A Systematic Review on the Technical Performance and Sustainability of 3D Printing Filaments Using Recycled Plastic

Abstract

Over the past 40 years, global plastic production has increased twenty-fold, prompting efforts to mitigate plastic waste. Recycling has emerged as the predominant strategy for sustainable plastic waste management. As additive manufacturing (AM) continues to evolve, integrating recycled plastics with various additives has gained significant attention. This systematic literature review, conducted in full accordance with the PRISMA guidelines, aims to evaluate and compare the properties and effects of recycled plastics and their additives in AM. Specifically, it examines the thermal, mechanical, and rheological properties of these materials, along with their life cycle environmental and economic implications. A total of 88 research publications, spanning from 2013 to 2023, were analyzed. The databases searched include Scopus, Web of Science, ProQuest, and Google Scholar, with the final search conducted in December 2023. Studies were selected through a four-stage process—identification, screening, eligibility, and inclusion—based on predefined inclusion and exclusion criteria. The risk of bias was assessed using five criteria: credibility, scope, clarity, methodology, and analysis quality. The results show that most research focuses on the mechanical properties of recycled plastics, with significant gaps in understanding their thermal and rheological properties. Additionally, there is limited research on the environmental and economic viability of these materials, highlighting the need for integrated life cycle assessments and eco-efficiency analyses. This review offers additive manufacturing professionals a comprehensive understanding of the thermal, mechanical, and rheological performance of recycled plastics and additives, supporting efforts to improve sustainability in the industry.

1. Introduction

The emergence of plastic materials in the early 20th century went through a groundbreaking development with the production of the first synthetic polymer, Bakelite, which was initially used in household goods such as telephones and radios [1]. The versatility and ubiquity of plastic products contributed to a massive increase in plastic production [2], from 30 Mt in 1988 to 270 Mt in 2010, then reaching 368 Mt in 2019 [3]. Figure 1 shows the global plastic production capacity until 2021, revealing a consistent upward trend in the annual production of plastic. Such exponential growth highlights the escalating concerns regarding its associated impacts.

The extensive reliance on disposable products has resulted in substantial volumes of plastic waste, around 80% of which ends up in natural environments or landfills [5]. As a result, plastic debris has been identified as a primary global conservation concern, adversely impacting soil quality, marine ecosystem, and human health [6,7]. Additionally, it is crucial to recognize that the production of plastic materials heavily relies on petrochemical resources, which are finite and destined for depletion [8]; the manufacturing of virgin plastics accounts for approximately 4% of global oil production [9].

Moreover, plastic production contributes to the growing release of greenhouse gases, exacerbating global environmental challenges. Persisting on the current trajectory of plastic emissions will contribute 15% of the worldwide carbon budget by 2050 [10]. Plastic pollution has reached a decisive phase, urging academia, industry, and society to collaborate with insistence and highlighting the necessity to develop practical and prompt solutions [11]. To mitigate this problem, recycling plastic waste has emerged as a pivotal and sustainable method to alleviate landfill burdens and conserve raw materials. According to Tulashie et al. (2020), between 1950 and 2020, a staggering 6.8 billion tons of plastic waste were generated; however, only 10% were recycled and 15% were incinerated [12].

Recycling plastic waste offers significant benefits by promoting the reuse of materials rather than their improper disposal. One notable advantage is the reduction of carbon dioxide and other detrimental gases released into the atmosphere; such emissions often arise from burning these waste materials [13]. As per the ASTMD5033 standard, the recycling process comprises three categories: primary, secondary, and tertiary [14]. Primary recycling involves re-extrusion, where waste materials are reintegrated into the extrusion process cycle. Given the significant need for substantial uniformity, the feasibility of primary recycling is confined to semi-clean waste materials [15]. Secondary recycling employs physical methods to reprocess plastic waste from post-consumer sources, producing pellets or granules [16]. This approach consists of a series of preliminary steps, including collection, sorting, aqueous purification, desiccation, fragmentation, pigmentation, coloring, bonding, and pelletizing/extrusion, leading to the creation of the final product [17].

Primary and secondary methods of recycling plastic waste face multiple obstacles due to the intricate composition of plastic waste and the inefficiencies observed in mechanical recycling processes. Several factors limit the effectiveness of mechanical recycling, such as insufficient quantities and inadequate quality [18]. On the other hand, tertiary pathways involve chemical recycling, wherein plastic chains are transformed into smaller molecules by applying chemical agents or processes [19]. Chemical recycling has the potential to overcome certain constraints of mechanical recycling, such as challenges related to polymer blending, contamination, and degradation [20]. Nevertheless, an inherent limitation of chemical recycling pertains to the emission of certain chemicals into the surrounding environment. As the procedure involves heating plastics to facilitate melting, it simultaneously releases sulfur, carbon, and other gases into the atmosphere [21]. Therefore, there is a pressing need for innovative and sustainable recycling solutions while maintaining high-quality products. Recently, additive manufacturing (AM), often known as 3D printing, has emerged as a promising technology that can achieve sustainable production and the recycling of plastics [22,23]. AM offers an eco-efficient solution to global plastic pollution by repurposing post-consumer plastic to manufacture 3D-printed products [22,24,25].

AM has evolved into a practical approach for reducing waste, mitigating global carbon emissions, and meeting economic, environmental, and social criteria [26,27]. This process has attracted significant attention for its capacity to fabricate complex geometrical products using recycled materials instead of traditional manufacturing methods [28,29,30]. Advancements in 3D printing and digitization technology have provided new opportunities to improve accessibility in cultural heritage sites, particularly for tourists with disabilities. At the Piraeus Archaeological Museum, Kantaros et al. (2023) illustrated the possibilities of these technologies in producing tactile copies of ancient items using recycled PLA [31]. The AM of the plastic waste process consists of multiple steps, as shown in Figure 2. First, waste plastic is separated, cleaned, and shredded. Then, the shredded material undergoes extrusion at elevated temperatures, depending on the type of plastic [32]. Next, the produced filament is fed into the 3D printer to build the printed object.

AM techniques use 3D modeling software to expedite the design process. Three-dimensional printing entails the sequential deposition of material layers, with thermoplastic filaments serving as the raw material for fused filament fabrication. Creating printable filaments from diverse materials is feasible, provided they exhibit thermoplastic characteristics [33]. Recycled plastics have become a viable substitute for virgin materials in AM applications, mainly 3D printing. The literature on recycled plastics in AM has expanded over the past few decades.

Table 1. Literature reviews on using plastic through AM.

Reference	Properties	Impacts	Target Plastic/Additive	Years Covered	Publication Year
[34]	Mechanical, thermal, and electrical	None	Virgin and recycled/nanomaterial and fiber	Not mentioned	2020
[35]	Mechanical and thermal	None	PLA/nanomaterial and fibers	Not mentioned	2020
[33]	Mechanical, thermal, and rheological	None	Virgin and recycled/different additives	Not mentioned	2020
[36]	Mechanical and thermal	None	Virgin and recycled/natural fiber	Not mentioned	2021
Current study	Thermal, rheological, mechanical, and morphology characterization	Economic and environmental	Recycled/different additives	2013–2023

presents the previous review articles conducted on using plastic through AM. As shown in the table, limited review articles have addressed different aspects of the AM process, such as novel composites of plastics and admixtures to enhance the final product properties. Previous evaluations have mainly concentrated on plastics and their additive upgrades in a general sense. Yet, they tend to overlook the distinct functions of various recycled plastic types and the additives employed to improve the qualities of filaments. Moreover, the previous reviews have not covered the technical and sustainability aspects of producing filaments from different types of recycled plastic by assessing.

This systematic literature review (SLR) focuses on filament materials made from various recycled plastics by investigating the effects of different additives on mechanical, rheological, and thermal performance. Furthermore, the review covers the cost-integrated lifecycle assessments to understand better the financial and environmental implications of recycling plastic within the AM framework. The objectives of this SLR will be realized through four core research questions, each designed to bridge the identified gaps in current knowledge:

(1)

What are the diverse classifications of recycled plastics employed in AM? This question aims to catalog the types of recycled plastics used, providing a comprehensive understanding lacking in previous studies.

(2)

Which additives are combined with plastics in AM, and what roles do they play? This question explores the additives that improve filament characteristics, a topic frequently overlooked in current research.

(3)

How does the performance of recycled plastics compare to that of their virgin counterparts in the AM? By comparing recycled and virgin materials, this question addresses the critical gap regarding material performance and its implications for broader adoption.

(4)

What are the environmental and financial implications of employing AM for recycling plastic? This question incorporates a sustainability perspective, merging economic and ecological factors to provide a thorough understanding of the impact of recycled plastics in AM.

By addressing these questions, this review aims to enrich the AM literature, providing valuable insights for enhancing the performance and sustainability of future manufacturing practices.

2. Materials and Methods

A comprehensive search strategy was designed for this systematic review consisting of four levels: identification, screening, eligibility, and inclusion [37]. The identification level includes the keyword used, databases incorporated, and records extraction. The screening level involves the inclusion and exclusion criteria, whereas the eligibility level entails the assessment of the full papers. Ultimately, the framework culminates by determining the research papers included in the SLR.

2.1. Search Strategy

SLRs involve diligently searching for relevant studies aligned with the research questions. To ensure a comprehensive investigation, a tailored search strategy was applied across multiple search engines: Scopus, ProQuest, Web of Science, and Google Scholar. The search strategy utilized keywords to target titles, abstracts, and topics within the selected databases. Based on the research questions, the search was structured around four key elements: techniques, materials, properties, and impacts. Substitutive expressions and synonyms are listed in

Table 2. Synonyms and alternatives terms for major search terms.

Techniques	Materials	Properties	Impact
Additive manufacturing 3D print * (3D printing, 3D printed)	Cycl * (Recycle, Cycle, Upcycle) Admixture Additive * (Additive, additives)	Mechanical Physical Thermal Rheolog * (Rheology, Rheological)	Environment Financ * (Finance, Financial) Econom * (Economic, Economy, Economical) Lifecycle Life cycle

Note: The asterisk (*) is used as a truncation symbol to capture all variations of a root word.

. From these terms, the following search string was employed as follows: (“3d print*” OR “additive manufacturing”) AND cycl* AND “plastic waste” AND (admixture OR additive*) AND filament AND (mechanical OR physical OR thermal OR rheolog* OR environment* OR financ* OR econom* OR “life cycle” OR lifecycle).

2.2. Selection Criteria

The primary objective of this review was to systematically map the existing literature on the utilization of recycled plastics in AM, focusing on publications in English from 2013 to December 2023. A comprehensive search strategy was developed by employing carefully selected keywords to identify relevant studies. This SLR was conducted in full accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines (PRISMA) (refer to PRISMA checklists in Supplementary Materials) to ensure a methodologically rigorous approach to the selection and synthesis of the literature. The selected document types were limited to articles, conference papers, and reviews. As per PRISMA guidelines, the inclusion and exclusion criteria for reviewed studies were conducted over four stages: identification, screening, eligibility, and inclusion, as shown in Figure 3. The first phase identified all possible relevant research based on the information present in the title, keywords, and abstract, retrieving a total of 913 papers after excluding the duplicates. Afterward, screening was performed based on the method section to exclude 545 records that did not match the scope of the review, which used recycled plastic or recycled and virgin plastic but not limited to virgin plastic only. Simultaneously, the articles in doubt or with unclear scope were forwarded for additional review, where a comprehensive evaluation was carried out through full-text examination, excluding another 99 entries.

2.3. Bias and Certainty

Following the screening phase, a quality assessment of each study was systematically conducted to further enhance the quality of the selected articles. The assessment was based on five distinct criteria: (1) credibility: evaluating whether the study is well-designed and supported by robust experimental work in AM or 3D printing, prioritizing studies published in high-impact journals and those with rigorous validation of results; (2) scope: examining the relevance of the application field and the comprehensiveness of the work, with a preference for studies that address significant challenges or novel applications in the field of recycled 3D printing filaments; (3) clarity: assessing the transparency and clarity of the research objectives and hypotheses, which is crucial for understanding the study’s contributions and limitations; (4) methodology: considering the appropriateness and rigor of the research methods and experimental procedures, favoring studies that use advanced or innovative methodologies that could advance the field; and (5) analysis quality: evaluating the depth and accuracy of data analysis, including the robustness of computations and the quality of critical discussions and conclusions. Each study was scored against these criteria, and only those that met a predefined threshold for high-impact research were included in the review. This selective approach resulted in a total of 88 studies that cover a broad range of topics and contribute significant, high-quality insights to the field. By being explicit about the influence of these criteria, the review aimed to focus on impactful research that advances the understanding and development of 3D printing filaments using recycled plastics.

2.4. Review Statistics

This review encompasses 88 research papers conducted worldwide between 2013 and 2023. As the timeline progressed, a noticeable interest in this study area increased, as shown in Figure 4. The studies were divided into three types: journal articles (78), conference papers (6), and reviews (4). Journal papers were published in various journals, including Polymers, Materials, Journal of Cleaner Production, Journal of Thermoplastic Composite Materials, and Recycling, among others.

The geographical distribution of research on this subject provides insight into global engagement and variances in academic contributions, revealing underlying trends, priorities, and capacities across different regions. Figure 5 depicts an international distribution of studies across 33 countries; the United States and India lead the list, each with 11 studies, followed closely by Italy with 9. A cluster of countries in the middle range, including Australia, Greece, Malaysia, and Spain, each contributing with five studies. Other countries listed have only conducted 1 or 2 studies each. This distribution suggests a concentration of research in a few countries, particularly North America, South Asia, and parts of Europe, with less representation from Africa.

3. Discussion and Critical Analysis

3.1. Plastic Types Used in Additive Manufacturing

AM technology has witnessed exponential attention, primarily due to its application in prototyping and cost-effective small-scale production projects. The end products are crafted through 3D printing, wherein filaments are meticulously shaped through extrusion techniques. The quantity of plastic filament derived from virgin or recycled plastic has notably increased. Developing a 3D-printed prototype from plastic waste involved four stages: the initial sorting of plastic types and the washing and shredding of plastic materials. The subsequent step entails extruding the material at a temperature suitable for the specific plastic type and additives utilized. Subsequently, the filament is used for the fabrication of the 3D-printed components. Acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) are the most popular types used to produce filaments for 3D printers [38]. ABS exhibits excellent mechanical properties of printed filament despite a few demerits related to biodegradability and toxicity [36,39,40]. On the other hand, PLA is a commonly used biodegradable polymer in filament fabrication [33,36,41,42].

A long list of polymers, including polycarbonate (PC), polyamides (PA), polypropylene (PP), polymethyl methacrylate (PMMA), polystyrene (PS), high-density polyethylene (HDPE), thermoplastic polyurethane (TPU), and iterations of polyethylene (PE), are conventionally employed in AM to produce an extensive range of products, e.g., automotive parts, medical tools, prototypes, packaging, garden features, and toys [33,35,36,43,44,45]. Various plastics have been reported in the literature to produce different filaments for AM. A comprehensive summary of these diverse plastic types and their corresponding melting points and extrusion temperatures is presented in

Table 3. Plastic types and corresponding manufacturing parameters for filament production.

Plastic-Type	Source of Plastic	Melting Temperature (°C)	Extrusion Temperature (°C)	Extrusion Speed (rpm)	Filament Diameter (mm)	Reference
PP	Post-industrial waste	165	190	9	1.75 ± 0.1	[47]
ABS	Toys	203	205	35	1.75 ± 0.02	[48]
PP	Noodle food packaging	156	185	-	1.75	[49]
PETG	-	-	235	20	1.75 ± 0.05	[50]
ABS	-	270	270	18	1.75	[51]
PET	Water bottles	250	-	-	2.875	[52]
ABS	-	-	350	20	1.75	[53]
PET	Seaside bottles	-	220	10		[54]
ABS	Automotive industry	240	240	-	2.85	[55]
PET	Post-consumer plastic	240	250	18	1.75	[56]
ABS	Electrical equipment waste	251	300	-	1.75	[57]
PP	-	-	180	20		[58]
PLA	-	165	170	60	1.75 ± 0.5	[58]
PLA	-	210	170	70	1.75	[59]
Nylon-6	-	235	260	60	1.75	[60]
PLA		240	240	20	1.78 ± 0.04	[61]
PP	Polypropylene bags	-	180	20	3	[62]
PET	-	250	260	-	3	[63]
HDPE	Detergent containers, shampoo bottles	136	190	-	2.93 ± 0.22	[44]

. The melting points and extrusion temperatures are essential parameters to consider during filament manufacturing due to their direct influence on the filament properties and performance in various applications. As a result of the multiple stages in the recycling process of waste polymers, they often experience a drop in properties [46], which is undesirable when reusing the waste material. Consequently, it becomes necessary to reinforce the plastic-based matrix and enhance the characteristics of the recycled polymer.

3.2. Additives Used in Plastic Composite Filament

Recycled plastics often change their physical attributes due to impurities, potential degradation from prior use, and the inherent variability in their composition. These alterations may not align with ideal reuse requirements, necessitating the need to reinforce the plastic matrix. Extensive research has investigated the efficiency of incorporating these additives into polymer composites, aiming to augment their characteristics and boost their molecular weight. Introducing these additives can improve the performance of 3D-printed components while promoting their sustainability. These additives can enhance composite polymers’ mechanical properties, flexibility, conductivity, and resistance [30,36,64]. Additives used with plastics in AM are typically categorized into two main groups: micro- and nanoparticle additives and fiber-based ones [33].

The use of micro- and nanoparticles in polymer modification is a well-established technique [46,65], known for its potential to significantly enhance material properties due to the increased surface area of particles. Moreover, nanoparticles are widely utilized in applications requiring significant thermal and electrical conductivity. The introduction of highly conductive additives, such as graphene nanoplates (GNP), carbon nanotubes (CNT), and multi-walled carbon nanotubes (MWCNT), can substantially augment the thermal conductivity of the polymer matrix [66,67,68]. Moreover, metal nanoparticles can significantly strengthen polymeric composites’ mechanical and thermal properties [35,66]. Among the metals utilized, aluminum and iron powders are effective reinforcing agents for polymers like ABS, PP, and PA [69,70,71].

An innovative approach involves the extrusion of nanocrystalline powders, including Fe, Si, Cr, and Al, in conjunction with PP, HDPE, and low-density polyethylene (LDPE) filaments [46,65]. Moreover, polymer ceramic composites have attracted significant interest, incorporating ceramic fillers into the polymeric matrices to impart enhanced characteristics. Notable ceramic choices include calcium ceramics, extensively employed for fortifying polymeric matrices such as PLA, PA, PP, Polycaprolactone (PCL), Polyetheretherketone (PEEK), PMMA, and styrene–butylene–styrene (SBS) [43,72,73]. Ahmed et al. (2020) explored the utilization of silica as an additive to augment the mechanical characteristics of composite materials derived from waste PLA generated through 3D printing [74]. The research results demonstrated an enhancement in the mechanical aspect of the combined material. The PLA/silica blend with 10% silica exhibited a 37.4% increase in ductility and 21% improved elasticity, along with 90% and 15.3% enhanced tensile strength and elongation, respectively. Similarly, there was an observed increase in toughness, progressing from 3.6 MPa at 0 wt% to 5.6 MPa at 10 wt%. These findings underscore silica’s role in augmenting the material’s inherent ability to dissipate and absorb energy.

In addition, wood is one of the natural materials used as a filler in AM. There are multiple advantages associated with wood filaments, such as biodegradability, nontoxicity, low deformability, and good elasticity [30]. Several studies have used wood powder from different sources and quantities to optimize the performance of wood filaments [40,75]. For instance, integrating fillers such as Bakelite powder (BP) and wood dust (WD) in ABS led to a notable enhancement in strength and thermal stability [76]. However, Gkartzou et al. (2017) blended a lignin additive with recycled PLA at an extrusion temperature range of 180–190 °C [77]. The addition of lignin resulted in a reduction of 18% in tensile strength and 6% in Young’s modulus compared to the pure PLA material. Natural fibers derived from harakeke and hemp fiber were also used to produce post-consumer recycled PP filament [62]. Filaments with 30% harakeke fibers enhanced the tensile strength, modulus, and shrinkage compared to recycled PP filament by 77%, 275%, and 84%, respectively.

Moreover, fibers play a crucial role in reinforcing recycled plastics by substantially enhancing their mechanical strength, thermal stability, and overall performance. This synergy between recycled plastics and fibers aids in developing more sustainable and advanced composite materials for various industries and applications. Adding carbon fiber (CF) to recycled HDPE improved tensile and flexural strengths, achieving values as high as 58 MPa and 48 MPa, respectively [78]. Another study used CF to reinforce both polyethylene terephthalate glycol (PETG) types: virgin and recycled. The results indicated an increased tensile strength attributed to the presence of CF, especially at a 25% concentration [79]. Also, CF significantly improved recycled HDPE’s tensile and flexural strength [78]. Other environmentally conscious alternatives have been used in AM, such as harakeke and hemp fibers, using recycled PP at different percentages. The results demonstrated an enhancement in tensile strength, Young’s modulus, and composite shrinkage. Cocoa bean shells (CBS) produced filament composite with recycled PP and showed an improved thermal property than neat recycled PP [47].

3.3. Properties of Modified Recycled Plastic Filament

3.3.1. Thermal Properties

Among several influencing factors, thermal properties are paramount in defining the performance of 3D printing filaments and their response and behavior under varying temperature conditions. Thermal characteristics are crucial in determining the printability, mechanical integrity, and overall quality of 3D-printed objects. A comprehensive investigation was performed on recent research that has explored the thermal characteristics and processing techniques of innovative plastic filaments incorporating various additives. By examining the filaments’ melting temperatures, degradation behavior, and crystallinity, valuable insights into their thermal performance could be gained. These findings promote thermally optimized filaments toward a circular economy and reduced ecological footprint of AM processes. Recent research demonstrated that introducing additives to various recycled plastics results in distinct thermal behaviors, as presented in

Table 4. Influence of additives on thermal properties of recycled plastic.

Plastic-Type	Additive	Thermal Property Affected	Key Findings	Reference(s)
PCL	CBS	Crystallinity	Minimal changes in PCL’s melting temperature upon the addition of CBS, a reduction in PCL crystallinity with higher CBS content.	[80]
PP	CBS	Thermal degradation	CBS exhibited three-phase degradation, while neat PP exhibited a two-stage degradation process.	[47]
PP	GP	Melting temperature	GP improved the thermal properties of PP filaments and enhanced 3D printing suitability.	[49]
ABS	BP/WD	Thermal stability	BP/WD Improved thermal stability of ABS composites	[76]
PETG	Carbon Filament Fibers	Thermal adhesion	The thermodynamic adhesion of PETG to CFF is more favorable than PETG.	[79]
PLA	Lignin	Melting temperature	Lignin promoted double melting behavior in PLA.	[77]
PLA	MD	Crystallization, crystallinity	MD improved the crystallinity of the composite.	[81]
PP	Carbon Black	Crystallinity, melt flow index	Carbon black influenced melting temperatures and crystallinity, enhancing the thermal stability of PP by absorbing more heat during melting.	[82]
PP	Talc	Melting enthalpy	The incorporation of r-PP into blends effectively reduced the total melting enthalpy.	[83]

.

In a study by Puglia et al. (2016), CBS was added into a PCL matrix at varying amounts (10, 20, and 30 wt%) [80]. Differential scanning calorimetry (DSC) indicated minimal changes in the melting temperature of PCL upon CBS addition. CBS’s thermogravimetric analysis (TGA) revealed distinct degradation peaks for hemicellulose, pectin, and cellulose, leaving a 33% residue at 900 °C. The crystallization temperature of the composites slightly increased compared to the neat matrix, suggesting challenges for PCL chains to rearrange in the presence of CBS residue. Furthermore, higher CBS content led to a reduction in the degree of the crystallinity of PCL. Morales et al. (2021) characterized recycled polypropylene (rPP)/CBS composites with 5 wt% CBS [47]. The study revealed a three-phase degradation process for CBS, involving water vaporization, hemicellulose, cellulose degradation, and lignin and remaining cellulose degradation, resulting in a residual char of approximately 35%.

In contrast, neat rPP exhibited a degradation process consisting of two successive stages: impurities and main polymer chain degradation (around 55% weight loss). The rPP/CBS composite exhibited a weight loss curve reflecting the thermal behavior of both fibers and fillers. The degradation of the composite involved fiber degradation and matrix decomposition, aligning with previous studies on natural composites [84,85,86,87]. Kristiawan et al. (2022) investigated the impact of glass powder (GP) additives on the thermal properties of rPP filaments [49]. Different GP fractions (2.5%, 5%, and 10%) were added into rPP filaments, showing the potential of GP as an effective thermal enhancer for rPP filaments in 3D printing. Incorporating GP enhanced the stability of mass alterations when exposed to heat and raised the melting temperature of rPP. Seibert et al. (2022) focused on using recycled polyethylene terephthalate (PET) bottle flakes as a filament for 3D printing [52]. The thermal characteristics of the recycled PET filament were assessed and compared to those of the virgin material, along with a commercially accessible PETG filament. The recycled PET filament exhibited higher sensitivity to moisture, leading to degradation during processing. For 3D printing and filament production, virgin and recycled PET showed robust thermal attributes, consistent thermal behavior, and a slightly different glass transition range, emphasizing their potential in heat-intensive applications. Chawla et al. (2022) aimed to recycle thermosetting plastic waste by incorporating waste BP and WD as reinforcements for 3D-printed thermoplastic composites based on ABS [76]. The results revealed that adding BP/WD improved the thermal stability of ABS composites. Reinforcement with 10% BP exhibited higher peak strength, energy-carrying capacity, and thermal stability. Chong et al. (2016) explored the use of recycled HDPE as a filament for 3D printers [44]. The filament demonstrated commendable water resistance, comparable extrusion rate, and satisfactory thermal stability.

In a study by Bex et al. (2021), thermodynamic adhesion analysis suggested a higher interfacial interaction between recycled PETG and continuous carbon filament (CCF) fibers than between PETG and CCF fibers, supporting rPETG’s potential as a matrix material for 3D-printed continuous fiber composites [79]. Jamnongkan et al. (2022) suggested that adding rPP and carbon black (CB) can benefit the thermal behavior of PP-based composites [82]. The study investigated the effect of blending rPP with regular PP and found that adding up to 40 wt% rPP into the PP matrix did not significantly impact the thermal properties. However, the melt flow index (MFI) of the blended PP increased with higher rPP content, and the degree of crystallinity of PP increased when rPP and CB were added, indicating their role as nucleating agents. Ferrari et al. (2020) reported changes in the crystalline structure of polymer waste from collected PET bottles due to different processing conditions and reduced thermal stability after repeated processing cycles during 3D printing [54]. Paszkiewicz et al. (2020) investigated environmentally friendly polymer 80/20 blends based on post-consumer glycol-modified PET-G foils and PEF [88]. These blends exhibited improved thermal stability and water absorption, attributed to interfacial interactions between the polymers, leading to a positive synergistic effect.

Furthermore, recycled PP blended with talc was formulated by Arrigo et al. (2022) for 3D printing filaments, and the optimized materials minimized the total melting enthalpy of the samples [89]. In another study by Turku et al. (2018), PS, ABS, and polyvinylchloride (PVC) wastes were characterized as potential extruded feedstock filaments for 3D printing [22]. The recycled materials showed comparable thermal properties to their virgin counterparts, making them suitable for 3D printing applications. The study concluded that recycling offers a viable solution for wasting plastics in AM. Farina et al. (2019) also developed sustainable nylon-6 fused deposition modeling (FDM) filaments [60]. The recycled nylon-6 filaments exhibited high thermal stability with no significant degradation during extrusion. This approach enables the use of recycled nylon-6 while maintaining its desirable thermal characteristics for FDM, contributing to the eco-friendly and cost-effective production of high-quality 3D-printed products.

Gkartzou et al. (2016) investigated the thermal behavior of PLA/lignin composites during 3D printing and found that lignin promoted the double-melting behavior of PLA [77]. TGA analysis showed a wide temperature range of thermal decomposition for lignin, indicating the potential applicability of PLA/lignin composites in 3D printing while considering their thermal stability during processing. Lendvai et al. (2021) highlighted the impact of waste marble dust on the thermal properties of PLA composites, providing valuable insights for practical applications [81]. The incorporation of marble dust in PLA-based bio-composites affected the thermal behavior by acting as a nucleating agent, lowering the cold crystallization peak, and favoring the formation of stable crystalline structures. The crystallinity ratio decreased at 5–10 wt% marble dust but increased at higher concentrations.

A study by Wuamprakhon et al. (2023) analyzed the thermal properties of bespoke PLA filaments using simultaneous TGA/DSC. They compared neat and recycled PLA with commercial CB/PLA. They found that the initial onset temperature for all materials was similar (≈300–305 °C), indicating no deterioration in thermal stability due to recycling or additional processing. The commercial sample showed a secondary onset of decomposition, speculated to be from a plasticizer [90]. Another case study by Ragab et al. (2023) utilized DSC and Dynamic Mechanical Analysis (DMA) to characterize blends of rHDPE and PET at an 80:20 ratio. The study modified the blends using maleic anhydride compatibilization, surface functionalization with sodium dodecyl sulfate, and a novel hybridization approach. The DSC analysis revealed that the melting temperatures of both HDPE and PET were unaffected by blending. However, the crystallinity of HDPE significantly decreased with compatibilizer addition, while PET crystallinity increased due to nucleating agents. PET’s glass transition temperature (Tg) remained unchanged [91].

The thermal stability of wood dust fiber-reinforced recycled PP composites using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) has been investigated by Nafis et al. (2023). TGA revealed that dehydration of the composites began at 35 °C, with significant mass loss between 350 and 480 °C due to the decomposition of hemicellulose, cellulose, and lignin. DSC analysis showed minimal changes in melting temperature and crystallinity between treated and untreated filaments. However, silane-treated filaments exhibited higher thermal stability. The optimal extrusion temperature for producing the composite filament was 167–170 °C [92]. Grønborg and Pedersen (2023) [93] used DSC to examine the thermal behavior of plastic filaments from ocean debris mixed with reactive compatibilizers. Single melting and crystallizing peaks were seen in the DSC, demonstrating consistent thermal behavior. Unfortunately, there was inadequate interlayer adhesion and considerable warping due to the high coefficient of thermal expansion (CTE). It restored melt strength with a 5–10 weight percent compatibilizer, allowing filament development.

Nevertheless, to preserve mechanical integrity and avoid overheating during printing, the low melting point necessitated high cooling fan speeds [93]. Tolcha et al. (2023) investigated the thermal properties of short glass fiber-reinforced rHDPE and rPET composite filaments using DSC. DSC analysis examined the thermal stability and behavior of pure rHDPE, rHDPE/rPET blends, and composites reinforced with 15% and 30% short glass fibers. The results showed that the filaments’ glass transition and melting temperatures increased with the addition of glass fibers. The melting peak of pure rHDPE was around 129.5 °C, while the Tm of rHDPE/rPET with 30% SGF was approximately 13 °C higher. The filaments’ crystallinity degree increased with filler loading, indicating improved thermal stability. The crystallinity of rHDPE was calculated to be 12.14%, with higher crystallinity values observed in the reinforced composites [94]. Composites of rHDPE have been investigated by Daniele et al. (2023) using TGA and DSC. TGA revealed that adding maleic anhydride reduced the thermal stability of HDPE composites. DSC showed that glass fibers improved the composite’s elastic modulus while PE-g-MA compatibilizer enhanced mechanical properties but increased brittleness [95]. Bergaliyeva et al. (2023) investigated the thermal properties of recycled PLA filaments with various additives for material extrusion additive manufacturing (MEX-AM). TGA showed enhanced thermal stability with higher decomposition temperatures. At the same time, DSC indicated minimal changes in glass transition and melting temperatures but reduced crystallization temperature and crystallinity. These additives maintain or enhance recycled PLA thermal stability, making it suitable for MEX-AM applications [96].

3.3.2. Mechanical Properties

It is imperative to rigorously investigate the mechanical properties of recycled plastic filaments and their resultant 3D-printed samples to optimize design and manufacturing processes. A comparison with virgin plastic is crucial for quality control, cost–benefit analysis, and sustainability considerations. The mechanical properties of these materials can profoundly affect their performance and range of applications. Factors such as the plastic type, printing parameters, and incorporated additives can cause variations in the mechanical properties [38,44]. Existing literature extensively examines these variances, particularly emphasizing their tensile strength and Young’s modulus. The following sections provide a comprehensive review of the literature on the mechanical properties of various plastic wastes.

ABS Plastic Waste

Among the various thermoplastics available for 3D printing, ABS is particularly prevalent due to its distinct characteristics [83,97,98,99]. Several studies have delved into the mechanical properties of 3D-printed recycled ABS, drawing intriguing comparisons with virgin ABS. For instance, multiple researchers have observed that 3D-printed specimens made from recycled ABS exhibit tensile strength and elasticity modulus comparable to those of virgin ABS [48,100,101]. In a study by Turku et al. (2018), the tensile strengths of filaments derived from recycled plastics ABS and PS were 14.5 MPa and 4.3 MPa, respectively. Notably, the recycled ABS filament demonstrated superior tensile strength performance than the other materials. Regarding Young’s modulus, the produced filament achieved 2.7 and 1.85 GPa for ABS and PS, respectively [22].

The finding from Di and Yang (2022) contrasts these observations, they found that the ultimate tensile strength of recycled ABS decreased from 34.181 MPa for virgin material to 17.043 MPa after three recycling rounds [102]. In addition, Maidin et al. (2021) reported a reduced mechanical performance in recycled ABS. However, to counteract this decline, they introduced a method of applying ultrasound vibrations at 20 kHz to the ABS samples, which was found to effectively enhance the mechanical properties of recycled ABS [51]. The results showed a 53% increase in flexural strength, a 59% increase in compression strength, and a 19% increase in tensile strength compared to a static loading condition. Mishra et al. (2023) further examined the addition of recycled ABS to virgin ABS. They discovered mechanical performance improved by adding up to 40% VABS to RABS. Compared to 100% recycled ABS, a 60% recycled ABS/40% virgin ABS blend printed at 240 °C increased the yield strength by 13.37%, the ultimate tensile strength by 15.64%, and the Young’s modulus by 13.69% [103].

Including fillers such as BP and WD in ABS enhanced tensile strength and thermal stability notably; these improvements were observed when the filler content reached 10% BP, as reported by Chawla et al. (2022) [76]. The incorporation of 2% by weight of GO into ABS has been studied by Aumnate et al. (2018). The results showed a significant increase in tensile strength and Young’s modulus, with a 29% improvement in tensile strength and a 28% improvement in Young’s modulus [104]. In addition, using ABS waste from electrical and electronic equipment produced a filament with mechanical properties very close to ABS commercial filaments [57].

PLA Plastic Waste

Anderson (2017) compared the mechanical characteristics of virgin and recycled PLA; the results indicated a reduction in tensile strength and hardness of recycled PLA by 10.9% and 2.4%, respectively [105]. This reduction aligns with the results reported by Lee et al. (2019) [100], which showed decreased tensile strength for recycled PLA. Also, Jayawardane et al. (2023) showed that recycled PLA lowers ultimate tensile strength by 12%, lowers density (1.09–1.14 g/cm³ vs. 1.18 g/cm³), and reduces fatigue strength (14.56 MPa vs. 17.20 MPa at 10⁶ cycles) [106]. However, the shear strength of recycled PLA demonstrated a 6.8% increase compared to virgin PLA [100]. The impact of a multi-extrusion process on the tensile strength and fracture morphology of PLA filament products was studied by Syaifuddin et al. (2021). The findings indicate that the mechanical recycling of PLA through a multi-extrusion process should not exceed six cycles. In addition, a significant deterioration in tensile strength occurred after the ninth cycle, with a 14% reduction observed after the twelfth cycle [99]. Therefore, it is not advisable to exceed six cycles when employing the mechanical recycling approach using a multi-extrusion process.

The impact of incorporating a chain extender into a thermoplastic composite has been investigated by Ju et al. (2022). The composites consisted of thermoplastic starch, PLA, and polybutylene adipate-co-terephthalate with a fixed ratio of 50:40:10% by weight. The findings revealed that a 1% chain extender led to a 3 MPa increase in the tensile strength compared to the composite blend. Furthermore, incorporating a chain extender enhanced the brittleness of the blends while improving the elongation at the break by 113% and increasing the composite materials’ impact strength by 15 kJ/m² [59]. According to Correia et al. (2022) [107], recycled PLA has lower tensile characteristics than virgin PLA, with a tensile modulus reduction of 43.60% and a tensile strength loss of 64.72%.

In comparison to recycled PLA without 1,3-Bis(4,5-dihydro-2-oxazolyl)benzene (PBO), the addition of PBO increased both the tensile strength (+121%) and the tensile modulus (+13%), partially recovering the mechanical performance of recycled PLA [107]. However, Grønborg and Pedersen (2023) investigated the conversion of plastic debris from the ocean into filaments. A compatibilizer (Acti-Tech) had to be added at 5 and 10 wt%. Tensile tests revealed that although the yield strength did not change from filament to printed component, the printed parts had a lower E-modulus and a lower yield strength at larger concentrations of the soft Acti-Tech compatibilizer. The material showed a high tendency to warp, a low melting temperature, and poor interlayer adhesion; these factors posed substantial obstacles. The warping was two to ten times greater than conventional PLA [93].

Li et al. (2023) demonstrated that continuous glass fiber (CGF) reinforcement can significantly enhance the mechanical properties of PLA. They achieved a maximum tensile strength of 186.4 MPa and a storage modulus of 17.5 GPa with 40 wt% CGF content. This represents a substantial increase over pure PLA, with tensile strength increasing 3.5 times at just 32 wt% CGF content [108]. Dash et al. (2022) employed a comprehensive factorial design of experiments to examine the impact of layer thickness and infill density on the mechanical properties of virgin and recycled PLA materials. The test results indicated that in the case of virgin PLA, layers with a thickness of 0.2 mm and a 100% infill percentage exhibited the most excellent tensile and flexural strength. Conversely, for recycled PLA, optimal tensile and flexural characteristics were observed in layers with a thickness of 0.1 mm and a 100% infill percentage [109]. Lee et al. (2022) examined recycled PLA’s tensile strength in horizontal and vertical orientations. The results retrieved that produced PLA exhibit anisotropic characteristics, whereby its tensile strength is higher when printed horizontally than vertically [38]. Moreover, Dobrzańska-Danikiewicz et al. (2023) reported that recycled PLA exhibited 37.6 MPa tensile strength when printed at 220 °C with vertical filament deposition and 67.8 MPa with horizontal filament deposition. The study revealed that the filament deposition method significantly impacted tensile strength [110]. These results are compatible with the findings of Perez 2014 et al. (2014) [111].

PET Plastic Waste

Seibert et al. (2022) conducted a comparative investigation that examined the tensile stress and Young’s modulus of virgin PET, recycled PET sourced from water bottles, and commercially available PETG filament. The study results revealed that recycled PET slightly decreased tensile strength while maintaining a similar modulus to virgin PET. However, recycled PET outperformed commercial PETG filament by approximately 15% [52]. Vidakis et al. (2020) investigated the impact of recycling PETG polymer for up to third cycles on various mechanical properties. The findings revealed that recycling PETG polymer for up to the third cycle improved mechanical properties compared to virgin PETG. These enhancements included a 15.8% increase in tensile strength and a 30.6% increase in elastic modulus [53]. Another study compared two types of PETG filaments: one composed of 50% recycled material and 50% virgin material, and the other comprised 100% recycled material. The findings disclosed a reduction of around 10 Mpa in tensile strength between the sample with 50% recycled PETG and 50% virgin PETG compared to the sample made entirely of recyclable PETG [50].

Another promising approach was used by Idrees et al. (2018). They derived a filament from PET and pyrolysis packing waste at concentrations of 0.5% and 5% by weight. The results showed a significant enhancement in tensile strength when compared to PET alone. The tensile strength increased by 32% and 60% for the respective biochar concentrations [112]. CF has been used as a reinforcement to recycled PET. Introducing CF with a 25% concentration to recycled PETG has resulted in a 13% increase in tensile strength [79]. The use of different cooling rates during the extrusion of PET filament was found to have a significant influence on the PET mechanical properties. Ferrar et al. (2020) extruded PET filaments collected from bottles using different cooling rates (slow and rapid). Under the slow cooling rate, the tensile stress and Young’s modulus were 63 MPa and 2.19 GPa, respectively. On the other hand, rapid cooling resulted in a tensile strength of 39 MPa and a modulus of 1.14 GPa [54]. These results align with the study conducted by Little et al. (2020) [56], which reported an approximate reduction of 50% in the tensile strength of PET samples when using cooling compared to samples without cooling.

The size and shape of the recycled PET affected the resulting filament tensile strength, as Little et al. (2020) reported. They stated that the tensile strength results of PET pellets exhibited a tensile strength of 29.62 MPa, whereas the PET flakes demonstrated a tensile strength of 20.35 MPa [56]. Rashwan et al. (2023) examined the mechanical characteristics of recycled PET (rPET) filaments mixed with a chain extender. Comparing plain recycled PET to recycled PET with a chain extender (PMDA) revealed a 25% increase in tensile strength and a 30% increase in elastic modulus [113]. Using recycled PET/HDPE blends (80:20) for 3D printing, Ragab et al. (2023) used three distinct approaches: compatibilization with maleic anhydride (GMA), surface functionalization of PET with sodium dodecyl sulfate (SDS), and hybridization by combining the two methods. The improved blends exhibited notable mechanical improvements: the glass fiber-reinforced blends (GMA-GF and SDS-GF) demonstrated an increased elastic modulus of 61% and 59%, respectively. The GMA blend had a 66% higher tensile strength and 231% higher elongation. There was an increase in impact strength of 633% (SDS), 509% (GMA-SDS), and 429% (GMA). These findings show that recycled plastic mixes with the right modifications can have mechanical qualities that are on par with or even better than those of virgin materials [91].

PP Plastic Waste

Recycled PP has been showing superior properties with AM [114]. Recycled PP can be added to virgin PP up to 40% content without affecting the tensile strength and modulus [82]. In their study, Morales et al. (2021) aimed to promote sustainability by developing a 3D printing filament from recycled PP and CBS. They further explored the impact of different 3D printing raster angles (0° and 90°) on this filament’s performance. The orientation of the raster significantly influenced the outcomes related to the mechanical properties. Tensile strength exhibited an elevation with a raster angle of 0° in contrast to samples printed at 90°. This was attributed to the direction of the applied load. For the recycled PP, the tensile strength was 26 MPa at 0° raster angle compared to 4.3 MPa at 90° raster angle. Also, the rPP/CBS filament showed a tensile strength of 15.2 MPa and 7.9 MPa with raster angles of 0° and 90°, respectively [47]. Recycled PP from food packaging was used with different percentages of GP (2.5%, 5%, and 10%) to improve its mechanical properties. The specimens containing 10% GP additive exhibited a 38% increase in the ultimate tensile strength and a 42% increase in Young’s modulus compared to the PP specimens [49]. In addition, it enhances recycled PP filament by incorporating 14 wt% CF, resulting in a rise in filament’s tensile strength from 22 MPa to 24 Mpa, along with an increase in Young’s modulus from 1209 Mpa to 1413 Mpa. Nevertheless, this improvement is accompanied by a decrease in elongation to failure from 9.83% to 6.58% [58].

Other Plastic Wastes

Singh et al. (2019) investigated the best setting of a 3D printer to print recycled HDPE reinforced with Al₂O₃. They concluded that infill 60%, raster angle 60°, and nozzle diameter 0.5 mm are the best settings for hardness [115]. Tolcha and Woldemichael (2023) examined the recycled HDPE and recycled PET blends reinforced with short glass fibers. They discovered that incorporating 30 wt% SGF into the rHDPE/rPET blend significantly enhanced the filament’s yield stress to 8.98 MPa and the elastic modulus to 761.96 MPa, compared to 6.92 MPa and 715.47 MPa for pure rHDPE filament [94]. Farina et al. (2019) developed a sustainable Nylon-6 filament from recycled Nylon-6. The result showed high tensile strength at around 86.91 MPa and a Young’s modulus of 1.64 GPa [60].

Overall, it is important to acknowledge that recycled filaments can display variations in their mechanical properties due to several factors. These factors include the type of plastic, size of plastic particles, printing parameters, inclusion of additives, and the number of recycling cycles. Notably, adding additives and fillers, such as biochar, GO, and CF, has shown considerable potential in enhancing recycled plastics’ tensile strength and other mechanical properties. Therefore, future research should emphasize optimizing recycling processes for different plastics to maximize their mechanical properties.

Table 5. Comparison between mechanical properties of different recycled plastic.

Plastic-Type	Additives	Printed Sample Standard	Printed Temp. (°C)	Printed Bed Temp. (°C)	Printed Speed mm/s	Tensile Strength (Mpa) or % Increase	Young Modulus (Mpa) or % Increase
PLA [109]	-	ISO 527 1	200	60	100	-	(46)
PP [47]	CBS (5%)	ASTM D3039 2	250	90	60	83%	-
ABS [48]	-	ASTM D638 3	230	100	25	(31.9)	-
PP [49]	GP (10%)	-	220	80	20	38%	42%
PET [52]	-	ASTM D638 3	270	90	33	(53)	(1720)
PETG [53]	-	ASTM D638 3	235	70	4	46.1	(247)
PETG [79]	CF (25%)	ISO 527-1BA 1	230	80	60	More than 100%	More than 100%
PLA [38]	-	ASTM D638-10 3	-	65	-	(57.73)	-
ABS [101]	-	Dog bone	220	90	3	(23.784)	(1150.5)
PLA [116]	-	ISO 527 1	195	60	200	(46.4)	(3093)
PP [89]		ISO 527 (Type 5A) 1	260	-	30	(14.2)	(654)
MRE [117]		ASTM D638 (Type IV) 3	285	-	-	(14.4)	(300)
HDPE [46]	Fe (10%)	ASTM D638 (Type IV) 3	235	-	90	(15)	-
LDPE [46]	Fe (10%)	ASTM D638 (Type IV) 3	235	-	90	(12)	-
PP [53]	-	ASTM D638 3	230	80	30	(32.4)	(10,500)
PET [56]	-	ASTM D638 Type I 3	240	100	50	(29.6)
PLA [74]	Silica (10%)	ASTM D638 3	190	-	0.083	92%	21%
ABS [98]	-	ASTM D638 (Type IV) 3	250	90	30	(26)	-
PET [98]	-	ASTM D638 (Type IV) 3	240	90	30	(40)	-
PP [98]	-	ASTM D638 (Type IV) 3	250	90	30	(24)	-
PP [58]	CF (14%)	ASTM D638 (Type I) 3	210	85	25	15%	40%
PP [62]	Harakeke (30%)	-	230		50	77%	More than 100%

¹ Determination of tensile properties [118]; ² Tensile test on composite materials [119]; ³ Standard test method for tensile properties of plastics [120].

summarizes the mechanical properties of different recycled plastics printed at different conditions using various additives.

3.3.3. Rheological Properties

Investigating the behavior of materials when subjected to stress-induced flow and deformation falls within the domain of rheological properties. These properties are essential due to their influence on the extrusion process and the ultimate quality of the printed object. Understanding the rheological response is crucial to comprehend recycled plastics’ structural arrangement and flow dynamics. Several techniques are used in the literature to evaluate the rheological properties of composite recycled plastics. The interaction between the polymer and the 3D printer can pose challenges in terms of compatibility. Thus, it becomes essential to pre-assess the rheological behavior of the polymeric composite before commencing the 3D printing process. This necessity underscores adopting standardized testing conditions to ensure precise and dependable results.

MFI and MFR Methods

The melt flow index (MFI) is one of the key rheological properties that significantly contributes to evaluating the flow characteristics of recycled plastics and 3D printing processes. MFI measures a plastic material’s flowability under certain conditions, typically in grams per 10 min. In the context of plastic waste recycling, MFI provides valuable information about the melt processing characteristics of the material. By determining the MFI, recyclers can assess the flowability and viscosity of the plastic waste, which is essential for selecting the appropriate recycling method. Different processes, such as extrusion or injection molding, require specific MFI ranges for optimal performance. Therefore, knowing the MFI allows recyclers to choose the most suitable recycling technique and ensure better overall recycling efficiency [82,121].

Similarly, MFI is crucial for 3D printing applications using recycled plastic materials. When recycled plastics are used as a feedstock for 3D printing, the MFI describes the material’s ability to flow through the printer nozzle accurately. This information helps select the most appropriate recycled plastic for 3D printing, ensuring smooth filament extrusion and precise layer deposition. Consequently, understanding the MFI of recycled plastics facilitates the creation of high-quality and functional 3D-printed objects from waste materials [73].

The Melting Flow Rate (MFR) is another rheological property of extrusion that plays a significant role in plastic waste recycling and 3D printing processes. MFR measures a polymer’s viscosity or flowability when melted under specific conditions. In plastic waste recycling, MFR determines the level of processing required to dissolve and transform the waste plastic into a new usable form. Different plastics have varying MFR values, some highly viscous and others more fluid. Sorting and selecting plastics based on their MFR aids in determining appropriate recycling techniques and equipment to optimize the process. Materials with a high MFR may require more energy and specialized machinery to achieve the desired melt consistency for further processing [76]. MFR is also crucial in 3D printing, where the plastic filament is melted and extruded to create complex three-dimensional objects. A filament with an appropriate MFR ensures smooth and consistent extrusion, leading to accurate and high-quality prints. If the MFR is too low, the filament may not flow freely, causing clogs and inconsistent material deposition. On the other hand, a filament with a higher MFR can extrude too quickly, resulting in imprecise prints with a poor surface finish.

According to a study conducted by Turku and his colleagues (2018), they observed that the MFI of recycled ABS was 8.9 g/10 min, which is noticeably lower than the MFI of the original ABS at 15 g/10 min [22]. This divergence suggests a considerable distinction in flow behavior between the recycled ABS and the virgin ABS. Mishra et al. (2023) found that adding virgin ABS to recycled ABS blends increased the MFI, suggesting enhanced flowability. The rise in the blend’s overall molecular weight and the induction of chain relaxation by virgin ABS have been associated with this improvement [103].

The outcomes revealed a noteworthy increase in MFI as Fe content increases [46]. This trend can be attributed to the heat retention capacity of Fe, where higher Fe content within the composite preserves heat during the extrusion process at elevated temperatures. As a result, the accumulated heat is transferred to the polymer chains, diminishing polymer viscosity and resulting in higher MFI values as the Fe content in the composite increases. The MFR serves as another indicator of rheological properties. Introducing either BP or WD to ABS showed an initial rise in the MFR of the ABS composite filament. Specifically, a 30% increase was observed at a 2.5% inclusion percentage, while a 70% increase occurred at a 5% inclusion percentage. However, exceeding a filler content of 5% resulted in a decline in the MFR; this decrease can be attributed to insufficient mixing of the reinforced particles within the ABS matrix, introducing resistance to heat flow [76]. Correia et al. (2022) also investigated the rheological behavior of PLA using melt flow rate (MFR) measurements. They found that recycled PLA had a 44% higher MFR than virgin PLA, indicating a drop in melt strength and viscosity. Interestingly, adding PBO increased the MFR for recycled PLA by 21% while having no discernible effect on the MFR of virgin PLA. This outcome showed that chain extenders increased MFR [107].

The significance of comparing the MFR of recycled PET material with that of virgin PET under both dried and undried conditions was investigated by Seibert et al. (2022). A substantial difference in more than 2.5 times for virgin PET was evident between the MFR of dried and undried samples. PET is highly sensitive to hydrolytic degradation and has too much humidity influence. However, measuring the MFR of rPET material was only viable post-drying, as undried tests yielded extremely low viscosity and inadequate residence time within the cylinder. These experimental results underscore the necessity of implementing pre-processing drying procedures for the material [52].

Rheometer Methods

The rheological properties of recycled plastics can also be evaluated using various tests conducted with different rheometers. Rheometers are highly instrumental devices used in rheology to measure and analyze the materials’ flow and deformation properties. These instruments evaluate specific rheological properties, such as viscosity, elasticity, and shear stress, to understand the behavior of materials under various conditions. By subjecting samples to controlled and precise flow or forces, rheometers provide crucial insights into a material’s flow characteristics, structural changes, and other important properties. This information is vital for plastic waste recycling and 3D printing processes to optimize product formulations and processes. Rheometers can perform a diverse range of relevant standard test methods and protocols. These measures capture various rheological properties of plastic waste for recycling and 3D printing applications.

To characterize the viscoelastic properties of molten polymers, Martey et al. (2022) utilized a small-amplitude oscillatory shear technique to examine the complex viscosity and storage modulus of various ocean plastics, specifically HDPE and PP mixed with virgin LDPE. Multiple additives were combined with these plastics, such as functionalized clay, styrene multi-block copolymer (SMB), and ethylene-propylene-based rubber (EPR) [122]. A frequency sweep test at 200 °C and a strain amplitude of 10% was conducted, which was within the linear viscoelastic region. The angular frequency range for the sweep spanned from 100 rad/s to 0.1 rad/s. It was found that PP exhibited the lowest viscosity, while the viscosity increased when a blend of HDPE and virgin LDPE was combined with EPR. This increase in viscosity can be attributed to the behavior of EPR, which acts as a filler and immobilizes the polymer chains at the interfaces [122]. The study did not observe significant differences in the storage modulus among the samples. Arrigo et al. (2022) assessed the yield stress of a blend consisting of PP modified with talc and varying percentages (10%, 20%, 30%, 40%, and 50%) of recycled PP. Using a strain control rheometer, they performed a frequency sweep test on the prescribed blends. The results indicated that the blends containing 30% and 40% recycled PP exhibited the highest yield stress values at 260 °C [89]. Another study comparing the frequency sweep test for the virgin PETG and recycled PETG showed that the virgin PETG displayed 40% higher viscosity than the recycled PETG at low frequencies. At high frequencies, the viscosity of both materials was found to be quite similar, suggesting a lower molecular weight for the recycled PETG [79].

Seibert et al. (2022) employed a relaxation test on virgin and recycled PET (vPET, rPET) for dried and undried samples. The relaxation test results demonstrated that the dried vPET exhibited the highest shear stress, followed by the dried rPET and the two undried samples [52]. This disparity can be attributed to moisture within the material, which reduced viscosity. The reduced viscosity of rPET suggests a generally diminished molecular weight distribution, which could potentially correspond to an accelerated degradation process due to shorter chains. Ferrari et al. (2020) assessed viscosity changes in PET, comparing rapid and slow cooling. The findings revealed that all the samples exhibited a moderate pseudo-plastic behavior, with a slight decrease in viscosity with an increasing shear rate. However, cooled PET filament and 3D-printed samples slowly demonstrated an almost Newtonian behavior with low viscosity, which can be attributed to a reduction in the average molecular weight [54]. This reduction is likely a result of ester bond hydrolysis caused by the high temperatures experienced during processing.

In 2021, Patti and colleagues discovered that fresh and reused PLA filaments displayed comparable complex viscosity changes over time and frequency. These patterns indicated a decrease of about 30% after a 10 min testing period. The stability of rheological characteristics over time was enhanced when samples underwent a vacuum-assisted drying process at 80 °C for 10 h [83]. Furthermore, the storage modulus (E’) and the dissipation factor (tan delta) of 3D-printed objects constructed from various PLA-based filaments were also found to be similar [83]. In their 2017 study, Cruz et al. examined the impact of recycling on the zero-shear viscosity of PLA by subjecting it to multiple cycles, ranging up to five cycles. The results showed a decrease in viscosity from 2729.21 Pa.s in extrusion 1 to 219.85 Pa.s in extrusion 5, which confirms the hypothesis of a significant reduction in the molecular weight of the recycled PLA [116].

Gudadhe et al. (2020) employed the ARES G2 instrument from TA Instruments to generate a time–temperature superposition master curve for HDPE derived from waste material. The experimental conditions included a frequency range of 0.1 to 100 and a strain of 5% to evaluate the molecular weight distribution. The results indicated an average molar mass of 9005 g/mol, a mass average molar mass of 98,987 g/mol, and a polydispersity index of 10.99 [123]. These studies highlight differences in melt flow behavior, viscoelastic properties, and molecular weight distribution, offering insights into how material composition, processing methods, and recycling practices impact these properties. The findings underscore the importance of understanding rheological characteristics for optimizing material performance in various applications, such as 3D printing and composite manufacturing [72].

Table 6. Different techniques used to evaluate rheological properties.

Plastic Types	Techniques Used	Main Finding	Reference
PET	Viscosity	Recycled PET samples showed almost Newtonian behavior with low viscosity due to reduced average molecular weight.	[54]
LDPE	MFI	Increasing Fe content to recycled LDPE leads to increased MFI.	[46]
PET	Relaxation	Recycled PET showed a significant increase in MFR by 93% compared to virgin PET and lower relaxation compared to virgin PET.	[52]
ABS	MFR	Adding small quantities of WD to recycled ABS leads to an increase in MFR; however, an increase in WD quantity of more than 5% leads to a decline in MFR due to poor particle mixing.	[76]
PP	Yield Stress	Adding talc power to recycled PP exhibited the highest yield stress at 260 °C.	[89]
PLA	Complex Viscosity	Recycled and virgin PLA showed similar patterns of complex viscosity changes.	[83]
PLA	Zero Shear Viscosity	Recycled PLA showed a viscosity decrease due to a reduction in molecular weight.	[116]
ABS/PS	MFI	Recycled ABS and PS decreased MFI by 40.6% and 4%, respectively, compared to virgin ones.	[22]
PP and EPR	Amplitude Oscillatory Shear	The complex viscosity and storage modulus of the EPR/PP composite increased.	[122]
HDPE	Time–Temperature Superposition	Adding Dimethyl-dibenzylidene sorbitol and LDPE to recycled HDPE, there is a 10-fold increase in dynamic elastic modulus (G’) at 160 °C.	[123]

summarizes different rheological techniques used to assess the rheological properties of recycled and 3D-printed plastic wastes.

3.4. Advanced Characterization of Recycled Plastic Composite

Advanced techniques for characterizing plastic composites at micro- and nanoscales play a significant role in revealing their complex behavior. These methods provide a deeper understanding of the structural, mechanical, and thermal properties of recycled waste plastic composites. These approaches go beyond traditional testing methods, offering valuable insights into the internal composition, interactions between different fractions, and overall material performance. These advanced characterization techniques include but are not limited to scanning electron microscopy (SEM), transmission electron microscope (TEM), and Fourier transform infrared (FTIR). By utilizing these advanced tools, researchers can obtain a comprehensive view of the morphology of these composites, as described in Table 7. This knowledge aids in developing customized plastic composites with enhanced properties suitable for a wide range of engineering and industrial applications.

Ivanov et al. (2019) investigated diverse polymer compositions comprising PLA and economically viable industrial GNP or MWCNT. This research delved into these formulations’ structural, electrical, and thermal attributes. Techniques such as SEM, TEM, and Raman spectroscopy were employed to confirm the uniform dispersion of nanoparticles within the polymer matrix. Notably, mono-filler configurations containing either MWCNT or GNP exhibited enhanced electrical conductivity. Moreover, a synergistic effect in hybrid bi-filler nanocomposites has produced more excellent electrical conductivity than individual CNTs and GNPs at identical filler concentrations [68]. The thermal conductivity showed improvement with higher filler content, particularly in PLA/GNP composites. Morphological analysis unveiled smaller, more uniformly distributed filler aggregates, facilitating improved electron and heat transfer [68]. In a related context, Abdul Haq et al. (2017) performed FTIR analysis on composites of PCL and PLA with varying PLA content (ranging from 10% to 50% by weight). This examination assessed their suitability as biomaterials for FDM. The analysis identified distinctive peaks corresponding to specific vibrational modes, including CH stretching vibration, C=O stretching vibration, and C-O-C vibration in esters. Alterations in peak intensity and position were observed as the PCL and PLA content varied, indicating changes in chemical bonding and molecular interactions. These changes were attributed to variations in the composite’s molecular structure and composition, influenced by the ratio of PCL to PLA and the processing conditions [124].

Kristiawan et al. (2021) conducted a supplementary FTIR analysis to explore the influence of GP on rPP material [49]. Their findings revealed that incorporating GP preserved the position of the conventional PP peak; concurrently, it introduced a distinct new peak. The distinctive peaks in the recycled HDPE filament were validated through FTIR spectroscopy. Additionally, SEM examination demonstrated that HDPE pellets exhibited smooth surfaces and uniform diameters [44]. Another study by Martey et al. (2022) investigated the effects of additives, including clay and rubber, on blends of ocean-bound (o-HDPE and o-PP) and virgin (v-LDPE and v-PS) polymers for 3D printing, utilizing recycled plastic waste. Their FTIR analysis revealed characteristic peaks in the recycled HDPE filament, with minor additional peaks in the filament derived from recycled HDPE flakes, potentially attributable to contaminants. Moreover, SEM imaging highlighted that blends containing PS were prone to brittleness, and those with clay exhibited rough surfaces. Notably, blends incorporating rubber exhibited improved elasticity and reduced void size [122].

Moreover, Bex et al. (2021) examined the application of recycled PETG as the matrix component in continuous CF composites fabricated through 3D printing. SEM analysis showed enhanced adhesion between rPETG and CF compared to the adhesion between PETG and CF. Despite the reduced tensile properties of the rPETG-based composites, their flexural characteristics were similar to those of the composites based on PET [79]. The thermodynamic adhesion work approach substantiated the conclusions, which revealed enhanced adhesion within the rPETG/CFF system. Jamnongkan et al. (2022) conducted an FTIR analysis to understand the chemical interactions in PP blends involving varying concentrations of rPP and composites. Distinct characteristic peaks corresponding to virgin PP and rPP samples were shown, signifying their similarity in chemical structure. Similarly, the spectra of rPP composites with CB revealed the presence of CB within the PP matrices [82].

In a study by Laoutid et al. (2021), vulcanized and devulcanized microparticles obtained from ground tire rubber were incorporated into ABS and thermoplastic polyolefins for utilization in 3D printing. The SEM analysis demonstrated the effective dispersion of rubber particles within the polymer matrix. These composite materials are well suited for producing filaments with precise composition and diameter, facilitating their practical application in 3D printing. Optimized blends exhibited intriguing mechanical properties, with certain ABS-rubber blends similar to thermoplastic polyolefin composite in impact resistance and elongation at break [55]. To investigate the molecular structure and morphology of PET-G/PEF blends, Paszkiewicz et al. (2020) conducted analyses involving FTIR and SEM. FTIR spectroscopy provided evidence of morphological changes and confirmed the calculation of miscibility parameters, indicating partial miscibility and phase separation between PET-G and PEF. SEM micrographs of fractured surfaces revealed a robust interface between PET-G and PEF, characterized by ambiguous phase boundaries [88]. This observation points to specific interconnections between polymer chains, which can be attributed to the elevated processing temperature.

Wuamprakhon et al. (2023) also employed advanced morphological analysis using SEM to investigate the recycled conductive PLA filaments. The SEM results demonstrated that the inclusion of CB as a conductive filler significantly influenced the microstructural properties of the filaments. The analysis revealed that the conductive filler was well-dispersed within the PLA matrix, contributing to the filaments’ enhanced electrical conductivity and mechanical properties [90]. Jayawardane et al. (2023) utilized SEM to analyze the fracture surfaces of tensile specimens made from 3D-printed recycled PLA, virgin PLA, and injection-molded PLA. The SEM images revealed that virgin PLA exhibited lower porosity and more uniform print lines than recycled PLA, which displayed significant voids and print line defects. These imperfections in recycled PLA led to increased crack nucleation and propagation, affecting its mechanical properties. This detailed microscopic analysis underscores the impact of recycling on the quality and performance of 3D-printed components [106].

Ragab et al. (2023) also conducted SEM analysis on the fractured impact specimens of HDPE and PET blends. The SEM images revealed that adding compatibilizers, such as maleic anhydride and sodium dodecyl sulfate, significantly reduced void formation and improved interfacial adhesion between the polymer components. These enhancements in microstructural properties contributed to the improved mechanical performance of the blends [91]. Nafis et al. (2023) conducted SEM and FTIR analyses on wood dust fiber-reinforced recycled PP composites. SEM images revealed that silane-treated filaments exhibited minimal voids and gaps, enhancing interfacial bonding and higher strength. In contrast, untreated filaments showed significant voids and weak adhesion between the fibers and the matrix, which correlated with lower pull test strength. FTIR analysis confirmed the functional groups introduced by the silane treatment, further validating the improvements in interfacial bonding and mechanical properties [92].

A study by Rodríguez et al. (2022) conducted a comprehensive sorting and characterization of dirty non-recyclable mixed plastic from municipal solid waste, which included PP (28.89%), PET (22.02%), PS (9.65%), and rigid PE (4.68%). FTIR analysis revealed cross-contamination with traces of other polymers, highlighting the heterogeneity of the recycled materials. These findings underscore the complexity of recycling mixed plastic waste and the challenges of achieving material purity [125]. Romani et al. (2023) conducted weather simulation tests on 3D-printed recycled polycarbonate (rPC) and rPC/ABS samples using a Xenon Arc apparatus. FTIR analysis revealed minor photooxidation phenomena on the surface of the samples, indicating limited photodegradation effects. The colorimetry tests showed slight discoloration and a minor decrease in lightness post-simulation, with no significant changes in gloss or mechanical properties. These results suggest that recycled materials maintain their structural integrity and aesthetic properties under simulated outdoor conditions, highlighting their potential for sustainable applications in the circular economy [126].

Mercado et al. (2023) conducted an in-depth fractology analysis of cylindrical specimens of recycled rPET using a high-resolution SEM. The SEM analysis revealed distinct fracture behaviors depending on the printing direction. Specimens printed in the Z direction exhibited ductile fractures characterized by buckling and shear forces, indicative of the material’s ability to undergo significant deformation before failure. In contrast, specimens printed in the X/Y direction demonstrated considerable plastic deformation and brittle fractures attributed to molecular diffusion and interlayer cohesion forces. These findings highlight the anisotropic nature of the recycled rPET material and underscore the importance of considering printing orientation in the design and analysis of FFF components [127]. Tolcha et al. (2023) investigated the fractured surfaces of short glass fiber-reinforced recycled HDPE and PET composite filaments through SEM. SEM images revealed that short glass fibers were uniformly distributed within the HDPE and PET matrix without aggregation, contributing to improved mechanical properties. The SEM analysis showed enhanced tensile strength and Young’s modulus by adding 30 wt% short glass fibers compared to pure recycled HDPE. This uniform dispersion and alignment of short glass fibers along the extrusion direction were achieved under optimal blending conditions, preventing nozzle clogging during the 3D printing [94].

Dobrzańska et al. (2023) used optical microscopy to compare rPLA and recycled r-PETG to their new raw material equivalents in FDM and FFF. Microscopic images showed that rPLA had smaller triangle-shaped holes, indicating better layer adhesion, while rPETG exhibited larger, uneven scales. Fracture analysis revealed that r-PETG was more ductile with visible plasticized threads, whereas rPLA had a flatter, evenly scaled surface. The results suggest that recycled materials can perform comparably to virgin materials in 3D printing [110]. SEM analysis was performed by Bergaliyeva et al. (2023) on fracture surfaces of tensile test samples to examine the dispersion and distribution of titanium dioxide nanoparticles (TiO₂) nanoparticles within the recycled PLA matrix. SEM images revealed that TiO₂ nanoparticles were homogeneously distributed within the PLA matrix but tended to form agglomerations. The analysis showed flat and smooth fracture surfaces, indicating a brittle nature. Energy-dispersive X-ray (EDX) analysis corroborated the presence of TiO₂ nanoparticles, confirming their integration into the polymer matrix [96].

Table 7. Findings of advanced characterization techniques.

Plastic Type	Additive	Characterization Method	Findings	Reference
TPU	Wood flour	FTIR, SEM	Improved compatibility between TPU composite	[128]
PLA	GNP/MWCNT	SEM, TEM, Raman	Better dispersion and transfer properties	[68]
PCL/PLA	-	FTIR	Changes in peak intensity and position as PCL and PLA content varied, indicating variations in molecular interactions and chemical bonding	[124]
PP	GP	FTIR, SEM	Introduced a new peak while preserving the conventional PP peak	[49]
HDPE, PP, LDPE, PS	Clay, rubber	FTIR, SEM	Blends with EPR exhibited improved elasticity and reduced void size	[122]
PETG	CF	SEM	Improved adhesion between PETG and CF	[79]
PP	Carbon black	FTIR	No chemical bonding was observed between carbon black and rPP30 molecules	[82]
ABS, TPO	Vulcanized/devulcanized rubber	SEM	Rubber microparticles effectively dispersed in polymer matrix	[55]

Table 7. Findings of advanced characterization techniques.

Plastic Type	Additive	Characterization Method	Findings	Reference
TPU	Wood flour	FTIR, SEM	Improved compatibility between TPU composite	[128]
PLA	GNP/MWCNT	SEM, TEM, Raman	Better dispersion and transfer properties	[68]
PCL/PLA	-	FTIR	Changes in peak intensity and position as PCL and PLA content varied, indicating variations in molecular interactions and chemical bonding	[124]
PP	GP	FTIR, SEM	Introduced a new peak while preserving the conventional PP peak	[49]
HDPE, PP, LDPE, PS	Clay, rubber	FTIR, SEM	Blends with EPR exhibited improved elasticity and reduced void size	[122]
PETG	CF	SEM	Improved adhesion between PETG and CF	[79]
PP	Carbon black	FTIR	No chemical bonding was observed between carbon black and rPP30 molecules	[82]
ABS, TPO	Vulcanized/devulcanized rubber	SEM	Rubber microparticles effectively dispersed in polymer matrix	[55]

3.5. Environmental and Economic Impacts

Using plastic waste as raw materials for creating innovative plastic products not only aligns with environmentally responsible practices but also presents tangible economic and social advantages [129]. Life cycle assessment investigations have explored the conversion of waste plastics into filaments suitable for 3D printing applications. Kreiger et al. (2014) studied the environmental impact of recycled HDPE through distributed AM and compared it to centralized recycling. The findings indicated a significant energy consumption reduction of approximately 80% when distributed recycling was implemented in scenarios with low population density [130]. Furthermore, when considering in-house extrusion in the context of distributed recycling of HDPE, a cost advantage of 89% was realized in contrast to the purchase of commercially available filament [45]. According to research by Gaikwad et al. (2018), producing 1 kg of new ABS was associated with emitting 4.5 kg of carbon. Conversely, shredding 1 kg of recycled ABS yielded 0.9 kg of carbon emissions. Consequently, adopting discarded plastics as the input for 3D printing, as opposed to new polymer materials, could potentially result in a 68% reduction in carbon emissions per kilogram [131].

In pursuit of a zero-waste manufacturing process and to exemplify a sustainable and circular economy-oriented initiative, an endeavor was undertaken to produce face shields during the COVID-19 pandemic. Kantaros et al. (2021) reported that using discarded PLA material resulted in a remarkable 86% reduction in raw material costs [132]. Aligned with the principles of the circular economy, Cañado et al. (2022) conducted a life cycle assessment to comprehensively evaluate the entire production process of 3D printing in creating marine litter products. This study investigated the viability of three newly developed bio-based polymers: PA, PLA, and polyhydroxybutyrate (PHB). The research findings underscored that employing marine plastic waste as the input material in 3D printing is more ecologically favorable than using fresh bioplastics sourced from renewable materials like bio-PA, PLA, and PHB. As a result, this approach leads to a significant reduction in the potential for global warming. Notably, this reduction is 3.7 times and 1.8 times more efficient when contrasted with bio-based polyamide and newly manufactured petroleum-based polyamide [11]. This reduction bears considerable significance in mitigating the environmental impact of the plastic industry and contributes to the pursuit of the targeted 55% reduction in emissions by the year 2030.

Wuamprakhon et al. (2022) aimed to develop high-value supercapacitors using recycled PLA feedstock. The study demonstrated the economic feasibility of the method by achieving a 13.7% reduction in material utilization and a low material cost per electrode (around £0.15). By lowering plastic waste and dependency on virgin resources, turning post-industrial plastic waste into energy storage devices provides a sustainable environmental option [90]. Furthermore, the utilization of waste plastics confers significant economic advantages. Nur-A-Tomal et al. (2020) documented that the average cost of producing 1 kg of original ABS in China was USD 4.2 in 2017 [48]. In contrast, waste plastics are currently obtainable at a minimal cost, with an electricity cost of approximately USD 0.2 to dispose of 1 kg of waste plastics. Hence, adopting an advanced method utilizing waste plastics can reduce the production cost (per kg) associated with the 3D printing process by approximately USD 4.0. This consideration assumes that labor, transportation, drying, filament production, and 3D printing expenses remain consistent for both approaches. The potential effects of utilizing virgin and recycled PLA for distributed recycling through AM were explored by Mendoza et al. (2022) [133].

The outcomes demonstrated a reduction of up to 97% in the production of recycled plastic filament compared to virgin plastic across various impact categories, including climate change, resource depletion, and eutrophication potential [133]. Jayawardane et al. (2023) assessed the life cycle of recycled PLA. Their assessment showed recycled PLA had 93% lower normalized environmental impact and 97.1% reduced global warming potential compared to virgin PLA. Energy consumption was also 91% lower for recycled PLA. At the same time, life cycle costs were 74% higher for recycled PLA due to shorter service life. These results highlight the potential of recycled materials to reduce the environmental footprint of AM despite economic challenges [106]. Ragab et al. (2023) assessed the environmental and economic impacts of recycled PET/HDPE blends for 3D printing. Their life cycle assessment showed recycled blends had a 24.6% lower environmental impact than virgin materials. Economically, they estimated a 50% cost reduction compared to commercial filaments [91]. Silva et al. (2023) offer insightful information on recycled marine plastic PP blends, which showed a 24.6% lower environmental effect than virgin materials. In terms of economics, the study calculated that switching to recycled filament from commercial options would save material costs by 50% [114]. It is evident that using recycled plastics in 3D printing upholds environmental responsibility and offers significant financial benefits, resulting in lower energy consumption and production costs. Additionally, incorporating used plastics into 3D printing can significantly reduce carbon emissions, furthering environmental goals. Therefore, recycling through AM has many advantages, constituting a substantial step towards a circular and environmentally responsible economy.

3.6. Emerging Trends and Future Perspectives

The conducted SLR revealed multiple insights and emerging trends in the potential applications of plastic waste in the AM field. Technologies for chemical recycling can produce higher-quality recycled plastic suitable for 3D-printed filaments. Chemical recycling decomposes plastics into their fundamental chemical constituents, enabling the removal of impurities and the regeneration of polymers with properties similar to virgin materials. A novel closed-loop recycling method for PET using a zinc-based catalyst has been demonstrated by Vollmer et al. (2020) [13]. This method efficiently breaks down PET into its monomers, making it easier to re-synthesize without sacrificing the integrity of the polymer, providing a technique to make premium PET filaments from recycled materials. Similarly, enzymatic degradation techniques for plastic recycling have advanced significantly. A designed enzyme created by Tournier et al. (2020) can break down PET quickly into its component monomers, which can then be depolymerized to develop new plastic polymers [134]. This biotechnological strategy offers a viable way to transform PET recycling, with potential uses in manufacturing filaments for 3D printing. Another area of active research to improve the qualities of recycled plastic filaments is the development of novel additives. The application of cellulose nanocrystals (CNCs) as reinforcing agents in regenerated PLA filaments was examined by Zander et al. (2019) [135]. The findings demonstrated that adding CNCs could considerably enhance the recycled filaments’ mechanical qualities and thermal stability and improve their performance to approach that of virgin PLA. Moreover, Chow et al. (2022) investigated the application of a novel compatibilizer in a different study to enhance the interfacial adhesion of several plastic types in mixed plastic waste [136].

Based on the aforementioned findings, there is ample scope for expanding the knowledge and capabilities of 3D-printed parts. Further investigations should focus on widening the range of materials, optimizing manufacturing processes, exploring composite materials with reinforcing agents, and developing surface textures for specific applications [89]. These advancements shall improve the properties and functionality of 3D printed parts, enabling their broader adoption in various industries and sectors. Efforts can be focused on optimizing the manufacturing processes to determine the ideal parameters for each plastic waste type to enhance properties. Extensive examinations could be dedicated to evaluating this technology’s environmental, financial, and societal repercussions. Moreover, further attention should be focused on post-processing techniques in AM. Those methods, including surface finishing, heat treatment, and electropolishing, are crucial for enhancing the product’s quality and functionality, with current research focusing on integration, automation, and standardization to address challenges in consistency and scalability across different materials and geometries [137]. Additionally, anisotropy in additively manufactured structures poses a significant challenge. This anisotropy property can lead to diverse mechanical characteristics in response to distinct loading circumstances [111]. A deeper understanding of the interlayer bonding phenomenon is paramount to harnessing this technology’s full potential on a larger scale. The ability of 3D-printed parts to maintain structural integrity and mechanical strength relies heavily on the quality of the bond formed between successive layers. Therefore, elucidating the factors that influence interlayer bonding, such as material properties, printing parameters, and post-processing techniques, is imperative. Thermal stresses represent another significant obstacle that needs to be overcome. The heating and cooling cycles inherent in the 3D printing process can induce residual stresses within the printed parts, leading to deformation, warping, or cracking [22,138]. Developing strategies to minimize or mitigate these thermal stresses, such as optimizing the printing parameters, implementing suitable support structures, or exploring advanced thermal management techniques, will improve printed components’ overall dimensional accuracy and quality.

Furthermore, achieving precise dimensional accuracy remains challenging in 3D printing [50]. Factors such as material shrinkage, thermal variations, and machine calibration can result in deviations from the intended design specifications. By investigating and refining the existing methodologies for dimensional control, including calibration techniques, in situ monitoring, and compensation algorithms, the accuracy of 3D printed parts can be significantly enhanced, allowing for greater consistency and reliability in their production. A critical challenge within AM is the protracted and time-intensive printing duration. Unless substantial progress is made in curtailing the printing time, AM cannot achieve a complete substitution of conventional manufacturing techniques.

4. Conclusions

AM is a progressive technology that shows promise as a viable alternative to recycling plastic waste over traditional manufacturing methods. AM technologies can handle intricate geometries and exhibit efficiency concerning material aspects and energy utilization. This systematic review focuses on assessing the various plastic wastes used in AM, which presents a promising and multifaceted domain that holds considerable promise for sustainable production. This comprehensive review was extracted from 88 studies published between 2013 and 2023. The study investigated the transformative potential of integrating recycled plastics into 3D printing processes, with a discernible emphasis on optimizing material properties, addressing inherent challenges, and realizing both environmental and economic benefits. The performance of different plastic wastes was compared, and the advantages of adding additives to improve the properties of each plastic type were discussed. The finding of this SLR study indicated that several plastic wastes, stand-alone or with additives, have been successfully utilized to produce a filament for 3D printing. The key findings of this study are as follows:

• Incorporating additives, from metal nanoparticles to natural materials, is crucial to enhance the properties of recycled plastics in 3D printing. Incorporating additives, such as CB, CF, harakeke, and silica, is crucial for enhancing the mechanical properties of recycled plastics in 3D printing, while GP, BP, and CB are particularly effective in improving thermal properties.
• Introducing additives into 3D printing filaments significantly modulates thermal properties, influencing printability, mechanical integrity, and environmental sustainability in AM.
• The mechanical properties of recycled plastic filaments, influenced by various factors, including plastic type and additive use, play a pivotal role in the performance, cost-effectiveness, and sustainable nature of 3D-printed items relative to their virgin counterparts.
• Variations in thermal and rheological properties can significantly impact the material’s performance, emphasizing the need for standardized testing using rheometers and other related devices for optimizing recycling processes.
• Advanced characterization techniques provide detailed insights into recycled plastic composites’ internal structure, interactions, and performance and help pave the way for further optimization and innovative applications.
• Adopting recycled plastics in 3D printing supports environmental sustainability and provides substantial economic advantages.
• The AM approach leads to decreased energy consumption, reduced production costs, and a significant cut in carbon emissions.

Three-dimensional printing offers vast opportunities, yet it is essential to address existing challenges to harness its full potential by optimizing manufacturing processes for each plastic. However, enhancing methodologies for dimensional control can lead to more accurate and reliable printed part types. Also, understanding interlayer bonding is crucial to mitigate this challenge and ensure consistent properties. Focusing on these challenges and continuously refining processes, 3D printing can revolutionize the manufacturing sector, paving the way for innovative solutions in various industries. Based on the findings of this SLR, multiple recommendations for future research in the field of AM using recycled plastics and composites are proposed: (1) future studies should focus on fine-tuning printing parameters specifically for recycled plastics and composites; (2) research is needed to explore the use of compatibilizers in recycled plastic filaments; (3) novel post-processing methods compatible with recycled plastic filaments should be developed and studied; and (4) the environmental footprint of products made from recycled plastic filaments should be evaluated by conducting a comprehensive life cycle assessment.