Influence of Solvent Type, Specimen Dimensions and Temperature on Chemical Debinding of Alumina Parts Fabricated Via Digital Light Processing

Abstract

Vat photopolymerization via digital light processing (DLP) enables the rapid fabrication of high-resolution ceramic components. However, careful post-processing is required to remove the hardened resin and consolidate the microstructure. Polymer removal can be achieved through chemical dissolution and/or thermal decomposition, though solvent debinding remains scarcely explored for DLP ceramics. This study assessed the efficiency of solvent debinding in alumina specimens produced by DLP, examining the effects of solvent type, specimen geometry, and temperature. Samples with three geometries (varying surface area/volume ratios) were printed from a photocurable suspension containing 40 vol.% solids and poly(ethylene glycol) diacrylate. They were immersed in isopropyl alcohol, acetone, or chloroform at 20, 40, or 60 °C, and polymer removal and porosity evolution were monitored. Chloroform achieved the highest debinding efficiency, though prolonged exposure (>240 min) caused chipping and cracking. The optimal condition, 40 °C for up to 30 min, enabled partial polymer removal while minimizing defects during subsequent heating. Combining solvent debinding with thermal debinding, particularly under N₂ flow, further improved densification, yielding defect-free parts with relative densities up to 97%, and enhanced surface finish. Thus, solvent debinding effectively minimized defects, ensured uniform binder removal, and improved the microstructural quality of DLP-fabricated alumina ceramics.

Keywords:
Photopolymerization; chloroform; debinding; solvent; thermal treatment

1. Introduction

The fabrication of parts with complex geometries remains a significant challenge for conventional ceramic processing methods, which often fall short in producing components with intricate shapes and high-quality surface finishes¹-⁴ In this context, photopolymerization-based techniques, stereolithography (SLA)⁴-⁶, and digital light processing (DLP)⁷-¹⁰, stand out and their application to ceramics has been expanding steadily¹¹-¹³. DLP typically enables faster printing and higher resolution, with fewer moving components¹⁴. This has led to its application in the fabrication of customized components¹⁵, scaffolds¹⁶, hydrogels, and bone implants¹⁷, as well as in the indirect production of dense or porous ceramic parts.

Despite its advantages, ensuring the structural integrity of ceramic parts produced by DLP remains a challenge, as defects may originate during printing, from the feedstock formulation, or in post-processing steps¹⁸-²². One of the most critical post-processing stages is polymer removal, which can be accomplished via chemical dissolution (solvent debinding) and/or thermal decomposition (thermal debinding) of the organic constituents (see Figure 1). Improper polymer removal during these steps often results in microstructural defects such as cracks, pores, and delaminations.

Figure 1
Schematic representation of the typical steps involved in the production of ceramic parts via additive manufacturing²².

The debinding stage precedes sintering and encompasses a series of transformations, including dissolution, evaporation, thermal degradation, and oxidative decomposition of organics⁷,²³,²⁴. Several procedures (e.g. solvent, catalytic, supercritical and/or thermal debinding) have been reported in the literature²⁴-³⁰. To preserve the structural integrity of the green body, it is essential to ensure the smooth mass transport of decomposition byproducts from the interior to the surface of the part⁷. Typically, the debinding process may involve: (i) dissolution of the organic binder using a solvent³¹; (ii) a two-step thermal treatment, consisting of vacuum heating to gradually remove the binder, followed by oxidation to eliminate residual carbon³,³²; or (iii) a multi-step heating and holding protocol, optimized based on thermogravimetric analysis (TGA) data of the resin binders employed⁹,³³.

However, each of these approaches presents significant challenges. Solvent-based debinding is often difficult to control due to limited penetration and uneven removal. The two-step thermal process requires separate heating cycles, resulting in increased energy consumption, higher operational costs, and greater process complexity. Meanwhile, the multi-step approach, typically conducted in air or argon (Ar) atmospheres, can cause microcracking or leave carbon residues in larger parts. Some reports even describe thermal treatments extending up to one week⁷, which may render the process impractical from both economic and energy-efficiency standpoints.

Various strategies have been proposed to optimize debinding, focusing on parameters such as part geometry⁸, gas atmosphere³,³⁴, dwell time and temperature⁹,³⁴, and heating rate control³⁵,³⁶. Among these, solvent debinding, where organic components are partially removed via solvent dissolution before thermal treatment, has gained attention³⁷,³⁸. It is well documented that solvent-based debinding facilitates the formation of an interconnected network of open pores³⁰, which aids in the efficient release of gases during subsequent thermal treatments, thus minimizing the occurrence of structural defects. However, the efficiency of this process depends strongly on factors such as part geometry (specifically the surface area-to-volume ratio), solvent temperature, and porosity evolution. Specimens with higher surface area-to-volume ratios tend to exhibit faster polymer removal compared to those with lower ratios³⁸. Additionally, higher solvent temperatures enhance polymer-solvent interactions, promoting more effective dissolution⁸,⁹. Despite its potential, few studies have systematically tracked the binder removal and the development of porosity during solvent debinding²⁷,³⁹. Most existing investigations have focused on injection-molded ceramic or metal parts²⁵,²⁶,³⁰,³⁸, often overlooking the influence of the resulting pore network on the subsequent thermal debinding stage³⁰.

In this sense, this study aims to evaluate the effectiveness of solvent debinding in alumina parts fabricated via DLP, with a focus on the influence of solvent type, sample geometry, and process temperature. The physical and microstructural properties of the samples following debinding were characterized to assess the quality of the final ceramic components and to identify the most favorable conditions for polymer removal.

2. Experimental

To prepare the ceramic suspension, the following materials were used: reactive alumina (A1000SG, d₉₀ =1.7 mm, d₅₀ = 0.52 µm and d₁₀ = 0.18 mm, Almatis), polyethylene glycol diacrylate (PEGDA, Mn 250, 1.11 g/cm³, Sigma Aldrich), the dispersant DISPERBYK-111 (BYK-111, BYK Chemie), and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Sigma Aldrich) as the photoinitiator. A formulation containing 40 vol.% alumina, 3 wt.% dispersant, and 2 wt.% photoinitiator was prepared by mixing the components in a ball mill for 24 hours to break up agglomerates and ensure homogeneity⁴⁰,⁴¹. The rheological behavior of this suspension was previously characterized⁴², revealing a viscosity of 190 mPa·s at a shear rate of 30 s⁻¹ and a temperature of 25 °C, which is suitable for digital light processing (DLP) 3D printing.

Ceramic parts were fabricated using a compact DLP 3D printer (LD-006, Creality). To investigate the influence of surface area-to-volume (As/V) ratio on the solvent debinding process, three sample geometries were produced (Table 1). The unit of the As/V ratio will be omitted for convenience throughout the text.

Table 1
Characteristics of the samples evaluated in this study.

Samples’ dimension (mm)	Surface area (mm2)	Surface area/ volume (As/V, mm-1)
3.7 × 9.0 × 25.0	701.6	0.84
3.7 × 5.5 × 12.0	261.5	1.07
1.8 × 12.3 × 18.0	551.9	1.38

Isothermal solvent debinding experiments were conducted using three specimens of each geometry. The samples were immersed in isopropanol, acetone, or chloroform at 20, 40, or 60 °C for 24 hours under slow and continuous stirring in a controlled bath. Solvents were continuously collected following distillation to minimize the accumulation of dissolved species. At specific time intervals, samples were retrieved, weighed, and the mass loss was calculated to determine the percentage removed during debinding. Additional specimens were dried after treatment and tested for apparent porosity in accordance with ASTM C830-00⁴³. Thermogravimetric analysis (TGA) was also performed using a Shimadzu TGA-50 system under a nitrogen flow (30 mL/min) at a heating rate of 10 °C/min to assess weight loss in samples subjected to solvent debinding.

To evaluate the effect of debinding treatments on the final properties of the ceramic parts, some samples underwent only thermal debinding (in oxidizing or inert atmosphere – nitrogen flux = 150 mL/min, as shown in Figure 2), while others were first chemically debound in chloroform at 40 °C for 30 minutes and then subjected to the same thermal treatments. All samples were subsequently sintered in a Lindberg Blue furnace up to 1600 °C, with a heating rate of 5 °C/min and a 2-hour dwell time. The sintered parts were characterized for apparent porosity and density (based on the Archimedes principle, ASTM C830-00 ⁴³), using water as the immersion medium. Linear shrinkage of the ceramic samples was also determined from length measurements taken before and after sintering.

Figure 2
Thermal treatments applied for sample debinding, conducted in electric furnaces under either oxidizing or inert (nitrogen flow) atmospheres. The temperature was raised up to 600 °C, with a total treatment time of approximately 1225 minutes (20.4 hours).

Green parts (after printing) and samples treated with chloroform were examined by scanning electron microscopy (SEM, Tescan-MIRA, LCE-UFSCar) to observe the layer stacking, interlayer connectivity, porosity, and potential defects in the printed structures.

3. Results and Discussion

3.1. Comparison of solvent effectiveness in polymer removal from printed parts

Three solvents were selected to interact with the polymeric fraction remaining in the samples after the photopolymerization-based printing process. Isopropyl alcohol is commonly used to clean parts and tools in DLP printing by removing unreacted resin. However, it proved ineffective for removing the polymer after crosslinking—a transformation resulting from the interaction between PEGDA and the photoinitiator upon exposure to UV radiation during printing⁴⁴. As shown in Figure 3b, even at 60 °C, isopropyl alcohol had little effect on polymer removal.

Figure 3
Removed binder fraction of specimens presenting As/V ratio = 1.38 and kept in contact with different solvents at (a) 40°C and (b) 60°C.

Acetone, a solvent frequently employed in the chemical debinding of parts fabricated with thermoplastic polymers⁴⁵, also demonstrated limited efficacy with the thermosetting polymer used in this study. Only when the experiments were conducted at 60 °C did acetone result in noticeable mass loss and partial polymer removal (Figure 3b).

In contrast, chloroform emerged as the most effective solvent. It enabled the removal of a significant portion of the polymeric fraction even at lower temperatures (Figure 3a). According to previous reports⁴⁶, the Hansen solubility parameter distance (Ra – see Table 2) between polyethylene glycol and chloroform is notably smaller than that between PEG and other solvents. This suggests a stronger affinity between the PEGDA network and chloroform, which likely facilitates a higher swelling rate and more efficient polymer extraction, as evidenced in Figure 3.

Table 2
Compatibility between PEG and selected solvents based on Hansen solubility parameters⁴⁶.

Solvents	dD2 (MPa1/2)	dP2 (MPa1/2)	dH2 (MPa1/2)	Ra (MPa1/2) a
Hexane	14.9	0	0	15.3
Acetone	15.5	10.4	7.0	10.2
Toluene	18.0	1.4	2.0	10.6
Water	15.5	16.0	42.3	41.3
Chloroform	17.8	3.1	5.7	9.8

• ^a
  Hansen distance: R_a = {4*(dD₁ - dD₂)² + (dP₁ - dP₂)² + (dH₁ – dH₂)²}^1/2
• PEG: dD₁ = 20.0, dP₁ = 11.2, dH₁ = 2.3 (MPa^1/2)

However, the relatively low boiling points of acetone (56.0 °C) and chloroform (61.2 °C) made it challenging to maintain stable and controlled debinding conditions at 60 °C. Although a distillation system was employed for solvent recovery and a heating mantle was used to regulate the system temperature, the results obtained at 20 °C and 40 °C proved to be more consistent and reliable. As a result, the subsequent experiments in this study were conducted exclusively at 20 °C and 40 °C.

3.2 . Evolution of binder fraction removal as a function of samples dimensions and temperature

Given the stronger interaction between chloroform and the polymers contained in the ceramic parts, the subsequent investigations in this study focused exclusively on this solvent. Figure 4 shows the fraction of polymer removed from samples with different geometries (As/V ratio = 0.84, 1.07, or 1.38) when exposed to chloroform at 20 °C or 40 °C for up to 24 hours (1440 minutes). A faster and more significant binder removal was observed in samples maintained at 40 °C and with higher As/V ratios (Figure 4).

Figure 4
Removed binder fraction of specimens kept in chloroform at different temperatures and presenting A_s/V ratios of (a) 0.84, (b) 1.07 and (c) 1.38.

An increased As/V ratio results in a greater contact area between the solvent and binder, which in turn reduces the debinding time. The removal of soluble components from specimens with a shorter internal diffusion path (As/V = 1.38, Figure 4c) highlights the influence of lower temperature on binder extraction.

Raising the temperature enhances the efficiency of the debinding process for all sample geometries, due to the increased solubility and diffusivity of the polymer in chloroform over time²⁸,³⁸. For specimens with comparable thicknesses (As/V = 0.84 and 1.07), higher temperatures led to removal times that were inversely proportional to specimen volume. This behavior reflects the competition between solubility and diffusivity processes, which are directly influenced by part dimensions²⁵,²⁸,³⁰. It is important to note that prolonged exposure to chloroform (> 240 minutes) resulted in chipping and cracking of the parts, underscoring the aggressive nature of the solvent and the need to limit contact times in order to preserve the physical integrity of the samples.

Thermogravimetric analysis (TGA) was conducted on samples with surface area-to-volume (As/V) ratios of 0.84 and 1.38 after exposure to chloroform at 40 °C for short durations (0–60 minutes, see Figure 5). The initial suspension used for printing consisted of approximately 70 wt.% alumina, 30 wt.% PEGDA, and an additional 5 wt.% of additives (dispersant and photoinitiator), resulting in a total organic content of about 35 wt.% expected to decompose during heating. The TGA curves reveal a primary weight-loss event corresponding to PEGDA degradation, occurring between 350 and 480 °C⁴⁰, consistent with the expected binder content of the feedstock. Specimens with an As/V ratio of 0.84 exhibited similar weight loss regardless of immersion time (Figura 5a). In contrast, samples with a higher As/V ratio showed a progressive decrease in weight loss with increased immersion time, indicating effective removal of soluble components during solvent exposure (Figura 5b). Based on the thermal analysis results, Table 3 provides an estimate of the binder fraction removed during the solvent debinding stage as a function of chloroform exposure time.

Figure 5
Thermogravimetric curves for specimens (a) As/V = 0.84 and (b) As/V = 1.38 after different immersion times in chloroform at 40 °C.

Table 3
Removed binder fraction and apparent porosity of printed samples after solvent debinding at 40 °C using chloroform.

Time (min)	As/V = 0.84		As/V = 1.38
	Removed binder fraction (%)	Apparent porosity (%)	Removed binder fraction (%)	Apparent porosity (%)
0	1.13	6.11 ± 0.44	0.77	8.46 ± 0.83
10	1.07	9.58 ± 0.50	0.86	8.70 ± 0.88
30	1.28	10.15 ± 0.35	1.67	11.93 ± 1.21
60	1.87	12.47 ± 0.83	3.70	13.37 ± 1.37

These experiments confirm the trend of increasing polymer removal with longer solvent exposure times, an effect that is particularly evident in samples with higher As/V ratios (Table 3). According to some authors, solvent debinding should proceed until a minimum mass loss of 6% is achieved to generate sufficient open porosity for effective thermal debinding⁴⁷. In the present study, the maximum polymer fraction removed was limited to 3.7% within the evaluated time range. Nonetheless, visual inspection revealed significant geometric alterations in the samples after 60 minutes of chloroform exposure (Figure 6). These included corner rounding, complete or partial detachment of the base layer, and partial delamination of the top layer. As a result, longer exposure durations were avoided to preserve the structural integrity of the printed parts.

Figure 6
Photographs of DLP-printed ceramic samples with As/V ratios of (a) 0.84 and (b) 1.38 after solvent debinding in chloroform at 40 °C for varying exposure durations.

The solvent debinding step results in the formation of a porous network within the green parts, originating from the removal of the primary binder. This open porosity facilitates the efficient release of gases generated by the backbone polymer during thermal debinding, thereby reducing the likelihood of structural defects. However, despite its critical role, the development of open porosity during solvent debinding is often overlooked, as assessments typically focus only on the weight fraction of the primary binder removed³⁰.

A marked increase in apparent porosity was observed in the samples following solvent exposure, particularly with longer immersion times (Table 3). For example, porosity values measured immediately after printing and initial cleaning with isopropyl alcohol (indicated as time 0 in Table 3) ranged from 6.11% to 8.46%. After 60 minutes of exposure to chloroform at 40 °C, the same samples exhibited porosity values between 12.47% and 13.37%. This represents an increase of up to 74% in porosity, attributed to the action of the solvent under the specified conditions.

Figure 7 presents SEM micrographs of specimens (As/V = 1.38, fractured cross-sections) after different solvent debinding durations. The micrographs of the as-printed specimens (reference) reveal a similar microstructure in regions near both the core and the external surface. The observed microstructure is homogeneous, with good interlayer adhesion across the printed parts. After 30 minutes of immersion in chloroform, the fracture surface near the external region exhibits a porous structure, attributed to the removal of soluble components. However, no noticeable changes were observed in the core region of the specimens. With increased immersion time (60 minutes), the microstructure near the surface becomes more irregular, further indicating the dissolution and removal of soluble phases. Although some variations in particle packing may occur, all specimens analyzed in this study exhibited similar behavior, with well-defined printed layers that were properly connected to one another (Figure 7).

Figure 7
SEM micrographs of the specimens (As/V = 1.38, fracture cross-section) before and after debinding in chloroform at 40 °C for 30 or 60 minutes.

To evaluate the influence of the pore network generated by solvent debinding on the subsequent thermal debinding and sintering stages, samples either exposed or not to chloroform at 40 °C for 30 min were selected for further characterization. Figure 8 presents the linear shrinkage, apparent porosity, and relative density of alumina samples with different As/V ratios, subjected either to thermal debinding (TD) alone or to solvent followed by thermal debinding (SD + TD), under the heating conditions described in Figure 2. The effect of the furnace atmosphere was also investigated, as previous studies have indicated that thermal treatments in an inert atmosphere can enhance ceramic quality and reduce defects²³,²⁴,⁴⁸. All samples were subsequently heated to 1600 °C to promote sintering and further densification of the microstructure.

Figure 8
(a) Linear shrinkage, (b) apparent porosity, (c) relative density, and (d) images of alumina samples subjected either to thermal debinding (TD) alone or to solvent debinding (chloroform exposure at 40 °C for 30 min) followed by thermal debinding (SD + TD), under oxidizing or inert atmospheres. All samples were subsequently sintered at 1600 °C for 2 h.

Specimens subjected solely to debinding (TD or SD+TD) in an oxidizing atmosphere, followed by sintering at 1600 °C, exhibited linear shrinkage of 17.96–19.02% (Figure 8a), apparent porosity of 15.55–19.71% (Figure 8b), and relative density up to 83.11% (Figure 8c). The limited densification is likely related to defect formation (cracks and delaminations, Figure 8d) caused by rapid oxidation of the organic phase and gas release during thermal debinding in an oxidizing environment³⁵. Conversely, heat treatment in an inert atmosphere enhanced the structural integrity of the printed ceramics, yielding defect-free samples (Figure 8d). After sintering, these specimens showed greater shrinkage (23.88–25.43%), lower porosity (2.90–11.94%), and higher relative density (up to ~97%) (Figure 8c). Specimens with a higher As/V ratio (1.38) exhibited greater densification at 1600 °C, regardless of the debinding route. The combined use of solvent and thermal debinding (SD+TD, with the latter performed under N₂ flow) further enhanced densification and reduced porosity compared to TD alone. Visually, solvent-treated samples displayed fewer flaws, along with a superior surface finish (Figure 8d). These findings demonstrate that solvent debinding improves the microstructural quality of DLP-fabricated alumina ceramics.

4. Conclusions

This study demonstrated that exposing alumina ceramics produced by digital light processing (DLP) to chloroform at 40 °C for 30 min is a simple and effective procedure for partially removing the polymeric fraction contained in such materials. Samples with a higher As/V ratio provided greater contact area between solvent and binder, reducing the debinding time. However, prolonged chloroform exposure (> 240 min) caused chipping and cracking of the parts, highlighting the solvent’s aggressive nature and the need to limit contact time to preserve sample integrity.

Both the debinding route and furnace atmosphere significantly influenced the microstructural quality and densification of the evaluated ceramics. Specimens subjected solely to thermal debinding (TD) in an oxidizing atmosphere showed limited densification, higher porosity, and visible defects, resulting from rapid polymer oxidation and gas release. In contrast, debinding under an inert atmosphere markedly improved densification and structural integrity. Among the tested geometries, those with a higher As/V ratio (1.38) showed the most effective polymer removal during chloroform exposure and achieved greater densification during sintering. The combination of solvent debinding followed by thermal debinding (SD+TD), particularly under N₂ flow, further improved densification (yielding defect-free samples with relative densities of up to 97%) while reducing porosity and enhancing surface finish compared to TD alone. These results demonstrate that solvent debinding, when used as a pre-treatment, is highly effective in minimizing defect formation and ensuring uniform, efficient binder removal.

Overall, integrating solvent debinding with controlled-atmosphere sintering offers a promising approach for fabricating high-quality, defect-free alumina ceramics via DLP, with enhanced mechanical and structural performance.

5. Acknowledgments

This study was funded by the São Paulo Research Foundation (FAPESP) under grant numbers 2024/03788-2 and 2024/04483-0. The authors would also like to thank Almatis and BYK for providing the raw materials used in this work.

Data Availability

The full dataset supporting the findings of this study is available upon request to the corresponding author.

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*

e-mail: [e-mail]

Edited by

Associate Editor: Celso Santilli.

Editor-in-Chief: Luiz Antonio Pessan.

Publication Dates

• Publication in this collection
  19 Jan 2026
• Date of issue
  2025

History

• Received
  14 Aug 2025
• Reviewed
  09 Oct 2025
• Accepted
  29 Nov 2025