10 -6 cm µm mm cm mm 10 -9 10 -3 10 -2 10 -12 nm pm pm µm mm 10 -9 nm nm nm nm Cover story 22 Filtration+Separation September 2005 Nanofibres: media at the nanoscale N anofibres can create excellent filtration media for both gases and liquids – but producing them can be a tricky business. Greg Ward of Nonwoven Technologies explores three methods... In order to discuss the esoteric field of nanofibres it is helpful to understand the relative size of nanofibres compared to other fibres and objects. ‘Nanofibres’ is becoming a very popular term today. It is rapidly reaching ‘buzz-word’ status in liquid and air filtration technology. Interestingly, nanofibres mean different things to different technical groups. Variously, nanofibres are described as fibres with diameters less than 1 micron, fibres with diameters less than 0.3 microns or some other arbitrary range. Obviously a fibre with a diameter of 500 nms is a nanofibre regardless of what convention is used. This article will define nanofibres as having a diameter between 0.001 and 1 micron – more accurately from 1 to 1000 nms. What does that mean from a practical perspective? The best way to understand what a nanofibre means is to compare it to ordinary fibres or other substances we can already measure. For example, we know a cotton fibre is about 18,000 nms in diameter compared to a human hair, which is approximately 30,000 nms. Ordinary meltblown fibres are 5,000 nms in diameter compared to 25,000 nms for typical spunbond. In the very small real world of bacteria and viruses we know that bacteria are typically around 900 nms compared to about 20 nms for a virus. Finally the diameter of a single molecule of polyethylene is about 1 nm. An interesting characteristic of nanofibres, especially in the 700 nm and lower ranges, is that they only appear as an undifferentiated mass; individual fibres are not discernable. You need an electron microscope to actually see what you have produced. Benefits of nanofibre media Fine diameter fibres have always been of interest in filtration applications. It is generally understood that the smaller the diameter fibres, the better are their filtration properties. The mechanisms of fine fibre filtration are fairly well understood and won’t be covered in depth here. However it is sufficient to point out that with nanofibres “a little bit goes a long way”. Nanofibre production methods for filtration media There are three major processes for producing nanofibres for fluid filtration media. They include electrostatic spinning, improved modular melt blowing and multi- component fibre spinning or the ‘islands-in-the-the sea’ method. Each process has its advantages and disadvantages and is described below. Melt blown systems (see figure 2, page 23) Standard Exxon type meltblown fabrics have been the main options available for companies interested in participating in markets where fine fibres are required. These meltblown fibres typically range from between 2-5 microns in diameter. Meltblown materials have limitations that have kept them from expanding their usage into a broad range of filtration technologies and applications. Meltblown fabrics are typically produced only from polypropylene. Fibres produced are in the narrow range of two to five Figure 1: Meltblown fibres, from 250-750 nms.
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Cover story22Filtration+Separation September 2005
Nanofibres:media at the nanoscaleN anofibres can create excellent filtration media for
both gases and liquids – but producing them can be a tricky business. Greg Ward of Nonwoven Technologiesexplores three methods...
In order to discuss the esoteric field ofnanofibres it is helpful to understand therelative size of nanofibres compared to otherfibres and objects.
‘Nanofibres’ is becoming a very popular termtoday. It is rapidly reaching ‘buzz-word’status in liquid and air filtration technology.Interestingly, nanofibres mean differentthings to different technical groups.Variously, nanofibres are described as fibreswith diameters less than 1micron, fibres with diametersless than 0.3 microns or someother arbitrary range.Obviously a fibre with adiameter of 500 nms is ananofibre regardless of whatconvention is used. Thisarticle will define nanofibresas having a diameter between0.001 and 1 micron – more accurately from 1 to1000 nms.
What does that mean from apractical perspective? The bestway to understand what ananofibre means is to compareit to ordinary fibres or othersubstances we can alreadymeasure.
For example, we know acotton fibre is about 18,000 nmsin diameter compared to a human hair,which is approximately 30,000 nms.Ordinary meltblown fibres are 5,000 nms indiameter compared to 25,000 nms for typicalspunbond. In the very small real world ofbacteria and viruses we know that bacteria
are typically around 900 nms compared toabout 20 nms for a virus. Finally thediameter of a single molecule ofpolyethylene is about 1 nm.
An interesting characteristic of nanofibres,especially in the 700 nm and lower ranges, isthat they only appear as an undifferentiatedmass; individual fibres are not discernable.You need an electron microscope to actuallysee what you have produced.
Benefits of nanofibre media
Fine diameter fibres have always been ofinterest in filtration applications. It isgenerally understood that the smaller thediameter fibres, the better are their filtration
properties. The mechanisms of fine fibrefiltration are fairly well understood andwon’t be covered in depth here. However itis sufficient to point out that withnanofibres “a little bit goes a long way”.
Nanofibre production methods forfiltration media
There are three major processes forproducing nanofibres for fluid filtration
media. They includeelectrostatic spinning,improved modular meltblowing and multi-component fibre spinning orthe ‘islands-in-the-the sea’method. Each process has itsadvantages and disadvantagesand is described below.
Melt blown systems(see figure 2, page 23)
Standard Exxon typemeltblown fabrics have beenthe main options availablefor companies interested inparticipating in marketswhere fine fibres are required.These meltblown fibrestypically range from between2-5 microns in diameter.
Meltblown materials havelimitations that have kept them fromexpanding their usage into a broad range offiltration technologies and applications.Meltblown fabrics are typically producedonly from polypropylene. Fibres producedare in the narrow range of two to five
Figure 1: Meltblown fibres, from 250-750 nms.
Cover story 23Filtration+Separation September 2005
microns (2000 to 5000 nm) fibre diameter.Although the fabric can be electrostatically-charged to increase its filtration efficiencythe fabrics are relatively weak and generally
must be used with support media. Normallymeltblown fabrics are layered to optimisetheir filtration properties. This requires thata number of layers of meltblown fabrics are
needed to provide a balance of penetration,solids holding capacity, and pressure drop.This becomes an expensive way to meet acustomer’s needs.
These deficiencies of standard or Exxon typemeltblown fabrics have been overcome by anew meltblown technology introduced byNonWoven Technologies (protected by USPatents 5,679,379 and 6,14,017 and EUPatent 0893517).
The modular dies which are constructedfrom thin stainless steel plates producemeltblown fabrics with average fibrediameters depending on the melt orifice size. For example, a melt orifice size of 0.003 inches (0.076 mm) will producepolypropylene fibres between 200 and 500nms in diameter. Modular die constructionpermits the use of any metal. Virtually any polymer can be blown with polymer side pressures reaching above 3500 psi (240 bar).
Because the generation of fibres that are lessthan 1.0 m (1000 nms) in diameter requiressmall orifices (0.09 inch or less), andpolymer flow of about 0.01 to 0.30grammemes per hole per minute (ghm,)serious concerns about the economicviability arise. The polymer flow rate isapproximately 10% of what is generally
A - Stream of Hot AirB - Molten Polymer FeedC - Cool AirD - Collector Screen
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Figure 2: Meltblowing the Exxon way.
Cover story24Filtration+Separation September 2005
considered to be an economically viablecommercial output of 0.8 to 1.2 ghm.
Consequently a commercially viablenanofibre process must compensate byhaving 10 to 20 times the number of orificesper metre of the standard commercialequipment. Since the typical commercialmelt blown die is constrained to a single rowof orifices, this would suggest that acommercial sub-micron meltblown linewould require a series of eight to twelvestandard meltblown dies.
At first glance this appears to be intrinsicallyuneconomical and is if standard Exxontechnology is used. However, NonwovenTechnologies’ meltblown fibre productionline can provide economical levels ofcommercial production. The structure andcomposition of such a line is described below,and is shown in figure 3 below.
This process can be economicallyconstructed and run using NonwovenTechnologies’ SpunBlown system, whichincludes novel equipment also identifiedbelow.
Modular meltblown systemattributes and description
The major attributes of the modular die meltblown system are
• unique modular die construction usinglaminated plates
• twin or triple orifice rows in each diemodule
• ultra high die operating pressures (1500 to3000 psi)
• narrow machine direction die width
• high hole density meter of line width
• unique air jet positioning options
• capability to use super-sonic air jet nozzles
• multiple spin pumps with multiple outlets
• multiple in-line dies can be used tofurther increase capacity.
As an example, a one m wide NTI die withthree rows of 0.005 polymer orifices wouldcontain a total of 4800 orifices. This wouldproduce 300 m3 per minute of 3grammeme/M2 meltblown web having anaverage fibre diameter of 500 to 600 nms.
Ultra-high surface area is a primary benefit ofusing nanofibres for filtration. The surfacearea per gramme difference for 300 nm fibresproduced by modular dies compared totypical 2-5 micron meltblown fibres is afactor of seven. Put in another context, thenanofibres above have more than seventimes the surface area per gramme as theExxon type system.
Electrostatic spinning
Electrostatic spinning is another method ofmaking nanofibres with fibre diameters inthe range of about 10 nm to 350 nm from apolymer solution through electrostatic force.The process has been described in literatureand in patents.
When a droplet of polymer solution issubject to a high electrical voltage the chargedrags the solution from the tip of a capillaryto a collector. A voltage is applied to thepolymer solution, which causes a jet of the
solution to be drawn to a grounded collector.The fine jet sprays can be collected on anonwoven carrier fabric.
Using compatible polymers and solventsystems, nanofibres can be made withdiameters in the range of 40 to 800 nm.These nanofibres have the potential ofnumerous applications including highefficiency filter media, protective clothingmaterial, medical membranes, etc.
A drawback of electrospun fibres is weakstrength. This is due in part to low drawingforces and the small diameter of thenanofibres. A further problem is that somepolymers are notoriously difficult to dissolve.Some researchers have been successful inelectrospinning polymer melt.
Splitting bicomponent fibres
A third technique that can be used toproduce nanofibres is spinning bicomponentfibres that split or dissolve. There are severalapproaches to using this technology to makenanofibres.
The most researched approach is theproduction of ‘islands-in-the-sea’ fibres usinga standard spunbond process with specialisedspinnerets. Hills describes a process where1120 islands were used and the compositefibre had a final drawn denier of one. Theproduction rate was approximately 5kilogrammes per hour at a take-up speed of2500 metres per minute. Polypropylene wasused for the island polymer with EVOH usedas the sea polymer.
After dissolving the sea polymer, theresulting nanofibre had a diameter ofapproximately 300 nm. Unlikeelectrospinning and melt spinning, thenanofibres produced with this technique hada very narrow diameter range.
Conclusion
Based on the manufacturing processesdescribed above for polymeric nanofibres,there is a definite long term potential forcommercial nanofibres. The ease ofmanufacturing may show that nanofibresproduced by modular die technology have anedge over electro-spinning and splittablefibres.•About the authorGreg Ward is Vice President of NonWovenTechnologies since 1995. He holds both anME and a Juris Doctorate degree. He has 11patents and 9 patents pending. During hiscareer he has worked with Shell Chemical,Johnson & Johnson, Scott Nonwovens andInternational Playtex in R&D Management.He can be contacted at:Nonwoven Technologies, Inc.Oyster BayNew YorkUSAFigure 3: 2 row modular die system.
