Recycling of Polylactic Acid 3D Printing Residues: Potential Use as a Matrix in Nanocomposites

Abstract

With the growth of the 3D printing industry, there are concerns regarding the PLA residues from the manufacturer and the temporary use of the material. To address these concerns, this work presented a sustainable approach to recycling polylactic acid (PLA) residues from 3D printing by incorporating graphene as a reinforcement material. Aiming to simulate a mixed material scenario, the residue is generated by processing on a Creality Ender 3 printer, heated to 200 °C at a speed of 25 mm/s. Then, PLA residues were ground and mixed using a co-rotating twin-screw extruder at 190 ºC with 0.18% by weight of graphene. In general, graphene was incorporated, resulting in a homogeneous distribution that does not contribute to increasing the tensile strength compared to recycled PLA; however, it is higher than that of the original PLA filament. Regarding the thermal properties, the reprocessed material retains its original thermal stability, with a slight decrease in the degradation temperature. The recycled PLA exhibited a crystallinity of 20.3%, which was higher than that of the original PLA filaments and recycled PLA with graphene (3.4%). In conclusion, PLA residues from 3D printing can be recycled to obtain PLA nanocomposites without compromising the mechanical and thermal properties.

Keywords:
Recycling; PLA; 3D printing; graphene; nanocomposites

1. Introduction

The advancement of technology has transformed manufacturing, shortened development, and improved industry connectivity. The growth of additive manufacturing is a key enabler of the industry 4.0 revolution. The concept is based on overlapping layers of one or more materials, varying their position in three dimensions, to form the desired geometry¹,². 3D printing is based on melting and material deposition (FDM), the most accessible technology for the public. In this process, a thermoplastic, in the form of a thread (filament), is heated to its melting temperature. In sequence, the material is extruded through the nozzle, which guides the material along the X, Y, and Z axes to form layers in the required position. 3D printing is easy to use, presents low cost, and has a wide variety of applications, making this production technique widespread in different areas of society. Additionally, with the increasing adherence to the FDM technique, the consumption of inputs for this technology also follows its growth².

Despite 3D printing being an additive manufacturing process often praised for reduced material waste compared to traditional methods, it still generates significant waste, including failed parts, print media, and unused materials³. An example is the support structures required for complex geometry parts. These supports are detached and discarded, representing a large percentage of the material used. The UNEP (United Nations Environment Program) report states that 3D printing waste can pollute the direct ocean with microplastics. The thermoplastic used and the method of disposal can influence the degree of impact caused by 3D printing in the oceans⁴.

Additionally, modern 3D printers with automatic filament changing systems generate waste during the "purging" process to clean the printer and prevent color mixing. The amount of purge material can significantly exceed the material used for the printed part, especially with numerous color changes, as demonstrated by a simulation where purge material was 112 grams compared to 99.8 grams for the model itself. This highlights a substantial and specific waste stream that requires effective management⁵.

The materials used in a 3D FDM printer are thermoplastic polymers, such as Acrylonitrile-butadiene-styrene (ABS), polylactic acid (PLA), Nylon, Polycarbonate (PC), and Polystyrene (PS), among which PLA is the most popular⁶. PLA is a semi-crystalline polymer produced with monomers from fermentation sugar or starch derived from renewable organic resources, such as corn, sugarcane, wheat, and potatoes⁷. In its polymeric chain are branches of oxygen double bonds and the carboxylic acid functional group. This polymer has good mechanical and thermal characteristics, and its applications range from plastic supermarket bags to medical prostheses⁸. PLA is a compostable polymer, which means that under favorable conditions, it undergoes a natural degradation process, thereby reducing the environmental damage caused by plastic accumulation in ecosystems⁷,⁹.

Although PLA is a biomaterial with a reduced environmental impact compared to polymers from fossil sources, reprocessing and recycling the material are essential to extend its life cycle. Reducing the consumption of renewable resources and energy makes reprocessing an attractive option for both industry and the environment¹⁰,¹¹. The reprocessing of thermoplastics can expose the material to high temperatures, degrading it and modifying its mechanical and thermal properties¹⁰. To mitigate the problem of mechanical property loss, one technique used in producing nanocomposites with PLA as a matrix and graphene as reinforcement¹²-¹⁴. Graphene in the nanocomposite can increase tensile strength by up to 25%¹⁵.

Nanomaterials are structures with at least one dimension on the nanometer scale. They have several applications, including textiles, cosmetics, toys, the medical field, advanced materials, and environmental uses¹⁶. Graphene is a carbon-based nanomaterial, consisting of a single layer of graphite. Graphene exhibits several interesting properties, including high electronic mobility, low resistivity, and high mechanical strength¹⁷.

Although PLA recycling is relevant, little data is available on the impact of the recycling process on PLA's overall properties. The improper disposal of PLA residues represents a significant environmental challenge. This study addresses this problem by investigating the viability of reusing specific PLA residues generated by 3D printing, transforming them into new materials such as nanocomposites, to contribute to more sustainable waste management in the 3D printing industry¹⁸. A recent literature¹⁹ concluded that a single reprocessing step does not significantly affect PLA performance. Meanwhile, another study concluded that recycled PLA samples exhibited over 25% higher mechanical strength than neat PLA samples²⁰.

This study proposes a sustainable approach for recycling polylactic acid (PLA) waste from 3D printing by reinforcing it with graphene. Unlike previous works that rely on polymer feedstocks, this research employs a post-processing of residues collected from a Creality Ender 3 printer, thereby simulating an actual mixed-material waste scenario. The residues were mechanically milled and reprocessed in a co-rotating twin-screw extruder with the controlled addition of 0.18 wt% graphene. Thus, the primary objective of this work is to assess the behavior and properties of PLA residues from 3D printing parts when used as a matrix with graphene, thereby contributing to the understanding of thermal and mechanical properties after reprocessing.

2. Materials and Methods

Figure 1 illustrates the steps for reprocessing 3D printing waste and adding graphene as reinforcement. Briefly, the PLA residues with different additives and types of pigments were aggregated, ground, extruded, granulated, and injected into the form of tensile specimens. The addition of graphene occurred in the extrusion stage. This work produced two types of materials: recycled PLA and Recycled PLA with 0.18 wt.% graphene.

Figure 1
The flowchart illustrates the stages for reusing 3D printing PLA.

2.1. Waste description and preparation

The PLA waste used in this work was from filaments produced by 3D LAB, supplied by NatureWorks (Ingeo™ 3D850 grade). The residue comes from processing on Creality Ender 3 printers. Most of the material was printed at 200°C in the printer at an average speed of 25 mm/s. No other polymer was added. The residues underwent a batch milling process using the analytical mill (Quimis Equipamentos Científicos LTDA). After milling, the material was sieved through a 5 mm sieve to remove fine particles and dried in an oven at 60 ^°C to remove moisture absorbed during storage. The mixture obtained and the milling steps are shown in Figure 2. An original filament provided from Ingeo™ 3D850 grade was used for comparison with the recycled materials. The technical data sheet²¹is as follows: T_m is 165 to 180 °C; tensile yield strength is 51 MPa; and tensile strain is 3.31%. The graphene used in this work was chemically synthesized from commercial graphite (Micrograf 99503UJ, 99% carbon) via an oxidation-reduction route²². The synthesis followed the Hummers method, which employed reagents such as sulfuric acid, potassium permanganate, and hydrogen peroxide. This graphene oxide was then subjected to a reduction process using ultraviolet radiation from a 300 W OSRAM Ultravitalux lamp for 5 hours, yielding the reduced graphene oxide²².

Figure 2
Milling process.

2.2. Graphene addition and extrusion

The extrusion was carried out using a Thermo Scientific Model Process 11 twin-screw extruder, rotating at 70 RPM. The temperature of the last stage of the extruder heating ramp was 190 °C. The equipment features three mixing zones, and graphene was manually added through an opening located before the previous mixing zone to ensure greater material homogeneity. Graphene was added at a concentration of 0.18 wt%. The concentration of 0.18 wt% graphene in composites was used because it represents an optimal balance between dispersion, mechanical reinforcement, and processability²³,²⁴. The heating chamber was separated into eight temperature zones: 50 ºC, 120 ºC, 130 ºC, 150 ºC, 160 ºC, 170 ºC, 180 ºC, and 190 ºC.

The milled 3D printing waste was constantly fed into the feeder. After the material left the heated zone, it passed through a tank containing water for cooling, and it was subsequently granulated and separated. In this work, we obtained two materials: Recycled PLA, which refers to 3D-printed PLA residues extruded without the addition of graphene, and Recycled PLA with graphene. Additionally, we worked with an original filament provided by NatureWorks (Ingeo 3D850) – the original PLA filament and 3D printed parts waste, specifically PLA residue, for comparison purposes with the samples obtained.

2.3. Injection of test specimens

The machine used was the Thermo Scientific MiniJet Pro, and the process was carried out at a pressure of 500 bar for 5 seconds, with the screw rotating at 70 RPM and a temperature of 175 ºC in a specimen mold (number V) according to ASTM D638 (Figure 3A). The holding time was 30 s at a pressure of 250 bar in the mold at room temperature.

Figure 3
The mold used for injection molding (A) and the injected specimens of recycled PLA (A) and recycled PLA with graphene (C).

In this work, we obtained two materials for tensile tests: Recycled PLA (Figure 3B), which refers to 3-D printed PLA residues extruded from filaments produced by 3D LAB, and Recycled PLA with graphene (Figure 3C).

2.4. Characterization of filaments by Fourier Transform Infrared Spectroscopy (FTIR)

The PLA samples were dissolved in chloroform, and a film was formed after solvent evaporation in an oven at 60 ^°C for 3 hours. FTIR spectra were collected using a Thermo Scientific Nicolet IS10 spectrophotometer with a diffuse reflectance accessory (DRIFT) configured to operate between 4000 and 400 cm^-1 at a 4 cm^-1 interval and 64 scans.

2.5. Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC)

The samples were analyzed from room temperature (25°C) to 600 °C at a heating rate of 10 °C/ min in a simultaneous (TGA-DSC) thermal analyzer (Q600 SDT, TA Instruments, USA) under a nitrogen atmosphere at a flow rate of 100 mL/min. Samples weighing 10 ± 0.5 mg were previously cut into suitable parts for the test and were deposited on an aluminum pan.

The degree of crystallinity of the polymers was determined using Equation 1. The calculation compares the melting enthalpy of 100% crystalline PLA, 93.1 J/g, with that obtained in the samples according to Equation 1, adapted from²⁵.

$\%X_c=\frac{\Delta H_m-\Delta H_{cc}}{w_{PLA}\times \Delta H_m^0}\times 100$ (1)

Where: % X _c = percentage of polymer crystallinity; ΔH _m = melting enthalpy of the sample; ΔH^o_fm= theoretical melting enthalpy of the polymer with 100% crystallinity; w_PLA = weight fraction of PLA in the material, and ΔH_cc = cold crystallinity enthalpy.

2.6. Mechanical characterization by tensile test

The tensile test was performed on a universal testing machine (Instron 8801, Instron, USA) equipped with a 100 kN load cell, a pressure of 20 Psi in the gripper, and a speed of 1 mm/min. Additionally, an extensometer Intron model 2026 was employed to measure specimen deformation. Fourteen (14) samples were analyzed, seven (7) from recycled PLA and seven (7) from recycled PLA with graphene. Tukey’s test was applied to identify significantly different groups at a 5% significance level for the mechanical results. The statistical analysis was performed using SPSS 20.0.

2.7. Characterization of electrical properties

The electrical properties test assesses whether the addition of graphene enhances the material's conductive properties. For this purpose, two HP-brand multimeters—Hewlett-Packard model 34401A and a direct current source from Agilent Technologies, model 6015A—were used, with a maximum voltage capacity of 500 volts (V) and a current of up to 5 amps.

2.8. Scanning Electron Microscopy (SEM)

SEM was used to investigate the cross-sectional surface obtained during the extrusion process. Characterization by microscopy was performed using a JEOL model JSM-7001F microscope equipped with a secondary electron detector, operating at 15 kV. The specimens were metalized using the EM SCD 500 equipment, which features a Sponge Coater and a gold plate.

3. Results and Discussion

3.1. Scanning electron microscopy for the extruded materials

In this work, a mixed PLA residue from 3D printers was processed by extrusion, as described in the Materials and Methods section. Surface images of the cross-sectional surface of the recycled PLA and recycled PLA with graphene are shown in Figure 4. It is possible to verify the presence of material agglomerate points in the recycled PLA with graphene sample, as shown in the red circle in Figure 4A. These intrusions are formed by the agglomeration of graphene embedded in the material¹¹. It is also possible to see gaps formed around the graphene clusters, indicating a lack of matrix adhesion to the reinforcement. Comparing the recycled PLA and the recycled PLA with graphene, Figures 4A and 4C (both at the same magnification), we can see the differences between the two materials. Still, in the 5000x zoom, it’s clear that the recycled PLA contains some different materials, possibly due to the addition of additives, dyes, and plasticizers, which are typically used in commercial filaments by the manufacturer. As Fico et al.²⁶ noted, manufacturers do not always report additives in their data sheets, which can pose a challenge in evaluating recycled materials.

Figure 4
SEM images: recycled PLA with graphene, 1000x zoom (A); recycled PLA at 1000x zoom (B); recycled PLA with graphene zoom 5000x (C) and recycled PLA zoom 5000x (D).

3.2. Characterization by FTIR and thermal properties

The results of the FTIR characterization test are shown in Figure 5. The x-axis values were set to start at 800 cm^-1 for better visualization of the results and to remove noise present in the graphene sample. In the graphene sample, it is possible to visualize the characteristic band of the C=C bond, which occurs at 1648 cm^-1. Other bands that appear correspond to the interaction of graphene with the ambient humidity, which is at 3200-3500 cm^-1 and 2900-3200 cm^-1, respectively, for OH and CH bonds²⁷.

Figure 5
FTIR spectra of samples: graphene sample (A); original PLA filament (B); recycled PLA (C), and recycled PLA with graphene.

The FTIR spectrum of PLA shows a band at approximately 862 cm⁻¹, which is associated with the stretching of C-C vibrations. Another band, around 900 cm⁻¹, is considered one of the most important and is linked to angular C-H vibrations in aromatic rings²⁸. These aromatic structures are also evident in the bands between 1600 and 1500 cm⁻¹ related to C=C vibrations. The band at 1020 cm⁻¹ is observed, which is related to the O-C stretching of PLA²⁹. Additionally, the bands at 1103 cm⁻¹ and around 1139-1140 cm⁻¹ are associated with stretching C-O bonds in the CHO group of PLA³⁰.

Additionally, as reported by these authors³¹, the band around 1749 cm⁻¹ is attributed to the amorphous phase of PLA, while the band at 1707 cm⁻¹ is associated with the crystalline phase. In the PLA spectrum, a band at 3493 cm⁻¹ is also observed, corresponding to the overtone of the hydroxyl group vibrations and the ester group, respectively³². The bands located at 2974, 2929, and 2850 cm⁻¹ are related to the asymmetric and symmetric vibrations of CH₃ in saturated hydrocarbons³³.

In the spectrum referring to the recycled PLA and the nanocomposite with graphene, in Figure 5C, it is possible to highlight the characteristic bands present in PLA. The highest intensities were found at 1780, 1465, 1380, 1280, and 1143 cm ^-1. The 1780 cm^-1 band corresponds to the -C=O stretch. This group is a striking feature of the PLA polymer, representing the main path for PLA degradation²⁷,³⁴. The 1380 cm ^-1 band is associated with symmetric CH₃ stretching. The band at 1465 cm ^-1 is associated with the asymmetric stretching of this same bond. Bands at 1284 and 1143 cm ^-1 are related to COC bonds²⁷,³⁵. These bands are relative to the polymeric matrix of the nanocomposite. The presence of graphene, although discreet, is perceptible. The band referring to the C=C bond is found at 1648 cm^-1. In the second spectrum of Figure 55D, it is possible to compare the samples with and without graphene, and there is an increase in the intensity of the C=C bond band in the sample with the nanomaterial.

TG/DTG curves of original PLA filament, PLA residue, recycled PLA, and recycled PLA with graphene are given in Figure 6. According to Figure 6 (TG curves), it is observed that the original PLA (230 °C) exhibits almost the same thermal stability (T_i) as the samples of recycled PLA and PLA residue (227 °C) and recycled PLA with graphene (224 °C). The recycling process was observed, and the addition of graphene did not result in a loss of thermal stability compared to the original PLA filament. Additionally, the TG/DTG curves reveal a minimal thermal difference between the recycled PLA and the recycled PLA with graphene nanocomposite samples. This is due to the low amount of graphene added, which, although it intensified the material's color, did not affect the thermal properties, a behavior also found in the literature¹¹.

Figure 6
TG/DTG curves for original PLA filament, PLA residue, recycled PLA, and recycled PLA with graphene.

Furthermore, according to the DTG curves (see Figure 6), the PLA filament sample presented a single degradation stage. Thus, the main degradation of PLA filament occurred in only a single step³⁶. However, the PLA residue, recycled PLA and recycled PLA with graphene samples showed very similar curves, with two degradation stages, the first one around 350 °C which corresponds to the degradation of the polymer itself, and the other stage that occurs at approximately 410 °C can be attributed to the degradation of the residues and/or addition of graphene in the structure of the samples³⁶,³⁷.

Figure 7 shows the first heating DSC curves of original PLA filament, PLA residue, recycled PLA, and recycled PLA with graphene. The DSC curves show similar overall thermal profiles but differ in the magnitude of the thermal events observed, such as glass transition, cold crystallization, and melting enthalpies.

Figure 7
The first heating DSC curves for original PLA filament, PLA residue, recycled PLA, and recycled PLA with graphene.

The first event observed in the range (50-60 °C) refers to the material's glass transition process, where it leaves the solid state for a malleable and moldable state. Similar behavior was observed in this study³⁸. The second exothermic event at 100 °C is associated with the cold crystallization of PLA. The third event that occurs between 157 and 169 °C corresponds to the typical region of the melting process, where the intermolecular bonds are broken, and the material has the characteristics of a viscous fluid. The fourth event (350-370 °C) is attributed to the characteristic region of the material degradation³⁹,⁴⁰. Table 1 shows the data obtained from the DSC analysis.

Table 1
Thermal properties from TG, DTG, and DSC curves.

Sample	Tg (ºC)	Tm (ºC)	Tdegradation (ºC)	$\Delta H_{cc}$ (J/g)	$\Delta H_m$ J/g	Xc (%)
Original PLA filament	60.00	150.00	319.22	8.91	12.08	3.4
PLA residue	52.32	168.91	276.00	10.88	15.34	4.8
Recycled PLA	54.77	157.27	270.51	0.24	19.17	20.3
Recycled PLA with graphene	55.81	159.74	275.95	10.19	13.31	3.4

When the samples were first heated, a cold crystallization peak was observed at around 100 °C for the original PLA filament, PLA residue, and Recycled PLA with graphene. However, this peak is minimal around this temperature for the Recycled PLA. This phenomenon typically occurs during the rearrangement of PLA macromolecules upon heating, resulting in an exothermic process before melting⁴¹. For the samples, the PLA residue presented the highest $\Delta H_{cc}$ value, while the recycled PLA presented the smallest value. This indicated that reprocessing stimulated an increase in the crystallinity of the polymer, resulting in the highest X_c value of 20.3% for the Recycled PLA. However, this trend is not observed after the addition of graphene, where the crystallinity remains unchanged from that of the original PLA filament.

The original PLA melting temperature (T_m) was 150 °C, which is lower than that of the other PLAs analyzed. For recycled PLA and recycled PLA with graphene addition, the T_m values were slightly lower than those reported in previous studies¹¹,¹⁵ However, these authors did not evaluate the behavior of 3D printing residues, which could cause the difference between the values. Regarding the T_g, all samples showed a decrease in values, from 60 °C to 52 °C for the PLA residue.

The incorporation of graphene into PLA significantly modifies its rheological properties, enhancing the material’s components and performance, thereby favoring processes such as 3D printing⁴².

3.3. Tensile behavior, electrical properties, and morphology of the surface

Figure 8 shows the stress-strain curves for recycled PLA and recycled PLA with graphene samples. The samples exhibited tensile strengths of 82.37 ± 0.01 MPa for the recycled PLA and 82.95 ± 0.01 MPa for the recycled PLA with graphene, as shown in Figure 8. The average strain at break of the samples was 1.48% and 1.38%, respectively, for the recycled PLA and the recycled PLA with graphene. The strength threshold values are slightly higher than those found in the reprocessed PLA behavior⁴³.

Figure 8
Representative stress-strain curves for recycled PLA and recycled PLA with graphene.

Comparing the tensile strength results of the samples from this study (see Table 2) with the tensile strength values of the original PLA filament (51MPa) and (53.5 MPa) as reported in the referenced publication³⁶, it is evident that the recycling process, along with the addition of graphene, significantly increased the tensile strength limit. According to this study⁴⁴, this increase can be attributed to shorter polymer chains caused by enhanced physical-chemical interactions during crystallization. This suggests that good miscibility between the shortened polymer chains may result in better tensile strength properties⁴⁵.

Table 2
Mechanical tensile properties for recycled materials and original PLA filament compared to the mechanical properties of PLA filament from the literature.

Material/Property	Tensile strength (MPa)	Strain at break (%)	Young's Modulus (GPa)
Original PLA filament (Ingeo 3D850)	51	3.31	2.31
Recycled PLA	82.37 ± 0.01a	1.48 ± 0.02a	3.51 ± 0.021a
Recycled PLA with Graphene (0.18 wt%)	82.95 ± 0.01a	1.38 ± 0.01a	3.57 ± 0.023a
Neat PLA 47	41.64	3.42	2.36
Neat PLA 48	33.69	-	-
PLA with Graphene 49	41.03	-	-
PLA with Graphene (5 wt%) 50	43.10	-	2.64
PLA with Carbon Fiber 51	49.41	7.81	1.26
PLA with Flax Fiber 52	43.43	6.0	-
PLA with Titania 53	31.44	2.30	2.30
PLA with Alumina 54	40.20	-	1.20

• ^a
  Letters indicate significant differences between samples according to Tukey’s test.

The samples showed brittle fracture characteristics. Despite the deformation above 1%, there was no formation of “necks” in the specimen (CP), nor was there plastic behavior before rupture. The Young's modulus (see Table 2) also showed similar behavior to the values of 3.5⁴⁶ and 3.58 GPa³⁹ reported by these studies for the original PLA filament samples.

Comparing the results obtained in this study (see Table 2) with those reported in the literature, it is evident that both Recycled PLA and Recycled PLA with Graphene exhibit substantially superior mechanical performance compared to PLA and most composites reinforced with different materials. While PLA composites containing natural fibers, carbon fibers, or metallic nanoparticles typically show tensile strengths ranging from 31 to 49 MPa, the materials developed in this work exceeded 82 MPa, demonstrating a remarkable improvement even when using a recycled matrix.

Additionally, Tukey’s test was applied in this study to identify significant differences in the tensile testing results. A 5% significance level was used to compare the average results of all the Recycled PLA with those of the Recycled PLA with Graphene composites, and the results revealed that adding graphene did not influence the tensile strength and Young’s modulus. Statistical analysis indicated no significant differences among the recycled materials composites, as all samples were grouped (group “a”).

Furthermore, the Young's Modulus achieved for Recycled PLA with Graphene (3.57 GPa) surpasses the values reported for PLA/graphene composites in Table 2, which rarely exceed 2.64 GPa. However, this high strength and stiffness are accompanied by a significant reduction in strain at break (~1.4%), indicating brittle behavior and a lower energy absorption capacity compared to fiber-reinforced composites, such as PLA reinforced with carbon or flax fibers, which can reach strain at break above 6%. Therefore, the proposed materials appear to be particularly promising for applications where rigidity and strength are the primary requirements. However, their use in components subjected to impact or repeated flexural loading may require further evaluation.

Beyond the mechanical benefits, the reuse of post-consumer or post-processing PLA waste offers both environmental and economic advantages. From an environmental perspective, this recovery reduces the volume of discarded waste and lowers the demand for virgin PLA production, which is energy-intensive and dependent on agricultural resources. Studies indicate that recycled PLA can generate fewer greenhouse gas emissions compared to fossil-based polymers⁵⁵.

Economically, transforming low-cost or zero-cost waste materials into high-performance engineering products adds considerable value to discarded resources and may reduce manufacturing costs in large-scale applications. Consequently, the results presented in this study not only expand the current knowledge on the mechanical enhancement of recycled polymers but also highlight a viable strategy for aligning the development of high-performance materials with circular economy principles⁵⁶.

Based on this result, the PLA matrix recycling process was unable to increase the material's toughness or reduce its brittleness. Moreover, the addition of graphene also did not cause significant changes to Young's modulus. Chatrath and coworkers⁵⁷ reported that this property may depend on the length of the polymer chains and that the recycling and extrusion processes can reduce the size of these chains, applying more stress to the PLA chains.

The electrical properties test was conducted as an investigation. The sample with graphene was tested at the maximum source voltage of 500 V, and no variation or electrical conduction signal was observed through the material. Graphene did not impart conductive properties to the nanocomposite in proportion to the amount of graphene added. In their study⁵⁸ with different graphene concentrations (0, 2, 2.5, 3, 4, 5, and 10 wt%) in the PLA matrix, the authors concluded that concentrations below 2 wt% did not exhibit measurable current. This finding is consistent with the results of this study, which utilized 0.18 wt% of graphene, suggesting that a higher concentration would be necessary to achieve a measurable current.

4. CONCLUSIONS

The twin screw extrusion process homogenized the samples, presenting homogeneous colors depending on the type of material: recycled PLA or recycled PLA with graphene. Although the resulting coloration of the materials differed, the low percentage of added graphene did not lead to electrical conductivity. However, the tensile strength increased by approximately 61.5% and 62.6% for the recycled PLA and graphene-recycled PLA samples, respectively, compared to the original PLA filament. Due to the high cold crystallization enthalpy compared to the melting enthalpy in the first heating on DSC, all samples presented low crystallinity, except for the Recycled PLA. Additionally, the thermal behavior, as determined by TG analysis, showed lower T _degradation for all processed materials compared to the original PLA filament. However, the thermal stability (T_i) was almost the same for all samples. Overall, the findings demonstrate that mechanical recycling not only preserves but also can substantially enhance the performance of PLA, significantly extending its useful life. These results reinforce the potential of recycled PLA, especially when combined with graphene, as a viable and sustainable alternative for engineering applications. Future studies exploring higher graphene concentrations could unlock additional functionalities, including electrical conductivity, paving the way for multifunctional recycled polymer composites.

5. Acknowledgments

The authors would like to thank Decanato de Pós-Graduação, University of Brasília (DPG-UnB), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for their financial support of this project.

• Data Availability
  The data presented in this study is presented in the manuscript.

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Edited by

• Associate Editor: Rodrigo Orefice.
  Editor-in-Chief: Luiz Antonio Pessan.

Data availability

The data presented in this study is presented in the manuscript.

Publication Dates

• Publication in this collection
  16 Feb 2026
• Date of issue
  2026

History

• Received
  21 July 2025
• Reviewed
  05 Nov 2025
• Accepted
  11 Jan 2026