In the food processing industry the trend is to replace metal cans with something more convenient and more efficient. To that end, retort pouches have been conceived.
A typical retort pouch is made of face-to-face laminate structures which are sealed together on three sides before filling, and they are sealed on the fourth side after filling. The sealed retort pouches usually are heated or retorted after final sealing to cook the contents of the pouch and to kill all bacteria within the pouch.
To laminate structures, typically, comprise an aluminum foil with a polyester adhesively attached to one side thereof, frequently by means of a polyester urethane adhesive, and a sealable lamina attached to the second side of the foil.
If all that was desired was the structuring of a pouch, the sealable lamina of the aluminum foil could also be attached in the same manner as the polyester lamina. However, polyester urethanes have not been approved by governmental authorities because of possible contamination of the contained food. Therefore, for retort pouches for enclosing food, a maleic anhydride polymer grafted onto polypropylene is coated on the second surface of the aluminum foil, and then it is heat-cured in line with coating operation. Such materials are known by the trade names Hercoprime and Morprime. An inner sealant layer of polypropylene is then attached onto the maleic anhydride polymer grafted onto polypropylene at high temperatures on the order of 500.degree. Fahrenheit. Unfortunately, the high temperature heating of the sealant layer of polypropylene appears to oxidize the surface of the polypropylene thereby increasing the difficulty of sealing the laminate structures together to form the pouch.