Interpretation of starch-based degradable plastics: crosslinking and modification
Introduction: Starch is a natural and renewable polymer compound. Because of its abundance, easy availability and low price, starch is widely used in the research of degradable plastics. At present, starch-based biodegradable plastics account for about 50% of the existing commercial biodegradable plastics, and they have been used in food packaging films, agricultural mulch films, foamed plastic lunch boxes, and medical bone tissue engineering scaffolds 41. However, due to the molecular structure of starch, the processability of starch itself is poor. Compared with traditional plastics, the mechanical properties and barrier properties of starch-based degradable plastics are greatly reduced, and they cannot be further promoted commercially. Therefore, the physical or chemical modification of starch is very important. Cross-linking is one of the main methods of starch modification. The tightly connected three-dimensional network structure formed by cross-linking enhances the intermolecular interaction, thereby obtaining a degradable material with good heat resistance, water resistance, high strength and flexibility.
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Cross-linking method of starch-based degradable plastic
Cross-linking is the process of linking linear or branched polymer chains into a networked polymer by forming covalent bonds. According to different cross-linking methods, cross-linked starch-based degradable plastics can be divided into chemically cross-linked starch-based degradable plastics and photo-cross-linked starch-based degradable plastics.
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Chemical crosslinking
Chemical cross-linking is the reaction of a cross-linking agent containing binary or multiple functional groups with the hydroxyl groups on the starch molecule to form groups such as diether bonds or diester groups, thereby cross-linking multiple starch molecules to form a spatial network Structure-like polymer method. Commonly used crosslinking agents are glutaraldehyde, epichlorohydrin (ECH), sodium trimetaphosphate (STMP), citric acid (CA) and malic acid. The properties of starch-based degradable plastics prepared with different crosslinking agents are also different.
In recent years, with the vigorous promotion of environmental protection concepts, CA-type "green" non-toxic crosslinking agents have become more and more popular among researchers and have become the main crosslinking agents for crosslinking starch-based degradable plastics. According to the time when the crosslinking agent is added, chemical crosslinking can be divided into crosslinking (that is, the crosslinking agent is added to react during the polymer molding process) and post-crosslinking (that is, after the material is formed, impregnated by the crosslinking agent solution) The method causes cross-linking between molecules).
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Photocrosslinking
Photocrosslinking is a method of adding a photosensitizer to the starch system to decompose it into free radicals under ultraviolet light (UV) irradiation, and use the photosensitizer to polymerize the hydroxyl groups in the starch to crosslink the starch molecules. . When photocrosslinking to prepare starch-based degradable plastics, the radiation dose and photosensitizer concentration are the most important factors affecting the crosslinking degree of the material. Compared with the chemical cross-linking method, the photo-cross-linking method does not require hydrothermal equipment and cross-linking reagents, is safer and more environmentally friendly, and is simple to operate, and the reaction is easy to control. It can be adapted to large-scale continuous production of materials and is suitable for bio-based hydrogels. , Preparation of drug delivery materials, etc.
The effect of cross-linking on the properties of starch-based degradable plastics
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Water resistance
Water resistance is one of the important conditions for testing the application standards of bio-based degradable membrane materials. However, due to the natural hydrophilicity of starch, starch-based film materials generally exhibit stronger hydrophilicity and higher permeability. Cross-linking modification makes starch have a tightly connected three-dimensional network structure, which can effectively prevent the entry and migration of water molecules. Water absorption, swelling and water vapor transmission rate (WVP) are often used to characterize the water resistance of starch-based materials.
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Mechanical behavior
In daily production and life, packaging film materials need to have a certain strength and flexibility to maintain their integrity during processing. Cross-linking establishes intermolecular and intramolecular connections, makes starch molecular chains longer, enhances intermolecular interactions, and results in increased tensile strength of the material and decreased elongation at break. Generally speaking, adding a small amount of crosslinking agent can meet the performance requirements of the product. When the degree of cross-linking is low, the length of starch molecules available for sliding increases. With the continuous increase of the degree of crosslinking, the intermolecular and intramolecular interactions are enhanced, and the tensile strength is increased, but at the same time the intermolecular slippage is also restricted, which leads to the decrease of the elongation at break of the material and the brittleness. Starch has strong hydrophilicity. If there is too much starch in the system, the intermolecular force will be weakened after the material absorbs water, which greatly reduces the tensile strength of the material. In addition to the degree of crosslinking and starch content, RH also has a greater impact on the mechanical properties of starch-based degradable plastics. When the RH is 40%, the mechanical properties of the starch-based sheet are the best. Too low RH may make the material brittle and break into pieces when stretched; when RH is too high, a large amount of water molecules enter the plastic sheet as a plasticizer, and the tensile strength decreases. The curing time and curing temperature are also important factors affecting its mechanical properties.
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Degradability
Degradability is the biggest advantage of starch-based materials. The biodegradation of starch-based materials is usually caused by the biological activities of fungi, bacteria and other microorganisms under natural conditions such as soil, or under certain specific conditions such as composting conditions or in aqueous culture solutions. The soil burial method uses microorganisms to erode the starch in it and secrete enzymes, which reduces the strength of the material. The plastic and the metal salt in the soil undergo auto-oxidation to generate peroxides, which promote the rupture of polymer molecular chains and become low molecular substances. , Which becomes H2O and CO2. The composting method uses microorganisms to control the conversion of degradable organic matter in solid waste into stable humus, H2O and CO2 under oxygen conditions. Cross-linking enhances the intermolecular and intra-molecular interaction force and reduces the degradation rate of the material. Under normal circumstances, the degradation degree of starch-based degradable plastics is positively correlated with starch content and soil burying time, and the degradation rate is positively correlated with starch content, environmental humidity, cross-linking degree, and plasticizer content.
Modification method of cross-linked starch-based degradable plastic
Due to the compact three-dimensional network structure formed by cross-linking, the performance of cross-linked starch-based degradable plastics has been improved to a certain extent, but it still does not meet the standards of general plastics. Therefore, it is necessary to further modify it, including blending with other polymer materials, nano-material enhancement, multiple modification, coating hydrophobic coating, etc.
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Natural polymer blending modification
Geng Shengrong and others used STMP to cross-link the blend system of starch and konjac fly powder, and prepared a degradable blend film with good water resistance and mechanical properties through the casting film forming method. The results of this study show that glucomannan with a network structure can inhibit starch settling and is beneficial to improve the compatibility of the blend system. Then an appropriate amount of PVA is added to the system to blend. PVA provides the film with better water resistance and mechanical properties, but the system compatibility becomes poor. Li et al. used glutaraldehyde as the cross-linking agent to prepare a potato/CS composite cross-linked film with better water resistance by the casting method. At the same time, CS gave the film unique antibacterial properties, enabling it to be used in biomedicine or food preservation. The field is widely used. Compared with pure starch-based films, the properties of starch-based films that are blended and cross-linked with other biopolymers are improved. At the same time, other biomaterials can also bring some additional properties to starch-based films. This method is to prepare The new trend of new starch-based membrane materials.
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Blending modification of degradable polyester
Blending starch with degradable polyester, with the help of polyester's excellent mechanical properties and water resistance, can effectively make up for the lack of performance of starch films. For blending systems, the compatibility of multiple substances is an important factor affecting the mechanical properties of materials. ECH and glycerin are used to modify the starch, and then the modified starch is blended with PLA, and the film can be prepared through the hot pressing process. The hydroxyl groups on the starch molecules are cross-linked by ECH molecules to form ether groups, thereby changing the hydrophilicity of starch. The results indicate that the cross-linking and plasticizing modification of starch improves its compatibility with PLA and enhances its mechanical properties. A ternary blend film can be prepared with different ratios of starch/PVA/CA.
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Nano filler reinforced modification
In addition to degradable polyesters, cellulose and nanoparticles are also fillers commonly used in recent years to study the mechanical properties of starch-based degradable plastics. Studies by Balakrishnan et al. have shown that cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) have a better strengthening effect on starch membranes, and CNFs have better strengthening effects than CNCs. This is mainly due to the fact that cellulose and starch are both polysaccharides, and the two have similar structures, and strong hydrogen bonds are easily formed between hydroxyl groups, resulting in extremely strong interfacial adhesion. CNCs have a needle-like morphology with high crystallinity; while CNFs have a network structure with a large aspect ratio, a high degree of entanglement with starch, and greater interaction between molecules. It improves the mechanical properties of starch-based film materials.
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Hydrophobic coating modification
In order to improve the water resistance of the starch-based film, it can also be surface treated, such as coating with a hydrophobic coating. Soak the prepared wheat gluten film and foaming starch tray in the triacetate-dichloromethane solution. The slow volatilization of the solvent will form a coating film on the surface of the material. The triacetate starch coating greatly enhances the material Water resistance. When using PLA as a raw material to coat starch-based foam, although the PLA coating has poor compatibility with the foam, it still improves the density, tensile strength and impact strength of the foam.
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Multiple chemical modification
The performance of cross-linked starch-based film materials prepared by comprehensively using multiple modification methods is better than that of a single cross-linked modified starch film. Multiple chemical modifications are the mainstream trend to improve the performance of starch-based degradable films. When the cross-linked oxidized starch film (C-OS film) and the oxidized cross-linked starch film (O-CS film) are respectively prepared with hydrogen peroxide and boric acid, the O-CS film has more advantages due to the partial degradation of starch molecules due to the first oxidation. High hydrophilicity makes the film more flexible; C_OS film has a higher degree of crosslinking due to the first crosslinking, and the film has better water resistance and strength. The hydrophobic and lipophilic composite polyol plasticized modified starch was prepared by successively performing STMP cross-linking, aluminate coupling agent surface treatment and compound plasticizer plasticization on corn starch, and then blending it with PCL and calendering it into Film, the hydrophobicity of the film can be greatly improved. Sukhija et al. used STMP as the cross-linking agent and sodium hypochlorite as the oxidant to prepare cross-linked oxidized double-modified lotus root starch, blended it with concentrated whey protein, glycerin, and plantain hulls, and prepared it by the casting method. The starch-based film is obtained, and the thermal stability, mechanical properties and water resistance of the film are greatly improved. Multiple modification not only improves the performance of the material, but also increases the production cost of starch-based materials. Therefore, choosing fewer modification methods to obtain better performance is the focus of future research on starch-based degradable plastics.
The future development direction of cross-linked starch-based degradable plastics
The use of cross-linked starch-based degradable plastics is one of the important methods to alleviate the problem of "white" pollution. However, after years of research and development, the material has not yet been mass-produced and applied, mainly due to its cost and performance. The fully cross-linked starch-based degradable plastic has excellent degradation properties, but its mechanical properties and water resistance are difficult to reach the standards of general plastics. In order to obtain starch-based fully degradable plastics that meet the requirements of use, simple cross-linking modification of starch is far from enough. It must be achieved through the comprehensive application of complex modification methods or blending with polyester. The resulting high costs will inevitably hinder the commercial promotion of materials. At present, the price of cross-linked starch-based fully degradable plastics produced on the market is 4-10 times that of general-purpose plastics. They are mainly used in the outsourcing and packaging of some high-end cosmetics and some medical supplies (such as surgical sutures and drug release capsules). ).
However, starch is one of the most competitive raw materials for bio-based plastics, and has broad application prospects in sustainable development strategies. At present, starch-degradable polyester blend plastics are the most mature and fully degradable products on the market. In the future, researchers should devote themselves to developing low-cost and high-performance starch-based degradable plastics. There are mainly the following research directions:
1. Safe and environmentally friendly modification methods should be used in the modification process, and green and pollution-free additives should be used as much as possible to facilitate their degradation or composting.
2. For blended starch-based degradable plastics, the amount of starch used should be increased as much as possible on the premise of meeting the performance requirements to reduce the production cost of the material; at the same time, the blends should have good compatibility to obtain Excellent mechanical properties.
3. Introduce new materials to give starch-based degradable plastics multifunctional properties, such as adding chitosan, PVA, etc. to give starch materials antibacterial properties, adding rosin, etc. to give starch-based materials anti-ultraviolet properties, so as to broaden the application of starch-based degradable plastics market.
4. Develop new processes to reduce the energy consumption of the production of materials, such as the use of improved molding methods, extrusion blow molding methods, etc. instead of casting film methods to promote the commercial continuous mass production of starch-based degradable plastics.
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