Frontal Polymerization in Fiber Reinforced Polymers

Abstract

Frontal polymerization (FP) has emerged as a promising alternative to traditional bulk curing methods in recent years for manufacturing high-performance fibre-reinforced polymer (FRP) composites. The energy utilized in this self-propagating curing strategy is solely derived from the exothermic enthalpy of polymerization, making it potentially more efficient than traditional curing methods, which are extremely energy-intensive and therefore unsustainable. Research in the field of FP has been primarily focused on studying the effectiveness of the FP formulations, particularly the relationship between the monomer types and initiator concentrations on front properties. However, the research on the use of FP for manufacturing FRPs has been limited to flat rectangular plates. Therefore, this research aims to investigate the behaviour of the propagating fronts in composites with varying fibre volume fractions (Vf) within the sample and varying geometries to assess the feasibility of using FP in structures that more closely resemble real-life applications. Several test setups were explored to determine the best method for maintaining the heat balance required to sustain a propagating front. Through these trials, a stainless steel-based closed mould test setup with Teflon sheets was conceptualized and manufactured, consistently allowing for the manufacturing of composite samples with a Vf below 36%. The influence of changing Vfs on the behaviour of the front was studied, and the results were quantified using temperature data captured via thermocouples placed at strategic locations. Through a series of experiments, a critical length was established that allowed the propagation of the front from the low Vf region to the high Vf region. When the length of the low Vf region with respect to the high Vf region was smaller than the critical length, the front was seen to quench at the interface. This phenomenon was attributed to the physical reduction in the volume of the resin, which implied a reduction in the generated heat through polymerization. This led to fronts with lower peak temperatures, which upon reaching the interface would quickly fall below the threshold temperature required to sustain the front due to the higher rate of heat loss resulting from the higher fibre content. This study was followed by the manufacturing of L-shaped composite samples with uniform Vf, which showed the propagation of the front in complex geometry. Finally, the Vf was varied within the L-shaped samples, and the behaviour of the front showed similarities to the rectangular samples with varied Vf, leading to the conclusion of the presence of a critical length irrespective of geometry. These findings effectively contribute to the knowledge base necessary for implementing FP as a curing strategy for FRPs. The results obtained from this study can inform the design of manufacturing processes and setups tailored for the use of FP, potentially leading to more efficient and sustainable manufacturing practices.