Improvement of Flame Retardancy on Polyamide 6 Composites via Melamine Polyphosphate-Modified Carbon Nanotubes

Abstract

Melamine polyphosphate (MPP) is a widely employed additive-type flame retardant for polyamide 6. Generally, a higher loading of MPP leads to improved flame retardancy of polyamide 6 composites. Nevertheless, excessive addition tends to cause problems such as flame-retardant migration, leakage, and exudation. Against this background, this work focuses on covalently grafting melamine polyphosphate onto the surface of carbon nanotubes via a facile chemical reaction, with the aim of alleviating the migration and leakage of the flame retardant in the polyamide 6 matrix. Carbon nanotubes (CNTs) were surface modified with a silane coupling agent (KH560) to obtain CNTs bearing epoxy groups (CNT-KH560). Subsequently, a ring-opening addition reaction was conducted between the CNT-KH560 and melamine polyphosphate (MPP) yielding carbon nanotubes with surface-bonded flame-retardant MPP (CNTM). Polyamide 6 composite slices (PA6/CNTM) were prepared via twin-screw extrusion blending and compounding and then by hot-press molding into test specimens. The modified carbon nanotubes were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetric analysis. The results confirmed the successful grafting of MPP onto the carbon nanotube surface, with a grafting degree of 9.1 g/100 g measured. The flame retardancy of the PA6/CNTM composites were evaluated through UL 94 vertical burning and limiting oxygen index (LOI) tests and cone calorimeter. These flame retardancy results indicated that when the content of flame-retardant-modified carbon nanotubes was 10 wt%, the PA6/CNTM10 composites achieved UL 94 V-2 and the limiting oxygen index increased from 24.5% of pure PA6 to 29.1%. The PHRR value of pure PA6 decreased from 750 kW/m² to 614 kW/m². This design of surface-grafted flame retardant provides a new strategy for the preparation and application of high-performance polyamide 6 flame-retardant composites.

1. Introduction

Polyamide 6 was first synthesized by ring-opening polymerization of caprolactam monomer, and it is one of the most commonly used engineering plastics [1,2,3]. Because of its excellent mechanical properties, wear resistance, electrical insulation and workability, it is widely used in automobile manufacturing, electronic appliances, building materials and other fields [4,5,6,7,8,9,10]. Its excellent performance brings many conveniences to people’s lives and work. However, PA6 itself is flammable and accompanied by a serious droplet phenomenon, so its use poses a fire safety hazard. Therefore, it is necessary to modify nylon 6 when it is used, and adding flame retardant to nylon 6 matrix to form a composite material is an effective method of flame-retardant modification [11,12,13]. Historically, the flame-retardant modification of nylon 6 has been achieved through the use of halogen-containing flame retardants. These retardants are characterized by high flame-retardant efficiency, superior thermal stability and excellent electrical properties, making them widely applied in flame-retardant systems for polyamide 6. Nevertheless, as international environmental protection requirements and relevant laws and regulations become increasingly stringent, halogen-containing flame retardants used in PA6 are gradually being phased out of the market [14,15,16]. In their place, new halogen-free intumescent green and high-efficiency composite flame retardants, such as melamine urate (MCA) and melamine polyphosphate (MPP), are steadily gaining market share. The flame-retardant mechanism of phosphorus-nitrogen flame retardants mainly involves two pathways: gas-phase flame retardancy and condensed-phase flame retardancy. For the gas-phase mechanism, melamine salt flame retardants undergo thermal decomposition to produce inert gases (including nitrogen, ammonia, water vapor, carbon dioxide, etc.). These gases lower the concentrations of oxygen and combustible substances in the combustion zone; at the same time, the generated high-density vapor forms a covering on the surface of the burning material, thereby suppressing combustion. In contrast, the condensed-phase flame-retardant mechanism works as follows: melamine salt flame retardants absorb heat during decomposition and generate strong acids (e.g., pyrophosphoric acid), which then cover the material’s surface. This process induces the dehydration and carbonization of the material, and a carbonization barrier layer is subsequently formed to hinder the combustion process of the combustible material [17,18,19].

Currently, the majority of research works adopt a direct addition approach: melamine salt flame retardants are first incorporated into the PA6 matrix to prepare flame-retardant masterbatches, which are then re-added to the PA6 matrix to form flame-retardant nylon chips. While this method resolves the issue of uneven dispersion of melamine salt flame retardants in the nylon 6 matrix, it fails to tackle the problems of migration, leakage, and exudation of melamine salt flame retardants within the nylon 6 matrix [20,21,22,23]. As the service life of nylon 6 composites extends, the leakage of flame retardants will become more pronounced. Such leakage can cause surface frosting on nylon 6 composites and contaminate the areas where these materials are applied, which in turn leads to a continuous decline in the flame-retardant performance of the composites. At the same time, the excessive addition of melamine salt flame retardants highlights the problem of sacrificing the mechanical properties of PA 6 composites while ensuring their flame-retardant performance. The balance between mechanical properties and flame retardancy is still a challenge in the research of nylon 6 composites.

Yuan Wencong et al. [24] melt-blended the synthesized melamine urate flame-retardant with PA6 to produce flame-retardant PA6-MCA composites. The test results showed that when the dosage of MCA flame retardant was 8%, the oxygen index (LOI) of the flame-retardant composite material reached 29%, and the vertical combustion grade UL94 reached a V-0 level. However, with the increase in flame-retardant dosage, the mechanical properties of PA6-MCA composites showed a downward trend. Our research group [25,26] has previously modified the surface of carbon nanotubes and then grafted glycidyl methacrylate onto the surface of carbon nanotubes through free radical polymerization. Then, flame-retardant MPP was grafted onto the surface of carbon nanotubes through a ring-opening addition reaction, and flame-retardant functionalized carbon nanotubes were added to the epoxy resin matrix. After curing, flame-retardant epoxy resin composite materials were prepared to achieve a good flame-retardant effect. However, in the face of the large demand for the preparation of flame-retardant fillers in PA6 composites, complex polymer polymerization chemical reactions are not suitable for large-scale industrial production.

Based on this background, this work proposed a simple preparation method for flame-retardant functionalized carbon nanotubes by surface modification reaction of a silane coupling agent and further graft bonding of flame-retardant MPP onto the surface of carbon nanotubes [27]. These flame-retardant-modified carbon nanotubes and PA6 slices were added to a twin-screw extruder to extrude and granulate them into flame-retardant slices. Then, the flame-retardant PA6 composites were prepared through hot-pressing molding. The ultimate objective of this design strategy and experimental protocol is to minimize or even completely eliminate the migration, leakage, and exudation of the MPP flame retardant within the polyamide 6 matrix. The developed flame-retardant PA6 composites will provide new design strategies and material choices with required thermal protection, such as power battery packs for new energy vehicles.

2. Materials and Methods

2.1. Main Reagents for the Experiment

Carbon nanotubes (CNTs), industrial grade, with a diameter of about 30 nm and a length of about 30–50 μm, were purchased from Henan GuoTan Nano Technology Co., Ltd., Pingdingshan, China. Silane coupling agent (KH560), analytical purity, was bought from Shanghai Yuanye Biotechnology Co., Ltd., Shanghai, China. Melamine polyphosphate (MPP), analytical grade, was supplied by Upper McLean Biochemical Technology Co., Ltd., Beijing, China. Nylon 6 (PA6) slices, B21G-35, were provided by Pingmei Shenma Group Engineering Plastics Co., Ltd., Pingdingshan, China. N-N dimethylformamide (DMF), analytical grade, was purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd., Beijing, China. Anhydrous ethanol, analytical grade, was provided by Tianjin Yongda Chemical Reagent Co., Ltd., Tianjing, China.

2.2. Experimental

2.2.1. Melamine Polyphosphate Modified Carbon Nanotubes

The schematic diagram of the modification process of carbon nanotubes is shown in Figure 1. Firstly, a carbon nanotube acidification treatment was prepared using an acidic mixed solution that contained concentrated sulfuric acid and concentrated nitric acid, to which an appropriate amount of carbon nanotubes (CNTs) was added. After the reaction time was over, the beaker was allowed to stand for 2 h. The upper clear liquid was removed, and then the sediment was washed with water and ethanol repeatedly three times to obtain acidified carbon nanotubes.

Secondly, the acidified carbon nanotubes were modified with coupling agents. Carbon nanotubes (30 g), an appropriate amount of silane coupling agent KH-560, and anhydrous ethanol were added to a three-necked flask, stirred magnetically, and heated in a water bath at 50 °C. After the reaction was complete, the mixed solution was removed, filtered, washed alternately with anhydrous ethanol and pure water, and dried in a vacuum oven to obtain carbon nanotubes modified with the coupling agent KH-560 (CNT-KH560).

Finally, the dried CNT-KH560 was taken out and placed in a three-necked flask. Then, DMF and an appropriate amount of melamine polyphosphoric acid (MPP) were added to the flask and stirred magnetically at 80 °C in an oil bath. After the reaction was completed, the mixed solution was filtered and washed alternately with acetone and anhydrous ethanol. Then it was dried in a vacuum oven to obtain carbon nanotubes loaded with flame retardants (CNT-KH560-MPP, CNTM).

2.2.2. Preparation of PA6/CNT Composites

The above-obtained products were added to a twin-screw extruder, where they were processed by mixing the flame-retardant-modified carbon nanotubes and polymer PA6 slices evenly in a blending machine according to the mass ratio (3:97, 5:95, 10:90, 15:85). Prior to twin-screw extrusion, PA6 chips and modified carbon nanotubes were subjected to vacuum drying at 80 °C for no less than 12 h. During the extrusion process, the twin-screw extruder was operated at a rotational speed of 300 rpm, with the control torque maintained within the range of 40–60 Nm and the die pressure kept below 50 bar. During the extrusion blending process, an intermediate sample was taken as the sample, which was granulated by a granulator and stored to obtain PA6/CNTM slices. The composite slices consisting of modified carbon nanotubes and polymer PA6 were designated as PA6/CNTM3, PA6/CNTM5, PA6/CNTM10, and PA6/CNTM15, corresponding to their respective mass ratios of 3:97, 5:95, 10:90, and 15:85. Subsequently, the PA6/carbon nanotube slices, which had been extruded and pelletized using a twin-screw extruder, were placed into the mold of a flat vulcanizing machine for hot pressing, yielding square sheets with dimensions of 300 mm × 300 mm × 2.7 mm. These square tablets were further cut into the required spline size for testing.

As a comparative experimental group, unmodified carbon nanotubes and polymer PA6 chips were blended and pelletized in the same mass ratios via a twin-screw extruder. The resultant composites were designated as PA6/CNT3, PA6/CNT5, PA6/CNT10, and PA6/CNT15, respectively.

2.3. Measurements and Characterization

Infrared spectroscopy analysis was conducted using Tensor 37 Fourier transform infrared spectroscopy from Bruker, Germany. The test substance was mixed with KBr and compressed into tablets for testing. The absorption spectrum scanning range was 400~4000 cm⁻¹. Thermogravimetric-differential thermal analysis was performed using a Q600 synchronous thermal analyzer manufactured by TA Instruments (New Castle, DE, USA). The tests were carried out under a nitrogen atmosphere, with the temperature ranging from ambient temperature to 600 °C and a heating rate of 10 °C per minute. The microstructural morphology of the samples was characterized via a SU8010 field-emission scanning electron microscope (FE-SEM, Hitachi Ltd., Toyama, Japan) operated at an accelerating voltage of 1.0 kV. For the test procedure, powder samples were affixed to the sample stage using conductive adhesive, while bulk samples were mounted on a dedicated sample stage with conductive adhesive to facilitate the observation of their microstructures. The flame retardancy grade of the samples was evaluated in accordance with the GB/T 2408-2008 standard (Beijing, China, 2008), employing a UL94-X horizontal and vertical burning tester supplied by Modis China Combustion Technology Co., Ltd., Nanjing, China. The samples used for this test had dimensions of 127 mm × 12.7 mm × 2.7 mm. The limiting oxygen index of the samples was determined based on the GB/T 2406-2009 standard, using an HC-2C oxygen index tester produced by Nanjing Shangyuan Analytical Instrument Co., Ltd., Nanjing, China. The test samples were prepared with a size of 100 mm × 10 mm × 2.7 mm. Based on the GB/T 16172-2007 standard (Beijing, China, 2007), the combustion performance of PA6 and its composite material samples was measured using a cone calorimeter (CC) from FTT in the UK. The sample size was 100 mm × 100 mm × 2.7 mmm and the radiant heat flux was 35 kW/m². The tensile properties of PA6 and its composite materials were evaluated in accordance with the GB/T 1040.2-2006 standard (Beijing, China, 2006), utilizing a UTM5504 electronic universal testing machine manufactured by Shenzhen Xinsansi Technology Co., Ltd., Shenzhen, China. The tests were conducted at a crosshead speed of 2 mm/min, with 6 specimens prepared for each group. The tensile strength of the materials was determined by calculating the average value of the test results. The dimensions of the tensile specimens were specified as 120 mm × 10 mm × 2.7 mm.

3. Results and Discussion

3.1. Characterization of Functionalized Carbon Nanotubes

3.1.1. SEM Images of CNT-KH560 and CNTM

Figure 2 shows microscopic morphology images of (a) coupling-agent-modified carbon nanotubes and (b) carbon nanotubes loaded with MPP measured by SEM. The carbon nanotube in Figure 2a had a diameter of about 30 nm and a length of about 30–50 μm. The surface boundary of the individual carbon nanotube was relatively distinct, while the carbon nanotube exhibited a rounded overall morphology. Figure 2b displayed carbon nanotubes loaded with MPP, and the overall morphology of the carbon nanotubes appeared rough, which may be due to the outer layer of the carbon nanotubes being coated with melamine polyphosphate. At the same time, the presence of MPP can reduce the surface energy of carbon nanotubes, thereby enhancing their dispersion and reducing the presence of agglomerates between carbon nanotubes, laying the foundation for their uniform dispersion in the polyamide 6 matrix.

3.1.2. Infrared Spectroscopy Analysis

Curves 1, 2, and 3 in Figure 3 represent the infrared absorption curves of the original CNTs, CNTs-KH560 and CNTs-KH560-MPP loaded with flame-retardant MPP. Comparing curves 1 and 2, it can be observed that the characteristic peaks of KH560 modification are present: a C-H stretching vibration absorption peak corresponding to 2995 cm⁻¹ and a small peak at 1059 cm⁻¹ representing the stretching vibration absorption peak of Si-O-C. Comparing curve 2 with curve 3, it can be concluded that curve 3 exhibits a characteristic peak of P-O bond stretching vibration at low wavenumbers of 576 cm⁻¹, a stretching vibration absorption peak of the C=N bond in the MPP molecule at 1680 cm⁻¹, and a stretching vibration absorption peak of the -NH₂ dual peaks at 3111 cm⁻¹. These absorption peaks indicated that curve 3 contains functional groups of MPP molecules. By integrating the absorption peaks of curves 1, 2, and 3, it can be confirmed that the modification strategy was successfully executed, with MPP molecules successfully loaded onto the surface of the carbon nanotubes.

3.1.3. TGA Analysis

Figure 4 exhibits the thermogravimetric curves of CNTs and modified carbon nanotubes measured by a synchronous thermal analyzer from room temperature to 700 °C. The curves 1, 2, 3, and 4 represent the thermogravimetric test curves of the original carbon nanotubes (CNTs), acidified carbon nanotubes (CNTs), coupling agent-modified carbon nanotubes CNTs-KH560, and carbon nanotubes CNTs-KH560-MPP loaded with flame-retardant MPP, respectively. It can be clearly observed that line 1 and line 2 basically overlap, and the weight loss curves of the original carbon nanotubes and the acidified carbon nanotubes show similar thermal stability within the tested temperature range.

The weight loss of curve 2 is slightly higher than that of curve 1, which may be due to the loss of hydroxyl groups on the surface of the acidified carbon nanotubes. At the same time, it can be observed that line 3 starts to accelerate and decreases at around 200 °C and approaches a plateau at 600 °C. Compared with line 1, it can be analyzed that the thermal decomposition of CNT-KH560 particles began at 200 °C and basically ended at 600 °C, resulting in a weight loss rate of 6.5% for the grafted CNT-KH560 particles. This indicates that the grafting degree of KH560 on the surface of carbon nanotubes in the grafted CNTs-KH560 particles was 6.5 g/100 g. Next, it is observed that curve 4 also begins to decline at 200 °C, sharply declines around 300 °C to 400 °C, began to steadily decline at 400 °C, and approaches stability around 600 °C. Through a comparison of curve 1 and curve 3, it can be inferred that the weight loss rate of the flame-retardant MPP-loaded carbon nanotube (CNT-KH560-MPP) is 15.6%, which suggests that the grafting degree of MPP onto the surface of the carbon nanotubes reaches 9.1 g/100 g.

3.2. Combustion Performance of Composites

The cone calorimeter is currently recognized as one of the effective laboratory-scale tools for characterizing the combustion performance parameters of materials under simulated real-fire conditions [28,29,30]. In this study, the flame-retardant properties of PA6 composites were investigated in detail, with a specific focus on energy variation, smoke generation, and mass loss rate during the combustion process.

3.2.1. Study from the Heat Release

Figure 5 presents the variation curves of total heat release (THR) and heat release rate (HRR) with combustion time for neat nylon 6 (PA6), PA6/CNT10 composite, and PA6/CNTM10 composite. As illustrated in Figure 5a, the peak heat release rates (PHRR) of neat PA6, PA6/CNT10, and PA6/CNTM10 were determined to be 750 kW/m², 579 kW/m², and 614 kW/m², respectively. Compared with neat PA6, the PHRR values of PA6/CNT10 and PA6/CNTM10 decreased by 23% and 18%, respectively. This finding demonstrates that the incorporation of carbon nanotubes (whether modified or unmodified) can reduce the PHRR of PA6 composites during combustion, exerting a positive effect on slowing down and delaying the combustion process of the composites. The flame-retardant MPP chemically bonds to the surface of modified carbon nanotubes (CNTM) and decomposes into non-combustible gases (e.g., nitrogen, ammonia, and carbon dioxide) when heated, which dilutes the oxygen concentration on the surface of the combustible material and reduces its flammability. Meanwhile, the polyphosphoric acid generated from MPP decomposition promotes the dehydration and carbonization of the PA6 matrix, forming a dense carbon layer that inhibits the continuous combustion of the combustible material. Additionally, carbon nanotubes (both modified and unmodified) can cover the surface of the combustible material, preventing the flame from further igniting the interior of the composite.

As shown in Figure 5b, the THR curves of neat nylon 6, PA6/CNT10, and PA6/CNTM10 were nearly consistent overall. Owing to the presence of carbon nanotubes, the regularity of nylon 6 molecular chain segments was disrupted, leading to a higher THR for the composites compared to neat nylon 6 within the first 200 s of combustion—with the THR of all three materials reaching 68 MJ/m² at this time point. It can also be observed from Figure 5b that within the initial 200 s of combustion, the THR of PA6/CNTM10 was higher than that of both PA6/CNT10 and neat PA6. This phenomenon may be attributed to the fact that the introduction of fillers into the neat nylon 6 matrix disrupts the aggregated morphology of the original resin matrix segments, thereby influencing the combustion stability of the resin material. After 200 s, the THR curves of the three materials almost overlapped; only at the final stage of combustion does the THR of PA6/CNT10 and PA6/CNTM10 become slightly lower than that of neat nylon 6.

3.2.2. Study in Terms of Smoke Production

Figure 6 depicts the variation curves of smoke production rate (SPR) and total smoke production (TSP) of neat nylon 6, PA6/CNT10 composite and PA6/CNTM10 composite as a function of combustion time. As presented in Figure 6a, the peak smoke production rate of neat nylon 6 was 0.041 m²/s, which was significantly higher than that of the two composite materials. At a filler content of 10 wt%, the peak smoke production rate values of PA6/CNT10 and PA6/CNTM10 were determined to be 0.043 m²/s and 0.035 m²/s, respectively. Notably, the peak smoke production rate of PA6/CNTM10 (modified carbon nanotube composite) was lower than that of neat nylon 6, whereas the peak smoke production rate of PA6/CNT10 (unmodified carbon nanotube composite) was slightly higher than that of neat nylon 6. These results illustrate that fillers grafted with the flame-retardant MPP exhibit superior performance in reducing the peak smoke production rate compared to unmodified fillers.

As observed in Figure 6b, the TSP values of PA6/CNT10 and PA6/CNTM10 composites were 2.50 m³ and 2.57 m³, respectively—both higher than the TSP of neat PA6 (1.39 m³). In terms of smoke generation, both composites emitted a substantial amount of smoke during combustion, which was unfavorable to their overall flame-retardant performance. Nevertheless, these data also confirmed that the incorporation of carbon nanotubes (whether unmodified or modified) can indeed inhibit the complete combustion of PA6, ultimately resulting in excessive smoke production during the combustion of the composites. This phenomenon can be attributed to the relatively low loading of the flame retardant. During the combustion process, the flame retardant decomposes preferentially and releases a large amount of non-flammable gas, which exerts a gas-phase flame-retardant effect. Meanwhile, owing to the insufficient addition level, the flame retardant cannot effectively suppress the combustion of the composite matrix. As a result, the composite undergoes incomplete combustion, leading to a significant increase in smoke release. Consequently, the experimental strategy for the surface modification of carbon nanotubes loaded with flame retardants requires further exploration and optimization.

3.2.3. Study from the Mass Loss Rate

Figure 7 shows the variation curves of mass loss rate (MLR) during combustion for neat PA6, PA6/CNT10 composites, and PA6/CNTM10 composite. As can be observed from the graph, the peak mass loss rates of PA6/CNT10 and PA6/CNTM10 were 0.23 g/s and 0.22 g/s, respectively—both lower than the PMLR of neat PA6 (0.28 g/s). This indicates that the incorporation of carbon nanotubes (either unmodified or modified) can reduce the mass loss rate of PA6. Meanwhile, the addition of carbon nanotubes also resulted in a sharp surge in the MLR of PA6/CNT10 and PA6/CNTM10 at 50 s, whereas the MLR of neat PA6 only started to increase rapidly at 90 s. This phenomenon suggests that the introduction of fillers may diminish the regularity of PA6 molecular segments, thereby leading to the degradation of the performance of PA6 composites.

In summary, the cone calorimeter was used to analyze the flame-retardant properties of PA6 composites from multiple aspects, including heat release rate, total heat release, total smoke production and smoke production rate, as well as mass loss rate. Compared with the PHRR of pure PA6, the values of the PA6/CNTM10 composites decreased by 18%, the peak MLR of the PA6/CNTM10 composites was reduced from 0.28 g/s of pure PA6 to 0.22 g/s, and the data have been reduced by 21%. These results showed that the addition of carbon nanotubes modified by flame-retardant MPP grafting can effectively suppress the combustion of nylon 6 composites, but the ideal flame-retardant effect had not yet been achieved. This experimental plan needs further improvement and refinement.

3.2.4. Carbon Residue Diagram and SEM of PA6 and Composites

Figure 8a–c show digital photos of residual carbon after the combustion of neat PA6, PA6/CNT composites, and PA6/CNTs, respectively. From the figure, it can be seen that the pure PA6 had a small amount of ash residue after irradiation combustion, the composite material PA6/CNT10 left a thin film after irradiation combustion, and the composite material PA6/CNTM10 left a shell-like wave layer after irradiation combustion. Compared with Figure 8b,c, there were indeed more residues retained after adding flame-retardant fillers, and there was also a significant difference in the thickness of the residue shell.

Figure 8d,e correspond to the scanning electron microscopy (SEM) micrographs of the residual char from the radiant combustion of the PA6/CNT10 and PA6/CNTM10 composites, respectively. Due to the addition of 10 wt% carbon nanotubes in both the PA6/CNT10 and PA6/CNTM10 composites, almost all of them were residual carbon nanotubes after irradiation combustion, and there was almost no difference between the two SEM micrographs. The residual carbon nanotube skeleton in Figure 8e appeared to be more detailed and compact than the skeleton of carbon nanotubes in Figure 8d. Meanwhile, some layers can also be observed in Figure 8e, which may be due to the presence of flame-retardant MPP causing incomplete combustion of polyamide 6 and leaving residual carbon.

3.3. Flame-Retardant Performance of Composites

Table 1. Limiting oxygen index (LOI) and flame-retardant grade of neat PA6 and its composites.

Samples	PA6	PA6/CNTM5	PA6/CNTM10	PA6/CNTM15	PA6/CNT15
LOI (vol%)	24.5	27.3	29.1	31.4	28.2
UL-94 rating	NR	NR	V-2	V-1	V-2

presents the LOI test results and UL-94 flame-retardant rating of neat PA6 and its composites. The experimental data indicated that neat PA6 had an LOI of 24.5%, classifying it as a combustible material. With the gradual increase in the amount of flame-retardant-modified carbon nanotubes (CNTM), the LOI of the composites continued to rise, increasing from 24.5% (neat PA6) to 31.4% (PA6/CNTM15), thus meeting the flame-retardant material standard. Meanwhile, the UL-94 flame-retardant rating of the PA6/CNTM composites improved progressively with the increase in CNTM content. When the CNTM content reached 10 wt%, the composite achieved a V-2 rating; upon increasing the CNTM content to 15 wt%, the PA6/CNTM15 composite attained a V-1 rating. For the comparative group, the PA6/CNT15 composite (incorporating unmodified carbon nanotubes) exhibited an LOI of 28.2%, slightly higher than that of neat PA6 (24.5%), and its UL-94 flame-retardant rating was V-2. These results confirm that carbon nanotubes themselves possess a certain flame-retardant effect and further validate the feasibility of the flame-retardant design scheme for nylon composites proposed in this study.

3.4. Tensile Properties of Composites

Figure 9 presents the tensile strength (a) and tensile modulus (b) of neat PA6 and its composites (PA6/CNT and PA6/CNTM). Analysis of Figure 9a reveals that the tensile strength of PA6/CNT reached its maximum value when the filler content was 5 wt%. However, as the filler content continued to increase, the tensile strength of PA6/CNT began to decrease progressively, indicating that only the addition of an appropriate amount of filler can effectively enhance the mechanical properties of the composite. For the PA6/CNTM composites, their tensile strength decreased continuously with the increase in CNTM content, suggesting that the incorporation of CNTM filler was not conducive to improving the tensile strength of PA6. Although the variation trends of tensile strength for PA6/CNT and PA6/CNTM composites were nearly identical, the tensile strength of PA6/CNT was slightly higher than that of PA6/CNTM. Thus, the introduction of flame-retardant MPP still reduced the mechanical properties of PA6 composites. As observed in Figure 9b, with the increasing filler content, the tensile modulus of both PA6/CNT and PA6/CNTM composites continued to rise, demonstrating that the addition of carbon nanotube fillers was beneficial for improving the tensile modulus of nylon 6 composites.

Traditional flame-retardant PA6 composites are typically prepared by directly blending melamine salt flame retardants into the nylon matrix. However, this method inevitably impairs the mechanical properties of the flame-retardant PA6 composites. In this study, melamine salt flame retardants were loaded onto the surface of carbon nanotubes via a chemical modification approach. While exerting the flame-retardant effect, this method aimed to minimize the impact on the mechanical properties of the flame-retardant composites. As reflected by the tensile test data of PA6 composites in Figure 9, the flame-retardant strategy proposed in this study can maintain the high mechanical properties of PA6 composites while ensuring their flame-retardant performance. Additionally, its simple preparation process is suitable for large-scale industrial production, providing a new feasible solution for the preparation and application of high-performance flame-retardant PA6 composites.

4. Conclusions

In this study, a facile coupling agent modification method was used to load melamine polyphosphate (MPP) onto carbon nanotubes (CNTs) to obtain modified CNTs (CNTM). PA6 chips and CNTM were blended and melt-extruded via a twin-screw extruder to prepare composite chips, which were then hot-pressed into PA6/CNTM composite samples. Cone calorimeter test results showed that the peak heat release rate (PHRR) of neat PA6 was 750 kW/m², while that of PA6/CNTM10 composite (10 wt% CNTM) decreased to 614 kW/m², a reduction of 18%, confirming the flame-retardant synergism between CNTM and PA6. Limiting oxygen index (LOI) tests showed that the PA6/CNTM15 composite (15 wt% CNTM) had an LOI of 31.4 vol%. UL-94 vertical burning tests indicated it achieved a V-1 flame-retardant rating, verifying the flame-retardant system’s feasibility. Mechanical tests revealed that at 10 wt% CNTM loading, the tensile strength and modulus of PA6/CNTM10 remained higher than those of neat PA6, avoiding significant mechanical deterioration. These results confirm the success of the designed flame-retardant scheme, laying a solid foundation for expanding the application and market prospects of flame-retardant PA6 composites. These composites will provide new options for flame-retardant nylon products in the field of electronics and electrical appliances.