Polyamide 6.6 Modified by DBD Plasma Treatment for Anionic Dyeing Processes

Plasmatic double barrier discharge (DBD) obtained in air at atmospheric conditions is widely used, among other non-thermal plasmatic alternatives, to modify chemical and physical properties of different textile polymers (Morent et al., 2007). The impacts of DBD on environmental aspects of textile processing rise to get high attention due to important reduction of costs in dyeing by savings in processing times, products, human resources, water and energy (Carneiro et al., 2001). All fibers, from natural to synthetics, can be submitted to several irradiation methods with diverse and significant meaning in different areas of textile processing (Sparavigna, 2001). The effects on surface are reported for cellulosic fibers (Carneiro et al., 2005; Souto et al., 1996), wool (Rakowski, 1992), polyester (Oktem et al., 2000, Leurox et al., 2009), polyamide 6.6 (Papas et al., 2006; Oliveira et al., 2009), polyamide 6 (Dumitrasku & Borcia, 2006), polytetrafluoroethylene (Liu et al., 2004), polyethylene (Oosterom et al., 2006), polypropylene (Yaman et al., 2009) and meta aramid (Chen et al., 2008), being roughness, microporosity and creation of polarity by oxidation mechanisms the main modifications induced by several types of irradiation techniques. Acid dyes are the most common in use for polyamide dyeing, but some problems are very well known, as difficulties to manage uniformity and fastness. The necessary pH to achieve a good exhaustion of dye in the fiber must be carefully controlled and sometimes is excessively low. Reactive dyes are very important for the dyeing of cellulosic and protein fibers, but in polyamide the results are not equivalent due to paler colors obtained (Soleimani et al., 2006). Reactive dyes for cellulose are similar to acid dyes in their chromophoric structure, but they possess reactive groups able to react chemically with the fiber in the presence of alkali. Only few of these dyes have been developed for polyamide application with ability to react with amino groups in fiber structure without the need of alkaline fixation. Stanalan (Dystar) and Eriofast (Ciba) are well known dyes for this purpose. Reactive dyes for cotton fibers, Procion (Dystar), Kayacelon (Nippon Kayaku), and Drimarene (Clariant) were tested for polyamide dyeing at boiling temperature and different pH showing distinct results. At pH 4, the most convenient result was obtained due to a high protonation of nucleophilic amino groups, contributing to electrostatic attraction between anionic dye and positively charged fiber (Soleimani et al., 2006).


Plasma treatment 2.2.1 DBD plasma machine
A double barrier discharge was produced in a semi-industrial machine (Softal/University of Minho) functioning with air at normal temperature and pressure, using a system of ceramic electrode and counter electrode with 50 cm effective width, and producing the discharge at high voltage and low frequency.

Plasma dosage
The power of discharge, velocity, number of passages of the fabric between electrodes were variable corresponding to calculated discharge dosages from 400 to 3600 W.min.m -2 ( Table 1). The dosage was calculated according to the following equation (Eq.1): Equation 1. Plasma dosage determination, where W = power (Watts); P = number of passages; V = velocity (m.min -1 ) and L = width of treatment (0.5m).

Characterization of DBD treated fabrics and structural analysis 2.3.1 Water drop absorption test
In order to evaluate the wettability of the untreated and of the plasmatically modified polyamide woven fabrics, a water-drop test was applied by measuring the time for its complete absorption into the material. This test is performed in order to check the efficiency of the plasmatic treatment in function of the experimented dosages.

Contact angle measurement
Dataphysics equipment using OCA software with video system for the capturing of images in static and dynamic modes was used for the measurement of contact angles of the water drops in polyamide 6.6 fabrics.

X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy analysis provides information about changes in chemical composition (elemental analysis) and chemical state (wave separation method) of atom types on the fiber surface. The VG Scientific ESCALAB 200A equipment was used to obtain a more detailed and complete analysis.

Scanning electron microscopy
Morphological modifications in samples taken from the polyamide 6.6 fabric were analysed with high resolution environmental scanning electronic microscope Schottky FEI Quanta 400FEG / EDAX Genesis X4M. The sample was mounted in aluminum specimen stubs and coated with a layer of gold.

Atomic force microscopy
The morphological and topographical characteristics of the polyamide surface before and after DBD plasma treatment were investigated, in a multimode SPM microscope controlled by a Nanoscope III -from Digital Instruments, equipped with an ultra light cantilever with 125 μm long by 30 μm large. The tips were silicon NHC type, with a resonance frequency from 280 to 365 kHz.

Dyeing methods
The dyeing properties were investigated using a dye-bath exhaustion process. The schemes of dyeing processes are shown in figure 2. Various dyeing tests were performed with differents parameters such as temperature, dye concentration (% w/w) and pH, in order to optimise dyeing method and conditions are presented in Table 2. Dyeings were carried out in a laboratorial "Ibelus" machine equipped with infra-red heating and the SIMCORT software was used for continuously assess dye exhaustion from the bath. The samples (each 5.0 g) were dyed with a liquor ratio of 40:1, using stainless steel dyepots with 200 cm 3 capacity each.

Fluorescence microscopy
The level of dye penetration into fibers was visualised by fluorescence microscopy on polyamide transversal cuts. The fibers were embedded into an epoxy resin and transversal cuts of the fibers with 15 mm were prepared using a microtome (Microtome Leitz). Fibers' cross sections were analyzed by a fluorescence microscope LEICA DM 5000B at 40x magnification.

Optimal plasma dosage
The color depth of dyed polyamide 6.6 was increased by using a reactive dye for wool dyeing (Lanasol Blue 3G), a reactive dye for cotton dyeing (Remazol Yellow Golden RNL) and a direct dye (Sirius Scarlet KCF), when different dosages of plasmatic discharge are used varying power of discharge and number of passages ( Figure 3). The K/S values in polyamide samples depend on plasma conditions, being more intense for higher dosages. This criterion was used to choose optimal plasma dosage. According to the results obtained, the dosage of 2400 W.min.m -2 is enough to obtain maximum values of K/S and minimum quantity of dye in residual bath for the three dyes, with the following working parameters in DBD machine: power of discharge -1500 W; speed -2,5 m.min -1 ; nº of passages -2. This dosage was further applied to all polyamide samples submitted to dyeing. Table 3 shows the difference between the wettability of the untreated and the plasmatically modified polyamide woven fabrics by measuring the time for complete water absorption into the material. CV(%) 6.3 5.5 Table 3. Wettability results

Contact angle measurement
The hydrophilicity of the polyamide fabric is highly improved by the plasmatic treatment in accordance with the results published by several authors for different synthetic and natural fibers, mentioning modifications in accessible polar groups at the surface and creation of microporosity (Pappas et al., 2006, Oliveira et al., 2009. This surface modification might transform the synthetic fiber from hydrophobic to hydrophilic which is key point for the absorption of aqueous dye solutions. The static and dynamic contact angle evaluation of a water droplet in the textile polyamide fabric is shown in figures 5, 6 and 7, corresponding to a mean value of five measurements.

a) b)
www.intechopen.com After DBD treatment, the wettability of the polyamide fabric considerably increases. This result may be attributed to the incorporation of polar groups onto the fabric surface.

X-ray photoelectron spectroscopy
The XPS analysis shows that the oxygen and nitrogen content level was increased after DBD treatment. This indicates a substantial incorporation of these atoms onto the fabric surface. Chemical states of atoms, represented by relative peak areas, can be obtained by wave separation method. The carbon component can be divided into peaks (280.5 to 294.0 eV), assigned as CH 2 , CH 2 CO, CH 2 NH and NHCO (Pappas et al., 2002). It can be observed that the relative peak areas of sub-components change significantly after DBD treatment.
Results reveal that the element C1s decreases while both N1s and O1s increase. DBD treatment can be responsible for the breaking of the long chain molecules of polyamide 6.6, causing an increase of carboxyl and amine end groups. The table 4 shows the elementary composition and the atomic ratio before and after plasma treatment.  Table 4. XPS results in samples with and without DBD treatment The figure 8 shows the increase of O 1s and N 1s atoms and consequently decrease of C 1s when DBD treatment is applied, which corresponds to an enhancement of hydrophilicity of fabrics due to an increase of polar groups in the surface of polyamide fabrics.

Scanning electron microscopy
The surface modification by DBD plasma on polyamide fabrics was detected by the scanning electron micrographs (Figure 9). Compared to untreated polyamide 6.6, the DBD plasma treated polyamide 6.6 presents small patches on the surface responsible for an increase of surface area. The comparison of the images obtained with and without DBD plasma treatment, shows a somewhat different surface morphology.

Atomic force microscopy
The increase in roughness can be better understood when AFM images are compared ( Figure  10). The following results were obtained for Ra (arithmetic average roughness ), Rq (root mean squared roughness) and Rmax (Table 5), regarding untreated and plasma treated samples.

Samples
Ra ( The surface of the sample without treatment is relatively smooth while the treated polyamide has rougher surfaces. The AFM analysis shows that the Ra, Rq and Rmax roughness increases with the DBD plasma treatment and the modification of the shape of surface features is quite evident. The applications of these results illustrate, for example, the increasing of wettability (as showed by contact angle measurements) and consequently polyamide dyeability properties.

Berger whiteness
The figure 11 shows the results obtained for Berger whiteness when different plasma dosages were applied to the polyamide fabric. Whiteness of polyamide fabric slightly decreases (2.8 degrees for a dosage of 3600 W.min.m -2 ) for higher dosages, although without noticeable effect in product characteristics.

Dyeing properties
Results show the effect of the plasma discharge on dyeing behavior of polyamide 6.6 with direct dyes, reactive dyes for cotton and wool and acid dyes. The surface modification of the polyamide fiber after DBD plasma treatment permits very intense and fast colors with dye exhaustion almost reaching 100% in every case. Fig. 12. Exhaustion results for a) direct dye, b) reactive dye for cotton, c) reactive dye for wool and d) acid dye in polyamide 6.6 samples with and without DBD treatment (dyeing conditions: 100ºC and 1% dye concentration).
The complete bath exhaustion was obtained in very short time during dyeing processes with remarkable positive difference when a plasmatic treatment is made on polyamide fabrics ( Figure 12). Forty minutes are enough to exhaust dye from dyebath which is a very attractive behavior regarding industrial application. The direct and acid dyes are anionic and water soluble and the increase of fiber hydrophilicity with plasmatic treatment is favorable to dye adsorption, essential to promote coulombic dye fixation to protonated amine groups of the polyamide. Reactive dyes are also water soluble and anionic, apart being able to form covalent bonding with reactive groups in the fiber, namely the amine groups of polyamide. The same factors promoting adsorption and diffusion of dye in the fiber for the acid dyes are present for reactive dyes, explaining complete exhaustion of dye in the fiber after forty minutes of dyeing. The table 6 shows the comparative results of K/S with reactive dyes, direct dyes and acid dyes in polyamide 6.6 with and without plasma (DBD) treatment. The results demonstrate www.intechopen.com that in dyeings carried with these anionic dyes, the color strength (K/S) is considerably higher for the fabric with DBD treatment, quantified by means of the percentage gain of the treated sample when compared to the non treated one.  Table 6. K/S values for dyeings of polyamide 6.6 fabric with and without DBD treatment

Dye Class
The gain in color yield obtained with DBD treatment is effective for all the dyes, although results are quite variable for the different colors of each commercial dye.

The influence of temperature in dyeing processes
The figure 13 shows the influence of the temperature in polyamide dyeing process with the following dyes: Levafix Brilliant Red E-BA (reactive dye for cotton), Sirius Orange 3GDL (direct dye,) Realan Blue EHF (reactive dye for wool) and Telon Blue M-GLW (acid dye). Comparisons were made between the samples with and without plasmatic treatment.
The increase in temperature of all dyeing processes ( Figure 13) leads to an increase of the color yield (K/S values) of the polyamide dyed samples. Nevertheless, it is evident that dye absorption mechanism is quite influenced by plasmatic treatment. In fact, a linear behavior of K/S with temperature is much more present when the fiber is plasma discharged, demonstrating that structural transitions are not the main driving force for dye penetration in polyamide.
Meanwhile it is quite clear that the untreated fiber is highly dependent on temperature to achieve high K/S values, being noticeable that structural changes may occur at temperature around 90ºC. Therefore, the reduction in energy demand can be considerable in the dyeing process when plasmatic pre treatment is made compromising the same color yield (K/S) at much lower temperatures. Fig. 13. K/S values of reactive dye for cotton (a), direct dye (b), reactive dye for wool (c) and acid dye (d) in polyamide 6.6 with different temperatures (dye concentration: 1% dye weight/fiber weight ).

The influence of dye concentration in dyeing processes
The figure 14 a, b, c and d shows the influence of the dye concentration in the polyamide dyeing.
Since polyamide has only a small number of amine end-groups, saturation is easily got and it is difficult to achieve darker colors by dyeing with anionic dyes Perkins, 1996). Figure 14 shows a considerable increase of color yield with the increase of dye concentration being that the total amount of dye in the fiber is always higher after DBD treatment. Darker colors in the fabric of polyamide 6.6 can be achieved when the plasmatic treatment is applied, with less dye concentration, meaning that is now possible to dye polyamide materials in darker colors by adopting a much more economic process. The form of the curves concerning polyamide dyeing with the reactive dye for cotton, direct dye and acid dye shows a much higher but limited saturation in plasma treated fabric, whenever the behavior of the reactive dye for wool demonstrates the formation of higher level of linkage groups in the plasmatically treated polyamide.

The influence of pH in dyeing processes
The figure 15 a, b, c and d shows the influence of the pH in the polyamide dyeing process, comparing results between the samples with and without plasmatic treatment. When acid is added to dye bath, the polyamide fiber develops an overall positive charge (-NH 3 + ). Thus, in acidic conditions the polyamide fiber becomes positively charged and strongly attracts the negative groups of the anionic dyes. At pH 3 all the studied dyes give similar dyeing yield, either plasma treated or not treated polyamide. However, at pH 3 the polyamide 6.6 fabrics can be degraded, which is very inconvenient for the final quality of dyed materials (Burkinshaw, 1995). When pH of the dye bath is increased, the color yield reduces considerably in the samples without DBD treatment. On the other hand, if samples are treated with plasma and dyed, a type of "buffer systems" can be observed for all the dyeings, propitiating a very important stabilization during the dyeing process in case of pH variations. Dyeing results are pH independent in the interval 3 to 7, which gives strong indications about the huge influence of plasmatic discharge in chemical composition of fiber surface. Table 7 shows the results of washing fastness for reactive (wool, cotton) and acid dyes in dyeing of polyamide 6.6. The results of washing fastness at 40 ºC are very good confirming the level of dye diffusion and fixation into the fiber. The surface modification of the polyamide fiber after DBD plasma treatment permits to obtain very fast colors, with the best result for washing fastness.

Washing and rubbing fastnesses
The results of rubbing fastness in dyed fabrics with and without treatment are very good. The value 5 in the gray scale was obtained for all the samples.

Sample AC CO PA PES PAC WO Color Change
Procion   Figure 16 shows the results obtained from fluorescence microscopy analysis in the case of polyamide dyed with reactive dyes for cotton (Levafix Red -EBA). The dye shows a perypherical distribution in the sample without treatment (c), while in the case of the plasma treated sample the dye presents a deeper diffusion into the core of the fiber (d).

Fluorescence microscopy
www.intechopen.com These results are very promising, because after the plasma treatment the dye is able to penetrate into the fiber so giving high guaranty of good fastness results.

Dyeing process optimization
The dyeing process of polyamide 6.6 is usually performed at boiling temperature and the time required for this process is around 120 minutes ( Figure 2a). However, after DBD plasma treatment (2400 W.min.m -2 ) a new process will be checked with lower temperature and time than those generally used in the polyamide dyeing. This process will start at 40ºC, the temperature is raised until 70°C with a gradient of 1ºC/min and remains in this temperature during 60 minutes.
The maximum values of exhaustion for samples with and without treatment (traditional process 100ºC) and for the samples dyed by optimized dyeing process at 70ºC are shown in table 8. In this optimized dyeing process, the temperature and dyeing time were reduced of 30°C and 25% respectively. The best results are obtained when DBD treatment was applied. The reduction of bath exhaustion, when compared the process (b) at 100ºC with process (c) at 70ºC were: 0.5% for the direct dye Sirius 3GDL, 2.6% for the acid dye Telon MGLW, 3.8% for Levafix Red EBA and 4.1% for Red dye Levafix EBA. This way, it is possible to dye polyamide 6.6, treated with DBD, at 70ºC with an excellent bath exhaustion. The figure 17 shows the bath exhaustion behavior of the reactive dye Levafix Red, for the samples prepared with DBD discharge (b, c) and dyed at 100ºC and 70ºC respectively, compared with the untreated sample conventionally dyed at 100ºC (a). A smoothest curve and excellent bath exhaustion of 93% are observed for the optimized process performed at 70ºC.  Table 9 shows the results of washing fastness for direct, acid and reactive dyes for the optimized dyeing of polyamide 6.6. The results of washing fastness are very good, confirming the level of dye fixation and diffusion into the fiber, despite of this process had been performed at 70ºC. The optimized process performed at lower temperature and time than conventional dyeing of untreated polyamide did not change the washing fastness results. The results of rubbing fastness in dyed fabrics with and without treatment are very good. The value of 4/5 or 5 in the gray scale was obtained for all the samples.

Cost of DBD plasma treatment
The DBD treatment used the following parameters: power -1500 W, number of passage -2, velocity -2.5 m.min -1 . The cost to treat one kilogram of polyamide 6.6 fabric (weight : 105 g.m -2 ) is calculated according to Despite the cost of the treatment DBD is around 0.036 €.kg -1 , several benefits are achieved such as: reduction of 30ºC in the temperature of dyeing with an excellent bath exhaustion; reduction of dyeing time of around 25%; reduction of the quantities of dyeing auxiliaries and the possibility to get more intense colors.

Conclusion
A relatively low plasma dosage (DBD), around 2400 W.min.m -2 , can be used to modify the surface of polyamide textile materials, leading to enhanced hydrophilicity and dyeability. Due to these modifications, dyeing of polyamide 6.6 fiber is now possible using nonconventional dyes for this fiber, such as direct and reactive dyes for cotton and wool. The results suggest that the change in exhaustion and dyeing yield in different dyeing conditions closely correspond with the roughness´s creation and the changes in chemical oxidative properties induced by DBD treatment in polyamide fabrics. These results reflect directly on dyeing ability of the fiber providing more terminal groups to make dye bonding.
The application of anionic dyes in discharged polyamide 6.6 shows extensive improvement of dye exhaustion from baths easily achieving 100% in shorter dyeing times. The kinetics of dyeing in every case is quicker, but leveled results are obtained. It should be noted that deeper dyeing is a great advantage for all anionic dyes since darker shades are obtainable using less amount of dyestuffs. Using plasmatic treatment in polyamide substrates it is possible to obtain decisive energetic gains by dyeing at lower temperatures, to have quite pH independent processes similar to a buffer effect and to reduce dyes and auxiliaries with deeper colors and less pollutant charges. The DBD plasma treatment has an high industrial potential, because it is an environmental friendly dry process. Including a plasma treatment in the processing of the substrate, dyeing properties obtained by using anionic dyes are improved, namely yield, dyebath exhaustion, washing and rubbing fastness, providing a cheap, clean and high quality option for the dyeing of polyamide materials. www.intechopen.com