Study On Surface Modification And Properties Of CNTs/UHMWPE Fiber

Ultra high molecular weight polyethylene （UHMWPE） fiber, which holds a molecular massmore than1.5million was developed as an excellent organic fiber in1970’s. The special chainstructure and processing technique endow the fiber with many excellent properties, such as lowdensity, high strength and modulus, corrosion resistance, impact resistance, outstandingstress-crack resistance, and self-lubricating properties. In spite of that, there are also variousconstraints including its poor creep resistance and low thermal instability.The carbon nanotubes/ultra high molecular weight polyethylene （CNTs/UHMWPE） fiber isobtained through gel-spinning method after adding carbon nanotubes into ultra-high molecularweight polyethylene material. Not only is the mechanical properties strengthened, but also thethermal properties and creep resistance are improved. However, due to the unique structure ofthe macromolecular chain and the special technology, the surface of this fiber is too smooth tobond with other materials, which limits its applications in some extend.In this paper, the crystallization properties and creep recovery behavior of CNTs/UHMWPEfiber were tested, and the two models were employed to fit these two processes, respectively.Then, the surface of the fiber was modified by chromic acid oxidation method and the besttechnological condition was also found by combining orthogonal experiment and single factormethod. Finally, the surface properties of the modified fiber were studied by using scanningelectron microscopy （SEM）, infrared spectrum analysis （FTIR） and wetting test. Besides, thebonding property between the modified fiber and epoxy resin was also characterized by thesingle fiber pull-out test. The results show that:1. The carbon nanotubes play a nucleating agent effect in CNTs/UHMWPE composite fiberwhich improves crystallinity degree of the material. The creep strain of both pure UHMWPEfiber and CNTs/UHMWPE fiber increased as the temperature rises, and the creep strain ofCNTs/UHMWPE fiber was less than the pure one. The Burger’s model was used to fit the creepbehavior and the results of the analysis of parameters （EM, EK, ηM, ηK） in the model showed theelasticity, rigidity and resistance to permanent deformation ability were all improved comparedwith the pure one. The Weibull distribution model was used to fit the recovery behavior and the results of the analysis of parameters （ε1, ε2, ε3） showed the three element of strain of theCNTs/UHMWPE fiber were lower than that of the pure fiber.2. The chromic acid oxidation method was used to modify the surface properties of theUHMWPE/CNTs fiber. By using orthogonal experiment, the influence of the chromic acid ratio,processing temperature and time on the mechanical properties and contact angle of the fiber wereanalyzed, and it was found that processing temperature and time were two major factors whilethe chromic acid ratio was a minor factor of the properties of the fiber. Furthermore, single factormethod was adopted to analyze the changes of the properties of fiber on temperature and time,and it was found that with the increase of temperature and time, both the mechanical propertiesand contact angle decrease. Finally, a best technological condition was obtained that: chromicacid ratio Kr₂Cr₂O₇:H₂SO₄:H2O equals to1:20:2; processing temperature was set to be60centigrade degree; and the processing time was set to be5minutes.3. The surface of CNTs/UHMWPE fiber was modified under the best condition and theresults of both SEM and FT-IR indicated that after modification, the surface become rough andmany oxygen groups were grafted on the fiber, which improved the surface polarity and enlargethe contact areas. The spreading process of epoxy resin on CNTs/UHMWPE fiber surfaceshowed that, the contact angle of the modified fiber and epoxy resin was much smaller than thatof the as-received fiber, namely the wetting properties was improved after modification. Finallythe bonding property between the modified fiber and epoxy resin was tested by single fiberpull-out test. The results showed that both the initial debonding force P1and ultimate debondingforce P₂were higher than those of the as-received fiber and interface shear strength was alsodoubled after modification. In addition, some resin fragments were adhered to the surface of thefiber, showing the failure process was a combination of interracial adhesive failure and epoxycohesive failure which also indicated the improvement of bonding property between fiber andepoxy resin.