| Cotton fabrics are favored for their soft,breathable and comfortable wearing properties.They are widely used in many fields such as clothing,home textiles,and transportation interiors.However,cotton fabrics are flammable and the flame spreads rapidly,making them the most dangerous textiles.Therefore,the flame retardant finishing of cotton fabrics is of great practical significance to ensure the safety of people's lives and property.At present,cotton fabric flame retardant finishing technology often has problems such as complicated processes,low flame retardant efficiency,poor durability,or the release of free formaldehyde.Therefore,research and development of halogen-free,formaldehyde-free,highly efficient and durable cotton fabric flame retardant finishing technology has been a research hotspot and difficulty in the field of flame retardant at home and abroad.In this paper,ammonium dihydrogen phosphate(NH4H2PO4)and melamine were used as flame retardants,and a highly efficient and durable phosphorus-nitrogen synergistic flame retardant cotton fabric(P/N-Cotton)was prepared by chemical grafting method.The effects of technological parameters such as NH4H2PO4,urea,cotton fabric dosage ratio,reaction temperature,and reaction time on the grafting efficiency of phosphorus and nitrogen were systematically investigated.Together,microstructure,thermal stability,mechanical properties,flame retardant properties and durability of modified cotton fabric were studied.Finally,the flame retardant mechanism was further explored by combining TG-IR,XPS,Raman and other analytical techniques.(1)Phosphorylation modification and performance of cotton fabric:Phosphorylated cotton fabric(P-Cotton)was prepared by chemical grafting in a mixed system of NH4H2PO4 and urea.The effects of process parameters such as the molar ratio of NH4H2PO4,urea,and cotton fabric(in terms of anhydroglucose units(AGU)),reaction temperature and reaction time on the grafting rate of phosphorus were considered.The best process for phosphorylation modification of cotton fabric was determined,namely AGU:NH4H2PO4:urea=1:2.5:15,reaction time was 1.5 h,and reaction temperature was 130?.At this time,the surface of P-Cotton was rich in phosphoric acid groups,WG was 2.2%,and its P content was 6.1%,and it exerted a good flame retardant effect during the combustion process,and its limit oxygen index(LOI)reached 50.9%,reaching the non-combustible level.SEM observation showed that the structure of cotton fiber was not damaged by phosphorylation modification,and P-Cotton retained the original fiber shape after burning.TG research found that the thermal stability of P-Cotton was significantly improved after modification,and the final residue was still as high as 39.4%under nitrogen atmosphere at 800?.Washing resistance tests show that P-Cotton's flame retardant properties had decreased after washing,but after 30 laundering cycles,its LOI value was 28.5%,which was still a flame retardant material.(2)Preparation and performance of phosphorus-nitrogen synergistic flame retardant cotton fabric:In order to further improve the mechanical properties and flame retardancy durability of phosphorylated cotton fabric,the aldehyde group produced during the phosphorylation process was used to graft melamine to the surface of cotton fabric through schiff base reaction to produce phosphorus-nitrogen synergistic retardant cotton fabric(P/N-Cotton).When the reaction temperature was 135? and the reaction time was 25 min,P/N-Cotton had the maximum weight gain rate,the WG was 25.8%,the P and N contents were 5.3 and 4.9%,and the LOI value reached 51.9%.Compared with P-Cotton,the thermal stability of P/N-Cotton was further improved,the fastest thermal degradation temperature was increased by 50?,and the final residue in the air atmosphere at 800? was increased by 16%.The vertical combustion test showed that the damage length of P/N-Cotton was only 8.0 cm,and it had a self-extinguishing effect.The cone calorimeter test showed that compared with untreated cotton fabrics,the peak heat release rate and total heat release decreased by 70.8 and 68.9%,respectively,and the final residue was 36.5%.After 50 laundering cycles,the LOI value of P/N-Cotton was 32.7%,which was a durable flame-retardant cotton fabric.TG-IR analysis showed that the flammable gas released by P/N-Cotton during thermal degradation was greatly reduced,and the amount of water vapor and non-combustible CO2 was significantly increased.The mechanical properties research found that after the cotton fabric was modified by phosphorylation and melamine,the mechanical strength of P/N-Cotton did not decrease significantly,and it still maintained 72.3%of the original cotton fabric.(3)The flame retardant mechanism of phosphorus-nitrogen synergistic flame retardant cotton fabric:The analysis of P/N-Cotton's final residue structure after combustion was performed using FTIR,SEM,XPS,and Raman spectroscopy.It was found that the P-O-C and and the C=N bond were broken during the combustion process,and the phosphate group was decomposed into phosphoric acid and other dehydrating agents to catalyze the dehydration of cellulose molecules to form a compact carbon layer,preventing cellulose from degrading into L-glucose,reducing the release of flammable gases,and simultaneously releasing a large amount of water vapor and Non-flammable CO2.The resulting carbon layer had a higher degree of graphitization,good thermal stability,and protected the substrate from further combustion.The surface grafted melamine was thermally decomposed into non-combustible gases such as NH3,diluted the concentration of oxygen and combustible gases,and was flame-retardant in the gas phase.Combining with TG-IR research,it was found that the flame retardant effect of P/N-Cotton mainly occurred in the solidified phase,and the flame retardant effect was achieved by promoting the dehydration and carbonization of cellulose to form a graphitized carbon layer.In this paper,NH4H2PO4 and melamine were used as flame retardants,and a highly efficient and durable phosphorus-nitrogen cooperative flame retardant cotton fabric was successfully prepared,and its flame retardant mechanism was clarified.The obtained flame retardant cotton fabric had the advantages of halogen-free and formaldehyde-free,efficient flame retardant,good durability and small reduction in mechanical properties.It solved the problems of large environmental pollution,low flame retardant efficiency and resistance to washing in conventional flame retardant finishing processes of cotton fabrics,and provides a theoretical basis and technical reference for the development of durable flame retardant cotton fabrics. |
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