2.2. Preparation of resols and adhesives
UF resin has a density of 1280–1290 kg/m3 at 20 ± 2 °C, pH value of about 8.5–9 at 20 ± 2 °C, a viscosity of 350–600 mPa s at 20 ± 2 °C and jell time of 40–50 s at 100 °C. The free formaldehyde ratio of UF is maximum 0.15% and the solution has a resin of 90 g + ammonium nitrate of 6 g and emulsion of 4 g.
The adhesives were prepared manually in advance by copolymerization at room temperature of 7% of corn flour (by weight) with the previously prepared resols [17].