Synergistic effects of BHDB-IPC with AlPi/MCA on flame retarding TPEE

Abstract

The deoxybenzoin-based copolyarylate BHDB-IPC with perfect charring ability was synthesized. The synergistic effects of BHDB-IPC as a charring agent in TPEE flame retarded by the phosphorous–nitrogen flame retardant (P–N FR) based on AlPi/MCA were studied using the limiting oxygen index (LOI), UL94 test, thermogravimetric analysis (TGA), temperature dependent FTIR, laser Raman spectroscopy (LRS), and scanning electron microscopy (SEM). The UL94 test showed that with BHDB-IPC added, TPEE/P–N FR systems had a synergistic FR effect in condensed phase with AlPi. The TGA data illustrated that BHDB-IPC increased the thermal stability of the TPEE/P–N FR systems and especially the residues at high temperature (T > 700 °C). The synergistic mechanisms between AlPi and BHDB-IPC were analyzed from the temperature dependent FTIR. The residues were analyzed with LRS and SEM, which could provide useful information on the carbonaceous microstructures and the morphological structures. It was demonstrated that BHDB-IPC promoted the formation of stable and compact carbonaceous char, which could prevent the melt dripping and improve the flame retardancy. Thus, BHDB-IPC was a promising synergist for the P–N flame retardant in TPEE.

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1. Introduction

Thermoplastic polyether ester elastomer (TPEE) consisting of hard segments (PBT chains) and soft segments (polyether chains) has the properties of rubber-flexibility, high elasticity and reversible deformation. Moreover, TPEE can be processed and modified using the thermoplastic resin processes and modification methods. Due to these excellent properties, in some areas, TPEE has replaced rubbers currently with more broad applications.¹ However, like most polymer materials, conventional TPEE composites are easy to burn with flaming dripping, which may limit the application of TPEE. Traditionally, TPEE has been mainly made flame retarded by adding halogen flame retardants, such as decabromodiphenyl ethane synchronized with antimony trioxide (DBDPE/Sb₂O₃);^2,3 however, during combustion, halogen flame retardants almost decompose and release dioxin, which endangers the safety of humans as well as the environment.^4,5 Thus, such halogen-containing flame retardants have been banned and are gradually being replaced.

Intumescent flame retardant (IFR) is fast developing as a novel type of non-halogen flame retardant with phosphorus and nitrogen as the main effective ingredients containing acid, gas and carbon sources simultaneously. To achieve the most ideal flame retardant effect, synergistic agents may be necessary.^6,7 Few researchers have reported that phosphorous–nitrogen flame retardants with synergistic agents, such as OMMT,⁸ novolac,⁹ CNTs or MoS₂,¹⁰ can be effective on flame retarding TPEE. Highly efficient and stable carbon agents are of great significance to improve the efficiency of IFRs, and thus research on carbon agents is becoming a hotspot in flame retardants.^11,12 Ranganathan^13,14 reported that deoxybenzoin-based copolymers can be used as highly efficient charring agents for ketone units as deoxybenzoin easily dehydrates into diphenylacetylene and subsequently forms phenanthrene polymers through cyclization. Some deoxybenzoin-based copolymers have been proved to be excellent charring agents from the TGA test with low heat release in the cone test.¹⁵ However, no research has reported the application of deoxybenzoin-based copolymers in thermoplastic, let alone TPEE.

In this research, the copolyarylate BHDB-IPC was synthesized from 4,4′-dihydroxy acetophenone (BHDB) and isophthaloyl chloride (IPC) by a solution polymerization process. Finally, the halogen-free flame retarded TPEE material that reached UL94 V-0 was successfully prepared by mixing BHDB-IPC, phosphorus–nitrogen flame retardant and TPEE. The fire behavior, the thermogravimetric behavior and the structure of residue chars were studied via LOI, UL94 test, TGA, LRS and SEM.

2. Experiment

2.1 Materials

Commercial TPEE (H2040, M_w = 45 000 g mol⁻¹, T_m = 190 °C) was purchased from Sunplas Company (China). Deoxybenzoin and pyridine hydrochloride were purchased from Sigma-Aldrich. Benzyltriethylammonium chloride and isophthaloyl chloride (IPC) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Aluminum diethylphosphinate (AlPi, OP935) was purchased from Clariant (Germany). Melamine cyanurate (MCA) was supplied by Xusen Company (Shanghai, China). The P–N flame retardant was pre-mixed from AlPi and MCA (5 : 1 w/w). TPEE and MCA were dried in an oven at 90 °C for 5 h before compounding. Others were used as received.

2.2 Synthesis of BHDB and BHDB-IPC

BHDB and BHDB-IPC were synthesized via a two-step reaction presented in Scheme 1 using a previously reported procedure.^13,14 The weight of carbon residue of BHDB-IPC at 800 °C can reach 43 wt% after the thermogravimetric analysis (TGA) test.

Scheme 1 Schematic of the synthesis of BHDB and BHDB-IPC.

2.3 Sample preparation

According to many experiments and our previous studies, the best flame retardancy for TPEE/P–N can be obtained when the mass ratio of phosphorous to nitrogen is kept at 5 : 1.⁹ OP935 and MCA were blended first at the mass ratio of 5 : 1 to obtain the phosphorus–nitrogen flame retardant (P–N FR). Based on numerous previous experiments, the total content of flame retardants in TPEE composites was kept at 18%, as shown in Table 1. Based on the formulations in Table 1, TPEE was melt-mixed with the other components in a torque rheometer (RM-200A, Harbin Harpro Electrical Technology Co. Ltd.) at 200 °C for 10 min. After mixing, all the samples were hot pressed under 12 MPa for 5 min at 205 °C to obtain suitable test specimens for analysis.

Table 1 Composition of formulations (wt%)

Formulation	TPEE	P–N	BHDB-IPC
1	100	0	0
2	82.0	18.0	0
3	82.0	16.5	1.5
4	82.0	15.0	3.0

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2.4 Characterization

Flammability. The flammability of the samples was determined by limiting oxygen index (LOI) on an HC-2 oxygen index meter (Jiangning, China) according to ASTM D2863 (specimen size: 130 mm × 6 mm × 3 mm) and using the vertical burning test (UL 94) on a CFZ-2 type instrument (Jiangning, China) according to ASTM D3801 (specimen size: 130 mm × 13 mm × 1.6 mm).

Thermogravimetric analysis. TGA was performed using a STA409 PC/PG (Netzsch, Germany) thermogravimetric analyzer with 5–8 mg samples under a N₂ flow rate of 20 ml min⁻¹ at a heating rate of 10 °C min⁻¹ from room temperature to 700 °C.

Real time FTIR spectra. The real time FTIR spectra were recorded using a Nicolet 5700 FTIR instrument equipped with a heating device having a temperature controller. The film samples were made by melting sample powders on a KBr disc and then placed in a ventilated oven at a heating rate of 10 °C min⁻¹ from room temperature to 400 °C for the dynamic measurement of FTIR in situ during the thermo-oxidative degradation.

Laser Raman spectroscopy. The LRS measurements were carried out at room temperature with a SPEX-1403 laser Raman spectrometer, with excitation provided in backscattering geometry by a 514.5 nm argon laser line focused at a micrometer spot on the sample surface.

Scanning electron microscopy. SEM was used to investigate the surface of char residues of the flame-retardant TPEE after the composites were burnt at 800 °C for 15 min in a muffle furnace to introduce the thermo-oxidation char during combustion.

3. Results and discussion

3.1 Effects of BHDB-IPC on LOI and UL94 values of TPEE/P–N blends

To investigate the flame retardancy of the flame-retardant TPEE samples, the LOI and vertical burning ratings (UL 94) of these samples were tested. The results are given in Table 2 for all the investigated samples.

Table 2 Results of LOI and UL94 testing for investigated samples

NO	Sample	LOI (%)	UL94, 1.6 mm bar
			t1/t2^a (s)	Dripping	Rating
a t1 and t2, average combustion time after the first and second applications of flame.b BC burns to clamp.c NR not rated.d No/yes corresponds to the first/second flame application.
1	TPEE	17.5 ± 0.5	BC^b	Yes/—	NR^c
2	TPEE/18P–N	29.0 ± 0.5	1.43/5.49	No/yes^d	V-2
3	TPEE/16.5P–N/1.5BHDB-IPC	28.9 ± 0.5	1.40/3.60	No/yes	V-2
4	TPEE/15.0P–N/3.0BHDB-IPC	28.2 ± 0.5	1.34/2.25	No/no	V-0

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The neat TPEE is easy to burn in air, which is shown by the LOI value of 17.5%. The LOI value of flame retarded TPEE with the loading of 18 wt% P–N FR composed of OP935 and MCA can improve greatly and reach 29.0%. According to the literature and our previous studies, the results of LOI testing are mainly related to the gas-phase flame retardant.¹⁶ Thanks to the non-flammable gas decomposing from the melamine cyanurate (MCA),¹⁷ the LOI values of all the P–N flame retarded TPEE samples increase to about 29.0%.

However, with the addition of extra BHDB-IPC, which functions mainly in the condensed phase, the flame retarded samples did not show significant changes in LOI values, whereas the results of the UL94 testing improved. Neat TPEE was easy to burn to the clamp with serious flaming dripping in the UL94 test. In comparison, with the loading of 18 wt% P–N FR, the combustion time after flame decreased to 1.43 s/5.49 s, as aluminum diethyl phosphinate and melamine cyanurate can be effective FRs for most polyester resins.¹⁸ However, without flame retardant synergistic agents, especially charring agents, the TPEE/18P–N FR sample dripped after the second flame application and the UL94 rating can only reach V-2, as the charred layers after burning was loose. After 1.5 wt% BHDB-IPC was added to the TPEE/16.5P–N sample, the combustion time was shortened and the carbonaceous char was enhanced to some extent but the dripping could not be inhibited completely. Only when the loading of BHDB-IPC was 3 wt%, the UL94 rating was enhanced to V-0 from V-2 with the decreased combustion and inhibited dripping. During combustion, BHDB-IPC acts as charring agents, which could provide a compact and dense protective char at the surface to prevent the flame dripping, which would be in accordance with the SEM observation.

3.2 Thermogravimetric behavior

To better understand the thermal stability of different formulations, the thermal stability of neat TPEE and flame-retard TPEE composites were analyzed by thermogravimetric analysis (TGA) under nitrogen at a heating rate of 10 °C min⁻¹. The mass loss and the mass loss rate curves are presented in Fig. 1 and the related data are summarized in Table 3.

Fig. 1 Mass and mass loss rate curves of all samples under nitrogen atmosphere, heating rate 10 °C min⁻¹.

Table 3 Results of thermogravimetric analysis^a

	Sample	T5% (°C)	Tonset (°C)	Tmax (°C)	dw/dt (max)	Residue (wt%)
					(wt% min^−1)	Experiment	Theory
a Note: T5% is the temperature of 5 wt% mass loss; Tonset is the temperature of 10 wt% loss; Tmax is the temperature of the maximum mass loss; dw/dt (max) is the maximum mass loss rate; residue is the weight of the residue at 700 °C after TGA test.
a	BHDB-IPC	395.55	445.21	499.57	−0.425	45.56	—
b	TPEE	390.83	401.36	425.47	−2.64	1.67	—
c	TPEE/18P–N	376.62	389.47	415.74	−2.08	7.20	—
d	TPEE/16.5P–N/1.5BHDB-IPC	374.31	388.52	414.62	−2.02	11.80	7.78
e	TPEE/15.0P–N/3.0BHDB-IPC	370.44	387.75	410.96	−1.84	16.05	8.35

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As can be seen in Table 3, the mass loss of BHDB-IPC begins at 445.21 °C and the mass loss is continuous with a broad mass loss rate peak appearing at about 499.57 °C. This can be explained by some dehydration reactions taking place as the aromatization of deoxybenzoin proceeds.¹⁹ The residue of BHDB-IPC at the end of the TGA test is 45.46%.

The decomposition of neat TPEE starts at 401.36 °C with the highest mass loss rate of 2.64 wt% min⁻¹ at 425.47 °C. There are almost no residues at 700 °C after the TGA test as the neat TPEE decomposed completely into gaseous molecules.⁹ For Sample c, with 18 wt% P–N flame retardant added into TPEE, the temperature of 5 wt% mass loss, initial degradation and maximum mass loss decreased by about 10 °C, while the loss rate was reduced and the weight of residue increased to 7.20%. The reason may be that the decomposition temperature of P–N flame retardant is lower than TPEE, but there is a catalytic role played by the aluminum diethyl phosphinate (OP935), which may accelerate the charring process during the degradation.²⁰ No significant changes were detected in the mass loss temperature on comparing Sample c with Samples d and e. Our focus was on the maximum mass loss rate and especially on the residue weight. With the loading of BHDB-IPC, the maximum loss rate decreased as BHDB-IPC may play the role of charring agents to promote the formation of carbon layers as well as enhance the carbon layers as mentioned during the UL94 testing. The carbon layers can slow down the degradation to a certain extent. At 700 °C, the residue amounts of TPEE/16.5P–N/1.5BHDB-IPC and TPEE/15.0P–N/3.0BHDB-IPC were 11.80% and 16.05%, respectively, which exceeded the calculated values and demonstrated that BHDB-IPC could strengthen the action of the condensed phase. All the results show that a strong interaction between the BHDB-IPC and P–N flame retardant system occurred during the thermal degradation process and thus improved the flame retardant properties of TPEE composites.

3.3 Changes of dynamic FTIR spectra with the pyrolysis temperature

Fig. 2 presents the changes of dynamic FTIR spectra obtained from the thermo-oxidative degradation of the BHDB-IPC, TPEE/18P–N and TPEE/15.0P–N/3.0BHDB-IPC samples, respectively, in the condensed phase with increasing pyrolysis temperature. The flame retardant mechanism of AlPi in TPEE has been studied previously.⁹ Herein, more attention was focused on the synergistic effects between BHDB-IPC and AlPi.

Fig. 2 Dynamic FTIR spectra at different pyrolysis temperatures (a) BHDB-IPC; (b) TPEE/18.0P–N; (c) TPEE/15.0P–N/3.0BHDB-IPC.

The most interesting region in Fig. 2 is located at 1600–1700 cm⁻¹. The peak in Fig. 2a and c at about 1660 cm⁻¹ has been assigned to the various –C=O– thermo-oxidation products, which can be linked to the thermo-oxidative degradation of BHDB-IPC.

For the pure BHDB-IPC, the peak at 1660 cm⁻¹ shows no significant changes until 390 °C. However, for the TPEE/18P–N/3.0BHDB-IPC blends, the decrease of the peak at 1660 cm⁻¹ begins at 350 °C; while compared with Fig. 2b, the peaks at 1150 cm⁻¹ increase, which can be assigned to the stretching mode of P–O–C in the intermediates. Based on these changes, the principle decomposition pathways for BHDB-IPC are illustrated in Fig. 3.

Fig. 3 Main decomposition mode of BHDB-IPC.

As can be seen in Fig. 3, with the nucleophilic reaction between diethylphosphinic acid, decomposing from AlPi, and carbonyls in the BHDB-IPC, the dehydration reaction of BHDB-IPC was accelerated and then the carbon layers formed faster at lower temperature. The sooner and faster the formation of carbon layers is, the weight loss will be reduced to some extent, which corresponds to the TGA results. In general, the synergistic effects can be attributed to the catalytic effects of AlPi on BHDB-IPC.

3.4 Microstructure characterization of carbonaceous charred layers

Raman scattering is especially sensitive to the structural disorder of graphite. In this work,^21,22 Raman spectroscopy was used to characterize the different types of carbonaceous structure formed in the intumescent char. Fig. 4 presents the laser Raman spectra of the three samples after being burnt in a muffle furnace at 800 °C for 15 min. It can be seen that the three have a common feature of two distinct broad peaks centered at 1580 cm⁻¹ (G peak) and 1340 cm⁻¹ (D peak), which have been assigned to the oriented and the disordered graphite structures.²³ The relative intensity ratio (I_D/I_G) of the D peak to the G peak is inversely proportional to the degree of graphitization of the residual chars.²⁴ That is to say, the lower the I_D/I_G ratio, the better the structure of the chars.

Fig. 4 Laser Raman spectra of the intumescent char residues obtained from the samples after burning in a muffle furnace at 800 °C for 15 min (a) TPEE/18P–N; (b) TPEE/16.5P–N/1.5BHDB-IPC; (c) TPEE/15.0P–N/3.0BHDB-IPC.

According to Fig. 4, the incorporation of 1.5 wt% BHDB-IPC into TPEE/16.5P–N decreases the I_D/I_G ratio from 2.47 to 2.25, which suggests an increase in the graphitization degree in the residual char. Furthermore, as the addition of BHDB-IPC reaches 3.0 wt%, the I_D/I_G ratio decreases from 2.25 to 2.02, which show the highest graphitization degree and the most thermally stable char structure of the TPEE/15.0P–N/3.0BHDB-IPC sample. The results of the laser Raman spectra indicated that the BHDB-IPC additives could promote the formation of more compact char layers, which is in good agreement with the TGA and UL94 results.

3.5 Morphological structures of residual chars

All the samples were burnt in a muffle furnace at 800 °C for 15 min. The neat TPEE burnt out without any residues left after the burning test. The digital images and the SEM images of residues for Formulations 2–4 are shown in Fig. 5 and 6, respectively.

Fig. 5 Digital images of the charred residue for Formulations 1–4 ((1) TPEE; (2) TPEE/18P–N; (3) TPEE/16.5P–N/1.5BHDB–IPC; (4) TPEE/15.0P–N/3.0BHDB-IPC).

Fig. 6 SEM images of the charred residue for Formulations 2–4 at (a) low magnification (500×) and (b) high magnification (2000×).

As can be seen in Fig. 5, the digital images demonstrate that the neat TPEE burns out with almost no residues left; however, there is much residual char for the flame-retardant TPEE composites. The char of 18P–N flame-retardant TPEE (Fig. 5(2)) is not integrated, whereas the char of flame-retardant TPEE with BHDB-IPC is compact and strong, especially as shown in Fig. 5(4), which could be very effective in reducing mass loss as mentioned in the TGA and inhibiting drippings as mentioned in the UL94 test. It means that the incorporation of BHDB-IPC with P–N FR could improve the char residue structures and during combustion, highly effective protective shields could be formed on the surface of char residues.

To further investigate the mechanism of BHDB-IPC on the char formation of flame-retardant TPEE during combustion, the morphologies of the char residues after the furnace-burning test were observed via SEM.

As can be seen in Fig. 6, flame-retardant TPEE composite with 18 wt% P–N FR is covered with discontinuous charred layer with many large cavities mainly due to the gas released from the gas phase flame retardant MCA.¹⁷ With the addition of 1.5 wt% BHDB-IPC, the cavities became smaller, but the char surface was not so strong as to prevent the penetration of gases and a few flaws still exist on the surface. However, the char surface of TPEE/15.0P–N/3.0BHDB-IPC is compact, smooth and tight without any flaws. Based on these results, it is easy to understand the results from the UL94 test and the TGA test.

4. Conclusions

(1) In this study, deoxybenzoin-based polyarylate BHDB-IPC was synthesized. In the TGA test, the residues at 700 °C can reach 45 wt%.

(2) A small amount of BHDB-IPC added into the phosphorous–nitrogen flame-retarded TPEE can play an important role in the UL94 test and lead to a V-0 classification for the TPEE/15.0P–N/3.0BHDB-IPC sample.

(3) The TGA and DTG data show that the addition of BHDB-IPC to the TPEE/P–N blends can improve the thermal stability. In particular, the weight of the solid residue increases several times more than that from neat TPEE.

(4) The temperature-dependent FTIR shows that AlPi can catalyze the dehydration of BHDB-IPC and accelerate the formation of carbon layers.

(5) The residue analyses via LRS and SEM provide positive evidence that the TPEE/P–N blends with BHDB-IPC can form more compact microstructures in charred layers and thus enhance the flame retardancy in the condensed phase.

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