Plasma treatment of textiles -II

Surface Modification Modes:

Ablation

The ability of plasma processing to break down weak covalent bonds in a polymer through bombardment with high-energy particles is known as ablation. This affects the outermost molecular layers of the substrate exposed to the plasma, which boils off and is removed by the vacuum. Because the chemistry of any layers of surface contamination is also generally made up of weak C-H bonds, plasma treatment can remove contaminants such as oil films or injection molding additives, thereby leaving behind a uniformly clean and active polymer surface.

Cross-linking

Cross-linking is the setting up of chemical links between the molecular chains of polymers. Plasma processing with inert gases can be used to cross-link polymers and produce a stronger and harder substrate micro surface. Under certain circumstances, cross-linking through plasma treatment can also lend additional wear or chemical resistance to a material.

Activation

The replacement of surface polymer groups with chemical groups from the plasma is called activation. During activation, the plasma breaks down weak bonds in the polymer and replaces them with highly reactive carbonyl, carboxyl, and hydroxyl groups. Activation can also be performed with amino groups or other functional groups. The resulting change in substrate characteristics will be determined by the type of chemical groups incorporated into the surface.

Deposition

In plasma deposition, a thin polymer coating is formed at the substrate surface through polymerization of the process gas. Depending on the selection of the gas and process parameters, these thin coatings can be deposited with various properties or physical characteristics. Coatings produced in this manner through plasma deposition exhibit different properties than films derived from conventional polymerization, including a high degree of cross-linking and extremely strong adherence to the substrate.

Functional Textile Finishing with Plasma:

  • Enhance mechanical properties:

Softening of cotton and other cellulose-based polymers, with a treatment by oxygen plasma. Reduced felting of wool with treatment by oxygen plasma. Top resistance in wool, cotton, silk fabrics with the following treatment: dipping in DMSO (Dimethyl sulfoxide) and subsequently N2-plasma.

  • Enhancement of both hydrophilic and hydrophobic proper ties:

This is one of the most widely studied plasma applications. Oxygen, ammonia, air, nitrogen, etc. plasmas have been used to increase the wettability of synthetic polymers (PA, PE, PP, PET, PTFE, etc.), while hydrophobic or oleo phobic finishing of natural fibers (cotton, wool, silk, etc.) has been obtained by using siloxanes, perfluorocarbons, SF6, acrylates, etc.

  • Adhesion enhancement:

Plasma treatment increases adhesive properties through the following mechanisms –

(1) Removal of a low surface energy layer through cleaning and etching

(2) Strengthening of the top layer through cross-linking

(3) Introduction of new functional groups by reaction of the activated surface with plasma species

(4) The creation of a high-energy coating tightly bonded to the substrate by grafting

  • Desizing:

Desizing through plasma treatment is a fairly recent and novel process. Several studies have shown that, the plasma aided size removal system is viable and eco-friendly compared to conventional desizing systems.

  • Dyeing and printing:

Several studies have shown that dyeability or printability of textiles can be markedly improved by plasma treatments. This effect can be obtained on both synthetic and natural fibres. Capillarity improvement, enhancement of surface area, reduction of external crystallinity, creation of reactive sites on the fibers and many other actions can contribute to the final effect depending on the operative condition.

  • Electrical properties:

Antistatic properties have been conferred to artificial or synthetic polymers. Moreover, studies have been carried out for the creation of fabrics with very high conductive properties, suitable for integrating electronic devices into fabrics.

  • Intelligent filtration properties:

Filtration of gases or liquids is one of the most common technical applications of fibers, fabrics or non-woven. Appropriate surface functionalization can enhance chemical selectivity of traditional filters based on more conventional adsorption/absorption or physical separation processes.

  • Applications in Biology and Medicine:

Fabric favoring overgrowth with cells for cell culture tests, fermentation or implants. Fabric not favoring overgrowth with cells for catheters, membranes, enzyme immobilization etc.

  • Sterilization/Antimicrobial Properties:

Although the conventional method of sterilization is very effective, these methods are often associated with damage to the material/ medium that is supporting to the microorganism. As a result, newer, more effective forms of sterilization are being researched, and have led to the discovery of plasma sterilization. The processes of sterilization with plasma not only kill microorganisms, but they have also been effective in destroying bacteria, spores, fungi, and viruses.

The plasma technologies are being used for achieving antimicrobial properties in textiles. Most importantly, plasma finishing can be applied to a wide range of materials including rubber and polyester. Plasma treatments have also shown promise for graft copolymerization of antimicrobial agents onto nonwovens.

  • Other properties:

The extreme versatility of the plasma processes is shown by a very large number of investigations concerned with a wide range of different properties of great importance for textiles, such as flame retardancy, crease resistance, antimicotic, biological compatibility, UV -protection, as well as ‘hand’ modification, softening and ant pilling.

Effect of plasma treatment on different fabric surface:

  • Plasma treatment for cotton:

Plasma treatment improves the wet-ability of grey cotton fabric by water and caustic soda solution. In a research work cotton has been treated with radio frequency plasma in air at different power levels and time intervals. In that experiment it has been found that the plasma treatment can lower the moisture content and decrease surface resistivity. In some studies plasma initiated grafting of cotton has also been carried out. As in the case of wool, the specific surface area of cotton after oxygen plasma treatment is increased. On the other hand, the treatment with a hexamethyldisiloxane (HMDSO) plasma leads to a smooth surface with increased contact angle of water (sessile drop method) up to a maximum of 130o. Thus, a strong effect of hydrophobisation is achieved. Similarly, when a hexafluoroethane plasma is used instead of an HMDSO plasma the surface composition of the fibers clearly indicates the presence of fluorine and the material becomes highly hydrophobic. Still, the water vapor transmission is not influenced by the hydrophobisation. Hydrophobisation in conjunction with increased specific surface area results in an effect generally known as Lotus effect: dirt particles are easily removed from the surface by water droplets.

  • Plasma treatment for wool and silk:

Lower temperature plasma treatment of wool has emerged as one of the environmental friendly surface modification method for wool substrate. The efficiency of the low temperature plasma treatment is governed by several operational parameters like

  • Nature of the gas used
  • System pressure
  • Discharge power
  • Duration of treatment

Plasma treatment can impart anti-felting effect decreasing, improved dyestuff absorption and increasing wetting properties. Other changes in wool properties are as follows

  • Plasma treatment increases fiber-fiber friction but reduces the differential friction effect.
  • Plasma treatment does not change the strength and the elongation, the breaking force in loop form is slightly reduce.
  • The fatty matter content in wool is reduced by about 1/3 due to plasma treatment.

The water content of the wool top is reduced by about 3% due to plasma treatment. 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 increases the content of oxidized sulfur spaces are the main factors responsible for improvements in dyeing and shrink proofing of plasma treated wool.

Silicon resins applied to plasma treated wool increase the shrinkage over that for untreated wool. The polymer after treatment reduces both relaxation and felting shrinkage almost independently of plasma treatment time. There is more even and quicker penetration of dyestuff and chemicals in plasma treated wool than the untreated reference sample. Surface analysis of wool fibers treated with different plasma gases reveals that the wettability, weakbilty, surface contact angle of the material are significantly changed in a direction that may lead to new uses for these materials. Plasma treatment increases the hydrophilic groups in the wool fibers and the cystine linkages present in the surface layer are converted to cystic acid. The endocuticle and the density of crosslink in the surface layer are decreased by the reactive spaces in the plasma gas and thus facilitate diffusion of dyes and chemicals.

Plasma treated wool may exhibit more or less firm or harsh handle because of surface roughing. This property is very important for hand knitting yarns or yarns for underwear fabrics. The enzyme treatment is capable of improving the handle of plasma treated wool. In case of silk fiber the N2 plasma pretreatment can increase its wettability.

Low temperature plasma (LTP) is regarded as an emerging technique when used to achieve the effect of an anti-felt finishing in wool. Cuticle cells of the wool fibers 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 micro craters N2 plasma treated fibers show higher advancing contact angle values than air plasma treated fibers, suggesting a minor presence of hydrophilic groups on the surface of the N2 plasma treated fiber.

That point confirms that at the treatment times studied, the main effect of air and N2 LTP is superficial chemical modification. Water vapor plasma produces an important increase in the hydrophilicity of the fiber. Oxygen plasma reveals as the most aggressive of the plasma gases studied, in terms of etching at shorter times. Scanning electron microscopy (SEM) can be used as a tool for the study of the topographical surface effects of plasma on fibers.

LTP treatment could influence not only the mechanical properties but also affect the air permeability and thermal properties of the wool fabrics.

Chen, 1996 studied the free radical formation on cotton and wool fibers treated with low Temperature plasmas of O2, N2, Ar, CO, CF4 at the RF generator, at the power of 300W and the pressures of 0.3-1.5 Torr. Free radicals play an important role in polymerization, grafting, cross-linking and implantation. Free radical intensities are different for various gases with the general rule that O2

  • Plasma treatment for Synthetic material (Polyester/PP):

Plasma may be used for removing the contaminants, finishing and sizing agents from the fabric. Desizing of polyester fabric that used polyvinyl alcohol as the sizing agents can be removed by plasma treatment. The wet-ability of polyester fabric also increases significantly. Polyester fiber can be effectively modified by low pressure plasma treatment. Treatment of polyester fiber by glow discharge in air or oxygen causes a partial degradation of the fiber surface together with an increase in the capillary sorption of Iodine or cation in aqueous solution. Wetting out properties of polyester can be achieved by treatment of polyester with plasma and corona discharge. The fabric can be processed without the use of wetting out agents.

Generally polyester has a very hydrophobic surface because the surface is Made up of ether oxygen (C-O-C) linkages while the hydrophilic ester oxygen (C=O) is facing towards the core of the fiber. When surface is treated by plasma either the ester oxygen causes closer to the surface as a result of etching or some new C=O bonds are forms due to oxygen ions present in the plasma chamber.

A small scale matched for the preparation of liner fabric is described, which practically eliminates the use of chemical reagents. The grey fabric is treated in a glow discharge plasma in air and then the process maintains the strength of the fabric. Maintaining the strength of the fabric does not affect the natural color of liner and does give fabric a high degree of hydrophilicity.

The barrier discharge or corona treatment of polypropylene significantly increases the hydrophilicity of the surface, the contact angle of water being decreased from 90o to 55o. Even after two weeks a sustained effect is observed, the contact angle of water being 60o. Instead of the contact angle of water, the oxygen/carbon ratio of the atomic composition of the surface can be used to follow the influence of a plasma treatment, in particular for polypropylene fleeces with layered-structure.

The oxygen/carbon ratio for the first layer is highest; but even at the tenth layer a significant effect is observed. The uptake of oxygen at a polypropylene surface is even more significantly demonstrated when maleic acid anhydride (MAH) is used as an assisting reagent. The incorporation of oxygen is permanent and a contact angle with water of 42o can be achieved.

Researches on plasma finishing on textiles:

In a lecture presented at the 15th International Symposium on Plasma Chemistry, Orleans, France, 9-13 July 2001, Hartwig Hocker has presented that, the plasma technology can be used to wool fibers to achieve shrinkage resistance. The Area Felting Shrinkage (%) is drastically reduces after plasma treatment to wool.

In case of cotton fiber, the treatment with HMDSO (hexamethyldisiloxane) plasma leads to a smooth surface with increased contact angle of water up to a maximum of 130°. Thus, a strong effect of hydrophobization is achieved. Still, the water vapor transmission is not influenced by the hydrophobization. Hydrophobization in conjunction with increased specific surface area results in an effect generally known as Lotus effect: dirt particles are easily removed from the surface by water droplets.

The barrier discharge or corona treatment of polypropylene significantly increases the hydrophilicity of the surface, the contact angle of water being decreased from 90° to 55°.

In a review article Kale et al. studied on the atmospheric plasma treatment on textiles using non-polymerizing gases and concluded that, atmospheric pressure plasma treatment can modify the textile surfaces in variety of ways and can impart desired functional properties to the textile substrate. Treatment of textiles with plasma generated from non-polymerizing gases improves wettability, hydrophilicity and adhesion. It brings about chemical, physical and morphological changes in the textiles. It is also concluded that, like other industries, plasma has not found the same success in the textile sector.

Nasadil et al. showed that the plasma pre-treated samples show brighter and deeper color shade. Used plasma helps the textile to absorb more dye from dyeing bath, but has only small impact on colour fastness of textile material. Plasma treatment shows good antibacterial effect and confirm that plasma is able to activate TiO2 (note: TiO2 without activation has no antibacterial, nor photocatalytical properties) for antibacterial effectiveness. Activated TiO2 can inhibit bacteria growth more than 5 hours after activation, even if it’s placed in dark.

Jahagirdar et al. showed that polyester fabric can be modified suitably by treating with DCDMS (Dichlorodimethylsilane) solution so as to make it water repellant without losing its original strength. The modified polyester fabrics do attain good water repellency even after washing with water for ten cycles. For DCDMS treatment, the optimized duration of plasma exposure for polyester fabric is 2 min and 30 s. They also concluded that, if the cost factor of plasma device could be eliminated, this technology would be valid and useful for the textile finishing industry.

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Posted on April 22, 2014, in Special Garments Production, Technical textiles and tagged , , , , , , . Bookmark the permalink. Leave a comment.

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