The effect of the carboxylation degree on cellulose nanofibers and waterborne polyurethane/cellulose nanofiber nanocomposites properties

There has been an exponential rise in the interest for waterborne polyurethanes (WBPU), due to the easy customizability of their properties and their ecofriendly nature. Moreover, their aqueous state facilitates the incorporation of hydrophilic reinforcements. Cellulose nanofibers (CNFs) have shown great potential, thanks to their renewability, large natural availability, low cost and great specific properties. However, CNFs often require some modification to obtain optimal compatibility. In this work, standard bleached hardwood kraft pulp has been subjected to a carboxylation process followed by mechanical disintegration. Varying treatment times and passes, CNF samples with different carboxylation degrees have been obtained. WBPU/CNF nanocomposites with different CNF content have been prepared. The effect of the carboxylation degree on the CNFs and on the nanocomposites properties has been studied. Although carboxylation damaged the cellulose structure, decreasing the crystallinity degree of CNF and reducing the thermal stability of fibers, composites showed better thermal and thermomechanical stability and improved mechanical properties than the unreinforced matrix counterpart. A maximum increase of 1670% in modulus, 377% in stress at yield and 86% in stress at break has been achieved for composites reinforced with carboxylated fibers. Therefore, it was observed that carboxylation improved matrix/reinforcement interactions.