H. Mahfuz, V. Prado Correia, T. Irons, L. Carlsson, O. Masory, and T. Langston
Florida Atlantic University,
Keywords: Ballistic Fiber, UHMWPE, Hybridization, Functionalized Nanotubes
Summary:UHMWPE is the current state of the art polymer precursor for ballistic fibers. Structure of UHMWPE is unique and characterized by a linear homopolymer with average molecular weight of about 3 million g/mole. Its molecular chain consists of as many as 200,000 ethylene repeat units. UHMWPE chains can rotate about C-C bonds and create chain folds, forming locally ordered crystalline lamellae structures embedded within the amorphous regions. UHMWPE fibers thus have highly extended chains providing high strength and modulus but low fracture strain. If the fracture strain can be increased significantly with minor loss in strength and stiffness, its overall energy absorption can be substantially enhanced. A hybridization approach with nylon 6 is undertaken to improve ductility of UHMWPE. The reason for choosing nylon 6 is its high fracture strain (~ 37%) which is one order higher than that of UHMWPE fiber. Since these two polymers are immiscible, there is a poor adhesion between the two phases. Two steps were taken to overcome this problem. Ratio of two polymers was controlled such that nylon became a minor phase while polyethylene served as the major phase. Ultrasonic cavitation and homogenization in presence of a spin solvent (paraffin oil) were utilized to break the spherical domains of nylon into smaller microspheres. After extrusion, the microspheres embedded within the UHMWPE ligaments allowed sliding of polymer chains and provided large fracture strain. To Increase strength and modulus, functionalized carbon nanotubes (CNT) were added in the hybrid mix and the composite was termed as Nano-Hybrid. The idea of nanoparticle inclusion was to share the load along with UHMWPE and recover the strength and modulus lost due to hybridization. Nanotubes used in the investigation were single-walled and functionalized with COOH (carboxylic) and ODA (octadecylamine). Nanotubes were first sonicated into a large volume of paraffin oil and then homogenized with UHMWPE and nylon 6. Ratio of UHMWPE to nylon 6 was 4:1 and concentration of nanotubes was 2.0 wt%. Fibers were extruded using a gel spinning process. After homogenization, the admixture was heated to form into a gel which was then fed into an extruder. Extruded filament was treated with hexane and heat-stretched in a convection oven to remove paraffin oil. Fibers were then tested under to failure under tension to determine modulus, strength, and fracture strain. Some of the fibers were strain hardened to further align the molecular chains of UHMWPE and nanotubes. During strain hardening, fiber was loaded and unloaded in a hysteresis loop with small strain-increments. Properties of strain-hardened fibers were also determined. A quality index called normalized velocity was calculated based on the measured properties. Normalized velocity is a product of fiber’s toughness and tensile wave speed divided by its density. Tensile test data revealed that normalizing velocity of neat UHMWPE fiber increased from 668 to 870 m/s due to hybridization and nanotube (SWCNT-ODA) reinforcement - indicating a 30% improvement. After strain hardening, normalized velocity increased to 1274 m/s demonstrating a phenomenal enhancement of 91%.