麦秆液化制备可降解聚氨酯泡沫材料方法及机理研究

【作者】 陈秋玲；

【导师】 孙可伟；

【作者基本信息】 昆明理工大学 ， 环境工程， 2010， 博士

【摘要】 麦秆是我国主要的农业废弃物之一,产量巨大。由于缺乏高附加值的利用,每年大量麦秆被直接焚烧于田间地头,不仅浪费资源而且严重污染环境。以麦秆为原料制备聚氨酯泡沫材料,既可缓解以石油为原料的聚氨酯所面临的原料紧缺、环境污染等问题,又为麦秆等农业废弃物的高附加值利用提供新途径,对我国发展循环经济、低碳经济具有重要意义。目前,以麦秆为原料制备聚氨酯泡沫材料鲜有研究,特别是植物纤维整体液化时纤维素、木质素等主要成分的液化反应机理还未见报道。因此,本论文着重研究以麦秆为主要原料,经微波预处理后进行多羟基醇液化并进一步制备可降解聚氨酯泡沫材料的方法和相关机理。论文首先以液化残渣率作为液化效果的评价指标,研究了在浓硫酸(H2SO4)催化下,麦秆纤维在聚乙二醇(Polyethylene glycol,PEG)和乙二醇(ethylene glycol, EG)混合溶液中液化的工艺,结果表明,麦秆纤维液化的最佳工艺条件为：麦秆纤维含水率约为10%,粒径2-12目,液固比为5：1,反应时间60min,温度160℃,EG用量为20%,浓硫酸用量3%。在该条件下所得残渣率为4.8%,可以用作生物质多元醇。为研究麦秆纤维液化机理,论文采用有机溶剂萃取和柱层析相结合的方法,对麦秆液化混合液进行逐级分离和提纯,得到三种由麦秆纤维液化反应生成的化合物(化合物1,2,3),通过NMR及MS等分析鉴定其结构,结果表明：化合物1主要是麦秆纤维降解得到的混合多糖组分；化合物2为乙酰丙酸与不同聚合度的乙二醇酯,是由纤维素降解而来；化合物3含有2分子苯环,是含有愈创木基木质素和对羟基苯基木质素结构单元的酯类化合物。根据化合物2的结构特征,结合液化过程中羟值、酸值和残渣率以及残渣化学结构的变化,提出麦秆中纤维素的液化机理：纤维素在多羟基醇中的降解同时存在醇解-酯化和水解-酯化两种反应过程,并且水解反应对麦秆纤维的整体液化具有一定的催化作用。根据化合物3的结构,提出了木质素的多羟基醇降解机理：木质素首先在酸、水和醇的作用下发生降解生成对香豆醇、芥子醇等结构单元,其后发生氧化和酯化反应,同时酚羟基与液化试剂多元醇形成醚。上述三种化合物均富含羟基,因此均可直接与异氰酸酯反应制备聚氨酯材料,并且由于在多元醇中引入大量酯基,所制备的聚氨酯材料具有较好的生物降解性。为进一步提高液化效率,论文以水为介质,将麦秆纤维在不同的微波条件下进行预处理,研究微波处理对液化效率的影响,并利用FTIR, SME、XRD、DSC等仪器分析方法对微波处理前后的麦秆纤维形貌、化学结构和聚集态结构等进行分析对比,研究了微波加速液化反应的机理。结果表明：微波作用可显著降低液化残渣率,缩短反应时间,在最佳工艺条件下(微波功率340w,处理时间3min),微波处理可将液化时间由60min缩短为40min,或将硫酸催化剂用量由3%减小为2%,对液化效率提高作用明显；研究发现微波只是加快了液化反应的速率,不会改变液化反应的历程,微波加速麦秆纤维液化可能是通过微波的“致热作用”和“非致热作用”,一方面溶解部分木质素和半纤维素,破坏其对纤维素的包裹作用；另一方面使纤维素的结晶度降低,可及度变大,从而增强反应活性。利用液化所得到的混合液化产物(LWS)制备了可降解聚氨酯泡沫材料(polyurethane foam made of liquefied wheat straw, LWSPUF),最佳工艺条件为：催化剂用量为2%辛酸亚锡和1%三乙烯二胺,硬泡硅油泡沫稳定剂加入量为3%,发泡剂水用量2%,异氰酸酯指数为1.3,在此条件下室温发泡制备的聚氨酯泡沫压缩强度为169.35kPa,密度为35kg/m3,比相同条件下用传统化工原料PEG400制备的聚氨酯泡沫材料PEGPUF性能更好；红外和DSC分析表明,LWSPUF具有聚醚酯型聚氨酯的特征,其热稳定性好于PEGPUF。采用土埋法和简化的受控堆肥法评价聚氨酯泡沫降解性,结果表明,LWSPUF在土埋或受控堆肥条件下降解都明显比PEGPUF快,土埋条件下,180天质量损失达7.5%,而在受控堆肥条件下,84天可降解8.5%；综合分析结果表明,LWSPUF的生物降解主要是由于植物液化时产生的大量多糖和聚醚酯多元醇通过与异氰酸酯反应而留在聚氨酯高分子骨架中,在土壤微生物作用下,残留的糖苷键和酯键易于断裂,从而引起聚氨酯大分子的降解。通过本论文的研究提出了一种利用麦秆液化制备生物质多元醇并进一步制备可降解聚氨酯泡沫材料的新方法,提出了麦秆主要成分的液化降解机理和微波预处理加速液化的作用机理,为麦秆高附加值利用提供理论和实验基础。更多还原

【Abstract】 Wheat straw is one of the major and output-huge agricultural residues in China, but in many rural areas, due to the lack of high value-added complex utilization, most of them have to be burned at the site of their growth every year, resulting in great resource waste and serious environment pollution. Wheat straw can be used as an alternative raw material of the oil product to prepare polyurethane foams (named LWSPUF), which would not only settle down the shortage of oil, but also reduce the environmental pollution of the traditional non-undegradation polyurethane foams. Meanwhile, it is a new way with high added value for utilization of the wheat straw, also plays an important role in recycling economy and low-carbon economy in China.Researches on synthesis of LWSPUF are rarely reported at present, especially on the mechanism of liquefaction of cellulose and lignin existed in plant fiber. Therefore, this dissertation focused on investigating the preparing method of LWSPUF and mechanism of polyhydroxy-alcohol liquefaction.In this dissertation, liquefaction residue content as an evaluation index was used to evaluate the polyhydroxy alcohol liquefaction reaction of wheat straw fiber with concentrated sulfuric acid (H2SO4) as the catalyst, polyethylene glycol (PEG) and ethylene glycol (EG) as the liquefaction reagent. The index values showed that the optimum reaction conditions were that, moisture content of wheat straw was about 10%, sizes of wheat straw fibers was 2-12 mesh, liquid-solid ratio was 5:1, temperature of liquefaction was 160℃, reaction time was 60min, the ratio of PEG to EG was 4:1, the percentage of concentrated sulfuric acid (H2SO4) was 3wt%. Under these conditions, the content of wheat residue was only 4.8%, which indicated that a lot of wheat straw fiber was liquefied into biological polyhydric alcohol.A method that combined organic solvent extraction with column chromatography was adopted to research the liquefaction mechanism. Using this method, three kinds of compounds (compounds 1,2,3) was separated from the liquid mixture of wheat straw. According to their structure identification by NMR and MS, compound 1 was mainly the mixed polysaccharide component produced in the process of degradation of wheat straw fibers; compound 2 was levulinic acid glycol (and its condensation) ester; compound 3 contains two molecules of benzene ring, which are guaiacyl lignin and hydroxy esters unit of lignin structural. According to the structure of compound 2, the changes of hydroxyl value, acid value and residual rate in the process of liquefaction, it was found that there were two degradation reactions of cellulose (alcoholysis-esterification and hydrolysis-esterification) occurred in the existence of polyhydric alcohols. Especially in the second reaction, hydrolysis played an important role in promoting the liquefaction of wheat straw fiber. In the other hand, when the structure of compound 3 and the degradation process of lignin were analyzed, the result showed that, at the presence of acid, alcohol and water, lignin was firstly degraded into structural units of p-coumaric alcohol, erucic alcohol, and so on, and then ether was formated from phenolic hydroxyl and the polyol by oxidation and esterification. Finally, the three compounds are rich in hydroxyl groups as shown by NMR and MS, therefore they can be directly prepared polyurethane material by reaction with isocyanate. So wheat straw polyurethane foam with a better biodegradability can be prepared from the viewpoint of liquefaction mechanism.Additionally, in order to further enhance the efficiency of liquefaction, the microwave pre-treatment of wheat straw fibers in water under different conditions was also used to study effect of microwave treatment on the efficiency of liquefaction. The results showed that microwave can significantly enhanced the efficiency of liquefaction by reducing residue content and shortening reaction time. Under the optimum conditions (microwave power 340w, processing time of 3min), the liquefaction time was shortened from 60min to 40min, the amount of sulfuric acid catalyst decreased from 3% to 2%. The results also showed that microwave can accelerate the liquefaction of wheat straw through microwave’s thermal effect and non-thermal effect. The reason for this might be the structure destruction of lignin and hemicellulose by microwave, or might be the crystal change of cellulose, inducing the degradation or liquefaction of wheat straw become easier.Through experimental investigation, the optimum conditions for preparing biological degradation of polyurethane foam were:the catalyst weight percentage of stannous octoate was 2%, triethylene diamine 1%, the foam stabilizing reagent percentage of hard foam silicone oil was 3%, the water percentage in foaming reagent was 2%, and the isocyanate index was 1.3. The compressive strength of polyurethane foam prepared under these conditions was 169.35kPa at room temperature; it also has a density of 35kg/m3, the performance of which was better than that of traditional polyurethane foam materials of PEGPUF prepared by PEG400 under the same conditions. The infrared and DSC analysis to LWSPUF showed that LWSPUF had the characteristics of polyether ester polyurethane, but its thermal stability was better than that of PEGPUF. The soil burial and composting experiments were also adopted to test mass loss of polyurethane foam. The results showed that no matter soil burial method or composting method, the degradation rate of LWSPUF was greater than that of PEGPUF. The mass loss of LWSPUF buried underground after 180 days was 7.5%, while it was 8.5%under a controlled composting condition after 84 days. The comprehensive analysis demonstrated that high degradability of LWSPUF was mainly resulted from a large number of polysaccharide and polyether polyols in the polyurethane polymer structure. When residues glycosidic bond and ester bond were exposed to soil microorganisms, it was broken down so easily that polyurethane macromolecules can be fast degraded.In a word, by this experimental investigation, a new method for preparing biodegradable polyurethane foam materials using wheat straw as raw material has been set up, and the liquefied mechanism of main components of wheat straw as well as the mechanism of action of microwave accelerating liquefaction has been proposed for the first time. The experiment results will provide a way, play a foundation for theoretical research in the near future.更多还原