Ultrasonic Welding of Glass Fiber Reinforced Vitrimer Composites

Abstract

The welding of composites has become a crucial step in the production of complex composite parts. Typically, various welding and joining techniques are used to manufacture composite assemblies. However, ultrasonic welding is one of the most popular methods for welding thermoplastic composites and cannot be used for composites with a thermosetting matrix. This study employed ultrasonic welding process for fiber-reinforced polymer composites with a dynamic covalent adaptive network (CAN). We used a thermosetting polymeric network with an associative dynamic CAN, commonly known as vitrimers; the dynamic nature of these polymers makes them reprocessable, recyclable, and reusable. These polymers can be crosslinked like thermosets but flow as thermoplastics at elevated temperatures. Thin composites were fabricated by reinforcing glass fibers into the vitrimer polymeric matrix and then welded together using ultrasonic welding. This work represents the first demonstration of welding vitrimer glass fiber-reinforced composites (vGFRC). The vGFRC matrix employed transesterification molecular exchange reactions to initiate chain mobility, leading to interlocking and rearrangement reactions on the bondline during the welding process. We evaluated the mechanical, chemical, and morphological properties of welded lap joints before and after mechanical testing. Shear strengths of 10 MPa were observed for vGFRC, indicating good weldability properties of vitrimer surfaces. The reprocessable nature of these composites was evaluated by repeated testing of lap shear strength followed by ultrasonic welding over five cycles. Additionally, we evaluated the surface functionality and atomic percentage on the fractured surface using FTIR and XPS, respectively. This exciting discovery shows that high frequency and pressure can be used to weld and reprocess polymers with dynamic molecular networks by exploiting molecular fusion and rearrangement reactions at the bondline.