Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-02-25
2001-06-05
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S318800, C428S319900, C428S910000
Reexamination Certificate
active
06242084
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an opaque biaxially oriented film which contains at least one layer of metallocene-catalyzed substantially isotactic propylene polymer. In one instance, the invention relates to a metallocene-catalyzed substantially isotactic propylene polymer-containing biaxially oriented film in which the metallocene-catalyzed propylene polymer of the core layer contains a strata of voids which renders the film opaque. In other manifestations, the invention relates to a metallocene-catalyzed substantially isotactic propylene polymer-containing biaxially oriented film in which the metallocene-catalyzed propylene polymer layer is on one or both surfaces of a core that contains a strata of voids which render the film opaque. That core polymer can be either a metallocene-catalyzed polymer or a Ziegler-Natta catalyzed polymer.
BACKGROUND OF THE INVENTION
Metallocene catalyzed propylene polymers and copolymers are known to have a low melting temperature, relative to conventional propylene polymers. For this reason, metallocene catalyzed propylene polymers and copolymers have been described as useful as the outer layer heat sealable material of multi-layer films in U.S. Pat. No. 5,468,440 (column 6, lines 32 to 41). Other multi-layer films in which the advantage of an outer layer of metallocene catalyzed propylene polymers are described are found in U.S. Pat. Nos. 5,529,843 and 5,462,807. Although these patents describe multi-layer films in which a skin layer is made from a metallocene catalyzed propylene polymer, the patents are silent on the use of metallocene catalyzed propylene polymers in inner layers of a multi-layer film. U.S. Pat. No. 5,254,394 to Bothe et al., discloses a clear polyolefin film for packaging comprising isotactic polypropylene base layer and a top layer of syndiotactic polypropylene which has high sealed-seam strength and high clarity. The top layer can contain lubricant additives, such as waxes at levels of up to 2% wt. relative to each other.
Opaque polypropylene films are described in U.S. Pat. No. 4,377,616 which teaches an opaque biaxially oriented polymeric film structure of lustrous satin appearance comprising a thermoplastic core matrix having a strata of voids which are created by void-initiating solid particles which are incompatible with the matrix material.
Oriented polypropylene films of the above-disclosed types are used extensively in the packaging of a variety of foods. These are often aimed at applications where it is desired to have as low a water vapor transmission (WVTR) and oxygen transmission (TO
2
) as possible. Other applications do exist for which these types of film structures are unacceptable. One of these is the wrapping of fresh cleaned and chopped produce, such a lettuce, carrots, celery, etc., from which salads can rapidly be prepared. For these applications, a higher level of WVTR and TO
2
is desirable to allow respiration through the package walls, while continuing to maintain package integrity.
The film of the present invention has a WVTR and TO
2
that have been increased by the use of novel means, as a way to address this need.
SUMMARY OF THE INVENTION
The invention is directed to an opaque oriented multi-layer film which comprises (a) a core voided layer comprising a substantially isotactic propylene polymer and void-initiating particles which are incompatible with the matrix material; and (b) at least one non-voided layer on at least one surface of core layer. Optionally, there is at least one skin layer on at least one surface of the core layer opposite to the surface having the non-voided layer or on the same surface as the non-voided layer or layers. The film of the present invention can also have at least one heat sealable layer on at least one outer surface of the film. The substantially isotactic propylene polymer of the core or of the non-voided layer or layers, or of both the core and non-voided layer or layers is a metallocene-catalyzed isotactic propylene polymer.
DETAILED DESCRIPTION OF THE INVENTION
Voided Layer
The voided layer has a polymer matrix material within which is located a stratum of voids. From this it is to be understood that the voids are integral to the matrix configuration. The term “stratum” is intended to convey the understanding that there are many voids creating the matrix. The matrix polymer and void initiating particles must be incompatible and this term is used in the sense that the materials are two distinct phases. Essentially spherical void initiating particles constitute a dispersed phase throughout the lower melting polymer which polymer will, ultimately, upon orientation, become a void-filled matrix with the spherical particles positioned somewhere in the voids. The void space occupied by the particle is substantially less than the volume of the void, the population of the voids in the voided matrix being such as to cause a significant degree of opacity. The voids themselves become oriented so that the two major dimensions are aligned in correspondence with the directions of orientation of the polymeric structure. After each void has been formed through the initiation of the described particle, the particle generally contributes little else to the system. This is because its refractive index can be close enough to the matrix material that it makes no contribution to opacity. When this is the case, the opacity is principally a function of the light scattering effect which occurs because of the existence of the voids in the system.
Void initiating particles contemplated include polybutylene terephthalate (PBT), PBT spheres, nylon-6, calcium carbonate, cyclic olefin copolymers, as well as other organic and inorganic materials of suitable melting points that are phase distinct from the polymer matrix material, having a size and quantity sufficient to form voids upon orientation of the extruded matrix material.
It is preferred that the average diameter of the void-initiating particles be from about 0.1 to about 10 microns. These particles may be of any desired shape although it is preferred that they be substantially spherical in shape. This does not mean that every void is the same size. It means that, generally speaking each void tends to be of like shape when like particles are used even though they vary in dimensions. These voids may assume a shape defined by opposed and edge contacting concave disks.
The substantially isotactic propylene matrix polymer can be produced by using Ziegler-Natta or metallocene, preferably metallocene catalysts. The broad molecular weight distribution of substantially Ziegler-Natta catalyzed isotactic propylene matrix polymer gives good operability. The WVTR and TO
2
levels of the final product made from Ziegler-Natta catalysts, however, are lower than desired for some applications, such as for the packaging of fresh produce, as described above. The preferred metallocene-catalyzed substantially isotactic propylene polymer can be used as the propylene matrix polymer in the core layer with satisfactory operability. This satisfactory operability is surprising in view of the narrow molecular weight distribution of the metallocene-catalyzed resin. By converting to the use of the metallocene-catalyzed polymer, we have been able to achieve up to about 35% higher WVTR and TO
2
levels in the film.
The propylene polymer is substantially isotactic. Metallocene-catalyzed isotactic polypropylenes made developmentally or commercially are EOD 96-21 and EOD 97-09, from Fina Oil and Chemical Co., EXPP-129, from Exxon Chemical Co., and Novalen M, from BASF GmbH., among others. The propylene polymers are usually predominantly comprised of propylene (at least 90% by weight) and have a melting point of about 140° C. or higher, more typically a melting point of 150° C. or higher. The melt flow rate usually ranges from about 0.5 g/10 min. to about 15 g/10 min. at 230° C., more typically about 1.5 g/10 min. to about 4 g/10 min. at 230° C. The melt flow is measured by ASTM 1238D.
The voided layer of the film is of sufficient thickness to provi
Copenheaver Blaine
Mobil Oil Corporation
Santini Dennis P.
Simmons T. Dean
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