Composites consisting of the same thermoplastic material for the matrix phase and reinforcement, the so-called ‘polymer unity composites’, are gaining increasing attention. These composites, typically those made from polypropylene (PP) and/or polyethylene (PE), possess distinct properties with potential unique applications. Unlike the conventional composites containing glass or carbon fibers embedded in polymer matrices, they can be easily recycled as they do not require the tedious process to separate the reinforcement fibers from the matrix before individual recycling. These composites can be called the ‘green composites’ as they are more environment-friendly than other coniposite materials in terms of ease of recycling....[ Read more ]

Composites consisting of the same thermoplastic material for the matrix phase and reinforcement, the so-called ‘polymer unity composites’, are gaining increasing attention. These composites, typically those made from polypropylene (PP) and/or polyethylene (PE), possess distinct properties with potential unique applications. Unlike the conventional composites containing glass or carbon fibers embedded in polymer matrices, they can be easily recycled as they do not require the tedious process to separate the reinforcement fibers from the matrix before individual recycling. These composites can be called the ‘green composites’ as they are more environment-friendly than other coniposite materials in terms of ease of recycling.

In the present study, the effects of various processing conditions, including different cooling rates and isothermal cooling, on the morphology and important mechanical properties of PP/PP composites consisting of homo-PP fibers and propylene-ethylene random copolymer matrix were characterized. The effect of service temperature was also evaluated. The interfacial adhesion was correlated to the degree of crystallinity, crystalline morphology and the bulk mechanical properties of the composite. Composite laminates of different cooling rates and thermal treatments were prepared using filming stacking and compression molding methods.

The single filament fragmentation test was carried out to characterize the PP fibre-PP matrix interfacial adhesion. It is shown that the slow cooled samples had a higher interfacial shear strength (IFSS) than the fast cooled ones. It is shown that the tensile properties generally increased with decreasing cooling rate. The isothermal treatment also improved the tensile strength and modulus as the slow cooling rate did. This can be attributed to the change in the crystallinity induced by different thermal processes. A higher crystallinity in the slow cooled samples gives rise to a high tensile strength and modulus and a relative brittle fracture mechanism, whereas a lower crystallinity introduced by the fast cooling process results in a relatively high ductility with lower tensile properties. There were functionally similar variations in IFSS and tensile properties with respect to cooling rate.

The Charpy impact test results indicate that the impact performance of the composites is highly dependent on the testing temperature. There was virtually no difference in impact fracture energy between the samples processed at different cooling rates when tested at room temperature. The sensitivity of cooling rate on impact fracture energy became significant when tested at sub-zero temperatures. The impact fracture energy were generally higher for the samples with slower cooling rates than the fast cooled samples.