Air Blast TNT Equivalency for Rolls of Paper Toy Caps
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An earlier version appeared in: Journal of Pyrotechnics, No. 21, 2005. Air Blast TNT Equivalency for Rolls of Paper Toy Caps K. R. Mniszewski, FX Engineering, Inc., Hinsdale, IL, USA and K. L. Kosanke, PyroLabs, Inc. Whitewater, CO, USA No quantitative information had been produced ABSTRACT by the primary investigating agencies regarding A study of the explosive output of rolls of paper the expected explosive output of bulk quantities of toy caps, in variously sized assemblages, was toy caps, and a literature search was unsuccessful conducted. The testing has shown that toy cap in locating such data. As part of the continuing rolls are clearly capable of producing a powerful accident reconstruction effort, an estimate of the explosive effect if initiated with a sufficiently en- effective amount of energetic material involved in ergetic event. TNT equivalencies based on toy cap the explosion was sought. The technique used was composition mass ranged from approximately 10 to determine the TNT equivalence of various to 80% for different sized configurations, with the quantities of the toy paper cap rolls. As with any largest equivalences being produced by the larg- condensed phase explosion, a number of effects est assemblages of toy caps tested. The results of are produced, including the production of an air this study are disturbing, considering that the toy blast wave, fireball, ground shock and projectiles. caps (even in bulk packaging) have a UN classifi- To estimate the yield of such an explosion, the cation of Explosive1.4S, which by definition most useful measure is the air blast wave. This should not produce significant blast or fireball paper reports on that study. effects when initiated. Thus perhaps it is appro- priate to consider whether the UN test protocol is adequate for this product. Paper Toy Cap Materials Toy cap composition is typically composed of Keywords: air blast, TNT equivalence, toy caps, Armstrong’s mixture, generally consisting of ap- Armstrong’s mixture, UN test proximately 67% potassium chlorate, 27% red phosphorus, 3% sulfur and 3% calcium carbonate by weight.[1] To form the toy caps, the composi- Introduction tion is prepared wet and extruded onto a strip of paper as a series of tiny dots, which are then lami- A few years ago, an accident occurred in a toy nated over with another thinner layer of paper and factory in California. Several workers were killed wound into rolls. The dry mixture is extremely and others were injured when a number of bulk sensitive to accidental ignition[1–2] and, even in cases of rolls of paper toy caps exploded with small quantities (1 gram), is reported to have sig- great violence, sufficient to produce traumatic nificant explosive strength (approximately 23% amputations of worker’s limbs. The workers in- TNT air blast equivalent when initiated using an volved were repacking the bulk cases of toy caps electric match).[3] at a workstation using a blister pack machine. A number of enforcement and regulatory agencies Careful weight audits of sample cap materials were involved in the accident investigation and in this case were conducted to determine the aver- reconstruction. However, while it seemed quite age mass of toy cap composition per cap. This clear that the toy caps were the cause of the acci- was determined through a comparison of: 1) the dent, this was hard to reconcile with the fact that mass of a collection of blank paper dots, taken the bulk cases of toy caps were classed as Explo- from the rolls of caps from the areas between the sive 1.4 S. individual caps using a paper punch; and 2) the mass of a collection of individual toy caps har- vested using the same paper punch that was used Selected Pyrotechnic Publications of K.L. and B.J. Kosanke Page 819 to produce the blank paper dots. The result was an accomplished using a so-called mass-scaled dis- average energetic material content of approxi- tance, Z, defined as mately 1.85 milligrams per cap. R Zf=⋅ (2) The bulk quantities of this particular brand of d M 1/3 cap were packaged 100 caps per roll, 12 rolls per thin-walled plastic tube, 12 tubes per paper pack- where R is the distance between the center of the age, and 100 packages per corrugated cardboard explosive charge and the point of measurement of case. Thus a case contained 1.44 million individu- its output, and fd (called the atmospheric transmis- al toy caps, estimated to contain a total mass of sion factor for distance) corrects for the effect of 2.66 kg (5.86 lb) of energetic material. differing air densities. This atmospheric transmis- sion factor is 13 PT⋅ TNT Equivalence Concept = ao fd (3) PT⋅ A blast wave from an explosion can damage oa structures and injure personnel in the area. From where P and T are the absolute atmospheric pres- an analysis of this damage an estimate of the sure and temperature. The subscript a denotes charge size involved in an explosion can be calcu- ambient conditions at the time of the measure- lated. While complicating factors must be consid- ment, and o denotes the standard conditions of the ered, such as reflections off structures in the area, TNT blast data, specifically 1.013 bars and 288 K the geometry of the charge, etc., the technique is (15 ºC). viable and quite useful.[4] However, when practi- cal, the direct measurement of explosive output is Procedurally, after one determines the peak air blast overpressure for the test explosive charge, it preferred. Since in this case there was a sufficient o (but not abundant) supply of the paper toy caps, is converted to a relative peak overpressure, p /Pa. the direct measurement approach was taken. Then using the data and method of reference 5, the scaled distance, Z, is determined for which a The information to follow is based on refer- standard charge of TNT (i.e., a spherical 1 kg ence 5; however, much the same information can charge of TNT exploded at 1.013 bars pressure [6,7] be found in other standard reference texts. The and a temperature of 15 ºC) is known to produce ability of explosives to cause damage is often stat- the same relative overpressure as did the test ex- ed in terms of its TNT equivalence (E), which can plosive charge. Then, using the value of Z just be defined as the ratio of the mass of TNT (trini- determined, and the values of R, T and P that ex- trotoluene) to the mass of a test explosive that isted for the overpressure measurement of the test produces the same explosive output under the explosive charge, equation 2 can be rearranged to same conditions, specifically solve for the mass of TNT, MTNT, which would M produce the same peak overpressure under the = TNT ETest (1) same conditions as did the test charge. At that MTest point, knowing both the masses in equation 1, the where M is charge mass, E is usually expressed in TNT equivalence can be calculated for the test terms of percent, and a common measure of ex- explosive. plosive output is peak air blast overpressure. In cases where a booster (or initiating charge) Using this technique typically begins with is used, the output from that charge may contrib- measuring the peak overpressure, po, produced at ute a significant portion of the overall explosive a measured distance from a test explosive charge output. When that is the case, it is necessary to of known mass. Then the amount of TNT that account for the booster’s contribution. This can be would be needed to produce the same peak over- done by measuring the explosive output of the pressure is determined using accepted “standard” booster exploding alone and calculating its TNT data for a charge of TNT under similar test geom- equivalence. Knowing the explosive mass of the etry. booster, MB, equation 1 can be used to calculate the booster’s equivalent mass of TNT, M(TNT)B. The comparison between the measured output =⋅ of a test explosive charge and that from TNT is M ()TNT BZM B B (4) Page 820 Selected Pyrotechnic Publications of K.L. and B.J. Kosanke Then in calculating the TNT equivalence for the test charge (less the contribution of the booster), Electric Match with Shroud the booster’s equivalent TNT mass, M(TNT)B, must Acryllic Plastic Tube be subtracted, and equation 1 becomes Flash Powder MM− = TNT() TNT B ETest (5) MTest Hot-melt Glue E-Match Leg Wires Paper Toy Cap Testing Program Figure 1. Sketch of the initiator chosen for use in First, a relatively soft initiator was devised for the testing. testing the cap materials. The intent was to pro- vide a relatively strong shock without producing much in the way of high density fragments that piezoelectric pressure gauges (PCB model could act as flyer plates. The first configuration 137A12) were used to measure the side-on pres- tried was simply to insert an electric match sure from the test devices. The distances to the (Daveyfire A/N 28 B) inside one of the tubes of gauges were chosen to be commensurate with the toy cap rolls. This initiator would have been pre- size of the charges being tested; however, in each ferred because the explosive charge would be a case the far gauge was at twice the distance of the single electric match with virtually zero explosive near gauge, see Table 1. Digital oscilloscope rec- output; however, in three tests this initiator was ords were made of the pressure-time history of unsuccessful in initiating a reaction of the toy each explosion.