ABSTRACT

The proliferation of single-use plastics in the foodware and packaging sector has stimulated interest in sustainable material substitutes that can be processed efficiently and which possess sufficient structural integrity. Herein, we study the structure, mechanical response and diffusion properties of leaf-sheath from a representative palm species—Areca catechu—widely cultivated in the Indian sub-continent and Southeast Asia. The selection of this eco-friendly material system is based on its formability attributes, that enable production of foodware (e.g., plates, bowls), directly, in a single step from the leaf-sheath by stretch forming analogous to sheet metals. Furthermore, the leaf-sheath biodegrades in ~ 100 days.  Analysis of the macro- and micro- structure of the leaf sheath, using various imaging techniques, shows the sheath to be a composite material, with structural characteristics intermediate between those of the palm leaf and stem. By measuring the mechanical response of the sheath to various types of 1D and 2D loading (e.g., uniaxial tension and compression, biaxial stretching, rolling), and hydration conditions, we show that the sheath material has high stretch-formability, especially when hydrated. This sheath formability is equal to or greater than that of the most ductile sheet metals.  The formability can be further enhanced by addition of small quantities of NaOH (~ 5%) during hydration. Local deformation measurements in biaxial stretching, based on analysis of distortion of grid-markers inscribed onto sheath samples, have enabled characterization of strain-field anisotropy and mode of failure in the sheath. By consolidating the mechanical test results, we present a forming limit diagram for the leaf-sheath.

The structural integrity and life of foodware products produced from the leaf-sheath are directly determined by diffusion of liquids (e.g., water, oils) through the sheath wall thickness. Water and oils are important constituents of semi-solid and solid foods.  Diffusion of water is also important for designing the hydration cycle to enhance formability. The diffusion of water through the sheath material process is studied using mass gain measurements and in situ imaging of water transport. We determine the diffusion coefficient for water, which is critical for estimating product life. The diffusion coefficient for the matrix is shown to be one order of magnitude greater than for the fiber. We vary salt concentration in the water by controlled additions of NaCl and note a non-monotonic dependence of the diffusion on concentration. By subjecting the leaf-sheath to a short-time (~ 3 minutes) thermal treatment (~ 800 C), a hydrophobic wax layer can be made to secrete onto the leaf surface. This wax coating is found to significantly reduce the water diffusion, enabling the sheath foodware life to be increased. Lastly, since the leaf-sheath is a “waste product” of the palm, we argue that it has negligible embodied energy (4 to 5 orders of magnitude smaller) compared to paper and plastics based foodware. 

We discuss the implications of the results for single-step forming of high-aspect ratio products and structures from the palm leaf-sheath, plant-material based foodware production by forming, and future research into mechanical behavior of plant leaf materials from a forming perspective.