China will soon join the WTO, and commodity circulation will further develop toward globalization. If China's export commodities can win more and more of my share in the globalized market, there are many factors affecting it. How can we increase the shelf life of exports? How to develop high-tech products with independent intellectual property rights? Are two very important topics. Specific to the packaging industry, the development and innovation of barrier packaging film materials has also attracted more extensive attention and attention.
Since the advent of high-polymers, it has been widely used in various types of packaging, especially in food packaging. Film-type plastics in particular have been the main source of flexible packaging materials. With the development of commodity economy and the improvement of people's living standards, the output of such plastic films has been continuously increasing, and its process technology has also been continuously improved, thus forming a type of packaging design, that is, barrier packaging design.
There are many packaging methods in practical use, including vacuum packaging, air-packing, moisture-proof packaging, deoxygenation packaging, self-adjusting atmosphere packaging, etc., all of which are inseparable from barrier-type film packaging materials and barrier packaging design. A complete barrier packaging design includes three major factors: the performance of the packaged product; the relevant parameters of the environmental conditions surrounding the package; and the characteristics of the barrier packaging material. In terms of food packaging, how the barrier properties of the package depends mainly on the permeability of the packaging material. If a soft packaging film material has low permeability to a gas, it indicates that the material has good barrier properties against such gases; otherwise, it has poor barrier properties.
According to the above three factors, a complete barrier packaging design can be formed, a reasonable mathematical model can be established, and computer scheduling can be used to solve the problem and optimize the design. However, for in-depth analysis, the characteristics of the packaged food are determined by the type of the food itself; and the surrounding environmental parameters of the package are determined by the product's circulation environment. In this way, the entire barrier packaging design process concentrates on the barrier properties of the barrier packaging material, and it is hoped that the optimization of the barrier packaging design will be achieved through the improvement of the packaging material barrier material. So far, although the barrier packaging film includes a variety of composite films, it still cannot satisfy people's requirements for various barrier properties of food packaging. Therefore, the international packaging industry has been working hard to develop functionally diversified packaging films, including film materials with high barrier properties, partial barrier properties, and high permeability. This article reviews the research and development process of barrier packaging films, discusses the rational ideas and methods for the continued development of such packaging materials, and proposes innovative principles and technology for alternative barrier packaging film materials.
1. High polymer single film and composite film, coating film
In general, a high barrier material means a gas transmission rate of 10 ml/100 in2days·atm below 22.8°C and a material density of 1 mil (25.4 μm).
For high barrier film materials, PVDC and EVOH are the most widely used.
PVDC-polyvinyl chloride films can be formed by copolymerizing vinylidene chloride monomers and acrylic monomers. As the content of these two monomers increases, the gas barrier properties of the film become better. However, the unfavorable factor is the deterioration of the thermal stability of the material. This problem was seen as a difficult step to further develop barrier packaging materials. In order to meet the needs of food packaging, consider the use of composite film method, that is, use PVDC airtight good as the middle layer of composite film, polyester, polypropylene, nylon or paper as its outer layer, polyethylene as heat The layer is melted to form a three-layer or multi-layer composite film. However, the cost of its manufacturing process is obviously increasing.
EVOH-polyethylene polyvinyl alcohol film also has good gas barrier properties and moisture resistance. This is mainly due to the improved performance of EVOH copolymerized with polyethylene and polyvinyl alcohol. Polyethylene has good moisture resistance, while polyvinyl alcohol has good gas barrier properties. However, EVOH is a hydrophilic film that adsorbs moisture, which reduces the gas barrier properties of EVOH. This shows that the good barrier property of EVOH is also unstable, and is greatly affected by the humidity of the environment. Therefore, it is best to use EVOH in the middle layer of multi-layer composite film, and then use other polyolefin materials with high moisture resistance to prevent it from absorbing moisture, so as to fully exert the strong gas-barrier property of EVOH.
Polyacrylonitrile (PAN) is also a widely used transparent high barrier film. In food packaging, films such as PET and PA with good barrier properties, and LDPE, PVC, and PE films with selective air permeability are also frequently used. In addition, PVDC-coated films of various types such as PP, PA, PE, and PT are also frequently used. Since each type of plastic film has its own merits and shortcomings, they should be used in combination in order to exert their respective strengths. This is the reason why there are a large variety of laminating composite films on the market. Now the number of composite films has exceeded 1,000, and the number of composite layers has grown from two to three layers to ten layers. Despite this, in some cases it is still difficult to meet the packaging requirements, and the process is complicated and increases costs. In addition, this "superimposed" composite method does not fully utilize resources and therefore has limitations.
2. "Polymer alloy"
The blended polymer is the application of the existing polymer species, through a suitable manufacturing process, made of polymer - polymer mixture, so that it has a comprehensive performance in many aspects. This blended polymer and alloy in the metallurgical industry is very similar, so the new varieties used to form blended polymers are known as "polymer alloys" or "polymer alloys." The blending method is a shortcut to the development of new polymer materials. At present, this method is not only applied to EVOH, PVDC and other materials, but also used in other barrier resins to make up for and eliminate defects in existing resins, improve performance, and expand the scope of use.
2.1EVOH/PA polymer alloy
When a small amount of polyamide (6-12PA) is added to EVOH, the molecular orientation is improved without impairing the gas barrier property of the latter, so that the thermal stability is improved and the thermal processability is also improved. The characteristics of this high molecular alloy material are very advantageous in the blow molding extension process and the hot forming process. Polyamides of 10-12 PA are added to EVOH, which can be thermoformed at temperatures as low as 150 °C. Otherwise, it is impossible to process at such a low temperature.
2.2EVOH/PET polymer alloy
When 20% EVOH is mixed in PET (polyester), its barrier properties are significantly improved without affecting the mechanical strength of PET. The higher the EVOH grade, the more the barrier properties of such polymer alloys are improved. The higher the level of EVOH, the more ethylene it contains. The reason for improving barrier properties is to increase the aspect ratio of the intermolecular channels in the polymer blend.
2.3 PVDC/PA polymer alloy high polymer, but its brittle temperature is quite high, generally higher than 0 °C. Therefore, the mechanical properties at low temperatures are rather poor, thus limiting the range of applications. Packaging containers made with PVDC tend to be brittle at low temperatures and often break if dropped. One of the measures taken to solve this problem is to blend PVDC with another polymer that has a very high brittle temperature. For example, PVDC and 6-12PA form a polymer alloy that significantly reduces the brittle temperature without affecting the gas barrier properties.
According to the above examples, it can be known that by mixing other polymers in one kind of polymer or adding additives to the polymer, the material properties can be improved and the application range can be expanded. This indicates one direction for the development of new barrier materials using the "physical" approach. This process technology for the development of new materials is obviously not limited to binary systems. If the aforementioned polymer single membrane, coating membrane and multi-layer composite membrane are called first-generation barrier materials, then this kind of “polymer alloy†material obtained by means of the blend polymerization method can be called the second generation. Barrier type material.
3. Sprayed and filled films
Sprayed aluminum film is a food high barrier film that has been widely used for a long time, but its biggest drawback is that it loses the transparency of the plastic film. A layer of high-purity silicon oxide with a thickness of 800-1500 Angstroms (Angstroms) is sprayed on the plastic film. This film not only has high barrier properties, but also has high transparency and high temperature resistance, and is suitable for microwave ovens. The processing method is electron beam vacuum screen spraying technology, and the spraying speed is about 1/3 of aluminum spraying. Due to the sublimation characteristics of silicon oxide, it is difficult to achieve a constant evaporation rate. Japan recently introduced a new process that uses a special thermal resistance spray method that solves the problem of uneven evaporation of silicon oxide in electron beam vacuum spray methods. The test results show that the gas barrier properties are almost equal to aluminum. However, this new spray barrier material is expensive and therefore difficult to promote in the market.
The addition of platelet-like inorganic fillers such as mica to the polymer can also improve gas barrier properties. The mica particles are mixed into EVOH, and the alignment direction is just perpendicular to the oxygen infiltration direction, so that the EVOH forms a secondary gas barrier layer, and the gas barrier properties of the mica-filled EVOH can be increased by three to five times. However, the film thickness should not be less than 0.5 mils, otherwise it will reduce the mechanical properties. In addition, plastic films filled with ceramic powders in LDPE have also been developed. The barrier properties of this film to ethylene gas are particularly low, which can reduce the content of ethylene gas in fresh fruit and vegetable packages, thereby delaying the ripening of fresh fruits and vegetables and extending them. The purpose of the preservation period.
This method of spraying or filling inorganic materials to change the barrier properties of polymer materials is very effective, but its manufacturing process remains to be further matured. Such films can be referred to as third generation barrier materials. This is another new idea for developing barrier packaging materials.
4. Chemical barrier material
The above-mentioned methods for improving the barrier properties of polymers all belong to the so-called "physical barrier method." It is very difficult to increase the barrier property to the level equivalent to glass and metal without affecting the processability and practicality. One of the ways to overcome this obstacle is to use the so-called "chemical barrier method" that corresponds to the "physical barrier method." Since the 1980s, there have been several such products available, which can be divided into the following two categories.
4.1 Through the oxidation reaction
In the late 1980s, the United Kingdom developed a chemically synthesized barrier material called OXBR. It consists of PET (main ingredient), MXD-6PA (combined oxidant) and organic cobalt salt (as catalyst). According to reports, this composite material has been processed into various types of containers by extrusion or blow molding, and its vacuum container can maintain its internal oxygen zero barrier level within two years. Japan has also been successfully developed. At present, this material has been tried on packaging containers such as fruit drinks.
The United States recently developed an oxygen-absorbing plastic called "longevity." The OH groups in the base polymer used may react chemically with the silicon element, may also react with the metal element, and may also be combined with the metal element. This material is insoluble in water, has a structure similar to hemoglobin, and therefore has a strong oxygen absorption. The United States has used such materials as liners for certain beverage caps, which can significantly extend the shelf life of certain oxygen-sensitive beverages.
4.2 Applied Biotechnology
This is an entirely new approach to the development of new barrier materials, which is the use of enzymes to remove oxygen from liquid foods or beverages and other substances, thereby increasing the shelf life of foods. According to this principle, a pharmaceutical company in the United States developed the library's "active barrier packaging technology." The specific method is to mix two types of enzymes in the packaging container: glucose oxidase and catalytic enzyme. When the oxygen in the air inside the container enters the liquid product to be packaged, these oxygen gases infiltrate into the container under the action of the catalytic enzyme to avoid the inside, with gluconate and water, thereby achieving the purpose of deoxidizing the liquid food (drug) product.
The high barrier polymer made by this chemical barrier method was successfully developed on the basis of the physical barrier method, and its barrier level reached a higher level, which is a fourth-generation barrier material.
5. Metallocene polymer
Recently, there is a new class of materials in the field of polymer development. That is, the use of metallocenes as catalysts in polyolefin-based materials increases the barrier properties of the polymers. Metallocene ethylene (MPE)
Since the advent of high-polymers, it has been widely used in various types of packaging, especially in food packaging. Film-type plastics in particular have been the main source of flexible packaging materials. With the development of commodity economy and the improvement of people's living standards, the output of such plastic films has been continuously increasing, and its process technology has also been continuously improved, thus forming a type of packaging design, that is, barrier packaging design.
There are many packaging methods in practical use, including vacuum packaging, air-packing, moisture-proof packaging, deoxygenation packaging, self-adjusting atmosphere packaging, etc., all of which are inseparable from barrier-type film packaging materials and barrier packaging design. A complete barrier packaging design includes three major factors: the performance of the packaged product; the relevant parameters of the environmental conditions surrounding the package; and the characteristics of the barrier packaging material. In terms of food packaging, how the barrier properties of the package depends mainly on the permeability of the packaging material. If a soft packaging film material has low permeability to a gas, it indicates that the material has good barrier properties against such gases; otherwise, it has poor barrier properties.
According to the above three factors, a complete barrier packaging design can be formed, a reasonable mathematical model can be established, and computer scheduling can be used to solve the problem and optimize the design. However, for in-depth analysis, the characteristics of the packaged food are determined by the type of the food itself; and the surrounding environmental parameters of the package are determined by the product's circulation environment. In this way, the entire barrier packaging design process concentrates on the barrier properties of the barrier packaging material, and it is hoped that the optimization of the barrier packaging design will be achieved through the improvement of the packaging material barrier material. So far, although the barrier packaging film includes a variety of composite films, it still cannot satisfy people's requirements for various barrier properties of food packaging. Therefore, the international packaging industry has been working hard to develop functionally diversified packaging films, including film materials with high barrier properties, partial barrier properties, and high permeability. This article reviews the research and development process of barrier packaging films, discusses the rational ideas and methods for the continued development of such packaging materials, and proposes innovative principles and technology for alternative barrier packaging film materials.
1. High polymer single film and composite film, coating film
In general, a high barrier material means a gas transmission rate of 10 ml/100 in2days·atm below 22.8°C and a material density of 1 mil (25.4 μm).
For high barrier film materials, PVDC and EVOH are the most widely used.
PVDC-polyvinyl chloride films can be formed by copolymerizing vinylidene chloride monomers and acrylic monomers. As the content of these two monomers increases, the gas barrier properties of the film become better. However, the unfavorable factor is the deterioration of the thermal stability of the material. This problem was seen as a difficult step to further develop barrier packaging materials. In order to meet the needs of food packaging, consider the use of composite film method, that is, use PVDC airtight good as the middle layer of composite film, polyester, polypropylene, nylon or paper as its outer layer, polyethylene as heat The layer is melted to form a three-layer or multi-layer composite film. However, the cost of its manufacturing process is obviously increasing.
EVOH-polyethylene polyvinyl alcohol film also has good gas barrier properties and moisture resistance. This is mainly due to the improved performance of EVOH copolymerized with polyethylene and polyvinyl alcohol. Polyethylene has good moisture resistance, while polyvinyl alcohol has good gas barrier properties. However, EVOH is a hydrophilic film that adsorbs moisture, which reduces the gas barrier properties of EVOH. This shows that the good barrier property of EVOH is also unstable, and is greatly affected by the humidity of the environment. Therefore, it is best to use EVOH in the middle layer of multi-layer composite film, and then use other polyolefin materials with high moisture resistance to prevent it from absorbing moisture, so as to fully exert the strong gas-barrier property of EVOH.
Polyacrylonitrile (PAN) is also a widely used transparent high barrier film. In food packaging, films such as PET and PA with good barrier properties, and LDPE, PVC, and PE films with selective air permeability are also frequently used. In addition, PVDC-coated films of various types such as PP, PA, PE, and PT are also frequently used. Since each type of plastic film has its own merits and shortcomings, they should be used in combination in order to exert their respective strengths. This is the reason why there are a large variety of laminating composite films on the market. Now the number of composite films has exceeded 1,000, and the number of composite layers has grown from two to three layers to ten layers. Despite this, in some cases it is still difficult to meet the packaging requirements, and the process is complicated and increases costs. In addition, this "superimposed" composite method does not fully utilize resources and therefore has limitations.
2. "Polymer alloy"
The blended polymer is the application of the existing polymer species, through a suitable manufacturing process, made of polymer - polymer mixture, so that it has a comprehensive performance in many aspects. This blended polymer and alloy in the metallurgical industry is very similar, so the new varieties used to form blended polymers are known as "polymer alloys" or "polymer alloys." The blending method is a shortcut to the development of new polymer materials. At present, this method is not only applied to EVOH, PVDC and other materials, but also used in other barrier resins to make up for and eliminate defects in existing resins, improve performance, and expand the scope of use.
2.1EVOH/PA polymer alloy
When a small amount of polyamide (6-12PA) is added to EVOH, the molecular orientation is improved without impairing the gas barrier property of the latter, so that the thermal stability is improved and the thermal processability is also improved. The characteristics of this high molecular alloy material are very advantageous in the blow molding extension process and the hot forming process. Polyamides of 10-12 PA are added to EVOH, which can be thermoformed at temperatures as low as 150 °C. Otherwise, it is impossible to process at such a low temperature.
2.2EVOH/PET polymer alloy
When 20% EVOH is mixed in PET (polyester), its barrier properties are significantly improved without affecting the mechanical strength of PET. The higher the EVOH grade, the more the barrier properties of such polymer alloys are improved. The higher the level of EVOH, the more ethylene it contains. The reason for improving barrier properties is to increase the aspect ratio of the intermolecular channels in the polymer blend.
2.3 PVDC/PA polymer alloy high polymer, but its brittle temperature is quite high, generally higher than 0 °C. Therefore, the mechanical properties at low temperatures are rather poor, thus limiting the range of applications. Packaging containers made with PVDC tend to be brittle at low temperatures and often break if dropped. One of the measures taken to solve this problem is to blend PVDC with another polymer that has a very high brittle temperature. For example, PVDC and 6-12PA form a polymer alloy that significantly reduces the brittle temperature without affecting the gas barrier properties.
According to the above examples, it can be known that by mixing other polymers in one kind of polymer or adding additives to the polymer, the material properties can be improved and the application range can be expanded. This indicates one direction for the development of new barrier materials using the "physical" approach. This process technology for the development of new materials is obviously not limited to binary systems. If the aforementioned polymer single membrane, coating membrane and multi-layer composite membrane are called first-generation barrier materials, then this kind of “polymer alloy†material obtained by means of the blend polymerization method can be called the second generation. Barrier type material.
3. Sprayed and filled films
Sprayed aluminum film is a food high barrier film that has been widely used for a long time, but its biggest drawback is that it loses the transparency of the plastic film. A layer of high-purity silicon oxide with a thickness of 800-1500 Angstroms (Angstroms) is sprayed on the plastic film. This film not only has high barrier properties, but also has high transparency and high temperature resistance, and is suitable for microwave ovens. The processing method is electron beam vacuum screen spraying technology, and the spraying speed is about 1/3 of aluminum spraying. Due to the sublimation characteristics of silicon oxide, it is difficult to achieve a constant evaporation rate. Japan recently introduced a new process that uses a special thermal resistance spray method that solves the problem of uneven evaporation of silicon oxide in electron beam vacuum spray methods. The test results show that the gas barrier properties are almost equal to aluminum. However, this new spray barrier material is expensive and therefore difficult to promote in the market.
The addition of platelet-like inorganic fillers such as mica to the polymer can also improve gas barrier properties. The mica particles are mixed into EVOH, and the alignment direction is just perpendicular to the oxygen infiltration direction, so that the EVOH forms a secondary gas barrier layer, and the gas barrier properties of the mica-filled EVOH can be increased by three to five times. However, the film thickness should not be less than 0.5 mils, otherwise it will reduce the mechanical properties. In addition, plastic films filled with ceramic powders in LDPE have also been developed. The barrier properties of this film to ethylene gas are particularly low, which can reduce the content of ethylene gas in fresh fruit and vegetable packages, thereby delaying the ripening of fresh fruits and vegetables and extending them. The purpose of the preservation period.
This method of spraying or filling inorganic materials to change the barrier properties of polymer materials is very effective, but its manufacturing process remains to be further matured. Such films can be referred to as third generation barrier materials. This is another new idea for developing barrier packaging materials.
4. Chemical barrier material
The above-mentioned methods for improving the barrier properties of polymers all belong to the so-called "physical barrier method." It is very difficult to increase the barrier property to the level equivalent to glass and metal without affecting the processability and practicality. One of the ways to overcome this obstacle is to use the so-called "chemical barrier method" that corresponds to the "physical barrier method." Since the 1980s, there have been several such products available, which can be divided into the following two categories.
4.1 Through the oxidation reaction
In the late 1980s, the United Kingdom developed a chemically synthesized barrier material called OXBR. It consists of PET (main ingredient), MXD-6PA (combined oxidant) and organic cobalt salt (as catalyst). According to reports, this composite material has been processed into various types of containers by extrusion or blow molding, and its vacuum container can maintain its internal oxygen zero barrier level within two years. Japan has also been successfully developed. At present, this material has been tried on packaging containers such as fruit drinks.
The United States recently developed an oxygen-absorbing plastic called "longevity." The OH groups in the base polymer used may react chemically with the silicon element, may also react with the metal element, and may also be combined with the metal element. This material is insoluble in water, has a structure similar to hemoglobin, and therefore has a strong oxygen absorption. The United States has used such materials as liners for certain beverage caps, which can significantly extend the shelf life of certain oxygen-sensitive beverages.
4.2 Applied Biotechnology
This is an entirely new approach to the development of new barrier materials, which is the use of enzymes to remove oxygen from liquid foods or beverages and other substances, thereby increasing the shelf life of foods. According to this principle, a pharmaceutical company in the United States developed the library's "active barrier packaging technology." The specific method is to mix two types of enzymes in the packaging container: glucose oxidase and catalytic enzyme. When the oxygen in the air inside the container enters the liquid product to be packaged, these oxygen gases infiltrate into the container under the action of the catalytic enzyme to avoid the inside, with gluconate and water, thereby achieving the purpose of deoxidizing the liquid food (drug) product.
The high barrier polymer made by this chemical barrier method was successfully developed on the basis of the physical barrier method, and its barrier level reached a higher level, which is a fourth-generation barrier material.
5. Metallocene polymer
Recently, there is a new class of materials in the field of polymer development. That is, the use of metallocenes as catalysts in polyolefin-based materials increases the barrier properties of the polymers. Metallocene ethylene (MPE)
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