Real-Time Gelation Point Analysis of a Standard Aerospace Epoxy System Using Oscillation Windowed Chirp System Rheological Technique

Thermosetting epoxy resins are widely used as matrices for high performance carbon fiber reinforced polymers (CFRPs) in aerospace industries. However, rapid crosslinking during high temperature molding causes fast viscosity escalation, making it difficult for conventional sequential frequency sweeps to precisely capture the frequency dependence of identical curing states without time lag artifacts. This study evaluates the rapid rheological behavior of a benchmark system consisting of Diglycidyl ether of bisphenol A (DGEBA) and Diethyltoluene diamine (DETDA), a latent aromatic amine curing agent. Capitalizing on DETDA room temperature latency, void free specimens were prepared without vacuum degassing and subjected to isothermal curing at 100 degrees Celsius. To capture the rapid phase transition, TA Instruments Oscillation Windowed Chirp System (Fast Frequency Sweep), which simultaneously applies multiple frequencies within a single excitation signal, was introduced. This technique acquired storage and loss modulus across a wide frequency range in ultra fast, 2 second snapshots. Consequently, the true isothermal viscoelastic spectrum was quantified without time delays. Furthermore, the precise physical gelation point (True Gel Point) was determined by capturing the exact crossover of multi frequency tangent delta curves according to the Winter Chambon criterion. These results demonstrate that the Chirp technique offers a powerful industrial solution for evaluating resin processability and optimizing manufacturing parameters in high performance composite applications with narrow processing windows.