模塑制品热压过程中木素结构变化研究

目的探讨以高得率浆(HYP)为原料制备模塑制品过程中木素结构的变化规律,为研究以HYP为代表的高木素含量原料制备模塑制品的强度形成机理提供理论依据,为逐步构建纸浆模塑技术纤维结合强度形成理论奠定基础。方法采用相对分子质量测定(GPC)、红外光谱分析(FT-IR)、二维核磁共振谱图(2D-HSQC)、定量磷谱分析(~(31)P-NMR)等手段,通过模拟纸浆模塑热压干燥过程前后木素结构的对比,研究热压过程对木素结构的影响。结果基于落叶松HYP为原料的热压过程,模塑浆板(MPB)中的磨木木素(MWL)分子质量由5216 g/mol提高到6132 g/mol;FT-IR分析证实MPB-MWL的非共轭羰基减少,共轭羰基增加;从HYP-MWL到MPB-MWL的过程中,2D-HSQC解析证实树脂醇(B)结构百分比由8.66%减少到6.68%(以芳环为内标,下同),而苯基香豆满(C)结构百分比由15.45%增加到20.88%;由~(31)P-NMR定量结果可知,MPB-MWL中脂肪族羟基、G型酚羟基结构、对-羟基酚羟基均较HYP-MWL有所降低。结论 MPB-MWL相对分子质量的提高说明木素在热压过程中发生了聚合反应;FT-IR分析证实非共轭羰基(C=O)化学键的断裂、共轭羰基(C=O)化学键的生成在热压过程中共存;2D-HSQC和~(31)P-NMR分析证实木素解聚反应和聚合反应同时发生,是一对竞争性的反应,但聚合反应的程度大于解聚反应。

Structural Changes of Lignin during Molding Hot Pressing

The work aims to discuss the structural changes of lignin in the process of preparing the molding products with the high yield pulp (HYP) as the raw material, provide theoretical basis for the study on the strength formation mechanism of the molded products prepared by the raw material with high content of lignin represented by HYP, and lay a foundation for gradually forming the theory of formation of fiber bonding strength based on the pulp molding technology. By means of gel permeation chromatography (GPC), Fourier transform infrared (FT-IR), two-dimensional heteronuclear signal quantum coherence (2D-HSQC) spectra and quantitative 31P-NMR, the comparison of the lignin structures before and after the pulp molding hot pressing and drying process was simulated. The influence of hot pressing process on the lignin structure was studied. Based on the hot pressing process with the dahurian larch HYP as raw material, the molecular weight of MWL in the molding pulp boards (MPB) increased from 5216 g/mol to 6132 g/mol. FT-IR analysis confirmed that the non-conjugated carbonyl groups of MPB-MWL decreased while the conjugated carbonyl groups increased. 2D-HSQC analysis confirmed that the resinol (B) structure percentage from HYP-MWL to MPB-MWL decreased from 8.66% to 6.68% (with aromatic ring as the internal standard, the same below), while that of the phenylcoumarans (C) structure increased from 15.45% to 20.88%. 31P-NMR quantitative results showed that, the structures of aliphatic hydroxyl groups, G type phenolic hydroxyl groups, and p-coumarates in MPB-MWL were lower than those in HYP-MWL. The increase in molecular weight of MPB-MWL illustrates that the polymerization of lignin occurs in the process of hot pressing. FT-IR analysis indicates that both the cleavage of non-conjugated (C=O) chemical bonds and the formation of con-jugated (C=O) chemical bonds coexist during the hot pressing. 2D-HSQC and 31P-NMR analysis confirms that depolymerization and polymerization occur simultaneously, which are a pair of competitive reactions, but the extent of polymerization is greater than depolymerization.