Comparative effects of oilborne and waterborne preservative treatments on the mechanical and dimensional stability of birch and spruce plywood composites

Plywood is a versatile composite material valued for its high strength-to-weight ratio and ease of fabrication, yet its structural use in exterior environments remains limited due to moisture sensitivity and biological degradation. This study investigates the influence of two industrial-scale preservative pressure treatments, waterborne Tanalith E3475 and oilborne Tanasote S40, on birch and spruce plywood composites. Thirteen full-size panels were treated using full-cell (Tanalith) and empty-cell (Tanasote) processes, followed by controlled drying. Retention and penetration were quantified by mass gain and copper mapping with the dithiooxamide reagent, while mechanical and physical performance were assessed by bending (MOR/MOE), in-plane compression, Brinell hardness, and dimensional stability observations. Retention increased systematically with decreasing panel size, reflecting the edge-to-volume ratio. Spruce panels generally achieved higher retention than birch, attributed to their thicker veneers and more permeable adhesive lines. Tanalith-treated panels exhibited notable warping and veneer cracking, whereas Tanasote-treated panels remained dimensionally stable. Relative to untreated controls, Tanalith reduced birch bending strength and stiffness by approximately 25%–30%, while Tanasote caused only minor strength losses (∼8%) and enhanced surface hardness (+35%–40%). The results indicate that oilborne treatment more effectively preserves both dimensional integrity and mechanical performance, highlighting its suitability for exterior-grade structural plywood.