Themoxidative stability and char formation mechanism for the introduction of CNTs and MoS₂ into halogen-free flame retarding TPEE

Abstract

In this study, we give an insight into the char formation mechanism for the addition of CNTs and MoS₂ into halogen-free flame retarding thermoplastic poly(ether ester) elastomers (TPEE). We used real-time Fourier transform infrared spectroscopy (FTIR) to analyze the change in the characteristic peaks of the formulations during the themoxidative process. Thermoxidative stability of the samples has been investigated by thermogravimetric analysis (TGA) under air atmosphere at a heating rate of 10 °C min⁻¹. The catalyzation of carbon nanotubes (CNTs) during the thermo-oxidative process has been demonstrated. The pyrolysis products for different formulations have been studied by pyrolysis/gas chromatography/mass spectroscopy (pyrolysis/GC/MS). Variations in the composition of volatile decomposition products (Mn < 100 g mol⁻¹) have been discussed. The morphology and the graphite degree of the residues remaining at high the temperature have been studied by scanning electron microscopy (SEM) and Raman spectroscopy, respectively. We found that the Mo element can effectively catalyze the char forming process through a cycloaddition interaction of volatile decomposition products. TPEE/P–N/CNTs/MoS₂ exhibited the best stable-char structure for the combination of the bone structure of CNTs and the cycloaddition reaction of the pyrolysis gas products with low molecular weight.

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1. Introduction

Thermoplastic poly(ether ester) elastomer (TPEE) is a potential polymer material that can replace natural and synthetic rubbers.¹ With the combination of advantages including the flexibility of rubber, the strength of plastics, and the processability of thermoplastics, TPEE has been widely used for various applications.² The development of multifunctional TPEE with excellent performances have been a hotspot research topic. Considerable efforts have been taken to realize this goal such as the incorporation of functional groups into the main chain of TPEE and^3–6 the preparation of TPEE alloys and TPEE nanocomposites.^7–9 Moreover, TPEE can be easily ignited and it burns with serious melt dripping behavior and the release of combustible decomposition products,¹⁰ which limit its application in many fields. Enhancement of the flame-retarding performance for meeting the requirements of industrial application is urgently needed. The formation of compact char during the burning process has proven to be an effective way to enhance the fire retardancy of polymers.^11–13 In recent years, the incorporation of nano-fillers with a high aspect ratio can obviously enhance the flame retardancy of polymers on account of the formation of a high-quality char during the burning process.^14,15

Carbon nanotubes (CNTs) have attracted considerable interest due to their unique structure, electronic, thermal and mechanical properties,¹⁶ and thus have been used in various applications, including drug delivery,^17,18 chemical sensing,^19,20 electronic sensing^21,22 and as reinforced fillers for polymers.^23,24 CNTs have been a promising candidate for enhancing the flame retardancy of polymer materials since 2002.²⁵ Published studies have shown that the introduction of CNTs into the polymer can effectively improve the thermal stability and flame retardancy of composites because it can provide a barrier effect for the matrix during the heating process.^26–29 Furthermore, the combination of CNTs and other nano-additives exhibit a synergistic effect in char formation at high temperatures.^30–33

Moreover, in recent years, molybdenum disulfide (MoS₂), a type of two-dimensional (2D) layered material, has attracted increasing attention for various applications, such as in transistors,³⁴ catalysts,³⁵ and composites,^36,37 due to its intrinsic structure and unique performances. MoS₂ is called as white graphite because of its structure similar to that of graphite. Different layers were associated by van der Waals forces.^38–40 Recently, MoS₂ was incorporated into polymer materials to enhance their fire resistance and thermal stability.^41–44

The dominating synergistic flame-retarding mechanism of CNTs and MoS₂ involves the formation of a three-dimensional (3D) network structure in the matrix that can provide a physical nano-barrier effect and act as framework structure during the burning process, resulting in an enhancement of the thermal stability and flame resistance of the nanocomposites. Nano-fillers play the physical role during the char formation process.

However, the chemical interaction of CNTs and MoS₂ in the char formation process has not yet been reported. The aim of this study is to give an insight into the thermoxidative decomposition and the char forming mechanism for the combination of CNTs and MoS₂ introduced into the halogen-free flame-retarding TPEE. MoS₂ can eliminate the negative effect of CNTs on the chemical char forming process. The new flame retarding mechanism would provide guidance in developing the next generation of binary synergistic systems.

2. Experimental

2.1 Materials

TPEE (H3303) was provided by Sunplas (China). P–N flame retardants consisting of aluminum diethylphosphinate (AlPi) and melamine polyphosphate (MPP) were provided by Clariant (Germany) with the mass ratio of AlPi to MPP of 2 : 1. Carbon nanotubes (CNTs, with purity > 95%, length: 0.5–40 μm, diameter: 10–30 nm, Ni = ∼2.4%, Fe = ∼0.4%) and molybdenum disulfide (MoS₂, AP, particle size = ∼3 μm, density: 5.06 g ml⁻¹) were purchased from Shenzhen Nanotech Port Co., LTD and Sigma-Aldrich, respectively. CNTs and MoS₂ were used as received without any chemical treatment. Table 1 shows the composition of the different formulations. All the formulations were melted in a twin-roll mill at 190 °C for 10 minutes with a screw speed of 60 rpm. All the materials were hot-pressed under 12 MPa for 5 min at about 190 °C to obtain a sheet of suitable thickness and size for analysis.

Table 1 Composition of formulations in wt%

	Material	TPEE	P–N	CNTs	MoS2
1	TPEE	100	—	—	—
2	TPEE/P–N	85	15	—	—
3	TPEE/P–N/CNTs	82	15	3	—
4	TPEE/P–N/CNTs/MoS2	81	15	3	1

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2.2 Measurements

2.2.1 Dripping behavior. Samples were measured by UL 94 (ASTM D3801, size of the sample: 130 mm × 13 mm × 1.6 mm).

2.2.2 Residue analysis. High temperature residues were collected from the samples after heating at 800 °C for 10 min. The morphologies of the residues were observed by SEM (S-4800, Hitachi, Japan). The degree of graphitization was calculated from the results of laser Raman spectroscopy (LRS) measurements. Raman spectra were recorded at room temperature on a SPEX-1403 laser Raman spectrometer with a 200 mW argon-ion laser at an excitation wavelength of 514.5 nm.

2.2.3 Thermoxidative stability. Thermogravimetric analysis (TGA) was carried out using a NETZSCH STA 409 PC/PG thermogravimetry analyzer in air atmosphere with a heating rate of 10 °C min⁻¹.

2.2.4 FTIR spectroscopy. The change in the characteristic peaks during the heating process was measured by Fourier transform infrared (FTIR) spectroscopy with a heating device. Samples filmed on a KBr disc were heated from room temperature to 400 °C with a heating rate of 10 °C min⁻¹.

2.2.5 Pyrolysis/GC/MS analysis. To investigate volatile products during the burning test, Formulations 1–4 were analyzed by pyrolysis/gas chromatogram/mass spectrometry (pyrolysis/GC/MS). The samples were pyrolyzed at 600 °C and the gaseous thermal decomposition products were identified by GC/MS (Agilent 6890/5073B). Mass range: 18–500 amu.

3. Result and discussion

3.1 Dripping behavior

The results of UL 94 tests of all the formulations are shown in Table 2. As presented in our previous study, TPEE burned with heavy melt dripping in the UL 94 test. Moreover, the P–N flame retardant could not form the compact char with enough strength that could protect the matrix from preventing the heat and mass transfer.¹⁰ Melt dripping behavior still existed by simply adding the P–N flame retardant into TPEE. The addition of 3 wt% CNTs into P–N flame retarding TPEE also could not provide desirable fire resistance without melt dripping behavior. However, the addition of the combination of CNTs and MoS₂ into TPEE with P–N flame retardants got the V-0 ranking. The significant difference in the UL 94 test between TPEE/P–N/CNTs and TPEE/P–N/CNTs/MoS₂ was highlighted by the result of the dripping behavior. TPEE/P–N/CNTs presented a melt dripping behavior during the first ignition process of the UL 94 tests. TPEE/P–N/CNTs/MoS₂ exhibits the best flame retardancy in terms of anti-dripping behavior. The reason for the different results for the UL 94 test for TPEE/P–N/CNTs and TPEE/P–N/CNTs/MoS₂ will be discussed in the following parts.

Table 2 Results of UL-94 tests for the investigated formulations

	Material	Ranking	Dripping
a The specimen burns out with dripping behavior.b No/yes corresponds to the first/second flame application.
1	TPEE	No rating	—^a
2	TPEE/P–N	V-2	No/yes^b
3	TPEE/P–N/CNTs	V-2	Yes/yes
4	TPEE/P–N/CNTs/MoS	V-0	No/no

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3.2 Thermoxidative decomposition and chain formation mechanism

3.2.1 Real time FTIR. Fig. 1 shows the alternation of intensity and wavenumber of characteristic solid phase peaks in the FTIR spectra at a heating rate of 10 °C min⁻¹ in air atmosphere. All the attributions of wavenumbers for the typical bands are presented in Table 3. The changes in the characteristic bands of TPEE during the decomposition process are presented in Fig. 1(a). The intensities of bands at 2956 and 2861 cm⁻¹ (–CH₂) decreased rapidly at high temperatures mainly resulting from chain scission of the α-methylene group in the soft segment, which has been demonstrated in our previous study.¹⁰ In Fig. 1(b), the characteristic band at 3413 cm⁻¹ is attributed to the action of the intermolecular hydrogen bond of MPP. As the temperature increased, peaks resulting from the release of non-flammable gases, such as ammonium, were not observed at 3300–3600 cm⁻¹. During the heating process, the metal ions interacted with the decomposition products containing carbonyl acid groups. The typical absorption bands of carbonyl (C=O) (1715 cm⁻¹) shifted to lower wavenumbers (1683 cm⁻¹) because of the migration of electrons for interactions with the metal ions in the introduced flame retardants. P–N flame retardants changed into metal polyphosphate salts that formed a brittle char with differently sized holes on the surface of the matrix.¹⁰

Fig. 1 FTIR spectra of solid phase TPEE (a), TPEE/P–N (b), TPEE/P–N/CNTs (c), and TPEE/P–N/CNTs/MoS₂ (d) at different temperatures.

Table 3 Assignment of wavenumbers to characteristic peaks obtained for all the formulations [7,19]

Wavenumbers of characteristic peaks (cm^−1)	Assignment
3413 cm^−1	–OH, MPP
2956 cm^−1, 2861 cm^−1	–CH2, stretching vibration
1715 cm^−1	–C=O, stretching vibration
1458 cm^−1	–CH2, scission vibration
1411 cm^−1	Aromatic ring
1270 cm^−1	–CO–O esters
1100 cm^−1	–CH2–O–CH2– ether
1150 cm^−1, 1079 cm^−1, 780 cm^−1	–P–O
1013 cm^−1	–PO4^3−
727 cm^−1	–CH bending vibration of aromatic ring
531 cm^−1	O–P–O, MPP
474 cm^−1	O=P–O, AlPi

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However, the characteristic band of CNTs and MoS₂ was difficult to be observed because of their low addition amounts and the method of sample preparation for the FTIR test. The changes in the typical peaks for Formulation 3 are shown in Fig. 1(c). With the addition of 3 wt% CNTs into TPEE/P–N composites, TPEE/P–N/CNTs failed the UL 94 tests for melt dripping behavior. The anti-dripping performance of TPEE/P–N/CNTs was even worse than that of TPEE/P–N for the appearance of melt dripping behavior at the first ignition process of UL 94 tests. As is well known, CNTs can effectively increase the viscosity with regards to the formation network structure.⁴⁵

Only the typical band at 1675 cm⁻¹ was attributed to the electric charge on the metal ion to the carboxyl group. There might be two reasons for the change in the peak. The first one is that the metal cation on the surface of the CNTs and the aluminum ion (Al³⁺) acts as a Lewis acid site, providing the basis for a coordinative interaction with the carbonyl group of TPEE. The introduction of CNTs can strengthen the Lewis based acid interaction between the additives and the decomposition products. The other reason is that the barrier effect can decrease the release rate of degradation products. Therefore, the aluminum ion (Al³⁺) can react with decomposition products containing the carbonyl group completely because of the barrier effect provided by the network structure of CNTs. There is another interesting zone at 2800–3000 cm⁻¹. The characteristic peaks of aliphatic groups completely disappear at high temperatures, indicating that the addition of CNTs accelerated the chain scission reaction resulting from the existence of the metal ion on the surface of CNTs; this is in agreement with the composition of CNTs. However, the burning process is a vigorous chemical and physical reaction with the release of heat and volatile gases. The barrier effect provided by the introduction of CNTs can enhance the thermal stability. Moreover, catalyzation by the metal ions on the surface of CNTs can accelerate the chain scission reaction. As a result of the UL 94 tests, we found that the catalyzation by CNTs is the main course of the melt dripping behavior. The introduction of CNTs also cannot result in the formation of compact stable chars to prevent the melt dripping behavior. Then, we moved on to Formulation 4. The alternation of characteristic bands of TPEE/P–N/CNTs/MoS₂ during the heating process is presented in Fig. 1(d). There are two obvious distinctive regions in the spectra in comparison with the spectrum for TPEE/P–N/CNTs. The intensity of the typical bands at 2800–3000 cm⁻¹ of the aliphatic groups at 400 °C, indicating the introduction of MoS₂, revealed that the decomposition rate of aliphatic groups of TPEE resulted from the decomposition catalyzed by the addition of CNTs. The second unique region was at 1600–1750 cm⁻¹.

As presented in Fig. 1(c), the characteristic peak of the carbonyl group completely shifted to 1675 cm⁻¹ because of the interaction between the metal ions and the carboxyl acid. The special structure of TPEE/P–N/CNTs/MoS₂ decreases the decomposition rate during the heating process. Because of the better nano-barrier effect provided by 1-dimensional CNTs and 2-dimensional MoS₂ layers, which can promote the interaction between the additive and the decomposition products of TPEE, the characteristic peak of the carbonyl group should shift to lower wavenumbers. The introduction of MoS₂ can promote char formation and improve the char residue, as reported by other researchers.^41,42

The mechanism has not been demonstrated. If the Mo element might promote char formation through Lewis acid based interactions (Fig. 2(a)), the characteristic peak of the carbonyl group should shift to 1675 cm⁻¹ completely. On the contrary, there are still two typical peaks in the 1630–1750 cm⁻¹ zone, indicating that the char formation process might be changed after the introduction of MoS₂. Double-bonded products undergo cycloaddition interactions due to the catalyzation by the Mo element, improving additional carbonaceous char at high temperatures. The mechanism is shown in Fig. 2(b). Thus, TPEE/P–N/CNTs/MoS₂ has the best stability, which is in agreement with the results from TGA. These results are attributed to the introduction of MoS₂: a better nano-barrier effect for the coexistence of CNTs with MoS₂, and MoS₂ can promote char formation to avoid the catalysis of decomposition for surface impurities on CNTs. The network structure reduces the release rate of volatile gases and can provide more time for the char formation process.

Fig. 2 Mechanism of Mo catalyzation of the char forming process.

3.2.2 Pyrolysis/GC/MS analysis. The thermal decompositions of Formulations 1–4 in the solid phase were investigated by real-time FTIR. Different types of products having carboxylic acid or hydroxyl end groups, such as benzoic acid, tetrahydrofuran and their derivatives, mainly resulted from the degradation step of neat TPEE by the ester-linkage breaking reaction, and are released at high temperatures.¹⁰

This study mainly focused on the change of the volatile products due to the introduction of P–N flame retardants, CNTs and MoS₂. As shown in Fig. 3, the introduction of the additives did not change the decomposition pathway of TPEE, as seen by comparing the peaks in the GC spectra. Characteristic peaks for all the different types of volatile products were analyzed using Wiley's library. Identification of the volatile products is presented in Table 4. Two characteristic peaks (7, 9) were found for the introduction of nitrogen containing flame retardants, which resulted from the gas flame retarding action of P–N flame retardants. The phosphorus containing products demonstrated that the phosphorus containing flame retardants did not exhibit flame retarding activity in the solid phase. The characteristic mass peaks (Mn < 100) and relative peak area of Formulations 1–4 are presented in Table 5. The relative peak areas of low molecular weight products were decreased after the introduction of additives. The value of the relative peak area of benzoic acid in TPEE/P–N/CNTs is 4.9, resulting from the Lewis-based interactions between the benzoic acid and Ni²⁺ on the surface of the CNTs; this is in agreement with the result from FTIR. The values of relative peak areas of 1,3-butadiene in TPEE/P–N/CNTs/MoS₂ was half of that in neat TPEE, which indicated more 1,3-butadiene remained in the solid phase. The combination of the CNTs and MoS₂ effectively decreased the release of 2,3-dihydrofuran and tetrahydrofuran resulting from chain scission of the α-methylene group in the soft segment due to the nano-barrier effect of the 3-dimensional network.

Fig. 3 Gas chromatogram of the gaseous pyrolysis products of TPEE (a), TPEE/P–N (b), TPEE/P–N/CNTs (c), and TPEE/P–N/CNTs/MoS₂ (d) obtained from heating at 600 °C.

Table 4 The pyrolysis products of Formulations 1–4 at 600 °C

No.	Molecular mass (m/z)	Retention time (min)	Compound
1	44	0.785	CO2
2	54	1.602	1,3-Butadiene
3	70	1.865	2,3-Dihydrofuran
4	72	2.037	Tetrahydro-furan
5	72	2.171	3-Buten-1-ol
6	127	4.351	4-Butoxy-1-butene
7	103	6.16	Benzonitrile
8	100	7.514	2,3-Dimethyl pentane
9	147	9.1	4-Cyanobenzoic acid
10	128	9.49	Benzoic acid
11	59	11.1	Guanidine
12	202	11.6	1,4-Butanediol diglycidyl ether
13	164	11.7	Benzenebutanoic acid
14	186	14.1	Heptan-2-yl butanoate
15	170	17.1	3-Butoxy-2,4-dimethyl-1-pentene
16	250	17.4	2-Hexan-3-yloxycarbonylbenzoic acid
17	100	19.2	cis-3-Methylpent-3-ene-5-ol
18	278	19.4	Dibutyl terephthalate
19	286	21.4	2,2,4-Trimethyl-1,3-pentanediol diisobutyrate
20	100	23.7	z-1-Methoxy-2-pentene
21	396	25.1	Bis(2-ethylhexyl) ester
22	206	26.7	3-Hydroxy-2-methyl-1-phenyl-hexan-1-one
23	334	28.8	2-Ethylhexyl-2-methylpropyl phthalate

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Table 5 Characteristic mass peaks (M_n < 100) and relative peak areas of Formulation 1–4

Products	Retention time	1 (%)	2 (%)	3 (%)	4 (%)
1,3-Butadiene	1.602	13.72	11.01	8.14	6.06
2,3-Dihydrofuran	1.865	5.762	2.429	2.152	1.927
Tetrahydrofuran	2.037	17.48	9.315	6.3	6.285
Benzoic acid	9.49	5.42	5.44	4.9	5.207

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The introduction of MoS₂ can decrease the release of volatile product into the gas phase for special nanostructures and the double-bond containing product remained in the condensed phase for the catalyzation of char forming processes. Therefore, TPEE/P–N/CNTs/MoS₂ has the best thermal stability and fire resistance.

3.3 Thermoxidative performance

The thermo-oxidative behaviors of all the formulations were investigated by TGA in air atmosphere at a heating rate of 10 °C min⁻¹; the results are presented in Fig. 4. As shown in the figure, all the samples decomposed by a two-step thermal decomposition process, which means the additives did not change the main decomposition pathway of TPEE. The thermo-oxidative behaviors of TPEE and TPEE/P–N have been demonstrated in our previous study.¹⁰

Fig. 4 Mass loss (a) and mass loss rate (b) curves of samples recorded under air atmosphere at the heating rate of 10 °C min⁻¹.

TPEE had no remaining residues at 700 °C after the complete decomposition process. A special shoulder peak was observed at 300–350 °C, which resulted from chain scission of the α-methylene group of soft segments; this is in agreement with the result from real-time FTIR testing. The temperature of the shoulder peak, the first maximum decomposition temperature and the second maximum temperature all shifted to higher temperatures than those for TPEE. The remaining residues from TPEE/P–N increased to 8.5 wt%. The decomposition products of P–N flame retardant interacted with the decomposition product of TPEE, which effectively decreased the release of volatile products and enhanced the thermal stability and residues at high temperatures.

After the addition of 3 wt% CNTs into P–N flame retarding TPEE, the initial decomposition temperature and the maximum decomposition temperature were higher than that of TPEE/P–N. This resulted from the barrier effect of CNTs. However, the residue of Formulation 3 at the end of the TGA test decreases plainly to 6.81 wt%. Moreover, the mass loss rate obviously increased since the introduction of CNTs. The results suggested that the metal cation on the surface of CNTs catalyze the decomposition process of TPEE during the thermo-oxidative heating process. After the addition of 1 wt% MoS₂ into TPEE/P–N/CNTs, the initial decomposition and the maximum decomposition temperature was shifted to higher temperatures. The residues of Formulation 4 increased to almost twice that of Formulation 3. The most interesting thing is that the mass loss rate decreased obviously by simply adding 1 wt% MoS₂. This result suggests that the combination of CNTs and MoS₂ can enhance the network structure to improve the thermal stability of the nanocomposites for an excellent barrier effect.⁴⁵

Besides, the increase in the amount of remaining residues at high temperature was attributed to the interaction of the pyrolysis products and MoS₂ during the heating process.

3.4 Characterization of residues

The residue after burning the samples in a muffle furnace at 800 °C for 10 min was collected. It was found that no residue remained after high temperature treatment for neat TPEE. This result is in agreement with the results of TGA. Fig. 5 presents the SEM images of the residues from Formulations 2–4. The residue morphology of TPEE/P–N is shown in Fig. 5(a). There are a lot of holes with different sizes present in the image. The inner part of the char has many porous structures, which could be destroyed easily. During the burning process, the release of the volatile products would be through the path of the permeable holes and flaws. These brittle chars cannot protect the materials from melt dripping behavior.

Fig. 5 SEM images of the residues from TPEE/P–N (a), TPEE/P–N/CNTs (b), and TPEE/P–N/CNTs/MoS₂ (c).

The residue morphology of TPEE/P–N/CNTs is presented in Fig. 5(b). We found that there are still different sized pores existing in the image. The char layer appears to be more compact and integrated than TPEE/P–N. The introduction of CNTs into the composites resulted in the formation of a network. The network structure can slow down the release rate of the composites for a barrier effect at low temperatures. At high temperatures, the CNTs can act as a framework structure for building compact chars. Nevertheless, there are still many small holes in the char, and the catalyzation of metal ions on the surface of CNTs results in melt dripping behavior that reached a V-2 ranking in the UL94 test.

The SEM images of the residues from TPEE/P–N/CNTs/MoS₂ are presented in Fig. 5(c). The introduction of MoS₂ has a clear improvement in the quality of the residues. The compact structure can effectively stop the melt dripping behavior because it is strong enough to protect the matrix from the burning process. These results indicated that CNTs and MoS₂ have a good synergistic char formation effect because the formation of better network structures provides an excellent nano-barrier effect during the burning process.⁴¹ The structure can decrease the release of volatile combustible products. Moreover, the CNTs can provide the framework structure and the Mo element can catalyze the char forming process, resulting in a compact and dense char layer on the surface of the matrix.

As is well known, laser Raman spectroscopy has been demonstrated as an easy way to characterize the degree of graphitization of carbonaceous structures after the burning process. As presented in the Fig. 6, there are two bands that exist for Formulation 2–4. The first one is at 1352 cm⁻¹, called as the D band, resulting from the amorphous carbon band, and the second one is at 1600 cm⁻¹, called as the G band, resulting from crystalline graphite. The degree of graphitization of the residual char could be calculated by the ratio of accumulated intensities of the D and G bands (I_D/I_G)⁴² where I_D and I_G are the integrated intensities of the D and G bands, respectively. The residue with the lower ratio of I_D/I_G indicates a higher degree of graphitization and the better compacted char. Residue char layers with a high degree of graphitization can efficiently decrease the rate of releasing volatile products from the integrated structure during the burning process. A compact and stable char can effectively stop heat and mass transfer and thus protect the materials under the char.

Fig. 6 Raman curves of the char residues from TPEE/P–N, TPEE/P–N/CNTs, and TPEE/P–N/CNTs/MoS₂ after calcination at 800 °C for 10 min.

The I_D/I_G ratio follows the order TPEE/P–N/CNTs/MoS₂ (1.76) < TPEE/P–N/CNTs (1.81) < TPEE/P–N (2.38). Formulation 4 has the highest I_D/I_G ratio value, indicating it has the most thermally stable char structure. This result corresponds with the morphology of the char residue. There is not much difference in the I_D/I_G ratio value between Formulation 3 and Formulation 4 because the Mo element can catalyze the char forming process, resulting in the production of more amorphous carbon on the surface than inside the graphited carbon. Adding the CNTs can enhance the graphitization of the char for the network structure and has a benefit for the inner pressure, which is important to the graphitization process. The combination of CNTs and MoS₂ can build a new network structure that can improve the degree of graphitization. Moreover, the Mo element catalyzed the char forming process. The graphited carbon was filled into the pores of the network structure and a large amount of amorphous carbon was dispersed on the surface of the layer, which effectively prevented the melt dripping behavior.

4. Conclusion

In this study, the melt dripping behavior disappeared after adding a combination of CNTs and MoS₂ into P–N flame retarding TPEE during the burning process. The TGA results suggested that the addition of CNTs obviously decreased the weight of char residues at high temperatures. Otherwise, the introduction of 1 wt% MoS₂ effectively increased the char residue. FTIR indicated that the metal ions on the surface of the CNTs can catalyze the decomposition process of TPEE, while the Mo element can catalyze the cycloaddition interaction of double-bonded product, which can promote additional carbonaceous char at high temperatures. SEM images and Raman spectra showed that a compact char was formed through the introduction of the combination of CNTs and MoS₂ into P–N flame retarding TPEE. During the burning process, the Mo element catalyzes the cycloaddition process of volatile decomposition products, which forms amorphous carbon to cover the surface of CNTs. TPEE/P–N/CNTs/MoS₂ can effectively build the network to decrease the decomposition rate and the addition of MoS₂ can eliminate harmful effects resulting from the introduction of CNTs. CNTs and MoS₂ had a synergistic effect on the char forming process and anti-dripping behavior.

Acknowledgements

Financial support from the East China University of Science and Technology is gratefully acknowledged.

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