WO2008029428A1

WO2008029428A1 - Absorbent web product, method of obtaining it and absorbent article utilizing this product

Info

Publication number: WO2008029428A1
Application number: PCT/IT2007/000601
Authority: WO
Inventors: Riccardo Cecconi; Chiara Allegrini; Barbara Bulleri
Original assignee: Fintex & Partners Italia S.P.A.
Priority date: 2006-09-06
Filing date: 2007-08-31
Publication date: 2008-03-13
Also published as: ITFI20060217A1

Abstract

The absorbent product comprises at least one layer of bonded textile fibers (F) and, inside said layer of textile fibers, a plurality of cells formed through expansion of at least one expandable substance (S).

Description

"Absorbent web product, method of obtaining it and absorbent article utilizing this product"

DESCRIPTION Technical field The present invention relates to an absorbent product, for example a semi-finished product in the form of a sheet or continuous web, intended in particular, although not exclusively, for use in the production of absorbent articles, such as diapers, feminine sanitary napkins, wet wipes, cleaning wipes and the like. State of the art

To produce sanitary napkins, children's diapers, incontinence diapers and the like, structures are normally used comprising an upper layer (topsheet), permeable to liquids and which comes into contact with the skin, and a lower layer (backsheet) opposite the upper layer, normally impermeable to liquids. Between the two upper and lower layers an absorbent structure is disposed, in turn constituted by one or more layers. This structure must be light and thin to increase the comfort of the person wearing the absorbent product, but at the same time it must ensure a high absorption capacity to incorporate body fluids

(menstrual fluid, urine or the like) in adequate quantities, without releasing them.

Absorbent structures of various types have been studied for this purpose, comprising or constituted by layers of nonwoven, dry paper or the like. In some cases, particles of superabsorbent polymers (SAP), such as sodium polyacrylate or the like, are added to the textile fibers or other absorbent fibrous material.

Examples of absorbent articles for sanitary use or the like are described in WO-A-99/48454 and US-A-5,728,083.

WO-A-03065951 describes a textile based product incorporating superabsorbent polymer particles applied with a specific method, based on the use of electrostatic fields. The textile structure is constituted by one or more layers of carded fibers or the like.

EP-A- 1154061 describes a superabsorbent composite material to produce sanitary napkins or the like. A mixture containing superabsorbent polymer particles is distributed between two layers of carded fibers. US-A-5,585,170 describes an electrostatic process to incorporate superabsorbent particles in the semi-finished product to be used in the production of sanitary napkins or the like.

EP-A-0719531 describes an absorbent article comprising superabsorbent polymers disposed between two layers of fibrous material.

US-A-4, 826,498 describes a system to produce a composite material constituted by several fibrous layers interconnected with one another, between which superabsorbent polymer particles are disposed.

US-A-4, 655, 757 describes a process to produce nonwovens incorporating superabsorbent particles, intended to produce sanitary absorbent articles.

EP-A-333228 describes a carding and hydro-entangling system to produce a layer of nonwoven forming a semi-finished product for use in the manufacture of absorbent articles. WO-A-2003/073971 describes a further method to produce a textile article comprising a plurality of layers, for example made of nonwoven, joined to one another, incorporating a superabsorbent polymer.

US-A-5,752,945 describes a multilayer structure for sanitary napkins, comprising a plurality of layers, between which there is included a liquid transfer layer, made of nonwoven, disposed between an actual absorbent structure and a topsheet.

DE-A-10008746 describes a carding system to produce a composite web material for the manufacture of sanitary napkins and similar articles.

WO-A-9828478 describes a fibrous absorbent web material based on dry paper, containing particles to absorb unpleasant odors.

The use of superabsorbent polymers has considerable drawbacks, as these are costly materials which cause difficulties and problems in the application and subsequent handling step of semi-finished products containing them, also due to the reactivity of superabsorbent materials to ambient humidity.

In general, current liquid absorption and storage systems to be used in sanitary napkins, diapers and the like are based on a combination of physical and chemical absorption. In physical absorption the body fluid (exudates, urine, menstrual fluid and the like) are stored between the fibers of the absorbent textile structure, without there being an actual reaction of chemical absorption between the liquid and the fibers. This physical absorption process can also be obtained with non-hydrophilic fibers, such as polypropylene.

In chemical absorption, on the other hand, a true chemical reaction takes place between the substance to be used for absorption (for example, sodium polyacrylate) and the body fluid. The superabsorbent polymer absorbs water on the surface and then forms a gel, retaining a quantity of liquid equal to a multiple of the base weight of the superabsorbent polymer. In other cases the chemical bond takes place between the fibers that form the absorbent structure and the water based liquid. In this case the hydrophilic groups located on the surface of the fibers react with the liquids and bond thereto preventing subsequent release.

Objects and summary of the invention

An object of the present invention is the implementation of an absorbing product, suitable to be produced in the form of semi-finished product to be used for the manufacture, for example, of sanitary napkins, sanitary products, children's diapers, but also wet wipes, cleaning wipes and the like.

A further object of a particularly advantageous embodiment of the invention is the implementation of an absorbent product with which it is possible to obtain a high level of physical retention of the liquid, i.e. retention obtained without a chemical reaction, for example, between the liquid and a superabsorbent substance, in order to attain a product with low cost and high efficiency in terms of absorption capacity.

According to a particular embodiment, an object of the invention is to produce a semi-finished absorbent article to be used for the manufacture of sanitary napkins, diapers and the like, which has high absorption capacities with limited costs and limited thicknesses of the finished product.

According to another aspect, an object of the present invention is the implementation of a method to produce absorbent articles which have high absorption capacities and limited costs.

According to a first aspect, the invention consists of an absorbent product comprising at least one layer of bonded textile fibers and, inside the layer of textile fibers, a plurality of cells formed through expansion of at least one expandable substance. In substance, according to the invention a plurality of microspheres expandable through heating are inserted into the textile structure formed by the fibers: these are microspheres whose walls are constituted by a thermoplastic material, inside which there is encapsulated a fluid expandable by heating, typically a liquid hydrocarbon. If subjected to suitable heating, the walls of the microspheres soften, the liquid contained therein (typically a hydrocarbon) changes to a gaseous state thereby causing expansion of the microsphere up to tens of times (typically up to 50 times) the initial volume thereof. The cells thus formed determine the generation of large interstices between the fibers and the bonding resin, i.e. large empty volumes in which the body fluid can be trapped and absorbed.

In practice, by inserting said expandable microspheres inside the initial textile structure a very high quantity of cells and intra-cell interstices can be formed between the fibers, with a consequent increase in the volume of the textile structure inside which a high quantity of liquid can be absorbed through the physical effect of the cells and interstices, which act as microreservoirs.

According to a practical embodiment of the invention, the expandable substance comprises an expanding polymer resin in the form of microspheres. Microspheres of this type are known and used in a plurality of applications. Expandable or expanding microspheres are, for example, marketed by Akzo Nobel (Sweden) with the trademark Expancel®. Microspheres and methods for the production and use thereof are also described in US-A-2006/0102307; EP- A-1059339; US-A-2001/0051666; US-A-2003/0114546; WO-A-2004113613; WO-A-9902589; EP-A-1288272; US-A-2003/0143399; US-A-2004/0157057; WO-A-0183100; WO-A-2004072160.

The use of microspheres in structures formed on the basis of a layer of consolidated fibers has been suggested. Examples of uses of microspheres for this purpose are described in US-A-4,483,889; WO-A-01/79600; US-A- 4,362,778; EP-B-1276934; US-A-6,497,787. However, in these applications the fibrous structure is impregnated with a hardening resin to obtain a finished product in the form of rigid and flexible layers and not to produce absorbent material. The microspheres are used as fillers, to obtain a solid and full structure, having a low specific weight.

According to the invention, the final structure is a flexible and absorbent textile material, suitable for use in a diaper or other absorbent product for hygienic, sanitary or similar use. The fibrous structure is consolidated by means of a crosslinkable resin, which is chosen and applied in such quantities as to maintain the characteristic of nonwoven in the finished product. According to a possible embodiment of the invention, the fibers that form the structure of the product are continuous fibers, or staple fibers, for example carded fibers, bonded together by means of any suitable bonding system, such as mechanical, chemical or thermal bonding, or combinations thereof. According to a preferred embodiment of the invention, bonding of the fibers takes place through a binder based on polymer resin and preferably a crosslinkable polymer resin. The crosslinking temperature of said resin is suitably above the T_max of the microcapsules or microspheres, i.e. the temperature at which the microspheres expand, so that this expansion is not prevented. In substance, heating causes the microspheres to expand and subsequently, upon reaching a higher temperature, the resin to crosslink with consequent bonding and stabilization of the fibrous structure, once this has been expanded as the effect consequent to expansion of the microspheres contained in the structure.

The resin is mixed with the expandable microspheres and applied to a layer of non-bonded textile fibers, for example carded fibers, and made to crosslink for example by supplying a suitable form of energy. Crosslinking can preferably take place through heating, although it would also be possible to use resins that crosslink as a consequence of supplying a different form of energy, such as UV irradiation. According to a modified embodiment, the fibers or part of the fibers used are thermoplastics and bonding takes place by softening at least part of the material forming the fibers. In the case of bicomponent fibers, bonding can be obtained by heating and consequent softening or partial fusion of the outer sheath of at least part of the fibers. According to a possible embodiment the microspheres can be heated until they swell, but without exploding or collapsing. According to a different embodiment, at least a part of the microspheres is heated to obtain breaking by collapse. However, according to a currently preferred embodiment, there is a tendency to limit the quantity of microspheres that collapse, or even to prevent them from collapsing, to obtain a more voluminous final structure.

It must be understood that, even if the process to collapse the microspheres is not necessarily advantageous, in general part of the microspheres will always have collapsed at the end of the heating process. In fact, to obtain maximum expansion of the gas inside, operations are carried out at a certain temperature, called maximum expansion temperature, beyond which the walls of the microspheres melt and collapse.

Preferably, the resin constituting the binder or part of the binder of the fibers forming the textile structure is a hydrophilic resin. In this way the product will have an absorption capacity through physical absorption on account of the large interstitial volumes between the fibers formed by expansion of the expanding microspheres and, moreover, it will also have a further absorption capacity through chemical absorption on account of the hydrophilic nature of the binding resin with which the fibers are bonded or consolidated. The fibers that form the structure can in turn be treated to become hydrophilic or hydrophobic. When the fibers are at least partly hydrophilic a greater effect of chemical absorption is obtained, which can be added to any hydrophilic effect of the binding resin, and which in any case is added to the effect of physical absorption obtained as a consequence of the interstices and hollow volumes inside the textile structure.

The textile structure can be formed by carding or using any other known method to produce plies of unbonded fibers. The fibers can be continuous or staple and bonded together preferably using a hot crosslinkable resin. This technique allows the use of a single heating operation to obtain on the one hand crosslinking of the resin and consequently bonding of the fibers thus forming the nonwoven, and on the other to cause expansion and possible collapse of part of the expandable microspheres inserted in the fibrous structure, with consequent formation of cells and interstices. The fibers can be synthetic, natural or mixed. They can be constituted by staple fibers or continuous filaments. The fibers can be viscose, cotton, PET, PP, PE, bicomponents or combinations thereof.

When reciprocal bonding between the fibers forming the basic textile structure of the absorbent product is obtained through simple heating of entirely or partly thermoplastic fibers, for example bicomponent fibers with an outer sheath having a low softening point, a layer of unbonded fibers can, for example, be produced inside which expandable microspheres are inserted (during or after formation). The semi-finished product thus obtained is then heated to cause partial bonding of the fibers and consequently consolidation thereof and simultaneously dilation and possible explosion or collapse of all or part of the microspheres contained in the structure.

When consolidation takes place by applying a binder, such as a crosslinkable resin, according to an advantageous embodiment of the invention the expandable microspheres of polymer material are added to the binder in advance. In this way insertion of the expandable microspheres into the textile structure takes place simultaneously to application of the crosslinkable resin or other binder.

The basis weight of the absorbent product can for example range from 20 to 100 g/m². It can contain a quantity of fibers ranging from 14 to 70 g/m² and a quantity of resin or other binder ranging from 6 to 30 g/m². In particular, the total quantity of microspheres can range from 0.5 to 10 g/m².

The thickness of the absorbent product can range from 0.2 to 5 mm. To measure the thickness of the semi-finished absorbent product the EDANA 30.5-9 method can be used. When the textile structure is made with staple fibers, these can have a length ranging from 38 to 60 mm and preferably a count ranging from 1.7 to 7 dtex.

The use of expandable microspheres makes it possible to obtain adequate absorption capacities through physical absorption of the final structure and consequently to avoid the use of superabsorbent materials, which have high costs and are difficult to handle. Nonetheless, it would also be possible to use, in a product according to the invention, variable quantities of superabsorbents, mainly to ensure liquid retention.

However, thanks to the presence of expanded microspheres in the finished structure and the cells and interstices of high volume that these generate between the fibers, it is possible to obtain high absorption capacities obtained with modest quantities of superabsorbent polymers, below the basis weights normally used in conventional articles. Therefore, even when the addition of superabsorbent polymers in the final product is desired or necessary, with the use of expanded microspheres it is in any case possible to obtain the advantage of reaching high absorption capacities with small quantities of added superabsorbent polymers.

Typically, the quantity in weight of superabsorbents with respect to the total weight of the finished product will preferably be below 70%. The superabsorbent material can be chosen from the group comprising

(co)polymers grafted with one or more hydrophilic monomers; cellulose derivatives; starch ethers; carboxymethyl cellulose derivatives.

The superabsorbents can be inserted into the product in the form of powders, fibers, granules or in another suitable physical form.

According to a preferred embodiment of the invention, the "foam bonded" production technique is used, i.e. impregnation of a layer of carded fibers, or other layer of non consolidated fibers, with a resin that is dried and then crosslinked in an oven. During heating, in addition to crosslinking of the binding resin, the expandable substance contained in the form of microspheres is swollen. This swelling creates cells inside the resin which, once stabilized, will remain inside the final structure. Intra-cell interstices in which the liquid remains trapped are created between the cells. The substance contained in the microspheres, which gives rise to expansion, can be a hydrocarbon liquid, the boiling point of which is at a temperature below the fusion or softening temperature of the polymer forming the wall of the microsphere. The materials that can be used to produce microspheres, and the production methods thereof are known per se and will not be described in greater detain herein.

Using a hydrophilic resin as binder and possibly a hydrophilic polymer to produce the microspheres, the structure that is obtained will have an even higher absorption capacity on account of the sum of the effect of physical absorption and of chemical absorption due to the hydrophilic capacities of the binding resin, of the polymer forming the microspheres and possibly of the fibers forming the base of the absorbent textile structure. The binding resin can be chosen from the group comprising: vinyl resins

(vinyl copolymers), hydrophobic acrylic resins (acrylic copolymers), styrene- butadiene polymers, polyurethane dispersions. Brief description of the drawings

In the accompanying drawings: Figures 1A and 1 B show microphotographs under the electronic scanning microscope with two different enlargement ratios of a textile structure formed by a layer of carded fibers consolidated with a crosslinked resin obtained with the conventional technique; Figures 2A and 2B show microphotographs under the electronic scanning microscope of absorbent structures based on textile fabrics consolidated with a crosslinked resin incorporating an expanding resin in the form of microspheres expanded by heating, according to the invention;

Figure 3 shows an absorbent product that uses the material according to the invention.

Detailed description

Figures 1A and 1 B show microphotographs obtained with the electronic microscope of a nonwoven structure formed of carded fibers F bonded, i.e. consolidated, through a resin subject to crosslinking. The empty volume, i.e. the interstices between the fibers, represents the volume for receiving and retaining the fluids that the nonwoven structure is intended to absorb, for example urine, menstrual fluid or the like. The fibers are coated with a binding resin which ensures bonding.

Figures 2A and 2B show a textile structure analogous to that in Figures 1A and 1 B, to which however, according to the invention, expanded microspheres S have been added. The empty volume between the fibers and inside the binding resin is substantially greater than the volume in Figures 1A and 1 B, with consequent greater absorption capacity through physical absorption. Examples of products according to the invention are listed below.

Example 1 : basis weight:25-30-60-80-95g/m² fibers: PET 6.7dtex/57mm + Vl 1.7/38mm expandable resin (expandable microspheres):5-10% in weight vinyl resin :20% in weight

Example 2: basis weight:25-30-60-80-95g/m² fibers: PET 1.5dtex/60mm + Vl 1.7/38mm expandable resin: 5-10% in weight acrylic resin:20% in weight Production parameters: - production speed used: 60-100m/min according to the basis weights; oven temperature: corresponding to the expansion temperature T_max of the microspheres (170⁰C) technology used: foam bonded, starting with one or two layers of carded fibers, the fiber is bonded through the addition of foamed resin and applied using a padder. In the subsequent step the layer is dried and the resin is subsequently crosslinked and expanded. The absorption capacity of a 25 g/m² product obtained with 5% in weight of expandable resin (microspheres) is around 1000% (EDANA 10.4-02 method), while the corresponding foam bonded product with the same composition, same production parameters but without expandable resin has an absorption capacity of 600%. Figure 3 schematically shows a feminine sanitary napkin with a topsheet

1 permeable to liquids, an impermeable backsheet 3 and an absorbent core 5, comprising at least one layer of absorbent product obtained as described above.

It is understood that the examples illustrated and described are purely some of the possible modes of implementation of the invention, which can vary in forms and arrangements without however departing from the scope of the concept on which the invention is based, as defined in the claims below.

Claims

I . An absorbent product comprising at least one layer of bonded textile fibers and, inside said layer of textile fibers, a plurality of cells formed through expansion of at least one expandable substance.

2. Absorbent product according to claim 1 , wherein said expandable substance comprises at least one expanding polymer resin.

3. Absorbent product according to claim 1 or 2, wherein said expanding substance comprises expanding polymer microspheres.

4. Absorbent product according to claim 3, wherein said microspheres are present in the layer of fibers at least partly in expanded and integral form.

5. Absorbent product according to one or more of the preceding claims, wherein said fibers are bonded by means of at least one binder comprising a polymer resin.

6. Absorbent product according to claim 5, wherein said polymer resin is a crosslinkable resin.

7. Absorbent product according to claim 5 or 6, wherein said polymer resin is a hydrophilic resin.

8. Absorbent product according to claims 5 to 7, wherein said polymer resin is a hydrophobic resin.

9. Absorbent product according to one or more of the preceding claims, wherein said textile fibers are at least in part hydrophilic fibers.

10. Absorbent product according to one or more of the preceding claims, wherein said fibers are at least in part hydrophobic.

I 1. Absorbent product according to one or more of the preceding claims, comprising: a base structure formed by said fibers bonded with a binding resin; in said base structure a plurality of cells formed by expanded polymer microspheres.

12. Absorbent product according to claim 11 , wherein at least a part of said expanded microspheres are collapsed.

13. Absorbent product according to one or more of the preceding claims, having a basis weight ranging from 20 to 100 g/m².

14. Absorbent product according to one or more of the preceding claims, with a thickness ranging from 0.2 to 5 mm.

15. Absorbent product according to one or more of the preceding claims, wherein said fibers are chosen from the group comprising: viscose (Vl); cotton; PET; PP; PE; bicomponent fibers.

16. Absorbent product according to one or more of the preceding claims, wherein said fibers are staple fibers.

17. Absorbent product according to claim 16, wherein said layer of fibers is a layer of carded fibers.

18. Absorbent product according to one or more of the preceding claims, wherein said fibers have a length ranging from 38 to 60 mm and preferably a count ranging from 1.7 to 7 dtex.

19. Absorbent product according to one or more of claims 1 to 15, wherein said fibers are constituted by substantially continuous filaments.

20. Absorbent product according to one or more of the preceding claims, comprising a quantity in weight of fibers ranging from 14 to 70 g/m².

21. Absorbent product according to one or more of the preceding claims, comprising a quantity in weight of binding resin ranging from 6 to 30 g/m².

22. Absorbing product according to one or more of the preceding claims, comprising a quantity in weight of microspheres ranging from 0.5 to 10 g/m².

23. Absorbent product according to one or more of the preceding claims, also comprising a superabsorbent.

24. Absorbent product according to claim 23, wherein said superabsorbent is a superabsorbent polymer.

25. Absorbent product according to claim 24, wherein said superabsorbent polymer is chosen from the group comprising: (co)polymers grafted with one or more hydrophilic monomers, cellulose derivatives, starch ethers, carboxymethyl cellulose derivatives or combinations thereof.

26. Absorbent product according to claim 23, 24 or 25, wherein the quantity in weight of said superabsorbent is equal to or less than 70% of the total weight of the absorbent product.

27. .Absorbent product according to one or more of claims 23 to 26, wherein the quantity of superabsorbent is equal to or less than 70 g/m².

28. A product comprising a topsheet permeable to body fluids, a backsheet substantially impermeable to body fluids; and between said topsheet and said backsheet, an intermediate absorbent layer comprising an absorbent product according to one or more of the preceding claims.

29. A method to produce an absorbent product, comprising the steps of: producing a structure of textile fibers inserting an expandable substance therein; bonding said structure of textile fibers, generating on the textile structures cells between the textile fibers, through expansion of said expandable substance.

30. Method according to claim 29, wherein said expandable substance is an expandable polymer resin.

31. Method according to claim 30, wherein said expandable polymer resin is in the form of expandable microspheres.

32. Method according to claim 29, 30 or 31 , wherein said textile fibers are bonded through a binder.

33. Method according to claim 32, wherein expandable microspheres are inserted in said binder.

34. Method according to claim 33, comprising the steps of: impregnating the structure of textile fibers with said binder to which said microspheres have been added; hot crosslinking said binder and expanding said microspheres forming said cells in the textile structure.

35. Method according to one or more of claims 29 to 34, wherein a part of said microspheres collapses after expansion.