Introduction: What Is Plasma?


Irving Langmuir first used the term plasma in 1926 to describe the inner region of an electrical discharge. Later, the definition was broadened to define a state of matter in which a significant number of atom and/or molecules are electrically charged or ionized. The components present will include ions, free electrons, photons, neutral atoms and molecules in ground and excited states and there is a high likelihood of surface interaction with organic substrates. In order to maintain a steady state, it is necessary to apply an electric field to the gas plasma, which is generated in a chamber at low pressure. (Kan, 1999; Ganapathy, 2000; Pane et al., 2001; Allan, et al. 2002). Plasma, as a very reactive material, can be used to modify the surface of a certain substrate (typically known as plasma activation or plasma modification), depositing chemical materials (Plasma polymerization or plasma grafting) to impart some desired properties, removing substances (plasma cleaning or plasma etching) which were previously deposited on the substrate (Pane et al. 2001)


Ways to Induce the Ionization of Gases:


  1. Glow-Discharge: It is produced at reduced pressure and assures the highest possible uniformity and flexibility of any plasma treatment. It is formed by applying a direct current, microwave, low frequency (50 Hz) or radio frequency (40 kHz, 13.56 MHz) voltage over a pair or a series of electrodes (Figure.1.a, 1.b, 1.c). Alternatively, a vacuum glow discharge can be made by using microwave (GHz) power supply (Figure.1.d).


  1. Corona Discharge: It is formed at atmospheric pressure by applying a low frequency or pulsed high voltage over an electrode pair, the configuration of which can be one of many types. Typically, both electrodes have a large difference in size (Figure.1.e). The corona consists of small lightning-type discharges, their inhomogeneity and the high local energy levels make the classical corona treatment of textiles problematic in many cases.


  1. Dielectric-Barrier Discharge: It is formed by applying a pulsed voltage over an electrode pair of which at least one is covered by a dielectric material (Figure.1.f). Though also here lightning-type discharges are created, a major advantage over corona discharges is the improved textile treatment uniformity (Verschuren & Kiekens, 2001).



Figure 1: Schematic principle of different plasma processes (Verschuren & Kiekens, 2001)


 

Plasma technologies offer a wide spectrum of possible treatments of materials. The plasma technology for industrial processes essentially uses two different types of plasma: The first one "thermal plasma", is produced at high pressure (>10kPa) by means of direct or alternating current (dc-ac) or radio-frequency (RF) or microwave sources. These devices produce plasmas with electron and ion temperatures of the order of 1-2 eV and with very low gas ionization. Thermal plasma can be used to destroy the solid, liquid and gaseous toxic halogenated and hazardous substances or to generate anti-corrosion, thermal barriers, antiwear coatings, etc. The second type of plasma, named cold or non-equilibrium plasma, is characterized by the electron temperature higher than the ion temperature; it is produced under vacuum conditions using low power rf or microwave or dc sources. Cold plasmas can be used for surface modifications, ranging from the simple topographical changes to the creation of surface chemistries and coatings that are radically different with respect to the bulk material (Bonizzoni & Vassallo, 2002).


Plasma Treatments in Textile Technology


Plasma treatment of textile fabrics and yarns is being investigated as an alternative to wet chemical fabric treatment and pretreatment processes, e.g., shrink resistant or water repellent finishing, which tent to alter fabric mechanical properties and are environmentally hazardous. The transfer of research results into the technological field would lead to non-polluting and very promising operating conditions. In the prospect of chemical finishing using plasma, two main methods can be considered: grafting of a compound on the fiber or surface modification by means of discharges. Plasma treatment modifies the uppermost atomic layers of a material surface and leaves the bulk characteristics unaffected. This treatment of textiles may result in desirable surface modifications, including but not limited to surface etching, surface activation, cross linking, chain scission, decrystallization, and oxidation. Treatment depends on the choice of working gas and plasma density and energy. Air, oxygen, argon, fluorine, helium, carbon dioxide or their mixtures can be used as plasma medium. The process result is affected by the type of the gas used. Although the gas the same, if the fiber type is different the result will be different (Pastore & Kiekens, 2001; Poll et al., 2001; McCord et al., 2002).3 Textiles can be treated between two electrodes (in fact in the plasma) or near the plasma region. In the Figure.2, there are a schematic view of plasma device and different reactive species.


Plasma-chemical conversion of the feed gas produces chemically active particles that are able to modify textile surface molecules via chemical reactions after impinging on the surface. The radicals generated inside the plasma region must be given the opportunity to move to the reaction place at the textile fiber surface. Thereby the path of radicals between the locations of generation and reaction is limited on the one hand by the



Figure 2: A Schematic view of plasma device and different reactive species

(Yousefi et al., 2003)


Distance between single fibers and on the other hand by the gas density, i.e. by the mean distance between gas particles. Assuming radicals react or recombine after several impacts with gas particles and at surface sites on fibers there is a relationship between penetration depth of the plasma effect inside the textile structure and process pressure as well as the textile structure itself (Kan et al., 2001; Poll et al., 2001).


 

Hydrophilicity and Wettability


There are a lot of investigations on plasma treatments of several fibers for changing their wettability characteristics. Cotton, polypropylene, polyester, flax and wool are the examples studied in this subject. Poll et al., 2001, studied the hydrophilization of cotton fabrics in oxygen plasma, taking the gas pressure as the most important process parameter at different treatment times. Hydrophilization was measured on both front and back sides of each fabric layer. Pressure of atmospheric plasma was above 100 mbar and of low pressure RF plasma was 0.6-8mbar (Figure.3). For the low pressure plasma, after approx. 700s the hydrophilization is complete, even for the most inner fabric layer and pressure increase results in a remarkable increase of depth and velocity of the penetrating hydrophilization front. This might be due to an increasing concentration of oxygen radicals per volume unit with increasing pressure. For the atmospheric plasma, besides increasing temperature and strong decomposition of the fabric (etching) a hydrophilization effect on the outer plasma facing fabric layer is observed. This behavior is traced back to the pressure far exceeding the optimum value and thus favoring deactivating collisions between the active species in the gas phase (Poll et al. 2001).


Molina et al., 2003, treated wool fabric with water vapor plasma, which was produced in a RF reactor with 100Pa and 100W at different treatment times of 10, 40 120 and 600s. The shrink resistance and contact angle were determined. To analyze the changes in surface chemical composition and the topographical changes, the XPS technique, and the SEM and Herbig sac formation were used respectively. They found that the water plasma confers hydrophilic properties to the fibers even in the time of 10s. The XPS results confirm the removal of the hydrophobic fatty acid layer and the generation of new hydrophilic surface groups. By the oxidation and removal of this layer, the wool shrink tendency is clearly reduced, even when the treatment time was 10 s. Plasma treatment time >600s leads to removal of epicuticle (Figure.4). The wetting decay after an elapsed time of plasma treatment was observed (Molina et al. 2003).



Figure 3: a) Electrode arrangement for low and medium pressure plasma treatment

b) Arrangement for plasma treatment at atmospheric pressure (Poll et al. 2001)



Yousefi et al., 2003 studied the surface modification of biaxially oriented polypropylene (BOPP) films by low-temperature, low-pressure oxygen plasma treatment. By the introduction of oxygen gas, the pressure was adjusted to 1-10 Pa in the plasma chamber.


Different samples were treated with low temperature oxygen plasma for 0.5, 1 and 2 minutes. The changes in surface energy were determined. Scanning electron microscopy images and attenuated total reflectance infrared spectra were obtained. Plasma treatments show a larger increase in surface energy compared with untreated surfaces in 2 min (Table.1). SEM images show that low-pressure glow discharge treatment caused mainly physical changes by a creating roughness on the surface. The appearance of treated films changed mainly towards low reflectance. The difference spectra show the changes in the chemical structure of the film and the degradation of the polymer.



Figure 4: SEM images of untreated and 600s H2O plasma treated wool fibers (Molina et al. 2003)


 

The bands that characterize the CH2 groups of polypropylene were reduced by hydrogen loss and polymer degradation due to oxygen plasma etching. The spectra represent new functional groups assigned to -CO and OH absorption bands. Creation of polar groups such as hydroxyl and carbonyl on the surface increased the surface energy and wettability. Although surface roughness is not the primary reason for the improved wettability it has a great effect.


Table 1: Surface energy of BOPP treated with low pressure glow discharge for different times

Treatment Time (min)

Surface Energy (mJ/m2)

0

24

0,5

60

1

65

2

71



Shrink Resistance of Wool Fibers


Low-temperature plasma treatment is claimed as an environmentally friendly treatment for improving the felting and dyeability properties of wool fiber (Kan et al., 2001).


Kan et al., 2001, studied the oxygen plasma treatment by using a glow discharge generator of four types of wool fiber having different diameters. The pressure, power and time of the treatment were 10Pa, 80W and 5 minutes, respectively. After the plasma treatment, on all fibers there are micropores and cleft lines on the scales parallel to the fiber axis. The feltability decreased but alkali solubility so the damage to fiber increased after plasma treatment for all fibers. Treated wool fibers dyed faster than the untreated fibers. The alteration of properties was quite similar in the different batches of wool fibers. Canonica & Mori 2001 studied the surface modification of wool fibers based on a combination of low-pressure air (100Pa) plasma treatment with enzyme treatment to enhance the handle and dye ability properties of the fiber. It was established that the fibers dyed deeper in color than the fibers which were treated only with enzymes. Also the handle was enhanced with addition of enzyme treatment on the plasma treatment.


Zuchairah et al. 1997 studied the combination of the glow discharge oxygen plasma pretreatment and polymer application to improve the shrink resistance of wool fabric compared to combined chlorine pretreatment with polymer application.


Functional Finishing of Textiles


Pane et al. 2001, investigated the effects of the plasma etching (cleaning) and plasma coating processes compared to traditional method of finishing (washing and coating) for the waterproofing of the acrylic fabrics for outdoor applications. The gases employed were oxygen (power 2000W, time: 3,5,10 min) for pretreatment, while for the hydrophobic coating mixtures of argon, fluorocarbon and hydrocarbon gases were applied. It was found that plasma treatment can take the place of the traditional washing and coating processes which gives the fabrics their waterproof behavior. Treated fabrics have characteristics in terms of handling and waterproofing, which in some case are superior to traditionally finished fabrics (Table.2). The abrasion (Martindale) and Weathering (Xenotest) tests indicate that plasma coatings have good wear and UV resistance.


 

Table 2: Effect of plasma coatings on the waterproof characteristics of fabrics with respect to traditional coating


Fabrics

Water Column (cm)

Contact Angle (o)

Traditionally finished

32

133

Washed and plasma coated (Fluorocarbon20%/Ar80%)

32

131

Washed and plasma coated (Fluorocarbon90%/methane10%)

26

123

Washed and plasma coated (Fluorocarbon93%/methane7%)

30

129

Plasma cleaned and plasma coated (Fluorocarbon20%/Ar80%)

33

133

Plasma cleaned and plasma coated Fluorocarbon90%/methane10%)

24

123

Plasma cleaned and plasma coated (Fluorocarbon93%/methane7%)

37

140

Plasma cleaned and plasma coated (Fluorocarbon90%/Ar10%)

34

134

Plasma cleaned and plasma grafted (Unsaturated Fluorocarbon3%/Ar97%)

35

136



Topographical Study of the Effects of Plasma Treatments


Wong et al., 2000 presented a study of the surface morphology and topography of oxygen and argon plasma treated flax fibers using ESEM, SEM and AFM techniques. They used a plasma system with a radio-frequency of 13,56 MHz. Fiber samples exposed to oxygen or argon plasma at a pressure of 15 Pa and 200W discharge power, and for 0, 2.5, 5, 10, 20, 40, and 60 minutes. Oxygen plasma shows a faster etching rate by exhibiting a progressively pitted and etched pattern after only 10 minutes of accumulated exposure, whereas argon plasma treatment shows a relatively slower etching rate, and the etched pattern appears after almost 40 minutes of accumulated exposure. Argon plasma etching results in a clearer visualization of fibrils parallel to the fiber axis, while oxygen plasma results in shrinkage, causing the corncob structure perpendicular to the fiber axis. Oxygen plasma causes more severe effects, with fiber contraction during plasma treatments. As the exposure time increases, the depth as well as the sizes of the micro-pores etched by the plasma increase. The dominant weight loss during plasma treatment is mainly due to surface etching of the fibers on the fabric surface. According to AFM results, in the relatively smooth surface of an untreated flax fiber, argon plasma creates pits with sizes (both depth and diameter) mainly in the submicrometer range, while the oxygen plasma creates pits of a few micrometers.


McCord et al. 2002, investigated the modifying nylon (exposure time 1-8min) and polypropylene (exposure time 0.5-4min) fabrics with atmospheric plasmas of He and He-O2 at capacitively coupled device operating at low frequencies from 1 to 12 kHz. SEM analysis showed no apparent changes in the plasma treated nylon, but significant surface


Morphological changes for polypropylene. Similarly, XPS analysis showed small difference in the surface C and O contents of untreated and treated nylon fabrics; while it showed a significant increase in O and N content of the polypropylene fabrics. There is a slight decrease in nylon fabric tensile after the He plasma treatment for 3 min., while there was no significant change in tensile strength of it treated with He-O2 after exposure times of up to 8min.


 

Graft Copolymerization


Plasma grafting is grafting molecules on the material surface after plasma activation. The effects of the plasma do not penetrate more than 100 from the surface. Because the bulk of the material is not affected by the treatment, desirable structural characteristics are maintained. Abidi & Hequet 2004 studied creating the active centers within the cellulose chains which were used to initiate copolymerization reactions with vinyl monomers to impart hydrophobic character to lightweight cotton fabric. N2, O2 and Ar plasmas were obtained using a microwave generator at 2.45GHz under vacuum. To monitor the changes UATR-FITR was used. Plasma treatment for 240 s with 500W was sufficient to create active carbonyl groups. Ar plasma generated the most active groups. Before the second plasma treatment, the fabrics were impregnated with vinyl laurite. According the results for maximum grafting efficiency the vinyl monomer concentration should be below 0,664 mol/l. Above this concentration, the homopolymerization reactions are likely to be dominant. Testing of treated fabric revealed that excellent water repellency was obtained (Abidi & Hequet, 2004)


Table 3: Relative free radical intensities detected by ESR after the plasma treatments (Chen, 1996).


Plasma gas

Cotton

Wool

O2

0.5

0.4

N2

0.6

0.5

Ar

1.6

0.6

H2

1.8

0.6

CO

2.9

0.7


Chen 1996 studied the free radical formation on cotton and wool fibers treated with low temperature plasmas of O2, N2, Ar, CO, CF4 at the RF generator, at the power of 300W and the pressures of 0.3-1.5Torr. Free radicals play an important role in polymerization, grafting, cross-linking and implantation. Table 3 shows that free radical intensities are different for various gases with the general rule that O2<N2<Ar<H2<CO<CF4 (Chen,1996). The free radical formation was increased with increasing time.


If the disadvantages of plasma treatments, such as the high cost of the plasma device, can be eliminated, this technology will be valid and very important method for the textile finishing industry.


References:


  1. Abidi, N. & Hequet, E. (2004). Cotton Fabric copolymerization Using Microwave Plasma.Universal Attenuated Total Reflectance-FITR Study, Journal of Applied Polymer Science, Vol.93, 145-154 (2004)
  2. Allan, G. et al. (2002). The Use of Plasma and Neural Modelling to Optimise the Application of a Repellent Coating to Disposable Surgical Garments, AUTEX Research Journal, Vol.2, No.2, June 2002
  3. Bonizzoni, G. & Vassallo, E. (2002). Plasma Physics and Technology; Industrial Applications, Vacuum, 64(2002), 327-336
  4. Ganapathy, R. (2000). Immobilization of Alpha Chymotrypsin and Papain on Plasma Functionalized Polymer Surfaces. Ph. D. Thesis, University of Wisconsin-Madison.
  5. Kan, C.W. (1999). The Effect of Descaling Process on the Properties of Wool Fibres. Ph. D.Thesis, Hong Kong Polytechnic University.
  6. Kan, C.W. et al. (2001). Development of Low Temperature Plasma Technology on Wool, The 6th Asian Textile Conference, Innovation & Globalization, Proceedings, August 22-24, 2001, Hong Kong
  7. Kan, C.W. et al. (2001). The Effect of Low Temperature Plasma Treatment on Different Types of Wool Fibre, The 6th Asian Textile Conference, Innovation & Globalization, Proceedings, August 22-24, 2001, Hong Kong
  8. McCord, M.G. et al. (2002). Modifying Nylon and Polypropylene Fabrics with Atmospheric Pressure Plasmas, Textile Research Journal, 72(6), 491-498 (2002)
  9. McCord, M.G. et al. (2003). Surface analysis of Cotton Fabrics Fluorinated in Radio-Frequency Plasma, Journal of Applied Polymer Science, Vol.88, 2038-2047
  10. Molina, R. et al.(2003). Surface Characterization of Keratin Fibres Treated by Water Vapour Plasma. Surface and Interface Analysis, 2003; 35; 128-135

 

  1. Pane, S. et al. (2001) Acrylic fabrics treated with plasma for outdoor applications. Journal of Industrial Textiles, Vol.31, No.2, October 2001, 135-145
  2. Pastore, C. M. & Kiekens, P. (Eds.) (2001). "Surface Characteristics of Fibers and Textiles". Marcel Dekker, Inc., New York, pp.203-218
  3. Poll, H.U. et al. (2001). Penetration of Plasma Effects into Textile Structures. Surface &Coatings Technology, 142-144 (2001), 489-493
  4. Verschuren, J. & Kiekens, P. (2001). Plasma Technology for Textiles: Where Are We? IX. International Izmir Textile & Apparel Symposium, October 2001, eme-Izmir, Turkey.
  5. Wong, K.K. et al. (2000). Topographical Study of Low Temperature Plasma Treated Flax Fibers. Textile research Journal, 70(10), 886-893
  6. Wong, K.K. et al. (2001). Wicking Properties of Linen with Low Temperature Plasma. Textileresearch Journal, 71(1), 49-56