This invention relates to innerseals for containers, and, more particularly, innerseals that are suitable for containers made of glass.
Heat-sealable innerseals have been found to be useful in sealing containers for liquid products, such as, for example, motor oil, brake fluid, antifreeze, household ammonia, liquid detergents, etc., which products present technical problems with respect to leakage. Heat-sealable innerseals have also become popular for providing the tamper-resistant innerseals that are required by the Food and Drug Administration for over-the-counter drugs.
Heat-sealable innerseals generally comprise a layer of aluminum foil bearing a coating of heat-sealable material thereon. The innerseals are inserted into caps, and the resulting assemblies are then supplied to the packager. The cap is then placed by the packager onto the filled container, with the coating of heat-sealable material being in contact with the lip or rim of the container. The container then passes under an induction heater which generates heat through the aluminum foil, thereby melting the heat-sealable coating, and causing the innerseal to bond to the container.
Heat-sealable innerseals applied by induction heating have been found to be especially useful with plastic caps and plastic container systems, as induction heating does not heat the plastic material excessively. Heat-sealable innerseals can also be used with metal caps in an induction heating system where the metal cap itself conducts heat to melt the heat-sealable material.
In the case of glass containers, however, heat-sealable innerseals will lose adhesion within days, even hours, however, upon exposure to moisture. This loss of adhesion occurs because the sodium ion, which is an essential component of many popular glass containers, gradually leaches out of the bulk of the glass and reacts with water in the air or in the container to form a layer of sodium hydroxide on the surface of the glass. It is this layer of sodium hydroxide that reduces the adhesion between the heat-sealed coating of the innerseal and the glass surface of the container.
One innerseal material in current use that overcomes the aforementioned problem is an ionomer sold under the trademark "Surlyn". However, this ionomer has a disagreeable odor and, at high temperatures, portions thereof leach into the liquid contents of the container.