Abstract:
A method for the fabrication of a fibrous web including the steps of loading fibers with a precipitant, thereby defining treated fibers. Creating at least a portion of the precipitant as crystalline precipitant particles. Supplying the treated fibers as a pumpable fiber stock suspension to a sheet forming process that forms the fibrous web. And, controlling filler distribution across a web cross section of the fibrous web by way of a vacuum supply in the sheet forming process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation of PCT application No. PCT/EP03/50032, entitled “METHOD FOR PRODUCING A FIBROUS WEB”, filed Feb. 25, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a method for the fabrication of a fibrous web, particularly a paper or cardboard web.  
         [0004]     2. Description of the Related Art  
         [0005]     Conventionally, precipitated calcium carbonate (PCC) is used as a filler during paper production. However, a uniform distribution of the filler particles throughout the formed sheet with the objective of achieving a constant ash content or level, as well as optimum printability, is at present technically not possible.  
         [0006]     Loading fibers with an additive, for example a filler, may occur through a chemical precipitation reaction, that is especially by way of a so-called “Fiber Loading” process, such as the one described, in U.S. Pat. No. 5,223,090. In such a fiber loading process at least one additive, especially a filler, is deposited on the wetted fiber surfaces of the fiber material. In this process the fibers may, for example, be loaded with calcium carbonate. Calcium oxide and/or calcium hydroxide is added to the moist disintegrated fiber material in such a way that at least some of it associates itself with the water that is contained in the fiber material. The so treated fiber material is subsequently treated with carbon dioxide.  
         [0007]     What is needed in the art is an improved method of distribution of filler particles.  
       SUMMARY OF THE INVENTION  
       [0008]     It is the objective of the current invention to create an improved method of the type mentioned above. The desired end result is a more uniform distribution of the filler particles, as well as improved printability should be achieved, over the methods.  
         [0009]     This objective is met by the current invention with a method for the fabrication of a fibrous web, especially a paper or cardboard web whereby the fibers are loaded with a precipitant, moreover creating crystalline precipitant particles. The so treated fibers are supplied to a sheet forming process in the form of a pumpable fiber suspension. During this sheet forming process, the filler distribution occurring across the web cross section is controlled and/or regulated by way of an appropriate vacuum supply.  
         [0010]     In accordance with one embodiment of the present invention, crystalline precipitant particles, in a size range of approximately 0.05 μm to approximately 0.5 μm, especially in a range of 0.1 μm to approximately 2.5 μm and preferably in a range of approximately 0.3 mm to approximately 0.8 μm are created. Crystalline precipitant particles can advantageously be produced in a size range of approximately 0.05 μm to approximately 0.1 μm, and preferably in a range of approximately 0.3 μm to approximately 0.8 μm. According to one embodiment of the inventive method the precipitant is calcium carbonate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     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 an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0012]     FIGS.  1  to  9  schematically illustrate examples of various filler or ash distributions for various former segments.  
         [0013]      FIG. 10  schematically illustrates the influence of certain factors upon the filler distribution in the z-direction.  
         [0014]      FIG. 11  is a schematic comparison of a total ash distribution in conventional paper, together with a possible total ash distribution in a fiber loading (FL) paper product. 
     
    
       [0015]     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Referring now to the drawings, it is especially advantageous if calcium oxide and/or calcium hydroxide are added to the fiber stock suspension for loading of the fibers with calcium carbonate, and the precipitation is triggered by treating the fiber stock suspension with carbon dioxide.  
         [0017]     When, for example, loading the fibers with a filler, calcium carbonate (CaCO 3 ) can be deposited on the wetted fiber surfaces by adding calcium oxide (CaO) and/or calcium hydroxide (Ca(OH) 2 ) to the moist fiber material, whereby at least some of it can associate itself with the water in the fiber volume. The fiber material treated in this manner can then be treated with carbon dioxide (CO 2 ).  
         [0018]     The term “wetted fiber surfaces” may include all wetted surfaces of the individual fibers. This especially also includes the instance where the fibers are loaded with calcium carbonate, or any desired other precipitant, on their outside surface as well as on their interior (lumen). According to this embodiment the fibers may, for example, be loaded with the filler calcium carbonate, whereby the deposit onto the wetted fiber surfaces occurs through a so-called “Fiber Loading” process, as described in U.S. Pat. No. 5,223,090. In this fiber loading process, for example, the carbon dioxide with the calcium hydroxide reacts to form water and calcium carbonate.  
         [0019]     The cited method can be utilized particularly advantageously in the production of newsprint, especially with an ash content of 5% to 20%, of SCA paper, of LWC (light weight coated) paper, of ULWC (ultra-light weight coated) paper, of wood-free non-coated paper, of wood-free coated paper, of white lined liner and/or bleached types of cardboard.  
         [0020]     The cited method enables the fabrication of a totally new grade of paper with uniform filler distribution across the entire cross profile of the paper, as well as on the surface. The paper manufacturer can therefore produce a sheet of paper whereby a more uniform distribution of the filler content occurs, leading to savings in raw material, most importantly wood or secondary fibers, as well as to an improvement on the paper machine side where fewer chemicals become necessary for the paper manufacturing process. It is now possible to produce the same paper grade on a substantially lighter basis, whereby the same gloss is achieved due to the more uniform distribution of the fillers. Overall, this leads to savings in fillers and fibers. On the side of the paper product, improvements, relative to the physical and optical paper characteristics, are achieved, thereby improving the paper quality. Improvements with regard to printability result from the fact that a more uniform distribution of printing ink particles, especially on the printable surface is made possible since the paper surface displays less roughness and a higher uniformity.  
         [0021]     One embodiment of the inventive method provides for better printability characteristics. The method relates to stock production, as well as to paper fabrication. During stock production, secondary fibers that were recovered from waste paper, or primary fibers are disintegrated, deflaked and cleaned. In this segment, relating to the stock production, the filler in the form of precipitated calcium carbonate (PCC) is added to the fiber preparation in such a manner that all fibers possess a more uniformly distributed PCC layer. This is due to the fact that the wood fibers or chemical pulp fibers are exposed to Ca(OH) 2  and everything is being mixed. The mixture is then exposed to CO 2  in a reactor where the CaCO 3  (PCCC)-crystals are formed. The size of the crystals may be in a range of approximately 0.05 μm to approximately 0.5 μm, especially in a range of approximately 0.1 μm to approximately 2.5 μm and preferably in a range of approximately 0.3 μm to approximately 0.8 μm. The crystals may be deposited on the inside and on the outside of the fibers, or they may be provided as free PCC particles, meaning they may be present as solids in the water of the fiber pulp. The fiber pulp, treated in this manner, can then be delivered as a pumpable fiber stock suspension to the sheet forming or paper forming process. Various additional steps are necessary for the formation of a fiber web or a paper sheet, for example, dewatering (thickening), compressing and calendering.  
         [0022]     While the stock is produced in the above described manner, a new paper product can be produced by modifying the manufacturing process on the papermachine side.  
         [0023]     Generally, the following machinery or devices are examples of equipment used in the production of a paper web: Fourdrinier machine, Hybrid-Former (Duo-Former™ D) and Gap-Former (Duo-Former™ CFD). The filler distribution, occurring over the cross profile, ensues from the drawing. Based on the utilization of any type of forming device a higher or lower modest ash content occurs on the paper surface side, which could negatively influence the printability of the paper. Even when a light weight surface coating is later applied to the paper, preferably no filler distribution would be present on the cover surface. The coating surface penetrates into the openings or gaps; however, it does not cover the paper surface. This makes it difficult to print the paper. The printing ink must cover the color of the fibers. Since white light consists of the sum total of all complementary rainbow colors, no white light radiation as such exists. This means that a certain pigment size is only desirable for one color. Other colors are reflected differently. Relative to the paper this means that a high level filler content is necessary in order to produce a higher level of whiteness, if the particles are not evenly distributed. The coating surface adds more white pigments to the paper, thereby making the white surface thicker so that the transit time of the light beam is longer, resulting in a white color. If for example, a room is painted brown or black a base of four or five layers of white paint would be necessary in order to cover the base color. The same applies to paper where more white pigments are necessary to cover black, in order to produce paper having a high opacity. The whiter the paper is, the less printing ink is required in order to achieve the same result. In other words, when the filler particles are uniformly spaced, less printing ink is required and that the ink may penetrate into the basis paper sheet in the Z-direction to a lesser extent. Adjustment of the vacuum at the formers can facilitate a better filler distribution, see especially the broken line curve in  FIG. 1 . When utilizing fiber stock, in whose production the fibers were loaded with a precipitant, in conjunction with any of the various forming devices, and combined with a low level, (generally −1.5 m to −4 m) or a high level of −4 m to −7.5 m or preferably a medium level vacuum of −2 m to −6 m, as well as together with a device for the application of a very light weight coating a much better filler distribution, as well as a better topographic paper surface that is a more uniform paper surface, can be achieved. This means essentially also a lower filler fluctuation, better running characteristics (runability) of the paper machine and a lower requirement for retention aids and opacity enhancing agents.  
         [0024]     As far as SC (supercalendered) papers are concerned where the fibers themselves are platicized during the calendering process through pressure and heat, such that the paper surface is smoothed, a substantially lower calendering expenditure is required to achieve a certain quality of calendered paper.  
         [0025]     With improved filler distribution in the paper sheet, less coating and therefore less drying is required, since the fibers are better covered with filler and the appropriate covering or ink layer (coating). In addition, the printability of the paper is also improved in that a multi-layer headbox is utilized with which the filler distribution (PCC) in the paper can be influenced in cross profile (X-section) direction. This, however, is only possible for paper grades having basis weights that allow utilization of multi-layer headboxes (&gt;80 g/m 2 ).  
         [0026]     Multi-layer headboxes cannot be utilized for extremely lightweight grades, such as newsprint (40-50 g/m 2 ) and telephone directory paper (28-40 g/m 2 ). It is however feasible to utilize multi-layer headboxes for basis weights higher than 50 g/m 2 .  
         [0027]     With the “Fiber Loading” (“FL”) technology, whether a modified multi-layer or conventional single layer headbox is used, the so-called “linting” is prevented. “Linting” refers to the extraction of the fillers during dewatering, that is a so-called “First Pass Retention” of the fillers. Improvements in a range of 5% to 50%, preferably in a range of 10% to 25% are possible. “FL” stock, that is stock that was treated according to the “FL” technology, possesses approximately 25-200 times greater freeness than conventionally produced stock, based on the refining process.  
         [0028]     The raw paper produced in accordance with the present invention has a higher level of thickening that can be influenced with modified dewatering (DuoFormer, Fourdrinier, Gap-Former). Due to the greater dewatering, a higher dry content is achieved after the press section. This means, for example, that the paper enters the press at a higher, or at the same dry content but exits the press having a higher dry content (1% higher dry content saves approximately four dryer cylinders). The improved dryness range is within a range of approximately 0.1% to approximately 5%, and preferably of about 0.5% to approximately 2%.  
         [0029]     Utilization of the “FL” technology described above is especially suitable for papers that require good printability and at the same time as high, as possible, a degree of filler, combined with a fiber content in the paper that is as low as possible. The advantage is in that the filler particles establish themselves on the fiber and not, as is the case with conventional fillers between the hollow fiber cavities. A better printability is thereby achieved since the printing ink is applied to the filler particles and does not have to cover the fiber first. With this arrangement the printing ink also penetrates the fibers to a lesser extent.  
         [0030]     The uniform filler distribution in a given paper sheet is therefore achieved by utilization of the so-called “Fiber Loading” process by way of which the filler particles, that are known as precipitated calcium carbonate (PCC), are deposited on, in and in-between the fibers. The “Fiber Loading” process is applied in the stock manufacturing device, known as stock preparation. The treated stock may be pre-refined or may be refined subsequently, in order to prepare it for the paper machine process.  
         [0031]     When the stock, that was treated by a “Fiber Loading” process, is brought onto a respective sheet forming wire, a paper sheet having uniform filler distribution is produced. The filler content is for example, approximately 50%, based on the solids mass weight. Since approximately up to 25% of the total ash content is deposited in and on the fibers, this has the effect that the sheet already possesses an ash content, in a cross direction, of approximately 25% of the desired total ash content. The filler retention in the paper machines is in a range of approximately 30%-60% relative to the total filler content. This means that the basis ash content, in the cross direction of an “FL” treated sheet, is in a range of approximately 50% to approximately 85%. In comparison, values achieved in a conventional process are 30% to 60%. In the subsequent press and dewatering process in the papermachine, the filler content can be improved in the cross direction. The objective, in the conventional sheet forming process, is around 30% to 70% uniformly distributed filler content in the cross direction, depending upon the paper manufacturing process. When utilizing “FL” treated stock in the paper machine, the filler content is distributed uniformly in the cross direction in the paper sheet and may be approximately 55% or even 95%, based on the paper manufacturing process.  
         [0032]     A uniform filler distribution results in improved gloss values. Improved gloss values mean greater whiteness in the sheet. Since white light is formed by the sum total of all complementary rainbow colors, no white light radiation as such exists. This means that a pigment size is only useful for one color. Other colors are reflected differently. Relative to paper this means that a high filler content is necessary in order to produce a higher level of whiteness, if the particles are not evenly distributed. Greater whiteness can be achieved with a lower filler content through uniform distribution of filler particles, since the filler particles are spaced evenly across the cross section of the paper, as well as distributed uniformly on the fibers. The optimum crystal size is in a range of approximately 0.5 μm to approximately 0.1 μm and preferably in a range of approximately 0.3 μm to approximately 0.8 μm.  
         [0033]     Less filler is required when utilizing the optimum crystal size, since the filler pigments are distributed evenly across the fibers in order to achieve optimum optical characteristics. An additional advantage is that more filler is retained through the paper wire forming process, since smaller particles permit better penetration within the sheet. This means that the sheet itself is filled from the inside toward the outside with filler, thereby achieving better filler retention throughout the sheet forming process, leading to a uniform distribution in the paper and achieving a higher paper production by treating the stock in the following manner:  
         [0034]     The fiber stock suspension, that was previously mixed with Ca(OH) 2 , is put into a crystallizing apparatus, for example a Fluffer, Refiner, Disperger or similar device, at a consistency or solids concentration in a range of approximately 5% to approximately 60%, preferably in a range of approximately 15% to approximately 35%. The Ca(OH) 2  may be added in liquid or dry form. The fiber pulp is treated with CO 2 . The CO 2  is added at temperatures within a range of approximately −15° C. and approximately 120° C., and preferably at temperatures within a range of approximately 20° C. and approximately 90° C.  
         [0035]     The fiber stock suspension comes into a gas zone where each individual fiber is exposed to a gas atmosphere, followed by the precipitation reaction, that immediately results in the CaCO 3  formation. The CaCO 3  crystals may be rhombohedral, scalenohedral or globular in their form, whereby especially the crystal mass depends upon the temperature range that is selected for the fiber stock suspension, as well as the CO 2  and Ca(OH) 2  content in the fiber stock suspension. After the fiber stock suspension, together with the formed crystals, has passed through the gas zone, the formed PCC or the fiber stock suspension with the crystals in the lumen, on the fibers and between the fibers, is routed through a rotor and a stator where the distribution of the crystals in the fiber stock suspension is concluded by mixing at a low shear action.  
         [0036]     When the fiber stock/crystal suspension passes through the rotor a shear distribution occurs that results in a size distribution of the crystals of approximately 0.05 μm to approximately 0.5 μm and preferably of approximately 0.3 μm to approximately 1.0 μm.  
         [0037]     The form of the utilized filler particles is, for example, rhombohedral with a respective cube size in a range of approximately 0.05 μm to approximately 1 μm, or scalenohedral with a respective length in a range of approximately 0.05 μm to approximately 1 μm and a respective diameter in a range of approximately 0.01 μm to approximately 0.5 μm, depending upon the paper grade that is to be produced.  
         [0038]     The longer the fiber stock suspension remains on the rotor plate the less will be the shearing, depending on the H 2 O that was added for thinning. The concentration of the fiber stock suspension passing over the rotor plate is approximately 0.1% to approximately 50% and preferably approximately 35% to approximately 50%.  
         [0039]     The pressure supplied to the CO 2  supply line is in a range of approximately 0.1 bar to approximately 6 bar and preferably in a range of approximately 0.5 bar to approximately 3 bar, in order to ensure a constant CO 2  supply to the gas ring to obtain the desired chemical reaction. The CO 2  supply and thereby the CaCO 3  creating precipitation reaction can be controlled and/or regulated through the pH value. pH values are in a range of 6.0 pH to approximately 10.0 pH, preferably in a range of approximately 7.0 pH to approximately 8.5 pH may be considered for the concluding reaction of the CaCO 3  crystals. The energy utilized for this process is within a range of approximately 0.3 kWh/t and approximately 8 kWh/t and preferably within a range of approximately 0.5 kWh/t and approximately 2.5 kWh/t. Dilution water may be added and mixed with the fiber stock suspension, in order to obtain a final dilution in which the produced fiber stock suspension, with the filler, has a consistency or solids concentration in a range, for example, of approximately 0.1% to approximately 16%, preferably in a range of approximately 2% to approximately 6%. The fiber stock suspension is then exposed to the atmosphere in a machine, a container or in the process equipment that follows in the process.  
         [0040]     The rotational speed of the rotor plate may especially be in a range of approximately 20 m/s to 100 m/s and preferably in a range of approximately 40 m/s to approximately 60 m/s on its outside diameter.  
         [0041]     The speed through the rotor and the stator is in a range of, for example, approximately 0.2 m/s to approximately 0.55 m/s and preferably in a range of approximately 0.05 m/s and approximately 0.2 m/s, depending upon the filler content and the crystal size.  
         [0042]     According to current knowledge, the crystal filler content, the crystal size and the speed are linearly linked. The gap between rotor and stator is approximately 0.5 mm to approximately 100 mm, and preferably approximately 25 mm to approximately 75 mm.  
         [0043]     The diameter of the rotor and the stator can be especially in a range of approximately 5 m to approximately 2 m.  
         [0044]     The reaction time is for example in the range of approximately 0.01 min. to 1 min, preferably in a range of approximately 0.1 sec. to approximately 10 sec.  
         [0045]     The method described above enables the production of individual particles that are equally spaced from each other and are deposited onto the fibers, whereby they cover the fibers in the desired manner.  
         [0046]     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.