Beech (Fagus sylvatica L.) veneers were treated with 11 different chemicals used in the wood, textile and paper industries. The treated veneers were investigated on water repellent effectiveness, water sorption and radial swelling in a humid environment, fixation of the chemicals after water extraction, color change and tensile strength retention after the treatments. In a further step, three chemicals: N‑methylol melamine solution (NMM‑1; 10% solid content), fatty acid modified Nmethylol melamine dispersion containing paraffin (mNMM‑2, 5% solid content) with an aluminium salt as a catalyst, and alkyl ketene dimer dispersion (AKD, 1% solid content) were selected for making modified plywood in two different processes. The impregnated veneers were cured in a hot press prior to gluing in the process 1, whereas curing of the impregnated veneers and hot pressing after gluing were combined in one step, in process 2. The plywood was produced with five beech veneers and glued with a phenol formaldehyde adhesive.
All the treatments in both processes, except for early delamination of mNMM‑2 treated plywood in process 1, reduced water uptake and thickness swelling for the plywood during the first 24 h of submersion, but only NMM‑1 treated plywood imparted reduced water uptake and thickness swelling at water saturation state. The water repellent effect was stable over 6 cycles of water submersion and oven-drying.
The bonding quality of the treated plywood reached the requirements for plywood used in noncovered exterior conditions according to the EN 314 (except for mNMM‑2 treated plywood in process 1). Shear strength of the plywood treated with mNMM‑2 and AKD in process 2 was higher than that in process 1 due to hydrophobic surfaces of the veneers after curing in process 1, while shear strength values of NMM‑1 treated plywood in both processes were similar. In both processes, shear strength and cohesive wood failure of mNMM‑2 and AKD treated plywood were lower as compared to those of the control plywood, whereas NMM‑1 treated plywood displayed higher shear strength and lower cohesive wood failure than the control plywood.
The modulus of elasticity (MOE), bending strength (MOR) and Brinell hardness of NMM‑1 treated plywood were slightly increased in comparison to those of the control plywood. Whereas, the treatments with mNMM‑2 and AKD did not result in reduction in MOE, MOR and Brinell hardness. With regard to water resistance, dimensional stability, bonding quality and the other mechanical properties, the plywood produced in process 2 (without curing of the impregnated veneers before vi gluing) showed more advantages than in process 1. Therefore, plywood produced in process 2 was selected for natural weathering and fungal decay tests.
During 18 months of natural weathering exposure, the treatments with NMM‑1, mNMM‑2 and AKD lowered the moisture content of the treated plywood panels in comparison to that of the control plywood. The cracks and discoloration on weathered surfaces, mould colonization on reverse surfaces of the treated plywood were slightly reduced. Application of a coating (paint or stain) resulted in less cracks and reduced discoloration on weathered surfaces, but did not effectively limit mould colonization on the reverse surfaces of the plywood panels. Over the weathering exposure, NMM‑1 treated plywood panels exhibited the greatest form stability and no delamination, followed by mNMM‑2 and AKD treated plywood panels, while the control plywood panels displayed both delamination and deformation. The reduced delamination and deformation of weathered treated plywood panels were in accordance with the number of delaminated specimens in the water submersion test.
The treatments with NMM‑1 imparted high resistance against decay and blue stain fungi for both the treated veneers and treated plywood, while the treatments with mNMM‑2 only brought about the efficient fungal resistance for the treated veneers. The AKD treatments almost failed in protecting the veneers and plywood from decay and blue stain fungi.