Nonwoven fabrics prepared using visbroken single-site...

Details Nonwoven fabrics prepared using visbroken single-site... Nonwoven fabrics prepared using visbroken single-site...

1999-11-12

2003-06-24

Cole, Elizabeth M. (Department: 1771)

Fabric (woven, knitted, or nonwoven textile or cloth, etc.)

Nonwoven fabric

Melt-blown nonwoven fabric

C442S327000, C442S401000

Reexamination Certificate

active

06583076

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to nonwoven fibers and fabrics prepared using visbroken single-site catalyzed polypropylene, wherein the ratio of the melt flow rate (MFR) of the polypropylene after visbreaking to its MFR before visbreaking is from about 1:1 to about 3:1.
BACKGROUND OF THE INVENTION
Conventional, Ziegler-Natta catalyzed polypropylenes have a relatively broad molecular weight distribution after leaving the reactor. It has long been known to partially degrade or “visbreak” propylene homopolymers and copolymers to narrow the molecular weight distributions. Visbreaking processes are disclosed, for instance, in U.S. Pat. No. 4,282,076, issued to Boynton et al. and U.S. Pat. No. 5,250,631, issued to McCullough, Jr., et al. Visbreaking techniques involve thermal degradation, radiation, and the use of peroxides and other catalysts. Visbreaking of conventional polypropylenes leads to narrower molecular weight distributions (as well as lower average molecular weights) because initially larger polypropylene molecules are more susceptible to visbreaking than initially smaller polypropylene molecules. U.S. Pat. No. 5,723,217 issued to Stahlet al., discloses that the melt spinning of conventional (Ziegler-Natta catalyzed) propylene polymers improves with the degradation of the polymers using peroxide. Stahl et al. further discloses the melt spinning of propylene polymers prepared using single-site metallocene-type catalysts. According to Stahl et al., reactor-grade (i.e. non-visbroken) single-site catalyzed propylene polymers provide the same fiber spinning advantages as visbroken Ziegler-Natta propylene polymers, because the single-site polymers have narrower molecular weight distributions leaving the reactor. These advantages include, for instance, better ease of processing and the ability to make small diameter, high strength fibers. Thus, the use of a metallocene catalyzed propylene polymer eliminates the need for a visbreaking process step.
Stahl et al. further discloses that metallocene catalyzed propylene polymers can be visbroken to a lower molecular weight to facilitate fiber production. Stahl et al. alleges that the visbreaking does not change the molecular weight distribution, defined as the ratio of weight average molecular weight to number average molecular weight. From the disclosure of Stahl et al., for instance, it would appear that a reactor grade 5 MFR polymer which is visbroken to 55 MFR, results in a polymer having the same narrow molecular weight distribution as before. For purposes of melt spinning, there is no apparent incentive to visbreak a lower MFR metallocene polymer to a higher MFR, as opposed to melt spinning a higher MFR reactor grade metallocene polymer which has not been visbroken.
SUMMARY OF THE INVENTION
It has been discovered, unexpectedly, that a single-site catalyzed propylene polymer visbroken to a higher MFR from a lower MFR exhibits better melt spinning properties than a similar single-site catalyzed reactor grade propylene polymer initially having the higher MFR, when the ratio of the higher MFR to the lower MFR is about 1:1 to about 3:1. Thus, contrary to the teachings of the prior art, there is an advantage to visbreaking a lower MFR single-site catalyzed propylene polymer to a higher MFR, instead of simply producing the higher MFR polymer in a polymerization reactor.
The present invention is directed to nonwoven fibers which are spun from a metallocene catalyzed propylene polymer which has been visbroken from a lower MFR to a higher MFR, wherein the ratio of the higher MFR to the lower MFR is about 1:1 to about 3:1. The present invention is also directed to nonwoven fabrics produced from the fibers, and to personal care and medical products that include the nonwoven fabrics.
It is a feature and advantage of the invention to provide a relatively strong, thin fiber made from a single-site catalyzed propylene polymer visbroken from a lower MFR to a higher MFR, and which may be thinner (finer) than would be produced under similar conditions using a reactor grade single-site propylene polymer having the higher MFR.
It is also a feature and advantage of the invention to produce a nonwoven web of fibers made using the visbroken single-site catalyzed propylene polymer, and various personal care and medical products incorporating the nonwoven web.
DEFINITIONS
The term “nonwoven fabric or web” means a web having a structure of individual fibers or threads which are interlaid, but not in a regular or identifiable manner as in a knitted fabric. Nonwoven fabrics or webs have been formed from many processes such as, for example, meltblowing processes, spunbonding processes, air laying processes, and bonded carded web processes. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91.) The term “microfibers” means small diameter fibers having an average diameter not greater than about 75 microns, for example, having an average diameter of from about 7 microns to about 30 microns. Another frequently used expression of fiber diameter is denier, which is defined as grams per 9000 meters of a fiber. For a fiber having circular cross-section, denier may be calculated as fiber diameter in microns squared, multiplied by the density in grams/cc, multiplied by 0.00707. A lower denier indicates a finer fiber and a higher denier indicates a thicker or heavier fiber. For example, the diameter of a polypropylene fiber given as 15 microns may be converted to denier by squaring, multiplying the result by 0.89 g/cc and multiplying by .00707. Thus, a 15 micron polypropylene fiber has a denier of about 1.42 calculated as (15
2
×0.89×.00707 =1.415). Outside the United States the unit of measurement is more commonly the “tex,” which is defined as the grams per kilometer of fiber. Tex may be calculated as denier /9.
The term “spunbonded fibers” refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine capillaries of a spinneret having a circular or other configuration, with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartman, U.S. Pat. No. 3,502,538 to Petersen, and U.S. Pat. No. 3,542,615 to Dobo et al., each of which is incorporated herein in its entirety by reference. Spunbond fibers are quenched and generally not tacky when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and often have average diameters larger than about 7 microns, more generally, between about 10 and 75 microns.
The term “meltblown fibers” means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity heated gas (e.g., air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed for example, in U.S. Pat. No. 3,849,241 to Butin. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in diameter, and are generally self bonding when deposited onto a collecting surface. Meltblown fibers used in the present invention are preferably substantially continuous in length.
The term “monocomponent” fibers refers to fibers formed from one or more extruders using only one polymer. This is not meant to exclude fibers formed from one polymer to which small amounts of add

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