Interlaminar bond strength development is crucial in the processing of continuous fiber reinforced thermoplastic composites. This research systematically analyzed the effect of cooling rate on the bond strength from the perspective of material properties and mechanics, respectively. Wedge peel tests on glass fiber reinforced polypropylene 0°4 laminates showed a significant bond strength increase (~100%) for fast-cooled samples compared with slowly-cooled counterparts. XRD and DSC analysis explained that fast cooling resulted in a lower crystallinity, finer spherulites and smaller spherulitic size, and the formation of β-PP. However, different delaminated surfaces for different cooling indicated an underlying but more direct explanation from the perspective of microstructures and mechanics. SEM observation on chemically etched samples revealed that fast cooling results in an interlocked grain boundary as the crack path, indicating a higher fiber/matrix adhesion in combination with more amorphous PP at the glass fiber surface; While slow cooling tends to form a transcrystalline structure and accordingly flat and smooth grain boundaries as the crack path, and worse fiber/matrix adhesion. In summary, this research illustrates the differences in the fiber/matrix morphology and adhesion induced by different cooling rates, and emphasizes the corresponding consequences on the bond strength.
Zhou et al. (Mon,) studied this question.