Easily openable closure for a retortable container having a metal end to which a membrane is sealed

Abstract

A retortable, easily openable closure of the type having a metal end and a membrane closure heat-sealed thereto, wherein the metal end has a coating of a heat-seal composition comprising a food-compatible metal-coating composition blended with polypropylene. The polypropylene disperses within the food-compatible metal-coating composition during curing of the coating on the metal end such that an outer surface of the coating defines a multitude of separate, spaced islands of polypropylene dispersed within the food-compatible metal-coating composition so as to form bonding sites. The membrane closure's lower surface has a heat-seal layer of polypropylene for sealing to the coating on the metal end.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a container having a closure formed by a metal end joined to a top end of the container and defining an opening through which contents of the container are dispensed, and a peelable membrane sealed to the metal end so as to hermetically close the opening. The invention relates more particularly to such a container closure that is capable of withstanding a retort sterilization process without failing, but that has a controlled peel strength that is sufficiently low to be readily openable by hand. BACKGROUND OF THE INVENTION [0002] A variety of food products are packaged in containers that, after filling and sealing, are subjected to a retort sterilization process to render the filled containers shelf-stable such that they do not require refrigeration until opened (referred to herein as “retort containers”). The retort process typically involves elevating the temperature of the filled and sealed container to about 250-260°...

AI Technical Summary

Benefits of technology

[0007] The present invention provides a retortable, easily openable closure of the type having a metal end and a membrane, wherein the failure mechanism during peeling of the membrane from the metal end is not a cohesive failure within the heat-seal layer on the membrane. The heat-seal layer on the membrane in accordance with the invention does not require incompatible polymers or fillers to be added, nor does it require a coextruded structure, because the failure mechanism involves a failure between the heat-seal layer on the membrane and a corresponding heat-seal coating on the metal end. Accordingly, the heat-seal layer on the membrane is simplified and therefore less costly.

[0008] In accordance with the invention, the metal end has a coating of a heat-seal composition comprising a food-compatible metal-coating composition with polypropylene dispersed therein. The polypropylene disperses within the food-compatible metal-coating composition during curing of the coating on the metal end such that an outer surface of the coating defines a multitude of separate, spaced islands of polypropylene dispersed within the food-compatible metal-coating composition so as to form bonding sites. The polypropylene islands are microscopic in scale. The membrane's lower surface has a heat-seal layer thereon, comprising a layer of polypropylene. The polypropylene heat-seal layer on the membrane is heat-sealed to the coating on the metal end, whereby the polypropylene heat-seal layer bonds to the bonding sites on the coating. The peel strength of the resulting bond depends on the fraction of the coating's surface area that is represented by the bonding sites. As the collective area of the bonding sites becomes a greater fraction of the coating's total surface area, the peel strength increases. In accordance with the invention, the heat-seal layer of polypropylene on the membrane bonds to the membrane with a strength exceeding that between the heat-seal layer and the coating on the metal end, such that when the membrane is pulled to open the container, the failure occurs between the heat-seal layer on the membrane and the coating on the metal end. A controlled low peel strength can be achieved by suitably formulating the polypropylene-filled metal-coating composition.

Problems solved by technology

However, the bond tends to be so strong that the peel force required to peel the membrane from the metal end is unacceptably high.

Such membranes having controlled defects are relatively complex and thus tend to be relatively expensive to produce.

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