Wettability of Hybrid Poplar Veneers with Cold Plasma Treatments in Relation to Drying Conditions

Veneer overdrying is a primary cause of poor plywood glue bond quality due to surface inactivation and its effect on glue transfer and penetration. The goal of this work is to investigate the feasibility of using atmospheric cold plasma to treat overdried hybrid poplar (Populus hybrids) veneers to improve wettability and plywood glue bond (shear) strength. The effectiveness of the plasma treatment using three veneer drying conditions, air drying, oven drying, and overdrying, were compared. It was found that the cold plasma treatment significantly enhanced veneer surface wettability under all three drying conditions. Compared to untreated controls, instantaneous surface contact angles of treated veneers decreased the most under the overdried condition, followed by the oven-dried and air-dried conditions, regardless of whether glycerin or urea formaldehyde (UF) was used. After a 5-s treatment, overdried veneers achieved complete wetting with both droplets. The results demonstrated that the atmospheric cold plasma was the most effective one to treat overdried poplar veneers to achieve the smallest contact angle, leading to the highest plywood glue bond (shear) strength. However, further research is still needed to understand the mechanism of the cold plasma treatment on veneers under different drying conditions.