Interface strength of glass fibers in polypropylene: Dependence on the cooling rate and the degree of crystallinity

The article experimentally investigates the interface strength of glass fibers in polypropylene (PP) with two different levels of crystallinity. The different degrees of crystallinity, 46.6% and 52.5%, were achieved using fast (quenching, ~4500°C/min) and slow (~2°C/min) cooling, respectively, during production of the PP film. The degree of crystallinity was measured using the differential scanning calorimetry. The mechanical properties of the films were characterized with the dynamic mechanical analysis and with a tensile test on Deben micro‐tester. Interfacial shear strength (IFSS) of glass fiber/PP was determined using fiber fragmentation test. Interfacial normal strength (IFNS) was determined using inverse identification based on finite‐element modeling of transverse tensile loading of a single fiber (Deben micro‐tester with digital image correlation‐based observation of the debonding). The measurements have confirmed the expected trend in mechanical properties of the film: increase of the storage and Young's moduli (room temperature) with the increase of degree of crystallinity, accompanied by the decrease in the loss modulus and tan δ. Interfacial strength followed the trend of the PP stiffness: both the IFSS and IFNS values for the PP with high crystallinity (slow cooling) are three (IFSS) and four (IFNS) times higher than those for the low crystallinity (fast cooling) case.