Monitoring the Surface Temperature on Curing Epoxy Resin Samples
High‐end thermography with the VarioCAM® HD research 800
Epoxy resin systems are mostly used as a matrix material in fibre composites. In a variety of manufacturing processes, the corresponding resin system is processed in a flowable state. The material only acquires its rigidity in a subsequent curing process. This is characterized by an exothermic chemical reaction with a pronounced temperature dependence.
During the curing process, a polymer with a strongly cross‐linked structure – also called a thermoset – is formed. Polymers generally have low thermal conductivity. Particularly in the case of thick‐walled components, these properties lead to an inhomogeneous temperature distribution with hotspots. This entails the risk that the material properties of the polymers deteriorate, for example, their strength decreases, porosity increases, or they even ignite. In addition, the cross‐linking curing reaction of epoxy resins is accompanied by a volumetric shrinkage of the material. This can sometimes cause strong mechanical residual stresses in the material, which can lead to the failure of the component before the actual loading. A precise numerical prediction of the temperature development in components is essential in order to develop suitable temperature controls in the manufacturing process of fibre composite components.
The Institute of Applied Mechanics of Clausthal University of Technology develops mathematical models based on a wide range of experimental studies. These material models reflect the mechanical, thermal, and in this case, the chemical behaviour of the polymer. Implemented in finite element software, they enable the prediction of the process or component behaviour. Against the background of the aforementioned challenges to process control in the production of fibre composite components, Dipl.‐Ing. Chris Leistner and his colleagues at the Institute of Applied Mechanics, among other topics, examined the pure epoxy resin system as part of their tests. They use temperature measurements on epoxy resin samples during curing in order to validate the model.
Self‐developed Measurement Scenario
The curing of epoxy resin systems is often monitored by a contact temperature measurement. Temperatures inside the component can be recorded by thermocouples, but can only be determined locally. In addition, such methods always have an influence on the measurement object. Thermography systems, on the other hand, measure non‐reactive and provide detailed surface temperatures. This qualifies them excellently for model validation when using hot‐curing epoxy resin samples.
"We have developed a special test setup for this application," says Dipl.‐Ing. Chris Leistner. "In addition, the geometry and the boundary conditions of the experiment must be specified as precisely as possible." The scientists use a cylindrical sample container made of aluminium as a cavity, which is filled with a previously mixed epoxy resin system (see Fig. 1). The heat supply required for the curing takes place via the outer surfaces of the aluminium container. To ensure a temperature that is as constant as possible on these surfaces, the sample container is embedded in an aluminium block preheated to a fixed temperature, which serves as a heat accumulator. Heat‐conducting paste, applied to the contact surfaces, lowers the thermal transfer resistance between the two parts. The thermography system is aligned at right angles to the surface of the epoxy resin sample by means of a tripod (see Fig. 1). The measurement of the surface temperature requires a clear view. Thus, convection and thermal radiation are present as a boundary condition of the surface.