Talking about the past, present and future of film laminated steel
As early as the early nineteenth century, tinplate began to be used in the food packaging industry. After 200 years of development, the industry is very mature. The so-called tinplate is a cold-rolled thin steel plate with a layer of metal tin electroplated on the surface. Depending on the application, the thickness of tin plating is about 0.5~2μm. The annual output of tin in the world is about 250,000 tons, of which 1/3 is used for tinplate surface electroplating. Tin is a rare strategic resource. In order to reduce the use of tin, people are constantly reducing the thickness of the tin coating, and on the other hand, they are also actively looking for alternative products. Chrome-plated iron is one of them. When it comes to chromium, some people are afraid, thinking that chromium is a very harmful substance. In fact, this understanding is incomplete. Hazardous chromium is hexavalent chromium, which is expressed in ionic form as Cr2O72-, while the one electroplated on cold-rolled sheet is metallic chromium, which has a chemical valence state of 0, which is completely safe.
Chrome-plated iron is a kind of tin-free steel (Tin free steel, referred to as TFS), which is a layer of metal chromium plated on the surface of cold-rolled thin steel. The difference is that the thickness of the chromium coating is only about 1% of the tin coating on the tinplate surface. Therefore, the production cost of chrome-plated iron is lower than that of tinplate (taking 0.2mm thickness of chrome-plated iron as an example, the price per ton is several hundred yuan lower than that of tinplate of the same specification). In comparison, the history of chromed iron is much shorter than that of tinplate.
In 1961, Japan's Toyo Steel Co., Ltd. established Japan's first chrome-plated iron production line.
In 1989, Japan's Toyo Steel Co., Ltd. successfully coated a layer of polyester film on both sides of chrome-plated iron, which can be used to produce food and beverage cans. This material is coated iron. At present, Japan is a global leader in the production technology of coated iron. In addition to Toyo Steel, Nippon Steel and JFE have their own coated iron production lines. Now, the actual output of coated iron in Japan has reached nearly one million tons, and the vast majority of Japanese food and beverage cans are coated cans.
In 1991, European coated iron was put into production, but so far, European coated iron is mainly used for chemical cans, aerosol cans, etc., and is rarely used in food and beverages.
At the beginning of this century, a company in Jiangsu,CHINA used the adhesive method to paste plastic film on chrome-plated iron at low temperature. Its product is also called "coated iron", but its performance is far from that of foreign coated iron, and it can only be used for Tea cans, moon cake boxes and other miscellaneous cans cannot be used as food cans and beverage cans.
Therefore, the "film laminated steel" mentioned in this article refers specifically to the coated iron used for food and beverages. According to Japan, coated iron is a composite material in which a plastic film is laminated on a cold-rolled steel sheet by a melting method, and it is a deep-processed product of steel. By covering the front and back sides of cold-rolled steel sheet or chrome-plated iron with multiple layers of polyester composite film, the laminated steel can have the dual characteristics of plastic film and metal sheet. Compared with the traditional metal packaging material tinplate, coated iron not only contains no harmful substances in the raw material itself, but also the subsequent production process is environmentally friendly, and the production cost is lower than that of tinplate. The best alternative in the industry.
Next, let's talk about some technical issues. Coated iron is not simply to find a film and paste it on the steel plate, it is called coated iron. The three words "film laminated steel" each represent a core technology.
lamination is the lamination of equipment and processes
Film refers to a special modified film that can be well combined with TFS;
Iron, preferably cold-rolled sheet with chrome-plated surface, should have as low a carbon content as possible.
This section will talk about the structure and raw materials of laminated steel.
1. Laminated Steel structure
The structure of the laminated Steel used for can making is divided into an inner wall and an outer wall, and the inner and outer walls are coated with two layers. The outer layer of the inner wall plays a blocking role, the inner layer of the inner wall plays the role of bonding with the chrome-plated iron and anti-corrosion; the outer layer of the outer wall plays the role of anti-scratch and facilitates printing, and the inner layer of the outer wall plays the role of bonding with the chrome-plated iron.
2. Laminated Steel Substrate
Laminated Steel often uses multi-layer polyester composite film (thickness is about 12-25μm), its ability to block the penetration of corrosive media is close to "complete inhibition", so the coated iron substrate can theoretically directly use cold-rolled thin steel substrates However, the adhesion of the film directly on the cold-rolled sheet is not strong, so in practice, all countries use chrome-plated iron as the base material. Since there is a layer of chromium hydrated oxide film on the surface of TFS products, it can be combined with the organic film in the form of hydrogen bonds, and the organic film has extremely high fastness.
There are also clear requirements for the content of metal chromium and chromium oxide on the surface of TFS. Generally speaking, the content of chromium is 50~150mg/㎡, while the content of chromium oxide is roughly 5~15mg/㎡. The specific indicators are also different according to the different film formulations. The surface roughness after chrome plating is also required, usually between Ra 03~0.5μm, too high or too low coating effect is not the best.
3. Special polymer film for laminated steel
For laminated steel, the development of film is its core technology. Japanese companies are extremely strict in the confidentiality of the film, and the Chinese can only rely on their own ingenuity.
The polymer film used in the manufacture of laminated steel needs not only the ability of thermoplastic bonding, but also compatibility with the physical and chemical properties of different canned contents, so as to ensure good resistance to chrome-plated substrates and contents; The flavor of the packaged food must be maintained during the shelf life of the contents; in addition, it should not discolor and fall off during the high-temperature cooking and sterilization process of the food, so as to maintain a high humidity and airtightness, and can withstand high temperature and high temperature during the can-forming process. Due to the effect of composite forces such as friction and bending, some are still printed and need to withstand high temperature baking. Therefore, the film must have good adhesion.
Theoretically, plastic films that can be used for compounding with TFS include PP, PET, PC, PE, etc. Among them, PET film is the most common and has the best overall performance. PET film has the characteristics of formability, sterilization, high hardness, wear resistance, and printability after special treatment.
From the color, the PET polyester film for coated iron can be divided into white film, transparent film, golden film and laser film. In terms of application, it can be divided into film for deep drawing, general chemical tank film and food tank film. From a structural point of view, the PET polyester film for coated iron is a multi-layer structure (generally 2~3 layers), which needs to be produced by multi-layer co-extrusion. Among them, the layer attached to TFS is modified PET, whose melting point is lower than that of ordinary PET, and can be quickly attached to TFS after reaching its melting point.
Japan also has the practice of using tin-plated tinplate as the base material for processing. Due to the low melting point of tin, a modified polyester film with a lower melting point must be used.
With qualified raw materials, how to process them is also very important. This requires good equipment and proper craftsmanship.
The production of laminated steel adopts a composite process of high temperature, high pressure and rapid cooling.Seeing "high temperature", many people will take it for granted that it is so easy, isn't it just to add a temperature? There are various heating methods. However, one of the most important issues is how to make the temperature of the steel plate not only uniform in the transverse direction, but also at the speed of 100~200m/min, the temperature of the steel plate is always uniform when it is composited. In the past few years, when domestic companies developed coated iron production lines, they used various heating methods, some used oven heating, some used gas heating, some used heat transfer oil roller heating, and some used induction coil heating directly. None of these requirements can be met. In fact, Japanese companies use electromagnetic induction heating rollers as the heating source for steel plates. Only in this way can it be ensured that the temperature of the steel plate is always stable and uniform, thus ensuring good product consistency.
The production line of laminated steel is mainly composed of chrome-plated iron uncoiling part, welding and iron appearance quality inspection, substrate heating, lamination unit, cooling unit, product inspection and oil coating coiling, etc. The core part is the heating of the substrate. unit and lamination unit. The steel belt is heated to a temperature above the melting point of the film by the preheating part of the electromagnetic induction heating roller, and the substrate and the film are hot-melt bonded by the extrusion of two symmetrical pressure rubber rollers. The hot-bonded steel strip is rapidly cooled by a cooling device to ensure that the molten polymer is rapidly recrystallized.
At present, the world's major laminated steel production lines use electromagnetic induction heating rollers to preheat steel plates. This heating roller can not only heat the TFS to a temperature above 250°C, but also has a very good temperature uniformity (which can be controlled within ±1°C). However, it is far from enough that the temperature of the heating roller has high uniformity. The temperature uniformity of the lamination roller is the key to the consistency of the lamination of the steel plate and the film. The surface of the lamination roller is covered with hard rubber, 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 the rubber roller from being too high, cooling rollers are often designed to cool it. But at the same time, due to the extremely poor thermal conductivity of rubber, if the temperature of the cooling roller is not uniform enough and accumulated for a long time, it will also cause uneven temperature on the surface of the rubber roller, thus affecting the consistency of the lamination.
Now, our country is also actively formulating various policies to support the development of the laminated steel industry, some of which have been introduced in the past two years. China's film laminated steel industry will soon usher in the spring of rapid development after more than ten years of brewing.