Crystallinity of On-Line Consolidated Thermoplastic Composites

Abstract

Processing of high-quality thermoplastic composite parts by laser-assisted tape consolidation involves in-situ melting and solidification of the thermoplastics matrix material. In contrast to autoclave processing, fusion of the matrix material occurs locally, preventing both the development of residual stresses and fiber buckling in the inner layer. High temperature thermoplastics 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 present study the effects of these parameters on the thermal properties, such as the melting and cold crystallization point, are determined utilizing Differential Scanning Calorimetry (DSC). These thermal properties can give information about possible degradation of the polymer induced during the manufacturing process. The material selected for this study (PEEK/Carbon composite) exhibits a decrease of melt and cold crystallization temperature for increasing laser powers where the latter is more pronounced. The crystallinity, calculated from regular DSC data, showed wide variations. Therefore, modulated DSC measurements were also performed, which are considered to be more accurate 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 believed to be caused by a molecular weight reduction due to heat fragmentation during processing. The reduction of molecular weight is expected to be responsible for the increase in crystallinity with laser power, since the glass transition temperatures were lower at higher laser power settings.