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(Nanowerk
Spotlight) Nanotechnology has brought a new area of
finishing applications to the textile industry. Coating the
surface of textiles and clothing with nanoparticles has
become a common approach for the production of highly active
surfaces to have UV blocking, antimicrobial, flame
retardant, water repellant or self-cleaning properties.
While antimicrobial properties are exerted by nanosilver, UV
blocking, self-cleaning and flame-retardant properties are
imparted by coatings containing zinc oxide or titanium oxide
nanoparticles (see for instance
"Nanotechnology finishing in textiles"). Going even
further, future smart textiles could actually have fully
integrated electronic capabilities that will open a whole
new range of applications (see
"Nanotechnology e-textiles for biomonitoring and wearable
electronics"). |
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Besides the exciting
functionalities that smart textiles will offer its wearer,
comfort is also a fundamental and universal need. Clothing
comfort not only has an impact on the performances of the
people wearing it but ultimately will have a decisive impact
on the commercial success or failure of these textiles.
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One important aspect
of clothing comfort is thermo-physiological comfort. By
adjusting the transport of heat and moisture through a
fabric, thermo-physiological comfort can keep people
comfortable with regard to temperature and moisture.
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Some hydrophobic
fabrics have deficiencies in this area. Take wool. Wool is
one of the best insulating fibers known to man ¨C while at
the same time being light and soft. The quality that
distinguishes wool fibers is the presence of a fatty,
water-repellent outer layer that surrounds each fiber.
Therefore, the water absorption and sweat venting properties
of wool fiber are not very good, which affects the wearing
comfort of wool textiles. The wool hydrophobic surface layer
is also a barrier to anticrease finishing, dyeing, and
grafting of hydrophilic agents, which is an issue in trying
to add smart functionalities to wool fabrics. |
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Researchers in China
have now developed a simple method for fabricating
environmentally stable superhydrophilic wool fabrics. They
applied silica sols to natural wool fibers to form an
ultrathin layer on the surface of the fibers, increasing
both the surface roughness and surface energy of the wool
fabrics. That way, functionalized fabrics can be obtained by
further modification of the surface of the wool fibers with
bioactive agents or stimuli-responsive molecules.
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"A lot of research has
been conducted to improve the hydrophilic properties of wool
fabrics, for instance by nonthermal plasma or enzyme
treatment of wool fibers," Dong Chen explains to Nanowerk.
"However, the hydrophilicity of fabrics treated with
nonthermal plasma is not stable during storage in air; and
enzyme treatments destroy the scale layer of the wool fiber
and lead to irreversible damage to the surface structure of
wool fiber." |
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Chen is a scientists
at the Chinese Academy of Sciences (CAS) in Beijing, who is
conducting this research while he pursues his PhD degree at
The Institute of Textiles and Clothing at Hong Kong
Polytechnic University. The project has been led by
professors
Fangqiong Tang from the
Technical Institute of Physics and Chemistry at CAS and
Yi Li from The Institute of Textiles and Clothing.
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"We adopted a new and
simple method, similar to dyeing, for fabricating
environmentally stable superhydrophilic wool fabrics"
explains Chen. "A transparent and thin layer of silica
nanoparticles changes both the surface energy and surface
roughness of the wool fibers. Thanks to the silica layer's
optical transparence and chemical stability, we obtained
functionalized fabrics without changing the color and fabric
morphology." |
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A and B are
photographs of the absorption of water droplets on pristine
wool fabric and superhydrophilic wool fabric, respectively;
C and D are SEM images of pristine wool fiber and
superhydrophilic wool fiber. The inset in C is water contact
angle on pristine wool fabric; the insets in D are water
contact angle on superhydrophilic wool fabric and a
high-magnification SEM image of silica coating layer on
superhydrophilic wool fiber. (Image: Dr. CHEN Dong)
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The researchers have
reported their findings in a recent online issue of
Langmuir ("Fabricating
Superhydrophilic Wool Fabrics"). |
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Chen explains the
possible mechanism for their relatively simple coating
technique: "Coating an ultrathin silica layer onto wool
fibers could be dependent on electrostatic interactions
between wool fibers and silica sols. Silica sols behave in a
similar manner to dye molecules during wet chemical
processing. Wool fiber has been known to have free amino and
carboxyl groups in the wool backbone as important reactive
sites. At pH 3 to 4, a considerable number of internal amino
groups are protonated but the carboxyl groups are not
substantially protonated until the pH approaches 2, leading
to the wool fiber having a positive surface charge.
Therefore, acid solution swells wool and will benefit silica
sols coating the wool surface." |
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With fast moisture
absorption, diffusion and dry qualities, this breathable
wool fabric can keep your skin dry while not clinging to it.
Not surprisingly, the researchers see a great potential in
applying this novel coated wool fiber to all kinds of
sportswear, travel wear, underwear, bedroom fabrics and
summer garments for humid climates. |
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There could also be
quite a substantial environmental benefit from these coated
wool fibers. Today, the biggest problem of washing wool
clothing lies in the shrinkage of the textiles.
Consequently, wool garments are often dry cleaned, bringing
with it all the environmentally harmful consequences of dry
cleaning processes. By developing nanotechnology wool
textiles that can absorb a large amount of moisture, dry
quickly and are machine-washable without shrinking,
household washing machines could largely replace dry
cleaners for cleaning wool textiles. |
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Right now, the
stability of the modified wool fibers during washing does
not yet meet the expectations of the researchers. "To fully
realize the everyday application of functional textiles
modified by nanoparticles, the coalescing force between the
fiber and the nanoparticles must be enhanced" notes Chen.
"In theory, this problem can be resolved by surface
modification of the fiber to enhance the chemical bonding
between particles and fiber. Right now, this is something we
are working on in our lab." |
