Utilization of Birch Bark as an Eco-Friendly Filler in Urea-Formaldehyde Adhesives for Plywood Manufacturing

Abstract

The potential of using ground birch (Betula verrucosa Ehrh.) bark as an eco-friendly additive in urea-formaldehyde (UF) adhesives for plywood manufacturing was investigated in this work. Five-ply plywood panels were fabricated in the laboratory from beech (Fagus sylvatica L.) veneers bonded with UF adhesive formulations comprising three addition levels of birch bark (BB) as a filler (10%, 15%, and 20%). Two UF resin formulations filled with 10% and 20% wheat flour (WF) were used as reference samples. The mechanical properties (bending strength, modulus of elasticity and shear strength) of the laboratory-fabricated plywood panels, bonded with the addition of BB in the adhesive mixture, were evaluated and compared with the European standard requirements (EN 310 and EN 314-2). The mechanical strength of the plywood with the addition of BB in the adhesive mixture is acceptable and met the European standard requirements. Markedly, the positive effect of BB in the UF adhesive mixture on the reduction of formaldehyde emission from plywood panels was also confirmed. Initially, the most significant decrease in formaldehyde release (up to 14%) was measured for the plywood sample, produced with 15% BB. After four weeks, the decrease in formaldehyde was estimated up to 51% for the sample manufactured with 20% BB. The performed differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and derivative thermogravimetry (DTG), also confirmed the findings of the study. As this research demonstrated, BB as a waste or by-product of wood processing industry, can be efficiently utilized as an environmentally friendly, inexpensive alternative to WF as a filler in UF adhesive formulations for plywood manufacturing.

Keywords: UF resin; adhesive fillers; beech plywood; eco-friendly fillers; formaldehyde emission; ground birch bark.

Figure 1

Free formaldehyde emission (mg·L⁻¹) in five-layer plywood panels produced with urea-formaldehyde (UF) resin and three addition levels (10%, 15%, and 20%) of BB as adhesive filler. Different letters denote a significant difference. Means followed by the same letter do not statistically differ from each other (p ≤ 0.05).

Figure 2

Free formaldehyde emission (mg·L⁻¹) in five-layer plywood panels produced with UF resin and three addition levels (10%, 15%, and 20%) of BB as adhesive filler, measured after four weeks. Different letters denote a significant difference. Means followed by the same letter do not statistically differ from each other (p ≤ 0.05).

Figure 3

Differential scanning calorimetry (DSC) thermograph for (a) wheat flour (WF) at concentration 10% (red) and 20% (black) and (b) BB concentration 10% (red), 15% (blue) and 20% (black). Right half of the figure: the isothermal section at the temperature of 105 °C.

Figure 4

Thermal gravimetric analysis (TGA) curves (top) and derivative thermogravimetry (DTG) curves (bottom) of Ref10 (red) and BB15 (blue).

Figure 5

Left part: TGA curves to 105 °C of Ref10 (red dashed line), BB10 (red), BB15 (blue), BB20 (black). Right part: the isothermal section at the temperature of 105 °C.