Multi-level Surface Treatments Of UHMWPE Fibers And Interfacial Properties Of Their Composites

Ultra high molecular weight polyethylene(UHMWPE)fiber reinforced composites materials have broad application prospects due to their higher impact strength and energy absorption.In composites,the bonding strength of the interface between the fiber reinforcement and the resin matrix is the key factor influencing its mechanical properties.In this paper,the surface treatment of UHMWPE fiber is carried out by a multi-level treatment process inclouding oxidation pretreatment,polydopamine(PDA)coating,diisocyanate grafting and crosslinking.First,the surface of the fiber is oxidized in liquid phase by acid potassium permanganate(KMnO4),adding oxygen-containing functional groups with higher activity,and improving its surface morphology;then PDA coating through oxidative polymerization,increase the surface roughness,resin wettability and chemical activity of UHMWPE fiber;and then grafting isophorone diisocyanate(IPDI)on the surface of PDA and make it cross-linked with the thermoplastic polyurethane(TPU)matrix to improve the interface bonding strength of UHMWPE fiber reinforced TPU composites.Using scanning electron microscope,atomic force microscope,Fourier transform infrared spectrometer,X-ray diffractometer,thermal analyzer,fiber tenacity tester,universal testing machine and other analysis instruments,the UHMWPE fiber and its composites with TPU are tested and analyzed.We also executed the characterization of changes in surface morphology,crystal structure,chemical composition and mechanical properties.After UHMWPE fiber is oxidized,a uniformly distributed microporous structure appears on the surface,and the specific surface area increases.If the degree of oxidation is too high,it will cause the micropores on the surface of the UHMWPE fiber to expand and produce micron-level cracks,which will damage the crystal structure inside the fiber,reduce the crystallinity,and decline its mechanical properties.Compared with liquid oxidants such as chromic acid solution and hydrogen peroxide solution of the same concentration,the acid KMnO4 solution has a moderate oxidation rate on UHMWPE fibers,and the degree of oxidation is well controllable.The formation of crack structures can be prevented by adjusting the oxidation treatment time.The oxidation treatment of KMnO4 solution with a duration of 30 minutes can maximize the integrity of the fiber crystal structure and avoid a large drop in mechanical properties.At the same time,the microporous structure formed on the surface of the UHMWPE fiber can promote the infiltration and penetration of the PDA coating and the matrix resin.An anchored chimeric structure is formed,and the introduction of polar groups can increase the intermolecular interaction force and improve the bonding strength of the PDA coating.Dopamine(DA)can spontaneously oxidize and polymerize in alkaline oxygen-containing solution to form PDA,and form a dense coating on the surface of UHMWPE fiber.The dissolved oxygen content in the solution is one of the factors that determine the polymerization reaction rate of PDA.Increasing the dissolved oxygen content in the solution by high-speed stirring can effectively increase the polymerization rate of PDA,so that PDA can be quickly and uniformly coated on the surface of UHMWPE fibers;the polymerization reaction of PDA affected by pH of the solution.When the pH is 8.5,the polymerization and adhesion rate of PDA is relatively highest,which can realize rapid and uniform coating on the fiber surface and form a random concave-convex structure,which significantly increases the surface roughness of the fiber and further increases the interface sliding resistance of the material of composites,thereby enhancing the mechanical properties such as the interface shear strength of the composites.In addition,the active functional groups such as hydroxyl and amino on the surface of PDA also create conditions for graft modification on the fiber surface.The isocyanate group in the diisocyanate molecule has extremely high reactivity and can react with the hydroxyl and amino active hydrogen on the surface of PDA and the carbamate group in the TPU matrix,so it can graft IPDI on the PDA coating on the surface of UHMWPE fiber,and during the composites preparing by compression molding process,the isocyanate group on the fiber surface is cross-linked with the TPU matrix through heating and catalysis.Compared with MDI,IPDI has a lower melting point,lower molecular structure steric hindrance,higher reactivity,higher grafting rate and grafting amount of IPDI,and an effective cross-linking structure can be produced to improve the interface of composites.Covalent bond bridging enhances its microscopic interface shear strength and macroscopic interlayer shear strength.After KMnO4 oxidation-PDA coating-IPDI grafting and crosslinking multi-level surface treatment,the surface roughness of UHMWPE fiber is significantly improved,and the physical bonding force between fiber and resin matrix is increased;PDA coating increases polar functional groups on the surface,Improve the wettability of the fiber and provide the grafting reaction substrate;the covalent bond connection between the UHMWPE fiber and the TPU matrix is introduced through the IPDI grafting and cross-linking,and the UHMWPE fiber is synergistically improved from the micro-chemical structure and the macro-physical structure at multiple scales.Enhance the bonding strength of the TPU composites interface,so that it can play a higher impact strength and other mechanical properties.