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

Saturday, 7 October 2006

INTC, Houston , Sept 25th – 28th 2006

Key Points

• Peptides have biocidal activity against bacteria and fungi in microgram quantities and are safe for the environment.
• Antimicrobial filter layers have been made by electrospinning inherently antimicrobial polymers.
• Copper containing nonwoven fabrics possess potent antibacterial, antifungal and antiviral properties which are unaffected by washing.
• The simulation of nonwoven geometry, the fluid-flow associated with that geometry, and particle deposition on the fibers, makes feasible the virtual design of a filter.
• The deviation of the surface coefficient of friction correlated best with the subjective assessment of handle of diaper and pantiliner topsheets.
• Nylon filaments with a diameter of 360 nm are liberated when a 360 island/sea fiber with a 25/75 nylon/PLA ratio has the PLA sea removed. They have high tenacity (5 gf/denier) and modulus (140 gf/den) and nonwovens made from them have very high tear strength.
• Nordson's bico melt-blow process is giving webs with an average fiber diameter of 0.75 micron, yielding double the usual hydrohead.

  This years INTC, excluding the student papers and the tutorial session, comprised 74 papers in 15 separate sessions arranged in 3 concurrent streams. Jointly organised by INDA and TAPPI, this meeting had a “research academy” feel - like the old TAPPI conferences - 42 of the papers being from universities or research institutes. Only the 25 papers which could be attended are summarised here.

Charged Nonwoven Materials

Kyung-Ju Choi, AAF International said the point of electrically charging filters was to achieve higher efficiency without increasing air resistance. The common ways of charging were:

• Corona discharge
• Triboelectric charging
• Induction charging (as in electrospinning)

He has characterised the structure and thermal properties of charged nonwovens from homo and hetero-polymers using optical microscopy, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The charge distribution was checked with an electrostatic fieldmeter and filtration efficiency and resistance measured.

He concluded:

• Fiber orientation was random for dry-laid, wet-laid and spunbond nonwovens except for non-crosslapped dry lay.
• Fiber sizes ranged from 9.5 to 40 micrometers for dry-laid and wet-laid nonwovens.
• The residual static voltages on a polypropylene meltblown web ranged from -0.38 to +0.53KV.
• These static voltages moved around slowly with slight changes of maximum and minimum voltages.
• The filtration efficiency of polypropylene meltblown nonwovens increased with increasing voltage applied.
• The filtration efficiency of charged polyester spunbond, dry-laid, and wet-laid materials doubled c.f. the controls but the efficiency dropped completely in 3 days.
• PP spunbond loses charge during the first day but is then stable.
• Electrospun PTFE holds its charge for a month.

Can the microstructure of charges be measured? Not yet – he's working on it.

Feather Fiber in Filters

Roy Broughton replaced Xiuling Fan of Auburn University to present this paper. The USA produces 2-3 billion lbs (dry weight) of dirty chicken feathers annually and these are usually fed to chickens because they are not easily disposed of in other ways. However the US government may follow the EU and ban this practice so funds are available to find other uses for these feathers. Possibilities include insulation, reinforcement, papermaking and protein fiber production. AU is cleaning them, stripping out the coarse backbone and air-laying a polyester blend of the small branch feathers on a scrim prior to needlepunching using 4 passes to give a total of 1000 punches per inch. Fabric physical and filtration properties were measured.

Although feather fiber could be recycled into air-filter fabrics, its fineness and the tree/fan-like structure of the feather did not offer a high level of performance advantages over conventional fibers. The use of feather fiber in air filtration applications must rely primarily on a favorable cost and weight differential in favor of the feather fiber, assuming there is one after the cleaning and dequilling process. Prof Broughton alluded to a chicken and egg situation. No one was prepared to clean up the feathers until a market had been found, and vice versa. Cleaning would yield about 40% of nonwoven-processable feather, so there would still be nearly a couple of billion pounds of waste and its dirty water to dispose of.

Aerosol Filter Tests

Sven Schütz, Palas GmbH (Germany ) reviewed round robin testing of car cabin air filters in 6 labs in the USA and Europe during 1999-2004 using ISO test methods (A2 Dust Test and DIS111155-1 KCl test). Electreted and electreted-discharged filters were included in Oct 2004. The results were “disillusioning”, showing deviations between labs of 50-75%. Mr Schütz showed that the while the test stands could deliver reproducible results, the aerosol generators and the detectors used after the filters needed improvement. A future standard (ISO 21501) describes both the aerosol spectrometer detection method and the clean-room particle counter method. The latter is suspect because it is designed for use with very low particle levels, much lower than those obtained downstream of a cabin air filter.

Nanofibers for Defence

Seshastri Ramkumar of Texas Tech University has been applying nanofibers in the area of national defense and value-added products such as decontamination wipes, chemical protective clothing and highly efficient filters. Polyurethane and polyethylene oxide nanofibers in the range of 50 to 300 nm were electrospun and the first known self-assembling phenomenon was observed with the PU spun from 50/50 DMF/THF. These honeycomb-like nanofiber microstructures could be of value in chemical countermeasures substrates such as facemasks and chemical protective clothing liners. They appear when a nonconductive collector is used and seem to be the consequence of charged fibres repelling each other. Metal oxides embedded in polyethylene oxide nanofiber webs allow them to behave as self-detoxifying substrates due to their reactivity against chemical warfare agents, organophosphorous pesticides, and industrial toxic chemicals. PEO with Magnesium Oxide appears to break the P-O and P-F bonds in these chemicals. As an aside Mr Ramkumar commented that nanoparticles of gold are, unlike the bulk metal, reactive.

Simulating Nonwoven Filters

Andreas Wiegmann of the Fraunhofer Institute ( Germany ) described the development of a geometric model for the 3 dimensional structure of nonwoven. It contains the porosity, fiber diameters and fiber anisotropy as parameters and uses a random number generator to prevent no two realizations looking alike. In these structures, fluid flow and then particle deposition can be simulated. Initial pressure drop, filter efficiency and filter lifetime can be computed based on the structure, the mean flow velocity and the particle size distribution. Electric charges, adhesion forces and in case of very small particles, diffusion can also be modelled. With this simulation chain from nonwoven geometry to filtration properties, the virtual design of a nonwoven filter becomes feasible. Nonwoven deformations, strengths and acoustic absorption can also be simulated. Comparisons with measurements on real media show that the simulations are capable of reproducing real effects and have the potential to provide insights on new filter media design.

Simulating Permeable Nonwovens

Sabrina Zobel of NCRC has attempted to simulate a calendered spunbond using software that lays down a random array of rectangular fibres horizontally and vertically (90 degrees being the only angle of intersection present). In order to simulate bending on compression, each fibre is assumed to be made up of 14 micron cubes which shear by a predetermined percentage according to pressure applied. Validation against real spunbonds (2 denier fils) was carried out over a range of basis weights from 20 to 100 gsm. However all were calendared identically, so all had similar solid volume fractions. The simulation and the experimental results for permeability were indeed in the same region, but only over a narrow range of basis weights

Peptides as Biocides

Steve Arcidiacono of the US Army Natick R&D Center pointed out that t raditional biocides for protecting fabrics against degradation (e.g. metal and phenolic compounds) are neither eco-friendly nor selective toward the target organisms. Antimicrobial peptides, however, have biocidal activity against bacteria and fungi in microgram quantities and are safe for the environment, so Natick are checking them for biocidal activity in polymeric films and coatings. The antimicrobial peptide cecropin P1 has been shown to be active against bacteria and fungi in thin films and coatings even after exposure to organic solvents and high temperatures. These 20-40 angstom particles are obtained from toads, frogs, pigs and cows and when they come into contact with a bacteria they form an alpha helix, several of which self-assemble into a circle and burrow through the bacterial cell wall, puncturing it. The activity of treated films is diminished by washing, so Natick are now exploring grafting the peptides onto a surfactant molecule, one end of which will bond to the polymer, the other being free to form a helix and cooperate with its neighbors to kill bacteria. Asked about the likely shelf-life of peptides, this had yet to be checked, but informally, they've been in use for weeks without apparent loss of activity.

Hydrophilic Silane Quats

Tim Byers of AEGIS Environmental Management argued that the use of antimicrobials would soon be extended from their niche in socks, shoes and underwear into the more challenging fields of military uniforms, protective clothing and sportswear. Here reductions in odor were marketable, because odor leads to discomfort, and in the military case smelly soldiers can easily be detected at night. Conventional antimicrobials rely on leaching for their activity, but AEGIS have developed silane quats where the alkoxysilane bonds the quat to the fabric to give a “kill on contact” non-leaching activity. Unfortunately these compounds are normally hydrophobic and spoil the comfort of a garment, so AEGIS has developed a hydrophilic version for this application. The new version allows a treated cotton to behave like the untreated fabric in drop and wicking tests. On polyester it improves the wettability. Both cotton and polyesters with the new finish remain active for at least 25 washes. Asked about the durability of the original hydrophobic material, Mr Byers said it would do 1200 washes and was in reality only lost from the surface by abrasion.

Antimicrobial Nanofibers

Ilse Tuinman of TNO Defense Security and Safety Division ( Holland ) is developing antimicrobial filter layers by electrospinning inherently antimicrobial polymers. These filter layers will be incorporated into medical devices, military clothing and aerosol filters. The polymers used were modified quaternized acrylic polymers based on methacrylates and on polystyrene maleic acid anhydride. The methacrylates were prepared by copolymerization of dimethylaminoethylmethacrylate with butylmethacrylate in acetone at 85°C. After quaternization with alkylbromides C5 to C16 in acetone for 24 hours these polymers have shown a log 4 reduction and better against E.coli, Klebsiella pneumoniae and Staphylococcus aureus according to JIS 1902. The highest reduction obtained was log 5.5 for a polymer consisting of 84% butylmethylacrylate and an undisclosed compound developed by TNO (patent pending). PVA was also electrospun from water, and soaked in Dow 5700 antimicrobial. This showed almost no activity. Work continues with attempts to improve the uniformity and hence the filtration performance of the electrospun webs.

Copper Oxide Antimicrobials

Gadi Borkow of Cupron Inc. reminded us that copper and brass doorknobs kill E.coli where stainless steel has no effect. Copper water pipes kill Legionella bacteria and copper-bottomed ships go faster because of its natural anti-fouling properties. Copper ions damage cell walls and bacterial DNA without allowing tolerant strains to develop, so Cupron has developed copper oxide master batches to allow the production of fibers and films containing this safe biocide. Because the copper is a non-leaching kill-on-contact biocide, it has to be present at the polymer surface and this means in the skin of a bicomponent fibre. The resulting copper containing nonwoven fabrics possess potent antibacterial, antifungal and antiviral properties which are unaffected by washing. The metal has been used as a spermicidal contraceptive which can remain implanted for decades (as Inter-Uterine Devices) and fibers containing it are non-toxic, not irritant and non-allergenic. Impressive photos of diabetic ulcers cured by wearing Cupron socks were shown, and Mr Borkow went on to claim that Cupron face masks and pillow cases reduced skin wrinkles due to copper ions being stabilisers of skin proteins. Applications foreseen are:

• Disposable hospital textiles to reduce hospital acquired infections.
• Barrier fabrics and suits to protect the wearer and immediate environment from pathogens.
• Air filters to reduce the passage of viable microbes.
• Filters to reduce transmission of bacteria and viruses during transfusion of blood or blood related products.
• Diaper topsheet to reduce diaper rash infections.
• Masks to protect the wearer from aerosol transmitted pathogens, such as the avian flu.

Asked if the valency of copper was important, Mr Borkow said the actual additive was a proprietary mixture of Cu-1 and Cu-2.

Performance Evaluation

Ning Pan of the University of California at Davis has been using the PhabrOmeter he invented to get an index of the softness, smoothness and stiffness of a wide range of textile fabrics. The PhabrOmeter records the forces generated as a disc of fabric pinned down by a similar sized weight is pushed through a hole in the center the support disc. Fabric friction, stiffness, creaseability, stretch, stretch recovery and drape all affect the result and an analysis of the resulting force/time plot (“extraction curve”) can be used to quantify several aspects of fabric handle. If a fabric is put through the machine twice, the difference between the curves contains information on crease recovery. The curves obtained appear remarkably consistent for repeated testing of the same fabric. With regard to the thermal characteristics of the fabric – whether it is warm or cool to the touch being an important part of the subjective assessment - the density of the material was the key, and this affected the PhabrOmeter result. (Density or air content dominates the sensation of warmth c.f. the thermal conductivity or specific heat of the polymers used)

Evaluating Fabric Texture

Sachiko Sukigara of Niigata University ( Japan ) has correlated panel assessments of the handle of diaper topsheets, toilet papers and some electrospun nonwovens with results from the Kawabata method normally used to quantify the hand of conventional textiles. 40 samples of spunbond and 22 samples of double-ply virgin pulp toilet paper were assessed by 40 female judges using a 5-point scale. Then measures of tensile strength, shear, bending, friction, roughness and compression were obtained from the Kawabata system. Because finer fibers give more smoothness, two electospun nanofiber materials were also tested. These were 100nm silk fibers electrospun from fibroin in formic acid and PLA electrospun from DMF. Of all the measurements taken, one result, surface smoothness or to be precise the deviation of the coefficient of friction of the fabric surface, correlated best with the subjective assessment of handle of all the materials. For the electrospun materials the coefficient of friction was not as low as expected, maybe due to some stick-slip effects as the piano-wire probe was dragged over the surface. The deviation of the coefficient of friction was however very low. In response to questions, the electospun webs were 20 microns thick and lower than normal test pressure had to be used because they were easily damaged.

Absorbent Product Texture

Hiroko Yokura of Shiga University ( Japan ) has carried out a similar study, this time panel testing the surface of diapers and pantiliners both dry and wet and correlating the results with the Kawabata and surface dryness measurements. 68 different diapers were evaluated by mothers and students for softness, smoothness and wetness. Best correlation was between subjective assessment of wetness and the surface dryness results. For dry diapers and pantiliners, the deviation of the coefficient of friction was the best predictor of good handle. Ms Yokura thought this measurement could be used in quality control, where values below 0.045 could be regarded as acceptable.

PhabrOmeter Hand Assessment

Shulin Zhang of P&G's Fabric Care Division has also been comparing the PhabrOmeter with expert panel testing and the Kawabata system for fabric hand assessment. Tactile properties of fabrics and fibres (including hair) are important to several P&G operations so any method which can improve on panel testing or the complex Kawabata evaluation, or give similar data more quickly, would be valuable. Terry towelling treated with various fabric softeners provided a range of textures to be assessed by expert panel and Kawabata. The fabrics were then evaluated on the PhabrOmeter and the “extraction curves” analysed to find the best correlations. Correlations between the Kawabata coefficient of friction and both the maximum curve height and the area under the curve (“extraction energy”) were excellent, but the curve height correlated less well with the panel assessment of softness. Here the initial slope of the curve and the extraction energy corellated best. If “Extraction Energy Reduction” was calculated by subtracting the EE result of a treated fabric from the EE result of the untreated (unsoftened) control, this EER correlated best of all. Dr Zhang concluded that the PhabrOmeter provided a valuable additional tool for tactile assessment, but should not be thought of as a replacement for panel testing or Kawabata. In response to questions:

• Wet fabrics could be tested on the PhabrOmeter but stiffness and friction may lead to high forces.
• The extraction curve has a maximum because the more the fabric is pushed into the hole, less is gripped by the weight.
• It works well with stretchy materials (Most materials are stretched in extraction)

Shouldn't hand be assessed against a soft material rather than smooth metal? Dr Zhang works mainly with cotton so these effects are not important. Anyway, the PhabrOmeter is not a replacement for hand testing.

Hydroentangled Delivery Systems

Steve Russell of the University of Leeds (UK) recycled the paper on 3D Nonwovens given at Pira's Future of Wipes Conference (July 2006)

Future of Spunlaid

Hans Geus, Technical Director of Reifenhäuser GmbH ( Germany ) went through the usual introductions to the technology and market, the statistics for which showed that spunlaid had lost share to drylaid between 2003 and 2004 in Europe . He reminded us that if we thought calendered spunlaid fabrics were flat and thin we should remember we could increase their thickness by several means:

• Through-air bonding instead of calendering
• Hydroentangling either after or instead of calendering
• Mechanincal softening

So, if the market requires the bulk and softness of dry laid, the future of spunlaid will be in different bonding systems. Furthermore if squarer fabrics are required for technical applications, these can be made by altering the aerodynamics of the laydown technique. In response to a question, spunlaid output was still growing at 9% per year, 5% of this being in the developed world.

Nanofibers from Spunbond

Nataliya Fedorova of NCSU has spun islands-in-a-sea bicomponent fibers with 360 Nylon islands in a PLA sea. If the ratio of nylon to PLA is 25/75, and the PLA is dissolved in caustic soda, nylon filaments with a diameter of 360 nm are liberated. At a more reasonable 75/25 nylon/PLA ratio they are half a micron in diameter. Tenacity (5 gf/denier) and modulus (140 gf/den) of the finest fibers is surprisingly good compared with the 100% nylon 16 micron homofil obtained from the same system. As for regular fibers, the strengths increased as the filaments got finer. Fabric properties after hydroentangling followed by PLA removal showed the nanofibre nonwovens to have substantially higher tear strengths (x3) than the 16 micron homofil nylon control. Tensiles were however only about half that of the control. Calendering the hydroentangled webs before PLA removal resulted in large strength losses, presumably because the PLA interfered with the nylon/nylon bonding. (Fabrics were tested after PLA removal). As Ms Fedorova concluded, this does indeed look like a promising technique for making tough microfibre fabrics for industrial applications.

Nanofibers as Sensors

Kris Senecal of the US Army Natick Soldier Center described the generation high surface area nanofibrous membranes with covalently attached Molecular Recognition Elements (e.g. antibodies, peptides/DNA) for the selective binding/capture of biological agents or food pathogens. The association of MREs with high surface area electrospun fiber membranes could allow the development of sampling devices tailored to specific hazards. Polymers with carboxylate or amine functionality (PVC-COOH or Polyurethane) were electrospun onto a stainless steel mesh, and the MRE's attached. For example, Avidin was chemically crosslinked to the membrane and coupled with biotinylated anti-staph antibodies. Membranes thus treated were shown to capture a 1 ng/ml concentration of Staph Enterotoxin B, despite the use of a non-optimised system. For the future, Natick intend to develop nanofibers not only for capture, but also to allow a capture event to generate an electronic signal through the membrane. To this end conductive versions of the sensors mentioned above have been created by polypyrrole coating of the nanofibre web.

Temperature effects in Meltblowing

Randall Bresee of the University of Tennessee recalled the contradictory reports on the effects of temperature on fiber diameter in melt blowing and has therefore conducted a series of experiments on a 15 cm lab line and a 115 cm commercial line. He has now shown:

• Increasing process temperature reduced the mean, minimum and maximum fiber diameters in webs but increased fiber diameter variability.
• The influence of temperature on fiber diameter was less at higher process temperatures than at lower process temperatures.
• Fiber contact/fusion depended on process temperature.
• Contact/fusion was thought to contribute to the increased diameter variability that was observed when process temperature was increased.
• Primary air temperature was more important than die block temperature. (On the commercial line the die block was heated from 180 to 280C without affecting fibre diameter – air temperature being constant at 200C)
• Die tip temperature was more important than die block temperature.
• Sensitivity to temperature change reduces as throughput increases. (more hot polymer damps out air temperature effects)

In response to questions:

• There was no shot formed at low temperatures.
• There was no fly (fine fibres being lost) even at 280C.
• There was no degradation of the PP even at 280C (residence time too low).
• MFR's used were 1100 to 1200. At higher levels degradation would probably occur at the higher temperatures.

Improving Filtration Performance

Mark Snider of Nordson Corporation described a new melt blowing process intended (eventually) to allow the production of nanofibres. In essence this is a bico-meltblown process producing segmented fibers which separate as a result of careful choice of polymers and optimal setting of temperature and air flows. At present the average fiber diameter (~0.75 micron) appears to be about half that achievable with conventional melt blown so maybe we are looking at side-by-side bicomponency. If the line is set to produce 1-2 micron average diameter, throughput can be double that of conventional melt-blown. Measurements of hydrohead on a range from 10-20 gsm fabrics showed that the new system delivered double that of the old. For the future, Nordson will be trying to get the average diameter down to 0.5 microns and will be optimizing polymer additives and electreting to allow the production of high efficiency filters. Asked about the throughput from the new system, Mr Snider said that 0.17 to 2 gms/hole/min were now possible compared with 0.1 g/hole/min on the old system (for 1-2 micron fibers)

Hydroentanglement optimization

Prof. Memis Acar from Loughborough University (UK) reviewed the last few years of work aimed at optimizing the nozzle geometry, process conditions and energy efficiency of the hydroentanglement process. His conclusions were as follows:

• Cone-up nozzles with a wide range of cone angles are sensitive to water pressure and give lower jet velocities but high discharge coefficients compared with cone-down nozzles. Cone-down nozzles give the longest intact water column length and are more efficient.
• Parallel laid webs are stronger in the machine direction.
• Tensile strength increases with pressure up to a maximum dependent on web density and fibre type (the critical pressure) and then declines with further pressure increases.
• At or above the critical pressure, process speed has little effect on the strength. Below the critical pressure, higher speeds give weaker fabrics especially for high modulus (hard to entangle) fibers.
• Fabric strengths increase sharply when the web is entangled from the opposite side.
• Prolonged entanglement from one side is less efficient than briefer entanglement from alternate sides.
• Choice of pressure profile is crucial for efficient entanglement, and this will vary with fibre type and web structure.

Image analysis of the orientation and distribution of straight fibre segments in entangled webs correlates with mechanical properties .

CFD simulation of HE nozzles

Nagendra Anantharamaiah of NCRC reminded us that hydroentanglement efficiency depends on the waterjets maintaining their intact length for an appreciable distance downstream of the nozzle. This is achieved by using a cone-down nozzle where the sharp 90 degree inlet forces the water to detach from the nozzle wall (the “hydraulic jump”) and remain unaffected by cavitation or wall-induced turbulence. As the sharpness of the inlet is eroded by the high pressure water, efficiency diminishes and eventually the hydraulic jump collapses allowing higher discharge coefficients but lower intact lengths. In practise, visible (under the microscope) rounding of the inlet is evident after 48 hours running at 3000psi. In his current work, Dr Anantharamaiah has simulated the steady-state in the nozzles using CFD and has investigated the effects of increasing inlet roundness on the nozzle's discharge coefficient. Discharge coefficients obtained from these simulations were indeed found to increase with increasing the inlet roundness and agreed with practical data obtained from measuring the inlet roundness with 3D profilometry over the range 0 to 0.2 r/d. A simple equation for predicting the nozzle inlet roundness by measuring its discharge coefficient at high pressures was therefore obtained.

Removing jet streaks

Katharina Rompert of Freudenberg ( Germany ) showed how adding a second row of holes to a hydroentanglement nozzle strip could eliminate the jet streaks commonly seen on hydroentangled fabrics. This second row could use smaller holes, and they had to be slightly offset so that the water columns impacted the fiber “peaks” raised at the sides of the first row's jet streaks. The improved surface uniformity was clearly visible in the photographs shown and Ms Rompert had also carried out a thorough image analysis to prove it and quantify it. The effect of the second row of holes was most pronounced when entangling on a belt at 200 bar but on a drum some stripiness was still visible. Various 2 nd row hole sizes were tried, 110 microns (following a 130 micron 1 st row) being best. Moving to 4 rows of holes from two gave no further benefit. With 2 rows, improved tear strengths were obtained but tensiles were unaffected.

3D Modelling of HE

Ping Xiang of NCSU presented a model of the hydroentanglement process based on the first principles of fluid mechanics. The CFD domain used in the simulation was a rectangular space with a width of the distance between the centres of two adjacent jet holes and a length of 0.45 mms in the MD. Three layers were assumed in the vertical direction, the first a layer of water of depth equal to twice the fibre diameter, the second being the fibre web (0.7mm) and the third being the forming wire. The forming wire is assumed to move at 2 m/s. Impact between the turbulent jets and these layers creates further turbulence and the fibres are entangled in the resulting vorticity. Tensile strengths obtained from fabrics entangled at different pressures were plotted against the vorticity calculated by the CFD package at these pressures. The correlations were near perfect, and said to validate the initial assumptions that tensiles would be proportional to degree of entanglement, and that entanglement was caused by fibres moving around in the vorticity. Furthermore, simulations of the effects of different hole sizes gave 0.127mm as the optimum size for maximum vorticity, this size being said to be pretty close to that adopted in industrial practise.

Dyeing of HE nonwovens

Andrew Hewitt of Leeds University (UK) recycled the paper he gave at EDANA's Nonwovens Research Academy in April 2006. The same concerns related to the lightly entangled nonwovens ability to survive commercial dyeing conditions were raised in questions.

Calvin Woodings - 3 rd October 2006