Plastic packaging multilayer composite barrier technology
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In order to improve the barrier properties 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. Due to the rapid spread of barrier technologies such as multi-layer compounding, blending, surface plating, surface coating and stretch orientation, the barrier properties of plastic packaging materials have been greatly improved. Among them, composite packaging film is the most dynamic and fastest growing packaging material in plastic packaging, representing the development direction of today's plastic packaging materials. Since each component in the multilayer composite structure is a continuous phase, it can be processed by wet compounding, dry compounding, coextrusion compounding and the like.
1, multi-layer dry composite
Dry compound is also called dry compound. That is, a layer of solvent-based adhesive (a single-component hot-melt adhesive and a two-component reactive adhesive) is coated on the plastic film, and the solvent is volatilized and dried after being dried in the compounding machine, and then heated. Bonded with other composite materials under pressure to form a smooth composite. Almost all films are currently produced by dry compounding. The composite film combines the excellent properties of each film, such as moisture resistance, barrier properties, heat resistance and heat sealability, to meet the packaging needs of various commodities.
In the dry composite production process, the adhesive is one of the key factors affecting the quality of the composite film. Most of the adhesives used in dry compounding are solvent-based, such as polyurethane, modified polyester, epoxy resin, polyvinyl acetate and natural rubber. Among them, polyurethane type adhesive has excellent comprehensive performance, good elasticity, low temperature resistance, boiling resistance, flexibility, strength and aging resistance. It does not peel off in long-term application in a wide range of temperature and humidity, and has good Heat resistance and chemical resistance, excellent resistance to cooking, heat sterilization, thermoforming and heat sealing, excellent adhesion to various materials, and the main glue used in composite film production. And generally use a two-component solvent-based adhesive.
The choice of adhesive is determined by the nature of the package (eg, frozen or cooked, food or non-food). Before compounding, the adhesive solution is first uniformly applied to the surface of the film, then sent to a hot drying tunnel to remove the solvent, and then composited with another film by a composite roll at a certain temperature and pressure. At present, dispersion adhesives are also popular, such as a suspension of a modified polyolefin in a solvent to a polar material such as polyolefin and aluminum foil. The solvent type and the dispersion type adhesive have in common that they are liquid when coated in a coater, and a large amount of energy must be consumed after coating to remove moisture or a solvent, which is likely to cause air pollution and is costly. Solvent-free adhesives are the future development trend.
Multi-layer dry composite was first used in the production of barrier packaging. It is made of high barrier properties and low-cost other packaging materials. The process can use traditional barrier materials - aluminum foil. In addition to aluminum foil, barrier materials can also be coated. Film or other medium-high barrier film (such as PDVC film, PA film, EVOH film). The structure is often 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 be used, and adhesives are adhered to each other.
The barrier properties of dry composite membranes are related to the use of membranes and adhesives. The surface of the composite base film must be clean, dry, flat, dust-free, and oil-free. The non-polar, dense and smooth polyolefin material must be corona treated in advance. As the composite substrate, the aluminum foil requires less pinholes and the pinhole aperture is as small as possible, and it is often necessary to remove the residual grease in the aluminum foil processing before compounding.
2, multi-layer coextrusion compound
Multi-layer coextrusion compound starts late compared with dry composite, but it has the advantages of saving raw materials, diversifying raw materials, adapting to environmental protection requirements, and eliminating the need for toxic adhesives, and the barrier effect is very ideal, and with the number of layers The better the effect of the increase, the better the polymer bonding between the layers. At present, the number of composite layers has been developed to more than ten layers, and has applications in packaging films and hollow containers.
Multi-layer coextrusion of PVDC and PS, HIPS, PP and other resins, can be used for vacuum packaging of dairy products, jams, etc.; composite sheets of PVDC and PE, PVC are suitable for disintegration, deliquescent, and volatile drugs. package. The PET/EVOH/PET packaging materials obtained by the British Bass company and the Miller Company of the United States using the multi-layer coextrusion method are used for the packaging of beer, and the barrier property is good, and the shelf life of the beer can reach 6-9 months.
The co-extrusion compounding process uses two or more extruders to separately melt and extrude various resins of different functions, and through the respective flow channels in the die or the die for foreign exchange, and then inflated and cooled to recombine. together. The process not only greatly simplifies the production process, but also reduces the use of materials, and at the same time reduces the consumption of raw materials and production costs; the use of this process to produce composite packaging materials of the same structure can save 30% of production costs compared to other processes.
Coextrusion compounding combines two or more materials in a molten state in a single die. Since the compatibility between the barrier material and the heat sealing material is generally poor, it is necessary to first consider the solvent. For example, when co-extruding a composite film of nylon-6 and low-density polyethylene, a nylon/Bynel/LDPE three-layer composite film can be extruded by using a Bynel resin and using a single extruder to share a composite die.
At present, the typical composite membrane structure is: main force layer / barrier layer / heat seal layer / peelable layer. The main stress layers are PS, HIPS, PP, PET, etc., the barrier layer is EVOH, PA, PVDC, etc., the heat seal layer is HDPE, LDPE, LLDPE, CPP, etc., and the peelable layer is PP. If the resin is poorly compatible between adjacent layers, a bonding layer is required. According to different purposes, different materials are used to make composite membranes with different structures. The following are commonly used: PET/PVDC/PE, OPP/PVDC/EVA, PVDC/PET/PE, EVOH/PVDC/EVA, PVC/PVDC /EVA, NY/EVA/PVDC/EVA, etc.
The multi-layer co-extrusion composite film mostly adopts ABCBA five-layer symmetrical structure, with PA or EVOH as the barrier layer, and PE as the heat seal layer. From the perspective of its functional combination, there are mainly three functions of blocking, heat sealing and bonding. The combination of different polymers meets the requirements of anti-oxidation and moisture resistance of packaging materials, usually composed of four kinds of polymers, but there are seven, nine, eleven or even more layers of co-extruded composite film on the market. . Among them, the co-extruded composite membrane with PVDC as the intermediate barrier layer has the fastest development and the largest yield. 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 resins require better processing properties to accommodate the need for good fluidity in co-extruded composite heads. Poor fluidity or too much fluidity between several resins will reduce the barrier properties of the composite membrane due to laminar flow formation. The main structure is PE / Adhesive / Nylon / EVOH / Nylon / PE, the structure of the food flavor leakage rate reduced from 5% to 1%. Other structures include HDPE (PP) / Adhesive / EVOH / Adhesive / HDPE (PP), PP / Adhesive / PVDC / Adhesive / HDPE.
In the co-extruded composite film, the thickness of each layer is controlled by adjusting the rotation speed of the extruder. Since the speed control of the extruder is not very precise, the thickness of the co-extruded composite film is often not well controlled, and high worker quality is required. More sophisticated machinery and equipment. In addition, the design of the machine head is also very important. Generally speaking, the barrier film and composite strength of the packaging film formed by the head of ABCBA five-layer and ABCDCBA seven-layer structure are the best. The main reason is that the material flow of the symmetrical structure is easier to form. flow. The disadvantage of coextrusion is that the different speeds of the extruder will cause confusion between the material and the material, and only surface printing.
3, multi-layer coextrusion casting
The high-barrier co-extruded cast film is a plastic packaging material successfully developed in the late 1980s. In recent years, with the advent of multi-layer co-extruded cast film, its barrier properties, aroma retention, moisture resistance, oil resistance, retortability and heat sealing properties have been further improved, and can be widely used in meat frozen products, cooking Packaging of meat, convenience foods, aquatic products, fruits and dairy products, edible oils, alcohol, soy sauce, etc., greatly extending the shelf life of the products.
The high-barrier multi-layer co-extruded cast film is made of high-barrier material. It can be divided into three categories according to other composite materials and bonding resin. The raw materials can be divided into three categories, namely high barrier materials and composite materials. And bonding materials. The performance of high barrier materials directly affects the high barrier properties of coextruded cast films. At present, the commonly used high barrier materials include PA, EVOH and PVDC. Since these materials are all polar materials, the moisture absorption is strong, and the moisture in the material has a great influence on the production, and the moisture itself can be degraded during heating. The effect, while the water content is too high, will generate bubbles during melt extrusion, causing the high barrier material to form a fault, which seriously affects the quality of the product. Therefore, the moisture content of the high barrier material is very high, generally not exceeding 0.06%. Therefore, in order to prevent moisture absorption of raw materials, it is required to use moisture-proof paper-aluminum composite packaging, and to ensure the integrity of the packaging during transportation; manufacturers with conditions can install dryers and dry the purchased raw materials before use.
According to the use of composite materials, cooking grade CPP pellets, composite grade CPP pellets, LDPE, LLDPE, metallocene LLDPE can be used. The MI value is required to be in the range of 2-8, the melt extrusion performance is good, and the heat sealing performance is good.
The size of the bond strength directly affects the quality of the coextruded film. Therefore, according to different high barrier materials and composite materials, a bonding resin with strong bonding strength is selected, and the MI value is between 2 and 6.
Under the premise of raw material quality assurance, the production process is the key to control product quality, including temperature, production line speed, thickness control, stratified shunt control, surface treatment and so on.
The high-barrier multilayer co-extruded cast film can be divided into two major categories, namely, the symmetric structure A/B/C/B/A and the asymmetric structure C/B/A. A is a composite material, B is a bonding material, and C is a high barrier material. For example, 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. Symmetrical structures are PA/Tie/PE, PA/Tie/PP, PA/EVOH/Tie/PE, etc.
Since the high-barrier multi-layer co-extruded cast film is co-extruded from several materials, it combines the advantages of a variety of materials, and thus has excellent performance and can be widely applied to various food packaging. However, since this film is currently not recyclable and relatively increases the production cost, accelerating technological innovation and optimizing the process has become an inevitable choice for its large-scale production application.