Effect of AKD Content on Wettability, Water Absorption Kinetics, and Interfacial Properties of Poplar Wood Flour/PLA Biocomposites

In this study, poplar wood flour was modified with alkyl ketene dimer (AKD), and poplar wood flour/polylactic acid (PLA) composites with 0–5% AKD were prepared. Effects of AKD content on wettability, water absorption kinetics, dimensional stability, flexural properties, and microscopic interface were investigated. AKD likely consumed hydroxyl groups through esterification and/or hydrogen bonding, as reported in the literature, and introduced hydrophobic alkyl chains. The 5% AKD sample exhibited the highest contact angle, while the 3% AKD sample showed the lowest total surface free energy (24.73 mJ·m−2). AKD content significantly changed the water diffusion mode: at 0–3% AKD, water absorption followed non-Fickian diffusion, with 2% AKD showing the highest n value (0.7132), indicating swelling-controlled diffusion and the lowest apparent saturation water absorption. At 4% AKD, n decreased to 0.4786, approaching Fickian diffusion, and agglomerated pores accelerated water transport. The 2% AKD sample exhibited optimal interfacial bonding and the highest flexural strength retention (46.29%). The 5% AKD sample showed the lowest steady-state thickness swelling coefficient (10.03%), indicating the best dimensional stability, but agglomeration defects weakened mechanical properties. Interfacial bonding strength, rather than low water absorption or low swelling, was key to mechanical retention. An optimal AKD content range exists when “optimal” is defined by the balance between mechanical retention and dimensional stability: 2% AKD is preferred for mechanical retention, while 5% AKD is preferred for dimensional stability under humid service scenarios. These findings support the water-resistant modification of PLA-based wood–plastic composites.