Flame Retardancy and Mechanism of Bismaleimide Resins Based on

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Flame Retardancy and Mechanism of Bismaleimide Resins Based on a Unique Inorganic−Organic Hybridized Intumescent Flame Retardant Chengwu Yang, Guozheng Liang,* Aijuan Gu,* and Li Yuan Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application. Department of Materials Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: To completely overcome three critical disadvantages of traditional intumescent flame retardants, a unique hybridized intumescent flame retardant (HPSi-IFR) with three-dimensional structure was synthesized, which has amine groups and Si and P elements. HPSi-IFR has very high thermal stability under either a nitrogen or air atmosphere. With a small addition of HPSi-IFR, the modified bismaleimide/diallyl bisphenol A (BDM/DBA) resin has significantly improved flame retardancy. With only 5 wt % addition of HPSi-IFR into BDM/DBA resin, the residue at 800 °C increases 23.1 wt %; meanwhile, the heat release capacity, the total heat release, and the maximum heat release are only 67, 55, and 63% of those of BDM/DBA resin, respectively. To reveal the mechanism behind the attractive flame retarding effect of HPSi-IFR, the char formation chemistry and thermodegradation kinetics were intensively studied. Results show that HPSi-IFR greatly improves the ability of producing a stable and condensed barrier that prevents the heat and mass transfer. than 280 °C.13,14,17 As we know, the initial degradation temperature (Tdi) of thermally resistant resins, such as bismaleimide (BMI) and cyanate ester (CE) resins, is usually higher than 400 °C,18 meaning that the carbonaceous layer around the Tdi value of the resin cannot be well formed. Second, traditional IFRs generally have low molecular weights, and lack chemical action with the polymer, so they tend to migrate to the surface of the polymer and prematurely release the acid and carbon sources at low temperatures.19,20 Obviously, this phenomenon does not fit the high Tdi values of thermally resistant resins. Third, a big loading of a traditional IFR should be added to get the satisfactory flame retardancy;21,22 this tends to bring a negative influence on processing characteristics, mechanical proerties, and/or dielectric properties. Therefore, developing unique IFRs for thermally resistant resins is of great interest. Silicone resin is famous for its outstanding integrated performances, including good heat resistance, high toughness, environmental protection, and some flame retardancy,23−25 so some researchers tried to overcome the disadvantage of IFRs by utilizing silicone resin. However, opposite conclusions were obtained from different investigations. Typically, Li’s group added polysiloxane into a polypropylene/IFR system and found that the presence of polysiloxane does not effectively improve the flame retardancy; moreover, the loading of IFR still should be as high as 20 wt % and hence the resultant system has decreased thermal stability.26 In other words, the outstanding thermal stability of polysiloxane does not play a role in increasing the thermal stability of the IFR. By contrast, our

1. INTRODUCTION Good flame retardancy is a feature of present and future high performance thermosetting resins (HPTRs), especially for those applied in cutting-edge fields including the aerospace, electric, and electronic industries;1−3 however, traditional thermosetting resins generally have poor flame retardancy.4−6 At present, adding a flame retardant into a polymer is an effective method for preparing flame retarding polymers,7−9 so designing and synthesizing high performance flame retardants for HPTRs is one important topic with great significance in science and application. In recent years, intumescent flame retardants (IFRs) have attracted much attention from scientists and engineers worldwide, because IFRs have attractive merits including that they have high efficiency, are halogen-free, are nontoxic, and protect the environment.10−12 The flame retarding mechanism of an IFR is the formation of a swollen multicellular char layer, which not only can reduce the heat transfer between the heat source and the polymer, but also can limit the fuel transfer from the polymer toward the flame and the diffusion of oxygen into the material. Therefore, the quality of the char layer is the key for an IFR to play its flame retarding role. To develop flame retarding polymers, it is important to take into account the moment (usually refers to the temperature) of forming the carbonaceous layer as well as the stability and quality of the layer.13,14 Note that when traditional IFRs are added into thermally resistant thermosetting resins, three critical problems will appear. First, an IFR system consists of an acid source, a carbon source, and a blowing agent. The acid source is usually a phosphorus-containing compound (polymer). However, the phosphorus bonds (P−C, P−O, P−N, etc.) have relatively low bond energies and are easy to break down,15,16 so the charring process of traditional IFRs takes place at temperatures lower © 2013 American Chemical Society

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June 30, 2013 October 3, 2013 October 4, 2013 October 4, 2013 dx.doi.org/10.1021/ie402047v | Ind. Eng. Chem. Res. 2013, 52, 15075−15087

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°C and maintained at that temperature with stirring for 8 h under a dry N2 atmosphere. The obtained crude product was filtered and washed with N,N-dimethylformamide and ethanol three times, followed by drying at 80 °C under vacuum. The resultant product was a pale yellow powder, denoted as HPSiIFR. 2.3. Preparation of BDM/DBA and BDM/DBA/HPSi-IFR Prepolymers and Cured Resins. Appropriate amounts of BDM and DBA with a weight ratio of 1:0.73 were thoroughly blended at 135 °C for 30 min to obtain a transparent liquid, which was BDM/DBA prepolymer. The prepolymer was put into a preheated mold and thoroughly degassed to remove entrapped air at 135 °C in a vacuum oven. Subsequently, the mold was put into an oven for curing and postcuring following the protocol of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 230 °C/4 h, successively. The resultant resin was cured BDM/DBA resin. BDM (57.8 g) and DBA (42.2 g) were blended with HPSiIFR and then heated to 135 °C for 30 min with vigorous stirring to get a prepolymer, denoted as BDM/DBA/nHPSiIFR, where n represents the weight loading of HPSi-IFR in the BDM/DBA/nHPSi-IFR system, taking values of 5, 10, 15, and 20. Subsequently, the prepolymer was put into a preheated mold and thoroughly degassed to remove entrapped air at 135 °C in a vacuum oven, and then the mold was put into an oven for curing and postcuring following the protocol of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 230 °C/4 h, successively, to obtain a cured resin. 2.4. Measurements. FTIR spectra were recorded between 400 and 4000 cm−1 with a resolution of 2 cm−1 on a Prostar LC 240 infrared spectrometer (USA). 1 H NMR spectra were recorded on a Bruker WM300 (Germany) with CDCl3 (for testing AHPSi) or H2SO4 (for testing HPSi-IFR) as the solvent and internal standard. 29 Si NMR spectra were acquired at 99.36 MHz using a Bruker 500 MHz instrument (Germany). A 5 s delay time and 128 scans were used. 29Si NMR spectra were acquired with samples in 5 mm (outer diameter) glass cells at 25 ± 2 °C. Chromium(III) acetylacetonate at a concentration of 1% was used as a paramagnetic relaxation agent to promote rapid relaxation of the 29Si nucleus. Powder X-ray diffraction (XRD) reflectance patterns were obtained on a MERCURY CCD X-ray diffractometer (RIGAKU, Japan) with Cu Kα radiation. The 2θ angle ranged from 5 to 60°, and the scanning rate was 2° min−1. A scanning electron microscope (SEM) (Hitachi S-4700, Japan) coupled with an X-ray energy dispersive spectrometer (EDS) was employed to observe the morphologies of samples. All samples should be dried at 100 °C for 6 h before tests. Thermogravimetric (TG) analyses were performed on a TGA SDTQ600 (TA Instruments, USA) in the range from 25 to 800 °C with a flow rate of 100 mL min−1 and a heating rate of 10, 20, 30, or 40 °C min−1. The temperature at which the weight loss of a sample reaches 5 wt % is regarded as the Tdi. Raman spectra were recorded with a Raman spectrometer (JYHR800, France). The excitation wavelength was 633 nm from a He−Ne laser with a laser power of ca. 15 mW at the surface of the sample. Thermogravimetric analysis infrared (TG-IR) spectra were recorded using a TGA F1 (Netzsch, Germany) IR thermogravimetric analyzer that was interfaced to a TENSOR 27 (Bruker, Germany) FTIR spectrophotometer. About 10.0 mg of the sample was put in an alumina crucible and heated from

group recently synthesized a hyperbranched polysiloxane (PHSi) containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide (DOPO), which was used to prepare flame retardant CE resin.27 Results show that the Tdi values of all P-HSi/CE resins are larger than that of CE resin. However, note that DOPO is not an IFR, so whether this technique can be used for developing IFRs is still a mystery. In this paper, BMI resin was chosen as the polymer model because BMI resin is the representative of thermally resistant thermosetting resins which has wide applications in electric information, aerospace and aviation industries, etc.28−30 Therefore, developing flame retardant BMI resins is of great interest. According to the structural feature of BMI, a unique hybridized IFR (HPSi-IFR) was designed and synthesized through chemical condensation between spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride (SPDPC) and an amino-terminated hyperbranched polysiloxane. The effect of HPSi-IFR on the structure and flame retardancy of BMI resin is discussed. Results show that HPSi-IFR completely overcomes the critical disadvantages of traditional IFRs, and the modified BMI resin has super flame retardancy. The flame retarding mechanism was revealed by thoroughly studying the combustion process and thermal degradation kinetics.

2. EXPERIMENTAL SECTION 2.1. Materials. The BMI used was 4,4′-bismaleimidophenylmethane (BDM), which was obtained from Northwestern Institute of Chemical Engineering (China). 2,2′-Diallyl bisphenol A (DBA) was purchased from Laiyu Chemical Factory (China). γ-Aminopropyl triethoxysilane was bought from Jingzhou Jianghan Fine Chemical Ltd. (China). Phosphorus oxychloride (POCl3, AR) was purchased from Kelong Chemical Ltd., China. Pentaerythritol (PER, AR) was obtained from Sinopharm Chemical Reagent Co., Ltd., China. Other reagents were commercial products of analytical grade and were used without further treatment. SPDPC was synthesized and characterized using the method reported.31 2.2. Synthesis of HPSi-IFR. A total weight of 221 g of γaminopropyl triethoxysilane, 72 g of distilled water, 5 drops of tetraethylammonium hydroxide (20 wt % water solution), and 50 mL of ethanol were put into a three-necked flask with thorough stirring to form a solution, which was heated to 50 °C and maintained at that temperature with stirring for 12 h. Then 162 g of hexamethyldisilazane was added into the flask with stirring and maintained at 50 °C for 6 h. After that the resultant product was put into an evaporator to give off ethanol and water. Finally, a transparent and viscous liquid (172.8 g, the content of amino group is 0.96 mol) was obtained, which was amino-terminated hyperbranched polysiloxane, denoted as AHPSi. Fourier transform infrared (FTIR) (KBr, cm−1): 3435, 3370, 1600 (−NH2); 1130 (Si−O−Si); 2930, 2860, 1397 (−CH2−, −CH3). 1H nuclear magnetic resonance (1H NMR) (CDCl3, 300 MHz, ppm): δ = 2.0 (1H, s, NH−C), 2.65 (2H, s, N−CH2−C), 1.58 (2H, s, C−CH2−C), 0.58 (2H, s, C− CH2−Si), 3.60−3.68 (2H, q, O−CH2−C), 1.17−1.68 (3H, t, O−C−CH3), 0.1 (3nH, s, Si(CH3)3). 29Si NMR (CDCl3, 500 MHz, ppm): δ = −53.28 (terminal units), −60.73 (liner units), −68.77 (dendritic units). SPDPC (30 g, 0.1 mol), 1 drop of pyridine (catalyst), and 50 mL of acetonitrile were added into a glass flask. AHPSi (54 g) was blended with acetonitrile (30 mL) to form an AHPSi solution. The solution was dropped into the flask with vigorous stirring within 30 min. After that, the mixture was heated to 60 15076

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30 to 800 °C with a heating rate of 20 °C min−1 under a nitrogen atmosphere, and the flow rate was 45 mL min−1. Limited oxygen index (OI) values were measured on a Stanton Redcrat Flame Meter (England) according to ASTM D2863-2009; at least five samples for each formulation were tested. The dimensions of each sample were (100 ± 0.02) × (6.5 ± 0.02) × (3 ± 0.02) mm3. The flammability of resins was characterized using an MCC2 type microscale combustion calorimeter (MCC, Govmark Organization, Inc., USA) according to the standard ASTM D 7309. In this MCC instrument, the sample (about 5 mg) was heated under a nitrogen stream flow (80 mL min−1) in a pyrolyzer (the maximum pyrolysis temperature was 750 °C) at a certain heating rate (typically 1−5 K s−1) and suffered thermal decomposition. A sample (625 ± 5 mg) was put on the central place of the heating platform in a muffle furnace (SX2-6-13, Jiangsu, China) with a preset temperature and maintained at that temperature for 15 min. The heating transfer mechanism was thermal radiation, and the heating was uniform. The dimensions of each sample were (10 ± 0.02) × (10 ± 0.02) × (2 ± 0.02) mm3.

Figure 2. FTIR spectra of SPDPC, A-HPSi, and HPSi-IFR.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of HPSi-IFR. HPSiIFR is synthesized through the condensation reaction between P−Cl in SPDPC and an amino group in AHPSi, as shown in Figure 1. As a hyperbranched polymer usually shows a three-

Figure 3. 1H NMR spectrum of HPSi-IFR.

Figure 1. Synthesis mechanism of HPSi-IFR.

dimensional distribution,32,33 and the molecular chains of SPDPC act as bridges for linking the dendritic or linear units of hyperbranched polysiloxane, consequently, HPSi-IFR shows a three-dimensional structure with a high molar mass. Figure 2 shows FTIR spectra of SPDPC, AHPSi, and HPSiIFR. The spectrum of HPSi-IFR looks like a combined spectrum of those of SPDPC and AHPSi, indicating that HPSi-IFR simultaneously contains polysiloxane and spiral phosphate units; moreover, a new absorption peak at 1087 cm−1 (P−N) appears in the spectrum of HPSi-IFR, manifesting the occurrence of the condensation between SPDPC and AHPSi.16,31 The above trends also appear in the 1H NMR spectrum of HPSi-IFR as shown in Figure 3, and can be confirmed by the 31 P NMR spectrum of HPSi-IFR (Figure 4). In detail, only one

Figure 4. 31P NMR spectrum of HPSi-IFR.

sharp signal attributed to the OPNH− group is observed (−7.88 ppm) in Figure 4, indicating that the chemical environment of P is singularity, and SPDPC has completely reacted with the amino groups of AHPSi. 15077

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The elemental analysis shows that the element compositions of C, O, N, P, and Si in HPSi-IFR are 67.70, 13.16, 5.65, 5.12, and 8.37, respectively. The P atom has a smaller concentration than the N atom. This is an expected result because AHPSi is a hyperbranched polymer, so it is not possible for all amine groups to react with SPDPC; in other words, there are some −NH2 groups in the molecule of HPSi-IFR. This makes HPSiIFR have good reactivity with resins. 3.2. Thermal Oxide Stability of HPSi-IFR. It is known that the thermal stability of a material is closely related to the atmosphere that the material has suffered. Generally, the stability of a material under oxygen-containing conditions is usually lower than that under inert conditions due to the oxidation effect.34 When a polymer is heated, the molecules of the polymer will undergo a series of changes, of which degradation and oxidation are very important stages, through which flammable gases will be released, and thus leading to the occurrence of the flame. Therefore, it has been generally accepted that the thermal oxide stability of a polymer is closely related to the flame retardancy, and high thermal oxide stability is very important for developing flame retardant materials.35,36 Figure 5 shows the TG and DTG curves of HPSi-IFR at the heating rate of 10 °C/min. It is interesting to find that the Tdi

values in air of poly(4, 4-diaminodiphenylmethane spirocyclic pentaerythritol bisphosphonate) are about 230 and 288 °C, respectively.31 Therefore, the attractively high thermal oxide resistance of HPSi-IFR provides the solid base for preparing high performance flame retarding thermosetting resins. The char yield (Yc) of HPSi-IFR in the air or N2 atmosphere is as high as 62.4 or 53.5 wt %, much higher than the corresponding values of traditional IFRs reported in the literature,26,31 suggesting that HPSi-IFR has excellent char formation ability. 3.3. Char Formation Ability of HPSi-IFR. According to the flame retarding mechanism of IFRs, the char formation ability reflects the flame retardancy of an IFR,40−42 so it is necessary to address this issue by evaluating the structure change during the thermal decomposition of HPSi-IFR. Figure 6 shows FTIR spectra of HPSi-IFR samples after maintenance a set temperature (25, 280, 350, 400, or 450 °C)

Figure 6. FTIR spectra of HPSi-IFR samples after degradation at different temperatures for 10 min in an air atmosphere.

for 10 min in an air atmosphere (coded as HPSi-IFR-25, HPSiIFR-280, HPSi-IFR-350, HPSi-IFR-400, and HPSi-IFR-450). Compared with the spectrum of HPSi-IFR-25, that of HPSiIFR-280 has a weakened peak assigned to the P−N (1087 cm−1) bond, indicating that some P−N bonds break. When the set temperature is 350 °C, the spectrum has a great change. Specifically, besides the peak of P−N bonds, some new peaks representing the P−O−C complex structure (963 and 806 cm−1) and the vibration of the CC bond in the aromatic complex structure (1640 cm−1) appear, reflecting that the char has formed through the acid-catalyzed polymerization reactions, while the acid results from the phosphoric acid produced in the phosphorate pyrolysis.17,43 In addition, the peaks assigned to the −NH2 group (3450, 1620 cm−1) and the −CH2− group (2950, 1460 cm−1) also disappear, indicating that aminopropyl decomposes completely to produce a great amount of nonflammable gases (such as NH3 and CO2) for swelling; this is in good agreement with the Tmax value as shown in Figure 5. In the spectra of HPSi-IFR-400 and HPSi-IFR-450, the peaks representing P−O (806 cm−1), P−O−C (965 cm−1), Si−O−Si (1100 cm−1), and CC (1640 cm−1) can still be observed, demonstrating that the final char has a hybridized structure that contains P−O−Ph, Si−O−Si, and aromatic/ graphitic structures. The P−O−Ph and aromatic/graphitic

Figure 5. TG and DTG curves of HPSi-IFR in an air or a N2 atmosphere at a heating rate of 10 °C/min.

value of HPSi-IFR in the N2 atmosphere is almost equal to that in the air atmosphere. With regard to the DTG curves, each curve has two peaks, suggesting that the decomposition of HPSi-IFR shows the maximum rate at two temperatures (Tmax1 and Tmax2). The Tmax1 value under the N2 atmosphere (at about 325 °C) is almost the same as that under the air atmosphere, which is thought to be assigned to the scission of the aminopropyl and phosphate ester bonds. 37,38 As the intumescent char begins to be formed, the degradation rate decreases. The Tmax2 value of HPSi-IFR is related to the nature of the atmosphere, and is about 365 °C in air and 429 °C in the N2 atmosphere, respectively. This phenomenon is attributed to the further pyrolysis of unorganized carbon structure.39 Because oxygen accelerates the formation of char, the Tmax2 value in air is weaker and appears at a lower temperature than that in the N2 atmosphere. In addition, HPSi-IFR shows much higher thermal stability and thermal oxide stability than traditional IFRs. For example, the Tdi in air of ammonium polyphosphate/pentaerythritol (PER) ranges from 200 to 250 °C,11,12 while the Tdi and Tmax 15078

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structure are thought to be obtained from the acid-catalyzed polymerization and “Diels−Alder” reactions in a condensed phase.43,44 With regard to HPSi-IFR, the Si−O−Si chains also participate in the formation of char. The TG-IR technique is useful for revealing the thermal degradation mechanism of polymers by detecting the volatile degradation products.45,46 According to the TG and DTG curves of the HPSi-IFR, several representative temperatures (such as 288, 300, 312, 337, 361, 443, 478, and 551 °C) are selected to study the volatile components released during the thermal degradation process. Figure 7 shows the TG-IR spectra

Figure 8. XRD spectra of HPSi-IFR samples after degradation at different temperatures for 10 min in an air atmosphere.

hand, as the set temperature increases, the peak at 19° gradually shifts toward a higher 2θ degree, and approaches 24.8° for HPSi-IFR-450, indicating that a graphitic structure has been formed.50 Figure 9 shows Raman spectra of HPSi-IFR samples after they are maintained at different temperatures for 10 min in an

Figure 7. TG-IR spectra of volatilized products at typical temperatures during the thermal degradation of the HPSi-IFR under a N 2 atmosphere.

of volatilized products at typical temperatures during the thermal degradation of the HPSi-IFR. At 288 °C, the absorption peaks (670, 2328, 2360 cm−1) representing CO2 can be clearly observed in the spectrum, which is proved to be resulted from the break of the unstable P−O−C structure.47 The peak becomes very strong when the temperature is 337 °C, suggesting that a large amount of CO2 has been released. As the temperature increases, besides the peaks for CO2, multiple intensive absorption peaks can be found at 3500−3900 and 1300−1650 cm−1, suggesting that the volatilized products contain H2O48,49 and aromatic rings.48,49 H2O results from the dehydration esterification reactions and the self-condensation reaction of phosphoric acid,43,44 while compounds containing aromatic ring are produced through repeated “Diels−Alder” reactions. Note that these complex reactions will lead to the formation of the aromatic/graphitic char layer, so the appearances of these peaks also reflect the process of the char formation. When the temperature further increases to 361 °C, new peaks representing the NH3 (3280, 3350 cm−1) and aliphatic components (2980 cm−1)48,49 appear, indicating the decomposition of the aminopropyl for swelling and the formation of a stable char layer. Figure 8 shows XRD spectra of HPSi-IFR samples after maintenance at different temperatures for 10 min in an air atmosphere. HPSi-IFR-25 exhibits two characteristic peaks centered at 2θ = 7 and 19°, respectively. They do not obviously change when the set temperature is lower than 350 °C; however, as the set temperature increases, the shape changes and only a single peak can be observed, demonstrating that the structure of the HPSi-IFR has been greatly varied. On the other

Figure 9. Raman spectra of HPSi-IFR samples after degradation at different temperatures for 10 min in an air atmosphere.

air atmosphere. The original HPSi-IFR has a flat curve, while a flat and small peak over a wide wavenumber appears in the spectrum of HPSi-IFR-280, which gradually becomes two overlaid peaks centered at 1360 and 1580 cm−1, representing the D band (the unorganized carbon structure) and G band (graphitic structure), respectively, as the maintaining temperature increases. The ratio of the integrated intensities of the two peaks, I1580 /I1360, coded as RI, reflects the degree of graphitization.50,51 The RI value decreases from 1.45 to 1.22 when the temperature increases from 350 to 400 °C, while the RI value is lower than 1 when the temperature is 450 °C, suggesting that HPSi-IFR tends to form char with big graphitic degree at low temperature (