Rationally designed zinc borate@ZIF-8 core-shell nanorods for curing epoxy resins along with low flammability and high mechanical property

https://doi.org/10.1016/j.compositesb.2020.108349Get rights and content

Highlights

  • ZB@ZIF-8 core-shell nanorods were constructed through self-sacrifice strategy.

  • ZB@ZIF-8 nanorods were used as the curing agent and modifier of epoxy resins.

  • EP/ZB@ZIF-8 nanocomposite exhibited higher fire safety than EP/ZIF-8.

  • The tensile strength and modulus of EP/ZB@ZIF-8 were higher than EP/ZIF-8.

Abstract

Here zinc borate@ZIF-8 (ZB@ZIF-8) hybrid nanorods were constructed through self-template strategy using zinc borate as the template, so as to realize the multifunctionalization of epoxy resins. By virtue of this smart design, it rationally combined the structural and functional advantages of the curing capability of ZIF-8 and the flame-retardant efficiency of ZB into core-shell nanoarchitecture, which could facilitate the curing of epoxy resins, improve the fire safety and enhance the physical-mechanical property of epoxy thermosets. Benefiting from this design, the apparent activation energy for ZB@ZIF-8 nanorods to cure epoxy resins is lower than ZIF-8, and epoxy thermosets cured by ZB@ZIF-8 presented lower flammability and higher mechanical properties as compared to epoxy thermosets cured by ZIF-8 individually.

Introduction

Today, polymer-based nanocomposites are rapidly developed for high performance so as to meet various requirements in the aspect of flame retardancy [1,2], dielectric constant [3,4], thermal conductivity [5,6], mechanical properties [7,8], etc. Following on this, they have been used in the fields of electronic & electrical, structural, energy storage, automotive, aerospace, etc [[9], [10], [11]]. As one of the commercially obtained nanofillers, zinc borates (ZB) are widely used as flame retardants and synergistic agents in a majority of polymers, such as polypropylene (PP) [12,13], EVA copolymer [[14], [15], [16]], polyamide (PA) [17,18], epoxy resins (EP) [19,20], etc. Due to the low cost, environmentally friendly nature, smoke-suppression behavior and flame-retardant synergism [21], they have been considered one of the most promising alternatives of halogen-containing flame retardants. However, the challenge is the low compatibility between the matrix and nanofillers, which has profound impact on the properties at the nanoscale. Liu et al. reported zinc borate nanorods with a low aspect ratio hardly maintained the mechanical properties of PP [22]. In this case, the functionalization of zinc borates is very important for obtaining polymers with satisfied comprehensive performance. But to date, multifunctional modification of zinc borates is seldom reported.

Metal–organic frameworks (MOFs) with an ordered and well-defined porous structure nowadays are broadly employed for scientific researches in the fields of gas adsorption/separation, sensors, ion conductors, catalysis, etc [23]. Among them, ZIF-8 has been well studied as well, and apart from the aforementioned applications, ZIF-8 has been developed in other fields requiring the performance of anti-corrosion [24,25], flame retardancy [26,27], low dielectric constant [28,29], etc., owing to its element constitution, relative chemical stability and high thermal stability. Recently, ZIF-8 is reported to possess the curing ability to harden epoxy resins, simultaneously, it is regarded as nano fillers in the epoxy-based nanocomposites. It is confirmed that the curing ability of ZIF-8 is ascribed to the imidazole groups of ZIF-8 which initiates the cross-linking reaction of epoxy groups [30]. In this case, we put forward a feasible method to deposit ZIF-8 on the surface of ZB nanorods, so as to construct ZB@ZIF-8 core-shell nanorods. Thereby, ZB@ZIF-8 nanorods have the potential to realize the multifunctionalization of polymers.

To date, although the flammability of polymers can be relieved even solved through incorporating reactive or addition-type flame retardants, it is still a challenge to develop versatile flame retardants for multifunctional polymers [31,32]. Epoxy resin (EP) one of the popular thermosetting polymers has a great deal of applications in the fields of electronics & electrical, automobile, structural, etc. owing to adhesion, electrical insulation, chemical resistance, high mechanical strength, etc. [33,34] As mentioned, EP suffers from the drawback of flammability owing to the constitution of hydrocarbon, which limits its application in the field compulsively requiring high flame retardancy [35,36]. So far, a lot of researchers and their groups around the world focus on the work related to epoxy-based nanocomposites, and have made a big progress on multifunctional epoxy resins [1,37]. For example, Mu etc. [38] prepared phosphorus and nitrogen containing macromolecular covalent organic nanosheets, which could be used to enhance the flame retardancy and mechanical property of epoxy resin. Meanwhile, Fang etc. [39] Found the introduction of functionalized graphene oxide (PPGO) modified by the self assemble of the supermolecular aggregates of piperazine (PiP) and phytic acid (PA) also played positive effect on improving the flame retardancy and mechanical properties of epoxy resins. However, it is noted, up to now, none of the used nanofillers are reported possessing the capability of curing epoxy resins. More importantly, nanofillers as hardener are beneficial to improve the interfacial force with matrix, which will show profound effect on the mechanical properties of nanocomposites. Based on the above, it is expected that ZIF-8 decorated ZB nanorods (ZB@ZIF-8) are suitable for multi-functional epoxy resins with desired flame retardancy and satisfied mechanical property.

For the first time, the above-mentioned ZB@ZIF-8 core-shell nanorods are prepared through a self-template method etching ZB by 2-methyl imidazole in the DMF/H2O solvent. Besides, the formation mechanism of ZB@ZIF-8 is studied in detail. Besides, it is proven that ZB@ZIF-8 actually presents curing activity for epoxy resins, and the relevant epoxy thermosets show lower heat release rate and higher mechanical strength as compared to epoxy thermosets cured by ZIF-8. This work offers a brand-new perspective on the functionalization of nanofillers and the fabrication of high-performance nanocomposites.

Section snippets

Materials

Zinc borate (4ZnO·B2O3·H2O, ZB) nanorods are free provided from Qinghai Institute of Salt Lakes, China. Epoxy resin (L20, 0.56 mol/100 g) is sourced from R&G Faserverbundwerkstoffe GmbH, Germany. 2-Methyl imidazole (MIM) and zinc nitrate hexahydrate are bought from Sigma Aldrich, Spain. Deionized water, dimethyl formamide (DMF), methanol and ethanol are all used as received.

Synthetic procedure

ZB@ZIF-8 core-shell nanorods: Typically, zinc borate (155 mg) was firstly dispersed in a mixed solvent with 25 mL DMF and

Characterization

The SEM images of ZB, ZB@ZIF-8 and ZIF-8 are presented in Fig. 1. ZB consists of rodlike nanostructure with smooth surface, and ZIF-8 exhibits a regular hexagonal faceted shape, agreeing well with the previous report [40]. After reaction in mixed solvent of DMF/H2O at 70 °C for 20 h, ZB becomes rough and the diameter seems to be widened to some extent, as a result of the deposition of ZIF-8 particles. In addition, the growth of ZIF-8 does not deteriorate the overall morphology of ZB and it

Conclusions

Through the core-shell construction of ZB@ZIF-8 nanorods through a self-template way, it exhibited the dual function including the curing behavior of ZIF-8 and the flame-retardant efficiency of ZB. Accordingly, ZB@ZIF-8 could be used as a curing agent and flame retardant simultaneously. It was found that ZB@ZIF-8 performed lower apparent activation energy in the ring-opening reaction with epoxy resins owing to the formation of more structure defects, although the curing temperature was higher

CRediT author statement

Rong-Kun Jian: Conceptualization, Methodology, Investigation, Writing - original draft. Xue-Bao Lin: Methodology, Investigation, Formal analysis, Writing - review & editing. Zhi-Qi Liu: Resources. Wen Zhang: Formal analysis. Jing Zhang: Investigation, Formal analysis. Lu Zhang: Investigation, Methodology. Zhi Li: Investigation, Methodology. De-Yi Wang: Supervision, Methodology, Writing - review & editing.

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.

Acknowledgment

Financial supports from the National Natural Science Foundation of China (Grant No. 21504015) and the Natural Science Foundation of Fujian Province of China (Grant No. 2019J01061338), and the Fujian Normal University (FNU) Training Program of Innovation and Entrepreneurship for Undergraduates (Grand No. cxxl-2017162) would be sincerely acknowledged.

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