Microfoamed Strands by 3D Foam Printing

We report the design, production, and characterization of microfoamed strands by means of a green and sustainable technology that makes use of CO₂ to create ad-hoc innovative bubble morphologies. 3D foam-printing technology has been recently developed; thus, the foaming mechanism in the printer nozzle is not yet fully understood and controlled. We study the effects of the operating parameters of the 3D foam-printing process to control and optimize CO₂ utilization through a maximization of the foaming efficiency. The strands' mechanical properties were measured as a function of the foam density and explained by means of an innovative model that takes into consideration the polymer's crystallinity content. The innovative microfoamed morphologies were produced using a bio-based and compostable polymer as well as polylactic acid and were then blown with CO₂. The results of the extensive experimental campaigns show insightful maps of the bubble size, density, and crystallinity as a function of the process parameters, i.e., the CO₂ concentration and temperature. A CO₂ content of 15 wt% enables the acquirement of an incredibly low foam density of 40 kg/m³ and porosities from the macro-scale (100-900 μm) to the micro-scale (1-10 μm), depending on the temperature. The foam crystallinity content varied from 5% (using a low concentration of CO₂) to 45% (using a high concentration of CO₂). Indeed, we determined that the crystallinity content changes linearly with the CO₂ concentration. In turn, the foamed strand's elastic modulus is strongly affected by the crystallinity content. Hence, a corrected Egli's equation was proposed to fit the strand mechanical properties as a function of foam density.