Effects of Plasma Treatment on Wool

Plasma Treatment of Wool:
Wool is a protein fiber. It is obtained from the fleece of the sheep or lamb or hair of the Angora or Cashmere goat. The effects of a plasma treatment on wool has revolutionary changed such as anti-felting effect, degreasing, improved dyestuff absorption and increase in wetting properties have been discussed in this article.

Effects of Plasma Treatment on Wool:
  1. Plasma treatment increases the fibre/fibre friction as measured by Roder method, but reduces the differential friction effect (DFE) as defined by Mercer and Lindberg.
  2. Plasma treatment does not change the strength and elongation; the breaking force in loop form is slightly reduced.
  3. The plasma treatment increases the top cohesion by a factor of 1.5-2.0; this increased cohesion remains stable after prolonged storage.
  4. The specific electrical resistivity does not change considerably after plasma treatment.
  5. The fatty matter content in wool is reduced by about one-third due to plasma treatment.
  6. The water content of the wool top is reduced by about 3% due to plasma treatment.
  7. There is changes in spinning behavior of plasma treated wool. The spinning aids applied on the first drawing frame are carefully selected. The rubbing intensity or twist of the slubbing should be increased. Reduction in breaks rate at ring spinning frame is usually observed and an increase in yam tenacity by 20-25% is observed for all yarns. 
The normal process of preparing light weight woolen fabrics has involved a chlorination operation. However, this leads to difficult working conditions, rapid con-osion of equipment and has a bad effect on the local ecology. Plasma treatment is a good alternative for chlorination treatment although two problems remain : namely the efficiency of plasma/polymer system itself and the ways and means to improve the fabric handle. However, plasma treatment considerably reduces the felting potential for any product obtained from the modified wool. The reduction in the content of covalently bound highly hydrophobic methylicosanoic acid and increase in content of oxidized sulphur species are the main factors responsible for improvements in dyeing and shrink proofing of plasma treated wool.

Treated and untreated of wool
Plasma treatment of wool followed by polymer application has also been studied. Almost all polymers used currently on pre-chlorinated wool cannot be used on plasma-treated top. Silicone resins applied to plasma-treated wool increase the shrinkage over that for untreated wool. However, the combined plasma/PMS/Hercosett treatment encompassing the top treatment gives excellent shrink resistance. The polymer after-treatment reduces both relaxation and felting shrinkage almost independently of plasma treatment time.

There is more even and quicker penetration of dyestuffs and chemicals on plasma treated wool than the untreated reference sample. The increased dyes and chemicals affinity is presumably attributed to the plasma induced oxidation of the cystine in the layer of the exocuticula and thereby to a reduction of the wetting bridge density in the fibre surface.

Surface analyses of wool fibres treated with different plasma gases reveal that the wettability, wickability, printability and surface contact angle of the materials are significantly changed in a direction that may lead to new uses for these materials. Several aspects affect the web wettability, such as pore size, fibre diameter, fibre surface roughness and fibre surface chemical composition. Chemical composition of the fibre surface is most important as it determines the surface bonding forces with water, i.e. disruption force, polar force, and H-bonding force. Surface roughness is not a primary reason for improved wettability, but it may increase it.

Plasma treatment increases the hydrophillic groups in the wool fibre and the cystine present in the surface layer is converted to cysteic acid. The endocuticle and the intercell membrane complex and the density of cross-links in the surface layer is decreased by the reactive species in the plasma gas and thus facilitate diffusion of dyes and chemicals. The internal lipids of cell membrane complex are also modified to a certain extent. These changes in the interior of the fibre are presumably caused by the short wave ultra-violet radiation which is produced by the low temperature glow-discharge plasma apart from the chemical active species such as electrons, radicals etc.

Woollen sliver and yarn have been treated in low temperature plasma in a vacuum chamber for times from 20 to 30 min. There is a significant increase in the strength which lead to better stability of the material during subsequent processing. Fabrics made from treated wool do not felt and also the shrinkage is reduced e.g. from 37% to 3-5%.

Plasma treated wool may exhibit more or less firm or harsh handle because of surface roughening. This property is very important for hand-knitting yarns or yarns for underwear fabrics. Softeners generally deteriorate the shrink resistance imparted by plasma treatment or plasma plus polymer after-treatment quite heavily. The enzyme treatment is capable of improving the handle of plasma treated wool as well as plasma treated and polymer after-treated hand-knitting yarns without imparting their shrink resistance.

Conclusion:
A lot of changes occur after plasma treatment on wool. Besides, atmospheric pressure plasma treatment of wool fabric, with a relatively short exposure time, effectively removed the covalently bonded lipid layer from the wool surface. The plasma‐treated fabric showed increased wettability and the fibres showed greater roughness. X‐ray photoelectron spectroscopy analysis showed a much more hydrophilic surface with significant increases in oxygen and nitrogen concentrations and a decrease in carbon concentration.

References:

  1. Chemical Technology in the Pre-treatment Processes of Textiles by S.R. Karmakar
  2. Study of atmospheric plasma treatment of wool fibers by Illya Kulyk, Marco Scapinello, Matteo Stefan
  3. Ageing effect of plasma‐treated wool by Maryam Naebe, Ron Denning, Mickey Huson, Peter G. Cookson & Xungai Wang
Special Thanks to:
Tariq Mehmood, Deakin University, Australia 
 

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