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EFFECT OF VOID SIZE ON THE DETONATIONPRESSURE OF EMULSION EXPLOSIVES.Abstract . To study the effect of void size, detonation pressure as well as detonation velocity was measured using PVDF pressure gauge for the emulsion explosives sensitized with plastic balloons of five different size ranging from 0.05mm to 2.42mm. The experimental results were compared with the detonation pressure and velocity calculated using KHT code. ) The experimental results showed that the detonation pressure and velocity were strongly affected by void size, and that the fraction of ammonium nitrate reacted in the reaction zone was strongly dependent on void size.INTRODUCTIONExplosives for civil use, such as emulsion explosives,slurry explosives and ANFO (Ammonium Nitrate-Fuel Oil), are usually ammonium nitrate (AN)-based. Those explosives are well known to show non-ideal detonation behavior clue to its slow reaction rate compared to high explosives for military use.The non-ideal detonation wave propagates steadily but its characteristics are significantly affected by the conditions such as charge diameter and confinement.Detonation pressure and detonation velocity are much lower than those for infinite charge diameter .This behavior is depending on the long reaction zone length due to slow reaction rate of AN. It is well known that detonation behavior can be widely controlled by the size and quantity of void contained in the emulsion explosives. The effect of void size andquantity on the detonation velocity, critical diameter and sensitivity of emulsion explosives were reported in many investigations [1-4].In those studies,glassmicroballoons of size smaller than 0.15mm were used as void In this paper, to study the effect of void size, detonation pressure as well as detonation velocity was measured using PVDF (polyvinylidenefluoride)pressure gauge for the emulsion explosives sensitized with plastic balloons of five different size ranging from 0.05mm to 2.42mm.EXPERIMENTALSSamplesThe formulation of the emulsion matrix used in this study is ammonium nitrate /sodium nitrate /water /wax and emulsifier = 77.66/4.68/11.22/5.40 in weight ratio. The oxygen balance of the emulsion matrix is 0.4g/100g, and the density of it is 1.39gfan. A certain amount of plastic balloons of mono-cell or multi-cell structure shown in Table 1 were added into the emulsion matrix to achieve the desired explosivedensity . Five different sizes of balloons were used . PB-1 is the smallest balloon of mono-cell structure with the average diameter of 0.05mm , and others are the balloons of multi-cell structure with average diameter ranging from 0.47 to 2.42mm.Microscopic photographs of both structures are shown in Fig.l as examples.The particle density of each balloon was determined from the densities of explosives that contain different amount of balloons.The size of balloon was optically measured.Detonation Pressure MeasurementThe emulsion explosive loaded into PVC pipe of 51mm in inner diameter, 60mm in outer diameter and 200mm in length was placed on a PMMA block as shown in Fig.2.A PVDF film of 10 m in thicknessand 5mm squares was sandwiched with polyimide films together with electrodes made of copper foil. The PVDF gauge was put onto a PMMA block and then covered and glued with a PMMA plate of 1mm inthickness . The emulsion explosive was initiated with an electric detonator. Additional 30grams of emulsion explosive was also used as a booster explosive, if necessary. The output of the pressure gauge was recorded with a digital oscilloscope at sampling rate of five nanoseconds.Calibration of PVDF pressure gauge was carried out by measuring electric chaige created under hydraulic pressure and by comparing with the pressure measured with a manganin pressure gauge that has preliminary been calibrated. The pressure profile observed by PVDF pressure gauge is that transmittedinto PMMA plate, which exists among the explosive and a PVDF gauge.Detonation velocity was measured with ionization gaps that were placed at points 130mm and 180mmapart from the upper end of the charge.RESULTS AND DISCUSSIONSFig. 3 indicates pressure profiles observed for the3 emulsion explosives of density 1.05g/cm3sensitized with balloons of five different size. The pressure rises up sharply to reach its peak pressure within about 75 nanoseconds in the explosives sensitized with balloons smaller than 0.80mm in diameter: PB-1, PB-2 and PB-3. The pressure rise time of about 75 nanoseconds can be explained by the detonation front curvature measured by optical observation [5] and shock transition time in PVDF film of 10 mthick . Pressure decrease in the reaction zone behind leading shock and following pressure decay in Taylor wave can be observed in the emulsion explosives sensitized with balloons PB-1 and PB-2 . Whereas the emulsion explosives sensitized with larger balloons such as PB-4 or PB-5 require longer time to reach its peak pressure. This is due to the important irregularity of detonation front of the emulsion explosives containing large balloons, which was measured in optical observation [5]. The peak pressure is fairly low compared with that for the emulsion explosives containing smaller balloons.The pressure observed in this experiment is that transmitted into PMMA plate of1mm in thickness.The detonation pressure was therefore determined from the impedance match method. The Hugoniot of PMMA was based on the reference [6], and that of the unreacted emulsion explosive was supposed to be same as Universal Hugoniot [7],[8] for AN solution.The reaction zone length estimated from the measured pressure profile is about1.5mm for the emulsion explosives sensitized with void of 0.05mm. This value agrees well with the reaction zone length estimated from the diameter effect of detonation velocity in the same emulsion explosives [4] .The theoretical detonation pressure and velocity were calculated with ahydro-thermodynamic code KHT in which the ingredients other than AN were assumed to be completely reactive .The measured and calculated detonation pressure as well as detonation velocity were summarized in Table 2 . The fraction of AN reacted in the reaction zone was evaluated based on the comparison between the measured and calculated detonation pressure and velocity. The fraction of AN reacted in the reaction zone evaluated from pressure agrees well with that estimated fromdetonation velocity. The fraction of AN reacted in the reaction zone is as high as 0.87 for the emulsion explosives sensitized with void of 0.05mm. On the other hand, the fraction of AN reacted is as low as 0.30 for the emulsion explosives sensitized with void of 2.42mm. This result is due to small number of void which act as hot spot, and this leads to longer reaction zone length and poor reactivity of AN. The poor reactivity of AN in the emulsion explosives containing large voids is due not only to the lateral rarefaction waves but also to the rarefaction waves from void itselfCONCLUSIONSTo examine the effects of void size, detonation pressure and detonation velocity were measured for the emulsion explosives sensitized with plastic balloons of five different size ranging from 0.05mm to 2.42mm in average diameter.When void size was increased, the difference between the measured and calculated values was increased both for detonation pressure and velocity. The fraction of AN reacted in the reaction zone was as high as 0.87 for the emulsion explosives sensitized with voids of 0.05mm. On the other hand, the fraction of AN reacted is as low as 0.30 for the large void of 2.42mm.in the case of the emulsion explosives sensitized with void smaller than 0.47mm, pressure decrease behind leading shock and following decay in Taylor wave were observed . Whereas, in the case of the emulsion explosives sensitized with void larger than 1.73mm,detonation pressure rise time is larger than 0.5 microsecond due to the important irregularity of detonation front.。