Hybrid polymers based on bio-based benzoxazines with inorganic siloxane linkage to confer impressive thermal performance

Abstract

Thermal performance of bio-based materials continues to be a vital concern in its path to commercialization and can be alleviated by performing hybridization of organic polymer with inorganic functionalities. The objective of the present work is to hybridize bio-based benzoxazine monomers with inorganic siloxane (Si-O-Si) linkage with an aim to develop hybrid thermosets possessing an impressive thermal performance. Hybrid bio-based benzoxazine monomers have been synthesized via Mannich like condensation of 1,3-bis(3-aminopropyl)tetramethyldisiloxane (APTMDS) with phenols of natural origin (cardanol, eugenol, guaiacol and vanillin) and paraformaldehyde using ethanol as environmentally preferable reaction medium. The structural characterization of the monomers has been performed using Fourier transform infrared (FT-IR) and nuclear magnetic resonance (1H, 13C NMR) spectroscopy. The polymerization behavior of the monomer has been evidenced using differential scanning calorimetry (DSC). In addition, rheological measurements were performed to ensure solvent-less processing of the benzoxazine resins. The contact angle measurements has been performed and surface free energy values have been calculated to study the effect of siloxane linkages on the hydrophobicity of hybrid polybenzoxazine. The influence of siloxane linkages on the thermal performance of bio-based polybenzoxazine network has been investigated by thermogravimetric analysis (TGA) which reveals Tmax to be in the range of 490–570 °C and 49–70% char yield for the thermosets. Furthermore, char yield obtained has been utilized to calculate Limiting Oxygen Index (LOI) value, which is an indicative of flame retardancy and was obtained in the range of 37–45 for thermosets. DSC investigation has been performed in order to study the glass transition temperature of the synthesized materials. Moreover, hybrid resins has been explored for adhesive application using standard procedures.