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Microfiber and electrostatically charged nonwovens in filtration.

Microfiber And Electrostatically Charged Nonwovens In Filtration Although microfiber technology is not new by any means, it has become a much larger factor in many infiltration segments. This move has been triggered primarily by melt blown technology, but also by micro stape filber and microfier spunbonded materials.

An entirely new family of electrostatically charged nonwoven materials has captured specific applications rather dramatically and continues to become a major portion of filtration materials.

The Growth of the Cabin Filter

One of the latest filtration applications has been developed in Europe and pertains to a car interior or cabin filter. This concept has been developed for a couple of years and was first introduced to the public at the International Automobile Show in Frankfurt in September, 1989. In 1990 it was estimated that only about 50,000 such cabin filters have been supplied, mostly through new models made by Mercedes and BMW, although Volkswagen offered this filter as an option for which the customer could pay extra.

This market, however, will change dramatically to at least five-six million units by the end of 1995. This is not even taking into account the replacement market, which might amount to another five million units in Europe alone.

The developments for this new application in North America and Japan are in their infancy; however, it is expected that starting in 1992 some models will have such filters built into new cars. American and Japanese adoption of this new concept will probably depend on the success in Europe.

Not surprisingly, the two market leaders, Freudenberg and the European operations of 3M, are challenged by a number of companies that want to enter this interesting new filtration application. Different filter media are currently being developed; most of them, however, are based on electronically charged nonwoven media. A filter media "standard specification" in Germany is expected in 1991.

Current filter material is based on "Microdon" ("Micron-Air") from Freudenberg, as well as "Filtrete" from 3M. There are a number of other developed materials currently under investigation and there will certainly be more than two suppliers as early as this year.

Typical dust particles that have to be addressed and filtered in such applications are listed in Table 1.

Early on Freudenberg probably made the largest commitment and produced a filter material that allows a relatively lightweight construction of its complete fabricated filters. 3M, however, is expected to modify its current material in a so-called second generation Filtrete and it seems as if it has also developed a much stronger commitment towards this very promising and fast growing filtration segment.

Figure 1 shows typical materials that are filtered to improve the atmosphere, particularly for people who are allergic and prone to asthma attacks, individuals often unknowingly affected by unfiltered cabin air.

Melt Blowns In Filtration

Melt blown nonwovens have been used in filtration applications for at least 10-15 years. In this market, however, the significant growth went along with the rapidly increasing number of melt blown manufacturers.

It was reported that in 1980 less than 20 million pounds of melt blown materials were produced worldwide. By 1984 that number had grown to 46 million pounds. By the end of 1990, the world market for all melt blown products has been estimated at approximately 100 million pounds, of which 19-20 million (19%) are produced into different filtration materials for dry and liquid filtration.

One of the most interesting developments is the high efficiency ASHRAE extended pocket filter for HVAC (heating, ventilation, air conditioning) applications. Currently, the majority of filters manufactured for this application are based on microglass blankets; however, Freudenberg introduced new synthetic microfiber filter materials into the market approximately 10 years ago. After Freudenberg succeeded in Europe, the company decided to enter the North American market in the mid-1980s and now produces such products in a modern plant in Hopkinsville, KY. The total market for such products has grown in the U.S. alone to $6-8 million in just the last couple of years. Now most of the major HVAC manufacturers offer the filter concept based primarily on melt blown microfiber products.

There are some interesting developments based on a combination of high loft microfiber, needled felt microfiber and composite sandwich constructions, all geared to accomplish ASHRAE efficiency of 60-95%, depending on the product type (Eurovent Class 6-9).

Although the price for such filters is significantly higher than the conventional microglass blanket construction, potential fiber shedding of glass fibers is eliminated. The jury is still out as to whether the synthetic fiber-based material may cause a similar concern with regard to very fine fibers, which, if mineral-based, are suspected of causing cancer.

Freudenberg has spent considerable effort to produce evidence with regard to such fiber penetration and demonstrated that glass microfibers have a tendency to fracture, thereby creating an undesirable contamination downstream of the filter. The synthetic microfiber, on the other hand, is more elastic by nature and is unlikely to suffer much similar breakage. From their company brochure, the table in Figure 2 is quite interesting.

It is expected that other companies will also develop such technical literature, but Freudenberg is definitely the undisputed leader at this time.

Recent developments of microfiber high ASHRAE/Eurovent efficient filters have led to similar developments in China. At least from one Chinese source electrostatically charged melt blown materials are manufactured into such extended surface filters.

Relatively few developments in this regard are so far reported from Japan. It is expected, however, that worldwide such microfiber-based, high efficiency ASHRAE/Eurovent filters will be used in increasing quantities.

HEPA Filtration Developments

HEPA filtration is based on microglass paper media and there are few manufacturers worldwide. The microglass paper manufactured by a wet laid paper-type process is based on different formulas utilizing microfiber glass to produce the highest obtainable efficiencies. The latest developments are ULPA (Ultra Low Penetration Air filters) to meet the increasing demand of even cleaner air in pharmaceutical, biomedical and electronic computer related applications.

HEPA filters, because of their construction, create a certain differential pressure. Since ULPA filters have to be more efficient according to DOP conditions (0.3 micron particles), normally their differential pressure is even higher.

The Japan-ese have developed new electrostatically charged melt blown materials that are currently tested and evaluated to replace conventional microglass paper. This product from Toray Industries, called "Toraymicron-E," is detailed in Table 2. Although these materials are significantly more expensive, the most important feature is the potential for greater flow, thus reducing the differential pressure for a given filter area.

One of the leading Japanese manufacturers will present in great detail a paper at the upcoming INDA Filtration Conference (March 12-14, Philadelphia, PA). This is a very interesting development considering that the HEPA filtration market worldwide is huge and is expected to grow particularly with the increasing demand from the electronics industry in this special field.

Some rather shy developments are also underway in the U.S. The Chinese, however, in 1989 developed the first HEPA filters that are already being used; these are based on electrostatically charged melt blown materials. Although their production is relatively small and the workmanship of their final filter elements is not necessarily comparable to Western design, it is remarkable that such new filters are currently being used.

It is predicted that this segment of the super high efficiency filtration market will grow rapidly. The availability of suitable media will be its only limitation. Market developments are going to be most aggressively pursued in Japan because of the strong electronics industry there and the increasing demand for such products by domestic manufacturers. With the largest HEPA/ULPA market in North America, however, it is expected that perhaps with some delay such filter concepts will also be seen here on our home turf.

Blood Filtration Applications

Another interesting specialty market is blood filtration. Proprietary developments in melt blown media have been successfully produced and are growing rapidly; in this application the melt blown media has an adsorbing task to fulfill and is used as a disposable filter in blood transfusions. With the widespread concern over AIDS and other HIV diseases, the importance of blood filtration technology and its demand on super high efficiency and performance will increase. Melt blown microfiber products will play a major role in these developments.

It is further expected that conventional nonwoven materials such as needled felts will be affected to some extent. One area in which that has already been the case for the past five years are in micron rated bags.

3M first introduced its 100 Series of products for this application, which has been succeeded by the Series 500 and now 700 and 300 to offer new, partially revolutionary filter configurations, particularly with regard to the radial pleat design (Series 300 and 700).

Other companies are also offering melt blown/microfiber-based materials for this application.

The much larger market for dust bags has not yet been addressed. It is expected, however, that very interesting developments where needled felts have been combined with microfiber webs will challenge the previously used needled felt concept. Microfiber filter media is expected to be the fastest growing segment within nonwovens for filtration, closely followed by "electrets," or the electrostatically charged filter materials.
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Author:Bergmann, Lutz
Publication:Nonwovens Industry
Date:Feb 1, 1991
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