Introduction: In recent years, research fields including aerospace, marine, biomedical and sports have made use of innovative and advanced materials. Among the many new materials developed recently, polymer foam materials stand out because of their good mechanical and physical properties.
Polymer foams are microporous materials based on polymers (plastics, rubber, elastomers or natural polymers) that have numerous air bubbles inside, and can also be considered as composite materials with gas as filler.
In August 2022, a team from Italy used carbon dioxide for 3D printing to make microfoamed wires. Their work has been published online in the journal Polymer under the title Microfoamed Strands by 3D Foam Printing.
These new microfoam materials have a hierarchical porous internal structure that controls their structure-property relationship. These materials have enhanced mechanical properties and more active surfaces compared to non-hierarchical structures. In nature, many organic structures with hierarchical internal organization exist. These structures are capable of achieving the best efficiency and performance with the least amount of material. These include honeycomb, bone and bamboo. These natural structures have properties such as high stiffness-to-weight ratio, good energy reflection and low thermal conductivity, and are now of great interest to scientists, who are also conducting a variety of studies for this purpose.
The fabrication of 3D polymer microfoams is currently the focus of research. 3D printing has been widely used for a variety of technological solutions and offers several advantages over traditional manufacturing techniques. Researchers have made several efforts to form layered lattices and foam structures from polymers. Filamentary struts with macro- and micro-scale pores can be 3D printed. Limitations in the density and inter-chain porosity of extruded struts have been encountered in the current study due to limitations in printing resolution.
Researchers have attempted to improve the process by foaming each single strand to produce foams with high intra- and inter-strand porosity structures. A two-stage approach in which interstrand pores are printed on the structure and the material is then bulk foamed or free dried to produce intrastrand pores.
One of the latest proposed methods is the discontinuous or in-line solubilization using foaming agents in a one-step process. However, designing 3D polymeric microfoams with innovative and fine-tuned morphologies remains problematic due to current technological and process limitations. 3D foam printing itself is a recent technological innovation in additive manufacturing that is still poorly understood and faces several technical challenges before achieving full commercialization. The foaming mechanism in the printer nozzle is not yet properly understood and there are difficulties in controlling the process.
Designing polymeric microfoams with finely controlled morphology and pore structure is key to their commercial application in several frontier research and engineering fields. Achieving this will help scientists take full advantage of the rich potential of layered structures and provide a wide design space for materials scientists.
Researchers have used carbon dioxide to synthesize porous, layered 3D polymer microfoams to produce materials with specific bubble morphologies. The design, production and characterization of the materials were explored in depth in the study. The relationship between bubble morphology and process parameters, namely CO2 concentration and temperature, was highlighted. Microfoams were produced using biodegradable and sustainable polymers and PLA. These polymers were then blown into CO2 to create a green and sustainable synthesis process.15 wt.% CO2 produced low density foams (40 kg/m 3 ) with micro- and macro-scale porosity. The crystallinity content of the produced foams ranged from 5% to 45% depending on the CO2 concentration, showing a linear relationship.
The authors observed that the elastic modulus of the foamed wires was strongly influenced by the crystallinity content. The authors propose a modified Egli equation, which explains the relationship between mechanical properties and foam density. Thus, an innovative model is shown in the study which demonstrates the relationship between crystallinity and properties.
The researchers found that adiabatic cooling is the main solidification mechanism of polymer strands. This cooling is due to the rapid adiabatic expansion of the polymer microfoam during its formation. The authors suggest that understanding this effect will help to adjust the operating parameters of the process to optimize the final foam density.
Polymeric microfoams are innovative bionic materials that can provide numerous benefits to industries such as biomedical, aerospace and textile. They have a complex, porous, layered internal morphology and are superior to traditional non-layered materials. There is growing interest in applying 3D printing methods to produce these materials, but the field is still in its infancy and poorly understood, making the process difficult to control. New research in polymers has demonstrated a simple green synthetic route that could revolutionize the field of 3D printed polymer microfoams. While many challenges remain, this is an area that researchers should continue to explore.