Overview of natural bio-based polymer materials

1. Introduction to bio-based polymer materials

â—†1. Bio-based polymer material composition

At present, bio-based polymer materials include bio-based platform compounds, bio-plastics, functional sugar products, wood-plastic composite materials, etc., and their products include daily necessities that are often seen in daily life, such as packaging materials, disposable daily necessities, etc. It also includes high-tech, high-value drug controlled release materials and bone fixation materials, as well as biomedical materials such as human tissue repair materials. Mainly can be divided into the following three categories:

(1) Bio-based thermoplastic polymer materials: such as PLA, thermoplastic starch, fiber and composite materials;

(2) Bio-based thermosetting polymer materials: such as epoxy resin, unsaturated resin, adhesive, etc.;

(3) Bio-based polymer additives: such as flame retardants, plasticizers, nucleating agents, modifiers, etc.

â—†2. Development purpose and significance of bio-based polymer materials

According to the data, nearly 99% of the polymer materials are derived from petrochemical resources, but now petrochemical resources are facing an increasingly depleted crisis, and environmental problems are becoming more serious. If bio-based polymer materials are used instead of ordinary plastic products, it can reduce oil consumption by 30% to 50% and reduce carbon dioxide emissions by 50% to 80%. Under such a background, it is of urgent practical significance to research and develop biodegradable bio-based polymer materials to replace petroleum-based polymer materials. In recent years, with the active efforts of governments and related companies, bio-based polymer materials have made great progress.

Second, the classification and introduction of natural bio-based polymer materials

As one of the important components of bio-based polymer materials, natural bio-based polymer materials refer to polymer materials that are blended and modified by natural organisms (including animals, plants, microorganisms, etc.) or other resources. It is rich in reserves, renewable, recycled or degradable, but it is difficult to form and process. The main categories are as follows:

â—†1. Plastic wood composite material

Plastic-wood composites mainly use plastics (polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), etc. and their recycled waste plastics) as raw materials, by adding wood flour, rice husks, straw and other waste plant fibers. A sheet or profile produced by mixing into a new wood material and then subjecting it to plastic processing such as extrusion, molding, and injection molding. The main features are: raw material resources, plasticization of products, environmental protection, cost-effective, recycling and recycling.

At the same time, because of the plastic water resistance and the texture of the wood, the wood is a good and durable outdoor building material (floor, fence, chair, garden or waterfront landscape, etc.); Among the various types of templates used in the project, the wood-plastic formwork is currently the most suitable material for recycling and environmental protection. It has been used in many key constructions. In 2015, the application of wood-plastic formwork has exceeded 100 million square meters.

â—†2. Starch-based plastic

Starch is a natural high molecular polymer widely found in plants such as corn, wheat, rice, potatoes, and cassava. Due to the large amount of hydroxyl groups in the molecule, the interaction between the starch macromolecules is strong, which makes the original starch difficult to melt process, and also has poor compatibility with other polymers in blending with other polymers. However, these hydroxyl groups can undergo chemical reactions such as esterification, etherification, grafting, crosslinking, etc., and these chemical reactions are used to chemically modify the starch to reduce the hydroxyl group of the starch and change its original structure, thereby changing the corresponding properties of the starch. Turn the original starch into a thermoplastic starch. Generally, there are physical modification, esterification, transesterification or etherification reaction, crosslinking reaction, blending modified composite material, blending modified copolymerization and the like.

Starch-based plastics are widely available, inexpensive, and renewable. They can be completely and rapidly degraded in soil and natural environment. They are non-toxic, pollution-free, and have no odor. They do not destroy the soil structure after degradation. At present, typical thermoplastic starch plastic products are films, which are transparent, soft and non-toxic. A typical starch/degradable polymer composite is a starch/PVA alloy.

â—†3. Cellulose and its derivative plastics

Cellulose chemistry and industry began more than 160 years ago and was the main research object in the birth and development of polymer chemistry. The base ring of cellulose macromolecule is a macromolecular polysaccharide composed of D-1 glucose with β-1,4 glycosidic bond. Its chemical composition is 44.44% carbon, 6.17% hydrogen, and oxygen 29.39%. Cotton is high purity (98%). Cellulose. As one of the most tough plastics in thermoplastics, cellulosic plastic has good gloss, good transparency, high hardness, strong mechanical strength, good dimensional stability, and excellent heat resistance, electrical insulation and weather resistance. Sex and chemical.

Cellulose derivatives are generally in the form of white fibers, granules or flakes, and various cellulose derivatives are obtained by various methods of high purity cellulose. There are usually nitrocellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, ethyl cellulose, cyanoethyl cellulose, hydroxyethyl cellulose, and the like.

The preparation of cellulose plastic is a kind of thermoplastic which is obtained by chemically treating a natural polymer compound of cellulose or cellulose in the above plant, chemically reacting, adding various additives and physically modifying. . Commonly used additives are: plasticizers, stabilizers, lubricants, fillers, colorants, solvents, etc.

Cellulose plastics can be produced by injection, extrusion, molding, blow molding, machining, and the like. Can be made into automotive windshield, stationery, packaging film, military safety glass, daily necessities, camera parts, radio casings, military supplies, electrical insulation parts and medical and health supplies.

â—†4. Protein plastic

Protein plastic is one of the most widely studied natural bio-based polymer plastics in the field of biodegradable materials. At present, the plant proteins that have been applied to biodegradable materials at home and abroad mainly include soy protein, zein, wheat protein, and sunflower seeds, among which soy protein is the most studied. At present, there are two methods for processing soy protein biodegradable materials: one is wet processing, that is, the modified protein is formulated into a solution, cast into a film, naturally dried or heated and dried; the other is dried. The method of processing, that is, the modified soybean protein is uniformly mixed with a certain amount of plasticizer, and then the product of the degradation material is obtained by extrusion, molding, blow molding or injection molding by a mechanical force and adopting a suitable mold. The molding conditions are mainly: molding pressure, molding time and molding temperature.

Since the main chain of the soy protein molecule contains a large amount of amide bond (–CO–NH–), the molecular side chain contains more water-absorbent amino acid residues (–NH2, –COOH), and the material is hard, brittle and highly absorbent. Characteristics, so the material should be modified first when preparing the material. At present, commonly used protein material modification methods include physical modification, chemical modification, small molecule plasticization modification, and blend modification. These modification methods only change the high-order structure or conformation of the protein molecule, and have no effect on the amino acid sequence of the primary structure. The main purposes of modification are two: to improve the hydrophobicity and mechanical properties of the material; to improve the plasticity and processing fluidity of the material.

At present, the factors restricting the development of biodegradable soy protein plastics are mainly expensive and difficult to promote; the degradation mechanism of materials is not yet clear, and the precise time control of degradation needs to be improved; there is no uniform evaluation method for degradation performance. standard. The focus of research at home and abroad is mainly on improving the processing technology and improving the mechanical properties of the materials. Some scholars have used soy protein to prepare biodegradable materials with good mechanical properties and certain water resistance.

â—†5. Lignin plastic

Lignin plastic is a composite material prepared by blending wood plastic with resin, plasticizer, inorganic filler, compatibilizer and pigment. Lignin is a renewable natural polymer. In nature, the storage of lignin is second only to cellulose. The annual output is 150 billion tons. It has the advantages of being degradable, renewable, low-cost and non-toxic. It is derived from papermaking black liquor.

The plastics industry needs to use a large amount of fillers and reinforcing agents every year. Compared with ordinary inorganic fillers, lignin has the greatest advantage in that it has a highly reactive functional group on the molecule, and it is convenient to chemically modify other functional groups required. Based on the commercial application value of lignin in physical, chemical properties, industrial production and plastics, it is very meaningful to develop lignin plastics. Common lignin plastics include lignin/PVC, lignin/phenolic resin (PF), lignin/polyurethane (PU), lignin/polypropylene (PP), lignin/polyethylene (PE), and the like.

At present, the focus of research on lignin plastics is still in the aspect of compatibilization technology. How to improve the compatibility between lignin and resin easily and effectively is the key to the large-scale use of lignin in the plastics industry; The matrix is ​​produced by graft polymerization to produce a fully degradable polymer material such as lignin-grafted methyl methacrylate, and lignin is directly involved in the reaction as a monomer, and phenolic resin, polyurethane, polyester and polyimide are also synthesized. Hot spots of development in recent years.

â—†6. Chitin and derivative chitosan plastic

â—‡6.1 chitin

Chitin, also known as chitin, is widely found in lower plant fungi, algae cells, arthropod shrimp, crab, maggot and insect shells, shellfish, mollusks (such as salmon, squid) shells and Cartilage, the cell wall of higher plants, etc., its annual biosynthesis resources amount from 10 billion tons to 100 billion tons, which is the second largest biological resource on earth after plant fiber. Chitin is involved in the carbon and nitrogen cycle of the ecosystem through the complete biodegradation of chitinase, lysozyme and chitosanase in nature, and plays an important role in regulating the earth's ecological environment.

It is known from the chemical structure analysis of chitin that chitin is the only natural high molecular polymer with a positive charge in nature. Most of the chitin in nature is always tightly bound to water-insoluble inorganic salts and proteins. In order to obtain chitin, the shell of the crustacean is often prepared by chemical or microbiological methods. At present, industrial production often uses chemical methods, acid-base treatment, removal of calcium salts and proteins, and then deacetylation with a strong base under heating conditions can be used to obtain a wide range of soluble chitin (chitosan).

At present, chitin is often extracted from discarded shrimp and crab shells at home and abroad. The content of chitin in shrimp and crab shell is 20~30%, the content of inorganic matter (mainly calcium carbonate) is 40%, and the content of other organic matter (mainly protein) is about 30%. China is a large country of chitin resources. The annual production of sea shrimp in the coastal area of ​​Zhejiang Province is 670,000 tons. According to the calculation of 40% waste, more than 10,000 tons of chitin can be obtained, and the resource potential is huge. Chitin has high barrier properties with polyvinyl alcohol, and its film properties can reach the performance of ordinary plastic films and can be biodegraded.

â—‡6.2 Chitosan Plastics

Chitosan (chitosan) is a water-soluble product formed by the removal of acetyl from chitosan by concentrated alkali water. The scientific name is (1,4)-2-amino-2-deoxy-β-D-glucan. The product is white, slightly pearly, and is a translucent sheet-like solid. Chitosan is a cationic polymer with good chemical stability, decomposition at about 185 ° C, non-toxic, insoluble in water and alkali, soluble in sulfuric acid, organic acids (such as 1% acetic acid solution) and weak acid aqueous solution. Soluble in dilute acid to form a viscous and transparent chitosan salt colloidal solution. At this time, H+ in the solution combines with the amino group in the molecule to form a positively charged polymer substance, which can be acylated, carboxylated, hydroxylated. , alkylation, esterification (sulfation), aldimine, azide, salt formation, hydrolysis, chelation, oxidation, chlorination, grafting and cross-linking reactions. Chitosan can be identified by two indicators: appearance (whiter appearance, better) and deacetylation degree (the higher the degree of deacetylation, the better).

Chitosan can be blended with other natural bio-based polymer materials into chitosan plastics, such as chitosan and cellulose blend composites, which can be used in the production of packaging materials, agricultural films, nursery pots and other products. The film produced by the chitosan and starch blend composite is insoluble in water and has high tensile strength and can be used for packaging foods.

In the face of increasingly depleted petrochemical resources and increasingly serious environmental problems, the use of degradable natural bio-based polymer materials instead of petroleum-based polymer materials is an important way to solve energy substitution and an effective means to improve the ecological environment. Very practical. The research and development of natural bio-based polymer materials has a long way to go, which requires everyone to work together to actively promote its development.

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