| High strength aramid fiber reinforced composites have been widely used in bulletproof armor,aerospace and other fields because of their high strength,high modulus,low density and excellent impact resistance.However,the surface of aramid fiber is smooth and lacks of chemical active groups,resulting in its low surface reaction activity and poor interfacial adhesion of the composites.It is noted that the interface is the necessary "path" to realize the effective transfer of the stress from the matrix to the reinforcement,which has an important impact on the mechanical properties of the composites.Weak interfacial bonding between aramid fiber and the matrix limits the application of the composites to a certain extent.How to improve the interfacial properties of the aramid reinforced composites has become the focus of the research in recent years.From the perspective of fiber modification,according to the surface characteristics of para-aromatic heterocyclic fiber and para-aromatic fiber,different active groups were introduced onto the fiber surface through surface grafting and surface coating modification,so as to increase the surface activity of the aramid,change the fiber surface morphology and improve the interfacial adhesion and mechanical properties of the aramid reinforced composites.Also,the interfacial strengthening mechanism of the composites was studied.Firstly,through the coordination reaction between Cu2+ and benzimidazole in para-aromatic heterocyclic fiber,a "Cu2+ bridge" was established between heterocyclic aramid and polyethyleneimine（PEI）,and-NH2 was grafted onto the fiber surface.The coordination reaction conditions and PEI grafting concentration were optimized.The surface characteristics of the fiber and the mechanical properties of the composites were studied.The results show that the interfacial adhesion of the composites modified through Cu2+ coordination and PEI grafting method was significantly improved.Compared with the unmodified composites,the interfacial shear strength（IFSS）and interlaminar shear strength（ILSS）of the modified composites were increased by 49% and 51%,respectively,and the flexural strength and tensile strength were increased by41% and 29% respectively.At the same time,the mode I fracture toughness of the modified composites increased by 16%,showing excellent delamination resistance,and the interlaminar crack propagation evolved from weak interface propagation to cohesive failure.The coordination-grafting method is only suitable for heterocyclic aramid fibers with specific structure.The para-aramid fibers,which are widely used at present,cannot be modified by coordination-grafting method.In addition,surface grafting has great damage to the strength of the fiber itself,and the process conditions are relatively harsh,while the coating modification of the aramid fiber is simple and easy.Therefore,the aromatic polyamide nano-coatings were developed to functionalize para-aramid fiber.The metallization/substitution reaction was carried out in sodium hydride/dimethyl sulfoxide（Na H/DMSO）system using PPTA polymer to prepare five coatings with amphiphilic structure and realize the surface functionalization of para-aramid fiber and its fabric through dip-coating methods.The structure of the coatings was confirmed.All coating polymers showed the structure of aromatic polyamide,and there were epoxy group,C-O-C,-NH2,-COO,-CH3/-CH2-and other active groups in the polymer structure.The diameter of aramid nanofibers（ANF）prepared by the metallization reaction was about 15 nm,and the coating polymers which was grafted with epichlorohydrin（ECH）,polyether amine（PEA）,tris（2-aminoethyl）amine（TAEA）and shellac（SLC）were nanospheres with different diameters.The effects of the coating on the surface structure of the aramid fiber and the interfacial properties of the composites were investigated.After modification,the surface of aramid fiber became rougher and the surface free energy of all coated fiber increased.It was found that the contribution of polarity component played a leading role in the surface free energy.After modified by PPTA coatings,the aramid reinforced composites showed better interfacial adhesion.Compared with the unmodified samples,IFSS and ILSS of the modified composites increased by 6%~31% and8%~25%,respectively.At the same time,the mode I fracture toughness of the composites was improved,indicating that the ability of the composites to resist open crack propagation was improved.The failure mode was changed from bond failure to cohesive failure.After coating modification,due to the good interfacial adhesion,the ultra-high strength conversion was realized,and the flexural strength and tensile strength of the modified aramid reinforced composites were improved.In order to further improve the modification effect of coating polymer,the structure of coating polymer was optimized.Sulfone functionalized copolymers（SPPTA）were prepared by low-temperature polycondensation from the perspective of molecular design.The effects of the introduction of sulfone group on the structure and properties of SPPTA polymer were studied.It was found that sulfone group and meta-benzene ring structure appeared in the SPPTA polymer.Compared with PPTA polymer,the regular arrangement of SPPTA molecular chains was broken while the surface free energy increased significantly.At the same time,SPPTA polymer showed good solubility and thermal stability.The sulfone modified SPPTA coating polymers were prepared by metallization/substitution reaction with SPPTA polymer as precursor.Infrared spectra and X-ray photoelectron spectra showed that there were sulfone groups and meta-benzene ring in the structure of SPPTA coating polymer.Sulfone functionalized SPPTA coating polymers were utilized to modify para-aramid fiber.The effect of the introduction of sulfone group in the coating polymer on the surface of aramid and the interface of the composites was investigated.After modification,the fiber surface became rougher and the surface free energy increased.Compared with the unmodified composites,IFSS,ILSS,interlaminar fracture toughness,flexural strength and tensile strength of the modified aramid reinforced composites increased.And IFSS,tensile strength and flexural strength of the composites modified by SPPTA coating polymer containing PEA were increased by 32%,261%and 62%,respectively.While ILSS and mode I fracture toughness of the composites modified by SPPTA coating polymer containing SLC were increased by 46% and 38%,respectively.Also,compared with the modification of the PPTA coatings,when introducing the sulfone group in the main chain of the coating polymer,IFSS and ILSS of the composites further increased.The IFSS of the composites modified by SPPTA coating polymer containing ECH increased the most,which was 8%,while the ILSS of the composites which was modified by SPPTA coating polymer containing PEA increased the most,which was 20%.At the same time,SPPTA coating polymer showed the same or better inter-layer toughening effect as PPTA coating polymer.In order to expand the application field of SPPTA coating polymers,the effect of SPPTA coating polymers on the surface modification of carbon fiber and its composites was studied.After modified by the sulfone functionalized aromatic polyamide coating,the surface structure of carbon fiber became coarser while the surface activity of carbon fiber increased.The surface free energy of carbon fiber increased from 32.7 m J/m2 to more than 39.7 m J/m2.Also,the modified composites showed excellent interfacial and mechanical properties.The ILSS and flexural strength of the modified composites increased by 48% and 20%,respectively,compared with those without modification.The mode I fracture toughness of the composites increased from 611J/m2 to more than 795 J/m2.When SPPTA coating polymer containing PEA was used to modify carbon fiber,the mode I fracture toughness of the composites increased most significantly,with an increase of 64%.The self-made sulfone functionalized aromatic polyamide coatings have broad development and application prospects in the field of reinforced high-performance fiber composites. |
PDF Full Text Request