Method of producing a nonwoven material

Abstract

Method of producing a nonwoven material, wherein a fibrous web containing continuous filaments and natural fibers and/or staple fibers is formed on a forming member (12) and subsequently hydroentangled to form the nonwoven material. The fibrous web is transferred to an entangling member (16) while subjecting the fibrous web to foreshortening and subsequently hydroentangling the foreshortened fibrous web, thus forming a composite material (19) wherein the continuous filaments are well integrated with the rest of the fibers.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the 35 U.S.C. 119(e) benefit of prior provisional application Ser. No. 60/515,640 filed on Oct. 31, 2003.

FIELD OF THE INVENTION

The present invention refers to a method of producing a nonwoven material, wherein a fibrous web containing continuous filaments and natural fibers and/or synthetic staple fibers is formed on a forming member and subsequently hydroentangled to form said nonwoven material.

BACKGROUND OF THE INVENTION

Hydroentangling or spunlacing is a technique introduced during the 1970's, see e.g. CA patent no. 841 938. The method involves forming a fiber web which is either drylaid or wetlaid, after which the fibers are entangled by means of very fine water jets under high pressure. The water jets twist the fibers around each other giving the web strength. Several rows of water jets are directed against the fiber web which is supported by a movable wire. The entangled fiber web is then dried. The fibers that are used in the material can be synthetic or regenerated staple fibers, e.g. polyester, polyamide, polypropylene, rayon or the like, pulp fibers or mixtures of pulp fibers and staple fibers. Spunlace materials can be produced in high quality at a reasonable cost and have a high absorption capacity. They can e.g. be used as wiping material for household or industrial use, as disposable materials in medical care and for hygiene purposes etc. Through e.g. EP-A-0 333 211 and EP-A-0 333 228 it is known to hydroentangle a fiber mixture in which one of the fiber components is meltblown fibers. The base material, i.e. the fibrous material which is exerted to hydroentangling, either consists of at least two preformed fibrous layers where one layer is composed of meltblown fibers or of a “coform material” where an essentially homogeneous mixture of meltblown fibers and other fibers is airlaid on a wire and after that is exerted to hydroentangling.

Through EP-A-0 308 320 it is known to bring together a web of continuous filaments with a wetlaid fibrous material containing pulp fibers and staple fibers and hydroentangle together the separately formed fibrous webs to a laminate. In such a material the fibers of the different fibrous webs will not be integrated with each other since the fibers already before the hydroentangling are bonded to each other and only have a very limited mobility.

EP-A-0 938 601 discloses a method of producing a nonwoven material by hydroentangling a fiber mixture containing continuous filaments, e.g. meltblown and/or spunbond fibers, and other fibers The method is characterized by foamforming a fibrous web of natural fibers and/or synthetic staple fibers and hydroentangling together the foamed fiber dispersion with the continuous filaments for forming a composite material, in which the continuous filaments are well integrated with the rest of the fibers.

OBJECT AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for producing a hydroentangled nonwoven material of a fibrous mixture of continuous filaments, for example in the form of meltblown and/or spunbond fibers and natural fibers and/or synthetic or regenerated staple fibers, in which there is given a high freedom in the choice of fibers and fiber lengths and where the continuous filaments are well integrated with the rest of the fibers. This has according to the invention been obtained by transferring the fibrous web to an entangling member while subjecting said fibrous web to foreshortening and subsequently hydroentangling the foreshortened fibrous web, thus forming a composite material wherein the continuous filaments are well integrated with the rest of the fibers.

Other features of the invention are disclosed in the dependent claims and in the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will below be closer described with reference to an embodiment shown in the accompanying drawings.

FIG. 1 shows schematically an embodiment of a process for producing a hydroentangled nonwoven material according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The hydroentangled composite material according to the invention comprises a mixture of continuous filaments and natural fibers and/or synthetic staple fibers. These different types of fibers are defined as follows.

Continuous Filaments

The continuous filaments are fibers that in proportion to their diameter are very long, in principle endless. They can be produced by extruding a molten thermoplastic polymer through fine nozzles, whereafter the polymer will be cooled and drawn, preferably by the action of an air flow blown at and along the polymer streams, and solidified into strands that can be treated by drawing, stretching or crimping. Chemicals for additional functions can be added to the surface.

Filaments can also be regenerated fibers produced by chemical reaction of a solution of fiber-forming reactants entering a reagent medium, for example by spinning of regenerated cellulose fibers from a cellulose xanthate solution into sulphuric acid. Examples of regenerated cellulose fibers are rayon, viscose or lyocell fibers.

Continuous filaments may be in the form of spunlaid filaments or meltblown filaments. Spunlaid filaments are produced by extruding a molten polymer, cool and stretch to an appropriate diameter. The fiber diameter is usually above 10 μm, e.g. between 10 and 100 μm. Production of spunlaid filaments is e.g. described in U.S. Pat. Nos. 4,813,864 and 5,545,371.

Meltblown filaments are formed by means of a meltblown equipment 10, for example of the kind shown in the U.S. Pat. Nos. 3,849,241 or 4,048,364. The method shortly involves that a molten polymer is extruded through a nozzle in very fine streams and converging hot air streams are directed towards the polymer streams so that they are drawn out into continuous filaments with a very small diameter. The filaments can be microfibers or macrofibers depending on their dimension. Microfibers have a diameter of up to 20 μm, but usually are in the interval between 2 and 12 μm in diameter. Macrofibers have a diameter of over 20 μm, e.g. between 20 and 100 μm.

All thermoplastic polymers can in principle be used for producing spunlaid and meltblown filaments. Examples of useful polymers are polyolefins, such as polyethylene and polypropylene, polyamides, polyesters and polylactides. Copolymers of these polymers may of course also be used.

Tow is another type of filaments, which normally are the starting material in the production of staple fibers, but which also is sold and used as a product of its own. In the same way as in the production of spunlaid fibers, tow is produced from fine polymer streams that are drawn out and stretched, but instead of being laid down on a moving surface to form a web, they are kept in a bundle to finalize drawing and stretching. When staple fibers are produced, this bundle of filaments is then treated with spin finish chemicals, are often crimped and then fed into a cutting stage where a wheel with knives will cut the filaments into distinct fiber lengths that are packed into bales to be shipped and used as staple fibers. When tow is produced, the filament bundles are packed, with or without spin finish chemicals, into bales or boxes.

The continuous filaments will in the following be described as spunlaid fibers, but it is understood that also other types of continuous filaments, e.g. meltblown fibers, can be used.

Natural Fibers

The natural fibers are usually cellulose fibers, such as pulp fibers or fibers from grass or straw. Pulp fibers are the most commonly used natural fibers and are used in the material for their tendency to absorb water and for their tendency to create a coherent sheet. Both softwood fibers and hardwood fibers are suitable, and also recycled fibers can be used. The fiber lengths will vary from around 2-3 mm for softwood fibers and around 1-1.5 mm for hardwood fibers, and even shorter for recycled fibers as well as blends of these. Other natural fibers that are commonly used in nonwoven materials are cotton and hemp.

Staple Fibers

The staple fibers used can be produced from the same substances and by the same processes as the filaments discussed above. They may either be synthetic fibers or regenerated cellulose fibers, such as rayon, viscose or lyocell. The cutting of the fiber bundles is normally done to result in a single cut length, which can be altered by varying the distances between the knives of the cutting wheel. The fiber lengths of conventional wetlaid hydroentangled nonwovens are usually in the interval 12-18 mm. However according to the present invention also shorter fiber lengths, from about 2-3 mm, can be used.

The Process

According to the embodiment shown in FIG. 1 continuous filaments 11 in the form of spunlaid fibers are produced by extruding a molten polymer, cool and stretch to an appropriate diameter. The fiber diameter is usually above 10 μm, e.g. between 10 and 100 μm. The spunlaid filaments are laid down directly on a forming wire 12 where they are allowed to form a relatively loose, open web structure in which the fibers are relatively free from each other. This is achieved either by making the distance between the spunlaying nozzle and the wire relatively large, so that the filaments are allowed to cool down before they land on the wire 12, at which their stickiness is reduced. The basis weight of the formed spunlaid layer should be between 2 and 50 g/m² and the bulk between 5 and 15 cm³/g.

In an alternative embodiment meltblown fibers are formed by means of a meltblown equipment. The meltblown technique shortly involves that a molten polymer is extruded through a nozzle in very fine streams and converging air streams are directed towards the polymer streams so that they are drawn out into continuous filaments with a very small diameter. The fibers can be microfibers or macrofibers depending on their dimension. Microfibers have a diameter of up to 20 μm, but usually are in the interval between 2 and 12 μm in diameter. Macrofibers have a diameter of over 20 μm, e.g. between 20 and 100 μm.

An aqueous or a foamed fibrous dispersion 13 from a headbox 14 is laid on top of the spunlaid filaments. In wet laying technique the fibers are dispersed in water, with optional additives, and the fiber dispersion is dewatered on a forming fabric to form a wet laid fibrous web. In foam forming technique a fibrous web is formed from a dispersion of fibers in a foamed liquid containing water and a tenside. The foamforming technique is described in for example GB 1,329,409, U.S. Pat. No. 4,443,297, WO 96/02701 and EP-A-0 938 601. A foam-formed fibrous web has a very uniform fiber formation. For a more detailed description of the foamforming technique reference is made to the above mentioned documents.

Prior to the headbox 14 the spunlaid filaments are according to one embodiment wetted in a spraybar 23 or gentle shower. The wettening of the filaments takes place at a very low pressure so that no substantial bonding of sideways displacement of the fibers take place. The surface tension of the water will adhere the filaments to the wire so the formation will not distort while entering the headbox. In some cases, when hydrophobic polymers are used for forming the spunlaid filaments, a tenside may be added in the spraybar 23 to wet the fibers.

Fibers of many different kinds and in different mixing proportions can be used for making the wet laid or foam formed fibrous web. Thus there can be used pulp fibers or mixtures of pulp fibers and synthetic staple fibers, e.g. polyester, polyethylene, polypropylene, polyamide, polylactide, rayon, viscose, lyocell etc. Other natural fibers than pulp fibers may further be used, such as seed hair fibers, e.g. cotton, kapok and milkweed; leaf fibers e.g. sisal, abaca, pineapple, New Zealand hamp, or bast fibers, e.g. flax, hemp, ramie, jute, kenaf. Varying fiber lengths can be used. However, according to the invention, it is of advantage to use relatively short staple fibers, below 10 mm, preferably in the interval 2 to 8 mm and more preferably 3 to 7 mm. This is because short fibers will more easily mix and integrate with the spunlaid filaments than longer fibers. There will also be more fiber ends sticking out form the material, which increases softness and textile feeling of the material. For short staple fibers both wet laying and foam forming techniques may be used.

In foam forming technique longer fibers can be used than what is possible with wetlaying technique. Long fibers, around 18-30 mm, may be an advantage in hydroentangling, since they increase the strength of the material in dry as well as in wet condition.

It is preferred that the fibrous web contains between 5 and 50% by weight, preferably between 5 and 20% by weight staple fibers. As stated above, for many applications it is advantageous to use short staple fibers, between 3 and 7 mm. In one embodiment a major part have a length in the interval 3 to 7 mm, wherein a major part refers to at least 50, preferably at least 70, more preferably at least 90 and most preferably at least 100% by weight of the staple fibers present in the material have a length in said interval.

As a substitute for pulp fibers other natural fibers with a short fiber length may be used, e.g. esparto grass, phalaris arundinacea and straw from crop seed.

It is preferred that the fibrous web comprises between 20 and 85% by weight, preferably between 40 and 75% by weight natural fibers, such as pulp fibers or substitutes therefore. It is further preferred that the fibrous web contains between 0.5 and 50% by weight, preferably between 15 and 30% by weight, continuous filaments, especially in the form of spunlaid or meltblown filaments.

The fiber dispersion laid on top of the spunlaid filaments is dewatered by suction boxes arranged under the wire 12. This provides the possibility to control the moisture content of the web before entering the subsequent foreshortening step. A higher moisture content increases the mobility of the fibers and their ability to rearrange and vice versa.

A spray station 15 may according to one embodiment be a pre-entangling station including one or several rows of nozzles from which very fine water jets under high pressure are directed against the fibrous web to provide a pre-entangling of the fibrous web. This pre-entangling binds the fibrous web to a certain degree, which should however not be higher than to allow a certain rearrangement of the structure in the subsequent foreshortening step. The pre-entangling step may in an alternative embodiment be eliminated.

The fibrous web is then transferred to an entangling wire 16 via a transfer wire 17. The entangling wire 16 is driven at a lower speed than the forming wire 12, and the transfer wire 17 is preferably driven at a speed intermediate that of the forming and entangling wires. Suction boxes 21 and 22 are arranged at the points of transfer between the wires.

Due to the speed difference, which normally is below 20%, the fibrous web is braked at the transfer between the wires, resulting in a foreshortening or compacting effect. This technique, sometimes called rush transfer, of transferring a fibrous web between wires driven with different speed, in order to provide a foreshortening effect of the fibrous web, is known from the papermaking field, especially tissue paper making. It is for example referred to U.S. Pat. No. 5,607,551.

At this so called rush transfer the fibrous web will in some sense be stuffed into the second wire. Because of the suction box 21 the fibrous web will be drained from water at the same time as it is stuffed in the surface of the transfer wire 17. Free from water the short fibers will to a certain degree rearrange to a more three-dimensional structure and the spunlaid filaments will catch some curls, bights and loops. The formation of curls will be eased if a three-dimensional structure is created already by the forming wire 12. These curls will ease the formation of loops in the entangling process and increase the penetration of the pulp into the spunlaid web. The increased mobility of the fibers will facilitate the intertwining of the fibers and will result in a structure where the pulp fibers are more firmly caught in the material.

The transfer fabric 17 may be replaced by a transfer roll.

The angles between the wires in the points of transfer should preferably be adjustable.

The type of foreshortening the fibrous web is exerted to by transferring it between wires driven at different speeds as described above, may be replaced by any other appropriate type of foreshortening a fibrous web, such as creping or micro creping, which e.g. is disclosed in U.S. Pat. No. 3,260,778 and U.S. Pat. No. 4,432,927, or through the so called “Clupak”-method, according to which a wet paper web is compacted by being placed on a rubber belt and be exerted to a varying tensile stress as is disclosed in U.S. Pat. No. 2,264,245.

After having been transferred to the entangling wire 16 the fibrous web is hydroentangled in an entangling station 18 including several rows of nozzles from which very fine water jets under high pressure are directed against the fibrous web to provide an entangling of the web. For a further description of the hydroentangling technique or, as it is also called, the spunlace technique, reference is made to e.g. CA patent 841,938. The entangling wire may optionally be patterned in order to form a patterned nonwoven material.

The foreshortening of the fibrous web creates a structure that more easily will mix and entangle in the subsequent hydroentangling step, which results in a composite nonwoven having a good integration between the spunlaid filaments, pulp and staple fibers. During the foreshortening when the web is compacted a part of the short pulp fibers and staple fibers will take a position oriented more in the z-direction of the web than would otherwise be obtained. This will result in improved absorption characteristics of the material. It will also improve the textile feeling of the material due to an increased amount of fiber ends sticking out.

The forming wire 12 and/or the entangling wire 16 may of course be substituted for another appropriate forming and entangling member respectively, such as an apertured belt, an apertured drum etc.

After the hydroentangling the material 19 is dried and wound up. The material is then converted in a known manner to a suitable format and is packed.

Claims

1. In a method of producing a nonwoven material, which comprises:

forming a fibrous web containing continuous filaments and natural fibers and/or staple fibers on a forming member and subsequently hydroentangling to form said nonwoven material;

the improvement which comprises transferring the fibrous web to an entangling member while subjecting said fibrous web to foreshortening and subsequently hydroentangling the foreshortened fibrous web, thus forming a composite material wherein the continuous filaments are well integrated with the rest of the fibers.

2. The method as claimed in claim 1, wherein the fibrous web is exerted to a pre-hydroentangling step before being foreshortened.

3. The method as claimed in claim 1, further comprising driving the forming member at a higher speed than the entangling member thus providing a foreshortening effect of the fibrous web when transferring the web between said members.

4. The method as claimed in claim 3, wherein the fibrous web is transferred to the entangling member via a transfer member, the entangling member is driven at a lower speed than the forming member, and the transfer member is driven at a speed intermediate that of the forming and entangling members.

5. The method as claimed in claim 1, wherein the fibrous web is wet-formed or foam-formed.

6. The method as claimed in claim 1, wherein the fibrous web comprises between 20 and 85% by weight natural fibers.

7. The method as claimed in claim 1, wherein the natural fibers are pulp fibers.

8. The method as claimed in claim 1, wherein the fibrous web contains between 5 and 50% by weight synthetic or regenerated staple fibers.

9. The method as claimed in claim 8, wherein at least a major part of the synthetic staple fibers have a fiber length between 3 and 7 mm.

10. The method as claimed in claim 1, wherein the fibrous web contains between 0.5 and 50% by weight continuous filaments.

11. The method as claimed in claim 10, wherein the continuous filaments are in the form of spunlaid or meltblown filaments.

12. The method as claimed in claim 1, wherein the fibrous web comprises between 40 and 75% by weight natural fibers.

13. The method as claimed in claim 1, wherein the fibrous web contains between 5 and 20% by weight synthetic or regenerated staple fibers.

14. The method as claimed in claim 1, wherein the fibrous web contains between 15 and 30% by weight continuous filaments.