The Effect of the Addition of Copper Particles in High-Density Recycled Polyethylene Matrices by Extrusion

In this study, the effect of the recycling process and copper particle incorporation on virgin and recycled pellet HDPE were investigated by thermo-chemical analysis, mechanical characterization, and antibacterial analysis. Copper particles were added to pellet HDPE, virgin and recycled, using a tabletop single screw extruder. Some copper particles, called copper nano-particles (Cu-NPs), had a spherical morphology and an average particle size near 20 nm. The others had a cubic morphology and an average particle size close to 300 nm, labeled copper nano-cubes (Cu-NCs). The thermo-chemical analysis revealed that the degree of crystallization was not influenced by the recycling process: 55.38 % for virgin HDPE and 56.01% for recycled HDPE. The degree of crystallization decreased with the addition of the copper particles. Possibly due to a modification in the structure, packaging organization, and crystalline ordering, the recycled HDPE reached a degree of crystallization close to 44.78% with 0.5 wt.% copper nano-particles and close to 36.57% for the recycled HDPE modified with 0.7 wt.% Cu-NCs. Tensile tests revealed a slight reduction in the tensile strength related to the recycling process, being close to 26 MPa for the virgin HDPE and 15.99 MPa for the recycled HDPE, which was improved by adding copper particles, which were near 25.39 MPa for 0.7 wt.% copper nano-cubes. Antibacterial analysis showed a reduction in the viability of E. coli in virgin HDPE samples, which was close to 8% for HDPE containing copper nano-particles and lower than 2% for HDPE having copper nano-cubes. In contrast, the recycled HDPE revealed viability close to 95% for HDPE with copper nano-particles and nearly 50% for HDPE with copper nano-cubes. The viability of S. aureus for HDPE was lower than containing copper nano-particles and copper nano-cubes, which increased dramatically close to 80% for recycled HDPE with copper nano-particles 80% and 75% with copper nano-cubes.


Introduction
Global polymer production is increasing yearly, reaching 335 million tons in 2016 [1] and 368 million tons in 2019 [2]. With this increasing trend, global production is estimated to reach 1.1 billion tons by 2050 [3]. In general, a large part of these plastics is discarded after a single use, generating a severe environmental problem [4,5] that could accumulate 12 billion tons of plastic waste in landfills and the environment by 2050 [3,6,7]. Furthermore, it is

Synthesis of Copper Particles
Cu-NPs were synthesized by mixing a solution of copper chloride (100 mL, 0.05 M) and 50 mL of ascorbic acid (0.1 M) at 80 • C while being stirred for 20 h (900 rpm). The suspension was filtered and stored at room temperature. Morphological characterization and high-magnification images (>5000) were acquired through TEM microscopy (Hitachi HT7700, Tokyo, Japan). The transmission electron microscopy (TEM) sample preparation was carried out by dipping Lacey carbon formvar-coated copper grids (300 Square, Pelco, CA, USA) in a diluted suspension (ethanol/H 2 O solution (3:1)) of samples. Then, the sample was air-dried before observation.
Cu-NCs were synthesized following a method previously reported by Gou et al. [34]. In brief, the copper (II) sulfate salt was reduced in an ascorbic acid solution in the presence of polyethylene glycol (PEG) and sodium hydroxide at 25 • C for 20 min. Later, the suspension was centrifugated, washed twice, and dried for 24 h at 70 • C. Cu-NCs were characterized by a scanning electron microscope (SEM, ZEISS, model EVO MA10, Jena, Germany) operated at 20 kV in a high vacuum. Figure 1 reveals a spherical morphology for Cu-NPs, with an average particle size near 20 nm, and a cubic morphology for Cu-NCs, with a more significant average particle size close to 300 nm. Cu-NPs and Cu-NCs were first centrifuged for 1 min at 15,000 rpm. After that, the supernatant solvent was extracted, keeping the Cu-NPs and Cu-NCs moist. The precipitate was deposited and massed on Petri dishes. polymers were extruded between 400 and 450 kg/h using a cylindrical screw (SCM-440 5 Hr) with a diameter of 150 mm. Samples were cooled in water at room temperature he in a stainless-steel container. Then, both samples were pelletized using a 10 Horse-Pow (HP) cutter equipped with a 24-piece blade (Fuji) and a steel guillotine.

Synthesis of Copper Particles
Cu-NPs were synthesized by mixing a solution of copper chloride (100 mL, 0.05 M and 50 mL of ascorbic acid (0.1 M) at 80 °C while being stirred for 20 h (900 rpm). Th suspension was filtered and stored at room temperature. Morphological characterizatio and high-magnification images (>5000) were acquired through TEM microscopy (Hitac HT7700, Tokyo, Japan). The transmission electron microscopy (TEM) sample preparatio was carried out by dipping Lacey carbon formvar-coated copper grids (300 Square, Pelc CA, USA) in a diluted suspension (ethanol/H2O solution (3:1)) of samples. Then, the sam ple was air-dried before observation.
Cu-NCs were synthesized following a method previously reported by Gou et al. [34 In brief, the copper (II) sulfate salt was reduced in an ascorbic acid solution in the presen of polyethylene glycol (PEG) and sodium hydroxide at 25 °C for 20 min. Later, the su pension was centrifugated, washed twice, and dried for 24 h at 70 °C. Cu-NCs were cha acterized by a scanning electron microscope (SEM, ZEISS, model EVO MA10, Jena, Ge many) operated at 20 kV in a high vacuum. Figure 1 reveals a spherical morphology for Cu-NPs, with an average particle si near 20 nm, and a cubic morphology for Cu-NCs, with a more significant average partic size close to 300 nm. Cu-NPs and Cu-NCs were first centrifuged for 1 min at 15,000 rpm After that, the supernatant solvent was extracted, keeping the Cu-NPs and Cu-NCs mois The precipitate was deposited and massed on Petri dishes.

Incorporation of Copper Particles on HDPE by Extrusion.
Virgin and recycled HDPE samples were pelletized and mixed with Cu-NPs and C NCs by extrusion, using a tabletop single screw extruder (Thera Instruments, BROX Sp Santiago, Chile). HDPE samples were obtained with a concentration close to 0.5 wt.% C NPs in HDPE and rHDPE and close to 0.7 wt.% Cu-NCs in HDPE and rHDPE.

Incorporation of Copper Particles on HDPE by Extrusion
Virgin and recycled HDPE samples were pelletized and mixed with Cu-NPs and Cu-NCs by extrusion, using a tabletop single screw extruder (Thera Instruments, BROX Spa, Santiago, Chile). HDPE samples were obtained with a concentration close to 0.5 wt.% Cu-NPs in HDPE and rHDPE and close to 0.7 wt.% Cu-NCs in HDPE and rHDPE. Figure 2 shows digital images of virgin and recycled HDPE samples.

Morphological Characterization
The morphology and size of the pure and modified samples were analyzed using an optical microscope (OM), model OLYMPUS BX60. QCapture Pro (version 6, Teledyne Imaging, TUS, USA) this software was used to perform OM image analysis to estimate the particle sizes. Furthermore, the HDPE samples were analyzed by field-emission scanning electron microscopy (FE-SEM, QUANTA 250 FEG, Hillsboro, OR, USA) which was equipped with an energy dispersion spectrometer.

Incorporation of Copper Particles on HDPE by Extrusion.
Virgin and recycled HDPE samples were pelletized and mixed with Cu-NPs and C NCs by extrusion, using a tabletop single screw extruder (Thera Instruments, BROX Sp Santiago, Chile). HDPE samples were obtained with a concentration close to 0.5 wt.% C NPs in HDPE and rHDPE and close to 0.7 wt.% Cu-NCs in HDPE and rHDPE. Figure shows digital images of virgin and recycled HDPE samples.

Differential Scanning Calorimetry and Thermogravimetric Measurements
The effect of the recycling process and the addition of Cu-NPs and Cu-NCs on HDPE samples were analyzed using differential scanning calorimetry (DSC). First, 12 to 16 mg of samples placed in an Al holder and sealed in a microcapsule by DSC (Perkin Elmer 6000, Waltham, MA, USA). The samples were heated to 280 • C, then cooled to 20 • C, and analyzed at a rate of 20 • C/min under a nitrogen atmosphere of 20 mL/min. Thermogravimetric analysis (TGA) was carried out using 10 mg of samples, which were exposed to a nitrogen atmosphere at 50 mL/min, starting at room temperature until reaching 800 • C at 10 • C/min, using a TGA 851e (Mettler-Toledo, Hong Kong, China). Both analyses were performed in duplicate.

Tensile Testing
The effect of the recycling process and the addition of Cu-NPs and Cu-NCs on HDPE were also investigated using a traction machine (Tinius & Olsen Mod. Super L, Shanghai, China), with a displacement speed of 50 mm/min, which was applied until the samples ruptured, as indicated in ASTM D638 [34]. Measurements were carried out five times each. Moreover, the hardness of the samples was measured using a durometer with a Shore D hardness scale, as described by the ASTM D 2240 [35] and ISO 868 standard [36]. The samples were molded using a hydraulic press (HP Industrial Instruments, Karnataka, India) under a pressure of 50 bar at 140 • C, which then were cooled under pressure with water circulation, obtaining 1 mm-thick samples, as shown in Figure 3. Each measurement was performed in triplicate.

Morphological Characterization
The morphology and size of the pure and modified samples were analyzed us optical microscope (OM), model OLYMPUS BX60. QCapture Pro (version 6, Teledy aging, TUS, USA) this software was used to perform OM image analysis to estima particle sizes. Furthermore, the HDPE samples were analyzed by field-emission sca electron microscopy (FE-SEM, QUANTA 250 FEG, Hillsboro, OR, USA) whic equipped with an energy dispersion spectrometer.

Differential Scanning Calorimetry and Thermogravimetric Measurements
The effect of the recycling process and the addition of Cu-NPs and Cu-NCs on samples were analyzed using differential scanning calorimetry (DSC). First, 12 to of samples placed in an Al holder and sealed in a microcapsule by DSC (Perkin 6000, Waltham, MA, USA). The samples were heated to 280 °C, then cooled to 20 °C analyzed at a rate of 20 °C/min under a nitrogen atmosphere of 20 mL/min. Thermo imetric analysis (TGA) was carried out using 10 mg of samples, which were expose nitrogen atmosphere at 50 mL/min, starting at room temperature until reaching 800 10 °C/min, using a TGA 851e (Mettler-Toledo, Hong Kong, China). Both analyses performed in duplicate.

Tensile Testing
The effect of the recycling process and the addition of Cu-NPs and Cu-NCs on were also investigated using a traction machine (Tinius & Olsen Mod. Super L, Sha China), with a displacement speed of 50 mm/min, which was applied until the sa ruptured, as indicated in ASTM D638 [34]. Measurements were carried out five each. Moreover, the hardness of the samples was measured using a durometer w Shore D hardness scale, as described by the ASTM D 2240 [35] and ISO 868 standar The samples were molded using a hydraulic press (HP Industrial Instruments, Karn India) under a pressure of 50 bar at 140 °C, which then were cooled under pressur water circulation, obtaining 1 mm-thick samples, as shown in Figure 3. Each measur was performed in triplicate.

Antibacterial Properties
Samples of virgin and recycled HDPE which had been modified with 0.5 wt. NPs and 0.7 wt.% Cu-NCs were analyzed to evaluate their antibacterial potential a Escherichia coli BW 25,113 and Staphylococcus aureus ATCC 2538. The analysis was on the E2180-01 [37] standard protocol, using Mueller Hinton agar and incubated at

Antibacterial Properties
Samples of virgin and recycled HDPE which had been modified with 0.5 wt.% Cu-NPs and 0.7 wt.% Cu-NCs were analyzed to evaluate their antibacterial potential against Escherichia coli BW 25,113 and Staphylococcus aureus ATCC 2538. The analysis was based on the E2180-01 [37] standard protocol, using Mueller Hinton agar and incubated at 25 • C, which was performed in duplicate. Figure 4 shows FE-SEM images of the HDPE and rHDPE samples modified with Cu-NPs and Cu-NCs, revealing the formation of cracks on the surface that had a lower density for samples modified with Cu-NPs. In addition, the rHDPE showed the incorporation of metal particles more clearly, with more homogenous dispersion for HDPE and rHDPE modified with Cu-NPs. Figure 4 also reveals an agglomerated distribution of Cu-NCs in rHDPE, with the size of the agglomerates being close to 4 µm, as reported by Gao et al. [38]. Figure 4 shows FE-SEM images of the HDPE with a higher magnification.  Figure 4 shows FE-SEM images of the HDPE and rHDPE samples modified with Cu-NPs and Cu-NCs, revealing the formation of cracks on the surface that had a lower density for samples modified with Cu-NPs. In addition, the rHDPE showed the incorporation of metal particles more clearly, with more homogenous dispersion for HDPE and rHDPE modified with Cu-NPs. Figure 4 also reveals an agglomerated distribution of Cu-NCs in rHDPE, with the size of the agglomerates being close to 4 µm, as reported by Gao et al. [38]. Figure 4 shows FE-SEM images of the HDPE with a higher magnification.

Differential Scanning Calorimetry and Thermogravimetric Measurements
Figure 5a-f shows the differential scanning calorimetry (DSC) heating and cooling curves of HDPE and rHDPE modified with Cu-NPs and Cu-NCs. From the DSC heating curves, the melting temperature ( ) and melting enthalpy ( ) were obtained, and from the DSC cooling curves, the crystallization temperature ( ) and the crystallization enthalpy ( ) were estimated, as also shown in Table 1 and Figure 5 DSC curves revealed a slight reduction in with the recycling process, from 141.1 °C for HDPE to 138.1 °C for the rHDPE. For a virgin HDPE, Cuadri and Martín-Alfonso [39] reported a close to 133.8 ± 1 °C. Nevertheless, Muniyandi et al. [40] found a near 115.8 °C, which decreased to 108.2 °C with the recycling process. Therefore, a direct relationship could be established between the decrease in and the recycling process of HDPE. The incorporation of Cu-NPs in the virgin and recycled matrix of HDPE decreases the between 1 and 2 °C, which was more drastic with Cu-NCs, reducing by 3 °C. According to Gao et al. [38] the did not decrease their value with the addition of nanoparticles in the virgin HDPE matrix, such as graphene oxide [41], ZrO2 [42], titanate nanotubes (Y2W3O12) [41], SnO2 [43], and ZnO [38]. Therefore, the effect on due to incorporating Cu nanoparticles suggests an enhancement in the thermal properties of the HDPE matrix.

Differential Scanning Calorimetry and Thermogravimetric Measurements
Figure 5a-f shows the differential scanning calorimetry (DSC) heating and cooling curves of HDPE and rHDPE modified with Cu-NPs and Cu-NCs. From the DSC heating curves, the melting temperature (T m ) and melting enthalpy (∆H m ) were obtained, and from the DSC cooling curves, the crystallization temperature (T c ) and the crystallization enthalpy (∆H c ) were estimated, as also shown in Table 1 and Figure 5.  Based on the DSC analysis, the ΔH of the polymer was estimated by integrating th area under the endothermic curve (in J/g). This allowed us to estimate the degree of cry tallization (X ), according to equation 1 [44], where the enthalpy of fusion used for 100 HDPE crystallization was 293 J mol −1 [45]: The was close to 49.7 and 42.4 for the HDPE and rHDPE samples, respectivel as shown in Table 1. This slight decrease can be attributed to the effect of the recyclin process on the structure and high packaging organization [46][47][48][49]. Some researchers hav proposed that the shear forces during the processing can induce the cleavage of the pol mer backbone leading primarily to chain shortening due to the recycling process [50]. Th also decreases for rHDPE with the addition of Cu-NPs and Cu-NCs compared HDPE, in agreement with Jeziorska et al. [51,52], who previously reported that the inco poration of silica nanoparticles (SiO2) and silica-containing nano copper (Cu-SiO2) cou decrease the crystallinity degree of the HDPE. For example, the X for virgin HDPE w 62.5% that reduced to 53% by adding SiO2. The addition of Cu-SiO2 allowed us to obta 59.9% of crystallinity [50,51]. In contrast, the X increases for HDPE with Cu-NPs and C NCS are shown in Table 1. Similarly, Gao et al. [38] reported that the addition of Zn nano-particles increases the X in concentrations between 1 and 5 wt. %. Mahmoud et a [53] found that when the content of nano-particles increased in the polymer with a un form distribution, the crystalline regions increase, causing higher crystallinity in HDPE  DSC curves revealed a slight reduction in T m with the recycling process, from 141.1 • C for HDPE to 138.1 • C for the rHDPE. For a virgin HDPE, Cuadri and Martín-Alfonso [39] reported a T m close to 133.8 ± 1 • C. Nevertheless, Muniyandi et al. [40] found a T m near 115.8 • C, which decreased to 108.2 • C with the recycling process. Therefore, a direct relationship could be established between the decrease in T m and the recycling process of HDPE. The incorporation of Cu-NPs in the virgin and recycled matrix of HDPE decreases the T m between 1 and 2 • C, which was more drastic with Cu-NCs, reducing by 3 • C. According to Gao et al. [38] the T m did not decrease their value with the addition of nanoparticles in the virgin HDPE matrix, such as graphene oxide [41], ZrO 2 [42], titanate nanotubes (Y 2 W 3 O 12 ) [41], SnO 2 [43], and ZnO [38]. Therefore, the effect on T m due to incorporating Cu nanoparticles suggests an enhancement in the thermal properties of the HDPE matrix.
Based on the DSC analysis, the ∆H m of the polymer was estimated by integrating the area under the endothermic curve (in J/g). This allowed us to estimate the degree of crystallization (X c ), according to equation 1 [44], where the enthalpy of fusion used for 100% HDPE crystallization was 293 J mol −1 [45]: The X c was close to 49.7 and 42.4 for the HDPE and rHDPE samples, respectively, as shown in Table 1. This slight decrease can be attributed to the effect of the recycling process on the structure and high packaging organization [46][47][48][49]. Some researchers have proposed that the shear forces during the processing can induce the cleavage of the polymer backbone leading primarily to chain shortening due to the recycling process [50]. The X c also decreases for rHDPE with the addition of Cu-NPs and Cu-NCs compared to HDPE, in agreement with Jeziorska et al. [51,52], who previously reported that the incorporation of silica nanoparticles (SiO 2 ) and silica-containing nano copper (Cu-SiO 2 ) could decrease the crystallinity degree of the HDPE. For example, the X c for virgin HDPE was 62.5% that reduced to 53% by adding SiO 2 . The addition of Cu-SiO 2 allowed us to obtain 59.9% of Polymers 2022, 14, 5220 7 of 13 crystallinity [50,51]. In contrast, the X c increases for HDPE with Cu-NPs and Cu-NCS are shown in Table 1. Similarly, Gao et al. [38] reported that the addition of ZnO nano-particles increases the X c in concentrations between 1 and 5 wt. %. Mahmoud et al. [53] found that when the content of nano-particles increased in the polymer with a uniform distribution, the crystalline regions increase, causing higher crystallinity in HDPE. Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample. mers 2022, 14, x FOR PEER REVIEW 7 o Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE sampl both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenc the initial temperature of HDPE. Thermal degradation is associated with the recycli process, which can be attributed to a change in the molecular weight without formi volatile products, resulting from the cleavage of weak links, such as oxygen bridges a lateral chains [20]. Several authors have reported that the heating applied during the cycling and pelletizing process can favor the formation of shorter chains in the HD samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 a shows minor differences in the thermal degradation profiles, especially in the maximu thermal degradation rate (minimum of the curve), which is slightly lower for recyc polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are p sibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of t HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechani properties without destroying the other polymeric characteristics. In this context, the fect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechani behavior of HDPE was studied using stress-strain curves and hardness measurements shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from recycling process, such as the maximum tensile strength (σ ), the tensile strength break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table  The properties of the virgin HDPE were degraded by the recycling process but enhanc by the addition of Cu particles, which can be associated with a modification of comp polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that maximum strength σ decreased slightly with the recycling process compared to t virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper pa  Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.  Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.  Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.  Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.  Figure 6 shows both modified with the initial temperat process, which can volatile products, re lateral chains [20]. S cycling and pelletiz samples, which are shows minor differe thermal degradatio polymers with Cu-N sibly related to the c

Mechanical Char
It has been rep HDPE matrix's nan properties without fect of the recycling behavior of HDPE w shown in Figure 7.  Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. al. [55] also reported a similar effect. The increase in the elongation suggests that th per particles could increase the ductility of HDPE, changing the crystalline/amor zones or increasing the local and linear defects. Table 2 also shows that the shore D ness was slightly reduced with the recycling process and addition of Cu-NCs, whic more drastic with Cu-NPs; therefore, the size particle and/or morphology slightly ence the mechanical properties of HDPE. Therefore, the incorporation of nano-particles improved the mechanical respon recycled and virgin HDPE matrices, which can be associated with the high surface of the nano-particles [56,[58][59][60], which can influence the movement of defects and mer chains [61,62], even with a 0.5% Cu-NP content and a 0.7% Cu-NC content.  Figure 8 shows the bacterial viability of virgin and recycled HDPE samples ex to a medium inoculated with E. coli and S. aureus. The virgin HDPE showed a redu in the viability of E. coli in samples containing Cu-NPs and Cu-NCs, the latter rev the lowest percentage of viability. The virgin HDPE sample exposed to a medium i lated with S. aureus also showed that the viability of the bacteria was lower in the sa  Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.
Singh et al. [56] studied the mechanical properties of a polymeric matrix composed of (50/50) virgin and recycled HDPE, obtaining a σ value of 12.27 MPa, which reduced the σ value of virgin HDPE by 30%, which was close to 17.39 MPa. Figure 7 and Table   ) HDPE, (   Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.
Singh et al. [56] studied the mechanical properties of a polymeric matrix composed of (50/50) virgin and recycled HDPE, obtaining a σ value of 12.27 MPa, which reduced the σ value of virgin HDPE by 30%, which was close to 17.39 MPa. Figure 7 and Table   ) HDPE 0.5 wt.% Cu-NPs, (   Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.
Singh et al. [56] studied the mechanical properties of a polymeric matrix composed of (50/50) virgin and recycled HDPE, obtaining a σ value of 12.27 MPa, which reduced the σ value of virgin HDPE by 30%, which was close to 17.39 MPa. Figure 7 and Table   ) HDPE 0.7 wt.% Cu-NCs, (   Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.
Singh et al. [56] studied the mechanical properties of a polymeric matrix composed of (50/50) virgin and recycled HDPE, obtaining a σ value of 12.27 MPa, which reduced the σ value of virgin HDPE by 30%, which was close to 17.39 MPa. Figure 7 and Table   ) rHDPE, (   Figure 6 shows both modified with the initial temperat process, which can volatile products, re lateral chains [20]. S cycling and pelletiz samples, which are shows minor differe thermal degradatio polymers with Cu-N sibly related to the c Figure 6. Effect of the of ( ) HDPE, ( ( ) rHDPE 0.5 wt.% imum thermal degrad

Mechanical Char
It has been rep HDPE matrix's nan properties without fect of the recycling behavior of HDPE w shown in Figure 7.  Figure 6 shows the thermogravimetry analysis (TGA) for HDPE and rHDPE samples, both modified with Cu-NPs and Cu-NCs, revealing that the recycling process influenced the initial temperature of HDPE. Thermal degradation is associated with the recycling process, which can be attributed to a change in the molecular weight without forming volatile products, resulting from the cleavage of weak links, such as oxygen bridges and lateral chains [20]. Several authors have reported that the heating applied during the recycling and pelletizing process can favor the formation of shorter chains in the HDPE samples, which are less thermally stable than virgin material [46,51,54]. Figure 6 also shows minor differences in the thermal degradation profiles, especially in the maximum thermal degradation rate (minimum of the curve), which is slightly lower for recycled polymers with Cu-NPs than virgin polymers with Cu NPs. The slight differences are possibly related to the content of Cu as NPs and NCs in each sample.

Mechanical Characterization
It has been reported that the mechanical properties decrease for high loading of the HDPE matrix's nano-filler [55]. Therefore, the challenge was to improve the mechanical properties without destroying the other polymeric characteristics. In this context, the effect of the recycling process and incorporation of Cu-NPs and Cu-NCs on the mechanical behavior of HDPE was studied using stress-strain curves and hardness measurements, as shown in Figure 7. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ ), the tensile strength at break (σ ), elongation at break (e ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE.
Singh et al. [56] studied the mechanical properties of a polymeric matrix composed of (50/50) virgin and recycled HDPE, obtaining a σ value of 12.27 MPa, which reduced the σ value of virgin HDPE by 30%, which was close to 17.39 MPa. Figure 7 and Table   ) rHDPE 0.7 wt.% Cu-NCs. Figure 7 reveals a drastic effect on the mechanical properties of the HDPE from the recycling process, such as the maximum tensile strength (σ max ), the tensile strength at break (σ B ), elongation at break (e B ), and shore D hardness (D), as also shown in Table 2. The properties of the virgin HDPE were degraded by the recycling process but enhanced by the addition of Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes [54,56]. Figure 7 and Table 2 show that the maximum strength σ max decreased slightly with the recycling process compared to the virgin sample that increased with the addition of Cu-NPs and Cu-NCs. The copper particles could change the crystallinity of samples, possibly enhancing the defects, such as vacancies and dislocations. Furthermore, adding Cu-NPs and Cu-NCs into the virgin and recycled HDPE increases the Yield strength and shows a paradoxical effect on the strain, growing slightly in the virgin HDPE samples and decreasing mildly in the recycled HDPE. Singh et al. [56] studied the mechanical properties of a polymeric matrix composed of (50/50) virgin and recycled HDPE, obtaining a σ max value of 12.27 MPa, which reduced the σ max value of virgin HDPE by 30%, which was close to 17.39 MPa. Figure 7 and Table 2 show that the σ B of virgin HDPE decreased almost four times with the addition of Cu-NPs and Cu-NCs in the samples. The e B of the polymeric matrix increased with the recycling process and incorporation of copper particles, in some cases more than the double. Tesfaw et al. [57] determined a tension of 20.67 MPa and an elongation of 25.13 mm for a virgin HDPE and a tension of 7.73 MPa and 9.97 mm of elongation for r-HDPE; Beigloo et al. [55] also reported a similar effect. The increase in the elongation suggests that the copper particles could increase the ductility of HDPE, changing the crystalline/amorphous zones or increasing the local and linear defects. Table 2 also shows that the shore D hardness was slightly reduced with the recycling process and addition of Cu-NCs, which was more drastic with Cu-NPs; therefore, the size particle and/or morphology slightly influence the mechanical properties of HDPE.
Therefore, the incorporation of nano-particles improved the mechanical responses of recycled and virgin HDPE matrices, which can be associated with the high surface areas of the nano-particles [56,[58][59][60], which can influence the movement of defects and polymer chains [61,62], even with a 0.5% Cu-NP content and a 0.7% Cu-NC content. Figure 8 shows the bacterial viability of virgin and recycled HDPE samples exposed to a medium inoculated with E. coli and S. aureus. The virgin HDPE showed a reduction in the viability of E. coli in samples containing Cu-NPs and Cu-NCs, the latter revealing the lowest percentage of viability. The virgin HDPE sample exposed to a medium inoculated with S. aureus also showed that the viability of the bacteria was lower in the samples with Cu-NPs and Cu-NCs, the latter having the lowest percentage of viability, being the best in terms of antibacterial activity. In addition, the recycled polymer showed that the best antibacterial activity was obtained by the sample containing Cu-NCs. The rHDPE sample showed much higher viability percentages, as shown in Tables S1 and S2 in the Supplementary Material. Furthermore, the different effects of the Cu-NPs and Cu-NCs can be attributed to their dispersion in the polymeric matrix, as shown in Figure 4. with Cu-NPs and Cu-NCs, the latter having the lowest percentage of viability, being th best in terms of antibacterial activity. In addition, the recycled polymer showed that th best antibacterial activity was obtained by the sample containing Cu-NCs. The rHDP sample showed much higher viability percentages, as shown in Tables S1 and S2 in th Supplementary Material. Furthermore, the different effects of the Cu-NPs and Cu-NC can be attributed to their dispersion in the polymeric matrix, as shown in Figure 4. Vásquez et al. [63] studied the antibacterial effect of incorporating ZnO-NPs in recy cled polyethylene terephthalate (r-PET) polymeric fibers, and they measured the diamete of inhibition (in mm) for the bacteria E. coli and S. aureus. The antibacterial properties o r-PET were lower than virgin PET, using the same quantity of added ZnO-NPs, sugges ing that the recycling process affects the antibacterial behavior of the polymers. Howeve only the antibacterial properties of virgin HDPE with the Cu-NPs and nano-fibers hav been reported [17,[64][65][66]. The authors reported a decrease in CFU (colony-forming unit by increasing the concentration of NPs in the polymeric matrix, which was associated wit the release of copper ions that can penetrate the membrane of the bacteria and affect the viability. Shankar et al. [65], Arfat et al. [66], and Chatterjee et al. [67] proposed that th antibacterial effect of Cu-NPs was related to its toxicity to reactive oxygen species. T mayo et al. [17] reported that the antibacterial behavior depends on three factors: the syn ergy between the polymer and the Cu-NPs, the capacity of the polymer for long-term io release, and the dispersion of the Cu-NPs in the polymer matrix. with Cu-NPs and Cu-NCs, the latter having the lowest percentage of viability, being the best in terms of antibacterial activity. In addition, the recycled polymer showed that the best antibacterial activity was obtained by the sample containing Cu-NCs. The rHDPE sample showed much higher viability percentages, as shown in Tables S1 and S2 in the Supplementary Material. Furthermore, the different effects of the Cu-NPs and Cu-NCs can be attributed to their dispersion in the polymeric matrix, as shown in Figure 4. Vásquez et al. [63] studied the antibacterial effect of incorporating ZnO-NPs in recycled polyethylene terephthalate (r-PET) polymeric fibers, and they measured the diameter of inhibition (in mm) for the bacteria E. coli and S. aureus. The antibacterial properties of r-PET were lower than virgin PET, using the same quantity of added ZnO-NPs, suggesting that the recycling process affects the antibacterial behavior of the polymers. However, only the antibacterial properties of virgin HDPE with the Cu-NPs and nano-fibers have been reported [17,[64][65][66]. The authors reported a decrease in CFU (colony-forming units) by increasing the concentration of NPs in the polymeric matrix, which was associated with the release of copper ions that can penetrate the membrane of the bacteria and affect their viability. Shankar et al. [65], Arfat et al. [66], and Chatterjee et al. [67] proposed that the antibacterial effect of Cu-NPs was related to its toxicity to reactive oxygen species. Tamayo et al. [17] reported that the antibacterial behavior depends on three factors: the synergy between the polymer and the Cu-NPs, the capacity of the polymer for long-term ion release, and the dispersion of the Cu-NPs in the polymer matrix. with Cu-NPs and Cu-NCs, the latter having the lowest percentage of viability, being the best in terms of antibacterial activity. In addition, the recycled polymer showed that the best antibacterial activity was obtained by the sample containing Cu-NCs. The rHDPE sample showed much higher viability percentages, as shown in Tables S1 and S2 in the Supplementary Material. Furthermore, the different effects of the Cu-NPs and Cu-NCs can be attributed to their dispersion in the polymeric matrix, as shown in Figure 4. Vásquez et al. [63] studied the antibacterial effect of incorporating ZnO-NPs in recycled polyethylene terephthalate (r-PET) polymeric fibers, and they measured the diameter of inhibition (in mm) for the bacteria E. coli and S. aureus. The antibacterial properties of r-PET were lower than virgin PET, using the same quantity of added ZnO-NPs, suggesting that the recycling process affects the antibacterial behavior of the polymers. However, only the antibacterial properties of virgin HDPE with the Cu-NPs and nano-fibers have been reported [17,[64][65][66]. The authors reported a decrease in CFU (colony-forming units) by increasing the concentration of NPs in the polymeric matrix, which was associated with the release of copper ions that can penetrate the membrane of the bacteria and affect their viability. Shankar et al. [65], Arfat et al. [66], and Chatterjee et al. [67] proposed that the antibacterial effect of Cu-NPs was related to its toxicity to reactive oxygen species. Tamayo et al. [17] reported that the antibacterial behavior depends on three factors: the synergy between the polymer and the Cu-NPs, the capacity of the polymer for long-term ion release, and the dispersion of the Cu-NPs in the polymer matrix. Vásquez et al. [63] studied the antibacterial effect of incorporating ZnO-NPs in recycled polyethylene terephthalate (r-PET) polymeric fibers, and they measured the diameter of inhibition (in mm) for the bacteria E. coli and S. aureus. The antibacterial properties of r-PET were lower than virgin PET, using the same quantity of added ZnO-NPs, suggesting that the recycling process affects the antibacterial behavior of the polymers. However, only the antibacterial properties of virgin HDPE with the Cu-NPs and nano-fibers have been reported [17,[64][65][66]. The authors reported a decrease in CFU (colony-forming units) by increasing the concentration of NPs in the polymeric matrix, which was associated with the release of copper ions that can penetrate the membrane of the bacteria and affect their viability. Shankar et al. [65], Arfat et al. [66], and Chatterjee et al. [67] proposed that the antibacterial effect of Cu-NPs was related to its toxicity to reactive oxygen species. Tamayo et al. [17] reported that the antibacterial behavior depends on three factors: the synergy between the polymer and the Cu-NPs, the capacity of the polymer for long-term ion release, and the dispersion of the Cu-NPs in the polymer matrix.

Conclusions
In this work, the effect of Cu particles on HDPE was studied by a surface analysis, thermo-chemical analysis, mechanical tests, and viability essays.
DSC analysis revealed that the recycling process decreased the melting temperature, being 139.57 • C for virgin HDPE and 132.6 • C for the recycled HDPE. Nevertheless, incorporating Cu-NPs and Cu-NCs increased the melting temperature for the recycled HDPE, reaching almost the melting temperature of a virgin HDPE, being 134.67 • C for recycled HDPE with Cu-NPs, and 136.36 • C for recycled HDPE with Cu-NCs.
Thermogravimetric analysis revealed that the recycling process influenced the initial decomposition temperature, reaching 420 • C for virgin HDPE and 398 • C for the recycled HDPE. This effect was associated with the heating applied during the recycling and pelletizing process. However, the decomposition temperature was not modified at 50 %, being close to 494 • C for virgin HDPE and 490 • C for recycled HDPE. A similar response was determined at 100%, near 524 • C for virgin HDPE and 518 • C for recycled HDPE.
The tensile tests revealed that the properties of the virgin HDPE were degraded by the recycling process but enhanced by adding Cu particles, which can be associated with a modification of complex polymeric chains during the tension processes. Nevertheless, the addition of copper particles increased the tensile strength of the recycled polymers. For instance, the maximum tensile strength was nearly 26.97 MPa for the virgin HDPE, which decreased to 21.51 MPa for the recycled HDPE. However, it was nearly 24.38 MPa with the addition of 0.5 wt.% Cu-NPs and close to 25.39 MPa with the incorporation of 0.7 wt.% Cu-NCs. In addition, the elongation at break increased with the incorporation of copper particles. For example, it was near 104.80 % for the recycled HDPE, 62.41 % with 0.5 wt.% Cu-NPs, and close to 105.87 % with 0.7 wt.% Cu-NCs.
A reduction in the viability for E. coli of 8% was seen in virgin HDPE samples containing Cu-NPs, and the viability in samples with Cu-NCs was lower than 2%. The recycled HDPE samples showed much higher viability percentages, being close to 95% with Cu-NPs and 45% with Cu-NCs. The viability of S. aureus for HDPE with Cu-NPs was lower than 2% and less than 1% with Cu-NCs. However, it was nearly 80% for the recycled HDPE samples with Cu-NPs and close to 75% Cu-NCs. Therefore, the antibacterial activity of the recycled polymer was improved with the addition of copper particles, mainly with the sample containing Cu-NCs, which has a more substantial antibacterial effect against E. coli.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/polym14235220/s1, Table S1: Variation of colonies number and fraction, and viability of E. coli.; Table S2: Variation of colonies number and fraction, and viability of S. aureus.  Data Availability Statement: The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.