Plasma treatment of textiles -I
Plasma is defined as an ionized gas containing both charged and neutral species, including free electrons, positive and/or negative ions, atoms, and molecules. The overall state of plasma is considered neutral with the density of electrons and negative ions being equal to the density of the positively charged ions, known as plasma quasi-neutrality. In order to form and sustain plasma, an energy source capable of producing the required degree of ionization must be used. Either direct current (DC) or alternating current (AC) power supplies may be used to generate the electric field required for plasma generation. For many industrial types of plasma, radio frequency (RF) power supplies are used, usually at a standard frequency of 13.56 MHz. Plasma generation may also be performed at various pressures, including low (vacuum), atmospheric, or high pressure.
Basic Principle of plasma: In elementary physics reference is often made to the three states of matter: Solid, Liquid and Gas. However, there is a fourth state: Plasma.
To convert a solid to a liquid energy is imparted to the solid, usually in the form of heat; similarly to convert a liquid to a gas. It is not therefore surprising that to convert a gas to a plasma, energy also needs to be imparted to the gas. The energy disassociated electrons from the gas atoms through atomic particle collisions. This occurs randomly, which means the energized gas is a mixture of ions, freed electrons, photons and neutral atoms (those yet to lose electrons). If a solid or liquid substance is introduced into the plasma, the high energy gas particles of the plasma will penetrate and collide with atoms or molecules several nano-meters into the solid or liquid, dissociating those electrons and bringing those atoms or molecules to an excited state to be part of the plasma. This means that the high energy particles of the plasma will continuously etch away a several nano-meter layer of a solid or liquid substrate for as long as the material is in contact with the energized gas.
Since there is an equal number of dissociated species (ions /electrons, etc.) in the gas volume, the gas remains electrically neutral. Thus, any ionized gas that is composed of nearly equal numbers of negative and positive ions may be called a plasma.
Historical Background of Plasma Technology:
- 1780: Georg Christoph Lichtenberg, a mathematics professor at Göttingen University in Germany, observed beautiful brush-like patterns on insulating surfaces following discharges from a pointed electrode.
- 1820: Michael Faraday prior to his groundbreaking discovery of electromagnetism, he spent the years investigating the properties of heated matter. Michael Faraday discussed the possibility of a fourth state of matter.
- 1857: Werner von Siemens had made use of technical plasmas in his apparatus for producing ozone. However, he did not recognize this as a plasma phenomenon.
- 1880: Sir William Crookes discovered the fourth state as ‘radiating matter’ in discharge tubes.
- 1900: Joseph John Thomson revealed the nature of cathode rays. Eugen Goldstein demonstrated the existence of ‘canal rays’.
- 1923: New York chemist Irving Langmuir discovered ‘plasma oscillations’.
- 1938: Fluorescent lamps became commercially available.
- 1953: Werner Schmellenmeier discovered diamonds as a product of acetylene gas discharges.
- 1983: In Japan, substrates were coated with polycrystalline diamond layers in microwave plasma.
- 2000: In Germany alone, well over 200 companies were active in the field of low temperature plasma technology.
In the textile field, significant research work has been going on since the early 1980s in many laboratories across the world dealing with low-pressure plasma treatments of a variety of fibrous materials showing very promising results regarding the improvements in various functional properties in plasma-treated textiles. A variety of commercial low-pressure plasma machines, mostly in prototype form, have been offered for batch/in-line processing of textiles for more than 15 years. In recent times, some companies have also started to offer commercial systems for atmospheric pressure plasma processing of textiles, both in-line and on-line.
Various application of plasma in textile:
|Hydrophilic finish||PP, PET, PE||Oxygen plasma, Air plasma|
|Hydrophobic finish||Cotton, P-C blend||Siloxane plasma|
|Antistatic finish||Rayon, PET||Plasma consisting of dimethyl silane|
|Reduced felting||Wool||Oxygen plasma|
|Crease resistance||Wool, cotton||Nitrogen plasma|
|Improved capillarity||Wool, cotton||Oxygen plasma|
|Improved dyeing||PET||SiCl4 plasma|
|Improved depth of shed||Polyamide||Air plasma|
|UV protection||Cotton/PET||HMDSO plasma|
|Flame retardancy||PAN, Cotton, Rayon||Plasma containing phosphorus|
Various plasma technologies used in textile:
There are many different ways to induce the ionization of gases. They are
(1) Glow discharge
(2) Corona discharge
(3) Dielectric Barrier discharge
(4) Atmospheric pressure plasma technique.
It is the oldest type of plasma technique. It is produced at reduced pressure (low-pressure plasma technique) and provides the highest possible uniformity and flexibility of any plasma treatment. The plasma is formed by applying a DC, low-frequency (50 Hz) or radio frequency (40 kHz, 13.56 MHz) voltage over a pair or a series of electrodes. Alternatively, a vacuum glow discharge can be made by using microwave (GHz) power supply.
It is formed at atmospheric pressure by applying a low-frequency or pulsed high voltage over an electrode pair, the configuration of which can be one of many types. Typically, both electrodes have a large difference in size. The corona consists of a series of small lightning-type discharges; their in homogeneity and the high local energy levels make the classical corona treatment of textiles problematic in many cases.
DBD is formed by applying a pulsed voltage over an electrode pair of which at least one is covered by a dielectric material. Though also here lightning-type discharges are created, a major advantage over corona discharges is the improved textile treatment uniformity.
Atmospheric pressure plasma technique:
As discussed earlier, there are various forms of plasma depending on the range of temperature and electron density. Generally, high plasma densities are desirable, because electrons impact gas molecules and create the excited-state species used for textile treatment. Having more electrons generally equates to faster treatment time. However, very high plasma densities (greater than 1013 electrons cm-3) can only exist with very high gas temperature (Thermal Plasma). This extremely high level of plasma density is unsuitable for textile treatment, because the plasma’s energy will burn almost any material. Hence for textile processing, the plasma needs to do their job at room temperature, thus the name ‘cold plasma’.
This is due to the fact that the energy of the plasma is mainly confined to the energy of low mass electrons. Non-thermal plasma or cold plasma is characterized by a large difference in the temperature of the electrons relative to the ions and neutrals. Thus, Te >> Ti >> Tn. As the electrons are extremely light, they move quickly and have almost no heat capacity. Ionization is maintained by the impact of electrons with neutral species. These plasmas are maintained by passing electrical current through a gas. The low temperature makes them suitable for textile processing. However, non-thermal plasmas generally require low-pressure or vacuum conditions.
To overcome these restraints, Atmospheric Pressure Plasma Techniques are being developed. This technique provides the highest possible plasma density (in the range of 1 to 5 x 1012 electrons cm-3), without the associated high gas temperatures and the cold plasma chemically treats fabric and other substrates without subjecting them to damaging high temperatures. The Atmospheric Pressure Plasma is a unique, non-thermal, glow-discharge plasma operating at atmospheric pressure. The discharge uses a high-flow feed-gas consisting primarily of an inert carrier gas, like He, and small amount of additive to be activated, such as O2, H2O or CF4.
The benefits of plasma over classical wet chemistry finishing:
- Endless chemical modifications are possible by choosing appropriate gasses or chemicals.
- Optimization of surface properties of materials without alternation of bulk characteristics.
- Polymers which are unable or very difficult to modify with wet chemicals, their surface properties can also easily be changed.
- Applicable to all substrate suitable for vacuum processes i.e. almost free choice of substrate material.
- In most cases it can be a dry process, reducing water consumption and energy to dry the treated materials.
- Reduction in the amount of water usage results in reducing the amount of waste water and the waste water treatment cost.
- It has an economical advantage over the conventional wet processing due to its low chemical consumption and reduction in chemical and water costs.
- It shortens the time of dye fixation.
- Although for the above reasons all plasma processing is more environmentally friendly in comparison to the textile wet processing, the closed plasma treatment systems are an even more environmentally friendly process because the plasma byproducts can be trapped rather than being released into the environment.
- Pore-free, uniform thin films with superior properties that can’t be achieved with conventional chemistry can be deposited on almost any substrate.
- The process is performed in dry, closed system. So, higher safety can be ensured.
Disadvantages of plasma finishing on textiles:
- System dependency is one of the most important disadvantages of the plasma treatment. This means that the same flow rate, gas pressure and power input may not produce the same level of the needed reacting species.
- Optimal process parameters must be established for each process and equipment. However, it is not too difficult to overcome these challenges
- It may produce harmful gases such as ozone and nitrogen oxides during operation. This happens due to the formation of free radicals, which react with atmospheric gases to form harmful byproducts.
- Although initial investments such as purchasing expensive plasma equipment and high vacuum pumps are considered to be limiting factors and could be considered as a disadvantage, these costs can be recovered by savings that were mentioned above.
- Scaling up and converting pilot batch process into a continuous process could also present some technical challenges.
- Treating thin surface layers without changing the bulk could also be a disadvantage for some end-use and an advantage for when the objective is to keep the bulk untreated and only thin surface treatment is needed.
- Textile materials are made from yarns or directly from fibers. In either case the fibers are covering each other, especially when they are in high twist yarns. This creates a shadow effect and the shadowed areas are generally protected from plasma treatment.