Manufacturing, Processing and Mechanics of Fiber Reinforced Vitrimer Composites

Abstract

Fiber-reinforced polymers (FRPs) are increasingly replacing metals in industries such as automotive and aerospace due to their excellent strength-to-weight ratios. This trend is expected to grow over the next decade as FRPs offer significant performance advantages. Typically, FRPs are composed of reinforcing fibers (carbon or glass) embedded in a polymeric matrix, which can be thermoplastic or thermosetting. The matrix and the inclusions in composite play a critical role in determining their thermal and mechanical properties. FRPs face limitations such as brittleness, lower fatigue and fracture life, and poor recyclability, which can hinder their broader adoption. Addressing these issues, research has recently shifted towards enhancing the sustainability and circularity of FRPs, aligning with the United Nations Sustainable Development Goals. A promising solution lies in the development of vitrimers, a novel class of polymers with repeated healing capabilities through external stimulus, which addresses the above shortcomings of conventional thermosets and thermoplastics. Along these lines, vitrimers are an exciting new type of polymer with a dynamic polymeric network; this enables these polymers to rearrange their molecular structure to achieve repeated healing. Although various chemistries have been developed for vitrimers, little work has been done on manufacturing vitrimer composites and their resulting mechanical properties. This research explores the manufacturing, mechanics, and processing of vitrimer-based composites through two key approaches. The first focuses on ultrasonic welding as a sustainable and cost-effective method for joining vitrimer composites, while the second investigates the low velocity impact (LVI) and compression after impact (CAI) behavior of vitrimer-based carbon fiber composites. The first part of the dissertation proposes a hypothesis that ultrasonic welding can be used to assemble and weld fiber-reinforced vitrimer composites with thermosetting backbones. Ultrasonic welding was used to weld vitrimer-based composites by leveraging the dynamic bond exchange reactions in transesterification vitrimer systems. Mechanical vibrational energy facilitates these bond exchanges, resulting in strong welds with promising mechanical properties. Ultrasonic welding offers significant advantages over traditional oven curing methods, requiring less energy and time, making it a green and efficient assembly technique. Experimental results revealed that vitrimer-based composites exhibit excellent weld strength and cohesive failure at lap shear interfaces, with performance influenced by catalyst concentration. Carbon fiber-reinforced vitrimer composites outperformed glass fiber-reinforced ones, demonstrating superior weldability and mechanical properties. This work provided insights into the mechanisms of ultrasonic welding, including polymer chain diffusion and bond exchange reactions, paving the way for energy-efficient and sustainable industrial applications. The second part of this dissertation focuses on the hypothesis that low energy, out-of-plane impact damage in fiber composites can be healed by reversing matrix related damage in fiber composites, and residual strength can be retained. To examine this hypothesis, the second aspect of the dissertation focuses on the compression after impact (CAI) behavior of vitrimer-based carbon fiber composites, addressing a critical limitation of FRPs: their susceptibility to damage from barely visible low-velocity (BVID) impacts. Such impacts, common during manufacturing or service, cause delamination at the fiber-matrix interface, significantly reducing residual compressive strength and potentially leading to catastrophic failures while in service. Conventional approaches, such as adding nanomaterials, microcapsules, or Z-pinning, improve damage tolerance but often increase weight and reduce sustainability. This research explores the inherent viscoelasticity of vitrimer matrices to heal impact damage and restore residual strength when heated beyond topology freezing temperature. Healing has been carried out using a heat press. Post impact and after healing damage assessment of composites has been done using computed tomography following CAI. 3D Digital image correlation to understand the out of plane buckling characteristics is also carried out in parallel along with CAI. Results, assessed via CAI testing indicate promising strength retention compared to pristine composites for heat press healed composites, demonstrating the potential of vitrimers for sustainable and high-performance composite applications. Apart from the two key research areas on vitrimer based fiber reinforced composites as discussed above a chapter is also dedicated to the manufacturing of vitrimer based foams using solid state foaming process, using carbon dioxide as a blowing agent. This novel processing route enables the tuning of foam microstructure via catalyst content, and showcases a recyclable, reprocessable thermoset foam concept. The findings open opportunities for lightweight vitrimer materials where microcellular architecture is combined with the advantages of dynamic covalent chemistry, relevant to sustainable foams and advanced dynamic polymer networks.