Synergistic Effects of Ladder and Cage Structured Phosphorus-Containing POSS with Tetrabutyl Titanate on Flame Retardancy of Vinyl Epoxy Resins

The cage and ladder structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) have been synthesized through the hydrolytic condensation of 9,10-dihydro-9-oxa-10-phosphenanthrene-10-oxide (DOPO)-vinyl triethoxysilane (VTES). The unique ladder and cage–ladder structured components in DOPO-POSS endowed it with good solubility in vinyl epoxy resin (VE), and it was used with tetrabutyl titanate (TBT) to construct a phosphorus-silicon-titanium synergy system for the flame retardation of VE. Thermal stabilities, mechanical properties, and flame retardancy of the resultant VE composites were investigated by thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), three-point bending tests, limiting oxygen index (LOI) measurement, and cone calorimetry. The experimental results showed that with the addition of only 4 wt% DOPO-POSS and 0.5 wt% TBT, the limiting oxygen index value (LOI) increased from 19.5 of pure VE to 24.2. With the addition of DOPO-POSS and TBT, the peak heat release rate (PHRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) were decreased significantly compared to VE-0. In addition, the VE composites showed improved thermal stabilities and mechanical properties comparable to that of the VE-0. The investigations on pyrolysis volatiles of cured VE further revealed that DOPO-POSS and TBT exerted flame retardant effects in gas phase. The results of char residue of the VE composites by SEM and XPS showed that TBT and DOPO-POSS can accelerate the char formation during the combustion, forming an interior char layer with the honeycomb cavity structure and dense exterior char layer, making the char strong with the formation of Si-O-Ti and Ti-O-P structures.


Introduction
Vinyl ester resin (VE) are generally prepared by the reaction of unsaturated carboxylic acid and epoxy resin. The unique chemical structures endow them with the outstanding characteristics of epoxy resins and the advantages of unsaturated polyester resins [1,2]. Due to its excellent adhesion, mechanical properties, chemical corrosion resistance and easy processability, VE has been widely used in various applications, such as composites, anticorrosion pipelines, adhesives, automotive, etc [3,4]. However, VE is easy to burn, which greatly limits its application in aviation, shipping, and other special fields [5][6][7]. Therefore, the preparation of efficient flame-retardant vinyl epoxy resin has become a hot topic of academic research [8][9][10].

Preparation of the VE Composites
The formulations of the VE composites are listed in Table 1. Firstly, DOPO-POSS was dispersed in MFE-711 resin by mechanical stirring at 70 °C for 30 min to get a clear liquid.
After the mixture was cooled to room temperature, the curing agent (LPT-IN) and accelerator (P002) were added and stirred at room temperature for 10 min. After degassing under reduced pressure, the mixture was poured into the steel mold and cured at 120 °C for 2 h in a convection oven. After curing, all samples were cooled to room temperature. The schematic diagram of process as shown in Scheme 2.
The samples containing TBT were prepared by the same method, except that different amounts of TBT were added with the addition of 4 wt% DOPO-POSS. In addition, a group of pure VE was prepared as control group.

Synthesis of DOPO-VTES
To a 250 mL three-necked flask equipped with a mechanical stirrer, a reflux condenser, a thermometer and a nitrogen inlet, DOPO (32.4 g, 0.15 mol), VTES (28.5 g, 0.15 mol) and AIBN (1.476 g, 0.009 mol) were added and stirred gently until temperature was increased to 80 • C. Then, the reaction mixture was stirred at 80 • C for 6 h to get a light-yellow viscous liquid (DOPO-VTES) (

Preparation of the VE Composites
The formulations of the VE composites are listed in Table 1. Firstly, DOPO-POSS was dispersed in MFE-711 resin by mechanical stirring at 70 • C for 30 min to get a clear liquid. After the mixture was cooled to room temperature, the curing agent (LPT-IN) and accelerator (P002) were added and stirred at room temperature for 10 min. After degassing under reduced pressure, the mixture was poured into the steel mold and cured at 120 • C for 2 h in a convection oven. After curing, all samples were cooled to room temperature. The schematic diagram of process as shown in Scheme 2.

Characterization
FTIR spectra were recorded in the range of 4000~400 cm −1 on Bruker tensor 27 FTIR instruments (Bruker, Germany). 1 H-NMR and 29 Si-NMR spectra were obtained with Bruker AVANCE 400 MHZ NMR instrument (Bruker, Germany) using CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard. Wide-angle X-ray diffraction (XRD) measurements (Bruker, Germany) were performed at room temperature on EMPY-REAN X-ray diffractometer at 40 kv and 40 mA with CuKαradiation (λ = 0.1541 nm), Scheme 2. The schematic diagram of process of VE composites.
The samples containing TBT were prepared by the same method, except that different amounts of TBT were added with the addition of 4 wt% DOPO-POSS. In addition, a group of pure VE was prepared as control group.
The limiting oxygen index (LOI) values were evaluated on a JF-3 Oxygen index instrument according to GB/T 2406. . The size of the samples was 100 × 10 × 3 mm 3 , fifteen samples were taken from each group. Cone calorimetry measurements were performed on a FTT cone calorimetry according to the ISO 5660 standard under an external heat flux of 35 KWm −2 . The size of the VE thermosets was 100 × 100 × 10 mm 3 and three specimens were tested for every sample.
Thermo gravimetric analysis (TGA) was determined on a STA449F5 thermal analyzer (NETZSCH, Germany) under N 2 and air atmosphere at 10 • C/min from 25 to 800 • C. Dynamic mechanical analysis (DMA) was measured on TA Q800 with the following conditions: frequency 1 Hz, heating rate 3 • C/min, temperature range of 30~150 • C. Threepoint bending test was investigated on an AGS-X electronic testing machine. The size of the VE thermosets was 80 × 10 × 4 mm 3 .
Scanning electron microscope (SEM) was recorded with a Hitachi S-4800(Hitachi, Japan) at an acceleration voltage of 10 KV. Prior to SEM measurements, the surfaces were coated with thin layers of gold of about 100 Å. Raman spectroscopy was determined on LABRAM HR Evolution to further investigate the residual char samples after cone calorimetry test. X-ray photoelectron spectroscopy (XPS) measurement was performed using an ESCALAB250XI instrument (Thermo Fisher Scientific, America). The obtained data were calibrated by C 1s standard peak and analyzed by PEAK XPS software.
TGA was coupled with FTIR (Bruker tensor 27), and the measurements were carried out in air atmosphere at 10 • C/min from 40 to 600 • C Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analysis was carried out with Exactive GC Orbitrap GC-MS (Thermo Fisher Scientific, America). High temperature cracker: CDS PYROPROBE 6200; Gas chromatograph TRACE 1310; Mass spectrometer equipment ISQ 7000. The temperature of GC/MS interface was 300 • C and the pyrolysis temperature was 900 • C.

Characterization of DOPO-VTES and DOPO-POSS
Supplementary Figure S1 shows the FTIR spectra of DOPO, VTES and DOPO-VTES. As shown in Supplementary Figure S1c, the characteristic absorption peak at 3066 cm −1 is assigned to C-H stretching vibration of the aromatic ring. The absorption bands at 2975 cm −1 , 2926 cm −1 and 2892 cm −1 from the C-H stretching vibration of the alkyl group are also detected. The absorption bands at 1478 cm −1 , 1207 cm −1 and 910 cm −1 are attributed to P-Ph, P=O, P-O-C groups. In addition, the absorption bands at 2437 cm −1 corresponding to the P-H characteristic peak disappeared, indicating the successful addition reaction between the P-H groups of DOPO and C=C groups of VTES.
The 1 H-NMR of DOPO, VTES, and DOPO-VTES are shown in Supplementary Figure S2. It can be seen that the characteristic peak at 8.90 ppm assigned to the protons in P-H of DOPO (Supplementary Figure S2a) and the signals at 5.80-5.95 and 6.07-6.14 ppm assigned to the protons of -CH=CH 2 of VETS (Supplementary Figure S2b) disappeared in DOPO-VTES ( Figure S2c), confirming the successful addition reaction between DOPO and VTES. Figure S3 shows the FTIR spectra of DOPO-VTES and DOPO-POSS. In Supplementary Figure S3a successful hydrolysis condensation of the ethoxy group. The absorption bands at 3472 cm −1 were attributed to Si-OH stretching vibration, and the peak at 2908 cm −1 was attributed to the -P-CH 2 -CH 2 group. In addition, the peaks at 1000-1200 cm −1 were assigned to Si-O-Si absorptions, in particular, the 1116 cm −1 band was assigned to the symmetrical cage structure, while the 1080 cm −1 band was assigned to the random structure [47], indicating that the ladder and cage structured DOPO-POSS has been successfully synthesized.
Supplementary Figure S4 shows the 1 H-NMR spectra of DOPO-POSS. It can be seen that the signals at 0.86-1.26 and 3.58-3.88 ppm assigned to the -CH 3  The 29 Si-NMR spectra of DOPO-POSS are showed in Supplementary Figure S5. The signals at -65 ppm were ascribed to Si atoms of complete condensation [48], and the signals at -55 ppm were assigned to the Si-OH in DOPO-POSS. Figure S6 shows the XRD profile of DOPO-POSS. It can be seen that DOPO-POSS exhibits three peaks at 2θ = 5.46 o , 13.18 o and 20.68 o corresponding to repeat distances of approximately 16.1 Å, 6.71 Å, and 4.29 Å, indicating the amorphous nature of the polymer. The narrow peak at 2θ = 5.46 o corresponds to the intermolecular distance and the wide peaks at 2θ = 13.18 o and 2θ = 20.68 o corresponds to the intramolecular distance. Compared with the reported polyphenylsiloxane (PPSQ) which has the chain-to-chain distance of 12.5 Å and the intramolecular distance of 4.6 Å [15,49], DOPO-POSS has two kinds of intramolecular distances (6.71 Å and 4.29 Å), which may be attributed to the cage and ladder structure of DOPO-POSS.
The MALDI-TOF mass spectrum of DOPO-POSS is shown in Supplementary Figure S7. In the spectrum, peaks in the m/z range 2000-3000 were detected, and the assignments of the major components are shown in Table 2. It can be seen from Table 2 that the differences in m/z between the two adjacent ladder structure peaks were 304 (RSiO 2 H) and 286 (RSiO), and two adjacent cage and cage-ladder structure were 304 (RSiO 2 H), respectively. The chemical structures of the representative cage and ladder structured components are illustrated in Figure 1. It can be seen that DOPO-POSS are composed of cage, ladder and cage-ladder structured components, and most of the components are ladder structure [50], which is consistent with the results of FTIR and XRD. The ladder and cage-ladder structures are beneficial to reduce the crystallinity of POSS, which improves the compatibility between DOPO-POSS and vinyl epoxy resin [36]. The presence of Si-OH groups also increases the affinity of synthesized DOPO-POSS to polar VE resin. structures are beneficial to reduce the crystallinity of POSS, which improves the compatibility between DOPO-POSS and vinyl epoxy resin [36]. The presence of Si-OH groups also increases the affinity of synthesized DOPO-POSS to polar VE resin.   Figure 2 shows the SEM micrographs of the fractured surfaces of the VE composites. It can be seen that at low loadings (≤4 wt%), no obvious particles or agglomeration are observed from the fractured surfaces of both VE-0, VE-1, and VE-2, indicating that the DOPO-POSS can be well dispersed in the VE matrix. At high loadings (5 wt%), aggregates  Figure 2 shows the SEM micrographs of the fractured surfaces of the VE composites. It can be seen that at low loadings (≤4 wt%), no obvious particles or agglomeration are observed from the fractured surfaces of both VE-0, VE-1, and VE-2, indicating that the DOPO-POSS can be well dispersed in the VE matrix. At high loadings (5 wt%), aggregates of micrometer size could be observed on the fractured surface of VE-3, which is attributed to the separation of DOPO-POSS from VE matrix. The good solubility of DOPO-POSS in VE matrix was due to the presence of the ladder and cage-ladder structure components. Figure 3 shows the σ-ε curves of the VE composites tested by three-point bending and the results are shown in Table 3. It can be seen that flexural strength of VE composites first increased and then decreased with the increase of DOPO-POSS in VE matrix. Generally speaking, the fracture strain of materials depends on both stiffness and toughness [35]. The improved fracture strain of VE composites with the addition DOPO-POSS may be attributed to the increased content of rigid benzene ring structure and cage structure of DOPO-POSS which greatly inhibit the movement of polymer molecular chain in thermoset networks [51][52][53].  of micrometer size could be observed on the fractured surface of VE-3, which is attributed to the separation of DOPO-POSS from VE matrix. The good solubility of DOPO-POSS in VE matrix was due to the presence of the ladder and cage-ladder structure components.   Table 3. It can be seen that flexural strength of VE composites first increased and then decreased with the increase of DOPO-POSS in VE matrix. Generally speaking, the fracture strain of materials depends on both stiffness and toughness [35]. The improved fracture strain of VE composites with the addition DOPO-POSS may be attributed to the increased content of rigid benzene ring structure and cage structure of DOPO-POSS which greatly inhibit the movement of polymer molecular chain in thermoset networks [51][52][53].

Thermal Stability
TGA and DTG curves of DOPO, DOPO-POSS and the VE composites under nitrogen and air atmospheres are shown in Figure 4. And the relevant thermal decomposition data,

Thermal Stability
TGA and DTG curves of DOPO, DOPO-POSS and the VE composites under nitrogen and air atmospheres are shown in Figure 4. And the relevant thermal decomposition data, including T 5% which is defined as the temperature at 5 wt% weight loss, T max which is defined as the temperature at maximum weight loss rate, and the char residues at 800°C are summarized in Table 4.

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As shown in Figure 4c,d, under the air atmosphere, it can be observed that a threestage degradation process occurred in all samples, the first and second decomposition process occurred at around 360 °C , and reached Tmax at about 410 °C , which may be attributed to the degradation of aromatic ring and alkyl chain [16]. The third decomposition process appeared at around 520 °C , mainly due to the further thermal oxidative degradation of the unstable char layer formed by aromatic ring and alkyl chain.      As shown in Figure 4a,b, under the nitrogen atmosphere, the T 5 % and T max of DOPO-POSS and DOPO are 384.0 and 479.4, 267.0, and 332.3 • C, respectively, and their char residues at 800 • C are 41.1% and 3.92%. Figure 4c,d show the results under the air atmosphere, the T 5 % and T max of DOPO-POSS and DOPO are 304.3 and 483.7, 234.7 and 271.7 • C, respectively, and their char residues at 800 • C are 37.71% and 3.07%. It can be observed that a two-stage degradation process occurred in DOPO, the first degradation process occurred at around 267 • C, which may be attributed to decomposition of the weak P-O-C bonds [9,16,54]. The second degradation process occurred at around 340 • C, which may be attributed to the degradation of aromatic ring [16,55]. Moreover, DOPO-POSS exhibited high thermal stability because it has SiO 2 cage core and rigid Si-O-Si structure could change the thermal decomposition process through the formation of thermal stable SiO 2 ceramic phase [30,34,56].
The T 5% and the char residues of VE-0 were around 321.2°C and 7.07%. With the addition of DOPO-POSS into VE matrix, the T 5% and the char residues of VE composites increased significantly. Compared to VE-2, with the addition of 0.5 wt% TBT, the char residues of VE-2-2 increased from 9.35% to 14.38%. This may be due to titanium-containing TBT having acted as a catalyst to promote the formation of char at high temperature [18].
As shown in Figure 4c,d, under the air atmosphere, it can be observed that a three-stage degradation process occurred in all samples, the first and second decomposition process occurred at around 360 • C, and reached T max at about 410 • C, which may be attributed to the degradation of aromatic ring and alkyl chain [16]. The third decomposition process appeared at around 520 • C, mainly due to the further thermal oxidative degradation of the unstable char layer formed by aromatic ring and alkyl chain.
Supplementary Figure S8 shows the curves of the storage modulus and tan δ of the pure VE and the VE composites. The glass transition temperature (Tg) of the samples is determined from the peak temperature of tan δ curves. The storage modulus at 40 (E 40 ) and 150 • C (E 150 ), tan δ and Tg are summarized in Supplementary Table S1. In general, T g is determined by the cross-linking density of the resin, the rigidity of the chain structure and the segmental motion freedom, etc. As can be seen from Supplementary Table S1, the E 150 of all the VE composites are lower than that of VE-0, indicating that the crosslinking density of the VE composites are reduced [57]. On the one hand, the large volume of rigid benzene ring structure and cage structure of DOPO-POSS can greatly inhibit the movement of polymer molecular chain, which increased T g ; on the other hand, the addition of bulky DOPO-POSS decreased the cross-linking density of VE resins, which decreased T g . Compared to VE-0, the glass transition temperature of VE-2 and VE-2-2 were slightly increased due to the good dispersion of DOPO-POSS in VE matrix. In addition, the glass transition temperature of VE-3 decreased compared to VE-0, which was attributed to the separation of DOPO-POSS from VE matrix at high loadings, which is consistent with the results of SEM.

Flame Retardant Properties of the VE Composites
LOI test was used to evaluate the flame retardancy of the VE composites, and the detailed data are shown in Table 5. With the increase of DOPO-POSS, the LOI values of the composites increased from 19.5 to 22.1. It is interesting that with the introduction of DOPO-POSS and TBT, the LOI values of VE composites were improved significantly. When 4 wt% DOPO-POSS and 0.5 wt% TBT were incorporated into VE, the LOI value reached 24.2, which was improved by 24.1% compared with that of the pure VE. The results revealed that TBT played a key role in increasing the LOI value of the vinyl epoxy resin [38][39][40], and phosphorus, silicon, and titanium showed a good synergistic effect in the flame retardancy of VE composites. The combustion behavior of the polymer was further investigated by cone calorimetry. Figures 5 and 6 show the total heat release (THR), the heat release rate (HRR) curves of the VE composites, and the key combustion parameters are summarized in Table 6. As shown in Figure 5a, after cone calorimetry test, the amount of residual char of the VE composites increased gradually in the order of VE-0, VE-2 and VE-2-2. From Figure 5b,c, it can be seen that pure VE burns quickly after ignition and the peak heat release rate (PHRR) and total heat release (THR) were 616.7 Kwm −2 and 395.8 MJm −2 , while those of VE-2 were 299.9 Kwm −2 and 267.1 MJm −2 , which were reduced by 51.4% and 32.5% compared with those of the pure VE. As for VE-2-2, the PHRR decreased to 264.5 Kwm −2 while THR increased slightly to 289.2 MJm −2 . As can be seen from Figure 6, smoke production rate (SPR), total smoke production (TSP), average of CO 2 Yield (av-CO 2 Y), average of CO Yield (av-COY) of VE-2 and VE-2-2 are all lower than those of VE-0. The peak of SPR, TSP, av-CO 2 Y and av-COY of pure VE were 0.127 m 2 s −1 , 85.15 m 2 , 2.15 kgkg −1 and 0.06 kgkg −1 , while those of the VE-2-2 decreased significantly to 0.104 m 2 s −1 , 73.45 m 2 , 0.69 kgkg −1 and 0.002 kgkg −1 by 18.1%, 13.7%, 67.9%, and 96.7%, respectively. This illustrated that DOPO-POSS and TBT have a good synergistic effect, could efficiently facilitate the generation of compact residual char layer that prevented further degtadation of the matrix into organic volatiles or gases and inhibited the burning effectively [52].     Figure 7 shows the photographs of the VE composites after heated at 450°C for 30 min in the muffle furnace under the air atmosphere. It can be seen that the char residue of the pure VE have been completely destroyed to thin and fragile fragments. With the addition of 4 wt% DOPO-POSS, the char residues of VE-2 and VE-2-2 retained intact [58].   Figure 7 shows the photographs of the VE composites after heated at 450 ℃ for 30 min in the muffle furnace under the air atmosphere. It can be seen that the char residue of the pure VE have been completely destroyed to thin and fragile fragments. With the addition of 4 wt% DOPO-POSS, the char residues of VE-2 and VE-2-2 retained intact [58]. The morphology of char layers after cone calorimetry testing was further investigated by SEM. As shown in Figure 8a,a', the exterior and interior char of VE-0 is porous, thin and brittle. In contrast, in the Figure 8b,b',c,c', after adding DOPO-POSS, the exterior residual char became dense and continuous, and the interior char layer had the characteris-  The morphology of char layers after cone calorimetry testing was further investigated by SEM. As shown in Figure 8a,a', the exterior and interior char of VE-0 is porous, thin and brittle. In contrast, in the Figure 8b,b',c,c', after adding DOPO-POSS, the exterior residual char became dense and continuous, and the interior char layer had the characteristic of honeycomb cavity structure, which could hamper the heat flow and mass transport [59].    Figure 9 shows the Raman spectrum of residual char after cone calorimetry. The characteristic peaks of 1350 cm −1 (D band) and 1590 cm −1 (G band) are disordered and ordered char, respectively [60]. The compactness of the char layer can be measured by the ratio of ID/IG peak intensity. The ID/IG value decreases in the order of VE-0 (0.85) > VE-2 (0.76) > VE-2-2 (0.74), indicating that the incorporation of DOPO-POSS increased the compactness of the residual chars, which is in accordance with the results from SEM. With the addition of TBT, more compact char residue was formed in sample VE-2-2 [39,40], which could provide better physical barrier effect, improving the flame retardancy of the VE composites. To further explore the flame retardant mechanism in condensed phase, the exterior and interior residual chars of VE-2 and VE-2-2 are studied by EDX analysis (Figure 10). It can be seen that for VE-2 and VE-2-2, the main elements in the char layers are carbon and oxygen, and a small amount of P and Si. Compared with VE-2, new signals belonging to Ti element appeared in VE-2-2, and the content of P and Si elements increased evidently. To further explore the flame retardant mechanism in condensed phase, the exterior and interior residual chars of VE-2 and VE-2-2 are studied by EDX analysis (Figure 10). It can be seen that for VE-2 and VE-2-2, the main elements in the char layers are carbon and oxygen, and a small amount of P and Si. Compared with VE-2, new signals belonging to Ti element appeared in VE-2-2, and the content of P and Si elements increased evidently. It can be inferred that the addition of Ti element into the VE composites was beneficial to the formation of dense and stable char layer, and different Ti containing compounds may be formed during combustion [53,61,62], which was helpful to improve the fire resistance. To further explore the flame retardant mechanism in condensed phase, the exterior and interior residual chars of VE-2 and VE-2-2 are studied by EDX analysis (Figure 10). It can be seen that for VE-2 and VE-2-2, the main elements in the char layers are carbon and oxygen, and a small amount of P and Si. Compared with VE-2, new signals belonging to Ti element appeared in VE-2-2, and the content of P and Si elements increased evidently It can be inferred that the addition of Ti element into the VE composites was beneficial to the formation of dense and stable char layer, and different Ti containing compounds may be formed during combustion [53,61,62], which was helpful to improve the fire resistance The exterior residual char VE-2 and VE-2-2 were studied by XPS to further investigate the mechanism of char formation. Figure 11a   The exterior residual char VE-2 and VE-2-2 were studied by XPS to further investigate the mechanism of char formation. Figure 11a [53,70], respectively. In the O1s spectrum of VE-2-2 (Figure 11f), there are four peaks at 530.9 eV, 531.8 eV, 532.6 eV, and 533.5 eV, which are attributed to Ti-O, C=O, P-C-O and P-O-P groups [63,65], respectively. The above results indicated that the addition of TBT was conducive to the formation of different Ti containing compounds in the char, which is helpful to improve flame retardancy of VE composites. Figure 12 shows the 3D TG-FTIR and FTIR spectra of the gas phase at different temperatures of pyrolysis of VE-0 and VE-2. As shown in Figure 12a,b, the pyrolysis products of VE-0 and VE-2 are obviously different. The evolved gas components of VE-0 and VE-2 in air at 322, 421 and 560 • C are shown in Figure 12c,d. With the increase of pyrolysis temperature, the number of absorption peaks first increased then decreased.

Pyrolysis Behaviors of the VE Composites
To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO 2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.        To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353. 3   To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353. 3   To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353. 3   To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353. 3   To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,  To sum up, the mechanism of flame retardancy was as follows: the radical quenching effect of phosphorus in the gas phase; the stable SiO2 ceramic phase formed by Si and the synergistic effect between phosphorus and Ti accelerated the formation of residue char, which had the characteristic of honeycomb cavity in the interior layers and compact exterior layers, preventing the heat flow and transfer to improve the flame retardancy of the composites.

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T5% increased from 321.2 to 353.3 °C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%,

Conclusions
In this work, a ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) was synthesized and characterized. The unique cage and ladder structure of DOPO-POSS facilitated its good solubility in the VE composites. DOPO-POSS and TBT was used as flame retardant additives to improve the flame retardancy of cured vinyl epoxy resin. Under the nitrogen atmosphere, T 5% increased from 321.2 to 353.3 • C and char residue increased from 7.07 to 14.38% compared to pure VE. With the incorporation 4 wt% DOPO-POSS and 0.5 wt% TBT, the LOI value of the VE composites increased from 19.5 to 24.2, and the PHRR, THR, SPR, and TSP were reduced by 57.1%, 26.9%, 18.1%, and 13.7%, respectively. In addition, the VE composites showed comparable mechanical properties to that of the pure VE. The flame retardant mechanism was mainly due to the radical quenching effect of phosphorus, the formation of stable SiO 2 ceramic phase, the catalytic char generation of Ti and the char forming of phosphorus. All the results indicated that DOPO-POSS and TBT combination have great potential applications in the future.