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Browsing by Author "Yu, T. H."

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    Influence Of Molecular Weight Distribution on The Melt Extrusion Of High Density Polyethylene (HDPE): Effects Of Melt Relaxation Behavior on Morphology And Orientation in HDPE Extruded Tubular Films
    Yu, T. H.; Wilkes, Garth L. (AIP Publishing, 1996-11-01)
    The influence of molecular weight distribution and extrusion processing variables on the morphological features and orientation of high density polyethylene (HDPE) uniaxially extruded tubular films was investigated. In order to gain a better understanding of the orientation-crystallization behavior occurring during extrusion processing, the melt flow properties of the two HDPE resins with identical <(M)over bar (n)> (14 600) values but different molecular weight distributions (<(M)over bar (w)>/<(M)over bar (n)> = 10.3, 15.1), utilized in our previous study, were characterized by dynamic rheological experiments over the temperature range from 150 degrees C to 230 degrees C within the angular frequency range from 0.1 to 100 rad/s. The experimental data were shifted to produce master flow curves. The how activation energy calculated from the shifting process was found to be 25.9 kJ/mol for resin 1 and 29.1 kJ/mol for resin 2. The characteristic relaxation time at 190 degrees C obtained by use of a Carreau-Yasuda analysis for resin 2 having the broader molecular weight distribution was found to be 6.5 times greater than that of resin 1. This observation further supports our previous conjecture that the prominence of the fibril nuclei in resin 2 is due to its longer melt relaxation time behavior. The extrusion processing variables of melt temperature at the die exit, quench height (which is the distance from the exit of the die to the cooling ring), flow rate of the air through the cooling ring, film line speed, and die gap were varied to control the melt relaxation time of HDPE resins and the processing time frame for cooling. The results show that a longer melt relaxation time and a shorter cooling processing time can enhance the formation of fibril nuclei. The importance of melt relaxation behavior in influencing the final morphological structure in HDPE extruded films and their associated properties is clearly made evident in this paper. (C) 1996 Society of Rheology.
  • Use of U.S. croplands for biofuels increases greenhouse gases through emissions from land-use change
    Searchinger, T.; Heimlich, R.; Houghton, R. A.; Dong, F.; Elobeid, A.; Fabiosa, J.; Tokgoz, S.; Hayes, D.; Yu, T. H. (Sciencexpress, 2008)
    Most prior studies have found that substituting biofuels for gasoline will reduce greenhouse gases because biofuels sequester carbon through the growth of the feedstock. These analyses have failed to count the carbon emissions that occur as farmers worldwide respond to higher prices and convert forest and grassland to new cropland to replace the grain (or cropland) diverted to biofuels. By using a worldwide agricultural model to estimate emissions from land-use change, we found that corn-based ethanol, instead of producing a 20% savings, nearly doubles greenhouse emissions over 30 years and increases greenhouse gases for 167 years. Biofuels from switchgrass, if grown on U.S. corn lands, increase emissions by 50%. This result raises concerns about large biofuel mandates and highlights the value of using waste products.