Optical Properties of Highly Transparent Polypropylene Cast Films: Influence of Material Structure, Additives, and Processing Conditions Katharina Resch Polymer Competence Center Leoben GmbH, Parkstrasse 11, 8700 Leoben, Austria Gernot M. Wallner Institute of Materials Science and Testing of Plastics, University of Leoben, Franz-Josef Strasse 18, 8700 Leoben, Austria Christian Teichert Institute of Physics, University of Leoben, Franz-Josef Strasse 18, 8700 Leoben, Austria Gu¨ nther Maier Material Center Leoben Forschung GmbH, Franz-Josef Strasse 13, 8700 Leoben, Austria Markus Gahleitner Borealis GmbH, St. Peter-Strasse 25, 4021 Linz, Austria Polypropylene homopolymers and ethylene/pro- pylene-random-copolymers formulated with and with- out anti-blocking additives were extruded to cast films with an industrial scale extruder equipped with a soft box, a specific air knife that expels a higher volume of air at lower velocity. The films were analyzed as to their optical properties haze and clarity. A comprehen- sive topographical characterization was performed using atomic force microscopy (AFM) and confocal microscopy. To obtain morphological information on a nanometer scale AFM phase imaging, micro-thermal analysis and small angle X-ray scattering (SAXS) were done. A significant effect of additives and processing conditions on the film topography and the optical properties was revealed. The films without anti-block- ing and antacid aids showed the best optical proper- ties and lowest vertical roughness fluctuations. In contrast, the addition of anti-blocking and antacid aids reduced the optical properties associated with an increase in surface roughness. While for films without anti-blocking aids an enhanced soft box condition re- sulted in lower vertical fluctuations of roughness and better optical properties, the behavior was in reverse for films with anti-blocking aid. By means of SAXS crystalline lamellae with a thickness of about 2.5 nm were detected. High-resolution phase imaging AFM yielded thicker crystal lamellae on the film surface. POLYM. ENG. SCI., 46:520 –531, 2006. © 2006 Society of Plas- tics Engineers INTRODUCTION Polymer films used in the packaging industry often have to fulfill high requirements as to their optical properties. Particularly in medical applications, special optical proper- ties such as low haze and high clarity are key issues. Polypropylene (PP) cast films are in use for packaging of drugs and medical substances. Because of technical and economic reasons, such films should be produced in a high-speed extrusion process. The disturbances affecting the optical properties of melt- processed films may be due to the structure of the bulk of the polymer as well as to the structure of the film surface, whereas the latter is predominant. Earlier studies [1– 4], especially by Ashizawa et al. [2], Bheda and Spruiell [3], and White et al. [4], who investigated various blown films of semicrystalline and amorphous polymers, established the surface as primary location of scattering by determining excellent correlations between surface roughness and opti- Correspondence to: Katharina Resch; e-mail: resch@pccl.at DOI 10.1002/pen.20503 Published online in Wiley InterScience (www.interscience.wiley. com). © 2006 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2006