US 7387706 B2
A method of forming a structured web including the steps of providing a fiber slurry through a headbox to a nip formed by a structured fabric and a forming fabric and collecting fibers from the fiber slurry in at least one valley of the structured fabric.
1. A method of forming a structured web with a paper machine, comprising the steps of:
providing a fiber slurry through a headbox to a nip formed by a structured fabric and a forming fabric;
collecting fibers from said fiber slurry predominately in a plurality of valleys of said structured fabric; and
dewatering in a forming area of the paper machine said fiber slurry through said forming fabric and not through said structured fabric.
2. The method of
3. The method of
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8. The method of
removing said forming fabric from the structured web;
contacting the structured web with a dewatering fabric; and
applying pressure to the structured web through said dewatering fabric.
9. The method of
10. The method of
transferring the structured web to a Yankee dryer at a transfer point; and
retaining the structured web with said structured fabric until reaching said transfer point.
11. The method of
12. A method of forming a structured web in a papermaking machine, comprising the steps of:
supplying a fiber slurry to a nip, said nip formed by a structured fabric and a forming fabric;
dewatering in a forming area of the paper machine said fiber slurry through said forming fabric and not through said structured fabric, thereby creating the web; and
retaining the web with said structured fabric through at least one dewatering process.
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1. Field of the Invention
The present invention relates to a method of forming a structured fiber web on a paper machine, and, more particularly, to a method and apparatus of forming a structured fiber web on a structured fabric in a paper machine.
2. Description of the Related Art
In a wet molding process, a structured fabric in a Crescent Former configuration impresses a three dimensional surface on a web while the fibrous web is still wet. Such an invention is disclosed in International Publication No. WO 03/062528 A1. A suction box is disclosed for the purpose of shaping the fibrous web while wet to generate the three dimensional structure by removing air through the structural fabric. It is a physical displacement of portions of the fibrous web that leads to the three dimensional surface. Similar to the aforementioned method, a through air drying (TAD) technique is disclosed in U.S. Pat. No. 4,191,609. The TAD technique discloses how an already formed web is transferred and molded into an impression fabric. The transformation takes place on a web having a sheet solids level greater that 15%. This results in a low density pillow area in the fibrous web. These pillow areas are of a low basis weight since the already formed web is expanded to fill the valleys thereof. The impression of the fibrous web into a pattern, on an impression fabric, is carried out by passing a vacuum through the impression fabric to mold the fibrous web.
What is needed in the art is a method to produce a fibrous web with a high basis weight pillow area of low density to thereby increase the absorption and bulk characteristics of the finished fibrous web.
The present invention provides a method of producing a structured fibrous web having a high basis weight pillow area of low density on a paper machine using a structured fabric.
The invention comprises, in one form thereof, a method of forming a structured web including the steps of providing a fiber slurry through a headbox to a nip formed by a structured fabric and a forming fabric and collecting fibers from the fiber slurry in at least one valley of the structured fabric.
An advantage of the present invention is that the low density pillow areas have a relatively higher fiber basis weight than that provided with other methods.
Another advantage is that the ratio of the uncompressed fiber mass to the compressed fiber mass is much higher, with the same overall basis weight than was achievable in the prior art.
Yet another advantage is that the fibrous web formed by the method of the present invention allows for a superior transfer of the web to a Yankee drying surface.
Still yet another advantage of the present invention is that hood associated with the Yankee dryer can utilize a higher temperature for drying the pillow portions of the fibrous web, without burning the pillow portions.
An additional advantage of the present invention is that the structured fabric can have deeper valleys or pockets than a prior art fabric, since the pillow portions of the fibrous web are thicker and have a higher basis weight, eliminating the pin hole problems associated with prior art methods, which results in a thicker more absorbent web.
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:
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.
Referring now to the drawings, and more particularly to
Structured fabric 28 includes warp and weft yarns interwoven on a textile loom. Structured fabric 28 may be woven flat or in an endless form. The final mesh count of structured fabric 28 lies between 95×120 and 26×20. For the manufacture of toilet tissue, the preferred mesh count is 51×36 or higher and more preferably 58×44 or higher. For the manufacturer of paper towels, the preferred mesh count is 42×31 or lower, and more preferably 36×30 or lower. Structured fabric 28 may have a repeated pattern of 4 shed and above repeats, preferably 5 shed or greater repeats. The warp yarns of structured fabric 28 have diameters of between 0.12 mm and 0.70 mm, and weft yarns have diameters of between 0.15 mm and 0.60 mm. The pocket depth, which is the offset between peak 28 a and valley 28 b is between approximately 0.07 mm and 0.60 mm. Yarns utilized in structured fabric 28 may be of any cross-sectional shape, for example, round, oval or flat. The yarns of structured fabric 28 can be made of thermaplastic or thermaset polymeric materials of any color. The surface of structured fabric 28 can be treated to provide a desired surface energy, thermal resistance, abrasion resistance and/or hydrolysis resistance. A printed design, such as a screen printed design, of polymeric material can be applied to structured fabric 28 to enhance its ability to impart an aesthetic pattern into web 38 or to enhance the quality of web 38. Such a design may be in the form of an elastomeric cast structure similar to the Spectra® membrane described in another patent application. Structured fabric 28 has a top surface plane contact area at peak 28 a of 10% or higher, preferably 20% or higher, and more preferably 30% depending upon the particular product being made. The contact area on structured web 28 at peak 28 a can be increased by abrading the top surface of structured fabric 28 or an elastomeric cast structure can be formed thereon having a flat top surface. The top surface may also be hot calendered to increase the flatness.
Forming roll 34 is preferably solid. Moisture travels through forming fiber 26 but not through structured fabric 28. This advantageously forms structured fibrous web 38 into a more bulky or absorbent web than the prior art.
Prior art methods of moisture removal, remove moisture through a structured fabric by way of negative pressure. It results in a cross-sectional view as seen in
In contrast, structured web 38, as illustrated in
According to prior art an already formed web is vacuum transferred into a structured fabric. The sheet must then expand to fill the contour of the structured fabric. In doing so, fibers must move apart. Thus the basis weight is lower in these pillow areas and therefore the thickness is less than the sheet at point A.
As shown in
The prior art web shown in
The increased mass ratio of the present invention, particularly the higher basis weight in the pillow areas carries more water than the compressed areas, resulting in at least two positive aspects of the present invention over the prior art, as illustrated in
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A shoe press 56 is placed adjacent to structured fabric 28, holding it in a position proximate Yankee roll 52. Structured web 38 comes into contact with Yankee roll 52 and transfers to a surface thereof, for further drying and subsequent creping.
A vacuum box 58 is placed adjacent to structured fabric 28 to achieve a solids level of 15-25% on a nominal 20 gsm web running at −0.2 to −0.8 bar vacuum with a preferred operating level of −0.4 to −0.6 bar. Web 38, which is carried by structured fabric 28, contacts dewatering fabric 82 and proceeds toward vacuum roll 60. Vacuum roll 60 operates at a vacuum level of −0.2 to −0.8 bar with a preferred operating level of at least −0.4 bar. Hot air hood 62 is optionally fit over vacuum roll 60 to improve dewatering. The length of the vacuum zone inside the vacuum roll can be from 200 mm to 2,500 mm, with a preferable length of 300 mm to 1,200 mm and an even more preferable length of between 400 mm to 800 mm. The solids level of web 38 leaving suction roll 60 is 25% to 55% depending on installed options. A vacuum box 67 and hot air supply 65 can be used to increase web 38 solids after vacuum roll 60 and prior to Yankee roll 52. Wire turning roll 69 can also be a suction roll with a hot air supply hood. Roll 56 includes a shoe press with a shoe width of 80 mm or higher, preferably 120 mm or higher, with a maximum peak pressure of less than 2.5 MPa. To create an even longer nip to facilitate the transfer of web 38 to Yankee 52, web 38 carried on structured fabric 28 can be brought into contact with the surface of Yankee roll 52 prior to the press nip associated with shoe press 56. Further, the contact can be maintained after structured fabric 28 travels beyond press 56.
Dewatering fabric 82 may have a permeable woven base fabric connected to a batt layer. The base fabric includes machine direction yarns and cross-directional yarns. The machine direction yarn is a 3 ply multifilament twisted yarn. The cross-direction yarn is a monofilament yarn. The machine direction yarn can also be a monofilament yarn and the construction can be of a typical multilayer design. In either case, the base fabric is needled with a fine batt fiber having a weight of less than or equal to 700 gsm, preferably less than or equal to 150 gsm and more preferably less than or equal to 135 gsm. The batt fiber encapsulates the base structure giving it sufficient stability. The needling process can be such that straight through channels are created. The sheet contacting surface is heated to improve its surface smoothness. The cross-sectional area of the machine direction yarns is larger than the cross-sectional area of the cross-direction yarns. The machine direction yarn is a multifilament yarn that may include thousands of fibers. The base fabric is connected to a batt layer by a needling process that results in straight through drainage channels.
In another embodiment of dewatering fabric 82 there is included a fabric layer, at least two batt layers, an anti-rewetting layer and an adhesive. The base fabric is substantially similar to the previous description. At least one of the batt layers include a low melt bi-compound fiber to supplement fiber to fiber bonding upon heating. On one side of the base fabric, there is attached an anti-rewetting layer, which may be attached to the base fabric by an adhesive, a melting process or needling wherein the material contained in the anti-rewet layer is connected to the base fabric layer and a batt layer. The anti-rewetting layer is made of an elastomeric material thereby forming elastomeric membrane, which has openings therethrough.
The batt layers are needled to thereby hold dewatering fabric 82 together. This advantageously leaves the batt layers with many needled holes therethrough. The anti-rewetting layer is porous having water channels or straight through pores therethrough.
In yet an other embodiment of dewatering fabric 82, there is a construct substantially similar to that previously discussed with an addition of a hydrophobic layer to at least one side of de-watering fabric 82. The hydrophobic layer does not absorb water, but it does direct water through pores therein.
In yet another embodiment of dewatering fabric 82, the base fabric has attached thereto a lattice grid made of a polymer, such as polyurethane, that is put on top of the base fabric. The grid may be put on to the base fabric by utilizing various known procedures, such as, for example, an extrusion technique or a screen-printing technique. The lattice grid may be put on the base fabric with an angular orientation relative to the machine direction yarns and the cross direction yarns. Although this orientation is such that no part of the lattice is aligned with the machine direction yarns, other orientations can also be utilized. The lattice can have a uniform grid pattern, which can be discontinuous in part. Further, the material between the interconnections of the lattice structure may take a circuitous path rather than being substantially straight. The lattice grid is made of a synthetic, such as a polymer or specifically a polyurethane, which attaches itself to the base fabric by its natural adhesion properties.
In yet another embodiment of dewatering fabric 82 there is included a permeable base fabric having machine direction yarns and cross-direction yarns, that are adhered to a grid. The grid is made of a composite material the may be the same as that discussed relative to a previous embodiment of dewatering fabric 82. The grid includes machine direction yarns with a composite material formed therearound. The grid is a composite structure formed of composite material and machine direction yarns. The machine direction yarns may be pre-coated with a composite before being placed in rows that are substantially parallel in a mold that is used to reheat the composite material causing it to re-flow into a pattern. Additional composite material may be put into the mold as well. The grid structure, also known as a composite layer, is then connected to the base fabric by one of many techniques including laminating the grid to the permeable fabric, melting the composite coated yarn as it is held in position against the permeable fabric or by re-melting the grid onto the base fabric. Additionally, an adhesive may be utilized to attach the grid to permeable fabric.
The batt fiber may include two layers, an upper and a lower layer. The batt fiber is needled into the base fabric and the composite layer, thereby forming a dewatering fabric 82 having at least one outer batt layer surface. Batt material is porous by its nature, additionally the needling process not only connects the layers together, it also creates numerous small porous cavities extending into or completely through the structure of dewatering fabric 82.
Dewatering fabric 82 has an air permeability of from 5 to 100 cubic feet/minute preferably 19 cubic feet/minute or higher and more preferably 35 cubic feet/minute or higher. Mean pore diameters in dewatering fabric 82 are from 5 to 75 microns, preferably 25 microns or higher and more preferably 35 microns or higher. The hydrophobic layers can be made from a synthetic polymeric material, a wool or a polyamide, for example, nylon 6. The anti-rewet layer and the composite layer may be made of a thin elastomeric permeable membrane made from a synthetic polymeric material or a polyamide that is laminated to the base fabric.
The batt fiber layers are made from fibers ranging from 0.5 d-tex to 22 d-tex and may contain a low melt bi-compound fiber to supplement fiber to fiber bonding in each of the layers upon heating. The bonding may result from the use of a low temperature meltable fiber, particles and/or resin. The dewatering fabric is less than 2.0 millimeters, preferably less than 1.50 millimeters, and more preferably less than 1.25 millimeters and even more preferably less than 1.0 millimeter thick.
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Belt 66 is a specially designed Extended Nip Press Belt 66, made of, for example reinforced polyurethane and/or a spiral link fabric. Belt 66 is permeable thereby allowing air to flow therethrough to enhance the moisture removing capability of belt press 64. Moisture is drawn from web 38 through dewatering fabric 82 and into vacuum roll 60.
Belt 66 provides a low level of pressing in the range of 50-300 KPa and preferably greater than 100 KPa. This allows a suction roll with a 1.2 meter diameter to have a fabric tension of greater than 30 KN/m and preferably greater than 60 KN/m. The pressing length of permeable belt 66 against fabric 28, which is indirectly supported by vacuum roll 60, is at least as long as a suction zone in roll 60. Although the contact portion of belt 66 can be shorter than the suction zone.
Permeable belt 66 has a pattern of holes therethrough, which may, for example, be drilled, laser cut, etched formed or woven therein. Permeable belt 66 may be monoplanar without grooves. In one embodiment, the surface of belt 66 has grooves and is placed in contact with fabric 28 along a portion of the travel of permeable belt 66 in belt press 64. Each groove connects with a set of the holes to allow the passage and distribution of air in belt 66. Air is distributed along the grooves, which constitutes an open area adjacent to contact areas, where the surface of belt 66 applies pressure against web 38. Air enters permeable belt 66 through the holes and then migrates along the grooves, passing through fabric 28, web 38 and fabric 82. The diameter of the holes may be larger than the width of the grooves. The grooves may have a cross-section contour that is generally rectangular, triangular, trapezoidal, semi-circular or semi-elliptical. The combination of permeable belt 66, associated with vacuum roll 60, is a combination that has been shown to increase sheet solids by at least 15%.
An example of another structure of belt 66 is that of a thin spiral link fabric, which can be a reinforcing structure within belt 66 or the spiral link fabric will itself serve as belt 66. Within fabric 28 there is a three dimensional structure that is reflected in web 38. Web 38 has thicker pillow areas, which are protected during pressing as they are within the body of structured fabric 28. As such the pressing imparted by belt press assembly 64 upon web 38 does not negatively impact web quality, while it increases the dewatering rate of vacuum roll 60.
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Advantages of the HPTAD process are in the areas of improved sheet dewatering without a significant loss in sheet quality, compactness in size and energy efficency. Additionally, it enables higher pre-Yankee solids, which increase the speed potential of the invention. Further, the compact size of the HPTAD allows for easy retrofit to an existing machine. The compact size of the HPTAD and the fact that it is a closed system means that it cam be easily insulated and optimized as a unit to increase energy efficiency.
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The fiber distribution of web 38 in this invention is opposite that of the prior art, which is a result of removing moisture through the forming fabric and not through the structured fabric. The low density pillow areas are of relatively higher basis weight than the surrounding compressed zones, which is opposite of conventional TAD paper. This allows a high percentage of the fibers to remain uncompressed during the process. The sheet absorbency capacity as measured by the basket method, for a nominal 20 gsm web is equal to or greater than 12 grams water per gram of fiber and often exceeds 15 grams of water per gram fiber. The sheet bulk is equal to or greater than 10 cm3/gm and preferably greater than 13 cm3/gm. The sheet bulk of toilet tissue is expected to be equal to or greater than 13 cm3/gm before calendering.
With the basket method of measuring absorbency, five (5) grams of paper are placed into a basket. The basket containing the paper is then weighted and introduced into a small vessel of water at 20° C. for 60 seconds. After 60 seconds of soak time, the basket is removed from the water and allowed to drain for 60 seconds and then weighted again. The weight difference is then divided by the paper weight to yield the grams of water held per gram of fibers being absorbed and held in the paper.
Web 38 is formed from fibrous slurry 24 that headbox 22 discharges between forming fabric 26 and structured fabric 28. Roll 34 rotates and supports fabrics 26 and 28 as web 38 forms. Moisture M flows through fabric 26 and is captured in save all 36. It is the removal of moisture in this manner that serves to allow pillow areas of web 38 to retain a greater basis weight and therefore thickness than if the moisture were to be removed through structured fabric 28. Sufficient moisture is removed from web 38 to allow fabric 26 to be removed from web 38 to allow web 38 to proceed to a drying stage. Web 38 retains the pattern of structured fabric 28 and any zonal permeability effects from fabric 26 that may be present.
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.
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