Multi layer composite barrier technology of the ho

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Plastic packaging multi-layer composite barrier technology

in order to improve the barrier performance of plastics, in recent years, the industry has been researching and developing new high barrier plastics on the one hand, and modifying existing plastic packaging materials on the other hand. The barrier properties of plastic packaging materials have been greatly improved due to the rapid promotion of barrier technologies such as multi-layer composite, blending, surface plating, surface coating and tensile orientation. Among them, composite packaging film is the most dynamic and fastest-growing type of packaging materials in plastic packaging, representing the development direction of plastic packaging materials today. Because each component in the multilayer composite structure is continuous phase, it can be produced and processed by wet composite, dry composite, CO extrusion composite and other methods

1. Multilayer dry compounding

dry compounding is also called dry compounding. That is, a layer of solvent-based adhesive (one component hot-melt adhesive and two-component reactive adhesive) is coated on the plastic film, and the solvent is volatilized and dried through the drying channel of the compounding machine, and then it is bonded with other composite materials in the hot pressing state to form a smooth composite material. At present, almost all films can be produced and processed by dry lamination. The composite film integrates the excellent properties of each layer of film, such as moisture resistance, barrier, heat resistance and heat sealing, so as to meet the packaging needs of various commodities

in the dry composite production process, adhesive is one of the key factors affecting the quality of composite film. The adhesives used in dry compounding are mostly solvent based, such as polyurethane, modified polyester, epoxy resin, polyvinyl acetate and natural rubber. Among them, polyurethane adhesive has excellent comprehensive properties, good elasticity, low temperature resistance, boiling resistance, outstanding flexibility, strength, aging resistance, no peeling in a wide range of temperature and humidity for long-term application, good heat resistance and chemical medium resistance, excellent resistance to steaming, heating sterilization, thermoforming and heat sealing, and excellent adhesion to various materials, It is the main adhesive used in the production of composite films, and generally uses two-component solvent adhesives

the choice of adhesive depends on the nature of the package (such as freezing or cooking, food or non food conditions). Before compounding, the adhesive solution is first evenly coated on the surface of the film, then sent to the hot oven to remove the solvent, and then compounded with another layer of film at a certain temperature and pressure through the compounding roller. At present, dispersed adhesives are also popular, such as compounding polar materials such as polyolefin and aluminum foil with the suspension of modified polyolefin in solvent. The common point of solvent-based and dispersed adhesives is that if a manipulator is used in one unit of the coater to automatically process the sample, the coating is liquid, and a lot of energy must be consumed to remove moisture or solvent after coating, which is easy to cause air pollution and high cost. Therefore, solvent-free adhesives are the development trend in the future

multilayer dry compounding was first used in the production of barrier packaging. It uses materials with high barrier properties to compound with other low-cost packaging materials. This process can use traditional barrier materials - aluminum foil. In addition to aluminum foil, barrier materials can also choose coating, coating or other medium and high barrier films (such as pdvc film, PA film, EVOH film). Its structure is often outer BOPP, BOPET, the middle barrier layer can be PA, pdvc, EVOH or aluminum foil, and the inner heat seal layer is CPP ". If high temperature resistance is not required, PE can also be used to bond with each other with adhesive

the barrier performance of dry composite film is related to the film and adhesive. The surface of the composite base film must be clean, dry, flat, free of dust and oil. Corona treatment must be carried out in advance for non-polar polyolefin materials with dense and smooth surface. As a composite substrate, aluminum foil requires less pinholes, and the pinhole diameter should be as small as possible. Before compounding, it is often necessary to remove the residual grease in aluminum foil processing

2. Multilayer coextrusion

compared with dry composite, multilayer coextrusion composite started late, but it has the advantages of saving raw materials, diversifying raw materials, adapting to environmental protection requirements, and not using toxic adhesives. Moreover, the barrier effect is very ideal. With the increase of the number of layers, the effect is better, and the polymer between layers is well combined. At present, the number of composite layers has developed to more than ten layers, which are applied in packaging membranes and hollow containers

the multilayer coextrusion of PVDC with PS, hips, PP and other resins can be used for vacuum packaging of dairy products, jam, etc; The composite sheets of PVDC, PE and PVC are suitable for the packaging of fast disintegrating, hygroscopic and volatile drugs. The pet/evoh/pet packaging material prepared by bass company in Britain and Miller Company in the United States by multi-layer coextrusion is used for beer packaging. Its barrier performance is good, and the shelf life of acrylic stadium beer can reach 6-9 months

coextrusion compounding process is to use two or several extruders to melt and extrude various resins with different functions respectively, and then combine them in the die head or in the die head through their respective flow channels, and then compound them through blowing and cooling. The process not only greatly simplifies the production process, but also uses less materials, and can reduce raw material consumption and production costs at the same time; Using this process to produce composite packaging materials with the same structure can save 30% of the production cost compared with other processes

coextrusion refers to the composite welding of more than two materials in one die in the molten state. Since the compatibility between barrier materials and heat sealing materials is generally poor, it must first consider the selection of a good solvent. For example, when CO extruding the composite film of nylon-6 and LDPE, the nylon/bynel/ldpe three-layer structure composite film can be extruded with bynel resin and three extruders sharing a composite die

at present, the typical composite membrane structure is: main stress layer/barrier layer/thermal seal layer/strippable layer. The main stress layers are PS, hips, PP, pet, etc., the barrier layers are EVOH, PA, PVDC, etc., the heat sealing layers are HDPE, LDPE, LLDPE, CPP, etc., and the peelable layers are PP, etc. If the resin compatibility between adjacent layers is poor, an adhesive layer shall be added. According to different uses, different materials are used to make composite films with different structures. The commonly used ones are pet/pvdc/pe, opp/pvdc/eva, pvdc/pet/pe, evoh/pvdc/eva, pvc/pvdc/eva, ny/eva/pvdc/eva, etc

multilayer coextrusion composite films mostly adopt abcba five layer symmetrical structure, with PA or EVOH as the barrier layer and PE as the heat sealing layer. From the perspective of its function combination, it mainly has three functions: barrier, heat sealing and bonding. Through the combination of different polymers to meet the requirements of oxygen and moisture resistance of packaging materials, it is usually composed of four kinds of polymers, but seven, nine, eleven or even more layers of coextruded composite films have appeared on the market. Among them, the coextrusion composite membrane with PVDC as the intermediate barrier layer has the fastest development and the largest output. This is because the elution technology of DCI has been successfully developed and applied to the new spray drying elution tower, which can effectively control the product pollution

barrier resin requires good processability to meet the needs of good fluidity of coextrusion composite die. If the fluidity is too poor or the fluidity difference between several resins is too large, the barrier performance of the composite film will be reduced due to the formation of laminar flow. The main structure is pe/adhesive/nylon/evoh/nylon/pe, which reduces the leakage rate of food flavor from 5% to 1%. Other structures include HDPE (PP)/adhesive/evoh/adhesive/hdpe (PP), pp/adhesive/pvdc/adhesive/hdpe, etc

in the coextrusion composite film, the thickness of each layer is controlled by adjusting the speed of the extruder. Because the speed control of the extruder is not very accurate, the thickness of the coextrusion composite film is often difficult to control, requiring higher worker quality and more precise machinery and equipment. In addition, the design of the machine head is also very critical. Generally speaking, the barrier and composite strength of the packaging film formed by the machine head designed according to the abcba five layer and abcdcba seven layer structures are the best, mainly because the material flow of the symmetrical structure is easier to form laminar flow. The disadvantage of coextrusion is that different extruder speeds will cause confusion between materials, and it can only be printed on the surface

3. Multi layer coextrusion casting

high barrier coextrusion casting film is a plastic packaging material successfully developed in the late 1980s. In recent years, with the advent of multi-layer coextrusion casting film, its barrier property, fragrance retention, moisture resistance, oil resistance, digestibility and heat sealing performance have been further improved. It can be widely used in the packaging of frozen meat products, cooked meat food, convenience food, aquatic products, fruits and dairy products, edible oil, alcohol, soy sauce and other products, greatly extending the shelf life of goods

high barrier multi-layer coextrusion cast film is made of high barrier material as the main material, combined with other composite materials and bonding resin through one-time extrusion. Its raw materials can be divided into three categories, namely, high barrier material, composite material and bonding material. The performance of high barrier material directly affects the high barrier property of coextrusion casting film. At present, the commonly used high barrier materials include PA, EVOH and PVDC. Because these materials are polar materials with strong hygroscopic power, and the moisture in the materials has a great impact on production, the moisture itself can degrade during the heating process, and too high moisture content will produce bubbles during melting and extrusion, which will make the high barrier materials form faults and seriously affect the quality of products, so the requirements for the moisture content of high barrier materials are very high, Generally, it cannot exceed 0.06%. Therefore, in order to prevent moisture absorption of raw materials, it is required to adopt moisture-proof paper aluminum composite packaging, and ensure the integrity of the packaging during transportation; Conditional manufacturers can install dryers to dry the purchased raw materials before use

according to the application, the composite materials can be cooking grade CPP pellets, composite grade CPP pellets, LDPE, LLDPE, metallocene LLDPE. The MI value is required to be in the range of 2~8, with good melt extrusion performance and good heat sealing performance

the bonding strength directly affects the quality of coextrusion film. Therefore, according to different high barrier materials and composites, the greenhouse gas emission with strong adhesion is becoming more and more serious, and the MI value of bonding resin is between 2 and 6

on the premise of raw material quality assurance, production process is the key to control product quality, mainly including temperature, production line speed, thickness control, layered and shunt control, surface treatment, etc

the structure of high barrier multilayer coextrusion cast films can be divided into two categories, namely, symmetrical structure a/b/c/b/a and asymmetric structure c/b/a. A is composite material, B is bonding material, and C is high barrier material. For example, the commonly used symmetrical structures are pe/tie/pa/tie/pe, pp/tie/pa/tie/pp, pe/tie/evoh/tie/pe, pe/tie/pvdc/tie/pe, etc; Commonly used asymmetric structures include pa/tie/pe, pa/tie/pp, pa/evoh/tie/pe, etc

because the high barrier multi-layer coextrusion cast film is formed by coextrusion of several materials, it combines the advantages of many materials, so it has good performance and can be widely used in all kinds of food packaging. However, this kind of film can not be recycled at present, which increases the production cost relatively. Therefore, accelerating scientific and technological innovation and optimizing the process flow have become the inevitable choice for its large-scale production and application

author/Liu Guoxin

source: Jiagong electromechanical

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