CHAPTER 2 SURVEY OF LITERATURE ON POLYVINYL NITRATE Polyvinyl nitrate, (C„H 0 N) , is the nitrate ester of polyvinyl alcohol (PVA), which is obtained by hydrolysis of polyvinyl acetate (PVAc). The reaction scheme may be represented as follows: ™2-f - CH^-CfH Hydrolysis OH c=o CH„ Polyvinyl acetate Polyvinyl alcohol Nitration CH^-CfH - 0N0„ Polyvinyl nitrate Polyvinyl nitrate (PVN) is one of the few known polymeric explosives, and bears a formal resemblance to the better known polymeric nitrate explosive called cellulose nitrate or nitrocellulose (NC). Polyvinyl nitrate (PVN) was first prepared in Germany in 1929 by the nitration of polyvinyl alcohol (PVA) with a mixture of 1 nitric acid and sulphuric acid . The product contained only 10% N, against a theoretical value of 15.73°/ N. It detonated when ignited. The work of several investigators on the preparation and properties of PVN has been reported in open as well as patent literatur1,*. ^ e 2-21 A considerable portion of the work has originated in Germany and France. In general, polyvinyl alcohol was used as the starting material, but the composition and experimental conditions were varied with a view to obtaining a better yield and quality of PVN. For example, the methods which were used by investigators for the preparation of PVN Eire summarized below. 30 mesh pulverized (10% moistured) polyvinyl alcohol was slowly added to cold (-8°C) nitric acid with constant stirring, and 2 the temperature was maintained for 3 hrs . The slurry thus obtained was poured into axi equal volume of water with vigorous stirring. After filtering off, the white ppt was washed with water and kept for further 12 hrs in pure water. It was then treated with 95% ethyl alcohol followed by 12% sodium bicarbonate solution. The product was dried in vacuum. It contained 96% PVN with 14.48% N (lower viscosity sample) ajid 13.43% N (higher viscocity sample). The product was soluble in acetone, nitroglycerine (NG), diethylene glycoldinitrate (DEGDN), and insoluble in 95% alcohol. Some workers recommended a classical nitration method (due to simultaneous hydrolysis problem), viz. nitration of polyvinyl alcohol either with a mixture of nitric acid and acetic anhydride or with a solution of nitric acid in carbon tetrachloride, where-by a higher nitrated product, softening at a temperature of 40 - 50°C, 3 was obtained . Nitric acid was directly added to polyvinyl alcohol in inert atmosphere (N or CO ). A product with as high as 15% N 4 5 content was obtained . It has been reported that as polyvinyl alcohol was gradually added to concentrated (>95%) nitric acid, the desired ratio of acid to polyvinyl alcohol was more than 20: 1, otherwise the viscosity of the solution in nitric acid would be 12 very high, maicing it difficult to have an efficient mixing. The temperature was maintained at 0 C for 15 min (for cooling) ajid 15 min for addition of polyvinyl alcohol to nitric acid, 30 min for nitration and 15 min ai'ter nitration at 20°C. The yield of product was 90% with 14.8% N (the degree of nitration was 88.7%). This 5 5 method was not economical . The same investigator has also reported that the nitration of polyvinyl alcohol was best when the concentration of H-SO in the mixed acid was 10-30%. The viscosity of the product, measured by Ostwald viscometer, in 0.5% acetone solution, was found to be 0.45 cps. A 'reverse addition' procedure was followed by some workers for their preparation of PVN in which nitric acid was added to polyvinyl alcohol+acetic anhydride mixture in cold (-5°C) and the reaction temperature was maintained below 20 C. This procedure of adding nitric acid to PVA-slurry permitted greater control, flexibility, and safety. The white product obtained was purified by boiling in frequent change of water. The product was found to contain 14.86% nitrogen . The esterification of polyvinyl alcohol by nitric acid or nitrating mixture has been described in which the nitrogen content and Theological properties of PVN depend on the experimental conditions of manufacture and the degree of polymerization of the polyvinyl alcohol. The yellow white product was thermoplastic in nature with softening point of 30-45°C . A white powder of PVN was obtained with a high degree of nitration and its colour also varied g from white to buff depending on the method of preparation . The o 9 softening point of the product was 30-50 C. Other workers prepared PVN with nitrogen content ranging between 13-15% and found it to be unstable decomposing at room temperature. An extensive preparative method of PVN has been reported by investigators who claimed that 13 the product had a nitrogen content as high as 15.7%. A stabilized PVN has been prepared by some workers who reported that the nitration of polyvinyl alcohol by nitric acid was dependent on the temperature of the reacting mixture, stirring rate and quajitity of occluded acid. A process for preparation of stabilized PVN, having more than 15% N content, produced in the form of free flowing and non-caking 17 pxjwder from the crude product, has been described in a U.S patent . The product was soluble, in all proportions, in acetone, 17 nitrobenzene and liquid nitrate esters Tacticity of polyvinyl nitrate (PVN) depends on the configuration (orientation) of H-atoms and nitric ester (0N0„) groups in space, and, therefore, on the tacticity of polyvinyl alcohol used. The degree of stereoregularity also influences the crystallinity of the polymeric material. Hence, on the basis of spatial arrangement of H atoms and 0N0„ groups in PVN, it can be divided into the following types : (1) Atactic This has irregular or random stereochemical arrangement of hydrogen atoms and 0N0„ groups in the polymeric chain, eg:— H H H H H 0N0_ H 0N0„ I I I I I I ' I I ^ -C— C — C— C— C— C C — C H ONO^ H ONOg H H H H This caji be prepared by nitration of commercially available polyvinyl alcohol. (2) Syndiotactic This has a stereoregular arrangement of H atoms and ONO groups in the polymeric chain, but the alternate ONO groups occur 14 in mutually opposite orientation, as follows H H H 0N0„ H H H 0N0_ I I I 1^1 I I I ^ -c — c — c — c — c — c — c — c — H ONO^ H H H ONO H H (3) Isotactic This also has a stereoregular arrangement of H atoms ajid ONO groups in the polymeric chain, but all ONO^ groups occur on the same side as follows HHHHHHHH I I i I I I I I -c — c — c — c — c — c — c — c — I I I I I I I I H ONO^ H ONO^ H ONO H ONO The above two stereoregular types of PVN have been prepared by some workers using special methods and these are reported to be 18 19 generally superior to atactic PVN ' 20 Some workers followed a method involving nitration of polyvinyl acetate using epichlorhydrin as a crosslinking agent in their preparation of PVN. Determination of nitrogen content in PVN using different methods like Dumas', Kjeldahl's, Nitrometers, Titanium chloride and R in 11 SL'^ 1 Devarda's alloy etc has been reported ' ' . Modified Devarda's 11 method was also employed to determine the nitrogen content in PVN . Because of unreliable results from nitrometer or KJeldahl methods, these have been replaced by modified Dumas' method for estimation 21 of nitrogen content in PVN . Molecular weight of PVN has been 5 7 22 determined by several investigators ' ' using different methods. For exajnple, for PVN (15.15% N), the ultracentrifuge method was 5 - 7 used and Mn found to be 60,000 - 100,000 . Some workers reported 22 - Mw as 200,000 whereas some others reported Mw as 150,000-320,000 15 and Mn as 120,000-140,000. The infra-red (IR) spectrum of thin film of PVN (15.1% N) has 23 -1 been studied by investigators in the extended region of 70 cm to 3600 cm, and polarized infra-red measurements were made on oriented specimens in the range of 350 to 3600 cm . Films were prepared by casting from acetone solution. X-ray diffraction patterns indicated no crystallinity, which suggested atactic configuration and which was also concerned about the local configuration of the nitrate group, i.e. ONO in either rocking or bending mode. It was indicated that although the specimen was not in crystalline portion of the chains, it had a configuration not far from a zig-zag chains. The nature of plasticized PVN has been studied by pulsed 24 NMR spectroscopy . It indicated the presence of rigid and mobile 24 ptolymer phases in mixtures of PVN and dibutyl phthalate . Behaviour of nitrocellulose and polyvinyl nitrate (PVN) in solvents has also 25 been reported PVN is a highly energetic substance capable of detonation under suitable conditions. Explosive and energetic properties of PVN and PVN-containing formulations as well as their likely applications have been reported in literature ' . For 2 exajnple, the detonation of PVN was found to propagate at a rate similar to that of nitrocellulose (NC) of the same N content. The rate of detonation of PVN containing 13.4% N in 30 mm dia card­ board cartridge was found to vary with its density as follows :— Density, gm/cc Detonation Velocity, m/sec. 0.3 2030 0.6 3450-3520 1.0 4920-5020 1.4 6090 1.5 6560 16 2 Also, PVN was recommended for use in double base propellajits e.g. in combinations of NC + NG + PVN or NC + EGDN + PVN.