# A comparative study of CMC and HPMC binders for direct ink writing of ceramics

Flavie Lebas, Frédérine Marie, Sylvain Marinel, Charles Manière

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Flavie Lebas, Frédérine Marie, Sylvain Marinel, Charles Manière. A comparative study of CMC and HPMC binders for direct ink writing of ceramics. Journal of the American Ceramic Society, 2026, 109 (2), pp.e70565. 10.1111/jace.70565. hal-05510310

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# A Comparative Study of CMC and HPMC Binders
## for Direct Ink Writing of Ceramics

Flavie Lebas<fnref n="1" />*, Frédérine Marie<fnref n="1" />, Sylvain Marinel<fnref n="1" />, Charles Manière<fnref n="1" />*

## Abstract

The selection of binders in ceramic additive manufacturing plays a high role in determining the feasibility and quality of printed components. This study investigates the performance of sodium carboxymethylcellulose (CMC) and hydroxypropylmethylcellulose (HPMC) as bio-based binders in aqueous alumina suspensions for direct ink writing (DIW). CMC, characterized by its polyelectrolyte nature, demonstrated rapid dissolution, exceptional dispersion stability, and consistent rheological properties, facilitating smooth extrusion and the formation of high-quality surfaces. These characteristics are critical for DIW, where ink homogeneity and stability directly impact printing resolution and part integrity. HPMC, by contrast, exhibited slow dissolution, thermally induced gelation, and printing inconsistencies, likely due to its lack of electrostatic stabilization. Microstructural and mechanical evaluations of sintered parts confirmed that CMC-based systems achieved higher density, homogeneity, and microhardness. This study highlights the significance of CMC as a binder, providing a pathway to overcome common challenges in DIW, such as agglomeration, foaming, and poor sintered properties, while enabling the production of high-performance ceramic components.

## Graphical abstract

<figure><img src="image_2.png" /></figure>

## Keywords

Carboxymethylcellulose; Direct Ink Writing; Gelation; Rheology; Milling, Ceramics

<footnote n="1">¹Université Caen Normandie, ENSICAEN, CNRS, Normandie Univ, CRISMAT UMR6508, F-14000 CAEN, France</footnote>

<footnote>* Corresponding authors: flavie.lebas@ensicaen.fr, charles.maniere@ensicaen.fr</footnote>
# 1. Introduction

In direct ink writing (DIW) of ceramic suspensions such as alumina, the binder system plays a central role in rheology control determining printability, shape retention, and microstructural integrity. Conventional DIW formulations often rely on synthetic polymers like polyethyleneimine (PEI) [1], ISOBAM-104 [2] or Pluronic® F127 [3] to provide viscosity control, interparticle bridging, and cohesion. While these additives ensure excellent rheological tenability, clean burnout, they are petroleum-derived polymers and raise concerns related to biodegradability, biocompatibility and environmental impact [4–6]. Beyond these widely used systems, numerous alternative binders have been investigated in ceramic DIW, spanning synthetic polymers, polysaccharide derivatives, and hydrogel-based formulations. These materials exhibit distinct chemical structures and processing windows, leading to different rheological responses, drying behavior, and debinding constraints. To contextualize the present work, a comparative summary of representative binder systems used in ceramic DIW, highlighting their main advantages and limitations, is presented in Table 1.

Following this comparative overview, cellulose-based binders stand out as promising sustainable alternatives, which motivates the detailed comparison presented in this study. In particular, bio-based cellulose derivatives such as hydroxypropyl methylcellulose (HPMC) and carboxymethyl cellulose (CMC) have emerged as sustainable alternatives. Both polymers share a cellulose backbone, but they differ in the nature of their functional side groups. Derived from partial etherification or carboxymethylation of cellulose, these polysaccharides are water-soluble, biodegradable, and capable of generating highly pseudoplastic behavior through hydrogen bonding and polymer entanglement [7,14]. CMC is an anionic cellulose derivative with high degree of hydrophilicity and electrostatic stabilization in aqueous media. In contrast, HPMC is a nonionic cellulose ether, where some hydroxyl groups are replaced by methoxy and hydroxypropyl groups. These substitutions make HPMC water-soluble but less ionically active, leading to different rheological and gelation above a certain temperature [15–17].

The gelation behavior can be advantageous for ceramic DIW inks if the temperature of the printed object is elevated after extrusion. Pluronic systems can maintain low viscosity during the extrusion at moderate temperatures but solidify rapidly as temperature increases, preserving the printed geometry without requiring high polymer content. However, this requires a careful control of the temperature during the printing. Another challenge is the heat generated during milling, which may induce premature solidification and compromise homogeneity in gelation-sensitive binders. In contrast, the highly anionic groups of CMC render the molecule very stable and strongly hydrophilic. Consequently, CMC dissolves readily at room temperature, simplifying ink preparation, preventing gelation during milling, and allowing partial particle dispersion through mechanisms. PEI-based formulation also use such strategy utilizing both long and short polymer chains but with cationic groups [1,18] CMC has been successfully applied in various DIW suspensions [8,19,20], although its use is less common than that of synthetic polymers or HPMC [1,21,22].
<table><caption>Table 1: Overview of commonly used binder systems in ceramic direct ink writing (DIW), summarizing their main advantages, disadvantages, and representative references.</caption><thead><tr><td>Binder</td><td>Advantages</td><td>Disadvantages</td><td>Ref.</td></tr></thead><tbody><tr><td>Carboxylmethylcellulose (CMC)</td><td>Water-soluble; biodegradable; strong shearing behavior; high suspension stability due to anionic groups; easy ink preparation at room temperature</td><td>Sodium present</td><td>[7-9]</td></tr><tr><td>Hydroxylpropylmethylcellulose (HPMC)</td><td>Water-soluble; biodegradable; good shape retention after extrusion</td><td>Temperature-sensitive rheology; thermally induced gelation; High sensitivity to processing; Long dissolution</td><td>[1,9,10]</td></tr><tr><td>Methylcellulose (MC)</td><td>Thermoreversible gelation; strong viscosity enhancement at low concentration; good filament stability</td><td>Narrow processing temperature window; slower dissolution; limited use at high solid loading</td><td>[9,11]</td></tr><tr><td>Pluronic® F127</td><td>Thermoreversible hydrogel; excellent print fidelity; low viscosity during extrusion; clean burnout</td><td>Requires precise temperature control; sensitive to heat generated during milling; petroleum-derived; limited long-term ink stability</td><td>[9,12]</td></tr><tr><td>Polyvinyl alcohol (PVA)</td><td>Water-soluble; good green strength; easy processing; widely used in ceramic shaping</td><td>Shrinkage during debinding; petroleum-derived; limited shearing alone; slower burnout</td><td>[9,13]</td></tr><tr><td>ISOBAM-104</td><td>Combines dispersant and binder functions; strong interparticle bridging; good rheological tunability; clean burnout</td><td>Synthetic petroleum-derived polymer; limited biodegradability; relatively high cost</td><td>[2,9]</td></tr></tbody></table>

For one-step high energy ball milling of DIW aqueous pastes, the binder must meet several key criteria: (i) it should not foam due to amphiphilic properties and mechanical agitation, where CMC's anionic charge and higher solubility provide a distinct advantage; (ii) it must dissolve easily in water, unlike HPMC, which requires several hours and low temperatures for complete dissolution. (iii) Additionally, gelation and thermal sensitivity must be avoided during prolonged ball milling, as heating disrupts hydrogen bonds, causing the initially hydrated chains to collapse and aggregate. While controlled gelation can be beneficial for printing on heated plates, it is not recommended for high-energy ball milling due to the risk of premature gelation.
In this study, we compare the rheological behavior, ink preparation, aging, printing, and sintering characteristics of CMC and HPMC binders. These cellulose derivatives are bio-based but present certain challenges when used with the milling process. Special focus is placed on the feasibility of one-step ball milling and how to manage milling-induced heating, gelation behavior, and the conditions required to obtain stable suspensions. The impact on the sintering microstructure is also analyzed.

## 2. Materials and methods

### 2.1. Raw materials

Two binders were used in this study: a medium-viscosity (viscosity: 400-800 cp) sodium carboxymethylcellulose (CMC, Sigma-Aldrich–Merck, Germany) and an industrial-grade METHOCEL™ F4M hydroxypropylmethylcellulose (HPMC, Nutrition & Biosciences Switzerland GmbH, Switzerland) of medium molecular weight (viscosity: 2 600-5600 cp). Their chemical structures are illustrated in Fig.1a. The ceramic powder used to prepare the aqueous suspensions was BMA15 alumina (Baikowski, France), characterized by a BET specific surface area of 16 m²g⁻¹ and a median particle size (D₅₀) of 0.12 µm. The dispersant employed was Dolapix CE64 (Zschimmer & Schwarz, Germany), which was added to improve powder dispersion and suspension stability.

### 2.2 Preparation of binder solutions and ceramic aqueous suspensions

In order to characterize the gelation behavior of the binders, two aqueous mixtures were prepared by dispersing 3 wt% of binder (either CMC or HPMC) in water. The binders were mixed with water using a ball milling (4 minutes at 250 rpm) and stored in sealed glass bottles to ensure complete dissolution prior to thermal gelation and viscosity measurements. The 3 wt% binder concentration was selected based on the formulation of the printable aqueous ceramic suspension (BMA15) developed in ref[20], which used 3 wt% CMC relative to water. The objective of the present work is to compare the influence of CMC and HPMC under similar conditions. Accordingly, the main aqueous ceramic suspensions were prepared following the same procedure as the CMC-based BMA15 system. HPMC was mixed simultaneously with all components (ceramic powder, dispersant, and water) in a 100 mL bowl containing 20 mm balls and processed for 1h30 min using a planetary mono mill (Pulverisette 6, Fritsch, Germany) rotating at 350 rpm. To optimize the formulation of the HPMC-based BMA15 suspensions, an additional batch containing 1.5 wt% HPMC (relative to water) was also prepared.

### 2.3. 3D printing and sintering

All suspensions were printed using a pneumatic extrusion-based robocasting printer (Eazao Bio). Simple curved geometries were first printed at different speeds (20mm/s to 100 mm/s) to evaluate the printability and flow behavior of the HPMC-based BMA15 suspension. The ability of the material to maintain its shape after deposition was further assessed by printing taller cylindrical structures to examine its self-supporting capability. All printing tests were performed under identical conditions and compared with the reference[20] CMC-based BMA15 suspension. Similarly, the sintering cycle is 1450 °C for 3 hours, with a heating rate of 100 °C/h. Given the low organic content and the slow heating rate, binder
removal occurs in *situ* during the programmed sintering cycle, allowing gradual decomposition of HPMC or CMC while the ceramic particles densify.

## 2.4 Suspensions and sintered part characterization

### 2.4.1 Rheology measurements

The thermal gelation and rheological behavior of the suspensions were evaluated using a rotational rheometer (AR1000, TA Instruments, USA) using a plate-plate geometry. The viscosity of the two different suspensions was measured from 25 °C to 80 °C at a heating rate of 2.5 °Cmin⁻¹ and a shear rate of 0.1 s⁻¹. Under these parameters, the gel network forms without disruption from the applied shear rate. The heating rate of 2.5 °Cmin⁻¹ ensures a gradual temperature increase, allowing adequate time for gelation while limiting water evaporation. Viscosity measurements were also performed over a shear rate range of 10–100 s⁻¹, corresponding to the conditions encountered during printing at ambient temperature. In addition, oscillatory measurements at a fixed angular frequency of 1 Hz were conducted to determine the complex modulus components, G' and G'', providing insights into the suspensions' viscoelastic properties.

### 2.4.2 Microstructural and hardness characterization

The microstructures of the sintered samples were examined using a scanning electron microscope (SEM, JSM-7200F, JEOL, Japan) equipped with a backscattered electron (BSE) detector. Vickers microhardness measurements were carried out with a Falcon500G2 tester (Innovatest®, Netherlands) under a load of 1 kgf. The average grain size was assessed using the linear intercept method and the open-source software ImageJ, yielding an average value that is multiplied by 1.56 [23]. Porosity measurement is indeed challenging, as printing defects can create cavities that lead to erroneous values of sintering advancement when using the Archimedes method. The porosity of the different parts was measured by image analysis using an open-source image processing software (ImageJ). For the evaluation of porosity, a thresholding procedure was applied to the images in order to determine the proportion of porosity in the parts. This method makes it possible to eliminate the influence of defects sometimes present in the parts due to the printing process and to more accurately assess the level of material densification.

## 3. Results and discussions

### 3.1. Thermal gelation and viscosity properties of binder solutions

The first observation concerns the difficulty of obtaining a homogeneous binder solution with HPMC. In contrast to CMC, the HPMC powder did not disperse readily in water after ball-milling, flocs and numerous air bubbles were still visible. Only after several hours of rest did the flocs gradually dissolve, and after standing overnight, the mixture transformed into a uniform and viscous binder solution. HPMC tends to foam during ball milling because its hydrophobic (methoxy and hydroxypropoxy) and hydrophilic (–OH) groups give it an amphiphilic nature, which promotes air entrapment; a problematic effect in ball milling. As shown in Fig.1.b, the inset graph presents the viscosity as a function of shear rate for both the CMC and HPMC solutions. It is evident that, at the same concentration (3 wt% relative to water), the HPMC solution exhibits a significantly higher viscosity than the medium viscosity CMC one, confirming its stronger thickening ability [24].
Regarding their thermal gelation behavior, the HPMC and CMC aqueous solutions exhibited distinct responses to temperature (Fig.1.b). As the temperature increased from 30°C to 90°C, HPMC showed a sharp, thermoreversible sol–gel transition at approximately 70°C, characterized by a pronounced increase in viscosity. At the end of the heating cycle, a white thermally induced gel was formed, indicating the transition from a viscous solution to a solid- like gel, clearly reflected in the viscosity rise. This gelation is attributed to the establishment of hydrophobic interactions between methoxylated polymer chains. The phenomenon is well- documented and arises from HPMC's amphiphilic nature: heating promotes phase separation and subsequent network formation, which reverses upon cooling [10,16,25]. In contrast, CMC solutions exhibited a gradual decrease in viscosity with increasing temperature and did not form a gel, resulting in a homogeneous transparent viscous solution at the end of the test. The reduction in viscosity is due to the disruption of polymer-solvent interactions, particularly hydrogen bonds between the polymer hydroxyl groups and water, facilitated by thermal agitation. Moreover, the lack of sufficient hydrophobic methoxy groups in CMC prevents the formation of a rigid three-dimensional network, in contrast to HPMC, whose gelation is driven by hydrophobic associations between polymer chains at elevated temperatures [10,16,25].
<figure><img src="image_3.png" /><figcaption>Fig. 1. Rheological properties and thermal behavior of carboxymethyl cellulose (CMC) and hydroxypropylmethylcellulose (HPMC) solutions and suspensions: a) Chemical structures of CMC and HPMC, b) Viscosity as a function of temperature for CMC and HPMC solutions. Inset compare the viscosity. c) Viscosity of the ceramic suspensions with CMC and HPMC and images illustrate the physical state of suspensions after ball milling, d) and e) Storage modulus (G') and loss modulus (G") as a function of oscillation stress for BMA15-CMC and HPMC</figcaption></figure>
## 3.2 Rheological behavior of the suspensions

The compositions of the CMC and HPMC-based BMA15 suspensions are presented in Table 2.

The first noticeable difference between the CMC and HPMC-based suspensions concerns their quality after ball milling. The HPMC-based suspension was far from homogeneous, containing numerous agglomerates, whereas the CMC-based suspension was smooth, glossy, and free of aggregates (Fig.1.c). These observations are consistent with the behavior of HPMC in aqueous solution: as noted during the preparation of water-HPMC mixtures, dissolution is slow and requires several hours to achieve a uniform solution. This confirms that HPMC is not suitable as a binder for a rapid, one-step preparation of ceramic suspensions. Furthermore, the presence of a gelation temperature for HPMC introduces additional challenges. Frictional heating during ball milling can locally raise the temperature, potentially triggering the transition of the viscous HPMC solution into a solid-like gel, which is then broken into agglomerates by the milling process. Such behavior explains the observed heterogeneity and confirms that HPMC is less practical than CMC for the fast preparation of well-dispersed ceramic suspensions in our case.

Fig.1.c shows a comparison of the viscosities of the two suspensions. Overall, the viscosities appear similar to those of the BMA15 suspensions. The data points for the HPMC-based suspension are more unstable, likely due to the presence of agglomerates. Regarding the viscoelastic behavior (Fig.1.d and e), similar trends were observed for both suspensions. The solid-like nature remained stable under increasing oscillatory stress, indicating a broad linear viscoelastic region (LVR). No transition from solid-like to liquid-like behavior (typically characterized by the loss modulus (G") approaching or exceeding the storage modulus (G') was observed for either suspension. The absence of a crossover between G' and G" confirms that both suspensions maintain a stable gel structure under small oscillatory deformations. The slightly less linear response observed for the HPMC suspension (Fig.1.e) is likely due to the presence of residual agglomerates, which can locally increase stiffness confirming the heterogeneities in the suspension. These results suggest that, both CMC and HPMC-based suspensions possess sufficient mechanical strength to maintain their shape during handling or printing but HPMC is slightly more heterogeneous.

<table><caption>Table 2: Compositions (in vol%) of the BMA15 ceramic suspensions formulated with carboxymethylcellulose (CMC) or hydroxypropylmethylcellulose (HPMC) as viscosifying/binding agents. The volume fractions of ceramic powder, water, dispersant, KOH, and cellulose derivative are indicated. For the HPMC-based formulations, two polymer contents were investigated, including a reduced HPMC concentration (1.5 wt%)</caption><thead><tr><th>(vol%)</th><th>Powder</th><th>H<sub>2</sub>O</th><th>Dispersant</th><th>KOH</th><th>Viscosifying agent</th></tr></thead><tbody><tr><td>BMA15 CMC</td><td>46.80</td><td>48.71</td><td>3.10</td><td>0.48</td><td>0.91 (CMC)</td></tr><tr><td>BMA15 HPMC</td><td>46.80</td><td>48.71</td><td>3.10</td><td>0.48</td><td>0.91 (MC)</td></tr><tr><td>BMA15 HPMC (1.5wt%)</td><td>47.01</td><td>48.93</td><td>3.12</td><td>0.48</td><td>0.46 (MC)</td></tr></tbody></table>
## 3.3 Printing results

The results of the printing tests, particularly the print speed tests, are presented in Fig.2.a and Fig.2.b. It can be observed that the CMC-based BMA15 suspension performed very well regardless of the printing speed. The filament was properly deposited and accurately followed the designed paths of the printed part. This behavior contrasts with the HPMC 3 wt% suspension, which showed poorer printability due to the heterogeneity of the suspension, as seen previously in Fig.1.c. For the HPMC 3 wt% ink, printing was challenging, it required a larger nozzle (1.2 mm instead of 0.8 mm), and even then, as shown in the magnified image at 60 mm/s (Fig.2.b), the filament was not fully continuous and exhibited visible agglomerates. In the second test (Fig.2.e), the printing of tall cylindrical structures proved difficult (22.4 mm of height) because these agglomerates prevented the formation of a solid and coherent filament capable of supporting subsequent layers. To improve the printability, the HPMC content was reduced from 3 wt% to 1.5 wt% to account for its higher inherent viscosity (relative to the water content), the composition is done in Table 2. Fig.2.f show this modification which led to a significant improvement: the printed cylinder (with this time a 0.8 mm nozzle diameter) was noticeably taller (32 mm of height) and nearly comparable to that obtained with the BMA15 formulation (31 mm of height) (Fig.2.d), although it began to lean slightly before collapsing near the end of the build. Some surface defects were still observed on the cylinder. Regarding high-speed printing (Fig.2.c), the optimized HPMC 1.5 wt% suspension was also able to print parts rapidly, with only minor degradation in filament quality at the highest speeds (80–100 mm/s). In conclusion, although reducing the HPMC content improved printability, the results clearly show that the CMC-based formulation (BMA15) remains superior. A qualitative comparison of the printing performance of the CMC- and HPMC-based suspensions was conducted based on surface finish, filament definition, and shape fidelity as presented Fig.2.d,e and f. High-magnification observations using a binocular microscope revealed clear differences between the formulations. The CMC-based ink exhibited a smoother surface with regular and well-defined filament stacking, indicating better shape retention during printing. In contrast, the HPMC-based inks showed increased surface irregularities and local defects, particularly at 3 wt.%, while the formulation at 1.5 wt.% displayed a quasi-smooth surface with localized defects. These differences also impacted the maximum printable height and overhanging capability, highlighting the influence of binder type and concentration on the effective printing resolution and structural stability.
<figure><img src="image_4.png" /><figcaption>Fig. 2. 3D printing performance and structural integrity of BMA15 CMC and HPMC suspensions: a), b) and c) printing speeds test, d), e) and f) max height printable by suspensions at 20mm/s and high-magnification binocular microscope images highlighting the surface finish and filament definition of the printed parts.</figcaption></figure>
### 3.4. Microstructure and microhardness

The characterization results of the HPMC-sintered parts are compared with those of the CMC-sintered parts. Table 3 presents a detailed comparison of porosity, grain size, and microhardness. The BMA15–CMC sample exhibits a fine and homogeneous microstructure (Fig.3.c), characterized by a low porosity of 0.34% and a small average grain size of 2.78 µm, which contribute to its higher hardness of 1874 HV₁. A very correct value knowing for 3 µm, the expected Hall Petch hardness is near 1660 HV₁ [26]. In contrast, Fig.3.d, the BMA15 containing 1.5 wt% HPMC shows a coarser microstructure, with larger grains (3.98 µm) and a higher porosity of 1.20%. These microstructural differences lead to a slightly lower hardness of 1830 HV₁. In addition, some localized porous regions were observed in the microstructure of the HPMC-based BMA15 sample (Fig.3.b), which were not present in the CMC-based formulation (Fig.3.a). This could be attributed to binder-induced residual porosity originated from heterogeneous HPMC concentration in the suspension.

However the Fig.3.g further situates the hardness values obtained in this study within the broader literature. Both CMC and HPMC-based samples exhibit Vickers hardness values that are comparable to or higher than those reported for similar direct ink writing alumina sparts processed by various sintering routes, including conventional sintering, HIP, and SPS-based techniques. The HPMC-based sample also falls within the upper range of reported values, despite its slightly coarser grain size and higher residual porosity. Overall, this comparison confirms that the mechanical performance achieved using both binder systems is competitive with state-of-the-art processing routes reported in the literature.

In Fig.3.e and Fig.3.f, both microstructures exhibit needle-like features highlighted in yellow. Since these needles appear in both suspensions, their origin is unlikely to be related to the sodium from CMC or the binder, which is completely removed during processing. They are most probably associated with the KOH used to impose a basic pH. To confirm their chemical origin, a suspension was prepared with three times the usual amount of KOH to obtain a clearer EDS signal. As expected, the EDS analysis detected approximately 2 at.% of potassium in the needles, while none was found in the surrounding grains.

<table><caption>Table 3: Details of characterization of BMA15 CMC and BMA15 HPMC</caption><thead><tr><td></td><td>Porosity (%)</td><td>Grain size (µm)</td><td>Hardness (HV<sub>1</sub>)</td></tr></thead><tbody><tr><td>BMA15 CMC</td><td>0.34 ± 0.06</td><td>2.78</td><td>1874 ± 48</td></tr><tr><td>BMA15 HPMC 1.5wt%</td><td>1.20 ± 0.35</td><td>3.98</td><td>1830 ± 34</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 3. SEM microstructures: (a), (c), and (e) correspond to BMA15 CMC, (b), (d), and (f) to BMA15 HPMC. A table presents the atomic concentrations of different zones (needle and grain), g) Comparison of Vickers hardness at various sintering temperatures with literature values [12,20,26–31].</figcaption></figure>
### 3.5. Aging of CMC-based alumina suspension

CMC is clearly more favorable than HPMC for preparation by ball milling. However, the long-term stability of these CMC-based suspensions remains to be determined. Sedimentation can occur through slow particle rearrangement under gravity, and the weak-bond network of CMC may evolve over time. To evaluate this effect, an aging study of the shear-thinning rheological behavior and a sedimentation test were conducted. To accelerate possible aging phenomena, an APA-grade alumina powder with slightly larger grains than the BMA15 grade [20] was used. The results, reported in Fig. 4, show that the rheological curves remain almost unchanged over time. Similarly, the sedimentation test revealed no visible sedimentation front even after two months. In addition to the advantages mentioned above, the anionic nature of CMC further contributes to the long-term stability of the suspension.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Aging of CMC-based APA alumina suspension.</figcaption></figure>

### 4. Conclusion

This study compared carboxymethylcellulose (CMC) and hydroxypropylmethylcellulose (HPMC) as bio-based binders in aqueous alumina suspensions for extrusion-based 3D printing. Clear differences were observed in their dissolution behavior, rheology, printability, and the quality of sintered parts. The polyelectrolyte nature of CMC is highly favorable, as it dissolves rapidly, forms homogeneous and stable suspensions without bubbles, and provides consistent viscoelastic properties and excellent printability, even at high speeds. In contrast, HPMC dissolved slowly, foamed, formed agglomerates, and exhibited poor homogeneity and print quality due to its thermal gelation behavior. Reducing its content improved results slightly but remained less effective than CMC. CMC also provides high suspension stability against aging effects.

After sintering, CMC-based parts were denser, more homogeneous, and slightly harder. Overall, CMC proved to be the most suitable binder, enabling fast preparation of stable, printable suspensions and ensuring high-quality, reproducible alumina components.
# ACKNOWLEDGEMENTS

## Funding

This work has been funded by UNICAEN PhD funding number: "Allocations Etablissements 2023 (2023) 2023050211502900001220000257".

All data are given in the graph of the article

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