Synthetic polymer materials have the advantages of light weight, high strength, good chemical stability and low price. They have become the four pillars of the national economy alongside steel, wood, and cement, and are widely used in product packaging. However, a large amount of waste after its use is also increasing day by day, becoming a source of white pollution, seriously endangering the environment, causing sewage and soil pollution, endangering human survival and health, and causing a non-negligible negative impact on the environment on which mankind depends. In addition, the raw material for the production of synthetic polymer materials-petroleum is always used up in one day. Therefore, it is urgent to find new environmentally friendly materials and develop non-petroleum-based polymers. Biodegradable materials are the solution to this problem. Effective Ways.
Definition and degradation mechanism of biodegradable materials
Biodegradable materials, also known as "green ecological materials", refer to materials that can be degraded under the action of soil microorganisms and enzymes. Specifically, it refers to a polymer material that can cause biodegradation under certain conditions under the action of natural microorganisms such as bacteria, molds, and algae.
The ideal biodegradable material is a polymer material that has excellent performance and can be completely decomposed by environmental microorganisms after being discarded, and finally converted into CO2 and H2O to become a component of the carbon cycle in nature.
The decomposition of biodegradable materials is mainly through the action of microorganisms. Therefore, the degradation mechanism of biodegradable materials is the process of digestion and absorption of materials by bacteria and molds. First, the microorganisms secrete hydrolase in vitro to bind to the surface of the material, and cut off the surface polymer chains by hydrolysis to generate small molecular weight compounds, and then the degraded products are taken into the body by the microorganisms, and undergo various metabolic routes to synthesize microorganisms or transform. The energy for microbial activities is finally converted into water and carbon dioxide. According to the chemical nature of its degradation, it can be divided into hydrolysis and enzymatic hydrolysis.
01 Hydrolysis mechanism
The degradation of the material is essentially a process in which the internal polymer segments break into low-molecular-weight oligomers under certain conditions, and finally decompose into monomers. The "corrosion" of the material refers to the process in which the water-soluble small molecular substances formed due to the breaking of the molecular chain leave the polymer material, resulting in the reduction of the mechanical properties of the material and the complete disappearance of the material. The dissolution can be surface dissolution and overall dissolution.
If the degradation rate of the molecular chain is faster than the diffusion rate of water molecules in the material, the hydrolysis of the chain is limited to the surface of the material, and it is difficult to enter the inside of the material. This method belongs to surface corrosion or heterogeneous corrosion. When the diffusion rate of the material is faster than the hydrolysis rate of the polymer segment, the degradation of the surface and the interior of the material will proceed simultaneously, so it belongs to the overall dissolution.
02 Enzymatic hydrolysis mechanism
Enzymatic hydrolysis mechanism
For polymers that are easily hydrolyzed, there may be both simple hydrolysis and enzymatic hydrolysis in the body. Lipase can promote the decomposition of polyester, while hydrolase can promote the degradation of easily hydrolyzable polymers. Lipase R.delemer lipase, Rhizopus arrhizus lipase, and Pseudomnas lipase are specific degrading enzymes of PCL. In the presence of these enzymes, PCL degradation speeds up. Under normal circumstances, it takes 2-3 years to completely degrade, but in the presence of enzymes The complete degradation time is shortened to a few days.
For some non-hydrolyzable polymers, the possible degradation mechanism is the enzymatic oxidation mechanism. Immunohistological studies have confirmed that the material is finally absorbed and metabolized in the body through the endocytosis of phagocytes. After polymer biomaterials are implanted in the body, they will cause local acute inflammation to varying degrees. When the tissue is injured, the permeability of the surrounding blood vessels will change, and the multinucleated leukocytes will quickly move to the inflammation site, and the activated neutrophils Can make monocytes differentiate into macrophages. The metabolism of polymorphonuclear leukocytes and macrophages produces a large amount of peroxy anions (O2), and this unstable intermediate is converted into a stronger oxidant hydrogen peroxide. The reduced coenzyme (NADPH) and oxidase in the body are all involved in this conversion reaction, while superoxide dismutase (SOD) plays a role in accelerating the conversion. Hydrogen peroxide may initiate the decomposition reaction of the polymer itself at the implantation site; at the same time, hydrogen peroxide can be further converted into hypochlorous acid under the action of muscle peroxidase (MPO). Hypochlorous acid is also a strong oxidant for biological materials. It can oxidize the amino groups in polyamide, polyurea, and polyurethane, and break the polymer chain, thereby achieving the effect of degradation.
Among the biodegradable materials, microorganisms such as bacteria, molds, fungi and actinomycetes play a major role in degradation. According to the form of their degradation, they can be divided into three types:
1. The physical function of organisms, the material is destroyed mechanically due to the growth of biological cells; 2. The biochemical effects of organisms, the effects of microorganisms on materials produce new substances; 3. With the direct action of enzymes, microbes erode some components of the material products and then cause the material to decompose or oxidatively collapse.
Features of biodegradable materials Biodegradable materials have the following characteristics:
1. It can be treated together with garbage, or it can be made into compost to return to nature;
2. The volume of the landfill is reduced due to degradation, and the service life of the landfill is prolonged;
3. There is no problem that ordinary plastics need to be incinerated, which can suppress the emission of harmful gases such as dioxin;
4. It can reduce the harm to wild animals and plants caused by arbitrary discarding;
5. Convenient for storage and transportation, as long as it is kept dry and does not need to be protected from light;
6. It has a wide range of applications, not only in agriculture and packaging industry, but also in medical industry.
Classification of biodegradable materials
Biodegradable materials can be divided into completely biodegradable and biodestructive materials according to degradation mechanism and destruction methods:
01Completely biodegradable materials
Completely biodegradable materials refer to biodegradable materials that can be completely decomposed by bacteria, fungi, actinomycetes and other microorganisms. It can finally be decomposed into carbon dioxide and water under the action of bacteria or its hydrolase to return to nature, so it is called "green material". From the preparation method, it can be divided into three kinds: microbial fermentation method, chemical synthesis and natural polymer blending.
02 Biodestructive materials
Biodestructive materials are at the material level, mainly degradable materials made by blending or copolymerizing natural polymers and general-purpose synthetic polymers. There are several combinations:
1. Use melting and solution blending method;
2. Disperse a polymer material in an aqueous solution of another polymer to form a suspension system, and finally make various composites;
3. Disperse or dissolve natural polymer materials in a system that can undergo polymerization, and conduct homopolymerization and copolymerization reactions to polymerize monomers in the system to obtain composite materials containing natural polymers;
4. Properly degrade natural macromolecules under appropriate conditions (such as acidic or alkaline, etc.), and polymerize the degraded molecular segments with other monomers to prepare new copolymers with biodegradable properties.
Application of biodegradable materials
Biodegradable materials are new types of materials developed after the 1980s with the emergence of environmental and energy contradictions, which can partially replace general-purpose plastics. Currently, biodegradable materials are mainly used in environmental protection and medical fields.
Problems facing the development of biodegradable materials In recent years, biodegradable materials at home and abroad have developed rapidly. In particular, disposable material products, such as biodegradable food packaging bags, beverage bottles, and agricultural films, have achieved industrial production. However, the current development and application of biodegradable materials still have the following problems:
Market application: Due to the high cost of producing degradable materials, the price in the market is relatively high, which has a great impact on the promotion of degradable materials.
2. Technology and technology: Compared with traditional plastics, degradable materials have the problems of poor water resistance, poor mechanical properties and poor processing properties, which are difficult to meet the requirements of industrial production. In addition, the degradable materials have accurate degradation time control after use. Rapid degradability, complete degradability, and scrap recycling technology need to be further improved and perfected.
3. Standards and experimental evaluation methods for degradable materials: For biodegradable materials, there is no unified experimental evaluation method, identification mark and product detection technology in the world, resulting in the lack of a correct and unified understanding and accurate evaluation, and the product market is rather chaotic. False is hard to argue.
Prospects for biodegradable materials
In recent years, with the advancement of raw material production and product processing technology, biodegradable materials have attracted much attention and have become the focus of sustainable and circular economy development. It is of great significance whether it is from energy substitution, carbon dioxide reduction, environmental protection and solving the "three rural" issues. At present, in the development and application of biodegradable materials in my country, the R&D capabilities and investment in independent intellectual property rights, innovative products, etc. all need to be improved, and the recycling and processing system of biodegradable materials still needs to be improved.
