Eco-friendly halogen-free flame retardant cardanol polyphosphazene

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Eco-friendly halogen-free flame retardant cardanol polyphosphazene polybenzoxazine networks Nagarjuna Amarnath, Divambal Appavoo, and Bimlesh Lochab ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02657 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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ACS Sustainable Chemistry & Engineering

Eco-friendly halogen-free flame retardant cardanol polyphosphazene polybenzoxazine networks Nagarjuna Amarnath, Divambal Appavoo, Bimlesh Lochab* Materials Chemistry Laboratory, Department of Chemistry, School of Natural Sciences, Shiv Nadar University, Gautam Buddha Nagar, Uttar Pradesh 201314, India. *[email protected]

ABSTRACT

We report on the preparation of hexa-functional cardanol (renewable phenolic compound) benzoxazine with a phosphazene core (CPN) for use as a greener eco-friendly halogen-free flame retardant reactive additive for the formation of sustainable polyphosphazene polybenzoxazine networks for flame resistant applications. The structure and purity of the monomer was confirmed by fourier transform infrared (FTIR), nuclear magnetic resonance (1H-, 13C-, 31P-NMR) and gel permeation chromatography (GPC) studies. The CPN monomer showed good compatibility with benzoxazine monomer (CPN0) as suggested by the co-curing studies. The thermal properties of the copolymer can be directly tuned by altering the composition of the monomer blend. The occurrence of phosphazene-phosphazane thermal rearrangement is also suggested for the thermal behaviour (thermogravimetry analysis, TGA) at higher loading of CPN in the monomer feed ratio. An improvement in mechanical properties of the copolymer with increase in glass transition

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temperature was confirmed by enhancement in crosslink density as compared to neat polybenzoxazine. The reactive nature and presence of phosphazene core improved both the smoke density rating, vertical burning rating and led to higher limiting oxygen index (LOI). The FTIR and scanning electron microscopy (SEM) studies of residual char supported the formation of functionalities and morphologies favorable to support the flame resistance behavior of polymer by incorporation of reactive benzoxazine with phosphazene core. Finally, we demonstrate that incorporation of cardanol phosphazene network has good compatibility with the polybenzoxazine phenolic thermosets with improvement in flame retardancy. The higher cardanol (65.7%) and phosphorous content (3.4%), and reactive nature of synthesised compound is attractive as a sustainable additive with the scope of their utilisation with other polymeric resins.

KEYWORDS: cardanol, sustainable benzoxazine, halogen-free flame retardant, eco-friendly, phosphazene

INTRODUCTION Flame retardant (FR) properties in polymers are crucial for several applications such as advanced composite matrices, coatings, semiconductor packaging and fabrication of copper-clad laminates, printed circuit boards, and barrier materials. The resistance to flame initiation, propagation and finally its extinguishment are affected by the presence of bromine, phosphorous, antimony, tin, boron in FR compounds. Although the effectiveness of flame retardancy for halogenated additives showed a higher potential, however, they pose a possible threat to the environment and mankind due to release of highly toxic gases and formation of potentially carcinogenic substances during combustion. The European Union, World Health Organization and the US Environmental


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Protection Agency pose a restriction of certain hazardous substances (RoHS, 2002/95/EC) such as polybrominated compounds and heavy metals both on their use and discard as waste of electric and electronic (E&E) equipment (WEEE, 2002/96/EC).1 Moreover, the recent vote by the Stockholm Convention on persistent organic pollutants is to globally ban halogenated FRs. Simultaneously, to suffice the increase in demand of FRs (2.2 MMT, since 2011) with safer alternatives research interest has boosted amongst scientific community for designing and implementation of halogen-free and environmentally safe FRs worldwide.2,3 The advantages such as enhanced char yield and intumescence associated with the organophosphorus compounds, are suggestive as a good alternative to the polybrominated derivatives.4,5 In comparison to halogen containing FR polymers, phosphorous compounds do not release harmful decomposition products upon catching fire. The disadvantage of physical blending of organophosphorus compounds in monomer/polymer is that it demands large dosage which may pose both physical and chemical compatible issues accounting to a subsequent deterioration in thermo-electrical-mechanical properties of the modified resin. Such problems can be circumvented by specific designing of FRs with chemically linked phosphorous atoms which have a required reactive functionality to act both as

a

monomer

and

additive.

The

phosphorous

containing

compound

hexachlorocyclotriphosphazene, N3P3Cl6 is an attractive intermediate due to its commercial availability, existence of multi-armed rigid ring, highly reactive P-Cl bond that allows ease of chemical substitution to give phosphazene derivatives. The introduction of phosphazene ring in polymer network is found to enhance thermal and hydrolytic stability.6 Amongst recently developed phenolic thermosets, polybenzoxazines, flame resistance is improved by incorporation of phosphorous in monomers such as 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide


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(DOPO),7-9 dopotriamine,10 phosphinated benzoxazine,11 and phosphazene based salt and derivatives.12-14 Cardanol is a non-edible phenolic oil generated at low cost from cashew nut agro-waste with exportation reached to 11,442 MT (in 2016) by India.15 It is widely used in frictional materials and adhesive formulations, compounding with rubber and for surface coatings. Understanding the synthetic requirement of benzoxazine monomer, cardanol efficiently substitutes petro-phenol (including bisphenol-A, BPA which is an endocrine disruptor) to form BPA-free polybenzoxazine. There are several promising initiatives for substituting the phenolic source with cardanol to form cardanol polybenzoxazines.16-19 Our group is actively involved in exploration of cardanol as a renewable feedstock for both the solventless-synthesis and processing of sustainable benzoxazines with high thermal stability (> 300 °C) and demonstrated their potential as adhesives and in energy storage applications.20-24 The polybenzoxazines based on cardanol are reported with much lower char yields than the petroleum origin phenols which is due to the lower crosslinking density. Besides higher thermal stability, the lower char yields (the solid content obtained after burning) of cardanol polybenzoxazines is an issue to advocate their usage for FR applications. Several groups tackled this problem in cardanol based polybenzoxazine by molecular engineering at amine component either by enhancing the functionality in the benzoxazine monomer25,26 or by incorporating a moiety which offers a higher crosslinking reaction sites during thermal curing of the monomer such as trityl27 and furan.28,29 Rationally, an enhancement in FR properties is the most essential and practical demand to encourage cardanol polybenzoxazines utility further at industrial scale. It will be interesting to study the covalently-linked polyphosphazene network with polybenzoxazines based on cardanol. Herein, we report the synthesis and exploration of cardanol benzoxazine tethered to phosphazene


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core as sustainable and relatively environmentally safe fire retardant polymers. The monomers synthesised were characterised by 1H-,

13C-, 31P-

NMR spectroscopy and gel permeation

chromatography (GPC). The curing studies and formation of catalyst-free thermally mediated crosslinked network formation were studied by fourier transform infrared (FTIR), differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) respectively. The extent of crosslinking network formation and mechanical properties of the polymers was studied by dynamic mechanical thermal analysis (DMTA). The flame retardancy of the polymers was confirmed by limiting oxygen index (LOI), values, vertical burning tests and smoke density analysis. The char residue of composites was analysed by digital photos, scanning electron microscopy (SEM), and FTIR spectroscopy, to enable an understanding in the flame retardant behaviour of prepared composites. EXPERIMENTAL SECTION Materials 4-Acetomidophenol (98%), 3-pentadecylphenol (> 90%) were purchased from Alfa Aesar and Tokyo Chemical Industry (TCI) Co. respectively. Sodium borohydride (10-40 mesh, 98%) and phosphonitrilic chloride trimer (99%) were purchased from Sigma Aldrich, magnesium chloride (anhydrous, 99%,) from Spectrochem, sodium hydroxide (97%), triethylamine, paraformaldehyde (98%) from Rankem, potassium carbonate (anhydrous, 99%) and ammonium chloride from Chemlabs and Qualigens respectively. All solvents used were AR grade and purified by standard procedures. Measurements


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The structures of the compounds were verified by proton ( 1H), carbon (13C) and phosphorus (31P) nuclear magnetic resonance spectroscopy (NMR) using Bruker AV400 NMR spectrometer at proton frequency of 400 MHz as well as the corresponding carbon and phosphorus frequencies at room temperature using deuterated solvents with internal references tetramethylsilane and H3PO4. Signals were averaged from 16 transients for 1H- and 31P-NMR, and 256 transients for 13CNMR to yield spectra with sufficient signal-to-noise ratio. FTIR spectra were recorded on a Nicolet iS5 spectrometer equipped with attenuated total reflectance (iD5-ATR) accessory, in the range of 4000–400 cm−1. Mass spectrometry analysis was carried out using Agilent HRMS Q-ToF 6540 Series equipped with ESI mode. GPC was recorded on a Viscotek Model 305 TDA max with refractive index detector. GPC system was calibrated with polystyrene standards and data were analysed using Omnisec software. Sample was dissolved in tetrahydrofuran solvent (4–5 mg/mL) and filtered through 0.2 μm polytetrafluoroethylene filter before injection. Thermal transitions were monitored with a DSC, Model Q20 from TA instruments, at a scan rate of 10 °C/min. under nitrogen flow rate of 50 mL/min with a sample (5 ± 2 mg) enclosed in hermetic aluminium pans. Prior to the experiments, the instrument was calibrated for temperature and enthalpy using standard indium and zinc. The curing behavior of monomers was studied as the exothermic transition and was considered to be complete when the recorder signal levelled off to the baseline. The total area under the exothermic curve was determined to quantify the heat of curing reaction (∆H cure). TGA of cured monomers was performed with a Perkin Elmer Diamond STG-DTA in the temperature range 30 to 800 oC and a heating rate of 10 oC/min under nitrogen and air atmosphere at a flow rate of 50 mL/min. The limiting oxygen index (LOI) of the polymers was calculated from char yield obtained in the TGA data using Van Krevelen and Hoftyzer equation (1)30 𝐿𝑂𝐼 = 17.5 + 0.4 ∗ 𝐶ℎ𝑎𝑟 𝑦𝑖𝑒𝑙𝑑

(1)


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The relative amounts of smoke obscuration produced by the burning or decomposition of the polymer samples prepared was measured and observed by fire-test-response test using ‘Smoke Density instrument’ (according to ASTM international standards- Designation: D2843-16). 𝑆𝑚𝑜𝑘𝑒 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑟𝑎𝑡𝑖𝑛𝑔 =

𝐴𝑟𝑒𝑎 𝑢𝑛𝑑𝑒𝑟 𝑡ℎ𝑒 𝑐𝑢𝑟𝑣𝑒 𝑇𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎

∗ 100

(2)

The vertical burning tests (UL-94) was conducted according to ASTM D 3801 standard. Anton Paar Rheometer MCR302-CTD450 was used to perform dynamic mechanical thermal analysis (DMTA) of the crosslinked polymer sample obtained on a parallel plate size of diameter 8 mm. The polymer sample was analysed by applying oscillatory mode with amplitude of 0.5%, angular frequency at 10 rad/s and a temperature ramp rate of 2 °C/min. The surface morphology of samples was studied using a Scanning Electron Microscope (SEM) (ZEISS, EVO-MA10) under an acceleration voltage of 20 kV. Synthesis of [N3P3(OC6H4{NH(CO)CH3}-4)6] (1)31: A mixture of 4-acetamidophenol (6.51 g, 43.14 mmol), and K2CO3 (8.58 g, 62.12 mmol) in anhydrous acetone (150 mL) was stirred at room temperature for 10 min. under nitrogen atmosphere. To this, N 3P3Cl6 (2.00 g, 5.75 mmol) was added and the reaction mixture was heated at reflux under stirring for four days under inert atmosphere. The solvent in the reaction mixture was evaporated under reduced pressure and the residue obtained was filtered and washed with water, ethanol, and hexane. The resulting white powder, 1, was dried under vacuo at 40 °C for 48 h. Yield: 5.35 g (90%). 1H NMR (DMSO-d6, 400 MHz, ppm): 2.04 (18H, s, -CH3), 6.80 (12H, d, J = 8.8 Hz, N-C-CH), 7.44 (12H, d, J = 8.8 Hz, P-O-C-CH), 9.94 (6H, s, NH); 13C NMR (DMSO-d6, 100 MHz, ppm): 23.92 (CH3), 120.09


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(ArCH), 120.57 (ArCH), 136.46 (C-N), 145.07 (C-O), 168.15 (C=O). 31P NMR (DMSO-d6, 162 MHz, ppm): 9.20. Synthesis of [N3P3(OC6H4{NH2}-4)6] (2)31: To a solution of 1 (0.51 g, 0.49 mmol) in methanol (20 mL) was added an aqueous solution of NaOH (20 M, 3 mL) and the mixture was heated at reflux under vigorous stirring for 24 h. The solvent was evaporated under reduced pressure and the residue obtained was filtered and washed with large excess of water, ethanol and hexane. The resulting brown solid was dried under vacuo at 40 °C for 48 h to give 2 as a brown powder solid. Yield: 0.32 g (85%). 1H NMR (DMSO-d6, 400 MHz, ppm): 4.90 (12H, s, NH2), 6.43 (12H, d, J = 8.7 Hz, N-C-CH), 6.51 (12H, d, J = 8.7 Hz, P-O-C-CH); 13C (DMSO-d6, 100 MHz, ppm): 114.25 (ArC), 120.96 (ArC), 140.82 (C-O), 145.64 (C-N). 31P NMR (DMSO-d6, 162 MHz, ppm): 10.03. Synthesis of 4-pentadecylsalicylaldehyde (3)32: Paraformaldehyde (3.45 g, 114.9 mmol) was added to a mixture of the pentadecyl phenol (5.00 g, 16.41 mmol), anhydrous MgCl2 (2.34 g, 24.62 mmol) and triethylamine (6.64 g, 65.67 mmol) in acetonitrile (100 mL) and the mixture was heated at reflux for 1.5 h under inert atmosphere. After cooling to room temperature, aqueous HCl (5%) solution was added to the reaction mixture until neutral pH was obtained. The organic layer was extracted with diethyl ether and the combined organic phase was dried over anhydrous MgSO 4. The solvent was evaporated under reduced pressure and the residue was purified by column chromatography on silica gel using hexane as eluent to give 3 as a white solid. Yield: 5.22 g (92%). 1H

NMR (CDCl3, 400 MHz, ppm): 0.87 (3H, t, J = 6.6 Hz, -CH3), 1.25 (24H, s, Alk H), 1.62 (2H,

m, Ar-CH2-CH2); 2.63 (2H, t, J = 7.5 Hz, Ar-CH2-CH2 ), 6.80 (1H, s, OH-C-CH), 6.83 (1H, d, J = 9.12 Hz, Alk-C-CH(Ar)), 7.44 (1H, d, J = 7.8 Hz, CHO-C-CH), 9.83 (1H, s, CHO), 11.04 (1H, s, OH); 13C NMR (CDCl3, 100 MHz, ppm): 13.10 (CH3), 21.67-35.42 (Alk C), 116.06 (ArC), 117.82


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(ArC), 119.50 (ArC), 132.57 (ArC), 152.85 (ArC-OH), 160.77 (ArC-C=O), 194.81 (C=O). LCMS (ESI Interface- positive ions): [M+H+]: 333.2777 (332.2715 calculated). Synthesis of [N3P3(OC6H4{NCH(4-pentadecyl-2-OH)}-4)6] (4): 2 (0.10 g, 0.12 mmol) and 3 (0.29 g, 0.89 mmol) were taken in THF (5 mL) and the mixture was heated at reflux with vigorous stirring for 24 h under inert atmosphere. The solvent was evaporated under reduced pressure to obtain a yellow solid, which was filtered and washed with water, ethanol and hexane to afford 4 in the form of a yellow solid. Yield: 0.30 g (95%). 1H NMR (CDCl3, 400 MHz, ppm): 0.87 (18H, t, J = 5.0 Hz, -CH3), 1.25 (132H, s, Alk H), 1.62 (12H, m, Ar-CH2-CH2); 2.58 (12H, t, J = 6.0 Hz, Ar-CH2-CH2), 6.65 (6H, d, J = 6.16 Hz, Alk-C-CH), 6.78 (6H, s, OH-C-CH), 7.00 (12H, d, J = 6.8 Hz, P-O-C-CH), 7.09 (12H, d, J = 5.0 Hz, N-C-CH), 7.12 (6H, d, J = 6.0 Hz, N=CH-C-CH), 8.45 (6H, s, N-CH), 13.09 (6H, s, OH); 13C NMR (CDCl3, 100 MHz, ppm): 14.13 (CH3), 22.7036.29 (Alk C), 116.79 (ArC), 119.69 (ArC), 121.92 (ArC), 122.08 (ArC), 132.33 (ArC), 145.64 (ArC), 149.43 (OH-ArC), 161.06 (N=C-ArC), 162.25 (N=C), 31P NMR (CDCl3, 162 MHz, ppm): 9.22. Synthesis of [N3P3(OC6H4{NHCH2C6H4(4-pentadecyl-2-OH)}-4)6] (5): Sodium borohydride (5 mg, 0.13 mmol) was added to the mixture of 4 (0.100 g, 0.03 mmol) in THF (5 mL) at 0 °C and the mixture was stirred for 24 h at room temperature under inert atmosphere. Ammonium chloride was added to the reaction mixture until no more evolution of gas was observed. The solvent was evaporated under reduced pressure, CH2Cl2 and water was added to the residue. The organic layer was separated and dried over anhydrous Na2SO4, the solvent was evaporated under reduced pressure to give yellow solid 5. Yield: 0.07 g (87%).

1H

NMR (CDCl3, 400 MHz, ppm): 0.88

(18H, t, J = 8.0 Hz, -CH3), 1.26 (132H, s, Alk H), 1.56 (12H, m, Ar-CH2-CH2); 2.50 (12H, t, J = 9.5 Hz, Ar-CH2-CH2 ), 4.28 (12H, s, NH-CH2), 6.53 (12H, d, J = 8.8 Hz, NH-C-CH), 6.61 (6H, s,


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OH-C-CH), 6.64 (6H, d, J = 7.6 Hz, Alk-C-CH), 6.70 (12H, d, J = 8.6 Hz, P-O-C-CH), 6.99 (6H, d, J = 7.6 Hz, NH-CH2-C-CH); 13C NMR (CDCl3, 100 MHz, ppm): 13.96 (CH3), 22.53-35.50 (Alk C), 116.20 (ArC), 116.30 (ArC), 119.92 (ArC), 120.06 (ArC), 121.68 (ArC), 128.31 (C-CH2), 144.10 (C-O), 144.32 (ArC-N) 156.20 (C-OH); 31P NMR (CDCl3, 162 MHz, ppm): 10.30. Synthesis of [N3P3(OC6H4{NCH2CH2C6H4(4-pentadecyl-2-O)}-4)6] (6) (CPN): A mixture of 5 (1.00 g, 0.37 mmol) and formaldehyde solution (37% w/v in water, 0.30 g, 3.73 mmol) in 1,4dioxane (40 mL) was heated at reflux for four days under inert atmosphere. The reaction mixture was evaporated under reduced pressure and the resulting solid was dissolved in CH 2Cl2 and washed with water. The organic layer was dried over anhydrous Na2SO4, the solvent was evaporated under reduced pressure to obtain yellow crude material, which was purified by filtration through silica plug using CH2Cl2 as eluent to obtain 6 as a yellow solid. Yield: 0.63 g (62%). 1H NMR (CDCl3, 400 MHz, ppm): 0.87 (18H, t, J = 6.5 Hz, -CH3), 1.25 (132H, s, Alk H), 1.58 (12H, m, Ar-CH2-CH2); 2.49 (12H, t, J = 7.3 Hz, Ar-CH2-CH2), 4.49 (12H, s, N-CH2-Ar), 5.23 (12H, s, N-CH2-O), 6.62 (6H, s, O-C-CH), 6.70-6.87 (36H, m, Ar-H); 13C NMR (CDCl3, 100 MHz, ppm): 14.14 (CH3), 22.71-31.95 (Alk C), 50.32 (N-CH2-Ar), 79.67 (N-CH2-O), 116.65 (ArC), 117.84 (ArC), 118.97 (ArC), 121.17 (ArC), 121.73 (ArC), 126.48 (ArC), 143.11 (ArC), 144.92 (C-O), 145.39 (ArC-N), 154.10 (P-O-C);

31P

NMR (CDCl3, 162 MHz, ppm): 9.75. FTIR-

ATR (diamond crystal/cm-1): 2922, 2852, 1508, 1265, 1205, 1040, 950. Blend preparation and curing of monomers: The cardanol monomer (CPN0) was synthesised as per the reported procedure.25 The CPN0 monomer possess no chemically attached phosphazene unit containing P and N to impart the flame-retardance in the so formed polymer i.e. poly(CPN0). Therefore, to improve the resistance towards flame, C PN0 is blended with the synthesised CPN monomer in different weight ratios using tetrahydrofuran as the solvent. The monomer blends are


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abbreviated as CPNx, where x is the weight percent of CPN added in the monomer CPN0 and blends are represented as CPN10, and CPN80. The polymerisation of monomers CPN, CPN80, CPN10, and CPN0 was performed by first heating in an air oven at temperature at 50 oC (1h), 100 oC (1h), 120 oC (1h), 150 oC (1h), 180 oC (1h), 200 oC (1h), 220 oC (1h), 240 oC (1h) followed by additional heating at 240 for 0 h, 1 h, 1.5 h and 2 h respectively.

RESULTS AND DISCUSSIONS Synthesis and Characterisation of Monomer (CPN) Cardanol, a bio-sourced phenol obtained from agricultural waste is considered as a versatile building block for several chemical intermediates 33,34 for formation of sustainable polymers. The general synthetic requirements for benzoxazine ring formation are i) presence of phenolic-OH, ii) free ortho-position to phenolic-OH, iii) formalin source and iv) amine compound. Benzoxazine monomers can be synthesised either via one-step condensation reaction using above requirements or by multi-step reaction, where the latter strategy requires aldehyde group at ortho-position to the phenolic-OH. Previously our group has utilised the advantage of low viscosity of cardanol for one-step solventless synthetic route for cardanol benzoxazine monomer.25 The same methodology was implemented to synthesise cardanol based phosphazene benzoxazine (C PN) monomer 6 (Scheme 2) via a Mannich-like condensation reaction of amine 2 with cardanol and formaldehyde to form hexa-substituted benzoxazine derivative. However, one-step condensation reaction in both solventless as well as in presence of solvent did not yield the C PN monomer. In view of the same, multi-step synthetic strategy was adapted and cardanol structure was first chemically modified to afford aldehyde group at ortho-position.


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An aldehyde group was introduced by the electrophilic substitution reaction of cardanol with paraformaldehyde in the presence of magnesium chloride and triethylamine. The aldehyde functionality at ortho- position to the phenolic –OH group in cardanol, was introduced via magnesium salt, to aid the direction for the incoming electrophile. The reaction pathway towards the formation of compound 3 is believed to proceed as shown in Scheme 1. The attack of formyl group can be guided at C-2 and C-6 position, but preferred position is C-6 as former position is opposed by the steric hindrance due to the long flexible alkylene chain at C-3 position. The selective formylation at C-6 position was supported by formation of compound 3 by 1H- and 13CNMR spectroscopy, Figure 1 and Figure S1 respectively. The phosphazene core bearing hexa-amine functionality 2 was synthesised followed by its condensation reaction with the cardanol aldehyde 3 and then with formaldehyde to form cardanol based phosphazene benzoxazine (CPN) monomer (Scheme 2). The phosphazene core containing amine 2 was synthesised from hexachlorocyclotriphosphazene, N 3P3Cl6, by base mediated reaction with excess 4-acetomidophenol to yield compound 1 and, which upon hydrolysis, afforded the amine 2. Therefore, an indirect route was adopted, the so formed amine 2 was reacted with 3 to form the Schiff base 4 which is then reduced to compound 5. The treatment of amine tethered with cardanol and phosphazene core 5 undergoes intramolecular cyclization reaction to form hexacardanol benzoxazine substituted monomer 6. The renewable cardanol and phosphorous content in the CPN monomer is 65.7% and 3.4% respectively. The progress of the adopted synthetic route was followed by 1H-,

31P-

and

13C-NMR

spectroscopy, confirming the formation of the

synthesised structures. The formation of CPN was followed by 31P-NMR spectroscopy to ease the interpretation of each synthetic step. The existence of highly reactive Cl atoms in the phosphazene core are susceptible


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to react with moisture and acidic conditions to form undesirable side products containing P-OH bonds which can be easily noticed in

31P-NMR

spectroscopy.35 Such side reactions in the

phosphazene core during the formation of 1 may ultimately decrease the number of amine functionality in aromatic amine 2 than desired, hence affecting the NMR environment of P leading to appearance of several phosphorous signals. This will finally lead to several products and affect the purity of synthesised CPN monomer. Figure 2 shows the stacking of the 31P NMR spectra of the starting material and its conversion to the final monomer 6. As a preliminary assistance in structural analysis of the synthesised structures, the 31P-NMR spectra of all synthesised compounds showed a singlet (except compound 4) confirming the equivalence of magnetic environment across all the P atoms of the central core. The formation of hexa-adduct was further confirmed along with the 1H- and 13C-NMR analysis. A major upfield chemical shift in 31P (from 19.97 to 9.20 ppm) is observed upon reaction of N3P3Cl6 with 4-acetomidophenol which is accounted to the difference in electronegativity (Cl vs. O) and O-linkage with the benzene ring. In the consecutive synthetic steps, the change in chemical shifts are insignificant (≤ 1 ppm) due to subtle change in the phosphorus environments. The structures of the intermediates (presented in supporting information) and the C PN monomer was confirmed by 1H NMR and 13C NMR spectroscopy. The NH proton signal of 1 was observed at 9.93 ppm and the acetyl proton at 2.04 ppm (Figure S2). The absence of these two signals in the spectrum of compound 2 with the new signal at 4.89 ppm (NH2 protons) confirms its formation (Figure S4). The signal at 8.45 ppm in the spectrum of compound 4 corresponds to the HC=N proton of Schiff base (Figure S6). The successful reduction of 4 to 5 was confirmed by the signals at 4.29 ppm assigned to the NCH2 protons (Figure S8). The condensation reaction of 5 with formalin forms benzoxazine ring of CPN monomer 6. The 1H- and

13C-

NMR spectra of CPN is


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shown in Figure 3a and b. The formation of CPN monomer 6 is evidenced by the characteristic resonances at 4.49 and 5.23 ppm for the NCH 2Ar and NCH2O protons respectively (Figure 3a). The absence of characteristic NMR signals due to amine (2, 4.89 ppm, inset of Figure 3a) and cardanol (Figure 1) suggests the purity of the CPN monomer. It is known that the synthesis of benzoxazine monomers is often accompanied by the formation of oligomers and their occurrence can be confirmed by appearance of signals in 3-4 ppm region. The absence of both such impurities in CPN monomer is confirmed by 1H-NMR spectroscopy. The formation of the benzoxazine ring in CPN monomer was also proven by the appearance of signals at 50.32 and 79.67 ppm in the 13C NMR spectrum, as shown in Figure 3b. GPC trace, Figure S10, of monomer showed a Gaussian peak with a retention volume of 21.4 min. There is no peak observed at lower retention times suggesting absence of oligomers which further corroborates with 1H-NMR results. The numberaverage molecular weight of monomer was found to be 3008 Da and polydispersity index of 1.1. The observed molecular weight of the CPN which is slightly higher than the theoretical 2756 Da as former is based on hydrodynamic radii. It must be noted that the observed molecular weight is not a true molecular mass of CPN monomer and is a relative molecular weight w.r.t. polystyrene standards. Curing studies of monomer using FTIR studies The curing of monomer was studied by FTIR spectroscopy under non-isothermal (Figure S11) conditions. The FTIR spectrum of the sample was recorded after it has been heated at different temperatures: 50 °C (1 h), 100 °C (1 h) followed by heating in temperature range of 120-240 °C at a heating rate of 10 °C/h. The FTIR spectrum (Figure S11a) of CPN monomer showed the characteristic absorption bands at 1205 and 1040 cm-1 corresponding to the asymmetric and symmetric stretching of C-O-C benzoxazine ring.36 The P=N stretch varies from 1180 to 1280 cm-


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1

depending upon the nature of the substituent and the degree of substitution. 37 The conjugation of

phosphazene core in the monomer was supported by the absorptions centred at 1165 and 950 cm 1

due to P=N and P-O-Ar stretching respectively. The ring opening polymerisation (ROP) reaction

of the benzoxazine ring involves the cleavage of –O-CH2 bond in the monomer to primarily form phenolic-OH and Mannich-type (-CH2-NR-CH2-) linkages during thermal curing reactions. Therefore, curing of the monomer can be monitored by changes in the intensity of C-O-C stretches (1040 cm-1 in Figure S11a and 1205 cm-1 in Figure S11b). The intensity of the characteristic benzoxazine peaks are found to decrease substantially with increasing temperature (from 150 to 240 °C), suggesting that monomer 6 undergoes nearly completion of polymerisation reaction at 240 °C. In addition, new peak at 1680 cm-1 developed above 150 °C which could be attributed to the formation of Schiff’s base due to ring opening reaction of benzoxazine ring. It is reported that substituted phosphazene core undergoes ring opening polymerisation to form polyphosphazene at 250 °C.38 The proposed structure of crosslinked polymer network formed upon thermal treatment of C PN is shown in Scheme 3. Curing studies of monomer using DSC studies The curing behaviour of benzoxazines in general is dictated by the nature of phenol (additional reactive functionality), amine (ammonia, aliphatic, benzene and furan), number of benzoxazine rings in the monomer, besides catalysts and other reactive co-monomers.39-44Cardanol benzoxazines showed a higher ROP temperature than the phenol based benzoxazines, which is attributed to the dilution effect due to presence of C15-long alkylene chain.18 The change in properties of cardanol based benzoxazine monomer (C PN0) and polymer after blending with CPN monomer was studied to understand the beneficial effect of incorporation of


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phosphazene core in the polymer network. CPN0 (structure as inset in Figure S12) is an attractive sustainable monomer in terms of cost of reagents (cardanol and tris-p-aminophenylmethane) and the performance of the thermally cured polymer. The C PN0 monomer was synthesised and its formation is supported by 1H-NMR (Figure S12) spectroscopy. The percentage of blends were chosen so as to understand the variation of the weight percent of phosphorous content and the effect of changing the blend ratio on thermal behaviour of monomers and polymers. The temperature of ROP reaction in monomers and blends was studied by DSC. The characteristic curing parameters namely, initiation temperature (Ti), onset temperature (To), exothermic peak temperature (Tp) and heat of the curing reaction (∆H) of curing profile of the monomer are shown in Figure 4. From Figure 4, the Ti of pristine CPN0 and CPN monomer is 207 °C and 176 °C respectively. The addition of 80 wt% CPN monomer to CPN0 substantially decreased the Ti to 181 °C. The purity of CPN monomer was apparent from NMR spectra. The absence of any amine, cardanol, oligomeric impurity in CPN and CPN0 is suggestive that DSC profile is a true representation of curing parameters. It is well known that such impurities in benzoxazine monomer mediate ROP reaction and lower the curing temperatures. This suggests that the co-curing of CPN with CPN0 assisted in its curing reaction in a different manner. Structurally, CPN0 has three benzoxazine rings while CPN has six benzoxazine rings accounting for a lower temperature of polymerisation, corroborates well with the literature.27,45 Since curing temperature of CPN is lower than CPN0, it is expected that the benzoxazine ring opens first for the former monomer to generate the phenolic moiety which further facilitates the ring opening reaction of other benzoxazine rings. Surprisingly, the heat of curing reaction was found to be nearly unaffected upon blending the monomers as compared to the neat monomers. Curing


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of monomer blends confirmed the copolymerisation compatibility as noted by a single curing exotherm from DSC. It could be correlated to their similar curing temperature range and accounts for the formation of polymer network in 3-D manner as both have functionality >2. Simultaneously the free ortho- and para- positions in the benzene rings are also involved in enhancing the crosslink density, as shown in Scheme 3. To understand the effect of curing behavior of CPN0 by blending with CPN, the curing kinetics of the synthesised monomers for ROP reaction was determined using Kissinger–Akahira–Sunose (KAS), and Flynn-Wall-Ozawa method. The degree of conversion and isoconversional plots of benzoxazine monomers and their blends, and the apparent activation energies (E) determined at different conversions are shown in Figure S13, S14 and Table S1. Both the methods utilised for determination of activation energy (Ea) are in good agreement and the Ea values was found to follow the order as: CPN< CPN80 100 is categorised as intrinsically non-flammable class.59 In our case, the enhancement in LOI values of cardanol polymers was obtained with co-curing of cardanol phosphazene monomer. An increase in the value is found to be dependent on the amount


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of phosphorous introduced suggesting the role played by phosphazene core in flame retardancy. It is known that phosphorus-nitrogen containing flame retardants either act in the vapour phase or in the condensed phase to inhibit the exothermic processes and suppress combustion by altering the degradation path to enhance the formation of char with the minimal evolution of volatiles. The vertical burning test (UL-94) was used to determine the burning characteristics of polymers in upward direction and ratings are presented in Table 1. The poly(CPN0) sample was burnt instantaneously with a much shorter combustion time along with fire drippings failing the UL-94 V-0 rating. The flame resistance is evident form the digital images of phosphazene containing polymers before and after burning are shown in Figure S15. The flame resistant characteristics was found to increase with an increase in phosphorus content which corroborates with LOI results. A smoke density test was performed to determine the relative amounts of smoke produced during burning of polymer samples i.e. poly(CPN0), poly(CPN10), poly(CPN80), and poly(CPN). The digital images of thermally cured samples before and after smoke density analysis are shown in Figure 7A and 7B. The pure poly(CPN0) without any phosphazene unit showed almost very little residue char which crumbled down the mesh plate, while phosphazene containing polymer poly(C PN10), poly(CPN80), and poly(CPN) showed residue as shown in the digital image, Figure 7B. The polymer samples were burnt to measure the smoke released with time. The amount/density of smoke produced is measured in terms of variation of light absorption by the sensor with time (Figure 7C). The larger the area under the curve the more is the smoke produced by the sample when burnt. It is observed from the figure 7B that the incorporation of phosphorus reduced smoke released from the polymer. The observed smoke density rating of poly(CPN0), poly(CPN10), poly(CPN80), and poly(CPN) showed a rating of 77.7, 70.8, 34.3 and 33.9 respectively. The relative difference in smoke density was found to be 6.9 and 43.4 with 10wt.% and 80 wt.% incorporation of CPN


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monomer suggesting a significant role played by phosphazene core in reduction of smoke in poly(CPN0). Interestingly, after application of a flame, the poly(CPN10) sample burnt without flaming drips, suggesting a relatively higher non-flammable nature than poly(CPN0). Therefore, from above studies of smoke density rating, we conclude that incorporation of phosphorus in the blend reduced the smoke released. The residue char obtained after smoke density measurements was investigated by SEM (Figure 8) and FTIR (Figure S16) technique. The morphology of both exterior and interior surface was recorded. Several undulations and ripple with poly(C PN0), and poly(CPN10) which is accounted to the presence of a higher content of flammable matrix in it (Figure 8a and b). The formation of larger cracks on exterior surface of the order of 20-30 µm was observed in char of poly(CPN0), Figure 8a. The cracks reduced substantially with the incorporation of CPN, as noticed from the exterior surfaces of poly(CPN10), poly(CPN80), and poly(CPN). Both 80 wt.% and neat polybenzoxazine polyphosphazene i.e. poly(CPN80), and poly(CPN) produced a highly compact, thick charred layers with integrated swollen char residue. The above results are consistent with intumescent flame retarding mechanism as indicated by an intensive expansion of the burnt matrix with a simultaneous formation of protective charred layered architecture, corroborates well with digital images in Figure 7B. Analysis of interior morphology showed formation of big porous microstructures with softer surface while a numerous honeycombed structures with bubbles separated by very thin layers was observed with increase in phosphorous content in the char. The coexistence of several small lacunae with a compact surface of the char residue inhibit the release of free radicals generated via oxidative mechanism. This may prevent flame propagation and account for generation of several tiny holes which are evident form the surface of poly(CPN80),


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and poly(CPN). The higher surface area provided by such interconnected network of lacunae mitigate exchange of heat and air (oxygen) thereby resist advancement of flame. Furthermore, to elucidate the nature of functionalities generated in the intumescent char residues left after the smoke density analysis of polymer samples was examined by FTIR spectra (Figure S16). A substantial change is observed in the region of 850 to 1330 cm -1 with the increase in P% in the polymer samples. There is nearly no significant FTIR peak was observed for poly(CPN0), and poly(CPN10). However, both poly(CPN80), and poly(CPN) showed development of new FTIR peaks. The existence of peak at 880, 1008, 1034, and 1055 cm-1 was observed for poly(CPN80), and the signals became further broadened and with increase in %P content (from 2.7 to 3.4%). A prominence of FTIR peak at 880 and 1114 cm-1 for neat poly(CPN) charred samples is observed, which is in accordance to the literature.60 The peaks in the region 1150 - 1330 cm-1 are assigned to the to P-O-C bonds of phosphate-carbon complex architecture.61 The peak at 880 cm-1 is attributed to the stretching vibrations for P-O-P bonds formed via linking of phosphate units of polyphosphoric acids.62 The so-formed acids acts as dehydrating agents thereby mediating the carbonization process and enabling the formation of more heat-resistant carbonaceous char. The synergism of development of a morphology along with formation of phosphorous rich compounds by incorporation of phosphazene co-monomer effectively mitigate the flammable behaviour of poly(CPN0) matrix.

CONCLUSIONS A greener chemically-reactive flame retardant based on cardanol was synthesised and characterised. The presence of benzoxazine functionality, renewable sourced, phosphazene core is motivational and illustrated its usage in the new class of thermosets polybenzoxazines, with a


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scope to be utilised in other resins. The incorporation of cardanol phosphazene has assisted the curing behavior with improved the thermal stability, LOI value, V-0 rating with a simultaneous lowering in smoke density ratings. In addition, the improved mechanical properties results indicate the beneficiary nature of their incorporation with prospects of better processability. The present studies revealed the importance of molecular designing to introduce phosphorous-nitrogen synergistic effect towards efficient flame retardants as supported by SEM and FTIR studies. The present work is the first report on phosphazene core containing cardanol benzoxazines as reactive type flame retardants which exhibits high flame retardation efficiency and may be useful in demonstrating the developments of high performance polymers. A simultaneous improvement in both flame retardant behaviour with enhancement in mechanical properties using sustainable feedstock is a way forward for designing future generation reactive, safer, greener eco-friendly reactive flame retardants. ASSOCIATED CONTENT Supporting Information. 1H-,

13C-NMR

spectra of all synthesised intermediates (1-5, CPN0),

FTIR spectra of CPN curing study; GPC trace; degree of conversion vs temperature; isoconversional plots; apparent activation energy (E) at different conversion; digital images of the samples which passed the vertical burning test; FTIR spectra of residual char. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: (+91-120) 3819 100 Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS NA would like to acknowledge the financial support from Shiv Nadar University. The authors (DA, BL) would like to acknowledge the financial support from Department of Science and Technology, India DST/BL/2014/15 (Grant No. SB/FTP/ETA-0069/2014). We are thankful to SCCPL®, India for providing cardanol for research purpose. REFERENCES (1) http://data.europa.eu/eli/dir_del/2015/863/oj, Commission Delegated Directive (EU) 2015/863 of 31 March 2015 amending Annex II to Directive 2011/65/EU of the European Parliament and of the Council as regards the list of restricted substances C/2015/2067 (2) Shaw, S. D.; Blum, A.; Weber, R.; Kannan, K.; Rich, D.; Lucas, D.; Koshland, C. P.; Dobraca, D.; Hanson, S.; Birnbaum, L. S. Halogenated flame retardants: do the fire safety benefits justify the risks? Rev. Environ. Health 2010, 25, 261-305, DOI: 10.1515/REVEH.2010.25.4.261. (3) Schartel, B. Phosphorus-based flame retardancy mechanisms—old hat or a starting point for future development? Materials 2010, 3, 4710-4745, DOI: 10.3390/ma3104710. (4) Bourbigot, S.; Le Bras, M.; Duquesne, S.; Rochery, M. Recent advances for intumescent polymers. Macromol. Mater. Eng. 2004, 289, 499–511, DOI: 10.1002/mame.200400007. (5) Schartel, B.; Kunze, R.; Neubert, D. Red phosphorus–controlled decomposition for fire retardant PA 66. J. Appl. Polym. Sci. 2002, 83, 2060–2071, DOI: 10.1002/app.10144.


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For Table of Contents Use Only

Chemically linked renewable cardanol (65.7%) and phosphazene (P 3.4%): Greener eco-friendly halogen-free flame retardant reactive additive for sustainable polymer networks.


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Figure 1. 1H NMR spectra of cardanol and its aldehyde derivative 3



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Figure 2. 31P NMR spectra of N3P3Cl6 and 1, 2, 4-6 (solvent: CDCl3 except for 1 and 2 solvent is DMSO-d6)


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a)

b)

Figure 3. NMR spectra of CPN monomer 6 a) 1H b) 13C (Solvent: CDCl3)



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Figure 4. DSC thermograms of benzoxazine monomers (a) CPN0; (b) CPN10; (c) CPN80 and, (d) CPN at a heating rate of 10 oC/min.


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Figure 5. TGA (A, Aꞌ), and DTG (B, Bꞌ) traces of polybenzoxazines a) poly(C PN0); b) poly(CPN10) c) poly(CPN80), and d) poly(CPN) in nitrogen and air atmosphere respectively at a heating rate of 20 oC/min.



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Figure 6. (A) Extent of crosslinking in neat polymer resins; Mechanical properties of polybenzoxazines (B) storage modulus; (C) damping factor of poly(CPN0) (a) and poly(CPN10) (b).


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Figure 7. Digital images of cured samples [l x w x h: (25. ± 0.1) x (25.5 ± 0.1) x 3.0 mm] of a) poly(CPN0); b) poly(CPN10) c) poly(CPN80), and d) poly(CPN) (A) before; (B) after smoke density test; (C) measured from the plot of light absorption by sensor with time during burning of the sample.



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Figure 8. SEM images of exterior (a, b, c d) and interior (aꞌ, bꞌ, cꞌ, dꞌ) surfaces of residual char (a, aꞌ) poly(CPN0), (b, bꞌ) poly(CPN10),(c, cꞌ) poly(C 80), and (d, dꞌ) poly(CPN) samples.

SCHEMES

Scheme 1. Proposed mechanism for formation of compound 3


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Scheme 2. Divergent approach for the synthesis of cardanol phosphazene benzoxazine monomer (CPN).



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Scheme 3. Ring opening polymerisation of CPN monomer to form polyphosphazene polybenzoxazine

Scheme 4. Heat mediated phosphazene–phosphazane rearrangement


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TABLES Table 1.Thermal properties of polybenzoxazines

Samples

CPN0

CPN10

CPN80

CPN

Ti

To

Tp

H

(oC)

(oC)

(oC)

(J/g)

207

254

271

108

203

181

176

254

231

214

271

260

246

108

99

96

Tg

T10%

(oC)

(oC)

100

120

n.d.

66

Tmax

Char

P

Yield

Cardanol

LOI

(%)

(oC)

(%)

(%)

410

482

14

0

(380)

(368;491;684)

(0)

426

463

24

(389)

(403;477;709)

(7)

398

406; 482

32

(390)

(421;489;709)

(26)

405

398;479

39

(395)

(411;494;709)

(28)

70.4

23

UL-

Smoke

94

density

V-2

77.7

V-1

70.7

V0

34.3

V0

33.9

(18) 0.3

70.0

27 (20)

2.7

66.8

30 (28)

3.4

65.7

33 (29)

Values in parenthesis are determined under air atmosphere

Table 2. Crosslink density analysis of cardanol based benzoxazine monomers Sample

Tg (oC)

T

Storage

density

(oC)

modulus

()

(MPa)

νa

Mca

(x 103 mol/cm3) (g/mol)

polyCPN0

100

66

73.3

1.042

26.007

40.06

polyCPN10

120

66

83.3

1.026

29.555

34.71

using rubber elasticity equation i.e. Go Mc)RT, where R = universal gas constant, Go = Storage modulus, Mc = Molecular weight between crosslinks, Mc) = crosslink density (ν) i.e. number of moles of network chains per unit volume the cured polymer or concentration of network chains molecular weight between crosslinks. aCalculated


Eco-friendly halogen-free flame retardant cardanol polyphosphazene

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