Comparative Study on the Microwave-Assisted and Conventional Dyeing of Polyamide Fabric with Acid Dyes

1. Introduction

Polyamide fibers, particularly nylon 6 and nylon 6.6, represent a cornerstone of the contemporary textile industry. Their widespread application, ranging from high-performance apparel to advanced technical textiles, stems from their exceptional physiomechanical properties, including superior tensile strength, elasticity, and remarkable abrasion resistance [1,2,3,4]. The coloration of these versatile materials is predominantly achieved through the use of acid dyes, which form robust ionic interactions with the protonated amino groups inherent in the polyamide fiber structure. This interaction ensures excellent dye affinity, colorfastness, and the production of vibrant, durable shades [5,6].

In response to the growing demand for sustainable and efficient manufacturing paradigms, microwave-assisted dyeing has emerged as a revolutionary alternative technology [7,8,9,10]. The fundamental principle underpinning microwave heating is dielectric heating, whereby electromagnetic radiation, commonly at a frequency of 2.45 GHz, induces rapid molecular vibration and rotation of polar molecules, predominantly water, within the dyebath [11,12,13]. This unique mechanism results in rapid, uniform, and volumetric heating throughout the entire dyeing system, a stark contrast to the slower, less uniform, and surface-to-core heat transfer characteristic of conventional heating methods [14,15]. This internal generation of heat promises to dramatically curtail processing times, reduce energy consumption, and enhance the kinetics of dye diffusion and fixation within the textile substrate [16,17].

Recent scholarly investigations have increasingly underscored the transformative potential of microwave technology across diverse textile applications [18,19,20] For instance, studies have reported substantial reductions in dyeing time and notable improvements in color yield for microwave-assisted dyeing of cotton with reactive dyes [21,22,23,24,25].

This study is designed to address the existing research gaps by conducting a detailed comparative analysis of both conventional and microwave-assisted dyeing techniques for polyamide 6 fabric utilizing a model acid dye (Telon Blue M-2R) [26,27,28,29,30]. The research will systematically investigate the effects of key process parameters specifically temperature, dye concentration, dyeing time, and pH on the dyeing kinetics, ultimate dye uptake, and final color strength. By rigorously fitting the experimental data to established kinetic and thermodynamic models, this work aims to elucidate the fundamental underlying mechanisms and quantitatively assess the advantages conferred by microwave heating. The overarching goal is to furnish a robust scientific foundation that will facilitate the development of more efficient, economically viable, and environmentally sustainable dyeing protocols for polyamide textiles, thereby contributing to the advancement of green chemistry in the textile industry [31,32,33,34,35,36,37,38,39,40,41].

2. Materials and Methods

2.1. Materials

The substrate used for all dyeing experiments was a 100% polyamide 6 (PA6) fabric. The fabric was procured from Nurel Group (Turkey). Poliamide obtained from poly condensation of hexametylene diamine with adipic acid, so the nylon contains amide groups, carboxylic end groups, amino end groups, a greater part of the polar groups are amide groups. There are few strongly hydrophilic groups, fiber swelling is little and dye penetration is difficult [2].

The acid dye selected for this investigation was C.I. Acid Blue 324, commercially known as Telon Blue M-2R, supplied by DyStar (Germany). This dye possesses a molecular weight of 766.53 g/mol and was used as received with a purification. The chemical structure of the dye is depicted in Figure 1.

For pH control and adjustment of the dyebaths, analytical grade acetic acid (glacial, 99.8%) was used to prepare appropriate buffer solutions.

2.2. Dyeing Procedures

2.2.1. Conventional Dyeing Methodology

Conventional dyeing experiments were conducted in a laboratory-scale. The dyebath was prepared with a liquor ratio of 1:50. The dyeing process was initiated at a starting temperature of 30°C. The temperature was then uniformly raised to the target dyeing temperature (50, 60, 70, 80, or 95°C) at a controlled rate of 1°C/min. The dyeing was maintained at the final temperature for a duration of target reaction time (5-10-15-20-25-30 minutes) for comparative kinetic studies. Upon completion of the dyeing duration, the fabric samples were immediately removed, thoroughly rinsed with cold deionized water to remove any unfixed surface dye, and subsequently air-dried. In addition, the effects of other reaction parameters such as pH (3, 3.5, 4, 4.5, 5, 6) and dye concentrations (0.1%, 0.25%, 0.5%, 0.75%, 1%, 1.5%) on dyeing were investigated [42,43,44,45,46,47,48].

2.2.2. Microwave-Assisted Dyeing Methodology

Dyeing experiments were conducted using a modified microwave oven (Milestone Start D Microwave Digestion System). Dyeing baths were prepared in microwave-transparent 100 ml capacity tubes to ensure uniform microwave energy absorption.

The dyebath was prepared using the same liquor ratio (1:50) and dye concentrations as in the conventional dyeing protocol. The microwave power was systematically varied and set to 160 W. The dyeing duration was varied from 30 to 270 seconds, depending on the specific experimental conditions. The temperature of the dyebath was continuously and precisely monitored. Post-dyeing, the fabric samples were treated in the same manner as the conventionally dyed samples, involving a thorough rinsing and air-drying process [42,43,44,45,46,47,48].

2.3. Analytical Procedures

Colorimetric Measurement

Dye uptake was determined by measuring the color strengt in the fabric at predetermined time intervals using a Data Color SF 600 spectrophotometer at the maximum absorption wavelength of Telon Blue M2R (λ_max = 630 nm). A calibration curve was established using known concentrations of the dye.

Color strength (K/S value) of the dyed fabric samples was measured using a reflectance spectrophotometer (Data Color 1000 spectrophotometer.) according to the Kubelka-Munk equation:

$${\displaystyle \frac{K}{S}}={\displaystyle \frac{\left(1-R\right)²}{2R}}$$

(1)

where R is the reflectance value at the maximum absorption wavelength [31,32,33,34,35,36,37,38,39,40,41].

3. Results and Discussions

3.1. Conventional Dyeing

3.1.1. Effect of PH

Figure 2 shows the relationship between the initial pH of the dye bath and the resulting color strength (K/S) of the dyed material. The experiment was conducted under constant conditions of time (20 minutes) and temperature (80°C), with a dye concentration of 1% in conventional media. The results, presented in Figure 2, clearly demonstrate a strong inverse correlation between the initial dye bath pH and the resulting K/S value.

The data in Figure 2 shows that the highest color strength was achieved at the lowest initial pH of 3.0, while the lowest color strength was recorded at the highest initial pH of 6.0. This represents a significant decrease of approximately 47.5 % in color strength as the initial pH is increased from 3.0 to 6.0.

The plot of K/S versus pH exhibits a distinct negative exponential relationship. As the pH increases, the K/S value decreases sharply, particularly in the highly acidic range pH 3.0 to 4.0, before the rate of decrease moderates at higher pH values (above 4.5).

The data shows a progressive decrease in K/S as pH increases, with the steepest decline occurring between pH 3.0 and 4.0 (13.9% decrease per 0.5 pH unit). This indicates that the pH range 3.0–4.0 is critical for maximizing dye fixation on polyamide fibers.

3.1.2. Effect of Time andTemperature

The K/S value is a measure that indicates the color depth or dye uptake of a material The Kubelka-Munk function is widely used in color science to quantify the color yield of a dye on a substrate, where a higher K/S value indicates greater color depth or strength. [2,3,4,5,6,7,8,9,10].

The analysis were conducted to understand the effects of the different dyeing times and the different temperatures at a constant pH 6 and a dye concentration of 1 %. The dye samples at 1% concentration were heated to target temperatures (50°C, 60°C, 70°C, 80°C, and 95°C). Figure 3 shows the change in K/S values over time (minutes) for five different temperatures. K/S values at 0, 5, 10, 15, 20, 25, and 30 minutes were measured for each temperature. Generally, K/S values show an increase over time at all temperatures. As the temperature increases, both the final level of K/S values and the rate of reaching saturation increase. Analysis of conventional dyeing curves clearly demonstrates that temperature and time play critical roles in K/S values and dyeing kinetics. At the highest temperature of 95°C, the fastest dye uptake and the highest final K/S values were obtained.

3.1.3. Effect of Dye Concentration according to Different Temperatures

The dyeing process was initiated at a starting temperature of 30°C. The temperature was then uniformly raised to the target dyeing temperature (50, 60, 70, 80, 90 or 95°C) at a controlled rate of 1°C/min. Experiments were proceeded in the pH 5-6 range of approximately and a reaction time of 10 minutes to investigate the effects of different dye concentrations and temperature. Figure 4 illustrates the relationship between color strength, dye concentration, and dyeing temperature for a conventional dyeing process.

For any given dye concentration from Figure 4, an increase in temperature from 50°C to 95°C results in a substantial increase in the K/S value. These data shows that the dyeing process has a critical temperature zone where the kinetics of dyeing are dramatically accelerated.

The K/S value under these conditions is approximately 3.5 times greater than the value achieved at the same concentration but at 50°C. This underscores the synergistic effect where a high dye concentration and high thermal energy (temperature) work in concert to maximize dye adsorption and fixation.

The Conventional Dyeing Curve presented in Figure 5 illustrates the relationship between temperature and color strength (expressed as K/S values) for varying dye concentrations ranging from 0.1% to 1%. As observed, K/S values increase with rising temperature across all dye concentrations, indicating enhanced dye uptake and fixation at elevated temperatures.

3.2. Microwave Dyeing

3.2.1. Effect of PH

Figure 6 shows the relationship between the initial pH of the dye bath and the resulting color strength (K/S) of the dyed material. The experiment was conducted under constant conditions of time (4 minutes) and temperature (85°C), with a dye concentration of 1% in microwave media. The primary objective is to identify the optimal pH range for maximizing color yield in microwave media.

The results from Figure 6 demonstrate a strong inverse correlation between the initial pH of the dye bath and the color strength. The highest K/S value was achieved at the most acidic condition tested pH 3.0.

3.2.2. Effect of Time and Temperature

Figure 7 shows the effect of temperature (50– 95 °C) on the measured parameter over a 10 minute heating period. At all tested temperatures, the parameter increased continuously with time, indicating a temperature-dependent process. However, the rate of increase varied considerably with temperature.

Overall, the data clearly demonstrate a positive correlation between temperature and the rate of the observed change. The temperature dependence observed here is consistent with thermally activated processes that follow the Arrhenius rate equation.

3.2.3. Effect of Dye Concentration According to Different Temperatures

The presented data in Figure 8 systematically investigates the interplay between two critical parameters in microwave-assisted dyeing: dye concentration and temperature. At any temperature, a higher dye concentration in the bath yields a higher K/S value on the substrate. The influence of concentration is visible. Figure 8 shows the effect of temperature (50 – 95 °C) on the measured parameter over a 270 second heating period.

For any given dye concentration from Figure 8, an increase in temperature from 50°C to 95°C results in a substantial increase in the K/S value. These data shows that the dyeing process has a critical temperature zone where the kinetics of dyeing are accelerated. The most effective dyeing conditions, yielding the maximum color strength, are achieved at the highest combination of dye concentration (1.50%) and temperature (95 °C). The K/S value under these conditions is approximately 5.5 times greater than the value achieved at the same concentration but at 50°C.

4. Conclusions

This comprehensive study investigated the kinetics of acid dyeing of Polyamide 6 (PA6) fabrics, providing a detailed comparison between microwave-assisted heating and conventional dyeing methodologies.

This research successfully conducted a comprehensive comparative kinetic study on the acid dyeing of PA-6 fabric, systematically evaluating the performance of conventional heating against the innovative microwave-assisted technique using C.I. Acid Blue 324. The investigation analyzed the influence of key parameters pH, temperature, dyeing time, and dye concentration on the resulting color strength (K/S) and the underlying kinetic mechanisms.

The study confirmed that the conventional dyeing process adheres to established principles, with the highest dye uptake achieved at the most acidic condition (pH 3.0) and the highest temperature tested (95°C). However, the process is inherently time consuming, requiring a duration of 30 minutes to achieve maximum color yield, which is a significant factor in industrial throughput and energy consumption.

This comprehensive study investigated the comparative kinetics and efficiency of conventional heating and microwave-assisted dyeing techniques used for the acid dyeing of PA6 fabric. The findings clearly demonstrate that the microwave-assisted method provides a robust scientific basis for developing faster, more energy-efficient, and more sustainable dyeing protocols in the textile industry.

Figure 8. Comparison of Kinetic and Conventional Dyeing.

Figure 8. Comparison of Kinetic and Conventional Dyeing.

Key Findings are as below.

Effect of pH: In both dyeing methods, the optimum pH value for maximum dye uptake (K/S value) was determined to be 3.0. This confirms the critical importance of an acidic environment to maximize the ionic interaction between the protonated amino groups in the PA6 fiber and the dye molecules.

Conventional Dyeing Kinetics: In the conventional method, dye uptake increased proportionally with time and temperature, with the highest K/S values obtained after 30 minutes of dyeing at 95°C. This indicates that the rate-determining step of dye diffusion and fixation is dependent on thermal energy.

Superiority of Microwave-Assisted Dyeing: The microwave-assisted dyeing method provided a remarkable acceleration in the dyeing time. Thanks to the dielectric heating mechanism (160 W), the time required for dye uptake was drastically reduced compared to the conventional method (seconds instead of minutes).

Mechanistic Explanation: Kinetic analyses proved that the internal, volumetric heating mechanism of microwave energy accelerates the rate-determining step of dye diffusion. This is attributed to the microwave's ability to more effectively increase the molecular mobility of the dye and the swelling of the PA6 fiber structure compared to conventional external heating.

Figure 9. Comparison of Kinetic and Conventional Dyeing.

Figure 9. Comparison of Kinetic and Conventional Dyeing.