Isotactic polypropylene (iPP) foils : correlation of core and shell crystallinity with mechanical properties obtained by nanoindentation

Huskić, Miroslav (author), Slemenik Perše, Lidija (author), Orel, Boris (author), Mihelčič, Mohor (author)

This study investigates the correlation between the crystallinity and mechanical properties of calendered isotactic polypropylene (iPP) foils, focusing on the influence of haul-off speed and additive type. Two groups of iPP foils produced on an industrial scale were compared: (i) foils containing 10 wt.% recycled PP at haul-off speeds of 2 and 10 m/min; and (ii) foils with different additives (neat PP, 10 wt.% recycled PP, and PP ran- dom copolymer) at a constant haul-off speed of 10 m/min. All foils exhibited a pronounced skin–core structure, with the inner surface showing higher crystallinity (up to 10%) due to slower cooling rates, as determined by Flash Differential Scanning Calorimetry (Flash DSC). Nanoindentation tests correlated these differences in crystallinity with variations in the hardness and elastic modulus across the cross-section of the foil. Higher haul-off speeds (10 m/min) resulted in increased crystallinity, a higher elastic modulus and higher hardness. Polarized optical microscopy (POM) confirmed the morphological differences and highlighted the presence of highly oriented skin layers and stratified crystalline struc- tures. These findings emphasize the significant influence of processing conditions, such as hauling speed and the addition of recycled polypropylene or a random copolymer, on the mechanical and optical properties of iPP foils. This comprehensive approach to characterizing complex structure–property relationships is valuable for optimizing the production and performance of polypropylene-based packaging foils on an industrial scale.

• Language: English
• Keywords: isotactic polypropylene (iPP), foils, calendering, flash differential scanning calorimetry (Flash DSC), nanoindentation, polarized optical microscopy, crystallinity, haze
• Typology: 1.01 - Original Scientific Article
• Organization: FS - Faculty of Mechanical Engineering
• Publication status: Published
• Publication version: Version of Record
• Publication date: 11.03.2025
• Year: 2025
• Number of pages: 18 str.
• Numbering: Vol. 17, no. 6, art. 736
• PID: 20.500.12556/RUL-167831
• UDC: 621
• ISSN on article: 2073-4360
• DOI: 10.3390/polym17060736
• COBISS.SI-ID: 229024771
• Publication date in RUL: 14.03.2025

Record is a part of a journal

• Title: Polymers
• Shortened title: Polymers
• Publisher: MDPI
• ISSN: 2073-4360
• COBISS.SI-ID: 517951257

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: izotaktični polipropilen (iPP), folije, kalandriranje, hitra diferenčna dinamična kalorimetrija (Flash DSC), nanoindentacija, polarizacijska optična mikroskopija, kristaliničnost, motnost

Projects

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: P2-0264-2020
• Name: Trajnostni Polimerni materiali in Tehnologije