Major cause of strength improvement of a ramie/PA composite with low fiber contents

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Abstract The tensile strength of injection-molded semi-crystalline resin (polyamide) matrix composites was improved to higher than the values predicted from the rule of mixture merely by adding 1.0–5.0 wt% of ramie fibers to the resin. This improvement was attributed to faster crystallization on the ramie fiber surface: so-called ‘transcrystalline’. The resin strength became equivalent to the composite strength after isothermal heat treatment. To elucidate this phenomenon, we observed transcrystallization on the fiber surface in a single fiber composite specimen as a function of heating time. Results show that, despite the short heating time, the transcrystalline layer occurs earlier than spherulites. However, with longer heating time, spherulite radius reached a level equivalent to the transcrystalline layer thickness, eventually exceeding it. Although the heat treatment applied for the low-fiber-content composites or the neat resin improves their strength, premature crystallization on the fiber surface can efficiently enhance the composites’ tensile strength without heat treatment.