Synthesis And Application Of Intumescent Flame Retardants For Polyethylene And Their Action Mechanism

Polyethylene (PE) is widely used in many fields such as cable and wire, electrical and electronics industry, etc. However PE has only a limiting oxygen index (LOI) value of 17.4, and is flammable. Therefore, the flame retardancy of PE is very important in order to widen its application areas.At present, the main flame-retardant systems for PE are based on halogen, phosphorus and inorganic hydroxides. For halogenated flame retardants, there are potential damaging effects on environment, and for inorganic hydroxides the properties of the resulting flame-retardant PE materials are deteriorated due to their high loading. Recently, intumescent flame retardants (IFRs) have received a considerable attention because IFRs are of non-halogen, low toxicity, low smoke release and environmentally friendly. The IFRs are designed to swell and form a porous carbonaceous char layers on the surface of materials, which act as a barrier to heat, air and flammable pyrolysis products.Carbon nanotubes (CNTs) are potential nanofillers for flame retardancy. They have a good compatibility with polyolefin due to their non-polarity, so CNT are organophilic and can be dispersed directly into the polymer matrix. A CNT can be regarded as a cylinder rolled by sheet graphitene, so CNTs have many characteristics of graphite, such as heat resistance, good chemical resistance and high strength etc.In order to address the issue of halogen-free flame retardancy of PE, several kinds of novel flame retardants have been designed and synthesized based on 3,9-disubstitued-2,4,6,8-tetraoxo-3,9-diphosphaspiro- [5,5]-undecane-3,9 -dioxide (SPDPC) and triazine, and they exhibit good flame retardancy and synergism when combined with ammonium polyphosphate (APP) and multi walls carbon nanotube(MWNT). The main results in this dissertation are summarized below:A new flame retardant (SPS) was synthesized by melt polymerization from SPDPC and 4, 4'-sulfonyldiphenol(SDP). The other two novel flame retardants PTS and PTEN were synthesized by solution polymerization from the reactions of 2-methoxy-4, 6-dichloro-triazine with 4, 4'-sulfonyldiphenol (SDP) and diethylenetriamine, respectively. The effects of reaction conditions on the viscosities of SPS, PTS and PTEN have been investigated in detail. The reaction mechanisms have been analyzed, and the structures of all the products have been characterized via Fourier transform infrared (FTIR) spectroscopy, MS and 'H-NMR, which confirmed the formation of the object compounds. SPS, PTS and PTEN were oligomers. The result of TGA shows that both SPS and PTEN have good thermo-stability and high residue of 42% and 20% at 600°C, respectively.In this paper, a large mount of flame retardant LDPE formulations based on SPS, PTS, PTEN, APP and MWNT have been studied. Based on the results of LOI and UL-94 tests, two formulations with best flame retardancy were obtained: LDPE+30%IFR and LDPE+29%IFR+1%MWNT, whose LOI values are 29.2 and 30.6, respectively. In the above case, the optimal mass proportion of various components in IFR is APP:SPS:PTEN=3:1:1. The results of CONE showed that heat release rate (HRR), mass lost rate (MLR), peak heat release rate (pk-HRR), peak mass loss rate (pk-MLR) and peak smoke production rate (pk-SPR) decreased largely compared to neat LDPE, indicating that the IFR was very efficient for the flame retardancy of LDPE, and that there existed a synergistic effect between IFR and MWNT.The results of TGA proved that the thermal degradation behavior of LDPE had been changed due to the addition of IFR. The thermal degradation of LDPE+30%IFR system appeared at lower temperature than that of LDPE, but the final residue is much higher than that of LDPE, higher than the summation of residue of neat LDPE, APP, SPS and PTEN from 290 °C to 520°C, respectively, which means that there is a synergism between them. The thermal degradation activation energy of DFR/LDPE increased due to the addition of MWNT, showing that MWNT changed thechar-forming process of IFR/LDPE system.The fire-retarding mechanism of intumescent flame retardant LDPE has been investigated by means of SEM, XRD, FTIR and XPS methods. The results show that an effective foam char layer, which is amorphous, can form while burning for LDPE/IFR and LDPE/IFR/MWNT. The variety of P/C, O/C, N/C atomic concentration ratio shows that MWNT does change the char-forming process, and a better intumescent char layer can be observed from SEM than that of LDPE/IFR, which means a better flame retardancy.By means of SEM, XRD tests, the compatibilization of LDPE/IFR blends was studied. Results showed that IFR had no effect on the LDPE crystalline structure, and the tensile strength and elongation at break decreased largely. But the addition of MWNT can improve the mechanical properties and flame retardancy of LDPE/IFR systems. The addition of IFR did not change the pseudoplastic flow behavior of LDPE, but the viscosity of the system increased.The addition of compatibilizer, PE-g-MAH, did not changed the pseudoplastic flow behavior of LDPE/IFR systems, but the interfacial adhesion between flame retardant particles and matrix increased, so the viscosity increased, which means the processing temperature will have to be raised. However, the higher tensile strength and elongation can be obtained without an obvious change in the flame retardancy when PE-g-MAH was added.