Extrusion and Injection Molding of Polyethylene Loaded with Recycled Textiles: Mechanical Performance and Thermal Conductivity

Abstract

The aim of this project was to assess the thermal conductivity of polyethylene (PE) filled with carbon black (CB), specifically for geothermal pipes. The project explored the potential modification of PE’s thermal conductivity by incorporating recycled textile fibers. Different types of shredded recycled fibers were tested, including two types of polyamide fibers with varying contaminations and one type of polyester fiber. Following several preparation steps, various composite materials were manufactured and compared to bulk PE using various testing methods: Differential Scanning Calorimetry analysis (DSC), mechanical testing (flexural and tensile), and laser flash analysis (LFA). The results revealed alterations in the mechanical properties of the composite materials in comparison to PE filled with CB. The LFA tests demonstrated the effectiveness in reducing polymer thermal diffusivity at higher temperatures, particularly when the material was loaded with recycled polyester fillers.

1. Introduction

Among the products made of polymers, pipes are one of the biggest consumers. They can have different purposes, such as being part of an electrical system, protecting electrical wire, or conducting fluid through different distances and conditions [1,2]. In this last application, polymers are chosen for their mechanical properties, thermal insulating properties, and high production rates made possible by the extrusion process. Yet, polymers cannot offer perfect adiabatic barriers. The thermal loss within the pipes is not zero and provokes a loss of energy proportionate to the length of the pipe, the difference in temperature between the inside and the outside of the pipe, and the thermal conductivity of the material. Hence, one of the challenges associated with this application is how to modify the thermal insulating properties of polymers without compromising their mechanical properties.

This question has been investigated in many ways and mostly to decrease the insulating properties of electronic devices. Moreover, different studies have shown the influence of fibers [3] to increase the conductivity of the polymer. Since fibers can decrease thermal conductivity, they could also improve it. Thus, the project was to use textile waste fibers to investigate the thermal conductivity, mechanical properties, and processability of these materials.

Polymers can be differentiated based on various characteristics, including density, structural organization, cost, and other factors.

Table 1. Thermal conductivity for various polymers [4].

Materials	Aggregation Structure	Thermal Conductivity (W/(m.K))	Materials	Aggregation Structure	Thermal Conductivity (W/(m.K))
PPTA	High DoG	0.1197	PAI	A	0.3
PP	SC	0.147	POM	High DoG	0.31
PVC	Low DoG	0.16	PA.6 (Nylon)	High DoG	0.31
PC	A	0.19	PES	A	0.315
PEI	A	0.22	LDPE	Low DoG	0.34
ABS	A	0.226	PE	High DoG	0.48
PEEK	SC	0.25
PTFE	High DoG	0.25	General plastic
PBT	SC	0.274	Engineering plastic
PET	SC	0.29	Special Engineering plastic

displays different types of polymers arranged by increasing thermal conductivity [4]. The colors provide information about the typical applications of the polymer and offer an insight into its cost. The general plastic is shown in green, the engineering plastic in yellow, and the special engineering plastic in orange. General plastics are employed for single-use or low-cost parts, characterized by good processability and a broad range of applications. Engineering plastics represent the “middle class” polymers, featuring good processability and superior specific characteristics compared to general plastics. They are typically more expensive than general plastics. Lastly, special engineering plastics, such as PTFE, are chosen for their unique properties, like excellent aging resistance in highly polluted environments. These are utilized in specific domains like motor parts and come with a higher cost.

With the exception of a few electrically conductive polymers, the majority of polymers lack freely moving electrons owing to their saturated molecular structure. In materials where electron transport is absent, heat flow primarily occurs through phonon or lattice vibrations [4]. Consequently, the low thermal conductivity of bulk polymers is attributed to intrinsic mechanisms influencing phonon scattering. These mechanisms include weak chain bonds, chain twisting, entanglement, and chain ends, as well as extrinsic factors like voids, defects, etc. [5].

When exposed to heat, atoms within any material undergo vibrations. This thermal energy propagates from one point to another through discrete units known as phonons [6]. Phonons represent the quantized energy associated with lattice vibrations, and various mechanisms of thermal transport at the micro- and nano-level can be comprehended through the concept of phonons. Imperfections in the lattice structure lead to phonon scattering, influencing the thermal properties of the material. In the context of polymers, thermal conductivity is predominantly determined by the material’s ability to transfer phonons without encountering scattering. The efficiency of phonon transport directly correlates with a material’s capacity to conduct heat and, consequently, its thermal conductivity. Consequently, reducing thermal conductivity involves disrupting the preferential passage of phonons, essentially inducing phonon scattering. To achieve this, various parameters of the polymer structure can be modified to enhance phonon scattering.

In semi-crystalline polymers, the atoms within the crystals are interconnected, and they exhibit slight vibrations near their equilibrium positions [7]. In semi-crystalline polymers, rapid phonon transfer occurs along the molecular chain. However, challenges arise in forming complete crystals due to factors such as random entanglement, large molecule mass, and polydispersity. Phonon scattering can occur at interfaces, crystal boundaries, and defects. In contrast, amorphous polymers exhibit characteristic scattering regions due to their intrinsic disordered structure and poor chain alignment. The internal structure of polymers in terms of thermal conductivity can be conceptualized as multi-phase materials, considering both crystalline and amorphous phases and the interfaces between them [8]. Therefore, amorphous polymers are more inclined to exhibit lower thermal conductivity [9]. The thermal conductivity varies between two polymers of the same nature but with different molecular chain lengths. Research has shown that the thermal conductivity of polyethylene increases with an increase in chain length [10]. Notably, the beginning and end of the polymer chain should be considered phonon scattering areas where heat transfers are interrupted.

The addition of fillers to the matrix affects phonon scattering due to the characteristics of the fillers, the interfaces (fillers/matrix and fillers/fillers), and the thermally conductive networks formed by the fillers [8,11]. Additionally, the impact of the interfaces may be more significant in terms of thermal conductivity due to interface conductance. Accordingly, the effective thermal conductivity of the composite could be lower than the thermal conductivity of the filler, contingent on the nature of the interface [9]. In the presence of strong bonding, phonon scattering, and local thermal resistance may decrease, consequently improving the thermal conductivity of the composite. In addition to the bond matrix/fillers, the quantity of fillers, and their orientation, the shape and size of the fillers can also influence the thermal conductivity. In fact, rigid cylindrical fillers can hardly disperse isotopically at high concentrations and for thermal conduction, and composites with smaller fillers (larger interfacial area) have severe phonon scattering, resulting in lower thermal conductivity [9].

The literature offers various solutions for producing insulating PE pipes. Foam polymer processing is one such solution that has demonstrated excellent results. This material has the advantage of forming seamless insulating shells, effectively minimizing heat loss at joints and preventing the migration of vapor–air mixtures or droplet moisture through insulated surfaces [4,7,12,13]. Alternatively, a multilayer approach, such as a sandwich structure, presents another solution. This method aimed to maintain the mechanical properties of the already-used PE while introducing additional layers to reduce thermal conductivity [1,14].

This work is distinguished by its innovative approach to repurposing recycled waste from the paper machine industry and polyester filters sourced from Swedish companies. This innovative concept revolves around repurposing end-of-life materials as fillers in PE-based composites specifically designed for geothermal pipes. The potential of these end-of-life materials in the composite industry is substantial, offering versatility in shaping and incorporating them into composite structures. The anticipation is that these materials will bring about significant alterations in the thermal and mechanical performance of the resulting composites. This underscores the critical need to investigate the impact of waste materials as fillers or reinforcements on the properties of the resulting materials. Adopting such a strategy is pivotal for developing sustainable material systems, allowing for the customization of material combinations to achieve desired properties. Studies indicated that a weight ratio of 20% in recycled fibers enables the maintenance or improvement of the mechanical properties of the PE matrix [15]. The objective of this study was to develop multiple PE-based composites by incorporating three types of recycled fibers, namely two different types of polyamide (PA) with different contaminations, as well as polyester waste. Polyamide fabrics were used as a substrate for the printing of lightweight coated paper (LWC), possibly containing traces of sizing agents such as calcium carbonate (CaCO₃) and alkyl ketene dimer (AKD), and they originated from Albany International AB. The PA textiles are specifically designed for challenging environments and are composed of complexly engineered yarn structures. Over time, the felts compact, leading to a reduction in their dewatering capacity, which primarily accounts for their frequent replacement. Additionally, the felts undergo various conditioning treatments that cause wear and tear, resulting in the loss of fibers. Initially white, the felts discolor after processing due to the chemicals involved. For example, the presence of blue in felts can be traced back to mills that use UV light absorbents or blue-whitening agents [16]. Subsequently, the goal was to examine the impact of these fibers on various properties of the PE. Differential Scanning Calorimetry analysis (DSC), mechanical testing (flexural and tensile), laser flash analysis (LFA), and microscopic analysis of the fractured tensile test specimens were conducted on all variations of PE-based composites.

2. Materials and Methods

The polymer used as a matrix was polyethylene (PE) sourced from the Sabic company in Sweden. It was a high-density polyethylene with a density of 959 kg/m³ filled with 2.25% carbon black (CB). The material had a melting temperature of 128 °C and a tensile modulus of 1.05 GPa. Various types of fabrics were collected, including some contaminated fabric waste of polyamide (PA) and some textile fabrics of polyester. The polyester fibers for the experiments came from industrial waste and were supposed to have almost no contamination, while the PA fabrics were more likely to be contaminated.

Table 2. Details regarding polyamide (PA) end-of-life felts.

ID	Position	Mill	Felt	Possible Contamination
360	Third Press	Printing	Seam press point II	CaCO3, cellulose fiber, whitening agent
321	First press	Packaging	Dynavent II	CaCO3 sizing AKD or ASA, cellulose fiber

displays the possible contamination for each PA fabric. Position refers to the stage or location within the production process where a specific felt is used. Mill indicates the section of the mill where the felt is employed. In this case, “Printing” and “Packaging” refer to different parts of the production process. Third Press (Printing) refers to a stage in the printing process, where a felt called “Seam press point II” is used. This felt is likely involved in pressing and dewatering the paper during printing. First Press (Packaging) is part of the packaging process, where a felt named “Dynavent II” is utilized. This felt might be used in the initial pressing phase to remove moisture or to help shape the paper during packaging.

2.1. Preparation of the Composite Specimens

2.1.1. Decontamination Process

Upon receipt, the fabrics underwent an initial cleaning process to eliminate potential contamination. Various methods for cleaning and washing recycled synthetic fibers are documented in the literature. However, the chosen cleaning protocol should be practical for industrial applications. Therefore, methods involving hazardous chemical content and/or extensive manipulations, as well as excessively lengthy cleaning processes, are excluded. Moreover, the aim of the cleaning process is not to establish the ideal fiber–matrix interface but to minimize contamination as much as possible [17]. Therefore, the cleaning process involved washing the textile fabrics with lukewarm water in a continuous flow. Each fabric piece, approximately 100 g in weight, was washed solely by hand or with other fabrics of the same material for a duration of 2 min each. Subsequently, the fabrics were dried in an oven at 70 °C for 24 h. The extended drying time was essential to ensure complete water elimination from the material. The chosen drying temperature is aimed to prevent fiber damage or water boiling. Following the cleaning, all fabrics exhibited a weight loss (see

Table 3. The change in weight of the fabrics as a result of the cleaning process.

End-of-Life Textile	Initial Mass (g)	Mass after Cleaning/ Drying (g)	Weight Loss (%)
PA 321	95.45	93.10	2.5
PA 360	106.80	105.06	1.6
Polyester	102.88	102.23	0.6

), which could indicate contamination quantity.

2.1.2. Shredding

To facilitate proper material blending, the length of the fibers should not exceed 10 to 20 mm, necessitating the shredding of fabrics. A common shredding technique was employed for polyester fabric (Figure 1a). However, in the case of PA types, the presence of long fibers hindered the machine from producing fibers of the required shorter length. To address these challenges, various techniques were explored, ranging from different shredding devices to using a butcher knife. Ultimately, the PA fabrics underwent a two-step shredding process: initially with the primary shredding machine and subsequently with a grinder (Retsch SM100). Once the textile fibers reached the desired short length, they were blended with the PE matrix.

The results after shredding of the PA fibers are displayed in Figure 1b. As shown in Figure 1b, the blue circle represents a small type of PA fiber, while the red circle indicates longer and thicker fibers within the material that could not be shredded further due to their hardness.

2.1.3. Processing

The samples were created using a twin-screw micro-compounder (Micro 15cc Twin Screw Compounder) (which was manufactured by Thermo Fisher Scientific. The company’s headquarters is located in Waltham, MA, USA.) and a microinjection instrument. The injection process enabled the production of tensile samples (dogbone) and bending samples (Figure 2). Twenty wt% shredded textiles were compounded with the PA, followed by injection molding. What is shown in Figure 2 is the dogbone specimen resulting from the compounding and injection of PE and PA. In the case of PE, the shredding process was very tough because the PA polymer fibers were difficult to shred. The PA yarns were also very strong, and some pieces were hard to cut. However, a sieving process was used to select only very fine and short PA fibers for the PE. The presence of some speckles in the final composite was inevitable. Compounding was tested with different times, speeds, and temperatures; however, the results were not visually improved beyond what is shown in Figure 2.

Those devices allowed for good control of the different parameters for such experiments. Additionally, a significant aspect of sample production involved gaining experimental knowledge about the processing of each material to ensure valid samples. One primary parameter to determine was the processing temperature. Although the melting temperature is 128 °C, the PE needed to be melted at 175 °C to enhance production rates and reduce viscosity sufficiently for the injection molding of the samples. This temperature difference between the process temperature and the actual melting temperature of the material can be attributed to the presence of carbon black (~2.14% wt). The addition of any solid fillers increases the viscosity of the material. The melting temperature characterizes the temperature at which the polymer chains have gained enough freedom of movement to flow. Adding fillers to the material creates obstacles to that freedom of movement, requiring a higher temperature to achieve the same viscosity.

Another parameter is the rotating speed of the screw, measured in rounds per minute (rpm). This parameter affects the melting temperature and the structure before the injection process. The higher the rotating speed, the higher the shear rate. In this case, the PE exhibits rheofluidizing behavior, meaning that the viscosity of the material decreases with an increase in the applied shear strength. However, the speed is limited by the high viscosity of the material and the mechanical constraints of the equipment. Therefore, the screw speed must create a shear rate sufficient to achieve a homogeneous melted structure without putting excessive strain on the machine.

The compounding time is a third parameter crucial for ensuring a good final part. This is the time that the material spends in the screw area before the injection process. This parameter influences the homogeneity of the material before injection molding. Too little time will not allow the material to be completely melted or properly disperse the fillers, while too much time will result in a low production rate. Therefore, the right amount of compounding time should ensure a homogeneous structure for well-melted material and be kept as low as possible. Obviously, these three parameters (temperature, screw speed, and compounding time) are interdependent. The selected parameters are detailed in

Table 4. Processing parameters for micro-compounding.

Processing Parameters	Values
Processing temperature	175 °C
Screw speed	50 rpm
Compounding time	3 min
Amount of material	9 ± 1 g

.

The injection machine enabled the control of temperature in both the injection sleeve and the mold. The temperature of the injection sleeve needed to be consistent with that of the compounders, while the mold temperature was set at 30% of the sleeve temperature. Once the injection process was complete, it was crucial to carefully remove the samples from the mold and place them on a flat surface with a weight to prevent any shrinkage during air cooling. Afterward, the samples were separated and were ready for testing. Figure 3 illustrates a schematic representation of the composite production process, starting from textiles and compounding them into polymer composites.

2.2. Characterizations

2.2.1. Laser Flash Analysis (LFA) and Differential Scanning Calorimetry (DSC)

The thermal diffusivity, a crucial property in heat transfer studies, was meticulously determined using laser flash analysis (LFA), produced by NETZSCH Instruments, based in Germany, in strict adherence to the ASTM E1461 standard [18]. This widely recognized standard outlines the procedure for accurately measuring the rate at which heat diffuses through a material. LFA involves generating a short-duration heat pulse onto the specimen’s surface and monitoring the resulting temperature rise on the opposite side. By precisely recording the time-dependent temperature profile, the thermal diffusivity of each material was derived, providing invaluable insights into their thermal behavior. This method, as per the ASTM E1461 standard, ensures consistency, reliability, and scientific rigor in thermal diffusivity assessments.

The comprehensive evaluation of the materials involved in this study was carried out using Differential Scanning Calorimetry (DSC), a powerful analytical technique. The experiments were conducted using a DSC-2000 device manufactured by TA Instruments, based in the New Castle, Delaware, USA. This technique was employed to characterize not only the recycled fibers but also the commercial polypropylene (PP) filled with carbon black (CB). The DSC testing was conducted over a temperature range spanning from −25 °C to 250 °C, providing a thorough exploration of the thermal properties of the materials. This temperature range was carefully chosen to encompass critical temperature events, allowing for the observation of essential parameters such as the melting temperature of polyethylene (PE) and polyamide (PA). The temperature was raised at a rate of 10 °C/min in a nitrogen environment, with nitrogen flowing at 50 mL/min. Data were collected and analyzed using TA Instruments software from the second run.

2.2.2. Mechanical Testing and Physical Properties of the Composites

For the evaluation of mechanical properties, tensile and flexural tests were conducted, following the standards ISO 178 [19] and ISO 14125 [20], respectively. These tests were conducted using a Tinius Olsen H10KT testing machine, manufactured in Horsham, PA, USA.

Microscopic analysis of the fractured tensile test specimens focused on the breakage area of the middle-reduced cross-section for reinforced materials. A single broken sample was selected from the composite filled with shredded fibers and one from PE containing only CB and was then carefully sectioned into small rectangular pieces from the breakage region to ensure a compact fit under the microscope. The analysis employed a Nikon Industrial Microscope Eclipse LV100ND, manufactured by Nikon Corporation, which is based in Tokyo, Japan, connected to a computer for capturing and storing focused images. The microscopy aimed to identify fiber breakage and examine the fracture surface.

The impact of filler inclusion during the extrusion process on the macrostructure of polyethylene (PE) composites was evaluated. The potential introduction of air into the matrix was assessed by examining the macrostructure for the presence of holes. The density of specimens was calculated theoretically and was compared with the actual samples. The results to identify any weight differences were observed between the theoretical and experimental density values. Any observed weight difference was attributed to the presence of air within the composites.

3. Results and Discussion

3.1. Mechanical Properties of the PE-Based Composites

This section discusses the mechanical properties of PE composites filled with carbon black and those filled with shredded textiles.

Table 5. The tensile test results and the average values for composite.

		Elastic Modulus “E” (MPa)	Break Stress (MPa)	Max Strain (Extensometer %)	Max Force (N)
PE filled with CB	Average value	1322 ± 369.3	82.10 ± 8.95	27.30 ± 0.97	656.4 ± 48.7
PE + PA321	Average value	1133 ± 198.5	70.10 ± 3.53	17.7 ± 1.19	560.7 ± 22.78
PE + PA360	Average value	1356 ± 581.35	67.0 ± 4.10	16.7 ± 2.54	536.3 ± 43.83
PE + Polyester	Average value	1594 ± 204.32	77.90 ± 4.14	18.6 ± 2.28	623.0 ± 33.48

displays the average behaviors and confidence intervals (±) for each material and Figure 4 presents the stress–strain curves of each PE composite. The tensile tests revealed similarities in the tensile modulus of the materials. The composites exhibit modulus values ranging between E = 1133 MPa for the PE + PA321 composite and E = 1594 MPa for the PE + Polyester composite. In comparison, the PE filled with carbon black (CB) had a modulus of E = 1322 MPa, nearly equal to the modulus of the PE + PA360 (E = 1356).

The tensile strength of PE (polyethylene) filled with carbon black showed the highest value, reaching 82.10 MPa. The composites filled with shredded textiles exhibited tensile strengths ranging between 67 and approximately 78 MPa, demonstrating lower strength compared to the polymer loaded with carbon black. This is attributed to shredded textiles acting as impurities, leading to points of crack initiation and fracture. This can be supported by the microscopic images (Figure 5). The tensile tests resulted in specimen breakage. The difference in rupture profiles between the composite materials and the PE highlighted the influence of the shredded textile fibers. As depicted in Figure 5, the rupture profile of the PE filled with CB displays a clean break (Figure 5b), whereas the loaded composite exhibits a discontinuous profile (Figure 5a), which includes both fiber breakage and pull-out.

However, there were significant differences in maximum strain values. The composite values were consistent around 17 ± 1% but lower than that of PE-filled CB (max strain (PE) = 27.28%). The maximum strain values indicated a gap of approximately 10% strain between the composites and PE (loaded with CB).

The test results revealed a distinct contrast between the composite materials and PE. These differences may be attributed to the relatively weak interfacial bond between the matrix and the recycled fibers, as well as the existence of internal defects within the matrix, such as air pockets. Indeed, these outcomes were anticipated, given the lack of functionalization at the interfaces between the fibers and the matrix [9].

The flexural test provided crucial mechanical insights into materials subjected to bending stress. Standard testing procedures [19] were adhered to, with specimens undergoing testing up to a maximum deflection of 25 mm, as displayed in Figure 6. There was no complete breakage due to the high ductility of the materials during the tests.

The flexural modulus of the reinforced PE composite materials ranged from 1245 MPa (PE + Polyester) to 1265 MPa (PE + PA321), while the modulus of the PE/CB was 903 MPa (

Table 6. The average bending properties of the composites.

	Flexural Modulus of Elasticity (MPa)	Maximum Force (N)
PE filled with CB	904 ± 50.18	44.15 ± 2.03
PE + PA321	1265 ± 54.48	57.80 ± 3.09
PE + PA360	1235 ± 119.90	55.5 ± 3.43
PE + Polyester	1245 ± 77.78	55.50 ± 3.77

). This pattern is also evident in the maximal bending force values, with composite materials loaded with the shredded textile reaching a maximum force of approximately 55.00 N, surpassing the PE’s maximum bending force of 44.55 N. The ± values in the table represent the confidence intervals.

3.2. Physical Properties of Composites (Variations of Density)

The inclusion of fibers during the extrusion process has the potential to introduce air into the matrix. This aeration manifests as holes, and the presence of such defects in the macrostructure can significantly diminish the mechanical properties. To assess the extent of additional air in the specimens, the density of samples from the bending test was measured and compared to the theoretical density provided by Equation (1).

$$\mathsf{\rho }={\displaystyle \frac{1}{\frac{M_m}{{\mathsf{\rho }}_{\mathrm{matrix}}}+\frac{M_f}{{\mathsf{\rho }}_{\mathrm{fibers}}}}}$$

(1)

Consider (ρ) as the density of the composites, and (M_m) and (M_f) as the respective percentages of the matrix and fibers within the composite, where the composite was loaded with 20% wt of fibers. The densities of the matrix and fibers were denoted as ρ(matrix) = 959 kg/m³, ρ(PA fibers) = 1084 kg/m³, and ρ(polyester fibers) = 1156 kg/m³.

Upon comparison of the theoretical density calculated using the equation with the experimental values, a slight difference in weight was observed, as exhibited in

Table 7. Density measurements of the composite systems.

Composites	Density (kg/m3)
	Theoretical	Experimental	Weight Discrepancy (%)
PE + PA 321	981.60	978.97	0.27%
PE + PA 360	983.80	971.81	1.21%
PE + Polyester	992.80	972.99	2.00%

. This weight difference may be attributed to the presence of air within the composites.

3.3. Thermal Diffusivity and the Thermal Behaviors of the PE Composite Materials

The melting temperatures of the raw materials, before undergoing processing into composites, were measured using Differential Scanning Calorimetry (DSC) equipment. Some specific temperatures, such as the melting temperature of polyester (Tm = 255 °C) and the glass transition temperature of PE (T_g = −110 °C), exceeded the scale range, leading to the reporting of only the melting temperatures in this analysis. The DSC analysis of the polyamides (PAs) indicated comparable melting temperatures, with differences within ±1 °C. As DSC analysis does not induce any chemical changes, it is normal to observe no alterations in the melting temperatures. The melting temperatures for PE, PA321, PA360, and Polyester were recorded as 131.5 °C, 222.1 °C, 220.9 °C, and 255.0 °C, respectively.

The measurement of thermal diffusivity using the flash method entails exposing the surface of a flat sample to a brief pulse of heat flux, often referred to as a flash. The subsequent observation involves tracking the change in temperature over time at one or more specific points on the sample. The outcome of this process is referred to as a thermogram. To determine the thermal diffusivity, the experimental thermogram is subsequently compared to a theoretical model. This comparative analysis serves to ascertain the thermal diffusivity of the material under investigation [21]. The thermal diffusivity results are presented in

Table 8. The laser flash analysis (LFA) test results, measuring the thermal diffusivity for each composite material.

PE		PE PA360		PE PA321		PE Polyester
Temperature	Diffusivity	Temperature	Diffusivity	Temperature	Diffusivity	Temperature	Diffusivity
(°C)	(mm2/s)	(°C)	(mm2/s)	(°C)	(mm2/s)	(°C)	(mm2/s)
25.6	0.216	25.3	0.231	25.3	0.229	25.3	0.225
60.2	0.178	59.9	0.183	60.1	0.182	60	0.178
89.2	0.144	90.1	0.139	89.7	0.147	90	0.136

and Figure 7.

At 25 °C, the thermal diffusivity of the composites containing the shredded textiles (average value = 0.228 ± 0.003 mm²/s) exceeded that of PE (0.216 mm²/s). However, this difference diminished at 60 °C, with a common thermal diffusivity value of around 0.180 mm²/s ± 0.002. At 90 °C, the thermal diffusivity became more varied among the different materials, ranging from 0.136 mm²/s for PE + Polyester to 0.147 mm²/s for PE + PA321. Despite PE exhibiting a lower thermal diffusivity at room temperature, it became evident that the polyester PE composite had the lowest thermal diffusivity as the temperature increased. Incorporating fibers into the matrix should theoretically induce phonon scattering effects if the fibers are more insulating than the matrix, especially when the interfacial bond is weak [7]. The thermal diffusivity of the loaded composites exhibited different values compared to the PE polymer. The inclusion of polyester textile fillers into PE was more effective in decreasing thermal diffusivity, particularly at higher temperatures, specifically at 90 °C.

4. Limitations of This Current Study and Future Research Directions

While this study provides valuable insights into the use of recycled textile fibers in PE composites for geothermal pipes, there are several limitations and areas for future research. The findings revealed a weak interfacial bond between the shredded textiles and the PE matrix, which impacted the mechanical properties of the composites. Additionally, the introduction of air due to the textile fillers and the lack of functionalization at the interfaces were identified as factors affecting the overall performance. Future research should focus on improving the interfacial bonding between the fillers and the matrix, exploring methods to minimize the introduction of air, and investigating the functionalization of recycled fibers to enhance their compatibility with the PE matrix. These improvements could lead to more effective and efficient use of recycled materials in composite applications.

The comparison between carbon particles and textile fibers highlights a critical point about the relative effectiveness and economic feasibility of using different types of fillers. The primary motivation for this study was to explore the potential of using recycled textile fibers as a “sustainable alternative” to conventional fillers like carbon black. While it is true that carbon particles generally offer better performance in terms of mechanical properties and thermal conductivity, our study aimed to investigate the feasibility of repurposing waste materials to contribute to environmental sustainability. The economic feasibility of recycling raw fabrics for reinforcement does present challenges. However, our research underscores the importance of integrating recycled materials into composite applications to reduce waste and promote circular economy practices.

There have been many studies on improving interfacial adhesion between different types of polymers [22,23]. However, this has largely led to enhanced thermal conductivity in polymer systems, which is contrary to the objective of this work, where the goal was to lower the thermal conductivity. Furthermore, the removal of air due to enhanced adhesion is expected to improve thermal conductivity, not reduce it. The removal of air will likely involve a trade-off between reducing thermal conductivity and improving mechanical performance and durability.

5. Conclusions

These high-density PE pipes offer lightweight, long life, excellent impact resistance, resistance to large deformations, good hydraulic properties, and corrosion resistance. Additional thermal insulation is crucial for optimizing heating and cooling systems, potentially leading to significant energy savings. This study focused on exploring the thermal conductivity and mechanical performance of polyethylene (PE) intended for geothermal pipes by incorporating recycled textile fibers. In the first place, recycled/waste fabrics were shredded into small pieces in such a dimension that was suitable for extrusion or injection molding processes. Subsequently, these materials were incorporated into the PE thermoplastic system using a compounder. Eventually, the PE loaded with shredded textiles was extruded and then injection molded. The findings indicated changes in the mechanical characteristics of the composite materials compared to the PE filled with CB. These alterations result from a weak interfacial bond between the shredded textiles and the matrix, as well as the introduction of air due to the inclusion of textile fillers, as indicated by the density calculations for different composites. Additionally, the LFA tests illustrated the efficiency of reducing polymer thermal diffusivity, especially at elevated temperatures, particularly when incorporating recycled polyester fillers into the material. These observed effects of incorporating shredded textiles may be attributed to factors such as the mass fraction of recycled textile fillers, the type of fillers utilized, and the lack of functionalization in the interfacial bond.