If it is repeatedly creased or stretched (soft foil only has a ductility of about 3.5 per cent), which is often unavoidable in the manufacture of packaging, aluminium foil becomes damaged. However, using the aluminium together with materials that are more resistant to kinking and tearing (and more economical) strengthens it to such an extent that this risk is eliminated; the aluminium foil ensures that the composite material is impermeable to gases, liquids and aromas.
There are various types of composite packaging materials available, in particular for food packaging:
- After coating aluminium foil with plastics, mostly thermoplastics which become soft on heating, the latter also act as an adhesive that can be used, for example, to seal lightweight containers for foodstuffs.
- Lamination (a form of coating) of foil, which means bonding it with paper, paperboard or plastic film (using automatic machines), produces kink-resistant packaging materials (for chocolate products) or composite cans (with paperboard; lid and bottom partly made from strip or foil). As adhesives one uses either starch adhesives that are odour- and heat-resistant and water-soluble, or waxes and plastics that are waterproof but non-heat-resistant.
- Single-sided lamination with thin plastic foil makes aluminium foil up to 20 per cent more stretchable while maintaining complete impermeability, while with double-sided lamination there is an improvement of up to 35 per cent; such products are mostly used as flexible packaging.
- For greater corrosion resistance, both sides are lacquered and serve as sealing strips, such as for yoghurt pots.
- Laminated foil is also used as a barrier against moisture in housebuilding (for example as the backing on rolls of thermal insulation).