Study on Modified Dealkaline Lignin as Visible Light Macromolecular Photoinitiator for 3D Printing

Abstract

Under natural light, lignin can produce a small amount of free radicals, resulting in yellowing of lignin-related products. However, the activity of these free radicals is very low, so that few studies on exploring the application potential of lignin as photoinitiator (PI) in photopolymerization have been reported. In this study, based on dealkaline lignin (DAL), two alkylated DAL (DAL-11ene and DAL-12ane) were synthesized through facile one-step esterification reactions between DAL and undecanoyl chloride or dodecanoyl chloride. The photoinitiation properties of DAL, DAL-11ene, and DAL-12ane were investigated by real-time FTIR spectroscopy and NIR-photorheology using 1,6-hexanediol diacrylate as a monomer. The formation of reactive species was elucidated by laser flash photolysis and electron spin resonance. The results indicated that DAL itself showed a certain photoinitiating ability, especially when it was combined with an amine co-initiator, while both DAL-11ene and DAL-12ane presented much higher photoinitiating efficiencies compared with DAL. Moreover, DAL-11ene containing a polymerizable group could be bonded into the network of final polymer products, which can increase their biosafety. 3D structures of polyacrylates were fabricated successfully using DAL-11ene and DAL-12ane as PIs through digital light processing 3D printing upon 405 nm exposure, indicating their large potentials in photopolymerization.