Manufacturing and Recycling of 3D-Printed All-Polymer Composites

Abstract

The reinforcement of polymers with carbon or glass fibers is the reason for their incredible success as ideal lightweight construction materials. However, one challenge with these materials is their recyclability. True recycling, meaning achieving the same performance level as virgin material, is impossible, especially with mechanical recycling processes, because the reinforcement structure is destroyed. Additionally, thermoplastics undergo molecular degradation and changes in the properties of the materials. Therefore, polymer fiber-reinforced plastics may have an advantage here, as polymer fibers are much more flexible than glass or carbon fibers. We investigated the production and recyclability of microfibrillar composites (MFCs) made of polypropylene (PP) and polyethylene terephthalate (PET). The samples were produced using extrusion-based 3D printing with different parameters, and their morphology and mechanical properties were examined. The samples were crushed, and the residue was fed back into the production line. The process was repeated with the samples produced from regenerate. The results prove that the printing process can be controlled to ensure the presence of fibers in samples made from recycled material. However, it is important to note that the mechanical properties decrease with each additional processing cycle. The choice of manufacturing parameters, especially in 3D printing, is crucial for achieving good properties.

1. Introduction

Petroleum-based materials such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) serve as single-use plastics (SUPs) and are frequently used in the packaging industry due to their attractive properties such as lightweight, affordable prices, and simplicity of manufacturing. Nonetheless, the main environmental concern associated with petroleum-derived plastic lies in the disposal of end-of-life, which typically results in landfills or incineration. The inappropriate disposal of those plastics has been associated with significant environmental issues, including greenhouse gas (GHG) emissions, global warming, and contamination of terrestrial and marine ecosystems. To address the current plastic waste crisis, the European Union (EU) has directed that single-use plastics should be reduced, reused, and recycled [1,2]. Indeed, recycling is considered one of the most effective waste management strategies for conserving resources and minimizing the impact on the environment, as evidenced by several studies [3,4,5]. In addition, the recycling approach presents economic advantages such as savings on expenses in waste management and potential financial creation by the marketability of recovered products. In industry, the processes of thermoplastic waste recycling include mechanical, chemical, and energy recycling [6]. The most common method of thermoplastic recycling is mechanical recycling, which involves a series of steps such as sorting, washing, drying, grinding, and compounding [7]. However, the effectiveness of the recycling strategy is dependent on the physical structure of materials, processing conditions, and the number of reprocessing cycles. These factors mainly influence the mechanical, thermal, and rheological performances of the final products, as stated in several literature reviews [8,9].

Fiber-reinforced plastic (FRP) technologies have received growing attention over the past decades. This method produces a composite material made from a polymer matrix reinforced with fibers. In addition, synthetic reinforcements such as glass fibers, carbon fibers, and aramid fibers are commonly employed in fiber-reinforced composites [10]. Unfortunately, these inorganic fibers significantly raise greenhouse gas emissions, which contribute to climate change throughout their lifespan, including raw material extraction, processing, manufacturing, usage, and disposal [11]. Moreover, these fibers also limit the potential for recycling since the reinforcing effect is dependent on fiber length, which is unavoidably shortened during mechanical recycling in screw machines, as exposed by various researchers [12,13].

One interesting class of polymer–polymer composites known as microfibrillar-reinforced composites (MFCs) has been proposed as a viable solution for fiber-reinforced plastics in the upgrading and manufacturing of high-performance thermoplastics as well as a more sustainable material [14]. The essential steps of this technology are as follows [15]:

• The mixing step is the melt blending of immiscible polymers at a processing temperature higher than the melting temperature of both components.
• The fibrillation step is a hot or cold drawing of the polymer blends to facilitate the highly oriented molecule structures. In this stage, the draw ratio is an important factor in determining the fiber morphology (e.g., shape, length, and diameter) and aspect ratio of fibers [16,17].
• The isotropization step is the thermal treatment of the microfiber blends at a processing temperature higher than the melting temperature of the polymeric matrix but lower than the melting temperature of the reinforcing component to create an isotropic matrix while the second component remains in fiber form.

Since the temperature plays a critical role in this technology, the requirements to select the blend components are the following: the melting temperatures of each component must differ by at least 40 °C to ensure fiber preservation during matrix consolidation, both components must be processed at a single temperature without degradation in either component, and both components must be sufficiently drawable to allow fiber formation [14]. Based on those processes, the microstructure of the fiber, which determines the performance of the composite, is dependent on various parameters such as the composition ratio, processing conditions, viscosity ratio between the components, and draw ratio [18,19]. The most commonly studied MFC system is the combination of engineering polymers with the commodity thermoplastic polyolefin, due to their accessibility of manufacturing and low cost [17]. Correspondingly, PET and PP are the materials most frequently encountered in waste streams and thus the most available feedstock for this system.

In addition, fused filament fabrication (FFF) is the most widely used three-dimensional printing technique for thermoplastics, in which a thermoplastic filament is extruded through a heated nozzle and deposited semi-molten filament layer by layer in a predetermined path [20,21]. This technology offers multiple advantages, including cost-effectiveness, design flexibility, the availability of recycled materials, and application in a wide range of applications. This methodology additionally provides opportunities for fiber-reinforced polymer composites where there is excellent local control of the reinforcing component within the composites [22,23]. The potentiality of fiber alignment, which was determined by the elongational and shear flow characteristics in the 3D printing nozzle, has a significant impact on the performance of 3D-printed products [24,25]. The shear and elongational flow were primarily influenced by the 3D printing nozzle geometry and printing parameters, including nozzle temperature, printing direction, layer height, and printing speed, particularly mass flow rate. Simultaneously, the printing parameters influence the weld seams between distinct strands and layers, a crucial factor in assessing the quality of 3D-printed components [26]. Given these considerations, integrating fused filament fabrication (FFF) with microfibrillar composite (MFC) technology and recycled feedstocks offers a promising pathway to enhance mechanical performance through in situ fiber orientation while addressing the urgent demand for sustainable material utilization. This synergy enables improved weld properties, material circularity, and reduced environmental impact.

This study investigated the recyclability of MFCs with respect to their mechanical properties. As the reinforcement phase is essentially decisive for the structural properties of a composite material, the focus was placed on possible changes in the shape, length, and diameter of the reinforcing fibers. The samples were mechanically recycled as is standard practice for thermoplastics. After the components had been shredded, filaments were produced gently in the single-screw extruder, i.e., at the lowest possible temperature. Two scenarios were selected for printing:

(1)

As gently as possible, i.e., slowly at a low temperature;

(2)

Optimal for support adhesion and thus transverse tensile properties, i.e., fast at a higher temperature.

2. Results and Discussion

2.1. Morphology

As can be seen in Figure 1, the fracture surface of the filament produced from virgin material (F0) perpendicular (-|) to the extrusion direction shows a two-phase system with finely distributed circular inclusions in a coherent phase. The fracture surfaces parallel to the extrusion/stretching direction (||) show that these inclusions are fibrous. After etching out the polypropylene, only a huge bundle of fibers is visible. The fibers are therefore PET fibers with diameters well below those of technically relevant glass or carbon fibers.

The picture changes only gradually after 3D printing. Under the low-temperature printing scenario (L), the inclusions are less evenly distributed and appear larger overall. However, as the parallel fracture surface shows, these are predominantly fibers. Elliptical PET particles are also visible. Under high-temperature conditions (H), there are no significant differences in the appearance and distribution of the fibers compared to the filament. However, the fracture surface in the coherent matrix is terraced, probably due to different sample geometry compared to the thin filament, as well as additional thermo-mechanical stress during printing.

The picture changes significantly in the first recycling cycle (see Figure 2). While the PET inclusions in the filament cross-section (F1-|) remain homogeneously distributed, the longitudinal section (F1||) appears to contain significantly shorter and thicker fibers, i.e., inclusions with a significantly lower aspect ratio that are almost spherical. This trend towards shorter, thicker inclusions is reinforced by 3D printing in the low-temperature scenario (L), where a poorer distribution similar to that of printing with virgin material filament can also be observed. The picture is completely different in the high-temperature scenario (H). Here, the overview images show an abundance of fibers, i.e., no difference to hot-drawn filaments made from virgin material (F0) or printed samples of these filaments under the same conditions (P0H). However, the individual fiber images reveal a difference. The fibers produced with virgin material have a uniform cross-section over the fiber length. In contrast, fibers produced from recycled material exhibit variations in diameter in certain areas and substantial thickening at the fiber ends. These fibers were not produced by hot stretching, but by melt deformation processes during 3D printing. Therefore, the appearance of the fibers is due to stretching and recovery processes during 3D printing.

The second recycling loop shows exactly the same qualitative picture as the first recycling cycle. The fibers in the filament (F2) and in samples printed under low-temperature conditions (Figure 3, P2L) are comparatively short and thick. They also have varying diameters along their length. In contrast, the fibers in samples printed under high-temperature conditions are thin and long. However, these fibers also exhibit differences in diameter along their length, as indicated by the red arrows.

The characteristic dimensions of the PET component, including fiber length, diameter, aspect ratio, and mean single-fiber volume, were determined from the images shown in Figure 1, Figure 2 and Figure 3. The results are listed chronologically in

Table 1. Characteristic dimensions of the PET component in the composites.

Designation	Fiber Length		Fiber Diameter		Aspect Ratio	Single Fiber Volume	PET in Fibrous Form
	µm		µm		1	µm3	vol.-%
	M	SD	M	SD	M	M	M
F0	1633	268	0.74	0.26	2207	702	30
P0L	800	407	1.07	0.57	748	719	30
P0H	1816	329	0.63	0.46	2883	566	30
F1	958	360	1.87	0.26	512	2630	7.4
P1L	337	104	3.83	1.52	88	3881	6.9
P1H	1667	240	0.69	0.31	2416	623	28
F2	907	166	2.00	0.56	454	2848	4.3
P2L	251	91	4.67	2.04	54	4297	4.0
P2H	1644	191	0.82	0.47	2005	868	27

according to the processing steps. The data already reveals an interesting phenomenon in the first processing cycle (F0, P0). The initial fiber length in the filament changes significantly during 3D printing, i.e., after compounding and hot stretching. Under low-temperature printing conditions (L), the fibers shorten and increase in diameter. After printing under high-temperature conditions, even slightly longer but thinner fibers are found in the sample. However, the calculated average single-fiber volume remains almost consistent in cycle 0, regardless of the printing conditions. Therefore, it can be concluded that hot-stretched fibers shrink under low-temperature printing conditions and are slightly stretched and cut under high-temperature conditions.

In the first recycling loop (F1, P1), the average fiber length in the single-screw extruder decreases by around 30%. At the same time, the fiber diameter increases by a factor of 2.5. The increase in mean single-fiber volume by a factor of 4.6 is particularly noteworthy. This suggests that during filament production in the single-screw extruder, the fibers melt at relatively low extrusion temperatures and that dispersed PET islands partially flow together, i.e., coalescence occurs. In samples printed under L conditions, the average fiber length is two-thirds shorter than the fiber length in the filament. At the same time, the fiber diameter increases so much that the mean single-fiber volume is one third larger than that of the filament, which also indicates coalescence. Overall, these samples have an average volume fraction of only 7% PET in fibrous form. The remaining 23% are homogeneously distributed in the polypropylene matrix as predominantly elliptical inclusions. A different picture emerges when filaments made from recycled material are printed under H conditions. In this case, all fiber geometric parameters are at the same level as the extruded filaments made from virgin material, and thus also at the same level as the samples printed from virgin filaments under H conditions.

In the second recycling loop (F2, P2), the situation is almost identical to that in the first recycling cycle (F1, P1), albeit with a slightly stronger tendency towards coalescence. This is probably due to a change in viscosity caused by thermal/mechanical degradation in the plastics.

2.2. Mechanical Properties and Fracture Behavior

Figure 4 shows the mechanical properties of the test specimens printed with the two MFC components, PP and PET, individually, as well as with the MFCs after different processing runs.

The following trends can be observed:

• The MFC values, in general, lie between the PP and PET component values. In the best case, a stiffness level like that of a compact PET sample can be achieved with only 30% PET by weight in PP. The reinforcing effect of the fibers is evident.
• The properties of the MFCs depend on the number of processing runs and the process conditions during 3D printing.
• Generally, the mechanical properties decrease as the number of processing cycles increases. Initial viscosity measurements (see Figure S1) indicate a significant decrease in viscosity with each recycling cycle. This is likely due to thermo-oxidative degradation. Further investigation is needed to determine which polymer undergoes this degradation. Then, the virgin material can be stabilized specifically for mechanical recycling.
• The properties parallel to the printing direction are generally significantly higher than those perpendicular to it. Since fiber orientation is determined by the printing direction (please see Figure 1, Figure 2 and Figure 3), these results confirm the reinforcing effect of the PET component’s fiber shape.
• The properties of samples printed under H conditions are significantly higher than those printed under L conditions. For example, with an MFC consisting of 30 wt.% PET and 70 wt.% PP, stiffness can be achieved at the level of samples printed entirely from PET, when the testing orientation aligns with the fiber direction.
• PET that is not in fibrous form contributes slightly to increasing stiffness when the load is applied parallel to the longitudinal axis of the PET ellipses. Perpendicular to the longitudinal axis of the ellipses, all mechanical properties decline compared to pure PP.

The SEM pictures of the fracture surfaces shown in Figure 5 confirm that all components manufactured under low-printing conditions (L) have significantly thicker fibers/PET inclusions than samples generated under high-printing (H) conditions. In addition, these images reveal a significant number of holes, which can be attributed to fiber or particle pull-out. The comparatively low mechanical properties of the samples produced under L conditions are therefore due to the quality of the fibers and the fiber/matrix adhesion, as well as the low fiber volume fraction. Furthermore, these images demonstrate a significant number of holes, which can be attributed to fiber pull-out. The comparatively low mechanical properties are therefore also due to weak fiber/matrix adhesion.

Irrespective of the preceding history, the samples printed under H conditions exhibit a high fiber density in the images, which in this view obscures the view of the fracture surface. However, the presence of visible fibers indicates that the fracture process is predominantly characterized by matrix fracture and fiber pull-out. It is only at high magnification that the individual fibers appear to be broken.

3. Materials and Methods

3.1. Materials

A commercial polypropylene (PP, HD601CF) with a melt flow rate (MFR) of 8.0 g/10 min (230 °C, 2.16 kg) was supplied by Borealis Polyolefine GmbH, Linz, Austria, and poly (ethylene terephthalate) (PET, Lighter^TM C93) with an intrinsic viscosity of 0.80 dl/g was provided by Equipolymers GmbH, Schkopau, Germany. PP and PET in this work were introduced as a polymeric matrix and reinforcing component, respectively.

3.2. Filament Preparation

PET pellets were dried in the oven at 80 °C for 18 h. PP was mixed with dried PET in a weight ratio of 70/30. The composites were compounded on a twin-screw extruder (ZSE 18 MAXX, Leistritz AG, Nuremberg, Germany). The temperature profile was 120/170/220/245/260/260/260/260/240/235/230 °C from the hopper to the die. The screw rotation speed was set at 120 rpm. The filament’s diameter was reduced from 4.5 ± 0.10 mm to 1.75 ± 0.10 mm by continuous stretching over a 110 mm distance between the extruder die and the water bath inlet. Shortly after exiting the die, the filament’s surface temperature was 230 °C; at the inlet to the water bath, it was 210 °C. The stretched filament was defined as F0, which served as a baseline for comparison to the properties of the reprocessed filament. The draw ratio ($DR$), which affects the shape, length, diameter, and aspect ratio of fiber, was calculated by using the following equation:

$$DR={\displaystyle \frac{A_0}{A}}={\displaystyle \frac{\pi \times {\frac{D_d}{4}}^2}{\pi \times {\frac{D_f}{4}}^2}}$$

(1)

where $A_0$ is the cross-section area of the filament at the die, and $A$ is the cross-section area of the filament after stretching.

To analyze the influence of recyclability on the fiber microstructure and ultimately determine the characteristics of the final product, the F0 filament was used to prepare the specimen using an FFF 3D printer (Ultimaker 2 Extended, Ultimaker B.V., Geldermalsen, The Netherlands). The nozzle and platform temperatures were set at 260 °C and 100 °C, respectively, with a layup speed of 30 mm/s. To avoid issues about the warpage of printed samples, which is an important characteristic of semi-crystalline thermoplastic, the first layer of the specimen was printed with a brim on PP tape (Tesa 64, 014). Subsequently, the printed specimen was shredded into granules and dried in the oven at 80 °C for 18 h. Those shredded granules were finally reprocessed with a single-screw extruder (EX6, Filabot, Barre, VT, USA) to produce a printable filament with a diameter of approximately 1.75 ± 0.10 mm, designated F1. To preserve the fiber post reprocessing, the barrel temperatures were established below the melting temperature of the reinforcing component at 60, 160, 230, and 235 °C, with a screw speed of 5.0 rpm. The summary of processing conditions is listed in

Table 2. Summary of the processing conditions.

Designation	Extruder	Temperatures	Screw Speed	Feeding Rate	Average Diameter of Filament at the Die	Average Diameter of Stretched Filament	Draw Ratio
		-	-	-	${\mathit{D}}_{\mathit{d}}$	${\mathit{D}}_{\mathit{f}}$	$\mathit{D}\mathit{R}$
		°C	rpm	kg/h	mm	mm	-
F0	Twin-screw extruder	120/170/220/245/260/260/ 260/260/240/235/230	120	3.0	4.6	1.65	8.0
F1	Single-screw extruder	60/160/230/235	5.0	0.08	2.4	1.65	2.1
F2	Single-screw extruder	60/160/230/235	5.0	0.08	2.4	1.65	2.1

. The entire procedure was reiterated using identical processing and printing conditions, with the second recycled filament labeled as F2. The production line used in this work is illustrated in Figure 6.

3.3. Sample Preparation

The samples were printed on the FFF 3D printer (Ultimaker 2 Extended, Ultimaker B.V., Geldermalsen, The Netherlands) with a commercial E3D V6 nozzle with an inlet diameter of 2.0 mm, an outlet diameter of 0.8 mm, and a length of the conical area of 1.04 mm in the longitudinal direction (0° infill line direction) to study the impact of the reinforcing effect and the interfacial adhesion between the reinforcing component and polymeric matrix on the performance of the samples. The printing sequence started with the printing of an outer wall (shell), which was followed by a one-directional filling of the inner infill as shown in Figure 7. The samples were printed with two different sets of parameters as listed in

Table 3. Summary of the 3D printing parameters.

Designation	Nozzle Temperature	Platform Temperature	Mass Flow Rate	Contact Temperature on the y-Plane	Contact Temperature on the z-Plane	Layup Speed	Layer Height
	${\mathit{T}}_{\mathit{n}}$	${\mathit{T}}_{\mathit{p}}$	$\dot{\mathit{m}}$	${\mathit{T}}_{\mathit{c}\mathit{o}\mathit{n}\mathit{t}\mathit{a}\mathit{c}\mathit{t},\mathit{y}}$	${\mathit{T}}_{\mathit{c}\mathit{o}\mathit{n}\mathit{t}\mathit{a}\mathit{c}\mathit{t},\mathit{z}}$	$\mathit{v}$	${\mathit{L}}_{\mathit{h}}$
	°C	°C	g/h	°C	°C	mm/s	mm
L	245	100	3	172.5	172.5	10	0.1
H	260	100	9	192.9	177.5	30	0.1

: L and H. The L set was chosen based on the assumption that PET fibers are better preserved at low processing temperatures, allowing optimization of the fiber-parallel properties in the composite. The H set was chosen to optimize perpendicular properties, as good adhesion between the printed strands was anticipated. With the lower nozzle temperature, continuous melt discharge from the printer was barely possible. The nozzle temperature was adjusted in line with the printing speed to ensure that the contact temperature exceeds the melting temperature of polypropylene [27], allowing sufficient time for molecular diffusion [28] to ensure a strong bond between the strands and layers. To address warpage limitations, the initial layer of the sample was printed with a brim on PP tape.

In addition, neat materials, including PP and PET, were also printed for comparison purposes. The PP samples, described as P0-PP, were printed at nozzle and platform temperatures of 245 °C and 100 °C, respectively, and at a layup speed of 10 mm/s. While the PET samples marked as the P0-PET were prepared using a nozzle and platform temperature at 260 °C and 100 °C, respectively, and a layup speed of 10 mm/s.

3.4. Morphological Observation

The cryogenic fracture surfaces in the cross-extrusion and extrusion directions were examined by immersing the filaments and printed samples in liquid nitrogen. To clearly investigate the microfiber structure of the composite, the filaments and printed samples at different processing cycles were etched with xylene at 130 °C for 1 h to eliminate the polymeric matrix. The fracture surface, as well as the etched filament and printed sample morphology, was imaged using a laser scanning microscope (LSM, VK-X1050, Keyence Deutschland GmbH, Neu-Isenburg, Germany). The geometric dimensions of the fibers were determined from twenty microscopic images of at least two etched filaments/samples, and the mean values and standard deviations were calculated from these. The measurement was performed using the application software developed by Keyence for multi-file analysis. Following the micro-tensile testing procedure, the fracture surfaces of the specimens were subjected to investigation by means of a scanning electron microscope (SEM, SU8000, Hitachi, Japan).

3.5. Micro-Tensile Testing

To determine the tensile properties, dumbbell-shaped samples were cut from the 20 × 20 × 1 mm³ printed samples, as illustrated in Figure 7. The tensile specimens were tested using a micro tensile testing machine (Tension & Compression Module, Kammrath & Weiss GmbH, Dortmund, Germany) at a crosshead speed of 10 μm/s. The initial gauge length was L₀ = 5 mm. The tensile modulus was calculated using the slope of the stress–strain curve between 0.5% and 1.5% strain. Then, the surface morphology of the fracture surface was examined using a laser scanning microscope. At least five samples were tested, and the mean values and standard deviation were calculated.

4. Conclusions

• Microfibrillar polypropylene/polyethylene terephthalate composites (MFCs) can be recycled from 3D-printed components using the same mechanical recycling processes employed for thermoplastics. However, even under favorable printing conditions, each new processing cycle is associated with a decrease in properties in the direction of the fibers and perpendicular to them. This phenomenon is most likely due to the plastic material undergoing thermal/mechanical degradation. It is therefore vital that virgin material be subjected to adequate stabilization prior to undergoing several recycling cycles. As the phenomenon of sample failure is essentially characterized by fiber pull-out and matrix breakage, there is potential for optimization at the fiber/matrix interface. The utilization of compatibilizers may offer a solution, although their impact on processing must be duly considered.
• Contrary to the working hypothesis, however, the PET fibers from the shredded samples are destroyed when filaments are produced again for 3D printing by extrusion. Therefore, from this point of view, the thin and tough PET fibers offer no advantages over the much thicker, brittle glass or carbon fibers. However, under certain conditions, flow conditions are realized in the printer nozzle that transform PET domains formed from the coalescence of broken fibers back into fiber form. Producing MFCs in a nozzle without the detour of stretching represents an extremely time- and cost-efficient new method of producing polymer fiber-reinforced components. This processing strategy facilitates the optimization of the potential inherent in plastics, thereby enabling the minimization of production resources and the enhancement of material and environmental efficiency.