Polylactic Acid (PLA)

The relative property levels of PLA.

PLA (Polylactic Acid) is a commonly used thermoplastic filament in FDM 3D printing. It is recyclable, non-toxic, and the primary filament used by the MILL's processing team.

Specific Properties

• Printing Temperature (^oC): 190 - 220
• Hot bed Temperature (^oC): 60
• Printing Speed (mm/s): 30 - 70
• Density (g/cm³): 1.21
• Bending Strength (MPa): 68 ± 2

The atomic structure of PLA.

On the Atomic Level

Formula: (C₃H₄O₂)_n

Synthesis:

• Condensation of Lactic Acid (hence the name) with the loss of water.
  • Temperature control: Must be carried out below 200 °C; above that, lactide monomers will form due the increase in entropy..
  • Reaction type: Condensation or esterification reaction.
  • Reversibility: Reaction is reversible and reaches equilibrium; therefore, water removal is necessary to drive polymerization (need the product side of the reaction to be favored.
  • Water removal methods: Apply vacuum or perform azeotropic distillation to shift equilibrium toward polymer formation.
  • Molecular weight: This method yields polymers up to ~130 kDa initially.
  • Post-treatment: Crystallization from the melt concentrates reactive end groups (–COOH and –OH) in amorphous regions, allowing further chain growth.
  • Final result: Achieves high molecular weights (128–152 kDa) after solid-state polymerization.
• Ring-opening polymerization of lactide with various metal catalyst in solution.
  • Temperature control: Polymerization generally occurs at 130–180 °C, which balances high conversion with minimal side reactions.
  • Reaction type: The ring-opening polymerization (ROP) of lactide is the primary industrial method for synthesizing high–molecular weight PLA.
  • Monomers used: Two key monomers produce PLA — lactic acid and the cyclic di-ester lactide.
  • Catalysts: Commonly uses metal catalysts like tin(II) ethylhexanoate for example.
  • Reaction medium: Typically carried out in solution or as a suspension.
  • Yield: The ROP route achieves high yields and molecular weights exceeding 100–200 kDa, depending on catalyst efficiency and purification.
  • Drawback: The metal-catalyzed process can cause racemization, lowering the stereoregularity and crystallinity of PLA relative to optically pure feedstocks (e.g., corn starch–derived lactic acid).

The reaction formula for PLA through condensation of Lactic Acid or ring-opening polymerization of lactide.