Wettability and thermomechanical influence of the photocatalytically depolymerized and epoxidized lignin on the...

Lignin, a renewable aromatic biopolymer, has limited use in commercial epoxy systems due to its complex structure and strong intermolecular interactions that hinder dispersion and compatibility. In this work, lignin was photocatalytically depolymerized prior to epoxidation using epichlorohydrin to enhance its reactivity and dispersion within epoxy matrices. The successful formation of epoxidized lignin (EL) was qualitatively confirmed by the presence of characteristic oxirane and glycidyl ether bands in the Fourier transform infrared (FTIR) spectra. To improve the performance of the commercial epoxy for microelectronic underfill and structural adhesive applications, silane-modified calcium carbonate (CCNP) was also incorporated into the epoxy matrices in addition to EL. The modified epoxy systems demonstrated reduced contact angles on substrates after modification, decreasing from 23.00° to 18.80° on the ball grid array package with 20 wt% EL substitution and from 42.20° to 16.92° on glass substrates with 6 wt% CCNP loading. The improved wettability indicates enhanced interfacial adhesion. Furthermore, the lap shear strength increased by 102%, from 9.85 MPa to 19.88 MPa, when the EL loading was increased to 30 wt% of the resin blend before adding the curing agent. Fracture toughness peaked at 2.62 MPa·m^½ at 10 wt% EL. The thermal conductivity increased by 53% with 30 wt% EL. The simultaneous incorporation of CCNP resulted in a maximum thermal conductivity of 0.392 W·m^-1·K^-1. Both EL and CCNP improved thermal resistance due to increased crosslinking density and restricted chain mobility. The cured EL epoxy surface exhibited crack deflection and improved fracture morphology at low EL loading.