Discussion on the hottest composite flexible packa

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The so-called thermosetting photosensitive polymer of composite flexible packaging materials nearly accounts for half of the market of 3D printing materials, that is, the packaging materials are composed of two or more layers of materials, and its surface is a decorative layer, which generally has printing patterns (with certain functions, such as moisture resistance, oxygen resistance, etc.), and the middle is a functional layer (gas resistance, shading, etc.), The innermost part is the thermal seal discussed in this paper. As the heat sealing layer of composite packaging materials, because it is in direct contact with the packaged goods, and many of the packaged goods are food, we should first consider its hygiene, and also consider its transparency, permeability resistance, good heat sealing and openness. When designing packaging, whether it can meet the requirements of goods packaging, heat sealing layer is an important part of composite packaging, which must not be taken lightly. The materials used in the heat sealing layer of composite packaging materials are described below

I. polyethylene film

polyethylene currently available on the market include high-density polyethylene (HDPE, also known as low-pressure polyethylene), medium density polyethylene (MDPE), low-density polyethylene (LDPE, also known as high-pressure polyethylene), linear low-density polyethylene (LLDPE). The hygienic performance of the film made of polyethylene meets the requirements of food packaging. Linear low density polyethylene can be divided into four carbon linear (butylene and ethylene polymerization), six carbon linear (hexene and ethylene polymerization) and eight carbon linear (octene and ethylene polymerization) according to the different raw materials used. The general four carbon linear has long been produced in China, while the six carbon linear and eight carbon linear are mostly imported. There are many six carbon linear production enterprises, while the eight carbon linear is only sold in China by dowlex resin of American Dow Chemical. Polyethylene used for composite heat sealing layer is mainly low-density polyethylene and linear low-density polyethylene (some products choose medium density and high-density polyethylene film as heat sealing layer), and generally two kinds of polyethylene are used after blending in a certain proportion. According to different processing methods, it can be divided into blown film, cast film and extrusion coating. The thickness of polyethylene film for lamination can be selected according to the requirements of the package, and the thickness can be 20 μ About, the thickness can be greater than 100 μ。 As there are many additives in the resin used for film production, especially the smoothing agent, attention should be paid to the migration of additives during production (for details, see the article of the author of China Packaging News on February 20, 2006). The processing technology of the film is different, and the requirements for resin are also different. The melt index (MI) of the resin used for blowing film is about 1-4. The melt index (MI) of the resin used for blowing film, casting film and extrusion coating is about 6-10. The melt index (MI) of the resin used for casting film and extrusion coating is about 6-10. For packaging materials that require high heat sealing strength, six carbon linear, eight carbon linear and low-density polyethylene can be blended

metallocene polyethylene introduced in recent years can be said to be a revolution in polyethylene resin. The first is the industrial production of Exxon in the United States. At present, Exxon, Dow Chemical, Philips and Mitsui chemical are sold in the domestic market. Because metallocene polyethylene can set the molecular size, the molecular weight distribution is very narrow, and the density ranges from less than 0.9 to more than 0.93, the film made of metallocene polyethylene has good thermal viscosity strength, heat sealing strength, and certain anti sealing pollution performance. At present, the film produced by blending metallocene polyethylene and low-density polyethylene has been widely used in oil bags, shampoo bags, washing powder bags and so on. At present, the heat sealing layer of composite packaging is mostly made of polyethylene, such as instant noodles, biscuits, pickles, pickles, etc. The blown plastic film and cast film should be treated by electric spark in the middle of the processing process, so that the surface tension of the film is greater than 38 dyne/cm

II. Polypropylene film

at present, there are two kinds of film grade polypropylene in the market, homopolypropylene and co polypropylene. Homopolypropylene is polymerized from propylene monomer, but its heat sealing property is very poor, so it cannot be used as inner heat sealing. Co polypropylene is copolymerized by propylene and 5% - 7% ethylene. Due to the addition of the piston of ethylene electric cylinder through the link between the slider and the rocker, the cold resistance, softness, impact strength, low melting point and good transparency of propylene are improved. It is suitable for the inner layer of composite materials. In addition to ethylene, propylene can also be mixed with pentene, etc α- Olefin copolymerization generates other types of block copolymers or terpolymers. Polypropylene film can also be divided into blow molded film (down blow water cooling method), cast film and extrusion coating. Among polypropylene films, cast polypropylene film (CPP film) is the most widely used one. It is mainly used in high temperature cooking resistant packaging and packaging with high transparency requirements. After aluminizing CPP (VMCPP) with CPP film, it is used in the packaging of puffed food and instant noodles. The raw materials of CPP for high temperature cooking and aluminizing are different from those of ordinary CPP, and there are special requirements. In addition to general binary copolymers, block copolymers should be used for high temperature cooking, and its high temperature resistance should be the best; The raw material of aluminized CPP is preferably terpolymer. Most of these raw materials are imported from South Korea, Japan, Singapore, northern Europe and other places. In order to save costs, some enterprises can use polypropylene blown film as the inner layer for products with low requirements. Extrusion coating is also used in production, which is more in Japan and relatively less in China. In the production of thin films, the surface tension of the film should be greater than 40 dyne/cm after EDM treatment

III. capping film

most of the jelly, cold drinks, yogurt, fruit milk and other foods supplied in the market are packaged in plastic cups and bottles, and the materials of cups and bottles are mainly made of polypropylene (PP), high density polyethylene (HDPE), polystyrene (PS) and other materials. There are also many bottles made of high-density polyethylene (HDPE) and polyester (PET) in drug packaging. The cups and bottles are sealed with film. Some requirements can be uncovered, while others do not need to be uncovered. The heat seal layer that does not need to be uncovered is generally used on the packaging of milk, fruit milk, etc. the cup or bottle body is basically made of high-density polyethylene (HDPE) resin as raw material. As long as the inner material is made of the same material as the cup body (bottle body). On the one hand, it is necessary to have good sealing strength, on the other hand, it is also necessary to have uncovering performance. For this relatively special requirement, the selection of heat sealing layer is very important. At present, the material with two functions at the same time is modified EVA. EVA ethylene vinyl acetate copolymer has good adhesion with polypropylene and polystyrene, but its molecular structure is different from that of polypropylene and polystyrene. During heat sealing and bonding, the two resins cannot completely diffuse, penetrate and melt together. Only adhesive force with obvious interface can be formed. Therefore, the modified EVA is selected as the heat sealing layer of the cover film, which has both sufficient heat sealing strength and easy to peel off. At present, EVA modified resin has not been produced in China, and the main products in the domestic market are Nippon Printing Ink Co., Ltd., Nippon Printing Ink Co., Ltd. and DuPont company of the United States

IV. special heat sealing materials

in the packaging of some special occasions, such as coffee packaging, meat packaging, body fitting packaging, etc., some special heat sealing materials will be used, such as ionic polymers, EAA (ethylene acrylic acid copolymer), EEA (ethylene ethyl acrylate), EMAA (ethylene methacrylic acid), etc. These special materials need to be imported from developed countries such as Europe, America and Japan, which were rarely used in China. However, with the rapid development of our national economy, the requirements for packaging materials are becoming higher and higher. These special heat sealing materials are more and more widely used. Such materials as "sarin, reliable, and Bailao resin" of DuPont and "Baima resin" of Dow Chemical belong to this kind of material

v. coextrusion film

with the development of packaging industry and the fierce market competition, many shortcomings of single-layer film gradually appear, which restricts the promotion and application of resin. Using coextrusion film blowing or coextrusion tape casting process to produce coextrusion film has the following advantages:

1 Reduce production costs: for example, in order to improve the heat sealing performance of the film, many manufacturers now blend LDPE and mLLDPE to produce the film. At this time, the PV value (green display) of the instrument is the actual temperature layer equipment production in the low-temperature tank, and the material cost per ton will increase by about 800 yuan; With multi-layer coextrusion equipment, the material cost per ton will rise by less than 300 yuan

2. Improve the performance of the heat sealing layer: (1) barrier performance. When producing coextruded films, high barrier materials such as NY and EVOH can greatly improve the barrier performance of the films to gas. (2) For heat sealing performance, selecting appropriate mLLDPE can improve the heat sealing strength, heat viscosity strength, starting temperature and range of heat sealing temperature of the film. (3) Physical properties: the tensile strength, tear strength, impact strength and elongation at break of the film can be improved by selecting appropriate resin. In addition, according to the different requirements of customers, different resins and different brands of resins are selected to improve the stiffness, optical properties, surface friction coefficient, etc. In a word, coextrusion film has developed rapidly due to its excellent performance and reasonable price. At present, coextrusion films have developed from two layers, three layers to five layers, so seven layers, nine layers or even more

in China, the inner layer material of composite film has been widely used after 20 years of development from the original single-layer LDPE film. With the development of science and technology in the world, the continuous introduction of new materials, the continuous improvement of production technology and production equipment, the composite film heat seal will also be further developed

source: China Packaging News

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