Design and Synthesis of Efficient Phosphorus Flame Retardant for

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Design and Synthesis of Efficient Phosphorus Flame Retardant for Polycarbonate Chuanchuan Liu, and Qiang Yao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01915 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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Industrial & Engineering Chemistry Research

Design and Synthesis of Efficient Phosphorus Flame Retardant for Polycarbonate Chuanchuan Liu,†,‡ Qiang Yao*,†,‡ †

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, P.R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P.R. China

* Corresponding author: Tel: +86 574 87911135; Tax: +86 574 86685186; E-mail address: [email protected]

ABSTRACT: Phosphorus flame retardants have been considered to be ineffective for polycarbonates. In light of the recognition that the high efficiency of sulfonate salts is due to the base catalyzed decomposition of polycarbonate, a tertiary amine group is incorporated into a 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivative which was synthesized from DOPO, paraformaldehyde, and piperazine (DFPZ). DFPZ shows excellent flame retardancy and enables PC to achieve a UL-94 V0 rating at only 3%. In contrast, in spite of the close similarities in the structures, DPZ with a direct P-N bond synthesized from DOPO and piperazine is much less effective. It requires 10% of DPZ for PC to gain a UL-94 V0 rating. Based on the study of diphenyl carbonate/flame retardants and the results of TGA, FTIR, ICP, and morphology of PC/flame retardants, the base catalyzed decomposition of PC that

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leads to severe dripping is the key factor for the high efficiency of DFPZ.

KEY WORDS: DOPO; Base catalyst; Polycarbonate; Flame retardancy

1. INTRODUCTION Bisphenol A based polycarbonate (PC) is widely used in the fields of electric and electronics, automotive and construction due to its outstanding merits such as good hardness, stiffness, impact strength, transparency, dimensional stability, and thermal stability.1 Although PC shows a V-2 rating in the UL-94 test inasmuch as PC is a naturally high charring polymer,2-3 a higher rating of flame retardancy is often required in the electric and electronic application.4 Consequently, many flame retardants for polycarbonate have been developed, especially those based on halogen free alternatives in consideration of environmental problems associated with brominated flame retardants and related regulations.5-15 For example, sulfonate salts and silicon based flame retardants have achieved great commercial successes due to their remarkable effectiveness in PC at very low levels (typically less than 1 wt% of loading). Notably, in spite of phosphorus flame retardants’ wide application in PC/ABS alloy, they have been commercially disfavored in pure PC, primarily owing to their poor efficiency when compared to sulfonate salts or silicon based flame retardants.3, 15-19 To boost performance, phosphorus flame retardants with a significant vapor phase action have been sought and found to be beneficial.5 However, their loading levels were still several times higher than sulfonate or silicon based flame retardants. Since the high efficiency of sulfonate salts is largely attributed to the alkaline catalyzed


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decomposition of polycarbonate in the condensed phase,20 we speculated that an introduction of an amino group into phosphorus compounds with a vapor phase action could take the best of both worlds to raise the efficiency. Thus, a 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivative was first considered since it is well known that the DOPO moiety can produce PO• during its decomposition which is able to quench the flame in the vapor phase.21-24 Additionally, to avoid the potential aminolysis reaction of carbonate and the premature breakdown of PC chain, primary or secondary amino groups were excluded. In the end, DOPO derivatives with tertiary amine groups in the molecule were designed. Since an amine group immediately adjacent to the phosphinyl group is stabilized, its basicity is greatly reduced. Hence, the comparison of the amine group in different positions of molecules might give an insight of the effect of the base catalyzed reactions on the efficiency. In this paper, we report the flame retardancy of a condensation product of DOPO-formaldehyde-piperazine 25

DOPO-piperazine (DPZ)

(DFPZ)

and

a

known

compound

of

in polycarbonate. The comparison of DFPZ/PC and

DPZ/PC enables us to gain the understanding of the mechanism of the flame retardancy by the basic amino groups and serves as the basis to design better phosphorus based flame retardants in the future.

2. EXPERIMENTAL 2.1. Materials Piperazine, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and


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diphenyl carbonate (DPC) were purchased from Aldrich. Paraformaldehyde, triethylamine, tetrachloromethane, chloroform, dichloromethane, Toluene, methanol, and ethanol were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). All materials were used without further purification. Polycarbonate (141R) was purchased from GE plastic. Prior to compounding, the polymer pellets were dried at least 4 h at 120 °C in air. 2.2. Synthesis of Flame Retardants The preparation routes of the flame retardants are presented in Scheme 1. DFPZ was prepared in two steps according to a method adapted from the reference,26 and DPZ was synthesized according to the procedure reported in the literature.25 Scheme 1. Synthesis of DFPZ and DPZ.

Synthesis of the intermediate N, N′-bis(methoxymethyl)piperazine (BMP): To a 250 mL three-necked flask equipped with a temperature controller, a magnetic stirrer, and a reflux condenser were charged 8.60 g (0.10 mol) of piperazine, 6.00 g (0.20 mol) of paraformaldehyde, and 100 mL of methanol. The mixture was heated to reflux and maintained for 2 h. Then 7.00 g (0.050 mol) of K2CO3 was added under stirring and the mixture was left for 12 h. The solution was filtered and the solvent was removed under a reduced pressure. The residue was recrystallized from diethyl ether. The white


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crystals were dried to a constant weight under vacuum at room temperature (yield: 87%, Mp = 40 °C). 1H NMR (CDCl3): δ=4.01 (s, -CH2-O), 3.32 (s, CH3-O), 2.70 (s, N-CH2-CH2) ppm. Synthesis of DFPZ: a mixture of BMP (8.70 g, 0.050 mol), DOPO (21.60 g, 0.10 mol), and toluene (150 mL) in a 250 mL flask equipped with a temperature controller, a magnetic stirrer, and an outlet condenser was heated gradually to 80 °C under N2 atmosphere until the distillation of methanol was completed. The mixture was filtered and the residue was washed three times with ethanol to give an off-white powder which was dried to a constant weight in vacuum at 80 °C (yield: 76%, Mp = 224 °C). 1

H NMR (CDCl3): δ=8.00-7.94 (m, 4H), 7.90 (d, 3JHH=7.7Hz, 2H, O-C-C-CH) 7.69 (t,

2H, P-C-C-CH-CH), 7.49 (td, 3JHH=7.5Hz, 4JPH=2.9Hz, 2H, P-C-CH-CH), 7.34 (t, 2H, O-C-CH-CH), 7.22 (t, 2H, O-C-C-CH-CH), 7.15 (d, 3JHH=7.7Hz, 2H, O-C-CH), 2.91-2.88 (m, 4H, P-CH2), 2.17-2.10 (m, 8H, N-CH2-CH2) ppm.

31

P NMR (CDCl3):

δ=31.90 (s, 0.261), 31.71 (s, 0.739) ppm. 13C NMR (CDCl3): δ=150.1 (d, 2JPC=8.7Hz, O-C), 136.2 (d, 2JPC=6.0Hz, P-C-C), 133.3 (s, P-C-C-CH-CH), 130.9 (d, 2JPC=10.4Hz, P-C-CH), 130.4 (s, O-C-CH-CH), 128.3 (d, 3JPC=12.8Hz, P-C-CH-CH), 124.9 (s, O-C-C-CH), 124.3 (s, O-C-C-CH-CH), 124.1(d, 3

JPC=9.4Hz, P-C-C-CH), 122.3 (d,

3

1

JPC=117.8, P-C),123.3 (d,

JPH=9.8Hz,O-C-C), 119.4 (d,

3

JPC=4.3Hz,

O-C-CH), 56.4 (d, 1JPC =122.7Hz, P-CH2), 54.5 (d, 3J=8.3Hz, N-CH2-CH2) ppm (Figure S1-S3). Synthesis of DPZ: DOPO (21.60 g, 0.10 mol), piperazine (8.60 g, 0.050 mol) and triethylamine (10.10 g, 0.10 mol) were dissolved in 100 mL of dichloromethane,


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stirred and cooled to 0 °C. Then, carbon tetrachloride (15.40 g, 0.10 mol) was added dropwise at such a rate that the reaction temperature did not exceed 15 °C. The mixture was allowed to warm up to room temperature and maintained overnight after the complete addition. The solution was filtered and the precipitate was washed intensively with water to remove the triethylamine hydrochloride. The resulting white solid was washed twice with ethanol and then dried to a constant weight in vacuum at 60 °C (yield: 79%, Mp ＞ 300 °C). 1H NMR (CDCl3): δ=8.02 (t, 2H, P-C-C-CH), 7.95 (d,

3

JHH=8.3Hz, 2H, O-C-C-CH), 7.85 (q, 2H, P-C-CH), 7.71 (t, 2H,

P-C-C-CH-CH), 7.53 (td, 3JHH=7.5Hz, 4JPH=2.9Hz, 2H, P-C-CH-CH), 7.40 (t, 2H, O-C-CH), 7.27-7.24 (m, 4H), 3.25-3.23 (m, 8H, N-CH2) ppm. δ=14.70 (s, 0.645), 14.68 (s, 0.355) ppm.

31

P NMR (CDCl3):

31

C NMR (CDCl3): 149.9 (d, 3JPC=7.8Hz,

O-C), 137.6 (d, 2JPC=7.3Hz, P-C-C), 133.0 (d, 4JPC=1.9Hz, P-C-C-CH-CH), 130.4 (s, O-C-CH-CH), 129.8 (d, 2JPC=7.3Hz, P-C-CH), 128.3 (d, 3JPC=14.7Hz, P-C-CH-CH), 124.9 (s, O-C-C-CH), 124.4 (s, O-C-C-CH-CH), 123.7 (d, 3JPC=11.5Hz, P-C-C-CH), 123.3 (d, 1JPC=167.6 Hz, P-C), 121.6 (d, 3JPC=12.1Hz, O-C-C), 120.4 (d, 3JPH=6.2Hz, O-C-CH), 44.2 (m, N-CH2) ppm (Figure S4-S6). These results are in accord with the literature. 2.3. Preparation of PC/PFR Blends Flame retardant PC blends were prepared via melt compounding at 250 °C in a Brabender mixer with a roller speed of 50 rpm. The mixing time was 5 min for all PC/PFR samples. The composites prepared were transferred into a mold which had been preheated at 245 °C, then pressed at 10 MPa for 5 min, followed by pressing at


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room temperature for 10 min. The sample plaques obtained were cut into specific dimensions and stored for further tests. 2.4. Study of Diphenyl Carbonate with Flame Retardants Into a flask equipped with a condenser attached by a collection bottle were charged 2.14 g (0.010 mol) of DPC and 0.0050 mol of DFPZ or DPZ under nitrogen. The mixture was heated to 230 °C in 20 min, and then kept at 230 °C for 15 min. After cooling down, the white solid condensed on the condenser was collected and weighed. The compositions were analyzed by GC-MS. In the case of DPC/DFPZ, a total of 0.47 g of white solid was obtained. The analysis of GC-MS indicated that it was composed of 89.7 wt% phenol and 10.3 wt% DPC. Thus，the yield of phenol was 0.46 mol/mol based on DPC. On the other hand, no solid was obtained in the condenser and only raw materials were detected in the reaction mixture when DPC/DPZ was heated under the same conditions. 2.5. Characterization The NMR analyses of the newly synthesized flame retardants were performed on a Bruker 400 AVANCE spectrometer. The 1H, 31P and 13C NMR measurements were run at a frequency of 400, 162 and 100 MHz, respectively. Trichloromethane-d1 (CDCl3) was used as solvent. Thermal gravimetric analysis (TGA) experiments of the flame retardants and their PC composites were performed on a Mettler Toledo TGA/DSC Analyzer. 3~5 mg samples were heated in aluminum crucible from 50 °C to 600 °C in a nitrogen atmosphere (50 mL/min) at a heating rate of 10 °C/min.


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GC-MS analyses of the condensed products from the reactions between DPC and PFR were conducted on an Agilent 7890B-5977A. The helium (He) was utilized as a carrier gas for the volatile products. The temperatures of the injector and GC/MS interface were 280 °C. UL-94 vertical burning tests of the flame retardant composites were conducted on an AG5100B vertical burning tester (Zhuhai Angui Testing Equipment Company, China) with sample dimensions of 100mm × 13mm × 3.2mm according to ASTM D3801. The limiting oxygen index (LOI) tests of the flame retardant composites were performed on a 5801 digital oxygen index analyzer (Kunshan Yang Yi test Instrument Co., Ltd.) with sample dimensions of 100mm × 6.5mm × 3.2mm according to ASTM D2863-97. Fourier Transform infrared spectra (FTIR) of PC or PFRs at specific temperatures were recorded on a Nicolet 6700 FTIR spectrometer at 4 cm-1 resolution. KBr pellets containing samples were used.28 The attenuated total reflectance infrared spectra (ATR-IR) of the residues obtained from UL-94 experiments were recorded on a Cary660+620 micro-FTIR spectrometer (Agilent, USA). TGA-IR experiments of the flame retardant composites were conducted on the same equipment above, the temperature of the transferring line between TGA and FTIR was set at 200 °C. About 8 mg samples were heated from 50 °C to 700 °C in a nitrogen atmosphere (50 mL/min) at a heating rate of 10 °C/min. The spectra were collected every 40 s for 70 min.


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Scanning electron microscopy (SEM) experiments of the residues obtained from UL-94 burning were performed in an S-4800 scanning electron microscope (Hitachi Corp., Japan). All samples for SEM were sputtered with a thin layer of gold before examination. The phosphorus contents in charred residues were measured by the inductively coupled plasma optical emission spectrometry method (ICP-OES) on a Perkin Elmer Optima 2100DV apparatus. The charred residues were obtained in the oxygen index analyzer at an oxygen concentration of 3% above LOI in order to support the complete burning of the sample without the afterburning of char.27 The char yield is the weight ratio of sample after and before burning in LOI apparatus. Sample preparation for ICP-OES consisted of mixing the residue with aqua regia (6 mL per 0.10 g of sample), followed by digestion in a microwave. The suspensions were then filtered and diluted to 100 ml with HNO3 for analysis. Heat distortion temperatures (HDT) of the flame retardant composites were measured with sample dimensions of 80mm × 10mm × 4mm according to GB/T1634.2-04 in a Zwick/Roell Z020 testing machine (Ceast, Italy). Gel permeation chromatography (GPC) experiments were performed on a PL-GPC/220 (Polymer, UK) to determine the molecular weight (Mw) of samples. About 20 mg samples were dissolved in 3.0 mL chloroform (chromatographic grade), and then filtered to remove the insoluble. 1.5 mL sample solution was used for GPC test and the temperature of chromatographic column was kept at 40 ºC. 3. Results and Discussion


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3.1. TGA Analysis Figure 1 shows the TGA curves of two flame retardants and the experimental as well as the calculated curves of PC/flame retardants in nitrogen. Clearly DFPZ displays two steps of decomposition with the first one in the temperature range of 300-370 °C and the second step between 370-450 °C. In contrast, DPZ exhibits a single degradation step and a high thermal stability with Td5% = 349.2 °C as shown in Table S1. However, both DFPZ and DPZ essentially lose all of their masses to the vapor phase with R600 = 3.1% and 0.8% at 600 °C, respectively.

Figure 1. The experimental (exp.) and calculated (cal.) TGA curves of PC/PFRs in nitrogen. The relatively low thermal stability of DFPZ can be attributed to the weak bond of P-CH2N.29 Although it is generally accepted that the P-C bond is chemically and thermally stable, the presence of α-amino group can stabilize the C• after the homolysis of P-C bond and thus the cleavage of P-CH2N bonds takes place at the reduced temperature.30 Since the tertiary amine group formed after a hydrogen abstraction is a light fraction, it will vaporize in the first decomposition stage of DFPZ.


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Therefore, its effect on the decomposition of polycarbonate, if any, will be limited to the low temperature. The easy loss of the tertiary amine group is further confirmed by the study of the degradation of DFPZ at different temperatures via infrared spectroscopy. Figure 2 shows the IR spectra of DFPZ with temperatures. It is noted that DFPZ almost loses all of its aliphatic C-H (2800-3000 cm-1) and P-C (821 cm-1) structures at 330 °C，but the peaks associated with the DOPO moiety can be still seen at 370 °C and higher. Therefore, the first stage of decomposition of DFPZ must involve the cleavage of P-CH2N and loss of piperazine structure as illustrated in Scheme 2. However, since the total weight fraction of piperazine moiety and methylene is only about 21%, some DOPO fraction has to be lost in order to account for the 50% mass loss in the first stage. The second step mainly involves the decomposition of secondary products containing the DOPO moiety which have been produced from the first degradation step.

Figure 2. FTIR spectra of DFPZ at different temperatures.


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In addition, the experimental curve of PC/3%DFPZ in Figure 1 is notably lower than the calculated one, indicating that DFPZ promotes the degradation of PC. However，since Tdmax of PC/3%DFPZ is close to that of the pure PC, the major aromatization chemistry leading to the generation of char does not change significantly. Thus, the main effect of DFPZ is to initiate the decomposition of PC. In the case of PC/3%DPZ, it has an experimental TGA curve that goes across the calculated one after a theoretical mass loss attributed to DPZ, suggesting that PC has a stabilizing action on the decomposition of DPZ; however, this stabilization turns to accelerate the degradation of PC itself. With the increase of DPZ to 10%, the stabilization of PC on DPZ in the early stage and the destabilization of DPZ on PC in the late stage become evident with a difference of Tdmax value of about 25 ºC between PC and PC/10%DPZ. This stabilization-destabilization likely stems from the exchange reaction of carbonate and phosphoramide at high temperatures. Scheme 2. The proposed degradation paths of DFPZ.

3.2. Study of Diphenyl Carbonate/FR The degradation of diphenyl carbonate has been investigated in details and the results have been used to interpret the decomposition of polycarbonate.31-32 It has been


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reported that the degradation of diphenyl carbonate catalyzed by inorganic bases begins with the rearrangement to 2-phenoxybenzoic acid, from which phenol is produced via an acid-ester exchange reaction as shown in Scheme 3. Since there are no other foreseeable base catalyzed routes to the generation of hydroxyl group, which leads to the production of phenol, the detection of phenol strongly suggests the rearrangement of diphenyl carbonate to the branching carboxylic acids and the subsequent acid-ester exchange reactions. This has significance since the presence of aromatic alcohols can be used to deduce the similar reactions in the case of polycarbonate. When diphenyl carbonate was treated with DFPZ at 230 °C, a high yield (92%) of phenol was generated, indicating that DFPZ is a powerful catalyst for the rearrangement of diphenyl carbonate. Since diphenyl carbonate is a good model compound for polycarbonate, it is reasonable to assume that DFPZ works in a similar way in polycarbonate. This assumption is consistent with the results shown in Figure 1 where the degradation of polycarbonate commences at a much lower temperature in the presence of DFPZ than it should be. In contradiction to DFPZ that acted as a catalyst in the degradation of diphenyl carbonate, DPZ showed little chemical interactions with diphenyl carbonate at 230 ºC. Accordingly, a theoretical quantity of diphenyl carbonate was recovered when it was subjected to the reaction with DPZ at the same conditions as those used in DPC/DFPZ. This indicates that the amino group adjacent to the phosphinyl group essentially is inactive toward the rearrangement of diphenyl carbonate, and likely polycarbonate


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too. Scheme 3. Rearrangement of diphenyl carbonate (DPC).

3.3. Flammability The vertical burning classifications (UL-94) and limiting oxygen index (LOI) values for PC and the PC/FR composites are listed in Table 1. It can be seen that with an addition of only 3% DFPZ, PC remarkably achieves a UL-94 V0 rating. In comparison, a 10% of DPZ is required to get the same rating even though it has higher phosphorus content than DFPZ. Also, a 3% of DFPZ works as much as a 7% DPZ in the LOI experiment. Considering the close similarities of their structures, the vast difference in their flame retardancy is highly fascinating. Table 1. the Flame retardancy of PC and its composites with different loadings of PFRs.

Formulations

t1/t2 (s)

UL 94 (3.2mm) Cotton Dripping* ignited

LOI Rating

%

PC

1.1/9.2

NDI, DAI

Y

V2

26.3

PC/1%DFPZ

0.4/0.9

DI

Y

V2

26.8

PC/3%DFPZ

0/0

SDI

N

V0

32.3

PC/5%DFPZ

0/0

SDI

N

V0

36.8

PC/1%DPZ

3.9/45

NDI, DAI

Y

NR

27.5

PC/3%DPZ

0/32

NDI, DAI

Y

NR

27.9

PC/5%DPZ

0/3.4

DI

Y

V2

28.5

PC/7%DPZ

0/0

SDI

Y

V2

32.7


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PC/10%DPZ

0/0

SDI

N

V0

34.5

PC/3%DFPZ +0.5%PTFE

3.7/14

NDI, DAI

Y

V2

29.1

0/4.5

ND

N

V0

34.7

PC/10%DPZ +0.5%PTFE

* DAI = dripping after the removal of the ignition source; DI = dripping during ignition; NDI = no dripping during ignition; SDI=severe dripping during ignition; ND = no dripping.

One possible explanation to the distinctly different flame retardancy is the enormous dissimilarity in the degree of the dripping. Upon applying an ignition source, PC/3%DFPZ exhibits a severe dripping while there are no drips collected during the ignition of PC/3%DPZ or PC itself as shown in Table 1. Consequently, the latter two lack the good mean timely to remove the heat and continue to burn after the removal of the ignition source. The dripping suggests that the melt-away might operate as a key mechanism for the excellent flame retardancy of DFPZ. This presumption is proved by adding 0.5wt% of PTFE to PC/3%DFPZ. The presence of PTFE completely eliminates the dripping of PC/3%DFPZ during the ignition stage and largely suppresses it after the removal of the ignition source. In the end, weak flame retardancy of a UL-94 V-2 rating is obtained and a reduced LOI value of 29.1% from 32.3% of the original PC/3%DFPZ is observed as seen in Table 1. The poor flame retardancy in the presence of PTFE strongly supports the melt-away to be primarily responsible for the excellent flame retardancy of DFPZ. On the other hand, although the addition of 0.5wt% of PTFE prolongs the combustion time of PC/10%DPZ, which suggests the mechanism of the melt-away to some degree since PTFE eliminates the dripping, the LOI value of PC/10%DPZ/PTFE changes a little


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from that of PC/10%DPZ. Thus, other modes of the flame retardancy of DPZ must operate. Furthermore, the observations of the severe drippings in the UL-94 test of PC/3%DFPZ point to an enhanced degradation of PC that has been proven to be crucial to gain a high efficiency of flame retardancy.20, 33 To examine the degradation of PC and further understand the difference in the flame retardancy, TGA-FTIR of the flame retardant PC were performed at different temperatures and the compositions of char residues were analyzed. 3.4. TGA-FTIR Analysis of Gaseous Products Released from PC and Its Blends The FTIR spectra of PC, PC/3%DFPZ, and PC/3%DPZ with time reported here have been normalized by the mass of each sample. Figure 3a-c shows that all samples produce similar gaseous products during their degradations with aromatic alcohols at 3653 cm-1 and 1176 cm-1, methane at 3015 cm-1, CO2 at 2358 cm-1, CO at 2181 cm-1, carbonates at 1775 cm-1, and aromatic ethers at 1252 cm-1. However, the time (temperature) to generate the comparable concentrations of aromatic alcohols or CO2 are much shorter (lower) for PC/DFPZ than PC/DPZ or PC itself as shown in Figure 4. These results are the reflectance of the based catalyzed decomposition of carbonates, and signify that DFPZ catalyzes the rearrangement of PC to the branching carboxylic acids which undergo the decarboxylation or generate the aromatic alcohols through the known acid-ester exchange reaction as illustrated in Scheme 3.


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Figure 3. 3D image of TGA-FTIR results of PC (a), PC/3%DFPZ (b), PC/3%DPZ (c) and Total absorptions of the gaseous products (d). In contrast to the evolution of aromatic alcohols and CO2, the upward curves of methane or CO are overlapped in all samples, suggesting that DFPZ does not interact with methyl group. However, the relative amount of CH4 and CO generated is much less for DFPZ than DPZ, which is still less than the virgin PC. This is likely caused by the condensation of some heavy molecules on the wall of the transferring line between the IR cell and the TGA equipment. Oily materials were found in the transferring line and collected via acetone flush after TGA-IR experiments. The characterization by FTIR of the oily materials proved that they were largely similar to polycarbonate, thus they were mainly oligomeric species. Due to the small size of the samples used in the TGA-IR experiments, the amounts of the oily materials were hard to be quantified. However, the total absorptions of the gaseous products decrease in


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the order of PC > DPZ > DFPZ as seen in Figure 3d, suggesting that the quantities of the oily materials in the transferring line are decreased in the reverse order. Therefore, the amount of CH4 is reduced for DFPZ. The lessened amount of CH4 also supports that DFPZ catalyzes the decomposition of polycarbonate so that the latter generates a significant amount of the heavy oligomeric species which are volatile but condense in the transferring line.

Figure 4. Evolved gases from PC and PC/flame retardants measured by TGA-FTIR. 3.5. ICP analysis of Char Residues Table 2 shows that about half of total phosphorus volatilizes into the vapor phase for PC/3%DFPZ while only 36% of phosphorus of PC/3%DPZ remains in the condensed phase, suggesting that there are some vapor phase actions for both flame retardants but the vapor phase action should be not the most important factor. Otherwise, DPZ would have worked more efficiently than DFPZ. However, the vapor


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phase action of DPZ may explain the good flame retardancy for PC/10%DPZ in the presence of 0.5% PTFE as shown in Table 1. Table 2. Phosphorus distribution in the condensed phase after the forced LOI tests. P in PC/PFRs (%) PC/3%DFPZ 0.34 PC/3%DPZ 0.36 Sample

Char yield (%) 41.67 44.90

P in char (%) 0.42 0.29

P retained (%) 50.91 36.04

3.6. Morphology Analysis

Figure 5. SEM photographs of residues after UL-94 tests: (a) PC, (b) PC/3%DFPZ, (c) PC/3%DPZ. Figure 5 shows the morphology of the residues of virgin PC and PC/flame retardants after burning. The residue produced by the combustion of PC is continuous as seen in Figure 5a. Besides, its ATR-IR spectrum in Figure 6 displays the ambiguous peaks of the virgin PC, suggesting the formation of char. It is likely that the continuous layer of char helps PC to gain a V2 rating.


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Figure 6. The ATR-IR spectra of residues after UL-94 test and virgin PC. In the case of the PC/3%DFPZ, a smooth surface is observed as demonstrated in Figure 5b. Surprisingly, a close examination of both the residue and the drips collected during the UL-94 tests by the ATR-IR spectrum reveals the presence of all the characteristic peaks of virgin PC as shown in Figure 6 (an ATR-IR spectrum of the residue of PC/3%DFPZ shown). These results indicate that the amount of the char is insignificant and hence not a decisive factor in the flame retardancy. More importantly, the ATR-IR results suggest that the surface temperatures of both the residue and the drips are not higher enough during the ignition stage so that PC keeps most of its structures relatively intact. In view of the severe drippings observed during the UL-94 test, the low surface temperature of the residue is reasonably assumed to result from the fast heat removal by the dripping low melts which have been produced by the intensified breakdown of PC chain. For the char of PC/3%DPZ, it displays a scorching pattern with many cracks as seen in Figure 5c. Also, the ATR-IR spectrum of the residue’s surface in Figure 6


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essentially shows the absence of virgin PC. These results imply that its surface must have experienced high temperatures, a result of the reduced degree of drippings. 3.7. Mechanism of Fire Retardancy Based on the above results, the mechanism of flame retardancy of DFPZ is proposed as follows: first it catalyzes the rearrangement of PC through the active tertiary amine group in a very efficient way as evidenced by the DPC/DFPZ experiment. The rearrangement generates carboxylic acids which are subject to the acid-ester exchange reaction. This exchange reaction is normally slow under neutral or acidic conditions, but is highly facilitated by the base catalysts.34 The base catalyzed exchange reaction causes the effective breakdown of the PC chain and generates oligomeric species that produce the severe dripping as observed. The dripping carries away the heat and lowers the surface temperature. A low surface temperature was also observed in the sulfonate salt’s flame retarded PC.35 However, an intumescent effect was credited in that case. Considering the chemical structures of the residue and the drips that are largely similar to those of the virgin PC as evidenced by ATR-IR as well as the reduced flame retardancy in the presence of PTFE, it is reasonable to propose that the dripping is the key factor for the high efficiency of DFPZ. 3.8. Heat Distortion Temperature (HDT) Characterization of PC/PFRs PC with 3wt% DFPZ has a heat distortion temperature of 121.5 °C and the HDT of PC/3wt%DPZ is 125.1 °C seen in Table 3. Both of these values are lower than that of the virgin PC (130.5 °C), suggesting that the flame retardants have significant


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plasticizing actions on PC. However，in the case of PC/3%DFPZ, a reduced molecular weight is noted after processing as seen in Table 3. GPC analyses indicate that the Mw of PC/3wt%DFPZ is 1.09×104, which is a bit lower than that of the processed PC of 1.39×104. This should partially contribute to the reduced HDT for PC/3wt%DFPZ and also supports the accelerated degradation of PC by DFPZ as discussed before. Table 3. Heat distortion temperature and number-average molecular weight of PC/PFR composites Sample HDT/°C 4

Mw/(×10 )

PC

PC/3%DFPZ

PC/3%DPZ

PC/10%DPZ

130.5

121.5

125.1

118.7

1.39

1.09

1.32

1.01

4. CONCLUSION In an effort to design competent phosphorus based flame retardants for polycarbonate, DFPZ and DPZ were synthesized and their efficiencies of flame retardancy were compared. Despite the close similarities in the structures, DFPZ and DPZ showed drastically different flame retardancy in polycarbonate. DFPZ is more than two times effective than DPZ. Although both flame retardants display some vapor phase actions, the excellent flame retardancy of DFPZ primarily stems from its ability to accelerate the degradation of PC through the active tertiary amine group. DFPZ is proven to be a rare example that shows the core element of the flame retardancy of phosphorus flame retardant does not come from the action of phosphorus, but from those of its auxiliary group. In the future, an organic dormant base that becomes active at just below combustion temperatures will be considered in order to keep good physical properties of PC.


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ASSOCIATED CONTENT Supporting Information The related TGA data of PC and PC/PFR composites; The NMR spectra (1H, 31P, 13C) of newly synthesized PFRs.

AUTHOR INFORMATION Corresponding Author *Tel:

+86

574

87911135,

Tax:

+86

574

86685186,

E-mail

address:

[email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Financial supports from the Ningbo Science and Technology Innovation Team (No. 2015B11005)

and

Guangdong

Provincial

Science

and

Technology

(No.

2015B090925011) were greatly acknowledged.

REFERENCES (1) Bendler, J. T. Handbook of Polycarbonate Science and Technology; Crc Press: New York, 1999; pp 341-343. (2) Jang, B. N.; Wilkie, C. A. A TGA/FTIR and mass spectral study on the thermal degradation of bisphenol A polycarbonate. Polym. Degrad. Stab. 2004, 86 (3), 419-430. (3) Feng, J.; Hao, J. W.; Du, J. X.; Yang, R. J. Flame retardancy and thermal properties of solid bisphenol A bis(diphenyl phosphate) combined with montmorillonite in polycarbonate. Polym. Degrad. Stab. 2010, 95 (10), 2041-2048. (4) Itagaki, H. Flame-retardant polycarbonate resin composition and electrical and electronic components made by molding the same. U.S. Patent 6,423,766, July 23, 2002. (5) Huang, K.; Yao, Q. Rigid and steric hindering bisphosphate flame retardants for polycarbonate. Polym. Degrad. Stab. 2015, 113, 86-94. (6) Hu, Z.; Li, C.; Zhao, B.; Luo, Y.; Wang, D. Y.; Wang, Y. Z. A novel efficient


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

halogen-free flame retardant system for polycarbonate. Polym. Degrad. Stab. 2011, 96, 320-327. (7) Hou, S. J.; Zhang, Y. J.; Jiang, P. K. Phosphonium sulfonates as flame retardants for polycarbonate. Polym. Degrad. Stab. 2016, 130, 165-172. (8) Son, S. M.; Lee, R.; Lee, S. H.; etc. Non-halogen flame retardant and high rigidity polycarbonate resin composition. U.S. Patent 9,056,978, June 16, 2015. (9) Lu, S. Y.; Hamerton, I. Recent developments in the chemistry of halogen-free flame retardant polymers. Prog. Polym. Sci. 2002, 27, 1661-1712. (10) Levchik, S. V.; Weil, E. D. Overview of recent developments in the flame retardancy of polycarbonates. Polym. Int. 2005, 54, 981–998. (11) Innes, J.; Innes, A. Flame retardants for polycarbonate - new and classical solutions. Plast. Add. & Comp. 2006, 8, 26-29. (12) Weil, E. D.; Levchik, S. V. Flame Retardants in Commercial Use or Advanced Development in Polycarbonates and Polycarbonate Blends. Flame Retardants 2009, 26, 121-140. (13) Lanzinger, D.; Salzinger, S.; Soller, B. S.; Rieger, B. Poly(vinylphosphonate)s as Macromolecular Flame Retardants for Polycarbonate. Ind. Eng. Chem. Res. 2015, 54, 1703-1712. (14) Zhang W. C.; Li. X. M.; Yang R. J. Flame retardant mechanisms of phosphorus-containing polyhedral oligomeric silsesquioxane (DOPO-POSS) in polycarbonate composites. J. Appl. Polym. Sci. 2010, 124, 1848-1857. (15) Cheng B. F.; L. X. M.; Hao J. W. The effect of pyrolysis gaseous and condensed char of PC/PPSQ composite on combustion behavior. Prog. Polym. Sci. 2016, 129, 47-55. (16) Levchik, S. V.; Weil, E. D. Flame Retardants in Commercial Use or in Advanced Development in Polycarbonates and Polycarbonate Blends. J. Fire Sci. 2006, 24, 137-151. (17) Pawlowski, K. H.; Schartel, B.; Fichera, M. A.; Jäger, C. Flame retardancy mechanisms of bisphenol A bis(diphenyl phosphate) in combination with zinc borate in bisphenol A polycarbonate/acrylonitrile–butadiene–styrene blends. Thermochim Acta 2010, 498, 92-99. (18) Pawlowski, K. H.; Schartel, B. Flame retardancy mechanisms of aryl phosphates in combination with boehmite in bisphenol A polycarbonate/acrylonitrile–butadiene– styrene blends. Polym. Degrad. Stab. 2008, 93, 657-667. (19) Pawlowski, K. H.; Schartel, B. Flame retardancy mechanisms of triphenyl phosphate, resorcinol bis(diphenyl phosphate) and bisphenol A bis(diphenyl phosphate) in polycarbonate/acrylonitrile–butadiene–styrene blends. Polym. Int. 2007, 56, 1404-1414. (20) Nodera, A.; Kanai, T. Thermal decomposition behavior and flame retardancy of polycarbonate containing organic metal salts: effect of salt composition. J. Appl. Polym. Sci. 2004, 94, 2131-2139. (21) Gaan, S.; Liang, S. Y.; Mispreuve, H.; Perler, H.; Naescher, R.; Neisius, M. Flame retardant flexible polyurethane foams from novel DOPO-phosphonamidate additives. Polym. Degrad. Stab. 2015, 113, 180-188.


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Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Yang, S.; Wang, J.; Huo, S. Q.; Wang, J. P.; Tang, Y. S. Synthesis of a phosphorus/nitrogen-containing compound based on maleimide and cyclotriphosphazene and its flame-retardant mechanism on epoxy resin. Polym. Degrad. Stab. 2016, 126, 9-16. (23) Jian, R. K.; Wang, P.; Duan, W. S.; Wang, J. S.; Zheng, X. L.; Weng, J. B. Synthesis of a Novel P/N/S-Containing Flame Retardant and Its Application in Epoxy Resin: Thermal Property, Flame Retardance, and Pyrolysis Behavior. Ind. Eng. Chem. Res. 2016, 55, 11520-11527. (24) Salmeia, K. A.; Gaan, S. An overview of some recent advances in DOPO-derivatives: Chemistry and flame retardant applications. Polym. Degrad. Stab. 2015, 113, 119-134. (25) Neisius, N. M.; Lutz, M.; Rentsch, D.; Hemberger, P.; Gaan, S. Synthesis of DOPO-Based Phosphonamidates and their Thermal Properties. Ind. Eng. Chem. Res. 2014, 53, 2889-2896. (26) Prishchenko, A. A.; Livantsov, M. V.; Novikova, O. P.; Livantsova, L. I.; Petrosyan, V. S. Synthesis of bis‐ and tris‐organophosphorus substituted amines and amino acids with PCH2N Fragments. Heteroat. Chem. 2010, 21, 430-440. (27) Weil, E. D.; Zhu, W. M.; Patel, N.; Mukhopadhyay, S. M. Fire Retardant PolymersA systems approach to flame retardancy and comments on modes of action. Polym. Degrad. Stab. 1996, 54, 125-136. (28) Song, L.; He, Q. L.; Hu, Y.; Chen, H.; Liu, L. Study on thermal degradation and combustion behaviors of PC/POSS hybrids. Polym. Degrad. Stab. 2008, 93, 627-639. (29) Jiang, S. X.; Shi, Y. Q.; Qian, X. D.; Zhou, K. Q.; Xu, H. Y.; Lo, S.; Gui, Z.; Hu, Y. Synthesis of a Novel Phosphorus- and Nitrogen-Containing Acrylate and Its Performance as an Intumescent Flame Retardant for Epoxy Acrylate. Ind. Eng. Chem. Res. 2013, 52, 17442-17450. (30) Barrett, K. A.; McBride, M. B. Oxidative degradation of glyphosate and aminomethylphosphonate by manganese oxide. Environ. Sci. Technol. 2005, 39, 9223-9228. (31) Davis, A.; Golden, J. H. Thermal rearrangement of diphenyl carbonate. J. Chem. Soc. B: Phys. Org. 1968, 72, 40-45. (32) Davis, A.; Golden, J. H. Thermal degradation of polycarbonate. J. Chem. Soc. B: Phys. Org. 1968, 72, 45-47. (33) Dong, Q. X.; Gao, C.; Ding, Y. F.; Wang, F.; Wen, B.; Zhang, S. M.; Wang, T.; Yang, M. S. A polycarbonate/magnesium oxide nanocomposite with high flame retardancy. J. Appl. Polym. Sci. 2012, 123, 1085-1093. (34) Cooper, G. D.; Williams, B. Hydrolysis of simple aromatic esters and carbonates. J. org. Chem. 1962, 27, 3717-3720. (35) Ballistreri, A.; Montaudo, G.; Scamporrino, E.; Puglisi, C.; Vitalini, D.; Cucinella, S. Intumescent flame retardants for polymers. IV. The polycarbonate–aromatic sulfonates system. J. Polym. Sci. Part A: Polym. Chem. 1988, 26, 2113-2127.


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Table of contents (TOC)

Design and Synthesis of Efficient Phosphorus Flame Retardant for Polycarbonate The active basic group imparts excellent flame retardancy to DFPZ through an accelerated decomposition of polycarbonate.

(For Table of Contents Only)


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Design and Synthesis of Efficient Phosphorus Flame Retardant for

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