Abstract: The degradation mechanism of biodegradable waterborne polyurethane is briefly introduced. The research status and latest developments of biodegradable waterborne polyurethane modified by cellulose, lignin, starch and vegetable oil are reviewed. Finally, the biodegradable water is available. Prospects for research and application of polyurethane。
　　
       Key words: waterborne polyurethane; biodegradation; research progress　　
　　0. Preface　　
　　Waterborne polyurethane (WPU) is a polyurethane emulsion using water as a dispersion medium. It is widely used in coatings, adhesives, leather, textile industry, etc. because of its environmentally friendly, non-flammable, non-toxic and solvent-based polyurethane. Field [1-2]. However, due to the difficulty in the degradation and recycling of polyurethane in nature, the use and quantity of its products will increase the pollution of the environment. At present, the main raw material source of synthetic waterborne polyurethane is still petrochemical products, and the increasingly serious oil crisis will limit the source of raw materials in the polyurethane industry and affect its development. Therefore, research and development of degradable polyurethane materials using renewable natural polymers as raw materials can not only reduce environmental pollution damage, but also alleviate the dependence of the polyurethane industry on petroleum products. Natural polymers include cellulose, starch, lignin, etc., and these structures are rich in hydroxyl groups. In theory, they have the potential to replace polyurethanes with isocyanate groups to prepare polyurethane materials. The use of natural polymers to make a variety of biodegradable waterborne polyurethane materials can reduce the amount of petroleum-based polyols and impart biodegradability to the products. This will develop the polyurethane industry and alleviate the energy and environmental crisis facing humanity today. Are of great significance。
　　
　　1. Degradation mechanism and characterization of biodegradable polymer materials　　
　　Any material that does not cause damage to the Earth's ecological environment can be called a recyclable material. There are many kinds of recyclable materials, and the most popular materials are biodegradable polymer materials. The biodegradation of polymer materials refers to the process of degradation and assimilation of polymers under the action of organisms (mainly fungi, bacteria, etc.). The degradation mechanism of polymers is very complicated. It is generally believed that biodegradation is not a single mechanism. It is a complex biophysical and biochemical effect, accompanied by other physical and chemical effects, such as hydrolysis and oxidation. Biological and physical interactions promote each other. Synergistic effect [3]. The natural polymer compound-modified polyurethane material is roughly divided into two stages in biodegradation: (1) Degradation of the natural polymer compound causes the polyurethane to form many micropores on the surface. As the pores gradually increase, the urethane bond is first decomposed by microorganisms; (2) as the micropores are generated, the microorganisms easily invade the internal natural polymer compound, forming micropores inside, and accelerating until the polyurethane is completely degraded [4]. At present, the commonly used biodegradability characterization methods are: residual mass and relative molecular mass determination method [5], structural change method, decomposition product detection, mechanical strength method, appearance method, mold method. There are still some shortcomings in various analytical characterization methods related to biodegradability. Therefore, in the research process, it is often necessary to adopt a variety of methods to comprehensively test according to the specific situation [6].　　
　　2. Structural characteristics of biodegradable waterborne polyurethane　　
　　Biodegradation mainly depends on the size and structure of the polymer molecules, the type of microorganisms and environmental factors, and the molecular structure and composition of the materials are the decisive factors [3]. In general, [7], an aliphatic ester bond, a peptide bond > a carbamate > an aliphatic ether bond > a methylene group. In addition, polymer materials with large molecular weight, regular molecular arrangement, and high hydrophobicity are not conducive to the erosion and growth of microorganisms, and are not conducive to biodegradation. Polyurethanes are high polymers containing repeating urethane segments in the macromolecular backbone. The main chain of the polyurethane is usually composed of a soft segment (soft segment) having a glass transition temperature lower than room temperature and a rigid segment (hard segment) higher than room temperature, and the soft segment is composed of an oligomer polyol (such as polyester). The polyether is composed of a diisocyanate and a low molecular chain extender such as a diamine and a diol.　　
　　Due to the strong polarity of the hard segments, the mutual attraction is large, and the hard segments and the soft segments have a tendency to spontaneously separate thermally. Therefore, the hard segments are easy to gather together, forming many microdomains, which are distributed in the soft segment phase to form a microphase separation structure [8]. The microphase surface structure of polyurethane is very similar to biofilm, and has good biocompatibility. The chain structure of polyurethane determines its biodegradability. In the synthesis of polyurethane, polyurethane can be designed by selecting different blocks and adjusting the ratio between soft and hard segments to synthesize different chemical structures (such as linear, branched, crosslinked) and mechanical properties. (Rigid, flexible) and thermal properties of polyurethane to meet different application requirements [9].　　
　　3. Principle and type of preparation of biodegradable waterborne polyurethane　　
　　The biodegradable polyurethane material is prepared by utilizing the high activity of the isocyanate group of the polyisocyanate component and the biodegradability of the natural polymer compound, and theoretically, the natural polymer compound containing a plurality of hydroxyl groups can be used as the polyol component. First, a molecular chain decomposable by microorganisms is introduced into the polyurethane material by a reaction between the polyol component and the isocyanate component [10]. When treated by the soil-buried method, the material undergoes hydrolysis and oxidation under the action of microbial enzymes. These molecular chains are broken into fragments of low relative molecular mass. After the microorganisms absorb or consume these fragments of low molecular weight, they undergo metabolism. The formation of carbon dioxide, water and bio-energy, and ultimately achieve the purpose of degradation [5]. Because polyurethane has certain sensitivity to common microorganisms, it is an ideal raw material for the production of biodegradable materials, and its structure can be freely designed. Therefore, there are many types of biodegradable polyurethanes currently studied, mainly polyether type and polyester type. Polyurethane and natural biodegradable polymer modified polyurethane. Polyethers for synthesizing polyether polyurethanes include polyoxyethylene (PEO), polytetramethylene ether (PTMO), polyoxypropylene (PPO), and the like. Compared with the polyether type, the polyester type polyurethane is more susceptible to degradation, mainly because the polyester is easily hydrolyzed in the living body. Commonly used polyesters are PCL, PLA, PGA and copolymers thereof such as lactic acid/glycolic acid copolymer (PLGA). Numerous studies have shown that these polyester soft segments are also very biodegradable and can be safely degraded in living organisms [11]. YOUNGDUKKIM et al. [12] synthesized several polyester polyurethanes with different chemical structures, and studied their biodegradability by hydrolysis, enzymatic degradation and heaping. It has been found that polyurethanes synthesized using aliphatic diisocyanates and polyester polyols having more soft segments have good biodegradability.　　
　　4. Research progress of biodegradable waterborne polyurethane　　
　　4.1 Cellulose Preparation of Biodegradable Waterborne Polyurethane　　
　　Cellulose is the most abundant natural polymer on the earth. It is mainly derived from trees, cotton, hemp, cereals and other higher plants. It can also be produced by the enzymatic hydrolysis process of bacteria. It is inexhaustible and inexhaustible in nature. A renewable resource that produces about 200 billion tons of plant cellulose per year on Earth. Cellulose contains a large amount of hydroxyl groups, which easily form intramolecular and intermolecular hydrogen bonds, making it difficult to dissolve and refractory. Therefore, it is usually treated specially for the synthesis of degradable polymer materials [13]. Cellulose is a polyhydroxy compound that reacts with isocyanate groups in a manner to form biodegradable polyurethane materials. IVANAMAROVA et al [14] used different concentrations of carboxymethyl cellulose as a biopolyol to partially replace the commercial polyether polyol to synthesize polyurethane foam. The test results show that the sample is biodegradable and indicates the degree of degradation and modification. The type of (filler) is related to its concentration. The method for preparing biodegradable polyurethane materials from cellulose has been relatively mature, but there are few reports on the modification of water-based polyurethane with cellulose. On the one hand, cellulose cannot be dissolved in water and general organic solvents, and it needs to be chemically modified. On the other hand, although the solid form of cellulose is rich in hydroxyl groups, only a small part of the hydroxyl groups on the surface can participate in the reaction, only liquefaction. It can then be used as a polyol. YIXIANGWANG et al [15] successfully prepared starch nanocrystals (SN) and cellulose whiskers (CW) by using sulfuric acid hydrolysis to treat waxy corn starch granules and cotton linter pulp.　　
　　SN and CW were characterized by transmission electron microscopy, atomic force microscopy and wide-angle X-ray diffraction, respectively. Starch nanocrystals and cellulose whiskers are embedded in an aqueous polyurethane matrix using casting/evaporation techniques. The study found that when the mass fraction was 1% SN and 0.4% CW, the tensile strength, Young's modulus and fracture energy of the composites were significantly increased by 135%, 252% and compared with the unmodified waterborne polyurethane. 136%, the elongation at break is basically flat. The addition of water-based polyurethane composites with mass fraction of 1% SN and 0.4% CW was significantly stronger than that of SN and CW, respectively, indicating that SN and CW have synergistic enhancement effects on waterborne polyurethane. In addition, the waterborne polyurethane-based nanocomposites prepared by SN and CW have a large thermal resistance. The results show that different polysaccharide nanocrystals and whiskers combine to form a strong hydrogen bond network, which has synergistic enhancement effect on waterborne polyurethane. This work provides a new eco-friendly approach to the preparation of high performance polymer nanocomposites by using natural nanocrystals and whiskers to simultaneously modify the polymer.　　
　　4.2 Preparation of biodegradable waterborne polyurethane by lignin　　
　　Lignin is the most abundant and important organic polymer in the plant world, which is second only to cellulose. It is widely distributed in higher plants above the fern plants with vascular bundles, and is unique to gymnosperms and angiosperms. Chemical composition [16]. At present, lignin raw materials used in polymer materials are mainly derived from derivatized products of by-products of the paper industry, such as lignosulfonates. In the molecular structure of lignin, active groups such as an aromatic group, a phenolic hydroxyl group, an alcoholic hydroxyl group, a carboxyl group, and a conjugated double bond are present, and various types of chemical reactions can be performed. The lignin-modified polyurethane mainly utilizes various groups such as a carboxyl group, a phenolic hydroxyl group and an alcoholic hydroxyl group in the molecule, and these groups can react with the isocyanate group. Therefore, lignin and its derivatives can be used to replace some or all of the polyether or polyester polyols for the preparation of polyurethanes. The research on the preparation of polyurethane materials with lignin is relatively early and mature. Polyurethanes prepared from lignin and its derivatives can be used as engineering plastics, adhesives, films, etc. according to their properties [13]. There are few research work on lignin-modified water-based polyurethanes. Because of the large number of reactive groups contained in the structure, it is usually required to be processed to meet the requirements of synthetic polymer composites. GUOJUANCUI et al [17] used calcium lignosulfonate (LS) to modify waterborne polyurethane. LS is mainly introduced into aqueous polyurethane by chemical grafting or physical action to prepare a star network structure and a complex of LS as a center of supramolecular complex. When the addition amount of LS is 1.5%, the strength and elongation simultaneously increase, and the center of the polymer network is controlled by the spheroid of the lignin. With the increase of LS content, the content of WPU/LS composites is increasing, but the elongation is decreasing continuously. Because of the increase of LS content, it aggregates itself to form supramolecular complexes. The polymer network center is gradually replaced by LS supramolecular complexes. The structural changes of WPU/LS blends were characterized by FTIR, DSC and DMA. The results show that LS is mainly fused in the hard segment of waterborne polyurethane. The affinity between the hard segment and the LS and the chemical grafting of the LS promote a physical interaction, especially hydrogen bonding. In order to simultaneously increase the strength and elongation at break of WPU/LS composites, GUOJUANCUI et al. [18] used nitrated lignin (NL) to modify aqueous polyurethane to form a similar star network structure. The study found that when 3.0% NL was added, the mechanical properties of the composite were the best, and the mechanical strength of the composite was 71.3 MPa, which was 3.6 times that of the pure waterborne polyurethane. The simultaneous increase in mechanical strength and elongation depends on the formation of a star network structure in the composite. The rigidity of the lignin contributes to an increase in mechanical strength, and the crosslinked structure in the polymer can increase the elongation. However, too high a content of NL will aggregate on its own, preventing the formation of a star network structure, thereby reducing the mechanical strength and elongation of the material.　　
　　4.3 Preparation of biodegradable waterborne polyurethane　　
　　Starch is the most abundant carbohydrate in nature. It is widely found in plant seeds, leaves, tubers, roots, fruits and pollen with tiny, cold water-insoluble particles.　　
　　At the end of the 20th century, the world's total starch production reached 30 million tons per year. The commercial starch used was mainly corn starch and waxy corn starch, potato starch, wheat starch and tapioca starch [19]. In recent years, the preparation of starch-modified waterborne polyurethane is mainly based on blending modification, and the starch is physically or chemically modified. Researchers at Wuhan University in China have made some explorations on the preparation of modified waterborne polyurethane as a blending component and made some progress. Literature [20] reported that polyester waterborne polyurethane (WPU) and thermoplastic starch (TPS) were directly blended and thermoformed into sheets. Through mechanical properties research, it is found that when the WPU mass fraction of the blend is 5% to 30%, the tensile strength of the material is higher than that of TPS and WPU, and the elongation at break is between the two and much higher than TPS. This shows that the addition of WPU can effectively improve the mechanical properties of TPS. By studying the water resistance of the system, it was found that the R value of the TPS sheet (R = σb (wet) / σb (dry), σb is tensile strength) is 0.01 and the R value is 0.24 when 30% WPU is contained, and the water absorption of TPS The rate is 317%, which is much higher than 34% to 70% after joining WPU. At the same time, as the content of WPU in the system increases, the moisture content of the material decreases significantly, which indicates that the addition of WPU can improve the water resistance of TPS. Through infrared spectroscopy, X-ray diffraction and differential scanning calorimetry and scanning electron microscopy, it was found that there is a certain compatibility between TPS and WPU, which is the main reason for the significant improvement of the mechanical properties of the blended system. In addition, they also formed the blending process of the two into a film, and obtained the same conclusion by studying the mechanical properties, water resistance and compatibility of the blended materials [21]. GUANGJUNCHEN [22] and the like treated the potato starch with an aqueous solution of sulfuric acid, and a part of the obtained starch nanocrystal (StN) precipitate was uniformly dispersed in water by ultrasonic wave, and a part of StN was dispersed in methyl ethyl ketone. The modified waterborne polyurethane is prepared by three methods: Method 1, adding StN aqueous dispersion to WPU emulsion; Method 2, adding StN aqueous dispersion in the emulsification process; Method 3, adding butanone dispersed StN and DMPA in The chain extension stage of the polyurethane prepolymer is added. The tensile strength, elongation at break and Young's modulus of the StN/WPU composite prepared by the method 1 were improved in comparison with the unmodified WPU. When StN mass fraction is 2%, tensile strength and elongation at break are 28.6 MPa and 1406.6%, respectively, which are 2.6 times and 1.7 times that of unmodified WPU; when StN mass fraction is 5%, tensile strength and yang The modulus is 51.5 MPa and 5.2 MPa, respectively, which is 370% and 93% higher than that of unmodified WPU. The results show that starch nanocrystals can simultaneously toughen and enhance WPU, and the strong interfacial interaction between StN and WPU matrix is related to the transfer of rigid starch nanocrystal stress. Method 1 does not change the structure and interaction of WPU. one. Different from the mechanical properties of nanocrystals to improve WPU, the nanocrystals in such nanocomposites have low content, but the mechanical properties of WPU are obviously improved. PETERR.CHANG et al [23] obtained the grafted polycaprolactone (PCL) molecular chain by ring-opening polymerization of caprolactone on the surface of starch nanocrystals (StN) particles under microwave conditions to obtain StN-graft-PCL. The graft modified StN was introduced into the WPU matrix by blending. The structure and performance test and analysis of the WPU/StN-graft-PCL nanocomposite showed that the mass fraction was 5 compared with the unmodified WPU. The %StN-graft-PCL added amount of WPU showed a significant increase in tensile strength and elongation at break. The results show that the simultaneous increase of tensile strength and elongation at break is attributed to the uniform dispersion of StN-graft-PCL in the WPU matrix, as well as the strengthening of hard StN and the entanglement of grafted PCL molecular chains. However, as the content of StN-graft-PCL increases, the self-polymerization tends to increase to form a crystal region. Although the Young's modulus of the composite material is significantly increased, the tensile strength and elongation at break are not improved. According to reports in the literature, this is the first study on polymer-grafted natural nanocrystalline modified waterborne polyurethane. The soft segment in WPU and the macromolecular chain grafted on StN play a key role in enhancing the mechanical properties of the film. Such new high mechanical properties nanocomposites based on biodegradable materials have great application prospects.　　
　　4.4 Preparation of Biodegradable Waterborne Polyurethane from Vegetable Oil　　
　　Vegetable oil is a renewable raw material. The preparation of polyurethane by using hydroxyl-containing vegetable oil or hydroxylated vegetable oil as a polyol completely meets the requirements of environmental protection, and the fatty acid glyceride which is the main component of vegetable oil has hydrophobicity, and the polyurethane prepared by using vegetable oil polyol has good Chemical and physical properties, especially with better hydrolysis resistance and thermal stability [24]. The vegetable oil-modified water-based polyurethane mainly utilizes the reaction of the hydroxyl group in the vegetable oil or the modified vegetable oil with the isocyanate group, imparts some special properties to the water-based polyurethane material, and increases the biodegradability of the material. The structural advantages of vegetable oil itself also confirm these advantages: 1 natural vegetable oil contains unsaturated double bonds, so that the modified waterborne polyurethane can be photocured and further modified by polymerization with acrylate monomers to obtain excellent properties. product. 2 The long-chain structure of the vegetable oil contains a reactive group such as a hydroxyl group and an ester group and a double bond, so that the structure of the vegetable oil is manipulated, and the functionalized water-based polyurethane material is prepared by functionalizing the vegetable oil. 3 natural vegetable oil contains polyhydroxy ester structure, and the polyurethane obtained by reacting with isocyanate group has micro-crosslinking and semi-interpenetrating network structure. The increase of crosslink density can effectively increase the cohesive energy density of polyurethane material, thereby enhancing the material. Mechanical properties [25], dense molecular structure is also beneficial to improve the resistance of materials to corrosive media. Internationally, the YONGSHANGLU research team has done a lot of research work on vegetable oil modified waterborne polyurethane. YONGSHANGLU et al [26] blended with castor oil modified waterborne polyurethane and thermoplastic starch, the test shows that the two have good compatibility, this modification makes up for the lack of water resistance and physical and mechanical properties of thermoplastic starch. It provides theoretical support for the study of high performance degradable starch plastics. In 2005, the research team published a research paper on the synthesis of a new type of plasticized starch material by using hydrochloric acid ring-opened chlorinated rapeseed oil to synthesize water-based polyurethane [27]. The compatibility of the two was investigated by scanning electron microscopy. When the mass fraction of polyurethane emulsion is less than 20%, the compatibility between the two is better. The results of differential scanning calorimetry show that microphase separation occurs. The introduction of chlorinated rapeseed oil modified waterborne polyurethane enhances the hardness and mechanical properties of the starch film and improves the water resistance of the plasticized starch. In the field of modified soybean oil to synthesize waterborne polyurethane, the research team used emulsion polymerization to synthesize soybean oil-based waterborne polyurethane-acrylate blend emulsion. It has been found that different acrylate additions have a direct effect on the conversion rate of emulsion polymerized monomers. The addition of acrylate can destroy the hydrogen bonding force between waterborne polyurethane molecules; the crosslinked structure of soybean oil-based waterborne polyurethane improves the acrylate. The heat resistance and mechanical properties, the ratio of waterborne polyurethane to acrylate has little effect on the size of the emulsion particles, indicating that the particle growth mechanism of this system is different from that of the general seed emulsion polymerization [28]. The above work provides a new method for the use of renewable soybean oil to synthesize blended emulsions that can be used in coatings. The team also used hydrogen peroxide and formic acid to epoxidize the double bonds in soybean oil, and confirmed the synthesis of polyhydroxy soybean oil with different functionalities by nuclear magnetic resonance spectroscopy, and synthesized with modified soybean oil as raw material. A series of waterborne polyurethane emulsions, the functionality of soybean oil and the content of soybean oil in the resin have a significant effect on resin particle size, phase transition temperature and heat resistance. The increase of functionality and the increase of soybean oil content will make The emulsion particle size increases significantly and the crosslink density increases, making the emulsion coating film change from elastomer to rigid plastic [29].　　
　　5 Conclusion　　
　　Waterborne polyurethane is environmentally friendly than solvent-based polyurethane, and its use and application fields are expanding and consumption will increase. Polyurethanes used in light industry, electrical packaging and insulation materials are required to be biodegradable, and biodegradable waterborne polyurethanes will become one of the development goals of the polyurethane industry. China's agricultural resources are abundant. These renewable natural resources have opened up a new field for the preparation of waterborne polyurethane materials, which not only solves the problem of environmental pollution caused by waste polyurethane materials, but also reduces the dependence on increasingly depleted petroleum products. In China, it has a very broad development prospect. At present, the use of renewable natural resources to modify waterborne polyurethane is still in the research stage. On the one hand, the purification and modification of natural polymer itself is complicated. On the other hand, it is suitable for the modification of natural polymers and derivatives of waterborne polyurethane. Certain structural features. Therefore, the preparation of biodegradable composite materials by natural polymer modified waterborne polyurethane has yet to be further researched and developed to realize its industrial application.。

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