Swelling Pressure of Selected Tropical Hardwoods and Plantation-Grown Paulownia

Wood is a hygroscopic material, and water uptake causes dimensional changes associated with swelling. When swelling is restrained, internal stresses and swelling pressure develop, which may contribute to cracking, deformation and damage to wood elements. Knowledge of swelling pressure is therefore important for assessing the dimensional stability and appropriate application of exotic wood, particularly under variable humidity conditions. Unfortunately, comparative data for swelling pressure of tropical hardwoods and plantation-grown paulownia remain limited. Therefore, this study examined the magnitude, anisotropy, and time-dependent development of swelling pressure in sapele, khaya, framiré, tauari, abura, mansonia, and paulownia wood submerged in water while the wood was restrained from swelling. Six replicates of each wood species in each direction were examined. Maximum tangential and radial pressures were measured and related to wood density and compressive modulus of elasticity (MOE). Swelling pressure was measured from oven-dry starting conditions until the maximum swelling pressure was reached, as indicated by stabilisation or a decrease in the measured force. Maximum swelling pressure ranged from 1.81 to 7.00 MPa tangentially and from 1.17 to 8.85 MPa radially. Sapele exhibited the highest pressures in both directions (7.00 and 8.85 MPa), whereas paulownia showed the lowest (1.81 and 1.17 MPa). Pressure development differed among species: paulownia, abura, and mansonia reached comparatively low maxima rapidly, while sapele and khaya developed higher pressures more slowly. Tauari combined rapid initial development with a high maximum pressure. Following their maxima, pressures declined slightly. Swelling pressure increased significantly with wood density in both tangential (R² = 0.68) and radial (R² = 0.62) directions. Its relationship with MOE was significant tangentially (R² = 0.61), but not radially. Substantial differences among species of similar density demonstrate that density and stiffness alone cannot reliably predict swelling pressure. The results indicate that swelling pressure is governed by a combination of species-specific characteristics, anatomical direction, and time-dependent behaviour, with wood density and compressive stiffness providing only partial explanations for the observed variability. These wood features need to be considered while designing moisture-exposed, dimensionally constrained wood products. Further research focusing on the detailed characterisation of wood anatomy and moisture transport properties is necessary to clarify the mechanisms responsible for differences in swelling pressure between species and anatomical directions, and to improve prediction under restrained moisture-induced swelling conditions.