Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Producing multilayer work or article
Reexamination Certificate
2001-07-03
2004-02-03
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Producing multilayer work or article
C264S563000, C264S171260, C264S173160, C264S209800
Reexamination Certificate
active
06685872
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
If an encapsulated layer of a barrier material such as polyvinylidene chloride is extruded in this way, the polyvinylidene chloride core does not recombine along the weld line. Only the encapsulating material recombines at the weld line. Since the encapsulating material has a much higher gas transmission rate than polyvinylidene chloride, the tubular film has higher gas transmission properties at the weld line than in the rest of the tubular film. This is unacceptable in many applications.
U.S. Pat. No. 4,643,927, to Luecke et al., which is incorporated herein by reference, suggests one solution to this problem. Luecke discloses a multilayer film having a central layer of barrier material which overlaps itself by a substantial distance along the weld line. The patent states that an overlap of two thirds of one inch in the barrier layer along the weld line is sufficient to provide a film in which the oxygen transmission rate along the weld line is no greater than in other portion of the film.
While Luecke represents a significant improvement in the manufacture of tubular barrier films, problems still remain. Blown films containing polyvinylidene chloride can only be produced on small dies (those having a diameter of less than about 8 inches).
Furthermore, even these small blown film dies can only be operated for about 1 to 4 weeks before the line must be shut down and cleaned. Because of its thermal degradability, polyvinylidene chloride has a tendency to “carbonize” in the extrusion equipment. Carbonization results in the formation of small carbon particles in the molten extrudate. Blown film dies have a large surface area where the molten polymer is exposed to long residence time, and polyvinylidene chloride has a tendency to adhere to the metal. The long residence time results in degradation of the polyvinylidene chloride. Black, degraded polymer may form, which can then break loose and contaminate the film. This is an even bigger problem on large dies (those having a diameter of greater than about 8 inches) due to the increased surface area and higher metal temperature as a result of higher temperature skin polymers conducting heat to the die mandrel. The carbon build-up requires the manufacturer to shut down and clean the extrusion apparatus. The shutdown and cleaning of the extrusion apparatus results in high maintenance costs and lost production time.
Thus, it would be desirable to make a coextruded blown film containing a barrier material, to produce such a film on large dies, and to operate for long periods of time without shutdowns due to carbon formation.
SUMMARY OF THE INVENTION
These needs are met by the tubular, multilayer film, methods and apparatus of the present invention. The tubular, multilayer film includes a central barrier layer and a pair of adhesive layers on opposite sides of the central barrier layer. The adhesive layers completely cover the central barrier layer. Opposing edges of the central barrier layer overlap longitudinally along the tubular, multilayer film. The total thickness of the central barrier layers in the overlapping portion is substantially the same as the thickness of the central barrier layer in the non-overlapping portion. The tubular, multilayer film also includes inner and outer surface layers. The inner surface layer extends completely around the interior of the tubular, multilayer film, and the outer surface layer extends completely around the exterior of the tubular, multilayer film. This arrangement covers the encapsulated barrier layer and protects it from degradation. Additional layers may be included as needed.
The invention also involves a tubular film including a central barrier layer overlapping by at least an amount determined by Equation 1 along a weld line which extends longitudinally along the tubular film. The central barrier layer has substantially the same total thickness in the overlapping portion as in the non-overlapping portion. An inner adhesive layer and an outer adhesive layer are positioned on opposite sides of the central barrier layer. The adhesive layers completely encapsulate the central barrier layer. An inner surface layer is positioned inside the inner adhesive layer, and an outer surface layer is positioned outside the outer adhesive layer.
The central barrier layer is preferably made from a polymer selected from vinylidene chloride polymers and copolymers, ethylene vinyl alcohol polymers and copolymers, polyamide (Nylon) polymers and copolymers, and acrylonitrile polymers and copolymers. The adhesive layers are preferably made from a polymer selected from ethylene vinyl acetate (EVA) polymers and copolymers, ethylene methyl acrylate (EMA) polymers and copolymers, ethylene acrylic acid (EAA) polymers and copolymers, ionomers, and maleic anhydride grafted olefin polymers and copolymers. The surface layers are preferably made from a polymer selected from polyethylene polymers and copolymers, nylon and K-resins (styrene/butadiene block copolymers), ethylene vinyl acetate copolymer (EVA), polypropylene (PP) and polyethylene terephthalate (PET).
The present invention also includes a method of making a tubular, multilayer film. The method includes extruding a block of material having a barrier core and an adhesive covering the barrier layer core into a first stream having a generally annular cross-section. The first stream has a central barrier layer which overlaps longitudinally along the tubular, multilayer film, such that the total thickness of the central barrier layers in the overlapping portion is substantially the same as the thickness of the central barrier layer in the non-overlapping portion. An inner surface layer is extruded into a second stream having a generally annular cross-section. The second stream is positioned within the first stream and is joined thereto by the adhesive. An outer surface layer is extruded into a third stream having a generally annular cross-section. The third stream is positioned to surround the first stream and is joined thereto by the adhesive. The first stream is preferably extruded such that the opposing longitudinally extending edges of the central barrier layer overlap. The present invention also includes a method for coextruding a multilayer tubular film having a barrier material. A core extrudate of barrier material is extruded with a core extruder. A preencapsular extrudate of preencapsular material is extruded and directed to a preencapsulation die provided adjacent to the outlet of the core extruder. The core extrudate and the preencapsular extrudate are joined in the preencapsulation die in a coaxial relationship wherein the preencapsular extrudate is disposed radially outwardly of the core extrudate to form
Dooley Joseph
Jenkins Steve R.
Naumovitz John A.
Dow Global Technologies Inc.
Eashoo Mark
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