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Polylactic acid, also known as poly( lactic acid) or polylactide ( PLA), is a plastic material. As a thermoplastic polyester (or polyhydroxyalkanoate) it has the backbone formula or . PLA is formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by ring-opening polymerization of lactide , the cyclic dimer of the basic repeating unit. Often PLA is blended with other polymers. PLA can be biodegradable or long-lasting, depending on the manufacturing process, additives and Copolymer.
PLA has become a popular material due to it being economically produced from renewable resources and the possibility to use it for Compost products. In 2022, PLA had the highest consumption volume of any bioplastic of the world, with a share of ca. 26 % of total bioplastic demand. Although its production is growing, PLA is still not as important as traditional commodity polymers like PET or PVC. Its widespread application has been hindered by numerous physical and processing shortcomings. PLA is the most widely used plastic filament material in FDM 3D printing, due to its low melting point, high strength, low thermal expansion, and good layer adhesion, although it possesses poor heat resistance unless annealed.
Although the name "polylactic acid" is widely used, it does not comply with IUPAC standard nomenclature, which is "poly(lactic acid)". The name "polylactic acid" is potentially ambiguous or confusing, because PLA is not a polyacid (polyelectrolyte), but rather a polyester.
Several industrial routes afford usable (i.e. high molecular weight) PLA. Two main monomers are used: lactic acid, and the cyclic di-ester, lactide. The most common route to PLA is the ring-opening polymerization of lactide with various metal (typically tin ethylhexanoate) in solution or as a suspension. The metal-catalyzed reaction tends to cause racemization of the PLA, reducing its stereoregularity compared to the starting material (usually corn starch). (2025). 9780470649848 ISBN 9780470649848
The direct condensation of lactic acid monomers can also be used to produce PLA. This process needs to be carried out at less than 200 °C; above that temperature, the entropically favored lactide monomer is generated. This reaction generates one equivalent of water for every condensation (esterification) step. The condensation reaction is reversible and subject to equilibrium, so removal of water is required to generate high molecular weight species. Water removal by application of a vacuum or by azeotropic distillation is required to drive the reaction toward polycondensation. Molecular weights of 130 kDa can be obtained this way. Even higher molecular weights can be attained by carefully crystallizing the crude polymer from the melt. Carboxylic acid and alcohol end groups are thus concentrated in the amorphous region of the solid polymer, and so they can react. Molecular weights of 128–152 kDa are obtainable thus.
Polymerization of a racemic mixture of L- and D-lactides usually leads to the synthesis of poly-DL-lactide ( PDLLA), which is amorphous. Use of stereospecific catalysts can lead to heterotactic PLA which has been found to show crystallinity. The degree of crystallinity, and hence many important properties, is largely controlled by the ratio of D to L enantiomers used, and to a lesser extent on the type of catalyst used. Apart from lactic acid and lactide, lactic acid O-carboxyanhydride ("lac-OCA"), a five-membered cyclic compound has been used academically as well. This compound is more reactive than lactide, because its polymerization is driven by the loss of one equivalent of carbon dioxide per equivalent of lactic acid. Water is not a co-product.
The direct biosynthesis of PLA, in a manner similar to production of poly(hydroxyalkanoate)s, has been reported.
Although PLA performs mechanically similar to PET for properties of tensile strength and elastic modulus, the material is very brittle and results in less than 10% elongation at break. Furthermore, this limits PLA's use in applications that require some level of plastic deformation at high stress levels. An effort to increase the elongation at break for PLA has been underway, especially to bolster PLA's presence as a commodity plastic and improve the bioplastics landscape. For example, PLLA biocomposites have been of interest to improve these mechanical properties. By mixing PLLA with poly (3-hydroxy butyrate) (PHB), cellulose nano crystal (CNC) and a plasticizer (TBC), a drastic improvement of mechanical properties were shown. Using polarized optical microscopy (POM), the PLLA biocomposites had smaller spherulites compared to pure PLLA, indicating improved nucleation density and also contributing to an increase of elongation at break from 6% in pure PLLA to 140-190% in the biocomposites. Biocomposites such as these are of great interest for food packaging because of their improved strength and biodegradability.
Several technologies such as annealing, adding Nucleation agents, forming composites with fibers or Nanoparticle, chain extending and introducing crosslink structures have been used to enhance the mechanical properties of PLA polymers. Annealing has been shown to significantly increase the degree of crystallinity of PLA polymers. In one study, increasing the duration of annealing directly affected thermal conductivity, density, and the glass transition temperature. Structural changes from this treatment further improved characteristics such as compressive strength and rigidity by nearly 80%. Processes such as this may boost PLA's presence in the plastics market, as improving the mechanical properties will be important to replace current petroleum-derived plastics. It has also been demonstrated that the addition of a PLA-based, cross-linked nucleating agent improved the degree of crystallinity of the final PLA material. Alongside the use of the nucleating agent, annealing was shown to further improve the degree of crystallinity and, therefore, the toughness and flexural modulus of the material. This example reveals the ability to utilize multiple of these processes to reinforce the mechanical properties of PLA. Polylactic acid can be processed like most thermoplastics into fiber (for example, using conventional melt spinning processes) and film. PLA has similar mechanical properties to PETE polymer, but has a significantly lower maximum continuous use temperature.
Backbone architecture of PLA and its effect on crystallization kinetics has also been investigated, specifically to better understand the most suitable processing conditions for PLA. The molecular weight of polymer chains can play a significant role in the mechanical properties. One method of increasing molecular weight is by introducing branches of the same polymer chain onto the backbone. Through characterization of a branched and linear grade PLA, branched PLA leads to faster crystallization. Furthermore, the branched PLA experiences much longer relaxation times at low shear rates, contributing to higher viscosity than the linear grade. This is presumed to be from high molecular weight regions within the branched PLA. However, the branched PLA was observed to shear thin more strongly, leading to a much lower viscosity at high shear rates. Understanding properties such as these are crucial when determining optimal processing conditions for materials, and that simple changes to the structure can alter its behavior dramatically.
Racemic PLA and pure PLLA have low glass transition temperatures, making them undesirable because of low strength and melting point. A stereocomplex of PDLA and PLLA has a higher glass transition temperature, lending it more mechanical strength.
The high surface energy of PLA results in good printability, making it widely used in 3D printing. The tensile strength for 3D printed PLA was previously determined.
Other "green solvents" include propylene carbonate. Pyridine can be used, but it has a distinct fish odor and is less safe than ethyl acetate. PLA is also soluble in hot benzene, tetrahydrofuran, and dioxane.
PLA is used as a feedstock material in desktop fused filament fabrication by 3D printers, such as RepRap printers.
PLA can be solvent welded using dichloromethane. Acetone also softens the surface of PLA, making it sticky without dissolving it, for welding to another PLA surface.
PLA-printed solids can be encased in plaster-like moulding materials, then burned out in a furnace, so that the resulting void can be filled with molten metal. This is known as "lost PLA casting", a type of investment casting.
PLA has applications in engineering plastics, where the stereocomplex is blended with a rubber-like polymer such as ABS. Such 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 inferior to the widely used polypropylene (PP), but its properties are improved by means such as capping of the end groups to reduce hydrolysis.
PLA is also one of the most common filaments used in 3D printing.
Thanks to its bio-compatibility and biodegradability, PLA found interest 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 main ingredient in Sculptra, a facial volume enhancer used for treating lipoatrophy of the cheeks.
PLLA is used to stimulate collagen synthesis in fibroblasts via foreign body reaction in the presence of macrophages. Macrophages act as a stimulant in secretion of cytokines and mediators such as TGF-β, which stimulate the fibroblast to secrete collagen into the surrounding tissue. Therefore, PLLA has potential applications in the dermatological studies.
PLLA is under investigation as a scaffold that can generate a small amount of electric current via the piezoelectric effect that stimulates the growth of mechanically robust cartilage in multiple animal models.
Pure PLA foams are selectively hydrolysed in Dulbecco's modified Eagle's medium (DMEM) supplemented with fetal bovine serum (FBS) (a solution mimicking body fluid). After 30 days of submersion in DMEM+FBS, a PLLA scaffold lost about 20% of its weight.
PLA samples of various molecular weights were degraded into methyl lactate (a green solvent) by using a metal complex catalyst.
PLA can also be degraded by some bacteria, such as Amycolatopsis and Saccharothrix. A purified protease from Amycolatopsis sp., PLA depolymerase, can also degrade PLA. Enzymes such as pronase and most effectively proteinase K from Tritirachium album degrade PLA.
In situ experiments have shown that PLA does not fully disintegrate under normal marine conditions at the sea surface or seafloor after 231 and 196 days, respectively, or after 428 days in an open-circuit natural seawater aquarium. PLA also exhibited similar rates of degradation in the ocean as polyethylene terephthalate (PET), a petroleum-based plastic used for similar applications. Once PLA is introduced to the marine environment, it can contribute to microplastic production through mechanical shearing and UV induced photodegradation.
PLA has been shown to have varying levels of toxicity to marine organisms. An appropriate LC50 has not been calculated for most species, but existing published values are generally not environmentally relevant. However, PLA has displayed significant acute and chronic non-lethal effects on many organisms across the food chain, including marine Amphipoda, sea urchin larvae, zebrafish, and mussels. Effects vary across organisms but notably include oxidative stress and differentiated behavior and morphology in zebrafish and suppressed gene expression related to Byssus in mussels. Many studies have shown that aged PLA (via mechanical weathering and photodegradation) has a significantly greater toxic effect on marine organisms likely due to toxic transformation products and the production of nanoplastics.
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