Design,Preparation,and Flame-Retardant Application Of DOPO-Derived Acid Modified Sol-Gel System

The development and application of flame-retardant polymers and their composites is of great significance in the field.Low toxicity,simplicity,and high efficiency of flame retardants are the mainstream in the recent years.Sol-gel technology,as a simple and environmentally friendly approach,can form inorganic or organic-inorganic hybrid coatings on the surface of materials as a heat-resistant and fire-resistant barrier to endow materials with flame retardancy;however,the system is suffering from the problem of relatively low flame retardant efficiency.For this,sol-gel systems with halogen-rich,phosphorus-rich and/or nitrogen-rich molecules have been applied as flame retardant synergists.Among them,9,10-dihydro-9,10-oxo-10-phosphorus-phenanthrene-10-oxide（DOPO）and the derivatives have attracted much attention due to their environmental friendliness,high efficiency,and versatile applications.In this work,a novel DOPO-derived acid-modified sol-gel system was developed,by which DOPO was simply and conveniently introduced into the sol-gel system to prepare flame-retardant polyester fabrics and interfacial flame-retardant glass fiber/carbon fiber epoxy resin composites.The flame retardant properties and flame retardant mechanism were systematically studied,and the influence of the flame retardant system on the heat resistance and mechanical properties of the material was discussed,and the effectiveness of the system in the application of surface/interface flame retardant was proved.The main research results of this work are as follows:1.Using 3-aminopropyltriethoxysilane（APTES）and DOPO as raw materials,through the neutralization between polyγ-aminopropyl silsesquioxane（PAPS）and the DOPO-derived acid（DOPA）,a novel DOPO-modified sol-gel system（PAPSDOP）was obtained.The sol solution of PAPSDOP showed superior storage stability in room temperature and a colorless,uniform and transparent gel film was achieved after evaporation of the solvent.Moreover,the size distribution of PAPSDOP sol was studied by a nanoparticle size potential analyzer;the chemical structure of PAPSDOP was confirmed by Fourier transform infrared spectroscopy（FTIR）and nuclear magnetic resonance（NMR）;the thermal stability of PAPSDOP was studied by thermogravimetric test（TG）.2.The PAPSDOP coating was constructed on the surface of polyester fabrics by dip-coating technology.With an ultra-low loading（1 wt.%）,PAPSDOP could reduce the flammability completely stop melt-dripping of polyester fabrics.8 wt.%PAPSDOP could drastically improve the limiting oxygen index（LOI）of the fabrics to 32%from 21.5%;in the cone calorimetry test（CCT）,the ignition time of the flame retardant sample delayed compared with the reference sample and the heat release rate decreased.PAPSDOP coated polyester fabrics kept high thermal stability with an initial weight loss temperature of 393℃（that of the reference sample was 391℃）.Thermogravimetric infrared spectrometer（TG-IR）and thermal pyrolysis-chromatography/mass spectrometry（Py-GC/MS）were employed to studied the thermal decomposition behavior of the fabrics;besides,the morphology,element content,and chemical state of the char residues were studied by scanning electron microscopy-energy dispersive X-ray spectroscopy（SEM-EDX）and X-ray photoelectron spectroscopy（XPS）.It was thus found that PAPSDOP showed dominating gas-phase flame-retardant effect for the polyester fabrics accompanied with condensed-phase flame-retardant activities.3.Glass fiber blankets were coated by PAPSDOP coating through the dip-coating technology,and applied to prepare flame-retardant epoxy resin composites.SEM-EDX and XPS were applied to investigate the surface morphology and chemical composition of glass fibers;XPS and differential scanning calorimetry（DSC）were conducted to study the influence of PAPSDOP on the curing behavior of epoxy resins.Accordingly,it was confirmed that PAPSDOP deposited on the surface of glass fibers in a form of well-distributed and continuous coating and can promote curing of epoxy resin in the composites.With the weight gain of PAPSDOP of 10 wt.%,the composite(PAPSDOP₁₀@GF/EP)achieve the UL 94 V-0 rating with an LOI as high as 48.6%（that of the reference sample was 29.1%）.In the CCT,the peak of heat release rate（PHRR）,total heat release（THR）,and total smoke release（TSP）of the modified composite simultaneously decreased compared to control sample.Besides,the obtained composites showed relatively high thermal stability and mechanical performance.Especially,the Flexural Modulus of the PAPSDOP₁₀@GF/EP was even 29%higher than the unmodified sample.4.Carbon fibers were coated by PAPSDOP coating through the dip-coating technology,and applied to prepare flame-retardant epoxy resin composites.Accordingly,it was confirmed that PAPSDOP deposited on the surface of Carbon fibers in a form of well-distributed and continuous gel coating.With the weight gain of PAPSDOP of 10 wt.%,the LOI of carbon fiber composites increased from 30.2%of the reference sample to38.3%,and passed UL-94 V-0 rating.PHRR,THR and TSP are decreased in the CCT test.In the mechanical property test,it is found that the introduction of DOPA in the interface enables the carbon fiber composites to maintain excellent flexural strength and flexural modulus,and the notch izod impact strength is increased by 66.4%compared with the reference sample.