Development of swelling pressure in paulownia and Norway spruce during moisture absorption

Wood hybrid materials have gained significant attention for advanced technical applications over the past decade. In contrast to other materials, wood’s hygroscopic nature causes swelling and shrinkage, leading to differential expansion in hybrid systems under varying humidity conditions. When wood’s swelling is restrained by surrounding materials, stresses develop within both the wood and its adjacent components. To study this phenomenon, swelling tests were conducted on kiln-dried paulownia (Paulownia elongata) and Norway spruce (Picea abies) in a climate chamber at 20 °C and 98% relative humidity, with expansion restricted in one direction. Moisture absorption initially exhibited a steep, linear increase, levelling off after approximately 2 h. Swelling pressures rose sharply, peaking after 30 h for paulownia and 19 h for spruce, before gradually decreasing due to increased moisture content and relaxation. The measured stresses were lower than the compressive strengths of both wood species at their respective moisture contents. Microscopic examinations showed no cellular damage in paulownia during moisture absorption due to swelling pressure. In contrast, spruce wood displayed cell wall deformations and ray’s kinking in the early wood region of radial samples, as well as cell wall bending in tangential samples. This indicates that maximum stress is determined by the localised failure of the wood’s cellular structure rather than its overall properties. Such local effects were more pronounced in spruce than in paulownia due to their different structure. As a result, paulownia shows excellent potential for use in hybrid structures due to its low swelling and shrinkage properties and uniform structure.