| Carbon fibers have been paid more and more attentions as an important reinforcing component for preparing lightweight composite materials due to their high specific strength,high specific modulus,good temperature resistance and corrosion resistance.Especially,polyacrylontrile(PAN)based carbon fibers,with a best comprehensive performance,have occupied more than 90% of the world total carbon fiber production and been widely used in aerospace,defense areas and civil engineering.To prepare carbon fibers,PAN precursor fibers need go through stabilization(also known as pre-oxidation)and carbonization processes.Stabilization is usually conducted in the temperature range of 150 to 300 oC in air,during which linear PAN molecular chains are transformed to ladder structure to enable them to undergo processing at higher temperatures along with a great number of heat release.However,during stabilization,thermal shrinkage of PAN molecular chains usually makes the orientated molecules tend to disorient inevitably,and oxygen diffusion resistance tends to give rise to a non-uniform oxidation across the fiber section which can cause an apparent skin-core structure during carbonization.In addition,traditional stabilization process usually employs a gradient temperature program with a small fiber speed,which consumes a mountain of time and energy,and gives rise to a high cost in the preparation of carbon fibers.For carbon fibers,higher preferred orientation of graphene layers along fiber axis and interaction force between graphene layers,and fewer defects on the surface and inside fibers certainly result in high tensile strength and high modulus.Recently,? ray has been a hot area of research because of its high energy and penetrability which can modify polymer uniformly.In order to enhance the mechanical properties of carbon fibers and reduce the producing cost,the fibers derived from different processing steps in the preparation of carbon fibers were irradiated by using Co60 ? ray.The results indicated that ? ray can induce PAN molecules to cyclize and crosslink to enhance the thermal stability of molecule chains,especially,the cyclized structure can inhibit the disorientation of chains during heating treatment and accelerate pre-oxidation.In addition,crosslinking and cyclization can also be further induced by irradiating stabilized PAN fibers with ? ray to improve the structure uniformity on the cross section.Finally,the tensile strength and Young's modulus of carbon fibers derived from irradiated PAN or stabilized fibers were increased compared with the corresponding unirradiated samples.Firstly,PAN precursor fibers were irradiated in air by ? ray at different doses,and the structure changes and mechanisms of irradiated samples were investigated.It was confirmed that the cyclization and crosslinking reactions were occurred during irradiation by analyzing the solubility of PAN fibers in 75 wt.% sulfuric acid aqueous solution and dimethyl sulfoxide(DMSO)combining the thermal mechanical behaviors of fibers.Cyclization of nitrile groups was generated mainly at lower doses(?100 k Gy),however,mainly intermolecular crosslinking occurred at high doses which was most produced in amorphous regions.At the same time,the quantitative relationship of cyclized and crosslinked structure and absorbed dose was established.Additionally,the crystallite structure of irradiated PAN fibers was also investigated.It was noted that ? ray irradiation didn't destroy crystal form,but could impact the crystallinity of PAN and damage the smaller crystallite located at boundary of crystal regions when absorbed dose was higher than 200 k Gy to increase the average crystal dimensions.Crosslinked structure induced by ? ray could cure PAN molecule chains at room temperature which could be exerted larger drawing tension during stabilization to restrain disorientation of PAN molecule chains.Untreated and irradiated PAN precursor fibers were stabilized and carbonized on a continual production line and the structure and properties of stabilized and carbonized fibers were investigated.The results showed that the disorientation of irradiated PAN fibers at 100 k Gy was inhibited effectively during heating treatment because of the generation of semi-rigid structure at room temperature.Whereas,excessive irradiation could make the molecular chains break leading to more facile disorientation during heat treatment.After carbonization,the carbon fibers produced from 100 k Gy irradiated PAN fibers have an about 1.1% increase of the preferred orientation of graphene layers along fiber axis comprared with unirradiated ones,besides,have lower micro porosities,higher crystallite regularity and more crosslinkages between graphene layers.Finally,tensile strength and Young's modulus were increased by 13.2% and 2.3% respectively.What's more,stabilization time was shortened with increasing absorbed doses in the case of keeping the stabilization degree same.In other words,? ray irradiation can accelerate stabilization,when the absorbed dose was 400 k Gy,the stabilization time was reduced from 60 min for untreated PAN fibers to 43 min,with a decrease of about 30%.PAN fibers firstly were stabilized at different temperatures and then were irradiated with ? ray in air.We analyzed the structure changes and thermal properties of irradiated fibers.After some samples being carbonized,the structure and properties of carbon fibers were also studied.Further crosslinking and cyclization in stabilized PAN fibers induced by ? ray irradiation can enhance structure uniformity inside fibers.Carbon yields at 900 oC of both slightly and deeply stabilized fibers were enhanced with increasing the absorbed dose,the increased carbon yield was mainly attributed to the irreversible crosslinking induced by ? ray which included carbon-carbon crosslinked bonds and ether crosslinked bonds,whereas,irradiation might mainly generate more carbon-carbon crosslinked bonds.The crosslinking mechanisms of slightly and deeply stabilized fibers had a little difference.In slightly stabilized fibers,crosslinking was mainly produced by the reaction of alkyl radicals and free oxygen radicals from ionized oxygen in air.In addition to the oxygen containing crosslinking,carbon-carbon crosslinking was mainly generated by the combination of alkyl radicals on two adjacent chains.The oxygen containing crosslinking and carbon-carbon crosslinking in deeply stabilized fibers were mainly produced by the condensation of polar oxygen containing groups.Besides crosslinking in slightly stabilized fibers,there were two characteristic transformations: one was the reproduction of C?N,another was the increase of –CH2.Deeply stabilized fibers were carbonized in nitrogen.The resulting carbon fibers derived from irradiated stabilized fibers had the larger crystallite dimensions,and fewer defects and porosities compared with the corresponding unirradiated samples.Resultantly,the tensile strength and modulus were both increased to varying extents,especially,the increase was larger for the carbon fibers underwent the lower stabilization temperature,by about 23.3% and 12.1% increases of tensile strength for undergoing 260 and 270 oC heat stabilization respectively.In summary,? ray irradiation can effectively assist thermal stabilization of PAN fibers to inhibite disorientation of PAN molecules,enhance structure uniformity inside fibers,reduce stabilization time and increase the tensile strength and Young's modulus of carbon fibers. |
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