Optical properties of blown and cast polyethylene films: Surface versus bulk structural considerations

In this article we report on some surprising, and we believe new, findings regarding the factors affecting the optical properties (haze) of polyethyle ne blown and cast films. A comprehensive investigation of blown and cast fi lms made from conventional Ziegler-Natta catalyzed linear low density polye thylene (LLDPE) as well as metallocene-catalyzed LLDPE (mLLDPE) resins was conducted. The large majority of the contribution to the total haze in the blown and cast films was observed to come from the surface roughness of the films, with the bulk (internal) contribution being relatively minor. Using a variety of analysis and characterization methods, including atomic force microscopy, small angle light scattering, and wide angle X-ray scattering, we determined that the surface roughness in these films was a result of th e development of distinct spherulitic-like superstructures formed during th e blown or cast film processing. Furthermore, these superstructures were ob served only in the mLLDPE blown films, and not in the LLDPE blown films pro cessed at similar conditions. Analysis of the rheological and molecular cha racteristics of these various mLLDPE and LLDPE resins revealed that the mLL DPE resins exhibited considerably lower molecular weight, narrower molecula r weight distribution, lower zero shear viscosity, and lower melt elasticit y compared with the LLDPE resins of similar melt index. These observations support our general finding and primary conclusion from this work that in p olyethylene blown and cast films made using typical processing conditions, the optical haze properties are adversely affected because of enhanced surf ace roughness caused by the formation of spherulitic-like superstructures i n polymer melts that possess fast relaxing and low melt elasticity rheologi cal characteristics. (C) 2000 John Wiley & Sons, Inc.