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From earliest times, woven fabrics formed the bulk of filter media. Beginning in the 1940s, with the production of a suitably bonded felt, nonwoven materials started to be used for filtration, and now they dominate the business. One reason for this is
the continuing demand for finer filtration, of both liquids and gases, which can be met by very finely spun fibres, assembled into ever more complex forms of nonwoven materials.

Woollen felt is probably the oldest form of textile, and for many years was the only practical nonwoven fabric, produced by the combined action of moisture and heat on carded wool fibres. The development first of a strong adhesive-bonded felt,
and then of the multiplicity of forms of dry-laid synthetic fibres, has transformed the spectrum of nonwoven media, both in format and basic material.

A nonwoven fabric, then, is one that is made up from an agglomeration of fibres, and sometimes of continuous filaments, which are held together by some form of bonding, to create a more or less flexible sheet of fabric. This will be as wide as the
bed upon which the nonwoven material is laid down, and as long as the receiving rolls can accept. The chemical properties of a nonwoven fabric are dictated almost entirely by the nature of the basic fibre – unless there is a binding adhesive of significantly different properties (such as melting or softening temperature).

There are two broad classifications of nonwoven materials, into which almost all types will fall. These two classes are, to a large extent, divided by the means utilized to hold the loose fibres together:

● felts, which use the basic characteristics of the fibre to provide mechanical integrity, or which use mechanical processing (especially needling) to create a fabric
● bonded fabrics, which use some additional adhesive material to hold the fibres together, or, more commonly, rely upon the thermoplastic nature of the polymer to provide adhesion when properly heated.

This second group is then further divided in two, according to whether the formation of the basic fibre is an integral part of the manufacture of the medium (the drylaid spun media), or not (resin and thermal bonding).

The basic felt has no added binders: some fibres, wool especially, have the ability to cling together to form a coherent mass, because of protrusions from the fibre surface. Most others can be made to adhere by suitable processing. The first step
in any felt making, or dry-laying, process is the carding of the fibres, whereby they are drawn out into a thin web, which has its fibre content roughly aligned in one direction. Pieces of such web can then be placed one above the other to provide a
felt of the required thickness. The successive layers can be aligned with the fibres all lying in the same direction, or in different directions to give equal directional strengths. When sufficient thickness has been achieved, the felt is compressed and heated, often after dampening, to produce its final structure. It is a fundamentally weak structure, in terms of tensile strength, and many felts are strengthened by the inclusion within their thickness of a layer of woven material, called a scrim.

The fibres in a felt are not securely locked into the mass of the fabric, and a simple felt used as a filter medium would be liable to significant loss of the fibres into the clean filtrate. Hence the need for bonding techniques to hold the fibres, such as various adhesive techniques, including the use of adhesive dispersions within the felt, the integral bonding of thermoplastic fibres, and several mechanical bonding processes, based on needling or stitch knitting, with or without the use of binding threads.

Modern felts are produced from synthetic fibres or mixtures of synthetic and natural fibres, bonded with adhesive or held together mechanically, with close control of manufacture to yield consistent density, pore size and mesh geometry, so that the cut-off performance is reasonably predictable. The structure of felts is considerably more open than papers so that whilst filtering in greater depth, specific resistance is lower and high rates of flow can be achieved with smaller element areas and low pressure drop.

High temperature resistant meta-aramid fibre has helped hot gas filtration technology to move a step closer to the industry’s goal of zero emissions by providing a combination of high separation efficiency and low differential pressure.

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