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In order to improve its water resistance and compatibility with polymer matrix, ammonium polyphosphate (APP) is modified with melamine-trimesic acid (MEL-TA) aggregates by supramolecular self-assembly technology. Chemical structure and morphology of APP@MEL-TA are investigated by Fourier transform infrared spectroscopy and scanning electron microscopy (SEM), respectively. Intumescent flame retardant system of APP@MEL-TA and charring-foaming agent is introduced into polypropylene (PP) matrix. The flammability and combustion behavior of PP composites are investigated by limiting oxygen index (LOI), UL-94 vertical burning, and cone calorimetry tests. In terms of LOI values and cone combustion results, APP@MEL-TA performs better than pristine APP. Char residue of PP composites is investigated by SEM and Raman spectra. Flame retardant mechanisms are proposed based on thermal decomposition, combustion results, and analysis on char residue. K E Y W O R D S ammonium polyphosphate, flame retardant, polypropylene, supramolecular self-assembly, thermal stability 1 | INTRODUCTION Polypropylene (PP) is widely applied in a variety of applications, such as transportation, cables, housing, and electric industry. However, PP burns very rapidly with the release of a great deal of heat, which limits its applications in many fields. Thus, the improvement in flame retardant properties of PP has become an important research. Zhang et al 1 reviewed the research progress of halogen-containing flame-retarded PP. The obvious disadvantages of traditional halogen-based flame retardants are high fire toxicity and releasing of smoke during the combustion. 2 In addition, environment amity is the theme of whole society, and some halogen-containing flame retardants are undoubtedly damaging the environment. 3-6 Recently, halogen-containing flame retardants have been replaced by halogen-free counterparts, 7-9 which are characterized by their low fire toxicity. Intumescent flame retardants (IFRs) are the most popular halogen-free retardants because of their advantages of low smoke and toxicity during combustion. 10,11 Wang et al 12 prepared a novel IFR by the reaction of melamine (MEL) with methyl hydrogen siloxane, and this flame retardant system can effectively improve flame retardancy of PP.Acid catalyst, blowing source, and charring agent are three main ingredients for IFR system. 13,14 According to its elemental composition, ammonium polyphosphate (APP) acts as acid catalyst and blowing source because of its high phosphorus and nitrogen contents, both of which are primary flame retardant elements. 15,16 However, as an important component of IFR, APP has obvious disadvantages of weak water resistance and poor compatibility with polymer matrix. 3,17,18 Hence, it is of great significance to modify the surface of APP and improve flame retardant properties of resulting composites. In view of these deficiencies, many researches on surface modification of APP have been carried out. The surface characteristics of APP are modified by other flame retard...

Piperazine cyanurate (PCA) is designed and synthesized via hydrogen-bonding selfassembly reactions between piperazine and cyanuric acid. Chemical structure and morphology of PCA are investigated by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The prepared PCA is combined with ammonium polyphosphate (APP) to prepare flame-retardant polypropylene (PP) composites. Thermostability, flammability, and combustion characteristics of PP composites are analyzed. The maximum thermal decomposition rate of flameretarded PP composites has an apparent reduction compared with that of pure PP, and obvious char is left for this intumescent flame retardant (IFR) system of APP and PCA. A high limiting oxygen index value and UL-94 V-0 rating are achieved with addition of APP and PCA. In cone calorimetry test, heat and smoke releases of PP are significantly decreased by this IFR system. Gaseous decomposition products during the thermal decomposition of flame-retardant composites are studied. Chemical structure and morphology of char residues are analyzed. The results illustrate that APP and PCA have a superb synergistic action in the aspect of improvement in fire safety of PP. A possible flame-retardant mechanism is concluded to reveal the synergism between APP and PCA. K E Y W O R D S intumescent flame retardant, mechanism, polypropylene, synergism 1 | INTRODUCTION Polymeric materials, which are now universally used in many areas, have been important to modern life for many years. Polypropylene (PP) is one of the universal polymers and has many advantages, such as easy processability, corrosion resistance, and low cost. However, it is a kind of easily combustible material, which seriously restricts its applications. Because of its emission of poisonous gases while burning, PP has high fire hazard potential for human life. 1,2 Therefore, PP materials must be modified with flame-retardant additives in order to achieve desired flame-retardant performance and meet the requirements in some application fields. Currently, considerable effort has been devoted to explore methods of reducing the inflammability of PP. Various flame retardants, such as metallic hydroxide, expandable graphite, halogencontaining flame retardants, and phosphorous-containing compounds, have been successfully developed to endow PP with flame retardancy. 3 For instance, metal hydroxides, including aluminum hydroxide and magnesium hydroxide, are used in PP, but mechanical properties of PP materials are easily destroyed by high loading of fillers. 4 As a kind of halogen flame retardants, bromine-containing compound has been regarded as the most effective flame retardant for PP. 5 However, some halogen-containing flame retardants are being phased out because of their health and environmental hazards,

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This research proposes a simple and feasible method to prepare hydrophobic structures and it is a fortuitous finding. In our previous publication, triethyl phosphate is adsorbed on porous metal-organic framework...