Flame-Retardant Modification Of Polyacrylonitrile Filament Fabrics By Layer-by-Layer Self-Assembiy

The polyacrylonitrile（PAN）filament is usually composed of 50 to 200 monofilaments,with low total denier,and it has high and uniform mechanical properties.PAN filaments can be woven directly,the fabric drape good,and has high added value of products.However,it has poor flame-retardant performance,and its Limiting Oxygen Index（LOI）is only 17.5%,it will release HCN,CO and other toxic gas when it burns.Its fire hazard is great,which limited the expansion of PAN fabrics’practical application.In this paper,the phytic acid（PA）/polyethylene imine（PEI）binary self-assembly system and PA/PEI/3-aminopropyltriethoxysilane（APTES）terpolymer self-assembly system are be used to prepared the flame-retardant PAN filament fabrics respectively,with Layer-by-Layer self-assembly method,and research the corresponding flame-retardant mechanism.（1）The PAN filament fabric was pretreated by the strong acidity of PA.PAN was hydrolyzed for 1 H at 95℃.COO^-and PO₄^2-peaks were added to the infrared spectrum and XPS spectrum,and the content of P element was 8.61 wt%.（2）In the PA/PEI binary self-assembly system,the PAN filament fabric was prepared with 5,10,15 PA/PEI flame retardant film Layers（BL）,by the electrostatic adsorption of polycations such as NH₄⁺and polyanions such as HPO₄^-and COO^-.The changes of weight gain rates and Zeta potential indicated that PA/PEI were successfully attached to the surface of the fabrics.The flame retardant coating was observed in the SEM photo.The infrared and XPS maps show that the peaks strength of HPO₄^-and NH₄⁺were increased.And on PAN-15BL fabrics,the contents of N and P elements increased to 10.5 wt%and 9.59 wt%respectively.Accordingly,with the increase of the number of self-assembled layers,the thermal and flame-retardant properties of the fabrics are improved.The carbon residue at 900℃increases and the LOI also increases.However,in view of the lowest Fire Growth Rate index（FIGRA）（8.8m²s/k W）and the highest Flame Retardant Index（FRI）（7.7）,PAN-5BL has the best fire performance.The strength of PAN-5BL fabric was 140.1N and the elongation at break was 39.2%,which meets the application requirements.Take the mechanical properties and the fire properties into account,for further research on the combination formula of flame retardant,the five layers in Layers-by-Layers self-assembled system is selected to improve the flame-retardant effect.（3）Based on the study of PA/PEI binary Layers-by-Layers self-assembly system,the flame retardant fabrics of PA/PEI/APTES ternary system with different APTES concentrations（1.0wt%,2.0 wt%,3.0 wt%）and two different self-assembly processes were prepared.Through the electrostatic adsorption of polycations such as HPO₄^-polyanion and NH₄⁺,as well as the adsorption of-OH on phosphoric acid and Si-OH,the flame retardant film was successfully self-assembled on the fabric.The infrared and XPS spectra show that there was a new peak of Si-O/Si-OCH₃ appeared at 102.1 e V.The content of Si in PAN-3Si 2BL is the highest（4.01 wt%）.In SEM images,the overlay connection also can be observed,but the surface of the PAN-3Si 1BL and PAN-3Si 2BL fabrics appeared cracks.After PA/PEI/APTES Layers-by-Layers self-assembly,the thermal and flame-retardant properties of the fabrics are significantly improved.The hightest carbon residue at 900℃.The LOI of PAN-2Si 2BL,PAN-3Si 1BL and PAN-3Si 2BL are all higher than 26%,and the LOI of PAN-3Si 2BL is as high as 35%,reaching the non-combustible standard.The FIGRA values of self-assembled fabrics were lower than those of untreated PAN fabrics（<12.6 m²s/k W）,and the FRI values were also maintained at about 4.Meanwhile the breaking strength and elongation at break of PAN-2Si 2BL fabrics are 115.9 N and 34.6%,which meet the application requirements of the fabrics.Combined with the performance characterizations,we can know:PA,as an acid source,decomposes to produce phosphoric acid groups,adsorbs free radicals,and promotes the dehydrating cyclization of PAN and APTES.PAN formed a relatively stable trapezoidal cyclization structure.APTES formed a heat-resistant silicon-oxygen structure for dense carbon layer which can block oxygen,and playing a solid-phase flame retardant role.Meanwhile the amino groups in PEI and APTES are decomposed into nitrogen,which plays a flame-retardant role in the gas phase.The whole components play the role of synergistic flame-retardant effect.