Reserch On Continuous/Long Carbon Fiber Reinforced PA66 Composites

Continuous carbon fiber reinforced thermoplastic(CCFRTP)composites have high specific strength,high specific modulus,low density and recyclability,which are important materials in aerospace and military fields.With the development of times,the application field of CCFRTP composites has gradually expand.Therefore,it is of great significance to study the CCFRTP composites.In this paper,continuous carbon fiber-reinforced nylon-66(CCF/PA66)prepreg tapes were prepared by melting-impregnation method,and composites laminates were prepared by hot pressing molding.The tensile properties,crystallization behaviors and morphology,fracture morphology,dynamic mechanical behavior and thermodynamics properties of composites were characterized by universal material testing machine,X-ray diffraction,differential scanning calorimetry,scanning electron microscope(SEM),dynamic thermomechanical analysis(DMA)and thermogravimetric analysis.According to the problem of low bonding strength of composites,interface modifier-MAH grafted polyethylene terephthalate(PET-g-MAH)were prepared by free radical melting graft with the maleic anhydride(MAH)as grafting monomerr and 2,3-Dimethyl-2,3-diphenylbutane(DMDPB)as initiator.And the graft state of PET-gMAH was confirmed by infrared spectroscopy(IR).Through the study of the preparation process of CCF/PA66 prepreg tapes,it can be concluded the optimal ratio of adding antioxidants,mold temperature and traction speed,which were 0.75 wt%,290 °C and 10 r/min,respectively.The heat treatment of CF has a great influence on the mechanical properties of the prepreg tapes.With the increase of heat treatment temperature,the sizing agent containing epoxy group on the surface of CF was burned,the interface strength of the prepreg tapes was lowered,and the tensile strength was decreased.The heat treatment temperature continues to increase,the acidic groups and hydroxyl groups on the CF surface increase,the interfacial adhesion between CF and PA66 was enhanced,and the mechanical properties were further improved.The crystalline structure of the alpha form of PA66 were changed with adding CF,which promoted the heterogeneous nucleation of PA66,resulting in increasin g crystallization temperature.The phenomenon of double melting of PA66 became apparent with adding of CF.The higher melting temperatures decreased firstly and then increased,since there is CF played an induced promote nucleation effect role in PA66.Due to the growth of crystal imperfect,it occurred grain refinement and melting temperature decreasing.Further increasing the fiber content,the promotion of nucleation was saturation,and the grain were gradually grow tending to perfect,and the melting temperature was increased.The addition of CF had obviously reinforcment effect on PA66.When the fiber content was 55 wt%,the tensile strengths of the prepreg tapes reached 1174 MPa.The storage modulus(E?),loss modulus(E??),glass transition temperature and initial degradation temperature of the prepreg tapes improved with the increasing content of fibers.The optimum conditions for the preparation of compatibilizer PET-g-MAH were obtained by orthogonal test: DMDPB content(0.5 wt%)> mixing t ime(6 min)> mixing speed(60 r/min)> MAH content(4 wt%).The IR shows that MAH had been successfully grafted onto the molecular chains of PET.Experiments have shown that the tensile strength of the prepreg tapes were increased significantly with the addition of compatibilizer.The tensile fracture were observed by SEM,which shows that the prepreg tapes material without the compatibilizer had poor interfacial adhesion,there were a large gap between the resin and the fiber,and the fiber surface was smooth.The interface of the prepreg tapes were improved after the addition of the compatibilizer,and the surface of the fiber were wrapped by the resin,no gap was observed at the interface,which proved that the preparation of the compatibilizer PET-g-MAH were successful.The optimum molding temperatures and holding pressure time for preparing composite laminates were 280 °C and 20 min.Due to the poor heat and oxygen aging resistance of PP,the compatibilizer PP-g-MAH had no reinforced effect on composite laminates.However,the mechanical properties of the composite laminates by the compatibilizer PET-g-MAH were increased obviously.The DMA results show that the E? and E?? of the unidirectional composite laminates were significantly higher than 0°×90° and LFTT composite laminates.There were continuous fiber in interior of the unidirectional composite laminates and the fibers were in the direction of the force,so the E? of the unidirectional composite laminates were the highest.There were no transver se fiber in in the unidirectional composite laminates,which were smaller hindrance in the movement of the resin molecular chain,and the movement of the molecular chain can consume a large amount of energy,so the E?? was high.