This invention relates to a biaxially oriented, coextruded barrier film comprising polyolefin skin or outer layers, a core barrier layer of vinylidene chloride-acrylate copolymer, and adhesive layers therebetween.
Numerous coextruded cast barrier films are known in the art such as Saranex.RTM. (trademark of The Dow Chemical Company). Other barrier films are seen in U.S. Pat. Nos. 4,561,920, 3,524,795, 3,579,416, and 4,714,638. A typical barrier film comprises one or more barrier layers of a gas or liquid impermeable thermoplastic material, and one or more layers of a water and/or water vapor impermeable second thermoplastic material. Thermoplastics useful in barrier layers include polyvinylidene chloride and copolymers thereof.
Desired is a coextruded barrier film providing improved strength and durability, increased modulus, non-extensibility, improved optical clarity, and improved barrier properties. Such improvements in films in general have been effected by uniaxially or biaxially orienting the barrier film. Biaxial orientation of a film may be carried out simultaneously or sequentially. In simultaneous orientation, the film is stretched in two dimensions at the same time. The film may also be oriented simultaneously by stretching in both dimensions but at unequal rates of stretching. Simultaneous biaxial stretching may be effected in a conventional blown film process or in a tentering process. In sequential orientation, the film is stretched in two steps or completely in one dimension and then completely in the other dimension, and is usually accomplished in a tentering apparatus. Sequential orientation may also be carried out alternately by stretching in one dimension, stretching in the other dimension, and then again in the original dimension in an iterative manner for any number of cycles. The extent of sequential stretching in the two dimensions may be equal or unequal.
Previous attempts at biaxially orienting coextruded cast barrier films utilizing polyvinylidene chloride or copolymers of polyvinylidene chloride and polyvinyl chloride in the barrier layer have been unsuccessful because such orientation deleteriously affected the barrier properties of film. Biaxial orientation, particularly two-step or sequential biaxial orientation, can deleteriously affect the barrier properties of the film by altering the crystalline structure of the polymer or copolymer comprising the barrier layer resulting in microvoids therein. Microvoids in the barrier layer substantially increase the degree of oxygen permeability, an undesirable property in most applications utilizing barrier films.
Two-step, sequential biaxial orientation exacerbates the formation of microvoids in a multilayer coextruded barrier film more than simultaneous biaxial orientation because the crystalline structure of the barrier layer is altered twice instead of once.
In commercial and industrial applications, biaxial orientation of films is usually accomplished by sequential stretching in an on-line tentering apparatus. Simultaneous biaxial orientation, though useful in imparting as desirable if not more desirable characteristics to films than sequential biaxial orientation, is not as mechanically adaptable to industrial processes as sequential orientation. Since sequential biaxial orientation is prevalent in industrial processes for making oriented film, a particularly desirable oriented, coextruded barrier film would substantially maintain barrier properties upon sequential orientation.
The creation of microvoids can be avoided or substantially reduced by orienting the multilayer coextruded cast barrier film at a temperature as close as possible to the melting point of the crystalline or semi-crystalline polymer or copolymer comprising the barrier layer. Orientation of the cast coextruded film at such a temperature ensures that the degree of crystallinity of the polymer or copolymer comprising the barrier layer will be minimal, and, thus, that void formation will be minimized.
Orienting a coextruded, cast barrier film at temperature as close as possible to that of the melting point of the polymer or copolymer comprising the barrier layer is difficult because the melting point of most conventional thermoplastic materials which comprise barrier layers is usually greater than the melting point of the thermoplastic materials which conventionally comprise outer or skin layers. The temperature at which the orientation occurs must be lower than the melting point of the outer layers of the film because the tentering apparatus which imparts the desired orientation to the film physically contacts and interfaces with the film at the outer layers. Processing conditions at this interface usually dictate that orientation be carried out at a temperature 10.degree.-30.degree. C. below the melting point of the thermoplastic materials comprising the outer layers. If the temperature of orientation is substantially below the melting point of thermoplastic materials comprising the barrier layer, significant formation of microvoids may result due to a substantial degree of crystallization in the barrier layer at that temperature.
Accordingly, there exists a need for an improved coextruded barrier film which retains a substantial degree of oxygen impermeability upon biaxial orientation. There exists a further need for a coextruded barrier film which retains a substantial degree of oxygen impermeability upon sequential biaxial orientation. There exists a further need for a coextruded barrier film utilizing a barrier layer of polyvinylidene chloride or copolymers thereof which retains a substantial degree of oxygen impermeability upon sequential biaxial orientation.