Ultra-oriented high density pOlyethylene (HDPE)

SynopsisThe elastic and viscous properties of five cellulose acetateacetone solutions, varying from 19.9% to 28.6% solids concentration, are independently determined at 60°C by capillary rheometry techniques. The viscous flow behavior of the solutions is determined over four decades of shear rate. The Bagley analysis is used to determine the entrance pressure drop and the true shear stress at various shear rates. A plot of the entrance pressure drop at the maximum experimental shear rate versus solution concentration undergoes a rapid increase in slope at 24.0y0 solids concentration, the significance of which is discussed with respect to the development of an elastically deformable chain entanglement network. The die swell behavior of the solutions at 60°C is determined on a commercial-type dry-spinning apparatus. When the die swell ratio is plotted versus volumetric flow rate, all five solutions are found to possess a characteristic curve with a distinct maximum. Photographs illustrating the variation of die swell with volumetric flow rate are shown. Die swell measurements are also shown to correlate well with entrance pressure drop measurements. The degree of spinnability of each cellulose acetate-acetone solution at 60°C is found by determining first godet speed at which one or more threads break abruptly. Spinnability is found to go through a maximum at 24.0% solids concentration. The rheological measurements and spinnability results are discussed through the aid of a single rheological parameter incorporating both elastic and viscous solution responses.

High density polyethylene has been ultradrawn at 80°C by a novel radial-compression method which involves the gradual buildup of highly-stretched elastic (spandex) filaments around the circumference of a solid polymer cylinder. The sheath of tightly-wound filaments generates a high-radial pressure which forces the polyethylene cylinder to neck down, uniaxially extend, and ultradraw. The physical and mechanical properties of radially-compressed polyethylene of maximum draw ratio, 40-45X, were evaluated and compared with the same polymer ultradrawn in other stress fields. In most physical property categories, the radially-compressed samples are virtually indistinguishable from samples prepared by solidstate extrusion at higher temperature and pressure. The transparency and the maximum tensile modulus, 62 GPa, further confirm the effectiveness of radial compression in achieving ultradraw. An experimental technique for measuring the radial pressure generated by the winding of the highly-stretched elastic filament is presented. The upper-bound pressure in the radial-compression experiments was less than 100 MPa.

An unusual crystalline aggregate, which is toroidal in shape, has been formed at the entrance region to capillary dies during the high‐pressure extrusion of poly(ethylene terephthalate) melts in an Instron capillary rheometer. The toroidal remnant was discovered in capillary dies removed from the rheometer after the occurrence of oscillating flow. Detailed observations of the rheological factors involved in the formation of the crystalline toroid were made. Examination of recovered entrance plugs disclosed that the crystalline toroid was a hard, stagnant zone which throttled the bulk flow of polymer melt into the capillary during the extrusion experiments. Hot‐stage optical microscopy, differential scanning calorimetry, and wide‐angle x‐ray techniques were used to initiate a structural study of the crystalline toroid. The toroid contained clusters of fibrous crystals which persisted to high temperatures. The fibrous clusters were randomly oriented in a matrix polymer which melted normally. Based upon the rheological and structural evidence, a mechanism of crystalline toroid formation is proposed.

Ultra‐thin films of high density polyethylene of high orientation have been produced by the recently developed technique of solid state coextrusion. The films were prepared under moderate conditions, without lubricant in continuous lengths by extruding through conical dies of extrusion draw ratio up to 36. This is a draw ratio higher than achievable by conventional solid state extrusion at comparable processing conditions through slit dies. The ultra‐thin films of high orientation were transparent and exhibited dead bend. The physical and mechanical properties were evaluated and compared with the properties of the same high density polyethylene extruded through a slit die. The increase in the melt point, crystallinity, tensile modulus, and birefringence indicates that the method is very efficient for the production of ultra‐thin and highly oriented films. An experimental technique is also presented for preparing billets of controlled and uniform initial morphology and free of voids.