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Chapter 3 Preparation, Characterization And CHAPTER 3 PREPARATION, CHARACTERIZATION AND PHYSICAL PROPERTIES OF POLYVINYL NITRATE For the present study, it vra.s intended to prepare samples of polyvinyl nitrate (PVN) differing in their nitrogen content, so that their properties could be determined and correlated with nitrogen content. In a nitrated polymer like PVN, it is expected that higher degree of nitration would ensure greater availability of oxygen (that is, higher oxygen balance), which, in turn, should improve the performance potential of PVN as a high energy material. Literature survey has shown scanty reports of investigation in this area. MATERIALS AND METHOD Polyvinyl alcohol (PVA) Commercial variety of polyvinyl alcohol, procured from trade and having the following characteristics, was used for preparation of PVN : Designation of PVA : Gohsenol NM-11 Manufacturer : M/S Nippon (Japan) Supplied by M/S Merubeni Corporation, Bombay. Degree of polymerization : 1100 (approx.) Degree of hydrolysis : 99 - 100°/ Viscosity (of 4% aqueous solution at 20°C) : 13-16 cp Volatiles , max : 5% 24 Nitric acid The following three grades of nitric acid were used for nitration of PVA, viz. (a) 98-99% HNO ("Strong nitric acid" or SNA, supplied by High Explosives Factory, Khadki, Pune) (b) 70.7+1% HNO ("Concentrated Nitric Acid", supplied by M/S Glaxo) (c) 63.7+ 1% HNO„ (Moderately Cone. Nitric Acid, prepared locally by diluting above acid). Acetic anhydride Glaxo, Special quality (SQ) grade Density (at 20°C ) : 1.080 gm/cc Preparation of PVN Several small batches of polyvinyl nitrate (PVN) were prepared by nitration of granular or powdered polyvinyl alcohol (PVA) 50 gm, by addition of excess of cooled strong nitric acid (SNA) to PVA at a temperature of -10 + 2°C. The reaction, being highly exothermic and fast, needs careful attention to avoid local ignition of PVA particles. A brief outline of the procedure is given below. SNA (300gm), initially cooled to 0 c, was gradually added to a pre-cooled suspension of Gohsenol NM-11 grade of PVA (50 gm), in acetic anhydride (572.4gm) in a three-necked flask. The reaction mixture was maintained at -10 to -5°C, with constant stirring, for approx 2 hrs (acylation of alcoholic group is then avoided). Then the mixture was kept at -5° to 10°C for approx 2 hrs and at 10°C to 20°C for another 3 hrs, with constant stirring, the final temperature was gradually raised to 20°C, which was required for complete nitration. Thus, a total reaction time of 7 hrs was 25 allowed, after which the yellowish reaction mixture was poured slowly to excess of ice cold water. The resulting white fibrous solid was filtered off . After filtration, the product was washed repeatedly with plenty of hot water (at 60°C) to remove acidity. It was then washed with 12% solution of sodium bicarbonate and followed by washing with cold water to neutrality. After filtering off, the moist product was dissolved in acetone and reprecipitated with 47. solution of sodium carbonate and again washed with water until neutral, and finally washed with distilled water. After drying, the product was dissolved in acetone and the solution diluted with water to regenerate PVN in a pure white fibrous form. The yield of PVN was approximately 86% of theory. Several batches of PVN, prepared by this method, were mixed thoroughly to form a "lot" and used for the present study. The above mentioned method was used for preparation of PVN with relatively high percentage (> 15.0%) of nitrogen. Due to very high reactivity of PVA with HNO , it was very difficult to prepare lower nitrated grades of PVN. Therefore, to obtain different percentages of nitrogen in PVN, starting with 13 gm of PVA in all experiments, some reaction conditions were changed, namely, reaction temperature, reaction time and strength (concentration) of acid. A batch of PVN was prepared by addition of excess of ntrated nitric acid (70,7 + 1% HNO„) into PVA+acetic anhydride conce 3 suspension at a temperature of -10 to -5°C, followed by gradual rise to -5 to 15°C (2 hrs) and 15 to 20°C (2 hrs). Thus, a total reaction time of 6 hrs was allowed. After necessary processing 16.17% yield of PVN was obtained with 14.95% N content. In another experiment, PVN was prepared by addition of 26 strong nitric acid (98-99% HNO ) to PVA+acetic anhydride suspension at a temperature of -10 to -5°C, followed by a regulated rise to 19 C. A reduced total reaction time of 5 hrs was allowed. Yield of 70% PVN was obtained with 13.34% nitrogen content. In another experiment, PVN was prepared by addition of diluted commercial nitric acid (63.7+1% HNO ) to PVA+acetic anhydride suspension at a temperature of -10 to -5°C, followed by a regulated rise to 20°C. A total reaction time of 6 hrs was allowed. 15.4% yield of PVN was obtained with 11.76% nitrogen content. A summary of experimental variables for preparation of PVN lots, which were subsequently used for studies under the present dissertation, is given in Table 3.1. Table 3.1. Reaction conditions for preparation of PVN samples having different Nitrogen content Condition during Condition after Overal1 Nitro Reagents add it;: on of addition of reaction gen in reagents reagents time PVN * Temperature Time Temperature Time hrs. °C hrs. hrs. % PVA + -10 to -5 2 -5 to 10 2 98-99% HNO 7 15.71 10 to 20 3 PVA + -10 to -5 2 -5 to 15 2 70.7+1% HNO 6 14.95 15 to 20 2 PVA + -10 to -5 2 -5 to 8 2 5 13.34 98-99% HNO^ 8 to 19 1 PVA + -10 to -5 2 -5 to 15 2 63.7+1% HNO 6 11.76 o 15 to 20 2 * details given on page 29 27 Before standardizing the above procedure, other possible procedures were also used on the basis of trial and error. For example, addition of polyvinyl alcohol (PVA) into cooled strong nitric acid (SNA), or addition of PVA+ acetic anhydride slurry into to cooled nitric acid, or addition of nitric acid into a suspension of PVA+carbon tetrachloride were tried out. However, the addition of nitric acid into a suspension of PVA+acetic anhydride (which was 18 essentially similar to that metioned in U.S. Patent) was better than other procedures due to the following advantages : better control over the rate of mixing of reactants, avoidance of local ignition of polyvinyl alcohol, use of small excess quantity of SNA, better yield of product, and better quality of product. CHARACTERIZATION OF PVN The above product (PVN) was characterized by following studies: (a) Nitrogen content (b) Infra-red (IR) spectra (c) Scanning electron microscopy (SEM) (d) X-ray diffraction (a) Nitrogen Content Nitrogen content in PVN is a convenient indicator of the degree of substitution (DOS) of hydroxyl groups in the original PVA chain by nitric ester (ONO ) groups. PVA contains secondary alcoholic groups, and when nitrated completely, it should form PVN with a theoretically maximum nitrogen content of 15.73%. It is reasonable to expect that several properties of PVN, especially its energetic and explosive properties, would bear correlation with its "nitrate" content, or, simply, its % N. However, it should be remembered that " % N " indicates only the 28 average nitrogen content in the PVN chains and not the actual distribution pattern of -ONO groups. Amongst 13 "lots" of PVM, prepared for the present work, eight were micro-analysed for nitrogen content only, while the other five were analysed for carbon and hydrogen also. The results showed values of % N varying from 11,76% to 15.73%, as shown in Table 3.2. IK Table 3.2. Elemental composition of PVN Lot No. Carbon % Hydrogen % Oxygen % Nitrogen % I 30.35 3.79 51.88 13.98 II - - - 15.62 III - - - 14.74 IV - - - 14.51 V - - - 14.76 VI - - - 15. 13 VII - - - 15.73 VIII - - - 15.35 IX - - - 15.33 X 27.05 3.42 53.82 15.71 XI 28.38 3.55 53. 12 14.95 XII 31.80 3.97 50.89 13.34 XIII 35.05 5. 16 48.03 11,76 Note (a) Lot I was prepared by adding PVA to SNA + CCl^, and Lot II was prepared by adding PVA + Ac 0 slurry to SNA. Lots III to X and XII were prepared by adding SNA to PVA + Ac 0 slurry. Lot XI was prepared by adding concentrated HNO^ (70,7+1%) to PVA + Ac^O slurry, while Lot XIII was prepared by adding moderately concentrated HNO„ (63.3 + 1%) to PVA + Ac 0 slurry. Lots IV to IX represent mixtures of different batches.. 29 (b) The analysis of PVN samples was carried out partly at National Chemical Laboratory (NCL), Pune and partly at Explosive Research and Development Laboratory (ERDL), Pune. (c) Lot Nos. X, XI, XII and XIII were prepared in sufficient quantities and used for detailed studies. (b) Intra-red Spectra Infra-red spectra of thin film of PVN samples were taken in a Perkin Elmer double beam spectrophotometer. Typical IR spectra of PVN samples containing 15.71%, 13.34%, and 11.76% N are shown in Fig 3.1. The spectra show absorption peaks at the following frequencies which are assigned to characteristic structural po "^c^ '^R features in accordance with literature ' ' Frequency cm Relative intensity Assignments 690 strong -NO , bending 750 strong -NO , wagging 850-•870 broad -NO, streching 1275 very strong -0N0„, bending 1430 strong -CH , bending 1675 very strong -0N0„, streching 1700 weak impurities As can be seen in IR graphs, the absorption peaks due to various structural features depend upon % N in PVN samples.
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