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Fabrics can be woven from yarns of many sorts. It is usually the case that warp yarns (those running lengthways on the loom) are the stronger, while the weft yarns  (those running across the loom) may be bulkier and less tightly twisted – weft
yarns are often called filler yarns. It is quite common for the warp to be a single, relatively stout, filament, while the weft is a yarn of some very different material. Equally, it is quite normal for both warp and weft to be made of the same filament
or yarn.

The properties of a fabric, especially as regards its behaviour as a filter medium, depend very much on the way in which the yarns are woven together. Many properties, however, are intrinsic in the nature of the basic fibre or filament, and of the way in which it is made up into a yarn. There are three basic types of yarn in wide use for filter media: monofi lament, which is a single continuous filament of synthetic material (or silk); multifi lament, which comprises a bundle of identical continuous filaments that may or may not be twisted together; and staple, which is made from spun and twisted short fibres, either natural materials such as cotton and wool, or synthetic ones, which have been cut from extruded filaments. There is a
fourth, but much less common, type of yarn, made from fibrillated, or split-film, tape.
The key feature of yarn type that affects filtration performance is that, with monofilament fabrics, filtration occurs in the spaces between the filaments, while, with multifilament and staple yarns, filtration can also occur within the yarns as
well as between them – so the tightness of the twist in the yarn becomes important.

The physical and chemical properties of a yarn are largely those of the fibres or filaments making up the yarn. In addition to the natural fibres (mainly cotton, but with some wool and silk), and a small, but growing, number of inorganic fibres, the
bulk of filter fabrics is based upon an increasingly wide range of synthetic polymer fibres. The physical and chemical properties can then be tailored to the filtration application by choosing the appropriate polymer for the fibre.

The basic material of a woven fabric (filament or fibre) and the way that this material is formed into a yarn are major parameters in the choice of a fabric as a filter medium. The variety of available woven fabrics is virtually unlimited even if
only the materials from which the filaments or yarns are made, and the complexity of the yarn, are considered. To these must then be added the structure of the woven fabric itself: the way in which the yarns are woven together, and the finishing process (if any) applied to the fabric after weaving.

Woven fabrics are made up from yarns that are interlaced in a particular and regular order called a weave. The component yarns, warp and weft, need not be parallel to each other nor cross at right angles, but this is the case in most fabrics, and certainly it is so in filter media. The key features of a woven fabric come from the geometrical regularity of its components, and because these components are held in place, not by any rigid bonding, but by friction at their points of contact.

The binding system, or weave, is the basic factor that determines the character of the woven fabric. There are three main types of weave (plain, twill and satin) that are used in industrial textiles, although there are many other more complex systems. The differences among the weaves depend upon the pattern formed as the weft yarns are woven over or under the longitudinal warp yarns.

In plain weave, the weft yarn passes over and then under each succeeding warp yarn across the loom. The return weft then passes the opposite way, under then over succeeding warps, such that each weft is held securely in place by the interlocking
of the warp yarns. Plain weaves can give the tightest fabric, with the highest filtration efficiency, as well as the most rigid.

Twill weaves are characterized by a strong diagonal pattern. They are formed by the passage of the weft yarn over two or more warps at a time, and then under one or more, in a regular pattern across the loom. The next weft thread follows the same pattern of over-and-under, but displaced by one warp yarn. The essential feature of a twill weave is its regularity, leading to its diagonal pattern. In a twill weave, more weft threads can be crammed in to the fabric per unit length, which gives the fabric more bulk. Compared to a plain weave with the same yarns, twill fabrics are more flexible, and therefore easier to fit into a filter.

Satin weave extends further the concept of the twill weave, by having wider spacings between points of interlacing. Satin weave does not have the regular shift of weave pattern that twill has, and the result is an irregular appearance, smooth
faced, with relatively long floating warp yarns. Most satin fabrics are made from smooth, lightly twisted yarns, thereby enhancing the visual effects. Fabrics with a satin weave are still more flexible than the other two types of weave, because of the increased ease of yarn-to-yarn movement: this reduces the likelihood of particles becoming trapped in the structure. The longer floats allow insertion of proportionally more warp threads, thereby further improving the surface smoothness, resulting in easier cake discharge. However, unless the threads in both warp and weft directions are packed tightly together, satin weaves do not generally achieve high filtration efficiencies, while the long floats are more susceptible to abrasive wear.

In addition to cleaning, fabrics of all kinds will usually undergo some kind of finishing process after weaving, in order to ensure stability of the fabric, to modify the surface characteristics, and to regulate the permeability of the fabric. Calendering
and singeing are two familiar surface treatment processes, which also modify the permeability.

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