Polylactic Acid (PLA) is a widely used bioplastic that has become a staple material in fields like 3D printing, packaging, and biomedical applications. This polymer is derived from renewable sources such as corn starch or sugarcane, offering a more sustainable alternative to traditional petroleum-based plastics. As a thermoplastic, PLA can be repeatedly softened by heating and solidified by cooling. Understanding its thermal properties is fundamental to its utility and performance, dictating how it must be processed and setting limits for its real-world application.
Melting Point Versus Glass Transition Temperature
The question of PLA’s “melting point” is complicated because, unlike metals, polymers do not transition from a solid to a liquid at a single, precise temperature. Instead, PLA has two distinct thermal thresholds that define its behavior: the glass transition temperature (Tg) and the melting temperature (Tm).
The Tg is the lower of the two, typically falling within the range of 50°C to 65°C. At this point, the material changes from a hard, glassy state into a softer, rubbery, and more pliable material. The Tg represents the practical limit for a product’s heat resistance before it begins to deform.
The true Tm is significantly higher, generally ranging from 150°C to 180°C for most standard PLA formulations. This is the temperature range where the crystalline regions within the polymer completely transition into a true liquid state. This higher temperature is important for processing, but it is the Tg that defines the material’s structural stability in common use.
How Polymer Structure Affects Thermal Behavior
The specific thermal values for any sample of PLA are not fixed but vary based on its internal chemical and physical structure, meaning that “PLA” is better understood as a family of materials. A primary factor influencing both the Tg and Tm is the degree of crystallinity, which is the proportion of the material that has an ordered molecular arrangement. Higher crystallinity results in a higher, more defined melting point and improved thermal stability.
The manufacturing process, such as annealing, can intentionally increase a part’s crystallinity by holding it at a temperature between its Tg and Tm for a specific time. This heat treatment promotes crystal formation and growth, leading to enhanced heat deflection temperatures. Conversely, the presence of additives or impurities and a lower molecular weight—meaning shorter polymer chains—can both shift the Tg and Tm downward.
Stereochemistry, or the spatial arrangement of the lactic acid monomers, also plays a crucial role in determining the thermal profile. PLA can exist as Poly(L-lactic acid) (PLLA) or Poly(D-lactic acid) (PDLA). PLLA tends to have higher crystallinity and a melting temperature around 170°C to 180°C. A blend of PLLA and PDLA forms a highly stable structure called a stereocomplex, which can exhibit a significantly higher melting point, sometimes exceeding 225°C.
Real-World Limits for PLA Use
The glass transition temperature of 50°C to 65°C is the most significant practical limitation for items made from standard PLA. Any product exposed to temperatures at or above this range for an extended period will begin to lose its shape and structural integrity, leading to warping or slumping. This is why leaving PLA objects inside a vehicle on a hot day, where interior temperatures can easily surpass 65°C, often results in deformation.
In 3D printing, the thermal properties directly dictate the necessary machine settings. The extruder nozzle must be heated well above the material’s Tm, typically between 180°C and 220°C, to ensure the polymer melts and flows smoothly for proper extrusion. A heated print bed is often used to maintain the base of the print at or just below the Tg to prevent warping, which is caused by the first layers cooling too quickly and shrinking.
While the melting point is relevant for processing, thermal degradation, or pyrolysis, is the temperature at which the PLA polymer chains begin to chemically decompose. This onset of degradation typically begins around 290°C to 300°C. For this reason, 3D printing temperatures are kept below the 220°C range to ensure the material remains chemically stable and does not degrade into unwanted compounds inside the printer’s hot end.