Stable liquid melamine urea formaldehyde resins, hardeners, adhesive compositions, and methods for making same

A method of preparing a stable melamine urea formaldehyde resin, comprising:

• providing a first aqueous solution of formaldehyde;
• adding in a single step about 0.35 to 0.65 molar equivalents of urea based on the amount of formaldehyde provided in the first aqueous solution to form a UF mixture;
• adding acid to the UF mixture to adjust the pH to a value of about 4 to 7 to provide an acidified UF mixture;
• monitoring the viscosity of the acidified UF mixture until it reaches a value of about 150 cps to 1000 cps;
• adjusting the pH of the acidified UF mixture to a value of about 8 to 10 to provide a basic UF mixture;
• adding a second aqueous solution comprising about 0.24 to 1.27 molar equivalents of formaldehyde based on the amount of formaldehyde provided in the first aqueous solution to the basic UF mixture;
• adjusting the pH of the basic UF mixture to a value of about 8 to 10;
• adding in a single step about 0.15 to 0.55 molar equivalents of melamine based on the amount of formaldehyde provided in the first aqueous solution to the basic UF mixture to form an MUF mixture;
• monitoring the viscosity of the MUF mixture until it reaches a value of about 150 to 1000 cps.