采用超临界水中氧气回收碳纤维增强树脂基复合材料中的碳纤维.pdf

Short CommunicationChemical recycling of carbon fibers reinforced epoxy resin compositesin oxygen in supercritical waterYongping Baia,Zhi Wangb,*,Liqun FengaaSchool of Chemical Engineering and Technology,Harbin Institute of Technology,Harbin 150001,ChinabCenter for Composite Materials,Harbin Institute of Technology,Harbin 150001,Chinaa r t i c l ei n f oArticle history:Received 29 June 2009Accepted 30 July 2009Available online 3 August 2009a b s t r a c tThe carbon fibers in carbon fibers reinforced epoxy resin composites were recovered in oxygen in super-critical water at 30 1 MPa and 440 10?C.The microstructure of the recovered carbon fibers wasobserved using scanning electron microscopy(SEM)and atom force microscopy(AFM).The resultsrevealed that the clean carbon fibers were recovered and had higher tensile strength relative to the virgincarbon fibers when the decomposition rate was above 85 wt.%,although the recovered carbon fibers haveclean surface,the epoxy resin on the surface of the recovered carbon fibers was readily observed.As thedecomposition rate increased to above 96 wt.%,no epoxy resin was observed on the surface of the carbonfibers and the oxidation of the recovered carbon fibers was readily measured by X-ray photoelectronspectroscopy(XPS)analysis.The carbon fibers were ideally recovered and have original strength whenthe decomposition rates were between 94 and 97 wt.%.This study clearly showed the oxygen in super-critical water is a promising way for recycling the carbon fibers in carbon fibers reinforced resincomposites.?2009 Elsevier Ltd.All rights reserved.1.IntroductionCarbon fibers reinforced resin matrix composites are a class ofadvanced materials that have been developed for a variety of appli-cations in high-technology fields,such as the aerospace and auto-mobile industries,as well as in sporting goods 15.Since mostcarbon fibers are utilized with epoxy matrix in the form of compos-ites,a high scrap rate and significant amounts of off-cuts and re-jects are generated during the manufacture of these thermosetcomposites in contrast to the manufacturing of thermoplastic com-posites 5,6.Carbon fibers reinforced resin matrix composites thatwere destroyed in the course of use will become waste.Togetherwith end of life components,the amount of waste carbon fibersreinforced resin matrix composites has reached a significant level.At present time,the waste composites are mostly disposed of inlandfill because there is no economic means of recycling the wastecomposites.This results in the increase in the non-degradablewastes and also the loss of high value carbon fibers.An effectiverecycling process is therefore highly desirable in order to reclaimthe high value carbon fibers and reduce the pressure on the sur-rounding environment.Recycling the thermoset resin matrixcomposites has been extensively studied.For example,the carbonfibers reinforced resin matrix composites were degraded by pyro-lysis in the absence of oxygen 79,by chemical method and byusing catalyststoforma mixture oforganic compositions10,11.Unfortunately,the above-mentioned methods result inthe formation of the layer char or non-decomposed resin matrixon the recovered carbon fibers.Supercriticalfluidsandespeciallysupercriticalwater(T 647.3 K,P 22.1 MPa)are also potential media for the recy-cling of fibers and resin since they can be inexpensive reactionmedia,recyclable,non-toxic and relatively easy to handle 12.The supercritical water also possesses an interesting combinationof properties such as low viscosity,high mass transport coeffi-cients,high diffusivity and solvation power.Furthermore,thesupercritical water has a high solubility for organic compoundand oxygen,especially oxygen and water can form a single andhomogeneous phase,which allows oxidation to proceed rapidlyby an elimination of the potential interface mass transport limita-tions.These properties allow using water as reaction medium in-stead of hazardous volatile organic solvents 13.Although extensive research has been performed on the decom-position of the carbon fibers reinforced resin matrix compositesand the applications of the supercritical water in industry,thereare a few of the literature on the supercritical water decompositionof the carbon fibers reinforced resin matrix composites.In the present work,the oxygen was added into water in orderto increase the decomposition ability of the supercritical water.The carbon fibers reinforced resin matrix composite was decom-posed by the oxygen in supercritical water.The surface microstruc-ture of the recovered carbon fibers was investigated using atomic0261-3069/$-see front matter?2009 Elsevier Ltd.All rights reserved.doi:10.1016/j.matdes.2009.07.057*Corresponding author.Tel/fax:+86 451 86413711.E-mail address:(Z.Wang).Materials and Design 31(2010)9991002Contents lists available at ScienceDirectMaterials and Designjournal homepage: microscopy and scanning electron microscopy.Furthermore,the tensile strength of the recovered carbon fibers was evaluatedby single fiber strength.2.ExperimentalCommercially available T-300TMcarbon fibers with a diameter ofabout 67lm,purchased from Japan Toray,was used as reinforcefiller in the present work.E-51 epoxy resin was obtained fromYueyang Chemical Co.Ltd.,China.The phthalic anhydride was cho-senascuringagent.Epoxyresin/phthalicanhydride/benzyldimethylamine were 100,70 and 1 parts by weight,respectively.The prepreg that was fabricated by the carbon fibers and epoxy re-sin was put unidirectional into a mold to manufacture composites.The prepreg was pressed and cured under 5 MPa pressure for 2 h at90?C,then under 10 MPa pressure for 2 h at 120?C and last under10 MPa pressure for 4 h at 150?C by hotpress machine and hot-pressed composite with fibers mass fraction of 65(1.5%).Thedecomposition pressure and temperature in the reactor was main-tained at 30 1 MPa and 440 10?C.The composite was decom-posed in oxygen in supercritical water from 25 to 35 min.Themass ratio of the composite to water was below 10%.The surfacemicrostructure of the carbon fibers was examined on a Russian sol-ver P47 atom force microscopy(AFM)and Hitachi S-4700 scanningelectron microscopy(SEM).The surface element concentration ofthe carbon fibers was measured using the X-ray photoelectronspectroscopy(XPS)along with an X-ray photoelectron spectrome-ter(Perkin-Elmer,PHI 5300)equipped with magnesium X-raysource.Furthermore,the tensile strength of the recovered carbonfibers was evaluated by single fiber strength.A minimum numberof 120 specimens were tested for each treatment condition.Thesingle fiber specimens were prepared in paper frames with dimen-sions of 20 mm?100 mm.The free fiber length was approxi-mately 30 mm.A single fiber tensile test was carried out usingan interfacial micro-bond evaluation instrument made by ToheiSanyon Corporatin of Japan.The decomposition rate of epoxy resinin composites was calculated according to the amount of solidcomposition after treatment using the following formula:Dr Mc?MrMe?100%where Dr.is the decomposition rate(wt.%),Mc is the mass of com-posite before decomposition treatment,Mr.is the mass of solidcomposition after decomposition treatment and Me is the mass ofepoxy resin in composite before decomposition treatment.3.Results and discussion3.1.Surface microstructure and element compositionsFig.1 shows the SEM micrographs of the surface of the carbonfibers before and after treatment.It is recognized 14 that thecommercially available T-300TMcarbon fibers have many surfacegrooves that was produced in the process of manufacture of thecarbon fibers as shown Fig.1A.The clean carbon fibers were recov-ered when the decomposition rate was above 85 wt.%.Althoughthe recovered carbon fibers have clean surface,the epoxy resinon the surface of the recovered carbon fibers was readily observedby SEM observation as shown in Fig.1B.As the decomposition rateincreased to above 96 wt.%,no epoxy resin was observed on thesurface of the carbon fibers compared with the commercial carbonfibers.The atom force microscopy(AFM)was used in order to observethe surface microstructure of the carbon fibers.The virgin carbonfibers emerged after the commercial carbon fibers were treatedby the acetone extraction for 100 h 14.The surface of the virgincarbon fibers is relative smooth as shown in Fig.2A.The recoveredcarbon fibers were treated by the acetone extraction for 100 h.Theepoxy resin on the surface of the recovered carbon fibers was notextracted by the acetone when the decomposition rate was below92 wt.%as shown in Fig.2B,which was presumably attributed tothat the epoxy resin was not completely decomposed.When thedecomposition rate was above 94 wt.%as shown in Fig.2C,theepoxy resin was decomposed completely.Compared with the vir-gin carbon fiber surface,the amount of the surface groove in-creasedasthedecompositionrateincreasedwhenthedecomposition rate was above 96 wt.%,especially many grooveson the surface of the carbon fibers were incorporated into a deepfurrow when the decomposition rate was 100.2 wt.%as shown inFig.2D.This was attributed to the excessive oxidation of recoveredcarbon fibers 15.The overall XPS spectra of different specimens are shown inFig.3.There are significant differences between the virgin andrecovered carbon fibers.Table 1 summarizes the surface elementconcentration of different specimens obtained from XPS analysis.From these data,it was found that the surface elements of the vir-gin carbon fibers were mainly composed of the carbon,oxygen andnitrogen.The carbon and oxygen contents of the virgin carbon fi-bers were 90.13 and 7.44%and the O/C ratio was 0.0825.For therecovered carbon fiber of the decomposition rate of 100.2 wt.%,the carbon and oxygen contents were 69.09 and 28.65%,respec-tively,and the O/C ratio of 0.4147 was significantly higher thanthat of the virgin carbon fibers.This also revealed that the recov-ered carbon fibers were oxidized in oxygen in supercritical water15.3.2.Mechanical propertiesThe curve of tensile strength versus decomposition rate isshown in Fig.4.When the decomposition rate was below94 wt.%,the tensile strength of recovered carbon fibers was signif-icantly higher than 3.11 GPa of the virgin carbon fibers,which wasattributed to the incompletely decomposed resin that could healthe surface flaws of the carbon fibers and avoid the effect of stressFig.1.The SEM micrographs of the surface of the carbon fibers before(A)and after(B)treatment.1000Y.Bai et al./Materials and Design 31(2010)9991002concentration on the strength of the carbon fibers 14.When thedecomposition rates were 94.4 and 96.5 wt.%,the tensile strengthof the recovered carbon fibers were 3.15 and 3.13 GPa,respec-tively,which were statistically identical at 3.11GPa of the virgincarbon fibers.The tensile strength of the recovered carbon fibersdecreased rapidly as the decomposition rate increased from96.5%to 100.2%,which was presumably attributed to the damageof the carbon fibers due to the excessive oxidation of recoveredcarbon fibers 15.From these results,it could be detected thatthe carbon fibers were ideally recovered and have original strengthwhen the decomposition rates were between 94 and 97 wt.%.Fig.2.The AFM micrographs of the surface of the carbon fibers;A,virgin carbon fibers;the decomposition rates of B,C,D were 91.2%,96.5%and 100.2%,respectively.Fig.3.The overall XPS spectra of different specimens.Table 1Atomic concentration of different specimens obtained from XPS spectra.SpecimensC(%)O(%)N(%)O/CVirgin fiber90.137.442.380.0825Recovered fiber69.0928.652.230.4147Fig.4.The curve of tensile strength versus decomposition rate.Y.Bai et al./Materials and Design 31(2010)999100210014.ConclusionsThe clean carbon fibers were recovered in oxygen in supercrit-ical water at 30 1 MPa and 440 10?C for 30 5 min and hadhigher tensile strength relative to the virgin carbon fibers whenthe decomposition rate was above 85 wt.%.However,the tensilestrength 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