Add paraformaldehyde to an aqueous urea solution (first portion), heat to 60°C, and adjust the pH to 7 using sodium hydroxide. Reflux the mixture for 20 minutes, adjust the pH to 5 with formic acid, and reflux for another 30 minutes. Subsequently, neutralize the mixture with a 40% sodium hydroxide solution, add urea (second portion), and adjust the pH to 5.8 using formic acid solution. Finally, heat under reflux until the viscosity reaches 11.5 cP (at 20°C) to obtain the adhesive.
To use the adhesive, first roughen the bonding surfaces using a roughening machine or sandpaper. Then, apply the adhesive to the surfaces to be bonded; after a brief interval, join the two surfaces together and press them firmly using a heavy object.
