Film-Laminated Steel is a kind of composite material which is composed of plastic film and cold-rolled steel sheet by melting method. It is a kind of steel deep-processing product. By covering the cold rolled steel sheet or Chromium-Laminated Steel with multi-layer polyester composite film on both sides, the Laminated Steel can have the dual characteristics of plastic film and metal plate. Compared with the traditional metal packaging material tinplate, Laminated Steel not only does not contain harmful substances, but also has environmental protection in the subsequent production process, and the production cost is lower than tinplate. Therefore, Laminated Steel has been widely considered as the best substitute product in the packaging industry.
I. Development History of Laminated Steel
As early as the early nineteenth century, tinplate began to be used in food packaging industry. After 200 years of development, the industry has become very mature. The so-called tinplate is a cold rolled steel sheet with a layer of tin plated on the surface. Depending on the application, the thickness of tin plating layer is between 0.5 and 2 microns. The world produces about 250,000 tons of tin annually, of which 1/3 is used for tin plating on tinplate surface. Tin is a rare strategic resource. In order to reduce the use of tin, people are constantly thinning the tin coating and actively looking for alternative products. Chromium-plated iron is one of them.
Ferro-chromium plating is a kind of Tin-free steel, which is coated with metal chromium on the surface of cold-rolled sheet steel. The difference is that the thickness of chromium plating is only 1% of tin plating on the surface of tinplate. Therefore, the production cost of ferrochrome plating is lower than that of tinplate (taking 0.2mm thickness ferrochrome plating as an example, its price is about 1,000/ton lower than that of tinplate of the same thickness). The history of ferrochrome plating is much shorter than that of tinplate.
In 1953, Bethlehem Steel Works of the United States completed the first production line of chrome-plated iron.
In 1961, Japan Toyo Steel Company established Japan's first chrome-plated iron production line.
In 1989, the Laminated Steel based on ferrochrome plating was successfully produced by Japanese Toyo Steel Plate. At present, Japan is in the leading position in the production technology of Laminated Steel in the world. Besides Dongyang steel plate, Nippon Iron and JFE have their own Laminated Steel production lines. In 2005, the actual production of Laminated Steel in Japan reached 490,000 tons, accounting for 60% of TFS production.
In 1991, Laminated Steel was put into production in Europe. Germany, Britain, the United States, France and other countries have developed Laminated Steel which can completely replace tinplate. However, the total output of Laminated Steel is still far behind that of Japan.
At the beginning of this century, a company in Wuxi pasted plastic film on chromium-plated iron at low temperature by adhesive method. Its product is also called "Laminated Steel". However, its performance is far from that of Laminated Steel abroad. It can only be used in tea cans, moon cake boxes and other miscellaneous cans, not as food cans and beverage cans.
At present, there is no large-scale production capacity of TFS-based Laminated Steel in China, which is mainly dependent on imports.
II. Raw materials and structures
1. Film-Laminated Steel substrates
Multilayer polyester composite films (about 20-25 microns in thickness) are often used in Laminated Steel, and their ability to block the penetration of corrosive media is close to "complete repression". In theory, cold rolled thin steel substrates can be directly used, i. e. leaving blank Laminated Steel. But when blank Laminated Steel is used to make cans, the welding seams of cans are prone to rust and the adhesion is not strong. Therefore, in practice, all countries use TFS as the base material. Because there is a layer of chromium hydrate oxide film on the surface of TFS products, it can be combined with organic film in the form of hydrogen bond, and its organic film has a very high fastness.
2. Polymer Films for Laminated Steel
For Laminated Steel, the development of film is its core technology. Polymer films used in Laminated Steel manufacturing need not only thermoplastic bonding ability, but also compatibility with physical and chemical properties of different canned contents to ensure good repression of chromium-plated substrates and contents; at the same time, the flavor of packaged foods should be maintained during the shelf life of various canned contents; in addition, they should not be discolored and peeled during high-temperature cooking and sterilization of foods in order to keep them stable. It has high wettability and airtightness, and can withstand the effects of high temperature, high friction and bending forces in the process of tank forming, so it has good adhesion.
In theory, there are PP, PET, PC, PE and other plastic films which can be used to compound with TFS. Among them, PET film is the most common and has the best comprehensive performance. PET film has the characteristics of formability, bactericidal, high hardness, wear resistance and printing after special treatment.
PET polyester film for Laminated Steel can be divided into white film, transparent film, gold film and laser film. From the application, it can be divided into deep drawing film, general chemical canned film and food canned film. From the structure point of view, the PET polyester film for coated iron is multi-layer structure (generally 2-3 layers), which needs to be produced by multi-layer co-extrusion. The melting point of modified PET is lower than that of ordinary PET, and it can quickly adhere to TFS after reaching its melting point.
3. Film-coated iron structure
The structure of the Laminated Steel used for tank making is divided into inner wall and outer wall. Both inner and outer walls have two layers of Laminated Steel. The outer layer of the inner wall acts as a deterrent, and the inner layer of the inner wall acts as a bond with chromium-plated iron and an anticorrosion agent. The outer layer of the outer wall acts as a scratch-proof and Printing-proof agent, while the inner layer of the outer wall acts as a bond with chromium-plated iron.
III. PROCESS TECHNOLOGY
The composite process of high temperature, high pressure and rapid cooling is adopted in the production of coated iron. The production line of coated iron is mainly composed of uncoiling part of chromium-plated iron, welding and iron apparent quality inspection, substrate heating, film-cladding unit, cooling unit, product testing and oil-coating coiling, and the core part is the heating area of the substrate and film-cladding unit. The preheating part of the electromagnetic induction heating roller is used to heat the steel strip to the temperature above the melting point of the film, and the hot fusion bonding between the substrate and the film is carried out by the extrusion of two symmetrical pressure cots. The hot-bonded steel strip is cooled rapidly through a cooling device to ensure the rapid recrystallization of molten polymer.
At present, electromagnetic induction heating rolls are used in the preheating of steel plates in the main film-Laminated Steel production lines all over the world. This kind of heating roll can not only provide the temperature above 250 C, but also has a very good uniformity of temperature (can be controlled within < 1 C). But only uniform heating roll is far from enough. The uniformity of temperature of film roll is the key to the consistency between sheet metal and film. The hard rubber is coated on the surface of the film roller, which can form a high pressure on the steel plate and film under the action of the hydraulic cylinder. In order to keep the surface temperature of COTS not too high, cooling rollers are often designed to cool them. But at the same time, because of the poor thermal conductivity of rubber, if the temperature of the cooling roll is not uniform enough and accumulated over a long period of time, it will also cause the surface temperature of the rubber roll to be uneven, thus affecting the consistency of the film pressing. In a word, the key to the composite of steel plate and film lies in the following points:
1. Heating source with high temperature (above 250 C) and good uniformity (+1 C) - electromagnetic induction heating rolls are used in existing mature coated iron production lines.
2. Hard cots can provide high pressure stably to ensure the uniform combination of steel plate and film. At the same time, in order to ensure that the waste rate in the early stage of production can be reduced as much as possible, cots need to be heated evenly so that the surface of cots can reach the required temperature (about 110-140 C) from the beginning of production.
3. Cot deformation consistency is good to ensure uniform force on steel plate.
4. The surface temperature difference of the cooling roll with good temperature uniformity is less than (+1). Only in this way can the surface temperature uniformity of the cot be guaranteed.
5. Clean working environment to ensure that no dust and other impurities will be entrained when the steel plate and the film are laminated, so as to improve the product qualification.
IV. Application Prospects
Compared with traditional iron stamping process, Laminated Steel has more advantages in performance, material cost, production cost, environmental protection and energy consumption. Its application range is very wide.
1. Application in food metal cans: food cans, beverage cans, four-screw caps, six-screw caps, candy cans, tea cans, easy-to-open caps, etc.
2. Applications in chemical tanks: chemical tanks, paint tanks, paint tanks, two aerosol tanks and various top and bottom covers. Especially the technology of blank film processing can be applied to all kinds of three-piece cans.
3. Application in decorative cans: portable ash box, gift box, stationery box, jewelry box, cigarette box, wine box, etc.
4. Applications in other industries: building panels, walls, ceiling, anti-theft doors, floors, shades, etc.
With the rapid expansion of the application of two-piece cans in the world, the market demand of two-piece cans in China has been greatly promoted. At present, the annual consumption of two pieces of cans (steel and aluminium) in China is about 10 billion. With an annual growth rate of 10%, the demand for two cans in China will reach 16 billion in five years, a net increase of 6 billion. With the increasingly stringent requirements of environmental protection and the increasingly scarce resources, recyclable and recyclable metal packaging materials are favored by people. Therefore, it can be foreseen that the domestic market prospects for the production of two-piece cans with green and environmentally friendly Laminated Steel are very broad.
At present, the tinplate used for food packaging in China consumes about 4 million tons per year. If the technology of Laminated Steel is mature, it can completely replace the existing tinplate in theory. Therefore, the future development potential of Laminated Steel in China is very huge.
V. Domestic Development Situation
High temperature Laminated Steel is still in its infancy in China. At present, only a few enterprises have made efforts in the production of Laminated Steel.
1. A Wuxi company mentioned above is also trying to develop the production line of high-temperature melting Laminated Steel after producing bonded Laminated Steel. Due to the lack of many key technologies mentioned in the third chapter of this paper, the development of high temperature melting Laminated Steel production line is doomed to be uneven.
2. A listed company from Beijing has spent several years in Shaoxing to build a small test line of Laminated Steel, which is still in the process of perfection. At present, the company is also investing heavily in the construction of a pilot line. This pilot line also faces the same problems mentioned above.
3. TFS cold-rolled sheet, built by Baosteel in 2007, is currently the most advanced production line in China. The company hopes to introduce mature production technology of Laminated Steel from abroad, but the negotiations have failed for many years.
4. Hunan Qijiexin Electronic Technology Co., Ltd. has the necessary electromagnetic induction heating rollers for the production of Laminated Steel, and also has the special cot technology for composite requirements and the leading international cooling roll technology. The combination of the three technologies can ensure that the temperature deviation of steel sheet and film is maintained within (+1 C) at all times. Relying on these technologies, Hunan Qijiexin Electronic Technology Co., Ltd. has applied for many patents, and now has produced qualified Laminated Steel products, and has been recognized by many canning enterprises. Although the company is still small in scale, it has been favored by huge external investment because it has mastered many core technologies. Over time, it is bound to cause a great disturbance in the industry.
In the production of key materials of Laminated Steel, there are still some gaps between domestic enterprises and Japan, but the gap is not large. Domestic TFS manufacturers mainly include Baosteel, Zhongshan, Zhongguang, Donghai sheet metal, Unified tinplate and so on. Among them, only Baosteel can produce two pieces of tinplate at present. East China Sea Sheet introduced a continuous annealing cold-rolled sheet production line from JFE in Japan. It is expected to be put into operation in May 2014, and its TFS performance is said to be comparable to that of Baosteel products.
At present, many film manufacturers in China can provide qualified film for the production of Laminated Steel, but only three cans, top and bottom covers and other products, which can be used for deep drawing two cans film has not yet been produced. With the construction of film-Laminated Steel production line, it is believed that domestic film manufacturers will increase the development of film-Laminated Steel film, and the film used for deep drawing two-piece can will be developed successfully sooner or later.
In short, Laminated Steel is a sunrise industry in China. Because of its superior performance, environmental protection of products and processes, and competitive price, it will gradually replace tinplate in the metal packaging industry in the near future.
