The most widely produced bioplastic in the world, and the one most people have already held: clear cups, food trays, 3D-printer filament. It's made by fermenting plant sugar into lactic acid, then stitching that into a polymer.
| Property | Typical value | Notes |
|---|---|---|
| Glass transition (Tg) | 55–60 °C | Why PLA softens in a hot car, the key limit |
| Melting point (Tm) | 150–180 °C | Depends on L/D isomer ratio and crystallinity |
| Tensile strength | 50–70 MPa | Comparable to PET; strong but rigid |
| Elongation at break | 3–10 % | Low, this is the brittleness showing up |
| Young's modulus | 3.0–3.5 GPa | Stiff, glassy feel |
| Density | 1.24–1.25 g/cm³ | Slightly denser than water, it sinks |
| Crystallinity | 0–40 % | Tunable by annealing; raises heat resistance |
That ~55–60 °C glass transition is the single number to remember. Below it PLA is a stiff, glassy solid; above it, it goes soft. It's why PLA excels in cold and room-temperature uses (cold cups, produce trays) and fails hot ones (coffee, dishwashers), unless crystallinity is raised by annealing, which pushes usable heat resistance higher at the cost of clarity.
PLA starts in a field, not an oil refinery. The raw material is sugar, typically from corn, sugarcane, sugar beet, or cassava. Microbes ferment that sugar into lactic acidthe same molecule your muscles produce during exercise and that gives yoghurt its tang. Because the carbon comes from plants that recently pulled CO₂ out of the air, PLA's starting carbon footprint is lower than a fossil plastic's.
There's a subtlety that matters industrially: lactic acid comes in two mirror-image forms, L-lactic acid and D-lactic acid. The ratio between them in the final polymer controls how crystalline, and therefore how heat-resistant and how clear, the PLA turns out. Nearly pure L-PLA (PLLA) is more crystalline and heat-tolerant; a mix is more transparent but softer. This is the lever producers pull to make grades for very different jobs from the same feedstock.
That feedstock flexibility matters commercially: a producer in Thailand can run on cassava, one in Brazil on sugarcane, one in the US Midwest on corn. The polymer is the same; only the farm changes.
Turning lactic acid into a useful plastic takes two steps. First, lactic acid molecules are joined into lactidea ring-shaped pair of lactic acid units. Then that ring is opened and chained into long polymer molecules through a process called ring-opening polymerization. Controlling this step lets manufacturers dial in molecular weight and crystallinity, which is how the same base resin becomes either a rigid cup or a heat-tolerant fibre.
You can polymerize lactic acid directly, but it's hard to reach the high molecular weights that give strong, useful plastic. Going via lactide is the industry-standard workaround, it's what produces commercial-grade PLA at scale.
PLA's standout trait is clarityit's glass-clear, which is why it dominates clear cups, deli containers, and produce trays where consumers want to see the product. It's rigid, glossy, holds fine detail (hence its popularity in 3D printing), has low odour, and is approved for food contact. It's also one of the few bioplastics produced at genuine industrial scale, so it's available and relatively affordable compared with newer alternatives.
"Compostable" has fine print. Most commercial PLA needs an industrial composting facility, sustained temperatures around 55–60°C, to break down in a reasonable time. In a home compost heap or a landfill it can persist for a very long time. PLA is also brittle and softens around 60°Cso it fails the "hot coffee in a car" test without modification. And because PLA looks like conventional PET, it can contaminate PET recycling streams if sorted wrongly. None of this makes PLA bad, it makes it a material with a correct use and a wrong use.
PLA's best-case ending is industrial composting, where it returns to CO₂, water, and biomass. But the mechanism explains why the conditions are so specific: PLA degrades first by hydrolysiswater molecules cleaving the ester bonds in the polymer backbone, and only then by microbial assimilation of the resulting fragments. That hydrolysis step needs heat and moisture to run at any useful speed, which is exactly why an industrial composter's ~58 °C and high humidity work and a cool, dry landfill does not. It's not that PLA "won't" break down elsewhere; it's that the first chemical step nearly stalls without heat.
Beyond composting, PLA has two recycling routes worth knowing. Mechanical recycling (re-melting) works but degrades molecular weight each cycle. Chemical recycling is the more elegant fit: PLA can be depolymerized back to lactic acid or lactide and re-polymerized to virgin-quality material, effectively a closed loop. Both are growing but collection infrastructure is still thin in most regions. The practical takeaway: PLA only delivers its environmental promise where the right disposal system actually exists, which is a systems problem as much as a materials one.
Related: PHA / PHB (the home- and marine-degradable cousin) · PBAT (often blended with PLA for flexibility) · Compostability standards
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