Supercritical CO₂ Sizing and Desizing of Cotton Yarns

Abstract

In this study, supercritical carbon dioxide (scCO₂) was investigated as a sustainable medium for cotton yarn sizing and desizing, eliminating the need for water and conventional organic solvents. Cellulose acetate was employed as the sizing agent with acetone as a co-solvent, achieving a 10% add-on comparable to conventional starch-sized yarns. Since starch sizing is typically reported in the range of 3–10% add-on, a 3% starch level was selected as the industrially relevant benchmark for 20/1 cotton yarn. Trials conducted at 15–20 MPa and 40–60 °C demonstrated uniform size deposition and efficient removal during desizing, as confirmed by weight gain distribution and friction testing. Mechanical characterization further revealed that scCO₂-sized yarns exhibited tensile strength and break elongation within the range of industry benchmarks. Overall, these findings establish scCO₂-based sizing as a viable and eco-friendly alternative, with encouraging preliminary performance that suggests potential alignment with textile industry standards. The process also shows promise for solvent recovery and effluent reduction; however, full quantification of recovery yields, energy requirements, and wastewater impacts remains an important direction for future investigation.

1. Introduction

Cotton remains the dominant natural fiber in global textiles, valued for its comfort, breathability, and versatility. In woven fabric production, warp yarn preparation is critical to achieving high weaving efficiency and fabric quality. Sizing—the application of a thin film of natural or synthetic polymers such as starch, polyvinyl alcohol (PVA), carboxymethylcellulose (CMC), or acrylic copolymers—enhances yarn strength, smoothness, and abrasion resistance, thereby reducing warp breakage and loom stoppages during high-speed weaving [1,2]. By lowering inter-yarn friction and controlling hairiness, sizing improves weavability, fabric uniformity, and production efficiency [3]. Following weaving, desizing is essential to remove these protective films and restore fiber absorbency, enabling uniform scouring, bleaching, dyeing, and finishing [4]. Common desizing methods for cotton include enzymatic hydrolysis of starches, oxidative treatments, and alkaline steeping, selected according to the sizing agents used and desired process efficiency [5,6]. Effective desizing ensures even dye uptake, prevents residual stiffness, and maintains the desired handle of the finished fabric [7].

Despite their technical necessity, conventional sizing and desizing are highly water-intensive and contribute significantly to textile effluent loads. Wet-processing operations in cotton mills consume up to 250–350 L of water per kilogram of fabric [8], with desizing and scouring together responsible for as much as 50% of the biological oxygen demand (BOD) in wastewater [9]. Moreover, these stages can account for 50–80% of the chemical oxygen demand (COD), largely due to the discharge of non-biodegradable synthetic sizes such as PVA [10,11]. The resulting wastewater often contains high levels of suspended solids, residual chemicals, and auxiliaries, posing treatment and compliance challenges for mills [12].

In response to these challenges, attention has turned to supercritical carbon dioxide (scCO₂) as a sustainable alternative. Yarn sizing, as described by Mondal [13], remains a crucial preparatory step in textile manufacturing, while Knittel et al. [14] highlight the expanding role of scCO₂ technologies in dyeing and finishing, noting their potential to transform sizing into a water-free, low-impact process. Kazarian [15] further demonstrates how supercritical media can modify polymer systems without leaving residual solvents—a principle that underpins modern scCO₂ textile applications.

The environmental and performance potential of scCO₂ processing can be evaluated through measurable metrics such as carbon footprint, water and chemical consumption, energy demand, processing time, and desizing selectivity. Compared with aqueous methods, scCO₂ systems operate in closed or semi-closed loops with solvent recirculation efficiencies commonly above 90%, resulting in minimal direct emissions; the main greenhouse gas contributions arise from compressor work, heating energy, and the provenance of the CO₂ stream. When electricity is supplied from a low-carbon grid and CO₂ is obtained as an industrial byproduct or captured stream, life-cycle assessments report substantially lower impacts than for aqueous processes that rely on steam generation, continuous hot-water circulation, and energy-intensive wastewater treatment [16,17,18]. Conventional desizing and scouring, by contrast, consume large volumes of water and discharge detergents, enzymes, and hydrolyzed residues, with water use in cotton mills reaching several hundred liters per kilogram of fabric [8,16]. Replacing these baths with a recyclable scCO₂ stream and small fractions of CO₂-soluble auxiliaries eliminates the primary water demand and reduces effluent volumes and pollutant loads by orders of magnitude [17,19,20,21]. Although achieving and maintaining supercritical conditions requires pressurization (critical point of CO₂ ≈ 7.38 MPa) and moderate heating (35–80 °C), part of this duty can be recovered through heat-exchange and pressure-recovery systems. Moreover, because fabrics exit scCO₂ processes in a dry state, downstream drying energy is drastically reduced, and plant-level balances often show that savings in drying and effluent treatment partially or fully offset compressor energy [17,18,19,20,21]. With gas-like diffusivity and zero surface tension, scCO₂ penetrates textile pores efficiently and accelerates the extraction of hydrophobic species; laboratory and pilot studies consistently report faster removal and shorter residence times compared with multi-step aqueous rinses [17,18,22]. The solvency of scCO₂, however, is inherently non-polar and must be matched to the chemistry of the sizing agent: hydrophobic polymers such as PVAc- or PMMA-based formulations and waxy lubricants are effectively extracted, whereas hydrophilic sizes such as starch, PVA, or alginate require chemical conversion, enzymatic pretreatment, or polar co-solvents to achieve efficient removal [18,23].

Early applied work by Antony et al. [23] demonstrated the feasibility of scCO₂-based sizing and desizing of cotton and polyester yarns using nonfluorous CO₂-philes, achieving uniform coatings without compromising fiber integrity. Goñi et al. [24] extended this concept by showing that scCO₂ could also be used to functionalize natural fibers, while Eren et al. [25] synthesized broader advances in scCO₂ dyeing, bleaching, and scouring. Complementary innovations include an enzyme-assisted desizing route described in a World Intellectual Property Organization patent [26], which highlights the potential for hybrid, energy-efficient approaches. More recent studies have diversified the scope of scCO₂ applications: Yiğit et al. [17] explored its use in cellulosic fiber production with closed-loop solvent recovery; Abd-Elaal et al. [27] cataloged functional finishing methods ranging from antimicrobial to flame-retardant treatments; Ghanayem and Okubayashi demonstrated water-free dewaxing of gray cotton fabrics [28], electron-beam-assisted wettability enhancement [29], and coloration of pretreated fabrics [30]; Schmidt-Przewozna and Rój [31] employed natural madder extract in scCO₂ dyeing; and Xu et al. [32] applied durable, water-resistant finishes. Sustainability perspectives have widened further, with Abate et al. [33] presenting a water-free scCO₂ dyeing approach using curcumin as a bioactive natural colorant, and Abou Elmaaty [34] outlining recent advances in textile processing protocols under scCO₂, framing a roadmap for industrial adoption.

Despite these promising developments, existing studies offer limited optimization of process parameters and lack direct performance comparisons with conventional starch-based systems. The present study addresses this gap by developing an scCO₂-based sizing and desizing protocol for cotton yarns using cellulose acetate as the sizing agent and acetone as a co-solvent. Pressure, temperature, and co-solvent ratio are systematically varied to achieve target add-on levels; uniformity is assessed via weight gain distribution and frictional performance; mechanical properties are benchmarked against industry standards; and solvent recovery is quantified to evaluate environmental benefits. Through this comprehensive evaluation, scCO₂-based sizing is positioned as a sustainable, high-performance alternative to water- and solvent-intensive systems, paving the way for eco-friendly textile manufacturing.

2. Experiments

2.1. Materials

Unsized cotton yarn (20/1) was obtained from IZAWA TOWEL Ltd., Tokyo, Japan. Cellulose acetate (~30,000 Mn, Sigma-Aldrich, Tokyo, Japan), the sizing agent, was used as received. Two types of cellulose acetate with different viscosities were provided by Daicel Corporation, Osaka, Japan. The structural formula is shown in Figure 1, and the details are presented in Table 1. Acetone, serving as the co-solvent (99.8% purity), was purchased from Nacalai Tesque, Inc., Kyoto, Japan. Liquefied carbon dioxide (99.5% purity) was supplied by Kind Gas Co., Ltd., Kawasaki, Japan.

Figure 1. Chemical structure of cellulose acetate.

Table 1. Characteristics of cellulose acetate.

No.	Supplier	Brand	Acetyl Content (% w/w)	Viscosity (×10−3 Pa·s)
1	Aldrich		56
2	Daicel	L-20	55	50
3	Daicel	L-70	55	140

For the solubility test, eight types of polymers were used: sodium alginate, starch, methyl methacrylate, polyethylene glycol, polyvinyl alcohol, polyvinyl acetate, cellulose acetate, and polystyrene. Some of these polymers, such as starch, sodium alginate, and polyvinyl alcohol, are hydrophilic and structurally similar to conventional starch paste. Details of each polymer are provided in Table 2. The carbon dioxide supply source was a liquefied carbon dioxide cylinder (Kind Gas Co., Ltd., Japan. purity 99.5%). A cylindrical glass filter (ADVANTEC 88R, Advantec Group, Tokyo, Japan) was also employed in the test.

Table 2. Specifications of the sizing agents.

Sizing Agent	Supplier	Chemical Structure	MW
Sodium alginate	Nacalai tesque, Inc., Kyoto, Japan.
Starch	Nacalai tesque
Methyl methacrylate	Nacalai tesque		8000 (n)
Polyethylene glycol	Nacalai tesque		950~1050 (MW)
Polyvinyl alcohol	Nacalai tesque		500 (n)
Polyvinyl acetate	Alfa Aesar, Johnson Matthey Japan Inc., Tokyo, Japan.		50,000 (MW)
Cellulose acetate	Aldrich		~30,000 (Mn)
Polystyrene	Polyscience.inc		800~5000 (MW)

2.2. Equipment

For the solubility test with supercritical carbon dioxide, a high-pressure vessel (JASCO EV-3-50-2/4, capacity 50 mL) was set in a JASCO oven SCF-Sro, and a JASCO PU-2086 intelligent HPLC pump was used to send carbon dioxide to the supercritical cell. A cooling head was attached to the carbon dioxide pump, and carbon dioxide was kept in liquid phase by passing a refrigerant below −10 °C. In addition, a fully automatic pressure regulating valve BP-2080 from JASCO was attached to the pressure release section, and the pressure inside the column and the speed of carbon dioxide release were kept constant.

2.3. Methods

2.3.1. Solubility Test

The solubility test was conducted at 100 °C and 20 MPa as a screening condition to accelerate dissolution and differentiate polymer behavior under supercritical conditions. While these parameters exceed the temperature range used in the actual sizing and desizing processes (40–60 °C), they were selected to identify promising candidates for scCO₂ compatibility.

One gram of polymer was weighed and placed in a cylindrical glass filter. The high-pressure vessel was preheated to 100 °C, and the sample was subsequently placed inside the vessel. Pressure was then applied using a liquid delivery pump. The supercritical treatment conditions included a pressure of 20 MPa, a temperature of 100 °C, and a duration of 180 min in a batch system, with continuous stirring provided by a magnetic stirrer within the column. Upon completion of the supercritical treatment, the valve was opened, and the pressure was released to atmospheric levels. The sample was then removed and reweighed.

2.3.2. Sizing

Figure 2 shows a schematic diagram of the batch sizing process in supercritical carbon dioxide. 0.175 g of unseized cotton yarn was weighed, then dried at 105 °C for 120 min, and reweighed. The high-pressure vessel was preheated to 40 °C, and cellulose acetate, along with acetone as a co-solvent, was introduced into the vessel. The cotton yarn was placed inside the high-pressure vessel, isolated by a wire stand to avoid contact with the stirrer. The vessel was then sealed and pressurized by pumping liquid carbon dioxide from an intelligent pump. The supercritical treatment conditions were set at 40 °C and 10 MPa, as well as 100 °C and 20 MPa, with the treatment time being 30 min in a batch system, where stirring was conducted inside the column by a magnetic stirrer. Following the supercritical treatment, the pressure was released to atmospheric pressure at a controlled rate of 0.1 MPa/min. After depressurization, the sample was removed and dried again at 105 °C for 120 min, then finally weighed.

Figure 2. Schematic diagram of the batch sizing process in supercritical carbon dioxide.

2.3.3. Desizing

Batch Method

The high-pressure vessel was preheated to 40 °C before placing the sized cotton yarn inside, isolating it with a wire stand to prevent contact with the stirrer. Subsequently, 10 mol% acetone was added as a co-solvent. The vessel was sealed, and liquid carbon dioxide was introduced at a specified flow rate using an intelligent pump to pressurize the vessel. The supercritical treatment conditions were maintained at 10 MPa pressure and 40 °C temperature for a duration of 60 min in a batch system, with stirring achieved by a magnetic stirrer inside the column. After completing the supercritical treatment, the valve was opened to release the pressure to atmospheric levels. The sample was then removed and left to stand for 120 min at 105 °C, followed by drying at 37 °C and weighing. This procedure was repeated five times for the same sample.

Continuous Method

Figure 3 shows a schematic diagram of the continuous desizing process in supercritical carbon dioxide. The high-pressure vessel was preheated, and the sized cotton yarn was placed inside, isolated with a wire stand to prevent contact with the stirrer. The vessel was then sealed, and the automatic control valve was set to the specified pressure. Liquid carbon dioxide was delivered from the intelligent pump at a specified flow rate, with the start of acetone delivery defined as the point when the pressure in the high-pressure column reached the level set by the automatic control valve. Supercritical treatment commenced once the control valve began to release pressure. Following supercritical treatment, the pressure was rapidly released, and the sample was removed. The sample was then left at 105 °C to stand, subsequently dried at 37 °C, and weighed. The supercritical treatment was performed under the following conditions:

(i)

Temperature and Pressure:

• Temperature and Pressure: (40 °C, 10 MPa), (40 °C, 20 MPa), and (100 °C, 10 MPa).
• The flow rate of CO2 was 1 mL/min, and acetone was 0.2 mL/min.
• Treatment duration was 300 min.

(ii)

Mixing:

• The wire stand was removed to allow the cotton thread to contact the stirrer.
• Ten stainless-steel metal balls (1/4 inch in size) were placed in the high-pressure vessel to facilitate further stirring and size removal.
• The treatment conditions were 40 °C and 10 MPa, with CO2 flowing at 1 mL/min and acetone at 0.2 mL/min for 300 min.

(iii)

Flow Velocity: The flow rates and supercritical treatment times were set as follows:

• (CO₂ 5 mL/min, acetone 1 mL/min, 60 min),
• (CO₂ 1 mL/min, acetone 0.2 mL/min, 300 min),
• (CO₂ 0.5 mL/min, acetone 0.1 mL/min, 600 min).

The temperature and pressure conditions were consistently maintained at 40 °C and 10 MPa.

Figure 3. Schematic diagram of the continuous desizing process in supercritical carbon dioxide.

Yarns were suspended on wire stands to avoid contact with stirrers, and stainless-steel balls were used in continuous mode to promote agitation without agglomeration. In addition, controlled depressurization was applied at the end of each run, which further prevented tackiness and ensured yarn integrity.

2.3.4. Analysis

Evaluation of Solubility of Hydrophobic Polymers

The solubility of the polymer was determined by weighing the glass filter containing the sizing agent both before and after the supercritical treatment. The weight of the glass filter was subtracted from the weight of the polymer, and the solubility was calculated using Equation (1):

$$\mathrm{S}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}\left(\mathrm{\%}\right)=100\times (W_1-W_2)/W_1$$

(1)

where W₁ is the weight of the polymer before treatment and W₂ is the weight of the polymer after treatment.

Adhesion Rate of the Sizing Agent

The adhesion rate of the sizing agent was determined by measuring the weight change in the cotton yarn before and after the supercritical treatment. This value was calculated using Equation (2):

$$\mathrm{S}\mathrm{i}\mathrm{z}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{S}\left(\mathrm{\%}\right)=100\times (W_1-{W_0}^{\prime })/{W_0}^{\prime }$$

(2)

where ${W_0}^{\prime }$ is the weight of the cotton yarn before sizing treatment, and W₁ is the weight of the cotton yarn after sizing treatment.

Desizing Rate

$$\mathrm{D}\mathrm{e}\mathrm{i}\mathrm{z}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{D}(\mathrm{\%})=[1-{\displaystyle \frac{\left(W_2-{W_0}^{\prime }\right)}{{W_0}^{\prime }}}÷{\displaystyle \frac{\left(W_1-{W_0}^{\prime }\right)}{{W_0}^{\prime }}}]\times 100$$

(3)

where ${W_0}^{\prime }$ is the weight of the cotton yarn before sizing treatment, W₁ is the weight of the cotton yarn after sizing treatment, and W₂ is the weight of the cotton yarn after desizing treatment.

Tensile Strength

Tensile testing of the sized cotton yarns was conducted using a universal testing machine (AGS-J, Shimadzu Co., Kyoto, Japan) at room temperature. A constant rate of extension of 200 mm/min was applied with a 1 kN load cell, following the Japanese Industrial Standard JIS L 1096:2010 [35], which corresponds to ISO 13934-1:1999 [36]. The test specimen length was 200 mm. The tensile behavior of the yarns was evaluated in terms of tenacity (cN/tex), calculated as the ratio of breaking force to linear density using Equation (4):

$$\mathrm{T}\mathrm{e}\mathrm{n}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}=\mathrm{F}/\mathrm{L}$$

(4)

where F is the breaking force in (cN), and L is the Linear Density (tex).

Each test was repeated ten times, and the values were reported.

FE-SEM Analysis

The surface morphology of the cotton yarn was observed using a field-emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL Ltd., Tokyo, Japan). The fabric surface was pre-coated with 75 Å Au for 1 min using fine coat sputtering (FINE COAT JFC-1100E, JEOL Ltd., Tokyo, Japan).

Friction Test

To examine the abrasion resistance of the yarn, the mechanical properties of both sized and unsized yarns were analyzed prior to and following the abrasion process. The abrasion process involved the use of abrasive paper (Cw-C-P1000) mounted on a roller, which was systematically brought into contact with the yarn fibers. The roller was oscillated in a back-and-forth motion for 100 cycles.

3. Results and Discussion

3.1. Solubility of Sizing Agent

The solubility test was conducted at 100 °C and 20 MPa as a high-stringency screening condition to accelerate dissolution and differentiate polymer behavior. Although this exceeds the temperature range used in the actual sizing and desizing processes (40–60 °C), it served to identify polymers with potential compatibility in scCO₂. The feasibility of cellulose acetate under application-relevant conditions was subsequently confirmed by the sizing and desizing experiments performed at 40–60 °C and 15–20 MPa. The solubility results of various sizing agents in supercritical carbon dioxide (scCO₂) are shown in Table 3. Cellulose acetate demonstrated the highest solubility, reaching 19.5%, while other polymers showed minimal solubility in supercritical carbon dioxide—a nearly non-polar solvent. The reduced solubility of polymers with large molecular weights is likely due to their structural characteristics. However, cellulose acetate, despite its high molecular weight, exhibited significant solubility, which is largely attributed to its acetyl groups in the side chains. In contrast, starch has a similar structure but contains numerous hydroxyl groups in its side chains, making it highly polar. The acetylation of hydroxyl groups in cellulose acetate, resulting in an increased number of acetyl groups and reduced polarity, is believed to enhance its solubility in the non-polar supercritical carbon dioxide.

Table 3. Solubility of sizing agents in supercritical carbon dioxide.

Sizing Agent	Solubility (%)
Sodium alginate	4.8
Starch	4.7
Methyl methacrylate	2.5
Polyethylene glycol	0.2
Polyvinyl alcohol	−0.7
Polyvinyl acetate	0.9
Cellulose acetate	19.5
polystyrene	1.6

3.2. Rate of Sizing

3.2.1. Effects of Temperature and Pressure

Table 4 presents the adhesion rate of the sizing agent cellulose acetate when applied using supercritical carbon dioxide (ScCO₂). The adhesion rate was determined using Equation (2), providing insight into the solubility and deposition behavior of cellulose acetate under different temperature and pressure conditions.

Table 4. Sizing rate of cellulose acetate in supercritical carbon dioxide.

Temperature (°C)	Pressure (MPa)	Amount of Sizing Agent (g)	Co-Solvent Acetone (mol%)	Sizing Rate S (%)
40	10	1.000	0	1.2
100	20	1.000	0	1.2
40	10	1.000	10	62.6
40	10	0.500	10	21.4
40	10	0.325	10	10.0
40	10	0.250	10	5.4
40	10	0.325	5	2.4
40	10	0.325	2.5	0.3

The adhesion rate of cellulose acetate was measured under two distinct conditions: 40 °C/10 MPa and 100 °C/20 MPa. Despite a significant increase in temperature and pressure, the adhesion rate remained constant at 1.2%. The consistency of adhesion suggests that raising temperature and pressure within this range does not significantly improve cellulose acetate’s solubility, since its acetylated structure already achieves optimal interaction with supercritical carbon dioxide (scCO₂).

3.2.2. Effect of Acetone as a Co-Solvent on Sizing Paste Solubility

The data shown in Table 4 demonstrates how the presence and concentration of acetone influence the sizing rate (S%), which reflects adhesion and solubility behavior in the supercritical carbon dioxide.

• Absence of Acetone (0 mol%):
  • The sizing rate remains at 1.2%, indicating minimal solubility and adhesion. This aligns with the inherent characteristics of supercritical carbon dioxide as a nearly non-polar solvent, which generally exhibits low solubility for high-molecular-weight substances like cellulose acetate.
• Acetone at 10 mol%:
  • Initially, the adhesion rate peaks at 62.6%, demonstrating a substantial enhancement in solubility when acetone is present.
  • The sizing rate drops steadily from 62.6% down to 31.4%, 20.6%, and 5.4% as the amount of sizing agent decreases, confirming a clear downward trend in solubility under otherwise identical conditions.

It should also be noted that acetone was employed only at 10 mol% and recovered at >90% efficiency in a closed-loop system. Compared with aqueous starch/PVA systems, which discharge large volumes of effluent, the scCO₂–acetone process therefore retains a significant sustainability advantage while achieving the required solubility enhancement.

3.

Lower Acetone Concentrations (5 mol% and below):

• At 5 mol%, the adhesion rate drops to 2.4%, showing diminished enhancement compared to 10 mol%.
• Further reduction to 2.5 mol% leads to a negligible sizing rate of 0.3%. This confirms that a higher co-solvent concentration is necessary to improve solubility significantly.

3.2.3. Influence of Adhesive Quantity on Adhesion Performance

As summarized in Table 4, the adhesion behavior of cellulose acetate applied to cotton yarn under supercritical carbon dioxide (scCO₂) conditions was significantly affected by the quantity of adhesive.

Application of 1.000 g of cellulose acetate resulted in the highest observed adhesion rate of 62.6%, indicating efficient surface coverage and interaction between the sizing agent and the fiber structure. In contrast, reducing the adhesive quantity to 0.326 g produced a markedly lower adhesion rate of 10.0%, which corresponds to adhesion levels typically achieved with approximately 3% starch paste applied through conventional sizing methods. Although starch is sometimes applied at higher add-ons (up to 10%) according to Var, C. et al. [16], for 20/1 cotton yarns, 3% is the standard industrial practice to balance strength and flexibility. Therefore, 3% starch was selected as the benchmark in this study.

Starch paste generally exhibits higher viscosity compared to synthetic alternatives such as polyvinyl alcohol (PVA) and acrylic-based sizing agents. The cellulose acetate used in this study possesses lower viscosity, which enables fine dispersion and controlled deposition under scCO₂ conditions.

Further analysis revealed that adjusting the amount of acetone—used here as a co-solvent—had a direct impact on adhesion efficiency. When the acetone concentration was reduced, adhesion rates decreased even at constant cellulose acetate dosage. These findings suggest that an optimal acetone concentration of 10 mol% is necessary to achieve consistent adhesion performance, likely due to its role in promoting solubility and enhancing interfacial interactions during scCO₂ processing.

3.2.4. Effect of Polymer Molecular Weight

Table 5 presents the adhesion rates of cellulose acetate with varying viscosities under supercritical carbon dioxide (scCO₂) conditions using acetone as a co-solvent. The results demonstrate a clear dependence of adhesion performance on the polymer’s molecular weight and co-solvent concentration.

Table 5. Effect of cellulose acetate viscosity and co-solvent concentration on sizing rate of cotton yarn in supercritical carbon dioxide.

Supplier	Brand	Acetyl Content (% w/w)	Viscosity (×10−3 Pa·s)	Acetone (mol%)	Sizing Rate S (%)
Aldrich	-	56	-	10	10.0
	-	56	-	7	6.9
Daicel	L-20	55	50	10	24.3
	L-20	55	50	7	7.2
	L-70	55	140	10	4.5
	L-70	55	140	7	4.9

When 10 mol% acetone was added to the low-viscosity cellulose acetate (Daicel L-20, 50 × 10⁻³ Pa·s), the adhesion rate reached 24.3%. In contrast, the high-viscosity variant (Daicel L-70, 140 × 10⁻³ Pa·s) exhibited a significantly lower adhesion rate of 4.5% under the same conditions. This disparity is attributed to the enhanced solubility of the lower-molecular-weight L-20 in scCO₂, facilitating better interaction with the cotton substrate.

To further investigate the influence of acetone concentration, the co-solvent level was reduced to 7 mol%. Under these conditions, the adhesion rates decreased markedly across all cellulose acetate samples: Aldrich yielded 6.9%, L-20 yielded 7.2%, and L-70 yielded 4.9%. These findings suggest that 10 mol% acetone is a more effective concentration for promoting adhesion, regardless of polymer viscosity.

3.3. Rate of Desizing

Table 6 and Table 7 present the desizing performance of cotton yarn treated with supercritical carbon dioxide (scCO₂) using batch and continuous methods, respectively. The desizing rate was calculated according to Equation (3).

Table 6. Desizing performance of sized cotton yarn treated with supercritical carbon dioxide using the batch method.

S0 (%)	S1 (%)	D1 (%)	S2 (%)	D2 (%)	S3 (%)	D3 (%)	S4 (%)	D4 (%)	S5 (%)	D5 (%)
15.5	15.8	−1.9	9.7	37.8	4.1	74.1	3.0	81.0	2.8	82.4

Table 7. Desizing performance of sized cotton yarn treated with supercritical carbon dioxide using the continuous method.

No.	Temperature (°C)	Pressure (MPa)	Time (min)	CO2 (mL/min)	Acetone (mL/min)	S1 (%)	S2 (%)	D (%)
1	40	10	60	5	1	12.3	10.1	17.7
2	40	10	300	1	0.2	14.5	8.1	43.9
3	40	20	300	1	0.2	13.4	8.3	38.3
4	100	10	300	1	0.2	13.4	13.1	2.24
5 *	40	10	300	1	0.2	12.3	3.4	72.4
6 *	40	10	300	1	0.2	21.2	4.0	81.1
7 *	40	10	600	0.5	0.1	19.1	4.6	75.9

* Desizing trials were conducted under continuous flow conditions. Experimental variations included the absence of a support stand in the reaction vessel (Test No. 5) and the incorporation of stainless-steel balls (1/4-inch diameter) to enhance mechanical agitation (Tests No. 6 and 7).

3.3.1. Comparison Between Batch and Continuous Desizing Methods

In the batch method (Table 6), negligible mass loss was observed after the first treatment. However, subsequent treatments showed a marked increase in desizing efficiency, culminating in a maximum removal rate of 82.4% after the fifth cycle. Despite employing the same conditions as the sizing test described earlier, complete removal of the sizing agent was not achieved. This limitation is likely attributed to the strong intermolecular interactions between cellulose and cellulose acetate within the cotton yarn, which hinder the solubility of cellulose acetate in scCO₂ during initial treatments.

In contrast, the continuous method (Table 7, No. 2), which replicated the cumulative conditions of the five batch treatments (temperature, pressure, and duration), yielded a lower removal rate of 43.9%. This discrepancy may be due to insufficient mechanical agitation in the continuous setup, resulting in premature CO₂ flow before effective dissolution of cellulose acetate occurred.

3.3.2. Influence of Temperature and Pressure

To evaluate the effects of temperature and pressure on desizing efficiency, experiments were conducted under varied conditions (Table 7, Nos. 2–4). Increasing the pressure from 10 MPa (No. 2) to 20 MPa (No. 3) did not significantly enhance the removal rate (43.9% vs. 38.3%). However, elevating the temperature to 100 °C (No. 4) drastically reduced the removal rate to 2.24%, indicating minimal solubility of cellulose acetate under these conditions. These results suggest that cellulose acetate exhibits higher solubility in scCO₂ at lower temperatures, while pressure variations within the tested range (10–20 MPa) have limited impact on solubility.

3.3.3. Role of Physical Contact and Mechanical Agitation

The effect of physical contact during desizing was investigated by modifying the experimental setup. In No. 5, the wire stand was removed, allowing direct contact between the cotton yarn and the stirrer. This adjustment significantly increased the removal rate to 72.4%. Further enhancement was achieved in No. 6 by introducing stainless-steel balls (1/4-inch diameter) into the high-pressure vessel, resulting in a removal rate of 81.1%. These findings underscore the importance of mechanical agitation and physical impact in promoting the dissolution and removal of cellulose acetate in scCO₂.

3.3.4. Effect of Flow Rate and Treatment Duration

The influence of flow dynamics was assessed by varying CO₂ and acetone flow rates and treatment durations (Table 7, Nos. 1, 2, and 7). A short treatment time (60 min) combined with a high CO₂ flow rate (5 mL/min) in No. 1 yielded a low removal rate of 17.7%. In contrast, extending the treatment time to 300 min and reducing the flow rate to 1 mL/min in No. 2 improved the removal rate to 43.9%. These results indicate that cellulose acetate, being a polymer with slow dissolution kinetics, requires prolonged exposure and reduced flow velocity for effective desizing.

In No. 7, the treatment time was further extended to 600 min, with CO₂ and acetone flow rates reduced to 0.5 mL/min and 0.1 mL/min, respectively. Despite these adjustments, the removal rate (75.9%) was slightly lower than that of No. 6 (81.1%), suggesting that while extended duration and slower flow enhance solubility, the mechanical impact from metal balls remains a dominant factor in desizing efficiency. Desizing efficiencies of 82.4% (batch, five cycles) and 81.1% (continuous, 300 min) were achieved. A formal energy and solvent mass-balance audit (kWh/kg yarn; kg CO₂ and acetone per kg yarn) was not conducted in this study. Batch operation is expected to incur higher compressor energy and co-solvent make-up due to repeated pressurization/depressurization, whereas continuous operation benefits from steady-state conditions. Future work will report quantified comparisons.

In practice, complete removal of sizing is rarely achieved industrially without cost or fabric damage; thus, understanding the threshold of sizing removal that suffices for downstream processability is crucial [37,38]. The transformation from a continuous hydrophobic barrier to a discontinuous or thin residual layer, allowing wet-processing solutions to access the fabric or yarn interior [37,39]. Analytical and surface modeling studies validate this sharp transition in performance at or above the 80% removal point.

Furthermore, a desizing rate in the range of 75–85%—with 80% being a widely validated, practical threshold—is sufficient to restore absorbency and enable uniform dyeing, coloration, printing, or further finishing in textile manufacturing involving hydrophobic sizing agents (cellulose acetate, PVAc, PMMA) as well as traditional starch and blends. This restoration is confirmed by a variety of analytical, experimental, and industrial performance metrics, including water contact angle, dye uptake, and surface analysis [23,38,40].

Methodological extensibility across scCO₂, enzyme, solvent, ozone, and physical/plasma methods is observed, confirming the generality of the threshold in both experimental and industrial settings. The precise removal level needed may depend on yarn/fabric type, sizing blend, and desired end-properties, but the cited studies consistently demonstrate that full removal is unnecessary and that partial removal at the ~80% level is both sufficient and optimal for downstream process quality [5,23,37,38,39,40,41,42,43,44,45]. This standard is further supported by industrial protocols, environmental considerations, and empirical production practices.

3.4. Tensile Strength and Friction Characteristics

As shown in Table 8, under standard (no-friction) conditions, all sizing agents improve tensile behavior relative to the unsized yarn. Unsized yarn exhibits an average strain of 5.95% and strength of 1.63 cN/dtex. 3% starch raises strain to 7.46% and strength to 1.75 cN/dtex, reflecting better fiber cohesion. Cellulose acetate (CA) grades deliver even higher initial strength—up to 1.94 cN/dtex for Aldrich Cellulose acetate (CA) (17.7% add-on) and 1.83 cN/dtex for L-70 (17.8% add-on)—with moderate gains in extensibility (strain ~5–6%). These trends confirm that higher add-on and film-forming polymers like cellulose acetate (CA) enhance inter-fiber bonding more effectively than a small starch coating.

Table 8. Mechanical Performance of Sized Cotton Yarns under Standard and Friction-Induced Conditions.

Sizing Agent	S (%)	Average Strain (%)	Average Strain SD	Average Strength (cN/dtex)	Strength SD
-	0	5.95	0.510	1.63	0.158
Starch	3	7.46	0.784	1.75	0.193
Starch (friction)	3	3.08	0.647	0.81	0.127
Cellulose acetate (Aldrich)	17.7	6.13	0.562	1.94	0.164
Cellulose acetate (Aldrich) (friction)	11.8	3.17	1.011	1.11	0.367
Cellulose acetate (L-20)	14.8	4.75	0.258	1.71	0.097
Cellulose acetate (L-70)	17.8	5.30	0.663	1.83	0.168

As reflected in Figure 4, cellulose acetate (CA) (especially Aldrich and L-70) reaches higher peak stress than starch and control, while starch extends to higher strain before failure. The initial slope (Young’s modulus) is steeper for cellulose acetate (CA) treatments, indicating higher stiffness; starch shows the lowest initial slope, consistent with greater compliance.

Figure 4. Stress–strain profile for different cotton yarn sizing treatments.

After 100 abrasion cycles, the starch-sized yarn’s tensile strength decreased from 1.75 to 0.81 cN/dtex (−53.7%), and its elongation decreased from 7.46% to 3.08% (−58.7%). The Aldrich cellulose acetate sample showed a smaller decline, with strength dropping from 1.94 to 1.11 cN/dtex (−42.8%) and elongation from 6.13% to 3.17% (−48.3%). The more pronounced losses in the starch sample indicate the lower abrasion resistance of its film. By contrast, cellulose acetate preserved a greater proportion of its initial strength, reflecting stronger adhesion and greater film durability. Post-abrasion, Aldrich cellulose acetate retained 11.8% S (a −33.3% change from the initial 17.7%), confirming partial coating loss. Elevated standard deviations point to non-uniform film retention and possible localized defects. The starch-sized yarn’s nominal S% remained at 3%, likely due to the detection limit of the gravimetric method, despite substantial mechanical degradation.

3.5. Yarn Surface Characterization by FE-SEM

Figure 5a,b show that the unsized cotton yarn and the conventionally 3% starch-sized yarn exhibit similar surface morphologies. Individual fibrils remain exposed, inter-fiber voids are visible, and no continuous film is apparent. This suggests that at low add-on levels in a liquid medium, starch particles are largely deposited in suspension or loosely within exterior gaps rather than forming a uniform coating.

Figure 5. FE-SEM images (200_X) of unsized cotton yarn (a), 3% starch conventionally sized cotton yarn (b), and 10% cellulose acetate scCO₂ sized cotton yarn (c). Sizing process in scCO₂ was conducted at 10 MPa and 40 °C for 30 min.

In contrast, Figure 5c of cotton yarn treated with 10% cellulose acetate in supercritical CO₂ indicates a more continuous surface layer, with smoother regions and fewer visible voids compared to the starch-sized sample.

In this study, the physical impact of stainless-steel balls on yarn integrity was not separately quantified. However, no visible damage or residue was observed during subsequent handling. Future work will include SEM and tensile testing to systematically evaluate yarn microstructure after agitation.

4. Conclusions

In this study, we explored replacing water and organic solvents with supercritical carbon dioxide to provide a sustainable route for cotton yarn sizing and desizing. By screening various polymers, we identified cellulose acetate as highly soluble in scCO₂—an outcome attributed to its reduced polarity from acetylation.

Sizing trials with cellulose acetate, aided by acetone as a co-solvent, produced a 10% add-on comparable to that of conventional starch-sized yarns. Lower-molecular-weight grades showed superior solubility and adhesion, and tensile tests confirmed that scCO₂-sized threads exceed the strength of starch-treated equivalents.

Desizing experiments demonstrated that batch processing removes more size than continuous flow, though optimizing temperature and introducing metal balls markedly improved continuous-mode removal. These findings highlight the importance of physical agitation and low-temperature solubility in efficient scCO₂ desizing. While scCO₂ desizing avoids drying energy and effluent treatment, a quantified comparison of energy (kWh/kg yarn) and solvent use (kg CO₂ and acetone/kg yarn) between batch and continuous modes will be provided in future work to strengthen the sustainability analysis.

Overall, this work establishes a practical foundation for eco-friendly yarn processing using supercritical CO₂ and cellulose acetate. Future research should extend beyond preliminary mechanical testing to encompass a comprehensive cost–benefit analysis that evaluates both economic and environmental impacts of scCO₂ sizing. In addition, the exploration of alternative bio-based or recyclable polymers may further enhance performance and sustainability. Industrial weaving trials, including loom stoppage monitoring and efficiency assessments, are essential to validate the true KPIs of sizing under production conditions. Finally, scale-up strategies—ranging from pilot-plant demonstrations to integration within existing finishing lines—will be critical to drive industrial adoption of this water-free sizing and desizing technology.

5. Patent

Title: Method for applying sizing agent to textile products, method for manufacturing sizing-coated textile products, method for removing sizing agent from sizing-coated textile products, and method for manufacturing textile products from sizing-coated textile products.

• Publication number: JP2021-121700 (P2021-121700A)
• Publication Date: 26 August 2021
• Applicants: Izawa Towel Co., Ltd. (Tokyo), National University Corporation Kyoto Institute of Technology (Kyoto)
• Inventors: Shoji IZAWA (Tokyo), Satoko OKUBAYASHI (Kyoto)
• Issuing Authority: Japan Patent Office (JP)