Fabrics for Paper and Board Production

Almost throughout the whole paper machine the paper web is in contact with fabrics on one or both sides. Hereby quality issues such as surface characteristics of the paper or board are influenced as well as economic parameters, e. g. by ensuring continuous production by secure web guiding throughout the paper machine:
. • In the forming section the forming wire is the auxiliary means of filtration when the suspension is dewatered, resulting in a wet web largely showing the surface characteristics of the wire.
. • In the press section this formed web is mechanically dewatered under pressure which can be more than 100 bar. Here low flow resistance of the felt and mini¬mized rewetting are economic factors. Again the surface of the press felt and its local elastic behavior will show up on the web surface.
. • In the dryer section the dryer fabric has to ensure the undisturbed web transfer through the section, to enhance contact heat transfer from the cylinder surface to the web and to reduce web shrinkage in CD.

6.3.1
Forming Fabrics
Arved Westerkamp
6.3.1.1 Requirements
Forming fabrics are used in the forming section of the paper machine to dewater the fiber suspension and hence build up a continuous paper web. As web forming is the most critical phase in paper manufacturing, specialised fabrics have to be used to achieve the required paper sheet properties such as smoothness, filler distribution, or printability. The fabrics used for forming are woven on looms, where the machine direction yarns (warps) and cross direction yarns (weft) are interlaced with each other. Dependent on the paper machine, forming fabrics have a length between approximately 25 and 105 m, typically in a gap former the length of the wires would be between 25 and 30 m. As paper machine width has been increasing significantly in the past 10 years, the widest fabrics nowadays would be up to 11 m. The lifetime is between 30 and 120 days, most typically between 30 and 50 days.

The main requirements forming fabrics have to meet are:
. • uniform dewatering of suspension exiting the headbox and uniform build up of the paper web
. • gentle and uniform web support during intensified web dewatering at foils and vacuum suction boxes
. • safe web transport to the couch position
. • easy web release
. • sufficient hold up, controlled transport and release of the filtrate, the so-called white water.

This means that the forming fabrics have in part to meet contradictory require¬ments, as for instance:
. • the wire surface should be very smooth for uniform paper web support whilst having a large open area for uniform dewatering with low flow resistance
. • the free volume in the wire should be large, resulting from thick weft diameters, but this leads to severe water carrying.

All these properties should be constant over the width and length of the individual fabric and over the whole lifetime, which should be as long as possible.

6.3.1.2 Forming Fabric Design and History
Historically phosphor-bronze and stainless steel were used to weave forming fab¬rics. Increasing paper machine speeds as well as demand for improved paper properties led to a change in the raw materials employed which started in the early 1960s with the use of synthetic materials, primarily polyester and polyamide.
By using plastics, product lifetimes increased significantly, while at the same time problems arose such as stretching of the fabric in the machine direction (MD) and narrowing in the cross machine direction (CD). So it became necessary to introduce new manufacturing processes such as heatsetting of the woven struc¬ture. Due to the change to plastic materials, welding technology could no longer be used so an entirely new seaming technology also had to be developed .
Since this time, the fundamental manufacturing process chain has remained the same.
It consists of:
. • warping (winding up MD yarns on sectional warp beams)
. • weaving (interlacing MD and CD yarns)
. • heatsetting (locking the knuckles in the weave)
. • seaming (forming an endless loop)
. • finishing/packing (sanding the surface, width determination, marking).

Increasing paper quality demands (such as improved printability) as well as eco¬nomical reasons (such as lifetime) and operational reasons (such as water carry¬ing) led to a variety of new forming fabric designs, mainly during the 1970s and 1980s. Starting from the original single layer forming fabrics (forming fabrics with only one warp and one weft system in only one layer), the growing demand for, in particular, increased lifetime led to more sophisticated constructions having a highly abrasion-resistant wear side and a fine paper side (see Fig. 6.15).

As the quality requirements of papermakers were increasing, an improved weave was developed having an additional weft on the paper side, giving more support to the paper fiber (see Fig. 6.16).
Extended width and increased speed of paper machines called additionally for higher cross-direction fabric stability. This was provided by adding another weft system in the center of the fabric (see Fig. 6.17).

6.3 Fabrics for Paper and Board Production
 
Fig. 6.16 8-Shaft forming fabric with 2:1 paper side/wear side weft ratio and 1 warp system (source: Voith).
 Fig. 6.17 14-Shaft forming fabric with 2:1:1 paper side/center/wear side weft ratio and 1 warp system (source: Voith).
As printing technology and quality demands were advancing rapidly this con¬struction was no longer sufficient to fulfill the printability requirements of the paper industry’s customers. Consequently even finer surfaces were developed (see Table 6.2), by using two or more warp systems with a fine top warp and a thicker bottom warp yarn to facilitate sufficient crimping of the wear side weft yarns during heatsetting. This development gave improved printability with similar or even longer fabric life.
Table 6.2 Sales development of forming fabric types by product type – global (source PCA report March 2004).
1990 (%)  1995 (%)  2000 (%)  2002 (%)  2003 (%)
Triple weft, SSB  –  2  7  21  30
(sheet support binders)     
Triple layer  5  12  16  12  10
Double layer  69  77  70  61  55
Single layer  26  10  8  6  5

 Fig. 6.18 20-Shaft forming fabric with
1:1 paper side/wear side weft ratio and2 warp systems (source: Voith).
Nowadays modern forming fabrics for use on twinwire formers need to have a low void volume to reduce water carrying, extremely fine and planar surfaces for reduction of surface marking and a homogenous structure to give uniform dewa¬tering across the whole width of the fabric. These designs use weft diameters on the paper side which are between 0.12 and 0.15 mm, on wear side diameters be¬tween 0.20 and 0.30 mm. The weft count per cm exceeds 70 as does the warp count (see Fig. 6.18).

6.3.1.3 Manufacturing Technology
Forming fabrics are manufactured on weaving looms with a width of up to 15 m, where CD yarns (wefts) and MD yarns (warps) are interlaced with each other. The weft insertion is typically made by using shuttles, projectiles or band rapier sys¬tems.
The weft insertion speed on the looms is up to 1200 m min–1. The working principle of a loom is shown in Fig. 6.19.
After the fabrics are woven they are heatset. Here their final properties are deter¬mined by applying heat to the fabric while simultaneously stretching it in the MD
6.3 Fabrics for Paper and Board Production
 
 
and allowing a controlled CD shrinkage (see Fig. 6.20). The temperatures depend on the materials used but are usually between 180 and 210 °C. During heatsetting the temperature is increased following a time sequence. The stretch achieved in the machine direction is normally between 8 and 12 %, narrowing is up to 9 %. As the fabric is only partially heated in the heating zone, total dwell time depends on the speed (2–4 m min–1) and the number of revolutions.
After being cut for seaming, the fabric is made endless by using a single thread selection weaving technology called the Jacquard system, where the weave is re¬built in the seam area by turning it through 90°.
In the final finishing, moderate temperature is applied to the fabric in order to stretch out creases. After any protruding yarn ends in the seam area are cut off, the fabric will usually be sealed at the edges (and sometimes ground to enhance smoothness) before it is packed for dispatch as defined by the customers (usually wound on poles in wooden boxes).

Usually forming fabrics are quality checked during manufacture, following the manufacturing process. As scrapping a completed fabric would be far too ex¬pensive, qualifying criteria are defined for each manufacturing step. Typical tests would be hysteresis/stress strain tests, profile testing (i. e. mass distribution, cfm, calliper).
During development, other criteria such as bending stiffness, sheer, warp burial, abrasion resistance, etc. are tested.
Comparability of data from all competitors is achieved by using similar test/ calculation methods as defined in the standard test procedures by PCA (Paper Machine Clothing Association).

6.3.2
Press Felts
6.3.2.1 Requirements
Press felts are used in the press section of the paper machine and are tailor-made for every particular position in the different press nips. They are in direct contact with the paper surface and strongly influence both the quality of the paper and the economy of the papermaking process. Press felts are porous laminates, composed of a base weave layer with nonwoven layers on either side, which are assembled by a needling process. The caliper of a press felt is in the range of 3 to 4 mm, the length between 15 and 75 m, the weight per area between 800 and 1500 g m–2, and the air permeability between 15 and 450 l dm–2 min–1 (5 and 150 cfm). The total production of press felts in Western Europe is about 4000 to 5000 tons per year (2003). The average value of press felts is in the range of 55 to 85 € kg–1 (2003), depending on the design. The lifetime of a press felt e. g. in a graphical paper machine is between 3 and 4 weeks.
The functions of press felts are
. • to pick up the formed paper web from the wire in the forming section and guide it through the press section of the machine
 • to support the wet paper web in the press nip and to store the water squeezed out of the web.
 The resulting requirements press felts have to meet are as follows:
. • smooth paper side surface for good printability of the produced paper and low rewetting (rewetting: water flows back from the press felt into the paper web after the press nip)
. • low abrasion at roll side surface
. • high storage volume to store the water removed from the paper web
. • “constant” dewatering behavior over lifetime including quick start-up behavior (full operating capability within a short time after installation)
. • very uniform distribution of nonwoven layers (base weave) in MD and CD for uniform dewatering conditions and in MD to prevent vibrations
. • good dimension stability (no width change, no permanent elongation) over life¬time (12 m wide press felts running with a speed of close to 2000 m min–1 and with a press load of 120 bar).

The actual requirements put on each individual felt depend on the particular posi¬tion of its application in the press section and thus focus and felt design may vary.
6.3 Fabrics for Paper and Board Production
All press nips in modern paper machines are single or double felted. Shoe presses are mainly double felted. The pick-up felt is in the first press felt position; it has to transfer the wet paper web from the forming wire to the press section. Whilst the loads of the press nips increase from press position to press position of the press section, the diameter of the felt fibers on the paper side surface decreases. Chang¬ing from a coarser press felt surface to a finer one enables the transfer of the paper web, due to the increased adhesion by capillary forces and larger contact area.
During its lifetime the press felt runs several million times through the press nip. Reduction in felt thickness, abrasion and contamination are the main reasons why a press felt has to be replaced. Fillers such as calcium carbonate, clay/kaolin, other papermaking additives, and adhesives e. g. from recovered paper deposit in the press felt structure, impeding or even locally preventing water flow. In some cases not even alkaline and acids can re-open the press felt structure.

6.3.2.2 Press Felt Design and History
At the beginning of industrial paper making, simple cloths made out of wool were used. In the early sixties these felts were replaced by improved designs based on high-tech textiles. This replacement was necessitated due to the increasing de¬mands (life time more than six days, use of abrasive fillers, higher machine speed).
Figure 6.21 illustrates the principle of a typical press felt structure. The base weave is found in the middle layer of the structure. The base weave is circularly woven; this means that the weft yarn during weaving is the yarn in the length direction of the finished press felt. The warp yarn is the yarn in the cross direction. The weaving loom produces a seamless hose, which can be up to 12 m in width and 70 m in length. Weaving looms can be up to 33 m in width. These endless felts are also called seamless felts and can be used in all applications.
Seamed felts, in contrast, are used mainly in the production of board and pack¬aging and are not endless. They are closed in the paper machine with a seam, produced in a special variation of the weaving process. They are very common in North America (about 60 % of all press felts in NA are seamed felts) and are easier
 
B: first base weave, C: second baseweave, D: paper side fiber layer (source: Voith).
and safer to install in the paper machine. A disadvantage is the marking of the seam, a small stripe in the cross machine direction which may negatively affect the printability of the produced packaging paper or board.
The nonwoven layers on the top and bottom of the base weave consist of staple fibers with different yarn count (3.3 to 100 dtex which means a diameter of 20 to 100 mm). The nonwoven layer on the bottom is in contact with the roll covers and protects the press felt against abrasion. The nonwoven layer on top is in contact with the paper surface and ensures a low and uniform water flow resistance dur¬ing dewatering of the paper web. A coarser nonwoven layer is used in the first press nips with a higher amount of water removed from the paper web, finer nonwoven layers have use in the further press nips with a lower amount of water extraction.
Polyamide 6 and polyamide 66 are the raw materials used for both the base weave and nonwoven layers. In some cases polyamide 6.10 is used to increase the cross machine stability during the felt lifetime, due to its reduced water absorp¬tion.
6.3.2.3 Manufacturing
The fiber batt for the nonwoven layers on both sides of the base weave is produced from compressed staple fibers by carding lines. The fiber batt is stabilized in a pre-needling machine which makes it manageable for the further production steps. On the finishing needling machine (Fig. 6.22), the top and the bottom fiber batt are connected to the stretched base weave. After 5 to 15 revolutions in the needling stage, the press felt is washed to remove the spin finishing, and decontaminated. The following hot air drying causes both base weave and fibers to shrink, which results in a dense and stable press felt.
 
6.3 Fabrics for Paper and Board Production
 
Some press felts have additional special layers, like membranes or punched plastic films as one of the middle layers. These have been introduced to control the water flow and to prevent rewetting or to act as an elastic damper against vibra¬tions (Fig. 6.23).
Quality control during production includes caliper, weight per area and air per¬meability. As the allowed tolerances are small high production accuracy and uni¬formity are required.
6.3.2.4 Transfer Belts
Transfer belts are a specialty used in the second bottom position of a double shoe press replacing a press felt. These transfer belts guide the paper web from the press section to the dryer section. The requirements transfer belts have to meet are as follows:
. • smooth paper side surface for good printability of the produced paper
. • impermeable to water to prevent rewetting
. • easy pick-up and release behavior of the paper web
. • long lifetime (90 to 180 days)

The manufacture of a transfer belt is based on an endless press felt which is covered with a polyurethane or rubber layer on the paper or both sides. Figure 6.24 shows the SEM image of the cross section of a transfer belt with a smooth paper side.
 
6.3.3
Dryer Fabrics
6.3.3.1 Requirements
Dryer fabrics are utilised as the medium to transport the paper web through the dryer section of a papermaking machine and to press the web to the hot cylinder surface to increase the drying rate. The resulting requirements are :
. • Paper contacting surface to have a high contact density for optimisation of heat transfer from cylinder to paper web.
. • Smooth paper contacting surface and in-line seam for non-marking of the pa¬per.
. • Sufficient vapor permeability to allow evaporative drying to occur freely. The permeability profile must be uniform across the full fabric width to ensure that it does not influence the moisture profile of the paper web. Permeability must not be too high or this will adversely affect web runnability. For example, for high speed single tier type dryer sections web transport and control is critical. For heavier weight packaging grades running at slower speeds heat transfer may be of primary importance.
. • Aerodynamic surfaces and low void volume for reduced air carrying to maintain good web runnability and tail feeding.
. • Assist in safe transfer of the web between one dryer section and another and within a conventional dryer section.
. • Uncomplicated structure to allow effective cleaning.
. • High performance and quality running lifetime in hot, humid conditions.
. • Dimensional stability and high performance and quality during running life¬time (typically 12–18 months) in hot, humid conditions.
. • Safe, fast installation. All dryer fabrics are joined on the paper machine.

6.3 Fabrics for Paper and Board Production
 
 
6.3.3.2 Fabric Design and History
Before the invention of “man-made” fibers, dryer fabrics were made from natural materials such as cotton.
Advances in chemistry saw the invention of polyesters and polyamides. These materials could be spun and formed into monofilaments. Polyester quickly be¬came the preferred choice for dryer fabrics.
State of the art dryer fabrics are made from either polyester based polymer derivatives or polyphenylene sulfide (PPS). PPS material is inert to the environ¬mental conditions encountered in a paper machine. This material would be used for the construction of fabrics running on the hotter, more humid machines, e. g. packaging machines.
Dryer fabrics comprise machine direction (warp) yarns and cross machine direc¬tion (weft). These yarns may comprise monofilament, multifilament, spun, plied (twisted) etc. Figure 6.25 shows a dryer fabric suitable for high speed, state of the art paper machines.
Dryer fabrics may also be produced by linking a series of alternating left- and right-handed spiral monofilaments. The spirals are linked together by a connect¬ing monofilament. Figure 6.26 shows a spiral fabric design.
6.3.3.3 Dryer Fabric Manufacture
Dryer fabrics are woven as a continuous, flat fabric. The machine direction warp yarns are held on canister spools at the back of the loom and passed through a series of “reeds” and “sheds” (spacers and pattern makers). During the weaving process sheds are raised and lowered according to the desired final weave pattern.
At each shed motion a weft system is fired through the so formed “warp tunnel”. The reed then pushes the newly formed section tightly into line with the rest of the fabric body.
The fabric is removed from the loom and relaxed and stabilised through the use of heat and tension. A typical heat setting temperature is 180–200 °C for dryer fabrics. The tension is set according to the running tension on the paper machine section to which the fabric will be applied.
A seam is then created either from the machine direction yarns looped around themselves, or by insertion of a spiral yarn around looped warp yarns.
In the case of a spiral link fabric there is no weaving to be done. The left- and right-handed spirals are formed around a hot “mandril”. These are then meshed together and joining wires used to link adjacent spirals. Spiral fabrics may or may not be “stuffed” with filler material in order to control their permeability.
Dryer fabrics typically range between 1.2 and 2.5 mm in thickness, depending on the fabric design required for the application. Their weight is typically 800–1500 g m–2.
Dryer fabrics are typically supplied through the application range 60 cfm to around 100 cfm.