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Nonwovens revolutionizing European filtration.

Nonwovens Revolutionizing European Filtration The significance of nonwovens and nonwoven composite materials onthe worldwide market for filter media will lead to decisive changes by the year 2000. Nonwovens currently account for worldwide sales of more than $800 million, with approximately $560 million of that total in the European market. Estimates of worldwide sales place the market at more than $1 billion by the mid-1990's.

The technology with the most growth potential is melt blown nonwovens, especially in the substitution of filter media made from microglass fibers. The importance of composite materials incorporating nonwovens will also increase. Combinations with foils that have good breathability. Combinations with foils that woven fabrics are currently in development.

The filtration market is expected to be a significant factor at the upcoming INDEX '90 International Nonwovens Fair and Congress in Geneva, Switzerland, April 3-6. What follows are some insights provided by companies that will be exhibiting nonwovens specifically for filtration media at INDEX '90. these technological developments, as well as a number of others in the filtration field, will be one of the focuses of the largest European nonwovens show ever.

Nonwovens As Third Dimension

In the course of constantly rising demands on filter media, the performance limits of natural fibers have long been exceeded. Synthetic fibers, therefore, have become today's standards. They are inherently more resistant to chemical, mechanical and thermal loads.

Needled nonwovens have looked for and found their chance with these fibers. They fulfill the requirements of environmental protection and process engineering as well as the properties of the newer generations of filter plants. For years, needled nonwovens have set the standard for high filter sharpness and load capacity, low power consumption and long service life.

In contrast to their woven competitors, nonwovens consist of layers of fine single fibers, of which some are needled with a loadable carrier fabric for stabilization and bonding. The thickness of the nonwoven enlarges as a third dimension on the active total surface. In this way, they are specifically of higher load capacity and offer more design possibilities for production process requirements.

These needled nonwovens are made up into filter tubes, filter bags, band filters and filter cloths, among other end uses. They are utilized by the chemical industry, in mining, in refuse incineration plants, in sewage purification plants and in power stations.

Finer PTFE Fibers

The finer PTFE filtration products are manufactured from nearly all fibers suitable for wet and dry filtration, such as glass, polyvinyl sulfide, PTFE, polyacrylonitrile, polyester, polypropylene, aramids, polyamide, cotton and wool. PTFE fibers with a fineness of 3.5 dtex are new on the market. They should enable more dense, finer and less porous nonwoven filter media to be manufactured.

Compared with conventional PTFE fiber types, a fiber surface of up to 30% larger should result at the same weight. Because of the more compact filter felt structure, dust separation should be reduced. The filtration of flue gases in combustion plants, power stations and boilers of every type with coal firing are some of the areas of application.

Special Finishes

Thanks to their special structures, nonwovens combine high air permeability with good dust separation that can be further improved by special treatments. For example, to promote the separation of the filter cake, needled webs are smoothed by special flame passages or calendering. The filter action can be further increased by chemical, thermal or mechanical treatments.

As a further example, contact temperatures up to 3000[degrees]C can be coped with by the use of a heat shield made from carbon fibers on the inflow side of the filter medium. The heat shield provides effective protection against sparks and flames, smoldering, glowing and burning particles. The continuous temperature load capacity is around 260[degrees]C. Areas of application for this structure are wherever there is a risk of fire by flight of sparks in the filter plant.

Electrostatic Filter Effect

Apart from needled webs in wet and dry filtration, microfiber nonwovens are used for particle filtration. Their main areas of application are room ventilation, machine process ventilation and surface engineering. A specially developed process for the manufacture of synthetic microfiber nonwovens, in which the microfibers are spun and simultaneously electrostatistically charged in a high voltage field, was developed by a West German supplier.

The filter medium consists basically of three layers--a covering web, a microfiber web as core and a carrier web. Here the two outer nonwoven layers can be varied in weight, fiber composition and structure. The microfiber layer is spun from different polymers, such as polycarbonate and polysulfone.

This nonwoven filter represents an optimum combination of electrostatic filter effect and mechanical action. Because of its filter engineering properties and its material structure, it achieves low transmittances at low pressure differences. Solutions to filtering problems that could not be achieved--or could be achieved only at high cost--with previously available filter media, such as pressure surge-proof suspended matter filters, can be realized with this medium.

However, some nonwovens are used not only by themselves as filter medium, but also as aids to other membrane technology. One of the most urgent tasks of physical-chemical research is to gain pure drinking water from contaminated ground water or to clean industrial offgases. Success has recently been achieved in separating organic contamination better with innovative separating techniques.

The manufacture of such ultrathin membrane layers (with a thickness of a few thousandth millimeters) is not only complex at its basic level. An effective separation over lengthy operating times can be achieved only if the thin membrane is supported mechanically by a stable underlayer. So-called assymetrical "composite membranes" consist of several layers. An ultrathin membrane film is applied to a highly porous but strong supporting framework made from nonwoven material. Only then can this high performance membrane be put into action.
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Publication:Nonwovens Industry
Date:Feb 1, 1990
Previous Article:Spunbondeds as filter media.
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