Additive-Free One-Step Hot Pressing of Radiata Pine for Simultaneous Surface Densification, Resin Exudation Suppression, and Dimensional Stabilization

Radiata pine (Pinus radiata D. Don) is widely used but suffers from resin exudation, low surface hardness, and poor dimensional stability. Conventional modifications rely on a two-step “degreasing and reinforcement” sequence, requiring exogenous chemicals or fixation treatments. This study presents a one-step strategy without additives that integrates surface compression, resin degreasing, and deformation fixation to enhance surface hardness, antiresin exudation property, and dimensional stability. Surface-compressed and fixed (SCF) wood exhibited a 100.0% increase in average density within the compressed layers, a 40–60° improvement in water contact angle (WCA), and a 179.1–193.1% enhancement in surface hardness versus control wood. Prolonged treatment darkened the surface color, reduced set recovery induced by immersion in water to 9.0%, and suppressed resin exudation. Microscopic analysis revealed buckling of tracheid cell walls and a reduction in lumen volume. Chemically, treatment promoted hemicellulose deacetylation and increased the relative crystallinity and crystalline thickness of cellulose by 7.8–61.4 and 8.4–30.2%, respectively. These structural and chemical changes contributed to improved surface properties and dimensional stability. Compared with SCF-2 (2 h treatment), extraction-pretreated SCF-2 (E-SCF-2) showed lower dimensional stability and WCA, but the differences were not significant, indicating that resin deposition and redistribution, which fill cell lumens, contribute positively but not dominantly to water-related properties. The resin content in the lower compressed layer (LCL) was 5.1–19.5% higher than that in the upper compressed layer (UCL), leading to lower surface wettability and a more yellowish appearance on the LCL. This study provides an efficient method for transforming radiata pine into a high-performance material for furniture and flooring applications.