Straw Liquefied Preparation Of Biodegradable Polyurethane Foam Method And Mechanism

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.