AD

"TECHNICAL REPORT ARBRL-TR-02308 V

Qi THE INFLUENCE OF PHYSICAL PROPERTIES

ON BLACK POWDER COMBUSTION

Ronald A. Sasse'

SDTICMarch 1981 SJUECTEJUN 1 6 1981,1

B

US ARMY ARMAMENT RESEARCH AND DEVELOPMENT COMMANDBALLISTIC RESEARCH LABORATORY

ABERDEEN PROVING GROUND, MARYLAND

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TEClINICAl, REPORT ARBRL-TR-02308 - r 347 TITLE (and Subtitle) S, TYPE OF REPORT & PERIOD COVERED

Tfil: INFIUENCIE OF PHYSICAL PROPERTIES ON BRL Technical ReportBLACK POWDE~R COMBUSTION____________________

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Ronald A. Sasse1

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Black powderPropel lantPhysi cal properties of black powder

ý20 ANST1 ACT (Cauth eM venoshbNne...eemeavyan idsnty 67 block numober) (c it)It has been observed that black powder made from ingredients of the same loi

and fabricated using the same manufacturing procedure results in material exhibiting different burning rates. The current study was undertaken to determine ifthe physical properties of black powder affected burning rate and establish ifth•-se properties varied from one production batch to another.

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20. ABSTRACT: (Continued)

Physical properties of black powder were measured including surface area,pore size distribution, and internal free volume. Black powder derived from oakand maple charcoals was compared, and the effects of processing the material bya jet mill or ball mill were contrasted. Scanning electron microscopy revealedthat applied pressure, a manufacturing step used in making black powder, inducedconsiderable plastic flow that resultedi in a fused conglomerate, cohesive masscontaining interconnecting passageways of about 0.1 micron diameter. Density,free volume, and surface area were found to be functions of burning rate. Basedon these relationships, the conclusion was offered that the degree of openness ofa grain of black powder is a significant factor affecting its burning rate.

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TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS ............ ................... 5

I. INTRODUCTION .................. ....................... 7

II. EXPERIMENTAL ........................................ 12

A. Black Powder Samples ............. ................. 12

B. Scanning Electron Microscopy of Black Powder andCharcoal .... ........... ............. 15

C. Compression of Sulfur, Potassium Nitrate and Charcoal 15

D. Mercury Intrusion Porosimetry ..... ............. .. 16

E. B.E.T. Surface Area Measurements .... ........... ... 17

F. Density Measurements. ................. 17/

III. RESULTS ................ .......................... ... 17

A. Black Powder and Charcoal S.E.M. Microphotographs . . 17

B. Compression of Sulfur, Potassium Nitrate, and Charcoal. 18

C. Mercury Intrusion FPorosimetry . . ........... 28

D. B.E.T. Surface Area Measurements ........... 28

E. Density Measurements ................ .............. 33

IV. DISCUSSION ................................. 33

A. Black Powder and Charcoal S.E.M. Measurements ... ..... 33

B. Compression of Sulfur, Potassium Nitrate and Charcoal . 35

C. Mercury Intrusion Porosimetry ..... ............. ... 38

D. B.E.T. Surface Area Measurements ....... ....... ... 40

E. Density Measurements .............................. 44

V. FUTURE PLANS ............................... .... ... 47

VI. SUMMARY ....................... .................. .. 49

ACKNOWLEDGEMENTS ................ .................. 49

REFERENCES ...................... .............. ...... 51

APPENDIX ............................................ 55

DISTRIBUTION LIST ................ ................. ... 59

3/.f

LIST OF ILLUSTRATIONS

Figure Page

1. Comparison of burning rates as measured by flame spread,0 (ref. 16); closed bomb, 03 (ref. 14); and open airflamt spread, A (ref. 13) ...... ................ .. 14

2. Black Powder, Lot 1 . .. ................ 19

3. Black Powder, Lot 6 .. . . . ..... ................ 20

4. Black Powder, Lot 10 .................. 21

5. Black Powder, Lot 11 ...................... . 22

6. Oak and Maple Charcoals . . ............... 23

7. Oak and Maple Charcoals ......... . . . 24

8. Oak Charcoal Extracted from Lot 1 ............ 25

9. Maple Charcoal Extracted from Lot 11 .......... 26

10. Eiffect of pressure on potassium nitrate, sulfur, andcharcoal ........ ............. . ....... . . . . 27

11. Volume of mercury entering three samples of black powder-Lot 6 .......... ..................... . . . . . 29

12. Volume of mercury entering three samples of black powder-Lot 10 .............................. 30

13. Volume of mercury entering whole grains of black powder:

A-lot 11, burn rate O.60 m/see; *-lot 10, burn rate 0.51m/sec; U-lot 1, burn rate 0.32 m/sec; and V-lot 6, burnrate 0.38 in/see ... ............. . ........ 31

14. Volume of mercury entering half grains of black powder:A-lot 11; burn rate 0.60 m/sec; 0-lot 10, burn rate 0.51m/sec; O3-lot 1, burn rate 0.32 m/sec; and'*-lot 6, burnrate 0.38 n/sec.... ........... ............ 32

15. Rclationship between burning rate and density of blackpowder reproduced from reference 1. D~imensions refer topellet size ............ ...................... .... 37

1 16. Relationship between flame spread rate and density.Subscripts refer to deviant lot number .... ....... ... 39

* pF&Ch~jjjjU t.&Q 1LAL4KNOr FILAMiL

LIST OF ILLUSTRATIONS (Cont'd)

Figure Page

17. Passageway diameter distribution for whole grains of blackpowder. Dark portion repressed by a factor of 10 .. .. 41

18. Passageway diameter distribution for half grains of blackpowder. Dark portion repressed by a factor of 10 .. .. 42

19. Relationship between flame spread rate and surface area.Subscripts refer to lot number: 1-10 deviant lots; II,75-2, 75-61, 75-44 are GOEX material; 7-11 is CIL material;and M subscript samples were measured by MicrumeriticsCorp., others by NSW . . . . . ...... .. .............. 43

20. Rel,.tionship between flame spread rate and free volume.Subscripts refer to lot number: +, values calculated byequation (3) and , vwtlues calculated by equation (4) 46

21. Relationship between burning tir,.e and moisture content 48

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I. INTRODUCTION

Black powder was made in production mills in Europe as early as1340 in Augsburg, 1344 in Spandau and 1348 in Legnica. The process,traceable to the early B.C. effort of the Chinese, evolved as an "artform" having historical rather than scientific origins. The develop-ment of the craft has been reconstructed by various authors and excel-lent accounts have beeni given by Urbanski 1 and Marshall. 2 The subjecthas been reviewed by Fedoroff and Sheffield 3 who provide numerous refer-ences.

In essence, black powder has been made by grinding potassium nitrate,sulfur and charcoal into a very fine powder. This material is trans-formed into a cohesive conglomerate mass by either forming a paste whichis forced through and divided by a screen, or the material is pressedinto a cake. Both forms are subsequently broken into pieces. The smalllumps or grains of black powder are then coated with graphite to retardthe absorption of moisture and to prevent the grains from adhering.The grains are then screened to a particular size. This process hasbeen standardized to some degree, in accordance with prevailing tech-nology, but the operator adds water, adjusts temperature, selects

grinding time, or changes compression pressure based on personal experi-ence which is guided by individual judgment of color, odor, or generalp)erception of the condition of black powder as it undergoes its varioustransformat ions. Such judgments are exchanged from experienced operatorto I ou-,1neyman.

Although the production of black powder was as great as 10.6 millionpounds 4 during World War I, the advent of smokeless powder diminisheddemland. This fact together with the inherent dangers of production,marked by occasional explosions, has resulted in closing most manufac-turing facilities in North America. Because black powder still plays animportant role in fuses and ignition devices the U. S. Army Corps ofEngineers has built an Ammunition Plant in Charles':own, IN and equipmentwas installed by ICI Americas, Inc. In designing the plant every effortwas made to modernize the black powder manufacturing process and to makeit as safe as possible. In the planning stage various studies were

1 Tadeuvt Urbanaki, ,iemictry and Technology of Explosives. Vol 3,Pergamon Preau, NY, pp. 322-340, (1967).

ýArthur Marshall, E.2ploaiveo Sec. Ed. Vol 1, Ilictory and Manufacture,P. Bla•aton'o Son an( Co., Philadelphia, (1917).

3B. T. Fedorof'f and 0. E. Sheffield, •nyclopedia of E.xplosives and

Related It ems, Vol 2., PATR 2700, Picatinny Arsenal, Dover, NJ,pp. R165-B178, (1962).

4ArthuŽ' P. Vangelder and Hugo S hatter, Hiotory of the Explosive Industry

in America, NY Coliwmbia University Press, NY (1927).

conducted to: (1) elucidate and evaluate the ball and wheel mill process,(2) to investigate new techniques and equipment and (3) to compare theballistic properties of black powder originating from this and othercountries. The accumulated knowledge was used as a foundation for thedesign of the Indiana Plant. This work was reported in part by theChromalloy Corporation 5 Battelle Memorial Institute, 6 Olin Corporation 7

and IGI Americas, Inc. Battelle published abstracts of 50 articlesand 60 patents describing the wheel mill process in fine detail. Thisprocess, sometimes referred to as the "standard process", and used in the1860's can be currently witnessed at the Belin Powder Works in Moosic, PA.The plant was built in 1911-12 and operated for many years by theE. I. du Pont de Nemours ti Co. The plant is now owned and operated byGOEX, Inc., and supplies most of the military black powder requirements.

Particle size distributions obtained by "the standard process" weremeasured, using optical techniques, by Battelle Memorial Institute, 6 as afunction of both grinding time and moisture content. These results werecompared to black powder made by another technique, the jet mill process,in which material is ground by impaction created by high velocity airstreams. The jet mill process was developed by Kjell Lovold 9 and isused in Norway to make black powder. The technique was incorporated inthe Indiana Plant. Using the jet mill, particles are generally one thirdsmaller than produced by the wheel mill. Also, J. C. Alle:i 10 compared the

5Chromalloy Corporation, "A Study of Modernized Techniques for theManufacture of Black Powder", Propellex Chemical Division, Final ReportNo. DAI-23-072-501-ORD-P-43, Jan 60, Chromalloy Corp., IL.

6H. E. Carlton, B. B. Bohrer, and H. Nack, '"atte Le Memorial Institute

Final Report on Advisory Serviced on Conceptual Deeign and Develop-ment of New and Improved Processes for the Manufacture of Black Powder",Olin Corporation, Indianz Army Ammunition Plant, 20 Oct 1970, Charles-town, IN.

J. R. Pleseinger and L. W. Braniff, "Final Report on Development ofImproved Process for the Manufacture of Black Powder", RCS AMURE-109,Olin Corporation, Indiana Army Ammunition Plant, 31 Dec 1971, Charles-town, IN.

8Indiana Annun'tion Plant, "Black Powder Manufacturing Facility", Vol.1 and 2, Indiana Army Ammunition Plant, 1975, Charlestown, IN.

9YKjell Lovold, U.S. Patent No. 3660546, "Process for the Preparation ofBlack Powder", 2 May 1972.

10 j. C. Allen, "The Adequacy of Military Specification MIL-P-223B toAssure Functionally Reliable Black Powder", Report No. ASRSD-QA-A-P-60,Ammunition System- ReZlability and Safety Div, Product Assurance Direc-torate, June 1974, Picatinny Arsenal, Dover, NJ.

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effects of different milling procedures at various production facilitiesused in this and other countries.

Other types cf preparation have been attempted which include dis-solving sulfurS by either carbon disulfido or alcohol and mixing thissolution, or slurry, with charcoal and potassium nitrate before evaporating.Another innovation of note was pursued by VoigtII in which potassiumnitrate was precipitated from solution by adding alcohol in a controlledmanner to produce fine particles.

From the extensive background one general observation made in regard

to the production of black powder is that the same mabnufacturing proce-dure, using the same ingredients, will result in lots exhibiting differentburning rates. This has been the experience of GOEX, Inc. where fastand slow lots are blended to obtain a particular result. Although someof the factors which effect burn rate are known, the lot-to-lot variationshave not been identified. One attempt to study the effect of small per-turbations was supported by the Product Assurance D)irectorate, under thedirection of J. C. Allen1 2 where planned and slight deviations from a nor-mal production run were purposefully and systematically introduced inten lots of black powder made by the Indiana pilot plant. These sampleswere used to determine what manufacturing parameters are important ineffecting th performance and combustion of black powder. These lotswere termed - "deviant lots" and will be described later. The burningcharacteristics of these and other standard lots were measured by fourdifferent laboratories: ARRADCOM (BRL 13 , LCWSL 1 4), ICI Americas Corp. 15

aH. William Voigt, Jr., Pell W. Lawrence, and Jean P. Picard, "Processfor Preparing Modified Black Powder Pellets", U.S. Patent, 3937771,Feb 10J 1976, see also H. W. Voigt and D. S. Downs, "Simplified Pro-cessing of Black Powder and Pyrotechnical igniter Pills Including LowResidue Versions", Seventh International Pyrotechnics Seminar, Vol. 2,ITT Research Inst., Chicago I.L, July 2980.

J. C. Allen, "Concept Scope of Work For MMHTE Project 5764303 Accept-ance of Continuously Produced Black Powder". Report No. SARPA-QA-X-10•,November 1975, Picatinny Arsenal Dover, NJ.

13K. J. White, H. E. Holmes, J. R. Kelso Jr., A. W. Horst, and I. W. May,

"IIgnition Train and Laboratory Testing of Black Powder", ARRADCOCA-BRT,Report No. IMR-64•, April 1979., APG, MD. See also K. J. White, H. E.Holmes., and J. R. Kelso, "Effect of Black Powder Combustion on High andLow Pressure Igniter Systems", 16th JANNAF Combustion Meeting, CPIAPublication 308, Dec 1979.

' 4 L. Shulman, private communication, (data quoted in reference 13), 15Jan 9 78, Picatinny Arsenal, Dover, NJ.

15 Hugh Fitler, private communication - Draft report, "Acceptance ofContinuously Produced Black Powder", 8 March 1979, XCI America Corp.,Charlestown, IN.

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and Princeton Combustion Research Laboratories.16 These results werecompared by Kevin White 1 3 and will be presented later; it was concludedthat the same trends were observed in each laboratory. Although therewere differences in burning rates between various lots, the conclusionwas that small perturbations in manufacturing procedures do not accountfor the difference of a factor of two in the observed flame spread rates.When these lots were compared with some GOEX, Inc. black powder, it wasinferred that some other unidentified variable must be operative inthese samples.

The performance of black powder, in contrast to its preparation,has been characteri.-ed in the classic papers of Noble and Abell7"18.Recently, Williams summarized burning characteristics and includedseveral foreign references. Rose 20 studied the performance of blackpowder inade from charcoal produced by various manufacturers and concludedthat response could b(, traced to volatile content; he also presented anexcellent review article. 2 1 Kirshenbaum"2 concluded from thermal DTAanalysis that even when volatiles were removed from charcoal the same

order of ignition was maintained and some other parameter besides vola-tiles affected the ignition of black powder. lie also found that Ignitiontemperature was not a function of carbon surface area, ash content norof sulfur content. Further, activation energy was not a function ofvolatile content. Blackwood and Bowden 2 3 suggested that sulfur reactswith volatiles as an initiation step but Kirshenbaum showed that allthree compononts, sulfur, potassium nitrate and charcoal, must be

l6Neale A. Messina, Larry S. Ingram and Martin Sunmerfield, '"ackPowder Quality Asourance Flame Spread Tester", Final Report No.PCRL-TR-78-101, Dee 1978, Princeton Combustion Laboratoriea Inc.,Princeton NJ.

R. A. Noble and F. Abel, Phil. Trans. Roy. Soo.. London, Seriee AVol. 265, 49-155, (1875).

18R. A. Noble and F. Abel, Phil. Trans. Roy. Soo., London, Series A203-279, (2880).

2 9 F. WilZlczae, "The Role of Black Powder in Propelling Charges", Pica-tinnyi Arsenal Tech. Report No. 4770, May 1975, Picatinny Arsenal,Dover, NW.

James F. Rove, "Investigation on Black Powder and Charcoal", ITR-433,Sept. 1975, Naval Ordnance Station, Indian Head, MD.

2 1 James E. Ro.c, "B l3ack Powder - A Modern Comnentary", Proc. of the 10thSymposium on Explosives and Pyrotechnics, 14-16, 1979, FranklinResearch Tnot. Philadelphia, PA.

2 2 Abraham D. Kirshenbaum, Thermochimica Acta 18, 113, (1977).

A 2 3 J. D. Blackwood and F. P. Bowden, Proc. Roy. Soc., London, A213, 285,(21952).

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present to induce a low temperature pre-ignition exotherm. Perhaps oneof the most persuasive experiments on the importance of volttiles wasthat of Blackwood and Bowden in which they dissolved volatiles from char-coal with acetone and applied them to charcoal that was 9S percent carbon.Black powder made from this material had the same behavior as standardpowder.

24Clearly, volatiles affect burning rate and Hintze shows the rela-tionship. lie recommends 82.5 percent carbon content for charcoal andBlackwood and Bowden recommended 70 percent. However, neither workrelates these percentages to a standard initial form of water or ash-free basis and it is not clear that the recommendations can be compareddirectly. Both authors do recommend a rather high volatile content.However, this parameter cannot be the only property that affects burningrate for the deviant lots have diverse burning rates incorporating thesame charcoal.

In addition to the affect of volatiles, Shulman 2 5 ' 2 6 studied the

influence of small amounts of water, to two percent on ýhe burning rateof black powder; this study was expanded by Plessinger. These investi-gations quantify a well-known parameter.

From this discussion it is seen that the properties of charcoal areimportant, but the exact nature of the requirements necessary to make agood ballistic product are elusive. Present practice is that charcoalbe made from porous light wood, maple being currently selected, and be oflow ash content. No specification exists citing volatile content butcurrent practice is that it be 20-30%. This specification is broad inthat it requires charcoal which results in a proper performing blackpowder. The specification does not cite the wood, particulars relatingto distillation, or physical properties. For black powder the amountof potassium nitrate, sulfur, and charcoal is given but compaction pres-sure is not. Only a density range for black powder is given. Suchcriteria result in a large latitude in allowable variance in the materialsused to make black powder.

It appeared to this author that rather large variations in materialsor in preparation of black powder resulted in rather small but measurable

?4 Woldomar Hintze, ExpLosivstoffe, 2, 41, (1968).

25L. Shulman, C. Lenchitz, L. Bottei, R. Young and P. Casiano, "An Analyseisof the Role of Mloieture, Grain Size, and Surface Mlaze on the Combustionand Functioning of Black Powder", Picatinny Arsenal Report No. 75-FR-

G-B-15, Oct. 1975, Picatinny Arsenal, Dover, NJ.

2 6 L. Shulman, R. Young, E. Hayes and C. Lenchitz, "The Ignition Charac-

teristics of Black Powder", Picatinny Arsenat TechnicaZ Report No. 1805,Sept. 1967, F.catinny Areenal, Dover, NJ.

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variations in burning rate; lot-to-lot differences. using the sea.materials and techniques, was a dominant feature. It was for thesere.ssons that another sensitive parameter was sought and a caidate hy-pothasis was that the depee of openness of a grain could influence burn-ing rate. Soon the study evolved into an investigation of the physical Iproperties of black powder and their relation to burning rate.

In an attempt to characterize black powder, scanning electron micro-scope photographs were obtained which showed extensive fusing of material.This immediately suggested that compaction studies be performed on pureingredierlts to determine what material flowed and the functional relation-ship b,:cween flow and applied pressure. In addition the free volume wasmeasured by mercury intrusion techniques. Further, the internal surfacearea was obtaird from the Brunauer, Emmett, and Teller (B.E.T.) gasabsorption technique. From these several measurements a physical descrip-tion of black powder developed.

II. EXPERIMENTAL

A. Black Powder Samples

Deviant lots, mentioned earlier, were chosen for study as they were:(1) well characterized ballistically by four laboratories, (2) made fromingredients supplied by the same manufacturer, (3) fabricated by the jetmill process, and (4) representative pilot plant samples typical of futureproduction techniques. Such samples were compared to black powder madeby the "standard" wheel mill process at GOEX, Inc. The deviant lots weremade from oak charcoal and GOEX, Inc., used maple. Equivalent compactionpressures were used by both manufacturers.

Due to the limited capacity of the Indiana pilot plant, their jet-milled meal was moisturized in a wheel mill at Picatinny Arsenal andpressurized at GOEX, Inc., to not less than 3500 psi, or 246 kg/cm2 .Fabrication has been described in detaillS and composition given in Table1. For this table the carbon content has been recalculated on a moistureand ash-free basis. Also to obtain the high and low carbon content char-coals, three different lo'zs of oak charcoal were used. The high carboncontent charccal was but eight percent greater than that normally usedand the low carbon material was 17 percent below standard. The spreadin these values is significant but small. Such samples were compared toGOEX, Inc., and CIL black powder produced by the standard process andmade %.tLh maple charcoal. The burning rate data from various laboratoriesARRADCOiA (BRL, 13 LrWSL1 4), and Princeton Combustion Research Laboratories 16

are compared in Figure 1 reproduced from reference 13. The results arenormalized to the burning rate data for a standard GOEX, Inc., lot, 75-44.

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0111

o 2 4 6 8 10 12

Figure 1. Comparl son of burning rates as measured by flame spread,0 (rf. 16); 1osed bombs (ref. 14); and open airflame spread, (ref. 13).

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B. Scanning Electron Microscopy of Black Powder and Charcoal.

S.r.M. photographs of black powder were obtained at two magnifica-tions, 300 and l000X, by John Gavel of the Analytical and PhysicalMeasurements Services Section, E.I. du Pont de Nemours & Co., Wilmington,Di. Class one graini, those passing through a number 8 but not a number4 sieve, were examined. They were cleaved in half by breaking the grains,much like snapping a twig, to insure that no tool smeared the surface.The new surface was coated with a 20 nm gold and palladium film; 30 keVelectrons were used for analysis.

Another imaging technique was attempted using an electron microprtbe,model MS-64 made by the Acton Laboratories Inc., of Acton, MA. In theseexperiments the -ample was moved normal to the oscillating beam by a motordrive. The experiments were performed by Ralph Benck of the PenetrationMechanics Branch (PMB) of BRL. Both sulfur and potassium x-ray maps wereobtained sNimultaneously using two detectors. Benck also analyzed thecharcoal ash, from charcoal extracted from lots 1 and 11.

Otber S.L.M. photographs of oak and maple charcoals were taken atthe BHR, by Gould Gibbons Jr., of the Explosive Effects Branch (EEB), usingan International Scientific Corp. S.E.M. model ISI-40 made ir. Avon, CT.Oak charcoal was evaluated as received from the Indiana Ammunition Plant;however, it was pre-screened. Charcoal passing through an 80 but held ona 10t) mesh screen was examined. The maple charcoal was of a larger parti-cle size paissing through a 10 but not a 20 mesh screen and, therefore, itwas ground in a mortar and pestle to obtain particles of the same size asthe oak. Both samples were obtained by the Indiana group from the HardwoodCha1c,1Cl Co. located in Steelville, MO. S.E.M, views were obtained at 100,ISO, 500 an~d _00ORX.

In addition, o•ak charcoal was extracted from lot 1 and maple fromlot 11 black powder samples. They were extracted with water and carbondisulfide. For lot 11 ,magnifications of 150, 350, 750, and 150OX wereemployed; similar values were used for lot I and they were: 100, 250,500, and 1000X.

C. ,ompression of Sulfur, Potassium Nitrate and Charcoal

Nureý sul'tir, potaissiumi nitrate, and charcoal were each ground to atfinc powde, usink, a mortar and pestle. Samples between 1 and 2 grams wereloaded into a I'vrkin filmer Corp. potassium bromide infrared die, model180-0025, 1adC in Newark, CT. The powders were individually compressedusi an Instron Corp. material testing instrument, model 2T'L[M, made inCanton, MA. The press's cross heads move at various, constant, pre-selcted rates ;uld the small compressive movement of 0.254 mm per minutewa1s selected to insure some degree of pressure equilibrium. A totalload of 3,0t)0 kg cm- 2 tr 40,000 psi was applied slowly. At maximum,• pressure the directionof the cross heads was reversed, at the same rate, j

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to obtain a decompression curve. In some cases the process was repeatedtwice to define the permanent effect of the first compression.

One sample of potassium nitrate was made damp by adding four percentcolored water and grinding to a constant hue. The experiments were per-formed by Dominic Di Berardo of the PMB Branch of BRL.

D. Mercury Intrusion Porosit,.

"Internal volume and pore size distribution of either whole or cleavedgrains of black powder were measured by Jean Owens of Micromeritics Corp.located in Norcro s, GA. The technique subjected samples to mercury pres-sures to 3.5 X 103 kg cm'e or 50,000 psi forcing liquid into pores havingdiaweters grvater than 30A. The grains of black powder were degassed bya 1006C flowing nitrogen stream for 40 minutes. The technique has beendescribed by Orr 2 7 and Adamson. 2 8

Mercury, due to its high surface tension, does not wet most surfacesand pressure is required to force the fluid into small pores. Hence, thepore size is directly related to applied pressure and the relationshipis given by equation 1.

Pr a -2 o cos 0 (1)

where P is pressure, r fs the radius, a is the surface tension of mercury(taken as 474 dynes cm ) and 0 is the contact angle (taken as 130* formost substances).

The distribution of such openings is given by a distribution func-tion, D(r),

D(r) = (2)

where VT is the total penetraition volume and V Is the penetration volumewith openings greater than r.

.7Clyde Orr Jr., Powder Technol., 3, 117 (1969/1970).

" Arthur W. Adamson, Physical Chemistry of Surfaces 2nd Ed., Inter-scienoe, NY, pp. 546-549, (1967).

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E. B.E.T. Surface Area Measurements

In addition to the mercury intrusion measurements, MicromeriticsCorp. also determired surface area by a single point B.E.T. techniqueusing degassed whole grain samples and argon gas. This technique hasbeen described in several texts, for example Flood, 2 9 where surface

* area is related to the number of argon atoms required to form a liquidfilm on a given surface. These measurements were extended to othersamples by Eleonore Kayser of the Naval Surface Weapons Laboratory, NSW,in Silver Springs, MD, using the same type of Micromeritics equipment,model 2205.

F. Density Measurements

liven though density is defined clearly as gm/cm , the measurement ofvolume for powders and compressed masses can be ambiguous. For thisreason some experimental approaches will be discussed.

1. "Tap Density" is a term that has been used to describe theweight of a powder -r objects poured into a known geometry and allowedto settle by one or more gentle means such at "tapping". Such valuesare not used in this study.

"2. "Bulk t)ensity" depicts the volume element inscribed by its outersurface. The volume is measured by sinking a powder or object into mercuryat atmospheric pressure. Such values do not include pore volume andare given in Table I as reported by Fit ler.

"3. "True I)ensity" is obtained by ineasuring the volume of vI objectas equal to the amount of non-absorbed gases it displaces. In suchmeasurements helium is usually introduced into an evacuated sample andfills small pores. Similar values are obtained by using ether As thedisplacement fluid for evacuated samples, 3 '( or using mercury dk:,placementat very high pressures of mercury. In this study an ether disp',acementmeasurement was made on non- vacuated samples to estimate the samples'true density. Flow of ether into the sample was aided by placing thepycnometer filled with black powL,ýr and ether in an ultrasonic bath.

Ill . RILSJLT,'s

A. Black 'owder and Charcoal S.Ii.NI NMicrojl 2 j'raips

Four bilack powder samples were chosen from the deviant lots forexaminat ion. Lots I and o arc samplus having slow flame spread rates of

'. Alvoon Plood, Ed., The Sol•d-Gao 1nterqaqcg Vol. 1, Marcel fDekker

Inc., NY, (1967).

3 R. M. McTnto•h and A. P. Otuart, can. J. Reo. B.4, 124, (1946).

0.32 and 0.39 m/s as contr.sted to lots 10 and 11 which have faster

flame spread rates of 0.51 and 0.61 n/s. Further, lots 1, 6, and 10are jet-milled oak products whereas lot 11 is a standard wheel-milledmaterial of GOEX, Inc. The S.E.M. microphotographs are given in Figures2. 3, 4, and 5. Stereo views were made but add little to the detailalready shown and therefore are not presented. Potassium and sulfurx-ray maps were obtained but the concentration profiles did not showsharp boundaries as prominent features. The problem is the averageparticle size is about 15 microns or less and the x-ray maps depicted anaverage concentration to a depth of approximately 30 microns. Thereforesuch analysis represented an average concentration throughout the slab.For this reason those views have not been included.

The microprobe experiment was abandoned for black powder surfaceanalysis, for although the sulfur and potassium x-rays were recordedsimultaneously, and gave strong signals, the electron beam created areaction path and the data could not be relied upon to depict theoriginal sample. However, the microprobe did analyze the ash contentof both extracted charcoals and calcium was found to be the dominantspecies and traces of silicon and phosphorous were noted. No othermetals were detected. The ash content of maple charcoal was 2.3 percentand 11.9 percent was found for oak.

The S.U.M. microphotographs of the raw materials, oak and maplecharcoals, are given in Figures b and 7. Extracted charcoals from blackpowder are shown for lot 8 in Figure 8 and for lot 11 in Figure 9.

B. Coýmression of Sulfur, Potassium Nitrate, and Charcoal

In the manufacture of black powder, damp meal typically containingmaterials of less than 25 microns, is pressed into a cake where variousfabricators have used different compaction pressures. In general, apressure is selected which will result in a finished grain density ofapprojimately 1.7 and GOEX, Inc. employs a pressure no smaller than 24(6kg cm- or 3,500 psi. All of the S.E.M. microphotographs, Figures 2-5,show considerable plastic flow and fusing among particles formingextensive agglomeration larger than the original particle size. Thereforea compaction experiment was performed to determine the relo'tionshipbetween flow and applied pressure.

The results of compressing pure ground sulfur, puossium nitrateand charcoal are given in Figure 10. The relationships siown can belinearized by presenting the logarithm of applied pressure is a functionof conalaction. The density of this sulfur cake was 2.00 gm/cm3 and thatof th.- damp and dry l)OtSs1Sitm nitrate cakes were both 2.08 gm/cm3 .Literatuire, values-"' are 2.07 and 2.109 gm/cm3 for t.he-,e compounds andthus, at the maximum pressure used, no significant voids exist in eithe,'material. In these cases the decompression curve is almost verticalshowing little elasticity and recompression forms a small envelope aboutthe decompression curve. The oak and maple charcoal compression curves

, 18

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-J

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--

CL

- - - - - -- -' I i a m t ~~-,~-- ~---.--- -~-4 ~ -.

9C

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were identical, but a compressed density of 1.01 gm/cm3 was measured forthe maple charcoal and a density of l.16 gm/cm 3 was measured for oak toan accuracy of 0.5 percent. Values were obtained from the sample weightand die dimensions at maxtumum pressure. The decompression curves forboth samples showed considerable elasticity. Interestingly, each samplewas removed as a powder from the die and obviously the organics did notict as a binder.

In an attempt to interpret the compression density vaiucs, thedensities of both charcoals were measured by an ether displacement techniqueusing a pycnometer. The density values were 0.99 and 1.17 gm/cm3 andwere accurate to 1.0 percent. They were equal to those measured in thecompression experiments. After compression, the charcoal originallypassing through a1i 80 hut not a 100 mesh screen, was agaI'in sieved andthe results are given in Table 2. The values show that many pieces ofcharcoal were broken by the large applied force,

'rARLLU 2. CIIARCOAL S1IIVIVI AWI'l'lT COMPRESSION

Screeii Si:e Oak* Mapl e*Mesh Units- 11

100 24 51120 12 8200 38 2b340 26 is

Z by weight

C. Mercury Intrusion Porosimetry

Three different samples of 1-2 grains of whole grain black powder,lots 6 and 10, were evaluated to determine the reproducibility of boththe sampling and measuring techniques. Data are given in Figure 11 and12 showing the cumulative amount of mercury forced into pores of diff'erentradii by various pressures. The d. A have been normalized at 200 psi or14 kg/cm2 and shows a good degree of reproducibility. One sample eachof lots I and 11 was evaluated and this data is given together with arepresentative penetration curve for lots I and 6 in Figure 13. These

same four lots were also evaluated by cutting the grains in half. Thesedata are given in Figure 14, In all cases the cut or whole grains wereintact after pressure was released.

: . .L.T. Surface Area Measurements

The surface area of whole grains was measured by 4 ,.".T. analysis.All of the deviant lots were evaluated as well as GOEX, Inc. and CIL

}• " 28

i i ,I

PORE DIAMETER (microns)0 1.0 0.1 .01 .002.06 111, U• U 1 • . . I.. yi , ," U " i ,

.06.05'

"• .04 '9

2

.03

U.02 9

.01 9

9*,9

S 'V

100 ainslO a aO ap0 api a 0000

a, ma ti_ nill, .I , I a i , 11i1

10100 '2 1 0000*PRESSURiE |•,':2

Fijure 11. Volume of inercury entering three samples of black pounder -

tLot 6

!2

.. .4

PORE DIAMETER (microns)1.0 0.1 .01 .002

t0

.05 0S

S °OS.04-

- .C3 - *

_z

, -02

010+

100 1000 1 0-0-00 psi) 100000I I I IAi- I A A Ibi lk & &

10 100 1000 10000PRESSURE (kg/cm2 )

Fioure 12. Volume of mercury entering three samples of black powder

* Lot 10

30

" .4

-~ - -D

LUQoLf

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _C'

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black powder. One lot, lot 6, was examined in detail to determine thewvriance and precision associated with sampling and measurement. Thedata are given in Table 3 and surface areas ranged from 0.5 to 1.0m2 gm" 1 . Each individual sample was evaluated at least twice.

E. Density Measurements

The density of whole grains of black powder was measured by an etherdisplacement technique and values are given in Table 3. They weremeasured to an accuracy of one tenth of a percent.

The ether displacement density of charcoal extracted from lot 11 was1.43. The densities of the maple and oak charcoals supplied from theHardwood Charcoal Co., were 1.01 and 1.16. Reproducibility was as notedabove.

IV. DISCUSSION

A. Black Powder and Charcoal S.E.M. Measurements

The microphotographs of deviant lots 1, 6, and 10, made by the jetmill process, look much alike. They are shown in Figures 2-4. EachI• figure shows two different views of a particular grain and these viewsare shown at two different magnifications. The view at 300X was attemptedto get a general representation. A smaller subsection is shown at lOOOXto provide greater detail. Higher magnifications were recorded but theylose the general nature of the features shown. Sulfur and potassiumnitrate cannot be individually discerned; however from the relative abun-dance most of the features represent potassium nitrate.

Figure 2 shows a large isolated charcoal particle; clearly, compres-sion did not fill its pores nor did it crush the charcoal. Thus in thisinstance incorporation of material into the charcoal pores, a processthat has been ascribed to the wheel mill process,S did not take place.In all figures considerable plastic flow and particle deformation areshown and many consolidated regions exist that are much larger than theoriginal particle size of 10-15 microns. Figure 4, deviant lot 10, showsparticles which appear more intact and are individually more discernable.Also the degree of plastic flow seems less. This leads to a greaterdegree of openness between particles.

The S.E.M. photograrh of the GOEX, Inc. ball milled material, lot11, Figure 5, is difforet from the three-jet milled samples. Largeclumps of material are (,bserved as well as large void regions. Againone piece of charcoal i•i evident and its pores are not filled, nor is itcrusned. The pores are :2.8 ± 0.9 microns wide. In general, materialseems to have flowed around the charcoal and the photograph shows almostgeological folding. Stress lines and spherical pockets are noted in

33

TABLE 3. SURFACE AREA OF BLACK POWDER

AND ETHER DISPLACEMENT DENSITY

Surface Area* Surface Area** Densit

Lot Number m2 lm-1 m2 ipi-'I Cifl-

1 0.50 O.588

0.53 0.588 1.91

2 0.73 1.870.80

3 0.54 1.880.58

4 0.060 1.890. 060

5 0.580.58

1.85

0.5b 0.5880.55 0.587 1.86

0.520.570.530.510.620.52

7 0.550.58

1.85

0.5280.52 1.830.52

9 0.67 1.82o.b3

10 0.bb 0.639

0.62 0.630 1.85

O.b6

GOIEX - Lot 11 0.82 0.8010.76 0.796 2.02

GOKX 75-44 0.850.850.82

GOEX 75-b1 0.710.73

GOI-X 75-2 0.710.69

CIL 7-'.1 0.580.57

NSW

**Mio.mertice Corp.

At•erage io 0.55 ± 0.04.

34

consolidated regions. The grain is not as homogeneous in appearanceas the jet-milled gr&in.

In summarizing the S.E.M. microphotographs, compaction seems to forma fused conglomerate mixture laced with a maze of interconnecting passage-ways.

The S.E.M. microphotographs of oak and maple charcoals are given inFigures 6 and 7. Oak charcoal appears as separate lumps having a clearlydefined cell structure. The maple sample looks torn apart, perhaps bythe grinding action of the mortar and pestle. However, both samplesshow extensive cell structure and broken and chipped pieces dominate.

One series of S.E.M. microphotographs of oak, Figure 8, which wasground in the jet mill and extracted from lot 1 black powder, Figure 9,looks similar to maple charcoal obtained in the same manner from lot 11ground in a wheel mill. The particles seem tc be of similar size and theonly difference is the maple appears more rounded. No large charcoalpieces are evident in the maple photograph as was seen in its black powdercounterpart but the view may not be totally representative.

In summary, it appears that grinding by either method produces afine powder which has a multiplicity of edges, broken pieces and con-voluted surfaces. The grinding process breaks down the gross structureof charcoal, forming pieces and fragments such that the two charcoalslook much alike.

B. Compression of Sulfur, Potassium Nitrate and Charcoal

Noting the extensive flow in grains it became desirable to quantifythe effect of pressure on black powder meal. Damp meal, that is generally4 percent in moisture, has been pressed into a cake using various compac-tion pressures. Values of 1900 psi 5 (134 kg/cm2 ) to at least 3,500 psi 8

(24b kg/cm2 ) 8 has been employed and some work was done at 8,000 psi 3 '

(563 kg/cm2 ). The range of these values raised the questions, "do thesecompaction pressures span the individual yield points of the ingredientsand is a particular pressure threshold required to induce plastic flow?"To answer these questions an experiment was performed to determine ifthe change in volume, or plastic flow, was uniform as the applied pres-sure was increased.

The effect of pressure on the pure materials was determined andshown in Figure 10. From the curvature, it can be seen that eachmaterial flows throughout the applied pressure range. This is areminder that the applied pressure is a global value but the microscopicpressure exerted between particles can be much greater. One compression

31 Memoriat Dee Poudree Et Salpetres, VoZ. 6, Chapter II by M. VisilZe,Gaut'ier-Vallare Et Fil, 1mprfmeure-Librairje, Pa'ie, pp. 256-391, (1893).

£ 35

-.-

curve using damp potassium nitrate, four percent moisture, lies wellunder its dry counterpart showing that water reduces friction, allowing

* particles to slide. Thus, applied pressure and water content are coupledphei.3menon which govern grain density.

It was noted that both potassium nitrate and sulfur formed smoothglass like surfaces at the die faces. Such a surface could seal the pelletand this effect could explain why black powder pelletized in a pellet pressexhibits slow relative quickness values.? Additional descriptions ofthis effect can be found in the Ordnance Explosive Train Handbook 3 2 thatdescribes the burnring of a train of several pellets as a succession ofpuffs caused by burning through the "glazed interface". In such appli-cations, these effects are reduced by pressing pellets as interlockingcones to minimize the effects of burning through a sharp boundary.

In addition to the surface effect of pressed pellets, there is adensity profile that should be considered when pressing to a given graindensity.

The charcoal compression curve is similar to that of potassiumnitrate and sulfur. From the facts that bulk density at maximum compres-sion and ether displacement density are equal and that compression frac-tures charcoal (Table 2), it is clear that pressure fractures some ofthe charcoal into smaller fragments which then bend elastically to fillthe die leaving very little space between particles. With a density of0.99 for maple and 1.133 for oak charcoal, both far removed from theamorphous carbon value of 1.8 - 2.1, it is concluded that the density

of charcoal reflects an open structure.

The compression results show a gradual change in volume with appliedpressure. This reflects that applied pressure is a global value and theassociated microscopic pressure between small areas of two touchingparticles is much greater. The smooth exponential curves show no sharptransitions that could be associated with yield points nor are anythreshold values indicated.

The effect of compression on black powder meal is yet to be done butthe effect of pressure is to increase density. Vieille31 in 1893 demon-strated the relationship between black powder bulk donsity and burn rate.The data was reproduced by Urbanskil and his graph is given in Figure 15.Obviously the data relate to a process and materials different from thoseused today, but the trends should still persist. This same type of

3Na7.aZ Ordnance Laboratory, Ordnance ltoaive Train Desianere Handbook-* NOLR 1111, April 1962, Naval Ordnance Laboratory, Mite Oak, MD.

~Robert C. West, ed., Handbook of Chwnietvij and Phy eice,5ltdTeChemical Rubber Co., Cleveland., Ohio (2970-1972).

36

[-A

100

10.5mm

80

"-g SLOW BURNINGIN PARALLEL LAYERS

60LU

S40 3.5 m m

20

0 I I

1 6 1.7 1.8 1.9 2.0

DENSITY

Figure 15. Relationship between burning rate and density of blackpowder reproduced from reference 1. Dimensions refer to pellet size.

317

relationship was shown to be operative in pyrotechnic mihtures whereburning rate was given as a function of compaction pressure. 3 4

Vieille investigated the large density neighborhood of 1.44 to 1.85.In contrast, present experiments, using material that was available,define a more restrictive neighborhood and the parameter is not illus-trated as clearly. Since Vieillc's reference is not readily available,the liberty has been taken to translate his summary. This translationwas undertaken by Eli Freedman of our laboratory (BRL-Applied BallisticsBranch) and is given as an appendix.

'The flame spread rate and densities for the deviant lots, given inTable I are shown as a scattered presentation in Figure 16. A functionalrelationship appears to exist but its exact nature is not given by theselimited data. 'The scatter leads to the belief that compression condi-tions were not identical; however, it is clear that flame spread ratedecreases as density increases.

C. Mercury Intrusion 1Porosimetry

Equation (1) relating pore size and applied pressure presumes thatthe pores are a collection of right cylinders of different radii. Inpractice small openings exist leading to a larger volume element. Sucha situation has been termed an "ink bottle effect" and when encounteredwill lead to an overestimate of the number of pores that exist at aparticular radius. For loose powders, corrections are attempted bymeasuring the mercury ,xtruded as a sample is depressurized. In accor-dance with equation (1) the neck of the "ink bottle" will empty as wellas the "bottle" itself but at different pressures. For compactedmaterial, such a black powder equilibrium may be too slow to attempt thiscorrection depressurizing technique. These problems should be consideredwhen examining the intrusion data and the indicated radii should be con-sidered as approximate values. However, the mercury volume entering agrain is exact.

In examining the S.E.M. microphotographs of black powder many voidsare apparent with radii of 10 microns or less. For mercury to pene-trate the grain and reach these voids it must intrude through passage-ways that are small and are not particularly apparent in the view of theplane presented. Therefore, in filling such a void through a smallpassageway the mercury will fill the system indicating a pore volumehaving the radius of the passageway. The mercury penetration data,including this effect, are given in Figures 11, 12, 13, and 14. All ofthe data have been normalized to 200 psi or 14 kg/cm2 . Three different

34 A. A. Shidlovskii, "Principtee of Pyrotechnios", Air Force SystemsComnand, Report No. AD-AO01859, 23 Oct 1974, Wright Patterson AFBJ OR,Translation of .zd Masimoetroyeniye, Oxnovy Pirotekhniki, 3rd Ed.,Moscow (1964).

38i:1JI

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al-Mmm 96 mm

whole grain samples of lots 6 and 10 were evaluated, Figures 11 and 12,to determine the reproducibility of the sampling and measuring technique.The penetration data for these samples shows fair agreement and most ofthe mercury entered through passageways between 0.1 and 1.0 micron r'

meter. The experiment was expanded to include lots I and 11 and the..data are presented in Figure 13 for whole grains. The experiment wasrepeated on new samples of these lots where the grains were first frac-tured in two. The penetration volume is presented for half grains inFigure 14. The data for lot 10 appears to be in error, and for thissample some of the grains may have been crushed by the applied pressure.

The mercury intrusion data for the four lots of black powder arenearly the same for either whole or cut grains; neither group) showingmajor differences in penetration pattern. On this basis, it is concludedthat the graph-Ite coating does not seal the grain to pressurized mercury.The two experiments also show that mercury must be able to reach the in-terior of a grain since cut and whole grains host the same v4,lume ofpene'ntration. If mercury intrusion had been related to the black powdersurface, then penetration between whole and cut grains would have beenapproximately proportional to the ratio of the surface area of a sphere to

two corresponding hemispheres or 2/3. This was not found to be so andthe conclusion is that the mercury penetrated throughout the grain.I

The distribution of pass-Ageways was calculated using equation (2)and the data of Figures 13 and 14. The calculation is primarily depen-I dent on the slopes shown; therefore, it is not affected by the normali-zaltion. The distribution is given in Figures 17 and 18. A correlationexists between the number density of passageways between 0.2 and 0.1micron and flame spread rate. No such relationship was evident for thesmallest of radii and this range will be discussed later.

D. B.E.T. Surface Area Measurements

The intrusion data can be used to calculate internal surface areabut because of the "link bottle effect" the result would be suspect. Forthis reason the surface area was measured by the B.E.T. technique andthe results are given in Table 3 and plotted as a function of burningrate in Figure 19. The standard deviation for four samples of lot 6 is6.6, percent and similar agreement on replicates of the same sample wasfound. It is clear that an increase of internal surface area increasesburaing rate. The graph, Figure 19, contains several markedly differentblack powder samples including the jet-milled oak deviant lots, mapleGOEX Inc., powder and one CIL sample. The latter two producers used theball-wheel mill process. Overall, the samples appear to follow the same

. . function even in the small neighborhood tested where the density of blackpowder does not vary to any great extent. it remains to the future todevelop a broader neighborhood for :nvestigation. Under these constraintsthe relationship between burning rate and the degree of openness of agrain of black powder seems reasonable.

40

CLC

uii

¶i ann

a Cc

I-A

CUL

II 41

p-C

C4

IA

0

- U a

(8) 0 NlDn Nlnii

LU~ &ALU

_ 41.

MINE

41..

4.1.

+ ~NO1-

U,

10+- 414* 9

I-+-

;+ wl 41

o 43

The surface area of charcoal 22 is about 1000 22 gm- 1 and sincecharcoal consititues 15.6 percent of black powder, an upper value of 156m gm-I would be expected. The measured values are much less, about0.5 to 1.0 m2 gm" 1 . Therefore, it is concluded that almost all of thecharcoal particles are sufficiently sealed to greatly reduce gas absorp-tion. This could explain why burning rate was not found to be a functionof the original surface area of charcoal or other carbon materials. 2 2

It also should be pointed out that the surface area measurementswere extended from the original four samples to include all of the deviantlots. This was performed as a matter of convenience rather than prefer-ence over the pore size measurements, for at the time of analysis it wasmore expedient to extend the surface area measurements.

E. Density Measurements

'rhe total free volume can also be calculated from the bulk densityof black powder, (IB p,), the density of the spparate ingi .dients, (DI),aind percent compositIon. Such values were taken from Table 1 and thefollowing relationship, equation 3, was applied to a 100 gm sample. Ashand moisture content were not included.

100 ~KNOI+IS100 ++ free volume, (3)1)BP. 1)KNO D S DCJ

Using a density of 1.17 for oak and 1.43 for maple charcoal (determinedfrom charcoal extracted from lot 11) the total free volume was calculated.Values are given in Table 4 and Figure 20. They incorporate both isola-ted voids and accessible volume elements. Equation (3) can be used toshow that the density of charcoal is a sensitive parameter that affectsthe free volume of black powder when a particular compression pressureis selected for manufacture. Moreover, compressing black powder to aparticular density using different density charcoals will produce grainswith varying degrees of free volume. One problem yet to be addressed isthat no knowledge exists on the density variations for lot-to-lot pro-curement of charcoal. If a variance is found, compaction pressure andgrain density will have to be adjusted accordingly to obtain the samefree volume or a particular density.

In an attempt to measure the volume accessible from outside of thegrain, the true density (DT) was measured by an ether displacement tech-nique. These values and the bulk density (DB.p.) from Table 1, wererelated to the free volume for a 100 gm sample by the equation:

100 100 - free volume& (4)

BP T44

44

4IA

tFM to 0

$.00

-j c.

-L r2. 000e~

~ I-I 45

IA

090

0-

a

46J

I 1

These values are listed in Table 4 and in Figure 20. Comparison of thefree volumes obtained from equation (3) and (4) shows that in all cases,except lot 4 and lot 11, the total free volume including voids is largerthan that accessible from outside of the grain. These volumes can becompared to the mercury intrusion values. One comparison is to relatethe volume of' mercury penetrating through passageways larger than 0.1micron, a plateau in Figure 13, to the density derived values. In this

case, the mercury values are smaller and this is reasonable as small dia-meter pores have not been penetrated. However, examining the smallestradii of 0.0)03 microns, the volumes are too large for lots I and 6. Itis therefore concluded that the sharp rising asymptote occuring at thehighest mercury pressures either elastically and/or inelastically con-stricts the black powder grain. For this reas~n, the mercury intrusionSdat a i s suspec t beyond l 10,000 psi or 70 kg crm -

lrom the density measurements, both estimates of free volume are.1 showtn as a tfuction of flame spread rate in Figure 20. The better cor-

relation is given by the ether displacement volume reflecting the volumeaccessible from outside the grain.

If the volume occupied by moisture is considered, then it is clearthat a small percentage of water could be significant in relation to themeasured free volume of black powder. This could bg the physical effect

of water on burning rate. Shulman- 5 and Plessinger have measured theeffect of water on burning rate and their data Is reproduced in Figure21. Since there could be a difference in equipment, these data setsshould be examined independently, but in either case the trend is clearand it is suggested here, based on inference alone, that water plugs andoccupies passageways to retard burning rate.

V. FUTURE PLANS

,The suggestion is mtade that for future work a density range andvolatile content for charcoal be established. Also, compaction pressurefor making blacK powder shcald be selected at.' it is proposed that surfac(earea and porosity measurements guide this study. In selecting compactionpre•ssure, dic dimensions should be considered and the density distributiondetermined, rhe density of biack powder is an indirect nea,cure of grossstructure being sensitive to the density of charcoal and particle sizeof ingrdient•. From present work, it is proposed that the internalstructure be measured and controlled. In such a study it i.s proposedto generate a standard meal and form black powder samples that providLa greater variation in density, porosity and surface area to betterSdocument the effect of th',se viriables.

*1 i

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VI. SUMMARY

1. It was found that compaction of black powder meal produces con-siderable plastic flow and forms a conglomerate matrix of interconnectingpassageways to the extent that the internal free volume is but a fewpercent.

C2. orrelations between internal surface area, pore volume, anddensity were made with burning rate.

3. It was suggested that the adverse effect of water is the resultof occupying the free volume of black powder and retarding burning rate.

4. From the above relationships and S. P.M. nicrophotographs thehypothesis was established that an increase in the degree of openness ofblack powder grains increases burning rate.

AC KNOW LEDGEMEINTS

Assistance from several people was required to complete this study.The hel1 p and cooperation received was gratifying. I also drew extei-sively upon Kevin White's (ABB) experience with black powder; his coinseland advice are a hallmark of this study. Other credit is given in thetext where appropriate; however I would like to mention here thoseindividuals who assisted in this work: Ralph Benck (PMIB), DominicDi Berardo (PNIB), John Gavel (Dupont), Gould Gibbons, Jr. (LEB), tileanoreKayser (NSW), and Jean Owens (Micromeritics). Perspective was gained byconversations with Hugh Kerr (GOEX, Inc.) and tHugh Fitler (ICIA) and Ithank both men for allowing me to tour their facilities. I also expressmy appreciation to Eli Freedman (ABB) for his translation of part of aFrench reference which is given as an appendix. I also thank Lois Smithfor helping prepare this manuscript.

4/S

- 7~J -.

REFERENCES

I. Tadeusz Urbanski, Chemistry and Technology of IExplosives, Vol 3,Pergamon Press, NY, pp. 322-346, (1967).

2. Arthur Marshall, Explosives, Sec. lHd. Vol 1, History and Manufacture,P. Blakiston's Son and Co., Philadelphia, (1917).

3. B. T'. Fedoroff and 0. 1. Sheffield, incyclopedia of Explosivcs andRe'lated Items, Vol 2., PATR 2700, Piic-inny Arscna'l, Dover, NJ,p. B'IbS-B178 (1962).

4. Arthur P. Vangelder and Hugo Schatter, ilistory of the ExplosiveIndustry in America, NY Columbia University Press, NY (1927).

q. Chromalloy Corporation, "A Study of Modernized Techniques for theManufacture of Black Powder", Propcllex Chemical Division, FinalReport No. L)AI-23-072-5Ol-ORD-P-43, Jan 60, Chromalloy Corp., IL.

0. II. E. Carlton, B. B. Bohrer, and II. Nack, "Battelle Memorial Insti-tute Final Report on Advisory Services on Conceptual Design andDevelopment of New and Improved Processes for the Manufacture ofBlack Powder", Olin Corporation, Indiana Army Ammunition Plant, 20Oct 1970, Charlesiown, IN.

7. J. R. Plessinger and L. W. Braniff, "Final Report on Development ofImproved Process for the Maunfacture of Black Powder", RCS AMURE-109, Olin Corporation, Indiana Army Ammunition Plant, 31 Dec 1971,Charlestown, IN.

8. Indiana Ammunition Plant, "Black Powder Manufacturing Facility",Vol. I and 2, Indiana Army Ammunition Plant, 1975, Charlestown, IN.

9. Kjell Lovold, U.S. Patent No. 3660546, "Process for the Preparationof Black Powder", 2 May 1972.

10. J. Craig Allen, "The Adequacy of Military Specification MIL-P-223Bto Assure Functionally Reliable Black Powder", Report No. ASRSD-QA-A-P-bO, Ammunition Systems Reliability and Safety Div, ProductAssurance Directorate, June 1974, Picatinny Arsenal, Dover, NJ.

11. II. William Voigt, Jr., Pell W. Lawrence and Jean P. Picard, "Processfor Preparing Modified Black Powder Pellets", U.S. Patent, 3937771,Feb 10, 1976, see also II. W. Voigt and D. S. Downs, "Simplified Pro-cessing of Black Powder and Pyrotechnical Igniter Pills IncludingLow Residue Versions", Seventeenth International Pyrotecinics SeminarVol. 2, ITT Research Inst., Chicago, IL, July 1980.

12. J. C. Allen, "Concept Scope of Work For MM4TE Project 57b4303Acceptance of Continuously Produced Black Powder", Report No. SARPA-QA-X-10, November 1975, Picatinny Arsenal Dover, NJ.

51

[. -,

"A • • .2•T •••- ... . " _ - .

REFERENCES (Continued)

13. K. J. White, H. U. Holmes, J. R. Kelso Jr., A. W. Ilorst, and I. W.May, "Ignition Train and Laboratory Testing of Black Powder",ARRADCOM-BRL Report No. IMR-b42, April 1979, APG, MD. See alsoK. J. White, H. H. lolmes, and J. R. Kelso, "Effect of Black PowderCombustion on High and Low Pressure Igniter Systems", 16th JANNAFCombustion Meeting, CPTA Publication 308, Dec 1979.

14. L. Shulman, private communication, (data quoted in reference 13),15 Jan 1978, Picatinny Arsenal, Dover, NJ,

15. Hugh Fitler, private communication - Draft report, "Accuptance ofContinuously Produced Black Powder", 8 March 1979, ICI America Corp.,Charlestown, IN.

16. Neale A. Messina, Larry S. Ingram and Martin Summerfield, "BlackPowder Quality Assurance Flame Spread Tester", Dec 1978, PrincetonCombustion Laboratories, Inc., Princeton, NJ.

17. R. A. Noble and R. Abel, Phil. Trans. Roy. Soc., London, Series AVol. 165, 49-155, (1875).

18. R. A. Noble and R. Abel, Phil. Trans. Roy. Soc., London, Series A203-279 (1880).

19. F. A. Williams, "The Role of Black Powder in Propelling Charges",Picatinny Arsenal Tech. Report No. 4770, May 1975, PicatinnyArsenal, Dover, NJ.

20. James E. Rose, "Investigation on Black Powder and Charcoal", IHTR-433, Sept. 1975, Naval Ordnance Station, Indian Head, MD.

21. James E. Rose, "Black Powder - A Modern Commentary", Proc. of the10th Symposium on Explosives and Pyrotechnics, 14-16, 1979, FranklinResearch Inst. Philadelphia, Pa.

22. Abraham 0. Kirshenbaum, Thermochimica, 18, 113, (1977).

23. J. D. Blackwood and F. P. Bowden, Proc. Roy. Soc., London, A213,285, (1952).

"4. Woldemar Hintze, Explosivstoffe, 2, 41, (1968).

25. L. Shulman, C. Lenchtiz, L. Bottei, R. Young and P. Casiano, "AnAnalysis of the Role of Moisture, Grain Size, and Surface Glaze onthe Combusiton and Functioning of Black Powder", Picatinny ArsenalReport No. 75-FR-G-B-15, Oct. 1975, Picatinny Arsenal, Dover, NJ.

26. L. Shulman, R. Young, E. Hayes and C. Lenchitz, "The Ignition

52

REFERENCES (Continued)

Characteristics of Black Powder", Picatinny Arsenal TechnicalReport No. 1805, Sept, 1967, Picatinny Arsenal, Dover, NJ.

27. Clyde Orr Jr., Powder Technol., 3, 117 (1969/1970).

28. Arthur W. Auamson, Physical Chemistry of Surfaces, 2nd lid., Inter-science, NY, pp. 54o-549 (1967).

29. Ui. Alison Flood, Ed., The Solid-Gas Interface, Vol. 1, Marcel DekkerInc., NY, (1907).

30. R. NI. cilntosh and A. P. Stuart, Can. J. Res.., 1324, 124 (1946).

Il. Memorial Des Poudres lit Salpetres, Vol. 6, Chapter UI by M. Vieille,Gatuthier-Vallars lit Fils, Imprimeurs-Libraires, Paris, pp. 256-391,(189•3).

32. Naval Ordnance Laboratory, Ordnance Explosive Train Designers Hland-book, NOLR 1111, April 1952, Naval Ordnance Laboratory, White Oak,

.I33. Rocbrt C. Weast, ed., Handbook of Chemistry and Physics, Slst lid.,The Chemical Rubber Co., Cleveland, Ohio, (1970-1971).

3.v. A. A. Shidlovskii, "Principles of Pyrotechnics", Air Force SystemsCommand, Report No. AD-AO01859, 23 Oct 1974, Wright Patterson AFB,o0H, Translation of Izd Vo Masimostroyeniye, Osnovy Pirotekhniki,5rd Ed., Moscow (1964).

5/

~J

APPENDIXPARTIAL TRANSLATION OF VIELLE ARTICLE-REFERENCE 30 PAGES 306-325

by Eli Freedman

4 TI. First Series--Agglomeration of Pulverized Materials

We have used the materials of RWP brown powder. Those materials,ground and sieved through a silk gauze, were agglomerated dry by compres-sion with a hydraolic press, using the steel mold already described.

We thus obtained pastilles of 20-mm diamet.-r, about 3.5 mm high; andblocks of triple weight, 10.5 mm high. Six pastilles or 2 blocks of thesame density constituted a charge of loading density 0.6; 3 pastilles orI block permitted the corrying out of comparative experiments at a loadingdensity of 0.3.

The sect ons of the pistons used for the measurement of pressurewere configured in a way to furnish similar crushings so as to make thetracings comparable: a piston of 0.5 cm2 for the density 0.6, and theolle of 1.33 cm2 for the density of 0.3.

The crushings were carried to 3.5 mm and 4 mm by the use of coppercylinders similar to the regulation cylinders, but with a partially-reduced crajss section, ratio of similitude /1V71.

Table 11, gives the results of the tests (only one half of the

table is reproduced here given as illustration).*

TABLE 11

Duration of CombustionDensity of Density of

Density Thickness Loading 0.6 Loading 0.3

1.922 10.44 98.07 125.01.785 11.19 87.13 108.801.7b0 11.47 48.33 74.571.724 11.65 17.36 28.781.635 12.09 5.22 12.01

The results o5 these 3 determinations are represented in Figure 15(of text), whose abscissas represent the real densities of the pastilles,and the ordinates, the durations of combustion. [Curve reproduced iiiSassc's text as Figure 2].*

. rackets enctose tranesator'e commente cr notea.

P A4 J /

These four curves have a common shape. At low densities up to avalue of about 1.720, the duration of combustion is effectively inde-pendent of thickness whether the combustion occurs at a charge densityof 0.3 under a maximum pressure of 1200 kg, or at a charge density of0.6 under a pressure reaching 3000 kg. However, combustion time slowlyincreases up to four times the combustion time of elementary dust; then,suddenly, between densities of 1.720 and 1.800, the influence of thicknesssuddenly manifests itself and the combustion duration increases withextreme rapidity, tending to the limits characteristic of the compactmaterials; i.e., effectively proportional to the thicknesses.

This effect of compression is hardly unique to brown powders. Thesame functioning is observed with black powder materials reduced to thepowdered state.

V. General Mode of Action of Compression

In summary, for all of the materials that we have reviewed here,the influence of compression on the duration of combustion occurs in anidentical way.

Four regimes of compressinn can bc distinguished which correspondto four very different combustion modes.

In the first regime, with the weakest compression, the combustionduration of the materials does not differ from that of the elements whichcompose it placed next to each other. It is consequently independent ofthickness.

In the second compression regime, the combustion duration pro-gressively rises up to values reaching 4 or 5 times those that corre-spond to the grains of elementary dust, but this duration neverthelessremains independent of the thickness. It ia thie type of materiaZ thatnormaZly conetitutee the bZack or brown powdere actuaZZy used by artiZZery.

The third regime is characterized by an extraordinarily rapidvariation of combustion duration with density; the influence of thicknessappears and is accentuated more and more. This type of w.aterial issometimes introduced during manufacture, when compression is required toachieve excessive slowing-up of the duration of combustion of theelementary grain; but the extreme irregularity of the products does notnormally permit these types of materials to be used.

In the fourth regime, the materials, now virtually compact, showdurations of combustion slowly increasing with density and proportionalto their thicknesses.

, This succession of phenomena can be very simply explained, based onthe known laws of gas flows through capillary orifices. It is known that

451

the discharge, Q, of fluids across capillary canals of all kinds is givenby expressions of the type of Poisuille's law,

Q = KPI)k/I.

where I) -- diameter; P, pressure; L, length; and K, a coefficent charac-teristic of the ýluid; and that consequently, the flow velocities, pro-portional to Q/I)-, decrease with extreme rapidity with the dimension ofthe orifices.

it is conceivable that in the phenomenon of the penetration ofincandescent [flamingJ gas across a heterogeneous mass, the intersticesof a certain order of magnitude, which will be called the first order,alone play an effective role; the orifices of lower order, e.g., 10 timesless, permit the flame to propagate only with a velocity 100 times less.

In relation to the above ideas consider the material formed by agglo-meration of grains at the lowest degree of compression. A system of inter-stices exists in the charge which assures flame spreading to all of thegrains without a noticeable delay, or at least for a time duration negli-gible with respect to the combustion time of the grains themselves. Thismode of combustion is made evident in the numerous examples given above,showing the identity of the durations of combustion of the free grainwith the weakly-compressed blocks. This is the first phase of compressionat work.

Under stronger compression the interstices are reduced by a sort oftangling of the grains, whose surfaces in contact become married in amore and n.,)re complete way while blocking or reducing to a very smalldimension the entire grid of primitive interstices.

There results a new system of canals, whose dimensions remain of

the original order; rather thin limiting on the average each elementarygrain, as in the primitive state, they limit a polyhedral kernel formedby the partial agglomeration of several grains; and it is readily con-ceivable that the average duration of combustion of these kernels canvary continuously with compression up to values double, triple, etc.,the duration of combustion of the prir ,e grains without, however, aresidual system of first-order canals Leasing to exist in the mass, thusassuring a combustion duration independent of the external dimension ofthe grain. This is the working of the second phase of compression.

Under increasing compression, the number of first-order canals isreduced enough so that the dimension of the kernels which they limitbecome of the order of a milled grain. One falls therefore into theirregularities which we have observed in the third phase of compressionof the raw materials forming the grains at the moment where the influenceof thickness is starting to be noticed. These irregularities continue

"57

until all of the first-order canals have disappeared and the influenceof thickness definitely shows up.

The fourth phase of compression starts at this moment. The materialsapproach the limiting state corresponding to a perfect continuum and thevelocities of combustion vary only very slowly with density. It is infact conceivable that this velocity results from the conductibility ofthe materials for the heat accruing from a slowa vetoo'tty of infiltrationof glow~ing gao across the second-order interstices, whose dimensions areprogressively reduced. From there on, there is a slow decrease of com-bustion velocity, while the density rises up to the practically realiziblelimits.

VI. Influence of Pressure on the Duration of Combustion

The results in Tables XI, XII, and XIII relating to the influenceof compression on the mode of combustion of the pulverized raw materialspermit the deduction of several qualitative conclusions about the influ-ence that pressure exerts on combustion velocity.

If one considers materials of density greater than 1.780, for whichthe proportionality of the duration of combustion to thickness is effec-tively realized, i.e., the materials that function as co at one ob-tains the following numbers [in the table] for the rat te durationsof combustion of the same elements to the charge densities 0.3 and 0.6

(maximum pressures 3200 and 1300 kg).

The general average of these numbers is about 1.25, and the exponentof the pressure that takes account of this variation of the duration ofcombustion is about 0.25. This value was previously noted in Chap. 11,as resulting from the comparison of the durations of combustion of com-pacted materials of 30/40 powder burning at densities 0.3 and 0.6.

When one considers materials of density less than 1.780, the ratiorises rapidly and attains values greater than 2.00. Whatever the errorswhich lead to irregularities in the ratios obtained in some of the pairsof experiments, we can certainly conclude that the influence of presstmreincreases significantly when the compactness of the materials diminishes.The ratio 2.5 effectively recapitulate~s the proportionality of thedurations of combustion to pressures.

The discrepancy of the pressure exponents given by experimenters torepresent the influence of pressure on the combustion velocity is easilyexplained by the variable nature of the raw materials on which theyworked.

In any case, it is evident that this effect of pressure cannot be,a prior'i, regarded as identical for powders of different factories, andthat it is important to determine it for each particular case.

(---- page 325----)

58

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