Phase State and Thermal Transitions of Greases M. J. YOLD, G . S. HATTIANGDI, AND R. D. VOLD University of Southern Calgornia, Los Angeles 7, Calif.
+
solved in hot carbon dioxide-free 95% ethyl alcohol and X-ray diffraction patterns at room temperature and neutralized to the phenolphthalein end point Kith an aqueous temperature to the differentialheating curves from solution of the appropriate base for barium, lithium, and sodium 1iWefaction Point have been obtained for numerous soaps, Calcium tallowate was pre &red by mrtathosis of the samples of aluminum, barium, calcium, lithium, sodium, sodium soap with calcium chlorife as dcscribud by Vold, Hattiangdi, and Vold (19). Pure stearic acid [5R3 and 5R4 of and mixed base commercial lubricating greases and for (1911was also employed to make samples of lithium and sodium the corresponding oil-free soaps. At room temperature stearate. Lithium 12-hydroxystearate was recovered from its the greases contain principally finely divided crystalline grease by extraction of the oil with n-hcptane. soap in oil, but the crystallites may be modified by in All of the soaps were vacuum dried to constant weight a t 90" C. except lithium and sodium stearates which were air dried a t situ formation as compared with their structure when 110 c. and calcium soap hydrates which were air dried at 50" c. prepared in the oil-free The nature and extent of and subsequently analyzed for water content. such effects vary with the soap cation and with the ineorX-ray diffraction patterns were obtained with a North Ameriporation of additives. Lithium soap crystallites are alcan Philips Company self-recording x-ray spectrometer as previmost unaffected by in situ formation except in size, while at the other extreme, greases stabilized with barium acetate contain crysTABLEI. CHARACTERIZING DATAFOR COMMDRCIAL GREASES tallites of structure entirely different from soap that of either soap or salt. On heating, the Congreases undergo thermal transformations tent sample wt.' Oil Additional which are in some cases closely related to the No. % Soap Anion Characteristics H?O Data polymorphic transformations of the oil-free Al-1 14 Teohnical stearate" 0.0 Presumed dintearate 9 Technical stearate" 0.0 Presumed distearate AI-2 soaps. In other cases there exist more comAI-3 11 50% P, 40% Str, 1200 S.S.U.; 100 F. 0.0 plex solubility relations involving swelling of 1 0 7 016 60% 8 40% Str 350 S.S.U., looo F. Trace A1-4 12 the soap to form liquid crystalline solutions ... 0.3 Described' i s basio, Ba-1 20 Tallowhe 2% glycerol of oil in soap.
...
Ba-2
..
tallowate acetate (mole basis) 18 63.% tallowate; Wijs 22 viscosity index iodine value, 45 17% acetate (wt. 604 8.9 U looo F. basis) Californ& naphthenic 17 Tallowate, 42 titer ... 8 Tallowate, 42 titer 18 30% P , 20% Str, 320 S.S.U.: '100. F. 5 0 7 01 10 30% %, 20% Str, 310 S.S.U.,100O F. 50% 01 15 30% P, 20% Str, 310 S.S.U.,100' F. 50% 01 18.6 Tallowate; Wijsio- 30 viscosity index dine value, 45 California naphthenic 219 S.S.U. at l o O D F. 20
*/a
I/P
Ba-3
T
HIS paper presents the results of a study of
the x-ray diffraction patterns and differential heating curves of samples of different types of commercial greases and of oil-free soaps of similar composition. It was desired to determine to what extent greases of different types exhibit comparable behavior, whether there is any evidence that greases are other than mechanical dispersions of soap in oil a t room temperature, whether the crystal structure of the soap is affected by its in situ formation in oil during grease manufacture in contrast with its preparation by precipitation from oil-free aqueous media, what type of solubility relations exist at temperatures above room temperature, and whether or not these are simply related to the thermal transitions of the oil-free soap. EXPERIMENTAL PART
All of the greases were commercial samples. Characterizing data of these greases are presented in Table I. The soa s used for comparison with the greases were usuafly synthesized in t h i s l a b o r a t o r y . Aluniinum distearate was prepared from pure stearic acid by the method of Smith, Pomeroy, McGee, and Mysels (16). "Tallowates" were prepared from a mixture of 50% technical oleic acid (Eastman T343), 30% stearic acid (Armour Chemical Company, Neofat 1-65), and 20% palmitic acid (Neofat 1-56). The acids were dis-
Ca-1 Ca-2 Ca-3 Ca-4 Ca-6
Ca-6 Ca-7
..
CaR-1 11 CaR-2 (0 Li-1 14 Li-2
'
...
Abietate (unknown purity) Resin Technical stearate Technical palmitateo Tallowate, 42 titer
...
0.0
...
0.01
0 . 5 . q oxidation inhibitor
1.5 0.3 Trace
No glycerol
...
.. I
.
.
...
Trace Contains graphite 1.5
... 3
1
Li-5
0.0 13.7 50'7 P 40% Str, 59 S.S.U. a t 100° F. 18% '01 index, ... 12-Hydroxystearate 56108viscosity cs. at 100e .'E 0.5 20 Tallowate, 42 titer 16 50% P 40% Str, 420 S.S.W.'at 100' F. 0.0 10% bl 12 30% P, 20% Str, 1100 S.S.U.a t 100" F. 0 . 0 50% 0 1
Li-6
Na-1 Na-2 Na-3 Na-4 Na-5 M-1 M-2
...
6
..
...
... ... .
Hydrogedated fish oil acids
...
I
.
... ..* ... ...
mole excess LiOH Low temp. grease Li soap base 0.3% AI soap 1/a
..,
...
... ...
No. 3 lime 8081) base cup grease
0.0
14
...
1.7% aly~er?l,O.O5% oxidation inhibitor
... 0.0
Li-3 I,i-4
...
... ...
Trace
...
..
1.5% glycerol
... 1.8% glycerol
... ...
... ... 0.0
No. 3 fiber greaie soda soap base Driving journal opd. AI-Na soap
..
Lime soda soap base
a Probably about JO% oleate, 50% stearate, and 40% palmitate. 1, P refers t o palmitate, Str to stearate, and 01 to oleate, though this may represent total unsaturates calculated as oleate. 0 Probably contains substantial amounts of CIS, C I P ,and Cis acid radicals as well a8 palmitate.
2539
INDUSTRIAL AND ENGINEERING CHEMISTRY
2540 DIFFRACTION
d/n
=
&,
DIFFRACTION ANGLE (269
ANGLE (28)
ANGSTROM
Vol. 41, No. 11
UNITS
Figure 1. X-Ray Diffraction Patterns of Aluminum Stearates I Technical aluminum monostearate, 9.71 % AliOa; 11, laboratory ;reparation, aluminum distearate from pure stearic acid, 9.09% Ale@; 111, sample I1 after cooling slowly from 200’ C.; IV, technical aluminum distearate, 9.16yo AlzOi; V, terhuical aluminum tristearate, 6.71 % AlaOi. Line at 4.2 6.belongs t o stearic acid
ously described (22). The patterns obtained all consist of a more or less sharply crystalline soap attern superposed on a broad halo due to oil. Resolution of t l e one from the other is often a matter of some difficulty, particularly where it is sought to determine whether a relatively weak line characteristic of the oil-free soap is or is not present in the grease. Extraction of the oil from the grease in order to obtain sharper patterns, however, would defeat the purpose of the investigation by giving the soap an opportunity to disgregate or aggregate further or even partially recrystallize, and so it was not attempted. Differential heating curves were obtained against white mineral oil (Kujol) at a heating rate of 1.5 degrees per minute using the calorimeter and procedure already described (16). In a polycomponent system the occurrence of heat absorption over a range of temperature may result from the existence of a eutectoid a t the lowest temperature of the range, or from changing proportions of two or more phases which coexist over the whole range or its lower portion, as already discussed (21). In the absence of experiments a t varying heating rates on the same sample, distinction between these two types of phase change can often be made on the basis that the latter will generally yield peaks of more gradual slope. However, it cannot be assumed that the temperature of the sample at the time when the absorption of heat ends is coincident with the equilibrium value of the upper temperature limit of such a range. In addition it should be remembered that a sloping differential heating curve a t the beginning of a run or a t the end of a transition is merely the drift of the sample and reference cells toward a thermal steady state determined by their difference in heat capacity, their thermal conductivitv, and the heating- rate, and conseauentlv has no phase significance. In view of these considerations the differential heating curves have the following significance: Increases in slope correspond to temperatures a t which phase changes begin in samples being heated a t the given rate and which may or may not be in phase equilibrium a t that temperature. Relatively sharp increases probably correspond to first order changes (eutectoid temperatures) while more gradual changes mark the crossing of a phase boundary into a region containing two phases of varying composition, but the temperature range during which heat is absorbed is quite possibly larger than the equilibrium temperature range of coexistence of the two phases.
-~
RESULTS AND DISCUSSION
ALUXINUM SOAPSA N D GREASE. Figure 1 gives the x-ray diffraction patterns of three technical aluminum “stearates” andof a sample of aluminum “distearate” prepared from high quality stearic acid [stearic acid A of ( M )by ] the method of Smith et al. (16) before and after slow cooling (approximately 0.5 degree per minute) from 200” C. Despite the manufacturer’s designation, as “mono-,” “di-,” and “tri-”stearate, the ash values of the technical soaps do not correspond to the theoretical values for these substances, being 9.71, 9.16, and 6.71% aluminum oxide, respectively,
dfn * &e,
ANGSTROM
UNITS
Figure 2. X-Ray Diffraction Patterns of Aluminum Soap Greases Compared with Those of Aluminum Distearate and of Oil I, Aluminum distearate (111 of Figure 1); 11, sample .&I; 111, sample A1-2; IV, sample AI-3; V, sample A1-4; VI, naphthenic lubricating oil, Waterman analysisa 20% aromatic, 32% naphthenic, 48% paraffinic
compared with theoretical values of 15.6, 8.1, and 5.8% while the laboratory preparation of distearate gave 9.09%, Except for superposition of lines due to stearic acid in the alleged tristearate and for varying degrees of resolution between overlapping diffraction regions, all the x-ray patterns are rather similar to each other and to that published by Ross and McBain (12) for aluminum distearate. This means that regardless of whether or not aluminum distearate exists as a stoichiometric entity (6, 12, 16), all of the preparations have the same basic crystallographic structure. A pattern similar to Figure 1 is thus to be expected for any aluminum stearate grease if the structure is unmodified by the oil. Figure 2 gives the diffraction patterns of four samples of aluminum greases with one pattern for aluminum distearate (curve I11 of Figure 1) and one sample of a typical oil for comparison. T h e oil is naphthenic l u b r i c a t i n g oil, S.B.E. 20, with a viscosity index of -4. The strong line of the soap a t 3.98 8. is evident in the grease patt e r n s while t h e Figure 3. Differential Heating marked asymmetry Curves of Aluminum Stearates and of the broad halo Aluminum Soap Greases $entering around E I Technical aluminum monostearate, A. and arising fron 9:7l% Alios; 11, grease A1-4; 111, technical aluminum diatearate, 9.16c/, AlsOg; diffraction by thc IV, V, laborators preparation, aluminum distearate, 9.09% A1208 as originally preoil suggests thepree pared and after slow cooling from 200’ C . ; ence of the othe VI, grease A1-3; 1‘11, grease AI-2. This sample contains only 9 90’ aoap and thermal lines of the soap i effects are too small t o be definitive. VIII, Technical aluminurn tristearate. Peak at the vicinity of 4. 56’ is probably due to liquefaction of to 4.7 b. Likeais stearic acid. IX, Grease AI-1. Arahic numbers on curves are temperatures (“ C.) although the resol of the sample at times indicated by arrowm
November 1949
r*
*
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
tion is poor, all the greases show evidence of the existence -of a spacing of 41 to 44 b., and 7.8 t o 8.0 A. as found also ih the soap [The halo in the oil pattern in this region (26 = 4" t o 10")is due to the polystyrene film with which the liquid oil was cavered during exposure. The greases were not covered. ] Therefore, it appears t h a t aluminum greases contain crystallites of aluminum stearate of the same structure, though possibly modified size, shape, and degree of perfection, as the ,oil-free soap. Differential hettting curves of aluminum stearates and aluminum greases are presented in Figure 3. The five soap samples give widely divergent behavior. The arrows mark the positions where heat begins t o be absorbed by the soap at a greater rate than by the reference material of initially equal heat capacity a n d similar thermal conductivity. The behavior suggests that described by Mysels and McBain ( 9 ) . While the behavior of the aluminum soaps is in itself a matter of some interest, its variability makes it quite impossible to state with assurance t h a t heat absorption by the grease, which is accompanied by a visible increase in transparency, loss of grease texture, and eventual liquefaction, is directly associated with the transition of aluminum distearate occurring fairly sharply a t 76" C. for the most fully crystalline preparation. The sharp dips (heat evolution) in some of the curves are belielied t o be due to rapid, soap-catalyzed air oxidation in the samples which were not exhaustively deaerated before enclosing them in the sealed calorimeter cells. Sheffer (18)hae suggested that aluminum soaps be regarded as polymers of varying molecular weight, the most fully crystallized being of the highest molecular weight, and the most fully lyophilized or finely dispersed of the lowest molecular weight. Aging soap dispersions a t high temperatures tend t o increase the degree of polymerization yielding more viscous fluids which are gels rather than $rue greases after cooling. On this hypothesis the structural implications of the existence of a first order polymorphic transition are not clear, but the speculation t h a t the temperature of occurrence of such a change might increase with increasing degree of polymerization appears a t least plausible. BARIUMSOAPS AND GREASES. Figure 4 gives the diffraction patterns for a barium tallow soap and for three barium greases, two of which are stabilized by barium acetate and one of which
Figure 4. X-Ray Diffraction Patterns of Barium Soap Greases and of a Barium Soap of Mixed Fatty Acids Approximating Barium Tallowate I, Baqium tallowate. 11, grease Ba-1; 111, grease Sa-2; IV, grease 5Ba-3. Peaks marked A are t h e third orders of t h e Ion spacing which tis appreciably reater on 111 and IV t h a n on 1 and If. B, C, and D a r e lines on Ib and IV not duplicated i n curve I, although peaks contain Borne contribution from the fourth, fifth, and sixth orders of the long spacing. Halo near 5.0 %. is due to oil. No linea coreesponding to groups E, F G appear i n t h e soa pattern (I) Nuazaerical values of d/a: A: 17.33, 16.31, 15.21; f;, 12.90, 1 2 . i l i C . 80.48, 9.59; D, 7.86; E, 3.07, 3.04; F, 2.74,2.71; G, 2.35, 2.31
2541
is stabilized by the incorporation of excess barium hydroxide during saponification of the tallow (IO). The low total intensity of these patterns is presumably due t o excessive absorption of diffracted radiation by the heavy barium ions. Despite the low intensities it is apparent t h a t the crystallites present in the greases contain neither soap nor stabilizer separately in significant amount, but t h a t instead, the several components have interacted to produce a new crystallite of different structure.
TIME
Figure 5. Differential Heating Curves of Barium Soap Greases and of a Barium Soap of Mixed Fatty Acids Approximating Barium Tallowate I Barium tallowate. 11, grease Ba-33 111, da-2; IV, Ba-1. NuAbers on curves are temperatures ( O C.) of sample at times indicated by arrows
The diffraction patterns of the two acetate-containing greases differ enough t o suggest that the "complex" between soap and salt (and possibly oil) is not a stoichiometric compound. The long spacing is increased fromo46 A. for tallow soap t o 49 A. in one sample of grease and 52 A. in th? other. Both show additional spacings ( a t 14.9 A., 14.2 A., 9.6 A,, 7.9 A. for one, 12.9 A. for the other) in a diffraction region where only higher orders of the long spacing appear for the soap alone. None of these lines are characteristic of barium acetate, whose pattern was determined using the same technique for comparative purposes. A number of broad halolike diffraction bands occur a t smaller values of d / n which are similar but not identical for the two grease samples and grossly different from the patterns of either soap or salt. The so-called "basic" grease (which is not alkaline t o indicators) gave a pattern of very low intensity showing only a-single order of a long spacing a t 47 A. and a halolike peak a t 3.78 A. Figure 5 gives differential heating curves for barium tallowate and the three greases. Anhydrous barium acetate exhibits no thermal transitions between room temperature and 250 O C. The soap has a pronounced transition near 100' C. This transition is related t o that of calcium soaps (6) [at 123 O C. for calcium stearate (28)] near which calcium greases lose their grease texture. I n the barium greases this transition is absent, the first marked heat absorption occurring at much higher temperatures (140" t o 163" C.) Microscopic observation of the greases by the hot-wire techhique (17)showed t h a t the temperature of marked heat absorption coincides with the appearance of a strongly birefringent liquid crystalline phase. I n all three cases, the differential heating curves show that the formation of this new liquid crystalline phase is preceded by a region of more gradual heat absorption, which cannot be a t present fully interpreted in terms of the equilibrium phase behavior of the ternary system soap-salt-oil. It appears, however, t h a t in addition t o stabilizing barium soap greases against liquid loss a t room temperature barium acetate and the stabilizers present in "basic" barium grease also increase the temperature of liquefaction of the grease through the formation of a liquid crystalline phase a t about 150" C.
2542
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY 4.13 o&
VOl. 41, No. 11
is even more intense, so that the n$w b o :it
3.7 A. in the greases, for which a line a t 4.13 A. is eithr.1 missing or weak, is believed not to be due to form V I A
Figure 6. X-Ray Diffraction Curves of Calciuni Soap Greases, of an Oil, and of a Calcium Soap of Mixed Fatty Acids Approximating Calcium 'l'allowato Calcium tallowate; 11, grease Ca-2; 111, Ca-6; IV. Ca-1; V, (:a=?; VI, CR-3; VII, (:a-4; VIII, Ca-5; Ix, oil. IL, 56; viwosiqy ,+i 100° F., 108 osI
which itself does not l i q u d y until even inuch higher tempcmtures. CALCIUM Soaps AND GREASES. Figure 6 gives the x-ray diffraction patterns of seven calcium soap base cup greases and of calcium tallowate monohydrate. Anhydrous calcium tallowate has a diffraction pattern indistinguishnble from that of the monohydrate. Ail of the greases sho! at least one order (thc 3rd) of a crystal spacing of about 47.5 A., a promi;ent halo due to oil, and reasonably well resolved peaks at 4.4 A. and 3.4 A. (The intense line at 3.4 in the pattern of sample Cn-5 is due to graphite.) However, the line a t 4.13 I . , prominent for the oilfree soap, is either absent or muoh reduced in relative intensity. Moreover, instead of, or (in sample Ca-5) in addition to, a line at 3.97A. a weak line at 3.7A. is found in some samples. It is not possible to state on the basis of the x-ray results whether or not the calcium tallowate is hydrated because the hydrate and dry soap give the same pattern. They do, however, show conclusively that the structure of t h e crystallites formed in the grease differs somewhat from that of crystallites formed in tlie absence of oil. The polymorphism of calcium palmitate and calcium stearate and Lhcir hydrates Bas been investigated (19, %?), and since calciuni talTIME lowate hydrate has a diffraction pattern almost' Figure 7. Differential Heatidentical with those of caling Curves of Calcium Soap Greases and of Hydrated cium palmitate monohyand Anhydrous Calcium drate and calcium stearatr Soap of Mixed Fatty Acids moriohgdrate it might b r i p p r o x i m a t i n g CaIcium expected to behave simiTallowu te larly. Both the pure anI, Calcium tallowate inonoh?,drate; 11, grease Ca-1; 111, Ca-6; hydrous saturated soaps IV, Ca-7; V, Ca-3; VI, Ca-5; VII, Ca-4: VIII, Ca-2: I%, anhydrous may crystallize in a form calcium taliowate. Numbers o n [VI A of ( 1 9 ) ] in the curve9 are temperature (" C.) at times indicated h y arrowq. SliQht pattern of ~ h i c ha line at irregularities in heating rate mako perfect alignment of eurvca 3.7 A. is prominent. HOT$impossihlc. A Is ahout 100° C. ever, in these the linc a t and B abnut 40° C.
of the dry soap. Figure 7 gives differential heating curves for calcium taY'loq atv, i t s monohydrate, and for the acmes, arranged in order of decreasing water content (80 far zs known). The samples x w heated in sealed containers under a pressure of one atmosphcm of air (initially) but the pressure increases on heating due t o wcape of dissolved air from the grcmc and the vapor prc>wi(toi water. Little if any water esmppsfrom the oell up to 140" C, The anhydrous soap does not t how any very large heat t~ljsoipiion, whereas the hydrate showb a large heat absorption bcgiiiriing R t about 96" C. (probably associated with its decomporition ) All of the greases (except that eontaiiiing only 8% soap iind i n which, therefore, thermal ebrcts are almost outside the limit of bililv of the apparntw) show it gradual increase i n ilopc of their differential heating viirves, culminating in 8 prnk w a 2 1)eyond which no further heat _._ __ - 1 IS :ibsorbed and the tempcra? I - - - -I--t lire difference declines Lo it.: s t e a d y s t a t e value. Tho I magnitude of the peak i s I d t6 /-----?--------' * 1 5 h * I ioughly proportional t o t h e 2 - --_ I'L J TIME IL water content of the greasi' 0 so far as this is known. From Figure 8. Differential these observations it can be Heating Curves Of c*oncludedthat at least a polGreases tiori of the soap is present nq 1% liegreases are heated is not uniform, and marked inficbotions occur in some cases, it doc8 not seem profitable at this point to speculate about the nature of the araociated phase changer. The axle greases (containing calciuni resinate and unt meted calcium hydroxide) do not contain sufficient crystalline soap to yield zt diffraction pattern resolvable from that of liqiircl oi), xlthough lines due to calcium hydroxide w e found. Purified cdcium resinates apparcntiy do not form any hydrates, bccuuav the air-dry soaps precipitated from aqueous alcohol lose no M eight at 110' C. in air (18). They are thermally inert at least up t a 260 ' C. Nevertheless, the greases liquefy, with accompti nyiiy er, at about the same temperature as calcium tall cs. Diffmential h ~ a t i n gcurves for the two grea
:Ire'*b-
:-
r-
' 1
jL
9. X-Kay Diffraction Patterns of Lithium Stearate and o f Greases Containing Dominant13 Saturated Lithium Soaps of n-Fatty Acids
Figure
1, I'ure lithium stearate:
L i 4 c VI. oil:
X .
I#, gppAae Ei-P; PII, Li-2; IV. Li-3; 6 . 5h.r li ireoaitv at 100° F., 108 cs.
.
T
November 1949
-
A, ANGSTROM UNITS 2 SIN e Figure 10. X-Ray Diffraction Patterns of Grease Li-3 (TI), Its Soap (Lithium-12-hydroxystearate) (I), and Its Oil (111) dm
I
2543
INDUSTRIAL AND ENGINEERING CHEMISTRY
are given in Figure 8. The initial decrease in slope is due to drift toward a thermal steady state and is of no consequence. The significant feature of the curves is the extreme smallness of any thermal effect, despite their water content of about 3%, in contrast with the large peaks found for the tallow greases. This confirms the interpretation for the tallow greases t h a t the peak is associated with hydrate decomposition rather than simple loss of water. LITHIUMGREASES. The six lithium greases investigated differ appreciably in the chemical nature of the soap. Most rontained substantially saturated soap ( < 10% unsaturates, calculated as oleic acid), X-ray diffraction patterns for these and for anhydrous oil-free lithium stearate and for oil are given in Figure 9. The soap pattern, though much weaker because of the low soap content of the grease, does not differ significantly from that of the soap and oil superposed except in the diminution of intensity of the long spacings. There is thus no evidence t o suggest that these greases are other than a mechanical dispersion of soap in oil at room temperature. The same is true for the grease containing the lithium soap of 12-hydroxystearic acid (Figure 10) although the peaks in the grease are rather weak, probably due to a low soap content for the grease. Lithium palmitate and stearate exhibit three successive transitions on heating, the first being apparently a tmnsformation from one crystal form to another, the second between crystal and a soft waxy mesomorphic form, and the third, true melting. These occur at comparable temperatures for pure lithium palmitate ($3) and lithium stearate (101' and 113', 191' and 185', 223' and 224" C,), and a t somewhat lower temperatures in technical lithium stearates (91', 177", 219" C. in one such soap). The lithium lshydroxystearate has a similar behavior (temperatures 179' and 218' C.) but lacks the intercrystalline transformation. Differential heating curves for technical lithium stearate and for four lithium greases are given in Figure 11. The appearance in the greases of a sharp peak at 80"to 90' C. much like that of oil-free soap supports the inference from x-ray data that the soaps are inert toward oil a t room temperature. At more elevated temperatures each of the greases exhibits two further peaks, the higher of which is much the larger and is accompanied by liquefaction. I t is dangerous t o attempt a phase interpretation of the intermediate peak without a more complete study of the phase behavior of the system. However, the incidental observation that lithium greases heated to temperatures above this transformation but below their liquefaction temperature are often rubbery gels on cooling strongly suggests that the waxy form of the soap has its temperature of formation lowered by oil and that the peak corresponds t o a eutectoid in which the three phases are the crystalline soap, waxy soap containing dissolved oil, and dilute soap solution in oil.
L.-.J 100.
TIME
Figure 12. Differential Heating Curves of Technical Lithium Stearate and of Lithium Grease Containing Dominantly Saturated n-Fatty Acids I, Technical lithium stearate; 11, grease Li-5; 111, Li-4%Iv. Li-2; v, Li-1. Number o n curves are t e m peratme6 (" C.) of samples a t times indicated by arrows
Figure 12 gives differential heating curves of a grease containing lithium 12-hydroxystearate and for the soap obtained from the grease by extraction of the oil with n-heptane. Except for the absence of any transition for either soap or grease a t about 100' C., the curves are similar in type and consequently in intespretation t o those for the greases containing normal soaps.
1
i
TIME
Figure 12. Differential Heating Curves of Grease Containing Lithium-12Hydroxystearate and of Oil-Free Soap I, Soap; 11, grease. Numbers o n curves are temperatures (" C.) of samples at times indicated hy arrow8
Sample Li-3 was made from the lithium soap of tallow fatby acids with an excess of lithium hydroxide. Figure 13 shows that the pattern of the soap, while identifiable in the grease as such, is much less intense than is the case for other lithium greases. Unlike the barium greases stabilized by heating with excess barium hydroxide, the lithium hydroxide remains in the greasc as such, as shown by its strong diffraction line a t 2.76 h. and by the alkalinity of the grease. The differential heating curve for this grease, compared with that for a similar soap, is given in Figure 14. No transition is found in the grease corresponding to the transition of the soap a t 88" C. T h a t this absence is a real property of the system and not due to lack of adequate sensitivity of the apparatus is indicated by the fact that the corresponding transition for grease Li-1 containing only half ao much soap is quite conspicuous. At more elevated temperatures, heat evolution possibly indicates alkali-catalyzed oxidation, Despite the absence of positive indications of complex formation, the lithium hydroxide ha6 markedly hampered the formation of well-developed normal soap crystallites.
Vol. 41, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
2544
highly crystalline powders, is due to their exceedingly fine state of subdivision. Figure 16 gives differential heating curves for sodium stearate, sodium tallow soap, and the five greases. The sodium stearate sample had been cooled from the molten state (>300° C.). Freshly precipitated and dried samples give a somewhat different behavior. Thermal transitions of the soap in the region of 100 C. are repeated in the greases without any large qualitative differences. This is taken as confirmatory evidence that the greases u p to this temperature are purely physical dispersions of soap in oil. Phase diagrams for sodium stearate in hydrocarbons have been published b y Doscher and Vold ( a ) and by Smith and McBain (14). These are qualitatively different, the one set (for light hydrocarbons) showing a liquid crystalline phase involving a solution of hydrocarbon in soap of very high melting point existing over wide composition ranges, while the other (for cetane) shows considerable soluw bilityof the hydrocarbon in the superwaxy phase of thc oil-free soap, comw ,I plete liquefaction a t much lower temperatures, and the formation "e 5.C of a liquid crystalline $1L JVM+ p h a s e a n a l o g o u s to n TIME aqueous middle soap (3) Figure 14. Differential Heatover a much more limited ing Curves of a Lithium Soap composition range. of Mixed Fatty Acids ApproxiThe differential heatmating Tallow (I) and o f a ing curves of Figure 16 Grease (I1 sample Li-3) Conshow heat absorptions taining a Similar Soap and Excess Lithium Hydroxide apparently o c c u r r i n g Numbers of curves are temperature over a considerable tem(" C.) of samples a t times indicated perature range beginning by arrows a t temperatures as low as 139" C . in some cases. The peaks are irregular in form, suggesting successive transformations overlapping each other a t the high heating rate. Sample Na-2, made from a technical sodium stearate, rather than tallow, shows two distinct peaks, one a t 164" and the other a t 208' C. These are not difficult to reconcile with the Doscher-Vold diagram for sodium stearate and cetane if i t is assumed that the peak a t 164' C. corresponds to a eutectoid temperature at which oil, nonaqueous middle soap, and the oil solution in superwaxy soap are a t equilibO
I
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-
L
6 6
d/n
i
-
487
. -x-,
2 Slh
-
376358
e
*UGSTROM
178
e
43
UYITS
Figure 13. X-Ray Diffraction Patterns o f Lithium Soap of RIixed Fatty Acids Approximating Lithium Tallowate and of a Lithium Tallowate Grease Containing Lithium Hydroxide I, Soap; 11, grease Li-3; 111, oil, V.I., 56; viscosity a t 100' F . , 108 cs.
SODIUM SOAPSAND GREASES. Of the five sodium greases examined, one contains technical sodium stearate, two sodium tallowate, while the soap base of the remaining two is unspecifi$d, although the long value of the "long spacin?" (44.4 to 44.7 A.) compared with that of the others (41 t o 42 A . ) suggests soaps of higher molecular weight, possibly of hydrogenated fish oil. A considerable volume of work has been done in an effort to establish elementary facts as to how many different crystal forms of sodium soaps can be realized a t room temperature and how they may be characterized ( I , 4). This work is far from complete and the published papers are highly contradictory. Moreover, for soaps containing an appreciable proportion of unsaturates, the diffraction pattern of even oil-free soap is unsharp, so that inteipretation of the grease patterns in the region where the wide bands of soap and oil overlap cannot be made with assurance. Figure 15 shows the diffraction patterns of a sodium tallow soap, a technical sodium stearate, and of the five greases. The positions of the principal diffraction maxima for soap and oil aeparately are marked on each curve t o indicate the extent t o which correspondence between the grease pattern and a superposition of oil and soap patterns might be inferred. The position of the oil halo will of course vary somewhat from one oil to another tending toward shorter values for more paraffinic oils, byt lying in the range of 4.66 A. (value for cetane) to about 8.3 A. (value for a highly naphthenic fraction of very low viscosity index). Sodium soaps exhibit considerable variation in diffraction pattern with varying thermal history. Powder-type patterns for a number of the forms that can be realized a t room tempsrature are given by Buerger et al. ( 1 ) . The fact that the same form of soap is obtained in the grease in all the samples examined as in the oil-free soap is particularly strong evidence for the concept t h a t the soap and oil are inert t o each other in this case where numerous alternative soap phases have been realized by relatively small variations in processing conditions. hlany of the allegedly distinct forms of sodium soaps have quite similar x-ray diffraction patterns, so that the poor resolution here encountered due t o diffraction by oil would render interpretation difficult except for the circumstanceothat, both the soap and the greases have a etrong line at 2.95 A. well beyond the oil halo. The fact that this line is so relatively prominent in the grease patterns, with no other prominent lines at smaller values of d/n, suggests that the same single phase (probably sigma) is present in all the greases examined despite the multiplicity of possible phases. It seems likely that the weakening and broadening of the diffraction maxima due to soap in the case of these sodium soaps whose chemically pure counterparts have been obtained as
' 1 . 7
2
DILFRAGTION
d/n
:
ANGLE
ceei
h, ANGSTROM 2 SIN e
UNITS
Figure 15. X-Ray Diffraction Patterns of Sodium Soap Greases, Technical Sodium Stearate, and a Sodium Soap of Mixed Fatty Acids Approximating Sodium TalIowate I, Sodium tallowate. I1 technical sodium stearate' 111, grease Na-5; I V , Nh;' Y, Na-4%VI, Na-3; \'IT,
'V
101
194
4.
J
4-
l
101
V
IPI
rium, and 208' C. corresp onds to t h e beginning of rapid melting lof the superwaxy form to liquid. It a p p e a r s , therefore, possible that some residue to the complicated
2545
STRUCTURE OF SOAPCRYSTALLITES. For aluminum and sodium greases no pronounced modification of soap pattern was detected due to incorporation in oil. For lithium greases, the evidence is fairly conclusive that the crystallites are essentially unmodified by oil except in size. For calcium greases on the other hand, the relative sharpness of different lines in the diffraction pattern varies from grease t o grease and between grease and oil-free soap. This means that the shape of the crystallites, or the spatial extent of ordered regions in the soap particles in different crystallographic directions, is subject t o modification by processing conditions with attendant possible effects on the physical properties of the resultant grease. For greases stabilized by salts (such as barium acetate) the structure of the soap crystallite is entirely changed by the additive. A true understanding of the properties of these greases must be based on consideration of the grease as a three component system of soap, salt, and oil rather than a two component soapoil system somewhat modified by additive. SOAPSOLUBILITY RELATIONS. Each of the soaps so far examined (6, 18) exhibits a sequence of polymorphic transitions as it is heated. The simplest conceivable behavior for soap in oil would involve equilibria between each of these phases in turn and liquid soap solution in oil over the same ranges of temperature, and the next simplest a similar set of equilibria but with the transition temperatures of the soap somewhat lowered t o give a series of eutectoid temperatures displaced from the transition temperatures of the oil-free soap. DIFFRACTION
d/n
s
&,
hNGCE
ANGSTROM
k2B)
UNITS
Figure 17. X-Ray Diffraction Patterns for Sodium and Calcihm Soaps of Mixed Fatty Acids Approximating Tallow Fatty Acids and of Mixtures of the Soaps at Room Temperature before and after Melting and Cooling I, Sodium tallowate; 11, 50% mechanical mixture after cooling from melt; 111, same before melting; IV, calcium tallowate
GENERAL DISCUSSION
PHASESTATEOF GREASES AT ROOMTEMPERATURE. No evidence has been found to indicate that commercial greases a t room temperature are other than a mechanical dispersion of soap crystallites in oil. This conclusion is based principally on the facts that the x-ray diffraction patterns can be interpreted as arising from crystallites of varying degrees of size and perfection and that the differential heating curves show peaks corresponding to the first polymorphic transformation to be expected on heating the oil-lree soap for all except the barium greases and the alkaline lithium greases. A relatively small amount of amorphous or mesomorphic soap, however, would not be detected by the techniques employed, and, therefore, might also be present in addition t o crystalline soap.
Such simple behavior probably is realized for sodium soaps at their first transitions [to form supercurd and subwaxy soap (go)] and for lithium soaps all the way up to the transition to a waxy type of soap phase (at 185' C. for pure lithium stearate). More complex solubility behavior involves swelling of the soap in oil of sufficient magnitude to destroy the sequence of separate mesomorphic structures, replacing them with a single liquid crystalline solution of oil in soap or a sequence of such phases which may or may not be identical in structure with any of the oil-free forms. This type of behavior is suspected for sodium greases a t high temperatures and for cafcium greases, but the precise nature of the relationships has not yet been established. THEPHASESTUDY APPROACH TO GREASE PROBLEMS. Several investigators (8, 13, 14, 24) have pointed out the utility of phase studies of greases. From the standpoint of practical developments, the following general topics seem worthy of study. 1. Modifications of grease structure a t room temperature might be achieved by producing greases in time-persistent but
2546
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
metastable phase states by appropriate control of concentratioiis, Temperature, and mechanical treatment during processing. The! results of this survey indicate that existing manufacturing conditions do not in general result in a phase state for the grease' greatly difierent from simple dispersion of normal soap crystals in oil, except for modifications due t o additives such as barium neetate, etc. 2. The origin of the varying deyrecs of dispersion or pcrfcction o r crystallization in greases processed in various ways may be traceable to the phase state of the eoap in oil a t more elcvnted rcmperatures. Exploration of this possibility for the system talcium stearate-cetane (24) has shown that such effccts are real in soap-oil systems and such studies could, therefore, probably be profitably exploited. 3. The utility of a soap as a grease former is probably limited at, high temperatures by the formation of solutions of oil in soap in which the soap particles lose their discretr status and become u. ooherent sticky inass rather than a fibious matrix with cnrneslied fluid lubricant. That oil solubility in soap can Iw extensiveIy modified by additives has already been found empirically (see imium acetate). Thc systematic study of such effccts, related to the general fields of salting out and solubilization, seems to be R useful way of correlating and interpreting such empirical rcsults. 4. The temperatures of formation of oil-swollen sticky masst+ Luther than greases by soaps in oil seem to be associated closelv Kith the temperatures at which the oil-frce soaps exist in mesomorphic rather than crystalline form, so that the polymorphic behavior of a given soap provides a useful indication of its probable performance as a grease former in respect, t o lempcrntm(~ -\ability. ACKNOWLEDLRIENT
The greases were supplied by various manufactui cis, iiicludiiig B~ttenfeldGrease and Oil Company, California Research Company, Shell Development Company, and Union Oil Compaiiy 1\ few samples were taken from a display of petroleum produels furnished by the Texas Company. The authors wish to ~cknow.1edge the cooperation of these companies in furnishing samplcs Together with defining data. This research was carried out part of a project, "Phase Studies of Greases," supported by thf, Office of Naval Research, Contract No. N6-onr-2387'0-2, XR0.57057.
Vol. 41, No. 11
LITERATURE CITED
(I) Buerger, M. J., Smith, L. B., Ryer, F. V., and Spike, J. E., Jr., Proc. Natl. Acad. Sci. U.S., 31, 226 (1948). (2) Doseher, T. M., and Vold, R.D., J. Colloid Sci., 1, 299 (1946). (3) Doscher, T. M., and Vold, It. D., 9.I'hys. h Colloid Chcm., 52, 97 (1948). (4) Verguson, R. H., Oil & Soap, 21,6 (1944). (5) Gallay, W., and Puddington, I. E., Can. J. Reseamh, B26, 166 (1948). (6) Hattiangdi, G. S., Vold. M. J., and &'old, R. D., IKD.Exvm. CHEM.,41, 2320-4 (1949). (7) Kilemgard, E. N., "Lubricating Greases; Their lllanufaoture and Use," pp. 584ff, New York, Reinhold Publishing Corp., 1937. (8) I,awrence, A. S.C., J . Inst, Petroleum, 31,303 (1945). (9) Mvsels. K. J.. and McBain. J. WeeJ. Phus. & Colloid Chem 52, 1471 (1948). 110) Ott, T. F., Clarke, P. 8., and Van Marter, C,TI., U. 8.Patent 2,033,148 (March 10, 1936). (12) Ross, R . , and McBain, J. W., OaL & Soap, 23,214 (1946). (12) Sheffer, H., Can. J . Research, B26, 481 (1948). (13) Smith, G. H., J . Am. Oil Chem. Soc., 24,353 (1947). (14) Smith, G. H., and McRain, J. W,, J. Rhys. & Colloid Chem 51, 1189 (1947). (18) Smith, Go H., Pomeroy, H. H., iMcGee, C.G., and M y s e h , IC J . , J . Am. Chem.Soc., 70,1053 (1948). (16) \'old, M. J., Anal. Chem., 21, 683 (1949)* (17) Vold, M. J., J . Am. Chem. Soc., 63,160 (1941). (18) Vold, M. J., Hattiangdl, C . S., and Vold, R.D., 9.Am, Oil Chernists' Soc,, 26, to be published. it91 Vold. M . J.. Hattiangdi. 0. S,, and Vold, R, ID., J . ColZoid 9cl 4 , 9 3 (1949). ( 2 0 ) T'old, R. D., J. Am. C'hehem. Soc., 63, 2915 (1941).
