Fused Filament Fabrication of COC/Aluminum Composites for Structured Reactor Components

Abstract

The development of 3D-printable polymer–metal composites offers new opportunities for structured catalytic reactor design and process intensification. Here, cyclic olefin copolymer (COC) composites filled with micron-scale aluminum particles (1–15 wt%, 160 µm) were prepared via a two-step compounding and extrusion process to produce filaments suitable for fused filament fabrication (FFF). Thermal analysis confirmed that aluminum incorporation does not significantly alter the glass transition (T_g = 76–77 °C) or thermal stability of the polymer. Melt flow rate measurements indicated processable viscosity (MFR 3.82–4.57 g/10 min), while tensile testing revealed Young’s modulus of 1277 MPa–1783 MPa, maximum stress of 27 MPa–39 MPa, and enhanced strain at break for the 1 wt% Al composite (ε_B = 5.33%). Composites containing up to 15 wt% Al were successfully printed into mechanically robust static mixers, demonstrating complex geometries without particle sedimentation issues. The incorporation of aluminum particles introduces potential functionalities related to thermal management, surface modification, and future catalytic or photocatalytic applications. This work establishes a scalable polymer–metal platform integrating structural stability, geometric complexity, and prospective multifunctional behavior for advanced flow-reactor applications.

1. Introduction

The design of structured reactor internals is a key strategy for process intensification in modern chemical engineering, offering improved mass and heat transfer, enhanced mixing efficiency, and reduced reactor volume compared to conventional packed-bed systems. Among structured internals, static mixers have emerged as versatile platforms, enabling controlled flow, residence times, and efficient fluid mixing [1,2,3,4]. However, the fabrication of complex mixer geometries using conventional manufacturing methods is often challenging due to limitations in molding, machining, or assembly [5,6].

Recent advances in additive manufacturing (3D printing) have enabled the production of intricate reactor components with geometries that are difficult or impossible to achieve using traditional methods [7,8]. Several studies have demonstrated the use of 3D-printed structures as reactor internals or functional supports, highlighting improvements in flow distribution, surface area control, and structural precision. Additive manufacturing allows for rapid prototyping and scalable production of customized components, providing a unique route to integrate design and functional performance in a single part [7,9,10,11,12]. However, despite these advances, existing studies largely treat material development and reactor functionality separately. In particular, there is a lack of systematic studies addressing polymer–metal composites that simultaneously meet the processing requirements of fused filament fabrication and the thermal and mechanical demands of structured reactor applications [13,14,15].

To address this gap, we present a COC/aluminum composite specifically engineered for fused filament fabrication (FFF) of structured reactor components. COC was selected due to its excellent thermal stability, chemical resistance, and mechanical robustness, making it suitable for high-performance reactor applications [16,17]. Incorporation of aluminum particles into COC can provide opportunities for additional functionalities, such as thermal management or plasmonic interactions, while maintaining processability for fused filament fabrication. Unlike prior studies on polymer–metal composites, which primarily focus on bulk property enhancement or general additive manufacturing applications, this work targets the simultaneous optimization of printability, thermal behavior, and mechanical integrity required for static mixer performance. The novelty of this work lies in leveraging the native Al₂O₃ layer that spontaneously forms on aluminum surfaces under ambient conditions, which acts as a stable interface for catalyst immobilization. Moreover, aluminum’s plasmonic properties could be exploited in future photocatalytic applications, enhancing light absorption and charge carrier generation when combined with semiconductors such as TiO₂ [18,19,20,21]. This strategy integrates structural support, thermal conductivity, and catalytically relevant surface chemistry within a single printable material, providing a platform for flow-reactor applications that require both process intensification and catalyst functionality.

To the best of our knowledge, this study represents the first systematic investigation of COC/aluminum composites for fused filament fabrication that establishes the relationship between filler content, melt processability, and the resulting thermal and mechanical properties in the context of structured reactor applications. This approach opens new avenues for customizable reactor internals that bridge materials design, additive manufacturing, and heterogeneous catalysis, enabling compact and efficient reactors for a variety of liquid- and gas-phase flow reactions. The incorporation of aluminum particles in the COC matrix was not primarily intended to improve mechanical reinforcement but rather to develop multifunctional composite materials suitable for additively manufactured process-intensification components and structured reactor internals. Specifically, this work develops a printable COC/Al composite tailored for FFF, evaluates the effect of aluminum loading on melt flow, thermal, and mechanical properties, and demonstrates its suitability for structured reactor components with integrated functional potential.

2. Materials and Methods

2.1. Materials

Granular cyclic olefin copolymer (COC) (CAS No. 26007-43-2, TOPAS Advanced Polymers GmbH, Raunheim, Germany) was employed as the polymer matrix. Spherical aluminum (Al) powder with an average particle size of 160 µm (≥99% purity, batch no. 194342989, Carl Roth GmbH, Karlsruhe, Germany) was used as the conductive filler. Both constituents were utilized as supplied, without additional purification or surface treatment. COC/Al composites were produced via a two-step compounding approach. Initially, the polymer granules and aluminum particles were dry-mixed to achieve a preliminary homogeneous distribution. The required amounts of each component were weighed according to the target compositions and mechanically blended until a visually uniform mixture was obtained (

Table 1. Composition of COC/Al samples.

Sample	m (COC) [g]	m (Al) [g]	Al Content [wt%]	Al Content [vol%]
COC	200.0	0.0	0.0	0
COC/Al 1%	198.0	2.0	1.0	0.4
COC/Al 5%	190.0	10.0	5.0	1.9
COC/Al 10%	180.0	20.0	10.0	4.0
COC/Al 15%	170.0	30.0	15.0	6.3
COC/Al 20%	160.0	40.0	20.0	8.6

).

2.2. Composite Preparation and Filament Extrusion

The pre-mixed powders were first compounded using a Rondol LAB TWIN 21 mm co-rotating twin-screw extruder (Rondol, Nancy, France) operating at 70 rpm. The barrel temperature profile, from feed zone to die, was set to 235 °C, 235 °C, 230 °C, 230 °C, and 230 °C. This step ensured adequate dispersive and distributive mixing of the aluminum particles within the molten COC matrix. The extrudate was then cooled and pelletized. In the second stage, the compounded pellets were processed into filament using a Filament Maker Composer 450 (3devo, Utrecht, The Netherlands). Four heating zones were set to 200 °C, 195 °C, 185 °C, and 185 °C, providing stable melt flow and uniform filament formation. The material was extruded through a circular die and cooled with integrated fans. Filament diameter was continuously monitored by an inline laser gauge, with automatic adjustment of the haul-off speed to maintain a tolerance of ±0.08 mm (±5% of 1.75 mm). The produced filaments were subsequently used for the fabrication of static mixers via additive manufacturing. This two-step process yielded filaments with consistent geometry and surface quality.

2.3. Thermal Characterization

Thermal properties of the composites were evaluated using a Mettler Toledo DSC 3 differential scanning calorimeter (Mettler Toledo, Greifensee, Switzerland) under a nitrogen atmosphere (50 mL/min). Each specimen underwent two heating cycles to eliminate prior thermal history effects. The applied temperature program included:

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Heating from 25 °C to 260 °C at 10 °C/min;

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Cooling from 260 °C to 0 °C at 10 °C/min, followed by a 1 min isothermal hold;

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Reheating from 0 °C to 260 °C at 10 °C/min, with a 1 min hold;

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Final cooling from 260 °C to 25 °C at 10 °C/min.

The glass transition temperature (T_g) was determined from the second heating run.

Thermal stability was assessed using a Mettler Toledo TGA/DSC 3+ instrument (Mettler Toledo, Greifensee, Switzerland). Samples were heated from 25 °C to 800 °C at a rate of 10 °C/min under nitrogen flow (50 mL/min), enabling evaluation of degradation behavior in an inert environment.

2.4. Melt Flow Rate (MFR) Measurement

The flow characteristics of the composite granules were determined via melt flow rate (MFR) testing. Measurements were performed at 250 °C under a standard load of 2.16 kg using a die with a diameter of 2.095 mm. Approximately 5–6 g of material was loaded into the preheated barrel, compacted to remove air, and conditioned for 3 min. After conditioning, the load was applied to initiate extrusion. The extrudate was manually cut into segments (5–8 cm), and the time between cuts was recorded. Each segment was weighed, and the MFR was calculated as

MFR = (m × 600)/t

where m is the mass (g) and t is the time (s). This method allowed comparison of processability across formulations and assessment of the influence of aluminum content on melt viscosity. The obtained values also indicate suitability for fused filament fabrication (FFF), particularly regarding extrusion stability and print continuity.

2.5. Mechanical Testing

Mechanical performance was evaluated through tensile testing of 3D-printed specimens. Samples were fabricated using a Prusa i3 MK3S+ FFF printer (Prague, Czech Republic). Printing was performed at 25 mm/s without support structures, using a 0.6 mm nozzle and a textured build plate. The nozzle and bed temperatures were set to 285 °C and 90 °C, respectively. A layer height of 0.2 mm and 100% rectilinear infill were applied. Tensile specimens were printed in the XY plane on the build plate, with the specimen’s long axis positioned parallel to the X-axis and raster lines oriented perpendicular to the tensile loading axis. Specimens were designed in Autodesk Fusion in accordance with ISO 527-1:2012 (Type 5 geometry, Figure 1) [22] and processed using PrusaSlicer (version 2.9.4.) with modified PETG settings adapted for the composite material. Tensile tests were conducted on a Shimadzu AGS-X universal testing machine (50 kN capacity) equipped with a 5 kN load cell and a stand-alone clip-on DSES-1000 extensometer (Shimadzu, Kyoto, Japan), which was directly attached to the specimen gauge section during testing. Strain values were determined directly from extensometer measurements rather than from crosshead displacement in order to minimize the influence of machine compliance and grip slippage. Measurements were performed at a crosshead speed of 5 mm/min, with a preload of 1 N applied at 1 mm/min. These tests enabled evaluation of the effect of aluminum content and processing conditions on the mechanical properties of the printed composites.

2.6. Scanning Electron Microscopy (SEM)

Fracture surfaces of tensile specimens were examined using scanning electron microscopy to analyze morphology and failure mechanisms. Imaging was carried out on a Tescan Vega 3 microscope (Tescan, Brno, Czech Republic) at an accelerating voltage of 20 kV using a secondary electron detector. Prior to observation, samples were mounted on aluminum stubs and sputter-coated with a thin gold/platinum layer in argon plasma for 60 s to enhance electrical conductivity and image clarity.

3. Results and Discussion

3.1. Thermal Properties

The thermal behavior of the prepared COC/Al composites was evaluated using differential scanning calorimetry (DSC) in order to determine the glass transition temperature (T_g) and assess the influence of aluminum particles on the thermal transitions of the polymer matrix. Representative DSC curves obtained during the second heating cycle are shown in Figure 2. The value of 0.2 in Figure 2 refers to the DSC heat flow scale (0.2 W/g). The DSC curves were obtained from single measurements per composition. The manufacturer-reported temperature accuracy of the DSC 3+ instrument (±0.2 °C) is provided as the instrumental uncertainty associated with the thermal measurements.

The neat COC exhibited a characteristic glass transition temperature of 77.2 °C, which is consistent with reported values for cyclic olefin copolymers. After incorporation of aluminum particles, the T_g values of the composites remained within a narrow range. The measured T_g values are shown in

Table 2. DSC-derived thermal properties of COC/Al composites (n = 1). The manufacturer-reported temperature accuracy of the instrument is ±0.2 °C.

Sample	Tg (°C)
COC	77.2
COC/Al 1%	76.1
COC/Al 5%	76.6
COC/Al 10%	76.5
COC/Al 15%	76.3
COC/Al 20%	76.7

.

The incorporation of aluminum particles did not introduce additional thermal transitions within the investigated temperature range, confirming that the metallic filler does not undergo phase changes under the applied conditions. The slight decrease in T_g compared to pure COC can be attributed to the limited interfacial interaction between the polymer matrix and the aluminum particles. Due to the relatively large particle size (160 µm), the specific surface area of the filler is low, which reduces the extent of polymer–filler interfacial interactions. As a result, in some cases, no significant restriction of polymer chain mobility occurs. In some cases, weak interfacial adhesion or the presence of interfacial voids may even introduce additional space at the polymer–filler interface, leading to a minor reduction in T_g. This behavior is consistent with particulate–filler polymer systems in which large, non-interacting fillers primarily act as inert inclusions rather than as effective constraints on segmental dynamics [23,24]. The small changes in T_g suggest that, at the investigated filler loadings, aluminum particles do not significantly alter the segmental mobility of the COC matrix within the resolution of DSC. This stability of T_g with filler addition is beneficial for maintaining a consistent thermal processing window during filament extrusion and FFF printing.

Thermal stability of the composites was further evaluated using thermogravimetric analysis (TGA). The onset degradation temperature was determined as T₉₅, i.e., the temperature at which 5% mass loss occurs. The TGA curves of the composites are presented in Figure 3. Pure COC showed a typical single-step thermal degradation profile, corresponding to the decomposition of the polymer backbone. The onset degradation temperature of pure COC was 448.4 °C, with a residual mass of 0.2%. The incorporation of aluminum particles did not significantly alter the degradation temperature of the polymer matrix. The measured degradation temperatures are shown in

Table 3. Thermal stability of COC/Al composites.

Sample	Tonset (°C)	Residue (%)
COC	448.4	0.2
COC/Al 1%	442.5	2.2
COC/Al 5%	445.0	2.9
COC/Al 10%	445.4	4.8
COC/Al 15%	445.6	10.4
COC/Al 20%	446.0	12.9

and Figure 3, indicating that the composites maintain thermal stability comparable to that of the pure polymer. The residual mass increased with increasing aluminum content, reflecting the presence of the non-volatile metallic filler. This trend confirms the successful incorporation of aluminum particles into the polymer matrix and their thermal stability under the applied inert conditions. However, the observed residue values represent approximately 50–65% of the theoretical aluminum content based on nominal formulations, indicating partial deviation between the intended and actual filler loading. This deviation is attributed primarily to processing-induced variations in filler distribution. During melt processing, partial retention of aluminum particles on processing equipment surfaces (e.g., extruder screw and barrel) can reduce the effective filler content in the final filament. In addition, the relatively large particle size and high density of aluminum compared to the polymer matrix may lead to non-uniform dispersion and local compositional heterogeneity along the filament length. Since TGA measurements are performed on small sample masses, the analyzed specimens may not fully represent the average bulk composition, particularly in the absence of replicate sampling across different filament regions. Overall, the TGA results demonstrate that the addition of aluminum particles does not significantly affect the thermal stability of the COC matrix while introducing a thermally stable inorganic phase. These findings further confirm that the prepared composites are suitable for melt processing and fused filament fabrication (FFF), where elevated processing temperatures are required.

3.2. Melt Flow Rate of COC/Al Composites

The melt flow rate (MFR) of the prepared composites was measured to evaluate the influence of aluminum particles on the melt processability of the material. MFR is an important parameter for fused filament fabrication (FFF), as it reflects the viscosity of the molten polymer and its ability to flow through the printer nozzle during extrusion. All measurements were performed in triplicate, and the reported values represent the mean values with corresponding standard deviations. The measured MFR values are summarized in

Table 4. Melt flow rate (MFR) values of COC/Al composites.

Sample	MFR (g/10 min)
COC	4.21 ± 0.17
COC/Al 1%	4.57 ± 0.28
COC/Al 5%	4.34 ± 0.01
COC/Al 10%	4.44 ± 0.15
COC/Al 15%	4.01 ± 0.20
COC/Al 20%	3.82 ± 0.32

and Figure 4. Pure COC exhibited an MFR of 4.21 ± 0.17 g/10 min. After the incorporation of aluminum particles, the composites showed MFR values within a relatively narrow range. At lower aluminum contents (1–10%), the MFR values remained comparable to or slightly higher than those of pure COC, indicating that melt processability is largely preserved. At higher filler contents (15–20%), a gradual decrease in MFR was observed, indicating an increase in melt viscosity due to the presence of rigid aluminum particles that hinder polymer chain mobility and increase resistance to flow.

Despite this reduction, the MFR values remain within a range compatible with filament extrusion and FFF 3D printing, suggesting that the addition of aluminum particles does not significantly impair melt processability. The composites therefore retain adequate rheological behavior for stable extrusion through the printer nozzle and for the fabrication of 3D-printed static mixers. In addition, the results should be interpreted with caution, as the apparent viscosity of such a composite system can also depend on the nozzle diameter used during 3D printing.

3.3. Mechanical Properties of 3D-Printed COC/Al Composites

The mechanical performance of the 3D-printed COC/Al composites was evaluated via tensile testing. All reported values represent the mean ± standard deviation of five independent specimens, ensuring the reproducibility of the observed trends. Key parameters extracted from the stress–strain curves include Young’s modulus (E), maximum stress (σ_M), strain at maximum stress (ε_M), stress at break (σ_B), strain at break (ε_B), and toughness (W). All values represent the mean ± standard deviation of five specimens, as summarized in

Table 5. Mechanical properties of COC/Al composites (values represent mean ± standard deviation, n = 5 specimens for each composition).

Sample	E (MPa)	σM (MPa)	εM (%)	σB (MPa)	εB (%)	W (J)
COC	1783 ± 376	38.8 ± 6.2	2.95 ± 0.41	34.5 ± 0.4	3.21 ± 1.46	0.22 ± 0.07
COC/Al 1%	1277 ± 132	36.1 ± 2.4	3.10 ± 0.67	27.8 ± 8.1	5.33 ± 2.63	0.27 ± 0.21
COC/Al 5%	1169 ± 62	33.5 ± 2.2	3.29 ± 0.58	28.6 ± 2.7	4.81 ± 1.13	0.20 ± 0.11
COC/Al 10%	1086 ± 88	29.7 ± 3.3	3.27 ±0.49	28.4 ± 2.0	2.79 ± 0.38	0.11 ± 0.02
COC/Al 15%	1177 ± 97	27.4 ± 3.3	2.28 ± 0.90	25.6 ± 2.7	2.40 ± 1.00	0.08 ± 0.07

and Figure 5. Pure COC exhibited a Young’s modulus of 1783 ± 376 MPa and a maximum stress of 38.8 ± 6.2 MPa, with a corresponding strain at maximum stress of 2.95 ± 0.41% and a strain at break of 3.21 ± 1.46%. The work of fracture was 0.22 ± 0.07 J, indicating moderate toughness for the 3D-printed polymer. The addition of aluminum particles resulted in a gradual decrease in stiffness and strength with increasing filler content. The Young’s modulus decreased to 1277 ± 132 MPa for COC/Al 1 wt% and to 1177 ± 97 MPa for COC/Al 15 wt%, while the maximum stress decreased from 38.8 ± 6.2 MPa for pure COC to 27.4 ± 3.3 MPa for COC/Al 15 wt%. This trend is consistent with the presence of rigid filler particles influencing the deformation behavior of the polymer matrix. It should be emphasized that the incorporation of aluminum particles was not primarily intended to mechanically reinforce the COC matrix, but rather to introduce functional metallic content while maintaining sufficient printability and structural integrity for potential structured reactor applications. The strain at maximum stress (ε_M) and strain at break did not show a clear monotonic trend. For composites containing 1–10 wt% aluminum, the maximum strain remained similar to that of the pure COC matrix. Notably, the COC/Al 1% composite exhibited the highest strain at break (5.33 ± 2.63%), which may indicate more efficient stress distribution at low filler content. However, at higher filler contents (15 wt%), a slight decrease in strain at break (2.40 ± 1.00%) was observed, indicating a tendency toward more brittle behavior, likely due to local particle agglomeration or weaker interfacial adhesion between the polymer matrix and the aluminum particles. The work of fracture decreased with increasing filler loading, from 0.22 ± 0.07 J for pure COC to 0.08 ± 0.07 J for the COC/Al 15 wt% composite, reflecting a general reduction in ductility at higher aluminum loadings.

This non-monotonic behavior suggests a change in the dominant deformation and failure mechanisms with filler content. At low aluminum loading (1 wt%), the presence of sparsely distributed particles may promote localized stress redistribution and activate energy-dissipating mechanisms such as shear yielding or localized crazing, which can delay failure and result in increased strain at break. In contrast, at higher filler contents, the increased likelihood of particle–particle interactions and agglomeration, combined with locally reduced interfacial adhesion, may introduce preferential sites for stress concentration and crack initiation, leading to earlier failure and reduced ductility. It should be noted that the COC/Al 20 wt% composite could not be successfully printed using the FFF process, as repeated nozzle clogging occurred during printing, preventing stable filament extrusion. Consequently, no tensile test specimens were produced for this composition, and mechanical properties were not determined. The inability to successfully print the COC/Al 20 wt% composite is consistent with the lower MFR values observed for this composite, indicating reduced melt flow and increased risk of extrusion instability.

Based on the results obtained from the thermal, rheological, and mechanical characterization, the COC/Al composites containing up to 15 wt% aluminum were considered suitable for fused filament fabrication (FFF). To demonstrate the practical applicability of the developed material, a static mixer geometry intended for use in continuous flow reactors was fabricated using the optimized composite filament. A photograph of the printed static mixer and its technical drawing and key geometric dimensions are presented in Figure 6. The mixer design was selected to promote efficient fluid mixing and enhanced surface interaction, which are relevant for potential future structured flow applications. The successful fabrication of this geometry demonstrates that the developed composite can be used to produce complex reactor internals with integrated metallic filler, combining the advantages of additive manufacturing with the functional properties of composite materials.

SEM micrographs of the fractured tensile specimens were used to evaluate aluminum particle dispersion, interfacial adhesion between the COC matrix and aluminum filler, and the quality of the additively manufactured structures. The images (Figure 7) show a relatively uniform distribution of aluminum particles within the polymer matrix, with no severe agglomeration observed. This is attributed to the spherical morphology of the aluminum particles, which promotes stable dispersion during melt processing. However, evidence of particle pull-out and interfacial voids indicates poor adhesion between aluminum particles and the COC matrix, confirming weak interfacial bonding and limited stress transfer capability. No surface-active agents or compatibilizers were used during composite preparation, which further contributes to the observed weak interfacial adhesion. Despite this, the fracture surfaces reveal good interlayer fusion during fused filament fabrication. It should be noted that the SEM analysis was limited to fracture surfaces of tensile specimens, while polished cross-sectional SEM imaging of the printed static mixer was not performed. Therefore, direct evaluation of internal bead morphology, interlayer void distribution, and possible particle orientation effects induced by shear flow during extrusion through the nozzle could not be assessed in the present study. Overall, while dispersion is satisfactory, the smooth surface of the aluminum particles combined with the absence of interfacial modification limits reinforcement efficiency, suggesting that future surface treatment could improve load transfer and mechanical performance.

The mechanical behavior of aluminum-filled COC composites has previously been investigated in the literature. For example, Sinan Köse [25] examined the influence of aluminum powder on the mechanical and tribological properties of COC-based composites produced using conventional processing methods. That study demonstrated that the addition of aluminum particles can influence the stiffness, strength, and wear behavior of the polymer matrix, highlighting the potential of such composites for engineering applications. In contrast to these earlier investigations, the present work focuses on the processability of COC/Al composites in fused filament fabrication (FFF) and their suitability for the fabrication of 3D-printed functional reactor components. Additive manufacturing introduces additional considerations compared to conventionally processed composites, including filament extrusion stability, melt flow behavior, layer adhesion, and printability limits related to filler loading. The results obtained in this study demonstrate that COC/Al composites containing up to 15 wt% aluminum can be successfully processed into filaments and printed into mechanically stable structures, while higher filler loadings lead to extrusion instability and nozzle clogging (0.6 mm nozzle). An important advantage of the approach presented here is the ability to directly fabricate complex geometries, such as static mixers for flow reactors, which are difficult to achieve using conventional composite processing techniques. In this context, the integration of metallic particles within a 3D-printable polymer matrix provides not only structural integrity sufficient for printing but also a potential platform for catalytically active reactor internals. Consequently, the present work extends previous studies on COC/Al composites by demonstrating their applicability in additive manufacturing and structured reactor design, thereby opening new opportunities for multifunctional materials in process intensification.

While previous studies have primarily focused on the mechanical and tribological behavior of aluminum-filled COC composites prepared via conventional processing routes, their potential use in catalytically active structured materials has received considerably less attention. In particular, earlier work has largely treated aluminum particles as passive fillers used to modify the mechanical performance or wear resistance of the polymer matrix. In contrast, the present study explores the integration of aluminum particles within a 3D-printable COC matrix as a strategy to create multifunctional materials that combine structural integrity with potential catalytic functionality. Recent developments in additive manufacturing have demonstrated that 3D printing can be used to fabricate structured catalysts with precisely controlled geometries and improved transport properties. For example, recent work by Mastroianni et al. [26] reported the fabrication of porous γ-Al₂O₃-based catalytic structures using high-accuracy additive manufacturing, demonstrating that complex catalytic architectures can be produced with well-defined porosity and high structural fidelity. While these ceramic systems are highly effective as catalysts, their fabrication typically requires specialized printing techniques and high-temperature post-processing, and in photopolymer-based approaches such as digital light processing, particle sedimentation can lead to inhomogeneous filler distribution and defects. These limitations constrain the rapid prototyping of complex geometries like static mixers or microreactors. In contrast, the aluminum-filled COC composites presented in this work offer a complementary and more versatile approach. Using fused filament fabrication (FFF), complex geometries such as static mixers can be printed directly with a polymer–metal composite that combines mechanical robustness with embedded metallic particles. Unlike liquid-based printing of ceramics, FFF avoids sedimentation issues, ensuring a more uniform particle distribution throughout the printed structure. Moreover, FFF processing occurs at moderate temperatures, eliminating the need for high-temperature post-processing while enabling rapid and flexible production of structured reactor components. This positions the aluminum-filled COC composite as a versatile platform that can potentially bridge the gap between conventional ceramic catalysts and polymer-based additive manufacturing while also offering a route toward future catalytic functionalization (

Table 6. Comparison of this study with recent 3D-printed catalytic materials [25,26].

Property	COC/Al Composites (Conventional Processing)	γ-Al2O3 3D-Printed Catalysts	This Study
Material type	polymer–metal (COC + Al)	ceramic (γ-Al2O3)	polymer–metal (COC + Al)
Fabrication method	conventional composite processing (molding and extrusion)	high-accuracy AM (ceramic)	fused filament fabrication
Processing temperature	moderate	high-temperature sintering	low-to-moderate (no high-temperature sintering required)
Particle distribution	can be homogeneous, but dependent on mixing	often uniform in printed pores, but DLP can suffer from particle sedimentation	can be homogeneous if filament preparation is done correctly; FFF avoids sedimentation issues
Structural complexity	limited to molded shapes	complex porous architectures	complex geometries
Mechanical robustness	moderate, improved by Al	brittle	high, mechanically stable for manipulation and flow applications
Scalability	readily scalable	limited by ceramic AM equipment	readily scalable using standard FFF printer
Novelty	demonstrates mechanical reinforcement in conventional composites	high-fidelity ceramic AM for catalysis	demonstration of melt-compounded polymer–metal filament enabling FFF fabrication of complex geometries relevant to structured flow applications

).

4. Conclusions

Aluminum-filled COC composites were systematically developed and evaluated for thermal, rheological, and mechanical properties, demonstrating their suitability for fused filament fabrication of structured reactor components. Composites containing up to 15% aluminum maintained thermal stability, adequate melt flow, and mechanical robustness, while higher filler loadings caused extrusion issues, likely due to increased melt viscosity. Tensile testing revealed that low aluminum content (1 wt%) can enhance strain at break, possibly due to improved stress distribution at low particle loadings, while higher loadings reduce ductility and strength. However, the processability–performance trade-off at higher aluminum loadings remains a limitation that must be addressed for further material optimization. The practical applicability of these composites was demonstrated by printing a static mixer geometry for flow reactors, highlighting the potential applicability of such geometries in future flow-reactor systems. This work demonstrates the successful fabrication of aluminum-filled COC composites suitable for fused filament fabrication. Composites containing up to 15 wt% aluminum maintain adequate thermal stability, melt processability, and mechanical integrity for extrusion-based additive manufacturing. The printed static mixer demonstrates the ability to fabricate complex geometries using metal-filled polymer filaments. While functional catalytic or sensing performance is not investigated in this study, the presented approach establishes a scalable platform for future development of multifunctional structured materials.