Catalytic effect of mixed oxides in phosphine oxide epoxy-based covalent adaptable networks: Recyclability, fire protection, and smoke suppression

The non-recyclable nature and high flammability are two significant shortcomings of epoxy thermosets. Designing inherently flame-retardant epoxy-based composites with covalent adaptable networks (CANs) can counteract these issues. A new diphenyl phosphine oxide containing dicarboxylic acid (DPPOIPA) is synthesized and used with citric acid as hardeners to prepare inherently flame retardant epoxy polyester vitrimers. Sol-gel derived binary (Nb2O-SiO2, SiNb; P2O5-SiO2, SiP) and ternary (Nb2O5-P2O5-SiO2, SiNbP) acid solids are employed as transesterification catalysts and co-flame retardants to promote dynamic bond exchange and flame retardancy of the thermoset matrix. These mixed oxides lower the activation energy for the crosslinking reactions, resulting in efficient esterification during initial curing. The epoxy polyester vitrimers, containing a very low P amount (∼2 wt%), exhibit thermomechanical recyclability, flame self-extinguishing (V-0 rating at the UL 94 vertical burning test), and no dripping capability. However, vitrimers containing conventional zinc acetate only achieve a V-1 class, even with the same P content. Among the prepared catalysts, SiNbP allows for a substantial reduction in the peak heat release rate (pHRR), which is approximately 63 % lower than that of the pristine thermoset, accompanied by a remarkable increase (∼70 %) in the time to ignition, clearly indicating its intense condensed-phase flame retardant action. It is worth mentioning that a notable decrease in the total smoke release (up to 62 %) occurs in all samples containing mixed oxides, either in the presence or absence of DPPOIPA. Due to the suitable distribution of acid sites, SiNbP also enhances the release of phosphorus radicals, which is attributed to the decomposition of DPPOIPA, resulting in a strong inhibition effect in the gas phase. Strikingly, despite this effect in the gas phase, the release of smoke in the fire does not increase, thanks to the catalytic effect of the mixed oxides. This work reveals that acidic mixed oxides can be effective and sustainable bifunctional additives in epoxy polyester vitrimers, acting as solid catalysts for their recycling, smoke suppression, and fire protection.