Advances in foam extrusion of micro- and nanocellular polymers: Strategies for controlling cell size and open-cell content

Abstract

Extrusion foaming is a key industrial technology for the production of lightweight polymer foams widely used in many markets. Recently, growing interest has focused on micro- and nanocellular polymer foams due to their outstanding combination of properties. To date, the fabrication of such materials has been restricted to laboratory-scale processes. Among foaming technologies, foam extrusion stands out as the most promising route for scaling up the production of micro- and nanocellular polymers. However, achieving fine cellular structures while maintaining low density and process stability remains a major challenge due to the intrinsic characteristics of the extrusion process. Therefore, extending extrusion foaming towards micro- and nanoscale structures requires a deep understanding of the underlying physical mechanisms to design production process able to achieve such structures. This review provides a comprehensive overview of the strategies reported in the literature to control cellular architecture in extrusion foaming, particularly the cell size and cell interconnectivity. First, the fundamental principles of extrusion foaming are summarized. Then, approaches for cell size reduction are discussed, including blowing agent selection, processing conditions, and the use of nucleating agents, ranging from conventional mineral fillers to nanoscale and self-assembling additives. Special attention is given to mechanisms governing cell nucleation, growth, coalescence, and stabilization, highlighting the trade-offs between cell refinement and foam density. In addition, strategies aimed at promoting controlled open-cell structures are reviewed, given their growing relevance for some advanced applications. The effects of melt strength modification and the introduction of heterogeneities are examined. Finally, a critical outlook on current limitations and the underlying causes of process and material constraints is discussed. Open challenges and future research directions are also presented. Overall, this review provides a unified framework linking formulation, processing, and cellular structure in extrusion foaming. By identifying the key factors governing cell size reduction and open-cell formation, it aims to guide future design of formulations and processes for the manufacturing of high-performance polymer foams with controlled micro- and nanocellular open-cell structures.