Hydrophobicity improvement of cellulose nanofibrils films by stearic acid and modified precipitated calcium carbonate coating

Abstract

The development of a cellulose nanofibrils film with permanent hydrophobicity using green processes, avoiding hazardous solvents, through easy procedures, is a great challenge. The hydrophobicity of a layer of calcium carbonate modified with stearic acid has already been presented. However, the combination of a cellulose nanofibrils film with a layer of modified calcium carbonate to develop a permanent hydrophobic cellulose-based material rises the additional issue of adhesion between layers. In the present study, a set of cellulose nanofibrils films was coated with a layer of stearic acid and another set was additionally covered with modified precipitated calcium carbonate (0.4–6 µm sized particles with above 50% aragonite crystalline form), previously modified with a stearic acid suspension using ultrasounds. To investigate the issue of adhesion between layers, some films were subjected to heat treatments at 68 and 105 °C. Structural and physical analysis of the films, as well as barrier properties and static/dynamic contact angle measurements were performed. Results show that overall mechanical performance of the films was not substantially affected by the coating and posterior heat treatments. Heat treatments decreased the water vapor transmission rate of stearic acid coated films from 91.9 to 31.6 g m−2 day−1 and the oxygen permeability of stearic acid and modified calcium carbonate coated films from 26.4 to 2.6 cm3 µm/(m2 day kPa). The double layered coated cellulose nanofibrils films attained contact angle hysteresis of 3.1° and 5° and static contact angles of 150° and 140° with no heat treatment and with a heat treatment of 68 °C, respectively. The heat treatment enabled to permanently adhere modified calcium carbonate particles on the film, providing it with persistent hydrophobicity.