Renewable Cardanol-Based Phosphate as a Flame Retardant

Research Article pubs.acs.org/journal/ascecg

Renewable Cardanol-Based Phosphate as a Flame Retardant Toughening Agent for Epoxy Resins Xin Wang, Shun Zhou, Wen-Wen Guo, Pei-Long Wang, Weiyi Xing, Lei Song,* and Yuan Hu*

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State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ABSTRACT: A biobased flame retardant toughening agent, phosphaphenanthrene groups-containing triscardanyl phosphate (PTCP), was successfully synthesized via debydrochlorination, epoxidation and ring opening reaction from renewable resource cardanol. The chemical structure of PTCP was confirmed by the proton and phosphorus nuclear magnetic resonance. Epoxy resins (EPs) with different contents of PTCP were prepared through a simple mixing method. Thermogravimetric analysis results indicated that the earlier degradation of PTCP catalyzed the char formation of epoxy resins that was beneficial to protecting underlying polymers from further decomposition. The flame retardant properties were enhanced with the increase of the PTCP content. The EP composite containing 30 wt % PTCP showed a limiting oxygen index of 30.5%. Meanwhile, its peak heat release rate, total heat release and average effective heat of combustion values were decreased by 50%, 27% and 32%, respectively, in comparison to those of neat EP. The enhanced flame retardant behavior was attributed to the improved quality of char residue, which effectively inhibited the flammable volatiles, oxygen and heat transfer between degradation zone and flame zone. The impact strength was increased to 19.14 kJ/m2 for EP/ PTCP-30% composite from 14.85 kJ/m2 for neat EP, indicating the toughening effect of PTCP on EP. The findings in this study demonstrated that PTCP could be used as a promising flame retardant toughening agent for epoxy resins to overcome their drawbacks of intrinsic brittle and high flammability. KEYWORDS: Cardanol, Flame retardant, Toughening agent, Epoxy resins

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INTRODUCTION In the industrial community, development of new products from renewable resources has received increasing attention due to concerns over environmental issues and depletion of fossil resources.1 Among various renewable resources, cardanol is one kind of most commonly used, inexpensive and easily available agricultural byproduct, which is extracted from cashew nut shell liquid.2 With the reactive phenolic hydroxyl group and unsaturated long alkyl chain, cardanol is regarded as a versatile platform for chemical modification. Cardanol and its derivatives have been widely studied in the fields of biocomposites,3,4 coatings,5−8 curing agents9,10 and antioxidants.11 Recently, the use of cardanol and its derivatives as plasticizer has become a new strategy to contribute to sustainable chemistry in the polymer and rubber industries. Mohapatra and co-workers have grafted cardanol onto natural rubber, resulting in significant plasticizing effect.12 Furthermore, cardanol acetate and epoxidated cardanol acetate,13 and epoxidized cardanol glycidyl ether14 have also been reported to be efficient plasticizers for poly(vinyl chloride). Epoxy resins (EPs) constitute a prominent class of thermosetting polymers that are widely used as paints, adhesives, insulating materials as well as structural composites.15,16 EPs are well-known for their superior engineering © 2017 American Chemical Society

performances, such as good mechanical properties, outstanding chemical resistance, excellent adhesion and relatively low cure shrinkage. However, epoxy resins possess intrinsic poor impact resistance owing to their highly cross-linked structure.17 As the most popular commercial epoxy resin, bisphenol A diglycidyl ether-type epoxy exhibits brittle behavior with relatively poor resistance to crack initiation and growth, which restricts the engineering applications. Therefore, considerable efforts have been made to develop toughening epoxy products. It was reported that nanoparticle modification of epoxy resins is an effective strategy to improve the impact resistance of the resultant epoxy composites.18−20 However, the increased impact resistance depends strongly on the dispersion state of nanoparticles, and it is thereby not suitable for industrial applications. In addition to the intrinsic poor impact resistance, inherently high flammability is another shortcoming of EPs similar to the majority of synthetic organic polymeric materials, which is a great potential trigger for fire accidents in household or industrial applications. To minimize or even eliminate fire Received: January 7, 2017 Revised: February 28, 2017 Published: March 7, 2017 3409

DOI: 10.1021/acssuschemeng.7b00062 ACS Sustainable Chem. Eng. 2017, 5, 3409−3416

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Diagrammatic illustration of synthetic route of PTCP.

hazards of EPs, flame retardant treatment has been becoming an increasing important issue. Over the past few decades, a wide variety of techniques such as phosphorus-,21,22 nitrogen-,23 silicon-,24 boron-containing25 compounds as well as nanomaterials26−28 have been developed to improve the flame retardant property of EPs. Among these compounds, 9,10-dihydro-9-oxa10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives are considered to be a class of most promising and efficient flame retardant for commercial epoxy products.29,30 To overcome the intrinsic poor impact resistance and high flammability of epoxy resins, it is proposed to design a new agent with flame retardant and toughening effect simultaneously. As mentioned above, cardanol is a versatile renewable platform for chemical modification whereas DOPO is a reactive and effective flame retardant for epoxy resins. It is anticipated to combine cardanol with DOPO to obtain a biobased flame retardant toughening agent for epoxy materials. Until now, very few literature has reported on the synthesis and applications of biobased flame retardant toughening agents for polymeric materials; furthermore, those for thermosetting resins are even more rare. In this work, a phosphaphenanthrene groups-containing triscardanyl phosphate (PTCP) was synthesized via debydrochlorination, epoxidation and ring opening reaction from renewable resource cardanol. The chemical structure of PTCP was confirmed by the proton and phosphorus nuclear magnetic resonance. The thermal, flame retardant and mechanical properties of epoxy composites with PTCP were

investigated. It is expected that PTCP obtained herein will pave the way for development of flame retardant toughening agents for industrial applications in future.

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EXPERIMENTAL SECTION

Materials. Cardanol was purchased from Cardolite Corporation (Zhuhai, China). The product obtained was pale yellow in color, which consisted of 41% triene, 36% diene, 20% monoene and 3% saturated compound. DGEBA type epoxy resin (commercial code: E44, EEW: 227 g/equivalent) was supplied by Hefei Jiangfeng Chemical Industry Co. Ltd. (Anhui, China). 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) (97%) was supplied by Aladdin Chemicals Co. Ltd. (Shanghai, China). 3-Chloroperbenzoic acid was purchased from Anhui Wotu Chemicals Co., Ltd. (Anhui, China). Phosphorus oxychloride, dichloromethane, chloroform, triethylamine, sodium carbonate, sodium sulfate and 4,4′-diamino-diphenylmethane (DDM) were all reagent grade and purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Synthesis of Triscardanyl Phosphate (TCP). In a three-necked and round-bottom flask equipped with a reflux condenser and a magnetic stirrer were dissolved cardanol (0.30 mol), phosphorus oxychloride (0.10 mol) and triethylamine (0.30 mol) in dry chloroform (200 mL). The reaction mixture was heated under reflux (60 °C) for 4 h. After cooling down to room temperature, the white precipitate was filtered and the filtrate was washed with deionized water (100 mL × 3). The organic layer was dried with sodium sulfate, filtered and then the solvent was removed under reduced pressure. The brown liquid obtained was tris(3-pentadecylphenyl) phosphate. Epoxidization of the Unsaturated Side Chain in Triscardanyl Phosphate. TCP (0.05 mol), 3-chloroperbenzoic acid (0.30 mol), 3410


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ACS Sustainable Chemistry & Engineering and dichloromethane (200 mL) were charged into a round-bottomed flask equipped with a mechanical stirrer. The mixture was cooled to 0 °C by ice bath and stirred for 3 h. Then, the byproduct was removed by filtration, and the filtrate was washed with 10% sodium carbonate solution (100 mL × 3), deionized water (100 mL × 3) and dried with sodium sulfate. The solvent was then removed by rotary evaporation to afford epoxidized triscardanyl phosphate (ETCP). Synthesis of Phosphaphenanthrene Groups-Containing Triscardanyl Phosphate (PTCP). ETCP (0.03 mol) and DOPO (0.18 mol) were introduced into a round-bottomed flask equipped with a mechanical stirrer, condenser pipe and thermometer. The mixture was heated to 160 °C and maintained for 6 h. The brown and viscous liquid was obtained. The synthetic route of phosphaphenanthrene groups-containing triscardanyl phosphate is illustrated in Figure 1. Preparation of PTCP Toughened Epoxy Resins. PTCP was first mixed into DGEBA adequately by mechanical stirring at 80 °C for 1 h, followed by the mixture was degassed in vacuum oven at 100 °C for 15 min. Subsequently, the curing agent DDM was added to the mixture with an epoxide/amino molar ratio of 1/1. Then, the mixture was poured into the mold, cured at 100 °C for 2 h and postcured at 150 °C for 2 h. Finally, all samples were cooled down to room temperature. The EP/PTCP composites with 10, 20 and 30 wt % PTCP were prepared using this similar procedure. Characterization and Instruments. 1H- and 31P NMR spectra of all samples were measured on a Bruker AVANCE-400 NMR spectrometer operating in the Fourier transform mode using CDCl3 as solvent. The glass transition behaviors of the samples were studied using a Q200 differential scanning calorimetry (DSC) analyzer (TA Instruments, USA). The sample was crimped in an aluminum pan and heated at a rate of 5 °C min−1 from ambient temperature to 200 °C under nitrogen atmosphere. Thermogravimetric analysis (TGA) of the samples was performed on a Q5000 thermal analyzer (TA Instruments, USA) under air atmosphere, from room temperature to 800 °C at a heating rate of 20 °C min−1. Cone calorimeter tests were carried out on a dual cone calorimeter (FTT, UK), following the standard in ISO 5660-1. Each specimen with dimensions of 100 mm × 100 mm × 4 mm was irradiated horizontally at a heat flux of 50 kW/m2. The samples were mounted in aluminum foil. Limiting oxygen index (LOI) measurements were conducted on an oxygen index model instrument (Jiangning Instruments, China) according to ASTM D 2863-97 (the test specimen: 100 mm × 6.5 mm × 3.2 mm). The morphology of the char residue after cone calorimeter tests was observed using an XL-30 environmental scanning electron microscope (FEI, Netherlands). Prior to SEM observation, the samples were sputter-coated with gold in order to improve the conductive. The Charpy impact tests of the un-notched samples were carried out on a ZBC1400-A pendulum impact testing machine (MTS Systems, China). The impact energy used was 4 J and the dimensions of the rectangular shaped samples were 80 mm × 10 mm × 4 mm, according to the GB/T 1043.1-2008 standard. The tensile tests were performed on a 5966 dual column tabletop universal testing system (Instron, USA) according to ASTM D3039-08 method, at a crosshead speed of 1.0 mm/min. The samples with dumbbell shape were used. At least five runs were repeated for each sample, and the average value was reported.

Figure 2. 1H NMR spectrum of (a) TCP, (b) ETCP and (c) PTCP.

(labeled b, c and a, d) attached to the benzene ring of cardanol, respectively. The signal at 5.05 ppm is ascribed to the terminal vinyl group (labeled l) of the triene moiety. The signal at 5.40 ppm is related to the double-bond protons (labeled (i) of the triene, diene and monoene moieties. The signals of allylic protons (labeled h) of the triene, diene and monoene moieties are observed at 2.05 ppm. The signals at 2.55, 1.59 and 1.30 ppm are assigned to methylene groups (labeled e, f and g) from the long side chain of cardanol unit, whereas the signals of terminal methyl groups for the diene, monoene and pentadecyl moieties are observed at 0.90 ppm. Comparing to the 1H NMR spectrum of TCP (Figure 2a), the characteristic peaks of double bonds (labeled k, i and l) in the 1H NMR spectrum of ETCP (Figure 2b) disappear; instead, a new peak (labeled 9) appears at 2.9 ppm that is due to protons of the epoxy groups, indicating the successful epoxidization of the unsaturated side chain in triscardanyl phosphate. For the 1H NMR spectrum of PTCP (Figure 2c), the protons of the epoxy groups (labeled 9 in Figure 2b) disappear and instead the appearance of new multi peaks (labeled l′ in Figure 2c) at 7.3−8.0 ppm are assigned to protons on the phosphaphenanthrene groups, implying the successful reaction between ETCP and DOPO.31 31 P NMR spectrum was also measured to demonstrate the chemical structure and purity of PTCP, as displayed in Figure 3. Two intensive signals were observed at −16.8 and 34.6 ppm in the 31P NMR spectrum of PTCP, respectively. The former signal is attributed to the phosphorus atom originated from phosphate moiety (labeled a), whereas the latter one is assigned to the P-CH derived from DOPO moiety (labeled b). These

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RESULTS AND DISCUSSION Structural Characterization. To validate the successful synthesis of phosphaphenanthrene groups-containing triscardanyl phosphate, 1H NMR and 31P NMR measurements are employed to characterize the intermediates and the target product. Figure 2 gives 1H NMR spectra of (a) TCP, (b) ETCP and (c) PTCP. In the 1H NMR spectrum of TCP, the signals at 7.15, 6.76 and 6.65 ppm are assigned to the protons 3411


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Figure 3. 31P NMR spectrum of PTCP.

results are consistent with the chemical structure as expected, indicating the target product has been successfully synthesized. Thermal Properties. The glass transition behavior of neat epoxy and EP/PTCP composites was investigated by DSC. Figure 4 displays DSC thermograms of EP and EP/PTCP

Figure 5. TGA and DTG curves of PTCP, neat epoxy and EP/PTCP composites under air.

Table 1. TGA, DTG and LOI data for PTCP, Neat Epoxy and EP/PTCP Composites Sample PTCP EP EP/PTCP10% EP/PTCP20% EP/PTCP30%

T−10% (°C)

Tmax1 (°C)

Tmax2 (°C)

Char residue (%)

LOI (%)

286 361 336

322 367 344

500 554 554

2.8 1.4 4.2

23.0 26.5

328

344

575

5.7

28.0

311

335

577

8.3

30.5

is related to the scission of the cured epoxy macromolecular networks; the second one in the temperature range of 460−650 °C can be attributed to the thermal oxidative degradation of the char formed at the end of the first step. In the case of EP/ PTCP composites, the thermal degradation process exhibits the similar two-stage behavior as the neat one. However, comparing to the neat EP, the T−10% and Tmax1 of the EP/PTCP composites are lowered due to the earlier decomposition of PTCP. Generally, the earlier degradation of the phosphoruscontaining additives could promote the decomposition of polymers to form the protective char. The char could shield polymers from flame; thereby, the lowered T−10% and Tmax1 is likely essential rather than a defect of the EP/PTCP composites.26 This finding could be supported by the char yield at 800 °C (Table 1). The char residue is increased significantly to 8.3% for EP/PTCP-30% from 1.4% for the neat EP. Furthermore, from DTG profiles, it can be observed that the maximum mass loss rate the EP/PTCP composites is reduced with increasing the PTCP content, which is much lower than that of the neat epoxy. This finding agrees well with the condensed-phase flame retardant mechanism:33 the addition of PTCP is beneficial to forming a protective layer that inhibits the oxygen and heat permeation and retards the mass loss rate, and thus suppresses the thermal degradation process. Flame Retardant Behaviors. The flame resistant properties of the neat epoxy and EP/PTCP composites were investigated by measuring their LOI value, as summarized in Table 1. The results showed that the LOI value increases gradually with the increment of PTCP content in EP composites. The LOI value of neat EP is 23.0%, implying an easily flammable material. The LOI value increases significantly

Figure 4. DSC thermograms of EP and EP/PTCP composites.

composites. Neat EP exhibits a glass transition temperature (Tg) of 132 °C. After PTCP was incorporated, the Tg of EP composites shows a decreased trend with the increase of PTCP content, demonstrating the plasticization effect of PTCP on EP inherently. The reduced Tg of EP/PTCP composites may be attributed to the lower cross-linking density originated from the long alkyl chains and abundant aromatic structures in PTCP molecules. The large volume of PTCP molecules restricts the formation of high cross-linking networks, and thus Tg shifts toward a lower temperature. Thermogravimetric analysis was performed to evaluate the thermal degradation behaviors of PTCP, neat epoxy and EP/ PTCP composites. Figure 5 presents the TG/DTG profiles for the PTCP, neat epoxy and EP/PTCP composites. Some typical temperatures such as the temperature at 10% mass loss (T−10%), the first maximum thermal degradation temperature (Tmax1) and the second one (Tmax2) are listed in Table 1. As can be observed, the PTCP has lower initial degradation temperature in contrast to neat epoxy resin. It starts to degrade at about 286 °C (T−10%), which might be attributed to that the OPO bond in PTCP is less stable than the common CC bond.32 The earlier thermal degradation of phosphorus-containing groups is usually essential to promote the char formation at the beginning of degradation. In the case of neat EP, its thermal degradation process could be mainly divided into two steps: the first step locates the temperature range of 330−460 °C, which 3412


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the presence of charring process occurs upon ignition and thus postpones the appearance of peak heat release rate. The THR versus time curves (Figure 6b) shows that neat EP releases heat very fast, with a maximum value of 86.3 MJ/m2 achieved after combustion. The incorporation of PTCP suppresses the THR gradually as the PTCP content increases. The THR value of EP/PTCP-30% composite is decreased to 63.4 MJ/m2, about 27% reduction compared to that of neat EP. The reduction in the THR clarifies that more EP moieties participate into the carbonization process and thereby, less volatile products serve as “fuel” to feed the flame. A similar trend is also observed for the av-EHC (Table 2). The addition of PTCP leads to a reduction in the av-EHC values, implying the lower combustion heat generated from the burning of volatiles (fuels) in gaseous phase. From Table 2, it can be found that incorporation of PTCP results in a lowered TTI. As mentioned in TGA results, the earlier degradation of PTCP catalyzes the epoxy matrix to form the protective char, which is responsible for the lowered TTI. With regard to the FIGRA that is derived from the ratio of PHRR/time to PHRR,26 all the EP/PTCP composites show decreased FIGRA in comparison to pure EP. The FIGRA value is reduced dramatically from 13.5 kW/(m2·s) for neat EP to 6.5 kW/(m2·s) for EP/PTCP-30%, a 52% reduction in FIGRA relative to neat EP, implying the significantly improved fire retardancy of the material. Photographs of the residual char from neat EP and EP/ PTCP composites after cone calorimeter tests are displayed in Figure 7. EP burnt severely, leaving a fragmentary residue that disclosed the underlying foils after the cone calorimeter test (Figure 7a). In contrast, EP/PTCP composites present a larger volume of char residue (Figure 7b−d). The larger amount of char residue can provide better shielding effect of the underlying materials from heat irradiation. The microstructure of the char residue was further investigated by SEM. Figure 8 gives the SEM images of neat EP and EP/PTCP composites after cone calorimeter tests. As can be seen, the neat EP (Figure 8a) exhibits a fragmentary and porous residue after burning, which cannot prevent the flammable volatiles escaping from the decomposition zone to flame zone. Also, the heat and oxygen can easily penetrate through these big holes and gaps, and thus the neat EP exhibits the highest PHRR, THR and av-EHC values. With the addition of 10% and 20% PTCP, the char residue shows more integrated feature but still presents many holes and cracks on the surface. In the case of the EP/PTCP-30% composite (Figure 8d), a much more compact and continuous residue is observed. In general, the quality of residue correlates close to the flame retardant effect. The significantly improved flame retardant properties of the EP/PTCP-30% composite, observed from LOI and cone calorimeter tests, are probably attributed to this compact and continuous char that serves as an effective thermal insulating layer to prevent flammable volatiles escaping from

to 30.5% while incorporating 30 wt % PTCP. Generally, polymeric materials with LOI value greater than 26% are considered to be of good flame resistance.34 The LOI results demonstrated that PTCP imparts superior flame retardant behavior to EP composites. A cone calorimeter is a widely used bench-scale tool to study the fire behavior of materials, which can provide abundant firerelated parameters including time to ignition (TTI), peak heat release rate (PHRR), total heat release (THR), average effective heat of combustion (av-EHC) and so on. Figure 6

Figure 6. (a) Heat release rate and (b) total heat release versus time curves of neat EP and EP/PTCP composites.

depicts the heat release rate and the total heat release versus time curves of neat EP and EP/PTCP composites, and the detailed data is summarized in Table 2. As can be seen, neat EP burns fiercely upon ignition, displaying a sharp PHRR (1484 kW/m2). The PHRR values of EP/PTCP composites are decreased gradually with the increase of the PTCP content. The maximal PHRR reduction is observed in the case of EP/ PTCP-30%, an approximately 50% reduction in contrast to neat EP. Meanwhile, the addition of PTCP changes the shape of HRR curves, from a typical noncharring material (neat EP) to a charring one (EP/PTCP composite).35 A shoulder peak is observed obviously in the case of EP/PTCP-30%, meaning that

Table 2. Cone Calorimeter Data of Neat EP and EP/PTCP Composites Sample EP EP/PTCP-10% EP/PTCP-20% EP/PTCP-30%

TTI (s) 53 49 46 40

± ± ± ±

2 1 2 3

PHRR (kW/m2) 1484 1211 1110 748

± ± ± ±

52 25 37 14

THR (MJ/m2) 86.3 70.1 60.5 63.4

± ± ± ±

2.4 1.7 3.2 1.9 3413

av-EHC (MJ/kg) 23.60 20.45 18.82 16.14

± ± ± ±

0.65 0.47 0.38 0.26

Time to PHRR (s) 110 95 90 115

± ± ± ±

5 3 2 3

FIGRA (kW/(m2·s)) 13.5 12.7 12.3 6.5


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Table 3. Tensile Properties of Neat Epoxy and EP/PTCP Composites Sample EP EP/ PTCP10% EP/ PTCP20% EP/ PTCP30%

Impact strength (kJ/m2)

Tensile modulus (GPa)

Tensile strength (MPa)

Elongation at break (%)

14.9 ± 1.1 16.8 ± 1.0

1.56 ± 0.10 1.35 ± 0.10

40.6 ± 2.5 46.3 ± 3.4

2.0 ± 0.8 5.5 ± 0.1

18.1 ± 1.4

1.46 ± 0.03

60.8 ± 4.4

7.7 ± 1.8

19.1 ± 0.5

1.09 ± 0.09

46.7 ± 1.1

8.2 ± 0.4

phase that has a better impact resistance than stiff epoxy matrix phase. Static tensile loading tests were also performed to evaluate the influence of PTCP on the tensile properties such as tensile strength, elongation at break and tensile modulus of EP/PTCP composites. The stress−strain curves of neat EP and EP/PTCP composites are depicted in Figure 9 and the detailed data are Figure 7. Photographs of the residual char from (a) neat EP, (b) EP/ PTCP-10%, (c) EP/PTCP-20% and (d) EP/PTCP-30% after cone calorimeter tests.

the degradation zone to flame zone and also shield the underlying polymers from heat irradiation. Mechanical Properties. The impact properties are measured to evaluate the toughening effect of PTCP on epoxy resins. Table 3 lists the impact results of pure EP and EP/PTCP composites. Neat EP shows the impact strength of 14.85 kJ/m2, and the introduction of PTCP into EP composites causes an obvious increase in impact strength. Furthermore, the impact strength increases with the increment of PTCP content. The impact strength of EP/PTCP-30% composite is increased to 19.14 kJ/m2, a 29% improvement relative to neat EP, demonstrating a toughening effect of PTCP on EP. The toughening effect of PTCP originates from the long alkyl chains of cardanol moieties.36 These long alkyl chains serve as soft

Figure 9. Stress−strain curves of neat EP and EP/PTCP composites with various loadings of PTCP.

Figure 8. SEM micrographs of surfaces residues of (a) neat EP, (b) EP/PTCP-10%, (c) EP/PTCP-20% and (d) EP/PTCP-30% after cone calorimeter tests. 3414


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and the China Postdoctoral Science Foundation (Grant No. 2016M602035).

listed in Table 3. Neat EP shows a tensile strength of 40.6 MPa and a low elongation at break of 2.0%, indicating a relatively brittle material. The tensile strength of EP/PTCP-10% is increased to 46.3 MPa, whereas the elongation at break is improved to 5.5%. When the PTCP content is increased to 20%, the tensile strength reaches the maximum value of 60.8 MPa (approximately a 50% increase over neat EP). The improved tensile strength is attributed to the high compatibility between EP and PTCP. The elongation at break gradually increases with increasing the PTCP content, demonstrating the good toughening effect of PTCP on EP, which is consistent with the impact results. In the case of EP/PTCP-30%, the tensile strength is reduced to 46.7 MPa compared to EP/ PTCP-20%, but still higher than that of neat EP. The fluctuated trend in the tensile strength is attributed to the balance between the reactivity and the plasticizing effect of PTCP. When the PTCP content does not exceed 20 wt %, the hydroxyl groups in PTCP can react with epoxy matrix, which enables the binding forces between PTCP and epoxy become stronger. As the PTCP content increases to 30 wt %, the plasticizing effect of PTCP originated from the long alkyl chains of cardanol moieties exceeds the reinforcing effect, and thus the tensile strength decreases. Regarding the tensile modulus, it gradually decreases with increasing the PTCP content, which is ascribed to that the plasticizing effect of PTCP results in the reduced rigidity of the resultant composites.

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CONCLUSIONS Phosphaphenanthrene groups-containing triscardanyl phosphate (PTCP) was successfully synthesized and well characterized. The incorporation of PTCP into EP accelerated the thermal degradation process and improved the char yield. The higher char yield was beneficial to improving the flame retardant properties of epoxy resins. The LOI value of EP/ PTCP-30% was increased to 30.5% from 23.0% for neat EP. Also, the cone calorimeter results showed that the peak heat release rate, total heat release and average effective heat of combustion values of EP/PTCP-30% were decreased by 50%, 27% and 32%, respectively, in comparison to those of neat EP. The compact and continuous char could inhibit the mass and heat exchange between degradation zone and flame zone, which was responsible for the reduced fire hazards of epoxy composites. The impact strength of EP/PTCP-30% was increased by 29% relative to that of neat EP, indicating the obvious toughening effect of PTCP on EP. The findings in this study enabled PTCP to be a promising biobased flame retardant toughening agent for polymeric materials in the future.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Yuan Hu. Tel./Fax: +86-551-63601664. E-mail: yuanhu@ustc. edu.cn. *Lei Song. E-mail: [email protected]. ORCID

Yuan Hu: 0000-0003-0753-5430 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors want to acknowledge the National Natural Science Foundation of China (Grant No. 21604081 and 51303165), 3415


Research Article

ACS Sustainable Chemistry & Engineering (21) Wang, X.; Hu, Y.; Song, L.; Xing, W. Y.; Lu, H. D. Preparation, mechanical properties, and thermal degradation of flame retarded epoxy resins with an organophosphorus oligomer. Polym. Bull. 2011, 67, 859−873. (22) Wang, X.; Hu, Y.; Song, L.; Yang, H. Y.; Xing, W. Y.; Lu, H. D. Synthesis and characterization of a DOPO-substitued organophosphorus oligomer and its application in flame retardant epoxy resins. Prog. Org. Coat. 2011, 71, 72−82. (23) Wang, X.; Hu, Y.; Song, L.; Xing, W. Y.; Lu, H. D.; Lv, P.; Jie, G. X. Effect of a triazine ring-containing charring agent on fire retardancy and thermal degradation of intumescent flame retardant epoxy resins. Polym. Adv. Technol. 2011, 22, 2480−2487. (24) Mercado, L. A.; Galià, M.; Reina, J. A. Silicon-containing flame retardant epoxy resins: Synthesis, characterization and properties. Polym. Degrad. Stab. 2006, 91, 2588−2594. (25) Dogan, M.; Murat Unlu, S. Flame retardant effect of boron compounds on red phosphorus containing epoxy resins. Polym. Degrad. Stab. 2014, 99, 12−17. (26) Wang, X.; Kalali, E. N.; Wang, D. Y. Renewable cardanol-based surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustainable Chem. Eng. 2015, 3, 3281−3290. (27) 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, 2147−2157. (28) Wang, X.; Xing, W. Y.; Feng, X. M.; Yu, B.; Song, L.; Hu, Y. Functionalization of graphene with grafted polyphosphamide for flame retardant epoxy composites: Synthesis, flammability and mechanism. Polym. Chem. 2014, 5, 1145−1154. (29) Rakotomalala, M.; Wagner, S.; Döring, M. Recent developments in halogen free flame retardants for epoxy resins for electrical and electronic applications. Materials 2010, 3, 4300−4327. (30) Salmeia, K. A.; Gaan, S. An overview of some recent advances in DOPO-derivatives: Chemistry and flame retardant applications. Polym. Degrad. Stab. 2015, 113, 119−134. (31) Lin, C. H.; Wang, Y. R.; Feng, Y. R.; Wang, M. W.; Juang, T. Y. An approach of modifying poly(aryl ether ketone) to phenolcontaining poly(aryl ether) and its application in preparing highperformance epoxy thermosets. Polymer 2013, 54, 1612−1620. (32) Wang, X.; Song, L.; Xing, W. Y.; Lu, H. D.; Hu, Y. A effective flame retardant for epoxy resins based on poly(DOPO substituted dihydroxyl phenyl pentaerythritol diphosphonate). Mater. Chem. Phys. 2011, 125, 536−541. (33) Wang, X.; Song, L.; Yang, H. Y.; Xing, W. Y.; Kandola, B.; Hu, Y. Simultaneous reduction and surface functionalization of graphene oxide with POSS for reducing fire hazards in epoxy composites. J. Mater. Chem. 2012, 22, 22037−22043. (34) Pan, L. L.; Li, G. Y.; Su, Y. C.; Lian, J. S. Fire retardant mechanism analysis between ammonium polyphosphate and triphenyl phosphate in unsaturated polyester resin. Polym. Degrad. Stab. 2012, 97, 1801−1806. (35) Schartel, B.; Bartholmai, M.; Knoll, U. Some comments on the main fire retardancy mechanisms in polymer nanocomposites. Polym. Adv. Technol. 2006, 17, 772−777. (36) Chen, J.; Li, X. Y.; Wang, Y. G.; Li, K.; Huang, J. R.; Jiang, J. C.; Nie, X. A. Synthesis and application of a novel environmental plasticizer based on cardanol for poly(vinyl chloride). J. Taiwan Inst. Chem. Eng. 2016, 65, 488−497.

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Renewable Cardanol-Based Phosphate as a Flame Retardant

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