Preservative-treated plywood : Effects on mechanical properties, preservative uptake and dimensional stability

Plywood is a widely used engineered wood product known for its strength, dimensional stability, and versatility. Birch plywood, in particular, has shown promising structural properties, making it a potential alternative to traditional materials in demanding applications. However, its use outdoors is limited by vulnerability to moisture and biological degradation. This thesis investigates the effects of waterborne (Tanalith) and oilborne (Tanasote) preservative treatments on birch and spruce plywood, focusing on retention and penetration of wood preservative, dimensional stability, and mechanical performance. The aim is to assess the feasibility of these treatments for structural use and identify critical factors influencing treatment effectiveness. 13 full-size plywood boards (ten birch and three spruce) were prepared, cut into various sizes, and treated using industrial-scale pressure impregnation. A full-cell process was used for the waterborne Tanalith treatment and an empty-cell process for the oilborne Tanasote. After treatment, samples were dried under controlled conditions and analyzed for retention, penetration depth, and mechanical performance. Retention was calculated based on the mass increase after treatment in relation to the preservative concentration. Penetration depth was assessed using a copper reagent that caused a visible color change, showing how far the preservative had reached into the plywood. Mechanical performance was evaluated using four-point bending, compression, and Brinell hardness tests. Retention increased as panel size decreased, with smaller specimens absorbing more solution per unit volume. Spruce plywood exhibited higher retention than birch, potentially due to differences in veneer thickness, number and composition of glue lines. While Tanasote-treated boards showed higher apparent retention, these values are not directly comparable, as Tanasote was applied in undiluted delivery form whereas Tanalith was applied as a 4.3% concentration water solution. Tanalith-treated boards displayed more visible deformation, including warping and veneer cracking, especially in spruce. Tanasote-treated boards remained dimensionally stable. In mechanical testing, Tanalith-treated birch showed around 30% strength loss in compression and bending strength, while Tanasote-treated samples retained most strength and exhibited higher surface hardness. In conclusion, panel size had a strong effect on preservative retention, with smaller samples achieving sufficient retention levels, likely due to their higher edge-to-volume ratio. Tanasote-treated specimens generally maintained better dimensional stability and integrity, suggesting oilborne preservatives may be more suitable for structural plywood in exterior environments. However, further studies are needed to assess long-term performance and ensure treatment consistency.