Ultra-high Strength Polyethylene Fibers Research Articles

The fracture process of ultra-high strength polyethylene fibres

The fracture behaviour of ultra-high strength polyethylene fibres has been investigated in dead load tests as well as by electron microscopical observation of the fracture surfaces. It was found that the fracture process in the fibres involves an activation energy of about 60 to 75 kJ mol−1, which implies that the strength is mainly determined by the lateral bond strength between the molecules. Fracture is initiated at surface irregularities, such as kink bands, which leads to the formation of cracks with a fibrillated fracture surface. In this process the individual fibrils are cut through at topological defect regions in such fibrils, containing a relatively high concentration of trapped entanglements and chain ends. The ultimate strength of the polyethylene fibres was found to be inversely proportional to the square root of its diameter. Extrapolation to zero diameter yields a strength of 26 GPa for flawless fibres, which equals the theoretical strength of polyethylene.

The effect of pendant chains on the elasticity behaviour of polyethylene networks

Ultra-high strength polyethylene fibres were crosslinked by means of60Coγ-radiation. The equilibrium elasticity behaviour of the thus obtained networks was studied. The elastic modulus of the dry network, from which the sol-fraction was extracted, was found to be 10 times larger than the modulus of the same network in the swollen state or in the dry state prior to the extraction. This effect, that could not be attributed to oxidative degradation, is explained by the presence of a large amount of pendant chains in the fibre-networks. In the melt, after removal of the sol-fraction, the pendant chains are thought to establish severe constraints on the network chains, resulting in an increment of the elastic modulus of the polyethylene network.

Preparation of ultra-high strength polyethylene fibres by gel-spinning/hot-drawing at high spinning rates

The addition of 1 wt.% Al-stearate to the spinning-solution of ultra-high molecular weight polyethylene in paraffin-oil allows takeup speeds of the extrudate of 270 m/min. These as-spun filaments could still be hot-drawn, resulting in a tensile strength of 2.8 GPa. The overall draw ratio's encountered after spinning and hot-drawing are of the order of 1500 suggesting a resemblance between these flow phenomena and superplasticity.

Process of preparation and properties of ultra-high strength polyethylene fibers

Abstract

Longitudinal growth of polymer crystals from flowing solutions. X. Conditions for continuous growth on the rotor surface

AbstractUltrahigh strength polyethylene fibers can be generated by stress‐induced crystallization from a supercooled solution subjected to Couette flow, usually referred to as the “surface‐growth” process. Under appropriate conditions, continuous fiber production can be realized for a period as long as 19 days, whereas under other circumstances a rapid interruption of the growth process is met. The present investigation deals with the origin of fiber fracture during “surface growth.” The limiting values of process variables required to maintain continuous growth have been established. Interruption of the continuous growth can occur in three different ways: (1) formation of a closed fiberloop around the rotor; (2) limited crystal growth rate; (3) rapid crystallization, leading to depletion of the gel on the rotor surface. The gel layer is being formed by adsorption of long molecules on to the rotor surface and subsequent “reptation,” resulting in a dense entanglement network of these molecules. These factors determine the boundaries of the triangularly shaped domain for continuous growth in a graph of the two main variables, namely the takeup speed and the rotor speed. Furthermore, it was noticed that the introduction of a wedge‐shaped groove in the surface of the Couette rotor leads to a substantial reduction of failure. Continuous growth could be established in the temperature range from 103–125°C when p‐xylene was used as a solvent. For p‐xylene solutions at a crystallization temperature of 110°C and using a teflon rotor of 115 mm diameter, a maximum takeup speed and rotor speed were 16 and 180 mm/s, respectively. Basically the restrictions of the process appeared to be due to the limited rate of crystallization and rate of adsorption of polyethylene molecules on the surface of the rotor.

Crosslinking of ultra-high strength polyethylene fibers by means of γ-radiation

Crosslinking of ultra-high strength polyethylene fibers by means of γ-radiation

Crosslinking of ultra-high strength polyethylene fibers by means of γ-radiation

Ultra-high strength polyethylene fibers, with a tensile strength at break varying from 1.6 up to 3.5 GPa, were irradiated, at room temperature under vacuum by means of 60Co γ-radiation. Sol-gel analysis showed a ratio of crosslinking to main-chain scissioning of about 1. The tensile strength at break decreased upon irradiation. The decrease of tensile strength at break depended on initial fiber strength and could be attributed to main-chain scissioning. It was concluded that stressed chains break preferentially upon irradiation. “Fiber” networks exhibited a deformation ratio of 16 during stress-strain measurements.