WO2009077001A1

WO2009077001A1 - Method for pulp preparation and pulp treatment and a paper, especially a tissue paper

Info

Publication number: WO2009077001A1
Application number: PCT/EP2007/064064
Authority: WO
Inventors: Hans-Jürgen Lamb
Original assignee: Sca Hygiene Products Gmbh
Priority date: 2007-12-17
Filing date: 2007-12-17
Publication date: 2009-06-25

Abstract

The invention relates to pulp treatment by pulp fractionation such that the pulp is fractionated into a short fiber fraction and a long fiber fraction, wherein the short fiber fraction bypasses the refiner and the long fiber fraction is mechanically treated in the refiner, whereafter the untreated short fiber fraction and the mechanically treated long fiber fraction are mixed together or are used separately for paper production. By this fiber fractionation before refining fiber lengths distribution is optimized in order to maximize the strengths properties and to minimize the fines content. According to this mode the long fiber fraction will have a mechanical treatment which is more effective.

Description

Method for pulp preparation and pulp treatment and a paper, especially a tissue paper

Technical field

The technical field to which the present invention relates is the field of making absorbent structures such as paper and especially tissue paper, including pulp or furnish preparation and paper-making on a paper machine as generally known in the art .

A tissue paper is defined as a soft absorbent paper having a low basis weight. One generally selects a basis weight of 8 to 40 g/m², especially 10 to 30 g/m² per ply. The total basis weight of multiple-ply tissue products is preferably equal to a maximum of 100 g/m², more preferably to a maximum of 80 g/m². Its density is typically below 0.6 g/cm³, preferably below 0.30 g/cm^ and more preferably between 0,08 and 0.20 g/cm³.

The production of tissue is distinguished from paper production by its extremely low basis weight and its much higher tensile energy absorption index (see DIN EKf 12625-4 and DIN EN 12625-5} . Paper and tissue paper also differ in general with regard to the modulus of elasticity that characterizes the stress-strain properties of these planar products as a material parameter.

A tissue's high tensile energy absorption index results from the outer or inner creping. The former is produced by compression of the paper web adhering to a drying cylinder as a result of the action of a creping doctor or in the latter instance as a result of a difference e.g. in speed between two wires ("fabrics") . This causes the still moist, plastically deformable paper web to be internally broken up by compression and shearing, thereby rendering it more stretchable under load than an uncreped paper.

Typical properties of tissue paper include the ability to absorb tensile stress energy, their drapability, good textile-like flexibility, a high specific volume with a perceptible thickness, as high a liquid absorbency as possible and a suitable wet and dry strength as well as an interesting optical appearance of the outer product surface.

Background art

From US 2003/0121629 Al is known the use of fractionated fiber furnishes in the manufacture of tissue products. Therein it is mentioned that strength and softness are important attributes in consumer paper products such as bathroom tissue, towels and napkins. Strength and softness are strongly influenced by the sheet structure of a paper product. The type and arrangement of fibers employed in the manufacture of paper products are important factors in determining the strength and softness of products made from such fibers. Strength and softness are usually inversely related. That is, the stronger a given sheet, the less softness that sheet is likely to provide. Likewise, a softer sheet is usually not as strong. Thus, this inverse relationship between strength and softness results in a constant endeavour in the industry to produce a sheet having a strength which is at least as great as conventional prior art sheets, but with improved softness. Also, a sheet which is at least as soft as known sheets, but with improved strength, is desirable.

It is common in the manufacture of paper products to provide two or more furnishes of fiber. Sometimes, a two-furnish system is used in which the first furnish is comprised of hard wood eucalyptus wood fibers, the second furnish is made of soft wood fibers. Hard wood fibers made from eucalyptus tend to be softer and more "fuzzy" to the touch and, therefore, often these fiber types are provided on outer surfaces of a paper product.

Fractionation is the process by which cellulosic fibers are separated according to their properties. US 6,024,834 is directed to a process of separating, by fractionation, cellulosic fibers that exhibit desired properties such as fiber length and fiber coarseness values.

US 4,781,793 discloses a method in a paper manufacturing process for improving the properties of paper using fractionation. In the disclosure of the patent, the fiber slurry is separated into two components which are stated to contain substantially all of the fibers to be used for paper manufacture. One component contains mainly fiber longer than the average distribution of fiber length in the basic stock, and the other component contains primarily shorter fibers and fines .

Fines and short fibers usually are regarded as the least desirable fibers in most fiber slurries. Fines comprise short portions of cellulosic material that do not appreciably contribute to paper softness.

US 2002/0162635 Al teaches a product and process of making an absorbent paper article such as paper products, towels, napkins and the like. It is mentioned that one may supply a single furnish or slurry of cellulose fibers. Then, it is possible to separate or fractionate the slurry into at least two portions based upon fiber length in the slurry. Fines are employed in the process of manufacturing the products, and fines are specifically incorporated into an inner layer of the final paper products. Fines, and/or short fibers are used in the process so they may contribute in a positive manner to the final paper product. This use of fines in the inner layers of the products reduces the manufacturing costs and waste produced in the process. A soft paper product with good strength characteristics results from the process. According to the process, in one step, a cellulosic fiber mixture is provided for fractionating the first cellulosic fiber mixture into a second fiber mixture having relatively short fibers and fines, and a third fiber mixture having relatively long fibers . The third fiber mixture may be treated with chemical agents to soften the fibers. In a next step, the third fiber mixture is provided to a paper machine, thereby forming a paper sheet from the third fiber mixture on a wire former. Then, the step of adding the second fiber mixture to the upper surface of the paper sheet is provided.

According to US 2003/0121629 Al a method of making a paper product comprises:

providing a first furnish of fibers; providing a second furnish of fibers; fractionating the first furnish into a long fiber fraction and a short fiber fraction; diverting the short fiber fraction to the second furnish to form a third furnish; forming a first and second exterior layer using the third furnish; forming a first interior layer using the long fiber fraction of the first furnish; and combining at least the first and second exterior layers with the first interior layer to form a first ply. Here, the first furnish comprises softwood fibers and the second furnish comprises hardwood fibers. The first furnish is used for a strength layer and the second furnish for a softness layer of the paper. The first furnish is fractionated and the separated short fiber fraction is added to the second furnish for the softness layer.

Therewith, a paper product or a tissue is provided having reduced levels of slough, with about the same or comparable level of softness. Disclosure of the invention

Softness, haptic, absorption speed, absorption capacity, thickness, etc. are negatively influenced by fines. The positive impact fines have this on strength. The fines are coming with the pulp or are generated by refining the pulp. Normally, the pulp is refined as it is delivered and disintegrated in the pulper . As a consequence of that, all fines and short fibers are mechanically treated in the refiner and are even more shortened. The mechanically treatment of these fines and short fibers is not contributing to a further strengths increase.

It is the problem (object) of the invention to provide a method by which an absorbent structure as a multi- layer paper and especially a tissue paper can be manufactured in which the at least two layers comprise a strength layer of improved strength and a softness layer of improved softness.

This problem is solved by a method for pulp preparation and treatment including refining of the pulp in a refiner, characterized in that the pulp is fiber-fractionated upstream the refiner in that the short fiber fraction of the pulp containing most of the fines and short fibers is separated and bypass the refiner without mechanical treatment and that the remaining long fiber fraction is mechanically treated in the refiner.

The method for pulp preparation and treatment according to the invention can be modified by using a pulp for the specific treatment which has been already fractionated and mechanically treated at least one time upstream of the refiner .

A variety of papermaking machines have more than one refiners usually connected in cascade so that fiber fractions leaving the first refiner enter the second refiner and after being mechanically treated in such second refiner such fiber fractions enter the third refiner and so on. Such papermaking machines may have five, six, seven, eight or even more of such refiners . The method according to the invention may be used at one, at two, at three or more or even at all of such refiners being connected in cascade.

By fiber fractionation before refining fiber lengths distribution is optimized in order to maximize the strengths properties and to minimize the fines content. The "short fiber" fraction, roughly 20% to 70% of the pulp, especially 40% to 60% of the pulp containing most of the fines and short fibers, will be separated before the refiner and will bypass it without any mechanical treatment. Only the "long fiber" fraction will have a mechanical treatment, which is therefore more effective. After refining, the "long fiber" is mixed again with the "short fiber" fraction. Optionally, the "short fiber" and the "long fiber" fractions are used in different layers .

From a theoretical point of view, the fractionated refining will in comparison with a "standard" refining reduce the total percentage of fines and will increase the strengths at similar Schopper-Riegler values (measured according to DIN ISO 5267) . Estimating that a normal Kraft pulp mixture has approximately 10% of fines and approximately 25% of short fibers, there is a reasonable amount of fibers in the pulp, where a further mechanical treatment is not desirable, especially, because the refining of this fraction will not increase or probably decrease the strengths of this fraction. In addition, the refining will shorten the fines and short fibers further with a negative impact on freeness . As a consequence, the freeness of the total pulp is higher than necessary and probably the "long fiber" fraction needs to be treaued more than necessary to compensate the strengths loss in the "short fiber" fraction. Both effects have negative impact on tissue properties. At similar strengths values, a fractionated refining is increasing the air permeability, reducing the stiffness with a positive impact on softness, reducing the water retention value and also increasing absorption speed and absorption capacity.

Fractionated refining is most beneficial, when using sulfite pulp, hardwood pulp or any kind of mechanical pulp.

A multi-layer paper product inay be produced using the short fiber fraction and the long fiber fraction mechanically treated in the refiner in different layers of such a product. Alternatively, the mixture of the short fiber fraction and the long fiber fraction mechanically treated in the refiner can be used for a single layer paper product.

Tissue paper products such as handkerchiefs, cosmetic wipes, toilet papers, serviettes/napkins or kitchen towels are being distinguished from other paper products by their low basis weight and its significantly improved tensile energy absorption index.

Preferably the tissue paper contains as main component (in particular at least 80% by- weight, relative to the dry weight of the fibrous web) cellulosic fibres, in particular pulp, although a proportional use of modified pulp fibers (e.g. from 10 to 50 weight %, relative to the total weight of the fibers) or the use of synthetic fibers suitable for web making (e.g. from 10 to 30% by weight, relative to the total weight of the fibers) is covered by the invention. The origin of these fibers may be native or recycled. The tissue paper may be creped or uncreped, the use of creped tissue paper being preferred. The tissue paper (or the final tissue paper product obtained therefrom) can be single-ply or multi-ply with two, three, four, five, six or even more plies. Preferred tissue paper products comprise between two and four plies . The tissue paper may be homogeneous or layered, wet-pressed or blow-dried (TAD-dried) . The tissue paper includes, but is not limited to, felt-pressed tissue paper, pattern-densified tissue paper, uncompacted tissue paper or compacted tissue paper.

The starting material for the production of the tissue paper usually is a fibrous cellulosic material, in particular pulp.

The starting pulps used may relate to primary fibrous materials (raw pulps) e.g. wood pulps, non-wood pulps, linters, cotton or to secondary fibrous materials, whereby a secondary fibrous material is defined as a fibrous raw material recovered from a recycling process. The primary fibrous materials may relate both to a chemically digested pulp and to mechanical pulp such as thermo mechanical pulp (TMP) , chemothermo mechanical pulp (CTMP) or high temperature chemithermomechanical pulp (HTCTMP) . Synthetic cellulose- containing fibres can also be used. Preference is especially given to the use of pulp from non wood material, particularly synthetic cellulose-containing fibers or pulp from non wood forming plants. Fibers of softwood (usually originating from conifers) , hardwood (usually originating from deciduous trees) or from cotton linters can be used for example. Fibres from non wood forming plants such as esparto (alfa) grass, bagasse (cereal straw, rice straw, bamboo, hemp) , kemp fibers, flax, and other woody and cellulosic fiber sources can also be used as raw materials. The corresponding fiber source is chosen in accordance with the desired properties of the end product in a manner known from the prior art. For example, the fibers present in hardwood, which are shorter than those of softwood, lend the final product a higher stability on account of the higher diameter/ length ratio. If softness of the product is to be promoted, which is important e.g. for tissue paper, eucalyptus wood is particularly suitable as a fiber source. With regard to softness of the products, the use of chemical raw pulps is also preferred, whereby it is possible to use completely bleached, partially bleached, and unbleached fibers. The chemical raw pulps suitable according to the invention include inter alia, sulphite pulps, kraft pulps (sulphate process) .

Before a raw pulp is used in the tissue making process, it may also be advantageous to allow further delignification to occur in a separate process step or employ a bleaching process to achieve a more extensive removal of lignin after the cooking process and to obtain a completely cooked pulp.

A preferred production process for tissue paper uses

a a forming section (for wet-laying a slurry of cellulosic fibrous material, typically pulp) comprising a headbox and wire section portion, and b the drying section (TAD (through air drying) and/or conventional drying on the Yankee cylinder) that also may include the crepe process for tissues .

This is typically followed by

c the converting area.

The tissue paper can be formed by placing the fibers, in an oriented or random manner, on one of between two continuously revolving wires of a paper-making machine while simultaneously removing the main quantity of water of dilution until dry-solid contents of usually between 8 and 50%, especially between 12 and 45% are obtained. It is possible to include additives in the paper furnish to improve the wet-strength or dry-strength or other properties of the finished tissue paper. Drying the formed primary fibrous web occurs in one or more steps by mechanical and thermal means until a final dry- solids content of usually about 93 to 97% is obtained. In the case of tissue making, this stage is followed by the crepe process which crucially influences the properties of the finished tissue product in conventional processes. The conventional dry crepe process involves creping on a drying cylinder having a diameter of usually 3.5 to 6 meter, the so- called Yankee cylinder, by means of a crepe doctor with the aforementioned final dry-solids content of the base ( "raw tissue") paper (wet creping can be used if lower demands are made of the tissue quality} . The creped, final dry base tissue paper is then available for further processing into the paper product or tissue paper product according to the invention.

In addition to this conventional tissue making process described above, tissue paper can also be manufactured by a special kind of drying within process section b and in this way an improvement in bulk and/or softness of the resulting tissue paper is achieved. This pre-drying process, which exists in a variety of subtypes, is termed the TAD (through air drying) technique. It is characterized by the fact that the "primary" fibrous web (like a non-woven) that leaves the sheet making stage is pre-dried to a dry-solids content of between 55 and 99% before final contact drying on the Yankee cylinder by blowing hot air onto the fibrous web and by contact drying of the web. The fibrous web is supported by an air-permeable wire or belt and during its transport is guided over the surface of an air-permeable rotating cylinder drum. Structuring the supporting imprinting fabric or belt makes it possible to produce any pattern of compressed and uncompressed zones achieved by deflection of the fibres in the moist state, followed by pre-drying of the fibres (TAD step} and leading the web through a pressure nip between a pressure roll and the Yankee cylinder surface, thereby resulting in increased mean specific volumes and consequently leading to an increase in bulk and/or softness without decisively decreasing the strength of the fibrous web.

Another possible influence on softness and strength of base tissue lies in the production of a layering in which the primary fibrous web to be formed is built up by a specially constructed headbox in the form of physically different layers of fibrous material, these layers being jointly supplied as a pulp jet to the forming stage.

The one-ply intermediate products originating from the paper- making machine and made of lightweight paper usually dry- creped on a Yankee cylinder by means of a crepe doctor are generally described as "tissue paper" or more accurately base tissue paper. The one-ply base tissue may be built up of one or a plurality of layers respectively.

All one-ply or multi-ply final products made of base tissue and tailored to the end user's needs, i.e. manufactured with a wide variety of requirements in mind, are known as "tissue products" .

When processing the fibrous web or base tissue paper into the final tissue product, the following procedural steps are normally used individually or in combination: cutting to size (longitudinally and/or cross cutting), producing a plurality of plies, producing mechanical and/or chemical ply adhesion, volumetric and structural embossing, folding, imprinting, perforating, smoothing, stacking, rolling up. These procedural steps are usually carried out in the converting part (c) of the papermaking process.

To produce multi-ply tissue paper products an intermediate step preferably occurs with so-called doubling in which the base tissue in the finished product's desired number of plies is usually gathered on a common multiply master roll. The processing step from the base tissue that has already been optionally wound up in several plies to the finished tissue product occurs in processing machines which include operations such as repeated smoothing of the tissue, edge embossing, to an extent combined with full area and/or local application of adhesive to produce ply adhesion of the individual plies (base tissue} to be combined together, as well as longitudinal cut, folding, cross cut, placement and bringing together a plurality of individual tissues and their packaging as well as bringing them together to form larger surrounding packaging or bundles . The individual paper ply webs can also be pre-embossed and then combined in a roll gap according to the foot-to-foot or nested methods.

As follows, in the wording "short fiber/fines fraction" shall be understood to mean only such a fiber fraction which provides the shorter mean fiber length after fractionating in comparison with the basic stock. The mean fiber length of the "short fiber/fines fraction" is preferably at < 1,0 mm, more preferably < 0,8 mm and most preferably < 0,6 mm. The mean fiber length can be determined by using so-called optical fiber length analysers (described in TAPPI method T271) . Optical fiber length analysers count the number of fibers that produce a discrete digital response from the measuring sensors. It is pointed out here that a "short fiber/ fines fraction" does not mean the same as the wording "short fiber pulp", by which usually "hardwood pulp" is to be understood. Moreover, the wording "short fiber/fines fraction" means such fibers which are present after fractionating in the "short fiber/ fines fraction" irrespective from the fiber quality used in the basic stock. Generally speaking, each fibrous material consists of fibers of different lengths, wherein, by fractionating each fibrous material is separated in fractions of different lengths. In determining the fiber length of different single fibers of a fibrous material as a result, there is in any case a fiber length distribution, since the length of the individual fibers is not even constant when the type of wood and the pulping methods are the same. "Long fibers" usually are fibers of a length > 1,8 mm and "short fibers" are fibers of a length of < 1,0 mm. "Fines" are of a length of about 0,2 mm and less.

Softness, haptic, absorption speed, absorption capacity, thickness etc. are negatively influenced by fines. The positive impact fines can have is on strength. The fines are coming with a pulp or are additionally generated by e.g. refining of the pulp. Tissue, especially when it is layered can have two different functional layers, one layer is supposed to deliver strength to the tissue, normally this is a so-called hood- layer, because it is positioned near the hoodside of the tissue, the other layer is the so-called Yankee-layer, because it is positioned near the Yankee-side of the tissue and should improve softness, haptic, absorption speed and capacity, and thickness. As a matter of fact, all kind of pulp contains fines even without any refining. Also the best pulps for softness (e.g. eucalyptus fibers) or absorption (CTMP-fibers) may have quite high fines levels. To keep the fines in the tissue is desirable for quality and economic reasons. Therefore, it is an advantage to change the sheet structure of a layered tissue by concentrating the fines in the hood-layer or strength layer and to reduce the fines level in the Yankee-layer or softness-layer. This is achieved by the above-mentioned fractionation and delivery of the short fiber/ fines fraction to the strength layer. Thus, softness, haptic, absorption speed and capacity, thickness and strength are improved in the tissue paper product.

These advantages are furthermore improved when the short fibers/ fines are concentrated in the strength layer and the short fibers/ fines are reduced in the softness-layer.

If white water originating especially from the forming section is transferred back to the furnishes upstream the headbox for dilution the afore-mentioned advantageous effect is again reduced. This is due to the fact that white water coming from a forming section is characterized by a high content of short fibers/fines (e.g. as a result of low retention} , which results in an undesirable increase of these short fibers /fines in the furnishes upstream the headbox. Therefore, a realistic and surprising effect is achieved for improving strength and softness when a retention-aid is added to at least one of the two furnishes upstream of the headbox, whereby the short fibers /fines content in the white water is reduced. Preferably the retention-aid is added to the strength layer to increase the content of the short fibers/ fines in this layer and to reduce the short fibers/fines content in the softness layer.

The term retention-aid refers to a process chemical compound supporting the retention of solid particles within a suspension on a sieve or wire, thereby improving the runability of the drying process and reducing abrasion of the material of the sieve or wire. Retention-aids are especially used during the process of papermaking in order to improve formation of the paper web. Useful retention-aids are e.g. inorganic products such as aluminium sulfate (Bentonit) , natural products such as cationic starch or synthetic organic polymers such as polyacrylic amide, polyethyleneimine, polyamido amine or polyethyleneoxide.

In this respect it is possible to add the retention-aid to both furnishes, however, it is more effective when added only to the first furnish.

Brief description of the drawings

Fig. 1 is a schematic view of pulp fractionation according to the invention,

Fig. 2 is a schematic view showing a method for manufacturing two-layer paper, Fig. 3 shows a similar schematic view of a method for manufacturing two- layer paper, and

Fig. 4-1, 4-2 and 4™3 graphs show by curves the fiber length distributions of long fiber furnish and short fiber furnish.

Fig. 2 and Fig. 3 are showing views being self-explanatory. As far as the methods for manufacturing paper and tissue are concerned the methods contain the usual paper and tissue manufacturing steps.

Embodiments o£ the invention

Pulp coming from the pulper is fractionated before refining such that only the long fiber fraction is mechanically treated in the refiner. The short fiber fraction is roughly 20% to 70% of the pulp, especially 40% to 60% of the pulp and contains most of the fines and short fibers. This short fiber fraction is separated upstream the refiner and bypasses the refiner without any mechanically treatment. Only the long fiber fraction will take a mechanical treatment in the refiner which is therefore more effective (see Fig. 1) .

Downstream the refiner either the mechanically treated long fiber fraction is mixed with the mechanically untreated short fiber fraction and this mixture is used for a single or multi layer paper product and especially tissue paper product. According to another possibility the short fiber fraction as such is used for one layer of a multi layer paper product and especially tissue paper product and the long fiber fraction mechanically treated in the refiner for another layer of such a multi layer paper product (see Fig. 1) .

The embodiments are illustrated for a two-layer paper and especially a two-layer tissue paper manufactured in a tissue paper-making machine optionally including wet creped or dry creped or through-air-drying (TAD) or any other manufacturing process. Normally two separate layers in the headbox are provided in the paper machine being supplied differently with furnish, e.g. by the first furnish and the second furnish as illustrated in Figs. 2 and 3. The first furnish is destined for the strength layer of the paper, independently on the fact whether a two-layer paper or a multi-layer paper with more than two layers is manufactured. The second furnish is destined for the softness layer being an outer layer. For the first furnish primarily softwood pulp is used and primarily for the second furnish hardwood pulp is used. The second furnish is fractionated and the such achieved short fibers/fines fraction is added to the first furnish in order to achieve in the manufactured paper a strength layer of higher strength. This causes simultaneously the manufacture of a softness layer of higher softness . Fractionating can be accomplished by standard processes, e.g. by screening in a screen cylinder. Fractionating should be carried out using a suitable fractionating device such as e.g. low-consistence washers, intermediate-consistency washers, high-consistency washers flotation cells, flotation machines, centrifugal cleaners, pressure screens and gravity screens.

In Fig. 4-1 is illustrated the fiber length distribution of a hardwood furnish (second furnish) and a softwood furnish (first furnish) . The hardwood furnish is fractionated leading to a short fiber/ fines fraction and a long fiber fraction (Fig. 4-2) . The short fiber/fines fraction is being transferred to the softwood furnish (first furnish) , thereby resulting in a mixture of softwood refined and hardwood short fiber/fines fraction (Fig, 4-3} .

The following examples are presented for average fiber length of hardwood furnish and softwood furnish.

In Figs. 2 and 3 with respect to the schematically illustrated two-layer paper the hood side and the Yankee side are indicated, which relates to the Yankee dryer and the position of the strength layer and the softness layer respectively in relation to the hood surrounding the Yankee cylinder and the Yankee cylinder itself. If a TAD-dryer is present in the paper machine the hood side is similar to the air side and the Yankee side is similar to the dryer side with respect to the TAD fabric running through the TAD drying section of the paper machine.

Experiments were made and the results are shown in Table 1.

The Table is self-explanatory and is not to be described in all detail. Strength is presented by dry tensile strength and dry breaking length and the Table 1 shows different results under different circumstances. The desintegrator time indicates the time used for disintegrating the cellulose pulp. Softwood refers to is long fibrous pine sulfate and eucalyptus is hardwood pulp.

Table 1 shows (lab number 26-2} that the addition of 10% hardwood short fiber/ fines fraction from the second furnish (softness layer) to the first furnish (strength layer) leads to higher strength.

By beating and disintegrating combined with beating time and disintegrating time strength can be changed for different kind of cellulose pulp, wherein generally softwood pulp is used for strength layer of paper and hardwood pulp for softness layer of a multi- layer paper, wherein strength layer means that strength is of priority and softness layer that softness is of priority.

Fig. 3 is a schematic view similar to the view in Fig. 2, added by two separate layers within one headbox for each furnish and a former of the paper machine. The white water from the former is returned to each furnish for dilution. A retention aid is added to the first furnish and optionally also to the second furnish. This is an essential feature in contrast to the method illustrated in Fig. 2.

If white water is returned to the furnishes the result with respect to strength and softness and especially strength achieved according to the method illustrated in Fig. 2 is lowered by return of white water to the furnishes, which will be explained later. This reduction of strength is compensated essentially by the retention-aid.

This is explained in more detail on the basis of Table 2

To keep the fines in a tissue paper is desirable for quality and economic reasons. Therefore, it is an advantage to change the sheet structure of a layered tissue by concentrating the fines in the so-called hood-layer and to reduce the fines level in the Yankee-layer. This is achieved by having the fractionation in the Yankee- layer and to add the short fiber/fines fraction to the hood-layer. As mentioned before, this is improving softness, haptic, absorption speed and capacity, thickness and strength. Part of this effect is decreased by diluting the pulps with the fines containing white water upstream the headbox. This dilution is increasing the fines content in both layers. Nevertheless, the fines content in the hood-layer is higher than the fines content in the Yankee-layer.

According to Table 2, calculating with a fines content of 3.3% in the Yankee layer (d) ) and a fines content of 11.8% in the Hood layer (c) ) both before dilution of the stock with white water I, and assuming a retention of 60%, there will be after dilution of the stock with white water I a fines content of 40.2% in the Yankee layer (now f) } and a fines content of 40.7% in the Hood layer (now e) ) . Layers a) to f) refer also to Fig. 3.

The increase of the fines content in the Yankee layer of from 3.3% to 40.2% (simultaneously of from 11.8% to 40.7% in the Hood layer) is due to the dilution of the stock with white water I, which contains high amounts of fines.

When increasing the retention from 60% to 95% by using retention-aids, the increase of the fines content in the Yankee layer caused by the dilution with white water I is from 3.3% (d) to 7.5% {f } ) . Accordingly, the fines content in the Hood layer increases from 11.8% (c) ) to 15.7% (e)) due to the dilution of the stock with white water I.

As it becomes apparent from the fines content of the final tissue paper, fractionating according to the invention results in an increasing difference of the fines content between the Hood layer and the Yankee layer of 0.1% {7.4% - 7.3% : no fractionation) to 1.1% (7.9% - 6.8% : fractionation) . This difference can further be increased by adding retention-aids thereby raising the retention of from 60% to 95%. The resulting difference is now 5.2% (IQ.1% - 4.9%) instead of 1.1% for a retention of 60%. According to Fig. 3, retention-aids can be added to the strength layer (c) ) as well as to the softness layer (d) ) . However, in order to enhance concentrating the fines content in the strength layer (c) ) retention-aids should preferably be added to the strength layer (c) ) . Retention-aids are normally added in an amount of approximately lOOg up to 50Og per ton of fibers.

The positive effect of pulp fractionation before refining according to the invention will become apparent in the following example:

A long fiber sulfite pulp was first fractionated into a long fiber fraction and a short fiber/ fines fraction. The resulting long fiber fraction was refined and finally mixed again with the earlier separated short fiber/ fines fraction (see Fig. 1) .

The resulting pulp mixture according to the present invention (A) was compared with a sulfite pulp before refining and fractionation (B') and another sulfite pulp wich was not fractioned before refining so that the whole pulp was directly transferred into the refiner (C) . The results of these comparisons are shown in Table 3.

Table 3

The results disclosed in Figure 3 indicate that a pulp treatment according to the present invention (A) produces already at low refining levels a pulp with reduced Schopper- Riegler Values (corresponding to lower water retention values) and enhanced values for thickness, tear strength and air permeability at similar strength levels compared with a pulp treatment omitting the fractionation step (C) - The resulting tissue products based on the pulp according to the present invention (A) are characterized by improved properties .

In addition it should be mentioned that the fractionated pulp according to the present invention (A) might be more resistant to any mechanical treatment thereby requiring an increased specific energy in the refiner compared with a pulp not being fractionated before refining (C ) ■ Nevertheless, when considering both fiber fractions and taking the refining energy of the total pulp quantities into account the specific energy for the fractionated pulp according to the invention should be lower. Due to this fact it should be possible to save energy when producing pulp according to the method of the present invention.

Claims

1. Method for pulp preparation and treatment including refining of the pulp in a refiner, characterized in that the pulp is fiber-fractionated upstream at least one refiner in that the short fiber fraction of the pulp containing most of the fines and short fibers is separated and bypass the refiner without mechanical treatment and that the remaining long fiber fraction is mechanically treated in the at least one refiner.

2. Method according to claim I, characterized in that a sulfite pulp is used.

3. Method according to claims 1 or 2 , characterized in that a pulp based on non wood materials is used.

4. Method according to claims 1 to 3 , characterized in that the non-treated short fiber fraction and the long fiber fraction mechanically treated in the at least one refiner are used in different layers of a multi-layer paper product .

5. Method according to claims 1 to 4, characterized in that downstream the refiner the short fiber fraction and the long fiber fraction mechanically treated in the refiner are mixed together.

6. Method according to claim 5, characterized in that the mixed fraction is used for a paper product.

7. Paper product produced as a multi layered product using a pulp achieved by the method according to claim 1 and 4.

8. Paper product produced as a single layer product using a pulp achieved by the method according to claims 1, 5 and 6.