Introduction to Dissertation on Insensitive Highly Energetic Materials by Theodore S. Ted Sumrall
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Introduction to Dissertation on Insensitive Highly Energetic Materials by Theodore S. Ted Sumrall

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Introduction to Dissertation on Insensitive Highly Energetic Materials by Theodore S. Ted Sumrall

Introduction to Dissertation on Insensitive Highly Energetic Materials by Theodore S. Ted Sumrall

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Introduction to Dissertation on Insensitive Highly Energetic Materials by Theodore S. Ted Sumrall Introduction to Dissertation on Insensitive Highly Energetic Materials by Theodore S. Ted Sumrall Document Transcript

  • CHAPTER – 1 INTRODUCTION 1.1 Background 1.1.1 Uses for Commercial Explosives Commercial explosives are used in a number of applications, including coal mining, blasting of rocks, etc. It is desirable that explosives combine their efficiency in executing certain work with economy and safety. Many of the older types of explosives are based on NG (such as Dynamites), but the trend in modern explosives has been to reduce the amount of NG by replacing NG with other, less expensive components. The majority of modern commercial explosives contain AN and some combustible material such as charcoal, coal, coal tar, or petroleum products. A typical example of a modern, inexpensive commercial explosive is the Ammonium Nitrate-Fuel Oil (ANFO) described by Cook [1]. Many commercial explosives designed for certain types of work may also be used for other purposes as detailed in Table 1-1 below. Table 1-1 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Commercial Blasting Applications for Explosives [2] Tunnel Construction Quarry Blasting Blasting Ice Jams Oil Well Stimulation and Penetration Seismic Prospecting Harbor/Channel Construction Construction Work inc. grade construction and pipelines Metallic Mines (Ore) Nonmetallic Mines: Limestone, clay, gypsum, salt, potash, talc, and phosphate. Open Pit Mining Worldwide, explosives used in coal mines are the most important and numerous of all commercial explosives. They are employed for breaking coal or rock in mines. Explosives used for breaking coal should be of low brisance, but possess “heaving properties”, so that the coal will be broken in rather large lumps without pulverizing it. Addition of aluminum powder has been demonstrated as a proven method for decreasing the brisance of a high explosive while increasing the pressure pulse duration and overall effectiveness of explosives requiring low brisance and high overall blast pressure. Although classified as a “non-permissible” explosive (i.e. not permissible for usage in underground coal mines which contain volatile gases) aluminized explosives are still quite suitable for blasting coals either containing no volatiles (such as anthracites) or containing little of them (such as semibituminous coals). These explosives are also suitable for use in blasting rock in coalmines. [2,3] One of the requirements of any explosive used in underground mines is to possess a positive oxygen balance to CO2. This is in order to avoid the formation, on explosion of CO, which is extremely toxic and can form with air explosive mists. In this respect, neither K or Na oxidizer is an ideal oxygen supplier for mining explosives, because the K or Na Radical fixes oxygen, forming K2O or Na2O [4]. Ammonium Nitrate (AN) and Ammonium Perchlorate (AP) are very
  • suitable due to the superior oxygen balance of both. The price for AN in the 19th Century was too high for use in mining explosives. Only after 1920, when the synthetic nitrogen industry had been established and the price of AN dropped, did it become possible to use AN in many “permissible” NG based coal-mining explosives. Currently, AP is more expensive than AN, however, research by the early part of the next century should drop the price of AP in the same manner as the price of AN has dropped. Explosives used in blasting operations (such as in quarry work, non-gaseous mines, strip-coal mines, metal mines, ditching, hole drilling or in the demolition of buildings, roads, bridges, railroad tracks, sunken ships, under water obstacles in channels, etc.) are classified as blasting explosives. TNT and NG /Gelatin based Dynamites are among the explosive materials used for commercial blasting and demolition work. The short duration and low temperature of detonation required for permissible explosives are not required in most blasting operations. The principle factors which govern the selection of a commercial blasting explosive are: power; detonation pressure (also called brisance); density; sensitivity; stability; water-repellence; and cost. These factors assume varying degrees of importance according to the circumstances under which the explosive is to be used. For blasting very hard rock (such as gold quartz) a very powerful and brisant explosive is required. As underground drilling is expensive, the density of the explosive should be high. TNT and blasting gelatin are suitable for such purposes. TNT is readily initiated with even a small booster. Blasting gelatin requires initiation with a detonator powerful enough to produce a detonation velocity of approximately of 7000 m/sec. In quarrying and strip-mine operations where the work is done in the open, large drills are used and explosive charges from ≥ 10-15 cm diameter are required. For such work, TNT, either alone or in composite explosives is very suitable. For under-water blasting (i.e. harbor/channel/port construction, channel clearing, and etc.), waterproof explosives such as TNT, Gelatin Dynamites, or Pentolite (50/50 TNT/PETN) has been used. It should be noted for underwater explosives, the effect produced is considerably less than it would be on land due to the pressure of the surrounding water. Thus, high performance explosives (such as TNT) are essential.
  • 1.1.2 Major Energetic Material Incidents 1900-1997 A number of explosive accidents have claimed an estimated 15,000+ lives during the 20th Century. Table 1-2 lists 13 major accidental explosions described in available literature [5,6,7]. It is important to note that a higher percentage of fatalities per mass of explosive is observed in high population density areas such as Japan. As an example, the majority of the people killed from an explosion which took place in Yokohama, Japan in 1959 were passengers in a train that was passing by a TNT plant at the moment of the explosion. Table 1-2 Some Energetic Material Related Fatalities (1900-Present) Year Place Lbs. (M) 1994 1991 1980 1962 1959 1950 IowaU Liaoning P DehbosorgI KanagawaJ YokohamaJ New JerseyU 11.4 0.1 Unknown 0.0002 ≈ 0.5 1.0 Type Damage Major/Minor ≈ 2/8 1.2km/3km Unknown <0.1 1 km/2.5 km 1.5km/7km AN TNT Dynamite Granular TNT TNT/Al Dynamite F 1947 Brest 6.6 AN 5km/16km U 1947 Texas 7 AN 2 km/8km U 1944 Nebraska 1.1 TNT/RDX 1.5km/7km 1944 CaliforniaU 4.3 TNT/RDX 1.5 km/40km U 1926 New Jersey 1.6 TNT 1.5km/6.5km G 1923 Oppau 9.0 AN 6.5/km U 1918 New Jersey 1.0 AN 1.5km C 1912 Nova Scotia 5.2 TNT, NC 3km/ G 1909 Steinfeld 5.5 NG 3km J = Japan, U = USA, C = Canada, G = Germany, P = PRC, F = France, Casualties Killed/Injured 4/18 17/107 150/Unknown 5/42 ≈ 175/500 26/400 21/100 560+/3000+ 10/50+ 300/885 >100/>150 1,100/1500 64/100 1800/8000 The most striking fact in this tabulation is that a number of these greatest explosions involved one of the least sensitive of all explosives, namely Ammonium Nitrate (AN). The Texas City explosion was actually two fertilizer-grade AN explosions, one on the steamship Grand Camp and the other the following day on the steamship High Flyer (Figures 1-1 through 1-6). The Brest, France explosion also involved another shipload of AN. These cases illustrate the hazards involved whenever the explosive properties
  • of even a very insensitive explosive are overlooked, especially under conditions where the charge mass exceeds a certain critical value. [1, 7, 8] 1.2 Previous Related Studies Early Attempts at Desensitizing Energetic Materials (Pre-1800) Prior to 1800, the overwhelming majority of energetic materials in use for mining applications consisted almost solely of black powder. According to Dutton a Hungarian engineer, Kaspar Weindl, experimented in blasting ore near the town of Selmeczbanya by means of Black Powder in 1627. [9]. From that time, use of Black Powder for breaking ores spread to Germany, Sweden and other countries. According to Marshall, blasting was probably introduced to England in 1670, and in 1689 Epsly started to use Black Powder in Cornish mines. [10] However, as black powder was a very weak explosive, attempts were made during the 19th Century to replace it with something more powerful. One of such attempts was the invention of Berthollet Powder which consisted of 75% potassium chlorate, 12.5% sulfur, and 12.5% charcoal. This mixture was more powerful than black powder, but it was extremely sensitive and for this reason was very dangerous. [11] Several other mixtures were proposed but none proved to be suitable for industrial purposes. Owing to the much greater desire for improved performance, insensitivity and safety were of much smaller concern and negligible attempt was made to improve the safety characteristics of black powders. Early Attempts at Desensitizing Energetic Materials (1800-1900) The 19th Century brought about the invention and/or use of other explosives, including Picric acid (PA), trinitrotoluene (TNT), nitrostarch (NS), nitrocellulose (NC), and the very powerful but sensitive nitroglycerin (NG). Owing to its very low cost, high power and ease of manufacture, NG literally dominated the latter half of the 19th Century as the explosive of choice. However, as is widely known, NG is very sensitive and significant attempts were made to decrease the sensitivity of NG as detailed below. The first important invention before the invention of NC or NG was the preparation in 1833 of impure nitrostarch by Braconnot. In 1846, NC was prepared with a high nitrogen content independently by C.F. Shonbein (1799-1866) and F. Bottger (1806-1886). [12] In 1846, NG was prepared by the Italian chemist Ascanio Sobrero (1807-1867). NG proved to be more powerful than any previously prepared explosive. Because of the extreme sensitivity of NG, its use on a large scale was considered to be dangerous and the fact that it was liquid made it difficult to transport or to handle. It was nearly impossible to use NG in horizontal boreholes, but when used in vertical boreholes, there was a danger that NG might run into surrounding fissures in the rock, escape the detonation of charge, and either burn (giving deleterious nitrogen oxides) or, if unburned, remain a grave source of danger during removal of the rock by mechanical means. Attempts were made to overcome this difficulty by lining the boreholes with wet clay, but this was time consuming and not always effective. [13] However, NG in liquid form found extensive use for blasting oil wells in Pennsylvania USA where the petroleum industry started to develop in the middle of the 19th Century. Straight NG was also used in construction of tunnels at this same time.
  • Before inventing his dynamite in 1863, Alfred B. Nobel (1833-1896) proposed a safer method of transporting liquid NG by mixing NG with 15-20 parts of anhydrous methanol and transporting the resulting non-explosive mixture in tanks to places of work. At the work site, NG was precipitated by adding water and the supernatant dilute methanol was removed by de-cantation. However, as this method was time-consuming and rather wasteful (because it did not pay to recover methanol), it was seldom used in the USA, where everything is done in a hurry and as cheaply as possible. It was therefore preferred to transport NG in the liquid form in the US, although, it was more dangerous. Many accidents occurred and many lives were lost in connection with NG. As long as most of the workmen were foreigners, the industrialists, who card only for profits, did not introduce any safety regulations until they were forced by the US Government after establishing the Bureau of Mines in 1910. As accidents with NG continued to occur, some countries, among them Great Britain, prohibited its manufacture and use beginning about the middle of the 1860’s. However, despite a number of accidents, Dynamite began production in the US in 1868. The first US Dynamite plant was constructed in California at a spot called Rock House Canyon, near San Francisco. It was operated by Chinese, but the supervisors were American. The plant, consisting only of shacks, was called the “Giant Powder Co”. Great opposition to Dynamite was encountered from a plant constructed in Santa Cruz, California, “The California Powder Works”, manufacturers of black powder. However, when the Great Comstock Gold Lode was discovered, black powder proved to be too weak to blow the hard rock of Mt. Davidson and only Dynamite could do it. The increased demand made the management of Giant Powder Co. push their Chinese workers harder and harder to meet the demand, and as a result of this hurry, the plant blew up in November 1869, killing two Americans and all of the Chinese employees. This accident did not discourage Nobel Co., and another larger and safer plant was constructed. A large Chinese crew was assembled and a German chemist was sent from Europe as Superintendent. The first 23 years of Giant Company exemplify in dramatic terms the introduction of Dynamite in the US. During that period, five successive plants, each larger than the pervious ones, were wrecked by explosions and in all 83 people (mostly Chinese) were killed. [14] Prior to the discoveries by Nobel, a number of researchers attempted various methods to desensitize NG. In 1854, introduction of the earliest Dynamite called Magnesial’nyi was produced by the Russian officer V.F. Petrushevskii. It consisted of NG absorbed by powdered magnesit or magnesia. It was used in Siberia for blasting in gold mines.[15]. Also, there was a claim in the literature [16] that previous to the discovery by Nobel of Guhrdynamit (1867), a German mining engineer, F. Schell of Clausthal, Oberharz, used as early as 1866, a mixture of NG and crushed “Pochsand” (ore sand) for blasting in mines of the Harz Mountains. After visiting Schell in 1866, Nobel found out that kieselguhr would make a better absorbent for NG than Pochsand. Prior to the discovery of Guhrdynamit, Nobel, who started to work as early as 1862 on the search for a suitable absorbent for NG, prepared a charge consisting of a cartridge made from Zinc foil filled with pulverized black powder impregnated with NG. Such cartridges were successfully detonated under water and British Patent No. 2359 was obtained in 1863 for their manufacture. Manufacture began at Emmanuel Nobel’s plant in Helenborg, Sweden but was stopped in the fall of 1864 when a great explosion took place, killing Alfred’s youngest brother and a chemist employee.
  • Because black powder was not a good absorbent for NG and the mixture was not safe in transportation, Nobel started to investigate other possible absorbents. In 1865, Nobel succeeded (after trying the following as absorbents for NG: charcoal; silica, powdered brick; and shredded paper) in finding a good porous material, known as kieselguhr, guhr, or infusorial earth. When pulverized, it could absorb up to four times its weight of NG. On the strength of this he compounded a mixture of 75% NG with 25% guhr. This mixture proved to be satisfactory not only from a safety standpoint but also in its explosive properties, being less sensitive to shock but more readily detonable than liquid NG. Alfred Nobel called the new explosive Dynamit, derived from the Greek word “dinamis”, which means force. This particular explosive became also known as “Guhrdynamite” and “Dynamit No 1” or “Kieselguhr Dynamit”. Theodore Winkler, an associate of Alfred Nobel, made (in 1868 near San Francisco, CA) 3 pounds of Dynamite by mixing 1 part of Kieselguhr with 3 parts of NG. He used the facilities of Judson & Sheppard Chemical Works of San Francisco and then demonstrated the strong action of this new explosive by blasting boulders along the line of Bay Shore Railroad. [2] None of the Dynamite plants constructed before 1873 in the USA belonged to DuPont & Co., the leading manufacturer of black powder. There were two reasons for this: dangers in transportation of NG; and higher prices of Dynamites in comparison with the new black powder formula introduced in 1857 by Lammot DuPont. Although Guhrdynamite proved to be a good, inexpensive explosive (and it is still used in some parts of the world, although discontinued in the USA since 1908) its disadvantage was that it contained 25% inert material (kieselguhr) which did not participate in the explosion. To overcome this disadvantage, Abel in England and Trauzl in Austria proposed in 1867 the usage of NC as an absorbent for NG. As the NC in use by then had a higher Nitrogen content than Collodin Cotton, it could not be coloided by NG and explosives prepared by this method were not of the gelatin-type but powdery. As their NC was not a good absorbent for NG, explosives of Abel and Trauzl were exudable and for this reason not successful. Schultze of Prussia patented in 1868 an explosive consisting of NG and partly nitrated wood (nitrolignin) but this explosive, known as Dualin also exuded NG and for this reason was considered unsuitable. During the time of Nobel’s research towards discovery of a suitable absorbent for NG, pulverized sand was proposed by Prof. Seely and mixtures of pulverized AN with sawdust or charcoal were proposed in 1867 by Swedish inventors Norrbin and Ohlsson and also by Bjorkmann. When these mixtures were combined with NG, Dynamite-like explosives Ammiakkrut and Seranin were obtained. [18]. The Seranin explosive (developed by Bjorkmann and patented in 1867) consisted of 18.12% NG, 72.46% AN, 8.7% sawdust or charcoal. This formula was distinct from the Seranine explosive developed by Horsley, which consisted of 27% NG absorbed by a mixture of pulverized potassium chlorate, gall nuts, alum and magnesium sulfate. [19] Although Nobel knew at the time of invention of his Dynamite about advantages of AN addition, he could not use it in his Dynamites because AN based dynamites were already patented by Norrbin and Ohlsson. So he decided to purchase their patent in 1870. This patent covered the explosives containing 80% AN, 10%-14% NG and 6%-10% charcoal. As this explosive was hygroscopic, he coated the particles of AN with paraffin (Stearin, ozokerite or naphthaline), thus practically eliminating its hygroscopicity. For this explosive Nobel was granted British Patent No. 1570 (1873).
  • Previous to this, Nobel worked on explosive mixtures consisting of NG partially absorbed by mixtures of combustible materials (such as woodmeal, charcoal, rosin, sugar, starch, etc.) with oxidizers (such as potassium or sodium nitrate) and was granted British Patent No.442 (1869). These explosives, as well as Amoniakrut and Seranin, served as prototypes for explosives known in the US as “Ammonia Dynamites” or ”Ammonium Nitrate Dynamites”. These Dynamites are still in use in the Western Hemisphere. [20] In 1870, an American chemist (James Howden) proposed a Dynamite consisting of 75% NG absorbed by 25% of a mixture consisting of pulverized sugar, potassium nitrate and magnesium carbonate. This Howden’s Dynamite was stronger than corresponding Guhrdynamite and was first employed in 1872 on the first big Dynamite job, which was the driving of the Musconetcong tunnel, 1 mile long, near Easton, Pennsylvania on the Easton and Amboy Railroad (currently the Lehigh Valley Railroad). [2,20] The next step in the Dynamite industry was replacement of straight NG with NG gelatinized with Collodin Cotton, which is NC consisting of approximately 12% Nitrogen rendering explosives known as Gelatin Dynamites or simply Gelatins [21]. The strongest of these explosives were Blasting Gelatins. British Patent 4179 was granted in 1875 for both types of Dynamites. The last patent granted to Nobel on Dynamites was in 1879. That was for Ammon-gelatin or Ammonium Nitrate Gelatin. [18] Meanwhile, Alfred Nobel, with the assistance of his father, Emmanuel (1801-1872), was engaged in attempts to utilize NG in explosives and improve its method of preparation, naming their product Pyroglycerin or Glonilol. It became later known as Nobel Sprengol. Other inventions of note during the 19th Century were nitroguanidine (NQ), which was first prepared by Jousselin in 1877. NQ did not begin to be widely used until the turn of the century at which time it was used more for propellant applications than as an explosive owing to its cool burning characteristics and insensitivity to shock [23]. In 1880, TNT was made in a very pure state by P. Hepp. [10] In 1888, a now well-known, very insensitive high explosive (IHE), tri-amino-trinitro-benzene (TATB) was first prepared by Jackson and Wing. In 1928, the process was further refined by Flurscheim and Holmes [23]. Unfortunately, high production costs prevent this almost ideal explosive from enjoying wide spread usage today. In 1899, cyclo-trimethylene-trinitramine (RDX) was first prepared by Henning of Germany who named it Hexogen and used it for medical purposes. Then Brunswig prepared RDX in 1916, but it was not until 1920 that its’ explosive value was recognized by Eric Von Herz. The first person who prepared it in fairly good yield was G.C. Hale in 1925. Little additional research was conducted until 1940 when J. Meissner of Germany developed a continuous method for RDX manufacture [3]. Though more powerful than NG, RDX is less sensitive than NG (but still considered sensitive, however, not as expensive as TATB). Contemporary Attempts at Desensitizing Energetic Materials (1900-1950) Commercial efforts to desensitize explosives during this period of time principally concentrated on research in the area of freezing point depressants for NG. The early high-explosives era was plagued with many accidents associated with thawing of frozen NG in dynamites. Freezing
  • renders dynamites relatively insensitive, and they therefore had to be remelted before they could be used in blasting operations. Since NG freezes at +13 °C, it is natural that Dynamites containing NG will freeze in cold weather. This is very undesirable because such Dynamites become so hard that a blasting cap cannot be inserted in cartridges and the NG becomes somewhat insensitive to initiation. Such cartridges cannot be detonated, even when using the strongest cap available, but require a booster. Although frozen cartridges can be detonated (with difficulty), they should not be used without defrosting prior to usage. Since all the prescribed methods of defrosting are time consuming, attempts to speed-up thawing almost always resulted in accidents. The safest method was therefore to incorporate a freezing point depressant in NG in order to produce a Dynamite which did not freeze in cold weather. Because many of the attempts with addition of antifreeze resulted in either failure or lower performance, manufacturers ignored this drawback and continued to use NG without antifreeze. This continued until 1902 when a severe explosion in Greisenau Mine, Germany occurred caused by frozen Dynamite. This disaster produced the renewal of research on antifreezes and 1903, the SA de Poudres et Dynamites of France introduced DNT and TNT as antifreezes. Various melting-point depressants for NG were studied, some of the most interesting of which were mono-nitrotoluene (MNT) and di-nitrotoluene (DNT). Other solutions to the problem were made by Kast who discovered in 1906 the isomeric forms of NG, and in 1911 by Woodbury, who disclosed the nitration of mixtures of glycerin and sugar [27]. While the addition of DNT and TNT did not result in performance loss, unfortunately it still did not suppress the freezing point of NG to desired ranges. Since this time, a number of antifreeze additives were investigated with the most successful being principally nitrated alcohols. The most important antifreeze (prior to 1950) has proven to be ethyleneglycoldinitrate (EGDN). The commercial introduction in 1911 of nitrated mixtures of glycerin and ethylene glycol (producing NG and EGDN) provided the most satisfactory solution of this important problem. Today commercial NG is manufactured as a mixture of NG-EGDN. The explosive properties of EGDN are almost identical to those of NG, as also are those of the low-freezing mixture, the only significant difference being the melting point. One other invention which bears mentioning is the “Continuous Nitration Process” (developed by Agne Nilsson and Bemt Brunnberg) which permitted the continuous manufacture of NG, rather than conventional “Batch Mixing”. This minimized the amount of NG in the plant at any one time such that if an explosion did occur, only a minimum amount of NG would be involved in the incident. [28] Modern Attempts at Desensitizing Energetic Materials (1950-1997) A number of explosives have been developed during the latter half of the 20th Century in order to help: increase performance; decrease cost; and decrease the sensitivity of explosives and thereby decrease the risk of fatalities. Four types of explosives are worthy of mention: Ammonium Nitrate/Fuel Oil (ANFO), Slurry Explosives, Emulsion Explosives and Plastic Bonded Explosives (PBX). Slurry Explosives and Emulsion Explosives both derive a higher degree of safety due to the presence of water in the compositions. Additionally, slurries and emulsions can be manufactured as both high explosives and blasting agents. The Bureau of Explosives and the Department of Transportation differentiate between high explosives and blasting agents as follows. A high explosive is one that detonates with a No. 8 blasting cap at 21 ºC. A blasting
  • agent is one that fails to detonate with a No. 8 cap at 21 ºC, and passes electrostatic discharge, burning and differential thermal analysis tests. AN is actually shipped as an oxidizer despite its explosive properties. Development of ANFO While slurries and emulsions are patented inventions, ANFO generated spontaneously outside the explosives industry, even under an initial strong opposition by the industry. Four stages of this evolution may be cited: The prilled AN or Fertilizer Grade AN “FGAN", unique to ANFO, was developed in the early 1940's as a fertilizer initially rejected for use in explosives as too coarse, high in moisture, and low in density. "Explosive-grade" AN is nonporous and nonfunctional in ANFO except in very fine mesh sizes. FGAN explosion catastrophes in 1947 at Texas City and Brest (France), and later that year in the Black Sea, left the lasting impression on the public, particularly mine operators then paying high prices for explosives, that AN is a low-cost, powerful explosive with good blasting potential which was somehow overlooked by the explosives industry. Under this erroneous impression, mining engineers attempted (in the mid-1950's) to use FGAN directly as a blasting agent in the large boreholes of the iron ranges of Michigan and northern Minnesota. The mining engineers did not realize that the product available to them was a slightly desensitized form of AN which was distinctly different from the (sensitized) FGAN involved in the catastrophes. These early attempts were still not entirely unsuccessful owing to the explosive ability of pure AN and its use in large boreholes with multiple boostering by large dynamite charges. Do-it-yourself efforts finally became successful when operators, after learning that fuel is needed to sensitize and strengthen AN, began to use about a gallon of fuel oil along with each bag of AN. An important symposium held in October 1956 at the University of Minnesota brought together mining engineers and explosives scientists who then joined forces in the characterization, mechanization, and standardization of the ANFO system. The products that detonated in the accidental explosion of FGAN were coated (for anti-caking purposes) with nearly 1.0 % wax which was good for the intended purpose but unfortunately is also a good fuel-sensitizer for FGAN. By strange coincidence, this amount of the coating gave a (sharp) maximum explosive sensitivity. Even the oxygen-balanced 94.5/5.5 AN/wax composition was appreciably less sensitive. The relatively high sensitivity of the wax-coated FGAN, along with the large charge size factor (the shipload explosions involved 3 x 106 to 4 x 106 kgs of FGAN in each case), explained how the FGAN was able to undergo sudden transition from deflagration to detonation in each of the shipload conflagrations. But it did not explain how the fires were initiated. The explanation developed in the litigation seems conclusive. The FGAN was bagged and shipped hot (at 33ºC ± 11ºC). the paper bags often became AN impregnated in exposure to moist and wet conditions, and, while wax coatings do not lower the thermal stability of AN, Findlay and Rosebourne had found 25 years earlier that celluloid materials in intimate mixtures with AN do lower its thermal stability. In fact, the AN-paper combination self-heats in the very temperature range used in bagging and shipping the FGAN. Indeed, it often arrived at the seaport appreciably hotter than when it was bagged. Bag
  • embrittlement and even charring by self-heating caused excessive spillage, re-bagging, and contamination. Furthermore, several spontaneous fires in rail transit of the product preceded the Texas City catastrophe. After this disaster the wax coating was replaced by kieselguhr, a somewhat inferior anticaking agent but a safe one. It was this (desensitized) guhr-coated FGAN that mine operators used in the ANFO development. When sensitized with about 6% paraffin or Fuel Oil (FO), ANFO is the cheapest source of explosive energy in the modern world. This has long been recognized, and considerable research has gone into finding how to use it even more effectively in ever-increasing amounts. While the annual consumption of AN in commercial explosives was relatively small in the period 1912 to 1928, it attained greater prominence in the industry with the invention of Nitramon, a high AN, non-NG explosive developed by Du Pont [29, 30]. Though contributing immensely to industry for two decades, Nitramon, like dynamites, quickly gave way to modern blasting agents ANFO and slurry. So significant have been the modern blasting agents, ANFO and slurry, to open-pit mining that the entire commercial explosives industry suddenly expanded some five times relative to the zenith of the dynamite era. The tremendous expansion was triggered by the new economies made possible by blasting agents and other open-pit mining technologies. Indeed, the great taconite industry probably would not have been economically sound without the slurry development, a claim made by leading taconite operators. Several important advantages are responsible for the great success of ANFO: (1) ANFO is yet the cheapest source (by weight) of explosive strength (explosive energy per gram) in the modem world; (2) It is orders of magnitude safer than dynamites, yet not as safe as slurries or emulsions; and (3) Porous, prilled AN readily absorbs and retains the proper amount (6%) of fuel oil. ANFO is easily mixed and loaded into the bore-hole by fast and efficient mechanical methods. A great advantage of ANFO, slurries, and emulsions is that as bulk rather than packaged explosives they fill the borehole completely, a feature now recognized to be of great importance in blasting efficiency. The remarkably simple ANFO mixture found use at first only in the large-diameter boreholes of openpit blasting but a few years later became popular in small-diameter, underground blasting as well. This was made possible by blowing it into the small boreholes under sufficient pressure to break down the prills and reduce the critical diameter below the (small-diameter) bore-hole size. (Before this high-pressure loading, ANFO has a critical diameter of four to five inches.) Only two deficiencies prevented ANFO from taking over almost the entire commercial explosives market: (1) It is readily desensitized by water and therefore generally cannot be used under adverse water conditions characteristic of an appreciable part of commercial blasting. (2) In spite of complete filling of the bore-hole, it is low in density and develops too low a pressure in the borehole (“bore-hole pressure") to break successfully the hardest rocks, particularly at the bottom or "toe" of the burden and in "tight" blasts where breakage is most difficult. [31] ANFO has a calculated detonation velocity of approximately 3900 meters/second at a density of 0.82 g/cm3. This translates to a detonation pressure of only 31 kbars. Despite the low costs associated with ANFO, the performance is almost 8 times below the target detonation pressure of 240 kbars. The AN currently used in blasting is formed into a porous pellet called a prill. A mixture of 94.5% AN and 5.5% fuel oil (by weight) was found to be the most efficient explosive and is termed ANFO. The addition of fuel oil allows all the available oxygen from the AN to be used effectively in the explosion. Unfortunately, ANFO is not without several drawbacks, including its low density and its hygroscopicitiy. Paradoxically, it was found that through the addition of water, with suitable stabilizing agents, and if desired, gelling agents, not only are most of these problems overcome, but the handling of the explosive is simplified. These water-based slurries
  • or gels vary in consistency from a heavy paste or jelly to a solid rubbery mass, depending on the gelling agents used. The most common agent is a gum such as guar. The gelling agent serves two purposes: 1) it ensures a homogeneous mixture by preventing settling of components, and 2) it facilitates handling. A simple slurry is one in which the fuel oil found in ANFO is replaced with another fuel which is compatible with a water gel. Most commercial slurries consist of an explosive base (such as AN or Sodium Nitrate (SN)) and a fuel such as carbon, Sulfur or aluminum powder. The addition of large quantities of aluminum produces a slurry with very high energy release at moderate detonation pressures. Advantages are ease of field use, less sensitivity to heat and shock, voidless filling of holes and low expense. Disadvantages are: low density and thus low performance and/or the need for a large booster charge for initiation, and long term storage limitations. These booster charges, however, are typically manufactured from much more sensitive explosives such as TNT or TNT/PETN. [32] Development of Slurry Explosives and Blasting Agents The fundamental concept of Slurry Explosives (SE) and Slurry Blasting Agents (SBA) was that a saturated aqueous oxidizer (primarily AN) solution is used as the continuous or dispersion phase of a slurry to disperse both excess solid oxidizer and the sensitizing fuel. While usually insensitive to commercial detonators, slurries propagate detonation satisfactorily, particularly in the large diameter boreholes of open pit mines, when initiated with sufficiently powerful boosters. "Fuel" is used here in the generic sense as any combustible material, whether explosive or not, that can be uniformly dispersed in the slurry medium. It may include oxygendeficient explosives like TNT, Composition B, or it may comprise non-explosive fuels such as hydrocarbons, carbonaceous and cellulose materials and combustible, heat-producing metals, i.e., Al and silicon. As they contain significant amounts of water, SE and SBA are among the safest of all explosives (in the final state) because they are not easily ignited and are somewhat insensitive to most types of shock (i.e. bullet impact and friction) that can initiate the explosion of dynamites. Furthermore, they are made water-resistant by means of the hydrophilic colloidal guar gum. However, because aluminized slurry explosives contain a significant amount of water, over time the water (in conjunction with AN) reacts with the Aluminum often ending with an unintended detonation. Additionally, as high temperatures are required to ensure that all of the oxidizer is dissolved, self heating type explosions have been known to occur with both slurry and emulsion explosives and blasting agents [33]. Also, while slurry explosives may be less sensitive than dynamites to shock, they will most certainly fail fragment impact due to the shock properties associated with the fragment impact test and the fact that slurry explosives are cap sensitive and therefore, Class 1.1 explosives. Blasting agents are those based entirely on non-explosive ingredients and are insensitive to commercial detonators or blasting caps and require boosters to detonate them. Nitramon was the first slurry blasting agent explosive. One grade consists of simply AN and paraffin and was thus similar to ANFO except for use of a solid rather than a liquid fuel and a metal container. The aluminized slurry blasting agent, or SBA, was discovered in 1956. TNT slurry explosives, which were discovered in 1957, gained immediate commercial importance by virtue of excellent reproducibility and ease of manufacture. SE-TNT is singularly useful in high pressure applications, i.e., deep well and underwater blasting. SBA-Al and SE-TNT/Al with high Al contents (up to 35%) are some of the most powerful of all commercial explosives.
  • SBA-fuel types are the lowest in density and strength of the slurries but also have the lowest ingredient costs. They are therefore useful mainly as toploads and for whole-column loads in relatively soft rock. They are also less sensitive than slurry explosives to shock. SE and SBA have captured much of the open-pit blasting market because they are singularly applicable in very hard rock as bottom loads and in all types of rock under difficult water conditions. High loading densities (relative to ANFO) and bulk strengths characteristic of SE and SBA, permit lower drilling costs, higher blasting efficiencies, and better rock fragmentation. Savings are affected therefore in labor, drilling, digging, hauling, crushing, and grinding. Scientific field-cost comparisons between SBA and ANFO conducted in even dry, soft rock where ANFO works best have sometimes even favored SBA [31]. However, slurries still suffer from a number of drawbacks. Due to the high water content of slurries, they have much lower densities that TNT based compositions. This translates to lower performance. Also, as stated previously, according to the inventor of slurry explosives, aluminized slurries and slurry blasting agents should not be stored for more than 1 year at ambient temperature due to the reaction between AN, water and aluminum. Additionally, continuous heat dissipation is necessary when manufacturing even non-aluminized slurries and emulsions to prevent self heating and detonation of slurry and emulsion explosives and blasting agents [33]. Slurry explosives are still considered shock sensitive (Class 1.1) and both slurry explosives and slurry blasting agents are 350% to 550 % lower in performance (with regards to detonation pressure) than the target goal. Development of Emulsion Explosives Summary The commercial explosives industry produces some I.5 Million metric tons of explosives each year in the United States, Traditionally, this has been made up of three types of explosives: 1. Nitroglycerin dynamites, which may be gelatinous, granular, or intermediate, depending on the ratios of solid and gelled liquid components, 2. Dry blasting agents, which are essentially dry mixtures of ammonium nitrate and a variety of fuels, including hydrocarbons and aluminum metal. ANFO, a mixture of porous AN prills and fuel oil, is by far the most extensively used industrial explosive. 3. Slurries and Emulsions, which are gelled or emulsified aqueous solutions of ammonium nitrate and other nitrate salts with undissolved salts, carbonaceous fuels, and sensitizer ingredients such as finely divided aluminum and molecular explosive ingredients such as TNT. By common usage the term emulsion explosive is taken to mean a high-internal-phase-water-inoil (or invert) emulsion of a concentrated solution of nitrate salts in water emulsified into an oil base. Current developments are aimed at resolving such practical problems as stability and prevention of crystallization, as well as attaining specific design objectives relating to cost, rheology, and detonation parameters.
  • Before proceeding to a discussion of the science and technology of emulsions, we must first digress to point out some of the pertinent features of explosives. Generally, the explosives of commerce consist of an intimate mixture of condensed oxidizers, almost invariably the nitrate salts of ammonia, sodium, and calcium, which are mixed with a fuel and usually other additives. The purpose of the latter is to control rheology and to provide the correct reactivity, physical form, and density to ensure reliable detonation. Such mixtures are used in three physical forms for three different methods of loading into drilled holes: as prilled solids which are poured or augured into boreholes; as pumpable liquids which are loaded from a truck in bulk directly into the ground; and in the form of stiff pastes or gels which are packaged into paper or plastic tubes and sold by the case. The latter packaged explosive type is by far the smallest tonnage of the three but represents the highest value. It is also the most technically sophisticated, as it must meet requirements of shelf life, resistance to handling, immunity to changes in temperature, etc. Emulsion explosives are also characterized by a number of unusual scientific features. Their detonation capabilities and particularly, their high velocities of detonation depend on maintaining a very Intimately mixed, all-liquid system. In practice, this requires droplet sizes on the order of 1 micron. Since water proves to be an effective detonation inhibitor, the desired sensitivities are achieved by controlling water levels in the formulation. Finally, the requirements of redox stoichiometry create a situation that can be resolved only by the use of a large volume of highly supersaturated (and therefore metastable (and sometimes unstable)) salt solution, finely dispersed in a small volume of hydrocarbon oil. As In most other industrial explosives, AN is the major ingredient, about 70% of the over all composition. The dispersed aqueous phase, which constitutes over 90% by weight of the liquid fraction, and generally contains other salts such as sodium and calcium nitrates, is emulsified into 7 to 10% of oil or oil/wax phase. Additional solid ingredients (generally under 15%) include glass or plastic microbubbles, aluminum powder as an energetic fuel, and some quantities of the particulate salts. A variety of other ingredients, i.e. sensitizers, may be added to perform specific functions. Stability requirements are particularly demanding for the packaged versions of these products and must withstand wide ranges of temperature for many months in storage and transportation without significant crystallization, particle growth or emulsion breakdown. All failures of the emulsion systems lead in turn to loss of the required explosive properties. Storage time is limited to 1 year at ambient temperatures. Emulsion Formulations In 1961, Egly and Neckar [34] of Commercial Solvents Corp. disclosed that a water-in-oil (W/O) emulsion of concentrated metal nitrate solutions in paraffin oil could be made by the use of a fatty acid-based oxazoline and that this emulsion material had utility in an explosive. The discovery does not seem to have been followed up, however, and the next advance was made by Atlas Powder Co. [35,36], in their patents of 1966-1969, where nitric acid wall emulsion into an oil phase to produce an explosive, albeit one of a rather corrosive nature. In a landmark patent of 1967, Bluhm [37] showed that commercially usable explosives could be made by forming a water-in-oil emulsion of about 90% internal phase (v/v) from a hot nitrate solution, a paraffinic oil or wax, and a number of emulsifiers, the preferred being fatty acid derivatives of sorbitol (Span or Arlacel type emulsifiers). With the addition of some gas microbubbles to lower the density (thereby ensuring detonability), such a system formed a very simple explosive. The stoichiometry of the reaction between ammonium nitrate and a Hydrocarbon is: 3nNH4 NO3+ (CH2) n  nCO2 + 7nH20 + 3nN2 Equiv. wt. 240:14 Equation 1.2-1
  • The stoichiometric ratio requires 17 times as much ammonium nitrate by mass or about 10 times as much ammonium nitrate solution by volume as oil. From the time of Bluhm’s invention, improvements only have been made to his original formulation. Although such improvements are essential for the commercial acceptability of the products they leave the basic concept unchanged. An emulsion explosive, then, is conventionally formulated with three components that influence the emulsion: aqueous phase, emulsifier, and oil phase. As has been mentioned, other additives, which have no real effect on the emulsion structure, are added to make these compositions detonable, and these will be described later. Aqueous Phase The aqueous phase makes up some 90% or more of the formulation and conventionally consists of a melt of ammonium nitrate, sodium nitrate, and calcium nitrate with a little water to lower the melting point, A typical composition contains from 10 to 20% water and is usually made up so that the composition is close to a eutectic point. For example, most packaged explosives are made from an ammonium nitrate, sodium nitrate/water melt of composition approximately 73/14/3.3 [4,5], although each manufacturer has a preferred ratio. Such a melt has a crystallization point (more properly called “fudge point”) of around 70°C and therefore must be heated before inclusion Into the emulsion, It is one of the startling features of this system that in a properly formulated emulsion explosive the droplets of this melt phase remain liquid down to temperatures as low or lower than -20°C. That they can be subjected to such supercooling is attributable to the large degree of subdivision, which ensures that the number of droplets far exceeds the number of heterogeneous nuclei present. It is essential for the proper functioning of emulsion explosives that these droplets remain liquid, and a great deal of research has been devoted to means of preventing crystal growth and its, propagation through the emulsion. Not all emulsions are made from such highly supersaturated solutions. Products to be loaded In bulk generally contain more water and even large amounts of calcium nitrate, which lower the crystallization temperature, making manufacture and handling simpler and sometimes lowering the detonation sensitivity of the product [40,41,42] . Some emulsion explosives have even been formulated with no water [43,44,45] at all, employing a eutectic mixture of metal and ammonium nitrates with urea. Oil Phase Early in the evolution of emulsion explosives it was realized that the oil phase (continuous phase) played a highly important role in stabilizing these systems. Indeed, Bluhm's early patent [37] calls for the use of highly refined (highly paraffinic or naphthalenic paraffin oils and waxes in order to make stable products, and Brockington [46] and Wade [39,47] showed that the use of such refined waxes was essential to obtaining long-term stability under a variety of conditions. A number of types of waxes have been claimed to be effective in this system, including microcrystalline waxes [39,47], insect waxes [47], candelila [48], polyolefin [49], and paraffin
  • [50]. Since stability is less of a concern in bulk delivered explosive, it has proved possible to make bulk products from less refined materials [51] or even waste products. There are also some indications in the patent literature that packaged products can be made similarly [52]. Storage limitations are still generally restricted to less than 24 months. Many Japanese authors exemplify their patents with formulations containing unpurified microcrystalline waxes [53]. Yorke et al. [54] have shown recently that a wide range of crude or partially refined petroleum products can give results as good as (or even better than) the refined products commonly used. Petroleum products can also be replaced totally by other oils, such as epoxidized soya bean oil [55]. Occasionally, the hydrocarbon oil/wax emulsions are insufficiently reactive for some industrial applications and some researchers [56,57,58] have attempted to employ more energetic oil phases. In particular, nitroglycerin, TNT, nitropropane, and di-nitrotoluene have been examined. It has proved impossible to make formulations of sufficient stability using these oils, and commercial products have not yet proved to be feasible. However, higher performance will undoubtedly lead to higher sensitivity. Surfactants Surfactants In emulsion explosives have two roles to play. They’re primary function Is to form and stabilize the high internal phase W/O emulsion (≈ 90% v/v Internal Phase) at processing temperatures. However, once formed, these systems are cooled, supercooling the internal phase. This requires that the emulsifier also be capable of maintaining this supermodel states which implies that the surfactant does not nucleate the nitrate salts. Also, the surfactant must prevent those droplets that are inevitably nucleated from crystallizing the rest of the emulsion. Many materials have been claimed to be effective for this task and they cover an enormous range of types and hydrophile-lipophile balances (HLBs). Stability of Emulsion Explosives Emulsions usually “fail-safe,” i.e., when stored for an excessive period of time they become almost impossible to detonate. This is an important safety feature; however, the manufacturer must ensure that products will still function at full performance after the maximum time they are likely to be stored in a customer’s magazine. Stability is related to this requirement, as described below . However, on more than one occasion, aluminized emulsion explosives have been known to spontaneously react and either burn, explode or detonate due to breakdown of the emulsion droplets and exposure of aluminum to water containing concentrated oxidizer [33]. Because or the presence of supersaturated droplets, emulsion explosives are an inherently metastable system. As noted above, this supersaturation may break down completely if the emulsion is cooled to a sufficiently low temperature. With many commercial formulations a spectacular demonstration of nucleation can result from a sharp blow to the emulsion. The product nucleates because of the shear, it crystallizes, and the temperature rises rapidly. The material hardens noticeably within minutes. Emulsion explosives
  • having large droplets are less sensitive and have larger critical diameters than those having smaller droplets. The fuel/oxidizer reaction is slowed by an increased diffusion length. Coalescence therefore leads to a deterioration in detonation properties. Also, large droplets have an increased chance of containing a heterogeneous nucleus, and therefore higher crystallization probability. It should be realized that crystals are not necessarily confined to their original droplet size and shape but can break through the films separating droplets. The patent to Bluhm [37] is generally regarded as the first to describe all the components together in the form of a W/O emulsion explosive as it would be considered today. The stability was stated to be "good" with no phase separation, however, the importance of super-saturation seems not to have been fully understood. In packages, the stability has been found to be inadequate and improvements have been required. Cameron and Cooper [58] described emulsions stabilized by an alkyd polymeric surfactant, while Nippon Oil and Fats [59] describe an ethylene oxide-propylene oxide copolymer. Wade [39] discovered increased stability when a blend of microcrystalline wax and paraffin wax is used, rather than either alone. Takeda et. al. [60] claim high stability for emulsions containing a high-melting-point wax which contains a high proportion of urea non-adduct component. Yorke et al. [54] showed the advantages of using a crude hydrocarbon mixture (e.g., slackwax) of a wide molecular weight distribution. The monomeric surfactant used may have important effects on stability. Sudweeks and Jessop [61] claim habit modification of any crystals formed in an emulsion containing a C14 to C22 fatty amine or ammonium salt as an emulsifier. This effect limits the growth and size of the crystals. The same authors [40] claim that cationic surfactants having unsaturated lipophilic chains give substantially improved stability over their saturated counterparts. Similarly others [62,63] have shown that phosphates are capable of crystal habit modification. Binet et. al. [64] claim increased stability in emulsions containing at least one conventional water-in-oil emulsifier and at least one amphipathic synthetic graft, block, or branch polymeric emulsifier. The increased stability was ascribed to the highly ordered and stable film formed in the presence of the polymeric emulsifier and this assertion was supported by calorimetry measurements. Detonation Properties of Emulsion Explosives The W/O emulsion of concentrated nitrate salt solution in oil is not, in itself, an explosive. To It must be added gas bubbles and (optionally) metal fuels or solid nitrates in order to produce an effective commercial explosive. The addition of metal fuels is simply to increase the power of the explosive. For example, the addition of aluminum results in about 100 kJ/kg of product for each 1% of aluminum incorporated. The addition of solid nitrates is used to reduce the cost of these materials. The presence of gas bubbles is necessary to ensure the detonability of the emulsion. These bubbles provide a mechanism (bubble collapse, jetting, adiabatic compression), for the conversion of the energy in the detonation shock wave to the heat energy necessary to produce chemical reaction and, therefore, sustain the detonation. Emulsion explosives differ from most other explosives of commerce in that they rely only on these bubbles for their sensitivity. Other, more traditional explosives have depended on chemical sensitizers as well. It is therefore not
  • surprising to find that type and distribution of these bubbles is critical in determining the detonation performance of emulsion explosives. A review of this effect has appeared from Nippon Oils and Fats [65,66]. The traditional void material is glass microballoons, chosen for the combination of good emulsion stability, high velocity of detonation (Vd), and resistance to static pressure effects. Glass microballoons amount to 30% of the raw material cost of a small diameter emulsion, and commercial introduction of a wide product range of emulsions may depend on at least partial replacement by some other void material. A large number have been tested: mechanically entrained gas [37], chemically generated gas [67], perlite [39], inorganic foam and expanded polystyrene. Perlite is advantageous regarding costs, but the small-diameter products have low Vd values and limited stability. The use of perlite in intermediate diameters would seem to be acceptable. The effect of void size on the detonation properties of emulsions is now well studied. Small voids (<30 µm) yield products of high detonation velocities but low sensitivity, the product behaving as pseudohomogeneous. Large voids yield products of lower detonation velocities but high sensitivity and are more like the classical hot spot model. Use of a large proportion of voids over 75 µm in diameter offers the possibility of detonation cord-sensitive products without added sensitizers. Gas bubbles and microballoons show ideal behavior in emulsion; that is, the velocities of detonation observed approach those predicted for thermodynamic-hydrodynamic computer codes. However, the coarser voids, such as perlite and polystyrene, produce highly non-ideal detonations in this type of analysis. It is not only the size (type) of void that is important in determining the detonation characteristics of emulsion explosives; there is also an effect of the quantity of these voids (i.e., the density of the product). With increasing density there are fewer hot spots per unit volume and a lower extend of reaction, which leads to a falling velocity of detonation. In the limit, this results in failure of the detonation at high densities. The "ideality" of non-aluminized emulsion explosives (i.e., the fact that their detonation velocities approach those predicted by theoretical models) is one of their attractions to the industry. These materials detonate with high velocities (i.e., very rapid reactions) and a large proportion of their thermodynamic energy contributes to the detonation. This is different from other commercial explosives, where much of the available energy is produced in slow burning reactions following the detonation. In certain blasting applications, this high velocity is an advantage producing high detonation pressures (brisance) and shattering the rock well. In summary, emulsions are of interest to the explosives industry because they offer the possibility of producing all types of products with one simple system. In particular, small diameter cap sensitive explosives can be formulated without added explosive sensitizers. The sensitivity is a function of the degree of contact of fuel and oxidizer and of the supersaturation, which ensures that a large amount of energy is concentrated in the liquid phase, where reaction is rapid. This Is in contrast to slurry explosives (which consist of a sensitizer, a continuous oxidizer solution phase, solid ammonium nitrate, and a gelling agent), where the oxidizer is present in solid form and where chemical reaction is about an order of magnitude slower. In emulsion explosives complete reaction can occur in about 1µs. In slurry explosives, complete reaction usually requires a considerably longer time.
  • Because of the supersaturation, emulsions maintain their cap sensitivity almost unchanged down to low temperatures (-20ºC or better). On the other hand, in slurries, crystallization from the liquid component occurs with falling temperature. As a result, there is a drop in the energy of the fast-reacting components and a decrease in sensitivity. By adjusting mixing and formulation variables, the detonation properties of these materials can be varied widely, further adding to their utility. Limitations of Emulsions Deficiencies of emulsions are as follows: Aluminized emulsions also suffer from the same long term storage problems associated with slurries. However, due to the instability of the emulsion itself, typically the emulsion breaks down and massive crystallization occurs before aluminum begins to thermally react with the AN and water. Additionally, due to the high temperature associated with ensuring high salt density, the same self heating concerns exist with emulsions as with slurries. Finally, and most importantly for the purposes of this project, emulsions still (by virtue of the relatively high concentration of water (which both decreases density and reaction temperatures), suffer from poor performance characteristics relative to the project goals. Emulsion explosives have a detonation pressure 360% below the target goal while emulsion blasting agents have a detonation pressure 500% below the target goal. Development of PBX Explosives Plastic Bonded Explosives (PBX) is a term applied to a variety of explosive materials which are characterized by high mechanical strength (above 10,000 psi compressive strength), good explosive properties (usually > 7800m/sec detonation velocity (Objective # 1)), excellent chemical stability, insensitivity to handling and shock (to extremes of from 10% to 40% above basic explosives (Objective #2)), and high thermal input insensitivity with the average autoignition temp > 250 ºC (Objective #2). These explosive mixtures contain a large percentage of basic explosives such as RDX, HMX, HNS or PETN in intimate mixture with a polymeric binder such as polyester, polyurethane, nylon, polystyrene, various types of rubbers, NC, or teflon. In some instances a plasticizer such as di-octylphthalate (DOP) or butydinitrophenylamine (BDNPA) is included in the ingredients as well as a fuel such as Al or Fe powder for blast enhancement. Plastic Bonded Explosives are also waterproof (Objective #3), are easily boostered (Objective #6) with low critical diameters (Objective #7) and high pressures, are non-TNT based and have excellent long term storage characteristics (Objective #10). However, while PBXs are shock insensitive to some degree, they are still susceptible to a number of other types of shock (such as projectile impact and slow cook-off) and therefore, while more shock insensitive than TNT based explosives, are not the final solution. [68] Much of the advantage stemming from the use of PBX compounds is in the simplicity of the technique of the end item manufactured. About half of the developed PBX compounds are used to directly cast end items such as for the lunar seismic experiments. The first attempts to desensitize RDX were reported by Frankel and Carleton [69-73] who made use of polymeric materials such as polyurethanes to coat explosive crystals by means of emulsion or solution techniques. The first true PBX was developed in 1952 at the University of California and consisted of RDX coated with polystyrene plasticized with DOP [74].
  • Lunar seismic explosives were evolved during the 1960's and early 1970's using Teflon as the binder [75,76] There are more than several PBX compounds developed outside of the USA or by USA nonmilitary organizations which are of interest and are presented here. The PBX developed by Wright [77] is a molding powder prepared by mixing a water dispersion of a binder such as a polyacrylate, a plasticizer such as paraffin oil, with a water slurry of an explosive such as RDX and a coagulant such as ethanol. Hard, well-formed granules are produced which contain 9098% explosive. Subsequent pressing at 25000 psig and 49ºC, yields pellets with a compressive strength of 10000 psi. In another explosive patented by Wright [78] much the same procedure is followed using, in this instance, polyhexamethyleneadipainide as the binder and diaminotrinitrobenzene (DATB) as the explosive . The product contains 90-98% explosive with a bulk density of 0.60.7g/cm3. Sato [79] reported the development of a PBX series using PETN and a combination of Epikote 871 (an epoxy resin) co-polymerized with diethylenetriamine in percent ratios of 70/30, 65/35 and 60/40 (explosive/binder). The compression strength of this explosive series averages 60 kg (F)/cm2. Vacek and Skrivanek [80] invented a series of PBXs using an aqueous dispersion of poly-vinyl acetate binder, PETN as the explosive, with dibutylphthalate (DBT) as the plasticizer in a typical wt % ratio of 36/60/4. A typical detonation rate is 5200 m/sec using a 28 mm charge diameter with a density of 0.92g/cm3. A self-supporting PBX series has been developed by Minekawa et. al. [81] which can be fabricated in plate, strand or tape form for use in metal forming by impulsive loading. The compound can include polyethylene as binder, a paraffinic hydrocarbon as plasticizer, and an explosive/oxidizer combination. The development of PBX compounds using a polyester as the binder and RDX as the explosive has been reported by Reichel [82]. Evans [83] invented a PBX series with compounds using, for example, PETN as the explosive, a polyethacrylate rubber as the binder, and dibutylphthalate as the plasticizer in wt % ratios of 6085/10-40/0-10. A typical product detonates with a No 6 electric blasting cap at a rate of 8180 m/sec. In another PBX series, that of Butler et. al. [84], β-hydroxyethylacrylate is the binder, ethylene glycol is a copolymer, benzoyl peroxide is the catalyst, while LiCIO4 is the oxidant, and RDX or HMX is the base explosive . A typical wt % ratio is 20/13/1/9/57. An invention of Kegler et al [85] has increased the pot life (before molding) of PBX compounds containing 5-15% polyester binder by several days through the use of gelatin microcapsules to enclose the benzoyl peroxide. The catalyst is released during the pressure molding operation. Frankel et. al. [86 & 87] developed a series of nitrofluoroalkyl epoxides (epoxy ether explosives) which are heat and impact stable and are used as binders with RDX (80%) as the explosive. A flexible, self-supporting PBX was patented by Rothenstein [88] which uses fine particulate explosive in an admixture with low density prepolymers. For example, using hydroxyl terminated polybutadiene (HTPB) as the binder, RDX as the explosive, toluene di-isocyanate
  • (TDI) as a co-polymer, sym-di (2-naphthyl)-p-phenylenediamine as an antioxidant, and polybutene as the plasticizer in a weight % ratio of 12.1/42.7/5/0.2/40, producing a castable explosive. Several manufacturing procedures are currently used to produce PBXs. One of these techniques is that of casting. This procedure, at first glance, merely involves combining a dried explosive , such as RDX, with binder constituents and curing initiators in a mixing vessel, blending to desired homogeneity followed by casting. Unfortunately, there are hazards associated with the drying of large quantities of explosives such as HMX or RDX. Hence, a desensitizing procedure must be added for production scale operations. This procedure involves coating the HMX or RDX (which is normally water wet for storage and shipping), with the alkyl or polyester portion of the binder. The resulting lacquer is added slowly to an aqueous slurry of the HMX (or RDX). Agitation at approximately 250 rpm in the presence of water causes the resin to precipitate onto the surface of the HMX (or RDX), producing an insensitive powder which may be safely dried, handled, shipped, and stored until ready for use in the final PBX compound. The pre-coated explosive is then combined in a mixing kettle with sufficient copolymer to constitute the final binder compound. An accelerator (such as co-naphthenate) is added at this point. The mixture is then stirred until homogeneous, at first under ambient pressure, and finally in a partial vacuum to remove entrapped air. The curing agent (such as methyl ethyl ketone peroxide) is then added. After another short mixing period (≈15 min) the explosive compound is poured into the desired molds. Vacuum casting has been found necessary to obtain optimum density. Plastic Bonded Explosives are used wherever high mechanical strength, high energy, high heat insensitivity and relatively low shock sensitivity are required. Unfortunately, there was no PBX available at the time that this research began which met the project objectives as defined in Section 1.4. However, the high performance PBX composition known as PBX-109 was used as a baseline PBX composition, although this composition is considered quite sensitive and is a Class 1.1 explosive. 1.3 Deficiencies of Modern Explosives Modern day commercial explosives, principally ANFO, slurries and emulsions, still have a number of deficiencies as follows. First and foremost, for the purposes of this project, commercial explosives have relatively low performance. As stated earlier, higher performance translates to a lower amount of explosive required to perform a job and increased safety. ANFO as well as typical slurry/emulsion explosives/blasting agents have detonation pressures from between 350% to 775% lower than the target goal of 240 kbars ± 10%. Due to the relatively low densities (i.e. 0.82 for ANFO) and/or presence of water, performance characteristics of these explosives are much lower than the project goals. Table 1.3-1 compares a number of characteristics of slurry, emulsion and ANFO explosives with H-6. Slurry, emulsion and ANFO explosives and blasting agents have a number of other deficiencies other than low performance [Objective #1]. Slurry and Emulsion explosives are still somewhat sensitive and are classified as Class 1.1 explosives because they are cap sensitive [Objective #2]. Due to the presence of water, emulsion and slurry explosives/blasting agents do not function well below temperatures below freezing (due to the presence of water) [Objective #7]. Also, due to the presence of water and/or hygroscopic nature of ingredients, typical shelf lifes
  • are less than 2 years at best and can be as low as a few months [Objective #9]. Objectives 4 (Steam Kettle Processability) and 5 (Less Than 24 Hour Cure) are not applicable due to entirely different manufacturing methods. Therefore, of the 8 applicable goals, half of the goals are not met with slurry or emulsion explosives/blasting agents or ANFO. Table 1.3-2 details Project objectives and compares Emulsion and Slurry Explosives/Blasting Agents, ANFO and H-6 with regards to the project objectives. 1.4 Project Objectives and Approach Project Objectives The project objectives were to develop an energetic material with the following characteristics. First, an explosive must perform in order to complete the designed task (i.e. break up of rock formations). As a goal, therefore, very high performance (comparable to the explosive H-6) was of primary importance. While the performance of H-6 is very high (relative to typical commercial explosives) it would not be very difficult to reduce performance (once achieved) so that insensitivity could be improved. Additionally, there is a significant amount of data available on H-6 which could permit its use for comparative purposes. Secondly, the explosive should be insensitive to accidental initiation. The explosives developed (assuming they qualified for large scale testing) were subjected to the most difficult shock and thermal test standards known. Third, it would be desirable for the explosive to be waterproof in order to permit blasting in areas where water is present (i.e. limestone blasting (dead coral) in low lying levels such as South Florida). Fourth, in order to take advantage of surplus explosive manufacturing equipment, the energetic material must be processable in standard TNT steam kettles. The closing of numerous government owned explosive processing facilities in the US (and subsequent conversion to private industry usage) has allowed enormous amounts of equipment to become available to commercial explosives manufacturers. It was therefore a primary goal to develop an energetic material which could be processed in existing TNT type processing equipment and which would be self curing or would solidify once cooled (in a manner similar to TNT). By meeting these goals, commercial energetic material manufacturers would be more likely to manufacture the energetic material and make the energetic material commercially available. Fifth, In order to reduce time and cost, the explosive must be curable in less than 24 hours Sixth and Seventh, in order to for the explosive to be easily initiated, the energetic material must have a low critical diameter and sufficiently shock sensitive to permit initiation with standard boosters Eighth, since TNT itself continues to be responsible for a high number of explosive accidents, development of an insensitive non-TNT based energetic material (with comparable cost and performance) would result in a significant safety improvement to the public.
  • Ninth, since most commercial explosives only have a shelf life of 1 year or less before they either become ineffective or become very sensitive. Indeed a number of aluminized slurry formulations have detonated on their own due to the slow reaction between water and aluminum. Most emulsion explosives are not sufficiently stable to ensure reliable performance after 1 year. Therefore, it was a goal of this project to develop an explosive with a shelf life of 10-20 years. Tenth, although, not a primary goal, in order for commercial explosive manufacturers to find the energetic material financially attractive, it would be desirable for the explosive to have low raw material cost with high raw material availability. Therefore, it was a goal to develop an insensitive energetic material which could be produced for a cost not more than 2-3 times more expensive than typical commercial explosives. However, cost should never take priority over proper application of explosives, particularly when safety may be affected. It is important to note that the price of the explosives is only one factor to consider in evaluating the cost of blasting. Inexpensive explosives that do not serve the purpose for which they are intended are no bargain. High-energy explosives may have a higher initial price, but savings in drilling, loading, and mucking the rock may offset this cost [89]. Additionally, the costs associated with explosive accidents (in terms of loss of and injury to human life, as well as monetary losses) far outweighs the additional costs which may be associated with the production of safer explosives. The project objectives are summarized in Table 1-3. Table 1-3 OBJECTIVE NUMBER 1 2 3 4 5 6 7 8 9 10 Summary of Project Objectives OBJECTIVE DESCRIPTION High Performance Low Sensitivity Waterproof Steam Kettle Processability (Viscosity < 2kp) Less Than 24 Hour Cure Boosterable with Standard Boosters Low Critical Diameter Non-TNT Based Ten to Twenty year Shelf Life Low Raw Material Cost and High Raw Material Availability Project Approach The Project Approach is detailed in Figure 1.4-1 (Theoretical) and 1.4-2 (Research). The theoretical approach for the project consisted of the following format (Figure 1.4-1). First, a thorough literature review occurred to determine the state of the art in the field of Insensitive High Explosives.
  • Second, a thorough theoretical analysis of the raw materials (Molecular Explosive, Reactive Metal Powders, Binder and Oxidizer) occurred from predicted sensitivity (based upon literature review and prior experience), performance (based on thermochemical calculations), cost/availability (based upon vendor quotes), cure property (based upon literature review and prior experience) and processing standpoints (based upon packing fraction calculations). Third, additional thermochemical calculations were conducted to optimize potential compositions. Fourth, theoretical solids particle size distribution and content was calculated to optimize processing, viscosity and solids loading. The research approach for the project consisted of the following format (Figure 1.4-2). First, small scale (450 gram) energetic material processing and testing (for sensitivity and performance) occurred. Second, once the processing, sensitivity and performance characteristics were defined and determined acceptable, scale-up to intermediate scale (4.5 kg) mixing occurred followed by additional performance and sensitivity testing. Third, once the intermediate scale processing, sensitivity, and performance characteristics were defined and determined acceptable, scale-up to large scale (23 kg) mixing occurred followed by large scale performance and sensitivity testing. Fourth, once the interim project objectives were achieved, then the project was terminated. If the interim project objectives were not achieved, then energetic material reformulation would occur as required. 1.5 References 1. Cook, Melvin A., “The Science of High Explosives”, Reinhold Publishing Corp., NY, 1958 2. E. I. DuPont de Nemours & Co., Inc., “Blaster’s Handbook”, Willmington, DE., 14th Edition, 1966 3. Fedoroff, Basil T. and Sheffield, Oliver E., “Encyclopedia of Explosives and Related Items”, (Vol. 3), Comptown Press, Inc., Morristown, N.J., 1966 4. Taylor, J.T. and Gay, E.M., “Development of Modern British Coal Mining Explosives”, 5. Assehton, R., “History of Explosives,” 2nd Edition, Institute of Makers of Explosives, NY, 1940 and Private Communication 1997. 6. Robinson, C.S., “Explosives, Their Anatomy and Destructiveness”, McGraw-Hill Book Company, NY, 1944
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