The mechanical properties of polypropylene/polypropylene (PP/PP) composites consisting of homo-polypropylene fiber and propylene-ethylene random copolymer matrix were investigated. Generally, it has been already recognized that a transcrystalline layer is formed at the fiber/crystalline matrix interface. However, it is still uncertain whether this transcrystalline layer affects the mechanical properties of the composite material. Therefore, two types of unidirectional PP/PP composites with different impregnation conditions were fabricated by using a film-stacking and compression-molding method, and the effects of the transcrystalline layer on mechanical properties of PP/PP composites were examined. It is possible to form a much more transcrystalline layer at fiber/matrix interfaces by improving the impregnation conditions. Tensile tests in the 0°, 90° and intermediate loading directions were carried out. In the tensile tests at 0° and 90°, the tensile moduli in both directions increased with improving impregnation conditions, and particularly in the 90° direction, the improvement of tensile modulus was remarkable. On the other hand, in the intermediate loading directions θ=45°∼60°, the tensile moduli of the good impregnation samples were lower than the tensile modulus in the 90° direction. It is believed that these results are related to the structure of the transcrystalline layer. The lamellae in the transcrystalline layer grow perpendicular to the fiber and are able to elongate in the 90° direction. Therefore, in the 90° direction, the lamellae can transfer stress to the reinforcing fiber directly. On the other hand, in the 45° direction, the lamellae are difficult to elongate and transfer of stress to the reinforcing fiber is difficult. In all loading directions, the tensile strengths were slightly increased with improving impregnation conditions.
