Study On The Synthesis Of Novel Flame Retardants Phosphorus-containing And Preparation/properties Of Halogen-free Flame Retardant Crosslinking Eva-based Composites

With the continued vigorous development of polymer materials, the resulting fire hidden trouble and fire hazards become important issues of concern. Currently, high-performance, efficient, environmentally friendly flame retardant polymer materials are ever increasing, fire-retardant evaluation methods continue to improve and become more scientific, the more stringent fire regulations, in particular, the EU issued two directives“West Electrical and Electronic Equipment Directive (WEEE)”and“Restriction of Hazardous Substances Directive (RoHS)”, the traditional halogens are faced with severe challenges as flame retardant system, research and development of new efficient halogen-free flame retardant system is very urgent and necessary. The polyolefin materials are widely used in the field of wire and cable. EVA is one important member of the polyolefin materials, its oxygen index is low (<19%), with higher heat release rate while combustion, and accompanied by melting dripping and a lot of smoke, so it must be modified through flame retardant. In this paper, the EVA with higher VA content (50%) was selected as the matrix resin, peroxide DCP was used as crosslinked agent. Firstly, spirocyclic pentaerythritol bisphosphorate disphosphoryl chloride (SPDPC) was synthesized through simple dehydrochlorination reaction of pentaerythritol (PER) and phosphorus oxychloride (POCl₃); Secondly, the novel flame retardant (SPDH) containing phosphorus and another novel flame retardant (SPDV) containing phosphorus and silicon with a different molecular structure were synthesized from SPDPC and DOPO derivatives, respectively. The synthesized SPDH and SPDV were used for the flame retardancy in EVA, respectively, and the flame retardant mechanism was investigated. On the other hand, the method of peroxide crosslinking was used, and the flame retardant, mechanical, electrical and thermal aging properties were investigated for the halogen-free flame retardant EVA composites filled with ATH and MDH, respectively. The synthesized SPDV containing phosphorus-silicon was used for the flame retardant optimizing of EVA/MDH cross-linked composites. Meanwhile, the effect of flame retardant properties was studies by OMMT and MWCNTs organically modified with SPDV for EVA-based cross-linked composites. And also, CaCO₃, nature graphite (NG) and expandable graphite (EG) was used respectively for the flame retardant optimizing of EVA/APP/PER/ZB cross-linked composites; NG, EG and graphite oxide (GO) was utilized for the flame retardant of EVA-based cross-linked composites. The relationship between charring and flame retardant properties was analyzed in detail, and the flame retardant mechanism was discussed. Finally, the possible correlation between heat conduction and flame retardancy was studied for EVA-based composites loaded with different inorganic fillers.?The synthesis and characterization of new phosphorus-containing flame retardants First, phosphorus oxychloride (POCl₃) was reacted with pentaerythritol (PER) to form a intermediate product named spirocyclic pentaerythritol bisphosphorate disphosphoryl chloride (SPDPC). After that, the novel flame retardant (SPDH) containing phosphorus and another novel flame retardant (SPDV) containing phosphorus and silicon with a different molecular structure were synthesized from SPDPC and DOPO derivatives. The structure of the intermediate products (SPDPC, DOPO-HQ and DOPO-VMDMS) and the end products (SPDH and SPDV) was characterized by Fourier transform infrared spectroscopy (FTIR),Nuclear Magnetic Resonance (IH NMR, 31P NMR). The thermal properties of SPDH and SPDV were investigated by thermogravimetric analysis (TGA) in both nitrogen and air. It was found that SPDV had better char yield in air (44.6%) than in nitrogen at 800?while SPDH had good char yield only in nitrogen at 800?, which indicates that oxygen can help the charring progress of silicon-containing compounds and the improvement of heat-resistance for char residue.?Research on the effect of flame-retardant cross-linked EVA-based composites by SPDH and SPDV The synthesized two flame retardants (SPDH and SPDV) were used in EVA respectively, and the corresponding halogen-free flame-retardant cross-linked EVA/SPDH and EVA/SPDV composites were prepared by melt blending. The results showed that the PHRR, THR, EHC, and MLR were significantly reduced, and FPI, char yield and LOI were increased for the addition of flame retardants SPDH and SPDV, respectively. UL-94 vertical test indicated that V-2 grade can be obtained by the addition of 20phr flame retardants. It was found that the flame retardant was significantly improved for the cross-linked EVA/SPDH and EVA/SPDV flame retardant composites. The initial decomposition temperature and ignition time was slightly shortened because of the addition of SPDH and SPDV. The smoke production rate (SPR) and total smoke release (TSR) of flame-retardant cross-linked EVA/SPDH composites increased, while TSR was reduced and char residue was enhanced more evidently for flame-retardant cross-linked EVA/SPDV composites, which of the reason was the introduction of silicon-containing compounds promoting the char formation and its thermal stability. It can be found that the char residue presented some small pores for flame retardant cross-linked EVA/SPDH composites while it showed more compact for flame retardant cross-linked EVA/SPDV composites from SEM photos, and EDS results showed that the outer surface of silicon content was higher than that of the inner because of the migration of silicon compounds, which enhanced the thermal stability of the carbon layer.?Research on the effect of flame-retardant cross-linked EVA-based composites by nano-flame retardants (MWCNTs, OMMT)The synthesized flame retardant SPDV was used to the organic modification of the MWCNTs to be MWCNTs-g-SPDV, and its structure and thermal stability was characterized by FTIR, NMR, TEM and TGA, respectively. Both MWCNTs and MWCNTs-g-SPDV was utilized to the flame retardant of EVA. It can be found that the thermal stability was improved for flame-retardant cross-linked EVA composites. The PHRR, AHRR were significantly decreased, and TTI, FPI and combustion residue was increased significantly, which indicating that the flame retardant was enhanced significantly; and also the more obvious improvement for the addition of MWCNTs-g-SPDV because of the dispersion of improving in the EVA matrix and promoting the charring while combustion. On the other hand, MMT and OMMT was used in EVA, respectively. XRD and TEM was utilized to the characterization of microstructure for flame-retardant cross-linked EVA/MMT and EVA/OMMT composites. The results showed that the flame resistance was significantly improved for flame-retardant cross-linked EVA/OMMT composites, while it was slight on the improvement of flame resistance for the addition of MMT. This reason was the formation of intercalated structure by OMMT and EVA matrix, and the inorganic montmorillonite layers can migrate to the surface in the combustion process and form similar to the "labyrinth" structure, which played a role in barrier protection and promoted charring.?Research on the effect of flame-retardant cross-linked EVA-based composites by inorganic metal hydroxide (ATH, MDH) and its optimization of flame retardant by SPDVIt was composed of different formulations by ATH modified with vinyl methyl dimethoxy silane (VMDMS) and MDH modified with amino silane as flame retardants, respectively, and DCP as a crosslinking agent, DDA as antioxidant. The flame retardant, mechanical, electrical, thermal aging and rheological properties was studied for flame-retardant cross-linked EVA-based composites. The results showed that: the flame resistance was significantly improved for flame retardant cross-linked EVA composites, while tensile strength and elongation at break was decreased, and breakdown strength and volume resistance was decreased slightly, but still maintained a high level, which can meet the requirements used as halogen-free flame retardant cable sheathing materials. It can be found that the temperature level used at long time was improved to 125?from 105?rating for the optimized flame-retardant cross-linked EVA/MDH aging formulation. Meanwhile, the synthetic fire retardant SPDV was used to the flame-retardant optimization for flame-retardant cross-linked EVA/MDH composites. The results showed that the flame retardant was further improved, and it presented obvious intumescent phenomenon. The filled MDH can resist the sample deformation and droplet caused by thermal radiation and flame in the combustion process, and the char can obtain a higher intensity and form a swelled structure. The screen barrier and phosphorus/silicon flame-retardant effect was improved, which enhanced the flame retardant efficiency.?Intumescent flame retardant cross-linked EVA-based composites and its mechanism of flame retardant propertiesCaCO₃?NG?EG was used to the flame retardant optimization of the EVA/APP/PER/ZB flame retardant cross-linked composites, respectively. The results showed that the PHRR, THR, SPR were further reduced, and FPI, TTI and combustion residues were further increased significantly for optimized composites, which indicating that the flame retardant and smoke suppression was further improved, showing a certain degree of flame-retardant synergistic effect. It can be found that the thermal stability and strength of char residue was increased duo to the addition of CaCO₃, NG, EG, which was the one of important reasons for the improvement of flame retardancy. Further investigation of the flame-retardant cross-linked EVA composites filled with CaCO₃, NG, EG, it can be found that a obvious improvement of flame retardancy and smoke suppression for flame-retardant cross-linked EVA/EG composites, while a relatively low increase of flame resistance for the flame-retardant cross-linked EVA/NG, EVA/GO composites and a slight enhancement of smoke suppression for the flame-retardant cross-linked EVA/GO composites. A very important reason was that a dramatic expansion can be formed to give a protective screen composed of many graphite layers with hollow structure, which can effectively delay the heat, oxygen transfer, reducing the fuel thermal decomposition and diffusion rate, and promote char formation, while there was almost no effect of intumescent flame retardant for flame-retardant cross-linked EVA/NG, EVA/GO composites. The effect of barrier and thermal stability was greatly reduced for EVA/GO composites because of the oxidation damage of GO.?Study on the relationship between thermal conductivity and flame-retardant properties of EVA-based composites loaded with inorganic fillersThe thermal conductivity and flame retardant properties were tested and analyzed for inorganic filled EVA/MDH, EVA/ATH, EVA/EG, EVA/MMT and EVA/Cu composites. The results showed that it was a directly proportional relationship between the thermal conductivity and ignition time for EVA based composites, and the improvement of thermal conductivity can improve the flame resistance.