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Aluminium foil

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Aluminium foil (known as aluminum foil in North America) is aluminium prepared in thin sheets with a thickness less than 0.2 mm / 0.008 in, although much thinner gauges down to 0.006 mm are commonly used ^[1]. As a result of this, the foil is extremely pliable, and can be bent or wrapped around objects with ease. However, thin foils are fragile and easily damaged, and are usually laminated to other materials such as plastics or paper to make them useful.

Millions of tonnes of aluminium foil are produced annually, with production of approximately 800,000 tonnes in Europe^[1] and 600,000 tonnes (1.3 billion lbs) in the USA^[2] in 2003. Approximately 75% of aluminium foil is used for packaging of foods, cosmetics and chemical products, and 25% used for industrial applications (eg. thermal insulation, cables and electronics) ^[1] ^[3].

Aluminium foil is sometimes known as al-foil or alu-foil. It is also often called tinfoil (although it is not made from tin), silver paper (although it is not made from silver), or in North America as Reynolds wrap after Reynolds Metals, the leading manufacturer when it was introduced on the American market (much to the chagrin of Alcoa, Reynolds' main competitor, which had its brand "Alcoa Wrap" referred to as "Alcoa Reynold's Wrap")^{[citation needed]}. Metallised films are sometimes mistaken for aluminium foil, but are actually polymer films coated with a thin layer of aluminium.

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History

Foil made from a thin leaf of tin was commercially available before its aluminium counterpart. In the late 19th century and early 20th century, tin foil was in common use, and some people continue to refer to the new product by the name of the old one. Tin foil is stiffer than aluminium foil. It tends to give a slight tin taste to food wrapped in it, which is one major reason it has largely been supplanted by aluminium and other materials for wrapping food.

The first audio recordings on phonograph cylinders were made on tin foil.

Tin was first replaced by aluminium in 1910, when the first aluminium foil rolling plant, “Dr. Lauber, Neher & Cie., Emmishofen.” was opened in Kreuzlingen, Switzerland. The plant, owned by J.G. Neher & Sons (aluminium manufacturers) started in 1886 in Schaffhausen, Switzerland, at the foot of the Rhine Falls - capturing the falls’ energy to produce aluminium. Neher's sons together with Dr. Lauber discovered the endless rolling process and the use of aluminium foil as a protective barrier ")^{[citation needed]}.

The first use of foil in the United States was in 1913 for wrapping Life Savers, candy bars and gum ^[4]. Processes evolved over time to include the use of print, colour, lacquer, laminate and the embossing of the aluminium.

Manufacture

Aluminium foil is produced by rolling sheet ingots cast from molten aluminium then re-rolling on sheet and foil rolling mills to the desired thickness, or by continuously casting and cold rolling. The continuous casting method is much less energy intensive and has become the preferred process ^[5]. For thicknesses below 0.025 mm (0.01 in), two layers are usually put together for the final pass which produces foil with one bright side and one matte side. The two sides in contact with each other are matte and the exterior sides become bright. This done to reduce tearing, increase production rates and to help control thickness.

Some lubrication is needed during the rolling stages, otherwise the foil surface can become marked with a herringbone pattern. These lubricants are sprayed on the foil surface before passing through the mill rolls. Kerosene based lubricants are commonly used, although oils approved for food contact must be used for foil intended for food packaging.

Aluminium becomes work hardened during the cold rolling process and is annealed for most purposes. The rolls of foil are heated until the degree of softness is reached, which may be up to 340°C for 12 hours. During this heating, the lubricating oils are burned off leaving a dry surface. Lubricant oils may not be completely burnt off for hard temper rolls, which can make subsequent coating or printing more difficult.

Properties

Aluminium foils thicker than 0.025 mm (0.001 in) are impermeable to light, gases (including oxygen) and water vapour ^[5]. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process.

Aluminium foil typically has a highly reflective side and a more matte side. This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe that the shiny side's reflective properties keep heat in when wrapped on the interior and keep heat out when facing exterior, the actual difference is imperceptible without instrumentation ^[6]. The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80% ^[4].

Uses

Food and pharmaceutical packaging

As aluminium foil acts as a complete barrier to light and oxygen (which cause fats to oxidise or become rancid), odours and flavours, moisture, and bacteria, it is used extensively in food and pharmaceutical packaging. Aluminium foil is used to make long life packs (aseptic packaging) for drinks and dairy products which enables storage without refrigeration . Aluminium foil laminates are also used to package many other oxygen or moisture sensitive foods, in the form of pouches, sachets and tubes, and as tamper evident closures. Aluminium foil containers and trays are used to bake pies and to pack takeaway meals, ready snacks and long life pet foods.

Aluminium foil is widely sold into the consumer market, usually in rolls of around 50 centimetres width and several metres in length. It is used for wrapping food in order to preserve it, for example when storing leftover food in a refrigerator (where it serves the additional purpose of preventing odour exchange), when taking sandwiches on a journey, or when selling some kinds of take-away or fast food. Tex-Mex restaurants in the United States, for example, typically provide take-away burritos wrapped in aluminium foil.

Insulation

Aluminium foil is also widely used for thermal insulation (barrier and reflectivity), heat exchangers (heat conduction) and cable liners (barrier and electrical conductivity). Foils in special alloys are even used for structural honeycomb components for aircraft. Aluminium foil's heat conductive qualities make it a common accessory in hookah smoking: a sheet of perforated aluminium foil is frequently placed between the coal and the tobacco, allowing the tobacco to be heated without coming into direct contact with the burning coal.

In one year's April Fool's joke, a Dutch television news station reported that the government had introduced a new way to detect hidden televisions (in many countries in Europe, one must pay a television licence to fund public broadcasting) by simply driving through the streets with a new detector, and that the only way to keep one's television from being detected was to wrap it in aluminium foil.^{[citation needed]}

Cooking

Aluminium foil is also used for barbecuing more delicate foods such as mushrooms and vegetables; food is wrapped in foil then placed on the grill, preventing loss of moisture that may result in a less appealing texture.

As is the case with all metallic items, aluminium foil reacts to being microwaved. This is due to the effect of electric fields of the microwaves causing a build up of charge to form between the sharp points in the aluminium; if enough charge accumulates it will discharge to a different place on the foil, creating a spark (i.e. arcing). Due to frequent use in food services, this commonly leads to kitchen fires^{[citation needed]}. The design of modern microwaves has been corrected so microwave energy cannot be reflected back into the magnetron, and aluminium packages designed for microwave heating are available ^[7]. However, many consumers remain reluctant to use metal containers in a microwave oven.

It is also unwise to cook a highly acidic food in an iron vessel, then cover the leftovers in aluminium foil, because if the foil touches the food, a simple battery is created. The foil rapidly dissolves into the food, leaving dull grey dregs.

Art and decoration

Heavier foils made of aluminium are used for art, decoration, and crafts, especially in bright metallic colours. Metallic aluminium, normally silvery in colour, can be made to take on other colours through anodization. Anodizing creates an oxide layer on the aluminium surface that can accept coloured dyes or metallic salts, depending on the process used. In this way, aluminium is used to create an inexpensive gold foil that actually contains no gold, and many other bright metallic colours. These foils are sometimes used in distinctive packaging.

Electronics

Foils made from pure aluminium are used in the production of capacitors in the electronics industry.

Deterring pets

Aluminium foil is also sometimes used in the training of cats; as cats have an inborn dislike of loud noise, like that caused by sheets of aluminium foil, it is possible to prevent cats from jumping on or otherwise damaging furniture by covering its surfaces.^{[citation needed]}.

Geochemical sampling

Foil is used by organic/petroleum geochemists for protecting rock samples taken from the field and in the lab, where the sample is subject to biomarker analysis. While plastic or cloth bags are normally used for a geological sampling exercise, cloth bags are permeable and may allow organic solvents or oils (such as oils imparted from the skin) to taint the sample, whereas traces of the plastics from plastic bags may also taint. Foil provides a seal to the ingress of organic solvents and does not taint the sample. Foil is also used extensively in a geochemical laboratory to provide a barrier to the geochemist and for sample storage.

Criticism

The extensive use of aluminium foil has been criticised by some environmentalists because of the high resource cost of extracting aluminium, primarily as a result of the large amount of electricity used to decompose bauxite. However, this cost is greatly reduced via recycling, reduced energy requirements during transport due to lighter weight packages, and the fact that many foods that would otherwise perish can be protected over long periods without refrigeration. Many aluminium foil products can be recycled at around 5% of the original energy cost^{[citation needed]}, although many aluminium laminates are not recycled due to difficulties in separating the components and low yield of aluminium metal.

References

^ ^a ^b ^c European Aluminium Foil Association
^ Aluminum Association (USA)
^ Aluminum Association (USA)
^ ^a ^b Hanlon, J. (1992). 1st ed. Handbook of Package Engineering, Lancaster, PA, Technomic Publishing: ISBN 0-87762-924-2. Chapter 3 Films and Foils
^ ^a ^b Robertson, G. (2006). 2nd ed. Food Packaging, Principles and Practise, Boca Raton, FL, Taylor & Francis Group: ISBN 0-8493-3775-5. Chapter 7 Metal Packaging Materials
^ Frequently Asked Questions. Retrieved on 2007-08-14.
^ Huss G. (1997) Microwaveable Packaging and Dual-Ovenable Materials in The Wiley Encyclopedia of Packaging Technology, 2nd ed, eds Brody A. Marsch K. New York, John Wiley and Sons