Application and development of the hottest barrier

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Application and development of barrier plastic packaging film

over the past 20 years of reform and opening up, China's plastic packaging industry has developed steadily and at a high speed. It has developed from an industry with sensitive applications in the initial stage to an independent modern industrial system with complete product categories. It is predicted that by 2005, the annual market demand in the field of plastic packaging materials in China will reach 5.5 million tons, with an average annual demand growth rate of 10%. It is the fastest growing industry in the plastic products industry and is a sunrise industry with broad development prospects. Among them, film is the most used plastic packaging material. Due to its non-toxic, light weight, beautiful packaging and low cost, its application field is constantly expanding, and it has almost penetrated into all aspects of industrial and agricultural products and daily necessities. The investment in the plastic packaging film industry is growing rapidly. Therefore, grasping the overall trend of international and domestic plastic packaging film technology and market development is of great practical significance for making forward-looking and correct decisions

1functionalized development trend of plastic film

with the development of society and the improvement of people's living standards, the classification of products is becoming more and more detailed. The packaging of products is not limited to visual effects, but should be developed in depth in the direction of functionalization and diversification according to the characteristics of products and market demand. In recent years, technological progress has made the functionalization development trend of plastic packaging film increasingly obvious. Plastic packaging film with high requirements and high-tech content is becoming the pillar industry and research and development goal of many enterprises. Its advantage is that it is easy to operate, and the packaging functions are diverse. In addition to the requirements for antistatic, anti adhesion and smoothness of general films, it mainly endows the packaging film with some special functions through the adjustment of raw materials, additives or processes, For example, the heat shrinkability required for the straightness and tightness of cigarette and beverage packaging, the air permeability required for the packaging of vegetables and fruits, the conductivity required for the packaging of electronic components, the high optical performance required for the perspective packaging, the rust resistance required for the packaging of metal equipment and instruments, and the barrier and antibacterial properties increasingly needed in food, cosmetics and medicine, The functionalization of the film improves the added value of the product [1]. Barrier plastic packaging film is one of the fastest growing functional films at present. According to a research report on the development trend of eight high profit packaging markets in Europe in the next five years recently released by the famous British Pira International Group, the report provides first-hand information about the development trend of the high profit packaging market of 300 relevant packaging multinational enterprises in Europe, and accurately quantifies the growth of the high profit market in the next two and five years. The research shows that high barrier materials rank fourth in high value-added products. In China, barrier packaging film is in the growth period of promotion and use. Most domestic barrier films are used in the packaging of low-end products. Barrier films with excellent performance still need to be imported in large quantities, so there is a lot of room for market development

2 barrier property

the barrier property of resin refers to the shielding ability to small molecular gases such as oxygen, water vapor, liquid and odor. The barrier film is mainly used in food and pharmaceutical packaging to ensure that the packaging material will not deteriorate during storage and transportation, so as to extend its shelf life and shelf life. With the acceleration of the pace of life and the improvement of living standards, the demand for various convenience foods, such as meat products, dairy products, halogenated products, is increasing, and the quality requirements for pharmaceutical products are also increasing. In addition, barrier plastic films are also widely used in the packaging of cosmetics, tea, chemical reagents, pesticides, spices, feed and other products. Therefore, barrier films have become a hot spot of competitive development in various countries [2]. The use of barrier resins in barrier plastic films is the key [1 ~ 3], mainly including polyvinyl alcohol (PVA), ethylene vinyl alcohol copolymer (EVOH), vinylidene chloride copolymer (PVDC), polyethylene naphthalate (pen), special nylon (mxd2), nylon (PA), polyethylene terephthalate (PET), etc. among these barrier resins, PVA, EVOH, PVDC Pen is a high barrier material, the most commonly used are PVDC and EVOH, while PA is similar to pet in barrier property, belonging to medium barrier material. The research shows that PVA has the best oxygen resistance, but because it contains a large number of hydroxyl groups, it has great moisture permeability and is sensitive to water; EVOH obtained by copolymerization of vinyl alcohol and ethylene (generally 25% - 45% ethylene content) reduces the hydroxyl group in the material, so its moisture resistance is greatly improved. Although the oxygen permeability is improved, it is still a high barrier material better than PVDC. The disadvantage of PVA and EVOH is that the hydroxyl group in the material is easy to combine with water in the environment to form hydrogen bonds, and its moisture resistance is greatly affected by humidity, so it can only be used in the core layer; PA has a certain barrier property, but the high moisture absorption affects the barrier property, so it can not be used as an outer layer generally; The modified special nylon mxd2 has better heat resistance, low moisture absorption, and the barrier property decreases little with the increase of temperature and humidity. Therefore, although it was developed late, its development and application are rapid

pvdc is the first developed high barrier material with a long history of use. It has been widely used in food packaging since the 1980s. It is mainly used as casing film in China. PVDC has excellent oxygen and humidity resistance, but it has been opposed by environmentalists because of its chlorine content, and it is banned by family members in some European countries. Pen is a kind of high barrier material with excellent comprehensive performance, but its price is expensive and its popularization and application are limited. In the development of new materials, professional production enterprises in the United States, Japan and other countries have done a lot of work. Through modification technology, various new brands of resins with new functional characteristics have been introduced, such as blow molding grade PVA and PVDC, stretchable EVOH, barrier metallocene polypropylene, etc. Therefore, domestic enterprises have a wider range of choices in the use of raw materials. 3 production process of barrier plastic packaging film [4 ~ 7] due to the limitations of the material's own characteristics or price factors, generally barrier materials are not used alone. In order to meet the barrier requirements of different commodities, soft plastic packaging has developed from the original production of single-layer film to multi variety and multi-functional composite packaging film. At present, there are four kinds of composite technologies of barrier plastic packaging film that are most commonly used: dry compounding method Coating composite method, coextrusion composite method and evaporation composite method

3.1 dry compounding method

the dry compounding method uses various sheets or films as the base material, uses the gravure roller to coat a layer of adhesive on the surface of the base material, and then pastes the composite on the composite roller after drying and tacking through the drying channel. This is the most commonly used production method of composite film in China at present. The characteristic of dry compounding is that it can be applied to a wide range of areas, and appropriate adhesives are selected. Any sheet or film material can be compounded, such as PE film, PP film, PET film, PA film, etc., and the compounding strength is high and the compounding speed is fast. However, among these methods, the cost of dry compounding is the largest. In addition, because the amount of adhesive is generally 2.5 ~ 5 (g · m-2) (dry solid content), there are solvent residues and environmental pollution problems

3.2 coating and compounding method

the process of coating and compounding method is relatively simple. Coating and compounding can be used for barrier resins that are difficult to be processed into films alone, such as PVA, PVDC, etc. The melting temperature of PVA is 220 ~ 240 ℃, and the decomposition temperature is 200 ℃. To process into a film, plasticizers and stabilizers need to be added to increase the thermal decomposition temperature and reduce the melting temperature. The equipment and technology for producing PVA polymer films are very expensive. China claims to have developed the film making technology of PVA and has begun large-scale production [6]. For the same reason, PVDC is difficult to form film alone. Therefore, at present, the more mature technology for the use of PVA and PVDC is the coating process. PVA is water-soluble. In practical use, the mixture of water and ethanol is used as the solvent to coat 4 ~ 6 on PE or PP film μ For PVA with a thickness of M, due to its poor water resistance, crosslinking agent can be added to the PVA solution to improve its water resistance. At the same time, the adhesion between PVA and PE and PP can be improved, and the primer can be omitted. For the convenience of bag making, PE or PP film coated with PVA can be dry compounded with other films to form a composite film coated with pva/pe (or PP)/ldpe structure. This film has good barrier performance, The vacuum pumping effect is better than pa/ldpe, and the cost is relatively low. PVDC used for coating is a copolymer of vinylidene chloride and acrylate monomer through lotion polymerization. After adding appropriate solvents and additives, it is coated on cellophane, Bo peel strength tester PP, nylon and polyester, so that it has good moisture and gas resistance and heat sealing performance. The biggest problem of PVDC is its safety and hygiene. It is generally believed that the monomer content should be less than 1 ×。

3.3 coextrusion compounding method

coextrusion compounding method is a technology that uses multiple extruders to produce multi-layer composite films through a multi channel compounding head. This method has high requirements for equipment, especially for head design and process control. In recent years, with the maturity of machining and manufacturing technology, coextrusion composite method has developed rapidly. From the earliest two-layer to the current nine layer composite film can be produced. According to the functional needs, different materials can be selected, such as a typical seven layer composite film, whose core layer is EVOH, sandwiched between two layers of nylon film, which improves the barrier performance and reduces the thickness of the film, The outer layer uses adhesive compound PE or EVA as the heat sealing layer, which ensures the need of packaging and reduces the cost. According to relevant investigations, coextrusion packaging films in developed countries account for 40% of the whole flexible plastic packaging materials, while China only accounts for 6%. Therefore, multi-layer coextrusion technology will have a lot of application space in China. There are many reasons. First, the equipment investment is relatively small. Compared with the biaxial stretching equipment of many domestic enterprises, coextrusion equipment is simple; Secondly, coextrusion technology is becoming more and more mature; Moreover, the selection range of multi-layer co extrusion materials is wide, the thickness of each layer can be adjusted, the adaptability of composite film products is strong, and multiple composite processes are saved, so the production cost is greatly reduced. It is said that the co extrusion composite process can save 30% of the cost of producing composite packaging materials with the same structure compared with other composite processes

with the development of film surface printing inks and the improvement of surface printing technology, it is expected that the coextrusion composite technology will get greater development. In terms of technology, coextrusion composite includes coextrusion film blowing and coextrusion tape casting [7]. According to the latest introduction of W & H company, the blown film production line used to produce multi-layer (3-layer, 5-layer, 7-layer) barrier composite films has a maximum output of 1000kg/h; The thickness can be controlled within 7 ~ 25 μ m. Under the condition of meeting the functional needs, the ultra-thin film saves resources, reduces packaging waste, and meets the requirements of environmental protection; Thanks to the new technology, the bubbles are more stable and the control of film thickness deviation is more accurate; At present, China has also developed a 5-layer co extrusion barrier film blowing equipment, but its products are generally suitable for the needs of the low-end market. Another process of coextrusion composite method is coextrusion casting. The cast film is a kind of stretch free flat extruded film produced by the polymer melt through the T-shaped slotted die head and quenching on the cooling roller. The production of cast film in China began with the single-layer cast production line introduced from Japan and Germany in the 1980s, and the 3-layer or 5-layer coextrusion cast equipment introduced in the mid-1990s

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