Mechanical development of thermosets during cure

Author / Creator

Puentes Gil, John Roberto, author

Available as

Online

Physical

Summary

Novel experimental and modeling techniques to quantify and correlate the kinetic and mechanical development of thermosets during cure are evaluated in this work. Commercial epoxy adhesives and prep...

Novel experimental and modeling techniques to quantify and correlate the kinetic and mechanical development of thermosets during cure are evaluated in this work. Commercial epoxy adhesives and prepregs are analyzed using standard protocols and improved protocols proposed here. First, the use of initial fast ramps in heat flux differential scanning calorimetry (DSC), up to 500 K/min, was used to follow the isothermal kinetic development of adhesives. More complete kinetics were captured while mitigating cure during the heating step. This was achieved with the optimization of the electronic furnace parameters. The enthalpy spike at the dynamic-to-isothermic transition remains an issue. Preliminary empirical shifts are proposed to compensate the signal lag. This method was used to represent the kinetics in the time-temperature-transformation (TTT) diagram including correction for filler, moisture, and pre-cure history. The formal kinetic method was used giving a simpler model compared to the conventional Kamal-Sourour equation. Second, the moduli development was captured from the gel point up to the fully cured state in a single DMA three-point bending test. A new sample was used by sandwiching a partially cured plate between two fully cured plates of adhesive. Kinetic, viscosity, gel point and shrinkage information were used to delimit and select parameters, the upper, and lower bounds of the moduli. Sandwich beam elastic theory was used to quantify the moduli of samples during cure. This method reduced beam compliance and eliminated signal noise. The previous techniques still measured kinetic and mechanical development using different instruments, masses, sample geometries, and conditions. Third, to solve this problem, an in-situ method attaching ultrasonic transducers and a Raman probe to a mold chamber was used. The degree of cure and moduli were measured simultaneously giving a closer interpretation of the cure-mechanical development interaction. Raman spectroscopy captured more reaction at the initial and propagation stages compared to DSC analyses. It also measured more accurately the end stage governed by diffusion mechanisms. This work concluded by exploring warpage measurements using a laser measurement device. The method allowed for the capture of a 3D rendering of the mold and the final component. More accurate warpage was quantified for the case of angle distortion in L-shaped parts. The angle distortion was found to strongly depend on the profile thickness. Steps towards implementing the results of this work in a numerical simulation of warpage of composite parts are detailed.

Show More