FirePhysChem (Mar 2025)
Synthesis of propellant grade HHTPB by hydrogenation of HTPB using Pd-activated charcoal as catalyst
Abstract
To derive more performance from the conventional composite propellant based on hydroxyl-terminated polybutadiene (HTPB) and ammonium perchlorate (AP), it is envisaged to hydrogenate HTPB, which increases the H/C (hydrogen to carbon ratio) of the base polymer. This paper attempts to describe the partial hydrogenation of HTPB using a catalytic method that uses palladium supported by activated charcoal as a catalyst and HTPB polymer as a precursor. HTPB has a hydroxyl value of 41.0 mg KOH/g with a number-average molecular weight (Mn) of 6150, polydispersity (PD) of 2.25 was used as a precursor. Iso-propyl alcohol (IPA) and toluene were used as a solvent media to disperse the HTPB polymer during hydrogenation. The estimate revealed an increase in specific impulse (ISP) up to 1.2 s with ∼37 % hydrogenated HTPB (HHTPB) as binder in composite solid propellants. Partial hydrogenation is only attempted to retain other process capabilities essential for realizing a defect-free solid propellant grain with good structural integrity. In contrast, others attempted to reach near saturation. A reactor capacity of 100 ml, operating at 60 bar pressure, was used to synthesize HHTPB, and the extent of hydrogenation was controlled based on the numerous experiments varying the reactor parameters like operating temperature, operating pressure, and solvent-to-polymer ratio for a given catalyst concentration. Detailed characterization of the end product by FTIR, 1H NMR and 13C NMR to reveal the degree of hydrogenation along with generic polymer characteristics. The measured hydroxyl value of HHTPB is comparable with HTPB; however, a marginal increase in molecular weight and polydispersity was noticed at 37 % conversion, wherein other researchers found a loss in -OH value. Thermogravimetry analysis revealed that no significant change in the gasification rate. A slight increase in the H/C ratio and calorific value was observed for HHTPB compared to HTPB. Despite the increase in viscosity of HHTPB, the increase in the vinyl-type functional distribution of HHTPB aids for better process-ability during propellant processing. However, the propellant formulation should be optimized with the help of plasticizers, solid loading, etc., to achieve the required properties.
