Indian Journal of Chemical Technology Vol. 7, May 2000, pp. 100-104

I e l - !

Process technology development for LOVA! gun propellant - --- --- - - ---- -- ,---_ A G S Pillai, ~ RySanghavi, MS V H Khire, P D Bombe &. J S Karir

( High Energy Materials Research Laboratory Sutarwadi, Pune-411 021, )ndia

Received 25 Octoberl999; accepted 27 March 2000 .-

F "LOV A" gun propellants are formulated with the use of a suitable inert binder and a cyclic nitramine as the energetic

ingr'edient. For the technology development of LOV A gun propellant a suitable manufacturing method was required to be developed. Manufacture of propellant formulation using cellulose acetate and RDX was tried by conventional solvent proc-ess by two different methods. In the first method the fine RDX was first desensitised by the plasticiser coating and the de-sensitised fine RDX was incorporated with the inert binder. In the second method a two stage process technology was adopted. In the first stage, the basic composition is prepared by wet mixing process and in the second stage the dry basic mix is solvent incorporated for extrusion into the required size and shape. The first method was termed as dry process and the second method as wet process. The comparative analysis of the ballistic aspects as determined by closed vessel firing indi-cated that the propellant batches made by the wet mix process gave consistent and reliable results, and has been adopted for the manufacture of'LOVA' gun propellants. ) r

Conventional gun propellants are generally classified based on the chemical formulation into various categories like single base, double base, triple base and nitramine base depending on the major ingredients such as nitrocellulose (NC), nitroglycerine (NG), nitroguanidine and nitramines which are used as major ingredients. For ballistic efficiency gun propellants are manufactured in suitable size and geometry depending on the ammunition in which it is used. These can be degressive burning, neutral burning or progressive burning. In general conventional gun propellants are highly sensitive to impact, friction and heat stimuli, while efforts are on for the development of more and more high energy material s in gun propellant family to achieve improved ballistics for advanced ammunition system . The vulnerability aspect of gun propellant has gained utmost importance since untimely accidental initiation of gun ammunition has caused great concern to am munit ion developer. Hence the necessity of low vulnerab il ity propellant has gained equal importance as in the case of high energy propellant.

Conventional single, double or triple base gun propellants processing is generally carried out by the solvent process. Process conditions and the type of solvent are decided on the basis of chemical formulation and the characteristics of ingredients. NC constitutes the main ingredient in conventional gun propellant and the process conditions to a great extent depend on the specification of nitrocellulose used .

Since conventional gun propellants based on NC and NG are highly vulnerable to initiat ion by spall or hyper velocity impact, inert binder-plasticizer combination along with cyclic nitramine (RDX or HMX) was employed in LOVA gun propellant to make it less vulnerable. To enhance the energy level and to increase the burning rates small percentage of NC is used in LOVA gun propellant formulations'.

Binders for LOV A propellant fo rmulations can be grouped into three processing types--chemically cross-linked (cured) elastomers, solvent processed plastics and thermoplastic elastomers2 The type of binder selected by the authors in the present development was solvent processed plastics (cellulose acetate) plasticised by triacetin .

Energetic components such as RDX or HMX of specific size (5 to 6 flm) are of considerable importance for. "LOVA" gun propellants. It is also important to be able to use RDX or HMX of small particle size in desensitised form for safe processing in a batch mixer or in a twin screw extruder3. Salient aspects of the process technology for LOV A gun propellant by solvent process employing cellulose acetate as inert binder and RDX as energetic ingredient has been explained in this paper.

Aim

Twin-screw extruded technolog/-6 has been reported for the manufacture of LOV A gun propellant. Gallant et aC reported production

PILLAI et al. : LOV A GUN PROPELLANT 101

mrARATlON OF DESEHsITlSED FINE RDX

EXTRUSION (VERTICAL HYDRAULIC PRESS)

CUTTING/GRANULATION (ROTARY CUTTING MACHINE)

DRYING (HOT AIR BLOWING)

Fig. I--Desensitised RDX route (Dry process) for the manufac-ture of LOVA gun propellant

efficiency and safety as the salient aspects of twin screw extruded technology. Hafstrand et al. 8 reported the two stage process for the manufacture of LOV A gun propellant involving precipitation of all the ingredients into a homogenous precipitated product in the first stage and extrusion by twin screw extruder in the second stage. In all these cases cellulose acetate butyrate (CAB) and acetyl triethyl citrate (A TEC) were used as binder and plasticiser respectively. Since raw materials like CAB and A TEC and twin screw extrusion technology were not readily available indigenously it was planned to develop the technology using indigenously available raw materials and existing propell~t process machines. Cellulose acetate and triacc;tin were then selected as promising binder and plasticiser respectively.

Juhasz et al.9 and Pillai et al. 10 reported that LOV A gun propellant in cord geometry realised the pressure exponent (a) value as 1.28, whereas the same propellant formulation in heptatubular geometry realised a much lower a value (reported value is 1.06). Accordingly the technological goal was aimed at the development of the process technology for the manufacture of LOV A gun propellant in multitubular geometry using inert cellulose binder and fine RDX

FINE IlDX MANUF ACTIJRE BY EDUCTION PROCESS

STAGLl

Fig. 2-Two stage wet mixing process route for the manufacture of LOV A gun propellant

by solvent process with the existing process equipments already in use for conventional gun propellants. Vide reference 10 above Pillai et al. have systematically carried out the studies on the effect of RDX particle size on the burning rate of gun propellants and based on their studies which was carried out with a large spectrum of 3 micron to 20 micron size fine RDX as well as exhaustive experiments carried out by the authors in respect of LOVA propellant using fine RDX as the major energetic ingredient it has revealed that fine RDX of about 5 microns particle size is the most promising choice as a gun propellant ingredient to meet satisfactory ballistics.

Experimental Procedure

Cellulose acetate (CA) (secondary) was selected as the inert binder and triacetin (T A) was used as plasticiser. Nitrocellulose of lower nitrogen content (N2 - 12.2 %) was used in small percentage along with the inert binder. Fine RDX of about 5 )..lm particle size was selected as the energetic ingredient. Carbamite served the purpose of stabilizer for nitrocellulose.

Two process methods were experimented and tried out for the manufacture of LOV A gun propellant. As

102 INDIAN J. CHEM. TECHNOL., MAY 2000

Table I

Chemical Formulation

Binder - i) Cellulose acetate - 12 %

(Secondary)

ii) Nitrocellulose - 4 %

(N2. 12.2%)

Plasticiser - Triacetin or - 5.8 %

Acetyl triethyl citrate

Oxidiser - RDX -78.0 %

(Approx. 5 J..lm)

Stabiliser - Carbamitc - 0.2 %

Solvents Percentage

I Pure acetone 19

2 Acetone : Water 19

(92.5 : 7.5)

3 Acetone : Alcohol 19

(80 : 20)

4 Acetone : Alcohol 19

(60 : 40)

5 Acetone : Alcohol 20

(60 : 40)

6 Ethy l Acctate 23

7 Ethyl Acetate : Alcohol 24

(80: 20)

Fig. 3--LOV A propeliant seven holed

already referred at (3) above fine RDX before incorporating into the propellant matrix was required to be desensitised from safety considerations. Accordingly fine RDX desensiti sed by T A coating was used in the first process Inethod. Various stages of this process method are given in Fig. I .

In the alternate method tried for the manufacture, a two stage process was adapted. Ln the first stage of

Table 2

Experiment - I

Dry Process Wet Process

Particle size of fine RDX (J..lm) 5.6 5.6

Propell ant Geometry Heptatubular Heptatubular

Prop. density (glcm3) 1.64 1.66

Force constant (JIg) 1115 1124

Linear burn ing rate coefficient 0. 136 0. 10 (131) (cm/s/MPa)

Pressure exponent (a) 0.95 0.87

Experiment - 2

Partic le size of fine RDX (J..lm) 5.0 5.0

Propellant Geometry I-Ieptatubular I-Ieptatubular

Prop. density (glcmJ) 1.64 1.63

force constant (JIg) 1112 11 08

Linear burning rate coefficient 0.130 0. 100 (PI) (cm/s/MPa)

Pressure exponent (a) 0.97 0.85

Experiment - 3

Particle size of fine RDX (um) 5.8 5. 8

Propellant Geometry I-I cptatubul ar Heptatubul ar

Prop. density (glcmJ) 1.650 1.653

fo rce Constant (J Ig) 1124 111 9

Li near burning rate coeffici ent O. II 0.099 (PI) (cm/s/MPa)

Pressure exponent (a) 0.89 0.85

this process technology, a bas ic compos ition of all the ingredients except carbamite was prepared in the wet stage and finally dried, and in the second stage thi s dried bas ic mix was used to prepare the dough and then extruded into propellant strands. In both the methods of manu facture, incorporation and further stages of man ufacture are similar to the conventiona l solvent propellant process ing. The process flowchart for the manufacture by wet mix route is given in Fig. 2 .

A solvent feas ibility study was carried out for the development of these process methods to finalise a suitable solvent fo r the manufacture and accordingly acetone : a lcohol 70 : 30 ratio was se lected for the process . Chemical formulation and the details of different solvents tried for incorporation are given in Table I .

Finished propellant III heptatubular geometry manufactured by both the process methods were ballistically assessed by c losed vessel fir ing in a 700

PILLAI el al. : LOV A GUN PROPELLANT 103

Table 3-Comparative vulnerability test results for various ingredients

Impact Sensitivity (Height for 50% explosion) cm

Friction Sensitivity (Insensitive upto)

kg

Fine RDX (dry)

T A coated fine RDX

LOVA Prop : Mix

Propellant NQ

NC-NG paste

(28 : 22.5)

RH - 95 %, RT - 27 deg C * = 2 kg falling weight

61.5

90.5

88.5

29

52.0@

65.0*

92.0*

97.5*

29

32.4

28.8

32.4

19.2

\9.2

@ = Test could not be conducted with 2 kg falling weight since the sample is more sensitive hence \ kg falling weight was used for this sample.

Fig. 4--LOV A propellant nineteen holed f

cm3 vessel at 0.2 g/cmJ density of loading. Comparative results of the C.V. firings are given in Table 2. To have the comparative evaluation from the safety consideration of the two process methods and also with the conventional triple base propellant, impact and friction sensitivity of various samples were determined. These results are given in Table 3. Manufacture of propellant by both the process methods was carried out repeatedly to select the suitable process method.

Due to the low burning rate achieved for LOV A gun propellant, it may become necessary to manufacture propellant in very low web size. This will pose problems in manufacture of propellant in heptatubular strand geometry. Manufacture of propellant in 19 holed geometry will resolve thi s problem since the propellant strand diameter will be very much higher than that of heptatubular geometry. Adapting the above mentioned two stage process, propellant in 19 holed geometry was successfully manufactured . Propellant samples manufactured in

heptatubular and 19 holed geometry are shown In Figs 3 and 4 respectively.

Results and Discussion.

Propellants manufactured by both the process methods looked alike with uniformity in dimensional aspects, density, etc. Closed vessel firing results indicated that propellant made by desensitised RDX route (dry process) gave higher burning rate with occasional variation. This may be attributed to the increase in particle size of RDX during the desensitisation process because of treatment with large quantity of alcohol. Variation in the particle size of RDX will affect the burning rate of the finished propellant. Increased burning rate invariably is affected by pressure exponent (a) value also. Higher burning rate resulted in a higher value of 131. The propellant manufactured by the two stage wet mixing process gave consistent burning rates. Lower burning rate was commensurate with the particle size of RDX used in the manufacture.

From the solvent feasibility study it could be concluded that ethyl acetate and ethyl acetate:alcohol mixture are not good solvents as compared' to other solvents. acetone: alcohol solvent mixture of suitable proportion (70 : 30) was selected as a promising solvent for CA binder based LOV A gun propellant manufacture, considering the solvent strength and least solubility for RDX.

The safety test results such as impact and friction sensitivity as reported in Table 3 indicated that the basic composition made by wet mix process in the dry stage is sufficiently insensitive for safe handling during manufacture.

104 INDIAN 1. CHEM. TECHNOL. , MAY 2000

The two stage wet mlxmg process route for the manufacture of LOY A gun propellant is considered to be cost effective, since the use of large quantity of alcohol for the dehydration of fine RDX is avoided. This method is considered reliable for ballistic consistency since the propellant burning rate is found to be not affected by the change in RDX particle size. The process is considered superior from the safety consideration also. Accordingly this process is recommended for adoption for the manufacture of LOV A gun propellant. The technology now developed is close to the one reported earlier8, but for the precipitation of all the ingredients the authors feel that the present technology should be considered superior since under precipitation condition as reported in literature the fine RDX particle size control again becomes difficult and this will affect the

propellant ballistics. In the wet process this is taken care of with no change in fine RDX particle size.

Conclusion

A two stage process method was developed sa ti sfactorily for the manufacture of LOVA gun propellant using CA as binder, which is considered to be safe and cost effective. The whole process was

carried out with the existing machinery in use for the conventional solvent type propellant manufacture .

References

Kirshenbaum M S, Avrami L & Strauss B. J Ballistics. 7 (2) (1983), 170 I

2 Wise S & Rocchio J J, International Jahrestagung. ( 1982) 539

3 Muller D & Helfrich M, Proceedings oj the joint Interna-tional Symposium on compatibility oj plasticizer and other materials with explosive propellants and processing oj explo-sive. propellant and ingredients, April 199 1

4 Gall an t F M, Thomas P & Remmers D, Proceedings oj Joint International Sympusium on Compatibility, Oct 1984, 325

5 David R, J Ballistics, Vol 12, No 3, 233 6 Halfstrand Jorgent , Proceedings oJ the Join t International

Symposium on Compatibility oj plastics and other materials with explosives. propellants and pyrotechnics and processing oj explosives. propellant and ingredients , Apri l 199 1

7 Gallant F M, Thomas P & Remmers D, Proceedings oj Joint International Symposium on compatibility, 325

8 I-Iafst rand Anders & Sandstroem Joergen, Proceedings oJ the Joint International Symposium on Compatibility oj plasticis and other materials with explosives. propel/ant and pyrotech-nics and processing oj explosives. propellant and ingredients. (ADPA publication, Californi a) April 199 1, 205

9 Juhasz i\ A & Rocchio J J, Paper presented at International JaiJrestagung (ICn on lise of plastic materials for propellan ts and explosives, Karlsruk, 1982. 545

10 Pillai A G S, Dayanandan C R, Joshi M M, Patgaonkar S S & Karir J S, DeJence Sci J. 46 (2), ( 1996). 83