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A Novel Imidazole-Core-Based Benzoxazine and Its Blends for HighPerformance Applications A. Hariharan, K. Srinivasan, C. Murthy, and M. Alagar* Centre of Excellence in Advanced Material Manufacturing, Processing and Characterization (CoExAMMPC), Vignan University, Vadlamudi, Guntur-522 213, India S Supporting Information *

ABSTRACT: In the present work, a novel imidazole-corebased bisphenol (IBP) was synthesized and characterized. The IBP and formaldehyde were reacted separately with aniline and N,N-dimethylaminopropylamine (DMAPA) under appropriate conditions to obtain benzoxazines, which were characterized for their molecular structure and thermal behavior using different analytical methods. Both types of benzoxazines, imidazole core−aniline-based benzoxazines (IBPA-Bz) and imidazole core−DMAPA-based benzoxazines (IBPD-Bz), possess better glass temperatures (Tg) and higher char yields than conventional benzoxazines (bisphenol-F-based benzoxazine (BPFb) and bisphenol-A-based benzoxazine (BPAb)). However, the curing temperature (Tp) of IBPD-Bz is lower than that of IBPABz. The blending of IBPA-Bz with conventional benzoxazines (BPAb and BPFb) improved their thermal stability to an appreciable extent. Furthermore, the addition of bismaleimide cross-linkers (4,4′-diaminodiphenylsulfone- and 4,4′diaminodiphenylmethane-based bismaleimides) to the blends of IBPA-Bz and conventional benzoxazines improved the thermal behavior according to their nature and concentration. Moreover, the selected blends of benzoxazines were incorporated with 10% loadings of catalysts (4-hydroxyphenylmaleimide, 4-aminophenol, and 4-hydroxyacetophenone), and it was observed that Tp was reduced without significant variation in the thermal behavior, i.e., a reduction of more than 35 °C was noticed for both conventional benzoxazines. On the basis of the data obtained from the different studies, it is concluded that the blends of IBPABz and conventional benzoxazines can be used in the form of sealants, encapsulants, adhesives, and matrixes in the fields of microelectronic and automobile applications for better performance with enhanced longevity.

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attractive flame retardant materials for the transportation industry.12−14 Conventional benzoxazines normally cure in the range of 200−260 °C, restricting their application in many fields.15−18 In recent years, various approaches have been attempted by a number of researchers to achieve benzoxazines with low curing temperature (Tp) by using an initiator, catalyst, or catalystcontaining benzoxazines or strong intermolecular association without the addition of any catalyst or initiator. In addition to this, tailor-made polybenzoxazines, which can have the advantage of designed molecular structure with required flexibility for specific applications, is an emerging area to reduce Tp in resins. In general, two approaches have been used to reduce Tp: (1) addition of catalysts such as acids or imidazoles and (2) synthesis of benzoxazines that contain catalytic groups or molecular structures that can act as inbuilt

INTRODUCTION

Novel thermosetting resins with integrated performances and amenable processing techniques are in great demand for cutting-edge applications, as they are intended to conserve energy and time. Benzoxazines are a class of phenolic resins developed for high-performance characteristics with improved processability to overcome the drawbacks of traditional phenolic resin.1−8 Polybenzoxazines can be used for a number of industrial and engineering applications because of their molecular design flexibility, compatibility with other highperformance resins, and inherent properties. For example, their high thermal stability, enhanced mechanical strength, and excellent fire, smoke, and toxicity (FST) behavior make them suitable for aerospace applications.9−11 Their low viscosity at melting, high glass transition temperature, low moisture absorption, low shrinkage upon curing, low coefficient of thermal expansion, high char yield, high value of the limiting oxygen index (LOI), and excellent dielectric behavior make them useful for electronic packaging materials. Their excellent processability, no release of toxic gases and dark smoke, selfextinguishing behavior, and low heat release rate make them © XXXX American Chemical Society

Received: April 30, 2017 Revised: July 14, 2017 Accepted: July 17, 2017

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DOI: 10.1021/acs.iecr.7b01816 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Synthesis of Imidazole-Core-Based Bisphenol (IBP)

Scheme 2. Synthesis of Imidazole-Core-Based Benzoxazines Using (a) Aniline (IBPA-Bz) and (b) DMAPA (IBPD-Bz)

catalysts.19−22 However, the addition of catalysts (acids or imidazoles) results in the formation of fragile materials.23 In the present work, a novel imidazole-core-based bisphenol (IBP) was synthesized by the Debus−Radziszewski reaction24 and in turn was transformed into low-Tp benzoxazines (BPIBz) using amino compounds and formaldehyde under appropriate conditions. The resulting benzoxazines were blended/copolymerized with conventional benzoxazines obtained from bisphenol A (BPAb) and bisphenol F (BPFb) in the presence or absence of cross-linkers (4,4′-diaminodiphenylsulfone-based bismaleimide (BMI-S) and 4,4′-diaminodiphenylmethane-based bismaleimide (BMI-M)) and catalysts (4-hydroxyphenylmaleimide (HPM), 4-aminophenol (AP), and 4-hydroxyacetophenone (HAP)) to enhance the thermal behavior of the conventional benzoxazines and to make them suitable for high-performance applications. Data obtained from thermal studies (viz., the glass transition behavior, thermal stability, and flame retardant behavior) of the newly developed benzoxazines are presented and discussed herein.

Measurements. FTIR spectra were obtained with an Agilent Cary 630 FTIR spectrometer. 1H NMR spectra were recorded with a Bruker 400 MHz NMR spectrometer using dimethyl sulfoxide (DMSO-d6) as the solvent and tetramethylsilane (TMS) as an internal standard. Mass spectra were recorded with an Agilent mass spectrometer. Differential scanning calorimetry (DSC) measurements were done using a Hitachi DSC7020 calorimeter under purging with N2 (60 mL/min) at a scanning rate of 10 °C/min. Thermogravimetric analysis (TGA) was done using a Hitachi STA7000 series analyzer with 7 mg samples under a flow of N2 (260 mL/min) at a controlled heating rate of 20 °C/min. Synthesis of Imidazole-Core-Based Bisphenol. The synthesis of IBP is shown in Scheme 1. In brief, a mixture of 0.1 mol each of benzil, p-aminophenol, and p-hydroxybenzaldehyde and 0.2 mol of ammonium acetate was stirred vigorously at 110 °C for 8 h in the presence of acetic acid. Then the reaction mixture was cooled to room temperature and repeatedly washed with water until the acetic acid was removed. The obtained white solid material was filtered, washed several times with distilled water, and dried under vacuum overnight. The prepared BPI was analyzed for its molecular structure by FTIR and 1H NMR spectroscopy and mass spectrometry. Synthesis of Imidazole-Core-Based Benzoxazines (IBP-Bzs). The syntheses of the imidazole-core-based benzoxazines (IBPA-Bz and IBPD-Bz) are shown in Scheme 2. In brief, paraformaldehyde (0.1 mol) and amine (aniline or

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EXPERIMENTAL SECTION Materials. Benzil, p-aminophenol, p-hydroxybenzaldehyde, ammonium acetate, acetic acid, N,N-dimethylaminopropylamine (DMAPA), 4,4′-diaminodiphenylsulfone (DDS), diaminodiphenylmethane (DDM), aniline, 1,4-dioxane, p-hydroxybenzoic acid, and paraformaldehyde were obtained from Sigma-Aldrich and used as such without further purification. B


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Industrial & Engineering Chemistry Research Scheme 3. Synthesis of Imidazole-Core−Aniline- and Imidazole-Core−DMAPA-Based Polybenzoxazine Derivatives

DMAPA) (0.05 mol) were mixed in a 250 mL three-neck round-bottom flask in 1,4 dioxane at 5 °C for 1 h. After that, 0.025 mol of IBP was added, the temperature was gradually raised to 110 °C, and the mixture was stirred continuously for 12 h. The crude product obtained was washed several times with 2 N sodium hydroxide to remove unreacted bisphenol. The organic layer was then dried with sodium sulfate, followed by removal of the solvent using a rotary evaporator. The residual amount of solvent (if any) was removed by drying the solid product under vacuum at 60 °C overnight (95% yield). The molecular structures of IBPA-Bz and IBPD-Bz were confirmed using FTIR and 1H NMR spectroscopy and mass spectrometry. Ring-Opening Polymerization of IBP-Bz. Thermal ringopening polymerizations of IBPA-Bz and IBPD-Bz were carried out according to the reported procedure (Scheme 3).25 A typical procedure for polymerization is as follows: the respective amine-based benzoxazine monomer was dissolved in DMF in order to get the homogeneous product, and then the solvent was evaporated at 80 °C for 8 h. After complete removal of the solvent, the temperature was raised to 200 °C at a rate of 20 °C/h. The heating was continued for another 3 h at 200 °C for complete curing. Upon thermal treatment, the bond between nitrogen and oxymethylene gets cleaved, and the reactive methylene group abstracts the ortho hydrogen present in the neighboring oxazine ring, leading to polymerization. The polymerization was confirmed by IR spectroscopy. The nomenclature of monomers, polymers, cross-linkers, and catalysts is presented in Table S1 in the Supporting Information.

Figure 1. FTIR spectra of imidazole-core-based bisphenol (IBP), imidazole-core−aniline-based benzoxazine (IBPA-Bz), imidazolecore−DMAPA-based benzoxazine (IBPD-Bz), and imidazole-corebased polybenzoxazines (IBPA-PBz and IBPD-PBz) after heating at 200 °C for 3 h.

Additionally, the formula weights were confirmed by the mass spectra (Figure S2). Spectral Analysis. The 1H NMR spectrum of IBP in DMSO-d6 is given in Figure S1a. The peaks appearing in the range of 6.4−7.5 ppm are assigned to the aromatic protons, and the resonance peaks appearing at 9.65 and 9.75 ppm are assigned to the phenolic protons. Figure S1b presents the 1H NMR spectrum of IBPA-Bz. The peaks appearing between 6.60 and 8.0 ppm are assigned to the aromatic protons. The appearance of resonance peaks at 4.58 (Ar−CH2−N) and 5.75 ppm (O−CH2−N) confirm the formation of the benzoxazine ring.27 Similarly, Figure S1c presents the 1H NMR spectrum of IBPD-Bz, which possesses a singlet at 4.0 ppm for Ar−CH2−N and a singlet at 5.0 ppm for O−CH2−N.28 The structures of IBP, IBPA-Bz, and IBPD-Bz were ascertained using FTIR spectra (Figure 1). The absorption bands appearing at 1234, 1007, and 1351 cm−1 correspond to C−O−C asymmetric stretching, C−O−C symmetric stretching, and CH2 wagging, respectively.29 The disubstituted benzene groups present in the benzoxazines (IBPA-Bz and IBPD-Bz) exhibit an absorption peak at 1514 cm−1. The absorption band appearing at 938 cm−1 corresponds to the out-of-plane bending vibration of C−H and is due to the characteristic mode of benzene with an attached

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RESULTS AND DISCUSSION The molecular structures of IBP and the different types of benzoxazine monomers (IBPA-Bz and IBPD-Bz) were ascertained using FTIR and 1H NMR spectroscopy and mass spectrometry. The 1H NMR spectra of the imidazole-corebased benzoxazines are shown in Figure S1 in the Supporting Information, and their FTIR spectra are shown in Figure 1. The bands appearing at 1601 and 990 cm−1 are related to the absorptions of CN and CC in the imidazole ring, respectively.26 The band appearing at 3448 cm−1 is attributed to the presence of hydroxyl groups in the molecule. C


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Industrial & Engineering Chemistry Research

Figure 2. DSC thermograms of (a) imidazole-core−aniline-based benzoxazine (IBPA-Bz) and (b) imidazole-core−DMAPA-based benzoxazine (IBPD-Bz).

Table 1. Thermal Stabilities of Imidazole-Core−Aniline-Based Benzoxazine (IBPA-Bz) and Imidazole-Core−DMAPA-Based Benzoxazine (IBPD-Bz) curing behavior entry benzoxazine 1 2

IBPA-Bz IBPD-Bz

Ti (°C)

Tp (°C)

Tf (°C)

ΔH (mJ/mg)

Tg (°C)

5% weight loss (°C)


Tmax (°C)

char yield at 800 °C (%)

LOI

180 117

231 170

278 217

119 103

274 244

417 380

478 433

580 570

66 56

44 40

oxazine ring.30 The band appearing at 1600 cm−1 is attributed to the absorption of CN in the imidazole ring. 26 Furthermore, the disappearance of the oxazine ring absorption peak at 938 cm−1 after thermal curing at 200 °C for 3 h substantiates the occurrence of complete ring-opening and cross-linking polymerization to form the polybenzoxazines (Figure 1). The molecular weights of the monomers (IBPA-Bz and IBPD-Bz) were confirmed by the mass spectra (Figure S2). Curing Behavior of Imidazole-Core-Based Benzoxazines. The curing behaviors of IBPA-Bz and IBPD-Bz were monitored by DSC analysis at a heating rate of 10 °C/min. The DSC thermograms of the imidazole-core-based benzoxazines are presented in Figure 2. The appearance of exothermic peaks at 231 and 170 °C for IBPA-Bz and IBPD-Bz, respectively, confirm the ring-opening polymerization of the benzoxazines. Comparatively, IBPD-Bz has a lower curing temperature than IBPA-Bz because of the structural variation imposed by the amino groups of DMAPA. The aliphatic amine derivative is comparatively more mobile than the aniline derivative at elevated temperature, which in turn reduces the curing temperature in the case of DMAPA. Furthermore, the second DSC scans of IBPA-Bz and IBPD-Bz did not show any exothermic peaks, confirming the formation of the cross-linked polymer. The degree of conversion was found to be relatively high. The values are presented in Table. 1. Thermal Behavior of Imidazole-Core-Based Polybenzoxazines. Figure 3 shows the thermal stability and degradation characteristics of cured IBPA-Bz and IBPD-Bz, studied using thermogravimetric analysis under nitrogen atmosphere. The onset temperature for 5% weight loss of IBPA-PBz was found to be 417 °C, whereas that of IBPD-PBz was 380 °C. The decomposition maxima for IBPA-PBz and IBPD-PBz were found to be 580 and 570 °C, respectively. In addition, the residual char yields for the polybenzoxazines were measured at 800 °C in the TGA curves and found to be 66% for IBPA-PBz and 57% for IBPD-PBz. The data on the thermal characteristics of the benzoxazines are presented in Table 1

Figure 3. TGA thermograms of (a) imidazole-core−aniline-based benzoxazine (IBPA-Bz) and (b) imidazole-core−DMAPA-based benzoxazine (IBPD-Bz).

along with the values of the glass transition temperature (Tg) obtained from simultaneous TGA−DTA analysis. The Tg values for IBPA-PBz (278 °C) and IBPD-PBz (244 °C) were found to be much higher than those of conventional polybenzoxazines,31 suggesting that these materials can be used as high-performance flame retardant materials. Moreover, it is also possible to assess the flame retardant behavior of these materials in terms of limiting oxygen index (LOI) values calculated from the char yields using the equation of van Krevelen and Hoftyzer (eq 1):

LOI = 17.5 + 0.4CR

(1)

where CR is the percentage char yield of polymer remaining at 800 °C. The LOI values for the cured IBPA-PBz and IBPD-PBz polymers are 44 and 40, respectively, which are high compared with those of conventional benzoxazines. From this it is inferred that these materials can be used as non-halogen and non-phosphorus flame retardant materials. It is experimentally D


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Industrial & Engineering Chemistry Research Table 2. Effect of IBPA-Bz on the Curing Behavior of BPAb and BPFb curing behavior entry BPb:IBPA-Bz

Ti (°C)

Tp (°C)

Tf (°C)

ΔH (mJ/mg)

1 2 3 4 5

100:0 75:25 50:50 25:75 0:100

165 153 170 180 190

223 218 218 219 231

284 280 280 283 280

254 211 211 203 119

6 7 8 9 10

100:0 75:25 50:50 25:75 0:100

159 104 144 152 180

212 214 213 213 231

278 274 274 274 278

310 301 240 206 113

Tg (°C)


BPAb/IBPA-Bz Blends 176 332 195 412 206 413 235 417 274 417 BPFb/IBPA-Bz Blends 182 316 211 370 222 405 231 387 274 417


Tmax (°C)

char yield at 800 °C (%)

LOI

351 429 434 452 478

419 477 480 482 580

28 48 50 56 66

29 37 38 40 44

343 393 451 444 477

440 443 515 515 580

25 43 53 63 66

28 35 39 43 44

Figure 4. TGA thermograms of (a) BPAb with various ratios of IBPA-Bz and BMI-M, (b) BPAb with various ratios of IBPA-Bz and BMI-S, (c) BPFb with various ratios of IBPA-Bz and BMI-M, and (d) BPFb with various ratios of IBPA-Bz and BMI-S.

no significant improvement in reduction of Tpas expected, only a marginal reduction in the value of Tp was noticed, as shown in Figure S4. This may be due to the steric hindrance and rigid structure of the blends. However, blending of IBPABz with BPAb and BPFb resulted in a significant improvement in thermal stability (Figure S5). For example, for the 50:50 blend of IBPA-Bz and BPAb, the char yield at 800 °C was 50%, whereas that of neat BPAb was 28%. Hence, an improvement of 22% in the char yield was achieved. Similarly, in the case of the 50:50 blend with BPFb, an improvement of 28% (25% to 53%) was observed. The data on the thermal stability of the 50:50 blends suggested that these materials can be used as excellent adhesives, sealants, and encapsulants where high thermal stability and flame retardant applications are warranted.

proved and generally accepted that the materials with high LOI values possess superior flame retardant properties.32 Effect of Blending on Curing Behavior of BPAb and BPFb. Varying amounts of IBPA-Bz were blended with conventional benzoxazines (BPAb and BPFb) in order to assess its effect on the curing temperature. The Tp values obtained in curing studies of 75:25, 50:50, and 25:75 BPAb/ IBPA-Bz blends are 218, 218, and 219 °C, respectively (Table 2). The value of Tp obtained for the 50:50 blend of BPAb and IBPA-Bz was 5 °C lower than that of the pure conventional benzoxazine BPAb (223 °C). The 50:50 blend of BPFb and IBPA-Bz has Tp = 213 °C, and there was no significant variation (Table 2). It was observed that when IBPA-Bz was blended with conventional benzoxazines (BPAb and BPFb), there was E


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Figure 5. DSC thermograms of (a) 100:50:50 BPAb/IBPA-Bz/BMI-S and (b) 100:50:50 BPFb/IBPA-Bz/BMI-S blends containing 10% catalyst loadings of HAP, AP, and HPM.

degradation of blends studied using TGA. The thermal degradation is mainly attributed to the decomposition of the backbone of the polymer network, which developed as a result of the addition of ether linkages of bismaleimide and tertiary amine linkages present in the aryl core. The degradation maxima obtained for the blends were found to occur at about 500 °C, and this may be attributed to the presence of steric hindrance in the molecule and the formation of the cross-linked network structure between the polymeric groups. Similarly, the LOI values for the blends were found to be greater than 35 (Tables S2 and S3). Furthermore, the values of Tg for the blends are also presented in Tables S2 and S3. It was observed that Tg increases with the incorporation of BMI-M/BMI-S as a result of the formation of the network structure. From the values of Tg observed for different blend compositions, it is concluded that the incorporation of cross-linkers influences the Tg values according to their concentration as a result of the formation of the network structure and the presence of polar imide moieties. Furthermore, the catalytic effects on the curing temperature were studied by incorporating 10% loadings of the catalysts HPM, AP, and HAP into the 100:50:50 blends of BPAb and BPFb with IBPA-Bz and the cross-linker (BMI-S) (Figure 5). It was found that in the case of BPAb/IBPA-Bz/BMI-S, the value of Tp was reduced to 196, 191, and 178 °C respectively, and the values of Tp observed for BPFb/IBPA-Bz/BMI-S blends with the catalysts were 198, 192, and 182 °C, respectively. From the data for 100:50:50 blends with 10% catalyst (Table 3), the Tp values were found to be lower than those of neat BPAb and BPFb and their blends. Furthermore, it was also noticed that there were no significant variations in the values of Tg and the char yield.

Furthermore, the LOI values calculated from the char yields of the blends of IBPA-Bz and conventional benzoxazines (BPAb and BPFb) are presented in Table 2. It can be seen that the LOI values for the 50:50 blends are higher than those of the conventional benzoxazines. When IBPA-Bz was blended with BPAb and BPFb, no significant improvement in the reduction of Tp was noticed. However, the Tg values are found to be increased by blending IBPA-Bz with BPAb and BPFb (Table 2). Tg increased from 176 °C for BPAb to 235 °C for 25:75 BPAb/IBPA-Bz, and for the 75:25 and 50:50 BPAb/IBPA-Bz blends, the values of Tg were 195 and 206 °C, respectively. Similarly, for the 75:25, 50:50, and 25:75 BPFb/IBPA-Bz blends, the Tg values are 211, 222, and 231 °C respectively (Figure 4). Hence, an attempt was made to incorporate varying percentages of different bismaleimide cross-linkers (BMI-M and BMI-S) and catalysts (HPM, AP, and HAP) in order to reduce the Tp of blends of IBPA-Bz with BPAb and BPFb. The effects of the cross-linkers (BMI-M and BMI-S) on the blends of IBPA-Bz with BPAb and BPFb are presented in Figure S6. In the case of 100:50:50 BPAb/IBPA-Bz/BMI-M and BPFb/IBPA-Bz/BMI-M blends, the Tp values were about 18 and 7 °C lower than those of BPAb and BPFb, respectively. Similarly, in the case of BMI-S blends, reductions of 7 and 2 °C, respectively, were obtained. BMI-M (205 °C) was found to be more effective than BMI-S (216 °C) for blends of IBPA-Bz with BPAb (Tables S2 and S3). To investigate the possible curing reaction of the blends, FT-IR measurements were carried out for the blends after curing (Figure S3). As a result of thermal treatment, the absorption at 938 cm−1 for the oxazine ring present in the benzoxazine molecule disappeared after curing. Similarly, the appearance of an absorption band at 1213 cm−1 indicates the generation of ether bonds, which may be formed due to the reaction between BMI and benzoxazine, as shown in Scheme S1. As a result, benzoxazine undergoes ring-opening polymerization, generating the phenolic hydroxyl group, which reacts with the bismaleimide through Michael addition. Consequently, the blends with ether linkages enhanced the network structure of the resulting benzoxazine/IBPA/BMI blends. From the results obtained, it can be concluded that the addition of bismaleimide with IBPA-Bz can efficiently improve the thermal properties of conventional benzoxazines. The TGA thermograms for the blends of BMI-M/BMI-S and IBPA-Bz with BPAb and BPFb provide information about the formation of the network structure and enhanced thermal stability of the polybenzoxazines. Figure 4 shows the thermal

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CONCLUSION Novel imidazole-core-based benzoxazines were successfully developed and characterized using different analytical methods. The results from thermal analysis indicate that both anilinebased (IBPA-Bz) and DMAPA-based (IBPD-Bz) benzoxazines exhibit high values of the glass transition temperature (Tg) and high char yields. Data obtained from thermal studies, namely, values of Tg and the degradation temperature (Td) as well as char yields, showed that the imidazole core influences these behaviors to an appreciable extent compared with those of conventional bisphenol-A- and bisphenol-F-based benzoxazines (BPAb and BPFb, respectively). For example, the char yield of IBPA-Bz (66%) is enhanced more than 2-fold compared with F


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Table 3. Effect of 10% Loading of the Catalysts 4Hydroxyacetophenone (HAP), 4-Aminophenol (AP), and pHydroxyphenylmaleimide (HPM) on the Curing Behavior of 100:50:50 BPAb/IBPA-Bz/BMI-S and BPFb/IBPA-Bz/ BMI-S Blends curing behavior entry 1 2 3 4 5 6

catalyst

Ti (°C)

Tp (°C)

100:50:50 BPAb/IBPA-Bz/BMI-S Blendsa HAP 119 178 AP 122 191 HPM 135 196 100:50:50 BPFb/IBPA-Bz/BMI-S Blendsb HAP 118 182 AP 124 192 HPM 146 198

Tf (°C) 251 254 258 252 252 264

Tp = 216 °C for the 100:50:50 BPAb/IBPA-Bz/BMI-S blend without catalyst. bTp = 210 °C for the 100:50:50 BPFb/IBPA-Bz/BMI-S blend without catalyst.

a

those of the conventional benzoxazines (25−28%). Thus, the presence of tertiary nitrogen atoms coupled with the benzene moiety in the imidazole core has a significant role in contributing to an enhanced char yield with improved flame retardant behavior. Further, blending IBPA-Bz with the conventional benzoxazines improves the thermal stability and flame retardant behavior to a significant extent according to the concentration. In addition, the incorporation of a cross-linker to the selected combination of blends also improved the thermal behavior to an appreciable extent without altering the value of Tp. Similarly, the incorporation of a catalyst in the blends lowers the values of Tp compared with those of blends of imidazole-core-based and conventional benzoxazines. From the results obtained, it is concluded that the blends of IBPA-Bz and conventional benzoxazines with an appropriate cross-linker and catalyst can be used as high-performance adhesives, sealants, encapsulants, and matrices for microelectronics and automobile applications where thermal stability, low outgassing, and flame retardancy are warranted.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01816. Tables S1−S3, Figures S1−S6, Scheme S1, and structures of cross-linkers and conventional benzoxazines (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

M. Alagar: 0000-0002-6073-4693 Notes

The authors declare no competing financial interest.

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REFERENCES

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A Novel Imidazole-Core-Based Benzoxazine and ... - ACS Publications

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