Patent Application
20070042663
Laminates having elasticity in the cross-direction have previously been prepared by bonding a neck-stretched (“necked”) nonwoven web, such as a spunbond web, typically formed of inelastic filaments, to an elastic film or nonwoven web. These laminates, referred to as neck-bonded laminates, are formed by bonding the necked nonwoven web to the elastic film or nonwoven web when the former is in the neck-stretched state.
Neck-bonded laminates are described in numerous patents to Morman et al. Representative patents include without limitation U.S. Pat. Nos. 5,226,992; 5,336,545; 5,514,470; 5,910,224 and 6,475,600, which are incorporated by reference.
While neck-bonded laminates provide suitable cross-direction elasticity for a wide variety of applications, they typically involve use of a structurally integrated elastic nonwoven web or film (i.e. a layer which is elastic in all directions) to provide elasticity to the laminate, even when elasticity is required in only one direction, such as the cross-direction of the laminate. A full coverage layer of elastic web or film has disadvantages when used to produce a laminate that is elastic in only one direction, such as the cross direction. First, the amount of elastic polymer used may be more than what is required for the cross-direction elastic laminate since the requirement to form the web or film will dictate the minimum basis weight of the elastic material. Second, an elastic film may limit the laminate to a low amount of water vapor transmission (breathability). Third, elastic webs or films (more than individual filaments) are influenced by the physics of Poisson's ratio which retracts the web or film perpendicular to the direction of stretch. Fourth, webs or films which stretch in both directions require special processing techniques in converting machines because a machine direction stretch causes control problems in tension-controlled sections of the machines. For applications where elasticity is only needed in the cross-direction, there is a need or desire for a neck-bonded laminate which limits the amount of elastic material to that needed to obtain the unidirectional elasticity, and a method for making such a laminate.
The present invention is directed to a cross-directional elasticized composite material, and a method of making it. The composite material includes at least one facing material, which can be a nonwoven web having a machine direction (which is a direction of manufacture of the nonwoven web) and a cross-direction (which is perpendicular to the direction of manufacture). The facing layer can be a necked nonwoven web, which has been neck-stretched in the machine direction to cause narrowing (necking) in the cross direction. The nonwoven web can be formed of inelastic filaments.
The composite material also includes elastic filaments which are disposed in the cross-direction of the facing layer, or significantly in the cross direction (i.e. within about 45 degrees of the cross direction) of the facing layer. The elastic filaments can be substantially parallel to each other, and can be spaced apart and non-intersecting with respect to each other. The elastic filaments are suitably bonded to the facing layer.
The elastic composite material can be prepared by a method which includes the steps of:
feeding a facing layer to a first mandrel at an angle relative to a longitudinal axis of the first mandrel;
rotating the first mandrel in a first direction around the axis, while conveying the facing layer axially forward, causing the facing layer to wrap around the first mandrel in a spiral fashion;
conveying the facing material between an extrusion die and an inner surface of a second mandrel;
rotating the extrusion die in a second direction opposite the first direction while simultaneously extruding elastic polymer filaments from the extrusion die onto the facing layer, to form a laminate; and
severing the elastic filaments along edges of the laminate to form the elastic composite material.
The elastic polymer filaments are suitably bonded to the facing material by melt bonding immediately following their extrusion, without requiring a separate adhesive material.
With the foregoing in mind, it is a feature and advantage of the invention to provide an improved, lower cost cross-direction elasticized composite material, and a method of making it.
“Bonded” and “bonding” refer to the joining, adhering, connecting, attaching, or the like, of two elements. Two elements will be considered to be bonded together when they are bonded directly to one another or indirectly to one another, such as when each is directly bonded to intermediate elements.
“Melt bonding” refers to mechanical and/or chemical bonding of elastic filaments to a facing material via contact and/or penetration of the molten filament polymer into the surface of the facing material, resulting in bonding without the use of separate adhesive materials (i.e. separate from inherent adhesive properties of the molten filament polymer and additives compounded into the polymer).
“Breathable film” or “breathable laminate” refers to a film or laminate having a water vapor transmission rate (“WVTR”) of at least about 500 grams/m2-24 hours, using the WVTR Test Procedure described herein.
“Elastomeric” or “elastic” refers to a material or a composite which can be elongated by at least 50 percent of its relaxed length in at least one direction and which will recover, upon release of the applied force, at least 40 percent of its elongation. An elastomeric material or composite may be capable of being elongated by at least 100 percent, or by at least 300 percent, of its relaxed length and recover, upon release of an applied force, at least 50 percent of its elongation, or substantially all of its elongation. An “elasticized” material or composite is one which has been rendered elastic, such as by affixing elastic strands to an inelastic facing layer.
“Inelastic” refers to materials that are not elastic, either because they cannot be sufficiently stretched, or because they do not sufficiently recover when a stretching force is removed.
“Machine direction” as applied to a facing layer, refers to the direction on the material that was parallel to the direction of travel of the material as it left the extrusion or forming apparatus. If the material passed between nip rollers or chill rollers, for instance, the machine direction is the direction on the material that was parallel to the surface movement of the rollers when in contact with the material. “Cross direction” refers to the direction perpendicular to the machine direction. Dimensions measured in the cross direction are referred to as “width” dimensions, while dimensions measured in the machine direction are referred to as “length” dimensions. As used herein, “machine direction” and “cross direction” of a laminate or composite refer to the “machine direction” and “cross direction,” respectively, of a facing layer in the laminate.
“Meltblown fibers” are 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 et al. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than about 1.0 denier, and are generally self bonding when deposited onto a collecting surface.
“Neck” or “neck stretch” interchangeably mean that the fabric, nonwoven web or laminate is stretched in its machine direction under conditions reducing its width or its transverse dimension and increasing its length. The controlled stretching may take place at room temperature or greater temperatures and is limited to an increase in overall length in the direction being stretched up to the elongation required to break the fabric, nonwoven web or laminate, which in most cases is about 1.1 to 1.6 times an original length, suitably about 1.2 to 1.5 times an original length. When relaxed, the fabric, nonwoven web or laminate does not return to its original dimensions unless it is then stretched in the cross direction. The necking process typically involves unwinding a sheet from a supply roll and passing it through a first nip roll assembly driven at a given linear speed. A second nip roll assembly, operating at a linear speed higher than the first nip roll assembly generates the tension needed to elongate and neck the fabric. U.S. Pat. No. 4,965,122 issued to Morman, and commonly assigned to the assignee of the present invention, discloses a reversibly necked nonwoven material which may be formed by necking the material, then heating the necked material, followed by cooling and is incorporated by reference. The heating of the necked material causes additional crystallization of the polymer giving it a partial heat set. If the necked material is a spunbond web, some of the fibers in the web may become crimped during the necking process, as explained in U.S. Pat. No. 4,965,122.
As used herein, the term “necked material” refers to any material which has been drawn in at least one dimension, (e.g. lengthwise), reducing the transverse dimension, (e.g. width), such that when the drawing force is removed, the material can be pulled in the cross direction back to its original width. The necked material generally has a higher basis weight per unit area than the pre-necked material. When the necked material is pulled back to its original width, it should have about the same basis weight as the pre-necked material. Materials suitable for necking include without limitation nonwoven webs formed from inelastic polymers.
“Neck-bonded laminate” refers to a laminate formed while a first layer, typically a nonwoven fabric, is in a necked condition. A second layer (e.g. an elastic layer) is bonded to the necked layer, and may be relaxed when the laminate is formed.
“Nonwoven” or “nonwoven web” refers to materials and webs of material having a structure of individual fibers or filaments which are interlaid, but not in an 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, coforming 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.)
“Polymers” include, but are 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” shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic and atactic symmetries.
“Retract” and “retractability” refer to a material's ability to recover a certain amount of its elongation upon release of an applied force.
“Spunbond fiber” refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine capillaries of a spinnerette 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 Hartmann, 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 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 deniers larger than about 0.3, more particularly, between about 0.6 and 10. These and other terms may be defined with additional language in the remaining portions of the specification.
Referring to
The facing layer 12 may be a necked nonwoven web which has been stretched in the machine direction 11 to narrow (neck) its width in the cross direction 13. When the necked nonwoven web is later stretched in the cross direction, it may extend at least back to its original (pre-necked) width. Suitable necked nonwoven webs may be formed from spunbond webs, meltblown webs, bonded carded webs, other webs where there has been at least some bonding between the fibers, and combinations thereof. The nonwoven webs used for necking are typically formed of inelastic polymers, including without limitation polyethylene, polypropylene, copolymers of ethylene or propylene with up to 10% by weight of an olefin comonomer, polyesters, polyamides and the like.
The facing layer 12 may also be a material that is inherently extensible in the cross direction 13 but does not independently possess sufficient retractive force to constitute an elastic material. Such materials need not be necked. Bonded carded webs, for instance, possess some inherent extensibility in the cross direction. Other inherently extensible materials include films and nonwoven webs formed from inherently extensible polymers, including without limitation single-site catalyzed ethylene-alpha olefin copolymers having relatively high (e.g. more than 10% by weight) comonomer contents, and blends thereof.
The elastic filaments 14 may be joined to the facing layer 12, and are suitably joined by melt-bonding which occurs when the elastic filaments 14 are deposited in a molten state on the facing layer 12. The melt-bonding may be a final bonding step, suitable for end use of the laminate, or an interim bonding step adequate to progress the elastic filaments and facing layer without distortion to a subsequent bonding station where the filaments and facing may be further bonded using a spray adhesive, slot adhesive, ultrasonic bonding, thermal bonding or the like. The elastic filaments 14 are disposed on the facing layer 12 significantly in the cross direction. This means that the elastic filaments 14 may be disposed precisely in the cross direction 11, or may be disposed at an angle of about −30 degrees to about +30 degrees relative to the cross direction 13. Suitably, the elastic filaments may be disposed between about −15 degrees and about +15 degrees relative to the cross direction 13, or between about −5 and about +5 degrees relative to the cross direction 13.
The elastic filaments 14 can be formed of elastic polymers, and blends containing sufficient amounts of elastic polymer to render the filaments elastic. Suitable elastic polymers include without limitation styrenic block copolymers, for example styrene-diene and styrene-olefin block copolymers sold under the trade name KRATON® by Kraton Polymers, LLC.
Suitable styrene-diene block copolymers include di-block, tri-block, tetra-block and other block copolymers, and may include without limitation styrene-isoprene, styrene-butadiene, styrene-isoprene-styrene, styrene-butadiene-styrene, styrene-isoprene-styrene-isoprene, and styrene-butadiene-styrene-butadiene block copolymers. Suitable styrene-olefin block polymers include without limitation styrene-diene block copolymers in which the diene groups have been totally or partially selectively hydrogenated, including without limitation styrene-(ethylene-propylene), styrene-(ethylene-butylene), styrene-(ethylene-propylene)-styrene, styrene-(ethylene-butylene)-styrene, styrene-(ethylene-propylene)-styrene-(ethylene-propylene), and styrene-(ethylene-butylene)-styrene-(ethylene-butylene) block copolymers. In the above formulas, the term “styrene” indicates a block sequence of styrene repeating units; the terms “isoprene” and “butadiene” indicate block sequences of diene units; the term “(ethylene-propylene)” indicates a block sequence of ethylene-propylene copolymer units, and the term “(ethylene-butylene)” indicates a block sequence of ethylene-butylene copolymer units. The styrene-diene or styrene-olefin block copolymer should have a styrene content of about 10 to about 50% by weight, suitably about 15 to about 25% by weight, and should have a number average molecular weight of at least about 40,000 grams/mol, suitably about 60,000 to about 110,000 grams/mol.
Other suitable elastic polymers include without limitation single-site catalyzed ethylene-alpha olefin copolymer resins having a density of about 0.915 grams/cm3 or less, suitably about 0.860-0.900 grams/cm3, particularly about 0.865-0.895 grams/cm3. The term “single-site catalyzed” includes without limitation ethylene-alpha olefin copolymers formed using metallocene or constrained geometry catalysts. Examples of single-site catalysts include bis(n-butylcyclopentadienyl)titanium dichloride, bis(n-butylcyclopentadienyl)zirconium dichloride, bis(cyclopentadienyl)scandium chloride, bis(indenyl)zirconium dichloride, bis(methylcyclopentadienyl)titanium dichloride, bis(methylcyclopentadienyl)zirconium dichloride, cobaltocene, cyclopentadienyltitanium trichloride, ferrocene, hafnocene dichloride, isopropyl(cyclopentadienyl,-1-flourenyl)zirconium dichloride, molybdocene dichloride, nickelocene, niobocene dichloride, ruthenocene, titanocene dichloride, zirconocene chloride hydride, zirconocene dichloride, among others. A more exhaustive list of such compounds is included in U.S. Pat. No. 5,374,696 to Rosen et al. and assigned to the Dow Chemical Company. Such compounds are also discussed in U.S. Pat. No. 5,064,802 to Stevens et al. and also assigned to Dow. However, numerous other single-site catalyst systems are known in the art; see for example, U.S. Pat. No. 5,539,124 to Etherton et al.; U.S. Pat. No. 5,554,775 to Krishnamurti et al.; U.S. Pat. No. 5,451,450 to Erderly et al. and The Encyclopedia of Chemical Technology, Kirk-Othemer, Fourth Edition, vol. 17, Olefinic Polymers, pp. 765-767 (John Wiley & Sons 1996); the entire contents of the aforesaid patents being incorporated herein by reference.
The single-site catalyzed ethylene-alpha olefin copolymer may be formed using a C3 to C12 alpha-olefin comonomer, and is suitably formed using a butene, hexene or octene comonomer. The amount of the comonomer is normally between about 5-25% by weight of the copolymer, and may vary depending on how much comonomer is needed to achieve the desired density. Normally, higher comonomer amounts and/or larger comonomer molecules result in lower densities. The low performance elastomer may have a number average molecular weight of at least about 30,000 grams/mol, suitably about 50,000 to about 110,000 grams/mol, and may have a melt index of about 0.5-30 grams/10 min. at 190° C., suitably about 2-15 grams/10 min, measured using ASTM D-1238, Condition E. Suitable single-site catalyzed ethylene-alpha olefin copolymers are made and sold by the Dow Chemical Company under the trade names AFFINITY and ENGAGE, and by the ExxonMobil Chemical Co. under the trade names EXACT and EXCEED.
The elastic filaments 14 impart cross-directional elasticity to the composite laminate 10. The facing layer 12 can be stretched in the cross direction, but need not have independent retractive force. As shown in
The diameter and spacing of elastic filaments 14 may vary depending on the type of elastic polymer, the end use application(s) for the composite laminate 10, the amount of retractive force desired, and other factors. For example, elastic filaments 14 may have individual diameters of about 50 microns to about 5 mm, suitably about 100 microns to about 1 mm, and may have uniform or nonuniform diameters. The elastic filaments 14 may be spaced apart by distances of about 1 mm to about 25 mm, suitably about 3 mm to about 12 mm, and may be uniformly or nonuniformly spaced. The elastic filaments 14 may cover about 5% to about 75% of a surface area of the facing layer 12, suitably about 10% to about 50%.
The facing layer 12 should have a cross-directional extensibility which is sufficient to permit the laminate 10 to behave as an elastic composite material. The facing layer 12 may have a cross-directional extensibility of at least about 50%, suitably at least about 100%, or at least about 200%, or at least about 300%. When the facing layer 12 is a necked nonwoven web, the cross-directional extensibility may be achieved by neck-stretching the precursor nonwoven web to about 1.1-1.6 times its initial length in the machine direction, suitably to about 1.2-1.5 times its initial length.
The composite elastic laminate 10 may include a second facing layer (not shown) which is subsequently applied over the elastic filaments 14 after they are bonded to the first facing layer 12. The second facing layer may be formed of the same or different materials as the first facing layer 12, and should be extensible in the cross direction to the same or a similar extent as the first facing layer 12. The second facing layer may be applied using known techniques of thermal calendar bonding, adhesive bonding, ultrasonic bonding or the like.
Referring to
The facing layer 12 is fed to a first mandrel 56, with the machine direction 11 of facing layer 12 approaching first mandrel 56 at an angle relative to its longitudinal axis 57. The angle between the machine direction 11 of facing layer 12 and longitudinal axis 57 of first mandrel 56 can be about 15-75 degrees, suitably about 30-60 degrees, or about 40-50 degrees. The first mandrel is rotated in a first direction 58, while simultaneously conveying the facing layer 12 forward in an axial direction 59, causing the facing layer 12 to wrap around first mandrel 56 in a spiral fashion.
As further illustrated in
The size and number of conveyor belts 60 around the circumference of the first mandrel 56 should be sufficient to control the forward axial movement of facing layer 12. If the inner mandrel 56 has a diameter of about 10-20 inches, for instance, the number of conveyor belts 60 may range from about 4-30, suitably about 6-24, or about 8-20. Each conveyor belt 60 is positioned with its outward facing portion 66 about even with an outer surface of first mandrel 56, and with its inward facing portion 68 extending through an interior of first mandrel 56.
The axial motion of conveyors 60 and rotational motion of first mandrel 56 can be synchronized using a crossed helical gear assembly 70 (
As best illustrated in
The extrusion die 86 includes a row of circumferentially spaced die openings 92 extending completely around its outer circumference 94. The extrusion die 86 is caused to rotate in a direction 96 (opposite the direction of rotation 58 of first mandrel 56) at an approximate speed relative to facing layer 12 and mandrel 56 so as to deposit filaments in direction 13 or substantially in that direction. At commercial manufacturing speeds the centrifugal force of polymer leaving die openings 92 is sufficient to cause a uniform, evenly spaced deposition of molten elastic filaments 14 on a surface of facing layer 12. The centrifugal force exerted on molten elastic filaments 14 by the rotation of extrusion die 86 is greater than gravitational force G, suitably greater than 2G, or about 4G to about 10G. By applying a reasonable multiple of gravitational force via rotation of die 86, the elastic filaments 14 can be applied substantially uniformly regardless of whether the filaments 14 are extruded near the top of the rotation (against the force of gravity) or near the bottom of the rotation (with assistance from gravity). However, if the rotation achieves a centrifugal force that is too high, the elastic filaments 14 may break following extrusion before they contact the facing layer 12.
First mandrel 56 is cantilevered from its first end and, without additional support, would be unstable at its second end while rotating at commercial speeds. Extrusion die 86 is therefore configured with support extension 120 so that when the die 86 is slid into operating position the support extension 120 (having tapered end 121) is accepted into opening 130 and pilot bearing 140 at the second end of mandrel 56, to provide sufficient support.
The process conditions (melt temperature, extruder rpm, etc.) are suitably adjusted so that the molten elastic filaments 14 “melt bond” (adhere) to the facing layer 12 without the aid of an adhesive. Alternatively, the facing layer 12 may be treated with an adhesive to assist in the bonding.
The optimum centrifugal force exerted by rotation of die 86 will depend partly on the elastomeric polymer type, molecular weight and viscosity, the extrusion temperatures, the size of die openings 92 and other process factors. To minimize breakage of molten elastic filaments 14, it is also desirable to maintain a relatively small spacing 98 between the die openings 92 and the facing layer 12, as shown in
As illustrated in
Because the first mandrel 56 is rotating in the first direction while conveying the facing layer 12 axially, the facing layer maintains the resulting spiral path throughout the deposition of elastic filaments 14 onto facing layer 12. The rotation of the extrusion die in the second, opposite direction, expressed by N0, as revolutions per minute, can be adjusted to extrude the elastic filaments 14 onto the facing layer 12 at a desired angle. In order to achieve a significantly cross-directional disposition of elastic filaments 14, they should be extruded onto the facing layer at an angle of about −30 degrees to about +30 degrees, or about −15 degrees to about +15 degrees, or about −5 degrees to about +5 degrees, relative to the cross direction of the facing layer.
In many instances, it may be desirable to achieve a perfect or near-perfect cross-directional disposition of elastic filaments 14 onto the facing layer 12. Referring to
where N0 is the rpm of the extrusion die,
Similarly, the linear (axial) speed of the conveyor belts 60 required to convey the facing layer can be designated as Vb and determined from the following equation:
In the embodiment shown in
It is desired to minimize friction between the facing layer 12 and the second mandrel 90, especially when the composite laminate 10 is conveyed from the inner surface 88, around the terminal edge 99, to the outer surface 101 of the second mandrel 90. As illustrated in
The minimization of friction is especially important due to variations in the length of contact between the second mandrel 90 and the laminate 10 across its lateral width. As illustrated in
In another variation, the entire apparatus 50 (
The embodiments of the invention described herein are exemplary. Various modifications and improvements can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated by the appended claims, and all equivalents are intended to be encompassed within their scope.
