Food

Nowadays, with the continuous development of science and technology, people have higher and higher requirements for the quality of life and greater demand for material life, which has led to a greater tension in resources and energy. At the same time, environmental issues are becoming increasingly important. In the packaging industry, plastic flexible packaging materials have achieved rapid development, but they have also experienced severe challenges. In addition to meeting the increasing requirements of packaging quality and efficiency of different products, plastic flexible packaging materials are further required to take resource conservation, energy conservation, easy recycling after use or easy degradation by the environment as the starting point of technological development. Therefore, plastic flexible packaging materials are developing in the direction of high performance, multi-function, environmental protection and expanding application fields.

Now, high-performance and multifunctional plastic flexible packaging materials are becoming a hot development packaging materials. Some products have been put into industrial production. Such materials include plastic flexible packaging materials with high barrier, selective permeability, heat resistance, resealability and easy unsealing. Among them, high barrier, selective permeability and heat-resistant packaging materials have developed more rapidly.

1. High barrier plastic packaging materials

High barrier plastic packaging materials have developed with the rapid development of the food industry. High barrier packaging materials have been rapidly developed and widely used in the packaging industry, especially in the food packaging industry. It plays a role in ensuring the quality, freshness, flavor and prolonging the shelf life of food. There are a variety of technologies to preserve food, such as vacuum packaging, gas replacement packaging, sealed deoxidizer packaging, food drying packaging, sterile filling packaging, cooking packaging, liquid hot filling packaging and so on. Although many plastic packaging materials used in these packaging technologies are required to have a variety of properties, it is important to have good barrier properties. The following is a brief introduction of barrier materials in various common flexible packaging forms.

Common high barrier film materials are as follows:

(1) PVDC materials

Polyvinylidene chloride (PVDC) resin, often used as composite or monomer materials and coextrusion film sheets, is the most used high barrier packaging material. Among them, PVDC coating film is used in a large amount. PVDC coating film uses polypropylene (OPP), polyethylene terephthalate (PET) and other materials as substrates. Because pure PVDC has high softening temperature, close to its decomposition temperature, and poor compatibility with general plasticizers, it is difficult to be directly applied due to heating and molding. PVDC films used in practice are mostly copolymers of vinylidene chloride (VDC) and vinyl chloride (VC), and films with particularly good barrier properties made by copolymerization with methyl acrylate (MA).

(2) Nylon packaging materials

Nylon packaging materials used to be "nylon 6". But the air tightness of "nylon 6" is not ideal. There is a nylon (MXD6) polycondensated from m-dimethylamine and adipic acid. Its air tightness is 10 times higher than that of "nylon 6", and it also has good transparency and puncture resistance. It is mainly used in high barrier packaging film for food soft packaging with high barrier requirements. Its food hygiene has also been approved by FDA. As a film, its greatest feature is that its barrier property does not decrease with the rise of humidity. In Europe, as a substitute for PVDC films, MXD6 nylon is used in a large amount due to outstanding environmental protection problems.

In addition, a new film with bidirectional extensibility, which is compounded by MXD6 nylon and EVOH, is used as a nylon film with high barrier. The methods of compounding include multilayer compounding and blending and stretching MXD6 nylon and EVOH.

(3) EVOH materials

EVOH is the most widely used high barrier material.

In addition to the non stretching type, the film type of this material also includes biaxial stretching type, aluminum evaporation type, adhesive coating type, etc. There are also heat-resistant products for aseptic packaging in the biaxial stretching type. EVOH resin and polyolefin, nylon and other resins coextrusion film is mainly used for animal products packaging.

(4) Inorganic oxide coating film

As a high barrier packaging material, PVDC is widely used. Because its waste will produce HCl during combustion treatment, which will lead to environmental pollution, there is a trend to be replaced by other packaging materials. For example, the so-called coating film made by coating SiOx (silicon oxide) on the film of other substrates has been paid attention to. In addition to the silicon oxide coating, there is also the aluminum oxide evaporation film. Its air tightness is the same as that of the silicon oxide coating obtained by the same method.

In recent years, multilayer composite, blending, copolymerization and evaporation technology have developed very rapidly. High barrier packaging materials, such as multilayer composites of ethylene vinyl alcohol copolymer (EVOH), polyvinylidene chloride (PVDC), polyamide (PA), polyethylene terephthalate (PET) and silicon oxide evaporated films, have been further developed, especially the following products: MXD6 polyamide packaging materials; Polyethylene naphthalate (pen); Silicon oxide evaporation film, etc.

2. New fresh-keeping film

Due to the specialization of agricultural production, more and more long-distance transportation, the long-distance transportation of fresh fruits and vegetables, and people's requirements for the improvement of quality of life, there is an increasing demand for fresh-keeping films that can prolong the storage life and shelf life of fresh fruits and vegetables. A common feature of fresh fruits and vegetables is that they continue to breathe after picking, and breathing is accompanied by metabolism, water evaporation and ethylene production, which promotes the further maturation of fruits and vegetables until they decline. At the same time, this environment also makes bacteria easy to breed and multiply rapidly, resulting in rapid decay of fruits and vegetables. In order to prolong the storage and shelf life of fruits and vegetables, the research and development of new multifunctional fresh-keeping films have been strengthened at home and abroad.

Fresh food packaging attaches great importance to the permeability of gas, and uses this to choose different plastic materials. With the development of food industry and the progress of material science, as food packaging materials, it is not only required to have high gas resistance, but also further required to develop the function of selective permeability, that is, according to the requirements of the packaging contents, only the necessary (or unnecessary) gas molecules are allowed to pass through, so as to establish a suitable climate control environment. This kind of selective permeable packaging materials has entered the practical stage abroad, mainly including films with dissolved gas substances, films with porous zeolite or silicon oxide and other powders, changes in film properties by gamma ray irradiation, the dependence of diffusion coefficient on moisture content, and the introduction of films containing hydroxyl groups and amide groups. In addition, some antibacterial agents and preservatives can be added to the film to inhibit the growth and reproduction of bacteria and molds.

In recent years, the preservation technology that has attracted much attention is to improve the formula structure of the film through micropores, improve the air conditioning environment (atmosphere) in the packaging bag, and use the combination of Deaerator and selective permeation of the film. Among them, the following products will receive greater attention:

(1) Functional fresh-keeping film

A Japanese company has introduced a kind of strong sealing packaging bag, which is made of a kind of plastic called "qikelun", which has excellent oxygen isolation effect. Using it to package new rice has a particularly good fresh-keeping effect, which can keep the color, aroma and taste of rice unchanged for a long time, and the carbon dioxide produced in the bag also has the effects of insect and mold prevention.

Japan has developed a disposable fruit and vegetable fresh-keeping film, which is composed of two layers of nylon transparent film with strong water permeability, and a substance with high osmotic pressure is installed between the two layers. Using this kind of fresh-keeping film to package fruits and vegetables can slowly absorb the water seeping from the surface of fruits and vegetables, so as to achieve the purpose of fresh-keeping.

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