Degradable plastics have developed rapidly since their inception. Among them, the types and amounts of starch-based plastics are the most, accounting for the vast majority of degradable plastics. Starch-degradable plastics have undergone three stages of technological development: filled starch plastics, starch-based plastics and full-starch thermoplastics. The early development of filled starch plastics, that is, the simple mixing of the original starch and resin to obtain the product, the starch content is about 5% to 10%; in the 1990s developed starch-based plastics, through physical or chemical modification of the original starch After the treatment, it is graft-blended with the resin to form a starch-based plastic. If a photosensitizer is added, a bio/optical double-degradable plastic is obtained, and the starch content is about 10% to 40%. This starch-based plastic has been popular for a while. Our project has passed the national scientific and technological achievements appraisal, won the national invention patent, and won many awards. In terms of reduction, the application of these two types of starch plastics has alleviated the pollution of plastics to the environment to some extent, but from the perspective of use, there is no fundamental solution to the problem of “white pollutionâ€, and there is still degradability. Insufficient, the product cannot be completely degraded.
In order to overcome the shortage of starch-based degradable plastics, research and development of all-starch thermoplastics has begun in recent years. The so-called whole-starch thermoplastic means that 80% to 90% of the material is composed of starch in addition to processing aids. It has the property of plastic resin, which can be used for thermoplastic processing and can be quickly and completely degraded in the natural environment. As a new type of fully biodegradable material, it is a hot topic in the field of degradable plastics. At present, Sumitomo Corporation of Japan, Warnerlambert of the United States and Ferrizz of Italy all claim to have successfully developed this all-starch thermoplastic, which can be used in the production of film and sheet, but has not yet achieved industrial production. This technology is still in the research stage in China. Jiangxi Yinghua Institute and Zhejiang University are doing research work in this area, but there are no reports on industrialization and product application. However, as a development direction of degradable plastics, all-starch thermoplastics have attractive market prospects.
1. Status of technology development
All-starch thermoplastic is a plastic that is physically or chemically treated with natural starch to provide thermoplastic processing properties, has the properties of a plastic resin, and can be rapidly degraded in the environment. It is a completely biodegradable material in the true sense. The advantages of the original starch-based plastics are:
1 It has complete biodegradability in various environments. After degradation or ashing of starch molecules in products, CO2 gas is formed, which does not cause toxicity to soil or air;
2 adopting a suitable process to make the starch thermoplasticized to achieve the mechanical properties for the manufacture of plastic materials;
3 Because all of the starch is used as raw material, the source is wide, and the cost is lower than that of starch-based plastics and traditional plastics;
4 The industrial application of a large amount of starch is conducive to the adjustment of rural economic development and industrial structure. In order for the native starch to be processable, it must be thermoplastically treated to form a plastic article by methods such as extrusion, casting, injection molding, tableting, and blistering. At present, there are three technical methods for starch thermoplastic treatment:
(1) Starch graft modification or miscible blending with resin
A polymer is grafted onto the active support of the starch, which will be both biodegradable and thermoplastic. When the starch is blended with the resin, the starch must be surface treated to achieve the desired interfacial bonding of the starch to the polymer to address the compatible blending of the starch with the polymer. The treatment technology is mainly denaturation reaction of oxidation, amination, esterification or etherification of starch, and the reaction product has a hydrophobic group, which can significantly reduce the water absorption rate of starch. The surface of the modified starch granules is covered with an alkyl group or the like, which weakens the role of hydrogen bonding, and the compatibility with a polymer such as polyethylene can be improved to various extents. In addition, the addition of a third component of a compatibilizing resin, such as EVA, EAA, etc., to the blend can significantly improve the compatibility of the starch with the polymer. Currently developed starch graft polymers are: starch / methacrylate, starch / methyl acrylate, starch / polystyrene, starch / butyl acrylate and the like. Such starch plastics can be processed into products such as films.
(2) Composite of modified starch and degradable polymer
Starch can be combined with natural macromolecules such as gum, galactose, chitin, etc. into a fully biodegradable material for the preparation of packaging materials or food containers. There are many reports on this aspect, but they have not yet reached the level of industrial production. More industrialization is the combination of photodegradation technology and biodegradation technology into a double-degradable starch plastic. The photodegradable plastics technology has been developed relatively early, combining it with biodegradation technology. On the one hand, it overcomes the problem that starch-based plastics are difficult to degrade in non-biological environments. On the other hand, it can be realized by using the composite ratio and dosage of photosensitive systems. The degradation time is artificially controllable.
(3) coextrusion of starch with plasticizers, lubricants, etc.
In order to increase the flowability of starch melting, it is necessary to add a plasticizer and a lubricant to make plasticization of the starch possible. The plasticized starch spherulite size becomes smaller, the number of spherulites increases, the hydrogen bond between the starch molecules is weakened, the molecular chain diffusion capacity is improved, and the glass transition temperature of the material is lowered, so that the micro-deformation is realized before the decomposition. The melting of the crystal changes from a double-helical conformation to a random coil conformation, which makes the starch have the possibility of thermoplastic processing [3]. This is currently the main research method for all-starch thermoplastics.
Through long-term research, the author has developed a process route that is easier to implement and industrialize. After the natural raw starch is pretreated, it is plasticized and extruded by a special twin-screw extruder. The high shear force and high temperature of the compression section of the special twin-screw extruder destroy the crystallites of the starch, causing the macromolecule to be disordered. The linear arrangement is such that the natural raw starch has the properties of a thermoplastic and can be processed into various plastic products. The process route is small, the process is short, and the cost is low. Now it has entered the eve of industrialization.
2. Disordering mechanism
The so-called disordering means that the natural starch crystallites are melted in a certain way, and the molecular structure is changed from a double-helical conformation to a random coil group conformation. Natural starch is a partially crystalline, natural macromolecule with a double helix structure with a decomposition temperature Td greater than the melting Tm. And because the natural starch is a polyhydroxy compound, the adjacent molecules often form a complete microcrystalline structure through hydrogen bond interaction, which makes the natural starch granules have high rigidity and is not easy to be pulverized [4]. From the DSC spectrum, a variety of natural starches have a very broad endothermic peak around 100 ° C. This is because the equilibrium water in the starch agglomerates loses as the temperature continues to rise and causes decomposition, so natural Starch is not thermoplastic and cannot be processed in plastic machinery. To make it thermoplastic, it is necessary to change the molecular structure, disordering its molecular structure, and changing from a double helix conformation to a random conformation. The usual method is to first refine the starch under strong mechanical action, destroy part of the crystallites, and then use the "fracture agent" and the anti-hydrogen bond between the natural raw starch and water, cellulose, plasticizer and hydroxyl. Auxiliaries such as oxygen agents are mixed at a high speed in a high-speed mixer, and then plasticized and extruded through a special twin-screw extruder, and the high-shear force and high temperature of the twin-screw extruder are used to destroy the crystallites of the starch. The macromolecules are arranged in a disordered linear shape, so that the natural raw starch has thermoplasticity. Among the auxiliaries used, the plasticizer plays the most important role, which can lower the glass transition temperature of the starch, increase the plasticity, and facilitate the molding process. There are two ways of the mechanism: one is the non-polar plasticizer, that is, the plasticizer inserts between the starch molecules to increase the distance of the macromolecular chain, weakens the van der Waals force, thereby reducing the melt viscosity of the starch; the second is the polar plasticizer, At high temperatures, the thermal motion of starch molecules is intense, the distance between molecules increases, and the interchain function is weakened. The plasticizer molecules enter between the molecular chains of the starch, and the polar group interaction between them replaces the interaction between the polar groups of the starch molecules, causing the starch to swell, and the non-polar part of the plasticizer will be the polarity of the starch molecules. Shielding, and increase the distance between the starch molecules, weaken the van der Waals force between the molecules, make the starch molecular chain easy to move, thereby reducing the melting temperature of the starch, making it easy to process. The starch treated in the above manner showed a distinct melting endothermic peak between 140 and 160 ° C, indicating that the hydrogen bonding between the starch molecules was weakened and destroyed, the molecular chain diffusion ability was improved, and the glass transition temperature of the material was lowered. The melting of the crystallites is realized before the decomposition, and the double-helical structure of the natural starch is converted into a random coil structure conformation, so that the starch has the possibility of thermoplastic processing [5, 6].
3, the performance of the material
(1) Thermoplastic
Due to the thermoplasticity, the treated starch can be repeatedly plasticized like ordinary plastic such as polyethylene, and various products can be obtained by extrusion, blow molding, casting, injection molding, etc. on a conventional plastic device. However, it has been found through research that this type of thermoplastic has a great relationship with the type of starch and plasticizer. High amylose starch is good in plasticity and easy to achieve because of its low crystallinity. The influence of plasticizer is mainly reflected in the molecular weight. Generally, the plasticization effect is relatively good with respect to the molecular weight, but the melt strength is worse.
(2) rheological properties
The solution of the whole starch thermoplastic in the viscous flow state is a pseudoplastic fluid, and the apparent viscosity depends on the shear rate. Therefore, the melt viscosity can be adjusted by adjusting the shear rate to optimize the processing performance. From the material itself, its apparent viscosity is related to plasticizers, lubricants, and the like. Increasing the plasticizing dose can sufficiently reduce the molecular chain motion resistance and reduce the apparent viscosity and shear stress of the solution. The use of plasticizers can reduce the viscous flow activation energy of the system, which in turn reduces the flow resistance of the melt. In addition, the temperature is also a big influence, because the melting of the starch crystal zone is related to the plasticizing effect [8].
(3) Mechanical properties
From the current technical level, the tensile strength of the whole starch thermoplastics is 8-10 MPa, and the elongation at break is 150%-200%, which can basically meet the needs of the products, but further improvement is needed compared with ordinary plastics.
(4) Degradation performance
All starch thermoplastics have a starch content of 80% to 90% or more, and the additives used therein are also degradable. Therefore, the whole starch plastic can be completely removed by light, microorganisms, etc. in a short time in the environment after being discarded after use. Degradation, the production of CO2 and water, does not cause any pollution to the environment, is a true degradation of plastics.
4, the cost price
The whole starch thermoplastic is based on natural starch. The average price of starch is about 2,500 yuan / t, which is included in the processing and other costs. The total production cost is generally below 5,000 yuan / t. It greatly reduces the production cost of plastics, which is 10%-20% lower than traditional plastics (about 7,000 yuan/t). Compared with other degradable plastics such as polylactic acid and PHB, the cost is lower than 50%, which has a price advantage and strong. Market competitiveness.
5. Prospects and applications
5.1 Application
As a fully biodegradable material, whole starch thermoplastics can be used in many places instead of ordinary plastics.
(1) Agricultural and horticultural materials are first agricultural mulch films, which should be applied preferentially from the perspective of environmental assessment. Because China is the world's largest country covering agricultural film, there are also multi-purpose composite membranes, pesticides and fertilizers for sustained release coating materials. Gardening supplies include nursery pots, nursery nurseries, nursery sheds, shade cloths, and fertilizer release materials.
(2) The application of the catering industry is mainly used as a snack box
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