UVC-Driven Postphotopolymerization: An Advanced Strategy for Reducing Residual Monomers in Waterborne Polymer Dispersions

Abstract

Waterborne polymer dispersions (latexes) are considered environmentally friendly and sustainable, yet they still contain residual monomers, raising significant health and environmental concerns. Reducing or eliminating residual monomers is critical for enhancing product sustainability. This study investigates a novel approach using short wavelength ultraviolet light (UVC, 250 nm) in a continuous tubular reactor to induce initiator-free postpolymerization of residual monomers in polymer latex. Key parameters such as residence time (0–30 min), type of purging gas (air or nitrogen), and monomer concentrations (400–7000 ppm) were explored. Results revealed that postphotopolymerization predominantly occurred in the aqueous phase and that there is a monomer diffusion limitation to full conversion, due to lack of agitation in a reactor under laminar flow of the latex. Adding Texanol, a high-boiling-point plasticizer, improved monomer partitioning between phases, achieving up to a 95% reduction of monomer concentration. EPR analysis showed that UVC irradiation generated OH* and OOH* radicals in the aqueous phase that initiate the polymerization. Despite prolonged irradiation, latex stability, molecular weights, and film mechanical properties remained unaffected. However, they present decreased water resistance, likely due to the newly created oligomers containing hydrophilic initiating moieties. This study highlights UVC irradiation in a tubular reactor as a promising technique for residual monomer removal while providing deeper insights into UVC-induced radical generation in emulsion photopolymerization without the use of initiators.