Study On Synthesis And Application Of Phosphorus-Nitrogen Synergistic Durable And Eco-Friendly Flame Retardant For Cotton

Cotton has the advantages of being soft,comfortable,easy to dye and biodegradable,which is widely used in the production of clothing,home decoration,bedding and industrial fabrics.However,cotton has a limit oxygen index（LOI）of around 18%,which makes it highly susceptible to combustion.Every year,fires caused by burning cotton fabrics result in huge losses of life and property.Therefore,it is very important to study cotton fabric flame retardants to give cotton fabric flame retardant properties.Among the cotton fabric flame retardants that have been studied,halogen flame retardants are highly effective,but they produce toxic gases and carcinogenic substances during combustion which have been banned in many countries.Proban and Pyrovatex CP are phosphorous-containing flame retardants which are widely used in the market today to impart efficient and durable flame retardancy to cotton fabrics,but they can produce carcinogenic formaldehyde during use.Biomass-based flame retardants are safe and environmentally friendly,but most of the cotton fabrics prepared from them are not resistant to washing.Cotton fabrics finished with ammonium phosphonate flame retardants have excellent flame retardancy and can withstand several gentle washes,but they are susceptible to the effects of metal ions during washing,resulting in reduced flame retardancy and failing the more stringent durability tests.Therefore,there is still a need to develop new halogen-free,formaldehyde-free,highly efficient and durable flame retardants for cotton.In this study,three phosphorus-nitrogen synergistic active flame retardants for cotton were designed and synthesized,respectively pentaethylenehexamine octamethylene tetraphosphonate dimethyl ester ammonium tetraphosphorus acid（APEHAP）,amino trimethylene phosphonic acid ethylene glycol ester ammonium phosphonate（AATMPEG）and pentaethylenehexaamine phosphoramidite ammonium phosphonate（APPEHA）.The flame retardants were finished onto cotton fabrics by dip-rolling and baking,a the structure and properties of the flame retardant finished cotton fabrics were tested and analyzed.The main research work and results are as follows:（1）The flame retardant APEHAP containing phosphonate groups and ammonium phosphonate active groups was synthesized from pentaethylenehexamine,dimethyl phosphite,phosphite,formaldehyde and ammonia.Fourier transform infrared spectroscopy（FT-IR）and X-ray photoelectron spectroscopy（XPS）results showed that APEHAP reacted with the cotton fabric to form P–O–C covalent bond.Energy dispersive X-ray spectroscopy（EDS）,scanning electron microscopy（SEM）and X-ray diffraction（XRD）confirmed that the flame retardant APEHAP was successfully introduced into cotton fabrics with little or no effect on the surface morphology and crystal structure.After 50 washes according to standard AATCC 61-2013 2A,the EDS test results showed that the total Na⁺,Mg²⁺and Ca²⁺content of the APEHAP flame retardant finishing cotton was significantly lower than that of the pentaethylenehexamine octamethylene-phosphonic ammonium acid（APHOMPA）flame retardant finishing cotton.This indicated that flame retardants prepared by replacing part of the ammonium phosphonate active group with closed phosphonate groups can reduce the influence of metal ions.The LOI of the 30 wt%APEHAP-finished cotton fabric reached 45.2%,remained at 33.6%after 50 washes,and showed no re-ignition or under-ignition in the vertical burn test.This indicated that APEHAP-finished cotton fabric has excellent flame retardancy and durability.Thermogravimetric（TG）,cone calorimetric and residual char analyses showed that APEHAP can alter the thermal degradation process of cotton fibers,decomposing at high temperatures to produce phosphoric acid or polyphosphoric acid to promote dehydration and charring of cotton fabrics,and releasing N₂ or NH₃ to form a char layer with a large number of bubbles in the ignition area,effectively stopping the spread of the flame.The flame retardant treated cotton fabric showed no detectable formaldehyde and maintained good mechanical properties.（2）The flame retardant AATMPEG containing phosphonate ester groups and ammonium phosphonate active groups was synthesized by using amino trimethylene phosphonic acid,ethylene glycol and urea as raw materials.EDS,FT-IR and XPS tests showed that the flame retardant AATMPEG was uniformly distributed in the cotton fabric and grafted with the cotton fibers to form P–O–C bonds.Flame retardant in the synthesis process through the esterification reaction formed phosphonate ester groups,reducing the number of ammonium phosphonate active groups,which can reduce the effect of metal ions in the washing process on the flame retardancy of cotton fabrics.Durability,LOI and vertical flame tests showed that AATMPEG endows cotton fabrics with excellent flame resistance durability.The LOI of the 25 wt%AATMPEG-finished cotton fabric was 45.0%and the LOI after 50 washes according to standard AATCC61-2013 2A was 33.0%,both of which were not ignited in the vertical burn test.SEM and EDS tests showed that the flame retardant successfully entered the interior of the fibers without affecting the surface morphology of the cotton fabric,and that flame retardant cotton formed an expanded charcoal layer rich in phosphorus and nitrogen elements in the ignition area.TG test showed that AATMPEG promoted dehydration charring of cotton fabric and that flame retardant cotton had high residual char in both nitrogen and air atmospheres.Cone calorimetry test showed that AATMPEG can reduce the heat release rate and fire growth rate of cotton fabric,which can effectively inhibit the flame spread.The treated cotton fabrics showed a slight increase in hardness and a slight decrease in breaking strength.AATMPEG has no formaldehyde used in the synthesis process,making flame retardant finishing cotton safer and more environmentally friendly.（3）The flame retardant APPEHA containing P（=O）–N bonds and ammonium phosphonate active groups was synthesized from pentaethylenehexamine,phosphorus pentoxide and urea.FT-IR and XPS tests showed that the flame retardant was grafted onto the cotton fabric via the P–O–C covalent bond.There was a p-πconjugation effect in P（=O）–N bond which enhanced the stability of P（=O）–N bond and P–O–C bond between the flame retardant and the cotton fiber.Cotton fabric treated with 30 wt%APPEHA passed the more stringent AATCC 61-2013 3A standard for 50 washes and the limiting oxygen index value only dropped from 48.9%to 42.7%.Analysis of the surface morphology,elemental content and crystal structure showed that the flame retardant successfully penetrated the cotton fabric with hardly any effect on the surface morphology and crystal structure of the cotton fiber.APPEHA flame retardant finishing cotton showed excellent flame retardancy in both the vertical flame test and the cone calorimetry test.Compared with the control cotton,the fire growth rate and CO₂/CO values of flame retardant cotton decreased significantly,which indicated that the fire risk of APPEHA flame retardant cotton was significantly reduced.TG test showed that APPEHA can promote the dehydration charring of cotton fabrics,exhibiting a cohesive phase flame retardant mechanism.The residual charcoal analysis showed that APPEHA exerted a synergistic phosphorus-nitrogen flame retardant effect,causing the cotton fabric to form an expanded charcoal layer containing a large amount of non-combustible gases,which isolated oxygen and heat preventing the cotton fabric from burning.After the flame retardant finishing,the softness and breaking strength of the cotton fabrics remained good.The results show that APPEHA can be used as an efficient,durable and environmentally friendly flame retardant for cotton.