The addition of urea in divided portions in steps can lower the free formaldehyde content obviously, and mono-and di-substituted ureas exist in the resin after the end of reaction.

• 分次加尿工艺能明显降低树脂中的游离甲醛含量，游离尿素并不存在，尿素主要以一取代脉和二取代脉的形式存在。