Polylactic acid, also called poly(lactic acid) or polylactide (PLA), is a type of plastic. It is a thermoplastic polyester (or polyhydroxyalkanoate) with a repeating unit formula of (C₃H₄O₂)ₙ or [–C(CH₃)HC(=O)O–]ₙ. PLA is created when lactic acid, C(CH₃)(OH)HCOOH, undergoes a chemical reaction that removes water. It can also be made by linking lactide molecules, which are cyclic dimers of the basic repeating unit. Often, PLA is mixed with other materials. Whether PLA is biodegradable or durable depends on the manufacturing process, added ingredients, and the types of polymers used.

PLA is made from renewable resources and is used in compostable products. In 2022, PLA was the most widely used bioplastic globally, accounting for about 26% of all bioplastic demand. However, PLA is not as common as traditional plastics like PET or PVC. Its use has been limited by some physical and processing challenges. PLA is the most popular material for 3D printing filaments because it melts at a low temperature, has strong properties, expands little when heated, and bonds well between layers. However, it does not resist heat well unless treated with a special process called annealing.

Although "polylactic acid" is a commonly used name, it does not follow the official scientific naming rules (IUPAC). The correct name is "poly(lactic acid)." The term "polylactic acid" might be confusing because PLA is a polyester, not a polyacid (polyelectrolyte).

Chemical properties

The monomer is usually made from fermented plant starch, such as corn, cassava, sugarcane, or sugar beet pulp.

Several industrial methods produce usable (meaning high molecular weight) PLA. Two main monomers are used: lactic acid and a cyclic di-ester called lactide. The most common method to make PLA involves a process called ring-opening polymerization of lactide. This reaction uses metal catalysts, often tin ethylhexanoate, in a solution or as a suspension. The use of metal catalysts can cause the PLA molecules to change their structure slightly, making them less regular in shape compared to the original material, such as corn starch.

Another method involves directly combining lactic acid monomers to form PLA. This process must occur below 200°C. At higher temperatures, a different molecule called lactide is formed instead. Each time lactic acid monomers join, one molecule of water is produced. Because this reaction can reverse itself, removing water is necessary to create long, high-molecular-weight chains. Water can be removed using a vacuum or a technique called azeotropic distillation. This method can produce PLA with molecular weights up to 130 kDa. Even higher molecular weights can be achieved by carefully crystallizing the raw polymer from a melted state. This process concentrates carboxylic acid and alcohol end groups in the amorphous part of the solid polymer, allowing them to react further. Molecular weights between 128–152 kDa can be reached this way.

A different method involves reacting lactic acid with a substance called zeolite. This process is simpler and occurs at a temperature about 100°C lower than other methods.

Because lactic acid is chiral, several forms of polylactide exist. Poly-L-lactide (PLLA) is made from the L-lactide monomer. Advances in biotechnology have enabled the commercial production of the D-enantiomer form of lactic acid.

When a mixture of L- and D-lactide monomers is used, the result is poly-DL-lactide (PDLLA), which is amorphous. Using specific catalysts can create heterotactic PLA, which shows some crystallinity. The level of crystallinity, and thus many of PLA’s properties, depends mostly on the ratio of D to L enantiomers used, and to a smaller extent on the type of catalyst. In addition to lactic acid and lactide, a compound called lactic acid O-carboxyanhydride ("lac-OCA") has been studied in academic research. This compound is more reactive than lactide because its polymerization releases carbon dioxide instead of water.

A direct biological method to produce PLA, similar to how poly(hydroxyalkanoate)s are made, has also been reported.

Physical properties

Polylactic acid (PLA) polymers can exist in different forms, such as non-crystalline (glassy) or semi-crystalline (with some ordered structure). These materials have a glass transition temperature between 60–65 °C, a melting temperature between 130–180 °C, and a stiffness (Young's modulus) between 2.7–16 GPa. Heat-resistant PLA can handle temperatures up to 110 °C. The mechanical properties of PLA are similar to those of polystyrene and PET. The melting temperature of PLLA can be raised by 40–50 °C, and its heat deflection temperature can increase from about 60 °C to 190 °C when mixed with PDLA (poly-D-lactide). PDLA and PLLA form a highly ordered structure with higher crystallinity. Using a 1:1 blend of PDLA and PLLA maximizes temperature stability, but even small amounts of PDLA (3–10%) improve properties. In these cases, PDLA acts as a nucleating agent, speeding up crystallization. PDLA degrades more slowly than regular PLA due to its higher crystallinity. The stiffness (flexural modulus) of PLA is greater than polystyrene, and PLA has good heat sealability.

PLA has mechanical properties similar to PET for tensile strength and stiffness, but it is very brittle, with less than 10% elongation at break. This brittleness limits its use in applications requiring plastic deformation under stress. Efforts to improve elongation at break are ongoing to increase PLA’s use as a commodity plastic. For example, mixing PLLA with materials like PHB, cellulose nanocrystals (CNC), and a plasticizer (TBC) significantly improves mechanical properties. Using polarized optical microscopy (POM), smaller spherulites (crystal structures) were observed in the biocomposites compared to pure PLLA, indicating better nucleation and a much higher elongation at break (from 6% to 140–190%). These biocomposites are promising for food packaging due to their improved strength and biodegradability.

Techniques such as annealing, adding nucleating agents, creating composites with fibers or nanoparticles, extending polymer chains, and introducing crosslinks have been used to improve PLA’s mechanical properties. Annealing increases the crystallinity of PLA. In one study, longer annealing times affected thermal conductivity, density, and the glass transition temperature. This treatment also improved compressive strength and rigidity by nearly 80%. Adding a cross-linked nucleating agent to PLA increases crystallinity, and combining this with annealing further improves toughness and stiffness. These methods show how combining techniques can enhance PLA’s properties. PLA can be processed into fibers and films like other thermoplastics. It has mechanical properties similar to PETE polymer but a lower maximum continuous use temperature.

The structure of PLA’s backbone and its effect on crystallization has been studied to determine the best processing conditions. The molecular weight of polymer chains influences mechanical properties. Introducing branches on the polymer chain increases molecular weight, leading to faster crystallization. Branched PLA has longer relaxation times at low shear rates, resulting in higher viscosity than linear PLA. However, branched PLA shows stronger shear thinning, reducing viscosity at high shear rates. Understanding these properties helps determine optimal processing conditions, as small structural changes can greatly affect material behavior.

Racemic PLA and pure PLLA have low glass transition temperatures, making them less desirable due to weak strength and low melting points. A stereocomplex of PDLA and PLLA has a higher glass transition temperature, providing greater mechanical strength.

PLA has high surface energy, which makes it easy to print, a reason it is widely used in 3D printing. The tensile strength of 3D-printed PLA has been measured.

PLA dissolves in various organic solvents. Ethyl acetate is commonly used because it is accessible and safe. It is useful in 3D printing for cleaning extruder heads and removing supports. Other "green solvents" include propylene carbonate. Pyridine can be used but has a strong fishy odor and is less safe than ethyl acetate. PLA also dissolves in hot benzene, tetrahydrofuran, and dioxane.

Fabrication

PLA objects can be made using 3D printing, casting, injection molding, extrusion, machining, and solvent welding.

PLA is used as a material in desktop 3D printers, such as RepRap printers, which create objects by melting and layering the material.

PLA can be joined using a liquid called dichloromethane. Acetone also makes the surface of PLA sticky without dissolving it, which helps attach two PLA pieces together.

PLA-printed objects can be covered with materials similar to plaster. When heated in a furnace, the PLA burns away, leaving a hollow space that can be filled with melted metal. This process is called "lost PLA casting," a type of investment casting.

Applications

PLA is mainly used for short-lived and disposable packaging. In 2022, about 35% of total PLA production was used for flexible packaging, such as films, bags, and labels. About 30% was used for rigid packaging, such as bottles, jars, and containers.

PLA is used in many consumer products, including disposable tableware, cutlery, housings for kitchen appliances and electronics like laptops and handheld devices, and microwavable trays. However, PLA is not suitable for microwavable containers because of its low glass transition temperature. It is used for compost bags, food packaging, and loose-fill packaging made by casting, injection molding, or spinning. In the form of a film, PLA shrinks when heated, allowing it to be used in shrink tunnels. In the form of fibers, it is used for monofilament fishing line and netting. In the form of nonwoven fabrics, it is used for upholstery, disposable garments, awnings, feminine hygiene products, and diapers.

PLA has applications in engineering plastics, where the stereocomplex is blended with a rubber-like polymer such as ABS. These blends have good form stability and visual transparency, making them useful in low-end packaging applications.

PLA is used for automotive parts such as floor mats, panels, and covers. Its heat resistance and durability are less than those of widely used polypropylene (PP), but its properties improve when the end groups are capped to reduce hydrolysis.

PLA was once used in some Sun Chips bags from 2008 through 2010, though these were discontinued because the bags made a loud noise, around 95 dB, when handled.

PLA is also one of the most common filaments used in 3D printing.

In the form of fibers, PLA is used for monofilament fishing line and netting for vegetation and weed prevention. It is used for sandbags, planting pots, binding tape, and ropes.

PLA can degrade into harmless lactic acid, making it suitable for use as medical implants in the form of anchors, screws, plates, pins, rods, mesh, and surgical sutures. Depending on the type used, it breaks down inside the body within 6 months to 2 years. This gradual degradation is useful for support structures because it transfers the load to the body as the area heals. The strength of PLA and PLLA implants and surgical sutures is well documented.

Because of its bio-compatibility and biodegradability, PLA is used as a polymeric scaffold for drug delivery purposes.

The composite blend of poly(L-lactide-co-D,L-lactide) (PLDLLA) with tricalcium phosphate (TCP) is used as PLDLLA/TCP scaffolds for bone engineering.

Poly-L-lactic acid (PLLA) is the active ingredient in Sculptra, an injectable filler used for adding volume to the face, especially for HIV-associated lipodystrophy.

PLLA stimulates collagen synthesis in fibroblasts through a foreign body reaction involving macrophages. Macrophages help release cytokines and mediators such as TGF-β, which encourage fibroblasts to secrete collagen into surrounding tissue. This makes PLLA useful for dermatological studies.

PLLA is being studied as a scaffold that can generate a small amount of electric current through the piezoelectric effect, which may stimulate the growth of strong cartilage in multiple animal models.

Examples of PLA uses include:
– Mulch film made from a PLA-blend called "bio-flex"
– Biodegradable PLA cups
– Tea bags made of PLA. Peppermint tea is enclosed.
– 3D printing of a microcoil using a conductive mixture of polylactide and carbon nanotubes
– 3D printed human skull using data from computed tomography. Transparent PLA.

Degradation

PLA is compostable. How quickly it breaks down depends on its specific form. In medicine, PLA mainly breaks down through a process called hydrolysis, which uses water. The result is harmless lactic acid. In the human body, complete breakdown takes between 6 months and 2 years. PLA also breaks down in seawater, but this is not fully understood. It breaks down slowly in landfills and home composts but breaks down more quickly in industrial composts that are very hot, usually above 60°C (140°F).

Pure PLA foams break down in a solution called Dulbecco's modified Eagle's medium (DMEM) mixed with fetal bovine serum (FBS), which is similar to body fluids. After 30 days in this solution, a type of PLA called PLLA lost about 20% of its weight.

PLA samples with different molecular weights were broken down into methyl lactate, a type of green solvent, using a special catalyst.

Some bacteria, such as Amycolatopsis and Saccharothrix, can break down PLA. A specific enzyme from Amycolatopsis sp., called PLA depolymerase, also breaks down PLA. Enzymes like pronase and proteinase K from Tritirachium album can break down PLA, with proteinase K working most effectively.

Environmental Impacts

PLA is a type of polymer made from natural materials, but calling it a bioplastic can be confusing because it does not always break down easily in the environment. While PLA can sometimes break down using enzymes, its main way of breaking down is through heat, which requires a temperature of at least 60°C. Other factors, like moisture and oxygen, also affect this process. Full breakdown of PLA usually needs conditions found only in industrial composting facilities. In environments like home compost piles or soil, it may take nearly 30 years for PLA to fully biodegrade.

Tests in real environments show that PLA does not completely break apart in marine settings, such as at the ocean surface or seafloor, even after 231 and 196 days, respectively. It also did not fully break down after 428 days in a seawater aquarium. PLA breaks down in the ocean at similar rates to polyethylene terephthalate (PET), a plastic made from petroleum used for similar purposes. When PLA enters the ocean, it can create microplastics through physical breaking and sunlight exposure.

PLA has shown different levels of harm to marine life. While LC50 values (a measure of toxicity) have not been calculated for most species, existing values are not typically relevant to real-world conditions. However, PLA has caused noticeable short-term and long-term effects on many marine organisms, including amphipods, sea urchin larvae, zebrafish, and mussels. These effects include cell damage from oxygen and changes in behavior and body shape in zebrafish, as well as reduced gene activity in mussels related to attaching to surfaces. Studies suggest that older PLA, which has been physically worn down or exposed to sunlight, is more harmful to marine life. This may be due to harmful chemicals produced during breakdown or the creation of tiny plastic particles.

End of life

Four common end-of-life options for materials are:

• Recycling: This can be done through chemical or mechanical methods. Currently, a code called 7 ("others") is used for identifying polylactic acid (PLA). In Belgium, a company named Galactic began testing a chemical recycling process for PLA called Loopla. Unlike mechanical recycling, which may leave waste with impurities, chemical recycling breaks down PLA into its basic building blocks through processes like heating or water-based reactions. These building blocks can be purified and used to make new PLA without losing its original qualities (called cradle-to-cradle recycling). Another chemical process called transesterification can turn end-of-life PLA into a substance called methyl lactate.

• Composting: PLA can break down in industrial composting facilities. This process starts with a chemical reaction called hydrolysis, followed by the action of microorganisms that fully decompose the material. At high temperatures (about 58°C or 136°F), PLA can decompose into water and carbon dioxide in about 60 days. After this, the remaining material breaks down much more slowly, depending on how crystalline the PLA is. In environments without proper composting conditions, PLA decomposes very slowly, similar to traditional plastics, taking hundreds or thousands of years to fully break down.

• Incineration: PLA can be burned without producing harmful chemicals like dioxins or hydrochloric acid because it is made only of carbon, oxygen, and hydrogen. Burning PLA releases carbon dioxide but leaves no toxic residue. This suggests that burning PLA is an environmentally safe way to dispose of it.

• Landfill: Landfilling is the least preferred option because PLA breaks down very slowly at normal temperatures, often as slowly as other types of plastic.