Analytical Methods for In-Depth Assessment of Recycled Plastics: A Review

Abstract

Assessing the detailed characteristics of recycled plastics is essential for evaluating their quality and suitability for high-value applications compared to virgin polymers. This review provides a comprehensive overview of advanced analytical techniques used for characterizing the chemical, structural, morphological, and physical properties of recycled polymeric materials. The techniques examined include Fourier Transform Infrared Spectroscopy (FTIR), Micro-Raman spectroscopy, X-ray Fluorescence (XRF), Inductively Coupled Plasma (ICP) techniques, X-ray Powder Diffraction (XRPD), Differential Scanning Calorimetry (DSC), and Scanning Electron Microscopy (SEM). These methods are critically assessed for their effectiveness in detecting polymer degradation, surface and structural alterations, and the presence of contaminants—factors frequently introduced during mechanical recycling processes. For each technique, this review outlines the working principles, sample preparation protocols, and illustrative case studies while discussing their advantages, limitations, and operational challenges. By synthesizing current knowledge and methodological advancements, this review aims to support the development of robust and standardized quality assessment protocols. Enhancing the reliability and precision of recycled plastic characterization will improve their acceptance as high-quality secondary raw materials, thereby facilitating their upcycling and contributing to the broader goals of the circular economy.

1. Introduction

Plastic waste can originate either at a post-consumer stage, i.e., plastic materials discarded by consumers after use, or at a post-industrial stage, i.e., from plastic scrap generated during the manufacturing phase. In recent years, extensive research has been devoted to improving recycling techniques for managing this plastic waste more effectively. In this context, we can consider two recycling approaches: “closed-loop”, aimed at processing plastic waste into the same type of product (upcycling), and “open-loop”, where plastic waste is converted to a different product, usually of lower quality (i.e., downcycling) [1]. However, according to the 2024 report from Plastics Europe, only 2% of new plastic articles/items currently come from recycled plastics, likely due to their perceived lower quality, often observed as a consequence of the mechanical recycling processes [2].

In mechanical recycling, the main steps are usually collection, sorting, pre-treatment (e.g., size reduction and melting), and post-processing, towards the production of either flakes, fragments, or pellets. First, the plastic waste is separately collected and further transported to recycling plants for sorting and processing. Sorting is an essential step in recycling because it permits the removal of all unwanted plastics based on their polymer type, colour, and additives. This aims to ensure material purity by minimizing unwanted materials before processing. A combination of different sorting techniques is used to obtain a controlled homogeneous stream of material: magnets or magnetic drums, eddy current separators, sink–float separation, induction sorting, X-ray techniques, and near-infrared (NIR) sensors [3]. The sorted material is then usually subjected to a size-reduction and washing step for an increase in surface area and dirt removal. For the production of pellets, after sorting, the material is melted at a temperature ranging from 200 to 275 °C. The final processing step is determined by the desired final product: injection moulding, blow moulding, film blowing or fibre extrusion.

Mechanical recycling can lead to a decline in the properties of recycled plastics compared to their virgin counterparts, primarily due to polymer degradation. During processing, the scission of long polymer chains reduces the molecular weight of the material, negatively impacting its mechanical performance. For example, studies have shown that the viscosity of recycled polyamide (PA6) decreases due to degradation, as determined through rheological analysis, indicating a reduction in molecular weight and an increased flow rate, which, in turn, affects the mechanical behaviour of the plastic [4]. Similarly, properties such as tensile strength, elongation, notched impact strength, and bending strength tend to deteriorate after the recycling of polyethylene and polypropylene. In the case of polypropylene (PP), its shorter polymer chains and higher number of chain ends lead to reduced molecular entanglement and weakened C-C bonding during stretching, ultimately affecting its elongation properties [5,6,7].

Alternatively, pyrolysis can be used to treat plastic waste for material recovery. First, plastic waste molecules are broken down into different monomers, which are then used as fuels or feedstocks for new plastics. However, the use of this treatment as a proper “recycling” technique is still debated as it deliberately alters the molecular structure of polymers [8].

In Europe, recycled plastics can be remarketed as secondary raw materials, whether compliant with the so-called “End-of-Waste” criteria [9]. These require that (i) the waste-derived material has undergone treatment (e.g., recycling), (ii) the outcome of recycling answers to a market need while (iii) meeting the performance standards of their virgin counterpart and (iv) avoiding adverse risk on environments or human health from its intended use. For recycled plastics, these general criteria were ultimately translated into specific requirements on input materials, allowing recycling treatments and quality of output plastics [8]. While lacking a definitive operational definition, the quality of recycled plastics is evaluated based on the content of hazardous and restricted substances, as well as their potential to substitute raw counterparts. This substitutability is assessed by measuring a set of polymer-specific characteristics outlined in industry-standard technical references, such as EN 15347, 15342, 15344-46, and 15348 [10,11,12,13,14,15]. These characteristics include physical parameters (e.g., colour, density, shape), chemical (e.g., ash content, volatile content, extraneous polymers), and mechanical properties (e.g., Melt Flow Rate, impact strength, tensile stress at yield). Although this set of parameters is part of the routine characterization of recycled plastics, it cannot provide a comprehensive understanding of the degraded quality of recycled plastics when compared with non-recycled ones. Consequently, there may be a need to expand this operational definition by incorporating less conventional material features to gain a more thorough insight into the impact of the mechanical recycling process.

In this context, recycled plastic polymers can be better investigated from a chemical, structural, and physical point of view (

Table 1. Summary of the properties and related analytical methods for an in-depth assessment of recycled plastics quality.

Properties	Characteristic	Analytical Method
Chemical	Elemental composition	X-ray Fluorescence (XRF)
	Trace metal concentrations	Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
	Organic compound concentrations	Gas Chromatography–Mass Spectrometry (GC-MS)
	Functional groups	Fourier Transform Infrared Spectroscopy (FTIR)
	Nonpolar molecular structures and conjugation	Raman Spectroscopy
Structural	Crystalline and amorphous phase quantification	X-ray Powdered Diffraction (XRPD)
	Molecular weight distribution	Gel Permeation Chromatography (GPC)
Morphological	Surface morphology and micro-void formation	Scanning Electron Microscopy (SEM)
	Nanoscale mechanical property mapping	Atomic Force Microscopy (AFM)
Physical	Melting/crystallisation behaviour	Differential Scanning Calorimetry (DSC)
	Thermal stability and decomposition temperature	Thermogravimetric Analysis (TGA)
	Detailed viscosity profile	Rotational Rheometry

).

Section 2 refer to the elemental composition of plastics in terms of different polymer types, trace metal concentrations, organic compound concentrations, functional groups, and nonpolar molecular structures [16]. In particular, the presence of unknown substances should be investigated, such as plasticizers, heat and UV stabilizers, flame retardants, pigments for colour, biocides, compatibilizers, fillers (glass and carbon fibres), and anti-sticking agents, which interact with the polymer matrix and alter the chemical behaviour of the recycled plastic [17]. While some of these additives are classified as hazardous, they may still be present in recycled plastic. However, since they no longer serve a functional purpose in the material’s second life, they are now regarded as contaminants [18,19]. To analyse these characteristics, specific analytical techniques are used.

To identify the presence of most of the elements in plastics (i.e., occurring in compounds of Ba, Cd, Pb, Sn, Hg, Cu, Sn, and Bi), X-ray Fluorescence (XRF) could be used [20]. XRF is a destructive, rapid analytical technique that provides both quantitative and qualitative analysis of the elemental composition of the recycled plastic [21]. To analyse trace metals, such as Pb, Cd, Cr, Hg, and Ni, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) or Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) are usually used [22]. The need to detect and quantify the concentrations of Volatile Organic Compounds (VOCs) is usually addressed by Gas Chromatography coupled with Mass Spectrometry (GC-MS). This technique detects and identifies most of the organic compounds that could be found in the plastic [23]. The presence of these VOCs could lead to the formation of peroxides or free radicals in the recycled plastic, thus accelerating degradation. Furthermore, these VOCs with reactive groups may induce oxidation, hydrolysis, or cross-linking in plastics [24]. Fourier-Transform Infrared Spectroscopy (FTIR) is an appropriate technique in the identification of these reactive groups and other functional groups present in recycled plastics [25]. FTIR also plays a crucial role in identifying and classifying polymers [26,27]. Polyethylene (PE) and polypropylene (PP), as nonpolar polymers, inherently possess low surface energy, which limits their compatibility with polar substrates, fillers, and additives. This characteristic affects both virgin and recycled materials. In recycling, however, this limitation may be further exacerbated by the presence of nonpolar, low-molecular-weight compounds—such as residual waxes, lubricants, or degradation by-products—that accumulate during use or processing [28]. These residues can migrate to the surface of the polymer, interfering with interfacial adhesion and, in some cases, contributing to reduced mechanical performance. Moreover, the degradation or loss of compatibilizers originally present in the virgin material can intensify phase separation or dispersion issues in recycled blends and composites. Raman spectroscopy, which is also a good tool for identifying polymers [29], can help to identify nonpolar molecules due to its ability to detect vibrational changes in polarizability [30]. Similar to FTIR, Raman spectroscopy detects vibrational transitions and can be used to identify polymers, additives, fillers, and their chemical modifications in recycled plastic materials. However, Raman is often considered complementary to FTIR because it is more responsive to vibrational modes involving nonpolar bonds, whereas FTIR is more sensitive to polar functional groups. In particular, Raman spectroscopy is highly effective in detecting chemical groups with high polarizability, such as C=C double bonds and aromatic rings. Additionally, Raman analysis is often more suitable than FTIR for identifying pigments and certain inorganic fillers present in recycled polymers. In both techniques, the use of spectral databases and Principal Component Analysis (PCA) can further support the interpretation and classification of complex spectra.

The Section 3 include those characteristics providing information on the polymer chain arrangement and features of the material at a microscopic/molecular level, such as crystallinity and molecular weight distribution. A polymer usually shows a dominant crystal type due to the thermodynamic stability of a molecular conformation, but this could be modified through additives, such as nucleating agents or flow-induced crystallization [31]. Plastics, such as polyolefins, are highly crystalline with a crystallinity percentage that can be higher than 60%. During the recycling of plastics, a reduction in molecular weight can be observed, leading to an increase in their crystallinity due to shorter macromolecules in the arrangement of the polymer chain. On the other hand, the degree of crystallinity could decrease due to the presence of contaminants and organics after recycling, which can form significant amorphous regions [32]. Crystallinity can also affect the morphology and the mechanical properties (e.g., viscosity). X-ray Powder Diffraction (XRPD) is a suitable analytical method for quantifying the crystallinity of recycled plastics. This method helps to understand (detect and quantify) the phase transitions (change in crystallinity) during processing, and it verifies the crystalline morphology of recycled plastics [32]. The extent to which the molecular weight is decreased during mechanical recycling can be assessed by Gel Permeation Chromatography (GPC). GPC allows for obtaining the number-average molecular weight (Mn), weight-average molecular weight (Mw), and the molecular weight distribution (Mw/Mn), which characterizes the recycled plastic [4].

The loss in molecular weight causes the polymer chain to be rearranged, which also influences the Section 4, i.e., the surface characteristics, of the recycled plastic, with, for instance, the development of filaments, the deposition of some contaminants, and the formation of micro-voids due to the discontinuity of polymers [33]. Here, the use of Scanning Electron Microscopy (SEM) permits obtaining detailed images of the morphology of the plastic at scales that can reach microns [34]. Mostly, it is used as a verification analysis to complement the results of other analysis techniques, such as FTIR, XRPD, and ICP-OES/ICP-MS. Knowing the molecular rearrangement and change in morphology at the small scale is of great importance. This is performed using Atomic Force Microscopy (AFM), which allows for mapping the nanoscale mechanical properties, uncovering information on the elasticity, hardness, and deformations of the plastic at the nanoscale level [35]. This visualization of the change in mechanical properties at this scale helps us to have a more detailed, comprehensive understanding of the mechanisms that happen during degradation [36].

Finally, the Section 5 refer to the response of recycled plastics to temperature and applied stresses, i.e., melting/crystallization behaviour, thermal stability, decomposition temperatures, and viscosity profiles. Differential Scanning Calorimetry (DSC) is used to analyse the melting/crystallization behaviour of recycled plastics. This thermal analysis technique is appropriate for fingerprinting the composition of most recycled polymer blends. Based on the known melting and crystallization degrees, it permits identifying the polymers present [37]. Also, this method provides information on the temperatures at the glass transition, onset of melting, peak of melting, and enthalpy of fusion, which are of great importance in measuring the crystallinity of recycled plastics [38]. Physical properties are also essential in the description of the material degradation and stiffness. One of the major analyses of the physical properties of these recycled plastics is the thermal stability and decomposition temperature, which is mostly performed using Thermogravimetric Analysis (TGA) [39]. With the addition of some compatibilizers, the thermal stability of some plastics can either increase or decrease during recycling; TGA evaluates these changes with a good interpretation, allowing us to determine how they affect degradation of recycled plastics [40]. As temperature affects recycled plastics, so do processing conditions and shear rate. The combination of these changes results in a resistance to flow of the polymer, known as the viscosity of the polymer. This property of the plastic gives insights into the molecular weight, degradation level, and processability [41]. To follow these changes during recycling, a detailed viscosity profile is needed, which is obtained using Rotational Rheometric Analysis [42].

In this context, this review aims to provide a comprehensive overview of the most commonly used analytical techniques for the in-depth assessment of the quality of recycled plastics, focusing on chemical, physical, structural, and morphological characteristics. It introduces the principles of each analytical technique and sample preparation and discusses case studies, emphasizing their applications in recycled plastics. Also, their advantages, limitations, and ongoing challenges are discussed in this review. While this review focuses on widely established analytical techniques, such as FTIR, Raman, XRF, ICP, XRPD, DSC, and SEM for the characterization of recycled plastics, it is important to acknowledge that other valuable methods are also used in this field. Techniques, such as Near-Infrared (NIR) and Mid-Infrared (MIR) [43,44], Dynamic Mechanical Thermal Analysis (DMTA), Laser-Induced Pyrolysis Spectroscopy [45], and conventional Gas Chromatography with Flame Ionization Detection (GC-FID), have all been reported in the literature as effective tools for gaining deeper insights into polymer composition, ageing, and contaminant profiling. However, these techniques are not included in the present review in order to maintain a focused scope on the methods most widely applied across chemical, structural, morphological, and thermal property assessment. Future reviews could expand on these or other emerging techniques to provide a broader comparative perspective. By synthesizing our current knowledge and identifying research gaps, this review seeks to contribute to the development of improved quality assessment methods, ultimately supporting the upcycling of recycled plastics as good-quality secondary raw materials.

2. Chemical Properties

Chemical characterization plays an important role in evaluating the composition and potential contaminants in recycled plastics. These methods provide essential insights into the presence of additives, residual monomers, degradation products, and trace metals that may affect the performance and reusability of recycled materials [46]. In this section, major chemical techniques, such as Fourier Transform Infrared Spectroscopy (FTIR), Micro-Raman spectroscopy, X-ray Fluorescence (XRF), and Inductively Coupled Plasma (ICP), are discussed, highlighting their principles, applications, and relevance in the quality assessment of recycled plastics.

2.1. Fourier Transform Infrared Spectroscopy (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique for the characterization of recycled plastics, providing essential chemical and structural information. It enables the identification of polymer types, additives, and degradation products by the detection of vibrational transition characteristics of chemical groups (marker bands, with characteristic group frequencies) and/or a given molecule [47]. This technique is very important in recycling applications, as it helps in the determination of the purity and composition of recovered plastics, ensuring their suitability for reuse. FTIR spectroscopy works by measuring the absorption of infrared light at specific wavelengths corresponding to the frequencies of the vibrational normal modes of a polymer. This produces a characteristic spectrum, often referred to as a molecular fingerprint, which can be compared against reference databases for material identification [48]. In recycled plastics, FTIR is particularly useful for detecting oxidation, hydrolysis, and other chemical modifications resulting from processing and environmental exposure, which are related to material degradation and guide the development of more sustainable recycling methods.

Proper sample preparation is critical for obtaining high-quality FTIR spectra. Recycled plastic samples can be analysed in various forms, including thin films, pressed pellets, and powders. ATR-FTIR (Attenuated Total Reflectance) is commonly used for plastics (Figure 1), as it allows direct analysis without extensive sample preparation. Cryomilling may be employed to produce fine powders from bulk recycled plastics, improving spectral homogeneity and ensuring consistent measurements. During FTIR measurements, the choice between transmission and reflection modes depends on sample thickness and transparency. Transmission mode is ideal for thin films, while ATR mode is preferred for bulk samples as it enhances surface sensitivity. Reflection techniques, such as specular and diffuse reflection, can also be applied to detect surface modifications in recycled plastics. Ensuring proper contact between the sample and the ATR crystal improves data quality and reproducibility. FTIR spectrometers typically consist of a broadband infrared light source, an interferometer, and a detector. The interferometer modulates the infrared light, and the resulting signal is processed using Fourier transformation to generate the absorption spectrum [49]. Modern FTIR instruments are equipped with highly sensitive detectors, such as mercury cadmium telluride (MCT) or deuterated triglycine sulphate (DTGS), which enable precise spectral acquisition.

The selection of spectral resolution and scanning parameters depends on the level of detail required. A resolution of 4 cm⁻¹ is commonly used for general polymer identification, while higher resolutions (e.g., 1 cm⁻¹) are preferred for detecting subtle chemical changes [50]. Data acquisition software such as: OMNIC Paradigm by Thermo Fisher scientific and LabSpec 6 by HORIBA facilitates spectrum processing, peak assignment, and quantitative analysis, helping researchers determine polymer composition and detect contaminants in recycled plastics.

Several literature studies discuss the results of FTIR analysis on recycled plastic samples. Dodi et al., in their recent study on substrate contamination identification in recycled polyethylene terephthalate (PET) using ATR-FTIR, used rat DNA samples to contaminate blends of recycled PET [51]. In their study, once fundamental transitions of the polymer were recognized, by comparison with the spectrum of a reference virgin polymer, or considering vibrational band assignment from the literature, IR bands of spurious species (i.e., contaminants) were safely identified, and a diagnosis of the polymer modification/pollution occurring in the recycled material was performed. PE degradation can lead to the formation of hydroxyl (–OH) and carbonyl (C=O) groups, which give rise to characteristic absorption bands in the 3300–3500 cm⁻¹ and 1680–1750 cm⁻¹ regions, respectively. The relative intensity of these bands, compared to the fundamental vibrations of the polymer backbone, provides useful information on the extent of degradation. In another study, an FTIR analysis revealed clear differences between virgin and recycled polyethylene (vPE and rPE), mostly due to the effects of oxidation, heat, and UV exposure during recycling [52]. In the rPE samples, a peak at 1240 cm⁻¹—associated with hydroxyl groups—was significantly more intense than in the vPE samples, suggesting chemical changes from degradation. A new carbonyl stretching band also appeared around 1720 cm⁻¹ in rPE, and a weak, broad band was observed between 3300 and 3500 cm⁻¹, likely linked to hydroxyl groups. However, care must be taken in interpreting this region, as moisture can also cause similar signals. These spectral changes point to polymer chain scission and the introduction of oxygen-containing functional groups. For polypropylene (rPP), a broad peak near 1744 cm⁻¹, absent in the virgin material, was noted. This band, attributed to carbonyl stretching, may have resulted from early oxidation during recycling or the presence of additives containing carbonyl groups. The broadness of the peak could indicate a variety of oxygenated species forming on the polymer chain. Once the characteristic bands of the base polymer were established—either by comparing to virgin spectra or using known reference data—these additional features helped detect degradation and contamination in the recycled material. In the study by [53] the FTIR spectra of two recycled polypropylene (PP) samples showed differences in absorption band intensities, which the authors linked to variations in mechanical performance [53]. While this suggests a correlation between spectral features and material quality, it is important to note that FTIR intensity can be influenced by factors such as sample thickness, contact pressure (in ATR), or overall setup configuration. Their study did not specify whether normalization to a reference band or internal standard was applied. Therefore, interpretations based on intensity differences should be made with caution or, ideally, supported by relative intensity analysis under controlled conditions [53]. FTIR has been successfully applied in plastic waste recycling to identify polymer types within mixed streams and to detect the presence of common fillers, including titanium dioxide, talc, clay, silica, and calcium carbonate [54]. These results highlight the versatile utility of FTIR in the characterization of recycled plastics.

FTIR faces some challenges in plastic analysis. Firstly, plastic samples have to be dry to avoid interference with moisture and false results. This is because FTIR is sensitive to moisture; indeed, water strongly absorbs infrared (IR) radiation, especially in the O-H stretching region (3200–3600 cm⁻¹) and bending region (approximately 1600 cm⁻¹). Secondly, it faces some challenges with irregular-shaped microplastics, as reflectance measurements can cause refractive [55]. It is also less effective for plastic samples smaller than 500 microns and can take time to analyse large samples with high spatial resolution [56].

2.2. Micro-Raman Spectroscopy

Micro-Raman spectroscopy is a sensitive and non-destructive analytical technique widely used for the characterization of recycled plastics, similar to FTIR. It provides molecular-level information on polymer structure, chemical composition, and material degradation. By analysing the vibrational modes of polymer chains, Micro-Raman spectroscopy enables the identification of different plastic types, distinguishing between polymers with similar elemental compositions but different molecular arrangements. One of the primary advantages of Micro-Raman spectroscopy is its ability to detect chemical modifications induced by recycling processes [57]. Variations in peak intensities and shifts in Raman spectra can indicate oxidative degradation, cross-linking, or chain scission, which are crucial for understanding material ageing and performance [58]. Also, the technique permits the analysis of polymer blends, fillers, and additives, making it a valuable tool for assessing the quality and consistency of recycled plastics.

Micro-Raman spectroscopy requires minimal sample preparation, making it an efficient technique for recycled plastic analysis. Solid plastic fragments, thin films, or even microplastics can be directly analysed without extensive pre-processing. However, for enhanced spectral quality, surface cleaning may be performed to remove contaminants that could interfere with Raman signals. Measurements are typically carried out using a Raman spectrometer equipped with a microscope, allowing for high spatial resolution and targeted analysis of microscopic regions. Different laser wavelengths (e.g., 532 nm, 785 nm, or 1064 nm) can be used depending on the polymer type and fluorescence interference. The choice of excitation wavelength is critical, as certain polymers exhibit fluorescence that may overshadow Raman signals, necessitating the use of longer-wavelength lasers [58]. Micro-Raman analysis is performed in either confocal or non-confocal mode, with confocal setups offering improved spatial resolution and depth profiling capabilities. The laser is focused on the sample using high-magnification objectives (Figure 2), ensuring precise Raman signal acquisition. The collected spectra are then processed using software, such as LabSpec 6, or Origin 2024b, for baseline correction, peak fitting, and spectral deconvolution [59].

Raman spectral databases, including those provided by commercial libraries or open-source platforms, are used for polymer identification and comparison.

In one study, Micro-Raman was used in the identification of nanoparticles (NPs) on the surface of recycled polyvinyl chloride (PVC) powder (RPP) from commercially available recycled plastics. The authors performed Raman single-spot analysis on the NPs obtained from the shedding of the powder. As a result, Raman spectral features with characteristic adsorption peaks at 639 and 697 cm⁻¹ were assigned to the C-CL stretching vibration in the PVC polymer, and adsorption peaks at 1178, 1432, and 2917 cm⁻¹ corresponded to the C-H rocking vibration, C-H bending vibration, and C-H stretching vibration, respectively [60]. These peaks matched with the spectrum of the RPP matrix, indicating that these NPs originate from the grinding of RPP. In 2022, Marica et al. studied vibrational changes after 5 years of degradation of low-density polyethylene (LDPE) plastic using Micro-Raman spectroscopy and compared them to a reference LDPE polymer before it could be recycled [61]. As a result, they observed a higher background in the waste’s spectra compared to that of the reference spectra, indicating a possible presence of micro-impurities, persistent even after the regular washing of the plastic waste. Also, they observed a change in the functional group orientation or interaction between adjacent polymer chains due to continuous photo-oxidation [61]. Micro-Raman spectroscopy is widely used in the identification of microplastics (MPPs). In one study, it was used to assess the quality of recycled plastic bottles. The polyethylene (PET) bottles made of polypropylene (PP) caps and PE seals were found to have about 242 ± 64 MPP/L after 11 openings of the bottle [62]. Overall, 80% of the MPPs were identified in the smallest size class investigated (10–50 µm). The above results show the importance of Micro-Raman in the specific chemical characterization of recycled plastics.

Micro-Raman is a technique that is easily affected by fluorescence interference, especially when samples contain dyes or additives, which mask the Raman signal; a common example is the total shadowing of Raman transitions by very strong Raman scattering of the carbon black present in some recycled plastics [63]. As compared to FTIR, it may require higher acquisition times, and sample preparation needs to be performed carefully to avoid any surface contamination [64].

2.3. X-Ray Fluorescence (XRF)

X-ray Fluorescence (XRF) is a widely used analytical technique for the elemental analysis of recycled plastics. This method provides qualitative and quantitative information on the elemental composition of materials, making it particularly useful in assessing the presence of additives, contaminants, and hazardous elements in recycled plastic waste [21]. By detecting elements ranging from sodium (Na) to uranium (U), XRF plays a critical role in ensuring compliance with regulatory standards, such as the Restriction of Hazardous Substances (RoHS) directive, which limits toxic elements like lead (Pb), cadmium (Cd), and mercury (Hg) in consumer products. XRF analysis provides essential insights into the composition of recycled plastics, including the identification of additives and fillers, the detection of heavy metals, the verification of polymers, and homogeneity and contamination assessments [65].

XRF analysis requires minimal sample preparation, making it highly efficient for the routine screening of recycled plastics. Solid samples, including plastic pellets, flakes, or films, can be directly analysed using handheld or benchtop XRF instruments. For better precision in quantitative analysis, samples are ground into fine powders and pressed into pellets to ensure uniform measurement conditions. Calibration with certified reference materials (CRMs) is crucial to obtain accurate elemental concentrations. XRF instruments operate in two main configurations: Energy Dispersive X-ray Fluorescence (EDXRF) and Wavelength Dispersive X-ray Fluorescence (WDXRF). EDXRF uses a solid-state detector to simultaneously detect multiple elements. It is widely used for rapid screening and portable applications, whereas WDXRF utilizes a crystal-based wavelength dispersion system to achieve higher resolution and sensitivity, making it ideal for detailed quantitative analysis [21]. A primary X-ray source, typically an X-ray tube, excites the sample, causing characteristic secondary X-rays to be emitted (Figure 3). These emissions are detected and analysed to determine the elemental composition. The choice of detection system and operating parameters depends on the required sensitivity and the specific elements of interest. Data processing is performed using specialized software, such as Bruker’s SPECTRA.ELEMENTS or PANalytical’s SuperQ 6, which facilitate elemental quantification, peak deconvolution, and background subtraction [66]. These software solutions enable the precise interpretation of XRF spectra, ensuring the reliable identification of elements even at trace levels.

Several studies have shown the use and importance of XRF in recycled plastic analysis. Kajiwara et al. used XRF to identify and quantify bromine in 540 recycled plastic consumer products, including children’s toys, and found that 19% of the samples contained bromine at a concentration of ≥30 mg/kg⁻¹ [67]. This XRF step was the first step in identifying the bromine-positive samples. These samples were further chemically analysed using combustion-ion chromatography to reveal the presence of polybrominated diphenyl ether (PBDE). With the aim of removing bromine from recycled plastic from waste electric and electronic equipment, similar to the previously mentioned study, Charitopoulou et al. used XRF to identify bromine-positive samples and reduce the bromine quantity in those samples, because they hypothesized that the bromine present represented Brominated flame retardants (BFRs) [68]. With the use of Soxhlet extraction, they were able to reduce the bromine incorporated into the samples and then studied their degradation behaviour using Evolved Gas Analysis (EGA). The results showed that the thermal degradation in most cases followed a one-step mechanism, verifying their hypothesis on BFRs. In these two cases, we see the use of XRF as a primary tool to screen samples for a deeper analysis in detecting contaminants in recycled plastics. Also, XRF was used to detect rare earth oxide markers in recycled polyoxymethylene (POM). These rare earth oxide markers are used to mark POM before it enters the recycling process for improvement in tracer-based sorting [69]. XRF showed the accumulation of these rare earth oxides into the mechanically recycled POM.

In all the above applications in recycled plastic analysis, XRF has some challenges in quantifying light elements, like carbon, nitrogen, and oxygen, which are fundamental components of plastic polymers due to their low fluorescence and absorption effects. These elements emit low-energy fluorescence signals that are easily absorbed by the air or the sample. Additionally, there is the problem of matrix effects, especially in heterogeneous recycled plastic samples, where different densities, compositions, and surface characteristics can impact the fluorescence intensities. If the instrument is not properly calibrated, it might lead to inaccurate readings.

2.4. Inductively Coupled Plasma (ICP)

Inductively Coupled Plasma (ICP) techniques, including ICP–Optical Emission Spectroscopy (ICP-OES) and ICP–Mass Spectrometry (ICP-MS), are good analytical tools for detecting and quantifying trace elements in recycled plastics. These techniques provide high sensitivity and precision on materials, in general, making them essential for assessing the presence of heavy metals and additives in plastic waste streams. ICP is particularly useful for ensuring compliance with environmental and safety regulations, such as the RoHS directive and the European Union’s REACH regulation [70]. ICP analysis enables the detection and quantification of a wide range of toxic metals, such as lead (Pb), cadmium (Cd), mercury (Hg), and arsenic (As), to assess the safety of recycled plastics [59]. It also helps in obtaining the additive composition through the identification of elements, like antimony (Sb), used as a catalyst in polyethylene terephthalate (PET) production, or zinc (Zn) and calcium (Ca) in stabilizers for polyvinyl chloride (PVC). Finally, ICP techniques can perform material purity verification, which consists of determining contamination levels and the presence of foreign elements that may have been introduced during the recycling process.

Unlike XRF, ICP requires extensive sample preparation. Plastic samples must first undergo digestion to convert solid material into a liquid form suitable for analysis. This process typically involves acid digestion, which is the use of strong acids, such as nitric acid (HNO₃) or hydrochloric acid (HCl), often combined with hydrogen peroxide (H₂O₂), to break down the polymer matrix and release elements into solution; microwave-assisted digestion, which is a more controlled and efficient method that speeds up the dissolution process and ensures complete digestion of the plastic sample [70]; and filtration and dilution, which ensure that the final solution is free from particulates and at a suitable concentration for analysis.

Once prepared, the liquid sample is introduced into the ICP instrument (Figure 4), where it is nebulized and transported into the high-temperature plasma (6000–10,000 K). The elements in the sample are ionized, and their emission spectra (ICP-OES) or mass-to-charge ratios (ICP-MS) are measured for quantitative analysis. ICP instruments operate in two main modes. The first is ICP–Optical Emission Spectroscopy (ICP-OES), which measures the light emitted by excited atoms and ions in the plasma to determine their elemental concentrations. This technique is well-suited for multi-element analysis, especially at moderate concentration levels. The second is ICP-Mass Spectrometry (ICP-MS), which detects ions based on their mass-to-charge ratio. ICP-MS offers extremely high sensitivity and is ideal for identifying trace elements, even at parts-per-billion (ppb) or parts-per-trillion (ppt) levels. A well-optimized setup includes appropriate calibration with certified reference materials (CRMs) and internal standards to correct for matrix effects and instrument drift. Data analysis is performed using specialized software, such as Agilent’s ICP Expert v7.7 or Thermo Fisher’s Qtegra ISDS, which allows for peak identification, quantitative calibration, and interference correction [37]. These tools ensure accurate element quantification and data validation, which are critical for quality control in recycled plastic applications.

To determine the exact heavy metal load in recycled plastics, Klingenberg et al. used ICP-MS for quantification [22]. They were able to determine the quantities of ten regulated metals (automotive regulatory basis; global automotive declarable substance list), including Hg, As, Cd, Cr, Co, Cu, Se, Pb, Sn, and Ni, in post-consumer (PCR), post-industrial (PIR), and virgin polypropylene. Cu and As showed the highest and lowest concentrations, respectively, in both PCR and the virgin material. Also, regarding health and regulatory concerns, the measured heavy metal content of the PCR material did not meet the threshold. The virgin PP was found to be less contaminated than the recycling samples. On the other hand, Chibwe et al. analysed 60 elements (metals and metalloids) from 21 recycled plastic samples, including high-density polyethylene (HDPE), LDPE, PP, and PE, using ICP-MS [71]. They detected at least 36 of the 60 elements in every sample, and their concentrations varied by almost 7 orders of magnitude (<0.005–2980 mg/kg). Ca, Na, and Fe had the highest concentrations, whereas Hg, Zn, Pb, Cr, Ca, Sb, Na, Sr, Fe, and Co were the most frequently detected. With the aim of determining different contaminants in recycled polyethylene (R-LDPE) samples of agricultural, post-commercial, post-industrial, and post-consumer origin, Núñez et al. used ICP-MS as a preferred technique for the determination of metals [71]. The samples were analysed for the following metals: Li, B, Na, Mg, Al, Ca, K, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Ag, Cd, In, Ba, Tl, Pb, and Bi. Amongst these metals, those present in most samples and in higher concentrations were Fe, Al, Cu, Pb, Zn, and Ti. The samples with the higher concentrations of these metals were found to be those that were recycled through traditional washing and were also the dirtiest, as they contained inks and were coloured samples.

In summary, ICP is a good elemental analytical technique, but it is a destructive technique and requires complex and time-consuming sample preparation. Cryomilling and acid digestion are necessary for plastic matrix breakdown, and this step is at risk of contamination and/or incomplete digestion [24]. Furthermore, ICP-MS is very sensitive to matrix effects and spectral interferences (e.g., polyatomic ions), which must be corrected through careful method development. Finally, certified reference materials are essential to ensure reliable results.

3. Structural Properties

For structural characterization, the techniques used are essential in understanding how recycling processes affect the internal arrangement of polymer chains, particularly in terms of crystallinity and phase composition. Many polymers are amorphous in nature and do not exhibit crystalline ordering—examples include PMMA and polystyrene, which are typically used as glasses or rubbers. In contrast, semi-crystalline polymers, such as polyethylene (PE) and isotactic polypropylene (iPP), are capable of forming ordered crystal structures and, in some cases, different polymorphs. These structural properties have a direct impact on the mechanical strength, thermal stability, and performance of recycled plastics. X-ray Powder Diffraction (XRPD) is a widely used technique in this context, giving information on crystalline phases, crystallite size, and the degree of crystallinity. In this section, the application of XRPD in analysing recycled plastics is explored, with attention to its role in evaluating structural changes and identifying phase transitions caused by degradation or reprocessing.

X-Ray Powder Diffraction (XRPD)

X-ray Powder Diffraction (XRPD) is a powerful analytical technique widely used in the characterization of semi-crystalline polymers, providing crucial insights into their structural properties. It enables the identification of crystalline phases and several inorganic fillers, determination of the degree of crystallinity, and estimation of crystalline domain sizes [72]. These aspects are essential in assessing the impact of recycling processes on polymer structure, as changes in crystallinity can influence the mechanical performance and thermal stability of the material. Beyond phase identification, XRPD also allows for the quantitative determination of crystalline and amorphous phases, offering a deeper understanding of the molecular organization within recycled plastics [73]. Structural refinement techniques further aid in determining atomic coordinates and crystallographic occupancies [74], providing detailed structural information that helps evaluate material consistency and degradation effects caused by recycling. This level of analysis is fundamental in optimizing recycling processes and improving the quality of secondary raw materials.

The accuracy of XRPD analysis depends on proper sample selection, preparation, and loading. Recycled plastic samples are typically processed into fine powders using cryomilling, a technique where materials are milled at extremely low temperatures to prevent thermal degradation and minimize structural alterations. This ensures a more homogeneous sample and enhances the quality of diffraction data. Proper sample loading into the XRPD instrument, such as ensuring even distribution and minimizing preferred orientation effects, is crucial for obtaining reliable diffraction patterns and precise structural information. XRPD measurements can be performed using diffractometers operating in either reflection or transmission geometry, depending on the sample characteristics and analytical requirements [75]. The reflection mode is more commonly used for bulk materials, while transmission geometry is advantageous for thin or low-density samples, as it enhances sensitivity to weakly scattering phases. The choice of geometry affects data quality and should be carefully considered based on the nature of the recycled plastic sample.

A monochromatic X-ray source is typically used (Figure 5) to improve the resolution and accuracy of diffraction patterns, reducing background noise and improving phase identification. Data collection involves scanning over a range of diffraction angles (2θ) to capture the characteristic diffraction peaks of the crystalline phases present [71]. For phase identification, XRPD data are compared against reference databases, such as the ICDD Powder Diffraction File (PDF), using software like Profex 5.2 and HighScore Plus 5.3.0 [76]. These programs facilitate peak fitting, phase quantification, and structural refinement, ensuring a thorough interpretation of the data.

XRPD has been widely used to evaluate the effects of fillers on crystallization and to identify the phases of filler materials. Świetlicki et al. detected brucite, periclase, and quartz in talc-filled recycled PP and observed an increase in the elasticity modulus with increasing talc content [77]. To investigate the interaction between calcite and plastic materials in plastic-reinforced mortar, Kane et al. used XRPD to compare two treatment techniques: microbially induced calcium carbonate precipitation (MICP) and enzymatically induced calcium carbonate precipitation (EICP). Their study found that EICP deposited more calcite on recycled plastic flakes—including PET, PVC, LDPE, PP, polystyrene (PS), and acrylonitrile butadiene styrene (ABS) [78]. For crystallite size evaluation, Mekprasart et al. saw a decrease in the crystallite size of recycled PET nanocomposites with increasing rutile content from 2% to 10% [79]. This reduction might be attributed to the photocatalytic activity of rutile, which could induce some changes in the PET crystal structure through light-induced reactions or stress relaxation.

XRPD is usually limited when analysing highly degraded plastics because it requires finely powdered samples and may not detect low levels of crystallinity or subtle structural changes without advanced data analysis. This technique is also not suitable for identifying chemical composition or non-crystalline additives. Finally, phase identification relies on comprehensive reference databases, and overlapping peaks in complex mixtures can complicate interpretation.

4. Morphological Properties

Morphological characterization provides valuable information about surface and microstructural features, such as individual layer thickness and polymer components of recycled plastics [33], revealing the physical effects of degradation, contamination, or filler distribution. Scanning Electron Microscopy (SEM) is a key technique in this domain, offering high-resolution imaging of surface texture and internal structural features. In this section, the role of SEM in evaluating the morphology of recycled plastics is discussed.

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is a technique used for analysing the surface morphology and microstructural properties of recycled plastics. By providing high-resolution imaging, SEM allows for detailed examination of polymer degradation, contamination, and structural modifications that occur during recycling processes [80]. It is particularly useful in assessing the impact of mechanical, thermal, and chemical treatments on plastic materials, helping to ensure quality control and optimize recycling techniques. SEM helps in observing the surface morphology, filler and additive distributions, and microstructural defects. Detailed imaging of surface textures, defects, and irregularities can be obtained, and the images can indicate the effects of mechanical recycling, degradation, or contamination. Also, the identification of inorganic fillers, reinforcements, and additives, such as calcium carbonate, glass fibres, and titanium dioxide dispersed within the polymer matrix, can be seen. Microstructural defects, such as the detection of voids, cracks, and delamination within recycled plastics, which may affect mechanical properties and processing performance, are also visible with the help of SEM [33].

Proper sample preparation is essential for obtaining high-quality SEM images of recycled plastics. Since most plastics are insulating, they require specific steps to avoid imaging artifacts. For internal structure analysis, samples are often cryo-fractured using liquid nitrogen, producing clean and undeformed fracture surfaces. To prevent charging during imaging, non-conductive samples are usually sputter-coated with a thin layer of gold (Au), platinum (Pt), or carbon (C). When cross-sectional analysis is needed, samples can be embedded in epoxy resin and sectioned with a microtome to expose the internal features [81]. To obtain accurate and detailed microstructural information of recycled plastics, careful selection of imaging and detection parameters should be performed during SEM analysis [82]. The choice of the electron beam source influences image quality, with the Field Emission Gun (FEG) scanning electron microscope offering a better resolution compared to conventional tungsten filament scanning electron microscopes. Acceleration voltage is mostly set between 1 and 20 kV, depending on the desired depth of penetration and image resolution. For surface imaging, lower voltages are preferred, whereas for deeper structural analysis, higher voltages are preferred. Scanning electron microscope systems have various detectors (Figure 6), including Secondary Electron Detector (SED) and Backscattered Electron (BSE) detectors, which highlight compositional differences by detecting contrasts in atomic numbers. With SEM, we can have varying magnification ranges going from 10× to over 100,000×, making it ideal for both low-resolution overview scans and high-resolution analysis of surface features and internal microstructures in recycled samples. SEM images are processed and analysed using specialized software, such as ImageJ 1.54p, Thermo Fisher’s Avizo 2024.2, and Oxford Instruments’ AZtecLive 6.2 [83]. These tools support a variety of analyses, including image enhancement, contrast adjustment, and particle size and shape characterization. When coupled with Energy Dispersive X-ray Spectroscopy (EDX), they also enable elemental mapping of the sample surface.

In the plastic industry, SEM has been used to detect impurities and assess the effectiveness of various recycling techniques. For example, in the dissolution and filtration of polypropylene composites with glass fibres, SEM images demonstrated that the recycling technique successfully recovered clean glass fibres with minimal organic matter [84]. Additionally, this technique has been used to monitor the recycling process of polymer composites filled with quartz. SEM analysis revealed that during the moulding process, quartz can redistribute and form agglomerates, particularly in areas where a polymer is depleted of filler [85]. To upcycle buffing leather fibres from industrial waste streams and explore their potential as flame-retardant additives for polypropylene, Sanchez-Olivares et al. used SEM to investigate the impact of these fibres on polypropylene morphology and its other properties [86]. Their findings revealed that the addition of buffing leather fibres significantly enhanced the flame retardancy of polypropylene without significantly altering its morphology. With the aim of evaluating the effects of mechanical recycling on the microstructure of recycled high-density polyethylene pellets and bottles, Zeng et al. evaluated the morphological properties of rHDPE and virgin HDPE using High-Resolution SEM (HR-SEM) at 25 and 1000 m magnifications [87]. The surface of the region of the rHDPE pellets exhibited relatively rough textures compared to the vHDPE, and with magnifications at 1000×, the rHDPE pellets had large pits and cracks in the wake of the formation of rugged textures. This shows how well SEM can present immediate differences between recycled and virgin plastics.

SEM identifies a limited chemical composition of a sample on its surface or fractured cross-section, providing only point information and not bulk information of heterogeneous recycled materials [88]. Moreover, non-conductive polymers, such as plastics, can accumulate charges on their surfaces, thus leading to image distortion after a period time. The interpretation of degradation mechanisms (e.g., oxidation) only through SEM imaging is often insufficient without supporting techniques.

5. Physical Properties

The understanding of how materials respond to temperature, mechanical stress, and processing conditions is discussed in this section. Amongst the techniques used, Differential Scanning Calorimetry (DSC) is particularly important for assessing thermal transitions, such as melting, crystallization, and glass transition temperatures. These properties provide insight into the thermal history, degree of crystallinity, and potential degradation of a plastic [89]. This section highlights the use of DSC in recycled plastic analysis, with emphasis on its ability to track thermal behaviour and support the assessment of material consistency.

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) is a thermal analysis technique used to characterize the thermal properties of recycled plastics. It measures the heat flow associated with phase transitions, providing critical information on the thermal behaviour of polymers. DSC plays a crucial role in assessing the quality and consistency of recycled plastics, ensuring they meet performance standards for reuse in various applications. DSC analysis provides information on several important properties of recycled plastics, including glass transition temperature (Tg), melting temperature (Tm), Crystallization Temperature (Tc), and degree of crystallinity. Tg indicates the temperature at which a polymer transitions from a rigid to a rubbery state, which is crucial for determining processing and application conditions. Tm helps identify the crystalline phase melting point, differentiate polymer types, and assess thermal stability, whereas the Crystallization Temperature (Tc) is the temperature at which polymer chains arrange into a crystalline structure upon cooling, which is useful in evaluating recrystallization behaviour [90]. The degree of crystallinity quantifies the proportion of crystalline material in a polymer, influencing mechanical properties like strength and rigidity. By conducting DSC under oxidative conditions, it is possible to study the degradation and stability of recycled plastics, which is essential for quality assurance [91].

DSC requires careful sample preparation to ensure accurate thermal analysis; the recycled plastic samples can be in the form of pellets, films, or powders. Samples are typically cryomilled to obtain fine powders, ensuring homogeneity and reproducibility in measurements. A small, precisely weighed amount of a sample (typically 5–10 mg) is placed in an aluminium pan, which is then sealed and loaded into the DSC instrument (Figure 7). A reference pan, usually empty, is placed alongside for comparison. DSC operates by heating or cooling the sample at a controlled rate while measuring the heat flow differences between the sample and reference [92]. The key parameters in DSC analysis include the heating rate, typically set between 5 and 20 °C/min to balance resolution and sensitivity [93]. The measurement is often carried out under a nitrogen atmosphere to prevent oxidation or, alternatively, in air to study degradation behaviour. Another important parameter is the temperature range, which generally spans from −50 °C to 300 °C, depending on the polymer type. Specialized software, such as TA Instruments’ Universal Analysis 2000 v5.5.3 or Netzsch Proteus 9, is used to process DSC thermograms [94]. These tools enable baseline correction, peak integration for enthalpy calculations, and curve fitting to accurately determine thermal transition points.

Petrovič et al. performed a comparative analysis of virgin and recycled plastics based on thermochemical characteristics to study the impact of recycling on the material properties [95]. The plastic chosen in their study was HDPE, and from the results obtained, it was shown that the ash content was much higher in recycled HDPE (5.18 wt%) than in virgin HDPE (0.03 wt%). Differences between the samples were seen in their different crystallinity degrees: 60.17% for virgin HDPE and 19% for recycled HDPE. R-HDPE exhibited lower degradation temperatures as compared to virgin HDPE, most likely due to the presence of impurities. Knowing that varying amounts of polypropylene are usually found in recycled high-density polyethylene (rHDPE), Juan et al. used DSC to determine the presence of PP in these blends [96]. The different melting temperature (Tm) values of both polymers allowed them to differentiate PP from HDPE. The Tm corresponding to rHDPE appeared around 131 °C, while a small peak was observed at 163 °C for PP impurities in the recyclates, thus confirming contamination of rHDPE. Along the same line as Juan et al., Scoppio et al. used DSC to evaluate the PE and low-density PE (LDPE) contents in recycled polyolefin blends [97]. Firstly, they performed DSC analysis on 29 virgin PE materials: HDPE, LDPE, linear low-density polyethylene (LLDPE), and metallocene linear low-density polyethylene (m-LLDPE). Secondly, some HDPE, LLDPE, and LDPE grades were blended with PP homopolymer and block copolymer in different concentrations. Finally, recycled grades from post-consumer feedstock streams were analysed with the virgin and blended materials. The results showed that when heating the samples at 10 °C/min after a first heating and subsequent cooling cycle, the different types of PE grades melted in separate temperature ranges, depending on their microstructures. The different PE melting points found for the PE types analysed with DSC were essential in determining the content of the corresponding grades in the polymer blends. Furthermore, the change in thermal behaviour of the recycled grades indicated a change in microstructure and possibly the presence of impurities other than PE and PP.

The presence of additives, plasticizers, and fillers can make the data interpretation of thermal transitions difficult because they could be affected by sample mass, heating rate, and preparation inconsistencies. With blends, analysing the polymer can be challenging due to the overlapping or interfering thermal transitions. When using DSC, which is a bulk technique, the localized degradation or surface-level differences in recycled plastics may go undetected.

6. Conclusions

In the context of a circular economy, the quality of recycled plastic is paramount. Therefore, the analytical methods used to assess recycled plastics must be precise and efficient. The techniques discussed in this review (FTIR, Micro-Raman, XRF, ICP, XRPD, SEM, and DSC) are essential for characterizing the physical, chemical, and structural properties of plastics. While each method has proven its value, none alone can provide a complete picture of plastic quality or degradation behaviour; thus, the combination of two or more of these techniques will give more accurate information on a recycled plastic. Although this review focused on a selected group of commonly applied techniques, future work could integrate other complementary methods, such as NIR, DMTA, and GC-FID, for a broader characterization strategy.

However, as specified in this review, each technique also faces specific limitations, whether related to sample preparation, sensitivity, matrix effects, or interpretation complexity. These challenges show the need for standardizing protocols, proper calibration, and complementary use of methods to overcome analytical gaps. This characterization is invaluable to polymer engineers in designing new products.