Environmental Engineering Reference
In-Depth Information
the reaction temperature is raised and the pressure reduced. This initiates a depo-
lymerization process that results in lactide, which in turn is evaporated
in situ
[194]. Lactide, like lactic acid, comes in different stereoisomeric forms: l- and
d-lactides derived from homo lactic acids and rac-lactide (meso-lactide) derived
from a combination of l- and d-lactic acids. Ring-opening polymerization of
lactide is possible via cationic, anionic, or coordination-insertion mechanisms;
the most common approach uses stannous octoate as catalyst with a coordination-
insertion mechanism [190].
The production of PGA is analogous to that of PLA; here, glycolic acid is used
first to produce glycolide, which then undergoes ROP to produce PGA. In fact, the
systems are similar enough to allow the copolymerization of glycolide and lactide
to produce copolymers with properties between those of the homopolymers.
In addition, the monomers are often copolymerized in ROP systems with
ε-caprolactone, a fossil-based biodegradable monomer [190].
5.7.2
Properties of PLA
The properties of PLA match those of other thermoplastic polyesters. Homopolymers
consisting of 93% or more l-lactide are semi-crystalline, potentially reaching 40%
crystallinity [190]. When the polymer composition contains between 50 and 93%
l-lactide, the system is amorphous [190]. The melt transition of the homopolymer
is between 130 and 180°C and glass transition is typically observed between 50 and
80°C [190]. A racemic mixture of poly(l-lactide) and poly(d-lactide) produces
stereo-complexation that can increase the mechanical and thermal properties of the
material; in this case the
T
m
can be as high as 230°C [190].
Because one of the most important applications of PLA is in flexible packag-
ing, the physical and barrier properties of the material have been extensively
studied [192]. PLA films exhibit carbon dioxide permeability of 1.76 × 10
-
17
kg m m
-2
s
-1
Pa
-1
, which ranges between that of polystyrene (PS) and PET [197].
Both PET and PLA are hydrophobic, that is, they absorb very little water and have
similar low water vapor permeability coefficients [192].
The mechanical properties of PLA depend on a number of parameters, includ-
ing crystallinity, polymer structure, molecular weight, and processing, which can
be tuned to meet the requirements of a given application. PLA is a brittle polymer,
with a typical elastic modulus of 2.1 GPa and elongation at break of 9% [192, 194,
198]. Plasticization, mixing, or copolymerization of lactide can be used to increase
elasticity of the resulting polymer. The importance of the production method was
demonstrated by a study where fiber tensile modulus was increased from between
6.5 and 9.3 GPa to between 9.6 and 16 GPa by changing from a melt spinning
system to a solution spinning system for poly(l-lactide) [194].
Degradation properties are important defining qualities of both PLA and PGA.
Both polymers undergo thermal and hydrolytic degradation. PLA begins to
decompose between 230 and 260°C [190]. The hydrolytic degradation of PLA
