Four major categories of starch-based degradable plastics and their research progress

Plastic products are being widely used in various fields of people's production and life. Plastics are favored by people for their excellent properties such as light weight, water resistance, corrosion resistance, and strength.

However, large amounts of discarded plastic products are plagued by "white pollution" because of their non-degradability. For this reason, since the 1970s, people have begun research and development of degradable plastics.

As a natural polymer compound, starch has a wide range of sources, many varieties, low cost, and can be completely degraded in various natural environments. It eventually decomposes into CO2 and H2O without causing any pollution to the environment, resulting in starch-based degradable plastics. It has become one of the most researched and developed biodegradable plastics at home and abroad.

So far, starch-based degradable plastics have four major categories: filled, photo/biodegradable, blended, and starch-based plastics.

Starch structure and properties

Natural starch is in the form of small particles with a crystalline structure inside, and its molecular structure is straight chain and branched. For different plant species, the starch granule morphology, size H, and ratio of amylose and amylopectin content are all different. Most of the starch granules have a particle size of 15 to 100 μm. Amylose glucose is a chain compound with α-D-1.4-glycosidic linkage, with a relative molecular mass of (20 to 200) × 104. In addition to the α-D-1,4-glucosidic linkages, the linkages of glucose units in amylopectin also existed as α-D-1,6-glycosidic linkages with a relative molecular mass of (100-400)×106. Starch properties are related to the relative molecular weight of the starch, the length of the branches, and the ratio of amylose and amylopectin. Experiments have shown that high linear content starches are more suitable for the preparation of plastics and the resulting articles have better mechanical properties.

Hydrogen chains exist between natural starch molecules, which have poor solubility and are hydrophilic but not easily soluble in water. There is no melting process when heated, and decomposition occurs above 300°C. However, starch can under certain conditions destroy the hydrogen bonds into gelatinized starch or destructurized starch through physical processes. The crystalline structure of the starch in this state is destroyed and the molecules become disordered.

There are two ways to make the starch lose its crystallinity: First, the starch is heated under conditions containing more than 90% of water, and the starch granules first swell when the temperature is 60°C to 70°C, and then the starch granules disappear when the temperature reaches 90°C or more. . The second method is to heat the starch in a sealed state under a water content of less than 28%, and plasticize and extrude the starch. At this time, the starch undergoes true melting. The starch under this condition is known as disaggregated starch, and some people call it gelatinized starch. This starch differs from natural granular starch in that it is heated and plasticized, so it is known as a thermoplastic starch.

Filled starch plastic

In 1973, Griffin patented a starch surface-impregnated plastic for the first time. By the 1980s, some countries developed starch-filled biodegradable plastics under the background of Griffin's patent. Filled starch plastic is also called bio-destructive plastic. Its manufacturing process is to add a certain amount of starch and other small amounts of additives in general plastics, and then processing and forming, the starch content does not exceed 30%. Filled starch plastic technology is mature, the production process is simple, and the existing processing equipment can be produced with a slight improvement, so at present most of domestic degradable starch plastic products are of this type.

Natural starch molecules contain a large number of hydroxyl groups to form a strong hydrogen bond in the molecule and between molecules, the polarity of the molecule is larger, and the synthetic resin has a smaller polarity and is a hydrophobic substance. Therefore, the natural starch must be surface-treated to increase the hydrophobicity and its compatibility with the polymer. At present, the two methods of physical modification and chemical modification are mainly adopted.

(I) Physical Modification

Physical modification refers to the miniaturization of starch, the destruction of starch structure through an extruder or the addition of coupling agents, plasticizers and other additives to increase the compatibility of starch and general plastics. Tianjin University’s Yu Jiujiu refined the starch granules, and then selected a coupling agent to form a single molecular coating layer on the surface of the starch granules to mask the hydroxyl groups on the surface of the starch granules, that is, the lipophilic modification of the starch granules, making the starch granules The amount of oil absorption is greatly increased while the amount of water absorbed is significantly reduced. The starch processed by this process significantly improves the compatibility between the starch and the synthetic resin. G. Griffin, etc. use siloxane and starch and water mixed drying, and then blended with the self-oxidizing agent and ordinary plastic extrusion, made of degradable plastic masterbatch. Canada's St. Lawarnce Starch uses this technology to industrially produce Ecostar degradable plastic masterbatches.
Greizerstein H B et al. performed a composting test on plastic bags made from PE/Ecostar Plus blends and found that the starch degradation agents used in this study did not effectively promote PE degradation inside the compost.

(II) Chemical modification

Chemical modification is usually the introduction of hydrophobic groups into the starch molecule, which acts to enhance compatibility between the starch and the synthetic resin. The modification methods are esterification, hydroxyalkylation or graft copolymerization, etherification, Crosslinking modification. At present, the varieties of starch plastics produced by chemical modification methods include starch-vinyl ester copolymers, PE9321 from Cabot Plastics GmbH in Germany, starch-propylene-acrylic plastic from Monte-Edison, Italy, esterified starch/PE, ether from Coloron, USA Starch/PE and Graft Copolymer/Starch/Resin, Agrl-Tech Gelatinized Starch/Polyester (or Polyethylene, Polyacrylate).

Light/Biological Degradation

Biodegradable plastics are difficult to degrade in some special areas such as drought or lack of soil, while photodegradable plastics cannot be degraded when they are buried in soil. For this reason, the United States, Japan, and other countries have taken the lead in the development of a class of both photodegradable and Biodegradable light/biological biodegradable plastic. The photo/biodegradable plastics are made of photosensitizers, starches, synthetic resins and small amounts of auxiliaries (solubilisers, plasticizers, crosslinkers, etc.), wherein the photosensitizers are transition metal organic compounds or salts. The degradation mechanism is that the starch is biodegraded, the polymer matrix is ​​loosened, and the specific surface area is increased. At the same time, photosensitizers are induced by sunlight, heat, oxygen, etc., resulting in chain breaking of the polymer and a decrease in the molecular weight.

China once listed the research of optical/biodegradable plastic film as the national “8th Five-Year Plan” key scientific and technological breakthrough plan. In starch-based photo/biodegradable plastic film research, breakthroughs have been made in technical problems such as the miniaturization of starch, water absorption of starch derivatives and masterbatches, compatibility of starch and its derivatives with PE, and controllable induction period. Its representative products are “PE+Fe(I)x•Fe(F)x Photosensitizer+Modified Starch” from Changchun Institute of Chemical Technology, Chinese Academy of Sciences, and “PE+ferrocene derivative photosensitizer+modified starch” from Shanghai Institute of Organic Chemistry. . Huang et al. [16] researched and developed "PE+FeDBC/FeDEC photosensitizer + photosensitive modifier NiDBC + aluminate modified starch-CaCO3" and "PE+RECOOR3 photosensitizer + aluminate modified starch-CaCO3" to improve The accurate timing and degradability of plastic degradation reduces the cost. Products developed abroad include "PolygradeIII" from American Amps, "Polyclean" from ADM, and St. Canada. Lawrence's "EcostarPlus" and so on. American Ecostar has developed “Ecostar PlusTM”. By modifying the starch, the surface of the starch is hydrophobic and the compatibility with the polymer is increased. The degradation rate of the degradation product in the biological environment is higher than that of ordinary plastics. 100 times more.

Blending type

Starch blended plastics are starch plastics made by blending starch with synthetic resins or other natural polymers. The main components are starch (30% to 60%), a small amount of synthetic resins of PE, and ethylene/acrylic acid (EAA) copolymers. Ethylene/vinyl alcohol (EVOH) copolymers, polyvinyl alcohol (PVA), cellulose, lignin, etc., characterized by high starch content, some products can be completely degraded.

Japan developed a blend of modified starch/EVOH copolymer with LDPE, dimethylsiloxane epoxy modified starch, and then blended with LDPE. The products of Mster-Bi Plastics [4] of Novamont, Italy, and the NoVon of Warner-lambert of the United States also belong to this category. Mster-Bi plastic is a polymer alloy formed by a continuous cross-linked network of EVOH phase and starch phase. Since both components contain a large amount of hydroxyl groups, the product is hydrophilic, and the mechanical properties after water absorption are reduced, but it is insoluble in water.

The above plastics are not actually completely biodegradable plastics. Starch and PVA blended plastics. Starch blended with aliphatic polyesters or other natural polymers can be made into truly biodegradable plastics, but due to its high sensitivity to humidity, The application surface is very narrow. Such as: Hosokawa pure and other mechanical comminuted fine starch granules and chitosan solution blending, and in the blending solution by adding a small amount of plasticizers, enhancers, foaming agents, etc., cast film and sheet obtained by the cast method Can be used as packaging material. Fu Xiujuan and others made the degradable plastics by blending modified starch with a small amount of PVA. The material has high transparency and good mechanical properties. In the soil with the moisture content of 30%, it lost 25% in one month.

Whole starch type

The starch molecules are allosterically and disordered to form a thermoplastic starch resin, followed by a small amount of additives such as plasticizers, so-called full starch plastics. The starch content is above 90%, and the small amount of other substances added are also non-toxic and can be completely degraded, so the whole starch is truly a fully degradable plastic. Almost all plastic processing methods can be applied to the processing of full-starch plastics, but traditional plastics processing requires almost no water, while the processing of full-starch plastics requires a certain amount of water to play a plasticizing role, and the water content during processing is 8% to 15%. % is appropriate and the temperature should not be too high to avoid scorching. All-starch plastic is currently the most promising starch plastic at home and abroad. Sumitomo Corporation of Japan, Wanlerlambert of the United States, and Ferruzzi of Italy declared that they have successfully developed full-starch plastics with a starch content of 90% to 100%. The product can be completely biodegraded within 1 year without leaving any traces and no pollution. It can be used for Manufacturing various containers, films and garbage bags. The Battelle Research Institute in Germany developed a biodegradable plastic with modified green pea starch with a high linear content, which can be processed and formed by conventional methods. As a substitute for PVC, it can be completely degraded in a moist natural environment.

Source: China Starch Trading Network