We are all familiar with the latest technical buzz words. In
the recent past, we become aware of micro-machines, dot coms,
genome, and so forth. Today the hottest new word in textiles is
nanofibers. We have read about nanotubes, nanomaterials, nanomachines
and now nanofibers. In its general definition, nano means one
millionth (1/106) of a millimeter or 10-9 meters. When the term
is applied to technology (nanotechnology), the common definition
is "the precise manipulation of individual atoms and molecules
to create layer structures".
Okay, so what is a nanofibers? To answer this question we first
need to put a workable definition on the terms that came before
such as microfiber. Table I is my attempt at quantifying the six
most common terms used to describe the size of fibers. It is obvious
from Table I there is a breakdown in the terms used to define
fibers when the fiber size is below 0.3 denier. The term micron
has not been commonly accepted and has been somewhat corrupted
by the over use and somewhat ambiguous term microfiber.
Unfortunately, the term nanofibers has also become somewhat ambiguous
even though the definition given in Table I is very precise. The
reason for the confusion is carbon nanotubes which are an ordered
array of carbon atom which can have tensile strength up to 15
x that of steel. These tubes or fibers are often called graphite
or carbon nanofibers as well as nanotubes. The technology for
manufacturing carbon nanotubes is very different from common fiber
production techniques and the end uses are not those commonly
associated with fibers. The nanofibers of interest to the fiber
industry are polymeric nanofibers made from conventional and newly
emerging polymers and with end uses typical of standard textiles.
These fibers are the subject of this article. For these types
of fibers the smallest practical size is approximately 50 nanometer
as a polyester crystallite has dimensions in the order of 40 nanometer
so structures approaching this size would begin to become an ordered
array of atoms and would not have typical fiber morphology.
Research into production of polymeric nanofibers has been funded
by the government for almost twenty years. Much of the early work
was done by Darryl Reneleer at the University of Akron. The agency
most interested in this area has been the U.S. Army and the Natick
Solider Center looking for improved barrier fabrics for clothing.
Natick recently reported that only small amounts of nanofibers
on the surface of meltblown fabrics greatly enhances liquid retention
and decreases water contact angle (Ref 2). Other factors such
as air resistance and breathability are also expected to be significantly
impacted as nanofibers are added to a nonwoven fabric (Ref. 3).
However, the fact that we can manufacture nanofibers and find
beneficial end uses does not sufficiently answer the question
are nanofibers fantasy or future? For the answer to this we need
to rephrase the question: Will the manufacturing costs of nanofibers
be less than the potential value of the benefits imparted? To
explore this questions we need to review potential manufacturing
techniques.
The manufacturing techniques most after associated with polymeric
nanofibers is electrospinning (Figure 1). In this technique a
polymer is dissolved in a solvent (polymer melts can also be used)
and placed in a glass pipet tube sealed at one end with a small
opening in a necked down portion at the other end. A high voltage
potential (>50kv) is then applied between the polymer solution
and a collector near the open end of the pipet. This process can
produce nanofibers with diameters as low as 50 nanometers although
the collected web usually contains fibers with varying diameters
from 50 nm to two microns. The production rate of this process
is measured in grams per hour. Therefore, unless the production
rate of this technique can be increased by several orders of magnitude,
the cost of nanofibers production will continue to relegate them
to mostly a laboratory curiosity.
Another technique to produce polymeric nanofibers has recently
been introduced by Nanofiber Technology Inc. of Aberdeen, NC.
In this scheme described in preferences 4 & 5, nanofibers
are created by melt blowing a fiber with a modular die. The fibers
produced are a mixture of both micron and submicron sizes. This
technique lends itself to the use of thermoplastic polymers in
a relatively inexpensive spinning process. The technique does
appear to have the potential to make large quantities of polymeric
nanofibers at a cost less than $10 per kilogram. However, there
are still several unknown and concerns. One concern is the broad
range of fiber diameters produced (this could be of advantage
in some applications), and the other is the cost of spinning equipment
versus the production rate. Despite these concerns, this technique,
if perfected, certainly takes nanofibers production from a laboratory
curiosity to a possible commercial future.
A third technique that can be used to produce nanofibers is spinning
bicomponent fibers that split or dissolve. There are several approaches
to using this technology to make nanofibers. The most researched
approach is the production of islands-in-the-sea (INS) fibers
(Figure 2) using a standard spin/draw process. 1120 islands were
used and the composite fiber had a final drawn denier of one.
The production rate was approximately 5 kilograms per hour at
a take-up speed of 2500 mpm. PP,PET and PA-6 were all used for
the island polymer with EVOH used as the sea polymer. The bicomponent
polymer ratio was 50/50. The resulting nanofiber after dissolving
the sea polymer had a diameter of approximately 300 nm. Unlike
electrospinning and melt spinning, the nanofibers produced with
this technique had a very narrow diameter range. The projected
cost of these fibers is in the range of $1 to $5 per kilogram
which should be low enough for most commercial applications particularly
since most applications will include a small percentage of nanofibers
combined with standard melt spun fibers.
Another possible approach to the use of bicomponent fiber spinning
to manufacture nanofibers is to make splittable fibers in a melt
spinning process (Ref. 6). The number of segments would need to
be sixteen or greater and the best approach might be to use a
water dissolvable polymer in a small ratio along with PET or PP
(Figure 3). The ultimate approach is to melt blow INS fibers that
contain > 600 island fibrils that would have diameters as low
as 50 nm and which act as a regular melt blown fiber through fabric
formation after which the sea polymer is dissolved and only the
nanofibers are left.
Based on this review of the existing manufacturing processes
for polymeric nanofibers, the potential to bring large quantities
of nanofibers to the market place at relatively inexpensive prices
appears feasible and can be achieved within the next five years.
There is still a lot of development to be concluded and end uses
must be proven; however, polymeric nanofibers do appear to be
in the future of the fiber industry.
References:
1 - "Dictionary of Fibers and Textile Technology",
Hoechst-Celanese, 1990
2 - Schreuder-Gibson, Heidi L., Gibson, P.,Hsieh, Y-L, "Transport
Properties of Electrospun Nonwoven Membranes," proceedings
of the
International Nonwovens Technical Conference (INTC), September
5-7
2001
3- K. Graham, etal, "Polymeric Nanofibers in Air Filtration
Applications",
presented at the 15th Annual Technical Conference of the American
Filtrations and Seperations Society, April 8-12, 2002, Galveston,
TX
4 - U.S. Patent 6,183,670: "Method and Apparatus for Producing
High Efficiency Fibrous Medin Incorporating Discontinous Sub-Micron
Diameter Fibers and Web Media Formed Thereby," L. Torobin
and
R. Findlow, February 6, 2001.
5 - U.S. Patent 6,114,017; "Micro-Denier Nonwoven Materials
Made Using Modular Die Units", A. Fabbricante, G. Ward and
T. Fabbricante,
September 5, 2000.
6 - U.S. Patent 5,935,883; "Super Fine Microfiber Nonwoven
Web",
R. Pike, August 10, 1999.
Table I
Common Terms Used to Describe the Size of Fibers
Term Definition
Monofilament
A single filament of fiber used individually with a denier generally
greater than 14 (Ref 1) The size of nanofibers are usually described
by the diameter in either microns or inches (mils)
Denier Weight-per-unit-length measurement of a liner material
defined as the number of grams per 9000 meters. Can refer to either
an individual filament or a bundle of filaments (yarn).
Decitex Similar to denier expect it is the weight in grams of
10,000 meters of a yarn or fiber.
Microfiber Primarily a marketing term used for multifilament yarns
where the individual filaments have a denier less than one. A
typical one denier polyester fiber has a diameter of 10 microns.
Micron
(-Sized Fibers) When fiber size is less the 0.3 denier it is best
to define the size is terms of its diameter in microns (10-6 meters)
Nanofibers Term used for fibers with diameters less than 0.5 microns.
Typical nanofibers have a diameters between 50 and 300 nanometers.
They can not be seen without visual amplification. (See Table
II).
Other terms often used are micro-denier, sub-micron and superfine.
Table II
Key Dimensions in Nanometers
Atom - 0.3 nm
Polymeric Nanofiber - 50 to 500 nm
Melt Blown Fibers - 2,000 to 5,000 nm
Blood Cell - 5,000 nm
1.5 Denier Fiber - 12,500 nm
Human Hair - 20,000 to 30,000 nm
