Patent Application
20050136757
The present invention relates to an improved press felt for use on a papermaking machine. More particularly, the invention relates to press felts that incorporate a scrim formed at least partially from a regenerated cellulosic material as a component of the batt materials attached to one or both sides of the base fabrics of the felts in order to improve the dewatering capability of the felt.
Press felts are endless belts which may contain a seam and which are used to convey an embryonic paper web from the forming section, through the press and into the dryer section of a papermaking machine so as to dewater and ultimately dry the paper product so that it is suitable for use. In the press section, at least one press nip is typically provided between either a pair of rotating cylindrical rollers, or a roller and concave shoe. The embryonic paper web passes through the at least one press nip laid either upon a single felt, or sandwiched between at least two press felts. As the web passes through the at least one press nip, water is expressed from it and passes into the at least one press felt.
Papermaker's press felts are well known. See, for example, U.S. Pat. No. 4,199,401 to Liu et al., U.S. Pat. No. 4,356,225 to Dufour, U.S. Pat. No. 4,414,263 to Miller et al., U.S. Pat. No. 4,806,413 to Penven, U.S. Pat. No. 5,360,656 to Rexfelt et al., and U.S. Pat. No. 5,864,931 to Best et al. These felts are usually comprised of a woven base fabric (typically formed of nylon or similar polymeric yarns) to which is attached, generally by needling, at least one layer of a pre-tacked staple fiber web, commonly referred to as a batt. Typical press felt batts will usually include between one and about 5 or more layers of a pre-tacked staple fiber web needled onto a first planar surface of the base fabric (usually the surface which, when in use, will be in contact with the paper sheet, and is hereafter referred to as the “PS”) to form a PS batt, and from none to one or more layers needled to the opposite planar surface (which when in use will be in contact with the equipment of the paper machine, and is hereafter referred to as the “MS”) to form the MS batt. The staple fibers used to form either or both the MS and PS batt are typically made from one or more nylons, polyesters or other polymeric materials such as are commonly employed in the manufacture of industrial textiles.
The batt provides a smooth surface for the paper web and a void volume into which water, which has been expressed from the paper web at the press nip, can be received. The base fabric provides some additional void volume, as well as a stable structure to which the batt can be attached. The base fabric is typically comprised of interwoven polymeric monofilament or multifilament yarns to which the batt is attached, generally by needling or other entanglement process such as is known in the art.
After the paper web has been pressed in at least one nip in the press section, it will still contain an appreciable amount of water, as much as from 30% to about 60% or more by weight. This remaining water must now be removed in the dryer section of the papermaking machine in order to provide a paper product. The final drying of the paper product is typically carried out by evaporative means, which requires a large amount of energy. This adds substantially to the cost of manufacturing the paper product. Generally, a 1% increase in the dryness of the sheet exiting the press section will translate into about a 4% energy savings in the dryer section. It is also possible that the speed of the paper machine may have to be reduced or at least limited due to the evaporative capacity of the dryer section.
Thus, it would be highly desirable if the water removal characteristics of the press felts could be improved so as to increase the amount of water they are capable of transporting away from the paper product as it passes through the press section.
It has been known to use regenerated cellulosics such as rayon as a component of papermaking fabric batt materials. However, such use has generally been restricted to certain specific circumstances. One known application provided an article of paper machine clothing for a press section of an impulse drying machine having a paper contacting surface layer which included a thermal barrier with sheet release properties, a base structure layer, and at least one intermediate layer. This intermediate layer could include fine denier fibers and/or hydrophilic fibers such as wool, cotton and regenerated cellulosics. Fabrics constructed in this manner and evaluated on a pilot scale impulse drying machine operating at 205° C. were reported to have achieved 4 to 5 percentage points of added dryness in the sheet. However, the intermediate layer was heat shielded, and the improved drying appears to have been mainly due to the high drying temperature of about 205° C. This was a press fabric for use at temperatures well above the normal operating temperature range of press sections, which typically run between about 40° C. and about 80° C., and clearly involved a different application.
Another known felt included a so-called “flow control” layer located between the batt and base to “impede rewetting of the paper web” as it exits the press nip. This flow control layer was reported to be formed of a spunbonded filamentary nylon material which is noncircular in cross-section (such as trilobal). It was also noted that the flow control layer could be formed from various materials, including rayon. However, a hydrophobic treatment was imparted to the flow control layer to prevent water absorption.
Another known press felt has been reported that includes a high proportion of fibrillatable fibers located in at least the PS surface of the batt so as to provide a relatively fine sheet supporting surface for the paper web. The PS surface was indicated as being formed from fibers which are as fine as possible (below 1 denier in size). These fine fibers occur as a result of the fibrillation of relatively larger regenerated cellulosic fibers (e.g. >1 denier in size) due to hydroentanglement or mechanical pressure.
A transfer fabric has also been known that includes a base structure and a fiber batt layer which is impregnated with a polymer matrix. The batt fibers differ from one another with respect to their surface properties so that the PS surface of the belt facing the web has both hydrophilic and hydrophobic areas.
The vast majority of press felts which are manufactured for, and are in use in paper mills today, consist of 100% nylon staple fiber in at least the batt, mainly due to its abrasion resistance, resiliency and tenacity.
In accordance with the present invention, it has been determined that, by incorporating a scrim of a regenerated cellulosic product, in particular viscose rayon, in the batt of a press felt formed from polymeric staple fibers, it is possible to achieve an improvement of from about 3% to about 8%, or more, in the felt's dewatering capability when compared to an equivalent felt which lacks the scrim of regenerated cellulosic fibers. As used herein, the term “scrim” refers to a woven or nonwoven light weight textile product in mesh or similar form.
Further, in accordance with the invention it is possible to incorporate the scrim into virtually any position in the batt stratification, but preferably relatively near the paper side surface of the batt, and still obtain similar improvements in dewatering. It is also possible to incorporate the scrim into the machine side batt, where present.
Preferably, the fibers used to form the scrim are from about 1.1 dtex to about 44 dtex or more in size. More preferably, the fibers are about 3-15 dtex in size and can be blended together with the polymeric fibers in the scrim, if desired. The scrim can be made by weaving or can be formed as a nonwoven mesh.
Certain terminology is used in the following description for convenience only and is not considered limiting. Words such as “up”, “down”, “top”, and “bottom” designate direction in the drawings to which reference is made. This terminology includes the words specifically noted above, derivatives thereof and words of similar input. Additionally, the terms “a” and “one” are defined as including one or more of the referenced item unless specifically noted. The following abbreviations are also used herein: MS—“machine side”; PS—“paper side”; MD—“machine direction”; and CD—“cross direction”. As used herein, “scrim” is defined as a light weight woven or nonwoven textile such as a mesh or a similar fabric.
Referring to
The base fabric layer 12 can be a flat or endlessly woven fabric of the type known to those skilled in the art. Alternatively, the base fabric layer can be a spirally wound construction in which a strip of material having a width less than an overall width of the press felt 10 is helically or spirally wound to achieve a desired width of the press felt 10, and the adjacent edges of the wound strip are connected together, for example, as disclosed in U.S. Pat. Nos. 5,360,656 or 5,268,076.
Preferably, the at least one layer of staple fiber batt 15 comprises a plurality of layers of staple fiber batt material 20, 22, 24, 26, 28 which are preferably located on the PS of the base fabric 12. One or more layers of staple fiber batt material 30 may also be located on the MS of the base fabric 12, as shown in
Located between at least two of the layers of staple fiber batt material 26, 28 is a scrim 27 formed at least partially of a regenerated cellulosic material. As shown in
In the preferred embodiment, the regenerated cellulosic fibers in the scrim 23, 27 have a dtex from at least about 1.1 to about 44. When a plurality of scrims 23, 27 are utilized, as shown in
As shown in
The scrim 23, 27 of woven material can be manufactured using the same techniques that are known to those skilled in the art by flat or endless weaving, and can be produced the full width of the press felt 10, or can be produced as a narrower band of material that is spirally wound onto the press felt 10 as it is being formed.
Alternatively, in another embodiment of the invention as shown in
It has been found in connection with the invention if the regenerated cellulosic staple fibers of the scrim and the polymeric staple fiber yarns of the batt have too small of a size that one or more performance characteristics of the fabric are adversely affected. Accordingly, in the most preferred embodiments of the invention, the regenerated cellulosic fibers of the scrim 23, 27 have a dtex of at least about 3, and the polymeric staple fibers of the batt also have a dtex of at least about 3.
The regenerated cellulosic fiber material is preferably viscose rayon, and may be solid, hollow or otherwise shaped, such as Viloft® available from Courtaulds. It has been found in experimental trials that regenerated cellulosic fibers which are resistant to fibrillation are preferred for this use.
Alternatively, the scrim 23, 27 can contain bi-component fibers having an outer sheath made of a low melting point polymer, and an inner core made of a higher melt point polymer. Upon heat treatment of the press felt 10, the low melt point sheath material at least partially melts and helps to retain the other fibers in the batt layers and scrim in place. Other types of bi-component yarns may also be used.
Preferably, a weight of the regenerated cellulosic fibers in the PS scrim(s) in the press felt is from about 75 to about 1000 gsm (grams per square meter). More preferably, a weight of the regenerated cellulosic staple fiber in the scrim(s) 23, 27 in the press felt is about 300 to about 700 gsm.
Additionally, one or more layers of staple fiber batt material 30 which may be comprised of polymeric fibers is/are provided on the MS 18. Additional scrim(s) of the regenerated cellulosic material can also be located between these MS batt material layer(s) 30 and the base fabric layer 12, depending on the particular application.
In accordance with the invention, further improvements in reducing rewetting of the paper web 16 after it has passed through the nip of the press rolls may be provided if a hydrophobic surface treatment is applied to at least one of the scrim 23, 27 and/or the regenerated cellulosic fibers in the scrim 23, 27 or to the press felt 10. While this appears counterintuitive, it is believed that improved capillary action for removing water from the PS of the press felt 10 results from the hydrophobic treatment. In testing conducted on press felts in accordance with the invention, hydrophobic treatment resulted in approximately 1% better moisture removal from the paper web 16.
Experimental Trials
Experimental trials were conducted to determine the dewatering capacity and performance characteristics of a felt constructed in accordance with the teachings of the present invention. The press felt consisted of two layers of a base fabric whose basis weight was about 600 gsm (grams per square meter), to which nylon batt materials having a basis weight as indicated in Table 1 below was needled. One or more layers of a viscose rayon scrim was placed in the batt, as indicated. Finally, one or more upper batt layers of nylon staple fibers were located on the PS. The felt was assembled using normal industrial textile assembly methods consistent with the manufacture of papermakers' press felts and then installed in the first press position (i.e. the press closest to the forming section) of a papermaking machine manufacturing. A control felt, which did not include any regenerated cellulosic fibers in the batt, was run before the experimental installation. The machine was run at a speed of about 2,750 fpm (feet per minute). The experimental and control fabrics were exposed to identical physical conditions of furnish, temperature, machine speed, etc.
The consistency of the sheet was measured immediately downstream of the press nip in the center of the sheet. Measurements were made by means of “grab sampling” portions of the pressed sheet whereby a metal cup was used to remove a sample of the sheet immediately following the first press nip. The samples were each weighed, then oven dried and weighed again to determine their moisture content. We found that, on average, the control felts provided about 45.2% consistency as compared to 51.15% consistency for the trial felt. This represents an improvement in sheet consistency following the nip of 5.95%. The consistency was measured at normal operating temperatures, between 40 and 80 degrees C, for the press environment.
The scrim used in the experimental tests was a commercially available fabric made from 100% rayon natural pulp fibers in 8 layers. The fabric was woven from multifilaments in a plain weave and had a mesh and knock of approximately 20×21 (warp×weft per inch). Each layer had an air permeability of about 1,000 cfm and the assembled scrim had an air permeability of about 495 cfm. The caliper of the scrim was approximately 2.0 mm and the caliper of each of the 8 layers was about 0.35 mm. The basis weight of each layer was about 56 gsm and the basis weight of the fabric was 454 gsm. The individual fiber size of the multifilament yarn components used to weave the fabric was about 1.8 dtex, and the multifilament was about 0.20 mm in diameter.
While the preferred embodiments of the invention have been described in detail, the invention is not limited to these specific embodiments described above which should be considered as merely exemplary. Further modifications and extensions of the present invention may be developed and all such modifications are deemed to be within the scope of the present invention as defined by the appended claims.
The present application claims the benefit of U.S. Provisional Application No. 60/532,194, filed on Dec. 23, 2003, which is incorporated by reference herein as if fully set forth.
| Number | Date | Country | |
|---|---|---|---|
| 60532194 | Dec 2003 | US |
