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(11) | EP 0 896 086 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Functional epoxy-silicone coatings for paper machine clothings and method of coating same |
| (57) The present invention is directed to fabrics for paper making machines that are rendered
contamination resistant and maintain good permeability as a result of a durable coating
that lasts the entire life of the fabric. A silicone-epoxy will render the fabric
contamination resistant over the entire fabric lifetime. |
FIELD OF THE INVENTION
[0001] The present invention is directed to fabrics for paper making machines that are rendered contamination resistant by a durable coating that lasts the entire life of the fabric, while not overly limiting the permeability of the fabric.
BACKGROUND OF THE INVENTION
[0002] The modern papermaker employs a highly sophisticated papermaking machine which is in essence a device for removing water from the paper furnish. The water is removed sequentially in three stages or sections of the machine. In the first or forming section, the furnish is deposited on a moving forming fabric and water drained through the fabric 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 press fabric section and passed through one or more nip presses on a moving press fabric to remove sufficient water to form a sheet having a solids content of about 36 to 50 percent by weight. This sheet is then transferred to the dryer section of the papermaking machine where dryer fabrics hold the paper sheet against hot, steam-heated dryer cylinders to obtain about 92 to 96 percent solids content. The papermaking fabrics employed on the papermaking machine must perform a diverse range of functions, according to the position on the machine, i.e., forming, press or dryer section.
[0003] Forming fabrics used in the papermaking process are a kind of papermaking fabric which are used in the forming section of a papermaking machine. Forming fabrics are generally constructed of synthetic yarns joined together, ordinarily by weaving, in a fabric construction that is characterized by a high degree of open spaces between the intersecting yarns. Forming fabrics must maintain a high degree of openness to insure that they permit removal of water from the fiber slurry deposited thereon.
[0004] Since water removal capability is a critical function of the forming fabric, it is necessary to insure that the fabric retains a high degree of openness over its lifetime.
[0005] However, the degree of openness of a fabric is continually reduced during its life. In addition to the fiber slurry, paper pulp ordinarily contains additives such as filler clay, pitch, and polymeric materials that clog the open spaces of the fabric. The use of recycled fibers has introduced considerable amounts of contaminants in the form of inks, adhesives, tars, and polymeric materials, which also clog the open spaces of the fabric. In addition, forming fabric designs now include multilayer fabrics that are more susceptible to contamination problems.
[0006] Accordingly, it is desirable to provide a fabric which exhibits an improved degree of contamination resistance. One proposed prior art solution is the use of contamination resistant yarns in the construction of the fabric. This has not proved to be wholly satisfactory since the contamination resistance provided by such yarns is short-lived and/or ineffective. Another proposed solution calls for coating or treating paper making fabrics in order to improve resistance to contaminants. Again, this method is not wholly successful because the contamination resistance provided by the coating is short-lived and/or ineffective.
[0007] U.S. Patent nos. 5,207,873 and 5,395,868 describe papermaking fabrics claimed to have permanent resistance to adhesion of contaminants. The fabrics are coated with solutions having as their primary components tetrafluoroethylene, urethane copolymer and polyacrylamide. This coating has not proven to be totally effective and/or permanent on paper machine clothing.
[0008] One problem generally inherent to coatings or treatments is that coatings per se are known to reduce the permeability of a fabric, an undesired result that inhibits water removal capabilities, the primary function of a forming fabric. It is therefore important that any coating applied to a forming fabric reduce permeability as little as possible.
SUMMARY OF THE INVENTION
[0009] An object of the present invention is to provide a fabric used in the forming, pressing, or drying section of a paper making machine that exhibits an improved resistance to contamination over the entire life of the fabric. These fabrics are referred to as papermachine clothings by those skilled in the art, and the terms are generally interchangeable.
[0010] It is a further object of the invention to provide a durable coating that lasts the entire life of the fabric.
[0011] It is a further object of the invention to provide a coating which does not significantly affect the permeability of the fabric.
[0012] It is a further object of the present invention to provide a coating for a fabric used in a papermaking machine that achieves the aforementioned objectives.
[0013] The present invention is a coated fabric used in a papermaking machine that has significantly enhanced resistance to contamination which lasts over the entire fabric lifetime. In another aspect, the invention is a method of coating a fabric used in a papermaking machine in order to enhance its resistance to contamination. The coating formulations disclosed herein have been shown to substantially improve the contamination resistance of a fabric while not significantly reducing the permeability of the fabric, and not increasing the mass of the fabric to any significant degree. That is, the present invention provides a thin, low weight coating for a papermachine clothings that adds limited mass to the fabric.
[0014] The applicants have found that a coating comprised of a silicon-epoxy resin will render a paper machine clothing contamination resistant over the entire fabric lifetime. Suitable silicon-epoxy coatings include aqueous mixtures that contain silicone epoxy resins. It is believed that the silicone-epoxy resins bond with the polyester or nylon filaments that form the clothing. A suitable line of silicone-epoxy resin compositions are available from Decora Industries, Fort Edward, New York under the trade name Wearlon®. A preferred formulation is Wearlon® 2020-98, a three component formulation of a resin containing a silicone-epoxy based emulsion, a curing agent and a crosslinking agent. The components are mixed in-situ in accordance with the manufacturer's recommendations and applied to the fabrics and can be further diluted as necessary.
[0015] Effective contamination resistant fabrics have been prepared where the fabric coating formulation contains 5% to about 50% solids on a weight-weight basis, with a mass add-on of 0.1 % to 10.0 % based on the weight of the uncoated fabric. The % mass add on is:
[0016] As a general matter, a greater degree of the original permeability of a coated fabric is retained when the solids content of the coating or mass add on of the coating is reduced. Water, a preferred diluent for aqueous based coatings, may be used to reduce solids content and consequently % mass add on. It has been found that fabrics having coating formulations of a solids content in the range of 10% to 15 % (w/w) or a mass add on of 1% to 3% maintain a high degree of their original permeability. That is, on the order of about 90% - 99% of their original permeability, which is preferred. In other words, permeability is reduced only about 1% - 10% as a result of the coating. The fabrics can be coated in any conventional manner, including immersion within a coating bath, blade or bar coating techniques, squeegee coating, transfer coating, spray application, kiss or applicator roll, slot applicator, and brush applicator. Application with a kiss roll has been effective. The coating can be applied in a single pass, or it may be applied in multiple passes. Subsequent processing requires removing excess material and then drying or curing the coating as directed by the manufacture of that particular material. These methods are well known by those skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
Fig. 1 is a photograph of an uncoated section of a contaminated fabric at 5X magnification.
Fig. 2 is a computer generated image of figure 1 with the fabric removed in order to show the contaminants.
Fig. 3 is a photograph of the coated section of a contaminated fabric at 5X magnification.
Fig. 4 is a computer generated image of figure 3 with the fabric removed in order to show contaminants, if any.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE 1
[0019] A triple layer forming fabric constructed of polyester filaments was coated on part of, but not all of its surface with a an aqueous solution of WEARLON® 2020-98 silicon-epoxy coating (49% solids w/w) available from Decora Industries, Inc. of Fort Edward, N.Y. The coating formulation was applied in a single layer with a Binks #7 spray gun available from Binks Mfg. Co. of Franklin Park, Illinois. Excess liquid was removed with a vacuum. Mass add on was determined as 8.3% based on the weight of the uncoated fabric. Portions of the belt were not coated in order to serve as a control.
[0020] The fabric was installed on a pilot machine and run for 3 days. The machine was running pulp consisting of 100% old corrugated container (OCC). The speed of the machine ranged from 460-670 m/min (1500-2200 ft/min). Needle showers were used at 13.8 bars (200 psi) with no cleaning chemicals. After the trial, the fabric was removed and analyzed. The following figures relate to the appearance and condition of the fabric after removal.
[0021] A contamination analysis was done on the sections of the fabric. Figures 1 and 3 are photographs taken at 5X magnification of each section. The pictures were then scanned into the computer with a gray color scale. The image was converted to a bitmap file (* bmp file), and opened in Microsoft Paintbrush (see Figs. 2 and 4), and the contaminants were colored in blue. The gray fabric was removed from the image leaving only the blue portion. This image was then imported into an image analysis program which counted the blue pixels and found a % contaminated area. The results are listed below:
[0022] All of the areas of the fabric that were coated exhibited an increase in contamination resistance over the areas of the fabric which were not coated.
EXAMPLE 2
[0023] Silicone epoxy coating formulations of WEARLON® 2020-98 were applied to fabrics at solids content ranging from 5% to 25% (w/w). Three coats were applied to a triple layer fabric via kiss roll applicator. The compositions of the formulations are provided in Table 1.
[0024] Three separate samples of the coated fabrics were measured for air permeability and mass add on as a result of coating and the average was taken. At a coating formulation having 15% solids, mass add on of the coating, when dry, was determined to be 11.18 g/m2, or 2.7 % based on the weight of the uncoated fabric.
[0025] The coatings were allowed to cure for a period of one week. Three samples were cut from each of the fabrics per given coating and subjected to high pressure showering for a period of 8 hrs (oscillating needle showers) under the conditions set forth below. Data is presented in Tables 2 and 3. Mass, as reported in Table 2, is the mass of a 2" diameter sample taken from the fabrics.
[0027] Because of the demanding environment existing in a high pressure shower test and its duration (in this case, 400 psig for 8 hours), the test is a good measure of coating durability.
[0028] Table 3 is based on the data of Table 2. Table 3 expresses the data of Table 2 in terms of the changes in air permeability and mass add-on between the uncoated fabrics and the coated fabrics exposed to high pressure showering. Since the air permeability and mass add on of the showered, coated fabrics never return to the original levels of uncoated fabrics, it is evident that the coatings remain after exposure to the high pressure showering.
1. A contamination resistant papermachine clothing for use in the forming, pressing or
drying section of a papermaking machine comprised of a fabric coated with at least
one layer of a coating containing a silicon epoxy resin.
2. The paper machine clothing of claim 1 wherein the coating is a silicon-epoxy resin
applied from an aqueous mixture.
3. The papermachine clothing as set forth in claim 1 wherein the mass add-on of the coating
when dry is 0.1% to 10.0 % based on the mass of the clothing.
4. The papermachine clothing as set forth in claim 1 wherein the mass add-on of the coating
when dry is 1.0 % to 3.0 % based on the mass of the clothing.
5. The papermachine clothing of claim 1 wherein the coating is applied in at least two
layers.
6. The papermachine clothing as set forth in claim 1 wherein the coating applied to the
clothing is cured.
7. The papermachine clothing as set forth in claim 1 wherein the clothing is a single
layer fabric.
8. The papermachine clothing as set forth in claim 1 wherein the clothing is a multi
layer fabric.
9. A method of rendering a papermachine clothing contamination resistant comprised of
the steps of applying at least one layer of a silicon epoxy resin to a paper machine
clothing.
10. The method as set forth in claim 9 wherein the coating is applied by spraying.
11. The method as set forth in claim 9 wherein the coating is applied by kiss roll applicator.
12. The method as set forth in claim 9 wherein the coating is applied in at least two
layers.
13. The method as set forth in claim 9 wherein the coating applied to the clothing is
cured.
14. The method as set forth in claim 9 wherein the fabric is a single-layer fabric.
15. The method as set forth in claim 9 wherein the fabric is a multi-layer fabric.
16. The method as set forth in claim 9 wherein the mass add-on of the coating, when dry,
is 0.1% to 10.0 % based on the mass of the clothing.
17. The method as set forth in claim 9 wherein the mass add-on of the coating, when dry,
is 1.0 % to 3.0 % based on the mass of the clothing.