Among various metal laminations, laminated iron has attracted significant attention in the packaging industry. Laminated iron can be found in packaging for beverages, food, and various other products. It boasts high wear resistance and corrosion resistance, as well as excellent appearance and moisture resistance. It addresses the risk of substances like bisphenol A and melamine entering food, making it widely applicable in the metal packaging industry.
Laminated iron typically involves laminating a polyester film with a thickness of about 15 to 40 μm onto chrome-plated iron or cold-rolled steel sheets. The selection and development of the film are crucial. The film must have good barrier properties to prevent corrosion of the metal sheet, strong adhesion to the substrate to prevent detachment during processes like cooking, and excellent moisture sealing properties. PET is a polymer film with excellent performance. In actual production, PET is usually modified to lower its melting point before being laminated with chrome-plated steel sheets.
Currently, there are mainly two processes for laminated iron: low-temperature adhesive lamination and high-temperature melt lamination. Low-temperature adhesive lamination typically uses polyurethane adhesive to bond the metal substrate with a polyester film and then laminates the film at room temperature. This method originated in the early stages of the domestic laminated iron industry. Compared to melt lamination, the bond strength between the polyester film and the substrate is weaker in this method. Currently, only applications like refrigerator surface film coating still use adhesive preparation.
High-temperature melt lamination, on the other hand, can be divided into two types: roll lamination and coating lamination. Roll lamination involves heating the metal steel sheet to above the melting point of the polyester film through a preheated belt. When the surface layer of the polyester film in contact with the steel substrate melts, the molten polymer at the interface between the film and the substrate fully flows, spreads, contacts, and cools and solidifies until the film is roll-laminated with the substrate and further cooled and solidified. During the roll lamination process, a three-layer structure of steel substrate-molten polymer-solid polymer film is formed at the contact interface between the substrate and the film.
Coating lamination involves directly bonding molten resin to the steel plate through coextrusion and then cooling it. The biggest feature of this lamination method is the integration of film production and lamination processes, eliminating the need for transportation and cutting steps, but it is only suitable for lamination methods with minimal tension.
Currently, the most mature laminated iron process is TULC (Toyo Ultimate Can) developed by Toyo Seikan of Japan. TULC laminates PET film on both the inner and outer surfaces of chrome-plated iron and processes it into a cup shape like a DI can. The process is simple and environmentally friendly, and the development of the next generation of products has been completed. The entire production process of TULC laminated iron cans does not use coolants, eliminates the need for wastewater treatment after forming, and does not require painting and baking the inner surface, greatly reducing CO2 emissions. Therefore, TULC is a truly green product.
