V O L U M E 19, NO. 9
630 ing in density and particle siz?, subject t o marked segregation and oonsequently difficult to sample representatively. D a t a for duplicate determinations are given in Tables I1 and I11 to demonstrate the precision of the methods. Composition tolerances permitted in the specifications vary, depending on the particle size of the ingredients and other factors, from maximum of *0.7% far iron oxide scale, *0.5% for barium nitrate and aluminum, and *O.lYo for sulfur. I n all the analyses carried out, the procedures of this paper gave results of sufficient refinement to he used to investigate compliance with specifications. The procedures described, in addition to being rapid, have an advantage over procedures whioh would require placing the iron oxide scale in solution, as iron oxide can he best estimated by weighing in its original condition. I n general, larger and thus more representative samples of a mixture may he used than in methods necessitating dissolving the entire sample. The procedure of separation of ingredienis of a mixture by t,he artion nf adl*nt,ive a o l v m t s hn,s hem nsed in t,he andvlvsis .......nlso . " . . "__I_"
~
of other pyrotechnic mixtures such as colored signal smokes. U. S. Army signal smokes (Z), which are either mixtures of organic dyes, potassium chlorate, sodium bicarbonate, and sulfur, or dyes, chlorate, and sucrose, and most of the German and Italian smokes, which are mixtures of dyes, lactose, potassium chlorate, and inert material such as kieselguhr, have been analyzed satisfactorily by procedures similar to those given for incendiary mixtures. LITERATURE C1
(1) Chemical Warfare Service, Teohnir.. vvLLLIIIIIIIu, yl.li,ll. yIIw New~,22.1990(1944). (2) Lunge, G., y d Keane. C. A,, "Technical Methods of Chemical Analysis, 2nd ed., Vol. IV. p. 550, London, Gurney and Jackson, 1940. (31 Tavlor. C. A,. and Rinkenbach. W. H., Bur. Mines, Bull. 219 f72 (1923).' (4) Taylor, C. A,, and Rinkenbach. W. H., Bur 282,22 (1922). ( 5 ) Wer Dept., Tech. Manual 9-2900,77-82 (l!
.,
Direct Determination of Bu lning Rates of Propellant Pow1 A simple and convenient method for the diirect measurement of the linear burning rate of propellant powders is describedI. The time required for a long, endburning powder strand to burn a measured distance is recorded eleotrically. Burning along the side surfaces of the grai n is prevented by a restrictive coating. The metbod is rapid and precise and use'5 small, easily prepared samples. In favorable eases the probable error of a measurement may be less than 0.25%. The burning rates when measured in this w a y are shown to be characteristic of the powder composition alone. lid&burning
rate 1s one
fine m o ~ Important t propemes 01 I t s measurement is an experiment of fundamental importance, both as a preliminary to the designing of propellant charges and in the study of the mechanism of the burning reaction. When i t hecame apparent almost a t the start of these studies of the fundament& of, propelIant burning that existing methods for the determination of the burning rate were not adequate for the purpose, the development of more suitahle experimental methods was immediately undertaken. The burning-rate method developed by this work is reported here. The results of the burningrate measurements and of other observations on the burning process of propellaat powders will he reported in another paper. Burning rates have usually been determined by firing the powder charge in a closed vessel or in a vented chamber simulating a rocket motor. The variation of chamber pressure with time is observed. From a ballistic viewpoint, this is 8. valuable prom dure, for the rise in pressure with time is precisely the interesting datum for bsllistios. As a laboratory method of determining burning rates, however-especially for purposes of kinetic interpretation-such methods are unsuitable, for the fallowing reasons: 01
an explosive designed for use as a propellant.
.
1. T h e burning rate, as measured in ballistic vessels, is a function of powder and chamber geometry as well 8s of powdel oompositian, temperature, and pressure. 2. Certain questionable assumptions are always involved in utilizing the pressure data to oltlculate the linear regression 01 the burning surface. I Present address. Rayon Teohnieal Diviision, Ji Company. Wwnesboro. Ya.
E. I. du Pont de Nernourn
inoh.
4. The precision of such measurements is generally too poor to show clearly slight differences between similar powders. 5. The apparatus required is eomplioated and expensive and requires elaborate safetv precautions.
Figure 1.
Burning Rate Apparatus
left. Gas-handling smem Gnter. Thermostat containing preesurs r e a s d Right. Controls
SEPTEMBER 1947
Figure
631
2, Pressure Vessel Disassembled; Showing Powder S t r a n d in Position for B u r n i n g
6. These methods usually require large powder samples and special grain form cannot be and safely on a laboratory sczle. I n considering possible methods for a simpler and more direct determination of the burning rate, the direct observation of the burning of long grains of powders, as described by Murmur, immediately suggested itself ( 1 ) . The authors' methods improve on Muraaur's by increasing the useful range ai pressures and by improving the regularity of burning. The observation of the time of burning of a given length of powder strand remains the basic operation. The fist experiments were k e d out in a heavy-walled glass cylinder a t pressures up to 100 pounds per square inch. Later observations a t pressures up to 2000 pounds per square inch were made in s steel pressure vessel fitted with a heavy glass window through whioh the burning strand could be photographed with a motion picture camera. Burning rates were obtained from an examination of the film record. These early experiments showed that the strands burned very irregularly. The flame spread along the sides of the strand, producing a conical burning surface. The observed huriing rate was the rate of propagation of the burning reaction over the surface of the grain, rather than the desired rate of regression of the powder in a direction normal to the burning surface. It wm apparent that it would be necessary to suppress this surf8ee renotion before useful data could he obtained. After tests on a variety of organic and inorganic coating materials, satisfactory thin coatings of polyvinyl alcohol and of a vinyl chloridevinyl acetate copolymer were developed. The surface reaction was completely suppressed and the burning surface remained flat and normal to the axis of the strand. A t the same time no troublesome residue was left behind from the coating. The burning appeared to be regular and burning rates could he precisely reproduced. With irregular burning largely eliminated, it was possible to dispense with the photographic observation of the burning strands and make use of a simple electrical .method for measuring the burning time. This was accomplished by placing fine fuse wires through small holes in the strand a t measured distances and connecting them, through an appropriate relay system, to an electric stopclock. The elimination of the glass window in the pressure vessel made it possible to work a t higher pressures with greater safety.
lid down. A gasket made from annealed No. 10 copper wire, seated in a shallow groove in the lip of the chamber, makes a gastight seal. (A more recent model of the apparatus, in use a t the California Institute of Technology, has an unsupported areatype closure. This may be more convenient in operation, although the closure described here is remarkably trouble-free.) Four electrical leads through the lid connect to the two fuse wires and ignition loop and to a common return lead. Nitrogen or other gas is admitted through a fitting in the base of the chamber. The eleetrioal leads and pressure fittings are of stsndazd design and were obtained from the American Instrument Company. The electric timer was obtained from the qtandmd Electric Time Cammnv. The dial is maduated in tenths of B second and the time may be estimated-to 0.01 second. It is connected to the 110-volt alternating current supply through two relays. These in turn are eonnecbed to the two 0.6ampere fuse wires in such fashion that when the upper one is melted as the burning powder surface reaches it, the clack is started, while the second One stops the after the strand has burned for a measured distance. Two toggle switches in series with the fuse wires permit testing of the timing circuit before the shot is fired. T h e ignition system consists of a loop of No, 20 Nichrome wire conthrough a Puahbutton and pilottolight. netted a
.
F i g u r e 3.
P r e s s u r e Vessel for B u r n i n g R a t e Measurements
APPARATUS
The apparatus used in carrying out the electrical burning rate measurements is shown in Figure 1. It consists of the burning chamber mounted in a thermostat (center), the control cabinet containing the ignition and timing systems (right), and a gashandling system to supply nitrogen st high pressures (left). The disassembled combustion chamber is shown in Figure 2. Figure 3 is a detailed drawing of the combustion chamber showing fittings and a sample mounted in position for firing. The body, lid, and cap of the chamber are made of stainless steel. A thmst,ring and thrust bolts of hardened steel press the
Nitrogen a t pressures up to 2000 pounds per square inoh is sup plied from commercial cylinders. A motor-driven bomter pump is svailrtble for work a t higher pressures. Measurements me normally made over the pressure range from 100 to 5000 pounds per square inch, although the burning chamber is designed to stand considerably higher pressures. I n order to minimize the pressure rise in .the system due to the burning of the sample a large ballast tank is connected to the burning ohamber through a valve and short length of large-bore tubing. A small nitrogen cylinder, shown in Figure 1, is suitable for t b k purpose a t p r e e
632
V O L U M E 19, NO. 9
sures up to 2000 pounds per square inch. h heavier cylinder has been constructed for use a t higher pressures. The pressuir is measured with a calibrated Bourdon gage. PREPARATION OF SAMPLES
The powder samples are cylindrical strands appi oximately 0.125 inch in diameter and 7 inches long. They are readily prepared in the laboratory either by extrusion, using a minimum quantity of acetone or other solvent, or by solventless extrusion. Samples may be prepared from large grains without reprocessing by rough-cutting a long strip from the grain and reducing i t to the desired shape and size with a series of hollow mills. The burning rate is very sensitive to the moisture and solvent contents of the powder. Careful conditioning of the samples is necessary if full advantage is to be taken of the precision possible with this method of measurement.
2.0
0.1
I-
'
I
100
' I
, n
200
I
I
I
I I I I I
I
I
500
1000 2000 Pressure, Pounds per Square Inch
I
I
lIl1_1
5000
Figure 4
The restrictive coating is applied to the conditioned strands by dipping them in a solution of the coating agent. A 5.' dispersion of polyvinyl alcohol (du Pont Elvanol, medium viscosity, type B is satisfactory) in water or a 5% dispersion of vinyl chloride-vinyl acetate copolymer (Vinylite VYLF, made by the Carbide and Carbon Chemicals Corporation, was used) in methylene chloride is regularly used. The polyvinyl alcohol appears to give slightly better restriction but the vinylite dispersion has the advantage of drying in a few seconds, so that measurements can be made immediately after coating. When the coating has dried, two small, accurately spaced holes, 5 inches apart, are drilled through the strand. I t is then returned t o the conditioning cabinet until the time when a measurement is to be made. EXPERIMENTAL MEASUREMENTS
Two short lengths of fuse wire are threaded through the holes in the prepared sample, which is then mounted in the pressure vessel. The vessel is closed, the timing circuit is tested, and nitrogen is admitted to give the desired pressure while the valve t o the ballast tank is opened. Ten minutes are usually allowed for the attainment of temperature equilibrium. Two burning chambers, a t the same or different temperatures, .sith a single control boa can be conveniently handled by a single operator. K h e n the shot is fired the pressure increases by only 2 or 370. The average pressure during the burning is recorded; the burning time is registered on the stopclock; thus the necessary data are immediately available and the burning rate can be determined without laborious calculations and interpretation of data. PRECISIOY AND ACCURACY OF MEASUREMENTS
Typical burning rate-pressure curves are shown in Figure 4. The powder used in the experiments reported here had the following composition: Nitrocellulose, 13.25% n i t r o g e n Nitroglycerin Diethyldiphenylurea
54.0Y0
43.07, 3 0%
Table I.
Precision of Burning Rate Xleasurements by Strand-Burning Method
(Series of 20 shots, 25' C , 1000 pounds pkr square inch) Burning Time for Burning Shot S o %Inch Strand Rate Sec Inch/sec 1 9 88 0 5061 2 9 82 0 5092 3 9 78 0 5112 4 9 84 0 5081 5 9 87 0 5066 6 9 82 0 5092 7 9 81 0 5097 8 9 86 0 5071 9 88 9 0 50'61 10 9 87 0 5066 11 12 13 14 15 16 17 18 19
9 82 9 77 9 86 9 87 9 70 9 88 9 88 9 82 9 86 9 89
20 Average Probable error (neglecting KO.15)
0 5092 ,5118
n
0 507i
0 0 0 0 0 0 0 0 0
5066 5155 5061 5061 5092 5071 5056 5082 0013 = 0 25%
The method has been used successfully, in these and other laboratories, with both single-base and double-base ponders, with composite powders containing a variety of insoluble organic and inorganic fillers, and n-ith a wide variety of experiments1 propellants. The precision of the measurements depends largely on the quality of the propellant strands. Voids and fissures !Tithin the strand, and sometimes surface irregularitics, will cause erratic burning. Such' irregular burning is best studied by the photographic method. Table I shows the results of a series of 20 successive measurements on propellant strands of good, but not exceptional, quality propellant. The probable error of a single measurement, neglecting shot 15, which probably contained a flaw in the grain, is 0,25y0. The principal source of error lies in irregular burning of the strand and this always caused the observed burning rate to be too great. I n cases where the precision is poor the lowest rate observed will usually be the most reliable. Considerable effort was made to show that the burning rates measured by this method depend on the powder composition alone. The effect of radiation, which depends on the geometry of the charge and combustion chamber as well, is minimized by the low loading density and small flame volume. Tests using strands of different diameters of a powder containing large amounts of potassium nitrate to increase the emissivity of the flame gave no evidence of a radiation effect. These tests and similar ones with low emissivity potyders also showed that the burning rate was independent,, at least within fairly wide limits, of the strand diameter. Erosion-Le., accelera$ed burning due to the flow of hot gases over the burning surface-obviously plays no part in the met,hod described here. Powder samples have been burned in helium, carbon dioxide, and argon as well as in nitrogen. Xo significant difference in the burning rate was found in t,hese chemically inert gases, which show a considerable range of thermal diffusivities. (However, if the propellant is allowed to stand for a long time in carbon dioxide before firing, the burnirig rate is decreased on account of solution of the gas in the propellant. before firing.) The observed burning rates agree, in order of magnitude, 'with those obtained from ballistic measurements where the powder grain burns in an atmosphere of its on-n hot decomposition products. The oxygen remaining in the burning chamber when it is closed and the small amount found in most commercial nitrogen have no effect on the burning rate, although higher concentrations of oxygen increase it greatly, as shown in Table 11. Under the conditions of the authors' measurement,s the burning rate appears to be independent of the nature of the surrounding gaseous atmosphere.
SEPTEMB.ER 1947 Table 11.
633
Effect of High Concentrations of Oxygen on Burning Rate
Oxygen conrentration, 7' b y
voluriie 0 Burningrate,inchpersecond 0 38
5 10 15 0 . 3 8 0 39 0 . 4 2
20 0.52
25 0.67
30 1.00
Table 111. Comparison of the Burning Rate of a Powder Coated with Polyvinyl Alcohol and with yinylite (25' C., 1000 pounds per square inch) Burning Rate, Inch per Second Polyvinyl alcohol Vinylite 0.512 0 511 0.512 0 509 0 513 0 508 0,509 0 512 0.507 0 510 Av. 0.510 0 511
Different coating materials applied from different types of solution give the same burning rate, as shown by the data in Table 111. The burning rate is independent of the coating thickness, nithin wide limits (Table IV). The coating thickness must exceed a certain minimum value necessary to prevent irregular burning; too thick a coating may leave a troublesome residue and cause new irregularities. These coating experiments indicate that the coating acts only to sup-
Tahle IV.
Effect of Coating Thickness on Burning Rate (25O C., 1000 pounds per square inch)
KO. of Coats
Burning R a t e , Inches per Second Polyvinyl alcohol Vinylite 1.11 1 11 0.678 0 529 0.523 0 512 0.516 0 513 0.521 0 511 0.516 0 511 0.523 0 508
press the surface propagation of the flame and has no specific effect on the linear burning rate of the p w d e r . This method of measuring burning rates has been used successfully in a number of laboratories. The simplicity of the equipment, speed and precision of the measurements, and direct and unambiguous nature of the results obtained recommend it both for research purposes and for product control. LITERATURE CITED
(1) Muraour, H . , and Schumacher, W., -Vim. poudres, 37, 87-97 (1937). BASEDon work done for the Office of Scientific Research a n d Development under contract OELMsr-716 with the Gniversity of Minnesota and contract OEMsr-762 with t h e University of Wisconsin. I n p a r t from a thesis submitted t o the Graduate Faculty of the University of Minnesota by Clayton Huggett in partial fulfillment of the requirements for the degree of doctor of philosophy.
Determination of Antioxidants in Gasoline
,
LOIS R. WILLIARIS A N I BARYEY R. STRICKLAND Esso Laboratories, Process Division, Standard Oil Development Company, Elizabeth,
-4ntioxidant compounds are added to gasoline to inhibit the formation of gum and precipitation of lead and to maintain over-all stability. Alkyl-substituted p-aminophenol and p-phenylenediamine derivatives are the most commonly used inhibitors of high potency. Since many inhibitors may be lost from gasoline upon contact w i t h acid, alkali, or water, it is often important to check the amount of inhibitor present. This determination proved of value during the war in investigating the stability of
F
OR some time antioxidant compounds have been added to gasoline to inhibit the formation of gum and precipitation of lead, and to maintain the over-all stability. Alkyl-substituted p-aminophenol and p-phenylenediamine derivatives are a t present the most commonly used inhibitors of high potency. These materials are usually added in the concentration range of 1 pound per 5000 gallons of gasoline (2.4 mg. per 100 ml. is equivalent to 1 pound per 5000 gallons). Since many inhibitors may be lost from gasoline upon contact with acid, alkali, or wat,er, it is often important to check the amount of inhibitor present in a sample of gasoline. The analytical method presented proved of considerable value during the war in investigating the stability of aviation gasoline supplies, and is particularly useful in studying loss of inhibitor fromgasoline. Thc method for quantitatively determining the amount of aminophenol- and phenylenediamine-type inhibitors in gasoline \ Y ~ Sdeveloped by the Esso Laboratories from a procedure suggested by E. I. du Pont de Nemours and Company, based.on a reagent used by Folin and Denis ( d ) for indicating uric acid. Cortain other inhibitors, such as alkyl phenol derivatives, which are not extractable with acid or will not reduce Folin-Denis reagent citnnot tw determind in this manner.
N. J .
aviation gasoline supplies and is particularly useful in studying loss of inhibitor from gasoline. It is a colorimetric technique based on extracting amino-, phenol-, or phenylenediamine-type inhibitors from gasoline with aqueous hydrochloric acid solution, the hydrochloridesbeing formed. When the extract is neutralized with sodium carbonate in the presence of phosphotungstic acid solution, a blue coloration is produced which is proportional to the concentration of inhibitor present in the gasoline.
It is a colorimetric technique based on extracting the amino-, phenol-, or phenylenediamine-type inhibitors from gasoline with aqueous hydrochloric acid solution, the respective hydrochlorides being formed. When the extract is neutralized with sodium carbonate in the presence of Folin and Denis reagent (phosphotungstic acid solution), a blue coloration is produced which is proportional to the concentration of the inhibitor originally present in the gasoline. APPARATUS AND REAGENTS
Electric pH Meter. Instruments of the direct-reading type are preferable. Photoelectric colorimeter. Centrifuge. Hydrochloric acid, 5 5 aqueous solution. Sodium carbonate solution, 180 grams per liter. Folin and Denis Reagent. ,4 mixture of 100 erams of sodium tungstate, 750 ml. of Astilled water, and 80 m?. of 85% phosphoric acid boiled under reflux for 2 hours, cooled and filtered if necessary, and diluted to 1 liter. Reagents used must be free of nitrates, since nitric acid interferes with the color formation. Dye Solvent. A 1 t o 1 mixture of toluene and alkylate or isooctane. Diethyl ether may also be used. The solvent must be free of inhibitor.
