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 textile industry processes a large quantity of fibers obtained from various animals of which wool is commercially the most important. Plasma treatment has been studied over many years for shrinkproofing and as a pretreatment for improving the dyeing properties of wool, including increased dyebath exhaustion and shorter dyeing cycles. The nature of plasma gas plays an important role in the alteration of the dyeing properties of the plasma-treated wool fibers. 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:
Plasma treated wool has different physical and chemical properties when compared with the untreated one. The changes in fiber properties alter the performance of the existing textile processes such as spinning, dyeing and finishing to produce a series of versatile wool products with superior quality. Major effects of plasma treated wool are point out below:
- Plasma treatment increases the fiber/fiber friction as measured by Roder method, but reduces the differential friction effect (DFE) as defined by Mercer and Lindberg.
- Plasma treatment does not change the strength and elongation; the breaking force in loop form is slightly reduced.
- The plasma treatment increases the top cohesion by a factor of 1.5-2.0; this increased cohesion remains stable after prolonged storage.
- The plasma treated wool can be subjected to antifelting finishing more easily without using resins. The application of resin to the plasma-modified wool is more effective than that carried out on the untreated fabric.
- The specific electrical resistivity does not change considerably after plasma treatment.
- The fatty matter content in wool is reduced by about one-third due to plasma treatment.
- The water content of the wool top is reduced by about 3% due to plasma treatment.
- There are 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.
- Significant improvement in shrink resistance after the subsequent plasma treatment. Clearly, the shrinkage will increase as the processing is changed from relaxation shrinkage to felting shrinkage.
- Felting is a special property of wool that occurs when wool is subjected to mechanical agitation in water. Plasma-treated wool shows a significantly diminished felting behavior.
- Plasma treatment can influence the dyeing behavior of wool fiber. The plasma-treated wool fibers can be easily penetrated by the dyes, which are evenly distributed over the cross-section of the fiber.
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.
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 fiber surface.
Surface analyses of wool fibers 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, fiber diameter, fiber surface roughness and fiber surface chemical composition. Chemical composition of the fiber 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 fiber 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 fiber 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.
Plasma-treated wool can be used in woollen spinning with favorable results being achieved both in spinnability and yarn quality. This may be attributed to the increase in both the fiber friction coefficient and the cohesion of the wool top by a factor of 2.0 to 2.2. When the tenacity of plasma treated wool yarn is increased, the evenness is also improved but the hairiness is decreased. Nevertheless, after plasma treatment, the spinning behaviour of wool is changed and can be summarized as follows:
- The spinning aids applied to the first drawing frame must be carefully selected.
- The rubbing intensity or twist of the slubbing should be increased.
- Reduction in break rate at the ring-spinning frame is usually observed.
- Increase in yarn tenacity by 10%–25% is observed for all yarns.
The high cohesion of wool top after plasma treatment can last for three to four weeks. The coefficient of variation of cohesion within a single spinning is lower after plasma treatment than before. The increased top cohesion affects the tensile strength values of both single and piled yarns to a definite extent. The tenacity of yarns spun from plasma–treated wool top is higher by about 25% and elongation at break point is also higher compared with standard yarns.
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 fibers 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.
Plasma treatment leads to selective modification of the outermost wool fiber and has many implications for improved dyeing, printing, shrinkproofing and subsequent chemical finishing. Plasma treatments of wool represent a most innovative approach to replacing chlorination stages in wool finishing and to improving existing processes with special regards to an economical– ecological optimization.
- Chemical Technology in the Pre-treatment Processes of Textiles by S.R. Karmakar
- A Novel Green Treatment for Textiles: Plasma Treatment as a Sustainable Technology By Chi-wai Kan
- Plasma Technologies for Textiles Edited by R. Shishoo
- The Coloration of Wool and other Keratin Fibres Edited by David M. Lewis and John A. Rippon
- Study of atmospheric plasma treatment of wool fibers by Illya Kulyk, Marco Scapinello, Matteo Stefan
- Ageing effect of plasma‐treated wool by Maryam Naebe, Ron Denning, Mickey Huson, Peter G. Cookson & Xungai Wang
- Witold Rakowski, Plasma treatment of wool today. Part 1 – Fibre properties, spinning and shrinkproofing, October 2008, Coloration Technology 113(9):250 – 255, DOI:10.1111/j.1478-4408.1997.tb01909.x
- Chi-wai Kan & Chun-wah Marcus Yuen “Plasma technology in wool” Pages 121-187 | Published online: 13 Dec 2007, https://doi.org/10.1080/00405160701628839
- Rahul Navik, Sameera Shafi, Md Miskatul Alam, Md Amjad Farooq Influence of dielectric barrier discharge treatment on mechanical and dyeing properties of wool, June 2018, Plasma Science and Technology 20(6):065504 DOI:10.1088/2058-6272/aaaadd
Special Thanks to:
Tariq Mehmood, Deakin University, Australia
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.