Biodegradable materials
Degradable materials are materials that can be degraded in the sense of thermodynamics and kinetics over a period of time. According to the external factors of degradation, it can be divided into: photodegradable materials, biodegradable materials, etc. The influencing factors mainly include temperature, molecular weight, and material structure.
Chinese name
Biodegradable materials
Definition
Degradable material in the thermodynamic sense
Influencing factors
The influence of temperature on the degradation of polymer materials
Classification
Photodegradable materials, biodegradable materials
table of Contents
1 category
According to the external factors of degradation, it can be divided into:
1. Photodegradable materials: degraded due to sunlight;
2. Biodegradable materials: due to the respiration or chemical energy synthesis of natural microorganisms such as fungi and bacteria, they are degraded and finally decomposed into carbon dioxide and water;
3. Environmentally degradable materials: degraded under natural environmental conditions such as light, heat, water, polluting compounds, microorganisms, insects, and mechanical forces.
2 Examples
Lactic acid
Polylactic acid (PLA), also known as polylactide, is chemically synthesized using lactic acid, a product of microbial fermentation, as a monomer. It can automatically degrade after use and will not pollute the environment.
Polylactic acid can be processed into fibers and films with excellent mechanical properties, and its strength is roughly equivalent to nylon fibers and polyester fibers. Polylactic acid can be hydrolyzed into lactic acid and acetic acid in the body, and metabolized into CO2 and H2O by enzymes, so it can be used as a medical material. Japan and the United States have used polylactic acid plastics to process surgical sutures, artificial bones, and artificial skins. Polylactic acid is also used to produce packaging containers, agricultural mulch, fiber sportswear and bedding.
Starch plastic
The starch content is more than 90%, and other components added can be completely degraded. Sumitomo Corporation of Japan, Wamer-Lamber Corporation of the United States, Ferrizz Corporation of Italy, etc. have claimed that they have successfully studied whole starch with a starch content of 90% to 100%. Plastics are completely biodegradable in (January to 1 year) without leaving any traces, no pollution, and can be used to manufacture various containers, bottles, films, garbage bags, etc.
The production principle of all-starch plastics is to deform and disorder starch molecules to form a starch resin with thermoplastic properties, so it is also called thermoplastic starch plastics. The molding process can continue to use traditional plastic processing equipment.
The potential advantages of using starch as raw material to develop biodegradable plastics are: starch has complete biodegradability in various environments; after the starch molecules in the plastic are degraded or ashed, carbon dioxide gas is formed, which is not harmful to the soil or air; Appropriate technology can make starch reach the mechanical properties used to make plastic materials after thermoplasticization; starch is a renewable resource, which is inexhaustible, and the exploitation of starch is beneficial to the development of rural economy.
It should be noted that the vast majority of starch plastics produced in my country are filled starch plastics, that is, a certain proportion of starch is added to non-biodegradable polymer materials. The biodegradation of starch causes the physical properties of the entire material to collapse, prompting a large number of The end groups are exposed to oxidative degradation, but the PE and PVC in the remaining part after this "collapse" cannot be degraded and remain in the soil. The accumulation of time will certainly cause pollution, so foreign countries classify such products as obsolete .
Photodegradable plastic
Photodegradable plastics refer to plastics that can be degraded under the action of light.
1. Examples of photodegradable plastics
According to the manufacturing method, photodegradable plastics can be divided into synthetic degradable plastics and additive degradable plastics.
(1) Synthetic degradable plastics
a. Ethylene/carbon monoxide copolymer (E/CO)
Photodegradation is characterized by main chain scission. The photodegradation speed and degree of E/CO are related to the amount of ketone groups contained in the chain. The higher the content, the faster the degradation speed and the greater the degree. Scientists in Texas, USA have conducted outdoor exposure experiments on E/CO. In June when the sun is full, E/CO can be degraded within a few days at the earliest.
b. Vinyl/vinyl ketone copolymer (Ecolyte)
The ketone group on the side chain of Ecolyte molecule can be decomposed under the action of natural light. Ecolyte's photodegradation performance is better than E/CO, but the cost is also higher.
The disadvantage of this type of polymer is that it begins to degrade once exposed to light, and there is almost no induction period. Antioxidants need to be added to adjust the induction period.
(2) Additive photodegradable plastic
Additive type photodegradable plastic is to add a small amount of photosensitizer to the polymer. At low concentration, it is a photooxidation degradation catalyst, which reacts by sunlight (ultraviolet light) irradiation to break the polyolefin polymer.
Adding ketones, amines and other photosensitizers to PE, PP and other polymers can achieve better photodegradability.
The additive photodegradable plastic has low cost, simple production process, and good effect as a covering film. However, its degradation characteristics are that the exposed surface is relatively completely degraded, and the part buried in the soil is poorly degraded. The degradation induction period of such photodegradable plastics can be controlled at more than two months. But the controllability of degradation time is poor.
3 influencing factors
There are many factors that affect the degradation performance of materials, specifically as follows:
The effect of pH on the degradation of polymer materials
Mader et al. believed that the change of pH value has a great influence on the hydrolysis rate of the copolymer chain, but the degradation rate does not differ greatly in different parts of the organism. The degradation of the copolymer can form an acidic microenvironment, which promotes the autocatalysis of the copolymer, which leads to the increase of its degradation.
The influence of temperature on the degradation of polymer materials
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