Development and application of coated iron for metal packaging
Text / Zeng Ke
Laminated Steel (Coated Steel) is a composite material obtained by laminating a color-printed plastic film on a cold-rolled steel sheet by a fusion method or a bonding method instead of a tin plate (tinplate), a chrome plate, The galvanized sheet and the cold-rolled sheet are printed by printing iron. The process of the coated iron production line changes the traditional one-piece, one-color, and one-time baking process. Instead, the whole roll is directly cleaned or bonded by multiple sets of printed films. On the plate, it can be cut separately according to the requirements. The production process is fully automated, and the printing and laminating can be completed at one time, realizing the industrial production of coated iron for metal packaging.
The coated iron is composed of a polymer resin film and an iron plate. The applied resins are: polyester (PET), polypropylene (PP), etc., and the PET/PP film is a material that can be directly contacted with food by the US FDA, and has a price. Low, good transparency, non-toxic and flawless, good barrier properties. PET/PP film as a packaging material does not need to add any plasticizers or additives during processing. The environmentally friendly process makes PET/PP very suitable for food and beverage packaging materials. The application of iron plates is: tin plate, chrome plate, plain iron plate and galvanized plate. Because coated iron has the characteristics of both resin and iron, it has the following advantages: external sightseeing, lubrication, good decoration, good hand feeling; strong abrasion resistance, corrosion resistance; strong chemical stability, corrosion resistance, resistance Aging; low surface capillary action; good formability, deep drawing; low adhesion to paint.
China's "Eleventh Five-Year Plan" "Outline" clearly states that building a conservation-oriented society and developing a circular economy as a national development strategy. And on August 29, 2008, the National People's Congress promulgated the Law of the People's Republic of China on the Promotion of Circular Economy. Reduction, reuse and recycling are the main directions for the advancement of industries and enterprises and the promotion of industrial upgrading. In February 2009, the National People's Congress passed the "Food Safety Law of the People's Republic of China". On June 12, 2009, the law was promulgated and implemented. The country has placed food safety at an important position in protecting people's livelihood. In the whole process of food safety, the packaging of food and beverage is the last link of the modern food production industry. Through the research and development of food, beverage packaging and packaging materials innovation, it can effectively improve the safety of food and protect the nationals. Diet safe. Therefore, food packaging safety has become an indispensable part of ensuring food safety. Improving the food safety through the innovation of packaging methods and packaging materials is one of the key tasks of the food packaging industry in the field of scientific research and development.
The production of coated iron does not have a large amount of solvent exhaust gas, and the production technology of dry molding, thereby omitting the coating drying process and establishing an environmentally friendly production system. In addition, the coated iron usually uses a tin-free steel plate (belt) such as chrome-plated iron, which greatly saves the consumption of tin resources compared with the tinplate, and eliminates the waste of non-renewable resources such as tin. Therefore, for the food and beverage packaging industry, the value of the coated iron project is to fundamentally solve the problem of a large amount of waste gas wastewater pollution and scarce tin resource consumption caused by the existing paint iron production, which is energy-saving, environmentally friendly and safe. The intrinsic quality fits the core elements of “green packagingâ€.
In the metal packaging industry, food and beverage packaging account for the vast majority of the market. We have successfully applied coated iron materials to two shallow-wash food cans and food can bottom covers, which provide extensive application and marketing for coated iron. A solid foundation. Regarding the authorization of patents, in terms of intellectual property rights, as of December 2014, there are 17 patents authorized for the development and application of coated iron, including 12 inventions, 4 utility models, 1 design, and 4 other items. The invention patent is in the application and one invention patent has been authorized.
The key technologies for the development of coated iron materials are as follows:
1. Polymer film
There are two choices for polymer film for coated iron, polyester (PET) or polypropylene (PP). Both polymers are crystalline polymers. Moderate crystallization contributes to material barrier and corrosion resistance. The improvement, but the crystallization is too high will lead to a decrease in the film's resistance to processing. Therefore, the control of film crystallinity is very important in the manufacture and application of coated iron. The control of the crystallinity of the film can be started from the process, but more importantly, it is to change the bulk properties of the polymer resin raw materials. For example, the polyester raw materials currently used in Japan have undergone special chemical modification.
In response to the above problems, we have completely designed the formulation of the special film for coated iron. Through the design of the new formula and the adjustment of the film layer structure, we have successfully solved the above various defects. The detailed technical solutions are as follows:
1. Inter-acid modified polyester resin
Ordinary BOPET film has a faster crystallization rate, which is not conducive to the subsequent process of filming, printing and sterilization of the finished product, and the film has a high melting point (about 255 ° C), so that the film melt requires high heat and consumes a lot of energy. In response to this situation, the resin chip of our special polyester film was introduced with isophthalic acid during the polymerization to obtain an acid-modified polyester resin. The introduction of isophthalic acid can lower the melting point of the polyester material and improve the adhesion of the polyester material to the metal substrate. In the production process of the film, the bonding layer is simply modified resin or blended with ordinary modified resin to ensure the adhesion and retort resistance of the film to the metal substrate.
2, high barrier coextruded film
Covered iron is used to make food cans or lids. When the wells are required to have high barrier requirements (such as high acid and high salt contents) or foods with harsh sterilization conditions (for example, Babao's usual sterilization conditions are 127 ° C for 30 minutes) The barrier property of the conventional PET film on the surface of the coated iron can usually not meet the requirements of canning. When the food can is stored in a sterilized or long-term shelf, the film may be resistant to food corrosion and the film may be broken. Rust deteriorates. In response to this situation, we have invented a high-barrier co-extruded film by improving the formulation and film structure design. The outer layer of the co-extruded film is in contact with the food layer. The layer structure has a higher melting point and rigidity than PET. And the barrier property ensures the hardness and high barrier property of the coated iron surface; the inner layer of the coextruded film is still conventional PET or acid-modified PET, ensuring a lower temperature when heat-bonded to the metal plate. The lowering is carried out while ensuring a higher adhesion to the metal sheet.
3. Low surface energy film
When coated with iron cans and canned food such as luncheon meat or ketchup, it is often difficult for consumers to dump the food directly out of the can due to the high adhesion of the food to the can wall. To improve the consumption experience, we designed a low surface. The film can be made of ordinary polyester resin + silicone masterbatch, and the inner layer is acid-modified polyester resin + ordinary polyester resin + additive.
4. Modified PP material
For polypropylene (PP), since it is a non-polar material, it has poor adhesion to metal substrates (polar surfaces). Therefore, special chemical modification of PP materials is required to improve the film and metal substrate. The adhesion fastness.
Second, the metal substrate
Steel is a traditional and relatively mature industry, and its performance research and development in various aspects is relatively sufficient. We have carried out research on metal substrates from the two dimensions of satisfying the processing and forming properties of food cans and their effects on the process performance of film coating. The main results are as follows:
1, microstructure
The microstructure indicators of chrome-plated plates are mainly grain size and inclusions, which directly affect the n value of the substrate. The results show that the grain size should be controlled at a grain size of about 9 to 12, that is, the grain size is 8 to 15 um.
2, mechanical properties
The degree of tempering is the most basic characteristic value for evaluating the strength of chrome plate materials. The chrome plate used for shallow-drawing DRD cans mainly adopts DR material, and the degree of tempering is usually between DR8 and DR9.5, and most of them are DR8. The tempering degree of the SR material is mainly T3 to T5. The degree of tempering is mainly related to the aspect ratio of the coated iron cans produced. The smaller the ratio of diameter to height, the lower the degree of tempering. The elongation of the chrome plate for the bottom cover and the shallow-drawing DRD can is generally between 2% and 5%. The chrome plate elongation for deep draw cans is generally greater than 20%. Plastic strain ratio r value, Δr value: Studies have confirmed that the r value depends on the texture of the thin plate. The stronger the {111} texture, the weaker the {100} texture, the higher the r value. When the sheet has {111} texture, it has strong resistance to thickness reduction during forming, so the stamping performance is good. The main means to improve the Δr value is achieved by optimizing the key process parameters such as control composition, hot rolling temperature, cold rolling deformation, annealing temperature and speed, so that the lug effect when the absolute value of Δr approaches 0.1 is the least.
3, chrome plate surface state
The surface condition of the chrome plate is the most important performance index that affects the filming effect and the properties of the coated iron 4. Generally, it mainly characterization of the total amount of chrome plating and surface roughness of the surface of the parameter packaging. The chrome plated chrome layer consists mainly of a metallic chromium layer and a chromium hydrated oxide layer. When the plating amount of the chromium water oxide layer is increased, the surface convexity and concave will change. The experimental results show that the hydrated chromium oxide content shows a good wettability and sealing property at 5-20 mg/m2, and the wettability of the sample of 20 mg/m2 or more is deteriorated. The reason is not the reason why the hydrated chromium oxide is too thick, but the surface morphology changes, and the surface exhibits irregular convex and concave. Due to the deep drawing process during the subsequent processing of the coated iron, the requirements for the metallic chromium layer are different according to the different processes used for the different functional cans. The research results show that the plating amount of the metal chromium layer above 80mg/m2 will help to ensure a good film coating effect.
Third, coated iron application process
The process of laminating will greatly affect the final properties of the coated iron, such as corrosion resistance, processing resistance and so on. Therefore, different coating processes may be adopted for different products, and it is necessary to precisely adjust the temperature, speed, pressure of the film and the mutual cooperation between the three. In the application of coated iron, the key technologies have the following three points:
1. Film-coated iron plate pre-deformation printing technology
The anisotropy of the material results in uneven deformation during drawing, and the pattern of deformation after printing is different from the design pattern. After the substrate is rolled, the deformation texture is produced, and the mechanical properties of the material in different directions are different. During the drawing process, the deformation degree of each direction is inconsistent, and the lugs are easily generated. The closer the DRD can is to the trimming part, the greater the amount of deformation and the more obvious the difference. Therefore, it is necessary to consider the problem of uneven deformation after molding in the design of the pattern deformation, or to use a material with low ear rate to reduce the influence of uneven deformation of the material on the appearance of the product.
2, color printing film iron food can making process
Due to the secondary crystallization of the polymer film material during the printing process, the inner and outer molds will produce white indentations on the film during the stamping process. How to eliminate white marks and produce a customer-satisfied product requires the process of canning Advance evaluation and optimization.
Fourth, film coating equipment
In order to meet the demand for pre-coating iron test, we designed and built a coated iron test unit. The test unit has a running speed of 60m/min and a strip width of 550mm. It has both test and small-scale production functions. A sample of 8 tons of iron oxide, and through various tests, has successfully prepared samples of coated iron cans and lids.
The coated iron test unit can not meet the large-scale production demand, and there are drawbacks such as adjustable pressure and uncontrollable, large temperature fluctuation. However, the coated iron materials in the early stage of material development and customer sample trial production are all prepared by test lines, which can meet the requirements of material development and customer samples. The successful introduction of test lines provides an effective guarantee for the smooth progress of the coated iron project. Made an indelible contribution. Considering the expansion of the company's production capacity, the coated iron test unit can not meet the needs of high-speed production. Therefore, the coated iron intermediate pilot unit project was launched, and a coated substrate was designed and successfully built to a maximum width of 1050m and a speed of 100m/min. About 50,000 tons / year of coated iron production line.
in conclusion:
The successful completion of the development of coated iron has broken through the foreign technology blockade for many years, and comprehensively improved the efficiency of resource utilization, and achieved sustainable development of the metal packaging industry. At the same time, it has created a superiority for the future product differentiation of the Chinese metal packaging industry and the international market. Conditions and effective material protection.
The success of coated iron shallow-washing food cans will promote the development of domestic metal packaging industry in the direction of “resource conservation and environmental friendliness†under the premise of promoting food safety of content, comprehensively improve the level of domestic metal packaging industry, and fill the domestic coated iron products. blank. Coated iron is a new type of metal coating material that is more environmentally friendly and safe after the wide application of tinplate. At the same time, the rapid development of China's metal packaging industry, the traditional metal packaging material - the amount of tinplate is increasing at a rate of 100,000 tons per year, but because of the cost, performance, energy saving, environmental protection, food safety of coated iron compared to traditional coating iron The huge advantage, so the application of coated iron instead of tinplate has become the main material for domestic metal packaging in the future, and is the development trend of the domestic metal packaging industry.
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