Low temperature air plasma can be used as an alternative to wet chemical fabric treatments, as a clean economic technology, to acquire wool/polyester blended fabrics the desired characteristics with the least pollution and energy, avers Dr Dalia Maamoun.

Blended fabrics of wool and polyester fibres are usually dyed with dyestuff mixtures with acid or metal complex dyes being used for the wool and the conventional disperse dyes for the polyester, ie, two classes of dyes, which are poorly compatible. To obtain a homogeneous shade on both substrates, it is necessary to use complicated dyestuff mixtures even for simple shades, since a single disperse dye almost never has exactly the shade of a specific wool dye. In addition, dyestuff mixtures can only be adjusted to a very specific mixture ratio of polyester/wool. If this ratio changes, differences in the depth of shade on both substrates result ^[1].

Over the past three decades, low temperature plasma technology has been the focus of much research for improving the surface properties of polymeric materials without changing the bulk properties ^[2]. Plasma surface modification does not require the use of water and chemicals, resulting in a more economical and ecological process. The enormous advantage of plasma is the drastic reduction in pollutants and a corresponding cost reduction for effluent treatments, so it can be considered as an environmentally benign technology ^[3]. In recent years, the study of dielectric barrier discharges (DBDs) has received much attention on account of numerous industrial applications. Dielectric barrier discharge (DBD) has been known for more than a century. It is highly transient, non-thermal discharge form, which exists in broad pressure ranges. Sometimes dielectric barrier discharges are also called silent discharges^[5].

In the present study, wool/polyester blended fabrics are treated with DBD prior to the printing process at different current values and exposure periods of time. According to economical and ecological demands, plasma is used as a clean technology and an alternative to wet chemical fabric treatment and pretreatment to modify the fabric and acquire its surface new properties and improve its printability. The fabrics are printed with two different dye mixtures and only urea is incorporated in the printing paste. The various parameters and measurements that involve the surface characteristics and printing properties of the prints are investigated in detail.

Experimental

Materials

Fabrics
The used fabric is a wool/polyester blend with a blending ratio of 45/55 and a weight of 154 g/m².

Dyestuffs
The following dyestuffs are selected and used throughout the present work and are kindly supplied and manufactured by Dystar Textilfarben, Germany: Realan Blue B/33 (a reactive vinyl sulphone dye) and Supralan Blue GLW (an acid milling dye), which are mixed separately with the Dianix Class Orange S-2R dye (an anthraquinone disperse dye) according to the blending ratio of the substrate.

Chemicals and Auxiliaries
The thickening agent used throughout this work is Monaprint P-MV, which is a depolymerised galactomannan (guar). It is kindly supplied and manufactured by Bayer, Germany.

Technical procedures

Fabric pretreatment with DBD
The wool/polyester blended fabric is exposed to atmospheric air plasma using dielectric barrier discharge (DBD) at different powers and times.

Plasma set-up
The experimental arrangement of DBD used in textile treatment is shown in the following figure. DBD cell consists of two electrodes of stainless steel disc, has a diameter of 25.5 cm and a thickness of 2 mm. The lower electrode is fixed to a Perspex base of 30 cm diameter and 2 cm thickness and connected to earth. The upper electrode is fixed to a Perspex disc of 30 cm diameter and 1 cm thickness and is connected to high voltage (HV) AC power supply of 50 Hz frequency and a variable voltage of 0 - 20 KV via a load resistance (R). A dielectric material of glass has a thickness of 1.7 mm is fasted to the upper electrode. The upper and lower Perspex discs are collected to each other via O-ring. The gap distance (d) between dielectric glass and the lower electrode is 3 mm. DBD treatment occurs completely in the atmospheric pressure of air ^[6].

Experimental setup of DBD

Printing technique
The printing pastes are prepared according to the following recipe after, which the substrate is printed using a manual silk screen, steamed at 103°C for 20 min and washed off at 60°C for 10 min:

20 g dye
80 g urea
700 g stock thickener (75 g/kg of thickening agent)
X cm³ water

Measurements and analysis

Colour strength
The colour strength of the printed specimens is evaluated by a light reflectance technique at maximum. The spectraphotometer used is of model ICS-Texicon Ltd, England.

Tensile mechanical testing

Tensile strength measurement is carried out using a Textile Tensile Strength tester No: 6202, 1987, type: Asano Machine MFG, Japan.

Wettability
The Wettability is evaluated by measuring the wetting time according to the AATCC method ^[7].

Scanning Electron Microscope (SEM)
The untreated and treated samples with plasma are investigated by a Scanning Electron Microscope (SEM) Philips XL 30 attached with EDX unit; with accelerating voltage of 30 KV, magnifications range of 1500-2000x and a resolution of 200 A. Before examinations, the fabric surface was prepared on an appropriate disk and coated randomly by a spray of gold.

Fastness properties
Fastness properties of wool print to rubbing, washing, perspiration and light are assessed according to standard methods ^[8].

Results and discussion

Effect of printing paste pH on the K/S values of wool/polyester prints
The pH control has received a considerable attention in dyeing and printing processes because of its critical role in quality assurance^[9]. Different printing paste pH values are used to investigate the influence of pH on the colour yield of wool/polyester blended fabric and the K/S results are represented in Figure (1). It is clearly noticed from the data that, best K/S results for the blended substrate may be obtained on applying pH 7 as it gave relatively high K/S results on printing the wool/polyester blended fabric using both Realan/Dianix and Supralan/Dianix dye mixtures.

Effect of urea concentration on the K/S values of wool/polyester prints
The action of urea in printing wool fabrics may be referred to the nature of both, the substrate and the used dye. Urea enhances the solubility of dyes in the printing paste due to its salvation and disaggregating action on dye molecules ^[10]. This action varies from one dye to another according to its ability to dissolve in the printing paste. Therefore, the hydrophobic/hydrophilic balance of the dye molecule will determine its ability to dissolve under the action of urea. Hydrophobic dyes such as disperse dyes are not affected by urea addition as the more hydrophilic dyes. Thus, increasing the hydrophobic character of the used acid or disperse dye may diminish the solvolysis effect of urea and reduces its role in the printing paste.

It is well noticed from the Figure 2 that, best K/S results may be obtained on adding urea to the printing pastes by a concentration of 80 g/kg for the wool/polyester blend printed with Realan/Dianix and Supralan/Dianix dye mixtures for both the untreated and pretreated samples with air plasma. For the blend printed with Realan/Dianix mixture, adding 80 g/kg urea to the printing paste caused increases in K/S results by 142% and 140% for the untreated and plasma treated fabrics, respectively, while increases by 89% and 67% are achieved, for the untreated and plasma treated samples respectively on printing with the Supralan/Dianix dye mixture all compared without adding urea to the printing pastes.

Effect of air plasma discharge current and exposure time on the K/S values of wool/polyester prints
The influence of air plasma discharge current and exposure time on the colour strength of the wool/polyester blended fabric printed using the two different dye mixtures is investigated through using different discharge powers and exposure periods of time. The K/S data are formulated in Figures 3 to 6 for 45/55 blend for both dye mixtures.

It is clearly observed from the data that, exposing the blended samples, to air plasma at lower discharge powers than 2 mA does not cause any improvement in K/S values. It is noticed that from the data that, best pretreatment conditions of the fabrics are at a discharge power of 2.5 mA and an exposure time of 3 min since they resulted in enhancing the K/S values by 173.6 and 68.9% for printing with both dye mixtures Realan/Dianix and Supralan/Dianix, respectively when compared with the untreated printed sample.

It can be concluded from the previous data that low temperature plasma ablation attack the chains on the crystalline surface and amorphous region and cause a significant change in fibre crystallinity ^[11]. For certain phenomena in which, the mobility of molecules in a noncrystalline phase plays a dominant role, such as dye absorption, the noncrystalline phase can no longer be treated as amorphous phase. The dyeability ^{[12, 13, 14]} study has shown that, there exist at least dyeable and nondyeable domains (areas) within the noncrystalline phase. From the view point of fibre dyeability, the nondyeable amorphous area consists of polymer segments that significantly restrict the mobility of molecules. Such a domain might be visualised (imagined) by analogy to a liquid crystalline phase ^[15].

Effect of steaming time on the K/S values of wool/polyester prints
Figure (7) represents the influence of steaming time on the K/S values of wool/polyester plasma pretreated printed fabrics using the two dye mixtures. Steaming is carried out using saturated steam, which is known to increase colour strength of the prints due to its swelling effect on wool substrate, since moisture reduces the binding forces between polymer chains of wool leading to swelling, which increases by increasing temperature. Also, steam condenses onto the fabric raising its temperature to 100°C, which swells the thickener film printed on polyester resulting in the absorption (build-up) of the disperse dye ^[16,17].

It can be concluded from the data that, best K/S results may be obtained on steaming the plasma treated printed wool/polyester blend with saturated steam for 20 min, which proves to bring about best dye fixation regardless of the kind of dye.

Effect of air plasma treatment on tensile strength of wool/polyester blended fabrics
Low temperature plasma can modify the surface of a polymer substrate by physical and chemical changes, eg, etching, grafting, cross linking, etc ^[18]. The influence of air plasma pretreatment on the tensile strength of wool/polyester substrates of both blending ratios is studied using different discharge powers as well as exposing times. Figure (8) represents tensile strength resulting data for the previous factor on the wool/polyester substrate. It can be concluded from Figure (8) that air plasma treatment has a negative influence on the tensile strength of the substrate, which can be observed more in prolonged plasma treatments since it caused decreases by 11.8, 23.5 and 9.8% of fabrics strength due to plasma treatment for 10 min at 0.6, 1.2 and 2.5 mA respectively, compared to the untreated sample.

Effect of air plasma treatment wettability of wool/polyester blended fabrics
The influence of air plasma pretreatment on the wettability (expressed as wetting time) of the two wool/polyester blended substrates for different levels of discharge powers and exposing times is investigated and the results are plotted in Figure 10. It is clearly observed from the data that longer air plasma treatment of fabrics shows significantly shorter wetting time and lead to a higher wettability compared to the untreated samples. This is may be explained by the fact that, longer plasma exposure times may induce more hydrophilic functional groups on the fabric surface due to longer duration of the chemical interaction of plasma and substrate ^[19], which increases the surface free energy of the fibre and increases the contact angle ^[20]. This is clearly proved as the wetting time decreases sharply from 6 min to 1 sec for the wool/polyester blended fabric.

Effect of air plasma treatment on wool/polyester morphology
The effect of air plasma treatment on the morphological changes of the wool/polyester blended fabric is studied using Scanning Electron Microscope (SEM) and the obtained micrographs are illustrated in Figure 11. It is clearly demonstrated in the figure that, the untreated polyester fibres show a relatively smooth surface with some grooves (presumably on artifact of the fibre production processes) also, wool fibre scales appear in their natural appearance. It may be concluded from the micrographs that air plasma treatment exhibits significant surface morphological changes. These changes in the surface morphology correlate well with the discharge power and time of treatment. Increased surface roughness can be produced by the etching effect of plasma active species bombardment of the fibre surface ^[21,22].

Effect of air plasma treatment on the fastness properties of wool/polyester prints
The durability of printing of the wool/polyester blended fabric with different dye mixtures, pretreated with air plasma at optimum conditions of discharge current and time is evaluated in terms of fastness towards washing, rubbing, perspiration and light and the measurements obtained are plotted in Tables 1 & 2. It can be concluded from the Tables that air plasma pretreated prints display a slightly lower level of fastness properties also, and the prints show a fair fastness towards rubbing, especially wet rubbing. This action is believed to be due to choosing an inappropriate pH printing medium, which should be slightly acidic to ensure obtaining better fastness levels.

Table 1: Effect of plasma treatment on the fastness properties of wool/polyester blended fabrics printed with Realan/Dianix dye mixture

Table 2: Effect of plasma treatment on the fastness properties of 45/55 wool/polyester blended fabrics printed with Supralan/Dianix dye mixture

Conclusion

The present study clearly demonstrates that pretreating wool/polyester 45/55 blended substrate with air plasma can enhance the K/S values of the printed samples (using two dye mixtures) by adding 80 g/kg urea to the printing pastes at pH 7 using a current of 2.5 mA for 3 min. Exposure of materials to suitable plasma treatment can cause both chemical and physical changes on the surface layers so as to provide a more reactive surface without interfering with the bulk properties simply because of the shallow depth of penetration. Hence, a thin surface layer can be formed by means of surface bombardment with ions, electrons and other high energy particles, which knock polymer material out of the surface.

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Note: For detailed version of this article please refer the print version of The Indian Textile Journal June 2012 issue.

Dr Dalia Maamoun,
Textile Printing, Dyeing and Finishing Department,
Faculty of Applied Arts,
Helwan University,
Cairo, Egypt.
Email: daliamaamoun@gmail.com.