Polylactic acid

Polylactic acid, or polylactide (PLA) is a thermoplastic polyester with backbone formula (C
3H
4O
2)
n or [–C(CH
3)HC(=O)O–]
n, formally obtained by condensation of lactic acid C(CH
3)(OH)HCOOH with loss of water (hence its name). It can also be prepared by ring-opening polymerization of lactide [–C(CH
3)HC(=O)O–]
2, the cyclic dimer of the basic repeating unit.

Polylactic acid
Identifiers
ChemSpider
  • None
Properties
Density 1.210–1.430 g·cm−3[1]
Melting point 150 to 160 °C (302 to 320 °F; 423 to 433 K)[1]
0 mg/ml [2]
Hazards
NFPA 704 (fire diamond)
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilHealth code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
0
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

PLA has become a popular material due to it being economically produced from renewable resources. In 2010, PLA had the second highest consumption volume of any bioplastic of the world,[3] although it is still not a commodity polymer. Its widespread application has been hindered by numerous physical and processing shortcomings.[4] PLA is the most widely used plastic filament material in 3D printing.

The name "polylactic acid" does not comply with IUPAC standard nomenclature, and is potentially ambiguous or confusing, because PLA is not a polyacid (polyelectrolyte), but rather a polyester.[5]

Production

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

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 catalysts (typically tin octoate) 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).[6]

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.[6]

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.[7]

The direct biosynthesis of PLA similar to the poly(hydroxyalkanoate)s has been reported as well.[8]

Another method devised is by contacting lactic acid with a zeolite. This condensation reaction is a one-step process, and runs about 100 °C lower in temperature.[9][10]

Properties

Chemical properties

Due to the chiral nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide (PLLA) is the product resulting from polymerization of L,L-lactide (also known as L-lactide). PLA is soluble in solvents, hot benzene, tetrahydrofuran, and dioxane.[11]

Physical and mechanical properties

PLA polymers range from amorphous glassy polymer to semi-crystalline and highly crystalline polymer with a glass transition 60–65 °C, a melting temperature 130-180 °C, and a tensile modulus 2.7–16 GPa.[12][13][14] Heat-resistant PLA can withstand temperatures of 110 °C.[15] The basic mechanical properties of PLA are between those of polystyrene and PET.[12] The melting temperature of PLLA can be increased by 40–50 °C and its heat deflection temperature can be increased from approximately 60 °C to up to 190 °C by physically blending the polymer with PDLA (poly-D-lactide). PDLA and PLLA form a highly regular stereocomplex with increased crystallinity. The temperature stability is maximised when a 1:1 blend is used, but even at lower concentrations of 3–10% of PDLA, there is still a substantial improvement. In the latter case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Biodegradation of PDLA is slower than for PLA due to the higher crystallinity of PDLA. The flexural modulus of PLA is higher than polystyrene and PLA has good heat sealability.

Several technologies such as annealing,[16][17][18] adding nucleating agents, forming composites with fibers or nano-particles,[19][20][21] chain extending[22][23] and introducing crosslink structures have been used to enhance the mechanical properties of PLA polymers. 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.[24] With high surface energy, PLA has easy printability which makes it widely used in 3-D printing. The tensile strength for 3-D printed PLA was previously determined.[25]

There is also poly(L-lactide-co-D,L-lactide) (PLDLLA) – used as PLDLLA/TCP scaffolds for bone engineering.[26][27]

Solvent welding

PLA can be solvent welded using dichloromethane.[28] Acetone also softens the surface of PLA, making it sticky without dissolving it, for welding to another PLA surface.[29]

Organic solvents for PLA

PLA is soluble in a range of organic solvents.[30] Ethylacetate, due to its ease of access and low risk of use, is of most interest. PLA 3D printer filament dissolves when soaked in ethylacetate, making it a useful solvent for cleaning 3D printing extruder heads or removing PLA supports. The boiling point of ethylacetate is low enough to also smooth PLA in a vapor chamber, similar to using acetone vapor to smooth ABS. Other safe solvents to use include propylene carbonate, which is safer than ethylacetate but is difficult to purchase commercially. Pyridine can also be used however this is less safe than ethylacetate and propylene carbonate. It also has a distinct bad fish odor.

Applications

PLA is used as a feedstock material in desktop fused filament fabrication 3D printers (e.g. RepRap).[31][32] 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.[33]

PLA can degrade into innocuous lactic acid, so it is used as medical implants in the form of anchors, screws, plates, pins, rods, and as a mesh.[34] Depending on the exact type used, it breaks down inside the body within 6 months to 2 years. This gradual degradation is desirable for a support structure, because it gradually transfers the load to the body (e.g. the bone) as that area heals. The strength characteristics of PLA and PLLA implants are well documented.[35]

PLA can also be used as a decomposable packaging material, either cast, injection-molded, or spun.[34] Cups and bags have been made from this material. In the form of a film, it shrinks upon heating, allowing it to be used in shrink tunnels. It is useful for producing loose-fill packaging, compost bags, food packaging, and disposable tableware. In the form of fibers and nonwoven fabrics, PLA also has many potential uses, for example as upholstery, disposable garments, awnings, feminine hygiene products, and diapers. Thanks to its bio-compatibility and biodegradability, PLA has also found ample interest as a polymeric scaffold for drug delivery purposes.

Racemic and regular PLLA has a low glass transition temperature, which is undesirable. A stereocomplex of PDLA and PLLA has a higher glass transition temperatures, lending it more mechanical strength.[36] It has a wide range of applications, such as woven shirts (ironability), microwavable trays, hot-fill applications and even engineering plastics (in this case, the stereocomplex is blended with a rubber-like polymer such as ABS). Such blends also have good form stability and visual transparency, making them useful for low-end packaging applications. Pure poly-L-lactic acid (PLLA), on the other hand, is the main ingredient in Sculptra, a long-lasting facial volume enhancer, primarily used for treating lipoatrophy of cheeks. Progress in biotechnology has resulted in the development of commercial production of the D enantiomer form, something that was not possible until recently.[37]

  • Mulch film made of PLA-blend "bio-flex"
  • Biodegradable PLA cups
  • Tea bags made of PLA. Peppermint tea is enclosed.
  • 3D printed human skull with data from computed tomography. Transparent PLA.

Degradation

PLA is degraded abiotically by three mechanisms:[39]

  1. Hydrolysis: The ester groups of the main chain are cleaved, thus reducing molecular weight.
  2. Thermal degradation: A complex phenomenon leading to the appearance of different compounds such as lighter molecules and linear and cyclic oligomers with different Mw, and lactide.
  3. Photodegradation: UV radiation induces degradation. This is a factor mainly where PLA is exposed to sunlight in its applications in plasticulture, packaging containers and films.

The hydrolytic reaction is:

The degradation rate is very slow in ambient temperatures. A 2017 study found that at 25 °C in seawater, PLA showed no degradation over a year.[40] As a result, it is poorly degraded in landfills and household composts, but is effectively digested in hotter industrial composts.

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.[41]

PLA samples of various molecular weights were degraded into methyl lactate (a green solvent) by using a metal complex catalyst.[42][43][44]

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.[45]

End of life
PLA has SPI resin ID code 7

Four possible end of life scenarios are the most common:

  1. Recycling: which can be either chemical or mechanical. Currently, the SPI resin identification code 7 ("others") is applicable for PLA. In Belgium, Galactic started the first pilot unit to chemically recycle PLA (Loopla) . Unlike mechanical recycling, waste material can hold various contaminants. Polylactic acid can be chemically recycled to monomer by thermal depolymerization or hydrolysis. When purified, the monomer can be used for the manufacturing of virgin PLA with no loss of original properties [46] (cradle-to-cradle recycling). End-of-life PLA can be chemically recycled to methyl lactate by transesterification.[44]
  2. Composting: PLA is biodegradable under industrial composting conditions, starting with chemical hydrolysis process, followed by the microbial digestion, to ultimately degrade the PLA.
  3. Incineration: PLA can be incinerated, leaving no residue and producing 19.5 MJ/kg (8,368 btu/lb) of energy .
  4. Landfill: the least preferable option is landfilling because PLA degrades very slowly in ambient temperatures.

See also

References
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