1.170 1.150 1.120 0.980 1.020 0.922 1.070 1,090
w t % phosphorus
O C
9.5 8.4 7.8 8.3 7.9 7.4 7.1 8.4 8.2 7.8 7.0 11.7 7.8 7.1 9.9 9.1
Tris(2-ethylhexyl). 0
II 0 - P -0-8
m
A
Table 11. Oxidation and Weight Loss of Phosphate Esters:" DSC/TGA Data. 10 "C/min. in Oxygen total weight loss at OC phosphate ester onset of oxidn, " C 1% 5% 10% triaryl PE TPP b 188 236 252 215 TCP 184 255 278 210 TXP 224 268 286 IPPP-22 200 215 239 263 IPPP-32 215 201 252 272 IPPP-46 202 210 265 287 IPPP-68 210 218 265 288 TBPP-22 295 213 262 280 TBPP-32 290 222 268 286 TBPP-46 227 272 292 300 TBPP-100 234 305 277 295 trialkyl PE 175 TBP 76 113 127 155 TBEP 61 169 187 122 160 TOP 189 203 alkyl/aryl PE EHDPP 200 90 220 229 IDDPP 165 93 213 235 ~~
-mm1Pboqbrr
0 II R-0-P-0-R
0
d
Ml-sglPboqbrr
R
= Calil7 +
EtIDPP
C 1 0 H Z 1 4 rnDPP
Figure 1. Phosphate ester types.
Oxidation Characteristics. The data from Table I1 indicate that triaryl phosphate esters exhibit far greater oxidative stability than trialkyl or alkyl-aryl-type phosphate esters. The triaryl phosphate esters (TCP, TXP, IPPP, and TBPP) oxidize at temperatures above 200 "C, whereas the trialkyl phosphate esters (TBP, TBEP, and TOP) begin to oxidize between 150 and 200 "C. The oxidative stability of a phosphate ester can be related to the stability of the hydrocarbon portion of the molecule. Phosphate esters prepared from aromatic alcohols exhibit higher stability than phosphate esters synthesized from aliphatic alcohols. In all cases, the hydrocarbon portion of the molecule degrades to form high molecular weight oxidative intermediates, which further oxidize to form polymeric products. Previous work indicates that the polymerization step is kinetically faster than the primary oxidation reaction (Cho and Klaus, 1979). The alkyl-aryl phosphate esters studied here exhibit oxidative stabilities that are intermediate to triaryl and trialkyl phosphate esters. These results are not surprising, as the compounds contain hydrocarbon chains of mixed stability. Tris(2-ethylhexyl)phosphate shows an onset of oxidation at 160 "C, which is 40 "C lower than 2-ethylhexyl diphenyl phosphate. Both of these compounds are less stable than triphenyl phosphate, which does not oxidize under these test conditions. This trend suggests that, as the ratio of alkyl/aryl groups increases in the phosphate ester molecule, oxidative stability decreases. Isodecyl diphenyl Phosphate (IDDPP) was found to have a lower onset of oxidation temperature, relative to 2-
~
" Average values of commercial samples from several manufacturers. *Does not oxidize under these conditions. ethylhexyl diphenyl phosphate (EHDPP). This can be expected, as the longer chained isodecyl groups are less oxidatively stable than the shorter ethylhexyl groups. tertButylpheny1 phosphate esters (TBPP) show greater oxidative stability than isopropylphenyl phosphate esters. The series of commercial IPPP products evaluated begin to oxidize at approximately 210 "C. A typical TBPP ester is significantly more stable, and does not show any signs of oxidation until 295 "C. The superior oxidative stability of TBPP esters is due to the stability of the tert-butyl group on the phenyl ring, which presents an unfavorable configuration for oxidation. Isopropyl and methyl structures attached to the phenyl ring each have a hydrogen present at the a-carbon position, which result in lower oxidation onset values for IPPP, TCP, and TXP products. The presence of a hydrogen atom in an a-carbon position to an aromatic ring facilitates oxidation (Chasan, 1990). Since a hydrogen atom is not present at the a-carbon site in TBPP, oxidation is hindered. DSC data generated under the described conditions indicate that triphenyl phosphate (TPP) does not show any signs of oxidation. The TGA scan demonstrates that the sample shows complete evaporation before oxidation can occur. DSC measurements conducted with a TPP sample in a sealed aluminum pan prevent evaporation and iden-
1812 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 rn I N OXYGEN
1+
Mk.!
10.0 'C/.l"
\
h \ \\
ISPP
TCP
, 100,
,
,
,
, 200.
,
,
,
,
,
,
,
300.
,
,
, I
'C
ioo.
'
.
"
I
'
'
200.
.
'
I
"
"
l
300.
'C
Figure 2. Representative phosphate ester DSC profiles.
Figure 3. Representative phosphate ester TGA profiles.
tified the onset of oxidation to be 360 "C. Under these conditions, the TPP is exposed to a limited oxygen environment. TPP has no hydrocarbon chains attached to ita phenyl rings, and thus is extremely resistant to oxidation. Typical DSC profiles of several phosphate esters in oxygen are shown in Figure 2. In all cases, the oxidation onset value is identified as the temperature at which the curve deviates from the base line in the form of an exotherm. The endotherms seen in the DSC profiles correspond to volatility. The authors wish to point out that oxidation onset temperatures in this study were developed to compare the relative stability of various phosphate esters, and should not be considered as the maximum temperatures of use. However, short-term exposure to high temperatures or a limited oxygen environment allow phosphate eaters to be used at temperatures well above the DSC onset temperatures with little or no decomposition. Weight Loss Characteristics. Phosphate ester weight loss measurements conducted in an oxygen atmosphere could be due to oxidation, or due to volatility. Oxidation is a chemical effect, unlike volatility, which is purely a physical manifestation. If weight loss is seen at a temperature below the onset of oxidation,it would suggest that this weight loss is due to the volatility of the phosphate ester, and not due to oxidation. At temperatures above the onset of oxidation, this weight loss is likely due to both oxidation and evaporation. The focus of this study is not to identify the mechanism of weight loss, but to understand some of the characteristics of weight loss data obtained by TGA measurements. Due to significant variations seen in the 1%weight loss data, a comparison of weight loss characteristics is best made from data obtained at 5% and 10%. The variability in 1% weight loss data could be due to instrument artifacta, but is most likely due to small differences in the isomeric composition of the commercial samples which were used. Beyond lo%, the rate of weight loss increases rapidly, until all the material has evaporated or decomposed. This suggests that 10% weight loss represents a critical point, beyond which the performance of the phosphate ester would be suspect. TGA analysis indicates that the volatility of trialkyl phosphate esters is related to the molecular weight of the species. The trialkyl phosphates studied are each comprised of a single molecular structure and have only one isomeric configuration. Triakyl phosphate esters such as TBP, TBEP, and TOP show weight loss due to volatility at lower temperatures, relative to triaryl phosphate esters. TBP has the lowest molecular weight (266) and shows a 10% weight loss at 127 "C. TBEP and TOP,which have higher molecular weights of 398 and 434, respectively, show
a 10% weight loss at 187 and 203 "C. The triaryl phosphate esters also show a relationship between weight loss and molecular weight. These commercial products of different viscosity grades are manufactured by altering the degree of phenyl ring alkylation. As the viscosity of tert-butylphenyl phosphates (TBPP) and isopropylphenyl phosphates (IPPP)is increased, their average molecular weights also increase, which effectively shows a decrease in volatility. The relative volatility of TXP, TCP, and TPP also follow the molecular weight trend. The weight loss data generated on TBPP indicates that 1% , 5 % , and 10% weight loss are due to volatility alone, and not due to oxidation. The data in Table I1 demonstrate that the onset of oxidation takes place at a higher temperature than 10% weight loss, for each of the TBPP products evaluated. TGA profiles of a few phosphate esters in oxygen is shown in Figure 3.
Conclusions DSC measurements indicate the following order of phosphate ester oxidative stability (from highest to lowest): triphenyl phosphate, tert-butylphenyl phosphates, isopropylphenyl phosphates, tricresyl phosphate, trixylyl phosphate, mixed alkyl-aryl phosphates, trialkyl phosphates. The oxidative stability of these compounds is related to the stability of the hydrocarbon portion of the molecule. TGA measurements indicate that weight loss in phosphate esters is due to a combination of oxidative degradation and volatility. The weight loss characteristics of phosphate esters are a function of their composition and molecular weight. Slight variations in the composition of commercial phosphate ester products can significantly affect the onset of weight loss (1%)in TGA measurements. This study demonstrates the importance of comparing DSC and TGA data in order to obtain a thorough understanding of the thermal properties of phosphate esters. Registry No. TPP, 115-86-6; TCP, 1330-78-5;TXP, 2515523-1;IPPP, 26967-76-0;TBPP, 28777-70-0;TBEP, 78-51-3; IDDPP, 1241-94-7; TBP, 126-73-8;TOP, 78-51-3;DPIP, 2976121-5.
Literature Cited ASTM E537. "Method for Assessing the Thermal Stability of Chemicals by Methods of Differential Thermal Analysis"; 14.02; American Society for Testing and Materials: Philadelphia, 1986. ASTM D3850. "Testing Method for Rapid Thermal Degradation of Solid Electrical Insulating Materials by Thermogravimetric Method"; 10.02; American Society for Testing and Materials: Philadelphia, 1984.
Ind. Eng. Chem. Res. 1992,31, 1813-1819 Chaean, D. Stabilization of Petroleum Products. In Ozidation Znhibition in Organic Materials; Pospisil, J., Klemchuk, P., Eds.; CRC Press: Boca Raton, FL, 1990; Vol. I, pp 291-302. Cho, L.; Klaus, E. E. Oxidative Degradation of Phosphate Esters. ASLE Trans. 1979, 24 (l),119. Gunsel, S.; Klaus, E. E.; Duda, J. L. High Temperature Decomposition Characteristics of Mineral Oil and Synthetic Lubricant Baeestocks. Lubr. Eng. 1988,44 (a), 703. Koski, L.; Saarela, K. Oxidation Stability of Polymeric Materials by
1813
Dynamic DSC/DTA Method. J. Thermal Anal. 1982,25, 167. Marino, M. P.; Placek, D. G. Phosphate Eaters. Submitted for publication in Synthetic Lubricants; Society of Tribologists and Lubrication Engineers, Handbook of Tribology and Lubrication; Boozer, E. R., Ed.; CRC Press: Boca Raton, FL, 1992; Part 111.
Received for review November 22, 1991 Revised manuscript received March 23, 1992 Accepted April 3, 1992
Estimation of Isosteric Heats of Adsorption of Single Gas and Multicomponent Gas Mixtures Shivaji Sircar Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501
Isosteric heats of adsorption of a single gas and the components of a multicomponent gas mixture as functions of adsorbate loadings are critical thermodynamic variables for design of practical adsorptive separation processes. The rigorous estimation of these variables is, however, not easy. A shortcut approximate method has been proposed by Bulow and Lorenz to circumvent this problem. A thermodynamic analysis of this method, however, shows that substantial errors can be introduced when the adsorbate loadings are high, the adsorbates are not very strongly adsorbed, and the adsorbent is energetically heterogeneous. A differential shortcut method is proposed which can be used to calculate the isosteric heats of adsorption as functions of adsorbate loadings provided that the total adsorbate loading (all components) isotherms for the pure gas and the binary gas mixture are known as functions of varying pressure at different constant values of the gas-phase mole fraction.
Introduction The isosteric heats of adsorption of a pure gas and the components of a gas mixture are critical thermodynamic variables for design of adsorptive separation processes (Sircar, 1991a). They are needed for estimating the heat evolved (consumed) during the adsorption (desorption) process and the subsequent changes in the adsorbent temperature inside the adsorber. The adsorbent temperature, in turn, is a key factor governing the local adsorption equilibria and dynamics within the adsorber. The isosteric heat of adsorption of an adsorbate i can be estimated from the pure or multicomponent gas adsorption isotherms at different temperatures by the following thermodynamic relationships (Sircar, 1985): 8 In P pure gas: qio = R P [ niD
F]
gas mixture:
7 1 S In Pyi
qi = RP[
ni
qio is the isosteric heat of adsorption of pure gas i at a pure gas specific adsorbate loading of nio (mol/kg). qi is the isosteric heat of adsorption of component i from a multicomponent gas mixture of N components (i = l, 2, ...,N) at a multicomponent specific adsorbate loading of ni (mol/kg) for component i of the mixture. P is the total gas-phase pressure and yi is the gas-phase mole fraction of component i in equilibrium with the adsorbed phase at temperature T of the system. yi is equal to unity for adsorption of pure gas i. R is the gas constant. The quantities nio and ni describing the adsorbate loadings in eqs 1 and 2 are Gibbsian surface excess variables for adsorption of component i per unit amount of the adsorbent (Sircar,1985). They are the only meaningful experimental variables for measuring adsorption from a pure gas and a multicomponent gas mixture. The surface 0888-5885/92/2631-1813$03.00JO
excess of component i is approximately equal to the actual amount adsorbed of that component only when (a) the gas pressure is very low for pure gas adsorption and (b) the gas pressure is low and the component i is very selectively adsorbed over the other components for multicomponent gas adsorption. These quantities are, however, generally referred to (wrongly) as actual amounts adsorbed in adsorption literature. Equation 1shows that qy can be calculatedas a function of nio if pure gas adsorption isotherms [nio= nio(P)at constant T'j are available at different temperatures. The common practice is to plot In P against reciprocal absolute temperature at constant n y , and the slope of the plot gives the quantity (-qio/R). The plots yield a straight line over a large range of T because qio is generally a very weak function of T. Thus qio(nio)can be obtained for pure gas adsorption. It is necessary to measure pure gas adsorption isotherms over a very large range of P and T in order to obtain qio as a complete function of nio. In particular, high-pressure adsorption isotherms are needed at larger temperatures. An alternative method to obtain qio from the pure gas adsorption isotherms is to use the following thermodynamic relationship which can be derived from eq 1by the chain rule of calculus:
Equation 3 can be used to calculate qio for a given P and T and, hence, for the corresponding nio by measuring the slopes of the adsorption isobar (niovs T a t constant P ) and the adsorption isotherm at the chosen values of P and T. This procedure may avoid the requirement for measuring high-pressure isotherms at larger T values, but the use of eq 3 for calculating qio is sensitive to experimental error in the isotherm data because the isotherm and isobar 0 1992 American Chemical Society
