DESCRIPTION

This invention relates to paper machine clothing and has particular reference to paper machine clothing for use in the pressing and drying sections of a paper machine, although the invention described herein is equally applicable in other sections of a paper making machine.

A conventional paper making machine forms a web by depositing a slurry of pulp fibres to be formed into a paper sheet onto, for example, a travelling Foudrinier wire. After initial dewatering on the Foudrinier wire, the forming paper sheet or web is transferred to a press section where the web passes through one or more nips comprised of either roll or shoe combinations. During this time, the sheet is consolidated and further properties of the sheet may be developed. Thereafter, the web passes over a series of heated dryer drums (and optionally through a calender) and is then wound onto a

roll. There are many variations in the various sections of the machine such as the forming or press sections. In such a machine, the web usually arrives at the press section with about 80% wet base moisture (a ratio of water to fibre + water) and leaves the press section with approximately 60% moisture ratio (or 40% dryness). The remaining moisture has to be removed by thermal evaporation in the dryer section as the web passes over a series of heated drums. In a typical paper making machine, such as that manufacturing newsprint, a significant number of dryer drums will be employed, sometimes on the order of 50 to 70 drums per machine. Each dryer drum is expensive to construct and to operate and requires a provision of steam fittings and a supply of steam or other heat source for each drum.

In recent years, the principle of impulse drying has introduced. Impulse drying is a method of dewatering a paper sheet by incorporating both high temperature and pressure in a press section of a paper making machine. During this process, the surface of the press roll is usually heated to a temperature of 150°C or greater, and is in direct contact with the paper. With such a

process, the resulting paper sheet dryness is significantly increased to 60% or higher. The effectiveness of such a system is such that the number of dryer drums in a paper making machine can be substantially reduced.

The principle of operation underlying impulse drying is believed to be that the heated roll forms a zone of steam in the paper sheet during pressing of the sheet which drives out liquid water as the steam passes through the sheet. Generation of this steam front, and high temperature of the press roll provides rather severe conditions for the press fabric.

In order to withstand this high temperature, the press fabric should have sufficient thermal resistance to allow continued exposure to high temperature steam and occasional exposure to the high temperature press roll directly during breaks in the paper sheet passing therethrough.

In addition, this press fabric should adequately release the paper sheet after the nip to provide for acceptable sheet runnability. Also the press fabric

should minimize rewet of the sheet as it exits the nip to produce a sheet with maximum dryness. These properties should be combined with sufficient mechanical durability to withstand the severe conditions of the impulse drying process. According to traditional pressing theory, a press fabric should also provide and maintain adequate permeability for the water leaving the sheet.

According to the present invention, therefore, there is provided an article of paper machine clothing which comprises a first woven base layer and a second sheet contacting layer carried thereby, characterised in that the sheet contacting surface of said second layer comprises a polyfluorocarbon polymer.

The polyfluorocarbon polymer may be in a fibrous form. The said second layer may comprise a fibrous structure of batt or staple fibre which is needled to the base layer. The second layer may typically comprise a batt of 100% polyfluorocarbon polymers or may be a blend of fibres of polyfluorocarbon with any high temperature resistant fibres such, for example, as polyamide, polyaramid, polyester, polyimide, polyetherketone.

polybenzimidazole or polyether imide. The polyfluorocarbon may be polytetrafluoroethylene of the type commercially available under the trade name "TEFLON" (PTFE) .

In an alternative aspect of the present invention, the sheet contacting surface may contain an effective amount of polyfluorocarbon to achieve the desirable properties for the impulse drying process. The effective amount of polyfluorocarbon may be in the form of a discrete surface layer, a coating on the surface on the fibres constituting the said second layer, a pre-formed nonwoven layer of polyfluorocarbon, or a finely woven polyfluorocarbon fabric, each of which can be needled or otherwise bonded to the surface to incorporate it as part of the surface layer.

In another aspect of the present invention, the surface layer incorporating polyfluorocarbon may be in a form other than a textile form. The sheet contacting surface of said second layer may comprise a film of polyfluorocarbon. The layer may for example be formed by bonding, laminating or otherwise attaching a

polyfluorocarbon polymer film, to the remainder of the second layer.

Either a monolithic, perforated or microporous film can be employed. A surface layer can also be formed by coating with a fluorocarbon emulsion or suspension, by spraying, dipping or other suitable methods. The surface layer may also be formed by direct application of a fluorocarbon in particulate form.

In the practice of this invention, it has been noted that after multiple compressions within the press nip press fabrics have significantly reduced air permeability, yet surprisingly still continue to produce high sheet dryness values; the more so since traditional press fabrics having such low air permeabilities do not normally dewater the sheet effectively.

Following is a description by way of example only and with reference to the accompanying drawings of methods of carrying the invention into effect.

In the drawings:-

Figures 1 (a), (b) and (c) are a series of graphs relating to Example 2.

Figure 2 is a photomicrograph of the sheet contacting surface of a PTFE surfaced press fabric of Example 3.

EXAMPLE 1 An experimental pilot press fabric, made up of 11 test sections, was prepared using conventional carding and needling processes. Each experimental section represented either a candidate fibre or a test structure. Each candidate fibre was prepared into a carded web and a total of eight card web layers were combined to construct each experimental section. All sections were needled onto a common woven base fabric. The experimental felt was installed on a pilot impulse drying machine and run at the following conditions:

Nip pressure: 375 pli Roll temperature: 160°C (320°F) Fabric speed: 107 fpm

The handsheets used during this test were a newsprint grade comprised of 71% groundwood, 29% high yield sulfite. Five of the eleven test sections in the experimental felt were constructed with a paper contacting surface comprised of at least 50% by weight Teflon. Of the 11 sections tested during this study, all five Teflon containing felts produced paper sheets with the highest dryness, ranging between 70% and 75% dryness. All of the other high temperature resistant materials tested (such as polyaramid, polyetheretherketone, and an aromatic polyamide 6T) , produced paper sheets with lower dryness values (between 61% and 69%).

EXAMPLE 2

A series of experimental press fabrics were constructed with varying percentages of Teflon fibre in the paper contacting layers of the fabric. The blends ranged from 0% Teflon to 100% Teflon, in increments of 25%. Each fabric sample was conditioned at 21°C for 500 cycles at 1000 psi, and then tested on a laboratory impulse dryer tester at the following conditions:

Pressure: 5500 KPa

Nip Residence: 45 msec

Upper Platen Temp: 190°C

The paper stock used for these tests was 100% Bleached Softwood Kraft with a 500 CSF prepared into 50 g/m ² handsheets with an ingoing dryness of 36%. The ingoing felt moisture ratio for each fabric was 0.35. During testing, final paper dryness and felt moisture gain were determined. In addition, each felt's air permeability after conditioning was also measured.

Results of the study clearly show that as the percentage of Teflon fibre in the paper contacting layer increases, both paper dryness and felt moisture gain increase significantly, while the press fabric's air permeability decreases. Traditionally, lower air permeability in a press fabric usually relates to lower dewatering capability; however, in this case, the trend is opposite. Under the test conditions discussed above, approximately 10% higher paper dryness values are achieved with the sample having a 100% Teflon paper contacting layer compared to the sample containing 0% Teflon, while at the same time air permeability

decreased from 12.2 cfm for the 0% Teflon sample to 4.2 cfm for the 100% Teflon sample.

The results of this study are shown below in Table 1 and are displayed in Figure 1 of the accompanying drawings.

Table 1. Teflon Blend Press Fabric Structure Study

An experimental press fabric containing a 100% Teflon paper contacting layer was tested on a pilot paper machine with an impulse drying roll and an extended nip press shoe. The conditions used for this trial were as follows:

Maximum nip pressure: 600 pli

Machine speed: 820 m/min (2500fpm)

Hot Roll Temperature: 177°C (350°F)

Paper basis weight: 45-50 g/m ² Ingoing paper dryness: 25-36%

During this trial, a final paper dryness greater than 60% was obtained. Subsequent to this trial, a sample of the used press fabric was returned to the laboratory and examined. Visual examination showed that the surface was heavily sealed and glazed. An SEM photomicrograph of this surface (see Figure 2 of the accompanying drawings) confirmed this condition. A specimen was removed from this area of the fabric and subsequently tested on a laboratory impulse dryer tester for dewatering using 50 g/m ² handsheets at 36% initial dryness. Regardless of this sealed surface, the results, surprisingly, showed paper dryness as high as 65%. Through our experience, any other press fabrics having a similar glazed or sealed surface would be expected to produce a very low level of dewatering, thus low paper dryness.