Polylactic acid
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From Wikipedia, the free encyclopedia
Polylactic acid
Identifiers
CAS Number 26100-51-6
ChemSpider None
CompTox DTXSID20904011
Dashboard (EPA)
Properties
Density 1210–1430 kg/m3[1]
Melting point 150 to 160 °C (302 to 320 °F; 423 to
433 K)[1]
Solubility in water 0 mg/ml [2]
Hazards
NFPA 704 (fire diamond)
0
1
0
Except where otherwise noted, data are given for materials in
their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Polylactic acid, also known as poly(lactic 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.
PLA has become a popular material due to it being economically produced
from renewable resources. In 2021, PLA had the 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 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.[5][6]
Although the name "polylactic acid" is widely used, it does not comply
with IUPAC standard nomenclature, which is "poly(lactic acid)".[7] The name "polylactic
acid" is potentially ambiguous or confusing, because PLA is not a polyacid
(polyelectrolyte), but rather a polyester.[8]
Chemical properties[edit]
Synthesis[edit]
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).[9]
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.
[9]
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.[10][11]
