WO1995021596A1 - Absorbent materials and preparation thereof - Google Patents

Abstract

An absorbent article formed of two fibrous webs with absorbent particles between the webs. The two webs are laminated together in a combination of permanent and semipermanent bonds. The semipermanent bonds define spaces containing absorbent particles. Upon absorption of liquid, the semipermanent bonds release as the absorbent particles expand. This ensures dimensional stability of the product after absorption of liquid.

Description

ABSORBENTMATERIALSAND PREPARATION THEREOF

Background and Field of the Invention

This invention relates to absorbent materials and to their manufacture. In particular the invention relates to absorbent materials comprising a laminate of two or more layers of fibres having absorbent material between the layers of fibres. In one aspect, the invention relates to materials suitable for use in diaper manufacture.

Diapers manufacturers have been looking for ways of using super absorbent polymers (SAP) in diaper products based on their performance to cost ratio and the reduction, in thickness of the products. The problem has been how to get the SAP powder/granules into a stable product so that liquid is absorbed effectively.

Some systems have had SAP distributed with a web of fibres. However, the web in these tend to be destroyed as the SAP swells on absorbing liquid and hence becomes very weak so requires additional supporting webs to be effective. Also the SAP is not kept away from the surface of the sorbent core and so feels wet unless other webs are used to cover the surface. U.S. Patents Nos. 4,994,053 and 5,149,335 disclose absorbent articles comprising two webs bonded together by bond lines which define a series of pockets for containing absorbent material, such as SAP. Laminated products comprising two webs bonded together with SAP therebetween can allow expansion if there are spaces between points of lamination but in these cases the SAP is free to migrate during processing or before getting wet giving uneven loading and reduced performance i.e. the SAP moves to places that do not get wet or it collects in lumps which are less effective at absorbing than when in thin layers due to a reduction in surface area in contact with the liquid. Our co-pending British Patent Application No. 9318971.0 discloses a liquid sorbent article comprising at least one layer of sorbent fibrous material and at least one hinge about which the article may be preferentially folded, the hinge being formed by permanently bonding the fibres of the layer under pressure along a line.

By providing sorbent sheet material with hinges formed by bonding fibres under pressure the strength characteristics of the article is improved and the use of the article in a variety of formats is facilitated. For example, suitable positioning of the hinges allows the article to be fan-folded for packaging and in use the article can be used in the form of a sheet, mini- pillow or boom by simple folding to increase the thickness of the article. The hinges enable the sheet material to adopt and maintain a folded configuration without the need for intermittent releasable bonds.

Preferably the hinges are formed by welding the fibres under pressure, more preferably by pressure welding without application of heat. The article is preferably constructed of at least two contiguous layers of sorbent material which are bonded together by a plurality of hinges and optionally at one or more edges. Such a two layer product has improved strength over a single layer article of the same basis weight.

The hinges are generally formed as a straight line extending across or along the length of the article. The hinges may be continuous or discontinuous, providing the article may be preferentially folded along the line. The hinge may be formed with perforations along the line to facilitate tearing the sheet into desired sizes. Preferably the articles comprise a plurality of parallel hinges.

However, welds need not be formed in a straight line and may adopt other configurations, e.g., sinusoidal or zig-zag paths. Such configurations are particularly suitable for use with multilayered articles since it allows contiguous layers to be bonded in a similar manner to quilting. Furthermore, by suitable positioning of the welds it is possible to form a series of pockets between two contiguous layers which can be filled with particles, e.g., absorbents, indicators, neutralizers, etc. The pockets prevent the particles from moving freely about the material. Such particles may be dropped onto the lower web prior to application of the upper web and pressure welding. The blunt slitters used for pressure welding may have working edges adopting the desired sinusoidal or zig-zag configuration and are conveniently arranged such that the re-entrant portions of adjacent welds are adjacent thereby dividing the web into substantially discrete islands.

EP 0146190 discloses an absorbent article comprising a layered absorbent structure having an upper surface and a lower surface, and comprising: (a) n webs of fibrous materials, n being an integer of two or more, said webs being layered such that there is an uppermost web, a lowermost web, n-2 intermediate webs, and n-1 interfaces of two opposed adjacent contacting surfaces of adjacent webs, each of the interfaces having a surface area; and (b) absorbent particles forming a discontinuous layer at one or more of the interfaces; the opposed adjacent contacting surfaces at each of the interfaces where the particles are present being substantially entirely frangibly bonded by fibre entanglement between the contacting surfaces, the particles being immobilized at the interface(s) substantially entirely by fibre entrapment. There is sufficient spacing between the particles to allow them to swell to maximum size before touching neighbouring particles. The particles are immobilized but free to absorb fluid as if they were unconstrained since the bonds between the contacting webs near a swelling particle will be easily broken allowing the particle to freely swell.

It has now been found that particularly useful absorbent products may be obtained by incorporating absorbents in spaces between layers of fibre webs which are laminated together with a combination of permanent and semi-permanent bonds. Description of the Invention

According to the present invention there is provided an absorbent- article comprising two contiguous layers of fibre webs having therebetween particles of absorbent material, the layers being laminated together by a combination of permanent bonds and semi-permanent bonds which are positioned to define spaces which contain particles of the absorbent material, the semi-permanent bonds being releasable upon expansion of the absorbent material as it absorbs liquid, the permanent bonds ensuring dimensional stability of the article before and after absorption of liquid. The absorbent articles of the invention provide a simple, effective arrangement for incorporation of particles of absorbent material within a laminated structure. A series of spaces or pockets is defined by the laminating bonds in which the particles are contained thereby restricting undue movement of the particles without having to immobilize individual particles. A portion of the laminating bonds are semi-permanent or releasable which break or release as the particles of absorbent swell thereby allowing the particles to retain maximum absorbing capacity. The invention does not require fibre entanglement to form the releasable bonds. Preferably the permanent bonds are positioned at least at the sides and/or ends of the article, more preferably completely around the periphery. The article may comprise other permanent bonds spaced across the layers, e.g., one or more lines of bonds. The permanent bonds may conveniently be formed by welding the layers, e.g., by heat, pressure, ultrasonics or any combination thereof. The result of the welding is that the fibres are completely fused together at the bond, the identity of individual fibres not being discernible in the bond. Adhesives may be used to form the permanent bonds although they are not preferred as more complex manufacturing equipment is required. The semi-permanent welds prevent unwanted migration of the particles of absorbent material and are preferably arranged to form a series of pockets in a similar manner to quilting. However, other arrangements are also possible, e.g., parallel lines of semi-permanent welds will restrict particle movement providing the lines are sufficiently close. The semi-permanent welds are preferably formed by cold pressure welding under conditions which are sufficient to cause fusing of the contacting surfaces of fibres thereby retaining identity of the fibres at the bond. During expansion of the SAP the pressure will be sufficient to break the bond between overlapping fibres without breakage of many of the fibres so that the article retains its structural integrity. Adhesives such as those which lose adhesion on wetting may be used, although these are not preferred as they require more complex manufacturing equipment.

In a preferred embodiment both the permanent and semi-permanent welds are formed by cold pressure welding using blunt slitters, the blunt slitters having working edges having a sinusoidal or zig-zag configuration. The articles of the present invention can be formed from a wide range of sorbent sheet materials which can be laminated. Such sheet material include, for example nonwoven, fibre webs such as microfibrous sheet material, and wood pulp based webs which are well known to those skilled in the art. The article generally has a basis weight of from 20 to 500g/m².

The articles of the present invention are preferably formed from at least one layer of sorbent microfibrous sheet materials, particularly melt blown microfibrous materials. Such materials are described, for example, in van Wente "Superfine Thermoplastic Fibres", Industrial Engineering Chemistry, vol. 48, pp. 1342 et seq. (1956), or in Report No. 4364, Naval Research Laboratories, May 25, 1954, "Manufacture of Superfine Organic Fibres" by van Wente, A. Boone, CD., and Fluharty, E.L.

More preferably, the articles of the present invention are formed from at least one layer of melt blown microfibrous materials containing a surfactant topically applied or incorporated by direct addition of the surfactant to the molten polymer stream. Sorbent articles based on these __ , _,___ 95/21596

materials are suitable for use on either oil or water based spills. Such materials having surfactant incorporated in the fibre are described in U.S. Patent No. 4,933,229 (Insley et al.).

Most preferably, the articles of the invention are formed from melt

5 blown microfibre webs which contain microfibre microwebs and crimped bulking staple fibres as described in U.S. Patent No. 4,813,948 (Insley).

Polymeric materials suitable for use in preparing the microfibrous sheet or web material include, but are not limited to polyolefins such as polyethylene and polypropylene, polyesters such as poly(ethylene 0 terephthalate) and polyamides such as nylons, blends or copolymers. The microfibrous sheet materials are preferably based on polyethylene, polypropylene, or blends of the two resins.

Other materials such as UV stabilizers, dyes, pigments, etc., can also be incorporated directly into the melt blown microfibrous sheet 5 materials during the melt blowing process by blending the additive with the polymer prior to extrusion.

Articles of the present invention which are formed from melt blown microfibrous materials typically have sufficient strength, even when saturated with liquid, that an outer facing is not required to maintain 0 integrity. An optional outer facing may, however, be provided by applying a scrim material, for example, to the microfibrous sheet materials prior to lamination to improve the appearance of the article and coincidentally produce a more durable article. This can be accomplished by collecting the melt blown microfibrous web material directly on the facing or 5 alternatively bonding the facing to the microfibrous material by using an intermittent adhesive pattern or pin laminating the materials together. Facing material can also be applied to the microfibrous sheet material as a coating, for example, latex, at a level which does not substantially affect the sorbency of the microfibrous sheet material. 0 Scrim materials suitable for use in the preparation of the articles of the present invention preferably have a relatively open structure, such as obtainable with nonwoven fabrics or open weave woven fabrics, so that they can be readily penetrated by both oil and aqueous based fluids. Additionally, suitable scrim materials are preferably chemically inert. Particularly preferred scrim materials include spunbond non-woven polypropylene fabrics such as 0.5 oz/yd² CELESTRA, available from Fiberweb North America, Inc.

The particles of absorbent material are preferably SAP materials, e.g., cross-linked acrylate polymers of the type commercially available from Dow under the trade designation X2 95889.01 , polyacrylamide polymers of the type commercially available from Allied Colloids under the trade mark VERSICOL W25 and sodium acrylates of the type commercially available from Atochem under the trade mark AQUAKEEP D50-65. Such materials are designed for absorption of water based fluids. Other SAP materials may also be employed, e.g., copolymers of aromatic alkenes and dialkenes that are designed to absorb oils, acids and alkalis. Other absorbent materials which may be employed include sorbent fibres, microfibres and microwebs, e.g., scrap blown microfibres from edge trims and other waste from the production of sorbent articles.

Other solid materials may also be incorporated between the layers, e.g., neutralizers for acids or bases, such as, sodium carbonate for products dealing with acid spills. Indicators, e.g., of the litmus type, disinfectants, etc., may also be employed.

The articles of the invention are particularly suitable as absorbent laminates for incorporation into a diaper or other personal hygiene product to act as the absorbent core.

Typically two layers of blown microfibre absorbent webs (50 to 150gsm each) are welded together at the edge and with a centre line to form two tubes. (Typical size of diaper = 14 x 44cm). These permanent welds can be achieved by heat, pressure, ultrasonics or combination of these. Super absorbent polymer (SAP) is spread evenly between the layers but not in areas of the permanent welds, prior to the welding process. (Typically maximum 14g per 14 x 44cm). Further semi¬ permanent welds are made over the SAP-bmf using pressure. These welds keep the SAP in place by both the pocketing design and closeness of the welds. The top layer of bmf (blown microfibre) is hydrophilic and absorbs the liquid, the bottom layer is hydrophobic and prevents the passage of liquid through the article (this latter layer could be a spunbond web on a film, rather than a bmf web). Both layers encase the SAP to act as a carrier. The top layer promotes the absorption and spread of liquid over a large surface area and allows a large surface area of SAP to quickly contact the liquid.

Once wet the SAP swells. The semi-permanent welds break allowing expansion without damage to the webs. The webs now act as a case encapsulating the absorbed liquid with sufficient strength not to break open. The SAP when wet is sticky and so does not migrate once the welds have opened.

The semi-permanent welds allow both expansion of the SAP without destruction of the web and also good anchoring of the SAP in places where it is needed. Therefore the SAP is used efficiently. The weld lines also help promote wicking throughout the product. Therefore the absorbent does not absorb in one place but is maximized over the full surface of the product. The SAP absorbs quite slowly and blown microfibre web quickly, so to pick up liquid quickly it is ideal to absorb liquid first into the blown microfibre web and then SAP begins to absorb the liquid out of the blown microfibre. This means that liquid can be quickly adsorbed by the web and also that the web tends to dry out after wetting which is a desirable feature in this type of application.

The product can be incorporated directly into standard wood pulp filled diaper lines without large redesign of the lines because it is strong enough to be unwound and pulled through the lines without breaking. The ,..,_..._tn, 95/21596

product may be supplied in long lengths, with the diaper manufacturer cutting the product to length (typically 44cm) and making the end welds.

The use of a hydrophobic bottom web gives soft feel, bulk and strength to the product, whilst preventing the penetration of liquid through 5 to the outside of the diaper.

Brief Description of the Drawings

The invention will now be described with reference to the accompanying drawings in which: 0 Figure 1 is a perspective view of an absorbent article in accordance with the invention,

Figure 2 is a cross-section through the article of Figure 1 , and Figure 3 is a cross-section through the article of Figure 1 after absorption of liquid. 5

Detailed Description of the Drawings and Preferred Modes

Figures 1 to 3 illustrate a sorbent article in accordance with the invention comprising two layers (2, 4) of fibre webs. At least one, preferably both layers, comprise blown microfibre web. Blown microfibre 0 web is formed by extruding fibres through a dye onto a collector. The side of the web that contacts the collector has a smooth relatively lint-free surface finish and the other side is usually more ragged. Preferably the blown microfibre webs are arranged with the rough surfaces in contact so that the outer surfaces of the article are smooth thereby providing a 5 satisfactory look and feel to the article. In the case of material for use in a diaper one layer (2) is preferably a hydrophilic sorbent layer and one layer

(4) is preferably hydrophobic.

The layers (2, 4) are permanently bonded to each other by permanent welds (6) positioned at the edges of the web and a permanent 0 weld (8) which is centrally positioned. These welds are conveniently formed by passing the composite layers through blunt slitter rolls such that the layers are welded together under pressure without being slit.

Alternatively, heat or ultrasonic welding may be employed or combinations of these methods.

The article additionally comprises a series of parallel semi- permanent welds (10) and sinusoidal semipermanent welds (12). These semi-permanent welds may be formed by cold pressure welding using blunt slitter rolls. It has been found that cold welding pressures of 200 to- 1000N/mm² provides good semi-permanent welds with many commercially available polyolefin non-wovens. In real terms, this is about 100 to 1000N force applied to a slitter wheel having a flattened face of from 0.5 to 2mm width. It is important that the flat face of the slitter wheel does not have sharp edges which would result in cutting of the webs.

The welds between the layers (2, 4) provide a series of pockets (14) which contain particulate material (16). The particulate material comprises an absorbent, preferably SAP material. The particulate material is deposited on the lower layer (4) prior to application of the upper layer (2) before the composite is passed through the slitter rolls to form the permanent and semipermanent welds. The arrangement of welds prevents the particulate material from readily moving through the article during transportation, handling and use thereby ensuring the article retains uniform properties throughout its length.

The pocket dimensions should be small enough to prevent unwanted movement of powder, but minimized in number so as to reduce the manufacturing difficulties and reduce the area of compressed web. Conveniently the pockets have length and width dimensions in the range 1.5 to 4cm. However, it is possible to control movement of powder by using straight parallel welds to form tubes preferably having a width of 1 to 2cm.

Figure 3 shows a cross-section through the sorbent article after it has been saturated with liquid. The absorbent (16) swells as it absorbs liquid bursting the 5 semi-permeable welds (10, 12). Dimensional stability of the article is maintained by the permanent welds (6, 8).

Claims

1. An absorbent article comprising two contiguous layers of fibre webs having therebetween particles of absorbent material, the layers being laminated together by a combination of permanent bonds and semi¬ permanent bonds which are positioned to define spaces which contain particles of the absorbent material, the semi-permanent bonds being releasable upon expansion of the absorbent material as it absorbs liquid, the permanent bonds ensuring dimensional stability of the article before and after absorption of liquid.

2. An absorbent article as claimed in Claim 1 having permanent welds along its major edges.

3. An absorbent article as claimed in Claim 1 or Claim 2 having a permanent weld substantially around its periphery.

4. An absorbent article as claimed in any preceding claim comprising a plurality of semi-permanent welds extending along the length thereof.

5. An absorbent article as claimed in any preceding claim comprising a plurality of sinusoidal or zig-zag semi-permanent welds extending along the length thereof.

6. An absorbent article as claimed in any preceding Claim in which at least one of the layers comprises blown microbfibres.

7. An absorbent article as claimed in claim 6 in which the fibre webs are of polypropylene.

8. An absorbent article as claimed in any preceding claim in which the permanent bonds are formed by heat, pressure and/or ultrasonic welding.

9. An absorbent article as claimed in any preceding Claim in which the semi-permanent bonds are formed by pressure welding.

10. An absorbent article as claimed in any preceding Claim in which the particles of absorbent material comprise super absorbent polymer.

11. An absorbent article as claimed in any preceding Claim in which one layer of fibres is hydrophilic and the other layer of fibres is hydrophobic.

12. An absorbent article as claimed in Claim 1 substantially as herein described with reference to the accompanying drawings.

13. A method of making an article as defined in Claim 1 which comprises: providing a first layer of fibre web, applying particles of absorbent material to a surface of said first layer, providing a second layer of fibre web over said particles of absorbent material, and bonding said first and second layer together by a combination of permanent and semi¬ permanent bonds.

14. A method of making an article as claimed in Claim 13 in which the permanent bonds are formed by heat, pressure, and/or ultrasonic welding.

15. A method of making an article as claimed in Claim 13 or claim 14 in which the semi-permanent bonds are formed by pressure welding.

16. A method of making an article as claimed in any one of

Claims 13 to 15 in which the semi-permanent welds are formed by passing the two layers between a blunt slitter roll and a solid support to effect a semi-permanent weld.

17. A method of making an article as claimed in Claim 16 in which the slitter roll exerts a pressure of from 200 to 1000 N/mm².

18. A method of making an article as claimed in Claim 16 or Claim 17 in which the two layers are passed through a slitter apparatus comprising two cutting slitter rolls spaced apart and at least one blunt slitter roll positioned therebetween.

19. A method of making an article as claimed in Claim 18 in which the slitter apparatus comprises a plurality of blunt slitter rolls spaced equidistant between the cutting slitter rolls.

20. A method of making -an article as claimed in any one of Claims 16 to 19 in which the blunt slitter roll(s) has a sinusoidal or zig-zag working edge.

21. A method of making an article as claimed in any one of Claims 13 to 20 in which at least one of the layers comprises blown microbfibres.

22. A method of making an article as claimed in Claim 21 in which the fibre webs are of polypropylene.

23. A method of making an article as claimed in any one of Claims 13 to 22 in which the particles of absorbent material comprise, super absorbent polymer.

24. A method of making an article as claimed in any one of Claims 13 to 23 in which one layer of fibres is hydrophilic and the other layer of fibres is hydrophobic.

25. A method of making an article as claimed in Claim 13 substantially as herein described with reference to the accompanying drawings.