Insight into the Dyeability of Bio-Based Polyamide 56 by Natural Dyes

Abstract

Bio-based polyamide 56 (PA56) is a new sustainable material in the polyamide family. In this study, dyes suitable for PA56 fibers were experimentally screened from natural plants rich in pigments. The results showed that the preferred natural dyes for PA56 fabric are turmeric for a yellow hue, madder for a red hue, catechu for a brown hue, and indigo for a blue hue. A green hue was achieved by the two-bath dyeing method using indigo and turmeric, respectively. For a dyability comparison with conventional PA6 and PA66, PA56, PA6, and PA66 fabrics were woven under identical conditions and dyed with turmeric, madder, catechu, and commercial indigo extracts. PA56 fabric exhibited the best dye uptake and the fastest dyeing rate (PA56 > PA6 > PA66). The reason for the excellent dyeability of PA56 fibers was analyzed in terms of differential scanning calorimetry measurement and molecular dynamics simulations. The results showed that the lowest crystallinity was exhibited by PA56 (PA56 < PA6 < PA66); in addition, PA56 displayed the largest fractional free volume (PA56 > PA6 > PA66). These structural characteristics contribute to the excellent dyeability of PA56 fibers. Therefore, PA56 fibers are promising materials, as they are derived from a sustainable source and have superior dyeing properties compared to PA6 and PA66 fibers.

1. Introduction

The dyeing of fibers and fabrics is an essential part of textile processing [1]. Natural dyes, generally derived from plants, animals, and minerals, have the advantages of being widely available, being non-toxic or low in toxicity, and having sustainable extraction methods compared to synthetic dyes [2,3]. Although natural dyes have not met the needs of the textile industry, the development of natural dyes has attracted the attention of many scientific researchers. They focus on the process of extracting dyes from natural plant roots, stems, leaves, and other organs that contain pigments, as well as their dyeability, antibacterial properties, and UV protection ability on various fibers [4].

The dyeing of textiles using natural dyes originally started with natural fibers of wool and cotton. In a recent study, the natural dye extracted from pterocarpus santalinus wood waste was used to dye yak wool fabrics [5]. The quinoa plant was used to dye wool fibers [6]. Wool dyed with different parts of the quinoa plant (e.g., flowers, leaves, and stems) showed a distinct yellow hue, and wool samples dyed with leaves showed good antimicrobial properties against Staphylococcus aureus. The dye extracted from the fruit of Cinnamomum verum J. Presl was used to dye wool fibers, and the highest color intensity on wool fibers was obtained under the conditions of pH 4, temperature 100 °C, and time 60 min [7]. In addition to wool, cotton fibers were dyed with many kinds of natural dyes, such as extracts from cacao husk, pomegranate peel, nutshell, orange tree leaf, and alkanet root. To improve the color fastness and color strength ($K/S$) of wool and cotton, dyeing with natural dyes usually requires mordants. Certain metallic mordants (e.g., alum) are permitted within specified maximum and minimum limits for inorganic element content in naturally dyed products. To address this problem, bio-mordants of tannic acid and acorn were investigated to create green coloration in cotton [8], and the bio-mordant of aloe vera was applied in dyeing wool with dalbergia sissoo dye [9]. The results indicated successful enhancement of the fixation of natural dyes on fibers using bio-mordants.

In addition to natural fibers, the possibility of dyeing synthetic fibers or materials using natural dyes is also being gradually explored. Poly(ethylene terephthalate) was dyed with a natural dye derived from madder [10]. Polyester fibers were dyed using dyes derived from curcumin, annatto seeds, chrysophanol, etc. [11].

As an important chemical material for the modern industry, polyamide (PA) is popular due to its excellent moisture absorption, abrasion resistance, and dyeability. Currently, PA66 and PA6 are the most widely used materials in the garment and plastics industry, making up about 90% of PA products [12]. It is well known that synthetic monomers of PA66 and PA6 are dependent on petroleum resources. With the depletion of oil resources, there is a need for eco-friendly materials to substitute petroleum-based fibers.

Bio-based PA56 is a new sustainable material in the polyamide family. It is produced through the polymerization reaction of pentodiamine monomers and adipic acid, where pentodiamine is the product of lysine decarboxylation by microorganism fermentation [13]. PA56 fibers have been demonstrated to have mechanical properties and heat resistance comparable to PA66 and superior to PA6 [14] and dyeability superior to PA66 using acid dyes [15]. In recent PA fiber-dyeing research, the main focus has been on improving dye uptake and minimizing raw material consumption through the use of synthetic dyes [16]. Additionally, PA6 and PA66 fibers have been carefully investigated for the dyeability of natural dyes, such as natural dyeing using dragon’s blood resin extract [17]. However, few studies have explored the dyeing of PA56 fibers with natural dyes, especially in experimental comparisons with the dyeing of PA6 and PA66 fabrics woven under identical fabric-processing conditions. Research on PA56 dyeing with natural dyes could have considerable environmental benefits because it is derived from a more sustainable source.

Therefore, this paper discussed the dyeability of PA56 fibers using natural dyes considering three aspects. Even if plants exhibit a wide range of colors, not all of these can be used as dyes. First, natural plants were purchased, and experiments were conducted to select preferred plant dyes for PA56 fibers in terms of the primary colors: red, yellow, brown, and blue. Second, to enrich the chromatography of natural dyes for PA fibers, a two-bath dyeing method was used to achieve the primary color green. Finally, parallel dyeing experiments were carried out to compare the dyeability of PA56, PA6, and PA66 fabrics by analyzing dye uptake and dyeing rate curves. The critical factors influencing the dyeing of PA fibers were examined: end group content measurement, crystallinity analysis with differential scanning calorimetry (DSC), and fractional free-volume calculation through molecular dynamics (MD) simulations using Materials Studio (MS) software.

2. Materials and Methods

2.1. Materials

For comparison, PA56, PA6, and PA66 filaments were prepared using the same spinning process. Next, PA56, PA6, and PA66 fabrics were woven under the same specifications: warp yarn, 70D/34f; weft yarn, 70D/34f; warp × weft density of the fabric, 300 × 640 roots/10 cm; weight of the fabric, 112 (g/m²); and plain weave.

The plants used for dye production in the experiment were purchased from Taobao Web, Dalian, China. The commercial dye indigo (AR, 90%) was purchased from Shanghai McLean Biochemical Technology Co., Ltd., Shanghai, China. Sodium hydroxide (NaOH) was purchased from Tianjin Komeo Chemical Reagent Co., Ltd., Tianjin, China. N-methylpyrrolidone (NMP) and Na₂S₂O₄ were purchased from Tianjin Damao Chemical Reagent Factory, Tianjin, China.

The botanical names (common names) of the plants and parts of plants used for dyes are as follows:

• Roots: Curcuma longa (turmeric), Scutellaria baicalensis georgi (radix scutellariae), Rheum rhabarbarum (rhubarb), Salvia miltiorrhiza bunge (radix salviae miltiorrhizae), Rubia tinctorum (madder), and Spatholobi caulis (caulis spatholobi)
• Fruit peels: Punica granatum (pomegranate), Physalis alkekengi L. (physalis alkekengi), Citrus sinensis (orange), Allium cepa (onion), and Piri fructus (pear)
• Stem: Thymus vulgaris (thyme)
• Peeled branches: Acacia catechu (catechu)
• Fruit shell: Castanea sativa (chestnut)
• Leaves: Olea europaea (olive tree leaf), Eucalyptus globulus (eucalyptus leaf), and Spinacia oleracea (spinach)

2.2. Preparation of Plant Dye Solutions

The preparation of plant dye solutions is shown in Figure 1a. First, plant pellets were put into beakers at a material-to-liquor ratio of 1:10. Next, the beakers were heated to 100 °C, the mixture was boiled for 1 h, and finally, the mixture was filtered through cotton gauze fabric to obtain plant dye solutions with pigments.

2.3. Dyeing of PA56 Fabric with Plant Dye Solutions

PA56 fabric samples were dyed with plant dye solutions prepared in Section 2.2 at a fabric-to-liquor ratio of 1:50. The dyeing solutions were heated from room temperature to 60 °C at a rate of 1 °C/min and held for 1 h. The dye powders were used without further purification.

2.4. Preparation of Dye Powders from Plant Dye Solutions

The preparation of dye powders from plant extracts is shown in Figure 1b. The plant dye solutions (turmeric, madder, and catechu) were dried in an oven at 100 °C and then ground to obtain dye powders.

2.5. Dyeing of PA56, PA6, and PA66 Fabrics with Dye Powders

PA56 fabric samples were dyed with the extracted turmeric, madder, and catechu. Dye baths with 20–90% (on weight of fabric (o.w.f.)) dye concentrations were prepared. PA56 fabric samples were placed in the dyeing solutions at a fabric-to-liquor ratio of 1:50. The dyeing bath was heated to 60 °C at a rate of 1 °C/min and held for 1 h. Indigo is an ancient blue dye. The dyeing of PA56 fabric with indigo is shown in Figure 1c. A solution containing indigo (0.5–6% o.w.f.), Na₂S₂O₄ (25 g/L), and NaOH (4 g/L) was heated in a water bath at 60 °C for 0.5 h. Next, PA56 fabric was dyed for 1 h at a fabric-to-liquor ratio of 1:50. Finally, the PA56 fabric sample was taken out and oxidized in the air for 20 min. The sample was then rinsed with running water for 5 min to remove floating colors. In this indigo dye, Na₂S₂O₄ was the reducing agent. Under alkaline conditions, Na₂S₂O₄ can transform indigo into soluble leucoindigo. After oxidation, leucoindigo is reconverted into an insoluble dye and fixed on the fibers. Mordant treatment was carried out to improve the light fastness of the fabric. Some dyed PA56 fabric samples were further treated in aloe vera gel solution at 70 °C for 30 min and then washed and dyed at 65 °C for 20 min [18].

To increase the color spectrum of natural dyes, turmeric, and indigo were used to obtain a green hue, as displayed in Figure 1d. Indigo dyeing needed an alkaline medium, so indigo and turmeric dyeing was performed using the two-bath dyeing method, where indigo was dyed first and turmeric was dyed second. After indigo dyeing, the PA56 fabric was washed for 5 min to remove the remaining NaOH. After turmeric dyeing, the dyed sample was washed again and dried for further measurements.

For comparing the dyeing effect of PA56, PA6, and PA66 fabrics, PA6 and PA66 fabrics were dyed with turmeric, madder, and catechu powder dyes at 80% (o.w.f.) and indigo at 6% (o.w.f.) using the same dyeing process as that for PA56 fabric.

2.6. Dyeing Rate of PA56 Fabric with Indigo Dye

Indigo is insoluble in water and soluble in H₂SO₄ and NMP solvent. Considering that NMP solvent is safer than H₂SO₄, NMP was used in this experiment. First, the standard curve of indigo was prepared with the absorbance-concentration curve fitting of NMP solutions (0.002, 0.004, 0.006, 0.008, and 0.010 g/L). The UV absorbance of the NMP solutions was measured at the maximum wavelength (UV-8000 spectrophotometer, Shanghai Yuanxi Instrument Co., Ltd., Shanghai, China). Second, PA56 fabric samples were dyed at 60 °C for 5, 10, 15, 20, 30, and 40 min, respectively. Next, the absorbance of the dye bath at time t was measured, and the dye concentration was obtained with the standard curve of indigo. Finally, the percentage of dye exhaustion was calculated with Equation (1):

$$E(\%)={\displaystyle \frac{C_0 - C_t}{C_0}} \times 100$$

(1)

where $E$ is the percentage of dye exhaustion, %; $C_0$ is the initial dye concentration; and $C_t$ is the dye concentration at time t.

2.7. Methods

2.7.1. Glass Transition Temperature and Crystallinity Measurements

DSC (DSC-Q2000, TA instrument, New Castle, DE, USA) was used to evaluate the glass transition temperature and crystallinity of PA56, PA6, and PA66 fabrics. Fabric samples (about 5.0 mg) were placed in a furnace chamber with nitrogen at a flow rate of 0.1 (mL/min). The samples were first heated from 30 to 300 °C at a heating rate of 20 °C/min, then held at 300 °C for 1 min to eliminate the thermal history, and subsequently cooled to 0 °C at a cooling rate of 20 °C/min to start a second heating phase. DSC curves for the first heating phase were used to calculate crystallinity using Equation (2). DSC curves for the second heating phase were used to determine the glass transition temperature.

$$X_c={\displaystyle \frac{\mathsf{\Delta }{H}_f}{\mathsf{\Delta }{H}_m^0}} \times 100$$

(2)

where $X_c$ is the crystallinity; $\mathsf{\Delta }{H}_f$ is the enthalpy of melting in the DSC curves, J·g⁻¹; and $\mathsf{\Delta }{H}_m^0$ is the standard enthalpy of fusion at 100% crystallinity, J·g⁻¹ (190 J·g⁻¹ for PA6, 188.68 g⁻¹ for PA56, and 194.69 J·g⁻¹ for PA66) [19].

2.7.2. End Group Content Measurement

An automatic potentiometric titrator (916 Ti-Touch, Metrohm Switzerland Wantong Co., Ltd., Shanghai, China) was used to measure the end group content of -NH₂ in PA56, PA6, and PA66 fibers. A trifluoroethanol/water solution (volume ratio 88/12) was used to dissolve the fibers.

2.7.3. X-Ray Diffraction Measurement

X-ray diffraction (XRD) measurements of PA56, PA6, and PA66 fabrics were carried out on a wide-angle X-ray diffractometer (D/max-3B, Rigaku Co., Ltd., Tokyo, Japan). Fabric samples were scanned at 2θ = 10–50° at an operating voltage of 40 kV and a current of 200 mA. The radiation was Ni-filtered Kα radiation.

2.7.4. Fractional Free-Volume Simulation Details

The fractional free-volume simulation of PA56, PA6, and PA66 macromolecules was analyzed using MD simulations. The MD simulations were performed in the Material Studio 2020 software package. To achieve the fractional free-volume simulation, four steps were carried out as follows:

Step 1: Establish a stable amorphous polymer model. Taking into account the efficiency and computational complexity of the simulation, we constructed an isotactic polymer chain with a degree of polymerization of 20 and then minimized the energy through the Discover process, applying the steepest descent method. An amorphous polymer with PA56 chains (PA6 or PA66) was constructed with an amorphous cell module. The density of polymer PA56 (PA6 or PA66) was set to 1.14 g/cm³, and the initial density was set to 0.1 g/cm³.

Step 2: Obtain an optimized molecular model with annealing treatment. In molecular dynamics optimization of five canonical ensemble cycles, the temperature was set to 300–600 K, the pressure was set to 0.001 GPa, and temperature and pressure control were obtained using Andersen and Berendsen methods, respectively.

Step 3: Dynamics simulation. After steps 1 and 2, the local force was reduced, the unreasonable structure was eliminated, and the model close to the real material was obtained. The dynamics simulation was completed through the Discover process. The structure file was optimized with a 50 ps constant pressure and constant temperature, and the generated trajectory file was optimized with 50 ps NVT dynamics, with a step size of 1.0 fs.

Step 4: Fractional free-volume calculation. The most common application of free-volume theory is describing the diffusion of molecules in polymers. Using the Atom Volume & Surface Calculation module in MS, the hard sphere probe method was used to make the spherical probe molecules roll continuously along the atomic surface of the polymer to form a Connolly surface [20]. The volume enclosed by these Connolly surfaces was the free volume. The Connolly radius was 1.00 Å. The grid interval was 0.75 Å. The volume occupied by polymer chains and the unoccupied free volume were calculated for each simulated cell using the Connolly surface tool. The fractional free volume was calculated as in Equation (3):

$$FFV(\%)={\displaystyle \frac{V_{free}}{V_{free}+V_{occupy}}} \times 100$$

(3)

where $FFV$ is the fractional free volume, %, and $V_{free}$ and $V_{occupy}$ represent the free volume and occupied volume, respectively.

2.7.5. Color Measurement

The colorimetric data (L* a* b*) and reflectance (%R) values of dyed samples were determined with a CM-3600d spectrophotometer (Konica Minolta Investment Co., Ltd., Shanghai, China). The $K/S$ value was obtained with Equation (4):

$$K/S={\displaystyle \frac{(1 - R^2)}{2R}}$$

(4)

where $R$ is the reflectance value at the maximum absorption wavelength, λ.

2.7.6. Color Fastness Testing for PA56 Fibers

The washing fastness (staining) of PA56 fabric was measured at 40 °C according to China’s GB/T 3921-2008 standard (“Tests for color fastness: Soaping colorfastness”). Fabric samples were sewn between two pieces of standard lining fabric (PA fabric) and placed in a washing machine (soap slices 5 g/L, bath ratio 1:50, temperature 40 °C), agitated for 0.5 h, and then washed and dried. The rubbing fastness of the dyed fabric samples was measured according to the AATCC-8-2007 standard (Y571B; Wenzhou Darong Textile Equipment Co., Ltd., Wenzhou, China). The light fastness of the dyed fabric samples was measured according to the AATCC-163-2014 standard (YG611S; Wenzhou Darong Textile Equipment Co., Ltd., Wenzhou, China).

3. Results and Discussion

3.1. Screening of Preferred Natural Plant Dyes for PA56 Fiber

Table 1. K/S, λ, L*, a*, b*, washing fastness, and photos of PA56 dyed with plant dye solutions.

Plants/Plant Parts	λ	L*	a*	b*	$\mathbf{K}/\mathbf{S}$	Washing Fastness	Digital Images
Pomegranate rind	400	65.03	3.98	35.99	19.17	5
Radix Salviae Miltiorrhizae	400	65.61	4.69	22.52	3.34	5
Radix scutellariae	400	67.14	2.82	36.77	12.98	4–5
Turmeric	420	67.41	8.35	73.77	17.26	4
Pear peel	400	67.65	5.26	22.43	7.89	5
Thyme	400	68.81	−0.36	27.53	4.67	4–5
Physalis alkekengi peel	400	72.79	3.36	21.69	3.10	4–5
Sweet orange peel	400	77.67	−0.95	20.92	2.57	5
Madder	420	30.95	29.37	29.18	15.04	4–5
Catechu	400	23.90	11.93	15.60	29.13	4
Onion peel	410	37.47	10.50	23.18	28.95	4
Olive tree leaf	400	52.98	6.23	22.43	14.08	5
Rhubarb	440	54.55	6.73	47.76	12.72	4
Eucalyptus leaf	400	55.54	8.21	24.54	15.30	5
Caulis spatholobi	400	57.13	13.69	21.69	2.48	5
Chestnut shell	400	60.47	5.28	26.40	6.83	4–5
Spinach	400	10.11	2.45	14.02	2.10	3

summarizes the $K/S$, λ, L*, a*, b*, and washing fastness (staining) values and photos of PA56 fibers dyed with natural plant extracts without mordants. The color classification was based on a combination of the samples’ visual appearance and the color position in the CIELAB color space.

From Figure 2b, it can be seen that almost all dyed samples, except for those dyed with madder, were concentrated in the yellow space. This is due to the visual characteristics of dyeing, where the chromaticity coordinates of some brown dyes are close to those of yellow dyes. The biggest difference is that brown usually has lower brightness, while yellow has higher brightness. Thus, based on visual appearance, we established the following classification: The yellow family of dyes included pomegranate rind, radix salviae miltiorrhizae, radix scutellariae, turmeric, thyme, pear peel, physalis alkekengi peel, and sweet orange peel extracts, while the brown family of dyes included catechu, onion peel, olive tree leaf, rhubarb, eucalyptus leaf, caulis spatholobi, and chestnut shell extracts.

Following the preliminary classification of plant pigments, the screening of the primary colors yellow, red, and brown was carried out. Fabric samples dyed with yellow colorants (turmeric and pomegranate rind extracts) demonstrated excellent color depth, with $K/S$ values all exceeding 15. However, in terms of yellowness, the turmeric-dyed samples showed the highest yellowness index (73.77), exhibiting distinct advantages compared to pomegranate-rind-dyed samples. The sample dyed with turmeric also displayed a visually clear yellow color. Thus, turmeric extract was the preferred yellow dye for PA56 fibers. The fabrics dyed with catechu extract had the highest $K/S$ (29.13) among plants in the brown family of dyes. The color fastness of catechu dyes was of 4 levels. In addition to $K/S$ and washing fastness values, additional functionality was another consideration. Catechu, along with natural substances with antibacterial and UV protection abilities, was easily available in the market. Therefore, catechu extract was the preferred brown dye for PA56 fibers. The red dye was a madder extract.

As we all know, PA polymers are macromolecules connected by amide bonds. Considering the chemical structures of PA56 fibers and plant pigments of turmeric, madder, and catechu [10] displayed in Figure 3a, PA fibers are more conducive to dyeing with these natural dyes due to the amide groups and relatively flexible hydrogen bonds between dyes and polymers. Therefore, the pigments of turmeric (yellow), madder (red), and catechu (brown) were selected.

3.2. Dyeing Results of Using Plant Dye Powders on PA56 Fabric

The extracted turmeric, madder, and catechu dye powders, as well as commercial indigo, were used to dye PA56 fabric samples to achieve yellow, red, brown, and blue hues at different concentrations. The photos of the dyed fabric samples are shown in Figure 3b₁. All PA56 fabric samples presented a range of hues from a bright hue at a low dye concentration to a deep hue at a high dye concentration. The color strength of the dyed samples increased gradually with an increase in concentration. The $K/S$ values of the samples after dyeing for each color family ranged from a minimum of 3.5 to a maximum of 15 (Figure 3b₂). Cross-sectional analysis of dyed samples using the Hary microtome demonstrated excellent dye penetration of dye molecules into the amorphous area of the fibers (Figure 3c).

Table 2. K/S and color fastness values of PA56 fabric after dyeing and mordant treatment.

PA56 Fabric			K/S	Washing Fastness	Rubbing Fastness		Light Fastness
Methods	Dye	Concentration (o.w.f.)			Dry	Wet
Dyeing	Turmeric	80%	14.69	4	4–5	4–5	2
	Madder	90%	11.43	4–5	4–5	4–5	2–3
	Catechu	90%	15.46	4	3–4	3–4	3–4
	Indigo	6%	16.5	4	3	2–3	4
Dyeing and post-mordant treatment	Turmeric	80%	16.74	4–5	5	5	3–4
	Madder	90%	14.51	4–5	5	5	3–4
	Catechu	90%	18.36	4	4	4	4–5
	Indigo	6%	19.76	4–5	3–4	3	5

shows the results of $K/S$ and color fastness values of the dyed PA56 fabric samples. It is clearly seen that the dyed samples had poor light color fastness. The turmeric-dyed fabric had a light fastness of 2 levels, the madder-dyed fabric had a light fastness of 2–3 levels, the catechu-dyed fabric had a light fastness of 3–4 levels, and the indigo-dyed fabric had a light fastness of 4 levels. Considering the poor light fastness of PA56 fibers, the bio-mordant of aloe vera was used to improve their light fastness. Post-mordant treatment, we saw an increase in the $K/S$ value of dyed PA56 fabric. The light fastness increased by 1 to 1.5 points. Light fastness represents a critical challenge in natural dye applications, primarily due to its dependence on the molecular characteristics of the dye. The stability of a dye or pigment against photochemical effects depends on the structure of the chromophore and piratically autochrome groups of the dye molecule. Although all PA56 fiber samples listed in Table 2 demonstrated light fastness of ≥3–4 levels, the color fastness of the PA56 fabric was still low. For these reasons, it is recommended that PA56 materials dyed with natural dyes be optimized for specific applications, particularly for textile products, excluding outerwear. The focus of subsequent experiments was to improve the light fastness of PA56 fibers. Importantly, the rubbing fastness of the samples also increased by 0.5 points post-mordant treatment.

Most natural green dyes are derived from plants like spinach and green tea. Green dyes present significant challenges in control due to chlorophyll being the predominant pigment in these sources. Notably, chlorophyll exhibits extreme sensitivity to pH, temperature, and oxidation, rendering it susceptible to thermal decomposition or oxidative fading (e.g., turning yellowish brown) during dye extraction and application [21]. In our spinach-dyeing experiment on PA56 fibers, the expected green coloration failed to materialize, instead yielding a yellow hue (in Table 1). External factors, including cultivation conditions and storage duration, can influence pigment content, resulting in notable variations in colors. Thus, to enrich the color palette of natural dyes and achieve green PA56 fabric, the two-bath dyeing method was followed using indigo and turmeric dyes on PA56 fabric. The fabric samples were first dyed in the indigo dye bath and then in the turmeric dye bath. Due to the higher uptake rate of indigo dye compared to turmeric dye, a lower concentration of indigo was used for this two-bath dyeing. The position distribution of the PA56 fabric samples after two-bath staining in the CIE LAB color space is shown in Figure 3d. The results showed that the dyed samples were located in the green area of the CIE LAB color space. The K/S value of the dyed PA56 fabric (sample 1) was 15.52 using 80% turmeric and 0.5% indigo. When the concentration of indigo was reduced, the K/S value of the dyed PA56 fabric (sample 2) was 4.97 using 80% turmeric and 0.2% indigo. In addition, the green-dyed samples showed washing fastness of 4 levels. Through the aforementioned dyeing experiments, we obtained PA56 fabric with primary hues of yellow, red, brown, blue, and green.

3.3. Comparison and Discussion of the Dye Ability of PA56, PA6, and PA66 Fibers

3.3.1. Comparison of the Dye Ability of PA56, PA6, and PA66 Fabrics

The aforementioned experiments showed that PA56 fabric can be dyed well with natural dyes with the primary colors red, yellow, blue, green, and brown. In this section, a dyeability comparison of PA56, PA6, and PA66 fabrics is carried out. Figure 4a shows the reflectance spectra of PA56, PA66, and PA6 fabrics dyed with natural dyes under the same dye conditions. As displayed in Figure 4a, the reflectance curves of PA56 fabric were lower than those of PA6 and PA66 fabrics for the study’s plant dyes (turmeric, madder, catechu, and indigo extracts). These findings indicated that PA56 fabric exhibits the best dye uptake compared to PA6 and PA66 fabrics. The dyeing rates of PA56, PA66, and PA6 fabrics using the indigo dye are shown in Figure 4b. Among these fabrics, the PA56 fabric displayed the fastest dyeing rate. Therefore, within the evaluated range, the dye uptake rates of the PA fabrics followed the order of PA56 > PA6 > PA66.

3.3.2. Comparison of the Free Volume of PA56, PA6, and PA66 Macromoleculars

The fractional free volume was estimated through molecular dynamics simulations, which serve as an essential tool for predicting structural and transport properties at the molecular level. Materials with a higher free volume exhibit superior permeability. Figure 5a₁–a₃ illustrates the molecular dynamics computational models and aggregated states of PA56, PA66, and PA6, respectively. At the same degree of polymerization, PA6 molecular chains exhibited sparser packing, whereas PA66 chains were the most densely packed. The aggregated structure of polymers significantly influences various physicochemical properties through the generation of intrinsic microporosity. Figure 5b₁–b₃ depicts the molecular models and experimental XRD patterns of PA56, PA66, and PA6 fibers, respectively. The XRD patterns of PA56, PA6, and PA66 fibers all showed strong diffraction peaks (α-form) near 2θ ≈ 20° and 23°, which are characteristic of polyamide materials. Additionally, PA6 may exhibit both α and β crystal forms (β-form near 2θ ≈ 21°). The XRD patterns of the molecular models for PA56, PA66, and PA6 also displayed similar strong diffraction peaks at corresponding angles (2θ ≈ 20°, 21°, and 23°), suggesting that the molecular models were reasonably consistent with actual materials.

Table 3. Crystallinity, glass transition temperature, fractional free volume, and end group content of PA56, PA6, and PA66.

Samples	Crystallinity (%)	Glass Transition Temperature (°C)	Free-Volume Simulation			-NH2 Content (mmol/kg)
			FFV (%)	${\mathit{V}}_{\mathit{o}\mathit{c}\mathit{c}\mathit{u}\mathit{p}\mathit{y}}$	${\mathit{V}}_{\mathit{f}\mathit{r}\mathit{e}\mathit{e}}$
PA56	32.46	50	9.3	5608	579	71.3
PA6	36.02	50	8.6	3041	287	76.4
PA66	39.03	50	8.4	6044	551	74.5

and Figure 5c₁–c₃ present the percentage and distribution of the FFV for the constructed models. The simulated FFV% of PA56, PA6, and PA66 was 9.3%, 8.6%, and 8.4%, respectively. Notably, the higher FFV% of the PA56 simulation unit implied a greater number of voids within the polymer, facilitating easier penetration of dye molecules into PA56. The FFV% trends followed the order of PA56 > PA66 > PA6, aligned with their respective dyeing rate and dye uptake.

3.3.3. Comparison of Glass Transition Temperatures, Melting Points, and Crystallinity of PA56, PA6, and PA66 Fibers

DSC measurements of PA56, PA6, and PA66 fibers were performed, and the results are shown in Figure 6. It is worth noting that the glass transition temperatures of these fabrics were close to 50 °C in the second heating curve of DSC. In the first heating curve of DSC, PA66 showed a wide asymmetric peak at 260.5 °C, and PA6 displayed a wide asymmetric peak at 222.8 °C. Compared to the PA66 and PA6 curves, the PA56 curve displayed a wide asymmetric peak at 253.6 °C and a small peak at 237.9 °C, indicating that the crystalline structures of PA56 are more complex than those of PA66 and PA6. These curves also showed the melting points of PA56, PA6, and PA66 fibers, which were 253.6 °C, 222.8 °C, and 260.5 °C, respectively. Calculated with Equation (2), the crystallinity of PA56, PA6, and PA66 fabrics was 32.46%, 36.02%, and 39.03%, respectively. PA66 fibers had the largest crystallinity, and PA56 fibers had the smallest crystallinity. The crystallinity degree followed the order of PA66 > PA6 > PA56, correlating well with the dyeing rate. The smallest crystallinity value suggested that more dye molecules can easily diffuse and rapidly enter the amorphous region of fibers.

3.3.4. Discussion

Dyeing is a particularly complex process and can be influenced by many factors. Based on the aforementioned measurements, the critical factors affecting the dyeing of PA fibers, including the glass transition temperature, crystallinity, and free-volume fraction, were analyzed. The results for the factors, as well as end group content, are summarized in Table 3.

There are two important aspects of dye uptake: adsorption and diffusion (Figure 7a). In the adsorption process, dye molecules are adsorbed on the surface of the fibers through ionic bonds, hydrogen bonds, van der Waals forces, etc. The functional groups of PA56, PA6, and PA66 macromolecules are amide bonds. The terminal groups of PA56, PA6, and PA66 macromolecules are -NH₂ and -COOH groups. For turmeric, madder, and catechu, hydrogen bonding, and van der Waals forces play a crucial role in the dye adsorption process. Indigo is an organic aromatic molecule that requires a reduction in an alkali environment to produce a soluble leuco form, which can be used for dyeing. For leucoindigo, ionic bonding and hydrogen bonding play a crucial role in the dye adsorption process. The ionic bonds occur between the -NH₃⁺ end groups of PA molecules and the ionic form of leucoindigo; the hydrogen bonds occur between the hydroxyl of leucoindigo and the carbonyl of PA molecules [22,23] (Figure 7b). As shown in Table 3, the difference in the amount of -NH₂ groups among PA56, PA6, and PA66 chains is small. In fact, the -NH₂ groups only exist at the ends of the PA chains. All these findings suggest a small impact of the amount of -NH₂ groups on the dyeing effect of PA fibers.

In the diffusion process, dye molecules can enter the amorphous regions of PA fibers. This diffusion ability of a dye can be characterized by its crystallinity and free-volume fraction. As the results presented in Table 3 showed, PA56 fibers exhibited the lowest crystallinity (PA56 < PA6 < PA66) and the maximum fractional free volume (PA56 > PA6 > PA66). The lower crystallinity of PA56 fibers means a less regular macromolecular arrangement. A higher free-volume fraction of PA56 fibers means slightly stronger segmental mobility of the molecular chains in the amorphous regions. Both these factors can be conducive to the easy entry of dyes into fibers and achieve good dye uptake and dyeing rates.

In addition, the glass transition temperatures of PA56, PA6, and PA66 fibers were all 50 °C, suggesting that the effect of the glass transition temperature on dyeing results can be ignored. Taking all these factors into consideration, we concluded that the smallest crystallinity value and the largest fractional free volume of PA56 made a major contribution to the superior dyeing effect of PA56 among PA56, PA6, and PA66 fibers.

4. Conclusions

Creating eco-friendly and versatile textiles is gaining increasing attention. This study demonstrated the potential dyeing effect of PA56 fibers using eco-friendly natural dyes and revealed the mechanism underlying the excellent dyeability of PA56 fibers in comparison to conventional PA6 and PA66 fibers. The main conclusions are specified next.

Primary colors suitable for PA fibers were successfully selected from natural plants. Preferred hues of red, yellow, brown, blue, and green were successfully obtained for PA56 fibers. Turmeric was the preferred natural dye material for achieving a yellow hue, madder for a red hue, catechu for a brown hue, and indigo for a blue hue. A satisfactory green hue was obtained through the two-bath dyeing process using indigo and turmeric. The light fastness of the samples improved by 1–1.5 points and the rubbing fastness by 0.5 points through aloe vera gel mordant treatment.

Compared to commercial PA6 and PA66 fabrics, PA56 fabric dyed with natural dye exhibits better dye uptake and the fastest dyeing rate (PA56 > PA6 > PA66). The reason for the excellent dyeability of PA56 fibers was analyzed in terms of dye adsorption and diffusion and investigated by measuring the end group content, crystallinity, and fractional free-volume simulation. PA56 chains exhibited the lowest crystallinity (PA56 < PA6 < PA66) and the maximum fractional free-volume percentage (PA56 > PA6 > PA66), correlating well with their respective dyeing rate and dye uptake. The amount of -NH₂ groups at the end of macromolecules and the glass transition temperatures of the PA56, PA6, and PA66 fibers had a small impact on the dyeing effect. Thus, crystallinity and the fractional free volume contribute to the excellent dyeability of PA56 fibers. Combined with the sustainability of bio-based resources and their superior dyeing properties compared to PA6 and PA66 fibers, PA56 fibers are a promising candidate for future textiles.