Linear low density polyethylene (LLDPE) inherently has a higher rigidity (modulus) than a high pressure low density polyethylene of the same density and, therefore, is restricted from some market areas that require lower rigidity. Very low density polyethylene (VLDPE), i.e., polyethylene having a density of less than 0.915 gram per cubic centimeter, has a lower rigidity than LLDPE because of its lower density. However, VLDPE also has its restrictions. The narrow molecular weight distribution makes VLDPE very difficult to process because of the higher melt viscosity. Increasing the melt index of VLDPE to improve the processing characteristics is helpful, but the extractables then increase and virtually eliminate VLDPE products from any Food and Drug Administration (FDA) end use application. A blend of the two, i.e., VLDPE and LLDPE, which would result in a resin that would be easy to process, have a lower rigidity, higher environmental stress cracking resistance, and lower extractables thus meeting FDA requirements, was suggested.
It was observed, however, that neither LLDPE nor VLDPE had adequate optical properties for certain applications which require film or sheet having a percent haze per millimeter of thickness of no greater than about 2.50.
USPTO Backgrounds
USPTO Backgrounds