Effect of the Phenyl Borate on the Properties of High-ortho Phenolic Fibers

Effect of Monophenyl Borate on Properties of High-ortho Phenolic Fibers

Mingli Jiaoa, Dongxue Rena, Quan Diaoa, R. Hugh Gongc, Kai Yangb,*, Jian Caoa,*

aSchool of Materials & Chemical Engineering, Zhongyuan University of Technology, Zhengzhou, 450007, China

bSchool of Fashion, Zhongyuan University of Technology, Zhengzhou, 450007, China

cTextiles and Paper, School of Materials, University of Manchester, Manchester M13 9PL, UK

*Corresponding author. E-mail address: Kaikai_422@163.com (K. Yang), biotech@126.com (J. Cao)

ABSTRACT

A series of high-ortho phenolic copolymer resins with different content of monophenyl borate (MPB) were prepared. The MPB modified high-ortho phenolic fibers (BOPFs) were melt-spun from the corresponding copolymers and cured in a combined solution of formaldehyde and hydrochloric acid. The resulting fibers were heat-treated in N2 at elevated temperatures. Fourier transform infrared spectrometer (FTIR), nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and electrical tensile strength apparatus were employed to characterize the change of functional groups, thermal performance, microstructure and mechanical properties of fibers after curing. The results show that the addition of MPB in the precursor resin can introduce B-O linkages into molecular chain, and the heat curing process increases the crosslinkage, thermal stability and mechanical properties. The peak of O/P value of fiber (1.94) and elongation (5.6%) were obtained when the BOPFs with 6 parts of MPB were heat-cured at 240 0C for 2h.

Keywords: high ortho; phenolic fiber; monophenyl borate; copolymerization

1. Introdution

Phenolic fibers with high flame resistance, outstanding resistance to corrosive environments, and high thermal insulation are applied widely in flame resistant textiles, composites, friction materials, precursors for the production of carbon and activated carbon fibers, and so on [1-4]. But the problems of brittle fracture and thermal stability have not been studied in detail [5]. High-ortho novolac resins display a high spinnibility and reactivity because of low viscosity, regular structure, high molecular weight and high reaction activity of para [6]. They have been used to prepare phenolic fibers by melt-spinning [7]. However, the high reactivity induces the formation of skin-core structure which has adverse effects on the mechanical properties. The fully dense skin layer caused by the highly active para of phenol hinders the +CH2OH to diffuse and reacts with phenolic molecular in the core layer of fiber which forms the skin-core structure with a low mechanical performance.

The boron-containing novolac resin has been synthesized and applied [8-12]. It shows a high thermal stability and good mechanical strength. Meanwhile the boron-containing phenolic fibers prepared by melt-spinning a mixture of novolac resin and boron acid show an increasing thermal stability and mechanical strength [8]. However, the preparation and spinning of MPB modified the high-ortho novolac resin have not been reported.

In this study, a MPB modified high-ortho phenolic copolymer was synthesized. The BOPFs were prepared by melt-spinning, crosslinking of as-spun fibers in a mixture of formaldehyde and hydrochloric acid, and heat curing from room temperature to 240 0C in nitrogen. The resultant BOPFs exhibit an excellent crosslinking structure, thermal and mechanical properties.

2. Experimental

2.1 Materials

All chemicals were of analytical grade and used without further purification. Phenol, hydrochloric acid and zinc acetate were supplied by Tianjin Fengchuan Chemical Reagent Co., Ltd (China). Formaldehyde (37 wt %) and sulfuric acid (98 wt%) were supplied by Xilong Chemical Co., Ltd (China). Paraformoldehyde, boric acid, oxalic acid, and absolute ethanol were supplied by Tianjin Hengxing Chemical Reagent Co., Ltd (China).

2.2 Preparation of MPB

MPB was prepared by the reaction of phenol and boric acid in the presence of absolute ethanol and oxalic acid. Phenol (1 mol), boric acid (1.1 mol), absolute ethanol (1.21 mol), and oxalic acid (1/30 mol), were added to a three-necked flask equipped with a reflux condenser, a stirrer, and a thermometer, and then the solution was heated and maintained at 1000C for 6 h, The reaction mixture was distilled under vacuum to remove absolute ethanol and oxalic acid at elevated temperature up to 180 0C for 2 h. The reaction is shown in Scheme 1. The resultant molten MPB was poured into a stainless steel plate and allowed to cool naturally.

2.3 Preparation of BOPFs

Phenol, MPB, paraformaldehyde (27.1 g, 0.85 mol per 1.0 mol equivalent phenol),and zinc acetate (2.0 g, 1 g per 50 g phenol) were added in a three-necked flask, equipped with a reflux condenser, stirrer, and thermometer. The mixture was heated and maintained at 94 0C for 4 h, and then sulfuric acid (0.3 mL, 0.3 mL per 100 g phenol) was added, followed by further heating under reflux for 50 min. The reaction mixture was distilled under vacuum to remove the water and unreacted phenol at elevated temperature up to 115 0C for 3 h. The reaction is shown in Scheme 2. A series of high-ortho novolac resins was obtained by changing the concentrations of MPB.

BOPFs were prepared by melt-spinning using the melting phenolic precursor resins around 125 0C with a winder speed of 400 m/min., A melt-spinning machine with a single hole spinneret plate was used. The as-spun fibers were cured in a combined solution of hydrochloric acid (12.5 wt %) and formaldehyde (18.0 wt %) in a bath equipped with a thermometer and stirrer. The solution was heated form room temperature to 95 0C at a heating rate of 15.4 0C/h. Then, the solution cured fibers were heated at a rate of 2.5 0C/min to 240 0C and held for 2 h in nitrogen. Finally, the cooled samples were washed with water and dried at room temperature.

The obtained resins an fibers containing phenol ((100-0.94×n)g) and MPB ((1.38×n)g) in the reaction flask, where n=0, 1, 2, 3, 4, 6, are denoted by BOPFs-0, BOPFs-1, BOPFs-2, BOPFs-3, BOPFs-4, and BOPFs-6. The BOPFs cured in solution and in heat curing are denoted by BOPFs-n-1S and BOPFs-n-2H, respectively.

2.4 Characterization

Fourier transform infrared spectra (FTIR) were recorded on a Nicolet Magna-FTIR 750 spectrometer (USA) using the KBr disk technique to investigate the functional groups of the resins and fibers. The fibers were pulverized and mixed with KBr before being pressed into a disk. The concentration of the samples in KBr was 1.0 wt%, and 0.2 g of KBr was used in the preparation of the reference and sample disks.

The structures of the MPB modified high-ortho phenolic resins were analyzed at 293 K on a Bruker Avance 400 MHz spectrometer. The sample solutions were prepared by dissolving about 30 mg of samples in 2.0 ml of CDCl3. 1H NMR spectra were recorded at 400.13 MHz; TMS was used as an internal standard.

Thermogravimetric analysis (TGA) up to 900 0C with a NETZSCH STA 409 PC/PG thermogravimetry analysis system (Germany) under an inner atmosphere of nitrogen was employed to investigate the high-temperature behavior of BOPFs. The heating rate was 20 0C/min.

The surface and cross-section structures were coated with 5-10 nm of gold and then observed with a JEOL JSM-6360LV scanning electron microscopy (Japan).

The tensile performance of HBPFs was measured by a XQ-1A fiber tensile tester (Shanghai New Fiber Instrument Co., Ltd, China). The gauge length was 20 mm and the tensile speed of 10 mm/min was used. The fiber diameters were observed using microscopy (LWT300LPT, Xian Cewei Optoelectronic Technology Co., Ltd, China).

3. Results and discussion

3.1 The synthesis and structure of BOPFs

Fig.1 shows IR spectra of the as-spun fibers with different MPB content, and the characteristic absorptions are listed in Table 1. The benzene ring absorption at 1600 cm-1 was used as an internal standard [8]. According to the Beer-Lambert Law, the ratios of characteristic group (cg) absorbance (Acg/A1600) were obtained and shown in Table 1. It can be seen that the ratios of methylene ether, B-O, and O-H groups increase with increasing MPB content. The results suggest that MPB is introduced into novolac main chain, which leads to an increase of the O-H, and B-O bonds. Whereas the large borate group attached to the phenolic molecular is benefit for the formation of the methylene ether CH2-O-CH2 group in the ortho of phenolic resin, the relative intensity of the band corresponding to the stretching of PhCH2-OH decreases with the addition of MPB. The decreasing PhCH2-OH indicates that MPB promotes the increasing of high molecular weight phenolic resin which reduces the content of terminal group, and the decrease in C=O groups shows that the addition of MPB decreases the activity of methylene bridge group and increases the thermal stability of the resin.

Fig.1. FTIR Spectra of as-spun BOPFs samples with different phenyl borate content

Table 1. The ratios of characteristic group of samples

Group

Wavenumbers /cm-1

BOPFs-0

BOPFs-2

BOPFs-4

BOPFs-6

BOPFs-4-1S

BOPFs-4-2H

PhCH2-OH

1042

0.90

0.44

0.44

0.59

0.58

0.63

C-O-C

1102

0.72

0.92

0.94

1.00

1.29

1.15

C-O-C

1233

1.70

1.82

1.91

1.68

2.29

1.38

B-O

1378

0

0.99

1.15

1.16

1.24

1.17

CH2

1457

1.64

1.61

1.80

1.67

2.31

1.61

C=O

1720

0.24

0.08

0.05

0.15

0.23

0.25

O-H

3428

1.31

2.07

2.04

2.40

3.52

2.67

The reactivity groups of as-spun filaments are bound to change during the crosslinking in the curing processes. The filament and the fibers cured in solution and heat oven were characterized by FTIR spectroscopy (Fig. 2). Possible curing reaction scheme is presented in Scheme 3. In comparison with the as-spun filaments, the relative intensities of solution curing fibers corresponding to hydroxymethyl, methylene ether, and methylene bridge increase with the crosslinking of +CH2OH in fiber. And then, the relative intensities of heat curing fibers to methylene ether, and methylene bridge decrease with the thermal crosslinking in fiber. The groups are converted to hydroxymethyl and carbonyl group, as shown in Table 1. At the same time, the B-O bond ratio shows small fluctuation after the solution and heat curing process, which may corresponds to the hydrolysis of B=O, a by-product in the MPB syntheses.

Fig.2. FTIR Spectra of BOPFs-4 with different curing process

The absorption peaks at 754 cm-1 and 825 cm-1 were attributed to 1,3- and 1,2,3-substituted benzene ring and 1,4- and 1,2,4-substitution. The changes in the characteristic absorption band intensities are listed in Table 2. The relative intensives of the absorptions at 825 cm-1 and 754 cm-1 (A754/A825) are considered a measure of the extent of benzene ring substitution [13,14]. The effect of the isomeric composition on the navolac resins is evaluated by the study of the substitution of the benzene ring. An approximate evaluation of ortho–ortho ratio of novolac resin can be obtained using the relative intensives.

Scheme 1. The reaction of phenol and boric acid to produce resoles monophenyl borate

Scheme 2. The possible reaction of copolymerization

Scheme 3. The typical network structure of solution curing fiber, and heat curing fiber

As shown in Table 2, the proportions of ortho links of phenolic fibers change with the MPB content, solution and heat curing process. The O/P values of as-spun fibers decrease from 3.61 to 2.31 with increasing content of MPB, which indicates that the addition of MPB reduces the activity of ortho carbon-hydrogen bonds because of the block of larger borate group and lower pH value.

Table 2. The O/P value of different MPB content and curing process

Sample

As-spun

Solution curing

Heat curing

BOPFs-0

3.61

2.60

1.66

BOPFs-2

3.35

1.54

1.89

BOPFs-4

2.71

2.06

1.94

BOPFs-6

2.31

1.64

1.87

The effect of curing process on O/P value is summarized in the Table 2. The values decrease gradually from 3.61 of the as-spun filaments to 2.60 of the solution cured fibers, and to 1.66 of the heat cured fibers. The same descending rule can be discovered in the others samples. The activity of para carbon-hydrogen bonds is higher than that of ortho in solution curing because of the low reaction energy barrier [15]. As a result, the values begin to decline from the ratios of filaments to solution curing fibers. On the other hand, the O/P values of heat cured fibers are less than 2. A phenol molecular has two orthos and one para carbon-hydrogen bonds to crosslink in a ideal crosslinking structure. The BOPFs-4-2H achieves a most proximate value of the ideal crosslinking structure, and shows good mechanical properties.

Fig.3. 1H NMR spectra of monophenyl borate (a), BOPFs-0 (b), and BOPFs-4 (c)

In the syntheses of phenolic resins, the formation of numerous chemical structures shows the complicated 1H NMR spectra[16]. In general, the 1H NMR spectra of the BOPFs-0 (Fig.3(b)) and BOPFs-4 (Fig.3(c)) resembles those of non-boron or boron containing phenolic resins. Compared to the 1H NMR spectrum of triphenoly borate [17], the triplet at 4.02 ppm and the doublet at 1.27 ppm (Fig.3(a)) are due to ethyl alcohol esters, which may indicate that some ethanol is esterified during the synthesis of MPB in the specutrum of MPB. The spectra of the resins with or without MPB show that substitution of methylol Ph-CH2OH groups or methylene ether bridges occurred at the positions on the ester phenyl rings (4.88 and 4.75 ppm). Absorption of characteristic chemical linkages appearing in traditional phenolic resins spectra are also observed in the spectra of BOPRs-0 and BOPFs-4. The strong resonance peaks at about 7.31 to 6.75 ppm are assigned to the aromactic hydrogens. The resonance at the regions from about 3.94 to 3.84 ppm are assigned to the ortho-ortho and ortho-para methylene bridges, and the resonance peaks at about 3.70 ppm assigned to the para-para methylene bridges is very weak, which shows that the ortho substitutions have dominant roles in the resins.

The barely visible peaks at around 10 and 11 ppm indicate the new aldehyde and carboxylic acid groups formed from the methylene ether [17]. At elevated temperatures during the synthesis of phenolic resins, substitution reactions revert to starting materials (formaldehyde and phenol); the carboxylic acid group results from oxidation of the methylol group to aldehyde and carboxylic acid. From the spectrum of the BOPRs-4 (Fig.3(c)), the peaks due to aromactic hydrogens broadening as a result of the decreasing para-substitute and addition of B-O bonds. Generally, the pattern of absorptions is comparable in BOPRs-4 and BOPRs-0. This result confirms that reaction of the MPB with paraformaldehyde to form the MPB modified resins occurs during the copolymerization.

3.2 Some propetites of BOPFs

Fig.4 shows the TGA curves in N2 of the solution cured and heat cured fiber. The heat curing process in oven was applied from room temperature to 240 0C slowly, and then kept at 240 0C for 2h. As shown in Fig.4, in the temperature range of 100-340 0C, small weight losses which may correspond to the loss of some small, end groups and weaker bonds in the chains of the BOPFs, are observed in the heat cured fibers. The residual mass of fibers cured in solution and oven decrease by 7.6 % and 2.1 %, respectively.

Fig.4. TGA curves of solution curing and heat curing BOPFs-4 in N2

It is reasonable that the high-ortho phenolic fibers, derived from the heat curing process, contain higher crosslinkages. The high crosslinkage, based on the greater amount of trisubstituted phenol compounds formed by the B-O bonds (Scheme 3), eliminates less amounts of the small molecular, unstable terminal groups and inner units within the temperature range of 100-340. At the same time, the heat curing process increases the thermal stability of BOPFs with the appearance of carbonyl groups converted from the methylene links (analysed in FTIR), which leads to lower weight loss. Obviously, the heat cured fibers show higher thermal stabilities than the solution cured fibers. It suggests that the 2 hours heat curing at 240 oC in oven induces the reactions mentioned above under 340 oC. The weight losses, corresponding to the occurrence of crosslinking and polymer branching and oxidation, in the temperature range of 340-900 oC, show a similar trend for the different curing processes. The results indicate that the crosslinking groups of phenol borate B–O linkage and carbonyl formed during heat curing can effectively stabilize the terminal benzene rings from scission under 340 oC.

Fig.5. TGA curves of heat curing BOPFs with different monophenyl borate content in N2

Fig.5 shows the TGA curves of the BOPFs, derived from novolacs with different MPB in N2. The initial decomposition temperature shows a slight increase around 7 oC with increasing MPB content, and the total char yield increases from 65.6 % for BOPFs-1 to 71.2 % for BOPFs-6. This is reasonable becasue the higher bond energy and increasing crosslinked network by the boron O-B-O bridges decrease the weight losses during the pyrolysis of the fibers. The weight loss of BOPFs-6 is lower than those of the other BOPFs.

Table 3 The residual mass of different PB content phenolic fiber

Sample

First Step

(340 0C -470 0C)

Second Step

(470 0C -840 0C)

900 0C

Mass at

340 0C (%)

Mass at

470 0C (%)

Weight loss (%)

Mass at

840 0C (%)

Weight loss (%)

Char yield (%)

BOPFs-2-2H

96.8

86.5

10.3

66.4

20.1

65.6

BOPFs-3-2H

97.3

88.7

8.6

68.3

20.4

67.6

BOPFs-4-2H

97.9

91.8

6.1

70.0

21.8

69.3

BOPFs-4-1S

92.4

85.6

6.8

66.6

19

65.9

BOPFs-6-2H

97.8

91.9

5.9

72.5

19.4

71.2

On the other hand, the TGA curves exhibit a two-step weight loss, as summarized in Table 3. The onset temperatures of the two steps are 340 oC and 470 oC, respectively. The first step corresponds to condensation reactions with the elimination of water between either phenolic hydroxyl groups, yielding diphenyl ether linkage, or phenlolic hydroxyl and methylene bridge, yieldying aliphatic methenyl [18]. The weight losses of the first step decrease from 10.3 % of BOPFs-2 to 5.9 % of BOPFs-6. The results can be explained in that the evolution of water have happened between B-OH and phenolic hydroxyl during the heat curing process, which depletes the phenolic hydroxyl in the fibers and reduces the weight loss from condensation reaction with increasing content of MPB.

Above 470 oC, the weight loss corresponds to carbonyl groups formed by the oxidation of methylene bridges, and the network collapsing with the formation of the sketch of polyaromatic domains. As shown in Table 3, the second step weight losses keep around 20%. It means that the boron bridges have litter influence on the formation of carbonyl groups and polyaromatic sketch. The weight loss of 470 oC decides the total char yield at 900 oC.

Fig.6. The effect of the MPB content on the mechanical properties

The influences of the MPB content on the tensile strength, elastic modulus and elongation are shown in Fig.6. The tensile strength and modulus of BOPFs increase with increasing PB content. This trend can be explained as follows. Phenolic fibers are known to have a three-dimensional structure with a methylene bridge between the benzene rings. The tensile strength and modulus are closely related to the degree of crosslinking of the fibers. Besides the traditional functional groups in high-ortho phenolic fibers, such as methylene bridges, methylene ether bridges, and carboxyls, the increased crosslinking of the B-O groups formed in heat curing plays a dominate role in the increase of tensile strength and modulus. At the same time, the elongation increases with increasing MPB. The B-O bonds increase the molecules chain flexibility because of the longer B-O bridge instead of methylene bridge.

The tensile modulus increases rapidly with increasing MPB content, but the increase of tensile strength slows when the MPB content exceeds 4 parts. In contrast, the elongation decreases suddenly after reaching 5.6 % of BOPFs-4-2H. This is mainly due to the fact that BOPFs with a higher degree of crosslinking exhibit higher elastic modulus which increases the brittleness of fibers. Correspondingly, BOPFs-6 shows a decreasing elongation.

Fig.7. Surface SEM micrographs of the as-spun phenolic fiber (a), solution curing fiber (b), and heat curing fiber (c).

Fig.7 shows SEM micrographs of BOPFs-4 surface after different curing processes. It can be seen that all of the fibers are round and continuous, and as-spun fiber has a smooth skin (Fig.7(a)). However, as shown in Fig.7(b) and 6(c), there are some axial grooves on the surface of fibers. This is because of the double diffusion in the solution curing process and the steam formed by condensation reactions when the fiber is heat-cured at high temperature. It is apparent that the heat cured fiber shows a flatter axial groove on the surface than the solution cured fiber because of the further contraction during the heat curing process.

Fig.8. Cross-section SEM micrographs of as-spun fiber (a), solution curing fiber (b), and heat curing fiber (c)

The SEM micrographs of the cross-section structure of BOPFs-4 are shown in Fig.8. The cross-section of the solution cured and heat cured fibers contract into oval shapes during curing, but there is no micro-pore in the inner part of fibers. As shown in Fig.8(a), the brittle rupture of as-spun fibers is characterized with smooth cross-section fracture morphology. However, some ductile rupture occurs in the solution cured fiber (Fig.8(b)). Finally, Fig.8(c) shows much more ductile rupture accurs in the heat cured fiber because of the crosslinking during heat curing.

4. Conclusions

The MPB modified high-ortho phenolic resins were prepared with zinc acetate and sulfuric acid catalysts. Melt-spinning was used to prepare the BOPFs with good spinnability around 125 0C. The fibers was cured in the crosslinking bath and then heat-treated in N2 at elevated temperatures. The phenol borate B-O bonds are introduced into the molecular chain during the copolymerization, and this content increases with increasing MPB. The heat curing process promotes the condensation reaction or dehydration in high temperatures, which improves the mechanical performances. The initial decomposition temperature is increased up to 340 0C. The B-O bonds play a dominant role in the first-step mass loss, and shows a direct effect on the total char yield. The total char yield at 900 0C increases with increasing content of phenyl borate, which reaches the maximum (71.2 %) for BOPFs-6. The tensile strength and modulus increase to 165 Mpa and 8.1GPa for BOPFs-6, but the elongation of BOPFs shows a peak at 5.6% for BOPFs-4 with continuous imcrease in crosslinking.

Acknowledgements

This work was financially supported be the National Natural Science Foundation of China (Grant No. U1404507) and China Scholarship Council (No. 201308410358 and 201508410397)

References

[1] Economy J, Clark R A. Fibers from novolacs: U.S. Patent 3,650,102[P]. 1972-3-21.

[2] Tzeng S S, Chr Y G. Evolution of microstructure and properties of phenolic resin-based carbon/carbon composites during pyrolysis[J]. Materials Chemistry and Physics, 2002, 73(2): 162-169.

[3] Patton R D, Pittman C U, Wang L, et al. Ablation, mechanical and thermal conductivity properties of vapor grown carbon fiber/phenolic matrix composites[J]. Composites Part A: Applied Science and Manufacturing, 2002, 33(2): 243-251.

[4] Zhang D, Shi J, Guo Q, et al. Studies on hollownesss regulation and properties of crosslinked hollow phenolic fibers[J]. Fibers and Polymers, 2012, 13(4): 495-500.

[5] Liu C L, Ying Y G, Feng H L, et al. Microwave promoted rapid curing reaction of phenolic fibers[J]. Polymer Degradation and Stability, 2008, 93(2): 507-512.

[6] Ying Y G, Pan Y P, Ren R, et al. Effect of the molecular structure of phenolic novolac precursor resins on the properties of phenolic fibers[J]. Materials Chemistry and Physics, 2013, 143(1): 455-460.

[7] Jiao M, Yang K, Cao J, et al. Influence of epichlorohydrin content on structure and properties of high‐ortho phenolic epoxy fibers[J]. Journal of Applied Polymer Science, 2016, 133(18).

[8] Zhang W F, Liu C L, Ying Y G, et al. The preparation and characterization of boron-containing phenolic fibers[J]. Materials Chemistry and Physics, 2010, 121(1): 89-94.

[9] Gao J, Xia L, Liu Y. Structure of a boron-containing bisphenol-F formaldehyde resin and kinetics of its thermal degradation[J]. Polymer Degradation and stability, 2004, 83(1): 71-77.

[10] Martin C, Ronda J C, Cadiz V. Boron-containing novolac resins as flame retardant materials[J]. Polymer Degradation and Stability, 2006, 91(4): 747-754.

[11] Wang D C, Chang G W, Chen Y. Preparation and thermal stability of boron-containing phenolic resin/clay nanocomposites[J]. Polymer Degradation and Stability, 2008, 93(1): 125-133.

[12] Ming-Li J, Kai Y, Jian C A O, et al. Effect of boron content of high-ortho phenolic fibers on thermal property[J]. Thermal Science, 2015, 19(4).

[13] Grenier-Loustalot M F, Larroque S, Grenier P. Phenolic resins: 5. Solid-state physicochemical study of resoles with variable FP ratios[J]. Polymer, 1996, 37(4): 639-650.

[14] Rao M R, Alwan S, Scariah K J, et al. Thermochemical characterization of phenolic resins[J]. Journal of thermal analysis, 1997, 49(1): 261-268.

[15] Nomoto M, Fujikawa Y, Komoto T, et al. Structure and curing mechanism of high-ortho and random novolac resins as studied by NMR[J]. Journal of Molecular Structure, 2010, 976(1): 419-426.

[16] Kawamoto A M, Pardini L C, Diniz M F, et al. Synthesis of a boron modified phenolic resin[J]. Journal of Aerospace Technology and Management, 2011, 2(2): 169-182.

[17] Abdalla M O, Ludwick A, Mitchell T. Boron-modified phenolic resins for high performance applications[J]. Polymer, 2003, 44(24): 7353-7359.

[18] Costa L, Di Montelera L R, Camino G, et al. Structure-charring relationship in phenol-formaldehyde type resins[J]. Polymer Degradation and Stability, 1997, 56(1): 23-35.

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Structure of Solution Curing Fiber

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