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Chapter 4 Explosive Properties of Polyvinyl

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Chapter 4 Explosive Properties of Polyvinyl CHAPTER 4 EXPLOSIVE PROPERTIES OF POLYVINYL NITRATE AND EFFECT OF ADDITIVES THEREON INTRODUCTION Several compounds containing C-NO_ groups (the nitro compxjunds), N-NOp groups (the nitramines), and 0-N0_ groups (the nitrates or nitric esters) are knovm to exhibit explosive properties, i.e. they decompose rapidly producing heat and gaseous products under the influence of suitable mechanical and/or thermal stimuli. Polyvinyl nitrate (PVN), being a nitrated polymer, is expected to exhibit explosive properties depending on its degree of nitration (expressed in terms of % N by weight). The propensity of PVN tovreu'ds explosive decomposition is attributable to the presence of the relatively weaJc N-0 bond (bond energy 53 kcal/mol) in large number throughout the chain. It vras, therefore, considered desirable, in this study, to investigate a few selected explosive properties of PVN which would have a bearing on its practical application, viz. the deflagration (or autoignition) temperature, sensitiveness to impact loads, heats of combustion and explosion, and thermal stability. (Detonation characteristics, like detonation velocity, critical diameter, power, etc, were not included in the study due to lack of experimental facilities). It is intended to investigate the influence of certain variables on the explosive properties of PVN, as, for example, the type (fibrous or gelatinized) as well as nitrogen content of PVN vis-a-vis its deflagration temperature; the type, nitrogen content and sensitizer additive in PVN vis-a-vis its impact sensitiveness; the type, stabilizer additive and its concentration in PVN vis-a-vis its heat stability. Thermochemical properties of PVN were determined, najnely, heats of combustion and explosion by calorlmetrlc experiments, and heat of formation by calculation, In order to obtain an Idea of Its energetic potential. MATERIALS AND METHOD PVN Fibrous Fibrous PVN with different nitrogen content (11.76%, 13.34%, 14.95% and 15.71%) was prepared by nitration of granular PVA (Gohsenol NM-11) at low temperature by varying the concentration of nitric acid and time - temperature conditions, as described In Chapter 3. PVN Gelatinized Fibrous PVN (15.71% N) was gelatinized by dissolving It in acetone. A thick slurry was obtained which was • dried at room temperature by using high vacuum. Additives The following compounds, In CP grade, were used ais additives by incorporating them separately with gelatinized PVN in weight concentrations ranging from 0.25% to 1% : (a) Additives used in 1% concentration for Impact Sensitivity test- (I) Ceu'bon black (II) Lead stearate (ill) Lead salicylate (b) Additives used in 0.25%, 0.5%, 0.75% and 1% concentrations for Stability test (1) Diphenylajnine (li) 2 Nitro diphenylamine (ill) Sym. diethyl diphenyl urea (carbamite) (iv) 1,3 dihydroxy benzene (resorcinol) The following properties of PVN samples, without and with additives, were determined using the staJidEird apparata and 38 procedures : (1) Deflagration (or ignition) temperature and activation energy. (2) Impact sensitivity. (3) Stability (Abel heat test and Vacuum stability test). (4) Calorific and calorimetric Values. All these properties are considered relevant to the potential usefulness of PVN as a high energy material. The experimental procedure (in brief outline) euid results are presented in the following paragraphs. RESULTS AND DISCUSSION / Deflagration temperature and activation energy Deflagration (or ignition) temperature tests of PVN sajnples, with various nitrogen contents, were carried out in a special appeiratus shown in Fig 4.1. It consists of a cylindrical copper block of diajneter 10 cm, which has a cavity of diajneter 2.5 cm ajid depth 7.5 cm, containing Wood's metal (composition : bismuth 50%, lead 25%, tin 12.5% and cadmium 12.5%) as the bath material. Wood's metal has a low melting point of 65 C. The molten metal bath can be heated upto a temperature of 300°C. A thermometer is inserted in the bath to measure the temperature. Approximately 5 mg sample was taken in an aluminium cup of dia 5 mm and height 2 mm. The cup was covered with an aluminium cap. The Wood's metal bath was electrically heated (the heating rate was controlled by a dimmerstat). The bath temperature was monitored by thermometer. When a desired temperature was reached, the current was controlled. The aluminium cup containing the sample was ^•^ ALUMINIUM LID ALUMINIUM SAMPLE CUP METALLIC CONE WOOD'S METAL KANTHAL WIRE 23 SWG COPPER BLOCK ASBESTOS BOX - ASBESTOS WOOL INSULATION Fig 4.1. APPARATUS FOR DETERMINATION OF IGNITION TEMPERATURE (SCHEMATIC) 54 introduced into the bath and a stop watch was started. After a certain time interval the sample ignited or exploded with flaSh and sound and the stop watch was instantly stopped. This reading was noted as the ignition delay or deflagration time lag. In the above majiner, readings were taken at successive temperature intervals of 5C till a temperature was reached at which the ignition delay was less than 5 sec. The temperature corresponding to 5 sec ignition delay is usually referred to as the "ignition temperature" (T ) or "explosion temperature" or "deflagraration temperature". The data on ignition delay corresponding to different temperatures, ranging from 468 K (195°C) to 518 K (245''C), for fibrous PVN (15.71% N) are presented in Table 4.1. Table 4.1. Ignition Delay of Fibrous PVN (15.71% N) at different temperatures SNo Temperature I/Txio"* Ignition delay Log of D K K D (sec) 1. 468 21.4 23.5 1.37 2. 473 21. 1 19.0 1.28 3. 478 20.9 15.5 1. 19 4. 483 20.7 13.0 1. 11 5. 488 20.5 12.5 1. 10 6. 493 20.3 9.0 0.95 7. 498 20. 1 7.5 0.88 8. 503 19.9 6.0 0.78 9. 508 19.7 5.5 0.74 10 513 19.5 5.0 0.69 11 518 19.3 4.6 0.66 similarly, three other sets of experiments were conducted on deflagration temperature vs ignition delay of fibrous PVN with 14.95%, 13.34% and 11.76% nitrogen content at temperature rsinging from 473 K (200°C) to 553 K (280°C). The results are presented in Table 4.2. Table 4.2. Ignition Delay of Fibrous PVN (14.95%, 13.34% & 11.76% N) at different temperatures PVN (14.95% N) PVN (13.34% N) PVN(11.76% N) Temp. 1/TxlO * Ign. Log of Ign. Log of Ign. log of delay delay delay D D D SNo. D D D K K sec sec sec sec sec sec 1 473 21. 1 - - 34.0 1.53 - - 2 478 20.9 14.0 1. 15 27.0 1.43 37.5 1.57 3 483 20.7 13.8 1.14 26.0 1.41 33.0 1.52 4 488 20.5 13.0 1.11 17.0 1.23 27.5 1.44 5 493 20.3 12.0 1.08 12.0 1.08 24.0 1.38 6 498 20.1 9.0 0.95 11.0 1.04 21.5 1.33 7 503 19.9 8.0 0.90 10.0 1.00 18.5 1.27 8 508 19.7 6.5 0.81 9.0 0.95 15.0 1. 18 9 513 19.5 6.0 0.78 8.5 0.93 13.5 1.13 10 518 19.3 5.5 0.74 8.0 0.90 11.0 1.04 11 523 19.1 5.0 0.69 7.0 0.84 10.0 1.00 12 528 18.9 4.5 0.65 6.5 0.81 9.0 0.95 13 533 18.8 - - 5.0 0.69 8.0 0.90 14 538 18.6 - - 4.5 0.65 7.0 0.84 15 543 18.4 - - - - 5.9 0.77 16 548 18.2 - - - - 5.0 0.70 17 553 18.1 - - - - 4.5 0.65 It would be seen from the data given in Table 4.1 and 4.2 that the ignition temperatures (correspxinding to 5 sec delaiy time, as per definition) for the fibrous PVN SEunples are as follows PVN (15.71% N) 513 K (240*C) PVN (14.95% N) 523 K (250°C) PVN (13.34% N) 533 K (260°C) PVN (11.76% N) 548 K (275°C) The ignition temperature range (240-275*'c) for PVN appears to be quite satisfactory for all practical purposes. Activation energy of PVN samples having different nitrogen contents, was calculated by using Semenov equation D = C e^/«T In D = In C + E / RT where, D = Deflagration time lag or ignition delay, E = Activation energy, T = Absolute temperature, C = a constant (depends on the compxssition of material), R = Universal gas constant (1.986 cal/g). The above expression cam be rewritten as log D = B + E/(2.303 x 1.986) T where B is a constant On plotting log D against 1/T, a straight line is obtained. The slope of the plot is equal to E/R, frpm which the activation energy (E) for self-ignition (deflagration) of PVN samples could -4 be found. Plots between log D vs 1/TxlO K for the present work are shown in Fig 4.2 to 4.5 . From these plots the following values of activation energy of PVN samples were obtained : PVN (15.71% N) = 16.076 kcal/mol, PVN (14.95% N) = 13.299 kcal/mol, PVN (13.3% N) = 16.017 kcal/mol. C7 Another standard equation was also used for calculating activation energy of PVN (log D -log D ) E = 4.576 = — (1/T -1/T ) (log Dg-log Dj) TjX Tg or, E = 4.576 (T - T ) where D and D„ are Ignition delays corresponding to the absolute temperatures T.
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