Addition of RDX/HMX on the Ignition Behaviour of Boron-Potassium Nitrate Pyrotechnic Charge

Abstract

Boron-potassium nitrate (B-KNO3) (25/75) is a well-known pyrotechnic composition which
finds application as energy-release system for small-calibre rockets and pyrogen igniters for
larger motors. The decomposition of the oxidiser in this composition is endothermic which can
be activated by the addition of high explosives, which decompose exothermically. This paper
describes the influence of two nitramine explosives, RDX and HMX, on the ignition characteristics
of B-KNO3 composition using thermogravimetry, differential scanning calorimetry, heat and
pressure output measurements. Different compositions were prepared by varying the amount of
RDX/HMX from 10 per cent to 50 per cent. Thermal studies on the B-KNO3/high explosive
mixtures reveal that these undergo two-stage decomposition. The first stage corresponds to the
decomposition of high explosive and the second stage corresponds to that of the reaction between
B and KNO3. Kinetic parameters were calculated for both the stages of TG curves using Coats-
Redfern and Mac Callum-Tanner methods. Ignition temperature of B-KNO3 decreases on the
addition of RDX/HMX while the onset of RDX or HMX decomposition is not significantly affected
by B-KNO3. The pressure output of B-KNO3 increases on adding RDX/HMX. The heat output
of B-KNO3 is not much affected by the addition of RDX or HMX, even though the heat of
explosion of RDX and HMX are low. This is due to the reaction between the combustion products
of RDX/HMX and reaction products of B-KNO3 to form more exothermic products like B2O3,
releasing extra heat. The flame temperature of the charge increases while the average molecular
weight of the products of combustion decreases as the RDX/HMX content increases. Thus, the
charge, on addition of RDX or HMX, produces higher pressure output, maintaining the heat
output at comparable levels.