500
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. CohlPARATIVE D A T A
Run No.
BY
Pestle-Stirred Kettle Steam-Heated Paddle-Stirred Kettle A.S.T.M. A.S.T.M. A.S.T.M. A.S.T.>l. penetration dropping Run penetration droppmg (77' F.Ia point, OF. No. (77' F,)a point, O F .
44 461 H 71 44 468 I 58 42 447 J 61 L, 44 467 Ii 68 E 44 468 L 74 F 40 454 hl 67 G 40 470 77 Av. 43 Av. 6 8 Unworked; worked penetrations were not obtained A
B
C
a
GREASES,P R E P A R E D COiYT-41N 30.07c S O A P
ON
I D E N T I C A L P R O C E D V R E S TO
450 426 436 444 442 447 449
Vol. 39, No. 4
nen- kettle is operated by a temperature controller which maintains a constant temperature ivithin close limits. Table I1 lists temperatures maintained in this kettle compared with those obtained in the Conventional, large, steam-heated kettle with manual control. I t is evident t h a t the automatically controlled kettle will operate a t more nearly constant temperature than the other. Small variations in the conventional steam-heated kettle n-ere particularly difficult t o avoid, partly because of slight varintions in steam pressure. OTHER GREASES AXD 1\IATERIALS
The grease whose preparation is described here is a fairly hard, dry-looking, medium-fiber material which becomes considerably softer and quite stringy when viorked a t room temperature. Greases of other types, such as stringy long-fibered soda grease, T h e 125-pound steam pressure available \vas sufficiently high t o short-fibered cup (calcium) grease, and conventional lithium reach a temperature of approximately 300" F., since 125 poundb stearate grease by gelling a t 440" F. (227' C.), have been preper square inch is equivalent to 353" F. (178" C.); therefore the pared successfully in this kettle. Because of the very viscous long time required t o reach dehydration temperature in the padnature of cup grease in the intermediate stages of preparation, dle-stirred kettle must be ascribed t o less efficient stirring in this the peculiar shearing action of t h e pestle caused considerable kettle as compared t o t h e pestle-atirred kettle. aeration. Once the grease base was aerated, it was impossible to prepare a satisfactory product. This difficulty v a s overcome b y maintaining the level of the batch ahove the top of the pestle TABLE 11. h f I S U F 4 C T U R I S G TIVECYCLE? 4 U D TEMPERITERE :it :ill times and removing portions of the unfinished batch occaR.4SGEs ~ionnllj-when it became too large. Other types of greases preAutomatic Control hlanual Control pared in this equipment have offered no difficulties and they can Pestle Stirrer Paddle Stirrer Operation Time, min. Range, ' F. Time, min. Range, ' F he prepared in a straight-forward manner. Caustic addition 179-180 60- 90 30 18&192 This equipment has been employed only for grease manufac179-180 30 178-187 Saponification 60- 90 ture, hut it should be useful in other n-ork in which pasty or very 120 18&300 185-290 Heating 30 150 299-300 Dehydration 15 290-296 viscous materials require blending, etc. One desirable property Oil addition 45 270 298-300 284-296 Stirring 300-301 292 30 90 of the equipment is its ability t o "knead out" fairly hard, some750-810 Total 180 what plastic lumps present in a softer mass, without breaking down as is the case viith other more violent types of misers.
-
T h e greater efficiency of t h e pestle-stirred kettle is s1ion.n by the much shorter processing time, rvhich is ascribed t o more efficient mixing; t h e result is more rapid dehydration, incorporation of oil, and other desirable features in grease manufacture. PRECISION OF TEMPERATURE CONTROL..is stated earlier, tlie
ACKNOWLEDGMENT
The author desires t o express his appreciation of the assistance rendered by P. A. Hargrave of t h e Engineering and Drafting Department and of the helpful advice of other memhers of t h e staff of the Beacon Research I.:ihoratory, The Tesas Conipmy.
Synthetic Low Temperature Greases from Aliphatic Diesters G . M. HAIN, D. T. JONES, R. L. MERICER,
~ V D W. -1.ZISMAN
iVaral Research Laboratory. K'ashingtort, D . C .
T
H I S investigation originated with attempts to make grexbe structures by dispersing various soaps in a number of aliphatic diester lubricating fluids described in other publications of this laboratory ( 1 , 2 ) . T h e improvements desired in the greitsr included decreased torque requirements a t temperatures below -40" F., much lower evaporation rates, improved storage stability, and greater reproducibility. Difficulties encountered during the war with all available greases in aircraft coiitrol bearings and in ordnance fire control equipment emphasized the need for improved greases. Upon completion of the laboratory development of these greases, the Bureau of Aeronautics and the Bureau of Ordnance requested the expediting of their utilizat,ion in naval equipment b y avoiding time-consuming mechanical laboratory tests; there-
fore, :I l u g e ~irunber(11' .-:impleh of several of the more Irromisiiig \vere distriliiited t o oi-er a hundred cooperating orgaiiizaTliii lrd t o tlir new specificatioris 1 K - 8 tion? t'or service tc.4.. nnti A S 4 b 2 5 . THEORETIC.4L CONSIDERATIONS
I,iii)i,icatiiig grease> are liomogeticiius di.per.*ioni of soalifiber gels ill lubricating fluids (4).Practicnlly all the commercially availnhle greases :ire made from petroleum fluids. 1Iuny varicties of 30aps are used as gelling agents: calcium, sodium, aluminum, :uid lead soaps made by saponifying commercial talloxvs or their fatty acids are the more common. Lithium soaps are n rrcent development (3),and their large scale use came with t,lie war. These greases offer certain advantages over the calcium
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1947
50 1
less variation in the oxidation stability and d i e r properties of different batches than with tlie use of petroleums. It ~v:is considered t h a t t,he fire kettle nietliod could be used more advantageously to prepare diester grenkes than the steam kettle method since the latter involved gr:ir'e difficulties with of the diester in the presence of a hot, concentrated, aqueous, alkaline solution. As the fire points of :ill diesters considered here are over 400" F. (Z), they are mole than 100" higher than those of the petroleum oils now used for low temperature greases. Hence, the flammability problem inhertiit in tlie fire kettle method is less serious than usual. Excessive osirlxtion of the oil during the kettle operation was prevent,ed by the early introduction of the antioxidant. Precautions requdiiig ventilation were found advisable t o minimize the effects of the rntlicr irritnting vapors from hot diesters.
series of greases suitable for lubrication a t temperatures from -40" t o -100' F. h a l e been prepared from aliphatic diesters thichened with lithium stearate and modified by the addition of small quantities of well defined chemicals. They resemble in texture and working properties the present l o w temperatnre petroleum greases specified b> the h n y and V a \ > . However, the high e\aporation rates and poor storage stability normall! encountered in low temperature greases haTe been eliminated without sacrificing any. operating characteristics. These greases ha\ e good lubricating properties arid excellent oxidation stability
.
greases they replace although information o n their commercial SYNTHETIC FLUIDS productioii has been closely held by the various manufacturers. Much evidence has been given (1, 2 ) for concluding t h a t ccrtain The calcium soap greases available on the market have, for the diesters made from the aliphatic dicarboxylic acids and aliphatic most part, such poor oxidation stabilities that antifriction bearbranched-chain alcohols were the most satisf:ictory for developings packed v-itli them cannot be used after as short a shelf storing lubricating oils. Table I is a compilation of data on eisliteen age period as 6 months. T h e S a v y has experienced difficulty diesters having freezing points beloiv -40" 17. Alllthese fluids n.ith calcium soap greases both in the Sperry gyrohorixon and in have pour points (after 72 hours of storage) of -80" F. or lorver gun-sight telescope trains. I n addition, most calcium soap except the tetradecyl and hept,adecyl diesters which hnvc values grmscs break d o n under service conditions above 150" F. and between -60" arid -80" F. The use of 4OCG or more of the dinielt a t temperxtures in the neighborhood of 200" F. Lithium (2-ethylhexyl) adipate or of 20y0 or more of the azelate is suffim i p grease.- > u c h 3s those supplied under Specification A?ri-G3a cient to depress permanently the freezing point of the di-(2appear t o have good self life, and they are not subject t o serious structural chnriges a t temperatures beloTv 300 F. except shear Iweakdon-n. Both ciilcium and lithium greases have good low FLNE WESH FINE ,KNURL temperature properties and both are water in?oluble. WIRE \ ,--I 1/2 INCHES-16 THREAD PLUNGER Tn o methods of preparing litliiuni soap greases are in comnion w e . In the fire kettle or high temperature method of obtaining a colloidal dispersion of lithium stearate, a suspension of lithium stearate soap in the fluid is heated t o approximately 400" F. until n clear solution is obtained. With a minimum delay to avoid oxidation and minimize fire hazard, the fluid is chilled rapidly t o form an elastic gel. This gel is then mechanically worked by I6 THREAD any of a number of extruding or milling processes t o the familiar WASHER smooth, buttery texture of a cup grease. In the steam kettle or l o w r temperature method the fatty matter is reacted n-ith the Figure 1. Miniature Grease Worker alkali in tlie 1ire;eiice of some of the miiieid oils, and the grrase is filii-hrd hy slowly \\-orking or kiiending the reTABLE I. PROPERTIE3 O F THE ALIPH.iTIC DIESTERS SUIT.IBLE FOR USE B E L O W -40" F. maining oil into tlie hot soap Kineand oil paste. Iii the latter Freezing VolaFlash Fire matic Viscosity, Centistokes Viscosity tility", Pointb, Point, Point, process the product is less sub1der.tification OF. 210' F. 100' F. 0' F. -40' F. Index yo F. OF. ~
subsequent use, provided the \vorking temperatures encountered iii service are not high enough to cause a cliange in the state of aggregation of the soap. The aliphatic diesters were considered promising for the fluid phase of greases for several reasons. The presence of the polar eater groups in the oil \vas believed lilrely t o assure good compntnbility with the soaps. The diesters used to develop oils (I, 2 ) had very low freezing points and low evaporation rates. Iiot only could they be efiectively inhibited against oxidation by the addition of properly chosen antioxidants ( I ) , but there would be
GLUTABATES Di-(?-ethylhexyl) Di-(undecyl) Di-(terradecyl) d
