It is well known that meltblown fabrics (with 2-5 micron fibres) can enhance liquid retention and decrease water contact angle (1).
Sub-micron fibres are expected to bring value to applications where properties such as sound and temperature insulation, fluid holding capacity, softness, barrier property enhancement, and filtration performance are needed.
Micro fibres are widely used in ultrasuede and other synthetic leather products; other possibilities are in the wipes and filtration field, for medical or semiconductor applications, which require the ultimate in superfine filtration (2,3 ). Currently, researches are also exploring the use of nanofibres in composite materials for protective gear such as facemasks, medical gowns and drapes, and protective clothing applications (4).
Micro and nanofibres lead to high specific surface area, a critical feature that emerging technologies offer compared to the more traditional fibre-based products.
Specific surface area is measured in terms of area per gram of material (m2/g). Most normal fibres are in the range of 1 to 2 denier and higher and therefore, the fibre diameter is relatively large. Consequently, regular fibres possess very low surface area. To achieve higher surface areas requires significantly smaller fibres.
It is not therefore, surprising that there has been a tremendous amount of activity dealing with various ways to form fibres that are much smaller in scale. The terms micro and nano are used interchangeably in the literature. The term nano is often referred to fibres having a diameter below one micron. There are several documents including patents that coin the term nano as referring to fibres having a fibre diameter of less than two microns. Today, these definitions are invalid. Meltblown products can be easily developed with a mean diameter of 0.5 micron. Some believe that one should use the term nano when the fibres are below 0.1 micron. Others believe that the term should be used when the fibre diameter falls below 0.5 microns.
However, one also cannot ignore the fibre diameter distribution. Can for example, one refer to a web as being composed of nanofibres where only a small percentage of the fibres are below one micron? Regardless of what we define as ‘nano’, it is critical to also determine and report the standard deviation.
Below, we review various recent efforts aimed at developing fibres below 1 micron.
Electrospinning
Electrospinning is no longer ‘new’ and a ‘secret’. While its history can be tracked to 1911 (5), its widespread commercial use is rather new. There are numerous commercially available machine producers and there exist now a large number of commercial products that rely on electrospun webs. Most notable are: Elmarco, ANSTCO, MECC, Kato Tech, Yflow among others.
The Elmarco system is different in that it is a needleless system that offers potential advantages over other nozzle based systems in terms of uniformity.
The literature is full of papers reporting fibres down to 200 nm range. It is however, possible to push the edge on this technology and 50 nm fibres are quite possible. This translates to specific surface areas higher than 20 m2/g. Compared to other materials, and other fibres, this is a revolutionary step in the right direction. High surface areas in the range of 20 and higher offer potential applications for replacing much more expensive systems such as chromatography beads.
One challenge with electrospinning remains that it is still incapable of dealing with polymers in melt. Melt electrospinning has not yet been demonstrated to be a viable alternative to meltblowing.
There are three key features of electrospun webs that make them unique:
1) Structure isotropy (the fibre diameter distribution is uniform)2) The fibre diameter distribution is normally incredibly good3) Excellent basis weight uniformity in terms of spatial distribution.
These together lead to webs that offer excellent performance in critical applications such as filtration. The use of electrospinning is today limited to applications requiring low basis weight to achieve desired performance – in filtration, for example, 0.2 or 0.3 grams per square metre would be sufficient to produce high efficiency aerosol filters. In these applications, electrospinning will compete well with other technologies that require significantly higher mass to achieve similar performance.
Other Systems
There are numerous other systems that have been developed that also produce sub-micron fibres. Notable examples are Fiberio, Xanoshear and Dienes. These systems are all reported to be able to produce sub-micron fibres. There is little data available on the properties of fibres (length and diameter distribution) and/or web properties in terms of fibre orientation distribution and mass uniformity. These are technologies to watch.
Meltblown Systems
Meltblowing has been the king of microfibres for many decades – it is hard to imagine what the nonwovens industry would be today in the absence of meltblowing. This has been an enabling technology leading to incredible innovations in medical, hygiene, filtration, adsorbents, and many other market segments. Typical meltblowing can produce fibres in the range of 2 microns at high throughputs ( 80 kg/metre/hour/beam). This is several orders of magnitude higher than electrospinning.
Recently, it was demonstrated that it is possible to produce fibres down to 300 nm or less by using novel dies and reduced throughput. At more than 10 kg/metre/hour/beam (6).
One notable advantage of meltblowing is that it is done in the melt (the use of solvents is frowned upon by many today due to environmental concerns) and uses readily available, well-developed polymer stock. For most applications, it is hard to imagine that there is a technology that can compete commercially with meltblowing.
And, meltblowing technology is not staying static. There are significant developments underway that are and will be resulting in major advances in the ability to produce large quantities of micro and nanofibres. Look for example at the developments enabled by systems that rely on polymer melt fracture to yield sub-micron fibres. A critical feature of melt fracture is the ability to use the technology to form fine fibres. The origin of the latest technologies being practiced is due to Leonard Torobin, and Richard Findlow in US patent 6,183,670, and Reneker in US patent 6,382,526. Both these patents use annular films that are fractured by air streams to form fine fibres. In a patent by Chhabra, et.al, (US patent 7,291,300), the first claim states:
“A method of forming a nonwoven web comprising the steps of:• forming fibres from a melt film fibrillation process in a first fluid stream• providing a coating substance in the first fluid stream,• applying the coating substance onto the surface of the fibres, and• depositing the coated fibres on a surface to form a web.”
The latest commercial technology developed and introduced by PGI (Arium Technology) is also based on similar concepts but extended to a curtain spinning. (7)
Nanofibre formation in bulk is already possible and will undoubtedly continue – meltblowing, we believe, will continue to lead in all but perhaps the most specialised of applications.
Spunbond Systems
Spunbonding has been the king of nonwovens for many decades – it produces fibres as low as one denier in incredibly high volumes. These filaments are highly drawn, are fully formed and offer high strength. They are often used to ‘protect’ meltblown (or electrospun) webs in a sandwich structure.
One critical feature of spunbond structures is that they produce fibres that have a tight fibre diameter distribution. Couple this with bicomponent technology and one can potentially produce structures composed of sub-micron fibres at high throughput. For example, the islands in the sea technology has the potential of reaching that goal of producing nanofibres with uniform size because the islands are metered and formed as fibres and are incredibly uniform is size. The challenge with the spunbond process remains in the polymers used and the way in which the sea is discarded. Watch out for future developments in EastOne (Eastman Chemical) – a water dispersible polyester that is an enabler for technologies such as islands in the sea.
A recent development at the Nonwovens Institute has led to the fibrillation of the sea – the sea remains in the structure but is fibrillated and the islands are released… this eliminates the need for the removal of the sea from the structure. The net result is a flexible, durable fabric containing fibres as small as 300 nm (8).
And, there are other methods for forming high surface area fibres. The concept goes back to Eastman Chemical and Procter and Gamble who developed shaped fibres known as Deep Grooved Fibers. 4DG is commercial product offering by Fiber Innovation Technologies (Tennessee) that offers significantly higher surface areas than regularly sized fibres. 4DG is produced by using special spinnerets and is therefore, limited to fibres that are larger than 6 denier per filament. A new development in this area uses bicomponent fibre technology to form a shaped multi-lobed fibre as a core wrapped by a sacrificial sheath (9). The fibre is known as the ‘winged’ fibre and contains anywhere from 12 to 32 lobes.
The ultimate question is what technology we should use. Each technology is unique in its own way. The answer surely depends on the final properties required, the cost and flexibility, and availability of the technology and the fit for the product or the business. And, as is the case with most structures, the final solution may be a hybrid one taking advantage of unique properties of more than one technology alone. The fact remains that no single technology can be the answer to all questions – we are faced with many choices and many opportunities. The one who wins is the one who will choose the right technology solution for the right reason. The world of fibres remains as exciting today as ever.
References
(1)Schreuder-Gibson, Heidi, Gibson, Hsiesh, Y-L, Transport properties of electrospun nonwoven membranes, proceedings of the International Nonwovens Technical Conference, September 5-7, (2001)
(2)Royal Kessick and Gary Tepper, Microscale electro spinning of polymer nanofiber interconnections, Applied Physics Letters, vol.83, no.3, pp. 557-559, (2003)
(3)Darrell H Reneker and Iksoo Chun, Nanometer diameter fibers of polymer, produced by electro spinning, Nanotechnology 7, pp.216-223, (1996).
(4)U.S. Patent No.3705226 Okamoto et al., Method for manufacturing fibrous configuration composed of a plurality of mutually entangled bundles of extremely fine fibers
(5)Physical Review 3 (2): 69–91. doi10.1103/PhysRev.3.69.
(6)M. A. Hassan a, B. Y. Yeom, A. Wilkie, B. Pourdeyhimi, and S. A. Khan“Fabrication of nanofiber meltblown membranes and their filtration properties”, Journal of Membrane Science 427, 336–344, (2013).(7)http://www.textileworld.com/Articles/2011/November/PGI_Debuts_Arium_Technology.html
(8)B. Pourdeyhimi, N. Fedorova, and S. Sharp, High Strength, Durable Micro and Nano-fiber Fabrics Produced by Fibrillating Bicomponent Islands in the Sea Fibers, US Patent 7,981,226, July 19, 2011.
(9)B. Pourdeyhimi and Walter J. Chappas, High Surface Area Fiber And Textiles Made From the Same, US Patent 8,129,019, March 6, 2012.