Process for the synthesis of highly stable sulfonated melamine-formaldehyde condensates as superplasticizing admixtures in concrete

A process for preparing a stable sulfonated melamine-urea-formaldehyde condensate solution, which comprises the steps of:

• (a) condensing a mixture of melamine and urea with formaldehyde in an aqueous reaction medium having a formaldehyde:amino group ratio of 1:1 to 1.33:1, said condensation being conducted at a pH of from about 10 to about 13 at a temperature of from about 60.degree. to about 85.degree. C.;
• (b) sulfonating the melamine-urea-formaldehyde condensate formed in step (a) by adding a sulfonating agent to the reaction medium obtained in step (a) and by heating said reaction mixture to a temperature of from about 70.degree. to about 90.degree. C., said sulfonating agent being capable of yielding bisulfite ions or sulfite ions under the reaction conditions and being present in an amount sufficient to provide a sulfite:melamine ratio of from about 0.8:1 to about 1.2:1, said sulfonation being carried out at a pH of from about 10 to about 13;
• (c) further condensing the sulfonated melamine-urea-formaldehyde condensate formed in step (b) by reducing the temperature of the reaction medium from step (b) to about 40.degree.-60.degree. C., adjusting the pH thereof to about 2.5-4.0, maintaining said temperature and pH conditions for a period of from about 5 to about 150 minutes, and stopping said further condensation prior to gelation of the further condensed sulfonated melamine-urea-formaldehyde condensate by adjusting the pH of the reaction medium to about 6.5-9.0;
• and
• (d) stabilizing the aqueous sulfonated melamine-urea-formaldehyde condensate solution formed in step (c) by heating the reaction mixture obtained in step (c) to a temperature of from about 70.degree. to about 100.degree. C. for a period of from about 30 to about 180 minutes.