Development and Characterization of Waste Polystyrene/Calcite Composites Prepared by Solution Casting

Abstract

This work reports the development of sustainable composites based on waste polystyrene (WPS) reinforced with natural calcium carbonate (CaCO₃), aiming at both plastic waste valorization and performance enhancement. The composites were fabricated via a solvent casting approach using recycled polystyrene cutlery and varying CaCO₃ loadings. Structural analyses performed by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and photoluminescence (PL) spectroscopy confirmed the effective incorporation of calcite within the polymer matrix, as evidenced by characteristic peak shifts and intensity variations indicative of interfacial interactions. PL spectra exhibited two emission bands centered around 365 and 400 nm for all samples, with enhanced emission intensity at low filler contents (2 and 6 wt%), followed by pronounced quenching at 10 wt%, highlighting a concentration-dependent interaction and uniform filler dispersion at lower loadings. Thermal analyses (DSC, TGA, and DTA) demonstrated a systematic improvement in thermal stability upon CaCO₃ addition, with decomposition temperatures increasing up to 432 °C compared to 429 °C for neat WPS, attributed to the barrier effect of the inorganic filler and restricted polymer chain mobility. Overall, these results demonstrate that natural calcite is an efficient, low-cost, and locally available reinforcing agent for recycled polystyrene, providing a viable and scalable strategy for the upcycling of plastic waste into value-added, thermally stable composite materials.

1. Introduction

Plastic pollution has become one of the most critical environmental issues of the 21st century, largely due to the persistence of non-biodegradable polymers such as polystyrene (PS) [1,2,3]. As a thermoplastic polymer, PS is extensively used in packaging, insulation, and disposable consumer products because of its lightweight nature, transparency, and low production cost [4,5]. However, its chemical stability and resistance to degradation make it particularly problematic at the end of its lifecycle, contributing significantly to environmental pollution, especially in the form of microplastics [6]. PS waste can persist in terrestrial and aquatic ecosystems for centuries, posing serious ecotoxicological risks to wildlife and natural habitats [7].

Global plastic production has increased dramatically over the last decades, reaching approximately 413.8 million tonnes in 2023, according to Plastics Europe. Polystyrene, including general-purpose polystyrene (GPPS), high-impact polystyrene (HIPS), and expanded polystyrene (EPS), represents an important share of commodity plastics because of its widespread use in food packaging, disposable utensils, electronic appliances, and building insulation. Despite these extensive applications, post-consumer polystyrene remains one of the least recycled thermoplastics due to its low bulk density, contamination by food residues, and the high transportation and sorting costs associated with waste collection. Consequently, a considerable proportion of PS waste is still disposed of in landfills or leaks into terrestrial and marine environments, where it fragments into persistent microplastics. These challenges clearly demonstrate the urgent need for efficient recycling and upcycling strategies capable of converting waste polystyrene into value-added materials while supporting circular economy objectives [8,9,10,11].

Despite these continuously increasing production volumes, the global recycling rate of polystyrene remains alarmingly low. This is primarily due to its low density, the logistical challenges of collection, and the limited economic viability of conventional recycling methods [8]. As a result, large volumes of PS waste accumulate in landfills and natural environments.

Recycling strategies for plastic waste are generally classified into three main categories: mechanical, chemical, and energy recovery (incineration) [9]. Mechanical recycling, which consists of collecting, cleaning, grinding, and remelting plastics, is cost-effective and widely applied. However, it often produces materials with degraded mechanical and thermal properties compared to virgin polymers, due to repeated thermal and mechanical stresses [10]. Chemical recycling, which includes processes such as pyrolysis, depolymerization, chemical and physical activation, and gasification, can regenerate valuable chemical feedstocks but is energy-intensive, costly, and requires complex purification steps in many cases [11,12,13,14,15]. Energy recovery through incineration enables volume reduction and energy production, but it generates greenhouse gases and toxic by-products, limiting its sustainability [16,17,18].

To overcome these drawbacks, solvent-based recycling has emerged as a promising alternative for PS valorization. This method involves dissolving polystyrene in an appropriate solvent, removing impurities, and re-precipitating or blending it with functional additives. Unlike conventional mechanical recycling, solvent-based approaches preserve polymer integrity, minimize thermal degradation, and allow the incorporation of reinforcements such as mineral fillers [19]. In this study, we employ this approach using discarded PS cutlery as the raw material, thereby combining environmental remediation with the functionalization of recycled PS [20].

In response to this challenge, recent research efforts have focused on developing advanced recycling and upcycling technologies aimed at converting PS waste into higher-value materials [4].

Recycled polystyrene (rPS) composites have attracted increasing attention as a promising route for simultaneously reducing plastic waste and improving material performance. A wide variety of fillers has been investigated in rPS matrices, including mineral fillers such as calcium carbonate (CaCO₃), talc, silica, fly ash, and clay minerals, as well as bio-based fillers such as wood flour, natural fibers, eggshell powder, and biochar. Mineral fillers are generally incorporated to increase stiffness, dimensional stability, and thermal resistance, whereas bio-based fillers contribute to weight reduction, cost reduction, and improved sustainability [7,16,21,22].

Different processing techniques have been reported for the fabrication of recycled PS composites, including melt extrusion, compression molding, injection molding, and solvent-assisted methods. Melt processing is the most widely used industrial approach; however, repeated thermal processing may induce chain scission and deterioration of polymer properties. In contrast, solvent-based dissolution and casting techniques can preserve polymer integrity, facilitate impurity removal, and promote a more homogeneous dispersion of fine mineral particles within the recycled polymer matrix [16,20].

The final performance of rPS composites strongly depends on the quality of the polymer–filler interface. Weak interfacial adhesion often results in poor stress transfer, filler agglomeration, and limited reinforcement efficiency. For this reason, several studies have employed surface-modified CaCO₃ particles using stearic acid, oleic acid, silane coupling agents, or titanate treatments to improve compatibility with hydrophobic polymer matrices. These interfacial modifications have been shown to enhance filler dispersion, reduce particle aggregation, and improve the mechanical and thermal properties of the resulting composites [22,23].

To evaluate these effects, previous investigations have commonly combined scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and mechanical testing. These techniques provide complementary information on morphology, crystallinity, chemical interactions, thermal transitions, degradation behavior, and reinforcement efficiency. Reported improvements in rPS composites include increased thermal stability, higher Young’s modulus, improved dimensional stability, enhanced hydrophobicity, and better resistance to thermal degradation, although excessive filler loading may lead to particle agglomeration, increased brittleness, and deterioration of mechanical performance [22,23,24].

Because of these combined advantages, recycled PS composites reinforced with low-cost mineral fillers are being considered for several applications, including packaging materials, thermal insulation panels, lightweight construction products, decorative components, and non-structural consumer goods. Nevertheless, achieving homogeneous filler dispersion while maintaining a favorable balance between thermal stability, processability, and interfacial compatibility remains a significant challenge, particularly when untreated natural mineral resources are employed [16,22,23,24].

Among the different categories of fillers investigated for recycled polystyrene, calcium carbonate remains one of the most attractive candidates because of its low cost, natural abundance, chemical stability, and ability to improve thermal and mechanical performance. Consequently, particular attention has been devoted to the development of rPS/CaCO₃ composites for sustainable material applications [16]. Calcium carbonate (CaCO₃), in particular, stands out due to its abundance, affordability, and compatibility with polymer systems. Its inclusion not only improves thermal stability, rigidity, and whiteness but also reduces production costs [21]. The incorporation of CaCO₃ as a filler in polymer matrices has been widely studied. For instance, Kurtz et al. studied the pyrolysis of polystyrene (PS) in the presence of various mineral fillers, including CaCO₃ [7]. They demonstrated that high concentrations of CaCO₃ significantly altered the thermal decomposition profile, thereby enhancing the thermal stability of the composites. More specifically, using thermogravimetric analysis (TGA), they observed that incorporating up to 50% CaCO₃ into the PS matrix caused a marked shift in the decomposition temperature to higher values, indicating improved thermal resistance of the composite material.

Similarly, Olowofoyeku et al. demonstrated that PS/CaCO₃ composites modified with citric and oleic acids exhibited tunable hydrophobicity and photoluminescence, with enhanced porosity and surface properties [22].

Ritonga et al. [23] demonstrated that organo-precipitated CaCO₃ (O-PCC) enhanced the mechanical and thermal performance of LLDPE/CNR composites. In their work, a 5% concentration of O-PCC yielded optimal results, with a tensile strength of 17.17 MPa and a Young’s modulus of 252.68 MPa, representing increases of 26.2% and 28.1%, respectively, compared to unfilled blends.

Moreover, thermal analysis confirms that CaCO₃ acts as a thermal stabilizer. Ritonga et al. reported a maximum degradation temperature of 481 °C for O-PCC composites, compared to 319 °C for unfilled blends.

Hayeemasae and Ismail [24] explored the synergistic effects of combining CaCO₃ with eggshell powder (ESP) in R-PS matrices, demonstrating that CaCO₃ significantly enhances mechanical and thermal properties when compared to ESP alone. Their findings revealed that composites containing 20 wt% CaCO₃ exhibited the highest tensile strength, Young’s modulus, and impact resistance, attributed to the ultrafine particle size and superior dispersion of CaCO₃ within the polymer matrix. Thermogravimetric analysis (TGA) in their study further supported the thermal benefits of CaCO₃, with the composite containing 20 wt% CaCO₃ reaching a degradation temperature of 425.93 °C, higher than that of ESP-filled composites. This thermal resistance is consistent with our findings, where the addition of calcite delayed the onset of decomposition and increased the overall stability of the PS matrix.

These innovations align with the principles of the circular economy, promoting the dual valorization of plastic waste and locally available mineral resources. In this study, we investigate the transformation of post-consumer polystyrene waste into functional composites by integrating natural calcite sourced from the Gabès region of Tunisia. The composites are synthesized via a solvent-based method using discarded PS cutlery, with the dual objective of reducing environmental impact and enhancing material performance.

To evaluate the structural and thermal behavior of the WPS/CaCO₃ composites, we employ a suite of characterization techniques including X-ray diffraction (XRD), infrared spectroscopy (IR), differential scanning calorimetry (DSC), differential thermal analysis (DTA), and photoluminescence (PL).

A comparative overview of the most relevant studies on recycled PS/CaCO₃ composites is presented in

Table 1. Comparative overview of previous recycled PS/CaCO₃ composite studies and the novelty of the present work.

Study	Matrix	Filler	Processing	Characterization	Main Focus	Limitation	Present Work
Hayeemasae & Ismail (2021) [24]	Recycled PS	Commercial CaCO3 + ESP	Melt mixing	Mechanical + TGA	Mechanical reinforcement	No PL analysis, no natural calcite	Natural calcite + PL
Olowofoyeku et al. (2025) [22]	Virgin PS	Modified CaCO3	Solution casting	PL + Surface properties	Optical properties	Virgin PS only	Recycled PS
Kurtz et al. [11]	PS	CaCO3	Pyrolysis	TGA	Thermal decomposition	Not composite development	Composite development
Present work	Post-consumer WPS	Untreated natural Tunisian calcite	Solution casting	XRF, SEM/EDS, XRD, FTIR, PL, DSC, TGA	Structure–PL–thermal correlations	=	First combined approach

to highlight the current research status and identify the remaining knowledge gaps addressed in this work.

As summarized in Table 1, previous studies have mainly focused on improving the mechanical and thermal performance of recycled or virgin PS composites using commercial or surface-modified CaCO₃. However, limited attention has been devoted to untreated natural calcite, post-consumer waste polystyrene, and the investigation of photoluminescence behaviour, which constitute the main research gaps addressed in the present study.

Although CaCO₃ is widely used as a filler in polymer composites, its potential as a multifunctional additive in recycled polystyrene remains largely unexplored. Beyond its filler role, natural calcite may act as a passive photostabilizer by providing physical UV shielding, thereby reducing photo-induced degradation while influencing the thermal behavior of the polymer matrix. Therefore, this study investigates the effect of natural calcite on the structural, morphological, and thermal properties of waste polystyrene (WPS) composites, highlighting their potential for sustainable plastic waste valorization.

Calcium carbonate has been extensively investigated as a reinforcing filler for polymer composites; most previous studies have focused on virgin polymers, commercial CaCO₃, surface-modified fillers, or conventional melt-processing techniques. Comparatively fewer investigations have addressed the valorization of post-consumer waste polystyrene using untreated natural calcite through solvent-assisted recycling routes. Moreover, previous studies have mainly concentrated on mechanical and thermal properties, while the optical behaviour and photoluminescence response of recycled PS/CaCO₃ composites remain largely unexplored.

The novelty of the present work therefore lies in the combined use of post-consumer waste polystyrene recovered from disposable cutlery and untreated natural calcite originating from the Gabès region of Tunisia as a sustainable mineral reinforcement. Unlike previously reported studies, the composites were prepared through a solution-casting process designed to preserve the integrity of the recycled polymer while promoting homogeneous filler dispersion. Furthermore, this study combines XRF, SEM/EDS, XRD, FTIR, photoluminescence, DSC and TGA analyses to establish correlations between filler dispersion, interfacial interactions, optical behaviour and thermal stability. To the best of our knowledge, the influence of untreated natural calcite on the photoluminescence behaviour of recycled polystyrene composites has not been previously reported. These findings provide new insights into the multifunctional role of natural calcite beyond its conventional use as an inexpensive mineral filler and contribute to the development of sustainable high-value materials within a circular economy framework.

2. Materials and Methods

2.1. Materials

This study employed waste polystyrene (WPS), recovered from post-consumer cups, as the polymer matrix. The collected material was thoroughly cleaned, rinsed with water, and dried before being cut into small fragments for subsequent characterization and processing. The mineral filler was sourced from the Gabes region in southern Tunisia and consists of a dark beige calcite powder with relatively high hardness. Before incorporation into the polymer matrix, the calcite was ground and sieved to obtain uniform particle fractions, thereby enhancing their dispersion within the composite. The particle size showed an average diameter of approximately 60 µm. The elemental composition of the natural calcite, including the presence of trace elements, was determined using X-ray fluorescence (XRF) spectroscopy.

2.2. Elaboration of WPS/CaCO₃ Composite

The WPS/CaCO₃ composites were synthesized using a solvent-based method. Three different loadings of calcite were reinforced in the WPS matrix, as summarized in

Table 2. Composition of WPS/CaCO₃ composites.

Sample	Mass of Calcite (g)	Mass of WPS (g)
WPS	0.0000	30.0000
WPS 2%	0.6021	29.4400
WPS 6%	1.8023	28.2573
WPS 10%	3.0004	27.0819

.

The WPS was first dissolved in chloroform (50 mL) at 50 °C (Figure 1). After complete dissolution, the predetermined amount of calcite was incorporated into the polymer solution, and the mixture was magnetically stirred for 2 h at 60 °C to ensure uniform dispersion of the filler within the matrix.

The resulting suspension was then left to dry at ambient temperature (approximately 25 °C) for one week to allow complete evaporation of the residual solvent. The solid composites were subsequently ground using a ball mill to produce fine powders appropriate for detailed structural and thermal characterization.

In addition, preliminary trials were undertaken to evaluate different solvent systems in order to identify the most effective medium for achieving optimal filler dispersion within the polymer matrix. Among the solvents initially investigated were ethanol, acetone, and ethanol/water mixtures. Although these preliminary experiments provided useful insights into the dispersion behavior of the fillers, they did not yield the most satisfactory dispersion and were therefore not selected for the final composite preparation.

Chloroform was selected as the solvent due to its high volatility and excellent capability to completely dissolve waste polystyrene at room temperature, ensuring optimal interfacial contact between the polymer chains and the calcite particles during the solution-casting process.

2.3. Composite and Filler Characterization

X-ray fluorescence spectroscopy (XRF) is used to determine the exact composition of the natural calcite filler, including its major and minor elemental constituents. The analysis was carried out using a Bruker S8 TIGER wavelength-dispersive X-ray fluorescence (WDXRF) spectrometer equipped with a 3 kW X-ray tube(Karlsruhe, Germany). The spectra were processed and quantified using the Geo-Quant software, which enables standardless semi-quantitative elemental analysis.

The particle size distribution (PSD) and the mean diameters of the microcapsules were determined using a Microstrac S3500 particle size analyzer (Haan, Germany).

The X-ray diffraction (XRD) patterns of the samples were recorded between 20° and 80° 2θ at a scanning speed of 2°/min, using a Panalytical X’Pert High Score Plus diffractometer with CuKα1 radiation (λ = 1.5406 Å), operated at 30 mA and 40 kV. Subsequently, different diffractograms were processed by X’pert High Score Plus software(version 3.0.0.).

The average crystallite size d was calculated using the Scherrer Equation (1):

$$d={\displaystyle \frac{K\lambda }{\beta \cos \theta }}$$

(1)

where K is the crystal shape factor (0.89 here), λ is the wavelength of the X-ray (0.1542 nm here), β is the width of the diffraction peak at its half height, θ is the angle between the incident X-ray and the plane perpendicular to the (hkl) plane, and d is the average crystallite size (nm) [25].

This approach was performed using Data analysis, and graphing was performed using OriginPro 2021 (OriginLab Corporation, Northampton, MA, USA).

FTIR-ATR analysis was performed on the samples at room temperature using a Nicolet 560 spectrophotometer equipped with a diamond ATR accessory. The spectral range covered 4000 to 400 cm⁻¹ with a resolution of 4 cm⁻¹.

The thermogravimetric analyses (TGA) were performed using an ATD/TGA Setsys Evolution 1750 (SETARAM) analyzer under an argon flow of 10 mL/min. For the calcite analysis, the sample was heated from 25 to 900 °C at a heating rate of 10 °C/min, whereas the composite samples were analyzed from 25 to 600 °C under the same experimental conditions.

Thermograms were recorded using a DSC Mettler Toledo 823 instrument. Calibration was performed with indium in the temperature range of 0–350 °C. Experiments were conducted under nitrogen gas (N₂), at a flow rate of 50 mL/min, from 20 °C to 500 °C at a heating rate of 10 °C/min.

The experimental photoluminescence device used is a Perkin–Elmer LS55 Spectrofluorometer. The excitation source is a 450 W xenon lamp that allows continuous excitation from 200 nm to 800 nm. The excitation and emission wavelengths are selected using double monochromators with a focal length of 22 cm.

3. Results and Discussion

3.1. Characterization of the Filler

3.1.1. X-Ray Fluorescence Result

The chemical composition of the filler (

Table 3. Elemental composition of natural calcite obtained by XRF.

Name	MgO	Al2O3	SiO2	SO3	CaO	SrO	Na2O	P2O5	K2O	TiO2	Cr2O3	Fe2O3	Mn2O3	Total
% in Calcite	0.142	0.054	0.251	0.012	55.957	0.010	0.004	0.024	0.015	0.009	Not detected	0.025	0.003	99.90

) shows that the composition is exclusively calcium oxide (CaO).

The XRF analysis showed that CaO is the predominant oxide (55.957 wt%), confirming the high purity of the natural calcite used as a mineral filler. Minor amounts of SiO₂ (0.251%), MgO (0.142%), and other oxides indicate negligible impurities. Since XRF results are expressed as oxide equivalents, the reported CaO corresponds to the calcium contained in CaCO₃, whereas the remaining mass is mainly attributed to CO₂, which is released during calcite decomposition and accounted for as the Loss on Ignition (LOI).

3.1.2. Particle Size Distribution of the Filler

The particle size distribution (PSD) of the calcite (Figure 2) exhibits a broad unimodal profile spanning from ~1 to 500 µm, centered around a median diameter of 59.62 µm. Such a particle size distribution is highly advantageous, as it facilitates the optimal dispersion of the inorganic filler throughout the polymer matrix. Consequently, this improves the macroscopic homogeneity of the composite and maximizes the interfacial contact area between the phases.

3.1.3. Thermogravimetric Result of the Filler

Figure 3 presents the thermogravimetric analysis (TGA) of the calcite sample. An endothermic peak appears at approximately 875 °C [26]. This stage corresponds to the decomposition of calcium carbonate (CaCO₃) into calcium oxide (CaO) and carbon dioxide (CO₂), as described by Equation (2):

CaCO₃ → CaO + CO₂

(2)

The mass loss observed within this temperature range aligns with the theoretical decomposition of CaCO₃, thereby confirming both the thermal purity and the characteristic stability of the mineral filler.

3.2. Morphological and Structural Properties of WPS/Ca Composites

Scanning electron microscopy (Figure 4) observations of WPS/calcite composites evidence pronounced morphological evolution at the surface as a function of filler content. At a low calcite loading of 2 wt% (Figure 4a), the particles are uniformly dispersed within the polymer matrix, leading to a relatively smooth and homogeneous surface morphology. The filler appears well embedded, indicating good interfacial adhesion and effective stress transfer. Increasing the filler content to 6 wt% (Figure 4b) results in a higher surface roughness and particle population, as expected. At the same time, the dispersion remains largely uniform, and no significant phase separation is detected.

In contrast, at 10 wt% calcite (Figure 4c), the microstructure becomes noticeably heterogeneous, characterized by the presence of particle agglomerates. This morphological transition suggests that the matrix capacity to accommodate additional filler is exceeded, leading to filler–filler interactions dominating over matrix–filler interactions. Such agglomeration is likely to generate localized stress concentrations, which may adversely affect the thermal and mechanical performance of the composite [19].

The EDS spectrum analysis showed that the majority of the particles were mainly composed of carbon and oxygen, confirming their predominantly polystyrene-based nature. Moreover, the persistent and pronounced calcium peak observed in the spectra indicates the successful incorporation of calcite as a filler within the WPS matrix.

The XRD pattern of pure calcite exhibits the characteristic reflections of rhombohedral CaCO₃, with the dominant peak at 2θ ≈ 29.4° corresponding to the (104) plane, which serves as the primary reference for phase identification (Figure 5). This same reflection is observed in all WPS/CaCO₃ composites (WPS/Ca₂, WPS/Ca₆, WPS/Ca₁₀), demonstrating that the calcite preserves its crystalline structure after incorporation into the polymer matrix, with no evidence of phase transformation, peak shifting, or the formation of new crystalline phases [27]. The progressive increase in the intensity of the (104) peak as the filler loading increases reflects the growing crystalline contribution of calcite within the composite, consistent with previous findings reporting a proportional enhancement in CaCO₃ diffraction intensity with increasing filler content. The persistent amorphous halo of WPS in all diffractograms confirms the coexistence of the polymer and mineral phases, indicating that calcite acts as an inert filler without undergoing interfacial reactions or structural modifications during composite processing.

The crystallite size of calcite (

Table 4. Crystallite size (d) of WPS/CaCO₃ composites and Calcite.

Sample	Crystallite Size d (nm)	Filler Loading (%)
Ca	54	100
WPS/Ca10	33	10
WPS/Ca6	23	6
WPS/Ca2	18	2

) decreases markedly from that of the pure mineral (~54 nm) to the low filler composite WPS/Ca₂ (~18 nm), a behavior resulting from the combined influence of mechanical shear during melt mixing, which induces partial fragmentation of the mineral crystallites, and the confinement imposed by the WPS matrix, which generates interfacial micro stresses responsible for additional peak broadening and a reduced apparent crystallite size according to the Scherrer relation [28].

At higher filler loadings, the increased tendency of calcite particles to aggregate reduces the confinement imposed on individual crystallites, resulting in diffraction patterns that more closely resemble those of bulk calcite. This behavior reflects the well-established propensity of CaCO₃ to form aggregates at elevated concentrations within polymer matrices.

The FTIR ATR spectrum of waste polystyrene (WPS) exhibits strong and well-defined characteristic absorption bands, as summarized in

Table 5. FTIR-ATR spectroscopic data of WPS.

Peak 1.	Functional Groups
755–694	C-H Deformation (δ C-H)
1157–1104–1027	C-H Deformation (δ C-H)
1453–1607	C=C Symmetrical Elongation (νsym C=C)
1453–1607	C-H aromatic Elongation (ν C-H aromatic)

.

The FTIR spectra of the WPS/CaCO₃ composites (Figure 6) display all the characteristic absorption bands of polystyrene, including the aromatic C–H stretching at 3028 cm⁻¹, the aliphatic C–H stretching at 2882 cm⁻¹, and the phenyl ring vibrations at 1610, 1446, 900, and 675 cm⁻¹, confirming that the polymer structure is preserved after incorporation of the mineral filler. The composites also exhibit the distinctive carbonate bands of calcite at 1426, 874, and 712 cm⁻¹, which appear slightly broadened due to overlap with the polymer signals, indicating the coexistence of both phases within the matrix [29]. A minor shift in the carbonate stretching bands (ν C–O) toward higher wavenumbers, together with an increase in their intensity as the filler loading increases, suggests weak but detectable interactions between the polymer chains and the mineral particles. The characteristic vibrational bands of polystyrene also undergo slight shifts to higher frequencies, particularly for the aromatic C–H stretching and in-plane deformation modes. Additionally, the emergence of a weak band near 1803 cm⁻¹, possibly associated with carbonyl-related vibrations induced by the presence of calcite, supports the presence of subtle chemical interactions or physical adsorption phenomena between CaCO₃ and the WPS matrix [30].

3.3. Photoluminescence Property of Composite WPS/CaCO₃

The photoluminescence (PL) spectra of pure WPS and WPS/CaCO₃ composites, recorded at room temperature, are presented in Figure 7. All samples exhibit two distinct emission bands: The first emission band (monomer), centered at approximately ~370 nm (FM), is assigned to the intrinsic fluorescence (monomer emission) of isolated phenyl groups in the polystyrene chains. This band originates from π→π electronic transitions associated with the aromatic rings of the polymer. The second, broader and red-shifted band at ~406 nm corresponds to excimer emission (FE), attributed to π→π* and n→π*; excited-state interactions between adjacent phenyl groups that adopt a face-to-face ("sandwich") configuration following local rotation of the polymer segments. Because both emission bands are also observed in neat WPS, their origin is primarily associated with the polystyrene matrix rather than with the calcite filler. The excimer forms through a local rotation of polymer segments, enabling a face-to-face “sandwich” arrangement between adjacent phenyl groups, consistent with intramolecular interactions reported in earlier studies [31,32,33].

Natural calcite is not expected to exhibit significant intrinsic photoluminescence in this spectral region. Although trace impurities or structural defects present in natural CaCO₃ may contribute weak luminescence, their contribution is considered negligible compared with that of the polymer matrix. Therefore, the role of calcite is mainly indirect, influencing the emission behavior through modifications of polymer chain mobility, filler dispersion, and polymer–filler interfacial interactions rather than acting as an independent luminescent center.

The incorporation of calcite into the WPS matrix induces a significant and concentration-dependent modification of the PL intensity. At low to moderate filler loadings, WPS/Ca₂ and WPS/Ca₆ composites exhibit a pronounced enhancement in emission intensity, reaching values of approximately 1100–1200 a.u. compared to ~600 a.u. recorded for pure WPS (

Table 6. Wavelengths of luminescence bands of WPS and WPS/CaCO₃ composites.

Sample	λ (1st Band) (nm)	λ (2nd Band) (nm)
WPS	371	406
WPS/Ca2	366	396
WPS/Ca6	366	396
WPS/Ca10	386	400

). Photoluminescence measurements in WPS/CaCO₃ systems have previously shown that the optical response is sensitive to both filler concentration and surface modification state [22], consistent with the role of well-dispersed CaCO₃ particles as optical scattering centers that increase the effective optical path length within the matrix. Furthermore, this intensity enhancement provides indirect but compelling evidence of a homogeneous dispersion of calcite particles within the polymer network and confirms the existence of effective interfacial interactions between the mineral filler and the WPS chains, in full agreement with the structural modifications observed in XRD and FTIR analyses. The increase in PL intensity observed at 2 and 6 wt% therefore reflects improved interfacial compatibility and restricted non-radiative relaxation pathways resulting from the homogeneous dispersion of calcite particles within the polymer matrix.

However, a reversal of this trend is observed at higher filler content, as WPS/Ca₁₀ displays a marked quenching of PL emission, with intensity dropping to ~400 a.u., significantly below that of pure WPS. This phenomenon is ascribed to concentration quenching, whereby the excessive presence of CaCO₃ particles promotes filler agglomeration, introducing structural heterogeneities that reduce the bonding strength between the matrix and the filler, and generating non-radiative recombination centers that compete with radiative emission pathways [34]. Luminescence techniques are sensitive probes of defects in the CaCO₃ crystal structure, further supporting the interpretation that beyond an optimal loading, the dispersion quality deteriorates and locally compromises the filler–matrix interfacial compatibility [35].

Overall, these results indicate that the observed emission bands originate predominantly from the polystyrene matrix, whereas the addition of calcite modulates their intensity through changes in morphology, interfacial interactions, and polymer chain dynamics, rather than through direct luminescent emission from the mineral filler itself.

Additionally, the presence of rigid inorganic particles locally restricts segmental mobility near the polymer–filler interface. This steric hindrance impedes the formation of the face-to-face conformation required for excimer emission, leading to lower excimer yield and reduced photochemical quenching efficiency, in agreement with Al-Ani et al. [36]. In this context, CaCO₃ behaves as a passive photostabilizer by providing physical UV shielding rather than participating in energy transfer processes.

These results further confirm the homogeneous dispersion of calcite within the polymer matrix, consistent with the trends observed in XRD and FTIR spectra.

Taken together, the structural, morphological, and photoluminescence results reveal a clear concentration-dependent effect of CaCO₃ incorporation into the WPS matrix. At low and intermediate filler contents (2 and 6 wt%), the relatively homogeneous dispersion of calcite particles observed by SEM is accompanied by an enhancement of the PL intensity, suggesting that a good filler dispersion favors effective polymer–filler interfacial interactions and modifies the local polymer chain environment. This behavior is consistent with the XRD and FTIR results, which confirm the incorporation of the calcite phase and reveal changes in the vibrational features of the polymer–filler system. In contrast, at 10 wt% CaCO₃, the SEM observations reveal more pronounced particle agglomeration, which coincides with a marked decrease in PL intensity. This concentration-dependent transition suggests that excessive filler loading promotes filler–filler interactions and morphological heterogeneity, thereby reducing the efficiency of the polymer–filler interface and favoring non-radiative relaxation processes. The thermal results follow the same general trend, with increasing CaCO₃ content progressively increasing the onset decomposition temperature and residual mass. Thus, the combined results indicate that CaCO₃ content governs the balance between filler dispersion, structural/interfacial interactions, photoluminescence response, and thermal behavior. Intermediate CaCO₃ contents (2–6 wt%) provide a favorable balance between dispersion and optical response, whereas the higher loading of 10 wt% promotes agglomeration and PL quenching, although it contributes to delaying the onset of thermal degradation.

3.4. Thermal Result

Thermal gravimetric analysis (TGA), coupled with differential thermal analysis (DTA) and differential scanning calorimetry (DSC), was employed to investigate the effect of calcite on the thermal stability of the WPS matrix. The corresponding thermograms are presented in Figure 8 and Figure 9, and the resulting characteristic parameters are summarized in

Table 7. Evolution of the decomposition start temperature of WPS and WPS/CaCO₃.

Sample	Tonset (°C)	Td (°C)
WPS	376	429
WPS/Ca2	376	432
WPS/Ca6	378	432
WPS/Ca10	385	432

.

The pure WPS matrix exhibits a monokinetic degradation profile, characterized by near-complete mass loss (Figure 8a), indicating total polymer pyrolysis. The onset decomposition temperature (Tonset) is 375 °C. The DTA thermogram (Figure 8b) shows a distinct endothermic peak centered at 429 °C (693 K), corresponding to the maximum decomposition temperature (Td), with a decomposition enthalpy of −766 J/g. This single peak is typical of a mechanism dominated by the chain of the polystyrene.

The incorporation of CaCO₃ significantly modifies the degradation kinetics. For filler loadings of 2 wt%, 6 wt%, and 10 wt%, Tonset shifts to 376 °C, 378 °C, and 385 °C, respectively. This shift towards higher temperatures, particularly pronounced at 10 wt% (+10 °C), reveals a stabilizing effect of the inorganic filler.

The evolution of the residual mass after pyrolysis (Figure 8a) confirms this trend. While pure WPS leaves no residue, the composites exhibit increasing char contents: 1 wt% (WPS/Ca₂), 3 wt% (WPS/Ca₆), and 12 wt% (WPS/Ca₁₀). The non-additive nature of these residues suggests physico-chemical interactions at the polymer–filler interface or, according to some studies, the presence of trapped residual solvents that influence the initial stages of pyrolysis.

Overall, these results highlight a modest improvement in the thermal stability of WPS induced by calcite. This stabilization is attributed to strong interfacial interactions between the polymer chains and the mineral particles, acting as physical cross-linking points. They restrict chain mobility, hinder the diffusion of volatile degradation products, and thus delay the comprehensive decomposition process.

Although the increase in the maximum decomposition temperature (Td) is relatively limited (approximately 3 °C), its practical relevance should be considered together with the progressive increase in the onset decomposition temperature (Tonset) and the higher residual char content observed as the calcite loading increases. These combined results indicate that natural calcite does not fundamentally modify the thermal degradation mechanism of waste polystyrene but rather delays the initiation of degradation through a physical barrier effect and the restriction of polymer chain mobility at the polymer–filler interface. Consequently, the primary contribution of CaCO₃ is a modest enhancement of thermal stability rather than a substantial increase in the ultimate decomposition temperature. Such behavior is consistent with that commonly reported for conventional mineral fillers, whose principal role is to retard thermal degradation and improve processing stability rather than dramatically increase the decomposition temperature.

Pure WPS exhibits a distinct glass transition (Tg) at approximately 60 °C, corresponding to the transition from the rigid glassy state to the amorphous rubbery state, a value consistent with reported data for polystyrene-based systems (Figure 9).

As reported in the literature, the addition of rigid fillers such as CaCO₃ can modify the polymer Tg [37]. In WPS/CaCO₃ composites, the presence of mineral particles restricts the cooperative movements of chain segments, particularly at the polymer–filler interface. This decrease in mobility, linked to the previously described interfacial interactions, results in a slight shift in Tg toward higher temperatures. This behavior confirms the existence of an interfacial structuring favorable to the thermomechanical stabilization of the composite [38].

4. Conclusions

This study demonstrated the feasibility of producing calcite-filled composites from recycled waste polystyrene (WPS) using a solution-casting approach. XRD, FTIR, SEM, and photoluminescence analyses confirmed the successful incorporation of natural calcite into the polymer matrix and revealed a concentration-dependent evolution of the composite structure and morphology. At low and intermediate CaCO₃ loadings (2–6 wt%), relatively homogeneous filler dispersion and enhanced photoluminescence intensity were observed, whereas the higher loading of 10 wt% promoted particle agglomeration and a pronounced decrease in PL intensity. These results indicate that the photoluminescence bands originate predominantly from the polystyrene matrix, while CaCO₃ mainly modulates the emission behavior through changes in filler dispersion, polymer–filler interfacial interactions, and polymer chain mobility.

The thermal analyses further showed a modest improvement in thermal stability with increasing CaCO₃ content, mainly reflected by the progressive increase in the onset decomposition temperature and residual mass, while the increase in the maximum decomposition temperature remained limited. Therefore, the incorporation of calcite should be considered as providing a moderate stabilization effect rather than a substantial enhancement of the ultimate thermal resistance of WPS. Overall, the combined structural, morphological, optical, and thermal results indicate that intermediate CaCO₃ contents (2–6 wt%) provide a favorable balance between filler dispersion, photoluminescence response, and thermal behavior, whereas excessive filler loading promotes agglomeration and limits the optical benefits of the mineral phase.

These findings highlight the potential of natural calcite as a low-cost mineral filler for the valorization of post-consumer polystyrene waste. However, the use of chloroform as a processing solvent constitutes an environmental limitation due to its potential VOC emissions, and mechanical properties could not be evaluated because of difficulties encountered during the fabrication of standardized test specimens. Future work should focus on optimizing specimen preparation, performing comprehensive mechanical characterization, and investigating greener processing routes and solvent-recovery strategies. Further studies should also investigate the effect of CaCO₃ particle size, surface modification, and filler dispersion on the structure–property relationships of the composites. Overall, this work provides a promising basis for the development of recycled polystyrene-based composites with tunable optical and moderately improved thermal properties within a circular economy framework.