| Carbon fiber reinforced plastics composites(CFRP)are a kind of composites with excellent mechanical properties,which are widely used in aerospace,wind power,transportation and sports equipment.With the large-scale production and use of CFRP,a large number of CFRP waste formed in the production process as well as after service will inevitably cause environmental pollution and huge waste of resources.Therefore,the treatment of CFRP waste has become one of the vital issues to be solved urgently for the sustainable development of CFRP industry chain.However,the research on recycling and reuse of CFRP waste has not really achieved the goal of high value recycling and reuse of carbon fiber at present,which means that more strategies of recycling and reuse of CFRP waste need to be innovated and tried.In this work,a new strategy to prepare carbon/ceramic composites from CFRP waste by forward regeneration-carbonization is proposed.First of all,CFRP waste is recovered in a way that carbonized by pyrolysis under the action of carburizing agent to harvest recycled carbon fiber(rCF)which containing a large amount of resin carbon.Second,the rCF was further densified to valuable recycled carbon fiber reinforced carbon(rCF/C)composites.Third,a gel reactive melting infiltration(GRMI)method for ceramic modification of C/C composites with large-sized and complex-shaped was proposed.Last,the rCF/C-SiC composites were modified by GRMI,and the as-obtained rCF/C-SiC composites are expected to be used as brake pads,high-temperature oxidation resistance and ablation protection materials.The main contents and conclusions of this paper include the following six aspects:(1)Four typical carburizers,hexa[p-(carboxyl)phenoxy]cyclotriphosphazene(HPCTP),9,10-dihydro-9-oxa-10-phenanthrene-10-oxide(DOPO),polycarbosilane(PCS)and ammonium phosphate(AP),were used to improve the carbon residue of epoxy resin(EP).The results show that HPCTP has a good carbon-increasing effect on EP.The residual carbon rate of EP has been increased from 9.7wt.%to 24.8wt.%,and can even be increased to?30wt.%in practical application.The content of carbon element in EP matrix is 71.20wt%,that is,the recovery of carbon in EP matrix is increased from 13.8%to 41.6wt%.The mechanism of HPCTP increasing EP char residue rate is as follows:HPCTP is decomposed into active phosphate molecules during thermal decomposition,which can capture H+easily,and produce strong dehydrated phosphoric acid.The resulting phosphoric acid can make the non-carbon groups(-OH,-O-)in EP decompose and remove ahead of time,thus changing the thermal decomposition pathway of EP,and ultimately making EP retain more pyrolytic carbon in the thermal decomposition process.In addition,the crystal structure and mechanical properties of rCF have not changed significantly after carbonization heat treatment.(2)The rCF was densified to recycled carbon fiber felt reinforced carbon(2.5D-rCF/C),layered recycled carbon fiber reinforced carbon(2D-rCF/C)and short-cut recycled carbon fiber reinforced carbon(S-rCF/C)composites using impregnation-pyrolysis method.The results show that the densification efficiency of rCF/C composites are approximately the same as that of the virgin carbon fiber(v CF)composites(v CF/C)composites.The flexural strengths of 2.5D-rCF/C,2D-rCF/C and S-rCF/C composites are 106.4 MPa,90.25 MPa and 34.0 MPa,respectively,while the flexural strengths of corresponding v CF/C composites are 111.5 MPa,95.45 MPa and38.7 MPa,respectively.There is little difference in the flexural strength between the two composites,and they have similar structure.However,there are some differences in fracture failure modes owing to that the EP pyrolytic carbon tends to form more pore defects in rCF/C composites.(3)A gel reactive melting infiltration(GRMI)method for ceramic modification of C/C composites with large-sized and complex-shaped was proposed.The feasibility and mechanism of the GRMI method were studied in detail.The results show that the PCS-Si gel has good reactive infiltration properties.After GRMI,the C/C composites are increased from 1.35g·cm-3 to 1.92g·cm-3.The main reasons are that the PCS has high residue rate,mild pyrolysis process,the pyrolysis residue SiC from PCS does not react with Si melt,and the gel coating can adhere to the surfaces of C/C composite steadily during the reactive infiltration process.Polycarbonitride borosilane(PCNBS)-Si,PCNBS-Si90Zr10 and PCS-Si90Zr10 gels exhibit good reactive infiltration properties similar to those of PCS-Si gels,which indicates that ceramic precursor polymers is the better choice for preparing gel infiltrators and the GRMI has a good universality.(4)The effects of reaction temperature and time on reactive infiltration of PCS-Si gel were studied.The results show that the bending strength of low density(1.05 g·cm-3)C/C composites decreases with the increase of reactive infiltration time or temperature.For the C/C composites with higher density(1.32 g·cm-3),the bending strength of the C/C-SiC composites increases first and then decreases with the increase of reaction infiltration time or temperature.The densities and mechanical properties of C/C-SiC composites increased with the increase of the thickness of PCS-Si gel coating.Compared C/C-SiC composite fabricated using the traditional RMI,due to the control of the amount of Si agent in GRMI method,the C/C-SiC composites avoid excessive ceramic-modified,and the thermal stress damage is effectively reduced.The corresponding bending strength and toughness of the C/C-SiC composites using GRMI are more excellent.(5)The effect of the density of C/C composites on the reactive infiltration of PCS-Si gel was studied.The results show that the bending strength of the C/C-SiC composites prepared by PCS-Si gel reactive infiltration increases first and then decreases with the increase of the density of the C/C composites,among them,the C/C-SiC composites fabricated by C/C composites with density of 1.32 g·cm-3 have the highest strength,?300 MPa.The complex-shaped C/C composites were successfully modified by PCS-Si90Zr10 gel,i.e.,the density of complex-shaped C/C composite was increased from 1.30 g·cm-3 to 1.96 g·cm-3,suggesting that the GRMI has great potential for ceramically modifying large-scaled and complex-shaped C/C composite components.(6)The rCF/C-SiC composites were manufactured by reactive infiltration of PCS-Si gel based on rCF/C composites,and microstructures,mechanical properties,friction properties and anti-oxidation ablation properties of the resulting composites were analyzed and compared.The results show that the flexural strength of the rCF/C-SiC and v CF/C-SiC composites are similar,253 MPa and 249 MPa respectively,and they have same fracture failure modes.The thermal diffusivity of rCF/C-SiC and v CF/C-SiC composites in different braiding directions of carbon felt is similar.The rCF/C-SiC and v CF/C-SiC composites have stable friction coefficients of about 0.4,and both of them exhibit low wear rates of 3.8?m·min-1 and 4.5?m·min-1,respectively.In addition,the mass ablation rates of rCF/C-SiC and v CF/C-SiC composites are nearly equal,0.62 mg·s-1 and 0.71 mg·s-1,respectively,and their high temperature oxidation resistance is also excellent.The rCF/C-SiC composites based on forward carbonization-regeneration of CF/EP composite waste have great potential to replace v CF/C-SiC composites and to be used in braking system and thermal protection system. |
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