The Effects of Sodium Antimonate on the Flame Retardancy and Mechanical Properties of Thermoplastic Polyurethane Composites

1. Introduction

Thermoplastic polyurethane elastomer (TPU) combines the high elasticity of rubber with the processability of plastics, making it irreplaceable in various fields [1,2]. However, TPU is a flammable material, which severely limits its application in fields with strict fire safety requirements [3,4,5,6]. The hydrogen halide gases generated by traditional halogenated flame retardants during combustion are highly corrosive and toxic [7,8]; consequently, their use is gradually being restricted. SA (NaSbO₃), as a typical inorganic flame retardant synergist, can promote char layer formation and inhibit combustion reactions [9,10,11]. SA is characterized by low addition levels and high flame-retardant efficiency, demonstrating promising application potential in the field of flame retardation for polymeric materials. In recent years, TPU flame retardant systems have primarily focused on phosphorus-based, nitrogen-based, inorganic, and composite systems [12,13,14]. When used alone, phosphorus-based flame retardants require high addition levels, which tends to result in a decline in the mechanical properties of TPU [15]; Synergistic Use of Nitrogen-Based Flame Retardants with Other Flame Retardants [16,17]. When inorganic flame retardants act synergistically with other flame retardants, they can significantly reduce smoke density [18].

When used synergistically with halogenated flame retardants, SA can significantly enhance the flame retardancy efficiency of materials [19,20]. Existing studies indicate that Sb₂O₃—generated from the high-temperature decomposition of antimony-based compounds—catalyzes the carbonization of polymers, forming a dense carbonaceous barrier that terminates chain reactions [7,21,22,23]. Inorganic acid salts can achieve enhanced flame retardancy by promoting char formation and reducing the heat release rate and smoke emission [7,18,21,22,24].

2. Results and Discussion

2.1. Effect of SA on the Thermal Stability of TPU

Figure 1 presents the DSC curves for pure TPU (TPU-SA0) and composite systems containing varying amounts of SA (TPU-SA1, TPU-SA3, and TPU-SA5). As shown in the figure, pure TPU exhibits a glass transition step associated with the soft segments in the range of 60–70°C, followed by an endothermic melting peak corresponding to the hard segments between 150°C and 170°C; this behavior is consistent with the typical thermal characteristics of TPU. Upon the addition of SA, the glass transition temperature of the composite systems shifts gradually toward higher temperatures as the additive content increases. This indicates that the SA establishes interfacial interactions with the TPU molecular chains, thereby restricting the molecular motion of the soft segments and enhancing the rigidity of the matrix. Simultaneously, the melting peaks of the hard segments progressively diminish in intensity and broaden in shape, suggesting that the SA inhibits the ordered crystallization of the TPU hard segments and effectively modulates the aggregated structure of the matrix [25]. All samples exhibit a single glass transition event devoid of any additional phase transition peaks, thereby confirming that the SA is uniformly dispersed within the TPU matrix and demonstrates excellent compatibility. This structural foundation serves as the basis for the synergistic enhancement of both the mechanical properties and flame retardancy of the composite systems [26].

Figure 2 presents the TGA thermogravimetric curves and characteristic thermal decomposition parameters for the SA/TPU composite system. As shown in the figure, the pure TPU-SA0 exhibits an initial thermal decomposition temperature of 301.84°C, a maximum thermal decomposition rate temperature of 419.5°C, and a residual carbon yield of only 9.84% at 600°C, indicating relatively poor thermal stability [27,28]. Upon the addition of SA, the thermal decomposition behavior of the composite system undergoes significant changes: as the loading of SA increases, the initial thermal decomposition temperature decreases slightly—a phenomenon attributed to the premature decomposition of SA triggering the catalytic degradation of the TPU. Concurrently, the temperature corresponding to the maximum thermal decomposition rate decreases significantly, indicating that SA effectively retards the thermal decomposition rate of the TPU, thereby preventing concentrated heat release [29,30,31]. Furthermore, the residual carbon yield increases substantially; for sample TPU-SA7, the residual carbon yield reaches 13.89%, representing a significant improvement over that of pure TPU [32,33]. This enhancement is attributed to the antimony oxides generated from the decomposition of SA, which catalyze the cross-linking of TPU molecular chains into char. This process reduces the generation of volatile products, thereby effectively enhancing the material's thermal stability and laying a solid foundation for improved flame retardancy.

2.2. Analysis of Infrared Spectroscopy Characterization Results

Figure 3 presents the 3D temperature-dependent infrared spectra of pure TPU and composite systems containing varying amounts of SA composites. As illustrated in the figure, TPU-SA0 exhibits distinct characteristic urethane peaks (e.g., N-H at 3326 cm⁻¹ and C=O at 1703 cm⁻¹) in the low-temperature range; however, upon heating, these characteristic peaks decay rapidly, leaving virtually no residual signals in the high-temperature range, indicating complete thermal decomposition [34,35,36]. With the addition of SA, the infrared spectra of the composite systems undergo significant changes: in the low-temperature range, the intensities of the N-H and C=O characteristic peaks decrease and shift toward lower wavenumbers, indicating the formation of hydrogen-bonding interactions between the SA and the TPU molecular chains, thereby enhancing interfacial compatibility[35,37]. In the medium-temperature range, the decay rate of the characteristic peaks is markedly retarded, demonstrating that SA effectively inhibits the thermal decomposition of the TPU molecular chains [38]. Furthermore, in the high-temperature range, the intensity of the aromatic C=C peak—located near 1600 cm⁻¹—increases significantly as the SA content rises; this confirms that the antimony oxides generated from the decomposition of SA catalyze the aromatization of the TPU molecular chains, thereby promoting the formation of a stable aromatic char layer. This finding aligns with the increased residual carbon yields observed in the TGA results, thereby elucidating—at the molecular level—the condensed-phase char-forming flame-retardant mechanism of SA.

2.3. Effect of SA on the Mechanical Properties of TPU

The mechanical properties of TPU materials serve as a crucial guarantee for their practical applications; as an inorganic filler, the loading level and dispersion of SA directly influence the mechanical properties of the composite system [39,40]. Figure 4 presents the stress-strain curves, maximum stress variation curves, and a bar chart of the maximum tensile force for composite systems containing varying amounts of SA.

As evident from the figure, TPU-SA0 exhibits a maximum tensile stress of approximately 5.6 MPa, a maximum tensile force of 39 N, and a strain at break approaching 2.8, demonstrating excellent mechanical properties and high toughness. Upon the addition of SA, the mechanical properties of the composite system display a continuous downward trend:

Low-addition group (TPU-SA0, TPU-SA1): The maximum stress decreases to approximately 3.6 MPa, the maximum tensile force to 24 N, and the strain at break shortens to around 0.6; while these values represent a decline compared to the TPU-SA0 group, the material still retains a certain degree of toughness.

Medium-addition group (TPU-SA03, TPU-SA05): The maximum stress drops to 2.1 MPa and 1.7 MPa, respectively, and the maximum tensile force falls to 14 N and 12 N; the strain at break shortens further, and toughness declines significantly.

High-addition group (TPU-SA7, TPU-SA10): The maximum stress stands at merely 1.4 MPa and 1.3 MPa, and the maximum tensile force at 9.5 N and 9 N; the strain at break is extremely low, and the material undergoes distinct brittle fracture.

The fundamental reason for this change is that SA, acting as a rigid inorganic filler, disrupts the continuous-phase structure of the TPU matrix. When added in appropriate amounts, the filler disperses relatively uniformly, and its impact on mechanical properties remains relatively controllable. However, when the addition level exceeds 5%, the SA particles tend to agglomerate, creating defects and stress concentration points within the matrix; this renders the material prone to fracture during tensile testing, resulting in a significant decline in both load-bearing capacity and toughness.

Overall, the incorporation of SA leads to a significant reduction in the mechanical properties of TPU; furthermore, the lower the additive content, the less pronounced the impact on these mechanical properties. If it is necessary to balance both flame retardancy and mechanical performance, it is recommended to limit the addition of SA to within 5%. At this level, the composite system retains a certain degree of mechanical integrity, thereby achieving a relative equilibrium between flame retardancy and mechanical properties that satisfies basic application requirements.

2.4. Effect of SA on the Combustion Performance of TPU

2.4.1. Cone Calorimeter Test Results

Figure 5 presents the Total Heat Release (THR) curves for TPU composite systems containing varying amounts of SA. Total heat release is a critical metric for assessing the fire hazard potential of a material; higher values indicate that the material releases a greater total amount of heat during combustion, thereby leading to more rapid fire propagation and more severe damage. As illustrated in the figure, TPU-SA0 exhibits an extremely rapid rate of heat release during the initial stages of combustion, reaching a final THR value of 32 MJ/m², which signifies a very high fire hazard. Upon the addition of SA, the slope of the THR curve for the composite system decreases significantly; furthermore, as the additive content increases, the final saturation value continues to decline. Among the tested samples, the TPU-SA10 group demonstrates the most superior performance, with its final THR value dropping to 19 MJ/m², a reduction of nearly 40% compared to pure TPU. This indicates that the incorporation of SA effectively inhibits the cumulative release of heat during the TPU combustion process, thereby substantially mitigating the material's fire hazard. The fundamental mechanism underlying this effect is that the antimony oxides generated from the high-temperature decomposition of SA catalyze the formation of a dense char layer on the TPU surface; this layer acts as a barrier to heat transfer, thereby fundamentally reducing the generation and release of combustion products.

Figure 6 presents the heat release rate (HRR) curves for TPU composite systems containing varying amounts of SA. The heat release rate serves as a critical metric for assessing the intensity of material combustion and its associated fire hazard; a higher peak value and a sharper peak profile indicate more intense combustion and a greater risk of flashover [41,42]. As illustrated in the figure, the peak heat release rate of TPU-SA0 reaches as high as 605 kW/m², characterized by a sharp peak profile; its combustion is extremely intense, posing a significant fire hazard. Upon the addition of SA, the peak HRR of the composite system decreases significantly as the additive content increases, while the peak profile becomes noticeably broader: for the TPU-SA10 group, the peak heat release rate drops to 310 kW/m²—a reduction of nearly 50% compared to pure TPU—indicating a significantly attenuated combustion process and a substantially reduced risk of flashover. This demonstrates that SA effectively suppresses the combustion intensity of TPU; its underlying mechanism involves catalyzing the formation of a dense char layer on the TPU surface at high temperatures—thereby blocking the transfer of heat and oxygen—while simultaneously scavenging active free radicals in the gas phase to terminate chain reactions, thereby achieving highly efficient flame retardancy.

2.4.2. Analysis of the Microscopic Morphology of the Carbon Layer

The structure and morphology of the char layer formed after combustion serve as crucial evidence for elucidating flame-retardant mechanisms; Figure 7 presents SEM images of the char layers of various samples after combustion. Following combustion, (TPU-SA0) formed almost no intact char layer, leaving behind only a small amount of loose, flocculent residue; its surface was riddled with pores and cracks, rendering it incapable of acting as a barrier against heat and oxygen (Figure 7a).

Upon the addition of SA, the structure of the char layer formed by the composite system after combustion showed gradual improvement: the char layer of the TPU-SA3 sample began to exhibit a continuous structure, though it still contained numerous micropores and cracks, resulting in poor compactness (Figure 7b); for the TPU-SA5 sample, porosity in the char layer decreased and the structure tended toward greater compactness, forming a rudimentary continuous char layer (Figure 7c); the compactness of the char layer in the TPU-SA7 sample improved significantly, presenting a relatively smooth surface with only a few minute pores (Figure 7d); finally, the char layer of the TPU-SA10 sample displayed a dense, continuous network structure devoid of obvious cracks or large pores (Figure 7e). Such a dense char layer effectively impedes heat transfer and oxygen diffusion, thereby inhibiting the further combustion of the TPU matrix while simultaneously reducing the release of smoke and volatile products, ultimately achieving the dual effects of flame retardation and smoke suppression. The changes in char layer morphology are consistent with the results of the preceding thermal stability and combustion performance tests, demonstrating that SA enhances the flame retardancy of TPU by promoting the formation of a dense char layer [19,43,44].

2.5. Flame Retardancy Mechanism of SP/TPU Composites

Based on the aforementioned results regarding thermal stability, mechanical properties, combustion behavior, and structural characterization, the flame-retardant mechanism of SA within the TPU system can be summarized as a synergistic effect involving both condensed-phase and gas-phase flame retardation.

Condensed-phase flame-retardant mechanism: At high temperatures, SA decomposes to generate antimony oxides—such as Sb₂O₃—which act as Lewis acid catalysts; these oxides catalyze cross-linking and aromatization reactions within the TPU molecular chains, thereby promoting the formation of a dense, continuous char layer. This char layer exhibits excellent thermal stability and barrier properties; on one hand, it impedes the transfer of heat into the interior of the TPU matrix, thereby delaying the thermal decomposition of the matrix; on the other hand, it isolates the matrix from contact with oxygen, thereby cutting off the oxygen supply to the combustion reaction. Simultaneously, the dense char layer also suppresses the dripping of molten TPU and reduces the dispersion of burning materials, thereby inhibiting the spread of fire. Furthermore, the inorganic residues generated from the decomposition of SA can fill the pores within the char layer, further enhancing the density and stability of the char layer and reinforcing its barrier effect [18,45,46].

Gas-phase flame retardancy mechanism: During the decomposition of SA, active free radicals—such as SbO·—are released. Within the gas-phase combustion zone, these radicals capture other active free radicals generated by the combustion reaction—such as HO· and H·—thereby terminating the free-radical chain reactions, reducing the rate of gas-phase combustion, and consequently minimizing the release of heat and smoke. Simultaneously, the non-combustible gases generated by the decomposition of SA (such as Na₂O-related products) can dilute the concentration of combustible gases within the combustion zone, thereby reducing combustion intensity and further enhancing the flame-retardant effect.

Figure 8. Flame Retardancy Mechanism Diagram.

Figure 8. Flame Retardancy Mechanism Diagram.

Through the synergistic action of two flame-retardant mechanisms, SA significantly enhances the thermal stability and flame-retardant properties of TPU; when added in appropriate amounts, it preserves the excellent mechanical properties of TPU, thereby achieving a balance between flame retardancy and mechanical performance.

3. Materials and Methods

3.1. Experimental Raw Materials and Reagents

The raw materials and reagents used in the experiments included: thermoplastic polyurethane (TPU), grade TPU-1180A, serving as the composite matrix; SA (NaSbO₃)—with a purity of no less than 99.5%, a particle size not exceeding 5 μm, and a whiteness of no less than 92%—used as an inorganic flame-retardant functional filler; Antioxidant 1010 (a hindered phenol)—with a purity of no less than 98%—selected to inhibit TPU degradation during processing and testing; and stearamide (SA)—with a purity of no less than 98%—serving as a dispersant to improve the dispersion of SA within the TPU matrix.

3.2. Experimental Instruments and Equipment

The experimental instruments and equipment utilized include: a vacuum drying oven (Model DZF-6050) for drying raw materials and removing moisture; a high-speed mixer (SHR-10A) for the premixing of raw materials; a twin-screw extruder (TE-35) for the melt blending and granulation of composite materials; an injection molding machine (HTF86X1) for the preparation of standard test specimens; a four-point probe tester (RTS-9) for measuring the volume resistivity of the composites to characterize their electrical conductivity; a scanning electron microscope (SU8010) for observing the cross-sectional microstructure, SA dispersion, and carbon layer structure of the composites; an electronic universal testing machine (WDW-10A) for measuring tensile strength and elongation at break in accordance with GB/T 1040-2006; a Shore hardness tester (LX-A) for measuring the Shore A hardness of the composites in accordance with GB/T 2411-2008; and a thermogravimetric analyzer (Q5000IR) for evaluating thermal stability under a nitrogen atmosphere, with a heating rate of 10°C/min, within the temperature range of 30–800°C.

3.3. Sample Preparation

Raw Material Pretreatment: Place the TPU pellets into a vacuum drying oven and dry them at 80°C for 12 hours to remove moisture; grind the SA and pass it through a 200-mesh sieve to ensure uniform particle size.

Batching and Mixing: In accordance with the formulation presented in Table 1, the TPU and SA were stirred at high speed in a high-speed mixer for 5 minutes at a rotational speed of 1500 r/min, ensuring the preliminary, uniform dispersion of the SA.

Melt Blending and Granulation: The mixed raw materials were fed into a twin-screw extruder for melt blending, with the extrusion temperature set to 170–180°C and the screw speed at 300 rpm. The extruded material was subsequently water-cooled and pelletized using a pelletizer to obtain the composite masterbatch. The masterbatch was then dried at 100°C for 4 hours to remove any moisture adsorbed during the processing stage.

Injection Molding of Specimens: The dried composite masterbatch was injection-molded into standard specimens using an injection molding machine, with a molding temperature of 200°C, a mold temperature of 40°C, and a holding pressure time of 30 s. Tensile specimens (Type 5 dumbbell-shaped, 1.0 mm thick), combustion specimens (100 mm × 100 mm × 3 mm), and oxygen index specimens (130 mm × 6.5 mm × 3.2 mm) were prepared.

3.4. Performance Testing and Characterization Methods

3.4.1. Thermal Stability Test (TGA)

A TG 209 thermogravimetric analyzer was employed for the measurements under the following conditions: a nitrogen atmosphere with a flow rate of 40 mL/min, and a heating rate of 10°C/min over a temperature range extending from room temperature to 800°C. The thermal decomposition curves of the samples were recorded to analyze the initial decomposition temperature (T₅%, the temperature at which 5% mass loss occurs), the temperature of maximum decomposition rate (Tₘₐₓ), and the residual carbon content at 800°C.

3.4.2. Infrared Spectroscopy Characterization (FT-IR)

A Nicolet iS50 Fourier Transform Infrared (FTIR) spectrometer was employed, covering a spectral range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹ and 32 scans. The sample was mixed and ground with KBr at a mass ratio of 1:100 and then pressed into a pellet to analyze the interactions between SA and TPU molecules, as well as the changes in chemical structure during thermal decomposition [47,48,49]. In the data analysis of infrared spectroscopy, the formula for processing raw data of the temperature axis is defined as follows: the temperature (T) equals the set starting temperature (T_S) plus the quotient of the difference between the actual termination temperature (T_E) and the starting temperature (T_S) divided by the experimentally set duration (S_V, in minutes), multiplied by the time corresponding to each data point (S_E), and then divided by the experimentally set duration (S_V, in minutes).

$\begin{array}{c}T=T_S+{\displaystyle \frac{\left(T_E-T_S\right)}{S_V}}\times {\displaystyle \frac{S_E}{S_V}}\hspace{1em}\left(1\right)\end{array}$

3.4.3. Mechanical Property Testing

In accordance with the standard GB/T 528-2009, *Vulcanized rubber or thermoplastic rubbers—Determination of tensile stress-strain properties*, tensile tests were conducted using an electronic universal testing machine at a tensile speed of 500 mm/min. Each sample was tested five times to obtain an average value, and parameters such as tensile strength and elongation at break were recorded. Here, strain (S_L) is defined as the displacement D during tensile testing divided by the sample's original parallel length L, while stress (S_P) is calculated as the applied force N during tensile testing divided by the sample's original width W.

$\begin{array}{c}{S}_L=\epsilon ={\displaystyle \frac{D}{L}}\hspace{1em}\left(2\right)\end{array}$$\begin{array}{c}{S}_P={\displaystyle \frac{N}{W}}\hspace{1em}\left(3\right)\end{array}$

3.4.4. Combustion Performance Testing

Cone Calorimeter Testing: In accordance with the ISO 5660-1 standard, a cone calorimeter was utilized with a radiant heat flux of 50 kW/m². Test specimens measuring 100 mm × 100 mm × 3 mm were wrapped in aluminum foil and positioned horizontally. Key combustion parameters—including Heat Release Rate (HRR), Peak Heat Release Rate (HRR), Total Heat Release (THR), Smoke Production Rate (SPR), and Total Smoke Production (TSR)—were measured and recorded [50,51].

Observation of Char Layer Morphology: Following combustion testing, the samples were sputter-coated with gold. A SU3800 scanning electron microscope was then employed to observe the microscopic morphology of the char layer's surface and cross-section, thereby analyzing its compactness and integrity.

4. Conclusions

Research Conclusions

In this study, SA/TPU composite systems with varying contents of SA were prepared via melt blending. The effects of SA on the thermal stability, mechanical properties, and combustion behavior of TPU were systematically investigated, and its flame-retardant mechanism was elucidated. The main conclusions are as follows:

SA significantly enhances the thermal stability of TPU. As the content of SA increases, both the temperature at the maximum thermal decomposition rate (T_max) and the char residue yield of the composite system gradually rise. For the TPU-SA10 sample, the T_max reached 423.6°C, and the char residue yield at 800°C was 12.3%, representing increases of 23.9°C and 61.5-fold, respectively, compared to pure TPU.

A moderate amount of SA (content ≤5%) has a negligible impact on the mechanical properties of TPU; specifically, the TPU-SA5 sample exhibited a tensile strength of 42.5 MPa and an elongation at break of 485%, retaining over 88% of the mechanical properties of pure TPU. However, at high addition levels (≥7%), the mechanical properties deteriorate significantly due to particle agglomeration.

SA effectively enhances the flame retardancy and smoke suppression properties of TPU; the LOI value of the TPU-SA10 group reached 32.8%, and it achieved a V-0 rating in the vertical burning test. Furthermore, the pHRR and TSR were reduced by 60.0% and 45.3%, respectively, compared to pure TPU, resulting in a significant reduction in fire hazard.

The flame retardancy mechanism of SA in TPU involves a synergistic effect between the condensed phase and the gas phase: in the condensed phase, it catalyzes the formation of a dense char layer that acts as a barrier against heat and oxygen; in the gas phase, it captures active free radicals to terminate chain reactions.