GB2269603A - Process for the production of fluff pulp - Google Patents

Abstract

A fluff pulp, suitable for use in the absorbent core of an absorbent product such as a diaper or the like, comprises a blend of cellulosic fibres and synthetic heat-shrunk fibres, e.g. of polyester. Preferred cellulosic fibres are wood pulp fibres that have been modified so as to impart a crimp or twist thereto. The fluff pulp may be made by heating a mixture of cellulosic fibres and synthetic heat-shrinkable fibres to a temperature sufficient to cause shrinkage of the synthetic fibres.

Description

IMPROVEMENTS IN OR RELATING TO FLUFF PULPS Field of the Invention The present invention relates to fluff pulp, especially fluff pulp that incorporates synthetic fibre, to a process for producing such fluff pulp and to products made from the fluff pulp.

Backaround to the Invention Fluff pulp is widely used to form the absorbent core in a wide range of disposable personal hygiene absorbent products, for example babies' diapers, adult incontinence pads and ladies' external sanitary products, as well as in absorbent products for use outside the personal hygiene field. Fluff pulp is commonly produced by forming a dispersion of cellulose fibres, commonly vegetable fibres and in particular wood fibres, depositing fibres on a forming device, drying the resultant board and passing the dried board to a hammermill or other shredding device to reduce the board to fluff pulp.

A number of characteristics are required of the fluff pulp in order to meet the demands of the intended end use.

In particular, the fluff pulp should have a good absorption capacity and high wet resilience. Furthermore, the fluff pulp should be capable of being formed into a pad which retains a high level of integrity despite the mechanical forces exerted on it during use, especially such forces due to the movement of the wearer of the personal hygiene product. There is a continuing need for fluff pulps having improved characteristics in these areas.

A problem also arises in connection with the use of 100% sulfate pulps, especially those which are favoured for environmental reasons, being chlorine-free; they are comparatively hard and cause increased wear in the apparatus used for the production of fluff pulp therefrom.

Furthermore, more energy is required for the shredding of fluff pulp from these 100% sulfate pulps and especially the high density pulps than from traditional pulps and the increased vigour with which the pulps are worked creates more fines and also more noise. Accordingly, there is a need for means by which 100% sulfate pulps could be converted into fluff pulp with reduced expenditure of energy.

Other pulps such as sulfite, CTMP, one-year renewable crop vegetables, e.g. kenaf or cotton, and waste paper also benefit from the addition of synthetic fibres but for other reasons.

Summarv of the Invention The present invention now provides a fluff pulp comprising cellulose fibres and synthetic, heat-shrinkable or heat-shrunk fibres.

The present invention also provides a process for the production of fluff pulp, which method comprises forming a blend of cellulose fibres and synthetic, heat-shrinkable fibres and heating the blend to a temperature sufficient to cause shrinkage of the said synthetic fibres.

The present invention also provides an absorbent product, e.g. a disposable, personal-hygiene absorbent product, comprising an absorbent structure, for example a pad, formed of fluff pulp according to the present invention. The absorbent core structure preferably is held in a frame or structural system that gives shape or strength to the core for its intended use.

Description of Preferred Embodiments The various pulp terms used below are defined in SIS Handbook 146 (Paper Vocabulary) issued by the Swedish Standards Institute, and in J.P. Casey, "Pulp and Paper", Volumes 1 and 2 (Wiley Interscience), the latter giving a clear description of the fibre lengths of the wood pulps that may be used.

The cellulose-fibre component may be any of the conventional wood pulps employed in the production of fluff pulp. However, it is advantageous to use wood pulp containing fibres that are crimped or twisted or that shrink and/or develop a crimp or twist during heat treatment.

Examples of such special pulps are: chemically treated wood pulp, such as that disclosed in US-A-4,898,642 (Moore et al) and US-A-4,935,022; flash-dried pulp (Chemothermal Mechanical Baled Pulp), such as that commercially available from Svenska Cellulosa; pulp that has been formed using a rewetting step and which can be made into fluff pulp in roll form (this being a completely chlorine-free Kraft material), such as that available under the trade name "Perox" from Raumacell; and the mechanically and chemically modified wood pulp known as High Bulk Additive ("HBA") commercially available from Weyerhaeuser.

A wide variety of cut textile staple fibres come into consideration for addition to the cellulose fibres.

Typically, in the production of cut fibres, the undrawn tow is passed through a draw zone and subsequently through a relaxer, wherein the drawn tow is subjected to an elevated temperature for heat setting to develop various fibre properties (as illustrated, by way of example only, in Table 1 for certain Dacron (registered trade mark) polyesters fibres), the tow then being conveyed to a cutting zone for the production of the cut fibres.

Table 1 Fibre Tenacity Elongation DHS (%) (CN/dtex) (8) A 5.7 20 12.0 B 5.5 24 8.4 C 4.1 40 5.3 D 4.0 40 3.8 In the production of crimped fibres, the drawn tow may be passed through a crimping zone before it is conveyed to the relaxer. As is known, variation of the draw ratio in the draw zone will give rise to a variation in the fibre properties, such as tensile strength, elongation-to-break and Young's modulus, owing to a change in the molecular orientation in the synthetic fibre. The shrinkage characteristics of the resultant cut fibre are also affected and partially drawn fibres constitute a preferred class of heat-shrinkable synthetic fibres herein. The choice of temperature in the relaxer will also have an effect on the fibre characteristics.Thus, although the relaxer temperature has a major effect on the resilience of the fibre product, there is also an effect on fibre shrinkage, as shown in the following Table 2, which shows typical dry heat shrinkage ("DHS") values and other properties for certain commercially available Dacron polyester fibres treated at different relaxation temperatures (the DHS values being for 100% textile yarns with the draw zone characteristics constant): Table 2 Fibre Relaxation Temp. (OC) DHS (%) 1 85 15-19 2 100 11-13 3 125 5-7 4 135 < 3 5 180 1 ("Dacron" is a registered trade mark of Du Pont).

The DHS values may be compared with a DHS of 35-40% which might obtain if the fibre were not exposed to an elevated temperature in the relaxer (i.e. if kept at the ambient temperature).

Another preferred class of heat-shrinkable fibres are the partially quenched or jet quenched fibres, in particular fibres of which one side has been cooled in a spinning cell to produce a spiral crimp; see U.S. Patents No. 3,050,321, No. 3,061,874 and No. 3,118,012. Such fibres are known for fibre-fill applications, such as the fillings in pillows, duvets, quilts, sleeping bags and anoraks and other articles of clothing. Commercially available fibres of this type are made by Du Pont.

Yet another preferred class of heat-shrinkable synthetic fibres are those having a latent crimp, for example such polyester fibres available under the registered trade mark Dacron.

The synthetic heat-shrinkable fibres may be any suitable synthetic organic fibre (which expression herein includes any suitable man-made fibre or regenerated fibres), including fibres made of polyamide (e.g. nylon), polyolefin (e.g. polyethylene or polypropylene), acrylic, cellulose acetates, viscous rayon, polyimide or aromatic polyamide.

Suitable aromatic polyamide fibres are those marketed under Du Pont's registered trade mark "Kevlar" which can be used in specialist industrial absorptive products requiring high strength. However, the presently preferred fibres are polyester fibres, e.g. poly (ethylene terephthalate), especially those marketed under Du Pont's registered trade mark "Dacron".

The undrawn fibres disclosed by Yamamoto (US-A4,496,583) may also be used herein.

Suitable synthetic fibres are also disclosed in R.

Hill, "Fibre From Synthetic Polymers", Elsevier (1953), e.g.

Chapters 14 (Fibres from melt extrusion), 17 (Polyamide fibres), 18 (Polyester fibres) and 19 (Vinyl and acrylic fibres).

The synthetic organic fibres may have a surface pretreatment before they are incorporated into the blend with the cellulose fibres. Synthetic fibres tend to be hydrophobic but can be rendered hydrophilic by appropriate treatment. Thus, one preferred pretreatment is such as to improve the dispersibility of the fibres in water, especially when a wet addition route is employed, and the application to the fibres of a surface coating containing polyoxyalkylene groups, notably polyoxyethylene groups, may be effected. Man-made organic fibres, in particular polyester fibres with a coating comprising segmented polyethylene terephthalate/polyethylene oxide block copolymer are preferred. The surface pretreatment may be carried out at any convenient stage in the production of the fibres, for example after drawing and before crimping (if such is effected) and cutting.

Suitable surface pretreatments have been disclosed by Ring et al in US Patent No. 4,007,083, by Hawkins in US Patents No. 4,137,183, No. 4,179,543 and No. 4,294,883, by McIntyre in U.S. Patents No. 3,416,952 and No. 3,619,269, by Jayne in U.S. Patent No. 3,702,260, by Newkirk in U.S.

Patent No. 4,312,966, by Yoshida et al in U.S. Patent No.

4,529,481, by Du Pont in EP-A-0,333,515 and U.S. Patent No.

4,707,407, by Viscose Suisse in British Patent No. 958,350 and by Teijin in Japanese Patent No. 58208499 and EP-A 159,882; see also Shiffler, U.S. Patent No. 4,713,289.

A preferred surface treatment is the application of a "Zelcon" (a Du Pont registered trade mark) finish, such a finish being described in GB-A-2,215,609.

The water-dispersibility and indeed other properties may be affected not only by the application of coatings but also by the selection of special configurations, e.g.

selected length-to-diameter ratios and various crosssectional shapes. Thus, use may be made not only of fibres having a circular cross-section but also fibres having noncircular cross-section, e.g. a hollow, star-shaped, cruciform or scalloped oval cross-section. Reference may be made in this regard to US patents No. 4,707,407 (Clark and Shiffler) and No. 4,713,289 (Shiffler), US patent No.

4,529,481 (Yoshida et al), EP-A-0,198,400, EP-A-0,198,401, EP-A-0,261,820 and the paper by Schiffler entitled "Characterizing the Dispersion Kinetics of Synthetic Fibers in Water" at the 1984 Nonwovens Symposium, April 1984, reported by and available from TAPPI. Examples of various fibre cross-sections are also described by G. Allen in "Cellulose Chemistry and Technology" (1968), page 890 and (1970) page 567; and by R.W. Moncrieff in "Man-Made Fibres", 6th edition (Newness & Butterworth).

The fibre length and the length-to-diameter ratio of the synthetic heat-shrinkable fibres may be selected according to the desired characteristics in the end product.

Typically, with a linear density of the order of one decitex, a fibre length of 4-15 mm is generally suitable, although lengths in excess of 15 mm come into consideration at higher linear densities. With a typical fibre cut length of up to 12 mm, suitable length: diameter ratios are 400:1 to 2000:1 e.g. 500:1 to 1200:1.

Purely by way of example, selected characteristics of typical polyester fibres that canoe used herein are shown in the following table: Table 3 Ratio - Length: Diameter (Dacron*) Solid Round Fibre X-Section Fibre Denier 1.5 3.0 6.0 9.0 15.0 Fibre Diameter 12.5 18 24 30 40 Microns Cut Length mm Ratio : 1 6 mm - i" 480 330 250 200 150 18 mm 890 550 420 330 250 12 mm - ll 960 660 500 400 300 15 mm 1200 820 620 500 380 18 mm - " 1400 1000 750 600 450 24 mm - 1" 1920 1330 1100 800 600 Explanation Dacron* - Du Pont R.T.M.

The fibre blend may, in general, contain up to 80% by weight of synthetic organic fibre, relative to the total fibre blend. Preferably, however, the blend contains up to 20 or 25% by weight of synthetic organic fibre, more preferably from 3 to 15% by weight.

As stated above, fluff pulp is commonly made by defibrising a board formed by wet-laying and subsequent drying of cellulose, especially wood, fibres, such defibrising being typically carried out in a hammermill or other shredding device. The synthetic heat-shrinkable fibres may be added at any convenient point or points in the preparation of the wet-laid material, e.g. by addition during the stock preparation, for example before the machine chest, in the head box, or even by spray application at the forming board of the Fourdrinier machine (i.e. integrally mixed with the cellulosic fibres). It is also possible to add the synthetic fibres in a multiple board forming system as one of the layers.It will, of course, be appreciated that the dispersion of the synthetic organic fibres in an aqueous pulp of cellulose fibres may result in at least a partial loss of any surface finish, especially the very hydrophilic treatments which also act as dispersants for the said fibres, that has been applied to the synthetic organic fibres. However, the benefits that may be imparted in terms of reduced energy requirement in defibrisation and the improvements in bulk, resilience, integrity, absorbtion capacity and sheet uniformity (i.e. uniformity of fibre dosing, sheet grammage, etc.) of the fluff pulp pad will generally outweigh any reduction in the contribution of the synthetic organic fibre surface treatment to moisture transport rates, i.e. wickability through the fluff pulp pad.

It is necessary to subject the blend of cellulose fibres and heat-shrinkable fibres to a heat treatment in order to cause shrinkage of the synthetic ' fibres.

Conveniently, the heat treatment is effected during the drying of the wet-laid fibrous or pulp material. The heat treatment or pulp drying step may be carried out at a temperature of, in general, up to 1800C, or even higher if a flash-drying process is employed. It is preferred, however, that the heat treatment/drying step be carried out at 120-1600C, in order to prevent yellowing of the cellulose. It will be understood that, when appropriately configured or treated cellulose fibres are used (e.g. the special wood pulp fibres discussed above), the heat treatment/drying step may also cause the cellulose fibres to acquire a spiral, crimped or twisted configuration.

The wet-laid board will commonly have a basis weight of 600 to 900 g/m2. The wet-laid board will also typically have a thickness of about 2 mm, which will allow the fibres therein to reach a substantially uniform temperature during the heat treatment or pulp drying step, which in turn will permit a uniform development of the intended fibre characteristics throughout the fluff pulp.

Without wishing to be bound by any theory, the applicant believes that the shrinkage of the synthetic organic fibres in the heat treatment step will give rise to the breaking of hydrogen bonds between the wood (other cellulose) fibres, with a consequent reduction in the energy required for defibrisation. This in turn allows the board to be defibrised with less fibre breakage, fewer fines and a reduced generation of noise from the hammermill or other shredding apparatus. The breaking of the hydrogen bonds also facilitates the change in configuration of the cellulose fibres, where appropriate. The change in configuration of the fibres may also give rise to a defibrised material (fluff pulp) having an improved bulk, pad resilience and absorption capacity.

It is possible to introduce a particulate filler material into the blend of cellulose and synthetic organic fibres; such filler may be incorporated by means of a polymer, e.g. carboxymethyl cellulose (CMC) or a polygalactomannan, that is capable of functioning as a coupling agent between the filler particles and the synthetic organic fibres, as described in EP-A-0,261,820 (Du Pont). A wide variety of particulate filler materials come into consideration, for example filler materials that will modify the moisture absorption characteristics of the fluff pulp or which have an antibacterial effect and which therefore may serve as a deodorant. It is also possible, for example, to include a filler that will modify the oil absorption characteristics of the fluff pulp, particularly where the fluff pulp is intended for applications outside the personal hygiene field.The polymer used as a coupling agent according to EP-A-0,261,820, may in appropriate cases also be selected so as to serve a function in the fluff pulp pad; for example, carboxymethyl cellulose may improve the absorption capacity or reduce absorption time. The said filler coating may improve the dispersion of the synthetic fibres through the pulp sheet.

It is also possible to incorporate the synthetic organic fibre in the cellulose fibre material by a dry addition route, for example by adding the synthetic organic fibre to the cellulose fibre material during or after defibrisation of the latter and before the resultant fluff pulp is made up into an absorbent pad or like product. An apparatus and a method for the dry addition of fibre to fluff pulp or a fluff pulp precursor is described in the present applicant's United Kingdom patent application filed on 13th August 1992 and entitled "Process for the Production Qopiltation qx (reference 3 of Fluff Pulp" (reference Gllz67P),.It will appreciated that in the dry addition route, the blend of cellulose and synthetic organic fibres will require a heat treatment such as thermal bonding in order to shrink the synthetic organic fibres and obtain the desired fluff pulp characteristics.

Heating to a temperature of 120 to 1800C is generally appropriate to achieve the required shrinkage, for example by developing a spiral crimp, and, where appropriate, to develop sufficient shrinkage in the pulp sheet to permit the cellulose to twist or crimp up.

The invention as described herein may be modified by replacing the cellulose fibre wholly or partially with synthetic pulp fibres such as microdenier fibres or fibrids such as fibrillated polyolefin.

The teaching in the patent and technical literature referred to above is incorporated herein by reference.

It will of course be understood that the present invention has been described above purely by way of example and that modifications of detail can be made within the scope of the invention.

Claims (10)

1. A fluff pulp comprising a blend of cellulosic fibres and synthetic, heat-shrunk fibres.

2. A fluff pulp according to claim 1 wherein the cellulosic fibres are wood pulp fibres.

3. A fluff pulp according to claim 2, comprising wood pulp fibres that have been modified so as to impart a crimp or twist.

4. A fluff pulp according to claim 1, 2 or 3, wherein the synthetic heat-shrunk fibres are of polyester.

5. A precursor of a fluff pulp according to any of claims 1-4, comprising cellulosic fibres and synthetic, heatshrinkable fibres.

6. A precursor according to claim 5 wherein the heatshrinkable fibres are partially drawn fibres, partially quenched fibres, fibres having a latent crimp or a mixture of two or more such fibres.

7. A precursor according to claim 5 or 6 in the form of a wet-laid sheet or pulp board.

8. A process for the production of fluff pulp, which method comprises forming a precursor according to claim 5, 6 or 7 and heating the precursor to a temperature sufficient to cause shrinkage of the said synthetic fibres.

9. A process according to claim 8, wherein the said precursor is heated to a temperature of from 120 to 1800C.

10. An absorbent product comprising an absorbent core formed from a fluff pulp according to any of claims 1-4, preferably having a structural frame for holding the absorbent core for its intended use.