Development of starch-based biodegradable plastics

Plastic products are 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 mainly four categories: filled type [1], photo/biological double degradable type, blended type and whole starch plastic [2].

1 Structure and properties of starch

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 [3] and the 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 in which α-D-1,4-glycosidically binds and has a relative molecular weight 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 weight of (100-400)×106. Starch properties are related to the relative molecular mass 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 through a physical process to become gelatinized starch or destructurized starch. 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 at 60-70°C, and then the starch granules disappear and gel at 90°C or higher. 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 (4).

2 filled starch plastic

In 1973, Griffin patented a starch surface-impregnated plastic for the first time [5]. 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 polarity of the synthetic resin is smaller, which 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 used [6,7].

2.1 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 [8]. Tianjin University’s Yu Jiuhao [9] has refined the starch granules and then selected a coupling agent to form a single molecular coating on the surface of starch granules to mask the hydroxyl groups on the surface of the starch granules, that is, to modify the starch granules with lipophilicity. As a result, the oil absorption of the starch granules is greatly increased, and 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 et al. [10] used siloxane and starch and water to dry, and then blended with self-oxidizing agent and ordinary plastic to produce degradable plastic masterbatch. Canada's St.Lawarnce starch company uses this technology to industrially produce Ecostar degradable plastic masterbatch. Greizer steinHB et al. [11] conducted a composting experiment on plastic bags made from PE/Ecostar Plus blends and found that the starch degradation agents used in the study did not effectively promote PE degradation inside the compost.

2.2 Chemical Modification

Chemical modification is usually the introduction of hydrophobic groups into the starch molecule, which serves to enhance the compatibility between the starch and the synthetic resin. The modification methods are esterification, hydroxyalkylation or graft copolymerization, etherification, Crosslinking modification [12]. Starch plastics currently produced by chemical modification methods include starch-ethylene/acrylic acid copolymers, PE9321 from Cabot Plastics GmbH in Germany, starch/polypropylene plastics from Monte Real Edison, Italy, esterified starch/PE, etherification from Coloron, USA Starch/PE and Graft Copolymer/Starch/Resin, Agri-Tech's Gelatinized Starch/Polyester (or Polyethylene, Polyacrylate) [13].

3 light/biological degradation type

Biodegradable plastics are difficult to degrade in some special areas such as drought or lack of soil, while photodegradable plastics cannot be degraded when buried in soil. For this reason, the United States, Japan, and other countries took the lead in the development of a class of both photodegradable and Biodegradable photo/biologically degradable plastic [14]. Photo/biodegradable plastics are made of photosensitizers, starches, synthetic resins and small amounts of additives (solubilisers, plasticizers, cross-linkers, etc.), where the photosensitizers are transition metal organic compounds or salts [15]. 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, the photosensitizer is 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 the technical problems of 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” of Changchun Institute of Chemical Technology, Chinese Academy of Sciences and “PE+ferrocene derivative photosensitizer+modified starch of Shanghai Organic Institute”. ". Huang Lizhen 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”. Improves the precise timing and degradation of plastic degradation and reduces costs. Products developed abroad include "PolygradeIII" from American Impact Corporation, "Poly clean" from ADM, and "EcostarPlus" from St. Lawrence, Canada. American Ecostar has developed "EcostarPlusTM", which modifies the starch and makes the starch surface hydrophobic, increasing its compatibility with polymers. The degradation rate of degradation products in biological environment is higher than that of ordinary plastics. More than times.

4 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 modified starch/EVOH copolymer blended with LDPE and a dimethylsiloxane epoxy modified starch, which was then blended with LDPE [17,18]. Mster-Bi Plastics [4] of Novamont, Italy, and NoVon [13] of Warner-lambert, USA 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 of water absorption are reduced, but it is insoluble in water.

The above plastics are not actually fully biodegradable plastics, starch and PVA blended plastics, starch and aliphatic polyesters or other natural polymers can be blended to make genuine biodegradable plastics, but due to its high sensitivity to humidity, The application surface is very narrow. Such as: Hosokawa pure [19] and other mechanical comminuted fine starch granules and chitosan solution blended, and in the blending solution by adding a small amount of plasticizers, enhancers, foaming agents, etc., by cast film obtained And sheets can be used as packaging materials. Fu Xiujuan [20] used modified starch and a small amount of PVA to prepare fully degradable plastics. The material had high transparency and good mechanical properties. In a soil with a moisture content of 30%, weight loss was 25% in one month.

5 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 the 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 moisture content during processing is 8% to 15%. % is appropriate, and the temperature should not be too high to avoid scorching [21]. All-starch plastic is currently the most promising starch plastic at home and abroad. Sumitomo Corporation of Japan, Warner-lambert Company of the United States and Ferruzzi of Italy, etc. declare that they have successfully developed full-starch plastics with a starch content of 90% to 100%. It can be completely biodegraded within 1 year without leaving any traces and no pollution. It can be used to make various containers, films and garbage bags [2]. 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.

At present, there is no production of whole starch plastics in China. Qiu Wei Yang et al. used cornstarch and cellulose to obtain an all-starch plastic. Its mechanical properties can basically meet the performance standards of traditional plastics, and the degradation performance is very good. It can reach three months and six months through the control of formulas. A year's different degradation rates, but if the material absorbs water, the mechanical properties are significantly reduced [21].

6 Problems

The main problems existing in the current starch-based degradable plastics are cost and performance, and the performance includes degradation performance and use performance.

(1) Price The cost of degrading plastics is generally higher than that of general plastics. For example, the price of filled starch plastics with the lowest cost is 15% higher than that of ordinary plastics, and the price of fully degradable full-starch plastics is 4 to 10 times higher. Therefore, fully degradable plastics are currently used abroad only in a few high-end cosmetic packaging containers and medical applications.

(2) Degradability The main component of filled and double-degraded plastics is synthetic resin, so it can only be incompletely degraded. The result of degradation is that the overall mechanical properties of the material are greatly reduced and collapse into fragments or network-like structures. Fragments are more difficult to collect and handle. For example, it is used in agricultural mulch, polyolefin products remain in the soil, long-term accumulation led to a substantial reduction in agricultural production. In addition, the study of induction period control and degradation rate of double degraded starch plastics and whole starch plastics is not satisfactory.

(3) Performance The mechanical properties of starch plastics have basically reached the standard of traditional plastics, but due to the inherent water absorption of starch, the material will absorb water in a humid environment.