Adam Běťák

This paper investigates the temperature resistance of the load-bearing elements of bonded spruce joints subjected to static tensile shear stress immediately after thermal exposure. Four types of adhesives were used: one-component polyurethane, melamine-urea-formaldehyde, resorcinol-phenol-formaldehyde, and epoxy. The joints were exposed to temperatures of 100, 120, 140, 160, and 180 ° C for 1 and 2 h. To evaluate the influence of temperature on potential wood degradation and its effect on the thermal resistance of bonded joints, the wood itself was subjected to the same conditions and tests as the bonded joints. To isolate the effect of the adhesives, thin films were prepared and their change in stiffness during heating up to 180 ° C was measured using dynamic mechanical analysis. None of the adhesives were fully cured, and post-curing led to an increase in storage modulus and, in the case of MUF and PRF adhesives, significant shrinkage of the films. The strength of all joints decreased with increasing temperature. A continuous decrease was observed in epoxy joints, whereas the other joints exhibited fluctuations in strength. With respect to the decrease in strength, a temperature of 100 ° C already appears to be critical due to glass transition of all adhesives. Adhesives began to show thermal degradation at 140 ° C, which led to an increased incidence of adhesive failure in the joints, however the strength of the wood was also reduced above 140 ° C (a significant drop after 2 h). 1 C PUR-bonded joints exhibited the highest strength at 20 ° C and after thermal exposure, but all adhesive joints except EP demonstrated considerable thermal resistance. A significant effect of the duration of temperature exposure was also demonstrated.

This experiment compared commercially available moisture meters (three capacitive metersand one resistance meter) and tested their predictive ability at different moisture conditions on selected beech (Fagus sylvatica) and spruce (Picea abies (L.) Karst.) wood samples. The measurements were carried out on the samples at specified moisture intervals ranging from 5% to 30% moisture content (MC). The resistance meter showed a close correlation to gravimetric MC values; the influence of the measuring direction for MC below 17% was found when higher MCs in the transverse direction for both species were archieved. The difference was 4.6 times higher for softwood and 1.6 times higher for hardwood. Differences between radial and tangential transverse measuring were not observed. The close correlation coefficient of MC measurements was also found for capacitive methods. The effect of the direction was found for all the tested meters when higher MC values in the longitudinal measurements were found. This effect was especially significant at an MC of wood higher than 20 wt.%. For two capacitive methods, the effect of annual ring deflection only in the spruce samples was found where higher MC values in the tangential direction were observed.

The highest resistance to the action of wood-decay fungi was achieved for boards bonded with cementitious binder, for which the mass loss (ML) was passive for the tested fungi at both 3.8% for Coniophora puteana and 3.6% for Pleurotus ostreatus due to the carbonation process. By contrast, the lowest bioresistance was shown for particleboard. The achieved ML values were related to the changes of the internal structure, which were proven by the vertical density profile parameters and the surface area parameter. The effects of the ML and delamination process by immersion in water on the internal bond strength were determined individually to distinguish both synergistic parameters. The results showed the effect of long-term water immersion on the loss of internal bond strength, especially for wood particleboard.