Facile synthesis of lithium containing polyhedral oligomeric phenyl silsesquioxane and its superior performance in transparency, smoke suppression and flame retardancy of epoxy resin

https://doi.org/10.1016/j.compscitech.2020.108004Get rights and content

Abstract

Massive release of toxic volatiles and smoke during combustion is one of the biggest problems facing epoxy resin composites, therefore, it is still a huge challenge to obtain a flame retardant that can simultaneously improve the flame retardancy and reduce the evolution of smoke and toxic gases. Herein, a novel lithium containing polyhedral oligomeric phenyl silsesquioxane (Li-Ph-POSS) has been prepared via hydrolysis condensation of phenyltriethoxysilane in the presence of LiOH·H2O. FTIR, NMR, and MALDI-TOF-MS were used to identify the structure of Li-Ph-POSS, which indicate that it is a hepta-phenyl POSS comprised of incompletely condensed Si–OH and Si–O–Li groups. The excellent compatibility and dispersion of Li-Ph-POSS in EP/Li-Ph-POSS nanocomposites were detected by SEM and TEM. The effective smoke suppression and flame retardancy of EP/Li-Ph-POSS nanocomposites were investigated using cone calorimeter, smoke density and limited oxygen index measurements. Compared with pure EP, the EP with 4 wt% Li-Ph-POSS made the peak of heat release rate (p-HRR), total heat release (THR), peak of smoke production rate (p-SPR) and peak of CO production rate (p-COP) reduce by 61%, 36%, 44%, and 72%, respectively. This work provides a new strategy of mitigating the fire hazard of EP using phosphorus- and halogen-free metal-POSS.

Introduction

Epoxy resin (EP) has been widely used in aviation, ship, rail transit, etc. due to its outstanding mechanical properties, chemical resistance, low manufacturing cost and strong adherence [[1], [2], [3]]. However, EP is high flammability and high smoke, which seriously restrict its application [[4], [5], [6]]. Preparing flame retardant EP with low smoke production is urgently needed to increase the fire safety of EP-based materials [[7], [8], [9], [10], [11]].

There are many ways to flame-retard epoxy resin. For example, halogen-containing [12,13] or organic phosphorus-containing [14,15] flame retardants are the most widely used flame retardant varieties for EP. However, although they have high flame retardant efficiency, they generally do not have smoke suppression function, even increase smoke density or release corrosive gases because they exert a significant gas phase flame retardant mechanism [16,17]. Therefore, many studies have primarily concentrated on adding flame retardants and smoke suppressants simultaneously into EP, in order to improve the flame retardancy and smoke suppression performance. Traditional smoke suppressants usually include molybdenum trioxide, ammonium octamolybdate, zinc borate, iron oxide, and copper oxide [18,19]. However, these smoke suppressants display poor compatibility, damage to the mechanical properties, color, and almost no flame retardant properties when used in EP matrix.

With the development of nanotechnology, several zero-dimensional nanodots [20], one-dimensional nanotubes [[21], [22], [23]], two-dimensional nanosheets [[24], [25], [26]], and three-dimensional nano-frameworks [27,28] with rigid structures and high thermal stability have been used to increase the strength, thermal stability and flame retardancy of polymer materials. Functionalized polyphosphazene nanotubes wrapped with a cross-linked DOPO-based flame retardant (FR@PZS) were facilely synthesized by Hu et al. which reduced the peak of heat release rate of EP by more than 45% at 3 wt% content [22]. Furthermore, his team proposed a metal cobalt (Co) organic framework (P-MOF) with a phosphorus-containing structure, when the P-MOF was added to epoxy resin, the generation of CO during the pyrolysis process of EP was apparently decreased [27]. Xu et al. used a hydrothermal method to prepare titanium dioxide nanotubes coated with molybdenum disulfide (MoS2-TNT), which could significantly reduce smoke and heat release of EP [23]. At the same time, his team proposed a functionalized reduced graphene oxide with Co-ZIF adsorbed borate ions (ZIF-67/RGO-B), which could distinctly decrease the diffusion of smoke and CO of EP composites during combustion [28]. Moreover, they fabricated a hybrid of RGO-LDH/CuMoO4 using a co-precipitation method, and introducing 2 wt% of RGO-LDH/CuMoO4 into EP significantly decreased the maximum smoke density (Ds, max) by 52% [29].

In recent years, due to the structural design ability and functional diversity, polyhedral oligomeric silsesquioxane (POSS) has gradually become one of the major interesting zero-dimensional nanodot flame retardants used to reinforce the flame retardancy of composites [20]. POSS not only has accurate and well defined nano-structures, but also the organic R group in the POSS molecule can also be designed to be reactive and non-reactive and it can have special functional hetero atoms such as nitrogen, sulfur, phosphorus, aluminum, zinc, vanadium or titanium as needed [30]. Our group has synthesized a series of phosphorus containing POSS applied in EP, and some positive flame retardant results were obtained [4]. Alberto Fina et al. [31] also investigated the effect of metallic polyhedral oligomeric silsesquioxane (M-POSS) on the flame retardancy of polypropylene. These M-POSS containing metal and silicon elements played a certain role in catalytic char formation and smoke suppression, but their flame retardant effects were not satisfactory.

In this work, a high yield of lithium containing polyhedral oligomeric hepta-phenyl silsesquioxane (Li-Ph-POSS) was prepared via one-pot method based on relatively inexpensive raw materials for the first time. Introducing Li-Ph-POSS into EP could achieve nano-dispersion and transparent EP/Li-Ph-POSS nanocomposites were obtained through simple mixture method. Meanwhile, incorporating Li-Ph-POSS into EP could significantly reduce the heat, smoke, and CO release during combustion, thereby effectively mitigate the fire hazards of EP nanocomposites. The catalytic charring of alkali metal lithium during combustion was discovered and revealed for the first time, unlike the catalytic carbonization action of Fe, Co, Ni [32,33]. Finally, the effective smoke suppression and flame retardancy mechanism of the EP/Li-Ph-POSS nanocomposites are discussed in detail.

Section snippets

Materials

Phenyltriethoxysilane (PTES) (>99%) was purchased from JiangHan Fine Chemical. Tetrahydrofuran (THF), acetone and LiOH·H2O were purchased from Beijing Chemical Works. Diglycidyl ether of bisphenol A (DGEBA, E44) was supplied by FeiCheng DeYuan Chemicals Co., Ltd. 4,4-diaminodiphenylsulphone (DDS, >98.0%) was purchased from Tianjin Guangfu Fine Chemical Research Institute.

Synthesis of Li-Ph-POSS

In a dry 2 L three-necked flask equipped with magnetic stirring, 1000 mL of tetrahydrofuran (THF) and 251.6 g of PTES were

Characterization of Li-Ph-POSS

The complete cage structure POSS compounds, such as octavinyl-POSS and octaphenyl-POSS, have a regular chemical structure, hence the vinyl/phenyl and Si–O–Si chemical environments in the structure are single and identical [4]. However, in an incomplete cage structure, the chemical environments of the organic groups and Si–O–Si are different. Fig. 2 displays the FTIR spectra of Li-Ph-POSS, PTES and LiOH·H2O. Compared to the FTIR spectra of PTES, the FTIR spectra of Li-Ph-POSS shows that the

Conclusions

Lithium containing polyhedral oligomeric phenyl silsesquioxane (Li-Ph-POSS) was prepared via hydrolysis condensation reaction of phenyltriethoxysilane in the presence of LiOH·H2O. Li-Ph-POSS is a hepta-phenyl POSS with an incompletely condensed cage structure, which shows excellent compatibility and dispersion in the EP/Li-Ph-POSS nanocomposites. EP/Li-Ph-POSS nanocomposites display effective smoke suppression and flame retardancy based on the cone calorimeter tests, smoke density and LOI

CRediT authorship contribution statement

Xinming Ye: Investigation, Methodology, Writing - original draft. Wenchao Zhang: Conceptualization, Supervision, Writing - review & editing. Rongjie Yang: Data curation, Funding acquisition, Writing - review & editing. Jiyu He: Supervision, Writing - review & editing. Jiarong Li: Supervision, Writing - review & editing. Fengqi Zhao: Writing - review & editing.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgments

This project was funded by the National Program on Key Research Project (2016YFB0302101).

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