Study On PAN-Based Flame Retardant Fiber

PAN fiber is one of the synthetic fibers which have big production and extensive using.It has been applied in the field of garments industry,decoration and industry sector and its domestic annual consumption reaches almost 1 million ton.Therefore,it is closely relevant to people's daily life and producing activity.However,the limit oxygen index of PAN fiber is only 18%.It is easy to burn and release hydrogen chloride,carbon monoxide,acetonitrile and other toxic gases,which cause fatal harm to human body again.Hence,it is significant to study and enhance the flame retardant property of PAN fiber.At the premise of the mechanical properties?tenacity?1.8cN/dtex?meeting the specification,we studied the procession conditions of chemical modification of hydrazine alkali and metal ion to prepare modified PAN fiber with good flame retardant property.Then,related characters of the modified fiber with metal ions such as Na⁺?Mg⁺?Zn⁺ as well as degradation kinetics is further studied and the flame-retardant mechanism of the modified fiber is discussed.In the present research,a part of the cyano on the PAN fiber was firstly modified by hydrazine hydrate to form a crosslinked cyclic structure among the linear macromolecules and then with sodium hydroxide?NaOH?aqueous solution further hydrolyzing the hydrazine modified fiber,with other hydrophilic groups such as carboxyl and acylamino,thereby the moisture absorption properties of Na⁺ modified fibers have been enhanced,which has the role of endothermic cooling when being in combustion.The experimental results show that when the reaction liquor ratio is 15: 1 and the concentration of hydrazine hydrate solution is 30wt%,the hydrazine reaction process is 110?×6h,the concentration of NaOH aqueous solution is 5wt%,and when the alkali hydrolysis process is 100?×1h,the breaking strength of modified fiber is2.13 c N/dtex,elongation at break is 17.7% and moisture regain is 23.18%.The analysis of FTIR spectroscopy,elemental analysis and X-ray diffraction of Fiber before and after the modification shows that when the hydrazine is modified,the cyano undergoes a partial hydrolysis reaction as well as the crosslinking reaction at the same time,however,a large amount of hydrophilic groups are introduced by hydrolysis reaction.As a result,the regularity of the fiber has been significantly decreased.And the scanning electron microscopy?SEM?photographs of modified fibers and residual carbon show that the surface roughness of the modified fibers are increased,and the fibers are swelling to some extent due to the increase of the moisture absorption.After being burnt,the fibers of raw materials shrink into spherical droplets,while hydrazine modified and Na⁺ modified fiber still retain relatively complete fiber morphology.The former swells and the forms a numberof bubble-like protrusions by section,and the latter residual carbon layer surface is densely covered with uneven rugged protrusions,accompanied by pores,whose cross profile is fluffy and porous.The TG-DTG test shows that the maximum temperatures of thermal degradation of the raw fiber,hydrazine modified and Na⁺ modified fiber are respectively 307.4 ?,438.1 ? and368.9 ?.This is because the more stable covalent bond formed by hydrazine crosslinking enhances the intermolecular force and improves the fiber thermal stability.In the case of further alkali hydrolysis,some of the weakly crosslinked structures are destroyed,leading to a decrease in the thermal degradation temperature.But in general,the cross-linked cyclization structure and large amount of hydrophilic groups in the modified fiber macromolecule structure make the whole thermal degradation rate of the fiber slow down and the thermal stability obviously improved.And the non-combustible gases such as vapour,CO₂,NH₃ produced by the thermal degradation of the modified fiber can delay or even stop the combustion process.The LOI values of hydrazine modified and Na⁺ modified fibers are increased respectively to 24% and 29%.The Na⁺ modified fibers were chelated with Mg²⁺ and Zn²⁺ metal ions in the presence of Na +in the hydrazine base reaction,and the effects on the flame retardancy were studied.By means of the energy dispersive spectrometer?EDS?,the scanning electron microscopy?SEM?and the test of antistatic property,etc,the result shows that Mg²⁺ and Zn²⁺ modified fibers have more metal ions than Na⁺ modified fibers and that the ratio of fiber volume to resistance value?Rv?is greatly reduced.The Rv of Na⁺,Mg²⁺ and Zn²⁺ modified fibers decreases to 1.7216×10⁹?·cm,8.4557×10⁸?·cm and 1.6873×10⁸?·cm respectively with good antistatic properties.After combustion,Mg²⁺and Zn²⁺ modified fibers were more compact in the same fiber morphology.The analysis of TG-DTG and residual carbon ratio shows that the temperatures for the maximum thermal degradation rate of Na⁺,Mg²⁺ and Zn²⁺ modified fibers are 368.9?,440.9? and 442.7?,respectively.To promote the generation of the ring structure in the relatively stable intramolecular or intermolecular environment,the chelating of Mg²⁺ and Zn²⁺ delayed the thermal degradation rate of the fiber,further improving the thermal stability.The residual carbon contents of the three increased from 33.87% to 42.76% at 700? respectively.Correspondingly,the LOI values of Na⁺,Mg²⁺ and Zn²⁺ were also increased from 29% to 33% and 37%.Furthermore,in this paper,the kinetics of thermal degradation of metal ion modified fibers was studied by TG-DTG method and Kissinger method.The result shows that the maximum thermal degradation rate of the fiber increase with the increase of the heating rate.Meanwhile,the test result of the activation energy of thermal degradation shows: zinc ion modified fiber?145.40kj/mol?>magnesium ion modified fiber?89.38kj/mol?>Sodium ion modified fiber?78.78kj/mol?,which is consistent with the flame-retardant properties of the three modified fibers.According to the research,the flame retardant mechanism of the above-mentioned modified fiber can be considered as follows: the main mechanism of hydrazine modifying fiber is tointerrupt heat exchanging flame retardant and condensed phase flame retardant,while the mechanism of sodium ion modifying fiber is to interrupt heat exchanging flame retardant and gas phase flame retardant,the flame retardant of magnesium iron and zinc ion modifying fiber showed the synergistic effects of the interruption of heat exchanging flame retardant,gas phase flame retardant and condensed phase,and the flame retardant effect is relatively good.However,it is synthetically assessed according to mechanical properties of modified fiber and properties of flame-retardant,the limiting oxygen index of magnesium modified fiber is up to 33%,the fracture strength of the fiber is 1.95 c N/dtex,moisture regain can also be maintained about 18.7%,therefore it is more suitable to further develop as the environment friendly polyacrylonitrile flame retardant fiber.