The Effect of Introducing B and N on Pyrolysis Process of High Ortho Novolac Resin

In this contribution, high ortho novolac resins modified with phenylboronic acid were synthesized. The thermal stability of novolac resins cured with hexamethylenetetramine (HMTA) and chemical states of B and N via a pyrolysis process were studied. For the cured o-novolac modified with phenylboronic acid, the temperature with maximum decomposition rate increased by 43.5 °C, and the char yield increased by 5.3% at 800 °C compared with cured o-novolac. Density functional theory (DFT) calculations show the existence of hydrogen bonding between N of HMTA and H of phenol in modified resin. Thus, N could still be found at high temperature and C=N structure could be formed via a pyrolysis process. B2O3 was obtained at 400 °C by the cleavage of B–O–C and B–C bonds and it reduces the oxygen loss which may take part in the formation of carbon oxides in the system. The melting B2O3 on the surface of the resin will prevent small molecules and carbon oxides from releasing. Moreover, introducing B into the system helps to decrease the interlayer distance and improve graphite structures via a pyrolysis process.


Introduction
Phenolic resins (PF) have been used since 1907 for a wide variety of applications such as thermal insulation materials, molding powders, laminating resins, adhesives, binders, surface coatings, and the matrix for carbon composite materials, because of its low cost, excellent ablative properties, and thermal stability [1][2][3][4][5][6][7][8][9]. However, PF was not able to completely meet the constantly developing requirements which most focus on the thermal properties [10]. To improve the thermal properties of PF , the addition of phosphorus, boron, and silicon compounds has been used [11]. Introducing boric compounds, such as B 4 C [12][13][14][15], boric acid, phenylboronic acid [16], BN [17], into resin to synthesize boron-modified resin (BPF) is one of the most successful ways to modify phenolic resin. Gao et al. prepared BPF by using boronic acid as boron source and base as catalyst and pointed out that there were two way to react in the system, the reactivity of boronic acid with hydroxymethyl groups was much higher than that of phenolic hydroxyl [18,19]. Wang et al. introduced boronic acid and phenylboronic acid into resoles to synthesized BPF [16]. However, there is little published research about boron-containing high ortho novolac resin (B-o-novolac), as well as their pyrolysis mechanisms.
Compared with conventional resoles, high ortho novolac resin (o-novolac) exhibits a much more rapid curing behavior with hexamethylenetetramine (HMTA) and novolac resin has lower viscosity [20,21]. Moreover, phenylboronic acid has one aromatic ring which can improve the thermal property and reduce the possibility of gelling when synthesized with o-novolac [22].
In this work, we synthesized B-o-novolac by introducing phenylboronic acid into novolac resins and analyzed the structure and thermal properties of cured B-o-novolac, comprehensively. In addition, we also evaluated the chemical state changes of B and N in pyrolysis process and their effects on char yields.

Synthesis of B-o-novolacs
Phenol (40.00 g), formaldehyde (24.15 g), and Catalyst I (0.80 g) were added into 250 mL three-necked flask equipped with a stirrer, a cooling condenser, and a thermometer. The system was slowly heated using an oil bath and reacted at 100˝C for 4 h. Then the water was extracted. After that, sodium hydroxide (0.40 g) and phenylboronic acid were added into the system and reacted for 4 h at 140˝C. The molar ratio of phenol to phenylboronic acid was 1:0. After that, methyl isobutyl ketone (60.00 g) was used to extract water and catalysts. Finally, subjecting the solution to a vacuum at 120˝C until the total extraction of water and solvent was completed.

Synthesis of o-Novolac
Adding the same dosage of phenol, formaldehyde, and Catalyst I into 250 mL three-necked flask equipped with a stirrer, a cooling condenser, and a thermometer. The system was slowly heated using an oil bath and reacted at 100˝C for 8 h. Then methyl isobutyl ketone was used to extract water and catalysts. Finally, subjecting the solution to a vacuum at 120˝C until the total extraction of water and solvent was completed.

Preparation of the Cured B-o-novolac
Blend sample was prepared by mixing B-o-novolac with 12% HMTA in ethanol under magnetic stirring at 50˝C for 2 h until a homogenous solution was obtained. Then, the solvent was evaporated at 60˝C under vacuum. The concentrated sample was cured in a vacuum oven at 135˝C for 2 h, 150˝C for 2 h, and 170˝C for 4 h.

Preparation of Carbonized Samples
To analyze the chemical state of B and N in pyrolysis process, about 2.0 g cured B 0.30 -o-novolac or cured o-novolac were placed in graphite crucible and heated from room temperature to the targeted temperature for 2 h with a heating rate of 10˝C/min in Ar. The cured resins were carbonized at 200˝C/2 h, 400˝C/2 h, 600˝C/2 h, 800˝C/2 h, 1000˝C/2 h, and 1200˝C/2 h, respectively. The tube furnace was cooled from targeted temperature to room temperature with a rate of 5˝C/min.

Characterization
The Fourier transform infrared (FT-IR) spectra were recorded with a width of 4000-500 cm´1 to confirm the structure of the B-o-novolacs and o-novolac by using FT-IR spectrometer (BRUKER, Ettlingen, Germany).
The 11 B nuclear magnetic resonance (NMR) spectra were obtained using a Bruker Avance 400 MHz apparatus in dimethylsulfoxid-d6.
The thermalgravimetric analysis (TGA) of the cured resins were conducted on a thermogravimetric analyzer Q600SDT (TA, New Castle, USA) under dry Ar gas. The relative mass loss of the samples was recorded from 25 to 1200˝C with speed of 10˝C/min and the carbon yields of cured resins were gotten.
The X-ray photoelectron spectroscopic (XPS) tests were carried out using a K-alpha spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) and the core level spectra were measured using a monochromatic Al K R X-ray source. Binding energies were referenced to the C1s peak at 284.80 eV and the curve fitting of the XPS spectra was performed using the least-squares method. The experimental data were deconvoluted by built-in software.
Raman spectroscopy spectra were recorded from 600 to 2000 cm´1 on Raman spectrometer (Renishaw 2000) with λ = 514.5 nm to calculate the I D /I G ratio (R) [24].
The overall morphologies of the B 0.30 -o-novolac were observed with a VEGA3 XMH scanning electron microscopy (SEM)(Tescan Co.,Brno,Czech Republic). Samples were measured after sputtering a thin layer of gold (1-2 nm).

Characterization of B-o-novolac and Cured B-o-novolac
The structures of B-o-novolacs are confirmed via FT-IR, 11 B NMR, and XPS. Figure 1a shows that the absorption bands assigned to the stretching vibration of B-O-C group appears at 1352 cm´1. The peak at 1235 cm´1 corresponds to the stretching vibration of phenolic hydroxyl C-O both in o-novolac and modified resin. With the increase of boric compound contents, the peak at 1235 cm´1 becomes weaker. The peak at 700 cm´1 is the stretching vibration of C-H in benzene ring in phenylboronic acid which has five adjacent H [10]. The peak at 1028 cm´1 is the unreacted hydroxymethyl groups in modified resin based on DFPT-135 spectra. The peak at 1441 cm´1 is assigned to the stretching vibration of B-O in phenylboronic acid and 640 cm´1 is the B-OH bending vibration which the o-novolac does not exist [25]. In Figure 1b, the peak at 63.5-58.8 ppm was assigned to the -CH 2 OH in B-o-navolac [11,22]. While for o-novolac, -CH 2 OH peaks (63.5-58.8 ppm) do not exist. Since boron is electron-deficient, peaks (115.1-114.9 ppm, 120.5-119.4 ppm) on the benzene ring linked to phenylboronic acid shift to low field. Peaks at 131.5, 129.0, and 128.2 ppm are assigned to the benzene ring in phenylboronic acid [26].
In Figure 2, the 11 B NMR spectrum of B 0.30 -o-novolac indicate that boron has at least three different compounds. The peak at 1.16 and 24.69 ppm are four-coordinate BO 4 and boronates [22], respectively. The peak at 18.26 ppm is interpreted as the trigonal boron [27]. In Figure 3c, the boron elements are detected from the full-scan XPS spectrum of the B 0.30 -o-novolac. The B1s XPS spectrum can mainly be separated into four peaks at 190. 95, 191.60, 192.70, and 193.20 eV [28,29] which are assigned to B-O, B-O-C, B-OH, and B-O-B. According to the study by Gao [19] et al., the reaction activity of the B-OH with phenolic hydroxyl groups was lower than that with hydroxymethyl groups, and high temperature could increase reaction activity. In this test, when the concentration of boron in the reactoin mixture was increased, the possibility of the reaction at 140˝C between boric compounds and phenolic hydroxyl groups would increase according to the FT-IR in Figure 1.

Thermal Properties of B-o-novolacs
Thermal gravimetric (TGA) curves and derivative thermogravimetric (DTG) curves of the cured B-o-novolacs and o-novolac are shown in Figure 6. They are used to analyze the thermal stability of the resins. The representative thermal analysis data are listed in Table 1. The T 5% and T 10% are the decomposition temperature at 5% and 10% weight loss, whereas the maximum weight loss temperature (Tmax) is taken from the peak value of the DTG thermograms in Figure 6b. The TGA shows that char yields at 800˝C (C 800 ) of cured B-o-novolacs modified with different boric compound contents are all higher than that of cured o-novolac. Except B 0.35 -o-novolac, C 800 increases according to the increase of phenylboronic acids contents. After 1000˝C, the residual weights at 1000˝C (C 1000 ) and at 1200˝C (C 1200 ) decreased rapidly, especially for o-novolac. The C 1000 and C 1200 of B 0.30 -o-novolac are 63.34% and 56.41%, respectively, and 52.21% and 39.39% for cured o-novolac. The T 5% and T 10% of B-o-novolacs are lower than that of o-navolac, but the Tmax values are higher. And the weight losses of B-o-novolac increase with the increase of phenylboronic acid in the first stage (Table 1). When the content of phenylboronic acid is higher, some hydroxyl groups of phenylboronic acid not only reacted with hydroxyl groups in resins, but also with hydroxyl groups in phenol monomers at 140˝C. This would increase the phenol consumption and impede the chains to grow and leave some hydroxymethyl groups. The main products in this stage are H 2 O and CO. Except the condensation reaction involving phenolic hydroxyl groups and methyl groups, residual hydroxymethyl groups in B-o-novolacs (1028 cm´1 for -CH 2 OH vibration) [10] would react continuously to release water or methanol. Parts of B-C bonds would also break to remove benzene rings in stage 1 (stage 1:200 to 400˝C; stage 2:400 to 800˝C; stage 3:800 to 1000˝C) in DTG spectra [35][36][37][38][39]. These may be partial reasons why weight loss in stage 1 becomes higher with the increase of phenylboronic acid contents.

Chemical State of B and N during Pyrolysis Process
In order to confirm the presence and chemical states of B and N in the cured resins suffering from heating at different temperatures (200˝C/2 h, 400˝C/2 h, 600˝C/2 h, 800˝C/2 h, 1000˝C/2 h, and 1200˝C/2 h), FT-IR and core-level XPS measurements were taken as shown in Figures 4 and 7-9. The effects of B and N on the pyrolysis process of modified resins are illustrated based on these tests.  [27,38,[40][41][42]. Moreover, the dehydration reaction between two hydroxyl groups in phenylboronic acid or between phenylboronic acid and hydroxyl groups in resins in the curing process can also form B-O-B structure.
For the cured B 0.30 -o-novolac, when the heating temperature increased to 400˝C, the intensity of peak at 1256 cm´1 decreased and the peak at 1649-1599 cm´1 became stronger. It is because that the appearance of C=N stretching vibration, the peak at 1637 cm´1, is overlapped by the N-C structure and C=C structure in benzene ring [43]. The formation of C=N in six-member ring has been shown in Figure 10. As to o-novolac, the intensity of band at 1599-1640 cm´1 began to reduce above 200˝C.  [47]. When the treating temperature increased to 1200˝C, boron almost disappeared.    Figure 8a, it is clear that N atom exists in mainly two different chemical environments that could be attributed to the presence of NR 3 and NC 2 H, which is in accordance with the FT-IR in Figure 4 below 200˝C. When the treating temperature rose from 400˝C to 1000˝C, a new peak at 398.60˘0.20 eV in XPS (Figure 8b-e) appeared. This was assigned to C=N, which may form as shown in Figure 10 and could bear higher temperature [42,[47][48][49]. in accordance with the FT-IR in Figure 4 below 200 °C. When the treating temperature rose from 400 °C to 1000 °C, a new peak at 398.60 ± 0.20 eV in XPS (Figure 8b-e) appeared. This was assigned to C=N, which may form as shown in Figure 10 and could bear higher temperature [42,[47][48][49].       (Table 3). While for o-novolac, the XPS in Figure 9 shows that two individual lines are used with peaks centered at 401.00 ± 0.20 eV and 399.60 ± 0.20 eV which are assigned to NR3 and NC2H from 170 to 400 °C. With the treating temperature rising, the proportion of NC2H became larger than NR3. But after treating at 600 °C, the resin system rarely has N from the XPS measurement which was consistent with the research of Bertsch's group [54]. So, from XPS analyses, N atoms in B0.30-o-novolac would slightly contribute to a higher char residual compared with o-novolac.   Table 2 shows the contents of C, O, N, and B atoms of B 0.30 -o-novolac and C, O, and N of o-novolac at different treating temperatures. Table 3 shows the proportion of different chemical structures of N from N1s XPS of B 0.30 -o-novolac and o-novolac at different heating temperatures. The contents of C=N structure increased from 36.82% at 400˝C to 70.72% at 800˝C with the decreasing of NR 3 and NC 2 H from 63.18% at 400˝C to 29.28% at 800˝C (Table 3). While for o-novolac, the XPS in Figure 9 shows that two individual lines are used with peaks centered at 401.00˘0.20 eV and 399.60˘0.20 eV which are assigned to NR 3 and NC 2 H from 170 to 400˝C. With the treating temperature rising, the proportion of NC 2 H became larger than NR 3 . But after treating at 600˝C, the resin system rarely has N from the XPS measurement which was consistent with the research of Bertsch's group [54]. So, from XPS analyses, N atoms in B 0.30 -o-novolac would slightly contribute to a higher char residual compared with o-novolac. In Figures 8 and 9 nitrogen in cured B 0.30 -o-novolac would disappear after heating at 1200˝C, whereas at 600˝C for cured o-novolac. When pheneylboronic acid reacted with phenyl hydroxyl groups in the resin, formaldehyde would react at para sites and ortho active sites are left over because of steric effect. Thus, HMTA will react at ortho sites in the curing process. In order to study why N in cured modified resin could still be found at high temperature in comparison to cured o-novolac, the structures of modified resin and pure resin in Figure 11 were optimized by using the density functional theory (DFT) method B3LYP with 6-311G basis set in the Gaussian 09 program [55]. After curing, N of HMTA and H of phenol would form hydrogen bonding. The N¨¨¨H distance of B 0.30 -o-novolac is 1.75 Å, which is shorter than that of o-novolac (4.29 Å) and the respective Wiberg bond index (WI) is 0.0973 which are larger than the WI of o-novolac (0.0002), implying that the interaction force of N¨¨¨H in modified resin is stronger [56]. That is the reason why the deformation vibration peak of N-H of cured modified novolac in Figure 4b shifted to a lower wavenumber which was at 1570 cm´1 compared with o-novoalac. This is also the reason why N-H could keep in the system at higher temperature and form C=N structure above 200˝C.  Figure 12a,c, it can be seen that some small particles which are B 2 O 3 are on the surface of modified resin at 800˝C and they almost disappeared at 1200˝C with some voids. As for o-novolac in Figure 12b,d, there are some cavities on the surface at 800˝C. After treating at 1200˝C for 2 h, the surface becomes rougher compared with Figure 12c. The melting point of B 2 O 3 is 450˝C, when the treating temperature is higher than 450˝C, B 2 O 3 particles begin to melt, the volume expansion and wettability properties will help to cover the voids and defects on the surface of the cured resin, which preventing small molecules and carbon oxides from releasing [57]. Thus, the formation of B 2 O 3 on the surface of resin can decrease the quantity of voids and contribute to the higher char yields.

Analysis of XRD
XRD analyses of B 0.30 -o-novolac heated at different temperatures were carried out to analyze the effects of B on the structural properties as shown in Figure 13. The XRD patterns of B modified resin in Figure 13a show the existence B at 400 and 600˝C. The peak at 28.06˝can illustrate the formation of B 2 O 3 starting at 400˝C [16,22,28]. Diffraction peaks of (002) and (100) are invisible in all XRD patterns when the treating temperature is above 600˝C. The (002) and (100) peaks are associated with the hexagonal graphite and the site and width may be different owing to the changeable interlayer spacing [58,59]. Due to the decrease of interlayer space., the (002) peak positions of B 0.30 -o-novolac are observed at 24.13˝, 24.18˝, and 24.50˝at different heating temperatures, which are higher than that of o-novolac. Moreover, XRD patterns in Figure 13a show the sharper diffraction compared with Figure 13b. Table 4 lists the crystallite parameters of B 0.30 -o-novolac and o-novolac. As can be seen in Table 4

Analysis of Raman Spectra
Raman spectra were used to determine the effects of B on the carbon crystallite in B 0.30 -o-novolac ( Figure 14). The results are shown in Table 4. There are two bands in the Raman spectra of B 0.30 -o-novolac and o-novolac. D-band located between 1338-1350 cm´1 is defect lattice vibration mode associated with disorder [62,63] and shows a small shift from the o-novolac to B 0.30 -o-novolac. G-band at about 1600 cm´1 representing graphite structure. The intensity of G-band elevates and D-band decreases slightly with the increasing treating temperature( Figure 14, Table 4). Moreover, the R of B 0.30 -o-novolac is lower than that of o-novolac at the same heating temperature. For B 0.30 -o-novolac, R decreases from 2.038 at 800˝C to 1.747 at 1200˝C, while the R for o-novolac is from 2.262 at 800˝C to 1.846 at 1200˝C. The changes in the Raman spectra show that introducing B into resins helps to decrease the disordered structure and to form graphite structure.

Conclusions
Modified o-novolacs are synthesized with different contents of phenylboronic acid. After introducing B into novolac resins the temperature of maximum decomposition rate increased by 43.5˝C, and the char yield increased by 5.3% at 800˝C, 11.13% at 1000˝C, and 17.02% at 1200˝C, respectively. Not only did the formation of B 2 O 3 under pyrolysis contribute to the higher char yield of modified resin, but also the existence of nitrogen in the system. Due to hydrogen bonding of N and H, N could be found at higher temperature compared with cured o-novolac and stable C=N structure can be formed, which would reduce nitrogen loss. The formation of B 2 O 3 from the cleavage of O-C and B-C at 400˝C helps to decrease the oxygen consumption, which may be the components of released gases, and the volume expansion of B 2 O 3 above 450˝C in the system inhibits the volatilization of small molecules. Moreover, boron can improve the crystallinity and promote the graphite structure.
Author Contributions: Jin Yun, Lixin Chen and Junjun Feng designed and performed experiments. Lixin Chen, Xiaofei Zhang and Linlin Liu analyzed data. Jin Yun wrote the paper.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: HMTA hexamethylenetetramine DFT Density functional theory PF Phenolic resin BPF boron-modified resin B-o-novolac boron-containing high ortho novolac resin o-novolac high ortho novalac resin NMR nuclear magnetic resonance FT-IR Fourier transform infrared TGA Thermalgravimetric analysis XRD X-ray diffraction XPS X-ray photoelectron spectroscopic