The Fire Performance And Mechanism Of Intumescent Flame Retardant Polystyrene Containing Novel Synergistic Agents

Halogen-containing flame retardants and intumescent flame retardant have been widely used in improving flame retardancy of PS. Halogen-containing flame retardants usually have high efficiency; however, they increase the release amount of smoke and toxic gas during combustion. An intumescent flame retardant (IFR) system has advantages of low smoke release and anti-dripping, however, high loading (25%) is usually required to achieve certain flame retardant level, which damages other properties of PS. We has tried to investigate the possible synergistic agents for PS/IFR composite in order to improve the flame retardant efficiency and reduce the load of IFR in this work. Four kinds of potential synergistic agents were investigated in this paper (Lewis acid-type transition metal chlorides (LATs), HA zeolite, compound oxides and SO42-/Fe2O3solid super acid) to improve the flame retardancy of PS/IFR. The chemical structures of synergistic agents were analyzed by X-ray diffraction (XRD) and Fourier transition infrared spectroscopy (FTIR). The effect of synergistic agents on thermal stability and flame retardancy of PS/IFR was investigated by thermogravimetric (TG), differential scanning calorimetry (DSC), limiting oxygen index (LOI) and cone calorimetry. The morphology and chemical constitution of residue char were investigated by scanning electron microscope (SEM) and FTIR. The synergistic mechanism was also proposed and discussed.1. Four kinds of transition metal chlorides (NiCl2, CuCl2, ZnCl2and FeCl3) have been introduced to the polystyrene (PS)/IFR composite to improve its fire performance. The flammability of PS/IFR/LATs composites was characterized by limiting oxygen index (LOI) and cone calorimetry. The thermal behavior of the composites was analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results showed that initial decomposition temperature (T5%) of PS could be increased through the addition of FeCl3, CuCl2and ZnCl2. However, NiCl2decreased T5%and the temperature of the maximum mass loss rate (Tm), and increased stability of residue char at high temperature range. Tm of PS/FeCl3has been increased about20℃comparing to that of PS. The results showed that the flame retardancy of PS/IFR can be significantly enhanced by the presence of LATs. The LOI value of PS/IFR containing1.0wt.%FeCl3reached up to26, which is14%higher than that of PS/IFR。Heat release rate (HRR) peak of PS/IFR containing1.0wt.%FeCl3was decreased to325.8kW/m2which was reduced68.5%comparing with that of pure PS. The carbonaceous residue of PS/IFR composite containing1.0wt.%ZnCl2was increased to13%which was78.1%higher than that of PS/IFR in cone test. The smoking density of PS/IFR was also reduced after the addition of LATs.2. The acidified4A zeolites (HA zeolite) containing Lewis acid sites (4A-L zeolite) and Bronsted-acid sites (4A-H+zeolite) were prepared, and their crystallinity and acidic site structure were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The thermal behavior and flammability of a PS composite containing different types of zeolite were investigated by TGA, volatile gas and condensed phase FTIR analysis, LOI, and cone calorimeter. Both4A-L and4A-H+zeolite could enhance the flame retardancy of PS composite significantly. It showed that the LOI value of PS/IFR composite (20wt.%IFR) was23.6%. The LOI values of PS/IFR composites firstly increased with4A-L zeolite loading and reached its maximal value of26.0%when4A-L content was2.0wt.%. The HRR of PS composite was decreased48%after adding2wt.%4A-L. It is suggested that Lewis acidic sites in HA zeolite can alter the degradation route and enhance the char formation of a PS composite. THR of PS/4A-L composites was only52%of that of pure PS. The PHRR of PS/IFR/4A and PS/IFR/4A-L composites was216.9and138.4kW/m2, respectively, which was decreased significantly compared with357.0kW/m2of PS/IFR. The4A-L zeolite can promote the decomposition process of PS/IFR in the temperature range of350-408℃and reduce the mass loss rate after428℃, leaving more char residue after heating. The Lewis acid on4A-L zeolite can catalyze cross-linking reactions of the PS/IFR/4A-L composite. The generated from the esterification of APP and PER was involved in the reaction with4A-L zeolite. The Lewis acid on4A-L zeolite converted into complexes through interaction with H2O, and the complexes continued to react with unsaturated-C=C-to form primary carbonium ion. This interaction resulted in acidic catalysis Friedel-Crafts alkylation, promoting char formation.3. Compound oxides (NiFeO and CoFeO) were obtained by calcining layered double hydroxides (LDHs) precursors which were prepared by microwave-hydrothermal method. The structure of LDHs and compound oxides was characterized by XRD. The thermal behavior and flammability of polypropylene (PP) composites containing different types of compound oxides were investigated. Both NiFeO and CoFeO can increase the LOI significantly. LOI value of PP/IFR was increased7-7.5by the addition of2.0wt.%NiFeO or CoFeO. The HRR of a PP/IFR composite could be decreased by12%and33%respectively at the presence of2.0wt.%NiFeO or CoFeO. More compact residue char could be obtained by the addition of NiFeO or CoFeO. The char residue of PP composites after cone test was characterized. NiFeO or CoFeO can induce the formation of graphitic structure. The adsorption ability of magnetic compound oxides on phosphate and might remain more O and P in char residue. It is suggested that complex oxides containing transition metal can enhance flame retardancy of a PP composite through trapping free radicals in gas phase and improving the char formation quality in the condensed phase. As for PS/IFR, CoFeO could increase char formation significantly under air atmosphere. More compact char can be observed for PS/IFR/NiFeO or PS/IFR/CoFeO sample in cone test. The LOI value of PS/IFR containing0.5wt.%NiFeO reached24.8, and the LOI value of PS/IFR containing1.0wt.%CoFeO reached24.3. However, compound oxides increased THR and total smoke product. LRS shows that NiFeO or CoFeO can induce the formation of graphitic structure. The adsorption ability of magnetic compound oxides on phosphate and might keep more O and P in char residue.4. Solid superacid (SO42-/Fe2O3) was introduced in PS/IFR as synergistic agent. The structure of SO42-/Fe2O3was characterized by XRD. The thermal behavior and flammability of PS and PS/IFR were investigated, and the char residue of PS/IFR/(SO42-/Fe2O3) was analyzed. The results showed that acid sites on the surface had positive effect on improving the flame retardancy of PS/IFR. The residue char of PS/SO42-/Fe203at500℃reached13%under air atmosphere in TG test. The pHRR value of PS/IFR/(SO42-/Fe2O3) was reduced to341kW/m2from435.6kW/m2of PS/IFR. It is proposed the Lewis acid on SO42-/Fe2O3has the similar mechanism to that of Lewis acid sites in4A-L zeolite, which can promote char formation through reacting with H2O generated from the esterification of APP and PER. LRS shows that SO42-/Fe2O3could promote formation of amorphous carbon. XPS shows that SO42-/Fe2O3can also keep more O and P in char residue.