1 

NASA Technical Memorandum 1021 16 

Determination of the Thermal Stability of 

Perfluoroalkylethers by Tensimetry : 

Instrumentation and Procedure ~ 

(NASA-TH-lo 21 16) DETEHPIXNATION OF TH E W89-26095 

THEBF4A.L STABILITY OF EERFLUOHOALKYLETEERS BY 

TENSXHETRY: INSTRUEENIATION AND PROCEDURE 

[NASA. Levis Research Center) 31 pCSCL 1lC Unclas 

G3/27 0219602 

-% Larry S. Helmick 

Cedarville College 

Cedarville, Ohio 

and 

William R. Jones, Jr. 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

Fort Lauderdale, Florida, October 16-19, 1989 

7 STLE-ASME Tribology Conference 

A 
DETERMINATION OF THE THERMAL STABILITY OF FLUIDS BY TENSIMETRY: 

INSTRUMENTATION AND PROCEDURE 

Larry S. Helmick 

Cedarville College 

Cedarville, Ohio 45314 

and 

William R. Jones, Jr. 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

ABSTRACT 

A computerized tensimeter and experimental procedure for 

determination of the thermal decomposition temperature (Td) of 

perfluoroalkylethers have been developed and tested. Both the I+ m apparatus and the procedure are described in detail. Results of Q) U testing with bis(2-ethylhexyl) phthlalate and trimethylolpropane 

w I triheptanoate demonstrate that the reciprocal of the decomposition 

temperature is a linear function of the logarithm of the gas 

volume/heated liquid volume ratio. The Td obtained for each 

compound at a gas volume/heated liquid volume ratio of one was 

similar to the value previously reported using an isoteniscope 

technique. Results of testing with a polymer of hexafluoropropylene oxide demonstate that this instrument and procedure can be 

used to determine the Td of perfluoroalkylethers. 

INTRODUCTION 

Perfluoroalkylethers (PFAE) is a class of fluids which 

normally exhibits excellent thermal and oxidative stability (Refs. 

1 to 3) and, when combined with good viscosity-temperature characteristics (Ref. 3), good elastohydrodynamic film forming capabilities (Ref. 5) and adequate boundary lubricating ability (Refs. 

3, 4, and 6) make these fluids promising candidates for high temperature lubricant and hydraulic fluid applications. 

In the past, only a few different PFAE chemical structures 

were available. One was a branched structure based on the polymerization of hexafluoropropylene oxide (HFPO) monomer (Refs. 7 and 

8). Another was an unbranched structure prepared by the poly1 
merization of tetrafluoroethylene (TFE) in the presence of oxygen 

(Ref. 9). A third, also an unbranched structure, now commercially 

available, is made by polymerization of tetrafluorooxetane followed 

by direct fluorination (Ref. 10). Recently, a variety of new 

\ 

structures based on direct fluorination techniques have been 

prepared (Ref. 11). 

This influx of new PFAE fluids has made it possible to 

correlate various properties with the different chemical structures. One of these properties, which is important in determining 

the maximum use temperature, is the thermal decomposition temperature. 

In the case of hydrocarbons and most other fluid classes, 

thermal decomposition (i.e., processes occurring in the absence of 

oxygen) proceeds through a free-radical reaction yielding many 

products. Some of the products are volatiie. Thus, in a closed 

system, a pressure rise accompanies decomposition. 

A device which has been used for many years to measure this 

pressure rise and thus thermal decomposition is the isoteniscope 

(Ref. 12). It has been used to correlate thermal stability with 

chemical structure (Ref. 13 and 14) and is now used in a standard 

ASTM test procedure (Ref. 15) for measuring the initial thermal 

decomposition temperature of fluids. 

Recently, a computerized tensimeter, based on the isoteniscope 

principle, has been developed which automates many of the tedious 

procedures. This computerized tensimeter was used to measure the 

isothermal rate of pressure rise from which the thermal 

2 
decomposition temperature can be determined. The purpose of this 

communication is to report the development and testing of this 

tensimeter, the procedure used to determine the thermal 

decomposition temperature of a perfluoroalkylether, and the 

determination of the significant dependence of the thermal 

decomposition temperature upon the gas volume/heated liquid volume 

ratio. The importance of this dependence has not been recognized 

previously, and is a principal finding, both experimentally and 

theoretically in this paper. It is anticipated that these new 

fluids and the computerized tensimeter will eventually aid in 

determining the ultimate thermal stability of perfluoroalkylethers 

in the absence of potential catalyzing agents such as metals, metal 

oxides, metal fluorides, and air. 

8 

Details of the tensimeter apparatus used in the present tests 

and its calibration that are relevant to its operation are given 

below. 

APPARATUS 

Tensimeter 

A computerizedtensimeterwas usedto measure isothermal rates 

of pressure rise from which thermal decomposition temperatures were 

determined. A schematic representation of this device is shown in 

Figure 1 and an isometric view appears in Figure 2. The device 

consists of a sample cell assembly, an oven, a temperature programmer, temperature and pressure measuring components, and a data 

acquisition system. 

Pressure and Temperature Calibration 

3 
The pressure transducer had a resolution of 3 torr and was 

calibrated by measuring the millivolt output as a function of pressure for several known pressures (+ 2 torr) from 10 to 1450 torr 

(from an absolute pressure gage) and had a linear response 

(correlation coefficient=0.9999). The Type J iron-constantan 

thermocouple in the oven was precalibrated and had an accuracy of 

- +3"C. A check was made to determine if there was any significant 

difference between the oven temperature measured by the thermocouple and the fluid temperature in the bulb of the cell. The 

temperature of the fluid in the bulb of the cell was measured with 

a thermistor thermometer with an accuracy of 23°C while following 

the temperature program used to test fluids. Differences in 

temperature between the thermocouple and the thermistor were less 

than 3°C. 

Valve 

\ 

A valve (constructed of 316 stainless steel) with two 6.3 mm 

(1/4") ferrule fittings was modified to accept a 1.7 x lo5 Pa (0- 

25 psia) pressure transducer and one 6.3 mm (1/4") O-ring fitting 

for mounting the cell (Figure 3). The internal volume of the valve 

(excluding the transducer cavity, which was prefilled with test 

fluid) was determined (0.30 + 0.02 mL) by weighing empty and 

reweighing filled with water. 

Maanetic Stirrins Bar 

The volume (0.10 f. 0.02 mL) of a aorosilicate glass covered 

magnetic stirring bar, with a small enough diameter to fit into the 

4 
stem of the cell, was determined by displacement of water in the 

graduated barrel of a 2 mL syringe. 

Sample Cell \ 

The 150 mm long sample cells with 23 mm diameter bulbs were 

made from 6.35 mm diameter borosilicate glass tubing with 1.2 mm 

walls and pressure tested to 1500 torr. 

SamDle Cell Assembly 

The cell containing the magnetic stirring bar was mounted on 

the valve with the transducer and sealed with the O-ring fitting 

(Fig. 3). The cell assembly was vacuum tested for leaks with 

helium and a quadrupole residual gas analyzer at torr with no 

helium detectable. It was pressure tested at 1400 torr for 1 hr. 

with the cell at 300°C with no reduction in pressure observable (+ 

2 torr). 

Test Fluids 

Trimethylolpropanetriheptanoate (TMPTH) andbis(2-ethylhexyl) 

phthalate ("dioctyl phthalate") (DOP) were vacuum distilled before 

use. The perfluoroalkylether (HFPO type) was used as received. 

Structures of these fluids appear in Figure 4. Some typical 

physical properties of the test fluids are tabulated in Table I. 

OUTLINE OF BASIC TEST PROCEDURE 

1. Fill transducer cavity with test fluid. 

2. Determine volume of cell. 

3. Clean and dry cell, then weigh cell assembly empty. 

4. Inject fluid into cell. 

5. Degas fluid, then weigh cell assembly with fluid. 

5 
6. Mount cell assembly in oven. 

7. Determine T,' with tensimeter. (Symbols defined in Appendix 

B- 1 \ 

8. Weigh cell with fluid. 

9. Repeat steps 2-8 with a different V,/VL ratio. 

10. Clean valve and transducer cavity. 

DETAILS OF BASIC TEST PROCEDURE 

Prefillins the Transducer Cavity 

In order to minimize the volume of fluid in the cell during 

degassing, the transducer cavity was prefilled with test fluid. 

This was done by mounting a cell containing the type of fluid to 

be tested (approximately 2 mL), evacuating to 0.1 torr with stirring for several minutes to degas the fluid, closing the valve, 

and then inverting the cell assembly so that the fluid could flow 

into the evacuated transducer cavity. 

Once filled, the cavity remained full due to the viscosity of 

the fluid and the small diameter (2 mm) of the transducer opening, 

as long as the transducer cavity was not inverted. After removal 

of the cell, the valve and transducer were ready for use. 

Determinins Volume of Cell 

The total volume of each cell (approx. 5 mL) was determined 

(2 0.002 mL) by weighing empty, filling with deionized water, reweighing, and converting the net weight to volume by dividing by 

the density of water (0.997g/mL). The volume of the stem alone was 

also determined (1.60 0.05 mL) and assumed to be constant for all 

cells. 

6 
Cleanina and Drvins the Cell 

In order to prevent nonreproducible pressure increases due to 

vaporization of moisture and thermally unstable material from the 

\ 

inside surfaces of the cell, each cell was cleaned and dried immediately before use. The cell containing the stirring bar was 

mounted on the O-ring fitting of the valve with the valve open. 

The stirring bar was moved into the valve to avoid heating the 

magnet. The bulb and stem of the cell were heated with a gas 

burner to a yellow flame temperature (approx. 800°K) in air at 

atmospheric pressure to oxidize or decompose any thermally unstable 

material on the glass. The cell was then evacuated to 0.1 torr 

while hot to remove adsorbed water from the glass. The valve was 

closed and the cell assembly was removed from the vacuum line and 

weighed (+O. 002s) . One cleaned cell was tested for remaining traces of volatile 

and thermally unstable material by heating in the tensimeter from 

573°K to 723°K (300°C to 450°C) following the temperature 

programming used to test fluids. The cell pressure remained 

constant (+ 2 torr), indicating that the cell contained no material 

that would cause a pressure increase during the testing of a fluid. 

Iniectina Fluid Into the Cell 

The evacuated cell assembly was placed in a dry box purged 

with cryopumped nitrogen. The cell was removed from the valve. 

The calculated volume of fluid (V,) required to give a specific 

7 
V,/Vt ratio (see Discussion Section) was measured (+ 0.02 mL) in a 

5 mL syringe and injected with a 16 cm long 3 mm diameter needle 

into the bulb of the cell. The cell was replaced on the valve and 

the assembly removed from the dry box and degassed. 

, 

Deqassinq Fluid 

The fluid in the cell was degassed with the cell assembly in 

a vertical position by magnetically stirring at room temperature 

and 0.1 torr on a 6.3 mm (1/4") vacuum line equipped with a liquid 

nitrogen trap until bubbling ceased (approx. 30 min). If the fluid 

was too viscous to stir well, the temperature was raised to 80°C 

by heating with a hot-plate stirrer. Foaming of fluid into the 

stem was minimized by mild heating of the stem with a hot air gun 

(maximum hot air temperature: 548°K). This prevented entraining 

of fluid into the valve and loss through the valve into the vacuum 

system. Degassing is improved by stirring the fluid, increasing 

the degassing time and temperature, and decreasing the degassing 

pressure. However, care must be exercised in selecting degassing 

conditions so that composition and properties of the test fluid are 

not inadvertently changed during degassing. 

After degassing, the cell assembly was again weighed (+ 0.002 

g) and the exact volume of fluid present was calculated by subtracting the weight of the empty cell assembly and dividing the 

remainder by the density of the fluid. 

Mountina Cell Assemblv 

The magnetic stirring bar was moved into the valve to avoid 

heating the magnet. Then the cell assembly was mounted in the oven 

8 
in a reproducible position with approximately 10" upward tilt 

toward the bulb of the cell. 

Tilting the cell assembly slightly when mounting in the tensimeter oven insures that the valve and stem of the cell always re- 

\ 

main filled with fluid. This prevents any possible refluxing of 

volatile starting materials or decomposition products in the cell 

assembly, and the corresponding erratic pressure fluctuations, as 

well as reduction in fluid temperature due to the heat of 

vaporization and condensation of the refluxing material. It also 

insures that a known and reproducible volume of fluid will always 

be present in the heated bulb of the cell so that V,/Vl can be 

calculated (Equation 1). 

Determininq T, with Tensimeter 

Instrument temperature control and data acquisition was 

handled automatically by the data acquisition system. The oven was 

preheatedto a temperature about 20°C below the expected decomposition temperature of the fluid. The oven temperature was held 

constant for 360 sec. for the fluid to reach thermal equilibrium 

and constant vapor pressure. Then the temperature was recorded. 

Any change in pressure due to decomposition was measured during a 

144 sec. interval at that temperature and also recorded. Finally, 

the actual cell pressure was recorded. Then the oven temperature 

was raised 5" during the next 60 sec. and the process repeated. 

When the pressure reached 1500 torr, or the change in pressure 

reached 100 torr in 144 sec., or the temperature reached 723°K 

(450"C), data acquisition was terminated and the oven was turned 

9 
decomposition temperatures, two 

were tested under a variety of 

10 

off. The data were tabulated and a plot of the log of the 

isothermal pressure change (log dP/dt) vs. the reciprocal absolute 

temperature was prepared as shown in Figure 5 for bis(2-ethylhexyl) 

phthalate. 

\ 

That 360 sec. was adequate time to reach thermal equilibrium 

was demonstrated by increasing the equilibration time to 600 sec. 

in one test and observing no change in results. 

Weishins Cell with Fluid 

At the end of the test, the cell assembly was cooled. The 

fluid and stirring bar in the valve and stem were allowed to flow 

back into the bulb of the cell. The cell was removed from the 

valve and, after removal of the stirring bar, was weighed (+ 

0.002s) with the fluid in it to confirm the volume of fluid 

actually in the cell during the test. 

Cleanins the Valve 

The valve and transducer unit was disassembled and cleaned 

whenever the type of fluid was changed by rinsing thoroughly with 

1,1,2-trichloro-1,2,2-trifluoroethane or acetone, blowing out with 

nitrogen, and drying at room temperature and 0.1 torr. 

RESULTS AND DISCUSSION 

Experimental Procedure 

In order to evaluate the computerized tensimeter and the 

corresponding experimental procedure used to determine thermal 

esters and one perfluoroalkylether 

experimental conditions. The two 
esters were chosen in order to compare results with those obtained 

previously using an isoteniscope technique (Ref. 13). The PFAE was 

chosen to insure that the conditions selected for the experimental 

procedure would be suitable for analysis of perfluoroalkylethers. 

Experimental conditions investigated included the test fluid 

purity, cell cleaning procedure, degassing procedure, the gas 

volume/heated liquid volume ratio, and the mounting angle of the 

cell in the tensimeter oven. 

* 

Effects of variations in several of these experimental 

conditions have previously been mentioned (Refs. 12, 13, and 15). 

Presence of volatile or thermally unstable material in the test 

fluid or cell can result in erratic isothermal pressure increases 

before the actual thermal decomposition temperature of the test 

fluid has been reached. These in turn can produce a curved 

Arrhenius plot and a corresponding reduction in the calculated Tdl. 

These problems can often be minimized by purification of test 

fluids (e.g., drying and distillation), by flame cleaning of the 

cell followed by drying under vacuum while hot, by filling the cell 

in a dry atmosphere, and by thorough degassing of the test fluid, 

as described in the Experimental Section. 

Data Analysis 

After discarding any low temperature data (log dP/dt<0.7) 

which appeared to be due to volatile or thermally unstable impurities as discussed above, the log of the isothermal change in 

pressure (log dP/dt) was plotted as a function of the reciprocal 

of the absolute temperature (Figure 6). The temperature at which 

11 
the line obtained from a linear regression of the data intersected 

the log 0.3 line was taken as the decomposition temperature (Tdl) 

for the fluid at that particular gas volume/heated liquid volume 

ratio. (Figure 6 

and Equation 4), a value related to the energy of activation (Ref. 

13) . El 

was estimated to be accurate to 17 kJ/mol (4 kcal/mol) and 

\ 

The slope of the line was used to calculate E' 

Decomposition temperatures were reproducible to & 2 "C. 

reproducible to 8 kJ/mol (2 kcal/mol) . 

Calculation of the Gas Volume/Heated Liauid Volume Ratio 

A major observation in the study of the reproducibility of the 

data was that the measured decomposition temperature (Tdl) of a 

given fluid depended significantly upon the ratio of the gas volume 

to the heated liquid volume (V,/V,) in the cell assembly. This 

ratio was calculated from the following equation, where all units 

are in mL. 

V,/V, = (V, - V,)/(V, - 1.00) [I1 

V, is the fluid volume. V, is the total internal volume of the cell 

assembly (excluding the transducer cavity). It is equal to the 

cell volume (V,) plus the valve volume (V,) (0.30 0.02 mL) minus 

the magnet volume (V,) (0.10 f. 0.02 mL) (Figure 3) : 

v, = v, + v, - v, [21 

The cell assembly constant (1.00) represents the total volume 

of unheated fluid in the valve (0.20 mL) and stem (0.80 mL) of the 

cell. Therefore, the gas volume is equal to the total internal 

volume of the cell assembly (excluding the transducer cavity) minus 

the volume occupied by the fluid (V,=V,-V,). The heated liquid 

12 
volume is equal to the volume of the fluid placed in the cell minus 

the volume of unheated fluid in the valve cavity and stem of the 

cell outside the oven (V,=V,-1.00) . * 

Equation 1 neglects the expansion of the fluid due to heating, 

as well as the solubility of the product gases in the fluid. 

Individually, these two factors could significantly affect the 

pressure in the cell, but they appear to partially cancel each 

other. A theoretical derivation of the relationship between T,' 

and log V,/V, appears below, and supports the experimental 

observations. 

Derivation of the Relationship Between T,' and Los V/v, 

The rate of thermal decomposition usually varies with temperature according to the Arrhenius rate law (Ref. 13) as shown in the 

following equation: 

r31 -E/RT k = Ae 

where k is the rate constant, A is the frequency factor, E is the 

activation energy, R the gas constant, and T the absolute temperature. According to this equation, a straight line should be 

obtained when log k is plotted as a function of the reciprocal of 

the absolute temperature. It has been shown experimentally (Ref. 

13) that for many organic compounds, log dP/dt versus 1/T is also 

a straight line. Then, by analogy, 

dt 

[41 

Using this equation, one can determine the rate constant for 

thermal decomposition by measuring the isothermal rate of pressure 

13 
rise (dP/dt) at several temperatures. However, it is more 

convenient to have a single parameter for thermal decomposition 

rather than to tabulate values of At and Et which actually define 

the rate constant for different fluids. 

, 

Therefore, an arbitrary thermal decomposition temperature (T,) 

is defined as the temperature at which the isothermal rate of vapor 

pressure rise is 1.85 Pa/sec (50 torr/hr) when the gas volume is 

equal to the heated liquid volume. An arbitrary pressure rise of 

50 torr/hr was originally chosen because it represented the minimum 

pressure rise easily detectable experimentally. A gas volume equal 

to the heated liquid volume was originally used by Blake, et al. 

(Ref. 13) and is included in the definition here because of our 

experimental observation that T, is significantly dependent upon 

this ratio. Then, the decomposition points for a series of organic 

compounds are the temperatures at which all have identical isothermal rates of pressure rise. 

k For the reaction BLiq. --> b Dgas, Blake, et al. (Ref. 13) , 

have shown that the rate of pressure rise (dP/dt) due to formation 

of product (D) is related to the rate constant (k) by the gas 

volume/heated liquid volume ratio (V,/VB) and the constants b, nBa, 

R, and T: 

k= V dP/dt 

V,bnTRT 

[51 

where b is the number of molecules of gas formed from one molecule 

of liquid B, nB is the concentration of B, a is the order of the 

reaction, R is the gas constant, and T is the absolute temperature. 

14 
Setting equations 3 and 5 equal to each other gives equation 6: 

Taking logs of equation 6 gives: 

log A- (E/2.3RT)=log Vg/V,+lOg l/bnBaR+log l/T+lOg dP/dt [ 71 

Rearranging and solving for log Vg/V, gives: 

log Vg/V,=-E/2. 3RT+log T+log A+log bnBaR-log dP/dt [81 

For values of log Vg/V, not equal to 1, when log dP/dt=O. 30, T=Tdl. 

Substituting these into equation 8 gives: 

log Vg/V,=-E/2. 3RTd'+log Td'+log A+log bnBaR-O. 30 [91 

For decomposition of a specific fluid with various gas 

volume/heated liquid volume ratios, log Vg/V, and T,' will be 

variables while the other quantities are constant. Since log T,' 

is small and nearly constant (for small changes in Tdl) compared 

with E/2.3RTd' (Appendix A), it can be included in the other 

constants. Therefore, under these experimental conditions, 

equation 9 reduces to equation 10, which is the equation of a 

straight line. Theoretically, then, a plot of log Vg/V, vs. l/Tdt 

should be linear. 

log Vg/V, = -E/2.3RTd'+C [lo1 

This has indeed been found to be the case experimentally, as shown 

in Figures 7, 8, and 9, where V, = V,. 

Furthermore, since the slope is equal to -E/2.3R, the Arrhenius energy of activation (E) can be calculated from this graph 

and compared with E' obtained from the log dP/dt vs. 1/T graph. 

Blake et al., (Ref. 13) have shown that E=EI-RT, where T, is the 

15 
mean temperature for the range of observations. Thus El and E 

should be similar in magnitude. When the effect of the experimental error in dP/dt and T,' on the magnitudes of E' and E are 

considered, the magnitudes of E' and E are indeed found to be 

similar. For TMPTH, E1=246+5 kJ/mol (Table 111), E=283+31 kJ/mol 

(Figure 8). This places the instrument and method on a firmer 

mathematical basis and allows for more reproducible results. 

Thermal Decomposition Temperature of Fluids 

\ 

Since the T, of the fluid is the temperature at which the rate 

of isothermal pressure rise is 1.85 Pa/s (50 torr/hr) at V,/VL=l, 

the T, must be experimentally determined at this ratio, or read 

from a graph of log Vg/V1 vs. l/Tdl where log Vg/Vl=O. The graphical 

method is probably the best. 

The thermal decomposition temperatures (T,) obtained for DOP 

(537.4"K, Table I1 and Figure 7)(264.2 2 2"C, Lit. 263.3 +ZoC) and 

TMPTH (589.9"K, Table I11 and Figure 8)(316.7 2 2"C, Lit. 317.2 

- +2"C) are similar to values previously obtained (Ref. 13), 

demonstrating that the properties being measured with this 

computerized tensimeter and procedure are similar to those 

previously measured with the isoteniscope technique. 

Analysis of Perfluoroalkvlethers 

Although it is apparent that this computerized tensimeter 

works well under conditions necessary to determine the T, of 

hydrocarbon esters, conditions necessary for analysis of 

perfluoroalkylethers are considerably more rigorous. The oven 

temperatures required may range as high as 723°K (450°C) and some 

16 
of the reaction products, such as carbonyl fluoride (COF,) are 

known to be corrosive. Therefore, it was important to test the 

instrument under these conditions as well. 8 

Tests with the HFPO perfluoroalkylether (Table IV and Figure 

9) indicated that the instrument is capable of operating under 

these conditions as well, although etching of the inside surface 

of the pyrex cell occurred. The results were very similar to those 

obtained for the calibration fluids, indicating that nothing 

unusual was occurring under these conditions. Furthermore, the T, 

obtained for the HFPO perfluoroalkylether of 628.4"K (355.2"C) 

demonstrates that this instrument and procedure can be used to 

determine the thermal decomposition temperatures of 

perfluoroalkylethers. 

Finally, it is anticipated that thermal decomposition 

temperatures for several perfluoroalkylethers will be reported 

shortly, and that these will ultimately result in a correlation of 

thermal stability with chemical structure. 

SUMMARY OF RESULTS 

A computerized tensimeter and experimental procedure for the 

determination of the thermal decomposition temperature of 

perfluoroalkylethers have been developed. Testing of the 

tensimeter and procedure with standard fluids (bis(2-ethylhexyl) 

phthalate and trimethylolpropane triheptanoate) gave thermal 

decomposition temperatures which compared well with values 

previously determined by an isoteniscope technique. The thermal 

decomposition temperature was found both experimentally and 

17 
theoretically to be significantly dependent upon the gas 

volume/heated liquid volume ratio in the cell assembly. 

Recognition of this relationship places the instrument and method 

on a firmer mathematical basis and allows for more reproducible 

* 

results. Finally, the instrument and procedure were found to give 

satisfactory results under the more severe operating conditions 

required for the analysis of the PFAE hexafluoropolypropylene 

oxide. 

CONCLUSION 

The computerized tensimeter and corresponding experimental 

procedure are satisfactory for the determination of the thermal 

stability of perfluoroalkylethers. 

18 
Table I 

Phvsical Properties of Test Fluids 

I Bis(2- 

ethylhexyl) 

Kinematic Viscosity, CS ~ Phthalate 

@ 20°C 

38°C 

50°C 

99°C 

100°C 

Molecular Wt 

Density, a/ mL 

@ 20°C 

25°C 

390 

- 

0.981 

Trimethylolpropane 

Triheptanoate 

- 

15.2 

3.5 

- 

- 

470 

- 

0.963 

HFPO 

Perfluoro 

alkvlether 

2717 

444 

63 

- 

11000* 

1.92 

- 

* 

Average 

19 
Table I1 

Experimental Data: Bis(2-ethvlhexvl) Dhthalate 

V,O 

2.297 

2.538 

3.040 

3.498 

4.514 

Run No. I V:(mL) 1/T, I LOG Vq/Vl E' (kJ/mol.) 

0.0018407 0.345244 201.3 

0.0018492 0.236453 186.6 

0.0018537 0.089350 195.4 

0.0018648 -0.05006 190.4 

0.0018731 -0.21094 186.6 

Mean 192+6 

15 

19 

14 

17 

16 

5.169 

5.189 

5.546 

5.724 

6.676 
Table I11 

ExDerimental Data: TrimethvlolDropane Triheptanoate 

Run No. V: (mL) 

26 

27 

24 

25 

28 

2.488 

2.818 

3.256 

3.641 

4.227 

5.401 

5.478 

5.664 

5.688 

0.0016782 

0.0016808 

0.0016959 

0.0017008 

6.107 0.0017115 

LOG Vs/V1 1 El (kJ/rnol. 1 

0.291737 238.5 

0*165287 I 246.4 

0.028317 I 248.5 

-0.11065 

-0.23464 

251.9 

243.9 

I Mean 246+5 

21 
Run No. 

* 

15 

16 

22 

25 

21 

32 

26 

27 

Table IV 

ExDerimental Data: HFPO Perfluoroalkvlether 

V,O 

5.223 

5.174 

5.304 

5.338 

5.586 

5.817 

6.074 

7.259 

v,m 

2.40 

2.40 

2.90 

3.10 

3.50 

3.74 

4.01 

5.08 

~~~~ 

1/T, 

0.0015719 

0.0015699 

0.0015821 

0.0015891 

0.0015965 

0.0016003 

0.0016034 

0.0016112 

~~ 

LOG Vs/Vl 

0.304582 

0.296978 

0.102180 

0.027640 

-0.07862 

-0.12031 

-0.16385 

-0.27240 

El (kJ/mol.) 

261.1 

263.6 

262.8 

254.0 

255.6 

251.0 

254.4 

241.4 

22 
APPENDIX A 

Example calculations of log Tdl and E/2.3RTd1 for DOP (where R = 8.31 

J/mol OK) : \ 

- Td' ( OK) 10s TJ' E (kJ/moll E/2.3RT " ,I 

534 2.728 192.46 18.825 

542 2.734 192.46 18.547 

Note that log Tdl is small compared with E/2.3RTd' and that the change i 

log Td' (0.006) is small compared with the change in E/2.3RTd' (0.278). 

23 
Appendix B 

Definition 

Thermal decomposition temperature at V,/Vl=l 

Thermal decomposition temperature at V,/Vl not 

equal to 1 

Rate constant for a chemical reaction 

Preexponential factor 

Energy of activation 

A value related to the energy of activation 

Gas constant 

Absolute temperature 

Time 

Pressure 

Gas volume 

Heated liquid volume 

Total internal volume of the cell assembly 

Fluid volume in cell 

Cell volume 

Magnet volume 

Valve volume 

Liquid reactant in a thermal decomposition 

Gaseous product of a thermal decomposition 

Concentration of reactant B 

Number of moles of product gas D 

Volume of reactant B 

Order of reaction 

24 
REFERENCES 

1. Gumprecht, W.H., "PR-143 - P, New Class of High-Temperature 

Fluids," ASLE Trans., 2, pp. 24-30 (1966). 

* 

2. Sianesi, D., Zamboni, V., Fclntanelli, R., and Binaghi, M., 

"Perfluoropolyethers. Their Physical Properties and Behavior 

at High and Low Temperatures," Wear, 48, pp. 85-100 (1971). 

3. Snyder, C.E., Jr., and Dolle, R.E., "Development of 

Polyperfluoroalkylethers as High Temperature Lubricants and 

Hydraulic Fluids," ASLE Trans., u, pp. 171-180 (1976). 

4. Snyder, C.E., Jr., Gschwender, L.J., and Tamborski, C., 

"Linear Polyperfluoroalkylether-Based Wide-Liquid-Range HighTemperature Fluids and Lubricants, '' Lubr . Eng . , =, pp. 

344-349 (1981). 

5. Jones, W.R., Jr., Johnson, R.L., Winer, W.O., and Sanborn, 

D.M., "Pressure Viscosity Measurements for Several Lubricants 

to 5.5~10~ Newtons per Square Meter (8x104 psi) and 149O 

(300 OF)," ASLE Trans., 1_8, pp. 249-262 (1975). 

6. Jones, W.R., Jr., and Snyder, C.E., Jr., "Boundary Lubrication, Thermal and Oxidation Stability of a Fluorinated 

Polyether and a Perfluoropolyether Triazine," ASLE Trans., 

23, - pp. 253-261 (1980). 

7. Gumprecht, W.H., "The Preparation and Thermal Behavior of 

Hexafluoropropylene Epoxide Polymers," presented at the 

Fourth International Symposium on Flourine Chemistry, Estes 

Parks, CO, July 1967. 

25 
8. DuBoisson, R.A., and Sellers, S.F., "Some Subtle But 

Significant Differences Among Perfluoropolyether Vacuum Pump 

Fluids," Microelectronic Manufacturing and Testing, July 

1986. 

\ 

9. Sianesi, D., Pasetti, A., Fontanelli, R., and Bernardi, 

G.C., "Perfluororpolyethers by Photooxidation of Fluoroolefins," Chim. Ind. (Milan), u, pp. 208-221 (1973). 

10. Ohsaka, Y., "Recent Advances in Synthetic Lubricating Oils. 

(8). Perfluorinated Polyethers," Petrotech (Tokyo), a, pp. 

840-843 (1985). 

11. Jones, W.R. , Jr., Bierschenk, T.R. , Juhlke, T.J., Kawa, H., 

and Lagow, R. J., "The Preparation of New Perf luoroether 

Fluids Exhibiting Excellent Thermal-Oxidative Stabilities," 

NASA TM-87284 (1986). 

12. Fowler, L., and Trump, W.N., "A Recording Tensimeter and 

Decomposition Temperature Detector, 'I Analysis 

Instrumentation, Vol. 7, B. Connelly, L. Fowler, and R. 

Krueger, eds., ISA, pp. 141-144 (1969). 

13. Blake, E.S., et al., "Thermal Stability as a Function of 

Chemical Structure," J. Chem. Eng. Data, 4, pp. 87-98 

(1961). 

14. Jones, W.R., Jr., "Friction, Wear, and Thermal Stability 

Studies of Some Organotin and Organosilicon Compounds, I' NASA 

TN D-7175 (1973). 

15. "Vapor-Pressure-Temperature Relationship and Initial 

Decomposition Temperature of Liquids by Isoteniscope," ASTM 

Standard D-2879-75, 1980. 

26 
I 

El0 - - - - 

TEMPERATURE OVEN 

SAMPLE - - - 

t 

SIGNAL 

CONDITION 

FIGURE 1. - A SCHEMATIC DIAGRAM OF COMPUTERIZED TENSIMETER. 

TO NV 

SWLE 

CELL 

CONVERTER 

THEf 

TEMPERATURE 1 

PROGRmER J 

0 

CM 

FIGURE 2. - TENSINETER. 

v 

ilALVE 

HEATED TEST FLUID (Vi) 

MAGNETIC STIRRING BAR (V,) 

CD-89-40168 

FIGURE 3. - CELL ASSEMBLY DURING TEST. 
CH2CH3 

(a) BIS(2-ETHYLHEXYL) PHTHALATE (WP). 

2.0- 

1.7 

1.3 

- 

U \ 

1.0- 

a 0 -I 

.7 

.3 

0 

.0022 

0 

CH3CH2C[CH20C(CH2)5CH313 

(b) TRIMETHYLOLPROPANE TRIHEPTANOATE (TNPTH). 

- 

- 

- 

- 

FttFCF20lnCF2CF3 

CF3 

tc) HFPO PERFLUOROALKYLETHER. 

FIGURE 4. - CHEMICAL STRAUCTURES OF THE TEST FLUIDS. 

LOG(dP/dt) 

0.2383 

.2383 

.oooo 

,2383 

,5393 

,4144 

.6362 

.7154 

.8915 

1.0512 

1.1677 

1.3522 

1.5055 

1.6615 

1 I 8237 

1.9459 

TOTAL P. 

TORR 

30,56 

37.48 

42.68 

50.47 

60.85 

74.70 

95.48 

120.58 

156.93 

205.40 

277.25 

377.25 

516.14 

712.62 

988.74 

1379.11 

NUMBER 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

.0020 

.EMPERATWE. 

OC 

240.7 

245.0 

249.8 

255.0 

259.6 

265.1 

270.2 

275.4 

280.2 

285.2 

290.6 

295.4 

300.3 

305.1 

310.3 

315.5 

.0016 .0014 

1 /T 

dP/dt 

- 

1.731 

1.731 

1.000 

1.731 

3.462 

2.596 

4.327 

5.193 

7.790 

11.252 

14.714 

!2.504 

52.025 

15,875 

i6.648 

<8. 287 

FIGURE 5. - THERMAL DECOMPOSITION DATA FOR BIS(2- 

ETHYLHEXYL) PHTHALATE. 

28 
2.0 

1.3 

k 1.0 

L 

-u \ 

I 

I 

1 018 

i 

1 I 

275.4 

280.2 

285.2 

290.6 

5 295.4 

6 300.2 

305.1 

8 310.3 

9 315.5 

A T n .0016 .0014 

1 /T 

dP/dt 

5.193 

7.790 

11.252 

14.714 

22.504 

32.025 

45.874 

66.648 

88.287 

LOG(dP/dt) TOTAL P, 

0.7154 120.58 

,8915 156.93 

1 .OS12 205.40 

1.1677 277.25 

1.3522 377.65 

1 .SO55 516.14 

1.6615 712.62 1 1.8237 988.74 

1.9459 1379.11 

SLOPE = -9957 

Y-INTERCEPT = 18.87 

Td" 263.08 OC = 536.2 K. 

E' = 45.56 KCAL/MOL = 190.6t6.3 KJ/MOL. 

FIGURE 6. - TYPICAL THERMAL DECOMPOSITION CURVE, LOGARITHM 

OF RATE OF VAPOR PRESSURE RISE AS A FUNCTION OF RECIPROCAL OF ABSOLUTE TEMPERATURE. FLUID: BIS(2-ETHYLHEXYL) 

PHTHALATE. 

1/Td' 

FIGURE 8. - DETERMINATION OF Td FOR TRIHETHYLOLPROPANE TRIHEPTANOATE. LIT. Td = 317.2t2 OC: EXP. Td = 316.7i2 OC 

(589.9 K), E = 283i31 KJ/MOL: COR. COEF. = 0.964. 

I I ,-4 

.bo183 .00185 1/Td .00187 .00189 

1/Td' 

FIGURE 7. - DETERHINATION OF Td FOR BIS(2-ETHYLHEXYL) 

PHTHALATE. LIT. Td = 263.3*2 OC: 

EXP. Td = 264.2t2 OC (537.4 K). E = 329+23 KJ/MOL: 

COR. COEF. = 0.986. 

29 
cw\sA National Aeronautics and Report Documentation Page 

1. Report No. 

NASA TM-102116 

2. Government Accession No. 

7. Author@) 

Larry S. Helmick and William R. Jones, Jr. 

17. Key Words (Suggested by Author(@) 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D. C . 20546-000 1 

18. Distribution Statement 

3. Recipient's Catalog No. 

9. Security Classif. (of this report) 

Unclassified 

5. Report Date 

20. Security Classif. (of this page) 21. No of ~~ pages 22. Price' 

Unclassified 30 A03 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-489 1 

10. Work Unit No. 

505-63- 1A 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Prepared for the STLE-ASME Tribology Conference, Fort Lauderdale, Florida, October 16-19, 1989. Larry S. 

Helmick, Cedarville College, Cedarville, Ohio 453 14. William R. Jones, Jr., NASA Lewis Research Center. 

16. Abstract 

A computerized tensimeter and experimental procedure for determination of the thermal decomposition temperature (Td) of perfluoroalkylethers have been developed and tested. Both the apparatus and the procedure are 

described in detail. Results of testing with bis(2-ethylhexyl) phthalate and trimethylolpropane triheptanoate demonstrate that the reciprocal of the decomposition temperature is a linear function of the logarithm of the gas volume/ 

heated liquid volume ratio. The Td obtained for each compound at a gas volume/heated liquid volume ratio of 

one was similar to the value previously reported using an isoteniscope technique. Results of testing with a polymer 

of hexafluoropropylene oxide demonstrate that this instrument and procedure can be used to determine the Td of 

perfluoroalkylethers. 

Lubricants 

Perfluoroalky lethers 

Thermal stability 

Unclassified - Unlimited 

Subject Category 27 

'For sale by the National Technical Information Service, Springfield, Virginia 221 61 NASA FORM 1626 OCT 86