Materials Focus: Aluminium foil
From keeping Easter eggs fresh to insulating hot water pipes, uses for aluminium foil are nothing if not diverse, as Maria Felice finds out.
Aluminium foil is a very thin sheet with a maximum thickness of 0.2mm. While many people refer to it as tin foil, tin was in fact replaced by aluminium as the metal used for the production of foil more than 100 years ago. It is more malleable than tin and avoids the pitfalls of imparting a metallic taste or black stain.
In 1910, Robert Victor Neher took out a patent for the continuous rolling process for aluminium and opened the first plant for this purpose in Switzerland. By 1911, Tobler began wrapping its chocolate bars, including the much-loved Toblerone, in aluminium foil, and by the mid 1930s aluminium foil was being sold to households in rolls and sheets. There have been various milestones since, including freezer affordability in the 1950s, which saw the advent of TV dinners in aluminium foil trays. In the 2000s, a weight saving of 30% was achieved, which made the use of aluminium foil more efficient than ever.
According to the European Aluminium Foil Association, 75% of European production of the material is used for packaging and household foil, and 25% for technical applications. Last year, 818,400 tonnes of aluminium foil was produced in Europe – an increase of 2.1% from 2011. Most aluminium foil is made from pure aluminium, but increasingly alloys are used to improve properties and reduce the thickness required. There are two main ways of making the foil – one is to hot roll ingots to coils of thicknesses of 2–4mm and then cold roll these. The other is to employ continuous casting and make the coils straightaway from molten aluminium, and then cold roll these. To obtain the thinnest kind of aluminium foil, two sheets are rolled simultaneously and then separated. You can tell which foil has been made like this because one side will be shiny and the other matt – the matt side is the one that is on the inside during the rolling process.
Aluminium foil is used in a variety of markets and one perhaps less familiar use is in honeycomb-cored panels and structures. These are used in aerospace, architecture and even road signs. Sandwiched between aluminium sheets, layers of aluminium foil are joined together and expanded into an open-celled honeycomb structure. This provides an extremely strong yet light core that outperforms competitive materials.
Two fantastic properties of aluminium foil that we often take for granted are its formability and its hygiene. When pressed into a dish, aluminium foil retains the shape of the mould, and the foil’s thickness, alloy and temper are carefully selected so that it does this as well as possible. Due to the high temperatures employed during annealing, aluminium foil is completely sterile at the point of production. In addition, due to its smooth surface it does not harbour or promote the growth of bacteria. During use, aluminium foil is a barrier to light, gases and moisture, making it ideal for covering food and drink.
Aluminium is fully recyclable and can be done so repeatedly without any loss of quality. This holds for aluminium foil, too, which must be cleaned before being collected for recycling. When unsure of whether certain wrappers and packaging are in fact aluminium foil, perform the scrunch test – if the material springs back when scrunched, then it cannot be recycled.
Most crisp packets spring back because they are made from plastic film coated in a very thin layer of metal. The recycling process for aluminium requires just 5% of the energy used in its primary production. Packagers realise the value of this and increasingly consumers are cooperating and separating aluminium foil for recycling. In 2010, for the first time, 50% of aluminium foil containers used in Europe were recycled. The estimated average recycling rate for all aluminium packaging (including beverage cans) is above 55% in Europe, varying from 30% to 80% in different countries.
Finally, aluminium foil prolongs the safe life of many products, preventing spoilage which in turn saves resources and energy. Not that Easter eggs last very long, anyway...