Tuesday, 31 October 2006

EDANA Filtrex: 24th and 25th October 2006: Munich

Key Points • Nanofibers are becoming more readily available and their use in a wider range of filters is beginning to be explored.
• Nanofiber webs can retain up to 10% of the solvents used to dissolve the polymers for electrospinning.
• Nanoval GmbH claims to produce 10-15 gms of submicron fibers per minute per 0.6mm spinneret hole and to require less than half of the energy of meltblowing.
• Centrifugal spinning produces fibers in the 100 to 1000nm range from polymer solutions at 500 times the throughput of electrospinning. A 1.5m pilot line with 24 heads is under construction.
• The need for pure potable water is increasing the demand for desalination plants and hence the demand for reverse osmosis membranes.
• Biopharmaceutical - especially antibody - production is growing faster than pharmaceutical production and these processes need filters or membranes to separate the cells from the antibodies.
• 0.08 gsm nanofibre layers are equivalent to 10 gsm of fine-fiber meltblown for improving filters.
• The adverse health effects of diesel soot are leading the EU to consider mandating the use of diesel exhaust filters for vehicles used in cities.


Lutz Bergmann of Filter Media Consulting (USA) opened the meeting with a breakdown of the major market segments…
• Commercial Heating, ventilation and air-conditioning (HVAC) worth $450-$600 million at the filter level (not roll goods) in the USA .
• Residential HVAC worth $450-$500 million in the USA .
• High and Ultra efficient air filters (HEPA/ULPA) worth $30 million in the USA .
• Face masks and respirators
• Vacuum cleaner bags
• Automotive filters (engine intake, exhaust air and cabin air, fuel and oil)

…and a breakdown of the technologies used to satisfy the market:

• High loft
• Melt-blown and composites
• Glass fibre webs and blankets
• Spunbond
• Membranes (e.g. PTFE)
• Nanofiber coated webs.
Market size estimates are plagued with inadequately precise definitions. The US market could be worth from as little as $300 million at the filter level to $4 billion at the system level.
We are now seeing the emergence of nanofibre coatings on nonwovens as real alternatives to PTFE membranes, giving similar filtration with higher permeability. These coatings can be made by:
• Flame-blowing of glass
• Electrospinning
• Splitting sea-island bico fibers .
• Modified melt-blowing
• Hydrostatic splitting of stressed films
• Wetlaying ceramic or carbon whiskers.
In fact, nanofibers are now becoming readily available and their use in a wide range of filtration products is beginning to be explored. The nanofibre market was currently worth about $8-10million and the productivity of commercial electrospinning units had reached about 15 kgs/hour.

Antimicrobial indoor air filters

Alain Langerock of Devan Chemicals ( Belgium ) made a case for using the Aegis non-migrating antimicrobial additives on fibers destined for indoor air filtration on the grounds that these filters provide ideal conditions for microbial growth. 3-(trihydroxysilyl) propyldimethyloctadecyl ammonium chloride bonds to the fiber surface by the silyl end leaving the positively charged nitrogen and the aliphatic chain free to kill micro-organisms by attraction and “stabbing”. A treated fibre has ~25,000 of these aliphatic “swords” available for each organism. The mechanism of coating was not given, but the fiber surface had to be reactive and the coating, a monomolecular layer, would henceforth be described as a nano-coating.
ASHRAE 52-76 testing of filters with the Aegis Micro-Shield coat show either reduced pressure drop or higher particle loadings at the same pressure drop: i.e. it increases filter efficiency while reducing the microbial content of the exhaust air to 50% of that of the inlet air on a clean filter. Once soiled, the reduction of microbial content in the exhaust reached 86%. In field trials at St Lukes Medical Center in Phoenix Arizona , the treated air filters reduced the levels of 3 atmospheric bacteria by 70% and completely destroyed aspergillus niger fungal spores. At Ohio State University Cancer Hospital , pre-filters, bag filters and post-chiller bag filters were treated with Aegis. The treated bags showed much reduced growth and gave lower pressure drops than the untreated bags while reducing the particle content of the exhaust air to a third of the untreated filter outlet levels. Conclusion: Aegis controls mildew and hence reduces the number of spores entering the hospital. Asked what type of fibers were used to make the bag filters, Mr Langerock said they were synthetic, not glass.

Synthetics v Glass ASHRAE media

Norman Lifshutz of Hollingsworth and Vose (USA) described how the ASHRAE bag or pocket filters, originally made out of resin-bonded microglass were being replaced by electret-charged meltblown PP, and this was now leading to concerns over the effects of electret discharge. Electrets are just dipoles. No overall charge is detectable on the fabric, but a separation of +ve and –ve charges at the micron level causes them to attract particles. They are created by corona discharge. Needling triboelectrically different fibers also creates a charged fabric. The charges are neutralized by ionizing radiation or particle capture and are especially sensitive to oily mists such as diesel fumes or tobacco smoke. Shelf life is nevertheless very good even in high humidity.
The fact that the charge level is not measurable directly means that the media has to be tested in the charged and discharged states to assess its efficiency. Discharge is achieved with iso-propyl alcohol (IPA) in some standards and potassium chloride in others. The initial efficiency of electretted meltblown is significantly higher than microglass, but after discharge, the best “nanomelt” meltblown webs prove no better than microglass.
Were the meltblown webs damaged by the IPA discharging treatment? Mr Lifshutz said they were not, adding that the IPA had to be 99% pure, the normal 75% IPA having no discharging effect. It was possible that the 1% impurity in the IPA was responsible for the discharge, not the IPA itself. The new nanomeltblowns cost more than the microglass.

HEPA Filters, charged and uncharged

Peter Tsai of the University of Tennessee Knoxville (USA) pointed out that filters do not sieve out the particles, they remove them because they collide with and stick to the fiber surface. Electretting works by increasing the collisions by attraction, and electrospun nanofiber webs are naturally charged by the spinning process. Comparisons of charged (ECMB) and uncharged meltblowns with microglass has yielded the following conclusions.
• ECMB can achieve the HEPA efficiency at much lower pressure drop than other HEPA media.
• ECMB had lower filtration efficiency (FE) when challenged by DOP (di-octyl phthalate) particles.
• ECMB performance with DOP can be improved by using the Tantret T-II electetting method or by using higher basis weights.
• The FE and the pressure drop of glass fiber paper was stable to DOP loading but both the FE and the pressure drop increased with the NaCl loading.
• For electrospun PTFE membranes the FE decreased and the pressure drop increased on loading with oily particles, but both the FE and the pressure drop increased on loading with NaCl particles.
So, glass filters remain the best for oily particles. Mr Tsai commented that the electrospun PTFE was not charged. He does not know why but he is trying to solve the problem. Furthermore he is developing a way to electret heavy basis weight fabrics. Heat treatment has no effect on charging.

Electrospun Media for Air Filtration

Jonathan George of Finetex Technology Inc (France) defined nanotechnology as engineering at scales below 100nm. Sizes between 100 and 1000 nm were “submicron” and so with a little license, fibers in the lower half of the submicron range could be regarded as nanofibers. This still excluded the finest meltblowns (650 nm) but included flame-blown microglass at 150 nm. Polymer choice was key for many filtration applications, and while the polymer properties were the same at nanoscale, the kinetics of any physical or chemical changes would be altered due to their very high surface area. Degradation would occur more quickly and the positioning of the nanofiber layer in the filter was very important. Used on the inlet face it would clog quickly and on the outlet side it could easily be disturbed (e.g. by vibrations) and shed contamination.
The next generation of nanofiber layers would be stronger and made at higher basis weights and widths. Finetex has a 10 gsm target and intends to use these in HEPA and ULPA products.

Cancellation of a later paper allowed more time for questions:
• Finetex make webs with between 50 and 500 nm fibers.
• Mr George would not say what range of polymers was available but it included nylons, polysulphones and polyurethanes.
• Finetex electrospun webs are electrically charged.
• The nanofiber web is predominantly a mechanical filter.
• The webs can contain as much as 10% solvent which is hard to remove.
• Nanofiber strength is immeasurable. However one member of the audience claimed he had measured nylon-6 nanofibers which were stronger than Kevlar. Typically however they were very weak.
Mr George was unaware of any studies of the health risks of inhalation of nanofiber dust. He felt the solvents were the main problem. The process produced no fly and if a single fiber does escape collection it tends to coil up into particles.

Nanovlisz® - Meltblown Nanofiber?

Lüder Gerking of Nanoval GmbH & Co KG ( Germany ) claimed that in his new Nanovlisz® process a single 0.6mm die hole surrounded by a concentric 4mm diameter Laval nozzle could produce 10 gms/min of PP nanofibers and 15 gms/min of titania-dulled PET nanofibers. The small fiber size arose by splitting of the single filament into numerous microfibers as it cooled and expanded on leaving the Laval nozzle. (These fibers had a mean diameter of 0.9 microns so by the previous speakers definition they were only just “submicron”.) Furthermore he claimed energy usage of one-third to one-half that of conventional meltblown due to the ability to use ambient air at only 2 bar. So, his equipment could use fewer, more widely spaced holes (easy engineering) and it could also use normal fiber-making melt flow polymers, not the very high MFI's required by conventional meltblown. In practice, the machine could produce 20-30 kgs/hour/metre.

Lyocell spunlaid webs have been made using only 0.6 bar air pressure and this system could process 2.7 gms/hole/minute of dope (10% cellulose) into 5 micron average diameter cellulose fiber.
Asked why the micrographs showed round-section filaments after splitting, Dr Gerking explained that the splitting occurs while the interior of the filament is still molten. Fiber size variation gets less as the original fiber diameter decreases. Practically, all filaments are continuous and with the high orientation, the properties are more akin to spunbond than meltblown. PLA polymer has been processed successfully.

Centrifugal Spinning

Martin Dauner, Head of Nonwovens Technology at ITV Denkendorf ( Germany ) , the Moderator of this session, stepped in to give this unlisted paper when the second of the session's speakers cancelled. In the ITV process a 300-500 mm “center bell” spinning horizontally at 15,000 to 50,000 rpm and fed with a polymer solution flings nanofibers radially in all directions. An airstream (1 to 3 bar) and a strong electric field (0.5-1 kV/cm) guides the fibers 100-1000nm fibers onto a vertical collection belt 0.3 to 0.5 metres away.

Examples illustrated in photographs were:
• 5% polyethylene oxide in water converted at 3 ccs/min into 100-300nm fibers.
• 7.5% PEO solutions in water also converted at 3 ccs/min into proportionately coarser fibers.
• Ditto, 15% PEO in DMAC/DMF.
• Polyurethane, polyacrylonitrile, and polyvinyl alcohol had also been spun.
Claimed advantages of the centrifugal system were the ability to handle non-melt processable materials, e.g biopolymers and heat sensitive polymers. However polymers had to be in solution in solvents which were sometimes toxic and/or explosive. Compared with electrospinning, the throughput was about 500 times greater, but it was still only about one-sixth of meltblowing productivity. Nevertheless, ITV had a client interested in scaling-up to 1.5 metres wide on a line which would use 24 center bells to produce nanofiber webs at 7 m/min.

Dr Dauner thought centrifugal spinning would ultimately replace Electrospinning. Asked how the uniformity of web would look when fed from 24 overlapping bells, he commented that the technology was well established in the paint-spraying field where multiple bells were commonly used. Asked privately if the process could be used to make coarser fibres at much higher productivity, he thought it could produce 10 micron filaments at very high throughput.

Nonwovens for Membrane Reinforcement

Thomas Miotk of Freudenberg Vliesstoffe KG ( Germany ) listed the uses of membranes in purification processes:

• Micro-, ultra- and nano-filtration (down to 100nm)
• Reverse Osmosis (down to 1 nm)
• Gas separation
• Pervaporation (membrane permeation followed by evaporation)
• Dialysis (esp. blood)
• Electrodialysis.
Purification of water for drinking and for semiconductor production is becoming more important both, so desalination of sea water is a growth area. One desalination plant in Israel uses 2 million m 2 of membranes. The membranes are made by casting a polymer film onto a nonwoven. The nonwovens used must be very uniform in weight and thickness and must have a very smooth surface with a complete absence of vertical fibers. Pore size distribution variability must be minimal and the nonwoven must have sufficient mechanical strength, thermal and chemical stability for the end-use. For water purification, it must also meet FDA standards. Reemay® polyester spunbond used to be the substrate of choice but calendered wetlaid polyester now achieves the highest uniformity and is preferred. Nonwovens with a +ve zeta potential, made from surface treated poly- butylterephthalate (PBT – Novatexx®) were especially good for their selectivity for molecules with a negative charge.
Asked how big the market for such membranes was, Mr Miotk said that information was available but confidential to Freudenberg. How much variability in nonwoven uniformity was allowed? +-3%.

Nonwovens for biopharmaceutical processes

Dr Loewe replaced Dr Lausch for this talk from Sartorius AG ( Germany ) . He said biopharmaceuticals are now growing faster than pharmaceuticals, and the main action is in antibody production; antibodies now accounting for 2/3rds of all biopharmaceuticals at the pre-clinical testing phase. Production processes use animal or plant cell cultures (not chemistry) and while US production is only about 6 tons per year, it is worth $18 billion. Nonwovens are used as filters to remove the cells once they've completed their work.

The European market for such cell harvesting and clarification filters was put at €200million and comprised:
• Cellulose paper loaded with diatomaceous earth (55% of the market, but these depth-filters had problems with possible contamination with endotoxins arising from the water used.)
• Wet-laid glass (12% of the market, but these filters also had binder-leaching problems)
• Meltblown (33% of the market, but these could not match the pore size distribution of glass)

Extruded PP membranes made using gas injection into the melt to produce an open-cell foam structure with <50 micron pores, were looking to be a promising new contender for this market and would do well if the pore size and its variation could be further reduced. Given surface activation allowing them to capture antibodies from unfiltered broth, these membranes could be an ideal alternative to filtration. The antibodies would then be harvested from the membrane by elution with saline.
Asked how long it could take to develop and qualify a new filter or membrane for this market, Dr Loewe thought 5 to 10 years should be allowed.

Automotive Air Filtration

Maura Roperto of Ahstrom Turin SpA ( Italy ) has been adding fine meltblown and nano-fiber layers to wet-laid air-filter media and measuring their effects on initial efficiency and dust holding capacity using NaCl, DEHS ( Di-Ethyl-Hexyl-Sebacate) and ISO-fine dust contaminants. Fine fiber layers were single or double layers of 50 gsm PP meltblown, and the nano-fiber layers were nylon 6 electospun to 0.08, 0.16, and 0.31 gsm. The fine fiber layers were added to the inlet face of the filter and the nanofiber layers to the exit side which was also the “wire-side” of the wet-laid nonwoven, - a HEPA grade microglass at of 80-90gsm.
In all cases filtration efficiency was improved by adding the fine/nano layers and the 0.08 gsm nanolayer was shown to be equivalent to 10 gsm of the fine MB PP. The nano-coatings reduced the rate of pressure build up during the filtration process, this indicating that improved filter lives would be possible.
Asked why the filter lives would be longer, Ms Roperto thought the nanolayer improved surface filtration rather than depth filtration. How were the nanofiber basis weights measured? They were calculated from polymer throughput. Ms Roperto thought direct measurement was impossible, but one member of the audience claimed to be able to do this. Would the nanolayer lead to a cleanable HEPA filter? No. HEPA is usually used for aerosols, not dust.

Diesel engine exhaust filtration

Andreas Mayer of TTM ( Switzerland ) said all internal combustion engines emit 20-300 nm particles, and diesel soot, being invisible, odorless and tasteless is particularly dangerous. However some of it does tend to agglomerate up to round 300 nm and at this size is more easily dealt with. 78nm polystyrene particles have been shown to pass easily through cell walls. 100nm soot particles, when inhaled, pass through the alveoli into the blood and through the blood/brain barrier into the brain. They are now being implicated in Alzheimers and Parkinsons diseases. A US study correlating the air particulates with life expectancy (15,000 cases) shows that those living in areas with 10 micrograms of nano-particles per cubic metre have a 90 year life expectancy while those living in areas with three times this level have a 65 year life expectancy. In fact weighing the particles (rather than counting them) underestimates the problem because the smallest are the most dangerous.
Internal combustion engines liberate 10 billion particles per liter of exhaust gas but these can be caught and mostly converted to carbon dioxide by catalytic-coated ceramic filters which are continuously regenerated due to their high operating temperature. Some of the particles are inorganic (from oil additives and engine wear) and these will ultimately block the filter. These filters were initially developed for diesel engines used in tunneling, but they are now being tested on 10,000 vehicles in Switzerland , where they are proving to be capable of functioning for the life of the engine. The costs are high - $50/kw of engine power or about $5000 per car, but they replace the silencer and the reductions in health care costs for city-dwellers means that the benefit is likely to be 5-10 times the costs. Air quality leaving the exhaust after filtration is better than the air-quality entering the engine, so vehicles fitted with these filters will do more than their fair share of cleaning the air in our cities. The EU is proposing to mandate the retrofitting of such filters to all diesels used in cities in the next few years. The filters are made of silicon carbide and operate between 500 and 1500 o C.
Asked if petrol engines would need filters, Mr Mayer said these emitted between a tenth and a fiftieth of the particles of a diesel engine when working properly, but were just as bad as diesel when out of tune. However most petrol engines now had catalytic converters and fitting additional filtration would be easy. Aero-engines were the biggest problem and here there was no solution as long as they used petroleum products for fuel.

Separating water from diesel fuel

Christophe Peuchot, MD of the Institute of Filtration and Techniques of Separation ( France ) described methods being developed to test the water separation efficiency of diesel fuel filters. (Traces of water are not a problem per se, but rust particles arising from it can block the fine injectors now used.)
Laser diffraction was used to assess the size distribution of water droplets in actual fuel pumping systems, and this method was validated using standard glass spheres in fuel and microscopy. The method showed water initially occurring as 40-180 micron droplets, with a D 50 of 105 microns. However on suction systems (fuel pump after filter), the average size increased to 300 microns, and on pressure systems (filter after pump) the size dropped to 60 microns. For laboratory testing purposes, a system which creates droplets in these size ranges in a test fuel has been developed. The method will be published in a new international standard in 2007 – ISO16332.
Asked how much water we buy with our fuel, Mr Peuchot said it was about 0.2% at the fuel depot, but condensation of humid air in the vehicle fuel tank increases this in some climates.

An ISO standard for General Ventilation Air Filters

Rolf Homburg of VTT Technical Research Center (Finland) reviewed the progress with ISO/CD 21220, based on EN 779:2002, and intended to be the single worldwide test for general ventilation air filters. After initial testing (Pressure Drop, Air Flow Rate and Initial Efficiency using a 0.3 to 3 micron DEHS or equivalent aerosol) the filters would be conditioned (to simulate real life conditions) and any electrets discharged with IPA, prior to loading with ISO 12103-1 A2 fine dust at 140 mg/m 3 in air. For coarse filters total dust capacity and arrestance up to 250 Pa pressure drop would be reported. For fine filters efficiency curves would be reported using both 0.3-3 and 0.4-2.5 micron dusts, along with total dust capacities at 250 and 375 Pa. Filter classification was excluded and this would remain the task of national bodies. In parallel with this ISO work, EN779:2002 was being revised, particle shedding testing was being subjected to a round robin test and a new air-conditioning test method was being developed.

A new Fractional Efficiency Tester

Sven Schütz of Palas GmbH ( Germany ) replaced Martin Schmidt to describe the problems arising when using light scattering techniques to measure particle size distribution before and after a filter. One counter alternating between upstream and downstream sampling points meant that short term variations in the upstream particle content could not be allowed for, and 2 independent counters would be expensive and would need calibrating against each other for meaningful results to be obtained. Using tubing to run the fluids from the sampling points to a single instrument caused “particle loss in tubing” errors, so the Palas method had two optic fibers channeling light from a single light source to each sampling point, and two more optic fibers to channel the scattered light back to a single photomultiplier. This allowed “quasi-simultaneous” particle counting in both inlet and outlet flows. Furthermore their patented “T-Aperture” technology meant they could eliminate the two other errors of particle sizing and counting, the border-zone error and the coincidence error.

Vacuum Plasma Treatment

Marc Pauwels of Europlasma N.V. ( Belgium ) replaced Paul Lippens to describe the evolution of vacuum plasma treatment (VPT) from a way of cleaning printed circuit boards through general plastics activation to a way of polymerizing gases onto surfaces.
• Oxygen can now be used to add carbonyl functionality to the surface of polypropylene thereby increasing its wettability.
• Fluorocarbon gases increase hydrophobicity or add oleophilic character and can now make a polyester needlefelt totally hydrophobic for use in automotive filtration.
• VPT can also be used to electret meltblown for face-masks where a single treatment converts a 95% removal product to a 99% removal product.
• NiMH batteries now use wettable PP/PE separators obtained by VPT.

In the last example, alkenes react with oxygen to form a coat of polyethyleneoxide-like polymer under VPT and this increases the surface energy of the polyolefin from ~30 to ~70 dynes/cm.

VPT is carried out roll-to-roll at about 5 m/min in massive vacuum chambers. The total cost is about €0.05/m 2 , or €0.01/m 2 for more simple hydrophobic treatments which can run up to 10 times faster. Does it work on glass? No. Does it improve the hydrolytic stability of polyester? No. How does it compare with atmospheric plasma treatment? Mr Pauwels said APT is used to graft materials onto a polymer surface, whereas VPT can also polymerize them.

Green Binders for Nonwovens

Barry Weinstein of Rohm and Haas (USA) promoted their Aquaset® acrylic thermoset binders as replacements for the urea/formaldehyde thermosets typically used on automotive filters:

• Aquaset® is an environmentally benign formulation using a phosphorous catalyst which achieves high degrees of polyester crosslinking.
• The catalyst (sodium hypophosphate) is also the chain transfer agent. It stays in the product as part of the polymer backbone, and bleaches it.
• Hot wet tensiles are excellent.
• The binder is stageable, i.e. it can be applied to a nonwoven, shipped to the converter and later cured after (say) pleating.
• No ammonia, formaldehyde, phenol or methanol evolves in curing. The only emission from Aquaset® curing is water.
• Curing can be monitored using a bromophenol blue indicator.

Asked what happens to the water liberated in the reaction, Mr Weinstein said it was not a discharge because it was recycled within the process.

Spunlaced Filter Media

André Lang of Jacob Holm Industries GmbH ( Germany ) has been developing scrim-reinforced spunlaced nonwovens as replacements for needled products in hot gas filtration. Spunlacing gives the following advantages:

• 10-15% less fiber is needed to obtain the same strength
• Reinforcing scrims are not damaged by spunlacing
• More uniform pore size distribution
• Higher permeability with the same filtration efficiency
• No dust channeling through needle holes
• Improved dust holding capacity
• Filter cloths can be made up to 1000 gsm (including scrim).
• Spunlacing can also be used on needlefelts to improve their uniformity.

Filtration data from a 650 gsm PTFE fiber spunlaced filter during several cycles of filtering and cleaning was presented. Technical details on the manufacture of the nonwoven were not provided, and the control fabric appeared to be a 500 gsm needled polyester.

Hycoknit® Gas Filters

Dr Elke Schmalz of the Sächsisches Textilforschungsinstitut e.V Chemnitz ( Germany ) described new routes to air filtration media. In the first a highly voluminous stitch bonded pile fabric with high dust holding capacity is laminated to a spunlaced backing which provides a high filtration efficiency. For removal of finer particles, the pile side of the stitchbonded layer can also be hydroentangled, and if this layer is made of splittable bico fibers, then even higher efficiencies can be obtained by splitting the fibers in hydroentanglement. When used in pocket filters which are automatically cleaned when the pressure reaches a predetermined level, Hycoknit® materials give much longer cycle times compared with the equivalent weight of needlefelt. They also give much reduced cleaned-gas dust levels. Asked about the price of these new filtercloths, Ms Schmaltz said the PET versions were only slightly more expensive than the PET needlefelts.

Testing Liquid Filters

Richard Wakeman of Loughborough University (UK) provided and overview of filter media and the ways of testing them developed by the national and international standards organization. For the future he thought pore size calibration using a sonic sifting device (The Gilsonic Autosiever”) which fluidizes glass microspheres using a 60 Hz vibration, rather than shaking the filter would give the most accurate challenge tests. Had porometry been correlated with challenge testing? No, but sonic-sifting versus PMI porometry had, and the agreement was good.

Modelling Filters

Prof Behnam Pourdeyhimi of NCRC (USA) provided a comprehensive review of filtration mechanisms and theory prior to describing their current work on modeling fiber lay down, calendering and the resulting nonwoven permeability. This was also covered in Sabrina Zobel's paper at INTC (Sept 2006)

Calvin Woodings – 1st November 2006

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