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Void Size Measurement in Emulsion Explosives: A Noninvasive Approach Using NMR Imaging V. Mohan Rao* and P. K. Ghosh*,# ICI India R&T Center, P.O. Box 155, Thane-Belapur Road, Thane 400601, India V. Vijayaraghavan and N. Chandrakumar CLRI, Adayaru, Chennai 600020, India Received September 13, 1999
Introduction The importance of voids or gas bubbles in initiation and propagation of detonation in a secondary explosive is wellknown.1 Under the sudden impact of a shock wave these voids collapse adiabatically giving rise to hot spots. These hot spots initiate the process of combustion in the explosive matrix leading to detonation.2 Both the population of voids as well as their size distribution are important in a detonation process. While bubble population is important for ensuring the continued propagation of detonation, bubble sizes in the range of 100-200 µm are crucial for initiation.3 Thus the optimum incorporation of voids in an explosive matrix is of considerable commercial importance as evident from the proprietary literature on the subject.4 Consequently, techniques for measurement of void size distribution in the explosive matrix are of interest to ensure the quality and performance of explosives. Voids can be incorporated into the explosive matrix both in the form of particulate material such as glass microballoons, perlite, or expanded corn, or can be generated in situ by conducting a chemical reaction involving ammonium nitrate and sodium nitrite to produce nitrogen gas.5 The latter process is of particular interest because it is inexpensive and offers the potential to vary the size distribution of voids. The two most common secondary explosives are the watergels6 and concentrated waterin-oil emulsions.7 The soft consistency of the explosive matrix makes it difficult to measure the gas bubble sizes using conventional microscopy techniques due to the easy escape of voids during sampling and artifacts arising from perturbation of the voids. As part of an overall program of work on concentrated emulsions,8-13 we have reported in an earlier paper11 the * Author to whom correspondence should be sent. # Present address: CSMCRI, Bhavnagar, 364002, Gujarat, India. (1) Bowden, F. P.; Yoffe, Y. D. Initiation and Growth of Explosion in Liquids and Solids; Cambridge University Press: London, 1952. (2) Mader, C. L. Phys. Fluids 1961, 8, 1811. (3) Cooper, J.; Leiper, G. A. J Energ. Mater. 1990, 7, 405. (4) For example, USP 4940497, 3886010; Canadian Patent Appl. 2093309, 2113945; UK Patent Appl. 2179035; World Patent WO 89/ 02881 and references therein. (5) (a) Ridd, J. H. Q. Rev (London). 1961, 15, 418; (b) Jocelyn Pare, J. R. Indian J Chem. 1981, A20, 1116. (6) US Patent 4141767. (7) US Patent 3447978. (8) Das, A. K.; Ghosh, P. K. Langmuir 1990, 6, 1668. (9) Mukesh, D.; Das, A. V.; Ghosh, P. K. Langmuir 1992, 8, 807. (10) Das, A. K.; Mukesh, D.; Swayambunathan, V.; Khakhar, D.; Ghosh, P. K. Langmuir 1992, 8, 2427. (11) Swayambunathan, V.; Mukesh, D.; Krishnan, S.; Ghosh, P. K. J. Colloid Interface Sci. 1993, 156, 66. (12) Khilar, K. C.; Khakhar, D. V.; Ghosh, P. K. J. Colloid Interface Sci. 1992, 153, 578. (13) Mohan Rao, V.; Ghosh, P. K. Proceedings of 27th International Annual Conference of ICT, Karlsruhe, FRG, 1996, P-131.
Figure 1. SEM pictures of (a) sample A containing no chemically generated gas bubbles. About 2% voidage is caused due to physical entrapment during mixing. A lone gas bubble could be seen here. (b) Sample B containing chemically generated gas bubbles; voidage is about 11%.
use of freeze fracture scanning electron microscopy (FFSEM) as a new technique for measurement of void sizes in such systems. While the technique allowed us to generate reliable data on the effect of various manufacturing parameters on void size, it was not possible to extend it to routine use because of the special needs of sample preparation including the need to prepare the sample stub prior to the onset of gassing in the matrix. We report here a new noninvasive technique based on proton magnetic resonance imaging. The technique has the potential to map voids in explosive cartridges, although in its present form only small quantities of material can be subjected to such analysis. Experimental Section The model system employed in the study was a water-in-oil type concentrated emulsion. The internal phase of the emulsion contained 36% NH4NO3, 17% Ca(NO3)2, 6% NaNO3, 14% NH4Cl, and 21% H2O. The external phase consisted of 4.5% paraffin oil and 1.5% emulsifier.11 The oil and the surfactant were preblended and placed in a cylindrical vessel and heated to 80 °C. A homogeneous solution of the internal phase was prepared by heating the ingredients to 80 °C. The internal phase was added gradually to the external phase under agitation and mixing continued for obtaining an emulsion having an approximate
10.1021/la991205s CCC: $19.00 © 2000 American Chemical Society Published on Web 02/19/2000
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Figure 2. NMR imaging pictures of (a, left) sample A and (b, right) sample B. viscosity of c.p. 9.5 × 105 at 80 °C at a shear rate of 0.125 s-1. Details of rheological measurements are provided elsewhere.10 For emulsions subjected to gassing, the emulsion matrix was cooled under stirring to 70 °C and an aqueous solution of sodium nitrite (0.05 wt. % with respect to the total emulsion) was added and mixing continued for a further 1 min to ensure uniform dispersion in the matrix. The matrix was immediately loaded into NMR tubes (5 mm diameter) as otherwise the voids tend to distort and escape during the filling process. The process of filling required the application of a gentle vacuum so that the emulsion could flow into the tube. Ample care has to be taken to ensure that there are no air pockets entrapped during the process as this could lead to serious errors in the results. The filled NMR tubes were plugged at both ends and immersed in the bulk emulsion matrix to ensure that the gassing profile in the tube mimics that of the bulk matrix. Two samples A and B were prepared. While sample A contained no chemically generated voids (an estimated 2% voidage is however present due to physical entrapment during mixing), sample B contained 11% voidage. Voidage is defined as percent volume occupied by voids in the matrix and is experimentally determined by float-and-sink density measurements.11 1H NMR spectra were recorded on a Bruker MSL 300 P FT NMR spectrometer. 1-D spectra were recorded initially to ascertain peak positions and relative intensities. For chemical shift selective imaging (CSSI) experiments,14 a 5 mm 1H RF insert was used on an 8.9 cm Bruker MSL 300 P FT NMR system with an actively shielded microimaging probe head. The field of view was 8 × 8 mm with a nominal slice thickness of 3 mm, resulting in a resolution of 63 × 63 × 3000 µm. The spectral width was 50 000 Hz and the number of scans was 16. The echo time was 10.05 ms and the repitition time was 1020 ms. For spin-echo imaging,14 a nominal slice thickness of 3 mm was used with an 8 × 8 mm field of view and a 128 × 128 data matrix size. Freeze fracture scanning electron micrographs were obtained by the technique described previously.11
Results and Discussion SEM pictures of samples A and B are shown in Figure 1. Sample A is not expected to contain gas voids except for those due to physical entrapment during mixing. One lone void could be seen in the photograph shown. Sample B showed considerable presence of voids as is expected. Measurement of void sizes in the photographs revealed the size range to be mostly 50 to 200 µm with very few having 400 to 500 µm diameter. (14) Callaghan, P. T. Principles of Nuclear Magnetic Resonance Microscopy; Clarendon Press: Oxford, 1991.
Three peaks were obtained in the one-dimensional NMR spectrum, at 5.76 and 3.42 ppm with respect to the highest peak assigned to 0 ppm (chemical shifts 6.77, 4.43, and 1.01 ppm respectively, measured with external benzene). These peaks correspond to the ammonium ion, water, and external phase (paraffin oil and emulsifier). The high field peak was an unresolved doublet with low intensity while the other two peaks were of higher intensity and comparable to each other. However, in CSS imaging, the high field peak turned out to be more intense than the other two. The 3 CSS images, obtained with sample A, had comparable information. This implies that there is no molecular level spatial inhomogeneity in these samples on this distance scale. Sample B, on the other hand, did not permit useful CSS images to be obtained, owing to poor signal-to-noise in this sample, compounded by an initial long shift selective pulse. To permit comparison of the voidage of both samples, therefore, we settled on the chemical shift nonselective spin-echo experiment. The images obtained are shown in Figure 2. As can be seen, this technique was found to give improved image clarity and demarcation of void boundaries. Sample A showed hardly any voids, much along the lines of what was seen in SEM pictures. Sample B showed voids in the similar size range as shown in SEM photographs. The trend was thus similar to what is seen in SEM pictures in Figure 1. Future work will aim at further improvements in the resolution of the NMR technique so as to permit quantification of the void size distribution. The technique will also need to be improved so as to enable larger samples such as actual explosives cartridges to be analyzed via this noninvasive technique. In conclusion, we have demonstrated through our preliminary studies the feasibility of NMR imaging of voids in concentrated emulsions. Quantitation of void size information and ability to subject much larger samples to such investigation will open up the possibility of using the technique for both understanding of shelf life and sensitivity of the explosives which was hitherto not possible due to nonavailability of noninvasive techniques for void measurements. Acknowledgment. We thank Mr. S. Krishnan for the assistance in carrying out the experimental work. We thank ICI India for their support. LA991205S
