Novel Cyclolinear Cyclotriphosphazene-Linked Epoxy Resin for

Oct 31, 2012 - A novel halogen-free flame-retardant epoxy resin was designed by introducing phosphazene rings into the backbone in a cyclolinear-linke...
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Novel Cyclolinear Cyclotriphosphazene-Linked Epoxy Resin for Halogen-Free Fire Resistance: Synthesis, Characterization, and Flammability Characteristics Yongwei Bai, Xiaodong Wang,* and Dezhen Wu State Key Laboratory of Organic−Inorganic Composite Materials, School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: A novel halogen-free flame-retardant epoxy resin was designed by introducing phosphazene rings into the backbone in a cyclolinear-linked mode, and then it was synthesized through a three-step synthetic route. The chemical structures and compositions of all the cyclotriphosphazene precursors and the final product were confirmed by 1H and 31P NMR spectroscopy, elemental analysis, and Fourier transform infrared spectroscopy. The thermal curing behaviors of the synthesized epoxy resin with dicyandiamide, 4,4′-diaminodiphenylmethane, and novolak as hardeners were investigated by differential scanning calorimetry (DSC), and the thermal properties were also evaluated by DSC and thermogravimetric analysis. These thermosets achieved high glass transition temperatures over 150 °C and simultaneously displayed good thermal stability with high char yields. Moreover, these thermosets have higher tensile and flexural strength but lower impact toughness in comparison with the conventional epoxy thermosets. The flammability characteristics of the thermosets obtained by curing this epoxy with three hardeners were studied on the basis of the limiting oxygen index (LOI) and UL−94 vertical burning experiments as well as the analysis of residual chars of the tested bars after vertical burring. The high LOI values and the V−0 classification for these epoxy thermosets indicate that the incorporation of phosphazene rings into the molecular backbone imparts flame retardancy on the epoxy resin. This may result from a unique combination of phosphorus and nitrogen in the phosphazene ring. The epoxy resin synthesized in this study is a green functional polymer and may become a potential candidate for fire- and heat-resistant applications in electronic and microelectronic fields.

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INTRODUCTION Phosphazenes, particularly polyphosphazenes, are a well-known class of versatile functional materials having an inorganic backbone structure. When appropriately substituted, the materials can be employed as flame resistant materials, elastomers, membranes, solid ionic conductors, and inert biomaterials.1 The beginning of the study of phosphazenes can be traced back more than 160 years, but very little progress was achieved until the 1960s. In recent years, phosphazenes have received considerable attention from both the academic and commercial communities.2,3 These compounds contain both phosphorus and nitrogen and may display enhanced flame retardancy when compared to similar compounds containing phosphorus alone. In addition, these materials yield relatively minor amounts of toxic combustion products in a fire situation. These properties may be ascribed to the unique molecular framework based on alternating phosphorus and nitrogen atoms. A rich substitution chemistry at the phosphorus center allows the introduction of the various functional moieties.4,5 With highly replaceable chlorine atoms linked to the phosphorus atoms of a phosphazene ring, phosphazenes offer a high degree of tailorability through variations in the synthesis procedures. They can be used to functionalize a broad range of polymers to impart autoextinguishability on these highly flammable organic materials.6,7 An alternative promising area of research involves the incorporation of phosphazene units into organic polymers. The properties of organic polymers can be modified significantly to improve their fire resistance, ionic © 2012 American Chemical Society

conductivity, biological compatibility, or other properties by the incorporation of a small amount of a specifically tailored phosphazene.8−11 This makes phosphazenes particularly good candidates for the development of fire-resistant materials for use in electric and electronic applications.12 Epoxy resins, an important family of functional materials, have achieved wide application in high-tech electric and electronic fields. In this case, the versatile epoxy resins with excellent flame retardancy as well as high performance have necessarily been required to reduce or to avoid the fire threats.13,14 Although the effectiveness of flame retardancy for the halogen-containing epoxy resins is absolutely undoubted, a possible threat to the environment and human health may not be negligible.15 This has prompted a re-evaluation of the hazards associated with the use of halogen-based epoxy resins and a search for halogen-free and environmentally friendly flame-retardant epoxy resins.16,17 Most of the halogen-free flame-retardant materials contain phosphorus compounds,18−21 because phosphorus can act in the condensed phase promoting char formation on the surface, which acts as a barrier to inhibit gaseous products from diffusing to the flame and to shield the polymer surface from heat and air. Nitrogen-containing compounds are another class of halogen-free flame retardants, Received: Revised: Accepted: Published: 15064

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from Beijing Chemical Reagent Co., China. THF was distilled from sodium benzophenone ketal prior to use, and the other chemicals and reagents were used as received. Dicyandiamide (DICY), 4,4′-diaminodiphenylmethane (DDM), and novolak were selected as hardeners and also purchased from Beijing Chemical Reagent Co., China. A conventional epoxy resin, diglycidyl ether of bisphenol A (DGEBA), was commercially supplied by Wuxi Epoxy Factory of BlueStar New Chemical Materials Co., Ltd., China. Characterization and Measurement. Nuclear Magnetic Resonance (NMR) Spectroscopy. 1H and 31P NMR spectra of synthesized cyclic phosphazene precursors and the final product were obtained using a Bruker AV-400 400 MHz spectrometer in dimethyl sulfoxide-d6 (DMSO-d6) or deuterochloroform (CDCl3) solution and were referenced to external tetramethylsilane (TMS) and external 85% H3PO4 with positive shifts recorded downfield from the reference, respectively. The 31P NMR spectrum was proton decoupled. Elemental Analysis. The elemental analysis was performed with a Vario−EL−cube CHNS elemental analyzer (Elementar Analysensysteme GmbH). Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra were obtained using a Bruker Tensor−27 FTIR spectrometer with a scanning number of 50. A finely ground, approximately 1% mixture of a solid sample in KBr powders is fused into a transparent disk for FTIR measurement using a hydraulic press. Epoxy Equivalent Weight (EEW) Measurement. The EEW of the synthesized cyclolinear phosphazene-based epoxy resin was determined using the HCl/acetone chemical titration method. Steric Exclusion Chromatography (SEC) Measurement. The molecular weight and its distribution were obtained through SEC using a Waters GPC515−2410 gel permeation chromatographer with THF as rhe solvent at a flow rate of 1.0 mL/min. The chromatographic column was calibrated with polystyrene standards. Differential Scanning Calorimetry (DSC). A thermal curing study of the synthesized epoxy resin with various hardeners was carried out on a TA Instruments Q20 differential scanning calorimeter equipped with a thermal analysis data station, operating at heating rates of 10 °C/min under a nitrogen atmosphere. Thermal transition temperatures were also determined by DSC. Thermogravimetric Analysis (TGA). A TGA experiment was performed under a nitrogen atmosphere using a TA Instruments Q50 thermal gravimetric analyzer. The samples with a mass of about 10 mg were placed in an aluminum crucible and ramped from room temperature up to about 800 °C at a heating rate of 10 °C/min, while the flow of nitrogen was maintained at 50 mL/min. Mechanical Property Test. Impact, tensile, and flexural test specimens were fabricated via a cast molding with the different shapes required for mechanical measurements. Notched Izod impact strength was measured with a SANS ZBC−1400A impact tester according to the ASTM D−256 standard. The thickness of the notched impact bars was 3.2 mm, and the impact energy was 1 J. The tensile and flexible properties were measured with a SANS CMT−4104 universal testing machine using a 10 000 Newton load transducer according to ASTM D−638 and D−790 standards, respectively. All the tests were done at room temperature, and the values reported reflected an average from five tests.

which can release nonflammable gases or decompose endothermically to cool the pyrolysis zone at the combustion surface. Incorporating these phosphorus- and nitrogencontaining flame retardants into polymeric materials by physical means obviously provides the most economical and expeditious way of promoting flame retardancy for commercial polymers.22,23 Nevertheless, a variety of problems, such as poor compatibility, leaching, and a reduction in mechanical properties of the polymer matrix, weaken the attraction. Therefore, the reactive approach is considered as a more effective route. Such an approach involves either the design of new, intrinsically flame retarding polymers or modification of existing polymers through copolymerization with a flame retarding unit either in the chain or as a pendent group. The main advantage of reactive flame retardants is to impart permanent flame retardancy as well as maintain the original physical properties of the epoxy resins in a better way.24 Redesign and synthesis of epoxy macromolecules in both backbone and side groups have attracted the most attention in order to improve both the flame retardancy and the physical properties of an epoxy resin. Many studies have been reported for the design and synthesis of flame retardant epoxy resins by incorporating phosphorus-containing flame retarding units in their backbone such as phosphine oxide, phosphates, and the other phosphorylated and phosphonylated derivatives.25−27 However, these phosphoruscontaining epoxy resins hardly gain a high weight fraction of phosphorus, which results in a low degree of flame retardancy. The chemical incorporation of phosphazene units into the epoxy resins is considered as a good idea to gain a dramatically high flame retarding efficiency due to the synergistic effect of phosphorus and nitrogen.6,28−31 Some reported investigations show that epoxy thermosetting resins containing cyclotriphosphazene units exhibit a potential application in the electric industry for their good flame retardancy.28−31 So it is desirable to use this synergistic approach to obtain novel halogen-free flame retardant epoxy materials with high performance. A novel cyclolinear-structured epoxy resin containing cyclotriphosphazene was reported herein, which can be readily modified with a variety of substituents via nucleophilic substitution. The good reactivity of the chlorine atoms of hexachlorocyclotriphosphazene toward nucleophiles offers synthetic adaptability to introduce the substituents with the expected functionality, which can subsequently be transformed into desired synthetic precursors.32,33 This makes it possible to build the epoxy macromolecules in a cyclolinear-structured mode as designed in advance. The introduction of cyclotriphosphazene moieties into the backbone is expected to enhance the thermal resistance, thermal stability, and most importantly, the fire resistance of the resulting epoxy materials. A complementary study on the curing properties and flammability characteristics of the synthesized epoxy resin was also performed and described in this study.

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EXPERIMENTAL SECTION Chemicals. Hexachlorocyclotriphosphazene (1) was commercially obtained from Shanghai Yagu Chemical Co. Ltd., China. It was recrystallized from n-heptane and sublimed at 40 °C with a vacuum degree of 0.1 mmHg before use. Phenol (2), bisphenol A (4), epichlorohydrin (6), sodium hydride (NaH) (70% suspension in mineral oil), hexadecyl trimethyl ammonium bromide, dichloromethane (CH2Cl2), n-hexane, tetrahydrofuran (THF), toluene, and acetone were purchased 15065

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Figure 1. Scheme of the synthetic route for cyclolinear cyclotriphosphazene-linked epoxy resin.

Limiting Oxygen Index (LOI) Test. The LOI test was performed using an HD-2 oxygen index apparatus with a magneto-dynamic oxygen analyzer, according to the ASTM D2863 standard. The average values of LOI were obtained from the results of five tests. UL−94 Vertical Burning Test. The UL−94 vertical burning test was carried out based on the testing method proposed by Underwriter Laboratory according to ASTM D−1356 standard. Scanning Electron Microscopy. The scanning electron microscopy (SEM) observation was performed on a Hitachi S−4700 scanning electron microscope to investigate the morphologies of the residual chars. The char samples for SEM were obtained after combustion in the vertical burning tests and were made electrically conductive by sputter coating with a thin layer of gold−palladium alloy. The images were taken in high vacuum mode with 20 kV acceleration voltage and a medium spot size. Synthesis and Reactions. Synthesis of Tetra-Substituted Cyclotriphosphazene Precursor [N3P3(OC6H5)4Cl2, 3]. Compound 1 (35 g, 0.101 mol) was dissolved in 150 mL of THF. Compound 2 (39.75g, 0.424 mol) was dissolved in 150 mL of THF and then added dropwise to the suspension of NaH (10.4g, 0.424 mol) in 100 mL of THF. This reaction mixture was stirred at room temperature for 24 h under a nitrogen atmosphere. The resultant sodium phenoxide solution was added dropwise to the stirred solution of 1 in THF at 0 °C. This mixture was allowed to warm to room temperature to perform the reaction for 12 h. After the reaction was completed, the solvent was evaporated at reduced pressure, and then the oily resultant was dissolved in 500 mL of CH2Cl2. This solution was washed four times with 500 mL of 3% aqueous NaHCO3, dried over Na2SO4, and concentrated by rotary evaporation. Column chromatography (silica gel, CH 2Cl 2/n-hexane = 3/2) was carried out for further

purification to give off-white oil as 3 (24.24g, 41.5 wt % yield). Elemental analysis found: C, 50.25; H, 3.16; N, 7.45; Cl, 12.24. Calculated for N3P3O4C24H20Cl2: C, 49.85; H, 3.49; O, 11.07; N, 7.27; P, 16.07; Cl, 12.3. Synthesis of Hexa-Substituted Cyclotriphosphazene Precursor [N3P3(OC6H5)4(OC6H5C(CH3)2C6H5OH, 5). A mixture of 4 (17.36 g, 0.076 mol) and NaH (3.82 g, 0.156 mol) in THF (200 mL) was stirred and refluxed at room temperature for 2 h, and afterward a solution of 3 (20 g, 0.034 mol) in 100 mL of freshly distilled THF was added dropwise to this reaction mixture to perform the reaction with vigorous stirring at room temperature for 24 h under a nitrogen atmosphere. After the reaction was completed, the white precipitate, NaCl, was filtered off, and then the solvent was removed from the reaction mixture by reduced pressure rotary evaporation. The residue was dissolved in CH2Cl2 and washed successively with water to remove NaCl. The solvent was removed by reduced pressure rotary evaporation, and a white solid was obtained. Column chromatography (silica gel, CH2Cl2/methanol = 3/2) was carried out to get the solution containing the product. Finally, the solvent was evaporated at reduced pressure to get an offwhite solid as 5 (17.56 g, 53.7% yield). Elemental analysis found: C, 68.29; H, 5.11; N, 4.25. Calculated for P3N3C54H50O8: C, 67.43; H, 5.24; O, 13.31; P, 9.66; N, 4.37. Synthesis of Cyclolinear Cyclotriphosphazene-Linked Epoxy Resin (7). Compounds 6 (25.09 mL, 0.320 mol) and 5 (15.0 g, 0.016 mol) were refluxed at 75 °C with vigorous stirring under a nitrogen atmosphere, and 1 wt % of hexadecyl trimethyl ammonium bromide as a catalyst was added. This reaction mixture was stirred for 2 h, and then a NaOH solution (5.0 mL, 28.57 wt %) was added. The reaction mixture was continuously stirred at 70 °C for 2 h, and then the residual 6 was removed at reduced pressure. The above reactant was dissolved in toluene, and then another NaOH solution (5 mL, 15066

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28.57 wt %) was added. The mixture was continuously stirred at 70 °C for 2 h to perform the ring-closure reaction. The mixture containing the final product was washed with hot distilled water three times. The organic layer separated was dried over magnesium sulfate and then was distilled at reduced pressure successively to remove toluene. Finally, a dark-brown liquid as 7 was obtained. Curing Procedure of Cyclolinear Cyclotriphosphazene-Linked Epoxy Resin. The synthesized cyclolinear cyclotriphosphazene-linked epoxy resin was cured with DICY, DDM, and novolak as hardeners. The cyclolinear phosphazenebased epoxy resin was dissolved in an appropriate amount of acetone, and then the hardener was added with an equivalent ratio to the epoxy resin of 1:1. 2-Methylimidazole (0.2 wt %) as a curing accelerator was also added into this solution. The mixture was stirred constantly to be a homogeneous solution and then was kept in a vacuum oven at 90 °C for 3 h to remove the solvent. A two-step curing procedure was carried out in a mold to obtain the thermosetting resins. The epoxy formulations containing DICY, DDM, and novolak were first precured at 150 °C for 2 h and then were further postcured at 180 °C for 3 h. At the end of the curing procedure, the cured system was cooled gradually to room temperature to avoid stress cracking.

Figure 2. 31P NMR spectrum of product 5.

(4-hydroxyphenyl)propan-2-yl)phenoxy groups. The 1H NMR spectrum of 5 exhibits two intensive singlet resonance signals at 1.62 and 5.34 ppm (see Figure 3S), which are well assigned to six methylic protons and two hydroxy protons, respectively. The other five sets of resonance frequencies are observed at 7.3−6.6 ppm corresponding to 36 aromatic protons. Compound 5 also shows a broad absorption band corresponding to the hydrogen-bonded O−H in its FTIR spectrum at around 3431 cm−1. As another important feature of the spectrum, a strong C−H stretching vibration of methyl groups at 2800− 3000 cm−1 is observed. The other characteristic peaks at 1211 and 1056 cm−1 are attributed to P−O−C bonding and an asymmetrical stretching C−O band, respectively. Moreover, the disappearance of characteristic P−Cl absorption bands at 528 and 601 cm−1 proves the whole substitution of chloride atoms. These results suggest that the two chloride atoms remaining on the tetra-substituted cyclotriphosphazene precursor have been successfully substituted by 4-(2-(4-hydroxyphenyl)propan-2yl)phenoxy groups. Additionally, the elemental analysis data are consistent with the calculated one based on the formula of 5, which further confirmed the predicted structure of 5. Synthesis and Characterization of 7. The novel cyclolinear cyclotriphosphazene-linked epoxy resin 7 was synthesized through the polycondensation of 5 with 6, and its chemical structure was also characterized by 1H and 31P NMR spectroscopy, FTIR spectroscopy, and SEC. Figure 3 illustrates the 1H NMR spectrum of this epoxy resin. This spectrum shows the expected resonance signals for the aromatic protons of both the substituted phenoxy groups on cyclotriphosphazenes and the phenoxy units on the backbone. A strong singlet resonance signal at 1.66 ppm can be clearly observed and is assigned to the protons (labeled g) of methyl on 4-(2-(4hydroxyphenyl)propan-2-yl)phenoxy units. As is shown by the inlet of Figure 3, two sets of multiplet resonance signals at around 3.95 and 4.20 ppm corresponding to the protons (labeled c) of the methylene connected with oxirane rings are an indication of the reaction at these sites to form the final epoxy resin. The other two sets of multiplet resonance signals at around 2.92 and 2.77 ppm are attributed to the protons (labeled a) of the methylene on the oxirane ring while a set of multiplet resonance signals appear at around 3.36 ppm as the assignment for the protons (labeled b) of methine on the oxirane ring. In addition, three sets of weak resonance signals at

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RESULTS AND DISCUSSION Synthesis and Characterization. Synthesis and Characterization of 3. The overall reaction sequences for the synthesis of cyclolinear cyclotriphosphazene-linked epoxy resin are depicted in Figure 1. A tetra-substituted cyclotriphosphazene precursor, 3, was first synthesized through the reaction of 1 with 2, and its chemical structure was characterized by 31P NMR. The change is found from a sharp singlet at 20.01 ppm for 1 to an A2B spin pattern of 3 (see Figure 1S), in which the chemical shifts at around 4.83 and 20.44 ppm are attributed respectively to a diphenoxy-substituted phosphorus atom and the other two phosphorus atoms bearing a phenoxy group and a chloride atom. The absorptions for these two different environmental phosphorus atoms, i.e., P*−(OC6H5)2 and P*− (OC6H5)Cl, gave an integration ratio of 1:2. The FTIR spectrum of 3 shows two absorption peaks at 877 and 1171 cm−1 (see Figure 2S), indicating the formation of a P−O−C bond due to the occurrence of a substitution reaction. Furthermore, a strong PN stretching vibration is also observed in the infrared spectrum at 1180−1250 cm−l, which is an indication that the phosphazene ring remained intact during the substitution reaction. In addition, the elemental analysis results are in good agreement with the data calculated in terms of the expected formula of 3. These results indicate that four phenoxy groups have been successfully substituted on this cyclotriphosphazene precursor, and no side products are detected. Synthesis and Characterization of 5. A hexa-substituted cyclotriphosphazene precursor 5 was synthesized from 3 and 4, and its structure was confirmed by 1H and 31P NMR spectroscopy. The 31P NMR spectrum of 5 shown by Figure 2 clearly displays the intensive doublet resonance signals at 9.50 and 9.34 ppm, corresponding to two different environmental phosphorus atoms, i.e., P*−(OC6H5)(OC6H5C(CH3)2C6H5OH) and P*−(OC6H5)2, on the phosphazene ring. The chemical shifts for these two types of phosphorus atom only demonstrate a slight gap of 0.16 ppm due to the similar substituent environments between phenoxy and 4-(215067

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confirmed the structure of 7. The GPC data demonstrate that 7 achieved a number-average molecular weight no more than 1156, as well as a weight-average molecular weight of 1267 with a polydispersity of 1.096. The GPC trace also indicates that most (89.1 wt %) of 7 only contains one cyclotriphosphazene unit, and a little (7.4 wt %) contains two units, but a very small amount contains tree ones or more. The EEW of 7 was determined by the HCl/acetone chemical titration method and showed a value of 624.78 g/equav. It is noteworthy that the EEW of 7 is very close to its theoretical values of 606.12 g/ equav based on the GPC data, indicating that the reaction between 5 and 6 has been completed. Curing Behaviors. The thermal curing behaviors of cyclolinear cyclotriphosphazene-linked epoxy resin with DICY, DDM, and novolak as hardeners were investigated by DSC. Dynamic scans were performed at a heating rate of 10 °C/min, and their thermograms were illustrated in Figure 5. All

Figure 3. 1H NMR spectrum of cyclolinear cyclotriphosphazenelinked epoxy resin.

4.21, 4.02, and 3.58 ppm can be distinguished from the 1H NMR spectrum, which are assigned to the protons of methylene [−CH2*−CH(OH)−CH2*−], methine [−CH2− CH*(OH)−CH 2 −], and hydroxy [−CH 2 −CH(OH*)− CH2−] on the molecular backbone, respectively. Moreover, the 31P NMR spectrum also supports the chemical structure of 7 and confirms the high purity of this epoxy resin (see Figure 4S). Figure 4 shows the FTIR spectrum of 7, in which a distinct absorption peak at 1243 cm−1 due to PN stretching indicates

Figure 5. DSC thermograms of thermal curing reactions of cyclolinear cyclotriphosphazene-linked epoxy resin with three hardeners.

of the curing systems show a single exothermic peak corresponding to the curing reactions of the synthesized epoxy resin with hardeners. This result implies that the thermal curing has performed completely for each curing systems without any postcuring as a result of homopolymerization. It is noted from Figure 5 that the onset exothermic temperature of the epoxy resin cured with novolak is lower than the other two curing systems, indicating that the thermal curing reaction of this epoxy resin with novolak occurs initially prior to the other two hardeners. However, the exothermic peak temperature (Tp) corresponding to the curing reaction of this epoxy resin with DDM is lower than those of the other two curing systems, and a variation trend of Tp stands by an order of DDM < novolak < DICY. This indicates that DDM has a higher chemical reactivity with the synthesized epoxy resin than the other two hardeners, and furthermore the chemical reactivity of these three hardeners toward this epoxy resin increases accordingly in the order of DDM > novolak > DICY. It is well-known that the epoxy curing is carried out through the nucleophilic substitution reaction, and the activation of oxiranering-opening is achieved by proton donors during the course of reaction. The amine group has much stronger basicity than the hydroxyl group, and the low steric hindrance of DDM can lead to a greater propensity toward nucleophilic attack in an oxirane

Figure 4. FTIR spectra of cyclolinear cyclotriphosphazene-linked epoxy resin.

the presence of the phosphazene rings. The characteristic peak at 1186 cm−1 is attributed to the P−O−C bonding while the asymmetrical stretching C−O band appears at 1035 cm−1. As an important feature of this spectrum, two absorption bands at 3443 and 2969 cm−1 are attributed to the hydrogen-bonded O−H and −CH2− stretching, respectively, and three intensive bands characteristic of oxiranic C−O−C stretching vibration appear at 1297, 912, and 833 cm−1. These results provided evidence for the introduction of epoxy groups and also further 15068

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Table 1. DSC and TGA Results for Cyclolinear Cyclotriphosphazene-Linked Epoxy Thermosets Cured with Three Hardeners nitrogen atmosphere thermoset sample a

CL-CTPN epoxy /DICY CL-CTPN epoxya/DDM CL-CTPN epoxya/novolak

air atmosphere

Tg (°C)

Tonsetb (°C)

Tmaxc (°C)

char yield at 750 °C (wt %)

Tonsetb (°C)

Tmaxc (°C)

char yield at 750 °C (wt %)

156.6 159.2 166.5

307.6 343.7 309.4

402.1 398.4 409.2

26.68 33.24 37.72

284.5 310.1 286.7

407.3 358.9 412.5

25.62 21.16 30.79

a

The abbreviation of the cyclolinear cyclotriphosphazene-linked epoxy resin. bThe onset decomposition temperature, at which the thermoset undergoes 3 wt % of weight loss. cThe characteristic temperature, at which a maximum rate of weight loss occurs.

molecules restricts the segmental motion and consequently results in an improvement in the Tg of the thermoset. The thermal stability of cyclolinear cyclotriphosphazenelinked epoxy thermosets cured with three hardeners was investigated by TGA under both air and nitrogen atmospheres. The TGA thermograms of these thermosets are illustrated in Figure 6, and the analysis data are summarized in Table 1. It is

ring as well. Therefore, DDM is most reactive toward the oxirane ring among these three hardeners. The lowest reactivity of DICY is probably due to an induction effect as a result of the decline in the nucleophilicity of nitrogen caused by the strong electron withdrawing cyano group on DICY molecules. Novolak contains a few proton donors like hydroxyl groups on its molecule and can catalyze the curing reaction by itself, resulting in a higher reactivity than DICY. In addition, it is interestingly found that the curing system with DDM shows both a much steeper initial slope and a more narrow width of the exothermic peak on its DSC trace compared to the other two systems. These results indicate that its curing reaction performs faster than the other two systems both in the initial stage and throughout the whole course. It is understandable that the reaction rate of the nucleophilic substitution reaction is determined by the electron density in the reaction site. The aromatic units in DDM have an electron-withdrawing naturen while the other two hardeners lack of this character. Therefore, the curing system with DDM underwent a faster reaction with the oxirane ring than those with DICY and novolak. Thermal Resistance and Stability. The thermal resistance is one of the most important properties of epoxy thermosets because it establishes the service environment for the epoxybased functional materials. Usually, the glass transition temperature (Tg) represents the thermal resistance of epoxy thermosets, which are only used well, in most cases, at a temperature below Tg. Therefore, it is very important for the epoxy resin to achieve a high Tg when designing its molecular structure. The Tg's of cyclolinear cyclotriphosphazene-linked epoxy thermosets cured with three hardeners were detected by DSC, and the obtained data are summarized in Table 1. It is noteworthy that all the thermosets show a high Tg more than 150 °C as expected in comparison with the conventional epoxy thermosets reported.34,35 It has already been known that glass transition is generally ascribed to the segmental motion of the polymeric networks, and Tg is determined by the degree of freedom for the segmental motion, cross-linking and entanglement constraints, and the packing density of the segments. In this work, phosphazene rings were incorporated into the backbone chain of the synthesized epoxy resin as well as the presence of aromatic substituents on the phosphazene ring, leading to a great steric hindrance, and thus the Tg's of cyclolinear cyclotriphosphazene-linked epoxy thermosets increased as a result of the confinement for segmental motion. It is also observed that the epoxy thermoset achieves a somewhat higher Tg when cured with novolak compared to the other two hardeners. As is well-known, novolak contains more reactive groups for cross-linking than DICY and DDM. This results in a high packing density for the networks, and thus, a high Tg can be gained. Nevertheless, the thermoset cured with DDM shows a higher Tg than that with DICY. It is reasonable to believe that the rotational hindrance of the phenyl groups in the DDM

Figure 6. TGA thermograms of cyclolinear cyclotriphosphazenelinked epoxy thermosets cured with three hardeners.

clearly observed that the TGA traces of thermosets cured with DICY and DDM exhibit a typical one-stage degradation in nitrogen as a result of the fact that the thermosets undergo onestage decompositions for the major components of the thermoset molecules. However, these two thermosets exhibit a two-stage thermal degradation in the air. Besides the first major degradation of the main chains, the second step one is ascribed to the minor decomposition of more thermally stable oxidation products of cyclotriphosphazene generated in the air. Furthermore, the thermoset cured with novolak was also observed to present a two-stage thermal decompositions both in nitrogen and in air atmospheres. The first stage of thermal degradation is caused by the hydrocarbon segments of the thermoset, and the second one is attributed to the decomposition of the major backbones of polymeric networks due to their higher thermal stability. The TGA results also demonstrate that the onset decomposition temperatures of all the thermosets corresponding to 3 wt % weight loss are in the ranges of 280−310 °C in the air and 300−350 °C in nitrogen. It is noteworthy that the main degradations start at a maximum decomposition temperature (Tmax) beyond 400 °C, at which the weight loss occurs at a maximum rate. These results indicate that the cyclolinear cyclotriphosphazene-linked epoxy 15069

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thermosets have good thermal stability in nature. It should be pointed out that the thermoset cured with novolak exhibits the highest Tmax among these three thermosets, and the thermal stability of the thermosets cured with three hardeners stands by an order of novolak > DICY > DDM in nitrogen but novolak > DDM > DICY in air according to the Tmax's. These results are attributed to the high cross-linking density of thermoset as well as the thermally stable aromatic units on the molecular backbone of novolak. As shown by the data in Table 1, the thermoset cured with novolak shows the highest char yields of 30.79 wt % in air and 37.72 wt % in nitrogen among these tree samples. The thermosets cured with DICY and DDM also obtained high char yields more than 21 wt % in the air and 26 wt % in nitrogen. The achievement of such high char yields may be explained by the highly thermally stable phosphazene rings in the thermosets, which promotes char formation during pyrolysis. Furthermore, all of the thermosets exhibit much higher char yields in nitrogen than those in the air. The oxygen in air can enhance the thermooxidative decomposition of the phosphazene rings as well as the other segments of these thermosets and thus results in lower char yields. In addition, the thermoset cured with novolak obtained the highest char yield in nitrogen among these three thermosets, which indicates that the high cross-linking density of the thermoset dominates the formation of char in the absence of oxygen. On the basis of these results, it can be deduced that the heat resistance of cyclolinear cyclotriphosphazene-linked epoxy resin is superior to other known phosphorus-containing epoxy resins that have been reported.14,25,26,28 Mechanical Properties. The mechanical performance of the cyclolinear cyclotriphosphazene-linked epoxy thermosets cured with three hardeners was investigated. Figures 7 and 8 demonstrate the tensile and flexural testing results of these thermosets, respectively, and the corresponding mechanical parameters of the DGEBA thermosets as control samples were also displayed in these two figures. It can be observed that most of the cyclolinear cyclotriphosphazene-linked epoxy thermosets show slightly higher tensile strength and Young’s modulus than the DGEBA thermosets, except that the one cured with DDM presents a lower tensile strength. Moreover, their flexural strength and modulus are also higher than those of the DGEBA thermosets. Such an increment in mechanical strength is due to the presence of cyclotriphosphazene on the backbone, which generates a notable steric hindrance. This may restrain the mobility of the chain and thus results in an increase in rigidity of the thermosets. Furthermore, from the impact toughness of these thermosets shown in Figure 9, it is also found that the notched Izod impact strength of the cyclolinear cyclotriphosphazene-linked epoxy thermosets is appreciably lower than that of the control samples, indicating the considerable brittleness in natural. These mechanical parameters suggest that the cyclolinear cyclotriphosphazene-linked epoxy resin has slightly higher rigidity but lower impact toughness than the conventional epoxy resin, DGEBA, as a result of the incorporation of phosphazene rings into the backbone in a cyclolinear mode. Additionally, it is notable that the hardeners also influenced the mechanical performance of both cyclolinear cyclotriphosphazene-linked epoxy thermosets and DGEBA ones. On the basis of the average values from five tests for each sample, the thermosets cured with novolak exhibit the highest tensile strength and Young’s modulus among the three types of the thermosets. A similar result can be found in flexural

Figure 7. Tensile strength and Young’s modulus of cyclolinear cyclotriphosphazene-linked epoxy (CL−CTPN) thermosets and DGEBA ones cured with three hardeners.

strength and modulus. These results may be ascribed to the high cross-linking density resulting from the curing of multifunctional novolak. Such a high cross-linking density also results in the lowest impact toughness of the thermosets cured with novolak among the three types of thermosets, as shown by Figure 9. Flammability Characteristics. The flammability characteristics of cyclolinear cyclotriphosphazene-linked epoxy thermosets cured with three hardeners were first evaluated by the LOI measurement, and the corresponding results were summarized in Table 2. It is expected to notice that all of the thermosets exhibit high values of LOI over 29 vol %, significantly greater than the oxygen concentration of air, 21 vol %. This means that these thermosets should be completely nonflammable in the air, though the LOI values vary a little when the thermosets cured with different hardeners. This indicates that incorporating phosphazene rings into the backbone of epoxy resin is very effective for the enhancement of flame retardancy and makes the epoxy resin become a nonflammable functional polymer. The UL−94 vertical burning experiment is considered another important means for determining the upward burning characteristics of cyclolinear phosphazene-based epoxy thermosets. The UL−94 vertical burning test results of these epoxy thermosets are summarized in Table 2. As was expected, all the thermosets achieved the V−0 classification in UL−94 test. It is surprisingly noted that these thermosets did not exhibit an aggressive combustion during the tests (see Figure 5S), and the emission of few smoke and no flaming drips were observed as well. One of the fascinating characteristics of combustion is that 15070

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Industrial & Engineering Chemistry Research

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Table 2. LOI and UL−94 Vertical Buring Testing Results for Cyclolinear Cyclotriphosphazene-Linked Epoxy Thermosets Cured with Three Hardeners flammability from vertical burning testing thermoset sample CL-CTPN epoxya/ DICY CL-CTPN epoxya/ DDM CL-CTPN epoxya/ novolak

LOI value (vol.%)

UL−94 classification

flaming drips

total flaming time (s)

maximal flaming time (s)

32.4

V−0

none

26.9

4.5

31.6

V−0

none

29.7

6.1

30.2

V−0

none

32.6

8.2

a

The abbreviation of the cyclolinear cyclotriphosphazene-linked epoxy resin.

flame spread during combustion. These phenomena give evidence of nonflammability for the cyclolinear cyclotriphosphazene-linked epoxy thermosets. It is also noteworthy that the thermosets show a different degree of flame retardancy when cured with three different hardeners and present an order of DICY > DDM > novolak according to the data listed in Table 2. This phenomenon can be explained by the contribution of a nitrogen element to the flame retardancy of thermosets when the two nitrogen-containing hardeners, i.e., DICY and DDM, are used to cure with cyclolinear cyclotriphosphazene-linked epoxy resin. On the basis of these results, it is evident that the cyclolinear cyclotriphosphazene-linked epoxy has a virtually nonflammable nature. Such good inherent flame retardancy is ascribed to the presence of a unique combination of phosphorus and nitrogen in the thermosets as a result of directly linking cyclotriphosphazene to the backbone chain of epoxy resin in a cyclolinear mode. The presence of cyclotriphosphazene moieties on the backbone can promote the formation of intumescent carbonaceous chars and thus enhance the flame retardancy in the way of a condensed phase.36−40 Such a category of a flame retarding mechanism is well-known as an “intumescent mechanism”. Following with this mechanism, an organic material can swell up when exposed to fire or heat to form a foamed mass, usually carbonaceous, which acts in the condensed phase promoting char formation on the surface as a barrier to inhibit gaseous products from diffusing to the flame and to shield the polymer surface from heat and air. Evidently, such a characteristic molecular structure is highly advantageous to achieve excellent reactive flame retardancy. Analysis of Residual Char. Usually, the residual chars formed during combustion can give some important information regarding the inherent flammability characteristics of a polymeric material, and they also reflect the fire-resistant mechanisms to some extent. Furthermore, the physical structure of the charring layer also plays a very important role in the performance of flame retardancy. Therefore, the morphology of the residual chars obtained from UL−94 vertical burning tests was investigated by SEM. Figure 10 illustrates the SEM images of these residual chars. As shown by Figure 10a, the outline of these residual chars exhibits some irregularshaped bulk, and the inside of the char shows a multiporous structure according to the magnified micrographs of each sample, indicating a typical morphology after the intumescent char formation. Additionally, all the residual chars are observed to display a very gassy surface with a few pores breaking

Figure 8. Flexural strength and flexural modulus of cyclolinear cyclotriphosphazene-linked epoxy (CL−CTPN) thermosets and DGEBA ones cured with three hardeners.

Figure 9. Notched Izod impact strength of cyclolinear cyclotriphosphazene-linked epoxy (CL−CTPN) thermosets and DGEBA ones cured with three hardeners.

most of the testing bars just combusted slightly with a small blaze and quenched within 5 s when the flame agitator was removed during the vertical burning test, indicating an autoextinguishable feature. At the end of burning experiments, the surfaces of these thermosets were covered with an expanded char network, indicating that the thermosets formed an effective char which was able to prevent the heat transfer and 15071

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Figure 10. SEM images of the residual chars for cyclolinear cyclotriphosphazene-linked epoxy thermosets. (a, b) The char of thermoset cured with DICY, (c) the char of thermoset cured with DDM, and (d) the char of thermoset cured with novolak.

through. These char layers formed during combustion present a rigid and compact texture in nature, and there are lots of integrated closed honeycomb pores inside. Such a structural form favors the temperature grads in the char layer and protects the matrix inside. Therefore, it is concluded that the thermooxidative reaction of cyclotriphosphazene moieties enhances the char formation during combustion, which results in a protective char layer formed on the surface of thermosets serving as a barrier against heat and oxygen diffusion, and consequently the flame retardancy of the thermosets is improved significantly. Moreover, the multiporous structure of residual char also depends on the other factors such as the condition of curing, gases released during combustion, the composition, and structural feature of thermosets. Because the three thermosetting systems were cured under the same conditions and underwent the postcuring for enough time, the effects of the curing conditions on the structure of residual chars were considered to be identical. Therefore, the gas releasing during combustion plays an important role in determining the multiporous structure of residual char. It is clear that the thermosets cured with DICY and DDM exhibit solid chars with the partial multiporous structure, indicating that these two thermosets only released a fraction of gaseous products during combustion. However, it should be highly noted that the char structure of the thermoset cured with novolak seems to be much more loose and porous than those with DICY and DDM, which leads to a collapse of some pore walls, as observed in Figure 10d. Usually the nitrogencontaining compound can act together with phosphoruscontaining compounds to form a high viscose char during

combustion. However, compared to the nitrogen-containing hardeners DICY and DDM, novolak is only a carbohydrate compound. This may cause a lower viscosity of char while the thermoset cured with novolak was burning. Therefore, the gaseous decomposition products easily evaporated through the low viscose char, resulting in a seriously multiporous char with many open bubbles. Such an excessively multiporous structure is disadvantageous for the prevention of heat transfer and flame spread during combustion when retarding flame. This phenomenon is in good agreement with the results of the LOI and UL-94 vertical burning tests. Although the above investigation on the microstructure of chars confirms the characteristics of intumescent char formation for cyclolinear cyclotriphosphazene-linked epoxy resin, the effect of the molecular structures of phosphazenebased epoxy functional polymers and their participation in all stages of combustion process are not fully understood yet. A further intensive study is still necessary to clarify their effecting mechanisms, so that the optimal molecular structure of the phosphazene-based epoxy functional polymers can be designed for flame retardancy and high performance.

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CONCLUSIONS A novel halogen-free flame-retardant epoxy resin having a cyclolinear cyclotriphosphazene-linked structure was synthesized successfully, and its chemical structure was confirmed by 1 H and 31P NMR spectroscopy, elemental analysis, and FTIR spectroscopy. The thermosets of this epoxy resin cured with DICY, DDM, and novolak as hardeners achieved a high thermal resistance due to high Tg's over 150 °C and also exhibited good 15072

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thermal stability with high char yields. Furthermore, the cyclolinear cyclotriphosphazene-linked epoxy thermosets achieved high LOI values and UL−94 V−0 classification, indicating excellent flame retardancy. This is attributed to the structural feature of this epoxy resin that the phosphazene rings are linked with the backbone in a cyclolinear mode. Such a unique combination of phosphorus and nitrogen in the phosphazene resulted in a synergistic flame retarding effect. In addition, the thermosets based on the cyclolinear cyclotriphosphazene-linked epoxy resin showed higher mechanical strength but lower impact toughness than the conventional epoxy thermosets, indicating acceptable mechanical performance for future application.

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ASSOCIATED CONTENT

S Supporting Information *

31

P NMR spectrum of product 3, FTIR spectra of product 3 and product 5, 1H NMR spectrum of product 5, 31P NMR spectrum of cyclolinear cyclotriphosphazene-linked epoxy resin, digital images of a burning procedure of cyclolinear cyclotriphosphazene-linked epoxy thermosets during the UL−94 vertical burning test. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 6441 0145. Fax: +86 10 6442 1693. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (Project Grant No.: 50973005) is gratefully acknowledged.

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REFERENCES

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