Peelably sealed plastic packaging is widely used and available.  For example, peelable seals are desirable in sealed, consumer and medical packaged articles, foodstuffs and the like wherein the package may be a sealed bag, cup, blister or other composite container.  Polypropylene homopolymer and copolymers and high density polyethylene are one preferred material for construction of plastic packaging because of their high strength, high melting point and durability.  However, although various plastics are known to form peelable seals with polypropylene and high density polyethylene, such seals involve shortcomings including low peel strength, or "legging" as the seal is pulled producing stringy fibers at the sealing interface. 
Polypropylene and high density polyethylene cups and containers are well suited for use in retort and microwaveable packaging and other packaging which is exposed to relatively high temperatures.  But these materials when sealed to themselves produce a fusion, i.e., non-peelable, seal.  Thus, when a peelable seal is desired in such packaging, a special adhesive, or a specially designed heat-sealable film must be provided.  The resultant seal must have strength at high temperatures, i.e., 180.degree.  F. or higher, in order that it retain integrity at hot-fill conditions or at retort conditions.  Additionally, in some instances the surface area of the seal will be small, as for example in the case of a lidding closure sealed to the lip of a rigid cup or other container.  These applications require unusually strong seals since the seal surface area is small.