Preparation, Propertiesand Crystallization Behaviors Of Thermoplastic Composites Based On Biodegradable Polymerand Recycled Carbon Fiber

With the continuous promotion of requirement to the physicalproperties of materials in diverse fields of advanced technology likeaerospace and bullet train, the development of materials scienceexperiences the process from simple single materials to the complexcomposites. As one of most important types of materials in advancedengineering fields, reinforced thermosetting composites with highperformance fiber have been widely used to substitute the conventionalmaterials. However, owing to a rapid growth of the worldwide usageamount of fiber-reinforced composites, there is a concern about the highenergy consumption and the potential tonnage of the waste frommanufacturing processes and end-of-life products. Therefore, the study onthe recovery and reuse of those high performance fibers, especiallycarbon fiber, has recently attracted a great interest since those fibers stillkeep their most of performance despite being used repeatedly and can be reused as universal materials. On the other hand, due in part to a growingemphasis on "green" chemical processes and low energy consumption,there has been increasing enthusiasm for the development ofenvironmentally friendly polymeric materials and their composites. Fullybiodegradable materials, a continuum of polymers ranging from thosethat are obtained from annually renewable feedstocks to those thatultimately degrade to benign by-products, have attracted increasinginterest and being developed rapidly. In this study, we prepared a series ofbio-based composites consisting of virgin poly(l-lactide)(PLLA),poly(butylene succinate)(PBS), andpoly(3-hydroxybutyrate-co-4-hydroxybutyrate)(P(3,4HB)) with recycledcarbon fiber (RCF) recycled from the waste thermosetting resin-basedcomposites via a melt extrusion. An intensive investigation was alsocarried out in terms of mechanical and thermal properties, morphologyand crystallization behaviors, so that a type of high-performancebio-based composite materials can be developed based on the results ofthis work.(1) Study on surface treatment of recycled carbon fiber and themechanical properties and micrographs of its composites with diversebiodegradable resins.To enhance the interfacial interaction between fiber and polymermatrix, the concentration nitric acid and 3-Glycidoxypropyltrimethoxysilane (KH560) was employed to modifythe surface of RCF. The results of fourier transform infrared spectroscopy(FTIR) and energy-dispersive X-ray analysis give the evidence that thesilane coupling agent has been introduced into the surface of RCFsuccessfully. It was found that some residues attach on the surface of RCFbefore any treatment was executed, which can be attributed to thecarbonaceous deposition and insufficiently thermooxidativedecomposition during the recovery process of RCF. A textured surface ofRCF was obtained after it was chemically surface-treated and few flawscan be observed.The incorporation of RCF extremely enhances the mechanicalproperties of polymer matrices. With increasingly increasing the RCFcontent, it was found that the tensile and flexible properties continuouslyimproved. The notched Izod impact strength of bio-based compositesshows a continuous increment with increasing the RCF loading except forPLLA/RCF composites. In the case of PLLA/RCF composites, themaximum of impact strength was obtained when11wt.%of RCF wasadded into the PLLA matrix and the impact strength decline with higherRCF loading being added. The biobased composites show a majority offiber breakage on the surface and most of fibers are found to be paralleland aligned with the melt shearing direction during the injection-moldingprocess, which is indicative of good compatibility between matrix and fiber and appropriate process parameters. According to the images of thefractography, a transformation of fracture mechanism of composites canbe deduced. The interfacial debonding, fiber pull-out and fracture mainlycontribute to the dissipation of impact energy with the low RCF loading,while the impact energy is absorbed by the interfacial debonding and thematrix deformation when high RCF loading was added. In the cases ofPBS/RCF and P(3,4HB)/RCF, due to the flexible chain structure, PBSand P(3,4HB) matrix show well capability of matrix deformation and canundertake more impact stress during the impact tests with the high RCFloading, and thus the impact strength keep continuously increasing. Oncontrary, the capability of matrix deformation of the rigid and brittlePLLA matrix is not enough to dissipate the most of impact energy andthus lead to the decrease of impact strength when excessive RCF loadingwas added into PLLA matrix.(2) The influence of RCF on the crystallization behaviors of polymermatrixAs typical semicrystalline polymers, the crystallinity andcrystallization behavior of PLLA, PBS and P(3,4HB) are correlated withthe diverse properties, such as mechanical properties, processability anddegradability. Therefore, the polarized optical microscopy (POM), X-raydiffraction (XRD), and differential scanning calorimeters (DSC)wereemployed to investigate the morphology, structure and crystallization behaviors of those three kinds of bio-based composites.In the cases of PLLA/RCF and PBS/RCF composites, the RCF playsa key role to facilitate the crystallization of polymer matrix due to itsheterogeneous nucleation effect. Diverse tests results give the evidence toprove it. From the images obtained from POM, the size of spherulitesdecreases, while the density of spherulites increases with increasing theRCF loading. Such a variation tendency gives the first evidence that theRCF has well nucleation effect on the crystallization process of polymermatrix. Then, crystallization temperature at peak obtained from thenonisothermal crystallization process by DSC shift to the high range oftemperature with increasing the RCF loading, which means the degree ofsupercooling required to nucleation decreased significantly. Thedevelopment of relative degree of crystallinity (Xt) as a function ofcrystallization time t and the classical Avrami isothermal crystallizationkinetics were also investigated by DSC in terms of isothermalcrystallization behaviors. The results reveal that the half time ofcrystallization (t0.5) derived from the Avrami equation decrease with theincrease of RCF loading. It's evident that the crystallinity of polymermatrix is enhanced with the presence of RCF in polymer matrix. Then,the quantitative analysis of heterogeneous nucleation effect of RCF wascalculated and the results show that RCF have the strong heterogeneousnucleation effect for the process of polymer crystallization. Furthermore, the subsequent melting behaviors of bio-based composites whencrystallized at various temperatures were also monitored. The multiplemelting behaviors were observed during the thermographs of both PLLAand PBS when the crystallization temperature is lower than a certainvalue. In the case of PLLA/RCF composite, the phenomenon can beattributed to the phase transition of α'-α form of crystal. The mechanismfor the multiple melting behaviors of PBS/RCF composite can beascribed to the melting, recrystallization, and remelting course during theheating process.In the case of P(3,4HB)/RCF composites, the nonisothermal coldcrystallization with different crystallization rates was monitored by DSC,which shows two exotherm crystallization peaks from the heatingthermograms. It is understandable that the crystalline ability of P(3,4HB)decrease as a result of copolymerization. Thus, an imperfect crystal firstlyformed at the range of low temperature during the heating process. Withthe temperature rising, the ability of thermal motion of P(3,4HB) segmentkeep a gradual increase and lead to a anneal phenomenon occur during anappropriate range of temperature. There is no distinct variation of meltingtemperature being observed from the subsequent heating thermogramsafter isothermal crystallization with the temperature range from64~76oC,which suggests that the crystalline parameters including crystallizationintegrity and lamellar thickness keep constant. An increasing tendency of melting temperature with increasing the isothermal crystallizationtemperature was detected when isothermally crystallized during thetemperature from90~102oC. Evidently, the crystallization integrity andlamellar thickness improve with increasing the crystallizationtemperature.(3) thermal properties of bio-based compositesTotally, the storage moduli of bio-based composites for differentsystem show the similar regular which is the moduli increase with theincrement of RCF loading. Such a tendency imply the well disperse offiber in matrix and strong interfacial interaction between fiber andpolymer matrix. Whereas, the loss factor (tan δ) shows individualvariation tendency according to the results. For the composites ofPLLA/RCF, the peaks at glass transition temperature (Tg) slightly shift tohigher temperature with increasing the RCF loading, which is the resultof inhibition effect for the segment motion due to the strong interfacialadhesion. On the other hand, the values of tan δ are higher for PLLA/RCFcomposites than that for neat PLLA. It can be interpreted by the frictionloss between two phases as well as the rigidity of PLLA backbone. TheTgs for neat PBS and its composites with RCF show no visible changeduring the whole range of RCF loading, while the values of tan δ decreasewith increasingly increasing RCF loading. This result may be ascribed tothe rigidity effect and orientation distribution of RCF in PBS matrix as well as the strong interfacial interaction between fibers and matrix, whichsynergistically contribute to the improvement in resilience for PBS/RCFcomposites. Two internal friction peaks denoted as α and β from high tolow temperatures respectively was observed from the DMA thermogramsof P(3,4HB)/RCF composites. The β relaxation is assigned to the TgofP(3,4HB). For classifing the type of α relaxation, DMA experiments werecarried out after the samples were annealed at120oC for5h. The positionof α relaxation shifting to high temperature and tending to disappearimplies that α relaxation can be assigned to the thermal motion ofmolecule of crystal surface. With the temperature rising, the free volumeof molecule of crystal surface increase and correspondingly the thermalmotion ability of segment rise, which leads to the increment of internalfriction. When the temperature reached to a certain value, the coldcrystallization owing to the annealing phenomenon leads to the crystalintegrity and lamellar thickness increased. Thus, the ability of segmentmotion decreased and finally the friction loss declined. In addition, thethermal gravimetric analysis was performed to evaluate the stability ofpolymer matrix and found that the carbon fiber shows a slight flameretardancy and carbonization effect.