Time-triggered calcium ion bridging in preparation of films of oxidized microfibrillated cellulose and pulp

One of the main trends in developing bio-based materials is to improve their mechanical and physical properties using MFC derived from sustainable natural sources and compatible low-cost chemicals. The strength of anionic MFC based materials can be increased with addition of multivalent cations. However, direct mixing of solutions of multivalent cations with oxidized MFC may result in immediate, uncontrollable fibril aggregation and flock formation. The aim of this study was to design a method where Ca ²⁺ ions liberate from solid CaCO ₃ particles on bleached hardwood (birch) kraft pulp, which was mixed with oxidized MFC and crosslink it to tailor the mechanical properties of the dried structure. In few minutes after adding acetic anhydride, pH of the wet film dropped from 7.3–4.8 through liberation of acetic acid and CaCO ₃ particles solubilized releasing Ca ²⁺ . The novel method could be applied on industrial scale for improving the performance of packaging materials.