Aluminium foil: Wikis

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Large roll of aluminium foil, with micrometer showing a thickness of 0.0005 inches (0.013mm)

Aluminium foil is aluminium prepared in thin metal leafs, with a thickness less than 0.2 mm / 0.008 in, although much thinner gauges down to 0.006 mm are commonly used.[1] Foils are commonly gauged in Mils. 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 often laminated to other materials such as plastics or paper to make them more useful. It replaced tin foil in the mid 20th century.

Annual production of aluminium foil was approximately 800,000 tonnes in Europe[1] and 600,000 tonnes (1.3 billion lbs) in the USA in 2003.[2] 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).[2]

In North America, aluminium foil is known as aluminum foil, and sometimes alternatively as al-foil or alu-foil. It is also often called Reynolds wrap after Reynolds Metals, the leading manufacturer in North America. Metallised films are sometimes mistaken for aluminium foil, but are actually polymer films coated with a thin layer of aluminium.

Contents

History

household aluminium foil
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Before aluminium foil

Foil made from a thin leaf of tin was commercially available before its aluminium counterpart. In the late nineteenth century and early twentieth century, tin foil was in common use, and some people continue to refer to the new product by the old name. Tin foil is much stiffer than aluminium foil. It tends to give a slight tin taste to the 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.

The first aluminium foil

Tin was first replaced by aluminium in 1910, when the first aluminium foil rolling plant, “Dr. Lauber, Neher & Cie. and Emmishofen”[3] was opened in Kreuzlingen, Switzerland. The plant, owned by J.G. Neher & Sons, the aluminium manufacturers, started in 1886 in Schaffhausen and 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 in December 1907.

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, color, 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. To maintain a constant thickness in aluminium foil production, beta radiation is passed through the foil to a sensor the other side. If the intensity becomes too high, then the rollers adjust, increasing the thickness. If the intensities become too low and the foil has become too thick, the rollers apply more pressure, causing the foil to be made thinner. The continuous casting method is much less energy intensive and has become the preferred process.[5] For thicknesses below 0.025 mm (0.001 in), two layers are usually put together for the final pass and afterwards separated which produces foil with one bright side and one matte side.[6] The two sides in contact with each other are matte and the exterior sides become bright, this is done to reduce tearing, increase production rates, control thickness, and get around the need for a smaller diameter roller.[6]

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 oxygen and water. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process.

Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass. It is nearly impossible to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny. This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe that the different properties keep heat out when wrapped with the matte finish facing out, and keep heat in with the matte finish facing inwards, the actual difference is imperceptible without instrumentation [7]. The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80% [4].

Uses

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, and tobacco, 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 [8]. 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.

EMF Shielding

The typical shielding effectiveness of aluminium foil for a 100MHz Spectrum is 80dB per standard Mil. It will block all broadcast radio waves when completely surrounding the receiver and absorb free standing waves when grounded.

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 microwave ovens has been corrected so microwave energy cannot be reflected back into the magnetron, and aluminium packages designed for microwave heating are available.[9]

Art and decoration

Aluminium foil 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.

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, and traces of the plastics from plastic bags may also taint the sample. Foil provides a seal to the ingress of organic solvents and does not taint the sample. Foil is also used extensively in geochemical laboratories to provide a barrier for the geochemist, and for sample storage.

Polishing steel

A simple and inexpensive way to remove rust from and polish steel surfaces by hand is to rub it with aluminium foil dipped in water. The aluminium foil is softer than steel, and will not scratch the surface. As heat is generated by rubbing friction, the aluminium will oxidize to produce aluminium oxide. Aluminium has a higher reduction potential than iron, and will therefore leach oxygen atoms away from any rust on the steel surface. Aluminium oxide is harder than steel, and the microscopic grains of aluminium oxide produced creates a fine metal polishing compound that smoothes the steel surface to a bright shine.

Ribbon Microphones

The material use in many ribbon microphones is aluminium leaf or "imitation silver leaf" as it is sometimes called. This is pure aluminium and is around .6-2.0 micron thick. It is in fact virtually the same material that the BBC used on Coles ribbons, with the exception that they also hand beat the leaf even more. They did this by sandwiching the ribbon between toilet paper and beating with a ball peen hammer. This "cold forges" the leaf. Corrugations must also be imparted into the ribbon, Coles used 25 per inch, RCA 44BX has 19 per inch (and is around 2" long) and the RCA 77 has 13 corrugations per inch. RCA ribbon material is around 1 - 1.5 micrometers (microns) or .00005 inch. The new Nady ribbon plus AEA both clearly advertise the fact they use 2 micron aluminium ribbon in their mics.

Environmental issues

The extensive use of aluminium foil has been criticized by some environmentalists because of the high resource cost of extracting aluminium, primarily as a result of the large amount of electricity used to the process bauxite ore. 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,[10] although many aluminium laminates are not recycled due to difficulties in separating the components and low yield of aluminium metal.

See also

References

  1. ^ a b European Aluminium Foil Association
  2. ^ a b Aluminum Association (USA)
  3. ^ http://inventors.about.com/library/inventors/blaluminum.htm
  4. ^ a b Hanlon, J. (1992). 1st ed. Handbook of Package Engineering, Lancaster, PA and Technomic Publishing: ISBN 0-87762-924-2. Chapter 3 Films and Foils.
  5. ^ 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.
  6. ^ a b Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, p. 386, ISBN 0-471-65653-4 .
  7. ^ "Frequently Asked Questions". http://www.reynoldspkg.com/reynoldskitchens/en/faq_detail.asp?info_page_id=743&prod_id=1789&cat_id=1337. Retrieved 2009-12-24. 
  8. ^ Examples of products
  9. ^ 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
  10. ^ Asia-Pacific Partnership on Clean Development and Climate. "Action Plan, page 5, table 2: 4.2 vs. 0.19". http://asiapacificpartnership.org/english/tf_aluminium.aspx. 

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