Time–temperature–transformation-layering diagrams: a design tool for lightweight multi-layered, single-material polymeric structures

In the design for recycling strategy, reducing the number of different materials is encouraged. However, multi-material structures like multi-layers are often used to optimize performance. Here, we demonstrate that, by engineering the coupled mass and heat transport during the processing of semi-crystalline polymeric components, it is possible to achieve a multilayered structure using a single material. Layering—in terms of crystallinity and foaming—has been accomplished by identifying the processing window in which the characteristic times are of the same order of magnitude for (i) mass transport of the foaming agent, (ii) heat transport and (iii) polymer crystallization. A time–temperature–transformation-layering diagram is thus constructed and exploited. This strategy has been validated using two different semi-crystalline polymers, poly(lactic acid) and poly(ethylene terephthalate), and CO2 as a foaming agent. We demonstrate that sustainability and performance need not be mutually exclusive. Possibility of leveraging this approach with other types of materials and/or processes for which layering is required significantly broadens the scope of this research.