The materials used for flexible packaging should be safe from different points of view. Food is not allowed to be in contact with external contaminant like bacteria, viruses, other microorganisms, dirt, so the microstructure of the package materials must not allow this dangers to pass through and contaminate food.
Polypropylene has a reticulated crystalline structure that is used for a good barrier for gas and contaminants. So the polymers of polypropylene are widely used in food packaging, conferring a good protection. But some migrants could pierce the macromolecular structure of polymers and could contaminate the food stuff. Such migrants could be mainly the components of printing inks and of polyurethane adhesives.
Polyurethane adhesives are a mixture of two polymers that reacts as temperatures around 30 to 50 degrees celsius. These two components suffer a polymerization process, and it strengthens.
So, this adhesive is used for bonding the printed polymer to metalized polymer.It is known that for package food in wraps, and bags, are used laminates. Preferred materials for food packaging often consist of a first substrate, such as a film, which is generally thin and transparent (about 30 microns), and can be printable, and a second substrate which can be another type of material often is thinner film about 20 microns, a metal foil, a metallised film or a transparent film.
The use of reactive polyurethane adhesives has in some cases been found to lead to contamination of the packaged food with un-reacted isocyanates and carcinogenic aromatic amines, formed by the reaction of adhesives components with moisture from the food. Thus, coating weights of the adhesive used in lamination will be not higher than 1.5-5g/m². food packaging materials may release migration compounds which result from the adhesives used in manufacturing such materails.
