脲醛树脂及其改性树脂的化学结构

利用13C-NMR分析了在传统合成工艺(碱—酸—碱)条件下,脲醛树脂及其改性树脂在反应不同时刻的结构差异。用于13C-NMR分析的样品分别在碱性阶段保温开始时、保温结束时和总反应结束时采集。通过对化学位移的对比分析,得到以下结论,脲醛树脂和聚乙烯醇改性脲醛树脂在相同的反应时刻,存在相同的反应特征和结构特征,因此,不能证明聚乙烯醇参与了反应。三聚氰胺的改性作用在于,它不仅能与甲醛反应,而且能与尿素甲醛发生共缩聚反应;在树脂的缩聚阶段除了有亚甲基化的反应外,还存在着醚键的分解反应,少量的醚键在反应结束后仍然存在。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,尿素主要以一取代脲和二取代脲的形式存在。

Chemical Structure of Urea-Formaldehyde Resin and Modified Urea-Formaldehyde Resin

The structures of UF resin, melamine modified UF resin, polyviol modified UF resin and melamine-polyviol modified UF resin synthesize by the traditional craft (alkali-acid-alkali) were compared by means of 13C-NMR at different reaction moments. The samples used for 13 C-NMR analysis are seperately collected at the beginning and the end of heat preservation in alkali stage as well as at the end of whole reaction. In contrast with the chemical shift, UF resin and polyviol modified UF resin have similar reaction features and structural characters at the same reaction moment, therefore, the polyvinyl alcohol do not take part in the reaction. The reaction character of melamine not only reacts with formaldehyde but also copolymerizes with urea and formaldehyde. At the end of reaction, besides methylenation reaction, there still exists a decomposition reaction of the ether linkage, and a little amount of ether linkage still occurs after the end of reaction. 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.