Influence of environmental factors on the surface condition of thermally modified ash wood in polyvinyl acetate adhesive joints

The aim of this work was to study polyvinyl acetate adhesive joints of “thermally modified ash/pine wood” concerning changes in the surface condition of thermally modified ash wood (correlated with changes in the strength of the samples) under prolonged exposure to external factors. Test samples of adhesive joints were placed on a test rack for external exposure. Every three months, they were removed to record destructive changes in the thermally modified wood surface using scanning electron microscopy, assess changes in wettability and the duration of water droplet penetration into the structure (by measuring changes in contact angles over time), and determine changes in the strength of the adhesive joints. It was established that photochemical damage to thermally modified ash wood is the “trigger mechanism” for its further degradation changes. Samples exposed during summer periods exhibited more pronounced destructive changes in the thermally modified surface and a significant decrease in the strength of adhesive joints (from 6.56 MPa to 6.05 MPa after the first cycle and from 5.93 MPa to 5.62 MPa after the second cycle). The study showed that due to a cascade of destructive mechanisms, the structure of thermally modified ash wood, after 24 months of exposure to natural conditions, sustained damage to a depth of 0.05-0.2 mm (while the strength of the adhesive joints decreased from 7.12 MPa to 5.13 MPa), the surface became more hydrophilic, which led to a reduction in the time required for water penetration into its structure. Accordingly, the contact angle on such the surface reached θ = 17° after 480 seconds, while on the surface of thermally modified ash wood, which was not exposed to natural factors, a similar value was reached only after 570 s