Effect Of Modification On Structure And Property Of Polyacrylonitrile Based Pre-oxidized Fibers

Because of excellent elasticity,warmth,and abrasion resistance,polyacrylonitrile?PAN?fibers is widely used in various fields such as clothing,decoration,and industry.However,the limiting oxygen index?LOI?of PAN fibers is only 18.0,which is extremely flammable.Therefore,the research,development and production of flame retardant fibers have been paid particular attention.Polyacrylonitrile based pre-oxidized fibers?OPF?is a kind of permanent flame retardant fibers,which can be used in the fields of flame retardant and high temperature resistance.However,during the pre-oxidation process,the cyclization reaction of PAN fibers is exothermic and concentrated,which causes great damage to the fibers.Therefore,we use PAN fibers as the raw material,select and optimize the chemical modification process through orthogonal experiments,prepare the chitosan?CS?-sodium tripolyphosphate?STPP?flame retardant coating by layer-by-layer?LbL?assembly method,and prepare silica?SiO₂?flame retardant coating by the sol-gel method.Finally,the OPF with good morphology,superior thermal stability and flame retardant properties is successfully prepared by the synergistic effect of three-step modification.In order to improve the degree of pre-oxidation and produce OPF fibers with better flame retardant properties,the PAN fibers are firstly pretreated by chemical modification,and the influence of the parameters such as concentration,temperature,time and pH value of the modified reagent on the flame retardant properties was studied.By designing orthogonal experiments and analyzing the results,it is found that the PAN_HA-MEA fibers have excellent properties,which are treated with hydroxylamine hydrochloride?HA?solution first,and then treated with monoethanolamine?MEA?solution.Before and after modification,the results of the Fourier transform infrared spectrometer?FT-IR?,scanning electron microscope?SEM?,X-ray spectrometer?EDS?and X-rays diffraction?XRD?show that the cyano group has undergone an amide oxime reaction.Compared with an individual reagent,the use of mixed reagent has a greater degree of fibers'modification.The results of LOI,thermogravimetric analyzer?TG?and micro combustion calorimeter?MCC?show that during the pre-oxidation process of modified fibers,part of the cyano group first reacts with the reagent,which leads to a reduction in the activation energy required for the cyclization reaction,which promotes the pre-oxidation process and increases the degree of cyclization.At the same time,it avoids the loss of fibers caused by concentrated heat release,so that the modified pre-oxidized fibers(OPF_HA-MEA)have good mechanical and flame retardant properties.All of this provide good support for subsequent flame retardant coating.On the basis of modification,we further selecte CS and STPP,prepare the CS-STPP intumescent flame retardant coating by LbL assembly method,and its effect on the flame retardant property of the fibers is studied.First,we prepare a certain mass fraction of CS solution and STPP solution,and use a simple dipping method to deposit it on PAN_HA-MEAA-MEA through electrostatic interaction to form a CS-STPP coating.Then it is pre-oxidized to obtain the OPF_HA-MEA-L?L represents the numbers of bilayers?.The morphology,mechanical property and flammability of the prepared OPF_HA-MEA-L are systematically studied.The results show that the PAN fibers by chemical pretreatment have a large number of hydrophilic groups,and the fibers show hydrophilicity,which is beneficial to bond the coating and the fibers.In the pre-oxidation process,the presence of the coating can reduce the damage caused by heat release.After pre-oxidation,there is still a part of the CS-STPP coating on the fibers'surface.During the combustion process,CS as a carbon source and blowing agent,can easily carbonize and release inert gases.STPP can generate more phosphorus-containing compounds and promote the formation of cross-linked carbon layers,thereby improving the thermal stability and flame retardant of the fibers.Finally,sol-gel technology was used to prepare SiO₂ sol with tetraethyl orthosilicate?TEOS?as the precursor.After gelation,a layer of SiO₂ gel network was formed on the surface of OPF_HA-MEA-L?L=10?fibers.The effect of SiO₂ concentration on the properties of OPF_HA-MEA-10 fibers was discussed.Through the analysis of its appearance,thermal stability and flame retardant property,it is found that the surface of the fibers is covered with a dense coating.As the concentration of SiO₂ loaded increases,the thermal stability and flame retardant of the fibers are significantly improved.when heated,the coating plays a certain role in supporting and filling the carbon layer,so as to achieve good thermal insulation and oxygen barrier,and increase the amount of residual carbon.The existence of SiO₂ gel network restricts the relative sliding between the fibers,thereby increasing their breaking strength.