Improvement of polyethylene fibers wettability and mechanical properties through an environmentally sustainable spinning process

Abstract

Thermoplastic polymer pyramid suggests that the performance of polymers advances as the molecular weight increases. Molecular weight is a decisive and limiting parameter in process selection. Melt spinning is a conventional method to spin fibers of thermoplastic resins, but it is limited to performing fiber formation to those polymers whose chain can be mobilized via thermal energy alone. For this reason, high-performance functional fibers can preferably be spun through solvent assisted, expensive systems such as gel spinning and wet spinning. These systems require the assistance of auxiliary chemicals to initiate chain mobility and orientation of those. Ultra-high molecular weight polyethylene is one these high performing polymeric fibers, and it finds an extensive application area from fishing nets, naval accessories, abrasion resistant fabrics, ropes to anti-ballistic vests and reinforcements as fiber, continuous filament and fabric forms occupying the major industries of automotive, energy, defense and composites. Key requirements of these industries involve superior mechanical performance, by means of high tensile strength and elastic modulus with low strain, and compatibility to the materials fibers are used with. Therefore, tensile properties and surface characteristics are at great importance, and they need to be improved simultaneously. This thesis aims to produce functional performance fibers from commodity polymers. For this purpose, polyethylene (PE) polymer is selected due to its semi-crystalline structure, theoretical mechanical potential and its ability to be spun into fiber with thermo-mechanical processes. Owing to its simple chain backbone and non-polar structure, polyethylene-based materials exhibit weak adhesion properties in both reinforcement and matrix form. Therefore, development of efficient production strategies is needed for polyethylene fibers with improved wettability and performance. The proposed melt spinning line is modified with in-line treatment zone (HiPER), which manipulates the fiber internal structure to generate unique precursors and to achieve high orientation depending on the treatment temperature and threadline dynamics acting within. Fibers were produced at lowest stable conditions and higher take-up speeds with various treatment conditions. Afterwards, they were exposed to hot drawing and zone annealing to enhance and ensure high tensile performance and chain orientation. PE fibers were characterized using optical, polarized and scanning electron microscopy, X-ray diffraction (XRD), differential scanning calorimeter (DSC) and universal tensile tester. It has been revealed that HiPER treatment manipulated internal structure of fibers in which crystalline, amorphous and chain orientations were achieved. The orientation was further enhanced by hot drawing with relatively small rates (1.6). The mechanical performance of the HiPER treated was superior to the control samples produced via conventional melt spinning. For linear low density (LLDPE) fibers taken-up 2250 m/min, HiPER samples had 290% and 163.5% higher modulus and tensile strength with final values of 1.5 GPa and 195 MPa. These samples showed nanofibrillated inner structure indicating successful chain orientation, which was supported by birefringence measurements. Drawn samples of HiPER fibers achieved 2.5 GPa elastic modulus, 345 MPa tensile strength and 13% elongation. HiPER system adopts the principles of melt spinning line with a performance modification, which consists of non-toxic and non-oily fluidic environment. The drag force acting within the treatment zone can be further manipulated by introduction of viscosity enhancers. In order to keep the system's sustainable characteristic, chosen hydrocolloids were polysaccharide-based tree gums (Arabic gum, AG). This addition was introduced to further increase the mechanical performance and create surface functionalization of fiber during spinning process. In the semi-molten state, fiber would interact with its surrounding liquid in the heated environment and drag force acting against the spinning direction. Fourier transform infrared spectroscopy (FTIR) demonstrated sharpened characteristic peaks of PE and weak peaks belonging to AG for AG-HiPER fibers. A continuous surface was created with fibers to conduct contact angle (CA) goniometry. The CA measurements revealed hydrophilic characteristic for AG-HiPER fibers. Viscosity manipulation resulted in fiber spinning at lower take-up speeds (750 m/min). AG-HiPER treatment increased the modulus and tensile strength of HiPER fibers by 14% and 16%. These fibers were hot drawn and annealed in the same conditions as HiPER fibers, and the resulting fibers had the modulus of 3 GPa and tensile strength of 272 MPa. To evaluate the compatibility of surface treated fibers with epoxy matrix, drop weight impact test was performed. AG-HiPER fiber reinforced epoxy composite showed the highest absorbed energy level among pristine epoxy, control- and HiPER-reinforced samples. The results indicate that surface functionalization can be achieved without compromising on mechanical performance. They can be improved in a single-step sustainable spinning approach, which has low production and environmental costs. Internal and surface characteristics of the produced fibers can completely be manipulated with the HiPER system, which has shown potential to be scaled-up to industrial purposes for manufacture of composites from medical to defense applications.