尿素改性木质素基酚醛树脂的性能

Performances of lignin-phenol-formaldehyde resin modified by urea

木质素基酚醛树脂(LPF)固化速度较慢,阻碍其推广与应用。在LPF树脂合成过程中添加尿素,并对合成的尿素改性木质素基酚醛树脂(LPUF)树脂的凝胶时间及所制胶合板的胶合强度进行了分析。同时,通过核磁共振(13C NMR)及差示扫描量热(DSC)技术对树脂分子特征结构、固化温度及固化反应热进行表征,研究尿素添加量对LPUF树脂化学结构、固化特性及其胶接性能的影响。结果表明,随着尿素添加量的增加,LPUF树脂中酚环与尿素单元之间的共缩聚亚甲基桥键含量明显增加。添加适量尿素可以提高LPUF树脂的缩合程度,实现树脂的快速固化,明显降低树脂游离甲醛含量。添加过量的尿素会生成低分子量取代脲,不利于固化后树脂的交联密度及所制胶合板的胶接性能。本研究范围内,当尿素添加量为6%时,LPUF树脂具有较快的固化速度,其所制胶合板的耐水胶合强度满足GB/T 9846—2015中I类胶合板的胶合强度要求。

Lignin-phenol-formaldehyde resin(LPF)has been widely accepted as a wood adhesive.However,the commercialization and application of LPF are substantially restricted by its slow curing property.It has been confirmed that divalent metal ions,such as Mg 2+and Zn 2+,could promote the curing of LPF.However,it still has not met the requirements of the practical application of LPF in the industrial scale.A further improvement in the curing rate of LPF is of great necessity.The curing rate of urea-formaldehyde resin is much higher than that of the phenol-formaldehyde resin,because of the special molecular structure of the urea-formaldehyde resin.Inspired by this reason,the curing rate of LPF could be improved by the incorporation of urea.In this study,the effects of urea amount on the chemical structure,curing characteristic and plywood bonding property of the lignin-phenol-urea-formaldehyde(LPUF)resin were investigated by characterizing the functional groups,gel time,curing temperature and curing enthalpy with the liquid 13 C nuclear magnetic resonance(13 C NMR)and different scanning calorimetry(DSC).The results indicated that,with the increase of the amount of urea,the formation of phenolic methylene linkages and co-polymer linkages in LPUF resin were increased.An appropriate amount of urea in the formulation could improve the condensation of the resin,promote the curing process,and decrease the free formaldehyde content of LPUF resin.However,excess urea could lead to the formation of a large amount of low molecular weight methylolurea,which was not beneficial for the curing and performance of the LPUF resin.Within the scope of this study,the LPUF resin with 6%urea amounts had relatively higher curing rate,and the water bonding strength of the plywood made by this LPUF resin can meet the bonding strength requirements of type I plywood according to China Industry Standard(GB/T 9846-2015).