Patent Application
20060288547
The present invention relates to the field of webs, web processing methods, and web processing apparatus. More particularly, the present invention provides apparatus and methods for stretching one or more zones of a web in the cross-web direction and webs so stretched.
It is desirable in many instances to stretch a web in the cross-web direction during processing. For example, webs including layers of inelastic materials, e.g., nonwoven webs, laminated or otherwise attached to elastic layers while the elastic is not extended typically require stretching to impart elasticity to the web. The web is stretched so that the inelastic layers, or bonds within the inelastic layer or layers, are broken or otherwise disrupted allowing the elastic to freely stretch which leaves the stretched web laminate elastic. Such stretching to impart elasticity to a web is commonly referred to as “activation” of the web (with the elasticity of the web being “activated” by the stretching). Activation can be done in the machine direction of the web or the cross direction of the web or both. Cross direction stretching or activation can be performed by a variety of known methods including, for example, tentering and ring rolling.
Tentering typically involves grasping the edges of a web and stretching the web in the cross-web direction while advancing the web in the down-web direction (i.e., along the length of the web). Although tentering does provide the ability to vary the amount of strain induced in the web, it also suffers from a number of disadvantages. For example, the edges of the web must often be discarded after tentering due to damage or inconsistent strain in the web at the edges. Another potential disadvantage is that it may be difficult or impossible to induce strain into selected portions or zones of a web using tentering. Further, tentering equipment can be both costly, complex, and may require significant amounts of floor space to operate as the web expands in the cross direction during the process.
Ring rolling is an alternative to tentering for stretching a web in the cross direction. Various ring rolling apparatus are described in, e.g., U.S. Pat. No. 4,223,059 (Schwarz); U.S. Pat. No. 4,968,313 (Sabee); U.S. Pat. No. 5,143,679 (Weber et al.); U.S. Pat. No. 5,156,793 (Buell et al.); and U.S. Pat. No. 5,167,897 (Weber). Ring rolling or incremental stretching refers generally to placing the web between rolls having interengaging teeth. The engaging teeth stretch the web based generally on the size, number and pitch of the teeth. Ring rolling can be used to stretch selected zones in a web and stretch only in the cross direction. However ring rolling teeth grip the web and this contact of the web by the ring rolling apparatus can tear the web and undesirably affect the web's appearance. The amount of strain that can be induced in a web using ring rolling is limited by the specific ring rolls used. Adjustment or change in the degree of stretch requires new ring rolls to be machined. This is of course costly and inflexible.
The present invention provides apparatus and methods for stretching one or more zones of an anisotropic web and anisotropic webs including one or more stretched zones. Each of the stretched zones in the web is stretched in the cross-web direction, i.e., the direction transverse to the down-web direction. The stretching can be performed continuously as the web is advancing through the apparatus in the down-web direction.
The method for stretching an extensible web in the cross direction generally is practiced on a substantially continuous, extensible anisotropic web. The web is traveling in a downweb direction at a first speed under tension in the web plane. The extensible anisotropic web has a width and substantially continuous length in the downweb direction. The cross web stretching occurs in an orientation zone established by an orientation unit. The orientation unit moves the web out of the plane of the web where the web is under tension, but without any side restraints. The web moves over the orientation unit where the degree of orientation is proportional to the cross direction displacement of a portion of the web by the orientation unit. The anisotropic web has a tensile strength in the downweb direction greater than the cross direction such that the web is preferentially displaced in the cross web direction by the orientation unit. This can be a downweb direction tensile strength at least 50 percent greater than the cross web direction.
a is a side view of an alternative diversion wheel.
a is a graph showing the tensile to break of activated and unactivated webs in machine direction and cross machine direction.
b is a graph showing the hysteresis properties of activated and unactivated webs.
As shown in
As shown in
As shown in
Generally the orientation provided by the orientation units 15 is created preferentially in the cross web direction by the overall anisotropic strength behavior of the web being oriented and without any longitudinal side restraints holding the web on either side of the orientation zone while the web is in the orientation unit. This anisotropic strength can be due to strengthening zones, as described above, or by providing the web, or one or more layers forming the web with anisotropic strength properties such as a strength in the downweb direction at least 50% greater than the cross web direction or greater than 100% or 200%. The web 2 being treated, generally has a significantly higher overall strength in the downweb direction Z than in the crossweb direction X. This tends to isolate the orientation induced by the orientation units 15 in the cross web or X direction orientation.
The overall anisotropic strength behavior of the web can be created for example by an anisotropic nonwoven web or layer where the fibers are preferentially oriented in the downweb direction. This could also be created by a film or film layer that has induced orientation in the downweb direction, which could be melt induced orientation or subsequent elongational orientation created by stretching the film. Elongational orientation can be used with other types of web or web layers as well. Anisotropic fibrous webs are described, for example, in U.S. Pat. No. 5,393,599, the substance of which is incorporated by reference in its entirety, where fibers are laid down in a carding machine to create a high ratio of fibers extending in the machine direction verses the cross direction. The webs described have a tensile strength ratio of at least 4/1 and up to at least 6/1 or higher. This web can then be joined to an elastic web, which could be a nonwoven elastic, a elastic net or a elastic film by hydroentangling, adhesives, heat bonding, ultrasonic bonding, extrusion bonding or the like. This laminate could then be joined to other layers such as a nonanisotropic nonwovens, films or the like and still have preferential strength properties in the machine direction. Spunbond webs can also be made anisotropic, for example by drawing the web in the machine direction during or after web formation or by directing the fibers downweb during web formation by use of the directionality of the spinning device, directional air streams or the orientation and speed of the forming wire, for example. Anisotropic melt blown webs can be formed, for example, as described in U.S. Pat. No. 5,366,793 by preferentially directing the stream of meltblown fibers at an angle to the forming surface or deflecting the stream of fibers relative to the forming surface. These anisotropic nonwoven webs could be directly formed onto other webs or films to directly form a multilayer anisotropic laminate. Anisotropic films can be formed directly in, for example, the melt as described in U.S. Pat. No. 6,270,910, the substance of which is incorporated by reference in its entirety. In this patent anisotropic behavior is created by use of a discontinuous phase of a higher strength material in a continuous phase. The discontinuous phase is aligned in the machine direction by melt shear forces in the extrusion device and/or by post formation stretching. This technique can also be used with co-extruded films or films having included continuous higher strength phases or layers such as described, for example, in U.S. Pat. Nos. 5,501,675; 5,462,708; 5,354,597 or 5,344,691, the disclosures of which are incorporated by reference in their entirety. In this case if an elastic layer or phase is included in the film, higher strength in the machine direction could be enhanced by stretching the film in the machine direction. If the film has a continuous elastic layer, heat treating the stretched film can be used to relax the elastic material, but retaining the orientation within the elongationally oriented inelastic material phase or layer. This would result in a film with elastic properties in the cross direction but high strength properties in the machine direction.
The orientation unit 15 has a web diversion device 25 shown in
The diversion device could be any shape or form and could be, for example, a ramp having a gradual increase to an apex. This ramp could be a solid stationary tool or be formed by one or more discrete elements, wheels, rollers or the like. The diversion device could also be provided as one or more adjacent units, which could be integral or mechanically isolated units.
The wheel type diversion device 25, shown in
a shows an alternative embodiment of a diversion device wheel 35, where the wheel is noncircular to create regions that have high degrees of diversion H′ and low degrees of diversion H° to allow for variable degrees of cross web orientation of the web in the downweb direction in a single orientation zone. This effect could also be created by eccentrically mounted wheels.
The extensible web is in a preferred embodiment a laminate of an elastically extensible web 22 and one or more relatively inelastic web 23 as shown in
An orientation unit used to stretch portions of a web in accordance with the invention can be used in-line with other web processing equipment or easily be placed in an existing multifunctional line such as a diaper line. For example, the orientation unit may be located downstream of an apparatus that may, for example, process a pre-existing web by, e.g., heating, cooling, calendaring, applying materials to an existing web (e.g., laminating a material by heat, ultrasonics, hot melt or pressure sensitive adhesives), etc. In some instances the apparatus may manufacture a web (by, e.g., extrusion, spun-bonding, carding, melt blowing, weaving, laminating a nonwoven or other inelastic web to an elastic web, etc.) that is then directed as is, or in a laminated form, into an orientation unit according to the present invention.
The orientation unit according to the present invention may also be located upstream of another processing apparatus that acts on the web after portions of the web have been stretched according to the principles of the present invention. For example, apparatus for slitting, perforating, and/or aperturing the web at one or more locations or apparatus for laminating materials to the web (e.g., such as attaching fastener materials such as hooks) die cutting, etc. An orientation unit, in accordance with the invention could easily be placed in an assembly line, such as a diaper assembly line; to specifically orient or activate certain predetermined cross direction zones.
The following examples are provided to enhance understanding of the present invention. The examples are not intended to limit the scope of the invention.
Tensile Strength/Hysteresis:
The tensile strength and hysteresis properties of the elastic/nonwoven laminates were measured. For tensile strength at break testing, a 50 mm wide by 100 mm long piece of laminate was mounted in a tensile testing machines (INSTRON Model 55R1122, available from Instron Corp.) with the upper and lower jaws 40 mm apart. Line contact jaws were used to minimize slip and breakage in the jaws. The jaws were then separated at a rate of 51 cm/minute until sample breakage occurred. The results are shown in
An elastic/nonwoven laminate web was prepared using the method disclosed in PCT publication WO 2004/082918.
A 40 mm diameter twin screw extruder fitted with a gear pump was used to deliver 75 grams/meter2 of a molten elastomeric polymer blend consisting of a styrene-ethylenebutylene-styrene block copolymer (70%, KRATON G-1657, Kraton Polymers Inc., Houston, Tex.) and ultra low density polyethylene (30%, Engage 8452, Exxon Polymers Inc., Houston, Tex.) at a melt temperature of approximately 246° C. to a die. The die was positioned such that a film of molten polymer was extruded vertically downward into the interface region of a heated doctor blade and a cooled forming roll. The doctor blade was maintained at a temperature of 246° C. and the forming roll was maintained at a temperature of 30° C. by circulating chilled water through the interior of the roll. The doctor blade was held against the forming roll with a pressure of 450 pounds per lineal inch (788 Newtons/lineal cm).
Approximately 10 cm in width of the exterior surface of the forming roll was chemically etched so as to have a series of elliptically shaped posts arranged around the periphery of the roll. The posts were 1.6 mm wide and spaced 3.2 mm apart circumferentially (downweb) around the roll and 5 mm apart axially (crossweb) along the roll. The height of the posts was 63 microns. The tops (or lands) of the posts were the same height as the non-machined outermost areas of the roll such that when the doctor blade wiped extrudate from the roll, no extrudate was left on the lands of the posts resulting in an apertured polymeric film 10 cm in width. The extrudate was transferred from the forming roll to a lightly bonded high extension carded (HEC) nonwoven polypropylene substrate (Product FPN 332D) with a basis weight of 27 grams/meter2 and a width of 22 cm from BBA Nonwovens (Simpsonville, S.C.) at a nip formed with a conformable backup roll (a steel core with a rubber cover having a durometer of 75 Shore A). The core of the backup roll was chilled by circulating water at a temperature of 5° C. The pressure exerted on the nip between the forming roll and the backup roll was 14 pounds per lineal inch (25 Newtons/lineal cm). To enhance the bond between the extrudate and the nonwoven, the nonwoven was sprayed in a swirl pattern with a hot melt adhesive (4.5 grams/meter2, H9388, Bostik, Wauwatosa, Wis.) across the full width (22 cm) of the nonwoven. The 10 cm of extrudate was centered onto the 22 cm wide nonwoven, resulting in approximately 6 cm of outermost edge zones without elastomer. A second layer (22 cm width) of the same type of nonwoven, also sprayed with adhesive, was then laminated to the elastomer side of the previously created laminate using a rubber roll/steel roll nip, resulting in a 3 layer laminate in the middle 10 cm of the web and a 2 layer laminate in the outermost 6 cm of the web.
The laminate was then stretched in the cross-direction using an apparatus similar to that shown in
As briefly addressed above, the present invention can be used to process any suitable extensible web, including homogenous webs, monolayer webs, multilayer webs and composite webs. This would include assembled articles, which had specific zones or regions that were extensible.
The preceding specific embodiments are illustrative of the practice of the invention. This invention may be suitably practiced in the absence of any element or item not specifically described in this document. The complete disclosures of all patents, patent applications, and publications are incorporated into this document by reference as if individually incorporated in total.
