Synthesis of N-methyl triazine-ethylenediamine copolymer charring foaming agent and its enhancement on flame retardancy and water resistance for polypropylene composites

A novel charring foaming agent N-methyl triazine-ethylenediamine copolymer defined as MTEC was synthesized from cyanuric chloride, methylamine and ethylenediamine through nucleophilic reaction. Its chemical structure was well characterized by Fourier transform infrared spectroscopy, elemental analysis and 13C solid-state nuclear magnetic resonance. The wettability of flame retardant was evaluated by water contact angle (CA) tests, and the synthesized MTEC present excellent hydrophobic property with the water CA of 117°. Meanwhile, the water CA of the prepared intumescent flame retardant (IFR) system containing MTEC, ammonium polyphosphate and silica reached 104° and also presented hydrophobic property. The obtained IFR was incorporated into polypropylene (PP) resin to prepare flame retardant PP composites, and the flame retardancy, thermal degradation behavior, water resistance and flammability behavior for IFR-PP were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter and thermogravimetric analysis (TGA) tests. Water resistant properties of IFR-PP composites were evaluated by soaking the samples into distilled water at 70 °C for 168 h. The results demonstrated that IFR-PP samples both passed UL-94 V-0 flammability rating before water treated and after hot water soaking and drying when the loading amount of IFR was 22 wt %, the LOI value of IFR-PP only decreased from 29.6 to 29.3% and the mass loss percent was only 0.17 and 0.69% for 3.2 and 1.6 mm samples after water soaking and drying. The TGA results indicated that the incorporation of IFR promoted PP matrix decomposition and charring at a relative low temperature, and then enhanced the char yield and thermal stability for IFR-PP composites at high temperature. The cone calorimeter tests revealed that the introduction of IFR greatly decreased the combustion parameters, such as heat release rate (HRR), smoke production rate (SPR) and so on. After water resistance test, the combustion parameter for IFR-PP was slightly increased. The scanning electron microscopy (SEM) tests indicated the introduction of IFR benefited to the formation of a sufficient, intumescent and homogeneous char layer on the materials surface during burning, which effectively prevented the underlying materials from further degradation and combustion. The structure and morphology of char layer for IFR-PP remained very well after water treated, consequently the water-treated IFR-PP presented excellent flame retardancy.