Enhancing Through-Thickness Electrical Conductivity in Recycled Carbon Fiber-Reinforced Polymer Composites Using Machining Waste

Abstract

CFRP (carbon fiber-reinforced polymer) production in Europe is approximately 10,000 metric tons annually, and according to the UK authorities, approximately 35% of end-of-life CFRP waste is currently landfilled. The authors propose a novel recycling process for industrial CFRP waste particles to produce the core of a sandwich CFRP panel through the direct molding method. Industrial CFRP powder from grinding operations was collected, sieved and molded into square panels with and without external skins of virgin CFRP prepreg. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analysis revealed thermal activation (~70 °C), indicating potential for reprocessing. This study proposes a novel recycling route that directly molds industrial CFRP grinding waste into the core of sandwich structures, with or without virgin CFRP prepreg skins. Key findings: thermal re-processability was confirmed through TGA and DSC, showing activation near 70 °C; electrical conductivity reached 0.045 S/cm through the thickness in sandwich panels, with recycled cores maintaining comparable conductivity (0.04 S/cm); mechanical performance was improved significantly with prepreg skins, as evidenced by three-point bending tests showing enhanced stiffness and strength. These results demonstrate the potential of recycled CFRP waste in multifunctional structural applications, supporting circular economy goals in composite materials engineering.

1. Introduction

Over the past few decades, carbon fiber-reinforced polymers (CFRPs) have gained widespread use across multiple industries due to their high strength-to-weight ratio, fatigue resistance, and durability. These properties make them highly suitable for structural applications in aerospace, automotive, marine, civil engineering, sports equipment, and renewable energy sectors [1,2]. In these contexts, CFRPs help enable lightweight structures that reduce fuel consumption, increase payload efficiency, and enhance design flexibility. Among CFRP systems, thermosetting composites—particularly epoxy-based CFRPs—are the most used, given their excellent mechanical strength, thermal stability, and chemical resistance [3]. Such materials are found extensively in aircraft fuselages, structural panels, wind turbine blades, and vehicle body components.

However, thermoplastic CFRPs are emerging as viable alternatives due to their shorter processing cycles, damage tolerance, and, importantly, recyclability. These developments reflect a growing shift in the materials industry toward circularity and sustainability, especially as environmental regulations become more stringent and industries face increasing pressure to reduce carbon footprints. In parallel, the accelerated demand for carbon fiber composites has led to a corresponding surge in CFRP waste generation, creating both a waste management challenge and an opportunity for material recovery and reuse.

Recent studies estimate that approximately 31,000 tons of CFRP waste are generated annually worldwide, a number projected to rise sharply as more CFRP-intensive components reach end-of-life (EoL) status. By 2050, Europe alone could accumulate an estimated 190,000 tons of CFRP waste, with the aerospace and wind energy sectors contributing the most. Despite growing awareness, current recycling and waste management practices remain limited in scope and efficiency. In some regions, over 35% of CFRP waste is still landfilled, and only 20% is recycled and 2% reused [4]. These figures underscore a significant materials and environmental gap: while carbon fiber is an expensive, high-performance material with high embodied energy, a large fraction is discarded, resulting in economic loss and environmental impact. Moreover, projections indicate that global carbon fiber demand will outpace supply by 2030, further emphasizing the need to reclaim and repurpose CFRP waste.

To address this issue, several recycling methods have been developed or are currently under evaluation. These include mechanical, thermal, and chemical recycling approaches, each with distinct advantages and challenges. Mechanical recycling is the most cost-effective and industrially accessible technique. It involves physically shredding, crushing, or grinding CFRP waste into smaller pieces without separating the fiber and matrix phases. Initial coarse shredding reduces the material to particle sizes of 50–100 mm, followed by fine grinding to produce powders ranging from sub-millimeter to tens of microns. These powders, although heterogeneous, retain functional characteristics—such as residual stiffness, conductivity, and thermal resistance—and can be reused as fillers or reinforcements in new composite systems [2,5].

Thermal recycling, notably pyrolysis, involves heating CFRP waste in an inert or oxygen-limited environment to decompose the polymer matrix and recover carbon fibers. This method is well-established in industrial settings and is capable of reclaiming long, continuous fibers. However, the process often leaves residual char on fiber surfaces, which negatively affects fiber–matrix adhesion in new composites [6]. More advanced thermal methods, such as microwave-assisted pyrolysis or fluidized bed systems, aim to reduce energy input and improve fiber quality but remain under evaluation for scalability and cost-effectiveness [7]. Chemical recycling, or solvolysis, offers a more refined approach by using high-temperature solvents or supercritical fluids to selectively depolymerize the resin matrix. While this method can yield high-purity recovered fibers and even recover matrix constituents, it often requires hazardous reagents and sophisticated equipment, limiting its commercial viability [8].

While most recycling techniques aim to extract clean, reusable fibers, another promising direction involves the functional reuse of the full CFRP waste, including both fiber and resin phases, as hybrid reinforcement systems. This approach leverages the fact that mechanically milled waste consists of fragmented carbon fibers embedded in thermoset resin particles. When ground to fine particle sizes and subjected to appropriate thermal and pressure conditions, this hybrid material can be reactivated—partially through softening or degradation of the residual matrix—and consolidated into dense, self-supporting forms [9]. This strategy eliminates the need for fiber separation or virgin additives and enables the design of new composite architectures with tailored multifunctional properties.

One critical functional property of interest is electrical conductivity, particularly through the thickness of composite laminates. In aerospace applications, such as lightning strike protection, structural health monitoring, and electromagnetic interference (EMI) shielding, through-thickness electrical conductivity is essential [1]. However, conventional CFRPs are inherently anisotropic and electrically insulating in the through-thickness direction due to the non-conductive polymer matrix and the alignment of carbon fibers primarily in-plane. Bridging this conductivity gap typically requires embedding conductive pathways across the laminate, which presents both technical and economic challenges.

Traditionally, this has been addressed by incorporating conductive nanofillers—such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), or graphene derivatives—into the matrix or interlaminar regions [10,11,12,13]. These fillers can be introduced via solution blending, vacuum-assisted resin transfer molding (VARTM), or electrostatic and magnetic field-assisted alignment [14,15]. While effective, these techniques often rely on costly virgin nanomaterials and complex processing conditions. Furthermore, the sustainability of using nanofillers remains questionable when considering the overall life cycle impact and resource constraints.

Recent studies suggest that recycled CFRP waste itself can serve as a conductive additive. For example, the use of chopped or milled recycled fibers has been shown to significantly enhance electrical conductivity in epoxy-based matrices, sometimes surpassing the performance of virgin fillers [16,17]. These gains are highly dependent on filler morphology, dispersion, alignment, and interfacial bonding quality [18,19]. Yet, much of the current literature focuses on using recycled fibers as additives rather than as bulk structural elements.

In this context, a sustainable and scalable alternative involves repurposing CFRP machining waste directly as a functional core material in sandwich composite structures. This approach not only reduces the need for virgin materials but also provides an opportunity to exploit the inherent conductivity and structural potential of recycled carbon-rich waste. Additionally, it bypasses the complexity of fiber recovery and purification by directly molding the waste material into usable forms.

Building upon this idea, the present study investigates the use of mechanically ground CFRP machining waste—collected from aerospace-grade composite machining operations—as the core layer in multifunctional sandwich panels. The waste, composed of carbon fibers, fragmented thermoset resin, and trace contaminants, was processed through direct compression molding without the use of additional binders, virgin fillers, or curing agents. This process, previously developed by the authors [20,21], leverages incipient thermal degradation, surface bond activation, and residual resin reactivity to achieve particle consolidation. This is primarily due to the mechanical shear and frictional heat generated during grinding, which can locally raise the temperature and mobilize residual unreacted epoxide or hardener groups. These conditions may induce secondary crosslinking reactions or softening of the network, particularly in systems that were initially under-cured or contain excess reactive functionalities [22,23].

The resulting molded plates were integrated with external skins made from recycled CFRP prepreg to create sandwich structures. The primary focus of this study is to evaluate the through-thickness electrical conductivity and mechanical performance of the fabricated panels and to assess the potential of this process as an environmentally responsible and industrially viable recycling pathway. By demonstrating comparable or superior conductivity to systems containing virgin nanofillers, this research contributes to advancing circular economy goals while enabling the development of multifunctional composite structures from waste-derived materials [24].

2. Materials and Methods

2.1. Supplied Materials

A carbon fiber-reinforced (CFR) sandwich panel was fabricated using recycled commercial materials. Ground carbon fiber-reinforced polymer (CFRP) scraps were recovered from an aerospace manufacturing facility, where they were generated during the dismantling and end-of-life processing of CFRP structures. As machining waste from certified aeronautical parts, the material is inherently heterogeneous, comprising carbon fibers and cured epoxy matrix fragments. Due to its complex and variable nature, detailed ultimate and proximate analyses were not conducted; however, the composition aligns with typical aerospace-grade CFRP materials. Specifically, the material was collected immediately after the grinding phase, during which both particle size and external contamination levels were highly variable. The machined CFRP components consisted of a thermosetting epoxy matrix reinforced with carbon fibers. Initially, the recovered scraps were sieved using a Retsch AS 200 (Retsch GmbH, Haan, Germany) basic sieve shaker equipped with two mesh sieves: the first with a 4 mm opening and the second with a 1 mm mesh. This procedure enabled the collection of powder with particle sizes smaller than 1 mm, achieving a yield of approximately 30%. The heterogeneity of the powder is clearly visible in the stereoscopic images acquired using a Leica S9i stereo microscope (Leica Camera AG, Wetzlar, Germany) after sieving. The particle size distribution of the sieved powder was further analyzed using the Leica S9i stereo microscope. Finally, Hexcel’s HexPly M49/42%/CHS-3K CFR prepregs (Hexcel Corporation, Stamford, CT 06901, USA), featuring a plain woven fabric with a nominal epoxy resin content of 42 wt%, an areal density of 200 g/m², and a thickness of approximately 350 μm, were used as the external prepreg skins in the sandwich panel. This high-performance 0°/90° woven cloth prepreg is commonly employed in aeronautical applications.

2.2. CFRP Panel Fabrication Procedure

Recycled CFRP specimens were fabricated via direct compression molding of the industrial CFRP powder, without the addition of virgin polymer or binding agents, following the processing scheme illustrated in Figure 1. Square samples (84 × 84 mm²) were molded and two different panels were produced: one made entirely of recycled CFRP powder (labelled core alone), and the other incorporating a single external CFRP prepreg ply on both the top and bottom surfaces (labelled sandwich). The molding process was carried out in a hot plate press by ATS FAAR (Cassina dè Pecchi, Italy) with a maximum load of 264 kN and the plate size of 300 × 300 mm², where the upper plate was maintained at 250 °C and the lower at 220 °C. This temperature difference between the upper and lower platens was required due to the specific molding setup and the type of mold used, for example, to account for the differing thickness in the aluminum mold’s upper and lower parts, creating a controlled temperature gradient. This gradient promotes resin softening and flow within the recycled CFRP powder, enhancing particle bonding and consolidation during compression molding. This parameter selection is consistent with the authors’ previous work on recycled CFRP processing [21]. The selected scraps from aeronautical CFRP components were placed into an aluminum mold featuring an internal cavity of 84 × 84 mm². A pressure of 1.5 bar was applied for 15 min. To prevent adhesion between the recycled material and the mold surfaces, a fluorinated ethylene propylene (FEP) release film was applied to both the top and bottom interfaces. Upon completion of the cycle, the mold was allowed to cool to ambient temperature inside the press before demolding the samples. In the direct molding of CFRP powder, elevated temperatures are essential to activate polymer segment mobility and facilitate particle bonding, while sufficient pressure improves interparticle contact and densification.

2.3. Thermal Analysis

The sifted powder and the hot-compacted samples were thoroughly characterized using thermal and thermomechanical analyses to determine the key thermal properties of the raw material and to highlight the effects of the manufacturing process.

Thermogravimetric analysis (TGA) was performed on the sifted powder using a Perkin Elmer TGA instrument (PerkinElmer, Shelton, CT, USA) under a nitrogen atmosphere. The sample was heated from 25 °C to 800 °C at a rate of 10 °C/min to quantify the inorganic fiber content present in the mixture. Differential scanning calorimetry (DSC) was conducted using a TA Q2000 system following a standard three-step protocol: an initial heating from 25 °C to 250 °C, a cooling phase back to 25 °C, and a second heating cycle identical to the first, all at a rate of 10 °C/min, with the Tg value determined from the second heating cycle to minimize the influence of the material’s thermal history. This analysis was carried out on the sifted powder.

2.4. Electrical Conductivity Measurement

To assess the improvements in through-thickness electrical conductivity of the fabricated composite structures, measurements were performed using a Valex digital multimeter with standard probes. Prior to each measurement, the probes were cleaned to ensure consistent contact quality. No additional surface treatments, such as oxide layer removal, were applied to the sample surfaces. Electrical resistance was recorded in both the in-plane and through-thickness directions, with the two probes positioned according to the configurations shown in Figure 2. For through-thickness (z-direction) measurements, the probe spacing corresponded to the panel thickness. The probe tips were placed perpendicularly to the surface and aligned along the same vertical axis to minimize misalignment and ensure measurement accuracy. For the in-plane (x–y plane) electrical conductivity measurements, the probes were placed directly on the sample surface, positioned perpendicular to the panel face, and spaced 15 mm apart along the surface. This configuration enabled many measurements—over 30 acquisitions—across the panel surface. Electrical conductivity values were then calculated by normalizing the measured conductance with respect to the probe tip spacing.

2.5. Microstructural Observation

To investigate the quality of internal adhesion, fiber embedding, and the formation of interfacial regions within the composite structure, a comprehensive morphological analysis was carried out using scanning electron microscopy (SEM) (FEI Quanta 200 F, Zurich, Switzerland. Samples were cryo-fractured in liquid nitrogen to preserve the microstructural integrity and reveal the fracture surfaces representative of the bulk material. Prior to imaging, all specimens were sputter-coated with a thin conductive layer of gold–palladium to prevent charging effects during SEM analysis. SEM observations were performed using a high-resolution field-emission scanning electron microscope) operating under high vacuum conditions at an accelerating voltage of 30 kV. The analysis focused on evaluating the dispersion and embedding of carbon fibers within the polymer matrix, the degree of interfacial bonding, and the presence of voids or delamination at the matrix–fiber interfaces.

In addition to cross-sectional imaging of the sandwich composites, SEM was also used to examine the morphology of the sieved powders, as well as powders produced by mechanical grinding of hot-compacted core samples. These comparative observations provided insight into the structural evolution of the recycled material from its raw state through the compaction and consolidation process.

2.6. Thermomechanical and Mechanical Testing

The density of the molded composite samples was determined by precisely measuring their physical dimensions and corresponding masses. These measurements were used to calculate the bulk density using the standard mass-to-volume ratio.

To assess the mechanical performance of the fabricated specimens, three-point bending tests were performed in accordance with the ASTM D7264 standard [25]. Testing was carried out using an MTS Insight 5 electromechanical universal testing machine (MTS Systems S.r.l, Torino, Italy), operated in displacement-control mode with a constant crosshead speed of 1 mm/min. The support span was set to 40 mm and a minimum preload (1 N) was applied. During each test, both the applied load and mid-span deflection were continuously recorded. These data were used to compute key mechanical parameters, including the flexural strength, maximum deformation at failure, and flexural modulus, following the equations provided by ASTM D7264. The results provided critical insights into the stiffness, load-bearing capacity, and overall structural integrity of the recycled and reference composite sandwich panels under flexural loading.

Moreover, dynamic mechanical analysis (DMA) was performed on both the standalone core (core alone) and the complete sandwich composite samples to evaluate their viscoelastic behavior and gain insights into the effect of the sandwich architecture on thermal–mechanical performance. The objective of this investigation was to characterize how the recycled core material and the outer CFRP skins influence the stiffness and damping capacity of the composite system over a broad temperature range, in addition to its glass transition temperature (Tg). The tests were carried out using a Perkin Elmer Pyris Diamond DMA instrument (PerkinElmer, Shelton, CT, USA) in dual cantilever bending mode. Five rectangular specimens (40 × 10 mm²) were cut from each panel type and subjected to a controlled temperature sweep from 25 °C to 250 °C at a constant heating rate of 3 °C/min. The storage modulus (E′) and tan delta (E″/E′, with E″ loss modulus) were continuously recorded as a function of temperature. These parameters provide crucial information on the stiffness, energy dissipation, and thermomechanical transitions of the materials, allowing for direct comparison between the performance of the recycled core and the reinforced sandwich configuration.

3. Results

3.1. Thermal and Thermomechanical Analysis

Figure 3 presents the gravimetric weight-loss curve and its corresponding derivative signal for the sieved powders. The TG curve shows the percentage of the sample’s mass remaining as the temperature increases from room temperature up to 800 °C. This temperature range captures the main decomposition processes of the epoxy matrix, while it does not fully reach a stable plateau, potentially due to slow oxidative degradation of residual char or carbon fibers at high temperatures. However, for this preliminary study, the range is useful for assessing the organic component for the subsequent compression-molding phases. Initial mass loss (~2%) begins around 200–250 °C, which may be attributed to moisture desorption or evaporation of light volatiles, to partial degradation of low-molecular-weight polymeric components. A major mass loss (~33%) occurs between ~250 °C and 450 °C, peaking around ~327 °C (as confirmed by the DTG curve). This is typically associated with thermal degradation of the epoxy matrix, which is the polymeric component in CFRP. After 450 °C, the mass loss rate slows considerably, suggesting that a great part of the organic matrix has decomposed and that the remaining mass is primarily inorganic—carbon fibers and metallic residues. The residual mass at 800 °C is approximately 65%, indicating a high content of thermally stable, inorganic material, such as carbon fibers, which do not decompose under inert conditions, and metallic particles or fillers, possibly from machining processes. The DTG peak at ~327 °C marks the maximum rate of decomposition, characteristic of the thermal breakdown of epoxy resin. Secondary peaks might indicate the presence of different resin formulations or varying degrees of cure, or the oxidation or degradation of additives or surface treatments on the fibers. Consequently, the recycled CFRP powder retains a significant proportion of carbon fibers and inorganic components (~65% residue); a main degradation event centered around 327 °C confirms the presence of an epoxy matrix, typical of aerospace-grade composites, and a relatively small initial mass loss (<2%) which suggests low moisture content and minimal contamination with volatiles. These results validate the thermal stability of the recycled material’s fibrous content and confirm the suitability of the powder for reuse in applications requiring good thermal and structural performance.

The residual reactivity of the recycled CFRP powder was investigated by differential scanning calorimetry (DSC), as shown in Figure 4. During the first heating scan, the material exhibited a softening behavior, indicating that thermal reactivation of the partially cured matrix is possible at elevated temperatures. The degree of residual reactivity is closely linked to the initial cure state of the thermoset resin, which, in turn, depends on the processing conditions of the original composite. However, the identification of a distinct curing exotherm in the DSC thermogram is often difficult, particularly in recycled systems. Despite the absence of a discernible curing peak, the residual reactivity can be inferred from the shift in the glass transition temperature (Tg) between the first and second heating scans. In this study, thermal activity is present in the range 75–130 °C in the first scan, which became a Tg at 130 °C in the second scan, suggesting that some post-curing occurred during the initial heating cycle. While the exact degree of residual reactivity and its effect on re-agglomeration cannot be quantitatively determined, the observed Tg shift confirms that further crosslinking reactions did take place. This shift in Tg is consistent with findings by Hardis et al., who highlighted that temperature-modulated DSC (TM-DSC) analysis allows the detection of the cure state of pre-cured carbon-fiber epoxy prepreg, demonstrating the progression of curing and its impact on Tg [26]. The fact that the first and second scans are superimposed at high temperatures indicates that there is no material loss during the first scan and that the material remains essentially stable throughout the second one.

To exploit this residual reactivity during the direct molding process, a forming temperature of 130 °C could potentially suffice. However, because re-agglomeration also depends on additional thermal mechanisms—such as localized flow, inter-diffusion, and incipient degradation—a higher processing temperature may be more effective. Based on these observations, the temperature of the parallel plates in the molding press was set to 250 °C. This choice aimed to both approach the threshold of incipient degradation—promoting better polymer mobility—and offset the initially cold state of the mold (room temperature at the end of powder filling). This thermal strategy proved effective, as evidenced by the satisfactory degree of particle agglomeration achieved in the final product.

3.2. Microstructural Investigations

The recycled CFRP-core-alone panel exhibited an average thickness of 3.7 ± 0.17 mm and a density of approximately 1.3 g/cm³, whereas the CFRP sandwich had a slightly higher thickness of 4 ± 0.2 mm and lower density 1.2 g/cm³. Following the direct molding process, the core alone displayed a generally smooth and continuous surface, with a uniform appearance and good structural cohesion. Only minor surface cracks, primarily due to demolding, were observed.

The overall quality and integrity of the standalone core, as confirmed by the microstructural observations presented in Figure 5, validate the effectiveness of the selected molding conditions and underscore the potential of the process for recycling and reprocessing CFRP waste into structurally reliable materials. The microscopic images in Figure 5 illustrate the effective particle agglomeration achieved through the direct molding of recycled machining waste. In the magnified views, randomly oriented agglomerates of chopped carbon fibers, resin fragments, and metallic residues from composite part milling are clearly visible. These metallic inclusions, originating from components embedded to enhance various performance attributes, contribute to the composite’s heterogeneity. Notably, the absence of macroscopic voids indicates a high degree of compaction and efficient waste utilization. The outer CFR layers, consisting of two plies, are clearly distinguishable, along with a thin resin-rich interface layer. Additionally, Figure 5 presents a 3D surface reconstruction of the panel’s top surface, demonstrating the effectiveness of the molding process, even when using recycled constituents.

Moreover, SEM micrographs of the sample cross-sections are presented in Figure 6. In particular, these provide a general overview of composite morphology, comparing the core-alone configuration (Figure 6a) with the full sandwich structure (Figure 6b). Both configurations exhibit a good degree of particle compaction; however, the sandwich panel demonstrates greater homogeneity and a more uniform internal structure. This improved distribution is likely due to the additional constraint imposed by the external CFR skins during molding. The skins act as structural face sheets that confine the shredded core, enhance interfacial bonding, and improve surface quality. More importantly, they provide additional mechanical restraint, contributing to improved flexural stiffness, dimensional stability, and stress distribution. Their function is thus not to increase molding pressure, but to enhance overall laminate performance and integrity [27]. At higher magnification (Figure 6c,d), both sample types reveal randomly dispersed fiber bundles embedded within a continuous matrix formed by the compacted thermoset powder. These bundles are typically composed of chopped carbon fibers with varying orientations, indicative of the stochastic nature of the recycled feedstock. Notably, the fiber–matrix adhesion appears strong, as no significant debonding or interfacial gaps are observed.

The interface between the skin and the core is clearly discernible. It is characterized by a transition from the highly aligned fibers of the CFR skin to the more randomly distributed, compacted fiber–resin matrix of the core. Importantly, no delamination or discontinuity is evident at this interface, confirming the mechanical integrity and quality of adhesion achieved during the molding process.

The compaction behavior of the recycled powders is further highlighted in Figure 7a,b. Prior to molding, the sieved powder appears loose and granular, with clearly distinguishable fiber and resin particles (Figure 7c). After compaction, however, the material exhibits tightly bonded agglomerates composed of polymer matrix and embedded fibers (Figure 7d), indicative of effective densification and cohesion achieved under heat and pressure. This trend is corroborated in Figure 7c,d, where the untreated powder shows a high content of free, incoherent fibers (Figure 7c), while the after-molding material demonstrates a matrix with well-embedded and interconnected fibers (Figure 7d). In other words, ii is obvious that the compaction process clearly enhances the structural integrity and continuity of the composite material.

Interestingly, SEM images of the raw, unprocessed sifted powder (Figure 8a) reveal the presence of pre-existing fiber bundles. These larger agglomerates, retained by the 1 mm sieve mesh, serve as effective reinforcement nuclei during molding. Their presence supports the appropriateness of the chosen mesh size, as they contribute significantly to the final mechanical performance by facilitating localized densification and improved load transfer.

Moreover, some foreign inclusions were detected in the sieved powders, as shown in Figure 8b. Energy-dispersive X-ray spectroscopy (EDX) analysis confirmed that these particulates are metallic in nature, with a composition consistent with aluminum alloys (

Table 1. EDX of the extraneous residues in the CFRP recycled sieved powder.

Element	Weight [%]
C	33.96
O	3.81
Mg	0.93
Al	58.44
Cu	2.85
Total	100

). These inclusions likely originate from machining processes involving hybrid composite materials and, while small in volume, could potentially influence local mechanical or electrical behavior.

3.3. Electrical Conductivity

The high-quality consolidation achieved for both composite panels provided a reliable basis for evaluating their electrical properties. Conventional CFRP laminates typically exhibit low through-thickness electrical conductivity due to the insulating nature of the resin matrix, particularly when resin bleeding occurs during processing. The literature also highlights the intrinsic anisotropy of carbon fibers with respect to electrical conductivity [14]: while they exhibit high conductivity (up to 10⁶ S/m) along their longitudinal axis, their performance in the transverse direction is significantly lower. In CFRP materials, the use of woven fiber structures allows a limited degree of through-thickness conductivity. However, in the ideal case of perfectly aligned unidirectional fibers, the material would behave as a near-perfect insulator in the through-thickness direction.

To address this limitation, various strategies have been explored to enhance conductivity, often by incorporating carbon-based additives uniformly throughout the laminate. In this study, a preliminary assessment of the electrical performance of the manufactured panels was carried out by measuring both in-plane and through-thickness electrical resistance. The results, presented in Figure 9, include surface maps that illustrate the spatial distribution of conductivity across the samples. In the core-alone panel, the in-plane electrical conductivity is relatively homogeneous and exhibits consistently low average values (Figure 9a). In contrast, the through-thickness conductivity is higher, with greater variability, primarily attributed to small leverage variations arising from the powder-based nature of the molded material (Figure 9b).

For the sandwich panel, small, localized reductions in in-plane conductivity (Figure 9c) are observed and can be attributed to resin face bleeding, which increases the insulating content at the surface. Conversely, the through-thickness conductivity (Figure 9d) is, on average, higher, with distinct peaks likely corresponding to areas where chopped carbon fibers formed conductive bridges between the two external CFRP skins.

In terms of electrical resistance, the sandwich composite panel exhibited average in-plane and through-thickness resistance values of 39 ± 6.4 Ω and 48 ± 15.2 Ω, respectively. In contrast, the core-alone panel displayed higher resistance values, with an average in-plane resistance of 66 ± 9.5 Ω and a through-thickness resistance of 72 ± 13.8 Ω. The higher in-plane resistance observed in the core-alone panel highlights the role of the surface prepreg plies in enhancing electrical conductivity; their absence significantly increases the overall resistance. As reported in the previous paragraph, EDX analysis indicated that the metal residues in the recycled CFRP material are primarily aluminum with minor traces of copper. The proportion of these metal elements is low compared to the carbon fiber content. Consequently, while metal particles possess higher intrinsic conductivity, their limited presence suggests that carbon fibers predominantly govern the electrical conductivity in the samples. The contribution of metal residues to the conductive pathways is therefore considered minimal.

To provide a more precise evaluation of the electrical performance, resistance values were converted into conductivity. The sandwich panel showed average electrical conductivity values of 0.045 ± 0.013 S/cm through the thickness and 0.02 ± 0.003 S/cm in-plane. Meanwhile, the core-alone panel exhibited a slightly lower in-plane conductivity of 0.01 ± 0.002 S/cm and a comparable through-thickness conductivity of 0.04 ± 0.011 S/cm (

Table 2. Average resistance and electrical conductivity of both composite panels.

	Resistance (ohm)		Conductivity (S/cm)
	in-plane	through-thickness	in-plane	through-thickness
Core alone	66.5 ± 9.48	73.2 ± 13.84	0.010 ± 0.0016	0.039 ± 0.0095
Sandwich	38.7 ± 6.36	47.9 ± 15.17	0.018 ± 0.0031	0.045 ± 0.0133

).

3.4. Thermomechanical and Mechanical Testing Results

An ideal composite architecture should combine functional properties—such as enhanced electrical conductivity—with robust thermomechanical and mechanical performance. To this end, additional investigations were conducted to evaluate the thermomechanical and mechanical behavior of the developed composite panels. Figure 10 shows the results of the DMA analysis for both composite panels. Specifically, Figure 10a presents the temperature dependence of the storage modulus (E’) for the two composite configurations: the core-alone panel (black curves) and the sandwich panel with external CFRP prepreg skins (red curves). The storage modulus was measured during heating from room temperature to 250 °C in dual cantilever mode. At room temperature (25 °C), the sandwich panel exhibits significantly higher E′ values, ranging from approximately 4500 to 5500 MPa, while the core-alone panel ranges between ~1600 and 1900 MPa. For both configurations, a progressive decline in E′ is observed as the temperature increases, with a sharp drop occurring between 100 and 160 °C, indicating the glass transition region of the matrix. Beyond 160 °C, both materials reach a rubbery plateau with E′ values stabilizing at lower levels: approximately 300–500 MPa for the sandwich panel and below 200 MPa for the core-alone panel. Figure 10b presents the temperature-dependent tan δ curves for both composite panels. The recycled material (core alone) exhibits a double thermal transition, which, however, disappears in the sandwich structure because it is overshadowed by that of the CFRP skin, which only shows the second, higher-temperature transition. For all samples, a main peak is observed in the range of 140–150 °C, corresponding to the glass transition temperature (Tg) of the epoxy-based matrix.

In general, it is also noteworthy that the variability between samples is more significant in the core-alone structure. This may be inferred from the heterogeneous nature of the re-processed powder, probable voids, and irregularities in fiber distribution and compaction during molding. Conversely, the sandwich structure benefits from a major structural uniformity and fiber alignment inherent in the prepreg layers, ensuring better repeatability of thermomechanical properties.

The results of the three-point bending tests are presented in Figure 11a as stress–strain curves for two representative samples for each type of composite panel. The sandwich samples exhibited a maximum stress of 31.6 ± 7.88 MPa, an average flexural modulus of 9.2 ± 1.78 GPa, significantly higher than the maximum stress 7.6 ± 1.79 MPa and the flexural modulus 1.7 ± 1.56 GPa measured for the core-alone samples, corresponding to an increase of over 81%. The contribution of the external CFR skins was also evaluated independently, showing a flexural modulus of 36 GPa. When embedded in the composite structure, these skins contributed to the major stiffness of the panel configuration. The stress–strain curves of the sandwich samples revealed multiple peaks, indicative of a failure mechanism. Specifically, the failure process involved both delamination of the CFR skins from the core—particularly at the intrados—and brittle cracking within the core. The core exhibited fragile behavior, with no significant plastic deformation or damage precursors observed prior to fracture. The progression of cracks along the core at maximum deflection is clearly visible in Figure 11b. These observations were confirmed by analyzing the curves of the core-alone samples.

4. Discussion

CFRP particles with a thermoset matrix can be effectively consolidated using the direct molding technique, and this study provides further confirmation of this technological principle. However, contributions in the scientific literature on this topic remain limited, particularly when it comes to rigid thermoset particles [26]. While the direct molding of elastomeric materials is facilitated by the deformability of the particles under pressure—leading to reduced final porosity—the scenario is different for rigid particles. Their limited deformability results in poor inter-particle contact during molding, often causing higher porosity and lower mechanical performance compared to elastomer-based systems (e.g., rubber from tires) [28]. Although rigid particles soften at elevated molding temperatures, aiding in agglomeration, this mechanism alone is insufficient for optimal compaction. Therefore, ensuring good contact between the poured powder and the mold punch surface at the end of filling is critical. If this contact is not uniform, some regions of the recycled part may become over-compressed, while others remain under-compacted and mechanically weak.

Specifically, the TGA and DTG analyses reveal that the recycled CFRP powder retains a high proportion of inorganic material (~65% at 800 °C), mainly carbon fibers and metallic residues. A major thermal degradation peak around 327 °C confirms the presence of epoxy resin, typical of aerospace-grade CFRPs. A minimal initial mass loss (~2%) suggests low moisture and minimal volatile contamination. DSC analysis indicates residual reactivity of the matrix. The increase in glass transition temperature (Tg) from ~75 °C to 130 °C between the first and second scans points to post-curing behavior, confirming that some reactive groups remain in the material. This justifies the selection of a 250 °C molding temperature to encourage polymer mobility and effective particle agglomeration.

At a first visual inspection of the molded panels, the surfaces were compacted and clean, with minimal or no particle shedding, indicating actual agglomeration of the direct molding process. These outcomes highlight the influence of the processing parameters—specifically molding pressure, temperature, and holding time—on the consolidation and quality of the recycled core material, even when starting from the same type and amount of composite waste with similar residual reactivity [21]. A higher molding pressure improves interparticle bonding and overall mechanical performance. However, the intrinsic compressibility of the material sets a limit, as excessive pressure may yield diminishing returns. Similarly, elevated molding temperatures enhance polymer flow and compaction due to reduced viscosity, yet excessively high temperatures can induce thermal degradation of the matrix. A controlled level of degradation during processing facilitates the formation of new bonds between particles. However, excessive degradation can lead to microcracking within the polymer matrix. Furthermore, elevated temperatures may result in the release of volatile organic compounds (VOCs) or even initiate combustion. For these reasons, an optimal combination of molding parameters—temperature, pressure, and dwell time—was carefully established. Microscopic analysis also confirmed the successful consolidation of the laminate at the end of the autoclave molding process. Strong adhesion between the carbon fiber-reinforced (CFR) outer skins and the recycled thermoset core is facilitated by resin bleeding, which fills surface cavities and micro-voids, enhancing interfacial bonding.

Microscopic and SEM analyses confirm successful compaction of the recycled powder under the selected processing conditions. The core-alone and sandwich structures both exhibit dense, well-bonded matrices with uniform fiber distribution and strong fiber–matrix adhesion, indicating effective consolidation. The sandwich panel shows greater microstructural homogeneity, likely due to the compressive effect of the outer CFR skins. No delamination is observed at the skin–core interface, indicating excellent mechanical integrity. Pre-existing fiber bundles and metallic inclusions (mainly aluminum) in the powder contribute to mechanical reinforcement and heterogeneity but may locally influence performance. In recycled CFRP composites with shredded or ground waste cores, interfacial adhesion is crucial for mechanical integrity. Recycling can cause surface contamination, fiber damage, and poor wetting, reducing load transfer efficiency. Although this study did not apply surface treatments, the literature shows that methods like plasma treatment [29] and chemical grafting with silane agents or oxidative treatments [30] can enhance fiber–matrix bonding. These approaches are especially relevant for compression-molded structures, where heterogeneous interfaces from thermoset residues and fiber fragmentation are common. Future work will systematically assess these surface treatments’ effects on interfacial strength using micromechanical tests such as interlaminar shear and fiber pull-out, aiming to optimize the recycled CFRP sandwich-structure performance.

These first results demonstrate that the recycled CFRP powder maintains high thermal stability and structural integrity and exhibits residual chemical reactivity useful for thermomechanical reprocessing. Consequently, this could be an effective way to reuse and to produce dense, cohesive composite panels with promising mechanical properties.

In terms of electrical conductivity, while good electrical conductors typically exhibit conductivities above 10² S/cm, the composite panel falls within the semiconductive range (10⁻⁸–10² S/cm), qualifying it as a functional material with potential for electromagnetic or antistatic applications. In the core-only structure, these networks are formed by randomly oriented, fragmented carbon fibers in direct contact with each other and with partially conductive thermoset resin remnants. However, the lack of long-range fiber alignment and limited compaction during molding restricts the efficiency of the conductive pathways, particularly in the through-thickness direction. In contrast, the sandwich structure, which incorporates external CFRP prepreg skins, exhibits improved conductivity due to enhanced fiber continuity at the interfaces and better compaction. The prepreg skins facilitate denser packing and improved interfacial bonding during compression molding, effectively reducing inter-particle gaps and increasing the number of conductive contact points across the thickness. Quantitatively, the core-only panel demonstrated an average through-thickness conductivity of 0.04 S/cm, while the sandwich structure achieved 0.045 S/cm. This represents a 12.5% increase in conductivity, highlighting the role of the external skins in facilitating more efficient electrical percolation through the laminate. This level of conductivity demonstrates that the recycled machining waste can impart functional electrical properties comparable to those achieved with more conventional conductive fillers, thereby supporting the use of recycled materials in applications such as EMI shielding and lightning strike protection.

These findings suggest that, beyond mechanical reinforcement, the addition of CFRP prepreg skins contributes to the development of more continuous and robust conductive networks, enhancing the multifunctionality of recycled CFRP sandwich panels. The enhancement in through-thickness conductivity is primarily attributed to the presence of short, randomly oriented recycled carbon fibers—originating from machining waste—which form conductive pathways during the hot press molding process. Further improvements could be achieved by aligning fibers preferentially in the through-thickness direction, although such an approach would introduce additional complexity to the manufacturing process.

Additionally, the thermomechanical and mechanical results underline the higher storage modulus and flexural modulus of the sandwich panel, showing the reinforcing effect of the external CFRP prepreg skins. The CFRP skins contribute to the overall stiffness and mechanical integrity of the structure. In particular, the enlarged modulus values in the glassy region settle the proper interaction between the co-molded core and prepreg skins. So, the core-alone panel displays a lower E′, more associated with the discontinuous structure of the initial material (CFRP powder from scraps) which contributes less resistance to deformation due to the absence of continuous fiber reinforcement. The Tg range is similar in both panels, suggesting that the matrix polymer is chemically comparable. However, the difference above Tg emphasizes the importance of configuration design. In fact, the core alone provides limited structural stiffness, but its integration into a sandwich structure enhances load-bearing capabilities due to the stiff outer layers and sandwich synergy. These findings emphasize the potential of co-molding recycled CFRP powder within a sandwich configuration to recover mechanical performance suitable for semi-structural applications. The higher tan δ peak in the core-alone panel suggests increased molecular mobility and energy dissipation capacity, likely resulting from a lower crosslink density or the more heterogeneous microstructure of the recycled CFRP powder. These features can arise due to incomplete polymerization or local variations in the recycled matrix. In general, the sandwich structure offers superior stiffness and thermal stability across the tested temperature range. The enhanced modulus and delayed transition behavior make it a more suitable candidate for structural applications where mechanical performance at elevated temperatures is critical. Meanwhile, the core-alone panel, although less stiff, still presents moderate viscoelastic performance, highlighting the potential of recycled CFRP powders for cost-effective and sustainable core materials. Thus, the material is well-suited for secondary structural applications, especially where good thermal resistance, semiconductor properties and mechanical strength are required.

In the end, to further validate the proposed process, it was applied on non-planar geometries. For the first time, this study presents the successful application of direct molding to particulate material shaped into a complex tubular form. As illustrated in Figure 12, a recycled CFRP tubular ring was fabricated, using an aluminum mold with an internal diameter of 20 mm and an external diameter of 24 mm. The mold was engineered to include a precise clearance between the inner and outer diameters, specifically intended to accommodate a hollow piston. This piston was designed to fit closely within the annular gap and apply uniform pressure to the recycled particulate CFRP, which was strategically distributed in a ring-shaped configuration inside the mold cavity The molding parameters used are the same as the previous molding phase (1.5 bar, 250 °C of the upper hot plate, 220 °C of the lower hot plate for 15 min).

Preliminary visual inspection, combined with stereomicroscopic analysis, highlights a uniform and good degree of compaction across the entire tubular section. As shown in Figure 13a,b, the cross-section of the recycled CFRP tubular sample exhibits a dense and consistent microstructure, with minimal visible porosity and strong cohesion between the particles. The absence of significant voids or interfacial gaps, particularly at the inner and outer surfaces of the tubular sample, suggests that the applied processing pressure and temperature were sufficient to promote particle rearrangement and thermomechanical reactivation. This level of compaction not only confirms the effectiveness of the selected reprocessing parameters, but also demonstrates that even complex geometries, such as annular sections, can be successfully fabricated via direct molding without the need for extensive preprocessing or additives.

These results validate the potential of this technique for broader application in the recycling of thermoset-based composite powders, also for non-planar components where uniform pressure distribution is more challenging to achieve.

5. Conclusions

Recent advancements in composite manufacturing have shifted toward the development of multifunctional structures that not only maintain excellent mechanical performance but also offer additional properties such as electrical and thermal conductivity, improved damping, reparability, and thermoformability. These multifunctional features are increasingly sought after in aerospace, automotive, and electronic applications where weight, durability, and functionality must be optimized simultaneously. Simultaneously, rising concerns around the environmental impact of composite waste and the global push toward sustainable manufacturing have placed greater emphasis on recycling, reuse, and remanufacturing—key pillars of the circular economy. Among various recycling methods for CFRPs, mechanical recycling and pyrolysis are the only techniques that have progressed to industrial-scale deployment. Mechanical recycling generates large quantities of carbon-rich powders from machining and trimming operations, which are often undervalued or landfilled despite their latent functionality. Recognizing this, the present study proposes a novel and scalable approach to utilize such CFRP machining waste to fabricate composite structures that are not only sustainable but also functionally enhanced. Specifically, we demonstrate the direct compression molding of ground CFRP waste into a rigid core material, which was subsequently integrated with CFRP prepreg skins to form a multifunctional sandwich structure.

For the first time, this recycled material was used to enhance through-thickness electrical conductivity in laminated composites. The compression-molded core—comprising carbon fibers and partially cured thermoset resin fragments—served as both a structural and conductive element. The resulting sandwich panel exhibited improved dimensional stability and structural cohesion, and is, to date, the largest sample successfully manufactured using this method. Electrical characterization confirmed measurable conductivity in both the core-only and sandwich panels, with the latter achieving an average through-thickness conductivity of approximately 0.045 S/cm—more than twice the in-plane conductivity (0.018 S/cm). This enhancement is attributed to the formation of conductive fiber networks across the thickness, promoted by fiber bridging between the skins and the compacted core. While the sandwich panels fabricated in this study demonstrated promising electrical conductivity, the mechanical properties of the sandwich panels, while improved over core-only samples, are currently below typical requirements for structural applications. For semi-structural composites, flexural strengths above 50 MPa and sufficient stiffness are generally needed. Further improvements in interfacial bonding and matrix reactivity are required to meet these mechanical standards for practical use. The lower ultimate strength and brittle failure observed in the current recycled CFRP panels indicate that further optimization—such as improving interfacial bonding and matrix reactivity—is necessary to meet these mechanical thresholds and enable broader practical use. However, despite being performed at laboratory scale, these results demonstrate the effectiveness of using recycled CFRP particles to achieve functional electrical performance. Additionally, dynamic mechanical analysis (DMA) revealed superior viscoelastic performance for the sandwich structure, including a higher storage modulus, elevated glass transition temperature, and enhanced damping behavior compared to the core-alone sample. Mechanical testing further confirmed the benefits of the sandwich design, with failure stresses of 31.6 MPa in the sandwich panel versus 7.6 MPa in the core-only configuration. Although delamination and brittle failure were observed, likely due to weak interfacial bonding and heterogeneous agglomeration, these issues can be mitigated by optimizing the molding parameters or improving residual matrix reactivity. Overall, this work demonstrates the feasibility of repurposing CFRP machining waste into functional, semi-structural composites using a process compatible with existing manufacturing technologies. The approach offers a promising pathway toward high-value applications requiring electrical conductivity, EMI shielding, or lightning strike protection, and establishes a foundation for sustainable, circular material design in advanced composite engineering.