Design On Sizing Agent Of Carbon Fiber And Study On The Interfacial Properties Of Its Reinforced Polyamide 66

Carbon fiber(CF)reinforced thermoplastic composite has efficient moulding character and unique welding manner,so that it has great advantages in scale,low cost and efficient repair and recovery.The mechanical performance of composites highly depends on the interfacial structure and properties between CF and matrix.However,the surface of bare CF from high temperature carbonization furnace is of low reactivity,little physical junction,and absence of bundling,making it difficult to satisfy the requirements of interface construction and subsequent processing.Therefore,the surface modification of bare CF must be performed.Sizing as one of surface modification technique has the unique advantage of satisfying simultaneously the interface properties of composite and bundle processing.However,currently,the sizing agents developed for thermoplastic resins,are not only in short supply and high price,but also have low chemical reactivity and poor compatibility with thermoplastic resin matrix.This leads to the lower interfacial properties of CF reinforced thermoplastic resin composite.Besides,some sizing procedures are complicated,time-consuming,and not environmentally friendly,making it difficult to match CF industrial production line.Therefore,in this paper,the chemical structure of sizing agents is designed to meet the needs of interfaces between CF and polyamide 66(PA66)resin.Meanwhile,new sizing technology is developed to satisfy the environmental protection and high efficiency of sizing process.(1)Based on the low cost and abundant supply of epoxy sized carbon fiber(ESCF)in the market,the surface modification technology that is oxidation in air atmosphere was developed to solve the problem of poor compatibility of ESCF with thermoplastic resin matrix.This paper is aimed to achieve the transformation of surface group on ESCF into the structure fitting the PA66resin.Main research results and conclusions are summarized as follow.The interlaminar shear strength(ILSS)of the composite reinforced with the oxidation modified ESCF for 15 min is remarkably enhanced to 80.4 MPa by 426%,compared to that reinforced with the pristine fiber,and by 93%compared to that reinforced with heat treated fiber in nitrogen.Further studies show heat treatment in nitrogen atmosphere can remove the epoxy groups with poor compatibility with the matrix.In the air,partial methylene,and hydroxyl groups on the epoxy sizing agent in the presence of oxygen were oxidized to form new carboxyl groups,and then these groups reacted with original carboxyl groups on CF by condensation,consequently forming strong chemical bonding at the interface of the fiber and sizing by generated anhydride group crosslinking.Moreover,these anhydride groups on the oxidation modified ESCF can react with amino groups on PA66 molecules,so that to significantly improve the covalent bonding between CF and PA66 resin.The influence of the oxidation modification on ESCF mechanical and bundling properties is negligible for protection of the reserved sizing.The results indicate that the surface groups on ESCF by the oxidation modification can be translated into the structure matching PA66 resin,which greatly improves the interfacial properties of the reinforced PA66 resin.Meanwhile,the bundling properties of the oxidation modified ESCF are reserved.(2)In order to adapt to the rapid development of thermoplastic resin composites,it is very important to develop sizing agents to match them.On the issues of the low reactivity and poor compatibility of polyurethane sizing agents with thermoplastic resin matrix,four structure polyurethanes with different flexible and reactive groups were selected,and correlation between their chemical structure and interfacial properties was analysed.Main research results and conclusions are summarized as follow.The ILSS of composite with cross-linkable polyether polyurethane sized CF is 47.8 MPa,reaching the level of oxidized CF.However,the ILSS of composites with the rest of polyurethane sized CFs is all lower than that with the oxidized CF.The ILSS of composite with polyester polyurethan sized CF decreases to 38.2 MPa,but increases by 14%than that with the bare CF.The ILSS with polycarbonate polyurethan and block isocyanate sized CFs is even lower than that with the bare CF.This is related to the flexibility of sizing agent molecule and chemical activity of its reactive groups.Cross-linkable polyether polyurethane has more flexible molecular chain and active isocyanate groups deblocked at high temperature,which not only improves the compatibility between the sizing and matrix,but also can achieve covalent bonding between CF surface and sizing.Therefore,the interfacial properties between the sized CF and matrix are enhanced.Other sizing agents have lower chain flexibility or chemical activity.Besides,the bundling property of cross-linkable polyether polyurethane sized CF reaches the level of commercial CF.These results indicate that sizing agents containing high active groups compatible with matrix are the key to enhancing the interfacial properties of reinforced PA66 resin.(3)This part is aimed to further improve the contents of high active groups in sizing agent molecules compatible with matrix,and satisfy the environmentally friendly and highly efficient requirements of sizing process.Water-soluble glycerol and maleic acid(MA)were used to in-situ polymerize onto CF surface to synthesize hyperbranched polyester(HBPE)sizing with lots of active hydroxy groups.The influence of CF oxidation extent on monomer polymerization mechanism,and the effect of the HBPE structure on the interfacial properties of reinforced PA66 composite were studied.Main research results and conclusions are summarized as follow.HBPE sizing is formed by the synergistic effects of addition reaction and esterification between glycerol and MA on CF at above 200?.After electrochemical oxidation of bare CF,the carboxyl groups on oxidized CF can prompt the ester reaction between carboxyl and hydroxyl,and consequently improve the extent of ester reaction of HBPE sizing.The more carboxyl groups on 6 A/m~2 oxidized CF contribute to the stronger catalysing of its surface monomer esterification,consequently forming higher branch point and more terminal hydroxyl groups of HBPE sizing structure,which enhances the interaction at the interface of CF/sizing and sizing/matrix by constructing many covalent and hydrogen bonds.The ILSS of PA66 composite reinforced with HBPE sized oxidized-CF under 6 A/m~2 current density is remarkably enhanced to 77.9 MPa by 46%compared to that with the unsized one.These results indicate that in-situ of polymerization of maleic acid and glycerol monomer on CF surface is an environmental and efficient alternative method to size CF and significantly enhance the interfacial property of CF/PA66 resins.(4)In order to balance the ability of chemical cross-linking and deformation of molecule chain at interface,and introduce more molecular entanglement and deformation,lower functionality,and higher molecular weight polyethylene glycol(PEG)was chosen to in situ polymerize with MA,to achieve the increase of chain length of HBPE sizing.Moreover,the molecular weight of PEG was changed,and the effects of the activity and chain length of HBPE on the interfacial properties were studied.Main research results and conclusions are summarized as follow.Firstly,HBPE sizing agent with abundant hydroxy groups was synthesized by optimizing in-situ polymerization temperature.Then the effects of in-situ polymerization of different molecular weight PEG and MA at 235 and 280°C on HBPE structure and interfacial properties of reinforced PA66 composites were studied.It is found that when the molecular weight of PEG is less than 2000,the formed HBPE sizing structure is not affected by the molecular weight of PEG,and there are abundant hydroxyl groups and branch points on the sizing.Moreover,with PEG molecular weight increasing,the chain length of the formed HBPE sizing increases.However,when the molecular weight of PEG is more than 2000,the hydroxyl groups and branch points on the HBPE sizing decrease due to the decrease of reaction activity of terminal groups on over-high molecule weight PEG.Therefore,the hydroxyl group content and molecule chain length on the HBPE sizing can achieve balance by choice of 2000molecular weight PEG and MA to in situ polymerize.As a result,the ILSS of PA66 composite reinforced with the sized oxidized-CF by in-situ polymerization of 2000 molecular weight PEG and MA at 280°C is remarkably enhanced to 90.5 MPa by 70%compared to unsized CF,and by 16%compared to that by in-situ polymerization of high functionality glycerol with MA.This is ascribed to the abundant activity groups and molecule chain of the formed HBPE sizing by in-situ polymerization of 2000 molecular weight PEG and MA,which enhances the interaction of CF/sizing and sizing/matrix by balancing the contribution of molecular entanglement,deformation,and covalent interconnection to interface.Besides,the sized oxidized-CF by in-situ polymerization of 2000 molecular weight PEG and MA can meet handleability in its application.These results indicate that in-situ polymerization of PEG and MA onto CF further achieves the optimization of the interfacial properties of CF reinforced PA66 composite.Though the surface modification of ESCF,and the adjusting and control of activity, flexibility and chain length of sizing agents,the interfacial properties between CF and PA66 resin can be enhanced significantly,as well as meeting the requirements of environmental protection and high efficiency of sizing process in industry.This provides a new thought or method for the structure design of CF sizing agent applied in thermoplastic resin matrix composite.