plasma technology in wool

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Plasma technology in woolChi-wai Kan a & Chun-wah Marcus Yuen aa Institute of Textiles and Clothing , The Hong Kong PolytechnicUniversity, Hung Hom , Kowloon, Hong Kong, People's Republic ofChinaPublished online: 13 Dec 2007.

To cite this article: Chi-wai Kan & Chun-wah Marcus Yuen (2007) Plasma technology in wool, TextileProgress, 39:3, 121-187, DOI: 10.1080/00405160701628839

To link to this article: http://dx.doi.org/10.1080/00405160701628839

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CONTENTS

1. Introduction 122

2. Review of different surface treatments for wool 123

3. What is plasma? 123

4. Mechanism of plasma treatments 1254.1. Polymerisation 1254.2. Plasma ablation (physical sputtering and chemical etching) 127

4.2.1 Sputtering 1274.2.2 Chemical etching 1284.2.3 Ion-enhanced energetic etching 1284.2.4 Ion-enhanced protective etching 129

4.3. Advantages of plasma treatment 1294.4. Disadvantages of plasma treatment 129

5. Generation of plasma 1305.1. DC glow discharge 1305.2. Radio-frequency discharge 1315.3. Microwave plasma 131

6. Factors affecting plasma treatment 1326.1. Nature of gas used 132

6.1.1 Inert gas 1326.1.2 Oxygen-containing gas 1326.1.3 Nitrogen-containing gas 1336.1.4 Fluorine-containing gas 1346.1.5 Hydrocarbon 1346.1.6 Halocarbon 1346.1.7 Organosilicon plasma 134

6.2. Flow rate 1356.3. System pressure 1356.4. Discharge power 1366.5. Duration of treatment 1376.6. Ageing of the plasma-treated surface 138

6.6.1 Effects of environment 1386.6.2 Effect of temperature 139

6.7. Temperature change during plasma treatment 139

7. Production of plasma species 1407.1. Ionisation and detachment 1417.2. Recombination, detachment and diffusion 141

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8. Application of plasma technology in textile industry 141

9. Application of plasma treatment of wool under industrial conditions 1429.1. Spinnability of plasma-modified wool tops 1429.2. Printing of fabric made from plasma-modified wool 1429.3. Machine-washable fabrics made from plasma-modified wool 1449.4. Plasma-modified wool for fully fashioned knitted fabrics 1469.5. Hand-knitting yarns made from plasma-treated wool tops 1469.6. Shrinkproofing of plasma-treated wool fabric 1469.7. Dyeing properties of plasma-modified wool substrate 148

9.7.1 Dyeability modification 1489.7.2 Fabric dyeability measurement 1509.7.3 Chrome dyeing 151

9.7.3.1. Hexavalent chromium determination 1529.7.3.2. Trivalent chromium determination 153

10. Surface analysis of plasma-treated wool 15410.1. Surface morphology by SEM 154

10.1.1 Untreated wool fibre 15510.1.2 Oxygen-plasma-treated wool fibre 15610.1.3 Nitrogen-plasma-treated wool fibre 15710.1.4 Gas mixture plasma-treated wool fibre 158

10.2. XPS surface analysis 15810.3. FTIR-ATR measurement 160

11. Serviceability of plasma-treated wool for industrial use 166

12. Fabric objective measurement of plasma-treated wool fabric 16812.1. Tensile properties 16812.2. Shearing properties 17012.3. Bending properties 17012.4. Compression properties 17112.5. Surface properties 17112.6. Air permeability 17112.7. Thermal properties 172

13. Development of plasma technology in wool industry 17213.1. Atmospheric plasma treatment 17213.2. Continuous plasma treatment 17313.3. Fibre identification 17513.4. Enhancing self-cleaning in couple with nano-technology 17613.5. Plasma sterilisation 176

14. Conclusion 176

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Textile ProgressVol. 39, No. 3, September 2007, 121–187

Plasma technology in wool

Chi-wai Kan∗ and Chun-wah Marcus Yuen

Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom,Kowloon, Hong Kong, People’s Republic of China

The textile industry processes a large quantity of fibres obtained from various animalsof which wool is commercially the most important. However, it has some technicalproblems which affect the quality and performance of the finished products such as felt-ing shrinkage, handle, lustre, pilling and dyeability. These problems may be attributedmainly to the presence of wool scales on the fibre surface. The scales are relatively hardand have sharp edges which are responsible for causing fibre directional movement andshrinkage during felting. Furthermore, the scales also serve as a barrier for diffusionprocesses which will adversely affect the sorption behaviour. In recent years, there hasbeen an increase in the modification of wool surface scales by physical means such asmechanical, thermal and ultrasonic treatments, and chemical methods such as oxida-tion, reduction, enzyme and ozone treatments which can solve the felting and sorptionproblems to a certain extent. Hitherto, chemical treatments are still the most commonlyused descaling methods in the industry.

Owing to the effect of pollution caused by various chemical treatments, physicaltreatments such as plasma treatment have been introduced recently as they are capableof achieving a similar descaling effect. Since the 1960s, scientists have successfullyexploited plasma techniques in materials science. The plasma technologies have beenfully utilised to improve the surface properties of fibres in many applications. The fibresthat can be modified by plasmas include almost all kinds of fibre such as textile fibres,metallic fibres, glass fibres, carbon fibres, fabrics and other organic fibres.

Plasma-treated wool has different physical and chemical properties when com-pared with the untreated one. The changes in fibre properties alter the performance ofthe existing textile processes such as spinning, dyeing and finishing to produce a seriesof versatile wool products with superior quality. Therefore, the aim of this monographis to give a critical appreciation of the latest developments of plasma treatment of wool.In this monograph, different surface treatments of wool including plasma treatment willbe precisely described. Since plasma treatment can be used to alter material surfacesby removing outer layers, thus the method of generation of plasma and the reactionmechanisms between material surface and plasma species will be highlighted in thismonograph. Similar to other chemical reactions, the factors such as (i) the nature of gasused, (ii) gas flow rate, (iii) system pressure and (iv) discharge power affecting the finalresults of plasma treatments will be described.

The main content of this monograph includes the application of plasma treat-ment on wool under different industrial conditions such as dyeing and shrinkproofingprocessing which will be reported and discussed respectively. In addition, the com-mon analytical methods such as Scanning Electron Microscopy, X-ray PhotoelectronSpectroscopy and Fourier Transform Infrared Spectroscopy with Attenuated Total In-ternal Reflectance mode analysis employed for characterising the surface properties ofplasma-treated wool will be discussed. Based on the surface characterisation results,more details about the mechanism of plasma treatment that affects the wool processingsuch as dyeing and shrinkproofing can be explored.

∗Corresponding author. Email: [email protected]

ISSN 0040-5167 print/ISSN 1754-2278 onlinec© 2007 The Textile Institute

DOI: 10.1080/00405160701628839http://www.informaworld.com

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122 C.-w. Kan and C.-w.M. Yuen

In the latter part of the monograph, the serviceability of plasma-treated wool fabricsis discussed and the possibility of applying the plasma-treated wool fabric to industrialuse is evaluated based on standard performance specification, e.g. ASTM. The fabricperformance in terms of tailorability and sewability are also discussed with referenceto the Kawabata Evaluation System for Fabric (KES-F) results. As the plasma processis a “dry” process, i.e. the water used in the plasma system can be recycled, thus it cansolve the industrial effluent problem resulting in providing an effective means for themodification of wool fabrics.

Keywords: wool, plasma treatment, dyeing, shrinkproofing, serviceability

1. Introduction

The textile industry uses large quantities of fibres obtained from various animals ofwhich the wool obtained from sheep is commercially the most important one. However,it has some technical problems which affect the quality and performance of the finishedproducts, such as felting shrinkage, handle, lustre, pilling and dyeability. These problemsmay be attributed mainly to the presence of wool scales on the fibre surface (Figure 1).These scales are relatively hard and have sharp edges, which are responsible for causingfibre directional movement and shrinkage during felting. In addition, the scales also serveas a barrier for diffusion processes, which will adversely affect the sorption behaviour(Figure 2). Recently, the elimination of the surface scales by physical means, such asmechanical, thermal and ultrasonic treatments, and chemical methods, such as oxidation,reduction, enzyme and ozone treatments, will help to solve the felting and sorptionproblems to a certain extent. Hitherto, chemical treatments are still the most commonlyused descaling methods in the industry. However, as a result of the pollution problem causedby various chemical treatments, physical treatments, such as plasma technology, have beenintroduced recently because they are similarly capable of achieving a descaling effect.

Starting in the 1960s, scientists have successfully exploited plasma technique in materi-als science. Since that time, plasma technologies have been utilized to improve the surface

Figure 1. Schematic diagram of the morphological component of a fine wool fibre [1].

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Textile Progress 123

Figure 2. Diffusion pathway of wool fibre [1].

properties of fibres in many applications [2,3]. The fibres that can be modified by plasmainclude almost all kinds of fibres, such as metallic fibres, glass fibres and carbon fibres[4–8], fabrics [9–13] and other organic fibres [14–29].

Plasma-treated wool fibres have different physical and chemical properties when com-pared with untreated wool fibres. These changes in properties alter the performance of theexisting textile processes, such as spinning, dyeing and finishing, to produce versatile woolproducts of superior quality. The aim of this monograph is to give a critical appreciation ofthe recent developments of the plasma treatment of wool.

2. Review of different surface treatments for wool

The wool surface treatments are mainly used for producing shrink-resist wool and im-proving the dyeing properties of wool fibre, but other properties may also be affected.The development of surface treatments is divided into chemical and physical methods assummarised in Table 1.

Among various chemical methods being developed, chlorination was the most com-monly used commercial method due to its ease of application. However, as a result ofincreasing environmental legislation restriction, absorbable organohalogen compounds(AOX) – free treatment is required for wool modification. Researchers in the past decade hadmade use of physical methods (Table 1) to modify the wool fibre. Of all the known physicalmethods, the plasma treatment has been developed most rapidly for surface modificationand so it will be the main focus discussed in this monograph.

3. What is plasma?

The concept of plasma was introduced by Langmuir [155–157]. Plasma is defined as a statewhere a significant number of atoms and/or molecules are either electrically, thermally ormagnetically changed or ionised. Plasma, in general, refers to the excited gaseous stateconsisting of atoms, molecules, ions, metastables, and excited state of these, and electronssuch that the concentration of positively and negatively charged species is roughly the same.The ionised gas system displays significantly different physical and chemical propertieswhen compared with its neutral condition. Theoretically, plasma is referred to as a ‘fourthstate of matter’ and is characterised in terms of the average electron temperature and thecharge density within the system [158–160].

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Table 1. Major chemical and physical methods.

Chemical and physical methods References

Chemical methodsWet chlorination free chlorination 30–36Wet chlorination – acid containing chlorine 37–40Wet chlorination with chloramines and chloramide 41–44Dry (gaseous) chlorination 45,46Halogenation – Fluorination, bromination and iodination 47–49Permanganate 50–59Oxide of sulphur 60–62Ozone 63–66Oxidising agent 67–69Reducing Agent 70–81Alcoholic–alkaline treatment 82–86The dry solvent process 87–89Enzyme process 90–99Shrink resistance by cross-linkage 100–104Treatment with aldehyde 105,106Treatment with resin/polymer application 107–128

Physical methodsMechanical treatment 129–131Corona discharge 132–140Plasma treatment 141–146Sputter etching 147,148Electron beam irradiation 149–151Ion implantation 152Ultrasonic irradiation 153,154

The physical phenomena called plasma can be divided into hot plasma (equilibrium) andcold plasma (low temperature, non-equilibrium). The low-temperature plasma is commonlyused in material modification. In low-temperature plasma, the electron temperature is 10–100 times higher than the gas temperature [161]. However, because of the very low densityand very low heat capacity of the electrons, the very high temperature of electrons does notimply that the plasma is hot. This means that although the electron temperature rises overseveral tens of thousands of kelvins, the gas temperature remains at 100 K. Therefore, thisexplains why plasma is termed as low-temperature plasma (thereafter called plasma) andis used for the modification of polymer surfaces.

Depending on the gas pressure, two different forms of electrical discharge in gases areknown, which are often referred to as the plasma treatment [162]:

(a) Corona discharge: This is generated at gas pressures equal to or near to the atmosphericpressure with an electromagnetic field at high voltage (>15 kV) and frequency in the20–40 kHz range for most practical applications.

(b) Glow discharge: This is generated at gas pressures in the 0.1–10 MPa range with anelectromagnetic field in a lower voltage range (0.4–8.0 kV) and a very broad frequencyrange (0–2.45 GHz).

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4. Mechanism of plasma treatments

As plasma is a gaseous mixture consisting of electrons, equally charged ions, moleculesand atoms, many reactions occur simultaneously in a plasma system. There are two majorprocesses with opposite effects, namely (i) polymer formation – leading to the deposition ofmaterials and is termed as plasma polymerisation and (ii) ablation – leading to the removalof materials. Besides the conditions of discharge, such as the energy density, the plasmagas mainly determines which of the two processes is dominant [161]. If the plasma gashas high proportions of carbon and hydrogen atoms in its composition, such as methane,ethylene and ethanol, the plasma will result in plasma polymerisation. The plasma polymerfilms are typically pinhole-free, highly cross-linked and insoluble. It is easy to obtain verythin films [162]. Ablation of materials by plasma can occur by means of two principalprocesses, one is physical sputtering and the other is chemical etching. The sputteringof materials by chemically non-reactive plasma, such as argon gas plasma, is a typicalexample of physical sputtering. Chemical etching occurs in chemically reactive types ofplasma. This type of plasma gas includes inorganic and organic molecular gases, such asO2, N2 and CF4, which are chemically reactive but do not deposit polymers in their puregas plasma. Plasma ablation competes with the polymer formation in almost all the caseswhen plasma is used to treat the surfaces of solid materials [161]. A scheme of interactionbetween a solid phase and a plasma phase is summarised in Figure 3 [161].

After plasma treatments, there are still a lot of free radicals remaining on the treatedfibre surfaces. These free radicals play an important role in forming functional groupsand bonds between the fibre and matrix. They will also be extinguished when exposed toatmosphere especially oxygen by decreasing the extent of bonding between the fibre andmatrix. Hence, the time lapse between plasma treatments and composite fabrication shouldbe as short as possible.

4.1 Polymerisation

Plasma polymerisation is a unique technique for modifying polymer and other materialsurfaces by depositing a thin polymer film [163–175]. Table 2 lists some examples showingvarious applications of this technology in polymer surface modification. Plasma-depositedfilms have many special advantages, which are as follows:

Figure 3. Polymerisation–ablation competition of the plasma treatment [161].

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(1) A thin conformal film of thickness of a few hundred angstroms to 1 µm can be easilyprepared.

(2) Films can be prepared with unique physical and chemical properties. Such films, highlycross-linked and pinhole-free, can be used as very effective barriers.

(3) Films can be formed on practically any kind of substrate including polymers, metal,glass and ceramics. Generally speaking, good adhesion between the film and substratecan be easily achieved.

Plasma polymerisation is a very complex process that is not well understood. Thestructure of plasma-deposited films is highly complex and depends on many factors in-cluding reactor design [176], power level [176], substrate temperature [177], frequency[178], monomer structure, monomer pressure [179] and monomer flow rate [180,181].Two types of polymerisation reactions can occur simultaneously, namely plasma-inducedpolymerisation and polymer-state polymerisation. In the former case, the plasma initiatespolymerisation at the surface of liquid or solid monomers [182–184]. For this to occur,monomers must contain polymerisable structures, such as double bonds, triple bonds orcyclic structures. In the latter case, polymerisation occurs in plasma in which electrons andother reactive species have enough energy to break any bond. Any organic compound andeven those without a polymerisable structure, needed for conventional type of polymerisa-tion, can be used in plasma-state polymerisation. The rates at which monomers polymeriseare relatively similar regardless of the structure of the monomer [176].

4.2 Plasma ablation (physical sputtering and chemical etching)

In the case of glow discharge, plasma with different ionisation extent can be produced.The active plasma species produced carry high-kinetic energy (from 1 eV to several eVs).This energy can initiate reactions of not only the saturated organic compounds but alsounsaturated ones. Although the kinetic energy is high, the temperature of the plasmais relatively low. The active species in plasma will lose the energy once they interactwith the polymer material. As a result, the penetration of the plasma into the polymermaterial is rather shallow (beyond 1000 A) [185], and the interior of the material is onlyslightly affected. Hence, the plasma treatment can be considered to be a surface treatment(Figure 4). The plasma species, carrying high-kinetic energy, bombards the polymer causinga sputtering or etching effect [186] on the surface. This bombardment, therefore, alters thesurface characteristics of the polymeric material.

4.2.1 Sputtering

In sputtering, ions accelerated across the sheath potential bombard a surface with highenergy. The sudden energy impulse can immediately eject surface atoms outward, or by abilliard ball-like collision cascade can even stimulate the ejection of subsurface species. Ifthere is to be net material removal, however, molecules sputtered from the surface must notreturn. This requires a low-gas pressure, or equivalently, a mean-free path that is comparableto the vessel dimensions. If the mean-free path is too short, collision in the gas phase willreflect and redeposit the sputtered species.

Sputtering requires plasma conditions with high-ion energies. These conditions existin low pressure (<50 mTorr), where mean-free paths are long as well. As a mechanicalprocess, sputtering lacks selectivity. It is sensitive to the magnitude of bonding forces andstructure of a surface rather than its chemical nature, and quite different materials can also

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Figure 4. Four basic methods of plasma etching: (1) sputtering, (2) chemical etching, (3) ion-enhanced energetic etching and (4) ion-enhanced protective etching [186].

sputter at similar rates. In a way, this is symptomatic of using ion bombardment with energyfar higher than the surface-binding energy.

4.2.2 Chemical etching

In chemical etching, gas-phase species merely react with a surface according to elementarychemistry. Fluorine atom etching of silicon is a good example of this mechanism. The key,and really the only requirement for this kind of process, is that a volatile reaction productcan be formed. In silicon/F-atom etching, spontaneous reactions between F-atoms and thesubstrate form SF4, a gas. The only purpose of the plasma in chemical etching is to make thereactive etchant species, for example F-atoms. The etchant species are formed through colli-sions between energetic free electrons and gas molecules, which stimulate dissociation andreaction of the feed gas, that is plasma feeds such as F2, NF3 and CF4/O2, all make F atoms.

Chemical etching is the most selective kind of process because it is inherently sensitiveto differences in bonds and the chemical consistency of a substrate. However, the processis usually isotropic or non-directional, which is sometimes a disadvantage. With isotropicetching, both vertical and horizontal material removal proceed at the same rate, making itimpossible to form the fine lines (less than from about 3 µm in the usual films to 1 µmthick films).

4.2.3 Ion-enhanced energetic etching

In ion-enhanced energetic etching, which is a directional etching mechanism, the impingingions damage the surface and increase its reactivity. For example an undoped single crystal

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silicon surface is not etched by Cl2 or Cl atoms at room temperature. When the surface issimultaneously exposed to a high-energy ion flux, the result is a rapid reaction that formssilicon chlorides and removes material much faster than the physical sputtering rate [187].The word ‘damage’ in this case can be referred to the partly dissociation of the surfacecompound. Whatever the microscopic details (which undoubtedly can vary greatly fromone surface/etchant system to another), the generic mechanism is one in which ions impartenergy to the surface, which serves to modify it and to render the impact zone as well as itsenvironment more reactive.

4.2.4 Ion-enhanced protective etching

This kind of etching mechanism can be classified as inhibitor ion-enhanced etching requir-ing two conceptually different species, that is etchants and inhibitors. The substrates andetchants in this mechanism will react spontaneously and etch isotropically, if it was not forthe inhibitor species. The inhibitors form very thin film on surfaces that cease little or noion bombardment. The film acts as a barrier to etchant and prevents the attack of the featuresidewalls, thereby making the process anisotropic.

4.3 Advantages of plasma treatment

The advantages of plasma treatment [176] include the following:

� Modification can be confined to the surface layer without modifying the bulk propertiesof the polymer. Typically, the depth of modification is restricted to a few hundredangstroms only.

� Excited species in a plasma can modify the surfaces of all polymers, irrespective of theirstructures and chemical reactivity.

� By the selection of the feed gas to plasma reactor, it is possible to achieve the desiredtype of chemical modification for the polymer surface.

� The use of plasma can avoid the problems encountered in wet chemical treatments suchas residual chemical in the effluent and swelling of the substrate.

� Modification is fairly uniform over the whole surface.

4.4 Disadvantages of plasma treatment

The disadvantages of the plasma treatment [176] are summarized as follows:

� The plasma treatment is normally carried out in vacuum although atmospheric type isbeing developed, thereby increasing the cost of the operation.

� The processing parameters are highly system dependent, that is the optimal parametersdeveloped and optimised for one system usually need to be modified for application toanother system.

� The scale-up of an experimental set-up to a large production reactor is not a simpleprocess.

� The plasma process is so complex that it is difficult to achieve a good understanding ofthe interactions between the plasma and the surface necessary for a good control of theplasma parameters, such as power level, gas flow rate, gas composition, gas pressureand sample temperature.

� It is very difficult to control precisely the amount of a specific functional group formedon the sample surface.

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5. Generation of plasma

Plasma is electrically neutral and generated by electrical discharge, high-frequency elec-tromagnetic oscillation, high-energy radiation such as a- and γ -rays etc. [188]. Electricaldischarge is commonly used in industrial application. Plasma is usually excited and sus-tained electrically by the methods of (i) direct current (DC), (ii) radio frequency (RF) or(iii) microwave (MW) applied to the gas. Plasma density is controlled mainly by electronenergy and gas temperature. Therefore, as long as identical energies and temperatures canbe achieved, the type of discharge used to create the plasma is of little importance. Thechoice of a specific method and equipment to produce discharges is determined by therequirements of flexibility, process uniformity, cost and process rate. Various methods usedfor the generation of plasma are described in the following.

5.1 DC glow discharge

A DC glow discharge is produced by applying a DC voltage between two conductiveelectrodes inserted into a gas at low pressure as shown in Figure 5. A high-impedancepower supply is used to provide the electrical field.

A small quantity of free electrons is always present in the gas as a result of ionisationby naturally occurring radioactivity or cosmic rays. Free electrons can be produced byphotoionisation or field emission. As the voltage applied to the gas in the discharge tubeis gradually increased, the available free electrons as produced by radioactivity or cosmicrays are accelerated in the electric field, thereby gaining kinetic energy. The free electronsmay lose energy upon inelastic collision with the atoms or molecules of the gas. Theseatoms or molecules will also be referred to as collision targets.

Initially, when the energy of the electrons is too low to excite or ionise a target, thecollision will necessarily be elastic. The average function of electron energy lost in an elasticcollision with a gas atom or molecule is –2me/M eV [188], where me and M are the massof the electron and the target, respectively. Hence, only a very small fraction of the totalkinetic energy of the electron, typically only 10−5, is lost per elastic collision. Meanwhile,the electron continues to gain energy between collisions until it attains sufficient energyto cause ionisation of the targets through inelastic collisions. Large amounts of energyare transferred to the target in the inelastic collisions, making those collisions an efficientmeans of energy transfer. The new electrons produced in the ionisation process are in turnaccelerated by the electric field to produce further ionisation.

Figure 5. DC glow-discharge set-up [188].

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When the number of electrons is sufficient to produce just enough ions to regenerate thenumber of lost electrons, a steady state is reached in which an equilibrium is establishedbetween the rate of formation of ions and the rate of their recombination with electrons. Atthis stage, the discharge is self-sustaining. Extensive breakdown occurs in the gas, and theglow discharge is thus established.

5.2 Radio-frequency discharge

Although a DC discharge may be initiated, it will be quickly extinguished as the electronsaccumulate on the insulator and recombine with the available ions. In some cases, it ispreferable to have the electrodes located outside the reactor, to avoid or minimise thecontamination of the process caused by the material removed from the electrodes. Suchproblems can be solved by alternating the polarity of the discharge.

When an alternating electric field of low frequency (<100 Hz) is applied betweenthe two electrodes of the discharge tube, each electrode acts alternately as cathode oranode. Once the breakdown potential is surpassed on each half cycle, a temporary DCglow discharge is obtained. When the voltage drops during the cycle below the breakdownvalue, the discharge is extinguished and sufficiently low frequencies are reinitiated withinverse polarity [188]. Hence, high frequency is used to maintain the discharge process. Thefrequencies used in the high-frequency discharges are in the range of radio transmissiongiving the high-frequency discharges the name of radio frequency, or RF discharge.

The elastic collision frequency, ν, in gases at glow discharge conditions is normallybetween 109 and 1011 collisions/ per second [189]. This makes the collision frequency muchhigher than the applied radio frequency even for 13.56 MHz discharges, and so electronswill experience many collisions during each applied field cycle. They will be generatedby impact ionisation in the body of plasma. Therefore, the loss of electrical carriers fromthe RF discharge is controlled by ambipolar diffusion and homogeneous recombination(recombination in the gas phase) but not by the electric field. Newly charged particles areproduced mainly through electron impact ionisation of neutral gas and molecules. If anelectron makes an elastic collision with an atom, reversing its motion at the same timemakes the electric field change direction, it will continue to gain speed and energy [190].Electrons in a RF discharge could thus accumulate enough energy to cause ionisation evenat low-electric field. As a result of this behaviour, the RF discharge is more efficient thanthe DC discharge in promoting ionisation and sustaining the discharge.

5.3 Microwave plasma

Microwave plasma is sustained by the power supply operating at a frequency of 2.45 GHz.This frequency, which is commonly used for industrial or home heating application, makessuitable power supply readily available. The excitation of plasma by microwaves is similarto the excitation with RF, whereas their differences result from the range of frequencies.However, microwave discharge is more difficult to sustain at low pressures (<1 Torr) thanDC or RF discharge [188].

Although the RF glow discharge can be made to extend virtually throughout the entirereactor, its dimensions are much smaller than the wavelength of the RF field (∼22 mat 13.56 MHz). The microwave plasma has its greatest glow intensity at the couplingmicrowave cavity and diminishes rapidly outside it because of the much smaller wavelengthof the microwave (λ = 12.24 cm for frequency of 2.45 GHz). In a microwave plasma, themagnitude of the electric field can vary within the reactor, and the dimensions have the

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same order of magnitude as the wavelength. One can thus find active species from thedischarge still persisting in a region free of the glow of the plasma, that is, in the afterglow.

6. Factors affecting plasma treatment

In the plasma treatment, the effectiveness of the treatment depends much on different factors[191]. Several factors are commonly governing the effect of the plasma treatment, namely(1) nature of gas used, (2) flow rate, (3) system pressure, (4) discharge power, (5) durationof treatment, (6) ageing of plasma-treated surface and (7) temperature change during theplasma treatment.

6.1 Nature of gas used

The result of plasma treatment depends strongly on the nature of the gas [191–193] or thevapour used in glow discharge. Most organic, organosilicone or organometallic vapours tendto form a thin film on the surfaces, which are subjected to glow discharge, and the depositionof these films is the main factor modifying the polymer surface in such a case. On the otherhand, glow discharges of non-polymerising gases, for example noble gas, nitrogen, oxygen,hydrogen, ammonia or water vapour, modify polymer surfaces through processes, such asoxidation, ablation, cross-linking and perhaps grafting. It should be recognised, however,that ablation supplies the gas phase with various chemical species, some of which maybe able to form deposits, especially in mixtures with such non-reactive gases as hydrogenor ammonia. In short, the characteristics of plasma are varied by changing the gas that isused for electrical discharge, for example oxygen plasma is oxidising in nature whereashydrogen is reductive in nature. Furthermore, the surface composition and characteristicsof the polymeric material also vary with gas feed of different natures.

6.1.1 Inert gas

Helium, neon, and argon are the three inert gases commonly used in plasma technology.Because of the relatively lower cost, argon is by far the most common inert gas being used.The direct and radioactive energy transfer (momentum transfer) processes created by inertgas plasma can cause physical modification of the surface. Inert gas plasma has been usedfor the pre-treatment of substrates for cleaning purposes before reactive gases are applied.If a plasma reaction is to be carried out with a high-system pressure but at a low-reactivegas flow rate, an inert gas can serve as a diluent. Treatment of polymer surfaces by exposureto inert gas plasma has been utilised to improve the adhesive characteristics of polymers.Polymers have been subjected to low-power plasma of noble gases for certain periods,typically from 1 second to several minutes. This exposure is sufficient to abstract hydrogenand to form free radicals at or near the surface, which then interact to form cross-linksand unsaturated groups with chain scission. The gas plasma also removes low-molecularweight materials or converts them to a high-molecular weight by cross-linking reactions.As a result, the weak boundary layer formed by the low-molecular weight materials isremoved. Consequently, greater adhesive joint strengths are observed. This treatment hasbeen known as CASING (cross-linking by activated species of inert gases) [194].

6.1.2 Oxygen-containing gas

Oxygen and oxygen-containing plasma are most commonly employed to modify polymersurfaces. It is well known that the oxygen plasma can react with a wide range of polymers to

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Table 3. Surface composition of oxygen-plasma-treated PTFE as a func-tion of treatment time [195].

Chemcial composition (at%)Treatment time(min) C F O

0 39.8 60.4 0.80.5 44.6 48.9 6.41.0 42.7 51.1 7.12.0 42.6 50.9 6.55.0 40.9 57.0 2.1

10.0 38.3 60.5 1.215.0 38.3 61.4 0.3

produce a variety of oxygen functional group, including C O, C O, O C O and C O Oat the surface. In oxygen plasma, two processes occur simultaneously including (i) etchingof the polymer surface through the reactions of atomic oxygen with the surface carbonatoms, giving volatile reaction products; and (ii) formation of oxygen functional groupsat the polymer surface through the reactions between the active species obtained from theplasma and surface atoms. The balance of these two processes depends on the operationparameters of a given experiment.

Oxygen-plasma treatment of PTFE illustrates the competitive nature of these two pro-cesses [195]. The surface chemical composition of oxygen-plasma-treated PTFE as a func-tion of treatment time is shown in Table 3. After a short treatment time of 0.5–2 minutes,the fluorine concentration decreased and the oxygen concentration increased, whereas aftera long-treatment time, the trend was reversed.

The interaction of microwave plasma of carbon dioxide and polypropylene leads totwo competitive reactions, namely (i) modification and (ii) degradation [196]. Surfacemodification produces ketone, acid and ester on the polymer surface, whereas degrada-tion generates volatile products and a layer of oxidised oligomers of polypropylene. Theconditions favouring surface modification are low gas pressure, power and treatment time.

Water plasma may be used to incorporate hydroxyl functionality into a material surface.Usually, oxidation reactions rather than reduction reactions are obtained in H2O plasma,for example to create a hydrophilic surface on PMMA by the incorporation of hydroxyland carbonyl functionalities [197].

6.1.3 Nitrogen-containing gas

Nitrogen-containing plasma is widely used to improve wettability, printability, bondabilityand biocompatibility of polymer surfaces. For example, to improve the interfacial strengthbetween polyethylene fibres and epoxy resins which are cured by amine cross-linking,amino groups are introduced on the fibre surface to promote covalent bonding [196]. Theintroduction of amino groups on the surface of polystyrene films with ammonia-plasmatreatment is reported to improve cell affinity [198]. Ammonia-plasma and nitrogen-plasmahave been used to provide surface amino binding sites for immobilisation of heparin ona variety of polymer surfaces [176]. Ammonia-plasma treatment is reported to increasethe peel strength between polytetrafluoroethylene and nitrile rubber, when a phenol-typeadhesive is used [176].

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Oxygen functionalities are always incorporated into nitrogen-plasma-treated polymersurfaces. It is a common phenomenon that oxygen is incorporated on the polymer surfacesafter and during non-oxygen-plasma treatments. Free radicals that are created on a polymersurface can react with oxygen during a plasma treatment. In addition, free radicals thatremain on a polymer surface after a plasma treatment will react with oxygen when thesurface is exposed to the atmosphere.

6.1.4 Fluorine-containing gas

In the fluorine-containing plasma, surface reactions, etching and plasma polymerisationcan occur simultaneously which reactions predominate will depend on the gas feed, theoperating parameters and the chemical nature of the polymer substrate and electrode.

CFx radicals play important roles as polymerisation promoters, etchants of silicon diox-ide and recombining species during anisotropic etching. Halogen atoms, especially fluorineand chlorine atoms, are the major etching species (i.e. non-polymerisable etching species)for a variety of materials. Ions and electrons can influence the plasma–substrate interactionprocess. Their bombardment of the surface can either alter the surface bonds of the latticeor promote the desorption of some chemisorbed species. The extent of bombardment caninfluence the etch rates, degree of anisotropy and polymerisation.

Tetrafluoromethane shows the highest relative etching characteristics for a materialreactive with fluorine atoms. Its decomposition in the plasma is characterised by the highestconcentration of fluorine atoms ([F]) and the lowest concentrations of CF and CF2 radicals([CF] and [CF2], respectively). If the F/C atomic ratio of the feed-in monomer decreases(e.g. tetrafluoroethylene), [CF] and [CF2] will be much higher at the expense of [F]; andthe fluorocarbon plasma becomes a polymerising plasma rather than the etching plasma[176].

6.1.5 Hydrocarbon

Hydrocarbons, such as methane, ethane, ethylene, acetylene and benzene, have been widelyused in the generation of plasma-polymerised hydrogenated carbon films. The outstandingphysical properties of these films, such as microhardness, optical refractive index andimpermeability, provide them with numerous potential applications such as anti-reflectionand abrasion-resistant coatings.

6.1.6 Halocarbon

Plasma of fluorine-containing inorganic gases, such as fluorine, hydrogen fluoride, NF3,bromine trifluoride, sulphur tetrafluoride and SF6 and monomers, is used to incorporatefluorine atoms into polymer surfaces to produce hydrophobic materials. The wide rangeof F/C ratios obtained by plasma polymerising various fluorocarbon monomers providestremendous potential for a variety of applications.

6.1.7 Organosilicon plasma

Plasma polymers obtained from organosilicon monomers have demonstrated excellentthermal and chemical resistance as well as outstanding electrical, optical and biomedi-cal properties. They may find uses in many branches of modern technology: for exampledielectric coatings or encapsulatants in microelectronics, anti-reflection coatings in con-ventional optics, thin-film light guides in integrated optics and biocompatible materials

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Figure 6. Effect of gas flow rate on weight loss of Nylon 6 treated by air plasma for 5 minutes:(o) plasma discharge at 100 W, ( �) 70 W, (�) 50 W and 30 W [192].

in medicine. Various organosilicon precursors frequently used include silanes, disilanes(SiSi), disiloxanes (SiOSi), disilazanes (SiNHSi) and disilthianes (SiSSi).

6.2 Flow rate

Figure 6 shows the effect of gas flow rate [191,192], for example on the weight loss of Nylon6 treated by means of air plasma for 5 minutes with different degrees of discharge power.The weight loss increases with the flow rate at every discharge wattage in the low-flow-rateregion. As the flow rate is further increased, the weight loss deviates from linearity andstarts to decrease. The initial increase with the flow rate can be explained by the increase inthe number of reactive species, particularly of O and O3 in air. The deviation from linearityand the further decrease occur at lower flow rates when a lower wattage is applied. It is clearfrom the plot that at the higher flow rate, the concentration of the active species decreasesdespite the increase in the gas flow rate.

6.3 System pressure

The system pressure is perhaps the least understood parameter of the plasma treatment.This misunderstanding stems largely from the lack of distinction between non-polymer-forming plasma and polymer-forming plasma. The polymerisation itself can change thesystem pressure [186,192, 199–201]. Another factor contributing to the misunderstandingis the failure to recognise the effect of the gas. In many cases, the system pressure observed

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Figure 7. Relationship between ion bombardment energy and pressure [186].

before the plasma treatment, P0, is cited as the system pressure throughout the treatment,Pg . However, it has been claimed that Pg is adjusted to P0 by controlling the pumping rate.Since Pg is dependent on the production rate of the plasma, such an operation is not alwayspossible. Furthermore, in view of the etching effect of gas, such an operation does not seemto have any advantage or significance in controlling the process.

In summary, the system pressure used may affect the energy of the plasma species. Ifthe pressure is high, the probability of collision between plasma species will be increasedleading to the loss of energy of the species before interacting with the material. At apressure below approximately 0.02 Torr, the transport of species becomes collisionless.Figure 7 summarises the relationship between the ion bombardment energy and the systempressure, providing a clear picture of the contribution of system pressure on the plasmatreatment.

6.4 Discharge power

The intensity of plasma is a combined factor of pressure and discharge power [192,199].The breakdown energy necessary to produce plasma varies from one gas to another. Hence,the initiating energy is not a constant but is primarily dependent on the nature of gas fed.Normally, the higher the discharge power applied, the more kinetic energy the plasmaspecies will carry, resulting in strong intensity of plasma action.

In general, there will be a change in the total amount of the excited particles inside theplasma and their energy level accordingly when the input power increases under a constantpressure [202], resulting in the increase in the charged ion concentration. Figure 8 showsan example of verifying the dependence of dyeing behaviour on the power of wool fabricstreated under several power conditions (25, 50, 100, 150 and 200 W) with the system

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Figure 8. The exhaustion curve of wool (with reactive dye) treated with plasma of different dischargepower [199].

pressure and treatment time being fixed at 25 Pa and 10 minutes, respectively. The uptakebecomes greatest when the power is around 100 W.

6.5 Duration of treatment

The duration of treatment [203,204] plays an important role in the plasma treatment.Generally speaking, the longer the duration of treatment, the more severe the modificationof the material surface, for example sputtering or etching, will be. A longer duration willnot only affect the material surface but also provide an opportunity for the plasma speciesto penetrate into the interior region of the material. This may alter the morphology of thepolymeric material. However, when the treatment duration is too long, this will adverselyaffect the material and therefore careful control of treatment duration is required.

The relationship between duration of the plasma treatment and dyeing behaviour ofreactive dyeing on wool is shown in Figure 9, which illustrates the percentage of exhaustion

Figure 9. Relationship between percentage of exhaustion and duration of the plasma treatment [199].

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against the rate of dyeing of plasma-treated wool. At 10–15 minutes of duration of thetreatment, there is an obvious increase in dyeing behaviour.

According to Figure 9, it is noted that the increased duration of treatment results in adecease in dyeing behaviour. This result confirms that long duration of treatment would notgive better effects, and there existed an optimum treatment time. In such a phenomenon,it is supposed that the substrate is considered to produce a plasma-like region, where allbonds are broken during the treatment. Most of these bonds tend to recombine withinthe same molecule rather than with the neighbouring molecules. This recombination caneventually lead to the carbon cluster formation [152]. Hence, the overmuch treatment timemight barrage and damage the dyeing path, resulting in affecting the dyeing behaviour.

6.6 Ageing of the plasma-treated surface

In general, the concentration of functional groups introduced to a polymer surface by theplasma treatment may change as a function of time depending on the environment andtemperature. This is because polymer chains have much greater mobility at the surfacethan in the bulk, allowing the surface to reorient in response to different environments.Surface orientation can be accomplished by the diffusion of low-molecular weight oxidisedmaterials into the bulk and the migration of polar function groups away from the surface.Ageing of plasma-treated polymer surfaces [205] can be minimised in a number of ways.An increase in the crystallinity and orientation of a polymer surface enhances the degreeof order and thus reduces mobility of polymer chains, resulting in slower ageing [205]. Ahighly cross-linked surface also restricts the mobility of polymer chains and helps to reducethe rate of ageing.

6.6.1 Effects of environment

When a polymer is exposed to the oxygen-containing plasma, the surface will change to ahigh-energy state, that is increase in surface tension, as a result of the formation of polargroups. Various surface studies indicate that when the treated surface is placed in a low-energy medium, such as air or vacuum, the decrease in surface energy is caused by therotation of the polar groups in the bulk or the migration of low-molecular weight fragmentsto the surface to reduce the interfacial energy [206–209]. When a low-energy surface formedby treating a polymer in fluorine-containing plasma is placed in a high-energy medium,such as water, the apolar groups will tend to minimise the interfacial energy by moving awayfrom the surface into the bulk. This phenomenon is usually described as ageing of a treatedsurface. The ageing of plasma-treated polymer surfaces is a very complex phenomenonthat is strongly affected by the treatment parameters, nature of the polymer and storageconditions. The contact-angle measurement, which is a very surface-sensitive technique,has been successfully used to study the dynamic characteristics of polymer surfaces invarious environments.

For example, when an oxygen-plasma-treated poly(dimethyl siloxane) surface is agedin air, the surface returns to a low-energy state [208]. In vacuum or air, the surface orientsits apolar groups towards the interface, minimising the interfacial free energy. When ageingis performed in water or an aqueous phase, the plasma-treated surface, which has a highconcentration of polar groups, maintains its polar groups at the surface, thereby minimisingthe interfacial free energy.

The ageing of the nitrogen functional groups on polyethylene has been studied [210].The surface chemical composition of a nitrogen-plasma-treated polyethylene surface ismonitored as a function of storage time in air. A rapid loss of nitrogen and a significant

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increase in oxygen can be observed during the first few days. Subsequent experimentsreveal no further loss of nitrogen, but a gradual increase in oxygen. The initial loss ofnitrogen and increase in oxygen can be explained by the hydrolysis of imines by means ofatmospheric water:

and the slower increase in oxygen is due to the following reaction:

6.6.2 Effect of temperature

Another important factor that affects the ageing characteristics of a plasma-treated polymersurface is temperature. A lower storage temperature reduces the rate of ageing. Figure 10shows the change in the water advancing contact angle on an oxygen-plasma-treatedpolypropylene surface as a function of ageing time at different temperatures [206]. Therapid change in the contact angle at high temperatures supports the idea that the changes inthe surface structure are caused by polymer chain motion, resulting in reorienting the polargroups into the bulk.

6.7 Temperature change during plasma treatment

The decomposition of fibres under plasma condition is a major concern during the plasmatreatment [192]. The temperature increase inside the plasma reactor is the possible way ofthe thermal decomposition of fibre as a side effect [192]. Figure 11 presents the change inthe temperature with time for air plasma. In most cases, the temperature inside the reactorreaches a saturation level, usually lower than 130◦C and remains relatively constant duringthe whole treatment. Air plasma treatment is accompanied with a slightly higher tempera-ture increase, which might be attributed to the evolution of heat of oxidation. Nevertheless,

20

40

60

80

100

0 1 2 3 4 5 6 7

Ageing time (h)

Con

tact

ang

le (

deg)

293K

333K

363K

393K

Figure 10. Contact angle of water on oxygen-plasma-treated polypropylene as a function of ageingtime at different temperatures [206].

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Figure 11. Temperature inside the reactor as a function of time for air plasma (discharge conditions:(◦) power 70 W, flow rate 180 sccm; (•) power = 50 W, flow rate = 120 sccm; (�) power = 30 W,flow rate = 100 sccm) [192].

the temperatures of the plasma treatment are always far below the thermal decompositionregions for the respective polymers. Hence, the possibility of the pyrolysis of fibres ac-companying their plasma degradation should be ruled out [192]. One should remember,however, that in certain cases the evolution of the low-molecular weight species from thebulk of the fibre may be expected at elevated temperatures. Although this phenomenon isnot significant, it may contribute somewhat to the total weight loss [211].

7. Production of plasma species

The production of plasma species is briefly shown below with oxygen gas being used as anexample [186]

(a) ion and electron formation

e + O2 → O+2 + 2e

(b) atom and radical formation

e + O2 → O + O and

(c) generation of heat and light

e + O2 → O∗2

O∗2 → hν

e + O → O∗

and

O∗ →hν

where O∗2 and O∗ are excited states of O2 and O.

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In the plasma, each formation step balances various loss processes. The equilibriumbetween formation and loss determines the steady concentration of species in a discharge.For the charged species in plasma, these formation and loss processes may be grouped intoa few categories as shown in the following.

7.1 Ionisation and detachment

Ionisation reactions are the main source of ions and electrons. The general form of thesereactions is

e+M → A++2e(+B),

where M is either a molecule (AB) or an atom. If the molecule dissociates in this processto yield the neutral fragrant B, it is called dissociative ionisation. When the species M is anegative ion, the process is called ‘detachment’, since the negatively charged electron is saidto be attached when a negative ion is formed in the first place. The detachment process, e +A− → A + 2e, is no less an ionisation and similarly creates a free electron. Less frequently,another process called the Penning ionisation can make an important contribution. In thePenning ionisation, an excited metastable state is formed by electron impact, e + C → C*+ e, and the excitation energy of the metastable state is enough to ionise a second speciesvia, C* + M → C + M++ e, or (where again M = AB), C∗ + AB → C + A+ + B + e orC* + M → CM++ e. The Penning ionisation has been found to be significant in mixtures,where C is a rare gas, for example neon, with a metastable state excited energy that is justabove the ionisation energy for M.

7.2 Recombination, detachment and diffusion

A series of loss processes balance the formation steps outlined before. Some of the mostimportant changes loss in mechanisms is electron–ion recombination, e + M+ → A +B, attachment, e + M → A−+ B and diffusion of ions and electrons to the walls of thereaction vessels. These reactions take place in a variety of ways, depending on the speciesinvolved. In the case of oxygen plasma, the dissociative recombination will be the mostrapid ion–electron recombination process, e + O+

2 → O + O, in a pure argon discharge,only simple electron–ion recombination is possible, e + A+ → A.

In highly exothermic reactions, reaction channels that form two or more product frag-ments with comparable mass are generally favoured because they make it easier to conserveboth energy and momentum.

8. Application of plasma technology in textile industry

Activities in plasma treatment have greatly increased in the past decade in the followingareas of the textile industry:

� enhancement of mechanical properties of advanced composite by plasma inert gastreatment,

� fibre treatment in a plasma to obtain desirable properties such as water absorbency, waterrepellency, improved yarn spinning properties, dirt release, dyeing, shrinking and formretention,

� metallisation of polyester fabrics by cathode sputtering,

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� grafting of acrylic acid and other monomers, such as poly(ethylene terephthalate), poly-acrylates and cellulosic, to fibres for surface modification,

� plasma etching followed by chromic acid treatment of polyethylene fibres to impartenhanced properties to epoxy–fibre composites,

� high-colour development of poly(ethylene terephthalate) by plasma etching followed byimmersion in acrylate solutions, and

� reduction in fibre shrinkage, especially wool, by air plasma at 80◦C and with traces ofchlorine and water.

9. Application of plasma treatment of wool under industrial conditions

The plasma treatment alters many different wool properties as shown in the following:

� spinnability of plasma-modified wool tops,� printing of fabrics made from plasma-modified wool,� machine-washable fabrics made from plasma-modified wool,� plasma-modified wool for fully fashioned knitted fabrics,� hand-knitting yarns made from plasma-treated wool tops,� shrinkproofing of plasma-treated wool fabric, and� dyeing properties of plasma-modified wool substrate.

9.1 Spinnability of plasma-modified wool tops

Plasma-treated wool can be used in woollen spinning with favourable results being achievedboth in spinnability and yarn quality. This may be attributed to the increase in both the fibrefriction coefficient and the cohesion of the wool top by a factor of 2.0–2.2 [212]. When theyarn tenacity of plasma-treated wool yarn is increased, the evenness is also improved butthe hairness is decreased [212,213]. Nevertheless, after the plasma treatment, the spinningbehaviour of wool is changed and can be summarised as follows [162]:

(a) The spinning aids applied to the first drawing frame must be carefully selected.(b) The rubbing intensity or twist of the slubbing should be increased.(c) The reduction in break rate at ring-spinning frame is usually observed.(d) An increase in yarn tenacity by 10%–25% is observed for all yarns.

The high cohesion of wool top after the plasma treatment can last for 2–4 weeks.The coefficient of variation of cohesion within a single spinning is lower after the plasmatreatment than before. The increased top cohesion affects the tensile strength values of bothsingle and piled yarns to a definite extent. For example, the tensile strength values of 32tex × 2 knitted yarns of these differences are shown in Table 4. In addition, the ‘handle’ ofthe plasma-treated yarns and knitted goods is far from being acceptable due to noticeableincrease in harshness.

9.2 Printing of fabric made from plasma-modified wool

The marked hydrophobic surface structure of wool, which is partly responsible for itsfelting properties, means that printing woven or knitted wool fabrics, without carrying outa special pre-treatment, is impossible in practice. The plasma treatment can improve thewettability of the wool fibre surface by printing paste and guarantee a certain felt resistanceduring the after washing stage following printing. Any felting that occurs on the fabric

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Table 4. Relative tensile strength values of 32 tex × 2 knitting yarns produced from differentlymodified top [212].

Parameters of plasma treatment Relative tensile strength ofRelative top

Processing time (s) Power supplied (kW) cohesion Single yarn Piled yarn

30 3.7 281 120 12020 3.7 268 118 12015 3.7 227 118.7 143.310 3.7 240 109.3 105

7.5 3.7 213 116.3 12010 5.6 227 123.6 13310 6.4 210 112.4 116.610 8.3 199 110.5 113.3

7.5 7.4 206 112.2 125

Values for standard top and yarn are 100.

during after washing causes noticeable distortion of the printed design and a reduction inquality [212].

Through plasma pre-treatment, deeper colour yields evenness of printing and improvedcolour fastness can be obtained [214–216]. Other important features arising from plasmapre-treatment for printing operations include elimination of chemicals and water consump-tion, reduction of energy consumption by 90% and less shade variation [212]. Table 5 showsthe depth of print for four pairs of head scarves made from plasma-treated and -chlorinatedfabrics.

As expected, the plasma-treated wool has a considerable improvement in the colouryield under the influence of the plasma treatment when compared with the untreated wool asshown in Table 5. Such improvement is due to the superficial modification of wool and theformation of new, polar groups that make the wool more accessible to water and dye [216].Moreover, improved swelling and wetting induce better adsorption and easier diffusion ofdye molecules, resulting in the increase of the colour yield of wool print. Apart from thecolour yield, the improvement in wool surface hydrophilicity also enhances the wettabilityof wool fibre [214,217]. During the printing process, the capillary effect of the wool fabricincreases the diffusion and penetration of print paste inside the fabric and so more printpaste remains in the fabric as shown in Table 6. In addition, as more print paste penetratesinto the fabric, the penetration ratio is reduced giving a higher colour yield.

Table 5. Depth of print for plasma-treated and -chlorinated wool fabrics [212].

Depth of printed dye

Ground colour Plasma treated Chlorination

Black 25.4 28.029.3 27.9

Red 30.4 26.823.1 23.6

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Table 6. The amount of print paste pick up, the penetration ratio and the colour yield of woolprinting with C.I acid blue 41 [215].

Sample Untreated Chlorinated Plasma treated

Print paste pick up (g) 0.041 0.045 0.089Penetration ratio 14.10 3.30 2.05Colour yield (K/S) 8.46 13.72 16.87

Yellowness is one of the main problems that appear during chlorination processingand steaming of printed wool. When compared, the plasma treatment does not deterio-rate wool whiteness, which mostly remains unchanged after the plasma treatment [216].Table 7 shows the dependence of wool whiteness on the pressure and treatment time ofoxygen plasma. It is well known that the existence of a certain level of wool whitenessprior to printing is essential for the achievement of desirable print, especially when printingfabrics are in pale, pastel shades. At the same time, the improved hydrophilicity is also oneof the reasons that make the plasma treatment particularly suitable for wool preparation forprinting. It seems that plasma treatment fulfils both conditions.

In general, the colour fastness ratings of prints produced on plasma-modified fabricsare considerably better than those produced on chlorinated fabrics as shown in Table 8.

9.3 Machine-washable fabrics made from plasma-modified wool

The ‘Superwash’ standard, which was introduced and promoted by International WoolSecretariat (IWS) as a quality concept, for manufacturers, commerce and consumers, actsas a guarantee for the customer. For example, a jumper labelled with this symbol can bewashed up to 100 times in a washing machine using the special programme without thealteration of its dimensions or colour.

Certain requirements related to felting resistance and colour fastness must meet beforethe Superwash label can be used. Nowadays, almost 80% of the world production ofSuperwash articles is carried out in a two-stage process on wool tops, that is chlorinationand synthetic resin application. The pre-chlorination in this two-stage process is importantbecause chlorination can affect the adhesion and uniformity of the resin coating on the woolfibres, thereby improving the felting resistance. On the other hand, the plasma-treated woolcan be subjected to anti-felting finishing more easily without using resins. Researchershave shown that the application of resin to the plasma-modified wool is more effectivethan that carried out on the untreated fabric [212,218]. In the research works, wool fabricsproduced from plasma-modified tops are impregnated with two synthetic resins, namely(i) Basolan SW (polyether with reactive groups) and (ii) Synthappret BAP + ImpranilDLN. For comparison, an identical fabric produced from the untreated wool is finished

Table 7. Whiteness of wool withdifferent treatments [212,216].

Sample Whiteness

Untreated 33.25Chlorinated 38.90Plasma treated 32.80

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Table 8. Colour fastness of plasma-modified and-chlorinated fabrics [216].

Printed fabric

Colour fastness Chlorinated Plasma modified

Perspiration, alkaline Shade change 3–4 4–5Staining, wool 2 4–5Staining, cotton 2–3 4–5

Water Shade change 3–4 4–5Staining, wool 2 4–5Staining, cotton 2–3 4–5

Rubbing Dry 4–5 4

using the same amount of finish. The results of a felting resistance test are carried outin accordance with the IWS TM 185 test method, which is applicable to the Superwashstandard as shown in Table 9. According to this method, the shrinkage permissible shouldnot exceed 5%–10% depending on the type of fabric.

The results presented in Table 9 shows quite clearly that both types of resin are con-siderably more effective on the plasma-modified fabric than on the untreated fabrics. Thelowest effective resin application, that is minimum effective treatment level (METL) is25% Basolan SW for the untreated fabric and only 10% for the plasma-modified fabric.Furthermore, it is not necessary to use Na2S2O5 with the plasma-treated fabric.

The METL value determined in the same way for open-width tubular fabric is 3%–4%Basolan SW for the untreated fabric and only 1.5%–2% for the plasma-treated knittedfabric. Lower METL values are better not only because of the reduced processing costs butalso due to less change in the fabric handle and reduced soiling during the washing process.

Table 9. Felting shrinkage of untreated and plasma-modified fabrics in accor-dance with IWS TM 185 [212].

Composition of the padding liquor (g/l) Shrinkage (%)

H2SO4 (5%) Na2S2O5 pH Untreated Plasma treated

Basolan SW25 25 10 8.6 −4.5 +6.425 25 — 8.6 −35.4 −1.620 20 10 8.6 −32.4 +4.620 20 — 8.6 −43.8 +4.615 15 10 8.6 −47.7 +4.815 15 — 8.6 −56.0 +3.320 20 — 8.6 −21.8 +0.410 10 10 8.6 −27.4 —10 10 — 8.6 — −0.8

5 5 — 8.6 −63.3 −36.4Synthappret BAP + Impranil DLN

10 12.5 7.3 −25.5 −3.880 10 7.3 −49.8 −33.4

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Table 10. Shrinkage of knitted fabrics of different weights (TM 7A)[212].

Shrinkage (%) of knittedfabrics made from

Needle gauge of theknitting machine (gg) Untreated wool LTP-treated wool

9 13.4 4.015 18.8 6.721 23.7 7.7

The improved resin application may be due to the enhanced surface tension of woolfabric after the plasma treatment [219,220], which in turn increases the adhesion propertyof the fabric during the coating process [218]. Hence, the application of resin to the plasma-treated wool fibre surface becomes easier and more effective. On the whole, fabrics madefrom the plasma-modified wool top can meet the Superwash requirements with regard tofelting resistance even without applying resins.

9.4 Plasma-modified wool for fully fashioned knitted fabrics

Products made from fully fashioned knitted wool fabrics, for example jumpers, cardigansand waistcoats, have to be washed fairly frequently. Since they are usually worn quite often,thus they need to undergo anti-felting finishing.

Table 10 shows the evaluation results of the shrinkage of three different knitted fabricsproduced from the plasma-modified wool and the untreated wool in accordance with IWSTM 7A. It is obvious that felting resistance of all the plasma-modified wool fabrics isconsiderably higher when compared with the untreated wool, meaning that they can meetthe industrial requirement. Furthermore, knitted fabrics made from the plasma-modifiedwool exhibit a higher abrasion resistance than those made from the untreated wool.

9.5 Hand-knitting yarns made from plasma-treated wool tops

It is not necessary for all fabrics to meet higher requirement of felting resistance. Hand-knitting yarns can be regarded as an example of a product requiring a lower felting resistancebut with a durable handle and colour.

Experiments have been carried out in conjunction with a knitting yarn that is producedby plasma-treated wool tops [162,212], which can have resistance to hand washing in water.However, there is one condition such that the knitting density (number of rows and walesper 10 cm) must conform to the yarns. Table 11 shows the results of a series of experimentscarried out on the knitting yarns of 64 tex × 4. The three results at the end are related toa second series of experiments involving 7.5, 10 and 15 seconds of the plasma treatmenttime.

9.6 Shrinkproofing of plasma-treated wool fabric

A number of studies [70,185,221] have shown that plasma treatment can improve thelaundering properties of the wool fabric. The improvement is attributed to the reduction

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Table 11. Felting resistance of knitted fabrics produced fromuntreated and plasma-treated knitting yarns, 64 tex × 4 in ac-cordance with TM 7A [212].

Felting shrinkage (%)Weight perunit area (g/m2) Untreated Plasma treated

190 46.7 27.1227 41.6 18.2260 34.6 13.4294 25.9 10.7300 (7.5 s) — 6.5300 (10 s) — 4.8300 (15 s) — 6.0

in the directional frictional coefficient (DFE) of the fabric, resulting in the decrease in thefelting tendency of the wool.

Table 12 shows that the plasma-treated wool can have a greater reduction in DFE thanthe other chemical treatments. This reduction in DFE implies that the felting tendency ofwool is lower. In addition, the value of the density (D) of the felt ball is an inverse measureof the degree of felting. It is observed that the untreated wool fibre has the greatest D values,whereas the plasma-treated wool shows the greatest reduction in D among the oxidised,reduced and polymer-deposited wool.

It can be seen in Table 13 that there is a significant decrease in area shrinkage afterthe subsequent plasma treatment. Clearly, area shrinkage will increase as the processing ischanged from relaxation shrinkage to felting shrinkage.

The wool fabric shrinkage is correlated with the frictional coefficient of the constituentwool fibres. It is also common knowledge that plasma treatment increases the dry andwet frictional coefficient in the scale and anti-scale directions. However, the effect ofthe plasma process is attributed to several changes in the wool surface, such as (i) theformation of new hydrophilic group, (ii) partial removal of covalently bonded fatty acids

Table 12. The DFE value and felt ball density of wool fibre under different treatments [70].

Result

Felt ball density,Sample DFE (%)a D (g/cm3)b

Untreated 41.8 0.064Potassium permanganate/salt 29.4 0.042 (↓34.4%)Sodium metabisulphite treatment 35.1 0.054 (↓15.6%)Plasma treatment (oxygen plasma) 23.7 0.022 (↓65.6%)Basolan DC (chlorination) + Basolan MW Micro

(polymer deposition) combined treatment29.3 0.039 (↓39.1%)

aDFE was calculated by Mercer’s equation: DFE = (µa − µw)/(µa + µw)(µa + µw) × 100%.bFelt ball density was calculated by D = g/V = 0.524d3, where D is density of felted ball (g/cm3), g isweight of the wool sample (g), i.e. 2 g, V is volume of the felting ball (cm3) d is average diameter of thefelted ball (cm).

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Table 13. Results of dimensional changes (lengthwise) of the samples [70].

Relaxation dimensional Consolidation dimensional Felting dimensionalchange (%) change (%) change (%)

Sample Warp Weft Warp Weft Warp Weft

Untreated 5.0 2.0 6.8 2.6 9.6 3.6LTP treated 0.6 0.2 0.8 0.3 1.1 0.4

belonging to the outermost surface of the fibre and (iii) the etching effect [162]. The first twochanges contribute mainly to the increased wettability, whereas the last basically reducesthe differential friction coefficients of the fibres leading to the decrease in the naturalshrinkage tendency [70].

9.7 Dyeing properties of plasma-modified wool substrate

9.7.1 Dyeability modification

The exhaustion curves of the plasma-treated samples with different plasma gases namelyoxygen (PO), nitrogen (PN) and nitrogen/hydrogen mixture (PM), and the untreated samplesare shown in Figure 12. Table 14 summarises the results of time of half dyeing (t1/2)and percentage of exhaustion at equilibrium (%E at Em) obtained from Figure 12. Thepercentage exhaustion curve shows the variation in dyebath concentrations against timefrom which the characteristics of a dyeing system, that is t1/2,%E at Em and the initial rateof dyeing (strike), can be determined.

Figure 12 shows that the slopes of the curves representing plasma-treated fibres at thestart of dyeing are steeper than that of the untreated fibre, implying that the initial dyeingrate of the plasma-treated samples is faster than the untreated fibre. This phenomenonis probably because the diffusion rate of dye molecules becomes relatively faster for theplasma-treated fibre as a result of surface modification. In addition, the time to reach thedyeing equilibrium also becomes significantly shorter for the plasma-treated samples, thatis the percentage exhaustion curve starts to flatten at a faster time than the untreated fibre.Of the three different gases used, PN has the greatest effect on the dyeing rate followed by

Figure 12. Percentage dyebath exhaustion of different samples (acid dye) [224].

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Table 14. Time of half dyeing (t1/2) and percentage exhaus-tion at equilibrium (%E at Em) of control and treated samples[224].

Sample t1/2 (minutes) %E at Em (%)

Untreated 23.43 70.98PO 6.43 (↓72.56%) 71.56 (↑0.83%)PN 4.98 (↓78.75%) 72.02 (↑1.47%)PM 8.39 (↓64.19%) 71.69 (↑1.00%)

The figures inside the bracket indicate the increase or decrease invalue when the treated sample is compared with the control (↑ meansincrease in percentage of value, and ↓ means decrease in percentagevalue).

PO and PM. According to this sequence, it is likely that the nature of the plasma gas usedcould influence the dyeing behaviour of a dyeing system [222–226].

A previous study showed that the nature of the plasma played an important role inaltering the surface composition of the fibre [226]. The changes in surface compositionare likely to affect the dyeing behaviour of a dyeing system. When nitrogen gas is usedfor the plasma treatment, it may have introduced the amino groups (–NH2) to the fibre[193,227,228]. The induced –NH2 groups might have become the dyesites on the woolfibre resulting in increased dye absorption. Unlike nitrogen gas, the oxygen plasma willincrease the cysteic acid content on the wool fibre surface [227,228]. The cysteic acidgroups will facilitate the hydrophilic and wetting character of the wool fibre to enhance thedyeability of the wool fibre. Although the composition of gas mixture plasma is quite similarto the nitrogen plasma, yet different results are still obtained for each kind of plasma. Thehydrogen gas in the gas mixture plasma becomes a very active species under the influenceof electrical discharge. This hydrogen species will not only perform an etching effect onthe wool fibre, but also have a strong reducing power which can generate free radicals ofcarbon on the fibre surface during the plasma process. The generated carbon free radicalson the fibre surface will have the possibility of combining together to form single-bondedcarbon chains [193], resulting in the formation of cross-linkages on the fibre surface, whichmay present a barrier to the dye absorption. Hence, the PM-treated wool fibres show theleast effect on the dyeing behaviour in the present dyeing system.

Furthermore, similar experimental results shown in Table 14 also indicate that theplasma treatment can alter the dyeing rates. The time of half dyeing (t1/2) defined as thetime required to reach half equilibrium is used as an effective value to quantify the rate ofdyeing in a dyeing system. The t1/2 values of the plasma-treated wool fabrics are found tobe greatly reduced, that is more than 64% decreased, when compared with the untreatedsample. However, the change in %E at Em is not significant, that is ranging merely from0.83% to 1.47%. This interesting observation may be due to the presence of a numberof available dyesites in the wool fibres, which will affect the percentage of exhaustion atequilibrium. The dyesites of the fibre are generally associated with the internal structure ofthe fibre, and so any change in the internal fibre structure may alter the amount of dyesites.However, the plasma species can only penetrate to a depth of 0.1 µm [185] at the fibresurface within the duration of the treatment time. This penetration of plasma species is notdeep enough to alter the whole or partial internal structure of the wool fibre. As a result,most of the available dyesites will remain unchanged after the plasma treatment havinglittle effect on the final dyebath exhaustion.

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Figure 13. Colour reflectance curves of each sample [223].

9.7.2 Fabric dyeability measurement

The fabric dyeability measurement is studied through the observation of the reflectancecurve. The reflectance curves of different plasma-treated fabric samples, namely (i) oxygenplasma (PO), nitrogen plasma (PN) and nitrogen/hydrogen plasma (PM), are shown inFigure 13.

The reflectance curve provides the information of the depth of shade of the materialin the visible spectrum. When the value of reflectance is large, the depth will be of paleshade and vice versa. Figure 13 shows that the depth of shade of different samples canbe compared. It is clear that the position of the reflectance curve of the control sampleis higher than the plasma-treated samples over the visible spectrum. This indicates thatthe shade of 1% depth of the untreated fabric is paler than the plasma-treated samples.For three plasma treatments with different nature of gas, namely oxygen, nitrogen andnitrogen/hydrogen gas mixture being used and their colour reflectance curves nearly coin-cide with each other, although there are still some differences between them. The oxygenplasma gives the highest colour reflectance of the three plasma treatments, whereas thecolour reflectance curves for both nitrogen plasma and gas mixture plasma are almost over-lapped. These differences may be due to the introduction of new functional groups, suchas –NH2, on the fibre surface in the case of nitrogen and gas mixture plasma treatments.In addition, the molecular chains present on the wool fibre surface might be broken intosmaller molecules during the plasma-etching process. Consequently, these low-molecularmaterials are likely to be ejected from the fibre surface, leaving polar molecules on thesurface and enhancing the dye absorption. Furthermore, the main factor contributing to theimproved dyeability of the plasma-treated wool fibre is the apparent increase in the overallsurface area, that is cracks and gaps [218], as a result of the morphological modificationinduced by the plasma treatment. The increased surface area provides more opportuni-ties for the dye to contact the fibre and thus increases the possibility for the dye to enterthe fibre. The dye concentration in the fibre may then be increased, resulting in a deepershade.

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9.7.3 Chrome dyeing

Microscopic studies elucidate that the plasma treatment can influence the dyeing behaviourof wool fibre [229]. Light microscopic studies demonstrate that the plasma-treated woolfibres can be easily penetrated by the dyes, which are evenly distributed over the crosssection of the fibre [229]. This phenomenon can be due to both the plasma-induced cystineoxidation in the A-layer of the exocuticle and the reduced number of cross-linkages at thefibre surface. These two surface morphology changes obviously facilitate a transcellular dyediffusion in addition to the intercellular dye diffusion. Transmission electron microscopyinvestigation [229] also shows that plasma treatment only modifies the A-layer of the cuticleto various extent due to sputtering resulting in a partial swelling of the A-layer. In addition,the etching of the A-layer leads to the formation of grooves in this layer. Because of apartial degradation of the A-layer which acts as a barrier to the diffusion of dyes and otherchemicals, the affinity of the fibre for dyes will be increased correspondingly. The increasein dye absorption is most probably caused by the modification of the endocuticle and theneighbouring cell membrane complex, resulting in a modification of the intercellular pathof diffusion [229].

Figure 14 shows the behaviour of dyebath exhaustion during the dyeing process withdifferent types of plasma gas, namely oxygen (PO), nitrogen (PN) and nitrogen/hydrogengas mixture (PM). The results demonstrate that the plasma treatment can influence thedyeing behaviour of wool to different extent. Table 15 shows the time of half-dyeing andfinal dyebath exhaustion derived from Figure 14.

It may be seen in Figure 14 that there is an increase in dyeing rate for both the plasma-treated wool fabrics, but the extent of increase is different. Obviously, the PM-treated fabricshows the fastest rate of dyeing among the fabrics followed by PN, PO and untreated fabrics.

In Table 15, the results of time of half-dyeing do provide a good support for thedetermination of rate of dyeing. There is a significant change in the time of half-dyeingfor all the plasma-treated fabrics, that is a drop from 6.52% to 44.43% for the plasma-treated fabrics when compared with the untreated wool fibre. If the nature of plasma gas istaken into consideration, the gas mixture shows the greatest effect among the other gases.The reduction in the time of half-dyeing indicates that all plasma treatments can lead to

Figure 14. Dyebath exhaustion of the plasma-treated wool sample with chrome dye (C.I. MordantBlack 11, 4% on weight of fibre (o.w.f.)) as compared to that of the untreated sample [223].

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Table 15. Time of half dyeing and final bath exhaustion forchrome dyeing [223].

Time of half-dyeing Final bathSample (minutes) exhaustion (%)

Untreated 22.53 97.16PO 21.06 (↓6.52%) 98.73 (↑1.62%)PN 17.34 (↓23.04%) 98.67 (↑1.55%)PM 12.52 (↓44.43%) 98.91 (↑1.80%)

The values in the bracket show either the increase or decrease in percentagewhen compared with the untreated sample (↑ means increase in percentageof value and ↓ means decrease in percentage value).

a considerable shortening of dyeing time, thereby reducing the energy consumption andhence improving the dyeing operation. Based on such results, it is suggested that the natureof plasma gas plays an important role in the alteration of the dyeing properties of theplasma-treated wool fibres. The final dyebath exhaustions shown in Table 15 do not showsignificant changes, that is the changes are within 2% with the greatest increase for thePM-treated fabric followed by PO and PN treatment. Although PO shows a slower rateof dyeing than PN, the final dyebath exhaustion is affected in an opposite way, that is thePO-treated fabric has better final dyebath exhaustion than the PN-treated fabric. The finaldyebath exhaustion depends very much on the available dyesites of the wool fibre. Sincethe penetration of plasma species to a depth of about 0.1 µm [185], thus the depth ofpenetration and etching are not sufficient enough to alter the internal structure of fibre or toinduce any new dyesites in the fibre. As a result, only a small increase in the final dyebathexhaustion is observed and the nature of plasma gas shows no significant alteration of thefinal dyebath exhaustion. Of the three plasma gases used, the gas mixture shows the fastestrate of dyeing followed by nitrogen and oxygen but the sequence of nitrogen and oxygenis interchanged in the case of final dyebath exhaustion.

9.7.3.1. Hexavalent chromium determination. C.I. Mordant Black 11 requires the removalof yellowish staining by means of an ammonia aftertreatment during which chromium canbe extracted from the wool fabrics. This will result in a further effluent load, which can beused for the determination of hexavalent chromium.

The change in hexavalent chromium level during the afterchroming process for boththe untreated and plasma-treated wool fabrics is shown in Figure 15. It is obvious thatthe hexavalent chromium concentration decreases during the afterchroming process. Thechromium uptake by the plasma-treated fabrics occurs more rapidly than the untreatedwool fabric. The effect is very similar to the results obtained in the rate of dyeing, thatis the hexavalent chromium concentration is the smallest in the PM treatment dyebathfollowed by the PN and PO dyebath. The first effluent sample collected at 5 minutes afterthe application of potassium dichromate at 50◦C (denoted as zero-treatment time in Fig-ure 16) shows that a large amount of the chromium has already been exhausted by theplasma-treated wool fabrics. On the contrary, the untreated wool fabric still shows a rela-tively lower affinity for chromium at the beginning of the afterchroming process. For the first10 minutes of afterchroming process, all the plasma-treated wool fabrics except the PM-treated fabric show a similar rate of chromium exhaustion as compared with the untreated

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Figure 15. Concentration of hexavalent chromium in afterchrome treatment bath [223].

wool fabric. After 10 minutes of the afterchroming process, it is clearly demonstrated thatall the plasma-treated fabrics have a faster rate of chromium exhaustion than the untreatedwool fabric. This phenomenon is maintained until the end of afterchroming process. Al-though the concentration of hexavalent chromium is dropping during the afterchromingprocess, the decrease still reaches a state of equilibrium, at the end of the afterchromingprocess.

9.7.3.2. Trivalent chromium determination. The trivalent chromium concentration ab-sorbed by different plasma-treated wool fabrics is shown in Figure 16 in which all theplasma-treated fabrics indicate an increase in the amount of absorbed trivalent chromiumcontent throughout the afterchroming process when compared with the untreated wool

Figure 16. Uptake of trivalent chromium by differently plasma-treated wool in the afterchromeprocess as a function of the plasma treatment time [223].

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fibre. Such results are related closely to the amount of hexavalent chromium exhaustedby the wool fibres. It seems that Figure 15 is almost an inverted graph of Figure 16. Sim-ilarly, the fixation of final trivalent chromium is slightly increased for all the plasma-treated wool fabrics and this phenomenon is similar to the result of final dyebathexhaustion.

The improved trivalent chromium fixation and hexavalent chromium exhaustion willreduce the amount of effluent load discharged to the environment. The experimental resultsshow that the plasma treatment of wool fibres will not only facilitate the uptake of chromiumby the fibre but also reduce its discharge in the effluent. In addition, the use of differentplasma gases has a definite influence on the afterchroming process, although the final uptakeis slightly increased. Of the three plasma gases used, the gas mixture shows the greatesteffect on the afterchroming process followed by nitrogen and oxygen.

10. Surface analysis of plasma-treated wool

Different surface characterisation methods such as infrared spectroscopy (IR), scanningelectron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), auger photoelectronspectroscopy, secondary-ion mass spectroscopy, small-angle X-ray diffraction, transmis-sion electron microscopy, scanning tunnel microscopy and atomic force microscopy can beused as a powerful method in characterizing the surface and morphological analysis of thesubstrate surface. In the following sections, three surface analysis technologies, which aresimple and commonly used in recent years, that is scanning electron microscope (SEM)[230], X-ray photoelectron spectroscopy (XPS) [231] and Fourier transform infrared withattenuated total internal reflectance mode analysis (FTIR-ATR) [232], are employed toexamine the composition of the plasma-treated wool surface at different levels. The depthof analysis of these surface technologies are summarised in Table 16.

Three different plasma gases PO, PN and /PM are illustrated as examples to show theeffect of the nature of gas on the modification of surface properties of the plasma-treatedwool.

10.1 Surface morphology by SEM

To study the modified surface features of the wool fibres, the terminology previously used[233] is adopted in this monograph. The external surface of the wool fibre is accordinglyconsidered to be divided into ‘faces’ contained in some of the following boundaries, that is‘clefts’, ‘escarpments’ and ‘ridges’. Clefts separate two faces (clearly two adjacent cells)at the same radial distance from the fibre axis. Escarpments may form boundaries betweenfaces of the same cuticular cell at different radial distances from the fibre axis, or at thedistal edge of cuticular cells. Ridges separate faces at the same or different radial distancesfrom the fibre axis. Figures 17–23 are the SEM pictures of the untreated and plasma-treated

Table 16. Surface analysis technique and analyt-ical depth [232].

Method Depth of study

SEM Surface morphologyXPS 5 nmFTIR-ATR 500 nm

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Figure 17. SEM picture of untreated wool fibre [234].

wool fibres (treated with three different types of plasma gas of a different nature using 5and 45 minutes treatment time as an illustrative example with the same discharge power(80 W) and system power (10 Pa)).

10.1.1 Untreated wool fibre

Figure 17 shows the SEM picture of the untreated wool fibre in which the escarpments areprominent and well defined. There is no evidence of separation between the neighbouringcuticular cells and the cleft lines are ill-defined. A smooth fibre surface can be pronouncedto the untreated fibre surface.

Figure 18. SEM picture of oxygen-plasma-treated wool fibre (5 minutes of the plasma treatment)[234].

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Figure 19. SEM picture of oxygen-plasma-treated wool fibre (45 minutes of the plasma treatment)[234].

10.1.2 Oxygen-plasma-treated wool fibre

Figures 18 and 19 are the SEM pictures of wool fibre after 5 and 45 minutes of treatmenttime with oxygen plasma, respectively. After 5 minutes of treatment with oxygen plasma,there is an appearance of some continuous cracks that are located parallel to the directionof the fibre axis and the scale edges are slightly eroded and rounded. After 45 minutes oftreatment, the wool scales are severely damaged resulting in the removal of escarpment butthe clefts still appear on the fibre surface. The scale shape still remains but it is indistinct,but the scale edge is eventually removed.

Figure 20. SEM picture of nitrogen-plasma-treated wool fibre (5 minutes of the plasma treatment)[234].

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Figure 21. SEM picture of nitrogen-plasma-treated wool fibre (45 minutes of the plasma treatment)[234].

10.1.3 Nitrogen-plasma-treated wool fibre

Figures 20 and 21 are the SEM pictures of wool fibre after 5 and 45 minutes of treatment withnitrogen plasma, respectively. After the plasma treatment for 5 minutes, the escarpmentsare lifted so that the clefts are clearly observed on the fibre surface. Apart from thesemorphological changes, isolated spots are evenly distributed over the fibre surface andthe cracks also appear along the fibre axis. After 45 minutes of the plasma treatment, thewool cuticular scale is partially lost producing a flatten region on the fibre surface, and theescarpments are almost completely removed. Ridges and cracks are clearly observed onthe surface in the orientation direction of the fibre axis. This confirms that wool fibre issubjected to serious damage after the plasma treatment.

Figure 22. SEM picture of gas mixture plasma treated wool fibre (5 minutes of the plasma treatment)[234].

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Figure 23. SEM picture of gas mixture plasma treated wool fibre (45 minutes of the plasma treatment)[234].

10.1.4 Gas mixture plasma-treated wool fibre

Figures 22 and 23 are the SEM pictures of wool fibre after 5 and 45 minutes of treatmentwith gas mixture plasma, respectively. After the plasma treatment for 5 minutes, the micro-pores are visible over the cuticular scale as shown in Figure 22. The escarpments are stillwell defined with some cracks being located perpendicularly to the scale edge. Figure 23shows the exposure of the cortical region after the gas mixture plasma treatment for45 minutes. Some part of the wool scale is completely removed, leaving a smooth fibresurface. The escarpments and clefts are removed, but the ridges are formed correspondingly.Consequently, the fibre surface is significantly altered.

Although the wool fabrics are treated with different plasma gases under the sameplasma treatment condition, the surface features of the wool fibres are not similar to eachother. It seems that oxygen plasma has the strongest effect followed by nitrogen plasmaand gas mixture plasma. This illustrates that the nature of plasma gas used can affect themorphological structure of the wool fibre.

10.2 XPS surface analysis

After 5 minutes of the plasma treatment with different plasma gases, the micro-analyticaldata of the surface elemental composition of different samples are collected and summarisedin Table 17.

It is obvious that the carbon content is significantly reduced after the plasma treatment.This reduction is probably due to the etching effect of the plasma treatment on the wool fibreresulting in the removal of fibre surface material. After the etching process, the inner surfaceof wool fibre is exposed and new functional groups are formed by the plasma species as aresult of chemical effect. The contribution of both factors will induce a change of the surfacecomposition. Scanning electron microscopic pictures [218,234] have clearly shown that theoxygen plasma imparts the most significant surface etching effect by introducing groovesalong the fibre axis. This grooving effect induced by oxygen plasma is most pronouncedfollowed by the nitrogen plasma and then the gas mixture plasma. This sequence agreeswith the order of reduction in carbon content as obtained by the XPS analysis.

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Table 17. Surface elemental analysis and atomic ratio of wool treated with different plasma gases[228].

Elemental concentration (wt%) Atomic ratio

Sample C1s N1s O1s S2p C/N O/C

Untreated 74.72 8.78 13.55 2.58 8.51 0.18PO 65.61 8.88 20.16 2.26 7.39 0.31PN 68.31 10.19 18.86 2.23 6.70 0.27PM 68.67 9.34 18.82 2.14 7.35 0.27

The nitrogen content of the wool fibre has increased to a different extent after differ-ent plasma treatments. The nitrogen plasma induces the highest amount of the nitrogencomponent inside the wool fibre, followed by the gas mixture and then the oxygen plasma.This enhancement of the nitrogen content on the wool fibre reflects the increase in the NHcontent of the wool fibre.

The increase in the oxygen content of the plasma-treated wool fibre is probably due tothe fact that oxidation has occurred during the plasma treatment with the oxygen plasmashows the strongest effect followed by nitrogen plasma and then the gas mixture plasma.When compared, both nitrogen plasma and gas mixture plasma produce quite a similaroxidation effect on the wool fibre. Since the gas mixture is composed of nitrogen andhydrogen, thus its oxidising effect will obviously be minimal, which may explain thesequence of results shown in Table 17. The increased amount of oxygen may enhance thehydrophilicity and wettability of the wool fibre. As a result, the dye uptake and polymeradhesion during finishing are also enhanced.

The content of sulphur after the plasma treatment decreases slightly. This is probablydue to the fact that plasma treatment might etch away the cuticle which contains a largenumber of disulphide bonds (–S–S–). Of the three plasma gases used, the gas mixtureplasma shows the largest reduction in sulphur content followed by nitrogen plasma andthen oxygen plasma. In addition, Figure 24 shows the XPS spectrum where two broad S2p

peaks at binding energy values of 163 and 168 eV can be seen. In the untreated woolfibre, the intensity at 163 eV peak is stronger than that of 168 eV peak. After the plasmatreatment, the intensity of 168 eV peak is stronger than that of 163 eV peak. This shift ofS2p peak to higher binding energy is an indicator of the increase in the oxidation state of thesulphur atoms at the fibre surface [235]. Thus it suggests the conversion of cystine residuesto cysteic acid residues [193,236], according to the following equation:

W–S(II)–S–W → W–S(VI)O3H,

where W is wool.Since the 168 eV peak is rather broad, it is possible that the intermediate oxidation

products of cystine may also be present.The XPS surface analysis can be used for monitoring the superficial chemical changes,

that is a depth of about 10 nm, after the plasma treatment [193]. The LTP treatmenthas shown a decrease in the relative atomic concentration of carbon and a correspondingincrease in the relative atomic concentration of oxygen. This suggests the oxidation of thefatty layer present on the outermost part of the epicuticle [193].

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Figure 24. Sulphur peak (S2p) spectra of wool before and after treatment with different plasma [220].

The C/N atomic ratio has decreased from 8.51 for the plasma-treated wool to 7.39(oxygen-plasma-treated wool), 6.70 (nitrogen-plasma-treated wool) and 7.35 (gas mixtureplasma-treated wool) for different gas-treated wool, whereas O/C atomic ratio increasessignificantly as compared to the untreated wool. These changes suggest that a partialoxidation of the hydrocarbon chains of the F-layer occurs without the epicuticle removal[193].

10.3 FTIR-ATR measurement

Since the amino acid analysis can cause the breakdown of intermediate cystine prod-ucts during the acid hydrolysis stage [193], thus the FTIR-ATR mode measurement withsecond-order derivative spectroscopic analysis can offer the benefits of non-destructivetesting. Comparison of the FTIR-ATR spectrum of zero order and second order reveals thatintensities obtained in the zero-order derivative spectra are inverted in the second-orderderivative spectra [193,227]. FTIR signal assignments to the functional groups are shownin Table 18. The intensity of each signal depends on the concentration of the number of theconcentration of the functional group. The FTIR-ATR technique can analyse a depth of 500nm that is good enough to detect the surface chemical components of wool fibre. Hence,the FTIR-ATR technique offers both qualitative and quantitative methods for measuringthe composition of wool surface. The absorbance of the selected band frequencies, that is1600 cm−1, 1121 cm−1, 1071 cm−1, 1040 cm−1, 1022 cm−1 and 1000 cm−1, is dividedby the absorbance of the peptide frequency (amide III, 1232 cm−1 which is used as aninternal reference), and the absorbance ratio is related to the concentration of the surfacecomponent.

In the past, determination of Bunte salt was based on a colorimetric technique, whichdid not allow direct determination. However, the Bunte salt concentration can now bedetermined non-destructively by means of the FTIR-ATR technique. Figure 25 illustrates

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Table 18. Characteristic IR absorbance frequencies [228].

Group Wavenumber (cm−1)

NH bending (–N–H) 1600Cystine dioxide (–SO2–S–) 1121Cystine monoxide (–SO–S–) 1071Cysteic acid (–SO−

3 ) 1040S-Sulphonate (Bunte salt) (–S–SO−

3 ) 1022Carbon–carbon (stretching) single bond (–C–C–) 1000

that the amount of Bunte salt increases with longer treatment time of plasma-treated woolusing three different plasma gases.

Of the three different plasma-treated wool fabrics, the highest absorbance ratio obtainedis the case of oxygen-plasma-treated fabric (PO) followed by nitrogen plasma (PN) andgas mixture plasma (PM). For the oxygen and nitrogen plasma treatments, the initial rate(1–5 minutes) of the formation of Bunte salt is much faster than that of the gas mixtureplasma treatment. However, in the case of the gas mixture plasma treatment, the formationrate of Bunte salt increases gradually throughout the duration of treatment. On the whole,the formation of Bunte salt is probably related to the improved shrink-resistance propertiesof wool [75].

Apart from Bunte salt formation, cysteic acid is also formed as a result of the cleavageof disulphide linkage [193]. The presence of cysteic acid on the polypeptide chain togetherwith Bunte salt provides the wool fabric with a polar surface, which in turn helps to improvethe wettability of wool fabric [237]. Furthermore, the cleavage of disulphide bonds helpsto remove the surface barrier of wool fibre. The absorbance ratio of cysteic acid againstfunction of time is shown in Figure 26.

Figure 26 clearly shows that the amount of cysteic acid content increases considerablyafter the plasma treatment. All the three plasma treatments demonstrate similar graphpatterns, that is after a rapid initial increase at the initial stage; the cysteic acid content

Figure 25. FTIR-ATR absorbance ratio of S-sulphonate (Bunte salt) content as a function of theplasma treatment time [228].

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Figure 26. FTIR-ATR absorbance ratio of cysteic acid content as a function of the plasma treatmenttime [228].

continues to rise gradually throughout the treatment time. When compared, oxygen plasmagives the largest absorbance ratio and the sequence is found to be the same as that of Buntesalt formation, that is PO > PN > PM.

Apart from the Bunte salt and cysteic acid, the other interesting cystine residues to bestudied are cystine monoxide and cystine dioxide. Both cystine residues are believed tobe intermediate cystine oxidation products, that is disulphide → monoxide → dioxide →sulphonic acid [238]. Cystine monoxide and cystine dioxide are interesting groups becausethey represent a more reactive form than the parent disulphide [193]. The formation ofcystine monoxide and cystine dioxide in wool thus generates a more reactive substrate,which provides a suitable site for introducing agents such as dyes and softeners carryingnucleophilic reactive groups [239]. Figure 27 illustrates the variation in the absorbanceratio of cystine monoxide and cystine dioxide to amide III as a function of treatment time,respectively.

Both graphs in Figure 27 exhibit a similar pattern, that is the absorbance ratio aftershowing an initial increase decreases gradually over a prolonged treatment. In both thecases, the absorbance ratio increases rapidly during the first 5 minutes but starts to decreasethereafter and reaches a nearly constant signal. Figure 27(b) also shows that in all casesthe amount of cystine dioxide is less in the treated wool fabrics than the untreated woolfabric after 20 minutes of the plasma treatment. The effect is more pronounced in the caseof PN and PM as compared to PO. The decreased amount of cystine dioxide content maybe due to the spontaneous conversion of cystine dioxide to cysteic acid. It can be seen fromFigure 26 that the rate of formation of cysteic acid for the nitrogen plasma and gas mixtureplasma is faster than that of the oxygen plasma within 5–8 minutes of the plasma treatment.This suggests that the wool fabrics treated with nitrogen plasma and gas mixture plasmahave lower cystine dioxide content than the untreated wool fabrics after 8 minutes of theplasma treatment.

Figure 28 demonstrates the conversion relationship between different cystine oxidationproducts as a function of the plasma treatment time with respect to oxygen plasma treatment.The amount of cystine monoxide and cystine dioxide increases rapidly to a maximum valuein the first 5 minutes of plasma treatment, and then starts to decrease until reaching anearly constant absorbance ratio. Consequently, the cysteic acid content starts to increase

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Figure 27. FTIR-ATR absorbance ratio of (a) cystine monoxide and (b) cystine dioxide content as afunction of the plasma treatment time [228].

after 5 minutes of plasma treatment. Since cystine oxides are not stable and may easily beconverted to cysteic acid [193], thus it is hard to quantify accurately the formation of eachproduct with respect to cysteic acid at different plasma treatment times. Cysteic acid isconsidered as the main oxidation product within the wool fibre. However, the presence ofcystine monoxide and cystine dioxide suggests that cysteic acid is probably formed fromthese intermediates as proposed in the previous model [238].

Similar graph patterns are found in Figures 25–28 suggesting that all types of gasplasma treatments can generate the same functional groups on the wool fabric surface butwith different concentrations. Of the three gases used, the oxygen plasma treatment hasthe highest absorbance ratio followed by the nitrogen plasma treatment and gas mixtureplasma treatment. This may be probably due to the fact that the functional groups beingstudied are mostly the oxidation products of cystine. Generally speaking, the nature of theplasma depends much on the nature of the gas used [228]. If oxygen gas, which is oxidativein nature, is used as the plasma gas, the wool surface will be oxidised to such an extent

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Figure 28. FTIR-ATR absorbance ratio of cysteic acid, cystine monoxide and cystine dioxide contentas a function of the plasma treatment time (oxygen plasma treatment) [228].

that the signal of oxidation product can be easily detected. On the other hand, the oxidisingpower of nitrogen is lower than that of oxygen. As a result, the signal intensities of thenitrogen-plasma-treated fabric are not as strong as oxygen. In the case of the gas mixtureplasma treatment, the signal is quite similar to that of nitrogen plasma treatment becausethe former is 100% nitrogen gas whereas the latter still contains 75% nitrogen gas. As thegas mixture plasma is mixed with 25% hydrogen gas, which is reductive in nature, thismight finally reduce the oxidation effect of the nitrogen gas resulting in the weakest signalintensity of the selected functional groups.

Although the wool fabric itself contains amino groups (–NH2), yet further introductionof amino groups by the plasma treatment may have enhanced the absorption of anionic dyeduring the dyeing process [193,228]. Figure 29 shows the variation of concentration of NHagainst the plasma treatment time.

Figure 29 shows the NH content of the plasma-treated wool fabrics as a function of theplasma treatment time. Obviously, the NH content of all cases being studied is increased.For oxygen plasma, the NH content increases only moderately whereas the increase is verypronounced in the case of nitrogen and gas mixture plasma treatment. When compared,the wool fabric samples treated with the nitrogen plasma show more NH content thanthose treated with gas mixture plasma. The increase in the NH content may provide anexplanation for the previous dyeing results [224,226], that is the nitrogen plasma and gasmixture plasma treated fibres have a higher percentage of exhaustion at equilibrium (%E

at Em) than the untreated fibre. This may be due to the increase in plasma-induced NHgroups on the wool fibre, leading to the introduction of more new dyesites and enhancingthe dye absorption ability of the wool fibre.

Hydrogen present in the gas mixture plasma may be changed to reactive hydrogen,such as H+ and H+

2 under the condition of electrical initiation. The reactive hydrogen has ahigh level of reactivity [193]. When these reactive species bombard the fibre surface, a freeradical may be formed by eliminating an atom from a saturated compound as shown in thefollowing equation:

–C–C–H + H+ → –C–C•+ H+2

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Figure 29. FTIR-ATR absorbance ratio of NH-bending group as a function of the plasma treatmenttime [228].

However, it is postulated that the free radical present on the polymer chain combinestogether as shown below:

2 –C–C• → –C–C −–C–C–

The above reaction demonstrates that a cross-linking reaction may occur as a consequenceof the plasma treatment as reflected by Figure 30, which shows the presence of carbon–carbon single bond content on the fibre surface after plasma treatments.

Figure 30 illustrates that the carbon–carbon (stretching) single bond content is increasedafter the plasma treatment. In the case of gas mixture plasma, the carbon–carbon single bond

Figure 30. FTIR absorption ratio of carbon–carbon (stretching) single bond as a function of theplasma treatment time [228].

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content increases considerably as compared to oxygen and nitrogen plasma. It is likely thatthe gas mixture plasma treatment enhances the carbon–carbon single bond formation onthe wool fibre surface as depicted above. These cross-linkages present on the fibre surfacemay impart hydrophobicity and hinder diffusion through the surface. On the other hand,the gas mixture plasma can improve the wettability and hence the dyeability by introducingthe amino groups to the fibre surface, causing the wool fibre to become more hydrophilic.The dyeing absorption behaviour of the gas mixture plasma-treated wool fibre may be thecompromise of these two opposing factors.

11. Serviceability of plasma-treated wool for industrial use

Comprehensive studies [240–242] have been conducted to study the serviceability of oxy-gen plasma-treated wool fabric for industrial use. Table 19 demonstrates the results of theplasma-treated wool fabric after comparing with the standard performance specifications,namely (i) ASTM D378–02: standard performance specification for men’s and boys’ wo-ven dress suit fabrics and (ii) ASTM D4155–01: standard performance specification forwomen’s and girls’ woven sportswear, shorts, slacks and suiting fabrics.

In the breaking strength values, both fabrics fulfil the standard requirements. Althoughthe untreated fabric can meet the requirements, its breaking strength can be further en-hanced after the plasma treatment. The breaking strength of the plasma-treated fabric iscomparatively larger than those of the untreated fabric. In the tensile strength test, a load isapplied to cause the fabric breakage. In general, the fabric breakage depends not only on thenature of fibre but also on the fabric construction. When considering the fabric construction,the inter-yarn and inter-fibre frictions play an important role in the tensile strength propertiesof the fabric. With the application of the plasma treatment, it is believed that there will be anincrease in inter-yarn and inter-fibre frictions, as confirmed by the roughening effect on thetextile surface [243]. Hence, more forces must be required to overcome the inter-yarn andinter-fibre frictions before the occurrence of fabric breakage, resulting in higher breakingload.

The tearing strength of the fabrics in the warp and weft directions fulfils the requirementof performance specification. Under the influence of the plasma treatment, there is areduction in the tearing strength of wool fabric in both warp and weft directions. Themechanism of tearing is explained by the appearance of del in the cut slit of the test fabricduring the tearing strength testing [244]. The formation of the del is probably due to therelative sliding of yarns during the tearing period. The del yarn breaks consecutively asthe load is applied, resulting in tearing of fabric. When the del is large, more yarns willexperience the same load, leading to the increase in sliding between the yarns during thetearing period and causing the tearing strength to increase. However, when the inter-yarnfriction is too large between the warp and weft yarns, there is a relative reduction in thesliding action of yarns, making the del become smaller and resulting in the decrease oftearing strength. In a previous study [243], it has been found that the inter-yarn frictionincreases after the plasma treatment. Hence, it is postulated that the inter-yarn friction canrestrict the sliding action of yarns during tearing, thereby reducing the tearing strength.

In the assessment of colour fastness to washing, the untreated wool fabric fails to meetthe minimum requirement in colour change but can just fulfil the minimum requirementafter plasma treatments. On the other hand, the staining colour fastness of both the untreatedand plasma-treated fabrics can satisfy the standard requirements. In the case of the colourfastness to perspiration, both the untreated and plasma-treated fabrics do not achieve theminimum specification requirement. The fastness ratings in both colour change and staining

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Tabl

e19

.Pe

rfor

man

cesp

ecifi

catio

nof

diff

eren

tpro

pert

ies

[240

–242

].

Prop

ertie

sA

STM

D37

80–0

2A

STM

D41

55–0

1U

ntre

ated

LTP

trea

ted

Bre

akin

gst

reng

th17

8Nm

inim

um15

5Nm

inim

um26

2.5N

(war

p),1

58N

(wef

t)32

7N(w

arp)

,185

N(w

eft)

Tear

stre

ngth

11N

min

imum

8.9N

min

imum

23.3

N(w

arp)

,10.

3N(w

eft)

16N

(war

p),9

.2N

(wef

t)D

imen

sion

alch

ange

a2%

max

imum

inea

chdi

rect

ion

3%m

axim

umin

each

dire

ctio

n9.

6%(w

arp)

,3.6

%(w

eft)

1.1%

(war

p),0

.4%

(wef

t)C

olou

rfa

stne

ssto

was

hing

(sha

dech

ange

)

Cla

ss4

min

imum

Cla

ss4

min

imum

Cla

ss3–

4C

lass

4

Col

our

fast

ness

tow

ashi

ng(s

tain

ing)

b

Cla

ss3

min

imum

Cla

ss3

min

imum

Cla

ss3

Cla

ss4

Col

our

fast

ness

toPe

rspi

ratio

n(s

hade

chan

ge)

Cla

ss4

min

imum

Cla

ss4

min

imum

Cla

ss2–

3C

lass

3–4

Col

our

fast

ness

tope

rspi

ratio

n(s

tain

ing)

b

Cla

ss3

min

imum

Cla

ss3

min

imum

Cla

ss2

Cla

ss2–

3

Col

our

fast

ness

tocr

ocki

ng(d

ry)

Cla

ss4

min

imum

Cla

ss4

min

imum

Cla

ss4

Cla

ss4–

5

Col

our

fast

ness

tocr

ocki

ng(w

et)

Cla

ss3

min

imum

Cla

ss3

min

imum

Cla

ss4

Cla

ss4

The

valu

esin

italic

ssh

owth

eim

prov

edpr

oper

ties

whe

nco

mpa

red

with

the

perf

orm

ance

spec

ifica

tion.

aFe

lting

dim

ensi

onal

chan

ge.

bT

helo

wes

tsta

inin

gra

ting

amon

gdi

ffer

entm

ultifi

bre

com

pone

nts.

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assessment are found to be slightly improved by the plasma treatment. All fabric samplescan fulfil the specification requirements in the colour fastness of both dry crocking andwet crocking tests. In the dry crocking test the staining rating of the plasma-treated fabricis merely improved by a half step, but no change is noted in the wet crocking test. It isobserved that the dyed plasma-treated fabric has a slightly improved colour fastness towashing in terms of both staining and colour change assessment.

When a comparison is made, the plasma-treated fabric has achieved a better perspirationfastness than the untreated fabric. In the colour change rating of washing, the plasmatreatment makes a positive improvement, implying that the plasma-treated fabric becomesfaster than the untreated wool fabric. In the dry crocking condition, the colour fastnessof the plasma-treated wool fabric is slightly improved whereas wet crocking shows nosignificant improvement in colour fastness. After the plasma treatment, the wool fibresurface is modified [193] and the extent of surface modification has been examined by thetransmission electron microscopy previously [229]. It is observed that the plasma treatmentonly modifies the A-layer of cuticle to varying degrees as parts of the A-layer have beensputtered off, leading to the formation of grooves in this layer [229]. Owing to the partialdegradation of the A-layer which acts as a barrier to the diffusion of dyes and other chemicalsinto the wool fibre due to the high number of cross-linkage and hydrophilisation of the fibresurface, the affinity of the fibre for dyes is significantly increased. Hence, more dyes canaccumulate in this layer and then diffuse faster into the fibre homogeneously. The facilitateddye absorption is probably caused by the modification of endocuticle and neighbouring cellmembrane complex causing a modification of intercellular path of diffusion. As a result,the colour fastness of the plasma-treated wool fabric is improved.

Generally speaking, the wool fabric shrinkage is correlated with the frictional coefficientof the constituent wool fibres, and it is a common knowledge that the plasma treatmentcan increase the dry and wet frictional coefficients in the scale and anti-scale directions.However, the effectiveness of the plasma treatment is attributed to several changes inthe wool surface such as (1) formation of new hydrophilic group, (2) partial removalof covalently bonded fatty acids belonging to the outermost surface of the fibre and (3)the etching effect [212,214]. The first two changes contribute mainly to the increasedwettability, whereas the last basically reduces the differential friction coefficients of thefibres and thus decreases the natural shrinkage tendency [215,216].

12. Fabric objective measurement of plasma-treated wool fabric

The increase in the competition has shifted the focus of the textile industry to the highquality of the finished products. The manufacturers throughout the world are striving to meetthe desired quality levels and standard specifications. The fabrics are tested objectively fortheir durability, comfort, aesthetics and tailorability along with properties such as structural,mechanical and stress mechanical. As there is a large demand of the objective measurement,the Kawabata Evaluation System for Fabric (KES-F) has been developed as a useful tool tomeasure the performance of fabric objectively. Several studies [245–248] concerning themeasurement of the plasma-treated wool fabric with KES-F have been reported. Table 20presents the results of the change in different testing parameters of KES-F as a function ofthe time on the plasma-treated wool fabrics.

12.1 Tensile properties

The tensile properties are composed of tensile energy (WT), tensile resilience (RT) andextensibility (EMT). WT is defined as the energy required for extending the fabric, which

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Table 20. Low-stress mechanical properties, air permeability and warm/cool feeling of oxygenplasma-treated wool fabric [246].

Plasma treatment time (minutes)

KES-F properties 0 5 10 20 30

Tensile WT (gf.cm/cm2) 11.61 12.72 12.75 12.79 12.84RT (%) 64.07 59.59 59.39 59.15 58.97EMT (%) 9.54 8.39 8.34 8.28 8.20

Shearing G (gf/cm.degree) 0.71 1.30 1.32 1.35 1.382HG (gf/cm) 0.60 2.00 2.05 2.06 2.082HG5 (gf/cm) 1.75 5.55 5.58 5.60 5.63

Bending B (gf.cm2/cm) 0.111 0.139 0.140 0.143 0.1462HB (gf.cm/cm) 0.038 0.083 0.087 0.089 0.095

Compression To(mm) 0.659 0.709 0.712 0.716 0.718Tm (mm) 0.531 0.557 0.559 0.563 0.568WC (gf.cm/cm2) 0.08 0.08 0.08 0.09 0.09RC (%) 46.63 19.21 19.16 19.08 19.02EMC (%) 19.42 21.44 21.48 21.52 21.55

Surface MIU 0.213 0.369 0.372 0.375 0.380SMD (µm) 3.88 4.20 4.32 4.43 4.53

Air permeability R (kPa. s/m) 6.79 7.70 7.75 7.82 7.90Warm / cool feeling qmax(W/cm2) 0.155 0.128 0.124 0.120 0.113

reflects the ability of fabric to withstand external stress during extension. A larger value ofWT implicates a better tensile strength of the fabric. After the plasma treatment, it is notedthat the WT increases steadily as the treatment time is prolonged. However, the incrementis not so great when compared with the untreated fabric.

Generally speaking, the tensile strength of fabric depends on a lot of factors such asfabric structure, yarn twist and yarn count. Since identical fabrics are used in the study,thus the major factor that affects the WT of fabric will be the fabric structure. However, theplasma treatment cannot alter the fabric structure as it is only a surface treatment methodthat causes an etching action resulting in a roughening effect on the fibre surface [234,240].Such a roughening effect may impart more contact points within the fibres microscopicallyand within yarns macroscopically [234]. The increment in a number of contact points willresult in enhancing the inter-yarn and inter-fibre friction, where a larger cohesive force willbe developed during the application of tensile stress. The increase in the value of WT isprobably due to a larger cohesive force being developed during the extension period so thata larger amount of energy is required for extending the fabrics.

RT refers to the ability of fabric to recover after applying the tensile stress. The reducedfabric RT value indicates that the fabric becomes very difficult to recover to the originalshape after removing the applied tensile stress. After the plasma treatment, the overall RTis decreased. It will be further decreased with the prolonged treatment time. The reductionin value of RT after the plasma treatment can be explained by the increment of cohesiveforce between the fibres and between the yarns. When the extension load is removed, thefrictional restraint will be created simultaneously by the increased cohesive force, whichwill hinder the extended fabrics to recover their original position. The recovery ability ofthe extended fabric is finally lowered resulting in a reduced value of RT. It is thereforeconcluded that the plasma-treated fabrics has difficulty to recover the original shape after

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removing the applied tensile stress. Moreover, with the treatment time prolonged from 5minutes to 30 minutes, the decrease in value of RT is not so great.

EMT is another interesting factor associated with the tensile properties of fibre. It is thepercentage of the extended length after applying a known tensile stress to the fabric whencompared with the initial length. The greater the value of EMT, the larger the elongation ofthe fabric under a known applied stress will be. Under the influence of the plasma treatment,the fabrics show a reduced EMT value and the reduction is further enhanced with theprolonged treatment time. This phenomenon is probably due to the increase in interactionforce between the fibres and yarns, which in turn reduces the relative movement of fibresand yarns during the extension period and also restricts the elongation of fabrics. From theviewpoint of shaping and sewing, the decrease in fabric extensibility can adversely affectthe tailorability. However, a high level of EMT can cause an excessive hygral expansionof wool fabric, leading to a puckering problem in the tailored garment at various ambientrelative humidity conditions. On the whole, the EMT of the plasma-treated fabric is stillacceptable in the laying-up process.

12.2 Shearing properties

The shearing properties consist of shear rigidity (G), shear stress at 0.5◦ (2HG) and at 5◦

(2HG5) shear angle, respectively. G reflects the ability of fabric to resist shear stress. Afterthe plasma treatment, there is a significant increase in the value of G of the wool fabricsbut the increment is enhanced slightly with the prolonged treatment time.

The fabric recovery ability after the application of shear stress can be reflected by theshear stress values at 0.5◦ and 5◦ shear angles. The greater the values of shear stress, theworse the recovery ability of fabric will be. It is observed that after the plasma treatment,there is a significant increase in 2HG and 2HG5 values, more than 200% is increased whencompared with the untreated fabric. However, the plasma treatment showed a similar resulton the shear stress properties with the prolonged treatment time.

Shear is an important determinant of the handle and drape of fabrics. The shear rigidityreflects the subjective handle of fabric, that is increasing the shearing rigidity will enhancethe subjective stiffness of fabric. After the plasma treatment, there is a very large increasein fabric shear rigidity and shear stress. The high-shear rigidity indicates that it will bevery difficult for draping and three-dimensional forming as required in the tailoring ofthe plasma-treated fabric. On the other hand, the plasma-treated fabric exhibited a higherdegree of inelasticity in shear as indicated by the extremely large values of shear stress.The shear rigidity of the fabric primarily depends on yarn interaction, that is an increasein yarn interaction will normally enhance shear rigidity. The increased value of G of theplasma-treated fabrics implies that there is an increase in inter-yarn friction of wool fabricsafter the plasma treatment. To overcome the rigid effect caused by the plasma treatment,finishing process such as softening should be applied to eliminate this deficiency.

12.3 Bending properties

The bending properties have important effects on both the handle and tailoring performanceof fabric. In Table 20, the bending properties of the plasma-treated fabric being studiedinclude the bending rigidity (B) and bending moment (2HB). There is an increase inoverall values of B of the plasma-treated fabric as the duration of treatment time increased.However, the extent of increment after 5 minutes of the plasma treatment is not so mucheven at the longest treatment time, that is 30 minutes.

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The plasma-treated fabrics have a dramatic increase in the values of 2HB, that is morethan 115% increase. 2HB reflects the recovery ability of fabric after bending. The smallerthe values of 2HB, the better the fabric bending recovery ability will be. When compared,the value of 2HB increases correspondingly as the treatment time is prolonged.

The increase in the values of B and 2HB of the plasma-treated fabric will greatlydecrease the fabric flexibility and elastic recovery from bending which in turn will affectthe fabric tailoring, draping and wear.

12.4 Compression properties

The compression properties contain the fabric thickness at 0.5 (T0) and 50 (Tm) gf/cm2

pressure, compressional energy (WC), compressional resilience (RC) and compressibility(EMC). It is obvious that after the plasma treatment, the fabric thickness at T0 and Tmisincreased and the degree of increment is enhanced with the prolonged treatment time,but the results obtained are quite similar. The increased fabric thickness reflects that theplasma-treated fabrics will become fuller than the untreated fabric in fabric handle.

Generally speaking, EMC indicates the change in the thickness of the plasma-treatedfabric. When the EMC value is increased, the fabric handle will become fuller. After theplasma treatment, the EMC values of the plasma-treated fabric with different treatmenttimes are increased to a similar extent.

On the other hand, the surface-raising effect caused by the plasma treatment shows nosignificant change in the value of WC. The WC value implies the fluffy feeling of the fabric.When the values of WC are increased, the fabric will appear fluffier.

Another important property obtained from the compressional hysteresis cure is RC. Thisproperty can help to determine the recoverability of fabric after compressional deformation.When the value is small, the retention ability of deformation after compression will be good.After the plasma treatment, there is a remarkable reduction in the RC of fabric. Such areduction in RC is probably associated with the increased cohesive forces existing betweenthe yarns as a result of the roughening effect imparted by the plasma treatment within theyarns. Hence, this will block the recovery of the extended fabrics. As the treatment time isprolonged, the values of RC are also decreased gradually but are not so significant.

12.5 Surface properties

Coefficient of friction (MIU) reflects the fabric smoothness, roughness and crispness. Afterthe plasma treatment, the values of MIU are increased significantly and the increment isfurther enhanced with the prolonged treatment time. The increment in the value of MIUindicates that the plasma-treated fabric surface becomes less smooth and rougher.

On the other hand, the geometrical roughness (SMD) shows the evenness characteristicsof the fabric surface. The greater the SMD value, the less evenness the fabric surface willbe. The plasma treatment obviously reduces the surface evenness of wool fabric. Theprolonged treatment time also plays an important role to alter the evenness of the woolfabric. It is evident that upon the plasma reaction, the plasma species will bombard on thefabric surface resulting in the etching effect. This can cause the changes in the evenness ofthe fabric surface and hence altering the surface properties of fabric [213].

12.6 Air permeability

The air permeability expressed as air resistance (R) of the plasma-treated fabric is sum-marised in Table 20. Plasma treatment increases the R value of fabric with the prolonged

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treatment time. The air permeability depends on the construction characteristics of yarnsand fibres in which a large proportion is occupied by air space. There are some factorsaffecting the air permeability of fabric, for example fabric structure, thickness and surfacecharacteristics. It is known that the plasma treatment does not have influence on the fabricstructure; therefore, the change in R values is regarded as being closely related to thefabric thickness and surface characteristics. Since the plasma treatment can increase thefabric thickness and alter the surface morphology, thus it is possible to say that the plasmatreatment can induce a certain degree of roughness [234] on the fabric surface. As a result,this will increase the fabric thickness and change the fabric surface characteristics. Thesechanges act as a boundary to hinder the airflow through the fabric, resulting in the reductionof the air permeability of fabrics.

12.7 Thermal properties

Table 20 shows the thermal properties of fabric expressed as the warm/cool feeling (qmax) offabrics with the variation of treatment time. The value of the qmax indicates the heat loss perunit area under the condition of 10◦C temperature difference. It reflects the instantaneouswarm/cool feeling sensed when there is initial contact of the fabric with the surface of theskin. A higher value of qmax denotes that there is a more rapid movement of heat from theskin to the fabric surface, which will provide a cooler feeling. It can be observed that theqmax value of the plasma-treated fabric shows a reduction with the prolonged treatment time.This implies that the plasma-treated fabric has a better warmth-retention property whencompared with the untreated fabric. The thermal properties of a textile fabric depend to agreat extent on the air trapped within it. As the plasma treatment provides an etching effecton the fibre surface, thus such an etching effect can enhance the fabric surface roughness,voids and space, which may increase the amount of the air trapped between the yarns andfibres [228]. In addition, the air permeability results indicate that the plasma-treated woolfabrics have such poor air permeability that the air trapped inside the fabric will not escapeeasily. The air so trapped inside the fabric can act as a good insulation medium and help toprevent the heat loss of the fabrics.

Although the plasma treatment shows significant influence on the properties of woolfabric, yet the prolonged exposure time in the plasma treatment has no further effect onthe mechanical properties, air permeability and thermal properties of the plasma-treatedwool fabrics. As a conclusion, the plasma treatment used for the modification of woolfabric has a high-industrial potential in view of the fabric properties. It is considered asan environmentally friendly dry process, which does not involve any of the solvents andreagents for the wet chemical process.

13. Development of plasma technology in wool industry

Although plasma technology has been developed and applied to textile material modificationfor more than 20 years, there is still room for further development of this technology tomeet the industrial environment.

13.1 Atmospheric plasma treatment

Plasma treatment is a technique useful for the modification of fibre or polymeric materialsin a dry system, that is without water. There have been some problems in productivityand equipment costs because the treatment needs to be conducted at low pressure (below

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Figure 31. Apparatus for atmospheric low-temperature plasma treatment: (1) glass jar, (2) brasselectrode, (3) dielectric layer (polyamide), (4) gas inlet, (5) gas outlet and (6) specimen for treatment[148].

1 Torr). Hence, practical application to textile process has been limited since a vacuumevacuation system and regular maintenance of the vacuum pipeline are necessary.

Recently, researchers [10,148,249–252] have found that plasma can be generated underatmospheric pressure. The discharge conditions of atmospheric plasma treatment are similarto that of corona discharge, and an example of this apparatus is shown in Figure 31.

In the plasma treatment, there are many parameters describing plasma; some are opera-tional such as power and pressure, whereas others may be considered diagnostic includingspecies densities, for example neutrals, ions and electrons. In combination, both sets ofparameters can assist in providing a better understanding of the plasma and the processthat it is being used for. Table 21 outlines the key similarities and differences of operationalparameters between four atmospheric pressure plasma and typical vacuum radio-frequencyplasma.

Table 22 contains a selection of the diagnostic plasma parameters, which are categorisedas either being from atmospheric or vacuum radio-frequency plasma.

When compared, the application of atmospheric pressure plasma to wool is moreuniform and stable causing less surface physical damage than vacuum plasma. In addition,the changes in surface properties and chemical composition caused by the atmosphericplasma and vacuum plasma are similar [254].

13.2 Continuous plasma treatment

Because of the requirement of vacuum plasma treatment, the plasma reactor usually operatesin a batch process. Moreover, the productivity of the plasma treatment will be limited bythe intermittent action of production. In recent researches [162,255], a continuous type of

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Table 21. Typical operational parameters for atmospheric and vacuum plasma [253].

Atmospheric pressure Atmospheric Cold remotenon-equilibrium glow nitrogen Vacuum

plasma discharge plasma Parallel radio-Property (APNEP) (APG) (CRNP) plate frequency

Operating gases Any Helium He, Ar, N2 He, Ar, air AnyFrequency (MHz) 2450 0.003, 13.56 2450 0.001–0.100 13.56Operating

pressure (kPa)Atmospheric Atmospheric 1.3–6.7

(atmosphericunder certainconditions)

Atmospheric >1.3

Power (W) 1000–3000 16–50 0–1500 0–180 >100Gas throughput

(dm3min−1)0–160 ∼5 10–30 100–150 >1

the plasma treatment instrument has been produced. The machine capacity is estimated tobe 40 kg/hour and the top breaks during processing are not a problem since the number ofbreaks is ≤0.12 breaks/1000 m of top. The plasma treatment of wool top does not damagethe fibres. In the research study, it is confirmed that the fibre-to-fibre friction is increasedwhereas the directional frictional effect is decreased. Other physical properties of woolremain unchanged with the exception of a slight decrease in loop breaking force. Thetenacity of yarns spun from the plasma-treated wool top is higher by approximately 25%,and elongation at break is also higher when compared with the standard yarns. Figures32 and 33 show a semi-continuous plasma reactor [161] and a continuous reactor [161],respectively.

The development of continuous atmospheric pressure plasma is still in progress. Thepreliminary investigation reveals that the cost of atmospheric pressure plasma treatment isminimal compared to both vacuum plasma treatments and conventional methods, and theproduct has a much more uniform colour and final properties [256,257]. Although industrialscale continuous plasma treatment is in the investigation stage, recently the atmosphericpressure plasma treatment equipment can be inserted ‘inside’ existing textile-processingmachine to carry out the plasma treatment in line immediately before the traditional processas an alternative to continuous plasma treatment.

Table 22. Similarities and differences between the atmospheric and vacuum plasmacharacteristics [253].

Parameter APNEP Vacuum radio frequency

Particle density (m−3) 2 × 1025 <1021

Electron density (m−3) 1017 ∼1015

Electron temperature (eV) 0.1–2 1−10

Elastic collision rate (s−1) 1011 2 × 109

Vibrational temperature (K) 4000–10000 ∼300–500Degree of ionisation 10−8 ∼10−6

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Figure 32. Semi-continuous plasma reactor [161].

13.3 Fibre identification

Plasma treatment coupled with the scanning electron microscope as a fibre identificationmethod is investigated [234,258–261]. It is found that the etched material has a surfacemorphology, which is related to each material structure and chemical composition, charac-terised by such features as cracks, spots, holes and fibrils. By examining the etched surfacecarefully, the fibre type can be identified. This feature makes plasma treatment a usefultechnique in identifying the nature and structure of unknown polymers in a complex sample,that is a fingerprinting technique. When used in the case of wool or forms of animal fibre,plasma etching has been found to be a successful technique for removing the cuticle of fibreto reveal the cortex [255,256], which can then be examined using the scanning electronmicroscope. Investigations have further shown that the nature of the revealed cortical cellscan be used as an aid for fibre identification [255,256].

Figure 33. Continuous plasma reactor [161].

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13.4 Enhancing self-cleaning in couple with nano-technology

As the plasma treatment can improve surface absorption properties of wool, it can be servedas a pre-treatment for wool fabric surface for allowing the loading of TiO2 by wet chemicaltechniques in the form of transparent coatings constituted of nano-particles of diverse size[262]. These loaded textiles show a significant photo-oxidative activity under visible lightin air under mild conditions, which discolours and mineralises persistent pigment stainscontained in wine and coffee. The observed discoloration of coloured stains seems toinvolve visible light sensitisation of the stain pigment on the TiO2-loaded wool textile.

13.5 Plasma sterilisation

Sterilisation is a process of killing or removing micro-organisms, including vegetable cells,spores and viruses, from an item or material of interest. Common sterilisation methodsinclude dry heat, moist heat (boiling and autoclaving), filtration, radiation and chemicalmeans, such as gaseous ethylene oxide sterilisation. With the help of the plasma treatment,the active species inside plasma are potential sterilising agents [263–265]. These activespecies include strong oxidising agents, such as atomic oxygen and ozone, as well asultraviolet photons, energetic ions and energetic electrons, which can break molecularbonds and denature organic protein. The active species can oxidise and remove layers ofhydrocarbon oils several micrometres thick in the exposure times of only a few minutes.These suggest potential killing mechanisms for micro-organisms for wool fabrics [263–265], which include the following actions:

(i) Destruction by UV irradiation of the genetic material of the micro-organism.(ii) Erosion of the micro-organism, atom by atom, through photodesorption. Photo-

induced desorption resulted from UV photons breaking chemical bonds in the micro-organism material leads to the formation of volatile compounds from atoms intrinsicto the micro-organism.

(iii) Erosion of micro-organism, atom by atom, through etching. Etching stems from thedesorption of reactive species from the plasma on the micro-organism with which theysubsequently undergo chemical reaction to form volatile compounds.

14. Conclusion

Developments in the plasma treatment on wool have been critically reviewed in this mono-graph. The plasma technology is a fast growing and emerging field of science which hasfound numerous applications for the processing of various materials. The application ofplasma treatment modifies the surface properties of the textile material especially wool.The modified wool surface acts favourably for dye uptake, finishes and adhesion properties.Plasma-treated woollen materials exhibit better dye pick-up and give more colour value forthe same strength of dyestuff. The plasma treatment enhances the surface tension of thewool fabric, which in turn increases adhesion property of the wool fabric during coating.The plasma treatment also significantly improves the shrinkproofing property of the woolfabric. Being a physical process, plasma treatment does not involve any industrial effluentand so it can eliminate the problem of pollution. As a result, the plasma treatment canprovide an effective means for the modification of wool fabrics in the industry.

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plasma technology in wool

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