Study On High-temperature Resistance In Aromatic Fibers

High-temperature resistant fibers are these fibers which can be used for extended periods of time at above 180℃.Aromatic fibers have excellent high-temperature resistance property because of containing benzene-cyclic or heterocyclic in their molecular backbone.Aromatic fibers are widely used for manufacture with thermal loads,including for military,aerospace,civil engineering, construction,protective apparel,geotextiles and electronic appliances.And the difference of linkage in various functional groups and aromatic rings causes their distinguishing features in temperature resistance and process technique.The studies of high-temperature resistance can provide valuable information to understand the mechanism of chemical change during the thermal suffering.Also, it is significant to preparation,modification and application of these fibers.In this paper,the mechanism of high-temperature resistance is discussed systemically,and the features and varieties of aromatic fibers are presented, especially,several typical kinds are emphasized.The high-temperature resistance and structures of aliphatic polyamides,aliphatic-aromatic polyamides and aromatic polyamides are compared,and on this base,the behaviors of pyrolysis and thermal degradation of Poly(m-phenylene isophthalamide)(PMIA)fiber,Poly(p-phenylene terephthalamide)(PPTA)fiber,Poly(p-phenylene-2,6-benzoxazole)(PBO)fiber, Polyimide(PI)fiber and Poly(ether imide)(PEI)fiber are investigated in inert gas and air by the means of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS)and thermogravimetric analysis coupled with mass spectrometry or Fourier transform infrared spectroscopy(TGA-DTA/MS(FTIR)),and their mass loss and volatile fragments are analyzed in detail.On the other hand,the main monomer synthesis,polymerization and spinning of PEI are presented,which is a modification kind of PI.And it is a great influence to add ether bonds in the macromolecular chain for process technique and high-temperature resistance.PMIA and PPTA are the most important high-temperature resistant fibers.And the difference in the linkage of functional groups and aromatic rings causes distinguishing features in temperature resistance and various processes of pyrolysis and thermal degradation.In inert gas,the results of pyrolysis and thermal degradation observed by Py-GC/MS and TGA-DTA/MS are consistent and complementary each other.Three-step degradation of PMIA and the single one of PPTA reflect in the diversities of their volatiles.In low degradation temperature,hydrolysis is a primary reaction for PMIA and causes a three-step mass loss and lower heat resistance.But with increasing temperature,homolysis is enhanced,especially for PPTA.When the temperature reaches 1000℃,PPTA takes higher mass loss than PMIA.In both inert gas and air,mass spectra of volatiles are obtained,and PMIA and PPTA mostly show the same mass signals. The temperatures of maximum abundances in ion currents are corresponding to the temperatures of mass loss peaks in DTG curves.Through proposed molecular structures of these ion currents,thermal degradation of PMIA and PPTA in different conditions can be deduced.In all degradation processes,small molecular ions such as 44(CO₂),18(H₂O),27(HCN),et al have large relative abundances, and the relative abundances which represent phenyl compounds have very small values,especially in air.With elevating temperature,the relative abundances of 44 (CO₂)improves greatly.On the other hand,the results of elemental analyses of decomposed PMIA and PPTA fibers are in agreement to the results of volatiles analysis.In both nitrogen and air,for both PMIA and PPTA,there aren't significant changes in the elemental composition until a number of volatiles appearing.The increase of C and the decrease of H show that degradation occurrs strongly.A synthetic kinetic model is introduced,and the analyses of kinetic parameters can explain the degradation progresses well.Hydrolysis,homolysis and oxidation are emphasized differently in stepwise thermal degradation.PBO fiber is so far the most high-temperature resistant polymer fiber.From room temperature to 1000℃,in inert gas,PBO fiber keeps the residues of 76.5%. The results of Py-GC/MS show that repeating unit of molecular chain is the most pyrolysis fragment and also CO₂ keeps a great yield in total pyrolysates.After 650℃,the volatiles,including HCN,NH₃,NO₂,NO,CO,CO₂,H₂O and aromatic compositions can be found by TGA-DTA/MS.But in air,PBO almost losses all mass when the temperature reaches 1000℃.Only several small molecular volatiles, such as CO₂,NO₂,HCN,H₂O,etc are searched by TGA-DTA/FTIR and TGA-DTA/MS,especially after 650℃,thermal degradation is taken intensively and CO₂ is produced mainly.The result presents a strong influence of oxygen for the thermal degradation.Thermal degradation mechanism of PBO is suggested.In high temperature,the bonds between benzene ring and heterocyclic ring are ruptured firstly,then the linkage of C=N or C─O in heterocyclic ring are broken, and free groups appear,which cause recomposition or intensive rupture.At the same time,some small molecular compounds are produced.When H₂O is produced, hydrolysis speeds the degradation progress.In air,oxygen is taken into the degradation reaction,and oxidation accelerates the degradation.As a result,lots of oxides are obtained.When the temperature get to 650℃,PBO begins to burn and the carbons in benzene ring are oxidized into CO₂,and PBO loses its mass rapidly.Pyrolysis and thermal degradation of PI and PEI take similar characters,but for PEI,the process is more complex than the one of PEI.PEI produces more fragments than PI in the results of Py-GC/MS,and ion currents of PEI fragments are stronger than the ones of PI fragments.In air,the results of TGA-DTA/MS show that the temperature of PEI appearing obvious mass loss is ahead of 50℃when comparing to the one of PI.This indicates that PEI also keeps a well heat resistance.But about the counterparts in MS,PI only emerges several small molecular ions including CO₂⁺,NO₂⁺,NH₃⁺ and H₂O⁺,and CO₂⁺ keeps the peak values.And PEI appears abundant fragments at the temperatures of 530℃,540℃and 580℃.Adding ether bonds and isopropyl leads to the non-symmetrical form in molecular chain,and causes complicated behaves in the thermal degradation of PEI.Four steps of synthesizing 2,2-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride(BPDEDA)was designed,which is a main monomer of polymerizing PEI.The PEI polymerization of BPDEDA and meta-phenyl diamine was carried by HAAKE torque rheometer over 320℃.Obtained samples and Ultem(?)resin are compared by infrared spectrums,C¹³-NMR,DSC and TG.The results show that polymerization have been carried well.And PEI polymerization can be controlled easily after reaching its reaction temperature.Rheological behavior of PEI melt is researched,and melt spinning of PEI is explored in the temperature range of 320～370℃.The titer of crude fiber is about 260 dtex.After drawing and thermal treatment,the strength can reach about 3.0 CN/dtex,the modulus get to about 55.8 CN/dtex,and break elongation is about 30%.Also,PEI fiber keeps a well heat resistance in lower temperature than 200℃.As a matter of fact,PEI can be easily adapted for processing and keeps a high thermal durability.It is successful as a modified kind of PI,and more researches need to be taken.Innovation of this paper:1.The knowledge of high-temperature resistance is summarized and the heat resistance of aramid fibers is analyzed.Heat resistance as a term is introduced to evaluate the ability of keeping mechanical properties involving the retention of strength,module and so one in a long time in high temperatures.And the term of thermal stability is referred to describe chemical varieties of macromolecule and capability against the varieties. In this paper,the change of heat resistance is presented with the transformation of molecular structure from aliphatic amides to aliphatic-aromatic amides and to wholly aromatic amides.The heat resistances of various aramid fibers are compared,and the results are explained.2.Combined analyzers are used to investigate the features and reaction mechanisms in aromatic fibersThe representative aromatic fibers such as PMIA,PPTA,PBO,PI and PEI are researched by the means of Py-GC/MS,TGA-DTA/MS and TGA-DTA/FTIR.The features of their mass losses are discussed by stages,and their volatiles are analyzed qualitatively and quantitatively.The changes of thermal stability which come from different atmospheres and various molecular structures are compared, and some novel properties and viewpoints have been found.As a result,the mechanisms of pyrolysis and thermal degradation can be deduced.Also,for PMIA and PPTA,elemental analyses of decomposed fibers provide the direct information for chemical and morphological evolutions.A synthetic kinetic model is introduced, and the analyses of kinetic parameters can explain the degradation progresses well.3.Exploring the techniques of PEI polymerization and spinning─one example of modifying heat-resistant polymerFour steps of synthesizing 2,2-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride(BPDEDA)was designed.Polymerization and spinning of PEI was carried,and PEI fiber keeps a well heat resistance.