Application of hottest starch and fiber in biodegr

Application of starch and fiber in biodegradable packaging materials

Abstract: starch and plant fiber are two natural polymer materials. It is widely studied and applied in biodegradable materials. This paper summarizes the current situation and development prospect of the utilization of starch and fiber in biodegradable packaging materials

key words: starch; Fiber; As we all know, the development of polymer industry has led to the continuous intensification of environmental pollution. Plastic is light and bulky, and it is difficult to dispose of waste. Landfill and incineration are not ideal solutions. "White pollution" has become a global fixture problem in such cases. In addition, oil resources are decreasing day by day, so it is urgent to find new non oil producing polymers. In recent decades, people have been seeking completely degradable materials that can replace traditional plastics through various ways. After repeated debate and years of practice, a consensus has been reached that degradable materials must be able to be completely degraded into harmless substances, such as carbon dioxide and water, in a relatively short time after abandonment. The utilization of natural degradable substances is of great practical significance for the preparation of degradable materials. Natural polymer materials based on starch, plant fiber, chitin, chitosan and so on are meeting the requirements and trends of completely environmental protection. As a natural polymer material, starch is rich in sources and cheap. It is decomposed into glucose under the action of microorganisms, and finally metabolized into water and carbon dioxide. It is an inexhaustible renewable resource. Natural starch is 15 ~ 100 μ M exists in the form of small particles, and there are polymer compounds with crystalline structure in the particles. Starch is composed of amylose and amylopectin. Amylose is glucose α－ D-1, 4 glycosidic bond linked chain compound with relative molecular weight of (20 ~ 200) × 104。 Preparation of standards for branched chain starch and polyurethane building insulation materials α－ Besides D-1 and 4 glycosidic bonds, there are also α－ D-1, 6 glycosidic bond, relative molecular weight (100 ~ 400) × 106。 Natural plant fiber is also a renewable resource that meets the requirements of sustainable development. Plants produce a large amount of fiber. It is the most abundant carbohydrate on the earth. In nature, it can be degraded by microbial decomposition enzymes and ingested by plants or microorganisms as a nutrient source

starch and plant fiber, two natural polymer materials, have been widely studied and applied in degradable materials, including disposable tableware, pharmaceutical packaging materials, molded products, foamed plastics and other fields. Starch and plant fiber are very similar in structure, but their configurations and compositions are different. Cellulose, the main component of fiber, is an isomer of amylose, and the glucose unit passes through β－ The 1,4-glycoside bond is connected, and the oxygen atom forms a trans structure with the hydroxyl group. This slight difference leads to the difference in the spatial configuration of amylose and cellulose. The configuration of amylose is relatively curved, while cellulose is relatively straight. The result is that the solubility and reactivity of the two isomers in solution are very different, which also leads to the great difference in the macro properties of starch and fiber to a great extent. Moreover, without chemical modification and specific process conditions, they are difficult to carry out general cross-linking, grafting and other chemical reactions in the molecular field. However, people always try to organically combine these two natural polymers to make them easy to process and have good performance and degradation

1. The current situation of the utilization of starch and fiber in biodegradable packaging materials

starch and some other natural macromolecules such as cellulose, hemicellulose, lignin, pectin, chitin, protein, etc. are compounded to make fully biodegradable materials, which is a kind of all natural biodegradable materials with rapid development in recent years. It has many advantages. The raw materials needed are renewable resources, and its unit price is much lower than that of traditional plastics. The average cost decreases with the increase of output; And it can be completely biodegradable, the degradation products are not harmful to the environment, and harmful gases will not be produced during combustion; At the same time, it also has a certain degree of thermoplastic, which can be formed by different methods. It is an ideal biodegradable material. Its film and sheet can be used as all kinds of packaging materials, and can also be processed into all kinds of products, which are widely used. At present, the comprehensive application of starch and fiber plays an important role in biodegradable materials with natural substances as raw materials, especially in the field of packaging. It can be roughly divided into the following two aspects:

1.1 application in disposable tableware, food packaging trays, packaging films and sheets

Li Jizhu and others studied the problems of biodegradable PS starch plastic foaming fast food utensils. By mixing plant fibers into PS starch resin, they promoted multi-component crosslinking, and transformed the molecular chain from the original linear or slightly branched chain structure to a three-dimensional structure, Form fiber reinforced starch plastic foam, so as to effectively improve the degradation performance, mechanical properties, processing properties of fast food tableware products, and reduce costs

the starch content in products has a significant impact on the production process and all aspects of performance. The higher the starch content is, the better the degradation performance is, but after reaching 25%, the physical properties of the products are significantly reduced, and its strength, rigidity, surface quality and use performance are significantly different from those of PS foam plastic tableware. In the process of extruding foamed sheet, the higher the starch content, the lower the melt strength, resulting in insufficient cell wall strength and difficult to bear bubble expansion. As a result, the foam is easy to crack, the product has a high opening rate, and the cell density and compression resistance are reduced; The higher the starch content is, the easier the butane in the inflation will escape and lose, resulting in cell collapse, obvious shrinkage of the sheet, and obvious reduction of foaming multiple, which not only increases the production cost, but also causes wrinkles on the surface of the product and affects the appearance quality; The fluidity of PS melt with high starch content is significantly reduced, and the viscoelasticity of the melt suitable for foaming is difficult to maintain stably, and the melt viscosity cannot be reduced by increasing the material temperature (it is easy to make the starch zoom and the collapse of the foam body intensify), so that the production capacity of the extruder is significantly reduced, the quality stability is poor, and sometimes horizontal ripples appear

plant fibers are mixed into PS starch resin as a means to improve the process and performance. Plant fibers have poor compatibility with plastics and have strong water absorption as starch; Therefore, when using, it is necessary to modify and add necessary compatible and dispersing additives to ensure uniform mixing. First of all, the particle size and addition amount of plant fiber should be considered. In order to form uniform and fine bubbles during extrusion foaming, the foaming process is continuous and stable, and the mechanical properties of products are significantly improved, the fiber raw materials must be refined, generally more than 100 mesh. The mass fraction of fiber in the formula has a great influence on the foaming quality and the physical and mechanical properties of products. It is found that the bubble expansion of starch plastic foamed sheet without fiber is easy to merge into large bubbles, with high opening rate and large bubble shrinkage, which is due to the weak melt strength. After adding fiber, the melt polymer is crosslinked to form a three-dimensional structure, which improves the melt strength, is conducive to the uniform expansion of bubbles to form dense and uniform bubbles, and also reduces the escape of gas during extrusion expansion, and the shrinkage of the sheet is significantly reduced; However, when the amount of cellulose is too large, the melt strength is too high, the elasticity is insufficient, and the bubbles are difficult to expand rapidly and evenly. Therefore, the bubbles are small and few, and the foaming multiple is significantly reduced. The blending of plant fibers also needs to be modified. Maleic anhydride is a good compatibilizer for polar and non-polar materials. After the fiber is treated with maleic anhydride and titanate coupling agent and mixed with starch PS resin, its compatibility will be significantly improved and the strength of the mixture will be improved. One end of maleic anhydride contains anhydride group, which can esterificate with the carboxyl group of the fiber, reduce the active carboxyl group in the fiber and reduce the polarity of the fiber; The other end is a macromolecular chain, which has good compatibility with PS matrix. Through its entanglement with PS, the fiber has a strong interface with PS matrix, so it also strengthens the three-dimensional structure of the melt mixture

Zhao Hua et al. Studied the process of preparing fully degradable plastic sheets with modified starch, fiber, light calcium carbonate and polyvinyl alcohol as the main raw materials. Modified starch is cationic starch, which is prepared by chemical reaction between starch and substances containing amino, imino, ammonium, matte and other groups. After the reaction, the starch group has a cationic charge. The interaction of ionic bonds between various groups, coupled with the role of the original hydrogen bond, increases the viscosity. At the same time, the cationic group has an adsorption effect on the cellulose with an anionic charge, so that the cationic starch acts as an ionic bridge between cellulose and calcium carbonate, which tightly combines the three. The reaction formula is:

the influence degree of each factor on the material properties and the tensile strength is obtained through orthogonal experiment: fiber PVA active calcium carbonate; Effect on water resistance: fiber activated calcium carbonate PVA. Calcium carbonate plays the role of filler in the ingredients in order to increase the stability of the product and reduce the production cost. With the increase of its dosage, the tensile strength of the product gradually decreases, and the heat resistance first increases and then decreases. Fiber has strong cycle test mechanical strength and chemical stability. It plays a skeleton role in the product and can significantly improve the tensile strength of the product. However, when the amount is too much, starch and calcium carbonate will be converted from continuous phase to discontinuous phase, affecting the full combination between them and fiber, resulting in the decline of water resistance. As an inorganic filler, there are holes of different sizes at the interface between calcium carbonate and matrix resin. These defects make the mechanical properties decline with the increase of the proportion of inorganic filler when subjected to external force; Inorganic filler itself has water resistance, and the increase of its dosage will improve the water resistance, but too much will affect the close combination between starch, calcium carbonate and fiber, resulting in the decline of water resistance

Feng Xiaozhong et al. Studied the composition, preparation method and molding process of degradable molding powder system with straw powder (mainly composed of plant fiber) and light calcium carbonate as the main body, PVA and starch as the adhesive. Using the molding powder and adopting the compression molding process, biodegradable tableware and egg packaging trays can be made, which meets the requirements of environmental protection. Among them, the amount of starch has a great impact on the material properties. The combination of starch and PVA has a synergistic effect on the polar material system, and the starch itself has a certain cementation effect on straw powder after gelatinization. The business strategy of "excellent service" provides customers with a perfect use environment. Appropriately increasing the starch content is beneficial to processability, product appearance and impact strength, but when the starch content is high, it has a strong adhesion to the equipment and has a certain impact on the plastic refining and molding operations. When the amount of plant fiber, i.e. straw powder, is too much, the fibers are easy to entangle with each other, causing difficulties in mixing and plasticizing. When the amount is too small, the fibers are difficult to overlap into a bridge, which has little effect on the impact strength

the Netherlands Wageningen Agricultural University has developed a biodegradable plastic without petrochemical products. This material is made of wheat, corn and potato starch, and mixed with human hemp fiber to improve its strength. It can be used as packaging coating, food storage bin, dustbin lining, shopping bag and agricultural film. This material can be completely dissolved in water and degraded into water and carbon dioxide. American scientists use the fiber of wheat straw and the starch in wheat grains to make a fast-food packaging box, which is a kind of