EVALUATION OF POLYAMIDE FIBER DYEING USING ALCOHOLIC SOLUTIONS

Purpose: This research aims to evaluate the effect of alcoholic solutions in the reduction of the dyeing temperature process and its effects on the properties of polyamide fiber. Theoretical framework: The textile industry is known for its high water demand and energy consumption. Fiber dyeing generally contributes to high energy demand and is a key process in the textile industry. Method: Solutions of ethanol, butanol, and phenylmethanol were used. Alcohol concentration during dyeing was set at 50 g/l, while the final dyeing temperature was 50°C. The samples obtained in these processes were compared with standard sample dyed at 5°C and 100°C. Results and conclusion: The results showed that the K/S value for the standard sample was 31.4. The use of ethanol and butanol reduced the K/S values to 6.9 and 18.2, respectively. The best color yield is achieved with the use of phenylmethanol presenting a K/S value equal to 28.7 at a dyeing temperature of 50°C. Research implications: The use of phenylmethanol contributed effectively to reducing the temperature of the dyeing process without causing significant changes in the chemical and morphological structure of the analyzed fibers. Originality/value: This study highlights the efficiency of phenylmethanol in reducing the temperature of the dyeing process of polyamide fiber contributing to reducing the use of energy in the textile industry.


INTRODUCTION
The textile industry is considered one of the most important industries in the world playing a vital role in the economic sector (Campos et al., 2023).However, leveraging the growth of the textile industry associated with sustainable development remains a challenge.The textile industry consumes higher amounts of fresh water and energy for the conventional wet processing of textiles.Conventional textile dyeing uses 5.8 trillion liters of water per annum, which contributes to one-fifth of world industrial water pollution (Pawar et al., 2019).Organic dyes are not readily biodegradable and can be found in the industrial effluent of textile industries (Kadim & Abd, 2022)On the other hand, the textile industry has low efficiency in energy utilization and is one of the major energy consuming industries ( (Pawar et al., 2019); (Pawar & Adivarekar, 2021); (Kale et al., 2015)).
Environmental concerns are the driving force behind the development of new dyeing process techniques to use fewer natural resources, such as water and energy (de Oliveira et al., 2012).Based on these environmental concerns in recent years, researchers are working on the development of sustainable dyeing techniques such as plasma processing, air-dye, solvent microemulsion, and supercritical carbon dioxide, etc to reduce water consumption in the textile industry (Ferrero & Periolatto, 2012;Oliveira et al., 2017;Radei et al., 2018).
Solvent dyeing could be an alternative to minimize water requirements in the textile industry.Glycerol, chlorinated hydrocarbons, non-ionic surfactant, n-octanol, alkane and ethanol/water and calcium chloride/ethanol/ water mixtures were used as a dyeing medium for dyeing cotton, polyester and polyamide fibers with reactive dye to reduce water usage, carriers and energy requirements and such dyeing showed comparable results with aqueous dyeing process (Ferrero & Periolatto, 2012;(Li et al., 2015); (Rietzler et al., 2018) ; (Opwis et al., 2020)).The usage of harmful solvents, such as dimethylformamide, was also tested to promote modifications in polyamide crystallinity (Shabana, 2006).Solvent-assisted dyeing is another possibility of dyeing to minimize water consumption.Pawar et al. (2019) obtained lowtemperature solvent-assisted dyeing of polyester fabric using microemulsion of n-butylacetate (Pawar et al., 2019).Xia and coworkers (2018) observed that compared with conventionally dyed samples, cotton yarns died with ethanol-water solution (90:10 v/v) exhibited enhanced exhaustion, total fixation, and color strength ( (Xia et al., 2018).Rietzler and Bechtold (2018) studied the effect of using calcium chloride/ethanol/ water mixtures to perform the surface modification of polyamide 6.6 (Rietzler et al., 2018).The authors verified that the treated fibers exhibit greater diameters and surface roughness with structural differences between an outer shell and inner core with higher water retention when comparable with non-treated fibers (Rietzler et al., 2018).
Technological innovation can represent the introduction of new products or processes that contribute to a more sustainable scenario for both the environment and the industries (Salgado & da Silva Franchi, 2023;Grejo & Lunkes, 2022).The solvent-assisted dyeing method could be an alternative for suitable tailoring of the treatment conditions reducing the usage of water, energy, and chemicals.In this way, this work evaluated the effect of ethanol, butanol, and phenylmethanol aqueous solutions to promote the dyeing of polyamide 6.6 through solventassisted dyeing to minimize the water, energy, and time consumption during the dyeing process.

Dyeing procedure
The standard sample was dyed at 100°C and 50°C using 2.5 g/l of dye and 1,0 g/l of glacial acetic acid.For the samples dyed at 100°C, identified was TEX 1, the dyeing process starts at a temperature of 40ºC, the dye bath is heated to 100ºC at a rate of 1ºC per minute, and remains at a temperature of 100ºC for 60 minutes, then it is cooled to 60ºC at a rate of 2ºC per minute.The dyed polyamide samples are washed with water at a ratio of 1:10 for 5 minutes, the washing process is repeated twice.Subsequently, the polyamide samples are centrifuged and dried in an Electrolux dryer for 30 minutes at 110 °C.For the sample dyed at 50°C, identified as TEX 2, the dyeing process is similar.The dyeing process starts at a temperature of 40ºC.The dye bath is heated to 50ºC at a rate of 1ºC per minute and remains at a temperature of 50ºC for 60 minutes, then it is cooled to 50ºC at a rate of 2ºC per minute.The dyed samples are washed with water at a ratio of 1:10 for 5 minutes.The washing process is repeated twice.Finally, the polyamide samples are centrifuged and dried in an Electrolux dryer for 30 minutes at 110 °C.The concentration of alcohol solutions was fixed at 50 g/l and the dyeing process was tested only at 50°C.The samples identified were TEX 3, TEX 4, and TEX 5 for samples dyed with ethanol, butanol, and phenylmethanol alcohols, respectively.

Samples characterization
The color strength (K/S values) was obtained by a Datacolor 600 Model spectrophotometer.Fourier-transform infrared (FTIR) analysis was carried out using the attenuated total reflectance (ATR) method on a Nicolet IS10 Thermo Scientific using 32 scans from 4000 to 400 cm -1 .Scanning electron microscopy was conducted with a SEM -FEG Mira Tescan 3 in sputtered with gold samples.X-ray diffraction analysis was performed on a Shimadzu XRD6000 instrument which is equipped with CuKα radiation.Scanning range of 2θ was kept between 5-50°, with a step size of 0.05°/min.

Dyeing of polyamide
Figure 1 shows the plot of K/S values for polyamide fibers dyed without using any alcohol as solvent-assisted dyeing at a maximum temperature of 100°C.The K/S values increased with the gradual increase of the temperature.This could occur because the dyeing is a diffusion-controlled process (Kale et al., 2015) and an increase in temperature and time may permit a major diffusion of the dye into the fiber structure.This increase in the value of K/S proves the dependence that this process has on the increase in temperature (Carrion-Fite & Radei, 2017).Wang et al. (2020) observed the same behavior in wool/acrylic blend fabrics and also attribute the increase in K/S values when temperature increased to fiber swelling and dye diffusion (M.Wang et al., 2020).
When the process reaches the temperature of 100 °C the K/S value is 28.2.The dyeing remains at this temperature for 60 minutes, and after this time the final K/S value is 31.4,that is, the K/S variation is about 11%, but this phase is very important for color uniformity across the entire fiber surface.

Effect of alcohols on the dye solutions
The K/S curves of polyamide fibers dyed using the alcohols-water system evaluated at 50 °C can be seen in Figure 2. The K/S value for the reference sample dyed at 50°C is reduced to 6.9 after 130 min of treatment.As discussed in the previous section, the K/S value for the sample treated at 100°C without alcoholic solution was 31.4.This result indicates that when treatment temperature is reduced from 100°C to 50°C the K/S value for the reference sample is reduced by 5 times.This behavior proves the great effect of temperature on the diffusion of the dyeing into the polymer fiber.The utilization of an alcohol-water solution promotes an increase in the K/S values for all alcohols tested, as can be seen in Figure 2. The usage of ethanol causes a slight increase in the K/S values, while the addition of butanol and phenylmethanol generates an increase of 3 and 4.5 times in the K/S values when compared with the standard sample.A possible explanation for this behavior may be associated with the chemical structure of the alcohol tested.
The ethanol-water mixture, rich in intermolecular hydrogen bonds, can act as a marginal solvent for the dye particles, which might promote the aggregation of the dye monomers to form dimmers via electrostatic interactions.The same behavior was previously observed by Xia and coworkers (2018) during the dyeing of cotton in an ethanol-water mixture (Xia et al., 2018).According to the authors, in the ethanol-water mixture, the formation of intermolecular hydrogen bonds between ethanol-ethanol molecules, as well as between water-ethanol molecules, plays an essential role in dye aggregation (Xia et al., 2018).In addition, polar solvents such as ethanol are more likely to be associated in solution because of dipole-dipole interactions and hydrogen bonding.Due to their strong hydrogen bond donor capabilities, these strong ethanol-ethanol interactions may also be a driving force for the dye-dye interactions, resulting in dye particle aggregation (A.Wang et al., 2020;Xia et al., 2018).
The better K/S value obtained using a butanol-water solution may be associated with the reduction of the butanol-butanol and butanol-water interactions due to the less polar characteristic of butanol.The dye particles may be better dispersed in a butanol-water solution which can result in a better diffusion into the polyamide even with the reduction of dyeing temperature.
The polyamide fiber dyed in a phenylmethanol-water system (TEX 4) at 50°C exhibits the highest K/S values when compared with all alcoholic solutions tested.The phenyl group presented in the structure of phenylmethanol may interact with the aromatic structure of the dye used and thus contribute to its dispersion in aqueous solution.In addition, the less polar characteristic of phenylmethanol also contributes to a decrease in the strong intermolecular interactions with water due to the formation of hydrogen bonding.As a result, the K/S value of this sample dyed at 50°C is equal to 28.7, which is a similar value found to the standard sample dyed at 100°C.

Surface characterization of polyamide fibers
Figure 3 shows the SEM micrographs of the dyed fibers without and with the utilization of alcoholic solutions treated at 50°C.The morphology of polyamide fibers after treatment remains the same as the fibers before treatment, as can be seen in Figure 3.In addition, the micrographs also indicate that the fiber diameter did not change significantly after using all the alcoholic water solutions tested, as presented in Table 1.The alcohols tested may promote swelling of the polyamide fiber, reducing the interactions between polymer chains which contribute to dye diffusion, and after the end of the 7 dyeing process, these alcohols are probably removed during the washed process.Figure 4 shows an energy dispersive spectroscopy (EDS) analysis of the fiber composition of the TEX 5 sample.The fibers are mainly composed of carbon and oxygen, with some traces of titanium.The same composition was observed for the other fibers studied.

FTIR analysis of polyamide fibers
FTIR spectra of all samples studied are illustrated in Figure 5.

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The bands assigned in the region of 3297-1638 cm -1 are related to the stretching of the groups -NH2 and C=O presented in the polyamide, respectively, which are present in all four samples analyzed (Krishna Prasad et al., 2017).In the region of 1638 cm -1 , stretching of the C=O groups, stretching of the N-C groups, flexion of the C-N-C groups, torsion of the C-C groups, and plane flexion of the C-C group can be observed (Koosha et al., 2017).
The behaviors of the four samples tested were similar when compared to FTIR characterization.In this way, the action of alcohols on the polyamide fiber does not substantially change the structural composition of the fiber.However, only a few differences were observed in the region of 1040 cm -1 , even so, the changes observed in Figure 5 are relative to nominal values of transmittance, and the vibration in this band is observed in all samples tested.

XRD analysis of polyamide fibers
The effect of alcoholic solvent on the physical structure of polyamide fiber was evaluated by XRD analysis as shown in Figure 6.X-ray diffraction curves indicate no major changes in the structure of polyamide fibers after using the alcoholic solutions.In all samples, two peaks at 20.5° and 23.2° can be observed.According to Leite (2009), these peaks located in the of 2θ from 20º to 24º are related to the formation of the crystalline phase of the polyamide and the crystal structure of the polyamide represented by the α phase (Leite et al., 2009).The formation of the α or γ phases depends on the crystallization conditions or the addition of specific loads at the time of fiber manufacture.The α form is characterized by 2θ peaks between 21° and 24° and the γ phase with 2θ peaks at 11°, 22° and 23° (Leite et al., 2009).A slight shift in some peaks was observed in 2θ and interplanar distances, as presented in Table 2. Probably the molecules of dye are entrapped physically in polyamide fibers (Pawar et al., 2019).The 2θ peaks exemplified in Table 2 belong to the monocyclic form of the polyamide as the XRD standard for nylon 6 and 6.6 shows the crystalline nature of these polyamides in peaks of 19.99° and 23.77° (Estrada-Flores et al., 2018).As described by Bezerra et al. (2010) pure polyamide 6 is characterized by peaks that demonstrate its crystalline structure, and these peaks are located in regions close to 20° and 24° in 2θ referring to the crystalline phase of polyamide is called the α-phase (Bezerra et al., 2010).

CONCLUSION
Based on the results of this work it can be verified that processes that seek to reduce the dyeing temperature of the polyamide fiber are viable from the point of view of the color performance of the dye on the fiber.
The influence of the use of alcoholic solutions for this purpose proved to be viable, especially with the use of benzyl alcohol for this purpose.Butyl alcohol also provides an increase in color yield in the low-temperature process compared to using standard process chemicals, but this increase is not of the same magnitude as benzyl alcohol and ethanol alcohol does not provide an increase in color yield.
Treatment with the tested alcohols did not lead to the formation of new bands in the FTIR spectra.Analyzes carried out using the SEM showed that the different dyeing processes tested did not change the surface structure of the polyamide fiber.XRD tests demonstrate that the samples had no change in their interplanar distances.The use of phenylmethanol contributed effectively to reducing the temperature of the dyeing bath without causing significant changes in the chemical and morphological structure of the analyzed fibers.

Table 1 :
Fiber dimensions obtained through SEM micrographs *Equal letters indicate no significant difference between samples by Turkey test at 5%.

Table 2 :
2θ and interplanar distance values for all samples studied