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

Editor's Note: This is the third in a series of three articles on the basics of machine clothing. To access the previous two articles, go to the "Additional Resources" box at left.

The primary functions of press fabrics are to absorb water removed from the sheet, provide uniform pressure to paper in press nips, impart desired surface finish to the sheet, transport the web through the press section, and act as a power transmission belt to turn undriven rolls in the press section. Water is removed from the web in the press section to increase solids content from 14-28% off the forming section to 38-55% entering the dryer section. Press fabrics must be capable of handling most of the water removed from the web in press section nips while meeting the other requirements.

It is much more economical to squeeze water from the web in the press section than it is to evaporate it in the dryer section, so it is critical that press section dewatering be maximized. Relative cost of removing water from the sheet in the forming, press, and dryer sections is shown in Figure 1.

Solids improvement of 0.5 to 1.5 percentage points is typical by optimizing press fabric application. These dryness improvements provide a 2 to 6% improvement in production rates or a 2 to 6% reduction in main section dryer steam consumption at the same production rate. Higher dryness after the press section also improves sheet transfer to the dryer section.

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Papermakers typically want press fabrics to permit operation at maximum paper machine speed from startup to removal, maximize sheet water removal, provide trouble-free sheet transfer, produce high quality paper, never change during life, have sufficient life to support scheduled maintenance schedules, and have all press fabrics perform the same way.

Press fabric application is a difficult challenge since papermaking variables and paper machine press sections change regularly. Challenges include fewer and wider press nips, higher machine speeds, higher temperatures, and increasing quality demands at lower basis weights. Press fabric designs and materials of construction are continually evolving to better meet these challenges. Each press position has different demands and fabrics must be customized to meet these requirements. Press section fabrics influence press section performance by their structure, openness, surface, and compression properties.

Press felts were historically woven using wool. Polyamides (better known as nylon) are the primary materials currently used to manufacture press fabrics. Polyamide is durable, strong, has good resistance to wear, and has replaced other materials in most press clothing applications. Wool and special polymers are still used to meet special requirements on some press positions. Papermakers often refer to press section clothing as "felts" although "fabrics" is a better term to describe modern clothing produced using different materials.

BASE FABRICS

Base fabrics provide stability to the fabric structure, control caliper and void volume, and distribute applied pressure. A typical press fabric structure is shown in Figure 2. Base fabrics are woven from spun multifilament or monofilament yarns. Spun yarns are made by joining short stable fibers. Multifilament yarns are made by twisting continuous filaments. Monofilament yarns can be used singularly or plied. Spun yarns are seldom used in modern fabric designs. The trend towards seamed fabrics has increased use of single monofilament yarns. Yarns used in base fabrics are shown in Figure 3 and Figure 4.

Yarns are woven together to form a base weave. The base weave can be woven as a piece of flat material that is later joined together with a seam or in an endless loop as a tube. The base weave can be made in a variety of different weaving patterns. The number of machine direction layers determines whether the base is a single-layer, multi-layer, or a combination of two separate base weaves that are laminated together.

The base weave provides void volume in the fabric structure to accept water removed from the wet sheet in press nips. Different types of yarns and base weave are used to achieve the proper water handling capacity and compressibility of each fabric. Base fabrics are usually heat-set to stabilize the fabric before batt is applied (see section below). There are also some special constructions called non-woven base fabrics where machine and cross direction yarns are not woven together but are laid on top of each other and then needled with fibers in the needling process.

Figures 5, 6, 7, 8 and 9 show some different types of weave patterns. Double and triple-layer fabrics are thicker and heavier than single-layer fabrics and accordingly have greater water handling capacity. Caliper and void volume increases with the number of layers, but multi-layer fabrics are also stiffer, harder to install, and more difficult to keep clean.

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BATT

Batt fibers cover the base fabric to prevent marking, provide pressure uniformity to improve pressing uniformity, and help control sheet pick-up and release. Batt fibers are laid on the base weave and mechanically bonded into the base weave by use of felting needles in needling machines. The needles have small barbs facing the needle point and are fastened on a vertically reciprocating plate. The barbs grasp a few batt fibers as they descend and force and lock them into the base weave.

The number of batt layers may be varied and applied to the paper side or both sides of the fabric. Type of needling equipment, batt characteristics, batt orientation, needle penetration, and number of passes through the needle loom determine finish, openness and other fabric characteristics. Surface characteristics and permeability can vary as much as 40% based on needling procedures and type of batt used.

Batt structure is often applied in stratified layers to provide good micro-scale sheet pressing support, improve sheet smoothness, and achieve optimum roll side drainage. Stratification is achieved by layering finer surface batt fibers over coarser batt layers under the fabric surface. Stratification optimizes surface contact area and overall drainage and openness. Different fineness of batt is used depending on sheet basis weight as shown in Figure 10. Finer batt is used on lightweight sheets to improve fabric surface to sheet surface contact to optimize sheet dewatering and surface smoothness. Fabric porosity, caliper, and resistance to compaction all increase as batt fiber diameter is increased. Batt fiber compacts and captures contaminants like a filter and must be properly conditioned to maintain good fabric performance.

FINISHING

Press fabric finishing includes applying a trade line, washing, chemical treatment, heat-setting, precompaction treatment, and final sizing to needled length and width. Various chemical treatments are used to improve fabric properties. Two types of treatments are used: those that internally modify molecular structure of the materials and external coatings on the fibers to protect them and bond them together. Use of reactive treatments is minimal due to environmental concerns, so most current treatments are topical.

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Treatment benefits include reduced fiber shedding, improved resistance to bacterial or chemical degradation, increased resilience, reduced filling tendency, increased wear resistance, improved startup properties, decreased stiffness to make installation easier, and improved stability. Various treatments may be used alone or in combination.

Heat-setting is important for achieving dimensional stability. Temperature relaxes stresses in fibers and establishes a new memory in the fiber. Precompaction involves applying pressure to the fabric in finishing to shorten initial break-in on the paper machine. Precompaction reduces caliper and permeability and has a smoothing effect on the surface of the fabric.

MODERN PRESS FABRIC STRUCTURES

Press fabric design has steadily improved and there are currently different groups of fabrics: single- and double-layer, laminated, and non-woven. Laminated fabrics are made with a combination of two or three base weaves and are used to improve fabric performance on the most demanding press positions. Laminated fabrics improve sheet quality, sheet dewatering, and fabric life. Multi-axial laminated fabrics have top and bottom base weaves made from a narrow flat-woven weave that is spiraled and joined together through spiraling two weaves in different directions.

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Yarns in multi-axial fabrics are oriented in at least four directions. This produces advantages on paper machines including better pressure uniformity, improved sheet moisture profiles, and better compaction resistance that improves dewatering through fabric life. Fabric structures with elastomeric construction improve moisture profiles, provide better sheet dewatering, and increase fabric life.

FABRIC TECHNICAL PROPERTIES

Press fabric variables related to water removal are pressure uniformity, compressibility, and flow resistance. Pressure uniformity refers to how evenly pressure is transferred from loaded rolls to the paper web. Fabric surface uniformity is a very significant variable in water removal efficiency and paper quality. Pressure uniformity is affected by fabric base structure and batt application. Large scale pressure uniformity is related to base fabric mesh count and yarn size. Small scale pressure uniformity is related to batt structure. Benefits of pressure uniformity are shown in Figure 11. Figure 12 and Figure 13 show significant differences in pressure distribution between fabric designs.

Fabric compressive properties affect nip width, peak pressure achieved in press nips, and fabric void volume. Void volume is the total water-handling capacity of the fabric under a given load and is equivalent to "empty space" in the fabric. Increasing press load decreases void volume. Void volume reduces with fabric age.

Compression is change in fabric caliper from out-of-nip to in-nip. Compressibility of press fabrics decrease as fabrics compact with age and become filled with contaminants. Flow resistance is the rate at which water can enter and exit the fabric. Low flow resistance allows water to move easily into and out of the fabric. The best way to evaluate fabric flow resistance under operating conditions is to measure water permeability under load.

A number of technical properties are measured to determine a fabric's suitability for a specific press position and to evaluate running conditions of each press section. There are no standardized quality control testing methods, but all fabric suppliers test basis weight, thickness, stiffness and tensile strength, compressibility, air permeability and flow resistance, and surface uniformity.

However, some of these measurements do not predict how fabrics will perform on paper machine press sections. For example, air permeability is a good measurement for fabric quality control but does not provide an indication of water removal capability. Table 1 shows typical property ranges of single and multilayer press fabrics.

SEAMED FABRICS

Seamed fabrics were introduced in the late 1980s and their use has increased significantly during the last 10 years. Primary factors in growing use of seamed press fabrics are safety, reduced installation time, and improvements in seam technology. The press section does not have to be dismantled to change seamed fabrics so the chances of changing press roll alignment and causing loading issues is minimized. A smaller crew is required to install seamed fabrics. Less machine room crane time is required for installation of seamed fabrics, so additional paper machine maintenance can be done on shutdowns.

Pin seam fabrics are woven flat with loops woven into the fabric in a separate process. These fabrics have crimp in machine direction yarns. Woven loop seamed fabrics are woven endless with loops formed during the weaving process. Woven loop designs have crimp in the cross machine direction like conventional endless fabrics. Both types of seams can be made with laminated fabrics on the surface. Figure 14 shows a pin seam fabric.

PRESS FABRIC APPLICATION

Factors that influence press fabric design selection include press configuration, roll type, press load, water load, machine speed, paper grade, sheet property requirements, fabric conditioning equipment, stationary elements, and life requirements. Fabric designers typically look for similar paper machines and press positions to improve fabric performance. Papermakers and press fabric designers have to work together closely to optimize fabric application since good press section dewatering is critical to efficient paper machine operation and energy consumption.

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REFERENCES

1. Reese, R. A., The Paper Machine Wet Press Manual, TAPPI PRESS, Atlanta, 1999, Chap. 5-6.

2. Paulapuro, H., Papermaking Part 1, Stock Preparation and Wet End, Fapet Dy, Helsinki, Finland, 2000, Chap. 9.

3. Adanur, S., Paper Machine Clothing, Technomic Publishing Co., Lancaster, PA., Chap. 3.2.

Contact Dick Reese at [phone] or email him at [e-mail].

All figures courtesy of Albany International and AstenJohnson.

WHAT YOU WILL LEARN

* The primary functions of press fabrics.

* The different base fabrics used in making press fabrics.

* How batt fibers are used in press fabrics and the properties they impart.

* Press fabric finishing, new fabric structures, and why use of seamed press fabrics is growing.

ADDITIONAL RESOURCES

* "Press fabric selection & performance improvement," by Richard Reese, Solutions!, October 2004. To access this article, type in the following product code in the search field on www.tappi.org:. 04OCTSO42. Or call TAPPI Member Connection at 1 800 332-8686 (US); 1 800 446-9431 (Canada); [phone] (International).

* "The Basics: What You Need to Know About Forming Fabrics," by Richard Reese, Solutions!, August 2005. Product Code: 05AUGSO33.

* "The Basics: What You Need to Know About Dryer Fabrics," Solutions!, October 2005. Product Code: 05OCTSO31.

Table 1(below): Typical property ranges of single and multilayer press
fabrics.

                                                           Air
Structure      Base Fabric Weight   Total Fabric Weight    Permeability

Units          gsm (oz/sq ft)       gsm (oz/sq ft)         cfm
Single Layer    275-549 (0.9-1.8)    702-1495 (2.3-4.9)     8-160
Two Layer       580-824 (1.9-2.7)   1129-1769 (3.7-5.8)    12-140
Three Layer     671-824 (2.2-2.7)   1220-1830 (4.0-6.0)    15-120
Laminated       763-824 (2.5-2.7)   1403-1586 (4.6-5.2)    12-90
  Two Layer
Laminated       671-824 (2.2-2.7)   1495-2135 (4.9-7.0)    15-130
  Three Layer
Laminated      1037-1159 (3.4-3.8)  1586-2135+ (5.2-7.0+)  15-130+
  Four Layer