Ferrocene-Based Nonphosphorus Copolymer: Synthesis, High

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Ferrocene-Based Non-phosphorus Copolymer: Synthesis, Highcharring Mechanism and Its Application in fire retardant Epoxy Resin Dui-Jun Liao, Qi-Kui Xu, Richard William McCabe, Heeralal Vignesh Babu, Xiao-Ping Hu, Ning Pan, De-Yi Wang, and T. Richard Hull Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02980 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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Ferrocene-Based Non-phosphorus Copolymer: Synthesis, High-charring Mechanism and Its Application in fire retardant Epoxy Resin Dui-Jun Liao†, Qi-Kui Xu†, Richard W. McCabe‡, Heeralal Vignesh Babu¶, Xiao-Ping Hu†‡*, Ning Panξ, De-Yi Wang¶, T. Richard Hull‡* †

School of materials Science and Engineering, Southwest University of Science and

Technology, Mianyang 621010, P.R. China ‡

Centre for Fire and Hazards Science, University of Central Lancashire, Preston PR1

2HE, UK. ¶IMDEA ξ

Materials Institute, C/Eric Kandel, 2, 28906 Getafe, Madrid, Spain

Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory,

Southwest University of Science and Technology, Mianyang 621010, P. R. China

* Corresponding authors: Prof. Xiao Ping Hu; Prof. T. Richard Hull E-mail addresses: [email protected] (Xiao Ping Hu); [email protected]

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ABSTRACT: A novel non-phosphorus copolymer (PDPFDE) was synthesized, with the aim of reducing the fire hazards of epoxy resin (EP), via an aza-Michael addition reaction and was well characterized. A high char yield of about 62.9 wt% was obtained for PDPFDE from TGA results and a charring mechanism has been suggested that involves iron-cored carbon nanotubes as catalysts. An EP composite containing 5.0 wt% PDPFDE reached a low LOI value of 29.1% and V-1 rating in the UL-94 test. On comparison with neat EP, the peak of the heat release rate and the total smoke production of the composite were reduced by 36.0% and 24.0%, respectively. SEM and EDX results indicated the formation of coherent, dense and nitrogen-rich char residues due to the incorporation of PDPFDE. Furthermore, the addition of an appropriate amount of PDPFDE improved the mechanical properties compared to pure EP.


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1. INTRODUCTION Due to its outstanding adhesion, dielectric properties, high tensile strength and modulus and excellent chemical and corrosion resistance, epoxy resin (EP), as one of the well-known thermosetting resins, has been applied widely in adhesives, coatings and electronic/electrical appliances, etc. However, like the most common plastic materials, its inherent drawback is its high flammability, which largely limits applications in fields requiring high flame-retardancy.1-4 In recent years, halogen flame retardants have been banned or voluntarily withdrawn due to their persistent, bio-accumulative and toxicity (PBT) problems.5 Hence, most researchers tend to look for halogen-free flame retardants for improving the flame retardancy of EP. Aluminum hydroxide (ATH),6 layered double hydroxides (LDH),7-10 montmorillonite (MMT),11 melamine,12,

13

phosphorus,14-17 silicon,18 carbon nanomaterials,19,

20

etc., have been

employed as flame retardants and/or synergistic flame retardants for epoxy resins resulting in high thermal stability and improved flame retardancy. Among these, nitrogen-containing compounds play a crucial role in terms of improved blowing properties.

During

thermal

decomposition

processes,

ammonia

and

other

nitrogen-containing non-flammable gases were formed. These gases could dilute the flammable gases or cover the surface of materials to exclude oxygen and improve the flame retardancy. In addition to that, nitrogen-containing compounds, such as melamine and imides could improve the heat resistance and contribute to the formation of char in the condensed phase.21, 22 However, nitrogen-containing flame retardants (NFRs) alone have low flame-retardant efficiency; hence high loadings or cooperation with other



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phosphorus-containing compounds is required. Many studies show that a V-0 rating in the UL-94 test could be achieved easily by introducing small amounts of nitrogen-phosphorus flame retardants into epoxy materials.15,

22-25

Yang et al.

synthesized a flame retardant based on maleimide and cyclotriphosphazene and achieved good flame retardant epoxy resin composites with an obvious intumescent char residue.26 Luo et al., have prepared a 5,10-dihydro-phenophosphazine-10-oxide (DPPA) and Schiff base based flame retardant that was applied to a diglycidyl ether of bisphenol A (DGEBA) epoxy resin, a V-0 rating in the UL-94 test was obtained even with an ultra-low phosphorus content of 0.19 wt%, where a blowing-out effect played a crucial role during the combustion process.27 As mentioned above, the efficiency of flame retardants can be greatly influenced by the synergy

between

nitrogen

and

phosphorus.

On

the

other

hand,

some

phosphorous-containing flame retardants (PFRs), particularly organic PFRs, can lead to potential environmental problems. I. Saito et al., have carried out environmental migration tests for organophosphate compounds, detecting triethylphosphate (TEP) and tributylphosphate (TBP) in the wall and ceiling coverings of a newly built house.28 Moreover, recent studies revealed that the overuse of some PFRs may cause potential health and ecological risks because of their bioaccumulation characteristics. Tris-(1,3-dichloropropyl) phosphate (TDCPP) and triphenyl phosphate have been detected with high frequency in dust collected from homes, offices and automobiles, indicating that many people receive chronic exposure to these compounds.29, 30 So, it is an interesting idea to design a new chemical structural


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unit with high-effective catalysis for char formation as a replacement for phosphorus with nitrogen included into the polymeric molecule to further reduce the use of phosphorous, thus reducing both potential health and ecological hazards further. Over the past decades, ferrocene and its derivative have attracted a great deal of attention due to their outstanding performance in many fields, such as electrochemistry,31 catalysts32 and magnetic materials.33 It has been reported that ferrocenyl compounds can be used to improve both the flame retardancy and decrease the smoke production for polymer materials simultaneously, as a result of their ability to catalyze the formation of chars.34, 35 Furthermore, the incorporation of ferrocene units into the polymer backbone can improve the thermal stability and flame retardancy of polymeric materials.36, 37 Carty synthesized some ferrocenyl derivatives and applied them in polyvinyl chloride (PVC), to give good smoke suppression and flame retardancy and the amount of char residue increased from 16.6% to 23.9%.38 Generally, molecular structures containing triazine or benzene rings possesses good thermal stability and char formation ability. For example, poly(ether ether ketone) (PEEK), composed of aryl ether and aryl ketone units, has good char forming ability (55% at 700oC under a nitrogen atmosphere) and remarkable flame retardancy (LOI 37.5% and UL-94 V-0).39,

40

Wang et al., synthesized a series of inherent flame-retardant

semi-aromatic polyesters containing special aryl ether and ketone structures (“Ar-CO-Ar”, “Ar-O-Ar”, “Ar-O-Ar-O-Ar” or “Ar-O-Ar-CO-Ar-O-Ar”). These polyesters provided high char residues of 32.8%, 25.4%, 34.6% and 41.5% at 700oC under a nitrogen atmosphere, respectively.41 Su et al., synthesized a novel oligomeric



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charring agent (PTCA) containing both triazine and diphenyl groups.42 TGA results showed that PTCA has good thermal stability and when combined with ammonium polyphosphate (APP) in polypropylene, can promote the formation of an intumescent char layer (26.6% at 700oC under a N2 atmosphere). In addition, Feng et al. also found that

a

charring

agent:

4,6-dichloro-N-phenyl-1,3,5-triazin-2-amine-diamido

(CNCA-DA) with benzene and triazine rings had good char forming ability (22.5% at 700oC) and a high initial thermal degradation temperature.43 Since ferrocene has a good catalytic char-forming effect and the benzene ring can inherently take part in the charring process, the combination of ferrocene and benzene rings is worthwhile to be developed. In addition, the amine group in ethylenediamine could contribute to the formation of an intumescent char layer due to its efficient blowing effect.44 Hence, a new copolymer which combines ferrocene, benzene and diamine groups could be largely expected to have good char forming properties while enhancing the flame retardancy of EP materials. In the present work, a novel non-phosphorous polymer poly((3,3’-diphenyl diacetylethylenediamino)-1,1’-ferrocene) (PDPFDE), based on ferrocene, benzene and amine groups, was synthesized via an aza-Michael addition reaction, and incorporated into an epoxy resin to improve the fire retardancy. The thermal stability and gaseous pyrolysis products of the PDPFDE and various EP/PDPFDE composites were investigated on a thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TG-FTIR). The volatile components released during thermal decomposition of the PDPFDE were also studied by gas chromatography-mass


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spectrometry coupled with a pyrolyzer (Py/GC-MS) to help propose a possible charring mechanism for PDPFDE. The burning behaviour of the EP/PDPFDE composites was studied comprehensively by: cone calorimeter, limiting oxygen index (LOI) and UL-94 measurements. Furthermore, the char residues after the cone calorimeter tests were analyzed in detail. The effect of incorporation of PDPFDE on the impact and tensile strength tests of the EP composites was also evaluated. 2. EXPERIMENTAL SECTION 2.1 Materials Epoxy resin (DGEBA, commercial name: E-44, with an epoxy value of 0.41-0.48) was supplied by Nantong Xingchen Synthetic Material Co., Ltd. (Jiangsu China). The curing agent m-phenylenediamine (m-PDA) was provided by the Tianjin Guangfu Fine Chemical Research Institute. Ferrocene (98%), dichloromethane (99.5%), sodium hydroxide (96%), ethylenediamine (99%) and benzaldehyde (98.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Acetyl Chloride (99%), ethanol and sodium ethoxide (98%) were purchased from Kelong Chemical Industries Reagent Co. (Chengdu, China). Anhydrous aluminum chloride (99%) was provided by Tianjin Zhiyuan Chemical Reagent Co., Ltd. All reagents were used as received without further purification. 2.2 Synthesis of 1,1’-diacetylferrocene (DAF) AlCl3 (46.7 g, 0.35 mol) and dry CH2Cl2 (125 ml) were introduced under nitrogen atmosphere into a 500 ml three-necked round-bottom flask equipped with a magnetic stirrer, a reflux condenser and an addition funnel and the mixture stirred at room



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temperature. As acetyl chloride (0.45 mol) was added slowly and dropwise, dissolution of the AlCl3 was observed. After addition of the acetyl chloride was complete the mixture was stirred at room temperature for a further 10 min. A solution of ferrocene (18.6 g, 0.1 mol, in 100 ml of dry CH2Cl2) was added dropwise to the reaction mixture and stirring at room temperature was continued for 2 h. The dark purple reaction mixture was then poured slowly onto ice (500 g) and the organic and water phases were separated with a separating funnel. The organic phase was washed with water and extracted repeatedly (three times) and the CH2Cl2 was removed on a rotary evaporator. The crude product was purified by silica gel column chromatography (petroleum ether/ethyl acetate = 3/1 as eluent). Recrystallization from water gave red crystals of 1,1’-diacetylferrocene (DAF) (yield: 20.3 g, 75.2%). FTIR (KBr, cm-1) ν: 3104-3075 (Cp-H); 1661 (-C=O); 2980, 1456 and 1375 (-CH3); 542,502,482 (Fe-Cp).1H NMR (600 MHz, CDCl3, δ, ppm): 2.35 (6H, s, CH3); 4.51 (4H, s, C5H4); 4.77 (4H, s, C5H4). 13C NMR (CDCl3, δ, ppm): 201.42 (C=O), 27.86 (CH3), 71.16, 73.79, 80.97 (C5H4). (The corresponding spectra are shown in Figure S1, Figure S2 and Figure S3, respectively.) 2.3 Synthesis of 1,1’-dicinnamoylferrocene (DCF) DAF (16.46 g, 0.06 mol) was dissolved in 95% ethanol (450 ml) and added to a 1000 ml three-necked round-bottom flask equipped with a mechanical stirrer and an addition funnel. Sodium hydroxide (18.0 g, 0.45 mol) in distilled water (200 ml) was transferred into the reaction vessel. Under ultrasound irradiation (150W), freshly distilled benzaldehyde (38.0 ml, 0.36 mol) was added dropwise over 30 min and the


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reaction mixture was stirred at 20oC for a further 1 h when a mass of red precipitate was formed. The precipitate was filtered, washed with copious distilled water until the washings obtained reached pH=7 and dried at 70oC to obtain the red coloured crude product, which was recrystallized from ethanol/distilled water (v/v = 1:1). Filtration, followed by drying at 70oC, yielded the red solid 1,1’-dicinnamoylferrocene (DCF) (yield: 90.0%). FTIR (KBr, cm-1) ν: 3118, 3057, 3029 (Cp-H); 1658 (-C=O); 1600 (C=C); 3085 (Ar-H);1576, 1494 (Ar ring); 978 (Cp).1H NMR (600 MHz, CDCl3) δ, ppm: 4.63 (4H, s, -C5H4); 4.69 (4H, s, -C5H4); 7.1 (2H, d, O=C-C=CH); 7.39-7.63 (10H, m, -C6H5); 7.8 (2H, d, C6H5-C=CH). The FTIR spectrum is shown in Figure 1 (a) and the 1H NMR spectrum in Figure S4. 2.4 Synthesis of copolymer PDPFDE DCF (4.46 g, 0.01 mol) was dispersed in ethanol (120 ml) and transferred into a 250 ml four-necked round-bottom flask equipped with a condenser, mechanical stirrer, an addition funnel and a nitrogen gas inlet and outlet. In a separate flask, sodium ethoxide (2.72 g, 0.04 mol) was dissolved in ethanol (40 ml) under ultrasound (150W); ethylenediamine (2.40 ml, 0.04 mol) was added dropwise and the reaction mixture sonicated for further 20 min, transferred to the addition funnel and added to the DCF/ethanol dropwise over 30 min. The reaction mixture was heated to 70oC under nitrogen and after 6 h, an orange solid product was precipitated. The product was filtered and washed with ethanol until pH=7 was obtained. The residue was dried at 60oC for 4h to give of the PDPFDE co-polymer (yield: 73.0%) as an orange solid. FTIR (KBr, cm-1) ν: 3116, 3060, 3030 (Cp-H); 3094 (Ar-H); 1657 (C=O); 1582,



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1492(Ar ring); 1042 (C-N); 982 (Cp); 705 (-NH-). 1H NMR (600 MHz DMSO-d6, ppm): 1.09 (1H, -NH-); 2.20 (2H, -CH2-CH2-); 3.20 (2H, -CH2-C=O); 4.01 (1H, -CH-); 4.71, 5.31 (8H, Cp); 7.47-7.27 (5H, -C6H5). The preparation steps, a, b and c, of PDPFDE are summarised in Scheme 1. The FTIR spectrum is shown in Figure 1 (a) and the 1H NMR spectrum is shown in Figure 1 (b).

Scheme 1 Synthetic route to PDPFDE.

2.5 Preparation of EP/PDPFDE composites All EP composites were prepared by thermal cross-linking in the presence of m-phenylenediamine (m-PDA) as a curing agent. Various mass ratios of PDPFDE and EP were blended with acetone under ultrasound (150W) for 20 min. After a homogeneous mixture was obtained, the acetone was evaporated at 60oC using a rotary evaporator. Subsequently, the appropriate amount of m-PDA was added into the mixture in the ratio of 10:1 (EP:m-PDA, the ratio corresponding to an epoxy group to amine equivalent of 1:1). The mixture was stirred with a magnetic stirrer at 80oC under reduced pressure until no bubbles emerged and then the mixture poured into a preheated standard polytetrafluoroethylene (PTFE) mold at 80oC. The curing


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procedure was conducted as follows: 3 h at 80oC, 2 h at 100oC and another 3 h at 120oC. The cured EP samples were cooled slowly to room temperature to prevent cracking. A schematic representation of the curing process of EP/PDPFDE composites is shown in Scheme 2; while the formulae of EP/PDPFDE composites are presented in Table 2.

Scheme 2 A schematic representation of the curing process of EP/PDPFDE composites.

2.6 Characterization Fourier Transform Infrared Spectroscopy (FTIR) spectra were recorded on a Perkin Elmer Spectrum One spectrophotometer.

13

C and 1H NMR (600 MHz)

spectra were recorded on a Bruker Avance 600 spectrometer. The molecular weights

and

their

distribution

were

determined

by

gel

permeation

chromatography (GPC) with an Agilent 1200 SERIES instrument using THF as



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eluent at a flow rate of 1.00 ml/min. The glass transition temperature (Tg) of PDPFDE was measured by differential scanning calorimetry (DSC) using Jupiter STA 449C thermal analyzer (Netzsch, Germany) with a heating rate of 10 oC/min ranging from 25 to 250 oC under a nitrogen atmosphere. The glass transition temperature (Tg) of EP and its composite were measured by differential scanning calorimetry (DSC) using a DSCQ2000 (TA Instrument company, USA). Thermogravimetric analysis (TGA) and TG-FTIR spectra were performed on a STA6000 simultaneous thermal analyzer (PerkinElmer, USA) with a heating rate of 10oC/min from 40 to 700oC under a nitrogen atmosphere. The pyrolysis behaviour (Py/GC-MS) was conducted on a Clarus SQ 8 (Clarus, USA) with a Perkin Elmer Clarus SQ8 Gas Chromatograph/Mass Spectrometer (GC/MS) under nitrogen. LOI tests were carried out with a JF-3 oxygen index meter (Nanjing Jionglei Instrument Equipment Co., Ltd) according to the ASTM D2863-97 standard with sample dimensions of 100.0×6.5×3.2 mm3. UL-94 vertical burning tests were performed on a vertical burning test instrument (Nanjing Jionglei Instrument Equipment Co., Ltd) based

on

the

ASTM

D3801

standard

with

sample

dimensions

of

130.0×13.0×3.0 mm3. The cone calorimeter test was carried out on a FTT Standard Cone Calorimeter (Fire Testing Technology, UK) according to ISO 5660-1 under an external heat flux of 35 KW/m2 with sample dimensions of 100.0×100.0×3.0 mm3. Char residues after the cone calorimeter test were investigated on an ULTRA55 (Carl ZeissNTS GmbH, Germany) scanning


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electron microscope (SEM) with energy dispersive X-ray microanalysis (EDX). A WSM-10 KV computer-controlled electronic universal testing machine (Changchun Intelligent Instrument Equipment Co., Ltd.) was utilized to measure the tensile strength on the dumbbell-shaped specimens at a 20 mm/min testing speed, complying with GB/T 1040-92. The non-notched impact tests were performed on a PIT501J LCD plastic Charpy impact testing machine (Shenzhen million Test Equipment Co., Ltd.) with sample dimensions of 80.0×10.0×4.0 mm3, according to GB/T 1043-2008.

3. RESULTS AND DISCUSSION 3.1 Structural characterization of PDPFDE

Figure 1 (a) FTIR spectra of DCF and PDPFDE, (b) 1H NMR spectrum of PDPFDE

The chemical structure of PDPFDE was confirmed by FTIR and 1H NMR spectroscopy. Figure 1 (a) shows the FTIR spectra of DCF and PDPFDE. Comparison of the PDPFDE spectrum with that of the DCF, it is obvious from the disappearance of the C=C stretching vibration band at 1600 cm-1 of DCF



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and the appearance of peaks at 1042 cm-1 (νC–N) and 705 cm-1(ωN-H) in PDPFDE, that the aza-Michael addition reaction between DCF and ethylenediamine has been successful. In addition, the peaks at 3116, 3060, 3030 cm-1 (νC–H, Cp), 1657 cm-1 (C=O), 3094, 1582cm-1 (benzene ring), 1492 cm-1 (Cp) and 2930, 1460cm-1 (–CH2–) are clearly observed in the FTIR spectrum of PDPFDE. Unfortunately, the 1H NMR spectrum of PDPFDE (Figure 1 (b)) is broad and ill-defined, as is common with many polymer spectra. However, some tentative assignment can be made. The peaks ranging from 7.47 to 7.27 ppm correspond to the protons in the benzene ring. The small peak at 1.09 ppm may belong to the protons of the chain-terminal –NH2 groups.45 The remaining –NH– peaks could either be part of the H2O peak, due to proton exchange, or contributing to the Cp peak at 4.71 ppm, which is larger than expected compared to the second mono-substituted Cp ring peak at 5.31 ppm. The peak at 4.01 ppm represents the –CH– proton and the protons in the –CH2–C=O and –CH2–CH2– groups appear at 3.20 ppm and 2.20 ppm, respectively. The GPC curve of PDPFDE is shown in Figure S5 in the Supporting Information. The copolymer PDPFDE has a weight average molecular weight (Mw) of 15710 g/mol and a number average molecular weight (Mn) of 9123 g/mol. The polydispersity index Mw/Mn was about 1.722. The glass transition temperature (Tg) is an important parameter for identifing polymeric aggregation states. The DSC curve (see Figure S6 in the Supporting Information) showed that the Tg of PDPFDE is around 115oC, suggesting a relatively high


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temperature resistance for PDPFDE. 3.2 Thermal stability of PDPFDE The thermal degradation of PDPFDE was investigated by TGA and DTG under a nitrogen atmosphere. Pyrolysis under a nitrogen atmosphere were used as they provided an anaerobic environment similar to that occurring at the surface of a burning polymer. The TG and DTG curves are shown in Figure 2. Four main stages of weight loss were found. The onset thermal degradation stage from 220oC to 310oC resulted in a weight loss of 6 wt%. The decomposition temperature at 5 wt% weight loss (T5 wt%) and first temperature of maximum

Figure 2 TG and DTG curves of PDPFDE under N2 atmosphere.

mass lost rate (Tmax1) are 278oC and 281oC, respectively, which might come from early thermal degradation of some low-molecular weight PDPFDE. The second thermal degradation stage with the largest weight loss of 17 wt% occurred between 320oC and 550oC (with Tmax2 = 440oC), was observed from the

DTG

curve.

According

to

the

TG-FTIR

data

(Figure

3),

ferrocene-containing compounds were seen to sublime out above 400 oC. At higher temperatures, the C-N bond is more facile to break up and thus the



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PDPFDE molecules degraded in a greater number. During the third thermal degradation stage (570oC to 611oC) with about 2.0 wt% weight loss over this temperature range, the FTIR data (Figure 3) shows that ferrocene-containing compounds are beginning to disappear from the volatiles. This may be due to the pyrolysis of ferrocene-containing compounds to iron-cored carbon nanotubes, as would be expected from the many publications in the literature. 46 These iron-cored nanotubes would have a high catalytic effect for the thermal degradation of polymer to char and then can account for the about 6.0 wt% weight loss observed in the fourth stage (from 660 to 750 oC). Eventually, by 800oC, a high char residue of 62.9 wt% was obtained in accord with other polymers containing metallic elements in the backbone.47,

48

The average

weight loss of the PDPFDE is pretty low up to 800oC, indicating it has a good thermal stability and meets the processing conditions of most polymers. In addition, referring to the literature,49 FDADO-TPC, containing aromatic ring and ferrocene unit in the main chain, has a 40 wt% char residue at 600 oC, which is much lower than that of PDPFDE. The main reasons may be described as following: (1) the ferrocene unit and the benzene ring in the molecular structure of PDPEFD are rigid structural regions, which are beneficial for the improvement of the thermal stability of the polymer. (2) The inorganic nature of the ferrocene unit plays a unique role in improving char yields. (3) In addition to the same C-C and the C-N bonds, the structural unit of the polymer FDADO-TPC (c.f. literature ref 48) has extra weak C-O-C bonds in the


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backbone compared with PDPFDE. In general, weak bonds are more easily destroyed at high temperatures and the presence of more weak bonds in the molecular structure means that the polymer is more susceptible to degradation under heating conditions. Additionally, the narrow width of the molecular weight distribution of PDPFDE could be another reason for enhancing the thermal stability, as this may help promote high crystallinity. 3.3 Charring mechanism of PDPFDE

Figure 3 The TG-FTIR spectra of volatilized products at various temperatures during thermal decomposition of PDPFDE under N2.

The TG-FTIR techniques have been widely used to study the thermal degradation mechanism of fire retarding materials by analyzing the volatilized products. FTIR spectra of the pyrolysis products of PDPFDE at different temperatures are shown in Figure 3. The main characteristic absorbance bands of water (3900-3600 cm-1), the Cp ring (3015 cm-1), CO2 (2400-2300 cm-1, 669 cm-1)50 and aromatic compounds (1700-1560 cm-1) were found. There is no obvious peak existing between 220 and 250oC, indicating that PDPFDE has a



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good thermal stability. However, when the temperature reached 300oC, the CO2 peaks at 2361, 2324 and 669 cm-1 appeared, which can be attributed to oxidative cleavage of the C=O group; the extra O atom probably coming from other C=O bonds in the polymer (c.f. depletion of C=O peak in Figure 5) or, possibly, from O2 desorbed from the surface of the furnace if has been used in air rather than under nitrogen. It was notable that the peaks for CO2 become much stronger and a new absorbing peak for the Cp ring of ferrocene at 3015 cm-1 appeared at 400oC. This phenomenon was ascribed to the sublimation out of ferrocene-containing compounds, which was in accord with the TGA data (Figure 2).

Figure 4 EDX spectrum and data of the surface of the char residue of PDPFDE after heat treatment at 700oC for 30 min under N2.


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Meanwhile, the Fe-containing carbon-nanotubes produced at the char surface might catalyze the decomposition of residual PDPFDE to produce more CO2. With increasing temperature, the intensity of the CO2 peaks at 2361, 2324 and 669 cm-1 decreased and the intensity of the peaks at 1700-1560 cm-1 maintained no distinct change, indicating that a large amount of aromatic compounds existed in the condensed phase. This result is completely in keeping with the high char residue value of about 62.9 wt% at 800oC during TGA analysis. Generally, aromatic structures present in the condensed phase can contribute greatly to the formation of the stable char residue. Additionally, there were little or no peaks due to NH3 observed in the FTIR spectra of the volatile products (Figure 3), indicating that most of the nitrogen had remained in the condensed phase and may be in the form of nitrogenous heterocyclic compounds, with bonds such as C=N and C≡N. In fact, the EDX data (Figure 4) identified a high content of carbon (59.99%) and nitrogen (32.11%) in the surface of the char residue from PDPFDE formed at 700oC.

Figure 5 The FTIR spectrum of the char residue of PDPFDE after heat treatment at 700oC for 30 min under N2.



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The char residue of PDPFDE was further investigated by FTIR and the results are shown in Figure 5. The peak at 2253 cm-1 can be assigned to the C≡N bonds. The peak at 1446 cm-1 can be assigned to the stretching vibration of C=N and the peak at 880 cm-1 corresponds to an out-of-plane bending vibration of Ar-H. These data actually proved that nitrogen atoms had been incorporated into the char residue as C≡N or C=N bonds. Moreover, only a weak C=O peak remained at 1630 cm-1, revealing that the majority of the carbonyl groups and the carbonyl oxygen atoms in the main chain of the PDPFDE might be eliminated as CO2 during the thermal decomposition process. Further investigation of the thermal degradation mechanism of PDPFDE was carried out using a Py/GC-MS technique at 700oC under nitrogen and the analysis data are shown in Figure 6 and Table S1. Various main pyrolysis products have been detected: cyclopentadiene, benzene, toluene, ethylbenzene, benzenemethanimine,

ferrocene,

1-acetylcyclopentadiene, (E)-stilbene,

biphenyl,

1-acetylcyclopentadiene,

diphenylmethane,

2,2’-diphenylethylamine

and

1-acetylferrocene,

1,1’-diacetylferrocene,

corresponding to the peaks 2, 4, 5, 7-9, 15-17, 21, 22, 25 and 26, respectively. It is notable that numerous aromatic and N-heterocyclic structures are formed, such as indene derivatives (peaks 10-12), naphthalene derivatives (peak 13, 14), biphenyl derivatives (peaks 16, 28) and nitrogen-containing heterocyclic compounds (peaks 6, 8, 18 and 25); originating from the condensation and rearrangement

reactions

of

benzene

rings,

ferrocene


rings

and

the

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nitrogen-containing fragments.51 These aromatic and nitrogenous heterocyclic structures should contribute greatly to the formation of a stable char residue at high temperatures.

Figure 6 Py/GC-MS chromatogram of PDPFDE at 700oC under N2.

According to the results of TGA, TG-FTIR, EDX, FTIR and Py/GC-MS, a probable charring mechanism of PDPFDE could be described as shown in Scheme

3.

When

the

temperature

is

between

400

and

550 oC,

ferrocene-containing compounds tend sublime out of the polymer and at about 600oC decompose to give Fe-cored carbon nanotubes. In fact, what appear to be carbon nanowires have been also observed in the char residue of PDPFDE in preliminary SEM experiments (see Fig. S7 in Supporting Information). Rearrangement

and

ring-expansion/rearrangement

reactions

between

cyclopentadiene and benzene rings may occur under the catalysis of the Fe at higher temperatures, leading to the generation of some indene (peak 10-12) and, possibly, azulene derivatives (peak 23). Meanwhile, an aromatic ring-scission and rearrangement reaction to include nitrogen also seems to occur.51


Thus, a


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large number of aromatic cross-linked structures and some nitrogen-containing aromatic cross-linked structures were formed. Generally, these cross-linked structures would have high thermal stability, indicating PDPFDE has an excellent charring capability.

Scheme 3 Possible charring scheme for PDPFDE under nitrogen at 700oC

3.4 Thermal behaviour of EP/PDPFDE composites

Figure 7 DSC curves of EP-0 and EP-5 composites.

The glass transition temperature (Tg) of EP-0 and EP-5 composites were measured using DSC. Approximately 3 to 5 mg of each sample were encapsulated in aluminium pans and heated under nitrogen (>99.999% pure)


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with a heat-cool-heat profile. All samples were first heated from 20 to 150 oC at a heating rate of 10 oC/min and kept at that temperature for 5 min. Subsequently, they were cooled at a rate of -20 oC/min to 20 oC. Following the cooling scan, a second scan cycle was conducted at the same heating/cooling rate as the first. Heat flow vs. temperature scans from the second heating run were plotted and the mid-point of inflexion curves was assigned as the Tg of the corresponding sample. The corresponding DSC curves are presented in Figure 7. The EP-5 composite exhibited a single Tg, indicating a good compatibility between PDPFDE and the EP matrix. It is well known that Tg is mainly dependent on the degree of freedom of molecular segmental motion, degree of cross-linking and the entanglement constraints, and the packing density of the segments.52,

53

Therefore, a slightly higher Tg value of the EP-5 composite, comparing to EP-0 was found, which is probably caused by the chemical cross-linking reaction between the -NH- group of PDPFDE and the epoxy resin, leading to a higher cross-link density. Figure 8 shows the TGA and DTG curves of EP-0 and EP-5 under a N2 atmosphere, with detailed data listed in Table 1. The calculated (cal) TGA curve of EP-5 is calculated based on the experimental (exp) TGA curves of PDPFDE (Figure 2) and EP-0 using the following Equation: Mcal(EP-5) = [Mexp(EP-0)  95 + Mexp(PDPFDE)  5]/100



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Figure 8 TGA (a) and DTG (b) curves of EP-0 and EP-5 under N2.

Compared to EP, the EP-5 composite showed only one thermal degradation stage, which can be ascribed to a predominantly homogeneous system. The 5 wt% mass loss temperature (T5wt%) and maximum mass loss rate temperature (Tmax) of EP-5 were much lower than that of EP-0. However, the char residue yield (exp) of EP-5 at 700oC was 25.6 wt%, which was 25.5 % higher than the theoretical value (cal) of 20.4 wt%. According to the TGA analysis data, the PDPFDE can promote the advanced decomposition of EP at about 347.6oC, probably due to the –OH groups on the EP providing O to help decompose the PDPFDE at a lower temperature, thus moving the catalytic effect of the ferrocene produced to lower temperatures. Meanwhile, the self-high-charring properties of the PDPFDE and the catalytic cross-linking function of ferrocene


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as it migrates to the surface, should produce more char layer on the surface of the EP-5 sample. These char residues would act as an effective insulating barrier, protecting the underlying polymeric matrix from further thermal decomposition. Table 1 TG and DTG data of EP-0 and EP-5 composites under N2.

Samples 1 T5wt%(oC)2 Tmax(oC)

Rate of Tmax

Char residues (wt%)

(wt%/min)

500oC 600oC 700oC

EP-0

363.4

385.3

16.3

21.0

19.0

17.9

EP-5 (cal)

362.9

383.3

15.7

23.8

21.8

20.4

EP-5 (exp)

314.8

347.6

12.0

29.7

27.5

25.6

3.5 Flame retardancy and combustion behaviour Table 2 Detailed formulation and flame retardancy of the composites.

UL-94 Test 3 Samples4 PDPFDE (wt%)5 LOI (%) Dripping

Grade

EP-0

0

24.0

No

NR

EP-3

3

27.7

No

V-2

EP-4

4

28.5

No

V-1

EP-5

5

29.1

No

V-1

EP-6

6

29.6

No

V-1

EP-7

7

28.2

No

V-2



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LOI and UL-94 vertical burning tests are convenient, visually useful measurements

for

evaluating

the

fire

retardancy

of

materials.

The

corresponding formulation and results of the EP composites are summarized in Table 2. Neat epoxy resin (EP-0) is easily flammable with an LOI value of 24.0% and is not classified under the UL-94 rating. A 3.0 wt% of PDPFDE composite with EP, increased the LOI value to 27.7% and the UL-94 rating reached V-2. The LOI value increased to a maximum of 29.6% and the UL-94 rating reached V-1 for EP composites with up to 6.0 wt% PDPFDE.

However, at 7.0 wt%

loading of PDPFDE lowered the fire retardancy of the EP composites; the LOI value decreased to 28.2% and the UL-94 rating could not reach V-1 rating. This effect is probably due the charring rate becoming slower than the decomposition rate with excessive PDPFDE. This phenomenon demonstrated that there exists an optimal balance point between the catalyzing decomposition and char-formation function of the PDPFDE.

Figure 9 Cone calorimetry results for the EP-0 and EP-5 composites at an external heat flux of 35 KW/m2: (a) heat release rate (HRR), (b) total heat release (THR) and (c) comparison of mass loss curves.


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To study the combustion behavior of EP-0 and EP-5, the heat release rate (HRR), total heat release (THR), Fire growth rate (FIGRA), total smoke production (TSP), time to ignition (TTI), mass loss, CO and CO 2 production were investigated using cone calorimeter; the corresponding data are shown in Figures 9 & 10 and Table 3. The peak of heat release rate (pHRR) is one of the most important fire behaviour parameters for fire retarding materials. In Figure 9 (a), it shows that EP-5 has two pHRR values during combustion. The TTI and first pHRR of EP-5 occurred before EP-0, probably due to the ferrocene catalyzing degradation. However, this initial combustion formed a protective surface char layer, leading to a delayed second pHRR for EP-5 and a clear decrease of the pHRR by about 36.0 % (from 1050.5 to 673.6 KW/m2). These data suggest that an appropriate loading of PDPFDE is helps the formation of an effective char layer at the early stages of burning, which then protects the composite. Figure 9 (c) shows the mass loss of EP-0 and EP-5 composites. A significantly higher char residue was obtained for EP-5, compared to EP-0, suggesting that a dense and stable char layer is produced by EP-5 during burning. The FIGRA is the most commonly used parameter for assessing the fire hazard risk of products. It has definite pertinence for assessing the time that fire begins to the flashover point in an enclosure space and is the paramount parameter for estimating fire escape times.54, 55 A lower FIGRA value indicates a longer time to the flashover point and provision of enough time to escape. Table 3 shows



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that the FIGRA value of the EP-5 composite (5.0 wt% PDPFDE loading), decreased by 46.9 % compared to EP-0, suggesting that PDPFDE could actually enhance fire safety.

Table 3 Detailed combustion results of EP-0 and EP-5 composites obtained from cone calorimetry tests

TTI

pHRR

TTpHRR

FIGRAa

THR

TSP

av-SEA

Residues

(s)

(KW/m2)

(s)

(KW/m2s)

(MJ/m2)

(m2)

(m2/kg)

(wt%)

EP-0

75

1050.5

165

6.4

101.0

74.0

1851.4

4.0

EP-5

72

673.6

200

3.4

101.2

56.6

1400.4

17.6

Samples

aFIGRA

is calculated by dividing the value of pHRR by the time to pHRR (TTpHRR).

Figure 10 (a) Total smoke production (TSP), (b) CO2 production and (c) CO production curves of EP-0 and EP-5 composites during cone calorimeter test.

Besides heat release rate, smoke-production and/or the presence of toxic gasses are the major factors that cause death and environmental hazard resulting in more and more attention being given to these factors. Figures 10 (a), (b) and (c)


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show the total smoke production (TSP), carbon dioxide (CO2) and carbon monoxide (CO) production curves of the EP-0 and EP-5 composites, respectively. The average specific extinction area (av-SEA) data, which indicates the intensity of smoke released, is also presented in Table 3. In comparison to EP-0, the TSP and av-SEA values of EP-5 decreased by 24.0% and 24.4%, respectively. The formation of CO depends primarily on the incomplete combustion of volatile pyrolysis products during early thermal decomposition stages. The peak CO2 and CO production for EP-5 were much lower than those for EP-0. Moreover, both the CO2 and CO curves for EP-5 were similar to its HRR curve, showing an obvious condensed phase fire retardant mechanism.

3.6 Flame retardant mechanism of PDPFDE in EP

Figure 11 FTIR spectra of pyrolysis products for (a) EP-0 and (b) EP-5 composites at different temperatures under N2.



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FTIR spectra of the gaseous decomposition products from EP-0 and the EP-5 composite at different temperatures were recorded during TG-FTIR testing and are shown in Figure 11. Similar gaseous products were observed for both EP-0 and EP-5; such as H2O (3600-4000 cm-1), CO2 (2307-2380 cm-1), CO (2180 cm-1), aromatic compounds (3010-3030 cm-1, 1510 cm-1 and 825 cm-1, 742 cm-1, 683 cm-1) and hydrocarbons (2980 cm-1).7,

56, 57

However, for EP-0, the

maximum absorbance for CO2 occurs at 350oC, which is higher than that of EP-5 (320oC), thus supporting the suggested catalyzing degradation function of the PDPFDE and is consistent with the TGA results (see Figure 2).

Figure 12 Gram-Schmidt (GS) curves of EP-0 and the EP-5 composite.

In order to investigate the change in tendencies of formation of pyrolysis products, exhaustively, the Gram-Schmidt (GS) curves and a comparison of the FTIR absorbance intensities of several gaseous products from EP-0 and the EP-5 composite was carried out. The Gram-Schmidt curves can be built up based on vector analysis of acquired interferograms, showing the total amount of evolved gases detected by the spectrometer.58 The GS curves (Figure 12) showed that the absorbance intensity of the total decomposition products from


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EP-5 was much lower than those from EP-0. The reason may be attributed to the early thermal decomposition of the EP in the EP-5 composite, caused by the catalysis and high-charring ability of the PDPFDE, which results in the protective char layer on the surface of the sample preventing further combustion and increasing the char residue.

Figure 13 The FTIR absorbance of pyrolysis products for EP-0 and EP-5 composites versus temperature: (a) H2O, (b) CO2, (c) hydrocarbons and (d) aromatic compounds.

Figure 13 (a, b, c, d) showed clearly that the absorbed peaks of all kinds of pyrolysis products from EP-5 appeared at lower temperatures than from EP-0, further indicating that the incorporation of PDPFDE really promotes the earlier thermal degradation of EP composites. Furthermore, the gaseous decomposition products fell mainly into two categories: one is the non-flammable gases, such



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as water vapour and CO2; the other is flammable gases, such as hydrocarbons and aromatic compounds. It was quite obvious that the absorbance intensities of H2O and CO2 from the EP-5 composite were higher than that from EP-0 up to 450oC, which might dilute the concentration of flammable gases. On the contrary, the productions of hydrocarbons and aromatic compounds from the EP-5 composite were significantly reduced, implying that more of the hydrocarbons and aromatic compounds had remained in the condensed phase to form the compact protective char layer allowing less fuel to be fed back to the flame.59 The incorporation of the PDPFDE actually enhances the smoke suppression and, hence, the fire safety of the EP composites.

Figure 14 Digital photographs of the char residues from EP-0 and the EP-5 composite after the cone calorimetry test.

Digital photographs of the char residues after the cone calorimeter test are shown in Figure 14. In the case of EP-0, there was virtually nothing left after burning. However, with the addition of 5.0 wt% PDPFDE, a red intumescent


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char layer was observed (Figure 14 (d)): the red colouring most probably arising from surface haematite (Fe2O3). The surface morphologies of the char residues after the cone calorimeter tests were also investigated by SEM (Figure 15). The non-swelling char residue from EP-0 was discontinuous and contained many cracks, which would not act as an effective barrier layer. In contrast, the exterior surface of the char residue from the EP-5 composite (Figure 15 (b and b1)), looked like “honeycomb” and “meteor craters”, while its interior structure (Figure 15 (c and c1)) was smooth, compact and dense. This swelling char structure can be significant in preventing the transfer of heat and oxygen to the underlying polymer matrix, thereby reducing the combustibility of the EP composites and increasing the mass of char residue to 17.6 wt% after burning (Table 3).

Figure 15 Different magnifications of SEM micrographs of the surfaces of char residues after the cone calorimetry tests: exterior surface of EP-0 (a), (a1); exterior surface of EP-5 composite (b), (b1) and interior surface of EP-5 composite (c), (c1).



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EDX analysis was utilized for investigating the elemental composition and content of the char residues from EP-0 and the EP-5 composite (Figure 16). It can be seen that the content of Fe in the exterior char residue from EP-5 was 12.67 wt% and there was almost no Fe existing in the interior char residue. This phenomenon is ascribed to the migration of volatile ferrocene derivatives to the surface, where they form iron oxides at the exterior surface during burning. This migration is driven by an abundant rising of bubbles of volatile degradation products, convection flow within the polymer melt and the lower surface free energy of the Fe oxides.60-62 It is noticeable that a high content of N remained in both the interior and exterior char of EP-5, indicating that the PDPFDE may be helpful in incorporating the N into the condensed phase and so producing a thermally stable char layer.


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Figure 16 EDX spectra and elemental composition of the char residues from EP-0 and EP-5 after cone calorimetry testing.

3.7 Mechanical properties

Figure 17 The impact strength and tensile strength at break of EP-0 and the EP-3, EP-4, EP-5 and EP-6 composites.

Figure 18 Stress-strain curves of EP-0 and the EP-3, EP-4, EP-5 and EP-6 composites.



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The impact and tensile test of EP and various EP composites were performed (Figure 17 and Table 4). The stress-strain curves of various EP composites are shown in Figure 18.

Table 4 The Young Modulus and elongation at break data of EP composites

Sample

Young’s Modulus (GPa)

Elongation at break (%)

EP-0

1.0±0.05

5.74±1.45

EP-3

0.85±0.04

7.93±0.25

EP-4

0.93±0.04

9.08±0.04

EP-5

0.87±0.03

7.12±0.29

EP-6

1.20±0.08

2.99±0.59

The values of both tensile and impact strength showed an initial tendency to increase with increasing loading of PDPFDE, followed by decrease after EP-4 (4 wt%). The EP-4 composite showed a pronounced improvement in tensile (~40%) and impact strength (~30%), compared with that of EP-0. The results could be explained by the introduction of rigid groups (ferrocene and benzene ring) into the polymer, the chemical cross-linking reaction between epoxy and -NH- groups in the PDPFDE molecules, as well as good compatibility of molecules. It is well known that internal stress will be formed during curing of EP, leading to micro-cracks and voids, leading to poor mechanical performance.63 Conversely, the internal stress could be decreased observably by the incorporation of soft segments into the EP matrix.64,

65

Hence, the soft segments (ethylenediamine


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groups) of PDPFDE will reduce internal stress in the EP composites, improving the impact strength. Up to the EP-6 composite, the Young’s Modulus of the EP/PDPFDE composites decreased. This may be related to the introduction of soft segments from PDPFDE into the EP-0 structure until their concentration begins to disrupt the structure at EP-6. In addition, the increased elongation at break, again up to EP-6, appears to show a good adhesion via a cross-linking chemical reaction between PDPFDE and EP,66 however, the decrease in elongation at break of EP-6 can be an indication of a poor dispersion state owing to excessive loading of the PDPFDE in the EP matrix.

4. CONCLUSIONS The

novel

ferrocene-based

non-phosphorus

copolymer

PDPFDE

was

synthesized successfully employing an Aza-Michael addition reaction and incorporated into epoxy resin composites. TGA data illustrated that the PDPFDE possessed good thermal stability. Investigation of the charring mechanism of PDPFDE demonstrated that large amounts of aromatic and N-containing heterocyclic

char residues were formed during thermal

degradation, which appears to be catalyzed by formation of Fe-cored carbon nano-tubes generated at the surface from volatile ferrocene compounds formed during the pyrolysis. Although the introduction of PDPFDE promoted slightly the earlier degradation of the EP composite, it increased the overall thermal stability even at higher temperatures. Compared with neat EP, the pHRR value



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and TSP value of the EP composite containing 5.0 wt% PDPFDE were reduced up to 36.0% and 24.0%, respectively. The intumescent and nitrogen-rich carbonaceous layer of EP-5 shows that the PDPFDE mainly acts as a flame retardant by a condensed phase mechanism, in which the synergistic effects among ferrocene, amine and benzene groups plays a key role. Most importantly, both the impact and tensile strengths of the EP composites were enhanced by introducing up to 5 wt% of PDPFDE, showing that the PDPFDE molecules could participate in the chemical cross-linking curing of the EP and is not just a physical mixture. This study should provide a new direction for developing eco-friendly, phosphorus-free polymeric additives possessing fire retardancy , low toxicity and enforcement functions.

ASSOCIATED CONTENT Supporting Information Chemical structures of pyrolysis compounds for PDPFDE at 700oC under N2; FTIR spectrum of DAF; 1H NMR spectra of DAF and DCF;

13

C NMR

spectrum of DAF; GPC curve of PDPFDE; DSC curve of PDPFDE under N2 atmosphere and SEM image showing carbon-nanowires at the surface of the char residue.


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AUTHOR INFORMATION Corresponding Author Xiaoping Hu, E-mail to: [email protected] T. Richard Hull, E-mail to: [email protected] OCRID: Xiao-Ping Hu: 0000-0002-7460-7441 T. Richard Hull: 0000-0002-7970-420

De-yi Wang: 000-0002-0499-6138

Notes The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (51373140 and 51673160), the China Scholarship Council (201508515104) and the Post-graduate Innovation Fund Project by Southwest University of Science and Technology (17ycx005).

REFERENCES (1) Kalali, E. N.; Wang, X.; Wang, D. Y. Multifunctional intercalation in layered double hydroxide: toward multifunctional nanohybrids for epoxy resin. J. Mater. Chem. A. 2016, 4, 2147. (2) Jiang, S. D.; Bai, Z. M.; Tang, G.; Song, L.; Stec, A. A.; Hull, T. R.; Zhan, J.; Hu, Y., Fabrication of Ce-doped MnO2 decorated graphene sheets for fire safety applications of epoxy



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The Table of Content (TOC) TOC figure


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Ferrocene-Based Nonphosphorus Copolymer: Synthesis, High

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