DE69503758T2 - METHOD AND DEVICE FOR PRODUCING A DRY FIBER web - Google Patents

Abstract

For the manufacturing of dry-formed webs of cellulose fibres there are two different ways of binding the fibres, viz. an application of glue to the web surfaces and, respectively, an admixing into the fibre charge of heat actuated binding fibres that are actuated by passing the web through a heating zone. The latter method is the better in practice, of course, in particular for liquid absorbing products, but the problem exists that the finished material is dusting with short, unbonded fibres. According to the invention it has been realized that it is possible to widely avoid this problem by auxiliary use of the first method, modified to the effect that both surfaces of the web are concurrently subjected to a supply of a very small amount of glue in foamed condition, e.g. with only 1 gram per square meter, whereby the surfaces remain permeable, but also sealing against extrusion of short fibres from inside the web.

Description

The present invention relates to a method for producing a dry-formed fibrous web material based on dry-formed cellulose fibers with a suitable weave. The laying out of the fibers to form a running web is normally effected by defibering a pulp material and mixing it in an air stream which brings the loose fibers to a distributor head over a moving perforated forming (long) wire, under which a suction chamber is arranged for sucking the fibers down against the wire where they are continuously deposited as a coherent fibrous pulp web with a desired web thickness. The products are typically used as liquid absorption sheets.

It is important that the dry-formed pulp web is stabilized or bound, and in practice this is achieved in two different ways, namely by applying a binder or by using binding fibers:

1. Binding: On a supporting screen the pulp web is passed through a binding station where a suitable binder, normally of the latex type, is sprayed onto the web by means of a number of nozzles, after which the fourdrinier wire is passed through a tunnel oven to dry out the binder. The web is then transferred to an overhead screen against which it is secured by suction from above so that the underside of the web is exposed, and the web is then passed through or over a further binding station where the binder is appropriately sprayed against the underside of the web. The web is then passed through a tunnel oven to dry out and finally set the binder, after which the web is self-supporting and can be wound up for storage or delivery.

This process can produce sheets of relatively high quality, but as far as machinery is concerned, it is rigorous and prone to irregularities in the binding stations. A further limitation is that the process is not suitable for thick paper webs because it is difficult to achieve a high penetration depth of the binder, whereby a relatively thick layer tends to segregate in the central area where the bond is weak or completely absent. The products usually contain un approximately 85% cellulose fibers and approximately 15% binder. Typical product weights can be 50-120 g/m, because heavier products delaminate easily. Furthermore, the products are poorly suited to containing superabsorbents.

2. Use of binding fibers: This process delivers a homogeneous mixture of heat-activated binding fibers, also called thermo-bonding fibers, into the cellulose fibers, thereby fixing the dry-formed sheet material merely by passing it through a heating zone. In terms of system and control, this is a simpler process, and it can also produce thicker sheets because the binding fibers are evenly present in the outer as well as the inner planes of the material. The weight of the product can typically be from 40-800 g/m², and a considerable mixture of superabsorbents or other additives can be used. The quality achievable is perfectly acceptable for many different applications, but there are certain disadvantages in that, for economic reasons alone, the blend percentage of thermo-bonding fibers must typically be limited to about 15 percent. Normally the cellulose fibres are between 0.5 and about 5 mm in length, whereas heat-thermobonding fibres are often 6 mm or longer. In practice this provides very good bonding of the longest cellulose fibres, whereas the very short cellulose fibres are incompletely bonded. This means that the material with short fibres can become dusty, which is generally a serious disadvantage, and the products are therefore mainly used as filling materials in external packaging. This can prevent the dust problem from escaping to the outside, but in return it occurs to a large extent in the factories where the products are processed as filling material. Here powerful exhaust systems must be used, which themselves cause problems, and to this it must be added that the associated blowing out of "dust" involves removing a significant part of the original material, which means that this part is added completely for nothing and thus represents a real waste.

The invention is intended to counteract this fibre dusting, which naturally leads to considerations regarding a higher percentage of binding fibres of different lengths. However, this is hardly a realistic possibility, and the invention has recognized that a combined use of the two methods is a better solution, but with the binding method in a modified form.

As mentioned, the bonding process achieves a bonding result which is best at the web surface and worse in the middle layer of the web, but if it is only a question of sealing against dusting from the web by a combined application of the thermo-bonding binding fibres, this is achievable by coating the web with very thin surface layers of adhesive. Because the cohesion in the web material is ensured by the activation of the thermo-bonding binding fibres, a surface adhesive can be applied simultaneously to both sides of the already self-supporting web and both applied adhesive layers, both of which can be very thin, can be cured by passing through a single additional heating zone of relatively low capacity. If the surfaces of the web material therefore appear with thin binder bonds, a seal against extrusion of the short cellulose fibres from the web material is effected and this is exactly the desired result. This implies that there are no considerations of any local bonding of the fibres by the invention, although the effect may depend in part on the exact occurrence of increased fibre bonds in the surface, whereas it is to a lesser extent a question of the actual coating of the surface.

EP-A-0 202 472 is an example of documents disclosing a process for producing an absorbent product by dry forming a fiber mat using an admixed amount of heat activated fiber binder material.

US-A-4 292 271 discloses the manufacture of a dry-formed fibrous web based on the use of a binder solution or emulsion sprayed first on one side and then on the other, but with a binder solids concentration of 15-25%. Good internal bonding is obtained by wicking some binder into the web.

It is therefore a main feature of the invention that in the fibre bonding use is made of the thermo-bonding fibres, whereas the web surfaces are sealed to a greater or lesser degree by adding a binder dispersion in small amounts, without any ambition of deep penetration of the binder into the material. It is It is then not crucial how the binder is applied, but for the invention it is an important sub-aspect to show how the binder can be applied in a new and very economical way to achieve the desired result.

Therefore, according to the invention, it is a preferred possibility that the binder dispersion is applied in a foamed state, whereby it can cover the surface with a minimum of dry matter contained therein. The effect has been observed that the liquid in the setting foam settles preferentially in the intersecting areas between the fibres rather than by a general settling on the surfaces of the fibres, and this effect is optimal for the desired result, namely that by using small amounts of both liquid and dry matter, a bonding of the short fibres in the surface is effected, so that these partly remain in place and partly assist in sealing against extrusion of the short and unbonded fibres from the inside of the web material. This remains light and porous, so that it is suitable for example for impregnated wet fabrics.

A good effect can be achieved by mixing only 1-2 g of binder per m² per side using a water-dispersed foam binder with only about 2 g of dry binder per litre of foam. The foam can be produced in a conventional foaming unit and the dry matter of the binder can then be present in the foamed volume in an amount of only a few 0.2% by volume. Accordingly, there is little water to be dried out of the product, whereby the drying and curing of the binder can be effected with a relatively low energy consumption in the downstream curing zone.

The application of the foam can be effected in any suitable way, for example by feeding the web vertically, uphill or horizontally through pairs of rolls with controlled foam feed to the nip on both sides of the web. A rolling process of the web gives an attractive result in that the surface fibres are bonded in a couched position, giving the surface a smoother and stronger character.

By adding a dampening agent it is possible to control the penetration depth of the pre-expanded binder. If the penetration depth is too small, the thin bandage layer in the surface wears away quickly, and if the depth is too great, this partly leads to an unnecessarily large binder consumption and partly gives the product a plastic-like character with sluggish water adsorption.

As mentioned above, the binder can be dried out and cured with a low energy consumption, but it can take place at such a high temperature that the binder fibers can be reactivated and, as explained below, this can give the exceptional results that the tensile strength of the web can be increased almost double.

The invention provides significant results with regard to planning the manufacture of products with desired requirements without the requirements being changed by a subsequent extrusion of the short fibres from the product. Because this quantity of fibres can be of considerable size, the associated result will be that it is possible to deposit in a much better way in order to achieve the desired requirement of the finished product.

The invention is shown schematically in a preferred embodiment in the drawing.

In the drawing a forming (long) screen 2 is shown with a conveyor stretch 4 under a pair of distributor heads 6 for feeding a mixture of loose cellulosic fibers and heat thermobonding fibers to the stretch 4 on which a loose web 8 is thus formed. The latter is compressed between rollers 10 and then transferred to a transport screen 12 via an overhead transfer station 14. The screen 12 takes the web through a heating zone 16 in which the thermobonding fibers are activated to bind the cellulosic fibers, normally at a temperature of 130-140ºC. Thereafter the now stabilized and self-supporting web is passed through pairs of rollers 18. Conventionally the web, now as a product web, is then taken for winding into rolls 34 of finished product.

In the invention, however, a station 20 is connected downstream in which the web 22 is provided with a binding foam on both sides, preferably by guiding the web downwards around a diversion roller 24, downwards to a so-called padding unit, which consists of a pair of rollers 26 which are arranged at an adjustable distance and with not shown upper means for adding impregnating compound 28 to the upper roll gap are arranged on both sides of the downwardly guided web 22.

It is therefore possible to control the supply of the impregnating compound partly by adjusting the pairwise gap and partly by regulating the flow of the compound to a higher or lower level above the roller gap, for example within an adjustment range of 2-20 cm. In addition, the supply in this case can be adjusted by the dry matter content in the binder (for example within 5-15%) and by adjusting the foam density, for example within the interval 20-100 g/l.

The hardness/softness of the product can be controlled by choosing a binder with a hard or soft skin, while the hydrophilic/hydrophobic properties of the product can be controlled by using suitable foaming agents or additives.

After passing through the unit 20, the web 22 is guided on a belt 30 through a heat tunnel 32 in which the web is heated, preferably by means of supplied hot air, for rapid evaporation of the aqueous portion of the foam 28, after which the web is wound up into the usual delivery roll 34.

Surprisingly, it has been proven that by using a suitably high temperature in the tunnel oven 32, preferably 130-140ºC, not only an efficient water evaporation is achieved but also an improvement in the tensile strength of the web product of as much as 25-100%. This cannot be explained by the very modest supply of binder on the outer sides of the web 22, but by other circumstances.

As the web 8 passes the rollers 10, it is pressed downwards against the supporting screen 2, which is normally a woven mesh screen formed of intersecting threads which, through the connected intersections, form local upwardly projecting or indented parts. At the projecting parts, the fibres in the web are particularly compressed as they pass the rollers 10, so that the fibres, as locally indented thereby, are extra sensitive to the welding effect which occurs in the heating zone 16. This However, this condition is weakened by the fact that the web is somewhat loosened as it passes through the transfer unit 14, influenced by the slight pulling force and web movement required to take the web off the forming wire and transfer it to the transfer wire, just as a corresponding loosening occurs in the transfer from the transfer wire to the oven wire. In addition, the compressed web has a certain expansion force and therefore fibers that were originally well compressed are not compressed well enough overall when they pass through the curing tunnel 16.

When the web 22 subsequently passes through the station 20, however, a renewed compression of the web and a certain increase in the coherence in the surface layers of the web occur, so that during the subsequent passage through the oven unit 32 not only this evaporation of the water from the foam binder is caused, but also a renewed thermal activation of the thermo-binding fibers, because one preferably works with approximately the same temperature as in the tunnel 16, for example approximately 140ºC. Thus, the binder treatment of the web is a kind of catalyst for achieving a subsequent significantly increased tensile force of the web material.

The immediate result of the invention, namely the binding or sealing of the "dust fibers" while maintaining an unsealed surface, is also advantageous in another respect, because a retained amount of the fibers just within the fixed surface layers has a noticeable fluid distribution effect, which is of great importance for absorbed products. With many of these products one knows almost in advance that the fluid supply during use will only occur in a sub-area of the product, and for a desired absorption capacity a superabsorbent must be dosed with this fact in mind. If the product can occur with a good distribution ability, the dosage of the superabsorbent can be minimized.

It is worth emphasizing that the aqueous foam can be added to both sides of a product with a middle layer of superabsorbent material, without the material having to be activated with water. For example, the product can have a fiber bottom layer of 70 g/m², a middle layer of superabsorbent of 30 g/m² and a fiber top layer of 30 g/m².

The invention is also advantageous with respect to filter materials, where both the significantly reduced dusting and the maintained content of short fibers represent a positive effect.

The invention makes it possible to use the binder foam as a carrier for colour pigments, which are added before foaming, thus making it possible to produce a certain colouring of the products in the simplest way. However, since the binder is normally used in very modest quantities, only colouring in pastel colours can be achieved, unless the process is specifically adapted for a stronger colouring.

With the invention it has been found possible to coat a thermally bonded air-laid product with foam containing only a few percent of binding fibres, for example 5%, by using an increased amount of binder, for example 10%, whereby it is possible to produce a predominantly "latex-bonded" product without having to add the binder in the conventional spraying technique. Hereby the same abrasion-resistant surface is achieved as by conventional spraying of the binder, but without the associated limitation of the maximum thickness of the web, which is normally 100/120 g/m². At such thicknesses it was necessary to impregnate the web to prevent delamination of the product, but this is not necessary in the present situation and this means that, among other things, products with relatively low density can be produced. According to the invention, when working with a significantly reduced amount of binder, strong products hold a noticeably increased amount of cellulose fibers that are not covered with a skin of binder and thus have optimal absorption capabilities.

From a process point of view, it is advantageous that the application of the binder is effected at only one place in the system and not at two places as in the prior art. Furthermore, it is particularly advantageous to avoid the conventional spray stations, which cause many problems, both in terms of operation and maintenance, while, for example, a padder gives a good and consistent result and is easy to maintain. However, other foam application techniques could be used, e.g. the so-called screen coating.

In principle, there is nothing to prevent the binder foam from being applied to the web before it is thermally fixed, and in this case it may be sufficient to use a single subsequent heating zone to activate both the binder fibers and the adhesive.

Finally, a few examples of the manufacture of specific products according to the invention are given:

A. Preparation of a foam coated hydrophilic product.

An air-laid product, 60 g/m², consisting of 8590 wood cellulose fibres with the highest possible long fibre content and 15% thermo-binding fibres, is foam coated in a padder whose roller gap is set at 0.6 mm. The pre-foamed binder is prepared in the following way:

Binder mix specification:

11.1% Sarpifan WRG (Stockhausen, DE)

0.2% Rohagal 10n (Röhm GmbH, DE)

88.7% water

The binder is foamed to a density of 20 g/l.

When the foam level in the padder is about 4 cm, about 1.5 g binder (100% dry matter) per m² is added on each side of the product. After foam coating, the product is dried in the usual way, for example at 140ºC.

B. Production of a foam-coated hydrophobic product.

The product is manufactured in the same way as mentioned above, but with a different binder mix specification:

Binder mix specification:

11.1% Sarpifan WRG (Stockhausen, DE)

2.0% Stokal STA (Stockhausen, DE)

86.9% water

Claims (16)

1. A method for producing a dry-formed paper material by successively laying a web of cellulose fibres on a forming belt, treating the web to provide bonding of the fibres therein, characterized in that the bonding of the fibres is effected by a combination of thermal activation of thermally bonding fibres, provided by prior mixing into the cellulose fibres, and application of a binder dispersion to the surfaces of the web, the application being carried out with a dry weight proportion of the binder dispersion of significantly below 15%, preferably 0.5 to 5%, with the intention that short cellulose fibres bond to the surfaces, the binder being applied in an amount of 0.5 to 20 g dry substance per square metre of web surface, preferably only 0.5 to 10 g per m². 2. Method according to claim 1, characterized in that the binder is applied to the surfaces of the web after it has been stabilized by thermal activation of the thermally bonding fibers, and in that the binder is cured by continuously heating the web at such a high temperature that the bonding fibers are reactivated. 3. Process according to claim 1, characterized in that the binder is applied in foamed state, i.e. with water/air as carrier. 4. Process according to claim 3, characterized in that the web material, despite the use of the aqueous binder, is produced with one or more middle layers which contain or consist of superabsorbent material. 5. Process according to claim 1, characterized in that the binder is used with pigment additive. 6. Process according to claim 1, characterized in that for the production of a predominantly thermally bonded material, a web with 10 to 25% thermally bonded fibers and a binder quantity of 0.5 to 10 g/m² surface is used. 7. Process according to claim 1, characterized in that for the production of a predominantly thermally bonded material, a web with 3 to 7% thermally bonded fibers and a binder quantity of 5 to 20 g/m² surface is used. 8. A dry formed paper web material consisting predominantly of cellulosic fibres and manufactured so that there is a bond of fibres therein, characterised in that this bond is a combination of - thermally activated binder fibres, which are generally evenly distributed throughout the thickness of the web, and - a surface distribution of a binder in an amount of 0.5 to 20 g/m² on at least one side of the web represents. 9. A web material according to claim 8 and consisting of a mixture of 90-75% cellulose fibers and 10-25% thermally binding fibers, wherein the binder is present in an amount of 0.5 to 10 g/m². 10. A web material according to claim 8 and consisting of a mixture of 97-93% cellulose fibers and 3-10% thermally bonding fibers, wherein the binder is present in an amount of 5 to 20 g/m². 11. A web material according to claim 8, in which the binder is present in an amount of 1 to 2 g/m². 12. A sheet material according to claim 8 appearing as an outer layer in a laminated product. 13. A web material according to claim 12, wherein the binder is the dry product of an aqueous binder dispersion, wherein the laminate contains a superabsorbent material. 14. A sheet material according to claim 8, in which the binder is pigmented. 15. Apparatus for carrying out the method according to claim 1, comprising means for laying the fibres dry on a forming belt to form a fibre web and means for activating means for binding the fibres, characterized by the combination of the laying device which is of the type operating on the basis of fibres with admixed thermally binding fibres, the activation device being a heating zone, with the presence of a station for applying relatively small amounts of binder to the surfaces of the web, namely sufficient to render the finished web material substantially non-dusting. 16. Device according to claim 15, characterized in that the binder station comprises means such as a padder for applying the binder in a foam carrier.