The basics: what you need to know about dryer fabrics.

Editor's Note: This article is the second of a three-part series of "back to basics" articles on machine clothing.

The primary function of dryer fabrics is to ensure even and efficient drying of the paper web by promoting efficient heat and mass transfer. Air handling is also an important task on high speed machines to maintain web stability throughout the dryer section. Aerodynamic features of fabric structures, fabric openness, geometry of dryer pockets, and machine speed determine air pumping and dragging effects.

Dryer fabrics influence the drying process by their openness and contact with the paper web. Fabric permeability has a large impact on ventilation of dryer pockets and evaporation rates. Contact pressure caused by fabric tension and surface structure influence heat transfer from dryer cylinders to the sheet. The combination of the heat transfer, mass transfer and stable transport of the sheet drive affect drying efficiency on a paper machine. Dryer fabrics are critical for all of these functions.

Fabric selection for each dryer position must balance fabric properties to provide maximum life at optimum production rates while maintaining sheet runnability. All this must be done without adversely affecting paper quality. Maintaining paper quality during drying requires undisturbed web-to-fabric contact and a nonmarking fabric surface and seam. Optimum fabric performance requires maintaining all characteristics throughout life.

Chemical hydrolysis, wear, damage and contaminant accumulation are the most common reasons for removing fabrics. Hydrolysis is decomposition of fibers when exposed to moisture at elevated temperatures and is a concern on many fabric positions. Hydrolysis is directly related to temperature and humidity level. Figure 1 shows an example of fabric hydrolysis. Increased use of chemical furnish additives and recycled furnish along with higher machine speeds are causing increased contamination accumulation and premature fabric removal. Fabric life is typically 6 to 18 months. Choice of fabric raw material has a strong influence on performance. Fabric cleanliness can be improved through alterations of surface chemical properties and fabric construction.

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Fabric wear is usually caused by abrasive filler particles in the furnish or poor condition of dryer and roll surfaces. Raw material properties and fabric surface structure influence abrasion resistance. The seam on a woven fabric is the weakest point and seam uniformity is critical on abrasion prone positions. Spiral fabrics are manufactured differently than woven fabrics and are made without a traditional seam, so there is no weak link. Spiral fabrics provide the best resistance to abrasion.

Dryer fabrics act as driving elements by supporting the web and driving dryer cylinders and rolls in modern silent drive configurations. Fabric durability, dimensional stability, and structural stability influence this function. Other fabric properties include providing trouble-free runnability, good guiding properties and ability to clean.

Good tensile strength and wear resistance are needed to withstand normal and high stresses, including wad resistance during web breaks. Dryer fabric materials must have excellent hydrolysis resistance in moist conditions and degradation resistance in dry heat to reach optimum economic life. Increasing use of chemicals in paper furnishes and for cleaning requires good chemical resistance. Fabric materials must also have limited water absorption to obtain good dimensional stability and optimal drying.

Different yarn structures and shapes are used to control air permeability and influence properties such as fabric-to-paper contact, aerodynamics, wear resistance, stability, and cleanliness. Some dryer fabric yarn structures are shown in Figure 2. Significant improvements in these fabric properties can be made by varying type and shape of MD and CD yarns. Weft strand density controls the air permeability of fabrics. The guideline for permeability is to run as high as possible without disturbing the sheet. Yarn density, caliper of yarns, and weave pattern provide stability and strength.

Monofilament is the most common yarn type. Most modern dryer fabric designs use single monofilaments for the complete permeability range. Structures with 100% monofilaments provide good stability, high strength, low boundary air flows, cleanliness, and less water absorption. Round and rectangular shape monofilament yarns are used for dryer fabrics. Round yarns are typically 0.40-0.80 mm. Rectangular yarns are 0.25 X 0.50 mm to 0.50 X 1.00 mm. Rectangular shaped yarns are used as machine and cross directional yarns to improve runnability, sheet handling, and wear potential on high speed machines.

Other types of yarns still used to control permeability that are less common in modern fabric designs are plied monofilaments, multifilaments, bi-component yarns, and spun yarns. Plied monofilaments are twisted, small-diameter monofilaments bundled and twisted together. A multifilament uses many small diameter continuous filaments. Multifilament yarn bundles may have 120-300 filaments. Spun yarns are made by twisting numerous small, short fibers to form a yarn. They are available in all synthetic fiber types. Special yarn surface properties can be achieved by extruding a special material such as a web on a standard core material. Different fabric colors are used to create contrast between the web and fabric surface. This contrast is necessary for optical noncontact sensors such as break detectors to function effectively.

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Heat stabilized polyester (PET) is the predominant material currently used in dryer fabrics. PET is used in applications where steam pressures are less than 70 psig because of hydrolytic degradation. Hydrolysis is decomposition of polyester fibers when exposed to moisture at elevated temperatures. The yarns become very brittle and will easily break when hydrolysis occurs. Yarns can almost disintegrate in extreme cases. Moisture attacks the carboxyl end group of the polymeric chain, causing chain cleavage and decomposition. Hydrolysis is related to temperature and humidity levels.

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Most dryer fabrics use PET because it has excellent strength properties and is available in monofilament, multifilament, and spun form. Introducing capping agents to the PET monofilament extrusion process typically improves hydrolysis resistance. Capping agents are inert materials that attach to the carboxyl group and prevent its reaction with water. PET will still hydrolyze in extreme conditions and is not typically used in positions that operate above 70 psig steam pressures.

Common dryer fabric materials used on positions operating above 70 psig steam pressure are polycyclohexanedimethanol terephtalate (PCTA) and polyphenylene sulfide (PPS). PCTA has considerably improved hydrolysis resistance properties. Monofilament form PPS is used for applications in hot and wet environments. It resists elevated temperatures and will not hydrolyze.

Polyamide, poly-m-phenyleneisophthalamide (Nomex), fiberglass, acrylic, poly-p-phenyleneterephthalamide (Kevlar), and polyetheretherkethone (PEEK) are materials used in very limited volumes for dryer fabrics. Use of nylon and acrylic has limitations because of poor stability in hot and wet conditions. High cost limits use of the other materials. PEEK is used primarily in the spirals and cables of seams. Table 1 shows a comparison of dryer fabric properties.

DRYER FABRIC STRUCTURES

Dryer fabric design has undergone major evolution since 1960. Heavy canvas fabrics made primarily from cotton and the needled fabrics of staple wool and polyester are now just history. Modern monofilament structures have almost completely replaced multifilament fabrics.

Basic weave structure for modern fabrics is 1, 1.5, or 2 layer fabrics. Side views of some fabric designs are shown in Figure 3. Different permeability ranges are achieved by altering yarn density in the cross-machine direction. Machine direction yarns in modern dryer fabric designs are predominantly flat monofilaments. Flat yarns provide good contact with the paper web and improve aerodynamic properties of the fabric surface and cleanliness. Monofilament fabrics typically cover a permeability range of 60-500 cfm.

Modern dryer fabrics for high speed machines are very sophisticated. Graphic representations of some current fabric designs are shown in Figure 4. Weave constructions are tested and optimized to reduce air drag and maximize sheet stability. For the most modern machines, lower fabric caliper and optimal neutral line help reduce draws and sheet stretching.

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Spiral fabric constructions are a departure from woven dryer fabrics. Spiral fabrics are an endless construction formed by joining helical formed loops of monofilament yarn. The joining yarn passes through the "eye" formed when the right-hand helical formed yarns merge into each other. The spirals are oriented in a cross-machine direction. Figure 5 shows how a spiral fabric is made. Spiral fabrics are available in a broad permeability range. Permeability reductions are achieved by inserting cross-directional yarns into the open area of the spiral. There is no weak link since spiral fabrics do not have traditional seams like woven fabrics. In addition, spiral constructions offer higher levels of wear and damage resistance and are very suitable for challenging positions. In the past, spiral designs were used extensively on brown paper applications. However, through processing and material improvements, spiral fabrics today have no application limitations. They run on some of the fastest publication grade machines.

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DRYER FABRIC PROCESSING

Dryer fabrics are open, air-permeable structures usually made from synthetic monofilaments and fibers. Following the weaving process (for woven designs) or assembly process (for spiral designs), fabrics are heat set with tension at temperatures of 320-400 degrees F depending on the raw materials used. The fabric reaches final permeability during heat setting. Heat setting eliminates stresses in the fabric and gives the fabric structure the stability needed for efficient guiding and consistent runnability. Seam manufacturing follows heat setting.

Seams are the most important part of dryer fabrics and face many demands. Seams must meet requirements set by paper grade, machine speed, and the dryer section. Runnability, nonmarking, and easy installation are the main criteria. Similarity of properties of the seam area and the fabric body are critical on marking sensitive positions.

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The majority of fabrics are supplied with pin seams or in-line spiral seams. Pin seams are formed by weaving each machine direction yarn back into the fabric and allowing that yarn to form a loop. The loop forms the "eye" for the seam. The in-line spiral seam is made like the pin seam except a small spiral coil is woven into the end of the fabric. The difference between the in-line seam and the pin seam is that the spiral coil can control the loop form and distribution. All machine direction yarns can be used to bind the spiral coil, which maximizes seam strength. The spiral material is usually PET or PEEK, depending on temperature requirements on the machine. Historically, pin seams were the best option for mark-sensitive positions. However, modern in-line spiral technology is available that can run mark-free on even the most mark-sensitive applications. As a result, inline spiral seams are seen in all positions, while there are few pin seams remaining.

Seam fineness and nonmarking properties are very important on fine paper machines producing special paper grades and on machines with single-tier dryers. Seam marking is usually the result of an excessively rough surface (mechanical marking), high variation in openness (evaporation marking), or uneven support (waves due to partially delayed shrinkage). Figure 6 shows an inline spiral seam on the left that caused sheet marking. The seam on the right in Figure 6 eliminated sheet marking.

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Dryer fabric application is important for optimal paper machine runnability and dryer section efficiency. Dryer fabrics affect all the processes that make this happen--heat transfer, mass transfer, and sheet transfer. There are a variety of materials, constructions and seams. Papermakers should work with suppliers to optimize applications.

Table 1: Comparison of dryer fabric properties.

              Heat     Hydrolytic     Acid       Alkali
           Resistance  Stability   Resistance  Resistance

Polyester      4           2           4           2
Polyamide      3           4           2           4
Acrylic        4           4           4           3
Nomex          5           4           3           4
PEEK           5           5           5           5
PPS            4           5           5           5
PCTA           4           4           4           3

              Wear     Strength
           Resistance

Polyester      3          5
Polyamide      5          5
Acrylic        2          2
Nomex          3          3
PEEK           4          4
PPS            3          4
PCTA           3          3

5=excellent
1=poor

WHAT YOU WILL LEARN

* The main functions of dryer fabrics.

* Raw materials for dryer fabrics.

* Dryer fabric structures.

* Dryer fabric processing.

ADDITIONAL RESOURCES

* "The Basics: What You Need to Know About Forming Fabrics," by Richard Reese, Solutions!, August 2005. To access this article, type the following Product Code in the search field on www.tappi.org: 05AUGS033. Or call TAPPI Member Connection at 1 800 332-8686 (US); 1 800 446-9431 (Canada); [phone] (International).

REFERENCES

(1.) Luciano, B. and Fagerholm, L., PaperAge 112(1):18 (1996).

(2.) Rhyne, M., "Modern dryer fabric design and application," TAPPI 1995 Practical Aspects of Pressing and Drying Short Course, TAPPI PRESS, Atlanta, p.403.

(3.) Luciano, B., Pulp & Paper 59(11):142(1985).

ABOUT THE AUTHOR

Dick Reese has worked on paper machines for more than 40 years in various production and engineering/technical support roles. He has been a TAPPI member since 1973 and is an active member of the Papermaking and Water Removal committees. He was named a TAPPI Fellow in 1992, received the Manufacturing Division Technical Award in 1997, and the Engineering Division Technical Award in 1999. He is currently an independent papermaking consultant in Norcross, Georgia, USA. He can be reached by phone at [phone] or email at [e-mail].