ATP-Funded Green Process Technologies - NIST Advanced ...

HOW DOES IT WORK?
Figure 3 illustrates the process of producing PLA from corn. Production starts with
corn growing, harvesting, and the separation of corn starch from other components
(gluten and fiber). With the addition of enzymes, the starch is converted into dextrose
or sugar. Using proprietary microbial cultures, dextrose is fermented into lactic acid.
Lactic acid is a chiral molecule with L-isomers and D-isomers. The chirality property
implies that the individual isomers, while identical in chemical composition, each
have a molecular structure that cannot be superimposed on a mirror-image isomer.
This is analogous to left and right hands not being superimposable.
Fermentation-derived lactic acid (in contrast to chemically synthesized lactic acid)
typically consists of predominantly L-isomers and low levels of D- isomers.
Lactic acid is then converted to a low-molecular-weight “pre-polymer” by
dehydration condensation. Continued heating leads to the formation of lactide, a
cyclic oligomer of lactic acid. Lactide is vacuum distilled to remove impurities and to
control the D-isomer content of product streams.
Ring opening polymerization of lactide streams completes the manufacturing process,
yielding different families of high-molecular-weight (longer-chain polymer) PLA resins
with varying L- and D-isomer content that influences crystallization and functional
properties.
The process yields resins in the form of small pellets. These are sold to “converters”
to be melted and shaped into thermoform and film products, injection molded
products, and woven and nonwoven textile goods. PLA resins are produced as part
of three families:
• High-crystallinity resin: Low D-isomer content for high-temperature fiber and film
applications
• Moderate-crystallinity resin: Medium D-isomer content, for thermoforms
(semirigid containers), bottles, and some fibers
• Amorphous-grade resin: High D-isomer content for room-temperature film and
foam applications
Technical Challenges and Risks of the ATP-Funded Project
The first objective of the ATP-funded project was to research the key physical and
chemical properties of PLA. Research would lead to fundamental knowledge that
could be used to improve heat resistance, stiffness, barrier properties, color stability,
and compostability. Alternative research strategies included:
20 ATP-FUNDED GREEN PROCESS TECHNOLOGIES