Plasma Technology in Textile: A step towards the green environment (Part-3)
M. Tech. Scholar
Department of Textile Technology,
NIT Jalandhar, India
Cell: +91- 7837-696041
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3. Effect of plasma on textile surface:
There are five major effects of which three will be described in detail: fine cleaning, surface activation, and etching, cross-linking and coating deposition.
3.1. Surface activation by plasma:
Surface activation by plasma is also referred to as chemical grafting (Terlingen, 1993). It never occurs alone, but always occurs during/after plasma cleaning. Indeed, in the case of a substrate subjected to a soft secondary plasma which contains reactive species (e.g. oxygen atoms), the effect of those atoms will be twofold: they will react with organic contamination which is present on the substrate surface. Such organic contamination consists, in many cases, of loosely bound hydrocarbons. Both H and C will react with oxygen and will leave the substrate surface in the form of volatile H2O and CO2. Once the surface molecules of a polymer are freed from contamination, they can react with the oxygen atoms which will form carbonyl-, carboxyl- or hydroxyl functional groups on the substrate surface. It is said that the polymer surface has been chemically functionalised.
The effect of grafting carbonyl-groups onto a surface of PP, polyethylene (PE), or polyesters such as polyethyleneterephthalate (PET) or polybutyleneterephthalate (PBT) gives rise to an increase in surface energy to levels higher than 68 mN/m immediately after the plasma treatment. This effect is, however, not permanent: it has a certain shelf-life. Once the substrate has been removed from the plasma, and depending on the storage conditions, oxygen atoms will be released again from the surface molecules. This will happen slowly over time. After several days or even several months, the original surface energy of the substrate will have returned. The rate at which this happens depends on the type of substrate: e.g. PP has a fairly good shelf-life of a couple of weeks, whereas silicones show a shelf-life of less than one day. It further depends on the plasma conditions: an intensive plasma treatment will create a higher surface density of functional groups and, as such, the shelf-life will be longer.
Plasma activation is being used in several fabric and nonwoven applications in the textile industry:
- Fabrics for automotive and medical applications
- Pre-treatment before dyeing
- Activation of transportation textile before application of flame-retardant chemistry
In order to perform an efficient etching process, a direct plasma is normally needed. In such a configuration, the substrate is bombarded with charged particles (ions and electrons) and apart from a purely chemical effect; the substrate is subjected also to a physical sputtering effect. In the case of textiles and nonwovens, this effect of plasma treatment is not often used.
However, there is a certain potential even for fabrics. The textile market is trying to make deep, dark colours and this is not easy to achieve. One way to do this is to reduce the specular component of reflection of the fabric surface after dyeing. A plasma etching leads to a controlled Nano- or micro-roughness, increasing diffuse reflectance and minimising the specular component. In consequence, the dyed fabric will have an intenser darker colour after plasma etching.
Etching requires the removal of several hundreds of nanometres and etching processes are therefore slow. Needless to say, this technique is only viable for very high-end textiles.
3.3. Thin film deposition by plasma polymerisation:A very important usage of low-pressure vacuum plasma technology is thin film coating deposition by plasma polymerisation. In this specific case, reactive precursor gases that can polymerise are being used as process gases (Yasuda, 1976). The precursor gases are broken into radicals that react with each other on the substrate surface. The nature of the precursor gases will very much determine the properties of the deposited coating. Coating thickness is normally in the 10–50 nm range (5–30 molecular layers).
The very first applications of plasma polymerisation were found in the medical device industry. There are many industrial applications of thin film deposition by plasma polymerisation in the technical textile and nonwoven industry. Roughly, the coatings deposited in those industries can be categorised under either (permanently) hydrophilic coatings or hydrophobic/oleo phobic coatings. In most cases, the deposited coatings give rise to unique products that are difficult or even impossible to produce using other technologies.
Application of deposition by plasma on textile:
- Hydrophobation of nonwovens for filtration applications
- Hydrophilic coatings on nonwoven PP for battery separators
Due to high restriction in the control of chemical processing of textile materials, the new and innovative textile treatments are demanded. In this regard, plasma technology shows distinct merits due to its environmental friendly and better treatment results.
Various eras where this technology can be explored includes pre-treatments, other wet processes of textiles, technical textile and non-woven. Plasma can modify the surface properties of textile materials, deposit chemical materials (Plasma polymerisation) to add up functionality, or remove substances (plasma etching) from the textile materials and used to produce innovative functional textiles.
4.1. Desizing of cotton fabric:
Plasma technology can be used to remove PVA sizing material from cotton fibres. In conventional desizing process we use chemicals and hot water to remove size. But desizing with plasma technology we can use either O2/He plasma or Air/He plasma. Firstly the treatment breaks down the chains of PVA making them smaller and more soluble. X-ray photoelectron microscopy results reveal that plasma treatment introduces oxygen and nitrogen groups on the surface of PVA which owing to greater polarity increase the solubility of PVA.
Of the two gas mixtures that were studied, the results also indicate that O2/He plasma has a greater effect on PVA surface chemical changes than Air/He plasma.
Several studies have shown that dye ability or printability of textiles can be markedly improved by plasma treatments. This effect can be obtained on both synthetic and natural fibres. Capillarity improvement, enhancement of surface area, reduction of external crystallinity, creation of reactive sites on the fibres and many other actions can contribute to the final effect depending on the operative conditions. Also production of colours on fibres exploiting diffraction effects has been attempted.
4.2.1. Dyeability of Natural Fibres:
It has been reported that plasma treatment on cotton in presence of air or argon gas increases its water absorbency which in turn increase both the rate of dyeing and the direct dye uptake in the absence of electrolyte in the dye bath.
The contributory factors leading to this increase in dye uptake can be: i. The change of the fabric surface area per unit volume due to the surface erosion. ii. The etching effect of the plasma effect on the fibred mages the fiber surface and also removes surface fiber impurities (e.g. cotton wax or any remaining warp size, etc.). iii. The chemical changes in the cotton fiber surface (leading to carbonyl and carboxyl groups in the fiber. iv. The possibility of the formation of free radicals on the cellulosic chains of cotton. v. Thus the action of oxygen and air plasma treatments modifies the surface properties of cotton and leads to an increase in the rate and extent of uptake of direct dye.
The dye exhaustion rate of plasma treated wool has been shown to increase by nearly 50%. It has been shown that O2 plasma treatment increases the wetability of wool fabric thus leading to a dramatic increase in its wicking properties. Also the disulphide linkages in the exocuticle layer oxidize to form sulphonate groups (which are act as active sites for reactive dyes) which also add to the wetability. The etching of the hydrophobic epicuticle and increase in surface area also contributes towards the improvement in the ability of the fibers to wet more easily.
4.2.2. Dyeability of Synthetic Fibres:
In the synthetic fibres, plasma causes etching of the fibre and the introduction of polar groups leading to improvement in dyeability. This has been evaluated through in situ polymerization of acrylic acid in case of polyester, polyamide and polypropylene fabrics. Plasma-induced surface modification of microdenier polyester produces cationic dyeable polyester fiber. The researchers believe that this technique can lead to a continuous flow system, low energy consumption, and more environmentally friendly consumption, low temperature dyeing technology on polyester substrates.
Polyamide (nylon6) fabrics have been treated with tetrafluoromethane low temperature plasma and then dyed with commercially available acid and dispersed dyes. Dyeing results showed that the plasma treatment slows down the rate of exhaustion but does not reduce the amount of absorption of acid dyes. The dyeing properties of disperse dyes on plasma treated nylon fabric charged markedly when compared with untreated fabric. A slight improvement in colorfastness was seen with the treated sample.