Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1999-02-25
2001-08-07
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S521000, C428S910000, C264S210100, C264S210700
Reexamination Certificate
active
06270912
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a biaxially oriented film which contains a core layer of metallocene-catalyzed substantially isotactic propylene polymer.
BACKGROUND OF THE INVENTION
Oriented polypropylene films are known for use in packaging. U.S. Pat. No. 4,604,324 to Nahmias et al. discloses multi-layer polypropylene film structures comprising coextruded layers of a base layer of high stereoregularity polypropylene and a skin layer of comparatively low stereoregularity which contains anti-stick additives. In addition, the skin layer can contain up to 10 wt. % of a wax to improve heat seal characteristics or optical properties of the film.
Metallocene-catalyzed propylene polymers are now available, which have low melting temperatures relative to conventional propylene polymers. For this reason, metallocene-catalyzed propylene polymers 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) to McAlpin et al. The McAlpin et al. patent discusses temperatures at which metallocene-catalyzed isotactic polypropylene plaques or cast samples can be stretched in laboratory equipment. Those temperatures, however, are not suitable for continuous orientation processing.
Other multi-layer films in which the advantages 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 the '843 and '807 patents describe multi-layer films in which an outer layer is made from a metallocene-catalyzed propylene polymer, these patents are silent on the use of metallocene-catalyzed propylene polymers in the core layer of a multi-layer film.
U.S. Pat. No. 5,254,394 to Both et al., discloses a polyolefin film for packaging comprising an isotactic polypropylene base layer and a top layer of syndiotactic polypropylene which has high sealed-seam strength and excellent optical properties. The top layer can contain lubricant additives, such as waxes at levels of up to 2 wt. % relative to each other.
Metallocene-catalyzed polypropylene polymers have been developed under the product names NOVALEN M by BASF, FINACENE by Fina, and EXCEED by Exxon, for example. These metallocene-catalyzed isotactic polypropylenes share the beneficial characteristics of having a very narrow composition distribution. Thus, a large portion of the molecules have a level of atactic defects within a compositionally narrow range, for example, rather than a broad range of atactic defect levels averaging out at the desired level.
These same metallocene-catalyzed isotactic polypropylenes, however, also share the characteristic of having a very narrow molecular weight distribution. That is, a large portion of their molecules have a particular chain length. On the other hand, the more traditionally used isotactic polypropylenes have a broad range of chain lengths, averaging out at the desired level. For good operability in the OPP (oriented polypropylene) process, it has been shown that a broad molecular weight distribution has been highly preferred, if not absolutely necessary. The high molecular weight tails of the Ziegler-Natta catalyzed resins tend to distribute the stretching forces more uniformly, preventing stress concentrations, and ultimately providing more uniform orientation and improved operability. Thus, the metallocene-catalyzed isotactic polypropylene polymers have a narrow composition distribution, which is looked upon favorably for film properties, and a narrow molecular weight distribution, which is looked upon negatively for operability in the OPP process.
For this reason, multi-layer films disclosed in the prior art, heretofore, have generally used traditional Ziegler-Natta catalyzed, broad molecular weight distribution polymers in the core, to distribute orientation stresses more uniformly and avoid film breakage during orientation, relegating the metallocene-catalyzed polypropylenes, with their desired properties, to layers on one or both sides of the Ziegler-Natta catalyzed core resin.
At conditions known to be in use within the general OPP industry, as recommended by sources recognized as knowledgeable in the operation of OPP orientation lines, orientation of an OPP line with metallocene-catalyzed isotactic polypropylene as the core resin would not be expected to proceed as well as orientation when using a broad molecular weight range Ziegler-Natta catalyzed resin.
Surprisingly, we have now discovered that films having metallocene-catalyzed isotactic polypropylene resin as the core material can be produced satisfactorily under certain conditions, discussed more fully below.
SUMMARY OF THE INVENTION
The present invention is directed to an oriented multi-layer film which comprises:
(a) a core layer comprising a metallocene-catalyzed substantially isotactic propylene polymer;
(b) a first additional layer on a side of the core layer; and
(c) a second additional layer on a side of the core layer which side is opposite to the side having the first additional layer.
In one modification, the film of the present invention has a Ziegler-Natta catalyzed polypropylene layer on one or both sides of the core to improve operability.
Further, the present invention is directed to a process for making a multi-layer film, wherein the process comprises:
(a) extruding a core layer comprising a metallocene-catalyzed substantially isotactic propylene polymer;
(b) coextruding a first additional layer on a side of the core layer;
(c) coextruding a second additional layer on the side opposite to the side having the first additional layer;
(d) cooling the coextruded multi-layer film; and then
(e) orienting the film in at least the machine direction (MD), wherein rolls operating at a temperature greater than about 110° C. are used in the machine direction orientation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
General conditions recommended for use within the OPP industry typically include running the MD stretch at temperatures in the 107° C. to 148° C. range and rolls following the MD stretch at a reduced temperature, such as 80-110° C. Attempts to run metallocene-catalyzed isotactic polypropylene core at these temperatures, however, resulted in a number of splits. The present invention is founded on the discovery that running the line with all the MD rolls at temperatures in the range of > about 110° C. resulted in satisfactory operability of the OPP process and the production of useful films.
The core layer of the multi-layer film comprises a metallocene-catalyzed substantially isotactic propylene polymer. The phrase “substantially isotactic propylene polymer” or “substantially isotactic polypropylene” as used in the present invention generally refers to propylene polymers which have an isotacticity of 85% or greater (as measured by solubility in xylene). Exemplary isotactic propylene polymers and/or methods for making such polymers are described in, e.g., the following patent documents: U.S. Pat. Nos. 5,529,843 (particularly column 4, lines 52 to 62); 5,162,278; 5,158,920; 5,155,080; 5,036,034; 4,975,403; 4,892,851; and 4,794,096, all of which are incorporated herein by reference in their entirety.
As indicated above, metallocene-catalyzed polypropylene polymers have been developed under the product names NOVALEN M by BASF, FINACENE by Fina, and EXCEED by Exxon, for example. These metallocene-catalyzed isotactic polypropylenes have relatively narrow composition and molecular weight distributions.
The propylene polymers used in the present invention can, optionally, contain other monomeric units as in the case of a copolymer of propylene and ethylene or &agr;-olefin having from 4 to 20 carbon atoms such as butene-1, pentene-1, hexene-1, heptene-1, 4-methylpentene-1, etc. Generally, the amount of monomer other than propylene is no more than 10% by weight of the entire polymer. In one commonly used modification, a copolymer of ethylene and propylene is used in the core layer of OPP film, which contains about 0.5% ethylene by weight in th
Copenheaver Blaine
Hickman William E.
Mobil Oil Corporation
Santini Dennis P.
Simmons T. Dean
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