Green neutralization of poly(ethylene-co-methacrylic acid) with sodium hydroxide in aqueous solution

Following the guidelines of the project Made in Italy Circolare e Sostenibile (MICS) funded by Next Generation EU, a green approach for the production of ionomers based on ethylene (E) – methacrylic acid (MAA) copolymers (EMAA) is investigated. These ionomers are used in the packaging industry for their toughness and low sealing temperatures. EMAA can be bulk-neutralized with different counterions in a melt compounder at temperatures above 150 °C. Then, ionomer films are obtained by extrusion. The second option consists of preparing an EMAA solution with organic solvents like tetrahydrofuran or mixtures of toluene and alcohol. The neutralization is conducted in the solution by adding the desired base, and then thin films or coatings are prepared by casting. In the present study, EMAA with 12wt% MAA is neutralized in an aqueous solution of NaOH 1M for 1 hr at 130 °C, employing a hydrothermal reactor with magnetic stirring. Chemical characterization is performed with FTIR Spectroscopy. First, a thin film of neat EMAA is prepared by hot pressing using an especially prepared mold at 130°C for a few minutes. The carbonyl functional groups absorb IR light at 1700 cm-1 due to the formation of carboxylic acid dimers. After processing in NaOH solution, its intensity decreases and a red-shift down to 1680 cm-1 is observed. At around 1550 cm-1 a new peak related to sodium carboxylate appears: its absorbance increases with the degree of neutralization (DN). These results are in agreement with the literature and confirm the partial neutralization of the copolymer precursor. DN is estimated equal to 90±4%, obtaining the ionomer EMAA-0.9Na. The adopted protocol prevents the formation of anhydrides in EMAA. Finally, thermal characterization is carried out, showing that neutralization leads to a decrease in the melting temperature of the polymer.