PAPERMAKERS DRYER FABRIC

Abstract: A papermakers dryer fabric (10) woven from yarns (11, 12, 13) consisting essentially of metal and/or other materials with a thermal conductivity of more than 5 W/m/ DEG C.

Description: PAPE MAKERS DRYER FABRIC The present invention relates to a papermakers dryer fabric, and particularly but not exclusively, to a high temperature dryer fabric for use, for example, in an impingement dryer section of a papermaking machine. The term "high temperature" is used herein to mean temperatures of 200°C or more. A major trend in papermaking has always been to increase the speed of production. One way in which this has been achieved has been to increase the rate of drying in the dryer section of the machine, namely by increasing the temperatures to which the moist paper sheet is exposed. Traditionally the sheet is dried by being brought into contact with the face of a roll heated to 100-130°C under pressure exerted by a porous fabric, with air being used both to transport the evaporated moisture away from the dryer section and to ventilate the region close to the sheet in order to enhance moisture evaporation. A recent development has been the emergence of the so- called impingement dryer process in which a drying gas such as superheated air or steam at temperatures greater than 200°C, typically of the order of 250-300 c C is used to dry the sheet. This is achieved either by impinging the drying gas directly onto the face of the web, or by blowing the drying gas through the dryer fabric and onto the surface of the sheet. A typical impingement dryer section will comprise a large diameter, grooved, perforated suction roll followed by a steam-heated cylinder, a small diameter suction roll and finally another steam-heated cylinder, with air impingement taking place at the large suction roll where a drying hood directs high velocity hot air jets onto the sheet. An alternative arrangement sees the sheet urged against a cylinder by the supporting dryer fabric, with the high velocity air jets impinging the sheet surface through the porous fabric. The sheet is supported on a dryer fabric within the entire dryer section. Machine speeds of up to 2400 metres per minute can be achieved, compared with maximum speeds in the region of 1600 metres per minute at present. Most conventional materials for dryer fabrics such as polyesters, copolyesters, or polyamides are not able to withstand this combination of very high temperature and humidity. Many other high temperature materials such as polyimides, aramids or glass content yarns are highly susceptible to hydroiytic degradation. Materials available which are suitable for use within the impingement dryer temperature range include PEEK, but as yet this is a very expensive material and a fabric composed entirely of the same is impractical on a cost basis. The present invention seeks to provide a fabric suitable for transporting a moist paper sheet through a high temperature dryer section, e.g. an impingement dryer arrangement, displaying good resistance to hydrolysis, thermal degradation and wear, whilst at the same time comprising relatively inexpensive material yarns. According to the present invention there is provided a papermakers dryer fabric woven from yarns, at least some of said yarns consisting essentially of material having a thermal conductivity of more than 5 W/m/°C. The said material preferably has a thermal conductivity of more than 10 W/m/°C and more preferably of more than 25 W/m/°C. Typically the thermal conductivity would be in the range from 10 to 450 W/m/°C and preferably from 25 to 100 W/m/°C. All of the above thermal conductivity figures are for the temperature range 100 to 500°C. The said material preferably comprises metal and/or material with a material with like dimensional stability to metal at differing temperature extremes; i.e. a change in width and/or length of material of no more than 5% and preferably 1%. Preferably at least some, and ideally all, of the cross- machine direction yarns of the fabric comprise said material. Preferably at least some, and ideally all, of the machine-direction yarns of the fabric comprise said material. All-metal fabrics are well known in the field of papermaking, having been well established as forming wires. These fabrics typically comprise a single layer structure in a simple weave and were employed in the forming section of the papermachine, wherein the initial stock of cellulosic fibre slurry is cast onto an endless belt to drain through, leaving a highly moist web of fibres which is subsequently dewatered by mechanical means in the press section and with the aid of heat in the dryer section. The requirements for these forming wires are totally different than those for dryer fabrics. The main criteria for a forming wire are a low marking surface with a large number of support points to capture the cellulosic fibres, together with a high resistance to abrasion due to the passage of abrasive filler particles through the fabric, whereas a dryer fabric requires good resistance to hydrolysis and thermal degradation due to the elevated temperature. Forming takes place under ambient conditions. Besides, metal forming wires are rarely used these days, with their high cost having lead to their replacement with synthetics such as polyester or polyamide. The use of metal in dryer fabrics is also known. European Patent application 0006316 and Canadian Patent application 896,316 both teach of coating a dryer fabric with a resin mixture containing metal particles to improve the thermal conductivity of the dryer fabric. European Patent application 0195835 teaches of a metal or synthetic spiral link dryer fabric whose paper contacting surface is flocked with chopped synthetic fibres. US Patent 4,839,213 and German Patent 3221255 discuss metal stuffer yarns being inserted into spiral link fabrics to reduce the air permeability. European Patent 0054206, US Patent 4,015,038 and Canadian Patent 1,324,907 all contain metal core yarns twisted, braided or wrapped with other yarns. None of these patents address the problem of providing a dryer fabric suitable for use in a high temperature dryer. A patent that does discuss a high temperature dryer fabric is US Patent 4,359,501 which teaches of so-called industrial fabrics comprising 100% PEEK, PEEK+ synthetic yarns or PEEK + metal yarns. However, no reference is made to any weave structure or the proportion of components. A major problem that has been found with impingement dryers is that for when the hot drying gas is blown onto the sheet, the supporting dryer fabric typically has a temperature of 60°C beneath the sheet, but in its lateral regions where the drying gas impinges directly onto the fabric the temperature is in the region of 300°C. This huge difference in temperature across the width of the fabric causes deformation of the fabric, resulting in marking of the paper and accelerated fabric wear. A similar problem occurs where the drying gas is blown through the underside of the dryer fabric and onto the supported sheet, since in practice the drying hood does not extend across the entire width of the dryer fabric. Thus the difference in temperature between the cool lateral edges and the extremely hot fabric centre is of the order of several hundred degrees, once again causing fabric deformation. The use of metal yarns in the cross- direction (weft) means that a material with a far superior thermal conductivity than synthetics is in place. Thus any temperature differences are rapidly equalised. It has also been found that deformation also occurs due to the normally crimped synthetic machine direction (warp) yarns softening or changing dimensions at the high temperatures. The use of metal warp would reduce this deformation considerably. Although a 100% metal dryer fabric may be suitable for some applications, there are certain disadvantages. First of all with respect to synthetic fabrics they are extremely heavy, making them difficult and somewhat dangerous to handle. Thus it is preferred to reduce the weight by substituting some of the metal yarns and/or materials with like dimensional stability with other types of yarns, such as synthetic monofilaments or multifilaments - circular, flat or profiled - of suitable high temperature synthetic materials such as PEEK or polyphenylene sulphide. For lower temperature applications more conventional materials such as polyester (eg. PEN) or phthalamides (eg. Amodel or Ultramid T) may be used. Preferably the synthetic yarns are employed in the warp. A further disadvantage of a 100% metal fabric is the problem of wear as it passes over dryer cylinder surfaces. With most, if not all, of the metal yarns in the weft, contact of the metal with the machine parts of a dryer section, or even the paper sheet itself, is reduced. Typically such a fabric will be 0.8 - 1.5mm in thickness and have an air permeability of up to or exceeding 1000 cfm. Any metal or alloys thereof, including stainless steel, brass or bronze, may be used either alone or combination. The metal yarns may be circular, flat or profiled. Preferred alternative materials comprise any of the following either alone or combination: high temperature synthetic polymer containing particles of a high thermal conductivity material such as carbon, graphite or metal, a synthetic polymer provided with a coating of said thermal conductivity material, a composite yarn at whose surfaces a high thermal conductivity material predominates, carbon fibres or a yarn comprising a combination of individual high thermal conductivity material filaments or a combination thereof. The fabric may be woven endless or alternatively woven flat and then joined by seaming. In order that the present invention may be more readily understood specific embodiments thereof will now be described by way of example only with reference to the accompanying drawings in which:- Fig.l is a schematic diagram of one fabric in accordance with the present invention; and Fig.2 is a schematic diagram of a second fabric in accordance with the present invention. Referring to Fig.l a woven dryer fabric 10 comprises warp yarns 11 and weft yarns 12,13. The warp yarns 11 are illustrated in a manner extending across the page. Two different sizes of weft 12,13 are provided, alternatively, in the fabric. The relatively small weft yarns 12 drop into the large voids between the larger weft yarns 13. All of the weft yarns are made of stainless steel. Every fourth warp yarn is metal, the others being synthetic. Fig.2 shows a second dryer fabric similar to that illustrated in Fig.l except in that the smaller weft yarns 13 are profiled yarns. The profiled yarns are rectangular in cross section. It is to be understood that the above described embodiments are by way of illustration only. Many modifications and variations are possible.

Claims: CIAIU£ 1. A papermakers dryer fabric woven from yarns, at least some of said yarns consisting essentially of material having a thermal conductivity of more than 5 W/m/°C. 2. A papermakers dryer fabric as claimed in claim 1, wherein the said material has a thermal conductivity of more than 10 W/m/°C. 3. A papermakers dryer fabric as claimed in claim 1 or claim 2, wherein the said material has a thermal conductivity of more than 25 W/m/°C. 4. A papermakers dryer fabric as claimed in any preceding claim, wherein at least some of the said yarns which extend in the cross-machine direction of the fabric comprise yarns consisting essentially of said material. 5. A papermakers dryer fabric as claimed in any preceding claim, wherein all of the said yarns which extend in the cross-machine direction of the fabric comprise yarns consisting essentially of said material. 6. A papermakers dryer fabric as claimed in any preceding claim, wherein at least some of the said yarns which extend in the machine-direction of the fabric comprise yarns consisting essentially of said material. 7. A papermakers dryer fabric as claimed in any preceding claim, wherein all of the said yarns which extend in the machine-direction of the fabric comprise yarns consisting essentially of said material. 8. A papermakers dryer fabric as claimed in any preceding claim, wherein the fabric further comprises further yarns which do not comprise said material. 9. A papermakers dryer fabric as claimed in claim 8, wherein the said further yarns comprise any of the following either alone or in combination: PEEK, polyphenylene sulphide, polyester or phthalamide. 10. A papermakers dryer fabric as claimed in any preceding claim, wherein the fabric has a thickness in the range from 0.8 to 1.5 mm. 11. A papermakers dryer fabric as claimed in any preceding claim, wherein the said material comprises metal. 12. A papermakers dryer fabric as claimed in any preceding claim, wherein the said material comprises any of the following either alone or in combination: stainless steel, brass or bronze. 13. A papermakers dryer fabric as claimed in any preceding claim, wherein the said material comprises any of the following either alone or in combination: high temperature synthetic polymer containing particles of a high thermal conductivity material such as carbon, graphite or metal; a synthetic polymer provided with a coating of said thermal conductivity material; a composite yarn at whose surfaces a high thermal conductivity material predominates; carbon fibres or a yarn comprising a combination of individual high thermal conductivity material filaments.

Inventors: ASHWORTH TIMOTHY, HODSON MARK, ASHWORTH, TIMOTHY, HODSON, MARK
Applicants: ASHWORTH TIMOTHY, HODSON MARK, SCAPA GROUP PLC, ASHWORTH, TIMOTHY, HODSON, MARK
Application year: 1996
Main classification: D21F1
IPC codes: D21F1