Preparation of layered organic-inorganic aluminum phosphonate for enhancing fire safety of polystyrene
Graphical abstract
Introduction
Polystyrene (PS) produces large amounts of soot particulates and polycyclic aromatic hydrocarbons during combustion process, which are potentially health hazardous [1], [2], [3]. To eliminate these potential hazards, abundant attention has been changed to halogen-free flame retardants, including inorganic metallic compounds and organic phosphorus- or nitrogen-containing compounds [4], [5], [6]. Metallic hydroxide has been used as flame retardants for preparing fire-proof PS. However, the superior fire resistance of polymers generally requires higher content of metallic compounds [7], [8], [9], which damages the physical and mechanical properties of polymeric matrix. Meanwhile, the organic compounds have an ageing issue in the application process, which impacts efficiency of additives and even possibly contaminates surroundings [10]. A new series of flame retardants for PS, which own the comprehensive advantage of organic and inorganic compounds, should be on the schedule.
Recent researches about layered organic/inorganic materials have been discussed about their different prosperities and potential applications in various fields, such as batteries [11], superconductor [12], ion exchange [13], super capacitors [14], [15], sensors [16], and semiconductor [17]. Nano-sized layers, obtained after exfoliation of layered materials, have been utilized as elementary units for preparing other nanostructure, such as graphene, molybdenum disulfide, zirconium phosphate, and boron nitride [18], [19], [20], [21]. Two-dimensional materials have been widely used in preparing functional polymer composites, biomaterials and functional materials [22], [23], [24], [25], [26]. Our group has conducted a lot of works about the application of layered materials to enhance fire safety of polymers [27], [28], [29]. Layered metal phosphonates significantly improved flame retardancy of polymer. Layered zirconium phosphonate and layered titanium phosphonate have been intensively studied on improving the combination properties of polymer composites [30], [31]. Aluminium hypophosphite has been commercially employed in engineering plastics. However, as other inorganic flame retardants, it shows inferior compatibility with polymers, which is adverse to its dispersion in matrix. It is well known that homogeneously dispersed nanosheets will enhance the isotropous blocking effect of additives [32], and thus improve the flame retardancy of polymers. Therefore, the compatibility between layered materials and matrix should be taken into account. Compared with extra organic modification of inorganic compounds in the post processing, a better design scheme should be proposed.
Based on the above discussions, we proposed a novel one-step method to synthesis layered organic aluminium hypophosphite and firstly incorporated into PS in this work. Due to its organic components, OAHPi theoretically possesses superior compatibility with polymer matrix than that of inorganic compounds. Layered structure will endow it barrier effect during thermal degradation procedure. Meanwhile, phosphorus-containing components will function in gas and condensed phases to promote the suppression of fire spread. TG and MCC were applied to study the influence of OAHPi on the thermostability and flame behaviors of PS. The pyrolysis and decomposition products of PS and its composites were detected by TG-IR and GC-MS, which was beneficial to the proposal of flame-retardant mechanism. Meanwhile, the smoke release was also studied in this work.
Section snippets
Materials
Polystyrene (PS) was obtained from Yangzi petrochemical co., LTD. (China). Carbonization precursor (spirocyclic pentaerythritol bisphosphorate disphosphoryl chloride, SPDPC) was prepared by our laboratory. Tetrahydrofuran (THF, AR), N, N-dimethylformamide (DMF, AR), triethylamine (TEA, AR), sodium hypophosphite hydrate (AR), paraformaldehyde (POM, AR), aluminium chloride hexahydrate (AR) and hydrochloric acid (HCl, 37%) were purchased from Sinopharm Chemical Regent Co., Ltd. (China). Sodium
Characterization of OAHPi
The FTIR spectra of the as-prepared samples are displayed in Fig. 2a the spectrum of PBPPD exhibited characteristic peaks at 2930, 2650, 1250, and 1033 cm−1, which were assigned to the stretching of CH, POH, PO, and PCO, respectively [35], [36]. After nucleophilic addition reaction, the peak of the POH functional groups almost disappeared and a new band appeared around 1185 cm−1, which was corresponding to the stretching of POAl, illustrating the formation of aluminum phosphonate [37].
The 1H
Conclusion
In this work, OAHPi was synthesized from aluminium chloride and a carbonization precursor by one-step. As prepared OAHPi was not only easy accessibility, but also possessed structural advantages, such as: organic layered structure and remarkable compatibility with polymer. The TGA results showed that the addition of OAHPi into PS composites increased the thermostability and improved the char yield. The flame retardancy of PS/OAHPi nanocomposites were significantly enhanced, which can be
Acknowledgements
The work was financially supported by the National High-tech R&D program, 2016YFB0302104 and National Natural Science Foundation of China (No. 51603200, 51303167) and the Fundamental Research Funds for the Central Universities (No. WK2320000037, WK2320000027).
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