Elmarco Nanofibre Conference: Prague17 th -18th October 2007 Key Points • Electrospinning (espinning) appears to be moving into the mainstream through its ability to create sub- 1 gsm fibrous layers with high functionality due to the very high surface areas achieved. Introduction IntroductionThis predominantly academic meeting, to Elmarco's surprise, attracted 140 visitors from all over the world. The papers were generally good, but the printed papers from the Universities often bore little resemblance to and contained less information than the slides presented. Techniques, Functions and ApplicationsProf Joachim Wendorff of the Phillips University in Marburg ( Germany ) illustrated the wide range of espin research underway with a collection of slides not available in the printed text, and some from unpublished papers: • 5-7 nm PLA filaments Applications in homogeneous catalysis, tissue engineering, inhalation therapies, agriculture and oceanography are being developed. • Catalysts when immobilised in the core of a polystyrene nanofibre allow synthesis of compounds completely free of the catalyst. 100% recovery of the catalyst is possible. (Scandium triflate mentioned as the catalyst for a Aza-Diels-Alder reaction). Prof Wendorff commented that scale-up issues were now the key development area, and waved a large, apparently heavyweight sample of nanofibre nonwovens (around 2 m long by 1.5m wide) to indicate progress was being made. Asked about current production rates he said his large sample took about a minute to make, but fibre speeds at laydown were about 50 m/sec. Solvent removal, even water, was not an issue. The surface area was so great, the fibres were dry before they reached the conveyor. As an aside he commented that Freudenberg have been spraying nanofibre polyamide from formic acid solutions for over 20 years. Espinning Molten PolymerPaul Dalton of the University of Southampton (UK) claimed this to be an under-researched area with only 10 papers published since 1981. He was working on this route for biomedical applications, earlier work having focussed on filtration, breathable barriers and tissue engineering. For tissue engineering, cell cultures are sprayed with a nanoweb which forms a scaffold to direct further growth. Because solvents tend to be toxic and kill the cells, he uses a polypropylene melt, the viscosity of which has been reduced to “treacle-like” (i.e. higher than the “honey-like” solution viscosities). He claimed to get down to 1-2 micron fibre diameter when spraying onto a single collector, and down to 0.25 microns when making aligned nanofibres by spraying into the gap between two parallel collectors. Here draw-down caused by stretching between the plates caused the further reduction. (The illustrative slide showed fibres from 190nm to 50 micron in diameter). Comparisons of solution espinning of polyethylene glycol/poly caprolactone block copolymers with melt spinning the same polymer at 10 times the viscosity showed that good quality solid fibres could be produce, but only at about one-twentieth of the productivity. Melt spinning gave benefits in more controllable focussed fibre laydown but only at the critical temperature which might cause degradation. Solvent espinning gave unfocussed laydown but did not degrade the polymers. Varying the relative size of the PEG/PCL blocks in the polymer allowed a wide range of hydrophilic/hydrophobic behaviour to be obtained. Nanosurface EngineeringProf Akihiko Tanioka of the Tokyo Institute of Technology (Japan) positioned nano-surface engineering amongst the other surface sciences and defined a nanofibre as having a diameter less than 100nm and a length to diameter ratio of more than 100 – considerably more stringent than those claiming nanofibres from melt-blowing. Nanowebs made from such fibres have pore sizes below 100nm also, can become effective virus filters. His laboratory is working on a variety of applications for electrospun nanofibres: • Ion exchange materials, either cationic or anionic with colossal surface areas. These could be for making ultra-pure water for semiconductor processing, or for purifying pharmaceuticals or biological products. For cationics, the polymer would be sulphonated, and for anioics, quats could be added. Catalysts could be added to create highly absorbtive materials with high catalytic power. Polystyrene spun from 10-17% solutions in THF/DMF was preferred for cationics, and poly-4-vinyl pyridine spun from ethanol/water for the anionics. Factors affecting fibre formationDr Tong Lin of Deakin University ( Australia ) has studied the formation of PAN nanofibres spun from different concentrations in dope, over various field strengths and collection distances, and with and without the use of an ethanol coagulation bath on the grounded electrode. He also used high speed video to investigate the draw down process and assessed the number of non-fibrous beads in the final web. He concluded: • 7% dope is better than 5% Nanofibre YarnsMarjeet Jassal of the Indian Institute of Textiles ( Delhi ) has been spinning PAN nanofibres into the gap between two conveyors, sometimes through a rotating annular magnet to insert false twist, and sometimes through an intermediate annular electode which was said to improve filament alignment. Both of these claims were hard to understand as only a single filament was present at this stage of the process. Both the verbal descriptions of the equipment and the slides were unclear. She was however positive that the electric field had to be twice the normal strength, this being achieved by the conveyors being at an equal but opposite voltage to the nozzle, and not simply grounded as is usual. While at one point 8% PAN appeared to be the best concentration, at the end she claimed 22% PAN spun into a 70C chamber with a field strength of 36kV. The resulting “yarn” collected from the conveyor is too delicate and shows too high surface friction ever to be used in a textile process, so reinforcement is the suggested application. The longest yarn produced to date? 10 inches. Engineering and Medical NanofibresMasaya Kotaki of the Kyoto Institute of Technology ( Japan ) described the Nanon laboratory scale electrospinner developed by MECC Co. Ltd. This produces an A4 size sheet of web on a rotating cylinder in a sealed chamber. The rotational speed of the cylinder defines the orientation of the fibres and SEM's of a range of random to fully oriented results were shown. The webs could be removed from the cylinder on a cardboard frame. So far he had made webs from: • Porous nanofibres which looked like an open-celled foam and must have a very high specific surface area. He has also measured the tensile strength of single nano-filaments made at cylinder surface speeds between 70 and 700 m/min and has shown that this speed change causes a big increase in orientation which can be further enhanced by hot drawing. The as-spun (700 m/min ) PLLA single filament broke at a stress of 175 MPa and a strain of 0.5mm/mm compared with 65MPa/1.6 mm/mm at 70m/min. Nozzle sizes have been reduced from around 0.1mm to 1 micron to further improve orientation. Target applications were coatings for stents (to reduce porosity), electronic devices and sensors, and a filter receptor for spectroscopic analysis, the latter being used to carry liquids into FTIR analysis where the high transparency of PAN allowed good spectra to be obtained. Espinning EmulsionsE. Klimov of BASF ( Germany ) explained how aqueous dispersions of polymers from emulsion suspension polymerisation could be espun into water resistant nanofibres. The polymer emulsion was first thickened with a water soluble polymer, preferably one with some fibre-forming capability, and then espun. The polymer particles coalesced as the water evaporated, then deformed and finally their surface molecules interdiffused to form a coherent, if fragile, fibre. 50nm emulsions could give 200nm fibres while 160nm emulsions could give 500nm fibres. The best fibres are obtained when the spinning temperature and the Tg of the polymer were matched, but cross-linking is also needed to increase strength and thermal stability. To achieve this allylmethacrylate is added as a co-monomer. Asked for the ratio of emulsion to thickener, Mr Klimov said they were spinning dopes with from 1 to 20% of emulsion solids. Disruptor™ Nanofibre filtersRodney Komlenic VP Business Development of Ahlstrom Filtration (USA) described their use of nanoalumina, in reality the naturally occurring mineral boehmite - AlO(OH), the major constituent of bauxite – as a filter aid. The mineral is refined using a proprietary technique into nanofibrils said to be 2nm in diameter and 250nm long. These fibrils have a specific surface area of 500m 2 /gm and are grafted onto microglass and wet-laid to make depth filters 0.8mm thick, with an average pore size of 2 microns. The filter papers have 42,000m 2 of nanofibre surface per m 2 of paper. The fibrils work well in water between 5 and 9 pH where they develop a “huge” electropositive charge and can attract particles which would normally miss an uncharged fibre by a micron. This means that the 2 micron pore size paper can filter out submicron particles with minor increases in pressure drop. Log 3 reduction values for removal of 27nm MS2 phage virus had been found and Log 4 reductions of 300nm bacteria (Diminutia) and cysts (simulated by 3 micron latex spheres). They are also capable of absorbing dissolved heavy metals such as copper, lead and iron. The main application envisaged is as a polishing filter for drinking water. Having hydroxyl functionality it can attach to cellulose and can be formed into 3D filters using the “egg-box” techniques. 32% of Boemite is typically added to the filters. Powdered activated carbon (27 micron) can also be added to the paper to remove halogens and soluble orgainics without reducing the filter's efficiency at removing viruses. Apparently sensitive to the fact that asbestos used to be fibrillated into nanofibres for filters, Mr Komlenic was keen to point out that the nanofibre emulsion version of Boehmite, “Alhydrogel” is approved by the FDA for use in formulating vaccines to treat diphtheria, tetanus, polio and pertussis. It is also commonly used as a thickener for gastric medicines and tablets. Asked about Boemite particles in the filtered water, they could be found, but only during the initial flushing out process. Cabin Air FiltersNico Behrendt of Helsa Automotive GmbH ( Germany ) mentioned their continuous bipolar nanofibre coating process used to treat both sides of a nonwoven simultaneously while being very cost effective and allowing great flexibility of polymer. The process was not described in any detail but from the schematic shown, it used a dip roll to pick up the polymer solution, this being scraped off the surface by a charged doctor blade, the other edge of which provided the electospinning edge. The benefits of the process were illustrated with reference to filtration efficiency versus pressure drop on cabin air filters with various coatings. At a mid-range pressure drop of 20 Pa, state of the art cabin-air filters were 25% efficient. This could be increased to 45% using single side coated uncharged nanofibres, and further to 65% with charged nanofibres. Their bipolar process, presumably with the electrical charge gave 90% efficiency at the same pressure drop. Ion Exchange NanofibresLudek Jelinek of the Institute of Chemical Technology (Czech Rep.) has been collaborating with Elmarco on producing ion-exchange nanofibres using the Nanospider equipment. Polystyrene was espun onto a PP spunbond, its stability increased by cross linking to allow mild sulphonation using 80% sulphuric acid at 25C for 5 minutes. Stronger sulphonation dissolved the nanofibre. Unfortunately the ion-exchange properties were too weak for industrial applications, so work continues with other polymers. Polymethacrylates, cellulose and chitosan were mentioned. Chitosan itself can adsorb heavy metals but uncrosslinked it dissolves in acids. It can however be phosphorylated and functionalised with other chelating groups. Inorganic NanofibresVit Chudoba of Elmarco (Czech Rep.) described Elmarco's development of inorganic nanofibres for electrochemical devices (batteries, capacitors, fuel cells etc), composites, thermal barriers, water purification and catalysts. The additives used, presumably loaded into a polymer solution were elements such as copper, carbon and nickel; metal oxides (NiO, TiO 2 , ZrO 2 , and CuO); ceramics (SiC, SiO 2 , Al 2 O 3 ) and spinels, particularly lithium titanate. The lithium titanate nanofibre webs were for anodes of high power Lithium ion batteries, the metal oxides were for composite reinforcement or thermal insulation, and the TiO 2 was for photocatalysis where the sub-500nm fibres had a surface area of 40-60m 2 /gm and outperformed Degussa's P25 TiO 2 powder. Solvent-free electospinningWiebke Voigt replaced Helga Thomas of the German Wool Research Institute (RWTH Aachen - Germany ) to talk about espinning water solutions of polymers. Clearly the waterbased process gives water-soluble nanofibres, and to be useful these have to be stabilised by cross-linking. Presumably this yields a nanofibre superabsorbent intermediate, but Ms Voigt did not mention this. • Lupamine, a poly(n-vinylamine) from BASF had been modified with BHBP and MAC so that it could be stabilised by UV-induced crosslinking to give a fibre with intrinsic antimicrobial activity. Needleless EspinningDavid Lukas of the Technical University of Liberec (Czech Rep.) pointed out that liquid surfaces deformed into multiple tiny cones when subjected to a strong electric field, and if the liquids contained polymers this deformation could lead to fibre formation. This was the basis of TUL's spinning of clouds of near invisible nanofibres from the surface of a smooth roller rotating in bath of polymer solution, a process which was now being commercialised by Elmarco and is the reason for the conference. Mr Lukas explained the physics involved in incomprehensible detail. Centrifugal SpinningMartin Dauner of ITV Denkendorf ( Germany ) observed that while molten glass, pitch (carbon-fibre precursor) and Basofil have been converted into micron sized fibres by centrifugal spinning for many years, little work has been done using the process to make polymeric fibres. Commercial centrifugal sprayers (“Center Bells”) have been developed for paint and varnish application by Reiter (not Rieter), these being hand held with rotors running at 45,000 rpm. These have now been adapted to handle fibre forming solutions, and each head can process about a third of a litre of solution per hour to give fibres in the range 0.1-0.7 micron diameter. Each head has coverage of about one-third of a metre, and they can easily be lined up to make wider widths. Gear pumps are used for accurate polymer feed. Fibrous webs shown were: • Polyethylene oxide nanofibres from a 5% solution in water. Compared with espinning, the centrifugal approach gave similar fibre sizes at higher productivity (0.5 kg/m.hr claimed at this point), easy cleaning of spinning heads, and easy scale up by replication of commercially available heads. The downside? Filament diameter variability was higher and the system worked less well with solvents of low boiling point (80C minimum). Furthermore, fibre diameters below 100nm appeared impossible, and melts were too viscous to work. A 1.5 metre line running at 7 m/min was being planned. Medical ApplicationsGary Wnek of the Case Western Reserve University ( Ohio USA ) is growing cells on espun nanofibre webs. He has shown that the inherent charge on such webs attracts cells and encourages tissue formation. Random webs form random tissues (e.g for skin) and oriented webs form oriented tissues (e.g for muscles). The cells can be grown separately on the opposite sides of the web and eventually link up to form a bi-layer structure – like skin. Webs tried were listed as: • Poly lactic and glycolic acids and blends. Collagen nanofibres were particularly successful for skin regeneration because they mimicked natural collagen scaffolding and exhibited excellent cell infiltration. The best solvent for espinning collagen was hexafluoroisopropanol. Asked if nanofibres were really necessary, Dr Wnek said that it depends on the cell types. For cartilage, micro fibres are best: for liver cells, even coarser fibre are better. If the cells are much larger than the fibres, the fibres are incorporated in the cells. If the cells are much smaller, they grow on the fibre surface. Multi functional scaffolds for retina regeneration were using suspensions (rather than solutions) of poly(lactic-co-glycollic) acid, the water phase carrying the MMP2 gelatinase protein. The protein domains could be seen along the nanofibres. Asked if the lack of transparency of PLGA webs was a problem, it was not. Once the cells had colonised the web, web opacity was irrelevant. Elmarco DevelopmentsJana Svobovoboda of Elmarco R&D (Czech Rep.) described the work being done with chitosan, collagen and alginate on the Nanospider™ roller espinner to make antimicrobial and antiviral filters for face masks. • Chitosan's haemostatic, bacteriocidal, fungistatic and antitumoric properties make it a natural for biomedical fabrics, but it has proved hard to espin on its own. Now, Elmarco has a patent on blending chitosan with a little (<10%) polyethylene oxide which makes it work better. PVA can also be used in this support role. Applications in waste water treatment, HVAC filters, cabin air filters and cleanroom filters are now being sought. 1 O 2 from NanofibresJ Mosinger of Charles University in Prague (Czech Rep.) is producing polyurethane nanofibre webs doped with 5,10,15,20-tetraphenylporphyrin (TPP) photosensitizer. The 2 gsm webs contain 0.12% of the TPP, which on exposure to light releases singlet oxygen, a highly cytotoxic bacteriocide at the fibre surface. E.coli can be grown on the webs in the dark, but exposure to light kills the colony and prevents further growth. Auto disinfecting, sterile materials are therefore possible. Asked how much light is needed to trigger the kill, Dr Mosinger said about 2 hours in daylight or half an hour under a 100w halogen lamp. What was the shelf life of the fabric? About a year if kept in the dark. How transparent was the web? 90%: it was only 0.03mm thick. Electrochemical Storage DevicesLukas Rubacek of Elmarco (Czech Rep.) has demonstrated that the Nanospider™ roller system can make lithium titanate spinel nanofibres (LTO) for use in batteries, capacitors and supercapacitors. As an anode in a lithium battery, the LTO fibre allows higher potential leading to better safety than the conventional lithium ion battery which can ignite if the charging circuit malfunctions. It's theoretical capacity is 176 mAh/g (c.f. 300 mAh/g of the lithium carbide anode batteries) but it can be discharged and charged at much higher rates safely and without damage. 3 minute recharge times with a 5000 cycle life were quoted. So, high power rapidly rechargeable batteries which will not catch fire can be developed. The very high surface area of the LTO webs also allows electrical double layer capacitors to be made at higher capacities with lower internal resistance. Air FiltersStanislav Petrik of Elmarco (Czech Rep.) has been investigating the effects of nanofibres spun from the Nanospider™ machine on the performance of cellulosic air filter media. He has discovered that the Relative Fibre Length, i.e. the number of kilometres of nanofilament per square metre of surface correlates well with filtration performance for a wide range of fibre sizes and basis weights. In fact RFL and Pressure Drop correlate better than Basis Weight and Pressure Drop, so image analysis to establish fibre diameter and RFL can predict filtration performance where basis weights in the 0.01 to 0.1 gsm range are much harder to determine. Filtration efficiencies of submicron particles is increased by >>100% while air permeability is only reduced by >10%. Sound AbsorbtionKlara Kalinova of the Technical University of Liberec (Czech Rep.) stressed that here we were considering absorbtion, not insulation, of sound. Absorbtion, being the difference between incident and reflected sound, is very dependent on sound frequency for any material. Most absorbtion occurs at the resonant frequency of the material, in reality the resonant frequency of the free fibres in the wadding or nonwoven. The remainder of absorbtion occurs due to friction between the vibrating air and the fibres. Layered nanofibres work best, and if a density gradient can be set up sound absorbtion is further improved. As fibre size is reduced the resonant frequency falls, so the finer the fibre, the better the absorbtion of usually problematical low frequencies. An acoustic nanofibre is now being developed as part of the recently announced collaboration between Elmarco and Oerlikon-Neumag. Aligning NanofibresYousef Mohammadi of the Stem Cell Technology Company, Tehran ( Iran ) is using the dynamic gap electospinning method to precisely control the alignment of filaments in a nanofibre web. In this method the electodes attracting the fibres are two vertical rotating discs space about 10 inches apart below the vertically downwards extruding nozzle. Both discs are at the same voltage and the nanofibres align themselves between them. As the synchronised rotation of the pair of discs carries the aligned filament away from the forming zone they are collected on the surface of a drum rotating between the lower half of the electrodes. The text referenced an EU patent application. (no details)
Calvin Woodings 29 th October 2007 |
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