More and more producers and manufacturers in search of environmentally packaging are using aluminium as it is one of the most recycled metals. 75% of aluminium that has so far been mined is still in use. It therefore fits well with European ambitions to move towards a circular economy. In this context it is important to find out the extent to which the coatings in aluminium impact its eco-friendly image.
Aluminium is an excellent packing material, providing a barrier against air, light, temperature, moisture micro-organisms, and odour. It is neutral in taste, and can contain volatile components and aroma. It is light in weight and is completely recyclable. 11% of all aluminium produced is used for packaging, of which, 8% are for cosmetics.
Aluminium packaging in Germany in % (Image Credits: GDA)
The film of oxide that aluminium forms with air acts as a protective layer that makes aluminium and its alloys resistant to corrosion. However, aluminium can be attacked by dilute acidic substances and Sulphur, while high concentrations of salts result in migration. Therefore, aluminium is usually coated to prevent migration in both directions.
It can be used without any coatings if the content is neutral in pH. Therefore aluminium without coatings is also used, especially in case of oils.
Only a small amount of aluminium can enter the body through dermal contact according to Agency For Toxic Substances and Disease Register, USA. However, since there are chances that on contact with human skin, it can lead to oxidative stress, it is advisable for aluminium cosmetic packages to be coated. Aluminium is also reported to be linked with various health problems when it gets accumulated over time through repeated exposure.
On the other hand aluminium is difficult to avoid as it is the third most abundant element on earth according to Scientific American. However, with an aim to maintain the best quality standards for consumers, aluminium cans, tins, tubes and aerosol cans are coated.
Coats are applied in molten form, a solution or as dispersion. The coating used is usually plastic polymers. The type and number of coatings depend on the product that has to be packaged.
Coatings can be applied to aluminium after the different forms like cans, tubes etc. are ready. Before container formation, lamination is the procedure used to apply the protective layer.
Lamination: The metal is laminated/extruded with thermoplastic, made of films of polypropylene, polyethylene terephthalate, polyamide (nylon) or a polypropylene/nylon combination.
Coating: In case of coating, different materials are used for different containers based on the flexibility of the package, but they are usually only 1 to 10 μm (microns) thick.
Aerosol Cans use a basecoat of epoxy phenol and a vinyl topcoat.
Three piece cans are coated with epoxy-anhydride.
Tube coatings are normally epoxy phenols, organosol or polyesters (e.g. DE5449-4).
Lids coatings are mainly polyacrylics, poly-methacrylis, polyvinyl chloride copolymers or polyolefin.
Epoxy resins contains BPA, however after its ill-effects on human health were discovered there has been a concerted effort to produce epoxy resins without BPA. According to EU directives BPA from varnishes and coatings applied to packaging should ‘not exceed a specific migration limit of 0,05 mg of BPA per kg of food (mg/kg).’
In general, though coatings and varnishes are not regulated by the European Union, contact substances must comply with EU, Framework Regulation (EC) No 1935/2004.
Aluminium is recyclable, though it gets classified as ‘hard-to-recycle-waste’, after being mixed with plastic coatings. There are many methods that can separate and recycle the two types of materials. Pyrolysis is the one that is most cost effective and is largely used. It can burn off the plastic coatings, leaving behind the aluminium packaging, which can be then melted to produce secondary aluminium. The method that is gaining traction is the thermo-chemical treatment, as it reduces the emission of carbon monoxide and carbon dioxide that is released when plastic waste is burnt through combustion and gasification. Pyrolysis manages to recover 99% of the aluminium with hardly any impurities. The metal extracted is 99.9% pure. This system of recycling produces the metal using only 5% of energy needed to make primary aluminium. Moreover, the plastic itself degrades to its constituent monomers, giving oil, thereby helping in recovering a part of it which is then reused.
With increasing levels of collection and better recycling of mixed waste, aluminium even with coatings is able to hold its appeal as an environment friendly packaging material.