Indian Journal of Chemical TechnologyVol. 1, September 1994, pp. 271-274

Influence of cobalt salts of organic acids on the burning ratebehaviour of double base propellants

S M Pundlik & Haridwar Singh"Explosives Research & Development Laboratory, Pune 411 021, India

Received 23 February 1994; accepted 20 June 1994

Influenceof cobalt salts of aliphatic and aromatic acids has been studied on the burning rates ofNC-NGbased double phase propellants in the pressure range of35-140 kg/crn-. Cobalt-ff-resorcylate has the highestburning rates, followed by cobalt salicylate. Both the compounds show plateau effect in 105-140kg/ern"pressure region. Cobalt stearate is found to be effectivein low pressure region, whereas cobalt benzoate ismost effective in higher pressure. Cobalt gallate is ineffectivetowards any catalytic or plateau effect. Theresults have been explained on the basis of thermal decomposition studies of catalysed double basepropellants.

Lead and copper salts of organic acids are incorpo-rated in double base matrix to achieve enhancedburning rates and low pressure index (n) values.(r= aP~where r is burning rate, Pc is chamber pressurewhile a and n are constants whose values depend onpropellant composition). Propellants with zero ornegative n values are generally temperature andpressure insensitive and, therefore, offer numerousadvantages over conventional double base propel-lants (DBP). Most of earlier studies pertain to leadand copper salts to obtain the desired plateau/mesaeffects 1- 6. Polyvalent metals like manganese, cobaltand iron, with and without polyacetylene have alsobeen studied to achieve super burn rates? Effect ofcertain barium and cobalt salts on the burningrate-pressure relationship of DBP has also beenstudied". Since lead salts inclusion creates environ-mental pollution problems, a systematic study hasbeen undertaken on the effect of cobalt salts ofaliphatic and aromatic acids on the burning rates ofDBP, in order to find out suitable substitutes for leadsalt as ballistic modifier.

Experimental ProcedureNitrocellulose (NC) of 12.2% N2 content made

from cotton linter, nitroglycerine (NG), dinitro-toluene (DNT), carbamite (stabilizer) and di-butylphthalate (DBP) of required purity were obtainedfrom an ordnance factory. Cobalt salts of organicacids were made by precipitation method? -12. Purityof the cobalt salts was checked from percentage metalcontent estimated.

"Author to whom correspondence should be addressed

Cobalt gallate [C6H2(OHhCOO.Oh.2Co wasprepared as reported earlierl3. Propellant strandswere made by the solventless extrusion techniqueinvolving mixing, maturing, rolling and extrusion. Atotal of 30 passes were given to all the compositionsduring rolling for proper distribution of ballisticmodifiers!". The burning rates at various pressureswere determined by the strand burner method innitrogen atmosphere 1 5 • The thermal decompositionof ballistic modifier and propellants was studied withthe help of Differential Thermal Analysis (DT A),model NETZCH (West Germany). The surfacetemperature of propellant samples was determinedwith the help of non-contact IR thermometer,Thermodot, Model TD-7, E2 Thermodot, (USA).

Results and DiscussionThe results on the influence of various cobalt salts,

viz., cobalt sterate, benzoate, salicylate, ~-resor-cylate, and gallate on the burn rates ofDBP are givenin Table I. Cobalt stearate produced higher burnrates in the 35-70 kg/ern" range. However, itscatalytic activity (catalysed burning rate/uncata-lysed burning rate) was insignificant beyond 90kg/em- pressure. n values were lowered to 0.33, 0.23and 0.18 in the pressure range of 70-90, 90-105 and105-140 kg/em", respectively. In case of cobaltbenzoate, burning rates were higher only in 105-140kg/ern? pressure range and n value was reduced to 0.36in 50-70 kg/ern- pressure range. With cobaltsalicylate, high catalytic effect was observed in theentire pressure region studied and burning rateenhancement was of the order of 20-30%. Cobaltsalicylate produced plateau effect in the 105-140

272 INDIAN J. CHEM. TECHNOL., SEPTEMBER 1994

Additive(2 partsper 100)

Table I-Effect of cobalt salts on the burning rates of double base propellantComposition: NC (2R) = 51.0%, NG = 37.0%, DNT = 3.5%, DBP = 5.5%, Carbamite = 3.0%

Cal Catalytic activity at pressure, kg/em- /I value over pressure range, kg/em?Val

caljg 35 50 70 90 105 140 35-50 50-70 70-90 90-105 105-140

Nil 954 0.47 0.74 0.79 0.56 0.35Cobalt stearate 1.03 1.17 1.13 1.01 0.96 0.98 0.82 0.64 0.33 0.23 0.18Cobalt benzoate 920 1.03 1.10 0.97 0.76 1.01 1.05 0.66 0.36 0.81 0.84 0.47

Cobalt salicylate 923 1.32 1.32 1.32 1.31 1.31 1.19 0.49 0.73 0.75 0.56 0.04Cobalt-ff-resorcylate 926 1.55 1.61 1.46 1.34 1.30 1.20 0.57 0.46 0.45 0.34 0.09Cobalt gallate 933 0.75 0.85 0.84 0.86 0.96 0.97 0.81 0.70 0.89 1.30 0.40

kgjcm- pressure range, where n value was reduced to0.04. Results on the effect of cobalt stearate andsalicylate are more or less similar to those reportedearlier", although source ofNC was different (NC oflower viscosity and hence low mol wt used).Cobalt-Bvresorcylate having two hydroxyl groups inthe benzene nucleus produced the highest catalyticactivity and burning rates were increased by 30-60%.Lower n values were obtained throughout, except inlow pressure region of35-50 kg/ern". Cobalt gallatedid not show any catalytic or plateau effect. On thecontrary, burning rates were lowered throughout andreduction of burning rate was of the order of 4-25%.Since catalytic/plateau effect depends on severalparameters particularly concentration of ballisticmodifier, the effect of higher catalyst concentrationwas also studied by increasing from 2 parts to 4 partsper 100 parts of the base composition. The com-parative results are shown in Fig. I. Increase in theconcentration of cobalt stearate reduced catalyticactivity. However, higher concentration reduced nvalue drastically in 70-105 kg/em- pressure range to0.07. In case of cobalt benzoate, higher concentrationgave higher burning rates. in the pressure region(70-140 kg/ern") and reduced n value to 0.30 from0.47. Four parts cobalt salicylate also gave higherburn rates in low pressure region (35-50 kg/ern") andmaintained the plateau effect in 105-140 kg/ern-pressure range. Higher concentration of cobalt-~-resorcylate produced much better catalytic effectthan the lower concentration and increase in burnrates was of the order of 40-80%. Significantly,higher concentration of cobalt-Bvresorcylateproduced mesa effect in 105-140 kgjcm- pressurerange, where n value was reduce to - 0.44. Cobaltgallate did not exhibit appreciable change either inburn rate or reduction in pressure index values.

These results suggest that cobalt salts exert veryspecific action in the combustion ofDBP and each salthas an optimum concentration, where it producesbest results. Cobalt-b-resorcylate appears to be the

best catalyst and platonizer followed by cobaltsalicylate among the various salts studied. Althoughthe calorimetric value of propellant composition waslowered by 50-70 cal/g, the cobalt salts were effectivehurn rate catalyst. This may he due to the fact thatwhile cal-val is proportional to flame temperature,the rate determining process for burn rate is thereactions in condensed phase, particularly the fizzzone.

In order to understand the probable role of cobaltsalts in the combustion of DBP, thermal decompo-sition studies of propellants containing cobalt saltshave been carried out. The results are given in Table 2.It has been observed that cobalt stearate decomposesexothermally in the temperature region of 430-543°C. Cobalt salicylate was found to decompose inthe temperature range of 325-42YC. In case ofcobalt-Bvresorcylate decomposition commenced at300°C and was completed at 360°C. Cobalt gallatedecomposed in an exothermic mode in the tempera-ture range of J60-380°C. These temperatures are veryclose to surface temperature of DBP (300-400°C)and, therefore, their exothermic decomposition, veryclose to burning surface, is likely to increase total heatoutput resulting in higher burn rates. The modifiedpropellants decomposed at higher temperatures(J6YC) than the control (156°C) which is indicative ofhigher thermal stability of modified propellantcompositions containing cobalt salts. The results ofthermogravimetric analysis (TG) of propellants aregiven in Table 3, which indicate that percentagedecomposition of modified propellants was higherparticuJarly in the temperature range of J60-J90°C.Higher percentage decomposition of modifiedpropellant except in the case of cobalt gallate may beresponsible for higher burn rates and thermaldecomposition pattern of cobalt salts also explain thereason of their relative efficiency and ineffectivenessof cobalt gallate as ballistic modifier. In order to findout more evidence in favour or otherwise ofcondensed phase reactions, surface temperature (Ts)

PUNDUK & SINGH: BURNING RATE OF DOUBLE BASE PROPELlANTS 273

18·016·014'0

(/) 12·0E-E 10·0

n 9.0~ 8·0a•.. 7·001.~ 6·0L..

~ 5·0

4' 0 L...---::;-I;:--"'--;:~--=l:::--+--:-:!:=---:-!-=-...J35 50 70 90 105 1 0

Pressure, kg/cm2

Fig. I Comparative effect of cobalt salts (2 parts & 4 parts) on theburning rates of double base propellants [----- Control,-0- Cobalt stearate (2 parts), -.-Cobalt stearate (4 parts).- 0- Cobalt salicylate (2 parts). Cobalt salicylate (4 parts),-~-. Cobalt-~-resorcylate (2 parts). and -A- Cobalt-~-

resorcylate (4 partsj]

Table 2- Thermal decomposition temperature of propellantsQty of sample = 5 mg, Heating rate = IO°C/min, Temp. = 500°C

Additive Exothermic decompositiontemperature. °C

Inception Peak Final

Nil 156 197 243Cobalt stearate 167 192 228Cobalt benzoate 164 189 192Cobalt salicylate 165 185 208Cobalt-f-resorcylate 167 186 232Cobalt gallate 168 194 234

of propellants were determined by IR thermometer.The results are given in Tabel 4. In general, Ts washigher for modified propellants containing cobaltsalts (460-520°C) than the control (435°C). Highersurface temperature for modified propellants is inagreement with earlier finding for lead salts modifiedDBP cornpositions'P'!". The results of this studybring out that cobalt salts can be effective used as burnrate catalyst and pressure index reducer, in place oflead salts.

A number of mechanisms have been suggested toexplain the site and mode of action oflead and coppersalts in the combustion of DBpls-23. A newmechanism for super rate burning and plateau effecthas also been proposed which is supported by a largenumber of experimental facts-". According to thismechanism, a limited amount of carbon is formedduring the combustion of DBP in view of negativeoxygen balance of all practical formulations and the

Table 4-Surface temperature (Ts) of propellantOscilloscope setting = 50 mV/div., Emissivity = 0.93

Additive Surface Maximum C/NOtemp.,"C temp., °C (Weight basis)± 10°C ± 10°C

Nil 435 550 1.4

Cobalt stearate 460 560 1.6

Cobalt benzoate 465 510 1.6

Cobalt salicylate 480 540 1.7

Cobalt -~-resorcylate 490 520 1.8Cobalt gallate 440 560 1.7

Table 3-Per cent thermal decomposition (TG) temperatureof propellants

Sample weight = 5 mg, Heating rate = IO°C/min

Additive Per cent decomposed at temperature, "C

ISO 160 170 180 190 200 210 220

Nil 3 6 13 29 48 73 81 86Cohalt stearate 4 7 IS 33 70 75 83 84Cobaltbenzoate 5 8 14 32 66 76 84 89Cobaltsalicylate 5 8 17 35 71 80 85 90Cobalt+resorcyla te 5 9 18 39 78 84 92 95Cobalt gallate 4 6 12 27 42 72 76 80

amount of carbon is increased in presence of ballisticmodifiers mainly due to the decomposition andoxidation of organic moiety of aliphatic and aromaticsalts. Super rate burning and platonization aredependent on Cj NO ratio in the fizz zone. So long asCjNO is greater than one, super rate burning isobtained and when this ratio becomes unity plateaubehaviour is seen. A similar mechanism is expected tobe operative in case of cobalt salts also. TheoreticalCjNO values for different formulations are given inTable 4. These results indicate that Cj NO, was higherfor modified propellants. Further, it is a known factthat reaction of carbon with hydroxyl radicals ishighly exothermic+ and, therefore, higher' catalyticactivity of cobalt salicylate and cobalt-B-resorcylatemay be due to fast and exothermic reaction of OHradicals present in base acid of these modifiers withcarbon produced due to decomposition of ballisticmodifier. The plateau/mesa behaviour observed witha few cobalt salts in certain pressure region may be dueto CjNO remaining unity in these regions.

ConclusionsI Cobalt salts of aromatic acids (salicylic and

~-resorcylate) produce high catalytic effect(40-80% increase of burn rates).

274 INDIAN J. CHEM. TECHNOL., SEPTEMBER 1994

2 Every catalyst has an optimum concentration,where it produces best catalytic effect and reduc-tion of pressure index values.

3 Propellants containing cobalt salts decompose atlower temperature than the control. Percentagedecomposition of modified propellants was higherin the temperature region of l60-190°C than thecontrol.

4 Surface temperature of modified propellants washigher (460-520°C) than the control (435°C).

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