This invention relates to a field of packaging containers, such as dished trays for products such as food or the like, which comprise a base of fibrous material such as molded pulp or pressed paperboard, having bonded to one side thereof an impervious liner of polymeric material, which is designed to have a lid of transparent flexible plastic material, such as a film of heat sealable polyester, hermetically sealed around the edges thereof by heat and/or pressure, and wherein the lid is to be physically pulled away and removed from the container by the ultimate consumer to expose for consumption the food or other product packaged in the container.
Trays of this type are replacing trays made of metal such as aluminum foil in the frozen meal industry, for instance, because of their superiority in several respects, particularly their utility with the increasingly prevalent home microwave ovens.
This invention is particularly useful with previously shaped ovenable molded pulp trays having a liner obtained from a film of polyester of the type described in Foster and Stowers U.S. Pat. No. 4,337,116 (June 1982), the complete disclosure of which is incorporated herein by this specific reference thereto. Many aspects of this invention also may be useful with ovenable trays mechanically shaped from paperboard previously coated or lined with polyester, of the type disclosed in Kane U.S. Pat. No. 3,924,013 (December 1975). While the aforesaid disclosures relate to ovenable containers useful with food for human consumption, many aspects of the present invention also will prove useful, it is now believed, with other packaging containers for other end uses where undesirable delamination of the container when attempting to remove the lid therefrom presents a problem.
The problem heretofore unresolved by the prior art is most severe and thus best explained with respect to containers which comprise a base molded to substantially finished shape of fibrous pulp material, to which a liner from a film of polyester is bonded by heat and pressure. To receive a lid, such containers conventionally have a marginal portion surrounding a central portion where the product is packaged, the marginal portion taking the form of a lateral flange defining the outer periphery of the container. When a lid of transparent flexible material, such as a thin heat sealable polyester film, is tightly sealed by heat and pressure to the liner of the container around the lateral flange, then that lid seal may be stronger than either the bond between the liner and the molded pulp base, or the interfelted bond between the fibers of the molded pulp base itself.
When the lid seal is stronger than either of these bonds, then attempts to physically pull the lid away and separate it from the container are defeated because the lid remains sealed to the liner around the packaged product, and the liner of the container delaminates from the molded pulp base, usually with some fibrous pulp still bonded to the underside thereof. This retains the sealed envelope relationship of the lid and the liner around the packaged product, without exposing the product for removal from the container as desired for its intended end use.
This delidding problem is not as critical with containers shaped from a base of flat paperboard having a polyester coating extruded thereon, at least at present, because the more densely interfelted bond between the fibers of the pressed paperboard resist pulling apart to the point where a lid of polyester film sealed thereto may be stripped from the polyester coating with less force than it takes to delaminate the coating from the paperboard or the fibers of the paperboard itself. The paperboard bases of such containers are of necessity densely compacted throughout, however, which in turn dictates that the overall container is quite flexible or flimsy, and is only marginally strong enough to provide a commercially acceptable container of an appropriate size to hold products as heavy as frozen meals. The development of polyester lined ovenable containers utilizing a molded pulp base as described in the aforesaid Foster and Stowers patent, which have much greater overall strength for a given weight than a tray utilizing a pressed paperboard base, however, may well require that manufacturers of paperboard-based containers will be compelled to provide a much thicker and less dense base to meet competitive strength requirements. This change inevitably will create delamination upon delidding problems in paperboard based trays analogous to those now encountered with molded pulp based containers, as explained above, and thus it is now believed that the present invention ultimately will find favor with manufacturers of containers shaped from polyester coated paperboard.
Thus, the problem heretofore unresolved by the prior art is to provide a container, such as a food tray, comprising a relatively strong base of fibrous material having bonded to one side thereof a liner of polymeric material, capable of withstanding freezer-to-oven temperatures and times, wherein a lid sealed to the marginal portion of the container can be peeled away from the liner and fully separated from the container manually (physically, with the hands) without adversely affecting the fibrous material of the base or the bond between the liner and the base, at any temperature within that range, to easily and cleanly expose the packaged food or other product for its intended end use.