Solvent Assisted Pressure-Free Surface Welding and Reprocessing of Malleable Epoxy Polymers

Covalent adaptable networks (CANs; also known as dynamic covalent networks or vitrimers) are appealing for developing simple and efficient techniques for recycling thermosetting polymers. In this paper, ethylene glycol (EG) is used as a solvent to enable pressure-free surface welding, surface repair, and recycling of a malleable epoxy where the transesterification-type bond exchange reaction (BER) imparts a dynamic nature to the covalent network. At a high temperature, the EG molecules participate in the BER, leading to dissolution of the epoxy network. If the EG is allowed to evaporate, the dissolved epoxy can re-form into a solid. The effects of EG content, temperature, and catalyst concentration on EG-assisted BERs are investigated. It is found that the amount of EG can be adjusted to tune the solution/solid transformation: An excessive amount of EG is required to dissolve the epoxy; on the other hand, a shortage of EG can shift the reaction back to re-form the polymer. Furthermore, the catalyst concentration defines the point at which dissolution initiates, while the degradation rate depends on temperature. This new EG-assisted method is further used for surface welding, surface damage repair, and powder-based reprocessing. The EG-assisted method does not require pressure and can achieve the properties of a fresh sample. It also provides potential opportunities to develop facile recycling techniques for thermosetting polymers.