Does the moisture content of mercerized wood influence the modulus of rupture of thermopressed polyurethane-based composites?

Wood residues hold great potential for the development of composites. However, reinforcing wood requires modifications to enhance its adhesion to the polymeric matrix. In thermopressing, composites reinforced with wood are processed over a wide range of moisture levels. Nevertheless, there are limited studies on how varying this parameter influences the mechanical properties of these composites. This research provides valuable insights into how the moisture content present in wood reinforcement during the thermopressing process affects the adhesion of the composite through the modulus of rupture, a contribution not previously found in the literature. Therefore, our study aims to investigate the effects of different NaOH concentrations (5% and 10%) and the moisture content levels (3% and 12%) during thermopressing on the modulus of rupture of polyurethane-based composites. We analyzed three types of tropical wood residues from the Amazon, both with or without mercerization treatment, using thermal (TG/DSC), structural (XDR and FTIR), and morphological (SEM) characterizations. Subsequently, we evaluated the impact of moisture content on the modulus of rupture. The concentration of NaOH modifies the thermal, structural, and morphological characteristics of the wood residues, but these modifications were more effective in the composite when the moisture content of the fibers was low (3%). Reducing the moisture content of the reinforcement increased the resistance and the modulus of rupture of the composites made from Louro Itauba (from 2 to 9 MPa), Louro Gamela (from 12 to 16 MPa), and Massaranduba (from 3 to 11 MPa). Therefore, mercerization alone was insufficient to boost the modulus of rupture of polymeric composites. Instead, the control of moisture content emerged as the crucial factor during the thermopressing process.