Steel and tin cans

An empty tin can

A steel can, tin can, tin (especially in British English, Australian English, Canadian English and South African English), or can is a container made of thin metal, for distribution or storage of goods. Some cans are opened by removing the top panel with a can opener or other tool; others have covers removable by hand without a tool. Cans can store a broad variety of contents: food, beverages, oil, chemicals, etc.

Steel cans were traditionally made of tinplate; the tin coating stopped the contents from rusting the steel. Tinned steel is still used, especially for fruit juices and pale canned fruit. Modern cans are often made from steel lined with transparent films made from assorted plastics, instead of tin. Some are made from aluminum. Early cans were often soldered with neurotoxic high-lead solders. High-lead solders were banned in the 1990s in the United States, but smaller amounts of lead were still often present in both the solder used to seal cans and in the mostly-tin linings.

Metal cans are still (inaccurately) widely called "tin cans"; in some dialects, even aluminium cans are called "tin cans".

Cans are highly recyclable with around 65% of steel cans are recycled.

History

The tin canning process was conceived by the Frenchman Philippe de Girard, who had British merchant Peter Durand patent the idea in 1810. The canning concept was based on experimental food preservation work in glass containers the year before by the French inventor Nicholas Appert. Durand did not pursue food canning, but, in 1812, sold his patent to two Englishmen, Bryan Donkin and John Hall, who refined the process and product, and set up the world's first commercial canning factory on Southwark Park Road, London. By 1813 they were producing their first tin canned goods for the Royal Navy. By 1820, tin canisters or cans were being used for gunpowder, seeds, and turpentine.

Early tin cans were sealed by soldering with a tin–lead alloy, which could lead to lead poisoning.

In 1901 in the United States, the American Can Company was founded, at the time producing 90% of the tin cans in the United States.

Canned food in tin cans was already quite popular in various countries when technological advancements in the 1920s lowered the cost of the cans even further.: 155–170, 265–280  In 1935, the first beer in metal cans was sold; it was an instant sales success.: 155–170, 265–280 

Description

Most cans are right circular cylinders with identical and parallel round tops and bottoms with vertical sides. However, in cans for small volumes or particularly-shaped contents, the top and bottom may be rounded-corner rectangles or ovals. Other contents may suit a can that is somewhat conical in shape.

Fabrication of most cans results in at least one rim—a narrow ring slightly larger than the outside diameter of the rest of the can. The flat surfaces of rimmed cans are recessed from the edge of any rim (toward the middle of the can) by about the width of the rim; the inside diameter of a rim, adjacent to this recessed surface, is slightly smaller than the inside diameter of the rest of the can.

Three-piece can construction results in top and bottom rims. In two-piece construction, one piece is a flat top and the other a deep-drawn cup-shaped piece that combines the (at least roughly) cylindrical wall and the round base. Transition between wall and base is usually gradual. Such cans have a single rim at the top. Some cans have a separate cover that slides onto the top or is hinged.

Two piece steel cans can be made by "drawing" to form the bottom and sides and adding an "end" at the top: these do not have side seams. Cans can be fabricated with separate slip-on, or friction fit covers and with covers attached by hinges. Various easy opening methods are available.

In the mid-20th century, a few milk products were packaged in nearly rimless cans, reflecting different construction; in this case, one flat surface had a hole (for filling the nearly complete can) that was sealed after filling with a quickly solidifying drop of molten solder. Concern arose that the milk contained unsafe levels of lead leached from this solder plug.

Advantages of steel cans

A number of factors make steel cans ideal containers for beverages. Steel cans are stronger than cartons or plastic, and less fragile than glass, protecting the product in transit and preventing leakage or spillage, while also reducing the need for secondary packaging.

Steel and aluminium packaging offer complete protection against light, water and air, and metal cans without resealable closures are among the most tamper-evident of all packaging materials. Food and drink packed in steel cans has equivalent vitamin content to freshly prepared, without needing preserving agents. Steel cans also extend the product's shelf-life, allowing longer sell-by and use-by dates and reducing waste.

As an ambient packaging medium, steel cans do not require cooling in the supply chain, simplifying logistics and storage, and saving energy and cost. At the same time, steel's relatively high thermal conductivity means canned drinks chill much more rapidly and easily than those in glass or plastic bottles.

A World Steel Association initiative, Choose Steel, is encouraging the use of steel for beverage cans.

Materials and health issues

Tin

No cans currently in wide use are composed primarily or wholly of tin. Until the second half of the 20th century, almost all cans were made of tinplate steel. The steel was cheap and structurally strong, but prone to rust; the tin coating prevented the wet food from corroding the steel. Corrosion-resistant coatings on almost all steel food cans are now made from plastic, not tin. Some manufacturers use Vitreous enamel, instead.[better source needed]

Dissolution of tin into the food

Tin is corrosion resistant, but acidic food like fruits and vegetables can corrode the tin layer. Nausea, vomiting, and diarrhea have been reported after ingesting canned food containing 200 mg/kg of tin. A 2002 study showed that 99.5% of 1200 tested cans contained below the UK regulatory limit of 200 mg/kg of tin, an improvement over most previous studies largely attributed to the increased use of fully lacquered cans for acidic foods, and concluded that the results do not raise any long term food safety concerns for consumers. The two non-compliant products were voluntarily recalled.

Evidence of tin impurities can be indicated by color, as in the case of pears, but lack of color change does not guarantee that a food is not tainted with tin.

Lead

Lead is harmful to health in any quantity, and more lead is more harmful than less lead. Infants and children are more severely affected, as lead harms brain deveolpment.

In November 1991, US can manufacturers voluntarily eliminated lead seams in food cans. Imported food cans continued to include lead soldered seams.

In 1995, the US FDA issued a rule prohibiting lead soldered food cans, including both domestic and imported food cans. Unfortunately, the FDA did not give a definition of "lead solder", or a quantitative limit to permissable lead levels, and some solders and tin linings used on tin cans still contained significant amounts of lead. In 2017, quantitative limits were set, but they are high enough to permit intentionally adding lead, and the FDA measurements show measurable levels of lead in many US canned foods in the 2010s.

Plastic linings

Many metal food cans are lined with plastic, to prevent the food from corroding the can. These linings can leach contaminants into the canned food. Some of these contaminants are substances with known health harms, though whether they are ingested in canned food in levels sufficient to cause harms is not known.

Among other substances, the plastic linings in food cans sometimes contain bisphenol A (BPA). Pregnant women who eat more canned food have higher levels of BPA in their urine.

Other constituents in can linings, including newer BPA-free can linings, have also been identified as having known health harms.

Bisphenol-A

The chemical compound Bisphenol A found in can linings "...is associated with organizational changes in the prostate, breast, testis, mammary glands, body size, brain structure and chemistry, and behavior of laboratory animals", unborn children and adults.

Bisphenol-A (BPA) is a controversial chemical compound present in commercially available tin can plastic linings and transferred to canned food. The inside of the can is coated with an epoxy coating, in an attempt to prevent food or beverage from coming into contact with the metal. The longer food is in a can, and the warmer and more acidic it is, the more BPA leaches into it. In September 2010, Canada became the first country to declare BPA a toxic substance. In the European Union and Canada, BPA use is banned in baby bottles. The FDA does not regulate BPA (see BPA controversy#Public health regulatory history in the United States). Several companies, like Campbell's Soup, announced plans to eliminate BPA from the linings of their cans, but have not said which chemical they plan to replace it with. (See BPA controversy#Chemical manufacturers reactions to bans.)[citation needed]

Canada

In 2016, BPA was common in food can linings in Canada. As of August 2008, Health Canada's Food Directorate concluded that "the current dietary exposure to BPA through food packaging uses is not expected to pose a health risk to the general population, including newborns and infants". They also stated that, as some animal studies had shown effects from low levels of BPA, they were seeking to make BPA levels in food packaged for infants and newborns (especially formula). They also cited a WHO review.

UK

In modern times, the majority of food cans in the UK have been lined with a plastic coating containing bisphenol A (BPA). The coating prevents acids and other substances from corroding the tin or aluminium of the can, but leaching of BPA into the cans contents was investigated as a potential health hazard. The UK Food STandards agency currently considers can-derived BPA levels to be acceptable, but is investigating its safe-level thresholds; it currently has a temproray threshold for BPS, as of 2024.

US

A 2016 market, survey using Fourier-transform infrared spectrums to identify materials, found BPA and other substances known to have health harms were common in food can linings in the US. A similar survey done by food can manufacturers in 2020 found BPA only in some imported cans; it did not discuss potential harms from lining substances other than BPA.

Labels

A can traditionally has a printed label glued to the outside of the curved surface, indicating its contents. Some labels contain additional information, such as recipes, on the reverse side. More recently labels are sometimes printed directly onto the metal before or after the metal sheet is formed into the individual cans.

Traditionally, labels were glued on with casein glue, which dissolved easily in hot water. Some other glues may make the label harder to remove for recycling.

Cans are also printed directly without a separate label.

Standard sizes

Cans come in a variety of shapes and sizes. Walls are often stiffened with rib bulges, especially on larger cans, to help the can resist dents that can cause seams to split.

Can sizes in the United States have an assortment of designations and sizes. For example, size 7/8 contains one serving of half a cup with an estimated weight of 4 ounces; size 1 "picnic" has two or three servings totalling one and a quarter cups with an estimated weight of 1012 ounces; size 303 has four servings totalling 2 cups weighing 1512 ounces; and size 10 cans, most widely used by food services selling to cafeterias and restaurants, have twenty-five servings totalling 13 cups with an estimated weight of 10312 ounces (size of a roughly 3 pound coffee can). These are U.S. customary cups, not British Imperial standard.

In the United States, cook books sometimes reference cans by size. The Can Manufacturers Institute defines these sizes, expressing them in three-digit numbers, as measured in whole and sixteenths of an inch for the container's nominal outside dimensions: a 307 × 512 would thus measure 3 and 7/16" in diameter by 5 and 3/4" (12/16") in height. Older can numbers are often expressed as single digits, their contents being calculated for room-temperature water as approximately eleven ounces (#1 "picnic" can), twenty ounces (#2), thirty-two ounces (#3), fifty-eight ounces (#5), and one-hundred-ten ounces (#10 "coffee" can).

Can Name Dimensions (inches) Capacity (U.S. fluid ounces) No. 2 can equivalent Typical products
6Z 2216 × 312 6.08 0.295
8Z Short 21116 × 3 7.93 0.386
8Z Tall 21116 × 328 8.68 0.422
No. I (Picnic) 21116 × 4 10.94 0.532
No. 211 Cylinder 21116 × 41416 13.56 0.660
No. 300 3 × 4716 15.22 0.741 Cranberry Sauce, Pork & Beans
No. 300 Cylinder 3 × 5916 19.40 0.945
No. I Tall 3116 × 41116 16.70 0.813
No. 303 3316 × 438 16.88 0.821 Fruits, Vegetables, Soups
No. 303 Cylinder 3316 × 5916 21.86 1.060
No. 2 Vacuum 3716 × 338 14.71 0.716
No. 2 3716 × 4916 20.55 1.000 Juices, Soups, Vegetables
Jumbo 3716 × 558 25.80 1.2537
No. 2 Cylinder 3716 × 568 26.40 1.284
No. 1.25 4116 × 238 13.81 0.672
No. 2.5 4116 × 41116 29.79 1.450 Fruits, Vegetables
No. 3 Vacuum 414 × 3716 23.90 1.162
No. 3 Cylinder 414 × 7 51.70 2.515
No. 5 518 × 558 59.10 2.8744 Fruit Juice, Soups
No. 10 6316 × 7 109.43 5.325 Fruits, Vegetables

In parts of the world using the metric system, tins are made in 250, 500, 750 ml (millilitre) and 1 L (litre) sizes (250 ml is approximately 1 cup or 8 ounces). Cans imported from the US often have odd sizes such as 3.8 L (1 US gallon), 1.9 L (1/2 US gallon), and 946 ml (2 US pints / 1 quart).

In the UK and Australia, cans are usually measured by net weight. A standard size tin can holds roughly 400 g; though the weight can vary between 385 g and 425 g depending on the density of the contents. The smaller half sized can holds roughly 200 g, typically varying between 170 g and 225 g.

Fabrication of cans

Inside of a tin can

Rimmed three-piece can construction involves several stages;

  • Forming a tube and welding or soldering the seam of the sides
  • Joining the bottom end to the tube
  • Printing or attaching labels to the can
  • Filling the can with content; sterilization or retorting is required for many food products
  • Joining the wall and top "end".

Double seam rims are crucial to the joining of the wall to a top or bottom surface. An extremely tight fit between the pieces must be accomplished to prevent leakage; the process of accomplishing this radically deforms the rims of the parts. Part of the tube that forms the wall is bent, almost at its end, turning outward through 90 degrees, and then bent further, toward the middle of the tube, until it is parallel to the rest of the tube, a total bend of 180 degrees.

The outer edge of the flat piece is bent against this toward the middle of the tubular wall, until parallel with the wall, turning inward through 90 degrees. The edge of bent portion is bent further through another 90 degrees, inward now toward the axis of the tube and parallel to the main portion of the flat piece, making a total bend of 180 degrees. It is bent far enough inward that its circular edge is now slightly smaller in diameter than the edge of the tube. Bending it yet further, until it is parallel with the tube's axis, gives it a total bend of 270 degrees. It now envelops the outward rim of the tube.

Looking outward from the axis of the tube, the first surface is the unbent portion of the tube. Slightly further out is a narrow portion of the top, including its edge. The outward-bent portion of the tube, including its edge, is still slightly further out. Furthest out is the 90-degree-bent portion of the flat surface.

The combined interacting forces, as the portion of the flat surface adjacent to the interior of the tube is indented toward the middle of the tube and then outward forward the axis of the tube, and the other bent portions of the flat piece and the tube are all forced toward the axis of the tube, drives these five thicknesses of metal against each other from inside and out, forming a "dry" joint so tight that welding or solder is not needed to strengthen or seal it. Illustrations of this process can be found on pages 20–22 of the FAO Fisheries Technical Paper 285 "Manual on fish canning".

Design and manufacture

Steel for can making

The majority of steel used in packaging is tinplate, which is steel that has been coated with a thin layer of tin, whose functionality is required for the production process. The tin layer is usually applied by electroplating.

Two-piece steel can design

Most steel beverage cans are two-piece designs, made from 1) a disc re-formed into a cylinder with an integral end, double-seamed after filling and 2) a loose end to close it. Steel cans are made in many different diameters and volumes, with opening mechanisms that vary from ring pulls and tab openers, to wide open mouths.

Drawn-and-ironed (DWI) steel cans

The process of re-forming sheet metal without changing its thickness is known as 'drawing'. Thinning the walls of a two-piece can by passing it through circular dies is called 'ironing'. Steel beverage cans are therefore generally referred to as drawn-and-ironed, or DWI, cans (sometimes D&I). The DWI process is used for making cans where the height is greater than the diameter, and is particularly suited to making large volumes of cans of the same basic specification.

Steel can wall thicknesses are now[when?] 30% thinner and weigh 40% less than 30 years ago, reducing the amounts of raw materials and energy required to make them. They are also up to 40% thinner than aluminium.

Magnetic properties

Steel is a ferrous metal and is therefore magnetic. For beverage packaging this is unique. This allows the use of magnetic conveyor systems to transfer empty cans through the filling and packing processes, increasing accuracy and reducing potential spillage and waste. In recycling facilities, steel cans may be readily separated from other waste using magnetic equipment including cross-belt separators, also known as overband magnets, and drum magnets.

Opening cans

The first cans were heavy-weight containers that required ingenuity to open, with implements such as knives. Not until several years later, after can manufacturers started using thinner metal sheets, were any dedicated can openers developed.

While beverage cans or cans of liquid such as broth can be punctured—as with a church key—to pour out the contents, solid or semisolid contents require removing one end of the can. This can be accomplished with a heavy knife or other sharp tool, but can openers are safer, easier, and more convenient.

Some cans, such as those used for sardines, have a specially scored lid so that the user can break out the metal by the leverage of winding it around a slotted twist-key. Until the mid-20th century, some sardine tins had solder-attached lids, and the twist-key worked by forcing the solder joint apart.

The advent of pull tabs in beverage cans spread to the canning of various food products, such as pet food or nuts (and non-food products such as motor oil and tennis balls). The ends are known as easy open lids because they open without any tools or implements. An additional innovation developed for specifically for food cans uses a tab that is bent slightly upwards, creating a larger surface area for easier finger access.

Cans can be made with easy open features. Some cans have screw caps for pouring liquids and resealing. Some have hinged covers or slip-on covers for easy access. Paint cans usually have a lid with an interference fit, removable and replaceable any number of times so the paint may be stored between uses.

Recycling and re-use

Steel from cans and other sources is the most recycled packaging material. Around 65% of steel cans are recycled. In the United States, 63% of steel cans are recycled, compared to 52% of aluminium cans. In Europe, the recycling rate in 2016 is 79.5%. Most can recycling occurs at the smelters, but individual consumers also directly reuse cans in various ways. For instance some people use two tin cans to form a camp or survival stove to cook small meals.[citation needed]

Food tin cans reused for art and storage

Sustainability and recycling of steel beverage cans

Steel recycling

Spike Jones with tin cans in the background

From an ecological perspective, steel may be regarded as a closed-loop material: post-consumer waste can be collected, recycled and used to make new cans or other products. Each tonne of scrap steel recycled saves 1.5 tonnes of CO2, 1.4 tonnes of iron ore and 740 kg of coal. Steel is the world's most recycled material, with more than 85% of all the world's steel products being recycled at the end of their life: an estimated 630 million tonnes of steel scrap were recycled in 2017, saving 945 million tonnes of CO2.

Steel can recycling

A steel can can be recycled again and again without loss of quality; however, for the food grade steel it's required to remove tin from the scrap metal, which is done by way of electrochemistry: the tin is leached from a high pH solution at low negative voltage.

Recycling a single can saves the equivalent power for one laundry load, 1 hour of TV or 24 hours of lighting (10W LED bulb).

Steel beverage cans are recycled by being melted down in an electric arc furnace or basic oxygen furnace.[citation needed]

Most steel cans also carry some form of recycling identification such as the Metal Recycles Forever Mark Recyclable Steel and the Choose Steel campaign logo. There is also a campaign in Europe called Every Can Counts, encouraging can recycling in the workplace

Smaller carbon footprint

All beverage packaging creates CO2 emissions at every stage in the production process, from raw material extraction, processing and manufacture through to recycling. However, steel cans are an ecological top performer, as cans can always be recycled. The steel industry needs the used cans and will use them in the production of new steel product. By recycling the cans and closing the loop, CO2 emissions are dramatically reduced. There is also the potential for higher global steel recycling rates as consumers become more aware of the benefits.[citation needed]

See also


This page was last updated at 2024-04-17 16:28 UTC. Update now. View original page.

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