Objective: To study the dissolution and hydrolysis properties of polyester sizes.

Methods: By immersing polyester sizes and size films in ultrapure water and alkaline solutions, their solubility and hydrolytic characteristics were observed.

Results: In ultrapure water, the dissolution time of polyester sizes and size films decreased as temperature increased, while increased water hardness led to longer dissolution times. In NaCl solutions, dissolution time increased with higher salt concentration. In alkaline solutions, dissolution time increased as the concentration of the alkali increased. In a 1.5 g/L benzalkonium chloride solution and a 30.0 mg/ml sodium hydroxide solution, the polyester sizes and size films achieved complete hydrolysis within a short period. The experiments demonstrate that the addition of appropriate solvents to polyester sizes helps improve their dissolution and hydrolysis performance.

Polyester size is a novel macromolecular size containing ester groups and water-soluble groups, synthesized through the polycondensation of benzenedicarboxylic acid, glycol, and hydrophilic organic monomers. Due to its chemical structure being similar to that of polyester fibers, it exhibits strong adhesion to polyester fibers based on the principle of “like dissolves like,” making it suitable for sizing. Furthermore, because it is biodegradable and causes minimal environmental pollution, it has received significant attention and recognition.

Polyester size was successfully developed by Eastman Chemical Company (USA). To date, countries such as the United States, France, and Germany are capable of producing mature products. Research on polyester sizing in China began in the 1990s. Based on the mechanism of transesterification polycondensation, an amorphous copolymer polyester size was prepared, which possesses excellent viscosity and good sizing performance. Studies on the sizing properties of polyester size and its blended sizes indicate that polyester size can be used as a substitute in the sizing of polyester blended yarns.

After years of development and refinement, although the sizing performance of polyester fiber fabrics has been improved, the issue of poor water solubility remains to be solved. During the desizing process, polyester size may form precipitates or become insoluble in strong electrolyte solutions, leading to incomplete desizing.

1. Experimental Section

1.1 Instruments and Materials

1.1.1 Materials: Polyester filament size, polyester staple fiber size, magnesium sulfate, benzalkonium chloride, sodium chloride, sodium hydroxide, sodium carbonate. 1.1.2 Instruments: MILLIPORE Reverse Osmosis Water System (RIOSS), TW20 Constant Temperature Water Bath, Programmable Constant Temperature and Humidity Chamber.

1.2 Test Methods

1.2.1 Preparation of Size Films Two types of sizes—polyester filament size and polyester staple fiber size—were prepared into 6% solutions. Once the temperature reached approximately 70°C, the solutions were poured uniformly onto photographic paper and leveled using a steel ruler. The films were dried in a constant temperature and humidity chamber (set at 20°C and 65% RH) and then left for 24 hours. The thickness and moisture content were 0.09 mm and 6.0% for the polyester filament film, and 0.11 mm and 6.5% for the polyester staple fiber film, respectively.

1.2.2 Determination of Solubility and Hydrolysis Polyester size particles are in the nanometer range, which is smaller than the wavelength of visible light. According to the Tyndall effect, when a beam of light passes through a fully dissolved size solution, a light column is visible. Polyester size is a polymer containing ester bonds; upon hydrolysis, the ester bonds break to form water-soluble dibasic acid and diol monomers, which disperse into the solution, resulting in a transparent liquid without precipitates.

A specific weight of size or size film was placed into a solution to create a 1% concentration. Different temperatures were set, and the time taken for complete dissolution/hydrolysis was recorded. If complete dissolution/hydrolysis did not occur within 1 hour, it was recorded as “insoluble.” Each sample was measured five times, and the average was taken as the final result.

2. Results and Discussion

2.1 Dissolution Performance

2.1.1 Temperature The dissolution process of polyester is divided into two stages: swelling and dissolution. Swelling occurs as water molecules diffuse into the interior, causing the volume to expand. As more water molecules penetrate, the swollen polymer disperses uniformly into the water, completing the dissolution process.

In the experiments, the temperature was set between 70°C and 95°C. The dissolution time of the polyester sizes and size films decreased as the temperature increased. The dissolution times for the polyester filament film and the polyester staple fiber size/film were basically the same, with complete dissolution achieved within 5 minutes. The polyester filament size, due to its smaller contact area with the ultrapure water, required a longer time to dissolve (as shown in Figure 1).

Figure 1 Dissolution of polyester pulp and membrane in ultrapure water

Figure 1 Dissolution of polyester pulp and membrane in ultrapure water

2.1.2 Hardness

The hardness of the ultrapure water also has a certain impact on the dissolution of polyester sizes and size films. The water hardness was adjusted by varying the concentration of MgSO4 solution, as shown in Table 1.

table 1

table 1

Figure 2. Solubility of Polyester Sizes and Size Films in MgSO4 Solutions of Different Concentrations

Figure 2. Solubility of Polyester Sizes and Size Films in MgSO4 Solutions of Different Concentrations

In the experiment, the concentration of the MgSO4 solution was set from 0.0 g/L to 0.36 g/L. As the solution concentration increased, the dissolution time of the polyester sizes and size films gradually lengthened. This is primarily because Mg2+ ions bind with the water-soluble groups, hindering ionization and thereby reducing water solubility. When the MgSO4 concentration reached ≥0.36g/L, the dissolution time exceeded 1 hour for all samples. The polyester staple fiber size demonstrated better resistance to hard water. This is shown in Figure 2.

2.1.3 Sodium Chloride Concentration

In the experiment, the concentration of the NaCl solution was set from 0.1 g/L to 3.0 g/L. In NaCl solutions, the dissolution time of the polyester sizes and size films gradually increased as the salt concentration rose. This occurs because NaCl inhibits the ionization of water-soluble groups, leading to a decrease in dissolving power. When the NaCl concentration was ≤ 1.0 g/L, the dissolution time for the polyester sizes and size films was less than 1 hour; however, when the NaCl concentration was ≥2.0g/L, the dissolution time for the polyester filament size and size film exceeded 1 hour. This is shown in Figure 3.

Figure 3. Solubility of Polyester Sizing Agents and Size Films in NaCl Solutions of Varying Concentrations

Figure 3. Solubility of Polyester Sizing Agents and Size Films in NaCl Solutions of Varying Concentrations

2.1.4 Types and Concentrations of Alkali

2.1.4.1 NaOH Solution as the Alkaline Medium

In the experiment, the NaOH solution concentration was set from 0.1 g/L to 10.0 g/L. It was observed that polyester sizes and size films could be completely dissolved within 9 minutes in a 0.1 g/L NaOH solution. The polyester staple fiber size film exhibited higher alkali resistance than its corresponding size. When the NaOH concentration exceeded 0.5 g/L, the dissolution time for the polyester filament size/film and the polyester staple fiber size all exceeded 1 hour.

2.1.4.2 Na2CO3 Solution as the Alkaline Medium

In the experiment, the Na2CO3 solution concentration was set from 0.1 g/L to 10.0 g/L. It was found that polyester staple fiber sizes and films could be completely dissolved within 9 minutes in ≤1.0 g/L Na2CO3 solutions. When the Na2CO3 concentration exceeded 5.0 g/L, the dissolution time for all polyester sizes and films exceeded 1 hour.

Comparing the dissolution experiments of the two alkaline solutions, it was observed that at the same alkali concentration (< 5.0 g/L), the dissolution time of polyester sizes and films in NaOH solution was longer than that in Na2CO3 solution.

2.2 Hydrolysis Properties

The hydrolysis of polyester sizes and films was observed in benzalkonium chloride solutions of 1.0 g/L, 1.5 g/L, and 2.0 g/L, with NaOH concentrations ranging from 0.1 g/L to 40.0 g/L. Polyester size is a polymer containing ester bonds; upon hydrolysis, these bonds break to form water-soluble dibasic acid and diol monomers. Under alkaline conditions, benzalkonium chloride is rapidly adsorbed; upon heating, an ion-exchange reaction occurs, causing OH- ions in the solution to aggregate at the surface, which accelerates the hydrolysis rate.

The experiments showed that when the benzalkonium chloride concentration is in the range of 1.0 g/L to 2.0 g/L and the NaOH concentration is > 30.0 g/L, the polyester sizes and films can be completely hydrolyzed within 1 hour. In a benzalkonium chloride solution of the same concentration, the hydrolysis time decreases as the NaOH concentration increases. Polyester filament size is more difficult to hydrolyze than its film, whereas polyester staple fiber size is easier to hydrolyze than its film. In the same concentration of NaOH solution, there was no significant change in hydrolysis performance as the concentration of benzalkonium chloride increased.

Conclusion and Technical Support

In summary, the dissolution and hydrolysis performance of polyester sizes are significantly influenced by factors such as temperature, water hardness, electrolyte concentrations, and alkaline conditions. By precisely controlling desizing process parameters—such as optimizing the temperature range, maintaining an appropriate alkaline environment, and utilizing effective chemical additives—manufacturers can overcome the challenges of poor water solubility and residual sizing, thereby achieving more efficient and environmentally friendly textile production.

As a leading supplier specializing in water-soluble polyester sizing agents, we are committed to providing our clients with comprehensive technical support, ranging from material selection to desizing process optimization. If you encounter technical challenges in your product development or production processes, please do not hesitate to contact us. We are ready to provide you with high-performance, customized sizing solutions.

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