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12 February 2026

16 Pages

Effect of Accelerated Thermal Aging on the Dispersion Stability of Fine-Denier Silicone Emulsions for Carbon Fiber Precursor Processing

,
,
and
1
Convergence Research Division, Korea Carbon Industry Promotion Agency (KCARBON), Jeonju 54852, Republic of Korea
2
Department of Organic Materials and Textile Engineering, Jeonbuk National University, Jeonju 54896, Republic of Korea
3
Department of Carbon-Nanomaterials Engineering, Jeonju University, Jeonju 55069, Republic of Korea
*
Authors to whom correspondence should be addressed.
Materials2026, 19(4), 702;https://doi.org/10.3390/ma19040702
This article belongs to the Special Issue Processing and Mechanical Properties of Polymer Composites
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Highlights

What are the main findings?
  • Fine-denier silicone emulsions were prepared using disperser and colloid mill methods.
  • Colloid mill emulsification produced smaller and more stable droplet sizes.
  • Thermal aging at 50–70 °C mainly affected dispersion stability, not chemical structure.
What are the implications of the main findings?
  • Emulsification method strongly governs thermal stability of silicone emulsions.
  • Stable dispersion at 3–5% concentration is suitable for PAN fiber processing.
  • Colloid mill processing improves process stability in carbon fiber manufacturing.

Abstract

Fine-denier silicone emulsions play an important role in the polyacrylonitrile (PAN) precursor treatment process by reducing surface tension and preventing fiber fusion during thermal stabilization and carbonization. These emulsions are typically prepared by dispersing polydimethylsiloxane (PDMS) polymers with various functional groups into water through different emulsification methods. In this study, two types of silicone emulsions—one prepared using a mechanical disperser and the other using a high-shear colloid mill—were manufactured on a pilot scale and systematically compared. Thermal aging was conducted at 50 °C and 70 °C for approximately one month, and changes in particle size, dispersion stability, and physicochemical properties were evaluated. The colloid-mill emulsification method produced smaller and more uniform silicone particles and exhibited superior thermal and dispersion stability relative to the mechanically dispersed emulsion. NMR relaxation, Turbiscan multiple light scattering, and viscosity measurements confirmed that the colloid-mill emulsion maintained a stable microstructure with minimal aggregation even under elevated-temperature storage. These results demonstrate that high-shear emulsification is an effective approach for producing fine-denier silicone emulsions with enhanced stability, making the colloid-mill method a more reliable and practical route for preparing silicone-based oiling agents used during PAN precursor processing in carbon fiber manufacturing.

1. Introduction

Carbon fiber is widely used in the aerospace, civil construction, military, automotive, and sports industries owing to its outstanding properties, including high tensile strength, low density, and a low coefficient of thermal expansion. In particular, carbon fiber-reinforced plastics (CFRPs) exhibit excellent rigidity, corrosion resistance, and lightweight characteristics, making them attractive for structural components in transportation equipment and industrial machinery. Among various CFRP processing routes, sheet molding compound (SMC)-based CFRPs have been extensively adopted in the automotive industry for exterior components such as bonnets and fenders [1,2,3,4,5]. The performance and process stability of carbon fibers are influenced not only by the precursor materials but also by surface treatment agents applied during fiber processing.
Carbon fibers are typically produced by converting precursor materials such as polyacrylonitrile (PAN), coal tar pitch, petroleum-based pitch, or rayon into flame-resistant fibers under an oxidizing atmosphere, followed by carbonization under a high-temperature inert atmosphere. During these high-temperature processes, individual filaments tend to fuse together, leading to deterioration in fiber quality and yield. To mitigate this issue, silicone-based oil agents with excellent thermal resistance and release properties have been widely applied in industry. In particular, amino-modified silicone oils are commonly used because their functional groups promote crosslinking reactions, thereby enhancing thermal stability. When applied to precursor fibers, these oils form a smooth and heat-resistant interfacial layer that suppresses filament fusion during carbonization. However, previous studies have reported that amino-modified silicone oils may exhibit phase separation and adhesive behavior under certain processing conditions, resulting in oil deposition on rollers or guides, fiber breakage, and overall process instability, which ultimately degrades the mechanical properties of the final carbon fibers. Consequently, silicone-based oil agents function not only as lubricants but also play a critical role in controlling surface adhesion and filament aggregation under high-temperature conditions [6,7,8,9,10,11].
To improve process stability and fiber quality, oil agents are commonly applied in the form of emulsions to prevent adhesion between individual filaments during carbon fiber manufacturing. In practice, silicone-based oil agents are introduced at specific stages of the PAN fiber processing route, including stabilization and carbonization, as schematically illustrated in Figure 1. During these stages, silicone emulsions are repeatedly exposed to elevated temperatures, making their dispersion stability a key factor governing continuous processing, coating uniformity, and defect suppression. Previous studies have demonstrated that dispersion stability is strongly influenced by droplet size distribution, emulsification method, and thermal exposure, which collectively determine process continuity and fiber performance [9,10,12,13,14]. Recent studies have further emphasized the necessity of establishing quantitative relationships between processing technology, microstructural design, and functional performance in composite-related systems, demonstrating that processing-induced microstructural control plays a decisive role in long-term stability and performance [15].
Figure 1. Pilot scale polyacrylonitrile (PAN) fiber manufacturing process.
Despite these advances, most existing studies on silicone emulsions for carbon fiber processing have primarily focused on short-term stability or ambient conditions. Systematic investigations that quantitatively compare different emulsification methods under long-term thermal aging conditions—particularly from a process–structure–performance perspective—remain limited. Moreover, the influence of emulsification technology on the evolution of droplet microstructure and its subsequent impact on thermal dispersion stability has not been sufficiently clarified.
Although fiber formation itself occurs over a relatively short time scale, silicone emulsions may be exposed to elevated temperatures for extended periods during storage, handling, and repeated processing. Such conditions can promote physical destabilization phenomena, including droplet coalescence and phase separation, underscoring the need for evaluating long-term thermal dispersion stability under conditions relevant to industrial practice.
Accordingly, this study aims to elucidate the quantitative relationship between emulsification method, droplet microstructure, and thermal dispersion stability of fine-denier silicone emulsions used in PAN-based carbon fiber processing. By systematically comparing different emulsification technologies under accelerated thermal aging conditions, this work seeks to provide mechanistic insight and practical guidance for the rational design of oil agents capable of maintaining stable performance during high-temperature fiber processing.

2. Materials and Methods

2.1. Manufacturing of Silicone Emulsions

A polydimethylsiloxane (PDMS)-based silicone oil with a viscosity of approximately 1000 cP was used as the dispersed phase. A commercially available nonionic surfactant suitable for silicone emulsion stabilization was employed as the emulsifying agent. Deionized water was used for all experiments. All materials were used as received without further purification. The total volume of the diamino functional silicone fluid and emulsifier was 2 L. Its temperature was maintained at 25 °C for 15 min and the mixture was stirred until the silicone fluid and emulsifier were uniformly dispersed. The first silicone emulsion was prepared by stirring the mixture with 500 mL of distilled water using a disperser operated at 500 rpm to form a liquid crystalline phase. For the second emulsion, approximately 4 L of deionized water was gradually added at a rate of 40 mL/min while processing through a colloid mill operating at 7200 rpm with a rotor–stator gap of 15–20/1000 inch. Emulsification was continued for 25 min to obtain a uniform emulsion. Finally, to enhance emulsion stability, acetic acid was added under continuous stirring until its concentration reached 10% and the pH reached 7. The emulsion prepared using the disperser was designated as Emulsion #1, while that prepared using the colloid mill was designated as Emulsion #2.

2.2. Silicone Emulsion Thermal Acceleration Experiment

In order to analyze the thermal stability of the prepared silicone emulsions, both samples were stored in a hot air oven at 50 °C for approximately one month and at 70 °C for the same amount of time. After one month, the thermal acceleration test data for the emulsions and the dispersion stability of the silicone particles were confirmed.

2.3. Characterization of Silicone Emulsions

Changes in the silicon particle sizes during the thermal acceleration experiment were measured using a particle size analyzer (ELSZ-2000ZS, Otsuka Electronics Co., Ltd., Osaka, Japan). For each silicone emulsion, particle size measurements were performed ten times, and the results are reported as mean values with corresponding standard deviations. The chemical composition of each silicone emulsion was evaluated using Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA).
The dispersion stability of each silicone emulsion was measured using pulse nuclear magnetic resonance (NMR; Acorn area, Xigo Nanotools). Each silicone emulsion was measured as a stock solution by placing 30 mL of the emulsion in an NMR tube to prevent bubble formation. The dispersion stability was further evaluated based on the principle of multiple light scattering using a Turbiscan (TURBISCAN Tower, Formulation). Each silicone emulsion was subjected to accelerated thermal aging by placing it in a 20 mL cell and storing it at 30 °C and 80 °C for three days. After thermal aging, samples were collected without redispersion. In cases where phase separation was observed, aliquots were taken from the central region of the container without disturbing the separated phases to minimize the influence of localized phase separation.
The viscosity of each silicone emulsion was measured using a rotational rheometer (MCR 302, Anton Paar, Graz, Austria) equipped with a cup-and-bob geometry in accordance with ISO 3219. Rheological measurements were conducted over a shear-rate range of 10–1000 s−1, and all measurements were performed at a controlled temperature of 25 °C. Although the emulsions were subjected to accelerated thermal aging at 50 °C and 70 °C, all rheological measurements were consistently carried out at 25 °C after aging to eliminate the direct influence of measurement temperature.

3. Results

3.1. Pilot-Scale Silicone Emulsion

The structure of each silicone emulsion manufactured in this study featured dimethylsiloxane as the basic main chain and an amine group combined in a pendant form. Each amine group was subdivided and had the structural formula shown in Figure 2. The rationale for this was that, in the case of primary di-amine, there was no alkyl group bonded to the amine structure that contained two terminal nitrogen atoms other than a small hydrogen group. That is, in a form without steric hindrance, binding with acetic acid added together in the emulsification process became easier, and this structure enabled amine to easily disperse in water in the form of quaternary cation ammonium combined with acid. This was changed to a structure that was easy to hydrate and in which the surroundings contained more water, and it was able to maintain a stable emulsion structure with a smaller particle size [15,16,17,18].
Figure 2. Chemical structures of polydimethylsiloxane (a) and diamino functional silicone fluid (b).

3.2. Importance of the Tanning Process

For carbon fiber production, the process of producing a silicone emulsion with the raw PAN precursor plays an important role in the spinning process that stretches the PAN precursor, the flame-resistant process in oxidative conditions, and the high-temperature carbonization process in an inert atmosphere. The differences between the structures of the amine group and the equivalent (content) of the amine group have a great influence on the final carbon fiber production process and the physical properties that result after each step in the treatment process. The chemical structure of di-amine has relatively small steric hindrance because there is no substituted functional group around the amine group. However, in the stretching process or the continuous oxidative atmosphere flameproofing process, the thermal oxidation and crosslinking of amine groups may cause gum-up and yellowing.
In addition, in the high-temperature flameproofing and carbonization process, depending on the equivalent (content) of the amine group, the polydimethysiloxane group (the silicone main chain of the amine group) promotes crosslinking and thermal decomposition. It affects friction and heat resistance reduction. These problems significantly influence various physical properties, such as the strength of the carbon fibers that form due to the thermal fusion or thermal decomposition of the fibers to be formed. Therefore, a pilot-scale emulsion preparation study was considered necessary.

3.3. Characteristics of Silicone Emulsions According to the Emulsification Process

Emulsion #2 was more stable than Emulsion #1 and was able to undergo emulsification with a small and homogeneous particle size.
In the emulsion process, the stator and rotor of a device with a narrow milling interval, such as a colloid mill, reach a high pressure and high speed, which enables the device to make more polymers than the existing mechanical mixing method. By exerting a stronger shear force on the emulsifier mixture, it was possible to prepare smaller and homogenized emulsified particles.
As shown in Figure 3, this was confirmed through the thermal acceleration experiment. Emulsion #2 was heat treated at 50 °C, and as the storage time passed, it became more transparent and exhibited excellent thermal stability compared to Emulsion #1. Emulsion #1 was heat treated at 70 °C, and as the storage time passed, it exhibited a phase separation phenomenon between the silicone oil and water due to the deterioration of the thermal stability. Although the emulsification method varies according to the type, viscosity, and equivalent weight of the amino silicone (which is the core raw material of carbon fiber emulsion) and is important, the dispersion method had the greatest effect on the change in viscosity, particle size formation, and emulsion stability of the final emulsions. In addition, the silicone emulsion prepared by the colloid mill method was uniform and the particle size was smaller, thereby securing a stable silicone emulsion. Based on these basic properties, when it was finally used as a carbon fiber pretreatment agent, it had a substantial influence on various performance metrics, such as the permeability between fibers and the uniform adhesion.
Figure 3. Images of Emulsions #1 and #2 for different heat treatment temperatures and storage times.

3.4. Analysis of Silicone Emulsion Particle Sizes

Figure 4 shows the dispersion stability and particle size evolution of the silicone emulsions at different concentrations (20%, 5%, and 3%) during accelerated thermal aging at 50 °C and 70 °C. In practical polyacrylonitrile (PAN) fiber processing, silicone emulsions are typically applied at concentrations of approximately 3–5%; therefore, these concentrations were selected to evaluate dispersion stability under industrially relevant conditions, while 20% was included for comparative analysis.
Figure 4. Particle size analysis of Emulsions #1 (a) and #2 (b) with different concentrations during accelerated thermal aging at 50 °C and 70 °C.
The particle size results indicate that, at concentrations of 3% and 5%, no significant changes in particle size were observed during thermal aging, indicating stable dispersion behavior within the practical application range. In contrast, at a concentration of 20%, more pronounced particle size fluctuations and macroscopic phase separation were observed as aging progressed, reflecting reduced dispersion stability at higher solid contents.
With increasing storage time and temperature, dispersion stability gradually deteriorated, leading to enhanced phase separation. To assess the spatial heterogeneity induced by this phase separation, particle size measurements were conducted using samples collected from the upper (“up”), central (“middle”), and lower (“down”) regions of the emulsion container, as well as after gentle homogenization (“mixed”). Day 7 was selected as a representative aging time point because distinct changes in dispersion behavior became evident beyond this duration.
Overall, Emulsion #1 exhibited larger concentration- and position-dependent variations in particle size, indicating reduced dispersion stability, whereas Emulsion #2 maintained relatively uniform particle size distributions even after prolonged thermal aging. These results demonstrate that the colloid mill-based emulsification process provides enhanced resistance to thermally induced aggregation and phase separation within the concentration range relevant to PAN fiber processing [19,20,21,22].

3.5. Chemical Composition According to the Type of Emulsion

Figure 5 presents the FT-IR spectra of silicone emulsions after thermal aging at 50 °C and 70 °C for approximately one month. All spectra exhibit characteristic absorption bands associated with the silicone backbone. The strong band observed at approximately 1094 cm−1 is attributed to the asymmetric stretching vibration of Si–O–Si linkages, while the band near 798 cm−1 corresponds to the symmetric stretching vibration of cross-linked Si–O–Si structures. In addition, a weak absorption band around 950 cm−1 is associated with Si–OH groups, which are commonly observed in silicone-based materials.
Figure 5. FT-IR spectra for Emulsion #1 (a) and Emulsion #2 (b) after thermal aging at 50 °C and 70 °C for approximately one month.
Comparison of the spectra before and after thermal aging reveals no noticeable shifts in the positions of these characteristic bands, indicating that the fundamental chemical structure of the silicone emulsions remained unchanged during prolonged exposure to elevated temperatures. Furthermore, no significant changes in relative band intensities were observed between Emulsion #1 and Emulsion #2 after aging, suggesting that thermal aging did not induce chemical degradation or additional crosslinking reactions detectable by FT-IR.
These results indicate that the differences in dispersion stability observed between the emulsions are not associated with changes in chemical composition but are instead primarily governed by physical factors such as droplet size distribution and emulsification method. Consequently, FT-IR analysis confirms that the colloid mill-based emulsification process enhances dispersion stability without altering the intrinsic chemical structure of the silicone emulsions under the applied thermal aging conditions [21,22].

3.6. Evaluation of Silicone Emulsion Dispersion Stability

Pulse nuclear magnetic resonance (NMR) was utilized to study the liquid relaxation phenomenon, aiming to understand various characteristics at the interface between particles and the liquid phase. Figure 6 illustrates the behavior of H-protons in a liquid sample, such as an emulsion, measured in the magnetic domain. When a magnetic field is applied, hydrogen atoms (H) in the suspension sample absorb energy and generate a strong resonance signal. Subsequently, when the energy is released, the magnetic field reaction measures the return of hydrogen atoms to their original state. Liquid relaxation provides information about the density of hydrogen atoms within the measurement range and details about the reaction rate [23,24,25,26].
Figure 6. T2 relaxation curves for Emulsions #1 (a) and #2 (b) for different heat treatment temperatures and storage times.
The liquid relaxation phenomenon allows the confirmation of particle agglomeration through the examination of the interfacial properties between the liquid phase and the particles. Resonance frequency (RF) was applied in the vertical direction relative to the static magnetic field (B0, permanent magnet), and the relaxation time was calculated using a decaying signal in which the maximum magnetization value of the hydrogen atoms decreased according to the RF pulses. Figure 7 shows the measurement results of the interfacial properties and dispersion stability of the particles over the time (T2, relaxation time) required for the energy of the transverse magnetization (Mxy) to decrease by 37% [27,28].
Figure 7. Relaxation times for Emulsions #1 (a) and #2 (b) as a function of heat treatment temperature and storage time.
Based on an analysis of the dispersion stability of the fine particles in each emulsion after being placed in an oven at 50 °C and 70 °C for one month at each temperature, the stability of the fine particles in each emulsion was confirmed. The dispersion stability of the particles in each emulsion decreased. The shorter the relaxation time, the better the dispersibility, and the more the particle size increased. Table 1 indicates that Emulsion #1, which had weak thermal stability, exhibited a rapid increase in the relaxation time from the beginning compared to Emulsion #2. This confirmed that the dispersibility stability of the silicon particles deteriorated [27,28,29].
Table 1. Relaxation time for Emulsions #1 and #2 for different heat treatment temperatures and storage times.
The change in the dispersion stability of each silicone emulsion sample as a function of time was analyzed for the entire height of the sample via the multiple light scattering principle using Turbiscan. In this context, the principle of multiple light scattering did not involve several different wavelengths used as light sources, but photons of a single wavelength of 880 nm, which are sent into the concentrated sample and cause multiple particles to scatter [30,31].
In this study, each silicone emulsion was tested for thermal acceleration analyzed at the actual concentration without diluting all formulations (emulsion, suspension, and foam). Figure 8 shows the results of the dispersion stability evaluation at 50 °C and 70 °C without any restrictions on the concentrations and particle sizes of the silicone emulsions via simultaneously analyzing the transmission/backscattering. Emulsions #1 and #2, which were kept at room temperature during the thermal acceleration experiments, were evaluated for their dispersion stability at 50 °C and 70 °C over three days at each temperature. As shown in Figure 8, all silicone emulsions were initially dispersed and emulsified, and Emulsion #1 had less dispersion stability than Emulsion #2.
Figure 8. Dispersion stability of Emulsions #1 (a) and #2 (b) for different heat treatment temperatures and storage times.
In general, the phenomenon of liquid dispersion deteriorating over time in constant temperature conditions can be largely divided into two categories: (1) particle migration (creaming, sedimentation) caused by the movement of particles in the dispersed phase and (2) the dispersion of particles in the dispersed phase [30].
For Emulsion #1 (50 °C), Figure 8a shows that Transmission (T) increased after the intensity decreased, opposite to backscattering (BS). In the profile of the middle region and at the beginning of the analysis, T exhibited an increase. That is, in terms of the effect on the particle size, as the particles corresponding to the right region of the low point aggregated, T increased with respect to the total height of the sample. In addition, the low point of T was different depending on the refractive index (np) of the particle and the refractive index (nf) of the solvent, which was not 0.6 μm as it was for BS [31,32,33,34]. In the profile after 20 h in one day, T did not increase any more, but decreased, and increased again after 10 h in two days. This was a phenomenon in which the small particles remaining after the large particles were suspended and precipitated were agglomerated again. As particles corresponding to the left area of the low point aggregated, T decreased with respect to the total height of the sample, and this was a phenomenon that passed to the right area of the low point due to the continued aggregation. The middle part of Emulsion #1 (70 °C) demonstrated an increase in parallel with the initial (t0) standard profile. That is, as the particles corresponding to the left area of the apex were aggregated, BS increased, and the particles were already aggregated at the actual concentration and had a size of 0.6 μm or smaller.
For Emulsion #2 (50 °C), Figure 8b shows that the BS profile decreased when looking at the middle part after evaluating the dispersion stability for 17 h. BS decreased as the particles corresponding to the right area of the apex were agglomerated. T exhibited a decrease in the middle region profile. T decreased with respect to the total height of the sample as the particles corresponding to the left area of the low point were aggregated. Emulsion #2, which was evaluated for its dispersion stability at 70 °C, behaved with an average particle size of 0.6 μm at the actual concentration before the 17-h analysis, but the average particle size was more than 0.6 μm after 17 h. In addition, when the middle part of Emulsion #2 was observed, the particles agglomerated and had a size of 0.6 μm or smaller.

3.7. Measurement of Viscosity of Silicone Emulsions

Figure 9 shows the viscosity of each silicone emulsion measured at 25 °C after accelerated thermal aging at 50 °C and 70 °C for different storage times. Despite prolonged thermal aging, the viscosities of both emulsions did not exhibit significant systematic changes. However, Emulsion #1 showed irregular viscosity fluctuations, which can be attributed to partial agglomeration of silicone particles during aging. In contrast, Emulsion #2 exhibited relatively constant viscosity over time, indicating superior dispersion stability compared to Emulsion #1.
Figure 9. Viscosities of emulsions after thermal aging at 50 °C (a) and 70 °C (b) for approximately one month.

4. Discussion

In this study, two types of fine-denier silicone emulsions prepared using different dispersion methods were systematically evaluated with respect to their thermal and dispersion stability under accelerated aging conditions. The results clearly demonstrate that the emulsification method plays a decisive role in particle size formation, interfacial stability, and long-term dispersion behavior [35,36]. In particular, colloid mill-based emulsification produced smaller and more uniform droplets, resulting in markedly enhanced resistance to thermal aging during prolonged storage at elevated temperatures.
During thermal acceleration tests, the mechanically dispersed emulsion (Emulsion #1) exhibited pronounced particle growth, phase separation, and irregular viscosity fluctuations, especially at 70 °C, indicating limited resistance to thermal stress. In contrast, the colloid mill-based emulsion (Emulsion #2) maintained a relatively constant particle size and viscosity over time, accompanied by increased transparency, suggesting that its internal dispersion structure remained stable. These trends were consistently supported by NMR relaxation and Turbiscan analyses, which revealed slower relaxation changes and reduced light-scattering variations for Emulsion #2 compared with Emulsion #1 [37,38].
FT-IR analysis further showed no significant shifts in characteristic Si–O–Si or Si–OH absorption bands after thermal aging, indicating that chemical degradation of the silicone phase was negligible. Accordingly, the observed instability is attributed primarily to physical destabilization mechanisms rather than chemical changes [39,40]. Prolonged exposure to elevated temperatures enhances Brownian motion and weakens steric stabilization provided by nonionic surfactants, thereby increasing droplet–droplet interactions, coalescence probability, and eventual phase separation. These effects were more pronounced in Emulsion #1 due to its broader initial particle size distribution.
The superior thermal stability of Emulsion #2 can be explained by its finer and narrower droplet size distribution established during high-shear colloid mill emulsification. Smaller droplets reduce the driving force for coalescence and suppress preferential growth of larger particles during aging, resulting in improved dispersion uniformity and delayed phase separation. This behavior highlights a clear structure–stability relationship, in which the initial microstructure determined by the emulsification process governs long-term thermal performance [41].
These findings are directly relevant to practical carbon fiber manufacturing. During PAN precursor processing, silicone emulsions are applied as oil agents to prevent filament adhesion during spinning, stabilization, and carbonization. Insufficient dispersion stability can lead to fiber fusion, roller contamination, non-uniform coating, and process interruptions. The enhanced stability of the colloid mill-based emulsion therefore indicates its greater suitability for maintaining consistent performance under industrial processing conditions involving repeated thermal exposure [42,43,44].
Overall, this study establishes a quantitative and mechanistic link between emulsification technology, droplet microstructure, and thermal dispersion stability. The results demonstrate that high-shear colloid mill emulsification is an effective strategy for designing fine-denier silicone emulsions with improved long-term stability, thereby contributing to more reliable and high-quality PAN-based carbon fiber production.

5. Conclusions

In this study, fine-denier silicone emulsions for carbon fiber applications were prepared on a pilot scale, and their thermal and dispersion stabilities were evaluated. Silicone polymers containing primary diamine functional groups were used as the main raw materials, and two different emulsification methods—a mechanical disperser and a colloid mill—were compared. Thermal aging at 50 °C and 70 °C for approximately one month revealed clear differences between the two emulsions.
The colloid-mill emulsion produced smaller particle sizes and showed superior dispersion stability, while the mechanically dispersed emulsion generated larger particles and exhibited reduced stability during thermal aging. These results indicate that high-shear emulsification is an effective approach for producing uniform and thermally stable silicone emulsions. Therefore, the colloid-mill method is considered a more reliable and practical strategy for manufacturing fine-denier silicone emulsions for PAN precursor treatment in carbon fiber production.

Author Contributions

Conceptualization, J.-Y.Y.; methodology, J.-Y.Y. and K.-W.K.; software, J.-Y.Y.; validation, J.-Y.Y., K.-W.K. and B.-J.K.; formal analysis, J.-Y.Y.; investigation, J.-Y.Y.; resources, D.-C.C., K.-W.K. and B.-J.K.; data curation, J.-Y.Y.; writing—original draft preparation, J.-Y.Y.; writing—review and editing, D.-C.C. and B.-J.K.; visualization, J.-Y.Y.; supervision, D.-C.C. and B.-J.K.; project administration, K.-W.K.; funding acquisition, K.-W.K.; contributed equally, co-first authors. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the “Project for the Development of Innovative Technologies for Plastic Value-Up in Response to Global Recycling Regulations (Project No. RS-2025-25447226)”, funded by the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea, and by the “Establishing a Process Base for the Localization of Wet Powder-Based Thermoplastic Prepreg Materials and Components for Urban Air Mobility (Project No. RS-2025-16063432)”, supported by the Korea Institute for Advancement of Technology (KIAT), Republic of Korea.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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