Processing and structure of Bopp film with nanoparticle-based additives

Conference Dates

November 10-14, 2019


This paper reports on biaxially oriented polypropylene nanocomposite film produced by two different processes: simultaneous equi-biaxial stretching and sequential asymmetric biaxial stretching of extruded sheets of polypropylene layered silicate nanocomposites with 2 wt%, 5 wt% and 10 wt% of nanolayers. The nanolayers were coupled to the matrix polymer through solvent-free silane treatment of the organically modified montmorillonite and by the addition of maleated polypropylene; a masterbatch was prepared first and let down with the polypropylene homopolymer (MFR=8; melting temperature=167°C) in the sheet extrusion line. The first process of simultaneous equi-biaxial stretching was carried out on square pieces of 1.2 mm thick extruded sheet at 156°C in a Karo IV apparatus. Area stretch ratios of 45-50 were obtained in both stretching processes without breaking the film. This was achieved with a linear stretch ratio of 6.5 along each direction for the simultaneous equi-biaxial process and the resulting film was clear for the nanocomposite. The sequential stretching was carried out in a continuous line where the stretch ratio along the machine direction was 5, followed by a stretch ratio of nearly 10 along the transverse direction. While the equi-biaxial stretching process produced clear film without defects, the asymmetric sequential stretching process resulted in cavitated regions on the film—see Figure 1. The latter effect appeared during the transverse stretching step around submicron-sized nanolayer stacks. The mean aspect ratio and orientation of the nanolayers in the BOPP nanocomposite films were evaluated from TEM micrographs. The average crystallite size was obtained from X-Ray diffraction scans on the various BOPP films. The permeability to water vapor at 23°C and 100% RH as well as 75% RH was obtained in g-mil/m2-day from the MOCON 3/34 apparatus, using masks with an area of 5 cm2. A 50% to 75% reduction was obtained in the permeability to water vapor, depending on the loading of nanoclay, the process used and the multilayer configuration. The crystallite size was consistently larger in the BOPP film containing the nanoclay and the crystallite size was greatest in the film with the lowest relative permeability to water vapor.

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