UV-curable aromatic polyurethanes from different sustainable resources: Protocatechuic and cinnamic acids

Abstract

An innovative aromatic monomer, 3,4-dihydroxybenzyl cinnamate (CDHB), was synthesized from protocatechuic and cinnamic acids. These latter can be obtained from different sustainable resources, such as plastic waste or biomass, by biotechnology and/or chemistry routes. From CDHB, used as chain extender, and aromatic or aliphatic diisocyanates, various aromatic thermoplastic polyurethanes (TPU) with cinnamic pendant chains were carefully synthesized and characterized. Different techniques such as NMR, FTIR, TGA and DSC have been used. The main results show that the TPUs exhibited good phase segregation and thermal stability, due to the aromaticity of the chain extender. The pendant cinnamic chains acted as internal plasticizer with a reduction of the glass transition temperature (Tg). The cinnamic groups were also able to undergo a [2 + 2] cycloaddition under UV irradiation, leading to the crosslinking of PU with a strong evolution of the properties. This reaction was followed by UV–Vis spectroscopy. 80 % of cycloaddition was reached in 20 h, significantly increasing PU gel fraction, Tg, and thermal stability. This study highlights the potential to create modulable sustainable and aromatic PU macromolecular architectures that can be UV curable and display enhanced properties.