The consumption of aluminium foil is increasing, driven by the availability of new alloys which improve the performance of the products, widening the areas of application and the efficiency of the production process. EAFA statistics and surveys confirm the dynamism of the industry
by Giuseppe Giordano
Foil stock is defined as rolled sheets of aluminium and its alloys with a thickness of less than 0.2 mm (200 microns). In spite of the limits imposed by the concise definition, the product appears different from other rolled metals. Indeed, it can be seen that the foil has an upper limit of thickness but no lower limit because this is a constantly evolving goal. Currently the minimum thickness limit of the industrial product is just over 0.006 mm (6 micron), but in some niche applications 5 micron thick sheet is already being used, and the most modern rolling mills have as a mechanical limit the possibility of rolling even thinner sheets.
The use of increasingly thin foil is the main contribution of the product to the development of an environment-friendly economy. The downgauging process has involved all applications, from packaging to foil for insulation board panels. For instance, polylaminate material containers for aseptic milk or beverage packaging, when they appeared on the market in the Sixties, used foil 0.012 mm (12 microns) thick. The same type of barrier effect to prolong shelf life is currently achieved with foil half as thick, that is, far less than the thickness of a hair.
Foil has another interesting property: its consumption grows as the general well-being in the target market increases. Consumption in more advanced countries is much higher than in less evolved markets and the development of these is accompanied by the growth of all applications of foil.
Foil production is growing in Europe
It is very interesting to analyse the data offered by EAFA- European Aluminium Foil Association taking the last five years as a reference. It should be remembered that this five-year period refers to a wider region than the countries of the European Union. EAFA statistics from 2014 cover the 28 EU countries plus Armenia, Bosnia-Herzegovina, Macedonia, Montenegro, Norway, Russia, Serbia, Switzerland and Turkey.
The overall figure for foil production in 2018 shows a figure of over 900 Kton and a continuous increase during the last 5 years of about 10% (Figure 1). In 2019, again according to EAFA quarterly surveys, growth will be more limited, due to the continuing general uncertainty on global markets and the continuation of the China/US tariff discussion.
The production structure of the industry includes 20 producers in the EAFA perimeter with production concentrated in around 30 plants in 15 different countries (Figure 2).
EAFA statistics allow us to deepen our analysis. First of all, about 10% of production is exported outside the already vast EAFA region. However, import of Chinese products is growing, and in recent years this has come to be similar in quantity to the export quota mentioned above. The Chinese product exported to Europe is of high quality and for the best part obtained using machinery made in Europe.
The main applications of foil are found in the packaging sector where thin and very thin foils are especially used. The graph in Figure 3 shows foil consumption in the EAFA area in the year 2017, subdivided into different end-use industries.
New alloys for high performance products
Foil stock is obtained by cold rolling a sheet between 300 and 600 microns thick. Two processes are used to achieve the desired foil thickness. In the most recent one, the sheet to be transformed into foil stock is produced using a continuous casting process and subsequent deformation. In the traditional process this sheet is produced starting from plates with an initial thickness of about 400 mm, which, after removal of oxides and other surface unevenness, are first heated and hot rolled and then cold rolled to the typical thickness of foil stock as seen before. From this point onwards the foil is subjected to a specific rolling process that develops through a deformation process on working cylinders with mirror ground surface. Rolling is carried out on a single sheet up to a thickness of about 50 microns (0.05 mm). From here up to the minimum thickness (for example 6 microns) the foil is rolled “doubled up” in order to have better dimensional allowances on the surface of the cylinders and on their relative positioning. Before the end of the working cycle, the sheet is “unfolded” and rewound as a single sheet.
The starting material for almost all products is not pure aluminium but its alloys. In foil production, the use of different alloys has recently developed to exploit the difference in chemical composition, so as to achieve specific performances both during the process and on the finished product. Even at the end of the Nineties only alloys of the 1xxx series (with a minimum aluminium content of 99.0%) were used for the production of foil without seeking special product performances by means of the development of specific alloys. Since the end of the last century, the development of the use of 8xxx (Al-Fe-Si) alloys has allowed to obtain both high product performance and improved process conditions, as mentioned before, for instance by enabling an increase in the average rolling speed at lower thicknesses.
The main difference in composition between 1xxx and 8xxx alloys is given by the iron content. It is well known that iron, together with silicon, is the most easily found impurity in aluminium. The solubility of iron in liquid aluminium is higher than in the solid state, when most of this element is present in intermetallic compounds. The number, shape and distribution of the particles affect the annealing and recrystallization processes. Particularly, as the iron content increases, the time required to achieve complete annealing also increases, an effect that may also be expressed as an increase in the recrystallization temperature. This behaviour of high Fe% alloys has been the key for the success of the ENAW 8079 alloy, one of the first 8xxx specifically used for foil production and possibly the most widely used at the moment. The advantages determined by the behaviour described above may be found in the possibility to operate at higher temperature in the final annealing furnaces, guaranteeing a better evaporation of the rolling fluid and therefore a cleaner surface of the final product. At the same time a recovery process is avoided, which could make the foil’s performances uneven. It is therefore possible to use physical states with a high level of hardening even in processes with phases at a temperature much higher than room temperature. The composition of the main 8xxx alloys currently used for foil are shown in Table 1. It should however be noted that 1xxx alloys, such as ENAW 1050 and ENAW 1235 alloys, remain in use, mainly for products requiring a particularly bright foil surface as their main feature.