PROPERTIES OF UREA-FORMALDEHYDE ADHESIVES FILLED BY WOOD FIBER SLUDGE
In order to improve the properties of urea-formaldehyde adhesives they were filled by wood fiber sludge. During the research used: the dry milled wood fiber sludge with particle size less than 0,2 mm and 10% of moisture content, urea-formaldehyde resin, ammonium chloride, rotary-cut birch veneer with thickness of 1,5 mm and 6% of moisture content were used in this study. The properties and chemical composition of the sludge were determined. It was found that content of holocellulose in wood fiber sludge was about 1,5 times less than in the wood. The content of lignin was 1,5 times higher than expected, the content of resinous extractives was several more times greater, and ash content was 10 times more than their averaged content in the wood. A significant difference in the chemical composition of wood fiber sludge compared to the chemical composition of wood can be explained by the fact that sludge particles can be subjected to the action of microorganisms that destroy wood during the storage in the basins for sedimentation. The slightly increased acidity of an aqueous suspension sludge (pH = 4,18) is in favor of this argument, which may be due to the presence of organic acids which are the result of life-activity of microorganisms in the basins for sedimentation. The increase of extractives are the result of reducing content of holocellulose, because the action of microorganisms aimed at transforming the macromolecular wood components in low molecular that can be dissolved in water or organic solvents. The high ash content of the sludge samples is probably due to the ability of wood to adsorb dissolved in natural water mineral salts and mechanical ingress of dust, sand, mud, abrasive materials etc. Quality of adhesives was assessed by the measurement of viscosity, solid content, curing time, hydrogen ion concentration, spot life and mass fraction of free formaldehyde in the adhesive compositions with different amounts of sludge, and shear strength of plywood, made on their basis. It was established that the addition of dry fiber sludge as a filler to adhesive slightly reduces the concentration of hydrogen ions and curing time, practically does not change the solid content, but increases the viscosity of adhesives, and all of these values don't exceed the allowable normative values. Mass fraction of free formaldehyde in the developed adhesive compositions is significantly less than in the used urea-formaldehyde resin. The fraction of free formaldehyde is reduced almost in 1,5 or 1,8-2,2 or 2,5 times by adding 1, 2-3 or 4 parts by weight of the sludge mass, respectively, compared with the adhesive without filler. Consequently, the filling of urea-formaldehyde adhesives by wood fiber sludge makes it possible to obtain low-toxic adhesives. Thus, the shear strength of plywood manufactured using sludge-filled adhesives, meets the requirements of current standard. The studies confirm the possibility of using wood fiber sludge as adhesives filler for manufacture of plywood. Herewith, its addition leads to a significant reduction of the mass fraction of free formaldehyde of applied adhesives.