Civil Engineering Reference
In-Depth Information
be adopted for preparing cellulosic nanoi bers. Electrospinning [65] and air-blowing
spinning [66] are the best known methods amongst the applied physical-chemical
methods. In electrospinning the i bers are formed by electrostatic i eld, whereas in
air-blowing spinning the i ber is formed in a stream of compressed air. Compared to
air-blowing spinning for nanoi ber preparation, electrospinning has attracted a great
amount of attention due to its simplicity and ei ciency. Due to strong inter- and
intramolecular interactions of hydrogen bonding and rigid backbone structure, cel-
lulose either does not dissolve in most of the conventional solvents or is melted; it
does dissolve in dimethylsulfoxide/paraformaldehyde or sulfur dioxide. But, these
solvents were not suitable for electrospinning. To i nd ef ective and eco-friendly sol-
vent systems for cellulose, many ef orts have been devoted for several decades. In
most of the solvents used for spinning cellulose is not easy to evaporate and attempts
to form the i ber according to the methods end up producing an extended stream
of the cellulose solution. Coagulation of the extended spinning solution stream is
still needed to form the i ber. During the coagulation, the solvent travels to a l uid
which does not dissolve cellulose and the remaining cellulose material is formed into
a i ber. h e i nal form of the cellulose in such i bers dif ers greatly in supermolecular
structure from native cellulose and is referred to as regenerated cellulose [67, 68].
Nonetheless, the advantageous properties of cellulose are maintained and the i ne
shape of the i ber adds new interesting features absent in natural cellulose i bers.
Investigations into the production of thin cellulosic i bers are not restricted to the
i ber formation but include the preparation of the spinning solution. h ough cellu-
lose is one of the oldest polymers applied in the manufacture of i bers, its processing
still poses environmental, work safety and economic problems. In many respects,
the formation of nano- and microscale i bers from cellulose via electrospinning has
mirrored the history of conventional cellulose i ber spinning. Just as cellulose was
derivitized to form the i rst manufactured i ber, it is also reported as the i rst electro-
spun i ber with patents dating back to Formhals in 1934 [17]. Over the last 50 years,
several direct solvents for cellulose have been discovered and utilized in wet- or dry-
jet wet spinning processes [69].
12.6
Solvents for Electrospinning of Cellulose
Based on the research work carried out world-wide, three promising solvents may
be indicated as suitable for the preparation of cellulose spinning solutions on a large
scale. h ey are, (i) NMMO (N-methylmorpholine-N-oxide) in Lyocell process [70,
71], (ii) NaOH (sodium hydroxide-water) in Celsol technology [68], and (iii) imidaz-
ole salts and other ionic liquids [72]. Ionic liquids that have better cellulose dissolving
potential and higher thermal resistance than NMMO still remain in the labs. It is only
the Lyocell process exploiting NMMO as a solvent (Tencel i bers, Lenzing Lyocell)
that has found its way into industrial manufacturing with an amount of well over one
hundred thousand tons of i bers turned out annually worldwide. However these are
used for conventional i ber spinning. Currently, a few successful solvent systems are
reported for the preparation of solution for electrospinning of cellulose to produce
