Improvement of flexural strengths of poly(L-lactic acid) by solid-state extrusion, 2: Extrusion through rectangular die

Solid-state extrusion of poly (L-lactic acid) (PLLA) through rectangular die was performed to produce high flexural strength plates that can be used as internal fixation devices. A single-angle wedge-shaped rectangular die was utilized having the die exit dimension of 4 mm x 1 mm. Billets were machined out from vacuum compression molded PLLA having different crystallinities to have various thicknesses and thus various imposed draw ratios. Solid-state extrusion of billets was performed at various drawing rates at 130°C, above glass transition and below melting temperature. Extruded plates had the width and thickness smaller than the die due to the further drawing outside the die. The decrease in width was larger than the decrease in thickness, and this became more prominent with increasing draw ratio and drawing rate, resulting in a significantly smaller aspect ratio. Contributions of die drawing and further drawing outside the die were estimated from the extruded plate dimensions, by which the drawing rate effect was attributed to the further drawing outside the die. As actual draw ratio increased, crystallinity, melting temperature, crystalline orientation factor, and birefringence increased. Throughout the whole process the decrease in molecular weight was largely suppressed to be about 10%. As billet crystallinity, draw ratio, and drawing rate increased, both flexural strength and flexural modulus increased up to the maxima of 202 MPa and 9.7 GPa, respectively. This enhancement in mechanical properties was correlated with structural developments.