Combined Microwave Pretreatment and MMA Impregnation for the Development of High-Performance Wood–Polymer Composites

Wood–polymer composites (WPCs) produced through monomer impregnation have attracted increasing interest as a strategy to improve the durability and performance of wood materials. However, the limited permeability of certain wood species often restricts the effectiveness of impregnation treatments. This study investigates the use of microwave (MW) pretreatment as a drying and microstructural modification step to enhance methyl methacrylate (MMA) impregnation and in situ polymerization in maritime pine (Pinus pinaster) heartwood specimens. Wood specimens were subjected to MW treatment of 700 W and 5 min cycles prior to vacuum-pressure impregnation with MMA and subsequent thermal polymerization. Scanning electron microscopy and treatability parameters confirmed that MW pretreatment increased wood impregnability by generating microcracks and improving monomer penetration, thereby resulting in higher polymer retention and a higher weight percentage gain. As a result, the combined MW+MMA treatment produced a more homogeneous distribution of polymethyl methacrylate within the wood structure. The modified specimens showed a substantial reduction in water absorption and the highest water repellence efficiency among the studied groups, while dimensional stability improved to a lesser extent. In addition, the combined treatment significantly increased bending strength and stiffness, indicating an effective reinforcement of the wood structure through polymer loading. These results demonstrate that MW pretreatment is an efficient strategy to improve the treatability of maritime pine heartwood and to enhance the performance of MMA-based WPCs.