(1) In major tinplate-producing nations worldwide, there is a general trend of no longer establishing new tinplate production units, including those for Tin-Free Steel (TFS). However, developing countries, particularly China, are still in the process of constructing new production lines, experiencing rapid growth. TFS products offer lower production costs and contribute to the conservation of scarce tin resources. Over the past 40 years, significant progress has been made, and the industry has now stabilized, with ongoing development in China.
(2) In addition to promoting TFS for tin resource conservation, another trend is the thinning of tinplate thickness. Secondary cold-rolled tinplate, including TFS, is increasingly being applied. Three-piece can bodies widely adopt tinplate with a thickness of 0.14 to 0.17mm. Two-piece steel cans, particularly in Europe, commonly use DI material with a thickness of 0.235mm (for 330mL cans, equivalent to 0.245mm for 355mL cans). Some are already using 0.205mm DI material (330mL), and 0.19mm DI material (330mL) is in the experimental phase. In Japan, TULC material has been reduced to a thickness of 0.18mm (though the can body’s thin wall thickness is 0.08mm, thicker than traditional DI cans).
(3) Clean Production, Reducing Environmental Pollution.
In North America and Europe, research on hexavalent chromium-free passivation is currently underway. Traditional PSA plating solutions, due to their high toxicity and substantial pollution, are gradually being replaced by MSA plating solutions. The passivation film on tinplate is composed of metal chromium, hydrated chromium oxide, tin oxide, and other components. The most commonly used passivation treatment for food and beverage cans is cathodic heavy sodium dichromate passivation. However, chromium salts cause severe environmental pollution, prompting countries to develop passivation techniques with minimal environmental impact.
In the UK, a working group consisting of tinplate producers, coating manufacturers, can makers, and chemical companies has been established to assess passivation techniques involving zirconium, titanium, cerium phosphates, silicates, and some organic coatings. Clariant, a leading electrochemical company in the UK, has developed a basic chemical substance, phenol sulfonic acid (PSA), adopted by American tinplate manufacturers. Its fundamental formula contains 2% free phenol, considered to be the most environmentally friendly. The improved Clariant formula (LPSR) reduces free phenol to 1% and diphenyl sulfone (DDS) from 1% to 0.7%. This formula is not only environmentally friendly but also has the lowest cost.
In other European countries, the focus is on evaluating chemicals such as polyacrylic salts and rosin acid siloxanes. In the United States, it is proposed that zirconium sulfate (ZOS) and fluorinated zirconium acid (F-Zr) could substitute for chromic acid.
In the Asia-Pacific region, experiments involving mixed metals such as cerium oxide, silicon oxide, and titanium salts are ongoing, with fluorinated zirconium acid proving to be the most successful.
In the field of tinplate printing, traditional solvent-based inks are being replaced by eco-friendly inks. In the iron printing sector, 70% of tinplate lines are using environmentally friendly inks, with the UK already reaching 90%. Coating applications are also actively promoting the use of healthy and safe coatings.
(4) Recycling of Metal Cans
Developed countries place significant emphasis on the recycling and reuse of post-consumer metal cans. The European Union and others have enacted legislation specifying clear targets for recycling and recycling rates. Germany, for instance, mandates recycling rates of 70% for tinplate cans and 50% for aluminum cans. In the United Kingdom, the 2008 recycling targets were set at 61.5% for steel cans and 35.5% for aluminum cans. In the United States, the recycling rates in 1998 had already reached 56% for both steel and aluminum cans. Japan achieved recycling rates of over 80% for both steel and aluminum cans in the year 2000.
(5) Co-Extruded Composite Coating
ToyoKohan, a Japanese company, has pioneered the development of co-extruded composite coating for tinplate. ToyoKohan’s strategic approach involves collaborative research with their partners in can manufacturing, leveraging their expertise in thin iron production. In 1997, ToyoKohan successfully produced the first TULC can (Toyo Ultimate Can). Between 1998 and 2001, the company achieved successful development of non-oriented pressure-sensitive film and double-sided co-extruded composite coating for tinplate. The introduction of the dry forming process in 2006 reduced the cost of TULC cans, improved production efficiency, and further minimized environmental impact.
The internal non-oriented pressure-sensitive film enhanced the tensile properties of the iron, achieving optimal can forming effects. This thin film requires lower preheating temperatures during the production process, proving effective in coating adhesion and reducing production costs.
Double-ended co-extrusion stretching (DEC) represents an advanced forming process, allowing the creation of stable films as thin as 10 micrometers or even thinner on the can wall. However, the production cycle for cans has been shortened, and the speed of production lines is limited. It is anticipated that by 2008, DEC will reach a certain speed threshold.
Cans made from polymeric-coated iron, such as those utilizing Protact technology from Corus Packaging Plus, demonstrate resilience against impacts and friction encountered throughout the entire transportation chain. The continuous growth of the recycling rate for lightweight and sturdy canned packaging is evident, with the recycling rate in the EU-15 countries reaching 63%. Iron packaging, through Protact’s polymeric-coated technology, aligns with EU regulations, contributing to reduced raw material consumption, energy usage, and environmental impact.
Protact, utilizing Corus Packaging Plus’ multi-polymer coated iron (Protact) rolling technology and its advancements, combines the characteristics of polymers and iron, offering a secure, versatile, and high-performance packaging material. Two processes are employed in Protact iron production: the film-coating process has limited production speeds, approximately 50–80m/min, while the extrusion process operates at speeds of 100m/min, reaching up to 300m/min. Protact iron can feature various interchangeable composite layers, with options for single or double-sided coating, different thicknesses of polymers (transparent or colored), and customization based on specific requirements, such as tension during production, polymer adhesion, printability, and lubrication.
According to ICI Coatings’ Global Research and Investigation Division, achieving optimal corrosion resistance and food safety for iron cans depends on an ideal coating system formulation. The coating should adapt to the entire can production chain, ensuring the quality of the empty cans. In comparison to traditional three-piece cans and draw-redraw cans (DWI), the development of draw-and-iron cans requires specialized polymer coatings to withstand strong stretching forces. As forming capabilities increase, it becomes crucial to enhance the density of polymer cross-links to improve the corrosion resistance of the can’s interior.
Different steel bases will impact the surface tension of the coating, the interaction between the coating and the steel base, and the adhesion and corrosion resistance of the coating. Temperature is a significant factor during bending, as higher temperatures may lead to coating shrinkage and loss of adhesion. The more uniform the coating surface humidity, the fewer humidifying additives are needed, reducing the solvent requirements for water-based coatings.
Technistan is a novel electrolytic process for the production of tinplate, developed by the American company Tehic. This work is currently underway in the United States. The formulation of this process consists of four components: a tin sulfate solution containing approximately 20g of tin metal per liter; 5% concentrated sulfuric acid; 5% additive Techristan TP; and an antioxidant (to prevent excessive deposits on the surface of iron due to the use of sulfuric acid). Tehic is pursuing this new process in response to the rising prices of cold-rolled coils, seeking lower production costs. However, for this process to be widely accepted in the market, an extended period of time is required, along with the validation of its environmental impact.