Study On The Preparation And Performance Of Self-Emulsifying Polyurethane Sizing Agent Modified By Carbon Nanotubes

Carbon fiber（CF）reinforced thermoplastic composites have the advantages of low cost,easy for repairing and recycling due to their high processing efficiency and reusability,which make them widely used in many fields.The properties of carbon fiber composites are closely related to the interface cohesiveness between fiber and matrix.However,the chemical activity of carbon fiber surface is low,results in the poor binding with resin matrix.Polyurethane is compatible with a variety of thermoplastic resins,due to its unique carbamate.The use of self-emulsified polyurethane sizing agent on carbon fiber can form a layer of polyurethane film on the surface,which will improve the interface properties of composite materials effectively.However,self-emulsified polyurethane sizing agent has a disadvantage of strong water absorption ability,which will lead to the hydrolysis of the main polymer in the sizing agent and the destruction of the interface.Using hydrophobic carbon nanotubes for modification can improve the hydrophobicity without affecting the interface properties.To solve the problems above,a self-emulsifying waterborne polyurethane sizing agent was prepared in this thesis.Three kinds of carbon nanotubes containing different active groups were applied to make a modification for prepared sizing agent.The reinforcing mechanism of the composite interface was discussed in addition.The surface properties of carbon fiber and composite interface properties were also studied.The results indicated that:（1）A waterborne polyurethane（WPU）was synthesized by prepolymerization method.The hydrophilic groups were introduced by DMPA.The polymer were characterized by Fourier infrared spectroscopy,GPC and TG.The results showed that WPU was successfully synthesized which molecular weight ranges from 15000-20000,and the temperature at 5%weight loss is 295℃.The particle size of the emulsion prepared by the WPU was 15.8nm at the minimum which has good stability.The particle size and stability of the emulsion reached the best when the DMPA content was 4%and the-NCO/-OH ratio was 1.（2）The contact angle and surface energy of carbon fiber after sizing by self-emulsified WPU emulsion were tested by the dynamic contact angle measuring instrument.The lowest contact angle of carbon fiber with water was 37.1°,the lowest contact angle with diiodomethane was 15.4°,and the highest surface energy could reach 68.8 m J/m²,which was 275%higher than untreated carbon fiber.The results showed that the sizing agent was effective in improving the surface wettability of carbon fiber.The interfacial shear strength（IFSS）of the composite material was measured.The results showed that the highest IFSS could reach to 69.2 MPa,which increased 42.7%compared with unsizing carbon fiber.It indicated that the sizing agent prepared by the self-emulsification method could spread on the surface of carbon fiber and enhance the wettability of carbon fiber effectively.As a result the interfacial property of the composite materials increased.（3）The chemical structure and physical morphology of the surface of carbon fiber modified by carbon nanotubes with different active groups were analyzed.The results showed that compared with CNT-COOH and CNT-NH₂,the surface of carbon fiber modified by CNT-OH could have more carbamate active groups,so the hydrogen bond between carbon fiber and resin matrix could be strengthened,and CNT-OH particles were evenly distributed on the surface of carbon fiber,which made a good effect on enhancing surface roughness of carbon fiber.（4）The water contact angle of carbon fiber surface after sizing with CNTs modified polyurethane sizing agent was tested.The water contact angle of carbon fiber treated by CNT-OH modified sizing agent was 82.3°,which was smaller than the other two kinds of CNTs modification.It indicated that CNT-OH can enhance hydrophobicity while maintaining certain wettability.The IFSS of composite materials was tested.When the content of CNT-OH was 75μg/g,the IFSS reached 72.5 MPa,which was 49.5%higher than unmodified carbon fiber.The surface energy of the carbon fiber was 42.8m J/m²,which was 37.8%lower than that of unmodified sizing agent.This indicates that the use of carbon nanotubes can enhance the hydrophobicity of the interface,which will lead to the decrease of the wettability of the interface.