Low temperature plasmas are ionised gases generated at pressures between 0.1 and 2 torr. These types of plasmas work within a vacuum chamber conditions using low power RF or DC sources where atmospheric gases have been evacuated typically below 0.1 torr, say G Ramakrishnan, J Srinivasan, Jothilinkam MD and T Ramachandran.
Plasma is a partially-ionised gas composed of many types of species such as positive and negative ions, electrons, neutrals, excited molecules, photons and UV light. Plasma is also considered as the fourth state of matter. The lightning bolt and solar corona are example of plasma present in nature. Artificial plasma is also available in different devices such as fluorescent lamp, neon sign, welding arcs, gas lasers and plasma reactor. Plasma is used for varieties of industrial applications ranging from arc welding, metal hardening, metal coating, nuclear fusion, television, synthesis of nano-sized materials, creation of nano-structure, surface cleaning, functional polymeric coating, and change in surface hydrophilicity and hydrophobicity of substrates.
Plasma is thus characterised by the following properties:
- Contains an increased number of charge carriers (conductivity),
- Quasi neutrality (same number of positive and negative charges in a unit volume),
- Increased energy and heat content,
- Creation of equilibrium states (charge carrier formation and recombination is stationary),
- Existence of speed and energy distribution functions (permit the characterisation of the plasma by temperature)
- On the basis of pressure in plasma chamber- atmospheric pressure and low pressure plasma.
- On the basis of degree of ionization and the temperature of electrons and ions-Hot and cold plasma.
- On the basis of frequency of the power supply DC and AC plasma(RF, Microwave, GHz Plasma).
- Depending upon the electron affinity of the process gases used-Electropositive and electronegative gas plasma.
Hot plasma (equilibrium plasma)
This type of plasma can be artificially generated using a high voltage, high temperature arc, which is the basis for the corona discharge process and for the plasma torch used to vaporise and redeposit metals. The range of operation for typical high temperature plasma is greater than 1,000°C and is used for cutting, welding, or coating metallic components. Production in universe is of hot plasma. Hot plasma is nearly fully ionized which is actually known as the fourth-state of matter.
A good example of naturally occurring high temperature plasma is lightning. The Sun and the stars in the universe consist entirely of hot plasma, and the space within the stars of a galaxy is filled with plasma.
Cold plasma (non-equilibrium plasma)
Low temperature plasmas are ionised gases generated at pressures between 0.1 and 2 torr. These types of plasmas work within a vacuum chamber conditions using low power RF or DC sources where atmospheric gases have been evacuated typically below 0.1 torr.
It is characterised by the electron temperature higher than the ion temperature. Cold plasmas are used in surface modification and organic cleaning or to deposit specific coatings onto organic and inorganic substrates.A small fraction of the gas molecules (e.g. 1 per cent) is ionised. It has already found uses in a variety of manufacturing processes.
For example, flat-screen televisions use cold plasma to radiate light and create images. Cold plasma can produce ozone as its secondary offspring.
Atmospheric pressure plasma
Traditional sources include transferred arcs, plasma torches, corona discharges and dielectric barrier discharges. In arcs and torches, the electron and neutral temperatures exceed 3000°C and the densities of charge species range from 1016 cm-3 to 1019 cm-3. Due to the high gas temperature, this plasma is used primarily in metallurgy. Corona and dielectric barrier discharges produce non-equilibrium plasma with gas temperatures between 50°C and 400°C. Higher voltages are required for gas breakdown at 760 torr and often arcing occurs between the electrodes. However, to prevent arcing and lower the gas temperature, several schemes have been devised, such as the use of pointed electrodes in corona discharges and insulating inserts in dielectric barrier discharges.
Low-pressure plasmas are a highly mature technology developed for the microelectronics industry. Vacuum vessel is pumped down to a pressure in the range of 10-3 to 10 mbar with the use of high vacuum pumps. The gas which is then introduced in the vessel is ionised with the help of a high frequency generator. The advantage of the low-pressure plasma method is that it is a well controlled and reproducible technique.
Corona discharge 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 their 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. It is generated at gas pressures equal to atmospheric pressure. The electromagnetic field is greater than 15>kV and has a frequency range of 20-40 kHz.
Dielectric barrier discharge (Silent discharge) This is an atmospheric-pressure plasma source. In this case a pulsed high voltage power ranging from low frequency AC to 100 kHz is applied between electrodes, one or both of which is covered by a dielectric layer.
The purpose of the dielectric layer is to terminate rapidly the arcs that form in the region between electrodes. The discharge consists of series of rapid micro discharges.
Glow discharge is the oldest type of plasma. It is an ionized gas consisting of equal concentrations of positive and negative charge and a large number of neutral species. It is produced at reduced pressure and assures the highest possible uniformity and flexibility of any plasma treatment. The plasma is generated at a gas pressure in the 0.1-10 MPa range. The electromagnetic field is in the range of 0.4-0.8 kV and the broad frequency range is 0-2.45 GHz.
Two electrodes placed in chamber. One is connected with high voltage and high frequency supply while other is grounded. When supply is on, conduction between two electrodes take place through ionisation of gas present between two electrodes. Thus plasma consisting of free electrons, radicals, ions, UV radiation and other excited particles is generated, depending upon the type of gases used. If textile substrate is placed between the two electrodes, plasma species reacts with it, leading to surface modification.
The act of plasma treatment on textile
According to requirements, the materials to be processed processing (foils, membranes, textiles, polymers) will be treated for seconds or some minutes with the plasma. Essentially, four main effects can be obtained depending on the treatment conditions.
- The cleaning effect is mostly combined with changes in the wettability and the surface texture (see Point 2.). This leads for example to an increase of quality printing, painting, dye-uptake, adhesion an so forth.
- Increase of microroughness. This effects, for example, an anti-pilling finishing of wool.
- Generation of radicals. The presence of free radicals induce secondary reactions like cross linking. Furthermore, graft polymerisation can be carried out as well as reaction with oxygen to generate hydrophilic surfaces.
- Plasma polymerisation. It enables the deposition of solid polymeric materials with desired properties onto the substrates.
- The advantage of such a treatment is, that the modification is restricted to the uppermost layers of the substrate, thus not affecting the overall desirable bulk properties of the substrate adherent. The research in the field of plasma applications for textile treatments is very wide and here are some of its recent developments.
Let us conclude telling the plasma treatment has more advantages. The finished textile shows better performance and improved colour fastness properties. Though currently not very relevant in produced amounts, this type of high-performance textile will certainly grow in economic importance. Plasma technology performed under atmospheric pressure or under reduced pressure (depending on the special needs) leads to a variety of processes to modify fiber or textile materials to fulfill additional highly desirable requirements. As a result of their high added value even small textile batches can be produced at high profit, although perfect process control is absolutely necessary. Typically, textiles for medical applications or uses in the sector of biotechnology are expected to increase in importance in future.
- Plasma treatment advantages for textiles by Amelia Sparavigna, Dipartimento di Fisica, Politecnico di Torino, Corso Duca Abruzzi 24, Torino, Italy.
- http://textilelearner.blogspot.com/2012/04/application-of-plasma-technology-in.html (2012).
The authors thank the management of Kumaraguru College of Technology,Coimbatore for permitting to use their Advanced Textile Laboratories and Information centre in KCT-TIFAC CORE Research Center established under DST, Government of India.
G Ramakrishnan and J Srinivasan are with the Department of Fashion Technology, (KCT-TIFAC CORE) Kumaraguru College of Technology, Coimbatore – 641 049, Tamil Nadu, India. Jothilinkam MD and T Ramachandran are with the Karpagam University, Coimbatore.