Fluff pulp is normally made of softwood pulp which can be obtained from sulphate or sulphite pulping. Depending on the pulping method, the properties of fluff pulp differ from each other. For example, the fibres differ in thickness in such a way that fibres obtained from sulphite pulping are thinner than those obtained from sulphate pulping. Raw materials for softwood pulp may be various softwood species, such as pine (Pinus sylvestris), spruce (Picea abies), or mixtures of pine and spruce.

The aim is to produce from fluff pulp a pulp that can be well defibrated when dry, wherein in connection with the manufacture, it is possible to add agents which prevent the formation of fibre-fibre bonds, to make shredding of the pulp easier at the production stage. Demands set on a final product made of fluff pulp, such as dry paper, normally include soft feel, high bulk, high absorbency, high absorption rate, and high wet strength.

Fluff pulps made of softwood pulps have the problem that the length distribution of softwood fibres is large, i. e., the pulp contains very long fibres but also a relatively high content of noil. Long fibres impair the formation of products made of fluff pulp, and noil produces dust. Noil refers to fibres whose length is less than 0. 4 mm.

It is not possible to manufacture thin dry paper grades of softwood fluff pulp made by sulphate pulping, because the fibres are not properly bonded to each other but there are problems in the webs, e. g. due to splitting. Thin grades must be made of sulphite pulp in which the fibres are thinner and are thus better capable of bonding to each other. How- ever, sulphite pulp has become less available. Moreover, the brightness of softwood pulps is not at the desired level.

Fluff pulp is characterized in that it comprises pulp obtained from hard- wood. The method for manufacturing fluff pulp is characterized in that softwood pulp and hardwood pulp are mixed into a homogeneous pulp before the headbox. The product made of fluff pulp is characterized in that at least part of the product consists of a homogeneous mixture of softwood pulp obtained from sulphate pulping and hardwood pulp.

The method for manufacturing chemical pulp according to the invention can be used to produce pulp whose properties can be controlled in a stepless manner for a desired final product even at the stage of producing the pulp. Final products having a better appearance are achieved with a shorter average fibre length. A soft and smooth surface is achieved for dry paper products. In the product made of pulp produced by the method of the invention, the opacity and brightness are increased and the strength is improved. It is possible to run thin grades from sulphate pulp in a dry web machine, because short fibres make more fibre-to-fibre bonds than long fibres. The pulp according to the invention can be used to replace sulphite pulp which is presently poorly available. For the manufacture of the pulp according to the invention, it is possible to use a known apparatus.

By mixing hardwood pulp into softwood pulp, a pulp is obtained in which the content of noil remains substantially at the same level as in a corresponding raw material made wholly of softwood material. The average fibre length is shorter and the fibre distribution is narrower in hardwood pulp than in softwood pulp, wherein hardwood pulp contains short fibres of relatively the same length but which are, however, longer than 0. 4 mm. For example in sulphate birch pulp, the average fibre length is about 0. 90 mm, but the noil content is low, due to the shape of the fibre and the fact that there are no tracheids in birch. The formation of short fibres in a product which contains wood fibres is good, wherein the product becomes smooth and soft to feel. Smoothness and the soft feel are necessary for example in hygienic products, such as baby diapers, sanitary towels, panty liners, and incontinence protections. The product is provided with a good appearance, which is important, for example, in table setting products, such as table cloths made of dry paper.

Furthermore, the product can be easily embossed.

The mixture of softwood pulp and hardwood pulp is made in such a way that hardwood pulp is added into softwood pulp in a suitable mixing ratio before the headbox. The hardwood pulp is sludged in a first pulper, and the sludged fibre pulp is led at a suitable consistency to a second pulper.

Softwood pulp is sludged in the second pulper, and the hardwood pulp sludged in the first pulper is mixed with the softwood pulp. To make mixed pulps containing wood fibres from several hardwood species, there can be several first pulpes, and the fibre flow coming from them can be combined before adding to the softwood pulp. The soft-and hardwood pulps mixed with each other in a suitable ratio are led, as an aqueous suspension to which the necessary chemicals have been added, to the headbox. From the headbox, the fibre suspension is let onto a wire section on which a mixed pulp web is formed, fluid running through the wire. Water is first pressed out of the web by conveying the web in a press section, for example, through pressing roll nips, after which the web is led to a drying section to be dried, for example, by means of hot drying cylinders. After the drying section, the fluff pulp web dried to a suitable moisture content, normally a moisture content of less than 10 % weight percent, is cooled and reeled up.

The raw material of the softwood pulp used for making the mixed pulp can be any suitable softwood, such as pine (Pinus sylvestris), spruce (Picea abies), or a mixture of pine and spruce. The raw material of the hardwood pulp is preferably birch (genus Betula, different species), but also other hardwood species, such as aspen or eucalyptus, are feasible. It is also possible to use mixtures of pulps obtained from different hardwood species. In addition to wood fibres obtained from sulphate pulping, the mixed pulp may contain mechanical pulp or other fibres than wood fibres, for example short textile fibres intended for the manufacture of nonwoven fabrics.

Fluff pulp, which is a mixed pulp of softwood and hardwood, can be made by an apparatus known as such, which is advantageous, because no investments will be needed to convert the apparatus to be applicable for the production of the mixed pulp. As the first pulper, it is possible to use a pulper which is normally used for re-slushing of broke produced in connection with pulp production. The second pulper can be a normal

pulper used in the manufacture of pulp, in which the raw material of pulp production is disintegrated and mixed, and from which the fibre suspension is led, through intermediate steps, to the headbox.

The pulp is made from wood raw material by chemical sulphate pulping, known as such, which can be continuous or batch digesting. After the digesting, the pulp can be led, after washing, sorting and cleaning, directly to the production process of fluff pulp, wherein it is mixed into an aqueous suspension with a suitable consistency, or the pulp can be in the form of sheets which are re-slushed into an aqueous suspension.

Dry fluff pulp is used as a raw material for further processing, for example in the manufacture of dry paper or in another product which is made of dry pulp. The production of dry paper comprises the following steps : -Defibration of dry pulp into single fibres, -formation of a paper web and possible feeding of thermobonding fibres, -calendering to a desired thickness and embossing, -possible application of a binder, if a binder is used instead of thermo- bonding fibres, -drying and crosslinking of the web when a binder is used, or activa- tion of thermobonding fibres, -calendering, if necessary, reeling up, and -slitting and packing.

In the following, the invention will be described with reference to the appended drawings, in which Fig. 1 shows the process of producing a mixed pulp in a schematic view, and Fig. 2 shows a part of the production line of dry paper.

Figure 1 shows a schematic chart on the production process of fluff pulp, using re-sludging of pulp in sheet format. Hardwood pulp is sludged into an aqueous suspension in a first pulper, from which it is dosed and, after being diluted to a suitable consistency, preferably a consistency of about

3 %, led to a second pulper. The first pulper can be, for example, a machine pulper which is used for repulping of broke in a normal process of producing softwood pulp. The second pulper can be a normal pulper in the process. in the second pulper, softwood pulp is sludged to a suitable consistency, and the hardwood pulp and the softwood pulp are mixed with each other in the second pulper. After suitable pulp processing steps known as such, the fibre/water suspension is led to a headbox. Before the headbox, e. g., additives which can be for example chemicals effective on the strength of the fibre-to-fibre bonds, are added into the suspension.

To make, for example, a mixed pulp containing 60 % of softwood pulp and 40 % of birch pulp, the pulp production rate being 20 tons per hour, 8 tons of birch pulp are sludged per hour in the first pulper. When the birch pulp has a consistency of about 3 % in the first pulper, the fibre/water suspension is fed to the second pulper at a rate of 75 litres per second.

From the headbox, the water/fibre suspension is let out onto a wire, on which a fibre web is formed. The fibre web is first led to a press section which comprises presses by which water is pressed out of the fibre web, and the web structure is compressed. The presses may comprise, for example, roll nips and drying wires extending therebetween.

After the press section, the fibre web is led to a drying section which comprises drying cylinders. After the drying, the fibre web is cooled and reeled up. The ready fibre web is slitted and packed in a finishing section, after which it is ready to be used as fluff pulp, e. g. as raw material for the manufacture of dry paper.

Figure 2 shows a part of a dry web machine line. The chemical pulp is supplied in the form of a dry web to a hammer mill (not shown in the figure), inside which so-called hammers defibrate the pulp into single fibres. The obtained pulp of single fibres is pneumatically led into drum formers 1. There are normally two drum formers 1 one after the other, wherein the ready fibre web consists of two fibre layers formed on top of each other. The drum formers 1 rotate around their horizontal shaft which

is parallel to the width direction of a forming wire 12, and the fibres are discharged through the perforated housing of the drum former 1 onto the forming wire 12, through which an underpressure is effective from underneath the forming wire 12. The underpressure keeps the fibres appropriately in position on the forming wire 12. The fibre web is calendered to a suitable thickness at a suitable compression pressure by rolls2, and is embossed by rolls 3. A transfer wire 4 is arranged to transfer the fibre web from the web forming section to the web bonding section. A binder is applied onto the fibre web by spraying it by nozzles 5 onto the upper surface of the web. The fibre web is led to drying, where the binder is dried and crosslinked by means of heat. The lower surface of the web is treated in a corresponding manner after the treatment of the upper surface. After this, the fibre web can still be finished by calendering.

The ready fibre web is reeled up and forwarded to further processing, which comprises, for example, slitting, printing or other suitable further processing steps.

Instead of a binder, it is possible to use thermobonding fibres which are mixed in a suitable mixing ratio into the fibre pulp, for example by com- bining thermobonding fibres with the pulp flow of wood fibres. The thermobonding fibres are partly molten when they are treated by heat, thereby bonding the web.

Example 1.

Fluff pulp was prepared by the above-described method for preparing a mixed pulp. The content of hardwood pulp in the fluff pulp varied from 13 to 40 wt-% of the mass of dry substance. The softwood pulp was a mixture of pine and spruce. The hardwood pulp was birch. The used plasticizer was a chemical whose purpose was to reduce the formation of fibre-to-fibre bonds. The test results are shown in Table 1.

Table 1. Properties of fluff pulp made of a softwood/hardwood mixture. Sample 1 2 3 4 5 6 Property ; Moisture content (%) 8. 3 7. 8 7. 2 7. 7 7. 2 7. 1 Dry matter (gm2) 672 694 679 699 691 692 Density (/cm3) 0. 528 0. 519 0. 529 0. 525 0. 533 0. 527 Brightness 87. 4 87. 0 88. 0 87. 5 88. 5 88. 5 Fibre length (mm) 1. 86 1. 86 1. 76 1. 64 1. 61 1. 60 Noil content (%) 6. 7 6. 0 6. 8 6. 4 6. 7 6. 4 Content of long fibres (%) 47. 7 45. 7 41. 6 35. 8 33. 5 33. 3 (fibre length exceeding 1. 85 mm) I Bursting strength (kPa) 470 482 449 464 457 418 Shredding energy (MJ/t) 129 137 133 114 119 124 Bulk (cm3/g) 20.2 19.2 19.5 19.0 20.0 19.0 Absorption time (s) 5.2 5.3 5.1 5.0 5.2 5.0 Absorption capacity 9. 8 9. 5 9. 8 9. 2 9. 4 9. 3 (g H2O / g of chemical pulp) Strength of fibre network (N) 5.23 5.66 5.75 5.72 4.92 5.49 Plasticizer 0. 70 0. 71 0. 59 0. 70 0. 55 0. 69 (weight-% of dry matter) Birch pulp (%) 13. 4 18. 2 24. 5 33. 9 38. 7 39. 4

The fines content remains substantially at the same level, even though the content of fibres longer than 1. 85 mm is radically reduced and the average length of the fibres is reduced. The brightness is increased as the content of hardwood pulp is increased.

Example 2.

Fluff pulp was prepared by the above-described method for preparing a mixed pulp. The content of hardwood pulp in the fluff pulp varied from 22 to 69 wt-% of the mass of dry substance. The softwood pulp was a mixture of pine and spruce. The hardwood pulp was birch. The used plasticizer was a chemical whose purpose was to reduce the formation of fibre-to-fibre bonds. The test results are shown in Table 2.

Table 2. Properties of fluff pulp made of a softwood/hardwood mixture.

Sample 7 8 9 10 11 Property >1 Moisture content (%) 7. 5 7. 4 7. 0 7. 0 7. 3 Brightness 83. 0 86. 7 88. 4-89. 3 Fibre length (mm) 1. 76 1. 48 1. 38 1. 34 1. 32 Noil content (%) 6. 8 7. 1 7. 1 6. 9 7., 0 Content of long fibres (%) 47.0 7.1 23.9 21.8 20.8 Bursting strength (kPa) - - 455 440 506 Bulk (cm3/)-19. 5 19. 5 19. 0 19. 0 Absorption time (s) 5. 1 4. 5 4. 8 4. 7 Absorption capacity-9. 0 9. 0 9. 0 8. 9 H20 of chemical ulp) Plasticizer (wt-% of dry 0. 70 0. 68 0. 59 0. 40 0. 36 matter) Birch pulp (%) 22. 9 45. 2 55. 8 61. 5 63. 4 Table 2. continued. Sample 12 13 14 15 16 Property 2 Moisture content (%) 7. 5 7. 4 7. 5 7. 5 7. 5 Brightness 89. 4--89. 0 Fibre length (mm) 1. 31 1. 30 1. 28 1. 30 1. 26 Noil content (%) 7. 0 7. 2 7. 3 6. 8 7. 5 Content of long fibres (%) 19. 9 19. 7 18. 9 19. 1 18. 0 Bursting strength (kPa) 498 493 536 466 500 Bulk (cm3/) 18. 5 19. 0 18. 5 19. 0 19. 0 Absorption time (s) 4. 4 4. 6 4. 4 4. 7 4. 5 Absorption capacity (gH20/9. 0 8. 7 9. 2 8. 9 9. 1 of chemical pulp) Plasticizer (wt-% of dry 0. 29 0. 35 0. 29 0. 29 0. 30 matter) Birch pulp (%) 66. 3 66. 6 67. 6 68. 0 69. 0

The results show that the fines content is not substantially increased, even though the average fibre length is short with high contents of hardwood pulp compared with softwood pulp. The brightness is very high.

Example 3.

The properties of final products made of hardwood/softwood pulp obtained from sulphate digestion, softwood pulp obtained from sulphate digestion, and softwood pulp obtained from sulphite digestion were compared with each other. The final product was dry paper whose grammage was 70 g/m2. The ratio of the hardwood/softwood mixture was 60 : 40 (in weight percent). The hardwood pulp was birch pulp. The results of the comparison are shown in Table 3.

Table 3. Properties of dry paper using different raw materials. Sample < Hardwood/Softwood pulp Softwood pulp Property softwood pulp (sulphate) (sulphite) (sulphate) thickness (mm) 0. 48 0. 60 0. 48 tensile strength 0. 60 0. 60 0. 40 (kN/m) tensile strength 0. 35 0. 35 0. 20 when wet (kN/m) brightness 83 79 83 % ISO) opacity (%) 75 70 70

The results show that the tensile strengths of dry paper made of hard- wood/softwood, both dry and wet, are on the same level as those of dry paper made of softwood pulp obtained from sulphate digestion, whereas the corresponding values of dry paper made of softwood pulp obtained from sulphite digestion are lower. The brightness of dry paper made of hardwood/softwood pulp is better than that of dry paper made of softwood pulp obtained from sulphate digestion and at the same level as that of dry paper made of softwood pulp obtained from sulphite digestion. The opacity of dry paper made of hardwood/softwood pulp is better than that of either of the other dry paper products.

Furthermore, from the above-mentioned reference products it can be stated that they have a good absorption rate, about 1 s/0. 1 ml for all grades. The absorption capacity is directly proportional to the thickness, wherein the absorption capacity of dry paper made of hardwood sulphate/ softwood sulphate pulp and sulphite pulp is lower than that of dry paper made of softwood sulphate pulp. However, the absorption capacity of dry paper made of hardwood/softwood pulp is sufficient for certain products, such as table setting and hygienic products.

When added in softwood pulp, hardwood pulp yields a dense structure in which there are a lot of contact points between fibres, thereby imprpoving the strength of the product. Furthermore, the product has an even,

smooth and soft surface, and its printability is good. The product also has an advantageous capillary structure, which is beneficial in liquid transfer, wherein the use of the products is advantageous, for example, in liquid diffusing layers of hygienic products.

Hygienic products, in which the fluff pulp according to the invention can be used, include, for example, disposable hygienic products, such as baby diapers, sanitary towels, panty liners, incontinence protections, and hospital beddings.

The above description does not restrict the invention, but the invention may vary within the scope of the claims. The first pulper does not necessarily need to be a machine pulper intended for sludging of broke, but it can also be a feeding tank constructed for the purpose. There can be several first pulpers in parallel in such a way that each provides a slightly different pulp in a suitable mixing ratio. The parallel first pulpers can also be arranged in such a way that softwood pulp is sludged into at least one first pulper. In the product made of fluff pulp, such as a dry paper product, at least one of the product layers can be a layer made of normal softwood pulp and the other layers made of mixed pulp. The product made of mixed pulp is not necessarily dry paper, but it can be another product to be made of fluff pulp, requiring properties that can be obtained with mixed pulp. The main idea is that the properties of fluff pulp obtained from sulphate digestion can be controlled by controlling the softwood/hardwood ratio of the fluff pulp, and from the obtained fluff pulp, it is possible to make products with improved properties.