IR Spectroscopy as a Diagnostic Tool in the Recycling Process and Evaluation of Recycled Polymeric Materials
Abstract
1. Introduction
- The recycled material is different from virgin polymer: while it may be similar in chemical characteristics and performance, it is effectively a new material.
- Recycled polymers can vary from batch to batch, due to inconsistent and hard-to-control input materials. In some cases, inhomogeneities may be present even from pellet to pellet, or within a single pellet (especially if foreign inclusions are present, such as metal flakes).
- Chemical identification, including additives: especially useful for studying unknown formulations, can achieve quantitative diagnostics through calibration curves based on reference samples of known composition.
- Diagnosis of chemical modifications: important for monitoring degradation during production (e.g., extrusion, molding, spinning) or aging under various environmental conditions.
- Structural characterization: identifying molecular conformation, degree of crystallinity, and structural defects in semi-crystalline polymers.
- Morphological analysis: distinguishing amorphous from crystalline phases, identifying crystalline polymorphs, quantifying their relative fractions, and recognizing polymer chain orientation, as in fibrous materials.
- Thermal transformation analysis: monitoring phase transitions (e.g., melting) and post-treatment effects (annealing, thermal cross-linking) by recording spectra during controlled heating/cooling cycles, thereby reconstructing the material’s thermal history.
2. Materials and Methods
2.1. Materials
2.2. Spectroscopic Characterization
3. Results and Discussion
3.1. General Characteristics of Polymeric Materials and Their IR Spectra
- A polymeric material is often a formulation: in addition to the polymer itself, it contains additives that meet processing requirements and ensure the durability of the final product. Additives may provide chemical stability, while fillers can modify mechanical properties (e.g., reinforcing glass fibers) or electrical properties (e.g., graphene particles). Many polymeric materials intended for specific technological applications consist of blends of two or more polymers or copolymers with variable composition. The resulting IR spectrum is therefore a superposition of the spectral responses of all components and cannot perfectly match that of a pure homopolymer or even samples of the same polymer/copolymer that differ in additive type and/or concentration.
- Automated identification (typically using spectral libraries) is an effective and valuable method for determining the polymer family, especially when combined with supervised analysis. This is commonly employed for plastic sorting in recycling processes. However, especially for semi-crystalline polymers, sample preparation can influence morphology [30,31], resulting in spectral differences compared to literature references or database entries. Furthermore, the measurement technique—such as transmission, attenuated total reflectance (ATR), or reflection—can significantly alter the spectral pattern (band shape and intensity), even when spectra are mathematically converted to a common absorbance (or transmittance) scale. Note that absorbance conversion—except in transmission mode—is based on theoretical models and assumptions that are never perfectly met in practice.
- Effective use of spectral libraries often requires data post-processing: baseline correction and subtraction of solvent, substrate, or contamination bands may be necessary. Spectral comparison also requires proper normalization procedures, typically based on normalizing absorbance intensity to a reference band. These processes are not automatic and require expertise, as they may introduce artifacts.
3.2. Techniques and Measurement Setups
- ATR: One of the most convenient techniques for polymeric materials. It detects absorption of the evanescent wave generated at the surface of a high-refractive-index crystal in contact with the sample. Since surface layers often contain additives (e.g., slip agents, release agents), for bulk analysis, it may be necessary to remove the surface layer or section the sample. With special ATR accessories, direct measurements on finished products are also possible.
- Specular reflection: Suitable for polymeric materials with good reflectivity (also depending on surface finish). The sample must be thick enough to prevent the collection of transmitted photons (as in double transmission setups).
- Diffuse Reflectance (DRIFT): Captures diffusely scattered IR radiation over a large solid angle, from which the IR spectrum is obtained. Applicable to samples with high IR scattering (e.g., powders or granular solids), where transmitted IR is negligible and specular reflection is weak.
3.3. Spectral Analysis
3.3.1. Sorting
3.3.2. Comparative Analysis of Recycled Polymers: R-HDPE
3.3.3. Detection and Quantification of Polypropylene in R-HDPE Samples
- Absorption intensities of overtone and combination bands are much weaker than fundamental transitions, reducing bands saturation effects even in relatively thick films. In fact, thicker samples can enhance the visibility of these weak bands.
- Additives and fillers exhibit very weak absorptions in the NIR, making their contribution negligible compared to the main polymer component. Hence, the obtained information primarily reflects the bulk polymer.
- Transmission mode ensures analysis of the bulk material, preventing overestimation of surface chemical species possibly present on pellets.
3.3.4. IR Detection of Polymer Degradation
4. Conclusions
- The use of IR for materials characterization provides a detailed description of chemical composition, molecular structure, and morphology. Different materials or issues may require choosing different experimental setups. Standardization of protocols remains a challenge.
- R-materials are different from virgin ones. Available IR techniques allow the identification of several differences between R-polymer materials and virgin polymers.
- Characterization of R-polymers can suggest strategies to bridge the gap and/or develop new structure–property correlations with application perspectives in mind.
- The durability of R-polymers should be evaluated through appropriate accelerated weathering or ageing tests, followed by structural analysis, e.g., using IR.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
HDPE | High-Density Polyethylene |
R-HDPE | Recycled High-Density Polyethylene |
LDPE | Low-Density Polyethylene |
PP | Polypropylene (isotactic) |
ATR | Attenuated Total Reflectance (spectroscopy) |
NIR | Near-Infrared (spectroscopy) |
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Sample | PP [mg] | HDPE [mg] | ||
---|---|---|---|---|
a | 0.0 | 160.5 | 0.00 | 0.067 |
b | 7.0 | 149.4 | 4.69 | 0.080 |
c | 13.8 | 151.5 | 9.11 | 0.105 |
d | 19.6 | 120.5 | 16.27 | 0.139 |
e | 27.8 | 136.2 | 20.41 | 0.152 |
f | 26.9 | 106.2 | 25.33 | 0.160 |
Sample | ||
---|---|---|
8 | 0.124 | 14.25 |
4 | 0.107 | 10.00 |
7 | 0.089 | 5.50 |
3 | 0.08 | 3.25 |
6 | 0.078 | 2.75 |
2 | 0.077 | 2.50 |
5 | 0.073 | 1.50 |
1 | 0.068 | 0.25 |
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Hu, K.; Brambilla, L.; Castiglioni, C. IR Spectroscopy as a Diagnostic Tool in the Recycling Process and Evaluation of Recycled Polymeric Materials. Sensors 2025, 25, 6205. https://doi.org/10.3390/s25196205
Hu K, Brambilla L, Castiglioni C. IR Spectroscopy as a Diagnostic Tool in the Recycling Process and Evaluation of Recycled Polymeric Materials. Sensors. 2025; 25(19):6205. https://doi.org/10.3390/s25196205
Chicago/Turabian StyleHu, Kaiyue, Luigi Brambilla, and Chiara Castiglioni. 2025. "IR Spectroscopy as a Diagnostic Tool in the Recycling Process and Evaluation of Recycled Polymeric Materials" Sensors 25, no. 19: 6205. https://doi.org/10.3390/s25196205
APA StyleHu, K., Brambilla, L., & Castiglioni, C. (2025). IR Spectroscopy as a Diagnostic Tool in the Recycling Process and Evaluation of Recycled Polymeric Materials. Sensors, 25(19), 6205. https://doi.org/10.3390/s25196205