BACKGROUND OF THE INVENTION

The present invention relates to a nonwoven material produced by hydro-entanglement of a wet- or foam-formed fibre web.

Hydro-entanglement or spunlacing is a method which was introduced in the 1970s, see for example Canadian patent no.841 938. The method involves forming either a dry-laid or wet-laid fibre web, whereafter the fibres are entangled by means of very fine water jets under high pressure. A plurality of rows of water jets are directed towards the fibre web which is carried on a moving wire. The entangled web is thereafter dried. Those fibres which are used in the material can be synthetic or regenerated staple fibres, e.g. polyester, polyamide, polypropylene, rayon and the like, pulp fibres or a mixture of pulp fibres and staple fibres. Spunlace material can be produced to a high quality at reasonable cost and display high absorption capability. They are used inter alia as wiping materials for household or industrial applications, as disposable materials within health care, etc.

The pulp fibres used in spunlace materials are mainly chemically exposed softwood pulp from different kinds of wood. The use of chemically exposed hardwood pulp and pulp produced from recycled fibres is also described in the litterature, see EP-A-0,492,554.

Chemical pulp is produced by impregnating wood chips with chemicals and by subsequent boiling of the chips so that lignine, resins and hemicellulose are transferred to the boiling liquid. When the boiling is completed, the pulp is filtered and washed before it is bleached. The lignine content of such pulp is very close to zero and the fibres.

content of such pulp is very close to zero and the fibres, which essentially consist of pure cellulose, are relatively long and slender. The fibres show a certain degree of flexibility, which is an advantage when the fibres are entangled by the hydro-entanglement process. Furthermore, the cellulose in the fibres form hydrogen bonds, which increases the strength of the finished material. A high degree of hydrogen bonding of the material does, however, impair the softness and decrease the bulk of the material.

THE OBJECT AND MOST IMPORTANT CHARACTERISTICS OF THE INVENTION

The object of the present invention is to produce a spunlace material which presents improved absorption properties, softness and bulk. In accordance with the invention, this is accomplished with a material containing a certain proportion of wood pulp of mechanical, thermo¬ mechanical, chemical-mechanical or chemical- thermomechanical type and/or chemical pulp fibres which have been chemically stiffened or cross-linked in a manner so as to substantially reduce the ability of the fibres to form hydrogen bonds when wet and that said fibres preferably have been mixed with other fibres, such as chemical pulp fibres, vegetable fibres, synthetic fibres or regenerated cellulosic fibres in a wet- or foam-formed fibre web which has been hydro-entangled with sufficient energy to produce a dense absorbent material.

The proportion of pulp fibres of mechanical, thermomechanical, chemical-mechanical or chemical- thermomechanical type and/or chemically stiffened or cross- linked pulp fibres should be at least 5 and preferably at least 10 % by weight of the total fibre weight. The material may additionally contain a wet strength agent or

a binding agent. The invention is also directed to a method for producing the nonwoven material in question.

BRIEF DESCRIPITION OF THE DRAWINGS

Fig. 1 shows in the form of a diagram the effect of the CTMP on the bulk and the total water absorption for some foam-formed spunlace materials.

DESCRIPITION OF THE INVENTION

The spunlace material according to the invention contains a certain proportion of pulp fibres of mechanical, thermomechanical, chemical-mechanical, or chemical- thermomechanical type and/or chemically stiffened or cross- linked pulp fibres.

Mechanical pulp is produced by grinding or refining and the principle for mechanical pulp production is to mechanically disintegrate the wood. All of the wood material is used and the lignine thus remains in the fibres, these being comparatively short and stiff. Production of thermomechanical pulp (TMP) is carried out by refining in a disc refiner at elevated steam pressure. Also in this instance the lignine remains in the fibres.

A thermomechanical pulp can be modified by addition of small amounts of chemicals, usually sulfite, which are added before the refining. Such pulp is referred to as chemical-mechanical pulp (CMP) or chemical-thermomechanical pulp (CTMP) . A variant of CTMP is described in the International patent application PCT/SE91/00091 and in the Swedish patent application no.9402101-1, these pulps also being included in the invention. An effect of the chemical treatment is that the fibres are more readily exposed. A chemical-mechanical or chemical-thermomechanical pulp

contains more unbroken fibres and less shives (fibre aggregates) than a mechanical or thermomechanical pulp. The properties of the chemical-mechanical or chemical- thermomechanical pulps are close to those of the chemical pulps, but some essential differences exist, i.a. due to the fact that the fibres in chemical-mechanical and chemical-thermomechanical pulp are coarser and contain a high proportion of lignine, resins and hemi-cellulose. The lignine gives the fibres more hydrophobic properties and a decreased ability to form hydrogen bonds.

These are properties which previously have not been considered desirable in the fibres used for production of spunlace materials, where flexible fibres, which easily hitch on to each other and are entangled into a strong material, have been sought.

It has now, surprisingly, been shown that by adding fibres of the above mentioned kind to a spunlace material, the absorption capacity, bulk and softness thereof will be considerably improved. The tensile strength of the material is indeed reduced, but will still be totally sufficient for a wide range of applications. The tensile strength can, however, been increased by the addition of a wet strength agent or a binding agent, preferably in an amount corresponding to between 0.1 and 10 % by weight, and most preferably between 0.2 and 5 % by weight calculated on the total weight of the material. Of the above mentioned pulps, the chemical-thermomechanical pulp (CTMP) is preferred.

In the place of mechanical, thermomechanical, chemical- mechanical and chemical-thermomechanical pulp, chemically stiffened cellulosic fibres of chemical pulp may also be used. Hereby is meant fibres which have been chemically stiffened to increase the stiffness of the fibre in both dry and wet condition. This involves the addition of

chemicals which, for instance, cover the outside of the fibre as a coating, or penetrate into the fibre. It further includes such treatment which aims at changing the chemical structure of the cellulose, for example by cross-linking between the cellulose molecules. The treatment with chemicals reduces the ability of the fibres to form hydrogen bonds, and gives them in some aspects properties similar to those of chemical-thermomechanical pulp fibres, particularly in respect of wet resiliency. The chemically stiffened cellulosic fibres may additionally have a so called curl value, i.e. they may be twisted along their axis.

Some examples of stiffening chemicals that can coat or impregnate cellulosic fibres are cationically modified starch having a ino groups, latex, wet strength agents such as polyamide-epichlorohydrin resins, urea-formaldehyde resins, melamine-formaldehyde resins, and polyethylene- imine resins. Examples of cross-linking chemicals that can be used to cross-link cellulose molecules are C2-C8 mono- aldehydes and C2-C8 di-aldehydes, for instance formaldehyde or glutaraldehyde, and poly-carboxylic acids, for instance citric acid.

Naturally, it is also possible to use a combination of chemical-thermomechanical (or another mechanical) pulp and chemically stiffened cellulosic fibres.

Although the spunlace material may only contain fibres of the above mentioned kind, it preferably further contains other kinds of fibres, such as chemical pulp fibres, vegetable fibres, synthetic fibres and/or regenerated cellulosic fibres, i.e. viscose or rayon. In this manner, the tensile strength of the material is increased. Some examples of suitable synthetic fibres are polyester, polypropene, and polyamide.

Examples of vegetable fibres which can be used are leaf fibres such as abaca, pineapple and phormium tenax, bast fibres such as flax, hemp and ramie and seed hair fibres such as cotton, kapok and milkweed. During the addition of such long hydrophillic vegetable fibres in wet- or foam- formed materials, it may be necessary to add a dispersion agent, for example a mixture of 75% bis(hydro-generated tallow-alkyl)dimethyl ammonium chloride and 25% propylene glycol. This is described in greater detail in Swedish patent application no.9403618-3.

The invention comprises wet- or foam-forming of a fibre web containing the desired fibre blend and dewatering of the web on a wire. By foam-forming, the fibres are dispersed in a foamed liquid containing a foam-forming surfactant and water, whereafter the fibre dispersion is dewatered on a wire in a manner corresponding to that used in connection with wet-forming. An example of a suitable such foam- forming process is found in Swedish patent application no. 9402470-0.

The fibre web formed in this manner is exposed to hydro- entanglement with an energy input which may suitably lie in the range of 200-800 kWh/ton. The hydro-enganglement is carried out by conventional techniques and using equipment supplied by machine manufacturers. Subsequent to the hydro- entanglement, the material is pressed and dried and is rolled up. The finished material is then converted by known methods into a suitable size, and is then packed.

Materials produced according to the invention have sufficiently good strength properties to enable them to be used as wiping materials, even in applications where comparatively high wet strengths are required. By addition of a suitable binding agent, or a wet strength agent, by impregnating, spraying, film application or other suitable

method of application, the properties of the material can be further improved. The binding agent or wet strength agent can either be added to the hydro-entangled material, or to the fiber stock before wet- or foam-forming of the fibre web. The material may be used as wiping material for household purposes or for large quantity consumers such as workshops, industries, hospitals and other public establishments. Due to its softness it is also suitable as disposable material within the health care sector, for example operation gowns, drapes, and the like. Due to its high absorption capacity, it is further highly suitable as a component in absorption products such as sanitary napkins, panty liners, diapers, incontinence-products, bed protectors, wound dressings, compresses and the like.

EXAMPLE

Several different materials with different fibre composition and varying content of CTMP-fibres have been produced and tested, whereby a comparison has been made with a reference material not containing CTMP-fibres. The CTMP-fibres consisted of commercially available chemical- thermomechanical pulp produced from softwood. The chemical pulp fibres consisted of bleached chemical softwood pulp. The synthetic fibres that were used consisted of polyester of 1.7 dtex x 12.7 mm and polypropylene 1.4 dtex x 18 mm, respectively. Fibre webs were either produced by wet- forming or by foam-forming and were subsequently hydro- entangled with an energy input of about 600 kWh/ton, were slightly pressed and dried by through-blowing at 130°. The properties of the materials are presented below in Table 1, with the accompanying Fig. 1.

The results show that the bulk and the absorption capacity of the materials were notably increased with increasing admixture of CTMP-fibres. The materials were further percieved as being softer. The strength of the materials did, however, fall with increasing admixture of CTMP- fibres. For numerable applications, these strength values are, howvever, totally sufficient and as mentioned above, the tensile strength can be increased by addition of a wet strength agent or a binding agent, preferably in an amount corresponding to between 0.1 and 10 % by weight, and most preferably between 0.2 and 5 % by weight calculated on the total weight of the material.