Polyether triazoles: An effective binder for ‘green’ gas generator solid propellants

Abstract

Hydroxyl telechelic binders are extensively used in composite solid propellant to impart structural integrity to the solid rocket motor and for functioning as a fuel. Amongst these, hydroxyl terminated polytetramethylene oxide (PTMO) is extensively used for gas generator applications. The hydroxyl groups are cured using isocyanates to polyurethanes. The incompatibility of isocyanates with chlorine free oxidisers like ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF) warrants use of alternate curing methodologies for processing ecofriendly gas generator solid propellants. For this, propargyloxy terminated polytetramethylene oxide (PTMP) was synthesised by the functional modification of PTMO and cured using an azide bearing polymer viz. glycidyl azide polymer (GAP) by ‘Click Chemistry’ approach to form triazole networks. The cure chemistry was studied in detail by differential scanning calorimetry (DSC). The kinetic parameters viz. activation energy was computed to be 106 kJ/mol, pre-exponential factor was 2 × 1011 s−1and rate constant at 60 °C was computed to be 4 × 10−6 s−1. The cure profile at a given temperature was predicted using the kinetic parameters. Rheological studies revealed that the gel time for curing through the 1, 3-dipolar addition is 220 min against 120 min for curing through the urethane route. The effect of a new alkyne crosslinker on the mechanical properties and efficiency of crosslinking of triazole mediated network was evaluated. The morphological characteristics of the crosslinked network were assessed by scanning electron microscopy (SEM) and Raman spectroscopy. The new binder and cure route provides insensitivity to moisture. The compatibility of the binder with oxidisers ammonium perchlorate (AP) and ADN was evaluated by thermo gravimetric analysis (TGA) according to STANAG 4147 and found to be compatible. Propellants based on PTMP triazole provides superior processability, good mechanical properties and improved ballistic properties in terms of large amount of gas generating species during combustion.