Reuther, Norbert (207 Country Glen Pelzer, SC, 29669, US)
Breuer, Hans-peter (Pliensbacher Strasse 16 Zell unter Aichelberg, D-73119, DE)
| 1. | A transfer fabric, for use in the press section of a papermaking machine having a paper support surface and a running surface, said support surface being operative to hold paper during transport through said press section with multiple intensities said transfer fabric comprising: a base fabric formed of interlaced continuous filament yarns and fibers, said fibers being interdispersed among said yarns evenly throughout; a latex coating dispersed throughout said base fabric coating all external surfaces of said yarns and fibers forming said support and running surfaces with nonuniform valleys and peaks throughout and said transfer fabric with an uncompressed porosity of between 2.5 and 3.5 CFM and a compressed porosity of 0 CFM. |
| 2. | 2 The transfer fabric of claim 1 wherein said yarns forming said base fabric is woven. |
| 3. | The transfer fabric of claim 1 wherein said paper holding intensity of said support surface of said transfer fabric when compressed is greater than said paper holding intensity of said transfer fabric when uncompressed. |
| 4. | The transfer fabric of claim 1 wherein said configuration of valleys and peaks diverge substantially equally from a median plane parallel of said support surface. |
| 5. | The transfer fabric of claim 4 wherein said valleys and peaks have a maximum extend from said median plane of about 0.066 mm 6. |
| 6. | The transfer fabric of claim 1 wherein at least certain of said yarns forming said base fabric have a crosssection which is one of round, oval, rectangular, triangular and star shaped. |
| 7. | The transfer fabric of claim 1 wherein at least certain of said yarns forming said base fabric are one of PA, PPS, PE, PET, PEEK, PPA, PCTA, and PU. |
| 8. | In combination with a papermaking machine having a forming section, a press section with press rolls and a dryer section having a vacuum drum, a continuous transfer fabric adapted, to receive paper on a support surface, delivered from said forming section and transport said paper through said press section, said support surface being adapted to hold said paper during transport through said press section and to release said paper upon reaching said dryer section, said transfer fabric comprising: a base fabric formed of interlaced longitudinal and transverse continuous filament yarns having fibers interspersed throughout; a latex coating dispersed evenly throughout said base fabric coating said yarns and fibers and forming said support surface and a running surface opposite said support surface with nonuniform peaks and valleys; said transfer fabric having a first porosity in an uncompressed condition which creates a first paper holding intensity and a second porosity in a compressed condition which creates a second and greater paper holding intensity; said transfer fabric acting to hold said paper with said first intensity when in said uncompressed condition during movement to and from said press rolls and to hold said paper with said second intensity when in said compressed condition during movement through said press rolls; whereby, said paper is held more snugly against said transfer fabric through said press rolls which maintainssaid paper evenly disposed over said support surface during transport through said press section. |
| 9. | The combination of claim 8 wherein said first porosity is no more than 3.5 CFM. |
| 10. | The combination of claim 8 wherein said second porosity is 0 CFM. |
| 11. | The combination of claim 8 wherein said first porosity is sufficient to allow air to be drawn through said transfer fabric in the area of said vacuum roll. |
| 12. | The combination of claim 8 wherein said peaks and valleys diverge from a median plane parallel with said support surface up to about 0.066 mm 13. The combination of claim 12 wherein said divergence from said median plane between said press rolls is 0 mm 14. The combination of claim 8 wherein said coating comprises one of nylon, silicone, PU, PET, PP, and PE. |
| 13. | 15 The combination of claim 8 wherein said base fabric is multilayer. |
| 14. | 16 The combination of claim 8 wherein said base fabric is woven. |
The prior art is replete with proposals for eliminating so-called open draws from paper machines. By definition, an open draw is one in which a paper sheet passes without support from one component of a paper machine to another over a distance which is greater than the length of the cellulose fibers in the sheet. All such proposals for eliminating open draws have as their object the removal of a major cause of unscheduled machine shut-down, the breakage of the sheet at such a point where it is temporarily unsupported by a felt or other sheet carrier. When disturbances in the normally stable flow of paper stock occur, the likelihood of such breakage is quite strong where the unsupported sheet is being transferred from one point to another within the press section, or from the final press in the press section to the dryer section. At such points, the sheet usually is at least 50% water, and, as a consequence is weak and readily broken. At present, then, an open draw will place a limitation on the maximum speed at which the paper machine may be run.
The prior art proposals for eliminating open draws include some form of transfer fabric to carry and support the paper sheet between components of the paper machine. In so doing, the transfer fabric may have to carry out several of the following separate functions: a) to take the paper sheet from a press roll or press fabric (felt); b) to carry the paper sheet into a press nip; c) to work cooperatively with a press felt in the press nip to dewater the paper sheet; d) to carry the paper sheet out of the press nip; e) to repeat functions b) through d) as necessary where the transfer fabric carries the paper sheet through more than one press; and, f) to transfer the paper sheet to another fabric or belt, such as, for example, a dryer fabric.
In general, and referring to the various functions of a transfer fabric identified above, where the transfer fabric removes the paper sheet from a press roll, a procedure rarely used in practice, it must overcome the strong adhesion the paper sheet will normally have for the roll, which may be very smooth. In the in-going side of a press nip, the paper is squeezed until it becomes fully saturated, at which point water will start to move out from the sheet into the water receptor, the press felt. As a consequence, there will always be a water film, perhaps partly broken, at the interface between the roll surface and the paper sheet. This film has to be broken before the paper sheet may be reliably transferred from the roll to the transfer fabric.
In the press nip, the transfer fabric must work cooperatively with a press felt to dewater and to densify the paper sheet. As a consequence, the surface topography and compression properties of the transfer fabric are critical for producing a paper sheet with a smooth, mark-free surface. Because, as is well known to those skilled in the art, even a high quality, well-broken-in press fabric may provide a very non-uniform pressure distribution in the nip, a transfer fabric having a smoother and harder paper-side surface than the press felt will provide a more uniform pressure distribution to the paper sheet being dewatered, and will impart a smoother surface to the sheet.
Further, a transfer fabric with suitable compression properties can in effect lengthen the press nip to increase the time and the paper sheet is exposed to pressure and to allow more time for water to leave the paper sheet under a given press load. In addition, a transfer fabric which is substantially impermeable to water and air will contribute to the dryness of the paper sheet by eliminating the possibility of re-wet after the press nip, as may occur when a conventional press felt carries the paper sheet out of the nip.
Clearly, a transfer fabric must be designed with the understanding that it will work cooperatively in the nip with a press felt as a functional pair in order to provide high dewatering efficiency and high paper quality.
Referring again to the various transfer fabric functions identified above, the transfer fabric should carry the paper sheet out of the press nip. That is to say, more precisely, the paper sheet should adhere to the surface of the transfer fabric upon exiting the nip, as opposed to following the press fabric out of the nip and then moving over to the transfer fabric after the nip. Not only does the latter permit re-wet while the paper sheet
remains in contact with the press fabric, but the moving of the paper sheet over to the transfer fabric after leaving the press nip would also constitute an open draw, the very problem the transfer fabric is intended to eliminate. Such a situation can lead to blistering or some other deformation of the paper sheet. A good adhesion of the sheet to the transfer fabric on the exit side of the nip is even more important in press configuration where the belt is run in the top position and the sheet is to be transferred on the underside of the belt.
As before, the paper-side surface and in fact the entire transfer fabric should be neither water-absorbent nor water-permeable at the nip, so that re-wet of the paper sheet by the transfer fabric may be avoided.
Where the transfer fabric carries the paper sheet through more than one press, the stability of the transfer fabric will become an important factor. The speed of consecutive presses in a press section can never be absolutely synchronized, and, normally, will increase somewhat downstream in the section. Under such conditions, the transfer fabric must be able to carry the paper sheet without blowing, blistering, or drop off. In addition, the transfer fabric itself must be of a durable design, capable of enduring the backside wear and high shear forces, which would attend its use through more than one press, without rapid degradation.
The final function of the transfer fabric is to effect a correct transfer of the paper sheet to the next section of the paper machine. In many applications, this will be a transfer to the first fabric in the dryer section. It is preferred that this first fabric should be of a design suitable for both paper drying and for the closed transfer of the paper sheet.
A typical dryer fabric in the first drying position may be a woven, all-polyester monofilament fabric. Fabrics used in first drying positions normally have a low air- permeability and a smooth, fine paper side. Hence, the surface to which the transfer fabric is to transfer the paper sheet may initially consist of smooth, hydrophobic monofilament knuckles.
The transfer from the transfer fabric to the first dryer fabric should be carried out with as low a contact pressure as possible in order to avoid the marking of the paper sheet by the knuckles. Since the dryer fabric is air-permeable, vacuum may be used to assist the transfer of the paper sheet from the transfer fabric. In order to avoid the marking of the paper sheet by the knuckles of the first dryer fabric, the vacuum level used at the transfer point must be as low as possible. It follows, then, that the transfer fabric must readily release the paper sheet at the transfer point so that the vacuum level required may be kept at a minimum level.
Summary of the Invention In view of the preceding discussion, it may be understood that a successful transfer fabric must be able to carry out several different functions as it carries a paper sheet from place to place in a paper machine. Correspondingly, the behavior of the transfer fabric must change in response to the conditions under which it is placed at different locations in the machine.
The most critical of these functions are: a) to remove the paper sheet from a press fabric without causing sheet instability problems; b) to cooperate with a press fabric
in one or more press nips to ensure optimal dewatering and high quality of the paper sheet; and, c) to transfer the paper sheet in a closed draw from one press in the press section to a sheet-receiving fabric or belt in the next press, or presses, in the press section, or to a dryer pick-up fabric in the dryer section.
The surface of the transfer fabric must have a topography or sculptured effect on a microscale providing a degree of uneveness which decreases, or smooths out, as the belt compresses under the levels of compression to which it is typically subjected in a press nip, but which restores itself as the belt expands after exit from a press nip, to carry out these functions. In other words, the surface topography of the transfer fabric must have a pressure-responsive, recoverable degree of roughness, so that, when under compression in a press nip, the degree of roughness will decrease, thereby enabling a thin continuous water film to be formed between the transfer fabric and a paper sheet to bond the paper sheet to the transfer fabric upon exit from the press nip, and so that, when the original topography is recovered after exit from the nip, the paper sheet may be released by the transfer fabric, perhaps with the assistance of a minimum amount of vacuum, to a permeable fabric such as a dryer pick-up fabric. At the same time, the transfer fabric must have the necessary compression and hardness properties to produce a mark-free paper.
In addition to having a surface topography with a pressure-responsive, recoverable degree of roughness, a successful transfer fabric must also have an optimal combination of the following additional functional properties: 1) surface energy, which will determine the interaction of the surface of the transfer fabric with water; 2) limited permeability to air or water; 3) compressional properties, both for the surface of the belt
and for its structures as a whole; 4) hardness; 5) modulus; 6) durability; and, 7) chemical, thermal and abrasion resistance.
The present invention is a transfer fabric for a papermaking, boardmaking, or similar machine having a caliper including a surface topography with the requisite pressure- responsive recoverable degree along with an optimal combination of the above noted additional functional properties. The pressure-responsive, recoverable degree of caliper remains a characteristic of the transfer fabric throughout its entire lifetime on the papermaking or boardmaking machine so that the transfer fabric will be capable of carrying out its intended function for that time.
The transfer fabric may be characterized as having a homogeneous coating throughout which creates a well-defined topography and a well-defined surface energy on its support surface, such a surface being favorable for taking a paper sheet from a press roll or press fabric, and carrying it into a press nip, where it cooperates with a press fabric.
The transfer fabric may be further characterized as having a sheet-facing surface, optimally only slightly permeable to water and air, with a pressure-responsive microscale topography. Under pressure, the microscale degree of roughness of this surface decreases, and the permeability disappears making the surface much smoother and allowing a thin, continuous film of water to be built up between the paper sheet and that surface.
Such a thin, continuous film of water provides much stronger adhesive forces between the paper sheet and transfer fabric than those between the paper sheet and the press felt, so that the paper sheet may consistently and reliably follow the transfer fabric when leaving the press nip. Even where the press fabric, by reason of structural expansion, creates a light
vacuum at the outgoing side of the press nip, the energy required to overcome the adhesive forces arising from the water film between the transfer fabric and paper sheet is greater than that required to overcome any adhesion the paper sheet may have for the press felt. In addition, the caliper regain of the paper sheet upon exit from a press nip is normally much slower than that of the press fabric. As a consequence, when a light vacuum arises in both the expanding press fabric and expanding paper sheet upon exit from the press nip, the latter holds its vacuum for a longer period of time and sticks to the transfer fabric by virtue of the thin, continuous water film disposed therebetween. As a consequence, the paper sheet will follow the transfer fabric.
Despite the strong adhesion the paper sheet has for the surface of the transfer fabric at the nip exit, the material composition of the paper side of the belt and its surface characteristics provide it with the necessary release properties to successfully transfer the paper sheet to another fabric or belt. These release properties are a direct consequence of the use of an appropriate polymer coating, and the slight permeability of the transfer fabric.
The slightly permeable belt, having a very low permeability to air and water, and having a polymer coating in accordance with the present invention, will be able to carry out the functions of the present invention quite well so long as it has an air permeability of less than 5 cubic feet per square foot per minute, when measured according to the procedure set forth in"Standard Test Method for Air Permeability of Textile Fabrics, ASTM D 737- 75, American Society of Testing and Materials, re-approved 1980. Such a low permeability in a saturated belt structure allows the transfer function of the belt in the course of use on
a paper machine to function uniformly as the pores allow for the uniform cleaning of paper fines and other materials.
The mechanism by which the water film is broken up during the span between the press nip and the point where the paper sheet is to be transferred to another carrier is thought to be primarily a function of the pressure-responsive microscale surface topography of the coating on the paper side of the transfer fabric. In this regard, in order to break up the water film, the recovered degree of sculpture of the surface topography of the transfer fabric should be at least equal to the minimum caliper of the water film.
As has been discussed above, the primary mechanism by which the present transfer fabric releases the paper sheet at a desired point is thought to be its pressure- responsive, recoverable surface topography, since the strength of the adhesive bond formed between the surfaces of the transfer fabric and the paper sheet depends upon the actual interfacial contact area and surface roughness of each.
The water film between the paper sheet and transfer fabric will tend to fill the low spots in the belt surface. As the pressure distribution changes in the interface between sheet and belt during expansion after exit from the sculptured effect of the nip, the belt will increase, after having been compressed to a substantially smooth condition in the nip. The increase of sculptured effect causes the water film to break. The work necessary to counteract the adhesion of the paper sheet to the transfer fabric and to separate the two from one another depends upon surface tension, which decreases with increasing water film thickness. Where there are low spots in the surface of the transfer fabric, the thickness of
the water film will be increased. This reduces the adhesion of the paper sheet to the transfer fabric at such locations and promotes sheet release.
It is also possible that air may be trapped in low spots on the surface of the transfer fabric as the transfer fabric, paper sheet, and press fabric are entering the nip. As the paper sheet is compressed in the nip, the air is compressed into such low spots. In the outgoing part of the nip, this compressed air expands, exerting a pressure which helps to break the water film.
The transfer fabric of the invention comprises a base fabric formed of interlaced continuous filament yarns with fibers being interdispersed evenly among the yarns throughout. A water base latex coating is dispersed throughout the base fabric coating all external services of the yarns and fibers and forming support and running surfaces with non- uniform valleys and peaks throughout. The transfer fabric so constructed has an uncompressed porosity of between 2.5 and 3.5 CFM and a compressed porosity of 0 CFM.
The paper holding intensity of the support surface is directly proportional to the porosity, the holding intensity being greatest when the fabric is compressed and less when uncompressed. This is primarily achieved by the existence of the peaks and valleys. Peaks diverge to about 0.066 mm substantially equally from a median plane when the fabric is uncompressed to being substantially eliminated creating a substantially planar support surface when compressed.
The transfer fabric of the invention may be formed which have a cross-section which is one of round, oval, rectangular, triangular, and star shaped. These yarns may be formed of one of PA, PPS, PE, PET, PEEK, PPA, PCTA, and PU or a combination
thereof. The latex coating is preferably formed of one of nylon, PU, PET, PP and PE or a combination thereof.
A specific embodiment of the present invention will now be described in more complete detail, with reference frequently being made to the figures identified as set forth below.
Description of the Drawings The construction designed to carry out the invention will hereinafter be described, together with other features thereof.
The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein an example of the invention is shown and wherein: Figure 1 is a diagrammatic view of a press section in a papermaking machine; Figure 2 is a sectional perspective view of the transfer fabric of the invention; Figure 3 is a sectional side view of the press fabric, press felt, and paper in the nip of the press rolls; Figure 4 is a sectional side view of the combination of Figure 3 showing the paper leaving the nip of the press rolls and transfer fabric as it approaches the vacuum roll; Figure 5 is a diagrammatic view showing height and depth of the sculpture of the surface of the transfer fabric; and, Figure 6 is a diagrammatic view of a coating method forming the fabric of the invention.
Description of a Preferred Embodiment Turning now to the drawings, Figure 1 is a representative arrangement of the press section of a papermaking machine. As shown, the paper sheet P is delivered from the forming section onto the press felt 12 and carried into engagement with the transfer fabric 14. From this point the paper sheet is sandwiched between the fabrics 12 and 14 as it is carried through the press rolls where the fluid in the paper is forced out and almost totally carried off by the press felt 12. After paper P moves past press rolls 16, it is held on the transfer fabric and carried to the dryer section. Here vacuum roll 18 acts to transfer the paper from the transfer fabric to the dryer section. The dryer section comprises the final phase of the paper forming process.
There are three phases in this process where the proper function of the transfer fabric is essential.
The first phase is where the press felt and transfer fabric come together. Here, the surface of the transfer fabric must be such as to take control of the paper sheet and hold it firmly as the paper is carried into the nip of press rolls 16. This is accomplished by the fact that the surface of the transfer fabric is less porous than that of the felt and as the wet paper engages with the surface of the transfer fabric a capillary effect takes place and secures the paper in place.
The second phase is when the paper emerges from the press rolls. As the press rolls are also not porous, they, along with the transfer fabric, grip the paper. If the paper continues to adhere to the press roll it will be placed under undue traction, stretched and marked or even broken. Therefore, the capillary effect of the transfer fabric must be
increased in this area so that the paper will be held with the transfer fabric as it passes through the nip of the press rolls.
Finally, the grip of the paper with the transfer fabric should be reduced prior to its engagement with the vacuum transfer roll so that it may be drawn from the grip of the transfer fabric without being stretched or broken.
The transfer fabric 14 shown in Figures 2-4 accomplishes each of these functions. Transfer fabric 14 comprises a base fabric 20 which may be woven, knitted, or non-woven. As shown, fabric 20 is woven with yarns 22 extending longitudinally and yarns 24 extending transversely of the machine direction of movement. When woven, the base fabric may be woven endless or flat and seamed to be endless.
Fabric 20 may be formed as a single layer as multi-layer fabric. When multi- layer, the layers may be formed together or united by needling or coating.
Fabric 20 has the fibers of a fiber batt 26 arranged over each outer surface and needled into the base fabric from each side so that the fibers of each fiber batt are evenly distributed or dispersed throughout fabric 20 forming a felt having substantially identical outer surfaces, i. e. the running surface and the support surface. Optionally, only a single fiber batt may be arranged over the support surface of fabric 20. In this instance, needling will be from only the support surface side dispersing the fibers evenly throughout fabric 20 and forming a support surface as described above. In this second process, the remaining or running surface will be slightly rougher than the support surface.
Fabric 20 with the fibers needled thereto or felted fabric 20 is formed with a porosity of between 40-160 CFM.
The yarns forming fabric 20 may be continuous filament monofilament or continuous filament multi-filament yarns. They may have a round, oval, rectangular, star or other shaped cross-section. They may be formed of any number of synthetic materials such as PA, PPS, PE, PET, PEEK, PPA, PCTA, PU, and combinations thereof to include minerals.
Felted forming fabric 20 is preferably saturated with a water based latex coating.
The coating as applied penetrates throughout felted fabric 20 so as to coat all internal and external fiber surfaces. The coating may be applied by way of a coating roll onto a surface of the fabric which is then moved to a vacuum dryer which causes the latex coating to saturate the fabric and be evenly distributed throughout. After coating the porosity of the fabric is reduced to about 3.5 CFM. Figure 6 shows an apparatus satisfactory for carrying out this method.
The coating produces sculptured outer surfaces 28 which have randomly arranged peaks 30 and valleys 32 formed thereover (see Figures 3 and 4). One of the surfaces 28 serves as the paper support surface 29, while the other functions as the roller surface. There peaks and valleys create substantially equally configurations, as indicated in Figure 5, which are about 0.066 mm in each direction from a median plane.
Any number of polymers are suitable for use as the coating such as PU, nylon, PET, PP, PE, and silicone.
Normally, transfer fabric 14 will have a caliper C'. It is with caliper C'the fabric has a porosity of about 3.5 CFM. The sculptured support surface 29 is effective to grip the wet paper delivered from the forming section because of the capillary effect created
by the water and the substantially non-porous support surface. As the transfer fabric is moved into nip of press rolls 16, transfer fabric 14 is compressed evenly throughout its thickness producing a reduced caliper C2. The compressive action forms the outer surfaces 28 and particularly support surface 29 when between the nip rolls to be substantially completely planar with the porosity of the fabric in the area of the nip being reduced to 0 CFM. The restructured or planar support surface intensifies the capillary action between the paper and surface 29 causing the paper to be more tightly engaged with or held by surface 28 during passage through and just after emerging from the nip of the press rolls.
This increased holding or gripping action is sufficient to draw the paper off the press felt and press roll smoothly and evenly thereby preventing undue stretching as it is carried onto the drying section.
As the press felt and transfer fabric leave the nip area, fabric 14 expands to return to substantially caliper C'with the peaks and valleys 30,32 reappearing on support surface 28. Again, the capillary action between the paper and support surface is reduced, bur remains sufficient to maintain the paper evenly positioned over the surface of fabric 14.
Finally, as the fabric 14 reaches vacuum drum 18, the vacuum forces of the drum draw the paper into contact with dryer fabric which carries it into the dryer section.
That transfer fabric 14 has remained its porosity of about 3.5 CFM as it reaches vacuum drum 18, air may be drawn through the fabric to further assist in releasing the paper from support surface 29 without stretching or tearing.
It is noted that caliper C'is always larger than caliper c2 even though it may decrease in size between passages through the press rolls.
A main advantage of the instant transfer fabric is that it is extremely durable due to the fact that it is coated throughout rather than having only a coating layer formed over the support surface. Coating layers become brittle in use, sometimes breaking off damaging the paper product and shortening wear life of the transfer fabric. Another advantage is that the sculptured support surface does not require an after treatment but is formed naturally during the saturation process further reducing cost. Finally, the coating operation by saturation is less costly than one in which a doctor blade is employed for coating a layer on a surface.