The present invention relates to a papermaking dryer fabric for use in a dryer section of a papermaking machine.

Modern dryer fabrics especially for use in high speed papermaking machines with production speeds of 1200 meter per minute or more need to have relatively low air permeability, low inner void volume and low caliper for low air carriage. The weaving structure of such dryer fabrics has to have as less as possible intersections for low affinity of contamination. Further on such dryer fabrics must guaranty sufficient bending stiffness in the cross-machine direction (CMD) of the fabric.

In the state of the art several attempts for dryer fabrics are known. EP1736595 discloses e.g. a dryer fabric with weft yarns grouped in pairs and with ungrouped warp yarns as well as with weft and warp yarns grouped in pairs. Such fabrics have improved performance due to less water and air carriage, but have the disadvantage that the weaving process of such fabrics is difficult to control because of the parallel insertion of weft yarns which sometimes tend to overly each other instead of weaving parallel to each other.

US5, 799,708 discloses a papermaking fabric with flat shaped warp yarns which are grouped in pairs and with round shaped weft yarns which are ungrouped. Such fabrics have the disadvantage that the forming of seaming loops is difficult, because of the rectangular shape of the warp yarns which tend to twist during the formation of the seam loops. During the joining process of such fabrics in the papermaking machine the twisted seaming loops are difficult to interdigitate and to put a pintle through them.

What is needed is a new dryer fabric design with low water and air carriage, which is easy and controllable to manufacture and easy to join during installation in the papermaking machine.

The problem is solved with a papermaking dryer fabric comprising a system of MD yarns interwoven with a system of CMD yarns in a single layer weave, said fabric comprising groups of MD yarns, wherein each group being formed by a plurality of adjacent MD yarns having a circular cross section and weaving in said group, like a single yarn, side-by-side the same weave path with said CMD yarns. The dryer fabric according to the invention further has only ungrouped CMD yarns which have a non-circular cross section with a width to height ratio of more than 1 . The dryer fabric according to the invention further has a air permeability between 25 and 200 cfm.

The CMD yarns are ungrouped or in other words, adjacent CMD yarns weave different weave paths with the MD yarns. Therefore the weaving process is controllable in a reliable manner. Due to use of flat CMD yarns or in other words, CMD yarns with a width to height ratio of more than one, a low calliper fabric can be achieved combined with a reduced number of CMD/MD yarn intersections to provide a fabric with a reduced contamination affinity. Further on the flat CMD yarns give the ability to control or reduce the air permeability of the fabric in a certain range based on the specific choice of the width of the CMD yarns. For a controllable weaving process the maximum width of the CMD yarns is limited 3-4mm. To further control or reduce the air permeability of the fabric, the invention proposes grouped MD yarns which have a circular cross section, to guaranty untwisted seaming loops and to make joining more easy.

Preferred embodiments of the present invention are further described in the sub-claims.

According to a preferred embodiment of the invention the single layer weave is a plain weave. A plain weave provides a homogenous weaving structure with low making issues to the paper web that comes into contact with the paper-side of the dryer fabric. A plain weave pattern further gives a highly stable fabric with a high rigidity.

To further influence the air permeability of the dryer fabric preferably all CMD yarns of the fabric have a non-circular cross section with a width to height ratio of more than 1 .

Preferably all MD yarns form said groups of MD yarns. This further increases the ability to reduce the air permeability to the lower edge of the claimed range and can e.g . lead to air permeability values of 150cfm or lower.

To achieve that the grouped MD yarns lie in parallel next to each other without overlapping each other, preferably each of the groups of MD yarns is formed by four or less adjacent MD yarns, preferably by two or three MD yarns weaving side-by-side the same weave path with said CMD yarns. The non-circular CMD yarns for example can have a bone-like cross section (remark: in some prior art documents also called bi-nodal cross section). It is further possible that the non-circular CMD yarns have a rectangular cross section. Further on it is possible that the non-circular CMD yarns have a double convex shaped cross section, wherein double convex shaped means that the cross section consists of two convex halfs which cross each other at an angle of more than 0° and less than 180°. The width to height ratio can be 1 ,5 or more. Preferably the width to height ratio is not more than 2,0 to 4,0.

One of the advantages of ungrouped CMD yarns with non-circular cross section compared to grouped CMD yarns with circular cross section is, that the air permeability can be further reduced without increasing the thickness of the yarns and therefore the thickness of the fabric. Another advantage is that one flat CMD yarn has a smooth surface whereas grouped CMD yarns always generate some gap between them which can serve as traps for contamination.

Especially for high speed applications it is advantageous if the dryer fabric has a high CMD stiffness. To achieve a high CMD stiffness the inventors found out, that the height of the CMD yarns with non-circular cross section preferably should not be below 0,5mm. To make a sufficiently thin dryer fabric with reduced water and air carriage it is further advantageous is the fabric has a thickness of not more than 1 ,1 mm. According to a preferred embodiment of the invention it is foreseen, that the thickness of the CMD yarns preferably is between 0,5mm and 1 ,1 mm, more preferably between 0,6mm and 1 ,0mm, most preferably between 0,7mm and 0,9mm.

To achieve a good balance between low fabric calliper and easy bending of the MD yarns to form the seam loops on the one hand and sufficient stability of the MD yarns to withstand the drive load the fabric is subjected to, the inventors found out that for the dryer fabric design of the present invention it is advantageous, if the MD yarns have a diameter between 0,5mm and 0,8mm, preferably between 0,55mm and 0,65mm.

To further reduce water and air carriage it is advantageous if the air permeability of the fabric is between 50 cfm and 200 cfm, preferably between 50 cfm and 150 cfm.

Table 1 shows a comparison of the properties between a dryer fabric known from EP1736595 (conventional fabric) and a preferred embodiment of a dryer fabric according to the present invention, were all design features except the one expressively indicated as being different are identical .

Table 1 :

Weave design: plain weave

Paired MD yarns: PET / diameter 0,6mm / 160 to 190 MD yarns per 100mm

CMD yarns: i) conventional fabric PET / diameter 0,8mm / paired

ii) invention fabric PET / height 0,6mm / width 1 ,4mm / ungrouped

The above disclosed embodiment of the fabric design according to the invention gives the possibility to reduce the air permeability by at least 40%.

The papermaking dryer fabric according to the present invention can have at least some

CMD yarns which comprise a two-phase polymeric material, with a first polymeric material that melts at a first temperature and a second polymeric material which is substantially immiscible with the first polymeric material and which melts at a second temperature which is lower than the first temperature, wherein the fabric has been subjected to a heat setting treatment at a temperature at or above the second temperature and below the first temperature. Dryer fabrics according to this embodiment can have a smoother paper contacting surface due to the fact that during heat setting crimp, which is in the MD or warp yarns after the weaving process can be more easily transferred to the CMD or weft yarns. This is especially the case, when the MD yarns are made from a material with a melting temperature above the heat setting temperature.

The second material can include alone or in combination: polyollefins, polyamides or fluoropolymers. The first material can include alone or in combination: homepolymers or copolymers of polyesters e.g. PET, polyamides or PPS.

According to a second aspect of the present invention a method for producing packaging, board or cardboard is provided with the following feature:

- Providing a fibrous slurry to at least one forming fabric in a forming section and dewater the slurry through the at least one fabric to form a fibrous web

- transferring the fibrous web to a at least one pressfelt in a pressing section and dewater the web through at least one press felt

- transferring the web to a dryer fabric according to one of the claims 1 -14 in a dryer section and drying the web when the web is sandwiched between the dryer fabric and a heated dryer cylinder of the dryer section.

Due to the fact that the dryer fabric according to one of the claims 1 -14 has a low internal void volume combined with a smooth surface with reduced number of yarn intersections the dryer fabric is very easy to clean and has an excellent stay clean performance and therefore preferably is usable in the production of packaging, board or cardboard due to the fact that the fibrous slurry for the production of packaging, board or cardboard web often has a high recycled content.

Preferably the dryer fabric according to one of the claims 1 -14 is used in at least one of the first, second or third dryer group of the dryer section.

The invention will be further illustrated with the following figures, wherein

Fig. 1 shows a top view onto a part of the dryer fabric according to the invention,

Fig. 2 shows a cross sectional view in CMD direction of the fabric of Fig. 1 ,

Fig. 3 shows a cross sectional view in MD direction of the fabric of Fig. 1 with CMD yarns having a rectangular cross section, Fig. 4 shows a cross sectional view in MD direction of an alternative fabric with CMD yarns having a bone-like cross section,

Fig. 5 shows a cross sectional view in MD direction of an alternative fabric with CMD

yarns having a double convex shaped cross section,

Figure 6 shows top view onto a part of the dryer fabric as seen in figure 1 with a first

variant of seam loop construction and

Figure 7 shows top view onto a part of the dryer fabric as seen in figure 1 with a second variant of seam loop construction.

Fig. 1 shows a top view onto a part of the dryer fabric 1 according to the invention. The fabric 1 has a system of MD yarns 2 interwoven with a system of CMD yarns 3 in a single layer weave. In the embodiment shown in the figures 1 -3 the system of MD yarns only consists of groups 4 of two adjacent MD yarns 3 which weave in each group 4 side-by-side the same weave path with the CMD yarns 3. The dryer fabric 1 is flat woven in a plain weave design.

As can be seen from figure 1 and 2 the MD yarns 2 have a circular cross section. As can be seen from figures 1 and 3 all the CMD yarns 3 are ungrouped - that means adjacent CMD yarns weave different weave paths with the MD yarns 2 - and have a rectangular cross section with a width to height ratio of 2. The CMD yarns 3 in the embodiment as shown in figures 1 -3 have a height of 0,6mm, wherein the MD yarns 2 have a diameter of 0,6mm. The dryer fabric shown can have a air permeability between 75 cfm and 140 cfm.

Figure 4 shows a cross sectional view in MD direction of an alternative fabric with CMD yarns 3a having a bone-like cross section. The rest of the fabric of figure 4 is identical to fabric shown in figure 1 and 2.

Figure 5 shows a cross sectional view in MD direction of an alternative fabric with CMD yarns 3b having a double convex shaped cross section. The rest of the fabric of figure 5 is identical to fabric shown in figures 1 and 2.

Figure 6 shows top view onto a part of the dryer fabric as seen in figure 1 with a first variant of seam loop construction. As can be seen part of the MD yarns 2 form seaming loops 5 which are arranged at the widthwise edges of the fabric 1 to make it endless. The other part of the MD yarns 2 form so called binder loops 6 to hold the widthwise fabric edges. As can be seen between adjacent seaming loops 5 a binder loop 6 is located and vice versa. In the embodiment as seen in figure 6 every seaming loop 5 and every binder loop 6 is formed by MD yarns 2 of different groups 4. By way of example one of the seaming loops 5 is formed by a MD yarn 2 of group 4 ^' and by a MD yarn 2 of group 4 ^" and adjacent binder loop 6 is formed by a MD yarn 2 of group 4 ^" and by a MD yarn 2 of group 4 ^"' .

Figure 7 shows top view onto a part of the dryer fabric as seen in figure 1 with a second variant of seam loop construction. As can be seen part of the MD yarns 2 form seaming loops 5 which are arranged at the widthwise edges of the fabric 1 to make it endless. The other part of the MD yarns 2 form so called binder loops 6 to hold the widthwise fabric edges. As can be seen between adjacent seaming loops 5 a binder loop 6 is located and vice versa. In the embodiment as seen in figure 7 every seaming loop 5 and every binder loop 6 is formed by MD yarns 2 of the same group 4. By way of example one of the seaming loops 5 is formed by the two MD yarns 2 of group 4 ^' and adjacent binder loop 6 is formed by the two MD yarns 2 of another group 4 ^" . To hold the widthwise fabric edges this seam construction further has an additional end yarn 7 which passes through the binder loops 6 and over or under the seaming loops 5 without passing through them.