WO2012051056A1

WO2012051056A1 - Highly uniform spunbonded nonwoven fabrics

Info

Publication number: WO2012051056A1
Application number: PCT/US2011/055193
Authority: WO
Inventors: Jr. John F. Baker
Original assignee: Fiberweb, Inc.
Priority date: 2010-10-14
Filing date: 2011-10-07
Publication date: 2012-04-19
Also published as: JP2013544975A; CN103154346A; KR20130117793A; MX2013004217A; CA2827950A1; EP2627812A1; IL225714A0

Abstract

Highly uniform spunbonded nonwoven fabrics, as well as related fibers, products, machines, and methods, are disclosed.

Description

Highly Uniform Spunbonded Nonwoven Fabrics

CROSS REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application claims priority to U.S. Provisional Application Serial No. 61/393,232, filed October 14, 2010, and U.S. Provisional

Application Serial No. 61/497,241, filed June 15, 2011. The contents of the parent applications are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to highly uniform spunbonded nonwoven fabrics, as well as related fibers, products, machines, and methods.

BACKGROUND

Nonwoven fabrics formed from fibers that are thermally bonded to each other have been produced for many years. Two common thermal bonding techniques are area bonding and point bonding. In area bonding, bonds are produced randomly throughout the entire nonwoven fabric at locations where the fibers of the nonwoven fabric come into contact with one another. This can be achieved in various ways, such as by passing heated air, steam or other gas through an unbonded web of fibers to cause the fibers to melt and fuse to one another at points of contact. Area bonding can also be achieved by passing a web of fibers through a calender composed of two smooth steel rollers heated to cause the fibers to soften and fuse. In point bonding, a web of fibers is passed through a heated calender nip having two nip rolls, at least one of which has a surface with a pattern of protrusions. Typically, one of the heated rolls is a patterned roll and the other roll is a cooperating roll having a smooth surface. As the web moves through the calender rolls, the individual fibers are thermally bonded together at discrete point bond sites where the fibers contact the protrusions of the patterned roll and the fibers are not bonded in the locations between these point bond sites. Accordingly, the fabric thus obtained includes a bonding pattern. SUMMARY

The inventors have unexpectedly discovered that spunbonded fibers containing a single polymer having a monomodel molecular weigh distribution (e.g., a polymer with a single intrinsic viscosity) can be area bonded to form a spunbonded nonwoven fabric having a high surface uniformity (e.g., having a M-4 web uniformity index of at most about 600) that is similar to or better than that of a wet laid nonwoven fabric, while having improved tensile strength (e.g., in the machine direction or cross-machine direction) and lower product cost compared to a wet laid nonwoven fabric. Thus, the spunbonded nonwoven fabric can be used to replace wet laid nonwoven fabrics in certain applications (e.g., in membrane filtration media).

In one aspect, this disclosure features an article that includes a nonwoven fabric comprising a plurality of continuous fibers. Each fiber of the plurality of continuous fibers includes a single polymer that contains a polyester. The continuous fibers are randomly bonded throughout the nonwoven substrate.

In another aspect, this disclosure features an article that includes a nonwoven fabric containing a plurality of fibers. The nonwoven fabric has a M-4 web uniformity index of at most about 600. When the nonwoven fabric has a unit weight of 34 gsm, the nonwoven fabric has a tensile strength of at least about 10 pounds in a cross-machine direction as measured according to ASTM D4595-09.

In another aspect, this disclosure features an article that includes a nonwoven fabric comprising a plurality of spunbonded fibers. The nonwoven fabric has a M-4 web uniformity index of at most about 600.

In another aspect, this disclosure features an article that includes a nonwoven fabric comprising a plurality of fibers. When the nonwoven fabric has a unit weight of 34 gsm, the nonwoven fabric has a tensile strength of at least about 10 pounds in a cross- machine direction as measured according to ASTM D4595-09. The nonwoven fabric does not comprise a polymer having an intrinsic viscosity higher than about 0.64 dl/g.

In another aspect, this disclosure features a membrane filtration medium that includes at least one of the above-mentioned articles.

In still another aspect, this disclosure features a method that extruding a composition containing a single polymer to form a plurality of unbonded continuous fibers; and area bonding the unbonded continuous fibers to form a nonwoven fabric comprising a plurality of bonded continuous fibers. The single polymer includes a polyester.

Embodiments can include one or more of the following optional features.

Each fiber can include a single polymer, which can include a polyester. For example, the single polymer can be a polyethylene terephtalate, a polybutylene terephthalate, a polytrimethylene terephthalate, a polyethylene naphthalate, a

polyglycolide, a polylactide, a polycaprolactone, a polyethylene adipate, a

polyhydroxyalkanoate, or a copolymer thereof.

The single polymer can have an intrinsic viscosity of at least about 0.5 dl/g and/or at most about 0.7 dl/g.

At least some of the fibers can have a circular cross-section. For example, the circular cross-section can have an average diameter of from about 6 μιη to about 20 μιη.

At least some of the fibers can have a trilobal, quadrulobal, pentalobal, or octalobal cross-section. For example, the cross-section such a fiber can have an average diameter of from about 1 μιη to about 6 μιη.

The fibers can be randomly bonded throughout the nonwoven substrate.

When the nonwoven fabric has a unit weight of 34 gsm, the nonwoven fabric can have a tensile strength of at least about 10 pounds in a cross-machine direction as measured according to ASTM D4595-09.

The nonwoven fabric can have a M-4 web uniformity index of at most about 600.

The nonwoven fabric can have a mean pore size of at least about 5 μιη and/or at most about 125 μιη.

The nonwoven fabric can have a thickness of at least about 50 μιη and/or at most about 550 μιη.

The nonwoven fabric can have a bubble point of at least about 25 μιη and/or at most about 200 μιη.

The membrane filtration medium can be a reverse osmosis filtration medium.

The area bonding can include through-air bonding the unbonded continuous fibers to form the nonwoven substrate. The area bonding can be carried out at a temperature of at least about 145°C and/or at most about 250°C.

Prior to area bonding the unbonded continuous fibers, the method can further include passing the unbonded continuous fibers through at least two draw rolls to form oriented fibers. For example, each of the two draw rolls can have a fiber speed of at least about 1,800 meters per minute.

After area bonding the unbonded continuous fibers, the method can further include calendering the nonwoven substrate to form a calendered product. The calendering can be carried out at a temperature of at least about 145°C and/or at most about 215°C. The calendered product can include a membrane filtration medium.

The bonded continuous fibers can include spunbonded fibers.

Embodiments can provide one or more of the following advantages.

Spunbonded nonwoven fabric formed by area bonding fibers containing a single polymer having a certain intrinsic viscosity value (e.g., 0.60 dl/g to 0.64 dl/g) can have a high surface uniformity similar to or better than that of a wet laid nonwoven fabric, while having improved tensile strength (e.g., in the machine direction or cross-machine direction) and lower product costs compared to a wet laid nonwoven fabric.

One advantage of using a single polymer to prepare a nonwoven fabric is that no additional binder polymer is needed to bond fibers, thereby reducing the manufacturing costs.

Other features and advantages of the invention will be apparent from the description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a spunbonded nonwoven fabric formed of continuous fibers made from a single polymer.

FIG. 2 is a schematic illustration of an apparatus for producing spunbonded nonwoven fabrics.

FIG. 3 is partial cut-away perspective view of a filter system including a nonwoven fabric shown in FIG. 1. FIG. 4 is a perspective view of a reverse osmosis membrane filter including a nonwoven fabric shown in FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of a spunbonded nonwoven fabric 10 formed of a plurality of continuous fibers 12, which are area bonded with each other through a plurality of intermittent bonds 14 random distributed throughout the entire non-woven fabric. As used herein, the term "nonwoven fabric" refers to a fabric containing one or more layers of fibers that are bonded together, but not in an identifiable manner as in a knitted or woven material. The term "continuous fiber" mentioned herein refers to fibers formed in a continuous process that are not shortened prior to being incorporated into a fabric containing the continuous fibers. A nonwoven fabric can be formed in a continuous process where a continuous fiber is provided in the fabric. The nonwoven fabric can then be cut to a particular size (having a distinct length and width) and the continuous fiber would generally have a length equal to or greater than the length or width of the non- woven fabric. By contrast, a staple fiber refers to a fiber shortened prior to being incorporated into a fabric and therefore has a distinct length independent of and different from the length or width of the fabric. The term "spunbonded fiber" mentioned herein refers to a fiber that has been extruded, drawn, and laid on a moving substrate (e.g., a move belt). In general, non- woven fabric 10 has a planar structure that is relatively flexible and porous.

In general, fibers 12 are formed from a single polymer. As used herein, the term "a single polymer" refers to a polymer containing molecules having the same chemical composition and having a monomodal molecular weigh distribution. A polymer having a monomodal molecular weigh distribution mentioned herein refers to a polymer that has only one distinguishable molecular weight peak in a chromatogram of Gel Permeation Chromatography (GPC). As one example, a fiber made from polymers having two different chemical compositions (e.g., a polyethylene and a polyester) is not "a single polymer" within the meaning of this term defined in this disclosure. As another example, a fiber made from a polymer containing molecules having the same chemical

composition but having a multimodal molecular weight distribution (e.g., a blend of two polyethylene terephthalate polymers having two distinguishable molecular weight peaks in a GPC chromatogram) is also not "a single polymer" within the meaning of this term defined in this disclosure. In addition, a polymer containing molecules having the same chemical composition but significantly different intrinsic viscosity values (e.g., 0.61 dl/g and 0.67 dl/g) has a multimodal molecular weight distribution. Thus, such as a polymer is also not "a single polymer" within its meaning as defined in this disclosure.

An example of the single polymer that can be used in fibers 12 is a polyester, such as a polyethylene terephtalate (PET), a polybutylene terephthalate (PBT), a

polytrimethylene terephthalate (PTT), a polyethylene naphthalate (PEN), a polyglycolide or polyglycolic acid (PGA), a polylactide or polylactic acid (PLA), a polycapro lactone (PCL), a polyethylene adipate (PEA), a polyhydroxyalkanoate (PHA), or a copolymer thereof. Without wishing to be bound by theory, it is believed that fibers formed from a polyester can provide suitable stiffness to the nonwoven fabric for use in a filter medium. The single polymer can be a homopolymer or a copolymer.

In general, fibers 12 are area bonded to form nonwoven fabric 10 with a high surface uniformity. Nonwoven fabric 10 thus formed containing intermittent bonds 14 randomly distributed throughout the entire nonwoven fabric (i.e., without a bonding pattern) at locations where the fibers come into contact with one another. As a result, such a nonwoven fabric can be distinguished from nonwoven fabrics having a repeating bonding pattern, such as those prepared by point bonding (e.g., by passing fibers through two heated nip rolls, at least one of which includes a pattern). In some embodiments, a nip roll used in point bonding may have a random pattern. However, as the random pattern repeats after the nip roll completes a cycle, the bonding in the nonwoven fabric thus formed is still considered as having a pattern within the meaning of this term in this disclosure. Without wishing to be bound by theory, it is believed that nonwoven fabric 12 prepared by area bonding can have a much higher surface uniformity (e.g., a much higher M-4 web uniformity index) than that prepared by point bonding as area bonding can result in a fabric with a smooth surface, while point bonding results in a fabric with indentations of fused fibers on the surface. Without wishing to be bound by theory, it is believed that a nonwoven fabric prepared by area bonding can be desirable for use in a membrane filtration medium as the fabric can be porous throughout the entire surface. By contrast, as fabrics prepared by point bonding include indentations on its surface, the indentations prevent filtration from occurring at those points and therefore reduce the available filtration area, which in turn can increase pressure drop and reduce filter life.

Area bonding can be carried out by various methods known in the art. As an example, area bonding can be achieved by through-air bonding, i.e., by passing heated air, steam or other gas through an unbonded web of fibers to cause the fibers to melt and fuse to one another at points of contact. As another example, area bonding can be achieved by calendar bonding, i.e., by passing a web of fibers through a calender composed of two smooth steel rollers heated to cause the fibers to soften and then bond together.

Area bonding is typically carried out at an elevated temperature. For example, fibers 12 can be area bonded at a temperature ranging from at least about 145°C (e.g., at least about 155°C, at least about 175°C, at least about 195°C, at least about 215°C, or at least about 225°C) to at most about 250°C (e.g., at most about 245°C, at most about 240°C, at most about 235°C, or at most about 230°C). Typically, the area bonding is carried out between from about 225°C and about 245°C by using through air bonding or from about 175°C and about 245°C by using flat calendering. Heating during the area bonding process can be achieved by conduction (e.g., by using heated rolls in a calendar bonding process), convection (e.g., by using heated air in a through-air bonding process), or vibration (e.g., by sonic welding). In general, heat is uniformly applied to the entire fabric of fibers. Without wishing to be bound by theory, as the single polymer used in fibers 12 has a relatively low intrinsic viscosity (e.g., from about 0.60 dl/g to about 0.64 dl/g), it is believed that the single polymer softens at the temperature used in area bonding and, as a result, fibers bonds to each other at any contact point and therefore form random bonds substantially uniformly throughout the entire fabric without any bonding pattern. By contrast, fabrics prepared by point bonding only forms bonds where the fibers contact the protrusions on a patterned nip roll and leave the fibers in the locations between the point bond sites unbonded. As a result, fabrics prepared by point bonding include a bonding pattern.

Nonwoven fabric 10 can have a relatively high Gurley stiffness in the machine direction (MD) and/or the cross-machine direction (CD). The term "machine direction" used herein refers to the direction of movement of an unbonded web, a bonded web, or a nonwoven fabric during the production processing. The length of the unbonded web, bonded web, or nonwoven fabric is typically a dimension in the machine direction. The term "cross-machine direction" used herein refers to the direction perpendicular to the direction of movement of an unbonded web, a bonded web, or a nonwoven fabric during production or processing. The width of the unbounded web, bonded web, or nonwoven fabric is typically a dimension in the cross-machine direction. For example, nonwoven fabric 10 can have a Gurley stiffness ranging from at least about 65 mg (e.g., at least about 100 mg, at least about 200 mg, at least about 300 mg, or at least about 500 mg) to at most about 1,800 mg (e.g., at most about 1,600 mg, at most about 1,400 mg, at most about 1,200 mg, or at most about 1,000 mg) in the machine direction. As another example, nonwoven fabric 10 can have a Gurley stiffness ranging from at least about 45 mg (e.g., at least about 80 mg, at least about 150 mg, at least about 250 mg, or at least about 400 mg) to at most about 850 mg (e.g., at most about 700 mg, at most about 6000 mg, at most about 500 mg, or at most about 400 mg) in the cross-machine direction. The stiffness of the nonwoven substrate mentioned herein refers to that measured by using a Gurley type tester according to ASTM D6125-97 (2002). Without wishing to be bound by theory, it is believed that nonwoven fabric 10, which is prepared by area bonding, can have a higher Gurly stiffness than that of a nonwoven fabric prepared by point bonding having the same unit weight.

The single polymer suitable for use in fibers 12 has a relative low intrinsic viscosity value. For example, the single polymer can have an intrinsic viscosity value of at most about 0.7 dl/g (e.g., at most about 0.69 dl/g, at most about 0.68 dl/g, at most about 0.67 dl/g, at most about 0.66 dl/g, at most about 0.65 dl/g, at most about 0.64 dl/g, at most about 0.63 dl/g, at most about 0.62 dl/g, at most about 0.61 dl/g, or at most about 0.60 dl/g) or at least about 0.5 dl/g (e.g., at least about 0.52 dl/g, at least about 0.54 dl/g, at least about 0.56 dl/g, at least about 0.58 dl/g, or at least about 0.60 dl/g). Typically, the single polymer has an intrinsic viscosity value from about 0.60 dl/g to about 0.64 dl/g (e.g., from about 0.61 dl/g to about 0.63 dl/g). As used herein, the intrinsic viscosity mentioned herein is measured according to the method described in ASTM D4603-03. Without wishing to be bound by theory, it is believed that if the single polymer has an intrinsic viscosity that is too high (e.g., greater than about 0.7 dl/g), the polymer does not soften at a sufficient level to form bonding between fibers at the temperature generally used in area bonding. Further, without wishing to be bound by theory, it is also believed that if the single polymer has an intrinsic viscosity that is too low (e.g., lower than about 0.5 dl/g), the polymer does not have a sufficient tensile strength to form a nonwoven fabric that is strong enough for use in a filter medium.

Conventionally, a spunbonded nonwoven fabric formed by area bonding is either made by fibers containing polymers with different chemical compositions (e.g., a polyethylene and a PET homopolymer or a PET copolymer and a PET homopolymer) or fibers containing polymers with the same chemical compositions but significantly different intrinsic viscosity values (e.g., polyethylene terephthalates having intrinsic viscosity values of 0.61 dl/g and 0.67 dl/g). In those fibers, at least one of the polymers is used to provide mechanical strength of the spunbonded nonwoven fabric (e.g., the polymer having a higher intrinsic viscosity or higher melting point) and at least one of the polymers that can readily soften at the bonding temperature is used as a binder to form bonds between fibers (e.g., the polymer having a lower intrinsic viscosity or lower melting point). Unexpectedly, the inventors have found that a single polymer having one intrinsic viscosity within the range discussed in the preceding paragraph (e.g., from about 0.60 dl/g to about 0.64 dl/g) can be area bonded to form a spunbonded nonwoven fabric with a high surface uniformity (i.e., having a highly smooth surface) and sufficient mechanical strength so that the fabric made from such nonwoven fabric is suitable for use as a filter medium. Without wishing to be bound by theory, it is believed that one advantage of using a single polymer to prepare a nonwoven fabric is that the area bonding and/or thermal calendering process used in manufacturing the fabric can be readily optimized (e.g., by adjust the bonding and/or calendering temperature) to produce a fabric with an improved web uniformity, while it can be difficult to optimize these processes if two or more polymers are used. In addition, without wishing to be bound by theory, it is believed that another advantage of using a single polymer to prepare a nonwoven fabric is that no additional binder polymer is needed to bond fibers, thereby reducing the manufacturing costs. The cross-section of fibers 12 can have different shapes as desired. In some embodiments, at least some of fibers 12 can have a circular cross-section (e.g., a circular transverse cross-section). In such embodiments, fibers 12 with a circular cross-section can have an average diameter ranging from at most about 20 μιη (e.g., at most about 18 μιη, at most about 16 μιη, or at most about 14 μιη) to at least about 4.5 μιη (e.g., at least about 5 μιη, at least about 6 μιη, at least about 7 μιη, at least about 8 μιη, or at least about

9 μιη). In general, the average diameter of fibers correlate proportionally to the linear mass density of the fibers and the correlation can depend on the polymer used to form the fibers. For example, fibers with a linear mass density of about 0.9 dtex (i.e., about 1 dpi) correspond to fibers having an average diameter of about 10 μιη when the fibers are made of a polyester and correspond to fibers having an average diameter about 12.5 μιη when the fibers are made of a polypropylene. Thus, fibers 12 having the average diameter described above can have a linear mass density ranging from at most about 1.8 dtex (e.g., at most about 1.6 dtex, at most about 1.44 dtex, or at most about 1.26 dtex) to at least about 0.4 dtex (e.g., at least about 0.45 dtex, at least about 0.54 dtex, at least about 0.63 dtex, at least about 0.72 dtex, or at least about 0.81 dtex) when fibers 12 are made by a polyester. Without wishing to be bound by theory, it is believed that fibers having a smaller average diameter (or a smaller linear mass density) results in a nonwoven fabric with a better filtration efficiency and a higher surface uniformity (i.e., a smoother surface). On the other hand, if the average diameter of the fibers in nonwoven fabric 10 is too small, such a fabric could have a very high pressure drop, which would reduce the filtration capacity of the fabric.

In some embodiments, fibers 12 can have a cross-section (e.g., a transverse cross- section) with a multilobal shape (e.g., a trilobal, quadrulobal, pentalobal, or octalobal shape). Such fibers can have an average diameter ranging from at most about 18 μιη (e.g., at most about 16 μιη, at most about 14 μιη, at most about 12 μιη, or at most about

10 μιη) to at least about 1 μιη (e.g., at least about 2 μιη, at least about 3 μιη, at least about 4 μιη, at least about 6 μιη, or at least about 8 μιη). As used herein, the diameter of a multilobal fiber refers to the distance from the tip of a lobe across the center of the cross- section to the end of the cross-section on the other side of the center. In addition, fibers 12 having the size described above can have a linear mass density ranging from at most about 2.4 dtex (e.g., at most about 2.2 dtex, at most about 2.0 dtex, at most about 1.8 dtex, or at most about 1.6 dtex) to at least about 0.45 dtex (e.g., at least about 0.54 dtex, at least about 0.63 dtex, at least about 0.72 dtex, at least about 0.81 dtex, or at least about 0.9 dtex). Typically, fibers 12 having a multilobal cross-section can have a linear mass density of between about 1.6 dtex and about 2.2 dtex. Without wishing to be bound by theory, it is believed that, as fibers with a mulitlobal cross-section have a larger surface area per unit weight than that of fibers with a circular cross-section, the former fibers with a relatively small dimension (e.g., a relatively small diameter or a relatively small linear mass density) can be used to prepare a nonwoven fabric with a surface uniformity similar to that of the nonwoven fabric prepared by the latter fibers with a relatively large dimension (e.g., a relatively large diameter or linear mass density). As a result, the nonwoven fabric prepared by the former fibers can have a better filtration efficiency and a higher tensile strength than those of the nonwoven fabric prepared by the latter fibers.

Spunbonded nonwoven fabric 10 (e.g., having a unit weight of 34 gram per square meter (gsm) and/or having an area of 32 square inches) can have a tensile strength in the machine direction ranging from at least about 10 pounds (e.g., at least about 15 pounds, at least about 20 pounds, at least about 25 pounds, or at least about 30 pounds) to at most about 50 pounds (e.g., at most about 45 pounds, at most about 40 pounds, at most about 35 pounds, or at most about 30 pounds). Typically, spunbonded nonwoven fabric 10 can have a tensile strength in the machine direction ranging from about 20 pounds to about 40 pounds. Spunbonded nonwoven fabric 10 (e.g., having a unit weight of 34 gsm and/or having an area of 32 square inches) can have a tensile strength in the cross-machine direction ranging from at least about 10 pounds (e.g., at least about 15 pounds, at least about 20 pounds, at least about 25 pounds, or at least about 30 pounds) to at most about 50 pounds (e.g., at most about 45 pounds, at most about 40 pounds, at most about 35 pounds, or at most about 30 pounds). Typically, spunbonded nonwoven fabric 10 can have a tensile strength in the machine direction ranging from about 18 pounds to about 24 pounds. As used herein, the tensile strength of nonwoven fabric 10 is measured by the Grab Tensile method according to ASTM D4595-09. In general, spunbonded nonwoven fabric having a higher unit weight has an increased tensile strength in both the machine direction and the cross-machine direction. Without wishing to be bound by theory, it is believed that spunbonded nonwoven fabric 10 has significantly higher tensile strength in the machine direction and/or the cross-machine direction per unit weight than that of a wet laid nonwoven fabric. In addition, without wishing to be bound by theory, it is believed that another advantage of spunbonded nonwoven fabric 10 is that it has a superior fuzz rating compared to that of a wet laid nonwoven fabric since the former fabric is made by continuous fibers (and therefore has a minimal amount of fiber ends on the fabric surface), while the latter fabric is made by staple fibers that have a typically fiber length of from about 0.5 inch to about 3 inches (and therefore has a significant amount of fiber ends at the fabric surface).

In some embodiments, spunbonded nonwoven fabric 10 can have a ratio between a tensile strength in a machine direction and a tensile strength in a cross-machine direction ranging from at most about 3 : 1 (e.g., at most about 2.5: 1, at most about 2: 1, at most about 1.5: 1, at most about 1.4: 1, at most about 1.2: 1) to at least about 1 : 1 (e.g., at least about 1.1 : 1 , at least about 1.3: 1, at least about 1.5 : 1 , or at least about 2: 1).

The mean pore size of nonwoven fabric 10 can vary depending on how the fabric is prepared and the intended use of the fabric. The "mean pore size" used herein is measured on one square inch (i.e., about 6.45 cm²) sample by Capillary Flow Porometer CFP 1200 AEX available from Porous Materials, Inc, Ithaca, NY. For example, nonwoven fabric 10 can have a mean pore size ranging from at least about 35 μιη (e.g., at least about 45 μιη, at least about 55 μιη, at least about 65 μιη, at least about 75 μιη, or at least about 85 μιη) to at most about 125 μιη (e.g., at most about 120 μιη, at most about 110 μιη, at most about 100 μιη, at most about 90 μιη, or at most about 80 μιη) after the fabric is area bonded but before it is calendered to form a calendered product (e.g., a filtration medium for pool or spa, or a dryer sheet) with a reduced pore size. As another example, nonwoven fabric 10 can have a mean pore size ranging from at least about 5 μιη (e.g., at least about 7 μιη, at least about 9 μιη, at least about 1 1 μιη, at least about 13 μιη, at least about 15 μιη, or at least about 17 μιη) to at most about 25 μιη (e.g., at most about 23 μιη, at most about 21 μιη, at most about 19 μιη, at most about 17 μιη, or at most about 15 μιη) after the fabric is both area bonded and calendered to form a calendered product (e.g., a filtration medium) with a reduced pore size. The bubble point of nonwoven fabric 10 can vary as desired. The term "bubble point" mentioned herein refers to the largest pore size in a one-square-inch (i.e., about 6.45 cm²) sample measured by Capillary Flow Porometer CFP 1200 AEX available from Porous Materials, Inc. For example, nonwoven fabric 10 can have a bubble point ranging from at least about 75 μιη (e.g., at least about 85 um, at least about 95 μιη, at least about 105 μιη, or at least about 1 15 μιη) to at most about 200 μιη (e.g., at most about 190 μιη, at most about 180 μιη, at most about 170 μιη, or at most about 160 μιη) after the fabric is area bonded but before it is calendered to form a calendered product with a reduced pore size. As another example, nonwoven fabric 10 can have a bubble point ranging from at least about 25 μιη (e.g., at least about 30 μιη, at least about 35 μιη, at least about 40 μιη, or at least about 45 μιη) to at most about 50 μιη (e.g., at most about 45 μιη, at most about 40 μιη, at most about 35 μιη, or at most about 30 μιη) after the fabric is both area bonded and calendered to form a calendered product (e.g., a filtration medium) with a reduced pore size.

The thickness of nonwoven fabric 10 can also vary as desired. For example, nonwoven fabric 10 can have a thickness ranging from at least about 200 μιη (e.g., at least about 250 μιη, at least about 300 μιη, at least about 350 μιη, or at least about 400 μιη) to at most about 550 μιη (e.g., at most about 500 μιη, at most about 450 μιη, at most about 400 μιη, or at most about 350 μιη) after the fabric is area bonded but before it is calendered to form a calendered product with a reduced pore size. As another example, nonwoven fabric 10 can have a thickness ranging from at least about 50 μιη (e.g., at least about 75 μιη, at least about 100 μιη, at least about 125 μιη, or at least about 150 μιη) to at most about 250 μιη (e.g., at most about 225 μιη, at most about 200 μιη, at most about 175 μιη, or at most about 150 μιη) after the fabric is both area bonded and calendered to form a calendered product (e.g., a filtration medium) with a reduced thickness.

Nonwoven fabric 10 can have various unit weights depending on the intended use. For example, nonwoven fabric 10 can have a unit weight ranging from at least about 15 grams per square meter (gsm) (e.g., at least about 34 gsm, at least about 51 gsm, at least about 68 gsm, at least about 85 gsm, at least about 102 gsm, or a least about 136 gsm) to at most about 260 gsm (e.g., at most about 255 gsm, at most about 238 gsm, at most about 221 gsm, at most about 204 gsm, at most about 187 gsm, at most about 170 gsm, at most about 153 gsm, at most about 136 gsm, or at most about 119 gsm). As used herein, the unit weight of nonwoven fabric 10 is measured according to ASTM D3776- 96.

In general, nonwoven fabric 10 has a high surface uniformity. The surface uniformity (i.e., smoothness) of nonwoven fabric 10 can be quantified by using a M-4 web uniformity index.

The M-4 web uniformity index used herein is determined based on four physical properties of a nonwoven fabric (i.e., the thickness, unit weight, bubble point, and mean pore size) and is obtained by the following general method: A certain number of samples (e.g., 30 samples) having a certain area (e.g., having an area of 1 square inch, i.e., about 6.45 cm²) are taken uniformly across a nonwoven fabric whose M-4 web uniformity index is to be determined. An even number of samples should be taken in the machine direction and the cross-machine direction. Samples should not be selected based on visual appearance. After the samples have been taken, the thickness, unit weight, bubble point, and mean pore size of each sample are measured. A mean value of each property is then obtained. For example, when 30 samples are used, the mean thickness (t) is obtained by dividing the sum of the thickness values of all samples (i.e., tl + 12 + 13 + ... + t30) by the number of samples (i.e., 30). The mean values of other properties are calculated in the same manner. A random value is pre-selected for each property and is referred hereinafter as "normalized value." The normalized values for the thickness, unit weight, bubble point, and mean pore size are 178 um, 34 gsm, 27.41 μιη, and 9.09 μιη, respectively. The M-4 web uniformity index values mentioned in this disclosure are all calculated based on the just-mentioned normalized values. However, for comparison purposes, other normalized values for these properties can be used as long as the same normalized value for each property is used for all samples being compared.

After the mean value of each property of the samples is obtained, a "normalized factor" for each property is calculated by dividing the normalized value selected for each property by its mean value. For example, if the mean thickness of the samples is 190.5 μιη, the normalized factor is 0.934 (i.e., 178 μιη / 190.5 μιη = 0.934).

A "normalized test value" for each property of each sample is then calculated by multiplying an actual measured value of a property (e.g., thickness) of each sample by the normalized factor obtained above. For example, in the example above where the normalized factor is 0.934 for the thickness of the samples, the normalized test value of the thickness of each sample is calculated by multiplying the actual measured value of the thickness of each sample by 0.934.

After the normalized test values of each property of all samples have been obtained, a "normalized standard deviation" (normalized STDEV) of the normalized test values of each property is calculated by using the following equation:

in which x_{l s} x₂, ... and X_N are the normalized test values of a property of all samples, N is the sample number, μ is the average of xi, x₂, ... and X_N and is calculated by (xi + x₂ + ... + X_N) N, and σ is the normalized STDEV of the property.

The M-4 web uniformity index is then calculated by the following equation:

M-4 web uniformity index = (normalized thickness STDEV + normalized unit weight STDEV + normalized bubble point STDEV + normalized mean pore size STDEV) x 100. A more detailed description of obtaining the M-4 web uniformity index for nonwoven fabric 10 is provided in the Examples section below.

In general, a fabric with a lower M-4 web uniformity index has a more uniform surface (i.e., a smoother surface) than a fabric with a higher M-4 web uniformity index. As an example, using the method described above, nonwoven fabric 10 can have a M-4 web uniformity index ranging from at most about 600 (e.g., at most about 575, at most about 550, at most about 525, at most about 500, at most about 475, at most about 450, at most about 425, or at most about 400) to at least about 75 (e.g., at least about 150, at least about 200, at least about 250, at least about 300, at least about 350, or at least about 400). For example, nonwoven fabric 10 can have an M-4 web uniformity index from about 100 to about 600 (e.g., from about 150 to about 575 or from about 200 to about 550).

Without wish to be bound by theory, it is believed that nonwoven fabric 10 can have a M- 4 web uniformity index similar to or better than that of a wet laid nonwoven fabric, which is generally considered as having the most uniform surface among the nonwoven fabrics made by currently known methods. Without wishing to be bound by theory, it is believed that one advantage of spunbonded nonwoven fabric 10 is that it has improved surface uniformity with reduced fibers sizes compared to a conventional spunbonded nonwoven fabric. As a result, spunbonded nonwoven fabric 10 has improved filtration efficiency when used as a membrane filtration medium compared to a conventional spunbonded nonwoven fabric. In addition, spunbonded nonwoven fabric 10 can have a surface uniformity similar to or better than that of a wet laid nonwoven fabric (which is generally superior to that of a conventional spunbonded nonwoven fabric), but have improved tensile strength and reduced manufacturing costs. Thus, spunbonded nonwoven fabric 10 can be used to replace wet laid nonwoven fabrics in certain applications (e.g., in membrane filtration media).

Spunbonded nonwoven fabric 10 can be made from one or more (e.g., two, three, four, or five) layers of nonwoven materials. For example, fabric 10 can be made from one layer of spunbonded fibers containing the single polymer mentioned above. As another example, fabric 10 can be made from more than one layer of spunbonded fibers, each of which contains fibers made from the same single polymer but has different physical properties such as pore size, fiber size, or bonding density.

FIG. 2 illustrates an apparatus for producing spunbonded nonwoven fabric 10. As shown in FIG. 2, the apparatus includes first and second successively arranged spin beams 22 mounted above an endless moving conveyor belt 24. While the illustrated apparatus has two spin beams, other configurations of apparatus with only one spin beam or with three or more spin beams could be employed. Each beam extends widthwise in the cross-machine direction, and the respective beams are successively arranged in the machine direction. Each beam is supplied with a molten polymer (e.g., a single polyester having one intrinsic viscosity) from one or more extruders (not shown in FIG. 2).

Spinnerets with orifices configured for producing continuous filaments are mounted on each of spin beams 22.

The freshly extruded filaments can then be cooled and solidified by contact with a flow of quench air. The filaments can then be attenuated and drawn by devices 26 by methods known in the art, such as mechanically drawn methods or pneumatically drawn methods (e.g., slot drawn methods). For example, when devices 26 can include draw rolls, the filaments can be mechanically attenuated. Methods of mechanically drawing filaments are known in the art and have been described in, e.g., U.S. Patent No.

5,665,300. As another example, when devices 26 include attenuator devices, the filaments can be pneumatically attenuated and drawn. For example, when devices 26 include slot-shaped attenuators, the filaments can be slot drawn. Methods of slot drawing filaments are known in the art and have been described in, e.g., U.S. Patent Nos.

3,338,992, 4,208,366, 4,233,014, and 5,368,913.

The attenuated and drawn filaments can then be deposited randomly onto advancing conveyor belt 24 to form a web. The filaments can then area bonded at an elevated temperature to give the web coherency and strength. Area bonding typically involves passing the web through a heated calender composed of two smooth steel rollers or passing heated steam, air or other gas through the web to cause the filaments to become tacky and fuse to one another.

Specifically, as shown in FIG. 2, the web of unbonded filaments can be directed through a steam consolidator 32, an example of which is shown in U.S. Patent No.

3,989,788. The web can be contacted with saturated steam, which serves to soften the filaments. The web can then be transferred to a hot air bonder 34 to be bonded. In general, the temperature used in the bonding process is considerably higher than that used in the consolidator and can depend on the tack temperature of the polymer used in the fibers and the properties desired in the product (e.g., strength, dimensional stability or stiffness). For example, when fibers contain polyethylene terephthalate, the consolidated web is typically exposed to air at 140 to 250°C (e.g., 215 to 250°C) during bonding. During the consolidation and bonding steps, the fibers soften and become tacky, producing fusion bonds where the fibers contact one another. The resulting nonwoven fabric is an area bonded fabric with random bonding sites substantially uniformly distributed throughout the entire fabric. The bonding sites provide the necessary sheet properties such as tear strength and tensile strength. After the web is bonded, it can pass over exit roll to a windup device 36.

The bonded web can be further calendered to form a calendered product. The calendering process can be carried out by passing the bonded web through a heated calender having three or more smooth rollers (e.g., steel and nylon rollers). For example, when fabric 10 is made from a polyester, the calender can be heated at a temperature ranging from at most about 215°C (e.g., at most about 205°C, at most about 195°C, at most about 185°C, or at most about 175°C) to at least about 145°C (e.g., at least about 150°C, at least about 160°C, at least about 170°C, or at least about 180°C). Such a process can have a calendering speed of from about 9.14 m/min to about 91.4 m/minute (i.e., from about 10 to about 100 yards/minute) and a pressure between rolls of from about 525 to about 4903 Newtons per centimeter of roll width (from about 300 to about 2,800 pounds per inch of roll width (PLI)). As an specific example, fabric 10 made from a polyethylene terephthalate having an intrinsic viscosity of 0.61 dl/g can be calendered at about 171°C under a pressure of about 1,962 Newtons per centimeter of roll width (i.e., about 1,120 PLI) at a calendering speed of about 50 m/minute (i.e., about 55

yards/minute) to produce a calendered product. Without wishing to be bound by theory, it is believed that the calendering process can further reduce the thickness and pore size of nonwoven fabric 10 and increase the surface uniformity (i.e., surface smoothness) of the fabric.

Nonwoven fabric 10 can be used in a wide variety of applications. For example, uncalendered nonwoven fabric 10 can be used as a filtration medium (e.g., a pool and spa filtration medium), a dryer sheet, or a support for fiber reinforced plastic. Exemplary filtration media include pool and spa filtration media and media used in heavy duty air filtration systems, gas turbine filtration systems, window covers (e.g., blinds), liquid filtration systems (e.g., waste water or drinking water filtration systems), HEPA filtration systems, vacuum bag filtration systems, fuel filtration systems, oil filtration systems, battery separators, and/or pulse cleaning applications. As another example, calendered nonwoven fabric 10 can be used in membrane filtration media (e.g., as a support in a filtration medium such as a reverse osmosis filtration medium), garments, dryer sheets, and towels. Exemplary filtration media include ultra-filtration media, micro-filtration media, and reverse osmosis filtration media. Such a filtration medium can include one or more layers (e.g., nonwoven layers, filtration membranes, or films) different from nonwoven fabric 10 to control dirt holding capacity or filtration efficiency. These layers can be calendered together with nonwoven fabric 10 (e.g., by using the calendering process described above) to form a filtration medium. For example, when calendered nonwoven fabric 10 is used as a support in a reverse osmosis filtration medium (e.g., for desalination or pharmaceuticals separation), a reverse osmosis membrane can be attached to fabric 10 to separate molecules with different sizes.

FIG. 3 shows a cut-away perspective of an exemplary filter system 100 including a filter housing 101, a filter cartridge 102, an inner screen 108 and an outer screen 103. Nonwoven fabric 10 is disposed in filter cartridge 102. During use, a gas or liquid enters system 100 via an opening 104 and then passes through inner screen 108, nonwoven fabric 10 and outer screen 103. The gas then exits filter assembly 100 via opening 106. Nonwoven fabric 10 can optionally be pleated into any of a variety of configurations (e.g., panel or cylindrical).

FIG. 4 is a perspective view of a reverse osmosis membrane filter 200 including a filter housing 202 and reverse osmosis membrane medium 204. Medium 204 includes a nonwoven fabric 10 and a reverse osmosis membrane supported by nonwoven fabric 10. Medium 204 and housing 202 together form a feed channel 206, through which a feed liquid passes through. Medium 204 also forms a permeate channel 208, through which a filtered liquid is collected. During use, a feed liquid can be delivered into feed channel 206 under a high pressure so that a permeate obtained by filtration through membrane medium 204 can be obtained through permeate channel 208.

The following example is illustrative and not intended to be limiting. The contents of all publications cited herein (e.g., patents, patent application publications, and articles) are hereby incorporated by reference in their entirety. In particular, this application incorporates by reference the contents of co-pending U.S. Provisional Application Serial No. 61/393,232. Example 1 : Nonwoven fabrics containing a single polymer

The following spunbonded nonwoven fabrics were made by using a single polyethylene terephthalate (PET) homopolymer having an intrinsic viscosity of 0.62 dl/g: (1) an uncalendered fabric containing 1.1 dpf fibers with a round cross-section; (2) an uncalendered fabric containing 1.9 dpf fibers with a trilobal cross-section; (3) an uncalendered fabric containing 2.4 dpf fibers with a trilobal cross-section; (4) a calendered fabric containing 1.1 dpf fibers with a round cross-section; (5) a calendered fabric containing 1.9 dpf fibers with a trilobal cross-section; and (6) a calendered fabric containing 2.4 dpf fibers with a trilobal cross-section.

Specifically, the polymer was dried at 140°C for 5 hours and then was extruded by using an extruder having spinnerets containing 2,310 round holes with a diameter of 0.009 inch to prepare fabric (1), and an extruder having spinnerets containing 1,080 trilobal holes with a dimension of 0.004 inch x 0.011 inch to prepare fabrics (2) and (3). Three extrusion throughputs, i.e., 95, 75, and 95 pounds per hour, were used to produce fabrics (1), (2), and (3). The spinning speed was held constant at the two throughputs at 2,733 yards per minute. The drawn fibers were dispersed on a moving laydown belt moving at a speed of 90 yards per minute. The web was then consolidated by partial bonding using hot steam. The filaments were subsequently area bonded together at 235°C to produce nonwoven fabrics (1), (2), and (3). Fabrics (1), (2), and (3) were then calendered using a 3-roll calendar (i.e., two steel rolls and a nylon roll) to form fabrics (4), (5), and (6), respectively. Specifically, the fabric was first drawn around one steel roll and nipped between the steel roll and the nylon roll on one side and then drawn around the nylon roll and was nipped again by the other steel roll.

The properties of fabrics (l)-(6) were measured by the methods described herein and are summarized in Tables 1 and 2 below. In Tables 1 and 2, Air perm was measured according to ASTM D737-04; thickness was measured according to ASTM D 1777-96 Mullen was measured according to ASTM D3786-09; unit weight was measured according to ASTM D3776-96; MD dry heat shrinkage (DHS) and CD DHS were measured according to ASTM D2259-02; MD tensile and CD tensile were measured over samples having an area of 32 square inch according to ASTM D4595-09; MD tear and CD tear were measured over samples having an area of 32 square inch according to ASTM D 1424-09; and M-4 web uniformity was measured based on the method described herein. Table 1. Physical Properties of Uncalendered Products

It is known that fabrics made from finer fibers (i.e., fibers with a smaller denier value) generally exhibits better surface smoothness. However, as shown in Tables 1 and 2, fabrics made from fibers with a trilobal cross-section and a higher denier value exhibited lower M-4 web uniformity indices and therefore better surface smoothness than that fabrics made from fibers with a round cross-section and a smaller denier value. In other words, the results above show that, using fibers with a trilobal cross-section to prepare a nonwoven fabric can significantly improve surface smoothness of the fabric without reducing the fiber size (which could reduce the tensile properties of the fabric). In addition, as shown in Table 1 , uncalendered fabrics (2) and (3) exhibited a M-4 web uniformity indices of about 555, which is similar to or better than that of a wet-laid nonwoven fabric made from the same material. However, uncalendered fabrics (2) and (3) had significantly higher tensile strength than that of a wet-laid nonwoven fabric made from the same material having the same unit weight.

The liquid filtration efficiencies of fabrics (l)-(6) were measured by LMS Technologies (Bloomington, MN) by using a liquid filtration efficiency test using latex beads as challenge particles and a liquid flow rate of 1 liter per minute. The air filtration efficiencies of fabrics (l)-(6) were measured by LMS Technologies (Bloomington, MN) by using an air fractional efficiency test using potassium chloride as challenge particles and an air flow rate of 100 liter per minute. Before the test, potassium chloride particles were neutralized in a radioactive chamber by creating equal amounts of positive and negatively charged particles to have net charge of zero. This process eliminates the variability caused by excess of negative or positively charged particles.

In addition, Reemay 2024 was prepared as a comparative example using the same method above except that the fabric was made by using a PET homopolymer having an intrinsic viscosity of 0.64 dl/g and a PET copolymer having an intrinsic viscosity of 0.71 dl/g. The liquid and air filtration efficiencies of Reemay 2024 were also measured by the above-mentioned tests. The results are summarized in Tables 3 and 4 below.

Table 3. Liquid Filtration Efficiencies (%) at 16.2 g/hr-6

Table 4. Air Filtration Efficiencies (%) at a velocity of 10 fpm and a pressure drop of

0.018 meter of water

As shown in Tables 3 and 4, fabrics (l)-(6) (which contain a single polymer) exhibited significantly higher filtration efficiencies than Reemay 2024 (which contains two polymers with different intrinsic viscosity values) except that the air filtration efficiencies of fabrics (l)-(3) to filter particles of 0.3-0.5 microns are somewhat less than the air filtration efficiencies of Reemay 2024 to filter particles of the same size.

Other embodiments are in the claims.

Claims

WHAT IS CLAIMED IS:

1. An article, comprising:

a nonwoven fabric comprising a plurality of continuous fibers;

wherein each fiber of the plurality of continuous fibers comprises a single polymer, the single polymer comprises a polyester, and the continuous fibers are randomly bonded throughout the nonwoven substrate.

2. An article, comprising:

a nonwoven fabric comprising a plurality of fibers;

wherein the nonwoven fabric has a M-4 web uniformity index of at most about 600 and, when the nonwoven fabric has a unit weight of 34 gsm, the nonwoven fabric has a tensile strength of at least about 10 pounds in a cross-machine direction as measured according to ASTM D4595-09.

3. An article, comprising:

a nonwoven fabric comprising a plurality of spunbonded fibers;

wherein the nonwoven fabric has a M-4 web uniformity index of at most about

600.

4. An article, comprising:

a nonwoven fabric comprising a plurality of fibers;

wherein, when the nonwoven fabric has a unit weight of 34 gsm, the nonwoven fabric has a tensile strength of at least about 10 pounds in a cross-machine direction as measured according to ASTM D4595-09, and the nonwoven fabric does not comprise a polymer having an intrinsic viscosity higher than about 0.64 dl/g.

5. The article of claim 2-4, wherein each fiber comprises a single polymer.

6. The article of claim 4, wherein the single polymer comprises a polyester.

7. The article of any of claims 1, 5, and 6, wherein the single polymer is a polyethylene terephtalate, a polybutylene terephthalate, a polytrimethylene terephthalate, a polyethylene naphthalate, a polyglycolide, a polylactide, a polycapro lactone, a polyethylene adipate, a polyhydroxyalkanoate, or a copolymer thereof.

8. The article of any of claims 1 and 5-7, wherein the single polymer has an intrinsic viscosity of at most about 0.7 dl/g.

9. The article of any of claims 1 and 5-8, wherein the single polymer has an intrinsic viscosity of at least about 0.5 dl/g.

10. The article of any of claims 1-9, wherein at least some of the fibers have a circular cross-section.

11. The article of claim 10, wherein the circular cross-section has an average diameter of from about 6 μιη to about 20 μιη.

12. The article of any of claims 1-9, wherein at least some of the fibers have a trilobal, quadrulobal, pentalobal, or octalobal cross-section.

13. The article of claim 12, wherein the cross-section has an average diameter of from about 1 μιη to about 6 μιη.

14. The article of any of claims 2-13, wherein the fibers are randomly bonded throughout the nonwoven substrate.

15. The article of any of claims 1, 3, and 5-13, wherein, when the nonwoven fabric has a unit weight of 34 gsm, the nonwoven fabric has a tensile strength of at least about 10 pounds in a cross-machine direction as measured according to ASTM D4595- 09.

16. The article of any of claims 1 and 4-15, wherein the nonwoven fabric has a M-4 web uniformity index of at most about 600.

17. The article of any of claims 1-16, wherein the nonwoven fabric has a mean pore size of at most about 125 μιη.

18. The article of any of claims 1-17, wherein the nonwoven fabric has a mean pore size of at least about 5 μιη.

19. The article of any of claims 1-18, wherein the nonwoven fabric has a thickness of at most about 550 μιη.

20. The article of any of claims 1-19, wherein the nonwoven fabric has a thickness of at least about 50 μιη.

21. The article of any of claims 1-20, wherein the nonwoven fabric has a bubble point of at most about 200 μιη.

22. The article of any of claims 1-21, wherein the nonwoven fabric has a bubble point of at least about 25 μιη.

23. A product, comprising the article of any of claims 1-22, wherein the product is a membrane filtration medium.

24. The product of claim 23, wherein the product is a reverse osmosis filtration medium.

25. A method, comprising:

extruding a composition containing a single polymer to form a plurality of unbonded continuous fibers, the single polymer comprising a polyester; and

area bonding the unbonded continuous fibers to form a nonwoven fabric comprising a plurality of bonded continuous fibers.

26. The method of claim 25, wherein the area bonding comprises through-air bonding the unbonded continuous fibers to form the nonwoven substrate.

27. The method of claim 25 or 26, wherein the area bonding is carried out at a temperature of at most about 250°C.

28. The method of any of claims 25-27, wherein the area bonding is carried out at a temperature of at least about 145°C.

29. The method of any of claims 25-28, wherein, prior to area bonding the unbonded continuous fibers, the method further comprises passing the unbonded continuous fibers through at least two draw rolls to form oriented fibers.

30. The method of claim 29, wherein each of the two draw rolls has a fiber speed of at least about 1,800 meters per minute.

31. The method of any of claims 25-30, wherein, after area bonding the unbonded continuous fibers, the method further comprises calendering the nonwoven substrate to form a calendered product.

32. The method of claim 31 , wherein the calendering is carried out at a temperature of at most about 215°C.

33. The method of claim 31 , wherein the calendering is carried out at a temperature of at least about 145°C.

34. The method of any of claims 31-33, wherein the calendered product comprises a membrane filtration medium.

35. The method of any of claims 25-34, wherein the bonded continuous fibers comprise spunbonded fibers.