Preparation and characterization of oligomeric char forming agent and its effect on the thermal degradation and flame retardant properties of LDPE with ammonium polyphosphate**

Abstract

An efficient oligomeric charring agent (CNCA-DA) containing a triazine and benzene rings was synthesized and characterized by the nuclear magnetic resonance spectroscopy (13C solid-state NMR), elemental analysis (EA) and thermal gravimetric analysis (TGA). The synergistic effects between Ammonium polyphosphate (APP) and CNCA-DA on the flame retardancy, thermal degradation and flame retardant mechanism of low density polyethylene (LDPE) were investigated. The 13C NMR and elemental analysis (EA) results indicated that the structure of CNCA-DA coincided with the expected design. The thermogravimetric analysis (TGA) analysis showed that CNCA-DA had a good char forming ability with a high initial temperature of thermal degradation. APP improved the char residue of the APP/CNCA-DA system with char residue reached 41.0% and 18.2at 600 and 700°C under air. The results from limited oxygen index (LOI) and vertical burning test (UL-94) showed that the intumescent flame retardant (IFR) was very effective in the flame retardancy of LDPE. When the mass ratio of APP and CNCA-DA was 2:1 with IFR loading at 30%, the LOI value of LDPE/IFR composite reached 30.9%. It was also found that when the IFR loading was only 25%, the flame retardancy of LDPE/IFR still passed V-0 rating in UL-94 tests with LOI value of 26.8%. The thermogravimetric analysis (TGA) and Thermogravimetric analyzer coupled to a mass spectrometer (TGA/MS) results indicated that IFR could reduce the thermal degradation rate at main stage of degradation and side chain fragmentation was probably the dominant mechanism of thermal degradation. The cone calorimeter teat test (CCT) results demonstrated that IFR clearly changed the decomposition behavior of LDPE and formed an intumescent char layer on the surface of the composites, subsequently resulting in the efficient reduction of the flammability parameters, such as the heat release rate (HRR), total heat release (THR), smoke production rate (SPR), total smoke production (TSP), mass loss (ML), CO production (COP) and CO2 production (CO2P). The morphological structures observed by digital photos and scanning electron microscopy (SEM) demonstrated that IFR promoted to form the continuous and compact intumescent char layer. The Fourier Transform infrared spectroscopy (FTIR), Laser Raman spectroscopy analysis (LRS) analysis results revealed that an appropriate graphitization degree and network structure of the residue char were formed to improve the strength and shield properties of the char layer to obtain better flame retardant properties of the composite.