IL101852A - Method for the purification of HNS I - Google Patents

Description

A METHOD FOR THE PURIFICATION OF HNS I HNS I TiDUi? W>w SUMMARY The invention relates to a method for the production of a 2 , 2 ' , 4 , 4 ' , 6 , 61 -hexanitrostilbene (HNS^EIwith an improved purity from a less pure raw-product with lower bulk density, generally referred to as HNS I. According to the invention, the raw-product is purified through recrystallisation from N-methylpyrrolidone . la Case 2918 IL Div A METHOD FOR THE PURIFICATION OP HNS II This invention relates to a method for the production of a 2,2' ,4,4' , 6 , 6 ' -hexanitrostilbene (HNS) ith an improved purity, from a less pure raw-product, usually referred to as HNS I.

The search for explosives which retain their performance characteristics after being subjected to high temperatures for long periods, for use in for example space technology and natural oil/gas recovery, has focused to a large extent upon 2 , 21 , 4 , 4 ', 6 , 61 -hexanitrostilbene (HNS) as a promising candi- . date. The purified explosive has a melting point of 319°C and exhibits virtually unchanged explosive properties after being heated at 232°C for 200h. Furthermore it is easily manufactured from the readily available 2 , 4 , 6-trinitrotoluene (TNT) according to the process described in US 3 505 413. The product so obtained, HNS I, is not however pure, containing small amounts of TNT, hexanitrobibenzyl , and other impurities, and consequently its properties are somewhat less than optimal. Its melting point is usually around 315°C and it s 1 thermal stability is significantly reduced from that of the purified explosive HNS II.

During the past 20 years considerable effort has been devoted to finding methods for purifying HNS I. All of these methods involve either recrystallisation or digestion of the HNS I using either an organic solvent system or nitric acid, sometimes followed by physical processing of the crystals.

Since HNS has a rather low solubility in most organic solvents, the choice of solvents for recrystallisation is rather limited.

Most work has focused on the use of dimethylformamide (DMF) , 90 % HN03, and to some extent acetonitrile . The use of DMF, alone or in conjunction with acetonitrile, gives HNS in the form of _3 long needles with a rather low bulk density (0.25-0.45gcm ).

I 2 However the handling characteristics can be improved by milling, the crystals being fractured and the bulk density -3 increased to around 0.55gcm . Unfortunately the main limitation of this method is the use of DMF as the solvent ; whilst most of the specification requirements for HNS II are satisfied, the use of DNF almost invariably leads to vacuum test results which exceed the specification limit (<0.6mlg~ for 20min at 260°C the value at 2h is usually within the specification limit) . The use of digestion rather than recrystallisation is also only partially successful. Whilst most of the impurities are removed by this method, the bulk -3 density cannot reliably be increased above 0.45gcm Recrystallisation from 90% H 03 is widely used in the manufactur of HNS II because it is relatively cheap and easy to perform. However it is generally felt that the resulting product is almost always contaminated with nitric acid. E.E. Kilmer (NSWC/WOL TR 78-209; also 75-142) has made a thorough study of this question and demonstrated that HNO^ can be effectively removed from the crystals by proper washing and drying (120°C in vacuo - not in air) . He concludes however that fulfillment of the vacuum test requirements is much easier when organic solvents such as acetonitrile/toluene or acetonitrile/-xylene are used. Kilmer also studied the performance of detonating cord filled with HNS II which had been recry-stallised from either 90% II 03 or acetonitrile/toluene or acetonitrile/xylene , after thermal treatment at 218°C (425°F) , or repeated cycling between -54°C (-65°F) and 177°C (350°F) . With HNS II which contined as little as 0.01% residual HN03 , he observed decreased detonation velocity after repeated thermal cycling, or even after normal storage for 4 years, and decreased detonation velocity (failure after 20h) and chemical degradation (<20 HNS remained after 20h) on heating at 218°C. By contrast, HNS II obtained by acetonitrile/toluene or acetonitrile/xylene recrystallisation, was resistent to thermal treatment viz. no decrease in the detonation velocity was observed after 264h at 218°C, or after 100 thermal cycles.

T e most sat s actory, reporte met o or pro uc ng HN with regard to product specification is that described by L.J. Syrop in US 3 699 176 and US 3 832 142. This consists of continuous extraction process, in which hot acetonitrile extracts HNS from solid HNS I and this solution then passes into a higher boiling, non-solvent for HNS eg. toluene or xylene. The HNS thus separates from the boiling acetonitrile/ toluene or acetonitrile/xylene mixture as HNS II, and the acetonitrile is returned to the extraction cycle. This method -3 gives excellent HNS II with bulk density around 0.5 gem , melting point 319°C, and good vacuum stability (260°C) , but is rather slow due to the low solubility of HNS in acetonitrile.

The present invention avoids the deficiencies of the above reported methods for producing HNS II by using N-methylpyrro-lidone (l-methyl-2-pyrrolidone) as the recrystallising solvent.

The resulting product satisfies all the specification limits for HNS II except for the density, and retains essentially unchanged explosive properties after prolonged heat treatment.

Table 1 HNS purified Starting HNS Specif cation according to (HNS I) for HNS II the invention Chemical analysis: Hexanitrobibenzyl (%) <0.01 22 <1.2 2 , , 6-Trinitrotoluene (%) <0.01 01 Others (%) undetectable 49 0.05 Surface moisture (vrt%) <0.01 <0.05 Solubility in water (w %) 0.02 0.1-0.2 <0.03 Insoluble in DMF (wt%) <0.01 0.09 <0.03 Acidity (Δ Η from control) <-0.2 >-0.2 <-0.2 Bulk density (gem ) 0.81 0.42* >0.45 _„Λ , _o . -1 .o DSC max at 5 in ( 320.5 317.5 m.p. 319 Vacuum stabilit (260 1st 20min (mlg ) _1 54 4.6 <0.6 additional 2h (mlg h 41 2.4 <0.6 * according to DIN 53 194 ** according to WS 5003F N-Methylpyrrolidone has previously been used by E.E. Gilbert as a solvent for the oxidation of hexanitrobibenzyl to HNS (Propellant and Explosives, 1980, 5 168; US 4 245 129; US 4 243 614; US 4 270 012; US 4 268 696), and for the conversion of TNT to hexanitrobibenzyl, replacing THF in the traditional Shipp-Kaplan process (Propellants and Explosives, 1980, 5 15) , but its use as a recrystallising solvent for HNS has not been reported. Although quite structurally similar to DMF, N-methyl-pyrrolidone has almost tv/ice the DMF-solvent capacity for HNS (solubility gHNS/lOOml N-methylpyrrolidone : 4.3g at 10°C, 11. Ig at 100°C, 17.8g at 125°C of 6.1g HNS/lOOml DMF at 100°C) , and gives a product which has good vacuum stability at 260°C (see Table 1) . The latter is the main limitation of DMF.

The solubility data indicate that N-methylpyrrolidone should give recovery yields of 61% and 76% when saturated solutions of 4 101852/2 HNS are cooled from 100°C and 125°C resp. to 10°C. Example 1 indicates that recoveries close to the theoretical are readily attainable. Recovery yields can be increased further by the addition of a solubility reducing cosolvent such as chloro-benzene (Examples 2 and 3; 85.5 and 85% resp.) or toluene (Example 4; 82%), without raising the vacuum stability at 260°C above the specification limit.

The skilled reader of this specification is further referred to the US Patent 4,604,489 which defines another process for the production of HNS-II. According to said process the starting material in the form of HNS-I is dissolved in hot dimethylsulfoxide and the solution thereby received is injected into methanol at a temperature of ° o -50 C to -80 C. No temperature for the hot dimethylsulfoxide is given in the text but said solvent has got a boiling point of about 189° C. The temperature range for this process is thus quite considerable, a fact that might have been one of the reasons why this process, as far as we know, never ha5 been used commercially.

The HNS obtained by recrystallisation from N-methylpyrro-lidone, with or without a cosolvent, is in the form of needles (see Fig 1) whose length to breadth ratio is dependent upon the temperature profile of the cooling cycle, and the rate of addition of the solubility reducing cosolvent when used.

Typical bulk densities are 0.3-0.45gcm -3 , with volume mean diameters (VMD) of 100-250um. As such, the bulk density is below the specification limit for HNS II (< 0.45gcm ).

Example 1 HNS (200g) was recrystallised by didssolution in N-methyl-pyrrolidone (1125ml) at 125°C and then slow cooling to 10°C. The solid was filtered off, was washed with MeOH (2X) and 3%MeOH in H90 (3X) , and then dried. Yield: 147g (73.5%). Bulk density 0.42gcm , VMD 251um. Vaccum test (260°C) : 0.44 mlg after 20min, 0.27mlg-1h_1 after 2h. - 5 - 101852/2 Example 2 HNS (400g) was dissolved in N-methylpyrrolidone (2250ml) at 125°C. PhCl (2250ml) was then added with mechanical stirring during 50min while keeping the temperature at 125°C. The mixture was cooled in air to 65°C and then in ice/water to 10°C. The solid was filtered off, was washed with MeOH (2X) and 3% MeOH in H„0 (3X) , and then dried. Yield: 342g (85.5.%). Bulk density 0.48gcm , VMD 184um. Vacuum test (260 C) : 0.38mlg~ after 20 min, 0.29 mlg~1h~1 after 2h.

Example 3 HNS (400g) was recrystallised from N-methylpyrrolidone and PhCl as in Example 2, except that cooling from 125°C was with ice/water throughout. Yield: 340g (85%). Bulk density 0.44gcm~3, VMD 193um. Vacuum test (260°C) : 0.51mlg-1 after 20 min, 0.12mlg~1h~1 after 2h.

Example 4 HNS (400g) was recrystallised as in Example 3, except that PhCH3 was used instead of PhCl. Yield: 328g (82%). Bulk density 0.30gcm~3, VMD 135um. Vacuum test (260°C) : O.Slmlg"1 after 20min, 0.10mlg~1h~1 after 2h.

Claims (1)

- 6 - Case 2918 IL Claim

1. A method for the production of a 2 , 21 , 4 , 41 , 6 , 6 ' -hexanitro- II stilbene (HNS) with an improved purity from a less pure raw-product, generally referred to as HNS I, comprising recry-stallisation of the above raw-product from N-methylpyrroli-done (l-methyl-2-pyrrolidone) . KJP/Fkb 1992-03-24