Preparation Of Mesophase Pitches For High-performance Carbon Fibers

The synthesis of high quality spinnable mesophase pitches is the foundation and a key step to prepare pitch-based carbon fiberes with high-performance.The chemical composition and molecular structure of raw materials,especially the naphthenic structure and the short aliphatic groups,play an important role in controlling the molecular structure and orientation of mesophase pitches.Therefore,the preparation of high quality spinnable mesophase pitches by regulating their molecular structure is of great significance to the manufacture of mesophase pitch-based carbon fiber with high-performance.In this thesis,the spinnable mesophase pitches and their carbon fibers were prepared from fluid catalytic cracking decant oil(FDO)or coal tar pitch(CP)as main raw materials and the naphthalene pitch(NP),petroleum pitch(PP)or tetrahydronaphthalene(THN)as co-carbonization or hydrogenation reagents.Based on the difference of different molecular structure characteristics of various raw materials,the effect of co-carbonization/hydrogenation on the molecular structure of mesophase pitches was discussed and the molecular structure and orientation difference of mesophase pitches influence on subsequent preparation carbon fiber properties were furher discussed.These are expected to provide theoretical and technical support for the preparation of mesophase potch-based carbon fibers with low cost and high performance.The main conclusions as follow:(1)Naphthalene pitch ANP and BNP were synthsised from naphthalene with AlCl₃ and HF/BF₃ as catalyst,respectively.Subsequently,their mesophase pitches ANP-MP and BNP-MP with lower softening point and nearly 100 vol%anisotropic component content were futher prepared by thermal polymerization process from ANP and BNP,respectively.The low softening point of naphthalene-based mesophase pitches mainly resulted from the rich naphthenic structures produced by catalytic polymerization.(2)The properties of mesophase pitches prepared via co-carbonization of ethylene tar pitch(ETP)/coal tar pitch(CP)and naphthalene pitch indicates that BNP is a better co-carbonizing agent than ANP owing to its higher H/C value and naphthenic structures content.The molecular homogeneity and molecular configuration of the choosen raw materials must be fully considered for co-carbonnization.(3)NP with abundant naphthenic structures was introduced to co-carbonize with FDO for solving the problems of high softening point,low solubility and poor optical texture of mesophase pitches prepared from FDO.Spinnable mesophase pitches with a softening point as low as 266°C and nearly 100 vol%domained anisotropic content were prepared and the properties of prepared mesophase pitches improved with the increase of NP addition.The properties improvement of mesophase pitches mainly comes from the introduction of naphthenic structure in NP.(4)Molecular structure analysis shows that FDO contains both abundant naphthenic structures and short alkyl groups,and NP contains rich naphthenic structures as well as a few short alkyl groups.The naphthenic structures on polyaromatic hydrocarbon in NP show higher thermal stability than these in FDO.The naphthenic structure in NP can effectively reduce the free radical reaction activity of FDO,promote the polymerization of asphalt molecules,and further improve the optical texture and molecular orientation of mesophase pitches.(5)The graphitized fibers prepared from FDO-NP(20)-MP show higher thermal conductivity and greater crystal size than those of K1100,which are ascribed to the more ordered structure and lower spinning viscosity of the mesophase pitch.(6)Mesophase pitches C-MP,P-MP and C/P-MP were synthesized from coal tar pitch,petroleum pitch and their co-carbonized pitches,respectively.The structure and nature of raw materials have significant influence on the molecular structures and properties of prepared mesophase pitches.(7)C-MP possesses a rigid molecular structure with high aromaticity and a very small amount of methyl groups,which leads to the minimum defects and maximum carbonization yield of their carbon fibers,thus makes C-MP-CFS with the highest mechanical properties of the three carbon fibers.P-MP contain abundant methyl groups which improve their molecular fluidity and molecular stacking,thus increase their molecular stacking height(Lc)and molecular size(La),and finally lead to P-MP-CFs possesses the highest thermal conductivity the three carbon fibers.(8)A certain amount of methyl groups was introduced in the molecular structure of CP by the co-carbonization of CP and PP,resulting in the C/P-MP take advantages of both the high aromaticity rigid molecular structure like C-MP and the abundant methyl groups like P-MP.These molecular structures characteristics lead to the C/P-MP possessing a medium molecular stacking size(Lc and La)and a tunable molecular orientation.The molecular structure control and the improvement of microcrystalline orientation of C/P-MP accomplished by co-carbonnization promote the C/P-MP-CFs with both good mechanical properties and ultrahigh thermal conductivity to compare with those of the commercial K13D2U carbon fibers.The tensile strength of C/P-MP-CFs is 3.65 GPa and its thermal conductivity is 912 W/(m·K).(9)In order to solve the proplem of the poor spinnablity and the low thermal conductivity of its carbon fibers caused by the high aromaticity and high softening point of C-MP,a certain amount of naphthenic structures and alkyl groups was introduced to tne molecular of CP by hydrogenation.Mesophse pitch C-MP and HC-MP was prepared by thermal polymerization with the unhydrogenated coal tar pitch(CP)and hydrotreated coal tar pitch(HCP)as feedstocks,repectively.Compared with C-MP,HC-MP possess larger optical texture size,lower aromaity and larger molecular weight,and its molecular orientation and microcrystalline size are significantly larger than C-MP.(10)The HC-MP based carbon fibers graphitized at 2800°C show significantly higher thermal conductivity than that of C-MP-based carbom fibers by 817 W/(m·K)and their combination properties are comparable to that of K13D2U.This proved that the preparation of carbon fibers with high performance based on the molecular regulation of mesophase pitch is feasibile.