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Bi-DOPO Structure Flame Retardants with or without Reactive Group: Their Effects on Thermal Stability and Flammability of Unsaturated Polyester Yuan Cao, Xiu-Li Wang, Wen-Qiang Zhang, Xue-Wu Yin, Yue-Quan Shi, and Yu-Zhong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 7, 2017

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Industrial & Engineering Chemistry Research

Bi-DOPO Structure Flame Retardants with or without Reactive Group: Their Effects on Thermal Stability and Flammability of Unsaturated Polyester

Yuan Cao, Xiu-Li Wang*, Wen-Qiang Zhang, Xue-Wu Yin, Yue-Quan Shi, Yu-Zhong Wang*

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China. *Corresponding

authors.

Tel.

&

Fax:

+86-28-85410755.

E-mail

[email protected] (X. L. Wang) and [email protected]. (Y. Z. Wang)

1

ACS Paragon Plus Environment

address:


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Abstract:

A

novel

reactive

phosphorus-containing

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flame

retardant

1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl) methylene) diacrylate (TDCAA-DOPO),

with

two

9,10-dihydro-9-oxa-10-phosphaphena-nthrene-10-oxide

structure

symmetrical (bi-DOPO),

was

synthesized and combined with unsaturated polyester resin (UPR) via cross-linking to prepare flame retardant UPR (FR-UPR). To make clear both the effects of the bi-DOPO structure and the cross-linking reaction on the thermal stability and flame retardancy of UPR, an additive flame retardant also owned bi-DOPO structure but without vinyl bond,1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl)

carbinol)

(TDCA-DOPO), was used as a contrast. The results showed that the incorporation of TDCAA-DOPO or TDCA-DOPO into UPR can reduce its peak heat release rate (PHRR) and total heat release (THR), as well as improve the limiting oxygen index (LOI) value. Compared to TDCA-DOPO, the reactive TDCAA-DOPO endowed FR-UPR with enhanced thermal stability, residual char and glass transition temperature (Tg), which were even higher than those of pure UPR. Besides, their flame retardant mechanism for UPR had been well investigated. Keywords: Bi-DOPO structure; reactive; additive; flame retardance; thermal stability

2


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Introduction Unsaturated polyester resin (UPR), as one of the most important thermosetting resins, has been extensively utilized as fiber-reinforced polymer composite materials for its most prominent fabricability, low cost, excellent chemical resistance and dielectric properties, etc.1-3 However, pure UPR resin has poor fire resistance, and will release dense fumes during burning,4-6 which restricts its wide application. To meet this challenge, great efforts have been dedicated to reducing the fire hazards of UPR over the past decades.7-12 Although the traditional flame-retardant additives are inexpensive and show desirable efficacy in fire resistance, migration from matrix can’t be ignored. Besides, the unfavorable interfacial interaction between matrix and additives will deteriorate the mechanical properties of polymer.13-16 In contrast, the reactive flame retardants become an integral part of the polymer chain, which make polymer materials show better mechanical properties and durability.17,

18

Among the reactive flame retardants, phosphorus-containing flame

retardants have been paid great attention since they are high-efficiency and generate no corrosive and toxic gases during the combustion.19, 20 As we all know, UPR is prepared by a radical polymerization of unsaturated diols with saturated

diacids in

the

presence

of

co-reactant

diluents

styrene.

Therefore,

phosphorus-containing flame retardants such as cyclicphosphate,21, 22 phosphine oxide23, 24

or 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) or its derivatives,25, 26

usually are designed to possess double bonds, which can be easily introduced into the backbone of UPR via the reaction with UPR and styrene. It was found that when caged phosphate monomer (PDAP) was used as a reactive flame retardant,22 UPR samples with high phosphorus content showed high limiting oxygen index (LOI) value (~27.5%) as well as half-reduced heat release capacity (HRC) and peak heat release rate (PHRR). Lin et al.23 3



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prepared a reactive phosphine oxide monomer (TAOPO) and used it as a co-curing agent to prepare intrinsic flame retardant UPR. Their results show that as the phosphorus content increased to 3 wt%, the peak PHRR and total heat release (THR) values of flame retardant UPR reduced by 45.7% and 45.5% compared to pure UPR. Unfortunately, the initial decomposition temperature at 5% weight loss (T5%) of the UPR resins decreases above 60 °C when PDAP or TAOPO are introduced. This is related to the poor thermal stability of P-C-O bond in PDAP or TAOPO. DOPO and its derivatives, as novel phosphorus-containing flame retardants, exhibit excellent thermal stability and flame extinguishing behavior with respect to other flame retardants.27-31 Bai et al.25 synthesized a reactive DOPO-based acrylate (ODOPB-AC) and prepared flame retardant UPR with various amounts of ODOPB-AC. Their results show that the flame retardancy was improved accordingly, and at the same time the initial decomposition temperatures of the flame-retardant UPR decreased slightly. In this study, another kind of DOPO-based reactive flame retardant with bi-DOPO structure (TDCAA-DOPO) was synthesized and incorporated into the UPR. Due to more aromatic rings and higher phosphorus contents in its chemical structure, its introduction will not only improve the flame retardancy of UPR, but also enhance the thermal stability. As a contrast, bi-DOPO-based flame retardant (TDCA-DOPO) without the functional group CH2=CH- was also synthesized and introduced into UPR resin as a flame retardant additive. A comparative study is performed to investigate both the influence of bi-DOPO structure and cross-linking behavior on the flammability, thermal properties, and flame-retardant mechanism.

Experimental Materials 4


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Unsaturated polyester resin (UPR) (type 196#, commercial grade) supplied by Shunxin Co., Ltd (Chengdu, China), was composed of phthalic anhydride, maleic anhydride, propan-1,2-diol, and styrene (35-40 wt%). The initiator methyl ethyl ketone peroxide (MEKP, CP) and the catalyst cobalt (II) naphthenate (CP) for UPR curing were purchased from

Shunxin

Co.,

Ltd.

(Chengdu,

China).

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO, AR) and acryloyl chloride (97%, stabilized with 400 ppm phenothiazine, AR) were provided by J&K Technology Co., Ltd. (Beijing, China). Terephthaldicarboxaldehyde (TDCA, AR) was purchased from Aladdin

Industrial

Corporation

(Shanghai,

China).

Triethylamine

(TEA,

AR),

dichloromethane (AR) and toluene (AR) were all supplied by Kelong Co., Ltd. (Chengdu, China) and dried over 4-Å molecular sieves prior to use. All other materials were used as received. All the abbreviations and symbols in the experimental part were shown in Table 1. Synthesis of 1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl)carbinol) (TDCA-DOPO) TDCA-DOPO was prepared according to the literature.32 TDCA (0.10 mol), DOPO (0.22 mol) was dissolved in 300 mL dried toluene in a 500 mL three-necked flask equipped with a N2 inlet. The mixture was stirred vigorously and heated at refluxed temperature for 5 h. After being cooled to room temperature, the precipitated white powder was collected by filtration, washed with toluene and dried in vacuum at 70°C. Yield = 91%. 1HNMR (400 MHz, DMSO-d6, ppm): 8.30-8.12 (m, 4H), 8.06 (m, 1H), 7.91-7.73 (m, 2H), 7.61 (m, 1H), 7.53-7.11 (m, 12H), 6.23-5.56 (m, 2H), 5.46-5.10 (m, 2H). FTIR (KBr, cm-1): 3245 (CH-OH), 1588 (P-Ph), 1211 (P=O), 1120, 932 (P-O-Ph). 31PNMR (161.9 MHz, DMSO-d6, ppm): 31.1. Synthesis

of 5



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1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl)methylene)diacrylate (TDCAA-DOPO) The synthetic procedures of TDCAA-DOPO are illustrated in Scheme 1. The detailed synthesis processes are shown as follows. TDCA-DOPO (0.10 mol), trithylamine (0.20 mol) were dissolved in 300 mL dichloromethane in a 500 mL three-necked flask equipped with a N2 inlet. Acryloyl chloride (0.20 mol) in 50 mL dichloromethane was added dropwise at 0~ -5 ˚C in 2 h with a dropping funnel. Then the mixture was heated to room temperature and stirred for 5 h. Subsequently, the precipitated triethylamine hydrochloride was removed by filtering and the filtrate was rotary evaporated to remove the solvent under reduced pressure. The residue was washed with distilled water and a light yellow powder was obtained. Yield = 81%. 1HNMR (400 MHz, DMSO-d6, ppm): 8.30-8.12 (m, 4H), 8.06 (m, 1H), 7.91-7.73 (m, 2H), 7.61 (m, 1H), 7.53-7.11 (m, 12H), 6.67-6.52 (1H, CH-CH2), 6.53-6.35 (1H, CH-CH2, trans), 6.10-5.88 (1H, CH-CH2, cis). FTIR (KBr, cm-1): 1735 (C=O), 1627 (C=C), 1587 (P-Ph), 1257 (P=O), 1155, 1204 (C-O-C), 1117, 928 (P-O-Ph). 31

PNMR (161.9 MHz, DMSO-d6, ppm): 26.2.

Preparation of FR-UPR samples TDCAA-DOPO with different weight ratio was added into UPR resin at room temperature with vigorous agitation for about 30 min. Then, the initiator MEKP (2 wt%, with respect to UPR) and the solution of cobalt naphthenate (1 wt%, based on UPR) as a catalyst, were added to the mixture and stirred for another 20 minutes. After the air bubbles were removed under vacuum for 5~10 min, all the homogenized samples were rapidly cast into polytetrafluoroethlene molds, cured at 80 °C for 2 h and post cured at 100 °C for another 3 h. In accordance with the weight content of TDCAA-DOPO in the resin, the cured FR-UPR samples were named as UPR5, UPR10, UPR15 and UPR20. The control 6


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sample named UPR15* was prepared by blending UPR resin with 15 wt% of TDCA-DOPO with the same cured method. The detailed phosphorus content in all the FR-UPR samples are presented in Table 2 and the schematic reaction route between TDCAA-DOPO and UPR resin is shown in Scheme 1.

Scheme 1. Synthesis route for TDCAA-DOPO and its curing reaction with UPR resin. Table 1. The abbreviations and symbols in the experimental part Abbreviation/ Definition Symbols TDCAA-DOPO

1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl)methylene)diacrylate

TDCA-DOPO

1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl) carbinol)

bi-DOPO

bi-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide

DOPO

9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide

UPR

unsaturated polyester resin

FR-UPR

flame retardant unsaturated polyester resin

TDCA

terephthaldicarboxaldehyde

MEKP

methyl ethyl ketone peroxide

TEA

triethylamine 7



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Table 2.The detailed phosphorus content in FR-UPR Flame retardant content

Phosphorus content

TDCAA-DOPO

TDCA-DOPO

Theoretical

Actuala

(wt%)

(wt%)

(wt%)

(wt%)

UPR

0

0

0

0

UPR5

5

0

0.46

0.40

UPR10

10

0

0.92

0.72

UPR15

15

0

1.39

1.26

UPR20

20

0

1.83

1.74

UPR15*

0

15

1.64

1.58

Samples

a: tested by ICP-AES.

Characterization The gel contents measurements of cured UPR samples were carried out using Soxhlet extraction. The samples were cut into small pieces with a diameter of 2 mm. Then these chippings were packed into a filter paper and refluxed in dimethylformamide for 48 h. Finally, the residue in the filter paper was dried to a constant weight under vacuum at 130 °C. The gel contents (G) were calculated using the following equation:33 W  G =  g  × 100%  W0 

W0: the weight of dry sample before extraction; Wg: the weight of dry sample after extraction. The Fourier transform infrared (FTIR) spectra were recorded with Nicolet 6700 Spectrometer using KBr pellets. Spectra in the range of 4000-400 cm-1were obtained by 32 scans at a resolution of 4 cm-1. 1

HNMR and

31

PNMR spectra were recorded with a Bruker AVANCENMR instruments

operating at 400 MHz for 1H and 161.9 MHz for 31P at room temperature, using DMSO-d6 as a solvent. 8


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The actual phosphorus content of the cured UPR samples was determined by oxygen flask combustion-inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Advantage, TJA solution) method. Thermogravimetric analysis (TGA) was carried out on a NETZSCH TG 209F1 thermal gravimetric analyzer. All the samples were weighted within 3-5 mg and heated from 40 °C to 700 °C at a heating rate of 20 °C/min under both nitrogen and air flows of 50 mLmin-1. The Limiting Oxygen Index (LOI) measurement was performed using a HC-2C oxygen index measurement on a CZF-2 instrument (Jiangning, China) according to ASTM D2863-97 testing procedure with a sample size of 120 mm × 6.5 mm × 3.2 mm. The Cone calorimeter (CONE) tests were performed according to ISO 5660 standard with a FTT (UK) cone calorimeter at a heat flux of 50 kW·m-2. The sample size was 100 mm × 100 mm × 3.2 mm. The experiment error of cone is about±10%. The surface morphology of the testing samples was examined by Scanning Electron Microscopy (SEM) using a JEOL JSM 5900LV scanning electron microscope at the accelerating voltage of 20 kV. The Raman spectroscopy (RS) measurement was conducted at room temperature using a Lab RAM HR laser Raman spectrometer with excitation wavelength of 532 nm. The X-ray photoelectron spectroscopy (XPS) of the char residue was recorded with by a XSAM80 spectrometer using an Al Kɑ excition radiation (hv-1486.6 ev). The dynamic mechanical analysis (DMA) of the flake specimens (20 mm× 10 mm ×4 mm) were tested via a dynamic mechanical analyzer (DMA TA Q800) in Multi-Frequency -Strain mode. The frequency was set at 1Hz at a heating rate of 5 C/min from 0 °C to 180 °C under 3-Point Bending tension clamp. The

pyrolysis

behavior

of

samples

was

investigated

9


with

Pyrolysis/Gas


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Chromatography/Mass Spectroscopy (Py/GC/MS). The test was carried out with a DANI MASTERGC-TOF-MS system combined with a pyrolyzer (CDS5200).The samples (∼0.3 mg), packed in a quartz tube capillary using the platinum coil attachment, were heated from ambient to 450°C at a rate of 1000 °C/s, with isothermal steps of 20 s. The Py/GC interface temperature was set at 200 °C. The transfer line temperature was set at 260 °C. The injector temperature was set at 280 °C and operated in the split mode (split ratio 1000 : 1) with helium as a carrier gas. The GC column was SLBTM-5 ms Fused Silica Capillary Column of 10 m length, 0.10 mm diameter, and 0.10 µm film thickness. For the operation, the temperature program was as following: 2 min at 45 °C, temperature increased to 280 °C at a rate of 15 °C/min then kept at 280 °C for 5 min. Mass spectra were recorded under electron impact ionization at 70 eV electron energy, and the ion source temperature was maintained at 180 °C. Data analyses were obtained using the NIST Mass Spectral Searc Program and the NIST library were also searched as the standard spectral library to match the volatile pyrolysis products recorded from the analysis.

Results and discussion Structure characterization of flame retardants The FTIR spectra of TDCA-DOPO and TDCAA-DOPO are shown in Figure 1 (a). For convenience of comparing, the FTIR spectrum of DOPO is also provided.19 For TDCA-DOPO, as the addition reaction between DOPO and TDCA, the absorption peak at 2385 cm-1 for P-H stretching vibration in DOPO disappeared while a new absorption peak at 3245 cm-1 for C-OH stretching vibration appeared. The absorption peaks at 3065 cm-1 (Ph-H), 1588 cm-1 (P-Ph), 1211 cm-1 (P=O), 1120 cm-1 and 932 cm-1 (P-O-Ph) were assigned to the cyclic DOPO structure which were well maintained.34 When the substitution 10


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reaction between TDCA-DOPO and acryloylchloride occurred, the peak at 3245 cm-1 for C-OH stretching vibration disappeared, and the new peaks at 1735 cm-1for C=O and 1627 cm-1for C=C stretching vibration can be observed clearly, which demonstrated the presence of acrylate group in TDCAA-DOPO. Besides these, the absorption peaks at 1204 cm-1 and 1155 cm-1related to the C-O-C symmetric and asymmetric vibration were also observed. The other characteristic absorption peaks belonged to cyclic DOPO structure were still well maintained in TDCAA-DOPO. The 1H NMR spectra of DOPO, TDCA-DOPO and TDCAA-DOPO are shown in Figure 1 (b). For TDCA-DOPO, the signals of Ph-H proton were observed in the range 8.30 - 7.11 ppm. The chemical shifts of the characteristic proton -C(OH)-H (a) and C-OH (b) were found at 6.56 - 6.23 ppm and 5.50 - 5.07 ppm, respectively,32, 34 confirming the expected monomer structure of TDCA-DOPO. Compared to TDCA-DOPO, the characteristic shifts of C-OH were completely disappeared in TDCAA-DOPO and the -CH=CH2 proton gave rise to signals at the range of 6.67-5.88 ppm, which demonstrated the existence of acrylate group. Additionally, the 31P NMR spectra of TDCA-DOPO and TDCAA-DOPO in Figure 2 showed single signals at 31.1 ppm and 26.2 ppm, respectively, because of the different chemical environment. All the above results confirmed the expected structure of TDCA-DOPO and TDCAA-DOPO.

11



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Figure 1. (a) FTIR spectra and (b) 1H NMR spectra of DOPO, TDCA-DOPO and TDCAA-DOPO.

Figure 2. 31P NMR spectra of (a) TDCA-DOPO and (b) TDCAA-DOPO.

Enhancement of cross-linking density and interfacial adhesion of FR-UPR by introducing TDCAA-DOPO To demonstrate the TDCAA-DOPO was successfully incorporated into the backbone of 12


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UPR resin by vinyl bond cross-linking reaction and TDCA-DOPO was not, their gel contents were tested by Soxhlet extraction，the detailed data was shown in Figure S1. The gel contents of UPR15 were 77 %, which was higher than that of UPR15* (69 %), indicating that TDCAA-DOPO involved the cross-linking reaction. Moreover, the corresponding fracture sections of UPR15 and UPR15* were also investigated by SEM characterization. As shown in Figure S2 the fracture section of UPR15 was smooth and almost no obvious interfaces were observed between TDCAA-DOPO and the UPR matrix. However, the interface of UPR15* was apparent and discontinuous because large TDCA-DOPO additives were dispersed on the fracture section. Due to the no chemical cross-linking interaction between UPR and TDCA-DOPO, the TDCA-DOPO existed as fillers and was prone to migrated from the matrix. This result was consistent well with the gel contents test. Besides these, to make clear what the difference between FR-UPR containing reactive or additive flame retardants, their thermal stability, flammability, and flame-retardant mechanism will be investigated and well discussed in the following parts.

Effect of reactive or additive FR on thermal stability of FR-UPR The thermal stability of FR-UPR is investigated by TGA. The TGA and DTG curves under both nitrogen and air atmosphere are shown in Figure 3 and the detailed data are listed in Table 3. Compared to pure UPR, a markedly increase of T5% (defined as the temperature of 5 wt% weight loss) was seen when TDCAA-DOPO was introduced, and the increase trend became more obvious for that containing more TDCAA-DOPO. For UPR20, it showed a highest T5% (310 °C), which was more than 30 °C than pure UPR. This can be due to that the higher initial degradation temperature of TDCAA-DOPO than pure UPR (shown in Figure S3 (a) and Table S1), resulting the improvement of thermal stability of 13



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FR-UPR. Additionally, with the TDCAA-DOPO contents increased, more rigid aromatic rings from the bi-DOPO group were incorporated into the backbone of UPR, corresponded to increased T5%.This result was similar to that reported by Hu et al.25 The temperature at maximum weight loss rate (Tmax) of UPR containing TDCAA was a bit higher than that of pure one with a strong DTG peak at 380 °C - 390 °C, which was due to the major degradation of TDCAA-DOPO and unsaturated polyester chain. The residual char at 700 °C of UPR20 was 13.8%, which was much higher than the limited 9.5% of pure UPR. This means that the TDCAA-DOPO promoted char formation of UPR. In air atmosphere, the decomposition behavior of all samples was different from that in nitrogen. Compared with the decomposition under nitrogen, a new second degradation process, appeared at the higher temperature region above 500 °C, was corresponding to further oxidation of the primary carbonaceous char.10 Just like in nitrogen, the T5% of TDCAA-DOPO modified UPR was much higher than pure one and the Tmax also had a slightly increase in first degradation process. Meanwhile, in the second decomposition stage, the FR-UPR still showed much higher Tmax, which was more than 34 °C higher than pure UPR. In addition, the residual char increased markedly as TDCAA-DOPO contents raised, indicating TDCAA-DOPO had stronger catalyzed char formation ability in air. To investigate the influence of participation of cross-linking reaction on the thermal stability of UPR resin, UPR contained 15wt% TDCA-DOPO, UPR15*, was chosen as a control and also investigated by TGA. Compared to UPR15, UPR15* showed three-stage degradation process whether in air or nitrogen atmosphere. The new small DTG peaks appeared earlier at 200 °C - 280 °C, accompanied by much lower T5%. Moreover, its residual char at 700 °C decreased by 27% in nitrogen and 54% in air atmosphere, respectively. These results were not only related to the thermal stability of the flame retardants themselves but also 14


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associated with the cross-linking density in UPR networks. As shown in Figure S3 (b) and Table S1, for TDCA-DOPO, the T5% was about 46 °C lower than TDCAA-DOPO, and the residual char also reduced by 20%in both nitrogen and air atmosphere. It was obvious that TDCAA-DOPO showed better thermal stability than TDCA-DOPO, resulting in preferable thermal stability of UPR15 than UPR15*.What’s more, the lower gel contents of UPR15*, indicated that the TDCA-DOPO was added only as a filler, which can’t change the thermal decomposition behaviors of UPR. To sum up, only by introducing TDCAA-DOPO into the UPR via cross-linking can change the thermal decomposition behavior and improve the thermal stability of the matrix.

Figure 3. TG and DTG curves of FR-UPR: (a), (b) in nitrogen atmosphere; (c), (d) in air atmosphere.

15



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Table 3. TGA data of FR-UPR in air and nitrogen atmosphere T5% (°C)

Sample

Tmax (°C)

Residual char (700°C,%)

air

N2

air

N2

air

N2

UPR

278

280

380/531

381

0.7

9.5

UPR5

296

293

386/566

388

4.0

11.3

UPR10

297

301

389/574

389

4.8

11.4

UPR15

298

306

390/568

390

6.5

11.5

UPR20

307

310

386/575

386

8.3

13.8

UPR15*

229

245

227/394/565

399

3.0

8.4

Effect of reactive or additive FR on flammability of FR-UPR LOI measurement is often used to evaluate the flammability of materials. The detailed data for all samples are listed in Table 4. As shown in Table 4, the FR-UPR showed higher LOI value than pure UPR and the value enhanced with increasing the content of TDCAA-DOPO. For UPR20 with the phosphorus content of 1.74 wt% presented the highest LOI value of 27.2%. Although this result is almost as same as that reported value,22, 23

the phosphorus content in UPR20 was lower than them. Besides, it was noteworthy that

the LOI value of UPR15 (25.8%) was higher than UPR15* (23.5%), which illustrated that cross-linking reactions between TDCAA-DOPO and styrene as well as unsaturated polyester are favorable to the improvement of UPR’s flame retardancy. Cone calorimetry was performed to evaluate the overall combustion performance of UPR and FR-UPR. The total heat release (THR) and peak heat release rate (PHRR) of all samples are shown in Figure 4 and the corresponding combustion data are also presented in Table 4. As we know, the PHRR value, was considered as the most important parameter in evaluating the fire safety of the materials, which determined the rate of fuel-feeding in the combustion and further flame spread rate. By incorporating TDCAA-DOPO into UPR, both the THR and PHRR values of FR-UPR decreased significantly compared to pure UPR. For 16


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UPR20, the PHRR and THR values were 442 kW·m-2 and 47 MJ·m-2, respectively, reduced as high as 44.3% and 42.7% compared to pure UPR. Meanwhile, its residual char increased to 12.7%, illustrating the incorporation of TDCAA-DOPO into UPR can promote the UPR resin carbonization during the combustion, which was consistent well with the TGA results. The formed cohesive char layers can act as a protective barrier to reduce heat release. Furthermore, we can see clearly from Table 3 that the PHRR and THR values of UPR15* were higher than UPR15, and its residual char was much lower than UPR15. These results demonstrated again that with the contribution of cross-linking reaction of TDCAA-DOPO in UPR15, more stable decomposition species instead of volatile products were formed during the decomposition, which led to high residual char and reduced mass transfer and exothermic reaction.35

Figure 4. Cone calorimetric curves of samples: (a) THR; (b) PHRR.

Table 4. Cone calorimetric and LOI data of FR-UPR. Sample

PHRR (kW/m2)

THR (MJ/ m2)

Residual char (%)

LOI (%)

UPR

794

82

1.4

22.0

UPR5

641

60

6.7

24.8

UPR10

582

54

8.2

25.3

UPR15

512

52

9.1

25.8

UPR20

442

47

12.7

27.2

UPR15*

609

65

4.3

23.5

17



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Effect of reactive or additive FR on char formation of FR-UPR The residual char of UPR, UPR15 and UPR15* after cone calorimetry test was evaluated by SEM. Figure 5 displays the SEM images of the outer and inner surface morphologies of residual char under different magnifications. As shown in Figure 5 (a) and (b), a flaky and fragile structure was observed on the inner surface of the residual char of pure UPR. The outer surface (Figure 5 (a') and (b')) exhibited loose and porous structure, which was caused by vigorous emission of volatile during the combustion. Compared to pure UPR, a rigid and integrate residual char was formed in inner surface of UPR15 (Figure 5 (c) and (d)). The outer surface of the residual char was rather compact and continuous (Figure 5 (c') and (d')). Thus, such residual char structure inhibited the diffusion of the combustible gaseous products from fire and suppressing combustion effectively. In contrast, for UPR15*, the residual char was partially continuous and some gas holes were displayed on the inner surface (Figure 5 (e) and (f)). Although it’s outer surface ((Figure 5 (e') and (f')) presented firmer structure than pure one, the char structure was not intact and numerous crack was left on it. Therefore, it was obviously that the UPR15 exhibited better residual char morphologies than UPR15*, corresponding to higher reduction in PHRR. Raman spectroscopy was also used to characterize the graphitization degree of a carbon material (Figure 6 (a-c)). The spectrum usually exhibited two strongly overlapping diffusion bands which were allowed to separate into G band (1580 cm-1) and D band (1360 cm-1). The intensity of G band correlated well with the ideal graphitic lattice of carbon material, whereas the D band intensity was related to a disordered graphite structure.36, 37 Finally, the graphitization was described by the G and D band intensity ratio. The higher IG/ID value signified a good structure with few defects. The IG/ID value follows the sequence 18


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of UPR (0.302)＜UPR15* (0.417) ＜UPR15 (0.429). The UPR15 possessed the highest graphitization degree and the best performance in char forming than UPR and UPR15*. This result illustrated that with the incorporation of flame retardants with bi-DOPO structure into UPR resin the better char morphology was formed. Meanwhile, the bi-DOPO structure flame retardant with reactive group promoted the formation of more stable graphitic structure for UPR.

UPR

UPR15

UPR15* Figure 5. SEM images of the inner residual char of samples: (a), (b) UPR; (c), (d) UPR15; (e), (f) UPR15* and their outer residual char: (a'), (b') UPR; (c'), (d') UPR15; (e'), (f') UPR15*.

19



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Figure 6. Raman spectra of residual char: (a) UPR; (b) UPR15; and (c) UPR15*.

To investigate the chemical component of the residual char, XPS analysis was used. Figure S4 shows comparatively the XPS spectra of C1s, O1s and P2p of the residual char of pure UPR, UPR15 and UPR15*, and the detailed contents of each element in the residual char are shown in Table S2. Besides, the detailed C1s XPS results were presented in Table 5. Cox stands for the oxidized carbons, while Ca denotes the aliphatic and aromatic carbons. The smaller Cox/Ca means the bigger condensation degree of the aromatic species.38, 39 From Table 4 we can see, the peak areas assigned to C-C/C=C in aliphatic and aromatic derivatives for UPR15 and UPR15* was both increased compared to pure one, corresponding to the decrease of Cox/Ca ratio. These results indicated that the incorporation of bi-DOPO group into UPR resin resulted in more polyaromatic species formation during the combustion process, which could play an important role in flame retardance. Additionally, compared to UPR15*, the Cox/Ca ratio of UPR15 was a bit lower, indicated that the UPR15 produced more polyaromatic species due to its increased cross-linking structures. These polyaromatic structures would finally turn to the residual char and benefit to improving the flame retardancy. Thus, UPR15 showed good char-forming performance in TGA and Raman spectroscopy tests.

20


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Table 5. C1s XPS data of residual char after cone calorimetry test for UPR, UPR15 and UPR15*. a

C-C

C=C

C-O-C

C=O

COOR

C/O

area (%)

area (%)

area (%)

area (%)

area (%)

(%)

(%)

UPR

58.4

21.1

14.9

3.0

2.7

3.9

0.26

UPR15

63.7

23.5

8.7

1.7

2.4

6.8

0.15

UPR15*

62.9

22.7

10.1

1.8

2.5

5.9

0.17

Sample

Cox/Ca

a: Cox stands for the oxidized carbons, Ca denotes the aliphatic and aromatic carbons.

Due to the presence of phosphorous compounds in the residual char of UPR15 and UPR15*, the XPS spectra of O1s and P2p showed differences to that of pure UPR. For the O1s spectra, the peak area at 531.4 eV and 532.7 eV increased slightly, corresponding to the chemical components of P=O and C-O-P/P-O-P groups, respectively. Combining the P2p spectra, these phosphorous compounds were mainly ascribed to polyphosphonate or polypyrophosphate in the residual char. Furthermore, it was obvious that the peak area at 134.3 eV for UPR15 (73.1%) was higher than UPR15* (65.0%). This result illustrated that more polyphosphonate or polypyrophosphate were formed in the residual char of UPR15 which meant the majority of phosphorus was remained in the condensed phase (shown in Table S2). These phosphorus compounds can promote carbonization to form phosphorus-rich residual char, which will inhibit the mass and energy transport between the gas and the materials, certainly lead to good fame retardancy of UPR resin.

Effect of reactive or additive FR on pyrolysis behaviors of FR-UPR In order to investigate the detailed decomposition processes based on the gaseous products, the pyrolysis experiments of UPR, UPR15 and UPR15* were performed using Py/GC/MS. The total ion chromatogram is shown in Figure 7, and the corresponding peaks of possible assignments are listed in Table S3. As shown in Table S3, for pure UPR, the 21



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main pyrolysis products (peak 1 - 24) were obtained from the decomposition of UPR matrix (including styrene) and rearrangement of free radicals. Based on the literature,4, 7the detailed pyrolysis processes of UPR resin was given in Figure S5. Compared to pure UPR, the introduction of TDCAA-DOPO into UPR didn’t change the main pyrolysis products of backbone significantly. However, some new peaks such as benzyl methyl ether radicals (peak 25, shown in Figure S7),40 terephthalaldehyde (peak 26), DOPO derivatives (peak 27 - 33), benzyl propionate derivatives (peak number 34 - 37) were observed in UPR15. Based on these data, the detailed pyrolysis process for TDCAA-DOPO units in UPR15 is deduced and shown in Figure 8. The pyrolysis process for TDCAA-DOPO units could be divided into two routes. One route was the cleavage at carbon–oxygen bonds41 to form prop-2-en-1-ol radicals and TDCA-DOPO groups. Part of the TDCA-DOPO dehydrated and decomposed to form dibenzofuran radicals (m/z 169, Figure S6), benzyl methyl ether radicals (m/z 119, Figure S7) and PO radicals (m/z 47, Figure S6).42 The others continued to degrade to form DOPO radicals and 1,4-phthalaldehyde. In the presence of active hydroxyl groups, the aldol condensation reaction for 1,4-phthalaldehyde was occurred and produced H2O molecules.40 Meanwhile, some of the DOPO radicals bound with methoxyl radicals, methyl radicals, ethyl radicals, hydroxyl radicals andprop-2-en-1-ol radicals to form phosphorus-containing organic compounds (peak 27 - 33), and remained in the condensed phase. Others were decomposed to form benzene radicals, PO radicals and dibenzofuran radicals. All of the released PO radicals can capture H and OH radicals to interrupt the chain reaction, which will retard the polymer degradation during combustion process.43

22


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Figure 7. Total ion chromatogram of UPR, UPR15 and UPR15*.

The other route was that the DOPO groups in TDCAA-DOPO were released first and then decomposed to form benzene radicals, PO radicals and dibenzofuran radicals like route 1. While the ester group didn’t cleave at carbon–oxygen bonds, instead, they rearranged to give vinyl compound (peak 37). The vinyl compound continued to re-combine with dibenzofuran, benzene or some other fragment ions degraded from UPR backbone, such as benzene and vinyl benzoate, to produce polycyclic aromatic hydrocarbons (Figure 9). These polycyclic aromatic hydrocarbons had higher molecular weight and more stable conjugated structure, which remained in condense phase and cannot be detected in Py/GC/MS.

23



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Figure 8. Proposed pyrolysis process for TDCAA-DOPO structure units in UPR15.

For UPR15*, considering TDCA-DOPO had partial common structure with TDCAA-DOPO, we deduced that UPR15* had a similar pyrolysis pathway with that of UPR15 (peak1 - 33). However, due to lack of vinyl bond in the ester group, some active fragment ions decomposed from the UPR matrix were not bound and rearranged to form polycyclic aromatic hydrocarbons structure. Instead, they would release small molecule free radicals and form combustible volatile, which played a negative role in char formation. That’s the reason why UPR15* showed lower residual char in the TGA test. Combining the results of TGA, SEM, Raman and XPS, the flame retardant mechanism of FR-UPR can be concluded. The introduction of flame retardants with bi-DOPO structure into UPR contributed to better flame-retardant effects both in condensed phase and gas phase. During burning, the bi-DOPO units will release PO or dibenzofuran radicals to capture H and O radicals and effectively inhibited the expansion of fire. In addition, part of 24


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phosphorus remained in the condensed phase to promote the carbonization, which made FR-UPR show higher residual char than pure UPR in TGA results. For UPR15, the reactive vinyl bonds in TDCAA-DOPO were inclined to form more conjugated polycyclic aromatic hydrocarbons structures in the pyrolysis process. These conjugated aromatic hydrocarbons structures had higher molecular weight, which led to lower volatility and had contributions to char formation in condense phase. Thus, UPR15 exhibited higher residual char and flame retardant efficiency than UPR15*.

Figure 9. The possible char formation mechanism for UPR15.

Dynamic mechanical property difference of two kinds of FR-UPR DMA is used to investigate the thermal mechanical properties of all FR-UPR. The results for the maximum peak temperature of tan δ and storage modulus (E') are revealed in Figure 10 (a) and (b). As shown in Figure 10 (a), the Tg of the pure UPR was 109.1 °C. When 5 wt% of TDCAA-DOPO was introduced, the Tg decreased to 103.7 °C. This result was 25



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ascribed to the plasticization effect of TDCAA-DOPO, which could increase the mobility of molecular chains, resulting in the reduction of Tg. With further increasing the amounts of TDCAA-DOPO, the Tg of FR-UPR moved to high temperature regions. For example, the Tg of UPR20 was up to 115.7 °C, which was higher 6.6 °C than pure UPR sample. As we all know, a higher cross-linked density or more-rigid chain segment for UPR will produce a higher Tg. It was clear that there were a lot of reactive vinyl bonds and rigid DOPO rings in TDCAA-DOPO, which eventually led to a relatively higher Tg. Notably, the Tg of the UPR15* was 105.8 °C, which was much lower than UPR15 (113.7 °C). Apparently, the lower cross-linking density of UPR15* had significantly effect on its Tg values. The E' of the FR-UPR (Figure 10 (b)) were also increased compared to the pure UPR resin, which were also related to the higher cross-linking density and the rigid molecular chain. Conclusively, the incorporation of TDCAA-DOPO significantly enhanced the Tg and storage modulus of the FR-UPR, which was benefit to its wide applications.

Figure 10. The tanδ (a) and storage modulus E' (b) curves of FR-UPR.

Conclusion A novel reactive phosphorus-containing flame retardant based on bi-DOPO structure (TDCAA-DOPO) was synthesized and incorporated into UPR via cross-linking reaction to 26


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obtain flame-retardant UPR (FR-UPR). Comparatively, an additive flame retardant without vinyl bond, also based on bi-DOPO structure (TDCA-DOPO), was used as a contrast. Both of them can endow UPR with good flame retardancy, i. e. FR-UPR show enhanced LOI values, lower values of PHRR and THR. The introduction of additive TDCA-DOPO resulted in poor thermal stability and decreased Tg for FR-UPR, while the addition of reactive TDCAA-DOPO enhanced the thermal stability and Tg of UPR. In addition, the flame retardant efficiency for TDCAA-DOPO was higher than TDCA-DOPO. These results revealed that the cross-linking reaction between TDCAA-DOPO and UPR resin played a positive role in improving the thermal properties and combustion behavior of FR-UPR. By investigating their flame retardant mechanism, it was found that that TDCAA-DOPO and TDCA-DOPO acted both in the gas phase via flame inhibition and in the condensed phase through charring. What’s more, more stable conjugated structures were produced by the rearrangement reaction of FR-UPR containing TDCAA-DOPO leaded to higher residual char, which acted as a protective barrier to reduce heat release during combustion. So, compared to FR-UPR containing additive TDCA-DOPO, the FR-UPR with reactive TDCAA-DOPO showed better flame retardance. This study reveals incorporation of reactive bi-DOPO based flame retardant into UPR, not only flame retardancy, thermal stability, but also Tg as well as storage modulus were improved. However, for that without reactive group, even it has similar structure with that reactive one, it cannot simultaneously enhance the flame retardancy, thermal stability and thermal mechanical properties of UPR.

27



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Page 28 of 32

Supporting Information The gel contents of cured UPR15 and UPR15*; SEM images of fracture section: (a), (b) UPR15 and (c), (d) UPR15*; TG and DTG curves of TDCAA-DOPO and TDCA-DOPO: (a) in nitrogen atmosphere; (b) in air atmosphere; TGA data of TDCA-DOPO and TDCAA-DOPO; XPS data analysis and element contents of residual char after cone calorimetry test; (a) C1s, and (b) O1s spectra of UPR residual char after cone calorimetry test; (c) C1s, (d) O1s, and (e) P2p spectra of UPR15 residual char after cone calorimetry test; (f) C1s, (j) O1s, and (h) P2p spectra of UPR15* residual char after cone calorimetry test; Peak numbers, retention time, molecular weight and major mass fragments for pyrolysis products; Proposed pyrolysis process for pure UPR; Mass spectrum assigned to 9,10-Dihydro-10-methyl-9-oxa-10-phosphaphenanthrene-10-oxide

(peak

28);

Mass

spectrum of benzyl methyl ether radical (peak 25).

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21634006, 51573104and 51421061), and the Sichuan Province Youth Science and Technology Innovation Team (No. 2017TD0006).

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Bi-DOPO Structure Flame Retardants with or ... - ACS Publications

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