Monitoring the shrinkage of reactive systems

As an application example, the upper figure shows the tracking of the reaction shrinkage of an epoxy resin model system of bispenol A diglycidyl ether (120g) and hexahydrophthalic anhydride (100g) as well as 2-ethyl-4-methyl-imidazole as accelerator (1.2g) at a curing temperature of 80°C. The volume of the sample decreases over time – and thus with the chemical conversion – until a stable value is reached. The measurement was reproduced twice and shows the accuracy of the measuring method.

The figure below shows the reaction shrinkage of the same epoxy resin system (but with 2.4g of the accelerator) for temperatures of 70°C, 80°C and 90°C. The reaction shrinkage – and thus the chemical conversion – is faster at higher temperatures and reaches higher values for longer reaction times. The latter is due to a total conversion that increases with temperature. At lower curing temperatures, the chemical reaction freezes earlier – i. e. at lower total conversion – due to lower chain mobility.

With supplementary methods such as infrared spectroscopy, dielectric spectroscopy, differential calorimetry, dynamic-mechanical analysis and microscopy, the dependencies of the shrinkage kinetics and the material properties of a reactive systems can be derived from the curing parameters (temperature, inert gas, irradiation) and the formulation (e. g. accelerator portion and ratio of resin and hardener component).