Improved flame retardancy of epoxy resin composites modified with a low additive content of silica-microencapsulated phosphazene flame retardant

Journal Pre-proof Improved flame retardancy of epoxy resin composites modified with a low additive content of silica-microencapsulated phosphazene flame retardant

Lijie Qu, Yanlong Sui, Chunling Zhang, Xueyan Dai, Peihong Li, Guoen Sun, Baosheng Xu, Daining Fang PII:

S1381-5148(19)31272-6

DOI:

https://doi.org/10.1016/j.reactfunctpolym.2020.104485

Reference:

REACT 104485

To appear in:

Reactive and Functional Polymers

Received date:

28 November 2019

Revised date:

4 January 2020

Accepted date:

6 January 2020

Please cite this article as: L. Qu, Y. Sui, C. Zhang, et al., Improved flame retardancy of epoxy resin composites modified with a low additive content of silica-microencapsulated phosphazene flame retardant, Reactive and Functional Polymers (2019), https://doi.org/ 10.1016/j.reactfunctpolym.2020.104485

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© 2019 Published by Elsevier.

Journal Pre-proof

Improved flame retardancy of epoxy resin composites modified with a low additive content of silica-microencapsulated phosphazene flame retardant Lijie Qua,b,c, Yanlong Suia, Chunling Zhanga,* [email protected], Xueyan Daia, Peihong Lia, Guoen Suna, Baosheng Xub,c,* [email protected], and Daining Fangb,c a

School of Materials Science and Engineering, Jilin University, Changchun 130025, P.

R. China. b

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing

Beijing Key Laboratory of Lightweight Multi- functional Composite Materials and

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c

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100081, P. R. China.

Structures, Beijing Institute of Technology, Beijing 100081, PR China Corresponding author.

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*

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Abstract

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Epoxy resin (EP) composites with improved flame retardancy were fabricated. To solve the problem of the large addition of phosphazene flame retardants, we designed a system based on phosphorus–nitrogen–silicon synergistic flame-retardant Si(H)

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microcapsules with silica as the shell and hexaphenoxycyclotriphosphazene as the

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core. Si(H)/EP composites were manufactured with two coupling agents to promote interactions. In cone calorimetric tests, heat and gas releases of Si(H)/EP composites

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were reduced, and fire hazard was minimized. Residual analysis indicated the strong rigidity and mechanical robustness. The mechanism included phosphorus quenching effect and nitrogenous diluting effect of flame inhibition effect in gas phase, and phosphorus charring effect and silicic barrier and protective effect in solid phase. This research provides an effective flame-retardant method for EP composites with a low additive content. Keywords Flame retardancy; EP composites; Microcapsules; Phosphazene; Additives

1. Introduction Epoxy resin (EP) is one of the most widely used thermoset polymer material in adhesives, laminations, coatings, and electronic and electrical applications due to its high adhesion, excellent dimension stability, good corrosion resistance, high electrical

Journal Pre-proof insulation, and outstanding mechanical stiffness.[1-3] Nevertheless, the inherent flammable property of EP typically restricts its further applications, such as in the electrical industry.[4] Traditionally developed halogenated compounds added EP systems can generate corrosive and toxic gases, which can seriously damage human health and degrade environmental cleanliness. Accordingly, the development of halogen-free flame retardants with anti- flaming elements, such as phosphorus, nitrogen, and silicon, has elicited considerable interest from the scientific and industrial fields.[5] Phosphazenes, which are cyclic compounds that contain phosphorus and nitrogen

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elements connected by a conjugative mode, have been reported to exhibit excellent flame retardancy due to the synergies in their solid and gas phases.[6, 7] Incorporating phosphazene compounds is an effective method for endowing EP with flame

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retardancy.[8, 9] Among them, phenoxyl terminally hexaphenoxycyclotriphosphazene

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(HPCTP) exhibited high flame retardancy when the addition was in excess of 5 wt.%. However, the comprehensive performance of EP composites lacks guarantee because

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of the large addition of HPCTP.[10-12] Therefore, incorporating excessive fillers only to achieve flame retardancy can be costly in terms of engineering. Searching for an

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effective means to utilize a small number of active phosphazenes for manufacturing flame-retardant EP composites is worth studying.

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As a promising and effective method, microencapsulation has been widely used to

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produce flame retardants with improved properties.[13] In our previous work[14], synthetic polymer polymethyl methacrylate (PMMA) was used as the shell material to synthesize microcapsules called P(H) with HPCTP as the core. 1 wt.% P(H) incorporated EP composites exhibited improved thermal properties. However, the flame-retardant effects were restricted by the flammable PMMA shells. By contrast, the inorganic material silica (SiO 2 ) has been extensively applied to encapsulate materials through the sol–gel process.[15] Silicon-containing compounds will migrate and concentrate on the surface of polymeric materials to form a protective silica layer. Therefore, the further thermal decomposition of residues will be prevented, which can exert synergetic effects with other flame-retardant elements.[13] Therefore, introducing silicon-containing component into phosphazenes compounds is identified as a feasible thought to solve the heavy addition problem on the issue of flame-retardant modification.

Journal Pre-proof In the current study, we designed a flame-retardant microcapsule, denoted as Si(H), with HPCTP as the core and SiO 2 as the shell. The prepared microcapsules were then characterized and used to fabricate EP composites. To improve the interactions of Si(H) microcapsules with the EP matrix, two silane coupling agents, namely, 3-aminopropyltriethoxysilane

(KH-550)

and

3- glycidoxypropyltrimethoxysilane

(KH-560), were also added and investigated. Si(H) microcapsules contain phosphorus, nitrogen, and silicon elements. They play an important part in enhancing the flame retardancy of EP composites with a low content.

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2. Experimental 2.1.Materials. HPCTP

was purchased

from

Zibo

Lanyin

Chemical Co.,

Ltd.,

China.

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Tetraethoxysilane (TEOS), ammonia, and ethanol were acquired from Beijing

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Chemical Reagent Co., Ltd., China. The silane coupling agents (KH-550 and KH-560) were obtained from Shanghai Aladdin Chemical Reagent Co., Ltd., China. Diglycidyl

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ether of bisphenol A (DGEBA; commercial name: E-51) was supplied by Wuxi Dic Epoxy Co., Ltd., China. The curing agent, i.e., 4,4ʹ-diamino diphenylmethane (DDM),

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was supplied by Sinopharm Chemical Reagent Co., Ltd., China. All the reagents were used without further purification.

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2.2.Synthesis of Si(H) microcapsules.

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Microcapsules with SiO 2 particles as shell and HPCTP as core were synthesized via a sol–gel process. First, HPCTP (2.0 g) was dispersed in a mixture of ammonia (5 mL), ethanol (160 mL), and deionized water (32 mL) in a three-necked flask with ultrasonication for 15 min. Then, TEOS (5 mL) was dripped slowly into the above reaction mixture. The system was subsequently stirred at room temperature for 12 h. After the reaction was completed, the product was centrifuged and dried overnight under vacuum. The formation of Si(H) microcapsules is illustrated in Fig. 1. 2.3 Preparation of Si(H)/EP composites. Si(H)/EP composites with a series numbers of Si(H) microcapsules and different silane coupling agents were prepared via a thermal curing procedure. Si(H) microcapsules (0.5 wt.%, 1.0 wt.%, or 1.5 wt.%) were first added to EP monomers (DGEBA) with mechanical stirring at 75 °C. Once the entire system became homogenous and transparent, the silane coupling agents KH-550 (5 mL, 4.73 g) or

Journal Pre-proof KH-560 (5 mL, 5.33 g) were added dropwise with subsequent stirring after a few drops of water were introduced for the prehydrolysis process. Then, a stoichiometric amount of the curing agent (DDM) was slowly added with co ntinuous stirring until a uniform solution was obtained. Afterward, the resulting mixture was degassed and transferred to preheated Teflon molds. The curing process was set as follows: 105 °C for 2 h, 155 °C for 2 h, and 199 °C for 2 h. Samples for the follow-up tests were then retrieved after the molds were cooled. The pure EP thermosets were also prepared as contrast. The cross- linking networks of EP composites are also shown in Fig. 1. All the formulations of EP composites and the proportions of phosphor us, nitrogen, and

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silicon elements were calculated and summarized in Table 1. 2.4 Characterization.

Fourier transform infrared (FTIR) spectroscopy (Bruker Vertex 70, Germany) was

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conducted at room temperature to detect the chemical compositions and functiona l

bromide powder and pressed into flasks.

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groups of HPCTP and Si(H) microcapsules. The samples were mixed with potassium

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Scanning electron microscopy (SEM; FEI XL30 ESEM FEG, USA) and transmission electron microscopy (TEM; FEI Tecnai F30 G2, USA) were performed

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to examine the surface characteristics and encapsulation morphologies of Si(H) microcapsules. The specimens were coated with a conductive layer of gold or

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dispersed in ethanol before testing.

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Differential scanning calorimetry (DSC; TA Q20, USA) and thermogravimetric analysis (TGA; Perkin- Elmer, USA) were conducted to analyze the thermal properties and thermal stabilities of Si(H)/EP composites. In all the cases, samples were heated under nitrogen atmosphere from ambient temperature to 250 °C or 800 °C at a heating rate of 10 °C/min.

To detect the flame retardancy of Si(H)/EP composites, limiting oxygen index (LOI) values were measured using a LOI meter (FESTEC JF-3, Korea) with sheet dimensions of 80 mm × 6.5 mm × 3.2 mm in accordance with ASTMD 2863-97. Five independent tests were conducted for each sample. The UL-94 vertical burning properties were examined on a vertical burning tester (Motis Fire Technology UL94-X, China) with sheet dimensions of 125 mm × 12.7 mm × 3.2 mm in accordance with ASTMD 3801. Cone calorimetric measurements were conducted on a cone calorimeter (Fire Testing Technology FTT0007, UK) with flat dimensions of

Journal Pre-proof 100 mm × 100 mm × 3 mm in accordance with ISO 5660. Three samples for each composition were tested. The residues of Si(H)/EP composites were observed via SEM and FTIR spectroscopy. The Raman spectra of the char were obtained using a LabRAM-HR confocal Raman microprobe (JobinYvon, France). TGA-FTIR spectroscopy (Nicolet 6700, USA) was performed under nitrogen atmosphere at a heating rate of 20 °C/min from ambient temperature to 800 °C. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS; Agilent 5975, USA) equipped with a pyrolyzer (Shimadzu, Pyr-4A, Japan). Cracker temperature was 500 °C.

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The mechanical properties of Si(H)/EP composites were measured using an electronic universal testing machine (WSM-5KN, China) with a dumbbell-shaped specimen in accordance with GBT 2567-2008. The fracture surface of EP composites was detected

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through SEM.

3. Results and discussion

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3.1.Compositions and morphologies of Si(H) microcapsules. Si(H) microcapsules were prepared by hydrolyzing TEOS on the surface of HPCTP

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particles via a sol–gel process. The chemical compositions of HPCTP cores and Si(H) microcapsules with SiO 2 shells were confirmed by FTIR spectra (Fig. 2). In the

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HPCTP spectrum, the presence of phenoxy groups was proved by the peak at 3063 cm−1 attributed to the C–H stretching vibration, whereas the peaks at 1592 cm−1 and

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1488 cm−1 belonged to the skeleton vibration, and the peak at 766 cm−1 indicated the out-of-plane bending vibration of benzene mono-substitution. The distinct peaks at 1269 cm−1 belonged to the asymmetrical stretching vibration of P=N in the phosphazene rings. The peaks at 1190, 953, and 879 cm−1 were assumed to be the stretching vibration of the P–O–C bonds.[16] The appearance of these characteristic peaks in the spectrum of Si(H) confirmed the intact structure of HPCTP in the microcapsules. Moreover, the typical peaks of SiO 2 were observed in the spectrum of Si(H) microcapsules. The broad peak at 3446 cm−1 was regarded as the hydroxy asymmetrical stretching vibration of structural water. The peak at 1638 cm −1 was attributed to the bending vibration of H–O–H bonds. The sharp absorption bands observed at 1099 cm−1 and 464 cm−1 were ascribed to Si–O–Si asymmetrical stretching and symmetrical stretching vibration, thereby further indicating the

Journal Pre-proof formation of SiO 2 in Si(H) microcapsules.[17] The FTIR results clearly indicated the structural formation of SiO 2 shell materials over the intact HPCTP core materials. Fig. 3 presents the microscopic morphologies of raw HPCTP and Si(H) microcapsules in SEM (Fig. 3a and b) and TEM (Fig. 3c and d) images. As shown in Fig. 3a, raw HPCTP blocks exhibit smooth cuboid crystal features with sharp ends; the particles can hardly disperse evenly throughout the matrix because of the different polarities and the shortage of active groups, which will reduce the flame retardancy and other properties of EP composites.[18] After being microencapsulated, HPCTP was evidently uniformly coated with minute SiO 2 particles (Fig. 3b). The sharp

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corners of HPCTP were reduced. Numerous spherical SiO 2 particles with even nanometer sizes and defined profiles (Fig. 3c and d)) covered the smooth surface of HPCTP via van der Waals interaction. These particles provided innumerable reaction

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sites for subsequent disposal and modification. The SEM and TEM results further

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verified that Si(H) microcapsules with a core-shell structure were prepared successfully. The combination of a phosphorus–nitrogen-containing flame retardant

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and a silicon-based modifier was probably conducive to improving the properties of

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EP composites, such as flame retardancy.

3.2.Thermal behavior of Si(H)/EP composites.

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Si(H) microcapsules were utilized as fillers to prepare EP composites with specific

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contents. Fig. 4 presents the DSC curves of pure EP and Si(H)/EP composites at 0.5– 1.5 wt.% additive amounts with silane coupling agents (KH-550 or KH-560). The Tg values of the samples are also marked on the graph and summarized in Table 2. As shown in the figure, all the EP samples in the experimental temperature range were observed to display one Tg step without any curing peak, which showed the fully cured status. The pure EP exhibited a Tg value at 165.3 °C. After blending with Si(H) microcapsules and coupling agents, all Si(H)/EP composites exhibited a distinct decrease with the additives. For KH-550-modified composites, the Tg values were higher than the KH-560-modified samples. They were initially reduced to 152.0 °C and then increased to 155.6 °C as the Si(H) microcapsules increased. For the system modified by an equal volume of KH-560, the T g values exhibited a continuous deterioration to 142.2 °C. Various factors affected Tg possibly. First, Tg will be decreased because that the

Journal Pre-proof enlarged free volume rooted from the bulky HPCTP cores, and the plasticizing effect from SiO 2 particles in the EP matrix will reduce the cross- linking density of resins as the fillers increased,.[19, 20] Besides, the interparticle distance in the EP matrix were smaller than the microcapsules when more particles were added, which might result in favorable interfaces that construct physical networks to restrict chain mobility and increase the Tg.[19, 21] Moreover, the organosilane coupling agents between SiO 2 shells and epoxy resin provided covalent bonding and change the operatives required for segmental molecular motions,[22] which effected the T g. As shown in Table 1, KH-550 generated higher nitrogen content (3.001 wt.% for 1.0Si(H)/EP-550) for EP

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composites compared with the KH-560-modified samples (2.685 wt.% for 1.0Si(H)/EP-560). By working as amino groups of DDM, the extra amino groups from KH-550 would be helpful in forming more efficient and stronger interfacial

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interactions for bridging the structural connection between fillers and EP resins.

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Therefore, KH-550 generated Si(H)/EP-550 samples with better thermal resistance than Si(H)/EP-560 samples. All in all, the Tg values of the samples were mainly

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influenced by the enlarged free volume and the plasticizing effect, which caused the decrease.

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Fig. 5 presents the thermal stability of pure EP and Si(H)/EP composites as examined via TGA test under nitrogen atmosphere. The TGA parameters and

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derivative TGA (DTG) parameters are also summarized in Table 2. As shown in the

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figure, all the samples displayed one sharp weight loss stage that corresponded to the breakdown of the primary chains of EP cross-linking networks. The weight loss temperatures that correspond to 5 wt.% (T5

wt.%)

of the Si(H)/EP composites were

lower than the data of pure EP (Table 2). The decrease of T5 wt.% was attributed to the structurally

unstable HPCTP, which primarily decomposed at the P–O–C

bonds.[23-25] Besides, the surface combinative water of SiO 2 will primarily generate weight loss and decomposition at low temperature. However, the temperature at the maximum decomposition rate (Tmax ) and the corresponding derivative weight (DWmax ) of the composites presented an improvement in thermal stability. This result was attributed to the heat-resistant SiO 2 shells of the Si(H) microcapsules and the stronger interfacial interactions through the coupling agents between the fillers and the EP resins.[26] The reduced peaks of the DTG curves implied that the Si–O–Si bonds in the EP networks effectively provided additional stability to the hybrid materials.

Journal Pre-proof Moreover, an improvement in the char yield of all the modified EPs at 800 °C was also noted, and the values were raised as the amounts of fillers increased. Higher char yields resulted from a better capacity for residue formation, which might originate in the silicic structures in the matrices to prevent their further thermal degradation. Overall, Si(H) microcapsules with SiO 2 shells and coupling agents as bonding bridges helped

maintain the thermal stability of EP composites, particularly the

KH-550-reinforced composites with higher contents of nitrogen (Table 1) to construct a more compact structure. This capacity will endow EPs with a flame-retardant

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property.[27]

Journal Pre-proof 3.3 Flame retardancy of Si(H)/EP composites. The flame retardancy of EP thermosets was evaluated via LOI, vertical burning tests (UL-94), and cone calorimeter tests. In Table 3, the LOI value of pure EP was 25.7%. Si(H)/EP-550 and Si(H)/EP-560 networks presented similar increased tendency of LOI values with an increase in Si(H) microcapsules. Similar relationships were observed in the UL-94 tests. After the first 10 s fire ignition, pure EP began to dramatically burn and continuously combusted until the entire specimen was consumed. Along with the drastic fire, a large amount of black smoke and continuous dripping of combustion products were also noted. With the addition of Si(H)

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microcapsules with coupling agents into the systems, the modified composites exhibited improved fire performance without dripping. Moreover, in the cases of 1.5Si(H)/EP networks with KH-550 or KH-560, the fire was subdued until it was

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extinguished after the first and second ignitions. The entire combustion lasted for less

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than the standard durations and reached a UL-94 V-1 rating. Given the outcome of the LOI and UL-94 tests, Si(H) microcapsules with coupling agents exerted a beneficial

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effect on EP networks in terms of flame-retardant properties at a low content. To detect the flame retardancy under simulated real- world fire conditions, cone

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calorimetric tests were conducted on pure EP, 1.0Si(H)/EP-550, and 1.0Si(H)/EP-560 composites. The tests could provide sufficient data, from which we selected the data

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on heat release rate (HRR), total heat release (THR), and carbon monoxide production

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(COP) for analysis (Fig. 6). The primary characteristic parameters, such as time to ignition (TTI), HRR peak (PHRR), time to PHRR (TPHRR), THR, average effective heat of combustion (av- EHC), and percentage of mass loss (ML), were summarized in Table 4.

Fig. 6a depicts the curves of HRR versus time. Pure EP was easily flammable, which showed as 1677.9 kW/m2 in PHRR. Compared with that of pure resin, the PHRR values Si(H) microcapsules-enhanced EP composites exhibited a distinct decrease to 1159.1 and 972.9 kW/m2 (Table 4), which corresponded to the decrement of 30.9% and 42.0%. These reductions indicated that Si(H) microcapsules were effective in improving the flame resistance o f EP composites.[28] As described in the previous work[29], the evident platforms in the HRR curves implied that protective residues were formed. In this work, the HRR slopes of 1.0Si(H)/EP-550 and 1.0Si(H)/EP-560 composites decreased apparently relative to those of pure EP after

Journal Pre-proof PHRR, thereby suggesting that the residues were partly facilitated by Si(H) microcapsules and played a protective role during burning. Similar results can be found in Fig. 6c for the CO production curves, which showed the decrease of decomposed toxic gas. Furthermore, the resultant values rooted in 1.0Si(H)/EP-560 composites demonstrated a more evident effect than the other fillers, thereby indicating the different flame-retardant mechanisms among these flame retardants. In Fig. 6b, the THR values of Si(H)/EP composites were 114.4 MJ/m2 and 122.9 MJ/m2 , which were distinctly decreased compared with that of pure EP (148.0 MJ/m2 ). The reduction in THR compared with that of raw EP was ascribed to the pyrolysis

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products of HPCTP from Si(H) microcapsules. Therefore, the distinct reduction indicated the flame-retardant effect of Si(H) microcapsules in the EP networks at a low content.

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To directly describe the possible flame-retardant synergistic effects of Si(H)

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microcapsules in the gas and solid phases, we referred to prior quantitative assessment methods[14, 30, 31] to analyze the results from the cone calorimetric tests. In

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accordance with several parameters obtained from the cone calorimetric tests of pure EP and flame-retardant EP (FREP) composites, the integrated flame retardancy

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properties were concluded to exhibit a flame inhibition effect (FIE) in the gas phase, and a charring effect (CE) and a barrier and protective effect (BAPE) in the solid

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phase. These three aspects can be quantified through Formulas (1)–(3) by using the

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parameters in Table 4, which also provides the results. The FIE obtained through Formula (1) signified the effect of pyrolysis volatile gases in the gas phase. The av-EHC values discloses the burning rate of volatile gases in gas phase during combustion.[25, 32] Si(H)/EP composites affected by KH-550 and KH-560 exerted 10.4% and 7.7% FIE, respectively. This result wa s ascribed to the gas components decomposed from the HPCTP structures with phosphorus- and nitrogen-containing fragments. The former can liberate PO 2 • and PO • free radicals, which will subsequently eliminate the flammable H• and OH• free radicals through a quenching effect and prevent further flames. The latter will further decompose into nonflammable gases, such as NH3 , which will decrease the concentration of flammable gases and produce a diluting effect.[33] The quenching and diluting effects endowed Si(H)/EP composites with FIE. As shown in Table 1, when phosphorus contents were equal (0.080 wt.%), the structural characteristics determined that the

Journal Pre-proof KH-550-modified network possessed abundant amino groups (3.001 wt.% for nitrogen elements), whereas the KH-560-containing sample contributed additional epoxy groups but less amino groups (2.685 wt.% for nitrogen elements). Therefore, under the same conditions, the networks modified by KH-550 offered a higher number of nitrogen-containing fragments when burning, thereby providing a stronger diluting effect in the gas phase, which was prominent in the FIE. 𝑭𝑰𝑬 = 𝟏 − 𝑬𝑯𝑪𝑭𝑹𝑬𝑷 /𝑬𝑯𝑪𝑬𝑷

(1)

The CE obtained from Formula (2) was derived from decomposed components in the solid phase. In this study, phosphorus-containing compounds may react with other

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char matters and catalyze and promote the formation of charring layers. As shown in Table 4, Si(H)/EP composites samples presented only 6.7% and 5.1% of CE, thereby indicating that the effective phosphorus-containing fragments from HPCTP in Si(H)

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microcapsules might predominantly serve FIE in the gas phase, apart from CE in the

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solid phase. 𝑪𝑬 = 𝟏 − 𝑴𝑳𝑭𝑹𝑬𝑷 /𝑴𝑳𝑬𝑷

(2)

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BAPE was another parameter rooted to decomposed fragments in the solid phase that was obtained from Formula (3). The distinct BAPE of Si(H)/EP composites was

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attributed to the SiO 2 shells and coupling agents with abundant silicon elements. The aforementioned silicon-containing components will be converted to numerous SiO 2 or

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Si-O-phenyl structures to improve the barrier properties of the char layer. The formed

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dense char structures may effectively prevent the transfer of heat and oxygen and may play a role in reducing the release of combustible gases.[34] Moreover, the 1.0Si(H)/EP-560 samples have a more significant BAPE value, thereby indicating that under the same conditions, KH-560 exhibited better capacity for forming a protective char layer than KH-550. The difference between the two was ascribed to the silicon contents (0.787 wt.% for 1.0Si(H)/EP-550 and 0.819wt.% for 1.0Si(H)/EP-560 in Table 1) in the structure of EP composites. This small disparity can considerably impact flame retardancy in the solid phase, which suggested the crucial role played by silicon-containing flame retardants. 𝑩𝑨𝑷𝑬 = 𝟏 − (

𝑷𝑯𝑹𝑹 𝑭𝑹𝑬𝑷 𝑷𝑯𝑹𝑹 𝑬𝑷

)/(

𝑻𝑯𝑹 𝑭𝑹𝑬𝑷 𝑻𝑯𝑹 𝑬𝑷

)

(3)

The fire performance index (FPI) and fire growth rate (FIGRA) were also utilized in this study to further evaluate the composites for fire safety considerations. FPI is

Journal Pre-proof defined as TTI divided by PHRR, and FIGRA is defined as PHRR divided by TPHRR. In general, a material with superior fire safety performance should have a high FPI value and a low FIGRA value.[35] As shown in Table 4, the FPI values of the modified composites were higher and the FIGRA values were lower than those of pure EP, thereby indicating the apparent flame retardancy caused by Si(H) microcapsules. Besides, the 1.0Si(H)/EP-560 samples exerted more remarkable effect on minimizing fire hazard.

3.4 Analysis of residues from the cone calorimeter tests.

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To further analyze the internal flame-retardant mechanisms of Si(H) microcapsules in the solid phase, the char residues produced after the cone calorimetric tests of pure EP, 1.0Si(H)/EP-550,

and

1.0Si(H)/EP-560

composites

were

comprehensively

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investigated. Fig. 7a–c shows the digital photos of the char residues. The residues of

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pure EP (Fig. 7a) presented a poor status with minimal residues, through which the bottom of the mold could be easily seen. EP was intrinsically flammable and its

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charring ability was weak.[30] Si(H) microcapsules notably contributed to forming a firm and rigid char with a dense and compact structure (Fig. 7b and c). The stiff char

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structure will function as a protective layer to restrain the exchange of heat, oxygen, and volatile gases. When gases increased to the maximum of the critical value of the

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capacity of the char layer, cracks formed on the char layer. Moreover, a superficial

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white powder on the char layer should be noted, which might be SiO 2 particles or other silicon-containing fragments that swelled onto the char to protect the inner matrix during combustion.

SEM analysis was also conducted. Fig. 7d–i shows the residue SEM images of pure EP,

1.0Si(H)/EP-550,

and

1.0Si(H)/EP-560

composites.

Numerous

small

through-holes (white block scheme) and cracks in the char of EP (Fig. 7d) will provide considerable channels for the combustible volatiles to the gas phase. Therefore, combustion intensity was increased, and pyrolysis was accelerated.[30] Si(H)/EP composites exhibited considerable differences from the pure EP sample in the char layers. As shown in Fig. 7e and f, honeycomb- like structures with integrated pores inside were observed, as shown in the red block scheme. This unique structure favored temperature gradients in the char layer, thereby avoiding the feedback of heat, oxygen diffusion, and subsequent decomposition. Therefore, char formation during

Journal Pre-proof combustion will be enhanced, which confirms the CE in Table 4.[12, 36] Besides, a bubble-like structure can be observed on the resulting char as suggested by the blue block schemes, indicating that gas products can be intercepted by the char structures. This physical barrier can effectively reduce heat, oxygen, and mass exchanges between the solid phase and the gas phase, which enhances the flame retardancy of the composites.[37] Moreover, the char structure of the 1.0Si(H)/EP-560 composite (Fig. 7i) exerted stronger rigidity and mechanical robustness than that of the 1.0Si(H)/EP-550 composite (Fig. 7h). A stronger char will provide resistance to breakage and fracture to maintain the original frame construction of EP networks, [38]

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which was attributed to the higher silicon content in Table 1 and confirmed the superior BAPE in Table 4.

Raman spectroscopy was performed to characterize the structure and component of

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the residues of the EP and its modified products. The Raman spectra (Fig. 8a–c)

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depicted two bands at 1349–1359 cm−1 and 1596–1605 cm−1 , which correspond to the D and G bands. The ratio of the integrated intensity of the D and G bands (I D /I G) is

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commonly used to assess the graphitization degree of the residual char. As shown in Fig. 8a–c, the 1.0Si(H)/EP-550 and 1.0Si(H)/EP-560 composites exhibited lower

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values (1.16 and 1.13) than that of pure EP (1.47), thereby implying a higher graphitization degree of carbonaceous materials and higher thermal stability. This

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result was induced by more stable SiO 2 shells in Si(H)/EP composites, thereby

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improving flame retardancy in the solid phase.[39-41] The lowest value of 1.0Si(H)/EP-560 indicated the most stable char structure and the best BAPE presented in the cone calorimetric tests. The FTIR tests (Fig. 8d) also showed the effective compositions of residues that played a role in flame retardancy in the solid phase. Apart from the specific peaks derived from the char of pure EP, some new peaks detected in the char of Si(H)/EP composites are as follows: 1179, 952, and 879 cm−1 (P−O−C) and 742 cm−1 (P−C), which were attributed to the phosphorus-containing groups. Some peaks at 1565 cm−1 and 1509 cm−1 also indicated that new aromatics and polyaromatics were formed.[29, 42] Moreover, 1081 cm−1 (Si−O−C) and 1045 cm−1 (Si−O−Si) were also observed in the Si(H)/EP composite.[43] The FTIR results further illustrated that the stable char residues were mostly rooted in the phosphorus-containing HPCTP cores and in the silicon-containing SiO 2 shells and coupling agents. The former facilitated the

Journal Pre-proof carbonization process by decomposing organic phosphonates that can react with decomposition products from the EP matrix, thereby facilitating the CE in Table 4. The latter moved to the surface of the char as compact SiO 2 particles (Fig. 7b and c) and played the role of a protective layer and isolate heat, oxygen, and combustible gases from transferring between the inner unburned matrix and the air. Thus, the aforementioned BAPE was also confirmed. Through a combination of the preceding analyses, we concluded that Si(H) microcapsules exerted effects in enhancing the

TGA-FTIR and

Py-GC/MS

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3.5 Gaseous product analysis of Si(H)/EP composites.

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flame retardancy of EP composites in the solid phase at a low content.

tests were conducted

to obtain the detailed

flame-retardant information of the gaseous products of pure EP and its composites in

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the gas phase. Fig. 9a presents the 3D TGA-FTIR plots of the decomposition products

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of Si(H) microcapsules in the gas phase. The main gas products were phenol derivatives and water (3652 cm−1 ), NH3 (3334 cm−1 ), aromatic compounds (3054,

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1601, 1496, and 1339 cm−1 ), CO 2 (2358 cm−1 ), P=N (1256 cm−1 ), PO 2 • (1182 cm−1 ), and PO •H (3652 cm−1 , overlapping signals),[28, 33, 34, 38] which are also shown in

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Fig. 9b. The peaks of NH3 and phosphorus-containing compounds indicated that Si(H) microcapsules decomposed during pyrolysis and released effective gases. The

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quenching and diluting effects were achieved respectively by PO 2 • and PO • radicals

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and NH3 gas. The flame retardancy of EP composites in the gas phase was concluded through gaseous FIE, which had been verified in the cone calorimetric tests. To confirm the effect of Si(H) microcapsules on the thermal degradation process of EP composites, the gas products of EP and Si(H)/EP composites were also detected via TGA-FTIR. As shown in Fig. 10a–c, the differences in the primary decomposition products between pure EP and its composites with Si(H) microcapsules, such as aromatics compounds, aliphatic compounds, CO 2 , and ether compounds, were inconspicuous. However, the new absorbance peaks of the phosphorus-containing volatiles at 1010–1030 cm−1 were still observed. Moreover, specific products were studied to understand the change tendency of the decomposed products during heating at different times. The intensities of the aromatic compounds (Fig. 11a), aliphatic compounds (Fig. 11b), ether compounds (Fig. 11c), and CO 2 (Fig. 11d) for Si(H)/EP composites were distinctly lower than those for pure EP. With the exception of CO 2

Journal Pre-proof gas, the reduction of other flammable gases was in accord with the BAPE that had been indicated in the solid phase, which further decreased HRR and THR, as confirmed by the cone calorimetric tests.[37] Furthermore, the reduction of CO 2 gas implied that incomplete combustion was successfully realized and enhanced in the burning process. Py-GC/MS analysis was also conducted to further determine the degradation behavior of EP and Si(H)/EP composites. Fig. 12 shows the deduced pyrolysis route of EP and Si(H)/EP composites, and the simplified mass fragmentations are listed in Table 5. As depicted in Fig. 12, the main pyrolysis process of EP was divided in three

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sections: phenol (d, 94 m/z) and its derivatives (e, h, i, j, l, and m) that were derived from the chain scission of bisphenol A moieties; CO 2 (a, 44 m/z) and its derivatives (b and c) that were rooted from amido linkages; and N, N-dimethylaniline (f, 121 m/z)

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and its derivatives (g, h, and n) that came from DDM portions.[44] After being

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incorporated with Si(H) microcapsules and coupling agents, the EP composites exhibited similar degradation behavior, except for several significant postponement of

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pyrolysis time and important reductions of large pyrolysis fragments, as indicated in Table 5. The delay and suppression of these pieces that evaporated into the gas phase

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implied the BAPE of silicon-containing products that played a part in enhancing large fragments of the char residue. In summary, the results of the TGA-FTIR and

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Py-GC/MS tests indicated that Si(H) microcapsules can efficiently reinforce the flame

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retardancy of EP composites at a low content.

3.6. Flame-retardant mechanism of Si(H)/EP composites. From all the discussed results, we concluded that Si(H)/EP composites with Si(H) microcapsules and coupling agents exerted a phosphorus–nitrogen–silicon synergistic flame-retardant mechanism in Fig. 13.[45] After ignition of the EP composites, the Si(H) microcapsules within the matrix first separated into SiO 2 and HPCTP particles, and the shell and core materials played a part in the flame-retardant process. Then, HPCTP decomposed the phosphazene structure and phenoxyl segments. Subsequently, the

phosphazene

structure

further

decomposed

into

phosphorous-

and

nitrogen-containing fragments. In the gas phase, the former subsequently liberated PO2 • and PO • free radicals that eliminated flammable OH• and H• free radicals, such that combustion opportunity was reduced and a quenching effect was realized. The

Journal Pre-proof latter further decomposed into nonflammable gases, such as NH3 , along with CO 2 , which decreased the concentration of flammable gases and produced a diluting effect. KH-550 molecules with nitrogen elements also played a part in the diluting effect. The quenching and the diluting effects generated considerable FIE for Si(H) microcapsules in EP composites. In the solid phase, the phosphorus-containing pyrolysis products from HPCTP cores and silicon-containing SiO 2 shells and coupling agents took effect. The phosphorus-containing pyrolysis products facilitated the carbonization process by decomposing organic phosphonates that reacted with decomposition products from the EP matrix, thereby facilitating CE. The other two

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f

moved to the surface of the char as compact SiO 2 particles, which played the role of a protective layer and isolated heat, oxygen, and combustible gases from transferring between the inner unburned matrix and the air. Thus, BAPE was also verified. Among

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all the samples, KH-560-modified samples yielded better BAPE. To summarize the

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preceding results, including the quenching and diluting effects of FIE in the gas phase, and CE and the BAPE in the solid phase, we concluded that Si(H)/EP composites with

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coupling agents exhibited effective flame retardancy at a small addition volume.

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3.7. Mechanical performance of Si(H)/EP composites. The mechanical performance of pure EP and Si(H)/EP composites with coupling

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agents was investigated via tensile tests. The results of tensile strength and elongation

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of break are depicted in Fig. 14. As shown in the figure, the fillers exerted different characteristics with the two coupling agents. As the contents were increased from 0.5 wt.% to 1.5 wt.%, the tensile strength values o f Si(H)/EP composites with coupling agents initially increased and then decreased. The large Si(H) capsules resulted in holes and microcracks that further generated concentration sites and secondary triggering sites, thereby providing additional chances for damage.[14, 46] Under the condition of appropriate Si(H) contents, the composites exhibited better mechanical properties because of the reinforcing effect of SiO 2 shells. Moreover, the chemical bonding between SiO 2 and the EP matrix from KH-550 and KH-560 could develop and fully utilize the reinforcing effect of SiO 2 particles.[47] Among all the samples, Si(H)/EP-560 samples were found to have better properties than the others in terms of tensile strength and elongation of break, and 1.0Si(H)/EP-560 composites achieved a maximum of all the specimens. This result was attributed to the structural

Journal Pre-proof characteristic differences between the two coupling agents. However, all the values were observed with a large standard deviation, which implied inevitable individual variation in EP composites. The fracture surfaces of EP composites after tensile tests were studied via SEM in Fig. 15. Pure EP exhibited smooth and river- like lines on its surface (Fig. 15a), thereby indicating a typical brittle fracture. After addition of Si(H) microcapsules, the surfaces of composites became rough with many microcracks and microsteps (Fig. 15b–g). These microcracks and microsteps would lead to a change in propagation directions and absorb considerably more energy in the fracture process to improve

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mechanical performance.[47] Too few or too many fillers can hardly uniformly dispersed in the EP matrix (Fig. 15b, d, e, and g), and thus, agglomeration occurred in the EP matrix that deteriorated the reinforcing effect. Therefore, 1.0Si(H)/EP

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composites with KH-550 or KH-560 yielded more microcracks and microsteps on the

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surface, thereby indicating better toughness than the other Si(H)/EP samples (Fig. 15c and f). Nevertheless, the reinforced effects were nearly invalid compared with the

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properties of pure EP.

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Conclusions

In this work, flame-retardant Si(H) microcapsules with SiO 2 as the shell and HPCTP

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as the core were synthesized and characterized. Si(H)/EP composites with various

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contents were manufactured with coupling agents KH-550 or KH-560 to promote interactions. Si(H)/EP composites reached a LOI value of 30.3% and a V-1 rating in the UL-94 tests. In the cone calorimetric tests, heat and gas releases were both reduced, and fire hazard was minimized. The analysis of residual chars indicated the strong rigidity and mechanical robustness of the char. Raman detections and FTIR tests confirmed the effects of Si(H) microcapsules on enhancing the flame retardancy of EP composites in the solid phase. Conversely, the TGA-FTIR and Py-GC/MS results indicated that Si(H) microcapsules efficiently reinforced the flame retardancy of EP composites in the gas phase analysis. We concluded that the flame-retardant mechanisms included the quenching and diluting effect of the FIE caused by the phosphorous-free radicals and the nonflammable pyrolysis gases in the gas phase. Meanwhile, the CE and

BAPE were ascribed to the formation of the

phosphorus-containing residual char layer and compact SiO 2 layer in the solid phase.

Journal Pre-proof Tensile tests confirmed that Si(H) microcapsules prevented serious damage to the tensile property. This work provides a feasible microencapsulation method for the phosphorus–nitrogen–silicon synergistic effect on the flame-retardant enhancement of EP composites with a low additive content.

Conflicts of interest There are no conflicts to declare.

Data availability statement

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The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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Acknowledgements

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This work was supported by the Natural Science Foundation of Jilin Province, China (No. 20180101197jc), the International Science and Technology Cooperation

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Programme, China (20190701001GH), the National Natural Science Foundation of China (No. 11602125), and Beijing Institute of Technology Research Fund Program

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rn

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for Young Scholars, China.

CRediT author statement 1.

Lijie Qu: Conceptualization, Methodology, Investigation, Writing- Original draft, Writing- Reviewing and Editing, Visualization

2.

Yanlong Sui.: Methodology, Investigation, Writing- Original draft

3.

Chunling

Zhang:

Conceptualization,

Resources,

Investigation,

Writing-

Reviewing and Editing, Visualization, Supervision, Project administration, Funding acquisition

Journal Pre-proof 4.

Xueyan Dai: Validation, Investigation, Supervision

5.

Peihong Li: Validation, Investigation, Visualization

6.

Guoen Sun: Resources, Writing- Reviewing and Editing, Supervision

7.

Baosheng Xu: Methodology, Resources, Writing- Original draft, Supervision, Project administration

8.

Daining Fang: Resources, Supervision, Funding acquisition

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Declarations of interest: none.

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Fig.1 Formation of Si(H) microcapsules and the cross- linking networks of EP

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Fig. 2 FTIR spectra of HPCTP and Si(H) microcapsules.

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composites.

Fig. 3 Morphologies of HPCTP and Si(H) microcapsules: (a) SEM image of HPCTP,

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(b) SEM image of Si(H) microcapsules, (c, d) TEM images of Si(H) microcapsules. Fig. 2 DSC curves of pure EP and Si(H)/EP composites.

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Fig. 5 (a) TGA and (b) DTG curves of pure EP and Si(H)/EP composites. Fig. 6 (a) Heat release rate curves, (b) total heat release curves, and (c) CO production

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curves of pure EP, 1.0 Si(H)/EP-550, and 1.0 Si(H)/EP-560 composites. Fig. 7 Digital photographs and SEM images of the char structures obtained via the

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cone calorimetric tests: (a, d, g) pure EP, (b, e, h) 1.0Si(H)/EP-550, and (c, f, i)

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1.0Si(H)/EP-560 composites.

Fig. 8 Raman spectra of the char residues: (a) pure EP, (b) 1.0Si(H)/EP-550, and (c)

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1.0Si(H)/EP-560 composites; and (d) FTIR spectra of the char residues obtained from the cone calorimetric tests.

Fig. 9 (a) 3D TGA-FTIR spectra and (b) absorbance of the main gas products of Si(H) microcapsules in the TGA-FTIR tests. Fig. 10 FTIR spectra of volatile products of (a) EP, (b) 1.0Si(H)/EP-550 composites, and (c) 1.0Si(H)/EP-560 composites. Fig. 11 Absorbance of the main gas products of EP, 1.0Si(H)/EP-550 composites, and 1.0Si(H)/EP-560 composites in the TGA-FTIR tests: (a) aromatics compounds, (b) aliphatic compounds, (c) ether compounds, and (d) CO 2 . Fig. 12 Simplified mass fragmentations of EP and Si(H)/EP composites.

Fig. 13 Proposed flame-retardant mechanism of Si(H) microcapsules in Si(H)/EP

Journal Pre-proof composites. Fig. 14 Tensile strength and elongation of break of pure EP and Si(H)/EP composites. Fig. 15 Tensile fracture surfaces of (a) pure EP, (b) 0.5Si(H)/EP-550, (c) 1.0Si(H)/EP-550, (d) 1.5Si(H)/EP-550, (e) 0.5Si(H)/EP-560, (f) 1.0Si(H)/EP-560, and

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(g) 1.5Si(H)/EP-560 composites.

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Fig. 3 Formation of Si(H) microcapsules and the cross-linking networks of EP composites.

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Fig. 2 FTIR spectra of HPCTP and Si(H) microcapsules.

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Fig. 3 Morphologies of HPCTP and Si(H) microcapsules: (a) SEM image of HPCTP,

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(b) SEM image of Si(H) microcapsules, (c, d) TEM images of Si(H) microcapsules.

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Fig. 4 DSC curves of pure EP and Si(H)/EP composites.

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Fig. 5 (a) TGA and (b) DTG curves of pure EP and Si(H)/EP composites.

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Fig. 6 (a) Heat release rate curves, (b) total heat release curves, and (c) CO production

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curves of pure EP, 1.0 Si(H)/EP-550, and 1.0 Si(H)/EP-560 composites.

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Fig. 7 Digital photographs and SEM images of the char structures obtained via the cone calorimetric tests: (a, d, g) pure EP, (b, e, h) 1.0Si(H)/EP-550, and (c, f, i)

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1.0Si(H)/EP-560 composites.

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Fig. 8 Raman spectra of the char residues: (a) pure EP, (b) 1.0Si(H)/EP-550, and (c) 1.0Si(H)/EP-560 composites; and (d) FTIR spectra of the char residues obtained from

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the cone calorimetric tests.

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Fig. 9 (a) 3D TGA-FTIR spectra and (b) absorbance of the main gas products of Si(H)

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microcapsules in the TGA-FTIR tests.

Journal Pre-proof Fig. 10 FTIR spectra of volatile products of (a) EP, (b) 1.0Si(H)/EP-550 composites,

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and (c) 1.0Si(H)/EP-560 composites.

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Fig. 11 Absorbance of the main gas products of EP, 1.0Si(H)/EP-550 composites, and

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1.0Si(H)/EP-560 composites in the TGA-FTIR tests: (a) aromatics compounds, (b)

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aliphatic compounds, (c) ether compounds, and (d) CO 2 .

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Fig. 12 Simplified mass fragmentations of EP and Si(H)/EP composites.

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Fig. 13 Proposed flame-retardant mechanism of Si(H) microcapsules in Si(H)/EP composites.

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Fig. 14 Tensile strength and elongation of break of pure EP and Si(H)/EP composites.

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Fig. 15 Tensile fracture surfaces of (a) pure EP, (b) 0.5Si(H)/EP-550, (c) 1.0Si(H)/EP-550, (d) 1.5Si(H)/EP-550, (e) 0.5Si(H)/EP-560, (f) 1.0Si(H)/EP-560, and (g) 1.5Si(H)/EP-560 composites.

Table captions:

Journal Pre-proof Table 1 Formulations of EP composites. Table 1 Thermal parameters from the DSC, TGA, and DTG analyses. Table 3 Flame retardancy parameters obtained from the LOI and UL-94 tests. Table 4 Cone calorimetry parameters of pure EP, 1.0Si(H)/EP-550, and 1.0Si(H)/EP-560 composites

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Table 5 Major pyrolysis components of EP and Si(H)/EP composites

Table 1 Formulations of EP composites.

Journal Pre-proof EP

DDM

Si(H)

KH-550

KH-560

P

(g)

(g)

(g)

(g)

(g)

(wt.%) (wt.%) (wt.%)

EP

80.00

20.00

-

-

-

-

2.828

-

0.5Si(H)/EP-550

75.82

19.00

0.50

4.73

-

0.040

3.002

0.693

1.0Si(H)/EP-550

75.42

18.85

1.00

4.73

-

0.080

3.001

0.787

1.5Si(H)/EP-550

75.02

18.75

1.50

4.73

-

0.121

3.005

0.880

0.5Si(H)/EP-560

75.35

18.83

0.50

-

5.33

0.040

2.681

0.726

1.0Si(H)/EP-560

74.74

18.73

1.00

-

5.33

0.080

2.685

0.819

1.5Si(H)/EP-560

73.60

18.40

1.50

-

5.33

0.121

2.656

0.913

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Samples

Table 2 Thermal parameters from the DSC, TGA, and DTG analyses.

N

Si

Journal Pre-proof Tg

Samples

(°C)

T5 wt% (°C)

Tmax

DWmax

Residue

(°C)

(%/min)

at 800 °C (wt%)

165.3

374.5

386.5

-1.97

17.3

0.5Si(H)/EP-550

155.2

368.1

388.7

-1.56

23.2

1.0Si(H)/EP-550

152.0

370.1

390.8

-1.67

23.3

1.5Si(H)/EP-550

155.6

371.0

390.8

-1.43

25.4

0.5Si(H)/EP-560

145.6

357.0

396.8

-1.59

19.0

1.0Si(H)/EP-560

143.6

364.1

394.8

-1.52

21.1

1.5Si(H)/EP-560

142.2

367.5

390.6

-1.44

25.8

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EP

Samples

LOI (%)

UL-94 (3mm)

Dripping

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25.7±0.5

No rating

Yes

0.5Si(H)/EP-550

28.3±0.4

No rating

No

1.0Si(H)/EP-550

29.4±0.5

No rating

No

1.5Si(H)/EP-550

30.3±0.4

V-1

No

0.5Si(H)/EP-560

26.6±0.3

No rating

No

1.0Si(H)/EP-560

27.6±0.5

No rating

No

1.5Si(H)/EP-560

28.7±0.4

V-1

No

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Table 3 Flame retardancy parameters obtained from the LOI and UL-94 tests.

Table 4 Cone calorimetry parameters of pure EP, 1.0Si(H)/EP-550, and

Journal Pre-proof 1.0Si(H)/EP-560 composites TTI Samples

T PHRR

PHRR

THR 2

2

av-EHC

M ass loss

FIE

CE

BAPE

FPI FIGRA

(s)

(kW/m )

(M J/m )

(M J/kg)

(wt.%)

(%)

(%)

(%)

(10-2)

EP

40±1

135±1

1677.9±150.5

148.0±3.5

33.7±1.2

93.3±0.2

-

-

-

2.4

12.4

1.0Si(H)/EP-550

43±2

120±2

1101.6±85.1

125.3±2.7

30.2±0.6

87.0±0.1

10.4

6.7

22.3

3.9

9.2

1.0Si(H)/EP-560

43±2

125±2

972.9±42.3

122.9±2.4

31.1±0.4

88.5±0.1

7.7

5.1

30.2

4.4

7.8

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(s)

Table 5 Major pyrolysis components of EP and Si(H)/EP composites

Journal Pre-proof EP No. m/z

Assigned

Retention

formula

time

Relative area (%)

1.0Si(H)/EP-560

Retention

Retention

time

Relative area (%)

(min)

time (min)

Relative area (%)

44

CO2

1.27

3.15

1.84

2.99

1.78

2.91

b

44

C2 H 4 O

1.76

2.14

1.89

4.45

1.84

4.66

c

58

C3 H 6 O

1.85

3.11

2.01

2.60

1.96

1.97

d

94

C6 H 6 O

6.59

11.57

7.80

7.3

7.79

3.39

e

108

C7 H 8 O

7.84

4.57

9.39

2.11

9.40

0.89

f

121

C8 H11 N

8.46

1.81

9.59

0.91

9.59

0.52

g

135

C9 H13 N

9.92

1.64

11.04

1.00

11.04

0.56

h

136

C9 H12 O

10.60

7.52

11.73

5.10

11.72

2.52

i

134

C9 H10 O

11.70

12.31

12.83

9.45

12.83

4.95

j

212

C15 H16 O

18.38

15.80

19.52

12.82

19.51

10.68

k

211

C15 H17 N

18.59

3.62

19.71

3.12

19.71

1.63

l

226

C16 H18 O

19.69

7.12

20.48

6.84

20.47

4.26

m

256

C17 H20 O2

21.69

1.75

21.48

0.76

n

254

C17 H22 N2

22.84

21.49

23.96

17.93

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Highlights:

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Graphical abstract:

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1.0Si(H)/EP-550

Flame-retardant EP composites were fabricated based on Si(H) microcapsules.



Only a low additive content realized an obvious improvement of flame retardancy.



Phosphorus–nitrogen–silicon synergistic flame-retardant effect was realized.



The flame-retardant mechanism included four effects in the gas and solid phases.

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