Recycled Polyolefin and Polymeric Waste for Industrial Applications: A Comprehensive Review

2.1 Literature search and selection

A systematic literature review was conducted in the key indexing databases (Scopus and Web of Science) and publisher databases (ScienceDirect), with the support of Google Scholar and backward/forward citation. The literature search was focused on the period 2000-2025, whereas the latest five years were considered in the discussion of the emerging advances. Titles/abstracts were screened, and then the records were evaluated in full text regarding eligibility. Peer-reviewed journal articles and authoritative reviews that reported (or critically assessed) morphology-processing-property relationships in recycled polyolefin blends and mixed-plastic recycling were eligible, whereas studies were excluded that focused on virgin polymers and did not recycle or studies that lacked enough detail on processing/processing/property relationships to be interpreted mechanistically.

2.2 Synthesis approach and central thesis

The variables that were extracted were blend composition, melt-processing route, compatibilizer/filler type and architecture, morphology descriptors (e.g. droplet size, co-continuity and interfacial features), degradation indicators (e.g. changes in melt flow rate and oxidation/chain scission signatures), and mechanical performance metrics (tensile strength/modulus, elongation at break, impact strength, and toughness). The literature is summarized under an overall thesis: to achieve predictable performance in recycled polyolefin blends, a careful design of interphase is necessary; consequently, compatibilization decisions and recycling paths are to be jointly optimized, instead of being considered as separate processes [18, 19].

2.3 Conceptual framework used in this review

The manuscript is structured according to an interphase-engineering map that connects (i) drivers of variability and incompatibility of feedstock, (ii) compatibilization methods (non-reactive, reactive, and filler-assisted), and (iii) recycling methods (mechanical, solvent-based purification, chemical/catalytic and emerging bio-based routes). This framework relates the morphology and interfacial chemistry talked about in the early part of the review to the mixed-waste case study and to technology-selection consideration in the latter part to provide a critical sense of what works, when, and why [18, 20].

The structure-property distinction of commodity polymers is critical to recycling of mixed wastes, since polarity, crystallinity, melt viscosity and thermal stability control interfacial adhesion, phase morphology and degradation in melt processing, the same factor that leads to the blend failures in Section 5. In this regard, this section provides a brief overview of the most typical plastics found in both municipal and industrial waste streams, their main characteristics, common uses, and limitations on recycling [30, 31]. The recycling practice classifies plastics broadly as thermoplastics (re-meltable and reprocessable) and thermosets (irreversibly crosslinked and non-re-meltable), which are highly influential in determining the viable end-of-life choice [32, 33]. Low-density polyethylene (LDPE), High-density polyethylene (HDPE), PP, Polystyrene (PS), polyethylene terephthalate (PET), and Poly(vinyl chloride) (PVC) are the most widely used polymer materials in industry [34, 35].

Figure 3. Repeat-unit structure of low-density polyethylene (LDPE)

Note: LDPE is a nonpolar, semicrystalline polyolefin; chain branching lowers crystallinity and density, influencing melt rheology and property retention during recycling [36].

4.1 Low-density polyethylene (Thermoplastic)

LDPE is a branched, low-crystallinity polyolefin, which results in lower density, high flexibility, and good chemical and electrical insulation; its repeat-unit structure is shown in Figure 3. These attributes make LDPE common in films, bags, liners, squeeze bottles, and protective packaging. In a recycling stream, LDPE commonly exists in both post-consumer recovered (PCR) pellets and as raw resin used for reprocessing; cleanliness and control of the thermal/oxidative history are critical to preserving ductility and processability [36].

4.2 High-density polyethylene (Thermoplastic)

HDPE has a linear backbone and significantly higher crystallinity than LDPE, which gives it greater stiffness and strength; the structure of its repeat unit is shown in Figure 4. Recycled HDPE is commonly obtained from post-consumer containers, particularly milk and detergent bottles, and is processed into products such as bottles, pipes, and FIBC regranulate. Because the physical properties of HDPE are highly dependent on grade and prior processing history, controlling contamination and melt history is essential for ensuring consistent performance in recycled products [37].

Figure 4. Repeat-unit structure of high-density polyethylene (HDPE)

Note: HDPE has a predominantly linear backbone, leading to higher crystallinity and stiffness; these features strongly affect melt viscosity, shrinkage, and recyclate performance [37].

4.3 Polypropylene (Thermoplastic)

PP is used in many applications such as packaging, textiles, automotive components, and home furnishings due to its good chemical resistance, low moisture absorption, and good electrical performance. The repeat unit of PP is shown in Figure 5. The plastic’s semi-crystalline structure provides a good balance between stiffness and toughness; plus, its recyclability fits with circular manufacturing goals. In recycled applications, PP is also used as fiber reinforcement (e.g., in recycled-aggregate concrete), where formulation and processing determine mechanical outcomes [38].

Figure 5. Repeat-unit structure of polypropylene (PP)

Note: PP is a nonpolar, semicrystalline polyolefin; tacticity and crystallinity govern stiffness, heat resistance, and sensitivity to degradation during repeated processing [38].

4.4 Polystyrene (Thermoplastic)

PS possesses a set of characteristics that support its widespread use across multiple sectors; its repeat-unit structure is shown in Figure 6. PS is used in disposable cutlery and food-service items, appliance housings, smoke-detector cases, and packaging. Expanded PS (EPS) recycling is practiced in several regions (notably in parts of Europe), but recovered PS often remains brittle, which can limit high-performance reuse. Recent work has also explored surface modification of recycled PS films for functional applications such as triboelectric energy-harvesting devices [39].

Figure 6. Repeat-unit structure of polystyrene (PS)

Note: The aromatic side group raises glass-transition temperature and stiffness, but also contributes to brittle fracture and challenges in mechanically recycled products [39].

4.5 Polyethylene terephthalate (Thermoplastic)

PET possesses a set of properties that make it suitable for a wide range of applications; its repeat-unit structure is shown in Figure 7, and it is commonly designated by the resin identification code #1. It is a dominant packaging polymer for beverage bottles and thermoformed containers and is also used in fibers, films, straps, and engineering components. In recycling, moisture control is critical because ester linkages can undergo hydrolysis at processing temperatures, contributing to molecular-weight loss and property drift if drying is insufficient [40, 41].

Figure 7. Repeat-unit structure of polyethylene terephthalate (PET)

Note: PET contains polar ester linkages that enable strong intermolecular interactions but also introduce hydrolysis sensitivity; moisture control is therefore critical during recycling [40].

4.6 Polyvinyl chloride (Thermoplastic)

PVC is a compound that has the properties that render it applicable in a large variety of applications as illustrated in Figure 8. Additive packages including plasticizers, stabilizers, lubricants and pigments have allowed it to be modified to its rigidity, flexibility and color and this has facilitated its wide application in construction, electrical products and consumer goods. PVC in mixed-plastic recycling is especially an issue as HCl and other acidic products emitted during the dehydrochlorination process can increase degradation reactions, corrode processing equipment, and complicate the further recycling routes. It has been demonstrated through pyrolysis based studies that PVC pollution at approximately 3 wt.% is a significant source of char generation during polyolefin waste pyrolysis [39, 42, 43]. When using PVC-specific recycling conditions, including processing plastisols, the addition of fractions of recycled PVC may result in a significant rise in viscosity. This can necessitate formulation modification, especially of plasticizer content, to ensure that there is an appropriate processing window, and acceptable properties are typically obtained within an application-specific range of recycled content [40].

Figure 8. Repeat-unit structure of polyvinyl chloride (PVC)

Note: The chlorine-bearing backbone can dehydrochlorinate during processing, releasing HCl and driving corrosion and degradation in mixed-waste recycling [42].

PVC production across the EU-27, Norway, the UK, and Switzerland is estimated at around 6.5 million tonnes per year, which reflects its economic importance and highlights the need to improve end-of-life management, as shown in Figure 9 [41]. PVC continues to be widely used in construction, automotive, and electrical applications because of its durability, resistance to acids, bases, and salts, and relatively low cost. However, the complexity of its additive formulations and its chlorine chemistry make high-quality recycling difficult, which has prompted continued research to develop recycling chains that are both safe and effective [44].

Figure 9. End-use distribution of the ~6.5 million tons of polyvinyl chloride (PVC) produced annually within the EU-27, Norway, the UK, and Switzerland [41]

Tables 1 and 2 are a summary of representative physical and mechanical properties of the main commodity plastics. Table 1 shows the morphology, density and thermal-transition temperatures, whereas Table 2 presents the degradation onset and major mechanical parameters (strength, modulus, and strain at break). Critical note on Tables 1 and 2. The values of densities, melting ranges, degradation initiating temperatures, and tensile strengths should be viewed as values, not constants, as they change depending on grade (molecular weight distribution), additives (stabilizers, plasticizers, fillers) and previous thermal/oxidative history. In the case of mixed-waste blends, contrasts that are most decision-relevant include: (i) differences in polarity and crystallinity that govern interfacial tension and phase separation (e.g., PET vs. polyolefins), (ii) mismatched melting/processing windows that facilitate selective melting and coalescence, and (iii) degradation thresholds that determine the maximum processing temperature (particularly in streams containing PVC). Where possible, future revisions of these tables should report ranges (or typical commodity-grade intervals) and explicitly indicate whether values correspond to virgin or recycled grades, as this improves the predictive link to the failure modes summarized in Section 5.

Table 1. Physical and mechanical properties of main plastic materials 1/2 [45]

Note: Low-density polyethylene; HDPE: High-density polyethylene; PP: Polypropylene; PS: Polystyrene; PET: Polyethylene terephthalate; PVC: Polyvinyl chloride.

Table 2. The primary plastic materials' mechanical and physical characteristics 2/2 [45]

Note: LDPE: Low-density polyethylene; HDPE: High-density polyethylene; PP: Polypropylene; PS: Polystyrene; PET: Polyethylene terephthalate; PVC: Polyvinyl chloride.

This section summarizes recent progress in polymer-recycling technologies, and the trends that shape industrial adoption are summarized in Figure 10.

Figure 10. Schematic overview of plastic waste management routes (primary to quaternary), highlighting mechanical recycling, energy recovery, and chemical recycling pathways [53]

Mechanical recycling is the most commonly implemented due to its rather low capital intensity and established infrastructure, although repeated melt processing causes thermo-oxidative degradation, chain scission and property losses, which are further enhanced by contamination and additive variability. Such limitations have increased interest in alternative paths, such as solvent-based purification and chemical recycling (depolymerization, catalytic upgrading and thermochemical conversion such as pyrolysis) to obtain monomers or feedstocks in heterogeneous streams [53-57]. Simultaneously, catalytic process intensification (homogeneous and heterogeneous) is intended to enhance selectivity and decrease power requirements, whereas enzymatic pathways (most of them mature in PET) are able to perform depolymerization at lower temperatures under mild conditions [56, 58]. Supercritical-fluid and related separation concepts are also being explored to enhance purification and fractionation of mixed-plastic streams while limiting additional degradation.

6.1 Principal (primary) mechanical recycling: Principles and processes

Primary mechanical recycling is most effective for relatively clean and well-sorted plastic waste streams [59]. Achieving closed-loop performance with post-consumer recycled (PCR) polymers requires strict sorting, cleaning, and decontamination to reduce incompatible polymers, inks, fillers, and oxidative damage. These additional steps increase operational costs and remain among the major barriers to broader implementation [60]. Mechanical cleaning, with or without washing, is therefore necessary to minimize defects, odor, gels, and inclusions in the final products [61]. The main benefits of implementing closed-loop strategies from cradle to grave (and material circulation) are schematically highlighted in Figure 11.

Figure 11. Schematic summary of key advantages of closed-loop and end-of-life recycling for automotive plastics (material-cost reduction, supply security, regulatory compliance, and footprint reduction) [62]

Most polymer recycling begins with a mechanical size reduction, such as shredding, crushing, or milling. This step leads to a feedstock that homogenizes with the new-product stream and is reintegrated into production cycles together with, for example, raw polymer or stabilizers, or even compatibilizers. Mechanical cleaning with or without washing is needed to minimize defects and odor, gels, and inclusions in final products [61].

After melting, recyclates can be reformed using standard thermoplastic processing methods such as injection molding, extrusion, rotational molding, and compression/heat pressing. These routes are widely applicable to PE, PP, PET, and (in controlled cases) PVC, which can be remelted and reshaped multiple times; however, each reprocessing cycle can still reduce molecular weight and alter rheology and mechanical performance [63].

To achieve high efficiency in closed-loop recycling systems, three conditions are particularly important:

• Recycled material is reintegrated into manufacturing rapidly to minimize additional aging and logistics-related contamination.
• Contaminants are removed or managed to levels compatible with the target application (including inks, paints, fillers, and incompatible polymers).
• The recyclate remains sufficiently compatible with the intended formulation (including virgin polymer and additives) to process reliably in conventional equipment [64, 65].

6.2 Secondary (open-loop) mechanical recycling: techniques and material integrity

Secondary (open-loop) mechanical recycling—so-called downcycling, as illustrated in Figure 12—transforms used thermoplastic materials into products of decreased value and/or performance compared to their original application. Unlike primary recycling, in which material quality can be maintained to some extent, secondary recycling leads to an accumulation of molecular and mechanical degradations; thus, the upcycling is often inappropriate for critical applications. Some common products are plastic composites, packaging, and construction items [66].

Figure 12. Principle of open-loop (secondary) recycling, where recyclates are typically downcycled into lower-performance applications due to contamination and cumulative degradation [66]

6.3 Chemical recycling for end-of-life plastics: Concepts and routes

In Europe, chemical recycling is increasingly regarded as a strategic complement to mechanical recycling within circular-economy and climate-policy initiatives such as the European Green Deal [67]. As shown in Figure 13, chemical recycling includes technologies that convert plastics that are difficult to recycle mechanically into monomers, feedstocks, or other chemical products. These technologies include depolymerization and catalytic routes, as well as thermochemical processes such as pyrolysis [54-56]. As a complement to mechanical recycling, these approaches can recover value from heterogeneous or contaminated streams that would otherwise be downcycled, while potentially generating higher-purity outputs for demanding applications [57]. In this review, chemical recycling refers to processes that alter the molecular structure of polymers through bond cleavage or functional transformation to produce reusable chemical building blocks [68].

Figure 13. Simplified flow diagram of chemical recycling routes for polymers, including depolymerization, solvent-based purification, and thermochemical conversion [69]

Tables 4 and 5 provide a comparative overview of the principal plastic recycling routes, including their suitable feedstocks, key advantages, and main limitations. Mechanical recycling continues to be the dominant route for relatively clean and homogeneous waste streams. In contrast, chemical and other advanced recycling technologies are receiving increasing attention because of their potential to handle mixed, complex, and contaminated plastic wastes.

Table 4. Comparison of key plastic recycling processes, plastic wastes to be recycled, their strengths, and weaknesses

Note: Polyethylene terephthalate (PET); High-density polyethylene (HDPE); Low-density polyethylene (LDPE); Polypropylene (PP); Polystyrene (PS); Poly(ethylene terephthalate) (PET); Polyethylene (PE); Polyurethane (PU); Polylactic Acid (PLA); Polyhydroxyalkanoates (PHA).

According to Table 4, it can be concluded that mechanical recycling will probably continue to be the most common path to PE and PP since the technology is relatively well-developed and, as a rule, does not consume as much energy as most other pathways. Its performance is, however limited by tolerance to contamination and accumulative degradation of the polymer.

Solvents based purification or dissolution/precipitation can sometimes be used to recover the quality of the material to near that of virgin polymers. However, these methods are constrained by solvent recovery requirements, safety and operating expenses.

More heterogeneous or contaminated plastic feedstocks can be processed by chemical and catalytic recycling. Their technical and economic feasibility; however is highly dependent on product selectivity, chlorine control in PVC containing streams, process scale and catalyst life.

In this respect, the use of technology must be considered as a portfolio decision and not a one-way decision. Mechanical recycling should take clean fractions, whereas purification or compatibilization should be used where it can avoid downcycling. Recycling of chemicals should be applied to residual streams that cannot be held in closed material loops. Techno-economic assessment and life-cycle assessment should guide such decisions [47, 48, 52].

Existing TEA/LCA data also show that the choice of routes is very sensitive to the assumptions about feed quality and process structure. Mechanical recycling is usually the least energy-demanding and greenhouse-gas-intensive of comparatively clean streams, and purification pathways heavily rely on solvent recovery efficiency and throughput. In comparison, chemical and catalytic reactions are more energy-intensive, upgrading-demanding, and catalyst-stable reactions. Comparative tests thus ought to embrace harmonized system limits and report material decision variables, including solvent recovery, energy source and contamination-related downtime, to back strong ranking of recycling options of mixed and multilayer plastic feedstocks [47, 48, 52].

6.4 Techno-economic and life-cycle considerations

In addition to the property recovery at the laboratory level, cost, throughput, regulatory compliance and supply-chain constraints are all factors that eventually dictate the adoption of industrial level. The least capital- and energy-intensive recycling path is usually mechanical recycling, although its value declines quickly in the presence of contamination which causes severe downcycling. In certain instances, purification pathways, including dissolution/precipitation, can recover the performance of materials near that of virgin polymers, but they require high solvent-recovery efficiency, process safety and reasonable operating costs. Chemical recycling has the potential to increase the scope of feedstocks to which it can be applied (especially when dealing with heterogeneous or contaminated streams), though its practicability remains highly scale-dependent, energy-price-dependent, product-upgrading-dependent, and catalyst-dependent [47, 48]. The implication of the techno-economic and life-cycle of material performance should therefore be included in future research or at least be raised in discussion. As an illustration, research must be conducted on the effect of residual moisture or chlorine-based contaminants on operating cost, downtimes and product quality [19, 52]. This reporting will be critical in coming up with solutions that are not only scientifically effective but also economically viable and scalable in circular polyolefin value chains.

Table 5. Recent developments in technologies and the research trends of each recycling process

Recent research is converging on three main directions: (i) improved mechanical recycling through more effective washing, decontamination, and energy-efficient processing; (ii) selective chemical approaches, including catalysis, controlled depolymerization, and dechlorination, to broaden the range of treatable waste; and (iii) hybrid schemes that combine physical separation with targeted chemical upgrading [19, 47]. However, many emerging methods are still evaluated using model feedstocks rather than realistic post-consumer mixtures. In addition, multi-cycle property retention is rarely assessed under industrially relevant residence times and shear histories [19, 73]. Future studies should therefore adopt standardized comparators, including virgin materials, recycled but uncompatibilized materials, and compatibilized systems where relevant. They should also report uncertainty ranges associated with feedstock variability. Taken together, these needs support the central perspective of this review: scale-up requires interphase-focused design, contamination-aware operation, and benchmarking frameworks that are directly useful for process-level decision-making.

The author gratefully acknowledges the supervisors for their continuous guidance, encouragement, and support throughout the preparation of this review. The author also extends sincere appreciation to the Dean of the College of Materials Engineering, the Head of the Department of Polymer Engineering, and all staff members for their support and valuable efforts. Appreciation is further extended to the staff and laboratories of the Department of Polymer Engineering, College of Materials Engineering, University of Babylon, for the facilities and assistance provided during the completion of this work.