C. Simon

Comparative creep characteristics of structural glulam wood adhesives

Materials and methods Two commercial phenol-resorcinolformaldehyde (PRF) adhesives and two commercial polyurethane (PU) adhesives, all four of them approved in Germany for exterior grade structural application to glulam and fingerjointing, were tested dynamically by thermomechanical analysis (TMA) on a Mettler apparatus. Six samples of beech wood alone, and of two beech wood plys each 0.6 mm thick were bonded with each adhesive system at ambient temperature (25 C) and under pressure, and then conditioned at ambient temperature and to a constant equilibrium moisture content of 9%. Samples dimensions were of 21 · 6 · 1.2 mm. Triplicate samples were tested in non-isothermal mode between 25 C and 250 C at a heating rate of 10 C/min with a Mettler 40 TMA apparatus in three points bending on a span of 18 mm exercising a dynamic force cycle of 0.1/0.5 N on the specimens with each force cycle of 12 seconds (6 s/6 s). Triplicate samples were also tested in isothermal mode at 40 C, 45 C, 50 C, 100 C and 150 C under identical conditions as above. The classical mechanics relation between force and deflection E 1⁄4 [L/(4bh)][F/(f)] allows the calculation of the Young’s modulus E for each case tested, and this was done to follow the increase of modulus (MOE) as a function of temperature and time. The results are shown in Figs. 1 and 2.

UF/pMDI wood adhesives: Networks blend versus copolymerization

Summary Kinetic evidence from thermomechanical analysis experiments and 13C NMR evidence were used to show that the strength of a joint bonded with UF/pMDI glue-mixtures is improved by coreaction between pMDI and UF resin methylol groups forming methylene cross-links. These are predominant, as opposed to the formation of urethane bridges which still appear to form, but are in great minority. The coreaction occurs in the presence of water and under the predominantly acidic hardening conditions characteristic of aminoplastic resins (in presence of a hardener). Coreaction occurs to a much lesser extent under alkaline conditions (without UF resins hardeners). The predominant reaction is therefore different in UF/pMDI and PF/pMDI adhesive systems. The same reaction observed for the UF/pMDI system at higher temperatures was also observed in PF/pMDI systems, but only at lower temperatures. The water introduced into the UF/pMDI mixture by addition of the UF resin solution was shown not to react with pMDI to increase the strength of the hardened adhesive to a significant degree.