1460 (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35)
LEO FISKELSTEIN
MICHAEL,AI. W.:TDMR 1209, January 2 , 1946. MYERS,K.F . : 1943.* Ray, A . B.: CWM S L I I I , Part I , ( l ) !pp. 36-46 (1918). RUSSELL, R . P. : OSRD Report S o . 382, February 7,1942. SOCTHERS, J. h.: 1943.* SOUTHERS, J . A . , ASD ROTH,L. J . : 1913.* SOUTHERS, J. A . , . 4 S D ROTH,L. J . : 1943.“ STASDARD OILDEVELOPXEST COXPASY: “The Development of Oil Incendiary Bombs,” NDRC Division 11, January 20,1942. S ~ a h - o . 4OIL ~ ~ DEVELOPXEST C o w a s y : “Effects of Thickener and GasoIine Quality on the Properties of Sapalni Fuels,” S D R C Division 11, July 6,1941. THOMPSOS, 5 . J . : OSRD Report S o . 1702, August 11, 1943. U. S. Army Specification 96-131-378, August 24, 1945. WHITE,E. R.:“The Inhibition of the Oxidation of XapalmSoap,” NDRC Division 11, August 30, 1943.
RHEOLOGICAL PROPERTIES O F ISCENDIARY GELS LEO F I S K E L S T E I S Technical Command, Army Chemical Center, Maryland Receaved June 25, 1948
Incendiary gels are essentially non-Semtonian liquids, whose viscosity varies with the stress. Three particular features are observed with such systems: a variable ratio of shearing stress to rate of shear; a finite relaxation procedure for suddenly applied stress or deformations; and a frequency-dependent dynamic viscosity in the case of alternating processes (8). MECHASICAL .kXALOGIES
Carver and Van Kazer ( 5 ) consider the LIaxn-ellian model, which separates elasticity and viscosity according to the thermodynamic definition, perhaps the most successful expedient for disentangling the combined elastic and viscous properties of a viscoelastic fluid. The theory of AIaxwell on elastoviscosity approximates the structure of an elastoviscous liquid by means of a mechanical model. Such a model consists of series and parallel combinations of dashpots and springs. The dashpots contain Sewtonian liquids and the springs obey Hooke’s law. The first element represents a purely viscous resistance, while the second element is a purely elastic resistance representing a model for an ideal solid. Both elements are connected in the model, since in a structurally viscous liquid both elements are present. When the springs and dashpots are connected in series, the total extension is the sum of the extensions across individual com1 Presented at the Symposium on Gel Formation, Detcrgencv, Emulsification and Film Formation in Son-Aqueous Colloidal Systems which was held under the Auspices of the Division of Colloid Chemistry and the Division of Petroleum Chemistry at the 113th Meeting of the American Chemical Society, Chicago, Illinois, Aipril.1948.
RHEOLOGY O F INCESDIART GELS
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poncnt \; when they are conncotctl in piirallel, the total force ib the sum of the forceh across the components. Xost rheological instruments run under constant force or at constant rate of extension. Carver and Tan Wazer (5) considered that the mechanical model of a spring with series and parallel dashpots offered a convenient qualitative approximation of the rheological properties of most incendiary gels, but that three-dimensional cross-bonded structures could not be treated by means of these analogies if the cross-bonds had elastic-viscous properties. Such a model is best studied according to the methods of statistical mechanics. QUAL1TA4TIVE EVALCATION O F A GEL
The physical measurements described below were designed to characterize qualitatively the specific properties of incendiary gels. To test for honzogeneitg, the sample was cut in half and separated so as t o expose a fresh surface, and examined visually and tactually. To determine friability and the rafe of heal, the gel was cut with a spatula and a sample withdrawn. If the sample and the area from which it was n-ithdrawn retained their shape and sharp outline, the gel was hard and friable. I n the most undesirable cases, gel shavings could be produced by scraping the surface with a spatula. If a cut healed immediately or a smooth surface re-formed as soon as a sample n-as \rithdrawn, the sample was soft and had insufficient body. The best gels were intermediate between these two extremes. The sharp edges of a cut or of a sample should become blurred almost immediately, and a cut should be completely healed in approximately 5 min. The body of the gel could be evaluated by placing a sample on a plane surface; a soft gel spread and lost its shape rapidly. The strength and resilience of a gel were determined by forcing the flat surface of a spatula through the gel. The resistance offered to its passage was dependent on the strength of the gel. h friable gel might offer considerable initial resistance and then break away. This was considered a rigid, u-eak gel. Khen motion of the spatula was stopped, a resilient or elastic gel would tend to force the spatula back out of the “compressed” area. When a gel sample was Trithdran-n on the end of a spatula, the length was qualitatively observed. The connection between the sample and the main body of the gel \vas maintained over a ronsiderable distance with a long gel. If the gel \vas friable, it would be very short. A long gel was either excessively fluid or very elastic. These properties led to inferior firing characteristics. -1gel that was atlhesiw ~ r a difficult s t o remove from a spatula completely, h test of adhesion to wood could be conducted by using a physician’s tonguedepressor as a standard wood strip. The strip in a vertical position was forced down into the gel, the gel allowed to heal around it for about 30 sec., and the strip then withdrawn vertically. With a good gel, at least 50 per cent of thc submerged surface of the strip would be coated n-ith the gel. Ailong elastic gel could be dran-n part way out and would then snap back, leaving the wood clean. If one dips his hand into a mass of thickened fluid and withdraws it rapidly,
the gcl may string out, sho\\.ing cwiisitlcrahle cstcnsihility, or it miiy hreal; off’, i n id t o he short. This pi’opcrty is vai-ioiisly lmo\\-nas stiiiz~qijirs~s, shortness, or c.nte~isi6ility. Short gels shatter, d o not, carry \\-ell, and show lack of adhesion. DPX”I’RhIIS.\TIOS
O F RHEOLOGICIL PROPERTIES
Alttemptsto characterize gels accurately by measurement of specific rheological properties \\-ere undert’aken t o correlate gel formulation \\-ark, to estahlish specification tests, and to obtain an insight, into the factors influencing gel performance in incendiary munitions. ()bviously, no test \\.auld fulfill all these functions. Plastic materials such as incendiary gels resist deformation 11y low shearing forces and tend to yield more easily to higher shearing forces. Sincc t,he over-all performance of :I gel \\-as dependent on its behavior under both conditions, it \\-as impoitant to churacterize a gel at hoth high and low shearing for THK P.IR.ILLEL
PL.ITE TEST
Ihis test’ \\-as used to determine the cwnsistency of incendiary oils containing isobut>ylmethacrylate polymers. I t consisted of measuring the diameter, in centimeters, to which 5 cc. of gel spread in 1 min. betn.een parallel plates of glass under a 2-kg. load. In the CKS tests the plates of glass were 9 in. square. Ot’her tests ivere conduct’ed 11-ith the plate-glass squares measuring 12 in. square. The test \\-as a modification of a consistency test, for putt’y ( G ) . plug syringe was used t’o measure the 3 cc. of incendiary oil t o be tested. This syringe \\-as composed of tivo concentric pieces of glass tubing, a seal between the tn-o lieing made by an annulus of Seoprene tubing. The tubing of larger diameter ser.vcd as I: cylinder, calibrat’ed to deliver the required amount of filling. The tubing of smaller diameter, t o which a section of Seoprene tuhing \\-as held hy friction, served as a piston to remove the incendiary filling from the syringe. -4 stridy of the various loads indicated that values in the most sensitive ranges \\-ere ohtained with the recommended 2-kg. load. The majority of the gels \\-auld nearly i,each an equilibrium spread in the I-min. period of stress. When the load \\-as applied for 5 min. to a typical isohiityl methacrylate, the parallel plate value \\-as increased from 10.4 to 11.0 (am. Straight lines \\-ere ohtained by plotting the diameter against t,he logarithm of the time for ivhich the load \vas applied. Tht. original technique involved inriwsing the applied load liy 2-kg. iriciwnents cvryv minute until ;1 total load of 10 kg. resiiltetl. When the tliametcw o 1 ) s e i ~ by ~l this method \\-ere plotted against the logarithm of the applied load, parallel straight lines irere ohtained for gels of quite diverse caomposition. The simpler technique involving a single measurement \\-as therefore iwommended (9. 7
,
IMPACT STRENGTH
T o determine con tency at L: high rate of shear, :I S(ahoppe~-l)ynst:~tplustic’.; impact tester \\-as modified to handle gels (9). A $-in. \vide blade was moiiritetl on a pendulum \vith a total length of 13 in. .I trough t o retain the gel samplc vas placed horizontnlly at the liottom of the s\\-ing of the pendulum. The
RHEOLOGY O F I S C E S D I A R I - GELS
1463
horizontal length of the path of the blatle through the gel was 2G cm. T'arious load, \\ere placed on the pentlulum arm. The pendulum \vas released from a horizontal position, allowed to s \ ing ~ don n through the gel, and the vertical height of the upward swing \\-as determined. When the gel trough as empty, the pendulum, being nearly frictionless, rose to the horizontal position. K h e n the gel was present, the energy consumed in forcing the blade through the gel cxured the pendulum to iwing through to a maximum height somewhat short of the horizontal (or zero) point. The decrease in the height of the maximum \I\ ing from the zero point was proportional to the work done and, therefore, to the strength of the gel. To cover all types of gels, it n a s necessary to use three dif'ferent loadings on the pendulum. These \\ere referred to in increasing order of gel strength as B, C, and D scales. The B scale readings for the weakest gels ranged from one to ten units. (The units are empirical but in a rough way correbpond to kilogram centimeters.) The C scale covered a range of one t o 20 units. I n the lower half of this range, the B and C scale readings were nearly equivalent. X feiy tough gels had values exceeding the C scale. These were measured by placing a small additional weight on the impact by raising the release point from 90" to 120" from the vertical. This increased angle of fall altered the zero point of the initrument. The D scale values, therefore, were not directlv comparable to those of the B and C scales. MODIFIED STORVER VISCOSIMETER
Because of its simplicity of operation and ease of cleaning, t.he Stormer Tm' c o simeter had long been used for consistency measurements of paint's and allied products. modified Stormer viscosimeter, as described by Geddes and Dan-son (?)? consisted of a sample (*upcontaining a paddle n-hich was driven by the gravitational pull of a falling n-eight. For a given load, the speed with which the paddle would rotate \vas inversely proportional to the viscosit'y of the material in the sample cup. ;1 true determination required that stirring be caontinued until a constant' rate \vas obtained. A4t l o i r shearing forces (low applied load) the rate of rotation u-as extremely slo\r, n-hile under a somewhat higher load the increase in R.P.31. 11-ith an increase in the applied load v a s rapid. For the isobutyl methacrylate gels, the most satisfact'ory hut time-consuming c.riterion of consistenf- \\-as the load required to produce 10 R . P . M . With many gels, especially those caontaining methacrylate interpolymers, the viscosity of the gel increased diuing the measurement, and results could be duplicated only \\-hen fresh sample \\-asused for each measurement. I t v-astherefore not, possible t u obtain a constant rate. -4s a rapid generalized technique, a procedure was adopted which consisted in measuring the average rate of rotation during 100 revolutions of the paddle after an acceleration period of 10 revolutions iindei- ~1 standard load that appeared suitable for the particular type of gel (9). G.\RDSEH MOBILOMETER
This instrument \\-as used to determine the consistency of incendiary gels of the Sapalm type. It is an extrusion-typP visrosimeter in which the rate of fall
1.164
L E O FISRELSTEIX
of :t loaded piston through thc siimplc (cont:iincxtl in a tight,-fit'ting cylintlrr) i? clctcrmincd (6). 'I'hc mol)ilomrt(~rconsistctl of a tiihe, four-holc disk, plunger rod assembly, and bearing mounted on a suitable base and support. The pertinent dimensions of the standard instrument as used for incendiary gels are show1 in table 1. Owing to work-hardening phenomena, the value observed with methacrylate gels was markedly dependent upon the t'reatment of the sample. Reproducible values were obtained only \\-hen freshly prepared gels were allowed to age 24 hr. in the Gardner cylinder, and a single measurement, \vas made subsequently on the unworked sample. TT. H. Bauer (2) recommended that the consistency of incendiary gels be measured a t one rate of shear by means of the Gardner mobilometer to specify a given gel for test purposes. Each type of Sapalm gel has a specified consistency range expressed in grams. I n report'ing results two successive loads, in grams, for more than and less than 100 sec. are not'ed. The load required to give a time of fall of 100 sec. is obt'ained by linear interpolation from the two loads.
iu.
Internal diameter of tubc Diameter of disk Thickness of disk Diameter of holes in disk Diameter of plunger rod
1.538 1.500 3%
0.250 0.250 TORSIOMETER METHOD
Barnard (1) developed a method which was used for determining the elasticity and consistency of incendiary oils of high consistency. The t,orsiometer is shown in figure 1 assembled for use in the consistency test. The principal components of the torsiometer include a disk calibrated in thirty-six ten-degree divisions, a combination pulley-indicator, a pulley mounted on the edge of t'he disk t o carry a pulley cord from the center pulley over the edge of the disk to a weight (two weights are used, one after the other), irhich serves as the actuating force for the test, a testing paddle, and a trigger mechanism to release t,hr pointcrindicator when making tests. In malting an elasticity test, the follo\ring procedure \ r a y used: TVhen the. pointer was released, the weight actuated the paddle, tlvisting it, through one revolution. On completing 3G0°, the loop of t'he cord slipped off the peg (set in the groove of the pulley-pointer), permitting the paddle to return in a clockwise direct'ion toward the original zero position. The extent of return, or recovery of original condition, was indicated in degrees, marked on the disk. The elasticity was reported as the average of two consecutive readings which agreed within 10". In det'ermining the consist'ency of the incendiary oil being tested, the pointer
RHEOLOGY O F IKCEKDISRY GELS
1467
and the torsion rod system has a period of vibration of only 0.011 see. JIinor features are the lower bearing which was found essential to prevent the inner cylinder from being dragged from its central position, a cover on top of the outer cylinder which was necessary t o keep the material from cravAing up the torsion rod, and a thermostatic cup placed around the outer cylinder. The material under investigation was placed in the cup and alloir-ed to stand for several hours t o remove any air bubbles. Rotation was then started and the deflection of the inner cylinder followed visually on a scale or photographically. The traces for a Sewtonian fluid show a sudden, sharp rise a t the moment the outer cylinder starts to rotate, followed by a constant deflection proportional to the viscosity of the fluid. Very different traces are given by Sapalm and methacrylate gels, a pronounced rise which is a function of the time the gel is allowed to remain undisturbed in the cup between runs being obtained. The trace obtained with the jeweler’s lathe viscosimeter is in essence a graph of force a t constant rate of extension. JIACAMICHAEL VISCOSIMETER
This instrument has a disk of 3-cm. radius suspended about 5 mm. from the bottom of a cylindrical cup which is fixed in a bath on a turntable driven a t constant speed (normally 20 R.P.M.) by an electric motor and suitable gears. The suspension wire (about 25 cm.) is inclosed in and fixed near the bottom of a tube or spindle t o which the disk is fastened by a bayonet catch; the top of the spindle carries a graduated dial, the deflection of \vhich beloiv a pointer indicates the twist produced in the wire when the cup (containing the liquid under test) is rotated. The circumference of the dial is divided into 300 equal parts (MacMichael degrees). The temperature is controlled by electric heating. A table is supplied so that errors due t o permanent set may not occur. Measurements are made of the torque and the R.P.M. from these, and from these a curve of apparent viscosity versus rate of shear can be obtained, calibration being made with the aid of standard oil. This viscosimeter covers a rate-of-shear range of approximately 3-100 set.-' EXTESSIOJIETER (EASTMAK KODAK METHOD)
This method was developed t o evaluate shortness (4). Two 2i-in. long glass tubes of 4-mm. inside diameter and having smooth flat ends were filled with the incendiary gel by gentle suction. The tubes were joined, by a short section of rubber tubing, as close to each other as possible to prevent inclusion of air in the gel column. The filled assembled tubes were placed vertically in a suitable support and the upper tube was secured to a line which ran over a pulley and was connected to a constant-speed motor arranged to pull the string a t a rate of 3 in. per second. The string had to be carefully adjusted for tautness so that it would just hold the top tube upright when the rubber tube had been removed. The assembly was allowed to stand untouched for 1 min. while gel healing occurred. The rubber tube was then carefully slipped down below- the division point and the motor started. The distance the upper tube had t o be
1468
LEO FINKELSTEIN
raised to cause the gel string to break was reported in inches as the “estensibility” of the fuel. STASDARD OIL DEVELOPlIEIUT MODIFIED EXTPh-SIOJIETER
’
Since some difficulty vas experienced in obtaining reproducible results, u modified method was developed by Betts and Myers (3). The mechanical features of the modified extensiometer comprised a motor and gear-reduction system which pulled a string over a pulley a t a rate of 3 in. per second. The other end of the string was attached to a glass tube of approximately 2 a- mm. inside diameter. The lon-er end of the glass titbe had a sintered glass disk to which a piece of blotting paper was held by suction applied at the other end of the tube. Other variations ivere permissihle In that a perforated disk could be substituted for the sintered glass. In opelation, suction was applied to the tube to hold the paper in place and the tiihe I\ as lowered until the blotting paper made complete contact with the gel hurface. .lftcr the Contact had hecn made for 1 min., the motor nab started and the tube raised at a cxonstant rate until thc string of gel broke. Thc dibtance traversed WL\ recorded a t the “extensibility” of the gel. The size of the gel container inarlmlly affec.ted the r e d t , but it \\a5 found that a 1-quart LIa-on-jar container I\ as satisfactory provided the gel surface was smooth and not above the shoulder of thc jar, and the zamplc \\ab substantially free of air hubhles. JICASUREMEXT O F CLASTICITY AT HIGH FREQGENCIES
Early in the ini-estigation of the rheologic>alproperties of thickened fiielb. it was thought that it might be not the static rigidity and relaxation time which would be of importance, but their values at high frequencies. The Ferry method is optical in nature and depends upon the solution under investigation being optically clear and becoming birefringent under strain. Hence, it wis poszible t o make measurements only upon the isobutyl methacrylate interpolymer gels (4). B.ILL
VISCOYIlIETER METHOD
This test wasdeveloped to determine the coiisistency of thickenedflame thro\\ r r fuels in the field. The viscosimeter consisted of a graduated transparent plastic tube (Tenite or similar), 10 in. in length and 2 in. in diameter, and a series of commercial steel ball-bearings, from 4 32 t o 8 ’32 in. in diameter, graded in thirtyseconds of an inch. The consistency \\ as meawred by timing the fall, through the gel, of the series of balls past t \ \ o successive graduation?. The “ball consistency” \vas reported as a number equal to the number of thirty-seconds of an inch corresponding to the diameter of the hall whose time of fall \vas nearest 30 sec.
h .s.T.AI. GRE.1SE
PESETROAIETER
This instrument determined the depth of penetration in hundredths of centimeters of a cone into the gel in 5 sec. The standard Ai.S.T.RI.procedure used a
RHEOLOGY OF INCESDIART GELS
1469
load of 150 g. To obtain penetrometer values in the most sensitive region of the instrument, the load n-as reduced by a counter-balance to 50 and 25 g. Even under these conditions, the correlation between penetrometer values and apparent stiffness was still not good. The roughness of the gel surfaces and the presence of air bubbles in the gels seemed to introduce errors which exceeded the normal differences between gels (9). SL-MX.%RY
Rapid and reliable evaluation of incendiary gels has been the ever-present problem since adoption of this type of filling for incendiary bombs and flame thrower fuel. 1-arious instruments have been tried, but only in a limited numher of cases have satisfactory correlations with test firing data been obtained. The Gardner mobilometer \I a5 the only instrument found generally satisfactory for testing and evaluating Sapalm-gasoline gels, hut it did not prove applicahle t o other incendiary fillings. Yery good correlation h i ~ sheen found betn-een the Gardner and Stormer instruments n-hen wed in testing gels prepared from a given soap. Either instrument may be used for e~aluationof gel consistency, but for reasons of availability and simplicity the Gardner instrument is preferred. The correlation bet\\-cen the Gardner and falling-ball consistencies is sufficiently precise to jiistify use of the latter method for control of the viscosities of flame throuer fuels i n the field. Incendiary fuels are ejected from the flame throver at rates of shear far in ewe-> of that obtained in the inytitiments ~isedfor the evaluation of gel con' twcy. -1 compari-on of the G:udner mohlometer 11ith the capillary viscoiimeter at shear rates obtained in the portable flame thrower indicates that the Gardner instrument appeal5 adequate for predicting the performance of flame throlver fuels (3). The parallel plate method, Tvhich has been useful in specifying methacrylate gels, could not be correlated I\ ith field tests. -4lthough the Stormer viscosimeter \vas oonsidered as a test instrument for methacrylate gels, it proved unsatisfactory because the stiffness (rigidity) of the mixture required the ube of greater weights for starting rotation of the test paddle than the instrument was designed to carry; when rotation was initiated the paddle quickly sheared through the material being tested and started racing. 1T. H. Bauer ( 2 ) determined that at very lon- rates of shear the Stormer instrument ~ a . 4satisfactory for methacrylate and Sapalm gels. For consistencies exhibited by 10-14 per cent Sapalm gels, the Stormer instrument \vas of no use, since the rotating paddle cut through the gel and spun in the space created. The penetrometer designed for grease testing was applied to methacrylate gels n ith limited success. Extremely soft gels, Ivhile distinguishable from medium or stiff gels, were outside the range of the instrument. In the medium range the penetrometer could not differentiate with sufficient precision betiveen satisfactory and unsatisfactory gels. The only instrument that has been found satisfactory for the evaluation, characterization, and specification for methacrylate incen-
1470
LEO FLNKELSTEIN
diary gels has been the torsiometer. The method developed for using the torsiometer appears adequate for characterizing quantitatively the elasticity and consistency of such incendiary gels. The elasticity test as conducted on the torsiometer appears to be in some respects and within certain limits also a measure of the fluidity and cohesiveness of the gel. The degree of flow is a function of time, and as yet no means of measuring the time required for the initial 360" rotation has been devised. When elasticity values are low, the cause may be other than high fluidity. Frequently a gel may be so brittle or short that a rupture occurs during the course of the 360" rotation. When this happens the elasticity will be markedly less. Gels which have low cohesive strength, low tensile strength, or shortness usually reflect these characteristics in terms of low elasticity (4). The apparent viscosity coefficient determined with the capillary tube, MacMichael, and Clark- Hodsman viscosimeters has been found to be the same as the equilibrium apparent viscosity coefficient as determined on the jeweler's lathe viscosimeter (4). REFERESCESz
BARKARI), H . : TDJIR 1260, August 15, 1946. BAUER,W. H . : March 20,1943. BETTS, R.L . , ASD MYERS,S .F . : April 15,1943. CbRVER, E.K., .4UD BROUGHTOX, G . : December 7,1942. CARVER, E . I
