Research Progress of Starch-Based Degradable Plastics

Since Griffin first obtained patents for surface-modified starch-filled plastics [1] in 1973, starch-based biodegradable plastics have developed rapidly and are currently the most widely used biodegradable plastics. Biodegradable plastics researched and developed at home and abroad can be roughly divided into two types [2]: one is a natural polymer type such as starch, cellulose, chitin, and the other is a chemical synthesis type, such as polycaprolactone, poly Lactic acid, poly-3-hydroxybutyrate and the like. Chemical synthetic degradable plastics limit their application range due to their high price and other reasons. Among natural polymers, starch-based plastics are favored because of their low price, simple processing equipment, and excellent degradation performance, and various products have been sold at home and abroad. Such as Canada's St.Lawrance company, the United States Ampacet company, Italian Ferruzzi company. Domestic Changchun Institute of Applied Chemistry, Tianjin University, Sichuan University and other units have also developed starch-based biodegradable plastics.

1 Research content of starch-based biodegradable plastics

1.1 Starch Surface Treatment Technology

Starch molecular chains contain a large number of hydroxyl groups, which are easily absorbed by water and have an equilibrium moisture content of 12% in the atmosphere. In addition, starch granules are prone to agglomeration due to hydroxyl interactions and form microcrystalline particles, which have poor compatibility with non-polar polymers. Therefore starch must be surface-treated before application to increase hydrophobicity and its compatibility with high polymers. The current application of the processing technology is mainly to make starch oxidation, amination, esterification or etherification and other denaturation reactions [3], the reaction product has a hydrophobic group, can significantly reduce the starch water absorption rate. At the same time, because the surface of the modified starch granules is covered by alkyl groups, the effect of hydrogen bonding is weakened, and the compatibility with polyethylene and other high polymers can be improved to varying degrees. Maddever and Chapman have demonstrated that filling polyethylene with modified starch is stronger than the original starch filling system [4].

1.2 Compatible starch technology

Although the modified starch after surface treatment has certain compatibility with the polymer, when the content of starch in the blend is more than 40%, the mechanical properties of the composite material are obviously decreased, and the application requirements cannot be satisfied. How to use compatibilizer technology to improve the compatibility of starch and polymer is still a hot topic. There have been many patents and research reports on starch compatibilization technology [5-7]. Adding the third component in the blending system, such as ethylene-acrylic acid copolymer (EAA), ethylene-vinyl acetate (EVA), etc., can significantly improve the compatibility of starch and polymer. In addition, the technology of grafting starch with thermoplastic monomers has been deeply studied [7,8]. Thermoplastic monomers mainly include acrylates, acrylonitriles, methacrylates, etc. This grafted starch can be directly linked with polymers. Blending can also be used as a compatibilizer to improve the compatibility of starch and polyethylene and other high polymers. However, graft modification of starch is complicated and the price is high, and some of the grafted products increase the diameter of the starch particles and are not suitable for industrial production. The use of unsaturated fatty acids and starch mixed grafting reaction, strict control of the mixing process, can achieve a more ideal purpose of modification, very suitable for industrial production [9].

1.3 Starch particle size

The particle diameter of common starch is mostly between 2 and 150 μm, and the size of starch directly affects the dispersion uniformity of the starch in the substrate and the mechanical properties of the material. Especially for the film products, the starch particle size is too large to satisfy the normality of the film product. Production process requirements. In general, the refinement of starch can be performed using various general-purpose pulverization techniques. At present, ultra-fine starch with a particle size of 215 μm can be obtained by using air jet comminution technology [10] and ball mill crushing technology [11]. From the perspective of dispersion, the particle size of the dispersed phase is the main factor that determines the performance of the multiphase system. The finer the starch particle size, the more uniform the dispersion and the better the mechanical properties of the material.

1.4 Thermoplastic Starch

Natural starch does not have thermoplastic processing properties and cannot be processed in plastic machinery. To make it thermoplastic, it must have its molecular structure disordered. The process of preparing thermoplastic starch can be roughly divided into four kinds [12,13]: (1) starch and polymer polysaccharide composite; (2) composite of active starch and degradable polymer; (3) preparation of thermoplastic starch by grafting technology (4) Coextruded starch and plasticizer. The most widely used in industrial production is the fourth route. The plasticizer is a polyol compound. After the starch is plasticized, a clear melting endotherm appears between 140 and 160°C, which translates into processability. Thermoplastic starch. The principle is that after the plasticization, the intermolecular hydrogen bonds between the starch molecules are weakened, the thermal movement of the molecular main chain is intensified, the diffusion force is increased, and the glass transition temperature of the material is lowered, so that the microcrystals are melted before the thermal decomposition, and the starch is doubled. The helical conformation transforms into a random coil configuration with thermoplastic processing properties. The Battele Institute, Tianjin University, and the Applied Chemistry Research Institute of the Jiangxi Provincial Academy of Sciences have conducted in-depth studies. According to the processing properties of thermoplastic starches, the United States WarnerLamber Co., Ltd. launched a brand name of "Novon" full-starch plastic, its starch content of up to 90%, the other 10% of the main components of petroleum by-products [14]. Qiu Wei Yang et al. also developed a full-starch plastic film, but the mechanical properties are poor [15]. In short, full-starch plastic is the ultimate result of the development of starch-based plastics, and it is an important topic for research and development at home and abroad in recent years.

1.5 Starch-based plastic processing performance

Werner Wiedmann and Edgar Strobel et al. studied the extrusion process of starch-based plastics [16]. Starch can undergo a certain degree of disorder in the extrusion process. Raising the temperature and prolonging the residence time of the material in the cylinder can promote the disorder of the starch. Increase the water content in the barrel, add other softeners or plasticizers, reduce the apparent viscosity of the material and reduce the mechanical work required for extrusion. The flow of starch during processing is typical of particle flow. From the rheological point of view, the starch-based plastic melt is a Bingham fluid. At a low shear rate, there is no Newtonian plateau, and the melt begins to flow only when a shear force higher than the yield stress is applied. The processing behavior is similar to that of synthetic polymers and can be processed using conventional plastic machinery [17]. Due to the prevalence of particles in the starch, torque is low at the moment of starting. This is because at the beginning, the polymer does not completely melt and flow, and the starch granules act as “rolls” after adding starch, which is beneficial to the movement of the polymer. When the polymer melts, the starch granules The retarding effect makes the torque force rise slightly, but the increase is not large, and it does not affect industrial production [17]. It is worth noting that starch-based plastics are subject to rapid degradation at higher temperatures, with a small range of processing temperatures. However, the processing temperature of thermoplastics is usually above 160°C. Therefore, the residence time of starch in the cylinder is required to be as short as possible, generally 4 ~15min[18]. The starch plasticization, compatibilization and other modification processes should be prepared before processing, so as to avoid the agglomeration of starch granules and reduce the energy consumption during processing and forming.

1.6 Composites of starch and other degradable materials

Starch can be combined with natural macromolecules such as glue, galactose, cellulose, chitin, etc. to form a completely biodegradable material for preparing packaging materials or food containers. There are many reports on this aspect, but they have not yet reached the level of industrial production. More industrialized is the combination of photodegradation technology and biodegradation technology of double degradable starch plastics. The technology of photodegradable plastics was developed earlier. Combined 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 utilize the compounding ratio and dosage of photosensitive systems to realize the degradation time. The purpose of the control. The currently used degradable materials are mostly based on this type.

1.7 Degradation Performance Study

The definition of biodegradable plastics is still a qualitative but non-quantitative description: under certain conditions, biodegradable macromolecules can be produced under the action of microorganisms that secrete enzymes (such as fungi, molds, etc.). There are many reports on the degradation mechanism of starch-based degradable plastics [19]. The degradation process can be roughly divided into the following processes: Starch-based plastics are decomposed into oligomers by polymers under the action of microorganisms, and the oligomers continue to decompose. For various organic intermediates, it is finally decomposed into carbon dioxide, water and other low-molecular compounds. However, in fact, any degradation material is not a single form of degradation in the soil, and it is simultaneously in various forms such as biodegradation, photodegradation, thermal oxidation degradation, etc., and the pH value of the soil, light intensity, temperature, humidity, and metal ions in the soil. The composition and content of a variety of factors.

There are many ways to characterize the biodegradability. Many industrial countries such as the United States, Western Europe and Japan have done a lot of work on the evaluation of degradable plastics [20]. In particular, the United States is very active in this research and development. The American Society for Testing and Materials (ASTM) announced in 1992 its evaluation standards for biodegradable plastics: ASTM D5209-92, ASTM D5210-92, ASTM D5247-92, ASTM D 5271-92, and ASTM D 5338-92. No matter what test method is used to detect starch-based plastics, the same conclusion can be drawn: The starch-based biodegradable plastics have good biodegradability.

2 Application of Degradable Materials

Starch-based biodegradable plastics are mainly used in the field of plastic film and packaging materials, mainly foam materials, packaging bags, fast food boxes, beverage cups, non-woven fabrics, plastic film, medical products and other materials. Degradable plastics, a brand-new technological approach to the management of plastic waste, has achieved satisfactory progress after years of research and development. According to the forecast of the Freedonia Group of the United States, the demand for degradable plastics in North America has increased significantly from 880 kt in 1989 to 1920 kt in 1994 and will reach 3,200 kt by the end of 2000. According to the statistics of China's authoritative department, in 1992, the amount of plastic film in China was 210 kt. The amount of packaging materials is 240kt. It is forecasted that in 2000, the use of plastic film and packaging materials will increase to 300kt and 400kt respectively [20]. Therefore, the development of biodegradable plastics is in line with domestic and international development forms and meets the requirements of “green environment”. Degradable plastics, a high-tech product, has broad prospects for development.



Source: China Plastics Industry Network