CO₂-derived non-isocyanate urethane bearing carboxyls as a self-etch adhesive monomer

Conventional urethane-based monomers used in dental adhesives are typically synthesized from toxic isocyanates, posing safety and environmental concerns. This study reports the design and synthesis of a sustainable, CO₂-derived non-isocyanate urethane self-etch monomer (NIU-BCM) for dental adhesive applications.

NIU-BCM was synthesized via CO₂ cycloaddition to bisphenol A diglycidyl ether, followed by aminolysis with n-butylamine and subsequent maleation to introduce acidic functionality. NIU-BCM was incorporated (0-20 phr) into a BisGMA/TEGDMA/HEMA adhesive resin. Physicochemical, mechanical, interfacial, durability, and biological properties were evaluated.

All formulations maintained low viscosity (130-340 mPa·s). NIU-BCM incorporation increased DC from 70% (control) to 80% and reduced sol fraction. Photo-DSC revealed a slight decrease in total polymerization enthalpy and a broadening of the exotherms, indicating delayed vitrification without affecting peak heat flow or time to peak. DMTA showed a Tg decrease from 160 to 108 °C at 20 phr NIU-BCM, indicating increased segmental mobility and a moderate reduction in crosslink density, while tan δ peak width analysis provided insight into the polymer network relaxation behavior. Fracture toughness (KIC) increased from 0.74 MPa·m¹ ᐟ² (control) to 1.02 MPa·m¹ ᐟ² (20 phr), while flexural strength and modulus remained unchanged. Microshear bond strength to enamel increased to 15.49 MPa, indicating effective adhesion within the range reported for functional self-etch adhesive systems. SEM/EDX confirmed controlled self-etch demineralization, characterized by honeycomb-type enamel patterns and partially opened dentinal tubules. Water sorption decreased from 94.22 to 76.96 µg/mm³ , with consistently low solubility. Cell viability remained ≥ 85% at 20 phr NIU-BCM.

This CO₂-based non-isocyanate monomer provides a sustainable platform for developing self-etch adhesive systems with promising physicochemical and biological performance.