4. Choose absorbent packaging material

Fresh food packaging attaches great importance to the permeability of gases and uses different plastic materials. With the development of the food industry and advances in the science of materials, as food packaging materials not only require high gas barrier properties, but also require the development of selective permeability functions, that is, allowing only the required (or not required) according to the contents of the package contents. The gas molecules pass through to establish a suitable atmosphere for atmosphere adjustment. This type of selective packaging material has entered the practical stage in foreign countries, mainly including the addition of dissolved gas substances in the film, adding porous zeolite or silicon oxide powder film, using γ-ray irradiation to change the nature of the film and the use of the diffusion coefficient of Dependence of moisture content, introduction of a film containing a hydroxyl group and an amide group, and the like. In addition, antibacterial agents and preservatives can be added to the film to inhibit the growth of bacteria and molds. For example, a U.S. company has invented a new method of reducing oxygen content and increasing nitrogen content in packaging boxes to keep fruits and vegetables fresh. This method uses a new type of packing box with air conditioning performance in transportation. This kind of packaging has a special film that can absorb oxygen molecules and allow nitrogen to pass through. In this way, after the air enters the packing box through the film, the content of nitrogen in the box can be as high as 98% or more, so that the respiration of fruits and vegetables can be slowed down and the purpose of freshness preservation can be achieved for a longer period of time.

Another kind is named silicon window modified atmosphere fresh-keeping bag. This kind of modified atmosphere fresh-keeping bag is a silicone rubber film with a suitable area on a plastic film bag. Silicone rubber membranes selectively permeate oxygen, carbon dioxide and nitrogen. This silicone window modified air-conditioned storage bag has achieved good results in fruit storage. Use it under a certain storage conditions, determine the appropriate area of ​​the silicon window, after a certain period of closure, you can automatically adjust the concentration of oxygen and carbon dioxide in the package at an automatically required level. Generally storing 1,000 kilograms of apples and setting the silicon window area of ​​0.6 square meters can maintain oxygen and carbon dioxide at appropriate levels. Excess carbon dioxide produced by respiration can be discharged through the silicon window. When the oxygen in the bag is insufficient, it can be penetrated by silicon windows. . The key to silicon window air-conditioning technology is the selection of a silicon window film to determine the area of ​​the silicon window and the appropriate storage and storage temperatures.

5. Decomposable packaging materials

Nowadays the world's annual plastic production has reached 1*10T, and plastic products are about 5*108T. Antiu calculates that its use exceeded metal materials in the 1980s. Plastic has many raw materials, has been produced, excellent performance, low cost, light weight, easy molding, non-corrosive, easy to color and other excellent features, is widely used in various fields of packaging, so that metal, wood, natural fibers, natural rubber Excellent alternatives.

As the output and quantity of plastics continue to increase, the problems that accompany them are also increasing, mainly due to the continuous increase of waste plastics. Generally speaking, there are mainly three aspects of waste plastics: one is polyethylene, which is mainly used for handbags, agricultural films, etc.; the other is polypropylene; it is generally used as packing tapes for packaging, etc.; there is also polyphenylene. Ethylene, used as a foam cushioning material for packaging, fast food lunch boxes, packing fillers, etc. These three types of components account for 70%-80% of waste plastics. Nearly half of the new plastic products will become waste plastics after two years. Waste plastics are stable in natural environments, not easily corroded, degraded, and have a very serious impact on the environment, constituting a typical white pollution. Therefore, in recent years, In the plastics industry, the emergence of functional plastics that can degrade is also taken for granted.

According to the degradation mechanism, degradation of plastics can be divided into: chemical degradation, thermal degradation, oxidative degradation, radiation degradation, photodegradation, mechanical degradation, and biodegradation.

Biodegradable plastics can only be completely degraded into low-molecular-weight plastic materials by natural microorganisms, such as bacteria, molds, etc. It has the following characteristics: it can be made into compost and return to nature; it is due to degradation The volume is reduced, the service life of the landfill site is prolonged; there is no problem of incineration of ordinary plastics; it can reduce the harm caused by wild animals and plants caused by random disposal; it is convenient for storage and transportation; it has a wide range of applications.

According to the degradation mechanism and form of destruction, biodegradable plastics suitable for the packaging industry can be divided into: completely biodegradable plastics and biologically destructive (or collapsed) plastics. Fully biodegradable plastics are completely decomposed into small molecule compounds such as carbon dioxide and water in a certain period of time under the action of microorganisms, and completely destructible plastics are only decomposed into scattered fragments. According to the manufacturing method, biodegradable plastics can be divided into: natural polymers, microbial synthesis and artificial synthesis.

Natural polymer materials can be used as materials to explain cellulose, starch, chitin, lignin and so on. From the natural world, visitors biodegradation and nature, but its province does not have plasticity, so through natural chemical modification or blending with polymers, copolymerization modified or added to the polymer to form a biodegradable polymer material.

The microbial synthesis class mainly uses bioengineering to synthesize biodegradable polymer materials, and has obtained cheaper degradable plastic products. Its main component is microbial polyester. For example, genetic engineering is used to dry poplar leaves, pulverize them into fine powder, and then extract chloroplasts to obtain the mother particles of polyhydroxybutyric acid (PHB) from the chloroplasts of poplars, thereby obtaining a PHB degradable plastic.
Synthetic degradable chemical synthetic polymer materials include biodegradable polymer materials and photodegradable polymer materials. Biodegradable polymer materials include polyvinyl alcohol (PVA), polyethylene glycol and other water-soluble polymers and caprolactone (PCL). Currently commercial development includes: polyester polyamide copolymer (CPAE), Aliphatic and aromatic copolyesters (CPE), polyamino acids, polyorthoesters, polyester polyurethanes, etc.

(to be continued)

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