Penetration and bondability of melamine-urea-formaldehyde adhesive
Šernek, Milan ; Resnik, Jože
The penetration of adhesive into wood depends on the adhesive mixture`s properties (viscosity, molecular weight, adhesive mix constituents, etc.), and on the wood characteristics (moisture content, ...permeability, surface properties, etc.). The main objective of the present research was to evaluate the effect of the moisture content (MC) of beech (Fagus sylvatica L.) veneer and the viscosity of melamine-urea-formaldehyde (MUF) adhesive on the later`s penetration into the porous network of wood cells. The correlation between MUF adhesive penetration and the shear strength of the adhesive bond was also studied. Three-layered plywood boards were made from the beech veneer with MC of 4, 6, 9 and 14%, respectively. Four adhesive mixtures with different dynamic viscosities were used for gluing. They differed in the percentages of melamine/urea, the molecular weight of these resins, and the different proportions of additives. A fluorescent microscopy technique was used to measure the effective adhesive penetration (EP). It was found that veneer MC affected the penetration of MUF adhesive. The lowest EP (19.2 mm) was observed at 4%. It then increased with MC, and it was the highest (25.6 mm) at 6% and 9%, but dropped to 22.6 mm at a MC of 14%. The MUF adhesive penetration decreased with increasing dynamic viscosity of the adhesive mixture. The observed correlation between MUF adhesive penetration and the shear strength of adhesive bond was statistically not significant, and was established only in the case of samples prepared in a standard climate prior to testing. The adhesive bond shear strength of the samples prepared according the standard EN 314-1, 5.1.3, did not correlate with the depth of the MUF adhesive penetration.
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