| 摘要 |
PCT No. PCT/EP87/00392 Sec. 371 Date Sep. 19, 1988 Sec. 102(e) Date Sep. 19, 1988 PCT Filed Jul. 18, 1987 PCT Pub. No. WO89/00587 PCT Pub. Date Jan. 26, 1989.Urea-formaldehyde resins which may be cured to give products having a low total extractable formaldehyde content are prepared by the following process: (i) aqueous 30-50% 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 50 DEG C., preferably 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 until the formaldehyde:urea molar ratio is within the range 0.8-8:1. 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. |
