US20090205794A1

US20090205794A1 - Method for the production of tissue paper

Info

Publication number: US20090205794A1
Application number: US12/429,692
Authority: US
Inventors: Thomas Scherb; Luiz Carlos Silva; Rogerio Berardi; Dailo Oyakawa
Original assignee: Voith Paper Patent GmbH
Current assignee: Voith Patent GmbH
Priority date: 2005-01-08
Filing date: 2009-04-24
Publication date: 2009-08-20
Also published as: DE102005036075A1; US20060266487A1; EP1749935A2; EP1749935A3

Abstract

A method for the production of a tissue paper web, which is produced from a pulp suspension comprised of fibers, the pulp suspension having a refining degree of less than 21°SR and being of such condition that it is possible to produce from said pulp suspension a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at least a specific refining degree (RD_specific) is greater than or equal to the value resulting from 0.55 (km/SR)*(RD_specific−10°SR), whereby the specific refining degree is selected from a refining range from 15°SR to less than 21°SR, in particular 15°SR to 19°SR.

Description

This is a division of U.S. patent application Ser. No. 11/497,061, entitled “METHOD FOR THE PRODUCTION OF TISSUE PAPER”, filed Aug. 1, 2006, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a method for the production of tissue paper and to a method for the production of a pulp suspension for use particularly for the production of tissue paper.
2. Description of the Related Art
Tissue paper ideally displays a high absorbency and a high water absorption capacity coupled with a high tear resistance. The absorbency and water absorption capacity are defined essentially by the volume and porosity of the tissue paper. To increase the volume it was already proposed in the prior art in WO03/062528 to press the tissue paper web during production only on a zone basis in order to obtain only slightly pressed or unpressed voluminous regions and pressed regions of greater tear resistance.
The porosity and the permeability of the tissue paper are co-defined essentially by the refining degree of the fibers in the pulp suspension from which the tissue paper is produced. Here a high refining degree gives rise to a high proportion of fines in the suspension, leading to a low porosity and permeability of the produced tissue paper web. Also, a high refining degree gives rise to a high water retention value for the fibers of the pulp suspension, making the tissue paper hard to dewater during its production process. Furthermore, a high refining degree, meaning a high proportion of fines, reduces through too low a porosity and permeability the dewaterability of the tissue paper web during its production, leading often to too low a dry content for high production speeds such as 1200 m/minute. For example, a certain dry content is required upstream from the Yankee drying cylinder in order to prevent the tissue paper web lifting off the Yankee drying cylinder due to the evaporation of water from the tissue paper web as the result of its contact with the hot circumferential surface of the tissue drying cylinder. This effect has a particularly strong impact particularly in the case of tissue paper of the type that is compressed only in some regions during its production (so-called bulky tissue), as the liquid remains stored particularly in the uncompressed or only slightly compressed regions when the tissue paper is only insufficiently dewatered due to too low a porosity and permeability.
On the other hand, the tissue paper has to be tear-resistant both in the unfinished state and as a finished product. The tear resistance is influenced by both the production process and the refining degree of the fibers in the pulp suspension. To increase the tear resistance the tissue paper has to be compacted, meaning compressed, during its production. Also, the proportion of fines must be high in order to obtain a high tear resistance. The requirements imposed on the tear resistance conflict in particular with the above mentioned requirements imposed on the water absorption capacity, the absorbency and the dewaterability.

SUMMARY OF THE INVENTION

The present invention is directed to a method for the production of a tissue paper web, with which it is possible to produce tear-resistant tissue paper with a high water absorption capacity and absorbency at a high level of productivity.
Also, the present invention provides a method for the production of a pulp suspension which can be used in the method according to the invention for the production of a tissue web.
The method according to the invention for the production of a tissue paper web, which is produced from a pulp suspension composed of fibers, provides for the pulp suspension to have a refining degree of less than 21°SR and to be of such condition that it is possible to produce from the pulp suspension a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at least a specific refining degree (RD_specific) is greater than or equal to the value resulting from 0.55 (km/SR)*(RD_specific−10°SR), whereby the specific refining degree is selected from a refining range from 15°SR to less than 21°SR, in particular 15°SR to 19°SR.
Tests have indicated that particularly in the case of tissue paper which during its production is more intensively compressed in some regions than in others, sufficient porosity and dewaterability exist when the refining degree of the fibers in the pulp suspension is less than 21°SR (Schopper Riegel). As tests have shown, sufficient tear resistance of the tissue paper web is provided when it is possible to produce from the pulp suspension a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at least a specific refining degree (RD_specific) is greater than or equal to the value resulting from 0.55 (km/SR)*(RD_specific−10°SR), whereby the specific refining degree is selected from a refining range from 15°SR to less than 21°SR, in particular 15°SR to 19°SR. The laboratory sheet produced according to TAPPI 205 SP 95 (Rapid Köthen) has a gsm substance of 60 g/m².
From the dependency of the breaking length on the refining degree in the function 0.55 (km/SR)*(RD_specific−10°SR) there results a sharp rise in the change of breaking length when the refining degree increases slightly. It has turned out that precisely those types of pulp suspensions in which the dependency of the breaking length on the refining degree is greater than or equal to the previously indicated functional dependency are particularly suitable for the production of tissue paper, in particular such tissue paper that is less intensively pressed in some regions than in others.
Also, it has turned out that only little refining energy is required to produce the pulp suspension with the above mentioned properties for obtaining the required strength values.
Tests have shown that a sufficient porosity of the produced tissue paper exists when the pulp suspension has a refining degree less than or equal to 19°SR, preferably less than or equal to 17°SR. Furthermore, tests have shown that the finished product has sufficient strength and can be produced at a high level of productivity, meaning at a high machine speed, when the breaking length of the laboratory sheet produced according to TAPPI 205 SP 95 (Rapid Köthen) at a specific refining degree (RD_specific) from 0.55°SR to 19°SR, preferably 15°SR to 17°SR is greater than or equal to the value which results from 0.55 (km/SR)*(RD_specific−10°SR).
The refining degree of the pulp suspension used is preferably 19°SR or less, in particular preferably 17°SR or less. Through the reduction of the refining degree it is also possible to increase the dry content by 3-4% upstream from the Yankee drying cylinder.
Tests have shown that the tissue paper can be produced at a high machine speed, meaning 1200 m/min or more, when it is possible to produce from the pulp suspension at a maximum refining degree of 17°SR a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) with a breaking length of 4.0 km or more, preferably 4.3 km or more, measured according to TAPPI 220 and TAPPI 494.
Tests have shown that the pulp suspension meets the requirements imposed on porosity, absorbency and tear resistance in particular when the fiber fraction of the pulp suspension is composed of cellulose for the greater part, preferably to 60% or more, in particular preferably 80% or more. Most particularly it is preferred for the fiber part of the pulp suspension to be formed from 100% cellulose.
Tests have shown that the tissue paper web can be effectively dewatered during its production in order to obtain a satisfactory dry content when the fibers of the pulp suspension at a refining degree of 17°SR have a water retention value of 1.5 g/g or less, preferably 1.4 g/g measured according to TAPPI UM 256. Depending on whether, for example, toilet paper or towel paper is to be produced with the method according to the invention, the pulp suspension used can be comprised of softwood and/or hardware alone or in various proportions. If toilet paper is to be produced with the method according to the invention, the fiber content of the pulp suspension used can be comprised of preferably around 30% softwood and around 70% hardware for example. If towel paper is to be produced with the method according to the invention, the fiber content of the pulp suspension comprises preferably around 70% softwood and around 30% hardware for example. Good results for towel paper are also obtained when the fiber content of the pulp suspension used comprises around 70% software and around 30% chemi-thermomechanical pulp (CTMP).
The method according to the invention is particularly effective with regard to increasing the dewaterability during production and the water absorption capacity and absorbency of the finished product at a satisfactory level of tear resistance when the tissue paper web is produced such that it includes regions which are more intensively compressed than others during production.
If the tissue paper web is to include regions compressed with various intensities, it makes sense for the tissue paper web to be formed already from the pulp suspension on a structured, in particular 3-dimensionally structured mesh.
On such a structured mesh the side facing the tissue paper web has at least in some areas depressed regions and, relative to the depressed areas, raised regions, whereby the tissue paper web is formed at least in areas in the depressed and raised regions of the structured mesh. The areas of the tissue paper web formed in the depressed regions of the structured mesh have a higher volume and gsm substance in this case than the areas formed in the raised regions of the mesh.
A 3-dimensional tissue paper web is formed as the result. Here the tissue paper web has voluminous pillow areas with a high gsm substance formed in the depressed regions of the structured mesh and less voluminous areas formed in the raised regions of the mesh.
The structure mesh can include a TAD mesh or a DSP mesh. The advantage of a TAD mesh is its high permeability, thus guaranteeing quick dewatering during the forming operation.
With regard to the structure of the structured mesh and with regard to the formation of the tissue paper web on the structured mesh, reference is made to PCT/EP2005/050203, which is incorporated by reference herein.
After the formation of the tissue paper web, the tissue paper web is conveyed preferably in a dewatering step between an upper structured, in particular 3-dimensionally structured, and permeable skin and a lower permeable skin, whereby pressure is exerted on the upper skin, the tissue paper web and the lower skin during the dewatering step along a dewatering section.
The pressure exerted here on the arrangement of an upper structured and permeable skin, tissue paper web and lower permeable skin can be generated by a gas flow. In addition or alternatively to this, the pressure exerted can be generated by a mechanical pressing force.
To compress the tissue paper web only in some regions by the application of pressure and thus produce a tissue paper with a high volume for good absorbency in some regions—in the unpressed or less pressed regions—and with high strength in other regions—in the more intensively pressed regions—it makes sense for the side of the structured skin facing the tissue paper web to include depressed regions and raised regions relative to the depressed regions. Consequently, as previously mentioned, the tissue paper web is compressed less intensively in the depressed regions than in the raised regions.
The upper structured and permeable skin is preferably a structured mesh, in particular a TAD mesh or DSP mesh, and the lower permeable skin is preferably a felt having a sufficiently high water absorption capacity for the water which is pressed out of the tissue paper web. With regard to the structure of the lower skin, reference is made to PCT/EP2005/050198, which is incorporated by reference herein.
The compressibility (change of thickness in mm upon application of force in N) of the upper skin is preferably smaller than the compressibility of the lower skin. The voluminous structure of the tissue paper web upon the application of pressure is thus retained.
Tests have shown that a particularly good and gentle dewatering is possible when the dynamic rigidity (K)—as a measure for the compressibility of the upper skin—is 3000N/mm or more.
Given a hard or excessively hard lower skin, the voluminous pillow areas of the tissue paper web would not be compressed at all. Because of the compressible structure of the lower skin the voluminous pillow areas of the tissue paper are slightly pressed and hence gently dewatered. Tests in this connection have shown that the dynamic rigidity (K)—as a measure for the compressibility of the lower skin—is 100000N/mm or less, preferably 90000N/mm, in particular preferably 70000N/mm or less. Similarly it is an advantage for the G modulus—as a measure for the elasticity of the lower skin—to be 2N/mm²or more, preferably 4N/mm²or more.
Also, tests have shown that the water stored in the lower skin, for example felt, can be expelled more easily with a gas flow when the permeability of the lower skin is not too high. It proves to be an advantage when the permeability of the lower skin is 80 cfm or less, preferably 40 cfm or less, in particular preferably 25 cfm or less. In the above mentioned ranges the rewetting of the tissue paper web by the lower skin is largely prevented.
In the dewatering step, preferably the upper skin is first charged with gas, then the tissue paper web and finally the lower skin. The dewatering of the paper web takes place in this case in the direction of the lower skin.
Optionally or in addition to gas charging of the above mentioned arrangement, provision can be made for the arrangement of upper skin, tissue paper web and lower skin to be conveyed during the dewatering step at least in some areas along the dewatering section between a tensioned press belt and a smooth surface, whereby the press belt acts on the upper skin and the lower skin rests on the smooth surface. In this case, too, the dewatering of the paper web takes place in the direction of the lower skin.
The arrangement of upper skin, tissue paper web and lower skin is preferably charged with the gas flow at least in some areas in the region of the dewatering section so that the dewatering takes place simultaneously by the pressing force of the press belt and the through-flow of gas.
Tests have shown that the gas flow through the tissue paper web amounts to approx. 150 m³per minute and meter length along the dewatering section. Here the gas flow can be generated by a suction zone in a roller. In this case the suction zone has a length in the range between 200 mm and 2500 mm, preferably between 800 mm and 1800 mm, in particular preferably between 1200 mm and 1600 mm, and the vacuum in the suction zone amounts to between −0.2 bar and −0.8 bar, preferably between −0.4 bar and −0.6 bar. Optionally or in addition to this, the gas flow can also be generated by an excess pressure hood arranged above the top skin.
In the latter case the temperature of the gas flow amounts to between 50° C. and 180° C., preferably between 120° C. and 150° C., and the excess pressure amounts to less than 0.2 bar, preferably less than 0.1 bar and in particular preferably less than 0.05 bar. The gas can be hot air or steam.
The pressing force can be increased by a high tension of the press belt. Tests have shown that sufficient dewatering particularly of the non-voluminous areas of the tissue paper is obtained when the press belt is under a tension of at least 30 kN/m, preferably at least 60 kN/m or 80 kN/m. Here the press belt can have a spiralized structure and be constructed as a so-called spiral link fabric for example. Furthermore it is possible for the press belt to have a woven structure.
To be able to obtain a good dewatering of the tissue paper web by the mechanical tensioning of the press belt and as the result of the gas flow through the press belt it makes sense for the press belt to have an open area of at least 25% and a contact area of at least 10% of its total area facing the upper skin. A uniform mechanical pressure is exerted on the arrangement of structured upper skin and lower skin by increasing the contact area of the press belt.
Satisfactory results are obtained with all the values stipulated below for the contact area and open area of the press belt.
Provision is made accordingly for the press belt to have an open area of between 75% and 85% and a contact area of between 15% and 25% of its total area facing the upper skin. Also, provision is made for the press belt to have an open area of between 68% and 76% and a contact area of between 24% and 32% of its total area facing the upper skin.
Very good results with regard to dry content and voluminosity of the tissue paper are obtained when the press belt has an open area of between 51% and 62% and a contact area of between 38% and 49% of its total area facing the upper skin.
In particular through the construction of the press belt with a woven structure it is possible for the press belt to have an open area of 50% or more and a contact area of 50% or more of its total area facing the upper skin. As such it is possible to provide for a good gas flow through the press belt as well as a homogeneous pressing force by means of the press belt.
The smooth surface is formed preferably by the circumferential surface of a roller.
Through the above described dewatering operation it is possible for the tissue paper web to leave the dewatering section with a dry content of between 25% and 55%.
To ensure that the voluminous areas of the tissue paper are only slightly pressed during the dewatering step it makes sense for the structured mesh in the dewatering step to be the same mesh as in the formation of the tissue paper web. As a result, the voluminous pillow areas of the tissue paper web remain in the depressed regions of the structured mesh during application of the pressure such that the voluminous areas are largely protected against the application of pressure and far less pressure is exerted on these areas than on the areas of the tissue paper web lying in between. The voluminous structure of the pillow areas is thus retained during the dewatering step.
After the dewatering step the tissue paper web is preferably conveyed together with the structured skin of the dewatering step through a press nip in a further dewatering step and dewatered further.
Furthermore, the tissue paper web in the press nip is preferably arranged between the structured and permeable upper skin and an in particular smooth and heated roller surface, in which case the heated and smooth surface is preferably formed by the circumferential surface of a Yankee drying cylinder.
Transferring the tissue paper web on the structured upper skin through the press nip ensures that the voluminous pillow areas of the tissue paper are less intensively pressed than the areas lying in between during this dewatering step as well.
The depressed and by comparison relatively raised areas of the structured and permeable skin are constructed and arranged in relation to each other such that only 35% or less, in particular only 25% or less of the tissue paper web is pressed in the press nip.
If, as previously mentioned, the structured upper skin is the same structured skin as that on which the tissue paper was formed, then the 3-dimensional structure of the tissue paper is created already during the formation. By contrast, with the method according to the prior art the 3-dimensional structure of the tissue paper is not formed until during a subsequent dewatering step by the tissue paper web being pressed into a structured mesh, thus forming an essentially two-sided corrugated tissue paper.
With the method according to the invention, the formation of the tissue paper between the structured skin and a forming mesh with a relatively smooth surface forms a tissue paper web which is essentially smooth on the side which was formed on the smooth forming mesh. On passing through the press nip this side comes into contact with the circumferential surface of the Yankee drying cylinder, in which case the relatively large contact area compared to the prior art prevents the tissue paper web from burning at the high temperatures of the Yankee drying cylinder. As the result, the temperature of the Yankee drying cylinder can be raised compared to the prior art, leading to a higher dry content of the tissue paper web produced.
In the interest of gentle pressing in the press nip it makes sense for the press nip to be an elongated press nip, meaning that it is formed by the roller surface and a shoe press unit.
If the aim is to increase the dry content, which occurs at the expense of the voluminosity of the produced tissue paper web, then provision can be also be made for the press nip to be formed by a suction press roller and the roller surface instead of by the shoe press unit and the roller surface.
To remove water, which is carried in the structured upper skin and which obstructs dewatering in the press nip, it makes sense for the tissue paper web to be conveyed together with the structured skin around an evacuated deflector roller, whereby the structured skin is arranged between the tissue paper web and the evacuated deflector roller.
Various possibilities for the composition of the pulp suspension are conceivable. According to a possible embodiment of the invention the pulp suspension used has a suspension fraction which was produced from a low-consistency feed pulp suspension of high strength with a consistency of less than 10%. In this case this suspension fraction has a refining degree of 15°SR or more and was produced by at least one refining pass from the low-consistency feed pulp suspension of high strength at a refining degree of less than 15°SR.
A low-consistency feed pulp suspension of high strength is understood to be one from which it is possible to produce a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at a refining degree of 15°SR is greater than 3.0 km.
Furthermore it is possible for the pulp suspension used to have a suspension fraction which was produced from a high-consistency feed pulp suspension of low strength with a consistency of 20% or more, preferably 20% to 40%, in particular preferably 25% to 35%.
In this case this suspension fraction has a refining degree of 15°SR or more and was produced by at least one refining pass from the high-consistency feed pulp suspension of low strength with a refining degree of less than 15°SR.
The high-consistency feed pulp suspension of high strength can be produced, for example, from a low-consistency feed pulp suspension of low strength through concentration of the same.
A low-consistency feed pulp suspension of low strength is understood to be one from which it is possible to produce a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at a refining degree of 15°SR is less than or equal to 3.0 km.
Disclosed in addition is a method for the production of a pulp suspension composed of fibers, with which at least one suspension fraction is produced with the following steps:
provision of a high-consistency feed pulp suspension comprised of fibers and having a consistency of more than 20% and a refining degree of less than 15°SR,
refinement of the feed pulp suspension to a refining degree of 15°SR or more to obtain the suspension fraction.
With the method according to the invention it is possible to produce from a pulp suspension a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at least a specific refining degree (RD_specific) is greater than or equal to the value resulting from 0.55 (km/SR)*(RD_specific−10°SR), whereby the specific refining degree is selected from a refining range from 14°SR to less than 21°SR, in particular 15°SR to 20°SR.
Accordingly, with the method according to the invention it is possible in particular to produce a pulp suspension from which tissue paper with a high tear resistance at high porosity and permeability can be produced using a production process in which the tissue paper web is compressed to only 35% or 25% during its production.
It is possible not only for the pulp suspension to be provided from a suspension produced from a high-consistency feed pulp suspension but also for this suspension fraction (first suspension fraction) to be mixed with a suspension fraction (second suspension fraction) produced from a low-consistency feed pulp suspension with a consistency of less than 10% in order to produce the pulp suspension.
In this case the second suspension fraction preferably has a higher refining degree than the first suspension fraction.
According to a concrete embodiment of the invention the high-consistency feed pulp suspension has a refining degree of 12°SR to 13°SR and the suspension fraction produced therefrom a refining degree of 15°SR to 19°SR, preferably 15°SR to 17°SR.
To obtain the required strength it can make sense to perform the refining operation several times in succession.
The best results with regard to the strength obtained with a low refining degree are obtained when the high-consistency feed pulp suspension is refined with a refining energy in the range from 150 kWh to 300 kWh, in particular 180 kWh to 250 kWh per ton.
Furthermore, tests have shown that a high strength can be obtained for the pulp suspension with a low refining degree when the high-consistency feed pulp suspension has a consistency in the range of more than 20% to 40%, preferably 25% to 35%.
Furthermore it make sense for enzymes and/or agents for increasing the dry strength, so-called “Dry Strength Agents” (DSAs) and/or agents for increasing the wet strength, so-called “Wet Strength Agents” (WSAs), to be added to the low-consistency feed pulp suspension. In such a case, for example with a low-consistency feed pulp suspension of high strength, it is possible to dispense completely with refining. By adding DSAs it is possible to reduce further the refining degree in the pulp suspension while retaining the tear resistance. Carbon methyl cellulose, for example, can be used as DSA.
The high-consistency feed pulp suspension can be obtained through concentration of a low-consistency feed suspension, whereby the concentrating can be performed by means of a worm extruder for example.
Here it has proven advantageous for the enzymes to be added to the low-consistency feed pulp suspension at a temperature in the range from 25° C. to 70° C., preferably 30° C. to 60° C., in particular preferably around 35° C. to 45° C., as their effectiveness is highest in this temperature range.
Similarly, to increase the effectiveness of the enzymes it makes sense for the enzymes to be added to the low-consistency feed pulp suspension with a pH-value in the range from 5 to 8, preferably 5.5 to 7.5, in particular preferably around 6.5 to 7.
Good results are obtained when the enzymes are allowed to work for a period of 1-2 hours, preferably 1.5 hours, on the low-consistency feed suspension. The enzymes can be added in the pulper, for example.
To control the above mentioned advantageous properties of the pulp suspension it makes sense for the high-consistency feed pulp suspension to be refined at a temperature in the range between 20° C. and 80° C., preferably at 40° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows an apparatus for the production of a pulp suspension according to the invention.

FIG. 2 shows a comparison between pulp suspensions according to the invention and others from the prior art.

FIG. 3 shows a partial representation of an apparatus for performing the method according to the invention for the production of tissue paper.

FIG. 4 shows the structure of a tissue paper web upon its formation with the method according to the invention.

FIG. 5 shows the structure of a tissue paper web upon its formation with one of the known methods according to the prior art,

FIG. 6 shows the structure of a tissue paper web upon its dewatering with the method according to the invention,

FIG. 7 shows the structure of a tissue paper web upon its 3-dimensional structuring with one of the known methods according to the prior art,

FIG. 8 shows the structure of a tissue paper web upon its dewatering in the press nip with the method according to the invention,

FIG. 9 shows the structure of a tissue paper web upon its dewatering with one of the known methods according to the prior art,

FIG. 10 shows a first apparatus for performing the method according to the invention,

FIG. 11 shows a second apparatus for performing the method according to the invention.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an apparatus 1 according to the invention for providing a pulp suspension which is subsequently used in the method according to the invention for the production of a tissue paper web.
Apparatus 1 includes a pulper 2 in which the dry raw stock, halfstuff and old paper is dissolved in water and converted into a pumpable state. The pulp thus formed is then conveyed to a mixing chest 3. At this stage the pulp has a consistency of less than 10%, i.e. as a rule 5% or less, and in this connection is referred to as low-consistency feed pulp.
Depending on whether the low-consistency feed pulp is one with high strength or one with low strength, the pulp is subjected to a refining process at low consistency or a refining process at high consistency.
If it is a low-consistency feed pulp of low strength, for example, it is possible to produce from the pulp at a refining degree of 15°SR a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) with only a small breaking length of less than or equal to 3.0 km. Such a feed pulp suspension is subjected to a refining process at high consistency. For this purpose the low-consistency feed pulp is conveyed to a concentrator 4, which can be constructed as a worm extruder for example, and is concentrated therein from a consistency of 5% to a consistency of 25% to 35% for example, ideally around 30%, thus producing a high-consistency feed pulp suspension.
In this case the high-consistency feed pulp suspension usually has a refining degree from 12°SR to 13°SR. Then the high-consistency feed pulp suspension is heated in a heating channel 5 to a temperature up to 80° C., ideally around 40° C., and finally conveyed to a refiner 6 for refining.
During the refining operation the high-consistency feed pulp suspension is refined to a refining degree of 15°SR or more. In this case the high-consistency feed pulp suspension thus produced usually has a refining degree in the range from 16°SR to 19°SR, preferably in the range from 16°SR to 18°SR.
To obtain the high-consistency pulp suspension, the high-consistency feed pulp suspension is refined with a total refining energy in the range from 150 kWh to 300 kWh, in particular 180 kWh to 250 kWh per ton, whereby it is conceivable for the refining operation to be performed in one step or in several refining steps in succession. Enzymes and agents for increasing the dry strength (DSAs) can be added to the pulp prior to the refining operation, for example in the pulper 2. Here it proves to be particularly advantageous with regard to the desired properties of the subsequently formed tissue paper web in terms of its porosity and permeability coupled with high tear strength for the enzymes to be added to the low-consistency feed pulp suspension at a temperature in the range from 25° C. to 70° C., preferably 30° C. to 60° C., in particular preferably around 35° C. to 45° C., whereby the low-consistency feed suspension has a pH-value in the range from 5 to 8, preferably 5.5 to 7.5, in particular preferably around 6.5 to 7, and the enzymes are allowed to work on the low-consistency feed suspension for 1-2 hours, preferably 1.5 hours.
The pulp suspension obtained from the high-consistency refining operation is then diluted in a dilution tank 7 with water which is obtained at least in part during concentration of the low-consistency feed pulp suspension in the concentrator 5.
The re-diluted pulp thus obtained is then conveyed to a stock chest 8. The pulp suspension obtained from the high-consistency refining operation can be mixed in the stock chest 8 with a suspension which was obtained by a refining operation at low pulp consistency, meaning at less than 20%.
In the stock chest 8 it is thus possible to form a pulp suspension which includes one suspension fraction produced with a high-consistency refining operation and another suspension fraction which was refined at a consistency of less than 20%. In this case the suspension fraction which was refined from a low-consistency feed pulp has a higher refining degree than the suspension fraction which was refined from a high-consistency feed pulp. Needless to say it is also possible for the pulp suspension to include only the suspension produced with the high-consistency refining operation. Furthermore, it is also conceivable for the low-consistency feed pulp already to have a high strength such that the pulp suspension comprises only one pulp suspension fraction produced with a refining operation at low consistency. A low-consistency feed pulp of low strength is understood in this connection to be a pulp from which it is possible at a refining degree of 15°SR to produce a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) with a breaking length of 3.0 km or more.
Downstream from stock chest 8 the pulp suspension is greatly diluted with mesh water 9 and conveyed to a headbox 10.
Regardless of how the pulp suspension is obtained it is important for the production of tissue paper for the pulp suspension emerging from headbox 10 to have a refining degree of less than 21°SR and to be of such condition that it is possible to produce from said pulp suspension a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at least a specific refining degree (RD_specific) is greater than or equal to the value resulting from 0.55 (km/SR)*(RD_specific−10°SR), whereby the specific refining degree is selected from a refining range from 15°SR to less than 21°SR, preferably 15°SR to 19°SR, in particular preferably 17°SR to 19°SR.
FIG. 2 shows the dependency of the breaking length on the refining degree for various pulps. In FIG. 2 two different softwood pulps, designated “Softwood 1” and “Softwood 2”, are compared prior to and after the refining operation. The coordinates for breaking length versus refining degree prior to refining are identified by the points A and the coordinates for breaking length versus refining degree after refining are identified by the points B.
The various points are explained in the following list:
A_SW1=feed suspension of “Softwood 1”, whereby “Softwood 1” has a high strength,
B_SW1LC=pulp suspension after a refining pass at low consistency of “Softwood 1”
A_SW2=feed suspension of “Softwood 2”, whereby “Softwood 2” has a low strength,
B_SW2LC=pulp suspension after a refining pass at low consistency of “Softwood 2”,
B_SW2HC=pulp suspension after a refining pass at high consistency of “Softwood 2”
On the connecting lines between the points A and B lie the breaking lengths which would result approximately at refining degrees lying in between.
As is evident from FIG. 2, both B_SW1LCand B_SW2HCare suitable at a refining degree of less than 21°SR for the method according to the invention for the production of tissue paper, as the property according to the invention exists for both pulp suspensions, namely that it is possible to produce from the suspensions a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at least a specific refining degree (RD_specific) is greater than or equal to the value resulting from 0.55 (km/SR)*(RD_specific−10°SR), whereby the specific refining degree is selected from a refining range from 15°SR to less than 21°SR.
The functional dependency according to the invention between breaking length and refining degree is represented in FIG. 2 by the dashed line.
As is evident from FIG. 2, the “Softwood 2” refined at high consistency and the “Softwood 1” refined at low consistency comply in particular with the property that it is possible to produce from said softwoods a laboratory sheet according to TAPPI 205 SP 95 (Rapid Köthen) whose breaking length measured according to TAPPI 220 and TAPPI 494 at least a specific refining degree (RD_specific) is greater than or equal to 4.0 km, whereby the specific refining degree (RD_specific) is selected from a refining range less than or equal to 17°SR.
The further procedure is now explained further in the following FIGS. 3 to 11, with FIGS. 10 and 11 presenting two embodiments of different apparatuses for performing the method.
A pulp suspension 11 with the above mentioned properties emerges from headbox 10 such that the suspension is injected into the ingoing nip between a forming mesh 12 and a structured, in particular 3-dimensionally structured mesh 13, as the result of which a tissue paper web 14 is formed.
Forming mesh 12 has a side facing tissue paper web 14, which relative to that of structured mesh 13 is smooth.
Here, side 15 of structured mesh 13 facing tissue paper web 14 has depressed regions 16 and, relative to depressed areas 16, raised regions 17 such that tissue paper web 14 is formed in depressed regions 16 and raised regions 17 of structured mesh 13. The difference in height between depressed regions 16 and raised regions 17 amounts to preferably 0.07 mm and 0.6 mm. The area formed by raised regions 16 amounts to preferably 10% or more, in particular preferably 20% or more and in particular preferably 25% to 30%. In the embodiment presented in FIG. 3, structured mesh 13 is shown as a TAD mesh 13.
In the embodiment presented in FIG. 3 the arrangement of TAD mesh 13, tissue paper web 14 and forming mesh 12 is directed around a forming roller 18 and tissue paper web 14 is dewatered essentially by forming mesh 12 before forming mesh 12 is taken off tissue paper web 14 and tissue paper web 14 is transported further on TAD mesh 13.
Evident in FIG. 4 is the structure of tissue paper web 14 formed between flat forming mesh 12 and TAD mesh 13. The voluminous pillow areas C′ of tissue paper web 14 formed in depressed regions 16 of TAD mesh 13 have a higher volume and a higher gsm substance than areas A′ of tissue paper web 14 formed in raised regions 17 of TAD mesh 13. Accordingly, tissue paper web 14 already has a 3-dimensional structure as the result of its forming on structured mesh 13.
Evident in FIG. 5 is a tissue paper web 114 which was formed between two flat forming meshes 112 and 112′. As the result of its forming between two smooth forming meshes 112 and 112′, tissue paper web 114 has an essentially smooth and non-3-dimensional structure.
In a dewatering step after the formation of tissue web 14, tissue paper web 14 is conveyed between structured mesh 13, which is arranged above, and a lower permeable skin 19, which is constructed as felt 19, whereby during the dewatering step along a dewatering section pressure is exerted on structured mesh 13, tissue paper web 14 and felt 19 such that tissue paper web 14 is dewatered in the direction of felt 19, as indicated by arrows 20 in the FIG. 6.
As the result of tissue paper web 14 being dewatered during this dewatering step in the direction of felt 19 and as the result of tissue paper web 14 being dewatered on structured mesh 13 on which it was previously formed, the voluminous areas C′ are less intensively compressed than the areas A′, thus resulting in the voluminous structure of the areas C′ being preserved.
Evident in FIG. 7 is the creation of a 3-dimensional structure of tissue paper web 114 formed in FIG. 5. To create the 3-dimensional structure, tissue paper web 114 must be pressed into a structured mesh 113. For this purpose the tissue paper web 114 in the areas C, which are pressed into depressed regions 116 of structured mesh 113, are stretched, as the result of which the gsm substance in areas C is reduced. Also, tissue paper web 114 in the areas C is intensively pressed, as the result of which the volume of areas C is reduced as well.
The pressure for dewatering tissue paper web 14 is generated during the dewatering step at least in some areas simultaneously by a gas flow and a mechanical pressing force. Here the gas flow passes first through the structured mesh, then tissue paper web 14 and finally the lower skin constructed as felt 19. The gas flow through tissue paper web 14 amounts to around 150 m³per minute and meter web length.
In the case under consideration, the gas flow is generated by a suction zone 25 in roller 24, suction zone 25 having a length in the region of between 200 mm and 2500 mm, preferably between 800 mm and 1800 mm, in particular preferably between 1200 mm and 1600 mm. The vacuum in the suction zone 25 amounts to between −0.2 bar and −0.8 bar, preferably between −0.4 bar and 0.6 bar.
With regard to performing the dewatering step by mechanical pressing force and, optionally or in addition to this, with a gas flow, and with regard to the various configurations of apparatus for performing such a dewatering step, PCT/EP2005/050198 is incorporated herein by reference.
The mechanical pressing force is generated during the dewatering step by conveying the arrangement of structured mesh 13, tissue paper web 14 and felt 19 to a dewatering section 21 between a tensioned press belt 22 and a smooth surface 23, in which case press belt 22 acts on structured mesh 13 and felt 19 rests on smooth surface 23. Smooth surface 23 is thus formed by the circumferential surface 23 of a roller 24.
The dewatering section 21 is defined essentially by the wrap zone of press belt 22 around the circumferential surface 23 of roller 24, whereby the wrap zone is defined by the distance between two deflector rollers 25 and 26. Press belt 22 is under a tension of at least 30 kN/m, preferably at least 60 kN/m or 80 kN/m, and has an open area of at least 25% and a contact area of at least 10% of its total area facing the upper skin. In this specific case, the press belt is constructed as a spiral link fabric and has an open area of between 51% and 62% and a contact area of between 38% and 49% of its total area facing the upper skin. With regard to the structure of the press belt, PCT/EP2005/050198 is incorporated herein by reference.
The tissue paper web 14 leaves the dewatering section 21 with a dry content of between 25% and 55%. After this dewatering step, tissue paper web 14 together with structured mesh 13 is conveyed in a further dewatering step through a press nip 27, whereby tissue paper web 14 in press nip 27 is arranged between structured mesh 13 and a smooth roller surface 28 of a Yankee drying cylinder. Here, press nip 27 is a press nip formed by the Yankee drying cylinder 29 and a shoe press 30. On one side, tissue paper web 14 lies with a relatively large area on the circumferential surface 28 of Yankee drying cylinder 29, while on the other side tissue paper web 14 lies on structured mesh 13. Here the depressed and by comparison relatively raised regions 17 of structured mesh 13 are constructed and arranged in relation to each other such that pillow areas C′ are essentially not pressed in press nip 27, the areas being 35% or less, in particular 25% or less of tissue paper web 14. By contrast, the areas A′ are pressed, as the result of which the strength of tissue paper web 14 is increased (FIG. 8).
The tissue paper web 114 known from the prior art comes to rest on the circumferential face 128 of the Yankee drying cylinder with a relatively small area compared to the tissue paper web 14. The disadvantage of this is that the tissue paper 114 might burn on the circumferential face, which is why the temperature of the Yankee cylinder has to be kept low in the methods known from the prior art. Consequently, a lower dry content is obtainable with the method known from the prior art (FIG. 9).
Between the two previously described dewatering steps it is possible to provide a further dewatering step which can be performed using an apparatus 31.
Provision can optionally be made for tissue paper web 14 to be conveyed together with structured mesh 13 around an evacuated roller before the web runs through press nip 27, in which case structured mesh 13 is arranged between tissue paper web 14 and the evacuated deflector roller (not illustrated).
From FIG. 11 it is evident that the gas flow can be generated in addition by an overpressure hood 31 arranged above structured mesh 13, whereby in this case the dewatering step is performed without mechanical pressing force, i.e., unlike in FIG. 10 no provision is made for a press belt 22 which wraps around roller 24 in some areas.
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

Claims

1. A method for the production of a pulp suspension comprised of fibers, with which at least one suspension fraction is produced with the following steps:

providing a high-consistency feed pulp suspension comprised of fibers and having a consistency of more than 20% and a refining degree of less than 15°SR; and

refining the high-consistency feed pulp suspension to a refining degree of 15°SR or more to obtain the suspension fraction.

2. The method according to claim 1, wherein the suspension fraction for producing the pulp suspension is mixed with another suspension fraction, whereby the other suspension fraction is refined from a low-consistency feed pulp suspension with a consistency of less than 10%.

3. The method according to claim 2, wherein the suspension fraction for producing the pulp suspension is mixed with another suspension fraction, whereby the other suspension fraction is refined from a low-consistency feed pulp suspension with a consistency of less than 5%.

4. The method according to claim 2, wherein the suspension fraction produced from a low-consistency feed pulp suspension has a higher refining degree than the suspension fraction produced from a high-consistency feed pulp suspension.

5. The method according to claim 4, wherein the high-consistency feed pulp suspension has a refining degree of 12°SR to 13°SR and the suspension fraction produced therefrom has a refining degree of 15°SR to 19°SR.

6. The method according to claim 5, wherein the high-consistency feed pulp suspension has a refining degree of 12°SR to 13°SR and the suspension fraction produced therefrom has a refining degree of 15°SR to 17°SR.

7. The method according to claim 1, wherein the refining operation is performed several times in succession.

8. The method according to claim 1, wherein the high-consistency feed pulp suspension is refined with a refining energy in the range from 150 kWh to 300 kWh.

9. The method according to claim 8, wherein the high-consistency feed pulp suspension has a consistency in the range from more than 20% to 40%.

10. The method according to claim 2, wherein enzymes are added to the low-consistency feed pulp suspension for refinement.

11. The method according to claim 1, wherein the high-consistency feed suspension has a low strength produced from a low-consistency feed suspension of low strength.

12. The method according to claim 11, wherein the concentration is performed by means of a worm extruder.

13. The method according to claim 2, wherein at least one of enzymes and agents for at least one of increasing the dry strength (dry strength agents) and increasing the wet strength (wet strength agents) are added to the low-consistency feed pulp suspension.

14. The method according to claim 13, wherein the dry strength agent is comprised of carbon methyl cellulose.

15. The method according to claim 10, wherein the enzymes are added to the low-consistency feed suspension at a temperature in the range from 25° C. to 70° C.

16. The method according to claim 15, wherein the enzymes are added to the low-consistency feed suspension with a pH-value in the range from 5 to 8.

17. The method according to claim 16, wherein the enzymes are allowed to work for a period of 1 to 2 hours on the low-consistency feed suspension.

18. The method according to claim 17, wherein the enzymes are added in the pulper.

19. The method according to claim 1, wherein the high-consistency feed pulp suspension is refined at a temperature up to 80° C.

20. A method for the production of a fibrous web, with a pulp suspension produced by the method according to claim 6.