Degradable packaging materials, how much do you know?
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.
★Classification
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.
★Example
●Lactic acid
Polylactic acid (PLA), also known as polylactide, is chemically synthesized using lactic acid, the 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 in the production of packaging containers, agricultural mulch, fiber sportswear and bedding.
●Starch Plastic
The starch content is more than 90%, and other components added can also be completely degraded. At present, Sumitomo Corporation of Japan, Wamer-Lamber Company of the United States, Ferrizz Company of Italy, etc. have claimed that they have successfully studied the starch content of 90% to 100%. Starch plastics are completely biodegradable in (January to 1 year) without leaving any traces, no pollution, and can be used to make various containers, bottles, films, garbage bags, etc.
The production principle of all-starch plastics is to deform and disorder starch molecules to form starch resins with thermoplastic properties, so they are also called thermoplastic starch plastics. The molding process can follow the 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 most of the starch plastics currently produced in my country are filled starch plastics, that is, adding a certain proportion of starch to non-biodegradable polymer materials, which causes the physical properties of the entire material to collapse through the biodegradation of starch, which promotes A large number of 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 eliminated type.
●Photodegradable plastic
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 plastics are made by adding a small amount of photosensitizer to the polymer. At low concentrations, it is a photooxidation degradation catalyst, which reacts when irradiated by sunlight (ultraviolet light) 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 when used 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.
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