Effect of Ammonium Perchlorate Particle Size on Flow, Ballistic, and Mechanical Properties of Composite Propellant

Composite propellant based on hydroxyl-terminated polybutadiene and ammonium perchlorate (AP) has become the workhorse propellant for modern-day missiles and space vehicles. AP is the main ingredient and is used as an oxidizer in composite propellant and accounts for approximately 70%–80% of the composition. AP plays a vital role in tailoring the burning rate of the propellant using multimodal particle size distributions and provides strength to the propellant as filler. AP is ground to different particle sizes for use in propellant formulations to achieve different burn rates and higher solid loading. Grinding of AP leads to generation of a large surface area with excess surface energy and there is a tendency for agglomeration, segregation, caking, bridging, and no flow in silo/bin/hopper, stockpile, feeder, chute, conveyor, etc. The propensity of the problems increases with increase in the time gap between grinding and mixing operations (with all propellant ingredients). Here, various grades (particle size fractions) of AP were used in propellant formulations and characterized with respect to physical and flow properties. Physical properties that were studied were particle size, shape, density, and moisture. Particle size distribution was determined using the sieve analysis and laser light scattering technique. Powder flowability was measured using shear strength, angle of repose, and tapped-to-bulk density measurements. The values of Hausner ratio and Carr index are highest for ultrafine AP, indicating that it is a highly compressible powder, whereas Hausner ratio and Carr index are lowest for the coarse AP, which indicates its free-flowing behavior.