Cure Monitoring of Epoxy Systems

Molecules containing the reactive epoxide functional group have been in industrial use for more than 80 years due to its remarkable features. The epoxide functional group reacts easily, selectively and efficiently in specific environments, while at the same time being surprisingly stable. The dual functional epoxide bisphenol A diglycidylether (BADGE) is one of the most commonly used epoxides, as its use often result in good mechanical properties. The performance of an epoxy material comes from a combination of material properties, and these material properties depend on the network structure and interactions on the molecular level in the material. This PhD project was therefore focused on identifying and developing a combination of relevant rheological and chemical testing methods, to measure the molecular structure of epoxy materials from their liquid state and as they react and cure into solid materials.

It was found that a new analytical method was needed in order to measure the evolution of mechanical properties of the epoxy system during cure, while allowing solvent to evaporate from the measured sample. A dual cantilever dynamic mechanical analysis (DCDMA) setup using a PET felt support matrix was developed, which allowed for the simultaneous measurement of curing from epoxy reactions and solvent evaporation from the sample. Using this method, the curing and material changes with respect to gelation and vitrification of the measured sample could be measured reproducibly, as the sample cured from the simultaneously occurring epoxide reactions and solvent evaporation. The DCDMA method was further combined with a palette of test methods investigating the rheological and chemical properties of epoxy systems, such as 3 point bending dynamic mechanical analysis (3ptbDMA), dual plate rheology (DPR), differential scanning calorimetry (DSC), and  carbon-13 nuclear magnetic resonance spectroscopy (¹³CNMR). Using these methods, the cure of epoxy rich systems containing BADGE and the amine curing agents 3diethylaminopropylamine (DEAPA) and Cardolite were measured and compared to epoxy systems with varying epoxy ratios, curing agents, addition of polymers, crosslinking agent, and solvent.
Of particular note, the addition of DEAPA to epoxy rich systems was found to increase the degree of crosslinking in  the resulting material. By use of 13 CNMR, this increased crosslinking was determined to be due to a catalytic effect of the tertiary amine of DEAPA, as the tertiary amine could catalyse etherification reactions. Furthermore, the use of DSC showed that approximately half of the potential cure originated from etherification, when DEAPA was added to the epoxy system.

The analytical methods were further used to develop adducts of BADGE and DEAPA, where the aim was to reduce the amount of residual unreacted amines while having a low viscosity. It was found that increasing the degree of adduction in resulted in an exponential increase in viscosity, while the amount of unreacted amines decreased linearly. Addition of the allyl glycidyl ether greatly reduced the viscosity of highly adducted systems, but it had a negative impact on the mechanical properties. Additionally, the azamichael addition reaction was investigated through the addition of 1,4butanedioldiacrylate (BDDA), which achieved a reduction in both viscosity and the amount of unreacted primary amines. The analytical methods used during the project illustrated the usefulness of combining analytical methods, to  determine the material properties in relation to the changes made to the epoxy system composition.