In-Plane Compression and Biopolymer Permeation Enable Super-stretchable Fiber Webs for Thermoforming toward 3-D Structures

The typically poor ductility of cellulosic fibers and ensuing bonded networks and paper webs set a limit in any effort to produce associated three-dimensional structures without relying on chemical, often unsustainable, approaches. To address this challenge, we report on a facile and green method that combines mechanical and biopolymer treatment: in-plane compression and aqueous solution permeation via spraying. The first enabled network extensibility while the second, which relied on the use of either food-grade gelatin, guar gum or poly(lactic acid), improved network strength and stiffness. As a result, an unprecedented elongation of ~30% was achieved after unrestrained drying of the fiber web. At the same time, the structures experienced a significant increase in tensile strength and stiffness (by ~306% and ~690%, respectively). Such simultaneous property improvement, otherwise very difficult to achieve, represent a substantial gain in material's toughness, which results from the synergistic effects associated with the mechanical response of the network under load, fiber intrinsic strength and inter-fiber bonding. The level of plasticity developed in fiber webs upon biaxial compaction (longitudinal followed by lateral compaction), which was performed to reduce property anisotropy, allowed the synthesis of 3-D packaging materials via direct thermoforming. Moreover, the formability was found to be temperature and humidity dependent (strain and creep compliance after creep/recovery cycles in dynamic mechanical analyses). Overall, an inexpensive, green and scalable approach is introduced to expand the properties spaces for paper and related nonwovens that allows 2D and 3D formability of in-plane compacted fiber networks.