PAPERMAKERS FABRIC

Abstract: A papermakers fabric (10) is woven from machine direction yarns (11, 19, 20) and one or more layers of cross machine direction yarns (12-18). The cross machine direction yarns (12-18) in at least one of the layers comprise relatively large (12, 13, 16, 17) and small (14, 15, 18) yarns. The machine and cross machine direction yarns (11, 19, 20) are arranged such that the machine direction yarns (11, 19, 20) define outer floats or knuckles at the surfaces of the fabric and inner floats or knuckles in the body of the fabric around small cross machine direction yarns (14, 15, 18) provided in one of the cross machine direction yarn layers. The parts of the machine direction yarns provided at one surface defining the inner floats or knuckles do not extend to the plane of the obverse surface of the fabric.

Description: PAPERMAKERS FABRIC The present invention relates to papermakers fabrics and particularly, but not exclusively, to a fabric for use in the dryer section of a papermaking machine. Generally speaking papermaking machines are made up of three sections, namely the forming, pressing and drying sections. In each section a fabric belt is used to transport a continuous paper sheet through the papermaking equipment as the paper is being manufactured. The structure of the fabrics for each section differs, as the requirements of each section of the papermachine are different. For example, a forming fabric must have both a precise permeability which provides the required water drainage and a surface matched to the fibre type being processed, whereas a press fabric must be capable of rapidly absorbing and expelling water while protecting the newly formed paper sheet. Dryer fabrics are designed for heat resistance, high durability in adverse chemical and physical conditions and permeability. Patent application WO 91/190-44 describes a papermakers dryer fabric incorporating flat monofilament machine direction yarns interwoven with the cross machine direction yarns in a manner such that floats are provided on the upper and lower surfaces of the fabric. The floats provide a smooth fabric surface. The system of machine direction yarns comprises upper and lower yarns which are vertically stacked in order to control the permeability of the fabric. The machine direction yarns are woven contiguous with each other to reduce the permeability of the fabric and to lock in the machine αirection alignment of the stacking pairs of machine direction yarns. In the papermachine dryer fabrics pass over dryer cylinders. Here the fabric is subjected to intense heat on the cylinder side, to steam degradation throughout, (but more predominantly on the paper carrying side) and to wear on either side due to the nature of the paper being formed and/or due to rusty or corroded felt rolls, degradation being a particular problem at the edges of the belt which are always in contact with hot dryer cylinders. It is an object of the invention to provide a papermakers fabric having superior resistance to degradation due to heat, hydrolysis and wear. According to the present invention there is provided a papermakers fabric woven from machine direction yarns and one or more layers of cross machine direction yarns, characterised in that the cross machine direction yarns in at least one of the layers comprise relatively large and small yarns and the machine and cross machine direction yarns are arranged such that the machine direction yarns define outer floats or knuckles on one of the faces of the fabric and inner floats or knuckles below said one face around small cross machine direction yarns in one of the cross machine direction yarn layers, wherein the parts of the machine direction yarns defining the inner floats or knuckles below said one face do not extend to the plane of the obverse face of the fabric. Ideally all of the layers of cross machine direction yarns comprise relatively large and small yarns. Preferably rloats are provided by machine direction yarns on both the said one and obverse surfaces of the fabric with all of the inner machine direction floats or knuckles being located within the body of the fabric between the said one and obverse face of the fabric. The fabric of the invention has particular, but not exclusive, application as a dryer fabric. As the floats or knuckles of the machine direction yarns do not extend to the fabric surface, the opposite face of the yarn is insulated by air from heat from a dryer cylinder. As air is a better thermal insulation medium than fabric the papermakers fabric of the invention offers exceptional resistance to heat degradation, whilst the increased remoteness of the yarns from the fabric surface increases the resistance to degradation by wear and hydrolysis. This is of upmost importance in the dryer section of a papermachine. The machine direction yarns floating on one face and the machine direction yarns floating on the obverse face of the fabric may be made from the same material or different materials. The same or different materials may be used for the cross machine direction yarns provided at said one face and the obverse face of the fabric and/or within the individual cross machine direction layers. Suitable materials for the machine direction yarns include polyesters, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), polyamides, polyphenylene sulphide (PPS), polyetheretherketone (PEEK) or PCTA (copolymers of cyclohexane di ethanol with terephthalic acid and isophthalic acid). One suitable fabric is made entirely from polyester (e.g. PET), whilst another has polyester machine direction yarns on the back surface of the fabric and high performance machine direction yarns (e.g. PPS, PEN or PEEK) on the opposite surface. As the machine direction yarns on one surface of the fabric do not come into contact with the opposite surface of the fabric in the dryer machine section, machine direction yarns at the paper side of the fabric will not come into contact with the dryer cylinders and will therefore not be subjected to such extreme temperatures. Thus there is no need for the yarn to exhibit exceptional resistance to degradation by heat or wear. The yarns provided at the opposite surface of the fabric may be chosen to exhibit improved resistance to abrasion and heat. Thus by selecting the appropriate materials for the machine direction yarns at the two surfaces of the fabric the working life of the fabric may be increased. The cross machine direction weft yarns are typically made from polyester (e.g. PEN, PCTA), PPS, PEEK or combinations thereof. A further novel aspect of the invention is the use of different combination patterns of cross machine direction yarn sizes in order to control permeability. Typical cross machine direction yarn sizes are 0.3mm diameter alternating with 0.6mm, 0.4mm or 0.2mm. WO 91/19044 achieves permeability control by altering the number of machine direction yarns and cross machine direction yarns in the fabric. This is αisadvantageous compared with altering the difference between the cross machine direction yarn diameters since it involves time consuming re-setting of the loom. Other alternative methods of permeability control include the insertion of stuffer cross machine direction yarns, altering the number of machine direction yarns, changing the cross machine direction yarn density and utilising non-circular or multifilament cross machine direction and/or machine direction yarns. Examples of cross machine direction yarn diameter combinations are 0.5mm and 0.7mm; 0.5mm and 0.8mm; 0.5mm and 0.9mm; and 0.4mm and 0.9mm. The difference in diameter of the cross machine direction yarns is preferably in the range from 0.1mm to 0.6mm. In a preferred embodiment of the invention the fabric comprises at least two layers of cross machine direction yarns. Preferably all of the layers of cross machine direction yarns comprise relatively large and small yarns. The large and small yarns are preferably provided in an alternating fashion both in any single cross machine direction yarn layer and vertically in adjacent layers. In a preferred embodiment of the invention "Stuffer" cross machine direction yarns are further included in order to control the permeability. These are incorporated by weaving cross machine direction yarns having a large surface area into the fabric. The incorporation of additional yarns would decrease the permeability of the fabric. These additional yarns are preferably profiled yarns. The machine direction yarns of the invention are p referably flat monofilaments preferably having an aspect ratio in the range from 2:1 to 4:1. However, other profiled ( i.e. non-circular cross section) yarns may be used such as triangular yarns. Alternatively circular cross-section yarns may be used. Especially preferred is the use of "D" shaped yarns. S uch yarns would be arranged such that the curved surface faces into the body of the fabric. The curved surface gives improved resistance to contamination and facilitates easy cleaning of the yarns (using high pressure air/water jets ) . The fabric may contain different cross sectional sizes and/or shapes of machine direction yarns, for example as an alternative means of keeping certain yarns out of contact with the sheet and/or dryer cylinders. Preferably the machine direction yarns float over at least two relatively large cross machine direction yarns and any small cross machine direction yarns therebetween thereby forming a float. In a multilayer fabric the knuckle from the machine direction yarn provided at the opposite surface of the fabric would be formed under a single cross machine direction yarn in the fabric. 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 side elevation of one fabric in accordance with the invention; and Fig.2 is a side elevation of a second fabric in accordance with the invention. Referring to Fig.l a fabric 10 comprises a single layer of relatively large and small cross machine direction yarns arranged alternately. A first machine direction yarn 11 extends above two large cross machine direction yarns 12,13 and a small cross machine direction yarn 14 located therebetween so as to form a float. The yarn then passes below a small cross machine direction yarn 15 to form a knuckle and then over a further pair of large cross machine direction yarns 16,17 and a small cross machine direction yarn 18. The majority of the machine direction yarn 11 is thus located at the upper surface of the fabric. The parts of the machine direction yarn 11 forming the knuckles do not extend as far as the lower surface of the fabric. The other machine direction yarns 19 provided at the upper surface of the fabric are arranged in the same manner. The machine direction yarns 20 provided at the lower surface of the fabric float below two large cross machine direction yarns and one small cross machine direction yarn located therebetween. The yarn then extends over a small cross machine direction yarn so as to form a knuckle and then over two further large cross machine direction yarns and an intermediate small cross machine direction yarn. The machine direction yarns provided at the lower surface of the fabric do not therefore extend to the upper surface of the fabric. In use the fabric is formed into a belt for a paper dryer machine. The yarns on one surface of the fabric support the paper and are typically made from PPS, PEN, PEEK or PCTA which exhibit good resistance to degradation by heat, steam and wear. The yarns at the other surface of the fabric are typically made from polyester which exhibits good resistance to dry heat degradation. Fig.2 shows a further fabric 21 in accordance with the invention having two layers 22,23 of cross machine direction yarns, each layer comprising relatively large and small cross machine direction yarns arranged alternately. The cross machine direction yarns of the two layers are preferably, but not exclusively stacked in a manner such that a small cross machine direction yarn is located above a large cross machine direction yarn and a large cross machine direction yarn is located above a small cross machine direction yarn. A machine direction yarn 24 floats over two large cross machine direction yarns 25,26 at the upper surface and an intermediate small cross machine direction yarn 27. The yarn then passes below a small cross machine direction yarn 28 in the upper layer 22, and a small cross machine direction yarn 29 in the lower layer 23 and a further small cross machine direction yarn in the upper layer 30. The machine direction yarn 24 then repeats this weave pattern along the length of the fabric. The other machine direction yarns 31 at the upper surface are similarly arranged. The machine direction yarns 32,33 provided at the lower surface are arranged in a mirror image to the upper cross machine direction yarns. As before the machine direction yarns defining floats at the base of the fabric do not extend to the upper surface and the machine direction yarns defining floats at the top surface of the fabric do not extend to the lower surface of the fabric. It is to be understood that the above described embodiments are by way of illustration only. Many modifications and variations are possible.

Claims: CLAIMS 1. A papermakers fabric woven from machine direction yarns and one or more layers of cross machine direction yarns, characterised in that the cross machine direction yarns in at least one of the layers comprise relatively large and small yarns and the machine and cross machine direction yarns are arranged such that the machine direction yarns define outer floats or knuckles on one of the faces of the fabric and inner floats or knuckles below said one face around small cross machine direction yarns in one of the cross machine direction yarn layers, wherein the parts of the machine direction yarns defining the inner floats or knuckles below said one face do not extend to the plane of the obverse face of the fabric. 2. A papermakers fabric as claimed in claim 1, wherein all of the layers of cross machine direction yarns comprise relatively large and small yarns. 3. A papermakers fabric as claimed in claim 1 or claim 2, wherein machine direction yarns floating on said one face of the fabric are made of the same material to machine direction yarns floating on the obverse face of the fabric. 4. A papermakers fabric as claimed in claim 1 or claim 2, wherein machine direction yarns floating on said one face of the fabric are made of a different material to machine direction yarns floating on the obverse face of the fabric. 5. A papermakers fabric as claimed in any preceding claim, wherein the fabric comprises a plurality of layers of cross machine direction yarns and different materials are used for the cross machine direction yarns provided at the said one face and obverse face of the fabric. 6. A papermakers fabric as claimed in any preceding claim, wherein machine direction yarns floating on one face of the fabric comprise polyester and machine direction yarns floating on the obverse face of the fabric comprise PPS, PEN or PEEK. 7. A papermakers fabric as claimed in any preceding claim, wherein there is a difference in the diameter of the relatively large and small cross machine direction yarns in the range from 0.1mm to 0.6mm. 8. A papermakers fabric as claimed in any preceding claim, wherein the relatively large and small cross machine direction yarns are provided in alternating fashion in any single cross machine direction yarn layer. 9. A papermakers fabric as claimed in any preceding claim, wherein the fabric comprises more than one cross machine direction yarn layer and the relatively large and small cross machine direction yarns are provided in alternating fashion vertically in adjacent layers. 10. A papermakers fabric as claimed in any preceding claim, wherein the fabric further comprises additional stuffer cross machine direction yarns. 11. A papermakers fabric as claimed in claim 10, wherein the stuffer yarns are non-circular in cross section. 12. A papermakers fabric as claimed in any preceding claim, wherein the machine direction yarns are non-circular in cross section. 13. A papermakers fabric as claimed in claim 12, wherein the machine direction yarns are flat monofilaments having an aspect ratio in the range from 2:1 to 4:1. 14. A papermakers fabric as claimed in claim 12, wherein the machine direction yarns are "D" shaped, the curved surfaces thereof facing into the body of the fabric. 15. A papermakers fabric as claimed in any preceding claim, wherein the fabric contains different machine direction yarns having different cross-sectional sizes and/or shapes. 16. A papermakers fabric as claimed in any preceding claim, wherein the fabric is a dryer fabric.

Inventors: HODSON MARK, HODSON, MARK
Applicants: HODSON MARK, SCAPA GROUP PLC, HODSON, MARK
Application year: 1995
Main classification: D21F1
IPC codes: D21F1, D21F7