1444
JT.ILTER H. C. RUEGGEBERG
(3) DOSCHER, T.)I., .+SI)\ 7 0 ~ , iIt. ~ , D . : J . Colloid Sei. 1, 299 (1016). (4)FARRISGTOS, B. 13.. ASII ~ 1 R l ) ~ A ID. ~ lH ~ .. : Irist. Spokcsrnai~( S a t l ,Lubricating Grease Inst.) 11,4 (194i); Oil Gas J . 45,26S (1947). ( 5 ) GARDISER. I-s. C h t m . 60, 39 (1946).
USE OF ALUMISU-11 SOAPS A S D OTHER FUEI, THICKEIXERS I S GELLISG GASOLISES’ WALTER H. C. RUEGGEBERG
Technical Command, Army Chemical Centev, .llavyland Received June 2 5 , 1948 INTRODUCTION
Liquid incendiaries such as petroleum oils, carbon dibulfide, TI ood-distillation products, and other inflammable liquids were tested during World War I. These materials all had the same drawback of excessive dispersion. To overcome this, the inflammable liquids n-ere absorbed in some material such as cotton waste, but this method was only faii,ly satisfactory (26). During Wodd War I1 this idea was revived and, based on development work, 14 per cent cotton waste saturated with 86 per cent of a 50/50 mixture of gasoline and fuel oil was suggested as a possible filling for the JIG9 incendiary bomb. Subsequently, it was suggested that cellocotton be employed because of its greater absorptive capacity and its availability in sheets, which permitted better packing and negligible scattering on ejection from the bomb (33). This type of filling appeared t o be equal t o gelled gasoline with the exception of lack of adhesion (9). The main advantage claimed for it was that commercial gasolines could be used and specifications would not nrcd to he closely drawn with respect t o Reidvapor pressure and aniline point as required for gelled gasoline (23). METHODS O F S O L I D I F T I S G LIQUID ISCENDI.%RIES
Owing t o the high degre? of dispersion and consequent flash burn of liquid incendiaries, many materials have been proposed for use in solidifying liquid incendiaries. These are (17) : 1 Presented a t the Syniposium on Gel Format ion, I)etergenc\ , 14hulsification arid Film Formation in Son-Aqueous Colloidal Systerns which was held under t h e auspices of t h e 1)ivision of Colloid Chemistry arid t h e Division of Petroleum Chemistry at thc 113th hleeting of the .4mrrican Chemical Socirty, Chicago, Illinois, April, 1948.
THICKENERS FOR INCENDIARY GELS
( I ) Fatty acid derivatives: (a) Aluminum, sodium, zinc, and ammonium salts (b) Lead salts of hydroxy acids (c) Sulfonated products (d) &Amides (e) Fatty acids p e r se (f) Katural waxes (1) Nitrated waxes (2) Sulfonated waxes (3) \Tiaxes per se (g) Anilides (2) Polyhydroxy derivatives: (a) Glycol compounds (1) Esters of fatty acids (b) Ethanolamine compounds (1) Esters of fatty acids (2) Compounds of mono-, di-, and tri-ethanolamine (c) Glycerol compounds (1) Saponified vegetable oil mixes (2) Sitrated vegetable oils (3) Vegetable oils per se (d) Polysaccharide compounds (1) Lactose anhydride (2) Dextrins (3) Pectins (e) Cellulose esters (1) Ethyl cellulose (7-10 per cent) (2) Pulp (3) Resinous derivatives: (a) Natural (1j dlkali-treated resin (2) Shellac (3) Damar (4) East India fossil (5) African fossil (6) Sen- Zealand fossil (h) Synthetic (1) Saponified polyacrylates (4) Hydrocarbon derivatives: (a) Paraffin (b) Synthetic rubber (c) Satural rubber (d) Salts of sulfonated petroleum fractions (e) Salts of naphthenic acid
1445
1446
WALTER H. C. RUEGGEBERG
(5) Inorganic derivatives: (a) Organosilicon compounds (1) Esters (b) Bentonite (e) Oil shale Of all the materials listed above only a few n-ere ever found practical for use in thickening incendiary liquids. These were : (1) Sodium stearate (2) Aluminum salts of mised fatty acids and naphthenic acid (3) Polyacrylates (4) Rubber (natural and synthetic)
This paper presents a review of the use of aluminum soaps of fatty acids as fuel thickeners. SO.\P-THICKESED
GhSOLISES
The first soaps considered for use as thickeners were the stringy aluminum ones. -4luminum naphthenate, oleate, and stearate n-ere tested, but proved difficult to manufacture. &illrequired compounding a t about 250°F. However, when the soap content was reduced to about 20 per cent these soaps could be mixed a t about 130°F. This mixing required the use of pressure equipment, or the addition of considerable heavy oil. The alternative method of cold ballmilling would be very cumbersome. The soap had to be made by double decomposition and had t o be washed for satisfactory results. 811 of these soaps u-ere found unreliable and difficult to use in grease making. In addition, the temperature susceptibility proved n-orse than that of rubber blends, causing the gum to melt and flash on ignition and to become very hard at -40°F. Calcium soaps were also considered, though the product was not a t all like a rubber blend. It n-as an old process to gel oils with a mixture of lime and rosin oil to produce “Sett” greases, and this could be done n-ith gasoline. However, this type of soap was very thixotropic, and the gel atomized completely on explosion. About 25 per cent of non-fuel material was required. Tests were also made on batches prepared by cutting back ordinary cup greases (made from the cooked soap of pork fat) to the consistency of n heavy oil. These products were found to be too fluid and gave a flash burn on explosion. -4process n-as developed for making a Sett type of grease from fatty acids. This type was much less thixotropic than the rosin-oil mixtures, but more difficult to compound. h gel was made n-ith 15 per cent preformed soap present, and the necessary lime was dispersed in it. This grease was then dispersed in gasoline by stirring, after which the fatty acid was added. Setting took place in about 2 hr. The non-fuel content was about 20 per cent. The tar-lime gels used by the British were not investigated, because the vertical gas retort coal tar required was not produced in sufficient quantities in this country (27).
1Ui
THICKENERS FOR ISCEXDIARY GELS
Sodium stearate-gelled iriceridiaries Among the first materials tried in an attempt to thicken the liquid incendiary material sufficiently to improve dispersion was sodium stearate. This soap ivould not give a satisfactory product except with turpentine. Seither was it possible t o solidify the incendiary liquid with oleic acid and sodium hydroxide, but xhen stearic acid dissolved in the incendiary liquid was saponified by the addition of sodium hydroxide dissolved in 85 per cent or purer alcohol, excellent gels were prepared. Two or three grams of stearic acid per 100 g. of liquid incendiary was required to solidify the material satisfactorily (26). This method was also found satisfactory in the development of solid incendiary oils carried out in World War 11, but the use of solid sodium hydroxide 11-as unsatisfactory (12). The melted type of sodium soap gel was studied by several investigators and showed a number of advantages: It was easy to prepare, was of low non-fuel content, and melted sharply to a thin fluid so that it could be poured into cases. TABLE 1 S O D jorinnla 122 IYGREDIEhT
.-
WIGHT
~per cenl
Stearic acid ( o r hydrofoil) Rosin Cottonseed oil (castor oil) Caustic soda Water Llotor gasoliiie
3.4 1.8 3.0 1.1 2.2 88.4
Such a material, hoivever, \vas a brittle gel which tended to bleed gasoline and required heating to about 130°F. during compounding. This required either pressure equipment or the use of a high-flash, low-vapor-pressure fuel. Large amounts of alcohol were also required to lower the melting point, and no readily available substitute \vas found. This gel was also found to shatter badly on explosion and gave a flash burn unless a binder was added (27). The most satisfactory sodium stearate-thickened gasoline, designated az SOD Formula 122, had the composition shown in table 1 (31). Gasoline, S o . 2 kerosene, fuel oil, or combinations of these could be simply and satisfactorily solidified in bombs n-ithout the use of heat by dividing the oil t o be solidified into two equal portions. In one portion, 6 per cent stearic acid was dissolved ; in the other portion was an equivalent amount of sodium hydroxide dissolved in 95 per cent ethyl alcohol. These two mixtures were poured simultaneously into the bomb, and solidification took place immediately upon contact between the two streams (17).
1448
WALTER H. C . Rt-CGGEBERG
A I m in ziin soap t hiclic.n rrs These thickeners are aluininum salts of the saturated soap-forming fatty acids having eight to fourteen carbon atoms, an unsaturated soap-forming fatty acid, or a carbo‘xylic acid coniaining a carbocylic ring (eg., naphthenic acid), or mixtures of these. Aluminum st’earat,ehas certain disadvantages a s il gelling agent because heat treatment is required for incorporation into the fuel and because the gels are hard and friable and lack the cohesive and adhesive character desired in an incendiary fuel. Soaps of acids of lower molecular Iveight (e.g., aluminum laurate or myristate) may solvate fairly rapidly at ordinary temperatures but give gels which lack body, which tend to undergo syneresis at low temperatures, and which are of generally inferior quality. As regards aluminum oleutc, gelation (minot be accomplished in a simple manner n-ithout heating. Furthermore, oleate soaps tend to be rather weak, and excessive amounts are required to produce a given viscosity. The aluminum naphthenates are either resinous gums or lon-melting solids. As thickening agents for hydrocarbon solvents these soaps have the disadvantage t.hat the gelation of t’hefuel is not easily accomplished. Thus, although a resinous aluminum naphthenate can be processed by alcohol \rashing to give a product which will solvate at oitdinary kmperatures, the process is costly and the yield poor. &inothermethod of processing a naphthenate to secure low-temperature gelation is by precipitation as the solid hydrosyaluminum soap. Hoivever, the melting point of the resulting product is so low that it may be difficult t o avoid agglomeration of the material (8). From tests covering :i \vide range of aluminum soap gels, made from saturat,ed and unsaturated fatty acids and nuphthenic acids, it was concluded that (1): ( I ) S o class of aluminum gels was found to be free from instability. The results indicated both stable arid unstable gels on every formula. ( 2 ) A geI of hi& concentration appeared t o he more stable than one of lower concentration of the same formula. (3) There \vas some indication that an easily oxidized fatty acid led to an unstable gel. This, obviously, was not the whole explanation, as naphthenates also showed inst’ability. However, the tendency t o return to unthickened gasoline was more marked in oleate, cottonseed, and soya bean soaps, except crude cottonseed which contained lecithin. It appeared that oxidation might be a factor in instability. Mixtures of aluminum soaps have been the most successful in producing incendiary gels. Among t,he most studied have been the “Tapalm,” “Steolate,” and “W’soaps. XAPALM TIIICKESER
The original Sapalm polymer consisted of aluminum naphthenate which was heated to a maximum of 212”F., and then milled with a slurry of 50 parts by weight of aluminum palmitate and 100 parts of kerosene until homogeneous.
1449
THICKESERS FOR IIiCESDIARY GELS
The resulting gum was then forced through a perforated iron plate with holes about in. in diameter. The extruded strands issued into a tank containing the gasoline. The mixture was then circulated in the tank for 10 to 15 min. after the addition was complete. The thickened but still fluid material set into a gel in 5-6 hr. (16). The Napalm thickener finally adopted for incendiary use consisted of a granular base aluminum soap of naphthenic, oleic, and coconut fatty acids. The sodium soap used for the precipitation of the aluminum soap contained 0.104.15 per cent a-naphthol. The recommended formula for the organic acids used in making the Sapalm thickener was a5 follo\vs:
+
Coconut fatty acids Xaphthenic acid Oleic acid
50 25 25
The aluminum content of the finished thickener should be from 5.4 to 3.8 per cent, and the moisture content not more than 0.8 nor less than 0.4 per rent (19). .I study of the effect of ran- materials led to the folloning cxonclusions: \.aiying the composition of Sapalm from the standard to :L2: 1 : I ratio of coconut fatty acids to oleic acid to naphthenic acid indicated that the viscosity of the gel increased primarily with increase in oleic acid and, t o a lesser extent, uith increase in coconut fatty acids above the normal composition. The acid numbri~ of the coconut fatty acidb \vas found important. Iron \ v : ~an undesirable impurity when found in the alum but not in the acid ( i ) . Impurities in Sapalm thickener which may cause partial or complete breakdown of gels formed with gasoline, or oxidation of the thickener, include: excess mater, lime, or caustic. soda; soaps of sodium, copper, lead, iron, mangitnehe, and cobalt; po\v(.derpcl or -heet zinc and lead, lead nitrate; rust preventives containing amines, alcohols, and all acids. Tetraethyllead, on the other hand, has no injuriolls eflects (19). The fundamental reactions may he expre5setl as follon-5: 6SaOH fiSaOH
-
+ I H R + -U,(SO,)j 2;11(OH)R? + 3Sa2804+ 4H,O + 2HR + -112jSOd)j - 2A41(OH)?R+ 3Sa2S04 + 2Hz0
(1)
(2)
The soaps may hydrolyze :
=U(OH)R, -4l(OH)zR
+ HzO + H,O
+ HR + HR
+ A41(OH),R -+
AIl(OH)j
(3) (4)
Because of hydrolysis, ,11R doe5 not form and fatty acaicl in excess of t h n t required remains as iuch: GSaOH
+ GHR + Al?(S04)j
-+
+ 2HK] + 3?\Ta2S04+ 4 H 2 0 ( 3 )
[2hl(OII)R?
In the above equationh HR denote? the mixturf. of fatty and naphthcnica acids (11).
1450
WALTER H. C. RUEGGEBERG
The possible formulas for aluminum soaps are complex. There is definite el-idence of three chemical compounds: ( I ) The disoaps -All(OH)R2 (2) The monos0ap.s -A41(OH)2R ( 3 ) The acid disoap-~-~l(OH)R?HR There appear;. to be no el-itienre for the existence of the simple normal soap AlR;. There m:Ly possibly be other soaps, and liken-ise there may be hasic compounds, tleriIred from :tliiminuin hydroxide or mixed ivith fatty acid in colloidal form (22). S \I’ \ L \ I
31 i T I - F I C T I - R I :
1 hree pi ocey-e. hni-c I m n i i d ~iicce~\fully for the m:iiiufacture of Snpalm. AUare Iiased on the aforementioned equations, but the mechmical details diffei. odium hydio\icle \I a\ added t o the mixed fatty and naphthenic acid- t o form S u R . In the .’tcmpered-nlum” method no more sodium hydroxide \ ! a s added than wii requiietl foi the foimatlori of S a R . The excess was added t o the alum solution, :i$ it> eqiiiralent in sodium c a i h n a t e . The alum iolution containing the sodium carhonate TI a? added t o the alum solution. In the “one-stream” and “t\ro--treani” methods all of the hodium hydroxide n-as added to the mixed fatty and naphthenic acid\. In the “one-htream” method the alum solution was then added t o the (SaOH S a R ) mixture. In the “tu-o-stream” method both the alum solution and the (SaOH S a R ) mixture \\ere run simultaneously into \\ater. The ”one-stream” method \\-as the one most widely used. If the mixture of coconut, oleic., and naphthenic acids has an acid number of 240, the average molecular weight of H R is 233. The monohydroxy soap of equation 1 then contains 5.31 per cent of aluminum, while the dihydroxy soap of equation 2 contains 9.22 per cent. The mixture of free acid and monohydroxy soap of equation 5 contains 3.64 per cent aluminum. Kapalm was specified in terms of the consistency of an 8 per cent gel in gasoline. Aluminum and the hydroxide parts of the soap are gasoline-insoluble, while R and H R are gasoline-soluble. The lou-er the aluminum content, the lower the OH content, and the higher the R and H R content. Hence, the lower the aluminum content, the weaker the gel. Many other variables besides aluminurn content affected gel strength. For instance, if some of the coconut acids \rere replaced by naphthenic acids, the acid number could be held a t 240 and the aluminum content n-ould remain unchanged, yet the gel would be weaker. Moisture also affected gel strength markedly, without measurably changing the aluminum content. Hydrolysis also affected consistency more than it did aluminum content. Equations 3 and 4 illustrate this. The per cent aluminum of the monohydroxy soap of equation 3 is 5.31, while that of the mixture of dihydroxy soap and free acid is 5.13. Even more marked is the result of equation 4, since the Al(OH), would not gel at all, yet would analyze the same as aluminum soap. The H R was always converted to the sodium soap before the alum was added. 7 7
+
+
1451
THICKESERS FOR ISCEXDIARY GELS
I n equation 1, for instance, 4 moles of sodium hydroxide are used to form S a R , while 2 moles remain free. In equation 2 , 2 moles of sodium hydroxide are combined, while 4 moles are free. Excess caustic is defined as follows: Excess caustic
=
free S a O H - - ~ ~combined S a O H ~
x
100
The excess caustic is zero per cent in equation 5 , 50 per cent in equation 1, and 200 per cent in equation 2. It is seen that the aluminum content of the soap was controlled by the excess caustic used in the preparation of the soap and that it may be calculated, since the t\vo are related by the expressions: Per cent A1 = 3.74 Per cent -11= 4.00
+ 0.0312, where r. is less than 50 per cent' + 0 . 0 2 6 ~where ~ .-c is greater than 50 per cent
Since excess caustic could be controlled by the formulation of the soda-soap solution, it \vas more useful than the aluminum content. This v a s especially true when the alum contained silicates, giving high aluminum content in the soaps which did not contribute to the gel strength. T o manufacture Sapalm n-ith a gel strength hetxeen 500 and 800 g., the midpoint, or 630 g.. was aimed for. Equations 1, 2 , and 5 require 1 mole of alum for G moles of sodium hydroside, regardless of the H R used. That is, the alum required is independent of the excess caustic used, and hence has no connection with the gel strength of the resultant Sapalm. The per cent of alum added waa calculated on the basis of 1 mole of alum for (i moles of sodium hydroxide, correcting for the AU(OH)S04in the alum. The curve (figure 1) shown was based on a plant run in which the excess caustic was 56 per cent. Because of the free caustic present, the first addition of alum to the sodium soap solution formed only soluble sodium aluminate : 8SaOH
+ Alz(S04),+ 2SaX102 + 3Sa2S04+ 4Hz0
(ti)
When all of the free sodium hydroxide had reacted in this manner, additional alum decomposed the aluminate, and precipitated Sapalm :
4H20
+ 2SaA1O2 + l 6 S a R + 3A412(S0,)3-+
8A1(OH)R2
+ 9Sa2S04 ( 7 )
It will be noted that equations 6 and '7, combined, are equivalent to equation 1. An excess caustic of 56 per cent means that 35.9 per cent of the total sodium hydroxide is free. While 6 moles of sodium hydroxide react completely with 1 mole of alum, 8 moles of sodium hydroxide, if free, react with 1 mole of alum to form aluminate. The break eventually occurred at this point. The first break also is found a t a pH of about 10.6. This also is the pH a t which sodium aluminate is decomposed by acid. In the aluminate section of the curve, no Napalm should form. The precipitation of Sapalm should commence after the pH drops below 10.6. This also agreed with observed behavior in plant production. However, when the 2 =
excess caustic.
1452
WALTER H. C'. RL-ICGGEBERG
agitation was inferior, or the alum was added rapidly or at the wrong point, a local region of lorn pH might exist a t the point of cmtact of alum solution and sodium soap solution, even when the over-all pH \\as above 10.6. Sapalm then precipitated in this local region and did not redissolve in the available time \\hen it was swept into the high-pH region. The Sapalm 50 formed was ubually a spherical shell containing mother. liquor. 'I'h isavoided for these reasons. Their formation could be minimized by improvement of the agitation and method of addition of the :dum iolution
PERCENT OF ALUM ADDED
FI~:. 1. Dependence of Kapalm precipitatioii on pcr
writ of aluin adtlcd i n the prcwnce of
56 per c e n t excess sodium hvdrouide.
Between pH 10.6 and about 7 . 3 ,the Sapalm ihould precipitate in fine particles well dispersed in the mother liquor. With proper agitation and addition of alum, the appearance of the charge in this region should he that of latex. Foaming was most serious in the first part of this region. Ails the pH dropped rapidly, however, the latex coagulated suddenly and tended t o float and cake on the surface. This coagulation was known a i the strike. The rapidity with which it occurred could be controlled by the rate of addition of the alum solution. Lnless the agitation mas excellent, and the rate of addition of alum ideal, occlusion of mother liquor occurred during the strike. This was evidenced by slight foam around the coagulated Sapalm. To insure complete reaction with the sodium soap, excess alum was added (11). Sapalm thickener had been manufactured generally from two parts coconut
THICKESERS FOR INCEXDIARP GELS
1453
fatty acids, one part napht'henic acid, and one part oleic acid. A threatened short,age of naphthenic acid made it desirable to find a formula using minimum quantities of naphthenic acid. The Sapalm thickener formula was somewhat flexible and changes could be made in the acid rat'io provided the average efl'ects of each acid were maintained. Coconut oil acid soaps had strength but', at the same time, had very fine particle size and required heat for aging t'o maximum strengt'h. Oleic acid soaps \\-ere soft and required oxidation inhibitors, hut the aging properties \\-ere good. Saphthenic acid soaps were gummy and had large particle size. They also contained natural inhibitors xhich made them very stable to oxidation. \Yit,h as little as 5 per cent naphthenic acid in the formula the coconut oil acid could not, exceed 30 per cent, else t'he particle size would be too small and aclversely affect t'he gelation rate. T h e reduction in naphthenic acid content could he made up by increasing the oleic acid content to 65 per cent and increasing the amount of inhibitor used. If less than 30 per cent coconut oil acid was used! the strength of the gel suffered. If more than 5 per cent naphthenic acid \\-as used the particle size \viis increased, thus permitting the use of more coconut oil acids and resulting in an increase of strength. The ne\\- formula developed used 30 p a r k of coconut fatty acids, 65 parts of oleic acid, and 5 part,s of naphthenic acid. The new-formula Sapalm thickener had a finer particle size and a higher iodine number t'han t'he standard-formula thickener. In other respects the chemical propert'ies were very similar. The thickening pon-er of the soap and the stability of the gels made therefrom were similar for both formulations. Aklded water affected both Snpalms similarly. It was c*oncliidedfrom t,est,s that the Sapalm obtained from the new-formula thickener was similar t o that made from the standard-formula thickener with respect to thickening po\ver, stability of gel, stability of dry soap, oxidation stability, and u-ater peptization. Gels made with the new formula \\-ere faster setting than gels made with the standard formula. It' \vas recommended that the ne\\- formula for Sapalm thickener be used in rase of need for :t formula ivith decreased naphthenic acid content (24). OXID.ITIOS STAUILIT'1- O F S . I P . I L l I
The unsaturated fatty acids and soaps of these acids present in Kapalm account for it,s oxidation susceptibility. The oxidation of these unsaturated substances is supposed to take place through the intermediate formation of organic peroxides. The general character of the reaction is autocatalytic, presumably of a chain-mechanism type. Such reactions exhibit an induction period during which t,he reaction proceeds very slo\dy. The end of the induction period is characterized by a very rapid rise in t'he reaction rate. Since the organic peroxides t>hatare formed react with potassium iodide to liberate iodine, the course of the reaction can be follon-ed by measuring the quantity of iodine liberated by a given weight of sample. The number of milligrams of iodine liberated by 1 g. of the material under rigidly controlled conditions is defined as the peroxide number.
1454
WllLTER H. C. RUEGGEBERG
By following the change in the peroxide number of a soap stored in an oxidizing atmosphere it is possible to obtain considerable information about the course of the oxidation reaction. Thus, it is found that for a length of time the peroxide number is very low, indicating an extremely loiv rate of oxidation. This is followed by a sudden increase in the peroxide number, signalizing the end of the induction period and the comparatively rapid rise in the rate of oxidation of the soap. The peroxide number finally reaches a maximum and then declines. This is a result of the further decomposition of the organic peroxide. The maximum peroxide value attained varies according to the amount of oxidizable material in the soap. The length of the induction period is a guide to the oxidation stability of the soap. It is also possible to follow the oxidation of a soap by measuring its degree of unsaturation by treatment with iodine solution and back-titration of the unused iodine with thiosulfate. The quantity of iodine consumed by a given weight of soap (iodine number) decreases as oxidation proceeds, and the end of the induction period is indicated by a more rapid drop in the iodine number. The initial value of the iodine number depends upon the proportion of unsaturated acids in the soap. The crucial test for oxidation susceptibility of a soap is the length of the induction period. r n t i l a soap has passed through this period, there is little change. After exposure t o the oxidizing action of the atmosphere, a portion of this induction period is consumed and this affects the time the soap can be stored or s u b jected to further oxidation hefore extensive brealtdonn sets in. A lon- peroxide number or high iodine number, therefore, does not necessarily mean that the soap will be satisfactory for long storage. The length of the induction period depends chiefly on the amount and kind of inhibitors that are present in the soap. This is shon-n most clearly by acetone extraction of the soap. Thus, after such extraction the induction period even for the most stable soaps is reduced to a small fraction of its original value. Furthermore, the induction period of a readily oxidizable soap can tie extended considerably by treatment with a small amount of certain \\-ell-recognized oxidation inhibitors ( 5 ) . Among those recommended have been I-.O.P. S o . .J or its more satisfactory homolog, dilauryl-p-phenylenediamine, a-o-toluidinostearic acid ( 3 5 ) , and cy-naphthol (19, 28). Using the same fatty acids, the length of the induction period appeared to be dependent upon the soluble iron content of the alum used in the precipitation. Soaps with a high iron content usually, but not invariably, had a low induction period (6). The presence of manganese in amounts greater than 0.01 per cent by weight of aluminum sulfate resulted in extensive oxidation, as measured by the oxidation susceptibility test. The presence of ferric iron in amounts greater than 0.03 per cent by weight of aluminum sulfate was likewise harmful. Ferrous iron, on the other hand, did not show a marked effect. The presence of iron and lead in the sodium soap solution showed no marked effect on oxidation susceptibility; copper and chromium salts in the aluminum sulfate solution likewise showed no effect (30). Cobalt in moderate amounts as an impurity was suffi,ciently active t o cause a temperature rise in the heat test (13).
THICKESERS FOR INCEKDISRT GELS
1455
EFFECT O F MOISTURE OK SL4P.4L11
Sapalm is essentially a hygroscopic material analogous to gelatin or paper in its affinity for moisture, and thus gains or loses moisture when exposed to the atmosphere, except at the one humidity with 11-hich it i. in equilibrium (inert point). Equilibrium at any given relative humidity appear3 to be attained fairly rapidly, being practically complete in 8 hr. in $-in. or $-in. layers under normal convection. The v-ater absorbed greatly affects the conbistency of the gel formed when the solid soap is dissolved in gasoline. The average of :I large number of tests indicated that the absorption of 0.1 per cent moisture measured by the vacuum-oven method caused the consistency in the 24-hr. 130°C. test t o drop approximately 40-50 g. (4, 18). The cause of the moisture effect probably lies in the pievention of the formation or possibly the breaking of the w a p chains hy preferential coordination of water molecules at the points of attachment hetneen the individual units. Osmotic pressure measurements indicated that the normal Snpalm in solution in gasoline had a molecular weight of lG0,000-200,000. I t 3eems probable that these large molecules \yere built up largely by the coordination of the aluminum atom of each soap unit n ith one of the carboxyl oxygens of the next unit. Such chains can obviously be broken if a molecule to which aluminum coordinate? more strongly than the carboxyl oxygen is introduced. This ~ e e m 5to be the case \then water, amines, or free fatty acids of lon- molecular weight are added. The aluminum atoms coordinate preferentially n-ith the amine or hydroxyl groups, the aluminum chains are broken, and there is a resultant drop in molecular weight and consistency (18). EFFECT O F GkSOLISE QUALITY O S T H E PROPERTIES OF S-4P.4LM FUELS
The properties of Sapalm fuels were found to be affected by the type of hydrocarbon employed. Cyclic hydrocarbons tended to give high consistencies, while the paraffin (n-heptane) gave the lowest consistency values. Eleven gasolines meeting the requirements for 80-octane, general-purpose motor fuels all gave thickened fuels having consistencies within relatively narrow limits. Based on a wide variety of automotive fuels exclusive of those meeting the requirements of 80-octane gasoline, highly naphthenic or aromatic gasolines may be encountered which would markedly increase fuel consistency. Oxidized gasolines may greatly decrease the consistency of Sapalm fuels. However, oxidation-susceptible gasoline can be inhibited at the source with suitable commercial inhibitors as specified for U.S. Army go-octane, general-purpose gasoline and should then be suitable for use, unless stored for unusually long periods with excessive esposure (32). The minimum oxidation stability had to be 5 hr. To obtain this stability, an approved inhibitor such as C.O.P. S o . 4, C.O.P. S o . 3 , du Pont Anti-Oxidant S o . 5 , or du Pont Anti-Oxidant S o . 6 could be used, in u-hich case concentration of the active ingredient had to he 20 j=15 pounds per thousand 40-gallon barrels of gasoline (34). Consistency studies of gels prepared with Sapalm soap in various trade gasolines having aniline points from 75' to 140°F. indicated that apparent
1456
\V.ILTER
H. C. RUEGGEBERG
viscosity and extensibility \Yere substantially independent of the gasoline. However, t,he rate of solvation \vas markedly affected, but with most of the gasolines likely to be encountered in practice t'he variation in setting time could be tolerated in field computing (25). The CWS specification stated that the aniline point of the gasoline should not be greater than 46°C. (115°F.) (34). The physical properties of gasolines n-hich were studied did not adequately predict' the gelation behavior of the hydrocarbon, but if consideration is limited to the common motor gasolines, the aniline point correlated fairly well \vith solvation rate and lo\v-temperat,ure stability. It appeared advisable to limit. the aniline point t'o 120°F. maximum in order to insure that the setting time of Napalm would not exceed 13 min. and that the thickened incendiary fuels \..i-oultl Jvithstand approximately three months' storage at 0-20°F. without excessive syneresis. Tests of Xapalm fuels in the 1469 incwdiary bomb, made njth the quantit'y of thickener employed in production (approximately 9 per cent soap), indicated that the ignitability of the gel \vas dependent on the vapor pressure. With fuels containing I I per cent or more of Sapalm soap, variation in gel Reid vapor pressure from 1.4 to 1 I .2 p.s.i. did not appear to affect ignition performance in the 1169, although the evidence pointed to lower over-all ignitability than for the thinner, 9 per cent Sapalm fuel (2). Sperifirations indicated that. the vapor pressure of the gasoline (Class A00 01' .$XI) should be not less than 0.0 p.s.i. or more than 10.5 p.s.i. (31). I t was possible to add a heavier oil t o Sapalm-gasolinc gels, in amounts lip t o 50 per cent of the solvent, \\-ithotit significantly altering the apparent viscosity of the fuel. In addition to the possibility of incmasing the h r n i n g temperutiiro and the thermal yield of the fuel, the ntldition of the hravicr oil produced an increase in t,he burning i'atcx and a decrease in the amount of u n h r n e d m:ttei%il remaining. In the case of gels cwntaining greater than 50 pel' cent oil in thc solvent, the ignition of the gel was slower. Once txirnirig had begun, ho\\.ci-ei.. the burning charactei ics \\-ere equally a s good as thos;c of the gels of lo\\.c.i~oil content (21). Preliminary tests indic~itetlthat gasolines cwntaining 20 per rent of :il(whol \\-oiilci not, yield gels of adeqiintc. viscosity ( 3 ) . 1' I L \ l E S L GI. 11
This incendiaiy gel \\-a\ composed of .even part6 of ,iluminum pa1mit:itc. (mono wilt; Metasap Chemiwl Company) and foiir part\ of Seo-Fat 3-R (mixture of 40 per cent oleic acid and GO pcr cent linoleic acid; A4i.mourand Company) in S$) parts of gasoline. The gum \\as prepared by .;imply adding the palmitate poi\ clrr to the gasoline, mixing by momentary .haking or paddling, adding the S e o fat, mixing briefly, anti allouing the mix t o stand at 23°C. In 43 min. to an hour it set to a \tifl'jelly, hiit after standing for 12 to 24 hi*.at 25°C. this changed to a stringy gum. The burning characteristics \!-ere improved by the addition of 0..i-l per cent lamp lilack; the filler wa5 merely added to the gasoline before adding the other ingredient5 (13).
THICKENERS FOR IIiCENDISRT GELS
1457
STEOLATE SOAPS
Early work on aluminum soaps revealed the fact that when prepared by usual precipitation methods, some free fatty acids remained in the soap. Thus, commercial aluminum stearate contained free acids, stearic acid among them. It x a s recognized that tallow-base, double-pressed stearic acid, from Tvhich aluminum stearate was made, was chiefly palmitic and stearic acid, the former 50-55 per cent, the latter 40-45 per cent, with other higher fatty acids present in small quantities. It has been shown that free stearic acid has deleterious effects on gels made from aluminum soaps. Soaps containing free stearic acid produced gels with gasoline but these gels were unstable a t -40°F. and broke don-n a t room temperature after several days’ standing. The effect of free oleic acid was quite different from that of free stearic acid. When free oleic acid was added to extracted aluminum stearate, the gel produced was more stable and had greater string than that produced from aluminum stearate containing free stearic acid. Aluminum oleate coprecipitated with aluminum stearate had a stabilizing influence. The effect was more evident if this coprecipitated aluminum stearate-aluminum oleate was extracted with acetone to rid it of free stearic and oleic acids, and then a gel was made with the extracted material and free oleic acid. Such a gel was stable a t -40°F. and at 150°F. and had considerable string. - i n improved technique was developed for producing an intimate mixture of aluminum stearate and aluminum oleate, without the undesirable free stearic acid but containing free oleic acid. It involved extracting the free acids from aluminum stearate, suspending this extracted material in water, after n-etting it with alcohol, and precipitating aluminum oleate and free oleic acid on the surface. Soaps made by this technique were called steolates. By this procedure it n-as possible to coat different soaps, thereby obtaining soaps v i t h different gelling and aging rates. The preparation of gels from the steolates involved mixing the soap with gasoline, shaking or stirring for a fen. minutes (usually two or three), allowing to stand for approsimately 15 hr., then aging for another 15 hr. a t 150°F. in an air oven. The gel became transparent in 3-5 hr. a t the aging temperature. The apparent viscosity of steolate gels changed with the oleic acid content. The effect was that the addition of free oleic acid gave leqs viscous gels (20). Steolate soaps which could be aged at room temperatures were produced by careful control of manufacture (29).
Jv
GUMS
Aluminum soaps of the “JT” fatty acid (tallow fatty acid by-products of the meat-packing industry) were prepared and investigated for their suitability as thickening agents for incendiary gels. Preliminary attempts to form an aluminum soap of the “W”fatty acids in gasoline directly resulted in failure. Likewise, the heating of the fatty acids directly with potassium hydroxide solution, followed by the addition of aqueous alum, resulted in a gum Tvhich was unsatis-
1458
WALTER H. C. RUEGGEBERG
factory. X huccessful soap ivas made by dispersing the fatty acids in water and heating to 125°F. The potassium hydroside solution \vas then added, followed by stirring in an aqueous solution of alum. T o prepare the gel, the soap had to be stirred in the gasoline and the temperature raised t o 125OF., or the soap and gasoline were stirred in the cold until an even dispersion (paste) resulted. The mixture \vas then heated at 30°C. for 3-1 hr. It \vas concluded that a sstisfactory, stable, thickened gasoline for use in the X I 4 i and 1169 incendiary bombs could be prepared from the ',IT" thickener (14). STC
LTE-C'APRYL ITE A S D STEARATE-PELARGONATE SOAPS
Steolate and Sapalm soaps are susceptible to oxidation; both of them contain a t least 20 per cent of the oleate radical in the soap. I t vas felt advisable to find a suitable substitute for the oleic acid. The gelling effect of the oleic acid was expected to be due either to the fact that it Tvas unsaturated or to the fact that it n-as a liquid, since stearic acid acted so differently. Liquid fatty acids of eight or nine carbon atoms could be used to test part of thi!: hypothesis. -450/50 stearate-caprylate or 50 '50 stearate-pelargonate soap should have approximately the desired gelling properties if the double bond Tvas not involved in gel formation. The 50 /50 stearate-caprylate soap v-odd withstand oxidation longer than Sapalm and, in gel tests, it was comparable to Kapalm. The difficulty in using this soap as an alternate for Sapalm was that the source of caprylic acid Tvas coconut oil fatty acids. The stearate-pelargonate soaps were made from r a v materials nhich nere available in limited quantities. S o stearate-pelargonate soap n-as made that would give a gel aging a t room temperature without the 1i.e of a peptizer. Llmong the peptizers te.ited, p-cresol and diethylene, glycol monobutyl ether gave the most desirable waiilts. -1 peptizer increayed the string of a gel as well as the rate of gelation. Tests showed that the stearate-caprylate sonp vas more resistant to oxidation than Sapalm. It contained no msatiiruted linkages but Tvould form a gel, aging a t room temperature, with very desirable properties. I t appeared that n stearatepelargonate soap shonld have siisceptibilities comparable to those of n qtearatecarprylate soap (10). STZIMAKT
The liquid incendiaries tested during World War I possessed the dran-back of excessive dispersion, which n-as, however, moderately 11-ell corrected by the absorption of the liquidi in some materials such ab cotton waste. Iluring World War I1 this gener'il idea was revived, resulting ultimately in the incendiary gels. Of the various iubstances tried for solidifying incendiary fuels, the following were found to be generally applicable: ( f ) sodium stearate; ( 2 ) aluminum salts of mixed fatty acids and naphthenic acid; (,3) polyacrylates; and ( 4 ) rubber (natural and synthetic). Sodium stearate would not give a satisfactory product with incendiary fuels. Stearic acid, however, when dissolved in the incendiary liquid, produced excellent gels after the addition of sodium hydroxide dissolved in 85 per cent (or purer)
THICKESERS FOR ISCESDIARY GELS
1459
alcohol. SOD 122, a product of the Standard Oil Company of SeTv Jersey, was the most’ promising gel of this type. Aluminum soap thickeners consist of either aluminum salts of the saturated soap-forming fatty acids having eight to fourteen carbon atoms, an unsat’urated soap-forming fatty acid, or a. carhoxylic acid containing a carbocyclic ring (e.g., naphthenic acid), or a mixture of these. Sapalm, the aluminum soap of an oleic, naphthenic, and coconut fatty acid mixture, falls into this class of thickeners. A revien- of this substance, from the standpoints of chemistry, manufacture, stability, effect’ of moisture, gasoline quality, etc., is presented. In lieu of soaps, natural and synthetic rubber as well as resins such as the polyacrylates can be iwrd as thickeners, yielding, hm-ever, gel-products possessing properties some\vhat, different in their characteristics from those of the soap-thicliened fuels.
The author desires to express his appreciation t o Drs. I-era Smullep and Leo Firilielsteiii of the Chemical Corps Technical Comniand for their a conipilat,ion of the bibliography and their interest in the preparation of this paper. REFERESCEF (1) BEERBOWER, -4.: 1943.” (2) B E w s , It. L . : 1943.” (3) BETTs, R.L . , ASD ~ I Y E R S. S , F . : OSRD Report S o . 1345, April 15,1943. (4) BIRSBALX, S . ,ASD EDXOSDS, S. RI.: CUAIR 36, September 2 5 , 1943. (5) BIRSBATX, K.,ASD EDMOSDS, S.AI. : 1943.* ( 0 ) RROL-GHTOS’, J . , .\SI) BYFIELI), A , : OSRD Ilrport S O 2030, . S o v e m l w r IT, 1913. ( 7 ) BROT-GHTOS, J . , ASU BYIXIJ, :I.: OSRD Report S o . 203Ga, JIarch 7 , 1944. (8) BI-IL, B . .I.: 1643.* (9) C A R V E R1,:., I
