740
Ind. Eng. Chem. Res. 1994,33, 740-743
Thermal Degradation and Weight Loss Characteristics of Commercial Phosphate Esters Sundeep G. Shankwalkar' and Coralia Cruzt Process Additives Division, FMC Corporation, P.O. Box 8, Princeton, New Jersey 08543
In the polymer industry, neutral phosphate esters are used as flame retardant plasticizers. As lubricants, they find wide use as fire-resistant functional fluids and as antiwear additives. In all these applications, thermal properties like degradation, oxidation, and volatility play a critical role in their performance. This study evaluates degradation and weight loss (volatility) characteristics of commercial phosphate esters in a nitrogen atmosphere, and is a follow-up to previous work performed in oxygen. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques have been used to evaluate thermal properties. Decomposition aspects in nitrogen relative to oxygen are compared and discussed in this paper.
Introduction Neutral phosphate esters find use in a variety of commercial applications. They are used as flame retardants in engineering and polymer plastic applications. Additionally, they are recognized in this industry for their plasticizing properties. As fire-resistant functional fluid basestocks they impart lubrication characteristics as a replacement for mineral oil in areas where molten metal and open flames do not permit use of hydrocarbon-type fluids. As ashless additives in mineral oils, poly01 esters, PAOs, and diesters, they provide antiwear protection for hydraulics, compressors, jet engines, and other systems. Depending on their basic molecular structure, commercial phosphate esters can be grouped as trialkyl phosphates, triaryl phosphates, and alkyl-aryl phosphates. Details of these type of compounds are presented in the experimental section. It can be expected that phosphate esters of varying molecular structure will respond differently to changes in temperature and environmental conditions; i.e., their thermal properties will not be the same. Information on thermal behavior of phosphate esters has significant practical importance. Data of this type can be used for assessment of product life, stability, and quality control, and for design and synthesis of phosphate esters to meet new or existing requirements (Madorsky, 1964; Shankwalkar and Placek, 1993). Chemical decomposition in phosphate esters can be categorizedas thermal degradation or oxidation depending on the type of gas environment (Widmann and Riesen, 1986). Thermal degradation implies that decomposition is taking place in an inert atmosphere like nitrogen or argon in which no oxygen is present. Under these conditions thermal properties are directly dependent on temperature, and are measured by observing endothermic effects due to breakage of chemical bonds. When decomposition takes place via oxidation, temperature and oxygen both play a role in the process. In this case, oxidation and thermal degradation can take place simultaneously. Decomposition under these conditions usually results in color change (Landis and Murphy, 1990). Volatility is a physical process characterized by weight loss and can take place along with thermal degradation and oxidation. Typically, temperature and type of gas atmosphere (N2 or 0 2 ) could have an effect on the volatility profile of phosphate esters.
* Author to whom correspondence should be addressed.
+ Permanent address: Department of Chemical Engineering, New Jersey Institute of Technology, Newark, NJ 07102.
0888-5885/94/2633-0740$Q4.5O/Q
Differential scanning calorimetry (DSC) has been used to study thermal properties of materials including phosphate esters (Shankwalkar and Placek, 1993; Perez et al., 1990). A non-isothermal DSC measurement allows rapid assessment of decomposition behavior over a temperature range. This type of measurement can give relative information on the materials maximum utility temperature. Similarity, thermogravimetric analysis (TGA)is used to profile weight loss due to chemical and physical phenomena taking place in the sample. A combination of these techniques present an ideal system to study thermal properties of phosphate esters. Based on DSC and TGA measurements in oxygen and nitrogen, and the knowledge that oxidation usually predominates thermal degradation (Gunsel et al., 1988), it is possible to distinguish oxidation from thermal degradation. This paper is a follow-upto previous work by Shankwalkar and Placek (1992) in which oxidation and weight loss behavior of phosphate esters was evaluated in an oxygen environment. In the present study, thermal degradation and weight loss (volatility) characteristics of commercial triaryl, trialkyl, and alkyl-aryl phosphate esters are presented and compared with previous data on oxidation of these compounds.
Experimental Technique Thermal degradation measurements were run by DSC using a Mettler DSC 25 system. In this technique (ASTM E537), heat is exchanged with the sample over a desired temperature range and measured as the difference in heat flow to the sample and reference cell. An exothermic or endothermic peak represents heat flow difference and is recorded as a DSC scan. Peaks on a DSC scan indicate physical or chemical changes in the material. Weight loss measurements were determined by TGA using a Mettler TG 50 system. In this technique (ASTM D3850), change in sample mass is recorded over a defined temperature range. Material weight loss could be due to thermal degradation or volatility, or due to both effects acting simultaneously. All DSC and TGA runs were made between 30 and 400 "C at a rate of 10 "C/min with the sample (10 pL) enclosed in a perforated (single hole) aluminum pan (40-pL net volume). Nitrogen at 50 mL/ min was used as purge gas. Additional DSC runs were performed (not shown) between 60 and 300 "C to distinguish between thermal degradation and volatility. Sealed DSC samples were loaded in a nitrogen-saturated atmosphere. Data were analyzed with a Mettler TA 72 thermal analysis software system. 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 741 Table 1. Physical Properties of Phosphate Ester ComDounds ~~~
phosphate esters tricresyl (TCP) trixylyl (TXP) isopropylphenyl (IPPP-22) isopropylphenyl (IPPP-32) isopropylphenyl (IPPP-46) tert-butylphenyl (TBPP-22) tert-butylphenyl (TBPP-32) terg- butylphenyl (TBPP-46) tributyl (TBP) tris(butoxyethy1) (TBEP) trioctvlu (TOP) 2-ethilhexyl diphenyl (EHDPP) isodecyl diphenyl (IDDPP) 0 Tris(2-ethylhexyl).
IS0 viscosity wt% viscosity at 20 "C, sp gr at phosgrade CP 20°/20 OC phorus 32 80 1.17 8.4 7.8 46 90 1.14 8.3 22 70 1.18 32
90
1.165
7.9
46
155
1.125
7.4
22
75
1.2
8.4
32
104
1.17
8.2
46
160
1.15
7.8
8
n
TXP
-
-b:z3 F
-c-CH3I
272 276 264 282
283 276 281
169 157 205
231
257 264
226 233
231 246
281
274 277 278
306 302 293 307 307 305 305 306 197 181
7
14.1 25.6
0.922 1.07
7.1 9.9
Average values of commercial samples from several manufacturers.
19.6
1.09
9.1
ature a t which color change is seen in unsealed phosphate ester DSC runs. Fromdata presented in Table 2, it is apparent that triaryl phosphate esters show increased thermal stability in comparison to trialkyl and alkyl-aryl phosphate esters. Triaryls (TCP, TXP, IPPP, and TBPP) show thermal degradation above 310 "C relative to trialkyls (TBP, TBEP, and TOP) and alkyl-aryls (EHDPP and IDDPP), which show degradation below 285 "C. The enhanced stability of triaryls compared to trialkyls and alkyl-aryls can be related to their basic structure (Figure 1); i.e., aromatic ring groups present in triaryl phosphates exhibit superior thermal stability relative to weaker straight-chain alkyl groups. Comparison of these data to oxidation onset values identifies the critical role oxygen plays in decomposition of phosphate esters. In the presence of oxygen, decomposition is catalyzed which results in a low oxidation onset temperature relative to the thermal degradation temperature. This feature is evident in all phosphate esters that were studied, and relates to increased susceptibility of organic species to oxygen. Comparison of various triaryl phosphate esters (TCP, TXP, IPPP, and TBPP) show minimum difference in thermal stability. This can be explained on the basis of the presence of alkyl-type C-H bonds which at increased temperatures are the weakest link. TCP, TXP, IPPP, and TBPP all have alkyl groups on the phenyl ring resulting in comparable thermal stability. Slightly higher (-25 "C) thermal degradation onset temperatures for TBPP relative to IPPP may be due to tert-butyl group attachments on the phenyl ring. This feature is more apparent in the oxidation study of triaryl phosphate esters where TBPP shows significantly better oxidative stability (-85 "C higher) relative to other phosphate esters. Differences in thermal degradation onset temperatures among triaryls could also be due to triphenyl phosphate (TPP) content in the phosphate ester. Typically, aryl phosphate esters re isomeric mixtures of different molecular weight compounds containing a varying amount of TPP which decreases with increasing viscosity. TPP has excellent thermal stability and its presence would add to the stability of the phosphate ester. Due to their alkyl nature, phosphate esters like TBP, TOP, and TBEP have DSC profiles similar to those of mineral oil type hydrocarbon liquids, and are susceptibile
0
II R-0-P-0-R
M 1 -
"2
333 311 314 311 311 347 345 338
11.7 7.8
R
--L
total weight loss at O C 5% 10%
0.98 1.02
d
+
therm degradn onset temp, OC
3.7 12.2
2
R
R=CHs 2CHg
phosphate ester triaryl PE TCP TXP IPPP-22 IPPP-32 IPPP-46 TBPP-22 TBPP-32 TBPP-46 trialkyl PE TBP TBEP TOP trialkyl/aryl PE EHDPP IDDPP
7
0
R G O - !-OeR
Table 2. Thermal Degradation Onset Temperatures and Weight Loss of Phosphate Esters* DSC/TGA Data, 10 OC/min in Nitrogen
R=ChWp
+ TBP
c4nooc2n4+ canl7 6
TBEP
R=CaH17
+
c10n21-
BHDPp
rnDPP
TOP
rn TBPP
CHS
Figure 1. Basic structure of phosphate esters.
Data presented in this study are the average of two runs of different samples of similar viscosity grade and composition. All commercial samples used are described in Table 1. Basic molecular structure of the phosphate esters are shown in Figure 1. A detailed review of phosphate ester types is presented in the work by Marino and Placek (1993).
Results and Discussion Table 2 summarizes thermal degradation onset temperatures and percent weight loss data of phosphate esters. These data are based on DSC and TGA measurements in nitrogen. All oxidation data comparison in this discussion are based on work by Shankwalkar and Placek (1992). Thermal Degradation Characteristics. The thermal degradation onset temperature is observed as the point at which the slope of the DSC endotherm increases sharply. On the basis of phosphate ester DSC profiles one cannot directly distinguish thermal degradation endotherms from volatility endotherms. Hence, at the end of DSC runs, samples were visually inspected to observe color change which takes place at the onset of thermal degradation. This was confirmed by DSC runs in hermatically sealed pans (no hole), which show color change. Under these conditions volatility effects are absent and only thermal degradation takes place. In this case, thermal degradation onset temperatures obtained were similar to the temper-
-
L 01
n
EHDPP
TOP 0-
1111
I
-~'-'-,---T-~-l--.T-l--(200.
-,--250.
3UO.
35u.
--I--? 400.
'C
I
100
200.
300.
'C
Figure 2. Representative phosphate ester DSC profiles in nitrogen (rate 10 OC/min).
Figure 3. Representative phosphate ester TGA profiles in nitrogen (rate 10 OC/min).
to high temperature. They show thermal degradation below 285 "C. Additional DSC work is required to better understand exothermic effects seen (if DSC scan is enlarged) prior to the degradation endotherm. Alkyl-aryl phosphate esters like EHDPP and IDDPP show thermal degradation at 257 and 264 "C, respectively. These compounds have structures intermediate to triaryl and trialkyl phosphate esters, and their DSC scans, like those of trialkyls, show endothermic effect prior to the degradation endotherm. Thermal degradation DSC profiles of several phosphate esters are shown in Figure 2. The authors wish to point out that phosphate ester onset temperatures are determined for comparison of relative stability and should not be considered as the maximum utility temperature. Weight Loss Characteristics. In an inert gas atmosphere, weight loss in phosphate esters could occur due to thermal degradation or volatility. Hence, phosphate esters would show weight loss depending on their inherent thermal stability and molecular weight. If weight loss is seen at a temperature below the onset of thermal degradation, it would suggest that this weight loss is due to volatility, and not due to thermal degradation. Similarly, weight loss above thermal degradation onset temperature would be a result of degradation along with volatility. As seen in Table 2, all triaryl phosphates show 5 % weight loss at similar temperatures (around 275 "C). Since thermal degradation occurs at temperatures well above 300 "C, weight loss would be due to volatility alone. However, no molecular weight trends are seen among the triaryl phosphates. In trialkyl phosphate esters, too, weight loss at 5 % is due to volatility, and not to thermal degradation. TBP, which has the lowest molecular weight among trialkyls, shows maximum volatility. Similarly, one would expect TBEP to have the lowest volatility among trialkyls, since it has the highest molecular weight. This is not seen in TGA measurements, and may be due to the presence of oxygen in the TBEP molecular which affects its thermal properties and in turn its volatility characteristics. EHDPP and IDDPP show intermediate weight loss characteristics relative to triaryl and trialkyl phosphates. This trend is similar to their thermal degradation characteristics. Five and ten percent weight loss in alkylaryls is a result of volatility and not thermal degradation. In general, comparison of weight loss characteristics among phosphate ester groups shows a molecular weight trend; i.e., triaryls show minimum weight loss followed by alkyl-aryls and then trialkyl phosphates. This result is similar to the behavior of phosphate esters in oxygen. All commercial samples within each phosphate ester group
showed comparable weight loss, with no particular trend. This feature is in contrast to the oxidation study which showed a relationship between weight loss and molecular weight. Figure 3 shows typical TGA profiles of several phosphate ester in nitrogen and illustrates their weight loss characteristics.
Conclusion DSC measurements in nitrogen show that triaryl phosphate esters like IPPP, TCP, and TXP have comparable thermal degradation characteristics. TBPP has relatively higher thermal stability (25 "C higher). This is in contrast to the oxidation study which showed an oxidative stability trend (TBPP > IPPP > TCP > TXP). Thermal degradation behavior of phosphate esters can be explained on the basis of their basic molecular structure and TPP content. Weight loss in phosphate esters, as determined by TGA is primarily due to volatility and not thermal degradation. Triaryl phosphate esters show minimum weight loss relative to trialkyl and alkyl-aryl phosphate esters. Trialkyl phosphate esters show maximum weight loss. This feature indicates a molecular weight relationship to weight loss. Within each phosphate ester group, no weight loss trends are observed. However, this feature may be related to sensitivity of the experimental method. This study in conjunction with oxidation data demonstrates the importance of comparing DSC and TGA profiles to obtain a complete understanding of phosphate ester thermal characteristics.
Literature Cited ASTM D3850. 'Test Method for Rapid Thermal Degradation of Solid Electrical Insulating Materials by Thermogravimetric Method"; 10.02; American Society for Testing and Materiala: Philadelphia, 1984. ASTM E537. "Method for Assessing the Thermal Stability of Chemicals by Method of Differential Thermal Analysis"; 14.02; American Society for Testing and Materiale: Philadelphia, 1986. Gunsel,S.;Klaus,E. E.;Duda, J. L. HighTemperatureDecomposition Characteristicsof MineralOil and SyntheticLubricantBasestocks. Lubr. Eng. 1988, 44, (8), 703. Landis, M. E.; Murphy, W. R. Analysis of Lubricant Components Associated with Oxidative Color Degradation. Lubr. Eng. 1990, 47 (7), 595-598. Madorsky,S.L. Introduction. In Thermal Degradation of Organic Polymers; John Wiley and Sons: New York, NY, 1964; pp 1-2. Marino, M.; Placek, D. G. Phosphate Esters. Submitted for publication In Synthetic Lubricants; Boozer, E. R., Ed.; Society
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 743 of Tribologistsand Lubrication Engineere,Handbook of Tribology and Lubrication; CRC Press: Boca Raton, FL, 1993; Part 111. Paciorek, K. J. L.; Kratzer, R. H.; Kaufman, J.; Nakahara, J. H.; Christoe, T.; Hartatein, A. M. Thermal Oxidative Degradation of Phosphate Esters. Am. Znd. Hyg. Assoc. J. 1978,39 (81, 633-663. Perez, J. M.;Ku, C. 5.;Hegemann,B. E.; HBU,S. M. Characterization of Tricreryl Phosphate Lubricating Films by Micro-Fourier Transform Infrared Spectroscopy. Tribol. Trans. 1990, 33 (l), 131-139. Rizvi, Q. A. Lubricant Additives and Their Functions. In Friction Lubrication and Wear Technology, ASM Handbook; Henry, S. D., Ed.; ASM International: Materiale Park, OH, 1992; Vol. 18, pp 98-112. Shankwalkar, 5. G.; Placek, D. G. Oxidative and Weight Loss Characteritaticeof CommercialPhosphate Esters. Znd. Eng. Chem. Res. 1992, 31, (7), pp 1810-1813.
Shankwalkar,S. G.; Placek, D. G. New High Stability Synthetic Phosphate Ester. Submitted for publication in J. Synth. Lubr. 1993a. Shankwalkar, 5.G.; Placek, D. G. Oxidation Kineti- of Tricresyl Phosphate (TCP) wing Differential ScanningCalorimetry (DSC). Accepted for publication in Lubr. Eng. 1993b. Widmann, G.; Rieaen, R. Decomposition. In Thermal Analysis; Oehme,F., Ed.; Huthig: Heidelberg, 1987; pp 26-26. Receiued for reuiew September 7, 1993 Reui8ed manuscript received December 9,1993 Accepted December 22,1993. Abstract published in Aduance ACS Abstracts, February
16, 1994.
