Interfacial effects on the dynamic mechanical behavior of weft-knitted glass fiber fabric-reinforced polypropylene composites produced of commingled yarns. Tensile and flexural response

Weft knitted glass fiber (GF) fabric-reinforced polypropylene (PP) composite sheets were produced by hot pressing of knit layers composed of a commingled yarn with a GF content of 70 wt.%(approximate to 45 vol.%). The effects of fiber/matrix adhesion and reinforcing knit-induced stiffness anisotropy were characterized by dynamic-mechanical thermoanalysis (DMTA). DMTA spectra were registered under tensile and flexural loading in both course (C) and wale (W) direction of the knit. The knit structure-related stiffness anisotropy (relation of the Young's moduli measured in W- and C-directions, respectively), which proved to depend also on the loading mode of the composites, were well reflected in the DMTA spectra. The detrimental effect of poor adhesion between GF and PP could be clearly resolved when specimens cut in the C-direction were investigated. This seems to be in analogy with the transverse mechanical response of undirectional (UD) fiber-reinforced composites with interfacial variations. It was suggested that the heating-induced lofting of the composites is relied on the bond quality between GF and PP.