Stress and Strain During the Process of Thermal Stabilization of Modified Pan Precursors

610Journal of Donghua University (Eng. Ed.) Vol. 24, No. 5 (2007)Stress and Strain During the Process of Thermal Stabilization ofModified Pan PrecursorsZHANG Wang-xi(张旺玺)，WANG Yan-zhi(王艳芝), PAN Wei(潘玮)School of Materials & Chemical Engineering, Zhongyuan Unieversiy of Technoloky, Zhengzhou 450007. ChinaAbstract: Thermal mechanical analysis, FT- IR, WAXD andcharacteristics of different PAN precursors have led to thesome conventional measurcments， such as densities andbusic understanding that these characteristics vary markeully formechanical properties, were used to characterize the effect ofdifferent precursors and the mechanical propertics of thethe mdification using KMnO, and SnCL on the thermalresultant carbon fibers also establish a direct correlation withmechanical behaviors and structural changes during the processcharacteritics of the starting precursor. There are, however,of themal stabilization of modified PAN precursors. Comparedsome limitations during spinning， which do not permit toto the umodified original PAN precursors, some conclusionsproduce a precursor with all the desired characteristicswere drawn that the thermal stabilization starts at a lowercombined together-sJ . Therefore, post spinning moificationstomperature for modified PAN fibers, for example, the peak ofon PAN have been uscd as usefy| methods which have resultedthermal stress changes for modified PAN precursors usingin certain improvements in the properties of resulting carbonKMnO, displays a decrcase of 20C and a increase of 30% infibers6. Post spinning modification using KMnO、has beenthe utimate thermal stress, that chemical modification makesstuxdied in detail and it is observed that with KMnO treatmentstnuctural transfomation perfect and increases by 25% of theof PAN fibers, resutant carbon fibers display an improvementthermal stress at the temperature range of 230C- 300C , thatof 20%- 40% in the tensile strength in comparison to thethe modified PAN fibers display an increase of 100% in thecarbon fibers developed from unmodified fibers because of thethermal strain, once after pre-oxidized, show an increase ofplasticizing efet, catalytic effect of KMnO,[n.7.8% in orientation index, and a decrease of 9. 9% in crystalHowever, it is not fully to understand the relationshipsize for identical preload in the region of 13.1 - 14.5 MPa. Itbetween different modification results and the structure andwas also concluoded that the modification using SnCl, wouldproperties of PAN precursor fibers or carbon fibers, especially,alleviate the changcs in physical and charnical stress regimesthe themmal stress and strain changes of modified fibers duringand result in improvement in stnucture and decrease in defects.the thermal-oxidation stabilization were not reported withinKey words: carbon fibers; carbon precursor; chemicalur knowledge. This puper is mainly concentrated ontreatment ; mechanical propertiescomparative studies on the changes of thermal stress and strainCLC number: TQ 342+.74Document code: Aof original and modified PAN precursor fibers upon differentArticle ID; 1672 - 5220(2007)05 - 0610- 04heat treatment temperature.1 ExperimentalIntroductionA spcial grade of polyacrylonitrile precursor with AN/Carbon fibers, especially for polyarylonitrile (PAN) -MA/IA (92. 8/1.2/6 wt. %) was selected, which was suppliedbased oncs, offer the highest specific strength and modulus ofby Courtaulds Ltd. (United Kingdom). The original, that isall reinforcing fibers, which are mainly selected asunmodified, PAN precursor fibers were named as U samples,reinforcements in composite materials. In order to meetthe mdified PAN precursor fibers using 5 wt. % aqucousexpanded use in some high-tech sectors, many novelKMnO, solution were named as K samples, and the modifiedapproaches, such as dry-wet spinning', steam drawing'23 ,PAN precursor fibers using 4 wt. % aqucous SnCl solutionincreasing the molecular weight of precursors polymef'were designated as S samples.modifying the precursors prior to stabilizatior, etc. haveThe thermal-oxidation stabilization of the original andbeen performed to increase the tensile strength of PAN- baseddifferent modified PAN precursor fibers were carried out incarbon fibers. Studies on the physical, mechanical and thermala 3 - temperature-zone furnace in air with a flow rate of中国煤化工Rcceived date; 2006-01 - 11Foundation item; HAIPURT (No.2006KYCX009) ;National Natural Scien.MYHCNMHG20050080Henan Innovation Project of China(No. 0523021300)Biography: ZHANG Wang-xi (1967一), male, prof. E-mail: zhangwx@zzi. edu. cnJoumal of Donghua University (Eng. Ed.) Vol.24. No.5 (2007) 6111 L/min from 50C to 350C.valley ut 200C. The lowest vallcy of thermul stress ofThe mechanical properties of the various fibers weremouificd s fibers tends to a higher temperature at 240C.determined with a YG001 - A tensile testing machine at aHowever, those modlified K fibers all the sume display variouscrosshead speed of 10 mm/ min and 20 mm of testing gauge.increase trends, the moulified K fibers show higher thermalIn each specimen, at least 50 filaments were tested andstress than that of S fibers. This is because of much inclusiontheir average value was reported here. Densities of variousof oxygen for modified K fibers during their moulificationfibers were obtained at 25C by the use of density gradientprocess, the oxygen content of modified K fibers has inreusedcolumn method. FT - IR measurements were mude for the KBrby 71% compared to original ones, as a result of activedisks (0. 5 mg sumple with 200 mg KBr) for the specimens byreaction site, much includcd oxygen makes the cyclization andthe use of a Nicolet750 Magna- IR. Elementil analysis (EA)oxidation sturt earlier at 175C , other than 210C for originalwas carried out with a Carto Erba Azione 1106 model clementalfibers.analyzer. A Rigaku X rny dffractonecter, providing Ni- fiteredOuK。(λ=0.154 178 nm) rdition, was usxd to roord thewide-angle diffrction pattern of the original fibers and theirmdified counterpurts. The stepscan method was selccted tomcasure the stacking size (Le) using the following equation;Lc=K Bcas0(1]where λ is the wavelength of Cu K. X-ray; B is the widthat half maximum intensity of the peak at 20= 17" and 25"100 150200 250300 350for PAN precursors and carbon fibers, respectively; and KTemperature(C)is the apparatus constant ( = 0.89). The step-interval wasFig. 1 The thermal stress of various fibers inset at 0. 02". The preferred orientation of the precursorsa fixed-length stbilization process(001) and the resultant carbon fibers (002 ) wereFig.2 is the FT-IR spectraofU, K, S fibers and theirdetermined by an X-ray diffractmneter with a special fiber-stabilized fibers. The vibration charateristis of PAN structurespecimen attachment. The width at half-maximumintensity was used as an index of orientation, which may beare those of CN nitrile groups at 2243 -2241 cm , the strongbund at 1 732 cm~1 is atributed toC- =O stretching due to theused to calculate the parallelim of the crstalline part ofpresence of ester or acid. The most prominent stnucturalstructure by the following equation:changes are the decrease in the intensities of C=N bund and. 180- H,π=8RO X 100%(2)the decrease of those for aliphatic C -H ones once theprecursor fibers are heuted to an adequate elevatedwhere π is the orientation index, H is the width at half-temperature, especially for mdified K and S fibers.maximum intensity.2 Results and Discussion2.1 The thermal stress change in a fixed-lengthstabilization processFig. 1 shows the changes of thermal stress of original andmodlified PAN precursor fibers upon dfferent heat treatmenttemperature in a fixd-length thermal-oxidation stabilization3500.2 5001 50050process. It has been shown that some easily discemnible reginxssWavenumber(cm-1)canbe seen. In the first regime, when the heat treatmentFig.2 The FT- IR spectra of 1.U, 3. K, 5.S and theirtemperature is u甲to Tg， no more than 120C， thestabilied fibers of2.U, 4.K, 6.S after 250Cmacromolecules are inclined to shrink with the result ofthermal motion of chains owing to their conformational2.2 The strain change in a fixed-preload stabilizationchanges. When the temperature is up to 150C- 210C, the .macromolecules can undergo enough stretching bxcause of a中国煤化工t U and modified Kgreat deal of chain motion resulting in sippagp among the PANfibers0HCNMH(? zuation proess. Bothmacomolcules, bhence, the thermal stress of original PANJ ano N 110010 sIUw lllg when a preloadd isfibers starts to lower and the thermal stress of U fibers has a1.75 MPa, and K fibers have higher shrinkage than612Joumal of Donghua University (Eng. Ed.) Vol. 24. No.5 (2007)original U fibers, which is in agreement with higher2.3 Effect of preload on the changes of thermalthermal stress for K fibers in a fixed-length stabilizationstressprocess when they undergo constraint heat treatment.Both U and K fibers undergo thermal stress relaxationWhen the preload is up to 13.1 MPa, bothU and K fiberswhen a higher preload (≥8. 73 MPa) is imposed at thedisplay elongation trends, the higher the preloud is, thetemperature no more than 90C, as ilustrated in Figs. 4larger the elongation of fibers is. But, the K fibers alwaysand 5. Moified K fibers undergo higher thermal stressdisplay lower elongation, especially in a higher temperaturerelaxation than original U fibers. When upon heating toregion. Because the K fibers have been modified for a 5%90C- 160C， thermal stress is increased owing to thestretch, the results of differences in element composition,physical chains mobility, the original U fibers show fasterlater order, crystal size and orientation index from theincrease in thermal stress. When the temperature is beyondoriginal U fibers, are listed in Table 1. The catalytic effect130C, the changes of thermal stress are mainly resultedof KMnO causes lower initiating cyclzation temperaturefrom chemical reaction, such cyclization and oxidation, theand much ladder structure, which results in lower tensilesizes of new formed ladder molecules mainly determinedstrength, as shown in Table 2.the changes of thermal stress. As a result, the thermal stressis almost independent on the imposed preloxd when the heat20rsttreat temperature is up to 225C. In the same thermal stress,modified K fibers need lower preloud than original U fibers atidentical thermal- oxidation stabilization conditions.另一-15H14 t坌12F50100150200250300350z 10[Temperature(C)Fig.3 The strain of U and K fibers in differentpreload vs. temperatureTable 1 The lateral structure index of9%0~ 100150200250 300modified and unmodified fibersOrientationCrystalFiber typeFig.4 The thermal stress of U fibers in differentindex (%)size (nm)82.67.31K84.77.12Stabilized fibers from U69.33.83+8.73 MPaStabilized fibers from K-74.73.45- 13.10 MPa一17.46 MPa1ofTable 2 The properties of sample U, K, S and theirstabilized fibers after different temperatureTensileDiameterDensityFiberstrength(4m)(g/cm')(GPa)100 150一 200 250 300Ambient temp.12.51.19710.66U25011.01.27880.36270 c10.81.36570.32Fig.5 The thermal stress of K fibers in30010.41. 44890.30different preload vs. temperature12.71.21280.58250 t11.21. 334 30.340.313 Conclusion300 c10.71.473 10.28中国煤化工bers, the moitied K12.91. 203 20.62250C11.51.31260. 35MH.C N M H Ggen content, which270 C11.31.3740makes une cnemrcal-reacuon-lnaucea changes of thermal300 C11.11.465 9stress start earlier by 20C. The modified K fibers displayJoumal of Donghua University (Eng. Ed.) Vol. 24. No. 5 (2007) 613easy shrinkage and difficult stretch. which can be imposed[ 2 ] Mitsubishi Rayon Co, Ltd. Acrylonitrile Based Precursorlarger preload. In the same thermal stress. modified KFiber for Carbon Fiber and Method for ProductionThereof; EP 1130140AI[P]. 2001 - 05 - 09.fibers need lower preload than original U fibers at identicalthermal-oxidation stabilizution conditions. The chiange[3] Wilkinson K. Process and Product of AcrylonitrileCopolymer: wO 96/39552[P]. 1996- 12- 12.inflexions of thermal stress and strain take place at similar[ 4 ] Wilkinson K. Process for the Preparation of Carbon Fiber:temperature regions, which means that they can be used asUS 6051214[P]. 2000-04-25.an indication of chemical and physical changes of PAN[5] Jean BD, Tong K W, Jimmy C M P. Carbon Fibers[M],precursor fibers upon heat treatment.3rd. New York: Marcel Dekker Press, 1998; 13.[ 6] Zhang Wang-xi, Wang Yan-zhi. Manufacture of Carbon FibersReferencesfrom PAN Precursors Trated with CoSO, [J]. Jounal ofApplied Polymer Science, 2002, 85(1), 153- 158.[1] Bajaj P, Streekumar T V, Sen K. Structure Development[7 ] Tse-hao Ko. The Influence of Pyrolysis on PhysicalProperties and Micostructure of Modified PAN FibersDuring Dry-Jet-Wet Spinning of Acrylonitrile/Vinyl Acidsnd Acryoitrile/MethyI Acrylate Copolymers [J].during Carbonization [J]. Journal of Applied PolymerJournal of Applied Polymer Science.2002, 86, 773 - 787.Seience ,1991, 43(7), 589- 600.中国煤化工MYHCNMHG