A new ketocoumarin as key Type II photoinitiator for reduction of yellowing on free radical photopolymers under LED irradiation

This paper introduces a new Type II ketocoumarin-based photoinitiator (ES3644), designed to replace traditional photoinitiator such as isopropylthioxanthone (ITX), which pose toxicity and yellowing issues in coatings. The study demonstrates that ES3644, when used with an aromatic amine as coinitiator (A198), delivers higher reactivity under LED irradiation, achieving nearly twice the conversion rates of ITX, particularly at 365 nm and 385 nm, while significantly reducing yellowing in coatings—a major challenge in the field. With higher molar absorption coefficients and enhanced thermal stability (up to 400 °C), ES3644 also stands out for its low toxicity (LD50 > 5000 mg/kg, non-corrosive, non-mutagenic). Spectroscopic analyses and molecular modeling reveal that its planar geometry in the triplet state facilitates hydrogen abstraction, optimizing its efficiency in the presence of coinitiators. Finally, coating tests confirm its potential for pigmented inks and clear coatings, where it outperforms ITX in terms of curing speed and visual quality while meeting current environmental requirements. These findings position ES3644 as a promising solution for safer, more efficient, and sustainable photopolymerization processes. • New Type II ketocoumarin-based photoinitiator (ES3644) with low toxicity and low yellowing issues. • Higher reactivity with coinitiator under LED irradiation (365; 385 nm), reducing yellowing, major challenge. • Pigmented inks and clear coatings, outperforms ITX in curing speed, visual quality and environmental requirements.