Intrinsically Noncombustible Thermosets from Sulfur-Containing Epoxy Resin and Benzoxazines: Evaluation of Thermal and Mechanical Properties

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Introduction
Polybenzoxazines are a special class of phenolic resin that have attracted attention because of their useful properties, such as near-zero shrinkage during polymerization, good resistance to chemicals, a high glass transition temperature (T g ), and excellent thermal stability and mechanical properties. They find applications in a wide range of areas, including the aerospace and automotive industries, oil and gas exploitation, and electronic manufacturing. Despite these advantages, polybenzoxazines have serious drawbacks, such as high polymerization temperature, poor processability at room temperature, and brittleness, which limit their use in many high-performance fields [1][2][3][4][5].
To address these problems, researchers have modified benzoxazine (BZ) resins using different methods, such as designing molecular structures by choosing particular phenols and amines [6][7][8][9], preparing composites with functional solid materials [10][11][12][13], and blending modification. The most common method is blending modification, which is usually achieved by adding rubber [14][15][16][17], polyurethane [18,19], phenolic resin [20], maleimide [21,22], and epoxy resin (EPS) [23]. The copolymerization products of this process usually exhibit superior properties to their parent substances. Polybenzoxazines' low crosslink network structures in comparison with ordinary thermosetting resins with similar properties may be tightened by copolymerization with an epoxy to achieve a higher crosslink density. This is because phenolic groups, which are produced by the ring-opening polymerization of BZ, react with EPSs at elevated temperatures [24,25]. Ishida and Allen [26] found that the addition of epoxy to the polybenzoxazine network increased the crosslink density and influenced its mechanical properties. Specifically, they noted an increase in glass-transition temperature, flexural stress, and flexural strain at break and a slight decrease in stiffness. Zhang et al. [27] found that the temperature at 5% weight loss (T 5% ) increased with increasing epoxy fraction, which improved the thermal stability, albeit with a reduction in char yield. He et al. [28] demonstrated that the addition of siloxane-containing EPS eliminated the high brittleness of polybenzoxazines and their poor adhesion properties. Liquid epoxy can be used as an active diluent of BZ in its typical solid powder form, which is beneficial for resin processing [29]. However, the addition of epoxy slows the polymerization reaction and, in general, slightly increases the exothermic peak temperature [23,27,30]. Nowadays, epoxy monomers have developed in the direction of bio-based with superior heat resistance [31,32], the same as BZs [4], so homopolymer thermosets from them combine the advantages of the constituent components and extend their areas of application.
Because of the great flexibility in the design of the molecular structure of the BZ monomer, flame-retardant elements (such as phosphorus or sulfur) can be incorporated into the molecular structure of the polybenzoxazines to further improve their thermal stability and flame-retardant performance [33][34][35]. In our previous work [36,37], we found that polybenzoxazines based on 4,4'-thiobisphenol (TBP) had superior thermal performance to that of common bisphenol A (BA)-based BZs, which we attributed to the higher structural stability imparted by the thioether bonds. We inferred that the derived thioether bond could enhance the thermal stability and flame retardancy of EPSs. In this study, we used TBP to synthesize an EPS, which we then blended with three sulfur-containing BZs to obtain copolymers. The polymerization behavior, thermal stability, flame retardancy, and mechanical properties of BZ/EPS blends with different epoxy contents were studied. We expect that the results will guide the future manufacturing of high-performance thermosets.

Synthesis of Sulfur-Containing Epoxy Resins (EPSs)
. TBP (0.020 mol, 4.36 g) and epichlorohydrin (0.200 mol, 18.50 g) were added to a 50 mL round-bottom flask. The mixture was heated to 75°C with constant stirring until the TBP had completely dissolved. After that, the temperature was lowered to 70°C. Then, NaOH solution (1 M) was slowly added and stirred for 4 hr at 80°C. The crude product was dissolved in 20 mL of toluene and washed with warm water five times. The final product-liquid EPS-was obtained by drying the organic layer with air overnight and then placing it in a vacuum oven for 24 hr. BA-based epoxy resin (EPBA) was synthesized from BA and epichlorohydrin (Scheme 1).

Preparation of BZ/EPS
Copolymers. BZ/EPS copolymers were prepared by mixing sulfur-containing BZ monomers with sulfur-containing EPS in various weight ratios. The BZ/EPS copolymers were cured at 180°C, 200°C, 220°C, 240°C, and 250°C for 1 hr each. The molecular structures of the three sulfur-containing BZ monomers synthesized are shown in Figure 1. These were synthesized according to the procedures in our previous work.

Results and Discussion
3.1. Analysis of the Copolymerization Process of EPS and BZ. The FTIR spectrum of EPS ( Figure 2) features a strong absorption peak at 914 cm −1 , which we assign to the epoxy group, and peaks at 1,029 and 1,244 cm −1 , which we assign to C-O-C. The absorption peak for C-S is observed at 1,174 cm −1 [38][39][40]. These results indicate that the synthesized product was a sulfur-containing EPS. The epoxy value of the copolymer was 0.476, which was close to that of the EPBA (0.468). Figure 3 shows the FTIR spectra of copolymers with a ratio of BZ:EPS = 1 : 1 (w/w) after curing for 1 hr at different temperatures. For all copolymers, the absorption peak (914 cm −1 ) of the epoxy group gradually decreased with increasing temperature and disappeared at 240°C. The oxazine rings of TBP-a/EPS and TBP-tma/EPS appeared at 930 cm −1 and that of TBP-fa/EPS appeared at 940 cm −1 . With increasing temperature, the intensity of this peak decreased, indicating the opening of the oxazine rings; this peak disappeared when the temperature reached 240°C. The disappearance of the epoxy ring and oxazine ring proves that they open simultaneously during curing and that cross-linking occurs (Scheme 2). The peak at 1,461 cm −1 , which represents tetra-substituted benzene, increased with temperature, whereas the peak at 1,479 cm −1 , which represents tri-substituted benzene, weakened. This indicates that the tri-substituted phenol rings participate in the cross-linking reaction during the ring-opening of BZ, resulting in tetra-substituted phenol rings. Although the curing process of the copolymers involves ring-opening and cross-linking of the oxazine and epoxy rings, the final temperature of ring-opening was found to be lower for the oxazine ring than for the epoxy ring. In Figures 3(b) and 3(c), the peak at 1,630 cm −1 , which we attribute to the furan ring and thiophene ring, broadened during the heating process, which demonstrates the formation of the double-substituted five-membered ring. In summary, the copolymerization of TBP-fa/EPS and TBP-tma/EPS involves ring-opening of the oxazine and epoxy rings, as well as cross-linking of the furan and thiophene rings in cross-linking reactions, to form double-substituted structures.
The DSC curves of the copolymers are shown in Figure 4. For TBP-a/EPS and TBP-fa/EPS, the melting point decreased with increasing epoxy content. When the EPS content was 70%, the resins were close to their liquid state, which is why the melting points were not determined. The TBP-tma/EPS blends were solid, and melting points can be observed in DSC curves even when the TBP-tma content was 70%. For all BZ/EPS blends, the initial polymerization temperatures decreased with increasing EPS content. Thus, the decrease in the initial polymerization temperature is due to the addition of epoxy groups. The active phenol groups in BZ promote the ring-opening of epoxy groups. This promotes the ringopening of oxazine, which decreases the onset temperature. For most BZ/EPS blends, the end polymerization temperature increased. The increase in the final polymerization temperatures is due to the tight cross-linked structure of the copolymers.
With the exception of TBP-fa/EPS, there is only one polymerization peak in each DSC curve. This proves that the ring-opening of the epoxy group and oxazine occurs at the same time [41]. However, at a weight ratio of TBP-fa: EPS = 5 : 5, the polymerization peak of the copolymer split. This implies that furan participates in the cross-linking reaction. The oxazine ring opens first to crosslink with the epoxy, and later, the reactive furan ring participates in the crosslinking reaction [42]. Table 1 lists the DSC data for all resin blends. The maximum polymerization temperatures of the three BZ/EPS O R SCHEME 2: The network structure of copolymers. 4 Advances in Polymer Technology   Figure 5. The three pure polybenzoxazines demonstrated the best thermal stability.
With the addition of EPS, the char yields gradually decrease. The initial temperatures of decomposition (T int ) are shown in Table 2. The addition of EPS increased the T int of the TBP-a/EPS copolymers. However, the T int of the TBP-fa/ EPS and TBP-tma/EPS copolymers initially decreased and then rose with increasing EPS content. This demonstrates that the introduction of epoxy groups blocked some of the originally formed double-substituted structures. By adding EPS, the 10% weight-loss temperature (T 10% ) and char yields decreased. For BZ:EPS = 1 : 9 (w/w), the T 10% of the copolymers based on TBP-a, TBP-fa, and TBP-tma decreased by 8°C, 78°C, and 54°C, respectively, compared with pure polybenzoxazines; the char yields decreased by 28%, 31%, and 31%, respectively. The flexible -C-O-C-C-Ogroups formed by epoxy groups extended the main chain of the copolymer resins. This, in turn, decreased the effective crosslink density, making it easier to decompose and leading to a decrease in the char yield [35,41,43].
The limiting oxygen index (LOI) is a commonly used value to quantify the flame retardancy of polymers. It can where CR is the char yield (%) at 800°C in a nitrogen atmosphere. All BZ/EPS copolymers had LOIs greater than 27, which is the critical LOI for self-extinguishing behavior. Thus, although the addition of EPS reduces the char yields, the BZ/EPS copolymers are self-extinguishing and can be used as flame-retardant materials. When the ratios of copolymers were TBP-a:EPS = 1 : 16 (w/w), TBP-fa:EPS = 1 : 18 (w/w), and TBP-tma:EPS = 1 : 17 (w/w), the char yields were all 24% (Figure 6), which corresponds to the critical LOI. We also mixed EPBA with TBP-fa, whose the thermal stability is the best among the three polybenzoxazines, for comparison with the BZ/EPS copolymers. The TBP-fa/EPBA copolymer with a ratio of TBP-fa:EPBA = 1 : 15 (w/w) also had a char yield of 24% ( Figure S1), while EPBA was verified with the FTIR spectrum ( Figure S2).
The combustibility of the BZ/EP copolymers was evaluated by the MCC. The MCC results are shown in Figure 7 and summarized in Table 3. Only one exothermic peak in the curves was observed for the three sulfur-containing copolymers. The exothermic temperature ranges were mostly within 350°C-450°C, and the three T PHRR vales (peak temperature of HRR) were similar. There are two exothermic peaks (390°C and 430°C) in the MCC curve of the TBP-fa/ EPBA copolymer ( Figure S3). We attribute the first peak, which approaches the T PHRR of the copolymers, to the breaking of phenolic groups during degradation. We attribute the peak at 430°C to the burning of a large number of    Materials with an heat release capacity (HRC) less than 300 J/g·K are considered to be self-extinguishing and to have excellent flame retardancy [33,45]. The HRC values of three copolymers were less than 300 J/g·K. Although the LOI values of four copolymers were 27, the TBP-a/EPS copolymer had a higher HRC value (320.6 J/g·K) and the TBP-fa/EPBA had a lower HRC value of 271 J/g·K (Table  S1). Thus, the calculated LOIs based on the char yield lead to an inaccurate conclusion about the flame-retardant behavior of polymers. We attribute the higher HRC of TBP-a/EPS to the aniline group in TBP-a, which is less stable than the thiophene and furan groups and degrades at 350°C-450°C. The lower HRC of TBP-fa/EPBA is due to the high ratio of BZ in the blends and the cross-linking system formed by furan rings.
The THR values of the copolymers of the sulfur-containing EPS were lower than that of the BA epoxy copolymer, which results from the higher temperature for the cleavage of the thioether bonds. Moreover, the S-C bonds generated by the cleavage further hinder combustion and reduce the amount of heat released. This also explains the narrow exothermic ranges of the sulfur-containing epoxy copolymers.
Furthermore, to explore the combustion characteristics of double substitution structures in the combustion process, conical calorimetric analysis was performed on the three sulfur-containing epoxy copolymers. Figure 8 presents the curves of HRR, THR, and total smoke production (TSP) versus time. In addition to these parameters, the longest time to ignition (TTI), the maximum average heat release rate (MARHE) of materials, and the smoke release rate (TSR) were recorded in Table 4. The poly(TBP-fa/EPS) showed the longest TTI (33 s) and gradually decreased along the sort of poly(TBP-fa/EPS) > poly(TBP-tma/EPS) > poly(TBP-a/EPS). The smaller PHRR reflects the better flame retardant property [46,47], while there was a decreasing trend of poly(TBP-fa/ EPS) < poly(TBP-tma/EPS) < poly(TBP-a/EPS). Besides, the THR, MARHE, TSR, and TSP also followed the same order. These results showed that double substitution structures based on two aromatic five-membered rings can efficiently restrain the heat release of copolymers. However, poly(TBP-tma/EPS) showed lower PHRR with higher THR, TSR, and TSP than poly (TBP-a/EPS), which proved that the thiophene ring burns violently during the final combustion process. Among all the copolymers, poly(TBP-fa/EPS) showed the best flame retardancy and smoke suppression performance simultaneously, which can effectively reduce fire hazards due to the introduction of furan ring.
In summary, these results demonstrate that the sulfurcontaining EPSs have excellent flame-retardant performance owing to the formation of cross-linking structures based on the main chain of diphenyl sulfide during polymerization and the formation of S-C bonds based on the cleavage of C-S-C. The double substitution structures also contributed to the flame retardancy of the copolymers prepared in this work.

Thermomechanical Properties of the Sulfur-Containing
Copolymers. We expected that the BZ/EPS copolymers would have good mechanical properties because of the high crosslinking density of epoxy and BZ. We selected the TBP-fa/EPS copolymer, which had the best thermal performance, for mechanical testing.
The DMA curves of the copolymers at different ratios of TBP-fa and EPS are shown in Figure 9, where E′ is the storage modulus, E″ is the loss modulus, and tan δ is the loss factor. The E′ of poly(TBP-fa) at 50°C was measured to be 2521 MPa. With an increasing ratio of sulfur-containing EPS, the E′ of the copolymers decreased, reaching 2206 MPa at 70% epoxy. In polybenzoxazines, oxygen atoms in phenolic hydroxyl groups and nitrogen atoms participate in intermolecular and intramolecular hydrogen bonding, which results in their high E′ [48]. In BZ/EP copolymers, the epoxy groups react with the phenolic hydroxyl group to reduce the extent of hydrogen bonding. Moreover, the rigidity in the glass state is related to the resistance of the movement of the molecular segments [27]. The cross-linking segments of the EPS cannot effectively store the modulus, resulting in a lower E′. Nevertheless, the copolymer with TBP-fa:EPS = 3 : 7 (w/w) had an E′ of 2206 MPa. This was the smallest value among the three copolymers, but it was equivalent to that of poly(BA-a) (2200 MPa) [26] and greater than that of poly (TBP-fa/EPBA) (1949 MPa), as shown in Table S2. These results demonstrate that copolymers based on sulfur-containing EPS have good mechanical properties.
The T g was determined according to the peak of the tan δ curve. For TBP-fa:EPS = 5 : 5 (w/w), the T g was 289°C. At a ratio of TBP-fa:EPS = 3 : 7 (w/w), the T g decreased to 273°C but was still superior to that of poly(TBP-fa) (265°C). The phenolic hydroxyl groups generated by the ring-opening of oxazine functioned as catalysts and were also consumed as reactants. There was an insufficient number of hydroxyl groups to react with the epoxy because of the high epoxy content. The remaining EPS acted as a plasticizer, lowering the T g and impairing the mechanical properties of the copolymers. The T g of the TBP-fa/EPBA copolymers followed the same trend with increasing EPBA content. At a weight ratio of 3 : 7, the T g of TBP-fa/EPBA was 239°C, which was lower than that of neat polybenzoxazine; this is in contrast with the aforementioned results for TBP-fa/EPS. These results The tan δ values of TBP-fa/EPS decreased with increasing EPS content, indicating an increase in the degree of cross-linking. The half-peak widths of the tan δ curves of the EPS-containing copolymers broadened because the EPS contains different graft groups and segments in very different sizes, which move at different temperatures. The peaks of the tan δ curves of the EPS copolymers showed no obvious phase separation, indicating that TBP-fa and EPS have good compatibility after polymerization. However, the tan δ peaks of poly(TBP-fa/EPBA) at a weight ratio of 3 : 7 tended to split ( Figure S4), indicating the compatibility for EPBA copolymer is poor for this ratio.
Furthermore, the crosslink density of thermosetting resin was calculated by the relationship of its elastic modulus, which can be well explained by the statistical theory of rubber elasticity. The following equation is used to obtain the crosslink density of TBP-fa/EPS copolymer [49,50]: where n is crosslink density, E′ is the equilibrium store modulus at T g + 50°C (Pa), R is the gas constant (8.314 L· kPa · K −1 · mol −1 ) and T is the absolute temperature (K). The calculated crosslink density results are displayed in Table 5. It can be seen that the addition of sulfurcontaining EPS significantly increases the crosslinking density of the TBP-fa /EPS copolymers. When the ratio is 3 : 7, the crosslinking density of TBP-fa/EPS reaches the highest (28.1 mol/L). With the increase in the content of EPS, the cross-linking density decreased, demonstrating that the Advances in Polymer Technology cross-linking enhancement effect of the epoxy group and hydroxyl group began to be weaker than the plasticizing effect of a large number of epoxy groups. However, it is still higher than that of pure BZ and BZ/EPBA (Table S2), which also proves that TBP-fa /EPS has a higher crosslink density.

Conclusions
In this work, we synthesized an EPS using a sulfur-containing phenol source (TBP), the epoxy value of which was close to that of a BA-based one. We found that the curing process of the copolymers of sulfur-containing BZs and sulfurcontaining EPS involved simultaneous ring-opening and cross-linking processes of the oxazine and epoxy rings. For TBP-fa/EPS and TBP-tma/EPS, the furan rings and thiophene were rings also cross-linked to form a double-substituted structure, which helped increase the thermal stability. However, the participation of EPS in copolymerization hindered the formation of the double-substituted structure, resulting in an increase in weight loss in the early stages of decomposition. The C-O-C-C-O groups produced by the epoxy ring-opening   reaction tended to break, decreasing the final char yield at 800°C. We prepared self-extinguishing copolymers (char yield = 24%) with EPS contents increasing in the order: TBP-fa/EPBA < TBP-a/EPS < TBP-tma/EPS < TBP-fa/EPS. The HRC for these materials (except TBP-a/EPS) was less than 300 J/g·K, which is indicative of strong flame-retardant properties. We attribute the excellent flame-retardant properties to the breaking of the thioester bonds at high temperatures. In cone calorimeter measurement, the TBP-fa/EPS copolymer showed the longest time of ignition (33 s), the lowest THR (43.9 MJ/m 2 ), and the total smoke production (9.8 m 2 ), which represented the best flame retardancy due to the formation of double-substituted structures. The T g of the TBP-fa/EPS copolymer with a BZ:EPS ratio of 5 : 5 (w/w) reached 289°C. In addition, the storage modulus of this copolymer reached 2206 MPa for an EPS content of 70%, which is equivalent to that of neat poly(BA-a). Compared with BZ/EPBA copolymers, sulfur-containing epoxy systems have a higher storage modulus, a more advantageous T g and crosslink density, and better compatibility with BZ resins.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.