I. Basic Types of Filter Cloth

Filter fabrics are categorized into several basic types, including woven fabrics, nonwoven fabrics, and composite fabrics.

1. Woven fabric

Because the warp and weft yarns in woven fabrics can be arranged in a wide variety of ways, resulting in different weave structures, and because different types of yarns and processing techniques can also be employed, the range of possible weave patterns for woven fabrics is virtually limitless. Woven fabrics are created by interlacing warp and weft yarns according to a specific pattern; this interlacing pattern is referred to as the fabric’s weave structure. The three basic weave structures commonly used in woven fabrics are plain weave, twill weave, and satin weave.

Plain weave is a weaving technique in which each warp thread interlaces with each weft thread, alternating above and below. Plain-weave fabrics are highly dense, resulting in excellent filtration performance and outstanding stiffness.

Twill weaving is a fabric-weaving technique in which a single weft yarn passes alternately over and under two or more warp threads, and moves horizontally in a regular pattern from one row to the next. This weaving method produces diagonal twill lines that typically run across the fabric surface, moving upward from left to right. The advantage of twill weaving is that it allows more weft yarns to be packed into a given length of fabric, resulting in a fabric with greater bulk. Compared to plain weave fabrics, twill fabrics are more flexible and easier to install on filtration equipment.

Satin weave further expands upon the concept of twill weaving by employing a wider spacing between interlacing points. This weave produces fabrics with a smooth surface and, lacking diagonal twill lines, results in a satin fabric that is exceptionally soft. In addition to being easy to remove filter cakes, this type of fabric also reduces the likelihood of particles becoming trapped within its structure. However, a drawback of satin fabrics is their poor resistance to friction.

1.1 Finishing of Woven Fabrics

The primary objectives of machine-weave fabric finishing are to ensure fabric stability, improve the fabric's surface characteristics, and adjust the fabric's permeability.

(1) Thermal setting. Throughout the entire production process, tension is continuously applied to the filter cloth, so the cloth must exhibit dimensional stability. Otherwise, once the tension is removed, the dimensions of the cloth may change, potentially causing the feed inlet on the filter press to no longer align with the corresponding holes in the filter cloth. To prevent such issues, the cloth should undergo hot-water treatment or dry heat-setting treatment; the temperature and duration of the treatment depend on the polymer material of the cloth. Another reason why the filter cloth needs to be dimensionally stable is that during operation, tension is already pre-applied to the cloth—for example, in belt filters and vertical filter presses, where tension is initially imposed on the cloth. In these cases, it’s crucial to carefully control the temperature during the stretching process. Pre-stretching not only helps reduce further elongation of the cloth during use but also ensures that the cloth maintains a better running trajectory, minimizing deviation and misalignment.

(2) Calendering. Calendering is the most commonly used method for improving the surface properties of filter cloths. During operation, the filter cloth is passed between heated, pressurized rollers, and the temperature, pressure, and roller speed are adjusted according to the specific type of cloth. After calendering, the filter cloth has a smooth surface, which facilitates the removal of filter cakes and also optimizes permeability, thereby enhancing filtration efficiency.

(3) Singeing. Singeing is a surface treatment process in which the short fibers on the surface of fabrics woven from short-fiber yarns are slightly charred. The short fiber fuzz on the fabric surface can hinder the removal of filter cakes; therefore, an air frame or a very hot narrow metal strip can be used to quickly contact the fabric surface and burn off the fuzz.

(4) Napping. Napping involves using a fine steel comb to gently raise soft fibers on one or both sides of the fabric. Napping not only enhances the filter cloth’s ability to capture fine particles but also improves its dust-holding capacity. The napping method is commonly used to treat dust-collection filter fabrics.

(5) Surface coating. Surface coating is a specialized method for treating the surface of filter fabrics. Both woven and nonwoven fabrics can undergo micro-porous polymer coating, which not only enhances filtration accuracy but also improves the conditions for removing the filter cake from the fabric. For example, tetrafluoroethylene trimer can form an exceptionally robust coating on the fabric surface, featuring an abundance of pores with diameters ranging from 5 to 8 μm.

2. Nonwoven fabric

The production of nonwoven fabrics involves six steps: fiber preparation, web formation, bonding and reinforcement, drying, post-processing, and winding. Among these six steps, web formation and bonding and reinforcement are the most critical.

2.1 Networking

Web formation can be categorized into dry-web formation, wet-web formation, and polymer extrusion web formation.

(1) Dry-laid web formation. Dry-laid web formation is further divided into mechanical web formation and air-laid web formation. After the dry-laid fiber web undergoes chemical, mechanical, solvent-based, or thermal bonding treatments, it becomes a dimensionally stable nonwoven fabric.

(2) Wet-laid web formation. In the wet-laid process, water serves as the medium, allowing short fibers to be uniformly suspended in water. Then, using the action of the water flow, the fibers are deposited onto a permeable belt or a porous cylinder, forming a wet fiber web.

(3) Polymer melt spinning into nonwoven fabrics. Polymer melt spinning into nonwoven fabrics utilizes the principles and equipment of polymer extrusion. Representative spinning methods include melt spinning, dry spinning, and wet spinning—each of which is a nonwoven fabric production process.

2.2 Reinforcement

Reinforcement can be categorized into three types: mechanical reinforcement, chemical bonding, and thermal bonding. These methods can be used individually or in combination.

2.2.1 Mechanical Reinforcement

Mechanical reinforcement does not rely on chemicals such as adhesives or solvents, nor does it involve hot-melt bonding. Instead, it uses mechanical methods to interlock the fibers within the fiber web, or to reinforce the fiber web by means of coil-shaped fiber bundles or yarns.

(1) Needle-punching reinforcement method. Among dry-laid nonwoven fabrics, needle-punched nonwovens are the most common and their applications are becoming increasingly diverse. They are now used in geotextiles, carpets, automotive interior materials, papermaking felts, filtration materials, base fabrics for synthetic leather, and high-temperature-resistant synthetic materials, among other applications.

Needle-punched nonwoven fabrics have numerous, uniform, and irregular internal pores. As a result, they exhibit excellent air permeability and also boast superior particle-retention performance—a significant advantage. However, once particles penetrate into the interior of the nonwoven fabric, they tend to clog the pores, and the fuzz on the fabric’s surface can make it difficult to remove the filter cake. To address these issues, the fabric is subjected to processes such as flame treatment, calendering, and surface resin coating. After undergoing these treatments, needle-punched nonwovens not only maintain outstanding particle-retention capabilities but also ensure easy removal of the filter cake. Furthermore, if a base fabric is placed between the upper and lower fiber webs, this can further enhance the fabric’s performance.