NOVEL FABRICS FOR HIGH TEMPERATURE PRESSING APPLICATIONS

Abstract: An article of paper machine clothing for use in the pressing or drying section of a paper making machine has a sheet-contacting layer having a glazed surface. The glazing of the surface may be effected through the application of heat and/or pressure, and provides the paper machine clothing with an extremely smooth, and almost completely sealed, surface. The article may be used on the press section of a paper making machine adapted to carry out an impulse drying process.

Description: NOVEL FABRICS FOR HIGH TEMPERATURE PRESSING APPLICATIONS Technical Field This invention relates to paper making machine fabrics, commonly referred to as paper machine clothing. Background Art The textile structures normally used as clothing in the pressing section of a paper making machine consist generally of a woven base structure covered by a layer of batt fibers needled thereto. The process of paper making comprises the deposit of a slurry of the paper making constituents on a forming fabric and then deliquifying the slurry to produce a self-coherent sheet. This sheet is then transported on a fabric to various stations at which deliguification is progressed to produce the final paper sheet. Deliguification is usually effected by a combination of pressing and drying at elevated temperatures together with various other treatments that may be desirable depending, of course, upon the final grade and type of paper to be produced. The combination of pressing and drying, together with the quality and nature of the fiber constituents, will determine the quality of the end product. The primary functions of the paper machine clothing and, in particular, of the press fabrics are to absorb paper expressed from the paper sheet in the press nip, to support the sheet in the press nip, to prevent crushing, to provide uniform pressure distribution over the paper in the nip between the press rolls, and to impart the desired surface finish to the sheet during its passage through the pressing and drying sections. At the same time, the paper machine clothing or press fabric in contact with the sheet serves to equalize pressure distribution during pressing, and to eliminate or to reduce shadow marking which may be caused by grooves or suction pressure rolls. The item of paper making machine clothing also serves to transfer the sheet from one position to another, and to act as a power transmission belt for driving all the undriven rolls in the machine. To perform these various functions, therefore, the paper making machine clothing requires demanding properties which are capable of being maintained on a continuous basis in a very rigorous chemical environment. A paper machine fabric, particularly a press fabric, should be capable of extended operations without substantial degradation of its performance. The high loading of the fabric in the press nip, however, produces over a period of time mechanical damage to components of the fabric, and this, in practice, limits the longevity of the fabric and the consistency of the desirable properties thereof. While many variations of the structure of base fabrics have been described and many combinations of fiber types and diameters have been used in the batt component, it is a matter of experience that a press fabric which has a more uniform distribution of material in its plane performs better in terms of sheet marking and dewatering, but it is also an observed fact that a press fabric which has fine fiber in the sheet-contact surface also performs its dewatering and sheet consolidation functions in a much more satisfactory manner. The use of fine fibers, however, is limited since the breaking load of the fibers themselves is self- evidently reduced in proportion to their cross- sectional area. As a result, extremely fine fiber of conventional materials such, for example, as nylon, do not have sufficient tensile strength and inherent integrity to withstand the rigors of extended use in the press and dryer sections of a paper making machine. Accordingly, it will be appreciated by the person skilled in the art that there is a trade-off between desirable surface properties of the fabric on the one hand and the drying/water absorbing properties of the fabric on the other hand in the press section of the paper making machine. The present Applicants have found, however, that they can provide an improved dewatering technique within a paper making machine by operating the press rolls at a higher temperature and using a modified press fabric. Disclosure of the Invention According to the present invention, therefore, there is provided a method of forming a paper sheet which method comprises depositing a slurry of paper forming ingredients on a forming fabric, dewatering said slurry to form a self-cohesive sheet and thereafter further dewatering said sheet by passing said sheet through the nip of a pair of press rolls characterized in that at least one roll is heated to a temperature in excess of 250"F, and in that the paper sheet is supported on a press fabric, the surface of which is glazed sufficiently to provide increased dewatering under the conditions prevailing at the roll temperature employed. The invention also includes an article of paper machine clothing for use in the pressing and/or drying section of a paper making machine which item comprises a sheet-contacting layer in which the sheet-contacting surface is glazed. Modes for Carrying out the Invention The sheet-contacting layer of the press fabric may be either partially glazed or more substantially glazed and the glazed surface is preferably smooth. In a typical embodiment of the present invention, an item of paper making machine clothing may comprise a woven support layer and a layer of batt fibers needled thereto; the batt layers serving as the paper support layer. The glazing of the exposed surface of the batt layer may be effected by the application of heat and/or pressure. In a particular embodiment of the present invention, the glazing may be effected by heating to a temperature in excess of 250°F. In a further aspect of the present invention, the batt layer may be composed of cross-linked nylon fibers. The cross-linking may be effected with an aqueous formaldehyde treatment of the individual nylon fibers or of the batt after formation of the batt and prior to formation of the press fabric, or of the press fabric after manufacture, or may be effected after installation of the item of paper making machine clothing on the machine. In a further aspect of the present invention, the press roll temperature employed may be at a temperature in excess of 250°F, the pressure in the nip of the press may be greater than 300 lbs/linear inch. Hitherto, the glazing or formation of an extremely smooth and also almost completely sealed press fabric surface has been considered detrimental to dewatering in paper making machines. It is well known, to those skilled in the art, and as described in the introduction to the specification, that the formation of an extremely uniform surface by fine denier fibers or by other methods is beneficial to other conventional dewatering as a result of improved uniformity of pressure application. However, when the surface becomes so smooth as to substantially seal over, dewatering generally decreases substantially. The present invention provides for an effective impulse drying process which operates by a mechanism different from that of conventional pressing. The use of a very high temperature press roll, i.e. greater than 250°F and normally greater than 300°F, is considered to form a steam pressure gradient which then drives a wall of water through the sheet. This produces extremely high levels of sheet dryness as it leaves the nip of the press rolls. Surprisingly, in this process, the present Applicants have discovered that a glazed press fabric surface is capable of providing exceptionally high sheet dryness values. In particular, the fabric types required to achieve the appropriate level of glazing either during a breaking-in period on a machine or by pre-treat ent may be selected from a number of individual fiber types or blends with adequate heat resistance, but with the property to flatten and deform into a glazed configuration when exposed to the nip pressure and temperature conditions. In the present invention, a blend of Nomex and Kevlar fibers was found to be particularly effective. Also effective were blends of Teflon with other fibers with adequate heat resistance. Nomex and Kevlar fibers are aromatic polyamide fibers. Nomex fibers are fibers of poly(meta- phenylene isophthalamide) ; Kevlar fibers are fibers of poly(para-phenylene terephthalamide) . Teflon is a fluoropolymer , and, specifically, is polytetrafluoroethylene (PTFE) . Other fluoropolymers and copolymers of fluoropolymers may be used in place of Teflon. The other fibers having adequate heat resistance may be fibers of polyaryletherketones or polyphenylene sulfide or cross-linked polyamides. In another aspect of the invention, the contacting surface of the press fabric may be composed of nylon fibers which are cross-linked by an aqueous formaldehyde treatment to form a glazed press fabric surface in accordance with the present invention. The advantage of this latter embodiment is that the imparted cross-linking also raises temperature resistance, but the fibers still retain a sufficient degree of deformability under moist hot conditions to provide a satisfactory glazed surface. Following is a description by way of example only of methods of carrying the invention into effect. EXAMPLE 1 A press fabric was produced comprising a batt layer formed of a 50% blend of Nomex and Kevlar fibers needled to a woven backing support fabric. The press fabric was operated on the press section of a machine with a heated roll temperature of 400°F under a pressure of 375 lbs/linear inch and at a speed sufficient to provide a 40 millisecond nip residence time at a felt moisture ratio of 0.2. Hand sheets of newsprint furnish, consisting of approximately 70% ground wood and 30% Kraft with added clay filler at a basis weight of 49 grams per square meter and an ingoing dryness of 45%, were fed through the nip. A non-glazed press fabric construction gave a sheet dryness of 60-65% and considerable post nip adherence to the press fabric. The press fabric surface was then glazed by operating the press at elevated temperature for a sufficient period to give a glazed surface and thereafter the treatment of the hand sheets described above was repeated. In the second test, the glazed surface gave values of 70-75% sheet dryness with good sheet release. EXAMPLE 2 Example 1 was repeated in that a non-glazed press fabric construction was employed in which the press fabric contacting layer was a batt structure of cross- linked nylon. Hand sheets of newsprint furnish were prepared as described in Example 1 and were then subjected to a pass through the heated press section of the machine with the non-glazed fabric construction. Again, a sheet dryness of 60-65% was obtained. The press fabric was then glazed as described in Example 1 to produce a glazed surface. A repeat of the experiment showed sheet dryness factors of 70-75% or more and good sheet release. EXAMPLE 3 A press fabric was produced comprising a surface batt layer of 85% Teflon with 15% polyphenylene sulfide. Several batt layers of a high temperature resistant polyamide fiber were located beneath the Teflon surface layer. The fabric was produced using a standard needling procedure with a standard woven polyamide base fabric. The press fabric was tested on a pilot press section of a paper machine operated at the following high temperature pressing conditions: heated roll temperature - 400°F, nip pressure - 500 psi, and a machine speed to yield a nip residence time of 35 milliseconds. Two types of handsheets were used to evaluate the press fabric: LWC (light weight coated) and newsprint. The LWC handsheets were comprised of 60% groundwood with 40% Kraft. The newsprint handsheets were comprised of 80% TMP (thermal mechanical pulp) with 20% Kraft. Both types of handsheets were prepared to a solids content of approximately 37%. Before testing, the Teflon surfaced fabric was cycled for approximately 500 compression cycles at room temperature. After cycling, the fabric surface appeared unscathed and the air permeability of the fabric was measured to be 20 in. of H 2 0 vac. using an HSPT (Huyck-Smith Porosity Tester) . With the HSPT, a value of 20 indicates an open fabric, while a value of 150 indicates a closed/sealed fabric. At this stage of cycling (-500 cycles) , pressing trials were conducted which produced LWC handsheets with a final sheet dryness of 64%, and newsprint handsheets with a final sheet dryness of 65%. Following this pressing trial, the press fabric was allowed to cycle through the heated nip. After approximately 3000 cycles, the press fabric's surface appeared glazed and sealed. At this time, fabric air permeability was measured to be 150 in. of H 2 0 vac. - indicating a sealed surface. Once the fabric became glazed and sealed, pressing trials were repeated. Results of the pressing trials showed that the glazed fabric produced handsheets with higher final dryness than the unglazed fabric. With the glazed fabric, final sheet dryness values of 69% to 74% were achieved with the LWC and 67% to 71% with the newsprint. The glazed fabric continued to produce these high sheet dryness values throughout the duration of the trial - 28,000 cycles. While particular embodiments of the present invention have been shown and described, it is clear that various changes and modifications may be made, and it is therefore intended in the following claims to cover all modifications and changes as may fall within the true spirit and scope of the invention.

Claims: What Is Claimed Is: 1. In an article of paper machine clothing for use in high temperature pressing applications, said applications being those in which a paper machine press roll temperature is greater than 250°F, said article comprising a woven support layer and a sheet- contacting surface layer, said sheet-contacting surface layer being a carded batt of staple fibers needled into said woven support layer, the improvement comprising a glazed surface on said sheet-contacting surface layer, said glazed surface being provided by subjecting said sheet-contacting surface layer to a temperature in excess of 250°F under elevated compression applied in a direction perpendicular thereto, whereby said article of paper machine clothing may provide exceptionally high sheet dryness values. 2. An article of paper machine clothing as claimed in claim 1 wherein said glazed surface is a microporous surface. 3. An article of paper machine clothing as claimed in claim 1 wherein said glazed surface is a partially sealed surface. 4. An article of paper machine clothing as claimed in claim 1 wherein said glazed surface is smooth. 5. An article of paper machine clothing as claimed in claim 1 wherein said staple fibers comprise aromatic polyamide fibers. 6. An article of paper machine clothing as claimed in claim 5 wherein said aromatic polyamide fibers are a mixture of poly(meta-phenylene isophthalamide) fibers and poly(para-phenylene terephthalamide) fibers. 7. An article of paper machine clothing as claimed in claim 1 wherein said staple fibers comprise a mixture of fibers of fluoropolymers or copolymers of fluoropolymers and other high temperature resistant fibers. 8. An article of paper machine clothing as claimed in claim 7 wherein said fluoropolymer is polytetrafluoroethylene and said other high temperature resistant fibers are fibers of polyaryletherketones or polyphenylene sulfide. 9. An article of paper machine clothing as claimed in claim 1 wherein said staple fibers are cross-linked polyamide fibers. 10. An article of paper machine clothing as claimed in claim 9 wherein said polyamide is cross-linked through a treatment with an aqueous formaldehyde solution.

Inventors: BARLOW SANDRA K, KENNEY MARYANN C, KONOPASEK LUDMILLA, MORIARTY MICHAEL G, BARLOW, SANDRA, K., KENNEY, MARYANN, C., KONOPASEK, LUDMILLA, MORIARTY, MICHAEL, G.
Applicants: ALBANY RESEARCH UK, BARLOW SANDRA K, KENNEY MARYANN C, KONOPASEK LUDMILLA, MORIARTY MICHAEL G, ALBANY RESEARCH (U.K.) LIMITED
Application year: 1993
Main classification: D04H1
IPC codes: D04H1, D04H13, D21F7