1. Field of the Invention
The present invention relates to laminar thermoplastic film structures which are formed by coextrusion of the molten laminar layers through a single die orifice and particularly such laminar structures that are solidified after extrusion and passed onto an orientation or stretching operation to produce a biaxially oriented laminate film. The film is characterized by having good heat seal strengths, broad heat temperature ranges and excellent impact resistance.
2. Description of the Prior Art
Oriented polypropylene films have become useful and widely employed packaging films primarily because of their good moisture barrier properties, stiffness, improved machineability, high strength characteristics and excellent optical properties such as high gloss and minimal haze. Such biaxially oriented polypropylene films are quite difficult to seal because of the tendency of such films to deorient, pucker or tear at the requisite sealing temperatures. Consequently, in order to achieve satisfactory heat sealability, coatings of various types have been applied to the polypropylene film surface to lower the requisite heat seal temperatures. Generally, such coatings are applied in a separate operation after the film has been formed and oriented. Many times the coatings must be applied from solution in an organic solvent media. This separate operation for application of relatively expensive coatings which dictate employment of solvent recovery systems is quite costly. Examples of such coatings which have been employed in the past to impart heat sealability and other desirable characteristics to the polypropylene film include saran and acrylic multipolymer coatings.
An alternate method for imparting improved heat sealability to oriented polypropylene film, and one that is less costly than post orientation coating, comprises the coextrusion of surface layers of a lower melting resin onto the surface of the higher melting polypropylene core prior to orientation. Following orientation a product is obtained which has a relatively thick polypropylene core sandwiched between two relatively thin layers or skins of the low melting resin. However, as practiced in the piror art, this procedure has associated with it certain deficiencies. For example, in an instance where it is desirable to form a laminar structure comprising a thermoplastic core sandwiched between layers of a lower melting point material such as an ethylene propylene copolymer to provide heat sealability, the resulting laminar structure exhibits desirable high heat seal strengths but because of the relatively high melting point of such a copolymer layer, the heat sealing range, defined by the temperature at which usable seal strengths are formed and that temperature where undesirable film shrinkage occurs, is narrow.
In other instances in the prior art when a coextruded film comprising a low melting skin material is employed as the coating or skin layer, e.g., a medium density polyethylene resin produced by the high pressure, free radical catalyst process, a product is produced which exhibits a much wider sealing range than that of the hereinabove described coextruded film, however, its seal strengths are undesirably low. Additionally, since very low machine direction stretch ratios must be employed, such films are oriented only to a slight degree in the machine direction and as a result the film's toughness or impact strength is not as high as would be desired. Improvement of the toughness characteristics of the film, by imparting a greater machine direction stretch to the film while maintaining the heat seal advantages of the polyethylene skin could not be achieved because the higher temperatures which would be required to permit greater machine direction stretching would also cause undesirable sticking of the polyethylene skin to the stretching rolls.