DIFFERENTIAL THERMAL ANALYSIS FOR ESTIMATION OF T H E RELATIVE THERMAL STABILITY OF LUBRICANTS A.
A.
K R A W E T Z A N D T H E O D O R E TOVROG
Phoenix Chemical Laboratory, Inc., 3953 West Shakespeare Ave., Chicago, Ill.
Experimental studies of samples representative of several distinctly different classes of materials have established the applicability of the techniques of differential thermal analysis to the study of the thermal decomposition of organic lubricant systems. The data illustrate effectiveness of differential thermal analysis measurements made a t ambient and a t elevated pressures as a screening tool for evaluation of the thermal stability of lubricating fluids.
thermal analysis (DTA) has long been used as investigation of geological systems. Its application to the characterization and analysis of clays and minerals is a matter of general knowledge. While the large majority of DTA applications are concerned solely with the determination of the temperatures a t which reactions occur, a number of attempts have been made to apply the method quantitatively. Since in the past several years over one thousand papers have been published on the qualitative and quantitative applications of DTA, no attempt has been made here to provide a complete list of references. For that purpose several very good reviews are available. Three of the most recent are by Murphy ( 7 4 , Wendlandt (76), and Garn ( 9 ) . T h e quarterly publication, Thermal Analysis Reaiew (75), is also an excellent source. The current research involves specifically the qualitative application of DTA to the study of the decomposition of organic IFFERESTIAL
D a tool for the
systems, particularly those of potential interest as liquid lubricants a t high temperatures. There have been relatively few other applications of DTA to this specific problem. Brown, Aftergut, and Blackington (7) and Behun et al. (3-5) suggest the applicability of DTA to the screening of thermal stability. Over the years many different methods have been devised for estimating the thermal stability of materials. The majority of these have been empirical in approach, yielding data of limited applicability. Macro (70, 7 7 ) and semimicro (3, 4) sealed tube methods, although successful, are of limited use for the rapid screening of the thermal stabilities of experimental materials, as they usually require a number of experimental runs a t various temperature levels to evaluate a given system completely, thereby leading to the consumption of quantities of scarce experimental materials. The use of the isoteniscope to obtain vapor pressure data for the determination of thermal stability is perhaps the most satisfactory technique currently used for screening ; however, its shortcomings are significant (756, 72). I n the application of DTA to the screening of organic systems for thermal stability, the question often arises as to the nature and origin of observed thermal effects. DTA, per se, provides
Table I. Thermal Decomposition of Some Organic lubricants
(Nitrogen atmosphere, 8.4 O F./min. scan rate)
Figure 1 . S c h e m a t i c diagram of differential thermal analysis cell A. 8.
C. D.
Monel cylinder, 1-inch diameter, 2.5 inches long Sample and reference wells, 0.073-inch diarneter, 1.3 inch deep, with nominal 2-mm. 0.d. glass liners Pressurizingtubes, 0.050inch 0.d. Drilled and tapped 0.125-inch NPT for thermocouple flttings
Compound MLO-63-24 MLO-63-25 FPPN 93-1 per-FPPN 91-1 OS-124 MLO-59-692 MLO-60-294 MLO-62-128 MLO-7277 MLO-57-637 MLO-59-287 MLO-59-98 MLO-60-50
Temp. of Beginning of Principal Thermal Effect Due to Decomposition (Excluding Decomposition of Residues Left after Boiling ) -1.0 atm. 5.1 atm. 810 813 ~~. 922 905 875 880 850 888 None observed 832 942 937 (618?) (673?) 824 None observed (636?) 842 740 725 832 823 625 630 767 625 818 748
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and are summarized in Table I . Some representative thermograms are shown graphically in Figures 2 to 9. While many of the features of the DTA curves cannot be identified positively from DTA data alone, the interpretations of the thermograms given below are, in so far as is known, consistent with available facts. In all DTA runs reported 170/230-mesh borosilicate glass microbeads were used as the reference standard. Glass microbeads were added to the samples to make their thermographic base lines as similar as possible to those of the blank determinations by equalizing the heat capacities and thermal conductivities of the sample and reference sides of the DTA cell. All runs were made under a nitrogen atmosphere. Within certain narrow limits the rates a t which the temperature range is scanned may be varied without greatly affecting the results of studies of this type. A scan rate of 8.4' F. per minute has been found satisfactory for the present studies. In the thermograms (Figures 2 to 9) the ordinate represents temperature difference in microvolts as measured by ironconstantan thermocouples. A simplified notation has becn adopted. An arrow in each figure indicates the endothermic direction in the ordinate, while the length of the arrow supplies the scale in microvolts. Temperature measured in degrees Fahrenheit is recorded on the abscissa. The scale of the abscissa is nearly linear, although small departures from linearity may arise because the temperature of the cell block may not always track exactly the program called for by the temperature programmer. Only temperatures of interest are noted on the abscissa. In the lower left-hand corner of each thermogram is the notation "T-1" or "T-2." This indicates which of the two blank runs described below is to be used as the base line for that thermogram. Some differences exist between the base lines obtained with different thermocouple pairs because perfect matching has not been possible. The data shown in Figures 2 to 9 are complete unsmoothed runs selected from the replicate runs made for each sample. The repeatability of the data varies from better than 1.5' F. for the determination of the temperatures a t which melting points, boiling points, and rapid decomposition reactions occur to more than +IO' F. for slow decomposition reactions which produce changes in slope rather than peaks in the thermograms.
only the magnitude and the temperature a t which the observed absorptions and/or evolutions of heat occur. A principal object of this work is to show that the techniques of DTA applied a t ambient as well as elevated pressures can be of invaluable assistance in the analysis of data by permitting a distinction to be made between pressure-dependent and -independent reactions, and by providing a means for the isolation of thermal effects due to decomposition reactions which occur a t or near the normal boiling point of the sample being studied. A DTA apparatus capable of performing the indicated investigations a t elevated pressure has been designed and DTA applied to the investigation of the thermal stabilities of several organic systems, with particular emphasis on its use as a screening tool. Experimental
Apparatus. T h e heart of the DTA apparatus is the Monel cell block shown schematically in Figure 1. The cell block is heated in a standard crucible furnace modified with supports to hold the cell assembly in the center of the furnace. The furnace heating program is supplied by a temperature programmer consisting of a motor-driven voltage run-up source, the output of which is opposed to that of a thermocouple located in the DTA cell. The difference in e.m.f. between the voltage source and the thermocouple is received by a zerocentered meter-relay combination which controls the temperature of the furnace a t the level required to keep the e.m.f. difference equal to zero. Thus, when the output of the voltage source is increased linearly, the temperature of the furnace also increases linearly. The only systematic deviation from linearity is caused by the nonlinear e.m.f.-temperature response curve of the thermocouple. This variation is not serious, however, and the system may be adjusted so that deviations from linearity are not greater than &5' F. over the range 100' to 1100' F. A matched pair of thermocouples is used to measure the temperature difference between the sample and the reference. This is contained in 2-mm. glass capillary tubes which fit snugly into the wells of the cell block. Normally 2 to 4 mg. of sample are required. The temperature of the cell block (measured in a dummy reference well) and the temperature difference, suitably preamplified, are recorded simultaneously on a dual-channel strip-chart recorder. Procedures and Results. Thermograms have been obtained a t 1.0 and 5.1 atm. for several materials. I n the following sections the results for each sample are described in detail
BLANK (BOROSILICATE GLASS MICROBEADS) THERMOCOUPLE PAIR T - l NITROGEN
1
8.4 OF./MIN.
.
IO pv ENDOTHERMIC
1.0 ATM.
-
5
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I
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T- I
Figure 2. 192
Representative thermograms
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
"E
"E
temperature up to about 850' F. Above that temperature, the endothermic tendency diminishes somewhat as the thermogram begins to flatten out slightly. No other specific features are observed in either thermogram. Random noise, presumably thermal in origin, is clearly discernible in both thermograms (Figure 3). TRIFLUOROMETHYLPHENOXY PHENOXY PHOSPHONITRILE (FPPN 93-1). FPPN 93-1 is an 85/15 mixture of derivatives of phosphonitrile trimer and tetramer. The substituents are phenoxy and trifluoromethylphenoxy groups in a 2 to 1 ratio, respectively, which are statistically distributed on both phosphonitrile rings. At 1.0 and 5.1 atm. an endothermic trend extends from approximately 571' to 880' F. This general endotherm undoubtedly represents the evaporation of the sample, which also causes the numerous small endothermic features observed in this region of the thermogram. There is some roughness in the region about 880" F. which is suggestive
Designation of the temperatures of some of the smaller, less dramatic thermal effects may a t times be difficult. Dashed lines have been added to some of the thermograms to aid in distinguishing such effects from the over-all trends of the base lines which represent the sums of the temperature differences due to imperfect thermocouple matching and to changes in the heat capacities and thermal conductivities of the samples. BLATK,THERMOCOUPLE PAIR T-1. Neither the run a t 1.0 atm. nor the run a t 5.1 atm. shows any special features other than a slight general drift in the exothermic direction. Random noise is somewhat more prominent a t the higher pressure. The general stability of the electrical measurement system under nonprogramming conditions suggests that the observed noise is thermal rather than electrical in origin (Figure 2). BLANK,THERMOCOUPLE PAIRT-2. At both 1.0- and 5.1atm. total pressure, the thermograms exhibit a definite drift in the endothermic direction. The trend is nearly linear with
BLANK (BOROSILICATE GLASS MICROBEADS) THERMOCOUPLE PAIR T-2 NITROGEN
1.0 ATM,
5 300
400
500
600
700
800
900
1000
OF.
5.1 ATM.
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3. Representative thermograms
Figure
FPPN
93-1
NITROGEN 8.4 V M I N. A
57I
880 925 898
571
875
909
T- 2
Figure 4.
0
IO00
F.
OF.
Representative thermograms VOL. 5
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MLO - 63- 2 5 NITROGEN 8.4" E/ M IN.
I
860
I
I
936 905
I
OF;
,
922 962
T- I
Figure 5.
OF.
Representative thermograms
O S - 124 NITROGEN 8.4 O E/ M IN.
987 \
5.1 A T M .
832
T-I Figure 6.
993
1046
OF,
Representative thermograms
of the presence of the beginning of an exothermic reaction. When the temperature reaches 898' F. an exotherm becomes apparent and the thermogram enters into a pronounced exothermic break which reaches its maximum intensity a t 925' F. At 5.1 atm. the initial endothermic process is somewhat suppressed, but the exothermic reaction occurring between 898" and 925' F. a t 1.0 atm. has been completely resolved, so that an intense exotherm begins a t 875' F. and passes through a sharp peak a t 909' F. This exotherm is clearly the result of the thermal degradation of the sample. An endothermic peak of unknown origin is found a t 1000° F. (Figure 4). PER-TRIFLUOROMETHYLPHENOXY PHOSPHONITRILE(PERFPPN 91-1). per-FPPN 91-1 is an 85/15 mixture of hexa(mtrifluoromethy1phenoxy)triphosphonitrile and octa (m-trifluoromethy1phenoxy)tetraphosphonitrile. The thermograms for this material in a general fashion resemble those for FPPN 93-1. A major decomposition exotherm begins a t 888" F. and passes through a peak a t 933" F. a t 1.0 atm. At 5.1 atm. the decomposition exotherm which begins about 850' F. produces a very sharp peak a t 897' F. For this material as well as for FPPN 93-1 the intensity of the decomposition exotherm is markedly less a t the lower pressure. Despite the similarities in the major decomposition exotherms for these materials, perFPPN 91-1 may be slightly less stable because a t 1.0 atm. a series of moderately intense endotherms and exotherms in the 194
OF.
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
temperature range 672' to 863' F. precedes the decomposition exotherm. At 5.1 atm. an endotherm is found a t 695' F. corresponding to the endotherm a t 672' F. in the thermogram a t 1.O atm. HEXAPHENYLTRIPHOSPHONITRILE, MLO-63-24. A small sharp endothermic peak a t 405' F. a t 1.0 atm. and 407' F. a t 5.1 atm. is due to a pressure-independent transition or reaction occurring in the solid phase. A prominent endotherm follows at the melting point (450' to 452' F.). A broad peak due to the boiling of the sample occurs at 978' F. a t 1.0 atm. At 5.1 atm. the corresponding peak is diffuse with a broad maximum a t about 999' F. An exothermic shift in the slope of the thermogram is found at about 810' F. a t both 1.0- and 5.1atm. pressure. It is believed that this feature is the result of the thermal decomposition of the liquid sample. PENTAPHENOXYMOXO - (m - PHEYOXYPHENOXY)TRIPHOSPHONITRILE, MLO-63-25. At 1.0 atm. a fairly strong boiling endotherm begins a t 860' F. with a peak a t 905' F., followed closely by an exothermic decomposition which leads to a peak a t 936' F. At 5.1 atm. only the very beginning of the boiling peak is found. A sharp exothermic deflection a t 922' F. marks the beginning of a decomposition exotherm with a peak a t 962O F . The exothermic feature seems to be pressuredependent. At the higher pressure it appears to be much enhanced. Presumably the suppression of boiling allows a
MLO - 62 - I 28 NITROGEN
8.4 O E/ M I N.
A
5pv. ENDOTHERMIC
-------\ 1.0 ATM.
5.1 ATM. II
636
842
\
T- I
I
1011
OF.
L
~~
Figure 7.
Representative thermograms
greater portion of the sample to remain in a liquid state in the cell, where its decompoTition may produce a detectable temperature effect (Figure 5). POLYPHENYL ETHER (MIXEDISOMERS), OS-124. At 1.0 atm. the thermogram of this sample shows no significant features other than a sharp, intense endothermic boiling peak a t 987' F. At 5.1 atm. the boiling peak a t 1046' F. is preceded by a broad exothermic feature beginning a t 832' F. This exothermic feature is not detected a t 1.0 atm., as it is apparently masked by the shoulder of the boiling endotherm and the vaporization which precedes it. O n the basis of the DTA data this material appears to be very stable. Only the gradual exothermic drift which a t 5.1 atm. starts a t 832' F. and leads into a broad peak a t about 993' F. suggests the presence of thermal decomposition (Figure 6). BIS-(PHENOXYPHENOXY)BENZENE, MLO-59-692. T h e thermogram obtained a t 1.0 atm. is very similar to that obtained for OS-124. The boiling of the sample produces a n intense endotherm with a peak a t 990' F. A very weak, broad exotherm beginning a t 942' F. marks the onset of decomposition. At 5.1-atm. pressure a similar exotherm beginning a t 937' F. becomes much stronger after 1065' F. Very little evidence of the boiling endotherm is seen a t 5.1 atm., for that process not only is greatly suppressed but the small thermal effect associated with residual boiling is apparently counterbalanced by the decomposition exotherm. The sample is apparently stable. DEEP DEWAXED PARAFFINIC MINERALOIL, MLO-60-294. The sample also contains the following additives: 1% tricresyl phosphate, 1% phenyl-1-naphthylamine, and O . O O l ~ o silicone additives. At 1.0 atm. a minor endqtherm a t 51 5' F. is the first evidence of evaporation in the sample. Additional endothermic peaks occurring a t 668' and 695' F. reflect continuing evaporation. The major boiling endotherm is found a t 720' F. A second major endotherm follows the first, beginning a t 762' F. with a broad peak a t 823' F. A major exotherm which is certain to be associated with the decomposition of the residues of the original material occurs a t 990' F.; however, a weak exothermic feature beginning a t 618' F. shows that this reaction is probably a continuation of a process that starts well before boiling occurs. At 5.1 atm. the first indication of evaporation is a small endotherm a t 673" F. T h e major boiling endotherm begins a t 780' F. with a peak a t 824' F. An exothermic reaction which seemingly occurs simultaneously first becomes apparent after 824' F. A small exo-
thermic peak is seen a t 862' F. T h e weak exotherm found a t lower temperatures a t 1.0 atm. is almost completely obscured by evaporation processes a t 5.1 atrn., although a slight trace still remains a t 673' F. DEEP DEWAXED PARAFFINIC MINERAL OIL, MLO-62-128. T h e sample also contains the following additives: 1% 4,4'methylenebis-(2,6-di-tert-butylphenol),1% tricresyl phosphate, and O . O O l ~ oantifoaming agent. At 1.0 atm. a boiling endotherm is found a t 730' F. A sharp exothermic break follows a t 802' F. This exotherm is due to the decomposition of sample residues left after boiling. At 5.1 atm. the boiling endotherm occurs a t 842' F. That endotherm is interrupted by a strong exotherm which is much more intense than the one observed a t 1.0 atm. At 5.1 atm. the principal decomposition of the sample probably occurs simultaneously with the boiling of the sample, although an exotherm seen a t 636" F. suggests the presence of some degree of decomposition a t lower temperatures (Figure 7). SUPERREFINED NAPHTHENIC MINERALOIL (COMMERCIAL G R A D E )MLO-7277. , At 1.0 atm. two endotherms a t 530' and 632' F. and a n exotherm a t 725' F. occur before the main boiling peak which begins a t 784' F. with peak a t 827' F. The two endotherms presumably represent various stages in the evaporation of the sample. (They are repressed a t 5.1 atm.) The initial exotherm a t 725' F. seems to be the forerunner of a small exothermic peak a t 930' F. Thus the exotherm a t 725' F. may be due to decomposition of the sample itself, while the following exotherm arises from the decomposition of residues left after boiling. The first exotherm is more pronounced a t 5.1 atm., where it is seen a t 740' F. The boiling endotherm a t 5.1 atm. appears a t 812' F. with a peak a t 870' F. A broad exotherm with a peak a t about 940' F. follows. DIPHENYLDI-~-DODECYLSILANE, MLO-57-637. At 1.O atm. two small endotherms a t 645' and 698' F. precede a broad endotherm beginning a t 758' F. which marks the onset of significant vaporization. An exotherm which occurs a t 823' F. just before the main boiling peak a t 844' F. is indicative of the beginning of thermal decomposition. A similar exotherm is found a t 832' F. a t 5.1 atm. At 1.0 atm. a broad exotherm between 872' and 1037' F. seems to be an extension of the effect which begins a t 823' F. At 5.1 atm. there is a suggestion of a n endotherm a t 692' F. The exothermic shift beginning a t 832' F. precedes the main boiling peak a t 885' F. but does not carry over into the region beyond the boiling VOL. 5
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MLO-59-287 NITROGEN 8.4 E/ M IN .
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I
630
890
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920
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1028 " F
T- 2
Figure 8.
I
Representative thermograms
720 767 7 9 8
T-2
Figure
9.
Representative thermograms
peak, although there may be a slight plateau just after the main boiling endotherm. CHLOROPHENYL METHYLSILICONE,MLO-59-287. At 1.0 atm. the thermogram exhibits a pronounced roughness of a general endothermic character beginning a t 630' F. At 5.1 atm. a similar roughness begins a t 625' F. I t is probable that this feature reflects the competition between an exothermic decomposition beginning a t about 625' F. and the endothermic volatilization of relatively low molecular weight products resulting from the decomposition. At 1.0 atm. a shift in the exothermic direction a t 890' F. is followed by a strong break a t 1005' F. At 5.1 atm. the first exotherm is more pronounced and occurs a t 920' F.; the second is seen a t 1028' F. These effects are undoubtedly related to the final breakdown and loss of the sample from the D T A cell (Figure 8). (50-50 BLEND)PHENYLMETHYLSILICONEAND ESTEROF TRIMETHYLOLPROPANE AND HEPTANOIC ACID, MLO-59-98. At 1.O-atm. total pressure, the first evidence of evaporation or boiling occurs a t 625' F. At 657" F. a series of fairly small but very sharp peaks begins and continues to 770' F. No single large boiling peak is found. The observed effect suggests that 196
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
the rapid boiling of the sample is accompanied by a pronounced exothermic reaction in the sample. The competition between the two produces the sawtooth pattern observed in the thermogram in the region 625' to 770' F. A small exotherm is seen a t 840' F. At 5.1 atm. the thermogram turns to\vard the endothermic direction a t 720' F. as the boiling peak begins. Before the boiling process can be completed, the second reaction intervenes. At 767' F. the thermogram reverses its trend and reveals a sharp exothermic peak a t 798' F. The simultaneous competitive processes observed in the thermogram for 1.0 atm. have been resolved into separate effects at the higher pressure (Figure 9). ESTER OF TRIMETHYLOLPROPANE AND HEPTANOIC ACID, MLO-60-50. A series of endothermic peaks starts at 700' F. and continues up to 748' F. a t 1.0 atm. The latter temperature marks the peak of the last and largest endotherm of the group. Above 748' F. a rapid exothermic reaction produces a sharp peak a t 760' F. A similar phenomenon occurs a t 5.1 atm. A broad endotherm beginning at 698' F. lvith peak a t 725' F. marks the beginning of boiling or rapid evaporation. At the elevated pressure the principal endotherm is not so sharp
as before. After its maximum a t 818' F . the thermogram breaks sharply in the exothermic direction and passes through an exothermic peak a t 836' F. in the same manner observed in the c a ~ of e the lo\v pressure run. Discussion
Phosphonitriles. Altogether four phosphonitriles have been studied by D T r l . Decomposition exotherms were found in every case. O n the basis of these data it is possible to list the four materials in order of their thermal stabilities: M L O 63-25 (most stable), FPPN 93-1 and per-FPPN 91-1 (about the same stability), MLO-63-24 (least stable). I n two instances (YPPN 93-1 and per-FPPN 91-1) extremely sharp decomposition exotherms are found at 5 atm. This evidence suggests a relatively rapid reaction which proceeds to completion. Polyphenyl Ethers. T h e two examples of this class of material rvhich have been studied are reported to be very nearly identical chemically (8). The samples, however, differ some\vhat in appearance: OS-124 is straw colored while MLO-59-692 is \vater white. Polyphenyl ethers develop a straiv color upon exposure to sunlight. Data for both samples are reported? to demonstrate the effect of such photochemical drcomposition upon their thermal stabilities. The thermograms are in the main very similar; however, OS-124 is the le.;>stable, h i c e the exotherm preceding its boiling peak a t 5.1 atni. begins approximately 100' F. before the corresponding esotherm for MLO-59-692. This evidence indicates that, ivhile the polyphenyl ethers are among the most stable of compounds. the action of light upon them may prove detrimental to thcir thermal stability. Superrefined Mineral Oils. Two of the three superrefined mineral oils studied are alike in that their thermograms show relatively little evidence of thermal decomposition below their apparent boiling points. T h e two samples, MLO-60-294 and 5ILO-62-128, consist mainly of a hydrocarbon fraction boiling a t approximately 720' to 730' F . They differ only in the additives used as oxidation inhibitors. Examination of the thermograms of these two samples reveals few really significant differences in the thermal stability of either the oils or the residues left after boiling. Thus, the activities of the additives in question are about the same with regard to their effect on thermal stability. The third sample, MLO-7277, is a petroleum fraction boiling at approximately 825' F. T h e first evidence found for the thermal decomposition of this material is an exothermic shift, in the region 725' to 740' F. The thermal decomposition of both MLO-60-294 and MLO62-128 begins during or just before boiling. At 5.1 atm. the boiling endotherms for both samples seem to be abruptly interrupted by a strong exothermic reaction a t 824' F. for XILO-60-294 and a t 842' F. for MLO-62-128. There is some evidence that this exotherm, \vhich could only be attributable to the thermal decomposition of the samples: actually begins a t somewhat lower temperatures as suggested by the exotherms found a t 673' F. for MLO-60-294 and a t 636' F. for MLO-62128. Similarly an exotherm a t 618' F. precedes the boiling peak of MLO-60-294 a t 1.0 atm. Holyever, in view of the relative strengths of all the observed exotherms it is probably more accurate to consider the ones occurring a t the higher temperatures as truer indications of relative thermal stabilities of the samples. Silicone Compounds. O n e silane (MLO-57-637) and one silicone Auid (MLO-59-287) have been studied. Silicone fluids typically do not exhibit sharp endothermic and exothrrmic effects such as are found for most other classes of compounds. T h e most pronounced effects generally observed
are relatively gradual changes in the slope of their thermograms. Thermal decomposition apparently is reflected in two competing phenomena: degradation of the polymer chain to produce volatile fragments, folio\\ ed by rapid evaporation of the fragments so produced. Both processes apparently continue over a wide temperature range. In the thermogram for MLO-59-287 the onset of this process is marked by an abrupt increase in the average base line noise level of the thermogram a t approximately 625' to 630' F. Initial thermal decomposition is thought to begin a t this temperature. The silane (MLO-57-637) is relatively more stable than the silicone fluid. Just before the peak of the boiling endotherm the slope of the thermogram briefly shifts from the endothermic trend characteristic of boiling. After the endotherm the thermogram returns, not to the original slope of the base line, but to a new slope related to the initial break. This break, occurring bet\veen 820' and 830' F., is believed to represent the beginning of thermal decomposition. Poly01 Esters. Two examples of this class of compounds were examined: MLO-59-98 \vhich is a blend with a silicone oil, and MLO-60-50 which is a pure ester. As expected, the thermogram of MLO-59-98 exhibits some of the characteristics of a silicone. At 625' F. a roughness becomes apparent in the thermographic base line (compare with a similar effect at 625' to 630' F . for MLO-59-287). However, the multiple boiling endotherms that follow at 1.0 arm. are undoubtedly due to the ester, as is the pronounced decomposition exotherm which starts a t 767' F. with a peak a t 798' F. The pure poly01 ester, MLO-60-50, also exhibits multiple boiling endotherms. Decomposition exotherms w,hich begin a t 760' F. a t 1.0 atm. and 836' F. a t 5.1 atm. are partly overshadowed by the larger endotherms. Conclusions
DTA provides a powerful tool for the study of thermal decomposition reactions. T h e technique of studying a given system a t two pressures has proved very useful in the difficult task of assigning interpretations to the often complex thermograms. In this manner pressure-dependent processes such as boiling may be distinguished from the pressure-independent processes such as melting and some decomposition reactions. More significantly, it has been demonstrated that this technique can effect the resolution of simultaneous processes such as boiling and thermal decomposition so that they may be studied separately. For the present Lvork it \vas found that elevated pressure data best complemented those obtained at 1.O atm. Hoivever, reduced-pressure DTA measurements ivhich have been found czry useful in the determination of the vapor pressures of small quantities of organic materials (2, 73) may find future application in the study of thermal decomposition reactions. There is much information to be gained from the study of the thermograms taken individually as !vel1 as collectively. Some aspects of the interpretation of the thermograms have been discussed. However, to summarize briefly the application of D T A as a screening tool, the principal thermal effects resulting from decomposition reactions of the samples studied are shown in Table I . Data such as these may be conveniently used to shape decisions regarding relative thermal stability of these materials. I t is also possible to supplement the content of D T A investigations. Several types of effluent gas analysis may be performed: Effluent gases may simply be monitored by a thermal conductivity, ionization, or electron-capture detector, or they may be collected and reserved for infrared or chromatographic VOL. 5
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Literature Cited
(4) Behun, J. D., Kan, P. T., Preprints of Symposium on Behavior of Petroleum Products and Derivatives in Space, Division of Petroleum Chemistry, 144th Meeting, ACS, Los Angeles, Calif., March 31-April 5 , 1963, Vol. 8, No. 2, pp. C-117C-136. (5) Behun, J. D., Lajiness, W. G., Lenk, C. T., Preprints of General Papers, Division of Petroleum Chemistry, 142nd Meeting, ACS, Atlantic City, N. J., Sept. 9-14, 1962, Vol. 7, No. 3, p. 69-74. B1ake, E. S., Hammann, W. C., Edwards, J. W., Reichard, T. E., Ort, M. R., J . Chem. Enp.. Data 6, 87-98 (1961). (7) Brown, G. P., Aftergut, S., Blackington, R. J., Zbzd., 6, 125-7 (1961). ( 8 ) Dolle, R. E., Project Engineer, Air Force Materials Laboratory, Research and Technology Division, Air Force Systems Command, United States Air Force, Wright-Patterson Air Force Base, Ohio, private communication, 1964. ( 9 ) Garn, P. D., “Thermoanalytical Methods of Investigation,” Academic Press, New York, 1965. (10) Klaus, E. E., Fenske, M. R., Tewksbury, E. J., Air Force Materials Laboratory, Research and Technology Dibision, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, AFML-TR-65-112 (April 1965). (11) Klaus, E. E., Fenske. M. R., Tewksbury, E. J., Directorate of Materials and Processes, Aeronautical Systems Division, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, WADD-TR-60-898, Parts I, 11, and I11 (February 1961, 1962, 1963). (12) Krawetz, A. A., Krawetz, J., Directorate of Materials and Processes, Aeronautical Systems Division, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, ASD-TDR63-220 (March 1963). (13) Krawetz, A. A., Tovrog, T., Reu. Scz. Instr. 33, 1465-6 (1962). (14) Murphy, C. B., Anal. Chem. 36,347R-54R (1964). (15) Redfern, J. P., ed., Thermal Analysrs Reurew, Stanton Instruments, Ltd., London, published quarterly. (16) Wendlandt, W. W., “Thermal Methods of Analysis,” LViley, New York, 1964.
(1) Ballentine, 0. M., Wright Air Development Center, WrightPatterson Air Force Base, Ohio, WADC Tech. Rept. 54-417 (March 1955). (2) Barrall, E. M., 11, Porter, R. S., Johnson, J. F., Anal. Chem. 37,1053-4 (1965). (3) Behun, J. D., Kan, P. T . , Preprints of General Papers, Division of Petroleum Chemistrv. 145th Meeting, ACS, New York, N. Y., Sept. 8-13, 1963, V o l . 8, No. 3, pp. y5-48.
RECEIVED for review November 23, 1964 RESUBMITTED September 16, 1965 ACCEPTED March 30, 1966 Work performed under Contract No. AF33( 657)8771 administered by the Directorate of Materials and Processes, Aeronautical Systems Division, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio.
analysis. Differential pressure measurement may also be added to the standard D T A apparatus. And, of course, thermal gravimetric analysis may be used. I n spite of the specific advantages offered by each of the above techniques, it is felt that simple DTA investigations can in themselves provide sufficient information to permit the screening of materials for thermal stability. DTA permits the achievement of this end with a minimum expenditure of sample. Hence, it is particularly adaptable to the study of substances which are available only in experimental quantities. I n cases \+here DTA has suggested the promising nature of an experimental compound, the various processes involved in its thermal degradation are most easily investigated through the use of properly designed experiments of the types suggested by Behun et al. (*?. 4) and Klaus, Fenske, and Tewksbury (70, 77). This manner of exploiting the advantages of DTA is an effective and economical means of accumulating the evidence necessary to justify the preparations preliminary to testing on a conventional scale. Acknowledgment
The authors acknowledge with appreciation the assistance of the following Air Force personnel who contributed to the attainment of the objectives of this research: R. E. Dolle, project engineer; F . J. Harsacky; and H. Schwenker.
DEPENDENCE OF LITHIUM GREASE DROPPING POINTS ON T H E SOLUBILITY PARAMETER
OF THE OIL COMPONENT R. N. BOLSTER A N D R. C. L I T T L E U. S. hraval Research Laboratory, Washington, D . C.
s
oAP-oil systems exhibit a variety of phase equilibria and solubility characteristics which depend upon both the soap and the solvent. Even upon consideration of a particular soap species. the effects of changes in solvent character are understood only qualitatively. These changes are not simply related to the usual physical properties such as density, boiling point, viscosity, or dielectric constant. T h e inability to correlate solvent effects in soap-oil systems with suitable solvent characteristics has resulted in a large body of rather poorly 198
I & E C P R O D U C T RESEARCH A N D DEVELOPMENT
organized information on soap-solvent systems. Moreover, estimation of solvent effects on the solubility and phase behavior of soaps in untried oils has remained more a r t than science. A substantial degree of order can be achieved if the effect of solvent in these systems is assessed by use of the Hildebrand solubility parameter concept ( 5 ) . T h e solubility parameter is a measure of the intermolecular forces in a liquid. More specifically, it is the square root of the energy of vaporization
