MOLECULAR ENGINEERING

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Low temperature plasma is considered as an alternative treatment for the anti-felt treatment of wool fabrics. By means of scanning electron microscopy the influence of the plasma gas, time and power source on the topography of the surface of wool fibres have been studied by researchers.

Long treatment times can alter the surface of wool, specially with oxygen or water vapour plasmas no matter whether the reactor used is radio frequency or microwave.

The wool fibre exhibits hydrophobic properties due to the presence of a thin fatty layer covalently bound
to the epicuticle, which surrounds each cuticle cell. From the scanning electron microscopy (SEM)
micrographs (Fig.1 a and b) of an untreated (UT) merino wool fabric, the diameter of the fibers are
measured, being around ¡Ö18-22 ¦Ìm. The cuticular cells (scales) of the surface are clearly
distinguished, overlapping one another like tiles on a roof, and revealing the proper state of the surface of
the UT wool fibres, which will ensure readiness for further processing. By magnifying the image, we can
notice that cuticular scales are flat, the only roughness of the wool fibre coming from the overlapping
of the cells.

Both hydrophobicity and the cuticle scales of wool fibre exert a considerable influence on the shrinkage
of wool fabrics during an aqueous washing process. Wool fabrics undergo felting shrinkage as a result of
small movements of individual fibres during laundering or milling. The movements always take place
towards the fibre root due to the difference in the friction coefficient of a wool fibre in the “with-scale”
and against-scale directions.

Low temperature plasma (LTP) is regarded as an emerging technique when used to achieve the effect of
an anti-felt finishing in wool and it is one of the most studied applications of plasma technology in the textile sector, being also widely used to modify polymer materials.

In general, the main advantages of plasma technology are the extremely short treatment time and the low
application temperature, along with the fact that it is regarded as an environmentally friendly process,since no chemicals are involved .Water and solvents can be avoided and no or less chemicals are required.

In addition, it is a selective process used to modify the chemical and topographical properties of fibre surfaces without affecting the bulk of the fibres.

The LOW THERMAL PLASMA is generated when a gas at low pressure and near ambient temperature is subjected to EXCITATIONS.

The chemistry of the plasma takes place in non-equilibrium conditions, and the chemical reactions can take place while the gas or parts of it remain at relatively low temperatures. The plasma contains radicals, ions, photons and other excited species. These species can interact either physically or chemically with the substrate surface to a depth of a few tens of nm due to their high reactivity.

As a result of the glow discharge (GD) plasma treatment, the surface may be oxidized (generating
new chemical groups), and/or degraded as a result of the etching effect (removal of surface material), whereas the bulk properties remain intact. By changing the plasma variables, such as the nature of
the gas, the discharge power, the pressure and the exposure time, a great variety of surface effects can be
obtained due to different combinations of MOLECULAR ENGINEERING OF MATERIALS..

The uniformity of surface treatments plays an important role in their effectiveness, so it should be carefully controlled in order to avoid fibre damage and achieve the desired effects. Scanning electron
microscopy (SEM) is used as a tool for the study of the topographical surface effects of plasma on
wool.

The imaging system of SEM, with its wide range of magnification and great depth of focus is remarkably
well suited to the needs of textile technology for the following reasons :

1. Fibres are small but are not microscopic and hence may be imaged easily and quickly.

2. Fibre features that determine performance in manufacturing and conversion to end-use are of
the same size range and are easily imaged.

3. Garment appearance is vital to consumer acceptance; fibre features that determine appearance
are easily imaged.

The effects of using THE PROCESS AND SYNTHESIZER FOR MOLECULAR ENGINEERING OF MATERIALS with different plasma gases and treatment times on the surface of wool fabrics and fibres by means of SEM are furnished .

An incident power of 100 W and gas pressure of 1 mbar with ambient air, nitrogen, water vapour and oxygen involving several treatment times yield good results.

Cuticle cells of the wool fibres treated with air plasma show a surface similar to that of the UT wool,
although the roughness of the surface seems to have slightly increased due to the presence of microcraters
(Fig. 2a), more evident the stronger the plasma conditions (Fig. 2 b).

While with SEM no morphological changes can be observed in air plasma treated wool fibres, at times
shorter than 60 s, by means of contact angle measurements it is possible to detect chemical modifications
on the surface of the fibre even after only 10 s. According to the contact angle values, the formation of
hydrophilic groups on the fibre surface takes place in the early stages of the plasma treatment. Therefore,
a short plasma exposure time is sufficient to confer shrink-resistance properties to wool.

The SEM pictures for nitrogen plasma treated wool fibres (Fig.3) reveal similar cuticular cell appearance as in UT fibres.

Although SEM of air and nitrogen plasma treated fibres exhibits surfaces with very slight alterations or
unaltered, the N2 plasma treated fibres give higher advancing contact angle values than air plasma treated fibres, suggesting a minor presence of hydrophilic groups on the surface of the N2 plasma treated fibre.

This confirms that at the treatment times studied, the main effect of air and N2 LTP plasma is superficial chemical modification, the etching effect being not relevant.

Water vapour plasma produces, at low treatment times (Fig. 4 a-c), some striations on the cuticle, but
when the plasma treatment increased to 600 s (Fig.4 d) more remarkable striations appear on the fibre
surface.

Like with air and nitrogen treatments, water vapour plasma produces an important increase in the hydrophilicity of the fibre

Oxygen plasma is more aggressive in terms of etching at shorter times. In Fig. 5a and 5b the surface of wool shows very light striations, similar to those observed in water vapour plasma treatments, although with the added presence of some microcraters on the cuticle. Long treatment times (Fig. 5 c) show increased roughness due to microcraters (ranging from 0.025 to 0.075 µm) uniformly distributed all along the surface, and to the redeposition of etched material on the surface (Fig. 5 d).

Hydrophilicity of wool fibres slightly increases with treatment time, but time has to be limited to avoid
excessive damage of the fibre by the etching of the reactive species of plasma. The increase in hydrophilicity is probably due to the generation of new oxygenated groups and the partial elimination of the fatty layer.