Monitoring the shrinkage of reactive systems

Epoxy resins, polyurethanes and other reactive systems are used, for example, in electrical engineering, automotive engineering or even in the construction industry as encapsulation materials, adhesives or coatings. The good service properties of such reactive systems are countered by the disadvantages of the reaction-related volume shrinkage that occurs due to chemical cross-linking during curing. For encapsulation of components, in lithography or for medical applications (e. g. dental fillings), low-shrinkage materials are required to achieve low stresses between substrate and cladding.

With the dilatometer developed at Fraunhofer LBF for monitoring reaction shrinkage, the volume of transparent or highly filled resin systems can be monitored isothermally during curing [1].

[1]    Holst, Marco: Reaktionsschwindung von Epoxidharz-Systemen. Dissertation, Technische Universität Darmstadt (2001)

The upper figure outlines the measuring principle of volume determination. A glass cuvette with the sample to be examined is located in the beam path of a laser. Light passing through the sample is refracted at the curved surface (meniscus) of the sample. The transitions from areas with high light intensity to areas with low light intensity are detected with spatial and temporal resolution by a photodiode. The sample volume is determined from the resulting time-resolved height positions of the sample menisci.

The lower figure shows the measuring station consisting of a laser scanner and the temperature control unit. Isothermal measurements can be performed at temperatures between about 25°C and 120°C.

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).