Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-07-27
2002-11-05
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S250000, C526S339000, C526S345000, C526S347000, C528S50200C, C428S364000, C264S210800, C264S290500, C264S291000, C264S331150
Reexamination Certificate
active
06476172
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to &agr;-olefin random copolymer fibers and, more particularly, to such fibers and processes for their preparation from propylene &agr;-olefin random copolymers manufactured using a metallocene catalyst.
BACKGROUND OF THE INVENTION
Thermoplastic olefin polymers, such as linear polyethylene, polypropylene, and olefin copolymers, such as propylene-ethylene copolymers, are conveniently formed in continuous loop-type polymerization reactors and thermoformed to arrive at granules or pellets of the polymers. For example, polypropylene and propylene-ethylene copolymers are polymerized in continuous polymerization reactors in which the monomer stream is introduced into a reactor and circulated with an appropriate catalyst to produce the olefin homopolymer or copolymer. The polymer is withdrawn from the catalyst reactor and subjected to appropriate processing steps and then extruded as a thermoplastic mass through an extruder and die mechanism to produce the polymer as a raw material in particulate form, usually as pellets or granules. The polymer particles are ultimately heated and processed in the formation of the desired end products.
Polypropylene and propylene copolymers, as used in various applications involving production of films, fibers, and similar products, are thermo-processed and shaped or oriented by uni-directional or bi-directional stresses. Such polymers are thermoplastic crystalline polymers. Isotactic polypropylene is conventionally used in the production of fibers in which the polypropylene is heated and then extruded through one or more dies to produce a fiber preform which is processed by a spinning and drawing operation to produce the desired fiber product.
Isotactic poly-&agr;-olefins traditionally have been catalyzed by well-known multi-site catalysts including Ziegler-Natta type catalysts such as titanium chloride. While such catalysts are useful for producing resins or polymers of &agr;-olefins, including polypropylene and propylene-ethylene random copolymers, they produce polymers with relatively broad molecular weight distributions or polydispersity which include significant fractions of polymer material with both higher and lower molecular weight than the average or nominal molecular weight of the polyolefin polymer. For example, U.S. Pat. No. 4,298,718 to Mayr et al., U.S. Pat. No. 4,560,735 to Fujishita and U.S. Pat. No. 5,318,734 to Kozulla disclose the formation of fibers by heating, extruding, melt spinning, and drawing from polypropylene produced by titanium tetrachloride-based isotactic polypropylene. Particularly, as disclosed in the patent to Kozulla, the preferred isotactic polypropylene for use in forming such fibers has a relatively broad molecular weight distribution (“MWD”), as determined by the ratio of the weight average molecular weight (“M
w
”) to the number average molecular weight (“M
n
”) of about 5.5 or above. Preferably, as disclosed in the Kozulla patent, the molecular weight distribution, M
w
/M
n
, is at least 7.
The high molecular weight fraction found in such Ziegler-Natta reactor-grade isotactic polymers causes processing difficulties for the maker of polypropylene fibrous or fiber-containing products. As explained in U.S. Pat. No. 6,010,588, the high molecular weight fraction contributes significantly to the melt strength of the molten polymer, diminishing the processibility of the polymer. Some of the processing problems involve the need for higher processing temperatures necessary to reduce the inherent melt strength and viscosity and cause the higher molecular weight chains to move. This requires higher energy input to move the polymer through the extruder or other processing equipment. High melt strength also leads to difficulty in forcing the molten resin through a small fiber-forming orifice. Within that restriction, the high molecular weight molecules cause significant drag and diminish flow. Those same molecules also cause significant die swelling of the polymer fibril upon its exit from the fiber-forming orifice due to their inherent tendency toward elastic response with recovery of their conformational bulk. Along with these processing difficulties for fiber manufacturers, the fibers resulting from traditionally produced polypropylene tend to be thick, due to the melt strength of the molten resin. Such fibers lead to formation of fairly coarse fabrics which lack “give”, limiting their use in garments and other applications where a pleasant feel or “hand” is desirable.
One solution for reducing “boardiness” and increasing “softness” and “give” of fabrics made from polyolefin fiber has been copolymerization of ethylene with propylene to make random copolymers. Small amounts of ethylene monomer are added in a reacting medium comprising propylene and a Zeigler-Natta catalyst capable of randomly incorporating the ethylene monomer into the macromolecule chain, reducing overall crystallinity and rigidity of the macromolecule. Propylene-ethylene random copolymers, because of their lower crystallinity and rigidity, are preferred over homopolymer isotactic polypropylene in fiber and fabric applications that require enhanced softness.
However, like the Ziegler-Natta isotactic polypropylene polymers, the Ziegler-Natta propylene-ethylene random copolymers have fiber processing difficulties. Further, there has been inability of existing fiber and fabric processes to economically draw fine diameter fibers from conventional high ethylene content random copolymers, in particular random copolymers having an ethylene content greater than about 3% by weight. In addition, as explained in U.S. Pat. No. 5,994,482, random copolyers having an ethylene content greater than about 5% by weight generally have not been feasibly produced in liquid reactor or hybrid reactor technologies. Liquid and hybrid reactor systems account for the most part of polypropylene manufacturing capacity worldwide. In a liquid reactor system, the liquid hydrocarbon solubilizes the atactic portion of the polymer, the level of which is enhanced by the high incidence of ethylene monomer in the polymer chain. The atactic material is tacky and creates flowability problems in the downstream equipment as soon as the liquid hydrocarbon is vaporized. Above an ethylene content of about 5% by weight, tacky copolymer granules agglomerate and/or stick to the metal walls of the process equipment.
The processing difficulties described above respecting Zeigler-Natta polymers and copolymers led to development of post-reactor treatment of Ziegler-Natta polymers to enhance processability. Most of these post-formation or post-reactor processes involve some sort of molecular chain scission of the polymer molecules, normally accomplished through the treatment of polyolefins, particularly, polypropylene, with heat and oxygen, or a source of free radicals such as organic peroxides. When organic peroxides are mixed with polypropylene in the melt phase, the polymer is caused to degrade to a narrower molecular weight distribution (“MWD”) and lower average molecular weight (“M
w
”) and exhibits a higher melt flow rate (“MFR”). The M
w
of the visbroken polyolefin is determined by the MFX test (ASTM D1238, Condition L). MFR is a characteristic well known in the art and is reported as grams/10 minutes or dg/min, at 230° C. The M
w
of a visbroken polyolefin determines the level of melt viscosity and the ultimate desirable physical properties of the fiber. Basically, since a higher MFR flows more melted polymer through an orifice, a lower M
w
polymer is more easily melt spun. Most melt spinning is at MFR's exceeding 35 dg/min.
Degradation of polypropylene polymer to a lower average M
w
and a narrower MWD dan the starting material has been termed “visbreaking” the polyolefin. The presence of the organic peroxides in the polypropylene resin results in what is known as “controlled rheology” or “CR” resin. A peroxide of choice in the polypropylene art in the production of CR polypropylene resins is 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, availa
Albe Lisa K.
Chevillard Cyril
Wachowicz Darek
Fina Technology, Inc.
Misley Bradley A.
Teskin Fred
The Matthews Firm
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