ARECA PALM SHEATH: A PLANT-BASED MATERIAL ALTERNATIVE TO PLASTICS FOR FOODWARE PRODUCTS
Creators: name: Mohanty, Debapriya Pinaki
Publisher: name: Purdue University Graduate School
Year: 2021
Subjects: Mechanical engineering not elsewhere classified
Biomaterials

description: 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—<i>Areca catechu</i>—widely cultivated in the Indian sub-continent and Southeast Asia. The study of this material system, and the specific attributes, are motivated by the use of this material in foodware applications. Foodware such as plates and bowls can be produced from the areca sheath, directly, in a single step, by stretch forming analogous to sheet metals. The material is eco-friendly, biodegrading in ~100 days. Formability and water diffusion are two key attributes of relevance for foodware, since the former attribute determines the shape change capability of the material and range of producible shapes; and the latter, foodware product (structural) integrity and life.<br><br>We characterize the morphology (external structure) and anatomy (internal structure) of the sheath using imaging techniques and composition analysis. The sheath is shown to resemble 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 formability is similar to that of the most ductile sheet metals. The formability is shown to 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.<br><br>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<i> in situ</i> imaging of water transport.<br><br>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 (~ 80<sup>0</sup>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. <br><br>Lastly, we argue, that since the leaf-sheath is a “waste product” of the palm, it has negligible embodied energy (4 to 5 orders of magnitude smaller) compared to paper and plastics based foodware.<br><br>We discuss the implications of the results for single-step forming of high-aspect ratio products and structures from the palm sheath, methods to reduce diffusion of liquids and improve foodware product life, and some directions for future research into mechanical behavior of plant leaf materials from a forming perspective.<br>