Environmental Engineering Reference
In-Depth Information
5.7.1
Production of PLA
The structures of glycolic acid and lactic acid with their respective homopolymers
are shown in Figure 5.7. Both glycolic acid and lactic acid are produced naturally.
Glycolic acid is commonly produced from fossil feedstocks; however, it can be
recovered directly from biowaste with high sugar content. Currently, research into
an economically feasible, fermentation-style production of glycolic acid is ongo-
ing [196]. Lactic acid is produced by fermentation. This section describes the
production of lactic acid, the process to produce lactide, and the polymerization of
lactide to PLA, which is analogous to the production of PGA.
Lactic acid is typically produced via bacterial fermentation using carbohydrate
sources such as corn cobs and stalks, lignocellulose/hemicellulose hydrolyzates,
cassava bagasse, and other industrial bio-by-product streams [189]. The most
widely used bacteria are
Lactobacilli
with several common strains:
L. amylophi-
lus
,
L. bavaricus
,
L. casei
,
L. delbrueckii
,
L. jensenii
, and
L. acidophilus
[195].
Lactic acid comes in two optically active configurations - L and D isomers - and
specific bacteria will predominately produce one of the two stereoisomers. l-lactic
acid is produced by
L. amylophilus
,
L. bavaricus
, or
L. casei;
d-lactic acid or a
mixture of l- and d-lactic acid is produced by
L. delbrueckii
,
L. jensenii
, or
L.
acidophilus
[190]. After fermentation, lactic acid is separated by a neutralization
of the fermentation bath, typically with a basic salt. The soluble lactic acid solution
H
O
O
H
3
C
O
O
OH
O
n
HO
HO
OH
O
O
O
L-Lactic acid
PLLA
O
O
CH
3
H
O
O
OH
O
HO
n
OH
HO
O
O
O
D-Lactic acid
PDLA
O
O
OH
O
O
HO
O
HO
n
OH
PGA
O
Glycolic acid
O
O
Figure 5.7
Molecular structures for
l
-lactic acid,
d
-lactic acid, and glycolic acid and the repre-
sentative polymer backbone for poly(
l
-lactic acid) (PLLA), poly(
d
-lactic acid) (PDLA), and PGA.
