Thermal Decomposition Kinetics of Polyamide 6 Composites containing Sulphur- and Phosphorus-based Flame Retardants

 

Sweety Monga*

Department of Chemistry, Government College, Hisar - 125001, Haryana, India.

 

Abstract:

The thermal degradation behaviour and degradation kinetics of polyamide 6(PA6) composites containing guanidine sulfamate–based flame-retardant systems were investigated using thermogravimetric techniques. Thermogravimetric (TG) and derivative thermogravimetric (DTG) measurements were performed in air at a constant heating rate of 10 °C min⁻¹ to evaluate degradation pathways, decomposition rates, and char-forming behaviour. The incorporation of flame-retardant additives alters the degradation pathway of PA6 by initiating early decomposition while enhancing thermal stability at elevated temperatures through improved char development. The GS–DP system exhibits effective condensed-phase action, with maximum char formation achieved at an OH:S ratio of 0.92, whereas the addition of montmorillonite reduces char yield due to its catalytic influence on polymer degradation. In contrast, the GS–OP system shows the most effective flame-retardant performance, producing the highest char residue through the combined action of condensed-phase stabilization and gas-phase inhibition. Thermal degradation kinetics was evaluated using model-based Coats–Redfern, Broido, and Horowitz–Metzger methods applied to single-heating-rate TG data. Kinetic analysis indicates that the degradation of PA6 and its composites follows a first-order reaction mechanism, with reduced activation energy values for flame-retarded systems reflecting facilitated yet controlled degradation that favours efficient char formation. Overall, the results demonstrate that flame-retardant formulation and its influence on degradation kinetics play a critical role in improving the fire resistance of PA6 composites.

 

 

 

1. INTRODUCTION:

Polyamide 6 (PA6) is a widely used engineering thermoplastic owing to its good mechanical strength, chemical resistance, wear resistance, and processability, which makes it suitable for applications in automotive, electrical, and structural components^1–11. Despite these advantages, PA6 is inherently flammable and undergoes rapid thermal degradation when exposed to heat, producing mainly volatile decomposition products and only a limited amount of protective char⁵. This behaviour restricts its application in areas where fire safety requirements are stringent.

 

The flame-retardant performance of PA6 based composites is strongly influenced by their thermal stability, degradation rate, degradation mechanism, and ability to form a stable char layer during thermal exposure¹². Flame-retardant additives are commonly incorporated to modify the degradation pathway of PA6, promote condensed-phase protection, and suppress the release of flammable volatiles^5,6,13,14. However, conventional flammability tests alone are insufficient to explain the effectiveness of such additives, as they do not provide insight into the underlying thermal degradation processes. Therefore, investigation of thermal degradation behaviour and degradation kinetics is essential for understanding and improving flame-retardant performance^15–24.

 

 

Thermogravimetric analysis (TG) is one of the most effective techniques for studying the thermal stability and decomposition behaviour of polymers and their composites^25–30. TG provides information on degradation stages, mass-loss rates, and char residue under controlled heating conditions. In flame-retarded PA6 composites containing multiple additives, thermal degradation often involves overlapping reactions due to the decomposition and interaction of different components at various temperatures. Derivative thermogravimetric (DTG) analysis is used to resolve such complex behaviour by examining the rate of mass loss as a function of temperature. While TG and DTG analyses offer valuable insight into degradation behaviour, a quantitative understanding of the decomposition process requires kinetic analysis. Thermal degradation kinetics allows determination of key parameters such as activation energy and degradation mechanism, which directly govern the rate of polymer decomposition and char formation^{15,17,18,31–33}. Kinetic analysis based on TG data therefore provides a fundamental framework for comparing different flame-retardant systems and for understanding how their degradation behaviour affects flame-retardant performance in PA6 composites.

 

Kinetic analysis of TG data can be performed using model-based or model-free approaches. In the present study, model-based kinetic methods were employed, as they are suitable for single heating-rate experiments and provide insight into the degradation mechanism. Model-based methods involve fitting experimental TG data to different solid-state reaction models and selecting the most appropriate mechanism based on statistical correlation and physical relevance.

 

The Coats–Redfern (CR) method³² is an integral model-fitting approach widely used for evaluating thermal degradation kinetics from TG data obtained at a constant heating rate. For a first-order reaction (n = 1), the Coats–Redfern equation is expressed as:

 

where  is the degree of conversion,  is the absolute temperature,  is the pre-exponential factor,  is the activation energy,  is the gas constant, and  is the heating rate. Different algebraic expressions of the integral function  correspond to different solid-state reaction mechanisms (Table 1).

 

The Broido method³⁴ is another commonly used model-based approach that assumes first-order kinetics and utilises single heating-rate TG data. The Broido equation is given by:

 

where  is the temperature corresponding to the maximum rate of degradation.

 

The Horowitz–Metzger (HM) method³⁵ is also applicable to single heating-rate TG data and makes use of the DTG peak temperature. For first-order reactions, the HM equation is expressed as:

 

where ,  is the temperature at a particular conversion, and  is the temperature of maximum degradation rate.

 

 

Comparison of kinetic parameters obtained from these model-based methods improves the reliability of the analysis and aids in identifying the most appropriate degradation mechanism.

 

 

 

 

Table 1: Algebraic expressions for integrated functions, g(α) for the most commonly used mechanisms of solid state processes.

 

In the present work, the thermal degradation behaviour and degradation kinetics of flame-retardant polyamide 6 composites were investigated using thermogravimetric analysis carried out in air atmosphere at a constant heating rate. TG and DTG data were analysed to evaluate mass-loss behaviour, degradation stages, and char formation. Model-based kinetic analysis using Coats–Redfern³², Broido³⁴, and Horowitz–Metzger method³⁵, was employed to determine activation energy and to elucidate the degradation mechanism governing the thermal decomposition process.    

 

2. EXPERIMENTAL:

2.1 Materials used:

Polyamide 6 (PA6) in pellet form, with a melting point of approximately 220 °C, was used as the polymer matrix. PA6 was procured from Sigma Aldrich Co., India and Gujarat State Fertilizers and Chemicals Limited, India. Sodium montmorillonite (MMT) clay was supplied by Southern Clay Products Inc., Germany, and was used as the inorganic additive. The phosphorus-based flame retardant Exolit OP 1312 was obtained from Clariant Inc., USA. Exolit OP 1312 consists of a mixture of aluminum diethylphosphinate [C₄H₁₀PO₂]₃Al and melamine polyphosphate in a weight ratio of 2:1, with a reported decomposition temperature of approximately 350 °C. Guanidine sulfamate (GS), with purity greater than 90% and molecular weight of 156.2g mol⁻¹, was purchased from Hind Associates Pvt. Ltd., Gurgaon, India, and was used as the sulfur-containing flame-retardant additive. Dipentaerythritol (DP), technical grade with a melting point in the range of 215–218°C, was obtained from Sigma Aldrich Co., India and employed as the carbon source in the flame-retardant system. All materials and additives were used as received, without any further purification.

 

2.2 Preparation of Flame-Retardant Composites:

Prior to processing, PA6 pellets were dried in a hot-air oven at 80 °C for sufficient time to remove absorbed moisture. The required amounts of PA6 and flame-retardant additives were weighed accurately to obtain a total additive loading of 5 wt% in the composite formulation. Melt blending was carried out using a co-rotating twin-screw extruder (L/D = 42, maximum rpm=600, feeder rpm=200). The processing temperature was maintained in the range of 235–240 °C along the barrel to ensure proper melting and dispersion of additives within the polymer matrix. The screw speed was adjusted to obtain uniform mixing. The extruded strands were cooled at room temperature and pelletized.

 

2.3 Thermogravimetric Study:

Thermogravimetric analysis (TG) was performed using a thermogravimetric analyser (EXSTAR TG/DTA 6300) to investigate the thermal degradation behaviour of the prepared composites. Approximately 5 mg of each sample was placed in a platinum pan and heated from room temperature to 600 ^oC at a constant heating rate of 10 °C min^-1. All experiments were carried out under air atmosphere to simulate oxidative degradation conditions. The mass change of the sample was continuously recorded as a function of temperature. Thermogravimetric (TG) curves were obtained to analyse mass-loss behaviour, number of degradation stages, onset of decomposition, and residual char yield at high temperatures. Derivative thermogravimetric (DTG) curves were obtained from the TG data by plotting the rate of mass loss (dW/dT) as a function of temperature. DTG curves were used to identify temperatures corresponding to the maximum degradation rates and to resolve overlapping degradation steps.

 

2.4 Kinetic Study:

Thermal degradation kinetics of the composites was evaluated using model-based kinetic methods applied to the TG data obtained at a single heating rate. The degree of conversion (α) was calculated using the relation:

 

where  is the initial mass of the sample,  is the mass at temperature , and  is the final residual mass. Kinetic calculations were performed over the main degradation region corresponding to the principal mass-loss stage. The Coats–Redfern³², Broido³⁴, and Horowitz–Metzger³⁵ methods were employed to determine activation energy and related kinetic parameters. Various solid-state reaction models were tested to identify the most appropriate degradation mechanism governing the thermal decomposition process.

 

3. RESULTS AND DISCUSSION:

3.1 Thermogravimetric Analysis:

The thermogravimetric (TG) curves of neat PA6 and the corresponding flame-retarded composites are presented in Figure 1, and the relevant TG parameters are summarized in Table 2. From the TG profiles, key thermal characteristics such as the number of degradation stages, temperatures corresponding to 10% and 50% mass loss (T_10wt% and T_50wt%), temperatures of maximum degradation rate (DTG peak maxima), and residual char content were evaluated.

 

A series of PA6 composites containing a total flame-retardant loading of 5 wt% were prepared using GS–DP and GS–OP flame-retardant systems independently. For the GS–DP system, composites were formulated with varying OH: S ratios in the range of 0.5–1.0 in order to investigate the effect of dipentaerythritol concentration on the thermal degradation behaviour of GS-modified PA6. Here, the OH: S ratio represents the molar ratio of hydroxyl groups contributed by dipentaerythritol to the sulfur content of guanidine sulfamate per gram of composite. In the case of the GS–OP system, Exolit OP 1312 (OP) was used as a co-flame retardant in combination with guanidine sulfamate, while maintaining the total additive content at 5 wt%. This formulation strategy was adopted to examine the influence of the GS/OP ratio on the thermal degradation behaviour and flame-retardant performance of PA6.

 

3.1.1. Thermogravimetric Behaviour of Neat PA6 :

The thermogravimetric (TG) curve of neat PA6 recorded in air (Figure 1) exhibits two-stage thermal degradation behaviour, indicating a relatively simple decomposition pathway. In the initial temperature region up to about 370 ^oC, only a minor mass loss is observed, which is attributed to the release of residual moisture and volatilization of low-molecular-weight species present in the polymer. This limited mass loss reflects the inherent thermal stability of PA6 under moderate heating conditions. The main degradation stage occurs in the temperature range of approximately 370–475 ^oC, during which a substantial mass loss is recorded, with the maximum degradation rate appearing near 445 ^oC. This stage corresponds to rapid decomposition of the polymer backbone, primarily driven by random scission of alkyl amide linkages, leading to extensive volatilization of degradation products. At higher temperatures, above 475 ^oC, only a small additional mass loss is observed, which is associated with oxidation of the transient residue formed during the main degradation step. As a result, a very low amount of char residue remains at elevated temperature, confirming the limited char-forming ability of neat PA6 under oxidative conditions.

 

Table 2: TG data of PA6 and its various composites with GS-DP and GS-OP systems.

* Composition in percent by weight.

**OH:S is the ratio of number of moles of OH groups of dipentaerythritol to number of moles of S of guanidine sulfamate per gram  of the mixture.

 

Fig. 1: TG curves of (1) PA6, (2) PA6/GS/DP^a, (3) PA6/GS/DP^b, (4) PA6/GS/DP^c, (5) PA6/GS/DP/MMT, (6) PA6/GS/OP^a, (7) PA6/GS/OP^b and (8) PA6/GS/OP^c.

 

3.1.2. Effect of GS–DP Flame-Retardant System on Thermal Decomposition:

The incorporation of the guanidine sulfamate–dipentaerythritol (GS–DP) flame-retardant system significantly modifies the thermal degradation behaviour of PA6. In contrast to neat PA6, which exhibits two-stage degradation, GS–DP-containing composites show three distinct degradation stages, with an additional mass-loss step appearing in the temperature range of 100–385°C and corresponding DTG maxima around 370–375°C. This early degradation stage is attributed to interactions between the flame-retardant additives and the polymer matrix.

 

The onset temperature of degradation (T_10wt%) decreases by approximately 10–40°C for the GS–DP composites, indicating early activation of degradation reactions. However, the temperatures corresponding to 50% mass loss (T_50wt%) and the DTG peak associated with the main degradation stage shift toward higher values, reflecting improved thermal stability at elevated temperatures. This behaviour suggests that GS and DP promote controlled early degradation while enhancing stabilization of the polymer matrix at higher temperatures through condensed-phase reactions that favour char formation. This is similar to the effect observed by Lewin et al³⁶ on addition of ammonium sulfamate (AS) and dipentaerythritol (DP) flame retardants to PA6.

3.1.3. Influence of GS: DP Ratio on Degradation Characteristics:

The thermal degradation behaviour of PA6/GS–DP composites strongly depends on the GS: DP ratio. Among the studied formulations, the composite with an OH:S ratio of 0.92 exhibits the most favourable performance, showing the highest char residue (6.4 wt% at 550 °C) along with maximum T_10wt%, T_50wt%, and DTG peak temperature for the main degradation stage. This composition provides an optimal balance between acid generation by GS and carbon contribution from DP, leading to efficient char formation. Deviations from this ratio result in reduced thermal stability and char yield, indicating less effective condensed-phase protection.

 

3.1.4. Role of Montmorillonite in Thermal Degradation: 

The incorporation of 1.5 wt% of sodium montmorillonite (MMT) into the PA6/GS/DP^b composite results in a noticeable reduction in thermal stability, as evidenced by decreases in T_10wt%, T_50wt% and the DTG peak temperature of the major degradation stage by approximately 21, 4, and 11°C, respectively (Figure 1). In addition, the char residue at 550 °C decreases significantly from 6.4 wt% for PA6/GS/DP^b to 3.2wt% for PA6/GS/DP/MMT, indicating a reduction in condensed-phase char formation.

 

This behaviour suggests an antagonistic effect of MMT when combined with the GS–DP flame-retardant system. Similar antagonism has been reported^7,37 previously for PA6 systems containing ammonium sulfamate–dipentaerythritol flame retardants in the presence of montmorillonite-based fillers. The reduction in thermal stability and char yield is attributed to the catalytic activity of layered silicates, which accelerates chain scission of the PA6 backbone at lower temperatures and interferes with sulfation and dehydration reactions essential for the development of a stable carbonized network. Consequently, the effectiveness of the GS–DP condensed-phase flame-retardant mechanism is partially diminished in the presence of MMT.

 

3.1.5. Thermal Behaviour of GS–OP based Composites:

The TG curves of PA6 composites containing the GS–OP flame-retardant system exhibit three distinct stages of thermal degradation, as shown in Figure 1. The incorporation of Exolit OP 1312 (OP) in combination with guanidine sulfamate results in a significant reduction in the onset temperature of degradation, by approximately 50 °C compared to neat PA6. This early degradation is attributed to the thermal decomposition of OP components, namely aluminum diethylphosphinate (Al-Pi) and melamine polyphosphate (MPP), which decompose at relatively low temperatures.

 

Exolit OP 1312 is a mixture of Al-Pi and MPP in a 2:1 (wt/wt) ratio, and its decomposition contributes to flame retardancy through both gaseous-phase and condensed-phase mechanisms³⁸. Thermal and hydrolytic decomposition of MPP leads to the formation of polyphosphoric acid, which undergoes condensation upon heating and releases water molecules. Simultaneously, hydrolysis of MPP generates volatile species that ultimately decompose into CO₂ and NH₃, which act as flame-diluting gases, thereby inhibiting combustion in the gas phase^38,39. In addition, melamine undergoes sublimation near 200 °C, producing a cooling effect, and subsequently participates in self-condensation reactions to form melam, melem, and melon-type structures, contributing to heterogeneous char formation in the condensed phase⁴⁰.

 

The decomposition of Al-Pi further enhances flame retardancy through multiple pathways. Hydrolytic degradation of Al-Pi produces diethylphosphinic acid and aluminum trihydroxide, the latter releasing water and forming alumina (Al₂O₃) as a stable residue. Diethylphosphinic acid generates phosphorus-centred radicals such as PO, PO₂, HOPO, and HOPO₂, which effectively quench flame-propagating radicals in the gaseous phase⁴¹. Under dry conditions, Al-Pi decomposes to form aluminum dihydrogen phosphinate in the condensed phase while releasing ethylene into the gas phase. The aluminum phosphinate species further catalyse the conversion of olefinic degradation products of PA6 into a polyaromatic carbonaceous char, thereby enhancing condensed-phase protection³⁸.

 

As a result of these combined mechanisms, PA6 composites containing the GS–OP system exhibit a substantial increase in char residue. As shown in Table 2, the maximum char yield of 8.7 wt% at 550 °C is obtained for the formulation containing 2 wt% GS and 3 wt% OP, indicating a strong synergistic interaction between GS and OP. This enhanced char formation effectively improves the thermal stability and flame-retardant performance of PA6 through the simultaneous action of gaseous-phase inhibition and condensed-phase stabilization.

 

3.1.6. Char Residue and Condensed-Phase Protection:

A systematic comparison of residual mass clearly demonstrates enhanced char formation upon incorporation of flame-retardant additives. Neat PA6 produces only a very small amount of char (≈3 wt%), confirming its poor carbonization ability under oxidative conditions. In contrast, GS-containing composites exhibit significantly higher char yields, indicating effective condensed-phase action. The PA6/GS–DP system shows improved char formation, with the maximum residue (≈5–7 wt%) obtained at the optimized OH:S ratio of 0.92, whereas the addition of MMT reduces the char yield due to its antagonistic catalytic effect. The GS–OP system exhibits the highest char residue (≈8–9 wt%), reflecting strong synergistic interactions between guanidine sulfamate and organic phosphinate.

The increased char residue forms a protective barrier that limits heat transfer, restricts oxygen diffusion, and suppresses the release of flammable degradation products. These results confirm that condensed-phase mechanisms play a dominant role in improving the thermal stability and fire resistance of PA6 composites, particularly in formulations containing synergistic GS–OP flame-retardant systems.

 

3.2. Degradation Kinetic Analysis:

3.2.1. Kinetic Methods and Degradation Region:

The thermal degradation kinetics of neat PA6 and the flame-retarded composites PA6/GS/DP^b, PA6/GS/DP/MMT, and PA6/GS/OP^b were investigated using model-based kinetic approaches, namely the Coats–Redfern (CR), Broido (BR), and Horowitz–Metzger (HM) methods. These methods assume first-order reaction kinetics (n=1) and were applied to single-heating-rate thermogravimetric data to evaluate the kinetic parameters governing the main degradation stage. Analysis of the TG curves indicates that the principal degradation stage for all samples occurs in the temperature range of approximately 375–485^oC, corresponding to a conversion degree range of α = 0.2–0.8. This region, associated with the dominant mass-loss process, was selected for kinetic evaluation. Activation energy (E), pre-exponential factor (A), and correlation coefficient (R²) were calculated over this conversion range at a constant heating rate of 10^oC min^-1.

 

3.2.2 Evaluation and Selection of Reaction Models:

Application of the Coats–Redfern method (Table 3 and 4) resulted in highly linear plots for all tested solid-state reaction models, with correlation coefficients exceeding 0.97, indicating good mathematical fitting. However, correlation coefficient alone was insufficient to identify the appropriate degradation mechanism. Therefore, model selection was based on a combined consideration of correlation coefficient, physical relevance of the frequency factor, and consistency of activation energy values with those obtained from the BR and HM methods (Table 5).

Reaction models based on nucleation and growth (A2, A3, and A4) and phase-boundary mechanisms (R1, R2, and R3) yielded unrealistically low values of the pre-exponential factor, far below the accepted theoretical range for solid-state reactions. This clearly indicates that these models are not suitable for describing the thermal degradation behaviour of PA6 and its composites. Diffusion-controlled models (D1, D2, and D3) and the first-order reaction model (F1) produced acceptable frequency factors and high correlation coefficients; however, diffusion-controlled models resulted in excessively high activation energy values that were not comparable with those obtained from the BR and HM methods, suggesting that diffusion is not the rate-controlling step during the main degradation stage. The kinetic parameters of samples obtained by BR and HM methods are given in Table 5. The plots obtained by BR and HM methods, respectively over the conversion degree range α = 0.2–0.8 are shown in (Figures 2 and 3)

 

Table 3: The kinetic parameters of PA6 and PA6/GS/DP obtained by using Coats-Redfern method.

 

 

Table 4: The kinetic parameters of PA6/GS/DP/MMT and PA6/GS/OP obtained by using Coats-Redfern method.

 

Table 5: Kinetic parameters of PA6 and its composites by Broido and Horowitz-Metzger methods.

 

 

Fig. 2: Plots obtained by Broido method for PA6 and its composites with GS-DP and GS-OP FR systems.

 

 

Fig. 3: Plots obtained by Horowitz-Metzger method for PA6 and its composites with GS-DP and GS-OP FR systems.

 

3.2.3. Activation Energy Trends and Mechanistic Implications:

Using the Coats–Redfern method and the F1 reaction model, the activation energy values for PA6, PA6/GS/DP^b, PA6/GS/DP/MMT, and PA6/GS/OP^b were determined to be 152.1, 138.6, 148.1, and 123.5 kJ mol⁻¹, respectively. A systematic decrease in activation energy is observed upon addition of flame-retardant additives, reflecting facilitated bond cleavage and enhanced chemical reactivity during degradation. This reduction is associated with acid-catalyzed reactions, increased radical formation, and chemical modification of the polymer backbone by decomposition products of the flame-retardant systems.

Importantly, lower activation energy does not imply reduced flame resistance; rather, it indicates controlled degradation that promotes rapid char formation and suppresses uncontrolled volatilization. The activation energy values obtained using the Broido and Horowitz–Metzger methods are in close agreement with those obtained from the Coats–Redfern analysis and follow the same trend, confirming the reliability of the kinetic evaluation.

 

3.2.4. Identification of Degradation Mechanism:

Among the various solid-state reaction models examined, the first-order reaction model (F1) consistently provides the best statistical fit for neat PA6 as well as all flame-retarded composites. The suitability of the F1 model indicates that degradation proceeds predominantly through random chain scission, rather than diffusion-limited or nucleation-controlled processes. The persistence of this mechanism across different formulations suggests that flame-retardant additives modify the degradation pathway quantitatively, by altering degradation rates and residue formation, without fundamentally changing the random nature of polymer decomposition.

 

3.3. Correlation between Kinetics and Flame-Retardant Performance:

The combined TG, DTG, and kinetic analyses demonstrate a strong correlation between degradation kinetics and flame-retardant efficiency. Composites exhibiting lower activation energy and higher char yield show improved resistance to thermal decomposition at elevated temperatures. Among the studied systems, PA6/GS/OP^b displays the lowest activation energy, indicating the strongest catalytic influence during degradation, which correlates well with its pronounced mass loss during the main degradation stage followed by the formation of a substantial char residue at higher temperatures.

 

This behaviour confirms that effective flame retardancy in PA6 systems arises from early chemical activation followed by stabilized condensed-phase protection, rather than merely delaying the onset of degradation. The kinetic results are therefore fully consistent with the thermogravimetric observations and provide strong support for the proposed flame-retardant mechanisms. The combined TG, DTG, and kinetic results demonstrate a strong link between the thermal degradation behaviour and flame-retardant performance of PA6 composites. The introduction of flame-retardant additives alters the degradation pathway by initiating early decomposition reactions, while simultaneously enhancing stability at higher temperatures through increased char formation. Although the activation energy values of the composites are lower than that of neat PA6, this reduction reflects facilitated and controlled degradation rather than loss of thermal resistance. Such behaviour promotes efficient development of a protective char layer, which limits further decomposition at elevated temperatures.

 

Among the investigated formulations, the PA6/GS/OP^b composite exhibits the lowest activation energy together with the highest char residue, indicating the strongest catalytic influence during the main degradation stage followed by effective stabilization of the residue. This trend is fully consistent with the TG and DTG results, which show pronounced mass loss during the primary degradation step accompanied by substantial char retention at higher temperatures. Overall, these observations confirm that improved flame retardancy in PA6 composites arises from early chemical activation coupled with dominant condensed-phase protection, rather than from a simple delay in the onset of degradation. The close agreement between kinetic parameters and thermogravimetric behaviour supports the proposed degradation mechanisms and highlights the critical role of char-forming processes in enhancing fire resistance.

 

4. CONCLUSION:

The present study clearly demonstrates that guanidine sulfamate–based flame-retardant systems significantly influence the thermal degradation behaviour of PA6 by modifying both the degradation pathway and char formation process. Although the incorporation of flame-retardant additives initiates degradation at lower temperatures, the composites exhibit enhanced thermal stability at higher temperatures due to the formation of a protective carbonaceous residue. In the GS–DP system, flame retardancy is governed mainly by condensed-phase reactions, with optimum performance achieved at an OH:S ratio of 0.92, where maximum char yield and improved high-temperature stability are obtained. The addition of montmorillonite to this system reduces char formation and thermal stability, indicating an antagonistic effect arising from its catalytic influence on polymer chain scission. In contrast, the GS–OP system provides the most effective flame-retardant performance, producing the highest char residue through a synergistic combination of condensed-phase stabilization and gas-phase inhibition mechanisms. Kinetic analysis confirms that the thermal degradation of PA6 and its flame-retarded composites follows a first-order reaction mechanism, with reduced activation energy values reflecting facilitated yet controlled degradation that promotes efficient char formation. Overall, the results demonstrate that careful selection and optimization of flame-retardant formulations play a decisive role in controlling degradation kinetics and enhancing the fire resistance of PA6 composites.

 

5. ACKNOWLEDGEMENT:

This work was carried out as part of the PhD research of the author at Guru Jambheshwar University of Science and Technology, Hisar. The author acknowledges the guidance and support received during the course of this research.

 

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Received on 09.01.2026      Revised on 06.03.2026 Accepted on 15.04.2026      Published on 25.04.2026 Available online from April 28, 2026 Research J. Science and Tech. 2026; 18(2):127-137. DOI: 10.52711/2349-2988.2026.00018
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