Table, Reduced Permeability Papermaker's Fabrics Containing Fibers With Fins Designed to Distort at Lower Force Levels by Having a Reduced Cross Sectional Area Within the Fin

In the preparation of paper, woven and spiral fabric belts are utilized to support the cellulosic pulp fibers as they are moved through the papermaking process and converted from a thin slurry into finished paper. It has been found that mechanical stability and permeability control of these belts is critical to the production of consistent, high quality paper. As paper machine speeds have increased, fabrics designed for use in the dryer sections of papermaking machines have had their targeted permeability reduced from 500 cubic feet per miracle per square foot; with a pressure differential of one half inch of water to 100 or less. There has also been a trend toward use of thinner fabric constructions to minimize differential forces on the paper as it passes over and under the belts in certain process steps. For woven fabrics, these two papermaker's fabric requirements are in conflict since the common way to reduce permeability is to increase size of the weft yarn or the number of picks per inch, both of which can result in increased fabric thickness. Increased beat-up forces are required to force wefts into these fabric designs For the desired low permeability products. These high beat-up forces lead to fiber and machine damage. For spiral fabrics, ribbon and X shaped yarns have been developed to insert into the open areas of the fabric design. These designs give satisfactory permeability results, but require very careful size control and relatively high force levels for insertion into the fabric and prevention of fabric distortion.

As the demand for papermaker's and industrial fabrics has moved toward thinner, reduced permeability fabrics, suppliers of such fabrics have shifted from use of round monofilament wefts to use of twisted and cabled yarn constructions which have more capability to conform into the interstitial spaces formed at the crossings of warp and weft yarns. This switch has been moderately successful in regards to production of lower permeability and improved fabric stability. Some negative results of this practice are that the smaller monofilaments used in cabled constructions are more easily damaged by severe environmental exposure and that cabled yarns tend to become contaminated with process "tars" faster than true monofilament wefts. The extra handling and processing stages required to produce these twisted and cabled yarns also makes their cost significantly higher than that of monofilament.

There has also been a shift toward use of more ribbon-like warp yarns. These warps give improved paper contact and decrease the number of interstices, thus resulting in reduced fabric permeability. Reduction in the number of interstices has had a negative impact on fabric stability since interactions at fiber intersections lock the fabric together. Wear due to the thin profile of ribbon warp yarns has also been a drawback to more widespread use of this concept.

Fabric stability is improved by increasing the interaction between warp and weft yarns. As each weft pick is inserted into the fabric, it is beaten against the warp so that the warp takes on a sinusoidal crimp, The weft remains relatively flat, and the distortion created at the mechanical intersection of the warp and weft contributes significantly to fabric stability. Current methods of improving stability include increasing pick count, use of multifilament warps and/or wefts, use of cabled weft yarns, and application of resinous fabric treatments. Each of the listed methods is acceptable in selected areas, but all carry a cost or performance penalty which prevent them from being generally acceptable.

In U.S. Pat. No. 5,097,872, Laine et, al. teach the use of an X shaped fiber to achieve improved fabric stability, but their application requires almost complete flattening of the fiber on one side by bending forces and the design use described would not contribute to improved permeability control. There is no mention or inferred concept for hinged or variable cross sections in the arms of the X. In contrast, the current patent application requires that some of the finned extensions have a decreased cross sectional area along their length to make them easier to bend arid distort during weaving. Forces due to both bending and fiber compression during "beat-up" are present in the weaving process. Where bending or contact forces are not present, the fins will remain erect to block fabric interstitial spaces.

In U.S. Pat. No. 4,633,596, Josef teaches the use of warp fibers having a center thinner than the edges and which improves fabric dimensional stability by minor distortion at warp and weft crossings. This is in marked contrast to the use of finned and hinged weft and stuffer fibers which run in the cross machine direction in woven and spiral fabrics. Designs shown would not crush or easily distort duping weaving. Josef also makes no claim or mention related to spiral fabrics. The drawings and discussion of Josef's patent tend to lead toward production of fabric designs targeted toward high permeability fabrics.

In U.S. Pat. No. 5,361,808, Bowen teaches the use of finned weft fibers which deflect at the intersections of the warp and weft, but which fins remain extended to block fabric interstitial space where not mechanically contacted by other fibers. Fin length and use of plasticizers are the mechanisms described to promote flexibility.

In U.S. Pat. No. 5,364,692, Bowen and Smith teach the use of T, X, or v shaped stuffer yarns to reduce permeability of spiral fabrics. No special shape of mechanism to produce "arms" with reduced stiffness to bending forces is disclosed. This application contrasts significantly by specifying that the arms or fins of the shaped fiber contain a reduced cross sectional area designed to promote bending at force levels one half or less of that which would be required if the fin had a uniform cross section.

In U.S. Pat. No. 4,381,612, Shank describes spiral fabrics containing one or more stuffer filaments, but describes no technology or intent to design the stuffer filaments to allow for easy distortion and conformation into the free spaces of the fabric during fabric sizing and heatsetting.

The present invention provides thin, stable, controlled permeability papermaking or industrial fabrics, especially dryer fabrics, with the capability of being easily produced on standard industrial looms or spiral fabric lines. Special advantage is achieved in manufacture of fabric designs with permeability targets of less than 175 cubic feet per minute per square foot with a pressure differential of one half inch of water. Fabrics utilizing this invention also have improved dimensional stability over that achieved by the now common use of twisted and plied monofilament wefts in weaving or simple unshaped fins in weaving and spiral fabric production.

Specifically, this invention provides, in a papermaker's fabric, the improvement wherein some or all of the yarns contain filaments designed to flex and distort at reduced fiber to fiber force levels by having two or more finned extensions, some of which extensions are characterized by incorporating a reduced cross section "hinge" area and/or a variable thickness from the center outward. For the purpose of this discussion, a yarn may consist of one or more filaments, but the preferred embodiment of this invention will be a monofilament. A reduction in cross sectional area of 20 per cent or more anywhere along the fin except for the normal radius at the fin terminus will be considered to meet the reduced cross section specification. Since the force required to obtain deformation is proportional to the cube of Fin thickness or width, a reduction of 20 per cent in fin thickness results in approximately a fifty per cent reduction in "beat up" force required to mold the yarn into the fabric. Advantage of the lower mechanical stress requirement can be taken to increase interlocking of the fibers while simultaneously reducing damage due to warp tensions and "beat up" forces. In production of spiral fabrics, increased flexibility of these shaped or hinged fins significantly reduce the forces required for insertion into the open segments of the design. When the spiral fabric shrinks in heatsetting, the flexible fin designs are interlocked into the fabric with minimum distortion of the fabric surface while simultaneously providing the desired reduced air permeability. For most fabric products, the best design will be a monofilament yarn between 400 and 3000 denier, but use multifilaments with uniform or mixed cross section designs is possible. Multiple filament yarn designs can be utilized to achieve specific fabric properties including permeability, thickness and stability. The filaments which make up these multifilament yarns will have a denier of more than 100, preferably 200 to 1500, and when combined into multifilaments will usually have low twist levels. These yarns can be utilized in the warp, weft or filling of industrial fabrics. Any appropriate polymer type and additive package used to produce yarns for papermaker's industrial fabrics may be used. Significant economic benefits are realized due to reduced denier of these yarns over other yarns previously used for this service.

FIG. 1 is a cross sectional perspective view of the preferred embodiment for a filament having fins with a radially reducing cross sectional area.

FIG. 2 is a cross sectional perspective view of the preferred embodiment for a filament having the "hinged" fin concept of the invention.

FIG. 3 shows a fin with stepped area reduction while FIG. 4 shows a combination of fins with and without reduced area cross sections.

FIGS. 5, 6, and 7 show fins with mid hinge, curved area reduction and straight area reduction respectively.

FIGS. 8 and 9 show distortion of the hinged and reducing cross section fins as the are woven into the fabric.

FIG. 10 shows a special "ball" fin design.

FIGS. 11 and 12 show two and eight fin fiber designs to indicate some of the variety which can be produced by this concept.

FIGS. 13, 14 and 15 show spiral fabric designs demonstrating the hinged fin concept.

FIGS. 16 and 17 show arched and crescent filament designs which would be especially useful in spiral fabrics, but which also could be used in woven products.

On FIGS. 1, 2 and 4, the fin thickness at T1 and T2 are those measurements where the reduction in thickness ratio would typically be calculated. If T2 divided by T1 is less than 0.8, the fin will fall within the specification of this invention. It is specifically pointed out that the normal rounding or radius effect at the end of such a fin is not considered to be a "manufactured" reduction except for a design such as that of the "ball" fin. For a special case such as this, the maximum radius will be used as the denominator in the calculation of fin cross sectional area reduction.

The finned filaments used in the present invention can be prepared from a variety of thermoplastic materials. Polyethylene terphthalate, polyphenylene sulphide and 1,4-polydicyclohexanol terphthlate are currently widely used but not the only materials which might be chosen. In order to obtain the desired fin flexibility, these new yarn designs often require addition of polymer specific plasticizing agents during the filament extrusion process. Use of standard additive recipes which may include heat and hydrolysis stabilizers, contaminant release agents, and other such processing aids common to production of papermaker's yarns is considered as standard.

Modifications to customary techniques for filament production are required in order to achieve acceptable filament smoothness and uniformity for these new yarn shapes. This is caused by the overall filament width to fin thickness ratio necessary to obtain flexibility without fracture. This is where the concept of hinged or variable cross section fins excell over prior art. The taper or hinge results in a mope flexible fin which reduces the need for excessive plasticizing agents or extra lone fins. If the shape factor can be characterized by the ratio of the overall filament diameter divided by the average fin thickness, then filaments with a shape factor of from 3.5 to 20 can be used for these fabrics. The average fin thickness can be calculated by standard mathematical techniques. One such example is given for FIG. 1 where TAVG=(T1+T2)/2. R divided by TAVG is the shape factor considered fop the purpose of this invention. It is desireable to have the lowest possible shape factor in order to reduce surface to volume ratio of the fiber and thereby decrease damage due to difussion controlled degradation processes. For reasons of practicality, the number of fins will range from two to twelve.

Fabrics are woven from these new variable fin cross section yarns in the same manner as with round, ribbon or twisted and plied fiber constructions currently in common use. In FIGS. 8 and 9, warp yarn is shown as 1 and weft yarn as 2. Bending of the weft fins by the warp during weaving is shown. FIG. 8 shows a fabric cross section showing use of all hinged tinned weft yarns. The important concept here is that the deformable fins easily conform to fill the available volume between the warp yarns and by so doing, lock the woven structure together and significantly reduce the openness of the fabric. Wefts containing less than four finned extensions are often found to be sensitive to the slight twist inserted into the weft as it is supplied to the process over the top of supply bobbins. This twist insertion results in small surface and permeability irregularities which can be significant in critical product areas. The X design has been found to very closely match the rectangular or diamond open area common to most weave patterns and is also a very good compromise for economy of material and ease of water removal during spinning. Use of more than four lobes reduces the importance of weft yarn orientation in the fabric, but at a cost of more monofilament extrusion difficulty. The physical size of the filament or fiber design will be determined by the cross sectional shape chosen, the flexibility designed into the fins, the target fabric thickness and size of the chosen companion warp or weft. FIG. 9 shows a fabric design similar to FIG. 8, but containing a weft yarn with a gradually reducing cross section rather than hinged weft yarns. The formation of large numbers of small fin distortions per inch of width during fabric production provides significant lateral stability and rigidity to products containing these flexibly finned fibers. Filaments of this type can be used in the warp, but their advantages are currently mote easily achieved in weft or stuffer yarn applications. Warp fibers with fins located to one side of the yarn so that the fins would be turned toward the interior of the woven fabric would be one example of a design which would be suitable for this patent concept.

FIG. 13 shows a hinged stuffer yarn inserted into the formerly open area of a spiral fabric design. Size of the yarn is carefully controlled so that it may be easily inserted into the fabric open areas prior to heatsetting. FIGS. 14 and 15 show preferred embodiments of the concept for spiral fabrics. A crescent shaped stuffer yarn has been inserted into the open area of the fabric design and has been compressed into the upper and lower surfaces during heatsetting where shrinkage of the spirals under tension flattens the fabric, distorts the fins and locks the fabric together. If the fins are easily distorted, the fabric surface will remain smooth and flat. Permeability control may be achieved by inserting these special stuffer yarns into selected open areas or for another example, by alternating between yarns of this invention and other yarn types. For economy of production and material, the use of arched and crescent shaped designs with hinge areas are the preferred embodiment of the invention for spiral products.

In summary, the fibers of the invention will preferably be monofilaments between 400 and 3000 denier, have two to twelve finlike extensions, some of which extensions have variable cross sectional area designed to promote bending at reduced force relative to the force which would have been required if the fin cross section were uniform. In order to make this force reduction significant, design criteria have been chosen to allow for a 50 per cent force decrease by having a cross sectional width reduction of more than 20 per cent. Multifilament fiber designs can be used, with the finned filaments of such designs having a denier of between 200 and 1500 and some of which filaments contain variable area fins designed to promote bending. The fibers can be used in either the warp, weft or filling of papermaker's or industrial fabrics which can be either woven or spiral designs.