BACKGROUND OF THE INVENTION
This invention relates to papermaker's fabrics and more particularly to a papermaker's fabric having high wear resistant cross machine direction monofilament yarns on the machine side or wear side of the fabric.
Forming fabrics for use in papermaking machines usually are in the form of a fine mesh cloth which has been woven endless or otherwise joined into an endless web. As a very basis of good quality paper resides in the web formation itself, the structure of the forming wire is of vital and decisive importance. At one time, all forming wires were manufactured from metal wires. These metal-wire cloths were useful in all kinds of papermaking machines and for all paper qualities.
Eventually, metal-wire cloths were replaced by single-layer cloths or wires of synthetic fiber threads. The advantage of synthetic threads beyond metal-wire threads primarily resides in their improved wear resistance. Single layer synthetic wires do, however, suffer from the disadvantage of having considerably higher elasticity and less stability than cloths made from metal-wires of corresponding coarseness.
With such synthetic materials, and in the case where the liquid suspension contains in addition to the cellulosic fibers, an abrasive filler material such as calcium carbonate, the problem arises that the mesh may become rapidly worn, and the working life thereof consequently appreciably reduced, due to abrasion occurring as the mesh moves against the rollers and suction devices.
The initial synthetic yarns used in papermaker's fabrics were multifilament yarns. These yarns which frequently were made out of nylon were chemically treated in order to improve the wear resistance of the fabric. (See U.S. Patent No. 3,032,441 issued to Beaumont et al.) The introduction of all monofilament fabrics further improved the wear resistance of forming fabrics by virtue of the higher apparent density of the monofilament yarns compared to the multifilament yarns.
Efforts to improve the wear resistance of the monofilament yarns have so far had only limited success. Attempts to improve the wear resistance of the monofilament yarns are taught by U.S. Patent No. 4,289,173 issued to Miller and West German Patent Application No. 25 02 466 in the name of Wandel. These patents disclose the use of alternative nylon and polyester yarns in the cross machine direction for improving the wear resistance of the fabric. The papermaking performance of such prior art fabrics has been reduced by a pronounced wire mark on the sheet side. Furthermore, the combining of nylon and polyester yarns have proven to be rather difficult due to the different bending stiffnesses exhibited by the two types of synthetic yarns. Such yarns will also tend to exhibit different crimp when used in the same fabric.
Attempts to improve the wear resistance by coating the fabric with an abrasion-resistant film as taught by U.S. Patent No. 4,421,819 issued to Baker have also met with limited success. The normal wear rates of the fabric will resume as soon as the thin wear resistant coating has worn through. In addition, if the coating applied to the fabric is too thick, the drainage passage through the coated fabrics is reduced by the thickness of the coating up to the point where the drainage passages become completely blocked.
The problem of reduced drainage capacity due to the coating of monofilament fabrics is a particularly serious problem with fine mesh fabrics used for fine paper applications and with dual layer---fabrrcs where the distance between surfaces of adjacent machine direction monofilament yarns is extremely small.
It is therefore a principal object of the present invention to provide a papermaker's fabric having improved abrasion resistance and therefore working life without appreciable adverse modification of texture and drainage characteristics of the fabric.
- SUMMARY OF THE INVENTION
A further object of the present invention is to provide a papermakers fabric in which the abrasion resistance of the fabric is improved without applying a coating to the fabric.
The papermaker's fabric of the present invention is woven from monofilament yarns manufactured from a high wear resistant polyester. The high-wear resistant yarns are produced from a very high viscosity polymer having an intrinsic viscosity higher than the intrinsic viscosity of the manufactured yarns which must be 0.84 or higher.
The high-wear resistant yarns are produced using normal extrusion conditions with the polymer temperature increased by about 10°F in order to handle the high viscosity polymer.
- BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and objects of the present invention will be more fully understood from the following detailed description which should be read in light of the accompanying drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Fig. 1 is a plan view of a single layer papermaker's fabric according to the present invention which diagrammatically shows the wear area of the fabric;
- Fig. 2 is a plan view of a double layer papermaker's fabric according to the present invention which diagrammatically shows the wear area of the fabric;
- Fig. 3 is a schematic representation of a yarn wire abrasion test;
- Fig. 4 is a schematic representation of a wet roll abrasion test;
- Figs. 5a and 5b are schematic diagrammatic representations of a double layer papermaker's fabric, as shown in Fig. 2, containing regular polyester yarns, before and after wet roll abrasion tests respectively;
- Figs. 5c and 5d are schematic diagrammatic representations of a double layer papermaker's fabric, as shown in Fig. 2, incorporating the high-wear resistant polyester yarns of the present invention, before and after abrasion resistance tests respectively; and
- Figs. 5e and 5f are schematic diagrammatic representations of a double layer papermaker's fabric, as shown in Fig. 2, containing alternating regular and nylon 6.6 cross machine direction yarns, before and after abrasion resistance tests respectively.
The papermakers fabric of the present invention, which may be a single (Fig. I), double (Fig. 2) or multi-layer fabric, includes high wear resistant polyester yarns in the bottom layer of the fabric. Preferably all or substantially all of the cross-machine direction yarns are composed of or formed of a high viscosity polyester monofilament. The high wear resistant yarns incorporated in the papermaker's fabrics of the present invention are produced from a very high viscosity polyester of intrinsic viscosity over 0.95 with the intrinsic viscosity of the yarns being 0.84 or higher, depending on the extrusion rates. A suitable high viscosity polyester is that marketed by the Goodyear Tire and Rubber Co. The polyester may be mixed with an imide stabilizer and/or Ti02 and extruded into a monofilament.
The high wear resistant yarns of the present invention may be produced under the same extrusion conditions as regular cross machine direction polyester yarns. The only small adjustment needed is to use a polymer temperature about 10°F higher than normal to handle the high viscosity polymer. Productivities from such production are normal, and the high viscosity polymer costs only a few cents more per pound than regular, normal viscosity polyester polymer.
Many other features and aspects of the present invention will become apparent from the following non-limiting examples.
The fabrics in the following examples underwent two types of tests: the yarn wire abrasion test and the wet roll abrasion test. In the yarn wire abrasion test, schematically shown in Fig. 3, fifteen piano wires are distributed uniformally around a circle having a 7 1/2-inch diameter. The yarn or monofil to be tested is in contact with 90° of the "Squirrel Cage." The yarns are tensioned with a 1/2 pound weight, and the wires comprising the squirrel cage are rotated at 34 r.p.m. towards the weight. There are five positions on the wire tester and each has a counter which records the number of revolutions or cycles of the squirrel cage required to break or fracture the yarn. When the yarn breaks, the counter automatically stops. In the device of Fig. 3, the yarn is stationary and the wires rub along the length of the yarn.
- EXAMPLE 1
To perform the wet roll abrasion test, shown in Fig. 4, a rectangular fabric sample, 4 inches by 1/4 inch, is held stationary on a wet rotating papermaker's roll with a 14-inch diameter. The water is added via a spray, and the fabric holder keeps the fabric strip tangential to the roll and provides a controlled pressure on the fabric. The surface speed of the roll is 1000 feet per minute. The test lasts for 10 or 20 minutes depending on fabric weight. The degree of abrasion is determined by the percent lost in tensile strength of the abraded fabric strip compared to the strength of the fabric before abrasion.
A 0.19 diameter high wear resistant (HWR) yarn of 0.87 Intrinsic Viscosity was produced from Goodyear polymer 1005A of Intrinsic Viscosity (I.V.) 1.0. The yarn was drawn 4.0 x and heat set in an oven at 420°F. on the extrusion line. This yarn had a low modulus of 0.65 gm/decitex for 1% extension and a free shrinkage of 12% in an oven at 392°F.
The abrasion of this 0.19mm HWR yarn was compared with a regular polyester 0.19mm yarn of I.V. 0.65 produced from Goodyear polymer 7201A with similar modulus of 0.63 and free shrinkage of 9%. The results of the yarn wire abrasion and wet roll abrasion tests were as follows:
- EXAMPLE 2
The regular and HWR 0.19mm yarns were woven as cross machine direction yarns in a single layer fabric of the type shown in Fig. 1. The strength losses in the fabrics after the same number of revolutions of the wet roll were:
- EXAMPLE 3
The 0.19mm HWR yarn of Example 1 was evaluated as the cross machine direction yarns in a double layer fabric of the type illustrated in Fig. 2. The strength losses in the fabrics after an identical number of revolutions of the wet roll were:
In additional tests, 0.17mm HWR and regular yarns were produced using the following conditions
These two yarns were evaluated in a double layer fabric similar but not identical to that shown in Fig. 2. In addition, a pick and pick fabric with alternating regular polyester and nylon 6.6 monofils on the bottom layer of the double fabric was woven across the same warp.
- EXAMPLE 4
The worn areas on each of these three double layer fabrics both before and after wet roll abrasion are shown in Figs. 5a and 5b, 5c and 5d, 5e and 5f.
A 0.23mm HWR yarn of 0.87 I.V. was produced from Goodyear polymer 1005A using similar extrusion draws and oven temperature as the yarn of Example 1. This 0.23mm yarn had a modulus of 0.64 and a free shrinkage of 13%.
- EXAMPLE 5
This 0.23 HWR yarn, along with regular 0.23mm polyester was woven as cross machine direction yarn in a monoplane fabric similar to the fabric of Fig. 1. The abrasion results were:
A coarse 0.36mm HWR yarn was obtained from a Goodyear polymer 9501A of I.V. 0.95. The 0.36mm HWR monofil had an I.V. of 0.84, a modulus of 0.61, and a free shrinkage of 15%. The yarn wire abrasion test results for this 0.36mm HWR monofil as compared to a regular 0.36mm monofil were:
Many features and aspects of the present invention can be seen from examples 1-5 set forth above. These examples show that the high-wear resistant yarns can withstand 75% to 125% more wire abrasion revolutions than similar regular polyester monofils of the same diameter. In addition, the fabrics composed of regular polyester yarns have strength losses on the wet roll from 25% to 50% more than fabrics utilizing the high-wear resistant yarns of the present invention. As seen in Fig. 5b a double layer fabric having regular polyester yarns in the machine direction shows some wear on these machine direction yarns. Figs. 5d and 5f, however, show that the use of high-wear resistant yarns or pick and pick regular polyester/nylon 6.6 yarns results in little or no wear of the machine direction yarns when the fabrics are subjected to the same number of revolutions of the wet roll. Finally, most of the physical properties of the high-wear resistant yarns are similar to regular polyester yarns so that few if any changes in winding, weaving or finishing are required to process the fabrics incorporating the high-wear resistant yarns. These high-wear resistant monofils do have slightly more free shrinkage then regular polyester monofils. Most importantly, however, as seen above, the main difference between regular and high-wear resistant yarns is that the high-wear resistant yarns have significantly higher abrasion resistance than the regular polyester yarns.
The foregoing invention has been described with reference to its preferred embodiments and a number of non-limiting examples. Although variations and modifications will occur to those skilled in the art, it is intended that such variations and modifications fall within the scope of the appended claims.
What is claimed is: