Indigenous Technology For Tatb: A Thermally Stable, Insensitive High Explosive Having Versatile Applications
Amiya Kumar Nandi Scientist E High Energy Materials Research Laboratory (HEMRL)
Dr Raj Kishore Pandey Scientist 'G', Associate Director
High Energy Materials Research Laboratory (HEMRL)

1,3,5-Triamino-2,4,6-trinitrobenzene (TATB) is an aromatic high explosive of special interest because of its insensitivity, thermal stability (>350C), and respectable performance. The large-scale production of TATB is still an adaptation of Benziger route (Figure 1) from starting raw material 1,3, -trichlorobenzene (TCB)[1].

TATB is a strategic material having applications in the warhead of long range missile, solid rocket propellant, and in Explosive Reactive Armour (ERA). Its import is restricted due to its application in critical defence system. HEMRL, a premier R & D laboratory under DRDO, has adopted Benziger route, and the process has been developed and established at a pilot plant[2]. TATB preparation by the Benziger method consists of two steps: TCB is first nitrated to 1,3,5-trichloro-2,4,6-trinitro benzene (TCTNB), and the product TATB is synthesised in the second step by amination of intermediate TCTNB in an organic solvent with ammonia gas. The results and lessons learned during indigenous development of TATB technology at pilot plant scale are summarised in this report. The application potentials of TATB are also discussed.

TATB Process
The preparation of TATB consists of two-step processes: nitration and amination. The different operations involved in TATB manufacture are summarised in the block diagram (Figure 2). A brief description of these two -unit processes is given below.

Nitration:
TCB is nitrated in a batch process using a mixture of fuming nitric acid (98%) and oleum (20% SO3 content) as the nitrating agent. The reaction is carried out in a glass-lined steel reactor. It is a high temperature (>125C ) nitration process having thermal hazards (runaway behaviour) close to operating temperature. Before scale-up, the thermal hazards were assessed using thermal screening unit (TSU), reaction calorimeter (RC) to define the safe operating parameters[3]. Automatic feeding system of three hazardous raw materials (TCB, nitric acid and oleum), temperature control system of the reaction mixture were developed indigenously and implemented in the pilot plant. The process yield is 90%.

Amination:
Amination of TCTNB is an isothermal, single-feed, semi-batch, gas-liquid, heterogeneous, reaction crystallisation process. The chemical reaction and crystallisation occur simultaneously in this process. The product TATB and the bi-product NH4Cl are formed at the gas-liquid interface. They are insoluble in the solvent selected for the present process, and hence, resulting in co-precipitation of crude TATB (TATB-NH4Cl crystals). Crude TATB is further purified (removal of NH4Cl by dissolution) by hot water digestion.

Unlike other military explosives (e.g. TNT, RDX, HMX etc), TATB is virtually insoluble in most common solvents. Thus, conventional crystallization is found to be unsuitable, and also uneconomical for further purification and for realizing TATB of different particle sizes. Hence, as a part of process development research, emphasis was given to the amination process for producing high purity TATB (low chloride impurity content) of reasonably large particle size (>50 m). TATB of larger particle size gives a higher density, better fluidity, castability and solid loading in high explosive formulations. The chloride impurity in TATB causes compatibility problems in certain ammunitions, and hence, is detrimental to the storage life.

HEMRL has developed a wet-amination method to realise TATB of particle size (>50 m) and low chloride content (~0.5%)[4,5]. An acid-recrystallization method has also been developed to realise chloride-free ultrafine TATB (UF -TATB) from production grade TATB [6].

Effluent Treatment
HEMRL has developed the treatment methodologies for TATB process effluent. The process generates three kinds of effluents: water-based nitration effluent (WNE), organic solvent based amination effluent (OSAE), and water -based amination effluent (WAE). Treatment methodologies for all these three effluent are developed[7]. WNE is neutralised by caustic soda and discharged it to effluent pit, where its natural evaporation resulted in the generation of solid waste, which is disposed of by land filling. Circulating WAE through an activated charcoal column was found effective to reduce the BOD and COD to an acceptable level. Solvent is recovered from OSAE, and the solid explosive waste generated from the residue of OSAE after solvent recovery may be used for preparation of moderately powerful explosive 1,3 ,5triamino-2-chloro-4,6-dinitrobenzene (TACDNB).

Applications of TATB
Application in Defence Sector: In a nuclear fission bomb, the high explosive shaped charges are arranged in a sphere which is called implosion device. Simultaneous detonation of shape charges creates an explosive lens which exert very high pressure at the core where nuclear fissile materials e.g. plutonium, uranium are kept [Figure 3]. This high pressure increases the density of the fissile material resulting in triggering of nuclear chain reaction. TATB is used to make the explosive shape charges for nuclear fission bomb. Its high thermal stability and low impact sensitivity improves the overall safety of the nuclear weapon.

High burning rate Composite Propellant (CP) compositions (AP/HTPB/Al) are generally used in gas generators to eject missile from canister. Because of high burning rate, pressure index of the composition increases during burning which lead to over pressure inside the rocket resulting in rocket bursting/explosion. Addition of TATB (up to 5%) in the standard CP composition is found to be very effective to reduce the pressure index, sensitivity and also improve the overall thermal stability of the CP[8].

Applications in Oil and Gas Industry: The hydrocarbon stock in the existing fields is getting depleted rapidly, and also, the easier targets are becoming scarce. Thus, the searches of oil and gas are undertaken at deeper reservoir at high risk environment i.e. extreme pressure and high temperature [commonly known as highpressure and high -temperature (HPHT) well]. HPHT wells contain high-strength formation rock which is difficult to penetrate. The high pressure means that relatively more hydrocarbon is contained in these field compared with normal pressure field.

High explosive shaped charges are shot in wells with perforating guns in downhole conditions that range from benign to hostile with temperatures approaching 250C or more [Figure 4]. Downhole temperatures limit the choice of high explosive that can be used to manufacture shaped charges. The most commonly used oilwell perforating explosive is RDX, which is limited to temperature 170C for a 1-hr exposure in a carrier gun. HMX (High Melting eXplosive, chemical name cyclotetramethylenetetranitramine) is used for temperature up to 200C for 1-hr. For more hostile wells, shaped charged ] with either HNS (Hexa-Nitro-Stiblene) or PYX [Chemical name 2, 6-bis (pycrylamine)-3, 5-dinitropyridine] explosives have been used. HNS has a 260C, 1-hr temperature rating, and PYX has a slightly higher temperature rating. However, both HNS and PYX high-temperature explosives show poor penetration, and hence, their relative performance is lower compared to lower temperature explosives (RDX and HMX).

A new high temperature explosive, called HTX (High Temperature eXplosive) has been developed for perforating industry. HTX is a high explosive formulation made with TATB and HNS. The high thermal stability, impact insensitivity and VOD (Velocity Of Detonation) of TATB give the HTX formulation high-temperature rating with improved performance. The formulation has overcome the performance disadvantages that exist with HNS or PYX. HTX shaped charges are rated at 260C for 1-hr and have been tested to 225C for 200-hr continuous operation - long enough for most of the perforating operations. Thus, HTX is most suitable explosive for perforation of HPHT wells.