Preparation and properties of benzoxazine blends with intumescent flame retardancy

Polymer Degradation and Stability 163 (2019) 15e24

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Preparation and properties of benzoxazine blends with intumescent flame retardancy Yu Liu, Qichao Ran*, Yi Gu College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, 610065, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 December 2018 Received in revised form 14 February 2019 Accepted 17 February 2019 Available online 2 March 2019

A benzoxazine containing both alkynyl and aldehyde groups (PHB-apa) was used to modify typical diamine type benzoxazine (PH-ddm) with different ratios to get good flame retardancy. The effects of PHB-apa on curing reaction behavior, heat resistance and thermal stability of the copolymers of PHB-apa and PH-ddm, poly(PH/PHB)s, were analyzed by DSC, DMA and TGA, respectively. The flame retardancy of poly(PH/PHB)s was investigated by vertical combustion test, limiting oxygen index (LOI) and cone calorimeter. The results suggested that the flame retardancy of PH-ddm was greatly improved by adding a small amount of PHB-apa. When the addition amount of PHB-apa was only 1:12 (the molar ratio of PHB-apa to PH-ddm), the LOI value of poly(PH/PHB-12) was as high as 33.8 and UL-94 test was improved from V-1 to V-0 rating. Moreover, the structures and morphologies of the residual carbons of poly(PHddm) and poly(PH/PHB) after combustion were studied by SEM. It was found that the pores in carbon layer of poly(PH/PHB) were dense and homogeneous. Furthermore, the pyrolytic volatile products of poly(PH-ddm) and poly(PH/PHB) were analyzed by TGA-IR and Py-GC/MS to discuss the degradation and flame retardant mechanism. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Polybenzoxazine Flame retardancy Heat resistance Cone calorimeter Py-GC/MS

1. Introduction Polybenzoxazines are a kind of high performance thermosetting resins produced from phenolic compounds, primary amines and formaldehyde. Polybenzoxazines have excellent thermal properties, good mechanical properties and low water absorption, which enable them to be applied in many fields [1e5]. High char yield and N element in the polymer structure capacitate polybenzoxazines to be applied potentially as flame retardant materials [6e9]. At present, the researches on the flame retardancy of polybenzoxazines mainly include the following aspects [10e24]. (1) Preparing P-containing polybenzoxazines by introducing P element into phenol source or amine source. (2) Adding inorganic flame retardants to reduce the heat and form the cover during the combustion. (3) Introducing special functional groups into polybenzoxazines to get crosslinking structures with n et al. and Lin et al. prepared a high thermal stability. M. Sponto DOPO-containing polybenzoxazine whose limiting oxygen index  (LOI) reached 40.5 and UL-94 V-0 can be obtained [17,18]. M. Galia

* Corresponding author. E-mail addresses: [email protected] (Y. Liu), [email protected] (Q. Ran), [email protected] (Y. Gu). https://doi.org/10.1016/j.polymdegradstab.2019.02.022 0141-3910/© 2019 Elsevier Ltd. All rights reserved.

et al. mixed glycidyl phosphinate into a benzoxazine-phenolic resin and made the flame retardant rating increase from V-1 to V-0 [19]. Many reports have suggested that the introduction of the P element can significantly improve the flame retardancy of polybenzoxazines. Furthermore, inorganic flame retardants such as Al(OH)3 and Mg(OH)2 can also improve the flame retardancy of resins like epoxy resin, polybenzoxazine and polypropylene [21e23]. However, inorganic flame retardants have low flame retardant efficiency, and a large addition amount is usually needed to get obvious modification effect, which will deteriorate mechanical properties of materials. In addition, polybenzoxazines with special functional group also have outstanding flame retardancy. For example, introducing phthalonitrile or triazine into polybenzoxazines can get high char yield and low combustibility [10,11,24]. Besides, Zhang et al. prepared an ortho-tetrahydrophthalimide functional polybenzoxazine containing alkynyl group, which showed low heat release capacity and high char yield [25]. These studies indicate that the enhancement of the char yield through the functionalization of polybenzoxazine is beneficial to improve the flame retardancy. It is known that the thermal stability and char yield of thermosetting resins can be improved by introducing alkynyl groups [26e29]. The previous researches also proved that thermal stability

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and Tgs of polybenzoxazines can be increased by using alkynyl as additional reactive sites [30]. Besides, the aldehyde group in polybenzoxazine can not only improve its thermal stability by extra crosslinking reactions, but also produce non-combustible gases such as CO and CO2 at high temperature [31,32]. Therefore, we speculated that the introduction of both alkynyl and aldehyde groups should exert a positive influence on the flame retardancy of polybenzoxazines. Diamine type benzoxazine (PH-ddm) is a kind of typical benzoxazine resin that has been applied in insulation field. However, its flame retardancy needs to be improved. In this paper, we employed a benzoxazine (PHB-apa) containing both alkynyl and aldehyde groups to modify PH-ddm. The results showed that the addition of a small amount of PHB-apa can significantly improve the thermal stability and flame retardancy of PH-ddm. Moreover, its flame retardant mechanism was further studied. 2. Experiments 2.1. Materials All chemicals were used as received, 4,40 -diaminodiphenyl methane was obtained from Yan Tai Wan Hua Chemistry Reagent Co. 3-Aminophenylacetylene (98%) was obtained from Shandong Jiaozhou Fine Chemical Co. P-hydroxybenzaldehyde (99%) was obtained from Shanghai Jiacheng Fine Chemicals Co. Paraformaldehyde (96%) was bought from Alcro chemical Co (Spain). Phenol (99%), sodium hydroxide (99%) and toluene (99%) were bought from Chengdu Kelong Fine Chemicals Co. 2.2. Syntheses of benzoxazine monomers PH-ddm was synthesized according to Ref. [33]. 1H NMR (400 MHz, DMSO‑d6, 298 K): d ¼ 6.68e7.08 (m, 16H, AreH), 5.37 (m, 4H, NeCH2eO), 4.58 (m, 4H, NeCH2eAr), 3.68 (m, 2H, AreCH2eAr). FTIR: 1225 cm1 (asymmetric stretching of CeOeC), 1033 cm1 (symmetric stretching of CeOeC), 946 cm1 (oxazine ring), 752 cm1 (bis-substitution benzene ring). PHB-apa was synthesized according to Ref. [34]. 1H NMR (400 MHz, DMSO‑d6, 298 K): d ¼ 9.82 (s, 1H, CHO), 6.91e7.74 (m, 8H, AreH), 5.61 (s, 2H, OeCH2eN), 4.80 (s, 2H, AreCH2eN), 4.15 (s, 1H, ≡CH). FTIR: 3286 cm1 (stretching of ≡CeH), 1683 cm1 (stretching of C]O), 1238 cm1 (asymmetric stretching of CeOeC), 937 cm1 (oxazine ring). The chemical structures of PH-ddm and PHB-apa are shown in Scheme 1. 2.3. Preparation of polybenzoxazines Certain molar ratios (12:1, 10:1, 8:1 and 5:1) of PH-ddm and PHB-apa were stirred at 110  C for 15 min, and the blends are

Scheme 1. Chemical structures of PH-ddm and PHB-apa.

named as PH/PHB-12, PH/PHB-10, PH/PHB-8 and PH/PHB-5, respectively. PH-ddm and these blends were cured in an oven as follows: 140  C (2 h), 160  C (2 h), 180  C (2 h), 200  C (2 h) and 220  C (2 h). The obtained polybenzoxazines are designed as poly(PH-ddm), poly(PH/PHB-12), poly(PH/PHB-10), poly(PH/PHB8) and poly(PH/PHB-5), respectively.

2.4. Characterization and measurements Fourier transform infrared (FTIR): FTIR spectra were obtained by using a Nicolet Magna 560 spectrometer with a resolution of 4 cm1 and a scanning mode of 32 times. The FTIR spectra of the precursors were taken by casting a film onto a flaky KBr crystal, and the cured samples were grounded with spectroscopy-grade KBr powder. Nuclear magnetic resonance spectroscopy (1H NMR): 1H NMR spectra were recorded using Bruker Avance 400 Hz NMR spectrometers in DMSO‑d6 as solvent with tetramethylsilane (TMS) as an internal reference. Differential scanning calorimeter (DSC): DSC was performed on a TA Instruments Q20 model using 3e6 mg of sample at a heating rate of 10  C$min1 from 40 to 350  C under nitrogen atmosphere. Dynamic mechanical analysis (DMA): The dynamic mechanical properties of the cured samples were obtained by using TA instruments DMA Q800 with frequency of 1 Hz, the sample dimensions were 20 mm  10 mm  3 mm in a three point bending mode from 40 to 350  C, the heat rate was 5  C$min1. Thermogravimetric analysis (TGA): The thermal stability of samples was measured by High Resolution 2950 Thermogravimetric Analyzer (TA Instruments) around 4 mg, region temperature ranging from 40 to 800  C at a heating rate of 10  C$min1 under nitrogen atmosphere. UL-94 vertical combustion testing: UL-94 tests were carried out with a CFZ-2 instrument purchased from Jiangning Analysis Instrument Factory according to ASTM D3801. The dimensions of each specimen were 125 mm  13 mm  3.2 mm, and 5 specimens were used for a test. Limiting oxygen index (LOI): The LOI test was performed on a HC-2 type oxygen index instrument purchased from Jiangning Analysis Instrument Factory according to ASTM D2863-08. The specimen dimensions were 100 mm  10 mm  4 mm, and more than 7 specimens were used for a test. Cone calorimeter testing (CC): Combustion behaviors were measured by using iCone Calorimeter of Fire Testing Technology Co. The samples with a dimension of 100 mm  100 mm  4 mm were exposed to a radiant cone at a heat flux of 50 kW/m2. Scanning electronic microscopy (SEM): Morphologies of char residues were observed by JEOL JSM-7500F, the char residues were recorded after gold coating surface treatment under a high vacuum at a voltage of 20 kV. Thermogravimetric analysis-infrared spectrometry (TGA-IR): TGA-IR was performed on a TA Instruments TA-Q500 thermogravimetric analyzer interfaced to a Nicolet 6700 Fourier transform infrared spectrometer from 50 to 800  C at a heating rate of 20  C$min1 under nitrogen atmosphere with a resolution of 8 cm1. Pyrolysis-Gas chromatography/Mass spectrometry (Py-GC/MS): Py-GC/MS analysis was carried out using a combination of singlepoint pyrolyzer PY-2020i (Frontier, Japan) and chromatographmass spectrometer GC/MS QP2010 (Shimadzu, Japan) equipped with a pyrolysis injection system. About 1 mg of samples were pyrolyzed under helium atmosphere.

Y. Liu et al. / Polymer Degradation and Stability 163 (2019) 15e24

3. Results and discussion 3.1. Curing behaviors of PH/PHB blends The curing behaviors of PH/PHB blends with different ratios were studied by DSC. Their curves are shown in Fig. 1 and the corresponding results are listed in Table 1. As seen in Fig. 1, the DSC curve of PH-ddm showed an exothermic peak around 241.0  C and had an enthalpy of 374.3 J/g. In comparison, the exothermic peak temperature of PHB-apa was as low as 205.3  C, and the enthalpy was 572.5 J/g that is much higher than that of PH-ddm. The reason should be attributed to the presence of aldehyde and alkyne groups in the structure of PHB-apa. The electron-withdrawing aldehyde group decreases the electron cloud density on the phenol ring and makes the oxazine ring much easy to open [35]. At the same time, the extra crosslinking reaction of the acetylene groups during the curing process results in a huge exothermic enthalpy of PHB-apa

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[28]. When PHB-apa was added into PH-ddm, the exothermic peak temperatures of the blends decreased as the increase of the addition amount, and the enthalpy increased accordingly. It should be noted that the curing exothermal peaks of the blends still had good symmetry with the addition of PHB-apa, which indicated that the synergetic copolymerization reaction between PHB-apa and PH-ddm occurred during the curing process of the blends.

3.2. Dynamic mechanical properties of cured blends The DMA tests were conducted to determine the dynamic mechanical properties of polybenzoxazines. The results are shown in Fig. 2 and Table 2. Their crosslinking densities were calculated by equation of rubbery elasticity [36].

r ¼ E0 =3FRT where F is the front factor, T (K) is the absolute temperature, R is the gas constant (8.3145 J mol1 K1), and E’ (MPa) is the storage modulus at temperature Tg tand þ 40 K. The Tg and crosslinking density of poly(PH-ddm) were 225.5  C and 2.0  103 mol m3, respectively. The Tgs and the crosslinking densities of poly(PH/PHB) systems increased with the addition of PHB-apa. Comparing with poly(PH-ddm), the Tg of poly(PH/PHB10) increased by 24.9  C though the addition amount was little. For poly(PH/PHB-5), its Tg reached as high as 271.2  C that is much higher than those of epoxy resin and phenolic resin. These results indicated that the thermal resistance of poly(PH-ddm) can be improved by copolymerizing with PHB-apa, which was mainly ascribed to the alkynyl groups. The cyclotrimerization reaction of the alkyne groups generated more conjugated structures in the copolymerization systems [34].

Table 2 DMA data of poly(PH/PHB) and poly(PH-ddm).

Fig. 1. DSC curves of PH/PHB blends.

Table 1 DSC results of PH/PHB blends. Sample

Tonset ( C)

Tpeak ( C)

△H/J$g

PH-ddm PH/PHB-10 PH/PHB-8 PH/PHB-5 PHB-apa

233.5 227.9 226.7 224.4 190.9

241.0 237.5 236.9 235.7 205.3

374.3 381.4 390.7 416.0 572.5

Sample

Storage Modulus Tg E’’ (GPa) ( C)

Tg tand ( C)

Crosslinking Density (  103 mol m3)

poly(PHddm) poly(PH/ PHB-10) poly(PH/ PHB-8) poly(PH/ PHB-5)

5.5

208.4

225.5

2.0

5.3

226.4

250.4

2.6

5.2

239.7

258.5

3.1

5.1

250.4

271.2

4.0

Fig. 2. DMA curves of poly(PH/PHB) and poly(PH-ddm).

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Y. Liu et al. / Polymer Degradation and Stability 163 (2019) 15e24 Table 4 Combustion test results of poly(PH/PHB)s.

Fig. 3. The TGA curves of polybenzoxazines.

3.3. Thermal stability of cured blends The effect of PHB-apa on the thermal stability of poly(PH/PHB) were evaluated by TGA and the results of poly(PH/PHB) are shown in Fig. 3 and Table 3. The decomposition temperature at 5% weight loss (Td5) of poly(PH-ddm) was 366.8  C, and the derivative of weight loss showed a two-stage process centered at 418.5  C and 490.6  C with the maximum rates of weight loss of 2.73%/min and 3.41%/min, respectively. Nevertheless, poly(PHB-apa) showed only one decomposition step centered at 492.3  C. The higher thermal stability of poly(PHB-apa) than poly(PH-ddm) was originated from more crosslinking structures formed by the reactions of aldehyde group and alkynyl group [34]. It should be noted that Td5 of poly(PHB-apa) was 347.3  C that is lower than that of poly(PHddm), which resulted from further reactions of residual aldehyde groups in poly(PHB-apa) [31]. In DTG curves, two pyrolysis peaks of poly(PH-ddm) corresponded to the release of amine derivatives (Peak 1) and phenol derivatives (Peak 2), respectively [37]. With the addition of PHBapa, the maximum degradation rate of Peak 1 decreased slightly, but that of Peak 2 decreased obviously, indicating that the release of phenolic derivatives reduced. This is because the aldehyde group attached to phenolic ring is incorporated into the crosslinking network, which can improve the stability of phenol structures. In addition, compared with 46.9% of poly(PH-ddm), the char yields of poly(PH/PHB)s at 800  C increased, which was attributed to increased crosslinking densities and more rigid structures. 3.4. Flame retardancy of cured blends LOI and UL-94 rating are basic and critical parameters to evaluate the flammability of polymeric materials. LOI values and total afterflame time for each set (t1 plus t2 for the five specimens) in UL-

Sample

LOI (%)

UL-94 (t1þt2)total/s

UL-94 Rating

poly(PH-ddm) poly(PH/PHB-12) poly(PH/PHB-10) poly(PH/PHB-8) poly(PH/PHB-5)

31.3 33.8 33.8 35.5 37.9

78.0 23.8 5.8 0 0

V-1 V-0 V-0 V-0 V-0

94 tests of polybenzoxazines are presented in Table 4. As seen from the results, the LOI value of poly(PH-ddm) was 31.3% and its UL-94 test was V-1 rating. After the addition of PHB-apa, the LOI value and UL-94 rating were improved significantly. Among them, the UL-94 test achieved V-0 rating for poly(PH/PHB-12). Additionally, the LOI value reached 35.5% for poly(PH/PHB-8), and further increased to 37.9% for poly(PH/PHB-5). Actually, it is difficult for polybenzoxazines to obtain such a high value without the involvement of halogen or P element. Moreover, a significant decrease in smoke release can be observed during the combustion process of poly(PH/ PHB)s. The enhancement of LOI value and UL-94 rating is mainly attributed to the increase of thermal stability and the char yields of the copolymers. 3.5. Fire behavior by cone calorimeter Cone calorimeter is usually used to quantitatively investigate the combustion performance of a polymeric material. To further investigate the influence of PHB-apa on combustion behavior of poly(PH-ddm), poly(PH/PHB-5) with excellent flame retardancy and thermal stability was selected for the CC test to compared with poly(PH-ddm). Many valid data can be obtained from the CC test, such as the time to initial (TTI), heat release rate (HRR), the peak of heat release rate (PHRR), time to peak of heat release rate (TTPHRR), total heat release (THR), the average of specific extinction area (AvSEA), total smoke production (TSP) and the average effective heat of combustion of volatiles (Av-EHC). These results are shown in Table 5. Compared with the TTI of 69.0 s for poly(PH-ddm), the TTI of poly(PH/PHB-5) decreased to 59.0 s. This was because the reactions of residual aldehyde groups in poly(PH/PHB-5) brought the release of small substances at a relative low temperature. The ignition of the released small substances leads specimens to combustion in CC test. HRR curves of poly(PH-ddm) and poly(PH/PHB-5) are shown in Fig. 4. The PHRR value of poly(PH/PHB-5) was 175.3 kw/m2 lower than 299.3 kw/m2 for poly(PH-ddm), which indicated that poly(PH/ PHB-5) had a lower heat release capacity than poly(PH-ddm), and the formation of the effective carbon layer inhibited the heat release [38]. But there was an abnormal strong peak at 175.0 s in poly(PH/PHB-5), it was the reflection of the expansion phenomenon during combustion of poly(PH/PHB-5) rather than PHRR value. Poly(PH/PHB-5) had a significant expansion during the combustion

Table 3 TGA results of polybenzoxazines. Sample

Td5( C)

Td10( C)

Yc (%)

Peak 1a 

poly(PH-ddm) poly(PH/PHB-10) poly(PH/PHB-8) poly(PH/PHB-5) poly(PHB-apa) a b

366.8 366.8 358.4 370.0 347.3

The first degradation peak. The second degradation peak.

394.8 394.0 390.4 397.7 456.1

46.9 49.9 48.5 52.2 68.6

Peak 2b

T( C)

DTG(%/min)

T( C)

DTG(%/min)

418.5 412.3 412.3 413.1 492.3

2.73 2.72 2.71 2.57 1.30

490.6 474.1 474.1 473.9 e

3.41 2.60 2.56 2.35 e

Y. Liu et al. / Polymer Degradation and Stability 163 (2019) 15e24

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Table 5 Cone calorimeter data of poly(PH-ddm) and poly(PH/PHB-5). Sample

TTI(s)

PHRR (kw/m2)

TTPHRR (s)

THR (MJ/kg)

Av-EHC (MJ/kg)

Av-SEA (m2$kg)

TSP (m2)

Residue (%)

poly(PH-ddm) poly(PH/PHB-5)

69.0 59.0

299.3 175.3

80.0 65.0

100.1 129.8

26.9 28.5

580.0 192.0

22.3 12.5

32.3 18.7

Fig. 4. HRR curves of poly(PH-ddm) and poly(PH/PHB-5).

(Fig. S1), making the surface of the sample close to the heater. Then, the temperature of the specimen rose suddenly and resulted in fiercely combustion. This explained why THR and Av-EHC value of poly(PH/PHB-5) was higher than poly(PH-ddm). THR curves of poly(PH-ddm) and poly(PH/PHB-5) are shown in Fig. 5. Besides, although the char yield of poly(PH/PHB-5) was higher than that of poly(PH-ddm), the residue mass of poly(PH/PHB-5) in CC test was lower than that of poly(PH-ddm), which was also attributed to the expansion and strong combustion of poly(PH/PHB-5). Moreover, the Av-SEA and TSP values of poly(PH/PHB-5) were approximately half of poly(PH-ddm). As shown in Fig. 6, the smoke production of poly(PH/PHB-5) was lower than poly(PH-ddm) after 85.0 s. The reduction of total smoke production was attributed to the formation of compact carbon layer during the combustion process and the absorption of the solid particles by micropores inside carbon layer. At the same time, the release of low molecular substances was reduced by the improvement of thermal stability. In

Fig. 5. THR curves of poly(PH-ddm) and poly(PH/PHB-5).

Fig. 6. TSP curves of poly(PH-ddm) and poly(PH/PHB-5).

summary, poly(PH/PHB-5) showed obvious intumescent flame retardancy. 3.6. Morphologies and structures of the chars after combustion To better understand the microstructures of the protective chars, the chars after vertical combustion test and CC test were investigated by SEM. As shown in Fig. 7 (a2 and b2), the surface of poly(PH-ddm) after vertical combustion was powdery carbon with cracks, which was different from the appearance of poly(PH/PHB-5) that had a certain expansion, and the interior of the crack was brown, indicating that the combustion didn't spread into the inner. SEM image (a3) showed there were many big through-holes with a size around 150 mm randomly distributed on the surface of poly(PH-ddm). On the contrary, the carbon layer of poly(PH/PHB-5) was consisted of closed-cell structures with about 5 mm in diameter which tended to be orderly distributed on the surface, and this sponge-like structure showed expansion effect. The existence of closed-cell structures has a positive barrier effect of heat and oxygen, which is in favor of inhibiting the spread of flame [39]. Moreover, as seen from the photo of the samples after CC test, the residual char of poly(PH/PHB-5) (b4 in Fig. 7) had a slightly expansion after combustion, and its char layer was more compact than needle-like structure of poly(PH-ddm) (a4 in Fig. 7). Additionally, SEM photo of poly(PH-ddm) after CC test (a5 in Fig. 7) showed that the char surface possessed big through-holes with a diameter of around 0.5 mm, which would facilitate the combustion by transmitting oxygen and flame. However, SEM photo of poly(PHddm) was totally different from poly(PH/PHB-5) with relatively continuous dense protective char layer. This difference should arise from the improvement of thermal stability and char yield by the introduction of PHB-apa. The expansion effect with closed-cell structures in poly(PH/ PHB-5) has a close relationship with aldehyde groups and alkynyl groups. Firstly, the carboxyl groups formed by the oxidation of aldehyde groups as acid sources resulted in the dehydration of phenolic hydroxyl groups and form the carbon [40]. Secondly, the

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Y. Liu et al. / Polymer Degradation and Stability 163 (2019) 15e24

Fig. 7. Photos of poly(PH-ddm) (a) and poly(PH/PHB-5) (b), resin cast (1), resin cast after vertical combustion (2), SEM image of resin cast after vertical combustion (3), carbon residue after CC test (4), SEM image of carbon residue after CC test (5).

pyrolysis of crosslinking structures formed by aldehyde groups released gases gently at high temperatures and tended to form intumescent carbon layer. Finally, the crosslinking structures from alkynyl groups limited the expansion of the pores by its cyclization reactions. 3.7. Chemical structures of solid phase FTIR analysis was carried out to verify the structure conversion of poly(PH-ddm) and poly(PH/PHB-5) during the pyrolysis process. Poly(PH-ddm) and poly(PH/PHB-5) were pyrolyzed by using a tube furnace under nitrogen atmosphere at 400  C and 600  C, and the FTIR spectra of pyrolysis residues after different temperatures are shown in Fig. 8. For poly(PH/PHB-5), the peak at 1708 cm1 ascribed to the aldehyde group indicated that a few aldehyde groups still existed. Moreover, there was an obviously broad peak appeared in 3431 cm1 because the aldehyde groups were oxidized to the carboxyl groups during the curing process. After treatment at 400  C, the decrease of the peak at 1262 cm1 (CeO stretching) was due to the removal of some phenolic hydroxyl groups in both poly(PH-ddm) and poly(PH/PHB-5). Besides, the peaks at 1474 cm1 and 811 cm1 due to tetra-substituted benzene ring increased, meaning more crosslinking reactions occurred. The peak at 1708 cm1 in poly(PH/PHB-5) disappeared, indicating that the aldehyde groups transferred to CO or CO2. After pyrolysis at 600  C, the intensity of the peak at 874 cm1 attributed to penta substituted benzene ring in poly(PH/PHB-5) was stronger than that of poly(PH-ddm). Also, the peak around 1500 cm1 to 1000 cm1 in poly(PH/PHB-5) became wider and blunter than that in poly(PHddm), suggesting that poly(PH/PHB-5) had higher aromatization degree. In addition, the peak of eCH2e at 2908 cm1 disappeared at 600  C, suggesting that the pyrolytic reaction of methylene bridge structures in diamine occurred. It's worthy noted that the benzene ring vibration of poly(PH-ddm) and poly(PH/PHB-5) at 1619 cm1 shifted to 1590 cm1 and 1577 cm1, respectively. Bigger changes took place in poly(PH/PHB-5) also suggested that the aromatization degree of poly(PH/PHB-5) was higher than that of poly(PH-ddm) and more conjugated benzene ring structures were formed. Besides, the peak at 1227 cm1 due to CeN bending vibration implied

Fig. 8. FTIR spectra of carbon residues of poly(PH-ddm) (a) and poly(PH/PHB-5) (b) at different pyrolysis temperatures under nitrogen.

Y. Liu et al. / Polymer Degradation and Stability 163 (2019) 15e24

the formation of stable N heterocycle conjugated structures. Based on these results, it was believed that more stable solid phase structures were formed during the combustion of poly(PH/PHB-5), such as the aromatic fused ring structures and the N heterocyclic structures. 3.8. TGA-IR analysis TGA-IR technique is often used to analyze gaseous products during thermal decomposition process. Fig. 9 shows FTIR spectra of the gaseous products of poly(PH-ddm) and poly(PH/PHB-5) at several temperatures. The gaseous products mainly consisted of benzene compounds (1621 cm1 and 1512 cm1), amine derivatives (3373 cm1) and phenolic derivatives (3653 cm1). It was noteworthy that no gaseous products released in poly(PH-ddm) at 340  C, but poly(PH/PHB-5) had obvious gas release, and these gaseous products were mainly composed of aniline derivatives, phenol derivatives as well as carboxyl group compounds (1751 cm1) resulted from the reaction of the aldehyde groups at high temperature [31]. Carboxyl groups formed by the oxidation of the aldehyde groups can promote the breakage of CeN bond and CeC bond and release aniline and phenol compounds. The release of these small molecule substances made poly(PH/PHB-5) relatively easy to ignition in CC test, but its solid phase structures got better thermal stability because of the oxidation of aldehyde groups and the decarboxylation reaction. At 480  C, poly(PH-ddm) and poly(PH/PHB-5) exhibited obvious gas release at this stage. The characteristic peaks at 929 cm1 and 964 cm1 (NH3) were observed in poly(PH-ddm). NH3 is a non-flammable gas that can dilute the concentration of oxygen, which makes poly(PH-ddm) has a certain flame retardancy. Unlike poly(PH-ddm), the characteristic peaks at 2173 cm1 due to CO and at 2305 cm1 due to CO2 were detected in poly(PH/PHB-5). They were originated from aldehyde groups and carboxyl groups, and enabled poly(PH/PHB-5) to show gas phase flame retardancy. When the temperature increased to 660  C, poly(PH-ddm) still kept significant absorption peaks including NH3, amine derivatives and phenolic derivatives. For poly(PH/PHB-5), the pyrolysis products including CO, CO2 and phenolic derivatives reduced significantly. Among them, CO and CO2 were non-flammable gas, and phenolic derivatives can be oxidized to benzoquinone that can inhibit the chain reaction of the combustion and improve the flame retardancy. After 800  C, poly(PH-ddm) still released amine derivatives that can generate amine radicals in the combustion and promote the chain reaction. But for poly(PH/PHB-5), a small amount of CO and CO2 were found,

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which suggested that poly(PH/PHB-5) had high thermal stability and followed gas phase flame retardancy mechanism. 3.9. Py-GC/MS analysis Stepwise temperature method was applied in Py-GC/MS test to study the transformation of chemical structures of polymer bulk during pyrolysis process at high temperatures [41,42]. The main pyrolysis stages of poly(PH-ddm) and poly(PH/PHB-5) occurred in range of 300e600  C based on the TGA result, so the pyrolysis products were analyzed at 400  C, 500  C and 600  C, respectively, and the results are shown in Table 6. The pyrolysis of polybenzoxazines is mainly composed of the cleavage of CeN bond and CeC bond [43], and its pyrolysis products are mainly divided into primary pyrolysis products and secondary pyrolysis products. Primary pyrolysis products are mainly composed of phenols and aniline compounds, and secondary pyrolysis products are formed by recombination reactions of primary pyrolysis products. At 400  C, the pyrolysis products of poly(PH-ddm) and poly(PH/ PHB-5) contained primary and secondary pyrolysis products. The cleavage of weak CeN bond affords the pyrolysis of polybenzoxazines, and the possible pyrolysis pathway is shown in Scheme 2. As seen in Table 6, the contents of different pyrolysis products for poly(PH-ddm) and poly(PH/PHB-5) were different. The release of phenol compounds in poly(PH/PHB-5) was 25.7% that is less than 45.5% of poly(PH-ddm). This is because the crosslinking reaction of the aldehyde groups attached to para position of phenolic hydroxyls in poly(PH/PHB-5) improved the thermal stability of the phenols in the crosslinking networks [35]. At the same time, the crosslinking reaction of the alkynyl group in poly(PH/ PHB-5) may prevent the release of amine. Therefore, although there was an aniline structure in poly(PH/PHB-5), the amount of aniline after pyrolysis was fairly low. Diamine was one of main pyrolysis components for both polybenzoxazines, because this diamine was main chain structure in crosslinking structure. Additionally, some dihydric phenol compounds formed by the reaction between phenol compounds with free radical were observed as secondary pyrolysis products for both poly(PH-ddm) and poly(PH/ PHB-5), and the methylene bridge of diphenol can be further oxidized to the carbonyl group, as presented in Scheme 2 (b1 and b3). Furthermore, poly(PH/PHB-5) had a small amount of compounds containing a carboxyl group due to the oxidation of the aldehyde group (b5 in Scheme 2). At 500  C, the amount of phenol compounds was still large for both poly(PH-ddm) and poly(PH/PHB-5). Moreover, the amount of

Fig. 9. FTIR spectra of volatiles of poly(PH-ddm) (a) and poly(PH/PHB-5) (b) at different temperatures.

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Table 6 Py-GC/MS results of poly(PH-ddm) and poly(PH/PHB-5).

amine compounds increased obviously and the amount of diamine were significantly reduced for poly(PH-ddm), indicating that the cleavage of CeC bonds occurred, as shown in pathway a2 in Scheme 2. However, diamine compounds were still the major pyrolysis products of poly(PH/PHB-5), which proved that the fracture of CeC bonds seldom occurred in poly(PH/PHB-5). Furthermore, amides in the solid phase may transform to cyano groups [44], and the cyclization reaction of cyano groups can form N-containing heterocyclic rings as shown in path b4, which would generate carbon layer at high temperatures. The five-membered and the sevenmembered N-containing heterocyclic ring products in poly(PHddm) were derived from the reaction of path b2 [45]. At 600  C, The secondary pyrolysis products became the main products from poly(PH-ddm), which indicated that the bulk structures of poly(PH-ddm) underwent degradation reactions. However, phenolic compounds and diamine compounds were still

the main pyrolysis products for poly(PH/PHB-5), and the compounds containing benzophenone or cynao groups appeared. The delayed release of the pyrolysis compounds demonstrated that the solid phase of poly(PH/PHB-5) was more stable than that of poly(PH-ddm). 4. Conclusions In conclusion, the flame retardancy and thermal stability of PHddm was improved greatly by adding PHB-apa. The combustion tests showed that UL-94 rating of poly(PH/PHB-5) upgraded from V-1 to V-0, and its LOI increased to 37.9%. Good flame retardancy of poly(PH/PHB-5) were derived from gas phase flame retardancy, high thermal stability of solid phase structure and the formation of expanded carbon layer. Poly(PH/PHB-5) can release non-flammable gases, CO and CO2, during pyrolysis process, and the solid phase

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Scheme 2. The proposed degradation pathway of poly(PH-ddm) and poly(PH/PHB-5).

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