Лако-красочные материалы — производство

Follow the same procedure as for formulation A, but at the end of the water vacuum distillation add 1.7 to 1.9 parts by mass of the second melamine and heat the reaction mixture to 95°C again and maintain this temperature for 5 to 6 min, then cool rapidly.

B. MUF Formulation for Exterior Particleboard

This sequential MUF formulation can be successfully manufactured at different (M + U):F molar ratios according to exactly the same procedure, and not only for the proportions as indicated in the example that follows. Thus the same formulation gives excellent results for example at (M + U):F molar ratios of 1:1.5 and 1:1.7, but not only these ratios. For more MUF formulations see references [8,11,38].

To 113 parts by weight of Formurea (a formaldehyde concentrate stabilized by urea, of mass content 57% formaldehyde and 23% urea. NB: Formurea comes in other con­centrations too) are added 13 parts of urea and 30 parts of water. The pH is set at 10 to 10.4 and the temperature brought to 92 to 93°C under continuous mechanical stirring. The pH is then lowered to 7.8 and the reaction continued at the same temperature, allowing the pH to fall by itself over a period of 1 h 30 min to 1 h 35 min to a pH of 5.2 (one should strictly prevent the pH falling under 5 to avoid both uncontrollable reactions taking hold as well as a decrease in the finished adhesive performance later). To bring the pH back to 9.5 or higher, 22% NaOH water solution was added, followed by 41 parts by weight of melamine premixed with 19 parts of water. One part of dimethylformamide and 2 parts of diethylene glycol are then added to the reaction mixture, maintaining a temperature of 93°C. The water tolerance is checked every 10 min while the pH is allowed to fall by itself. When the water tolerance reached is 180 to 200% (this is often reached after 35 to 40min, and the pH reached is of 7.2), 6.5 parts by weight of second urea is added and the pH is again brought up to 9.5. The reaction is continued until the water tolerance reached is lower than 150% (the pH has reached generally 7.7 at this stage). The pH is then corrected to 9-10.2 again and the reaction mixture cooled and stored. Resins produced using this procedure have solids contents of 58 to 65%, a density of 1.260 to 1.280 at 20°C, a viscosity of 70 to 150 cP, free formaldehyde of approximately 0.32, and gel times with 3% NH4Cl of 51 to 57 s at 100°C. Increasing and lowering of pH where the pHs indicated are not reached by the reaction time can be done by addition of 22 to 33% NaOH water solution (pH increases) and by addition of formic or acetic acid (pH decrease). The preparation diagram of this resin is shown in Fig. 4.