基于纳米纤维素的脲醛树脂微结构调控及增强机制研究

Molar ratio reduction of formaldehyde/urea is the most effective way for urea formaldehyde(UF) resin environmentally friendly modification, which also reduces the bond property of the adhesive. Based on the preliminary researches, cellulose nanofiber(CNF) is introduced into UF resin synthesis system to adjust microarchitecture of the resin. Three contents are investigated by aggregation morphology observation of CNF/UF resin and cured CNF/UF resin, curing characteristics evaluation, and cross section observation of the broken cured resin. The first one is to investigate the effect of the CNF preparation condition on aspect ratio, surfaceness, electric charge, and functional groups of the CNF. This process could make CNF size and surface character controllable. The second one is to investigate the effect of CNF size, surface character, and electric charge on UF resin crystallinity, crystal morphology, crystal size, and aggregation morphology. This process could implement UF resin microarchitecture regulation. The third one is to investigate the effect of CNF addition on curing behavior, aggregation morphology after curing, crack and crazing on the cross section of the curd resin. This process could help to understand the effect of UF resin microarchitecture variation on the resin curing behavior and cross section of the broken cured resin. The action mechanism of CNF on elevated properties of UF resin will be concluded and reinforcement mechanism of CNF on UF resin will be revealed, which could provide new way for properties elevation of the UF resin with low molar ratio and resulted products formaldehyde emission control.

降低甲醛/尿素摩尔比是脲醛树脂低毒化改性最有效的方法，但改性树脂的固化胶接性能变差。本项目在前期研究成果基础上，拟将纳米纤维素引入脲醛树脂合成体系，调控脲醛树脂微结构，通过观测纳米纤维素改性脲醛树脂胶液及固化后聚集形态、固化特性变化、胶接破坏形态变化，研究制备条件对纳米纤维素长径比、表面粗糙度、电荷、表面活性基团的影响，实现纳米纤维素形态尺寸及表面特性调控；研究纳米纤维素形态尺寸、表面特性及电荷对脲醛树脂结晶度、晶体形态、结晶尺寸、聚集形态等的影响关系，实现纳米纤维素改性脲醛树脂的微结构调控；研究纳米纤维素对脲醛树脂的固化热行为、固化树脂聚集形态、胶接破坏界面裂隙、银纹形态等的影响关系，探讨纳米纤维素改性脲醛树脂微结构变化对其固化行为及胶接破坏形态的影响规律；揭示纳米纤维素对脲醛树脂性能影响的机制，解明其增强机理，为低摩尔比脲醛树脂性能提高及胶接制品甲醛释放量控制提供新思路。

降低脲醛树脂（UF）摩尔比是降低制备人造板甲醛释放量最有效的方法，但随摩尔比降低，树脂更多表现出胶体性质，力学性能急剧下降，其改性与增强难以用经典缩聚理论指导。本研究在胶体理论基础上，从UF树脂微结构入手，利用改变甲醛/尿素摩尔比、与三聚氰胺共缩聚建立树脂微结构（结晶结构、聚集形态、结晶区尺寸和分布）衡量体系，利用纳米纤维素调控脲醛树脂微结构，通过调控前后UF树脂固化特性、热稳定性、水解稳定性、胶接性能以及人造板甲醛释放量等表征，揭示了低摩尔比脲醛树脂固化与胶接机理，解明了纳米纤维素对UF树脂微结构作用机制和UF树脂微结构变化与性能影响关系，完善了脲醛树脂的胶体理论，为低摩尔比UF树脂改性提供新思路和理论支持。主要研究结论如下：.⑴ 当F/U摩尔比低于1.2时，UF树脂分子主要以胶体粒子形态存在，固化过程中胶体粒子聚集形成球状聚集体和结晶区。当摩尔比从1.2降低到0.9时，树脂结晶度提高14.26%，球状聚集体直径增加612.93%，从而树脂水解稳定性和热稳定性提高，制品甲醛释放量降低。.⑵ 三聚氰胺改性后树脂分子支链羟甲基含量增加，固化时发生交联反应降低树脂结晶度。当添加量从5%提高至15%时，树脂结晶度降低6.36%，球形聚集体直径降低36.77%，结晶区直径增加61.64%，制备胶合板胶合强度提高19.27%，甲醛释放量降低46.05%。.⑶ 纳米纤维通过表面羟基与UF树脂分子间形成氢键，诱导树脂胶体粒子在其周围有序排列，提升树脂结晶度并将不规则分布的大球形结晶区转化成均匀分布的小球形结晶，实现树脂自增强。加入0.3%NCC使树脂结晶度提高14.89%，球状聚集体直径降低36.95%，树脂热稳定性降低，水解稳定性提高，胶合板胶合强度提高32.79%，甲醛释放量降低21.32%。纤维素纳米纤维（CNF）诱导UF树脂胶体粒子在其周围聚集，但其网状形态阻碍结晶结构的形成。加入0.3%CNF使树脂结晶度降低12.82%，球状聚集体和结晶区消失，导致树脂热稳定性和水解稳定性降低，胶合板胶合强度提高54.98%，甲醛释放量增加25.75%。