Moduli development of epoxy adhesives during cure

Abstract

Quantifying the modulus development during cure of adhesives and prepregs is complex due to the resin's behavior. First, each phase and transition need a careful selection of frequency, strain, and thermal cycles. Second, between the gel point and the degree of cure at which resins are typically demolded, the material has a high compliance, causing excessive noise in data. Third, samples are made using multiple layers, which can cause delamination. Lastly, the data analysis and finding of trends is difficult because most tests are performed at discrete cure values.A testing protocol to detect the moduli changes in one experiment is presented. Adhesive DA 409 was analyzed using the kinetic, viscosity, gel point, and shrinkage information to delimit and select the parameters for the moduli tests. The frequency and strain of each property were selected based on dynamic strain and frequency sweeps. First, the gel point was measured using three methods; the cross-over of storage and loss moduli, the point where the complex viscosity reached industry accepted values, and the intersection of the dissipation factor at multiple frequencies. The gel point was found between 56 and 63% cure. Second, the viscosity was measured in a parallel plate rheometer, under isothermal and non-isothermal cycles, and fitted using the Castro-Macosko model. Third, the linear shrinkage was measured in a DMA in compression using zero strain to eliminate displacement due to compression loads. The total shrinkage was 4.34%, with most of the shrinkage occurring in the liquid viscoelastic region. A linear relationship between shrinkage and cure was observed.Lastly, the modulus was measured in a DMA in a three-point bending test. With this new protocol, the modulus was measured from the gel point up to the fully cured state by sandwiching a partially cured beam between two fully cured plates. The high compliance and noise were mitigated with this sandwich sample. An effective modulus of the three plates was obtained. The modulus of the partially cured beam was extracted using the elastic sandwich beam theory. This method shows potential in the testing of thermoset composites and films.