In the past 50 years, the application of petroleum plastics and various polymers in packaging has grown tremendously. It is estimated that the annual global plastic consumption is about 200 million tons, and the annual increase rate is 5%. The ecological pollution caused by the use of waste plastic products after the use of these non-degradable plastics and the huge consumption of resources such as oil and natural gas are the most serious environmental and resource crisis encountered by countries in the world after industrialization. In order to solve the environmental and resource crisis, the direction of finding alternatives has turned to biopolymers. Biopolymers are polymers formed in the biological growth cycle in nature and are therefore also referred to as natural polymers. Typically such polymers are resource renewable, environmentally degradable materials. Among them, starch is considered to be one of the most promising biodegradable materials because of its wide source, low price, renewable resources and short regeneration cycle. After proper modification of starch, the properties of the material have been improved. In some specific application fields, such as food packaging, it is very promising to replace traditional plastics. 1. Analysis of problems in starch-based materials Because the crystal formed by the strong hydrogen bonding between starch makes the starch itself not have thermoplasticity, it is necessary to add a strong hydrogen bond between the small molecule plasticizer and the starch to reduce the crystallinity of the starch molecule, thereby reducing the glass transition of the starch. The temperature gives the starch a good thermoplasticity. The addition of plasticizer has a significant effect on the mechanical properties, moisture permeability and hygroscopicity of the material. Plasticizers generally contain a group capable of forming a hydrogen bond with a hydroxyl group in the starch, such as a hydroxyl group, an amino group or an amide group. The effects of polyhydric alcohols such as glycerin, sorbitol and xylose on the plasticization of starch are obvious, but the literature shows that they have some drawbacks when used as a single plasticizer. Due to the recrystallization of the starch molecules in the starch-based material, it becomes aging and brittle during storage, and the use performance is lowered. Famáa et al. studied the effect of storage time at room temperature on the physicochemical properties of glycerin-plasticized tapioca starch film. At the 8th week of storage, the water content of the film decreased while the crystallinity increased, and the mechanical properties of the material were significantly affected by the storage time. Krogars et al. have reported that films prepared with a single plasticizer over time, plasticizers (such as xylitol, sorbitol, etc.) begin to slowly crystallize resulting in a decrease in flexibility and continuity of the film. Talja et al. found that the plasticizer crystallization gradually reduced the content of plasticizer in the material, the elongation at break of the material decreased, and the plasticity and processing operability of the material decreased. At present, the main reason that affects the large-scale production of starch-based materials is that the mechanical properties and water resistance of thermoplastic starch are still far from the traditional plastic packaging materials, which also limits the application of such materials. At the same time, the material is highly sensitive to the humidity of the processing and storage environment, especially the hygroscopic properties exhibited under high humidity conditions, which greatly limits the application of such materials. If starch-based materials are to replace traditional plastics and achieve industrial production, it is necessary to improve the aging and brittleness of materials and reduce their sensitivity to humidity. 2. Research status at home and abroad At present, starch is mainly chemically modified. Although chemically modified starch can improve the water resistance and mechanical properties of thermoplastic starch to a certain extent, the modification of starch generally uses organic solvent, which causes difficulty in separation, or due to starch. The reaction causes the preparation cost of the thermoplastic starch material to increase. Therefore, when the chemically modified starch is applied to the packaging material, especially the food packaging material, there are drawbacks such as the safety of the packaging material; and the non-chemical modification of the starch is not There are residues of chemical reagents, which have the advantages of “greenâ€, “environmental protection†and “safetyâ€, and have become one of the research hotspots. In order to improve the physical properties of starch-based materials, the researchers have done a lot of research on the selection of plasticizers, the addition of hydrophobic additives, the type of starch and the processing technology. 2.1, the choice of different plasticization systems Due to the phenomenon of phase separation and plasticizer crystallization in the preparation of the starch-based film by the plasticizer, the molecules of the amylose and amylopectin are rearranged, and the starch molecules are recrystallized to cause aging and brittleness. Despite this, there are not many studies conducted by domestic and foreign scholars on this issue. From the literature, only a few researchers have tried some. A stable plasticization system helps to inhibit the recrystallization of starch molecules. From the report, it is a feasible method to try to use plasticizers for plasticization. Krogars believes that the plasticizers of different molecular sizes may induce more interaction between the components in the material, allowing the plasticizers to bond more closely together, thereby inhibiting their crystallization or from the material. Precipitated, therefore, the problem of aging and rejuvenation of starch after composite plasticization has different degrees of improvement than single plasticizer. Domestic Ma Yinfei et al. prepared thermoplastic starch with formamide and urea as composite plasticizers. The materials have certain resistance to retrogradation, and the thermal stability and water resistance are better than the commonly used glycerin plasticized thermoplastic starch. Talja et al. prepared a starch-based biodegradable film using a binary mixture of polyols as a composite plasticizer. It was found that the plasticizer did not show the precipitation of plasticizer after plasticization of the composite plasticizer. Krogars et al. used glycerin and sorbitol as composite plasticizers to prepare high-linear corn starch film. It was found that the film remained stable after 9 months of storage, and the moisture permeability and elongation at break did not change. X -Ray diffraction showed that no crystallization occurred. These provide a good theoretical basis for improving the aging and embrittlement of starch-based materials. 2.2, the application of hydrophobic additives However, domestic and foreign scholars have a wide range of research on the problems of poor water resistance and high moisture permeability of starch-based materials. A large amount of literature shows that the water resistance and mechanical properties of thermoplastic starch can be improved to some extent by adding natural organic materials such as cellulose and natural inorganic materials such as montmorillonite, silica or mica flakes. Since the permeation of water vapor is mainly achieved by the hydrophilic portion of the starch-based material, the moisture permeability is dependent on the ratio of the hydrophilic portion to the water-repellent portion of the material. Therefore, some hydrophobic additives such as lipids are added to hydrophilic materials, coated or dispersed into a film-forming solution, and solution casting methods are used to prepare materials, which are widely studied to improve the moisture barrier properties of starch-based materials. Methods. The additives tried by researchers in the literature mainly include hydrophobic substances such as lipids, beeswax and gelatin. The addition of these substances has some disadvantages such as high moisture permeability or poor water resistance. GarcÃa et al. studied the effect of the addition of lipids to starch-based materials on the moisture barrier properties of materials. Studies have shown that when the amount of sunflower oil added is closely related to the ratio of crystal to amorphous in the material and the hydrophilic-lipophilic balance. When the amount of sunflower oil added is about 2 g / L, the starch-based film shows good moisture resistance. When the amount is more than 2 g / L, although the hydrophobic ratio in the material is increased, the migration of oil and the proportion of the crystal zone The decline in the moisture resistance of the material begins to decrease. Chen et al. added sucrose esters with different hydrophilic-lipophilic balance (HLB) as surfactants to the tapioca starch-based film, which improved the moisture resistance of the material, but decreased the tensile strength and elongation at break. the trend of. Studies have found that as the hydrophilic-lipophilic balance increases, the moisture permeability of the material is also getting lower and lower. The authors believe that primarily the polar groups in the active agent form hydrogen bonds with the hydrophilic gel, thereby reducing the number of polar groups that form hydrogen bonds with water molecules. The effect of the active agent on the moisture permeability of the material is strongly dependent on the type and concentration of the active agent, and is closely related to the raw material properties of the film forming solution. Villalobos et al. added a mixture of emulsifier Span 60 and sucrose ester P-1570 to the film forming solution, which significantly reduced the equilibrium moisture content and moisture permeability of the carboxymethyl cellulose membrane. Alves et al. studied the effect of mica flakes on the moisture permeability and gas permeability of polyglycolic films. The study found that when the amount of mica added is 10% (accounting for dry matter mass fraction), the moisture permeability and oxygen permeability of the material are reduced by 40% and 27%, respectively. In addition, 10% of mica exhibits a critical volume fraction as an inorganic particulate filler, so the barrier properties of the material are improved [24]. Phan et al. first studied the starch film prepared from agar and tapioca starch. It was found that agar can provide a highly adhesive matrix, so agar can increase the tensile strength and elongation at break of tapioca starch, but in high humidity. Under the agar, the addition of agar does not improve the moisture resistance. 2.3, the effect of amylose content Since linear and branched content and relative molecular mass are the main reasons affecting the performance of different kinds of starch membranes, in addition to the selection of plasticizers and the addition of hydrophobic additives, the starch mechanics and mechanical properties of different amylose content The impact of water resistance has also attracted the attention of many researchers. According to the literature reviewed, there is no clear and clear conclusion about the influence of the content of amylose on the mechanics and barrier properties of materials. The feasibility of using high amylose or high amylopectin for the preparation of starch-based biodegradable materials There are also objections. Studying the difference in performance of different amylose content films, especially on the mechanical and water resistance of the film, it has a guiding role in the development and selection of starch types in the preparation of starch-based biodegradable materials. Mali et al. found that cassava starch has higher transparency and flexibility than corn and artichoke starch film due to lower linear content; however, the linear content of artichoke starch film is not significantly greater than linear chain. The content of the corn starch film in the middle, because the relative molecular mass of amylopectin in corn starch is smaller than that of artichoke starch, and it is easy to reorder to form a relatively compact structure. GarcÃa et al. also confirmed that the high amylose corn starch film has lower oxygen, carbon dioxide and water vapor transmission rates than ordinary corn starch films. However, alves et al. used glycerol as a plasticizer to prepare cassava starch-based films with different amylose content. It was found that the increase of amylose content helps to form a film with better mechanical properties and permeability, and glycerin and straight The effect of the amount of amylopectin on the mechanical properties and barrier properties of the material is significant. This is different from or even the opposite of the findings of Mali and GarcÃa et al. High amylose has long been regarded as an ideal raw material for the preparation of starch-based biodegradable materials. However, when Bader et al. used high amylose starch as a raw material to prepare a film, it was pointed out that high amylose starch is not suitable for the preparation of starch-based films. Various studies have shown that there are still some differences regarding the effects of amylose content on the mechanics and barrier properties of materials, which is urgently needed for analysis and resolution by researchers. 3. Outlook In the 21st century, environmental protection and resource conservation have become the consensus of people. Plastic products made from fossil fuels such as oil and natural gas have destroyed a lot of non-renewable energy while destroying the natural ecology, which is contrary to the strategy of sustainable development. In principle, research and development of biodegradable starch-based natural polymer green packaging materials has become one of the key directions in the field of packaging materials. Although starch-based materials still have some defects compared with traditional plastic packaging materials, it is believed that as the research progresses, the performance improvement of starch-based materials will surely meet the needs of everyone. In recent years, starch-based materials have gradually been applied to the packaging of food packaging, especially fresh fruits and vegetables. It is believed that as the physical properties of materials become more and more perfect, the use of natural polymer materials such as starch will become more and more common.
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