US20050022954A1

US20050022954A1 - Hydraulically entangled nonwoven material and method for making it

Info

Publication number: US20050022954A1
Application number: US10/923,787
Authority: US
Inventors: Mikael Strandqvist
Original assignee: SCA Hygiene Products AB
Current assignee: Essity Hygiene and Health AB
Priority date: 2002-03-28
Filing date: 2004-08-24
Publication date: 2005-02-03
Also published as: US7326318B2; US20030213108A1

Abstract

A wetlaid or foam formed hydraulically entangled nonwoven material containing at least 30%, by weight, pulp fibres and at least 20%, by weight, man-made fibres or filaments. The material has a basis weight variation in a non-random pattern in that it comprises a plurality of higher basis weight cushions protruding from one major surface of the material. The cushions as a main component comprise pulp fibres and are surrounded by a lower basis weight network which as a main component comprises the man-made fibres or filaments. The invention further refers to a method for making the material.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending application Ser. No. 10/400,673, filed on Mar. 28, 2003, which is a non-provisional of 60/367,712 filed Mar. 28, 2002 the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention refers to a wetlaid or foam formed hydraulically entangled nonwoven material containing at least 30%, by weight, pulp fibres and at least 20%, by weight, man-made fibres. It further refers to a method of making such a material.

BACKGROUND OF THE INVENTION

Hydroentangling or spunlacing is a technique introduced during the 1970'ies, see e g CA patent no. 841 938. The method involves forming a fibre web, which is either drylaid or wetlaid, after which the fibres are entangled by means of very fine water jets under high pressure. Several rows of water jets are directed against the fibre, web which is supported by a movable wire. The entangled fibre web is then dried. The fibres that are used in the material can be natural fibres, especially cellulosic pulp fibres, man-made staple fibres, which may be synthetic, e g polyester, polyamide, polyethylene, polypropylene, or regenerated staple fibres, eg viscose, rayon, lyocell or the like, and mixtures of pulp fibres and staple fibres. Spunlace materials can be produced in high quality to a reasonable cost and they possess 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 in hygiene purposes etc.
Through e g EP-B-0 333 211 and EP-B-0 333 228 it is known to hydroentangle a fibrous mixture in which one of the fibre components is continuous filaments in the form of meltblown fibres.
In WO 96/02701 there is disclosed hydroentangling of a foam formed fibrous web. The fibres included in the fibrous web can be pulp fibres and other natural fibres and man-made fibres.
During the hydroentanglement the fibre web is supported either by a wire or a perforated, cylindrical metal drum. An example of a hydroentanglement unit of this kind is disclosed in for example EP-A-0 223 614. However, supporting members in the form of wires of the type utilised in connection with paper production is the most frequently occurring type as for example is shown in EP-A-0 483 816. One disadvantage with using wires of this type is that the fibre web, during the hydroentanglement, is exerted to a strong action by the water jets and will penetrate into and get caught between the wire threads. It may then be difficult to separate the final product from the wire.
WO 01/88261 discloses the use of a moulded, close-meshed screen of thermoplastic material as supporting member during hydroentanglement of a fibrous web. The removal of the final product from such screen is simplified as compared to a wire.
When making a nonwoven material, especially a material that is intended to be used as a wiping material, there a many properties that are important, such as absorptive capacity, absorption speed, wet strength, softness, drapability, low linting, high cleaning ability etc. It is however difficult to combine all these properties in one and the same material. It is for example possible to make cloth like, soft, strong and low linting hydroentangled nonwoven material by using 100% synthetic fibres. However the absorption properties of such a material will be low. Materials containing a high amount of pulp fibres have a high absorptive capacity, but a poor wet strength and high linting. The wet strength and linting properties can be improved by the addition of chemicals, such as wet strength agents and binders.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a hydroentangled nonwoven material that combines properties like wet strength, absorptive capacity, softness and drapability. This has been achieved by a wetlaid or foam formed hydraulically entangled nonwoven material containing at least 30%, by weight, pulp fibres and at least 20%, by weight, man-made fibres or filaments, said nonwoven material having a basis weight variation in a non-random pattern in that it comprises a plurality of higher basis weight cushions protruding from one major surface of said material, said cushions as a main component comprises pulp fibres and are surrounded by a lower basis weight network which contains a relatively higher amount of man-made fibres or filaments as compared to the cushions.
It is believed that this specific structure provides:

- a cloth like appearance of the material;
- high strength due to the network of the man-made fibres;
- high absorptive capacity provided by the high pulp content cushions and the three-dimensional structure formed by these;
- high softness and drapability due to the plurality of bending indications provided by the network pattern.

The opposite major surface of the material is preferably substantially smooth. This will improve the capability of the material to wipe a surface dry from any remaining liquid.
The material preferably contains ⁴⁰%, and more preferably at least 50%, by weight, pulp fibres. Preferably it contains at least 30%, and more preferably at least 40%, by weight, man-made fibres or filaments. The man-made fibres are in one embodiment staple fibres of a length between 6 mm and 25 mm.
It is preferred that the material has an absorptive capacity of at least 5 g/g water.
It is further preferred that the material has an absorption speed, WAT, in MD of no more than 1.5 s/m, preferably no more than 1 s/m, and in CD of no more than 2.5 s/m, preferably no more than 2 s/m.
In a preferred embodiment the cushions have a length and width between 0.2 and 4 mm, preferably between 0.5 and 2 mm. It is further preferred that the distance between the adjacent cushions is between 0.2 and 4 mm, preferably between 0.5 and 2 mm.
The present invention also refers to a method of producing a nonwoven material as stated above, said method comprises wetlaying or foamforming a fibre dispersion to form a fibrous web containing at least 30%, by weight, pulp fibres and at least 20%, by weight, man-made fibres or filaments, calculated on the total weight of the fibres in said fibrous web, and hydroentangling the fibrous web followed by subsequent dewatering and drying, wherein

at least part of the hydroentangling step is performed on a foraminous support member in the form of a moulded, close-meshed screen of a thermoplastic material, said screen having apertures of the cross-dimensional size 0.2-4 mm the and the distance between the apertures being between 0.2-4 mm.

Preferably the apertures in said screen are of the size 0.5-2 mm and the distance between the apertures is between 0.5-2 mm.
In one embodiment the fibrous web is formed on a formation wire and is subjected to a first hydroentangling while supported on said formation wire, and is then transferred to said moulded, close-meshed screen where it is subjected to a further hydroentangling.
Preferably said further hydroentangling is performed from the opposite side of the fibrous web as compared to the first hydroentangling.
In a preferred embodiment the web is after dewatering subjected to non-compacting drying, such as through-air-drying, IR drying or the like. In order to maintain bulk and absorbency of the material preferably no pressing of the fibrous web takes place during dewatering and drying.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will below be described with reference to some embodiments described in the accompanying drawings.
FIG. 1 is a schematic view of a device for hydroentangling a fibrous web.
FIG. 2 shows a schematic perspective view, on an enlarged scale, of a screen used for supporting the fibrous web during the hydroentangling.
FIG. 3 is a picture taken of a nonwoven material according to the invention on a magnification of about 30 times.
FIGS. 4 and 5 are electron microscope (SEM) pictures of a nonwoven material according to the invention.

DESCRIPTION OF EMBODIMENTS

The device, which schematically is shown in FIG. 1, for manufacturing a so-called hydroentangled or spunlaced material, comprises a vessel 10, e g a pulper, in which a wet or foamed fibre dispersion is prepared, which via a headbox 11 is distributed on a foraminuous support member 12. This foraminuous support member 12 is preferably a wire of any conventional kind used in papermaking industry and which is suited for formation and for a first hydroentangling step to intertwine at least the man-made fibres present in the web The formed fibrous web 13 is then subjected to hydroentanglement from several rows of nozzles 14, from which water jets at a very high pressure are directed towards a fibrous web, while this is supported by the foraminous support member 12. The fibrous web is drained over suction boxes 15. Thereby, the water jets accomplish an entanglement of the fibrous web, i.e. an intertwining of the fibres. Appropriate pressures in the entanglement nozzles are adapted to the fibrous material, grammage of the fibrous web, etc. The water from the entanglement nozzles 14 is removed via the suction boxes 15 and is pumped to a water purification plant, and is then re-circulated to the entangling stations.
For a further description of the hydroentanglement or, as it is also called, spunlacing technology, reference is made e.g. to the above-mentioned CA patent No. 841 938.
The fibrous web 13 is either wet-laid or foam-formed. In a wet-laid process the fibres are dispersed in a liquid, normally water, in a similar way as in a papermaking process and the dilute fibre dispersion is deposited on the foraminous support member where it is dewatered to form a continuous web-like material. The fibre dispersion may be diluted to any consistency that is typically used in conventional papermaking process. A foam forming process is a variant of a wet-laying process and a surfactant is added to the fibre dispersion, which is foamed, and the foamed fibre dispersion is deposited on the foraminous support. A very even fibre distribution is achieved in a foam forming process and it is also possible to use longer fibres than in a conventional wet-laying process.
The fibres used to form the fibre dispersion is a mixture of cellulosic pulp fibres and man-made staple fibres or man-made filaments. The pulp fibres can be selected from any type of pulp and blends thereof. Preferably the pulp is characterized by being entirely natural cellulosic fibres and can include cotton as well as wood fibres. Preferred pulp fibres are softwood papermaking pulp, although hardwood pulp and non-wood pulp, such as hemp and sisal may be used. The length of pulp fibres may vary from less than 1 mm for hardwood pulp and recycled pulp, to up to 6 mm for certain types of softwood pulp. The fibre dispersion should contain at least 30% by weight, calculated on the total fibre weight, pulp fibres.
The man-made fibres may be any suitable synthetic fibres or regenerated cellulosic fibres. Examples of commonly used synthetic fibres are polyester, polyethylene, polypropylene, polyamide, polylactides and/or coplymers thereof. Examples of regenerated fibres are rayon, viscose, lyocell. The man-made fibres may be in the form of staple length fibres. A preferred length of staple fibres used in a wetlaying or foam forming process is between 6 mm and 25 mm. The fineness of the fibres can vary between 0.3 dtex and 6 dtex. The fibre dispersion should contain at least 20% by weight, calculated on the total fibre weight, man-made fibres.
The man-made filaments are preferably spunlaid or meltblown filaments of suitable thermoplastic polymers, such as polyethylene, polypropylene, polyamides, polyesters and polylactides. Copolymers of these polymers may of course also be used, as well as natural polymers with thermoplastic properties.
The web 13 is turned 1800 and transferred to a second foraminous support member 16, which in a preferred embodiment is constituted of a moulded, close-meshed plastic screen, as disclosed in WO 01/88261. The plastic screen according to the invention can consist of one layer, as shown in FIG. 2, or of two or several layers applied on top of each other. Possibly, the screen can be reinforced with reinforcement wires 17 which extend in the intended machine direction of the plastic screen/entanglement wire 16. Reinforcement wires can be arranged also in the transverse direction of the screen, or both in the longitudinal and the transverse direction. The production of the plastic screen can take place e.g. in the way described in U.S. Pat. No. 4,740,409. The plastic screen is provided with a plurality of apertures 18, which will be described in greater detail below.
The web is hydroentangled a second time from several rows of nozzles 19 while supported on the plastic screen member 16. The second hydroentanglement takes place from the opposite side of the fibrous web 13 as compared to the first hydroentanglement. The fibrous web is drained over suction boxes 20.
Further dewatering of the fibrous web may take place over suction boxes 21, while the web 13 has been transferred to a dewatering wire 22. This further dewatering may optionally take place while the fibrous web is still supported by the plastic screen member 16.
The entangled material is then brought to a drying station 23 for drying before the finished material is reeled up and converted. Drying can be performed by blowing hot air through the fibrous web, by IR dryers or other non-compacting drying technique. Preferably no pressing of the fibrous web takes place during dewatering and drying thereof. The material may before conversion be exerted to different kind of treatments, such as corona or plasma treatment 24, treatment with chemicals of any desired kind etc. Corona or plasma treatment is preferably made after drying while chemicals may be added either to the fibre dispersion or after dewatering of the web by spraying printing or the like.
In the embodiments shown in FIG. 2, the apertures 15 in the screen 16 exhibit a rectangular shape, but it is evident that this shape can be varied to any geometrical shape. The meshes in the screen suitably exhibit an aperture size within the interval 0.2-4 mm, preferably 0.5-2 mm. The aperture size is herein defined as the size between opposite side edges or corners. The apertures are either of substantially the same size or of different sizes, and are either uniformly distributed across the screen or arranged to form patterns with alternating groups of apertures of different sizes. Also the cross-sectional shape of the apertures in the z-direction can be varied, and can be e.g. substantially rectangular, alternatively convex or concave. The distance between the apertures may vary between 0.2-4 mm, preferably between 0.5 and 2 mm. The distance between the apertures is defined as the shortest distance between adjacent apertures.
In case the screen consists of two or several layers arranged on top of each other, the different layers can exhibit different aperture sizes among themselves, e.g. with larger apertures in an upper layer and smaller apertures in a lower layer. This is shown in WO 01/88261. In this way, fibres can penetrate down into the larger apertures in the upper layer but be retained by the lower layer during the entanglement.
The surface, which is intended to support the fibrous web, can be substantially smooth, or exhibit a three-dimensional structure in order to impart a corresponding three-dimensional structure to the hydroentangled material.
Other foraminous supports such as wires and other types of screens may also be used, which have apertures of the size stated above.
When hydroentangling the fibre dispersion through the apertured screen 16 the shorter pulp fibres, which are more easily mobile, will to a higher extent follow the water that is drained through the apertures 18 and be accumulated in said apertures, while the longer man-made fibres which are less mobile and more easily intertwined by the water jets, will to a higher extent stay in place on the screen 16 and build up a strong fibrous network.
This will result in a nonwoven material having a plurality of cushions 25 protruding from one major surface of the material, said cushions as a main component comprise pulp fibres 26 that during drainage have accumulated in the apertures 18 of the screen 16. The term “main component” in this respect means that more than 50% by weight, preferably more than 60% by weight and more preferably more than 70% by weight of the fibres present in said cushions are pulp fibres 26. A minor proportion of the fibres in the cushions 16 will of course be man-made fibres.
The pulp fibre cushions 25 are surrounded by a network 27, which contains a relatively higher amount of man-made fibres 28 as compared to the cushions 25. In a preferred embodiment more than 50% by weight, preferably more than 60% by weight and more preferably more than 70% by weight of the fibres present in said network are man-made fibres 28. The longer man-made fibres 28 are more easily entangled and will intertwine with each other to form a strong continuous network 27 which will impart high strength to the material. The pulp fibre cushions contribute to the absorbency of the material. This is shown in FIGS. 4 and 5 showing SEM-pictures of a material according to the invention, and in which the accumulation of pulp fibres 26 to form the cushions 25 can be seen. It is further seen how these cushions 25 are surrounded by a network. This is also seen from the light microscope picture in FIG. 3.
In order to provide a pronounced cushion effect at least 30% by weight and preferably at least 40% of the fibres in the material should be pulp fibres and in order to provide a strong network of man-made intertwined fibres at least 20% by weight and preferably at least 30% by weight of the fibres in the material should be man-made fibres.
The length and width dimensions of the cushions 25 will correspond to the size of the apertures 18 of the screen 16 and the width of the network strands 27 between the cushions 25 will correspond to the distance between adjacent apertures 18 of the screen 16.
The opposite major surface of the material is preferably substantially smooth as compared to the first surface having a pronounced three-dimensional structure provided by the plurality of protruding cushions. This gives the material a two-sidedness with one side the is more “rough” and adapted to remove and capture liquids, viscous fluids and solid particles from a surface. The opposite smooth surface is adapted to wipe a surface dry from liquid.
Tests have been performed on materials produced as described below. A foamformed fiber dispersion was made from water, surfactant and a mixture of pulp fibres and man-made staple length fibres. A surfactant was added to the water in an amount of 0.03% by weight. The foamed fibre dispersion was laid on a wire and the formation was made at an air content in the foam of 30-50% by volume. The fibrous web was hydroentangled on the same wire used for formation. The web was then transferred to a moulded, close-meshed plastic screen as disclosed above, having holes of the size 0.89×0.84 mm and a distance between the holes of 0.46 mm. The web was then hydroentangled from the opposite side. The main part of the hydroentangling was made on the first wire in order to give maximum strength to the material. The total energy supply at the hydroentangling was about 200 kWh/ton material.
The fibrous web was then dewatered by vacuum suction boxes and dried by so called through-air-drying (TAD).
The fibres used for forming the fibrous web had the following composition:

Ex. 1: 25 wt % polyester (PET) from KoSa, 1.7 dtex/19 mm;
- 17 wt % polypropylene (PP) from Fibervisions, 1.7 dtex/18 mm;
- 58 wt % bleached sulphate pulp fibres from Korsnas, Vigor Fluff.
Ex. 2: 40 wt % polypropylene (PP) from Fibervisions, 1.7 dtex/18 mm;
- 50 wt % bleached sulphate pulp fibres from Korsnas, Vigor Fluff.

As reference material was used a nonwoven wiping material produced by SCA Hygiene Products AB under the trade mark E-Tork Strong™. It is made by wet forming a fibre mixture and hydroentangling thereof. However there is not used any moulded, close-meshed plastic screen, but the hydroentangling process is performed on a conventional papermaking wire. The material does not have the patterned three-dimensional structure as claimed by the present invention, but a more uniform fibre distribution. The fibre composition in the reference material was as follows:

Ref.: 25 wt % polyester (PET) from KoSa, 1.7 dtex/19 mm;
- 17 wt % polypropylene (PP) from Steen, 1.7 dtex/18 mm;
- 58 wt % bleached sulphate pulp fibres from Korsnäs, Vigor Fluff.

Thus the fibre composition is the same as for Ex. 1 except that the PP fibres are from another manufacturer.

Evaluations concerning strength properties both in dry and wet conditions, absorbency, wicking rate were performed and gave the results presented in Table 1 below:

TABLE 1


Ex. 1	Ex. 2	Ref.

Grammage	g/m²	76.4	88.0	83.0
Thickness	μm	623	641	357
Bulk 2kPa	cm³/g	8.2	7.3	4.3
Tensile stiffness MD	N/m	30230	38385	57518
Tensile stiffness CD	N/m	2096	6488	6689
Tensile stiffness index	Nm/g	104	179	236
Tensile strength MD, dry	N/m	3126	3061	1499
Tensile strength CD, dry	N/m	672	745	630
Tensile Index, dry	Nm/g	19	17	12
Stretch MD	%		28	33	13
Stretch CD	%	71	45	44
Stretch sq root (MDCD)	%	45	39	24
Work to rupture MD	J/m²	647	714	251
Work to rupture CD	J/m²	347	238	261
Work to rupture index	J/g	6	5	3
Tensile strength MD, water	N/m	2066	3028	568
Tensile strength CD, water	N/m	330	619	185
Tensile index, water	Nm/g	10.8	15.6	3.9
Relative strength, water	%	57	91	33
Tensile str. MD, surfactant	N/m	1536	3002	407
Tensile str. CD, surfactant	N/m	330	647	122
Tensile index, surfactant	Nm/g	9.3	18.2	2.5
Rel strength, surfactant	%	49	96	15
Absorption DIN, water	g/g	6.0	5.1	3.9
Absorption speed WAT, x-dir.	s/m	0.4	0.7	1.7
(CD)
Absorption speed WAT, y-dir.	s/m	0.8	1.3	2.7
(MD)
Wet linting	part./m²	397	228	259

The tensile stiffness, tensile strength, work to rupture and stretch were measured according to the test method SCAN-P44:81.
The absorption DIN was measured according to the test method DIN 54 540, part 4, with the modification that the sample was suspended vertically during soaking and not in horizontal position as in the standard method.
The absorption speed WAT was measured according to the test method SCAN-P 62:88. However the following modification of the sample was made: Instead of aiming for a total grammage of between 100 and 150 g/m²of the sample sheaf, we have aimed for a total thickness of 1 mm. No measurements of the absorption speed in z-direction were made.
These results show superior strength properties both in dry and wet conditions for the nonwoven materials according to the invention. This is believed to be due to the strong network that is created by the man-made fibres present in the material, said network being more or less continuous. The choice of man-made fibres also plays an important role for the strength of the material, and it is seen from the test results that Ex. 2 which contains 40% by weight polypropylene fibres (1.7 dtex/18 mm), has improved strength properties as compared to Ex. 1 containing a mixture of polyester and polypropylene, 25% polyester (1.7 dtex/19 mm) and 17% polypropylene fibres (1.7 dtex/18 mm). However both materials have considerably higher strengths, i.e. tensile strength (dry, water, surfactant), stretch and work to rupture, as compared to the reference material.
The materials according to the invention are less stiff than the reference material.
The materials according to the invention further have improved absorption properties, both total absorbency and absorption or wicking speed (WAT), as compared to the reference material. This is believed to be due to a combination of the high concentration of pulp fibres present in the plurality of cushions protruding from one side of the material, said cushions of pulp fibres being capable of absorbing and holding liquid, and the network of predominantly man-made fibres, said network being adapted to distribute the liquid in the material.

Claims

1. A method of producing a nonwoven material comprising wetlaying or foamforming a fiber dispersion to form a fibrous web containing at least 30%, by weight, pulp fibres and at least 20%, by weight, man-made fibres or filaments, calculated on the total weight of the fibres in said fibrous web, and hydroentangling the fibrous web followed by subsequent dewatering and drying,

wherein at least part of the hydroentangling step is performed on a foraminous support member in the form of a moulded, close-meshed screen of a thermoplastic material, said screen having apertures of cross-dimensional size 0.2-4 mm and the distance between the apertures being between 0.2-4 mm.

2. The method as claimed in claim 1,

wherein the cross-dimensional size of the apertures in said screen is 0.5-2 mm and the distance between the apertures is between 0.5-2 mm.

3. The method as claimed in claim 1,

wherein the fibrous web is formed on a formation wire and is subjected to a first hydroentangling while supported on said formation wire, and is then transferred to said moulded, close-meshed screen where it is subjected to a further hydroentangling.

4. The method as claimed in claim 3,

wherein said further hydroentangling is performed from an opposite side of the fibrous web as compared to the first hydroentangling.

5. The method as claimed in claim 1,

wherein the web after dewatering is subjected to non-compacting drying.

6. The method as claimed in claim 5,

wherein the non-compacting drying is selected from through air drying and IR drying.

7. The method as claimed in claim 5,

wherein no pressing of the fibrous web takes place during dewatering and drying.