| 摘要 |
<p>Urea-formaldehyde resins which may be cured to give products having a low total extractable formaldehyde content are prepared by the following process: (i) an aqueous solution containing more than 50% formaldehyde and urea are mixed in a formaldehyde:urea molar ratio of 2-3:1 at pH 6-11, (ii) this mixture is heated to at least 80 DEG C, (iii) a mineral or organic acid is added to bring the mixture to pH 0.5-3.5, (iv) the mixture is heated at a temperature between 80 DEG C and the reflux temperature, preferably for 15 minutes to 1 hour, (v) keeping the temperature at or above 80 DEG C, a base is added to bring the mixture to pH 6.5-9 and (vi) urea is added to the mixture at a temperature up to 45 DEG C until the formaldehyde:urea ratio is within the range 0.8-1.8:1. In a modification of the process, solid particulate resins are obtained by reacting the mixture from step (v) with urea at a pH below 7 to increase the viscosity thereof, adding a base until the mixture reaches a pH of 6.5-9 and dehydrating the resulting mixture.</p> |
