Bio-based poly(ester-urethane) coatings for paper: Effect of cutin oligoester esterification degree on functional properties

The growing demand for sustainable alternatives to petroleum-based packaging materials has renewed interest in paper substrates. Cutin, a natural polyester found in plant cuticle, offers promising properties such as hydrophobicity and low gas permeability, making it an appealing component to formulate paper coatings. In this study, poly(ester-urethane) (PU) coatings derived from cutin extracted from tomato peels were investigated as sustainable barrier layers. The synthesis was accomplished in two steps: cutin monomers obtained by a hydrolytic extraction were at first esterified with 1,4-butanediol and then polymerized by addition with isophorone diisocyanate. Importantly, the reaction parameters, particularly the reaction time in the first esterification step, were systematically optimized to obtain three formulations (PU30, PU38, PU50) with distinct esterification degrees (30, 38, and 50 %, respectively). The thermal stability by TGA analysis of the resulting PUx was little affected by the preparation procedure, while the glass transition temperature (Tg) was found to decrease of several degrees when the esterification degree increased. All the three formulations could be applied onto two different paper substrates: a standard paper for printing (Pp) and a single face coated paper (Gp) for packaging application. SEM analysis showed superior coverage and adhesion on Gp, and barrier tests confirmed improved resistance to water and grease, especially with PU30. Biodegradability and recyclability assessments demonstrated that PU30 maintained a favorable balance between functional performance and environmental compatibility, in contrast to PU38 and PU50, which significantly hindered these properties. Overall, the results highlight the potential of PU30 coatings as a viable, bio-based solution for enhancing paper packaging sustainability. • Poly(ester-urethane)s were synthetized from cutin extracted from tomato peels. • Two-components formulations were coated on paper by a solventless technology. • Coated paper exhibits good coverage, adhesion and resistance to water and grease. • Coated paper can be recyclable and biodegradable depending on the polymer structure.