EDANA Filtrex 08: Cologne : 6-8th October 2008

Key Points• Fresh and waste water filtration is now the largest and fastest growing segment of the filter media market.
• The growing demand for cleaner water is attracting major new players into the water treatment market.
• H&V's Nanowave™ technology yields a filter with a wavy surface which provides half the pressure drop of the equivalent glass filter with 3 times the dust holding capacity and a higher filtration efficiency when discharged.
• Irema Filter is co-forming PP nanofibres with a standard PP meltblown to give a gradient structure which has higher capacity, lower pressure drop and longer charge retention than the usual two-layer structure.
• CamilFarr use a new form of active carbon with “Rapid Adsorption Dynamics” in an F7 filter to absorb low concentrations of pollutants quickly.
• Norafin's 420 gsm hydroentangled fabrics with a split bico microfiber surface can match the performance of a 500 gsm needlefelt with an ePTFE membrane surface.
• Europlasma use plasma to polymerise a monomer onto PP to render it permanently hydrophilic. The monomers used are secret and the process needs a vacuum.
• With air handling fans consuming 15% of all electricity consumption, the EU are expected to require energy efficiency labelling of air filters in about 3 years.
• The nanoalumina whiskers used in Ahlstrom's Dispruptor™ papers can immobilize 8 micron active carbon powder without the need for binders which can reduce efficiency. The resulting filters are excellent for cleaning up water for drinking.
• Dupont's HMT nonwoven uses direct-spun continuous nanofibers to achieve membrane-like filtration at lower pressure drop.

Keynote: Filter Media Markets

Ken Sutherland of Northdoe UK defined the filtration market to include deep-bed sand filtration, coarse porous materials, wound yarns, woven fabrics, felts, papers and nonwovens including glass and carbon materials, sintered metal and ceramic media and polymeric membranes. Strictly speaking membranes were not filters because they separated molecules by rate of diffusion, and were not intended simply to remove particulates.

His $ values were at the point of sale to the filter equipment maker or replacement user and included all industrial, commercial and residential filters, whether fitted as original equipment or sold as replacements. Data of most interest to the nonwovens industry – tonnes or square metres of roll-goods - was not available:

• The world market for filter media in 2007 was $21.7 billion.
• 38% of this was sold with the filter hardware, and 62% in the aftermarket.
• Nonwovens accounted for 27% of this and Paper (surprisingly) another 2.9%.
• Membranes were the biggest sector (35.5%) with spunmelt nonwovens (14.3%), metallics (10.4%), felts (9.9%) and constructed cartridges, i.e. filter media plus frame and end caps as in most automotive air and oil filters, at 7.1%, making up the top-five filter types.
• USA (24.4%), China (12.1%) and Japan (7.5%) were the main countries of use. China was growing much faster than the others and because of the difficulty of getting good information could already be much larger than this. (The USA had fallen from 50% a few years ago.)
• Fresh and waste water filtration (14.3%) is the largest individual sector, followed by Bulk Chemicals and Petrochemicals (13.6%), Fine Chemicals and Pharmaceuticals (9.6%), Food & Beverage (8.3%), Power Generation (7.4%), Medical and Health (6.3%) and Mining and Metals Processing (6.0%)
• The global market was growing at 5.6% annually, China at 10.1%. Water treatment was growing at 6.8%.
• Major growth drivers were:
• End user demands for cleaner and purer water and air. (“Everyone, all the major companies, are getting into water treatment”)
• Environmental regulations tightening
• Growth in China and India and other developing regions.
• “Most of the areas the World now needs to improve involve using better filtration.”

Technologically, the development of ever finer fibres, and materials with better heat/chemical resistance was leading to innovative new filters. Heat/chemical resistance was getting more important as companies tried to save energy by extracting heat from effluent: the heat exchangers now requiring pre-filters to keep them working properly.

Asked for more detail about the $ values, Mr Sutherland said the total filtration market (equipment plus media) was $44bn, and all his numbers were based on the exchange rate on July 1 st 2007.

Nanowave™

Andre Boni, Director of Marketing for Hollingsworth and Vose described their proprietary new technology to make an “extended surface synthetic media”. Nanowave™ was a development of Nanomelt™ introduced at Index in 2005, the latter having had problems due to its higher initial pressure drop, lower test dust capacity and premature blinding by fine dusts.

The unique feature of Nanowave™ was its wavy surface, and this gave the following benefits:
• Double the alpha value of conventional discharged melt-blown media (alpha=Efficiency/Pressure drop)
• Double to treble the dust holding capacity (wavy surface, like crepeing, gives twice the surface area per square metre of filter.)
• The nanofibres give a high discharged efficiency.
• The structure can also be charged for even higher efficiencies
• Wave height, density and polymer type, as well as fibre dimensions can be adjusted for specific applications.
• It is stiffer and more resistant to compression.

Initial focus is on an HVAC bag filter to meet the F5 to F9 classes of EN779. Here synthetics, usually charged and so liable to lose efficiency over time, are battling with glass fibre which is more consistent but might shed fibres. Nanowave™ gives half the pressure drop of the equivalent glass filter, with 3 times the dust holding capacity and a higher efficiency even when discharged.

Asked about price, Mr Boni said it would be a premium product, but H&V would position it competitively. It is not commercial but test quantities can be made available.

Integrating Nanofibres

Wolfgang Rupertseder, Plant Manager, Irema Filter ( Germany ) reminded us that diffusion is the main mechanism for fine particle capture, and then the particle must stick to the fibre. The force of adhesion is highest when the fibre diameter equals the particle size, and in a real world gradient filter you can see that the coarse fibres catch the big particles and the fine fibres catch the small ones. Once fibres get smaller than about half a micron, a “slip-flow effect” (the air velocity at the surface is no longer zero) occurs and this reduces the pressure drop to about 70% of that calculated using theory which works for larger fibres.

Rather than create a layered structure with nanofibres laid on top of coarser melt-blown fibres, Irema co-mingle the fibres by spraying the two types into one another before they land on the conveyor. A bulky gradient structure is obtained with the top surface being richer in nanofibres than the bottom. For some applications a third head will apply a protective microfiber layer to the top side. The heads look like conventional melt-blow heads, and the nanofibre size (0.1 to 0.5 micron claimed) is probably obtained by running the nanofibre head at very low productivity, and the whole line at low speed.

The product is commercial in automotive cabin air filters where it gives a 70% NaCl capture efficiency and longer filter life. Media for HEPA filters is under development.

Results shown were for charged media, but in response to questions Mr Rupertseder said the blend structure discharges more slowly than a layered structure. PP is the main polymer used but the process has been shown to work with PET.

Nanogate Technology

Yvonne Klattwitter, Product Manager for Nanogate ( Germany ) said their core competence was formulating products using nanomaterials and processes based on nanotechnology. Their N-Charge® coating had been designed to improve the ability of surfaces to store an electric charge and works as a dielectric which prevents discharging due to particle capture. Realising that this may have applications on fibres and fabrics, they developed Permastatic® as a spray-on or paddable water-based coating.

Nonwovens coated with Permastatic® have:
• The same permeability as untreated nonwovens. The coat is 100nm thick.
• Increased fractional filtration efficiency. Retention of submicron particles improves from 70% to 90% in vacuum cleaner bags as tested by Filz Fabric Fulda.
• Longer lasting electrostatic properties. Charge retention was up from 61% to 83% in a 5 months test. In a 2 year test 100% PP and 100% PET retained their initial charge of 3-4kV almost unchanged. (Were these fibres or films?)
• Resistance to water washing and IPA extraction

Permastatic® can be combined with other treatments to give improved cleanability, hydrophobicity or hydrophilicity, and is said to be easily incorporated into existing processes as a low-cost upgrade. Ms Klattwitter could not answer questions about cost.

Nano-pollution

Thore Bertilsson and Jörg Scharnberg of CamilFarr ( Sweden ) defined air pollution as “matter found in the atmosphere not belonging to its natural consistency, or matter found in indoor air with harmful effects on people and materials”. Examples included dust, soot, mist, fog, gases and vapours and radioactivity. They can be natural (forest fires, volcanos, dust storms) or man-made (incineration, fossil fuel combustion, factory effluent). Very small particles and gases presented particular problems. Normally polluted town air had a bimodal distribution with particle size peaks at around 0.2 and 8 microns. Modern diesel engine exhausts emitted particles below 0.3 microns and these could pass through the lungs into the bloodstream. However an F7 air filter with >50% discharged efficiency at 0.4 micron would give acceptable air quality, reducing PM10s by 87% and sub PM1s by 75%. To improve on this and get noticeably better air quality a filter meeting EN13779:2007 would be required.

Atmospheric polluting gases - NOX, SOX, NH3, HCl, VOCs – and indoor pollutants, formaldehyde from wood and carpets, ozone from photocopiers and various unpleasant odours from food and waste can only be removed by adsorbtion on active carbon, active alumina or ion exchange resins. Existing granular forms of active carbon are too slow to adsorb low concentrations to be useful in HVAC systems but an new form with Rapid Adsorption Dynamics is now available. RAD deals with really low concentrations of pollutant quickly by virtue of being a really small particle of active carbon, and it can be integrated into an F7 particle filter.

Other pollutants which are currently untreatable include CO 2 , CO, CH 4 , NO, He and Ra.

Ahlstrom Disruptor™ in Reverse Osmosis

Domingo Sanchez (R&D) of Ahlstrom Filtration (USA) replaced Rodney Komlenic, VP Business Development, to deliver the paper on the Dispruptor™ fabrics made under licence from the Argonide Corporation. Here, naturally occurring 2nm by 250nm boehmite whiskers are attached to submicron microglass fibres which can then be easily processed by Ahlstrom into wet-laid nonwovens. These nonwovens have a 2 micron average pore size and a 42,000 m 2 nanofibre surface area per square metre of nonwoven. At pH 7.2 they have a >50millivolt zeta potential which is effective for up to a micron from the whisker surface, this allowing them to capture particles below 0.5 micron electrostatically, while the large particles are retained mechanically. In some tests they have been shown to retain 6 times their own weight of contaminant. They can be used at operating temperatures above 400 o F, yet have filtration efficiencies similar to the UF/MF polymer membranes while incurring a fraction of their pressure drop. Examples of use covered products normally associated with reverse osmosis, nanofiltration, ultrafiltration and microfiltration:

• Compared with membranes having a range of pore sizes (0.025 to 1.2 microns) Disruptor™ media removed B. diminutia, MS2 virus, and an endotoxin with higher efficiency at lower pressure drop and higher flow rates.
• T4 phage was adsorbed from a concentrated solution by simply immersing Disruptor™ in the solution.
• Iron, tin, lead and copper particles, problematic in drinking water and waste from metal finishing, were all removed easily by Disruptor™. Dissolved iron was also removed.
• The nanoalumina whiskers will immobilise 8 micron powdered active carbon (PAC) powder without the need for binders which can reduce the PAC adsorption efficiency.
• The resulting Disruptor™PAC paper can remove 1gm of chlorine or iodine from water per gram of PAC.
• Disruptor™PAC successfully removed all the humic acid from drinking water spiked with 20 FTU and reduced the turbidity of the water to zero. Even standard Disruptor™ removed more of the humic acid than the tightest (0.22 micron) Millipore membrane.
• Disruptor™ remove trace amounts of organic matter from lake water at high flow rates and very low pressure drop.
• They effectively reduce water contaminants missed by GAC beds, flocculation, microfiltration and ultrafiltration processes.
• As prefilters on reverse osmosis processes, the membrane cleaning routines can be increased from monthly to six-monthly due to their ability to reduce the silt density index (SDI).
• Biofouling of membranes by proteins, lipids, carbohydrates and amino acids can be significantly reduced.

Disruptor™ nanoalumina whiskers have been used in medicine since 1926 and have FDA approval for use in human vaccines. They have now passed the NSF/ANSI standard 42 for water contact use and standard 53 for water contact and cyst reduction, USP class VI and endotoxin testing. In conclusion Ahlstrom believe this is a real breakthrough in reducing a wide range of reverse osmosis biofoulants via electroadhesion and mechanical entrapment.

Asked about the cost of the papers, Mr Sanchez simply said $20/m 2 for an average basis weight.

Microglass and Spunbond

Dr Werner Groh of Johns Manville ( Germany ) introduced their new CombiFil® Premium air filter medium made by flame-blowing 1-3 micron glass fibres between two layers of spunbond polyester. The process can be used to make filter efficiencies up to HEPA with high stiffness and pleatablility without the need for wire supports. It holds more dust at lower pressure drops and maintains a stable filtration performance over longer times than the charged synthetic media with which it competes. Specifically, an F8 Combifil®Premium filter holds more dust at a lower pressure drop than an F7 synthetic.

Price? Dr Groh said it would be ~€3/m2 or about twice the cost of a standard material. The microglass was spray bonded with phenolic resin, which also served to bond the polyesters in place.

Spunlaced for Membranes?

Victor Lorentz, Sales Manager of Norafin ( Germany ) described the versatility of their core technology, hydroentanglement, pointing out that it was capable of making filters to compete with needlefelts in all applications. Furthermore it gave filters with a more uniform pore structure over a wider range of basis weights (40-700gsm). For demanding applications, metal or glass scrims were used as reinforcement. A 320 gsm hydroentangled filter gave efficiencies comparable with a 500gsm needlefelt at a much lower pressure drop when the same fibre were used.

If splittable bicomponents were used, much finer filters could be made, but only over an 80-120 gsm range because lighter fabrics were too thin to withstand the splitting pressures, and heavier ones were too costly to split sufficiently. Because the splitting occurred mainly at the surface, a gradient structure with a membrane-like surface was obtained in a one-pass operation without the need for lamination and binders. The fibre splitting increases the filtration efficiency from 65 to 95% for 1 micron particles on 1.3 dtex fibres. If a 100 gsm “microsplit” fabric was laminated to the 320 gsm standard HE fabric the resulting filtration performance had been shown to match a 500 gsm needlefelt with an ePTFE membrane surface.

Asked about the mechanical properties of the spunlace, Mr Lorentz said they were more robust than the equivalent needlefelts and the membrane surface more durable than the ePTFE. Could such materials be used in HVAC? Yes, if the split-fibre layer was laminated to a more bulky base.

Pleatable PET for Depth Filters

Dr Ulrich Hornfleck Sales Director of Sandler AG ( Germany ) observed that carded polyester, chemically bonded in thicknesses up to 3mm can be easily pleated to make an excellent self-supporting coarse dust filter for use in filter classes from G3 to F5 depending on the fibre decitex. However the finest cardable fibre diameter (i.e. about 10 microns) prevented this construction from entering F6 and above.

A new 3-layer structure with a highloft base, an ordinary fine fibre carded polyester centre and a melt-blown polyester surface layer has been shown to maintain its pleatability, offer temperatures of up to 120 o C at constant load with 200 o C peaks, have high mechanical stability and a very high crack resistance. Air permeability and dust holding capacity was superior to F7 microglass media.

A questioner was concerned that the comparisons between microglass and such a thick polyester filter would be invalid, because the use of flat sheet test methods rather than pleated media tests would bias the conclusions in favour of the thicker material. In real filters the glass media would allow a higher pleat density.

Characterisation of Needlefelt Surfaces

Joseph Schuberth of the Vienna University of Technology ( Austria ) has developed a method for comparing the particle penetration and particle holding capacity of filter media. A light microscope and image analysis software was used to determine the surface porosity and the pore depth distribution of a range of felts. Combining these with the mean clean gas concentration and dust loading obtained from a VDI 3926 Type 2 tester, correlations between the optical and physical tests were obtained. These calibration graphs now allow the clean gas concentrations and dust holding capacities of other needelefelts to be estimated simply by using the microscope and image analysis software. The optical measurements can also be used to calculate the relative standard deviation of the pore-size and the average porosity on both clean and dust-loaded filters, and this will allow investigation of clogging mechanisms.

Engine Oil Filters

Claes Frennfelt of Volvo Powertrain ( Sweden ) underlined the importance of oil filtration in reducing the costs of operating a fleet of trucks and complying with the ever-tightening regulations on exhaust emissions. Oil lubricates, cleans and cools a truck diesel and is typically filtered in a bank of three large cartridges. Two of these operate at 60-80 litres/hour on the whole oil circuit, while the third, a finer “by-pass” filter operates at 8 litres min. The 40 liters of oil in the circuit is at 400 to 600 kPa.

Standard cellulose filters last 60,000 km while long life versions can do 100,000 km. For 150,000 km service intervals a synthetic blend filter is needed. These synthetic filter cartridges are much more expensive because they are not self-supporting. Other factors affecting oil service life are fuel quality (>500 ppm sulphur shortens the filter-life), exhaust gas recycling (reduces it by a third), biofuel use (a 30% biofuel blend halves it), and topography (hilly worse than flat). Compared with the 3 times more expensive synthetic oils, mineral oil works well up to 100,000km in normal climates. The low viscosity synthetics are needed in cold climates.

Plasma Coatings for Engine Air Filters

Marc Pauwels of Europlasma NV ( Belgium ) described how plasma could be used to polymerise a monomer onto the fibre surface to give a permanent coating. Filter media could be processed in a vacuum chamber on a roll-roll system, or complete cartridges could be processed batchwise. Hydrocarbon monomers give permanently hydrophilic coatings when treated in the presence of oxygen and fluorocarbon mixtures give oleophilic and hydrophobic coatings. The process is used to improve air filters for diesel engines by increasing the hydrophobicity of a PBT nonwoven filter so that it can pass the water decantation and repellency tests. The benefit is achieved without altering the dust holding capacity or filtration efficiency. It is also used on facemasks, air conditioner filters, battery separators and blood filters. The roll to roll process runs at 5 m/min and the current kit can take 60cm diameter rolls. Variable costs are €0.02/m 2 , said to be cheaper than a wet-chemistry approach and better for the environment. Asked what monomers were used to achieve permanent hydrophilicity on polypropylene, Mr Pauwels said this was a secret. Atmospheric plasma treatment did not work because the monomer loss proved too costly.

Predicting Filter Performance

Martin Lehmann, Head of Simulation Filter Elements at Mann and Hummel ( Germany ) said measuring the distribution of fibre diameters in a filter is the key to predicting filtration performance and Mann and Hummel had developed a new way of doing it. Using a single average diameter was clearly wrong and SEM or optical microscopes coupled with image analysis were poor. He used the new Matlab image analysis software but provided no details. Furthermore his work with cellulose filters assumed that the clearly flattened fibres in the filter were round with a diameter equivalent to their width. Asked about this, he felt the microscope “saw” the fibres as a dust particle would, so the assumption of roundness was OK. The technique could be used in QC.

Testing Single Pleats

Dr Harmut Sauter of Mahle Filtersystems ( Germany ) described the three scales of filter evaluation: micro, meso and macro where macro considers the whole filter in its housing, meso considers the filter alone and micro considers filtration at the interfibre level. Most automotive air filters are pleated so at the meso level, understanding the behaviour of a single pleat becomes fundamental to filter design. Furthermore, the single pleat lends itself to computational fluid dynamics (CFD) simulation of air velocities and hence comparisons of experimental with simulation data.

Air velocities were measured by particle image velocimetry (PIV) where the particles were too small to be separated by the filter. Static pressure measurements at one air velocity showed that the simulation technique provided roughly comparable data, the differences being attributed to the pleat changing shape under pressure. In fact in a real filter they were observed to stretch and vibrate and change the way the dust distributes itself. Maybe the use of a V shaped pleat was too simple. The N shape might represent the smallest filter element more exactly.

Classifying Air Filters

Dr Thomas Caesar, Manager Filter Technics, Freudenberg Nonwovens ( Germany ) observed that as energy prices and CO 2 emission concerns increased, the power consumption of air filters was being scrutinized. In the UK and Sweden where the data is available, 15% of all electricity consumption is by the fans used to handle air, and a third of this power was lost in pressure drops across the filters used to clean the air. So energy efficient air filters would help to save the planet and they therefore had to be classified according to their energy efficiency as well as their filtration efficiency. The target is to identify the types of filter which provide the highest possible particle removal efficiency at the lowest possible pressure drop, the complication being that both of these parameters varies with the life and dust loading of the filter.

Dr Caesar proposes the derivation of a “key energy performance” or kep number which is a function of the average efficiency from EN 779 using a 0.4 micron particle at 3400 m 3 /hr, and the average pressure drop derived from a loading of 800g of ISO A2 dust also at 3400 m 3 /hr. Kep numbers less than 1 are given “Energy Class 1” as the most efficient now possible, and kep numbers below 0.6 are given “Energy Class 5” being the least efficient. These 1 through 5 Energy Ratings would be used like refridgerator energy classifications so that filter buyers could choose the efficiency they could afford.

Asked if this was just for Freudenberg or would it become an official standard, Dr Caesar said he was working with ISO and would expect it to become an international standard in about 3 years. Freudenberg would be introducing energy efficiency labelling on their range by the end of next year. Is the market demanding this? “Basically yes”.

Nexylene PPS fibres

Dr Jens Neumann-Roderkich, Development Manager, Nexis Fibres (Germany) ran through their PPS brochure, reminding us that poly phenylene sulphide fibres offered exceptional thermal stability up to 200 o C (melting at 290 o C) and were therefore positioned, both from a performance and price viewpoint, between nylon 6 and the meta-aramids. Unlike the meta-aramids, they were melt spinnable and could be produced in crystalline and amorphous forms with differing properties. The stronger crystalline form exhibited a shrinkage of up to 8% on heating to 220 o C, and the amorphous form could be used as a stretchy binder fibre with identical thermal stability. They could be made into all forms of nonwovens, the needlefelts with 25% Nexylene woven scrim being offered as recyclable hot gas filters. There were several questions and one comment from the audience:

• The comment pointed out that this fibre could not be sold as a hot gas filter because it did not meet the required thermal stability at 250 o C.
• How did Nexylene compare with Japanese and Chinese sources of PPS? It was as good as the best Japanese and better than the Chinese, one supplier in China being poor for needlefelts. Furthermore neither the Chinese nor the US producers offer 1.3dtex
• Were special extruders, piping and spinpacks needed to handle the PPS melt? Yes, the chlorine residues necessitated this.
• Were there any problems with the gas in the workplace? No, it was within German limits without special ventilation.
• What were the wet-laid PPS nonwovens used for? Electrical insulation – in combination with other fibres.
• Was there still a PPS shortage. No, the market is now in balance and two or three producers have expansions underway.

Air Filtration Composite

Dr Carsten Heldmann, Global R&D Director of Fiberweb (Germany) is developing high performance air filtration media with a low pressure drop by combining fine-fibre meltblown (FFMB), spunbond and carded webs to form graduated filters. The FFMB has diameters between 0.6 and 1.4 micron and thus straddles the currently used boundary between micro and nano fibres. In one example of a 4-layer “TopFlow”composite, a top layer of coarse meltblown protected a layer of standard meltblown on top of a FFMB which had been laid on to a spunbond reinforcing layer.

Carded prefilters could be made from fibres of different denier on the same line to get an even coarser prefilter for use in HVAC. These could be laminated to the spunmelt structures using sonic bonding to minimise the loss of open area. The main message was that Fiberweb were now willing to combine their carding and spunmelt expertise to offer a wide range of air filtration products.

Needlefelts for Liquid Filtration

Heribert Kurt of Heimbach Filtration ( Germany ) said that scrim-reinforced needlefelts are generally better than woven fabrics in liquid filtration because they offer good capacity at lower pressure drop due to their bulk and permeability. In industrial process filters they can be designed to survive the often harsh chemical environments and have established themselves in vacuum belts filters, rotary drum filters and filter presses. Their main uses are in the production of:
• Minerals and zeolites, including ores, lime, diatomite, and active carbon.
• Chemicals, e.g. fertilisers
• Dyestuffs and pigments
• Pharmaceuticals and antibiotics
• Non-ferrous metals including copper, nickel and zinc and its oxides.
• Food additives

One example used a 300gsm woven monofil PET scrim with a microfiber needlefelt surface which had been densified to remove 2 micron particles of zeolite from the process liquor.

Asked how Heimbach deal with the “smile” and “frown” variations in basis weight across the width (that needlefelts are prone to and wovens not), Mr Kurt said they could compensate for these in careful calendering and heat setting to get uniform permeability. Fibre loss from needlefelts was another issue, and here again the surfaces could be stabilised in the calendar.

Liquid Filtration for a better society

Professor Richard Wakeman, a consulting engineer (UK) described the variety of filtration media now available and wondered whether they could help reduce the 2 million deaths that occur from preventable water borne diseases in regions of poor water quality. Delivering large quantities of water free from viruses and bacteria requires membrane-quality filtration, probably in the shape of membrane bioreactors (MBR). These are hybrid processes which combine active sludge anaerobic biotreatment with membrane filtration. Because the sludge fouls the membrane, bubble “aeration” is used to create turbulence and clean the membranes. Reuse of water in heavily populated regions is increasingly complicated by drug, herbicide and pesticide residues persisting through purification processes, and these require adsorbents like active carbon for their removal.

So where do the filters come in? Ahlstrom's Disruptor™ with active carbon was mentioned along with the industrialisation of nanofibre nonwovens.

Hybrid Membrane Technology

Dr Antoine Schelling of Dupont ( Switzerland ) described their new HMT filters designed to fill the gap between traditional filters and microporous films. HMT uses continuous filaments in the 0.2 to 0.6 micron range to achieve filtration efficiencies close to membranes with lower pressure drop. The filament diameters are, unlike meltblown or electrospun, controllable, and this yields controllable pore-size.

When tested according to ASTM-F795-88 with ISO ultrafine test dust in water against a cellulose/meltblown composite which targets removal of ~1 micron particles, HMT showed:
• An initial pressure drop between 0.25 and 1.2 kPa (for 5gsm and 31 gsm respectively) compared with 1.7 kPa for the composite.
• The 31 gsm HMT had an initial ability to remove 100% of 1 micron particles compared with 87% for the composite.
• The dust holding capacity of the 31 gsm HMT was 1.9 gms compared to 1.1 gms for the composite.

Applications in HVAC, bag, cabin air and cartridge filters are foreseen, as well as in battery separators. The technology is currently being used with a nylon 66 polymer on a 1.8m wide line and is commercially available in the 10 to 30+ gsm range as roll-goods.

Elmarco questions related to whether or not the process used electrospinning, what the productivity was, and what Dupont meant by high throughput did not yield any useful answers. Dr Schelling did however add that extra capacity would be added in the next year. The EDANA conference programme did describe HMT as “nanofibre from electrospinning”. The main diameter produced was 0.4 micron to match microglass.

Calvin Woodings

14 th October 2008