Thanks to their permeant network structure, thermoset polymers exhibit superior mechanical strength and excellent resistance against environmental factors such as heat, chemicals and weathering. Conversely, these desirable characteristics bring in environmental challenge with thermoset wastes. Typically, those go to land fill, mechanical recycling, energy recovery, etc. As an intrinsic approach to address this, reversible polymer networks are of interest. Reversible polymer networks adopt functional groups for a reversible reaction onto their backbone. As an example, Diels–Alder (DA) epoxies are a thermally reversible material system capable of transition between a mendable segment state and a strong network state through a (forward) DA reaction and a retro-DA reaction. While their mechanical properties under operating conditions are comparable to those of conventional epoxy in operation conditions, they offer advanced functionalities such as self-healing, 3D printing, and recyclability.
However, there is an unasked question about reversible network materials: How many times can they be recycled? They cannot be recycled infinitely, as reprocessing cycles lead to a loss of reprocessibility and a degradation of mechanical properties with each cycle. While one factor is the varying degree of crosslinking during recycle processes, the primary unavoidable cause is the side reaction occurring during the thermal cycle for reprocessing. At a high temperature (>120°C), free maleimide groups are released from a DA cycloadduct by the rDA reaction. A maleimide group may react with another maleimide group, forming a non-reversible bond, instead of regenerating the DA cycloadduct. The irreversible bonds are likely to accumulate during repeated recycle processes, eventually rendering further reprocessing impossible.
In this talk, the recyclability of DA epoxy will be presented over a dozen recycles. DA epoxy samples were prepared by combination of a furan precursor and a maleimide precursor. The recyclability was determined by repeated recycle processes through a consistent thermal cycle until a DA epoxy sample turned into no longer reprocessible. The recyclability was examined by adding a radical reaction inhibitor and by introducing an alternative maleimide precursor with alkyl groups on its phenyl groups. The onset of the side reaction weas monitored for DA epoxy samples of each cycle of reprocessing. Mechanical properties including tensile strength and elastic modulus were analyzed for each cycle of reprocessing. Addition of a radical reaction inhibitor exhibited a partial delay on the onset of the side reaction, as a result, allowed DA epoxy samples for the third cycle of reprocessing, compared to only twice from the neat samples. Remarkably, the alternative maleimide precursor retarded the onset of the side reaction by six times loner (i.e., 10 minutes for the neat sample versus 60+ minutes for the sample with the alternative maleimide). Consequently, the DA epoxy samples with the alternative maleimide allow for a dozen cycles of reprocessing without significant degradation in mechanical properties.
