Effect of the Recycled HIPS Surface Yellowing Phenomenon on Its Properties

Abstract

The paper investigates the effect of the degree of HIPS surface yellowness on its properties: colorimetric, surface, rheological, and mechanical. In order to prepare three naturally degraded samples, about 1 kg of white HIPS flakes, semi-yellow HIPS flakes, and yellow HIPS flakes, segregation based on colorimetric analysis was applied. Then, these samples were subjected to ATR-FTIR analysis, sessile drop contact angle measurements, and MFI analysis. These analyses were repeated for standardized specimens made of the segregated HIPS flakes. The average absorbances were determined for 50 HIPS samples of each type in the form flakes. Finally, mechanical tests were carried out on the standardized specimens. As follows from the research, yellowing of the HIPS surface affects the final color of the standardized specimens, which is confirmed by optical colorimetry. Moreover, material degradation demonstrated by yellowing of its surface and confirmed by a decrease in ATR-FTIR spectra absorbance, is associated with changes in mechanical and rheological properties, as well as in surface characteristics. The novelty of this study lies in the investigation of naturally degraded HIPS samples under laboratory conditions (the HIPS materials were not subjected to artificial aging using laboratory equipment), obtained from waste post-consumer cooling devices used in consumers’ homes, representing natural wear and tear of the material. The tests provide insight into both the visual and mechanical properties of components manufactured from recycled HIPS originating from degraded refrigeration equipment. They also constitute a valuable source of information for processors and manufacturers.

1. Introduction

Nowadays, the world without polymer materials is unimaginable. Their adaptable properties and suitability for large-scale production make an invaluable contribution to living standards, for example, applying them in electrical and electronic equipment [1]. One of the most commonly used polymer materials in electric and electronic devices, besides the ABS (Acrylonitrile-Butadiene-Styrene copolymer) and PP (Polypropylene), is the High-Impact Polystyrene (HIPS) [2]. It is common in such types of applications owing to its advantages such as a proper balance of properties, easy processing, and low cost [3].

The phenomenon of plastic yellowing is caused by polymer degradation, which is one of the polymer aging symptoms [4,5,6]. In HIPS, yellowing is primarily associated with photooxidative reactions initiated by ultraviolet radiation, leading to chemical modifications such as chain scission, oxidation of the polybutadiene phase, and formation of chromophoric groups [5,7,8].

The effects of thermal oxidation on the HIPS polymer are presented in [9]. This was a simulation of degradation and performed by multiple processing and thermo-oxidative aging. In paper [10], the degradation process of HIPS material was performed using the conventional extrusion machine at the extrusion temperatures from 183.5 to 200 °C. The studies presented in [1] describe the properties of the reprocessed HIPS material in eight-cycle injection molding (starting with the virgin HIPS). In turn, paper [11] describes the influence of five-fold post-consumer HIPS processing on its properties. Another research describes the influence of the number of processing cycles (6) on the post-consumer recycled high-impact polystyrene from disposable cups. Paper [12] reported the effects of polychromatic light (200 h) on HIPS yellowing at 55 °C in the air. In [3], samples were prepared by injection molding and then exposed to the ultraviolet laboratory radiation for periods of up to 60 days at room temperature. In the studies presented in [2], HIPS sheets were exposed to the solar radiation for 2000 h. In paper [13], HIPS plates were photodegraded in both accelerated and natural conditions. In the natural condition method, plates were exposed to the south on only one sample side for approximately 4 months. In the research [14], after a natural weathering period of 8760 h, applying already known meteorological parameters, changes in mechanical properties were investigated. Half of the sample number was placed on the building roof and was exposed to natural weathering. Untreated samples (the other half) were kept in a controlled laboratory atmosphere. HIPS were exposed to continuous UV radiation for periods up to 8 weeks in paper [15].

Previous studies investigating photodegradation of HIPS reported changes in FTIR spectra, development of oxidation products, color shifts, and evolution of mechanical properties under laboratory UV exposure, solar radiation, and accelerated aging conditions [2,3,12,13,14,15]. Natural weathering studies additionally demonstrated gradual modification of mechanical performance after long-term environmental exposure [14]. In parallel, several works examined thermo-oxidative aging and multiple processing of HIPS as model degradation routes [1,9,10,11]. These studies showed variations in melt flow behavior, stiffness, and impact performance resulting from chain scission and structural changes in the rubber phase. Although such approaches provide valuable mechanistic insight, they rely on controlled aging scenarios that may not fully reproduce the heterogeneous degradation pathways occurring in post-consumer materials exposed to real service conditions. Consequently, the relationship between visually observable yellowing and the functional properties of naturally aged HIPS recyclate remains insufficiently understood.

This paper studies HIPS with a degraded (yellowed) surface and its mechanical properties after melting. The novelty of this study, compared to studies [1,2,3,9,10,11,12,13,14,15], lies in the experimental investigation of HIPS samples derived from post-consumer refrigeration equipment used in households, thus representing naturally aged materials under real-life conditions. The HIPS materials were not subjected to aging using laboratory equipment, such as UV lamps or stable sunlight for a period of time, polychromatic light, multiple processing, or thermo-oxidative simulation, which further emphasizes the innovative character of this work.

Moreover, in contrast to studies [1,2,3,9,10,11,12,13,14,15], contact angle measurements were performed for visually differentiated (less and more degraded) HIPS in both flake form and standardized specimens. Additionally, the use of post-consumer HIPS from refrigeration equipment combined with detailed colorimetric analysis has not been previously reported in the literature.

The analysis was conducted on HIPS in the form of flakes (after grinding on a specialized line for the recycling of refrigeration equipment in the Polish Recycling Corporation, Lublin, Poland) and HIPS in the form of standardized specimens produced using a laboratory injection molding machine in cooperation with the Lublin University of Technology.

The research methods were: material identification and characteristic peak analysis using ATR-FTIR, analysis of wetting angles using the sessile drop method, as well as the advancing and receding contact angles, colorimetric analysis, melt flow index measurement, XRD analysis, microscopic analysis of the yellow layer thickness, and testing of mechanical properties performed on a standardized specimen.

The HIPS flakes were approximately 1.5 cm in size and originated from recycled refrigeration equipment. Samples for testing were obtained from a company specializing in the recycling of waste electrical and electronic equipment—the Polish Recycling Corporation.

The MFI measurement was done at the Research and Development Center of Polish Recycling Corporation. All analyses, except for the MFI (melt flow index) test, were performed at the Department of Interfacial Phenomena of Maria Curie-Skłodowska University. The research activities carried out at this department were conducted within the framework of the “Implementation Doctorate” program funded by the Ministry of Science and Higher Education of the Republic of Poland.

2. Materials and Methods

2.1. Sample Preparation: Methods of the Representative Samples Selection

Individual samples for testing (HIPS flakes: mixture of white, yellow, and semi-yellow flakes) were taken directly from the flotation line (from the chute). Single flakes of white, medium-yellow, and yellow colors were manually selected from the whole sample. Finally, three types of laboratory samples (white, semi-yellow, and yellow flakes) were prepared, each weighing approximately 1 kg.

2.1.1. Colorimetric Analysis—Sorting Samples According to Their Lightness (“L” Parameter)

The degree of lightness (“L” parameter) and the assignment to the appropriate group:

• white HIPS flakes (“L” > 90),
• medium-yellow HIPS flakes (“L” = 87–90),
• yellow HIPS flakes (“L” < 87)

were based on the colorimetric analysis using the colorimeter (PCE Instruments, Meschede, Germany).

2.1.2. Identification of HIPS by the ATR-FTIR Method

Laboratory samples (approximately 1 kg of HIPS flakes for each fraction: “L” > 90, “L” = 87–90, and “L” < 87) after the colorimetric analysis were subjected to the ATR-FTIR analysis (Thermo Scientific, Nicolet iS10, Waltham, MA, USA) to confirm the type of material (i.e., to verify that the material was HIPS). Additionally, 50 spectra were randomly collected for each type of sample for further analysis. Each spectrum was obtained by the performance scans between 4000–400 cm⁻¹.

2.1.3. Production of Standardized Specimens (Dumbbell-Shaped) Using a Laboratory Injection Molding Machine

Standardized dumbbell-shaped specimens (PN-EN ISO 527) [16] (Type 1A tensile bars) were manufactured from manually sorted and pre-milled HIPS flakes using the injection molding process in accordance with PN-EN ISO 3167 [17] and PN-EN ISO 294-1 [18] standards, employing an Engel Victory VC 200/50 Fast Track machine (Engel Austria GmbH, Schwertberg, Austria) equipped with a dual-cavity mold and a 25 mm screw (L/D ratio = 24.8).

The injection molding was carried out according to the following processing parameters:

• nozzle temperature: 190 °C;
• melt temperature (zones I, II, and III): 200–190–180 °C,
• feed throat temperature: 30 °C,
• injection speed: 30 cm³/s,
• injection pressure: 600 bars,
• holding pressure: 300 bar,
• holding time: 5 s,
• mold temperature: 50 °C,
• cooling time: 30 s,
• plasticizing pressure: 50 bar,
• clamping force: 300 kN.

Three series of samples for further analyses were prepared: dumbbell-shaped specimens made from HIPS flakes characterized by “L” > 90, “L” = 87–90, and “L” < 87. Only samples obtained from the same mold cavity were used for testing in order to minimize variability. The flow chart for obtaining all six laboratory samples is provided in Figure 1.

2.2. Melt Flow Index (MFI) Test

This test was carried out using a plastometer (Bagsik XLR 400B, Gliwice, Poland). After heating the plastometer to the appropriate temperature, about 10 g of the previously prepared sample (three types of HIPS flakes) was poured through the funnel, in the form of pieces cut to approximately 1 mm. Extrudates were obtained and subsequently weighed. The average weight of the samples was entered into the plastometer. The test results (mass flow rate at a given temperature, expressed as the melt flow index) were generated automatically by the plastometer. The temperature of 200 °C and the load of 5 kg were used (ISO 1133:1997) [19]. The tests were carried out three times for each type of flake sample.

2.3. Analysis of Contact Angles by the Sessile Drop Method

Using three types of flake samples and three types of standardized specimens, a wettability test was carried out using the sessile drop method. To measure contact angles, a drop was placed on the plastic surface, and with the help of VisionDrop, the contact angle was measured by repeating the action 20 times for each sample. Then, the average results and their standard deviations were calculated.

2.4. Analysis of Advancing and Receding Contact Angles

Using six laboratory samples, the wettability test was carried out using the sessile drop method, including measurements of advancing and receding contact angles. Deionized water and the contact angle meter (GBX, Rhône-Alpes, France) were applied. A drop was placed on the plastic surface. After measuring the advancing contact angle, the same drop was partially retracted with a needle. Then, the receding contact angle was measured. The same measurement parameters were applied to each droplet. Using VisionDrop software (Version: 15.09.07.01 GB), contact angles were measured by repeating the procedure 20 times for each sample. Then, the average results and their standard deviations were calculated.

The apparent surface free energy of the tested materials was determined from the approach proposed by Chibowski (CAH) based on the hysteresis of contact angles often referred to in the literature as “Hysteresis approach” [20]. The resulting values represent the apparent surface free energy of the heterogeneous polymer surfaces.

2.5. Colorimetric Analysis

The “L”, “a”, and “b” parameters were determined for three types of dumbbell samples using a colorimeter (PCE Instruments, Meschede, Germany). Twenty measurements were taken for each dumbbell type. Twenty previously recorded results for “L” > 90, “L” = 87–90, and “L” < 87 HIPS flakes obtained during sample preparation (Section 2.1.1) were also used for this purpose.

2.6. ATR-FTIR Study—Analysis of Absorbances of Spectra Characteristics for the HIPS

Using the FTIR spectrometer with the ATR attachment (Thermo Scientific, Nicolet iS10, Waltham, MA, USA), 50 spectral measurements were performed for each sample. As in the case of colorimetric analysis, the results for HIPS flakes obtained earlier during the preparation of samples for testing (Section 2.1.2) were also made use of. Applying the specialist program for FTIR spectra processing, the absorbances/heights of the 11 most important spectra describing the HIPS were read. Each spectrum was obtained by means of scans between 4000–400 cm⁻¹.

2.7. Mechanical Properties Testing

The specimens prepared according to the procedure described in Section 2.1.3 were used for mechanical properties tests. All tests were carried out under controlled laboratory conditions (23 ± 3 °C and 40 ± 10% RH) two days after sample production.

The tensile strength test was conducted using a ZwickRoell AllroundLine Z010 machine (ZwickRoell, Ulm, Germany) equipped with an Xforce HP 10 kN load cell, according to PN-EN ISO 527 standard. The preload force was equal to 0.1 MPa, the test speed for determining Young’s modulus was equal to 1 mm/min, and the rest of the measurements were performed at a constant crosshead speed equal to 50 mm/min until specimen failure. The applied force and corresponding elongation were continuously recorded. The tensile strength, Young’s modulus, and elongation at break were determined using the ZwickRoell testXpert software (testXpert II V3.5). Ten samples from each series were tested.

Charpy impact testing was carried out using a digital impact tester QC-639J-3 (Cometech, Taichung, Taiwan) according to PN-EN ISO 179-1 [21] standard, method 1eU. Unnotched specimens were tested edgewise using a 5 J pendulum with an impact velocity of 2.9 m/s. The absorbed impact energy was measured, and the Charpy unnotched impact strength was calculated and reported in kJ/m² as an average of 10 samples.

The Vicat softening temperature (VST) was determined using the CEAST HV3 apparatus (Instron, Norwood, MA, USA) in accordance with PN-EN ISO 306 [22] standard, method B50. The test was performed under a load of 50 N with a heating rate of 50 °C/h. Three specimens of each sample were tested simultaneously using the three measuring positions of the apparatus, and the reported values represent the average.

Shore hardness was measured using a digital Shore hardness tester (Affri, Induno Olona, Italy) in accordance with PN-EN ISO 868 [23] standard. The results are reported in Shore D units, as an average of 10 measurements.

Mechanical data were interpreted using a signal-to-noise approach in order to distinguish systematic effects related to material degradation from variability resulting from processing and specimen heterogeneity. For each mechanical parameter, mean values and dispersion between specimens were considered to evaluate whether differences between the color-segregated fractions exceeded expected experimental variability. Changes that followed a consistent monotonic trend with decreasing L parameter were treated as a degradation signal, whereas isolated fluctuations without directional consistency were interpreted as noise associated with recyclate heterogeneity and molding variability.

2.8. XRD Analysis of HIPS Flakes

XRD analysis was used to characterize the mass ratio of fillers contained in HIPS. It was performed using the X-ray diffractometer (Empyrean-Malvern Panalytical, Almelo, The Netherlands, 2012) with the CuKα radiation. The diffractograms were obtained at room temperature, in the angular range of 2θ from 6° to 95° with a step of 0.02°.

2.9. HIPS Yellowing Thickness Measurement

The thickness of the degraded HIPS surface was measured using a Nikon MA200 metallurgical optical microscope (Nikon, Tokyo, Japan). Previously characterized HIPS flakes were cut, and the obtained cross-sections were analyzed microscopically. The thickness of the yellowed layer was measured using dedicated built-in software.

3. Results and Discussion

3.1. Sample Preparation: Methods of Representative Samples Selection

3.1.1. Colorimetric Analysis—Sorting Samples According to Their Brightness (“L” Parameter)

Separation of plastics was conducted based on the “L” parameter. The “L” parameter has a range from 0 to 100 and describes the brightness of the tested surface [12]. The larger the “L” parameter, the brighter the surface. It was determined that the plastic flakes with “L” < 87 originated from the yellow flakes sample, whereas flakes with higher “L” values corresponded to the white fractions (Figure 2a–c).

3.1.2. Identification of HIPS by the ATR-FTIR Method

The ATR-FTIR method was applied to confirm the type of material (HIPS). During the analysis, special attention was paid to the presence or absence of the peak at around 2235 cm⁻¹. The absence of this band confirmed that the analyzed plastic flake was HIPS rather than ABS, which is also a common material in refrigerators (ABS exhibits a spectrum similar to HIPS; however, the difference is visible at ~2235 cm⁻¹ due to C–N bonding present in ABS).

It is worth noting that the tested material was clearly HIPS, not GPPS/PS (general-purpose polystyrene/polystyrene), because during a manual impact test, the flakes did not break but exhibited flexibility. Additionally, the ATR-FTIR analysis revealed the presence of a C=C peak at approximately 966 cm⁻¹, which originates from the rubber phase. The rubber phase exists only in HIPS and contributes to its flexibility, unlike GPPS.

According to [24,25,26,27,28,29,30,31,32,33], the most important peaks for HIPS occur at approximately: 537.1, 694.7, 1026.9, 2849.3, 2921.6, and 3024.8 cm⁻¹, corresponding to C–H bonds; peaks at approximately 1451.2 and 1492.2 cm⁻¹, corresponding to C–C bonds; and peaks at approximately 1600.6 and 753.5 cm⁻¹, corresponding to C=C bonds.

3.1.3. Production of Standardized Specimens (Dumbbells) Using the Laboratory Injection Molding Machine

The normalized specimens were made following the description in Section 2.1.3. The dumbbells and the manual sorted material to provide production of the dumbbells are visible in Figure 2.

3.2. Melt Flow Index (MFI) Test

MFI results were as follows: “L” > 90 HIPS flakes—4.12 g/10 min, “L” = 87–90 HIPS flakes—3.97 g/10 min, and “L” < 87 HIPS flakes—2.95 g/10 min.

The average melt flow rate is the largest for the sample made of “L” > 90 HIPS flakes (4.12 g/10 min) and the smallest for the sample made of “L” < 87 HIPS flakes (2.95 g/10 min). Additionally, the extrudate obtained from the plastometer for the “L” < 87 fraction contained visible yellow inclusions.

In paper [9], the melt flow rate (MFI) was measured for the reprocessed HIPS virgin material. Polymer material was subjected to thermo-oxidative aging at 90 °C in order to simulate its period of life. The MFI procedure was also followed according to the ISO 1133:1997 standard. After the first reprocessing step, the MFI value increased by only approximately 2%, and after nine extrusion cycles, the MFI value increased by approximately 45% (from approximately 4 g/10 min to approximately 5.5 g/10 min).

In studies [11,27,28], although different test parameters were applied, an increase in MFI with increasing number of extrusion cycles was also observed. In another study [1], MFI increased after several injection molding cycles, although a reduction after the first cycle was reported.

In paper [28], only the first extrusion presented a decrease in MFI result (from 2.8 for the virgin HIPS to 1.7 g/10 min). In a doctoral thesis [29], MFR increases slightly with reprocessing of HIPS samples subjected to up to nine extrusion cycles, especially for the first three extrusion cycles. The effects of UV radiation on MFI of HIPS have been published in papers [14,30]. In the first paper, there are no remarkable changes in MFI values of the materials after weathering (on the building roof) during the time up to 4320 h. Changes in MFI are only visible for longer weathering from 4320 h up to 8760 h. In the second study, after the photodegradation, MFI increases from 0.65 to 2.2 g/10 min. According to paper [11], the melt flow index for five consecutive steps of reprocessing post-consumer HIPS increases slightly.

In this study, the MFI results indicate that a lower “L” parameter corresponds to a lower melt flow rate, suggesting reduced processability. However, the literature data show that MFI may both increase or decrease depending on the degradation stage and mechanism. This may indicate that the HIPS flakes analyzed in this study were moderately rather than severely degraded.

3.3. Analysis of Contact Angles by the Sessile Drop Method

Contact angle results for the HIPS flakes and the HIPS dumbbells are given in

Table 1. Results of contact angles of the HIPS flakes.

Type of Sample	Average Contact Angle [°] ± SD
“L” > 90 HIPS flakes	88.3 ± 4.6
“L” = 87–90 HIPS flakes	70.1 ± 4.4
“L” < 87 HIPS flakes	68.2 ± 5.3
Dumbells made of “L” > 90 HIPS flakes	94.0 ± 4.2
Dumbells made of “L” = 87–90 HIPS flakes	86.9 ± 5.7
Dumbells made of “L” < 87 HIPS flakes	93.7 ± 1.9

.

The contact angle of “L” > 90 HIPS flakes was the largest and amounted to 88.3°. The smallest result was obtained by the sample with “L” < 87 HIPS plastics, and it amounted to 68.2°. In the case of dumbbells, for the plates with the highest yellowness, the average contact angle was 93.7°. However, dumbbells made of “L” = 87–90 HIPS flakes had even lower wettability compared to the white dumbbells (86.9°), although this difference should be interpreted with caution because, when standard deviations are taken into account, the contact angle ranges for all dumbbells overlap. Therefore, these values should be considered statistically comparable rather than strictly higher or lower.

The contact angles (before the HIPS degradation) in the studies [24,31,32,33,34] for HIPS were 81.0–102.5°, while the contact angle of “L” > 90 HIPS flakes was 88.3°. In this study, smaller contact angles were obtained for “L” = 87–90 and “L” < 87 HIPS flakes (after HIPS degradation) compared to [24,31,32,33,34]. No papers about contact angle testing after HIPS thermo-oxidative, photochemical, and weathering degradation were reported.

Contact angles decrease with the decrease in the “L” parameter of the tested flakes’ surface. This indicates that plastic flakes with “L” < 87 are characterized by larger wettability. However, when measurement uncertainty is considered, this trend is not strictly monotonic and should be treated as a qualitative tendency rather than a statistically confirmed relationship. This may be attributed to the uneven surface of more degraded (more yellowed) plastics.

Values of standard deviations for HIPS flakes can be attributed to surface-related factors, such as roughness, curvature of the flakes, and the presence of contaminants (e.g., PUR foam residues or dirt), which locally affect droplet behavior. In contrast, for dumbbell specimens, which are produced by injection molding, surface roughness is significantly reduced and more uniform. Therefore, the relatively high standard deviation values observed for dumbbells (on the order of 4–5°) cannot be explained by surface roughness. Instead, they are most likely associated with the inherent heterogeneity of the recycled material, including variations in composition, pigment distribution, and the effects of melt processing and specimen formation.

Wetting angle results calculated for dumbbells did not show any significant differences (the largest difference was 7.1°), and the observed variations remain within the range of experimental dispersion. This further supports the conclusion that, after homogenization during injection molding, the influence of the degraded surface layer is significantly reduced, and the material exhibits nearly uniform wettability behavior. All the results obtained for dumbbells are similar to the wetting angle results for “L” > 90 HIPS flakes. It should be noted that the processing itself can cause a change in the properties of the processed material (the dumbbells were made after injection molding).

Comparing contact angle tests with [24,31,32,33,34], where the HIPS contact angle was in the range from 81.0° to 102.6°, it is evident that “L” = 87–90 and “L” < 87 HIPS flakes were not included in this range. This proves the surface characteristic changes due to the yellowing phenomenon. However, due to the relatively large standard deviations and overlapping value ranges, this conclusion should be interpreted as indicative rather than definitive. The test results for the dumbbells are from 86.9° to 94°, being within the given range. The reason is the homogenization of 1 kg of plastic flakes samples, where only the surfaces were degraded (the remaining part of the flake was white).

Surface roughness plays a crucial role in determining the contact angle values obtained for test liquids on polymer surfaces. In Malinowska B. et al.’s previous study [35], optical profilometry was used to characterize the surface topography of the same HIPS material as that investigated in the present work, i.e., post-consumer HIPS originating from refrigeration equipment. Surface maps with dimensions of 0.90 × 1.30 mm were recorded, corresponding to the area covered by the droplet during contact angle measurements. Five plates from each color-based fraction were examined. Due to the heterogeneous nature of the technological recyclate, the calculation of a simple average surface map was considered non-representative. Therefore, only the extreme cases (smoothest and roughest surfaces) were presented.

In contrast to laboratory-grade polymers, whose surface roughness is typically in the nanometer range (e.g., untreated ABS exhibiting an average roughness of approximately 53.10 nm), the investigated recyclate exhibits micrometer-scale roughness reaching several tens of micrometers. This pronounced roughness results from the mechanical grinding of entire refrigeration appliances, which generates irregular and curved flake geometries. The curved surface of the flakes inherently increases the dispersion of contact angle results and makes it difficult to obtain low standard deviation values. Furthermore, the recyclate may contain residual polyurethane (PUR) foam particles or surface contaminants that remain adhered to the HIPS flakes despite washing procedures. These heterogeneities contribute to local variations in wettability and thus influence the final contact angle values.

Although surface irregularities and material heterogeneity introduce variability into the measurements, contact angle remains a highly relevant parameter in the context of this research. From a technological perspective, even small differences in contact angle—within the range of standard deviation—can be sufficient for effective flotation-based separation of polymers on an industrial scale, as demonstrated in previous studies and industrial practice. Wettability is directly related to the efficiency of flotation-based separation processes on an industrial scale (tons of materials). Therefore, despite the inherent variability associated with real recyclate materials, contact angle analysis provides valuable insight into the surface condition and separation potential of naturally degraded HIPS.

It is well known that wetting behavior is governed primarily by two factors: surface roughness and surface polarity [1,2,3]. In the present study, surface roughness results mainly from the mechanical grinding process and is therefore considered comparable across the analyzed fractions of the same recyclate stream. Consequently, differences observed in the contact angle values are interpreted mainly in terms of changes in surface chemistry caused by natural aging processes. Photo-oxidative degradation of HIPS leads to the formation of polar oxygen-containing functional groups on the polymer surface, which modifies surface polarity and thus affects wettability [9,10,11,12,13,14,15]. Under these conditions, the influence of roughness can be regarded as relatively constant, allowing only qualitative conclusions to be drawn regarding the role of aging-induced chemical modifications in the observed wetting behavior.

3.4. Analysis of Advancing and Receding Contact Angles

Advancing and receding contact angles and apparent surface free energy test results are presented in Figure 3.

The left graph shows advancing and receding contact angles. Six columns on the left represent the HIPS flakes, and the other six columns on the right represent the HIPS dumbbells. The column color corresponds to the “L” parameter (white—“L” > 90, light yellow—“L” = 87–90, and intense yellow—“L” < 87).

The right graph presents apparent surface free energy results. Three columns on the left correspond to HIPS flakes, and the other three to HIPS dumbbells.

Advancing and receding contact angles for HIPS flakes decreased with decreasing “L” parameter. Each of the receding contact angles for the HIPS flakes was smaller than the advancing ones. Advancing and receding contact angles for HIPS dumbbells were similar. In one case (dumbbells made of “L” = 87–90 HIPS flakes), the receding contact angle was larger than the advancing one.

Comparing the HIPS flakes’ receding contact angle results to those presented in [24,31,32,33,34], where the contact angle for HIPS was from 81.01° to 102.58°, it is apparent that they were not within the given range. However, the contact angles for HIPS dumbbells were within this range. No papers on receding contact angle testing for HIPS were reported.

Apparent surface free energy is used to characterize how solid materials interact with liquids. For the HIPS flakes, apparent surface free energy increased with decreasing “L” parameter, indicating improved wettability and adhesion [36]. However, the apparent surface free energy was similar for each dumbbell sample, and the measurements were close to those of the “L” > 90 HIPS flakes sample. It should be noted that the only dumbbell result which was larger than the apparent surface free energy for “L” > 90 HIPS flakes was that for the dumbbell made of “L” = 87–90 HIPS flakes. This can indicate that the semi-degraded HIPS flakes had a positive effect on the apparent surface free energy.

3.5. Colorimetric Analysis

Table 2. Results of the lab parameters of the HIPS flakes.

	Dumbbells Made of “L” > 90 HIPS Flakes	Dumbbells Made of “L” = 87–90 HIPS Flakes	Dumbbells Made of “L” < 87 HIPS Flakes
	“L” parameter
Average	88.13	89.19	88.88
Max.	88.38	89.79	89.38
Min.	87.77	88.97	88.45
Standard deviation	0.19	0.29	0.33
	“a” parameter
Average	−0.22	−0.13	0.16
Max.	−0.16	−0.02	0.29
Min.	−0.28	−0.28	0.06
Standard deviation	0.04	0.08	0.08
	“b” parameter
Average	1.02	3.04	5.04
Max.	1.20	3.32	5.38
Min.	0.78	2.72	4.67
Standard deviation	0.15	0.19	0.28

presents the results for three of the HIPS dumbbells samples.

The “L” parameter did not reveal any significant differences between the dumbbells made of “L” > 90, “L” = 87–90, and “L” < 87 HIPS flakes. The same applies to the “a” parameter (the range from −128 to 127).

The “b” parameter, which describes the colors from blue to yellow (the range from −128 to 127), increased with increasing surface yellowness [12].

The difference in the “b” parameter between the dumbbell made of “L” > 90 plastic flakes and that made of “L” < 87 plastic flakes was over 4.00. This means that the dumbbell made of flakes with a yellowed surface took on a color more similar to yellow, which confirms the obtained result of the “b” parameter.

In paper [12], all the samples suffer from yellowing as a result of photo-oxidative laboratory degradation. The colorimetric data show a relatively small change in the “L” parameter (from 86.40 to 85.00) and the “a” parameter (from −1.37 to −2.08), but a significant change in the “b” parameter (from 8.46 to 19.10), indicating that discoloration was not a mere change in lightness (“L” parameter). These differences refer to the comparison between the non-aged reference HIPS sample and the HIPS sample subjected to the longest photo-oxidative aging time reported in that study. According to paper [12], the calculated differences between these samples for the colorimetric parameters “L”, “a”, and “b” were 1.4, 3.45, and 10.46, respectively. The study [13] presented the evolution of coloration during weathering, and it was mainly along the “b” parameter—the yellow indicator. In that work, the reported differences correspond to the comparison between the initial (unaged) material and the sample exposed to the longest weathering period. The difference between the “L” parameters was 7, and that between the “b” parameters was 19 in paper [13]. In paper [15], HIPS was degraded by photo-oxidation for 8 weeks. The luminosities (“L” parameter) were measured, and the evolution of the yellow/blue hue (“b” parameter) was investigated. The lightness parameter decreased from about 67 to 60, and the “b” parameter increased from approximately −4 to 15.

According to paper [12], differences in parameters “L”, “a”, and “b” were 1.4, 3.45, and 10.46, respectively. The difference between the “L” parameters was 7, and that between the “b” parameters was 19 in paper [13]. Also, in the study [15], the largest increase in the parameter “b” was observed with the increasing HIPS degradation time. Comparing the above-mentioned research results, it is evident that the differences for all parameters were the smallest in this study (particularly for the “b” parameter). This may confirm that the HIPS analyzed in this work was less degraded than in the cited studies. Yellowing of only the top layer of HIPS is caused by the fact that oxygen does not have time to migrate deeper because the surface layers are too reactive to it and consume it quickly [13].

3.6. ATR-FTIR Study—Analysis of Spectra Absorbances Characteristic of the HIPS

Figure 4 shows the exemplary spectrum for the “L” > 90 and “L” < 87 HIPS flakes obtained by the ATR-FTIR spectrometry (carbonyl group at 1743 cm⁻¹ and rubber phase at 966 cm⁻¹ were included).

Table 3. FTIR results of the HIPS flakes.

Wavenumber [cm−1]	Type of the Bonding	Average Absorbances			Absorbances Difference (Between “L” > 90 and “L” < 87) [%]	Average Absorbances			Absorbances Difference (Between Dumbbells Made of “L” > 90 and “L” < 87 HIPS Flakes) [%]
		“L” > 90 HIPS Flakes	“L” = 87–90 HIPS Flakes	“L” > 87 HIPS Flakes		Dumbbells Made of “L” > 90 HIPS Flakes	Dumbbells Made of “L” = 87–90 HIPS Flakes	Dumbbells Made of “L” < 87 HIPS Flakes
3024.8	C-H	0.0378	0.0307	0.0276	26.98	0.0350	0.0350	0.0309	21.17
2921.6	C-H	0.0705	0.0503	0.0477	32.34	0.0598	0.0598	0.0558	10.00
2849.3	C-H	0.0290	0.0187	0.0180	37.93	0.0216	0.0216	0.0203	12.50
1600.6	C=C	0.0454	0.0362	0.0327	27.97	0.0384	0.0384	0.0338	18.36
1492.2	C-C	0.1061	0.0893	0.0815	23.19	0.1003	0.1003	0.0882	15.92
1451.2	C-C	0.1053	0.0868	0.0791	24.88	0.1010	0.1010	0.0895	14.27
1026.9	C-H	0.0433	0.0360	0.0345	20.32	0.0348	0.0348	0.0312	21.21
753.5	C=C	0.1460	0.1289	0.1190	18.49	0.1288	0.1288	0.1152	12.73
694.7	C-H	0.5387	0.4613	0.4261	20.90	0.4751	0.4751	0.4120	20.12
537.1	C-H	0.1272	0.1106	0.1050	17.45	0.1066	0.1066	0.0950	25.49

lists the average absorbance results for 10 characteristic HIPS peaks for flakes and dumbbells.

The analysis of the spectra for HIPS flakes indicated a decrease in absorbance with increasing surface yellowing (Table 3). This indicates that all bonds present in HIPS were degraded (C-H, C=C, and C-C). The largest difference in the spectrum height (37.93%) was noted for the spectrum with a wavenumber of about 2849 cm⁻¹ for the C-H bond. The band at approximately 2849 cm⁻¹ is assigned to symmetric aliphatic C–H stretching vibrations (–CH₂⁻ groups) in the polymer backbone. The smallest difference in absorbance was observed for the spectrum with a wavenumber of about 537 cm⁻¹. This difference amounted to 17.45%. The band near 537 cm⁻¹ can be attributed to out-of-plane C–H deformation vibrations characteristic of the aromatic ring structure of polystyrene. Additionally, Figure 4 shows the peak appearing for the carbonyl group (C=O) of “L” < 87 flakes at about 1743 cm⁻¹. The absorption band at approximately 1743 cm⁻¹ corresponds to C=O stretching vibrations associated with oxidative degradation products.

The analysis of HIPS dumbbells spectra presented a decrease in the absorbance of each of them, along with the increasing yellowing of the HIPS surface (Table 3). This means that all bonds were degraded (C-H, C=C, and C-C). The largest difference in the height of the spectrum (25.49%) was noted for that with a wavenumber of approximately 537 cm⁻¹ for the C-H bond. This region corresponds to out-of-plane C–H deformation vibrations of the aromatic structure, suggesting structural changes in the polystyrene phase. The smallest difference in absorbance was observed for the spectrum with a wavenumber of approximately 2921 cm⁻¹, being 10.00%. The 2921 cm⁻¹ band represents asymmetric aliphatic C–H stretching vibrations, indicating that aliphatic chain segments were comparatively less affected after homogenization during injection molding.

Comparing the absorbance results of flakes and dumbbells, it is clear that the average difference in absorbance is larger for the flakes. This is likely due to the surface-localized degradation of flakes, whereas dumbbells represent homogenized bulk material.

In paper [37], non-irradiated and irradiated HIPS exhibited similar FTIR spectra. In the study [38], after 15 and 30 days of HIPS UV degradation (0.5, 5, 10, and 21 h), the peaks at about 700 cm⁻¹, 750 cm⁻¹, 1450 cm⁻¹, 1500 cm⁻¹, and 1600 cm⁻¹ decreased compared to that of pristine (pure/unspoilt) HIPS. The wavenumber was 650–1650 cm⁻¹. In [8], FTIR-ATR spectra of aged and not aged HIPS samples under the accelerated and natural conditions presented an increasing peak at 850 cm⁻¹ and decreasing peaks at 1450 cm⁻¹ and 1500 cm⁻¹. Paper [9] presents the FTIR absorbance spectra in the region between 1800 and 1400 cm⁻¹, with an increase in the wavenumbers 1450 cm⁻¹, 1500 cm⁻¹, and 1600 cm⁻¹ after HIPS aging. The increase in the mentioned peaks was observed for the HIPS after multiple processing. According to the study [39], most peaks’ absorbances decreased significantly after natural aging (large sunlight radiation dose, dry, high temperature, and rainy weather).

The only research results where not irradiated and irradiated HIPS presented the same FTIR spectra are in [37]. In the study [38], the peaks at about 700 cm⁻¹, 750 cm⁻¹, 1450 cm⁻¹, 1500 cm⁻¹, and 1600 cm⁻¹ decreased after the HIPS degradation and were the same as in this study. The wavenumber was from 650 cm⁻¹ to 1650 cm⁻¹. This made it impossible to compare the remaining peaks in this study (400–4000 cm⁻¹). According to the study [39], each of the 10 characteristic peaks for HIPS mentioned in this paper (3030 cm⁻¹, 2900 cm⁻¹, 2850 cm⁻¹, 1600 cm⁻¹, 1500 cm⁻¹, 1450 cm⁻¹, 1030 cm⁻¹, 750 cm⁻¹, 700 cm⁻¹, and 550 cm⁻¹) decreased after natural aging. In paper [8], the FTIR-ATR spectra of aged and not aged HIPS samples presented an increasing peak for 850 cm⁻¹ and decreasing peaks for 1450 cm⁻¹ and 1500 cm⁻¹. Interestingly, degradation of polybutadiene at 754 cm⁻¹ and 966 cm⁻¹ is the first step in the thermo-oxidative aging of HIPS, and some of the radicals formed during that degradation can act as initiators for the polystyrene oxidation [9]. Scission of the polymer chains occurs, among others, in the rubber particles (in the double-bonded structure). The evidence that surface reactions related to material degradation occurred on the surface of “L” < 87 HIPS flakes is the appearance of the peak originating from the carbonyl group around the wavenumber of 1743 cm⁻¹ [15]. Increasing peaks after HIPS degradation are mentioned only in two papers. It should be noted that 50 FTIR analyses were performed for each sample type, and these analyses were averaged in this paper. This affected more accurate absorbance results. Furthermore, it was difficult to compare FTIR spectra due to the lack of consistent baseline correction and detailed spectral analysis in some studies.

3.7. Testing of Mechanical Properties

The results of the mechanical properties test for HIPS specimens are presented in

Table 4. Mechanical properties of tested specimens.

Mechanical Properties	Dumbbells Made of “L” > 90 HIPS Flakes	Dumbbells Made of “L” = 87–90 HIPS Flakes	Dumbbells Made of “L” < 87 HIPS Flakes
Shore hardness [0–100 ShD]	76.400 ± 0.660	76.700 ± 1.720	76.500 ± 0.990
Charpy impact strength [kJ/m2]	34.672 ± 10.535	32.231 ± 9.956	29.143 ± 9.309
Vicat softening temperature (VST) [°C]	88.600 ± 0.260	88.300 ± 0.300	88.100 ± 0.100
Average Young’s Modulus [MPa]	1566.80 ± 10.98	1665.40 ± 5.38	1653.80 ± 8.30
Average tensile strength [MPa]	18.70 ± 0.18	20.39 ± 0.29	21.17 ± 0.16
Average stress at failure [MPa]	18.08 ± 0.66	20.10 ± 0.56	20.51 ± 0.79
Average elongation at break [%]	1.52 ± 0.04	1.59 ± 0.03	1.70 ± 0.00

.

The larger the Young’s modulus value, the lower the deformation under applied stress [40]. This indicates that the dumbbells made of “L” > 90 materials are the least deformable (1566.80 MPa) compared to the dumbbells made of “L” = 87–90 (1665.40 MPa) and “L” < 87 materials (1653.80 MPa). However, taking into account the standard deviations, the differences between the “L” = 87–90 and “L” < 87 groups are small and partially overlapping, and therefore, these values should be considered statistically comparable.

Dumbbells made of “L” < 90 HIPS flakes showed the smallest tensile strength (18.70 MPa) and the smallest stress at failure (18.08 MPa). The tensile strength (21.17 MPa) and the stress at failure (20.51 MPa) are the largest for the dumbbells made of “L” < 87 HIPS flakes. The impact strength provides information about the material’s resistance to cracking [41]. The dumbbells made of “L” < 87 flakes showed the smallest impact strength (29.143 kJ/m²) and those made of “L” > 90 flakes the largest one (34.672). However, the standard deviations are high (approximately 9–10 kJ/m² in all cases), resulting in a substantial overlap of value ranges. Therefore, the impact strength of all analyzed samples should be regarded as statistically comparable, and no clear decreasing trend can be conclusively confirmed.

Hardness results decreased slightly (76.400 for the dumbbells made of “L” > 90, 76.700 for those made of “L” = 87–90, and 76.500 for the dumbbells made of “L” < 87 HIPS flakes), but the observed differences remain within the measurement uncertainty and do not indicate a meaningful trend.

The softening point was the smallest for the dumbbells made of “L” < 87 HIPS flakes (88.100 °C) and the largest for those made of “L” > 90 HIPS flakes (88.600 °C). However, the variation is minimal and within a narrow range, suggesting no significant dependence on the degree of yellowing.

Average elongation at break is the smallest in the case of the dumbbells made of “L” > 90 HIPS flakes (1.52%) and the largest for those made of “L” < 87 HIPS flakes (1.70%). Although this increase appears systematic, it should be treated cautiously due to the limited dispersion range and potential measurement sensitivity.

In paper [11], the impact strength of post-consumer HIPS in five cycles of processing increased slightly for the first four processings (from 47.2 J/m to 56.4 J/m). However, in the last fifth processing, the impact strength decreased from 56.4 J/m to 53.4 J/m. The other mechanical property, that is, tensile strength, decreased from 27.4 MPa for the first processing to 26.6 MPa for the fifth processing. The other parameter, the Young’s modulus (E-modulus of elasticity), decreased from 1874.0 MPa to 1686.9 MPa. Moreover, elongation at break decreased from 25.9% to 15.8%. In paper [42], the tensile strength and the strain to failure for unrecycled HIPS are larger than those for the recycled material, but the modulus E is larger for the recycled HIPS polymer. In the study [13], the Young’s modulus becomes larger with the aging time. Also, stress at failure, strain at break (elongation at break), and Charpy impact strength decreased with the increasing aging time. Tensile strength decreased slightly for HIPS after 60 UV exposure days, according to paper [3]. In paper [1], the stress at failure decreased slightly for HIPS after eight cycles of reprocessing. Contrary to stress at failure, impact resistance (Charpy impact test) and tensile modulus exhibited a small increase in the chart. In the same paper, after the HIPS UV exposure for 2000 h, the tensile modulus and stress at failure decreased according to the added charts. HIPS showed a significant reduction in failure strain even at 250 h exposure. There was only a slight reduction in the impact performance. Y. Wang et al. [39] stated that the Young’s modulus increased after 21 months of natural aging. However, the tensile strength and elongation at break decreased. In paper [10], just like the tensile strength (increased) and elongation at break (decreased), the impact strength and Vicat softening temperature did not show large variations with the number of extrusion cycles (5), but decreased noticeably. Shore hardness increased after 8760 h of HIPS weathering in paper [14]. In the study [43], the Young’s modulus and elongation at break decreased, but Shore hardness and impact strength increased with the increasing number of reprocessing cycles (6). The study [28] proved that the Izod impact strength decreases slightly after the first processing cycle (from 80 to 69 J/m). On the other hand, tensile stress at break increased. In paper [9], for HIPS 9-cycle processing, the elongation at break decreased, but stress at break and Young’s modulus increased (slightly for E modulus). The same situation was observed after thermo-oxidative aging; however, the last day (16th) of thermo-oxidation presented a lower E modulus compared to the sample before degradation. In paper [15], the impact strength, elongation at break, and tensile strength after 60 days of HIPS photo-oxidative aging decreased. However, the elastic modulus increased.

Hardness of the material defines the resistance of the sample surface to penetration of a harder body [27]. The hardness test suggests that the reprocessed material undergoes chain deterioration, as well as crosslinking with every cycle. This results in the material’s resistance to penetration. Shore hardness increased as much as presented in two papers [14,27].

Impact strength describes the material’s ability to resist a mechanical impact loading [41]. As in [13,15] studies, impact strength increased, while in [10], this value remained practically unchanged. Recycling decreased the impact strength of HIPS in paper [42], and it was probably due to the presence of TiO₂ pigment, causing the inhomogeneity (which could affect the impact strength in this paper too).

The Vicat softening point (VST) determines the temperature at which the plastic material begins to soften under a specified load [44]. VST decreased slightly as in paper [10]. The average Young’s modulus increased for dumbbells made of “L” < 87 HIPS flakes. However, considering the standard deviations, the differences are small and partially overlapping; therefore, they should be regarded as statistically comparable.

The Young’s modulus value also increased in papers [9,13,15,39,42]. According to the study [13], the increase in the Young’s modulus value can be due to polybutadiene, which appears as a result of degradation in the yellowed plastics, especially due to crosslinking [27]. Interestingly, in paper [9], in the case of multiple processing of HIPS, the Young’s modulus increased; however, after thermo-oxidation, it decreased. This can point out that, depending on the type of degradation, Young’s modulus will behave differently. The next two studies where the Young’s modulus decreased after aging of HIPS were [11,41]. In summary, in most cases (6 out of 9), the Young’s modulus increased as in this paper.

The maximum stress that the material can withstand before its failure is its tensile strength [45]. The average tensile strength increased in this study, as well as in the case presented in [1,10]. Considering the relatively small deviations, this increase should be interpreted as a tendency rather than a statistically strong effect. In the paper [1], a change in the tensile strength depended on the type of degradation to which HIPS was subjected—after processing, it increased, and after UV exposure, it decreased. In most studies [1,3,11,15,39,42], the tensile strength decreased, but it should be noted that the results obtained in [1] can indicate that tensile strength depends on the type of aging.

Stress at failure is the maximum stress the material can endure before breaking or fracturing. The average stress at failure increased to the same extent as in papers [9,28]. Nevertheless, the overlapping standard deviation ranges indicate that these differences may not be statistically significant. As follows from [1,13,42], stress at failure decreased after degradation.

Elongation at break is the ratio between the changed wavenumber and the initial wavenumber after breakage of the specimen [40]. Only in this study did the elongation at break increase compared to those in [9,10,11,13,15,39,43]. However, due to the very low dispersion values, this observation should be interpreted carefully. According to paper [9], elongation at break should determine recycling possibilities and the second-market application for HIPS.

The statistical interpretation indicates that not all observed differences represent meaningful degradation effects. When standard deviations are taken into account, most mechanical parameters do not exhibit statistically significant differences between the analyzed fractions. In particular, impact strength and Young’s modulus show overlapping value ranges, which prevents confirmation of clear increasing or decreasing trends.

Therefore, the previously suggested relationships, such as the decrease in impact strength or the increase in Young’s modulus with increasing yellowing, should be interpreted as qualitative tendencies rather than definitive trends.

Parameters showing apparent directional changes with increasing yellowing may be associated with structural evolution of HIPS during natural aging, but these observations remain within experimental variability and require cautious interpretation. In contrast, properties such as hardness and selected tensile parameters exhibited only minor differences between fractions, suggesting that these variations may fall within the noise level typical for post-consumer recyclates. Such variability can originate from differences in flake composition, pigment distribution, thermal history, and molding heterogeneity. Consequently, these parameters should be interpreted cautiously and not treated as primary indicators of degradation. The signal-to-noise perspective supports the conclusion that visually driven segregation may reflect certain degradation effects, but does not provide statistically robust differentiation of mechanical performance at this stage of material aging.

3.8. XRD Analysis of HIPS Flakes

The results of this study showed that the analyzed HIPS flakes contained the following crystalline phases: titanium oxide, talc, and calcium carbonate. Their mass ratio was 5.3:2.1:1.0.

The XRD analysis was consistent with the ATR-FTIR results, but it was enhanced by the mass ratios of the additives. Considering that the analyzed HIPS flakes came from different refrigerators (different manufacturers and different production dates), presenting the amounts of additives used in HIPS in the form of mass ratios is completely sufficient.

The study indicates that if the additives present in HIPS had any effect on other analytical results (e.g., mechanical properties or colorimetry), titanium oxide, present in the highest amount, would have the greatest impact.

The presence of inorganic additives such as titanium oxide, talc, and calcium carbonate may also influence the melt flow index (MFI), because mineral fillers can limit polymer chain mobility and increase melt viscosity. Consequently, small variations in filler content may contribute to differences in the measured rheological properties. However, these inorganic phases are chemically stable and are not expected to participate directly in the aging reactions responsible for yellowing. The discoloration of HIPS is mainly associated with photo-oxidative degradation of the polymer matrix, particularly the polybutadiene phase, which leads to the formation of oxidized chromophoric groups responsible for yellow coloration [1,2,3,9,10,11,12,13,14,15]. Titanium dioxide may additionally influence aging behavior by interacting with UV radiation and modifying the intensity of photo-oxidative processes occurring in the polymer matrix.

3.9. HIPS Yellowing Thickness Measurement

Figure 5 presents three selected microscopic images of cross-sections made for three types of HIPS: (a) white HIPS flakes (“L” > 90), (b) semi-yellow HIPS flakes (“L” = 87–90), and (c) yellow HIPS flakes (“L” < 87).

The results of the four (two semi-yellow and two yellow HIPS flakes) degraded layer thickness tests are as follows:

• medium yellow HIPS flakes (“L” = 87–90): 58.80 µm, 57.66 µm, 63.76 µm, 50.38 µm, 47.73 µm, and 51.89 µm,
• yellow HIPS flakes (“L” < 87): 71.57 µm, 76.88 µm, 79.48 µm, 116.44 µm, 113.97 µm, and 110.57 µm.

The obtained results show that the measured layer thicknesses vary significantly. The results were not reproducible across different samples. The layer thickness of the HIPS fraction with an “L” parameter value of 87–90 ranged between 47.73 and 63.76 µm. In contrast, the layer thickness of the HIPS fraction with an “L” parameter value less than 87 ranged between 76.88 and 116.44 µm. Even the same HIPS flake has a varying degraded layer thickness (e.g., Figure 5c: 71.57–79.48 µm). The lower the “L” parameter, the thicker the yellowed layer.

Because yellowing varies with layer thickness, it is difficult to estimate how the yellowing thickness affected the final results of the tests performed in this article. The varying layer thickness is due to the variety of materials used for grinding (different brands of refrigerators with different production dates). The refrigerators were subjected to degradation for different periods of time.

It should also be noted that the yellowed region represents only a surface layer of the HIPS flakes, while the inner part of the material may remain largely unaffected by photo-oxidative degradation. Consequently, the analyzed samples consist of a mixture of degraded surface material and relatively pristine polymer in the core. Since the flakes were melted and homogenized before further testing, the measured mechanical and rheological properties correspond to the bulk-averaged behavior of this mixed material rather than exclusively to the oxidized layer.

Moreover, microscopic observations indicate that the thickness of the yellowed layer is non-uniform across a single flake surface. Therefore, the results presented in this work should be interpreted with the understanding that the extent of degradation varies locally and that the fraction of oxidized material cannot be precisely quantified.

3.10. Summary

Mechanistically, the FTIR results suggest a degradation pathway involving initial oxidation of the PB (polybutadiene) phase, followed by radical-induced reactions in the PS matrix, leading to chain scission and possible crosslinking. Chain scission reduces molecular weight and may increase the melt flow index (MFI), as reported in multiple reprocessing studies [9,11,27,28,29,30], whereas crosslinking within the rubber phase may contribute to increased stiffness. The relatively limited spectral changes observed in this study indicate that the investigated flakes were subjected to an early-stage oxidative degradation, which is sufficient to induce yellowing and measurable changes in mechanical and rheological properties, but does not correspond to severe structural breakdown.

The mechanical trends observed with increasing yellowing indicate the coexistence of competing degradation mechanisms acting in different HIPS phases. The simultaneous increase in Young’s modulus, tensile strength, and elongation at break, together with decreasing impact strength, suggests that degradation is governed by a balance between chain scission in the polystyrene matrix and oxidation-induced crosslinking in the polybutadiene phase [1,13,15,28,29,39,42]. Crosslinking limits the ability of rubber particles to dissipate impact energy, explaining the reduction in impact resistance despite material stiffening.

In addition, oxidation may modify rubber particle morphology and interfacial adhesion, affecting cavitation behavior and stress transfer between phases. Such interfacial restructuring can enhance the quasi-static tensile response while simultaneously promoting embrittlement under dynamic loading [13,39,42]. Therefore, the counterintuitive mechanical behavior can be interpreted as a characteristic of an intermediate degradation stage typical for naturally aged post-consumer HIPS, where stiffening and embrittlement develop concurrently rather than sequentially [1,13,15,28,29,39,42].

The decrease in MFI with increasing yellowing (from 4.12 g/10 min for “L” > 90 to 2.95 g/10 min for “L” < 87 flakes) contrasts with many reports where HIPS degradation led to an increase in MFI due to dominant chain scission and molecular weight reduction [9,11,27,28,29,30]. However, the literature data also indicate that the rheological response of HIPS depends on the prevailing degradation mechanism. In some studies, an initial decrease in MFI after early processing cycles was observed [1,28,29], suggesting that chain scission does not always control material behavior at moderate degradation levels. Therefore, the results obtained in this work may reflect competing degradation pathways rather than a simple decrease in molecular weight.

FTIR results confirm oxidative reactions in the most yellowed fraction, including the formation of carbonyl groups (~1743 cm⁻¹) and degradation of the polybutadiene phase. Oxidation of PB can lead not only to chain scission but also to radical recombination and crosslinking within the rubber phase [9,27], which restricts chain mobility and increases melt viscosity, thereby reducing MFI. Additionally, morphological changes in rubber particles and possible structural heterogeneity (e.g., inclusions observed in the extruded material) may further hinder flow.

The FTIR analysis indicates that surface yellowing of recycled HIPS is directly associated with chemical degradation processes occurring in both phases of the material, i.e., the polystyrene (PS) matrix and the dispersed polybutadiene (PB) rubber phase. The observed decrease in the absorbance of characteristic bands (3030, 2920–2850, 1600, 1500, 1450, 1030, and 750–700 cm⁻¹) with increasing yellowing confirms progressive structural deterioration of C–H, C=C, and C–C bonds. In particular, the reduction in the band near 750–760 cm⁻¹ indicates degradation of unsaturated PB units, which are known to be the primary sites of thermo-oxidative attack in HIPS. As reported in [9], oxidation of the PB double bonds initiates radical reactions that may subsequently propagate into the PS matrix.

The appearance of a carbonyl band at approximately 1743 cm⁻¹ in the most yellowed (“L” < 87) flakes provides direct evidence of oxidative processes leading to the formation of oxygen-containing groups such as ketones, aldehydes, or esters [15]. This confirms that yellowing is not merely a physical aging phenomenon but is related to chemical oxidation at the material surface. The evolution of carbonyl species is consistent with oxidative chain scission reactions and oxygen incorporation, which are typical for thermo-oxidative and photo-oxidative degradation of HIPS [9,15,29,39]. The absence of a strong carbonyl signal in the less yellowed fractions suggests that the degradation level in these materials remains relatively moderate.

4. Conclusions

In summary, the obtained research results confirm that yellowing of the HIPS surface affects the final color of standardized specimens, i.e., the material originating from recycling. Additionally, changes in mechanical properties confirm that yellowed HIPS has been subjected to various degradation mechanisms. Changes found in properties and colors of materials obtained from recycled plastics provide valuable information for companies involved in plastic processing and production of plastic elements. Moreover, information about the MFI value, which also changes due to or not to the yellowing of the HIPS surface, will be helpful for recyclers. The surface changes due to aging were also confirmed by the ATR-FTIR as well as contact angle measurements.

The obtained results can indicate that “L” < 87 HIPS does not negatively affect all mechanical parameters of the material produced from it. As shown by the results, degradation leads to modifications in polymer properties, including both improvements and reductions in selected mechanical parameters. Relating the ATR-FTIR results to those of the mechanical properties of HIPS, deterioration of HIPS is associated with surface reactions, which consisted of carbonyl formation, chain scission, and crosslinking. Thus, the surface chemical modifications result probably in cracks or cavities, which weaken/strengthen the HIPS material. Consequently, there were some reductions in mechanical properties [15]. Moreover, improved adhesion (highest for dumbbells made of “L” = 87–90 HIPS flakes) may contribute to locally enhanced mechanical performance [45].

Although the effects of photo-oxidation and thermo-oxidation on HIPS—such as carbonyl formation, FTIR spectral evolution, MFI variation, and changes in mechanical performance—have been widely reported for artificially aged materials [1,2,3,9,10,11,12,13,14,15], the present study differs in the nature of the investigated material. Most literature studies employ controlled laboratory aging conditions (e.g., defined UV exposure, elevated temperature, or multiple processing cycles), where a dominant degradation mechanism can be isolated and monitored. In contrast, the HIPS analyzed in this work originates from post-consumer refrigeration equipment and was subjected to long-term natural aging under uncontrolled environmental conditions. Consequently, the degradation pattern reflects the combined action of photo-oxidation, thermo-oxidation, environmental fluctuations, and service-related factors, rather than a single, well-defined aging stimulus. This difference is reflected in the experimental results. The FTIR analysis revealed moderate but non-uniform spectral changes, with carbonyl formation observed primarily in the most yellowed fraction, indicating surface-localized oxidation. Simultaneously, the MFI decreased with increasing yellowing, suggesting the coexistence of competing mechanisms such as chain scission and crosslinking in the polybutadiene phase, rather than the predominantly chain-scission-controlled behavior often reported for laboratory-aged samples [9,11,27,28,29,30]. Moreover, segregation based on the “L” parameter demonstrated that visually distinguishable fractions of naturally aged recyclate exhibit measurable differences in rheological and mechanical properties. Thus, the novelty of this study lies not in identifying previously unknown degradation reactions but in demonstrating how complex, naturally induced aging manifests in real post-consumer HIPS streams and how it affects their processing and performance characteristics.

To translate the obtained results into practical guidance for recyclers and processors, the colorimetric “L” parameter can be treated as an operational sorting criterion reflecting the degree of natural degradation of post-consumer HIPS. The results indicate that fractions with L > 90 exhibit the highest MFI, the highest impact strength, and the highest Vicat softening temperature, suggesting the most stable processing behavior and the lowest degradation level. The L = 87–90 fraction shows intermediate behavior and may be considered suitable for reuse in applications with moderate mechanical requirements. In contrast, the L < 87 fraction demonstrates reduced MFI, visible oxidative FTIR features (carbonyl formation), the presence of yellow inclusions in the extrudate, and the lowest impact strength, indicating a higher risk of processing variability and structural heterogeneity.

From an applied perspective, these findings suggest that an L ≈ 87 value may serve as a preliminary practical threshold for reuse decisions. Materials above this limit can be processed without additives in applications similar to those investigated, while materials below this limit may require blending with less degraded or virgin recyclate, the use of stabilization additives, or application in products with lower impact requirements. Importantly, this study demonstrates that visually based segregation supported by quantitative colorimetry enables the prediction of rheological behavior and changes in mechanical performance. Thus, the proposed L-based classification provides a simple screening tool that can be implemented at the industrial sorting stage to improve batch consistency and reduce processing risk when handling naturally aged post-consumer HIPS streams.

Further investigations, including coloring of specimens produced from white, semi-yellow, and yellow HIPS fractions, are planned.