APPLICATION OF PLASMA IN TEXTILE

Introduction

Textiles have undergone chemical processing since time immemorial. The textile industry is searching for innovative production techniques to improve the product quality, as well as society requires new finishing techniques working in environmental respect. Over recent years, physicochemical techniques have become more commercially attractive and have begun to

overcome conventional wet chemical methods for property modification. The importance of surface modification of textile materials extends over a wide range of alterations or embedded selective additions, to provide desired single or multi features for various applications. It is a highly focused area of research in which alterations to physical and/or chemical properties lead to new textile products that provide new applications or satisfy specific needs.

These processes, however, can be introduced into the production line without major changes or system interruption, allowing for high Speed and continuous processing involve numerous chemicals, some of which are toxic to humans and hazardous to the environment. Additional problems also arise due to degradation and/or weakening of the treated material. Alternative techniques have been investigated over the past two decades to decrease or eliminate dependency on chemical treatments. One recent alternative, involving non-aqueous processing, is plasma treatment of textile materials. Appropriate choice of gas and control of plasma operation conditions provide a variety of effects on textiles (improvement of dyeability, printability and colour fastness, improvement of adhesion properties of coated fabrics, increase in hydrophobicity and water resistance, etc.

Surface modification via plasma treatment not only eliminates the need for wet processing, but also yields unique surface characteristics. Several modifications include, but are not limited to hydrophilicity / hydrophobicity alterations, surface roughening, grafting, flame retardant, antimicrobial, insect repellent, stain resistant, and single or multiple surface functionalisation. An ideal plasma treatment for textile applications is a plasma system that can be introduced into the production line without major changes or system interruption, allowing for high speed and continuous processing

What is plasma?

Irving Langmuir first used the term plasma in 1926 to describe the inner region of an electrical discharge. Later, the definition was broadened to define a state of matter in which a significant number of atom and/or molecules are electrically charged or ionized. The components present will include ions, free electrons, photons, neutral atoms and molecules in ground and excited states and there is a high likelihood of surface interaction with organic substrates. A gas becomes plasma when the kinetic energy of the gas particles rises to equal the ionization energy of the gas. When this level is reached, collisions of the gas particles cause a rapid cascading ionization, resulting in plasma. If the necessary energy is provided by heat, the threshold temperature is from 50,000 to 1, 00,000 K and the temperatures for maintaining plasma range up to hundreds of millions of degrees²

“Plasma” derived from the Greek and referring to “something molded or fabricated”

The plasma state can be generated by:

1. Electrical energy
2. Nuclear energy
3. Thermal energy
4. Mechanical energy
5. Radiant energy

The differentiation between plasmas can be made based on his main characteristics, namely by the charged particle density, temperature, pressure and the presence/absence of electrical and/or magnetic fields.¹

Types of plasma

Plasma is generally classified as thermal or non-thermal. In thermal plasma, temperature of several thousand degrees is reached which is of a destructive nature and no material can stand their action. Contrary to thermal plasmas, non-thermal plasmas are cold‟ plasmas where the chemically active environment is achieved at nearly room temperature and this one is used for surface modification of textiles. There are two types of cold plasma which can be used for application on textiles, namely vacuum pressure and atmospheric pressure. Since plasma cannot be generated in a complete vacuum the name vacuum pressure is somewhat misleading and only refers to the low working pressures of such systems.

Many authors, however, choose to classify Vacuum pressure plasmas into sub categories of low and medium pressures.^1,2  Due to very little difference between the sub classes of vacuum plasma will not differentiate between the two forms and discuss two main classes of plasma which are near vacuum pressure plasmas and atmospheric pressure plasmas. Both these forms are suitable for application on textiles and progress continues to determine their effect on textiles. More work has, however, been documented on characterization of vacuum pressure plasmas as compared to atmospheric pressure plasmas.

The coupling of electromagnetic power into a process gas volume generates the plasma medium comprising a dynamic mix of ions, electrons, neutrons, photons, free radicals, meta-stable excited species and molecular and polymeric fragments, the system overall being at room temperature. This allows the surface functionalisation of fibers and textiles without affecting their bulk properties. These species move under electromagnetic fields, diffusion gradients, etc. on the textile substrates placed in or passed through the plasma. This enables a variety of generic surface processes including surface activation by bond breaking to create reactive sites, grafting of chemical moieties and functional groups, material volatilization and removal (etching), dissociation of surface contaminants/layers (cleaning/scouring) and deposition of conformal coatings.

In all these processes a highly surface specific region of the material (<1000 A) is given new, desirable properties without negatively affecting the bulk properties of the constituent fibers. Plasmas are acknowledged to be uniquely effective surface engineering tools due to their unparalleled physical, chemical and thermal range, allowing the tailoring of surface properties to extraordinary precision. Their low temperature, thus avoiding sample destruction. Their non-equilibrium nature, offering new material and new research areas. Their dry, environmentally friendly nature.

Different types of power supply to generate the plasma are:

Low-frequency (LF, 50–450 kHz)

Radio-frequency (RF, 13.56 or 27.12 MHz)

Microwave (MW, 915 MHz or 2.45 GHz

The power required ranges from 10 to 5000 watts, depending on the size of the reactor and the desired treatment. ^1,2

Plasma Equipments

Plasma may be generated in the laboratory using non-electrical discharges, e.g. Thermal methods, shock waves, chemical reactions of high specific energy, nuclear radiation or irradiation by high-energy photons, gamma rays or alpha particles. However, for plasma treatment of textiles only electrical-discharge techniques are used. Plasma is a partially ionized gas containing ions, electrons, atoms and neutral species. To enable the gas to be ionized in a controlled and qualitative manner, the process is carried in vacuum conditions.

A vacuum vessel is first pumped down via rotary and roots blowers, sometimes in conjunction with high-vacuum pumps, to a low to medium vacuum pressure in the range of 10-2 to 10-3 mbar. The gas is then introduced into the vessel by means of mass flow controllers and valves. Although many gases can be used, commonly selected gases or mixture of gases for plasma treatment of polymers include oxygen, argon, nitro us oxide, tetra fluoro methane and air.^1,3

Principle of plasma application

The plasma atmosphere consists of free electrons, radicals, ions, uv-radiations and lot of different excited particles in dependence of the used gas. Different reactive species in plasma chamber interact with the substrate surface cleaning, modification or coating occurs dependent of the used parameter. Furthermore the plasma process can be carried out in different manners. The substrate can be treated directly in the plasma zone. The substrate can be positioned outside the plasma this process is called remote process. The substrate can be achieved in the plasma followed by a subsequent grafting. The substrate can be treated with a polymer solution or gas which will be fixed or polymerized by a subsequent plasma treatment.

Various plasma technologies used in textile

There are many different ways to induce the ionization of gases.

(1) Glow discharge,

(2) Corona discharge,

(3) Dielectric Barrier discharge,

(4) Atmospheric pressure plasma technique.

 Glow discharge

It is the oldest type of plasma technique. It is produced at reduced pressure (low-pressure plasma technique) and provides the highest possible uniformity and flexibility of any plasma treatment.

The plasma is formed by applying a DC, low frequency (50 Hz) or radio frequency (40 kHz, 13.56 MHz) voltage over a pair or a series of electrodes. (Figure A, B, C) Alternatively, a vacuum glow discharge can be made by using microwave (GHz) power supply²

Let us consider partially ionised gas. When a sufficient high potential difference is applied between two electrodes placed in the gas, a breakdown among electrons and positive ions produces a discharge. Excitation collisions, followed by de-excitation, give the characteristic luminescence that is seen as the glow of the discharge. Due to collision processes, a large number of different plasma species are then generated: electrons, atoms, molecules, several kinds of radicals, several kinds of (positive and negative) ions, excited species, etc. Different species are in interaction with each other, making the glow discharge plasma a complicated gas mixture

 (2) Corona discharge

It is formed at atmospheric pressure by applying a low frequency or pulsed high voltage over an electrode pair, the configuration of which can be one of many types. Typically, both electrodes have a large difference in size . The corona consists of a series of small lightning-type discharges their in homogeneity and the high local energy levels make the classical corona treatment of textiles problematic in many cases.

  (3) Dielectric Barrier discharge,

DBD is formed by applying a pulsed voltage over an electrode pair of which at least  one              is covered by a dielectric material Though also here lightning-type discharges are created, a major advantage over corona discharges is the improved textile treatment uniformity

From a supply reel, the film passes through the glow discharge, driven by the rotating drum. DBD devices are well known in the packaging industry, where they are used to increase the wettability of polymeric films. They are strongly related to the APGDs, which operate with an a.c. voltage of 1-100 kV at a frequency of few Hz to MHz. APGDs are devices having homogenous and uniform discharges across the electrodes, whereas the DBDs produces discharges with micro-discharge filaments and considerably less uniform^2,3,4

(4) Atmospheric pressure plasma technique.

As discussed earlier, there are various forms of plasma depending on the range of temperature and electron density. Generally, high plasma densities are desirable, because electrons impact gas molecules and create the excited-state species used for textile treatment. Having more electrons generally equates to faster treatment time. However, very high plasma densities (greater than 1013 electrons cm-3) can only exist with very high gas temperature (Thermal Plasma). This extremely high level of plasma density is unsuitable for textile treatment, because the plasma’s energy will burn almost any material. Hence for textile processing, the plasma needs to do their job at room temperature, thus the name ‘cold plasma’.

Effect of plasma on fibres and polymers

Textile materials subjected to plasma treatments undergo major chemical and physical transformations including

• Chemical changes in surface layers,
• Changes in surface layer structure, and
• Changes in physical properties of surface layers.

Plasmas create a high density of free radicals by disassociating molecules through electron collisions and photochemical processes. This causes disruption of the chemical bonds in the fibre polymer surface which results   in formation of new chemical species. Both the surface chemistry and surface topography are affected and the specific surface area of fibres is significantly increased. Plasma treatment on fibre and polymer surfaces results in the formation of new functional groups such as -OH, -COOH which affect fabric wettability as well as facilitate graft polymerization which, in turn, affects liquid repellence of treated textiles and nonwovens. In the plasma treatment of fibres and polymers, energetic particles and photons generated in the plasma interact strongly with the substrate surface, usually via free-radical chemistry. Four major effects on surfaces are normally observed. Each is always present to some degree, but one may be favored over the others, depending on the substrate and the gas chemistry, the reactor design, and the operating parameters. ¹

The four major effects are surface cleaning, ablation or etching, cross-linking of near surface molecules and modification of surface chemical structure. Plasma processes can be conveniently classified into four overall processes: cleaning, activation, grafting and Deposition

 cleaning

Plasma cleaning and etching means a removal of material (impurities or substrate material) from the exposed surface

In a cleaning process, inert (Ar, He) and oxygen plasmas are used. The plasma-cleaning process removes, via ablation, organic contaminates such as oils and other production releases on the surface of most industrial materials. These surface contaminants as polymers, undergo abstraction of hydrogen with free radical formation and repetitive chain scissions, under the influence of ions, free radicals and electrons of the plasma, until molecular weight is sufficiently low to boil away in the vacuum (see fig.4).

Fig.4: Free radicals formation by means of plasma action.

Plasma can abstract hydrogen from the polymeric chainor can split chains

Activation

Plasma activation consists of the introduction of new functional groups onto the treated surface. Properties of the surface then depend on the nature of the chemical groups.

Activation plasma processes happen when a surface is treated with a gas, such as oxygen, ammonia or nitrous oxide and others, that does not contain carbon. The primary result is the incorporation of different moieties of the process gas onto the surface of the material under treatment. Let us consider the surface of polyethylene, which normally consists solely of carbon and hydrogen: with a plasma treatment, the surface may be activated, anchoring on it functional groups such as hydroxyl, carbonyl, peroxyl, carboxylic, amino and amines (Fig.5). Hydrogen abstraction produces free radicals in the plasma gases and functional groups on the polymeric chain. Almost any fibre or polymeric surface may be modified to provide chemical functionality to specific adhesives or coatings, significantly enhancing the adhesion characteristics and permanency. For instance, polymers activated in such a manner provide greatly enhanced adhesive strength and permanency, and this is a great improvement in the production of technical fabrics.

Fig.5: An example of surface activation by substituting hydrogen in a polymeric chain

with other groups such as O, OH, COOH, NO3, NH2, etc.

Grafting

Plasma-assisted grafting is a two-step process in which the plasma activation is followed by the exposure to a liquid or gaseous precursor, e.g. a monomer. The monomer then undergoes a conventional free radical polymerization on the activated surface.

In grafting, an inert gas such as argon is employed as process gas, many free radicals shall be created on the material surface. If a monomer capable of reacting with the free radical is introduced into the chamber, the monomer shall become grafted. This is a procedure for low-pressure plasma treatment but grafting can be obtained with atmospheric plasma processing too. Typical monomers are acrylic acid, allyl amine and allyl alcohol (Fig. 6).

By means of the plasma processes some properties of the surface can be changed to obtain several applications. First of all, surface wetting changes. Polymeric surface are usually not wettable and adhesion is poor. After plasma treatment the surface energy increases and wettability and adhesion enhancements are produced.

Grafting of a monomer on the surface: argon plasma produces radicals

on the chain and monomers are grafted on the surface.

deposition

Plasma can also produce a material deposition: when a more complex molecule is employed as the process gas, a process known as plasma-enhanced chemical-vapour deposition (PECVD) may result. For instance, when methane or carbon tetrafluoride is employed, the gas undergoes fragmentations in the plasma, reacting with itself to combine into a polymer. Selecting the process conditions, pinholefree chemically unique films, may be deposited onto surfaces of materials within the plasma reactor. PECVD coatings alter in a permanent way the surface properties of the material onto which these have been deposited

Effect of plasma treatment on different fabric surface  

 Plasma treatment for cotton

It has been reported in literature that plasma treatment improves the wet-ability of grey cotton fabric by water and caustic soda solution. In another work cotton has been treated with radio frequency plasma in air at different power levels and time intervals. From the literature it has been seen 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 130°.

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 fibres clearly indicates the presence of fluorine and the material becomes highly hydrophobic. Still, the water vapour 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  ^2,6

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 govern by several operational parameters like

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

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

1. Plasma treatment increases fibre-fibre friction but reduces the differential friction effect.
2. Plasma treatment does not change the strength and the elongation, the breaking force in loop form is slightly reduce.
3. The fatty matter content in wool is reduced by about 1/3 due to plasma treatment.
4. 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 obtains from the modified wool. The reduction in the content of covalently bound highly hydrophobic methylicosanoic acid and increases the content of oxidised 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 fibres 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 fibres 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 chemical Plasma treated wool may exhibits 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 fibre the N2 plasma can be increase 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 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 micro craters N2 plasma treated fibres show higher advancing contact angle values than air plasma treated fibres, suggesting a minor persence of hydrophilic groups on the surface of the N2 plasma treated fibre  .That point confirms that at the treatment times studied, the main effect of air and N2 LTP is superficial chemical modification.

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°. Even after two weeks a sustained effect is observed, the contact angle of water being 60°. 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 42^° can be achieved (Figure 4). When polyethyleneterephthalate (PET) fibres are used as an enforcing material for a polyethylene (PE) matrix, the hydrophobisation of the PET fibres using an ethylene plasma is quite impressive since the adhesion strength can be increased from 1 to 2.5 N/mm. The fracture morphology of these composite materials clearly shows the tight adhesion of the matrix to the fibre.  Permanent hydrophilisation of PP by plasma induced grafting of MAH^2,

Various applications of plasma in textile process

 Desizing:

Atmospheric pressure plasma treatment on cotton grey fabric (sized by standard sizing recipe containing starch) with air and He gas mixture alters the surface morphology, gave rise to desizing effect and enhance the wettability and wicking action. The hitting of ions gave rise to loosening of the surfaces that were removed in subsequent process of washing. The surface roughness as well as formation of (-C=O, -OH or C-N) bonds created functional groups are responsible for improved hydrophilic properties. The loss of weight in desizing process was more for plasma treated fabric that too in initial part of treatment. These higher rates of desizing plasma pre treated fabric can save time, energy and water.⁷

Atmospheric plasma treatments are applied to Desizing the cotton (PVA were used for sizing) using air/He and air/He/O2 combinations. These treatments removed some PVA film and significantly improved PDR (percent desizing ratio) by washing, especially by cold water washing. The tensile strengths of cotton fabrics treated with atmospheric pressure plasma were the same as for the unsized fabric. Results of the plasma treated PVA films revealed surface chemical changes such as chain scission and formation of polar groups, which promoted the solubility of PVA in cold water. Air/He/O2 plasma is more effective than air/He plasma on PVA desizing (because of oxidation is more for Air/He/O2 plasma)⁸

 Scouring

Low temperature plasma treatment modified the surface of cotton fabrics. The contact angles between the liquid (scouring bath) and the low temperature plasma treated cotton fabric surfaces decreased significantly. Furthermore, the O2 plasma treatment by changing the surface properties dramatically increased the wicking rate of cotton fabrics, making them more absorbent. The results for scourability revealed that low temperature plasma treatment increased the scouring rate of cotton fabrics. O2 plasma caused changes in the oil, fat, and wax contents by etching where the topmost of the layer of the substrate is stripped off. This increased rate means that a shorter time have been chosen for scouring, the process was more environmentally friendly, and energy consumption decreased because less time was needed to reach the desirable state. For the scouring process, 25 minutes have been used for plasma treated instead of 40 minutes needed for scouring cotton fabrics.⁹

 Dyeing

The O2 plasma treatment dramatically increases the wicking rate of cotton fabrics, making them more absorbent hence increase the dyeing rate of cotton fabrics. Holes were visible on the O2 plasma treated cotton fabric surfaces, which were caused by the ablation effect of this nonpolymerizing reactive plasma gas. These holes provided a new pathway for the dye to enter the fiber and hence increased the dyeing rate. For the dyeing process 50 minutes have been chosen after plasma treatment instead of 90 minutes needed for dyeing untreated cotton fabrics.

The Modification of the Cuticle and Primary Wall of Cotton by Corona Treatment was studied in which the effects of corona treatment were limited to the cuticle and primary wall of cotton, although one or two experiments (radiation sensitivity, bundle strength) suggested possibility for deeper penetration through to the secondary wall. Some disturbance of the wax on cotton was indicated in air chlorine corona treatments, and both the wax and cellulose reacted with chlorine in an air chlorine corona to produce C-Cl covalent bonds. Air-chlorine corona treatments have greater effect than the air corona treatments and air-chlorine corona treated fabrics are more wettable.

A possible practical application of these results was to reduce the scouring or kier boiling required achieving a given dyeing level or dyeing uniformity.⁶ The effect of low pressure plasma treatment on bleached and mercerized cotton fabrics was investigated with water vapour as working gas. Though bleached and mercerized cotton fabrics were hydrophilic, the change in hydrophilicity after plasma treatment has been tested and higher concentration of oxygen was founded on the surface of water vapour plasma treated surface. These higher oxygen concentrated surfaces gained higher hydrophilic properties. An increase in hydrophilicity revealed deeper dyeability of plasma treated fabrics.

Dyeing and printing– Improvement of capillarity in wool and cotton, with treatment in oxygen plasma. Improved dyeing polyester with SiCl4- plasma and for polyamide with Ar-plasma.

 Finishing

 Hydrophobization

Plasma treatment is used to achieve the hydrophobic effect by varying the application gasses. Plasma Treatment of cotton fabric with hexamethyldisiloxane gas has been used to smoothen the surface of the fibers and has capable of increasing the contact angle on the fiber till up to 130o. Similarly, by using hexaflouroethane plasma, a strong effect of hydrophobization has been achieved by introducing fluorine groups on the surface of the fibers. It produces very good water repellent effect on its treated fabrics. Neither of these methods reduces the water vapor transmission ability of cotton.¹¹

Plasma treatment also has been used to achieve the lotus effect on cotton fabrics. The underlying principle is etching of the fiber to create nano sized peaks and then covering them with a hydrophobic layer using an appropriate gas such as hexafluoroethane.

Antimicrobial activity

The crosslinked cotton fabrics with the combined dimethyloldihydroxyethyleneurea (DMDHEU) and acrylic acid crosslinking agent under the pad-dry plasma- cure process were determined to study antibacterial properties of the treated fabrics. The plasma treatment increased the surface distribution of crosslinking agents and lets the metal ion stay on the surface of the treated fabrics. Such crosslinks, increased the antibacterial property of the pad-dryplasma- cure and pad-dry-plasma-cure with copper after-treatment fabrics compared to that of pad-dry cure and pad-dry-cure with copper after-treatment fabrics.

The microwave induced plasma technique was used to modify the cotton fabric to study the effect of onion skin & onion pulp extractions‟ grafting reaction for cotton dyeing and compare the washing fastness and anti microbial S.aureus ability of differtially treated cotton fabric. It was observed that the cotton fabric with direct grafting of onion skin or onion pulp extractions shows negative results , on the contrary cotton fabric with microwave O2 plasma pretreatment provides more polar functional groups on the cotton surface and makes onion skin or onion pulp grafting reaction with anti microbial S.aureus ability. Cotton fabric with longer onion grafting time does not show bigger anti microbial S.aureus inhibition zone. It might be so because some anti microbial S.aureus components of onion are not stable under long high temperature reaction.¹³ Flash fire resistance Flame-retardant textiles are designed to reduce the ease of ignition and also reduce the flame propagation rates.

Conventional textiles can be rendered flame retardant by chemical after-treatments as co-monomers in their structures or use of FR additives during extrusion. High performance fibers inherently have high levels of flame and heat resistance with the synthesis of all aromatic structures. Flash-fires generated from improvised explosive devices (IED) incident upon the target for up to 3 s (sec). Clothing with moderate levels of flame retardancy, while shielding the wearer from heat radiation, can ignite under such flash-fire conditions causing burn injuries even when the underlying garments have some level of flame retardancy.

It is therefore necessary to provide flash-fire resistance to underlying garments including protective clothing for up to 3–8 s. flame retartend textiles with flash fire resistance help in avoiding the injuries in such cases. Nanoceramic surface treatment offer flash fire resistance, where the plasma treatment offers possibility of selective modification of the surface by keeping the bulk characteristics unchanged. A. R. Horrocks et al studied that plasma treatment of fabric (flame retardant cotton) surfaces in the presence of functionalized clay produced an inorganic or even a micro ceramic coating having reduced flammability at the high heat fluxes and it was an indicative of increased resistance to flash-fire ignition.

Crease recovery finishing

The pad – dry – plasma – cure process with argon as working gas increased the cross linking effect between cross linking agent and cellulose molecules. The cross linking agent was a mixture of Dimethylol dihydroxy ethylene urea (DMDHEU) and acrylic acid (AA). The increased cross linking effect improved the physical properties of finished fabrics.

The DCRA- dry crease recovery angle, WCRA- wet crease recovery angle and TSR- tensile strength retention values of pad – dry – plasma – cure finished fabrics are higher than pad – dry – cure finished fabrics at same value of nitrogen content and CL/AGU – number of cross linking per anhydrous glucose unit. Vinyl group of AA excited at Ar plasma treatment and grafted to cellulose fiber and DMDHEU under cure treatment increased the physical properties.¹⁵

Biostoning

The influence of corona (COR) and RF low-pressure plasma (LPP) parameters on the decolorization of indigo-dyed denim fabric were examined. Low pressure plasma and corona treatments could be a viable alternative to conventional biostoning forobtaining the „worn out‟  look of indigo-dyed denim fabric. This could be associated with the production of chemically active molecules and radicals in gas mixtures containing oxygen, which consequently leads to an oxidation of dyes. In addition to satisfactory color change effects, the main advantages of these treatments are the lack of water consumption and shorter process duration.¹⁶

Plasma treatment of wool

There is an enormous potential in the plasma treatment of natural fibre fabrics. Plasma treatment has proved to be successful in the shrink-resist treatment of wool with a simultaneously positive

effect on the dyeing and printing. The morphology of wool is highly complex, not only in the fibre stem but also on the surface as well. It is in fact the surface morphology to play an important role in the wool processing. Unwanted effects such as shrinkage, felting and barrier of diffusion are most probably due to the presence of wool scales on the fibre surface.

In the past, the modification of wool surface morphology were conducted either by chemical degradation of scale (oxidative treatment using chlorination) or by deposition of polymers on the scale. However, in both processes, a large amount of chemicals generated from incomplete reactions polluted the effluent.

The oxidation is also required to reduce the hydrorepellance of wool to obtain good dyability. Wool is composed at 95% of a natural polymer, the keratin. In the outer part, the cuticle, the cells are in the form of scale (see a drawing in Fig. 4). Cuticle cells overlap to create a directional frictional coefficient: the scales are moved by water and they have the tendency to close and join together with the typical movement that is proper to have a good textile but it is also producing felting and shrinkage. Plasma treatment of wool has a two-fold effect on the surface.^15,16

First, the hydrophobic lipid layer on the surface is oxidised and partially removed. Since the exocuticle, that is the layer of the surface itself (epicuticle), is highly cross-linked via disulfide bridges, plasma treatment has a strong effect on oxidising the disulfide bonds and reducing the cross-link density.

Fig.: A wool fibre in covered by cuticle scales. Very

long cells are building the inner structure of the fibre.

The former plasma treatments on wool were done with the corona discharge but it was not giving a uniform treatment on the fabric: the cuticle is modified, being formed on the fibre microroughness and holes. The corona discharge, consisting of a series of small lightning-type discharges, has the advantage to be easily formed at atmospheric pressure by applying a low frequency high voltage over an electrode pair. Corona discharge is usually inhomogeneous and then problematic for textiles. Rakowsky compared corona and glow discharge plasma concluding that the second is better: both treatments are involving only a surface thickness (10-8 m) and then do not modify the wool structure. As the surface is oxidised, the hydrophobic character is changed to become increasingly hydrophilic. The chemical and physical surface modification results in decreased shrinkage behaviour of wool top; the felting density decreases from more than 0.2 grams per cubic centimetre to less than 0.1 grams per cubic centimetre.^1,3

After plasma treatment the fibre is more hydrophilic, then a layer of water can be formed during washing procedures with a reduction of friction among fibres and a consequent felt reduction. With respect to shrink-resistant treatment, this effect is too small compared with the state-of-theart chlorine/Hercosett treatment. Therefore additional resin coverage of the fibre surface is required. It should be mentioned that the plasma treatment brings additional advantages, in particular increasing dyeing kinetics, an enhanced depth of shade, and improved bath exhaustion. akowsky discussed treatments with plasma gases of O2, air, N2 at low pressure: he observed a regular abrasion of the surface, the removal of the fat acid layer, the reduction of aliphatic carbon (C-C, C-H) of about 20-30% and the appearing of carboxylic COOH groups.

With N2 plasma the better effect on the dyeing of wool is obtained: in fact, it is producing amine groups on the surface possessing dye affinity. To obtain measurements of the role of plasma on

the dyeing processes, the coloration of a dye bath is evaluated in order to obtain the so called “exhaustion curves”, that give the kinetic behavior of the dyes during a dyeing process as function of the dyeing process parameters. Figure 5 shows the exhaustion curves for different plasma treatments on wool.

Fig.: Exhaustion curves for different plasma treatments, as a function of treatment time . diamonds are

showing data for control sample without plasma treatment, + for O2 plasma for N2 and squares for the

mixture O2/N2.

Nylon 6 plasma processing

The research on the plasma treatment on polyamide is main1y dyeability, wettability  and surface properties.  Oxygen and air plasma are used to increase wettability and dyeability. On PA6, an air plasma treatment was performed in an industrial production process: an increase of bondability was observed changing the bond from adhesive to cohesive. Moreover, with the same industrial procedure, with an ammonia plasma, a slight increase of wettability was observed Nitrogen-containing plasmas are in fact widely used to improve wettability, printability, bondability and biocompatibility of polymer surfaces and many application of nitrogen containing plasmas for surface modification of different polymers have been investigated. For example, to improve the interfacial strength between polyethylene fibres and epoxy resins, which are cured by amino crosslinking, amino groups were introduced on the fibre surface to promote covalent bonding.

After plasma treatment, the properties of the fabric, including surface morphology, low-stress mechanical properties, air permeability and thermal properties, were measured by researches, observing that nylon fabrics treated with different plasma gases exhibited different morphological changes. Low-stress mechanical properties revealed that the surface friction, tensile, shearing, bending and compression properties altered after the treatments. The changes in these properties are believed to be related closely to theinter-fibre inter-yarn frictional force induced by the plasma treatment. It was also observed a slightly decrease in the air permeability of the treated fabrics, probably due to plasma changing the fabric surface morphology.

A change in the thermal properties is in agreement with the above findings and can be attributed to the amount of air trapped between the yarns. For what concerns CF4, it is a non-polymerizing gas that does not polymerize itself, but tends to form thin films on the fibre surface subjected to the glow discharge. examined CF4 plasma –treated fabrics and found that ablation was accompanied by the deposition of thin films on the fibre surface.  suggested that a shorted exposure time will favour polymerization while a longer exposure time will favour ablation

Activation of PP, PE, PET and PTFE.

Polypropylene (PP) is a very interesting material for plasma treatment: it is a very hydrophobic material with extreme low surface tension. On the other hand, PP is used in a large number of technical applications where an improved wettability or adhesion properties are advantageous. This is also the case of PP technical textile applications such as filters for medical applications.

Since PP nonwoven filters can be wetted only with liquids with surface tension <35mN/m, no water can pass through the PP-web without applying a high pressure: using an oxidative plasma with a short treatment time can greatly improve wettability. Varying treatment time, vacuum level and treatment power of a O2 plasma, it was observed   that the increase in surface tension of PP is not in correlation with the intensity of plasma treatment. An increase in wettability can be indeed observed but only at relatively low treatment intensities:

once the optimum is reached a sharp drop in wettability was obtained if the plasma treatment intensity is raised further. Improvement of wetting in PP has been observed for air and NH3-plasma. As in PP, also in polyethylene PE, polyethylene terephthalate PET and polytetrafluoroethylene PTFE, treatments in air-, O2- and NH3-plasma are usually performed. These treatments are able to increase wettability but also adhesion for these materials (hydrophobic finishing on cotton/PET and PET already mentioned, are produced with  siloxan- or perfluorocarbon – plasma.

In the Table , data on the modification of surface energy and contact angle of water on several polymeric substrates are shown. But let us talk about another very important problem, that is how to increase adhesion of different polymer-metal systems, namely, PET-Al, Kapton-Al, and Teflon-Cu. To this purpose both reactive (O2 and NH3) and inert (He) gases have been used for the plasma treatments. The results obtained with PET are particularly interesting from both the fundamental and the industrial points of view. NH3 plasma treatments show to be successful in obtaining higher PET-Al adhesion values at very short plasma duration: the shorter the NH3 treatment time the higher is the adhesion increase. A treatment of 0.1 s is sufficient to promote a 15-20 increase of adhesion ³

Other applications

1 Spinnability/ surface roughness

“Frictionizing” textile fibers by corona treatment produces larger effects than conventional chemical methods. Cotton roving cohesiveness is increased initially by chlorine-corona treatment to four times its original value. The residual cohesiveness increases cotton yarn tensile strength by 24%. It appears that the very high cohesiveness of cotton roving occur immediately after corona treatment, it improved further spinning process .

Cotton Spinnability, yarn and fabric strength, and abrasion resistance are increased by corona treatment. Injection of dilute chlorine gas into the corona cell during corona treatment increases the effects achieved, particularly at about 95oC .Since fiber tensile strength is unaffected by this treatment, the improvements probably largely result from the increased fiber cohesiveness. The marked improvement in Spinnability attained suggested a possible significant improvement in cotton processing economy .

Mechanical property

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 and subsequently N2-plasma.

Electrical Properties. Antistatic finish of rayon, with chloromethyl dimethylsilane in plasma.

 Wetting.

Improvement of surface wetting in synthetic polymers (PA, PE, PP, PET PTFE) with treatment in O2-, air-, NH3-plasma. Hydrophilic treatment serves also as dirt-repellent and antistatic finish. Hydrophobic finishing of cotton, cotton/PET, with treatment with siloxan- or perfluorocarbon- plasma. Oleophobic finish for cotton/polyester, by means of grafting of perfluoroacrylat.

Metal-Coated Organic Polymers.

Metal-coated organic polymers are used for a variety of applications. If the metallised polymer is expected to fulfil its function, it is essential that metal strongly adheres to the polymer substrate. This can be obtained with a plasma pre-treatment of the polymer.

 Composites and Laminates.

Good adhesion between layers in laminates depends upon the surface characteristics of fibres in layers and the interactions taking place at the interface. A prerequisite condition of good adhesion remains the surface energy of fibres, which can be modified with plasma treatments.

Applications in Biology and Medicine.

Fabric favouring overgrowth with cells for cell culture tests, fermentation or implants. Fabric not favouring overgrowth with cells for catheters, membranes, enzyme immobilisation, sterilisation.

Applications in Membrane and EnvironmentalTechnology.

Gas separation to obtain oxygen enrichment. Solution-Diffusion Membranes to obtain alcohol enrichment. Ultra filtration membranes to improve selectivity. Functionalized membranes such as affinity membranes, charged membranes, bipolar membranes. Let us start with a more specific discussion of some cases studies. First of all we will study the role of plasma treatment on natural fibres such as wool and cotton. Then Nylon 6 is discussed and a brief review of plasma treatment on other synthetic polymeric fabrics is performed.³

 Environmental benefits

The complexity of textile processing environmental impact starts with high water and energy

Consumption, high oxygen demand of several input materials being used as well as a generation of huge amounts of effluents with high chemical oxygen demand (COD), excessive colour, pH and toxicity. In general, desizing, dyeing, washing and finishing are the main sources of effluent pollution. The main advantage of plasma processing is that it is a dry treatment. Additionally, it is a very energy efficient and clean process

In general, the environmental benefits of plasma treatment can be summarized as:

1. Reduced amount of chemicals needed in conventional processing Better
2. exhaustion of chemicals from the bath reduced BOD/COD of effluents
3. shortening of the wet processing time
4. Decrease in needed wet processing temperature
5. Energy savings.

 Future Growth and Developments

Plasma will play many important roles in the future manufacturing of non-woven and textile products. The first of these will be meeting the need to custom-design products and develop highly technical products, by which the manufacturer must distinguish himself from the competition. Another will be by providing a solution to increasing regulation in the use of process water and in energy consumption. A third in meeting the need of environment ally friendly processes, as well as for a safe operator environment. The fact that new products can be designed, that quality can be improved and costs can be decreased, will give a further impetus to plasma growth.

Conclusion:

Plasma technology with all its challenges and opportunities is an unavoidable part of our future.

The possibilities with plasma technology are immense and numerous. It can rightly be said that plasma technology is slow, but steady in the industrial revolution. The substantial shortcoming of plasma treatment of textiles is that it cannot replace all wet processes, but it can be a viable pretreatment, which can provide plenty of environmental and economical benefits. Therefore, textile industry should consider the concept of higher initial investments in equipment for plasma treatment of textiles in all forms that will be paid off quickly with respect to environment related savings and the profit of the sale of high value added products.

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