Canned food originated after the French revolution, in order to preserve food from deterioration, the first use was of glass cans. The canning industry was born in 1810 when Durant, an Englishman, quickly replaced glass cans with tin-coated, empty ones. By 1896, German American immigrants man Amon had improved sanitary cans similar to modern cans.
With the advent of the new century, as a result of the great development of agriculture, forestry, animal husbandry and sideline fishery, people are in great need of reliable and economic preservation methods for the food that cannot be consumed temporarily. Canned food has been developed rapidly.
Tinplate can making was introduced into China in the late Qing dynasty. In 1978, Wisco completed the first 120,000-ton tinplate plating production line, and in 1998, Bao Steel 400,000 ton tinplate special cold continuous rolling tinplate plating plant (with an investment of 65 billion yuan) was put into production.In this way, since the beginning of the 21st century, the situation of China’s food canning industry, which mainly relies on imported tinplate to support itself, has been completely changed.
Quality of steel substrates
Tinned sheet, in short, is a relatively pure low carbon cold rolled mild steel substrate, two sides of tinned.Obviously, its quality is based on the quality of the steel substrate.And steel substrate value, will account for more than 80% of the total value, so it is the top priority of tinplate quality.
The national standards of various countries have put forward clear minimum requirements for the composition of commercial steel substrates.The most representative standard is ASTM A 623-2011.The minimum requirements for the composition of three steel substrates used in food canning are listed.Unfortunately, many people regard this ingredient list as the highest standard.
For example, this composition table requires that the lead content (Pb) in the steel substrate should not be more than 0.02%, that is, 200ppm.However, in the 21st century, countries further limit lead (Pb) must be less than 0.01%, which is the improvement of tin plate quality.Other components, especially the four major non-metallic components (sulfur, silicon, phosphorus, carbon), have a negative impact on corrosion resistance, countries are trying to control to make it smaller, the four major metal components (manganese, copper, aluminum, chromium) also need to be controlled as needed.In this way it is possible to produce good corrosion resistance and processing resistance of tinplate.
In recent years, China’s production of electroplated tinplate, from 10 years ago, the annual output of more than 1 million tons jumped to 4.84 million tons, but the vast majority of the steel substrate can not meet the minimum requirements for empty food cans. There are many reasons, but our country’s national standard GB/ t2520-2008 electroplated tinplate, there is no set minimum limit on the composition of various steel substrates, is also the cause.In the standard, the base plate composition is uncertain, in the regulation is: the inherent quality of the thin plate, the manufacturer is not clearly responsible for the user.
In this way, from a good point of view, it is required that the buyer of the sheet metal (empty tank manufacturers and canning factory personnel) must have a complete grasp of the steel, that is, the steel base composition and corrosion resistance of the latest scientific and technological information, but also urge users to establish a perfect detection system; On the bad side, as long as it is a thin sheet of iron coated with tin, it may be recognized, or even the inherent defects, and by the technical inspection before and after the use of cans, can not be recognized by both parties. Thus, if the standards are not strict (product standards should be the best technical guidance for the production and use of this product at that time), there will be no technical guarantee for users in the market economy, and at the same time, it will lead to the rise of backward technology dregs.In the absence of integrity, such standards may become a tool of deception and thus risk damaging the industrial system.
For decades, the global electroplated tinplate used in the food packaging industry, about 60% of the annual output, and the current situation in China, there is a total of about 70% of the tinplate can not be used in food cans.
Corrosion resistance of tinplate and “K plate”
The quality of tinned sheets is partly visible, that is, easily ascertained by visual and simple measurements. For example, flatness, brightness (reflecting plating level and storage time), thickness tolerance, etc. But more important internal qualities, such as corrosion resistance (which is determined by tin plating, iron dissolution value (ISV), acid leaching delay value (PLV), tin ferroalloy layer couple (ATC) test values, grain size (TCS) and even the quality of the anti-oxidation chromium layer and oil film) and mechanical properties (tempering degree) are integrated.It must rely on sophisticated testing to assess.And the performance (quality parameters) and steel base composition (this is the most important), cold rolling technology (for example, rolling process should not be too much skin effect of nonmetal impurity or tin plating process, tin ion will not be plated on the surface of the steel plate, these places, then formed the loophole that microporous coating, it destroyed the tin layer density, corrosion effect damage) has a very close and direct relationship.
Since the 1930s, cold-rolled strip steel has been tinned to reduce the unevenness of the surface layer of tin (which does not look shiny) in order to reduce its absorption of water molecules from the air. People put it oil (commonly used kwai diacid dioctyl ester DOS, 3-6 mg/m2), before an increase is on the production line, semi-finished products after plating tin, transient heating to 232 ° C (lasts 1 second), then quenching. The result is to get a very shiny coating surface, this process called “soft melting.”
By the early 1950s, an American scholar had discovered that tin, which melts in a soft melting, seeps into the lattice of iron on the surface of a steel base, creating a thin layer of tin ferroalloy. This layer has been proved by research to have excellent corrosion resistance. He also found out the relevant technical conditions for forming a good alloy layer, including the requirements for the composition of steel base (generally vacuum smelting) and the best temperature time for heating, and invented the “tin ferroalloy layer couple ATC test” to determine the quality of tin ferroalloy layer.In order to commemorate his invention, posterity named this kind of high corrosion resistance tin plate after his name “Kamm”, which in Chinese is called “K plate” or “K iron”.
Secondary cold rolled sheet and chrome plated sheet
Entering the 21st century, the tinplate can use in our country, has been gradually by the former 0.25 — 0.23 mm, thinning to 0.21 — 0.16 mm nowadays. In fact, in ten years ago, Europe production ? 99 of the 1000 ml cans of food, have all made of 0.14 mm sheet.From the perspective of cold rolling technology, modern five-frame (5-pair roller) continuous rolling equipment, combined with today’s steel making level, the minimum thickness of a cold rolling product can reach 0.16mm. But to get equal to and under the thickness of the thin plate, generally use secondary cold rolling, that is, first rolling out 0.20 — 0.18mm strip steel, and after annealing, cleaning the surface, and then use the double frame mill, in the amount of pressure is about 20% — 30%, the implementation of secondary cold rolling.0.16mm and thinner tinplate can be obtained. Currently, in order to compete with other food packaging materials, thin plates with a thickness of 0.14 — 0.12mm are commonly used. At present, even 0.05mm thin plates have appeared.
At present, the amount of tin coating, in most cases, has been reduced to 2.8g/m2 and 1.1g/m2. Electroplating method, the more refined, tin layer density increased. This more or less compensates for some of the loss of resistance to tin plating.
The food hygiene
On the other hand, in order to improve the corrosion resistance of empty cans and reduce the infiltration of heavy metals into food in the inner walls, people began to use natural or artificial resins to coat the inner walls of empty cans since the 1930s. This can also gradually reduce the amount of tin coating on the inner wall (due to the invention of two sides of different tin coating “different thickness of iron”, tin coating on the outer wall has long been reduced).This process has continued until now.
But since the 1980s, due to people’s more and more clear understanding of harmful infiltrating ingredients in food, the requirements for commodities and environmental protection have become higher and higher. The food and agriculture organization of the United Nations (FAO), the world health organization (WHO) and national authorities (often represented by the food and drug administration of the United States) have issued relevant regulations and standards.This from the inner wall of empty cans and printing coating film, may move into the food of various trace monomers and heavy metals, has more and more strict limits, which also forces people to find a better solution.
Environmental protection laminated steel
From the early 1990s, Germany and Japan, had found respectively identified which will be more clean and sanitary, slightly thicker (about 20 um) of polyester film, using the method of hot pressing, its compound on the surface of chrome plated sheet, instead of using the coating process of coating iron, in this way, greatly reduces the printing paint coating (mainly phenolic epoxy resin composition) of the harmful phenol, formaldehyde and other polymer monomer, migrate into the food. In addition, as a result of this laminated density and thickness have been improved, composite process and adhesion has also been increased by more than three times, so its corrosion resistance by practical and our detection of double proof, are far better than the internal coating film. Therefore, it has a great advantage in food hygiene.
In 2006, Japan has closed the country’s almost dozens of printing and coating production lines (only one of the largest), at the same time, part of the European tinplate factory also announced the supply of lamination electroplating tinplate.
As a result of printing and coating process of environmental pollution is extremely serious, but laminated steel do not only has excellent corrosion resistance and health effects, but also minimal environmental pollution. These advantages make it become the “star of tomorrow” of empty food cans.
Drawn can Aluminum sheet
If we also consider that deformation printing can be realized with the help of auxiliary design, and a large number of applications of high-speed precision punch, the large number of empty food cans from three pieces to a much simpler process of shallow flushing two pieces of cans, is another important trend. These advantages are more obvious in small cans, so it is unwise to buy a three-piece can production line now.
Another significant development in food canning is the extensive use of another important material: aluminum alloy sheets.This is because in recent years, the price of aluminum alloy thin plate used for canning has fallen rapidly, so the enterprise considers the price, corrosion resistance, beauty and other aspects comprehensively.Developed countries, not only aluminum deep – flush cupping, shallow – flush aluminum two – piece cans are more and more popular. At this point, if you take into account a lot of products, because there are good conditions for transport, then the use of easy peel off ends EPE(aluminum, iron), canning costs will be reduced, and the quality will be improved, also be lighter.
The manufacturing, and processing aluminum alloy sheet is always with advanced aviation industry, so whether to make all kinds of ingredients of aluminum magnesium or aluminum manganese alloy cold rolled sheet, or shallow, deep stamping of can body, or a variety of easy open lid or tank bottom, the body process,is more simple than the production of tinplate and more easy to control, also are more likely to get better results.
Easy peel off ends (EPE)
Tinplate or aluminum alloy plate EPE, because it needs to rush out of a rivet, at the same time do not destroy the local material, and ensure that the depth of open indentation is uniform and easy to open, so the requirements for the use of thin plate is very strict, especially when the use of secondary cold rolled plate (thickness of 0.16mm or less). Therefore, in recent 10 years, developed countries have gradually turned to the simplest aluminum foil or steel foil with a thickness of only about 50um. The core technology here, of course, is the method of sealing.
Specific sealing method, in addition to the common adhesion (with food grade adhesives) method, for the negative pressure of food cans, or positive pressure of beer and so on, volume sealing is a better method of sealing fastness.
The invention and improvement of ultra-thin tinplate and chrome plate as well as laminated steel and aluminum plates are innovations in canning materials. This shows that the innovation of materials is the basis of the improvement and innovation of food empty cans and canning technology. Therefore, the pursuit of more sanitary, safer, lighter, more beautiful materials, through the new process, new technology, new equipment, to make new empty cans, is the direction of the industry.