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