GB2092194A - Dryer felt fabric - Google Patents

Abstract

A method of stabilizing a dryer felt fabric for a papermaking machine, comprises including in the cross- machine direction synthetic polymeric yarns 18 of relatively low softening point. Upon heat-setting the fabric 10, the yarns 18 of low softening point relax and take on the configuration of the weave. Upon re-solidifying the softening yarns 18 lock in with the remaining yarns of the fabric at cross- over points. As shown, the fabric 10 comprises two layers A, B of cross- wire yarns 12 which are bound together by interwoven lengthwise yarns 14. Preferably, the yarns 12, 14 are monofilaments of polyesters, polyamides, polyaramids or polyolefins which do not absorb high proportions of moisture. Stuffer pick yarns 18 partially fill voids 16 between the layers A, B and are made of a thermoplastic polymer having a substantially lower softening point than the yarns 12, 14, e.g. spun yarns of polypropylene. Heat setting may be carried out at temperatures of 200-380 DEG F (93.3-193.3 DEG C) for 15-60 minutes. <IMAGE>

Description

SPECIFICATION Dryer felt fabric The invention relates to paper machine clothing and more particularly relates to fabrics useful as dryer felts, and their use and to methods of stabilizing them.

The art is replete with descriptions of dryer felts and dryer felt fabrics used to fabricate dryer belts for use in the dryer section of a papermaking machine. In spite of the wide variety of fabric materials available, the ideal dryer fabric is yet to be found, many of them suffering from dimensional instability.

One form of dryer belt commonly employed in the dryer section of a papermaking machine is referred to as a "screen" and is fabricated by weaving synthetic monofilaments or twisted multifilaments together in an open weave. Although not subjected to any form of miliing, and therefore not "felts" in the original sense of the term, these screen fabrics have also become known as "dryer felts".

The endless belts made therefrom are generally woven flat and the ends thereafter joined to form an endless belt. The weave selected may be for example, a two or three layer weave of synthetic yarns such as multifilament, spun or monofilament yarns.

It will be appreciated that the screen type of "dryer felt" fabric is relatively open in design, resulting in a relatively high fabric permeability, i.e.; air permeability of the order of from about 70 to 700 CFM/sq. ft. at 1/2" water. Such fabrics permit free vapor passage through the fabric during operation of the papermaking machine. This high degree of openness has some disadvantages. For example, the fabrics lack a high degree of dimensional stability in use, particularly in the cross-machine direction (the machine-direction being supported by tension on the dryer belt).

The invention comprises in a papermachine dryer fabric, which comprises woven textile yarns, the improvement, which comprises; including in the cross-machine direction of the weave heat-softened yarns of synthetic, thermoplastic, polymeric resin yarns.

The invention is also of a method of stabilizing dryer felt fabrics and dryer belts prepared from the stabilized dryer fabrics.

A dryer felt fabric embodying the present invention obviates a number of problems associated with the prior art dryer fabrics, particularly of the screen type. The fabric is characterized by a wide range of permeabilities of the order of from 30 to 700 CFM/sq. ft. at 1/2" water. However, even at high permeability it also exhibits a high degree of dimensional stability in use. The fabric is advantageously used to fabricate "dryer felts" for papermaking machines.

Fabrics embodying the invention also resist absorption of water, permitting excellent passage of moisture vapor from the paper sheet being dried, enhancing the drying rate. The openness of the fabric also resists filling of the fabric voids with contaminant particles from the paper sheet.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, of which: Fig. 1 is a cross-sectional side elevation along the machine-direction of a pre-cursor fabric embodying the invention.

Fig. 2 is a view as in Fig. 1 but of a fabric embodying the invention.

Fig. 3 is a view along lines 3-3 of Fig. 2.

Fig. 4 is an enlarged view of a portion of the fabric shown in Fig. 3.

Fig. 5 is a view-in-perspective of a dryer belt made from a fabric embodying the invention.

The present invention relates to dryer felt fabrics. In general, such fabrics comprise a woven structure of weft and warp yarns. The yarns may be monofilaments, multi-filaments or spun from staple fibers. The fabrics may be made in simple, monolayer weaves or they may be complex:, multilayer weaves depending on the particular properties required.

Fig. 1 shows a portion of a precursor dryer fabric embodying the invention in a ci-oss-sectional side elevation. The fabric 10 comprises a plurality of cross-wise yarns 1 2 disposed in an upper layer A and in a lower layer B. The layers A, B are parallel to the crosswise plane of the fabric 10 (crnsswise to the machine direction of felt travel during use, the machine direction M.D. being shown by tie. arrow in Fig. 1). The crosswise yarns 1 2 and the layers A and B are bound together by the interwoven lengthwise yarns 14. The yarns 12, 1 4 may be monofilament, multi-filament or spun yarns of any conventional denier. Preferably for a dryer felt, low absorption monofilament yarns are employed.Representative of preferred monofilament yarns 12, 14 are monofilament yarns of polyesters, polyamides, polyaramids, polyolefins and the like which do not absorb high proportions of moisture. Preferably the monofilaments will have a diameter of from about 0.008 to 0.04 inches to enhance stability and structura' integrity in the fabric.

It will be observed from Figure 1 that there are a plurality of interstices or voids 1 6 among the layers A, B separating adjacent crosswise yarns 12 at select points along the length of the crosswise yarns 1 2. These voids 1 6 are open areas within the body of fabric 1 0 which permit unimpeded flow of air through the fabric 10, accounting in part for the high permeability associated with these "screen" types of fabric.In the fabric 10, the voids 16 are partially filled with stuffer pick yarns 1 8 which act as fillers and are disposed in the crosswise plane of the fabric 10, substantially parallel to the crosswise yarns 1 2. The nature of the stuffer pick yarns 1 8 is critical and a dimension is selected suitable for partially filling the voids 16 without expanding the thickness of the fabric 10 or closing all void space 16. Representative of yarns which may be employed as the yarns 1 8 are monofilament, spun and multifilament yarns of synthetic thermoplastic polymers. Representative of such yarns are mono- and multifilaments of polyolefins such as polypropylene, polyethylene and the like.A feature of the fabric is the selection of the yarns 1 8 from a thermoplastic polymeric resin material having a softening point substantially lower than the yarns 12 and 14. The reason for this will be apparent from the following discussion. Preferably the yarns 18 are spun yarns of polypropylene textile fibers.

The fabric 10 may be woven by conventional weaving techniques, well known to those skilled in the art.

Following the weaving of fabrics embodying the invention, as exemplified by fabric 1 0, they are heat set to stabiiize the fabric and to draw the yarns into their desired relative positions. The lengthwise yarns 14 are drawn inwardly of the outer surfaces of the fabric 1 0 and this pressure "crimps" the yarns 12 and 18 to some extent The degree of heat-setting required to achieve the desired structure of the fabric 10 will of course vary depending on the nature of the yarns 12, 14 and 18. The temperature employed must be within a range lower than the melting point of the yarns 12, 14 but higher than the softening point of the yarns 18. During the heat setting step, the yarns 12, 14 do not appreciably soften or melt, while the yarns 1 8 do. In softening, the polymer chains relax and the yarns mold in the particular configuration of their weave.Upon cooling of the heat-set fabric 10 below the softening point of the yarns 1 8 they re-solidify in the weave configuration and stabilize in those positions. When the preferred spun polypropylene forms the polymeric substance of the preferred yarns 1 8, the fibers melt during heat-setting and upon re-solidification the yarns 1 8 take on the appearance of woven monofitaments. The re-solidified yarns 18 also pick up and retain less moisture than the precursor spun yarn 18. Fig. 2 is a view as in Fig. 1 but after heat-setting of the fabric 10. As shown, the yarns 1 8 have been transformed so as to take on the appearance of monofilaments; see also Fig. 3, a view along lines 3-3 of Fig. 2.

Fig. 4 is a view of an enlarged portion of yarns 1 8 following heat treatment and shows the yarn 1 8 now locked in place, due to its melting and resolidification, in the weave with lengthwise yarns 1 4. This "locking" stabilizes the fabric 10 in the cross-machine (C.D.) direction (see arrow in Fig. 3). The tendency of the fabric to exhibit dimensional instability in the C.D. direction is obviated. Optimum times, temperatures and tensions placed on the fabric during heat-setting can be determined by those skilled in the art, employing trial and error technique for the different yarn materials. In general, heat-setting may be carried out at temperatures of from about 2000 F. to 3800 F. for from 15 to 60 minutes.The fabrics may be woven flat and the ends joined by conventional seaming methods, known to those skilled in the art to form dryer belts 5 as shown in Fig. 5. The seamed belts so made are readily:employed as dryer felts in the dryer section of a paper making machine.

The following example sets forth a particular mode of making and using the invention but is not to be considered as limiting.

Permeability was determined with a Frazier type air permeability tester manufactured by the United States Testing Company. The measurements of this instrument are given in units which refer to the number of cubic feet of air which pass through one square foot of fabric at a pressure corresponding to 1/2 inch of water (abbreviated, cu. ft./min./sq. ft./at 1/2" H2O).

EXAMPLE 1 There is provided a quantity of 0.020 inch diameter polyester monofilament (melting point 4820 F.) and a quantity of 0.021 inch diameter polyamide (nylon) monofilament yarn (melting point 4200--4300 F.). There is also provided a quantity of 500 grain per 100 yard size spun yarns composed of polypropylene fibers (melting point 3200 to 3500 F.; softens at 2850--3200 F.).

The density of the polyester monofilament warp in the product is 80 ends to the inch. The number of polyester weft yarns in the product is 1 8 monofilaments and 8.3 polypropylene spun yarns (stuffer yarns) for a total of 26.3 wefts per inch.

The fabric of the Example is finished in a conventional manner, i.e. by heatsetting under tension at a temperature of 3800 F. for 1 5 minutes to offer specific properties of runnability and dimensional stability.

Upon completion of making the fabric, it is subjected to physical testing and found to have the following physical properties, in comparison to the same fabric before heat-setting: Before Heat Set After Heat Set Permeability 75 CFM 210 CFM Dimensional Stability 3.0% elongation with 0.2% elongation with 5 Ib. tension on diagonal 5 Ib. tension on diagonal An endless dryer felt is made from the fabric. When installed on a paper machine as a dryer felt, the fabric performs well in the manufacture of papers. The belt tracks well, is easily guided and exhibits a long life even after exposure to temperatures of circa 2500 F.

The felts of the invention may be finished in any conventional manner, i.e.: by chemical treatments to offer specific properties of runability and resistance to chemical and abrasive degradation.

Those skilled in the art will appreciate that many modifications to the above-described preferred embodiments may be made without departing from the spirit and the scope of the invention.

Claims (8)

1. In a paper machine dryer fabric which comprises woven textile yarns, the improvement which comprises; including in the cross-machine direction of the weave heat-softened yarns of synthetic, thermoplastic, polymeric resin yarns.

2. The improvement of claim 1 wherein the remnant yarns are stuffer yarns.

3. An endless dryer belt made from the fabric of claim 1.

4. A dimensionally stable dryer felt fabric, which comprises: a plurality of crosswise yarns disposed in a plurality of separate layers, each layer being on a plane parallel to the crosswise plane of the fabric; a plurality of lengthwise yarns interwoven with the crosswise yarns and binding the layers of crosswise yarns together to form a multi-layer textile fabric; the crosswise and lengthwise yarns being synthetic monofilaments and the crosswise yarns in a given layer being separated from adjacent crosswise yarns in the adjacent layer at points along their length by void spaces within the body of the woven fabric; a plurality of stuffer yarns running substantially parallel to the cross-wise yarns, between layers of crosswise yarns and partially filling a portion of the void spaces between the layers of crosswise yarns;; said stuffer yarns being softened by heat and resolidified in place.

5. A dimensionally stable dryer felt fabric substantially as herein described with reference to and illustrated by Figures 1 to 4 of the accompanying drawings.

6. An endless dryer belt made from the fabric of claim 4 or 5.

7. A method of stabilizing a dryer felt fabric, said fabric comprising interwoven first yarns of a synthetic, polymeric resin having a given melting point, which comprises; weaving synthetic thermoplastic, polymeric resin second yarns characterized by a softening point lower than said melting point, into the fabric; and heat-setting the fabric under conditions wherein the thermoplastic yarns soften and conform to the weave while the first yarns remain unmelted.

8. A method of stabilizing a dryer felt fabric, said fabric comprising interwoven first yarns of a synthetic, polymeric resin having a given melting point, the method being substantially as herein described with reference to and as illustrated by Figures 1 to 4 of the accompanying drawings.