Prepregs are high quality semi-finished products out of a textile reinforcement structure impregnated with a resin matrix. As a reactive resin system is concerned, the prepreg is typically kept cold until its finishing process. The storage life of cooled prepregs adds up to 6-24 months. Expired prepregs are usually disposed of. Alternatively, costly and time-consuming measurements are done to ensure the usability of the expired prepreg. Parameters used to ensure the usability are e.g., the degree of cure or bonding force to certain surfaces.
A fast, easy, and non-destructive technology to ensure the usability of prepregs already on site is Near-Infrared-Spectroscopy (NIR). Using light in the range from 800-2500 nm, characteristic molecular vibrations are created. The molecular vibrations result in characteristic spectra which can be used to determine the state of the prepreg. But to evaluate the spectra a valid calibration-model, based on reference data is mandatory.
In the last years so called NIR-Micro-Spectrometer have been developed. They differ from classical process spectrometers through low weight and volume as well as a significantly lower price. Equivalently there are some cutbacks e.g., in resolution. Nonetheless they seem suitable alternatives to the conventional spectrometers and enable the development of a hand-held tool, with which prepregs can be tested during their storage and processing. A fast, easy, and non-destructive check of the prepreg quality could initiate the renunciation of the time-based disposal of prepregs in favor of a quality-based disposal.
Spectra from prepreg samples at different age are recorded by a NIR-Micro-Spectrometer. As reference to develop a valid calibration model, the degree of curing and bonding force on copper are determined. Afterwards different approaches for calibration models are tested to predict the usability of the aged prepreg.