In general, ballots obtained from sorting centers are composed of three components: the desired polymer, polymeric contaminants, and some residual wastes. Contaminants can be classified into two categories volatile organic contaminants (VOCs) and solid contaminants. This last can be a polymeric contaminant or another material (metal, wood…).

Compared to virgin polymers, recycled resin presents heterogeneity due to the attacking environment during its lifecycle. Irreversible structural changes can happen at both molecular and morphological levels. This modification can be induced mechano-chemically or by irradiation. The oxidation of polymeric materials creates free radicals such as carbon-centered (alkyl) and oxygen-centered (alkoxyl). This transformation is enhanced by the formation of crosslinked structures caused by the radical recombination of low molecular fragments [31].

Impurities are present in high concentrations in post-consumed plastics. Some of them are VOCs and depend on the polymer type. They come from additives added during the polymerization, such as phenolic antioxidants, consumed during the stabilization process. Furthermore, residues of titanium and aluminum polymerization generate colored salt. Resin absorbs contaminants, and the migration of some products to the matrix of packaging influences the quality of the material after being recycled [5].

In general, decontamination is performed by a degassing system or/and filtration system that is linked to an extruder. The material is melted at a high temperature, which generates VOCs (Volatile Organic Compounds) [32]. Numerous studies confirmed the effect of extruder profile heating on the extrusion performance. The ratio of VOCs was very high when heating reached 250 °C compared to 150 °C [33].

To remove VOCs contained in plastics, extruders are equipped with a devolatilization system. The vacuum present inside the extruder helps to remove volatile particles. Several studies highlighted the influence of using single and multi-degassing systems in a screw extruder [34]. The concentration of the odor was measured by dynamic olfactometry. The result shows that odor intensity decreases after one degassing step from 373 to 279 OU/m³ and after the third degassing step to 235 OU/m³ [35].

The first type of chemical agents are adsorbent agents. They allow the control of polymer emission during extrusion. The addition of 0.30% of adsorbent based on silicate to HDPE virgin pellets reduces the amount of VOCs, and the intensities of odors also decreased [36]. Furthermore, some studies show that the introduction of these particles with post-consumer HPDE in the extruder decreases odor by 50% [34]. The second type of chemical agent is a stripping agent. Their addition to the melt improves the devolatilization of the VOCs contained in the polymer. Their role consists of creating a bubble inside the matrix so the free volume in the melt increases, which helps the diffusion of VOCs in the vapor so that they can easily quit the extruder through the degassing system [36]. The most used stripping agents are water, nitrogen, and air, which enhance the devolatilization of VOCs contained in polyolefins [37]. For example, the use of nitrogen with polyethylene during extrusion reduces VOCs by 50% [38]. In the same context, some researchers developed a mathematical model that showed that the uses of stripping agents such as methanol toluene decrease the time required for degassing, and this number can increase by rising solvent concentration. [39].

A screen changer is an important piece of equipment in a recycling line to remove solid particles from the melt. The most used screens in the extrusion process are manual and hydraulic. The common point between them is the interruption of the melt flow while changing the screen [40]. Large-area filtration systems are available in single vessel-discontinuous or dual vessel-continuous configurations, allowing to change screens without interrupting production [40]. The different types of screens are listed in Table 3.

After being extruded, the polymer can be decontaminated by a purifying process such as extraction by supercritical fluid. Cristancho & Guzman [42] studied the supercritical extraction of VOCs using CO₂ and ethane. The process was performed with pressure in the range (7.6–20.7) MPa and two temperatures, 36 and 60 °C (Figure 5).

The result showed that using ethane was as effective as multiple extractions using CO₂. At a low pressure (7.6 Mpa) and medium temperature 60 °C, the extraction with CO₂ is effective. Higher pressure improves the extraction, but it will increase the operational cost. Both supercritical fluid help to decrease VOC concentration, but CO₂ remain safer and environment friendly than ethane. For this reason, CO₂ is the most used for the extraction of VOCs from polyethylene pellets [42].

Contaminants can be classified into two categories: polymeric contaminants and volatile organic contaminants (VOCs). To identify and quantify polymeric contaminants present in the blends, Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) can be used. Concerning volatile organic compounds, chromatographic methods are the most adapted.

This method is adapted to determine the fraction of polymeric contaminants based on the recording of heat exchange during heating and cooling. Nevertheless, this approach is not suitable to identify LDPE/HDPE or even LLDPE/LDPE because of the similarities between their microstructures and melting temperatures. For example, to identify the fraction of HDPE in isotactic polypropylene, we can model the blend with a known ratio of the virgin polymer. These blends are extruded and analyzed by DSC (Figure 6) [43].

The presence of two melting peaks confirms the immiscibility of these polymers, which has been reported by many other authors [44].

The melting enthalpies were calculated using a linear peak integration and the results for each blend (Figure 7). This calibration curve can be used to determine the HDPE ratio in the PP/HPDE blend.

This technique is used to quantify polymeric contaminants. Light is used to track molecular translations, rotations, and vibrations. The absorbed energy is specific for each chemical bond, and a spectrum is obtained that can be used as a fingerprint to identify polymers [43]. The spectra of the different compositions of blend based on PP and PE are shown in Figure 8.

The calibration curve was plotted based on the ratio of the absorbance (integrated area) of two peaks, 1168 cm⁻¹ for methyl group in PP and a peak of 720 cm⁻¹ for methylene in HDPE (Figure 9). A1168/(A1168 + A720) was plotted as a function of PP content. The calibration curve can be used to determine the composition of the PP/HDPE blend.

For the chemical analysis of recycled plastics, gas chromatography coupled with mass spectrometry (GC-MS) can be used to identify and quantify volatile organic compounds (VOCs). To detect organic contaminants, gas chromatography (GC) is equipped with a mass selective detector (MS). A capillary column with a film can be used for chromatographic separation. The GC oven can be programmed from 40 to 180 °C at 15 °C min⁻¹ and then to 300 °C at 5 °C min⁻¹, held for 12 min [45]. Organic contaminants can be identified by consulting the mass spectra libraries. The quantification of the contaminants can be performed by using external and internal calibration curves. The external standard method creates a calibration curve for a standard sample, and unknown samples are quantified using calibration curves. The internal standard method consists of adding a fixed amount of internal standard substance to an unknown sample when creating a calibration curve using a standard sample, and a calibration curve is created with the concentration ratio vs. peak area ratio for quantification [46,47].