Exploration of Alkaline Degumming Printing Techniques for Silk Gauze Fabric: Alkaline Boiling, Alkaline Steaming, and Alkaline Gel

Abstract

As an important branch of ancient Chinese silk dyeing and printing technology, alkali degumming printing utilizes alkali agents to degum raw silk, creating differences in fiber water absorption, dye uptake, and optical characteristics between degummed and non-degummed areas to achieve localized pattern formation.Based on the differences in degumming processes of Silk Gauze using alkaline boiling, alkaline steaming, and alkaline gel, this study compares the effects of these three alkaline degumming techniques under different conditions of alkaline agent dosage, hot press temperature, and hot press duration. The degumming efficiency, fiber surface morphology, and infrared spectra of the degummed Silk Gauze were analyzed and compared. Through the analysis of the degumming mechanisms, it was found that the alkaline gel, within a localized micro-system, meets the conditions of alkali, water, and heat required for precise degumming of Silk Gauze. Combining the dual effects of alkaline boiling and alkaline steaming, the alkaline gel can achieve rapid degumming at a hot press temperature of 80 °C within 50 s, without significantly affecting the surface morphology or the primary structure of the Silk Gauze. The implementation of alkaline gel for precise degumming of Silk Gauze holds significant importance for expanding the application of traditional alkaline printing techniques in modern silk degumming and printing processes.

1. Introduction

The alkali agent degumming printing method of the Tang Dynasty was first discovered in four pieces of printed gauze unearthed from the Astana tombs in Xinjiang. Wu Min tentatively named this printing method “alkali agent printing method” [1]. In the classification of traditional textile printing processes, alkali agents are used in both direct printing and resist printing based on different printing principles [2]. In direct printing, after the fabric is dyed, the alkali agent is used for discharge printing to remove color, which is achieved through acid–base neutralization [3]. In resist printing, the alkali agent is used for physical resist printing to achieve a light pattern on a dark background, a technique referred to as “grey printing” in ancient times and later known as “medicine-stained cloth” [4]. During the Tang Dynasty, people also utilized the property of alkaline substances to dissolve sericin in silk, resulting in a special printing effect due to the difference between raw and degummed silk. This is the aforementioned “alkali agent printing method” described by Wu Min, which has a distinctly different principle from direct and resist printing that also use alkali agents. The “alkali agent printing method” is only applicable to Silk Gauze fabrics made from raw silk, and the plain-colored patterns obtained through this method can be permanently maintained without fading or disappearing due to dyeing, finding extensive applications in ancient costumes, decorative items, and religious artifacts. This technique had not been documented in previous records nor passed down in later generations and is considered a creative invention by silk textile and dyeing workers of the Tang Dynasty in China, holding significant traditional craft cultural value [5].

Alkali agent printing belongs to the category of partial degumming printing on raw silk. The four pieces of printed gauze patterns unearthed from Astana were all made through partial degumming, with patterns composed of neat and uniform fine lines of 1~1.5 mm, resulting in clear and bright, often figurative and relatively complex designs. Achieving smooth and delicate edges of the alkali agent printing patterns through precise degumming is one of the key aspects of restoring the Tang Dynasty’s alkali agent printing technique. Based on the flower-revealing principle of alkali agent printing, Wang Li combined it with traditional Chinese resist dyeing techniques such as tie-dyeing, utilizing the differences in texture, thickness, and dye uptake between raw and degummed silk to showcase the fabric’s unique three-dimensional printing effect. However, due to the lack of quantitative expression of process parameters, it is difficult to apply it precisely and widely in design and creation [5]. In the restoration of green hunting pattern printed gauze, Zhang Hongyuan introduced a method of preparing alkali paste for printing using raw soybean flour, slaked lime powder, and plant ash powder, followed by water boiling for degumming. However, as noted, the final pattern finesse was not satisfactory [6]. In previous studies of our research group, two methods of alkali boiling degumming and alkali steaming degumming were introduced, along with their differences in degumming processes. These studies actively explored the restoration of Tang Dynasty alkali agent degumming printing and achieved preliminary success: alkali boiling degumming involves placing the patterned raw silk fabric in alkali water at a certain temperature for degumming. This process is rapid and effective, achieving a degumming rate of 27% after 30 min of boiling at 80 °C. It can be combined with traditional Chinese techniques such as clamp-resist dyeing and tie-dyeing, resulting in abstract and variable patterns, but it is not suitable for creating detailed figurative patterns [7]. Alkali steaming degumming involves brushing starch alkali paste onto the prepared stencil pattern and then using steam for cooperative degumming. Alkali steaming degumming can be integrated with stencil printing and screen printing, offering advantages in the expression of pattern freedom and pattern finesse. The drawbacks include longer degumming times, with a degumming rate of around 25% after 2 h of steaming at 100 °C, and the pattern size is limited by the volume of the steaming chamber [8].

The silk fabric printing achieved by alkali degumming technology can be summarized through two key aspects. First, the substrate uses raw Silk Gauze fabric, which forms patterns by exploiting the differential properties between raw and degummed silk. Second, the degumming process relies on the combined effects of alkali, water, and heat to successfully remove sericin from raw silk, thereby creating degummed patterns [9]. To meet these requirements, various gel printing methods were explored, and sodium polyacrylate (PAAS) was ultimately selected as the alkali carrier. Dissolving alkali agents in PAAS gel enables rapid and precise degumming within seconds through hot pressing [10]. As a high-performance water-soluble polymer, PAAS is widely used in textile printing for paste formulation and process optimization [11]. According to Flory’s polymer gelation theory [12], the carboxyl groups (–COOH) on PAAS chains undergo near-complete deprotonation to carboxylate anions (–COO⁻) under alkaline conditions. This induces enhanced interchain electrostatic repulsion, resulting in extended molecular chains and pH/Na⁺-dependent crosslinking strength, ultimately forming a three-dimensional physical gel network that transitions from liquid to elastic gel state [13]. The resulting alkali gel retains sufficient water to trigger gel network relaxation under hot pressing, enabling directional alkali penetration into silk fibers while maintaining the essential “alkali-water-heat” environment. This demonstrates exceptional alkali sustained-release capability and thermal responsiveness.

The study found that alkali boiling degumming tends to cause edge diffusion of patterns under liquid alkaline medium conduction, which is advantageous for achieving abstract patterns. Alkali steaming degumming relies on steam to provide moisture, enhancing the precision of figurative patterns but is constrained by equipment capacity and energy consumption intensity. Alkali gel degumming meets the “alkali-water-heat” environment required for silk degumming under hot pressing conditions, demonstrating excellent degumming and printing performance for figurative patterns. This research compared the effects of alkali dosage, processing time, and hot pressing temperature on the degumming and printing process of Silk Gauze fabrics using alkali boiling, alkali steaming, and alkali gel degumming methods. The fiber morphology and infrared spectroscopy of degummed samples were analyzed. The study aims to explore the degumming effects of alkali agents under different process conditions through scientific quantification, thereby expanding the artistic expression and application of traditional Chinese alkali degumming techniques in modern silk degumming and printing.

2. Materials and Methods

2.1. Experimental Materials

Fabric: 8-momme Silk Gauze, with a weight of 32 g per square meter, 100% mulberry silk, plain weave, sourced from Huzhou Xingjiali Silk Weaving Factory (Huzhou Zhuohao Silk Co. Ltd., Huzhou, China).

Main Reagents: Sodium hydroxide (NaOH, analytical grade, Shenzhen, China, Xilong Scientific Co., Ltd.); food-grade sodium polyacrylate ((C₃H₃NaO₂)n, Zhejiang, Hangzhou, China, Eno Bio-Technology Co., Ltd.); slaked lime (Ca(OH)₂, analytical grade, Shanghai, China, Shanghai Chemical Reagent Co., Ltd.); wheat starch (food grade, Shanghai, China, Shanghai Baoding Brewing Co., Ltd.); urea (CH₄N₂O, analytical grade, Shanghai, China, Shanghai Chemical Reagent Co., Ltd.); distilled water (Suzhou, China, Suzhou Herishin Electromechanical Co., Ltd.).

Experimental Instruments: High-precision electronic balance (Kunshan, China, Kunshan Youkeweite Electronic Technology Co., Ltd.); ZQB400-S273 steam box (Hangzhou, China, Hangzhou Robam Appliances Co., Ltd.); 101-1B constant temperature electric blast drying oven (Yuyao, China, Yuyao Xingchen Instrument Factory); JB-80SH powerful electric mixer (Hangzhou, China, Hangzhou Xiniu Technology Co., Ltd.); hot press plate machine (Yiwu, China, Yiwu Yizhao Machinery Co., Ltd.); scanning electron microscope ZEISS Sigma 300 (Berlin, Germany); Fourier-transform infrared spectrometer, Thermo Fisher Scientific Nicolet iS5 (Waltham, MA, USA).

2.2. Experimental Methods

2.2.1. Preparation of Alkaline Water

A specific amount of slaked lime is dissolved in distilled water at room temperature. After the liquid clarifies, the supernatant is taken. The preparation method for saturated lime water involves adding more than 0.2 g of slaked lime to 100 mL of distilled water at room temperature. This mixture is stirred thoroughly to dissolve and then left to stand overnight. Insoluble lime precipitates at the bottom, and the supernatant is taken to obtain saturated lime alkaline water.

2.2.2. Preparation of Alkaline Paste

A mixture of 5% urea and 20% wheat starch is weighed and dissolved in a fixed concentration of lime water. This mixture is heated in a water bath to near boiling and cooked for 5 min with constant stirring to form a uniform, particle-free, semi-transparent paste. This alkaline steaming printing paste is set aside for later use.

2.2.3. Preparation of Alkaline Gel

Four grams of food-grade sodium polyacrylate is gradually and evenly added to 100 mL of sodium hydroxide solution with a fixed concentration. During the addition, an electric mixer is used to continuously and thoroughly stir the mixture. The mixture is left to stand overnight to ensure complete integration, forming an alkaline gel with a specific amount of alkaline agent.

2.2.4. Alkaline Boiling Degumming

Silk Gauze fabrics (all cut to 20 cm × 20 cm) are weighed and recorded using an electronic balance. The fabrics are then folded into four layers and clamped neatly between two 5 cm × 5 cm high-strength resin plates, secured with a G-clamp to apply constant clamping force. The clamped fabrics are immersed in alkaline water with different pH values at a bath ratio of 1:80 for alkaline boiling degumming. After treatment at various temperatures and times, the fabrics are repeatedly washed with distilled water until the wash solution is neutral in pH. Finally, the degummed fabrics are dried in an oven at 95 °C for 2 h, then left to equilibrate overnight, weighed, and stored. This experiment is repeated three times to test the degumming rate of the Silk Gauze.

2.2.5. Alkaline Steaming Degumming

The fabrics are weighed, recorded, and fixed onto a printing table. Using a flat screen with a single filament diameter of 106 μm (150 mesh), a large square pattern of 10 cm × 10 cm is printed on one side of the fabric. The printed fabric is placed in a high-temperature steam oven, treated for a specific temperature and time, then washed with warm water (40–55 °C) followed by cold running water until the paste is removed and the wash water is neutral. Finally, the fabrics are dried in an oven at 95 °C for 2 h, then left to equilibrate overnight, weighed, and stored. This experiment is repeated three times to test the degumming rate of the Silk Gauze.

2.2.6. Alkaline Gel Degumming

The fabrics are weighed, recorded, and fixed onto a printing table. Using a flat screen with a single filament diameter of 106 μm (150 mesh) and the prepared alkaline gel, a large square pattern of 10 cm × 10 cm is printed on one side of the fabric. The gel-printed fabric is placed between two layers of cotton cloth and processed in a hot press plate machine at a specified temperature and time for degumming. After degumming, the fabric is washed repeatedly with cold running water until the paste is removed and the wash water is neutral. The washed fabric is then dried in an oven at 95 °C for 2 h, left to equilibrate overnight, weighed, and stored. This experiment is repeated three times to test the degumming rate of the Silk Gauze.

2.3. Testing Methods

2.3.1. Calculation of Degumming Rate

The fabric is weighed accurately using an electronic balance. Each group consists of three degummed fabric samples, and the average degumming rate is calculated [14]. The formula for calculating the degumming rate is as follows:

$$\mathrm{W}/\%={\displaystyle \frac{{\mathrm{M}}_1-{\mathrm{M}}_2}{{\mathrm{M}}_1}}\times 100$$

where W is the degumming rate in percentage (%), M₁ is the dry weight of the fabric before degumming in grams (g), M₂ is the dry weight of the fabric after degumming in grams (g).

2.3.2. Water Retention Capacity Test

The water retention capacity of the alkaline gel is determined using strip-shaped quantitative filter paper (10 cm in length and 1 cm in width). A line is drawn 1 cm from one end of the filter paper with a pencil to serve as the baseline. The filter paper is then dipped into the prepared gel so that the baseline is level with the liquid surface. After a set period, the filter paper is removed, and the length of the wetted area above the baseline is measured using a vernier caliper. A shorter wetted length indicates a stronger water retention capacity of the original paste, resulting in clearer printed patterns when used in printing applications [15].

2.3.3. Scanning Electron Microscopy (SEM)

Samples of the Silk Gauze fabric, both before and after degumming, are prepared and gold-coated. The surface morphology and structure of the samples are observed using a ZEISS Sigma 300 scanning electron microscope (Berlin, Germany).

2.3.4. Fourier-Transform Infrared Spectroscopy (FTIR)

The samples are ground into a powder and prepared using the potassium bromide (KBr) pellet method. The samples are then analyzed using a Nicolet 5700 infrared spectrometer (Berlin, Germany).

3. Results and Analysis

3.1. Mechanism of Alkaline Agent Degumming

In the textile printing process, to obtain high-definition printed patterns, it is usually necessary to add appropriate thickeners to the printing paste to prevent bleeding that leads to blurred patterns [16]. Polyacrylic-acid-based thickeners, which contain carboxyl groups, are non-toxic, harmless, and environmentally friendly. These water-soluble inorganic polymer materials have hydrophilic groups such as carboxyl and hydroxyl on their molecular chains and possess a certain degree of cross-linking. They can slowly dissolve in water to form a highly viscous, transparent solution with excellent water absorption and retention capabilities [17]. In this study, we utilized the excellent water absorption and retention properties of sodium polyacrylate for alkali solvents to explore the application of sodium polyacrylate alkali gel in the rapid degumming and printing of Silk Gauze.

From

Table 1. Basic properties of alkaline gel.

Item	Concentration/%	Water Retention/mm		Viscosity/mPa∙s
Sodium polyacrylate	4	Distilled Water	5 g/L Alkaline Water	Distilled Water	5 g/L Alkaline Water
		0.9	0	9473	68,285

, it can be seen that when sodium polyacrylate polymer begins to disperse in distilled water, the water-holding capacity is 0.9 mm and the viscosity is only 9473 mPa∙s. At this point, some water molecules are free outside the polymer, and the distilled water is not fully absorbed by the polymer. However, when sodium polyacrylate is dispersed in prepared 5 g/L sodium hydroxide alkaline water, the viscosity of sodium polyacrylate increases sharply to 68,285 mPa∙s, and the water-holding capacity drops to 0 mm. This is because the carboxyl groups on the sodium polyacrylate molecules are neutralized by the alkaline water, ionizing in the aqueous solution to produce carboxylate anions.

On one hand, the electrostatic repulsion between carboxylate anions causes the originally coiled molecular chains to extend, forming a linear rod-like structure, leading to entanglement between molecular chains, which increases internal friction. The hydrodynamic volume of the macromolecular network increases, restricting the flow of free water [18]. On the other hand, the hydrophilicity of carboxylate anions is higher than that of carboxyl groups, allowing them to hydrate with free water molecules in the solution, reducing the number of free water molecules and greatly enhancing the resistance to movement between polymers, thereby hindering the flow of water molecules [19]. These two effects cause the viscosity of sodium polyacrylate in alkaline water to increase sharply, resulting in a gel state of saturated alkaline water. This alkaline gel state is highly favorable for forming fine alkali gel prints.

During alkali boiling degumming, the alkaline solution can fully soak and swell the sericin on the surface of the silk. When heated, the sericin is efficiently removed in the alkaline water. Therefore, alkali boiling degumming is efficient and rapid. Alkali steaming degumming relies on the gradual release of the alkali agent in the paste by the water molecules in the steam, which, due to the limited amount of steam, requires a long duration to proceed slowly. In contrast, alkali gel degumming utilizes the water retention properties of the gel to create a local degumming microenvironment that simultaneously meets the conditions of alkali, water, and heat required for the fine degumming and printing of Silk Gauze. Combining the dual effects of alkali water and paste alkali steam, it can achieve rapid degumming in a short time.

3.2. Alkali Gel Printing Process

Compared to the alkali-steaming method, which requires approximately 2 h of steaming post-printing, the alkali gel degumming–printing process drastically reduces processing time to about 1 min of hot pressing. As illustrated in Figure 1a, the prepared alkali gel is screen-printed onto the raw silk organza fabric. The printed fabric is then subjected to hot pressing for roughly 1 min using a heated platen press (Figure 1b). After removing residual gel through washing (Figure 1c), solid-color patterns emerge on the undyed fabric, remaining permanently intact without fading or disappearing over time. Subsequent dyeing (Figure 1d) exploits the differential dye uptake between degummed and raw silk areas: the printed regions exhibit soft, fluffy fibers with high luster, while unprinted areas retain stiffness, lightness, and transparency. By controlling the alkali gel printing parameters, the patterns achieve localized degumming with sharp, non-feathered edges, presenting new directions for reconstructing Tang Dynasty alkali-based textile printing techniques.

3.3. Comparison of Alkali Agent Degumming Processes

The degumming process of Silk Gauze fabric relies on the combined effects of alkali agents, water, and heat. This section mainly explores the impact of three key factors—alkali agent dosage, hot pressing temperature, and hot pressing time—on the degumming of Silk Gauze.

3.3.1. Effect of Alkali Agent Dosage on Degumming of Silk Gauze

Referring to Section 2.2.4 (alkali boiling degumming process) and Section 2.2.5 (alkali steaming degumming process), both processes use slaked lime as the alkaline auxiliary. When the alkali dosage is 0.5 g/L, at a temperature of 80 °C for 30 min, the degumming rate of alkali boiling reaches 27.43%. For alkali steaming, at a temperature of 100 °C for 120 min, the degumming rate reaches 25%. Referring to Section 2.2.6 (alkali gel degumming process), we set the hot pressing time to 90 s and the hot pressing temperature to 80 °C to investigate the effect of different sodium hydroxide dosages on the degumming of Silk Gauze. The results are shown in

Table 2. Effect of alkaline agent amount on degumming rate of silk gauze.

Alkaline Agent Amount g/L	Alkaline Boiling Degumming Rate/%	Alkaline Steaming Degumming Rate/%	Alkaline Gel Degumming Rate/%
0.05	11.32 ± 0.34	11.11 ± 2.75	\
0.1	16.57 ± 0.62	11.82 ± 2.13	\
0.2	24.51 ± 0.18	18.90 ± 1.52	\
0.3	27.17 ± 0.55	25.27 ± 0.54	\
0.4	27.43 ± 0.41	25.00 ± 0.45	\
0.5	28.13 ± 0.75	26.13 ± 0.62	\
1	\	\	6.98 ± 0.53
2	\	\	7.81 ± 0.21
3	\	\	9.79 ± 0.39
4	\	\	10.10 ± 0.39
5	\	\	10.31 ± 0.26
6	\	\	14.38 ± 0.26
7	\	\	22.19 ± 0.68
8	\	\	23.96 ± 0.64
9	\	\	24.90 ± 0.53
10	\	\	27.71 ± 0.39

“\” indicates that the expected experimental results were not achieved under the specified conditions and are, therefore, not presented.

.

From Figure 2, it can be visually observed that when the sodium hydroxide dosage is below 5 g/L, the degumming rate increases slowly, maintaining around 10% or less. When the dosage exceeds 6 g/L, the degumming rate gradually rises to about 25%. When the dosage continues to increase to 10 g/L, the degumming rate rises to 27.71%. It is evident that the alkali agent dosage has a significant impact on the degumming rate of the fabric; as the dosage increases, the degumming rate also increases.

In contrast, the alkali gel degumming process uses sodium hydroxide as the alkaline degumming auxiliary. The degumming amount only reaches 25% when the alkali dosage exceeds 6 g/L. Sodium hydroxide, being a strong alkali, has a much higher alkalinity than slaked lime, but its dosage required to achieve a 25% degumming rate is more than ten times that of slaked lime. The reason for this is closely related to the formation of sodium polyacrylate gel. According to preliminary experiments, when using saturated lime water or sodium hydroxide at dosages below 1 g/L, sodium polyacrylate disperses in a jelly-like state with low viscosity, showing almost no degumming ability. When the sodium hydroxide dosage exceeds 5 g/L, the viscosity of the dissolved sodium polyacrylate increases sharply, gradually transforming from a jelly-like state to a gel-like state. The alkaline solution is fully absorbed, forming an alkali gel, and the degumming ability gradually improves. Therefore, the high dosage of sodium hydroxide is related to the formation of sodium polyacrylate alkali gel, with part of it being neutralized by the carboxyl groups on the sodium polyacrylate molecules and part of it participating in the degumming printing of Silk Gauze.

3.3.2. Effect of Hot Pressing Temperature on Degumming of Silk Gauze

Referring to Section 2.2.6 (alkali gel degumming process), with a sodium hydroxide dosage of 8 g/L and a hot pressing time of 90 s, the effect of different hot pressing temperatures on the degumming of Silk Gauze was investigated. The results are shown in

Table 3. Effect of hot pressing temperature on degumming rate of silk gauze.

Hot Pressing Temperature/°C	Alkaline Boiling Degumming Rate/%	Alkaline Steaming Degumming Rate/%	Alkaline Gel Degumming Rate/%
40	\	\	7.4 ± 0.39
50	\	\	11.88 ± 0.68
60	20.28 ± 0.24	2.82 ± 2.11	16.56 ± 0.26
70	24.54 ± 0.11	3.01 ± 1.17	23.65 ± 0.39
80	27.12 ± 0.35	6.21 ± 1.46	23.96 ± 0.64
85	31.47 ± 0.16	10.61 ± 2.62	23.98 ± 0.32
90	34.01 ± 0.38	12.88 ± 1.31	22.5 ± 0.26
95	38.35 ± 0.52	18.79 ± 1.05	22.13 ± 0.45
100	42.14 ± 0.85	25.15 ± 0.26	21.77 ± 0.53
110	\	\	21.35 ± 0.82
120	\	\	21.25 ± 0.88
130	\	\	20.62 ± 0.68

“\” indicates that the expected experimental results were not achieved under the specified conditions and are, therefore, not presented.

. As the hot pressing temperature increased from 40 °C to 80 °C, the degumming rate rose from 7.4% to 23.96%. When the temperature continued to rise, the degumming rate gradually decreased, dropping to 20.62% at a hot pressing temperature of 130 °C.

As shown in Figure 3, whether using alkali boiling or alkali steaming degumming, the degumming rate continuously increases with rising temperature. However, during the alkali gel degumming process, when the hot pressing temperature exceeds 90 °C, the degumming rate of Silk Gauze decreases, which is significantly different from the degumming methods of alkali boiling and alkali steaming. During alkali steaming, the process relies on the gradual release of the alkali agent in the paste by the water molecules in the steam. When the temperature does not reach the boiling point of water, the amount of steam is limited, causing the degumming rate to increase slowly and never exceed 20%. At a temperature of 100 °C and steaming for 2 h, the degumming rate remains around 25%. In the alkali boiling degumming process, the sericin fully contacts the alkaline water solution, which better surrounds and removes the sericin from the surface of the silk, making alkali boiling degumming rapid and effective. At a boiling temperature of 80 °C for 30 min, the degumming rate reaches around 27%, significantly higher than that of alkali steaming [20].

During the alkali gel degumming process, the slow evaporation rate of water from the gel at lower temperatures allows the alkali agent to act continuously and steadily on the fabric for degumming. As the temperature continues to rise, the water in the gel evaporates faster. Once the gel is completely dehydrated and dried, the degumming of silk no longer proceeds. Therefore, in the alkali gel degumming process, excessively high temperatures should be avoided to prevent the impact of water evaporation from the gel.

3.3.3. Effect of Hot Pressing Time on Degumming of Silk Gauze

Referring to the alkaline gel degumming process described in Section 2.2.6, an amount of 8 g/L of sodium hydroxide and a hot pressing temperature of 80 °C were selected to examine the effect of different hot pressing times on the degumming of Silk Gauze. The results are shown in

Table 4. Effect of hot pressing time on degumming rate of silk gauze.

Hot Pressing Time	Alkaline Boiling Degumming Rate/%	Alkaline Steaming Degumming Rate/%	Alkaline Gel Degumming Rate/%
10 s	\	\	14.79 ± 0.82
30 s	\	\	22.81 ± 0.26
50 s	\	\	26.88 ± 0.26
70 s	\	\	25.52 ± 0.39
90 s	\	\	22.92 ± 0.39
110 s	\	\	21.46 ± 0.15
130 s	\	\	21.15 ± 0.97
150 s			21.25 ± 0.51
15 min	25.34 ± 0.56	16.54 ± 2.60	\
30 min	27.12 ± 0.35	20.00 ± 0.94	\
45 min	31.24 ± 0.81	21.43 ± 0.51	\
60 min	35.14 ± 0.62	22.53 ± 1.20	\
75 min	39.32 ± 1.17	23.72 ± 0.89	\
90 min	43.16 ± 0.62	24.41 ± 0.52	\
120 min	52.37 ± 0.54	25.27 ± 0.54	\
150 min	60.31 ± 0.78	25.54 ± 1.31	\

“\” indicates that the expected experimental results were not achieved under the specified conditions and are, therefore, not presented.

. It can be observed that the degumming rate increases from 8.98% at 10 s of hot pressing to 26.88% at 50 s of hot pressing. However, with further increases in hot pressing time, the degumming rate of Silk Gauze decreases and stabilizes at around 21%.

This trend of the degumming rate first increasing to a peak and then decreasing with time is different from the trends observed in the alkaline boiling and alkaline steaming degumming processes, which are displayed in Figure 4. During the alkaline boiling degumming process, at 80 °C, the degumming rate reaches 25.34% at 15 min and further increases to 27.12% at 30 min, indicating complete degumming. When the boiling time extends to 2 h, the fabric loses more than 50% of its weight, and the fibroin starts to degrade. In the alkaline steaming degumming process, at 100 °C, the degumming rate increases from 16.54% at 15 min to a stable rate of around 25% at 2 h. Although the degumming rate shows a slow increase with time, it does not decrease [21]. The differing trends in the degumming rate are significantly influenced by the change in water content during the degumming process with alkaline gel.

The alkaline gel provides the necessary conditions of alkali, water, and heat for the degumming of silk during localized printing. At the start of the degumming process, when these conditions are simultaneously met, the sericin swells and begins to separate from the fibroin. If heating is stopped at this point, the swollen sericin can be easily washed away with clean water, resulting in an increase in the degumming rate [22]. However, as time progresses, the water content in the alkaline gel gradually decreases, causing the swollen sericin to dry and shrink, re-adhering to the fibroin. This leads to a decrease in the degumming rate, which can be explained through Figure 5.

Figure 5 shows the surface morphology and fiber structure of Silk Gauze under different degumming times at 8 g/L sodium hydroxide and 80 °C. In Figure 5a, after 10 s of hot pressing degumming, partial degumming of the silk surface is observed, but the Silk Gauze yarns remain largely in their fiber bundle form. In Figure 5b, after 50 s of hot pressing degumming, the yarn fiber bundles appear fluffy, and the silk surface is completely degummed. As the degumming time continues to increase, the alkaline gel loses water and dries out, causing the separated sericin to re-adhere to the fibroin surface, as seen in Figure 5c. This results in a decreasing trend in the degumming rate over time.

Therefore, during the alkaline gel degumming process, it is crucial to perform the degumming wash before the gel completely dries out to prevent the re-adhesion of sericin.

3.4. Surface Morphology and Infrared Spectroscopy Analysis of Silk Gauze Under Different Alkaline Degumming Processes

To further observe the differences in degumming effects on the surface of Silk Gauze under different alkaline degumming processes and whether these processes affect fiber structure, the study conducted comparative analyses using SEM and infrared spectroscopy.

3.4.1. Surface Morphology Comparison of Silk Gauze After Degumming with Different Alkaline Agents

The study utilized SEM to further observe the surface morphology and structure of Silk Gauze under different process conditions. Figure 6a shows the undegummed Silk Gauze, where the surface of the raw silk fibers is rough, and sericin tightly bonds multiple fibroin strands together [23], with significant gaps between the warp and weft yarns. Figure 6b presents the surface morphology of Silk Gauze after high-temperature alkaline boiling treatment (complete degumming conditions: saturated lime water, bath ratio 1:80, temperature 80 °C, time 30 min). Figure 6c shows the surface morphology of Silk Gauze under conditions of lime-alkaline-paste–steam synergistic treatment (saturated lime water alkaline paste, steam temperature 100 °C, steam time 120 min). Figure 6d depicts the surface morphology of Silk Gauze under alkali gel degumming treatment (sodium hydroxide concentration 8 g/L, hot pressing temperature 80 °C, hot pressing time 50 s).

It can be observed that there are no significant differences in the surface morphology of Silk Gauze under the three degumming methods. The silk fibers achieved good degumming, with the fibroin exhibiting fine longitudinal stripes due to the microfibril structure. The fiber surfaces are smooth and free of sericin attachment, and the fabric structure appears fluffy [24].

3.4.2. Infrared Comparison of Silk Gauze After Degumming with Different Alkaline Agents

The differences in silk light transmittance and absorption peaks observed among different degumming processes primarily stem from the impact of degumming on silk’s molecular structure. Silk-I crystalline structure, a metastable form formed by stacking α-conformation fibroin chains, can transition to Silk-II (β-sheet) through hydrothermal treatment or mechanical stress. The three degumming methods showed slightly varied degumming rates under different processing conditions: (b) alkali boiling 27%, (c) alkali steaming 25%, and (d) alkali gel 26%. Figure 7 reveals that compared with undegummed sample (a), all degummed samples exhibited characteristic absorption peaks at 3275 cm⁻¹ (N-H stretching vibration) and at 1615, 1509, and 1221 cm⁻¹ (amide I, II, and III bands from β-sheet structures). Quantitative analysis of characteristic peaks in the 2000–1615 cm⁻¹ region using 3275 cm⁻¹ peak as an internal reference showed systematic enhancement of 1615 cm⁻¹ peak intensity (β-sheet, amide I) in degummed samples (b–d), with enhancement magnitude positively correlating with degumming rate, indicating increased β-sheet content. Notably, no distinct α-helix amide I peak was detected in the 1700–1650 cm⁻¹ region, and deconvolution analysis confirmed a significant reduction in α-helix components, supporting Silk-I to Silk-II conformational rearrangement.

All samples showed neither peak shifts nor new/absent peaks compared to the control, confirming that degumming processes preserved the primary chemical structure without generating new molecular configurations or functional groups [25]. The intensified absorption peaks at 1615 cm⁻¹ (amide I), 1509 cm⁻¹ (amide II), and 1221 cm⁻¹ (amide III) in degummed samples correlated with degumming rates, attributable to preferential removal of water-soluble amorphous Silk-I (α-helix) components with sericin elimination, while the stable water-insoluble Silk-II (β-sheet) proportion increased [26]. In the 700–1000 cm⁻¹ region (amide IV–VI bands and amino acid side-chain vibrations), no peak shifts or new peaks emerged. Calculated peak area ratios between amide bands (e.g., 950 cm⁻¹) and reference peak (760 cm⁻¹) showed no significant variations, confirming that degumming processes caused no disturbance to local protein backbone conformations, thereby excluding chemical bond cleavage or side reactions.

4. Conclusions

This study employed alkaline boiling, alkaline steaming, and alkaline gel methods to perform alkaline degumming on Silk Gauze fabrics. The degumming mechanisms, as well as the surface morphology and infrared spectra of the degummed Silk Gauze, were compared and analyzed. The following conclusions were drawn.

The alkaline degumming process of Silk Gauze fabrics relies on the combined actions of alkali, water, and heat. The study utilized the strong alkaline effect of sodium hydroxide to neutralize the carboxyl groups on sodium polyacrylate molecules, ionizing them into carboxylate anions, which dramatically increases viscosity and forms an alkaline gel. This alkaline gel, in the local degumming microenvironment, satisfies the conditions of alkali, water, and heat required for fine degumming of Silk Gauze, combining the dual effects of alkaline boiling and steaming. Rapid degumming can be achieved at a hot pressing temperature of 80 °C and a hot pressing time of 50 s. The study compared the effects of the three degumming processes (alkaline boiling, alkaline steaming, and alkaline gel) on Silk Gauze under different conditions of alkali concentration, hot pressing temperature, and hot pressing time. The surface morphology and infrared spectra of the degummed Silk Gauze were also compared. The results showed no significant differences in the surface morphology of Silk Gauze under the three degumming methods. The silk fibers’ surfaces were smooth and free of sericin attachment, and the main chemical structure was not significantly affected. The fine degumming of Silk Gauze achieved by the alkaline gel process has important implications for expanding the artistic expression and application of traditional Chinese alkaline printing techniques in modern silk degumming and printing.