Today, with the continuous development of science and technology, people have higher and higher requirements for the quality of life, and the demand for material life is increasing, which leads to more and more shortage of resources and energy. At the same time, environmental issues are becoming more and more important. In the packaging industry, plastic flexible packaging materials have developed rapidly, but they have also experienced severe challenges. In addition to the requirements for plastic flexible packaging materials that can meet the increasing requirements of packaging quality and efficiency of different products, it is also required to save resources, save energy, be easy to recycle after use or be easily degraded by the environment as the starting point of technology development. For this reason, plastic flexible packaging materials are developing in the direction of high performance, multi-function, environmental protection and broadening of application fields.
Now, high-performance, multi-functional plastic flexible packaging materials are becoming a hot development of packaging materials. And some products have been put into industrial production. Such materials include high barrier properties, selective permeability, heat resistance, and plastic flexible packaging materials such as resealable and easy-to-open properties. 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 the preservation of food quality, freshness, flavor and shelf life. There are various techniques for preserving food, such as vacuum packaging, gas replacement packaging, encapsulated deoxidizer packaging, food drying packaging, aseptic filling packaging, retort packaging , liquid hot filling packaging and so on. Many of the plastic packaging materials used in these packaging technologies are required to have a variety of properties, but the most important point is that they must have good barrier properties. The following is an introduction to the barrier materials of various common flexible packaging forms.
The more common high barrier film materials are as follows:
(1) PVDC material (polyvinylidene chloride)
Polyvinylidene chloride (PVDC) resin, often used as composite material or monomer material and co-extruded film sheet, is the most used high-barrier packaging material. Among them, the amount of PVDC coating film used is particularly large. PVDC coating films are made of polypropylene (OPP), polyethylene terephthalate (PET), etc. as substrates. Due to the high softening temperature of pure PVDC, close to its decomposition temperature, and poor compatibility with general plasticizers, it is difficult to heat and form and it is difficult to directly apply. Most of the PVDC films actually used are copolymers of vinylidene chloride (VDC) and vinyl chloride (VC), and films with particularly good barrier properties made from copolymerization with methyl acrylate (MA).
(2) Nylon packaging materials
Nylon packaging materials used to use "nylon 6". However, the air tightness of "nylon 6" is not ideal. There is a kind of nylon (MXD6) made from the polycondensation of m-dimethylamine and adipic acid whose air tightness is 10 times higher than that of "nylon 6", as well as good transparency and puncture resistance. It is mainly used for high-barrier packaging film for food flexible packaging with high barrier requirements. Its food hygiene is also approved by the FDA. Its biggest feature as a film is that the barrier property does not decrease with the increase of humidity. In Europe, due to outstanding environmental protection issues, MXD6 nylon is widely used as an alternative to PVDC films.
In addition, a new type of film with bidirectional extensibility, which is composed of MXD6 nylon and EVOH, is used as a nylon film with high barrier properties. The method of compounding includes multi-layer compounding, and there is also a method of blending and stretching MXD6 nylon and EVOH.
(3) EVOH materials
EVOH - has been the most widely used high-barrier material.
In addition to the non-stretching type, the film types of this material include biaxially oriented, aluminum vapor deposition, and adhesive-coated types. There are also heat-resistant products for aseptic packaging in the biaxially stretched type. Films made by co-extrusion of EVOH resin and polyolefin, nylon and other resins are mainly used for animal product packaging.
(4) Inorganic oxide coating film
PVDC, which is widely used as a high-barrier packaging material, has a tendency to be replaced by other packaging materials due to the problem of environmental pollution caused by the generation of HCl when its waste is incinerated. For example, the so-called plated film obtained by plating SiOX (silicon oxide) on the film of other substrates has received attention. In addition to the silicon oxide plated film, there is also an aluminum oxide vapor-deposited film. Its air tightness is the same as that of the silicon oxide coating obtained by the same method.
In recent years, multi-layer compounding, blending, copolymerization and evaporation technology have developed extremely rapidly. High barrier packaging materials such as multi-layer composites of ethylene vinyl alcohol copolymer (EVOH), polyvinylidene chloride (PVDC), polyamide (PA), polyethylene terephthalate (PET) and silicone compounds Evaporated films have been further developed, among which the following products are particularly noticeable: MXD6 polyamide packaging materials; polyethylene naphthalate (PEN); silicon oxide evaporated films, etc.
2. New type of plastic wrap
Due to the specialization of agricultural production, more and more long-distance transportation, long-distance transportation of fresh fruits and vegetables, and people's requirements for improving the quality of life, there is a demand for fresh-keeping films that can prolong the shelf life and shelf life of fresh fruits and vegetables. getting bigger. 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 further ripening of fruits and vegetables until they die. At the same time, this environment also makes it easy for bacteria 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, research and development of new multi-functional fresh-keeping films have been strengthened at home and abroad.
Fresh food packaging attaches great importance to the permeability of gas, and chooses different plastic materials accordingly. With the development of the food industry and the advancement of material science, as a food packaging material, not only high gas barrier properties are required, but the function of selective permeability is further required, that is, according to the requirements of the contents of the package, only the required (or not) required) gas molecules pass through to create a suitable atmosphere. This type of selective permeable packaging materials has entered the stage of practical use abroad, mainly including films with dissolved gaseous substances added, films with porous zeolite or silicon oxide and other powders added, gamma rays are used to change the properties of the film, and diffusion coefficients are used. Dependence on moisture content, introduction of films containing hydroxyl groups and amide groups, etc. 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 fresh-keeping technology that has attracted much attention is the method of combining microporous or improving the film formulation structure, improving the modified atmosphere (atmosphere) in the packaging bag, and using oxygen scavengers and selectively permeable films. Among them, the following products will receive greater attention:
(1) Functional plastic wrap
A Japanese company has launched a strong airtight packaging bag, which is made of a plastic called "chikron", which has excellent oxygen barrier effect. It is used to package new rice and has a particularly good fresh-keeping effect. It can keep the color, aroma and taste of rice unchanged for a long time, and the carbon dioxide generated in the bag also has the effect of preventing insects and mildew.
Japan has developed a one-time-use fresh-keeping film for fruits and vegetables, which consists of two layers of nylon semi-packaging machine's transparent film with strong water permeability, and a substance with high osmotic pressure is installed between the two layers. The fresh-keeping film is used to package fruits and vegetables, which can slowly absorb the water seeping out from the surface of the fruits and vegetables, so as to achieve the purpose of fresh-keeping.