This paper is the final part of a series reporting on the penetration (P) and bonding behaviour of urea-formaldehyde (UF) resins of different viscosities based on different degrees of condensation (DCs). The various resins were applied onto radial and tangential wood surfaces, and solid wood assemblies were prepared by hot pressing. Pt and Pr were observed by evaluation of photomicrographs of the bond lines. The mean and maximum depth of penetration was inversely proportional to the DC. The average depth of Pt was found to be slightly higher than Pr because of the abundance of pits on the radial walls. However, the maximum P was higher in radial direction, due to the elliptic shape of vessels in poplar as well as due to rays improving the resin flow. The lap shear strength (σm) of the same bonded assemblies was determined. Despite the fact that the wood failure always was very high, the σm increased with the better penetration of the resins with low viscosity. The higher strength values measured has been interpreted as a fortification effect of the interphase by bulk resin penetration, as cell wall penetration cannot be expected due to the large size of such resin molecules.
Rising demands for fire resistance properties of wood construction and elements matching new standards have been an important part of building codes during the last decade. On the other side, lack of more detailed research on interaction between wood species and selected fire retardant chemicals even with basically one is evident. This is particularly truth with domestic wood species. In this research, beech and poplar veneers were immersed in 25% solutions of monoammonium phosphate (MP) and sodium acetate (SA) and impregnated for different periods of time. To determine the preliminary level of fire retardancy achieved in veneers before manufacturing of finished plywood, thermo gravimetric (TG) and derivative thermo gravimetric (DTG) methods were used. TG and DTG analyses of treated and untreated wood, as well as of fire retardants alone, were performed. The next properties of impregnated and no impregnated veneers and plywood were determined: absorption of imp regnant solution (A), weight percent gain (WPG) of imp regnant, equilibrium moisture content (EMC), pH values, and in the case of plywood, strength and fire resistance. Fire resistance of plywood was tested in accordance with standard test for resistance to the effects of fire and the most efficient fire retardant, monoammonium phosphate, had the same result as TG/DTG analyses, which pointed out the validity of TG methods in predicting fire resistance of future products
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