METHOD FOR PROCESSING NONWOVEN SHEETS BY COMBINED ACTIVATION AND BONDING

Abstract

The invention relates to a method for manufacturing nonwoven sheets that includes a step of combined activation and bonding. The invention further relates to an application of this method to the making of elastically stretchable nonwoven laminate sheets, sheets produced by such method, and to uses for such sheets.

Claims

1. A method for manufacturing a nonwoven sheet, comprising: (a) providing a precursor sheet; and (b) concurrently activating and bonding the precursor sheet to obtain the nonwoven sheet, wherein the activating comprises passing the precursor sheet between a pair of activation rollers having a plurality of meshing ribs and grooves arranged in parallel on their collaborating surfaces, thereby stretching the precursor sheet in a direction perpendicular to the orientation of the ribs and grooves, and wherein the bonding comprises an embossing of bonding points into the precursor sheet while it is stretched, the embossing being effected by embossing projections that are arranged along the crests of the ribs and/or grooves on the activation roller surfaces.

2. The method of claim 1, wherein the method is a method for manufacturing an elastically stretchable nonwoven sheet, the sheet comprising a first nonwoven carrier layer, an elastic nonwoven layer comprising elastic fibers, and, optionally, a second nonwoven carrier layer, wherein the precursor sheet provided in step (a) comprises: a first carrier structure, which is a precursor of the first nonwoven carrier layer of the sheet to be formed, an elastic structure comprising elastic fibers, which is a precursor of the elastic nonwoven layer of the sheet to be formed, and, optionally, a second carrier structure, which is a precursor of the optional second nonwoven carrier layer of the sheet to be formed.

3. The method of claim 2, wherein the first carrier structure is an unbonded fibrous carrier web, wherein the elastic structure is an unbonded elastic layer precursor web comprising elastic fibers, and wherein the optional second carrier structure is a second unbonded fibrous carrier web, wherein all fibrous webs are not bonded by calender embossing.

4. The method of claim 2, wherein the first carrier structure is a bonded nonwoven carrier layer, wherein the elastic structure is a bonded nonwoven layer comprising elastic fibers, and wherein the optional second carrier structure is a bonded nonwoven carrier layer, wherein all nonwoven layers are bonded by calender embossing.

5. The method of claim 1, wherein the ribs and grooves are oriented in machine direction and extend annularly over the surfaces of the rollers, such that the precursor sheet is stretched in cross-machine direction.

6. The method of claim 1, wherein the ribs and grooves are oriented in cross-machine direction and extend axially over the surfaces of the rollers, such that the precursor sheet is stretched in machine direction.

7. The method of claim 1, wherein in step (b) the precursor sheet is stretched to 150% of its original dimension in the direction perpendicular to the orientation of the ribs and grooves.

8. The method of claim 1, wherein the embossing projections are heated and/or vibrated at ultrasonic frequency for melting together the fibers of the structures of the precursor sheet under local pressure.

9. The method of claim 1, wherein the surface of one or both of the activation rollers is coated by an inert material around the embossing projections, wherein the inert material is a fluoropolymer.

10. An elastically stretchable nonwoven sheet manufactured by the method of claim 2 and comprising a first nonwoven carrier layer, an elastic nonwoven layer comprising elastic fibers, and, optionally, a second nonwoven carrier layer; wherein the microscopic fiber configuration within the carrier layer(s) changes back and forth along a regularly repeating pattern of parallel stripes, which are oriented in machine direction or cross-machine direction; wherein the sheet comprises a number of embossed bonding points extending through the carrier and elastic layers, which are arranged in lines that follow stripes of the pattern along which the local fiber configuration is the same; and wherein the sheet is elastically stretchable to 150% of its original dimension in a direction perpendicular to the orientation of the stripes.

11. The elastically stretchable nonwoven sheet of claim 10, wherein the repeating pattern of stripes is such that the local fiber configuration is repeated every 2-15 mm, and/or wherein the distance between adjacent lines of bonding points is 2-15 mm.

12. The elastically stretchable nonwoven sheet of claim 10, wherein the elastic nonwoven layer is a spunbonded or meltblown fabric comprising spunbonded or meltblown elastic endless fibers formed from a thermoplastic elastomer polymer material.

13. The elastically stretchable nonwoven sheet of claim 10, wherein the carrier nonwoven layer(s) are spunbonded, meltblown or carded nonwoven fabrics.

14. The elastically stretchable nonwoven sheet of claim 10, wherein the sheet comprises first and second nonwoven carrier layers and wherein the first and second nonwoven carrier layers are of different nature, wherein one of the nonwoven carrier layers is a spunbonded nonwoven and the other nonwoven carrier layer is a different spunbonded nonwoven or a meltblown or carded nonwoven.

15. A hygiene article comprising the elastically stretchable nonwoven sheet of claim 10.

16. The method of claim 1, wherein in step (b) the precursor sheet is stretched to 200% of its original dimension in the direction perpendicular to the orientation of the ribs and grooves.

17. The method of claim 1, wherein the surface of one or both of the activation rollers is coated by an inert material around the embossing projections, wherein the inert material is PTFE.

18. The elastically stretchable nonwoven sheet of claim 10, wherein the sheet is elastically stretchable to 200% of its original dimension in a direction perpendicular to the orientation of the stripes.

19. The elastically stretchable nonwoven sheet of claim 10, wherein the repeating pattern of stripes is such that the local fiber configuration is repeated every 5-10 mm, and/or wherein the distance between adjacent lines of bonding points is 5-10 mm.

Description

[0050] Further details and advantages of the invention will become apparent from the figures and examples described in the following. The figures show:

[0051] FIG. 1: a schematic cross-section of an elastically stretchable nonwoven sheet according to the invention;

[0052] FIG. 2: an exemplary machine setup for carrying out a method of the invention;

[0053] FIG. 3: an schematic illustration of a unit for concurrently activating, by stretch in cross-machine direction, and bonding the precursor sheet;

[0054] FIG. 4: an schematic illustration of a unit for concurrently activating, by stretch in machine direction, and bonding the precursor sheet;

[0055] FIG. 5: a picture of a crest line of a rib of an activation roller as schematically illustrated in FIG. 4, the rib having embossing projections on its crest line; and

[0056] FIG. 6: a schematic illustration of a unit of FIG. 4 in operation.

[0057] FIG. 1 shows a schematic cross-section of an elastically stretchable nonwoven sheet 100 according to the invention, where an elastic nonwoven layer comprising elastic fibers 130 is sandwiched between first and second nonwoven carrier layers 120.

[0058] An exemplary machine setup for making an elastically stretchable nonwoven sheet 100 according to the invention is shown in FIG. 2.

[0059] The setup comprises a conveyor belt 10 and altogether three spunbonding machines 20, 30 and 40 arranged in line on the conveyor belt.

[0060] In each of the spunbonding machines, a molten thermoplastic polymer is extruded through the holes of a die. The extruded fiber strands are then quenched and drawn/stretched to form endless fibers, which are then laid onto the conveyor belt 10 or a web that has been previously deposited thereon.

[0061] The first spunbonding machine 20 deposits a first carrier precursor web of regular polypropylene endless fibers on the conveyor belt 10. The middle spunbonding machine 30 deposits an elastic carrier precursor web of endless fibers formed from a thermoplastic elastomer on the first carrier precursor web. The last spunbonding machine 40 deposits a second carrier precursor web of again regular polypropylene endless fibers on the elastic carrier precursor web.

[0062] Each of the spunbonding machines 20, 30 and 40 is followed by a pair of precompaction rollers 21, 31 and 41, respectively, for precompacting the respective webs.

[0063] The key feature of the inventive process is the unit 50 for concurrently activating and bonding the three-layered precursor sheet previously formed in the spunbonding machines. This unit comprises a pair of counter-rotating activation rollers 51, 52 having a plurality of meshing ribs and grooves arranged in parallel on their collaborating surfaces, with embossing projections being arranged along the crests of the ribs. The unit 50 will be described below in more detail.

[0064] At the end of the overall in-line process, the product sheet is collected on the product roll 60.

[0065] FIG. 3 shows an embodiment of activation rollers 51, 52 of a unit 50 that is configured for stretching the precursor sheet formed in the spunbonding machines 20, 30 and 40 in cross-machine direction. The right-hand side picture is an enlarged cross-section along an axial plane.

[0066] Both rollers 51 and 52 comprise a plurality of regularly spaced annular ribs 53 on their acting surfaces, between which grooves 54 are formed. The width of the ribs 53 is designated with letter a, the depth of engagement is labelled with letter b and the distance between adjacent ribs is labelled with letter c. The cross-machine directional stretch during activation is controlled by these three parameters. In an exemplary embodiment, the distance c between adjacent ribs 53 can be about 6 mm. The depth of engagement b can be about 5 mm. The width a of the ribs 53 can be about 1 mm.

[0067] Another embodiment of activation rollers 51, 52 is discussed with reference to FIG. 4. The embodiment of FIG. 4 is configured for stretching the precursor sheet formed in the spunbonding machines 20, 30 and 40 in machine direction. The picture of FIG. 4 is an enlarged cross-section along a radial plane perpendicular to the roller axis.

[0068] Also in this embodiment, both rollers 51 and 52 comprise a plurality of regularly spaced ribs 53 on their acting surfaces, between which grooves 54 are formed. In contrast to the embodiment of FIG. 3, however, the ribs 53 in this embodiment are oriented in cross-machine direction and extend axially over the surfaces of the rollers 51 and 52.

[0069] Also in this embodiment, the width a of the ribs 53, the depth of engagement b and the distance between adjacent ribs 53 c controls the extent of the, in this case, machine directional stretch during activation. Also in this case, in an exemplary embodiment, the distance c between adjacent ribs 53 can be about 6 mm, the depth of engagement b can be about 5 mm, and the width a of the ribs 53 can be about 1 mm.

[0070] In both the embodiments of FIGS. 3 and 4, at the crest lines of each rib 53 on both rollers 51, 52 there is a series of embossing projections 59 for bonding the precursor web while it is stretched, or in other words for bonding the fabric simultaneously with the stretch activation.

[0071] FIG. 5 shows a picture of a crest line of a rib 53 of an activation roller 51, 52 as schematically illustrated in FIG. 4, the rib 53 having embossing projections 59 on its crest line.

[0072] The embossing projections 59 can have a height of about 0.2 to 0.5 mm, preferably about 0.3 mm on the crest line of the ribs 53. They can be approximately square-shaped, as shown in FIG. 5, or alternatively also, circular, oval rectangular, diamond shaped, etc. The bond area can be approximately 1 mm.sup.2, so, for example, the square-shaped embossing projections 59 shown in FIG. 5 can be 1 mm by 1 mm. The distance between the bonding points can be approximately between 1-5 mm, preferably between 2-4 mm, and more preferably between 2.5-3 mm.

[0073] The resulting bond area in the pre-stretched web, considering the dimensions above, is preferably between about 4% and about 8%, distributed between 4-8 bonding points per cm.sup.2. These bond areas and numbers of bonding points per unit area are proportionally increased after the product sheet retracts from a pre-stretched state in a relaxed state. Each of the two rollers 51, 52 provides half of the bonding points by the embossing projections 59 mounted thereon.

[0074] FIG. 6 shows a unit as shown in FIG. 4 in operation. From left to right in FIG. 6, the unbonded and unactivated precursor sheet consisting of the two carrier precursor webs and the elastic layer precursor web sandwiched there between enters the combined activation and bonding process. As the precursor sheet enters the nip of the two rollers 51, 52 the activation process is be initiated with the meshing ribs 53. In an exemplary embodiment, the distance from one crest line of one rib 53 sitting on a first roller 51 to a crest line of the closest rib 53 sitting on the other roller 52 can be approximately 2 mm as the precursor sheet enters the activation process. Due to the progressing engagement of the ribs 53 in the grooves 54 this distance is steadily increases, in an exemplary embodiment to a distance of approximately 5.2 mm in the center position where the ribs 53 and grooves 54 are fully engaged. The elastic layer precursor web during this process will elongate due to its elastic capability. The essentially inelastic fibers in the carrier precursor webs on each side of the elastic precursor webs, on the other hand, will re-orient themselves, creating high-density zones A around the crest lines of each rib 53, and low-density zones B there between.

[0075] At the maximum engagement point in the center position of the two rollers 51, 52, the embossing projections 59 on a rib 53 of one roller (here: the roller 51) will be in contact with the bottom of the opposite groove 54 of the opposite roller (here: the roller 52) and form a series of bonding points along the crest line of the rib 53, i.e. along a stripe-shaped high-density zone A of the pre-stretched precursor sheet.

[0076] As the material pre-stretched and bonded as described again exits the unit 50, the elastic fibers in the elastic nonwoven layer 120 of the product sheet 100 contract and revert the stretch of the material, such that it goes back to its original length in machine direction. The sections of the fibers in the carrier layers 110 and 130 that are not attached in a bonding point, during this relaxation process, re-orient and/or crimp, but always maintain the ability to again stretch in machine direction to the extent they have been pre-stretched in unit 50.

[0077] The dimensions of the ribs 53 and grooves 54, specifically the parameters a, b and c can be varied as needed depending on the elongation property required in the product sheet. The values for these parameters exemplified in the description of FIGS. 4-6 result in a machine-directional pre-stretch by approximately 160% (from 2.0 mm to about 5.2 mm). As most hygiene product manufacturers presently require the ability to elastically stretch by approximately 100-150%, this design with a stretch property of 160% is suitable to match the demands in most cases.