Abstract
Transparent thermosetting polyurethanes (PU) with increased circular and bio-based content have been developed with an effective and efficient approach based on the Design of Experiment. Starting from a commercial PU formulation for light-emitting diode encapsulation, the fossil-based polyol blend has been partially replaced with potentially recycled bis(2-hydroxyethyl) terephthalate (BHET) diol and bio-based polyol. BHET has been directly implemented in the polyol blend without requiring any additional solvent or treatment. The multivariate approach allowed to study formulative (i.e. BHET, bio-based and fossil-based polyols) and process factors (i.e. mixing time and temperature of polyol blend and PU blend) of the polymer preparation by using optical spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The effect of each factor on the optical and thermal properties of the resulting polyurethane films have been observed and clearly quantified. Particularly, the implementation of BHET improved the thermal stability of PU but reduced the film transmittance as the concentration increases. Circular and bio-based components into the polyol blend have been increased up to 80%w/w and the formulation has been optimized by following a property-driven analysis. Polyurethane formulated with the selected polyol blend showed transparency, thermal behaviour, long-term stability, and cost comparable to the starting commercial formulation. Market-level properties and ease of preparation highlight that circular and bio-based PUs can be rapidly developed and implemented within current polyurethane production processes, promoting the shift towards Circular Economy with solid industrial symbiosis. At last, the present approach can be easily repurposed to further improve sustainability of polyurethanes, as well as to develop other classes of polymers.
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