Nonwovens in Filtration: Stuttgart 6th-7th March 2001

The conference was organised by Filter Media Consulting in co-operation with EDANA. Lutz Bergmann of Filter Media Consulting and Peter Meijer of Fiberweb-BBA Nonwovens chaired the sessions. Paul Dewingaerden of EDANA opened the meeting.

The 2000 Top 10

Karin Bitz, Editor of Nonwovens Industry Magazine listed the top 10 nonwoven industry events of 2000 in reverse order as:
• The forging of partnerships in all segments of the industry.
• Globalisation: the roll-goods producers moving into China , Latin America, Eastern Europe and Turkey .
• The growth of the filtration sector, now a $2bn market. INDA's conference and exhibition last year attracted 2400 visitors.
• The growth of internet marketing
• The growth of web-based commerce.
• Expansion at PGI: their link-up with Vateks in Turkey and with Nanhai in China , the $40m investment at North Little Rock in Miratec hydroentanglement technology.
• Expansion at BBA: the Chinese air-lay line, the acquisitions of Snow Filtration and AQF Technologies.
• Air-laid expansions: Buckeye/Walkisoft and their start-up this spring of the 50,000 tonne air-layer.
• Continued ferocious competition, especially in diapers between P&G and K-C.
• The slower growth of established global markets, leading producers to look to the emerging markets.

With regard to this last key issue and the increasing evidence of overcapacity in some technologies she concluded that the discovery of the next “great new nonwoven application” was now somewhat overdue.

Filters on the Web

Brandon Ost, the CEO of eFiltration Inc reminded us that despite the 54% plunge in the Nasdaq index, internet use was still doubling every 100 days and the Web now hosted 100 million business users. The internet economy accounted for one-fifth of US revenue generated in the last 6 months and globally accounted for $657 billion in sales last year. With many more people and companies researching on-line and buying off-line, e-Commerce sites were already having a much larger influence than was suggested by the actual sales figures. Could a website be set up to market and sell industrial filters? Mr Ost clearly thought so. However, the complexity of the product range and the supply chain, and the need for in-depth product and application knowledge, coupled with extensive after sales support meant that the site would be anything but ordinary. EFiltration had helped a small distributor of industrial air filters to set up a website. Their complete range could be displayed and the software allowed customers to configure complex products on-line and order them at “customer-specific” prices. Combined set-up and annual costs to the filter distributor were around one-third of the costs of a salesperson. For a large filter maker with thousands of SKU's, the only practical approach seemed to be to start quickly with a small investment on a flexible system, which could grow “daily” as new features were required and as new e-business tools were developed.

Nanofibres?

Michael Wehmann of JM Laboratories Europe (soon to be renamed to reflect Nordson's ownership) gave the first of several papers to mention nanofibres. In the quest for ever-finer fibres, JM were working with Hills Inc on modified melt blowing processes including the use of splittable bicomponent technology, and on an electro-spinning process in co-operation with Akron University and NC State. Electro-spinning appeared to be melt-blowing with a high voltage between the die and collector. They were also driving down the fibre size obtainable in melt-blowing by optimising air-plenums, forming boxes and quench air systems. A 120 o (rather than 60 o ) die-tip angle was being used to generate turbulent air-flow at the point of extrusion, this having been shown to allow finer fibres with any given polymer. They were also working with the polymer producers to get the cleaner more consistent resins needed to maintain throughput as fibre size diminished.

Finer Meltblown

One of these producers, Borealis Polymers NV ( Belgium ) took up the story, Nancy Noynaert reviewing the benefits of increasing the melt-flow rate of PP to 1200. In addition to finer melt-blown fibres, the new pelletised polymer allows improved electrostatic charging due to reduced peroxide content, better melt, process and fibre uniformity, longer die life, and FDA appoval. It also allows a given level of web air-permeability to be achieved with lower process air volumes. The finer fibres produced were said to allow 95% of all particles below 0.1micron in diameter to be retained on the melt-blown filter. Ms Noynaert also reviewed the current market for melt-blown nonwovens. 120,000 tonnes of meltblown were produced globally last year, 97,000 tonnes of this being PP. Meltblowns were consumed in filtration (26%) hygiene (46%) and oil sorbents (28%) Europe used 37,500 tonnes in 2000 and was expected to require 50-55,000 tonnes in 2004. Asia used 18,400 tonnes and the Americas used 47,100 tonnes.

Melt- versus solution-blown implants

Dr Martin Dauner of ITV Denkendorf compared melt-blowing with solution spinning routes to microfibres, but only in the context of their use with resorbable fibres for medical implants. Both technologies use similar equipment, with a solution of polymer replacing the melt in the case of solution spinning. The dilute solution of polymer is blown onto the former where it bonds due to incomplete removal of the solvent. (A J&J electrostatic spraying process was also mentioned: EP 0095940 and GB 2189758.) The melt-blown process used by ITV has 15 holes and is based on an NC State development. Polymers tested in both processes included poly-l-lactic acid, the d,l copolymer, and a segmented polyurethane. Poly glycolic acid could only be meltblown. Fibres and pore sizes tended to be finer and more uniform with solution spinning. The fact that on average the solvent route gave fibres about one-tenth of the diameter of the meltblown route was partly due to the solutions extruded containing only 2-15% of solids. Illustrations not included in the written text showed the spray-forming of tubular tracheal implants on shaped mandrels, and the creation of an artificial ear by spray forming the shape and then growing skin cells on the resulting fabric. Comparisons of MB, SS, and an Electrostatic Spinning process showed ES with an undefined polymer to give the finest, most regular fibres (down to 0.01 micron diameter with up to 3x variation), but the resulting fabrics were not self supporting. Solution spinning gave fibres down to 0.1 micron with up to 100x variation, while melt blowing gave down to 1 micron fibres with up to 100x variation. In private conversation Mr Dauner agreed to forward a selection of the many slides not included in the written paper. Asked about cellulosics in implants he said the body could not absorb cellulose and always tried to reject it.

Multi-layered Media and Markets

Kyung-Ju Choi of AAF International ( USA ) described experiments to optimise the filtration performance of multi-layered media and provided some data on the world air-filtration market that was not included in the written paper. Multiple layers do not perform as well as calculated from filtration data on the individual layers. He concluded that the layers must be separated for best results. With regard to the market he estimated world air-filter sales to be $826million with the heating, ventilation and air-conditioning (HVAC) filters accounting for 52% of this. Bag House filters on industrial processes were the second biggest category with 24%, automotive was next (cabin-air 6% and engine air 5%). 8% were respirators and face-masks, 3% vacuum cleaner bags and 2% were ultra and high efficiency particulate air filters. The USA used 39% of the total, Europe 33% and Asia 23%. Melt blowing and needling were the growth technologies, wet-lay was declining.

Naturally charged filters

Bernard Drouin of Texel Inc ( Canada ) described how their triboelectric filter (Tribo™) complemented the other routes to efficient electrostatic removal of submicron particles from air streams. Particles below 2.5 microns account for very little inhaled mass but represent 99.5% of the total number of particles inhaled. These are hard to filter conventionally, but can be collected on electrostatically charged surfaces. Charged fibres can be obtained by induction (extruding polymer in an electric field) or by corona discharge treatment of a fibrous web or, as anyone who has worn or walked on synthetic polymers knows, by friction between triboelectrically dissimilar materials. Texel has discovered and patented a blend of polypropylene and polymetaphenylene isophthamide fibres (PCJ/CA 9800470) which produces a large and stable permanent electrostatic charge. In comparison with another well known triboelectric blend (50/50 PP/Modacrylic fibre) the Tribo™ blend with only 25% of the expensive fibre gave better particle retention with less decay over time at a 25% lower air resistance.

Nanofibres!

Nigel Walker of Technical Fibre Products ( UK ) gave the Shrikant Awasthi (Hitco Carbon Composites Inc.) paper on carbon nanofiber-based filter media. TFP has collaborated with Hitco to produce wet-laid filters from carbon nanofibres bonded to standard chopped carbon fibre. The nanofibres are unrelated to the recently discovered “Buckytubes” and are already produced on an “industrial” scale by growing them in a burning gas (e.g. acetylene) stream. They appear as soot, but at high magnification they are revealed as a crimped or spiral fibrous structure with diameters in the 0.1 to 0.2 micron range. When wet laid with regular carbon fibre they can produce filters with the particle removal efficiency of membranes but at 35 times the permeability of a PVDF membrane of similar micron rating. Applications under development include clarifying beers and fruit juices (replacing diatomaceous earth) and the microfiltration of industrial process and waste waters.

Bio-soluble glass fibre

Johns-Manville has developed a glass fibre which is classed as no more than an irritant in the latest European air-filter regulations. Foster Harding explained that this has been achieved by optimising the formulation to increase bio-solubility without affecting product performance (USP 5945360). High silica and low aluminium oxide content appear to the key changes. EU biopersistence testing showed that after rats breathed airborne 902 glass fibres for 5 days, 50% of inhaled fibres longer than 20 microns cleared in 6.8 days. Since the test threshold between “irritant” and “carcinogen” is 10 days, the new fibre gets into the irritant category. In response to questions the new glass fibre was said to be unsuitable for wet-laying. While finer polyester fibres were challenging glass in some markets, Mr Harding said that there is little comparable data on the effects of synthetic fibres in lungs, the crucial “injection” studies never having been done.

Molecular contamination

The semiconductor industry has successfully removed particulate contamination from clean-rooms by using HEPA/ULPA filters, so emphasis is now shifting to control of molecular contaminants. Matthew Middlebrooks of AQF Technologies/BBA Nonwovens ( USA ) listed the problem molecules as Acids and Bases (electron acceptors and donors), Condensables (substances which could condense on a clean surface) and Dopants (elements used to modify the semiconductor properties). AQF's active carbon-loaded through-air bonded bico-fibre nonwovens were capable of absorbing examples of all of these substances. Acids were represented by sulphur dioxide and hydrogen chloride, Bases by ammonia, Condensables by toluene and Dopants by trimethyl phosphate and boron trifluoride. Acid or base adsorption could be enhanced by pre-impregnating the carbon with the bases or acids respectively, although in both cases the condensables and dopants absorption decreased. Longer term, Mr Middlebrooks foresaw the technology being used to control odours in cars and in hotel and airport HVAC systems. (Substituting SAP granules for the carbon would make an excellent absorbent)

Engine filtration

Robert Murphy of Hollingsworth and Vose ( USA ) compared advances in engine filtration, US versus Europe . In Europe, filter lives were being extended past 60,000km (air) and 30,000km (oil) as part of the attempt to keep motoring affordable in a high tax, high gas-price environment. Frequent servicing is however common in the USA and higher efficiency filters have been developed in preference to longer-lasting ones. Europe continues to prefer the low cost phenolic resin bonded filters despite the need for curing and the associated formaldehyde emission. In the USA , solvent-based advanced-cure resins already obviate the need for curing, reduce the time-in-process and protect the users from future capital costs to upgrade curing ovens for environmental compliance. Mr Murphy argued that the advanced cure system would inevitably replace the phenolics as environmental regulations tightened and the increasingly global motor industry demanded identical filters wherever they were fitted. Other developments to note: the gasketless Japanese filters that seal through compressed filter-media, and the all-plastic European Eco oil-filter that can be incinerated whole.

Developing engines to meet ever-reducing emission targets requires radical redesign of both petrol and diesel engines. For diesel, a key step is the use of direct fuel injection through a common rail at very high pressures, and for this the fuel needs to be filtered to remove particles above 2 microns at very high efficiency. Roberto Forna reported that Ahlstrom has achieved this by laminating a meltblown polyester nonwoven upstream of a conventional phenolic coated woodpulp filter paper. The new composite media holds 5 times the contaminant and has an initial efficiency of 90% for 2 micron particles.

Hot-gas filtration I

Control of particulate emissions from incinerators involves the use of cyclones, electrostatic precipitators, wet-scrubbers and fabric filters. Andrew Startin of Madison Filter Ltd. ( UK ) argued that their low-density ceramic fibre cartridge filters offer some significant benefits over fabric filters where the gas stream is at elevated temperatures and where consistent low emissions are mandated. Data was provided from installations of ceramic filters in clinical and chemical waste incinerators where sodium bicarbonate sorbent was added to the gas stream to control acids. They were also being tested by Peugeot as filters on diesel exhausts where the operating temperature was 700-800 0 C.

In a second paper from Madison Filter, Richard Lydon explained how their new Tuftex™ filters are coated to improve abrasion resistance and cake release. The polymer coating is supplied on a release paper and attached to a polyester or polypropylene needlefelt filter through a nip roll. The polymer reticulates in a controlled manner during curing to leave a tough porous surface which appeared to be about 50% covered in film. This nearly doubled the Martindale abrasion resistance and stops any fibrillation if the substrate was PP. In response to questions the coating would double the cost of the filter but would more than double its useful life.

It's an ill wind…

Karl Ott of BASF ( Germany ) gave a remarkably candid account of the “miserable” launch failure of their Basofil™ flame resistant melamine fibre in hot-gas filtration. Having in retrospect chosen the wrong partner, the first trials on this relatively weak fibre were conducted on needlefelt bag-house filters without scrim reinforcement and after a singeing process which had predictably little effect on improving its surface. Their filter-producing partner turned out to be more interested in establishing business for his other filters within BASF than developing Basofil™ and while Mr Ott did not say so, we guessed he failed at this objective also. Convinced of the merits of Basofil™, made “from a condensation reaction of methylol to form a 3D structure”, BASF reanalysed the market at targeted the customers of the leading supplier of aramids. Data from recent bag-house trials at Brox Industries asphalt plant and at LTV steel in the USA showed long filter lives, low pressure drops and excellent emission values compared with the aramid controls. In concluding Mr Ott commented that one of the biggest factors in the successful relaunch was a predictably unfavourable comparison with aramid in the competitor's brochure. The fact that the market leader found Basofil™ worthy of comparison with his aramid created intense interest in the new fibre.

Pleated bags

As particle emission regulations tighten the need to fit more filter area into existing bag-houses increases. Michael Konesky of Southern Felt Co. Inc. ( USA ) described how their pleated filter elements - Pleatlox™ - could be retrofitted to older installations to increase their capacity. In addition to the increased area obtained by pleating the cylindrical filters, the new bags could be cleaned at lower pressures and exhibited reduced abrasion damage.

Hot gas filtration II

Holga Blaha of BWF Textil GmbH and Co. KG ( Germany ) reviewed the choices available for removing dust from power station flue gases. The fibres to consider were acrylic, polyphenylenesulphide, polyimide and PTFE. However the filter fabrics would need surface treatments designed to suit the nature of the dust being removed. Singed surfaces were appropriate for non-agglomerating dusts whereas an agglomerating dust would need a glazed filter surface. PTFE coating and even impregnation would be needed for protection against chemicals, sparks and humidity. For instance, their Needlona® PAN fibre filter impregnated with fluorocarbon would give a 3 year life in a combined heat and power plant with flue gas desulphurisation after spray absorbtion whereas the more expensive polyphenylene sulphide felt with a microporous coating would last for 6 years in the same process.

Surface or in-depth?

Pulse cleaning of bag filters causes a peak in emissions to atmosphere associated with finesbeing dislodged from the depth of the filter and the need for a new cake to form on the surface prior to full efficiency being regained. One solution to this, proposed by Klaus Schumann of Albany International Inc (France) was to increase surface filtration (over depth filtration) by laminating a PTFE membrane or microfibre web to the surface of the needlefelt. Compared with a 1.7 dtex round fibre, a fully-split 16 segment pie-fibre would give 10 times the surface area. Furthermore when the PA and PES polymers separate they generate a natural electostatic charge.

Filtering fumes

Günter Weitz of Jacob Handte and Co GmbH ( Germany ) showed SEM's of the welding “fume” from laser welding and cutting of steel. This dust of submicron (~0.2) spheres of metal oxide blinds almost any filter unit very quickly and can be ignited by sparks from the laser process. A laser cutter for steel plate can generate about 100 kgs/day of such dust, so designing filters for these machines represents a real challenge to the filter producer. The solution employed by Handte involved a PTFE membrane on a PES bico needlefelt, following a spark-precipitator cyclone. The unit is fully automatic, self-cleaning, maintenance free and gives long filter lives.

Waste water filters

• Plate and frame filters clothed with textiles were first developed in 1830 for filtering liquid effluent from the ceramics industry. Fully automatic versions of these are still the main filter type for sewage sludge and industrial process and waste waters. Hubertus Schütt of Junker Filter GmbH reviewed the types of cloths and membranes now available. He showed how the use of membranes in filter presses now offers the best solution to industrial waste water cleaning by reference to trials at a Hoechst AG plant.

Calvin Woodings 14/3/01