Evaluation of Sulfonium Borate Initiators for Cationic Photopolymerization and Their Application in Hot Lithography

Abstract

Sulfonium hexafluorophosphates and perfluorinated tetraphenylborates are well established initiators in the field of cationic photopolymerization and have already been applied in 3D-printing at elevated temperatures, named hot lithography. The cyanide-ligated borane-based photoinitiator B2 shows lower molecular weight compared to state-of-the-art borates and is less expensive to synthesize from commercial starting materials. By comparing reactivity in the epoxy monomer BADGE, B2 outperforms commercial initiators in a broad range of temperatures with outstanding epoxy group conversions of up to 99%. Besides B2’s excellent performance as a cationic initiator, formulations containing B2 are thermally and storage stable and can be sensitized by anthracene derivatives. Hot lithography of B2 was carried out successfully at 90 °C and compared to existing sulfonium initiators. Layer quality and coloration of the fabricated and postcured parts are superior for B2. Overall, the cyanide-ligated sulfonium salt B2 represents an excellent initiator for cationic photopolymerization and application in hot lithography.