Process for the preparation of urea-formaldehyde resins

Claim 1

A process comprising mixing an aqueous solution of formaldehyde and urea in a mole ratio of from 2.0:1 to 2.2:1, adjusting the pH to from 6.1 to 7.0, heating the mixture at from 90° to 100° C. for from 20 to 40 minutes, while at this temperature adjusting the pH to from 4.5 to 5.5, adding from 0.06 to 0.13 mole of additional urea per mole of formaldehyde originally present, continuing heating the mixture until the viscosity of the reaction mixture is from about W to about Y on the Gardner-Holt scale, adjusting the pH of the reaction mixture to from 6.8 to 7.2, cooling the reaction mixture to from 30° to 70° C., adding from 0.1 to 0.3 mole urea per mole of formaldehyde originally present, cooling the reaction mixture to from 30° to 40° C., and adding from 0.05 to 0.2 mole of formaldehyde per mole of formaldehyde originally present and from 0.1 to 0.3 mole of urea per mole of formaldehyde originally present to bring the total mole ratio of formaldehyde to urea to from 1.05 to 1.15 whereby a urea-formaldehyde resin containing less than 0.2 ppm free formaldehyde is produced.