Gluing ability of plywood produced with DMDHEU-modified veneers of Fagus sp., Betula sp., and Picea sp.

The shear strength, the cohesive wood failure, and the delamination were determined in plywood constructed with veneers of Fagus sp., Betula sp., and Picea sp. impregnated with 0.8, 1.3, and 2.3 M 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU). The plywood production process consisted of two steps: firstly impregnation and curing of the veneers, and secondly assembly of the panel. The adhesive used was a phenolic resin. The values obtained for shear strength were above the standard limit of rejection (1 N mm−2), and therefore the samples met the European requirements of plywood designed for exterior conditions. The phenolic resin improved its adhesive ability when the samples were heated for 72 h at 95 °C. The highest shear strength for modified wood was 3.97±0.45 N mm−2 at 0.8 M, 3.39±0.26 N mm−2 at 1.3 M, and 2.35±0.23 N mm−2 at 2.3 M, for Betula sp., Fagus sp., and Picea sp., respectively. The samples constructed with unmodified veneers presented higher shear strengths and higher cohesive wood failures than the samples constructed with modified veneers. The possible effect of the two-steps production of the plywood is discussed. The delamination test of DMDHEU-modified Betula sp. and Fagus sp. plywood showed no open glue lines, while the samples of Picea sp. treated with the higher concentration of DMDHEU suffered delamination.