Method for preparing a nonwoven web

Abstract

The present invention relates to a method for producing a nonwoven fabric. The method according to the invention comprises forming a nonwoven web using a wet lay forming process, hydroentangling the web and providing the web with a high quality hydroembossed pattern. The invention also concerns a nonwoven fabric exhibiting a three dimensional pattern, which remains even in wet wipes prepared from the material.

Claims

1. A method for producing a nonwoven fabric comprising a three-dimensional pattern thereon, the method comprising: forming a first web from an aqueous suspension of fibers on a wire support; hydroentangling the first web to provide a self-supporting web; transferring the self-supporting web from the wire support to a hydroembossing roll in a way in which the self-supporting web supports itself during the transfer; hydroembossing the self-supporting web via the hydroembossing roll, the hydroembossing roll comprising a sleeve associated with the roll, the sleeve having a negative impression of the three-dimensional pattern within a body thereof, wherein the hydroembossing comprises moving a portion of the fibers of the self-supporting web into the negative impression of the sleeve to form the three-dimensional pattern on the self-supporting web; and drying the self-supporting web to provide the nonwoven fabric comprising the three-dimensional pattern thereon.

2. The method of claim 1, further comprising hydroentangling the first web with one or more additional unwound webs.

3. The method of claim 1, wherein the sleeve is detachable from the roll.

4. The method according to claim 1, wherein the aqueous suspension is laid on the wire support on top of a second unwound web of a spunbond, spunlace, wetlaid or tissue type.

5. The method according to claim 4, wherein a third unwound web of a spunbond, spunlace, wetlaid or tissue type is further laid on top of the second web.

6. The method according to claim 1, wherein the aqueous suspension comprises both short cut regenerated fibers and natural fibers.

7. The method according to claim 1, wherein a height of the three-dimensional pattern on the nonwoven fabric comprises at least 200 μm between a minimum and maximum thickness of the nonwoven fabric.

8. The method of claim 1, wherein the hydroentangling is done at a speed of 500-1100 ft/min, a nozzle pressure of 1200 psi, with a nozzle size of 80-150 micron, and at a vacuum pressure of 1-450 mbar.

9. The method of claim 1, wherein during the hydroembossing, excess water is removed through openings in the hydroembossing roll.

10. The method according to claim 1, wherein in the nonwoven fabric, an angle of inclination from a minimum thickness to a maximum thickness of the nonwoven fabric is in a range 48°-90°.

11. The method according to claim 1, wherein areas of the nonwoven fabric comprising the patterned area comprise at least 10% more fibers by weight than non-patterned areas of the nonwoven fabric.

12. The method according to claim 1, wherein the self-supporting web comprises a mixture of pulp and regenerated fibers.

13. The method according to claim 1, wherein the pattern comprises a machine readable code.

14. The method according to claim 1, wherein a tensile strength of the web following the hydroentangling and prior to removal from the wire support is at least 200 g/25 mm for single layer products.

15. The method according to claim 1, wherein a tensile strength of the web following the hydroentangling and prior to removal from the wire support is at least 200 g/25 mm for multilayered products.

16. A method for producing a nonwoven fabric comprising a three-dimensional pattern thereon, the method comprising: forming a first web from an aqueous suspension of fibers on a wire support; hydroentangling the first web to provide a self-supporting web; removing the self-supporting web from the wire support; hydroembossing the self-supporting web via a hydroembossing roll comprising a sleeve associated with the roll, the sleeve having a negative impression of the three-dimensional pattern within a body thereof, wherein the hydroembossing comprises moving a portion of the fibers of the self-supporting web into the negative impression of the sleeve to form the three-dimensional pattern on the self-supporting web; and drying the self-supporting web to provide the nonwoven fabric comprising the three-dimensional pattern thereon, wherein the hydroentangling is done at a speed of 500-1100 ft/min, a nozzle pressure of 1-1200 psi, with a nozzle size of 80-150 micron, and at a vacuum pressure of 1-450 mbar.

Description

DETAILED DESCRIPTION

(1) In an embodiment, the aqueous fiber suspension has a solids content in the range of 0.5% to 5% by weight of the suspension.

(2) In at least one embodiment, the aqueous fiber suspension to be subjected to wet laying according to the present invention comprises short cut fibers and natural fibers.

(3) In at least one embodiment, the suspension may be laid on the wire on top of a second, unwound web which may be, for example, of the spunbond, spunlace, wetlaid or tissue type. In at least one embodiment, a third layer of unwound material may be applied on top of the first, wetlaid layer.

(4) The tensile strength of the web following the consolidation should be at least 70 g/25 mm for webs used in multilayered products. For single layer products it should be at least 200 g/25 mm. This strength is sufficient to allow the removal of the web from the wire support.

(5) In an embodiment, the embossing pattern is on a sleeve which is detachable from the roll. The pattern resolution and the hydroembossing parameters in combination with the degree of consolidation in the web at the time it reaches the roll provide a pattern which is of sufficiently high definition and persistence to satisfy the requirements of information coding and/or permanency in the wet state. Exemplary nonwoven materials having encoded information thereon and methods for encoding information are disclosed, for example, in PCT publication No. WO2019/115875, the entirety of which is incorporated by reference herein. Suitable sleeves for use in the processes described herein are disclosed in PCT publication No. WO2020/021158, the entirety of which is incorporated by reference herein.

(6) The consolidation of the web following wet laying, carried out through hydroentangling, provides a tensile strength which preserves the web in a consistent shape as it is lifted from the forming wire. Too little consolidation may cause, apart from the obvious risk of web breakage, holes in the web. Excessive consolidation does not leave sufficient formability in the web for the subsequent hydroembossing step carried out on a roll. The appropriate degree of consolidation is acquired using the following parameters:

(7) TABLE-US-00001 Speed Nozzle Nozzle Vacuum (ft/min) psi size μm (mbar) 500-1100 1-1200 80 to 150 1-450

(8) Following hydroentanglement and dewatering on a wire, the tensile strength of the web is in the range of 80 to 120% of its final tensile strength.

(9) Following the hydroentanglement and dewatering step, the web is transferred to a hydroembossing roll, preferably provided with a sleeve, for imposing a three-dimensional high definition pattern to the web.

(10) In certain embodiments, inside the roll, there is provided a partial vacuum for removing excess water from the hydroembossing through provided openings. In an embodiment, there are from 1 to 5 hydroentanglement units hydroembossing the web.

(11) In the following discussion, the top surface of the fabric is the surface that faces the sleeve during the hydroembossing operation; the bottom surface is the one exposed to the water jets.

(12) For considering the three-dimensional structure of the web of the present invention, the base level is defined as the level of the bottoms of the deepest depressions in the fabric, i.e. the level of the thinnest areas in the fabric or a minimum thickness of the fabric. In an embodiment, the thickness of the fabric at its thinnest points or minimum thickness is at least 30 μm.

(13) The embossing level is a level on the top surface of the fabric which is farthest or at a maximum distance from the base level. Put another way, the embossing level comprises areas (elevated portions) with a maximum thickness of the fabric. In this context, a pattern in a nonwoven web is considered a three-dimensional high definition pattern if the embossing level or maximum thickness is at least 200 μm above the base level or minimum thickness. In the present context, this dimension may also be referred to as a height of the pattern. In an embodiment, the embossing level or maximum thickness is at least 250 μm; and in other embodiments is at least 300 μm above the base level of the fabric. The angle of inclination in a high definition three-dimensional pattern refers to the angle of the slope of the elevated portions of the fabric relative to a plane of the base level. In a three-dimensional high definition pattern, the angle of inclination of the rise from the base level to respective elevated portion(s) is in the range 48° to 90°; preferably in the range of 55°-88°. In certain embodiments, the 3D pattern is machine readable.

(14) Angles and dimensions in nonwoven fabric structures may be determined using laser triangulation technology. Further, angles and dimensions of the embossing tool may be determined.

(15) As a portion of the fibres are moved from their original location during hydroembossing using a sleeve, the grammage of the patterned portions of the fabric, i.e. the portions of the fabric receiving the displaced fiber increases at least 10% by weight, e.g., 10-30% by weight or 15-30% by weight. Thus, the patterned areas contain at least 10% by weight more fiber than the non-patterned areas.

(16) The web is subsequently subjected to drying using e.g. an air dryer, a can dryer, an omega dryer or combinations of these.

(17) In one embodiment of the invention, the web is biodegradable, i.e. contains no synthetic material. In this embodiment, the web comprises from 20 wt % to 80 wt % pulp and from 20 wt % to 80 wt % regenerated fibers. In this embodiment, the basis weight of the fabric is in the range 35 gsm to 80 gsm; the wet tensile strength is between 1000 to 4000 g/25 mm MD; and the thickness is in the range 400 to 1000 μm. Consolidation is carried out using between 4 and 15 hydroentanglement nozzle bars.

(18) In an embodiment, the wet tensile strength is in the range of 800 g/25 mm to 4500 g/25 mm and the water content is in the range of 85%-92% by weight when the web is lifted off the forming wire for subsequent hydroembossing. Hydroembossing is carried out using 1-4 nozzle bars.

(19) A three-dimensional nonwoven exhibits clear performance improvements relative to a flat nonwoven of the same composition and grammage.

(20) In one embodiment, 32% by volume of the nonwoven constitutes an elevated pattern of thin lines forming isolated patterns evenly distributed on the nonwoven. The embossing level on the patterned area is at least 200 μm above the base level and the angle of inclination from the base level to the elevated portions is in the range 48°-90°, preferably in the range 55°-88°.

(21) When comparing to a flat nonwoven, the limited area of contact with a wiped surface enhances the capillary effect of the nonwoven and thus increases the water absorption capacity by at least 50%. Having a three-dimensional nonwoven does not only increase the surface area of the product, but it will also create varying contact angles in respect of the wiped surface. The increased surface area resulting from the generated 3D structure and the varied contact angle can increase dirt pick-up at least 45%.

(22) Hydroentanglement and hydroembossing bring several advantages relative to belt patterning and thermocalendering. Changing an embossing sleeve is a rapid operation compared with the changing of a thermoembossing roll: The sleeve is light and easy to handle. Once a sleeve is installed, there is no need for additional adjustments but the line can be started immediately, whereas a belt requires adjustment and a thermoembossing roll requires oil handling and heating. A sleeve allows great flexibility as to the character and the degree of detail in the pattern. Since the pattern is generated through the “valleys” or impressions in the sleeve surface, blocking of dewatering holes does not occur. This allows sharper and more detailed patterns. As the web is pre-entangled, only part of the fibers will move into the pattern. Moving a portion of the fibers into the pattern is a more controllable process than moving them out of the pattern, and this reduces the risk of creating holes in the nonwoven. Although the distribution of fibers is no longer even, the reduction in CD and MD strength is minimal. Unlike in thermal calendering, hydroembossing moves the fibers, and therefore the pattern will be retained both in a dry and in a wet state. Instead of compressing the web, as occurs while calendering, the displacement of fibers in the pattern will increase the thickness in some areas, creating a feel of increased overall thickness in the entire fabric. The variation in thickness improves the overall water absorption capacity. As no significant mechanical pressure is applied to the web, fiber damage which can weaken the fabric is minimized. Although the web becomes thinner in some areas, this does not significantly reduce its performance in terms of tensile strength, burst strength, toughness or abrasion resistance.