Research Progress of Full Starch Degradable Plastics (1)

With the continuous increase of plastic production and the continuous expansion of its use, plastic brings convenience to people, but also brings a lot of solid waste to the environment to form severe white pollution, which has become a worldwide public hazard. The raw materials of current plastic products are non-renewable resources—oil, and the world’s oil reserves can only be used for about 40 years. The development of non-petroleum-based polymers and the development of renewable resources that can be degraded in the natural environment in place of petroleum to produce plastics have become hot topics. Biodegradable plastics can be roughly classified into two types: one is a natural polymer type such as starch, cellulose, and chitin, and the other is a chemical synthesis type such as polycaprolactone, polylactic acid, and poly-3-hydroxybutyrate. Chemically synthesized degradable plastics limit their development because of their high price. In natural polymers, the starch source is abundant and inexhaustible. The complete biodegradability of starch in various environments has been recognized by scholars from various countries. Therefore, starch-degradable plastic is an important aspect of biodegradable plastics research.

1 Research status

Biodegradable plastics refer to biodegradable high-molecular materials under the action of microorganisms capable of secreting enzymes (such as fungi, molds, etc.) under certain conditions, which can be divided into bio destructible plastics and completely biodegradable plastics. (biodegradable plastic) [1]. The starch-filled plastic (w (starch) = 7% to 30%) that was popular in China in the 1980s is a bio-destructive plastic. It can only be degraded by starch, and PE and PVC cannot be degraded and remain in the soil. In addition, environmental pollution may still occur in China and Japan. Such products have already been eliminated. Therefore, most of the starch-based degradable plastics currently produced in China are meaningless, and the real promising ones are all-starch plastics (w (starch) ≥ 90%), and a small amount of added plasticizers are also biodegradable. After the use of such plastics can be fully biodegradable, and ultimately generate carbon dioxide and water, does not pollute the environment, is the main direction of research on starch degradation plastics at home and abroad in recent years [2 ~ 4].

The production principle of all-starch plastics is to make the molecular structure of starch out of order, forming a starch resin with thermoplastic properties, and therefore it is also called a thermoplastic starch plastic. There are mainly four methods for preparing thermoplastic starch [5,6]: (1) starch and other polymer products; (2) starch and degradable polymer compound; (3) preparation of thermoplastic starch through chemical reaction; (4) Starch and plasticizer co-extruded. The fourth method is the most used in industry.

1.1 Composite of starch and other polymer products

Starch is blended with cellulose, lignin, pectin, chitin, and protein to prepare a fully biodegradable plastic, which has advantages in development. Funke et al. [7] used conventional extrusion and injection molding techniques to process different types of starch and fiber blend systems and to study the ratio of amylose to amylopectin in natural and extruded samples, relative The molecular mass distribution reveals the effect of the structure on product performance. The different types of starch, the types of additives, and the processing conditions make the performance of the product very different. Adding a small amount of fiber to the starch can significantly improve the performance of the product. It is reported in the literature [8] that the Wageningen Agricultural University of the Netherlands developed corn-, potato-, and wheat starch-derived degradable plastics that are completely free of petrochemicals, and that their strength is enhanced by the incorporation of hemp fibers. This plastic can be used as packaging materials, coatings, food storage boxes, shopping bags, and agricultural films. The comminuted fine starch granules are blended with the chitosan solution to make a biodegradable packaging material.

1.2 Compounding Starch with Degradable Polymers

In the early research of starch plastics, people mostly chemically modified starch and polyethylene (PE), polyvinyl chloride (PVC) intermix to prepare filled starch plastic sheet, the product can be partially degraded. Currently, the modified starch is blended with biodegradable polyesters such as polycaprolactone (PCL), polylactic acid (PLA), polyglycolic acid (PHA), etc. to form a film that fully biodegrades the product. Compared with the use of a single biodegradable polyester, it reduces production costs.

US Bloembergen et al. [9] prepared starch acetate, triglyceride (or linolenic acid ester, lactate and citric acid ester) as plasticizers, and PLA (or PLA) from starch raw materials such as corn, potato, wheat, and rice, respectively. PCL, PHA, etc.) are extruded and formed into films. The product has good water resistance, good transparency and strong flexibility. Both starch and PCL have poor mechanical properties, but Takagi et al. copolymerized starch and acetic anhydride and blended with PCL has better mechanical properties and degradation performance [10]. Wang Xichen and Zhao Hua et al. [11,12] using a cross-linked starch and cationic starch and cellulose, polyvinyl alcohol, light calcium carbonate, etc. in a double-roller blender for blending and plasticizing to obtain a foamed starch plastic, Instead of polystyrene (PS) used as snack boxes and other packaging materials. Wu Jun et al [13] conducted sodium metaphosphate cross-linking modification of starch and surface treatment of silane coupling agent, and the obtained hydrophobic starch was then mixed with polycaprolactone to obtain a fully degraded starch plastic film after polyalcohol plasticization treatment. . Based on this study, they first processed the cross-linked starch by air-flow pulverization and ball-milling (mean diameter of 3 μm), and then carried out coupling treatment. The ester-like and hydrophobic properties of starch were significantly improved, and the dispersibility of the starch was improved. And compatibility with polyester PX [14]. The micronization process also enables the thermoplastic starch sheet to have good water resistance and mechanical properties under a high starch mass fraction, and at the same time, the biodegradation speed is also significantly improved. The miniaturization of starch is one of the directions for the development of starch degradation plastics.

Starch may graft hydrophilic or hydrophobic monomers to render the modified starch hydrophilic or hydrophobic. The starch graft copolymer blended with the biodegradable polyester (PLA, PCL), on the one hand, improves the compatibility of the starch and the biodegradable polyester, and at the same time reduces the production cost. Narayan [15] graft-copolymerized high amylose (w (amylose) = 99%) and blended with aliphatic polyesters (such as polyester amide, polyurethane, polyethylene glycol, etc.). By injection molding, blow molding or extrusion molding, the product is completely biodegradable. Han Qingyuan [16] invented a starch polymer instead of traditional plastics, used in the production of shock-proof packaging materials and snack boxes and other disposable packaging products, including w (starch) = 80% to 90%. In addition, Sun et al. [17] studied the preparation of high-strength plastics by blending raw starch and PLA, and used polyethylene glycol, polypropylene alcohol, etc. as plasticizers to coextrude wheat and corn starch. Product performance is similar to PLA, but the cost of the product is significantly reduced.

1.3 Preparation of Thermoplastic Starch by Chemical Reaction

The starch can be further modified to improve its thermoplasticity. Modification methods include esterification, etherification, oxidation, cross-linking and grafting. Sagar et al. [18] pointed out that the higher the degree of substitution of starch acetate, the longer the side chains and the more obvious the change of thermoplasticity and hydrophilicity. High melting point starch acetate has a higher melting point. Brochols [19] added a plasticizer with a molecular weight of 100-1000, such as triglyceride, to lower the melting temperature to about 150°C. U.S.Amylum Chemical Co., Ltd. uses hydroxypropylation of starch with an amylose mass fraction of 70%, and directly extrusion molding to obtain thermoplastic starch instead of expanded polystyrene (PS) for protective packaging [20] . The average relative molecular mass of 500,000 produced by Zhang Haiguang et al. [21] and the new thermoplastic hydroxypropyl-hydroxyethyl modified starch are expected to be used as masterbatch to develop degradable functional materials. Wang Chuanyu et al [22] grafted starch with amino resin, modified starch has improved mechanical properties and waterproof properties, can replace general-purpose plastic products. The Battelle Institute of Germany uses modified starch and 10% other natural resources as additive to prepare biodegradable plastics that can be molded by injection molding or blow molding and can be decomposed in water or soil for several months [23]. At present, the grafting of lactones and cyclic esters by ring-opening reaction is a new research direction for the chemical modification of thermoplastic starches.

1.4 Coextruded starch and plasticizer

Starch and plasticizer together, the high temperature melting, extrusion thermoplastic starch preparation is the main direction of the current research of starch biodegradable plastics, most of the plasticizers used are polyols. In general, when the ratio of the relative quantity of the polar group and the non-polar group of the polymer and the plasticizer and the space ratio is appropriate, the plasticization effect is better [24]. In the literature [4,6], after DSC plasticization, DSC differential scanning calorimetry showed that there was a clear melting endotherm at 140-160°C. It shows that the hydrogen bonding between starch molecules is weakened and destroyed, the thermal movement of the molecular chains is intensified, the diffusion force is increased, the glass transition temperature of the material is lowered, and the microcrystalline melting before decomposition is realized. Starch molecules change from a double helix structure to a random coil structure, so that the starch has thermoplastic processing properties. Wiedmann et al. studied the extrusion process of starch plastics. Starch can undergo a certain degree of disorder during the extrusion process. Increasing the temperature and prolonging the time of the material in the barrel can all promote the disorder of the starch.[5] ].

The relationship between the mass fraction of amylose and the properties of the extrudate was studied in detail. It was found that the bulk density of the extrudate decreased with increasing the mass fraction of amylose, and the shear strength increased with the mass fraction of amylose. And increase. Therefore, the starch suitable for the preparation of biodegradable plastics generally has a mass fraction of 40% to 70% of amylose, and the resulting plastic products have good mechanical properties [8,25]. Lacourse et al. [26] produced packaging materials based on amylose which can be comparable to expanded polystyrene, being inexpensive and fully biodegradable. This material does not carry static electricity, as a protective packaging material, such as electronic devices, electronic components and other sensitive device cushion layer, has the unparalleled superiority of expanded polystyrene. The Battelle Research Institute in Germany successfully modified the pea varieties and developed a starch with a high amylose mass fraction, which can be directly molded by ordinary processing methods. The resulting film is transparent and soft, and can be widely used as a substitute for PVC in water or in humid environments. It can be completely decomposed [23].

Wang Peizhang et al. [27] studied the thermoplastic mechanism of starch and prepared thermoplastic starches using four plasticizers: glycerol, ethylene glycol, sorbitol, and polyvinyl alcohol. It was considered that the use of hydroxyl-containing high molecular weight increases. Plasticizers and low molecular weight plasticizers are plasticized to help improve the mechanical properties of the product. In the study of plasticization of cornstarch, tapioca starch and soluble starch, amylose was more easily plasticized and mixed with the resin than amylopectin. Xiong Hanguo et al [28,29] used water, glycerol, propylene glycol, and other small molecules as plasticizers. The study found that the number of crystallization peaks of plasticized starch was drastically reduced, indicating that the crystalline area of ​​starch was destroyed by plasticizers. With the increase of stereotypes, starch has been transformed into a thermoplastic polymer material. It is considered that water is the most effective plasticizer for starch, and the amount thereof is 15% of the starch quality. Loercks et al. [30,31] considered that in the process of thermoplastic starch extrusion, if the mass fraction of water in the starch is ≥ 5%, the disaggregated starch instead of the thermoplastic starch is produced, and the structure of the disaggregated starch is not completely destroyed. It is brittle, non-scalable and cannot be used to make degradable plastics. Loercks [30] uses hydrophobic biodegradable polymers (aliphatic polyesters, aliphatic and aromatic polyesters, etc.) as plasticizers in their patents, is added to starch melts, and is uniformly mixed to form starches. Masterbatch. It has been found that the use of a hydrophobic biodegradable polymer as a plasticizer can avoid the migration of plasticizers in the thermoplastic starch melt, so that the starch during the melting, mastication process is formed of a thermoplastic starch instead of a disaggregated starch. At the same time, it is pointed out that the successful conversion of natural starch to thermoplastic starch has two crucial factors: 1 When the original starch and the plasticizer are mixed, the melting point of the original starch should be reduced to a level that can stop starch decomposition; 2 The starch should be dry enough To inhibit the formation of disintegrating starch.