Preparation and properties of epoxy - modi ﬁ ed thermosetting phenolic ﬁ ber

: This article proposes the preparation and micro - wave thermal cured ( MTC ) epoxy - modi ﬁ ed phenolic ﬁ bers for the ﬁ rst time. Epoxy - modi ﬁ ed thermoplastic phenolic resin was ﬁ rst prepared in acidic condition using phenol, formaldehyde, and epichlorohydrin as the reactants, and then underwent additive reaction with formaldehyde to obtain epoxy - modi ﬁ ed thermosetting phenolic resin, which was converted into nascent ﬁ bers through wet spinning. Finally, epoxy - modi ﬁ ed phenolic ﬁ bers were obtained through di ﬀ erent curing methods including solution cured, solution thermal cured, microwave cured, MTC, and was characterized by infrared spectroscopy, micro - scopic infrared imaging, nuclear magnetic resonance, ther - mogravimetric analysis, and scanning electron microscopy. The experiment results show that MTC epoxy - modi ﬁ ed phe - nolic ﬁ bers have optimal mechanical property with ultimate elongation of 4% and breaking strength of 133 MPa.


Introduction
In 1872, German chemist A. Baeyer discovered for the first time phenols and aldehydes synthesized phenolic resins under acidic conditions.In 1910, American scientist Baekeland proposed a patent on "pressurization and heating" curing of phenolic resin.Phenolic resins can be divided into two categories, namely thermoplastic phenolic resins and thermosetting phenolic resins (1)(2)(3)(4).
Phenolic fiber refers to a flame-retardant organic fiber made of linear thermoplastic phenolic resin or thermosetting phenolic resin after acetalization or complexation.The fiber neither melts nor burns in a flame at 2,500°C, but only carbonizes and maintains the original state of the fiber (5,6).At the same time, it has good thermal insulation performance, and has good corrosion resistance to various acids, alkalis, and organic solvents.Phenolic fiber can be used in corrosion-resistant materials for space navigation, and phenolic fiber cloth can be used in fire prevention and flame-retardant decorative materials (7).
The phenolic epoxy resins obtained by introducing epoxy groups into the main of phenolic resins show higher thermal stability and better mechanical strength.The synthesis and application of the phenolic epoxy resin have been reported (8)(9)(10)(11)(12)(13)(14); however, there have been no reports on the preparation of phenolic epoxy fibers.The spinning of phenolic epoxy resins into phenolic epoxy fibers is rather difficult because the viscosity of the resin is difficult to control.In the present work, phenolic epoxy fibers were obtained by wet-spinning, and then the epoxymodified thermosetting phenolic fiber is mainly studied to obtain fibers with high elongation and strength through different curing methods (15)(16)(17).
Institute.Epichlorohydrin, zinc acetate, and polyvinyl butyral (PVB) were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd.Sodium hydroxide was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd.Concentrated sulfuric acid (98%) was obtained from Luoyang Haohua Chemical Reagent Co., Ltd.All the chemicals were used as received (analytically pure) without further purification.

Synthesis of epoxy-modified thermoplastic phenolic resin
Typically, 100 g of phenol, 27.13 g of paraformaldehyde, 2 g of zinc acetate, 12.5 g of epichlorohydrin, and 54.26 g of distilled water were added into a three-necked, roundbottomed flask, and then underwent boiling reaction for 4 h.Then, sulfuric acid solution (2 g of concentrated sulfuric acid in 3 g of distilled water) was added into the above solution and the reaction continued for 1 h.Thereafter, epoxy-modified thermoplastic phenolic resin was obtained through decompression dehydration (18).

Synthesis of epoxy-modified thermosetting phenolic resin
The thermoplastic phenolic resin was ground into fine powder and dissolved into absolute ethanol in a threenecked, round-bottomed flask.The molar ratio of high ortho-phenolic resin and paraformaldehyde was set as 1:5.Sodium hydroxide of 0.25 g was added, and the reaction continued for 6 h.Then, sulfuric acid solution was added to adjust the solution pH, and the spinning stock solution of epoxy-modified thermosetting phenolic resin was obtained by centrifugation.The specific experimental methods could be seen in reference.

Preparation of epoxy-modified phenolic fiber by wet spinning
PVB was added to the prepared thermosetting epoxymodified thermosetting phenolic resin (mass ratio 15:85) in a three-necked, round-bottomed flask under 45°C oil bath and the solution was stirred for 2 h.Then, the solution temperature was slowly increased to 70°C and the water was removed by vacuum filtration.When the solution became thick, it was poured into a cylindrical syringe and then underwent wet spinning to obtain epoxy-modified phenolic nascent fiber (NF).Specially, the temperature of the coagulating bath was set at 50°C.

Solution cured (SC)
The spinning drum was placed in formaldehyde concentration of 18.0%, hydrochloric acid concentration of 12.0%, and the solution temperature was increased to 90°C for 2 h with a heating rate of 5°C‧min −1 .Thereafter, the spinning drum was taken out and dried.

Solution thermal cured (STC)
The SC fibers were placed in a heat curing oven, and the oven was vacuumized to 0.09 MPa.Then, the temperature of the oven was increased to 50°C for 1 h and nitrogen gas was introduced into the oven afterward.
Thereafter, the temperature of the oven was increased to 150°C for 1 h with a heating rate of 12°C‧min −1 and the heating switch was turned off, and the nitrogen gas flow was stopped when the temperature approached 120°C.Finally, the fibers were taken out after cooling down to room temperature (19).

Microwave cured (MC)
Place the spinning drum with the same proportion of curing solution into the microwave machine, and turn on the condensate water and magnetic stirring apparatus with the temperature increased to 90°C with a heating rate of 2°C‧min −1 .The spinning drum was taken out after cooling down to room temperature (20).

Microwave thermal cured (MTC)
The MC fibers were cured again by thermal cure.

Performance testing and structural characterization 2.5.1 Performance test and structural characterization
The structure of phenolic epoxy fiber was characterized by Fourier transform infrared spectroscopy (FTIR) (NicoletZ10) of Beijing Albo Technology Co., LTD and potassium bromide (spectral purity) tablet method was used.

Infrared microspectrography
Infrared microscopic imaging was tested by a thermal scientific facility.The infrared characteristics of fiber surface and cross-section are displayed by regional imaging.

Nuclear magnetic analysis (NMR)
The 13 C NMR high-resolution solid-state NMR measurements were performed on the Bruker 400 MHz NMR spectrometer with solid attachment.The high-resolution solid-state spectra of solidified samples at room temperature were measured by cross-polarization magic angle rotation technique.

Thermogravimetry
Thermogravimetric analysis (TGA) is to use a thermogravimetric analyzer under certain temperature conditions, is the sample temperature at a certain rate, the relationship between temperature and weight loss was measured and the heat resistance of the sample was analyzed.

Scanning electron microscope (SEM) observation
Quanta 250 FEG SEM from FEI Company in the United States was used to test and observe the surface and cross-section of the fibers.The gold-sprayed fibers were placed in the SEM sample chamber, and the brightness and magnification were adjusted to observe the morphology and surface.

Analysis of mechanical properties
The fiber tensile strength tester of Shanghai Lipu Institute of Applied Science and Technology XQ-1A was used to test the fiber, the stretching speed was set at 10 mm‧min −1 , and the holding distance of single fiber was fixed at 20.0 mm.The test was repeated for 30 times in one group.In the normal test, the value of elongation at break and strength take the average value of each group of data.
3 Results and discussion

FTIR
The change of the relative intensity of the functional group peaks in IR spectrum could reflect the crosslinking process related to crosslinking bath treatment, heat treatment, and MC.Based on Lambert Beer's law, we chose the IR peak at 1,600 cm −1 of typical C]C stretching vibration as the standard peak.Specially, in the IR spectrum of phenolic epoxy fiber, the peaks at 1,460, 1,040, 913, 813, and 750 cm −1 were attributed to the stretching vibration of the methylene, the stretching vibration of the hydroxymethyl, the absorption of the epoxy ring, the vibration peak of the para-position substitution of hydroxybenzene and the ortho-hydroxyl substitution of phenol, respectively (Table 1).
In comparison with the NF, the relative strength of the bands corresponding to the methylene (1,460 cm −1 ), the absorption of the epoxy ring (913 cm −1 ), and the orthohydroxyl substitution of phenol (750 cm −1 ) increases.At the same time the relative strength of the bands corresponding to the stretching vibration of hydroxymethyl (1,040 cm −1 ) and the para-position substitution of hydroxybenzene (813 cm −1 ) decreases.
As shown in Figure 1, the relative strength of the peak at 1,040 cm −1 of the NF was greater than that of the cured fiber, indicating that the NF contained more hydroxymethyl.However, after curing in coagulating bath, the relative strength of this peak decreased obviously, indicating the loss of hydroxymethyl, which was because the acid environment promoted the etherification of hydroxymethyl in hydrochloric acid cured solution.Correspondingly, the relative strength of the peak at 1,460 cm −1 related to methylene group became prominent.The relative strength of the peak at 913 cm −1 was greater than that of the cured Preparation and properties of epoxy-modified thermosetting phenolic fiber  3 fiber indicating the epoxy ring in the cured fiber with the active hydrogen via ring-opening reaction of epoxy group.The relative strength of the peak at 813 cm −1 of the NF was greater than that of the cured fiber, and the relative strength of the peak at 750 cm −1 of the NF was lower than that of the cured fiber, indicating that the ortho position of phenol easily crosslinks with -OH because of the higher activity than the para position under acidic conditions.

Microscopic infrared
Figures 2 and 3 display the cross-section diagram of MTC the examined fiber, the analysis point lattice for microscopic infrared imaging, and the corresponding infrared spectra, respectively.Similarly, we chose the IR peak at 1,600 cm −1 of typical C]C stretching vibration as the standard peak.Specially, points 56, 57, 58, 59, 60, and 61 are located at various sites that are sequential from the fiber axis to the surface of the examined fiber cross-section.As shown in Figure 4, points 3, 4, and 5 correspond to the center, median, and surface of the epoxy-modified thermosetting phenolic microscope, respectively.We chose the different colors, and the characteristic peak values are represented.The red and blue color areas represent high and low intensity areas, respectively.From Table 2, it can be seen that the relative strength of the typical peaks at 1,460 and 1,040 cm −1 , indicating the number of the methylene and the hydroxymethyl gradually increased from the inside to the surface because    the phenolic hydroxyl groups in the phenolic resin had reacted to form hydroxymethyl groups.At the same time, the relative strength of the bands corresponding to the stretching vibration of methylene (1,460 cm −1 ) converted from the unstable methylene ether bridge under acidic conditions.

NMR
To further investigate the structural difference between NFs and various cured fibers, the solid-state 13 C MAS NMR spectra was performed (Figure 5).As can be seen, relative to NFs, the relative intensity of the peak at 148 ppm related to phenoxycarbon of both crosslinking bath cured fibers and thermal cured fibers became stronger, indicating that dehydration condensation of phenolic hydroxyl occurred with synchronous enhancement of the crosslinking degree of the fibers (22).The relative intensity of the peak at 126.3-134.1 ppm related to fibers of MC and SC were similar, and the strength increased slightly after thermal cured, but both were much stronger than the strength of NF.The orthosubstituted carbon in the phenolic ring increased via hydrochloric acid solution and thermal curing, indicating that more reactions occurred in the ortho-position.The relative intensity of the peak at 99 ppm showed that the fiber is contained in the free aldehyde and other products.MC strong fiber peak slightly smaller than SC fiber, while strong peaks were greater than the NF in the bath of crosslinking cured fiber.This is attributed to the free aldehyde reduction into aldehyde in acidic environment, leading to aldehyde carbon peak power in the bath of crosslinking cured fibers.It can be observed that the NF after heat cured shows no peaks here, indicating that the fiber contains no phenolic carbon at this time.The NF peaks everywhere at 69 ppm of hydroxymethyl carbon atoms, and the fiber contains a large amount of hydroxymethyl.The peak strength increases after crosslinking bath cured, and weakens near 75 ppm of thermal cured, indicating that the hydroxymethyl group generates the methylene ether bonds after being cured.Peak intensity at 29.5-31 and 34-35 ppm represents the ortho and ortho, ortho and para methylene bridges, and more ortho-ortho methylene bridges were generated after the hydrochloric acid solution was cured and thermally cured.The generation of epoxide carbon represents at 42.3 ppm which proved the prepared phenolic epoxy fiber.

TGA
3.4.1 TGA (nitrogen) Figure 6 and Table 3 show the TGA curves and the detailed TGA data of epoxy-modified thermoplastic   Preparation and properties of epoxy-modified thermosetting phenolic fiber  5 phenolic fiber after SC, STC, MC, and MTC in nitrogen atmosphere.It can be seen from Figure 6, all the fibers show macroscopic mass loss at the initial thermal stage of 100-300°C, which is attributed to the loss of some small end groups and weak bonds as well as the removal of absorbed water, bounded water, and free aldehyde (23).Specially, the mass losses of different cured fibers are nearly the same of about 10% at this stage.However, the mass loss of the NF is much higher and reaches 17%, which may result from that at this stage, the NF undergo crosslink cured as well as the dehydration and condensation of phenolic hydroxyl groups are completed.The weight loss rate of SC after hydrochloric acid solution is larger, because the solidification of fiber forms double diffusion under acidic conditions, hydrochloric acid enters the fiber through the epidermis, and small molecular water in the fiber is removed through the epidermis.Therefore, the reaction of SC in the early stage is faster than that of thermal cured and MC (24,25).
In the temperature range of 300-650°C, the total weight losses of the NF, SC fiber, STC fiber, and MTC fiber are 41%, 54%, 42%, 38%, and 36%, respectively.The mass loss at this stage is mainly due to the thermal conversion of the polymer, accompanied by the loss of decomposition products and the formation of thermal stable intermediate products, cyclization, and dehydration condensation of the methylene and phenol rings in the polymer.Therefore, the transformation in the benzene ring, for example, in the later stage, may oxidize the carbon atoms of the benzene ring, and further oxidation will lead to the destruction of the benzene ring.The maximum decomposition temperature of each fiber is about 600°C, which accords with the characteristics of high ortho-phenolic fiber with good heat resistance.With the increase of temperature, benzene ring and its derivatives break and the fiber is deeply carbonized when the temperature is higher than 800°C.Hydrochloric acid enters inside the fiber through the skin of the fiber, and the small water molecule inside the fiber is removed through the skin.With the further solidification of the skin of the fiber, a tight surface layer is formed.It is difficult for hydrochloric acid to enter inside the fiber and the water molecules inside the fiber.It is also difficult for the substance to come out, resulting in incomplete curing of the fibers, forming a "skin-core" structure.
The residual carbon yield of MC fibers is the highest, the molecules of the fiber resonate at the same time during the microwave curing process and small molecules of water can escape continuously as the fiber is cured from outside to inside.+CH 2 OH can easily enter the fiber and react with phenolic hydroxyl groups for dehydration and condensation with the help of MC.Methylene, benzene ring, and their derivatives in the main chain are carbonized at 650-800°C and 800-900°C.

TGA (air)
Figure 7 and Table 4 show the TGA curves and the detailed TGA data of epoxy-modified thermoplastic phenolic fiber after SC, STC, MC, and MTC in air atmosphere.
It can be seen from Figure 7 that all the fibers show macroscopic mass loss at the initial thermal stage of 100-300°C, which was attributed to the loss of some small end groups and weak bonds as well as the removal of absorbed water, bounded water, and free aldehyde, further curing between them.Such phenomenon corresponds to other experimental results reported elsewhere.
In the temperature range of 300-650°C, the total weight loss of the NF, SC fiber, STC fiber, MC, and MTC fiber are 84.76%,92.12%, 85.75%, 89.56%, and 65.77%, respectively.The mass loss at this stage is mainly due to the thermal conversion of the polymer, accompanied by the loss of decomposition products and the formation of Notes: T 5% , T 10% , T 20% , T max , and R 650°C correspond to 5%, 10%, 20% of material weight loss under nitrogen atmosphere, the temperature at the maximum weight loss rate, and the carbon residue rate at 650°C, respectively.
thermal stable intermediate products, cyclization and dehydration condensation of methylene and phenol rings in the polymer.Therefore, the carbon atoms of benzene ring may be oxidized in the later stage.The maximum decomposition temperature of each fiber is about 550°C, which is in line with the good heat resistance of thermosetting phenolic fibers.With the increase of temperature, benzene ring and its derivatives break, and the fiber is deeply carbonized when the temperature is higher than 650°C.The temperature of the NF is 163.4°C when the weight loss rate is 5%, and the corresponding temperature is 223.7°C when the weight loss rate is 10%.And after SC, heat cured, and MC, the weight loss rate of the fiber is significantly improved when the weight loss rate is 5% and 10%.The temperature of the fiber after microwave heat curing is 368.3°C when the weight loss rate is 5%.The temperature at which the weight loss rate is 10% is 415.5°C.Among them, the temperature corresponding to the maximum weight loss rate of the fiber after microwave heat cured is 556.2°C, which is also the highest temperature corresponding to other curing methods.It can be seen that after MC and thermal cured treatment, the decomposition temperature of fibers is greatly increased.The residual carbon rate at 650°C also reached a maximum of 18.92%.
As shown in Figure 8a 1 , the overall surface of the NF is relatively rough with burrs and the grooves, which are due to the fact that phenolic epoxy molecules are drawn in a saturated anhydrous sodium sulfate solution, resulting in uniform fiber orientation, which is a common phenomenon in wet-spinning.Figure 8a 2 demonstrates that the cross-section image of the NF is uniform and smooth, indicating that the phenolic resin exhibits fine and homogeneous spin ability.It can be seen that the fiber is brittle and cannot be tested for mechanical properties during the sample preparation, and the failure of NF shows a brittle nature corresponding to the SEM image (26).
In comparison with the NF, the surface layer of the cured fiber is smooth, and the cross-section shows a ductile fracture structure.The inhomogeneous distribution of linear structures implies slightly improved mechanical properties of the fibers.
There are many small pores on the surface of the cured fiber and the number of pores varies with different curing methods.The cross-section pores of SC fiber, STC fiber, MC, and MTC fibers are getting denser.As the fibers continued crosslinking reaction in the curing solution of  Notes: T 5% , T 10% , T 20% , T max , R 650°C correspond to 5%, 10%, 20% of material weight loss in air atmosphere, the temperature at the maximum weight loss rate, and the carbon residue rate at 650°C, respectively.
Preparation and properties of epoxy-modified thermosetting phenolic fiber  7 hydrochloric acid, the hydroxyl methyl group was further dehydrated and condensed.The condensed hydroxyl methyl groups spilled out from the fiber interior, resulting in the appearance of small holes in the fiber cross-section.It also confirmed that the organic solvent and water spilled out of the phenolic epoxy fiber under acidic environment and heating condition.After thermal cured and MC, the cured reaction between the epoxy group and the active hydrogen molecules is more intense, and the hydroxymethyl group further increases the dehydration condensation to generate water molecules, thereby generating denser pores.

Mechanical property analysis
As shown in Figure 9, the elongation and breaking strength of the cured phenolic epoxy fiber have a great relationship with the curing method.After SC, the elongation at break was 3%, and the breaking strength was 77 MPa.Compared with solution curing, the elongation at break increased by 49%, and the breaking strength increased by 28%.The elongation at break after microwave curing was 4%, and the breaking strength was 80 MPa.Compared with solution heat curing, the elongation at break after microwave heat curing increased by 7%, and the breaking strength increased by 67%.Overall, the MTC epoxy-modified phenolic fibers have optimal mechanical property.As can be seen, both the elongation and breaking strength of MC fibers are larger than that of SC ones, and this is due to the reason that phenolic epoxy fiber molecules undergo vibration reaction at the same time, and +CH 2 OH is easier to enter the fiber molecules for polycondensation reaction and dehydration crosslinking with phenolic ring with the assistance of microwave.The mechanical properties are more excellent after thermal curing treatment, which is mainly because the ether bonds  in the phenolic epoxy fiber molecules are converted into methylene bridges and the degree of cross-linking of the fibers is deepened, resulting in the enhancement of the fiber strength (27,28).
The elongation at break and breaking strength are important indicators of phenolic fibers.The relationship between the two is negatively correlated.By improving the flexibility of molecules, the bonding strength between molecules is weakened, the fiber strength is reduced, and the elongation at break is increased.The elongation at break of pure phenolic fiber is generally 1.5-2.5%.As shown in Table 5, other authors improved the properties of phenolic fibers by introducing phenyl molybdate and boron elements, but there was a shortcoming of low elongation at break.In this experiment, the flexibility of the fiber was improved by introducing epoxy groups on the phenolic molecule, and then by exploring the effects of different curing methods on the phenolic epoxy fiber, the fiber with the best performance after microwave thermal curing treatment was obtained.

Conclusions
Synthesis of epoxy-modified thermosetting phenolic resin in this article, and the resin was prepared by wet spinning to obtain NF.The properties of the cured fiber were studied through solution curing, solution thermal curing, microwave curing, and microwave thermal curing, and the main conclusions were as follows: 1.The appearance of the wavenumber of epoxy ring and the epoxy carbon peaks in 13 C spectrum through the infrared spectrum observation, which indicates that epoxy-modified phenolic fiber was prepared.2. The decomposition temperature of the maximum weight loss rate of microwave heat-cured epoxymodified phenolic fiber reaches a maximum of 584.1°C, and the residual carbon rate at 650°C is also as high as 59.38% under nitrogen atmosphere.3. The high brittleness and the poor mechanical properties of the NF were by mechanical properties test and SEM observation.The further improvement of the tensile properties were by SC and heat treatment.MTC epoxy-modified phenolic fibers have optimal mechanical property with ultimate elongation of 4% and breaking strength of 133 MPa.Preparation and properties of epoxy-modified thermosetting phenolic fiber  9

Figure 1 :
Figure 1: IR spectra of epichlorohydrin-modified thermosetting resins with different cured modes.

Figure 2 :
Figure 2: Cross-section diagram of MTC the examined fiber.

Figure 3 :
Figure 3: Infrared spectra and the corresponding analysis point lattice for microscopic infrared imaging.

Figure 4 :
Figure 4: Microscopic ATR-FTI image of the methylene of MTC phenolic fibers.

Figure 5 :
Figure 5: The 13 C NMR of epoxy-modified phenolic fibers.Figure 6: TGA diagram of different fibers in N 2 atmosphere.

Figure 6 :
Figure 5: The 13 C NMR of epoxy-modified phenolic fibers.Figure 6: TGA diagram of different fibers in N 2 atmosphere.

Figure 7 :
Figure 7: TGA diagram of different fibers in air atmosphere.

Figure 9 :
Figure 9: Comparison of elongation at break and strength of phenolic epoxy fibers after SC, MC, STC, and MTC.

Table 1 :
IR relative intensity values of functional groups of phenolic epoxy fibers

Table 2 :
IR relative intensity values of functional groups of MTC phenolic epoxy fibers

Table 3 :
TGA data of different phenolic fibers in N 2 atmosphere

Table 4 :
TGA data of different phenolic fibers in air atmosphere

Table 5 :
Comparison of performance parameters of different phenolic