What is plasma? Plasma treatment on different textiles or fabrics.

Introduction

The use of plasma in textile modification represents a great outlook for improvement of older, energetically demanding, slow, and environment polluting treatment technologies. The application of plasma is eco-friendly and reduces production costs due to energy savings and the reduction of processing times. Moreover, plasma treatment offers the possibility to obtain textile finishes without changing the key textile properties. The present review attempts to give an overview of plasmas for textile treatment from a broad perspective. Plasmas are ionized gases and gas is normally an electric insulator. However, when a sufficiently large voltage is applied across a gap containing a  gas or gas mixture,  it will breakdown and conduct electricity. The reason is that the electrically neutral atoms or molecules of the gas have been ionized, i.e. split into negatively charged electrons and positively charged ions. The resulting ionized gas is often called a  discharge or plasma.  The interaction, neutral gas, and contact of the electrically charged particles with each other produce the unique physical and chemical properties of the surface plasma environment. This environment is different from those found in solids, liquids, or gases; So plasmas are sometimes called the fourth state of matter.

What is plasma?

Plasma is a state of matter where an ionized gaseous substance becomes so highly electrically conductive that the electrical and magnetic fields at a distance dominate the behavior of the substance. Plasma states are seen in contrast to other states: solid, liquid, and gas.

Plasma is an electrically neutral medium that is unbound positive and negative particles. Although these particles are infinite, they are not "free" in terms of not giving experience to the forces. The stored charged particles create an electric current within the magnetic field and any movement of the charged plasma particles is affected by the fields created by other charges. Instead, it manages joint behavior with a variety of different levels.

Three factors define a plasma-

A. Plasma Estimation: When the plasma parameter represents the number of charges carriers in a sphere surrounding a given charged particle, the plasma estimation is applied when the particle outside the sphere is sufficient to withstand the electrical effect.

B. Bulk Interaction: The length of screening (as defined above) is much shorter than the physical size of the plasma. This criterion means that interactions in most parts of the plasma are more important than their edges where boundary effects can occur. If this criterion is satisfied, then the plasma is quasineutral.

C. Plasma Frequency: Electronic plasma frequency is larger than electron-neutral collision frequency When this condition is valid, electrostatic interactions predominate over the processes of general gas dynamics.

There are many ways to induce the ionization of gases. Such as-

a. Glow discharge,

b. Corona discharge,

c. Dielectric Barrier discharge,

d. Atmospheric pressure plasma technique.

a. Glow discharge:

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

b. Corona discharge:

Corona Discharge is made at atmospheric pressure by applying a low frequency or pulsed high voltage over an electrode pair, the configuration of that can be one of many types.

c. Dielectric Barrier Discharge:

The dielectric-barrier discharge is created by applying a vibrating voltage over an electrical joint that is covered by at least one dielectric element.

d. Atmospheric pressure plasma technique:

The atmospheric pressure better stability, control, and reproducibility. Since plasma cannot be generated in a complete vacuum the name vacuum pressure is somewhat misleading and only refers to the low working pressures of such systems.

However, choose to classify vacuum pressure plasmas into subcategories of low and medium pressures. Both these forms are suitable for application on textiles and progress continues to determine their effect on textiles.

Importance of low-temperature Plasmas in Textile Treatment

Low-temperature plasma (LTP) or cold plasma is produced almost exclusively in glow discharges in a gaseous environment. A key property of this type of plasma is the lack of equilibrium between the temperature of the electrons and the energy of the un-ionized gas particles. This allows the creation of conditions in which the temperature of the plasma gas remains close to that of the environment, while the energy of the electrons is sufficient to break intermolecular and covalent bonds. This property of LTP makes it useful for initiating modifications in polymers that are not resistant to high temperatures, that is, of almost all fiber-forming polymers.  Very high plasma densities can only exist with very high gas temperatures (Thermal Plasma). This extremely high level of plasma concentration is unsuitable for textile treatment because the energy of plasma will burn almost any material. For textile processing, plasma needs to work at room temperature, hence the name ‘cold plasma’. Another advantage of cold plasma is that it chemically treats fabric and other substrates without subjecting them to damaging high temperatures.

Artificial plasmas-

Most artificial plasmas are generated by the application of electric or magnetic fields through a gas. The plasma produced for a laboratory setting and industrial use can generally be classified as:

The type of power source used to generate the plasma—DC, AC (typically with radio frequency (RF)), and microwave

The pressure they operate at—vacuum pressure (< 10 m Torr or 1 Pa), moderate pressure (≈1 Torr or 100 Pa), atmospheric pressure (760 Torr or 100 kPa)

The degree of ionization

Within the plasma—fully, partially, or weakly ionized

The temperature relationships within the plasma—thermal plasma (Te = Ti = Tgas ), non-thermal or "cold" plasma (Te greater than Ti = Tgas)

The electrode configuration used to generate the plasma

The magnetization of the particles within the plasma—magnetized (both ion and electrons are trapped in Larmor orbits by the magnetic field), partially magnetized (the electrons but not the ions are trapped by the magnetic field), non-magnetized (the magnetic field is too weak to trap the particles in orbits but may generate Lorentz forces)

Effect of plasma Treatment on a different types of fabrics

Cotton

Cotton is the most widely used natural fiber has gone through quite a bit of experimentation in terms of plasma treatment.  Atmospheric plasma has been used to remove PVA-sizing material from cotton fibers. It has been noted that in contrast with conventional treatment which requires hot water for effective removal of size, plasma-treated cotton could be completely rid of PVA sizing material with a simple cold water wash.  The scouring and dyeing behavior has been shown to improve with O2 plasma treatment under low-pressure conditions. Sun et al, investigated the effect of low-pressure plasma treatment on the dyeing of cotton fabrics.

Wool

In wool, the surface character of plasma treatment is limited to a small part of the fiber, in relation both to its mass and volume. Physical properties such as the fiber friction coefficient,   dye diffusion, etc. depend on the highly-networked layer known as the epicuticle. Modifying this layer, without causing changes in the wool cortex, facilitates and accelerates some technological processes, including the dyeing of fibers.  Dorota Biniaś worked on the effect of LTP on the dyeing process of woolen fabrics and found that LTP-treated samples proved to have bound the dye better than the unmodified sample. In addition, LTP damages an ultra-thin hydrophobic layer on the protective surface of the fiber. This process occurs only on the surface and does not damage the inner structure of keratin. Wool fibers were treated with  LTP  with different gases,  namely oxygen,  nitrogen, and a  mixture of gases  (25%  hydrogen,  75%  nitrogen), and it was shown that the surface composition of the  LTP-treated wool fiber was found to vary differently with different plasma gases.  The results have been similar to the previous reports, in which it was explained that the concentration of the functional groups influences the properties of the wool fiber.

The effects of using either radiofrequency or microwave power sources on the generation of plasma,  different plasma gases, and treatment times on the surface of wool fabrics and fibers were analyzed by means of  SEM. Ar,  O2, and air plasma drastically improved the wettability of wool-knitted fabric.  Additionally, plasma treatment brought about enhanced swelling of wool fibers. The swelling was the most pronounced in the case of oxygen plasma, whereas air plasma performed a swelling degree of the same order as a conventionally chlorinated sample.  Improved wettability and swelling of wool are attributed to plasma modification of the layer of covalently bound fatty acids known as the F-layer, which is mostly responsible for the natural hydrophobicity of the wool fiber surface. The dye fastness of the wool fabrics was improved significantly after plasma treatment.

Flax and Hemp

The observed increase in water retention and wetting rate of flax is attributed to better susceptibility of fiber to water molecules as a result of plasma etching and plasma oxidation i.e. the formation of new carboxyl groups.  The effect of low-temperature plasma (LTP) on hemp has been studied. Radetic studied the influence of low-temperature air plasma and enzymatic treatment on the dyeing properties of hemp. In addition to determining color dynamics, color yield, and color, SEM analysis was performed on weight loss assessments, water retention, and degree of whiteness, as well as samples treated differently. Low-temperature plasma treatment of hemp fabric caused an increase in the dyeing rate, final dye exhaustion, and color yield of dyed samples.  The positive effect of plasma treatment was explained by plasma etching and oxidation effect on the hemp fiber surface. This could be attributed to the more pronounced digestion of the amorphous areas of the fiber that became considerably more accessible to enzymes after plasma etching. It is likely that plasma etching increased fiber porosity and induced minor topographical changes that make hemp fiber more susceptible to dye and water molecules. Easier diffusion of dye into the fiber caused by plasma treatment is not sufficient for an increase in dye exhaustion as it is also considerably influenced by the structure, molecular weight, and state of dye in the dyeing bath.

Synthetic Fabrics/Fibres

Atmospheric pressure has been used to control water retention and pigmentation of DBD surfaces and to improve the capillary of synthetic fabrics. Polyester fabric is a system of micro and macro-perforated. Plasma contains various active particles (e.g. electrons, radicals, ions, photons, etc.). These species may attach to or react with the polyester (PES) surface, altering the nature of the functional group present, capillary-porous structure, and, consequently, surface properties as high-performance fibers; Unfortunately, they are prone to hydrolysis. Thus, the application of an isolated barrier to the surface should reduce the tendency to hydrolyze in the corresponding media. Hexafluoroethane/hydrogen plasma is highly suitable for applying this type of expansion-barrier layer on the surface. Resistance to 85% H2SO4 results in fibers remaining completely intact when conventional fluorocarbons terminate within a given condition resulting in significant shrinkage of the fibers in association with property loss.

When polyethylene terephthalate (PET) fibers are used as the application material for polyethylene (PE) matrix, the hydrophobization of PET fibers using ethylene plasma is quite impressive, as the adhesive strength can be increased from 1 to 2.5 N / mm. The fracture morphology of these composite materials clearly shows the tight adherence of the fiber matrix.

Polyester-cotton blended fabrics are treated with oxygen plasma. The effect of plasma treatment on the water absorption behavior of polyester-cotton blended fabrics was analyzed and the results indicate that the water absorption behavior was significantly improved.

Instead of the angle of contact of water, the oxygen/carbon ratio of the atomic composition of the surface can be used to follow the effect of plasma treatment, especially for layered-structured polypropylene burns. The oxygen/carbon ratio is highest for the first layer; However, a significant effect is also observed in the tenth level. To build soil resistance and improve pigmentation, poly (ethylene terephthalate) (PET) and polyamide (PAM) fabrics are treated in low-temperature plasmas. Five separate modification types were applied. Fabrics are treated directly with acrylic acid, water, air, and 2, and argon plasma. All types of plasma polymerization improve wettability and therefore soil color and soil resistance.

Nonwovens

In experiments at the UTK (the University of Tennessee at Knoxville's  ) Plasma  Sciences Laboratory on the surface energy,  wettability,  and wick ability of melt-blown polymeric fabrics exposed to inert gas OAUGDP  (One  Atmosphere  Uniform  Glow Discharge  Plasma)   plasmas  (helium and argon),  it was found that durations ranging from 30 seconds to 5  minutes were required to produce significant improvements to these characteristics. The nonwoven fabric Sentara, commonly used for surgical gowns, is treated with antimicrobial finishes and plasma containing fluorocarbon gas. Treated samples are evaluated for changes in physical and functional characteristics. Plasma treatment does not change weight, density, stiffness, air permeability, breaking strength, and elongation.

Plasma-treated and water-repellent Sentara samples show higher blood and water resistance compared to other treatments. Plasma-treated samples also show a zone of inhibition for Staphylococcus aureus, thus providing a barrier against microbes. There is no zone of inhibition for the water-repellent Sentara, untreated, and wet control samples. This suggests that nonwoven fabrics treated with plasma may provide a better barrier against germs than commonly available surgical gown fabrics with a fluorocarbon finish.

Silk

The properties, integration structure, and properties of Bombyx mori silk treated by low-temperature oxygen plasma were studied. The weight of B-Mori silk yarn decreased after low-temperature oxygen plasma treatment and decreased further as treatment time increased. It was a result of an etching by oxygen plasma. Slight flutes appeared on the surface of the treated fiber and fibrillary units became more evident in the section of treated fiber. The β-sheet conformation increased and crystallinity decreased after plasma treatment. Demura et al. studied the Bombyx mori silk fibroin fabrics treated with low-temperature plasma using various gases. After a short period of plasma treatment, the strength of the silk yarn changed somewhat. Plasma treatment for the improvement of the hydrophobic of the silk fabrics was investigated by Shen Li and Dai Jinjin and the results showed much improved hydrophobic properties.

Denim Finishing

Recently, low-pressure plasma and corona treatments for obtaining a worn look effect on denim fabrics are proposed. In a study by Radetic et al., a CIE Lab colorimetric system was used for the determination of the color difference between untreated and plasma-treated denim fabrics. The lightness difference between untreated and corona and low-pressure argon plasma-treated denim fabrics is demonstrated. Apparently, the higher the power and number of passes in the case of corona treatment, the higher the lightness.  Similarly, in the case of argon plasma treatment, the prolongation of treatment time and increase in power brought about an increase in fabric lightness. In both cases, under severe treatment conditions samples became more yellow, but yellowness disappeared after washing. The mechanical properties of the material were not changed after the plasma treatments. The decolorization of denim fabrics after plasma treatment in oxygen or argon was studied by Ghoranneviss et al. The results showed a better-decolorized output for Ar than O2 after a 15 min treatment. However, after washing the treated denim, the O2-treated samples looked well than those treated by Ar.

Environmental Impact of Plasma treatment on Textile

Plasma treatment became a negligible part of many industrial processes. Despite the obvious intrinsic effects and an environmentally friendly approach, it is still claimed as a potential or promising treatment in the textile industry. It might be due to the too traditional and rigid textile industry, which finds an excuse in expansive vacuum pumps required for plasma processing at low pressures. However, treatments at atmospheric pressures are available as well. The substantial shortcoming of plasma treatment of textiles is that it cannot replace all wet processes, but it can be a viable pretreatment, which provides plenty of environmental and economic benefits. Therefore, the textile industry should consider the concept of higher initial investments in equipment that will be paid off quickly with respect to environment-related savings and the profit of the sale of high added value products.

The environmental benefits of plasma treatment were-

a. the reduced amount of chemicals needed in conventional processing,

b. better exhaustion of chemicals from the bath,

c. reduced BOD/COD  of effluents,

d. shortening of the wet processing time,

e. the decrease in needed wet processing temperature, and

f. energy savings.

The comparative cost analysis of conventional chlorination and plasma processing of wool was worked out and they demonstrated that energy costs for chlorination are 7 kWh/kg wool whereas for low-pressure plasma treatment only 0.3-0.6 kWh/kg wool. The application of low-pressure plasma for the modification of 120 t/year of wool can save 27000 m of water, 44 t of sodium hypochlorite, 16 t of sodium bisulfite, 11t of sulphuric acid, and 685 MWh of electrical energy.

Conclusion

Plasma processing is a dry and environmentally friendly practice. It does not require immense supplies of water, heating, and drying, and only infinitesimal amounts of chemicals are necessary to reach the preferred functionality. Because the desired material behavior is accomplished by modifying only the surface of fibers, the bulk characteristics of the material, such as its mechanical strength, are unchanged. Further, plasma treatment allows achieving surface characteristics that are ahead of the reach in the field of textiles.