Microcellular to nanocellular polymer foams: Progress (2004–2015) and future directions – A review

• There are no low-density-nanofoams but rather, progress made at best are still medium-density-nanofoams and were achieved without the use of nucleating agents.
• Nanofoams can be made by polymers which have high CO2 absorption capacities (>20 wt%).
• 20 years (1980–2000) it took to move from macrofoams to microfoams and 15 years (2001–2015) to move to nanofoams.
• Polymers with Tg > 130 °C yield foams with smaller cell sizes while those with Tg below 130 °C yield low density foams.

The importance of low-density microcellular closed-cell thermoplastic foams over conventional macrocellular ones has been demonstrated over the years in several areas of applications. But more advanced and future expectations in terms of material cost and superior properties (thermal, mechanical, electrical, etc.) have necessitated the need to move away from fabricating microcellular to nanocellular foams. A lot of studies in this direction have yielded results especially in terms of cell size reduction but their corresponding foam densities remain either too high or quite close to the density of the non-foamed material or are simply omitted in several published articles. This review article summarises the progress made in the last decade by revealing polymer foams with the smallest mean cell sizes and lowest foam densities using the conventional foam processing techniques (batch foaming, foam extrusion and foam injection moulding).

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