Synthesis and characterization of hydroxyl terminated epoxidized natural rubber as a binder for solid rocket propellant

Abstract

Hydroxyl terminated epoxidized natural rubber (HTENR) was synthesized via oxidative degradation method to produce a low molecular weight rubber intended for use as a binder for solid rocket propellant. Epoxidized natural rubber (ENR) was depolymerized via chemical degradation method using cobalt (II) acetylacetonate (CAA) as the oxidizing agent, followed by treatment with sodium borohydride as a reducing agent to introduce hydroxyl (OH) functional group at both chain ends. The reaction times and the amount of oxidizing agent were varied from 1 h to 20 h and 1% to 5% w/w of CAA respectively in order to obtain the low molecular weight of HTENR. The molecular weight of HTENR was determined using gel permeation chromatography (GPC). The lowest molecular weight of HTENR was achieved at 3% w/w of CAA after 5 hours reaction time at 60oC where Mn and Mw were 34000 g/mol and 62000 g/mol respectively. FTIR analysis confirmed that HTENR contained OH group with the appearance of a broad peak around 3200-3600 cm-1 after the reaction. The presence of OH end groups was verified by NMR analysis with the appearance of two peaks at 3.38 ppm and 3.88 ppm corresponding to methylene proton adjacent to hydroxyl group in HOCH2CH2CH2- and methine proton adjacent to OH group in –CH2CH2CH(OH)CH3, respectively. HTENR with different Mn were used to produce propellant and its burning rate and calorific value were compared with HTPB based propellant as a reference. Although HTENR based propellant produced a slightly lower burning rate compared to that of HTPB based propellant, its combustion energy produced was relatively similar to HTPB based propellant. Therefore, it is expected that HTENR is suitable to be used as a binder in solid rocket propellant.