Crystallinity of on-line consolidated thermoplastic composites

Processing of high-quality thermoplastic composite parts by laser-assisted tape consolidation involves in-situ melting and solidification of the therm oplastics matrix material. In contrast to autoclave processing, fusion of t he matrix material occurs locally, preventing both the development of resid ual stresses and fiber buckling in the inner layer. High temperature thermo plastics such as Polyetheretherketone (PEEK) or Polyetherketoneketone (PEKK ), are semicrystalline. The level of crystallinity depends strongly on the local thermal history as well as other processing parameters. In the presen t study the effects of these parameters on the thermal properties, such as the melting and cold crystallization point, are determined utilizing Differ ential Scanning Calorimetry (DSC). These thermal properties can give inform ation about possible degradation of the polymer induced during the manufact uring process. The material selected for this study (PEEK/Carbon composite) exhibits a decrease of melt and cold crystallization temperature for incre asing laser powers where the latter is more pronounced. The crystallinity, calculated from regular DSC data, showed wide variations. Therefore, modula ted DSC measurements were also performed, which are considered to be more a ccurate for crystallinity measurements due to their ability to record endo- and exothermal events during cold crystallization and melting respectively . The results of these measurements show a significant trend of increasing crystallinity with laser power. Furthermore, a significant decrease of the glass transition temperature of the material was also observed. This was be lieved to be caused by a molecular weight reduction due to heat fragmentati on during processing. The reduction of molecular weight is expected to be r esponsible for the increase in crystallinity with laser power, since the gl ass transition temperatures were lower at higher laser power settings.