Papermaker's Grooved Back Felt

This invention relates to felts for use on papermaking machines, and has to do more particularly with felts for removing water from a paper sheet while the sheet is supported on and carried by the felt.

Various techniques have hitherto been proposed for removing water from a paper sheet, the most common being by the use of a press, wherein the paper sheet and the felt on which it is carried are squeezed between rollers, the felt being formed in such a manner that the water extracted from the paper will pass through the felt for discharge. Desirably, the press felts are woven so as to have relatively large open areas or voids which will enhance their water-conveying capabilities so that the water may be removed from the felt upon passage over a suction box.

It has been proposed to enhance the water-conveying capacity of the felt by providing spaced apart monofilaments on the back or machine side of the fabric, the monofilaments lying in spaced apart relation to define water-conveying channels between. Such monofilaments are formed from a synthetic plastic material bonded to the base fabric either by extruding the monofilaments in hot melt form and applying them directly to the base fabric or by adhering them by an adhesive, or by the use of a solvent to render the filaments tacky and hence capable of bonding to the base fabric. While the use of such plastic monofilaments has enhanced the water carrying capability of the felt, particularly when operated under high speed conditions, the use of such monofilaments has involved a number of difficulties. For example, special equipment and handling techniques are required to extrude the plastic monofilaments in molten condition and apply them to the back surface of the base fabric, and this is also true where the filaments are adhesively secured to the base fabric or are softened by a solvent prior to their application to the fabric. It has also been found that after the plastic monofilaments have been in use, they tend to crack and peel away from the base fabric, and additionally, the monofilaments tend to flatten, the net result being a significant decrease in the effectiveness of the channels to perform their intended water conveying function.

More recently, U.S. Pat. No. 4,119,753 discloses felts having improved water conveying channels formed on their machine sides by means of bulky, rib forming yarns which define channels therebetween, the rib forming yarns being securely interlocked with the base fabric and treated with a resin to render them essentially incompressible. Although these felts are capable of being manufactured utilizing conventional equipment which is readily available in mills which manufacture conventional papermaking felts, the process of resin impregnating the rib forming yarns of these felts is difficult, costly, and time-consuming.

The problems of the prior art are overcome by the felt of the present invention which has improved water conveying channels formed on its machine side by means of bulky rib forming yarns which define channels therebetween, the rib forming yarns being securely interlocked with the base fabric and heat fused to render them essentially incompressible.

In accordance with the invention, the rib forming yarns are integrally woven with the base fabric, the base fabric containing what amounts to double sets of warp and filling yarns. The face or paper carrying side of the fabric is defined by a set of warp and filling yarns in a conventional pattern, such as a plain weave or a satin or twill weave in which the load bearing yarns extend in the machine direction. The bulky rib forming yarns lie in the back or machine side of the fabric and also extend in the machine direction. The rib defining yarns are initially interlocked with the face side of the fabric by means of cross machine direction holding yarns which are interwoven with the machine direction yarns on the face side of the fabric. The holding yarns are preferably quite light so as to minimize obstruction of the channels defined between the rib forming yarns.

The bulky rib forming yarns comprise a mixture of staple or multifilament fibers with relatively low and relatively high melting points. Preferably, about 50% of these fibers are polypropylene fibers with a low melting point. The rib forming yarns may be individually spaced apart by substantially equal distances, or they may be formed in pairs, the objective in either case being to provide channel forming grooves between the spaced apart rib forming yarns or set of yarns. Sets of yarns are advantageous in certain instances to minimize pressure nonuniformities.

Following formation of the base fabric, a non-woven batting material is needled to the face side of the fabric, the needled batt material serving the dual function of defining the paper supporting surface of the felt and also acting to tightly secure the rib defining yarns to the yarns defining the face side of the fabric. To this end, the needling operation is conducted in two stages, the first being from the face side of the fabric, the batt fibers being needled through the warp and filling yarns and also the rib forming yarns so that the rib forming yarns are firmly anchored to the fabric by a multiplicity of fibers. In the second stage, the fabric is needled from its back or machine side to remove needled batt fibers from the channels between the rib forming yarns, the projecting fibers being needled back into the face side of the fabric.

Following needling, the rib forming yarns are heated to a temperature above the melting temperature of the fibers with the relatively low melting point, but not above the melting temperature of the fibers with the relatively high melting point. The fibers with the low melting point melt and fuse with the other fibers, rendering the rib forming yarns essentially incompressible and also serving to further insure against separation of the rib forming yarns from the fabric by internally bonding them to the needled batt fibers.

In a modification of the invention, the holding yarns are formed from fibers which may be dissolved following needling, thereby eliminating the presence of yarns which obstruct the open flow area of the grooves, the holding yarns in such instance being formed from fibers which may be readily dissolved in a suitable solvent.

Accordingly, it is a principal object of the present invention to provide a papermaking felt having an enhanced capacity for removing water from a paper sheet supported on the face side of the felt, the back or machine side of the felt being of ribbed configuration defining water conveying channels therebetween.

A further object of the invention is the provision of a papermaking felt, the ribbed machine surface of which is defined by bulky yarns securely anchored to the felt fibers by needling, the rib forming yarns being heat fused to render them essentially incompressible, thereby maintaining the integrity of the channels defined by the rib forming yarns and enhancing the wear characteristics of the felt.

Still a further object of the invention is the provision of a papermaking felt in which the channel forming rib yarns are initially secured to the base fabric by means of holding yarns, and wherein the holding yarns may be of such character that they may be dissolved subsequent to the needling of the felt so as to further increase the void areas or channels defined by the rib forming yarns.

It is still a further object of the invention to provide an improved felt construction which, due to the increased void areas on its machine side, may be advantageously used on plain presses in a dry-nip operation.

Still a further object of the invention is the provision of felts characterized by water escape channels on their machine sides which significantly reduce shadow marking when the felts are used on suction presses.

Other objects and advantages of the invention will become apparent as the invention is more fully illustrated and set forth in the accompanying drawings and detailed description which follows.

FIG. 1 is a diagrammatic vertical sectional view illustrating a fabric in accordance with the present invention.

FIG. 2 is a diagrammatic vertical sectional view similar to FIG. 1 but enlarged illustrating the fabric subsequent to removal of the holding yarns.

FIG. 3 is a diagrammatic vertical sectional view illustrating a modification of the invention.

FIG. 4 is also a diagrammatic vertical sectional view illustrating a modification of the invention having a different weaving pattern.

It is to be understood that the figures are drawn to an enlarged scale and are diagrammatic in nature. For the purposes of the description, the surface of the felt adapted to support and carry the paper sheet will be referred to as the face or top of the fabric, while the opposite side will be referred to as the back or machine side of the fabric.

Referring first to FIG. 1 of the drawings, the base fabric comprises warp yarns 1 and filling yarns 2 woven together in conventional fashion, the weave illustrated being a plain weave. It is to be understood that when the fabric is woven endless, the warp yarns 1 will lie in the cross machine direction in use and the filling yarns 2 will lie in the machine direction. Other weave patterns may be employed, such as a satin or twill weave, depending upon the characteristics to be imparted to the face surface of the fabric. The warp and filling yarns 1 and 2, respectively, may be formed from any of the materials normally employed to manufacture press felts, such as wool or synthetic fibers, or combinations thereof.

In accordance with the invention, the back or machine surface of the fabric is defined by bulky rib forming yarns 3 extending in parallel relation to the filling yarns 2, the rib forming yarns being spaced apart to define grooves or channels 4 therebetween. Preferably the orientation of the rib forming yarns 3 will be such that they lie below and between an adjacent pair of the filling yarns 2, such as the yarns 2a and 2b, the filling yarn 2b in conjunction with the next adjacent filling yarn 2c and the warp yarns 1 providing effective bridging in the areas of the channels 4. The rib forming yarns comprise a mixture of fibers with relatively low melting points and relatively high melting points. The rib forming yarns are preferably formed from staple or multifilament fibers, since it is essential to the invention that the rib forming yarns are of a character which may be readily needled. Preferably, half of the fibers comprising the rib forming yarns have a relatively high melting point and the other half have a relatively low melting point. The fibers with the relatively low melting point preferably comprise fusible polypropylene fibers. The rib forming yarns will be bulky as compared to the warp and filling yarns. By way of example, the size of the rib forming yarns will normally vary between the diameters represented by the lines A--A and B--B shown in the right hand portion of FIG. 1, the size of the rib forming yarns varying with the size and spacing between adjacent filling yarns 2. In the embodiment illustrated, the rib forming yarns are close to minimum size.

The rib forming yarns 3 are joined to the fabric by means to holding yarns 5 which pass over a first of the filling yarns, such as the filling yarn 2a, then downwardly between the rib forming yarn 3a and then upwardly over the adjacent filling yarn 2b and also over the next adjacent filling yarn 2c, whereupon the holding yarn passes downwardly for passage beneath the next rib forming yarn 3b, the pattern being repeated throughout the cross machine dimension of the fabric. The holding yarns 5 are intended to initially anchor the rib forming yarns to the warp and filling yarns and hold them in position for subsequent needling. Preferably the holding yarns will be very light, such as 300-400 denier, so as to minimize interference with the open flow areas of the channels 4. The number of holding yarns may vary, but usually one holding yarn for every two or three warp yarns provides effective rib yarn retention. Where the holding yarns are to be retained as an integral part of the fabric, they may comprise any of the conventional yarns, such as a cotton yarn.

If the fabric is woven endless, it may be placed directly on a conventional needling machine; but if the fabric is woven flat, it will first be spliced to form an endless belt. Endless weaving is preferred in that it eliminates the necessity for splicing together the ends of the bulky rib forming yarns.

A conventional non-woven batting material 6 is placed on the face side of the fabric and needled thereto. The needling is performed in two stages, the first being from the face side of the fabric, a portion of the batt forming fibers being caused to pass through the warp and filling yarns 1 and 2 and into and even through the underlying rib forming yarns 3. Following the initial needling operation, the fabric is turned over and needled from its back or machine side so that the protruding batt fibers, particularly in the areas of the channels 4, will be pushed back into the overlying fabric, thereby maintaining the channels 4 essentially free from obstructions. The needling operation thus serves to provide the desired surface finish on the face side of the fabric and also securely anchors the rib forming yarns to the machine side of the fabric.

In accordance with the invention, the holding yarns 5 may be retained as an integral part of the fabric, or in the alternative they may be removed to maximize the size of the channels where even greater water conveying capacity is desired. If the holding yarns are to be removed, they will be formed from fibers which may be readily dissolved, preferably by submersion in water. For example, holding fibers formed from polyvinyl alcohol are readily soluble in water at a temperature of 140.degree.-150.degree. F. The alginate fibers, such as calcium alginate, are also readily soluble in water and may be used to form the holding yarns. Alternately, the holding yarns may be dissolved by other solvents, such as by the use of an inorganic salt, depending upon the nature of the fibers from which the holding yarns are formed. For example, if polyamide or acrylic fibers are used, they may be treated with a solution of calcium thiocyanate.

FIG. 2 illustrates the fabric after the holding fibers have been removed, and it will be evident that the open areas of the grooves 4 have been enlarged by reason of the elimination of the holding yarns.

Irrespective of whether or not the holding yarns are removed, the rib forming yarns 3 will be subjected to a heat treatment for the dual purpose of rendering the rib forming yarns effectively incompressible and effecting a tight bond between the rib forming yarns and the batt fibers which are needled to the rib forming yarns. The rib forming yarns are heated to a temperature above the melting temperature of the fibers with the relatively low melting point, but not above the melting temperature of the fibers with the relatively high melting point. Due to the heat treatment, the fibers comprising the rib forming yarns are partially bonded or fused together and after cooling, the rib forming yarns are rendered essentially incompressible. As used herein, the term "essentially incompressible" is intended to denote a condition wherein the compressibility of the rib forming yarns is reduced to the extent of providing enhanced wear characteristics on the machine side of the fabric, with particular reference to resistance against deformation of the rib forming yarns.

FIG. 3 illustrates a modification of the invention wherein the rib forming yarns are formed in pairs comprising yarns 3c and 3d, each such pair being initially secured to the filling yarns 2 by the holding yarns 5 which, in this instance, separately anchor the rib forming yarns in each pair. To this end, rib forming yarn 3c lies between overlying filling yarns 2d and 2e, and yarn 3d lies between overlying filling yarns 2e and 2f. Rib forming yarn 3a is anchored by holding yarn 5a which-passes over filling yarn 2d, beneath rib forming yarn 3c, then upwardly over filling yarns 2e and also over adjacent filling 2f and 2g, whereupon the pattern is repeated. The rib forming yarn 3d is anchored by holding yarn 5b which passes over filling yarn 2c, beneath rib forming yarn 3d and then upwardly over filling yarns 2f, 2g and 2h, whereupon the pattern is repeated. Channels 4 are thus formed between the adjacent sets of rib forming yarns, and while the number of channels per unit of felt width is decreased, the pairs of rib forming yarns provide enhanced wear characteristics on the machine side of the fabric and also enhance the integrity of the channels by enlarging the areas of increased incompressibility. It is preferred to individually anchor the pairs of rib forming yarns rather than anchor each pair to a single holding yarn, although both rib yarns in each pair may be anchored by a single holding yarn. The number of holding yarns may vary but one holding yarn for every two or three warp yarns provides effective retention.

FIG. 4 is illustrative of a modification of the invention wherein the weave pattern of the warp and filling yarns is altered, each of the warp yarns having two floats on the face side of the fabric, indicated at 7, the resultant fabric having the surface characteristics of twill weave. It will be understood that additional pattern variations may be readily achieved by altering the sequence of the yarns, as will be readily understood by the worker in the art. FIG. 4 also illustrates the use of rib forming yarns which are of substantially maximum diameter relative to the spacings between the filling yarns.

As should now be apparent, the present invention provides papermaking felts, the back or machine sides of which are provided with grooves or channels which materially enhance the fabric having enhanced wear characteristics due to the manner in which the rib forming yarns are anchored, as well as the manner in which they are rendered incompressible. The fabrics can be readily woven, needled and heat fused using equipment which is readily available in mills which manufacture conventional papermaking fabrics.

While this invention has been described with reference to its preferred embodiment, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and is intended to cover in the appended claims all such modifications and equivalents as filed in the true spirit and scope of this invention.