Elsevier

Chemical Engineering Journal
化学工程杂志

Volume 499, 1 November 2024, 156143
第 499 卷，2024 年 11 月 1 日，156143
Chemical Engineering Journal

Multi-functional flame-retardant epoxy resin featuring diverse crosslinking networks
具有多种交联网络的多功能阻燃环氧树脂

工程技术TOPESI学科分类：工程科学IF(5) 13.2EI检索SCI升级版 工程技术1区SCI基础版 工程技术1区IF 13.3
https://doi.org/10.1016/j.cej.2024.156143 Get rights and content 获取权利和内容

Highlights 突出

  • •
    Crosslinkable small molecules additives are designed as multifunctional flame retardant for EP.
    可交联小分子助剂被设计为 EP 的多功能阻燃剂。
  • •
    Multi-crosslinking networks endowing high fire safety without conventional flame-retardant elements.
    多交联网络，无需常规阻燃元素即可实现高防火安全性。
  • •
    Modified EP thermosets revealed excellent comprehensive properties especially in toughness and strength.
    改性极压热固性材料显示出优异的综合性能，尤其是在韧性和强度方面。
  • •
    Simultaneous achievement in transparency, UV shielding and enhanced dielectric properties.
    同时实现透明度、紫外线屏蔽和增强的介电性能。

Abstract 抽象

High-performance epoxy resins (EPs) with ultimate flame retardancy, particularly in terms of smoke suppression (as asphyxiation is the primary cause of death in fires), remain a significant challenge. To address this issue, we propose a feasible multiple crosslinking strategy by incorporating an additive flame retardant (abbreviated as BPPDN) containing self-crosslinking groups to construct multi-crosslinking networks during the curing of EPs. Using this strategy, the BPPDN/EP flame-retardant samples achieved a V-0 rating in the UL-94 test in the absence of conventional flame-retardant elements such as halogens or phosphorus. Among the rest, 15BPPDN/EP had remarkable low fire hazards, with total smoke production and peak heat release rate reduced by 38.7 % and 22.2 %, respectively. Surprisingly, the 15BPPDN/EP material maintained its high transparency, excellent ultraviolet shielding, and dielectric properties (specifically, the dielectric constant decreased from 5.19 to 3.46; the dielectric loss decreased from 0.0066 to 0.0045), as well as its ultrahigh tensile strength (increased by 63 %), flexural strength (by 48 %) and impact strength (by 75 %). This paper investigates in detail the effects of BPPDN on the curing process, thermal properties, mechanical properties, dielectric properties, and flame retardancy of EPs, providing a new direction for the design of highly transparent, multifunctional high-performance flame-retardant thermosetting resins without the need for conventional flame-retardant elements.
具有终极阻燃性的高性能环氧树脂 （EP），特别是在烟雾抑制方面（因为窒息是火灾中死亡的主要原因），仍然是一个重大挑战。为了解决这个问题，我们提出了一种可行的多重交联策略，通过加入含有自交联基团的添加阻燃剂（简称 BPPDN）来构建 EPs 固化过程中的多重交联网络。使用这种策略，BPPDN/EP 阻燃样品在没有卤素或磷等传统阻燃元素的情况下，在 UL-94 测试中获得了 V-0 等级。其中，15BPPDN/EP 的火灾隐患显著降低，总烟雾产生量和峰值热释放率分别降低了 38.7 % 和 22.2 %。令人惊讶的是，15BPPDN/EP 材料保持了其高透明度、优异的紫外线屏蔽和介电性能（具体来说，介电常数从 5.19 降低到 3.46;介电损耗从 0.0066 降低到 0.0045），以及超高的拉伸强度（增加 63%）、弯曲强度（增加 48%）和冲击强度（增加 75%）。本文详细研究了 BPPDN 对 EPs 的固化过程、热性能、机械性能、介电性能和阻燃性的影响，为设计无需常规阻燃元件的高透明、多功能高性能阻燃热固性树脂提供了新的方向。

Keywords 关键字

Epoxy resins
Flame retardancy
Multi-crosslinking
Smoke suppression
Comprehensive properties

环氧树脂
阻燃性
多重交联
抑烟
性能全面

1. Introduction 1. 引言

Epoxy resins (EPs) are a class of polymers in which more than two epoxy groups are present in the molecule; these polymers crosslink through curing agents (usually di-/poly-amines or anhydrides) to form a three-dimensional crosslinking network with good physicochemical properties and are widely applied in coatings and adhesives, electrical and electronic appliances, and composite materials [1], [2], [3], [4]. However, when applied to cutting-edge fields such as aviation, 5G communications, and electronic packaging, EPs are unable to meet the combined requirements of high fire safety particularly low smoke release, as well as service properties such as mechanical and dielectric properties, optical transparency, etc. The most crucial aspect is to reduce the smoke production at the fire scene, which is the primary culprit of fatalities in fires. Smoke is a complex mixture of respirable particles and other substances absorbed therein, which are produced during combustion [5], [6]. The primary hazards associated with smoke in fire disasters include: reduced visibility, asphyxiation, respiratory irritation and burn, and poisoning [7], [8]. To solve the abovementioned problems, the conventional method involves incorporating flame retardants (FRs) into the EP matrix. For ecological and environmental reasons, phosphorus-containing FRs have been considered the best alternatives to halogenated ones [9], [10], [11]. However, an increasing number of studies have reported the persistent negative ecological and environmental impacts of phosphorus-containing flame retardants, especially organic phosphorus flame retardants. It is worrisome that exposure to these residues and some of their raw materials poses significant risks, including biological toxicity, bioaccumulation, persistence, migration, endocrine disruption, and carcinogenicity [12].
环氧树脂 （EP） 是一类聚合物，其中分子中存在两个以上的环氧基团;这些聚合物通过固化剂（通常是二胺/多胺或酸酐）交联，形成具有良好物理化学性能的三维交联网络，广泛应用于涂料和胶粘剂、电气和电子电器以及复合材料[1]、[2]、[3]、[4].然而，当应用于航空、5G 通信和电子封装等前沿领域时，EP 无法满足高消防安全性（尤其是低烟雾释放）的综合要求，以及机械和介电性能、光学透明度等服务性能。最关键的方面是减少火灾现场的烟雾产生，这是火灾死亡的罪魁祸首。烟雾是可吸入颗粒和其中吸收的其他物质的复杂混合物，它们在燃烧过程中产生 [5]、[6]。火灾中与烟雾相关的主要危害包括：能见度降低、窒息、呼吸道刺激和烧伤以及中毒 [7]、[8]。为了解决上述问题，传统方法涉及将阻燃剂 （FR） 掺入 EP 基体中。出于生态和环境原因，含磷阻燃剂被认为是卤化阻燃剂的最佳替代品 [9]、[10]、[11]。 然而，越来越多的研究报道了含磷阻燃剂，尤其是有机磷阻燃剂对生态和环境的持续负面影响。令人担忧的是，暴露于这些残留物及其一些原材料会带来重大风险，包括生物毒性、生物蓄积、持久性、迁移、内分泌干扰和致癌性 [12]。
New ideas and theoretical guidance are needed to seek environmentally friendly alternatives to existing flame retardants from the source. In the last decade, self-crosslinking without conventional flame-retardant elements such as phosphorus and halogens has been established, and well provided a feasible and viable strategy for addressing the flammability and smoke release in thermoplastics such as polyethylene terephthalate in our group [13], [14], [15], [16], [17], [18]. However, to date, this strategy has rarely been reported or proven to be equally effective in thermosets (such as EP) [19]. It was also reported that highly conjugated Schiff-base groups increased the thermal-oxidative stabilities of EPs due to crosslinking between the imines (cyclization reactions) [20], [21], [22]; while bisphthalonitrile groups enabled to self-crosslink at high temperatures to form triazine ring or phthalocyanine to build stable charring layers [23], [24], [25]. Owing to the two types of crosslinking reactions, the reactions between Schiff base (–CHdouble bondN-) and cyano group (−Ctriple bondN), an ortho-positioned imine and cyano-containing monomer named dimethyl-5-[(2-cyanoben-zylidene)amino]isophthalate (CBAA) has been designed, synthesized, and successfully copolymerized into polyethylene terephthalate (PET) chains, which played important roles dominantly in the condensed phase via the process of cyclization to aromatization then carbonization, endowing PETs with excellent flame retardancy [26]. Tremendous efforts have been used to accelerate the cyclization of bisphthalonitrile groups [27], [28]. Xiao [29] performed experiments and simulations to reach a conclusion that polymers containing phenolphthalein structures had high char yields and enhance fire safety due to thermal rearrangement then crosslinking. All of the above findings provided evidence the crosslinking strategies to achieve good flame retardancy in polymers, particularly smoke suppression. However, there are almost no reports discussing the introduction of an additive, a crosslinkable small-molecule flame retardant without conventional flame-retardant elements, can simultaneously enhance flame retardancy, specifically smoke suppression, and mechanical properties, especially toughness of EPs.
需要新的思想和理论指导，从源头上寻找现有阻燃剂的环保替代品。在过去的十年中，已经建立了不含磷和卤素等常规阻燃元素的自交联，并为解决我们小组的聚对苯二甲酸乙二醇酯等热塑性塑料中的易燃性和烟雾释放问题提供了可行的策略 [13]， [14]， [15]， [16]， [17]， [18]。然而，迄今为止，这种策略很少被报道或证明在热固性塑料（如 EP）中同样有效 [19]。还报道，由于亚胺之间的交联（环化反应），高度共轭的希夫碱基提高了 EP 的热氧化稳定性 [20]、[21]、[22];而双邻苯腈基团能够在高温下自交联形成三嗪环或酞菁以形成稳定的炭化层 [23]、[24]、[25]。 由于两种类型的交联反应，即席夫碱 （–CH double bond N-） 和氰基 （-C triple bond N） 之间的反应，一种称为二甲基-5-[（2-氰苯-氮基）氨基]间苯二甲酸酯 （CBAA） 的邻位定位亚胺和含氰单体已被设计、合成并成功共聚成聚对苯二甲酸乙二醇酯 （PET） 链，其在缩合相中通过环化到芳构化的过程发挥了重要作用碳化，使PET具有优异的阻燃性[26]。人们已经付出了巨大的努力来加速双酈基团的环化[27]、[28]。Xiao [29] 进行了实验和模拟，得出结论，含有酚酞结构的聚合物具有较高的 Char 产率，并且由于热重排然后交联而提高了防火安全性。上述所有发现都为在聚合物中实现良好阻燃性，特别是烟雾抑制的交联策略提供了证据。然而，几乎没有报道讨论引入添加剂，一种不含常规阻燃元素的可交联小分子阻燃剂，可以同时增强阻燃性，特别是烟雾抑制和机械性能，尤其是 EP 的韧性。
Herein, we propose a convenient and feasible strategy to introduce additive small molecules (BPPDN) into EPs. The presence of the phenolphthalein structure promotes the cyclization of the bisphthalonitrile groups, which participate in the curing of the resin together with the phenol group [30], [31]. On the one hand, BPPDN is expected to quickly produce non-flammable gas to quench combustible free radicals; on the other hand, after crosslinking, the remaining part participates in carbonization and then forms a protective charring layer. The flame-retardant activities, whether in the gas phase or in the condensed phase, jointly prevent the transfer of heat, oxygen, and fuel, as well as smoke release. Utilizing crosslinking with the resin matrix as well as self-crosslinking between the additive BPPDN, BPPDN/EP attained a closer semi-interpenetrating network inner the EP matrix by the various crosslinking reactions. Thermal stability, mechanical properties, flame retardancy, dielectric properties, transparency and UV shielding of EPs were studied in detail. This work supplied a promising strategy for solving the dilemma of flame-retardant EP for applications in demanding areas.
在此，我们提出了一种方便可行的策略，将加性小分子 （BPPDN） 引入 EP 中。酚酞结构的存在促进了双邻苯二腈基团的环化，双邻苯二腈基团与苯酚基团一起参与树脂的固化[30]、[31]。一方面，BPPDN 有望快速产生不易燃气体来淬灭可燃自由基;另一方面，交联后，剩余部分参与碳化，然后形成保护性炭化层。无论是在气相还是冷凝相中，阻燃活性共同阻止了热量、氧气和燃料的传递以及烟雾的释放。利用与树脂基体的交联以及添加剂 BPPDN 之间的自交联，BPPDN/EP 通过各种交联反应在 EP 基体内部获得了更紧密的半互穿网络。详细研究了 EPs 的热稳定性、机械性能、阻燃性、介电性能、透明度和紫外线屏蔽性。这项工作为解决阻燃 EP 在要求苛刻的领域中的困境提供了一种有前途的策略。

2. Experimental section 2. 实验部分

2.1. Synthesis of 4-(3-aminophenoxy) phthalonitrile (3-APN)
2.1. 4-（3-氨基苯氧基）邻苯二甲腈 （3-APN） 的合成

As shown in Scheme 1(a), 3-aminophenol (12.01 g, 110 mmol) and 4-nitrophthalonitrile (17.31 g, 100 mmol) were reacted with K2CO3 (30.42 g, 165 mmol) in DMF (300 mL) at room temperature for 24 h. Afterward, the mixture was poured into a NaOH solution (0.1 M). The obtained precipitate was filtered and washed with deionized water three times. Finally, purified 3-APN was obtained after sufficient removal of the solvent at 80 °C in a vacuum oven (yield: 91 %). Fig. S1(a) shows the 1H NMR spectrum of 3-APN.
如方案 1（a） 所示，将 3-氨基苯酚（12.01 g，110 mmol）和 4-硝基邻苯二腈（17.31 g，100 mmol）与 K2CO3（30.42 g，165 mmol）在 DMF （300 mL） 中在室温下反应 24 小时。然后将混合物倒入 NaOH 溶液 （0.1 M） 中。所得沉淀过滤，用去离子水洗涤 3 次。最后，在真空烘箱中于 80 °C 充分去除溶剂后，获得纯化的 3-APN（产量：91 %）。图 S1（a） 显示了 3-APN 的 1H NMR 谱图。
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    下载： 下载高分辨率图像 （166KB）
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Scheme 1. Synthesis routes for (a) 3-APN, (b) BPP-D, and (c) BPPDN.
方案 1.（a） 3-APN、（b） BPP-D 和 （c） BPPDN 的合成路线。

2.2. Synthesis of 5,5′-(3-oxo-1,3-dihydroisobenzofuran-1,1-diyl) bis(2-hydroxybenzaldehyde) (BPP-D)
2.2. 5,5′-（3-氧代-1,3-二氢异苯并呋喃-1,1-二基）双（2-羟基苯甲醛）的合成 （BPP-D）

According to the Duff reaction [32], [33], [34], BPP-D was synthesized as illustrated in Scheme 1(b). BPP (15.00 g, 47 mmol) and CF3COOH (500 mL) were added to the flask, and then, hexamethylenetetramine (56.08 g, 400 mmol) was slowly added to a 1000 mL three-necked glass flask under stirring. Under the protection of nitrogen, the reaction proceeded fully at 90 °C for 17 h. The mixture was cooled slowly to room temperature and poured into HCl (3 M, 1000 mL) for 24 h of stirring, after which the mixture was extracted with DCM. The impurities were removed by washing with NaHCO3, deionized water and NaCl solution. The organic layer was dried over Na2SO4, and the solvent was removed using a rotary evaporator. Finally, a yellow BPP-D solid was obtained after drying (yield: 80 %). Fig. S1(b) shows the 1H NMR spectrum of BPP-D.
根据 Duff 反应 [32]、[33]、[34]，BPP-D 的合成如方案 1（b） 所示。向培养瓶中加入 BPP（15.00 g，47 mmol）和 CF3COOH （500 mL），然后在搅拌下将己亚甲基四胺（56.08 g，400 mmol）缓慢加入到 1000 mL 三口玻璃瓶中。在氮气保护下，反应在 90 °C 下充分进行 17 h。将混合物缓慢冷却至室温，倒入 HCl（3 M，1000 mL）中搅拌 24 小时，然后用 DCM 提取混合物。用 NaHCO3、去离子水和 NaCl 溶液洗涤除去杂质。在 Na2SO4 上干燥有机层，并使用旋转蒸发器去除溶剂。最后，干燥后得到黄色的 BPP-D 固体（产量：80%）。图 S1（b） 显示了 BPP-D 的 1H NMR 谱图。

2.3. Synthesis of BPPDN 2.3. BPPDN 的合成

The synthetic route of BPPDN is represented in Scheme 1 (c). 3-APN (11.75 g, 0.05 mol) and ethanol (500 mL) were added to the flask, and then BPP-D (9.35 g, 0.025 mol) was added to the flask equipped with a nitrogen inlet at 80 °C. The reaction proceeded for 12 h. After cooling to room temperature, the precipitate was filtered and washed with ethanol three times. The purified BPPDN was obtained after removing the solvent at 80 °C in a vacuum oven (yield: 97 %). Fig. S1(c) and Fig. S1(d) show the successful synthesis of the final target product BPPDN.
BPPDN 的合成路线如方案 1 （c） 所示。向培养瓶中加入 3-APN （11.75 g， 0.05 mol） 和乙醇 （500 mL），然后在 80 °C 下将 BPP-D （9.35 g， 0.025 mol） 加入配备氮气入口的培养瓶中。 反应持续 12 h。冷却至室温后，过滤沉淀，用乙醇洗涤 3 次。在真空烘箱中于 80 °C 去除溶剂后获得纯化的 BPPDN（产量：97 %）。图 S1（c） 和图 S1（d） 显示了最终目标产物 BPPDN 的成功合成。

2.4. Preparation of the flame-retardant EP thermosets
2.4. 阻燃 EP 热固性塑料的制备

Flame-retardant EPs (BPPDN/EP) and the control one were prepared based on the DGEBA/DDS curing system. First, DGEBA and stoichiometric flame retardant BPPDN were added to a 250 mL three-neck flask with magnetic stirring. The homogenous mixture was obtained after continuously and rapidly stirring at 120 °C. DDS was then added into the mixture, which was kept stirring for 5 min. Afterwards, the newly prepared mixture was degassed by evacuation for 5 min and was rapidly poured into a special preheated poly(tetrafluoroethylene) mold, which was gradually heated at 180 °C for 2 h, 210 °C for 2 h, and 230 °C for another 2 h. The control EP was prepared following the same procedure. Table 1 summarizes all the EP thermoset formulas. Briefly, EPs containing 5 wt%, 10 wt%, and 15 wt% BPPDN were labelled 5BPPDN/EP, 10BPPDN/EP, and 15BPPDN/EP, respectively.
基于 DGEBA/DDS 固化体系制备了阻燃 EPs （BPPDN/EP） 和对照 EPs。首先，在磁力搅拌下将 DGEBA 和化学计量阻燃剂 BPPDN 加入 250 mL 三颈烧瓶中。在 120 °C. DDS 下连续快速搅拌后得到均匀的混合物，然后加入混合物中，继续搅拌 5 分钟。然后将新制备的混合物抽空脱气 5 min，并迅速倒入特殊预热的聚（四氟乙烯）模具中，在 180 °C 下逐渐加热 2 h，在 210 °C 下加热 2 h，在 230 °C 下再加热 2 h。对照 EP 按照相同的程序制备。表 1 总结了所有 EP 热固性公式。简而言之，含有 5 wt%、10 wt% 和 15 wt% BPPDN 的 EP 分别标记为 5BPPDN/EP、10BPPDN/EP 和 15BPPDN/EP。

Table 1. Formulas of the EP thermosets.
表 1.EP 热固性塑料的公式。

Sample 样本DGEBA (g) DGEBA （克）DDS (g) DDS （克）BPPDN (wt%) BPPDN （wt%）
EP100330
5BPPDN/EP100335
10BPPDN/EP1003310
15BPPDN/EP1003315

3. Results and discussion
3. 结果和讨论

3.1. Characterization of BPPDN
3.1. BPPDN 的表征

The thermal decomposition process of the flame retardant BPPDN is shown in Fig. 1(a). BPPDN first decomposed at Tmax1 = 286.2 °C until T5%=413.9 °C, which corresponded to the crosslinking of the phenolphthalein structure to generate CO2, in agreement with the weight loss percentage obtained from the theoretical calculations. This was also in line with our initial design idea that the phenolphthalein structure crosslinked quickly to dilute flammable gas and burst flammable radicals, which was well evidenced in the subsequent vertical burning test results. In addition, thermal behavior of two intermediates and BPPDN were tested by DSC (Fig. 1(b)). The intermediates 3-APN and BPP-D showed sharp melting peaks, indicating their purity. At higher temperature, the exothermic peak for 3-APN indicated that the bisphthalonitrile groups started to crosslink at 222 °C and reached the peak at 245 °C. The DSC curve of BPPDN started to show an obvious exothermic peak at 250 °C, which was also attributed to the crosslinking. Therefore, BPPDN was isothermally treated in a muffle furnace at the temperatures of 230 °C, 250 °C and 300 °C for 5 min each, and then subjected to infrared spectrometry respectively. For comparison, the FT-IR spectrum of the original BPPDN was also recorded (Fig. 1(c)). At 230 °C, the characteristic absorption peak of N-H in the triazine ring and the absorption peak of –OH overlapped; while the new peak at 2890 cm−1 appeared, which was attributed to the generation of the triazine and phthalocyanine rings. Moreover, the peak ascribed to cyano rings at 2232 cm−1 gradually weakened after thermal treatment, further disappeared completely at 300 °C. The two newly observed peaks at 1638 cm−1 and 1618 cm−1 belonged to the Cdouble bondC and Cdouble bondN of the phthalocyanine ring, which were difficult to distinguish due to the conjugation of the two bonds and the similarity of their wavenumbers; as the temperature increased, the redshift of the peak at 300 °C was 3593 cm−1, revealing that the cyclization reaction occurred. The above results showed that heat treatment at 230 °C and higher temperature of bisphthalonitrile groups generated triazine rings or phthalocyanine rings.
阻燃剂 BPPDN 的热分解过程如图 1（a） 所示。BPPDN 首先在 Tmax1 = 286.2 °C 分解，直到 T5%=413.9 °C，这对应于酚酞结构的交联生成 CO2，与理论计算获得的失重百分比一致。这也符合我们最初的设计思路，即酚酞结构快速交联，稀释易燃气体，爆裂易燃自由基，这在随后的垂直燃烧测试结果中得到了很好的证明。此外，通过 DSC 测试了两种中间体和 BPPDN 的热行为 [图 1（b）]。中间体 3-APN 和 BPP-D 显示出尖锐的熔解峰，表明其纯度。在较高温度下，3-APN 的放热峰表明双苯二腈基团在 222 °C 开始交联，并在 245 °C 达到峰值。 BPPDN 的 DSC 曲线在 250 °C 时开始显示明显的放热峰，这也归因于交联。因此，BPPDN 在 230 °C、250 °C 和 300 °C 的马弗炉中分别等温处理 5 min，然后分别进行红外光谱测定。为了进行比较，还记录了原始 BPPDN 的 FT-IR 光谱 [图 1（c）]。在 230 °C 时，三嗪环中 N-H 的特征吸收峰与 –OH 的吸收峰重叠;而在 2890 cm-1 处出现新峰，这归因于三嗪和酞菁环的产生。 此外，2232 cm−1 处归因于氰基环的峰在热处理后逐渐减弱，在 300 °C 时进一步完全消失。 在 1638 cm-1 和 1618 cm-1 处新观察到的两个峰属于酞菁环的 C double bond C 和 C double bond N，由于两个键的共轭和波数的相似性，很难区分;随着温度的升高，300 °C 时峰的红移为 3593 cm-1，表明发生了环化反应。以上结果表明，在 230 °C 和较高温度下对双苯二腈基团进行热处理，生成三嗪环或酞菁环。
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Fig. 1. (a) TG and DTG curves of BPPDN, (b) DSC curves of 3-APN, BPP-D and BPPDN, (c) FTIR of BPPDN samples recorded at different temperatures, (d) curing process of EP (E51-DDS) and E51-BPPDN monitored via DSC, the molar ratio of epoxy: BPPDN=4: 1, (e) curing process of E51-DDS system accompanied by the participation of BPPDN.
图 1.（a） BPPDN 的 TG 和 DTG 曲线，（b） 3-APN、BPP-D 和 BPPDN 的 DSC 曲线，（c） 在不同温度下记录的 BPPDN 样品的 FTIR，（d） 通过 DSC 监测的 EP （E51-DDS） 和 E51-BPPDN 的固化过程，环氧树脂的摩尔比：BPPDN=4：1，（e） E51-DDS 系统的固化过程伴随着 BPPDN 的参与。

Once again, it has been previously documented that hydroxyl [30] and cyano [35] groups can participate in the curing of EPs. To further prove that the BPPDN participated in curing, we mixed BPPDN with E51 at an equivalence ratio of 1:4. The DSC curve showed that BPPDN/E51 indeed cured without DDS (Fig. 1(d)). Furthermore, as shown by the calculated curing activation energy [36] in Table S1, BPPDN accelerated the curing process in the presence of aromatic amines.
同样，先前已经证明羟基 [30] 和氰基 [35] 基团可以参与 EP 的固化。为了进一步证明 BPPDN 参与了固化，我们将 BPPDN 与 E51 以 1：4 的等效比混合。DSC 曲线显示 BPPDN/E51 确实在没有 DDS 的情况下固化[图 1（d）]。此外，如表 S1 中计算的固化活化能 [36] 所示，BPPDN 在芳香胺存在下加速了固化过程。
All of the above results demonstrated that BPPDN was indeed involved in the curing reaction of E51, meanwhile, the presence of the phenolphthalein sturcture promoted the ability of the bisphthalonitrile groups to undergo a cyclization reaction at lower temperatures and crosslinking with Cdouble bondN (Fig. 1(e)).
以上结果表明，BPPDN 确实参与了 E51 的固化反应，同时，酚酞结构的存在促进了双邻苯二腈基团在较低温度下发生环化反应并与 C double bond N 交联的能力[图 1（e）]。

3.2. Thermal properties 3.2. 热性能

The thermal stability of the EP thermosets was measured by TGA. The resulting curves are shown in Fig. 2; and the relevant data are listed in Table 2, including the temperature of mass loss for 5 wt% (T5%), the temperature of maximum mass loss rate (Tmax), the peak value of the DTG curve (characterize the mass loss rate) (v max) and the residue char at 700 °C (R700).
EP 热固性塑料的热稳定性通过 TGA 测量。生成的曲线如图 2 所示;相关数据见表 2，包括 5 wt% 的质量损失温度 （T5%）、最大质量损失率温度 （Tmax）、DTG 曲线的峰值（表征质量损失率）（vmax）和 700 °C 时的残余炭 （R700）。
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Fig. 2. TG and DTG curves of the EP thermosets in nitrogen (a, b) and in air atmosphere (c, d).
图 2.EP 热固性塑料在氮气 （a， b） 和空气气氛 （c， d） 中的 TG 和 DTG 曲线。

Table 2. Detailed TGA data of the EP thermosets.
表 2.EP 热固性塑料的详细 TGA 数据。

Sample 样本N2Air 空气
T5% (oC)
T5% （oC）		Tmax (oC)
最大 T （oC）		vmax (wt% min−1)
V最大值 （wt% min-1）		R700 (wt%)
R700 （重量%）		Calculated R700 (wt%)
计算的 R700 （wt%）		T5% (oC)
T5% （oC）		Tmax1 (oC)
最大 T 1 （oC）		Tmax2 (oC)
最大 T 2 （oC）
EP401.7425.5−20.215.7−383.1410.5547.5
5BPPDN/EP381.8409.9−16.420.018.7389.4403.6563.8
10BPPDN/EP374.7400.9−15.525.421.8370.5399.7561.5
15BPPDN/EP364.1399.6−12.830.224.8369.5398.2580.5
As shown in Fig. 2(a) and (b), there was only one DTG peak for all EP thermosets, which indicated that there was only one weight loss stage for all EP thermosets. The vmax of all BPPDN/EP decreased with the introduction of BPPDN. The vmax of the EP was as high as −20.2 wt% min−1, while that of 15BPPDN/EP reached −12.8 wt% min−1. Furthermore, the T5% and Tmax of all the BPPDN/EP samples were only slightly lower than those of the EP sample, whereas the R700 gradually increased and the experimental R700 were significantly greater than the calculated values.
如图 2（a） 和 （b） 所示，所有 EP 热固性塑料只有一个 DTG 峰，这表明所有 EP 热固性塑料只有一个失重阶段。所有 BPPDN/EP 的 vmax 随着 BPPDN 的引入而降低。EP 的 v max 高达 -20.2 wt% min-1，而 15BPPDN/EP 的 vmax 达到 -12.8 wt% min-1。此外，所有 BPPDN/EP 样品的 T5% 和 Tmax 仅略低于 EP 样品，而 R700 逐渐增加，实验 R700 显著大于计算值。
In air atmosphere (Fig. 2(c) and (d)), EP and BPPDN/EP exhibited a two-stage thermal degradation process: the first stage in the range of 380–400 °C (approx. 60 wt% mass loss) was attributed to the thermal decomposition of the network chains of the crosslinked EPs; whilst the oxidative degradation of the char formed during the first stage occurred in the second stage at 500–700 °C. The T5% of EP in air atmosphere decreased by nearly 20 °C compared with that in nitrogen atmosphere, while BPPDN/EP still kept a relatively high T5% and degraded at high temperatures with a lower thermal decomposition rate, indicating that the introduction of BPPDN enhanced the stability of the charring layer at high temperatures and slowed the maximum rate of weight loss, showing better thermo-oxidative stability. All of the above results prove that BPPDN promoted EPs to maintain most of the thermal decomposition products in the condensed phase, which was favourable for improving the flame retardancy.
在空气气氛中[图 2（c） 和 （d）]，EP 和 BPPDN/EP 表现出两阶段的热降解过程：第一阶段在 380–400 °C（约 60 wt% 质量损失）范围内归因于交联 EP 网络链的热分解;而第一阶段形成的焦炭的氧化降解发生在第二阶段，温度为 500–700 °C。 空气气氛中EP的T5%较氮气气氛下降低了近20 °C，而BPPDN/EP仍保持相对较高的T5%，并在高温下以较低的热分解速率降解，表明BPPDN的引入增强了炭化层在高温下的稳定性，减缓了失重的最大速率， 表现出较好的热氧化稳定性。以上结果表明，BPPDN 促进了 EPs 在缩聚相中保持了大部分热分解产物，有利于提高阻燃性。
The glass transition temperature (Tg) is an important index for evaluating the segmental mobility within the crosslinked networks of the EP thermosets. As shown in Fig. 3(a), the Tg of 5BPPDN/EP was 209 °C, higher than that of EP, probably due to the large number of rigid structures decreasing the flexibility within the crosslinks. However, the Tg of BPPDN/EP slightly decreased with increasing BPPDN content. It was possible that more flame retardants were involved in the curing process of DGEBA, and the –OH groups triggered anionic ring-opening polymerization of DGEBA to produce more ether bonds. An increase in the number of flexible functional group ether bonds improved the molecular chain movement of the EPs.
玻璃化转变温度 （Tg） 是评估 EP 热固性塑料交联网络内链段迁移率的重要指标。如图 3（a） 所示，5BPPDN/EP 的 Tg 为 209 °C，高于 EP，可能是由于大量刚性结构降低了交联内的柔韧性。然而，BPPDN/EP 的 Tg 随着 BPPDN 含量的增加而略有降低。DGEBA 的固化过程可能涉及更多的阻燃剂，–OH 基团触发 DGEBA 的阴离子开环聚合，产生更多的醚键。柔性官能团醚键数量的增加改善了 EP 的分子链运动。
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Fig. 3. (a) DSC curves, (b) DMA curves, (c) gel fraction and swelling ratio, (d) tensile strength and modulus, (e) flexural strength and modulus, and (f) impact strength of the EP thermosets.
图 3.（a） DSC 曲线，（b） DMA 曲线，（c） 凝胶分数和溶胀率，（d） 拉伸强度和模量，（e） 弯曲强度和模量，以及 （f） EP 热固性塑料的冲击强度。

The thermomechanical behavior of the EP thermosets was evaluated using dynamic mechanical analysis (DMA). The variations in the energy storage modulus (E′) and loss factor (tan δ) with temperature are shown in Fig. 3(b), and the corresponding data are summarized in Table S2. The E' at 25 °C showed the rigidity of the EP thermosets, and all the BPPDN/EP had a higher E' at 25 °C than did the EP. Its high rigidity was related to the numerous rigid groups within BPPDN, as well as the intermolecular hydrogen bonding provided by the imine bond from the aromatic Schiff base structure and bisphthalonitrile groups. In addition, the peak at tan δ was defined as the characteristic temperature of the α-relaxation transition (Tα). The narrower and greater peak shape of tan δ in Fig. 3(b) implied a more homogeneous network of BPPDN/EP. The E' of the EP thermosets decreased to a constant value after Tα, which is known as the “rubber plateau”. It is generally accepted that the rubbery modulus (E' at Tα + 30 °C) is usually used to describe the crosslink density (νe) and is calculated by the following equation [37]:ve=E′3RTwhere T refers to Tα + 30 °C and R is the gas constant.
使用动态机械分析 （DMA） 评估 EP 热固性塑料的热机械行为。储能模量 （E′） 和损耗因子 （tan δ） 随温度的变化如图 3（b） 所示，相应的数据总结在表 S2 中。25 °C 时的 E' 显示了 EP 热固性塑料的刚度，并且所有 BPPDN/EP 在 25 °C 时的 E' 都高于 EP。其高刚性与 BPPDN 内的众多刚性基团以及芳香族 Schiff 基结构和双邻苯腈基团的亚胺键提供的分子间氢键有关。此外，tan δ 处的峰值定义为 α-弛豫转变的特征温度 （Tα）。图 3（b） 中 tan δ 的峰形更窄和更大，这意味着 BPPDN/EP 网络更加均匀。EP 热固性塑料的 E' 在 Tα后下降到恒定值，这被称为“橡胶平台”。人们普遍认为，橡胶模量（Tα + 30 °C 时的 E'）通常用于描述交联密度 （νe），由以下公式计算 [37]： ve=E′3RT 其中 T 是指 Tα + 30 °C，R 是气体常数。
As shown in Table S2, the introduction of BPPDN reduced νe, implying that the crosslinked network formed by the bisphthalonitrile groups and Schiff base structure had affected the 3D network from the epoxy-amine reaction, which in turn reduced the calculated apparent crosslink density, as well as the Tg and Tα. To investigate whether the introduction of BPPDN affected the perfection of the 3D network, a dissolution gel test was performed with chloroform as the solvent. The gel contents of all the BPPDN/EP were determined, as shown in Fig. 3(c), which were greater than that of EP (99.8 %), and the swelling rate decreased with the incorporation of BPPDN. The above results demonstrated that the introduction of BPPDN resulted in complete crosslinked networks in BPPDN/EP. As expected, our proposed cyano-Schiff base crosslinked network and phenolphthalein structure contribute to the formation of a rigid-flexible network, which suggested positive influence on mechanical properties.
如表 S2 所示，BPPDN 的引入降低了 νe，这意味着由双邻苯二甲腈基团和 Schiff 基结构形成的交联网络影响了环氧-胺反应的 3D 网络，这反过来又降低了计算的表观交联密度，以及 Tg 和 Tα.为了研究 BPPDN 的引入是否影响 3D 网络的完美，以氯仿为溶剂进行了溶出凝胶测试。如图 3（c） 所示，测定所有 BPPDN/EP 的凝胶含量，均大于 EP 的 99.8 %，并且溶胀率随 BPPDN 的掺入而降低。上述结果表明，BPPDN 的引入导致 BPPDN/EP 中的完整交联网络。正如预期的那样，我们提出的氰基-希夫碱交联网络和酚酞结构有助于形成刚柔结合网络，这表明对机械性能有积极影响。

3.3. Mechanical properties
3.3. 机械性能

A high crosslinking density and the presence of rigid groups can usually lead to brittle nature. However, when flexible segments are introduced, the strength and modulus sacrifice, and simultaneously exhibiting excellent strength and toughness is of great challenge for EPs, especially when applied in booming electronic devices where a thin thickness and high toughness are needed [38]. To achieve superior mechanical properties, our structure was designed with a large number of rigid molecular chains, and in the presence of additional crosslinked networks, BPPDN can aggregate and react with the resin matrix to form an interpenetrating-analogue network driven by intermolecular hydrogen bonding, which results in the construction of highly crosslinked yet rigid-flexible combined networks. These interactions, including π-π interactions and hydrogen bonding interactions caused by groups in BPPDN [39], effectively strengthen the EPs. The more flexible network highly toughens the epoxy resin. As clearly observed in Fig. 3(d)–(f), the corresponding data are presented in Table S3. BPPDN/EP exhibited better tensile, flexural and impact properties than EP. In particular, the impact strength of 10BPPDN/EP exhibited a maximum 118 % improvement. Comprehensively, 15BPPDN/EP displayed the best mechanical properties: the tensile strength increased by 63 %, the flexural strength increased by 48 % and the impact strength increased by 77 %.
高交联密度和刚性基团的存在通常会导致脆性。然而，当引入柔性链段时，强度和模量会牺牲，同时表现出优异的强度和韧性，这对 EP 来说是一个巨大的挑战，尤其是当应用于需要薄厚度和高韧性的蓬勃发展的电子设备时 [38]。为了实现卓越的机械性能，我们的结构设计了大量刚性分子链，在存在额外交联网络的情况下，BPPDN 可以与树脂基体聚集并反应，形成由分子间氢键驱动的互穿类似物网络，从而构建高度交联但刚柔结合的网络。这些相互作用，包括 π-π 相互作用和 BPPDN 中基团引起的氢键相互作用 [39]，有效地加强了 EP。更灵活的网络使环氧树脂高度增韧。如图 3（d）–（f） 所示，相应的数据显示在表 S3 中。BPPDN/EP 表现出比 EP 更好的拉伸、弯曲和冲击性能。特别是，10BPPDN/EP 的冲击强度最高提高了 118%。综合而言，15BPPDN/EP 表现出最佳的机械性能：拉伸强度提高了 63 %，弯曲强度提高了 48 %，冲击强度提高了 77 %。
The increased ether bonding network actively mobilize the movement of the surrounding crosslinked network, strengthen the matrix to resist external impact, and effectively improve the toughness of the BPPDN/EP thermosets while providing the possibility of increasing the tensile elongation at break of the thermosets (Fig. S3) [40]. This effect can also be demonstrated in Fig. S4. The fracture surface of the original EP was relatively smooth with few folds, and the folds extended along the fracture direction, showing brittle fracture characteristics. With increasing the content of BPPDN, the fracture surface became rougher, and the cracks became interlaced and extended in all directions, indicating that BPPDN/EP can fully dissipate energy through the deflection and spreading of internal cracks when subjected to an external impact force.
增加的醚键网络积极调动周围交联网络的运动，加强基体以抵抗外部冲击，有效提高 BPPDN/EP 热固性材料 （BPPDN/EP） 热固性材料韧性，同时提供增加热固性材料断裂拉伸伸长率的可能性（图 S3） [40]。这种效果也可以在图 S4 中演示。原 EP 的断裂表面比较光滑，褶皱少，褶皱沿断裂方向延伸，呈现脆性断裂特性。随着BPPDN含量的增加，断裂表面变得更粗糙，裂纹交错并向各个方向延伸，表明BPPDN/EP在受到外力作用时，可以通过内部裂纹的偏转和扩散来充分耗散能量。

3.4. Flame retardancy and combustion behaviors
3.4. 阻燃性和燃烧行为

The UL-94 vertical burning test and LOI are two commonly used methods to evaluate the flame retardancy of polymers. The burning behavior during the test and the limiting oxygen index (LOI) values are depicted in Fig. 4(a), and the specific information is detailed in Table S4. EP burned vigorously and spread to the fixture after one ignition during the test and burned for more than 30 s; thus, it exhibited no rating (NR) and had a lower LOI value of 23.0 %. In contrast, 10BPPDN/EP with only 10 wt% of the flame retardant passed the V-0 grade; while 15BPPDN/EP extinguished immediately after leaving the fire during two ignitions. It should be mentioned that the self-crosslinking of BPPDN to produce CO2, a nonflammable gas, contributed to rapid burnout at low heat flux, while the production of protonic acid also promoted the cyclization of cyano groups as well as the crosslinking of Schiff base, generating ammonia to further prevent the combustion of the polymer.
UL-94 垂直燃烧测试和 LOI 是评估聚合物阻燃性的两种常用方法。图 4（a） 描述了测试过程中的燃烧行为和极限氧指数 （LOI） 值，具体信息详见表 S4。EP 在试验中一次点火后剧烈燃烧并扩散到夹具上，燃烧时间超过 30 s;因此，它表现出无评级 （NR） 并且具有较低的 LOI 值 23.0%。相比之下，10BPPDN/EP 的阻燃剂含量仅为 10 wt%，通过了 V-0 级;而 15BPPDN/EP 在两次点火期间离开火场后立即熄灭。值得一提的是，BPPDN 的自交联产生 CO2（一种不易燃气体）有助于在低热通量下快速燃尽，而质子酸的产生也促进了氰基的环化以及席夫碱的交联，生成氨以进一步阻止聚合物的燃烧。
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Fig. 4. (a) Digital photos of the sample burning process during the UL-94 test at different times, LOI values and UL-94 ratings, (b) HHR curves and (c) TSP curves from cone calorimetry, (d) Specific optical density of smoke curves of the EP thermosets.

The heat release and smoke release behaviors of the EP during burning were further investigated using cone calorimetry (CCT), as shown in Fig. 4(b) and (c). The characteristic parameters are listed in Table 3. The peak heat release rate (p-HRR) and the total heat release (THR) of the EPs decreased with increasing BPPDN content. Furthermore, the total smoke production (TSP) of 15BPPDN/EP was suppressed by 38.7 % in comparison with that of EP. Noticeably, the peak carbon monoxide yield (p-COY) and peak carbon dioxide yield (p-CO2Y) both dropped. Additionally, the residual mass after CCT increased with increasing BPPDN content, and for 15BPPDN/EP it greatly increased by 104 % compared with that of EP, which was consistent with the results of the TGA. Reasonably, the heat and smoke suppression performance were attributed to the good char formation effect of the crosslinking of the BPPDN under high-power thermal radiation.

Table 3. Cone calorimetric results of the EP thermosets.

SampleTTIp-HRRTHRp-SPRTSPp-COYp-CO2YResidueDsmax
(s)(kW m−2)(MJ m−2)(m2 s)(m2)(kg/kg)(kg/kg)(wt%)
EP631351.792.30.3533.62.075.258.3>1320
5BPPDN/EP671168.398.40.3831.61.404.9411.7>1320
10BPPDN/EP581133.682.00.3528.60.974.7816.81054
15BPPDN/EP431051.073.50.2520.60.934.3920.4906
Considering the application of EPs in diverse fields such as transportation, large amounts of smoke emissions in these confined spaces like railway carriage or aircraft cabin dramatically cause more severe injuries and deaths [41], [42]. A smoke density box further served to evaluate the smoke suppression effect: smoke production was quantified by two smoke volumes and extinction area (or optical density) [43]. The impediment to evacuation caused by smoke was evaluated by the smoke density (Ds), as shown in Fig. 4(d). The Ds of the EP increased dramatically after ignition and exceeded the monitoring range of the instrument, say, the Ds after 150 s was shown to be constant and remained stable during the test. In contrast, the Dsmax of 15BPPDN/EP decreased to 906. More importantly, the smoke production time of all BPPDN/EP greatly decreased, which provide more time for evacuation during fire disasters.

3.5. Mechanism of flame retardation and smoke suppression

First, the gas-phase products released from the pyrolysis process were monitored using a real-time TG-FTIR. The results are shown in Fig. 5(a), and the main volatilization products of the EPs were: 1, ammonia (3734 cm−1, 3450 cm−1), 2, hydrocarbons (3033 cm−1, 2960 cm−1), 3, carbon dioxide (2349 cm−1), 4, aromatic hydrocarbons and 5, their derivatives (1593 cm−1, 1502 cm−1, 810 cm−1), respectively. The gas-phase products of 15BPPDN/EP released earlier than that of EP, and more substances, such as ammonia, produced in advance, which diluted oxygen and the combustibles such as aromatics and phenolics.
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Fig. 5. Volatile products analyses. (a) TG-FTIR spectra for the degradation products of EP and 15BPPDN/EP recorded at different temperatures, (b) total ion chromatograms of EP and 15BPPDN/EP at 700 °C from Py-GC/MS and their representative pyrolysis products.

Py-GC/MS was applied to characterize the pyrolysis behavior of the EPs closer to the combustion temperature; and Fig. S5 presents a comparison of the total ion chromatograms (TICs) of EP and 15BPPDN/EP at 550 °C. The cleavage processes of the two samples at 550 °C were similar, revealing the thermal cleavage fragments of phenol and its derivatives (m/z = 94, 136, 212, 226, and 242), aromatic amines and their derivatives (m/z = 107, 121, and 134). However, to a great extent, the intensity of the combustible fragments was significantly suppressed in 15BPPDN/EP (e.g., the peak 4 and 7). The above results demonstrated that the incorporation of BPPDN effectively reduced flammable gases, consistent with TG-FTIR. In addition, nitrogen-containing heterocyclic ring (peak 1) and the fluorene ring structure (peak 10) appeared, which reaffirms the phenolphthalein structure in BPPDN to crosslink earlier to produce non-flammable gases. No obvious fragments of the triazine or phthalocyanine ring were found, it was speculated that the Schiff base as well as the bisphthalonitrile groups crosslinked to form a thermally stable nitrogen-containing heterocyclic ring.
To confirm the thoughts, the pyrolysis behavior of EP and 15BPPDN/EP at 700 °C were further investigated in Fig. 5(b) and Fig. S6. It was clearly seen that the relative peak intensities of the combustible fragments from the conventional chain breakage of the EPs were significantly reduced in the entire pyrolysis stages, such as aromatic amines (m/z = 121) (peak 1), phenol derivatives (m/z = 212) (peak 2) and the sulfur-containing aromatic amine derivatives (m/z = 290) (peak 8). More importantly, 15BPPDN/EP yielded unique fragments of phthalocyanine-like and triazine-like structures, such as peaks of 3 (m/z = 290), 4 (m/z = 262), 5 (m/z = 249), 6 (m/z = 288), and 7 (m/z = 312), which allowed weak bonds (e.g., peak 1 and 2) at the crosslinking point position in the EPs to also participate in the process of aromatization; meanwhile, these structures also improved the thermal stability of the aminodiphenyl sulfone and suppressed the formation of peak 8. In contrast, in EP, the sulfonyl group preferentially produced SO2 (in Fig. 5(a)), suggesting that it was difficult for the remaining groups to participate in the rearrangement to generate compounds with a higher degree of aromatization.
The residual char after CCT was surveyed. In Fig. 6(a), the EP burned up completely and even the aluminum foil burned through. The relevant SEM image revealed that many large cracks and holes appeared on the outside and the inner of the residual char. In contrast, with increasing BPPDN content, the macroscopic morphology of the residual char showed a noticeable trend toward a more complete and denser residual char layer. Similarly, SEM revealed that BPPDN/EP had no obvious cracks and few holes and became smoother and flatter. The internal residual char further proved that many air bubbles were produced, and hence, the dilution effect was due to the generation of nonflammable gas such as CO2, NH3. Fig. 6(b) also shows their respective Raman spectra. The intensity ratio of the two peaks D (1360 cm−1) and G (1580 cm−1) was used to determine the degree of graphitization of the residual char. A lower ID/IG ratio represented a higher degree of graphitization of the residue after burning, showing positive influence on flame retardancy in condensed phase [44]. Raman spectroscopy revealed that the residual carbon ID/IG of 15BPPDN/EP was 1.92, which was much lower than that of the EP (2.46). The above results proved that the dense graphite-like complexes greatly inhibited the release of gas and heat, ultimately providing better protection for the matrix [45], [46].
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Fig. 6. Condensed charring analyses. (a) Digital photos of the burning residues, SEM micrographs of exterior and interior char residues for the EP thermosets, (b) Raman spectra, (c) FT-IR measurements, XPS high-resolution spectra of (d) EP and (e) 15BPPDN/EP for N1s for the EP thermosets after cone calorimetry.

Similar conclusions can also be drawn from Fig. 6(c). The FTIR spectra of EP did not have any peaks in the range of 1000 cm−1 to 1400 cm−1; whereas, the residual char of 15BPPDN/EP manifested obvious peaks at 1265 cm−1and 1175 cm−1, attributed to the characteristic absorption of Csingle bondN, and two sharp peaks of N-H were found at 3444 cm−1 and 3364 cm−1. Furthermore, in the N1s spectra [47] (Fig. 6(d) and (e)), 15BPPDN/EP possessed more graphitic nitrogen as well as Cdouble bondNsingle bondC, due to the cyclization and crosslinking of BPPDN.
Consequently, a high-efficiency flame retardant mechanism of BPPDN was proposed, as shown in Fig. 7. In contrast to general additive, BPPDN participates in the curing reaction of EPs, and more importantly, multiple cross-linkages occur in BPPDN/EP. On the one hand, the crosslinking of the phenolphthalein structure can produce CO2, and promote cyano-trimeric annulation, generating many more nonflammable gases such as ammonia. They rapidly quench the flammable free radicals and dilute flammable gases in the combustion process. On the other hand, Cdouble bondN in the aromatic Schiff base and Ctriple bondN form a stable carbon layer through crosslinking at high temperature, which can provide remarkable thermal insulation and inhibit the escape of flammable gases in the condensed phase. Broadly speaking, BPPDN has showed the advantages of simultaneously improving flame retardancy and combining strength with toughness in EPs.
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Fig. 7. Schematic illustration of the mechanism (mode-of-action) for enhancing the fire safety of the EP matrix.

3.6. Multifunctional properties

3.6.1. Dielectric properties

The ability of EPs to be widely used as electrical insulating materials in aerospace, electrical and electronics and other high-end fields was attributed to their relatively low and stable dielectric constant (k) and dielectric loss (tanδ) [48]. Specifically, the large three-dimensional structures in the EPs effectively impeded the polarization orientation of the dipole in the electric field. However, as the miniaturization of electronic products leads to overheating phenomena and severe fire hazards, there is an increasing demand for flame-retardant epoxy resins in this field [49].
The relationship curves between the dielectric constant and dielectric loss and frequency for BPPDN/EP and EP are shown in Fig. 8(a) and (b), and the related data are listed in Table S5. All the BPPDN/EP had much lower k and tanδ than the EP at any frequency, showing outstanding dielectric properties, which was attributed to the introduction of phenolphthalein structure with large spatial resistances and crosslinking to form a macrocyclic structure with intramolecular cavities [40]. It was proposed that BPPDN effectively enhanced the free volume of the thermosets and reduce the number of dipoles per unit volume. Among the rest, 15BPPDN/EP exhibited the best dielectric properties, making it a potential flame-retardant material for high volume production in electrical and electronic applications such as bonding, coating and encapsulation, which requires reliable electrical insulation, excellent thermal stability, and specified flame retardancy standards (UL-94 V-0).
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Fig. 8. The variation in the (a) dielectric constant and (b) dielectric loss versus frequency for the EP thermosets; (c) Ultraviolet images and digital images of (d) EP and (e) 15BPPDN/EP.

3.6.2. Transparency of the EP thermosets

In many cases, the transparency of EPs was often jeopardized by the need for improved flame retardancy and mechanical strength. The application of EPs for optoelectronic and encapsulation materials was often restricted due to their poor overall performance when both flame retardancy and high transparency were needed.
As shown in Fig. 8(c), 0.15 mm sheets were pressed and tested for UV shielding to determine the transmittance of the EPs in the measured wavelength range. The transmittance of the EP at 800 nm was 87.8 %, while that of 15BPPDN/EP was 86.9 %. Fortunately, the good transparency of the EPs was maintained at 15 % addition. As shown in Fig. 8(d) and (e), the emblem of Sichuan University was clearly visible under the sheet, further revealing that the BPPDN was compatible with the matrix. Furthermore, the EP was resistant to UV radiation only at wavelength below 360 nm, while the transmittance of the 15BPPDN/EP film at 400 nm was still close to zero. The conclusions implied that the nitrogen-containing heterocyclic conjugated groups formed during crosslinking in 15BPPDN/EP effectively absorbed the longwave UV-A [39], [50]. Surprisingly, BPPDN provides additional material value and widens the scope of application.
At last, improvements of BPPDN for impact strength, tensile strength, and the suppression of TSP in the CCT were compared with those of the related literature previously reported [9], [40], [45], [51], [52], [53], [54], [55]. As summarised in Fig. 9., 15BPPDN/EP has the advantage of being a novel multi-functional thermoset.
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Fig. 9. Statistical results for flame retardancy and mechanical properties in Δ Tensile strength, Δ Impact strength, Δ TSP compared with previous literature which all passed V-0 in UL-94 test.

4. Conclusion

A novel crosslinkable additive (BPPDN) just containing C/N/O element yet in the absence of conventional flame-retardant elements such as halogens and phosphorus, was designed and integrated into EPs. The presence of multi-crosslinking networks, including the amine-cured EP networks between DGEBA and DDS, the ether-linked networks initiated by phenolic hydroxyl anions, the phthalocyanine rings formed by thermal cyclization of the phthalonitrile structures, and the imine-crosslinked structures formed by the ring addition of Schiff base in BPPDN, conferred excellent comprehensive properties to the cured thermosets. These multiple crosslinking effects, along with the unique molecular structure forming rigid-flexible networks, enhanced the strength and toughness of the EPs while maintaining good dielectric and optical properties. The construction of synergistic crosslinked networks in EPs facilitated more rearrangement and charring, reducing the formation of soot particles. The modified EPs achieved a UL-94 V-0 rating and demonstrated low heat and smoke release. This was evidenced by reductions of 22.2 %, 38.7 %, and 31.4 % in p-HRR, TSP, and Dsmax values, respectively, offering a promising strategy for developing high-performance thermosetting materials.

CRediT authorship contribution statement

Yi Wang: Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation. Lei Zhang: Investigation. Jing-Hong Liu: Investigation. Yan-Fang Xiao: Investigation. Chuan Liu: Investigation. Yu-Zhong Wang: Supervision, Project administration, Funding acquisition. Li Chen: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Financial supports by the National Key Research and Development Program of China (2021YFB3700204), the National Science Foundation of China (21975166, 51991351, 51991350), the 111 Center (B20001) and the Fundamental Research Funds for the Central Universities are sincerely acknowledged.

Appendix A. Supplementary data

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Supplementary Data 1.

Data availability

Data will be made available on request.

References

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