Quick Determination of Curing and the Degree of Cross-Linking
Cross-linking plastics is a common method to obtain mechanical, thermal and chemical properties in standard polymers, which are similar to those in high-performance plastics. The best-known examples are cross-linked polyethylene (PE-X) in pipes as well as cross-linked polyamide in the automotive sector. In addition, there is a wide range of material systems, which cure through cross-linking.
The German Plastic Center SKZ, Würzburg, Germany, is testing the use of single-sided nuclear magnetic resonance (NMR) for the determination of the degree of cross-linking of thermoplastics and thermosets. This method shall enable non-invasive material testing in a significantly shorter period of time compared with current standard testing and thus allow for inline process control.
Thermosets are used as matrices in fiber-reinforced composites or in thermoset injection molding and adhesives. The degree of cross-linking and curing is a decisive quality feature with this but it is still mainly determined by means of wet-chemical analysis. The long time period (more than 8 hours in the case of standard testing) until a quantitative result is received along with the elaborate sample preparation in this standard measuring procedure, however, do not allow for a 100% complete control.
One possible solution is measuring with a single-sided NMR system. In a research project conducted by the SKZ, potentials and limitations of the measurement method for a process-oriented, non-destructive determination of the degree of cross-linking are studied. This will be done for a broad range of cross-linked and cured material systems taking into account the respective distinctive features. The aim of the project is to determine if single-sided NMR is suitable as an alternative for the standard testing method due to the comparatively simple handling and greater mobility as well as the significantly shorter measuring times.