Layer-dependent properties of material extruded biodegradable polylactic acid

Polylactic acid (PLA) is a biodegradable, biocompatible and non-toxic biopolymer with good mechanical properties, and is commonly used for the additive manufacture of PLA-based biomedical devices. Such devices are available in a range of sizes and thicknesses, with smaller devices capable of being realised via additive manufacturing in just a few layers. Due to their thermal history and thermal degradation, the thermal, molecular weight and mechanical properties of each layer was different when the raw material was melted, and the in-course layer was deposited to the previous layer. This study investigated the effect of the number of layers on mechanical, thermal and molecular weight properties, and the relationship between them. Material extruded ISO 527–2 type 5A specimens with 1-, 2-, 3-, 4-, 5-, 7- and 10-layers were prepared with the cutting die. Results indicated that the degree of crystallinity was found to decrease from 8% to 0.5% with an increasing number of layers. This was likely due to different cooling rates, where the molecular weight was lowest for 1-layer and increased with the increasing number of layers until it almost reached that of the bulk material. Additionally, ultimate tensile strength and strain increased with an increasing number of layers, while Young's Modulus decreased due to heterogeneous material structure. Of all obtained results, there was no significant difference between 5- and 10-layer in terms of mechanical and thermal properties.