A Green P–N–Al Synergistic System for Eco-Friendly Flame-Retardant Polystyrene

Highlights

What are the main findings?

• Developed a green flame-retardant strategy using bio-based PA and halogen-free components for PS
• Achieved 28.5% LOI, UL-94 V-0, 73.8% pHRR reduction, and 78.4% tensile strength retention
• Revealed cooperative condensed- and gas-phase flame-retardant mechanisms via in situ P-N-Al network

What are the implications of the main findings?

• Offers an eco-friendly strategy for high-performance flame-retardant PS materials
• Provides a new approach to overcome the flame-retardancy/mechanics trade-off in polymers
• Demonstrates a design strategy for multifunctional composites with balanced safety and performance

Abstract

Polystyrene (PS) is widely used yet highly flammable, and developing halogen-free flame retardants that ensure both high fire safety and mechanical performance remains a challenge. A green intumescent system comprising ammonium dihydrogen phosphate (ADP) and phytic acid–triethylenetetramine (PA–TETA) was incorporated into PS powder via sequential solution grinding and hot pressing. The optimal formulation, PS/10ADP/15PA–TETA, achieved a limiting oxygen index of 28.5% with a UL-94 V-0 rating, and reduced the peak heat release rate and total heat release by 73.8% and 46.2%, respectively, while retaining 78.4% of the tensile strength of neat PS. The ADP/PA–TETA system operates via a cooperative condensed-phase charring and gas-phase dilution mechanism, achieving superior flame retardancy in PS composites. This work provides an effective and eco-friendly strategy for fabricating high-performance PS composites with balanced flame retardancy and mechanical properties.

1. Introduction

The frequent occurrence of major fire incidents worldwide has heightened concerns over fire safety and environmental protection, spurring intensive research into flame-retardant materials [1,2]. Polystyrene (PS), a widely used thermoplastic in packaging, construction, and other civilian sectors, presents a significant fire hazard due to its inherent flammability. Enhancing its flame retardancy has thus become an urgent priority [3]. However, traditional halogenated flame retardants often release toxic and corrosive gases during combustion, raising environmental and health concerns. This has driven a global shift toward developing halogen-free, environmentally benign alternatives [4]. In this context, bio-based phytic acid (PA), a natural compound extracted from plant seeds, has emerged as a promising green flame retardant. Its molecular structure contains six phosphate groups, endowing it with a high phosphorus content (~28 wt%) [5]. During thermal decomposition, PA releases phosphate derivatives that can catalyze polymer dehydration and char formation in the condensed phase while simultaneously scavenging radicals in the gas phase, thereby exerting efficient dual-phase flame-retardant action [6,7,8,9]. As a renewable phosphorus source, PA holds considerable promise for the development of environmentally friendly flame retardants [10,11,12,13].

Despite these advantages, the direct application of PA in polymers—especially in hydrophobic PS—faces two core challenges: its strong hydrophilicity leads to poor compatibility with the matrix and phase separation, while its acidic nature often causes significant deterioration of mechanical properties at high loadings. The long-standing trade-off between “flame retardancy” and “mechanical robustness” remains a critical bottleneck in this field [14,15,16]. To address these issues, researchers have primarily pursued two modification strategies.

The first strategy involves metal-ion complexation, which aims to construct a “synergistic char layer.” By combining PA with multivalent metal ions such as Zn²⁺ or Al³⁺, the acidity of PA can be neutralized, and metal phytates can be formed. For example, Zhang et al. [17] prepared flame-retardant wood by reacting ammonium phytate with Cu²⁺ and Zn²⁺ ions, achieving significant reductions in total heat release and smoke production. The key advantage of this approach lies in the ability of the metal components to efficiently catalyze the formation of a dense char layer during combustion, while the metal-phosphate network interpenetrates the char structure, substantially improving its thermal stability, compactness, and mechanical strength. Tian et al. [18] synthesized a novel flame retardant (MAZn) by tuning the ratio of zinc phytate (ZnPA) to aminotrimethylene phosphonic acid (ATMP) and evaluated its performance in polylactic acid (PLA). The sample, with an ATMP:ZnPA ratio of 10:5, exhibited the best flame retardancy: cone calorimetry showed reductions of 59.87% in peak heat release rate and 89.74% in total heat release compared to neat PLA, while UL-94 testing confirmed a V-0 rating with excellent self-extinguishing behavior. These findings provide important insights for constructing “acid-source/gas-source” synergistic systems in PS.

Similarly, aluminum phosphates are regarded as environmentally friendly flame retardants, capable of suppressing combustion by radical scavenging (e.g., PO·) in the gas phase and promoting stable phosphorus–aluminum char formation in the condensed phase [19]. Compatibility engineering between inorganic phosphates and polymers has thus become a key research direction for balancing flame retardancy and mechanical performance [20]. Wan et al. [21] used aluminum hypophosphite (AHP) to flame-retard wood–plastic composites based on high-density polyethylene (HDPE). Through cone calorimetry, vertical burning, limiting oxygen index (LOI), and mechanical tests, they demonstrated that AHP, wood flour (WF), and bound water in WF together constitute an intumescent system. At 30 wt% AHP, the composite achieved a UL-94 V-0 rating and an LOI of 25.5%, indicating significantly enhanced flame retardancy. Aluminum dihydrogen phosphate (Al(H₂PO₄)₃) also functions via radical trapping in the gas phase and promotes stable phosphorus–aluminum char formation in the condensed phase.

The second strategy leverages phosphorus–nitrogen synergy to build intumescent flame-retardant systems. By combining PA as an acid source with nitrogen-containing compounds (e.g., piperazine, melamine) as gas sources [7], a dense and porous intumescent char can be generated during combustion. For instance, piperazine phytate, when incorporated into polypropylene at 18 wt%, significantly increases the LOI and enables the material to achieve a UL-94 V-0 rating [22].

Nevertheless, most current studies on PS/PA composites remain limited to physical blending or pre-modification of PA (e.g., esterification, salt formation) before compounding [23]. While these methods can partially improve flame retardancy, they fail to fundamentally address the interfacial compatibility between PA and the PS matrix, often resulting in limited synergy between flame retardancy and mechanical performance. The underlying reason is the lack of a strategy that, based on chemical structure design, can in situ construct a stable, integrated network within the PS matrix that simultaneously provides both flame-retardant and reinforcing functions. Compatibility optimization is therefore a critical pathway toward achieving a balance between flame retardancy and mechanical properties. The combination of PA with inorganic phosphates, along with compatibility engineering, has become an effective approach to overcoming the traditional “loading-performance dilemma” of flame retardants.

To this end, the present study employs bio-based PA (acid source), triethylenetetramine (TETA, cross-linker/gas source), and aluminum diethylphosphinate (ADP, ceramifying agent) to construct a flame-retardant PS composite. Through solution-grinding and hot-pressing, in situ acid-base neutralization, amidation, and metal coordination yield a three-dimensional P-N-Al hybrid network. This network delivers dual performance: mechanical reinforcement under ambient conditions via stress transfer, and intrinsic intumescent flame retardancy upon fire exposure, where PA catalyzes charring, TETA supplies expanding gases, and Al³⁺ yields a reinforcing ceramic phase. This work thus presents a structure–function integrated in situ strategy to synergistically optimize both flame retardancy and mechanical retention in PS.

Building on this understanding, the present study employs bio-based PA as an efficient acid source, triethylenetetramine (TETA) as a multifunctional cross-linker and gas source, and introduces aluminum dihydrogen phosphate (ADP) as a ceramifying component. Through a solution-grinding/hot-pressing process, a series of in situ reactions among PA, TETA, and ADP, constructing a three-dimensional P-N-Al hybrid network within the PS matrix. Under ambient conditions, it reinforces the matrix via stress transfer; upon fire exposure, it acts as an intrinsic intumescent system: PA catalyzes charring, TETA releases expanding gases, and aluminum ions form a high-temperature ceramic phase that strengthens the char. This work offers a structure–function integrated in situ strategy to simultaneously enhance both flame retardancy and mechanical performance in PS materials.

2. Materials and Methods

2.1. Materials

PS powder (GPPS-80, 300 mesh) was supplied by Dongguan Hongcheng Plastic Material Co., Ltd. (Dongguan, China). PA was obtained as a 70 wt% aqueous solution from Laiyang Wanjiwei Bioengineering Co., Ltd. (Laiyang, China). Triethylenetetramine (TETA, 65% aqueous solution) and aluminum dihydrogen phosphate (ADP, 95% purity) were acquired from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification.

2.2. Synthesis of PA-TETA Flame Retardant

The PA-TETA flame retardant was synthesized as follows: PA (10 g) was first dissolved in deionized water (10 g) under magnetic stirring at room temperature. Then, TETA (5.4 g) was added dropwise to the solution. After complete addition, the mixture was stirred at room temperature for 1 h. The obtained product was diluted with deionized water to form a 40 wt% aqueous solution.

2.3. Preparation of ADP Solution

An appropriate amount of ADP was dissolved in deionized water under magnetic stirring at room temperature to obtain a 40 wt% aqueous solution.

2.4. Preparation of PS/ADP/PA-TETA Powder

A typical procedure for preparing PS/10ADP/15PA-TETA powder is: 50 g of PS was placed in a mortar. Subsequently, 12.5 g of the 40 wt% ADP solution and 18.75 g of the 40 wt% PA-TETA solution were sequentially added and thoroughly ground for homogenization. The mixture was then dried at 100 °C for 4 h to remove residual water, yielding the composite powder. Other samples with different compositions (

Table 1. Formulations of PS/ADP/PA-TETA composites (phr: parts per hundred parts of PS by weight).

Samples	ADP (phr)	PA-TETA (phr) *	PS (phr)
PS/10ADP	10	0	100
PS/20ADP	20	0	100
PS/30ADP	30	0	100
PS/5PA-TETA	0	5	100
PS/10PA-TETA	0	10	100
PS/15PA-TETA	0	15	100
PS/20PA-TETA	0	20	100
PS/10ADP/10PA-TETA	10	10	100
PS/10ADP/15PA-TETA	10	15	100
PS/10ADP/20PA-TETA	10	20	100
PS/10ADP/25PA-TETA	10	25	100

* PA:TETA = 2:1 (wt%).

) were prepared analogously.

2.5. Preparation of Flame-Retardant PS/ADP/PA-TETA Composites

The composites were fabricated via hot-pressing. The composite powder was placed into a mold and preheated at 200 °C for 10 min in a vacuum hot press (Model Y-002, Zhengzhou Craftsman Machinery Equipment Co., Ltd., Zhengzhou, China). A pressure of 4000 kg (corresponding to approximately 6 MPa based on the mold area) was then applied and maintained for 8 min. Finally, the mold was cooled under pressure to 80 °C before demolding, resulting in composite sheets. The sheets were cut into specimens for testing. The formulations are summarized in Table 1. A schematic of the preparation process is presented in Scheme 1.

2.6. Characterization

Fourier-transform infrared spectroscopy (FTIR): Chemical structures were analyzed on a Nicolet iS10 spectrometer (Thermo Scientific, Waltham, MA, USA) using KBr pellets over 400–4500 cm⁻¹.

Scanning electron microscopy (SEM): Morphology of composites and char residues was examined using a Phenom scanning electron microscope (Phenom Scientific, Eindhoven, The Netherlands).

Thermogravimetric analysis (TGA): Thermal stability was evaluated on an HTG-4 thermal analyzer (Beijing Henven, Beijing, China) under air atmosphere. Samples (~8 mg) were heated from 30 to 800 °C at 10 °C/min.

Limiting oxygen index (LOI): Tests were conducted according to ASTM D2863 on a TTech-GBT2406-1 instrument (TESTech Instrument (Suzhou) Technologies Co., Ltd., Suzhou, China) using specimens with dimensions of 100 × 10 × 4 mm³.

Vertical burning (UL-94): Tests were performed following the UL-94 standard on a TTech-GBT2408 instrument (TESTech Instrument (Suzhou) Technologies Co., Ltd., Suzhou, China) using specimens with dimensions of 125 × 13 × 4 mm³.

Cone calorimetry test (CCT): Fire performance was evaluated according to ISO 5660-1 [24] on a PX-07007 cone calorimeter (Suzhou Phoenix, Suzhou, China) at a heat flux of 35 kW/m². Specimens (100 × 100 × 3 mm³) were wrapped with aluminum foil on the sides and back.

X-ray diffraction (XRD): Crystalline structures were analyzed on a DX-2700 diffractometer (Dandong Haoyuan, Dandong, China) with Cu Kα radiation (λ = 0.154 nm), scanning from 5° to 90° at 2° min⁻¹.

X-ray photoelectron spectroscopy (XPS): Chemical compositions and elemental states were analyzed on a Thermo K-Alpha multifunctional imaging electron spectrometer (Thermo Scientific, Waltham, MA, USA) with monochromatic Al K_α radiation (hν = 1486.68 eV). The pass energy was 50 eV (step size 0.1 eV), and binding energies were referenced to C 1s at 284.80 eV.

Tensile testing: Tensile properties were measured on a 20 kN electronic universal testing machine (Jinan PuYe Mechanic & Electronics Technology Co., Ltd., Jinan, China) using specimens with dimensions of 120 × 10 × 4 mm³.

3. Results

3.1. Chemical Structure and Microstructure Characterization

3.1.1. FTIR Analysis of Flame-Retardant PS Composites

FTIR spectra confirmed the successful synthesis of the flame retardants (Figure 1a). The characteristic bands of the raw materials were identified: PA showed a broad hydrogen-bonded O–H stretch at 3200 cm⁻¹, as highlighted by the gray box on the right in Figure 1a. The bands at 1120 cm⁻¹ and 1049–982 cm⁻¹ are assigned to the P=O and P–O groups [25,26]; TETA exhibited a primary amine N–H stretch at 1663 cm⁻¹; ADP displayed Al-coordinated H₂PO₄⁻ vibrations at 938 and 893 cm⁻¹ [27], along with a P-O symmetric stretch at 982 cm⁻¹.

For PA-TETA, the –NH₂ band of TETA disappeared and the O–H band of PA became sharply narrowed. The characteristic phosphate peaks of both components merged into a broad band (1098–1044 cm⁻¹), verifying the formation of phosphate–ammonium ionic bonds. In the spectrum of the ADP/PA-TETA mixture, the Al-coordinated H₂PO₄⁻ peak of ADP at 893 cm⁻¹ disappeared, and the phosphate bands from PA−TETA exhibited significant shifts and broadening (gray box on the right in Figure 1a). These spectral features suggest a structural reorganization between the two components. New peaks at 1133, 1062, and 991 cm⁻¹ appeared, which can be attributed to the formation of Al–O–P coordination bonds.

The chemical structure of the flame-retardant PS composites was further investigated by FTIR (Figure 1b). All composites exhibited characteristic PS absorptions at 3016, 1615, 1400 and 746 cm⁻¹, confirming the PS matrix [28]. Characteristic peaks of ADP and PA–TETA were observed in their respective composites, demonstrating successful incorporation into the matrix. Upon the introduction of PA–TETA (PS/10ADP/15PA–TETA), the characteristic peak of ADP at 910 cm⁻¹ disappeared. Concurrently, the absorption bands at 1133, 1062, and 991 cm⁻¹, corresponding to the Al–O–P hybrid network, are markedly enhanced. These spectral changes demonstrate that during melt processing, Al³⁺ from ADP further reacts with the phosphate and ammonium groups of PA-TETA, leading to the formation of a more homogeneous P−N–Al synergistic phase. Collectively, these results confirm the successful integration of both ADP and PA–TETA into the PS matrix.

3.1.2. Surface Micromorphology of Flame-Retardant PS Composites

The morphological evolution of the composites with different flame retardants was investigated by SEM (Figure 2). Neat PS (Figure 2a) exhibits a smooth surface, which is characteristic of brittle fracture. The incorporation of 10 phr ADP (Figure 2b) introduces pores and a coarse morphology. This suggests poor interfacial adhesion between ADP and the PS matrix. In contrast, PA–TETA showed a distinctly different dispersion state. At 15 phr (Figure 2c), it formed well-integrated flakes within the PS matrix, indicating better compatibility with PS. The combination of ADP and PA–TETA (Figure 2e) resulted in a more homogeneous dispersion of rigid, plate-like structures, which implies improved interfacial interaction through complexation.

However, when the PA–TETA loading exceeded 25 phr, the morphology became coarser. As shown in Figure 2e,f, excessive PA-TETA forms large, continuous domains and agglomerates, creating a heterogeneous structure. This morphological deterioration shifts the fracture mechanism from matrix-dominated brittle failure to large-area interfacial debonding or brittle fracture within the agglomerated PA-TETA phase itself.

3.2. Thermal Stability of Flame-Retardant PS Composites

The thermal degradation behavior of the composites was investigated by TGA under air (Figure 3,

Table 2. Parameters extracted from the thermograms of PS and its composites.

Samples	T5% (°C)	Tmax (°C)	Char Residue (%)
PS	281.1	318.8	0
PS/10ADP	288.1	302.0	2.73
PS/5PA-TETA	324.8	349.5	1.33
PS/10PA-TETA	293.2	414.3	1.59
PS/15PA-TETA	296.1	412.0	2.33
PS/20PA-TETA	281.1	408.4	2.67
PS/10ADP/10PA-TETA	287.5	350.5	4.11
PS/10ADP/15PA-TETA	324.0	404.2	4.72
PS/10ADP/20PA-TETA	323.5	378.1	7.22
PS/10ADP/25PA-TETA	323.1	395.0	9.80

). In Figure 3a,b, Neat PS decomposed in a single sharp step with an initial decomposition temperature (T_5%) of 281.1 °C and a maximum mass-loss rate temperature (T_max) of 318.8 °C, leaving negligible residue.

The individual components exerted distinct influences. Adding 10 phr ADP slightly increased the T_5% to 288.1 °C but significantly reduced the T_max to 302.0 °C, yielding only 2.73% residue (Figure 3c,d). This indicates that ADP alone accelerates the main decomposition step while providing limited thermal shielding. In contrast, PA-TETA markedly enhanced the onset stability (T_5% = 324.8 °C for PS/5PA-TETA). This enhancement is attributed to its endothermic decomposition. However, PA–TETA alone did not promote char formation, as evidenced by the low residue yields.

The char-forming synergy between ADP and PA-TETA is threshold-dependent. At 10–15 phr PA-TETA, char yields (4.11–4.72%) remained additive, approximating the sum of individual contributions. A super-additive effect emerged at ≥20 phr, with char yield surging to 7.22% and 9.80%. Notably, even at 15 phr, the system already exhibited enhanced thermal stability, as evidenced by a T_5% of 324.0 °C and a T_max shift to 404.2 °C.

The derivative (DTG) curves (Figure 3d) provided further insight into the degradation mechanism. The synergistic formulations exhibited broader, flatter peaks. This peak shape suggests a transition from rapid volatilization to a slower, char-forming degradation pathway, which is likely driven by pre-reactions that catalyze the formation of a protective char layer.

3.3. Flame-Retardant Performance of Flame-Retardant PS Composites

3.3.1. Limiting Oxygen Index and Vertical Burning Tests

Representative snapshots of the UL-94 tested specimens are shown in Figure 4. The complete UL-94 test results and LOI for all 12 formulations are summarized in

Table 3. LOI values and UL-94 ratings of flame-retardant PS composites.

Samples	LOI (%)	UL94
PS	17.4	No rating
PS/10ADP	19.3	No rating
PS/20ADP	20.5	No rating
PS/30ADP	21.2	No rating
PS/5PA-TETA	20.4	No rating
PS/10PA-TETA	21.6	No rating
PS/15PA-TETA	23.9	No rating
PS/20PA-TETA	24.8	No rating
PS/10ADP/10PA-TETA	27.1	No rating
PS/10ADP/15PA-TETA	28.5	V0
PS/10ADP/20PA-TETA	30.0	V0
PS/10ADP/25PA-TETA	42.0	V0

. Neat PS exhibited an LOI of 17.4% and failed the UL-94 test due to rapid burning and severe dripping. The incorporation of ADP or PA-TETA alone provided only marginal improvement. With ADP loadings of 10–30 phr, the LOI reached only 19.3–21.2%, and none of the formulations achieved a UL-94 rating. With PA-TETA loadings of 5–20 phr, the LOI increased to 20.4–24.8%, but all samples still failed the UL-94 test.

When ADP and PA-TETA were combined at a fixed ADP loading of 10 phr, the LOI increased progressively with PA-TETA content. A distinct performance transition was observed at approximately 15 phr PA-TETA. Below this threshold (e.g., PS/10ADP/10PA-TETA, LOI = 27.1%), the composite still burned completely with dripping. At the threshold composition (PS/10ADP/15PA-TETA), the LOI rose sharply to 28.5%, and the material achieved self-extinguishing behavior with a UL-94 V-0 rating.

Further increasing the PA-TETA content to 20 and 25 phr elevates the LOI to 30.0% and 42.0%, respectively, while all formulations maintain the V-0 rating. The non-additive nature of this enhancement—far exceeding the performance of either component alone—confirms a robust synergistic mechanism [29].

3.3.2. Analysis of Char Residue

FTIR analysis of the char residues (Figure 5) elucidates the formation of a stable P−C−Al hybrid barrier. Characteristic peaks at 1618 cm⁻¹ (aromatic C=C/C=O) [25,30], 1276 cm⁻¹ (P=O/P−O−C), and 973/804 cm⁻¹ (P−O−P/Al−O−P) [31] confirm the coexistence of graphitized carbon and a cross-linked inorganic network. With increasing PA-TETA content, the progressive enhancement of these peaks indicates a simultaneous increase in inorganic phase content, cross-linking density, and carbon graphitization.

Figure 6 depicts the influence of PA-TETA content on the char-layer morphology of PS/ADP/PA-TETA composites after combustion. The char residue of PS/10ADP exhibits a porous and loosely structured morphology, with pore sizes ranging from approximately 1 to 10 μm (Figure 6a). Upon the incorporation of PA-TETA, the char structure undergoes a systematic evolution: transitioning from this porous and fragile form to a continuous honeycomb-like structure (Figure 6b), accompanied by the formation of numerous closed pores smaller than 1 μm. With further development, the structure transforms into a uniform and dense cellular foam (Figure 6c), and ultimately approaches a smooth and compact morphology (Figure 6d). This morphological evolution suggests that the synergistic effect of PA-TETA and ADP regulates the structure and integrity of the char layer.

3.4. X-Ray Diffraction Analysis of Char Structure Evolution

Figure 7 presents the XRD patterns of the combustion residues for PS/10ADP/PA-TETA composites with different PA-TETA loadings. The residue of PS/10ADP exhibits distinct diffraction peaks at 2θ = 16.2°, 20.1°, 25.4°, 30.9°, and 34.4°, which are indexed to cubic Al(PO₃)₃ [32,33,34]. With the addition of PA-TETA (10−25 phr), all crystalline diffraction peaks disappear, and only a broad diffuse hump appears in the 20−30°. The intensity of the Al(PO₃)₃ crystalline peaks diminishes with increasing PA-TETA content, while the diffuse carbonaceous signal strengthens. The amorphous structure originates from the rapid decomposition and cross-linking of the phytic acid–triethylenetetramine–aluminum system at high temperatures, resulting in a highly cross-linked but disordered network comprising P−O−Al and P−C−O bonds [19,35].

3.5. Burning Behavior of Flame-Retardant PS Composites

CCT was conducted to evaluate the combustion behavior of the composites (Figure 8,

Table 4. Cone calorimetry data of PS/ADP/PA-TETA composites.

Parameters	PS	PS/10ADP	PS/15PA-TETA	PS/10ADP/15PA-TETA
TTI (s)	36	33	53	54
PHRR (kW/m2)	716.4	391.7	481.9	187.4
THR (MJ/m2)	138.2	94.0	78.8	74.4
TSP (m2)	4.62	3.34	10.0	8.24
Av-EHC (MJ/kg)	28.6	28.5	23.9	20.8
Time of flameout (s)	440	414	459	753
Char residue rate (%)	0	10.0	5.3	19.9

). Neat PS exhibited high fire risk, with a peak heat release rate (PHRR) of 716.4 kW·m⁻². The incorporation of 10 phr ADP alone reduced the PHRR to 391.7 kW·m⁻² (a 45.3% decrease) and the total heat release (THR) to 94 MJ·m⁻², while increasing the char residue to 10%. The sample containing 15 phr PA-TETA alone decreased the PHRR to 481.9 kW·m⁻², but the char residue remained low (5.3%), and the total smoke production (TSP) increased markedly to 10 m². When ADP and PA-TETA were combined (PS/10ADP/15PA-TETA), the PHRR dropped sharply to 187.4 kW·m⁻² (a 73.8% reduction), the THR decreased to 74.4 MJ·m⁻², and the char residue jumped to 19.9%. The time to ignition (TTI) was extended to 54 s, and the flameout time to 753 s. The average effective heat of combustion (Av-EHC) decreased from 28.6 MJ·kg⁻¹ (neat PS) to 20.8 MJ·kg⁻¹ for the synergistic system.

Although the TSP of the synergistic system (8.24 m²) was higher than that of neat PS, it was considerably lower than that of the PA-TETA-only sample (10 m²), indicating that the introduction of ADP partially mitigates the smoke issue associated with PA-TETA, underscoring the role of condensed-phase char formation in restraining incomplete combustion.

The macroscopic morphology of the char residues after CCT provides direct visual evidence of the distinct flame-retardant mechanisms (Figure 9). Neat PS burned almost completely, leaving no coherent char layer (Figure 9(a1,a2)). PS/10ADP formed a small amount of dense but compact char (Figure 9(b1,b2)), while PS/15PA-TETA left a loose and porous char residue (Figure 9(c1,c2)). In striking contrast, PS/10ADP/15PA-TETA developed a well-expanded, cohesive char layer with a height of approximately 1.5 cm (Figure 9(d1,d2)), visually demonstrating the strong synergy between ADP and PA-TETA. In this system, PA-TETA promotes char expansion, whereas ADP reinforces its structural stability. Together, they construct an efficient thermal-insulating barrier. This macroscopic observation directly confirms that the synergistic system operates through a “catalytic charring–foam reinforcing” mechanism, which underlies its high flame-retardant efficiency.

Figure 10 presents the XPS spectra of the residual char from the PS/10ADP/15PA-TETA composite after CCT. The survey spectrum in Figure 10a reveals the presence of C, O, P, N, and Al elements. The C 1s spectrum (Figure 10b) shows C−C (284.8 eV), C−O (286.5 eV) and C=O (289.0 eV) [35], indicating a graphitized carbon matrix with phosphate-ester functionality. The O 1s peaks (Figure 10c) at 531.6 eV (P−O−Al) and 533.1 eV (P−O−P), together with P 2p at 134.6 eV (Figure 10d) and Al 2p at 75.3 eV (Figure 10e), along with an Al/P atomic ratio of approximately 0.20—notably lower than the 1:3 stoichiometry of crystalline Al(PO₃)₃—confirm aluminum-crosslinked polyphosphate structures [34]. N 1s peaks (Figure 10f) at 400.1 eV (N−C) and 402 eV (N−O) evidence TETA-derived pyrrolic nitrogen stabilized by polyphosphate interaction [36]. This Al-crosslinked polyphosphate–carbon hybrid barrier accounts for the synergistic flame retardancy.

3.6. Flame-Retardant Mechanism

The flame-retardant mechanism of the PS/ADP/PA-TETA composites is illustrated in Figure 11, integrating evidence from FTIR, XRD, SEM, TGA, and CCT (Section 3.1, Section 3.2, Section 3.3, Section 3.4 and Section 3.5).

Upon heating, ADP releases Al³⁺/H⁺ ions, which react with phosphate groups of PA-TETA to form an Al−O−P hybrid network (Section 3.1.1), catalyzing PS dehydration and aromatization into an aromatic carbon skeleton. Concurrently, PA-TETA decomposes to release NH₃, H₂O, and CO₂, providing gas-phase dilution [37,38] (Section 3.5).

At higher temperatures, the generated Al(PO₃)₃ melts to form an inorganic glassy film on the char surface (Section 3.4), while nitrogen-containing fragments cross-link and foam beneath, producing an expanded char (Section 3.3.2). This dual-layer architecture effectively insulates the underlying polymer. This dual-layer architecture effectively insulates the underlying polymer by combining condensed-phase charring with gas-phase radical quenching. The barrier efficiency is evidenced by markedly suppressed heat release, prolonged combustion duration, and enhanced char formation. The gas-phase activity reduces combustion efficiency, while ADP mitigates smoke generation associated with PA-TETA alone. This synergistic interplay achieves comprehensive flame retardancy through complementary physical insulation and chemical inhibition mechanisms (Section 3.5).

In summary, the ADP/PA-TETA system operates via a cooperative condensed-phase charring and gas-phase dilution mechanism, achieving superior flame retardancy in PS composites.

3.7. Mechanical Properties of Flame-Retardant PS/ADP/PA-TETA Composites

Figure 12 presents the mechanical properties of the PS composites. While enhancing flame retardancy, the incorporation of flame-retardant additives inevitably influences the mechanical behavior. With increasing ADP content from 10 to 30 phr, the tensile strength decreased from 39.9 MPa to 28.2 MPa and the elongation at break dropped from 3.5% to 3.0%, indicating that excessive rigid filler disrupts matrix continuity and introduces stress concentration. When PA-TETA was added alone, the tensile strength remained relatively high (ca. 41.4−39.0 MPa) at low loadings (5−10 phr) but declined to about 31.1 MPa at 20 phr.

The combination of ADP and PA-TETA exhibited a synergistic mechanical effect. At a fixed ADP content of 10 phr, the tensile strength peaked at 41.2 MPa with 5 phr PA-TETA. At 15 phr PA-TETA, the composite showed an optimal balance: a tensile strength of 39.7 MPa, an elongation at break of 3.80%, and a strength retention of 78.4% relative to neat PS. Thus, this optimal composite not only attained high flame retardancy (LOI = 28.5%, UL-94 V-0) but also maintained satisfactory mechanical integrity. In contrast, further increasing PA-TETA to 20 and 25 phr caused a sharp drop in both strength (to 32.5 and 29.8 MPa, respectively) and elongation (to 3.2% and 3.0%), indicating severe embrittlement.

This non-monotonic mechanical behavior is closely related to the formation of a “P-N-Al” hybrid network. An appropriate amount of this network can enhance stress transfer and inhibit crack propagation, whereas an excessive network leads to modulus mismatch and interfacial defects, resulting in embrittlement.

3.8. Comparison of Flame-Retardant Properties with Literature Reports

Table 5. Comparison of flame-retardant properties between reported polystyrene systems and PS/10ADP/15PA-TETA composite.

Sample	Loading	LOI (%)	Increase in LOI (%)	UL-94	Tensile Strength Retention (%)	References
PS/10ADP/15PA-TETA	25 phr	28.5	63.8	V-0	78.4	This work
PS/PAEI/MMT	26 phr	32	77.8	V-0	61.9	[16]
PS/PAE/EG	10 phr	27.7	54	V-0	91.8	[15]
PS/hBN/SBC	12 wt%	24	33.3	NR *	NR *	[39]
PS/10%PON/10%EG	20 wt%	25.8	42.5	NR *	NR *	[40]
PS/25%MAHPi	25 wt%	24	33.3	V-0	NR *	[41]
PS/MP/EG	20 wt%	28	55.6	V-0	83	[42]
PS/25%AP	25 wt%	25.6	42.5	V-0	NR *	[43]
PS/25%HEDPA/PER/MEL	25 wt%	28.3	57.5	V-0	NR *	[44]
HIPS/MH/MRP	48.7 wt%	26.6	48	V-0	NR *	[45]

* NR = Not reported.

compares the flame-retardant performance and mechanical properties of PS/10ADP/15PA-TETA with previously reported PS systems. The optimized composite achieves a well-balanced combination of high flame retardancy (LOI 28.5%, V-0 rating, 73.8% PHRR reduction) and good mechanical property retention (78.4% tensile strength) at a moderate loading of 25 phr. This balance, along with the bio-based nature of the PA component, distinguishes the ADP/PA-TETA system as an effective and eco-friendly strategy for high-performance flame-retardant PS composites.

4. Conclusions

This study successfully constructed a phosphorus–nitrogen–aluminum synergistic flame-retardant system for PS based on PA-TETA and ADP, which significantly enhanced the fire safety and overall performance of the composites. The main conclusions are summarized as follows:

Synergistic Flame Retardancy: A pronounced condensed-phase synergy between ADP and PA-TETA was demonstrated, operating through a catalytic charring–foam reinforcing–glassy barrier mechanism. This synergy enabled the optimized PS/10ADP/15PA-TETA composite to achieve an LOI value of 28.5%, a UL-94 V-0 rating, and significant reductions in PHRR (−73.8%) and THR (−46.2%).

Char Structure and Mechanism: The in situ formed P–N–Al coordination network during processing promoted the development of a dense, expanded, and ceramic-like amorphous char layer during combustion. This char acted as an effective thermal-insulating barrier, suppressing heat and mass transfer.

Balanced Mechanical Performance: Favored by the in situ constructed hybrid network, favorable interfacial compatibility effectively mitigated stress concentration. As a result, the PS/10ADP/15PA-TETA composite retained 78.4% of the tensile strength of neat PS, achieving a successful balance between high flame retardancy and mechanical integrity.

In summary, this work not only provides an efficient and environmentally benign flame-retardant strategy for PS, but also offers a valuable design principle—through the elucidated coordination ceramification mechanism—for developing high-performance, multifunctional polymer composites.