Synthesis And Application Of A Series Of Environmentally Friendly,Efficient And Durable Reactive Flame Retardant For Cotton

Cotton fabric has been widely applied in various areas such as clothing,bedclothes,indoor adornment and automotive interior etc.due to its excellent functions,for example,softness,comfort,excellent breathability and hygroscopicity,and it occupies a large proportion in the textile market.However,the wide use of cotton brings on the security risks in which a large number of fire accidents reasily occurred for cotton,because cotton is inflammable with a low oxygen index of 18.0%.Hence,it is very significant to investigate the novel flame retardant finishing of cotton.Flame retardants for cotton are mainly divided into two categories,including halogen-based flame retardant and phosphorous-flame retardant.Halogen-based flame retardant could exhibit an excellent flame retardancy.However,halogen-based flame retardants release toxic gas and volumes of smoke,easily leading to secondary pollution.Phosphorous-flame retardant draws attention from many researchers,owing to its abundant ingredients,moderate reaction condition,low smoke and low toxicity.At present,the most widely used flame retardants for cotton are Tetrakis Hydroxymethyl Phosphonium Chloride（THPC）and N-hydroxymethyl-3-（dimethoxy-phosphono）propionamide（Pyrovatex CP）.The flame retardant finishing technique of the two flame retardant is advanced and both of flame retardants could present expected flame retardancy.Nevertheless,a large amount of formaldehyde was dispersed on the surface of cotton fabrics treated by the two flame retardants.Therefore,it is of great importance to explore novel halogen-free and formaldehyde-free flame retardants for cotton.Synthesises and evaluations of three kinds of ammonium phosphate were described in this paper,including ammonium salt of pentaerythritol tetraphosphoric acid（APTTP）,ammonium salt of ethylene glycol diphosphoric acid（AEGDP）,and ammonium salt of1-hydroxyethylidene-1,1-diphosphonic acid（AHEDP）.This series of reactive flame retardants were firstly designed and synthesized without halogen and formaldehyde.Their structures were characterized by ¹H NMR,¹³C NMR,³¹P NMR and FT-IR spectrum.A certain concentration of flame retardant solution was prepared and was used to treat cotton cellulose matrix.Flammability property test and mechanical test of the cotton treated by different concentrations of flame retardants were conducted in this paper.Flammability test includes limiting oxygen index test（LOI）,vertical flammability test,cone calorimetry,thermogravimetric（TG）,and TG-IR.Mechanical tests including stiffness,breaking strength,and air permeability were performed.The related research was described as the following:（1）Synthesis and evaluation of reactive flame retardant ammonium salt of pentaerythritol tetraphosphoric acid（APTTP）for cottonAPTTP was prepared by a solventless synthesis method and characterized by Fourier-transform infrared（FT-IR）spectroscopy and nuclear magnetic resonance(¹H NMR,¹³C NMR,and ³¹P NMR).Limiting oxygen index（LOI）of cotton fabric treated by 140 g/L APTTP solution was 43.8%,and the LOI value of treated cotton after 50laundering cycles could remain at 26.9%（within flame retardant standard）.This result indicates that APTTP exhibits excellent flame retardancy and durability,and the cotto n fabric treated by 140 g/L APTTP solution could be used as durable flame retardant fabric.During vertical flammability test,no after flame and after glow were observed for all the treated cotton.Cone calorimetry at irradiative flux of 35 and 50 kW/m²revealed that peak heat release rate and total heat release of APTTP-treated cotton were significantly decreased compared with those of control cotton.Thermogravimetric results revealed that initial decomposition temperature and final decomposition temperature were decreased nearly 100 ^oC compared with those of control cotton,while the final residue of treated cotton was greater than that of control sample.These results also show APTTP act in condensed phase mechanism.Scanning electron spectroscopy（SEM）revealed that the shapes of treated cotton fibers remained well even after burning.（2）Synthesis and evaluation of reactive flame retardant ammonium salt of1-hydroxyethylidene-1,1-diphosphonic acid（AHEDP）for cottonBased on the restriction of preparation time and production condition,we designed and synthesized an ammonium salt of 1-hydroxyethylidene-1,1-diphosphonic acid（AHEDP）by a facile,one-step solvent-free methodology.The AHEDP flame retardant exhibits excellent flame resistance and durability without affecting cotton cellulose crystal structure.The superior flame retardancy and durability of AHEDP-treated cotton,accessible ingredients and facile synthesis method make it possible for AHEDP to be industrialized.The LOI test results indicated that cotton fabric treated with 30-40%AHEDP could be used as a durable flame retardant fabric with a LOI value of 26.2-29.5%after 50 laundering cycles.The cone calorimetry results showed that the peak heat release rate（pHRR）and total heat release（THR）of cotton fabric treated with 30%AHEDP solution was decreased by 95%and 71%compared with those of untreated cotton.No after-flame and after-glow phenomena for treated cotton were observed in the vertical flammable test.Thermogravimetric（TG）data both in nitrogen and in air atmosphere revealed that the incorporation of AHEDP notably increased the char yield of treated cotton by catalyzing the dehydration of cellulose,thus protecting the underlying matrix from heat and fuel.Scanning electron microscopy（SEM）indicated that no AHEDP coating appeared on the surface of cotton and no damaged cellulose structure was observed.The Fourier Transform Infrared Spectroscopy（FT-IR）of control cotton and treated cotton revealed that new bonds P-O-C formed between cellulose and AHEDP,which further verified that AHEDP was chemically bound to cotton instead of only mixing into the polymer.（3）Synthesis and evaluation of reactive flame retardant ammonium salt of ethylene glycol diphosphoric acid（AEGDP）for cottonTo enhance the phosphorous content of the synthesized flame retardant,a small molecule of ethylene glycol was employed to prepare flame retardant ammonium salt of ethylene glycol diphosphoric acid（AEGDP）.The LOI value of cotton fabric treated by120 g/L AEGDP solution could reach 41.0%,and even after 50 laundering cycles,LOI could also remain at 28.4%.In cone calorimetry,the treated cotton presented a lower heat release rate and total heat release compared with those of control cotton at both35and 50 kW/m² heat flux.X-ray diffraction spectra suggested that AEGDP hardly affected cotton cellulose crystal structure.Because some of the hydroxyl groups of cotton cellulose reacted with flame retardant AEGDP and the decrease of the-OH group reduces the density of hydrogen bonds,thus extending the amorphous region,leading to reduced degree of crystallinity and crystalline index for the treated cotton cellulos.P-O-C peak was observed on the FT-IR spectrum of treated cotton,and no flame retardant coating was covered on the treated cotton cellulose.These results indicates that AEGDP was a kind of reactive flame retardant.The lower surface tension of treated cotton suggests that treated cotton is able to self-clean to a certain extent.（4）Investigation on flame retardant mechanis m of three flame retardantsThe flame retardant mechanism of the above-mentioned three reactive flame retardants is the same because all of them contain the same reactive group(–P=O（O^-NH₄⁺）.Firstly,–P=O（O^-NH₄⁺）groups in the flame retardants react with C₆–OH groups of cotton cellulose in the presence of catalyst dicyandiamide at 150 ^oC.Then,during the combustion,phosphorous or polyphosphorous acid was formed onto the surface of cotton,which could not only catalyze dehydration of cotton cellulose,but also isolate the undamaged cotton cellulose from heat and oxygen,thus,leading to the self-extinguish of burning cotton fabric.Compared with another kind of flame retardant which acts by forming coating on the surface of cotton,the flame retardants synthesized in this paper reacts via forming P-O-C covalent bonds,which conferred cotton with the excellent durability and superior flame retardancy.