Method for preparing multilayer films

#The present invention relates generally to multilayer, tubular films and to methods and apparatus for making such films. More particularly, the invention relates to tubular, polymer films containing a barrier layer and having uniform barrier properties around the circumference of the tube.

#Film structures including a barrier layer such as polyvinylidene chloride have been difficult to extrude in tubular form. Extrusion of polyvinylidene chloride presents special problems. One problem results from the thermal degradation of the polymer which can occur in the extrusion die. To overcome this, polyvinylidene chloride has been encapsulated in an encapsulating material which does not exhibit the thermal degradation of polyvinylidene chloride.

#Another problem which arises in the formation of polyvinylidene chloride into a tubular film is the seam which is formed. The seam extends along a weld line on one side of the extrudate. Generally, the extrusion die used to produce a tubular extrudate defines an annular chamber into which the extrusion material is forced. The material separates into two substreams which flow in opposite directions around the annular chamber, meeting on the opposite side of the chamber where they recombine. The streams then exit from the annular opening, and define a weld line at the point of recombination.

#FIG. 1 is a cross-section of the multilayer, tubular extrudate of the present invention, with the individual layer thicknesses greatly exaggerated for purposes of illustration.

#FIG. 2 is a partial cross-section of the extrudate in the region of the weld line, greatly exaggerated.

#FIG. 3 is a diagram of the overall process of the present invention.

#FIGS. 1 and 2 show cross-sectional views of the tubular, multilayer films of the present invention. For purposes of clarity in illustration, the thicknesses of the layers are exaggerated with respect to the size of the tubular laminate. The film comprises a central barrier layer 10, and a pair of adhesive layers 12 and 14 which are positioned on opposite sides of the central barrier layer 10. The central barrier layer 10 can be any barrier polymer, including, but not limited to, vinylidene chloride polymers and copolymers, ethylene vinyl alcohol polymers and copolymers, nylon polymers and copolymers, and acrylonitrile polymers and copolymers. The barrier layer is preferably a vinylidene chloride polymer or copolymer.

#The adhesive layers 12 and 14 completely cover the central barrier layer 10. The adhesive layers 12 and 14 can be any one of a number of adhesives, including, but not limited to, EVA polymers and copolymers, EMA polymers and copolymers, EAA polymers and copolymers, ionomers, and maleic anhydride grafted olefin polymers and copolymers. When the central barrier layer is polyvinylidene chloride, the adhesive layer is preferably an ethylene vinyl acetate polymer or copolymer.

#As best seen in FIG. 2, the opposing edges of 16 and 18 of the central barrier layer 10 overlap along the weld line. The thickness of the central barrier layers 10 in the overlapping portion is substantially the same as the thickness of the central barrier layer 10 in the non-overlapping portion. As a result of this arrangement, the film exhibits a substantially uniform oxygen transmission rate at all points around its circumference.