Inverse vulcanization employing epoxy compounds as crosslinking agents for elemental sulfur in the preparation of sulfur-rich epoxy resins

Abstract

Inverse vulcanization is an effective approach for the utilization of waste sulfur as a feedstock in the preparation of sulfur-rich polymers. The organic compounds employed in inverse vulcanization play a key role in the properties of the sulfur-rich polymers. In this work, epoxy compounds are demonstrated to be effective reagents for inverse vulcanization and synthesis of sulfur-rich epoxy resins. The epoxy-sulfur inverse vulcanization is performed through the formation of thiol-containing polysulfanes via hydrogen abstraction by sulfur radicals and thiol-epoxide addition reactions. Hence, the highest sulfur contents of the resulting sulfur-epoxy resins are majorly dependent on epoxy values (epoxy equivalent weights) of reagents and minorly affected by the miscibility between molten sulfur and epoxy reagents. Aliphatic glycidyl ethers exhibit relatively higher reaction rates than aromatic analogues. The wide scope of epoxy compounds contributes to the high flexibility of the molecular designs of sulfur-rich epoxy polymers. Consequently, the properties of sulfur-epoxy polymers could be tailored conveniently and feasibly. Within 4,4′-methylene bis(N,N-diglycidylaniline) (MDGA) as the reagent, the obtained sulfur-rich epoxy resin (containing 55 wt% sulfur) demonstrates a storage modulus of about 2000 MPa at 50 °C and a glass transition temperature (T_g) above 197 °C, a record-high T_g for sulfur polymers from inverse vulcanization. This work brings about significant progress in the chemistry of inverse vulcanization and preparation of sulfur-rich polymers.