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Events 30 Filtration+Separation October 2006 Filter media: Current developments K en Sutherland looks at today’s filter media marketplace – in the run up to Filtrex 2006 in Germany, and the International Filtration Conference in Japan. October’s Filtrex 2006 event – organised by EDANA, the “voice of nonwovens” and following on from a successful inaugural filtration event in 2004 – almost entirely concerns itself with nonwoven materials. And Lutz Bergmann’s International Filtration Conference – taking place in Osaka in November 2006 – will also touch on various aspects of filter media technology. This article is a survey of developments in nonwoven filter media technology, sparked by the listing of papers given in the Filtrex Conference programme. The Filtrex Conference for 2006 has 20 papers from a variety of companies and other organisations, grouped into 6 sessions: Air filtration; • Nanofibres; Liquid filtration; Automotive filtration; • Testing; Speciality and dust filtration. All of these, obviously, have a major nonwoven material interest. The authors are drawn from eight companies involved in fibre or filter media fabrication, from eight universities and technical institutes, from three chemical companies, and one laboratory equipment company. Pervading the whole conference content is a strong feeling of exciting developments within the field of nonwoven media as a whole, and particularly of fine fibre media. It is apparent that manufacturing methods for nonwovens that were once on the cutting edge of new technology are now well established, with an awesome array of technologies available to the filter media maker. There are probably three key themes running through the Conference, despite the division into six sessions (which seems more for the organisers’ benefit than that of the attendees), these being the ever-present need for finer and finer filtration, the different kinds of very fine fibre materials, and the creation of performance enhanced materials by mean of surface treatments. Finer filtration The main driving force in the whole of the filtration industry, not just that part concerned with nonwovens, is the requirements of the end- user sectors for finer and finer degrees of filtration, whatever the application. In liquid filtration this need is being increasingly met by membrane processes, which have expanded their range of applications into the micro-filtration range to meet the need. In air filtration (and, of course, increasingly in liquid filtration) the requirement for finer filtration is being met by media with finer basic fibre construction. A leading marker for many uses is the ability to separate bacteria from a fluid stream, and, increasingly, viruses. Typically, for depth filtration processes, the size of the species to be separated needs to be matched by the size of the basic fibre, so that the removal of micro- organisms at diameters of less than 1 micrometre, i.e. tens to hundreds of nanometres, has led to the production of nanofibres for this purpose. The nanofibre is well in evidence at the Conference, with a whole session of four papers devoted to the topic – manufacture and properties – and a mention in at least one other paper. (See ‘Ahlstrom breaks the mould’, page 26). Types of media The extruded fibre (spun) nonwoven filter medium has been a standard material for decades now. Spun bonded materials, produced from continuous filaments extruded from an oscillating row of spinnerets, collected on a belt moving along under the nozzles, make a fairly coarse and strong medium, while meltblown materials, in which the continuous filaments are broken into long fibres by a transverse air stream and then collected in the same way, are used to make a finer (and correspondingly weaker) medium. These two types, together with their composite sandwich format (spun bond- meltblown-spun bond or SMS), constitute a major part of the nonwoven filter media industry, having taken a considerable share of the market from the felts and needlefelts into which extruded fibres were first made. To take such materials into the nanofibre range is possible with melt blowing, although the resultant material is lacking in mechanical strength. Two advanced techniques, covered by papers at the Conference, enable the production of media from nanofibres. The first of these, hydro-entanglement (also called spun lacing) is a mechanical bonding process applied to what is effectively a felt. Instead of the local stirring of the fibres of the felt by the barbed needles that form a needlefelt, the fibres are entangled by means of high pressure water jets. The consequent relatively high mechanical strength of the spun laced material gives hydro-entanglement a wide range of end-use properties, and hence a correspondingly wide range of applications. The difference in behaviour results from the broad range of fibres that can be hydroentangled, as well as the considerable adjustment possible in the processing parameters. One of the major advantages of the hydro- entanglement process (reported at the Conference by Jacob Helm) is that it can be used to combine ordinary felt webs with spun bonded or melt blown webs, or other textiles such as scrims, so as to achieve a combination of material properties that could not be achieved with a single material. Its products can also accept a range of surface finishing options, and they can easily be rendered hydrophobic or hydrophilic.

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Page 1: Filter media: Current developments

Events30Filtration+Separation October 2006

Filter media:

CurrentdevelopmentsK en Sutherland looks at today’s filter media marketplace – in the

run up to Filtrex 2006 in Germany, and the InternationalFiltration Conference in Japan.

October’s Filtrex 2006 event – organised byEDANA, the “voice of nonwovens” andfollowing on from a successful inauguralfiltration event in 2004 – almost entirelyconcerns itself with nonwoven materials. AndLutz Bergmann’s International FiltrationConference – taking place in Osaka inNovember 2006 – will also touch on variousaspects of filter media technology.

This article is a survey of developments innonwoven filter media technology, sparked bythe listing of papers given in the FiltrexConference programme.

The Filtrex Conference for 2006 has 20 papersfrom a variety of companies and otherorganisations, grouped into 6 sessions:

• Air filtration;

• Nanofibres;

• Liquid filtration;

• Automotive filtration;

• Testing;

• Speciality and dust filtration.

All of these, obviously, have a majornonwoven material interest. The authors aredrawn from eight companies involved in fibreor filter media fabrication, from eightuniversities and technical institutes, fromthree chemical companies, and one laboratoryequipment company.

Pervading the whole conference content is astrong feeling of exciting developments withinthe field of nonwoven media as a whole, andparticularly of fine fibre media. It is apparentthat manufacturing methods for nonwovensthat were once on the cutting edge of newtechnology are now well established, with anawesome array of technologies available to thefilter media maker.

There are probably three key themes runningthrough the Conference, despite the division

into six sessions (which seems more for theorganisers’ benefit than that of the attendees),these being the ever-present need for finer andfiner filtration, the different kinds of very finefibre materials, and the creation of performanceenhanced materials by mean of surfacetreatments.

Finer filtration

The main driving force in the whole of thefiltration industry, not just that part concernedwith nonwovens, is the requirements of the end-user sectors for finer and finer degrees offiltration, whatever the application. In liquidfiltration this need is being increasingly met bymembrane processes, which have expanded theirrange of applications into the micro-filtrationrange to meet the need. In air filtration (and, ofcourse, increasingly in liquid filtration) therequirement for finer filtration is being met bymedia with finer basic fibre construction.

A leading marker for many uses is the ability toseparate bacteria from a fluid stream, and,increasingly, viruses. Typically, for depthfiltration processes, the size of the species to beseparated needs to be matched by the size of thebasic fibre, so that the removal of micro-organisms at diameters of less than 1micrometre, i.e. tens to hundreds of nanometres,has led to the production of nanofibres for thispurpose.

The nanofibre is well in evidence at theConference, with a whole session of four papersdevoted to the topic – manufacture andproperties – and a mention in at least one otherpaper. (See ‘Ahlstrom breaks the mould’, page 26).

Types of media

The extruded fibre (spun) nonwoven filtermedium has been a standard material fordecades now. Spun bonded materials, producedfrom continuous filaments extruded from anoscillating row of spinnerets, collected on a beltmoving along under the nozzles, make a fairly

coarse and strong medium, while meltblownmaterials, in which the continuous filaments arebroken into long fibres by a transverse air streamand then collected in the same way, are used tomake a finer (and correspondingly weaker)medium. These two types, together with theircomposite sandwich format (spun bond-meltblown-spun bond or SMS), constitute amajor part of the nonwoven filter mediaindustry, having taken a considerable share ofthe market from the felts and needlefelts intowhich extruded fibres were first made.

To take such materials into the nanofibre rangeis possible with melt blowing, although theresultant material is lacking in mechanicalstrength. Two advanced techniques, covered bypapers at the Conference, enable the productionof media from nanofibres.

The first of these, hydro-entanglement (alsocalled spun lacing) is a mechanical bondingprocess applied to what is effectively a felt.Instead of the local stirring of the fibres of thefelt by the barbed needles that form a needlefelt,the fibres are entangled by means of highpressure water jets. The consequent relativelyhigh mechanical strength of the spun lacedmaterial gives hydro-entanglement a wide rangeof end-use properties, and hence acorrespondingly wide range of applications. Thedifference in behaviour results from the broadrange of fibres that can be hydroentangled, aswell as the considerable adjustment possible inthe processing parameters.

One of the major advantages of the hydro-entanglement process (reported at theConference by Jacob Helm) is that it can beused to combine ordinary felt webs with spunbonded or melt blown webs, or other textilessuch as scrims, so as to achieve a combination ofmaterial properties that could not be achievedwith a single material. Its products can alsoaccept a range of surface finishing options, andthey can easily be rendered hydrophobic orhydrophilic.

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Events 31Filtration+Separation October 2006

The second important new manufacturingtechnique, electrospinning, was actually firstpatented in the 1930s, but is only now beingused to produce filter media, because of its abilityto produce filaments in the sub-micrometrerange (reported at the Conference by FinetexTechnology). Basically, electrospinning extrudesa single filament from a nozzle from which it fallsto a collector plate, with an electric potentialbetween nozzle and plate. The extrudate is not apure polymer, but a solution of the requiredpolymer in a volatile organic solvent, such that,as it falls from nozzle to collector plate, thesolvent evaporates, leaving a fine filament of thebasic polymer.

At the exit from the nozzle, under the particularhydrostatic and electrostatic conditionsobtaining there, the solution forms a cone, theTaylor cone, with its apex pointing away fromthe nozzle. The jet of solution leaves from thisapex as a fine continuousfilament (if the conditionsare right – but as a streamof droplets if they are not),which follows ameandering path to thecollector, which is actuallya moving belt, as withother extrusiontechnologies. Dependingupon the speed of thecollector, the electric fieldconditions and theextrusion rate, a web ofmaterial based on onenozzle, or a small number ofnozzles, is formed, whichcan have both a highpermeability and a very finefiltration cut-off point. Tooweak to be self supporting,the electrospun web has tobe supported on a substrate.It can be laid down directlyon a support layer, or combined with severaldifferent materials to form a composite medium.

[It is noteworthy that Lutz Bergmann’s recent marketsurvey report: “Nanofiber Technology for Filtration”(Filter Media Consulting, Inc, 2005) dividesnanofibre production processes into just twocategories: “electro-spinning” with 8 companiesquoted, and “non-electro-spinning” with 10companies.]

A third nanofibre production technology isreported by Nanoval, in which a single filamentof molten plastic leaving the extrusion nozzle,immediately enters a converging/divergingnozzle, through which it is dragged by anairstream. The filament is attenuated until itsinternal pressure makes it split apart into anumber of much finer filaments. These are thenallowed to bed down to form a layer of materialwith individual filaments well below amicrometre in diameter.

Performance enhancement

The combination filter has become a wellknown processing tool, combining particleseparation with some other function, usuallychemical, such as the parallel removal of odoursor colour from the suspending fluid stream.Mainly, this has been done by embeddingparticles of a reagent, such as active carbon, inthe filter medium, but it is now apparent thatthe medium material itself can be modified bysurface treatment, or the fibres can be so treatedbefore being formed into the material.Specifically mentioned in Conference papers areDevan’s antimicrobial treatment and Rohm andHaas’ catalytic material.

This sort of surface modification could make thefilter equipped with such media into a veryimportant processing tool, akin to theimmobilised enzymes used in pharmaceutical and

biochemical processes. A wide range of chemicalprocesses would be attainable, limited only bytemperature range and corrosion resistance (andthat limitation would not be too serious if thetreatments can be applied to glass, metal orceramic fibres).

The use of electrostatic charges to enhancefiltration performance is well known, but workreported at the Conference from the Universityof Tennessee shows how this may be done in acontrolled fashion, while Europlasma can use gasplasma technology not only to create charges,but also to convert a surface to hydrophilic orhydrophobic character. Such processes, freelyavailable, would be of great benefit to mediamakers.

Other materials

Amazing developments continue to be made innonwoven media as described at the Conference,

with new and improved extrusion technologiesleading the way. To some extent, thesedevelopments are reflected in improved wovenmaterials, where the same fibres as are spun intononwovens can also be used, either asmonofilaments or multifilament fibres, woveninto structured media, with many of the sameimproved properties.

It must be noted, however, that the mostimportant developments in filter media are asmuch in the fields of membrane technology andhot gas filtration. There is some mention of thisat the Conference, with Freudenberg’s paper onnonwovens for membrane manufacture, but hotgas media are not covered, yet the production ofnanofibres in ceramics, glass or carbon is movingout of the research institutes’ laboratories.

Applications

There are some key applications deserving ofnote: fresh and waste watertreatment, internal air quality, aswell as hot gas filtration. Thesewill all benefit from theavailability of finer filter media,but the Conference highlightsjust two, the downstreamprocessing of biopharmaceuticals,and the filtration of diesel engineexhausts. This latter is anapplication of rapidly growinginterest, vital for improved urbanair quality, and operating at theupper end of the temperaturerange for nonwoven materials.

Testing

Another distinguishable topic atthe Conference is the testing offilters and filter media. Onesession of four papers is devotedto testing, as is one paper inanother session. There is nostrong connection between thesepapers and nonwoven media –

they could have been slotted in to anyconference on filtration – and one paper, indeed,sets out to discuss testing needs for all filtermedia, including woven materials.

Nevertheless, the inclusion of this topic doeslead on to the realisation of how difficult it isbecoming to test the very fine media on whichthis Conference is largely reporting. Very finemedia mean very fine degrees of filtration, i.e.high filtration efficiencies, with very littlematerial getting through, and that extremely fine– so how do you measure it?

To do so accurately, and preferably continuously,is going to tax the makers of analyticalinstruments. •Author:Ken SutherlandNorthdoeTel: +44 (0)1737 218868E: [email protected]

The Filtrex conference will highlight two important areas of filtration – the downstreamprocessing of biopharmaceuticals, and the filtration of diesel engine exhausts. This latter is anapplication of rapidly growing interest, vital for improved urban air quality, and operating at theupper end of the temperature range for nonwoven materials (the engine pictured is theCummins’ ISL engine, selected for 2010 emissions project by the California South Coast AirQuality Management District – SCAQMD).

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