WO1991017734A1

WO1991017734A1 - Method of manufacturing absorbent articles

Info

Publication number: WO1991017734A1
Application number: PCT/GB1991/000736
Authority: WO
Inventors: Arthur Roger Shipley
Original assignee: Breger Gibson Limited
Priority date: 1990-05-11
Filing date: 1991-05-09
Publication date: 1991-11-28
Also published as: GB2243785A; GB9010672D0; GB9100771D0

Abstract

The present invention relates to a method for forming absorbent sanitary pads, for example diapers, which method comprises forming a layer of fibrous material, bonding together at least some of the fibres in said layer by means of fusion of a particulate thermoplastic bonding agent admixed with said fibres, the bonding agent being made at least in part from a material which dissolves in or is disintegrated by an alkali. Preferably the layer incorporates a super absorbent material so as to enhance the fluid absorbing properties of the layer.

Description

METHOD OF MANUFACTURING ABSORBENT ARTICLES

The present invention relates to a method of manufacturing absorbent articles, notably to a method for manufacturing fluid absorbent sanitary pads and diapers.

BACKGROUND TO THE INVENTION

Diapers typically consist of an absorbent fibrous pad held within a water resistant outer layer and an inner layer which is to be against the skin of the wearer and through which urine can pass into the absorbent pad. Other forms of sanitary articles, for example potty liners, incontinence pads and sanitary pads also use an absorbent pad to absorb body fluids excreted by a wearer. For convenience, the term sanitary pad will be used herein to denote in general all forms of such absorbent pad, whether used to absorb bodily fluids from a wearer or to absorb fluids from another source, as when the pad is used within a surgical dressing or the like.

Such absorbent pads have typically been manufactured by a felting or air entrainment operation in which chopped lengths of a suitable fibre are formed into a non-woven sheet from which the required pads can be cut. Such a process is cumbersome and, in order to produce material of consistent composition and thickness, requires that only a comparatively narrow width of material be laid down. Such a process has therefore been restricted to the manufacture of a single width of the pad material, which inherently limits the overall speed of production of the pads and of finished articles made from them.

It has been proposed to form fibrous pads by bonding the fibres together by the application of heat and pressure to a mixture of the fibres and a fusible bonding agent in powder or fibre form. The bonding agent can be applied as a thermoplastic coating on the fibres or as a thermoplastic powder or fibre admixed with the staple fibres of the pad. Such a process can be carried out on a wide width of material so that a number of rows of pads could be cut from a single width of material. This process would thus be capable of producing pad material which could be fed to a number of diaper manufacturing operations operated in parallel and thus achieve a substantial increase in the rate of production of the diaper or other finished article from a single pad production line.

However, such a method of manufacture of the pads gives rise to a comparatively compacted material due to the pressure required during the fibre bonding operation and the fact that the bonds are formed whilst the fibres are in a compressed state. Such compacted material is not suitable for direct use in some applications to absorb large quantities of bodily fluids. It has been proposed to release at least some of the bonds between the fibres by heat treatment of the pad to cause melting of at least some of the material at the bonds between the fibres. This treatment causes re-bulking of the material and thus renders it more absorbent. However, in order to achieve practically useful fluid absorbent properties, the material must often be re-bulked to such an extent that it becomes unacceptably bulky for wearing upon the person.

In order to reduce these problems, it has been proposed in European Application No 0301,772 A2 to incorporate one or more functional powders into a pad of fibres which have been bonded together by a particulate thermoplastic bonding agent. The initial product from such a process is a compacted pad which then requires re-bulking in order to impart adequate fluid absorbent properties. One example of a functional powder is a particulate super absorbent polymer to enhance the fluid absorbent properties of the re-bulked pad. However, such a product cannot readily be disposed of since, even in the re-bulked state, the staple fibres of the pad are held together by thermal bonds.

We have now devised a method of manufacturing an absorbent sanitary pad which makes use of the thermal bonding of fibres to form wide widths of material which can be subsequently formed into finished articles having a high fluid absorbency, yet which can readily be disposed of by a user.

SUMMARY OF THE INVENTION:

Accordingly, the present invention provides a method for forming an alkali sensitive fluid absorbent sanitary pad, which method comprises forming a layer of fibrous material, bonding at least some of the fibres in said layer by means of the fusion of a thermoplastic bonding agent admixed with said fibres, characterised in that: a. the bonding agent is provided at least in part by a particulate material admixed with said fibrous material; and in that b. at least part of the particulate bonding material is provided by a material which is soluble in or disintegrated by an alkali.

Preferably, the fibrous layer also contains an additional fluid absorbent material so as to enhance the fluid absorbing properties of the layer. This additional material, when present, is in the form of fibres and/or particles admixed with the fibrous material.

From another aspect, the invention also provides an alkali sensitive fluid absorbent sanitary pad comprising a layer of fibrous material at least some of the fibres of which are bonded together by a thermoplastic bonding agent, characterised in that at least part of the bonding agent is provided by a thermoplastic material which is soluble and/or disintegrable in alkali. Preferably, the pad also contains a material which absorbs fluid applied to the pad, whereby the fluid absorbing properties of the pad are enhanced.

The staple fibres in the fibrous layer can be provided by a wide range of natural or synthetic materials, for example cotton or wood fibres; cellulose or a cellulose derivative, eg. carboxymethyl cellulose or rayon; polyamide; polyester, eg. poly(ethylene terephthalate) ; polyalkylene, eg. poly¬ ethylene, polypropylene; vinylic polymers, eg. polyvinyl chloride, polyvinyl acetate or polyvinylidene chloride; or any other suitable material used to form fibrous materials. Mixtures of different fibres, or fibres which comprise a coating or sheath of one polymer upon a core or filament of another may be used if desired.

As indicated below, part, for example up to 20% by weight, of the fibres may be provided by fibres of a super absorbent material so as to enhance the absorbent properties of the fibrous layer.

The particulate thermoplastic material required to bond the fibres together may be provided by any suitable material, which typically has a fusion point of from 80 to 180°C, in powder or granular form. The thermoplastic material can be applied in part as a coating to the fibres, for example as a solution of a suitable lower fusion point polymer or pre- polymer applied as a coating to a higher fusion point polymer fibre. However, at least part, preferably in excess of 50%, notably 75 to 100%, of the bonding agent is provided as a particulate material admixed with the fibrous layer. The bonding material is formed at least in part from a material which readily dissolves or disintegrates when exposed to alkali, preferably to an aqueous alkali solution having a pH value above 7.5, typically above 8.5, so that the bonding of the fibrous layer will disintegrate when exposed to such a solution in a toilet pan and can then be disposed of by flushing the toilet. Thus, the bonding agent can be formed from a homopolymer and/or co-polymer of acrylic acid and esters thereof, notably C, - C₅ alkyl esters of acrylic or raethacrylic acids, for example the methyl, ethyl or butyl esters, or acrylonitrile; homo- and co-polymers of carboxy ester lactones, styrene-monoethyl maleate or N,N' -diethyl- acrylamide; polymers of other types which have been chemically modified by the introduction of solubilising groups such as carboxylic acid, sulphonic acid, sulphite, sulphate, phenolic hydroxy, ester or diamide groups; or polymers which have been rendered soluble by the introduction of interpolymer units into the main polymer chain, for example the silane interpolymers of US Patents Nos 4 062 451 and 3 951 893. Particularly preferred alkali sensitive polymers for present use are the substantially linear co-polymers of from 2 to 6 parts by weight of one or more monomers selected from acrylic acid, ethyl acrylate, butyl acrylate or acrylonitrile with one part by weight of methacrylic acid monomer to give thermoplastic resin products having molecular weights of from 25,000 to 300,000.

Particularly preferred acrylic resins for present use are those thermoplastic resins which are substantially free from cross-linking, ie. are substantially linear polymers, and which have a carboxyl functionality of from 3 to 15 weight percent and an acid number in the range 30 to 90. If necessary, the resulting resin can be treated in known manner with an alkali or base, for example ammonia, sodium hydroxide, morpholine or an alkylamine (for example triethylamine or triethanol-amine) , to impart the desired water solubility under alkaline conditions.

In an alternative embodiment, the bonding agent particles can be formed from a water soluble substrate which has applied thereto an alkali sensitive coating so that the particles remain intact during normal use, but readily dissolve or disintegrate when placed in a toilet pan to which an alkali, for example domestic bleach solutions or ammonia, has been added to give a pH in the water of 7.5 or more. Suitable water soluble resins include, for example, a polymer or copolymer of vinyl alcohol, a pol (alkylene oxide) or polyvinylpyrrolidone. Particularly preferred water soluble polymers are vinyl alcohol polymers or co- polymers having a saponification value of from 80 to 90%.

In a further alternative, the water soluble polymer can be used to provide part of the bonding agent so as to provide a product in which part of the thermal bonds between the fibres are released when the product is wetted during use thus re-bulking the product, the remaining bonds not being released until the product is immersed in an alkali, thus retaining the integrity of the product during use, but allowing it to be disposed of in a toilet pan.

The optimum amount and particle size of the bonding agent required can be readily determined by simple trial and error tests for any given case, but will usually lie within the range 1 to 30% by weight of the dry fibres, with a mass median particle size of less than about 800 micrometres, preferably in the range 50 to 350 micrometres.

The bonding agent can be incorporated into the fibrous layer by any suitable means, for example by admixing the particles of the bonding agent with the fibrous material before or as the layer is formed so that the particles are substantially uniformly distributed throughout the layer.

The fibrous layer preferably also contains an additional material which enhances the ability of the fibrous layer to absorb fluids. Such additional materials are known as super absorbers and many forms of such materials are available commercially. Super absorbers typically comprise a material have a backbone of a natural or synthetic resin carrying hydrophilic groups or having polymers containing hydrophilic groups bonded thereto, or such a material in intimate admixture with such polymers. Suitable super absorber materials include modified and regenerated polymers such as polysaccharides, notably starch graft polymers, cross linked glycollate and cellulose ether polymers and polyacrylates, notably cross linked acrylic acid polymers. Such polymers are described for example in US Patent 4,105,033.

It is preferred to incorporate the super absorber into the fibrous layer as particles having an average particle size in the range 100 to 1000 micrometres and to incorporate those particles by admixing them with the fibrous material before or as the layer is formed so that the particles are substantially uniformly distributed throughout the layer. In some cases, the super absorber may also act as the thermoplastic bonding agent described above. However, it is within the scope of the present invention for the super absorber to be applied as a dispersion or emulsion of the material in water or other suitable carrier medium so as to form a coating on the staple fibres which is dried in situ within the fibrous layer after it has been formed. In this way an absorbent rich layer or area can be formed within the fibrous layer corresponding to areas where particularly high absorbency are required in the final article. Alternatively, the super absorber can be used in the form of a fibrous material which is admixed with the other fibrous materials forming the fibrous layer.

The super absorber is typically present in from 5 to 20% by dry weight of the fibrous material in the fibrous layer.

The fibrous layer is conveniently formed by blowing or otherwise feeding fibres onto a suitable support surface, for example a moving belt or the like, to form a bed of intertwined or tangled fibres. To aid re-bulking of the fibrous layer later, part or all of the fibres may be used in corrugated or crimped form. Where not already present, one or more of the other particulate or fibrous components or precursors thereof can be incorporated into the bed of fibrous material using any suitable technique, for example by a conventional spray or other technique followed by a drying or curing stage.

The fibres in the layer are then bonded together, at least in part, by applying heat and, preferably also, pressure to the layer, for example by passing the layer through the nip of a pair of heated rollers or an oven or any other suitable technique. The pressure and temperature applied to the layer will depend, as is known, inter alia upon the depth of the layer, the nature of the fibres and the bonding agent and the extent of bonding required within the layer. The optimum bonding conditions can readily be determined by simple trial and error tests for any given case.

The bonding can be carried out uniformly throughout the fibrous layer, as when the layer is passed through the nip of a pair of plain surfaced heated rollers. However, the bonding can be carried out selectively and non- continuously, as when the surface of the rollers is grooved, pitted, pimpled or ridged so as to apply localised lines or spots of heat and pressure. In this way, bonding of the fibres is carried out over selected positions, leaving the remainder of the layer unbonded and uncompressed. Alternatively, the fibrous mixture can be moulded under pressure and heat to give a shaped product, for example a tampon.

The product from the bonding process is a body of fibres bonded together and optionally containing the super absorber distributed therein and which is in a wholly or partially compressed state by virtue of the pressure applied during the bonding process. It may be desirable to retain this compressed state for storage and transport. However, the product will usually not possess adequate absorbent properties in the compressed state and it may be necessary to re-bulk the product prior to use. This can be done during manufacture of the finished article or immediately prior to use. Typically, the re-bulking will be achieved by heating the fibrous body to above the fusion point of the bonding agent, typically to from 5 to 30°C above the softening point of the bonding agent. Thus, the finished article can be heated in a microwave oven or the like immediately prior to use by the end user. Where part of the bonding agent is provided as a water soluble resin, it may not be necessary to carry out a separate re-bulking step, since the wetting of the fibrous body by a bodily fluid will dissolve such a resin and allow a measure of re- bulking to occur in situ during use of the product, for example a sanitary pad or tampon.

The fibrous body typically produced as a comparatively wide strip of material from which pads or other articles of the required size and shape can be cut and incorporated into finished articles using conventional techniques. Since the process for producing the bonded fibrous layer can produce a strip wide enough to accommodate several rows of pads, the invention provides a simple means by which several article forming lines can be fed from a single fibrous layer forming line, thus increasing the rate at which finished articles can be formed.

Typically, the finished articles will incorporate a fibrous layer of the invention sandwiched between a water pervious layer, for example a needled, woven or non-woven perforated layer, and a water impervious layer, for example a sheet of polyethylene or the like; or alkali dissolvable or disintegrate-able alternatives thereto. Such other layers in the composite material used in the fabrication of the finished article can be applied to the fibrous layer using conventional techniques. However, the method of the invention is of particular advantage in forming such composite fabrics in that, due to the presence of the thermoplastic bonding agent in the fibrous layer, the other layers can be readily bonded to the fibrous layer during the bonding of the fibres in the fibrous layer. Thus, by suitable selection of the particulate thermoplastic bonding agent, the outer polyethylene layer and/or the inner tissue or other water pervious layer can be bonded to the outer faces of the fibrous layer simultaneously with the bonding of the fibres by the application of heat and pressure. The method of the invention thus enables the composite fabric to be formed in a single operation, which composite fabric can then be formed into the required finished article using conventional techniques.

The finished article can be a diaper or any other suitable form of finished sanitary pad or shaped article and the finishing operations are appropriate to the desired form of the finished article. DESCRIPTION OF THE DRAWING:

The invention will now be illustrated by way of example with respect to the preferred form thereof as shown diagrammatically in the accompanying drawings, in which Figure 1 is a diagrammatic cross-section through a pad of the invention; and Figure 2 is a diagrammatic block diagram of a diaper production process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

A diaper comprises an outer polyethylene sheet layer 1, an intermediate cellulosic fibre pad layer 2 and a non-woven layer 3. Alternatively, the inner and outer layers 1 and 3 can be made from an alkali sensitive resin so that the whole structure can be dissolved or disintegrated when exposed to aqueous alkali.

The edges of the layers are bonded together by any suitable means, for example by using a heated cutter sealing bar to simultaneously cut and seal together the edges of the layers of the composite fabric when forming the required shaped pad from the sheet of the fabric.

The fibres in the pad layer 2 are typically cellulose fibres bonded together with from 2 to 20% by weight of an alkali soluble microground carboxylated acrylic resin having a saponification value of about 88% and a particle size of about 300 micrometres. Alternatively, the particles can be made from a polyvinylalcohol resin coated with a methylmethacrylate resin. The pad layer 2 also contains uniformly dispersed therethrough 2 to 20% by dry weight of the particulate super absorber sold under the trade name Drytech 2000. Alternatively, the super absorber is provided by replacing 10% by weight of the cellulose fibres with the fibres available under the trade name of Fibersorb.

The outer polyethylene layer 1 and the inner non-woven layer 3 are adhered to the exposed faces of the intermediate pad layer 2 by virtue of the thermoplastic bonding agent present in the outer surfaces of the pad layer during the heat bonding step described below.

A diaper formed from a pad as shown in Figure 1 can have other features, for example wetness indicators, tapes etc. normally present in a diaper.

The pad shown in Figure 1 can be formed by a method shown schematically in Figure 2, in which cellulosic or other fibres 20, admixed with the required proportions of thermoplastic bonding agent and super absorber, are fed by air blast or other means onto a conveyor means 21 to form a bed of randomly orientated interlocking fibres on the conveyor. The bed of fibres is passed through the nip of a pair of heated rollers 22, which apply heat and pressure to the bed of fibres. This causes the fibres to bond together. Where a non-woven sheet and a polyethylene sheet are applied as upper and lower layers on the conveyor as shown as items 23 and 24 respectively, these will also become bonded to the fibrous layer to form a composite fabric.

The bonded bed of fibres is in a compressed state and is then fed to a cutter, eg. a hot bar cutter 30, where pads of the desired shape are cut from the bonded bed. Due to the width of the bed which can be formed on conveyor 21, several rows of pads can be cut across the width of the bed. The pads are then fed in parallel to several finishing lines where the necessary operation to complete the construction of a diaper are carried out. The diaper produced retains the fibres in the pad layer in a compressed state and is thus advantageous for storage and transport over conventional bulky diapers. The compressed pad layer can be re-bulked if required during the construction of the diaper or, more preferably, immediately prior to use by heating in a microwave oven or by playing a hair dryer over the diaper.

From another aspect, the invention also provides a sanitary article having a composite construction comprising an outer water impervious layer; a layer of fibrous material at least some of the fibres of which are bonded together by a particulate thermoplastic bonding agent made from a material which is soluble and/or disintegrable in alkali; and a next the body layer of a fluid pervious material. Preferably the fibrous layer also contains a material which absorbs fluid applied to the fibrous layer, whereby the fluid absorbing properties of the fibrous layer are enhanced.

Preferably, the article is in the form of a diaper and the fibrous layer is retained in a compressed stated by the action of the bonding agent.

Claims

CLAIMS ;

1. A method for forming an alkali sensitive fluid absorbent sanitary pad, which method comprises forming a layer of fibrous material, bonding at least some of the fibres in said layer by means of the fusion of a thermoplastic bonding agent admixed with said fibres, characterised in that: a. the bonding agent is provided at least in part by a particulate material admixed with said fibrous material; and in that b. at least part of the particulate bonding agent is provided by a material which is soluble in or disintegrated by an alkali.

2. An alkali sensitive fluid absorbent sanitary pad comprising a layer of fibrous material, at least some of the fibres of which are bonded together by a particulate thermoplastic bonding agent, characterised in that at least part of the thermoplastic bonding agent is provided by a material which is soluble and/or disintegrable in an alkali.

3. A method as claimed in claim 1 or a pad as claimed in claim 2, characterised in that the fibrous layer is in the form of a composite construction comprising an outer water impervious layer; a layer of fibrous material at least some of the fibres of which are bonded together by a particulate thermoplastic bonding agent made from a material which is soluble and/or disintegrable in alkali; and a next the body layer of a fluid pervious material.

4. A method or a pad as claimed in any of the preceding claims, characterised in that the fibrous layer also contains a material which absorbs fluid applied to the layer, whereby the fluid absorbing properties of the fibrous layer are enhanced.

5. A method or pad as claimed in claim 4, characterised in that up to 20% by weight of the fibres in said fibrous layer are provided by fibres made at least in part from said material which absorbs fluid.

6. A method or pad as claimed in any one of the preceding claims, characterised in that the bonding material comprises a homo- or co-polymer of acrylic acid and esters thereof and/or a polyvinylalcohol.

7. A method or pad as claimed in any one of the preceding claims, characterised in that the material which absorbs fluid and which is incorporated into the said layer is a starch graft polymer or co-polymer, a cross-linked glycollate and cellulose ether polymer, or a polyacrylic polymer.

8. A method or pad as claimed in any one of the preceding claims, characterised in that the material which absorbs fluid and which is incorporated into the said layer is a particulate material.

9. A method as claimed in any one of the preceding claims, wherein the said layer of fibrous material is formed by blowing fibres onto a moving support so as to form a randomly orientated bed of interlocking fibres.

10. A method as claimed in any one of the preceding claims, characterised in that the fibrous layer is initially produced in a compressed form and this is expanded prior to use by heating the fibrous layer to above the fusion point of the bonding agent whereby part of the bonding of the fibres within the fibrous layer is released.

11. A method as claimed in claim 10, characterised in that the heating achieved in a microwave oven by the end user.