Solidifying process and flame retardancy of epoxy resin cured with boron-containing phenolic resin

Solidifying process and flame retardancy of epoxy resin cured with boron-containing phenolic resin

Accepted Manuscript Title: Solidifying Process and Flame Retardancy of Epoxy Resin Cured with Boron-containing Phenolic Resin Authors: Peng Deng, Yan ...

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Accepted Manuscript Title: Solidifying Process and Flame Retardancy of Epoxy Resin Cured with Boron-containing Phenolic Resin Authors: Peng Deng, Yan Shi, Yuansen Liu, Yuan Liu, Qi Wang PII: DOI: Reference:

S0169-4332(17)32278-X http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.278 APSUSC 36812

To appear in:

APSUSC

Received date: Revised date: Accepted date:

26-12-2016 10-7-2017 28-7-2017

Please cite this article as: Peng Deng, Yan Shi, Yuansen Liu, Yuan Liu, Qi Wang, Solidifying Process and Flame Retardancy of Epoxy Resin Cured with Boron-containing Phenolic Resin, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.278 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Solidifying Process and Flame Retardancy of Epoxy Resin Cured with Boron-containing Phenolic Resin

Peng Deng1, Yan Shi2, Yuansen Liu3, Yuan Liua1, Qi Wang1

1

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute

of Sichuan University, Chengdu 610065, China 2

Sichuan Provincial Key Lab of Process Equipment and Control, Sichuan University

of Science & Engineering, Zigong, 643000, China 3

Engineering Research Centre of Marine Biological Resource Comprehensive

Utilization, Third Institute of Oceanography, State Oceanic Administration

aCorresponding

author State Key Laboratory of Polymer Materials Engineering Sichuan University, Chengdu Sichuan, 610065, RP China. Tel: +86 028 85463909

E-mail address: [email protected]

Highlights

 Boron-containing phenolic resin as a flame retardant curing agent applied to the epoxy resin curing system.  Boron-containing phenolic resin showed much higher curing reactivity with epoxy group compared with conventional phenolic resin.  Compared with the phenolic resin cured epoxy resin, the curing

 system of boron-containing phenolic resin has higher glass transition temperature and lower heat release.

Abstract For the sake of improving the charring performance and flame retardancy of epoxy resin (EP), boron-containing phenolic resin (BPR) instead of a conventional curing agent, linear phenolic resin (LPR) was employed to cure EP. Of several possible chemical structures for BPR, the existence of benzyl hydroxy groups in BPR chains has been confirmed using 1H nuclear magnetic resonance spectroscopy. The resonance of these groups may reasonably explain the higher curing reactivity of BPR-cured EP than that of LPR-cured EP. Thermogravimetric analysis, observation of the morphologies of the char residues and X-ray photoelectron spectroscopic were performed to characterize the charring process. Due to the presence of B2O3 produced on the char surface from decomposition of phenyl borates and the facile high self-crosslinking reaction of BPR, a more continuous and stronger char barrier was formed for BPR-cured EP compared to that for the LPR-cured EP system. Therefore the former exhibited much better flame retardancy. In addition, BPR-cured EP also displayed better dynamic mechanical properties, than those observed for LPR-cured EP. It is not subject to the significant lowering the glass transition temperature of the polymer which accompanies curing with LPR. This suggests that BPR cured resin may meet the requirement for utilization at high temperature.

Key words: linear phenolic resin; boron-containing phenolic resin; flame-retardant; curing agent

INTRODUCTION Epoxy resin (EP) is the most widely used thermoset polymer, owing to its high strength and modulus, excellent electrical insulation, high adhesion, low shrinkage during cure and satisfactory corrosion resistance [1-5]. However, with a limiting oxygen index (LOI) of only 19.8 %, flammability is an obvious drawback for common EP, e.g., bisphenol A-EP, which excludes it from use in electronic and electrical applications [6]. Hence, endowing EP with flame retardance is essential for extending the breadth of application. Generally, there are two methods that may be used to obtain flame retardant EP: physically adding some flame retardants into EP, and chemically combining certain flame retardant elements with the EP macromolecular chains. In comparison, the latter with intrinsic flame retardant properties displays remarkable advantages in maintaining uniform dispersion of flame retardants, as well as improving the compatibility of the materials. Phosphorus-containing flame retardants have been preferably used in EP due to efficient catalysis of char formation and the suppression of the release of toxic or corrosive smoke during combustion is compared with traditional halogen flame retardants [7-9]. At present, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is the most representative reactive flame retardant commercially applied in EP [10-13]. Limitations of this reactive flame retardant are its high cost and increased

brittleness of EP caused by the rigid cyclic phosphaphenanthrene moiety [14]. In recent years, special curing agents have been adopted to modify EP. Taking advantage of the chemical activity of the epoxy group, may compounds with reactive groups including NH2-, -CONH-, and Ar-OH, etc., can serve as curing agents of EP. By designing and introducing some functional structure and elements, a curing agent may generated which can endow EP with special performance characteristics. It may be that developing novel curing agents is more important than preparing new kinds of EP. Curing agents with flame retardant properties have gained more and more attention because they can simultaneously promote curing and import flame retardancy. With a much lower loading level, the cost can be greatly decreased and performance of the EP can be well maintained, compared with conventional curing agents and flame retardants added respectively. Xie [15] synthesized a new reactive curing agent (DOPO-phenylimino-4-hydroxyphenyl-methane) which could effectively solidify EP, and meanwhile efficiently improved the flame retardance of the resin. A new type of flame-retarding curing agent for EP based on ammonium polyphosphate has been prepared using cation exchange with diethylenetriamine was designed and prepared by Tan [16]. Good flame retardance of the cured system was reflected by results from the vertical burning and LOI tests. Certainly, the presence of these aids probably leads to some negative effects on EP, such as enhanced brittleness and moisture absorption. In addition, the relatively high costs of these materials also restrict their commercial application to some extent. As a low cost macromolecular curing agent for EP [17], linear phenolic resin

(LPR) not only provides favorable adhesion but also satisfactory heat and water resistance. LPR cured EP show the great virtues in the fields of manufacturing adhesives, coatings, pouring and laminated composite materials [18]. Moreover, phenolic resins also possess certain self-carbonization capacity due to the condensation of the phenolic hydroxyl at high temperature, hence usually served as a precursor of carbon materials [19]. In fact, as the curing agent, LPR can increase the charring amount of EP, which is beneficial to improve the flame retardance in the condensed phase to a certain degree. However, for common LPR, the macromolecule chains contain a great number of ether bonds (-O-) and methylenes (-CH2-), which tends to decompose and release the corresponding hydrocarbon volatiles at elevated temperature, thus decreasing the thermal stability of the char [20]. For this reason, the actual contribution of LPR to improve the flame retardance of EP is limited. In comparison, boron-containing phenolic resin (BPR) obtained by introducing a certain amount of B into phenolic resin [21-23], can make up the above disadvantages of LPR. It is generally believed that the formation of borate linkage decreases the possibility of forming ether linkage. Due to much higher B-O bond energy compared with C-O bond, BPR displays better thermal stability at high temperature. On most occasions, as an independent thermoset resin through a self-crosslinking reaction to form 3-dimentional network, BPR has been successfully applied in structural materials including rockets, missiles, aircrafts, etc. Furthermore, BPR is usually used as a fire retardant coating additive, which can effectively improve the fire resistant time and reduce the toxic gas emissions of the coatings. Although the applications of

BPR are rapidly growing, few researches concerning BPR used as a flame-retarding curing agent for EP, have been reported [22, 24]. In the present paper, the corresponding curing process and mechanisms of BPR-cured EP, were systematically studied. Meanwhile, the thermal stability, charring performance, flame retardance and dynamic mechanical properties of BPR-cured EP were evaluated and compared with LPR-cured EP. 2. Experimental 2.1 Materials EP (diglycidyl ether of bisphenol-A type, epoxy equivalent weight: 489 g per eq.) was purchased from Huntsman Advanced Materials Corporation Limited, and phenolic novolac resin hardener (hydroxyl equivalent weight: 105 g per eq.) was supplied by Momentive Chemical Corporation Limited. Boron-modified phenolic resin (hydroxyl equivalent weight: 103 g per eq.) was purchased from Shanghai Tianyue Chemical Corporation Limited. 2-ethyl-4-methylimidazole (EMI) was purchased from Tianjin Weiyi chemical technology Corporation Limited. Dimethyl sulphoxide (DMSO-d6) was purchased from Shanghai Hansi Chemical Corporation Limited. 2.2 Sample preparation The calculated amount of the curing agent (LPR or BPR) and EP were mixed and stirred in a pot to form the homogeneous resin mixture. The resin was degassed in a degassing chamber under 100 kPa vacuum pressure and at room temperature until no small bubbles rose up to the surface (a proper amount of the accelerator EMI was added and stirred evenly for some systems). The obtained resin mixture was poured

into the mold slowly and heated in an oven at 110 °C for 1h, 150 °C for 1h and 180 °C for 3h. Taking out the mold from the oven until naturally cooling to room temperature, the cured EP bars were taken out from the mold. The specimens were obtained by machining cutting according to the standard size of the following tests. 2.3 Characterization and measurements Determination of gelation time of the resin by gelation disk experiment: pouring 1g resin mixture into the flat plate at a set temperature. The time of the conversion from the original flow state to the gel without flowability was recorded. DSC analyses of the curing processes were carried out in a TA Q20 DSC analyzer. The measurements were conducted with 6∼10 mg curing samples in the sample pans. The measurement method included: non-isothermal (programmed temperature elevation from room temperature to 320 °C with a heating rate of 5 °C/min) and isothermal DSC (at 180 °C for 15 min). 1H NMR spectra of LPR and BPR were recorded with an Avance II-600 MHz NMR spectrometer with dimethyl sulphoxide (DMSO-d6) as a solvent, and tetramethylsilane as an internal standard. The thermal stability and decomposition behavior of LPR, BPR, LPR/EP/EMI and BPR/EP/EMI product was studied using a TA Q50 thermal analyzer with a heating rate of 10 °C/min in the temperature range of 25-800 °C and a nitrogen flow rate of 50 mL/min. A microscale combustion colorimeter (GOVMARK MCC-2) was used to investigate the combustion of the samples. The heat release rate (HRR) and the total heat release (THR) were measured for studying the fire behaviors of the materials. About 5 mg of each sample was heated to 750 °C at a heating rate of 1 °C/s in a stream of nitrogen flowing at 80

cm3/min. X-ray photoelectron spectroscopic (XPS) measurements of the char yield of BPR-cured EP were performed by using a British XSAM800 Kratos multi-functional surface analysis spectrometer (Al Kα radiation, hν = 1486.6eV). The base vacuum of this system was 2×10-7 Pa and the binding energies were calibrated by using the containment carbon (C1s =284.8 eV). The morphologies of the char yields of LPR cured-EP and BPR cured-EP were observed using a FEI-INSPECT F scanning electron microscope with conductive gold coating and an acceleration voltage of 20 kV. The glass transition temperature of LPR-cured EP and BPR-cured EP using was measured by a dynamic mechanical thermal analyzer (TA Q800). The specimens with 30×10×4 mm3 were tested in a three-point-bending configuration (1.0Hz, 5 °C/min, 25-250 °C). Glass transition temperature of each sample was calculated according to the curves of the storage modulus, loss modulus and loss tangent with the increasing temperature. The LOI was measured by a Dynisco LOI instrument, with the bar dimensions of 140.0×3.0×1.6 mm3, according to ASTM D2863-97. 3. Results and discussion 3.1 Analysis of the curing actions and mechanisms LPR and BPR have close hydroxyl equivalent weight values but different chemical structure, but their curing actions on EPs are very different. Accordingly, revealing the detailed curing mechanisms is of great significance. The gelation time test is a convenient method to evaluate the curing rate, because the curing system loses its flow ability after gelation. The time corresponds to the irreversible transform from soluble resin into insoluble and infusible three-dimensional solid

network. The gelation tests of LPR-cured EP (LPR/EP) and BPR-cured EP (BPR/EP) were firstly conducted in the absence of the accelerator (EMI). It was found that the wiredrawing phenomenon was observed in only 3min, and complete solidification occurred in 5 min at 150 °C for BPR/EP system (Fig.1-a). In contrast, LPR/EP glue still remained the original fluid state even after 2h at higher curing temperature of 200 °C (Fig.1-b). In the case of adding the accelerator, the curing reaction between LPR and EP was greatly accelerated and the corresponding gelation occurred. With the increasing temperature, the gelation time of both the systems decreased (Fig. 2). Overall, in the temperature range of the test, the gelation time of BPR/EP/EMI was much shorter than that of LPR/EP/EMI at the same curing temperature. Obviously, the above results reflected the curing rate of the former was much higher. Furthermore, a simple calculation of the activation energy for the curing reaction can be performed from the gelation time data at different temperatures. According to Arrhenius model [25], the relation between the gelation time (tgel) and the activation energy can be given by the following equation: Ea ㏑(tgel) = C + RT where tgel is the gelation time, C is a constant, Ea is an apparent activation energy, R is the gas constant and T is the absolute temperature. Based on the equation, the plots of the natural logarithm of the gelation time (tgel) at different temperature versus 1000/T were obtained by linear fit as shown in Fig. 3. From the calculated slope value (Ea/R), the apparent activation energy of the two

systems was determined. Lower activation energy (57.7 kJ/mol) of BPR/EP/EMI indicated higher curing reaction activity than that (67.1 kJ/mol) of LPR/EP/EMI. As the solidifications of most thermoset resins belong to typical exothermic reactions, DSC can sensitively detect the heat generated by the curing actions to investigate the reaction process. Here, two DSC measurement means including non-isothermal and isothermal (Fig. 4) were employed to investigate the curing processes. In the absence of EMI, there was no exothermic peak observed for LPR/EP in the programmed temperature-rise (non-isothermal) curve, however, two obvious curing exothermic peaks for BPR/EP appeared at about 130 °C assigned to the exothermic peak of BPR-cured EP and 220 °C assigned to self-crosslinking reaction of BPR (Fig. 4-a). The isothermal DSC curves also reflected similar results: an exothermic peak was observed in only 3 min for BPR/EP but none appeared in the curves of LPR/EP. The above results showed that almost no noticeable curing reaction occurred for the LPR/EP system, but quick curing action for the BPR/EP system in the absence of EMI. With the accelerator involved, the reaction between LPR and EP was greatly promoted. As shown in Fig. 4-b, the exothermic peaks of LPR/EP/EMI were seen at 160 °C in the non-isothermal curve and at 3 min pinot-in-time in the isothermal one, respectively. By contrast, the exothermic peaks of BPR/EP/EMI appeared at lower temperature and shorter point-in-time, showing remarkably higher curing reaction activity than the former. Additionally, the bigger area of the exothermic peaks of LPR/EP/EMI compared to that of BPR/EP/EMI demonstrated that the former released

more curing heat. Higher curing reaction rate of BPR on EP should result from the different chemical structure, particularly different types of hydroxyl groups participated in the curing reactions. However, the fine structure of BPR is still disputed up to now. There are several chemical structure (a [26], b [27] and c [28]) provided according to the previous references (Fig. 5). In order to determine the real structure, 1H NMR analysis of LPR and BPR was performed. In Fig. 6, the multiple chemical shifts at 6.63-6.98 and 3.63-3.78 ppm referred to the aromatic protons (Ar-H) and the methylene bridges combined with the benzene ring. Both the resonances at 9.09-9.14 ppm in (a) and 9.11 ppm in (b) were assigned to the hydroxyl protons. Compared to the spectrum of LPR, the most marked difference lied in the resonances at 4.62 ppm assigned to the methylene groups of the benzyl alcohol for BPR, thus confirming the existence of Ar-CH2-OH structure. According to the 1H-NMR analysis results, the most possible structure of BPR is shown in Fig. 6(b). The benzyl hydroxyl in BPR can also reasonably explain its higher reactivity with EP compared with LPR-cured EP. As both the phenolic hydroxyl-epoxy group and the hydroxymethyl-epoxy group reactions belong to nucleophilic addition. According to the theory of organic chemistry, the stronger the nucleophilicity of the hydroxyl is, the faster the reaction is. The phenolic hydroxyl of LPR has the same nucleophilic atom (O) with the hydroxymethyl of BPR, and their nucleophilicity is in accordance with the alkalinity of the corresponding anions after losing H. For the phenolic hydroxyl

groups in LPR, the electron cloud disperses throughout the benzene ring because of the P-π conjugated effect between the oxygen atom and the benzene ring. Thereby, it weakens the combining power of the O-H bond. This makes the hydrogen ion easy to separate from the phenoxy (Ar-O). As a result, the alkalinity of the phenolic hydroxyl group is weakened to some extent. However, for benzyl hydroxyl, there is no such conjugation effect because the hydroxyl does not directly connect with the benzene ring by the interval of the methylene. In contrast, the benzyl served as an electron-donating group, further increasing the alkalinity. By this reason, the reactivity of benzyl hydroxyl is much higher than that of phenolic hydroxyl. Hence, BPR can react with epoxy groups more easily [Scheme 1-a], but the reaction between LPR and EP [Scheme 1-b] is much more difficult in the absence of the EMI. Only with help of the accelerator, the latter reaction can be conducted due to the contribution of EMI that promoted the open loop of the epoxy group to greatly decrease the energy barrier [Scheme 1-c]. 3.2 Charring performance of the cured EP TGA was performed to evaluate the charring performance of EP cured with different amounts of LPR and BPR (Fig. 7). Firstly, the self-carbonization performance of LPR and BPR were compared. The incorporation of boron considerably improved the char yield, and the final charring yield of BPR was as high as 66.3 %, much higher than that of LPR (38.9 %). Although LPR can be self-carbonized through the condensation of Ar-OH to a degree, a great number of ether bonds (-O-) and methylenes (-CH2-) easily results in the degradation of LPR

chains at the elevated temperature. For BPR, one hand the formation of the borate linkage decreases the possibility of forming an ether linkage. On the other hand, due to much higher B-O bond energy compared with C-O bond, BPR displayed better thermal stability at high temperatures. As shown in Fig. 7 and Table. 1, neat EP almost completely degraded at 800 °C without residual char. With an increase of LPR and BPR content, the char yield of the cured EP was correspondingly increased. The theoretical char yield (Ct) of curing system was calculated by the additive property. For example, a resin system has a number of components, of which the mass fraction of i component was αi, and then, under the same conditions to test its char yield (Ci) at a certain temperature. In that way: Ct=∑αi Ci. Afterwards, the actual char yield (Ca) of the whole system has been tested by TGA under the same conditions. And then, the calculated char yield from EP was equal to Ca-Ct. From the calculated results, it can be seen that the efficiency of BPR promote the carbonization of EP is much higher than that of LPR. Furthermore, we found that the char yield of the cured EP Vs BPR content possessed a linear relationship but there was no any rule for LPR-cured EP (Fig. 8). Additionally, it was found that the actual charring amount of the cured EP obviously exceeded the self-charring amount from the curing agents that was calculated according to the independent self-charring ratio of the curing resins and their mass fraction in the cured EP. It means that the phenolic resins not only directly converted to char, but also promote EP to form char. Furthermore, BPR indicated much efficiency to increase the char yield of EP compared with LPR. This is probably

related to the introduction of boron that these inorganic boron compounds promote char formation in pyrolysis. 3.3 Flame retardant properties of LPR-cured EP and BPR-cured EP Microscale combustion colorimeter (MCC) is a convenient and quantitative test method that can effectively evaluate the combustion behavior of materials. Fig. 9 showed the curves of the heat release rate (HRR) of LPR, BPR, LPR-cured EP and BPR-cured EP. In EP system cured with phenolic resin, a general content of the curing agent is 25 %, therefore, the systems with 25 % of the curing agent were chosen to evaluate their properties. It can be seen from Table. 2 that the peak of HRR (pHRR) of LPR and total heat release (THR) were 90.2 w/g and 9.6 kJ/g, much higher than 10.0 w/g and 0.9 kJ/g of BPR, indicating that BPR had much smaller contribution to the heat release of combustion. Furthermore, by comparing MCC data of LPR-cured EP and BPR-cured EP, the pHRR and THR of the latter also showed nearly 32 % and 33 % decline. Undoubtedly, the latter exhibited better flame retardance. Apart from the caloric analysis, LOI was also employed to evaluate the flame retardancy. Compared to MCC, LOI can more factually reflect the flame retardant properties in real fire (Table. 3). It can be seen that BPR had higher LOI than LPR, and BPR-cured EP showed obviously higher than LPR-cured EP. Therefore, LOI evaluation results were in accordance with MCC.

Fig. 10 showed the state of LPR-cured EP and BPR-cured EP before and after carbonization. Clearly, the char residue of the former completely broke but the latter still maintained relatively integrate shape, which reflected that the produced char in BPR system possessed better stability and higher mechanical strength. From the micro-morphology through SEM in Fig. 11, it could be seen the char of LPR cured-EP was loose and discontinuous, and the char layer were cracked into pieces. By contrast, the char layer of BPR cured-EP was denser and meanwhile kept better continuousness. Accordingly, the latter had higher barrier property to isolate the heat, oxygen and the flammable volatiles. The variation of the surface elemental compositions of BPR cured EP before and after carbonization was compared by XPS (Fig. 12). Before carbonization, C and O of BPR cured EP were detected but the characteristic absorption of B was not observed. This was attributed to relatively low B element (only 0.3 %) lower than that of the test sensitivity. After carbonized, except decreased C and increased O content, the peak at the bond energy of 192.6 eV assigned to B2O3 was found. On one hand, the relative content of B was increased because the volatilization of the decomposed hydrocarbons, and on the other hand, B2O3 with good fluidity at high temperature tended to migrate to the surface of the char layer, which also increased the surface boron elements. In fact, the flame retardant actions of the boron-containing compounds on polymeric materials involve chemical factors as well as physical ones. At the temperature of 400 °C, the C-O bonds of triphenyl borates broke and the B2O3 was produced. The formation of B2O3 reduced the consumption of oxygen in the

decomposition process, thereby reducing the formation of the volatile hydrocarbons. Otherwise, with good fluidity at 500 °C [22], B2O3 particles can form a non-penetrating glass layer on the surface of the char to prevent the entry of oxygen, and also the incorporation of the glass inorganic boron considerably strengthened the char layer, advantageous to maintain the integrate shape of the condensed barrier. 3.4 Dynamic mechanical properties of LPR-cured EP and BPR-cured EP DMA was used to evaluate the mechanical properties of the above cured EP. The curves of the storage modulus (E′); loss modulus (E″) and loss tangent (tanδ) with the increasing temperature were shown in Fig. 13. BPR-cured EP had higher E′, so it should behave higher stiffness. As glass transition temperature Tg is defined as the lowest temperature of the free movement of the macromolecule chain segment of the amorphous polymer, it reflects the transformation from glass state to a higher elastic state of a polymer. Here, Tg could be obtained by the curves of E′, E″ and tanδ, respectively. From the calculation results (Table. 4), BPR-cured EP had much higher Tg than that of the LPR-cured EP which shows BPR-cured EP system has better heat resistance. BPR not only cures EP at relatively low temperature, but also its self-crosslinking easily occurs at higher temperature, resulting in increased crosslinking density of the cured system. With higher Tg, BPR-cured EP can make up the shortcoming of LPR-cured EP, showing extended application fields, particularly in the hot environments. 4.

Conclusion In summary, BPR was used as a curing agent and charring agent of EP to prepare

the thermoset material with high flame resistance. Compared with conventional curing agent LPR, BPR showed much higher reactivity with epoxy group due to the benzyl hydroxy structure. The curing reaction of the former can occur in the absence of the accelerator but the latter only occurred in the presence of the accelerator. BPR not only had better self-charring ability than LPR, but also more effectively promoted EP resin to produce more char residue. The char yield of 25 % BPR-cured EP at 700 °C was as high as 32.69 % but the same content LPR-cured EP had only 15.33 %. Furthermore, the char morphology of BPR-cured EP was more continuous and denser, but that of LPR-cured EP was cracked into pieces. It was obvious that the former could better isolate heat of combustion, oxygen, and decomposed volatiles, hence more advantageous to improve the flame retardance. MCC showed that the pkHRR and THR of the BPR-cured EP decreased nearly 32 % and 33 % more than LPR-cured EP. Furthermore, compared with LOI of LPR-cured EP (24.6%), BPR-cured EP had higher value (27.1%). Due to the formation of self-crosslinking structure of BPR, start point of E’ (Tg) of BPR-cured EP showed 77.8% higher than those of LPR-cured EP. In comparison, BPR-cured EP is more suitable to use for materials needing better flame retardance and higher stiffness. Acknowledgments The authors acknowledge financial support from the National Natural Science Foundation of China (No. 51473095), the open project of Sichuan Provincial Key Lab of Process Equipment and Control (GK201711), the Program of Innovative Research Team for Young Scientists of Sichuan Province (2016TD0010), Xiamen Southern

Oceanographic Center Funds (14GZP004NF04, 14GQT61HJ31 and 15GZP023NF01), Xiamen Ocean Research and Development Institute (K160101), the Public Science and Technology Research Funds Projects of Ocean (No.201505029).

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Fig. 1 Different states of BPR/EP (a) and LPR/EP (b) systems (without accelerator) during the curing process

Fig. 2 The gelation time versus temperature of LPR/EP/EMI and BPR/EP/EMI

Fig. 3 ㏑(tgel) VS (1000/T) curve of LPR/EP/EMI (a) and BPR/EP/EMI (b) systems

(a) In the absence of the accelerator

(b) In the presence of the accelerator Fig. 4 Comparison of non-isothermal and isothermal DSC curves of the LPR/EP/EMI and BPR/EP/EMI systems

Fig. 5 Possible molecular structure of BPR provided by previous researches

Fig. 6 1H NMR spectra of LPR (a) and BPR (b)

Fig. 7 TGA curves of LPR, BPR, neat EP, LPR-cured EP (a) and BPR-cured EP (b) with different mass fraction

Fig. 8 Relationship between the content of curing agent and the char yield of LPR-cured EP (a) and BPR-cured EP (b)

Fig. 9 MCC curves of LPR, BPR, 25 % LPR cured-EP and 25 %BPR cured-EP

Fig.10 The appearance of LPR cured-EP (a) and BPR cured-EP (b) before and after carbonization

Fig. 11 Micro topography of LPR cured-EP (a) and BPR cured-EP (b) after carbonization by SEM

Fig. 12 XPS spectra of BPR-cured EP before (a) and after (b) carbonization

Fig. 13 Dynamic mechanical analysis spectra of LPR-cured EP and BPR-cured EP: (a) storage modulus, (b) loss modulus and (c) loss tangent

Scheme 1 Curing reaction mechanisms of BPR/EP (a), LPR/EP (b) and opening loop of EP promoted by EMI (c)

Table 1 The char yields of LPR, BPR, neat EP, LPR-cured EP (a) and BPR-cured EP

(b) with different mass fraction Samples

Actual char yields (wt %)

Theoretical char yield

Calculated char yield

(wt %)

from EP (%)

LPR(100 %)

38.90

38.90

--

BPR(100 %)

66.31

66.31

--

LPR(20 %)/EP

13.74

7.92

5.82

BPR(20 %)/EP

28.01

13.41

14.6

LPR(23 %)/EP

13.65

9.09

4.56

BPR(23 %)/EP

30.75

15.39

15.36

LPR(25 %)/EP

15.33

9.86

5.47

BPR(25 %)/EP

32.69

16.71

15.98

LPR(28 %)/EP

15.11

11.02

4.09

BPR(28 %)/EP

35.33

18.70

16.63

LPR(30 %)/EP

16.39

11.80

4.59

BPR(30 %)/EP

36.58

20.02

16.56

Neat EP

0.18

0.18

0.18

Table 2 The pkHRR and THR values of LPR, BPR, 25% LPR-cured EP and 25% BPR-cured EP Sample LPR BPR LPR/EP/EMI BPR/EP/EMI

pHRR (w/g) 90.2 10.0 409.9 281.1

THR (kJ/g) 9.6 0.9 21.4 14.3

Table 3 LOI test results of LPR, BPR, 25 % LPR-cured EP and 25 % BPR-cured EP Sample LPR BPR LPR/EP/EMI BPR/EP/EMI

LOI (%) 47.4 52.6 24.6 27.1

Table 4 Tg calculated by different parameters of DMA The calculation method Start point of E′ Peak of E″ Peak of tanδ

Tg/°C LPR-cured EP 90.2 91.3 122.6

BPR-cured EP 160.4 172.2 185.6