Manufacture of stable urea/formaldehyde resins in a three stage reaction

Claim 1

A PROCESS FOR THE MANUFACTURE OF IMPREGNATING UREA/FORMALDEHYDE RESINS, PARTICULARLY FOR USE IN THE SURFACE-TREATMENT OF WOOD MATERIALS, WHICH COMPRISES: A. REACTING UREA AND FORMALDEHYDE IN AQUEOUS ACID SOLUTION IN A MOLAR RATIO OF FROM 1:1.5 TO 1:2.5 AND AT A CONCENTRATION TO GIVE A 40 TO 70% SOLIDS CONTENT RESIN SOLUTION, THE PH BEING FROM 4.5 TO 6.5, THE REACTION BEING CONDUCTED TO AT LEAST SUCH AN EXTEND THAT A SAMPLE OF THE SOLUTION CAUSES TURBIDITY WHEN ADDED DROPWISE TO COLD 50% MAGNESIUM SULFATE SOLUTION, BUT NOT TO AN EXTENT EXCEEDING THE POINT AT WHICH THE RESING STILL REMAINS SOLULE IN THE SOLUTION AT TEMPERATURES ABOVE 60*C, B. ADDING AT LEAST 0.2 MOLES OF FORMALDEHYDE PER MOLE OF UREA TO OBTAIN A MOLAR RATIO OF UREA TO FORMALDEHYDE OF FROM 1:2.5 TO 1:4 FOLLOWED BY CONDENSING AT A TEMPERATURE OF BETWEEN 40*C AND THE BOILING POINT OF THE SOLUTION AND AT A PH OF BETWEEN 3.5 AND 9 UNTIL THE RESIN SOLUTION IS CAPABLE OF BEING DILUTED WITH AT LEAST 20 VOLUMES OF WATER AT ROOM TEMPERATURE TO FORM A CLEAR SOLUTION, AND C. ADDING UREA TO GIVE A FINAL MOLAR RATIO OF UREA TO FORMALDEHYDE OF FROM 1:1.5 TO 1:2.5 EFFECTING POST-CONDENSATION OF THE MIXTURE OF A PH OF BETWEEN 4.0 AND 6.5 UNTIL ALL FREE UREA HAS DISAPPEARED.