High elongation, low denier fibers using high extrusion rate spinning

FIELD OF THE INVENTION

[0002] The present invention relates to low denier, high extensible fibers, soft extensible nonwoven webs comprising such fibers, and disposable articles comprising such nonwoven webs.

BACKGROUND

[0003] Nonwoven webs formed by nonwoven extrusion processes such as, for example, meltblowing and spunbonding processes may be manufactured into products and components of products so inexpensively that the products could be viewed as disposable after only one or a few uses. Representatives of such products include disposable absorbent articles, such as diaper, incontinence briefs, training pants, feminine hygiene garments, wipes, and the like.

[0004] There is an existing consumer need for nonwovens that can deliver softness and extensibility when used in disposable products. Softer nonwovens are gentler to the skin and help to provide a more garment-like aesthetic for diapers. Nonwovens that are capable of high extensibility at relatively low force can be used to provide sustained fit in products such as disposable diapers, for example, as part of a stretch composite, and facilitate the use of various mechanical post-treatments such as stretching, aperturing, etc. Extensible materials are defined herein as those capable of elongating, but not necessarily recovering all or any of the applied strain. Elastic materials, on the other hand, by definition, must recover a substantial portion of their elongation after the load is removed.

[0005] There are several approaches that have been used in the art to create extensible nonwovens:

[0006] World Patent Application WO 00/04215 discloses a specific bond pattern designed to produce a high elongation nonwoven fabric, specifically for skin-core polypropylene staple fibers. The bond pattern has sites in adjacent rows staggered such that they do not overlap one another in the machine direction of manufacture (MD). Preferably the sites are rectangular in shape and cover a total bond area of <20%. They disclose that fibers at an angle of 35-55° from the MD will not be bonded and therefore allow for higher cross direction of manufacture elongation.

[0007] Fiber formulation is often used to achieve extensibility. U.S. Pat. Nos. 5,804,286 and 5,921,973 disclose blends of polyethylene and polypropylene with and without a miscible ethylene-propylene copolymer that produce soft, strong nonwovens with low fuzz and good elongation. World Patent Application WO 00/31385 discloses polypropylene blends with ethylene copolymers and U.S. Pat. No. 6,015,317 discloses blends of 2 different ethylene polymers, both for improved bonding and fabric elongation while maintaining good spinning performance. U.S. Pat. No. 5,616,412 disclosess filaments (2-4 denier per fiber) of polypropylene and higher molecular weight polystyrene that exhibit higher elongation as compared to filaments of only polypropylene. U.S. Pat. No. 5,322,728 discloses soft nonwovens with good elongation comprising ethylene copolymers, while U.S. Pat. No. 4,769,279 discloses soft nonwovens with good elongation comprising ethylene acrylic copolymers. U.S. Pat. Nos. 4,804,577 and 4,874,447 disclose extensible meltblown nonwovens comprising a blend of a polyolefin and an elastomeric copolymer of an isoolefin and a conjugated diolefin, for example an isobutylene-isoprene copolymer. U.S. Pat. No. 5,349,016 discloses drawn fibers of grafted propylene polymers (e.g. styrene or methyl methacrylate grafted onto the polypropylene backbone) that have higher bend recovery and modulus, and in some cases elongation over the neat polypropylene control. U.S. Pat. No. 6,080,818 discloses fibers for nonwovens comprising a blend of isotactic polypropylene and an atactic flexible polyolefin that have higher elongation than if the flexible polymer was not included.

[0008] All of these formulation approaches can increase the extensibility of moderate to low denier fibers to some degree, but not to the degree disclosed in the present invention. In addition, they generally involve blending in higher-cost materials and can involve special mixing requirements to ensure proper dispersion within the blends.

[0009] The way in which the nonwoven web is formed can also be used to maximize stretch properties. U.S. Pat. No. 5,494,736 discloses a high elongation carded nonwoven from high elongation fibers that are laid down to be more cross direction oriented than conventional carded fabrics. Bond areas claimed are in the range of 8-25%.

[0010] There exists within the industry today an unmet need for extensible nonwovens with moderate to low denier fibers that can be made from conventional resins without the need for high cost specialty polymers or elastic polymers. It is well known that as spinning attenuation velocities increase, molecular orientation increases and fiber elongation decreases. For strong, low denier fibers, this is not a problem, but producing low denier fibers with high elongation remains a significant challenge. It is therefore an object of the present invention to disclose a method for producing low denier fibers that have high elongation to break. It is a further object of the present invention to disclose soft extensible nonwoven webs comprising such low denier, high extensible fibers. It is still a further object of the present invention to disclose disposable articles comprising such soft extensible nonwoven webs.

SUMMARY OF THE INVENTION

[0011] A means for producing low denier fibers with high elongation to break is disclosed. This combination of properties is accomplished by altering the spinnerette design on fiber lines to have small capillary diameters that maintain the desireable high spinning speeds, but result in moderate to low drawdown ratios as compared to the high drawdown ratios of conventional spinning processes. A particular embodiment of the present invention encompasses fibers with a diameter in the range of 5 to 25 microns that are produced by melt spinning a polymer composition such that the drawdown ratio is less than 400, the mass throughput is in the range of 0.01 to 2.0 grams per minute per hole, and the spinnerette diameter is less than 200 microns. An alternative embodiment of the present invention encompasses fibers with a diameter in the range of 5 to 25 microns that are produced by melt spinning a polymer composition such that the drawdown ratio is less than 400, the fiber elongation to break is greater than 400 percent, and the spinnerette diameter is less than 200 microns.

[0012] While not being bound by theory, it is believed that as a result of the lower drawdown, the fibers are less oriented and thereby maintain higher residual elongations to break. In this manner, highly uniform fabrics consisting of fine diameter, high elongation fibers can be produced. Nonwoven webs with this combination of properties are particularly well suited for use in disposable absorbent articles such as diapers, incontinence briefs, training pants, feminine hygiene garments, wipes, and the like, as they are able to be used in portions of the article where extensibility and softness can aid in the article's comfort and overall performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]
FIG. 1 is a graph illustrating the percent elongation to break of a 400 melt flow rate polypropylene melt spun at 0.2 grams per minute per hole using an 86 micron diameter capillary and a 570 micron diameter capillary.

DETAILED DESCRIPTION OF THE INVENTION

[0014] As used herein, the term “absorbent article” refers to devices that absorb and contain body exudates, and, more specifically, refers to devices that are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body.

[0015] The term “disposable” is used herein to describe absorbent articles that are not intended to be laundered or otherwise restored or reused as an absorbent article (i.e., they are intended to be discarded after a single use and, preferably, to be recycled, composted or otherwise disposed of in an environmentally compatible manner). A “unitary” absorbent article refers to absorbent articles that are formed of separate parts united together to form a coordinated entity so that they do not require separate manipulative parts like a separate holder and liner.

[0016] As used herein, the term “nonwoven web”, refers to a web that has a structure of individual fibers or threads which are interlaid, but not in any regular, repeating manner. Nonwoven webs have been, in the past, formed by a variety of processes, such as, for example, air laying processes, meltblowing processes, spunbonding processes and carding processes, including bonded carded web processes.

[0017] As used herein, the term “microfibers” refers to small diameter fibers having an average diameter not greater than about 100 microns, and a length-to-diameter ratio of greater than about 10. Those trained in the art will appreciate that the diameter of the fibers comprising a nonwoven web impact its overall softness and comfort, and that the smaller denier fibers generally result in softer and more comfortable products than larger denier fibers. For fibers of the present invention, it is preferable that the diameters are in the range of about 5 to 25 microns to achieve suitable softness and comfort, more preferable in the range from about 10 to 25 microns in diameter, and even more preferable in the range from about 10 to 20 microns in diameter. The fiber diameter can be determined using, for example, an optical microscope calibrated with a 10 micrometer graticule.

[0018] As used herein, the term “meltblown fibers”, refers to fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into a high velocity gas (e.g., air),stream which attenuates the filaments of molten thermoplastic material to reduce their diameter, which may be to a 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.

[0019] As used herein, the term “spunbonded fibers” refers to small diameter fibers that are formed by extruding a molten thermoplastic material as filaments from a plurality of fine, usually circular, capillaries of a spinneret with the diameter of the extruded filaments then being rapidly reduced by drawing using conventional godet winding systems or through air drag attenuation devices. If a godet system is used, the fiber diameter can be further reduced through post extrusion drawing.

[0020] As used herein, the terms “consolidation” and “consolidated” refer to the bringing together of at least a portion of the fibers of a nonwoven web into closer proximity to form a site, or sites, which function to increase the resistance of the nonwoven to external forces, e.g., abrasion and tensile forces, as compared to the unconsolidated web. “Consolidated” can refer to an entire nonwoven web that has been processed such that at least a portion of the fibers are brought into closer proximity, such as by thermal point bonding. Such a web can be considered a “consolidated web”. In another sense, a specific, discrete region of fibers that is brought into close proximity, such as an individual thermal bond site, can be described as “consolidated”.

[0021] Consolidation can be achieved by methods that apply heat and/or pressure to the fibrous web, such as thermal spot (i.e., point) bonding. Thermal point bonding can be accomplished by passing the fibrous web through a pressure nip formed by two rolls, one of which is heated and contains a plurality of raised points on its surface, as is described in the aforementioned U.S. Pat. No. 3,855,046 issued to Hansen, et al.. Consolidation methods can also include, but are not limited to, ultrasonic bonding, through-air bonding, resin bonding, and hydroentanglement. Hydroentanglement typically involves treatment of the fibrous web with high pressure water jets to consolidate the web via mechanical fiber entanglement (friction) in the region desired to be consolidated, with the sites being formed in the area of fiber entanglement. The fibers can be hydroentangled as taught in U.S. Pat. Nos. 4,021,284 issued to Kalwaites on May 3, 1977 and 4,024,612 issued to Contrator et al. on May 24, 1977, both of which are hereby incorporated herein by reference.

[0022] Although the nonwoven web of the present invention can find beneficial use as a component of a disposable absorbent article, such as a diaper, its use is not limited to disposable absorbent articles. The nonwoven web of the present invention can be used in any application requiring, or benefiting from, softness and extensibility, such as wipes, polishing cloths, furniture linings, durable garments, and the like.

[0023] The extensible, soft nonwoven of the present invention may be in the form of a laminate. Laminates may be combined by any number of bonding methods known to those skilled in the art including, but not limited to, thermal bonding, adhesive bonding including, but not limited to spray adhesives, hot melt adhesives, latex based adhesives and the like, sonic and ultrasonic bonding, and extrusion laminating whereby a polymer is cast directly onto another nonwoven, and while still in a partially molten state, bonds to one side of the nonwoven, or by depositing melt blown fiber nonwoven directly onto a nonwoven. These and other suitable methods for making laminates are described in U.S. Pat. No. 6,013,151, Wu et al., issued Jan. 11, 2000, and U.S. Pat. No. 5932, 497, Morman et al., issued Aug. 3, 1999, both of which are incorporated by reference herein.

[0024] As used herein, the term “polymer composition” generally includes, but is not limited to, homopolymers, copolymers, such as, for example, block, graft, random and alternating copolymers, terpolymers, etc., and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer composition” shall include all possible geometrical configurations of the material. These configurations include, but are not limited to, isotactic, syndiotactic and random symmetries. Examples of suitable thermoplastic polymers for use in the present invention include, but are not limited to polyethylene, polypropylene, polyethylene-polypropylene copolymers, polyvinyl alcohol, polyesters, nylon, polylactides, polyhydroxyalkanoates, aliphatic ester polycondensates, and mixtures thereof. Preferred polymer compositions comprise polyolefins such as polyethylene and polypropylene, or polyesters such as poly(ethylene terephthalate) and copolymers thereof. Preferred additional polyesters include, but are not limited to, poly(lactic acid) (e.g., Lacea from Mitsui Chemicals, or EcoPLA from Dow Cargill), poly(caprolactone) (e.g., Tone P787 from Union Carbide), poly(butylene succinate) (e.g., Bionolle 1000 series from Showa Denko), poly(ethylene succinate) (e.g., Lunare SE from Nippon Shokubai), poly(butylene succinate adipate) (e.g., Bionolle 3000 series from Showa Denko), poly(ethylene succinate adipate), aliphatic polyester-based polyurethanes (e.g., Morthane PN03-204, PN03-214, and PN3429-100 from Morton International), copolyesters of adipic acid, terephthalic acid, and 1,4-butanediol (e.g., Eastar Bio from Eastman Chemical Company, and Ecoflex from BASF), polyester-amides (e.g., BAK series from Bayer Corporation), hydrolyzable aromatic/aliphatic copolyesters (e.g., Biomax from DuPont), cellulose esters (e.g., cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate from Eastman Chemical Company), combinations and copolymers thereof, and the like.

[0025] The polymer compositions may further include various nonpolymeric components including, among others, nucleating agents, antiblock agents, antistatic agents, slip agents, pro-heat stabilizers, antioxidants, pro-oxidant additives, pigments, fillers and the like. These additives may be employed in conventional amounts although, typically, such additives are not required in the composition in order to obtain the advantageous combination of softness and extensibility.

[0026] One skilled in the art will appreciate that the melt flow rate of the polymer composition is suitable for the fiber producing method of interest, for example, melt spinning or melt blowing. The melt flow rate of a polymer composition can be determined using, for example, the methods outlined in ASTM D1238.

[0027] As used herein, the term “extensible” refers to any fiber, which, upon application of a biasing force, is elongatable to at least about 400 percent without experiencing catastrophic failure, more preferable to at least 600 percent elongation without experiencing catastrophic failure, and even more preferable to at least 800 percent elongation without experiencing castastrophic failure. The percent elongation to break can be determined using, for example, the method outlined in ASTM D3822, and is defined as the expanded length at break minus the initial test gauge length divided by the initial test gauge length multiplied by 100.

[0028] Continuous fibers, staple fibers, hollow fibers, shaped fibers, such as multi-lobal fibers and multicomponent fibers can all be produced by using the methods of the present invention. Component, as used herein, is defined as a separate part of the fiber that has a spatial relationship to another part of the fiber. Multicomponent fibers, commonly a bicomponent fiber, may be in a side-by-side, sheath-core, segmented pie, ribbon, or islands-in-the-sea configuration. The sheath may be continuous or non-continuous around the core. The fibers of the present invention may have different geometries that include round, elliptical, star shaped, rectangular, and other various eccentricities. The fibers of the present invention may also be splittable fibers. Splitting may occur by rheological differences in the polymers or splitting may occur by a mechanical means and/or by fluid induced distortion. As used herein, the diameter of a noncircular cross section fiber is the equivalent diameter of a circle having the same cross-sectional area.

[0029] In conventional melt spinning processes, fiber velocities or spinning speeds are most often calculated from the Continuity Equation:

\begin{matrix} V_{x} = \frac{4 Q}{π ρ_{fiber} d^{2}} & (1) \end{matrix}

[0030] where Vx is the total fiber velocity, Q the mass throughput per spinnerette hole, ρfiber the density of the fiber, and d the diameter (or equivalent diameter) of the fiber. The total fiber velocity is composed of two main components

V

x

=V

o

+V

A
(2)

[0031] where Vo is the fiber exit velocity from the spinnerette, and VA the apparent velocity the fiber associated with attenuation of the filament. The most notable contributions to determining VA are the inertial, drag and rheological forces. The VA forces are what develop the orientation in the filament.

[0032] The exit velocity of the fiber is calculated according to Equation 3 and depends only on Q and the diameter (or equivalent diameter) of the capillary D

\begin{matrix} V_{o} = \frac{4 Q}{ρ_{melt} D^{2} π} & (3) \end{matrix}

[0033] where the density ρmelt in this case is the polymer melt density. For a given Q, D is the only variable and thus the exit velocity depends only on the diameter of the capillary.

[0034] It is well established that higher fiber attenuation speeds VA lead to higher orientation in the fiber, producing smaller diameter fibers with higher strength but lower elongation. While not being bound by theory, for high elongation fibers it is desired to have a lower degree of orientation in the fiber while maintaining a moderate to small diameter for comfort and softness. In other words, the attenuation velocity VA=Vx−Vo, or alternatively the drawdown ratio Vx/Vo, should be low. In order to maintain small diameter fibers for uniformity, coverage and softness, this would conventionally require that the throughput Q be lowered. This, however, reduces total output of material leading to a less desireable economic impact. Generally, for those trained in the art, mass throughputs of about 0.01 grams per minute per hole are considered minimal. Those trained in the art will further appreciate that mass throughputs greater than about 2.0 grams per minute per hole can lead to die flow instabilities, for example melt fracture or wall slippage, leading to difficulities in processing or in collecting product of suitable quality. Therefore, it is preferred that mass throughputs be in the range of 0.01 to 2.0 grams per minute per hole, more preferable in the range of 0.2 to 1.0 grams per minute per hole, and even more preferable in the range of 0.6 to 0.8 grams per minute per hole. The mass throughput per hole can be determined, for example, by collecting the extrudate for a given amount of time and then dividing the value of the total mass collected by the time interval over which it is collected and by the number of holes in the spinnerette for which filaments are exiting.

[0035] By the method of the present invention, the drawdown ratio Vx/Vo or attenuation velocity VA can be lowered without lowering throughput by increasing the fiber velocity as it exits the spinneret capillary (Vo). This can be accomplished by using a smaller capillary diameter. A useful characteristic spinning number that captures much of of the present disclosure is given by

\begin{matrix} S_{x} = (\frac{V_{x}}{V_{o}}) \frac{d}{Q} & (4) \end{matrix}

[0036] Analogous to the drawdown ratio, lower values of the spinning number Sx are preferred.

[0037] While not being bound by example, for example, given a mass throughput of 0.52 g/min/hole (typical for a conventional high speed melt spinning system), and a standard capillary diameter of 0.6 mm, the exit velocity Vo of a melt spinning grade polypropylene filament would be approximately 2.5 m/min. Generally, conventional high speed melt spinning systems must run at or above about 2000 m/min fiber velocities Vx/Vo to produce a good uniform nonwoven fabric, then the drawdown ratio Vx/Vo would be about 800 and the spinning number Sx would be about 7980 microns/(g/min/hole), with the properties associated with such a fiber (e.g., high orientation and low elongation). If instead a capillary diameter of 0.07 mm is used, the exit velocity of the filament would be much higher at about 183 m/min, and Vx/Vo would be much lower at approximately 11 and Sx would be much lower at about 403 microns/(g/min/hole), for similar spinning conditions. The resulting fibers would have lower orientation and higher residual elongation.

[0038] Those trained in the art will appreciate that the diameter of the fibers comprising a nonwoven web impact its overall softness and comfort, and that the smaller denier fibers generally result in softer and more comfortable products than larger denier fibers. For fibers of the present invention, it is preferable that the diameters are in the range of about 5 to 25 microns to achieve suitable softness and comfort, more preferable in the range from about 10 to 25 microns in diameter, and even more preferable in the range from about 10 to 20 microns in diameter. In order to maintain small diameter fibers for uniformity, coverage and softness, this would conventionally require that the throughput Q be lowered. This, however, reduces total output of material leading to a less desireable economic impact. Generally, for those trained in the art, mass throughputs of about 0.01 grams per minute per hole are considered minimal. Those trained in the art will further appreciate that mass throughputs greater than about 2.0 grams per minute per hole can lead to die flow instabilities, for example melt fracture or wall slippage, leading to difficulities in processing or in collecting product of suitable quality. Therefore, it is preferred that mass throughputs be in the range of 0.01 to 2.0 grams per minute per hole, more preferable in the range of 0.2 to 1.0 grams per minute per hole, and even more preferable in the range of 0.6 to 0.8 grams per minute per hole. Furthermore, those trained in the art will appreciate that to achieve good uniform nonwoven webs on older melt spinning systems, the fiber velocities Vx must generally be greater than about 500 m/min, on newer moderate speed systems the fiber velocities must generally be greater than about 2000 m/min, and on newer high spinning speed systems the fiber velocities must generally be greater than about 3000 m/min.

[0039] Additionally, those trained in the art will further appreciate that the lower drawdown ratios Vx/Vo will generally result in higher residual fiber elongation to break. We find that drawdown ratios of less than about 400 are generally sufficient to produce fibers with elongations to break suitable for producing the soft extensible nonwovens of the present invention, more preferable are drawdown ratios less than about 150, and even more preferable are drawdown ratios less than about 50. To achieve these low drawdowns and high fiber velocities within the context of the present invention, we find that spinnerette diameters of less than about 200 microns are generally sufficient, more preferable are spinnerette diameters of less than about 150 microns, and even more preferable are spinnerette diameters of less than about 100 microns. Additionally, as used herein, the term “extensible” refers to any fiber, which, upon application of a biasing force, is elongatable to at least about 400 percent without experiencing catastrophic failure, more preferable to at least 600 percent elongation without experiencing catastrophic failure, and even more preferable to at least 800 percent elongation without experiencing castastrophic failure.

[0040] The products and methods of the present invention are further exemplified in the following examples.

EXAMPLE 1

[0041] This example demonstrates the melt spinning of a polypropylene resin according to the invention. Specifically, a polypropylene resin with a melt flow rate of 400 (Valtech HH441 from Basell Polyolefins Company, Wilmington, Del.) is spun into fibers using a vertical single-screw extruder which is mounted on a platform that can be raised and lowered, and which is equipped with a single-hole capillary die and a capillary of about 86 microns in diameter. The molten filament exits the capillary die into ambient air at approximately 25° C., and is drawndown with a height adjustable air drag device that uses compressed air supplied at high pressures to produce a stream of air that surrounds and draws the filament. The extruder output is kept relatively constant at about 0.2 grams per minute per hole, the distance between the die exit and the air gun is held constant at about 41 inches, the distance between the air gun and the collection screen is held constant at about 25 inches, the extruder and die set temperatures are as follows—zone 1=380° F., zone 2=400° F., zone 3=420° F., die adapter=425° F., die=420° F., and the air gun pressure is varied to achieve and collect fiber diameters of less than about 25 microns in diameter. With these conditions, fiber samples with diameters in the preferred range of 10-30 microns are collected. This example demonstrates that a standard fiber forming resin is melt spinnable according to the invention.

EXAMPLE 2

[0042] This example demonstrates the high extensibility of fibers produced according to the invention. Specifically, fiber samples from Example 1 are tested according to ASTM standard D3822. Testing is performed on an MTS synergie 400 tensile testing machine (MTS Systems Corporation, Eden Prairie, Minn.) equipped with a 10 Newton load cell and pneumatic grips. Tests are conducted at a crosshead speed of 2 inches per minute on single fiber samples with a 1 inch gage length. Samples are pulled to break, and the percent elongation to break is recorded and averaged for 10 specimens collected at the same air gun pressure. The resulting elongations to break are shown in FIG. 1, where the spinning speed is calculated according to equation (1) using the fiber diameters measured by microscopy. This example demonstrates that fibers with small diameters (˜10-20 microns) can also have high elongations to break (>600%) when fabricated according to the invention.

COMPARATIVE EXAMPLE 3

[0043] This example compares the fiber extensibilities from Example 2 with those generated using a conventional sized spinnerette. Specifically, the polypropylene resin from Example 1 is melt spun into fibers using a capillary with a diameter of about 570 microns following the procedure and conditions outlined in Example 1, and the elongations to break are determined according to the method outline in Example 2. The resulting elongations to break are shown in FIG. 1, where the spinning speed is calculated according to equation (1) using the fiber diameters measured by microscopy. This example demonstrates the enhanced extensibility that is achievable at comparable fiber diameters or spinning speeds when the fibers are spun according to the invention.

[0044] The disclosures of all patents, patent applications (and any patents which issue thereon, as well as any corresponding published foreign patent applications), and publications mentioned throughout this description are hereby incorporated by reference herein. It is expressly not admitted, however, that any of the documents incorporated by reference herein teach or disclose the present invention.

[0045] While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is intended to cover in the appended claims all such changes and modifications that are within the scope of the invention.

High elongation, low denier fibers using high extrusion rate spinning

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