Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2000-02-09
2002-12-31
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C264S045300, C264S173150, C264S173190, C428S413000, C428S483000, C428S480000, C428S516000, C428S523000, C428S910000
Reexamination Certificate
active
06500533
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to opaque polymeric films. More particularly, this invention relates to oriented opaque polymeric films prepared with a cavitating agent comprising polybutylene terephthalate (PBT) and polycarbonate.
BACKGROUND OF THE INVENTION
Opaque polymeric films having a polyolefin core layer, e.g., of an oriented isotactic polypropylene (OPP), have been widely used in food and non-food packaging applications, because such films have desirable properties such as resistance to transmission of moisture, air, deleterious flavors, and the like, as well as outstanding mechanical properties.
During the production of these opaque polymeric films, cavitating agents may be used. In cases where polybutylene terephthalate (PBT) is used as the cavitating agent, extrusion plate out can be problem. In particular, PBT may degrade and build up in the film production equipment, forming deposits which further affect the flow patterns of molten polymer in the die. The use of PBT cavitating agents to prepare opaque polymeric films is described in U.S. Pat. No. 4,632,869 to Park et al.
SUMMARY OF THE INVENTION
There is provided an opaque polymeric film comprising:
(a) a base layer comprising a polymeric matrix and at least one cavitating agent; and
(b) at least one additional layer;
wherein said cavitating agent comprises polybutylene terephthalate (PBT) and polycarbonate.
There is also provided a method for producing an opaque polymeric film comprising:
(a) extruding a base layer comprising a polymeric matrix and at least one cavitating agent;
(b) coextruding at least one additional layer on at least one side of the base layer;
(c) cooling the coextruded multi-layer film; and then
(d) orienting the film in at least the machine direction (MD);
wherein said cavitating agent comprises polybutylene terephthalate (PBT) and polycarbonate.
Advantages of the present films indude (1) reduced extrusion plate out during manufacture (2) uniform opacity, and (3) resistance to distortion caused by film crease.
DETAILED DESCRIPTION OF THE INVENTION
The base layer of the opaque polymeric film comprises a polymeric matrix, which may be selected from any of the polymers previously used in the art for such purpose. In many cases, such a polymer is a polyolefin having a melting point, for example, of at least about 150° C. and up to, for example, about 167° C. Preferably, the polyolefin of the base layer has a relatively high degree of crystallinity. A particularly desirable polyolefin as the base layer polymer is an isotactic polypropylene homopolymer having a crystallinity of, for example, about 89 to 99% (as measured by
13
C NMR spectroscopy using meso pentads), a melting point of about 155 to about 165° C., and a melt index of about 0.5 to about 15 g/10 minutes (as measured by the standard ASTM D1238 methods).
Other suitable polymeric matrix materials for the base layer include, but are not limited to, syndiotactic polypropylene, ethylene-propylene copolymers, ethylene-propylene-butylene terpolymers, butylene-ethylene copolymers, functionally grafted copolymers, blends of polymers, etc.
At least one cavitating agent in the form of a dispersed phase is provided in the base layer polymeric matrix material before extrusion and orientation of the film. Such dispersed phase comprises particles of polybutylene terephthalate (PBT) and polycarbonate. This dispersed phase may also, optionally, comprise at least one additional cavitating agent.
Polybutylene terephthalate is a semicrystal polymer which has low viscosity and, as discussed above, tends to plate out during extrusion. This plate out can be observed by pulling out the screw of the extruder and seeing polybutylene terephthalate build-up on metal surfaces after, for example, a 48 hour run.
Polycarbonate is an amorphous, high viscosity polymer. Polycarbonate is partially miscible with polybutylene terephthalate. It is believed that polycarbonate traps polybutylene terephthalate inside of the matrix polymer, thereby avoiding or reducing plate-out. The present process, therefore, represents an improvement over the aforementioned process of U.S. Pat. No. 4,632,869, where PBT is used in the absence of polycarbonate.
The mixture of polycarbonate and polybutylene terephthalate inside the polymer matrix may be from 4 wt % to 20 wt % of the entire weight of the cavitated layer. The weight ratio of polybutylene terephthalate to polycarbonate may be from 1:1 to 4:1.
The optional additional cavitating agent may be a material having a melting point that is higher than the melting point of the polymeric matrix material of the base layer. The optional additional cavitating agent may also be immiscible with polymeric matrix material of the base layer. The optional additional cavitating agent may be any of those described in U.S. Pat. No. 4,377,616, the entire disclosure of which are incorporated herein by reference. Thus, the optional additional cavitating agent may be selected from a polymer, such as, for example, a polyester other than PBT, nylon (e.g., nylon-6), an acrylic resin, or an ethylene norborene copolymer; or an inorganic material, such as, glass, calcium carbonate, metal, or ceramic, or mixtures thereof.
The particle size of cavitating agents in the dispersed phase may be, for example, about 0.1 micron to about 5 microns, more preferably about 0.2 micron to about 2 microns. The dispersed phase may be present in the base layer in an amount of up to about 20 weight percent, for example, from about 4 to about 20 weight percent, based on the entire weight of the base layer.
To preserve the structural integrity of the dispersed phase-containing base layer, a thin layer of the base layer polymeric matrix material without the dispersed phase may be coextruded on one or both sides of the dispersed phase-containing base layer polymeric matrix material. In this case, the total of the dispersed phase-containing polymer layer and the non-dispersed phase-containing layer(s) may be considered the overall base layer of the film. When such a polymer substrate is subjected to uniaxial or biaxial orientation, a cavity forms around the distributed dispersed phase moieties, providing a film having an opaque appearance.
The base layer of the film is of sufficient thickness to provide bulk properties, such as barrier, stiffness, etc. that are desired for product protection and good performance on packaging equipment. Preferably, the thickness ranges from about 50% to about 100%, based on the thickness of the entire film structure.
Additional layer(s) of the present invention may be any of the coextrudable, biaxially orientable film-forming resins known in the art. Such materials include, but are not limited to, syndiotactic polypropylene, low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), ethylene-propylene copolymers, butylene-propylene copolymers, ethylene-butylene copolymers, ethylene-propylene-butylene terpolymers, ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, nylons, polymers grafted with functional groups, appropriate blends of these, and others known to those skilled in the art. Each additional layer preferably has a range in thickness from about 0.005 mil to about 0.15 mil, preferably from about 0.02 mil to about 0.10 mil.
There can be more than one additional layer coextruded on each side of the base layer. That is, one or more layers can be added to the surface of the base layer. Films having such a multi-layer structure are represented, in simplified form, as having a structure “ABCDE” where “C” represents a base layer, “B” represents an additional layer adjacent to the base layer and “A” represents a further additional layer or skin layer applied to the outer surface of layer “B.” In such a film structure, the additional layer “B” can be referred to as a “tie-layer” or an “intermediate layer.” Layers “A” and “B” can be the same or different. Similarly, “D” and “E” represent additional layers on the other side of the ba
Kong Dan-Cheng
Larter John A.
Mount, III Eldridge M.
Bell Keith A
ExxonMobil Oil Corporation
James Rick F.
Tarazano D. Lawrence
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