Paper extensibility is a crucial property when forming three-dimensional shapes that increase the functionality and attractiveness of packaging. Over the years, VTT researchers have worked extensively on paper extensibility, gained an understanding on the basic phenomena, and developed methods to greatly improve the extensibility. The latest findings are related to elongation of individual fibers, structure of inter-fiber contacts, and shrinkage of paper.
High paper extensibility is an important factor, especially in applications where paper or board material is strained to form 3D structures such a cups, trays or blister packages. By improving the extensibility, the materials can be formed without cracks or wrinkles into more challenging forms, thus creating new opportunities for paper-based packaging.
The factors affecting the straining potential of paper can be categorized into to three groups: fiber structure, inter-fiber bonding, and fiber network structure that can be affected by suitable treatments. The role of different factors have been elucidated in several doctoral dissertations, master's theses and projects, funded by FIBIC and CLIC Strategic Centers for Science, Technology & Innovation and Academy of Finland.
The recent work has concentrated on the interaction of the structural components in paper, paper elongation potential due to mechanical network compaction, and network phenomena during straining. Experimental results and simulations show that only if the inter-fiber bonding is at a good level, the considerable elongation potential of single fibers can be transferred to network elongation. The inter-fiber bonds have to be flexible and tolerate the large material deformations.
The fiber network elongation can be additionally boosted by introducing shrinkage either via drying shrinkage forces or via applying external compacting forces to the paper. Even the shrinkage/compaction alone offers a considerable extensibility potential. If paper can be shrunk by 20% during the papermaking process, the resulting paper elongation will be as high as 30%.
Jarmo Kouko, Senior Scientist, tel. +358 40 7243 809, Jarmo.Kouko@vtt.fi
Elias Retulainen, Principal Scientist, tel. +358 40 8668 190, email@example.com
Kouko, Jajcinovic, Fischer, Ketola, Hirn, Retulainen (2019) Effect of mechanically induced micro deformations on extensibility and strength of individual softwood pulp fibers and sheets. Cellulose 26(3):1995.
Kouko, Turpeinen, Kulachenko, Hirn, Retulainen (2019) Understanding Extensibility of Paper: Role of Fiber Elongation and Fiber Bonding. 2019 International Paper Physics Conference at PaperCon, May 5-8, 2019 Indianapolis, IN, USA.
Kouko and Retulainen (2018) The relationship between shrinkage and elongation of bleached softwood kraft pulp sheets, NPPJ, 33(3), 522-533.
Khakalo, Kouko, Filpponen, Retulainen, Rojas (2017), In-plane compression and biopolymer permeation enable super-stretchable fiber webs for thermoforming toward 3-D structures. ACS Sustainable Chemistry & Engineering, 5(10), pp 9114–9125