Ultrasound-assisted esterification of cotton cellulose with long chain free fatty acids

Abstract

This work covers the production of cellulose esters with varying degrees of substitution (DS) using ultrasound (US) power input, leveraging free fatty acids as esterification agent (EA) as a bio-based alternative to traditional chlorides, anhydrides and vinyl esters. The best conditions without US were achieved with oleic acid, with an EA/cellulose molar ratio of 6 and a temperature of 80 °C for 24 h, producing esters with a DS of 1.44. Applying US at 20 kHz and 4.39 W at room temperature, required <30 min to produce cellulose esters with a DS of 0.38. Then, the effects of the US input power, reaction volume and properties of cellulose solutions on the cavitation activity were investigated by simulations in COMSOL. The density, viscosity and speed of sound in the cellulose esters solutions were measured and defined in the simulations as 936.2 kg m^-3, 23.3·10^–3 Pa.s, and 1495.8 m s^-1 for 25 g L^-1. Simulations with conditions resulting in the highest DS with US were characterized by the smallest acoustic cavitation volume and the lowest u: 9.60·10^–8 m³ and 40.06 m s^-1. US-assisted esterification produced thermoplastic esters with an energy input of 18 W g^-1 of cellulose against 93 W g^-1 required by conventional esterification.