Spruce lamellas were modified in a vacuum chamber at five different temperatures, from 160 °C to 210 °C. The sixth group of blades was a control group and was not heat treated. We measured the mass loss, which was the result of thermal treatment. We confirmed that there is a correlation between the modification temperature and mass loss, while the amount of mass loss at a certain temperature cannot be asserted with certainty. Lamellas were bonded together in pairs by urea-formaldehyde and phenol- formaldehyde adhesive. From the pairs of lamellas we cut out test specimens, according to standard SIST - TS CEN / TS 133354:2004. Before sample testing the samples were exposed to three different conditions: (1) a standard condition at 65 % air humidity and 20 °C, (2) they were for 24 hours immersed in cold water with 20 °C, and (3) boiled for 6 hours, and then they were cooled down in water. We observed the shear strength depends on the degree of thermal modification and the adhesive type. We also determined the share of wood failure. We found out that the degree of modification affects on bonding of solid wood. How and to what extent the impact is, depends on the kind of adhesive and method of test samples preparation. We found out the level of modification has influence on bonding massive wood. We can claim phenol-formaldehyde adhesive is suitable for bonding products intended for indoor use in humid conditions, while urea-formaldehyde adhesive is limited to bonding products intended for indoor use with dry climate.
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