PAPER MACHINE CLOTHINGS CONSTRUCTED OF EXPANDED PTFE FIELD OF THE INVENTION This invention relates to paper machine clothing suitable for use in the forming, pressing or drying sections of a paper making machine. More particularly, it relates to such fabrics made from monofilaments of synthetic polymer resins, specifically expanded PTFE.

BACKGROUND OF THE INVENTION The modern papermaking machine is in essence a device for removing water from the paper furnish, which is a slurry of papermaking constituents. The water is removed sequentially in three stages or sections of the machine. In the first section, also known as the forming section, the furnish is deposited on a moving forming wire and water is drained through the wire to leave a paper sheet or web having a solids content of about 18 to 25 percent by weight. The formed web is carried into a wet press felt section and passed through one or more nip presses on a moving press felt to remove sufficient water to form a sheet having a solids content of 36 to 50 percent by weight. This sheet is transferred to the dryer section of the papermaking machine where dryer felts press the paper sheet against steam heated cylinders to obtain a 92 to 96 percent solids content.

On papermaking machines, endless belts are employed in the various sections to carry the sheet or web of paper. There are a wide variety of forms of the endless belts, some fabricated from metal and others from textile material such as cotton, cotton

and asbestos, asbestos and synthetic fibrous or filamentous materials. The selection of a given material is dependent to some degree upon the use to which the fabric will be put, i.e., as a forming fabric, dryer felt, etc.

One form of belt which has been used extensively as a forming wire in the forming section of the papermaking machine is one fabricated from an open weave of synthetic, polymeric resin monofilaments. Such fabrics generally perform well in the forming section although there are certain limitations. For example, the relatively open weaves, particularly when run at highest speeds, lack dimensional stability. This shortens the overall life of the forming wire which is subject to abrasion as it shifts in position on the machine.

In addition, the relatively open weaves are less than fully supportive of the furnish fibers deposited on the wire. Ideally, the fiber and sheet supporting properties of a wire should be increased without significant decrease of water removal through drainage.

Press fabrics operate in the press section of the paper making machine. In this section, the paper sheet is transported by the press fabric and the sheet and fabric are pressed between the nip of the press rolls, which exerts enormous pressure on the sheet and fabric, thereby dewatering the sheet by pressing out the liquid. Also, temperatures in this section of the machine are relatively high.

The press fabric generally consists of a base layer and a batt layer of staple fibers needled to the base. Synthetic materials have become the norm, with polyamide being a particularly favored material.

The fabric in the drying section of the machine together with its sheet of paper tends to be

subjected to elevated temperatures in a rigorous chemical environment. Dryer fabrics or "dryer screens" employed in the paper making industry have, traditionally, been formed from a variety of materials such as poly (ethylene terephthalate), polyphenylene sulfide and polypropylene. Each material has different properties and pricing. An important property for any material used as a dryer screen in a paper making machine is that the material should have good hydrolytic stability and good dimensional stability.

Polypropylene is the cheapest material presently available; it has excellent hydrolytic stability, but poor dimensional stability at elevated temperature, and as a result it has only limited use.

Poly(ethylene terephthalate) (PET) is moderately priced, has exceptional dimensional stability and reasonable hydrolytic stability.

Poly(ethylene terephthalate) is the predominant material currently used in the market place and in most cases, the hydrolytic stability of poly(ethylene terephthalate) can be improved by the addition of carbodiimide stabilizers. Polyphenylene sulfide has excellent dimensional and hydrolytic stability, but is extremely high priced, is more difficult to work, and tends to suffer from brittle fracture problems in the crystalline state due to normal flexing experienced on the paper machine.

SUMMARY OF THE INVENTION The present invention is an article of paper making machine clothing suitable for use in the forming, pressing or drying sections of a paper

making machine wherein the article is comprised of a fibre structure constructed of yarns of expanded poly(tetrafluoro ethylene) ("expanded PTFE").

Expanded PTFE is a material available from W.L.

Gore and Associates, Inc., Newark DE, USA. As described in U.S. Patent No. 3,953,566, expanded PTFE is a highly porous material characterized by a microstructure of nodes interconnected by fibrils.

The disclosure of the '566 patent is incorporated herein by reference. The material, in addition to its high porosity, exhibits high tensile strength.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows the results of an abrasion test performed on the preferred embodiment.

Figures 2a-2d show SEM's of the filament used in the preferred embodiment.

Figures 3a-3b show the degree of abrasion for the prior art.

Figures 4a-4b show the degree of abrasion for the preferred embodiment.

Figures 5a-5b show the degree of abrasion for the prior art.

Figures 6a-6b show the degree of abrasion for the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The Applicants have found that paper machine clothings comprised of expanded PTFE filaments provide industrial fabrics that exhibit excellent abrasion resistance, contamination resistance, and crease resistance. Tensile properties of expanded

PTFE woven goods equal or exceed that of currently available polyesters or polyamides. The enhanced tensile properties relative to these other materials are believed to improve the dimensional stability of the fabrics. Also, the use of expanded PTFE monofilaments allows higher use temperatures, up to 288"C, than temperatures at which conventional materials could be used.

With the fabrics of the present invention, many of the above-described shortcomings of the prior art are eliminated or reduced in severity. Forming wires constructed according to the invention may be fabricated from an all monofilament fabric which provide an exceptionally smooth surface to contact the paper sheet. A maximum degree of support is achieved. As a result, relatively mark free paper product is obtained, while the desired advantages of an all monofilament fabric are retained, such as an efficient degree of water drainage. In addition, the structure of the fabrics and the wires of the invention is such that a greater dimensional stability is achieved in both the machine and cross- machine directions. This reduces yarn abrasion and increases operating life. The overall operating life of the forming fabric is significantly increased over prior art fabric.

One of the more important features of paper machine clothing in accordance with the present invention is its potential use in high temperature sections of a paper making machine, in particular dryer fabrics and dryer screen fabrics, since the material from which it is made is not readily hydrolyzed. That is, it exhibits good resistance to hydrolysis. PTFE is known to be a highly inert, water repellant material.

It will be appreciated by the person skilled in the art that with the tendency towards ever higher

temperatures in the forming and pressing sections of a paper making machine, paper machine clothings constructed of expanded PTFE can be used in all sections of the paper making machine. In the forming section it is possible to form an open weave using monofilament materials which allow for adequate support of the solid materials in the furnish and yet allow sufficient dewatering to produce a coherent sheet preparatory to pressing.

In the pressing section, by providing both the support layer and at least a portion of the surface layer of the pressing fabric in accordance with the present invention, pressing fabrics much more tolerant of high temperature operation are produced.

The invention, therefore, is concerned not only with the production of paper machine clothing (PMC) materials which may be of woven or spiral or of other suitable monofilament structures, in which monofilaments may extend in both the machine direction and the cross direction of the fabric, but also include other PMC structures. Such material may be used to produce PMC fabrics comprised of staple, multifilament, and/or monofilament fibres.

Typical range of sizes of monofilaments used in press fabrics and dryer fabrics are 0.20 mm - 1.27 mm in diameter or the equivalent mass in cross- section in other cross-section shapes, e.g. square or oval.

For forming fabrics finer monofilaments are used, e.g. as small as 0.05 mm, while special Industrial applications may use monofilaments up to 3.8 mm.

Filaments of expanded PTFE can be formed by helically winding expanded PTFE sheet material. See U.S. Patent No. 5,281,475, the disclosure of which is incorporated herein by reference.

Filaments are woven into paper machine belts

according to conventional weaving techniques. The type of weave and density thereof will depend on the type of paper and papermaking operation for which the belt is to be used. Single layer and multilayered constructions are suitable, and when formed of a woven construction, the fabrics may be woven endless or in flat form. The following examples demonstrate the advantages of the present invention.

Example 1 A woven single layer fabric was constructed of expanded PTFE monofilaments (obtained from W.L. Gore & Associates) as the shute yarns and Hoescht 906Y polyethylene terephthalate material as the warp yarns. The woven fabric was a 62 x 46 five harness single layer straight twill fabric woven with 0.30 mm monofilament shute in a 2x3 configuration. For comparison, a woven fabric containing 0.30 mm Hoescht 906Y HMW PET monofilament as warp and shute was prepared in the same configuration as the expanded PTFE containing fabric. The coefficient of friction of the woven samples was calculated from the force needed to drag a steel plate over the samples, with the test being performed in the machine direction. The friction values obtained were 50% lower for the expanded PTFE material on both the short and long warp knuckle sides. It was observed that the expanded PTFE samples felt very slippery.

Friction Values Long Warp Short Warp Knuckle Down Knuckle Down Expanded PTFE 0.10 0.12 Hoechst PET 0.20 0.21 Example 2 The fabric samples described above were tested for abrasion resistance. The samples were tested with the long shute knuckle down to maximize the exposure of the expanded PTFE to the wear cylinder.

A force transducer was used to measure the force exerted on the fabric during the test.

In the first running of the test samples were run to failure. After 40 minutes the PET sample had lost all of the wear side exposed shutes, as seen in Fig.3. All that remains are short sections of shute on the paper side. The sample containing expanded PTFE was run for 240 minutes without failure.

Inspection of the expanded PTFE sample showed that it was entirely plugged by smeared, expanded PTFE together with a little of the calcium carbonate filler from the test, as seen in Figure 4. Although the sample appeared to be intact, there was no strength remaining in the cross-machine direction and the sample easily pulled apart.

A second run was made where the samples were removed for caliper measurement after 20, 30, and 40 minutes. As seen in Fig. 1, the PET fabric lost caliper much more quickly than the expanded PTFE sample.

Unlike the first test, the PET sample was still intact after 40 minutes, but the shute strands are heavily worn. Photos from the test are shown in Figures 5 and 6. The expanded PTFE strands have become somewhat frayed at the edges. There is no wear to the warp strands of the expanded PTFE sample after 40 minutes.