Study On The Formation Technology Of High Strengthand Porous Uhmwpe Fiber And Influence On Its Structure And Properties

Ultra-high molecular weight polyethylene(UHMWPE) fiber possesses various excellent properties and has been widely applied in many fields, which is always prepared via “gel spinning and ultra hot-drawing” method. It is worth noting that large amount of voids can be produced when UHMWPE is subjected to gel spinning, extraction and drying process. If the voids can be survived on the premise that fiber possesses high strength, a potential functional material with high adsorptivity and strength may be born which can be applied in harsh or corrosive environment, such as acting as adsorbent adsorbing uranium from seawater, adsorbing and filtrating heavy metals in rivers and leaking oil in sea. Therefore, successful preparation of high strength and porous UHMWPE fibers is expected to take a new step in the development of new energy and managing the environment pollution.In this paper, we innovate the drawing process, on the basis of preparing UHMWPE gel-spun fibers via gel spinning, which is first-stage drawing, extraction, drying and second-stage drawing process. Two routes are divided to prepare porous fibers with high strength and high specific surface area by first-stage cold-drawing and hot-drawing, respectively. Then extracting and drying fibers with constant length followed by second-stage hot-drawing. The tensile test, small-angle X ray scattering(SAXS), scanning electron microscope(SEM) and filling fibers with dimethylbenzene are used to analyze the strain rate(SR), draw ratio(DR) and draw temperature(T) on the effect of fibers’ microstructure, mechanical property and porosity. The main results are as follows:1. Fabricating a set of fiber continuous drawing apparatus and carrying out the first-stage cold-drawing of gel-spun fibers. In the process of drawing the random molecular chain networks orient along fiber axis accompanied by extrusion of solvent through fiber surface. The greater the SR1 and DR1, the higher degree the orientation of lamellae, the stronger the tensile strength and modulus of fibers. The fibers after extraction and drying exhibit skin-core structure where the skin layer shows oriented lamellae along fiber axis and the core layer shows porous networks.2. Second-stage hot-drawing of extracted fibers. With drawing processing, transition develops from lamellae breakage and reorganization to chain-folded lamellae transformation into microfibrils and microfibril slipping. The tensile strength and modulus of fibers increase monotonously while the porosity goes the opposite trend during the whole drawing process. With draw temperature increasing, lamellae melting and recrystalization appears. The higher the temperature, the more remarkable the melting of lamellae. The tensile strength and modulus of fibers increase at first and then decrease slightly, and the porosity decreases gradually. SEM results indicate that the skin layer of fibers shows shish-kebab structure with voids filling in between lamellae. The core layer appears interlinked fibrillar porous networks. Combining the experimental results, the drawing parameters for fibers with optimal mechanical strength and porosity are as follows: SR1 is 1 s-1, DR1 is 10; SR2 is 1 s-1, DR2 is 3, T2 is 120 ℃. The corresponding tensile strength is 1.31 GPa, modulus is 10.1 GPa and porosity is 35 %.3. First-stage hot-drawing of gel-spun fibers with different SR1 and DR1 at 110 ℃. As SR1 increases, the molecular chains are dominated from relaxation to orientation to slipping, respectively. With increasing DR1, the lamellae grow up at first then slip, break and recrystallize, finally leading to microfibril slipping. The two-dimensional(2D) SAXS intensity of extracted fibers is much higher than that of gel fibers implying that the scattering intensity of the former mainly comes from voids. The porosity of extracted fibers is about 60-70 %.4. Second-stage hot-drawing of extracted fibers with different SR2, DR2 and T2. As SR2 increases, the molecular chains orient along fiber axis at first then stacked lamellae break and recrystallize when SR2 is higher than 2 s-1. The corresponding tensile strength and modulus increases linearly with increase of SR2 then the increasing trend of tensile strength is slower and modulus slightly decreases when SR2 is higher than 2 s-1. At the same time, the porosity of fibers also decreases gradually. As DR2 increases, the molecular chains arrange densely and partial chain-folded crystals may transform into chain-extended crystals. The tensile strength and modulus increase almost linearly and the porosity increases at first then keeps constant. As T2 increases, the melting behavior of lamellae becomes stronger leading to decrease of tensile strength, modulus and porosity of fibers. SEM results indicate that the fiber surface appears shish-kebab structure and voids exist between lamellae. The core layer appears backbone network comprised of loosed packed lamellae and large amount of voids. Combing the experimental results, the drawing parameters for fibers possessing optimal mechanical strength and porosity are as follows: SR1 is 1 s-1, DR1 is 10, T1 is 110 ℃; SR2 is 1 s-1, DR2 is 3 and T2 is 100 ℃. The corresponding tensile strength is 1.63 GPa, modulus is 10.5 GPa and porosity is 47.7 %.