Dry Spinning Preparation And Ultraviolet Irradiation Crosslinking Of Polysilazane Fibers

Si CN ceramic fiber,as one of the reinforcement of high performance ceramic matrix composites,has been widely concerned by researchers because of its high strength,high temperature resistance,oxidation resistance and other excellent properties.At present,melt spinning is the main spinning process of Si CN fiber,but the polymer precursor with high softening point has the problems of poor melt fluidity and difficult defoaming.The precursor with larger molecular weight and higher softening point can be selected for the preparation of fibers by dry spinning process,which has certain significance for exploring new spinning process of polysilazane green fiber.In this paper,PSZ was prepared from MHS and TMS by ammonolysis and polymerization.PSZ was successfully transformed into Si CN ceramic fiber by dry spinning,UV irradiation crosslinking and high temperature pyrolysis.By means of FT-IR,GPC,XPS,rotary rheometer,TG and SEM,the elemental composition,structural changes and microscopic morphology of PSZ precursor and its spinning dope,cured fiber and ceramic fiber were analyzed.The synthesis of PSZ,dry spinning,UV irradiation crosslinking,high temperature pyrolysis and the vacuum temperature resistance and oxidation resistance of Si CN ceramic fiber were studied.The main conclusions are as follows:（1）The polymerization process of LSZ was studied.It was found that PSZ-8 was the PSZ corresponding to the optimal polymerization time,its Mw was 4936 g/mol,the softening point was 237℃,the ceramic yield was 52.4%,and it could be completely dissolved in xylene to prepare the spinning dope.Based on the FT-IR and related literature,it is inferred that the dehydrogenation reaction between Si-H bond and Si-H bond and between Si-H bond and N-H bond may have occurred during the polymerization reaction,forming Si-Si bond,Si-N bond,four-membered ring and six-membered ring;Si₃N crosslinked structures were also formed by transamination reaction.（2）PSZ spinning dope was prepared with xylene as solvent and its rheological properties were studied.In the experimental range,the apparent viscosity of PSZ spinning dope decreases with the increase of shear rate,there is an obvious thinning behavior of shear force,and the spinning dope needs a minimum shear stress before flowing,indicating that the PSZ spinning dope prepared is pseudoplastic fluid at room temperature.In addition,with the increase of oscillation frequency,η*of PSZ-87 wt%decreases gradually,G"increases first and then decreases,G"gradually increases,and G"is always greater than G",which indicates that PSZ-87 wt%has good spinning performance at room temperature.（3）The dry spinning process of PSZ was explored.With the increase of spinning dope concentration,the continuous spinning time of spinning dope first increased and then decreased.When the take-up rate ranges from 75 to 135 r/min,the higher the concentration of PSZ spinning dope,the faster the collection speed corresponding to the longest continuous fiber length.With the increase of the take-up rate,the continuous length of the fibers spun by PSZ-87 wt%increases first and then decreases.The longest continuous length of the fibers is 121.1 m,and the diameter is 1.5～30μm.The surface of the fibers is smooth and flat,without grooves,cracks and other defects,and the cross-sectional structure is dense,without obvious holes.（4）PSZ green fibers were cured by UV irradiation crosslinking method and its mechanism was analyzed.Under the irradiation of UV lamp with wavelength of 185 nm and total power of 106 W,the thickness of curing layer formed on the surface of the fiber gradually increases with the extension of UV irradiation time.PSZ-UV-7 is the PSZ cured fibers obtained by optimal UV irradiation time.The ceramic yield of PSZ cured fibers increases from 52.4%to 83.2%（N₂ atmosphere,800℃）,and the surface of the fiber is smooth and flat,and the cross-sectional structure is dense.The UV irradiation crosslinking experiments between dry spun fibers and melt spun fibers showed that under the same irradiation time,the curing layer thickness,ceramic yield and increase of ceramic yield of PSZ fibers obtained by dry spinning of PSZ precursor with high softening point were improved compared with those produced by melt spinning of PSZ precursor with low softening point.According to the test results of EDS and FT-IR,it is inferred that during UV irradiation,O₂ in the air is converted to O₃,and part of Si-H,N-H and C-H bonds on the fiber surface break due to the absorption of UV energy,and Si free radicals,N free radicals and C free radicals are generated.The three kinds of crosslinked structures,Si-O-Si,Si-N-Si and Si-C-Si,formed by the interaction between free radicals,to cure the PSZ fibers.（5）Si CN fiber was prepared by pyrolysis and its vacuum temperature resistance and oxidation resistance were studied.The Si CN fibers obtained by pyrolysis at 1100℃mainly contain Si-N,Si-C,Si-O,N-C and C-C bonds,and the morphology of the fibers is smooth and dense.Under vacuum condition:at 1300℃,the morphology of Si CN fiber is smooth and dense,and the tensile strength reaches 0.71 GPa;at 1500℃,α-Si₃N₄,β-Si₃N₄ and Si C crystal phases appear in the fibers due to the decomposition of Si C_xN_yO_zphase,which leads to the appearance of granular texture on the surface of the fibers,and its mechanical properties decrease sharply;at 1600℃,the fiber structure is destroyed,the fiber section appears a large number of holes,its mechanical properties lost.In the air:at 1300℃,Si O₂ crystal layer appeared on the surface of Si CN fiber;at 1400℃,the morphology of the fibers remained smooth and dense;at 1500℃,due to the high oxidation temperature,a large amount of oxygen elements are introduced into the fiber,and Si O₂ crystal layer becomes significantly thicker,which makes the fiber surface uneven heating in the subsequent cooling process,leading to the appearance of a large number of cracks.In summary,the continuous spinning length of PSZ-87 wt%prepared by PSZ-8 and xylene was 121.1 m and the fiber diameter was 1.5～30μm.The morphology of PSZ fiber prepared by PSZ-87 wt%is smooth and dense,the diameter of fibers is 1.5～30μm,and the longest continuous length is 121.1 m.The ceramic yield of PSZ fiber（PSZ-UV-7）increased from 52.4%to 83.2%after 7 h UV irradiation.Under the same irradiation time,the curing degree of dry spun fibers obtained from PSZ with high softening point is higher than that of melt spun fibers obtained from PSZ with low softening point.Under vacuum condition,Si CN fiber can maintain the tensile strength of is 0.71 GPa after treatment at1300℃,andα-Si₃N₄,β-Si₃N₄ and Si C crystal phases appear in the fiber at 1500℃.Under the condition of air:Si O₂ crystal layer appeared on the surface of Si CN fiber after treatment at 1300℃,and the fiber morphology was smooth and dense after treatment at1400℃.PSZ with high softening point was synthesized by ammonolysis and polymerization,and the dry spinning of PSZ and UV irradiation crosslinking of PSZ fiber were realized,which has certain reference significance for the preparation of Si CN fiber.