Although the nanofiber is defined as a fiber equal to or less than 100 nm in diameter, that of less than one micron in diameter is also considered to be that in general. Among the nanofiber nonwovens, those made through an electrospinning process attract the most attention, but materials made through meltblown are also common. In terms of fiber thinness, 500 nm is generally the lowest that can be made with meltblown whereas electrospinning can handle fibers less than 100 nm in diameter.
As for the productivity of the nonwovens, meltblown is said to be superior. In the electrospinning method, a polymer, dissolved with solvent is used as the crude material for fiber forming and the polymer level of the crude material is around 20%, therefore, the materials, spun to be fiber, would be around 20%, because about 80% of the residue are solvent and vaporizable. On the other hand, in the meltblown method, 100% of the crude material would turn into the fiber since the molten polymer would be spun in the condition as it is. Subsequently, meltblown has the competitive edge in the range of 500-1000 nm in fiber diameter. So, the competitive strength in business of the electrospinning method cannot conflict. Owing to these characteristics, nonwovens, equal to or less than 300 nm, are mainly produced by the electro spinning method. In other words, the fiber forming by the electro-spinning method is deployed in the field where the meltblown method cannot go into. Yet, many kinds of crude material for nonwovens could be used in the electrospinning method and the polymers, which could be spun by meltblown method, could be applied. Consequently, this method could have a competitive edge even in materials of more than 500 nm thickness.
Filters and the nanofiber nonwovens
Although, a lot of enterprises are manufacturing nonwovens by electrospinning, companies making the nanofiber nonwovens by a meltblown method are limited. Mistui Chemicals, Asahi Kasei, Kuraray and Tapyrus have been producing nanofiber nonwovens by the meltblown method in Japan. The leading end use applications are air filters and liquid filters. For filters, to achieve the required precision, the quality of the filter media is important. As for the meltblown nonwovens, the fiber diameters are around 500nm, and to obtain higher emulgent, electrospinning nonwovens are required.
The electrospinning nonwovens, equal to or less than 300nm in fiber diameter, could be substituted for glass fiber of the HEPA filter in the case of air filter and it could be used as a substitute in membranes for liquid filters. Although the nonwovens would not easily be substituted for glass fiber because of the high cost, there is a competitive edge for the membrane. Because of lower percentage of void in the membrane than nonwovens, the pressure drop rises when used for filter media. While high emulgent of the nanofiber nonwovens of the electrospinning could be balanced with low pressure drop, the liquid filter is in a field where the characteristic of nanofiber nonwovens could be shown well enough. In fact, it may well be said that the sector is more promising than the air filter section.