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The possibility of replacing the rye flour commonly used as a filler for urea formaldehyde resins was investigated for plywood production with birch bark characterized by various particle dimensions. The effects on the curing and rheological properties of the adhesives were investigated. Moreover, the plywood was tested to evaluate the formaldehyde emissions, bonding quality, modulus of elasticity, and bending strength. The results showed that the size of the bark particles had a significant effect on the viscosity of the adhesives but did not affect other properties of the resin. Replacing the flour with the bark significantly decreased the formaldehyde release, but there was no clear correlation with the dimensional fraction of the bark powder. The particle size had notable effects on bonding quality and mechanical properties. The best results were obtained for a sieve with square holes whose inside lengths were 0.315; however, all plywood samples achieved shear strength values exceeding the requirements of EN 314-2 (1993). [ABSTRACT FROM AUTHOR]

Various flours were added as fillers for urea-formaldehyde (UF) resin, and effects on the rheological behavior and the curing properties of the resin were evaluated. The plywood properties such as bonding quality, bending strength, modulus of elasticity, and formaldehyde-release were tested. Five types of flours were used, i.e., rye flour, hemp flour, coconut flour, rice flour, and pumpkin flour. The type of flour had a major influence on the properties of resin mixture such as gel time, solid content, and viscosity. The use of rye and pumpkin flour resulted in a longer gelling time, and the adhesive mixture filled with rice flour did not attain the desired viscosity value needed for the production of plywood. The best mechanical properties and bonding quality were achieved by addition of pumpkin flour. However, the bonding quality of plywood glued with an adhesive filled with every other flour retained good values exceeding 1 N/mm² as required by the standard EN 314-2 (1993). The use of hemp flour as a filler for UF resin led to a substantial decrease of free formaldehyde content. [ABSTRACT FROM AUTHOR]