Study on Application Technology of Biodegradable Thermoplastic Starch Packaging Materials

Plasticizing mechanism of thermoplastic starch

It is well known that starch is a partially crystalline, natural macromolecule with a double helix structure. The starch molecule contains a large amount of hydroxyl groups, which makes the intramolecular and intramolecular hydrogen bonding of the starch strong. Its melting temperature is higher than the decomposition temperature and it is difficult to form with thermoplastics. The method is processed, so that the original starch has thermoplasticity, and the starch molecules must be deformed and disordered to form a thermoplastic starch resin. The processing principle of thermoplastic starch plastics is to destroy the original spherulite structure of starch under the action of heat, shearing force and appropriate plasticizer, so that the starch undergoes glass transition and melt transformation to form an amorphous shape, which realizes the crystalline state. An irreversible transformation to an amorphous form, so that it melts before starting thermal decomposition to form a thermoplastic starch

Under the action of plasticizer, mechanical shearing force and heat, the starch molecules are completely melted by the process of particle swelling, crushing, double-helical structure opening and macromolecular release. During the conversion of the original starch into TPS, three different levels of structural changes occurred: the breakage of the starch granules; the hydrogen bonding between the intramolecular and intermolecular molecules of the starch was weakened, and some or all of the original crystals were lost and converted into no Form; partial degradation of the starch molecule results in a decrease in molecular weight. Therefore, depending on the degree of plasticization of the starch, the resulting material is often composed of residual expanded starch granules, partially melted, deformed and broken starch granules, amorphous form after complete melt plasticization, and complex recrystallization of starch. Phase system.

The complexity of the starch structure has made the processing of TPS much more complicated than the processing of general plastics. The processing of general-purpose plastics is mainly the process of melting-solidification, while the processing of TPS involves plasticization, melting, volume expansion, degradation and physical and chemical reactions of starch. Since the processing properties of starch-based materials are controlled by the degree of plasticization of starch, the plasticization process of starch is particularly important, and the degree of plasticization is affected by the content, type and processing parameters of plasticizers.

In the process of TPS, the use of plasticizer can significantly reduce the interaction force between starch macromolecules, destroy the high crystallinity of starch, and reduce the glass transition temperature of starch to have plasticity. Water is a commonly used plasticizer for Zui. The water content in starch-based plastic processing is generally controlled below 20%. However, since water is extremely unstable during processing and is easily volatilized, high-boiling plasticizers are often used instead of water. Such plasticizers are mainly polyhydric alcohols such as glycerin, sorbitol, and ethylene glycol, among which glycerin zui is commonly used. Such plasticizers have large solubility parameters and contain groups capable of interacting with hydroxyl groups in the starch molecule, which can effectively weaken the hydrogen bonding between the starch molecules, thereby lowering the glass transition temperature of the starch. The degree of reduction of the glass transition temperature of starch is not only related to the type of plasticizer, but also to the content of plasticizer. Therefore, the mechanical behavior of the material can be adjusted by changing the content of the plasticizer, thereby obtaining articles with different properties. The data show that the plasticized starch has a distinct melting endothermic peak between 140 and 160 °C, indicating that the hydrogen bonding between the starch molecules is weakened and destroyed, the molecular chain diffusion capacity is improved, and the glass transition of the material is obtained. The temperature is lowered. The double helix structure of the native starch transforms into a random coil structure conformation, and the starch realizes the microcrystalline molten state before decomposition, so that the original starch has the possibility of thermoplastic processing.

Properties of thermoplastic starch

First, thermoplastic

Thermoplasticity is one of the main indicators for measuring polymer materials, especially the glass transition temperature Tg can determine the temperature of use of materials and the flexibility of materials. The thermoplasticity of starch has a great relationship with plasticizers and starch types. The influence of plasticizer is mainly reflected in the molecular weight. Generally, plasticizer molecules with relatively small molecular weight are more likely to enter between starch molecules, and the ability to weaken the hydrogen bonding force between molecules is stronger, so the plasticizer is better. However, the strength of the resulting product is worse. The Tg of the original starch is higher than 100 ° C, but after plasticizing in the presence of a plasticizer, the Tg of the starch material is lowered to about 80 ° C, and gradually decreases as the plasticizer content increases. High amylose starch is more susceptible to plasticity due to its lower crystallinity.

Second, rheological properties

The rheological property of the melt is an important parameter to measure the processing properties of the material. It reflects the fluidity of the whole macromolecule, and the fluidity will affect the difficulty of the processing technology and the quality of the product. The thermoplastic starch plastic exhibits a pseudoplastic fluid in the viscous flow state, and the apparent viscosity depends on the shear rate. Therefore, the melt viscosity can be adjusted by changing the shear rate to optimize the processability. The apparent viscosity of the material is also related to plasticizers, temperature, lubricants, water content, and the like. The plasticizer can not only reduce the molecular chain motion resistance, but also reduce the apparent viscosity and shear stress of the melt, and also reduce the viscous flow activation energy of the system, thereby reducing the flow resistance of the melt. Temperature is also a very important factor because the melting of the starch crystal zone is related to the plasticizing effect. Starch melt meets the law of exponents:

τ=κγm, η=κγm-1
The index m increases with increasing temperature, but the constant k decreases with increasing temperature and water content.

Third, the mechanical properties

The mechanical properties of materials are another research focus of thermoplastic starch plastics. The special design of the equipment is an aspect of material performance optimization. On the other hand, the composition of the material also affects its mechanical properties. Relevant data indicate that small molecular polyol additives such as glycerin have a great influence on the mechanical properties of thermoplastic starch materials. Starch plasticized by glycerin has a higher elongation at break, and the higher the content, the higher the elongation at break, but the tensile strength decreases. Other polyol/starch systems have shown similar trends in mechanical properties. The effect of water on the mechanical properties of thermoplastic starch is similar to that of polyols, but the change in elongation at break is different. From the current technical level, the tensile strength of thermoplastic starch plastics is 8-10 MPa, and the elongation at break is 150%-200%, which can basically meet the needs of products, but it needs further improvement compared with ordinary plastics.

Fourth, degradation performance

Thermoplastic starch plastics are fully biodegradable. The biodegradability of starch can be determined by the ratio of biological oxygen demand (BOD) to chemical oxygen demand (COD), which decomposes within approximately 40 days. Another method to investigate the biodegradability of thermoplastic starch is to measure the molecular fragmentation of thermoplastic starch plastics under mixing conditions by gel permeation chromatography (GPC). Four weeks after mixing, the molecules formed a distinctly low molecular weight distribution.

Application and prospects

As a material with complete biodegradability, thermoplastic starch plastic can be used in many places instead of ordinary plastics. Thermoplastic starch plastics are widely used in packaging. In agriculture, they can be used as slow-release coating materials for pesticides and fertilizers. In food, they can meet the requirements of direct contact with food, and can be used for packaging of dry solid foods, such as Dairy products and confectionery, can also be used in fast food containers, disposable plastic cutlery, supermarket food packaging boxes and packaging bags, etc.; can also be used to replace the polyethylene bonding layer of paper / polyethylene / metal foil laminate, so that multiple layers Recycling of packaging materials becomes easy.
So far, there have been some difficulties in the development of thermoplastic starch plastics, such as the high price, water resistance and mechanical properties of this material. With the enhancement of environmental awareness and advancement of technology, various properties of degradable plastics will continue to improve, and it is expected to replace ordinary plastics in a wider range, especially in the fields of plastic film, packaging materials, medical materials and the like. There is no doubt that the potential market for biodegradable plastics is enormous. The development and application of this new material will replace the monopoly of synthetic materials in the past 40 years and open up a bright road to the packaging industry.

At present, the Qutle Institute of Frankfurt, Germany, Sumitomo Corporation of Japan, BIOTEC of Germany, the National Foam Company of the United States, Warner Lamert of the United States and Ferrizz of Italy all claim to have successfully developed this thermoplastic starch plastic. Production of films, sheets, and the like. Zhejiang University, Tianjin University, etc. have done some work on starch thermoplasticization, but they have not entered the development stage of thermoplastic starch plastic products. The Institute of Applied Sciences of Jiangxi Academy of Sciences has used four different processes to disorder the starch, and produced SP-98 thermoplastic starch plastics and processed into thin slices and films, which are further improving their related performance. As a development direction of degradable plastics, thermoplastic starch plastics have attractive market prospects.

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