Optical properties of highly transparent polypropylene cast films: Influence of material structure, additives, and processing conditions

Abstract

Polypropylene homopolymers and ethylene/propylene-random-copolymers formulated with and without 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 comprehensive 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-blocking and antacid aids showed the best optical properties 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 resulted 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.

Designing polymer crystallinity: An industrial perspective

“…PP cast film studies are closely related to the formation and stability of mesomorphic phase as a result of the high cooling rates in this specific process. This has been recognized already by Geil et al, while later studies confirmed the importance of processing parameters for optical and mechanical performance . A systematic modelling of the crystallization during processing is desired for understanding the connection between processing and final properties .…”

Nucleation of Polypropylene Homo- and Copolymers

“…The main difference between the crystallization processes in injection molding is that in this process besides the "classical" nucleation shear-induced crystallization can occur whereas in the blown film process besides the nucleation an orientation/elongation process happens. Another important aspect for films is the predominant role of surface roughness, which has been nicely demonstrated for the case of PP cast films (Resch et al, 2006;.…”

Flow-induced crystallization of propylene/ethylene random copolymers

“…Molecular and other physical properties are listed in Table 1. RACO2 differs from the iPP and the other two RACO's in that it contains some additives not present in the others like a slip agent to reduce the friction coefficient (for post-processing steps in film applications) and a synthetic silica anti-blocking aid to reduce the adhesion between the film layers [32,33]. The anti-blocking aid consists of both spherical and plate-like particles ranging in size between 100 nm up to 10 lm.…”