Food aluminum foils
100 years of aluminum packaging
An aluminum foil is aluminum rolled up to thickness 0,2 mm. It can be found literally in any home: sparkling and silvery. It is impossible to imagine how without this product it was possible: to keep food fresh, cook barbecue, bake potatoes in their skins. At the table you encounter a pack of butter in foil and cover yogurt. In the bathroom, almost all creams and lotions are also in aluminum packaging. Interesting, the first time was the aluminum foil in the packaging 1911 year - more 100 years ago, when it replaced the tin foil /1/.
Figure 1 – Packaging food grade aluminum foil [1]
Figure 2 – Product, packed in aluminum foil [1]
Advantages of aluminum foil as a packaging material
The benefits include [2]:
• Appearance - attractive.
• Stability - aluminum forms an oxide layer on contact with air, preventing further oxidation. It is also inert and does not form toxic compounds when exposed to most chemicals., including most food and cosmetics.
• Barrier properties - sufficient “thick” foils form a complete barrier to gases and water. Moreover, aluminum reflects light, making it a suitable material for protecting photosensitive products.
• Hygienic properties - the smooth metal surface of aluminum does not absorb moisture. It can be easily cleaned and sterilized
• Formability - Aluminum's ductility makes it easy to form.
• Conductivity - aluminum conducts electricity and heat, which makes it suitable for such applications, how to induction heat seal food containers.
• Recyclable – aluminum can be recycled at a relatively low cost. It takes about 5 % energy, necessary for the production of primary aluminum.
Disadvantages of aluminum foil as a packaging material
The disadvantages include [2]:
• Pure aluminum loses strength significantly at temperatures above 150°C. This means that a protective coating is required., if it is subjected to further heat treatment, for example, when cooking in the oven.
• Despite, that aluminum is well molded, its high plasticity means, that aluminum foil is easily torn or pierced.
• For thicknesses less than 17 micrometer foil can also have "punctures" - tiny holes, caused by impurities in the metal or technological deviations. Pin holes allow water and gas to penetrate the metal. Consequently, it may be necessary to add a protective coating or include foil as part of the laminate (composite layer) to increase strength, prevent flex cracking and resist punctures [2].
Aluminum alloys and their states
Commercial grade aluminum is used for most foil applications., which consists of more than 99 % Al with additives up to 0,8 % Fe and about 0.25–0.3 % Si. Fully annealed (O-tempered) such Al technical purity series 1xxx, including well-known variants AA1050 and AA1200, have relatively low strength.. Higher strength – for example, to reduce the thickness – can be achieved due to the increased content of Fe, for example, in 8xxx series alloys, such as 8079 [3].
Aluminum foil production
The starting materials for the production of foil are ingots weighing from 10 to 25 tonnes. These roll out to sheets with a thickness of about 3-6 mm. At the last stage of rolling, sheets are fed in rolls in the annealed state O, so that they can be easily processed.
Aluminium foil, usually, has a thickness of up to 150 mm. foil is only thick 6,3 mm. More “thick” foil (> 17 µm) provides an absolute barrier to gases and liquids. Note, what for 9 mm foil typical water vapor transfer rate is 0,3 g/m2 of 24 hours at 38 °C and 90% RH [2].
For the production of aluminum foil, two main processes are used [2]:
• The conventional rolling-mill method of producing reroll aluminium stock and ultimately, aluminium foil (Fig. 1)
• Continuous casting or ‘hot-strip’ casting to produce aluminium reroll stock and ultimately, aluminium foil (Fig. 2).
Figure 1 – The conventional rolling-mill method of producing reroll aluminium stock and ultimately, aluminium foil [2].
Figure 2 – Continuous casting or ‘hot-strip’ casting to produce aluminium reroll stock and ultimately, aluminium foil [2].
- Since the conventional rolling method of foil production was established much earlier, it is still widely used. When rolling into foil, intermediate rolled products must be annealed several times between passes of the rolling mill to overcome strain hardening and restore machinability [2].
- The most economical way to produce aluminum foil is continuous casting.. A typical continuous casting production line runs directly from the furnace to the winder.. The system continuously supplies, pours out, cools and winds the roll material. Because it heats up during production, continuous rolling does not require re-annealing in the production of foil [2].
On pic. 3 shows a typical temperature-time graph in the production of foil [3].
Fig. 3 Typical temperature/time schedules for the production of foil [3]
Why is one side of aluminum foil matte?
To increase productivity and reduce the risk of damage to extremely thin foils, at the last stage of rolling, usually less than 40 m, foil doubles, so that the two layers of foil pass through the rolls together. That's why these foils have a shiny polished surface., given by rotating cylinders on one side, while the other side has a matte surface. Most aluminum foil products are applied in O state, which means, that the foil should eventually be soft annealed. Beyond recrystallization, this is required to remove rolling lubricants from the foil [3].
Sources:
1. Aluminium Foil – Multitalanted Lightweight Packing /GDA, Proceedings of the, 2014
2. Aluminium foil packaging /J. Kerry // Packaging technology. Fundamentals, materials and processes – Woodhead Publishing Limited, 2012
3. Design of Aluminum Rolling Processes for Foil, Sheet, and Plate /J.H. Driver, O. Engler // Encyclopedia of Aluminum and Its Alloys – Ed. G.E. Totten, M. Tiryakioğlu, and O. Kessler – 2019
